C-C bond formation and related reactions at the CNC backbone in (smif)FeX (smif = 1,3-di-(2-pyridyl)-2-azaallyl): Dimerizations, 3 + 2 cyclization, and nucleophilic attack; Transfer hydrogenations and alkyne trimerization (X = N(TMS)2, dpma = (Di-(2-pyridyl-methyl)-amide))

Article

pubs.acs.org/IC

C−C Bond Formation and Related Reactions at the CNC Backbone in

(smif)FeX (smif = 1,3-Di-(2-pyridyl)-2-azaallyl): Dimerizations, 3 + 2

Cyclization, and Nucleophilic Attack; Transfer Hydrogenations and

Alkyne Trimerization (X = N(TMS)₂, dpma = (Di-(2-pyridyl-methyl)-

amide))

Brenda A. Frazier,^†Valerie A. Williams,^†Peter T. Wolczanski,*^,†Suzanne C. Bart,^‡,∥Karsten Meyer,^‡

Thomas R. Cundari,^§and Emil B. Lobkovsky^†

^†Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States

^‡Department of Chemistry & Pharmacy, University of Erlangen-Nuremberg, Egerlandstr. 1·D-91058 Erlangen, Germany

^§Department of Chemistry, Center for Advanced Scientiﬁc Computing and Modeling (CASCaM), University of North Texas, Box

305070, Denton, Texas 76203-5070, United States

S

* Supporting Information

ABSTRACT: Molecular orbital analysis depicts the CNC^nb

backbone of the smif (1,3-di-(2-pyridyl)-2-azaallyl) ligand as

having singlet diradical and/or ionic character where electro-

philic or nucleophilic attack is plausible. Reversible dimeriza-

tion of (smif)Fe{N(SiMe₃)₂} (1) to [{(Me₃Si)₂N}Fe]₂(μ-

κ³,κ³-N,py₂-smif,smif) (2) may be construed as diradical

coupling. A proton transfer within the backbone-methylated, and o-pyridine-methylated smif of putative (^bMe₂Me₂smif)Fe-

o

N(SiMe₃)₂(8) provides a route to [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif)H))₂(9). A 3 + 2 cyclization of

ditolyl-acetylene occurs with 1, leading to the dimer [{2,5-di(pyridin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN-

(SiMe₃)₂]₂(11), and the collateral discovery of alkyne cyclotrimerization led to a brief study that identiﬁed Fe(N(SiMe₃)₂(THF)

as an eﬀective catalyst. Nucleophilic attack by (smif)₂Fe (13) on ^tBuNCO and (2,6-ⁱPr₂C₆H₃)NCO aﬀorded (RNHCO-smif)₂Fe

t

(14a, R = Bu; 14b, 2,6-ⁱPrC₆H₃). Calculations suggested that (dpma)₂Fe (15) would favorably lose dihydrogen to aﬀord

(smif)₂Fe (13). H₂-transfer to alkynes, oleﬁns, imines, PhNNPh, and ketones was explored, but only stoichiometric reactions

were aﬀected. Some physical properties of the compounds were examined, and X-ray structural studies on several dinuclear

species were conducted.

component could direct C−C bond formation at the CNC

backbone of the smif ligand,^4,7,8such as a dimerization process.

Examples of C−C bond formation derived from backbone

coupling reactions have been seen in attempts to synthesize

(smif)₂Ti.⁷As Figure 2 reveals, (smif){Li(smif-smif)}Ti (Ti-1)

degraded at room temperature to aﬀord a mixture of

diamagnetic products. Spectroscopic analysis (K-edge XAS,

2D NMR) of Ti-1 and its degradation products suggested that

they are best described as d¹Ti(III) centers AF-coupled to

ligand radicals: (smif){Li(smif-smif)²⁻}Ti^III(Ti-1), [(smif²⁻)-

Ti^III]₂(μ-κ³,κ³-N,py₂-smif,smif) (Ti-2), [(smif²⁻)Ti^III](κ³-

N,N(py)₂-smif,(smif)H) (Ti-3), and (smif²⁻)Ti^III(dmpa) (Ti-

4). While it is likely that a bound Li(smif) unit succumbed to a

nucleophilic attack of a bound smif to generate the new C−C

bond in Ti-1, the C−C bonds in Ti-2 can be construed as

arising from nucleophilic/electrophilic attack or the coupling of

diradicals; the same is true for Ti-3, because it is likely to be a

1. INTRODUCTION

While examining polydentate chelates of ﬁrst-row transition

elements,¹⁻³a decomposition led to the synthesis of (smif)-

CrN(TMS)₂(smif = 1,3-di-(2-pyridyl)-2-azaallyl).¹Discovery

of the anionic azaallyl anion ligand prompted the synthesis of a

set of new Werner complexes, (smif)₂Mⁿ(n = 0, M = V, Cr,

Mn, Fe, Co, Ni, Ru; n = +1, M = Cr, Mn, Co, Rh, Ir).^4,5The

electronic properties of these complexes were investigated in

response to their unusual optical densities, which are derived

from intraligand (IL) transitions wherein charge is transferred

from the backbone CNC^nborbitals to those with pyridine π*-

character.^1,4,5

An abbreviated look at the CNC^nborbital in the smif

backbone, where modest contributions from the pyridine rings

are ignored, is given in Figure 1.⁵⁻⁷A decomposition of the

crude wavefunction renders two orbital components that are

distinct: an ionic part that suggests the potential for

nucleophilic/electrophilic activation of the backbone, and a

covalent part that possesses singlet diradical character. Either

Received: December 19, 2012

Published: February 28, 2013

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Figure 1. Expansion of a truncated smif CNC^nborbital showing ionic and covalent diradical components.

Figure 2. Various smif-containing complexes where C−C (red) or C−H bond formation has occurred via the CNC backbone of smif.

Figure 3. Green (smif)Fe{N(SiMe₃)₂} (1) is monomeric in solution, but in the solid state, it is a dimer, [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif)

(2, X-ray). A μ_effvs T (K) plot and a Mossbauer spectrum of 2 are shown.

̈

protonolysis product derived from Ti-2. Redox noninnocence

was previous observed in (smif)₂Cr, that is, (smif)(smif²⁻)-

Cr^III,^5,6but whether such electronic factors are important to the

C−C bond forming events is unclear.

Reversible carbon−carbon bond formations are often seen

accompanying redox events, and such systems have been

explored in the context of energy storage.⁹⁻²⁰It is important to

note that the transformations described herein are not triggered

by external redox events, nor are the majority coupled to redox

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changes at the metal. The chemistry is intrinsic to the nature of

the frontier orbitals of the smif ligand. Herein are described

eﬀorts to control the smif backbone reactivity: generating

carbon−carbon and carbon−hydrogen bonds at the CNC

positions, and examining the means to prevent such reactivity.

The ﬁrst example of C−C bond formation at the backbone of

smif was the case of [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif)

(2) illustrated in Figure 3, which was brieﬂy mentioned in a

communication.⁴The examination of smif reactivity starts with

this case.

solutions, crystals that precipitated were orange, and lacked

substantial intensity. An NMR spectral analysis²²aﬀorded an

equilibrium constant for 2 1 ⇄ 2 of K₂₉₇∼ 4 × 10⁻⁴, showing

that the amount of C,C-coupled dimer 2 was relatively

insigniﬁcant in typical solutions, with ΔG°₂₉₇∼ 5 kcal/mol.

SQUID magnetometry on 2 provided a μ_effof ∼6.9 μ_Bat 300

K, which is essentially the value predicted for a two S = 2

centers on one molecule that do not couple. There is a

substantial downturn of the μ_effbelow 75 K as the eﬀects of

zero ﬁeld splitting (ZFS) are observed. Contributions from

antiferromagnetic coupling cannot be ruled out, but the

d(Fe···Fe) of 6.158(2) Å renders this factor unlikely.

2. RESULTS

2.1. Smif Backbone C−C Coupling. 2.1.1. Synthesis of

[{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2). The generation of

[{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2) was achieved

via the addition of 1,3-di-(2-pyridyl)-2-azapropene (i.e., (smif)-

H)⁵to Fe{N(SiMe₃)₂}₂THF²¹in Et₂O over the course of 3 h

(eq 1). From an emerald green solution distinctive for the

Mossbauer spectroscopy on 2 provided parameters consistent

̈

with a low-symmetry high-spin iron(II) center.^23,24The isomer

shift is 0.86 mm/s, typical for a high-spin ferrous species, and

the quadrupole splitting of 1.07 mm/s suggests signiﬁcant

asymmetry in the electric ﬁeld, as expected for a pseudo-square-

planar geometry.

2.1.2. Structure of [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif)

(2). A molecular view of [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-

smif,smif) (2) is provided in Figure 4, which reveals the

approximate C_2hsymmetry of the dimer and provides selected

information regarding interatomic distances and angles. Limited

data collection and reﬁnement parameters are listed in Table 1.

Two pseudo-square-planar (Σ(NFeN angles) = 360.0°) iron

centers are coupled via rather long carbon−carbon bonds

(1.578(2) Å) connecting the two smif backbones, perhaps

suggestive of the reversible monomer/dimer equilibrium. These

linkages are pyramidal at the carbon, with the sum of the angles

about the C6 and C7 centers averaging 110.0(24)° and

110.0(27)°, respectively. The iron−pyridine distances are long

(2.1950(11), 2.1910(11) Å) in comparison to the iron

backbone−amide (1.9432(10) Å) and iron−bis-trimethylsilyla-

mide (1.9538(10) Å) bond lengths, and long relative to those

monomer (smif)Fe{N(SiMe₃)₂} (1), orange crystals were

isolated in 74% yield. The UV−vis spectrum of 1 manifests

intense absorptions at 625 nm (ε = 52 000 M⁻¹cm⁻¹) and 398

nm (ε = 38 000 M⁻¹cm⁻¹), which constitute the “red’ and

“blue” intraligand transitions common to the complexes

containing smif.^1,5In contrast to its dark emerald green

Figure 4. Molecular view of [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2). Selected interatomic distances (Å) and angles (deg): Fe−Fe, 6.158(2);

Fe−N1, 2.1950(11); Fe−N2, 1.9432(10); Fe−N3, 2.1910(11); Fe−N4, 1.9538(10); N2−C7, 1.4356(16); N2−C6, 1.4334(17); C7−C8,

1.4964(18); C5−C6, 1.5023(17); C6−C7′, 1.5781(18); N4−Si1, 1.6983(11); N4−Si2, 1.6966(11); N1−Fe−N2, 77.20(4); N1−Fe−N3, 154.48(4);

N1−Fe−N4, 101.94(4); N2−Fe−N3, 77.38(4); N2−Fe−N4, 178.53(4); N3−Fe−N4, 103.43(4); Fe−N2−C6, 120.95(8); Fe−N2−C7, 120.66(8);

C6−Fe−C7, 113.94(10; N2−C6−C5, 108.09(10); N2−C6−C7, 109.08(10); N2−C7−C6, 108.93(10); N2−C7−C8, 108.03(10); C5−C6−C7′,

112.72(10); C6′−C7-C8, 113.10(11).

