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Is the Electrophilicity of the Metal Nitrene the Sole Predictor of

Metal-Mediated Nitrene Transfer to Oleﬁns? Secondary

Contributing Factors as Revealed by a Library of High-Spin Co(II)

Reagents

Anshika Kalra, Vivek Bagchi, Patrina Paraskevopoulou, Purak Das, Lin Ai, Yiannis Sanakis,

Grigorios Raptopoulos, Sudip Mohapatra, Amitava Choudhury, Zhicheng Sun, Thomas R. Cundari,*

and Pericles Stavropoulos *

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ABSTRACT: Recent research has highlighted the key role played by the

electron aﬃnity of the active metal-nitrene/imido oxidant as the driving

force in nitrene additions to oleﬁns to aﬀord valuable aziridines. The

present work showcases a library of Co(II) reagents that, unlike the

previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity

trends in oleﬁn aziridinations that cannot be solely explained by the

electron aﬃnity criterion. A family of Co(II) catalysts (17 members) has

been synthesized with the assistance of a trisphenylamido-amine scaﬀold

decorated by various alkyl, aryl, and acyl groups attached to the equatorial

amidos. Single-crystal X-ray diﬀraction analysis, cyclic voltammetry and

EPR data reveal that the high-spin Co(II) sites (S = 3/2) feature a minimal

[N₃N] coordination and span a range of 1.4 V in redox potentials.

Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates

reactivity patterns that deviate from those anticipated by the relevant

electrophilicities of the putative metal nitrenes. The representative L⁴Co catalyst (−COCMe₃arm) is operating faster than the L⁸Co

analogue (−COCF₃arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol

studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C_batom

of styrene, followed by product-determining ring-closure), the L⁴Co catalyst is associated with lower energy barriers in both steps.

DFT calculations indicate that the putative [L⁴Co]NTs and [L⁸Co]NTs species are electronically distinct, inasmuch as the former

exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for

L⁴Co (due to the polarizability of the tert-Bu substituent) can provide signiﬁcant stabilization of the turnover-limiting transition

state. This study highlights how small ligand modiﬁcations can generate stereoelectronic variants that in certain cases are even

capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

Synthetic protocols for the generation of aziridines abound,

but largely rely on three major methodologies. The cyclization of

1,2-amino precursors constitutes a traditional approach that has

been more recently complemented by addition of either C₁

sources to imines or electrophilic N₁donors to alkenes.⁶The

latter “C₂+ N₁” addition is extensively implemented due to its

operational simplicity and availability of a wide range of suitable

INTRODUCTION

■

The role of aziridines¹as intermediates and end products of

synthetic and biological chemistry is hard to overstate. Not only

do aziridines aﬀord avenues for further structural development

by taking advantage of the energetic content and stereochemical

disposition of their strained three-atom ring (ring opening,

expansion, or rearrangement),²but also they constitute valuable

functionalities in the framework of several natural products

possessing antibiotic or antineoplastic activities.³In addition to

the central role exercised by aziridines as ﬁne chemicals and

pharmaceutical agents,⁴their contribution to the chemistry of

materials has been increasingly recognized,⁵especially as key

entities for the development and postmodiﬁcation of polymeric

scaﬀolds.

Received: May 1, 2021

Published: June 4, 2021

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substrates and catalysts. The N₁donors encompass a variety of

nitrene/nitrenoid precursor oxidants such as iminoiodanes

(ArINR),⁷haloamines (RNNaX, X = Cl, Br),⁸O/N-

substituted hydroxylamines and N-tosyloxycarbamates (RN-

(X)−OR′, X = H, leaving group)⁹or atom-economical organic

azides (RN₃).¹⁰As opposed to oxo-transfer chemistry, the

corresponding nitrene/nitrenoid transfer relies signiﬁcantly on

the choice of the attendant R group to control the electro-

philicity of the active moiety and provide activated (R = SO₂R,

CO₂R, COR, carbamoyl, sulfamoyl) or nonactivated aziridines

(R = H, alkyl, aryl, silyl) with diﬀerential reactivity.¹¹

A wide range of catalysts has been explored to inﬂuence

reactivity and selectivity outcomes in nitrene transfer to alkenes,

including several organocatalytic¹²and metal-mediated pro-

cesses.¹³In the latter case, the presumptive and rather elusive

metal-nitrene (MNR) active species are entities with rich and

variable stereoelectronic attributes, inherent and/or ligand

Cobalt(II) imidos are more recent additions to the repertoire

of cobalt reagents, and encompass both high-spin (S = 3/2)³⁶

and low-spin (S = 1/2)³⁷cases as two- and four-coordinate

compounds, respectively. The high-spin examples have been

reported to perform nitrene-transfer to ethylene to aﬀord RN

CHCH₃, presumably due to a [2π + 2π] activation mode.

Similarly, certain C(sp)−H and Si−H bonds are activated not

via H atom abstraction, but by means of [2π + 2σ]

interactions.^36bOn the other hand, the low-spin Co(II) imidos

are unreactive versus alkenes, although they engage in nitrene-

transfer and/or nitrene-exchange with O/S with respect to

substrates such as CO, PMe₃, PhCHO, and CS₂.³⁷Finally, two

examples of high-valent Co(IV) and Co(V) bis-imido

complexes ([IMes]Co(NDipp)₂]^0/+), possessing low-spin

ground states of S = 1/2 and 0, respectively, proved to be

rather unreactive.³⁸The open-shell Co(IV) congener is the only

one that exhibits intramolecular nitrene C−H insertion into the

o-Me group of the Mes residue, possibly via an ortho-cobaltation

intermediate (Co−C).²⁹

Whereas the catalytic formation of new C−N bonds by means

of the isolable cobalt imidos noted above is only rarely observed,

the advent of a library of Co^II(Por) complexes that give rise to

Co^III−nitrenoid radicals [(Por)Co^III−^•NR] or [(Por^•)Co^III−

(^•NR)₂] has provided numerous instances of highly eﬀective

catalytic systems for the stereo-, chemo-, and site-selective

aziridination of alkenes and amination of C−H bonds.¹⁹Starting

with Co(TPP), and electron-deﬁcient analogues, several

generations of Co^II(Por) reagents with richly decorated

porphyrins have been introduced in the past two decades to

facilitate the activation of various organic azides, leading to the

generation of well characterized low-spin (S = 1/2) Co^III−^•NR

moieties, with spin density largely localized on the N atom.^19f,39

These relatively long-lived Co^III−nitrene-radical intermediates

owe their stability to hydrogen-bonding interactions of the

nitrene moieties with porphyrin-appended amido residues

(−NHCOR*), which can further introduce and metal-orient

chiral auxiliaries via their R* functionality in D₂-symmetric

overall geometries. Detailed theoretical and experimental

studies^19f,39have established that the mode of operation of

̀

induced, whose operation vis-a-vis oleﬁnic substrates is a matter

of intense investigation. The variety of transition metals

employed, both from the ﬁrst-row (Mn, Fe, Co, Ni, Cu)¹⁴⁻²²

and from the heavier platinum-group²³⁻²⁵and coinage

elements,^26,27coupled with a range of ancillary ligand frame-

works (e.g., porphyrinoids, salens, bis-oxazolines, tetracarbox-

ylate paddlewheels, trispyrazolyl-borates/methanes, polypyr-

idines) is a testament to the vigorous activity in this ﬁeld and that

of the closely related C−H bond amination reactions.²⁸

Among the late 3d transition elements, the case of cobalt is

most intriguing, inasmuch as isolable or even putative CoNR

units have been invoked with a variety of oxidation states (from

II to V), electronic ground-state spins (S = 0, 1/2, 1, 3/2, 2), and

coordination numbers (from 2 to 5).²⁹The most common

conﬁguration is that of diamagnetic Co(III) imidos (S = 0),³⁰

mostly supported by C₃or C₂symmetric ligands. In a handful of

cases, open-shell spin-states were observed for Co(III) imidos,

as for instance with (trispyrazolylborato)Co^III(NAd) (S = 1, at T

> 280 K),³¹(dipyrrin)Co^III(NR) (S = 1 for R = Mes; S = 0 or 0

→ 2 transition, for R = ^tBu, 1-Ad, other alkyls),³²

[(hmds)₂Co^III(N^tBu)]⁻(S = 1; hmds = N(SiMe₃)₂),³³and

possibly bimetallic Zr(μ-NMes)Co^III(NMes) (S = 0 → 2

transition, near room temperature).³⁴None of these com-

pounds have been reported to mediate nitrene-transfer to

alkenes. Observable reactivity includes (i) nitrene-transfer to

carbon monoxide;^30g,31a,35(ii) insertion of nitrene into ligand-

derived carbene residues;^30e(iii) formal hydrogen-atom

III

•

̀

[(Por)Co − NR] metalloradicals vis-a-vis CC or C−H

bonds consists of a two-step process: initial formation of a new

N−C bond with alkenes and relocation of the spin density on the

distal carbon atom (Co^III−N(R)−C−^•C−) (or formation of a

Co^III−NHR amido and a substrate-bound radical via hydrogen-

atom abstraction from a C−H bond), followed by an essentially

barrierless collapse of the carbon-centered radical with the N

atom to generate the product of aziridination (or amination)

along with Co^II(Por).

t

abstraction from a Bu or Mes ligand moiety by open-shell

CoNR, presumably generating an amido Co−NHR unit and a

carbon centered radical; the latter can then recombine with the

amido,^31adimerize,^31bor generate a Co−C bond;^31b,32a(iv)

intramolecular C−H bond insertion into alkyl azides (source of

imido), mediated by (dipyrrin)Co^III(NR), to generate sub-

stituted N-heterocycles;^32b,cand (v) a rare instance of

intermolecular hydrogen-atom abstraction from C−H bonds

of various substrates with BDE_C−H≤ 92 kcal mol⁻¹by

[(hmds)₂Co^III(N^tBu)]⁻,³³leading to the corresponding Co(II)

amido; the amido can then react with another equivalent of

substrate (C−H) to perform either proton transfer (frequently

with the concomitant formation of Co^II−C organometallics) or

formal hydrogen-atom abstraction via stepwise proton/electron

transfer or direct HAT, giving rise to Co(I) and substrate

dehydrogenation product. In several instances noted above, the

carbophilic character of cobalt is notable as a product-

determining factor.

More recently, the structurally related [Co^III(TAML^red)]⁻

and [Co^III(TAML^sq)] compounds, featuring the tetraamido

macrocyclic ligand TAML in its intact reduced form TAML^red

and one-electron oxidized variant TAML^sq(sometimes denoted

as TAML^+•), have been shown to give rise to [Co^III(TAML^q)-

(^•NR)₂]⁻(S = 1) and [Co^III(TAML^q)(^•NR)] (S = 1/2),

respectively (TAML^q= doubly oxidized, diamagnetic ligand;

Co^IIIsite is low-spin, S = 0).⁴⁰These cobalt nitrenes have

emerged as capable catalysts for the aziridination of largely

styrene substrates by imidoiodinanes (PhINNs, PhINTs,

PhINTces).⁴¹Their mode of operation is considered to be

unique, inasmuch as the turnover-limiting, initial N−C bond

formation with styrenes features an asynchronous transition

state, encompassing a partial electron-transfer to form a styrenyl

radical cation, in turn undergoing a nucleophilic attack by the

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Figure 1. Ligands employed in the present study.

nitrene lone pair (see below for more details). This initial

charge-transfer has also emerged as a central component of the

operation of iron(IV) imido species developed by Latour and

co-workers for alkene aziridinations, further underscoring the

importance of the electron aﬃnity of the metal nitrene as a

commonly encountered driving force.⁴²

bond-forming step of the high-spin system is very similar to that

reported for the corresponding low-spin Co(Por) (ΔG^‡= 24.1

kcal mol⁻¹) or Co(AmidoPor) (ΔG^‡= 22.8 kcal mol⁻¹) with

respect to Co^III−^•NSO₂Ph/styrene.^19fThe present work

signiﬁcantly enlarges the scope of high-spin Co(II) compounds

as nitrene-transfer reagents, and provides insights in their

̀

operational characteristics, not only vis-a-vis the reported low-

Finally, of great interest are recently reported Co(II)

organoazides,⁴³which either thermally or photolytically can

extrude N₂to give rise to nitrenoids best described as iminyl

[Co^III−^•NR] units (R = aryl, alkyl). Although the electronics of

these ﬂeeting intermediates are not yet known, crystal structures

of such species (R = alkyl) have been determined following N₂

expulsion from single crystals of metal azides in the solid state.

For R = aryl, the cobalt(II) organoazide can promote nitrene-

transfer by means of unisolable [Co^III−^•NR], both intra-

molecularly ([3 + 2] annulation) and intermolecularly (C−H

allylic amination or styrene aziridination in modest yields). The

reactivity of the Co(II) aliphatic azides is more complex and

includes (i) α-H atom abstraction via the incipient

[Co^III−^•NCH₂R] to generate the imine (RCHNH), if strong

δ-C−H bonds (sec, prim) are present; (ii) δ-H atom abstraction

and amination of relatively weaker δ-C−H bonds (benzylic,

tertiary) by the cobalt alkyl azide itself (initial N₂extrusion is not

needed), leading to substituted pyrrolidines; and (iii) intra-

molecular 1,3-dipolar cycloaddition of cobalt-bound CH₂

CH(CH₂)₄N₃to aﬀord 1,2,3-dihydrotriazole.

In the present work, we examine a library of high-spin Co(II)

reagents (S = 3/2), supported by a modular trisphenylamido-

amine ligand framework, giving rise to a weak equatorial ﬁeld.

Previous DFT calculations⁴⁴on one member of this library of

reagents indicated that exposure to a nitrene source (PhI = NTs)

generates a Co(III)−nitrene radical (Co^III−^•NTs) with a high-

spin ground state (S = 5/2). The corresponding doublet and

quartet [Co]NTs states lie slightly higher than the sextet by ΔG

values of 0.4 and 1.5 kcal mol⁻¹, respectively. The computed spin

density for the S = 5/2 state places ∼1.1 unpaired electron on the

nitrene N atom, and ∼3.3 unpaired electrons on Co, with the

remaining spin density being distributed to other atoms.

Similarly to the low-spin porphyrin-supported Co^III−nitrene-

radical (S = 1/2) noted above, the high-spin congener is capable

of performing alkene aziridinations in a two-step process

(successive formation of two N−C bonds). Remarkably, the

computed transition-state barrier (ΔG^‡= 23.4 kcal mol⁻¹vs

Co^III−^•NTs/styrene) for the rate-determining, initial C_β···NTs

spin (Por)Co(II) paradigms, but also in comparison with the

previously examined libraries of Mn(II) and Fe(II) reagents,

supported by the same trisphenylamido-amine ligand frame-

work.⁴⁴Whereas the nitrene-transfer reactivity of the Mn(II)

reagents in alkene aziridinations largely depends on the

electrophilicity of the presumptive Mn^III−^•NR (S = 3/2)

moiety, underscoring the role of the electron aﬃnity of the metal

nitrene as a dominant factor, the reactivity of the corresponding,

more reactive, Co(II) reagents is aﬀected by additional subtle

electronic and steric factors. These most likely arise from the

tighter disposition of the reaction cavity, resulting in ligand-

coordination ﬂexibility, electronic rearrangement, and secon-

dary stabilizing interactions. In this publication we show that

even an otherwise small change in ligand substitution can have a

signiﬁcant eﬀect on nitrene-transfer reactivity in aziridination

reactions, occasionally overriding the preponderance of the

metal-nitrene electrophilicity as a driving force.

RESULTS AND DISCUSSION

■

Synthesis and Characterization of New Ligands and

Co(II) Complexes. The family of trisphenylamido-amine

ligands (L¹H₃−L¹⁷H₃) employed in this study is shown in

Figure 1. The majority of these ligands (L¹H₃−L¹⁵H₃) have been

used and reported in previous studies.⁴⁴⁻⁴⁸They are all

derivatives of the common 2,2′,2″-triaminotriphenylamine

framework,⁴⁵featuring carbonaceous arm substituents (alkyl,

aryl acyl). Ligand L¹⁶H₃is prepared by methylation of

deprotonated (KH) 2,2′,2″-triaminotriphenylamine by MeI in

THF, and ligand L¹⁷H₃is derived via condensation of the same

triamine with the corresponding chiral acyl chloride in the

presence of Et₃N in dichloromethane. The solid-state structures

of these two new ligands (Figure S1) are indicative of their

favorable preorganization for metalation, in a cavity that is

buttressed by alkyl and acyl arms, respectively.

Co^IIcomplexes were synthesized with all ligands, by reacting

the deprotonated (KH) ligand with anhydrous beads of CoCl₂

in THF (alkyl and aryl armed ligands) or N,N-dimethylaceta-

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Figure 2. Minimal coordination of Co^IIcompounds with ligands L¹−L¹⁷explored in this study.

mide (DMA) (acyl armed ligands). A subset of Co^IIcompounds,

namely L³Co (3), L⁵Co (5), L⁸Co (8a), L⁹Co (9), L¹⁰Co (10),

and L¹³Co (13), has been previously reported in a study that

examined the use of these catalysts in controlled radical

polymerization of oleﬁns.⁴⁸In addition, L⁸Co (8b) has been

explored in conjunction with the L⁸Mn and L⁸Fe congeners,

toward establishing metal-dependent trends in catalytic nitrene

transfer to oleﬁns.⁴⁴Figure 2 depicts representations of the

minimal coordination site of each Co^IIsite, derived from single-

crystal X-ray diﬀraction data. In all cases the ligand coordinates

in a trigonal pyramidal [N_3(amido)N_amine] mode, exhibiting

various degrees of distortion, although in two instances (7,

17) the coordination to the axial N_amineresidue can be best

described as a long contact. Moreover, compound 11 features a

noncoordinating amido residue that has been protonated. The

dominant four-coordinate [N₃N] pattern is retained as the sole

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Figure 3. ORTEP diagrams (from left to right) of {[K(L⁴)Co^II]·Diethyl Ether}_n(4), [K(MeCN)(L⁶)Co^II−NCMe]·2MeCN (6), [K₂(DMA)₄]-

[[K(L⁷)₂Co^II]₂·2DMA (7), [K₂(DMA)₃(L⁸)Co^II]₂(8c), [K(DMA)(L¹⁴)Co^II]·DMA (14), [K(THF)₃(L¹⁵)Co^II]·THF (15), and [K(THF)K-

2

(L¹⁷)₂Co^II]₂·3Pentane (17), drawn with 40% thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selective interatomic distances (Å) and

2

angles (deg): 4, Co(1)−N(1) = 2.151(11), Co(1)−N(2) = 1.986(6), Co(1)−[N(2), N(2), N(2)] = 0.32(2) (distance of Co from mean plane) Å,

N(2)−Co(1)−N(2) = 117.48(11), N(2)−Co(1)−N(1) = 80.8(2); 6, Co(1)−N(1) = 2.235(3), Co(1)−N(2) = 2.046(4), Co(1)−N(3) = 2.049(4),

Co(1)−N(4) = 2.053(4), Co(1)−N(5) = 2.063(4), Co(1)−[N(2), N(3), N(4)] = 0.48(2) (distance of Co from mean plane), N(2)−Co(1)−N(4) =

115.18(15), N(2)−Co(1)−N(3) = 117.17(15), N(3)−Co(1)−N(4) = 111.69(15), N(2)−Co(1)−N(1) = 76.70(14), N(4)−Co(1)−N(1) =

76.49(14), N(3)−Co(1)−N(1) = 76.15(14), N(3)−Co(1)−N(5) = 103.24(15), N(2)−Co(1)−N(5) = 104.55(14), N(4)−Co(1)−N(5) =

102.81(15), N(1)−Co(1)−N(5) = 178.75(14); 7, Co(1)−N(1) = 2.463(4), Co(1)−N(2) = 2.053(4), Co(1)−N(3) = 2.054(5), Co(1)−N(4) =

2.054(4), Co(1)−O(6) = 1.975(4), Co(1)−[N(2), N(3), N(4)] = 0.61(2) (distance of Co from mean plane), Co(2)−N(5) = 2.517(5), Co(2)−

N(6) = 2.081(5), Co(2)−N(7) = 2.033(5), Co(2)−N(8) = 2.069(5), Co(2)−O(3) = 1.984(4), Co(2)−[N(6), N(7), N(8)] = 0.67(2) (distance of

Co from mean plane), N(2)−Co(1)−N(4) = 109.51(18), N(2)−Co(1)−N(3) = 113.66(17), N(3)−Co(1)−N(4) = 111.52(18), N(2)−Co(1)−

N(1) = 73.14(16), N(4)−Co(1)−N(1) = 73.24(16), N(3)−Co(1)−N(1) = 71.83(16), N(3)−Co(1)−O(6) = 103.19(17), N(2)−Co(1)−O(6) =

