Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes

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Article

Electronic Structures and Reactivity Profiles

of Aryl Nitrenoid-Bridged Dicopper Complexes

Kurtis M. Carsch, James T. Lukens, Ida M. DiMucci, Diana A. Iovan,

Shao-Liang Zheng, Kyle M. Lancaster, and Theodore A. Betley

J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09616 • Publication Date (Web): 09 Jan 2020

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Electronic Structures and Reactivity Profiles of Aryl Nitrenoid−Bridged

Dicopper Complexes

Kurtis M. Carsch,^†James T. Lukens,^‡Ida M. DiMucci,^‡Diana A. Iovan,^†Shao-Liang Zheng,^†Kyle M. Lancaster,^‡* Theo-

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dore A. Betley^†*

†Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge MA 02138, United States

^‡Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca NY 14853, United States

ABSTRACT: Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds (^Mesdmx, ^tBudmx: dimethylxan-

thine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by

transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes

(^Rdmx)Cu₂(µ²−NAr) (R: Mes, ^tBu; Ar: 4-MeOC₆H₄, 3,5-(F₃C)₂C₆H₃) were synthesized by treatment of the corresponding dicuprous

complexes with aryl azides. While one-electron reduction of (^Mesdmx)Cu₂(µ²–N(C₆H₄OMe)) with potassium graphite initiates an

intramolecular, benzylic C–H amination at room temperature, chemical reduction of (^tBudmx)Cu₂(µ²−NAr) leads to isolable

[(^tBudmx)Cu₂(µ²−NAr)]^–product salts. The electronic structures of the thermally robust [(^tBudmx)Cu₂(µ²−NAr)]^0/–complexes were

assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L_2,3/K-edge, N

K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence

complexes of the type [(^Rdmx)Cu₂(µ²−NAr)]^–, featuring significant NAr−localized spin following reduction from electronic popula-

tion of the [Cu₂(µ²−NAr)] π* manifold and contrasting previous methods for engendering iminyl character through chemical oxida-

tion. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g.,

nitrene transfer, H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N−radical character

afforded from different aryl substituents.

To correlate the electronic structure with the reactivity

profile of bimetallic transition metal complexes, we have pre-

1.0 Introduction

The incorporation of heteroatom functionality into unac-

tivated C–H bonds remains a paramount challenge in both fine

chemical synthesis and the conversion of hydrocarbon feed-

stocks into commodity chemicals.^1-4Dicopper complexes fea-

turing bridging imido or oxo moieties have been implicated in

the amination and hydroxylation of aliphatic C−H bonds, albe-

it by two distinct mechanisms. The dicopper imido complexes

reported by Warren and coworkers are proposed to be in equi-

librium with a mononuclear copper complex and a terminal

copper nitrenoid species, to which the latter is attributed for

the observed C−H bond amination reactivity (Figure 1).^5-8

Steric profile tuning of the ancillary ligands for both -

diketiminate and dipyrrin platforms allowed for observation of

reductively coupled terminal nitrenoid complexes⁸as well as

the isolation of authentic terminal copper nitrene complexes,⁹

respectively. The amination reactivity can be contrasted with

the partial hydroxylation of methane to methanol by zeolite

Cu-ZSM-5,¹⁰ascribed to a bent, dicupric oxide core (Figure

pared a dinucleating, dimethylxanthene-tethered, bis(dipyrrin)

ligand scaffold which proximally orients two metals cofacially

to facilitate multi-electron chemistry. These architectures are

well suited to stabilizing bridged transition metal species as

evidenced by the isolation of bridging ferrous oxo and hy-

droxo complexes.¹³Furthermore, we hypothesize a semi-rigid

spacer minimizes entropic penalties and promotes a dinuclear

arrangement for the exploration of dinuclear bridging com-

plexes across distinct redox states. These design principles are

prevalent in tethered diiron porphyrin complexes,^14-16compe-

tent for catalytic hydroxylation of exogenous substrates upon

photolysis. In addition, the dipyrrin scaffold limits access to

the primary coordination sphere, rendering the metal ions

high-spin and electrophilic.^17-19

In this study, we examine nitrene group transfer to a di-

copper core. The electronic structure and reactivity profile of

the [Cu₂(µ²−NR)] core is assessed as a function of redox state.

Notably, we observe that one-electron reduction of the neutral

[Cu₂(µ²−NR)] complex induces spin accumulation at the

bridging nitrenoid fragment (Figure 1), leading to a diagnostic

iminyl signature via N K-edge spectroscopy that is indistin-

guishable from subvalent iminyl agents generated from oxida-

tion of the metal imido precursors. The reactivity profiles of

both the neutral and anionic [Cu₂(µ²−NR)] complexes are as-

sessed in the contexts of nitrene transfer to nucleophiles and

propensity for H−atom abstraction, with divergent pathways as

a function of the aryl substituent attributable to varying de-

grees of radical character at the nitrenoid bridgehead.

1).¹¹The bent geometry of the zeolite-confined Cu^II(µ²–O)

2

core has been proposed to endow oxyl-character on the ox-

enoid fragment in the transition state, facilitating low-barrier

H–atom abstraction and radical recombination yielding meth-

anol.¹²We were thus interested in synthesizing dicopper coor-

dination complexes featuring bridging moieties to establish

rigorous electronic structure to reactivity correlations. More

specifically, we seek to probe how redox changes to a

[Cu₂(µ²−E)]ⁿcore can induce spin accumulation at the bridge-

head ligand, promote radical reactivity across the Cu−E bond

vector, and achieve substrate functionalization.

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Heating a benzene solution of 1 with a minor excess (1.2

equiv.) of 4-methoxyphenyl azide (4-MeOC₆H₄N₃) or 3,5-

1

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bis(trifluoromethyl)phenyl azide (3,5-(F₃C)₂C₆H₃N₃) (45 ºC,

Mes-

16 h) afforded the dicopper imido complexes

(

dmx)Cu₂(µ²−N(C₆H₄OMe)) (2) and (^Mesdmx)Cu₂(µ²−N(3,5-

(F₃C)₂C₆H₃)) (3), respectively (Scheme 2). Both reactions

were accompanied by gradual effervescence with a marked

color change from red-pink (1) to deep violet (2) or purple-

pink (3). Trituration of imido complexes 2 and 3 with cold

acetonitrile allowed for isolation of analytically pure products

after filtration in excellent yields (2: 92%, 3: 79%). The dia-

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magnetic H NMR spectra, satisfactory elemental analyses,

and featureless perpendicular-mode EPR spectra indicate an

overall singlet spin ground state and establish bulk purity for 2

and 3 (Figure S–7, S–9).

The solid-state structures of 2 and 3 (Figure 2a, S–75)

determined by single crystal X-ray diffraction reveal contrac-

tions of the copper-copper distances (2: 2.822(1) Å, 3:

2.844(1) Å) as compared to 1 (6.3800(7) Å). These bond met-

rics are comparable to the geometric sum of the Van der

Waals radii (approx. 1.40 Å)²³and nearly 0.10 Å shorter than

reported dinuclear copper imido complexes, (Table 1).^5-7,24

This geometric contraction results in a more cofacial arrange-

ment of the dipyrrin units (2: 6.7(1)º torsion, 3: 7.0(1)º torsion)

and a relatively acute copper-imido-copper bond angle (2:

102.3(1)º, 3: 103.1(2)º). The displacement of the copper cen-

ters with respect to the dipyrrin plane (2: 0.879(6) Å, 0.854(5)

Å; 3: 0.848(3) Å, 0.848(3) Å) compared to 1 (0.242(4) Å,

0.407(3) Å) reflects the capability of the tethered bis(dipyrrin)

ligand platform to support diverse intermetallic distances.

Figure 1. Dicopper mediated C−H bond functionalization:

(top) monomer-dimer imido equilibrium for alkane amination

(Warren); (middle) [Cu₂(µ²–O)] polarization for methane hy-

droxylation (Solomon); (bottom) radical accumulation on im-

ido bridge for [Cu₂(µ²–NR)]^–(this work).