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Table 1. Selected Crystallographic and Reﬁnement Data for [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2),

[{(Me₃Si)₂N}Fe]₂(μ:κ³-N,py,py′,κ³-N′,py′,py-^oMe₂smif-^oMe₂smif) (6), [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-

(^bMe,^bCH₂,^oMe₂(smif)H))₂(9), [{2,5-Di(pyrindin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe₃)₂]₂(11),

(2,6-ⁱPrC₆H₃-NHCO-smif)₂Fe (14b), and (dpma)₂Fe (15)

a

b

2

6

9

11

14b

15

formula

formula wt

space group

Z

C₃₆H₅₆N₈Si₄Fe₂

824.94

C₄₀H₆₄N₈Si₄Fe₂

881.05

C₄₄H₇₂N₈Si₄Fe₂

937.16

C₆₈H₈₄N₈Si₄Fe₂

1237.49

P2₁/c

C₅₄H₆₂N₈O₃Fe

926.97

C₂₄H₂₄N₆Fe

452.34

P1

̅

1

P2₁/c

P1

̅

1

Pcca

P2₁2₁2₁

8

2

4

8

a, Å

8.0558(5)

10.8367(7)

14.3114(9)

72.491(3)

85.244(4)

81.614(4)

1177.73(13)

1.163

9.1297(5)

18.5891(9)

14.2734(9)

90

8.5974(6)

12.1386(9)

12.5960(9)

87.456(5)

81.005(5)

69.286(4)

1214.34(15)

1.282

13.8897(7)

35.0779(17)

15.5291(8)

90

34.168(5)

17.174(3)

18.038(3)

90

8.5577(4)

16.6991(6)

30.5536(12)

90

b, Å

c, Å

α, deg

β, deg

104.629(3)

90

90.361(2)

90

γ, deg

V, Å³

ρ_calc, g·cm⁻³

μ, mm⁻¹

temp, K

λ (Å)

90

2343.8(2)

1.248

7566.0(7)

1.086

10585(3)

1.163

4366.3(3)

1.376

0.749

0.757

0.735

0.487

0.333

0.714

173(2)

0.71073

R₁= 0.0431

wR₂= 0.1050

R₁= 0.0740

wR₂= 0.1141

1.016

0.71073

R₁= 0.0631

wR₂= 0.1469

R₁= 0.1284

wR₂= 0.1702

0.71073

R₁= 0.0388

wR₂= 0.0708

R₁= 0.0560

wR₂= 0.0769

1.035

R indices

R₁= 0.0389

wR₂= 0.0963

R₁= 0.0533

wR₂= 0.1018

1.048

R₁= 0.0366

wR₂= 0.0917

R₁= 0.0508

wR₂= 0.0991

R₁= 0.0384

wR₂= 0.0794

R₁= 0.0586

wR₂= 0.0858

1.014

b,c

[I > 2σ(I)]

R indices

b,c

(all data)

d

GOF

1.034

0.925

a

b

c

One-half of the molecule is the asymmetric unit. One molecule of THF per asymmetric unit. R₁= ∑||F_o| − |F_c||/∑|F_o|. wR₂= [∑w(|F_o| − |F_c|)²/

d

2

∑wF_o]^1/2GOF (all data) = [∑w(|F_o| − |F_c|)²/(n − p)]^1/2; n = number of independent reﬂections, p = number of parameters.

.

in (smif)₂Fe (1.9634(12) Å (ave)). The bite angle of the

dipyridyl-amide chelate (N1−Fe−N2, 77.20(4)°; N2−Fe−N3,

77.38(4)°; N1−Fe−N3, 154.48(4)°) is typical, and the

remaining core angles (N1−Fe−N4, 101.94(4)°; N2−Fe−

N4, 178.53(4)°; N3−Fe−N4, 103.43(4)°) are commensurate

with the nearly planar iron center.

2.1.3. (dpma)Fe{N(SiMe₃)₂} (3) as a Model for 2. To check

whether the spectral parameters and magnetic information

regarding [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2) were

consistent with the dimeric formulation, the di-2-pyridyl-

methyl-amide (dpma) ligand²⁵was examined as an analogue

with a saturated backbone. The reaction of (dpma)H to

Fe{N(SiMe₃)₂}₂THF in Et₂O over the course of 12 h at 23 °C

(eq 2) led to a red solution from which red crystals of

2.1.4. Pyridine-Substituted smif Derivatives, (smif′)Fe{N-

(SiMe₃)₂}. To probe whether subtle electronic or steric eﬀects

would impact the solid-state dimerization characteristic of 1 to

2, small-scale syntheses were conducted on variants of smif.

The addition of 1,3-(2-pyridyl,6-methyl-2-pyridyl)-2-azapro-

pene (i.e., (^oMesmif)H)⁵to Fe{N(SiMe₃)₂}₂THF²¹in Et₂O

resulted in a deep teal-green solution from which yellow-orange

crystals of [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-^oMesmif,^oMesmif)

(5) were isolated in 47% yield (eq 3). ¹H NMR spectral studies

revealed a one ^oMesimif and one SiMe₃environment,

consistent with (^oMesmif)FeN(SiMe₃)₂(4), which is likely to

be teal-colored.⁵Given the related color changes to those in eq

1, the solid-state structure is probably dimeric; whether the

expected two isomers exist as shown could not be discerned by

the NMR data.

Di-ortho-methylation of the smif ligand proved to change the

nature of the carbon−carbon bond-forming process, as shown

in eq 4. Treatment of Fe{N(SiMe₃)₂}₂THF with (^oMe₂smif)H⁵

provided another dark teal solution, but a dark green

microcrystalline material was eventually isolated in 33% yield.

(dpma)Fe{N(SiMe₃)₂} (3) were obtained in 64% yield. The

solubility properties of 3 and its H NMR spectrum, which

1

revealed C_2vsymmetry, were consistent with a momomeric

formulation, and its UV−vis spectrum was essentially

featureless except for a modest maximum at 549 nm (ε =

1500 M⁻¹cm⁻¹). Contemporaneous with this work, West-

erhausen et al. have prepared (dpma)Fe{N(SiMe₃)₂} (3) by an

analogous route, and have shown it to be a dimer, that is,

[{(Me₃Si)₂N}Fe(μ-N,κ³-N,py₂-dpma)]₂(3), in the solid state,

with a μ_eff= 7.5 μ_B(Gouy balance).²⁶For further discussion of

the magnetism of 3, see the Supporting Information.

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The complex proved to be unstable in both solution and the

solid state, and disproportionated to aﬀord the known

(^oMe₂smif)₂Fe, which has bands at ∼580 nm (ε = 58 000

M⁻¹cm⁻¹) and ∼400 nm (ε = 32 000 M⁻¹cm⁻¹) that

dominate the UV−vis spectrum.⁵The intensity of these

absorptions masked the true color of the major product,

[ { ( M e ₃S i ) ₂N } F e ] ₂( μ : κ ³- N , p y , p y ′ , κ ³- N ′ , p y ′ , -

py-^oMe₂smif-^oMe₂smif) (6), and hampered spectral analysis.

Initially fooled by the omnipresence of teal/gold (^oMe₂smif)₂-

Fe, careful crystallization aﬀorded pure 6, and its structure was

ascertained by X-ray crystallography.

Redox noninnocence plays a role in the carbon−carbon bond

formation, as the reductive coupling of the smifs in putative

(^oMe₂smif)FeN(TMS)₂aﬀords a formal Fe(I) center that

reduces the pyridine-imine fragments to render the centers

Fe(II). SQUID magnetometry was originally conducted on a

sample, but the thermal sensitivity of 6, and plausible

contamination from S = 2 (^oMe₂smif)₂Fe, rendered its

interpretation suspect. There are actually four spins to consider:

two Fe(II) centers that are likely to be S = 2, and two pyridine-

imine radicals each S = 1/2. Given the likelihood of strong AF

coupling between each pyridine-imine radical and its adjacent

iron(II) center, it is likely that the molecule consists of two S =

3/2 centers

coordinated by two o-Me-pyridine groups, an imine, and the

(TMS)₂N unit. A formal oxidation state based on this

coordination sphere would be Fe(I) as a consquence of the

reductive coupling of the two smif units, each of which now

spans two iron centers. Upon closer inspection, the pyridine-

imine unit displays the telltale bond distances of a

monoreduced entitly, with the imine CN elongated to

1.328(2) Å, C6−C7 shortened to 1.405(3) Å, and the pyridine

CN stretched to 1.381(2) Å.²⁷⁻³³The iron centers are each

ferrous, with two additional spins aﬃliated with the pyridine-

imines. The Fe−N3 distance of 2.1464(15) Å is slightly longer

than the iron−pyridine (2.0965(15) Å) and iron−imine

(2.0069(15) Å) nitrogen distances within the monoreduced

fragment, while the bis-TMS−amide is the shortest at

1.9489(15) Å.

2.1.6. Backbone-Substituted smif Derivatives, (smif″)Fe{N-

(SiMe₃)₂}. Methylation of the CNC backbone of (smif)H and

(^oMe₂smif)H was accomplished in a one-pot procedure

involving sequential deprotonation and methylation events,

the former via NaH, and the latter using MeI (eq 5). The

methylations were nearly quantitative, such that the crude

products (^bMe₂smif)H and (^bMe₂Me₂smif)H were used in

o

subsequent reactions.

2.1.5. Structure of [{(Me₃Si)₂N}Fe]₂(μ:κ³-N,py,py′,κ³-N′,py′,-

py-^oMe₂smif-^oMe₂smif) (6). A molecular view of [{(Me₃Si)₂-

N}Fe]₂(μ:κ³-N,py,py′,κ³-N′,py′,py-^oMe₂smif-^oMe₂smif) (6) is

presented in Figure 5, whose caption includes selected metric

parameters. Some details of the data collection and reﬁnement

can be found in Table 1. Two distorted tetrahedral N₄Fe

centers (core angles range from 79.19(6) to 129.78(6)°) are

centrosymmetric with respect to the center of the new carbon−

carbon bond (d(C8−C8) = 1.561(2) Å); hence, each iron is

Surprisingly, the addition of 1 equiv of (^bMe₂smif)H to

Fe{N(SiMe₃)₂}₂THF did not provide (^bMe₂smif)FeN(SiMe₃)₂

despite NMR tube scale reactions that showed a promising dark

1

green color, and one dominant paramagnetic species by H

b

NMR spectroscopy. To check whether Me₂smif was a viable

ligand, 2 equiv of the imine precursor was added to

Fe{N(SiMe₃)₂}₂THF, and diamagnetic, mossy green (^bMe₂-

smif)₂Fe (7) was isolated in 72% yield (eq 6).

Potential problems aﬃliated with the methylated backbone

o

were identiﬁed when a switch to (^bMe₂Me₂smif)H was

conducted. The standard protocol failed to generate the

o

desired monomeric product (^bMe₂Me₂smif)FeN(SiMe₃)₂(8),

although a dark green solution typical for smif-chelated iron

complexes was generated.⁵As eq 7 illustrates, a red-orange

Figure 5. Molecular view of [{(Me₃Si)₂N}Fe]₂(μ:κ³-N,py,py′,κ³-

N′,py′,py-^oMe₂smif-^oMe₂smif) (6). Selected interatomic distances

(Å) and angles (deg): Fe−Fe, 5.941(2); Fe−N1, 2.0965(15); Fe−

N2, 2.0069(15); Fe−N3, 2.1464(15); Fe−N4, 1.9489(15); N1−C6,

1.381(2); C6−C7, 1.405(3); N2−C7, 1.328(2); N2−C8, 1.448(2);

C8−C8, 1.561(2); N1−Fe−N2, 79.19(6); N1−Fe−N3, 108.95(6);

N1−Fe−N4, 118.19(7); N2−Fe−N3, 83.70(6); N2−Fe−N4,

129.78(6); N3−Fe−N4, 125.16(6); C7−N2−C8, 118.73(15); N2−

C8−C8, 107.79(17); C8−C8−C9, 109.72(17).

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dimer crystallized from benzene solution over a period of 2

days, but this species did not have any new C−C bonds. Figure

1 illustrated how the azaallyl anion can serve as a nucleophilic

or electrophilic (or Lewis base/Lewis acid) functionality, and

o

the dimer resulting from utilization of (^bMe₂Me₂smif) arises

from a proton transfer from a backbone methyl group to a basic

azaallyl carbon. The result of this process is an exomethylene

group ﬂanking an amide whose other adjacent site is a CHMe

segment. Reversible dimerization of 8 occurs as two Fe−C

bonds convert the two amides to imines, thereby retaining the

Fe(II) oxidation states of the two centers. Dimer [{(Me₃Si)₂-

N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif)H))₂(9) was

identiﬁed by single-crystal X-ray crystallography, while ¹H

NMR spectra of redissolved 9 were consistent with the

monomer, 8.

The ferrous centers of 9 are potentially close enough

(4.7420(5) Å) for a minimal interaction, and SQUID

magnetometry data (Figure 6) revealed a declining μ_efffrom

Figure 7. Molecular view of [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-

(^bMe,^bCH₂,^oMe₂(smif)H))₂(9) and plausible valence bond structures.

Selected interatomic distances (Å) and angles (deg): Fe−Fe,

4.7420(5); Fe−N1, 2.3192(17); Fe−N2, 2.0511(15); Fe−N3,

2.3538(17); Fe−N4, 1.9914(16); Fe−C15′, 2.336(2); Fe−C6′,

2.889(2); N2−C6, 1.331(2); N2−C7, 1.465(3); C6−C15, 1.375(3);

N1−Fe−N2, 75.18(6); N1−Fe−N3, 149.14(6); N1−Fe−N4,

102.19(6); N2−Fe−N3, 74.29(6); N2−Fe−N4, 136.76(7); N3−

Fe−N4, 102.96(6); N1−Fe−C15′, 91.07(7); N2−Fe−C15′,

106.59(7); N3−Fe−C15′, 93.39(7); N4−Fe−C15′, 116.64(7).

CN double bond (cf 1.28 Å), and the d(CCH₂) is 1.375(5)

Å, which is slightly long for a CC bond (cf 1.33 Å), and quite

short for a CC single bond. Consideration of the Fe−N2

interaction as an amide would suggest that the irons are ligated

by an oleﬁn, but in this case, the β-carbon is 2.889(2) Å away.