106.85(17), N(4)−Co(1)−O(6) = 111.92(16), N(1)−Co(1)−O(6) = 174.16(15), N(6)−Co(2)−N(7) = 111.14(19), N(6)−Co(2)−N(8) =

112.01(19), N(7)−Co(2)−N(8) = 106.34(19), N(6)−Co(2)−N(5) = 70.48(19), N(7)−Co(2)−N(5) = 70.69(16), N(8)−Co(2)−N(5) =

71.42(17), N(6)−Co(2)−O(3) = 105.77(19), N(7)−Co(2)−O(3) = 105.15(18), N(8)−Co(2)−O(3) = 116.27(17), N(5)−Co(2)−O(3) =

172.25(16); 8c, Co(1)−N(1) = 2.1422(13), Co(1)−N(2) = 1.9941(14), Co(1)−N(3) = 1.9899(14), Co(1)−N(4) = 1.9872(13), Co(1)−[N(2),

N(3), N(4)] = 0.306(4) (distance of Co from mean plane), K(1)−O(1) = 2.7298(14), N(2)−Co(1)−N(4) = 114.79(6), N(2)−Co(1)−N(3) =

118.04(6), N(3)−Co(1)−N(4) = 120.18(6), N(2)−Co(1)−N(1) = 80.83(5), N(4)−Co(1)−N(1) = 81.54(5), N(3)−Co(1)−N(1) = 81.06(5); 14,

Co(1)−N(1) = 2.224(3), Co(1)−N(2) = 2.018(4), Co(1)−N(3) = 2.031(4), Co(1)−N(4) = 2.021(4), Co(1)−O(3) = 2.162(3), Co(1)−[N(2),

N(3), N(4)] = 0.42(1) (distance of Co from mean plane), N(2)−Co(1)−N(4) = 110.16(14), N(2)−Co(1)−N(3) = 121.11(15), N(3)−Co(1)−

N(4) = 115.82(15), N(2)−Co(1)−N(1) = 76.31(14), N(4)−Co(1)−N(1) = 78.82(13), N(3)−Co(1)−N(1) = 78.58(14), N(3)−Co(1)−O(3) =

106.07(14), N(2)−Co(1)−O(3) = 91.35(13), N(4)−Co(1)−O(3) = 108.68(13), N(1)−Co(1)−O(3) = 167.33(12); 15, Co(1)−N(1) =

2.337(17), Co(1)−N(2) = 2.041(17), Co(1)−N(3) = 2.048(18), Co(1)−N(4) = 2.033(17), Co(1)−O(4) = 2.405(13), Co(1)−O(6) = 2.302(14),

Co(1)−[N(2), N(3), N(4)] = 0.53(2) (distance of Co from mean plane), N(2)−Co(1)−N(4) = 103.6(7), N(2)−Co(1)−N(3) = 118.1(7), N(3)−

Co(1)−N(4) = 117.9(7), N(2)−Co(1)−N(1) = 74.3(6), N(4)−Co(1)−N(1) = 76.4(7), N(3)−Co(1)−N(1) = 73.5(6), N(3)−Co(1)−O(6) =

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Figure 3. continued

107.6(6), N(2)−Co(1)−O(6) = 127.7(6), N(4)−Co(1)−O(6) = 73.7(6), N(1)−Co(1)−O(6) = 146.4(6), N(3)−Co(1)−O(4) = 72.9(6), N(2)−

Co(1)−O(4) = 89.5(6), N(4)−Co(1)−O(4) = 154.0(6), N(1)−Co(1)−O(4) = 129.3(6), O(4)−Co(1)−O(6) = 80.5(5); 17, Co(1)−N(1) =

2.428(5), Co(1)−N(2) = 2.055(5), Co(1)−N(3) = 2.038(5), Co(1)−N(4) = 2.047(5), Co(1)−O(6) = 1.990(4), Co(1)−[N(2), N(3), N(4)] =

0.58(1) (distance of Co from mean plane), Co(2)−N(5) = 2.375(5), Co(2)−N(6) = 2.049(5), Co(2)−N(7) = 2.026(5), Co(2)−N(8) = 2.100(5),

Co(2)−O(3) = 2.027(4), Co(2)−[N(6), N(7), N(8)] = 0.56(1) (distance of Co from mean plane), N(2)−Co(1)−N(4) = 110.2(2), N(2)−Co(1)−

N(3) = 118.9(2), N(3)−Co(1)−N(4) = 107.2(2), N(2)−Co(1)−N(1) = 73.94(18), N(4)−Co(1)−N(1) = 74.25(18), N(3)−Co(1)−N(1) =

72.09(18), N(3)−Co(1)−O(6) = 109.47(18), N(2)−Co(1)−O(6) = 99.64(18), N(4)−Co(1)−O(6) = 110.20(18), N(1)−Co(1)−O(6) =

172.96(17), N(6)−Co(2)−N(7) = 112.3(2), N(6)−Co(2)−N(8) = 113.4(2), N(7)−Co(2)−N(8) = 112.71(19), N(6)−Co(2)−N(5) = 74.71(18),

N(7)−Co(2)−N(5) = 74.52(19), N(8)−Co(2)−N(5) = 73.18(17), N(6)−Co(2)−O(3) = 97.07(18), N(7)−Co(2)−O(3) = 112.18(19), N(8)−

Co(2)−O(3) = 108.06(17), N(5)−Co(2)−O(3) = 171.18(17).

ligand ﬁeld of seven Co^IIcompounds (4, 5, 8a, 8c, 9, 10, 12).

Additional elements of metal coordination, essentially located

trans to the axial N_amineresidue, are observed with all other

compounds, and include solvent moieties, especially for

compounds crystallized from MeCN (3, 6, 13) and THF

(16), as well as carbonyl units (−C(R)O−Co^II) deriving

from acyl residues belonging to the ligand (7, 14, 17). In a single

case (15), a six-coordinate Co^IIsite arises from the presence of

two ligand-derived ether residues in the metal coordination

sphere, in addition to the usual [N₃N] framework. Importantly,

most structures are polymeric, largely due to an intricate

network of intermolecular interactions generated by K⁺ions.

Mononuclear (1, 2, 5, 12, 13) or oligonuclear (8c, 11, 17)

compounds (molecular or ionic) are only encountered in a

handful of cases. A more detailed description of the structural

features of the new Co^IIcompounds is provided below.

long axial Co−N_amineinteraction (av. 2.49 Å). In addition, each

Co(II) is coordinated by an oxygen atom (carbonyl) positioned

trans with respect to the axial Co−N_aminedirection (av. N_amine

−

Co−O_CO= 173.2°, Co−O_CO= 1.98 Å). Importantly, the

oxygen atom (carbonyl) coordinated to each Co^IIcenter belongs

to the ligand surrounding the partner Co^IIsite, hence giving rise

to a dimer. The K⁺ions interconnecting the dimers in a pseudo

1-D array are coordinated by two carbonyl and, more weakly,

two N_amidoresidues all belonging to one dimer, and by only a

single carbonyl moiety (O(10)) belonging to the adjacent

dimer. A much more simpliﬁed version of this structure is

adopted by [K(DMA)₃(L⁸)Co^II]₂(8c), exhibiting a dimeric

structure comprised of two inversion symmetry related

[N₃N]Co^IIunits connected via carbonyl O atoms of acyl

residues to a central K₂(μ-DMA)₂(DMA)₄core.

Compound [K(DMA)(L¹⁴)Co^II]·DMA (14) retains the

usual [N₃N] trigonal-pyramidal coordination but exhibits an

additional unique feature, inasmuch as one of the chiral arms

generates a seven-member loop by positioning the ester

carbonyl in the coordination sphere of Co^II, trans to the axial

Co^IICompounds with Acyl-Armed Ligands. The seven

new compounds (4, 6, 7, 8c, 14, 15, 17; Figure 3) that belong in

this category are polymeric, with the exception of [K-

(DMA)₃(L⁸)Co^II]₂(8c) and [K₂(THF)₂K₂(L¹⁷)₄Co^II] (17)

4

that feature a dimeric and tetrameric molecular unit,

respectively. The catalytically important {[K(L⁴)Co^II]·Diethyl

Ether}_n(4) exhibits higher symmetry than all other compounds,

consisting of a rigorous 3-fold axis along the Co−N_amine

direction as well as through K⁺ions relating three diﬀerent

molecules in the crystal lattice. This compound is characterized

by an exclusive four-coordinate [N₃N] ligand ﬁeld and an open

metalated cavity fortiﬁed by the three −CO^tBu arms. The

carbonyl residues are positioned exo with respect to the cavity

and are further engaged in contacts with K⁺ions, inasmuch as

each potassium is coordinated by three oxygen (carbonyl) atoms

belonging to diﬀerent molecules, and is also involved in K⁺−

arene π contacts. These structural features are largely retained in

the structure of [K(NCMe)(L⁶)Co^II−NCMe]·2MeCN (6), but

important deviations also apply, mostly because of the presence

of a coordinated MeCN molecule in a trans position versus the

N_amine(N_amine−Co−N_MeCN= 178.75(14)°) and the lack of a

strict 3-fold crystallographic symmetry. Otherwise, each K⁺ion

is still coordinated by three carbonyl residues belonging to

diﬀerent molecules in addition to a single MeCN, in lieu of any

K⁺−arene contacts.

N

_amineresidue (N_amine−Co−O_CO= 167.33(12)°, Co−O_CO

=

2.162(3) Å). The polymeric nature of the compound arises

again due to identical K⁺ions, coordinated by one DMA,

forming contacts with oxygen residues (amidato carbonyls,

MeC(O)O−) belonging to three diﬀerent Co^IIsites in the

crystal lattice. Compound [K(THF)₃(L¹⁵)Co^II]·THF (15) is

the only six-coordinate species observed, inasmuch as two acyl

arms generate ﬁve-membered loops that place two ether

residues (ROMe) in the coordination sphere of the

[N₃N]Co^IIsite (Co−O = 2.302(14), 2.405(13) Å; N_amine

−

Co−O = 146.4(6), 129.3(6)°). Identical K⁺ions are

coordinated by three THF molecules and two acyl residues,

each located in neighboring molecules, thus giving rise to

pseudo 1-D polymeric structures.

Finally, the structure of [K₂(THF)₂][K(L¹⁷)₂Co^II] (17) is

2 2

very similar to that observed for 7 with respect to the formation

of two interconnecting dimers, but the K⁺ions are organized

diﬀerently, to aﬀord a molecular (tetranuclear) rather than a

polymeric complex. First, two K⁺ions link the two dimers in 17,

by employing the same contact pattern noted for the single K⁺

ion connecting the two dimers in 7. Second, the remaining two

K⁺ions in 17 are terminated by THF molecules, and thus do not

provide connections that could generate an 1-D array of

repeating tetranuclear units as in 7. Otherwise, the coordination

and arrangement of the Co^IIsites in 17 and 7 is very similar, with

somewhat more pronounced contact for the N_amineresidue (av.

Co−N_amine= 2.40 Å), and concomitant weaker attachment of

the oxygen (carbonyl) moiety (av. Co−O_CO= 2.00 Å), along

the axial coordination of Co^IIsites in 17 versus that of 7.

The structure of [K₂(DMA)₄][K(L⁷)₂Co^II] ·2DMA (7) is

2 2

organized in a much more complex manner, featuring a “one-

dimensional” array of a repeating unit, [Co(1)/Co(2)−K(1)−

Co(3)/Co(4)−K(2)]_n, connected to identical arrays via lateral

links provided by a DMA solvated K(3)/K(4) dimer

(K₂(DMA)₄). Within the repeating unit, the Co(II) sites are

arranged in two similar dimers linked via K⁺contacts. The

dimeric unit is composed of two slightly diﬀerent Co(II) centers,

each featuring the usual [N₃N] ligand coordination, but with a

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Co^IICompounds with Alkyl-Armed Ligands. The new

methyl-substituted compound 16 and the previously reported

isopropyl congener (9)⁴⁸are the only members of the alkyl-

armed category of Co(II) compounds explored in this study.

Compound 16 (Figure 4) demonstrates the familiar [N₃N]

presence of 2.2.2-cryptand, in the form of green crystals of

[K(2.2.2-cryptand)][(L²)Co^II]·1.5Pentane (2) of marginal

quality. Its structure is almost identical with that of 1, with a

similarly strong equatorial ﬁeld (av. Co−N = 1.927 Å). The

corresponding 3,5-Me₂disubstituted compound [K-

(THF)₃(L¹²)Co^II]·THF (12) is monomeric and geometrically

analogous to 2, with a weaker equatorial ﬁeld (av. Co−N = 1.956

Å), but, as opposed to 1 and 2, can be isolated without the

assistance of 2.2.2-cryptand. The K⁺ion in 12 is supported by

three THF molecules and a host of contacts with aromatic

moieties and N atom residues.

Although both the 3,5-Me₂and 2,6-Me₂disubstituted

compounds 12 and 13, respectively, can be isolated and

characterized, the corresponding 2,4,6-Me₃trisubstituted

species (L¹¹)Co^IIhas proven to be diﬃcult to synthesize,

apparently due to extreme sensitivity to even traces of water. In

contrast, the analogous [K(THF)₃(L¹¹)Mn^II−THF] can be

readily prepared.⁴⁴Indeed, after initial formation of a deep green

species in the reaction of deprotonated L¹¹H₃and CoCl₂in

THF, the color soon fades and ligand can be recovered intact

along with separation of blue Co(OH)₂. In one instance, under

scrupulous water exclusion, a few green crystals of [K(THF)-

(L¹¹H)Co^II−OH)]₂(11) have been isolated, amounting to a

species that can be viewed as the formal product of water

addition to (L¹¹)Co^II. Indeed, 11 features protonation and

dissociation of one nitrogen residue from the equatorial ﬁeld,

with concomitant formation of a Co^II−OH moiety. The

hydroxide is further coordinated by two K(THF)⁺ions in an

overall dimeric structure that connects two inversion-related

(L¹¹H)Co^II−OH monomers by means of a K₂(OH)₂rhomb.

Structures Featuring Ligand Rearrangement. In two

instances, we have isolated a few compounds that exhibit a

characteristic oxidative ligand rearrangement in the presence of

traces of dioxygen or one-electron oxidants. Similarly

reorganized compounds, featuring electron-donor substituents,

have been previously studied in our lab and attributed to the

formation of an incipient aminyl radical.^45b,49Indeed, the

electron-rich (L²)Co^II(2) is highly sensitive to oxidative

rearrangement, and provided two crystallographically charac-

Figure 4. ORTEP diagram of [K(L¹⁶)Co^II−THF]·0.5Pentane (16)

drawn with 40% thermal ellipsoids. Hydrogen atoms are omitted for

clarity. Selective interatomic distances (Å) and angles (deg): Co(1)−

N(1) = 2.226(3), Co(1)−N(2) = 2.007(4), Co(1)−N(3) = 1.990(4),

Co(1)−N(4) = 2.004(4), Co(1)−O(1) = 2.207(3), Co(1)−[N(2),

N(3), N(4)] = 0.37(2) (distance of Co from mean plane), N(2)−

Co(1)−N(4) = 112.70(15), N(2)−Co(1)−N(3) = 114.04(15),

N(3)−Co(1)−N(4) = 122.94(15), N(2)−Co(1)−N(1) =

79.40(13), N(4)−Co(1)−N(1) = 78.61(14), N(3)−Co(1)−N(1) =

79.64(14), N(3)−Co(1)−O(1) = 100.45(14), N(2)−Co(1)−O(1) =

104.32(13), N(4)−Co(1)−O(1) = 97.91(14), N(1)−Co(1)−O(1) =

175.75(13).

coordination, but unlike the more bulky isopropyl analogue, it

exhibits a ﬁve-coordinate Co^IIsite due to the presence of a

coordinated THF molecule trans to the N_amineresidue (N_amine

−

Co−O_THF= 175.75(13)°). The electron-rich alkyl substitution

dictates a stronger equatorial ligand ﬁeld (av. Co−N_amido= 2.000

Å) by comparison to all other ﬁve-coordinate Co^IIsites

investigated in this study. The polymeric nature of the

compound arises by means of a repeating −[Co(1)−K(1)]−

sequence, which features K(1) ions engaging in K−N_amidoand

K⁺−arene contacts with both ligands of adjacent Co(1) sites.

Co^IICompounds with Aryl-Armed Ligands. Among the

seven aryl-supported Co(II) reagents shown in Figure 2 (1, 2,

11, 12 are new; 3, 5, 13 have been previously reported⁴⁸), only

those crystallized from MeCN solutions (3, 13) feature a solvent

molecule coordinated to the Co(II) center. All others,

crystallized from THF solutions, exhibit four-coordinate

[N₃N]Co(II) sites devoid of any axial THF residues, in sharp

contrast to analogous ﬁve-coordinate Mn and Fe reagents

previously reported.

terized species, [K(THF)₃(L²)Co^II−THF] (2b) and [(L²_re,ox)-

re

Co^II−THF]·0.5 Pentane (2c) (Scheme 1 and Figure S2; bonds

broken in 2 and formed in 2b and 2c are shown in red), formed

in comparable amounts. Compound 2c is not only ligand-

rearranged, but also one-electron oxidized, as noted by relevant

metrical parameters associated with the phenylene ring between

atoms N₁and N₂(Figure S2). A possible overall stoichiometry

for this reaction can be written as 2[(L²)Co^II]⁻→ [(L²)Co^II]⁻

re

+ [(L²_re,ox)Co^II] + e⁻.

Compound (L¹⁶)Co^II(16) is also highly sensitive to the same

type of ligand rearrangement in the presence of traces of

dioxygen, aﬀording the isolable dimer [K(THF)₂(L¹⁶_re)Co^II]₂

(16b, Figure S3), which is equivalent to 2b in terms of ligand

reorganization. In this case, we were not able to isolate any other

compound that might provide evidence for the location of the

oxidizing equivalent(s).

Among the four new Co(II) complexes (Figure 5), (L¹)Co

(1) provides nice green crystals from concentrated THF

solutions, but single-crystal specimens (albeit of low quality)

were only obtained in the presence of the exceptional K⁺binder

2.2.2-cryptand. The resulting ionic complex [K(2.2.2-

cryptand)][(L¹)Co^II]·3THF (1) exhibits a distorted [N₃N]

coordination with an equatorial ligand ﬁeld (av. Co−N = 1.927

Å) that is equal or stronger than that demonstrated by similar

four-coordinate Co(II) sites supported by aryl substituents (2,

5, 12), presumably due to the electron-rich character of the

4-^tBu-substituted phenyl arm. Similarly, the 3,5-^tBu₂disub-

stituted compound (L²)Co (2) proved to be isolable only in the

Electrochemistry. Ten Co(II) compounds possessing aryl

(L³Co, L⁵Co, L¹³Co), alkyl (L⁹Co), and acyl arms (L⁴Co, L⁶Co,

L⁷Co, L⁸Co, L¹⁰Co, L¹⁷Co) were selected as representative

examples for examination by cyclic voltammetry. Electro-

chemical data for a handful of these examples have been

previously reported.⁴⁸Figure 6 provides a collective presenta-

tion of the corresponding waves (ﬁrst oxidation event), and

Table S2 summarizes relevant electrochemical data (potentials

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Figure 5. ORTEP diagrams (from left to right) of [K(2.2.2-cryptand)][(L¹)Co^II]·3THF (1), [K(2.2.2-cryptand)][(L²)Co^II]·1.5Pentane (2),

[K(THF)(L¹¹H)Co^II−OH]₂(11), and [K(THF)₃(L¹²)Co^II]·THF (12) drawn with 40% thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Selective interatomic distances (Å) and angles (deg): 1, Co(1)−N(1) = 2.102(18), Co(1)−N(2) = 1.946(18), Co(1)−N(3) = 1.92(2), Co(1)−N(4)

= 1.915(19), Co(1)−[N(2), N(3), N(4)] = 0.17(2) (distance of Co from mean plane), N(2)−Co(1)−N(4) = 122.7(8), N(2)−Co(1)−N(3) =

118.0(8), N(3)−Co(1)−N(4) = 117.0(8), N(2)−Co(1)−N(1) = 84.0(8), N(4)−Co(1)−N(1) = 85.1(8), N(3)−Co(1)−N(1) = 85.6(8); 2,

Co(1)−N(1) = 2.085(8), Co(1)−N(2) = 1.921(7), Co(1)−N(3) = 1.933(8), Co(1)−N(4) = 1.926(7), Co(1)−[N(2), N(3), N(4)] = 0.14(1)

(distance of Co from mean plane), N(2)−Co(1)−N(4) = 120.4(3), N(2)−Co(1)−N(3) = 118.2(3), N(3)−Co(1)−N(4) = 119.8(3), N(2)−

Co(1)−N(1) = 85.9(3), N(4)−Co(1)−N(1) = 86.0(3), N(3)−Co(1)−N(1) = 85.5(3); 11, Co(1)−N(1) = 2.260(5), Co(1)−N(2) = 1.956(5),

Co(1)−N(3) = 1.939(5), Co(1)−O(1) = 1.947(4), K(1)−O(1) = 2.675(5), K(1)−O(2) = 2.677(5), Co(1)−[N(2), N(3), N(4)] = 0.62(2)

(distance of Co from mean plane), N(2)−Co(1)−N(3) = 118.7(2), N(2)−Co(1)−N(1) = 81.6(2), N(3)−Co(1)−N(1) = 80.1(2), N(1)−Co(1)−

O(1) = 139.41(18), N(2)−Co(1)−O(1) = 108.5(2), N(3)−Co(1)−O(1) = 122.9(2), Co(1)−O(1)−K(1) = 113.83(18), O(1)−K(1)−O(2) =

116.95(15); 12, Co(1)−N(1) = 2.121(7), Co(1)−N(2) = 1.950(8), Co(1)−N(3) = 1.964(8), Co(1)−N(4) = 1.955(7), Co(1)−[N(2), N(3),

N(4)] = 0.20(1) (distance of Co from mean plane), N(2)−Co(1)−N(4) = 119.2(3), N(2)−Co(1)−N(3) = 121.5(3), N(3)−Co(1)−N(4) =

116.2(3), N(2)−Co(1)−N(1) = 84.3(3), N(4)−Co(1)−N(1) = 84.0(2), N(3)−Co(1)−N(1) = 84.0(3).

