The reactivity patterns of low-coordinate iron-hydride complexes

Published on Web 04/30/2008

The Reactivity Patterns of Low-Coordinate Iron-Hydride

Complexes

Ying Yu, Azwana R. Sadique, Jeremy M. Smith, Thomas R. Dugan,

Ryan E. Cowley, William W. Brennessel, Christine J. Flaschenriem, Eckhard Bill,

Thomas R. Cundari,* and Patrick L. Holland*

Department of Chemistry, UniVersity of Rochester, Rochester, New York, 14627,

Max-Planck-Institut fu¨r Bioanorganische Chemie, D-45470 Mu¨lheim an der Ruhr, Germany,

Department of Chemistry and Center for AdVanced Scientiﬁc Computing and Modeling

(CASCaM), UniVersity of North Texas, Denton, Texas, 76203

Received November 28, 2007; E-mail: holland@chem.rochester.edu; tomc@unt.edu

Abstract: We report a survey of the reactivity of the ﬁrst isolable iron-hydride complexes with a coordination

number less than 5. The high-spin iron(II) complexes [(ꢀ-diketiminate)Fe(µ-H)]₂react rapidly with

representative cyanide, isocyanide, alkyne, N₂, alkene, diazene, azide, CO₂, carbodiimide, and Brønsted

acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across

a multiple bond of the substrate, (2) reductive elimination of H₂to form iron(I) products, and (3) protonation

of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido,

benzo[c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known

bond transformations at iron complexes. Additionally, they give insight into the elementary transformations

that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands

on high-spin, low-coordinate metal atoms.

Introduction

(ENDOR) studies of nitrogenase mutants freeze-trapped during

substrate turnover. For example, a Val70Ile mutant of A.

Organometallic chemists have long appreciated the many

reactions of transition-metal hydride complexes, especially

reductions.¹Recently, hydride chemistry has become established

in bioinorganic systems.²Nitrogenases are prodigious reductants

that cleave double and triple bonds in N₂, CO₂, N₂O, N₃^-, and

CN^-at iron-sulfur clusters (FeMoco in the molybdenum-iron

nitrogenases, FeVco in the vanadium-iron nitrogenases, and

FeFeco in the iron-only nitrogenases).³Because these substrates

are reduced by multiples of two electrons accompanied by the

addition of protons, chemists have often speculated about the

potential role of hydrides.⁴

Vinelandii molybdenum-iron nitrogenase freeze-trapped during

proton reduction shows two hydrogen nuclei with very strong

coupling to the S ) ¹/₂iron-sulfur cluster, strongly suggesting

the presence of Fe-H bonds.⁵This frozen species loses two

molecules of H₂upon annealing to -20 °C, raising the

possibility that these two hydride ligands can combine with

nearby protons to release H₂.⁶This species is thought to be in

the redox state that reacts directly with N₂in the wild-type MoFe

nitrogenase,⁷underscoring the importance of hydride-containing

intermediates in enabling the nitrogen reduction activity char-

acteristic of this enzyme.

Recently, the ﬁrst direct evidence for hydride intermediates

in nitrogenase emerged from electron-nuclear double resonance

From the perspective of the coordination chemist, there are

a number of ways that potential FeMoco-bound hydrides differ

from the majority of known synthetic transition-metal hydride

complexes. First, FeMoco hydride adducts could achieve a

number of different oxidation levels. One-electron redox changes

undoubtedly occur in the FeMoco because electrons are supplied

to the FeMoco one at a time by the Fe protein, which dissociates

and reassociates before each one-electron reduction of the

FeMoco.³However, most synthetic hydride complexes have

diamagnetic metal centers, and little one-electron chemistry has

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6624 J. AM. CHEM. SOC. 2008, 130, 6624–6638

10.1021/ja710669w CCC: $40.75

2008 American Chemical Society

Iron-Hydride Reactivity

A R T I C L E S

Figure 1. The iron-molybdenum cofactor (“FeMoco”) of iron-molybdenum

nitrogenase in the isolated M^Nform.⁸This form is reduced (with

incorporation of hydrides in an undisclosed location) to give the intermediate

that reacts with N₂and other nitrogenase substrates. X is C^4-, N^3-, or O^2-

.

Figure 2. Bulky ꢀ-diketiminate iron complexes used in this work. The

ligands are abbreviated L^R, where R is the group on the diketiminate

backbone. Both complexes have roughly tetrahedral geometry at each iron

atom.^20,21

been reported.^9,10Second, the iron ions in the FeMoco are

expected to be high-spin, based on ligand ﬁeld considerations

(weak-ﬁeld sulﬁde donor set, coordination number less than ﬁve)

and the results of computational studies.¹¹Synthetic hydride

complexes, on the other hand, typically have strong-ﬁeld

organometallic or phosphine coligands, which enforce a low-

spin electronic conﬁguration.^12,13These fundamental differences

motivate synthetic research aimed at the creation of iron-hydride

complexes with weak-ﬁeld ligands and low coordination

number, to determine their characteristic reactivity patterns and

reaction mechanisms, which can in turn be correlated with

nitrogenase reactions.

rearrangements upon reduction.^7,17Peters and Holland have

independently proposed that dissociation of X is important in

enabling substrate binding to iron, based on the binding of N₂

to synthetic iron complexes.¹⁸Computational studies come to

a variety of conclusions regarding the structural ﬂexibility of

the FeMoco during catalysis.¹¹In short, there are many questions

about the structure of the iron-hydride nitrogenase intermedi-

ates, and the available data are not sufﬁcient to provide guesses

about their structure(s).