Since this oleﬁn is really an ene-amide, a disproportionately

greater interaction with the α-carbon might be expected, and

diﬀerentiation between the two valence bond depictions is

moot.

Figure 6. Plot of μ_effvs T (K) for dimer [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-

N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif)H))₂(9).

6.3 μ_Bat 293 K to ∼3.0 μ_Bat 50 K where the eﬀects of ZFS

appear. A μ_effvalue of 6.93 μ_Bis expected for two

noninteracting Fe(II) S = 2 centers, and the decline observed

is consistent with weak antiferromagnetic coupling (AF). It is

also likely that the measurements reﬂect a high density of

magnetic states present in the system.³⁴

2.2. Reactivity of the smif Backbone with Unsaturated

Substrates. 2.2.1. 3 + 2 Cyclizations. Figure 1 suggests that

the CNC^nborbital of a smif ligand can be considered as a

reagent in C−C bond-forming reactions,³⁵⁻³⁸hence a variety of

substrates potentially subject to nucleophilic, electrophilic, and

radical reactivity were probed with limited success. Two distinct

types of substrates provided isolable products, one of which was

alkynes.³⁵Treatment of a benzene solution of (smif)Fe{N-

(SiMe₃)₂} (1) with 2 equiv of ditolyl-acetylene aﬀorded a dark

green solution containing a mixture of compounds that

included hexatolylbenzene (∼15%). Crystallization from

diethyl ether provided a red 3 + 2 cycloaddition product,

[{2,5-di(pyrindin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-

ide)}FeN(SiMe₃)₂]₂(11, 19%) that was identiﬁed by X-ray

crystallography. The yield of red crystals from a green solution

was suggestive of another monomer/dimer situation, but direct

evidence of a solution monomer, that is, {2,5-di(pyrindin-2-yl)-

3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe₃)₂(10),

proved diﬃcult to obtain due to the presence of other species.

Utilization of 4 equiv of tolCCtol on an NMR tube scale in

C₆D₆permitted some of the reaction complexity to be

unraveled, as one diamagnetic product was of high symmetry,

and was tentatively identiﬁed as {2,5-di(2-pyridyl)-3,4-di-(p-

tolyl)-pyrrolide}₂Fe (12).³⁹Accompanying this product were

cis- and trans-ditolyl-ethylene, which, along with 12, suggested

2.1.7. Structure of [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-

(^bMe,^bCH₂,^oMe₂(smif)H))₂(9). Figure 7 depicts a molecular

view of [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-(^bMe,^bCH₂,^oMe₂-

(smif)H))₂(9), where its inversion center can be readily

seen. Metric parameters are given in the caption, and some

structure solution details can be found in Table 1. The local

structure about each iron is a distorted square pyramid, with the

N(SiMe₃)₂group bent 116.64(7)° away from the pseudoapical,

elongated iron−carbon bond (2.336(2) Å), which is slightly

bent away from the imine (106.59(7)°) and pyridine

(91.07(7)°, 93.39(7)°) nitrogens. On the imine side of the

plane, the Fe−N_pydistance is shorter (2.3192(17) Å) than that

on the opposing arm of the ligand (2.3538(17) Å), and the

chelate bite of 149.14(6)° is correspondingly small as compared

to (smif)₂M derivatives, with ∠N1−Fe−N2 = 75.18(6)° and

∠N2−Fe−N3 = 74.29(6)°. The N_py−Fe−N_amangles are

102.19(6)° and 102.96(6)°, and reﬂect the cant of the

N(SiMe₃)₂group out of the pseudo-N₄Fe plane.

Distances found for the former smif backbone are in between

that of the imine-alkyl and amide-exo-CH₂forms depicted in

Figure 7. The CN distance is 1.331(2) Å, which is long for a

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that the CNC backbone of the dihydropyrrole unit of 10 could

serve to transfer dihydrogen to ditolyl-acetylene. A subsequent

disproportionation provided a plausible route to 12.

SQUID magnetometry was conducted on 11, but its

interpretation is nontrivial. At 300 K, the μ_effwas ∼5.3 μ_B,

which is already substantially below that expected for two

noninteracting Fe(II) centers (6.93), and well below the 8.94

μ_Bthat would represent an S_T= 4 system. A steady decline in

μ_effwas observed until 25 K, where a modest ﬂat discontinuity

surfaced ∼2.4 μ_Bprior to the eﬀects of ZFS. In systems, such as

this, the low-symmetry and high-spin nature of the Fe(II)

centers can lead to a high density of magnetic states, and a

discussion of the case and alternative views are given in the

Supporting Information. Nonetheless, the SQUID data are

consistent with interacting Fe(II) centers, as the d(Fe−Fe) of

3.054 Å would suggest.

2.2.2. Structure of [{2,5-Di(pyrindin-2-yl)-3,4-di-(p-tolyl-

2,5-dihydropyrrol-1-ide)}FeN(SiMe₃)₂]₂(11). Data aﬃliated

with the structure determination of 11 is given in Table 1,

while selected metric parameters are presented in the caption of

Figure 8, which provides molecular views of the molecule. The

amide of the dihydropyrrol unit asymmetrically bridges the two

irons with d(Fe−N) averaging 2.107(14) and 2.162(2) Å. The

diamond core of the dimer is nearly square, with the Fe−N−Fe

and N−Fe−N angles averaging 91.3(3)° and 88.3(4)°,

respectively. The pyridines, while occupying diﬀering sites,

have iron−nitrogen bonds that average 2.158(16) Å, and the

(Me₃Si)₂N groups have the shortest bonds to iron, with d(Fe−

N)_ave= 2.060(6) Å, which is typical. Each iron is roughly

embedded in a distorted square-pyramidal environment, with

the (Me₃Si)₂N, the two pyridines, and the μ-dihydropyrrol

occupying the basal sites, while the other, slightly shorter, μ-

dihydropyrrolyl linkage is axial. The irregularity of the structure

is noted through N_ax−Fe−N_basangles ranging from the μ-

dihydropyrrolyls at 88.57(5)° and 87.97(5)° to the (Me₃Si)₂N

groups at 114.51(6)° and 114.76(6)°.

2.2.3. Nucleophilic Attack of Isocyanates. While electro-

philic substrates, such as aldehydes, ketones, CO₂, etc.,³⁵⁻³⁸

failed to elicit reactivity from selected smif-containing

t

complexes, principally (smif)₂Fe (13), both BuNCO and

(2,6-ⁱPr₂C₆H₃)NCO reacted with the CNC backbone of the

smif ligands on 13. As eq 8 indicates, dark red diamagnetic

Figure 8. Molecular and truncated views of [{2,5-di(pyrindin-2-yl)-

3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe₃)₂]₂(11). Selected

interatomic distances (Å) and angles (deg): Fe1−Fe2, 3.054; Fe1−N1,

2.1411(15); Fe1−N2, 2.1610(15); Fe1−N3, 2.1632(15); Fe1−N4,

2.0560(15); Fe1−N6, 2.0966(14); Fe2−N2, 2.1167(14); Fe2−N5,

2.1775(15); Fe2−N6, 2.1638(15); Fe2−N7, 2.1500(15); Fe2−N8,

2.0646(15); Fe1−N2−Fe2, 91.10(5); Fe1−N6−Fe2, 91.57(5); N1−

Fe1−N2, 78.27(6); N1−Fe1−N3, 143.98(6); N1−Fe1−N4, 96.20(6);

N2−Fe1−Fe3, 76.49(6); N2−Fe1−N4, 156.89(6); N3−Fe1−N4,

97.02(6); N6−Fe1−N1, 98.10(5); N6−Fe1−N2, 88.57(5); N6−

Fe1−N3, 106.48(6); N6−Fe1−N4, 114.51(6); N5−Fe2−N6,

76.43(6); N5−Fe2−N7, 144.10(6); N5−Fe2−N8, 97.02(6); N6−

Fe2−N7, 77.88(5); N6−Fe2−N8, 157.21(6); N7−Fe2−N8, 97.05(6);

N2−Fe2−N5, 107.18(6); N2−Fe2−N6, 87.97(5); N2−Fe2−N7,

96.48(5); N2−Fe2−N8, 114.76(6).

carbon position on the smif backbone, followed by a proton

transfer to the nitrogen of the isocyanate fragment.

Since the azaallyl anion functionality should behave as a

nucleophile,³⁵Na(smif) was treated with ^tBuNCO, resulting in

a deep red solution from which Na(^tBuNHCO-smif) was

obtained as metallic green microcrystals in 91% yield (eq 9). As

products (RNHCO-smif)₂Fe (14a, R = ^tBu, 47%; 14b,

2,6-ⁱPrC₆H₃, 75%) resulted from nucleophilic attack from a

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an alternative route to (^tBuNHCO-smif)₂Fe (14a), the addition

of 2 equiv of Na(^tBuNHCO-smif) to FeBr₂(THF)₂in THF

resulted in a dark red-orange solution, from which the complex

was isolated in 61% yield as dark red crystals (eq 10).

Attachment of the amide groups to the smif backbone may

become important if these optically dense materials need to be

incorporated into a polymer for light-harvesting applications.

Condensation polymerization of 14a,14b with a diammine or

the elaboration of (smif)₂M with a diisocyanate constitute two

plausible paths based on eqs 8−10.

In regard to UV−vis spectroscopy, inclusion of backbone

amides causes a blue shift in the “red” intraligand (IL) bands

from that of (smif)₂Fe (13-Fe, λ_max∼ 603 nm), as the λ_maxfor

14a is at 525 nm (ε ∼ 21 000 M⁻¹cm⁻¹), and that of 14b is at

530 nm (ε ∼ 22 000 M⁻¹cm⁻¹). This shift changes the color of

the compounds from the dark green of 13 to the dark red hues

of 14a,14b, and unmasks bands at 638 nm (ε ∼ 7000 M⁻¹

cm⁻¹) and 644 nm (ε ∼ 6000 M⁻¹cm⁻¹) for 14a and 14b,

respectively, that are tentatively assigned as lower-symmetry

components of CNC^nb→ smif-π* transitions^5,6generated from

amide substitution. The “blue” IL bands of 14a at 431 nm (ε ∼

42 000 M⁻¹cm⁻¹) and 14b at 427 nm (ε ∼ 46 000 M⁻¹cm⁻¹)

change only slightly from the λ_max∼ 437 nm (ε ∼ 42 000 M⁻¹

cm⁻¹) for 13.

2.2.4. Structure of (2,6-iPrC₆H₃-NHCO-smif)₂Fe (14b). A

listing of particular structure reﬁnement and collection data can

be found in Table 1, while selected metric parameters are given

in the caption to Figure 9, which illustrates the C₂symmetry of

(2,6-ⁱPrC₆H₃-NHCO-smif)₂Fe (14b). Core distances attributed

to 14b are very similar to (smif)₂Fe (13),⁵with Fe−N_azabond

lengths averaging 1.908(11) Å, and d(Fe−N_py) averaging

1.955(14) Å. The FeN₆core is nearly D_2d, with ∠N_aza−Fe−N_aza

= 176.49(17)°, N_aza−Fe−N_pyangles averaging 83.1(4)°, N_py−

Fe−N_pyangles of 165.92(16)° (ave), N_py−Fe−N′_pyangles of

90.8(18)° (ave), and a very modest spread (94.00(16)−

99.96(16)°) in N_aza−Fe−N′_pyangles. The amide electron-

withdrawing group renders the CNC linkage asymmetric. The

d(CN) away from the amide is short (1.329(11) Å ave)

compared to the CN adjacent to the amide (1.366(8) Å ave),

and the CC bond connecting the amide to the smif backbones

is slightly shorter (1.446(7) Å) than expected (1.47 Å),

consistent with some delocalization of charge into the amide.

The remaining metric parameters in the molecule are ordinary.

2.3. Collateral Discovery − Alkyne Cyclotrimerization

by {(Me₃Si)₂N}₂Fe(THF). While investigating the 3 + 2

cyclization³⁵of ditolyl-acetylene to (smif)Fe{N(SiMe₃)₂} (1)

illustrated in Scheme 1, the presence of hexa-tolyl-benzene was

noted, an obvious consequence of alkyne cyclotrimeriza-

tion.⁴⁰⁻⁴⁸As a consequence, a variety of smif- and dpma-

containing compounds were tested for catalytic activity with 2-

butyne as the substrate. Of these compounds tested, 1 proved

to be the best (Table 2), but evidence of degradation during

cyclotrimerization suggested that other species might warrant

consideration. For example, roughly 20−30% of 1 remained

after 50 equiv of 2-butyne was cyclotrimerized, according to

several trials, and evidence of HNSi(Me₃)₂and (smif)₂Fe (13)

was obtained by ¹H NMR spectroscopy. Accordingly,

{(Me₃Si)₂N}₂Fe(THF) was considered as a potential catalyst⁴⁸

since the reactivity shown in Scheme 1 revealed that the CNC

backbone of smif would react with the alkyne, potentially

triggering disproportionation events. Table 1 shows that

{(Me₃Si)₂N}₂Fe(THF) is a superior catalyst for the conversion

of 2-butyne to hexamethylbenzene, with a TOF of ∼10 h⁻¹at 2

mol % catalyst loadings at 90 °C, with little or no apparent

degradation. The results are quite comparable to other

cyclotrimerization catalysts.^45,48Whereas the crude data

suggests that the cyclotrimerization is ﬁrst-order in the catalyst,

the order in the substrate is unclear at this juncture.