2. Results

2.1 Synthesis of Dinuclear Dicopper Imido Complexes

Synthesis of the mesityl-flanked, cofacial bis(dipyrrin)

ligand featuring a dimethylxanthene spacer (^Mesdmx)H₂was

modified from the procedure to prepare the reported

Scheme 2

(

^tBudmx)H₂analogue (see Supporting Information for experi-

mental details).¹³Acid-catalyzed condensation of 2-mesityl-

1H-pyrrole²⁰with dimethylxanthene bis(aldehyde),²¹followed

by oxidation of the bis(dipyrromethane) with excess 2,3-

dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 3.0 equiv.)

afforded (^Mesdmx)H₂in excellent yield (79%) on multi-gram

scales as a red-orange powder. Under an atmosphere of nitro-

gen, (^Mesdmx)H₂was deprotonated with mesitylcopper²²in a

thawing benzene/acetonitrile solvent mixture (20:1, 10 equiv.

acetonitrile) to afford (^Mesdmx)Cu₂(NCMe)₂(1) as a red-pink

powder (Scheme 1). Trituration of the crude reaction mixture

with cold acetonitrile precipitated 1 in analytically pure form

(82%). Single crystals of 1 suitable for X-ray diffraction were

prepared by layering acetonitrile with a concentrated diethyl

ether solution of 1 at –35 ºC overnight. The solid-state struc-

ture displays a large copper-copper distance (6.3800(7) Å) and

non-cofacial arrangement (28.7(1)° distortion with respect to

the dipyrrin planes), suggesting no intramolecular communica-

tion between individual Cu-dipyrrin units (Figure S–72).

We also explored how modifications of the dipyrrin

flanking units influence the resulting [Cu₂] core properties. In

contrast to metalation of the (^Mesdmx)H₂platform, metalation

of (^tBudmx)H₂in the presence of coordinating solvents with

mesitylcopper did not furnish the desired bis(cuprous)

synthon. Consequently, an alternative transmetalation protocol

was pursued. Exposure of (^tBudmx)H₂in benzene to potassium

bis(trimethylsilyl)amide (2.1 equiv.) rapidly precipitated an

insoluble orange powder, which was isolated by filtration and

rinsed with boiling toluene to furnish the putative dianionic

[(^tBudmx)K₂] (4). Due to the insolubility of 4 in organic media,

4 was not structurally characterized and used without further

purification. Treatment of 4 as a suspension in tetrahydrofuran

with excess copper bromide dimethylsulfide (2.2 equiv.) fol-

lowed by addition of excess aryl azide (1.8 equiv.; 4-

MeOC₆H₄N₃or, 3,5-(F₃C)₂C₆H₃N₃; 75 ºC, 16 h) afforded the

Scheme 1

corresponding

(

imido

complexes

^tBudmx)Cu₂(µ²−N(C₆H₄OMe)) (5) and (^tBudmx)Cu₂(µ²−N(3,5-

(F₃C)₂C₆H₃)) (6), respectively, as deep purple solids (Scheme

3). Both products were isolated by filtration in cold acetoni-

trile and recrystallized from a saturated acetonitrile/diethyl

ether solution at –35 ºC in moderate yield (5: 65 %, 6: 49 %).

Single crystal X-ray diffraction reveals short copper-copper

distances (5: 2.837(1) Å, 6: 2.875(1) Å) akin to those of 2 and

3 (Figure 2b, S–76), (Table 1).

2

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Table 1. Selected Bond Lengths (Å) and Angles (º) for isostructural complexes 2, 3, 5–9.

2

3^†

5^‡

6^‡

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8

9

Cu1–Cu2

2.822(1)

1.949(4)

1.930(4)

1.925(4)

1.962(3)

1.822(4)

1.802(3)

103.2(2)

1.375(6)

1.406(7)

1.424(7)

2.844(1)

1.923(2)

1.937(2)

1.923(2)

1.937(2)

1.814(4)

103.3(2)

1.406(4)

1.404(4)

2.837(1)

1.942(7)

1.958(7)

1.929(7)

1.977(6)

1.822(7)

1.821(7)

102.3(3)

1.383(10)

1.402(11)

1.391(12)

2.875(1)

1.934(8)

1.929(7)

1.931(8)

1.963(7)

1.830(8)

1.818(7)

104.0(4)

1.386(12)

1.410(12)

1.404(13)

2.9031(7)

1.972(3)

2.039(3)

2.058(3)

1.958(2)

1.852(3)

1.848(3)

103.4(1)

1.318(4)

1.438(5)

1.428(5)

2.9448 (7)

2.058(2)

1.985(2)

2.028(2)

2.004(2)

1.869(2)

1.860(2)

104.3(1)

1.351(3)

1.438(4)

1.417(3)

2.8401(5)

1.973(2)

2.043(2)

1.999(2)

2.006(2)

1.860(2)

1.853(2)

99.8(1)

1.342(3)

1.435(4)

1.420(4)

Cu1–N_dipyrrin

Cu2–N_dipyrrin

Cu1–N_Ar

Cu2–N_Ar

Cu1–N_Ar–Cu2

N_Ar–C_ipso

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C_ipso–C_ortho

^†Cu1 and Cu2 are symmetry-equivalent. ^‡Two molecules are present in the asymmetric unit.

strate, 0.2 M [ⁿBu₄N][PF₆]). One quasi-reversible reduction

was observed at moderate potentials for both p-OMe substitut-

ed imidos (2: –1.32 V, 5: –1.18 V vs. [Cp₂Fe]^+/0, Figure S–61),

indicating the [Cu₂(µ²−NAr)] core could be chemically re-

duced. Imido complexes 3 and 6 similarly reveal quasi-

reversible reductive processes at milder potentials consistent

with the imido electron withdrawing substituents (3: –0.95 V,

6: –0.70 V vs. [Cp₂Fe]^+/0, Figure S–61). Accordingly, treat-

ment of 2, 5, or 6 in thawing tetrahydrofuran with stoichio-

metric potassium graphite (KC₈) in the presence of Cryptand

222 (C₂₂₂, 1.2 equiv.) precipitated insoluble graphite and fur-

nished a rapid color change from deep purple to brown-pink

(2, 5) or brown-orange (6) (Scheme 4). For the reduction

product of 2, an isolated yield could not be obtained due to the

thermal instability of the product (vide infra); nonetheless,

rapid addition of stoichiometric silver trifluoromethanesul-

fonate (AgOTf) in situ following KC₈addition at low tempera-

ture afforded quantitative recovery of 2 with deposition of

metallic silver, suggesting the single-electron reduction pro-

ceeds in quantitative yield to a new paramagnetic species.

Single-crystals of 7 suitable for X-ray diffraction were ob-

tained by layering the crude reduction reaction of 2 in tetrahy-

drofuran with a thawing benzene/hexanes mixture at –35 ºC,

ensuring all manipulations were conducted at cryogenic tem-

peratures (see Supporting Information for experimental de-

tails). Identification of the brown-pink crystals by single crys-

tal X-ray diffraction revealed the mono-anionic bimetallic

Scheme 3

copper

imido

complex,

[K(C₂₂₂)][(^Mesdmx)Cu₂(µ²−N(C₆H₄OMe))] (7), (Figure 3a).

Similarly, bulk recrystallization of the reduction of 5 and 6

with stoichiometric KC₈in the presence of C₂₂₂from a tetrahy-

drofuran/diethyl ether solution layering at –35 ºC afforded

large crystals of [K(C₂₂₂)][(^tBudmx)Cu₂(µ²−NAr)] (Ar: 4-

MeOC₆H_4,8; 3,5-(F₃C)₂C₆H₃, 9). Unlike thermally sensitive 7,

anions 8 and 9 persist in solution at room temperature over

several days.