Scheme 1

are reported versus the ferrocenium/ferrocene (F_c⁺/F_c) couple).

All aryl- and alkyl-armed Co(II) compounds examined by cyclic

voltammetry feature semireversible waves at negative potentials,

ranging from −0.665 (L¹³Co) to −0.090 V (L³Co), in

accordance with the electron-rich nature of the corresponding

substituents. Speciﬁcally for the alkyl-armed L⁹Co, the two

closely spaced, semireversible waves observed (−0.654, −0.500

V), may represent the two slightly diﬀerent Co(II) sites in the

crystal structure. Given the almost identical wave currents for all

these aryl- and alkyl-armed Co(II) compounds (3.0 M), and

their anodic shifts with respect to the analogous Mn(II)/

Mn(III) and Fe(II)/Fe(III) couples^44,46aby approximately 0.65

and 0.25 V, respectively, we assign the corresponding semi-

reversible waves to essentially metal-centered Co(II)/Co(III)

cycles.

In sharp contrast, the acyl-armed Co(II) compounds

examined demonstrate irreversible anodic ways with variable

i_p,avalues (mostly large versus the aryl/alkyl-substituted

congeners), suggesting signiﬁcant ligand-centered contribu-

tions. In addition, the ﬁrst anodic wave shown in Figure 6

overlaps with a subsequent oxidation wave (not shown),

rendering any attempts to garner further information from

exhaustive electrolysis futile. Nevertheless, all initial anodic

waves for the acyl-armed Co(II) sites are shifted to positive

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centered oxidation events does not permit any secure

identiﬁcation of the solution structure of species such as L⁷Co

and L¹⁰Co, which exhibit single anodic waves even in the

presence of dinuclear units in their polymeric solid-state

structures. On the other hand, the dual anodic-wave feature

for L¹⁷Co (solid-state tetramer) may signify a dinuclear structure

in solution. Finally, the catalytically important L⁸Co (−COCF₃

arm) possesses the indisputably most electrochemically stable

Co(II) site encountered in this series of Co(II) compounds, in

agreement with previous ﬁndings for the L⁸Mn and L⁸Fe

analogues.^44,46a

EPR Spectroscopy. EPR spectra of selected Co(II)

compounds were recorded from frozen DMF solutions of 3, 4,

5, 6, 7, 8a, 9, 10, 13, 14, 15, and 17. In all cases, the samples give

rise to signals that are consistent with isolated Co^II(S = 3/2)

species.⁵⁰Spectra recorded at 10 K are shown in Figure 7. The

spectra can be interpreted within the framework of the spin

Hamiltonian:⁵¹

Figure 6. Cyclic voltammograms of compounds [K(L³)Co^II−NCMe]

(3) and [K(NCMe)₃(L¹³)Co^II−NCMe] (13) in MeCN/(ⁿBu₄N)PF₆,

[K(L⁴)Co^II]·Et₂O (4), [K(THF)₆][(L⁵)Co^II]·1.5THF (5), [K-

(MeCN)(L⁶)Co^II−NCMe]·2MeCN (6), [K₂(DMA)₄][K-

(L⁷)₂Co^II]₂·2DMA (7), [K(THF)₂(L⁸)Co^II] (8a), [K₂(L⁹)₂Co^II]

Ä

É

Å

Ñ

S(S + 1)

2

E

D

2

Å

Ñ

̂

H_zfs= DÅS_z−

+

(S_x− S_y)Ñ + S·A·I

Å

Ñ

3

Å

Ñ

Å

Ç

Ñ

Ö

̂

2

+ βB·g₀·S

(1)

(9), and [K(THF)K(L¹⁷)₂Co^II]₂·3Pentane (17) in DMF/(ⁿBu₄N)-

2

PF₆, and [K₂(DMA)₃(L¹⁰)₂Co^II]·0.5Et₂O (10) in DMA/(ⁿBu₄N)PF₆,

2

In eq 1, D and E are the zero ﬁeld splitting (zfs) parameters. A

is the hyperﬁne tensor relevant to the hyperﬁne interactions of

the electronic (S = 3/2) and nuclear (I = 7/2 for ⁵⁹Co) spins,

and g₀is the intrinsic g-tensor of the Co(II) ion. For simplicity

we assume that the principle axes of the tensors are parallel to

each other. Often, due to large spin orbit coupling eﬀects for Co^II

(S = 3/2), the zfs parameter |D| is quite large, whereas A and g₀

are characterized by signiﬁcant anisotropy. Under the inﬂuence

of zfs, the 4-fold degeneracy is partially lifted in zero magnetic

ﬁeld, yielding two Kramers’ doublets, | 1/2⟩ and | 3/2⟩

separated by 2|D|. The | 1/2⟩ (or | 3/2⟩) doublet is the ground

with a Au disk electrode (1.6 mm in diameter); scan rate 0.1 V/s.

potentials (0.032 (L¹⁷Co) to 0.719 V (L⁸Co)), reﬂecting the

eﬀect of the electron-withdrawing acyl arm on the Co(II) center,

albeit in an order not always consistent with the electronic

nature of each individual acyl group. The most signiﬁcant

deviation is observed for L¹⁰Co (E_p,a= 0.559 V), whose anodic

shift versus that of L¹⁰Mn (E_p,a= −0.108 V; tentatively assigned

to Mn(II)/Mn(III))⁴⁴betrays very little, if any, metal-centered

contributions to the anodic wave. The intervention of ligand-

Figure 7. Experimental (black line) and theoretical (red-dashed line) EPR spectra from frozen DMF solutions of complexes 3, 4, 5, 6, 7, 8a, 9, 10, 13,

14, 15, and 17. For 3, 4, 6, 7, 8a, 10, and 17, the subspectra from sites 1 (green), and 2 (blue) are also shown, as described in the text. A spike signal at ca.

3400 G in the spectra of 5, 9, and 10 is due to an impurity in the cavity. EPR conditions: Temperature, 10 K; modulated amplitude 10 Gpp; microwave

power, 2.0 mW; microwave frequency, 9.4 GHz.

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state for positive (or negative) D. For |D| ≫ hν (≈ 0.31 cm⁻¹at

X-band), each Kramers’ doublet can be described by an eﬀective

S_eff= 1/2, giving rise to an EPR spectrum characterized by an

anisotropic g_efftensor.

cases suggest that the coordination environment of the Co ion

can be quite ﬂexible in solution.

Catalytic Aziridinations of Oleﬁns. Styrene. The 17 Co^II

compounds shown in Figure 2 were ﬁrst explored as catalysts (5

mol %) for the aziridination of styrene (8.0 equiv) by PhINTs

(1.0 equiv) in chlorobenzene at room temperature (Table 1).

The EPR signals shown in Figure 7 are consistent with the |

1/2⟩ doublet. No detailed temperature dependence of the

spectra was pursued in the present work. However, spectra

recorded at 4.2 and/or 20 K (not shown) indicate that the

intensity of the signals (scaled as Intensity × Temperature)

decreases as temperature increases. This suggests that the | 1/

2⟩ doublet is the ground state, implying a positive value for D.

For |D| ≫ hν, the spectra observed do not depend on D but

rather on the rhombic zfs parameter E/D, the values of the

intrinsic g₀-tensor,⁵²and the hyperﬁne term. With the exception

of the spectrum for complex 15, the g₀-tensor was assumed axial

(g_0x= g_0y= g_0⊥≠ g_0z= g_∥). In several cases (3, 4, 6, 7, 8a, 10,

17), the spectra indicated the presence of two Co(II) species,

characterized by a diﬀerent value for the parameter E/D.

Assuming a common value for |D|, the simulations determine the

relative abundance of each Co(II) site. The speciﬁc line shape of

the spectra results from a combination of factors, including

distributions on the parameter E/D (E/D strain), unresolved

hyperﬁne interactions, and residual line-broadening mecha-

nisms.⁵³The EPR parameters for all samples, as well as the

relative ratios of the two species when applicable, are presented

in Table S3. Because an unequivocal deconvolution of the line

broadening mechanisms is not feasible, the quoted values for the

relevant parameters are indicative.

With the exception of the six-coordinate 15, all other

complexes (four- or ﬁve-coordinate) exhibit rhombic parame-

ters (E/D) that lie in the interval [0, 0.19], indicating diﬀerent

degrees of rhombicity. All complexes (except 15) demonstrate a

valley-shaped signal at g ∼ 2.0, which corresponds to the g_∥

component of the g_eff-tensor. This signal is broadened due to

hyperﬁne interactions, and in some cases (7, 9, 17) the hyperﬁne

lines are well resolved. The simulations indicate that A_∥values in

all cases are in the range of 200−300 MHz and that the hyperﬁne

term has to be taken into account in order to reproduce the low

ﬁeld region of the spectra, corresponding to the perpendicular

components of the tensors in eq 1. Due to restrictions in the

determination of the line-broadening mechanisms, it is not

possible to unambiguously evaluate the magnitudes of the A_x

and A_y. Therefore, an average value, A_⊥= (A_x+ A_y)/2, is quoted

in Table S3. This value ranges in the 30−90 MHz range.

Complex 15 exhibits a relatively sharp peak at g = 6.24 and an

extremely broad derivative feature with an apparent zero

crossing at ca. g = 2.07. This behavior indicates a rhombic

system with E/D = 0.33. From this point of view, complex 15

exhibits unique EPR properties in this series, most likely due to

its special, six-coordinate ligand ﬁeld, featuring two O residues in

addition to the familiar [N₃N] coordination. The origin of dual

EPR-active species in DMF solutions of some compounds

cannot be ascertained at the present time, especially since

structural data from crystals derived from DMF solutions are not

available. Possible sources are small deviations in the

coordination of Co(II) sites in polymeric species (as noted for

7, crystallized from DMA solutions), and also potential

diﬀerentiation arising from partial DMF coordination to Co(II)

and even K⁺sites.

Table 1. Yields of Styrene Aziridination Mediated by Co^II

a

Reagents 1−17

compound

yield (%)

[K(2.2.2-cryptand)][(L¹)Co^II]·3THF (1)

[K(2.2.2-cryptand)][(L²)Co^II]·1.5Pentane (2)

[K(L³)Co^II−NCMe] (3)

18

36

70

32

50

59

69

68

38

49

−

25

38

36

45

25

35

[K(L⁴)Co^II]·Diethyl Ether (4)

[K(THF)₆](L⁵)Co^II]·1.5THF (5)

[K(NCMe)(L⁶)Co^II−NCMe]·MeCN·0.5H₂O (6)

[K₂(DMA)₄][K(L⁷)₂Co^II₂]₂·2DMA (7)

[K(NCMe)(L⁸)Co^II−NCMe] (8b)

[K(DMA)₃(L⁸)Co^II]₂(8c)

[K₂(L⁹)₂Co^II₂]_n(9)

[K₂(DMA)₃(L¹⁰)₂Co^II₂]·0.5Et₂O (10)

[K(THF)(L¹¹H)Co^II−OH)]₂(11)

[K(THF)₃(L¹²)Co^II]·THF (12)

[K(NCMe)₃(L¹³)Co^II−NCMe] (13)

[K(DMA)(L¹⁴)Co^II]·DMA (14)

[K(THF)₃(L¹⁵)Co^II]·THF (15)

[K(L¹⁶)Co^II−THF] (16)

[K(THF)K(L¹⁷)₂Co^II₂]₂(17)

a

Conditions: Catalyst, 0.0125 mmol (5 mol %); PhINTs, 0.25 mmol;

styrene, 2.0 mmol; MS 5 Å, 20 mg; PhCl, 0.200 g; 30 °C; 12 h.

The high-yielding L⁴Co^IIwas ﬁrst investigated as catalyst in

several solvents (MeCN, 50%; 2,2,2-Triﬂuoroethanol, 62%;

PhCF₃, 61%; CH₂Cl₂, 61%; Benzene, 53%; PhCl, 70%) and

found to aﬀord superior levels of aziridine in chlorobenzene

(similar solvent-screening results were previously obtained with

Mn^IIreagents⁴⁴). The yield drops signiﬁcantly (25%) if

equimolar amounts of styrene and PhINTs are employed in

PhCl, whereas a 2-fold excess of styrene over PhINTs improves

the yield to 46%. Increasing the amount of the L⁴Co catalyst to

10 mol % suppresses the yield to 53%. The styrene aziridination

yields obtained with various Co^IIcatalysts (Table 1) vary

signiﬁcantly (18−70%) as a function of the ligand employed. As

opposed to the corresponding L^xMn^IIreagents (x = 1−15)⁴⁴

that reveal a dominant relationship between increasing styrene

aziridination yields with anodically shifting Mn^II/Mn^IIIredox

potentials (and, by extension, with increasing electrophilicity of

the putative Mn^III−^•NR), the Co^IIcatalysts provide yields that

indicate a more complex pattern of electronic and steric

contributions. For example, although the comparatively

electron-deﬁcient, acyl-armed reagents remain, on average,

more productive than aryl- or alkyl-armed congeners, a

comparison between the −COCMe₃(L⁴) and −COCF₃(L⁸)

supported Co^IIreagents furnishes essentially the same

aziridination yields, in spite of the fact that the Co(II)/Co(III)

couple for L⁸Co is anodically shifted by ∼500 mV versus L⁴Co.

In addition, L⁸Co is potentially less congested than L⁴Co. In

In summary, the EPR studies from frozen DMF solutions of

the complexes indicate that all complexes feature a Co(II) (S =

3/2) ion with a large and positive zfs term, D, and variable degree

of rhombicity. The existence of more than one species in some

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a

Table 2. Yields of Aziridination/Amination of Oleﬁns by [K(L⁴)Co^II]·Et₂O (4)

a

Conditions: 4, 0.0125 mmol (5 mol %); PhINTs, 0.25 mmol; oleﬁn, 2.0 mmol; MS 5 Å, 20 mg; solvent (MeCN/CH₂Cl₂/PhCl) 0.200 g; 30 °C;

b

12 h. In MeCN.

contrast, the Mn^IIand Fe^IIcongeners are associated with

signiﬁcantly diverging yields in favor of the more electron-

deﬁcient L⁸M reagents (L⁴Mn, 12%; L⁸Mn, 75%; L⁴Fe, 45%;

L⁸Fe, 73%), correlating with an anodic potential shift of

approximately 600 mV for both the L⁸Mn and L⁸Fe catalysts

with respect to their L⁴M analogues. More importantly, as

indicated below, L⁴Co (4) is faster than L⁸Co (8b, 8c) in

mediating styrene aziridination. The closely related L⁷Co

(−COⁱPr arm) is in principle slightly less electron rich and

sterically congested than the L⁴Co congener, but aﬀords lower

yields than L⁴Co in a slower reaction (the opposite is true for the

corresponding Mn(II) catalysts).⁴⁴Two other acyl-substituted

Co^IIcatalysts (L¹⁴Co, L¹⁷Co), exhibiting signiﬁcantly lower

yields, are indicative of how the metal-nitrene electron-aﬃnity

bias may be overridden by other electronic or steric factors in the

fairly restricted reaction cavity of Co reagents. The correspond-

ing L¹⁴Mn catalyst, by contrast, is among the most productive

Mn^IIaziridination reagents examined (yield: 67%).⁴⁴As

expected, the aryl-substituted ligands generate Co^IIreagents

that are poor mediators of styrene aziridination. These sites are

oxidatively and even hydrolytically sensitive and tend to

generate thermodynamic sinks. Nevertheless, the muted role

of the electron-aﬃnity criterion can still be discerned in the

series of the electron-rich, aryl-armed reagents (1, 2, 3, 5, 12,

13), inasmuch as the highest-yielding L¹³Co (13) is the most

electron rich member of the group. Finally, the alkyl-substituted

L⁹Co and L¹⁶Co reagents are only modestly productive, as

anticipated for electron-rich Co^IIsites, but again the more

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electron rich, isopropyl-substituted L⁹Co provides higher yields

than the methyl-substituted congener L¹⁶Co. However, the

latter undergoes facile oxidative ligand rearrangement that may

compromise its structural integrity.

Alkene Aziridinations by [K(L⁴)Co^II]·Diethyl Ether (4). The

highest yielding L⁴Co (4) catalyst was subsequently investigated

as a nitrene-transfer (NTs) mediator with a panel of aromatic,

cyclo (Table 2), and aliphatic alkenes (Table S4). Styrenes

substituted at the para position with both electron-donor and

electron-acceptor groups were examined ﬁrst (entries 1−9)

under the conditions noted in the previous section. Both

methylene chloride and chlorobenzene were employed (as well

as acetonitrile in a few instances), invariably resulting in better

aziridination yields in chlorobenzene. In the majority of cases,

yields above 70% were recorded irrespective of the electron-rich

or poor character of the substituent, with the exception of two

moderately yielding cases involving strong electron-withdrawing

groups (p-CF₃, p-NO₂). Moreover, the product of 4-MeO-

styrene aziridination is known to be unstable,⁵⁴and is thus

associated with modest yields. Overall, the aziridination yields

for these para-substituted styrenes, save for the parent styrene,

are comparable to those previously reported for L⁸Co. Ortho-

substitution (entry 10) aﬀects aziridination yields, presumably

interfering with nitrene-transfer both sterically and electroni-

cally (due to the orthogonal orientation of the aromatic versus

the oleﬁnic plane of the substrate⁵⁵). Similar steric inhibition is

also observed for α-substituted styrenes (methyl, phenyl; entries

11 and 12), especially for the bulkier α-phenyl-styrene. An allylic

amination product is also obtained in both cases, ascribed to

aziridine-ring opening,⁵⁶which is more pronounced for the α-

phenyl-substituted product.⁵⁴Steric hindrance is also evident in

the aziridination of β-substituted styrenes (entries 13−16),

especially for the bulky cis- and trans-stilbene (entries 15, 16). In

agreement with previous observations in the application of

L⁸Co,⁴⁴the cis congeners are more productive (entries 14, 15),

with signiﬁcant loss of stereochemical integrity. For all these

encumbered substrates, L⁸Co is on average more productive

than L⁴Co, presumably reﬂecting the somewhat more

voluminous reaction cavity of L⁸Co. Allylic or benzylic

aminations compete successfully with aziridinations (entries

17−22), unless cis (entry 18) and/or electron-rich (entry 21)

oleﬁns are involved. Beyond cycloalkenes, other terminal or

internal aliphatic oleﬁns exhibit low aziridination yields (Table

S4), in accordance with previous results pertaining to the

application of L⁸Co in aziridinations of aromatic and aliphatic

oleﬁns.⁴⁴A competitive styrene versus 1-hexene (1.0 mmol

each) aziridination by PhINTs (0.25 mmol) catalyzed by L⁴Co

(5 mol %) in chlorobenzene provided a ratio of 25:1 in favor of

the styrene aziridination product (combined aziridine yield:

72%), not unlike the L⁸Co catalyst (28:1, yield 73%).⁴⁴

Figure 8. Yield of aziridine (%) as a function of time (min) in the

reaction of styrene (2.0 mmol) by PhI = NTs (0.25 mmol) mediated by

L⁴Co (4), L⁸Co (8c), and L⁷Co (7) (0.0125 mmol with respect to Co)

in chlorobenzene (0.500 g) at 30 °C.