2.4. Collateral Discovery − Dihydrogen Transfer from

dpma. 2.4.1. Calculations on H₂Loss from dpma. The

tentative identiﬁcation of {2,5-di(2-pyridyl)-3,4-di-(p-tolyl)-

pyrrolide}₂Fe (12), accompanied by alkyne hydrogenation

products (Scheme 1) suggested that saturating a smif backbone,

that is, conversion to an amide, could lead to dihydrogen

transfer. The thermodynamics of such an event was investigated

via DFT calculations, as indicated in Scheme 2, where the

dehydrogenation of D_2d(dpma)₂Fe (15) to (smif)₂Fe (13) and

2 H₂was examined. The dehydrogenation is roughly

thermoneutral, as loss of the ﬁrst equivalent of dihydrogen is

slightly endothermic (1.1 kcal/mol), whereas the loss of the

second equivalent is slightly exothermic (−2.9 kcal/mol). The

favorable free energy of reaction (ΔG° = −12.5 kcal/mol) is

due mostly to the increase in entropy (ΔS° = 36 eu) as the 2

equiv of dihydrogen are released. Since 15 is a high-spin

complex, the stepwise conversion also reﬂects a change in spin

state en route to 13. According to the calculations, (dpma)-

(smif)Fe (16) is also S = 2; hence the spin state change to S = 0

occurs in the second dehydrogenation. The second entropy

change is likely attenuated due to the spin state change, and the

overall greater rigidity of the low-spin bis-smif derivative.

2.4.2. Synthesis and Characterization of (dpma)₂Fe (15).

Treatment of {(Me₃Si)₂N}₂Fe(THF) with 2 equiv of (dpma)H

in diethyl ether aﬀorded dark blue (dpma)₂Fe (15) in 75%

yield according to eq 11. An Evans’ method magnetic

measurement on 15 (μ_eff= 5.0 μ_B) and SQUID magnetometry

measurements(μ_eff(290 K) = 5.2 μ_B) are consistent with a

relatively symmetric S = 2 center, since the eﬀects of ZFS are

Figure 9. Molecular and truncated views of (2,6-ⁱPrC₆H₃-NHCO-

smif)₂Fe (14b). Selected interatomic distances (Å) and angles (deg):

Fe−N1, 1.948(4); Fe−N2, 1.900(4); Fe−N3, 1.956(4); Fe−N4,

1.941(4); Fe−N5, 1.916(4); Fe−N6, 1.974(4); N2−C6, 1.371(6);

N2−C7, 1.337(6); N5−C18, 1.360(6); N5−C19, 1.321(6); C6−C25,

1.446(7); C18−C38, 1.446(7); N1−Fe−N2, 83.07(16); N1−Fe−N3,

165.81(16); N1−Fe−N4, 90.01(15); N1−Fe−N5, 97.46(16); N1−

Fe−N6, 91.10(15); N2−Fe−N3, 83.17(16); N2−Fe−N4, 99.96(15);

N2−Fe−N5, 176.49(17); N2−Fe−N6, 94.00(16); N3−Fe−N4,

89.00(15); N3−Fe−N5, 96.48(16); N3−Fe−N6, 93.27(15); N4−

Fe−N5, 83.52(16); N4−Fe−N6, 166.03(17); N5−Fe−N6, 82.53(16);

Fe−N2−C6, 115.4(3); Fe−N2−C7, 115.8(3); Fe−N5−C18,

115.8(3); Fe−N5−C19, 115.5(3).

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Scheme 1

Table 2. Catalytic Cyclotrimerization Results for the Conversion of 2-Butyne to Hexamethylbenzene with (smif)Fe{N(SiMe₃)₂}

(1) or {(Me₃Si)₂N}₂Fe(THF)

a

trial

cat.

cat. mol. %

[cat.] × 10³M

T (°C)

TON

TOF (h⁻¹

)

1

2

3

4

{(Me₃Si)₂N}₂Fe(THF)

(smif)Fe{N(SiMe₃)₂} (1)

LiN(TMS)₂

1.6

2

4.5

9.0

13.5

4.8

9.7

14.5

36

75

90

75

90

60

50

52

14

12

50

60

0

4.4

3.7

10

2

10.4

0.31

1.1

7.4

8.9

0

b

5

2

c

6

2

d

7

e

2

8

1.6

2

9

10

FeBr₂(THF)₂

2

17

90

0

a

b

c

Assessed at short reaction times such that [2-butyne] complies with pseudo-ﬁrst-order conditions. 20% 1 remained at conclusion. 23% 1

d

e

remained at conclusion. 29% 1 remained at conclusion. 24% 1 remained at conclusion.

seen below 50 K (Figure 10). The Mossbauer spectrum of 15

Scheme 2

̈

supports this conclusion, as the isomer shift of 0.94 mm/s is

typical for high-spin Fe(II), and the quadrupole splitting value

is a modest 0.86 mm/s, indicative of a relatively symmetric

electronic environment.^23,24Crystals of (dpma)₂Fe (15) were

orthorhombic, with two independent, statistically equivalent

molecules in the asymmetric unit. A view of one molecule is

given in Figure 10, while selected details regarding data

collection and reﬁnement are given in Table 1. The aza

nitrogens average 2.001(13) Å from the iron, and are

175.9(16)° (ave) apart, whereas the pyridine nitrogens average

2.201(13) Å, and are 150.9(16)° (ave) apart. The N_aza−Fe−

N_pyintrachelate angles average 75.5(4)°, while signiﬁcant

variation is observed for the N_aza−Fe−N_py′ and N_py−Fe−N_py′

angles between opposing dpma ligands, which range from

99.8(2) to 109.5(2)° and 86.90(7) to 101.00(7)°, respectively.

The Fe−N_azaand Fe−N_pydistances are 0.10 and 0.24 Å greater

than the corresponding (smif)₂Fe (13) derivative, an indication

of the signiﬁcantly weaker ﬁeld aﬀected by dpma versus smif.

Subsequent to the initial structural reports of (smif)₂Fe (13)

and (dpma)₂Fe (15),⁴Westerhausen et al. also published the

results of X-ray crystallographic studies on both complexes.²⁶

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evaluations were attempted utilizing backbone-deuterated

(dpma-d₄)₂Fe (15-d₈; see the Supporting Information for a

summary of the labeling studies).⁴⁹

The addition of benzophenone to (dpma)₂Fe (15) proved to

be complicated, as at least two paramagnetic iron species

formed in addition to (smif)₂Fe (13). Treatment of (smif)-

FeN(SiMe₃)₂(1) with (dpma)H or exposure of (dpma)FeN-

(SiMe₃)₂(3) to (smif)H led to mixtures of (smif)(dpma)Fe

(16) and (smif)₂Fe (13) due to disproportionation processes,

1

as shown in Scheme 4. The H NMR spectrum of 16 was

readily identiﬁed, yet it was not deemed to be a byproduct in

the benzophenone reaction with 15 (16 was present in a

number of shorter duration studies with alkynes and imines; see

the Supporting Information). It was logical that the products

derived from protonation of iron amide bonds by Ph₂CHOH

were present, since the free alcohol was not observed.

Treatment of 3 with Ph₂CHOH permitted isolation of a

1

plum-colored solid whose main H NMR spectral resonances

were consistent with [(dpma)FeOCHPh₂]_n(17). Likewise,

when Ph₂CHOH was added to 1, a green solid was produced,

and ¹H NMR spectral analysis indicated a rough 1:1 mixture of

[(smif)FeOCHPh₂]_n(18) and ubiquitous 13. Aqueous

quenches of 17 and 18 aﬀorded Ph₂CHOH as a major

product. Spectral comparisons showed that 17 was a product of

Ph₂CO reduction by 15, and a rough balancing of the observed

products suggested that “(Ph₂CHO)₂Fe” may also be

produced.⁵⁰

When (dpma)₂Fe (15) was exposed to excess 2-butyne or

ditolyl-acetylene, hydrogenation to cis-alkenes were observed in

addition to trimerization products hexamethylbenzene and

hexatolylbenzene, and (smif)₂Fe (13) was the identiﬁable iron-

containing product. Trans-alkenes are also observed in the

reaction mixtures, but subsequent studies showed that they

could be generated in a post-hydrogenation isomerization

event. The cyclotrimerization activity of 15 was determined to

be worse than the catalysts tested in Table 2, but there is clearly

a commonality of iron-amide functionalities.⁴⁶⁻⁴⁸¹H NMR

assays of product mixtures derived from the use of (d₄-

dpma)₂Fe (15-d₈) proved frustratingly diﬃcult to decipher, and

clean transfer of dideuterium to alkyne was not noted in any

instance.

Figure 10. Plot of μ_effvs T (K) for (dpma)₂Fe (15) showing its S = 2

GS. Mossbauer spectrum that is consistent with an S = 2 GS, and a

̈

view of one molecule of the asymmetric unit. Selected interatomic

distances (Å) and angles (deg): Fe1−N1, 2.197(2); Fe1−N2,

2.0184(18); Fe1−N3, 2.194(2); Fe1−N4, 2.195(2); Fe1−N5,

1.9921(18); Fe1−N6, 2.197(2); N2−C6, 1.421(3); N2−C7,

1.417(3); N5−C18, 1.417(3); N5−C19, 1.418(3); N1−Fe1−N2,

75.44(8); N1−Fe1−N3, 150.82(7); N1−Fe1−N4, 98.88(8); N1−

Fe1−N5, 105.07(8); N1−Fe1−N6, 89.65(8); N2−Fe1−N3, 75.40(8);

N2−Fe1−N4, 109.47(8); N2−Fe1−N5, 174.72(8); N2−Fe1−N6,

99.75(8); N3−Fe1−N4, 91.76(8); N3−Fe1−N5, 103.87(8); N3−

Fe1−N6, 94.20(7); N4−Fe1−N5, 75.73(7); N4−Fe1−N6, 150.75(7);

N5−Fe1−N6, 75.04(8). Related distances and angles of the second

molecule are statistically indistinguishable.

When (dpma)₂Fe (15) was heated in the presence of cis-2-

butene for an extended period (50 °C, ∼7 days), hydrogenation

to n-butane was noted and some of the residual oleﬁn was

isomerized to trans-2-butene. Once again, the only iron-

1

containing product identiﬁed by H NMR spectroscopy was

(smif)₂Fe (12). Butadiene was also hydrogenated to a mix of 1-

butene, and cis- and trans-2-butene, and 13 was again produced.

Unsaturated nitrogen-containing substrates proved to be

some of the best in terms of conversion, as azobenzene and 3,6-

mesityl-imino-1,4-cyclohexadiene⁵¹were smoothly converted

by (dpma)₂Fe (15) to 1,2-diphenylhydrazine and 1,4-dimesityl-

imino-benzene as (smif)₂Fe (13) formed. When (d₄-dpma)₂Fe

(15-d₈) was used, azobenzene was converted to PhND-NDPh,

but the iron product, expected to be (d₂-smif)₂Fe (13-d₄), had

2−6% H in its backbone. In addition, treatment of 15-d₈with

There are no substantial conﬂicts when the metric parameters

of the two independent studies are compared.

2.4.3. dpma Backbone Dihydrogen Transfers. The

potential to dehydrogenate the −CH₂-N-CH₂− backbone of

the dpma ligands in (dpma)₂Fe (15) was tested via a series of

NMR tube scale experiments where the products could be

readily assayed. As shown in Scheme 3, numerous transfers

were observed, but the (smif)₂Fe (13) product was unable to

be recycled back to 15 via H₂or other dihydrogen sources,

thereby preventing catalytic applications. In addition, (dpma)H

(not shown) was often noted at the end of the reactions,

perhaps indicative of protolytic degradation paths. Nonetheless,

a variety of hydrogenations were noted, and mechanistic

52

1,4-dimesitylamino-benzene aﬀorded 1,4-(MesND/H)₂-C₆H₄

with 25−32% H in the amine positions. Silating the glassware

lowered the amount to 16% H, but clearly, clean labeling

studies were diﬃcult to obtain, even with the better substrates.