For these reasons, we have created one- and two-iron

compounds that focus on two key features of the nitrogenase-

hydride intermediates: the weak-ﬁeld ligands and the coordina-

tion number less than ﬁve.^18aExtremely bulky bidentate

ꢀ-diketiminate ligands (Figure 2) have π-donating nitrogen

atoms that lead to a weak ligand ﬁeld.¹⁹The bulky diiso-

propylphenyl groups maintain a low coordination number that

mimics the ligand-poor environment of the iron atoms in the

FeMoco. Using these ligands, we have isolated the only

examples of iron-hydride complexes with a coordination number

less than ﬁVe (Figure 2).^20,21

Often the design of functional models of an enzyme is driven

by an attempt to achieve structural similarity to the enzyme’s

active site.¹⁴In the case of nitrogenase, an accurate structural

mimic of the FeMoco hydride species is elusive for several

reasons. First, the FeMoco of iron-molybdenum nitrogenase

(Figure 1) has eight transition metals in a cluster type (M₈S₉C

or M₈S₉N or M₈S₉O) that is unknown in synthetic chemistry.^15,16

Second, the crystallographically characterized form of the

FeMoco (M^N) does not have hydrides, as shown by ENDOR:

hydrides are only incorporated concurrent with catalytic turn-

over.⁵In any case, X-ray crystal structures of proteins do not

have sufﬁcient resolution to distinguish hydrogen atoms. Third,

the N₂-binding form of the FeMoco is reduced by three to four

electrons from M^N, and the FeMoco may undergo structural

Our studies on low-coordinate iron are complemented by

those of Peters and co-workers, who use tridentate, strong-ﬁeld

tris(phosphino)borate (BP₃) supporting ligands that contain

“soft” phosphine donors, and more often give low-spin electronic

conﬁgurations.²²In the BP₃systems, it has not yet been possible

to isolate a low-coordinate iron hydride, but the presence of a

terminal hydride in four-coordinate complexes is strongly

(9) Poli, R. Paramagnetic Mono- and Polyhydrides of the Transition

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J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6625

A R T I C L E S

Yu et al.

implied by the isolation of products that result from activation

of solvent or the supporting ligand.²³Bridging hydrides have

recently been observed in a few electronically unsaturated

dinuclear complexes,^24,25and borohydrides have been studied

on iron-sulfur clusters.²⁶

Recent contributions from our group have described the

reaction of [L^tBuFeH]₂with azobenzene to cleave the NdN

bond,^20,27and the reaction of [L^MeFeH]₂with boranes to cleave

B-C bonds.²⁸This manuscript reports a wider variety of

reactions with unsaturated small molecules that are nitrogenase

substrates or organic-soluble mimics of these substrates. By

examining the reactions with representative compounds, one

may begin to understand some of the unusual reaction patterns

available to hydride complexes with a high-spin electronic

conﬁguration. In addition, a number of the products have novel

structural and/or electronic features that are interesting from a

fundamental perspective.

In contrast, the Mo¨ssbauer parameters of the major component,

particularly the isomer shift, are in the range of high-spin iron(II)

diketiminate complexes (δ ) 0.62-0.86 mm/s), and so it is

assigned to 1a.^19,29Magnetic Mo¨ssbauer spectra measured at

4.2 K with applied ﬁelds up to 7 T show that the major

component is diamagnetic, suggesting antiferromagnetic cou-

pling between the two iron(II) ions. In sum, the spectroscopic

data support the formulation of 1a as an exchange-coupled pair

of high-spin iron(II) ions with a ground state of S ) 0.

1

The H NMR spectrum of 1b has at least 17 overlapping

peaks, because of extreme steric crowding in the dimer that

presumably renders some ligand bond rotations slow on the

NMR time scale.²⁷Upon heating a solution of 1b in C₆D₆, there

is growth of a simple seven-line ¹H NMR spectrum with

chemical shifts much like trigonal-planar alkyl complexes

L^tBuFeR.²⁰This behavior is ascribable to equilibration between

dimeric [L^tBuFeH]₂and monomeric L^tBuFeH that is slow on the

NMR time scale (ms) but rapid on the time scale of the

equilibration (min). Therefore, in the discussion below it should

be understood that 1b does not refer speciﬁcally to the monomer

or dimer, but the equilibrating mixture of the two.³⁰

Results

Iron-Hydride Starting Materials. The synthesis and charac-

terization of [L^MeFeH]₂(1a) and [L^tBuFeH]₂(1b) have been

presented previously.^20,21,27,28They are each synthesized from

iron(II) chloride complexes ([L^MeFeCl]₂or L^tBuFeCl) by reaction

with potassium triethylborohydride for 0.5 h in toluene, followed

by the prompt removal of solvent and BEt₃, then extraction with

pentane and crystallization to give brown powders or crystals.

Crude reaction mixtures are contaminated with the chloride

starting material and the dihydridoborate complex L^RFe(µ-

H)₂BEt₂.²⁸Multiple crystallizations are typically necessary to

remove these impurities, and ¹H NMR spectroscopy is used to

judge purity of the hydride complexes. The good yields of many

of the products derived therefrom support the purity of 1a and

1b.

Reactions with Ct N Triple Bonds, and with N₂and CO.

Carbon monoxide (CO) is an inhibitor of N₂reduction by

nitrogenase.³Methyl isocyanide (CH₃NC) and cyanide (CN^-)

are also inhibitors but are substrates as well: CN^-is reduced to

CH₃NH₂, NH₃, and CH₄, while CH₃NC is reduced to CH₃NH₂,

(CH₃)₂NH, and CH₄.³¹Therefore, it is of interest to examine

the reactivity of small molecules containing CO and CN triple

bonds with synthetic hydride complexes that mimic potential

activated forms of the FeMoco. We use alkyl cyanides as

surrogates for CN^-and tert-butyl isocyanide in place of CH₃NC.

The addition of CO or isocyanide to 1a results in the rapid

release of H₂. Adding an excess of CO to 1a gives L^MeFe(CO)₃

through formal reduction of the iron. Because L^MeFe(CO)₃has

been characterized previously, the reader is referred to the earlier

paper for its properties.⁴⁷

1

The H NMR spectrum of 1a consists of seven resonances;

the number of signals and their integrations are characteristic

of the diketiminate ligand in a local C_2Vsymmetry environment.

It has no unusual temperature- or concentration-dependent

changes in its ¹H NMR or UV-vis spectra, suggesting that the

dimeric structure in the solid state is always maintained. To

learn more about the electronic structure of 1a, we examined a

solid sample using Mo¨ssbauer spectroscopy. The zero-ﬁeld

Mo¨ssbauer spectrum at 80 K (Supporting Information, Figure

S-6) exhibits a superposition of two quadrupole doublets with

72% and 28% relative intensities. The major component has δ

) 0.70(2) mm/s and ∆E_Q) 0.86(2) mm/s, whereas the minor

component shows δ ) 0.49(2) mm/s and ∆E_Q) 2.06(2) mm/

s. The values for the minor component are relatively similar to

those of a high-spin Fe(I) diketiminate complex (∆E_Q) 2.02

mm/s, δ ) 0.41 mm/s at 180 K).^19cTherefore, we assign the

component to contamination from an unidentiﬁed Fe(I) impurity.