NCMe] (8b), has been previously shown⁴⁴to be signiﬁcantly

slower; only 14% of the reaction is complete after 1.0 h in d⁵-

PhCl, indicating potential interference by the strongly

coordinating acetonitrile. However, 8b is still much faster than

the corresponding [K(NCMe)(L⁸)M^II−NCMe] (M = Mn, Fe)

reagents, presumably reﬂecting the superior electrophilicity of

Co^III−^•NR as discerned from E_p,avalues associated with the M^II/

M^IIIcouple of the divalent catalysts (Fe: 0.228 V, Mn: 0.518 V,

Co: 0.837 V).⁴⁴

Hammett Analysis. Several para-substituted styrenes (para

t

substituent: Me, Bu, F, Cl, CF₃, NO₂; 1.0 mmol each) were

subjected to competitive aziridination (PhINTs, 0.25 mmol)

versus styrene (1.0 mmol), mediated by L⁴Co (5 mol %) in

chlorobenzene (0.200 g), in the presence of 5 Å molecular sieves

1

(25 mg). Hammett plots of log(k_X/k_H) (determined by H

NMR from the ratio of the corresponding aziridines) as a

function of the substituent polar parameter σ_Por even the

resonance-responsive parameter σ⁺did not provide any reliable

linear free-energy correlations (Figure S4, Table S5). In

contrast, Jiang’s dual-parameter correlation that incorporates

both polar (σ_mb) and spin-responsive (σ_JJ*) parameters (log(k_X/

k_H) = ρ_mbσ_mb+ ρ_JJ*σ_JJ* + C)⁵⁷provides a reasonable linear

correlation (R²= 0.98; Figure 9). The negative ρ_mbvalue is

Mechanistic Studies. Comparative Reaction Proﬁle. The

formation of the product of styrene aziridination was monitored

as a function of time (Figure 8) for catalysts L⁴Co (4), L⁷Co (7),

and L⁸Co (8c) (all crystallized from DMA/ether), under the

conditions noted above (Table 1), with the exception of the

amount of PhCl used (500 mg). Yields were determined after

quenching the reaction at various time intervals. Surprisingly,

the more electron rich and sterically congested L⁴Co (4)

exhibits faster product generation than L⁸Co (8c) during the

ﬁrst hour, whereas L⁷Co (7) is kinetically comparable to 8c. At

the 1.0 h mark, the reaction is complete by 88% for L⁴Co (4),

65% for L⁸Co (8c), and 76% for L⁷Co (7). Interestingly, the

MeCN-crystallized version of L⁸Co, [K(NCMe)(L⁸)Co^II−

Figure 9. Linear free energy correlation of log(k_X/k_H) vs σ_mb, σ_JJ* for

the aziridination of para-substituted styrenes (X = Me, ^tBu, F, Cl, CF₃,

NO₂) mediated by L⁴Co (4).

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consistent with a small positive charge developing at the benzylic

carbon, whereas the always positive ρ_JJ* value denotes an

incipient radical character for the same site. The ratio |ρ_mb/ρ_JJ*|

= 1.12 is similar to the one previously observed for L⁸Co

catalyzed aziridinations (|ρ_mb/ρ_JJ*| = 1.0),⁴⁴and indicates

competitive contributions of polar and spin-delocalization

eﬀects. Polar eﬀects are dominant in many Rh,^24dCu,^24d,58

and Fe^18g,59,60catalyzed aziridinations, for which Hammett plots

can be ﬁt with the assistance of polar parameters alone (σ_p,σ⁺),

but the need for incorporating spin-delocalization parameters

(σ*, σ_JJ*)^57,61with a wide range of |ρ_mb/ρ_JJ*| values (0.04−2.02)

is also evident in many other metal-catalyzed aziridination-

s.^{18e,22d,g,23f,g}More recently, larger |ρ⁺/ρ_JJ*| values have been

reported for the aziridination of styrene by PhINNs mediated by

[Co^III(TAML^red)]⁻(5.71) and [Co^III(TAML^sq)] (8.64), in

accordance with a novel mechanism that involves a partial

single-electron transfer from the styrene to the metal-nitrene as a

component of the turnover-determining step.⁴¹A similar

mechanistic scenario has also been advanced for Fe-mediated

aziridinations, but in this case Hammett correlations can be

successfully accommodated with polar parameters alone (σ⁺).⁴²

Kinetic Isotope Eﬀect and Stereochemical Integrity.

Evaluation of the secondary kinetic isotope eﬀect was

resulting aziridines (cis/trans partitioning due to C_α−C_βbond

rotation; Scheme 2) in competition with N−C_αbond formation.

The ratio of cis/trans aziridine (94:6) and trans/cis aziridine

(92:8) resulting from the L⁴Co-mediated aziridination of cis-β-

d-styrene and trans-β-d-styrene, respectively, signiﬁes the

interference of very small energy barriers in aziridine-ring

closure. A slightly larger barrier is indicated for the aziridination

of the more sensitive cis-β-d-styrene by L⁸Co (cis/trans

aziridine: 89/11). On the other hand, the stereochemical

scrambling observed in the aziridination of cis-β-methyl-styrene

is more pronounced with L⁴Co than L⁸Co.

Computational Studies. The structure and electronic

description of the presumptive [L⁴Co]NTs intermediate were

explored by DFT calculations at the B3LYP/6-31+G(d) level of

theory. Free energy calculations suggest that the intermediate-

spin quartet ground state (S = 3/2) lies only 0.1 kcal mol⁻¹lower

than the high-spin sextet state (S = 5/2), and 2.8 kcal mol⁻¹

below the doublet state (S = 1/2). As mentioned above, the

calculated free energies for [L⁸Co]NTs indicate that the high-

spin sextet is the ground state, in agreement with the weaker

ligand ﬁeld provided by the L⁸versus L⁴ligand.

Calculated structures for the three spin-states of [L⁴Co]NTs

along with key metrical parameters are presented in Figure 10.

The most conspicuous feature of these structures is the

dissociation of one arm from the equatorial coordination sphere

of the metal (Co−N = 3.93 (quartet), 3.67 (sextet), 4.08

(doublet) Å). The axial Co−N_aminebond is also elongated (Co−

N = 2.43 (quartet), 2.63 (sextet), 2.99 (doublet) Å), if not

dissociated, by comparison to that of L⁴Co (2.151(11) Å).

These features have been previously noted in the DFT structure

of [L⁸Co]NTs, although the latter exhibits an additional Co−F

equatorial contact (Co−F = 2.37 Å).⁴⁴

1

2

accomplished by H and H NMR with the assistance of

deuterated styrenes (α-d-styrene, cis- and trans-β-d-styrene; 1.0

mmol each) in competitive aziridinations (PhINTs, 0.25 mmol)

with styrene (1.0 mmol) catalyzed by L⁴Co (4) or L⁸Co (8c) (5

mol %) in chlorobenzene (Table 3). KIE values close to 1.0 were

Table 3. Secondary KIE Values in Aziridination of

Deuterated Styrenes vs Styrene

Most importantly, the calculated spin densities for all three

spin states of [L⁴Co]NTs place a full oxidizing equivalent over

the dissociated arm, hence generating a widely delocalized N-

aryl amidyl radical (Figure 11). For the ground-state quartet, the

computed spin density consists of ∼3.2 unpaired e⁻on Co, 0.79

e⁻on the nitrene N atom and −1.0 unpaired e⁻on the

noncoordinating arm. Similar spin densities are calculated for

the sextet (Co: 2.9 e⁻, N: 0.9 e⁻, ligand arm: 1.0 e⁻) and the

doublet state (Co: 2.8 e⁻, N: −0.66 e⁻, ligand arm: −1.0 e⁻).

This spin density distribution is accommodated by an electronic

structure such as [(^•L⁴)Co(II)−^•NTs]⁻, featuring a high-spin

Co(II) center (S = 3/2) and two oxidizing equivalents on the

noncoordinating ligand arm and the N atom of the nitrene

residue, respectively. In sharp contrast, the spin density of the

ground-state sextet of [L⁸Co]NTs (Figure 11) is largely

distributed on Co (3.3 e⁻) and the N atom of the nitrene (1.1

e⁻). In this case, the noncoordinating arm is redox innocent, and

the residual spin density is centered over ligating N atoms in a

typical spin polarization fashion. Hence, the sextet state of

[L⁸Co]NTs is better accommodated with an [(L⁸)Co-

(III)−^•NTs]⁻electronic description (S_Co= 2).

obtained with α-d-styrene for both catalysts, indicating that the

α-styrenyl is unlikely to be involved in the initial nitrene attack to

styrene. In contrast, the β-styrenes are associated with inverse

KIE values for L⁴Co (cis: 0.90 ( 0.02), trans: 0.92 ( 0.02))

that can be attributed to a limited sp²→ sp³rehybridization of

styrene’s C_βsite upon development of the initial N−C_βbond

(Scheme 2). More modest inverse KIE values are also noted in

cis- and trans-β-d-styrene aziridinations mediated by L⁸Co (8b⁴⁴

or 8c) (cis: 0.96 ( 0.02), trans: 0.98 ( 0.02)), suggesting only

minimal N−C_βbond formation in the transition state.

The kinetics of the aziridine ring closure (formation of the

second N−C_αbond) was further evaluated by ²H NMR in the

aziridinations of cis- and trans-β-d-styrene (Table 4), by

examining the degree of stereochemical scrambling in the

Scheme 2

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Table 4. Exploration of Stereochemical Integrity in the Aziridination of cis- and trans-β-d-Styrene

Figure 10. DFT structures (minimal metal coordination) for [L⁴Co]NTs active species in diﬀerent spin states (from left to right: quartet, sextet,

doublet) optimized at the B3LYP/6-31+G(d) level of theory. Hydrogen atoms were omitted from the ﬁgure for clarity.

FURTHER DISCUSSION AND CONCLUSIONS

■

In a rigorous recent account, Latour and co-workers⁴²highlight

the importance of the electron aﬃnity (EA) of iron-nitrene/

imido species as a guiding principle for predicting their reactivity

in a wide range of iron-mediated aziridinations. In these catalytic

reactions, the iconic substrate styrene undergoes aziridinations

by various iron-nitrene compounds (Fe = NR), under a general

mechanistic scheme that designates the formation of the initial

N−C_bbond as the rate-determining step, usually encountered in

aziridinations with a two-step mechanism ([M]NR radical

addition to styrene, ring-closure radical rebound). More

Figure 11. Spin density on the calculated lowest-energy spin state of the

putative cobalt nitrenoid intermediates: quartet [L⁴Co]NTs (left) and

sextet [L⁸Co]NTs (right).

importantly, this ﬁrst step is front-loaded by signiﬁcant charge

transfer from styrene to the iron-nitrene and, thus, is crucially

inﬂuenced by the EA of the active oxidant. The applicability of

the EA as a general predictor of reactivity seems to be wide, but

at the present time is largely conﬁned within the realm of

catalysts that provide Hammett correlations for the aziridination

of para-substituted styrenes that can be accommodated with

polar parameters alone (σ_P, σ⁺), or by a combination of polar and

spin-delocalization parameters (σ*, σ_JJ*) with dominant polar

contribution.

In an almost concurrent publication, de Bruin and co-

workers⁴¹advance a similar argument with the assistance of

electrophilic Co(III)-nitrene radical aziridination reagents,

generated from the reaction of anionic [Co^III(TAML^red)]⁻or

the one-electron oxidized and neutral [Co^III(TAML^sq)] with

PhINNs. Hammett plots for para-substituted styrene aziridina-

tions are ﬁtted with both polar (σ⁺) and spin-delocalization

parameters (σ_JJ*) with large |ρ⁺/ρ_JJ*| values (5.71 for

[Co^III(TAML^red)]⁻and 8.64 for [Co^III(TAML^sq)]), hence

these systems can also be considered as good candidates for

testing the EA criterion. Indeed, the larger |ρ⁺/ρ_JJ*| value for

[Co^III(TAML^sq)] and DFT calculations indicate that the energy

barrier for the initial, rate-limiting reaction of the incipient

[Co^III]NNs and styrene to generate the N−C_bbond is lower for

[Co^III(TAML^sq)] versus [Co^III(TAML^red)]⁻, in agreement with

an anticipated higher EA value for the nitrene species generated

from [Co^III(TAML^sq)]. Surprisingly, the experimental rate for

A global electrophilicy index (GEI) was also computed for

[L⁴Co]NTs and [L⁸Co]NTs by employing Stephan’s improved

methodology.⁶²For the quartet spin state, GEI is calculated to

be 5.0 eV for [L⁴Co]NTs and 5.7 eV for [L⁸Co]NTs. The

corresponding values for the sextet spin state are 4.7 and 6.1 eV

for [L⁴Co]NTs and [L⁸Co]NTs, respectively. Thus,

[L⁸Co]NTs is more electrophilic than [L⁴Co]NTs on the

basis of the GEI criterion.

Unfortunately, all eﬀorts to map the aziridination reaction

coordinate starting from [L⁴Co]NTs and styrene have not been

successful in locating an acceptable transition state for the initial

N−C_bbond formation. All three spin states (quartet, sextet,

doublet) of [L⁴Co]NTs generated large activation barriers for

this initial step (∼50 kcal/mol). However, when dispersion-

corrected DFT was applied,⁶³as appropriate for polarizable

bulky groups such as the tert-Bu, the corresponding barriers were

reduced by approximately 20 kcal/mol. These barriers are still

signiﬁcant by comparison to those we have previously identiﬁed

for the reaction of [L⁸Co]NTs (sextet) and styrene (23.4 kcal/

mol for the turnover-limiting N−C_bbond formation).⁴⁴Further

experimentation and attendant DFT calculations will be

required to unravel reliable trends and contributing factors

with the assistance of catalysts that feature substituents spanning

the CF₃to CMe₃range.

1987

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the aziridination of styrene by these two catalysts favors

[Co^III(TAML^red)]⁻versus [Co^III(TAML^sq)], but this has been

attributed largely to the instability of the latter reagent. A

distinctive feature of the Co(III) systems is associated with the

redox noninnocent character of the tetraanionic ligand

(TAML^red)⁴⁻, which can be successively oxidized in one-

electron steps to (TAML^sq)³⁻and (TAML^q)²⁻. The authors

argue convincingly that the emerging radical nitrene species

[Co^III(TAML^sq)(^•NNs)]⁻/Co^III(TAML^q)(^•NNs)₂]⁻and

[Co^III(TAML^q)(^•NNs)], resulting from [Co^III(TAML^red)]⁻

and [Co^III(TAML^sq)], respectively, interact with styrene in the

rate-limiting step by means of an asynchronous transition state,

encompassing signiﬁcant single-electron transfer from styrene to

the oxidized TAML ligand and a nucleophilic attack by the

nitrene lone pair (in lieu of the ^•NNs p radical) at the incipient

styrenyl radical cation. The attendant single-electron relocation

(TAML → Co(III) → ^•NNs) reestablishes the N lone pair and

retains the Co(III) oxidation state. Similar participation of

charge transfer in the rate-limiting transition state between high-

valent metal nitrenes/imidos and sulﬁdes has been recently

showcased for many other nitrene-transfer catalysts⁶⁴and is now

established as a common mechanistic feature.

the Mn(II) reagents.⁴⁴Indeed, a closer inspection of the acyl-

substituted subset of the Co(II) library of reagents reveals a wide

range of yields in the aziridination of styrene that cannot be

correlated with the anticipated electrophilicities of the

corresponding cobalt-nitrene moieties. To further pinpoint the

provenance of these disparities, we selected the high-yielding

L⁸Co (−COCF₃arm) and L⁴Co (−COCMe₃arm) for further

investigation. The L⁸Co was previously studied⁴⁴in tandem with

the L⁸Mn and L⁸Fe congeners and found to be more reactive and

selective than the other two base metal analogues. Mechanistic

and computational studies showed that all three L⁸M reagents

follow a two-step styrene aziridination path (turnover-limiting

addition of [L⁸M^III]−^•NTs to the β-styrenyl carbon followed by

product-determining ring-closure via radical rebound), with

activation barriers in the order Fe > Mn > Co for both steps. The

trend is consistent with the anticipated metal-nitrene electro-

philicities (ﬁrst step) and ease of reduction from M(III) to

M(II) (second step), hence highlighting the dominant role of

EA in both steps of styrene aziridination (aliphatic oleﬁns do not

follow the same trend for the second step).

The representative case of L⁴Co, however, presents a

conundrum, inasmuch as its reactivity in terms of styrene

aziridination yields is comparable to that provided by L⁸Co.

More importantly, the rate of product buildup in the ﬁrst hour of

the reaction mediated by L⁴Co is superior to that of L⁸Co

(Figure 8). These results are diﬃcult to reconcile for a reagent

such as L⁴Co, whose Co(II/III) couple is cathodically shifted by

500 mV versus that of L⁸Co, and its nitrene derivative

[L⁴Co]NTs is computed to have a lower global electrophilicity

index (GEI) than that of [L⁸Co]NTs, in agreement with the

electronic nature of the CMe₃and CF₃substituents. In addition,

the enhanced reactivity of L⁴Co deviates from that of the

corresponding L⁴Mn and L⁴Fe reagents, which exhibit

signiﬁcantly lower yields (and rates) in styrene aziridinations

by comparison to the L⁸Mn and L⁸Fe analogues, in line with

their electrophilic characteristics.

In a previous comprehensive study from our lab,⁴⁴we have

shown that a library of Mn(II) catalysts, supported by the vast

majority of the ligands used in the present work (L¹⁻¹⁵H₃),

mediates alkene aziridinations with reactivity that increases in

parallel with increasing electrophilicity of the putative

[Mn^III]−^•NTs active oxidant. Moreover, the electrophilicity

criterion holds across the base metals, inasmuch as the reactivity

of the best performing Mn(II) reagent L⁸Mn is inferior to that of

the more acidic L⁸Co. Although the electron aﬃnity of the

metal-nitrene reigns supreme for all these reagents, the

molecular interaction between [M^III]−^•NR and styrene in the

rate-limiting formation of the initial N−C_bbond, is quite distinct

with respect to the reagents explored by Latour and de Bruin.

Indeed, Hammett correlations for the aziridination of styrenes

mediated by our L⁸M reagents (M = Mn, Fe, Co) reveal rather

modest positive charge buildup on the α-styrenyl carbon

(increasing with metal acidity in the expected order: Fe < Mn

< Co), and require the inclusion of competitive spin-

delocalization contributions (|ρ_mb/ρ_JJ*| = 0.75 (Mn), 1.17

(Fe), 1.00 (Co); the correlation for Fe was rather weak). The

fact that these reagents demonstrate more modest charge-

transfer characteristics is consistent with the operation of

presumptive metal nitrenes ([M^III]−^•NR) resting at a lower

oxidizing level than the high-valent iron and cobalt nitrenes of

Latour and de Bruin, respectively. Overall, these Mn(II)

reagents and congeners can also be accommodated under the

general EA criterion advanced by Latour (after all, they are

catalysts engaged in typical electrophilic radical reactions),

although they are not characterized by a dominant charge-

transfer component. Incidentally, a strongly enhanced radical

contribution, as in the case of Betley’s iron dipyrrinato

complexes (|ρ_mb/ρ_JJ*| = 0.04 for NAd),^18ehas been

interpreted⁴²as the result of a competitive energy barrier for

the second, ring-closing step (radical rebound).

Mechanistic analysis of the operation of L⁴Co in styrene

aziridination indicate that both polar (σ_mb) and spin-

delocalization (σ_JJ*) parameters are needed to ﬁt Hammett

plots, suggesting that both modest positive charge buildup and

radical stabilization participate in the turnover-limiting step. The

unexpected preponderance of the polar eﬀect for L⁴Co by

comparison to L⁸Co can be traced both in the slightly higher

values of absolute ρ_mb(−0.58) and relative |ρ_mb/ρ_JJ*| (1.17)

than those observed for L⁸Co (ρ_mb= −0.56, |ρ_mb/ρ_JJ*| = 1.0).

The secondary KIE values obtained from the competition

between styrene and selectively deuterated styrene in aziridina-

tions conﬁrm that both catalysts operate via an initial, turnover-

limiting N−C_bbond formation step, but also indicate that the

L⁴Co mediated pathway incorporates more signiﬁcant rehy-

bridization of the β-styrenyl carbon in the transition state, hence

placing this TS energetically closer to the resulting radical

intermediate [L⁴Co]N(Ts)−CH₂−^•CH₂Ph. Moreover, the

ring-closing step (radical rebound) seems to operate via a

miniscule energy barrier for both L⁴Co and L⁸Co styrene

aziridinations, but the one for L⁴Co is even more suppressed

than that for L⁸Co, as judged by the superior retention of

stereochemistry in the aziridination of the sensitive substrate cis-

β-styrene. This runs counter to what is usually the main driving

force for the aziridine-ring closure of styrene, namely the ease of

reduction from Co(III) to Co(II).^22g,44,65All these mechanistic

observations would have been perfectly in line, had the

supporting L⁴ligand been more electron-withdrawing than L⁸.