Figure 2. Solid state molecular structures of (a) (^Mes-

dmx)Cu₂(µ²−N(C₆H₄OMe)) (2), (b) (^tBudmx)Cu₂(µ²−N(3,5-

(F₃C)₂C₆H₃)) (6) at 50 % ellipsoid probability. Color scheme:

Cu (cobalt blue), F (yellow-green), N (blue), O (red). Solvent

molecules, solid-state disorder, and hydrogen atoms are omit-

ted for clarity.

Scheme 4

2.3 Redox Chemistry of [Cu₂(µ²–NR)]

To assess the redox properties of the dicopper imido

complexes, we collected cyclic voltammetry (CV) on imido

complexes 2, 3, 5, and 6 in tetrahydrofuran (0.01 mM sub-

3

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Single crystal X-ray diffraction confirmed the composi-

tion of anionic imido complexes 7−9, representative structures

of which are provided in Figure 3 for 7 and 9. All three anion-

ic complexes display elongation of the Cu−N_imidebonds (aver-

age, Å, 2: 1.812(4), 7: 1.850(3); 5: 1.822(7), 8: 1.865(2); 6:

1.824(8), 9: 1.857(2), all relevant bond lengths are provided in

Table 1). Complexes 7 and 9 display elongated Cu–Cu bonds

(7: 2.9031(7) Å; 9: 2.8401(5) Å) whereas 8 reveals a Cu–Cu

bond contraction (8: 2.9448(7) Å). The reduction similarity

proceeds with elongation of the dipyrrin-copper bond lengths

(average, Å, 2: 1.942(4), 7: 2.007(3); 5: 1.952(8), 8: 2.019(2);

6: 1.939(8), 9: 2.005(2)), in accord with more electron-rich Cu

ion. Further bond metrics analysis reveals a contraction of the

N_imide–C_ispobond (average, Å, 2: 1.375(6); 7: 1.318(4); 5:

1.383(10); 8: 1.351(3); 6: 1.386(12); 9: 1.342(3)) and elonga-

tion of the C_ipso–C_orthobond (average/Å, 2: 1.415(7), 7:

1.433(5); 5: 1.397(12), 8: 1.427(3); 6: 1.407(13), 9: 1.428(4)).

detected for both the neutral (5, 6) and anionic (8, 9). This

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3

4

5

6

7

8

largely precludes assignment of any of these NIR bands as

intervalence charge transfer (Figure S–60). This assertion is

supported by TDDFT calculations, as no bands in the NIR or

visible region present as intervalence charge transfer bands

(IVCT; Figure S–72, Table S–1). Moreover, variable tempera-

ture UV–vis absorption measurements on

8

in 2-

methyltetrahydrofuran indicate no substantial electronic rear-

rangements as a function of temperature (Figure S–59).

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Figure 4. (a) Solution 2-methyltetrahydrofuran (9.843 GHz)

EPR spectrum of 7 obtained at 298 K expanded to show an

isotropic S = ¹/₂signal (red) simulated (black) with the follow-

ing parameters: g_iso= 2.033, g_x= 0.0277, g_y= 0.0039, g_z=

0.0051; ⁶³Cu₂A_iso= 87.4 MHz; ¹⁴N A_iso= 17.8 MHz. (b) Fro-

zen 2-methyltetrahydrofuran (9.378 GHz) EPR spectrum 7

obtained at 4 K expanded to show an anisotropic S = ¹/₂signal

(red) simulated (black) with the following parameters: g_x=

1.979, g_y= 2.032, g_z= 2.074; g_x= 0.0080, g_y= 0.0014, g_z

= 0.0098; ⁶³Cu₂A_x= 9.3 MHz, A_y= 43.9 MHz, A_z= 9.7 MHz;

¹⁴N A_x= 86.6 MHz, A_y= 125.6 MHz, A_z= 130.3 MHz.

2.4 X-ray Absorption Spectroscopy

Multi-edge X-ray absorption spectroscopy (XAS) com-

prising Cu K-edge, Cu L_2,3-edge, and N K-edge measurements

were carried out to probe and differentiate the electronic struc-

Figure 3. Solid state molecular structures of (a)

[K(C₂₂₂)][(^Mesdmx)Cu₂(µ²−N(C₆H₄OMe))] (7) at 35 % ellip-

soid probability and (b) [K(C₂₂₂)][ (^tBudmx)Cu₂(µ²–N(3,5-

(F₃C)₂C₆H₃))] (9) at 50 % ellipsoid probability. Color scheme:

Cu (cobalt blue), F (yellow-green), N (blue), O (red), K (pink).

Solvent molecules, solid-state disorder, and hydrogen atoms

are omitted for clarity.

tures of neutral 5,

(

6

and anionic 8, 9. Cuprous

^tBuL)Cu(NCMe) (10) and cupric [(^tBuL)CuCl]₂(11) complexes

were prepared and measured as reference dipyrrin-supported

copper complexes. Previously, Lancaster and coworkers illus-

trated that high energy Cu K-edge pre-edge features (~8981 ±

0.5 eV), typically assigned as Cu 1s → 3d excitations, can

instead be indicative of a ligand-based acceptor orbital with a

more reduced Cu center in lieu of a high-valent Cu^IIIoxidation

state assignment.²⁵In the present instance, the presence of pre-

edge features around ~8979 eV ± 0.5 eV traditionally assigned

to a Cu^IIcenter mirrors the case of TEMPO^•coordinated to

Cu^IIdihalides, in which again the acceptor orbital is dominated

by ligand character.²⁶Moreover, pre-edge transitions at

~8980.5 eV have been observed in the Cu K-edge XAS spec-

trum of Cu^Ispecies with low lying ligand π* orbitals, such as

in Cu-bipyridine species.^27,28In summary, these data demon-

strate Cu K-edge XAS pre-edge peak energies are not alto-

gether diagnostic of physical oxidation states in these often-

covalent systems; nonetheless, they remain useful to discuss to

test the veracity of electronic structure calculations.

X-band electron paramagnetic resonance (EPR) spectros-

copy on a thawing 2-methyltetrahydrofuran solution of 7 dis-

plays a seven-line pattern, which was appropriately modeled

with predominant isotropic copper hyperfine coupling (⁶³Cu₂

A_iso= 87.4 MHz) and a minor contribution from nitrogen hy-

perfine coupling (¹⁴N A_iso= 17.8 MHz) (Figure 4a). At lower

temperatures (4 K), the hyperfine coupling becomes aniso-

tropic with copper hyperfine coupling values ⁶³Cu₂A_x= 9.3

MHz, A_y= 43.9 MHz, A_z= 9.7 MHz and ¹⁴N A_x= 86.6 MHz,

A_y= 125.6 MHz, A_z= 130.3 MHz (Figure 7b). Akin to 7, EPR

spectra of both 8 and 9 are indicative of an S = ¹/₂spin ground

state which display seven-line patterns at room temperature in

2-methyltetrahydrofuran (Figure S–21 for 8, S–24 for 9). The

large anisotropic nitrogen hyperfine coupling resolved at low

temperatures (Figure S–22 for 8, S–25 for 9) is consistent with

a significant contribution of the N 2p_xorbital to the now sin-

gly-occupied redox active molecular orbital (vida infra). Ab-

sorptions of near-infrared wavelengths (1000–2000 nm) were

In scrutinizing the Cu K-edge XAS spectra of all species,

a single pre-edge feature is observed at 8978.7 eV in the spec-

trum obtained for 11, the authentic cupric reference. This fea-

ture is assigned as a Cu 1s → 3d transition consistent with the

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expected Cu^IIoxidation state assignment (Figure 5a). Similar

pre-edge features are found in the Cu K-edge spectra of 5 and

1

2

3

4

5

6

7

8

9

6 at 8979.1 eV, although they appear at higher energy within

the region conventionally ascribed to Cu^II. These features rep-

resent transitions into the LUMO of these complexes, which

comprise an anti-bonding interaction between the N 2p_xof the

imide and the weakly π-bonding Cu–Cu MO formed by the in-

phase combination of Cu 3d_x2-y2. In contrast, the corresponding

one-electron reduced anions 8 and 9 exhibit XANES spectra

that are devoid of resolved pre-edge peaks (Figure 5b). These

spectra have rising edges shifted to slightly lower energy, con-

sistent with redox participation by Cu. Higher energy rising

edge transitions observed at ca. 8983 eV gain intensity upon

reduction from the neutral dicopper imido species. These in-

tense rising edge features have been previously observed in Cu

K-edge XAS spectra and are assigned primarily as Cu 1s → 4p

transitions that can contain contributions from ligand-to-metal

charge transfer (LMCT) shakedown transitions.²⁹Due to the

dipole-allowed nature of an s → p (Δl = +1) transition, larger

intensities are observed compared to the relative weak, lower

energy Cu 1s → 3d and Cu 1s → L π* features. Moreover, the

increase in the intensity of these Cu 1s → 4p rising-edge fea-

tures is suggestive of greater Cu^Icharacter. These spectra are

consistent with diminished Cu character in low-lying acceptor

orbitals upon reduction of 5 and 6 to yield 8 and 9, respective-

ly.