Addition of a large excess of tert-butyl isocyanide gives an

intractable mixture, but treatment of a solution of 1a in pentane

or toluene with 4 equiv (per dimer) of tert-butyl isocyanide

results in the formation of a mononuclear iron(I) complex,

L^MeFe(CN^tBu)₂(2a). Integration of the ¹H NMR spectrum

against an internal standard (L^tBuFeCl in a capillary) indicated

62% conversion to 2a, among other unidentiﬁed products. The

production of H₂was quantiﬁed by GC to be ca. 0.2 equiv per

mole of 1a. The low yield of dihydrogen may be due to

hydrogen incorporation into some of the unidentiﬁed products.

Because of the low conversion, samples of 2a for further

spectroscopic study were generated through an alternative

t

method, by adding 4 equiv of BuNC to L^MeFeNNFeL^Me. The

(23) (a) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538–

4539. (b) Lu, C. C.; Saouma, C. T.; Day, M. W.; Peters, J. C. J. Am.

Chem. Soc. 2007, 129, 4–5.

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3122. (b) Nowik, I.; Herber, R. H. J. Phys. Chem. Solids 2003, 64,

313–317.

(30) Differences between the behaviors of 1a and 1b are attributed to steric

effects : Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory,

E. A.; Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L. Organo-

metallics 2004, 23, 5226–5239.

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(31) (a) Li, J.-G.; Burgess, B. K.; Corbin, J. L. Biochemistry 1982, 21,

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Biochemistry 1983, 22, 6260–6268. (c) Lowe, D. J.; Fisher, K.;

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6626 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Iron-Hydride Reactivity

A R T I C L E S

show bent isocyanide ligands (CNC angles of 164.0(7)° and

148.3(7)°). The reﬁnement model required that geometrical

restraints be placed on the N^tBu portions of the two ligand

disorder components, speciﬁcally that the N-C(^tBu) bond

lengths and angles in both components be similar. The result is

a disorder model that conﬁrms the CNC bending, but that should

be used with caution with regard to the exact values of the

angles.

The ambiguous spectroscopic and crystallographic data left

unanswered questions about 2a. Therefore, hybrid quantum

mechanics/molecular mechanics (QM/MM) computations were

performed to study complete models of 2a. The optimized

geometry of the doublet state (E_relative) 0) is square planar about

iron, but the higher energy quartet state (E_relative) 3.9 kcal/

mol) has a tetrahedral coordination geometry upon QM/MM

geometry optimization. The calculated metrical parameters for

doublet L^MeFe(CN^tBu)₂(compared to experimental ones in

parentheses) are Fe-C ) 1.83 Å (1.82 Å); CN ) 1.18 Å (1.15,

1.17, 1.22 Å). Both the lower calculated energy and the

geometrical similarity to the experimental structure support the

Figure 3. Molecular structure of L^MeFe(CN^tBu)₂(2a), showing 50%

thermal ellipsoids. There is 50:50 disorder in the position of one isocyanide

ligand. Selected bond distances (Å) and angles (°): Fe-C14 1.817(1),

Fe-C15 1.821(1), C14-N14 1.222(9), C14-N14′ 1.153(9), C15-N15

1.173(2), C14-N14-C24 148.3(7), C14-N14′–C24′ 164.0(7), C15-N15-

C25 170.1(2).

1

contention that 2a has an S ) /₂ground state.

¹H NMR and X-band EPR spectra of samples prepared in this

way were identical to the spectra of material generated from

1a.

The computations also address the unusual difference between

the CNC angles of the two isocyanide ligands (we denote the

difference between these CNC angles as θ). The optimized

geometry had CNC angles of 171° (close to the crystallographic

angle of 170°) and 162° (between the observed 164° and 148°

disorder components). A search of the Cambridge Structural

Database for neutral, monometallic complexes with at least two

^tBuNC ligands indicated that the average θ is 6.9 ( 10.1° (170

examples) with a median value of 2.9°.³⁴Hence, our computed

value of θ ) 9°, while somewhat large, is not outside

experimental norms. Computations indicate that bending the

CNC angle from 180° to 150° requires 4 kcal/mol in free ^tBuNC

and only 0.4 kcal/mol in the QM/MM model of 2a.³⁵Consider-

ing the softness of this bending distortion, it is not surprising

that the isocyanide is unusually ﬂexible and can exist in multiple

geometries in the solid state.

Solutions of the isocyanide complex 2a decompose over

several hours in solution at room temperature, making the results

of bulk magnetic studies unreliable, but EPR spectra of frozen

mixtures of L^MeFeNNFeL^Meand 1-4 equiv of ^tBuNC in toluene

show a rhombic signal with g ) 2.45, 2.24, 1.98, suggesting

that 2a has an S ) ¹/₂ground state.^32,33In the solid-state infrared

spectrum of 2a, four stretching bands are observed at 2122,

2050, 1969, and 1948 cm^-1. Some bands almost certainly derive

from decomposition products, because bands at 2050, 1969, and

1948 cm^-1are observed in the IR spectrum of a sample from

allowing 2a to completely decompose. Therefore, only the peak

at 2122 cm^-1can be assigned conﬁdently as a Ct N stretching

vibration of 2a. Computations (see below for details) predict

the second stretching vibration at 2050 cm^-1after correction

for anharmonicity, so it is possible that the peak at 2050 cm^-1

derives from 2a as well as a decomposition product.

In the next section, we turn to reactions with nitriles, mimics

of cyanide with substituents on the carbon. Scheme 1 shows

t

the reduction products obtained from CH₃CN and BuCN.