The library of the Co(II) reagents (S = 3/2) reported in this

work showcases some surprising deviations from the EA

criterion. Although the importance of the electrophilicity of

the metal-nitrene can still be detected in the relative enhanced

yields provided by the Co(II) compounds possessing acyl-

versus aryl- or alkyl-substituted ligands, the trend is certainly not

as smooth and predictable as that previously encountered with

1988

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DFT calculations on the electronic and geometric disposition

expected to outcompete the metal-bound nitrene radical,

they might oﬀer stabilizing interactions not yet realized.

On the other hand, the similarity of the Hammett

parameters for L⁴Co and L⁸Co suggests that the electronic

diﬀerences in the ground states of [L⁴Co]NTs and

[L⁸Co]NTs may have only a small eﬀect on their

reactivities, but this point requires further elaboration

once more information is available for the corresponding

transition states.

4

̀

of the presumptive [L Co]NTs vis-a-vis the previously explored

[L⁸Co]NTs highlight how a small ligand modiﬁcation can result

in a major electronic rearrangement. First, the ground state of

[L⁴Co]NTs is computed to be the quartet (S = 3/2), positioned

slightly underneath the sextet (S = 5/2). The sextet is the clear

ground state of [L⁸Co]NTs, presumably due to the weaker

ligand ﬁeld provided by the L⁸ligand. Geometrically, both cobalt

nitrenes are quite similar, their most outstanding feature being

the elongation of one of the equatorial N residues to a

noncoordinating position. However, spin-density calculations

reveal that the noncoordinating arm of [L⁴Co]NTs is one-

electron oxidized, whereas the corresponding arm of

[L⁸Co]NTs is redox innocent. The single-electron distribution

of the resulting N-aryl amidyl radical in [L⁴Co]NTs is spread

throughout the noncoordinating arm, with almost half of the

spin density being localized on the N atom. Apparently, the

electron withdrawing CF₃residue protects the noncoordinating

arm of [L⁸Co]NTs from a similar one-electron oxidation. The

overall electronic picture for the ground state of the two cobalt

nitrenes is schematically summarized in Figure 12. As noted

above, the [L⁴Co]NTs sextet (α-spin on the N-aryl amidyl

radical) is calculated to be only 0.1 kcal/mol higher in free

energy relative to the quartet.

(iii) Multiple spin-state reactivity channels,^65c,69such as those

oﬀered by the almost isoenergetic quartet and sextet states

of [L⁴Co]NTs, may aﬀord enhanced reactivity proﬁles in

aziridinations⁷⁰by comparison to a potentially single

spin-state operation by the [L⁸Co]NTs sextet.

(iv) London dispersion (LD) interactions applying intra-

molecularly between highly polarizable alkyl substituents

(also known as σ−σ interactions) are now well established

stabilizing forces of sterically congested molecules in

solution, by means of favorable enthalpic contributions.⁷¹

The tert-Bu group and other conformationally rigid alkyl

groups (ﬂexible alkyl groups have an unfavorable entropic

impact)⁷²have been credited as “dispersion energy

donors”,⁷³and deemed responsible for stabilizing many

highly congested organic and inorganic compounds.⁷¹⁻⁷⁴

More importantly, LD forces have started receiving

recognition as contributors to observed chemical

reactivity and catalytic outcomes.⁷⁵In the more tight

reaction cavity of cobalt reagents, the stabilization oﬀered

by tert-Bu groups via LD interactions can play a signiﬁcant

role, as already noted in our initial dispersion-corrected

DFT calculations. Interestingly, the L⁷Co reagent, which

carries the less polarizable i-Pr substituent, demonstrates

lower aziridination rates, not unlike those of L⁸Co.

Future experimental and computational research will seek to

disentangle and quantify the factors contributing to the

enhancement of catalytic reactivity above and beyond the

underlying electrophilic character of the active oxidant, and

further explore whether reagents with other rigid alkyl

substituents can be superior mediators of nitrene-transfer

chemistry.

Figure 12. Schematic distribution of spins in the ground state of

[L⁴Co]NTs and [L⁸Co]NTs.

Whereas a deﬁnitive justiﬁcation for the higher reactivity of

L⁴Co, in spite of lower electrophilicity, versus L⁸Co cannot be

provided at the present time, the following observations should

be taken into account:

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.organomet.1c00267.

(i) Although it cannot be excluded, it is deemed rather

unlikely that the ease of formation of the cobalt-nitrene

itself (presumably favoring [L⁴Co]NTs) will be a

contributing factor, since our previous calculations for

the reaction of [L⁸M^II] (M = Mn, Fe, Co) and PhINTs

indicate almost instantaneous generation of [L⁸M]NTs.

Rate-limiting metal-nitrene formation is more common

with organic azides (RN₃).⁶⁶

■

sı

Experimental procedures; physicochemical character-

ization of new compounds; crystallographic information;

catalytic and mechanistic data (PDF)

(ii) The fact that L⁴M (M = Mn, Fe), as well as a wide range of

other Mn(II) reagents, exhibit reactivities consistent with

the EA criterion, whereas L⁴Co and other Co(II) reagents

demonstrate deviations, suggest that ligand-centered

contributions to the overall oxidizing ability of the reagent

may enable more favorable reactivity channels. Indeed,

[LCo^III]NTs is more likely to store oxidizing equivalents

on ligand residues, as inferred by the cyclic voltammo-

grams of the LCo^IIreagents, and anticipated due to the

superior oxidizing power of Co(III) versus Mn(III) or

Fe(III). Among other possibilities, N-aryl amidyl radicals

are known to add to alkenes, at least intramolecularly,⁶⁷

and more electrophilic N-aryl sulfonamidyl radicals can

even add intermolecularly.⁶⁸Although they are not

Accession Codes

CCDC 2069526−2069535 and 2069537−2069542 contain the

supplementary crystallographic data for this paper. These data

can be obtained free of charge via www.ccdc.cam.ac.uk/

data_request/cif, or by emailing data_request@ccdc.cam.ac.

uk, or by contacting The Cambridge Crystallographic Data

Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44

1223 336033.

AUTHOR INFORMATION

Corresponding Authors

■

Thomas R. Cundari − Department of Chemistry, Center for

Advanced Scientiﬁc Computing and Modeling (CASCaM),

University of North Texas, Denton, Texas 76203, United

1989

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pubs.acs.org/Organometallics

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R., Ramsden, C. A., Scriven, E. F. V., Taylor, R. J. K., Eds.; Elsevier

Science: Amsterdam, 2008; Vol. 1, pp 1−104. (c) Sweeney, J. B.

Aziridines: epoxides ’ ugly cousins? Chem. Soc. Rev. 2002, 31, 247−258.

(d) Lapinsky, D. J. Three-Membered Ring Systems. Prog. Heterocycl.

Chem. 2015, 27, 61−85. (e) Botuha, C.; Chemla, F.; Ferreira, F.; Pérez-

Luna, A. Aziridines in Natural Product Synthesis. In Heterocycles in

Natural Product Synthesis; Majumdar, K. C., Chattopadhyay, S. K., Eds.;

Wiley-VCH: Weinheim, 2011; pp 1−39. (f) Padwa, A.; Murphree, S. S.

Epoxides and aziridines − A mini review. ARKIVOC 2006, 2006, 6−33.

(2) (a) Ouyang, K.; Hao, W.; Zhang, W.-X.; Xi, Z. Transition-Metal-

Catalyzed Cleavage of C −N Single Bonds. Chem. Rev. 2015, 115,

12045−12090. (b) Rotstein, B. H.; Zaretsky, S.; Rai, V.; Yudin, A. K.

Small Heterocycles in Multicomponent Reactions. Chem. Rev. 2014,

114, 8323−8359. (c) Huang, C.-Y.; Doyle, A. G. The Chemistry of

Transition Metals with Three-Membered Ring Heterocycles. Chem.

Rev. 2014, 114, 8153−8198. (d) Stankovic, S.; D’hooghe, M.; Catak, S.;

Eum, H.; Waroquier, M.; Van Speybroeck, V.; De Kimpe, N.; Ha, H.-J.

Regioselectivity in the ring opening of non-activated aziridines. Chem.

Soc. Rev. 2012, 41, 643−665. (e) Cardoso, A. L.; Pinho e Melo, T. M. V.

D. Aziridines in Formal [3 + 2] Cycloadditions: Synthesis of Five-

Membered Heterocycles. Eur. J. Org. Chem. 2012, 6479−6501. (f) Lu,

P. Recent developments in regioselective ring opening of aziridines.

Tetrahedron 2010, 66, 2549−2560. (g) Schneider, C. Catalytic,

Enantioselective Ring Opening of Aziridines. Angew. Chem., Int. Ed.

2009, 48, 2082−2084. (h) Hu, X. E. Nucleophilic ring opening of

aziridines. Tetrahedron 2004, 60, 2701−2743. (i) McCoull, W.; Davis,

F. A. Recent Synthetic Applications of Chiral Aziridines. Synthesis 2000,

2000, 1347−1365.

(3) (a) Tomasz, M.; Mitomycin, C. Small, fast and deadly (but very

selective). Chem. Biol. 1995, 2, 575−579. (b) Coleman, R. S.; Li, J.;

Navarro, A. Total Synthesis of Azinomycin A. Angew. Chem., Int. Ed.

2001, 40, 1736−1739. (c) Zhao, Q.; He, Q.; Ding, W.; Tang, M.; Kang,

Q.; Yu, Y.; Deng, W.; Zhang, Q.; Fang, J.; Tang, G.; Liu, W.

Characterization of the Azinomycin B Biosynthetic Gene Cluster

Revealing a Different Iterative Type I Polyketide Synthase for

Naphthoate Biosynthesis. Chem. Biol. 2008, 15, 693−705. (d) Ogasa-

wara, Y.; Liu, H.-W. Biosynthetic Studies of Aziridine Formation in

Azicemicins. J. Am. Chem. Soc. 2009, 131, 18066−18068. (e) Nakao, Y.;

Fujita, M.; Warabi, K.; Matsunaga, S.; Fusetani, N. Miraziridine A, a

Novel Cysteine Protease Inhibitor from the Marine Sponge Theonella

aff. Mirabilis . J. Am. Chem. Soc. 2000, 122, 10462−10463. (f) Liu, Y.; Li,

M.; Mu, H.; Song, S.; Zhang, Y.; Chen, K.; He, X.; Wang, H.; Dai, Y.;

Lu, F.; Yan, Z.; Zhang, H. Identification and characterization of the

ficellomycin biosynthesis gene cluster from Streptomyces f i cellus . Appl.

Microbiol. Biotechnol. 2017, 101, 7589−7602. (g) Foulke-Abel, J.; Agbo,

H.; Zhang, H.; Mori, S.; Watanabe, C. M. H. Mode of action and

biosynthesis of the azabicycle-containing natural products azinomycin

and ficellomycin. Nat. Prod. Rep. 2011, 28, 693−704.

Email: thomas.cundari@unt.edu

Pericles Stavropoulos − Department of Chemistry, Missouri

University of Science and Technology, Rolla, Missouri 65409,

United States; Email: pericles@mst.edu

Authors

Anshika Kalra − Department of Chemistry, Missouri University

of Science and Technology, Rolla, Missouri 65409, United

States

Vivek Bagchi − Department of Chemistry, Missouri University

of Science and Technology, Rolla, Missouri 65409, United

States; Institute of Nano Science and Technology, Mohali,

Punjab 160062, India

Patrina Paraskevopoulou − Inorganic Chemistry Laboratory,

Department of Chemistry, National and Kapodistrian

University of Athens, Athens 15771, Greece; orcid.org/

0000-0002-5166-8946

Purak Das − Department of Chemistry, Missouri University of

Science and Technology, Rolla, Missouri 65409, United States

Lin Ai − Department of Chemistry, Missouri University of

Science and Technology, Rolla, Missouri 65409, United States;

College of Chemistry, Beijing Normal University, Beijing

100875, People’s Republic of China; orcid.org/0000-

0001-9721-1493

Yiannis Sanakis − Institute of Advanced Materials,

Physicochemical Processes, Nanotechnology and Microsystems,

NCSR “Demokritos”, Athens 15310, Greece

Grigorios Raptopoulos − Inorganic Chemistry Laboratory,

Department of Chemistry, National and Kapodistrian

University of Athens, Athens 15771, Greece

Sudip Mohapatra − Department of Chemistry, Missouri

University of Science and Technology, Rolla, Missouri 65409,

United States

Amitava Choudhury − Department of Chemistry, Missouri

University of Science and Technology, Rolla, Missouri 65409,

United States; orcid.org/0000-0001-5496-7346

Zhicheng Sun − Department of Chemistry, Center for Advanced

Scientiﬁc Computing and Modeling (CASCaM), University of

North Texas, Denton, Texas 76203, United States

Complete contact information is available at:

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(4) (a) Ismail, F. M. D.; Levitsky, D. O.; Dembitsky, V. M. Aziridine

alkaloids as potential therapeutic agents. Eur. J. Med. Chem. 2009, 44,

3373−3387. (b) Ballereau, S.; Andrieu-Abadie, N.; Saffon, N.;

Génisson, Y. Synthesis and biological evaluation of aziridine-containing

analogs of phytosphingosine. Tetrahedron 2011, 67, 2570−2578.

(c) Dorr, R. T.; Wisner, L.; Samulitis, B. K.; Landowski, T. H.;

Remers, W. A. Anti-tumor activity and mechanism of action for a

cyanoaziridine-derivative, AMP423. Cancer Chemother. Pharmacol.

2012, 69, 1039−1049. (d) Pillai, B.; Cherney, M. M.; Diaper, C. M.;

Sutherland, A.; Blanchard, J. S.; Vederas, J. C.; James, M. N. G.

Structural insights into stereochemical inversion by diaminopimelate

epimerase: An antibacterial drug target. Proc. Natl. Acad. Sci. U. S. A.

2006, 103, 8668−8673.

(5) (a) Ujjain, S. K.; Bhatia, R.; Ahuja, P. Aziridine-functionalized

graphene: Effect of aromaticity for aryl functional groups on enhanced

power conversion efficiency of organic photovoltaic cells. J. Saudi Chem.

Soc. 2019, 23, 655−665. (b) Tuci, G.; Filippi, J.; Ba, H.; Rossin, A.;

Luconi, L.; Pham-Huu, C.; Vizza, F.; Giambastiani, G. How to teach an

old dog new (electrochemical) tricks: aziridine-functionalized CNTs as

efficient electrocatalysts for the selective CO₂reduction to CO. J. Mater.

Chem. A 2018, 6, 16382−16389. (c) Moon, H. K.; Kang, S.; Yoon, H. J.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was generously supported by the National Institute of

General Medical Sciences of the National Institutes of Health

under Award Numbers R15GM117508 and R15GM139071 (to

P.S.), and in part by the National Science Foundation through

grant CHE-0412959 (to P.S.). Authors Z.S. and T.R.C. further

acknowledge the U.S. National Science Foundation for partial

support of this research through grants CHE-1464943 and

CHE-1953547. Authors G.R. and P.P. also thank the Special

Account of Research Grants of the National and Kapodistrian

University of Athens for partial support. We thank the reviewers

for many insightful comments.

REFERENCES

■

(1) (a) Aziridines and Epoxides in Organic Synthesis; Yudin, A. K., Ed.;

Wiley-VCH: Weinheim, 2006. (b) Padwa, A. Aziridines and Azirines:

Monocyclic. In Comprehensive Heterocyclic Chemistry III; Katritzky, A.

1990

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

Aziridine-functionalized polydimethylsiloxanes for tailorable polymeric

scaffolds: aziridine as a clickable moiety for structural modification of

materials. Polym. Chem. 2017, 8, 2287−2291. (d) Tuci, G.; Luconi, L.;

Rossin, A.; Berretti, E.; Ba, H.; Innocenti, M.; Yakhvarov, D.; Caporali,

S.; Pham-Huu, C.; Giambastiani, G. Aziridine-Functionalized Multi-

walled Carbon Nanotubes: Robust and Versatile Catalysts for the

Oxygen Reduction Reaction and Knoevenagel Condensation. ACS

Appl. Mater. Interfaces 2016, 8, 30099−30106. (e) Chen, R.; Chen, J. S.;

Zhang, C.; Kessler, M. R. Rapid room-temperature polymerization of

bio-based multiaziridine-containing compounds. RSC Adv. 2015, 5,

1557−1563. (f) Kim, H.-J.; Chaikittisilp, W.; Jang, K.-S.; Didas, S. A.;

Johnson, J. R.; Koros, W. J.; Nair, S.; Jones, C. W. Aziridine-

Functionalized Mesoporous Silica Membranes on Polymeric Hollow

Fibers: Synthesis and Single-Component CO₂and N₂Permeation

Properties. Ind. Eng. Chem. Res. 2015, 54, 4407−4413.

(6) (a) Degennaro, L.; Trinchera, P.; Luisi, R. Recent Advances in the

Stereoselective Synthesis of Aziridines. Chem. Rev. 2014, 114, 7881−

7929. (b) Pellissier, H. Recent Developments in Asymmetric

Aziridination. Adv. Synth. Catal. 2014, 356, 1899−1935. (c) Pellissier,

H. Recent developments in asymmetric aziridination. Tetrahedron

2010, 66, 1509−1555. (d) Singh, G. S.; D’hooghe, M.; De Kimpe, N.

Synthesis and Reactivity of C -Heteroatom-Substituted Aziridines.

Chem. Rev. 2007, 107, 2080−2135. (e) Watson, I. D. G.; Yu, L.;

Yudin, A. K. Advances in Nitrogen Transfer Reactions Involving

Aziridines. Acc. Chem. Res. 2006, 39, 194−206. (f) Osborn, H. M. I.;

Sweeney, J. The asymmetric synthesis of aziridines. Tetrahedron:

Asymmetry 1997, 8, 1693−1715. (g) Tanner, D. Chiral Aziridines −

Their Synthesis and Use in Stereoselective Transformations. Angew.

Chem., Int. Ed. Engl. 1994, 33, 599−619.

Bond Cleavage: Synthesis of Highly Substituted 3-Pyrrolines. Org. Lett.

2011, 13, 5940−5943. (d) Dauban, P.; Dodd, R. H. PhI = NSes: A New

Iminoiodinane Reagent for the Copper-Catalyzed Aziridination of

Olefins. J. Org. Chem. 1999, 64, 5304−5307. (e) Alonso, D. A.;

Andersson, P. G. Deprotection of Sulfonyl Aziridines. J. Org. Chem.

1998, 63, 9455−9461. (f) Bergmeier, S. C.; Seth, P. P. A general

method for deprotection of N -toluenesulfonyl aziridines using sodium

naphthalenide. Tetrahedron Lett. 1999, 40, 6181−6184. (g) Warner, D.

L.; Hibberd, A. M.; Kalman, M.; Klapars, A.; Vedejs, E. N -Silyl

Protecting Groups for Labile Aziridines: Application toward the

Synthesis of N -H Aziridinomitosenes. J. Org. Chem. 2007, 72, 8519−

8522.

(12) (a) Zhu, Y.; Wang, Q.; Cornwall, R. G.; Shi, Y. Organocatalytic

Asymmetric Epoxidation and Aziridination of Olefins and Their

Synthetic Applications. Chem. Rev. 2014, 114, 8199−8256. (b) Zhang,

Y.; Lu, Z.; Wulff, W. D. Catalytic Asymmetric Aziridination with

Catalysts Derived from VAPOL and VANOL. Synlett 2009, 2009,

2715−2739. (c) Yoshimura, A.; Middleton, K. R.; Zhu, C.; Nemykin, V.

N.; Zhdankin, V. V. Hypoiodite-Mediated Metal-Free catalytic

Aziridination of Alkenes. Angew. Chem., Int. Ed. 2012, 51, 8059−

8062. (d) Siu, T.; Yudin, A. K. Practical Olefin Aziridination with a

Broad Substrate Scope. J. Am. Chem. Soc. 2002, 124, 530−531.

(13) (a) Chandrachud, P. P.; Jenkins, D. M. Transition Metal

Aziridination Catalysts. In Encyclopedia of Inorganic and Bioinorganic

Chemistry; Wiley Online Library, 2017; pp 1−11. (b) Che, C.-M.; Lo,

V. K.-Y.; Zhou, C.-Y. Oxidation by Metals (Nitrene). In Comprehensive

Organic Synthesis II, 2nd ed.; Knochel, P., Molander, G. A., Eds.;

Elsevier: Amsterdam, 2014; Vol. 7, pp 26−85. (c) Jung, N.; Bräse, S.