Cu L_2,3-edges provides entry for a more quantitative anal-

ysis of the composition of frontier orbitals. This analysis is

possible because ligand orbital admixture into frontier Cu 3d-

based orbitals modulates the allowedness and thus, intensity,

of Cu 2p → 3d transitions. An inverse relationship between

the ligand orbital admixture coefficient and the summed L₃

(Cu 2p_3/2→ Cu 3d) and L₂(Cu 2p_1/2→ Cu 3d) intensities is

observed.³⁰The L₃mainline features occur at 931.3 eV (4) and

931.0 eV (5) while the L₃-edge of 11 occurs at 930.7 eV.

Analysis of the summed L₃+ L₂integrated area reveals 20 ± 1

% and 24 ± 1 % Cu 3d character (per Cu) in the LUMO orbit-

als of 5 and 6, respectively (Figure 6b). These areas translate

to 40% Cu 3d in the LUMO of 5 and 48% Cu 3d in the LUMO

of 6. Accounting for the two indistinguishable copper centers,

these experimental values accord well with those calculated

for the total Cu character (vide infra) at 40.0 % and 47.0 % for

5 and 6 respectively, representing a covalent ground-state

electronic structure in which significant mixing of ligand char-

acter is expected. The Cu L_2,3-edge spectra of the reduced spe-

cies show evidence of significant photo-damage, thus prevent-

ing a quantitative analysis of Cu character in the LUMO.

Nonetheless, upon comparison of the first (and thus, least-

damaged) scans of 8 and 9 (Figure 6b), the L₃feature is signif-

icantly diminished in intensity as compared to 5 and 6, and a

pronounced satellite feature around 934.8 eV (iso-energetic

with that observed in 10) is apparent. The weakness of the

absorbance features for 8 and 9 implicates a ligand-dominated

SOMO and suggests that that both anionic 8 and 9 contain

reduced copper centers relative to the unambiguous cupric

sample 11.

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Figure 5. Cu K-edge XAS spectra of (a) [(^tBudmx)Cu₂(µ²-

NAr)]ⁿfor n = 0, Ar: 3,5-(CF₃)₂-C₆H₃(6, red); n = −1, Ar: 3,5-

(F₃C)₂C₆H₃(9, black); and mononuclear (^tBuL)Cu^I(NCMe) (10,

purple); (^tBuL)Cu^IICl (11, gold); (b) n = 0, Ar: 4-MeOC₆H₄(5,

blue); Ar: 3,5-(F₃C)₂C₆H₃(6, red); n = −1, Ar: 4-MeOC₆H₄(8,

green); Ar: 3,5-(F₃C)₂C₆H₃(9, black).

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The diminished intensity of the 395.2 eV features in 8 and 9

implicates an MO containing NAr 2p_xcharacter participating

in the 5/8 and 6/9 redox couples.

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Figure 6. Cu L_2,3-edge XAS spectra of (a) [(^tBudmx)Cu₂(µ²-

NAr)]ⁿfor n = 0, Ar: 3,5-(CF₃)₂-C₆H₃(6, red); n = −1, Ar: 3,5-

(F₃C)₂C₆H₃(9, black); and mononuclear (^tBuL)Cu^I(NCMe) (10,

purple); (^tBuL)Cu^IICl (11, gold); (b) n = 0, Ar: 4-MeOC₆H₄(5,

blue); Ar: 3,5-(F₃C)₂C₆H₃(6, red); n = −1, Ar: 4-MeOC₆H₄(8,

green); Ar: 3,5-(F₃C)₂C₆H₃(9, black).

Figure 7. N K-edge XAS spectra of (a) [(^tBudmx)Cu₂(µ²-

NAr)]ⁿfor n = 0, Ar: 3,5-(CF₃)₂-C₆H₃(6, red); n = −1, Ar: 3,5-

(F₃C)₂C₆H₃(9, black); and mononuclear (^tBuL)Cu^I(NCMe) (10,

purple); (^tBuL)Cu^IICl (11, gold); (b) n = 0, Ar: 4-MeOC₆H₄(5,

blue); Ar: 3,5-(F₃C)₂C₆H₃(6, red); n = −1, Ar: 4-MeOC₆H₄(8,

green); Ar: 3,5-(F₃C)₂C₆H₃(9, black).

N K-edge XAS was obtained to assess the redox partici-

pation of the aryl nitrenoid moiety. Multiple features assigned

as transitions from N 1s to antibonding MOs comprised of N

and aryl character from the dipyrrin ligand are seen along the

rising edge of the N K-edge for all species, consistent with our

prior measurements of mononuclear dipyrrin-supported Fe

imide/iminyl complexes³¹as well as for analogous dipyrrin-

supported nickel complexes³²(Figure 7).

2.5 Nitrenoid reactivity

Transition metal bound imido complexes facilitate a

range of nitrene transfer chemistry.^33,34Electron-rich metal

imide complexes have been observed to undergo transfer to

nucleophiles (e.g., tertiary phosphines, isocyanides, CO),

whereas electrophilic imide complexes can exhibit olefin

azirdination or the amination of C−H bonds.^35-40To establish

the effect of the dinuclear cuprous proximal arrangement af-

forded by the dimethylxanthene spacer, 10 was treated with

substoichiometric aryl azides (0.5 equiv) at elevated tempera-

tures (60 ºC, C₆D₆) to promote acetonitrile displacement, re-

Additional, weak features at ca. 395-396 eV have been

previously assigned as excitations into subvalent NAr-based

MOs given the aforementioned Fe/Ni iminyl³¹,³²and copper

nitrene⁹precedent. These assignments are supported by elec-

tronic structure calculations (vide infra). In 5 and 6, these tran-

sitions are observed at 396.0 eV and are assigned as transitions

into a MO comprised of both Cu 3d 2 2 and NAr 2p character.

−

y

x

Upon one-electron reduction, these transitions are shifted to

lower energy at 395.2 eV, and the intensities are diminished.

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sulting in quantitative formation of azoarene and accompanied 2 or 3 with DMAP affords the corresponding aniline as well as

by formation of dimeric cuprous species (^tBuL)₂Cu₂(18) as the

major copper-containing species. Complex 18 displays sub-

stantial pyrrole torsion with respect to each pyrrole plane

(27.0(1)º, 30.9(1)º) to accommodate an exceptionally short

intermetallic distance (2.443(2) Å, Figure S–88). Complex 18

was similarly competent for azoarene formation from aryl

azides at elevated temperatures. The absence of spectroscopi-

cally observable intermediates prior to azoarene formation

corroborates the paramount functionality of the dimethylxan-

16 under more mild conditions (35 ºC, 6 h).

1

2

3

4

5

6

7

8

Scheme 7

thene spacer in stabilizing [(^Rdmx)Cu^II(µ²–NAr)] complexes

9

2

10

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against rapid azoarene formation.