The product has also been characterized by X-ray diffraction

of a crystal grown at low temperature. The solid-state structure

of 2a is illustrated in Figure 3. The iron atom has a square planar

geometry, with the sum of angles 360.4(5)°. The two isocyanides

are not identical: one CNC angle is almost linear (170.1(2)°)

while the other is bent. The bent isocyanide ligand is disordered;

two positions of the ^tBu group were observed, and the ratio of

components is exactly 50:50 owing to the presence of a

crystallographic inversion center. Both reﬁned ligand positions

Heating 1a with 2 equiv of CH₃CN in toluene at 45 °C for

Scheme 1

(32) The X-band EPR spectrum of the solutions are consistent with a

mixture of at least two compounds. In addition to the signal described

in the text, there is a less rhombic signal with g ) 2.08, 2.06, 2.00

(see Supporting Information). These signals are almost identical to

33

those previously identiﬁed as L^tBuFe(CO)₂and L^tBuFe(CO)₃,⁴⁷

supporting the assignment of the more rhombic signal as L^Me

-

Fe(CN^tBu)₂and the less rhombic signal as L^MeFe(CN^tBu)₃. The

solution behavior of iron(I) isocyanide complexes will be described

at length in a future publication.

(33) Sadique, A. R.; Brennessel, W. W.; Holland, P. L. Inorg. Chem. 2008,

47, 784–786.

(34) Cambridge Structural Database, 2006 update: Allen, F. H. Acta

Crystallogr. 2002, B58, 380. We used only high-quality (i.e., R <

10%) non-polymeric structures with no crystallographic disorder.

(35) Similarly soft bending potentials have been seen in metal-imido

complexes: Cundari, T. R.; Russo, M. J. Chem. Inf. Comput. Sci. 2001,

41, 281-287.

9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6627

A R T I C L E S

Yu et al.

Figure 5. Molecular structure of 5b, showing 50% thermal ellipsoids. Most

hydrogen atoms have been omitted for clarity. Selected bond distances (Å)

and angles (deg): Fe-N14, 1.857(2); Fe-N11, 1.977(2); Fe-N21, 1.966(2);

N14-C14, 1.256(3); N11-Fe-N21, 94.69(6); N11-Fe-N14, 126.53(7);

N21-Fe-N14, 138.70(7); Fe-N14-C14, 143.0(2).

Table 1. Comparisons among C≡N Insertion Products

property

3a

4a

5b

Symmetry from ¹H

NMR at RT

C_2h

C_s

C_2V

Fe· · ·Fe distance (Å)

NdC distance (Å)

Dihedral angle

3.0790(5)

1.258(2)

0^a

2.7816(8)

1.258(4)

81.09(9),

80.6 (1)

N/A

1.256(3)

N/A

between ꢀ-diketiminate

ligand planes (deg)

NdC stretching

Figure 4. Molecular structures of 3a (top) and 4a (bottom), showing 50%

thermal ellipsoids. The aryl isopropyl groups are omitted for clarity. Selected

bond distances (Å) and angles (deg) for 3a: Fe-N3, 2.041(2); Fe-N3′,

2.079(2); N3-C14, 1.258(2); Fe-N3-Fe′, 96.72(6); N3-C14-C24, 125.4

(2). For 4a: Fe1-Fe2, 2.7816(8); Fe1-N5, 2.000(3); Fe2-N5, 2.016(3);

Fe1-H1, 1.69(3); Fe2-H1, 1.67(3); N5-C15, 1.258(4); Fe1-N5-Fe2,

87.7(1); N5-C15-C25, 129.3(3).

1637

1629

1687

frequency (cm^-1

N-C-C angle in

imide (deg)

)

125.4(2)

129.3(3)

127.2(3)

1.857(2)

Fe-N(imide)

distance (Å)

2.041(2),

2.079(2)

2.000(3),

2.016(3)

18 h gives an orange-yellow precipitate in 88% yield. The X-ray

crystal structure shows that the product is L^MeFe(µ-

NdCHCH₃)₂FeL^Me(3a) (Figure 4, top), with a crystallographic

inversion center relating the two halves of the molecule. Each

nitrile molecule has been reduced to a bridging NdCHCH₃

ligand with an NdC distance of 1.258(2) Å. The iron centers

in 3a have roughly tetrahedral geometries. The ¹H NMR

spectrum of 3a in toluene-d₈has peaks from 50 to -40 ppm,

and the number and integrations are consistent with the C_2h

symmetry in the crystal structure.

^aThe two ꢀ-diketiminate ligand planes are related by a crystal-

lographic inversion center.

Another kind of insertion product is observed when 1b is

treated with ^tBuCN. In this case, the orange product is

monomeric L^tBuFe(NdCH^tBu) (5b) in 67% yield. The geometry

around the iron atom is trigonal planar (Figure 5). A CdN

double bond is indicated by the N14-C14 bond length of

1.256(3) Å and the CdN stretching band at 1687 cm^-1. The

Fe-N(imide) bond length is 1.857(2) Å, substantially shorter

than the Fe-N distances in the bridging imides of 3a and 4a.

t

When 1a is treated with the bulkier nitrile BuCN, brown

L^MeFe(µ-NdCH^tBu)(µ-H)FeL^Me(4a) is obtained in 85% yield.

Some triple bonds do not insert into the Fe-H bond. There

is no evidence for thermal reaction of hydride complexes 1a

and 1b with N₂: use of high pressures of dinitrogen (up to 800

psi at 60 °C) gave L^tBuFeOFeL^tBu, the product of reaction with

moisture, as the only product observable by ¹H NMR

spectroscopy.^29bHowever, irradiation of each hydride complex

under 1 atm of N₂with a high-pressure mercury lamp yields

quantitative conversion to L^RFeNNFeL^Rover the course of 18 h

The diiron complex incorporates only one equivalent of the

t

nitrile, even in the presence of excess BuCN. Addition across

an Nt C bond has formed a bridging NdCH^tBu ligand, while

the other bridging hydride remains untouched. The crystal

t

structure of 4a (Figure 4, bottom) indicates that the large Bu

group pushes the two ꢀ-diketiminate ligands toward the bridging

hydride, which signiﬁcantly reduces the dihedral angle between

the two ꢀ-diketiminate planes to around 81° in each of the two

independent molecules in the crystal structure. This structural

distortion makes the ¹H NMR spectrum more complicated than

in 3a. At ambient temperature, approximately 16 peaks are

observed. Severe overlap between peaks has made attempts to

assign peaks and measure the solution magnetic moment

unsuccessful. At -20 °C, these ∼16 peaks split into ∼25 peaks,

consistent with a reduction of symmetry from C_s(in a

conformation like that in the crystal structure, but with the two

ꢀ-diketiminate ligand planes coplanar on average) to C₁(the

symmetry in the solid state).