New Catalysts for the Transition-Metal-Catalyzed Synthesis of

Aziridines. Angew. Chem., Int. Ed. 2012, 51, 5538−5540. (d) Jiang,

H.; Zhang, X. P. Oxidation: C−N Bond Formation by Oxidation

(Aziridines). In Comprehensive Chirality; Carreira, E. M., Yamamoto,

H., Eds.; Elsevier: Amsterdam, 2012; Vol. 5, pp 168−182. (e) Mößner,

C.; Bolm, C. Catalyzed Asymmetric Aziridinations. In Transition Metals

for Organic Synthesis, 2nd ed.; Beller, M., Bolm, C., Eds.; Wiley-VCH:

Weinheim, Germany, 2004; pp 389−402. (f) Halfen, J. A. Recent

Advances in Metal-Mediated Carbon-Nitrogen Bond Formation

Reactions: Aziridination and Amidation. Curr. Org. Chem. 2005, 9,

(7) (a) Darses, B.; Rodrigues, R.; Neuville, L.; Mazurais, M.; Dauban,

P. Transition metal-catalyzed iodine(III)-mediated nitrene transfer

reactions: efficient tools for challenging syntheses. Chem. Commun.

2017, 53, 493−508. (b) Chang, J. W. W.; Ton, T. M. U.; Chan, P. W. H.

Transition-metal-catalyzed aminations and aziridinations of C −H and

C C bonds with iminoiodinanes. Chem. Rec. 2011, 11, 331−357.

(c) Karila, D.; Dodd, R. H. Recent Progress in Iminoiodane-Mediated

Aziridination of Olefins. Curr. Org. Chem. 2011, 15, 1507−1538.

(8) (a) Chanda, B. M.; Vyas, R.; Landge, S. S. Synthesis of aziridines

using new catalytic systems with bromamine-T as the nitrene source. J.

Mol. Catal. A: Chem. 2004, 223, 57−60. (b) Chanda, B. M.; Vyas, R.;

Bedekar, A. V. Investigations in the Transition Metal catalyzed

Aziridination of Olefins, Amination, and Other Insertion Reactions

with Bromamine-T as the Source of Nitrene. J. Org. Chem. 2001, 66,

30−34.

657−669. (g) Muller, P.; Fruit, C. Enantioselective Catalytic

̈

Aziridinations and Asymmetric Nitrene Insertions into CH Bonds.

Chem. Rev. 2003, 103, 2905−2919.

(14) Historic Mn/Fe porphyrinoid systems: (a) Mansuy, D.; Mahy, J.-

P.; Dureault, A.; Bedi, G.; Battioni, P. Iron- and manganese-porphyrin

catalysed aziridination of alkenes by tosyl- and acyl-iminoiodobenzene.

J. Chem. Soc., Chem. Commun. 1984, 1161−1163. (b) Mahy, J.-P.;

Battioni, P.; Mansuy, D. Formation of an Iron(III) Porphyrin Complex

with a Nitrene Moiety Inserted into a Fe −N Bond during Alkene

Aziridination by [(Tosylimido)iodo]benzene Catalyzed by Iron(III)

Porphyrins. J. Am. Chem. Soc. 1986, 108, 1079−1080. (c) Mahy, J.-P.;

Bedi, G.; Battioni, P.; Mansuy, D. Aziridination of alkenes catalysed by

porphyrinirons: Selection of catalysts for optimal efficiency and

stereospecificity. J. Chem. Soc., Perkin Trans. 2 1988, 2, 1517−1524.

(d) Groves, J. T.; Takahashi, T. Activation and Transfer of Nitrogen

from Nitridomanganese(V) Porphyrin Complex. The Aza Analogue of

Epoxidation. J. Am. Chem. Soc. 1983, 105, 2073−2074.

(9) (a) Ma, Z.; Zhou, Z.; Kurti, L. Direct and Stereospecific Synthesis

̈

of N-H and N-Alkyl Aziridines from Unactivated Olefins Using

Hydroxylamine-O-Sulfonic Acids. Angew. Chem., Int. Ed. 2017, 56,

9886−9890. (b) Jat, J. L.; Paudyal, M. P.; Gao, H.; Xu, Q.-L.;

Yousufuddin, M.; Devarajan, D.; Ess, D. H.; Kurti, L.; Falck, J. R. Direct

̈

Stereospecific Synthesis of Unprotected N-H and N-Me Aziridines

from Olefins. Science 2014, 343, 61−65. (c) Lebel, H.; Huard, K.;

Lectard, S. N -Tosyloxycarbamates as a Source of Metal Nitrenes:

Rhodium-Catalyzed C −H Insertion and Aziridination Reactions. J. Am.

Chem. Soc. 2005, 127, 14198−14199.

(10) (a) Driver, T. G. Recent advances in transition metal-catalyzed

N -atom transfer reactions of azides. Org. Biomol. Chem. 2010, 8, 3831−

3846. (b) Intrieri, D.; Zardi, P.; Caselli, A.; Gallo, E. Organic azides:

“energetic reagents ” for the inter molecular amination of C −H bonds.

Chem. Commun. 2014, 50, 11440−11453.

(15) For select reviews of metal porphyrinoid nitrene-transfer

systems, see: (a) Intrieri, D.; Carminati, D. M.; Gallo, E. Recent

Advances in Metal Porphyrinoid-Catalyzed Nitrene and Carbene

Transfer Reactions. In Handbook of Porphyrin Science; Kadish, K. M.,

Smith, K. M., Guilard, R., Eds.; World Scientiﬁc, 2016; Vol. 38, pp 1−

99. (b) Anding, B. J.; Woo, L. K. An Overview of Metalloporphyrin-

Catalyzed Carbon and Nitrogen Group Transfer Reactions. In

Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard,

R., Eds.; World Scientiﬁc, 2012; Vol. 21, pp 145−319. (c) Ruppel, J. V.;

Fields, K. B.; Snyder, N. L.; Zhang, X. P. Metalloporphyrin-Catalyzed

Asymmetric Atom/Group Transfer Reactions. In Handbook of

Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;

World Scientiﬁc, 2010; Vol. 10, pp 1−84. (d) Fantauzzi, S.; Caselli, A.;

(11) (a) Hao, W.; Wu, X.; Sun, J. Z.; Siu, J. C.; MacMillan, S. N.; Lin,

S. Radical Redox-Relay Catalysis: Formal [3 + 2] Cycloaddition of N -

Acylaziridines and Alkenes. J. Am. Chem. Soc. 2017, 139, 12141−12144.

́

(b) Goossens, H.; Vervisch, K.; Catak, S.; Stankovic, S.; D’hooghe, M.;

De Proft, F.; Geerlings, P.; De Kimpe, N.; Waroquier, M.; Van

Speybroeck, V. Reactivity of Activated versus Nonactivated 2-

(Bromomethyl)aziridines with respect to Sodium Methoxide: A

Combined Computational and Experimental Study. J. Org. Chem.

2011, 76, 8698−8709. (c) Li, L.; Zhang, J. Lewis Acid-catalyzed [3 + 2]

Cycloaddition of Alkynes with N-Tosyl-aziridines via Carbon −Carbon

1991

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

Gallo, E. Nitrene transfer reactions mediated by metallo-porphyrin

complexes. Dalton Trans. 2009, 5434−5443.

Iron Catalyst. Organometallics 2016, 35, 1652−1657. (m) Cramer, S.

A.; Jenkins, D. M. Synthesis of Aziridines from Alkenes and Aryl Azides

with a Reusable Macrocyclic Tetracarbene Iron Catalyst. J. Am. Chem.

(16) For select reviews of engineered hemoproteins for nitrene

transfer to alkenes, see: (a) Brandenberg, O. F.; Fasan, R.; Arnold, F. H.

Exploiting and engineering hemoproteins for abiological carbene and

nitrene transfer reactions. Curr. Opin. Biotechnol. 2017, 47, 102−111.

(b) Farwell, C. C.; Zhang, R. K.; McIntosh, J. A.; Hyster, T. K.; Arnold,

F. H. Enantioselective Enzyme-Catalyzed Aziridination Enabled by

Active-Site Evolution of a Cytochrome P450. ACS Cent. Sci. 2015, 1,

89−93. (c) Gober, J. G.; Brustad, E. M. Non-natural carbenoid and

nitrenoid insertion reactions catalyzed by heme proteins. Curr. Opin.

Chem. Biol. 2016, 35, 124−132.

Soc. 2011, 133, 19342−19345. (n) Munoz, S. B., Jr.; Lee, W.-T.; Dickie,

̃

D. A.; Scepaniak, J. J.; Subedi, D.; Pink, M.; Johnson, M. D.; Smith, J. M.

Styrene Aziridination by Iron(IV) Nitrides. Angew. Chem., Int. Ed.

2015, 54, 10600−10603. (o) Nakanishi, M.; Salit, A.-F.; Bolm, C. Iron-

Catalyzed Aziridination Reactions. Adv. Synth. Catal. 2008, 350, 1835−

1840. (p) Mayer, A. C.; Salit, A.-F.; Bolm, C. Iron-catalysed

aziridination reactions promoted by an ionic liquid. Chem. Commun.

2008, 5975−5977. (q) Heuss, B. D.; Mayer, M. F.; Dennis, S.; Hossain,

M. M. Iron mediated nitrenoid transfer: [(η⁵-C₅H₅)Fe-

(CO)₂(THF)]⁺[BF₄]⁻catalyzed aziridination of olefins. Inorg. Chim.

Acta 2003, 342, 301−304.

(17) For select examples of Mn-based nitrene-transfer to alkenes, see:

(a) Zdilla, M. J.; Abu-Omar, M. M. Mechanism of Catalytic

Aziridination with Manganese Corrole: The Often Postulated High-

Valent Mn(V) Imido Is Not the Group Transfer Reagent. J. Am. Chem.

Soc. 2006, 128, 16971−16979. (b) Nishikori, H.; Katsuki, T. Catalytic

and highly enantioselective aziridination of styrene derivatives.

Tetrahedron Lett. 1996, 37, 9245−9248. (c) Minakata, S.; Ando, T.;

Nishimura, M.; Ryu, I.; Komatsu, M. Novel Asymmetric and

Stereospecific Aziridination of Alkenes with a Chiral Nitridomanganese

Complex. Angew. Chem., Int. Ed. 1998, 37, 3392−3394. (d) Du Bois, J.;

Tomooka, C. S.; Hong, J.; Carreira, E. M. Nitridomanganese(V)

Complexes: Design, Preparation, and Use as Nitrogen Atom-Transfer

Reagents. Acc. Chem. Res. 1997, 30, 364−372. (e) Lai, T.-S.; Kwong, H.-

L.; Che, C.-M.; Peng, S.-M. Catalytic and asymmetric aziridination of

alkenes catalyzed by a chiral manganese porphyrin complex. Chem.

Commun. 1997, 2373−2374. (f) Liang, J.-L.; Huang, J.-S.; Yu, X.-Q.;

Zhu, N.; Che, C.-M. Metalloporphyrin-Mediated Asymmetric Nitro-

gen-Atom Transfer to Hydrocarbons: Aziridination of Alkenes and

Amidation of Saturated C −H Bonds Catalyzed by Chiral Ruthenium

and Manganese Porphyrins. Chem. - Eur. J. 2002, 8, 1563−1572.

(g) O’Connor, K. J.; Wey, S.-J.; Burrows, C. J. Alkene aziridination and

epoxidation catalyzed by chiral metal salen complexes. Tetrahedron Lett.

1992, 33, 1001−1004.

(19) For select examples of Co-based nitrene-transfer to alkenes, see:

(a) Jiang, H.; Lang, K.; Lu, H.; Wojtas, L.; Zhang, X. P. Asymmetric

Radical Bicyclization of Allyl Azidoformates via Cobalt(II)-Based

Metalloradical Catalysis. J. Am. Chem. Soc. 2017, 139, 9164−9167.

(b) Jiang, H.; Lang, K.; Lu, H.; Wojtas, L.; Zhang, X. P. Intramolecular

Radical Aziridination of Allylic Sulfamoyl Azides by Cobalt(II)-Based

Metalloradical Catalysis: Effective Construction of Strained Hetero-

bicyclic Structures. Angew. Chem., Int. Ed. 2016, 55, 11604−11608.

(c) Subbarayan, V.; Jin, L.-M.; Cui, X.; Zhang, X. P. Room temperature

activation of aryloxysulfonyl azides by [Co(II)(TPP)] for selective

radical aziridination of alkenes via metalloradical catalysis. Tetrahedron

Lett. 2015, 56, 3431−3434. (d) Tao, J.; Jin, L.-M.; Zhang, X. P.

Synthesis of chiral N-phosphoryl aziridines through enantioselective

aziridination of alkenes with phosphoryl azide via Co(II)-based

metalloradical catalysis. Beilstein J. Org. Chem. 2014, 10, 1282−1289.

(e) Jin, L.-M.; Xu, X.; Lu, H.; Cui, X.; Wojtas, L.; Zhang, X. P. Effective

Synthesis of Chiral N -Fluoroaryl Aziridines through Enantioselective

Aziridination of Alkenes with Fluoroaryl Azides. Angew. Chem., Int. Ed.

2013, 52, 5309−5313. (f) Suarez, A. I. O.; Jiang, H.; Zhang, X. P.; de

Bruin, B. The radical mechanism of cobalt(II) porphyrin-catalyzed

olefin aziridination and the importance of cooperative H-bonding.

Dalton Trans. 2011, 40, 5697−5705. (g) Lu, H.; Jiang, H.; Hu, Y.;

Wojtas, L.; Zhang, X. P. Chemoselective intramolecular allylic C −H

amination versus C C aziridination through Co(II)-based metal-

loradical catalysis. Chem. Sci. 2011, 2, 2361−2366. (h) Subbarayan, V.;

Ruppel, J. V.; Zhu, S.; Perman, J. A.; Zhang, X. P. Highly asymmetric

cobalt-catalyzed aziridination of alkenes with trichloroethoxysulfonyl

azide (TcesN₃). Chem. Commun. 2009, 4266−4268. (i) Jones, J. E.;

Ruppel, J. V.; Gao, G.-Y.; Moore, T. M.; Zhang, X. P. Cobalt-Catalyzed

Asymmetric Olefin Aziridination with Diphenylphosphoryl Azide. J.

Org. Chem. 2008, 73, 7260−7265. (j) Ruppel, J. V.; Jones, J. E.; Huff, C.

A.; Kamble, R. M.; Chen, Y.; Zhang, X. P. A Highly Effective Cobalt

Catalyst for Olefin Aziridination with Azides: Hydrogen Bonding

Guided Catalyst Design. Org. Lett. 2008, 10, 1995−1998. (k) Gao, G.-

Y.; Jones, J. E.; Vyas, R.; Harden, J. D.; Zhang, X. P. Cobalt-Catalyzed

Aziridination with Diphenylphosphoryl Azide (DPPA): Direct Syn-

thesis of N -Phosphorus-Substituted Aziridines from Alkenes. J. Org.

Chem. 2006, 71, 6655−6658. (l) Gao, G.-Y.; Harden, J. D.; Zhang, X. P.

Cobalt-Catalyzed Efficient Aziridination of Alkenes. Org. Lett. 2005, 7,

3191−3193. (m) Caselli, A.; Gallo, E.; Fantauzzi, S.; Morlacchi, S.;

Ragaini, F.; Cenini, S. Allylic Amination and Aziridination of Olefins by

Aryl Azides Catalyzed by Co^II(tpp): A Synthetic and Mechanistic

Study. Eur. J. Inorg. Chem. 2008, 2008, 3009−3019. (n) Caselli, A.;

Gallo, E.; Ragaini, F.; Ricatto, F.; Abbiati, G.; Cenini, S. Chiral

porphyrin complexes of cobalt(II) and ruthenium(II) in catalytic

cyclopropanation and amination reactions. Inorg. Chim. Acta 2006, 359,

2924−2932. (o) Cenini, S.; Tollari, S.; Penoni, A.; Cereda, C. Catalytic

amination of unsaturated hydrocarbons: reactions of p-nitrophenyla-

zide with alkenes catalyzed by metallo-porphyrins. J. Mol. Catal. A:

Chem. 1999, 137, 135−146.

(18) For select examples of Fe-based nitrene-transfer to alkenes, see:

(a) Damiano, C.; Intrieri, D.; Gallo, E. Aziridination of alkenes

promoted by iron or ruthenium complexes. Inorg. Chim. Acta 2018,

470, 51−67. (b) Fingerhut, A.; Serdyuk, O. V.; Tsogoeva, S. B. Non-

heme iron catalysts for epoxidation and aziridination reactions of

challenging terminal alkenes: towards sustainability. Green Chem. 2015,

17, 2042−2058. (c) Correa, A.; Mancheno, O. G.; Bolm, C. Iron-

̃

catalysed carbon-heteroatom and heteroatom-heteroatom bond

forming processes. Chem. Soc. Rev. 2008, 37, 1108−1117. (d) Iovan,

D. A.; Betley, T. A. Characterization of Iron-Imido Species Relevant for

N -Group Transfer Chemistry. J. Am. Chem. Soc. 2016, 138, 1983−1993.

(e) Hennessy, E. T.; Liu, R. Y.; Iovan, D. A.; Duncan, R. A.; Betley, T. A.

Iron-mediated intermolecular N-group transfer chemistry with olefinic

substrates. Chem. Sci. 2014, 5, 1526−1532. (f) King, E. R.; Hennessy, E.

T.; Betley, T. A. Catalytic C −H Bond Amination from High-Spin Iron

Imido Complexes. J. Am. Chem. Soc. 2011, 133, 4917−4923. (g) Patra,

R.; Coin, G.; Castro, L.; Dubourdeaux, P.; Clémancey, M.; Pécaut, J.;

Lebrun, C.; Maldivi, P.; Latour, J.-M. Rational design of Fe catalysts for

olefin aziridination through DFT-based mechanistic analysis. Catal. Sci.

Technol. 2017, 7, 4388−4400. (h) Avenier, F.; Latour, J.-M. Catalytic

aziridination of olefins and amidation of thioanisole by a non-heme iron

complex. Chem. Commun. 2004, 1544−1545. (i) Liu, Y.; Che, C. M.

[Fe^III(F₂₀-tpp)Cl] Is an Effective Catalyst for Nitrene Transfer

Reactions and Amination of Saturated Hydrocarbons with Sulfonyl

and Aryl Azides as Nitrogen Source under Thermal and Microwave-

Assisted Conditions. Chem. - Eur. J. 2010, 16, 10494−10501. (j) Klotz,

K. L.; Slominski, L. M.; Hull, A. V.; Gottsacker, V. M.; Mas-Ballesté, R.;

Que, L., Jr.; Halfen, J. A. Non-heme iron(II) complexes are efficient

olefin aziridination catalysts. Chem. Commun. 2007, 2063−2065.

(k) Klotz, K. L.; Slominski, L. M.; Riemer, M. E.; Phillips, J. A.;

Halfen, J. A. Mechanism of the Iron-Mediated Alkene Aziridination

reaction: Experimental and Computational Investigations. Inorg. Chem.

2009, 48, 801−803. (l) Chandrachud, P. P.; Bass, H. M.; Jenkins, D. M.

Synthesis of Fully Aliphatic Aziridines with a Macrocycle Tetracarbene

(20) For select examples of Ni-based nitrene-transfer to alkenes, see:

(a) Waterman, R.; Hillhouse, G. L. Group Transfer from Nickel Imido,

Phosphinidine, and Carbene Complexes to Ethylene with Formation of

Aziridine, Phosphirane, and Cyclopropane Products. J. Am. Chem. Soc.

2003, 125, 13350−13351. (b) Lin, B. L.; Clough, C. R.; Hillhouse, G. L.

1992

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

Interactions of Aziridines with Nickel Complexes: Oxidative-Addition

and Reductive-Elimination Reactions that Break and Make C −N

Bonds. J. Am. Chem. Soc. 2002, 124, 2890−2891.

1994, 116, 2742−2753. (q) Kwart, H.; Khan, A. A. Copper-Catalyzed

Decomposition of Benzenesulfonyl Azide in Cyclohexene Solution. J.

Am. Chem. Soc. 1967, 89, 1951−1953.

(21) For historic examples of Cu-based enantioselective aziridina-

tions, see: (a) Li, Z.; Quan, R. W.; Jacobsen, E. N. Mechanism of the

(Diimine)copper-Catalyzed Asymmetric Aziridination of Alkenes.

Nitrene Transfer via Ligand-Accelerated Catalysis. J. Am. Chem. Soc.

1995, 117, 5889−5890. (b) Li, Z.; Conser, K. R.; Jacobsen, E. N.

Asymmetric Alkene Aziridination with Readily Available Chiral

Diimine-Based Catalysts. J. Am. Chem. Soc. 1993, 115, 5326−5327.

(c) Evans, D. A.; Faul, M. M.; Bilodeau, M. T.; Anderson, B. A.; Barnes,

D. M. Bis(oxazoline)-copper complexes as chiral catalysts for the

enantioselective aziridination of olefins. J. Am. Chem. Soc. 1993, 115,

5328−5329.

(23) For select examples of Ru-based nitrene-transfer to alkenes, see:

(a) Uchida, T.; Katsuki, T. Asymmetric Nitrene Transfer Reactions:

Sulfimidation, Aziridination and C −H Amination Using Azide

Compounds as Nitrene Precursors. Chem. Rec. 2014, 14, 117−129.