The neutral (^Rdmx)Cu₂(µ²–NAr) complexes (2, 3, 5, 6) all

undergo nitrene transfer upon exposure to nucleophiles. For

example, treatment of the neutral complexes with excess ter-

tiary phosphine (PMe₃, 2 or 3; PPh₃, 5 or 6) afforded the cor-

responding phosphinimide products with concomitant for-

mation of the phosphine ligated [(^Rdmx)Cu^I₂(PR₃)₂] products,

None of the (^Rdmx)Cu₂(µ²–NAr) complexes (2, 3, 5, 6)

exhibited propensity towards H−atom abstraction at room

temperature, even upon exposure to weak H−atom sources

(e.g., 2-hydroxy-2-azaadamantane; BDE_O–H= 69 kcal mol^-1).

However, the anion 7 was observed to intramolecularly ami-

nate at room temperature. Allowing solutions of 7 to stand at

room temperature over several minutes generated several new

(

^Mesdmx)Cu₂(PMe₃)₂(12), (^tBudmx)Cu₂(PPh₃)₂(13), in good

yields (Scheme 5). The phosphine-ligated products were veri-

fied by independent synthesis and characterized by single

crystal X-ray diffraction (Figures S–47, S–58). The electron-

deficient imide complexes (^Rdmx)Cu₂(µ²−N(3,5-(F₃C)₂C₆H₃))

(3, 6) reacted rapidly with excess tert-butyl isocyanide (CN^t-

Bu) to afford the corresponding carbodiimide product. In con-

trast, the reaction of electron-rich imidos (^Rdmx)Cu₂(µ²–

N(C₆H₄OMe)) (2, 5) with CN^tBu yielded the resulting

azoarene product (Scheme 6). The copper-containing species

from these reactions was identified as (^Rdmx)Cu₂(CN^tBu)₂(R:

Mes, 14; ^tBu, 15) and confirmed by independent synthesis and

single crystal X-ray diffraction. The difference in reactivity

profiles can be attributed to varying radical-localization at the

nitrogen bridgehead (vide supra).

1

diamagnetic species by H NMR, with one species identified

as a di(cuprous) product in which intramolecular amination of

the proximal mesityl C–H bond has occurred (17, Scheme 7).

Single crystals of the reaction product 17 were grown by vapor

diffusion of diethyl ether into a concentrated solution of 17 in

tetrahydrofuran. The molecular structure obtained by crystal-

lography, while poor, was sufficient to establish the connectiv-

ity (Figures S–41). This reaction proceeds through formal loss

of a single H-atom, which may contribute to the presence of

1

multiple unidentified species in crude H NMR spectrum. At-

tempts to mitigate ligand functionalization by introduction of

external weak C–H or O–H bonds to 7 were unsuccessful.

In contrast to the reactivity exhibited by the

(^Rdmx)Cu₂(µ²–NAr) complexes (2, 3, 5, 6) complexes and the

anion 7, the more thermally robust imido anions

[(^tBuL)Cu₂(µ²–NAr)]^–8 and 9 were unreactive toward tri-

phenylphosphine or weak C–H bonds and H−atom donors.

Though containing comparable % N 2p covalency values (5/8:

15 %/22 % vs. 6/9: 17/25%), formation of the corresponding

iminyl upon reduction ultimately results in most of the spin

density of the frontier RAMO being delocalized over the aryl

ring of the NAr substituent.

Scheme 5

3. Discussion

Scheme 6

3.1 Synthesis and structure.

The pacman bis(dipyrrin) dicopper complexes are well

suited to undergo nitrene capture from aryl azides. While the

dicuprous precursors do not exhibit substantial intercopper

interactions (e.g., 13, d_Cu–Cu: > 6.20 Å), the two-electron oxi-

dized imido complexes have shorter interactions (d_Cu-Cu: 2.82 –

2.87 Å) enforced by the bridging imido motif. The Cu–Cu

separation is shortened compared to the bridging aryl imido

complexes reported by Warren (2.911(1) Å)⁵but longer than

the isoelectronic dicopper carbene complexes (2.464(1) Å).⁴¹

The Cu–N_imidedistances are consistent with reported µ²-imido

(1.794(5) − 1.808(5) Å)^5-8and µ³-copper imidos (1.842(5) −

1.893(6) Å).^42,43Upon reduction, each of the crystallograph-

ically characterized complexes 7−9 exhibit expansion of the

[Cu₂N]¹⁺core, consistent with increased electron richness. The

near-identical coordination environment about each copper ion

in 7−9 contrasts the apparent valence-localized oxo-bridged

Fe^IIFe^IIIcomplex previously isolated within this framework.¹³

Each of the reduced nitrenoid complexes also features a con-

tracted N_imido−C_ipsobond (7, 1.318(4) Å; 8, 1.351(3) Å; 9,

While imido complexes 2, 3, 5, and 6 were thermally stable

(120 ºC, 24 h), azoarene generation from the imido complexes

was promoted by treatment of the imide complexes with pyri-

dine, 4-(dimethylamino)pyridine (DMAP), or cyclohexene.

For example, 2 decays as a first-order process in the presence

of excess DMAP (9 equiv.) with heating (65 ºC, 10 h) to af-

ford (^Mesdmx)Cu₂(dmap)₂(16) and the corresponding azoarene

in excellent yield (87 %) by both ¹⁹F and H NMR with no

spectroscopically observable intermediates. In the presence of

weak H-atom donors such as diphenylhydrazine, treatment of

1

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Figure 8. Simplified qualitative frontier molecular orbital

framework in idealized C_2vsymmetry showcasing Cu₂–NAr π

bonding in 5/6, 8/9.

1.342(3) Å) which approximate as C−N double bonds akin to

those found in pyridine (Table 1). Thus, the nitrenoid units

take on more of a ketamide resonance form similar to those

previously reported for the bimolecularly coupled

[(^EMindL)Cu]₂(N(3,5-(MeO)₂C₆H₃)₂)₂(N_imido−C_ipsobond lengths:

1.257(5) Å, 1.276(6) Å).⁹Furthermore, redox non-innocence

behavior of the aryl fragment is apparent upon inspection with

elongation of the C_ipso–C_orthobond upon reduction. Such arene

distortions have been observed in previously isolated dipyrrin-

supported quintet iron iminyl complexes,^31,44,45doublet nickel

iminyl complexes,³²and triplet copper nitrene complexes,⁹

1

2

3

4

5

6

7

8

Scalar relativistic single point DFT calculations employ-

ing the B3LYP hybrid density function with the CP(PPP) basis

set on Cu and the scalar-relativistically recontracted ZORA-

def2-TZVP(-f) basis set on all other atoms were carried out on

all compounds for which XAS data were obtained. For neutral

5 and 6, broken symmetry (BS) singlet diradical (BS 1,1) solu-

tions were found to have lower energy relative to correspond-

ing conventional unrestricted Kohn-Sham (UKS) solutions.

For 8 and 9, no BS solutions were obtained. Rather, standard

doublet (S = ¹/₂) solutions emerged.

The aforementioned single point DFT solutions were used

as starting points for time-dependent DFT (TDDFT) calcula-

tions of Cu and N K-edge XAS. TDDFT calculations are now

well-established as a means to produce simulated XAS pre-

edge data with high fidelity to experiment after proper calibra-

tion ^27,46-48The present compounds do not present an exception

to this trend, however the underlying electronic structure in-

terpretation requires extended explanation.

Upon application of a linear energy correction produced

via correlation of TDDFT-calculated with experimental pre-

edge peak energies for all compounds under discussion, the Cu

K pre-edge and rising edge peak energies accord well with

experiment (Figure S–61, S–63). These calculations were then

used to assign acceptor MOs underlying given transitions. For

10, the observed 8982.4 eV transition is predicted at 8982.1

eV as a Cu 1s → 4p excitation, where the acceptor MO exhib-

its some acetonitrile C–N π* character. These calculations

predict a weak Cu 1s → LUMO feature at 8980.6 eV (dipyrrin

π* orbital) that is not resolved experimentally. The calculated

spectrum of 11 reproduces the 8987.7 eV Cu 1s → 3d feature

while also predicting an unresolved Cu 1s → L π* transition at

8983.4 eV. The 8979.0 eV Cu 1s → RAMO transitions are

reproduced for 5/6, and upon inspection of the calculated spec-

tra for 8/9 weak Cu 1s → RAMO transitions are observed at

slightly higher energy relative to their neutral counterparts

(8979.5 and 8979.9 eV respectively). In addition, 8 and 9 dis-

play weak pre-edge features at 8981.1 eV corresponding to Cu

1s → L π* transitions, shifted to lower energy compared to the

8982.9 eV features predicted features for the neutral complex-

es. The diminished intensity of the Cu 1s → 3d transitions and

observed energy shifts supported the contention that 8 and 9

possess RAMOs with diminishing copper character and N-

localized spin. Fewer acceptor holes are available in NAr, and

participation of NAr in the redox is supported by the energetic

red shift, indicating a more electron-rich NAr fragment.