1

(1a) or 3 d (1b) as shown by H NMR spectroscopy. These

dinitrogen complexes have a formal oxidation state of +1 at

each iron atom, and therefore result from reduction.³⁶Therefore,

it is possible to induce these hydride complexes to reductively

eliminate H₂and bind N₂, but this transformation requires

photolysis.

(36) Loss of H₂to give the dinitrogen complex is formally a reductive

elimination, but the iron atoms in L^MeFeNNFeL^Meare best described

as Fe²⁺-N₂^2--Fe²⁺because of charge transfer to the N₂ligand. See

ref 19d.

9

6628 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Iron-Hydride Reactivity

A R T I C L E S

Scheme 2

Figure 6. Molecular structure of L^MeFeCPh)C(H)Ph (6a), showing 50%

thermal ellipsoids. Selected bond distances (Å) and angles (deg): Fe-C14,

2.017(2); C14-C24, 1.347(2); C14-C24-C34, 129.0(2); C24-C14-C94,

121.6(2).

Table 2. Effect of [PhC≡CPh] on the Rate of Reaction with 1a To

Give 6a^a

[PhC≡CPh] (mM)

[PhC≡CPh]/[1a]

k_obs(s^-1

)

Reactions with Ct C Triple Bonds. Acetylene is the most

commonly studied unnatural substrate of nitrogenases, and it is

reduced to ethylene.³⁷Mutant nitrogenase enzymes can also

reduce longer-chain alkynes.^38,39Here, we examine the reactivity

of the low-coordinate iron hydride complexes with alkynes

diphenylacetylene and 3-hexyne.

184

362

725

10.5

20.6

41.2

1.7(1) × 10^-3

1.6(1) × 10^-3

1.7(2) × 10^-3

^a[1a] ) 17.6 mM in C₆D₆at 30.8 °C. Details of the reaction of 1b

with 3-hexyne are given in ref 20.

Complex 1a inserts into the C–C triple bond in PhCCPh to

form L^MeFeC(Ph)dC(H)Ph (6a) in 75% yield. This vinyl

product is analogous to L^tBuFeC(Et)dC(H)Et (6b), which was

previously reported from the reaction of 1b with 3-hexyne.²⁰

Figure 6 presents the crystal structure of 6a. The C-C distance

(1.347(2) Å) and C-C-C angles (129.0(2)°, 121.6(2)°) in the

vinyl ligand conﬁrm that both carbons of the CdC double bond

as proposed for the reaction of 1b with 3-hexyne.²⁰This pathway

is reasonable for 1b, because the monomer is rapidly accessed

at room temperature (see earlier). The rate of the reaction is

consistent with the rapid equilibrium of monomer and dimer

observed previously.²⁰So, although the evidence is not deﬁni-

tive, the kinetic data here and elsewhere are most indicative of

the pathway A in the reactions of alkynes with 1a, but pathway

B for alkyne reactions with 1b. This difference is consistent

with the greater steric demands of the diketiminate ligands in

1b than 1a.

1

have sp²hybridization. The H NMR spectrum of 6a shows

seven peaks for the ꢀ-diketiminate protons, indicating averaged

C

_2Vsymmetry from rapid rotation around the Fe-C14 bond on

the NMR time scale.

Reactions with CdC Bonds. We previously reported that

L^MeFe(alkyl) complexes with ꢀ-hydrogens can act as sources

of transient hydride species L^MeFe(H)(alkene). These react with

alkenes to give insertion products, and complexes L^MeFeR′

(R′ ) ethyl, 1-propyl, 2-propyl, 1-butyl, 2-butyl, cyclohexyl,

2-phenethyl) were fully characterized.³⁰We have also character-

ized numerous alkyl complexes of iron with the larger L^tBu

ligand.⁴⁰With isolated iron hydride complexes in hand, we

veriﬁed that the addition of alkenes to isolated 1a or 1b gives

alkyl complexes of the same type through [1,2] addition across

the double bond. For example, reaction of 1a with 1-hexene

Monitoring the reaction of 1a with PhCt CPh, or the reaction

of 1b with EtCt CEt, shows no dependence of the reaction rate

on [PhCt CPh] (Table 2). Therefore, the rate laws are rate )

k[1] with ﬁrst-order rate constants 1.7(2) × 10^-3s^-1(1a/

PhCCPh at 31 °C) and 5.0(5) × 10^-4s^-1(1b/EtCCEt at 15

°C). The ﬁrst-order rate law is inconsistent with the interaction

of alkyne with 1a or 1b during or prior to the rate-limiting step

of the reaction. Two mechanistic possibilities consistent with

the rate law are shown in Scheme 2. In pathway A, the opening

of a single Fe-H bond is the rate determining step. We consider

this Fe-H bond opening pathway to be more likely for the

reaction of alkyne with 1a because (1) there is no other evidence

for any monomer form of 1a by NMR or UV-vis spectroscopy;

(2) in the reaction of 1a with boranes, the rate law was ﬁrst-

order in [1a] and ﬁrst-order in [BEt₃] but independent of the

steric demands of the borane or iron complex.²⁸These data were

inconsistent with dissociation of 1a into monomers (which

predicts a half-order dependence on [1a]) and most consistent

with single Fe-H opening, as in the bottom reaction pathway

here. In pathway B, dimer cleavage is the rate determining step,

1

quantitatively gives a 1-hexyliron complex, as judged by H

NMR spectroscopy. It is worth noting that the transient iron

hydride L^MeFe(H)(alkene) also adds across the CdN bonds of

imines (forming L^MeFeNR′CHR₂from R₂C)NR′) and the CdO

bonds of ketones (forming L^MeFeOCHR₂from R₂CdO).³⁰

Because these reactions have already been characterized starting

from the alkyl complexes,³⁰the reactions with isolated hydride

complexes were not investigated further.

Reactions with Azides. The azide ion (N₃^-) is transformed

by nitrogenase into N₂, NH₃, and N₂H₄.⁴⁰Here, substituted

azides AdN₃(Ad ) 1-adamantyl) and Me₃SiN₃are used as

(37) Yates, M. G. “Molybdenum-Dependent Nitrogen Fixation”. In Biologi-

cal Nitrogen Fixation; Stacey, G., Burris, R. H., Evans, H. J., Eds.;

Chapman & Hall: New York, 1992; pp 685-735 .