The imine PhCHNBn and some simple variants were also

chosen as substrates for 15, but the reaction paths were less

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Scheme 3

Scheme 4

clean, as indicated by stoichiometry relative to a standard,

reproducibility, and incomplete label transfer from 15-d₈.

While a conclusive general path for the dehydrogenative

events could not be established, there is a favorite among the

three general processesillustrated via alkyne reduction

depicted in Scheme 5. A radical would likely transfer H atoms

in discrete steps to an alkyne, but the general lack of trans-oleﬁn

products in the early stages of reaction and the observation that

cis-products do eventually isomerize discredit this mechanism,

despite the stabilization of a metalaradical, such as 19, due to its

Fe(III) form. While a concerted dihydrogen transfer is

attractive in terms of the initial cis-geometry, and its

resemblance to the 2 + 3 cyclizations into the CNC smif

backbone, scrambling of hydrogen into the smif ligands of

product (smif)₂Fe (13), albeit typically a modest amount, has

been observed for certain substrates. The data, while diﬃcult to

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Scheme 5

interpret due to the presence of paramagnetic species,

overlapping resonances, etc., appears best accommodated by

a standard β-H-elimination process that leads to 20, followed

by insertion to give 21 or 22, and abstraction from the former

Fe(II) species or reductive elimination from the putative

Fe(IV) intermediate can lead to products.

Substrates 1,2-diphenylhydrazine and 1,4-dimesitylimino-

benzene, the ones that react most cleanly, may be

mechanistically diﬀerent in that the rate of H₂/D₂transfer is

demonstrably faster (<1 h, 23 °C). In the former case, a

substantial amount of (dpma)H is generated, and H (2−6%) is

found in the backbone of the modest amount of 13-d₄

produced. For the latter, (dpma)H is again produced, and the

amount of H found in the 1,4-(MesND/H)₂-C₆H₄product

ranged from 16% to 32% depending on the run. The

complexity of the process, as evident in virtually every trial,

greatly hampered the formulation of concrete mechanistic

conclusions (see the Supporting Information).

considered a radical coupling. A related aluminum complex has

been generated as a byproduct in the thermolysis of

(dpma)AlMe₂,⁵⁹and it is plausible that (smif)AlMe₂is a

short-lived intermediate in this process.

Another example that can be interpreted as evidence of the

covalent diradical character of the CNC^nbsmif backbone is the

3 + 2 cycloaddition of ditolyl-acetylene to (smif)Fe{N-

(SiMe₃)₂} (1), which eventually aﬀords the dimer [{2,5-

di(pyrindin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN-

(SiMe₃)₂]₂(11). The relatively mild conditions under which

the cyclization occurs and the lack of any dipolar nature to the

alkyne support the plausibility of a reaction with diradical

character. Furthermore, some of the dehydrogenations of

(dpma)₂Fe (15), and the dehydrogenation of 11, followed by

disproportionation to yield the tentatively identiﬁed {2,5-di(2-

pyridyl)-3,4-di-(p-tolyl)-pyrrolide}₂Fe (12, Scheme 1),³⁹may

be examples of H-atom abstraction that may point toward the

diradical character of the smif-like ligands in the ultimate

products.

3. DISCUSSION

Evidence of radical behavior pertaining to the CNC

backbone is also evident in the coupling of putative

3.1. The Covalent Diradical Component of the CNC

Backbone. The description of the CNC^nborbital in Figure 1 as

being composed of covalent diradical⁵³⁻⁵⁸and ionic

components has been supported by reactivity at the

coordinated smif ligand. A standard ambiguity persists when

characterizing related singlet diradicals,⁵³⁻⁵⁸whose products in

various reactions can also be ascribed to nucleophile/electro-

phile behavior. Similarly, dimerization of (smif)Fe{N(SiMe₃)₂}

(1) to [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2) can be

ascribed to a radical coupling, or an attack of the smif anion on

the electrophilic (imine) part of another, that is, the ionic view

of the backbone. Nonetheless, this reversible C−C bond

formation, perhaps hinted at in the elongated C−C bonds of

1.578(2) Å between the two CNC units, can justiﬁably be

o

(^oMe₂smif)FeN(SiMe₃)₂(eq 4), in which the Me₂smif ligand

ends up spanning two iron centers as a single C−C bond is

generated to form [{(Me₃Si)₂N}Fe]₂(μ:κ³-N,py,py′,κ³-

N′,py′,py-^oMe₂smif-^oMe₂smif) (6). Here, it is likely that the

ortho-methyl substituents on the smif labilize arms of the

chelate, allowing it to bind two irons in the course of ﬁnding

the conﬁguration in which C−C bond formation via radical

coupling can occur. Subsequently, the two Fe(I) cores

generated via the reductive coupling process each reduce a

pyridine-imine unit, and the ultimate oxidation state of each

becomes Fe(II). Counter to the other C−C bond-forming

processes, this is the only case in which metal redox events are

critical.

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Figure 11.

3.2. The Ionic Component of the CNC Backbone.

Attempts to prevent CNC backbone coupling reactions via

methylation brought about interference from proton transfer,

which can be interpreted as a consequence of the acid/base-

like, or nucleophile/electrophile-like character of the function-

H₂CN(+)HCH₂(−) suggest that ∼9% of its GS is a singlet

diradical.

4. CONCLUSIONS

Chemical reactivity of smif-containing iron compounds

supports the description of the CNC^nbbackbone-localized

orbital as having covalent diradical and/or ionic character. The

reactions also show that, while pseudo-square-planar iron(II)

complexes are becoming more common, without substantial

steric features, the formation of dimers is to be expected.

Collateral ﬁndings include the discovery that the common

starting material {(Me₃Si)₂N}₂Fe(THF) is a signiﬁcant alkyne

cyclotrimerization catalyst, and that the saturated backbone of

the related di-2-pyridyl-methyl-amide ligand can be readily

dehydrogenated. The mechanism of the dehyrogenation of

(dpma)₂Fe was not readily elucidated, and may depend on the

nature of the substrate.

o

ality. The internal proton transfer involving (^bMe₂Me₂smif)-

FeN(SiMe₃)₂(8), which implicates the ionic nature of the

CNC^nbbackbone, causes a dimerization to [{(Me₃Si)₂N}Fe]₂-

(μ-κ⁴,κ⁴-N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif)H))₂(9). A visualiza-

tion of the process given in Figure 7 provides two resonance

forms, an imine-alkyl and an amide-exomethylene, but the

metric parameters favor the former. The dark green solution

observed upon dissolving 9 is common for “(smif)Fe” moieties,

1

and the accompanying H NMR spectrum of the species in

solution suggests that 8 is regenerated replete with a reversal of

the proton transfer.

The most convincing evidence of the potential nucleophilic

character of the CNC^nborbital of the smif backbone is the clean

t

attack of isocyanates BuNCO and (2,6-ⁱPr₂C₆H₃)NCO by

5. EXPERIMENTAL SECTION

t

(smif)₂Fe (13) to produce (RNHCO-smif)₂Fe (14a, R = Bu;

5.1. General Considerations. All manipulations were performed

using either glovebox or high-vacuum line techniques. All glassware

was oven-dried. THF and ether were distilled under nitrogen from

purple sodium benzophenone ketyl and vacuum transferred from the

same prior to use. Hydrocarbon solvents were treated in the same

manner with the addition of 1−2 mL/L tetraglyme. Benzene-d₆and

toluene-d₈were dried over sodium, vacuum transferred, and stored

over activated 4 Å molecular sieves. THF-d₈was dried over sodium

and vacuum transferred from sodium benzophenone ketyl prior to use.

Li(smif), Na(smif), (smif)₂Zn, (^oMesmif)H, (^oMe₂smif)H,⁵and 1,3-

(2-pyridyl)-2-azapropene ((smif)H),²⁵were prepared according to

literature procedures. Lithium bis(trimethylsilyl)amide was purchased

from Aldrich and recrystallized from hexanes prior to use. All other

chemicals were commercially available and used as received.

NMR spectra were obtained using INOVA 400 and 500 MHz

spectrometers. Chemical shifts are reported relative to benzene-d₆(¹H

δ 7.16; ¹³C{¹H} δ 128.39), toluene-d₈(¹H δ 2.09; ¹³C{¹H} δ 20.4),

and THF-d₈(¹H δ 3.58; ¹³C{¹H} δ 67.57). Multidimensional

techniques were conducted using INOVA software aﬃliated with the

spectrometers. Solution magnetic measurements were conducted via

Evans’ method in toluene-d₈.⁶⁶Elemental analyses were performed by

Robertson Microlit Laboratories, Madison, New Jersey, and at the

University of Erlangen-Nuremberg, Erlangen, Germany.

14b, 2,6-ⁱPrC₆H₃). Not only is the reaction a clear indication of

the ionic character of the smif backbone, but it shows how smif-

containing complexes can be readily incorporated into

polymeric matrixes via cross-linking by diisocyanates.

3.3. Approximating the 2-Azaallyl Anion. In principle,

treatments of allyl anions should manifest the same non-

bonding orbital composition given in the simpliﬁed version of

smif illustrated in Figure 1. In the calculations, an appreciable

resonance stabilization is indicated,⁶⁰⁻⁶⁴and considerable ionic

character is implied. The 2-azaallyl anion is distinguished by its

central electronegative nitrogen, which changes the nature, and

substantially changes the energies, of the standard 3 orbital, 4

electron, 15 state system. Figure 11 shows that the lowest allyl

orbital is dominated by nitrogen character, whereas the

antibonding allyl orbital is virtually all carbon in character.

These features allow neglection of Ψ^b, and the approximation

of Ψ* permits the system to be assessed as a 2 orbital, 2

electron, 4 state system, albeit with the phases of the orbitals

switched from a standard diradical.⁶⁵The states arising from

the (Ψ^nb)²and (Ψ*)²conﬁgurations interact to modulate the

amount of ionic and covalent character in each, and the

resulting ground state (GS), as described in Figure 1, has

mostly ionic character, but retains some covalent diradical

composition. Estimates based on a CASSCF(4,3)/6-31G(d)

calculation of the ground state on the neutral model

5.2. Procedures. 5.2.1. (smif)Fe{N(SiMe₃)₂} (1) and [{(Me₃Si)₂N}-

Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2). A solution of (smif)H (0.700 g, 3.55

mmol) in 10 mL of Et₂O was added dropwise to a stirring solution of

Fe{N(SiMe₃)₂}₂(THF) (1.592 g, 3.55 mmol) in Et₂O (10 mL) at 23

°C. The solution immediately became dark green (1). The reaction

was degassed and allowed to stir at 23 °C for 3 h, and volatiles were

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removed. The orange crystalline solid was triturated with pentane,

stripped, and ﬁltered in Et₂O to produce 1.090 g of 2 (74%). ¹H NMR

(THF-d₈, 400 MHz): δ −0.01 (ν_1/2≈ 9 Hz, 1H), 34.57 (ν_1/2≈ 380

Hz, 9H), 46.70 (ν_1/2≈ 35 Hz, 1H), 46.99 (ν _/2≈ 110 Hz, 1H), 86.21

C³H), 121.49 (py^im-C³H), 122.05 (py-C⁵H), 124.38 (py^im-C⁵H),

136.08 (py-C⁴H), 136.50 (py^im-C⁴H), 148.81 (py-C⁶H), 149.66 (py^im-

C⁶H), 158.60 (im-CH), 165.95 (py^im-C²), 166.23 (py-C²).

5.2.6. (^bMe₂smif)₂Fe (7). To a 50 mL round-bottom ﬂask charged

with Fe{N(SiMe₃)₂}₂(THF) (0.996 g, 2.22 mmol) and (^bMe₂smif)H

(1.000 g, 4.44 mmol) was vacuum transferred 25 mL of THF at −78

°C. The reaction mixture became dark green and was stirred at 23 °C

for 2 d. The volatiles were removed in vacuo. The solid was ﬁrst

triturated with pentane (2 × 15 mL) and then Et₂O prior to ﬁltering

and washing with cold Et₂O to yield a mossy green solid (0.802 g,

1

(ν_1/2≈ 790 Hz, 1H), 165.40 (ν_1/2≈ 620 Hz, 1H). H NMR (C₆D₆,

400 MHz): δ −14.34 (ν_1/2≈ 270 Hz, 9H), −10.83 (ν_1/2≈100 Hz,

1H), 71.49 (ν_1/2≈ 130 Hz, 1H), 79.24 (ν_1/2≈ 200 Hz, 1H), 136.99

(ν_1/2≈ 500 Hz, 1H), 203.24 (ν_1/2≈ 430 Hz, 1H).