Agreement between experiment and theory is maintained

upon returning to the N K-edge XAS spectra (Figure S-64, S-

65). Examining the spectrum of 10, the 398.0 eV feature in the

experimental N K-edge is predicted at 397.4 eV as a transition

into the dipyrrin-based LUMO, while the 398.9 eV feature is

predicted as a combination of transitions into higher energy

MOs of Cu 4p and acetonitrile π* parentage. For 11, the ob-

served 397.2 eV and 398.0 eV lower-energy shoulders that

represent transitions into the predominantly Cu 3d SOMO,

containing mixing with Cl 3p and dipyrrin N 2p, and lower-

lying dipyrrin π* orbitals, are predicted at 396.3 eV and 397.8

eV. Lastly, examining the N K-edge XAS spectra of the

di(copper) species, the 396.0 eV feature of 5 is predicted at

396.3 eV as a transition into the RAMO containing 47% Cu

and 30% NAr (15% N 2p). The lower energy 395.2 eV feature

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2

suggesting a delocalized [NAr] vacancy is potentially engen-

dered upon one-electron reduction. Such a transformation is

unusual, as the reduction of 2 proceeds with oxidation of the

most electronegative atom of the [Cu₂(µ²−NAr)] core.

3.2 Calculated X-ray Absorption Spectra

Electronic structure calculations were carried out to facili-

tate discussion of the XAS data and to interrogate the nature of

the redox event linking the neutral and anionic dicopper

nitrenoid complexes. The electronic structures that emerge, at

least for the neutral species 5 and 6, are not altogether straight-

forward. Considering each dipyrrin-Cu center in isolation, the

highest Cu 3d-based orbital in each Cu-dipyrrin fragment is

predicted to be 3d_x2-y2(Figure 8). This affords a basis for two

linear combinations representing weakly interacting Cu₂π and

π* orbitals. Introduction of the NAr 2p_xperpendicular to the

Ar plane produces a 3-center MO framework representative of

a 3-center-4-electron bond. This simple picture predicts that

the redox-active molecular orbital (RAMO) is best described

as 휓^∗≈ (Cu₂π – NAr 2p_x), an overall π* MO, in complete

accord with observations from XAS. The Cu orbitals are de-

pressed compared to those of the nitrenoid motif, resulting in

an inverted ligand field with NAr 2p_xcharacter dominant in

the RAMO. We note that this is an idealized picture that as-

sumes C_2vsymmetry and neglects Cu–Cu interactions––in fact

the two LCu inner coordination spheres do not perfectly over-

lay as would be required for this picture to be exactly opera-

tive; each complex exhibits significantly non-zero dihedral

angles defined by the two Cu and two dipyrrin meso-C atoms.

This deviation from ideal symmetry becomes apparent in scru-

tinizing orbitals derived from electronic structure calculations

(vide infra).

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of 8 is once again reproduced, predicted at 395.6 eV as a tran- CASSCF calculation is given in Figure 9. The analogous dia-

1

2

3

4

5

6

7

8

sition into the RAMO that contains 34 % Cu and 44 % NAr

(17 % N 2p). Similar results were obtained for 6/9 (Figure S-

64, S-65).

gram for 5 is qualitatively similar and is included in the sup-

porting information (Figure S–68a). The ground state singlet

comprises two leading configurations with weights of ca. 50%

and 40%. These configurations reflect either double occupa-

tion of the Cu₂π* MO or the (Cu₂π – NAr 2p_x) π* RAMO,

respectively. These configurations can be effectively be aver-

aged to yield the BS picture.

A qualitative frontier MO diagram for 9 is given in Figure

9b for comparison to 6. The diagram for 8 is similar and is

included as figure S-68b. These single-configurational doublet

states possess an unpaired electron in the (Cu₂π – N Ar 2p_x)

π* RAMO in accord with predictions and the observed de-

creased intensities of the N K pre-edge peaks upon in the

monoanions relative to the neutral analogues.

Unfortunately, quantification⁴⁸of N 2p covalency from

the given spectra was unreliable due to photodamage and

hence rapid reduction in the intensity of the 396.0 eV and

395.2 eV features that is seen upon carrying out multiple scans

at the same sample location. However, returning to the Cu

L_2,3-edge data, the ca. 40% and 47% overall Cu 3d participa-

tion in the LUMOs of 5 and 6, respectively, accord with the

40% Cu 3d and 47% Cu 3d predicted by DFT. Together with

anisotropic ¹⁴N hyperfine coupling manifesting in the frozen-

solution EPR data obtained for 6 and 9 as well as the the well-

resolved NAr features in the corresponding N K-edge XAS

spectra, these support the contention that there is a high degree

of NAr 2p character in the RAMO of the 5/8 and 6/9 couples.

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3.4 Electronic structure of the bridged imidos

All of the dicopper imido complexes (2, 3, 5, and 6) fea-

ture highly multiconfigurational singlet ground states (Figure

9). The singlet configurations emerging from these calcula-

tions are consistent with the diamagnetic electronic configura-

tions observed at room temperature as assessed by multinucle-

ar NMR and a lack of any discernible EPR signal. Via

CASSCF analysis, we assess the most consistent electronic

description for all these complexes comprise singlet ground

states whose two major configurations are defined by (a) dou-

ble population of (Cu−Cu) π* with no contribution from the

3.3 Ab Initio Calculations

While the TDDFT calculations of the XAS data exhibit

high fidelity to experiment, a paradoxical picture emerges

when more deeply probing the electronic structure of neutral

complexes 5 and 6, for which BS(1,1) solutions are encoun-

tered via DFT. Examination of the acceptor MOs used to gen-

erate the TDDFT-calculated spectra following a quasi-

restricted orbital (QRO) transformation reveals that they are

[Cu₂π – NAr 2p] π*, as predicted qualitatively (Figure S-69).

However, these are the canonical orbitals generated during the

BS calculation. Inspection of the unrestricted corresponding

orbitals (UCOs), which reflect the magnetic orbitals of the

singlet diradical and are mandatory for TDDFT analysis⁴⁹

presents a problem (Figure S-69). These occupied spin orbitals

are predicted to have diminished overlap (S = 0.74 and S =

0.56 for 5 and 6, respectively), and overall are antiferromag-

netically coupled with J-values of –2631 cm^-1and –1802 cm^-1

for 5 and 6, respectively in accord with the experimentally-

established diamagnetic natures of these complexes. However,

these occupied UCOs are effectively of the same nature as the

unoccupied canonical MOs involved in the XAS pre-edge

excitations.

imido unit (48%); and (b) double population of a [(Cu₂

π

− (NAr)] π* interaction contributing 40%. Configuration 1

illustrates how the cupric character in the dicopper imidos is

diminished, consistent with the XAS data, (Figure 9a).