(38) Dos Santos, P. C.; Igarashi, R. Y.; Lee, H.-I.; Hoffman, B. M.; Seefeldt,

L. C.; Dean, D. R. Acc. Chem. Res. 2005, 38, 208–214.

(39) Lee, H.-I.; Igarashi, R. Y.; Laryukhin, M.; Doan, P. E.; Dos Santos,

P. C.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem.

Soc. 2004, 126, 9563–9569.

(40) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Organometallics 2002,

21, 4808–4814.

(41) (a) Scho¨llhorn, R.; Burris, R. H. Proc. Natl. Acad. Sci. U.S.A. 1967,

57, 1317–1323. (b) Dilworth, M. J.; Thorneley, R. N. F. Biochem. J.

1981, 193, 971–983.

9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6629

A R T I C L E S

Yu et al.

double bond character similar to N-N distances in other

bridging azide complexes.³⁴The solid-state IR spectrum of 7b

shows characteristic azide bands at 2129 and 2081 cm^{-1 45}

.

²⁹Si{¹H} NMR analysis of the volatile products in the reaction

mixture revealed a resonance at -22 ppm for Me₃SiSiMe₃, but

none for Me₃SiH. The production of Me₃SiSiMe₃implies that

two hydrogen atoms were lost. However, H₂was not detected

in the headspace by gas chromatography, so the fate of the

hydrogen atoms is unclear. The coupling of two trimethylsilyl

units strongly implies the intermediacy of Me₃Si• radicals. These

in turn might derive from the attack of an iron radical on

trimethylsilyl azide.⁴⁶To test this idea, the iron(I) sources

L^tBuFeNNFeL^{tBu 47}and L^tBuFeClK(solvent)_x⁴⁷were each treated

separately with Me₃SiN₃. In each case, [L^tBuFeN₃]₂was formed.

Therefore, iron(I) species of this type are reasonable intermedi-

ates that could be formed by homolysis of Fe-H bonds.

Homolysis of Fe-H bonds does not occur spontaneously in 1b

at room temperature, and therefore we infer that coordination

of Me₃SiN₃weakens the Fe-H bond and brings about Fe-H

homolysis.

Figure 7. Molecular structure of L^tBuFe(µ-N₃)₂FeL^tBu(7b), showing 50%

thermal ellipsoids. Selected bond distances (Å) and angles (deg): Fe-N15,

2.061(1); Fe-N35, 2.025(1); Fe-N11, 1.988 (1); Fe-N21, 1.989(1);

N15-N25, 1.167(2); N25-N35, 1.176(2); N11-Fe-N21, 97.10(4);

N15-Fe-N35, 95.17(4); N15-N25-N35, 177.0(1).

Scheme 3

Compound 1b undergoes an insertion reaction with AdN₃to

give 67% yield of a triazenido complex, L^tBuFe(η²-HNNNAd)

(8b), which is shown in Scheme 4 and Figure 8. The bidentate

triazenide and ꢀ-diketiminate ligands are perpendicular to each

other, which gives a distorted tetrahedral geometry around the

metal center. The presence of an N-H band in 8b is conﬁrmed

by the observation of a weak bond at ν_N-Hat 3371 cm^-1in

the infrared spectrum. The ¹H NMR spectrum of 8b is

indicative of C_2Vlocal symmetry at temperatures from room

temperature to -75 °C, despite the C_s-idealized symmetry of

the molecule in the solid state (each face of the diketiminate

ligand should be inequivalent). This observation implies that

there is rapid exchange between the two possible orientations

of the triazenido ligand. Considering the stability of three-

coordinate complexes L^tBuFeX and the large size of the

models. The reactions of organic azides with transition-metal

complexes have been reviewed recently.⁴²

Reactions of complex 1a with Me₃SiN₃give mixtures, as

1

judged from H NMR spectra. However, complex 1b, when

treated with 2 equiv of trimethylsilyl azide in diethyl ether, gives

a dimeric azide complex, L^tBuFe(µ-N₃)₂FeL^tBu(7b) in 79% yield

(Scheme 3). Compound 7b can be prepared independently by

reacting L^tBuFeCl⁴³with sodium azide.

The X-ray crystal structure of 7b (Figure 7) shows that the

compound is dinuclear in the solid state with an Fe · · ·Fe distance

of 5.0966(4) Å. Each iron atom is coordinated to two azide

ions, which bridge the iron atoms in a µ_1,3-end-to-end fashion

with inequivalent iron-nitrogen bond lengths of 2.061(1) and

2.025(1) Å. The eight atoms of the Fe₂N₆core are coplanar, in

contrast to the only other crystallographically characterized end-

to-end bridged iron azide complex, [(PhBP₃)Fe(µ-1,3-N₃)]₂

(PhBP₃) [PhB(CH₂PPh₂)₃]^-), which has a chair conforma-

tion.⁴⁴The dihedral angle between the Fe₂N₆plane and the

Fe₂N₂(diketiminate) plane is 87.41(4)°. The azide ligands are

almost linear (N-N-N angle of 177.0(1)°) and the N-N bond

lengths within the azide groups (1.167(2), 1.176(2) Å) show

Figure 8. Molecular structure of triazenido complex 8b, showing 50%

thermal ellipsoids. Selected bond distances (Å) and angles (deg): Fe-N1,

2.081(2); Fe-N3, 2.078(2); Fe-N11, 1.878(4); Fe-N21, 2.117(4); N1-N2,

1.310(3); N2-N3, 1.285(2); N3-C1, 1.463(7); N11-Fe-N21, 98.32(9);

N1-Fe-N3, 60.46(7); N1-N2-N3, 107.6(2).

(42) Cenini, S.; Gallo, E.; Caselli, A.; Ragaini, F.; Fantauzzi, S.; Piangiolino,

C. Coord. Chem. ReV. 2006, 250, 1234–1253.

(43) (a) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun.

2001, 1542–1543. (b) Smith, J. M.; Lachicotte, R. J.; Holland, P. L.

Organometallics 2002, 21, 4808–4814.

Scheme 4

(44) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 1913–1923.

(45) These are similar to the bands observed in literature compounds: Cortes,

R.; Drillon, M.; Solans, X.; Lezama, L.; Rojo, T. Inorg. Chem. 1997,

36, 677–683.