5.2.2. (dpma)Fe{N(SiMe₃)₂} (3).²⁶To a solution of Fe{N-

(SiMe₃)₂}₂(THF) (0.500 g, 1.11 mmol) in Et₂O (10 mL) at −78

°C was added di-(2-picolyl)amine (0.222 g, 1.11 mmol) in 10 mL of

Et₂O via syringe under argon. Upon warming to 23 °C, the solution

became red and continued to stir for 3 h. The reaction was degassed,

1

72%). H NMR (C₆D₆, 400 MHz): δ 2.54 (s, CH₃, 3H), 5.87 (t, py-

C⁴H, 1H, J = 8 Hz), 6.03 (d, py-C³H, 1H, J = 8.4 Hz), 6.49 (t, py-C⁵H,

1H, J = 4 Hz), 7.80 (d, py-C⁶H, 1H, J = 5.2 Hz). ¹³C{¹H} NMR

(C₆D₆, 100 MHz): δ 17.55 (CH₃), 111.83 (CCH₃), 115.80 (py-C⁵H),

119.79 (py-C³H), 134.70 (py-C⁴H), 152.12 (py-C⁶H), 165.98 (py-C²).

cooled to −78 °C, and ﬁltered. Red needles of 3 (0.297 g, 64%) were

1

washed with cold Et₂O. H NMR (C₆D₆, 400 MHz): δ 12.73 (ν_1/2

≈

o

5.2.7. (^bMe₂Me₂smif)FeN(SiMe₃)₂(8) and [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-

1900 Hz, Si(CH₃)₃, 18 H), 25.20 (ν_1/2≈ 520 Hz, CH₂, 2H), 36.57 (ν

≈ 1200 Hz, py-CH, 1H), 81.90 (ν_1/2≈ 2400 Hz, py-CH, 1H),

N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif) H))₂(9). A small tube ﬁtted to a 180°

needle valve was charged with Fe{N(SiMe₃)₂}₂(THF) (0.105 g, 0.23

mmol) and (^bMe₂smif)H (0.025 g, 0.30 mmol). A bell pepper green

solution appeared immediately upon addition of benzene. The

reaction was degassed, sealed under vacuum, and allowed to sit for

2 days at 23 °C. The tube was opened, and benzene was decanted.

/2

130.74 (ν_1/2≈ 2200 Hz, py-CH, 1H), 182.06 (ν_1/2≈ 3100 Hz, py-CH,

1H). UV−vis (benzene) = 549 nm (ε ∼ 1500 M⁻¹cm⁻¹). Anal. Calcd

for H₃₀C₁₈N₄Si₂Fe: C, 52.16; H, 7.30; N, 13.52. Found: C, 52.32; H,

7.32; N, 13.49. μ_eff(SQUID, 293 K) = 3.0 μ_B.

5.2.3. (^oMesmif)FeN(SiMe₃)₂(4) and [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-

N,py₂-^oMesmif,^oMesmif) (5). To a stirring solution of Fe{N-

(SiMe₃)₂}₂(THF) (0.500 g, 1.11 mmol) in 10 mL of Et₂O was slowly

added a solution of (^oMesmif)H (0.235 g, 1.11 mmol) in Et₂O (8 mL)

at 23 °C. The reaction mixture became emerald-teal green (4). The

reaction was degassed, warmed to 23 °C, and stirred for 20 h while

yellow-orange crystals precipitated from solution. The suspension was

1

Red-orange crystals of 9 were washed with Et₂O. (0.069 g, 63%). H

NMR (8, C₆D₆, 400 MHz): δ −50.62 (ν_1/2≈ 370 Hz, CH₃, 3H),

18.23 (ν_1/2≈ 600 Hz, Si(CH₃)₃, 9H), 21.94 (ν_1/2≈ 350 Hz, py-CH,

1H), 48.25 (ν_1/2≈ 44 Hz, py-CH, 1H), 57.31 (ν_1/2≈ 310 Hz, py-CH,

1H), 69.57 (ν_1/2≈ 440 Hz, CH₃, 3H). Anal. Calcd for H₂₂C₃₆N₄Si₂Fe:

C, 56.39; H, 7.74; N, 11.96. Found: C, 56.62; H, 7.84; N, 12.04. μ_eff

(SQUID, 293K) = 4.3 μ_B.

concentrated, and yellow-orange crystals were isolated by ﬁltration to

1

5.2.8. {2,5-Di(pyrindin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-

ide)}₂Fe (11). To a small glass bomb reactor charged with

[{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2, 0.600 g, 0.73 mmol)

and di(p-tolyl)acetylene (0.300 g, 1.45 mmol) was added 12 mL of

benzene, generating a dark emerald green solution ((smif)FeN(TMS)₂

(1)) that was degassed. The bomb was placed in a 50 °C oil bath for 3

days. Volatiles were removed in vacuo. Any unreacted 1 was washed

away in pentane. The ﬁlter cake was washed with Et₂O, and the

ﬁltrates were concentrated. Cooling the solution to −78 °C yielded

red crystals (0.175 g, 19%) of 11 that were ﬁltered, and washed with

cold Et₂O. ¹H NMR (C₆D₆, 400 MHz): δ 7.97 (ν_1/2≈ 105 Hz,

Si(CH₃)₃, 9H), 17.59 (ν_1/2≈ 40 Hz, py-CH/ArCH, 1H), 27.39 (ν_1/2

≈ 230 Hz, CH₃, 3H), 31.89 (ν_1/2≈ 77 Hz, py-CH/ArCH, 1H), 35.96

(ν_1/2≈ 52 Hz, py-CH/ArCH, 1H), 98.84 (ν_1/2≈ 1100 Hz, py-CH/

ArCH, 1H), 194.18 (ν_1/2≈ 1100 Hz, py-CH/ArCH, 1H). Anal. Calcd

for H₈₄C₆₈N₈Si₄Fe₂: C, 66.00; H, 6.84; N, 9.05. Found: C, 67.40; H,

6.45; N, 7.59. μ_eff(SQUID, 293 K) = 5.3 μ_B.

yield 5 (0.225, 47%). H NMR (C₆D₆, 400 MHz): δ −21.82 (υ_1/2

≈

38 Hz, CH, 1H), −26.10 (ν_1/2≈ 38 Hz, CH, 1H), −13.23 (ν_1/2≈ 413

Hz, CH₃, 3H), 0.75 (ν_1/2≈ 300 Hz, Si(CH₃)₃, 18H), 14.26 (ν_1/2

131 Hz, py-CH, 1H), 40.95 (ν_1/2≈ 23 Hz, py-CH, 1H), 46.13 (ν_1/2

374 Hz, py-CH, 1H), 47.74 (ν_1/2≈ 23 Hz, py-CH, 1H), 59.84 (ν_1/2

96 Hz, py-CH, 1H), 64.39 (ν_1/2≈ 96 Hz, py-CH, 1H), 252.29 (ν_1/2

1000 Hz, py-CH, 1H).

≈

5 . 2 . 4 . [ { ( M e ₃S i ) ₂N } F e ] ₂( μ : κ ³- N , p y , p y′ , κ³- N ′, p y ′ , -

py-^oMe₂smif-^oMe₂smif) (6). In a N₂drybox, a 5 mL scintillation vial

was charged with 50 mg of Fe[N(TMS)₂]₂(THF) (0.11 mmol) and

0.5 mL of Et₂O. A solution of ^oMe₂smifH (25 mg, 0.11 mmol) in Et₂O

(1.0 mL) was added dropwise at room temperature while stirring.

Upon addition, the solution changed from the light green character-

istic of Fe[N(TMS)₂]₂(THF) to a darker pine green. The reaction

mixture was allowed to stir an additional 20 min, and solvent was

removed in vacuo to yield a dark green powder. Evidence of highly

colored (^oMe₂smif)₂Fe (teal solution, dark teal/gold in solid state) was

noted >12 h in solution and after 2−3 days in the solid state.

Crystalline [(^oMe₂Smif)FeN(TMS)₂]₂(32 mg, 33%) was isolated

5.2.9. Na(^tBuNCO-smif). To a solution of Na(smif) (0.300 g, 1.37

mmol) in 20 mL of THF was added tert-butylisocyanate (156 μL, 1.37

mmol) via syringe at −78 °C under argon. The solution was warmed

to 23 °C and turned red. Volatiles were removed in vacuo after 2 h, and

the resulting ﬁlm was triturated with Et₂O to remove residual THF.

Na(^tBuNCO-smif) was isolated as a metallic green solid (0.396 g,

1

from a saturated pentane solution at −30 °C after 3 days. H NMR

(C₆D₆, 400 MHz): δ −13.11 (ν_1/2≈ 65 Hz, py-CH, 4H), −1.47 (ν_1/2

≈ 600 Hz, CH₃, 6H), 3.72 (ν_1/2= 50 Hz, im-CH, 2H), 11.60 (ν_1/2

=

382 Hz, Si(CH₃)₃, 36H), 16.00 (ν_1/2= 750 Hz, CH₃, 6Hz), 34.24 (ν_1/2

= 95 Hz, py-CH, 4H), 40.07 (ν_1/2= 91 Hz, py-CH, 4H), 121.39 (ν_1/2

= 493 Hz, CH, 2H). μ_eff(SQUID, 300 K) = 7.26 μ_B.

1

91%). H NMR (C₆D₆, 500 MHz): δ 1.39 (s, C(CH₃)₃, 9H), 6.33 (t,

py-C⁵H, 1H, J = 5.8 Hz), 6.43 (t, py′-C⁵H, 1H, J = 5.8 Hz), 6.55 (d,

py-C³H, 1H, J = 8.1 Hz), 6.94 (t, py-C⁴H, 1H, J = 7.0 Hz), 7.17 (t, py′-

C⁴H, 1H, J = 7.7 Hz), 7.81 (d, py′-C³H, 1H, J = 8.1 Hz), 8.18 (s, NH,

1H), 8.20 (d, py-C⁶H, 1H, J = 5.4 Hz), 8.44 (d, py′-C⁶H, 1H, J = 3.9

Hz), 10.58 (s, CH, 1H). ¹³C{¹H} NMR (C₆D₆, 125 MHz): δ 30.58

(C(CH₃)₃), 50.26 (C(CH₃)₃), 111.02 (py′-C³H), 116.24 (py′-C⁵H),

116.94 (py-C³H), 118.74 (py-C⁵H), 121.24 (C(CO)), 123.58 (CH),

135.18 (py-C⁴H), 136.52 (py′-C⁴H), 148.28 (py-C⁶H), 150.40 (py′-

C⁶H), 157.79 (CO), 160.47 (py′-C²), 169.93 (py-C²).

5.2.5. 2,4-Di-(2-pyridyl)-3-aza-2-pentene ((^bMe₂smif)H). A solu-

tion of (smif)H (1.000 g, 5.07 mmol) in 20 mL of THF was slowly

added to a suspension of NaH (0.243 g, 10.13 mmol) in 15 mL of

THF at 0 °C. The reaction mixture turned magenta and was stirred at

0 °C for 3 h prior to the addition of CH₃I (0.63 mL, 10.11 mmol).