(b) Kim, C.; Uchida, T.; Katsuki, T. Asymmetric olefin aziridination

using a newly designed Ru(CO)(salen) complex as the catalyst. Chem.

Commun. 2012, 48, 7188−7190. (c) Kawabata, H.; Omura, K.; Uchida,

T.; Katsuki, T. Construction of Robust Ruthenium(salen)(CO)

Complexes and Asymmetric Aziridination with Nitrene Precursors in

the Form of Azide Compounds That Bear Easily Removable N -Sulfonyl

Groups. Chem. - Asian J. 2007, 2, 248−256. (d) Harvey, M. E.; Musaev,

D.; Du Bois, J. A Diruthenium Catalyst for Selective, Intramolecular

Allylic C −H Amination: Reaction Development and Mechanistic

Insight Gained through Experiment and Theory. J. Am. Chem. Soc.

2011, 133, 17207−17216. (e) Chan, K.-H.; Guan, X.; Lo, V. K.-Y.; Che,

C.-M. Elevated Catalytic Activity of Ruthenium(II)−Porphyrin-

Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with

N-Heterocyclic Carbene Ligands. Angew. Chem., Int. Ed. 2014, 53,

2982−2987. (f) Leung, S. K.-Y.; Tsui, W.-M.; Huang, J.-S.; Che, C.-M.;

Liang, J.-L.; Zhu, N. Imido Transfer from Bis(imido)ruthenium(VI)

Porphyrins to Hydrocarbons: Effect of Imido Substituents, C −H Bond

Dissociation Energies, and Ru^VI/VReduction Potentials. J. Am. Chem.

Soc. 2005, 127, 16629−16640. (g) Au, S.-M.; Huang, J.-S.; Yu, W.-Y.;

Fung, W.-H.; Che, C.-M. Aziridination of Alkenes and Amidation of

Alkanes by Bis(tosylimido)ruthenium(VI) Porphyrins. A Mechanistic

Study. J. Am. Chem. Soc. 1999, 121, 9120−9132. (h) Fantauzzi, S.;

Gallo, E.; Caselli, A.; Piangiolino, C.; Ragaini, F.; Re, N.; Cenini, S.

Rearrangement of N -Aryl-2-Vinylaziridines to Benzoazepines and

Dihydropyrroles: A Synthetic and Theoretical Study. Chem. - Eur. J.

2009, 15, 1241−1251. (i) Fantauzzi, S.; Gallo, E.; Caselli, A.;

Piangiolino, C.; Ragaini, F.; Cenini, S. The (Porphyrin)ruthenium-

Catalyzed Aziridination of Olefins Using Aryl Azides as Nitrogen

Sources. Eur. J. Org. Chem. 2007, 2007, 6053−6059. (j) Fantauzzi, S.;

Gallo, E.; Caselli, A.; Ragaini, F.; Macchi, P.; Casati, N.; Cenini, S.

Origin of the Deactivation in Styrene Aziridination by Aryl Azides,

Catalyzed by Ruthenium Porphyrin Complexes. Structural Character-

ization of a Δ²-1,2,3-triazoline Ru^II(TPP)CO Complex. Organo-

metallics 2005, 24, 4710−4713. (k) Zardi, P.; Pozzoli, A.; Ferretti, F.;

Manca, G.; Mealli, C.; Gallo, E. A mechanistic investigation of the

ruthenium porphyrin catalyzed aziridination of olefins by aryl azides.

Dalton Trans. 2015, 44, 10479−10489. (l) Manca, G.; Gallo, E.;

Intrieri, D.; Mealli, C. DFT Mechanistic Proposal of the Ruthenium

Porphyrin-Catalyzed Allylic Amination by Organic Azides. ACS Catal.

2014, 4, 823−832.

(22) For select examples of Cu-based nitrene-transfer to alkenes, see:

(a) Gephart, R. T., III; Warren, T. H. Copper-Catalyzed sp³C−H

Amination. Organometallics 2012, 31, 7728−7752. (b) Lebel, H.;

Parmentier, M. Copper-catalyzed enantioselective aziridination of

styrenes. Pure Appl. Chem. 2010, 82, 1827−1833. (c) Lebel, H.;

Parmentier, M.; Leogane, O.; Ross, K.; Spitz, C. Copper bis(oxazolines)

as catalysts for stereoselective aziridination of styrenes with N -

tosyloxycarbamates. Tetrahedron 2012, 68, 3396−3409. (d) Maestre,

L.; Sameera, W. M. C.; Díaz-Requejo, M. M.; Maseras, F.; Pérez, P. J. A

General Mechanism for the Copper- and Silver-Catalyzed Olefin

Aziridination Reactions: Concomitant Involvement of the Singlet and

Triplet Pathways. J. Am. Chem. Soc. 2013, 135, 1338−1348.

(e) Mairena, M. A.; Díaz-Requejo, M. M.; Belderraín, T. R.; Nicasio,

M. C.; Trofimenko, S.; Pérez, P. J. Copper-Homoscorpionate

Complexes as Very Active Catalysts for the Olefin Aziridination

Reaction. Organometallics 2004, 23, 253−256. (f) Díaz-Requejo, M.

M.; Pérez, P. J.; Brookhart, M.; Templeton, J. L. Substituent Effects on

the Reaction Rates of Copper-Catalyzed Cyclopropanation and

Aziridination of para-Substituted Styrenes. Organometallics 1997, 16,

4399−4402. (g) Bagchi, V.; Paraskevopoulou, P.; Das, P.; Chi, L.;

Wang, Q.; Choudhury, A.; Mathieson, J. S.; Cronin, L.; Pardue, D. B.;

Cundari, T. R.; Mitrikas, G.; Sanakis, Y.; Stavropoulos, P. A Versatile

Tripodal Cu(I) Reagent for C −N Bond Construction via Nitrene-

Transfer Chemistry: Catalytic Perspectives and Mechanistic Insights on

C −H Aminations/Amidinations and Olefin Aziridinations. J. Am.

Chem. Soc. 2014, 136, 11362−11381. (h) Lam, T. L.; Tso, K. C.-H.;

Cao, B.; Yang, C.; Chen, D.; Chang, X.-Y.; Huang, J.-S.; Che, C.-M.

Tripodal S-Ligand Complexes of Copper(I) as Catalysts for Alkene

Aziridination, Sulfide Sulfimidation, and C −H Amination. Inorg. Chem.

2017, 56, 4253−4257. (i) Li, Y.; He, J.; Khankhoje, V.; Herdtweck, E.;

Köhler, K.; Storcheva, O.; Cokoja, M.; Kuhn, F. E. Copper(II)

̈

complexes incorporating poly/perfluorinated alkoxyaluminate-type

weakly coordinating anions: Syntheses, characterization and catalytic

application in stereoselective olefin aziridination. Dalton Trans. 2011,

40, 5746−5754. (j) Comba, P.; Haaf, C.; Lienke, A.; Muruganantham,

A.; Wadepohl, H. Synthesis, Structure, and Highly Efficient Copper-

Catalyzed Aziridination with a Tetraaza-Bispidine Ligand. Chem. - Eur.

J. 2009, 15, 10880−10887. (k) Wang, X.; Ding, K. One-Pot

Enantioselective Aziridination of Olefins Catalyzed by a Copper(I)

Complex of a Novel Diimine Ligand by Using PhI(OAc)₂and

Sulfonamide as Nitrene Precursors. Chem. - Eur. J. 2006, 12, 4568−

4575. (l) Robert-Peillard, F.; Di Chenna, P. H.; Liang, C.; Lescot, C.;

Collet, F.; Dodd, R. H.; Dauban, P. Catalytic stereoselective alkene

aziridination with sulfonimidamides. Tetrahedron: Asymmetry 2010, 21,

1447−1457. (m) Kundu, S.; Miceli, E.; Farquhar, E.; Pfaff, F. F.;

Kuhlmann, U.; Hildebrandt, P.; Braun, B.; Greco, C.; Ray, K. Lewis

Acid Trapping of an Elusive Copper −Tosylnitrene Intermediate Using

Scandium Triflate. J. Am. Chem. Soc. 2012, 134, 14710−14713. (n) Ma,

L.; Jiao, P.; Zhang, Q.; Du, D.-M.; Xu, J. Ligand and substrate π -stacking

interaction controlled enantioselectivity in the asymmetric aziridina-

tion. Tetrahedron: Asymmetry 2007, 18, 878−884. (o) Vedernikov, A.

N.; Caulton, K. G. Angular Ligand Constraint Yields an Improved

Olefin Aziridination Catalyst. Org. Lett. 2003, 5, 2591−2594.

(p) Evans, D. A.; Faul, M. M.; Bilodeau, M. T. Development of the

Copper-Catalyzed Olefin Aziridination Reaction. J. Am. Chem. Soc.

(24) For select examples of Rh-based nitrene-transfer to alkenes, see

ref 9 and the following: (a) Roizen, J. L.; Harvey, M. E.; Du Bois, J.

Metal-Catalyzed Nitrogen-Atom Transfer Methods for the Oxidation

of Aliphatic C −H Bonds. Acc. Chem. Res. 2012, 45, 911−922. (b) Fiori,

K. W.; Espino, C. G.; Brodsky, B. H.; Du Bois, J. A mechanistic analysis

of the Rh-catalyzed intramolecular C −H amination reaction.

Tetrahedron 2009, 65, 3042−3051. (c) Fruit, C.; Robert-Peillard, F.;

̈

Bernardinelli, G.; Muller, P.; Dodd, R. H.; Dauban, P. Diastereose-

lective rhodium-catalyzed nitrene transfer starting from chiral

sulfonimidamide-derived iminoiodanes. Tetrahedron: Asymmetry

̈

2005, 16, 3484−3487. (d) Muller, P.; Baud, C.; Jacquier, Y. The

rhodium(III)-catalyzed aziridination of olefins with {[(4-Nitrophenyl)-

sulfonyl]imino}phenyl-λ³-iodane. Can. J. Chem. 1998, 76, 738−750.

(e) Nageli, I.; Baud, C.; Bernardinelli, G.; Jacquier, Y.; Moraon, M.;

Mullet, P. Rhodium(II)-Catalyzed CH Insertions with {[(4-Nitro-

phenyl)-sulfonyl]imino}phenyl-λ³-iodane. Helv. Chim. Acta 1997, 80,

1087−1105. (f) Catino, A. J.; Nichols, J. M.; Forslund, R. E.; Doyle, M.

P. Efficient Aziridination of Olefins Catalyzed by Mixed-Valent

Dirhodium(II, III) Caprolactamate. Org. Lett. 2005, 7, 2787−2790.

(g) Liang, J.-L.; Yuan, S.-X.; Chan, P. W. H.; Che, C.-M. Rhodium-

(II,II) Dimer as an Efficient Catalyst for Aziridination of Sulfonamides

and Amidation of Steroids. Org. Lett. 2002, 4, 4507−4510.

1993

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

(25) For select examples of Pd-based nitrene-transfer to alkenes, see:

(a) Okamoto, K.; Oda, T.; Kohigashi, S.; Ohe, K. Palladium-catalyzed

Decarboxylative Intramolecular Aziridination from 4 H -Isoxazol-5-ones

Leading to 1-Azabicyclo[3.1.0]hex-2-enes. Angew. Chem., Int. Ed. 2011,

50, 11470−11473. (b) Han, J.; Li, Y.; Zhi, S.; Pan, Y.; Timmons, C.; Li,

G. Palladium-catalyzed aziridination of alkenes using N ,N -dichloro-p-

toluenesulfonamide as nitrogen source. Tetrahedron Lett. 2006, 47,

7225−7228.

of a Cobalt(III) Imido from a Cobalt(II) Amido Complex. Evidence for

Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2007, 129, 2424−

2425. (c) Mehn, M. P.; Brown, S. D.; Jenkins, D. M.; Peters, J. C.; Que,

L., Jr Vibrational Spectroscopy and Analysis of Pseudo-tetrahedral

Complexes with Metal Imido Bonds. Inorg. Chem. 2006, 45, 7417−

7427. (d) Dai, X.; Kapoor, P.; Warren, T. H. [Me₂NN]Co(η⁶-toluene):

O = O, N = N, and O = N Bond Cleavage Provides β -Diketiminato

Cobalt μ -Oxo and Imido Complexes. J. Am. Chem. Soc. 2004, 126,

4798−4799. (e) Hu, X.; Meyer, K. Terminal Cobalt(III) Imido

Complexes Supported by Tris(Carbene) Ligands: Imido Insertion into

the Cobalt −Carbene Bond. J. Am. Chem. Soc. 2004, 126, 16322−

16323. (f) Betley, T. A.; Peters, J. C. Dinitrogen Chemistry from

Trigonally Coordinated Iron and Cobalt Platforms. J. Am. Chem. Soc.

2003, 125, 10782−10783. (g) Jenkins, D. M.; Betley, T. A.; Peters, J. C.

Oxidative Group Transfer to Co(I) Affords a Terminal Co(III) Imido

Complex. J. Am. Chem. Soc. 2002, 124, 11238−11239.

(31) (a) Shay, D. T.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.;

Theopold, K. H. Intramolecular C −H Activation by an Open-Shell

Cobalt(III) Imido Complex. Angew. Chem., Int. Ed. 2005, 44, 1508−

1510. (b) Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold, A.

L.; Theopold, K. H. Intramolecular C −H Activation by Inferred

Terminal Cobalt Imido Intermediates. J. Am. Chem. Soc. 2003, 125,

4440−4441.

(32) (a) King, E. R.; Sazama, G. T.; Betley, T. A. Co(III) Imidos

Exhibiting Spin Crossover and C −H Bond Activation. J. Am. Chem. Soc.

2012, 134, 17858−17861. (b) Baek, Y.; Betley, T. A. Catalytic C −H

Amination Mediated by Dipyrrin Cobalt Imidos. J. Am. Chem. Soc.

2019, 141, 7797−7806. (c) Baek, Y.; Hennessy, E. T.; Betley, T. A.

Direct Manipulation of Metal Imido Geometry: Key Principles to

Enhance C −H Amination Efficacy. J. Am. Chem. Soc. 2019, 141,

16944−16953.

(33) Reckziegel, A.; Pietzonka, C.; Kraus, F.; Werncke, C. G. C −H

Bond Activation by an Imido Cobalt(III) and the Resulting Amido

Cobalt(II) Complex. Angew. Chem., Int. Ed. 2020, 59, 8527−8531.

(34) Wu, B.; Sánchez, R. H.; Bezpalko, M. W.; Foxman, B. M.;

Thomas, C. M. Formation of Heterobimetallic Zirconium/Cobalt

Diimido Complexes via a Four-Electron Transformation. Inorg. Chem.

2014, 53, 10021−10023.

(35) Chomitz, W. A.; Arnold, J. Reactivity of a Co(I) [N₂P₂] complex

with azides: evidence for a transient Co(III) imido species. Chem.

Commun. 2008, 3648−3650.

(36) (a) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang, M.-W.;

Jiang, S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W.; Gao,

S. Two-Coordinate Co(II) Imido Complexes as Outstanding Single-

Molecule Magnets. J. Am. Chem. Soc. 2017, 139, 373−380. (b) Du, J.;

Wang, L.; Xie, M.; Deng, L. A Two-Coordinate Cobalt(II) Imido

Complex with NHC Ligation: Synthesis, Structure, and Reactivity.

Angew. Chem., Int. Ed. 2015, 54, 12640−12644.

(37) Liu, Y.; Du, J.; Deng, L. Synthesis, Structure, and Reactivity of

Low-Spin Cobalt(II) Imido Complexes [(Me₃P)₃Co(NAr)]. Inorg.

Chem. 2017, 56, 8278−8286.

(38) Zhang, L.; Liu, Y.; Deng, L. Three-Coordinate Cobalt(IV) and

Cobalt(V) Imido Complexes with N-Heterocyclic Carbene Ligation:

Synthesis, Structure, and Their Distinct Reactivity in C −H Bond

Amination. J. Am. Chem. Soc. 2014, 136, 15525−15528.

(39) (a) Goswami, M.; Rebreyend, C.; de Bruin, B. Porphyrin

Cobalt(III) “Nitrene Radical ” Reactivity; Hydrogen Atom Transfer

from Ortho -YH Substituents to the Nitrene Moiety of Cobalt-Bound

Aryl Nitrene Intermediates (Y = O, NH). Molecules 2016, 21, 242.

(b) Goswami, M.; Lyaskovskyy, V.; Domingos, S. R.; Buma, W. J.;

(26) For select examples of Ag-based nitrene-transfer to alkenes, see:

(a) Mat Lani, A. S.; Schomaker, J. M. Site-Selective, Catalyst-

Controlled Alkene Aziridination. Synthesis 2018, 50, 4462−4470.

(b) Ju, M.; Weatherly, C. D.; Guzei, I. A.; Schomaker, J. M. Chemo- and

Enantioselective Intramolecular Silver-Catalyzed Aziridinations.

Angew. Chem., Int. Ed. 2017, 56, 9944−9948. (c) Dolan, N. S.;

Scamp, R. J.; Yang, T.; Berry, J. F.; Schomaker, J. M. Catalyst-

Controlled and Tunable, Chemoselective Silver-catalyzed Intermolec-

ular Nitrene Transfer: Experimental and Computational Studies. J. Am.

Chem. Soc. 2016, 138, 14658−14667. (d) Rigoli, J. W.; Weatherly, C.

D.; Alderson, J. M.; Vo, B. T.; Schomaker, J. M. Tunable,

Chemoselective Amination via Silver Catalysis. J. Am. Chem. Soc.

2013, 135, 17238−17241. (e) Huang, M.; Corbin, J. R.; Dolan, N. S.;

Fry, C. G.; Vinokur, A. I.; Guzei, I. A.; Schomaker, J. M. Synthesis,

Characterization, and Variable-Temperature NMR Studies of Silver(I)

Complexes for Selective Nitrene Transfer. Inorg. Chem. 2017, 56,

6725−6733. (f) Weatherly, C.; Alderson, J. M.; Berry, J. F.; Hein, J. E.;

Schomaker, J. M. Catalyst-Controlled Nitrene Transfer by Tuning

Metal:Ligand Ratios: Inshight into the Mechanisms of Chemo-

selectivity. Organometallics 2017, 36, 1649−1661. (g) Scamp, R. J.;

Rigoli, J. W.; Schomaker, J. M. Chemoselective silver-catalyzed nitrene

insertion reactions. Pure Appl. Chem. 2014, 86, 381−393. (h) Rigoli, J.

W.; Weatherly, C. D.; Vo, B. T.; Neale, S.; Meis, A. R.; Schomaker, J. M.

Chemoselective Allene Aziridination via Ag(I) Catalysis. Org. Lett.

́

2013, 15, 290−293. (i) Llaveria, J.; Beltrán, A.; Díaz-Requejo, M. M.;

Matheu, M. I.; Castillón, S.; Pérez, P. J. Efficient Silver-Catalyzed Regio-

and Stereospecific Aziridination of Dienes. Angew. Chem., Int. Ed. 2010,

49, 7092−7095. (j) Li, Z.; He, C. Recent Advances in Silver-Catalyzed

Nitrene, Carbene, and Silylene-Transfer Reactions. Eur. J. Org. Chem.

2006, 2006, 4313−4322. (k) Cui, Y.; He, C. Efficient Aziridination of

Olefins Catalyzed by a Unique Disilver(I) Compound. J. Am. Chem.

Soc. 2003, 125, 16202−16203.

(27) For select examples of Au-based nitrene-transfer to alkenes, see:

(a) Li, Z.; Ding, X.; He, C. Nitrene Transfer Reactions Catalyzed by

Gold Complexes. J. Org. Chem. 2006, 71, 5876−5880. (b) Deng, X.;

Baker, T. A.; Friend, C. M. A Pathway for NH Addition to Styrene

Promoted by Gold. Angew. Chem., Int. Ed. 2006, 45, 7075−7078.

(28) (a) Singh, R.; Mukherjee, A. Metalloporphyrin Catalyzed C −H

Amination. ACS Catal. 2019, 9, 3604−3617. (b) Park, Y.; Kim, Y.;

Chang, S. Transition Metal-Catalyzed C −H Amination: Scope,

Mechanism, and Applications. Chem. Rev. 2017, 117, 9247−9301.

(c) Stavropoulos, P. Metal-Catalyzed and Metal-Free Intermolecular

Amination of Light Alkanes and Benzenes. Comments Inorg. Chem.

2017, 37, 1−57. (d) Hazelard, D.; Nocquet, P.-A.; Compain, P.

Catalytic C −H amination at its limits: challenges and solutions. Org.

Chem. Front. 2017, 4, 2500−2521. (e) Collet, F.; Lescot, C.; Dauban, P.

Catalytic C −H amination: the stereoselectivity issue. Chem. Soc. Rev.