(46) The homolytic Si-N bond energy in Me₃SiN₃is 76 kcal/mol: Adeosun,

S. O. Inorg. Nucl. Chem. Lett. 1976, 12, 301–305.

(47) Smith, J. M.; Sadique, A. R.; Cundari, T. R.; Rodgers, K. R.; Lukat-

Rodgers, G.; Lachicotte, R. J.; Flaschenriem, C. J.; Vela, J.; Holland,

P. L. J. Am. Chem. Soc. 2006, 128, 756–769.

9

6630 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Iron-Hydride Reactivity

A R T I C L E S

adamantyl group, it seems most reasonable to attribute this

behavior to a rapid, reversible isomerization of the triazenido

ligand from η²to η¹, followed by rotation and recoordination.

Although disubstituted triazenido complexes are well-known

in the literature, there are few examples of triazenido complexes

bearing H as one substituent; they come from insertion of azide

into a hydride complex,^48,49and from protonation of an azide

complex.^50,51Free H₂NNNR compounds are extremely unstable

with respect to loss of N₂. Although compound 8b is thermally

stable for hours in solution at room temperature, it eventually

decomposes to the Fe(II) amido complex L^tBuFeNHAd with loss

of N₂. (For identiﬁcation, L^tBuFeNHAd was synthesized inde-

pendently from L^tBuFeCl and LiNHAd by analogy to other

known iron(II) amido complexes.⁵²) Quantitative transformation

1

from triazenido to amido was observed by H NMR spectros-

copy after heating a sample of 8b in C₆D₆at 80 °C for 5 h.

Reactions with NdN Double Bonds. Diazene (HNdNH) is a

possible intermediate of N₂ﬁxation by nitrogenase. Consistent

with this idea, both HNdNH and CH₃NdNH are nitrogenase

substrates,⁵³and CH₃NdNCH₃is reduced by nitrogenase to give

ammonia, methane, and methylamine.⁵⁴Very recently, hydra-

zine (N₂H₄), methyldiazene (HNdNCH₃) and N₂derived

nitrogenase intermediates have been freeze-trapped.⁵⁵Free

diazenes bearing hydrogen substituents are extremely difﬁcult

to handle, because they decompose in seconds or minutes. As

an alternative approach, we have used the stable diazenes

azobenzene (PhNdNPh) and benzo[c]cinnoline.

Figure 9. Two views of the molecular structure of 9a, showing 50%

thermal ellipsoids. In the picture on the right, aryl groups are removed for

clarity. Selected bond distances (Å) and angles (deg): Fe-N2, 1.967(1);

N2-N2′, 1.385(2); N2-Fe-N2′, 41.22(7). The dihedral angle between the

ꢀ-diketiminate FeNCCNFe plane and the benzo[c]cinnoline plane is

55.35(5)°.

Scheme 5

We recently reported a detailed study of the reaction of 1b

with PhNdNPh, which leads ﬁrst to the [1,2]-addition product

L^tBuFeNPhNHPh, and subsequently to the amido complex

L^tBuFeNHPh.²⁷Mechanistic studies were most consistent with

a radical chain mechanism, mediated by an iron(I) carrier.²⁷

Reaction of the smaller 1a with PhNdNPh at ambient temper-

9a could be isolated in 91% yield from the reaction of

L^MeFeNNFeL^Meand benzo[c]cinnoline.

1

ature gives a mixture of products as judged by the H NMR

The crystal structure of 9a (Figure 9) shows that benzo-

[c]cinnoline binds face-on to iron(I) through the NdN π-bond,

and the N-N distance is 1.138(5) Å. The dihedral angle between

the benzo[c]cinnoline and the ꢀ-diketiminate ligand is 55.35(5)°.

A side-on interaction between iron and an NdN double bond

has been crystallographically conﬁrmed in only two other

compounds. In [Fe(NO)₂{PPh₂CH₂CH₂PPh₂NNAr}][PF₆], the

diazene (NdN 1.403(5)) is constrained to bind using a bidentate

phosphine;⁵⁶and our recently reported L^tBuFe(PhNNPh) has an

NdN distance of 1.398(2) Å.²⁷

spectrum. It has not been possible to purify and isolate these

compounds, but they may be tentatively assigned as L^MeFeN-

PhNHPh and L^MeFeNHPh on the basis of the similarity of their

¹H NMR spectra with the L^tBuanalogues.²⁷Longer reaction

time or heating does not drive the reaction mixture to L^MeFeN-

HPh. Instead decomposition occurs, probably due to the

instability of L^MeFeNHPh.⁵²

We also investigated the reaction of 1a with 2 equiv of

benzo[c]cinnoline, which gave 90% conversion to deep green

9a. The detection of 1.07(3) equiv of H₂by GC is consistent

with the reaction stoichiometry shown in Scheme 5. In this case,

there is no addition across the NdN double bond; rather, H₂is

lost and the benzo[c]cinnoline coordinates to iron. Alternatively,

1

The peaks in the H NMR spectra of C₆D₆solutions of 9a

are unusually broad. Mixing different ratios of 1a and benzo[c]-

cinnoline indicates that the broadness of the peaks increases

with a greater concentration of benzo[c]cinnoline, and becomes

substantially sharper at a 1:1 ratio of 1a to benzo[c]cinnoline

(see Supporting Information for spectra). This behavior suggests

that free and bound benzo[c]cinnoline exchange at a rate near

the NMR time scale (milliseconds). The spectra are substantially

sharper in C₆D₁₂, indicating that exchange of the aromatic with

C₆D₆is part of this process. We have previously characterized

L^MeFe(η⁶–C₆H₆), in which the benzene ligand is bound relatively

weakly (it is displaced rapidly by phosphines, alkenes, and

alkynes).⁵⁷Since benzene competes effectively with benzo[c]-

cinnoline as a ligand, it shows that the heteroaromatic π-ligand

is bound weakly as well.

(48) (a) Burgess, K.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R. J.

Organomet. Chem. 1982, 224, C40–C44. (b) Johnson, B. F. G.; Lewis,

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2006, 45, 5742–5751.

9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6631

A R T I C L E S

Yu et al.