The formally mixed valence anionic imidos (7−9) possess

doublet ground states exhibiting well-defined EPR signals

between 4−295 K (Figures S−21-22, 24-25). Solution-phase

EPR spectra display isotropic hyperfine coupling (i.e., elec-

tronic contribution from the 2s orbital), dominated by Cu

character with minimal N contribution and consistent with

spectra observed for fully delocalized, Class IIIA mixed va-

lence dicopper complexes^51-53in the Robin-Day

classification.⁵⁴This observation is rationalized by considering

the nitrogen contribution to the RAMO arises from the 2p_x

component, leading to an anisotropic component only apparent

at lower temperatures.⁵⁵In the frozen solution EPR spectrum,

a high degree of unpaired electron localization to N 2p_xis ap-

parent, contributing to the hyperfine coupling alongside the

copper centers. The temperature-dependence of the EPR spec-

tra are superficially inconsistent with a Class IIIA mixed va-

lence assignment and more reminiscent of Class II species,^56-59

but are accounted for when considering that the temperature

dependence arises not from changes in electronic structure but

from the participation of anisotropic hyperfine contributions.⁵⁵

While complexes 7−9 do feature optical transitions in the near

IR region (Figure S–59), their corresponding neutral precur-

sors also feature prominent transitions at similar energies.

Indeed, both (^tBudmx)Cu₂(µ²–N(C₆H₄OMe)) (5) and its anionic

analogue 8 feature absorptions at the same energy (~1300 nm)

with near equal intensity ( ~ 500 M^-1cm^-1), whereas the NIR

band for [(^tBudmx)Cu₂(µ²–N(3,5-(F₃C)₂C₆H₃))]^–(9) does shift

to lower energy (1750 nm) than its neutral precursor (6, 1450

nm).

This paradox plausibly arises due to the multiconfigura-

tional nature of the ground state electronic structures of 5 and

6. Ab initio, complete active space self-consistent field

(CASSCF) calculations were carried out on these complexes

were carried out these complexes using CAS(10,9) active

spaces.⁵⁰These active spaces were chosen to balance flexibil-

ity with computational expense. Truncated structural models

t

5 and 6 were generated by removing Bu substituents from

the pyrrole rings and optimizing H-atom positions for similar

reasons of economy. Hybrid DFT-calculated orbitals were

used as starting points for the CASSCF calculations, with or-

bitals included in the active space comprising those involved

in the Cu₂–NAr π interaction as well as some dipyrrin MOs.

These calculations were carried out using the ZORA-def2-

TZVP(-f) basis on Cu and N, with ZORA-def2-SVP on all

other atoms. For all cases, state averaging was used to facili-

tate convergence, with five singlet and five triplet roots calcu-

lated. The strongly contracted N-electron valence perturbation

theory 2 (NEVPT2) correction was applied to CASSCF state

energies to account for dynamical electron correlation. The

calculated singlet-triplet gaps after NEVPT2 correction are

1341.3 and 808.6 cm^–1for 5 and 6 in line with experimental

magnetic behavior. A molecular orbital diagram reflecting the

leading ground state configurations for 6 resulting from the

9

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Figure 9. (a) The two leading configurations making up the singlet ground state resulting from a CAS(10,9) calculation using the

truncated model [(^tBudmx)Cu₂(µ²−N(3,5-(F₃C)₂C₆H₃))]ⁿ(n = 0, 6). These calculations employed the ZORA-def2-TZVP(-f) basis set

on Cu and N with the ZORA-def2-SVP basis set on all other atoms. Occupation of MOs 213 and 214 differentiate the two configu-

rations; these orbitals are printed beneath the diagram. Panel (b) depicts the single configuration defining the doublet ground state

of 9. The MOs comprise QROs generated produced following an unrestricted B3LYP calculation employing the CP(PPP) basis set

on Cu and def2-TZVP(-f) on all other atoms. Orbital plots are depicted at an isovalue of 0.03 au. All orbital labels are based on

dominant interaction(s).

based LUMO in 5/6 by reduction, resulting in a SOMO with

Thus, the origins of the NIR absorption bands cannot exclu-

sively arise from IVCT between the two Cu centers. Using

calculated J parameters of –2631 cm^-1(5) and –1802 cm^-1(6)

yields 2H_ABvalues^60,61in the visible region between ~580 nm

and ~700 nm, in accord with observed IVCTs for the mixed

valence Cu_Aspecies.⁶²We note the IVCT transition may be

obscured by the intense dipyrrin Soret band transition. None-

theless, TDDFT analysis of the NIR absorptions indicates the

transitions are representative of metal-to-ligand charge trans-

fer (MLCT) and not intervalence charge transfer (Figure S–71,

S–72). Inspection of individual transitions within this region

(300−1200 nm, Table S–1) calculated via TDDFT did not

reveal any excitations that could clearly be ascribed to IVCT

transitions. Thus, the highly covalent, three-centered bonding

interaction of the [Cu₂(NAr)] core leads to full electronic de-

localization reserved for mixed valence complexes in the

Class IIIA regime.

significant N character.

The foregoing data indicate the reduced [Cu₂(µ²−NAr)]^–

complexes largely exhibit iminyl character. The copper cen-

ters in the anionic imido complexes exhibit diminished Cu

character in acceptor orbitals based on the Cu L_2,3-edge data

and Lowdin spin density analysis. These data together are

typical of bridged Cu complexes exhibiting highly covalent

cores with substantial ligand participation in the electroactive

orbitals.^63-67The RAMO of 8 and 9 display comparable Cu

character to other mixed valence Cu species such as Cu_A(44

% Cu),⁶⁷[(L^iPrdacoSCu)₂]⁺(38 % Cu),⁶⁷and [{(^tBuPNP)Cu}₂]

+

2

(24 % Cu).⁶³Nonetheless, we note 8 and 9 are lower coordi-

nate (tri-coordinate instead of tetra-coordinate) and contain

strictly hard nitrogen atom in the primary coordination sphere

in lieu of polarizable sulfur and phosphorous. Indeed, the N K-

edge data reveal common features between the reduced imidos

8 and 9 with other N−centered, radical species.^9,31,32Whereas

the previously reported metal-supported iminyl structures ex-

hibit bond-length distortions within the iminyl fragment con-

sistent with hole delocalization, complexes 8 and 9 do not

exhibit substantial distortions aside from enhanced N−C_ipsoπ-

bonding upon NAr reduction. The electronic description is

corroborated with the calculated electronic structure which

reveals a highly (NAr)−centered RAMO (Figure 9b). Insight

into the increasing cuprous character can be gleaned from

comparing the frontier orbitals of 6 and 9. While chemical

reduction of 6 occurs by population of the LUMO. the ground

state of imido 6 is multiconfigurational with nearly equal con-

tributions from (Cu−Cu) π* (213) and [(Cu₂π − (NAr)] π*

(214) singlet configurations. The frontier orbital configuration

of 9 leads to double population of (Cu−Cu) π* (which was

only partially occupied in 6), and partial population of [(Cu₂π

− (NAr)] π* (Figure 9b) − effectively reducing the overall

Cu₂−N bond order by 1.5. Thus, the bridging ligand radical

character was quite remarkably achieved through reduction,

not oxidation, of the [Cu₂(µ²–NAr)] core.

The clearest picture of the electronic structure for the

anionic imidos arises from the combined Cu (L_2,3, K-edges)

and N (K-edge) XAS spectra. While neutral imidos 5 and 6

feature prominent pre-edge absorptions in the Cu K-edge

spectra, the anionic imidos (8, 9) lack clear experimental pre-

edge features (Figure 5, inset) similar to cuprous standard

(

^tBuL)Cu^I(NCMe) (10). The weak pre-edge features observed

in TDDFT calculations can be attributed to addition of an

electron to the Cu and N based LUMO in 5/6. Both neutral 5

and 6 exhibit intense L₃main line features at energies (~930

eV) consistent with mononuclear analogues ((^tBuL)Cu^IICl, 11),

while the L₃feature is significantly diminished in the reduced

nitrenoids with prominent satellites at higher energy (935 eV)

(Figure 6b) akin to the mononuclear cuprous analogue

(

^tBuL)Cu^I(NCMe) (10). Lastly, the N K-edge reveals a system-

atic shift of the pre-edge absorptions as the singly-occupied

molecular orbital becomes more localized on the (NAr) frag-

ment, suggesting increased iminyl radical character exists for

the anions 8 and 9. In addition, the decreased intensity of the

N K-edge pre-edge feature is indicative of population of the N

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Neutral imido complexes of the type (^Rdmx)Cu₂(µ²−NAr)