After stirring at 23 °C for 12 h, the solution appeared orange-red. The

volatiles were removed in vacuo, and the residue was ﬁltered in

CH₂Cl₂. Methylene chloride was removed, and the solid was triturated

with Et₂O and ﬁltered to yield an orange solid (1.138 g, 99%). H

5.2.10. (^tBuNCO-smif)₂Fe (14a). a. To a small glass bomb reactor

charged with (smif)₂Fe (13) (0.150 g, 0.33 mmol) was added 10 mL

of C₆H₆. The bomb was cooled to −78 °C, and tert-butylisocyanate

(76 μL, 0.66 mmol) was added via GC syringe under argon. The

reaction was degassed and slowly warmed to 23 °C. The solution

turned deep red-orange after stirring at 23 °C for 18 h. The reaction

was stirred at 23 °C for 10 days. Volatiles were removed in vacuo, and

the reaction mixture was triturated with pentane (3 × 5 mL). After

1

NMR (C₆D₆, 500 MHz): δ 1.67 (d, CH₃, 3H, J = 7 Hz), 2.37 (s, im-

CH₃, 3H), 5.19 (q, CH, 1H, J = 6.5 Hz), 6.67 (t, py-C⁵H, 1H, J = 5.5

Hz), 6.68 (t, py^im-C⁵H, 1H, J = 5.5 Hz), 7.17 (t, py-C⁴H, 1H, J = 7.5

Hz), 7.19 (t, py^im-C⁴H, 1H, J = 7.5 Hz), 7.63 (d, py-C³H, 1H, J = 8

Hz), 8.39 (d, py^im-C³H, 1H, J = 8 Hz), 8.44 (d, py-C⁶H, 1H, J = 3.5

Hz), 8.54 (d, py^im-C⁶H, 1H, J = 4 Hz). ¹³C{¹H} NMR (C₆D₆, 125

MHz): δ 13.97 (CH₃), 23.52 (im-CH₃), 62.94 (im-CH), 121.37 (py-

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ﬁltering and washing with pentane, 14a was isolated as a dark red solid

(0.102 g, 47%). b. To a 25 mL round-bottom ﬂask charged with

FeBr₂(THF)₂(0.141 g, 0.39 mmol) and Na(^tBuNCO-smif) (0.250 g,

0.79 mmol) was vacuum transferred 20 mL of THF at −78 °C. The

solution turned red-orange upon warming to 23 °C and was stirred for

18 h. Volatiles were removed in vacuo, and the resulting solid was

triturated with Et₂O prior to ﬁltering in toluene. A dark red

The suspension immediately changed from emerald green to red. The

reaction was degassed and allowed to stir at 23 °C for 3 h while dark

red crystals precipitated from solution. The reaction mixture was

concentrated, ﬁltered cold, and washed with cold Et₂O to yield a

mixture of 16 and (smif)₂Fe (13) (16:1, 0.225 g, 63%). b. To a stirred

solution of (dpma)FeN(TMS)₂(3) 15-Fe (0.300 g, 0.72 mmol) in 20

mL of Et₂O at −78 °C was slowly added a solution of (smif)H (0.143

g, 0.72 mmol) in Et₂O (10 mL) under argon. The reaction was

degassed. The reaction mixture changed color from cherry red to dark

blue to purple to dark red as the solution warmed to 23 °C. Dark red

crystals precipitated from solution while it stirred at 23 °C for 3 h. The

reaction mixture was concentrated, ﬁltered cold, and washed with cold

1

microcrystalline solid, 14a, was obtained (0.154 g, 61%). H NMR

(C₆D₆, 500 MHz): δ 1.53 (s, C(CH₃)₃, 9H), 5.37 (s, NH, 1H), 5.71 (t,

py-C⁵H, 1H, J = 6.2 Hz), 5.76 (t, py′-C⁵H, 1H, J = 6.3 Hz), 6.32 (t, py-

C⁴H, 1H, J = 7.3 Hz), 6.36 (d, py-C³H, 1H, J = 7.9 Hz), 6.48 (t, py′-

C⁴H, 1H, J = 6.3 Hz), 7.03 (d, py′-C³H, 1H, J = 8.5 Hz), 7.75 (d, py-

C⁶H, 1H, J = 5.3 Hz), 7.98 (d, py′-C⁶H, 1H, J = 5.1 Hz), 9.91 (s, CH,

1H). ¹³C{¹H} NMR (C₆D₆, 125 MHz): δ 30.08 (C(CH₃)₃), 51.35

(C(CH₃)₃), 113.33 (py′-C³H), 116.31 (py′-C⁵H), 116.79 (py-C³H),

119.27 (py-C⁵H), 119.95 (C(CO)), 130.04 (CH), 135.01 (py-C⁴H),

135.42 (py′-C⁴H), 151.70 (py-C⁶H), 152.06 (py′-C⁶H), 164.40 (py′-

C²), 164.47 (CO), 165.62 (py-C²). UV−vis (benzene) = 383 nm (ε

∼ 28 000 M⁻¹cm⁻¹), 431 nm (ε ∼ 42 000 M⁻¹cm⁻¹), 534 nm (ε ∼

19 000 M⁻¹cm⁻¹), 644 nm (ε ∼ 6000 M⁻¹cm⁻¹). Anal. Calcd for

H₃₈C₃₄N₈O₂Fe: C, 63.16; H, 5.92; N, 17.33. Found: C, 63.24; H, 5.48;

N, 16.01.

1

Et₂O to yield a mixture of 16 and 13 (2:1, 0.260 g, 51%). H NMR

(C₆D₆, 400 MHz): δ 8.70 (ν_1/2≈ 19 Hz, CH₂, 2H), 14.98 (ν_1/2≈ 40

Hz, py-CH, 1H), 16.95 (ν_1/2≈ 40 Hz, py-CH, 1 H), 19.48 (ν_1/2≈ 60

Hz, py-CH, 1H), 21.80 (ν_1/2≈ 80 Hz, py-CH, 1H), 30.37 (ν_1/2≈ 187

Hz, py-CH, 1H), 32.02 (ν_1/2≈ 234 Hz, py-CH, 1H), 47.34 (ν_1/2

388 Hz, CH, 1H), 81.73 (ν_1/2≈ 1900 Hz, py-CH, 1H), 183.55 (ν_1/2

3500 Hz, py-CH, 1H).

≈

5.2.14. (dpma)FeOCHPh₂(17). To a 50 mL round-bottom ﬂask

charged with 3 (0.212 g, 0.256 mmol) and diphenylmethanol (0.095 g,

0.52 mmol) was vacuum transferred 15 mL of Et₂O at −78 °C,

resulting in a red solution. The solution darkened to red-purple and

was stirred at 23 °C for 3 h. The solution was concentrated, ﬁltered,

and washed with cold Et₂O to yield a purple solid (0.194 g). ¹H NMR

spectral analysis indicated 17 as the major product, but repeated

syntheses showed variable amounts of impurities that could not be

removed. A quench of the product with H₂O aﬀorded a 1.06:1.00 ratio

of di-(2-picolyl)amine/diphenylmethanol. ¹H NMR (signals attributed

to 17, C₆D₆, 400 MHz): δ 7.62 (1H, ν_1/2= 20 Hz), 8.33 (2H, ν_1/2= 27

Hz), 12.76 (2H, ν_1/2= 144 Hz), 13.75 (1H, ν_1/2= 52 Hz), 20.84 (1H,

ν_1/2= 83 Hz), 24.32 (1H, ν_1/2= 61 Hz), 65.56 (1H, ν_1/2= 691 Hz),

135.50 (1H, ν_1/2= 972 Hz).

5.2.11. (2,6-ⁱPr₂-C₆H₃-NCO-smif)₂Fe (14b). To a 25 mL round-

bottom ﬂask charged with (smif)₂Fe (13) (0.500 g, 1.12 mmol) was

added 20 mL of C₆H₆. The ﬂask was cooled to −78 °C, and 2,6-

diisopropylphenylisocyanate (0.48 mL, 2.24 mmol) was added via

syringe under argon. The reaction was degassed and slowly warmed to

23 °C as the solution turned deep red-orange. The reaction stirred at

23 °C for 17 h. Volatiles were removed in vacuo, and the reaction

mixture was triturated with pentane (3 × 5 mL). After ﬁltering and

washing with pentane, 14b was isolated as a dark red solid (0.719 g,

1

75%). H NMR (C₆D₆, 400 MHz): δ 1.38 (s, CH(CH₃)₂, 12H), 3.56

(sept, CH(CH₃)₂, 2H, J = 6.9 Hz), 5.84 (t, py-C⁵H, py′-C⁵H, 2H, J =

5.3 Hz), 6.33 (d, Ar−C³H, 2H, J = 3.8 Hz), 6.54 (t, Ar-C⁴H, 1H, J =

7.3 Hz), 6.98 (s, NH, 1H), 7.31 (m, py-C³H, py-C⁴H, py′-C⁴H, 3H),

7.44 (d, py′-C³H, 1H, J = 8.4 Hz), 7.77 (d, py-C⁶H, 1H, J = 5.4 Hz),

7.99 (d, py′-C⁶H, 1H, J = 5.2 Hz), 10.15 (s, CH, 1H). ¹³C{¹H} NMR

(C₆D₆, 100 MHz): δ 24.36 (CH(CH₃)₂), 24.38 (CH(CH₃)₂), 30.07

(CH(CH₃)₂), 114.11 (py′-C³H), 117.00 (py′-C⁵H), 117.98 (py-C³H),

118.34 (py-C⁵H), 119.89 (C(CO)), 124.07 (CH), 127.92 (Ar-

C³H), 133.02 (Ar-C⁴H), 134.18 (py-C⁴H), 135.24 (py′-C⁴H), 136.08

(Ar-C¹), 145.41 (Ar-C²), 151.42 (py-C⁶H), 151.88 (py′-C⁶H), 163.97

(py′-C²), 164.55 (CO), 165.51 (py-C²). UV−vis (benzene) = 381

nm (ε ∼ 33 000 M⁻¹cm⁻¹), 427 nm (ε ∼ 46 000 M⁻¹cm⁻¹), 525 nm

(ε ∼ 21 000 M⁻¹cm⁻¹), 638 nm (ε ∼ 7000 M⁻¹cm⁻¹). Anal. Calcd

for H₅₄C₅₀N₈O₂Fe: C, 70.25; H, 6.37; N, 13.11. Found: C, 69.86; H,

6.34; N, 13.00.

5.2.15. (smif)FeOCHPh₂(18). To a 25 mL round-bottom ﬂask

charged with [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2, 0.200 g,

0.24 mmol) and diphenylmethanol (0.089 g, 0.48 mmol) was vacuum

transferred 15 mL of THF at −78 °C, resulting in an emerald green

solution. After stirring at 23 °C for 2.5 days, volatiles were removed in

vacuo from the dark kelly green solution. The solid was washed with

1

pentane to remove excess diphenylmethanol. The H NMR spectrum

of the green, microcrystalline sample revealed a 1:1 mixture of

(smif)₂Fe and 18. Attempts to purify the sample were unsuccessful,

but diphenylmethanol was identiﬁed from hydrolysis by ¹H NMR

1

spectra analysis in C₆D₆. H NMR (assignments tentative, C₆D₆, 400

MHz): δ −29.46 (ν_1/2≈ 365 Hz, Ar-CH, 4H), −0.77 (ν_1/2≈ 61 Hz,

Ar-CH, 4H), 1.49 (ν_1/2≈ 20 Hz, CH,1H), 3.40 (ν_1/2≈ 38 Hz, Ar-CH,

2H), 3.74 (ν_1/2≈ 30 Hz, CH, 1H), 35.55 (ν_1/2≈ 47 Hz, py-CH, 1H),

66.47 (ν_1/2≈ 73 Hz, py-CH, 1H), 125.50 (ν_1/2≈ 861 Hz, py-CH,

1H), 254.92 (ν_1/2≈ 554 Hz, py-CH, 1H).

5.2.12. (dpma)₂Fe (15). a. To a solution of Fe{N(SiMe₃)₂}₂(THF)

(0.250 g, 0.56 mmol) in Et₂O (10 mL) at −78 °C was added [(2-

py)CH₂]₂NH (0.222 g, 1.11 mmol) in 5 mL of Et₂O via syringe under

argon. The solution became dark blue after 30 min at 23 °C and was

stirred for 12 h. The volatiles were removed in vacuo. The residue was

dissolved in benzene and ﬁltered through Celite. Addition of pentane

facilitated the crystallization of 15 as dark blue-metallic purple crystals

5.2.16. (d₄-dpma)H. To a 50 mL round-bottom ﬂask containing a

solution of 2-cyanopyridine (1.658 g, 15.93 mmol) in 16 mL of

CH₃OD was added sodium borodeuteride (2.000 g, 47.78 mmol). The

resulting cloudy suspension was reﬂuxed under argon for 16 h. The

yellow suspension was cooled to 23 °C, ﬁltered, and washed with

CH₃OD. Volatiles were removed from the yellow ﬁltrates in vacuo,

yielding a yellow solid that was dissolved in 10 mL of D₂O, stirred for

1 h at 23 °C, and extracted with CH₂Cl₂(3 × 10 mL). Organic layers

were combined, dried over Na₂SO₄, and ﬁltered, and volatiles were

removed in vacuo to yield an orange-yellow liquid containing both (2-

pyridyl)CD₂ND₂and (d₄-dpma)D, which were separated via

distillation under dynamic vacuum: (2-pyridyl)CD₂ND₂(0.479g,

28%) and (d₄-dpma)D (0.698 g, 44%). (2-pyridyl)CD₂ND₂: ¹H

NMR (C₆D₆, 400 MHz): δ 6.62 (dd, py-C⁵H, 1H, J = 7.5, 4.8 Hz),

6.88 (dt, py-C³H, 1H, J = 7.8, 1.2 Hz), 7.05 (td, py-C⁴H, 1H, J = 7.7,

1.8 Hz), 8.47 (d, py-C⁶H, 1H, J = 4.8 Hz). ¹³C{¹H} NMR (C₆D₆, 100

MHz): δ 121.31 (py-C³H), 121.79 (py-C⁵H), 136.24 (py-C⁴H),

1

(0.188 g, 75%). H NMR (C₆D₆, 400 MHz): δ 8.47 (ν_1/2≈ 40 Hz,

1H), 41.31 (ν_1/2≈ 70 Hz, 1H), 50.37 (ν_1/2≈ 100 Hz, 2H), 56.60 (ν_1/2

≈ 180 Hz, 1H), 163.05 (ν_1/2≈ 2900 Hz, 1H). Anal. Calcd for

H₂₄C₂₄N₆Fe: C, 63.73; H, 5.35; N, 18.58. Found: C, 63.53; H, 5.05; N,

17.69. b. (d₄-dpma)₂Fe (15-d₈). To a solution of Fe{N(SiMe₃)₂}₂-

(THF) (0.329 g, 0.73 mmol) in Et₂O (10 mL) at −78 °C was added

(d₄-dpma)D (0.300 g, 1.47 mmol) in 9 mL of Et₂O via syringe under

argon. The solution changed color from pale green to dark blue after

30 min at 23 °C and was stirred for 20 h. The volatiles were removed

in vacuo. The solid was taken up and ﬁltered in Et₂O. The ﬁltrates were

concentrated, cooled to −78 °C, and ﬁltered, and the resulting dark

blue crystals, 15-d₈, were washed with cold Et₂O (0.220 g, 66%).