2011, 40, 1926−1936. (f) Zalatan, D. N.; Du Bois, J. Metal-Catalyzed

Oxidations of C −H to C −N Bonds. Top. Curr. Chem. 2009, 292, 347−

378. (g) Díaz-Requejo, M. M.; Pérez, P. J. Coinage Metal Catalyzed C −

H Bond Functionalization of Hydrocarbons. Chem. Rev. 2008, 108,

3379−3394.

(29) Hu, L.; Chen, H. What Factors Control the Reactivity of Cobalt −

Imido Complexes in C −H Bond Activation via Hydrogen Abstraction?

ACS Catal. 2017, 7, 285−292.

(30) (a) Jones, C.; Schulten, C.; Rose, R. P.; Stasch, A.; Aldridge, S.;

Woodul, W. D.; Murray, K. S.; Moubaraki, B.; Brynda, M.; La Macchia,

G.; Gagliardi, L. Amidinato − and Guanidinato −Cobalt(I) Complexes:

Characterization of Exceptionally Short Co −Co Interactions. Angew.

Chem., Int. Ed. 2009, 48, 7406−7410. (b) Cowley, R. E.; Bontchev, R.

P.; Sorrell, J.; Sarracino, O.; Feng, Y.; Wang, H.; Smith, J. M. Formation

́

Woutersen, S.; Troeppner, O.; Ivanovic-Burmazovic, I.; Lu, H.; Cui, X.;

Zhang, X. P.; Reijerse, E. J.; DeBeer, S.; van Schooneveld, M. M.; Pfaff,

F. F.; Ray, K.; de Bruin, B. Characterization of Porphyrin-Co(III)-

′Nitrene Radical ′ Species Relevant in Catalytic Nitrene Transfer

Reactions. J. Am. Chem. Soc. 2015, 137, 5468−5479. (c) Hopmann, K.

H.; Ghosh, A. Mechanism of Cobalt-Porphyrin −Catalyzed Aziridina-

tion. ACS Catal. 2011, 1, 597−600.

(40) van Leest, N. P.; Tepaske, M. A.; Oudsen, J.-P. H.; Venderbosch,

B.; Rietdijk, N. R.; Siegler, M. A.; Tromp, M.; van der Vlugt, J. I.; de

1994

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

Bruin, B. Ligand Redox Noninnocence in [Co^III(TAML)]^0/−

Complexes Affects Nitrene Formation. J. Am. Chem. Soc. 2020, 142,

552−563.

(41) van Leest, N. P.; Tepaske, M. A.; Venderbosch, B.; Oudsen, J.-P.

H.; Tromp, M.; van der Vlugt, J. I.; de Bruin, B. Electronically

Asynchronous Transition States for C −N Bond Formation by

Electrophilic [Co^III(TAML)]-Nitrene Radical Complexes Involving

Substrate-to-Ligand Single-Electron Transfer and a Cobalt-Centered

Spin Shuttle. ACS Catal. 2020, 10, 7449−7463.

(42) Coin, G.; Patra, R.; Rana, S.; Biswas, J. P.; Dubourdeaux, P.;

Clémancey, M.; de Visser, S. P.; Maiti, D.; Maldivi, P.; Latour, J.-M. Fe-

Catalyzed Aziridination Is Governed by the Electron Affinity of the

Active Imido-Iron Species. ACS Catal. 2020, 10, 10010−10020.

(43) Baek, Y.; Das, A.; Zheng, S.-L.; Reibenspies, J. H.; Powers, D. C.;

Betley, T. A. C −H Amination Mediated by Cobalt Organoazide

Adducts and the Corresponding Cobalt Nitrenoid Intermediates. J. Am.

Chem. Soc. 2020, 142, 11232−11243.

Tetrachlorocobaltate(II) Dihydrate Single Crystal. Inorg. Chem.

1985, 24, 4009−4012. (d) Makinen, M. W.; Kuo, L. C.; Yim, M. B.;

Wells, G. B.; Fukuyama, J. M.; Kim, J. E. Ground Term Splitting of

High-Spin Co²⁺as a Probe of Coordination Structure. 1. Dependence

of the Splitting on Coordination Geometry. J. Am. Chem. Soc. 1985,

107, 5245−5255. (e) Bennett, B. EPR of Cobalt-Substituted Zinc

Enzymes. In Metals in Biology: Applications of High-Resolution EPR to

Metalloenzymes, Biological Magnetic Resonance; Hanson, G., Berliner, L.,

Eds.; Springer-Verlag: New York, 2010; Vol. 29, pp 345−366.

(f) Zolnhofer, E. M.; Käß, M.; Khusniyarov, M. M.; Heinemann, F.

W.; Maron, L.; van Gastel, M.; Bill, E.; Meyer, K. An Intermediate

Cobalt(IV) Nitrido Complex and its N-Migratory Insertion Product. J.

Am. Chem. Soc. 2014, 136, 15072−15078. (g) Antholine, W. E.

Resolved Hyperfine at L-band for High-Spin CoEDTA, A Model for Co

Sites in Proteins. Int. J. Mol. Sci. 2019, 20, 2385.

(51) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of

Transition Ions; Clarendon Press: Oxford, 1970.

(52) Pilbrow, J. R. Effective g values for S = 3/2 and S = 5/2. J. Magn.

Reson. 1978, 31, 479−490.

(53) Petasis, D. T.; Hendrich, M. P. Quantitative Interpretation of

Multifrequency Multimode EPR Spectra of Metal Containing Proteins,

Enzymes, and Biomimetic Complexes. In Electron Paramagnetic

Resonance Investigations of Biological Systems by Using Spin Labels, Spin

Probes, and Intrinsic Metal Ions, Part A; Qin, P. Z., Warncke, K., Eds.;

Methods in Enzymology; Elsevier, 2015; Vol. 563, Chapter 8, pp 171−

208.

(44) Bagchi, V.; Kalra, A.; Das, P.; Paraskevopoulou, P.; Gorla, S.; Ai,

L.; Wang, Q.; Mohapatra, S.; Choudhury, A.; Sun, Z.; Cundari, T. R.;

Stavropoulos, P. Comparative Nitrene-Transfer Chemistry to Olefinic

Substrates Mediated by a Library of Anionic Mn(II) Triphenylamido-

Amine Reagents and M(II) Congeners (M = Fe, Co, Ni) Favoring

Aromatic over Aliphatic Alkenes. ACS Catal. 2018, 8, 9183−9206.

(45) (a) Jones, M. B.; MacBeth, C. E. Tripodal Phenylamine-Based

Ligands and Their Co^IIComplexes. Inorg. Chem. 2007, 46, 8117−8119.

(b) Çelenligil-Çetin, R.; Paraskevopoulou, P.; Dinda, R.; Staples, R. J.;

Sinn, E.; Rath, N. P.; Stavropoulos, P. Synthesis, Characterization, and

Reactivity of Iron Trisamidoamine Complexes that Undergo both

Metal- and Ligand-Centered Oxidative Transformations. Inorg. Chem.

(54) Muller, P.; Baud, C.; Jacquier, Y. A Method for Rhodium(II)-

̈

Catalyzed Aziridination of Olefins. Tetrahedron 1996, 52, 1543−1548.

(55) Al-Ajlouni, A.; Espenson, J. H. Epoxidation of Styrenes by

Hydrogen Peroxide As Catalyzed by Methylrhenium Trioxide. J. Am.

Chem. Soc. 1995, 117, 9243−9250.

2008, 47, 1165−1172. (c) Çelenligil-Çetin, R. Synthesis, Character-

ization, and Reactivity of Iron Complexes with N-Donor Ligands in

Relation to Oxygenation of Hydrocarbons, Ph.D. Thesis, Boston

University, Boston, MA, 2004.

(56) Souto, J. A.; Zian, D.; Muniz, K. Iodine(III)-Mediated

̃

Intermolecular Allylic Amination under Metal-Free Conditions. J.

Am. Chem. Soc. 2012, 134, 7242−7245.

(46) (a) Paraskevopoulou, P.; Lin, A.; Wang, Q.; Pinnapareddy, D.;

Acharrya, R.; Dinda, R.; C

̧

elenligil-C

̧

etin, R.; Floros, G.; Sanakis, Y.;

(57) Jiang, X.-K. Establishment and Successful Application of the σ_JJ•

Scale of Spin-Delocalization Substituent Constants. Acc. Chem. Res.

1997, 30, 283−289.

Choudhury, A.; Rath, N. P.; Stavropoulos, P. Synthesis and Character-

ization of a Series of Structurally and Electronically Diverse Fe(II)

Complexes Featuring a Family of Triphenylamido-amine Ligands.

Inorg. Chem. 2010, 49, 108−122. (b) Pinnapareddy, D. Synthesis and

Characterization of Metal Reagents Mediating C−X Activation (X = Cl,

H) of Hydrocarbons, Ph.D. Thesis, University of Missouri−Rolla, Rolla,

MO, 2007.

(58) Han, H.; Park, S. B.; Kim, S. K.; Chang, S. Copper −Nitrenoid

Formation and Transfer in Catalytic Olefin Aziridination Utilizing

Chelating 2-Pyridylsulfonyl Moieties. J. Org. Chem. 2008, 73, 2862−

2870.

(59) Liang, S.; Jensen, M. P. Half-Sandwich Scorpionates as Nitrene

Transfer Catalysts. Organometallics 2012, 31, 8055−8058.

(60) Liu, P.; Wong, E. L.-M.; Yuen, A. W.-H.; Che, C.-M. Highly

Efficient Alkene Epoxidation and Aziridination catalyzed by Iron(II)

Salt + 4,4 ′,4 ″-Trichloro-2,2 ′:6 ′,2 ″-terpyridine/4,4 ″-Dichloro-4 ′-O -

PEG-OCH₃-2,2′:6 ′,2 ″-terpyridine. Org. Lett. 2008, 10, 3275−3278.

(47) (a) Jones, M. B.; Hardcastle, K. I.; MacBeth, C. E. Synthetic,

spectral and structural studies of mononuclear tris(κ²-amidate)

aluminium complexes supported by tripodal ligands. Polyhedron

2010, 29, 116−119. (b) Jones, M. B.; Hardcastle, K. I.; Hagen, K. S.;

MacBeth, C. E. Oxygen Activation and Intramolecular C −H Bond

Activation by an Amidate-Bridged Diiron(II) Complex. Inorg. Chem.

2011, 50, 6402−6404. (c) Villanueva, O.; Weldy, N. M.; Blakey, S. B.;

MacBeth, C. E. Cobalt catalyzed sp³C−H amination utilizing aryl

azides. Chem. Sci. 2015, 6, 6672−6675.

(61) Dincţ urk, S.; Jackson, R. A. Free radical reactions in solution. Part

̈

7. Substituent effects on free radical reactions: comparison of the σ •

scale with other measures of radical stabilization. J. Chem. Soc., Perkin

Trans. 2 1981, 2, 1127−1131.

(48) Bagchi, V.; Raptopoulos, G.; Das, P.; Christodoulou, S.; Wang,

Q.; Ai, L.; Choudhury, A.; Pitsikalis, M.; Paraskevopoulou, P.;

Stavropoulos, P. Synthesis and characterization of a family of Co(II)

triphenylamido-amine complexes and catalytic activity in controlled

radical polymerization of olefins. Polyhedron 2013, 52, 78−90.

(62) Jupp, A. R.; Johnstone, T. C.; Stephan, D. W. Improving the

Global Electrophilicity Index (GEI) as a Measure of Lewis Acidity.

Inorg. Chem. 2018, 57, 14764−14771.

(63) (a) Johnson, E. R.; Becke, A. D. A post-Hartree −Fock model of

intermolecular interactions. J. Chem. Phys. 2005, 123, No. 024101.

(b) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function

in Dispersion Corrected Density Functional Theory. J. Comput. Chem.

2011, 32, 1456−1465. (c) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H.

A consistent and accurate ab initio parametrization of density functional

dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem.

Phys. 2010, 132, 154104.

(49) Çelenligil-Çetin, R.; Paraskevopoulou, P.; Lalioti, N.; Sanakis, Y.;

Staples, R. J.; Rath, N. P.; Stavropoulos, P. Metalloradical Complexes of

Manganese and Chromium Featuring an Oxidatively Rearranged

Ligand. Inorg. Chem. 2008, 47, 10998−11009.

(50) (a) Bencini, A.; Benelli, C.; Gatteschi, D.; Zanchini, C. ESR

Spectra of Low-Symmetry High-Spin Cobalt(II) Complexes. 2.

Pseudotetrahedral Dichlorobis(triphenylphospine oxide)cobalt(II).

Inorg. Chem. 1979, 18, 2137−2140. (b) Benelli, C.; Gatteschi, D.

ESR Spectra of Low-Symmetry High-Spin Cobalt(II) Complexes. 10.

Five-Coordinated Trigonal-Bipyramidal Complexes. Inorg. Chem.

1982, 21, 1788−1790. (c) Drulis, H.; Dyrek, K.; Hoffmann, K. P.;

Hoffmann, S. K.; Weselucha-Birczynska, A. ESR Spectra of Low-

Symmetry Tetrahedral High-Spin Cobalt(II) in a Cinchoninium

(64) (a) Lu, X.; Li, X.-X.; Lee, Y.-M.; Jang, Y.; Seo, M. S.; Hong, S.;

Cho, K.-B.; Fukuzumi, S.; Nam, W. Electron-Transfer and Redox

Reactivity of High-Valent Iron Imido and Oxo Complexes with the

Formal Oxidation States of Five and Six. J. Am. Chem. Soc. 2020, 142,

3891−3904. (b) Sabenya, G.; Gamba, I.; Gómez, L.; Clémancey, M.;

Frisch, J. R.; Klinker, E. J.; Blondin, G.; Torelli, S.; Que, L., Jr.; Martin-

Diaconescu, V.; Latour, J.-M.; Lloret-Fillol, J.; Costas, M. Octahedral

1995

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Organometallics

pubs.acs.org/Organometallics

Article

iron(iv)−tosylimido complexes exhibiting single electron-oxidation

reactivity. Chem. Sci. 2019, 10, 9513−9529. (c) Hong, S.; Lu, X.; Lee,

Y.-M.; Seo, M. S.; Ohta, T.; Ogura, T.; Clémancey, M.; Maldivi, P.;

Latour, J.-M.; Sarangi, R.; Nam, W. Achieving One-Electron Oxidation

of a Mononuclear Nonheme Iron(V)-Imido Complex. J. Am. Chem. Soc.

2017, 139, 14372−14375. (d) Kumar, S.; Faponle, A. S.; Barman, P.;

Vardhaman, A. K.; Sastri, C. V.; Kumar, D.; de Visser, S. P. Long-range

Electron Transfer Triggers Mechanistic Differences between Iron(IV)-

Oxo and Iron(IV)-Imido Oxidants. J. Am. Chem. Soc. 2014, 136,

17102−17115.

(65) (a) Ogliaro, F.; Harris, N.; Cohen, S.; Filatov, M.; de Visser, S. P.;

Shaik, S. A Model “Rebound ” Mechanism of Hydroxylation by

Cytochrome P450: Stepwise and Effectively Concerted Pathways,

and Their Reactivity Patterns. J. Am. Chem. Soc. 2000, 122, 8977−8989.

(b) Shaik, S.; de Visser, S. P.; Ogliaro, F.; Schwarz, H.; Schröder, D.

Two-state reactivity mechanisms of hydroxylation and epoxidation by

cytochrome P-450 revealed by theory. Curr. Opin. Chem. Biol. 2002, 6,

556−567. (c) Moreau, Y.; Chen, H.; Derat, E.; Hirao, H.; Bolm, C.;

Shaik, S. NR Transfer Reactivity of Azo-Compound I of P450. How

Does the Nitrogen Substituent Tune the Reactivity of the Species

toward C −H and C C Activation? J. Phys. Chem. B 2007, 111,

10288−10299.

Importance of the Dispersion Energy for the Thermodynamic Stability

of Molecules. ChemPhysChem 2011, 12, 1258−1261.

(74) (a) Power, P. P. An Update on Multiple Bonding between

Heavier Main Group Elements: The Importance of Pauli Repulsion,

Charge-Shift Character, and London Dispersion Force Effects.

Organometallics 2020, 39, 4127−4138. (b) Liptrot, D.; Power, P. P.

London dispersion forces in sterically crowded inorganic and

organometallic molecules. Nat. Rev. Chem. 2017, 1, No. 0004.

(75) Wagner, J. P.; Schreiner, P. R. London Dispersion in Molecular

Chemistry − Reconsidering Steric Effects. Angew. Chem., Int. Ed. 2015,

54, 12274−12296.

(66) (a) Kuijpers, P. F.; van der Vlugt, J. I.; Schneider, S.; de Bruin, B.

Nitrene Radical Intermediates in Catalytic Synthesis. Chem. - Eur. J.

2017, 23, 13819−13829. (b) Xiong, T.; Zhang, Q. New amination

strategies based on nitrogen-centered radical chemistry. Chem. Soc. Rev.

2016, 45, 3069−3087.

(67) (a) Xiong, P.; Xu, H.-C. Chemistry with Electrochemically

Generated N-Centered Radicals. Acc. Chem. Res. 2019, 52, 3339−3350.

(b) Wang, P.; Zhao, Q.; Xiao, W.; Chen, J. Recent advances in visible-

light photoredox-catalyzed nitrogen radical cyclization. Green Syn-

́

th.Catal. 2020, 1, 42−51. (c) Zheng, S.; Gutiérrez-Bonet, A.; Molander,

G. A. Merging Photoredox PCET with Ni-Catalyzed Cross-Coupling:

Cascade Amidoarylation of Unactivated Olefins. Chem. 2019, 5, 339−

352. (d) Davies, J.; Svejstrup, T. D.; Reina, D. F.; Sheikh, N. S.; Leonori,

D. Visible-Light-Mediated Synthesis of Amidyl Radicals: Transition-

Metal-Free Hydroamination and N -Arylation Reactions. J. Am. Chem.

Soc. 2016, 138, 8092−8095.

(68) Wang, Q.; Wang, P.; Gao, X.; Wang, D.; Wang, S.; Liang, X.;

Wang, L.; Zhang, H.; Lei, A. Regioselective/electro-oxidative

intermolecular [3 + 2] annulation for the preparation of indolines.

Chem. Sci. 2020, 11, 2181−2186.

(69) (a) Usharani, D.; Janardanan, D.; Li, C.; Shaik, S. A Theory for

Bioinorganic Chemical Reactivity of Oxometal Complexes and

Analogous Oxidants: The Exchange and Orbital-Selection Rules. Acc.

Chem. Res. 2013, 46, 471−482. (b) Swart, M.; Gruden, M. Spinning

around in Transition-Metal Chemistry. Acc. Chem. Res. 2016, 49,

2690−2697.

(70) Ren, Y.; Cheaib, K.; Jacquet, J.; Vezin, H.; Fensterbank, L.; Orio,

M.; Blanchard, S.; Desage-El Murr, M. Copper-Catalyzed Aziridination

with Redox-Active Ligands: Molecular Spin Catalysis. Chem. - Eur. J.

2018, 24, 5086−5090.

(71) Schumann, J. M.; Wagner, J. P.; Eckhardt, A. K.; Quanz, H.;

̈

Schreiner, P. R. Intramolecular London Dispersion Interactions Do

Not Cancel in Solution. J. Am. Chem. Soc. 2021, 143, 41−45.

(72) (a) Rösel, S.; Becker, J.; Allen, W. D.; Schreiner, P. R. Probing the

Delicate Balance between Pauli Repulsion and London Dispersion with

Triphenylmethyl Derivatives. J. Am. Chem. Soc. 2018, 140, 14421−

14432. (b) Rösel, S.; Balestrieri, C.; Schreiner, P. R. Sizing the role of

London dispersion in the dissociation of all-meta tert -butyl hexaphenyl-

ethane. Chem. Sci. 2017, 8, 405−410. (c) Grimme, S.; Schreiner, P. R.

Steric Crowding Can Stabilize a Labile Molecule: Solving the

Hexaphenylethane Riddle. Angew. Chem., Int. Ed. 2011, 50, 12639−

12642.

(73) (a) Bursch, M.; Caldeweyher, E.; Hansen, A.; Neugebauer, H.;

Ehlert, S.; Grimme, S. Understanding and Quantifying London

Dispersion Effects in Organometallic Complexes. Acc. Chem. Res.

2019, 52, 258−266. (b) Grimme, S.; Huenerbein, R.; Ehrlich, S. On the

1996

https://doi.org/10.1021/acs.organomet.1c00267

Organometallics 2021, 40, 1974−1996

Article Doi

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents

DOI: 10.1021/acs.organomet.1c00267

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Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents

DOI: 10.1021/acs.organomet.1c00267

Source and publish data:

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Full text of DOI:10.1021/acs.organomet.1c00267

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