3.5 Reactivity of the imido complexes

t

1

2

3

4

5

6

7

8

(R: Mes, Bu; Ar: 4-MeOC₆H₄, 3,5-(F₃C)₂C₆H₃) were synthe-

The neutral imidos (2, 3, 5, 6) exhibit reaction chemistry

reminiscent of the electrophilic imido complexes. The nitrene

motif can be transferred to incoming nucleophiles (e.g., ter-

tiary phosphines, alkyl isocyanides), but the aryl-substituent

on the imide influences the reaction trajectory. The more elec-

trophilic [Cu₂(µ²–N(3,5-(F₃C)₂C₆H₃))] imido complexes 3 and

6 undergo clean nitrene transfer to nucleophilic reagents (PR₃,

CN^tBu), while the [Cu₂(µ²–N(C₆H₃OMe))] imido complexes 2

and 5 produce azoarene from nitrene coupling upon reaction

sized by treatment of the corresponding dicuprous precursors

with aryl azides. Chemical reduction of the imido complexes

afforded dicopper iminyl products as determined by EPR,

Cu/N XAS, crystallography, and computations. The iminyl

ligand is distinct from previously reported subvalent nitrogen

motifs which arise from radical delocalization following ni-

tride,^69,70imide,^31,44,45,71or amide^72-75oxidation.^72,76The iminyl

complexes reported herein show modest disruption of the aryl

units, arising from population of the NAr lowest-lying π^*or-

bital. The iminyl density is predominantly localized on the N

2p_xorbital, normal to the [Cu₂(µ²–NAr)] plane, optimally lo-

cated to aminate the benzylic C−H bonds of the (^Mesdmx) lig-

and but remain sterically occluded in the (^tBudmx) framework

and are thus robust. Despite the presence of bona fide radical

character on the bridging iminyl, the [(^tBudmx)Cu₂(µ²−NAr)]^–

complexes are resistant to nitrene transfer and H-atom abstrac-

tion reactivity. Thus, while we were successful in generating

dicopper complexes with substantive radical character accu-

mulation at the bridgehead ligand, these complexes were

found to be remarkably stable. Combined, our results contrib-

ute understanding of electronic structure, and identifying the

precise localization of frontier orbital radical density, can aid

in understanding group transfer reactions such as C−H bond

functionalization and nitrene insertion.

9

t

with BuNC. Several simple ligands (e.g., pyridine, DMAP)

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also can induce azoarene formation at elevated temperatures,

which do not require direct attack of substrate on the imido

fragment. Indeed, each of the nitrene transfer reactions ob-

served can occur from substrate attack on [Cu₂(µ²–NAr)] core,

potentially activating the core via ligation (e.g., [Cu₂(µ²–

NAr)(L)], L = substrate) or inducing Cu−N cleavage to form a

transient terminal imido complex. The activated dicopper core

can then undergo rapid bimolecular coupling or undergo free

nitrene expulsion. However, the complete selectivity of nitrene

coupling to yield azoarene argues against free nitrene expul-

sion without the detection of free nitrene products (e.g., aziri-

dine, C−H amination, nitrene aryl-ring expansion). The ob-

served difference in reactivity between electron rich and elec-

tron poor imido complexes may arise from the relative stabili-

ties of the triplet and singlet nitrene states on the transient

[Cu(L)Cu(NAr)] (L = PR₃, CN^tBu) formed. The Cu-bound

³[N(C₆H₄OMe)] would favor reacting with triplet configura-

tion as opposed to added singlet reactants. The absence of

intramolecular benzylic amination, intermolecular H-atom

abstraction, or ring expansion of the nitrene aryl ring argue

against dissociation of free nitrene in solution. Nonetheless,

the release of singlet nitrene cannot be fully discounted, noting

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the

ACS Publications website, including synthesis of 1–18, spectro-

scopic characterization, computational details, CheckCIF results,

solid-state molecular structures, and supplementary spectra.

1

. AUTHOR INFORMATION

Corresponding Author

that whereas [N(3,5-(F₃C)₂C₆H₃)] may have an appreciable

1

singlet lifetime in solution, intersystem crossing from [N(4-

MeOC₆H₃)] to a triplet ground state is exceptionally fast,⁶⁸

perhaps explaining the discrepancies in reactivity upon CN^tBu

treatment.

*kml236@cornell.edu

*betley@chemistry.harvard.edu

Aside from the intramolecular amination exhibited by

thermally unstable 7, the reduced imido complexes show no

propensity for nitrene transfer to substrate or proclivity to-

wards H−atom abstraction chemistry. These results suggest

the presence of radical character at the nitrogen bridgehead of

the imide motif enables amination of proximal ligand, ben-

zylic C–H bonds at room temperature. Despite the presence of

bona fide radical character at the bridgehead, 7 fails to per-

form intermolecular amination chemistry. This observation

may be attributed to the relatively reducing potential of 7,

rendering the resulting N–H bond excessively weak. Moreo-

ver, the inability of the ligand to support a zero-valent copper

center renders two-electron reduction chemistry from 7 unlike-

ORCID

Kurtis M. Carsch: 0000-0003-4432-7518

Ida M. DiMucci: 0000-0003-3440-7856

Diana A. Iovan: 0000-0001-9889-7183

Shao-Liang Zheng: 0000-0002-6432-9943

Kyle M. Lancaster: 0000-0001-7296-128X

Theodore A. Betley: 0000-0001-5946-9629

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

We thank E. Johnson (Harvard) for helpful discussions and the

initial synthesis of (^Mesdmx)H₂. We thank S. Sproules (Glasgow)

for helpful guidance with EPR interpretation. T.A.B. gratefully

acknowledges support by grants from NIH (GM-115815), the

Dreyfus Foundation in the form of a Teacher-Scholar Award, and

Harvard University. K.M.C. acknowledges the Fannie & John

Hertz Foundation and the National Science Foundation for finan-

cial support of this research. XAS data were obtained at SSRL,

which is supported by the U.S. Department of Energy, Office of

Science, Office of Basic Energy Sciences under Contract No. DE-

AC02-76SF00515. The SSRL Structural Molecular Biology Pro-

gram is supported by the Department of Energy’s Office of Bio-

logical and Environmental Research, and by NIH/HIGMS (in-

cluding P41GM103393). ChemMatCARS Sector 15 is principally

supported by the National Science Foundation/Department of

t

ly. The dipyrrin ligand substituents (Mes in 7, Bu in 8 and 9)

may sterically shield the iminyl fragment from incoming sub-

strate, especially given the SOMO character is localized on the

N_pxorbital normal to the [Cu₂(µ²−NAr)] plane. Thus, the ami-

nation exhibited by 7 may result from the optimally oriented,

mesityl benzylic methyl substituents, while the C−H bonds of

the ^tBu groups of 8 and 9 are too strong to be activated by the

(²NAr) and sterically restrict access to the [Cu₂(µ²–NAr)] core.

Furthermore, the anionic complexes 8 and 9 are less electro-

philic and show diminished reactivity observed for the neutral

analogues.

4. Conclusions

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Energy under grant number NSF/CHE-0822838. Use of the Ad-

(19) King, E. R.; Sazama, G. T.; Betley, T. A., Co(III) Imidos

1

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vanced Photon Source was supported by the U.S. Department of

Energy, Office of Science, Office of Basic Energy Sciences, un-

der Contract No. DE-AC02-06CH11357. Collection of low-

temperature UV/Vis spectra were obtained at the Center for Na-

noscale Systems (CNS, Harvard University), a member of the

National Nanotechnology Coordinated Infrastructure Network

(NNCI), which is supported by the National Science Foundation

under NSF award no. 1541959.

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[PPh₃(C₆H₄)CuNHMes],

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Article Doi

Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes

DOI: 10.1021/jacs.9b09616

Source and publish data:

Authors:

Article abstract of DOI:10.1021/jacs.9b09616

Full text of DOI:10.1021/jacs.9b09616

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