5.2.13. (dpma)(smif)Fe (16). a. To a stirred suspension of

[{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2, 0.300 g, 0.36 mmol)

in 20 mL of Et₂O at −78 °C was slowly added a solution of di-(2-

picolyl)amine (0.145 g, 0.73 mmol) in Et₂O (10 mL) under argon.

1

149.77 (py-C⁶H), 163.44 (py-C²). (d₄-dpma)D: H NMR (C₆D₆, 400

MHz): δ 6.62 (t, py-C⁵H, 1H, J = 5.6 Hz), 7.08 (m, py-C³H, py-C⁴H,

2H), 8.47 (d, py-C⁶H, 1H, J = 4.7 Hz). ¹³C{¹H} NMR (C₆D₆, 100

3309

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MHz): δ 122.98 (py-C³H), 123.32 (py-C⁵H), 137.21 (py-C⁴H),

150.82 (py-C⁶H), 162.27 (py-C²).

ﬁt using the least-squares method developed for determining the NMR

monomer shift and equilibrium constant for self-associating systems.²²

5.5. Hydrogenation and Deuteration Studies. Flame-dried

NMR tubes, sealed to 14/20 ground glass joints, were charged with

(dpma)₂Fe 15 (0.015 g, 0.033 mmol), 2 equiv (0.066 mmol) of the

appropriate organic substrate, and 0.5 mL of C₆D₆. The tube was ﬁtted

with a 180° needle valve and freeze−pump−thaw degassed three times

before sealing. Reaction progress was monitored via ¹H NMR

spectroscopy. Deuteration studies were performed in ﬂame-dried J-

Young NMR tubes. Several deuteration experiments were performed

in ﬂame-dried and silylated J-Young tubes. Results are summarized in

Scheme 3 and listed in the Supporting Information.

5.2.17. [{(Me₃Si)₂N}Fe]₂(μ-κ³,κ³-N,py₂-smif,smif) (2). An orange

parallelepiped crystal (0.40 × 0.20 × 0.15 mm) of 2 was obtained from

a pentane solution cooled from 80 °C. A total of 32 284 reﬂections

were collected with 7293 determined to be symmetry-independent

(R_int= 0.0488), and 5519 were greater than 2σ(I). A semiempirical

absorption correction from equivalents was applied, and the

reﬁnement utilized w⁻¹= σ²(F_o²) + (0.0511p)²+ 0.0000p, where p

2

= ((F_o+ 2F_c²)/3).

5. 2. 18. [{(Me₃Si)₂N}Fe]₂(μ:κ³-N, py, py′, κ³-N′, py′, -

py-^oMe₂smif-^oMe₂smif) (6). A green block (0.40 × 0.20 × 0.15

mm) was obtained from a benzene/pentane mixture at 23 °C. A total

of 22 647 reﬂections were collected with 5817 determined to be

symmetry-independent (R_int= 0.0295), and 4615 were greater than

2σ(I). A semiempirical absorption correction from equivalents was

5.6. Cyclotrimerization Studies. Flame-dried NMR tubes, sealed

to 14/20 ground glass joints, were charged with 0.5 mL solutions of

known concentrations of an iron compound in the glovebox. The

tubes were attached to a calibrated gas bulb and degassed on the

vacuum line via freeze−pump−thaw cycle. After condensing 2-butyne

at 77 K, the tubes were sealed with a torch. Reaction progress was

2

applied, and the reﬁnement utilized w⁻¹= σ²(F_o) + (0.0491p)²+

2

0.8812p, where p = ((F_o+ 2F_c²)/3). One SiMe₃showed a slight

disorder and was modeled accordingly.

1

5.2.19. [{(Me₃Si)₂N}Fe]₂(μ-κ⁴,κ⁴-N,py₂,C-(^bMe,^bCH₂,^oMe₂(smif)H))₂

(9). A red needle (0.60 × 0.10 × 0.05 mm) of 8 was obtained from

a 1:1 benzene/pentane mixture at 23 °C. A total of 20 055 reﬂections

were collected with 4095 determined to be symmetry-independent

(R_int= 0.0609), and 3-085 were greater than 2σ(I). A semiempirical

absorption correction from equivalents was applied, and the

monitored via H NMR spectroscopy by observing the disappearance

of 2-butyne (δ 1.50 ppm) and appearance of hexamethylbenzene (δ

2.13 ppm). Upon completion, the tube was opened in the glovebox

and the contents were transferred into a J Young tube containing 0.010

1

g of ferrocene. A H NMR spectrum was obtained, and integrations

permitted quantiﬁcation of hexamethylbenzene produced, turnover

number, and turnover frequency. The C₆D₆stock solutions (4.5 ×

10⁻³to 1.3 × 10⁻²M for {Fe{N(TMS)₂}₂(THF) and 4.8 × 10⁻³to

1.5 × 10⁻²M for 1) were prepared using 2 or 5 mL volumetric ﬂasks.

5.7. Magnetic Susceptibility Measurements. Magnetic suscept-

ibility measurements of crystalline powdered samples (10−30 mg)

were performed on a Quantum Design MPMS-5 SQUID magneto-

meter at 10 kOe (1 T) between 5 and 300 K for all samples. All sample

preparations and manipulations were performed under an inert

atmosphere due to the air sensitivity of the samples. The samples were

measured in gelatin capsules, and the diamagnetic contribution from

the sample container was subtracted from the experimental data.

Pascal’s constants⁶⁷were used to subtract diamagnetic contributions,

yielding paramagnetic susceptibilities. The program julX written by E.

Bill was used for (elements of) the simulation and analysis of magnetic

susceptibiltiy data.⁶⁸

2

reﬁnement utilized w⁻¹= σ²(F_o) + (0.0368p)²+ 0.3358p, where p

2

= ((F_o+ 2F_c²)/3). One of the SiMe₃groups was disordered and

modeled accordingly.

5.2.20. [{2,5-Di(pyrindin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-

ide)}FeN(SiMe₃)₂]₂(11). A red plate (0.60 × 0.30 × 0.05 mm) was

obtained from diethyl ether at 23 °C. A total of 69 998 reﬂections were

collected with 15 480 determined to be symmetry-independent (R_int

=

0.0467), and 10 139 were greater than 2σ(I). A semiempirical

absorption correction from equivalents was applied, and the

2

reﬁnement utilized w⁻¹= σ²(F_o) + (0.0606p)²+ 0.0000p, where p

2

= ((F_o+ 2F_c²)/3). SQUEEZE was applied to a disordered solvent

molecule.

5.2.21. (2,6-ⁱPrC₆H₃-NCO-smif)₂Fe (14b). A dark black-red needle

(0.45 × 0.10 × 0.03 mm) was obtained from a mixture of THF and

pentane at 23 °C. A total of 57 325 reﬂections were collected with

5564 determined to be symmetry-independent (R_int= 0.1560), and

2984 were greater than 2σ(I). A semiempirical absorption correction

5.8. Mossbauer Spectroscopy. ⁵⁷Fe Mossbauer spectra were

̈

from equivalents was applied, and the reﬁnement utilized w⁻¹

=

recorded on a WissEl Mossbauer spectrometer (MRG-500) at 77 K in

̈

2

σ²(F_o) + (0.0911p)²+ 0.0000p, where p = ((F_o+ 2F_c²)/3).

constant acceleration mode. ⁵⁷Co/Rh was used as the radiation source.

WinNormos for Igor Pro software has been used for the quantitative

evaluation of the spectral parameters (least-squares ﬁtting to

Lorentzian peaks). The minimum experimental line widths were

0.20 mms⁻¹. The temperature of the samples was controlled by an

SQUEEZE was applied to a disordered solvent molecule.

5.2.22. (dpma)₂Fe (15). A thin blue plate (0.60 × 0.15 × 0.05 mm)

of 14 was obtained from 1:1 benzene/pentane at 23 °C. The crystal

was determined to be a merohedral twin. A total of 37 723 reﬂections

were collected, with 7419 determined to be symmetry-independent

(R_int= 0.0594), and 6031 were greater than 2σ(I). A semiempirical

absorption correction from equivalents was applied, and the

̈

MBBC-HE0106 MOSSBAUER He/N₂cryostat within an accuracy of

0.3 K. Isomer shifts were determined relative to α-iron at 298 K.

5.9. Computational Methods. B3LYP⁶⁹⁻⁷³geometry optimiza-

tion utilized the Gaussian03 suite of programs; the 6-31G(d) basis set

was employed. Tests with the larger 6-311+G(d) basis set did not

reveal signiﬁcant diﬀerences in the optimized geometries. No

symmetry constraints were employed in geometry optimization.

Geometry optimizations were started from both a pseudo-C_2v

structure. Calculation of the energy Hessian was performed to conﬁrm

species as minima on their respective potential energy surfaces at this

level of theory.

2

reﬁnement utilized w⁻¹= σ²(F_o) + (0.0307p)²+ 0.4679p, where p

2

= ((F_o+ 2F_c²)/3).

5.3. Single-Crystal X-Ray Diﬀraction Studies. Upon isolation,

the crystals were covered in polyisobutenes and placed under a 173 K

N₂stream on the goniometer head of a Siemens P4 SMART CCD

area detector (graphite-monochromated MoKα radiation, λ = 0.71073

Å). The structures were solved by direct methods (SHELXS). All non-

hydrogen atoms were reﬁned anisotropically unless stated, and

hydrogen atoms were treated as idealized contributions (Riding

model). Any deviation from this protocol will be noted for the

individual descriptions below.

5.4. Equilibrium Study: 2 (smif)FeN(SiMe₃)₂(1) ⇄ [(TMS)₂-

NFe]₂(smif)₂(2). A series of ﬁve NMR tubes were charged with

known concentrations of (smif)FeN(SiMe₃)₂(1). The solutions (3.4

× 10⁻³to 2.4 × 10⁻²M) of 2 in THF-d₈were prepared in a 5 mL

volumetric ﬂask, and ¹H NMR spectra were obtained at ambient

temperature (∼23 °C). The chemical shift associated with the smif

“backbone” CH was monitored as it exhibited the largest change as

concentrations varied. The equilibrium constant, ∼ 4 × 10⁻⁴M⁻¹, was

ASSOCIATED CONTENT

■

S

* Supporting Information

Hydrogen-transfer studies from (dpma)₂Fe (15) and magnet-

ism studies and ﬁts for 2, 3, 9, 11, and 15. CIF ﬁles for 6, 9, 11,

and 14b.This material is available free of charge via the Internet

at http://pubs.acs.org. CIF ﬁles for 2 and 15 are available via

ref 4.

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dx.doi.org/10.1021/ic302783y | Inorg. Chem. 2013, 52, 3295−3312

Inorganic Chemistry

Article

Haaland, A.; Lappert, M. F.; Leung, W. P.; Rypdal, K. Inorg. Chem.

1988, 27, 1782−1786.

AUTHOR INFORMATION

Corresponding Author

*Fax: 607 255 4173. E-mail: ptw2@cornell.edu.

■

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3488.

Present Address

^∥Department of Chemistry, Purdue University, West Lafayette,

Indiana 47907, United States.

Notes

The authors declare no competing ﬁnancial interest.

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P.T.W. thanks the NSF (CHE-0718030) and Cornell

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Article Doi

C-C bond formation and related reactions at the CNC backbone in (smif)FeX (smif = 1,3-di-(2-pyridyl)-2-azaallyl): Dimerizations, 3 + 2 cyclization, and nucleophilic attack; Transfer hydrogenations and alkyne trimerization (X = N(TMS)2, dpma = (Di-(2-pyridyl-methyl)-amide))

DOI: 10.1021/ic302783y

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