Figure 10. Molecular structure of L^MeFe(µ-OCHO)₂FeL^Me(10a) showing

50% thermal ellipsoids. Isopropyl groups on the aryl rings are omitted for

clarity. The structure of 10b is similar and is shown in the Supporting

Information. Selected bond distances (Å) and angles (deg) for 10a: Fe-O,

1.978(2), 2.006(2); C-O, 1.245(3), 1.246(3); O-Fe-O, 115.57(7); O-C-O,

127.9(2). For 10b: Fe-O, 1.967(1), 1.994(1); C-O 1.248(2), 1.245(2);

O-Fe-O, 106.98(6); O-C-O, 128.2(2).

Figure 11. Molecular structure of 11b, showing 50% thermal ellipsoids.

Selected bond distances (Å) and angles (deg): Fe-N1, 2.1358(9); Fe-N2,

2.0675(8); Fe-N11, 2.0663(8); Fe-N21, 1.9908(8); N1-C1, 1.318 (1);

N2-C1, 1.323(1); N1-Fe-N2, 64.50(3); N11-Fe-N21, 97.44(3);

N1-C1-N2, 116.27(8).

These peaks are consistent with splitting of a isopropyl methine

and one isopropyl methyl resonance, which correspond to the

protons closest to one another in the dimer. Unfortunately, the

poor solubility of 10b, especially at low temperature, prevents

us from fully characterizing this phenomenon. The available

data are consistent with either a monomer-dimer equilibrium,

or a change from slow to rapid ﬂipping of the diketiminate

planes to each side of a dimer (change from C_2hto effective

D_2hpoint group symmetry).

To overcome the issues caused by the low solubility of these

products, we investigated the analogous reaction of diisopropyl

carbodiimide (ⁱPrNdCdNⁱPr) with 1b. This reaction proceeded

rapidly and quantitatively at room temperature to give the

formamidinate complex L^tBuFe(ⁱPrNCHNⁱPr) (11b) in 89%

yield. This compound is monomeric, in contrast to the dimeric

formate complexes derived from CO₂, a result that is most

reasonably attributed to the steric hindrance of the isopropyl

groups on the carbodiimide. In the solid state structure of 11b

(Figure 11) both the diketiminate ligand and the formamidinate

ligand are coordinated in an η²fashion to the Fe atom, giving

a distorted tetrahedral geometry around the metal center. The

N-Fe-N bite angle of each ligand is typical.³⁴

Reactions with Brønsted Acids. As described above, EPR

investigations suggest that an FeMoco species with two hydrides

can lose two equivalents of H₂to return to the hydride-free

resting state.⁶This suggests that some low-coordinate iron

hydride species could be protonated by nearby Brønsted acids

to give H₂. To investigate the susceptibility of synthetic Fe-H

compounds to protonation of the hydride, we explored the

reactions of 1a and 1b with weak acids.

In the presence of excess water, the low-coordinate iron

hydride complexes decompose to intractable mixtures that

contain free ꢀ-diketimine, suggesting that the acid protonates

the R position of the ꢀ-diketiminate ligand. Reaction with

smaller amounts of water often gives mixtures as well. In a

few cases, using solutions of H₂O in tetrahydrofuran gives

mixtures in which one compound predominates, enabling

isolation. For example, we reported that addition of 1 equiv of

H₂O to 1b (0.5 equiv per iron) gives the unique oxodiiron(II)

complex L^tBuFeOFeL^tBuin 71% yield.^29bIn another reaction,

the addition of 2 equiv of water to 1a gives a green iron

complex, L^MeFe(µ-OH)₂FeL^Me(12a) in 67% yield (Scheme 6).

The solid state structure of 12a (Figure 12) has pseudo-D_2h

symmetry, and half of the molecule is related to the other half-

Reactions with Carbon Dioxide and Carbodiimide. After

azide, CO₂is another heterocumulene that is a substrate of

nitrogenase. Seefeldt and co-workers have shown that CO₂is

slowly reduced to CO by the nitrogenase enzyme.⁵⁸

Complexes 1a and 1b each react with 2 equiv of carbon

dioxide at room temperature, generating yellow solids that are

formulated as formate-bridging diiron complexes (10a, 10b)

based on X-ray crystallography (Figure 10). Only a few

crystallographically characterized η²-formato bridging iron

complexes have been reported in the literature. These complexes

were synthesized from iron formate, Fe(O₂CH)₂·2H₂O,⁵⁹reac-

tion of iron metal and formic acid,⁶⁰or Fe(ClO₄)·10H₂O with

NaO₂CH.⁶¹Reactions of CO₂with iron hydride complexes are

uncommon.⁶¹

The formate-bridged diiron complexes are insoluble in

pentane, and only somewhat soluble in aromatic solvents, a

problem that was especially severe for 10b. The solution

magnetic moment of 10a is 8.8(2) µ_B, consistent with two nearly

uncoupled high-spin iron(II) ions, and 10b was too insoluble

1

to derive a reliable value. The H NMR spectrum of 10a at

room temperature has six peaks with chemical shifts that range

from 18 ppm to -60 ppm, suggesting idealized D_2hsymmetry.

Although this observation is consistent with either monomer

1

or dimer in solution, no decoalescence is seen in the H NMR

spectrum of 10a in toluene from -35 to 120 °C, indicating that

the crystallographically observed dimer is the most likely

1

solution species. In 10b, the H NMR spectrum at 85 °C has

only seven peaks that integrate as expected for a fully symmetric

diketiminate ligand, and the chemical shifts are similar to those

in 10a. Interestingly, new peaks appear at lower temperatures.

(58) Seefeldt, L. C.; Rasche, M. E.; Ensign, S. A. Biochemistry 1995, 34,

5382–5389.

(59) (a) Tolman, W. B.; Bino, A.; Lippard, S. J. J. Am. Chem. Soc. 1989,

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S. J. J. Am. Chem. Soc. 1991, 113, 152–164. (c) Sessler, J. L.; Hugdahl,

J. D.; Lynch, V.; Davis, B. Inorg. Chem. 1991, 30, 334–336.

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Dalton Trans. 2002, 2981–2986.

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Lippard, S. J. J. Am. Chem. Soc. 1984, 106, 3653–3667.

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2000, 39, 5632–5638. (b) Field, L. D.; Shaw, W. J.; Turner, P. Chem.

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9

6632 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Iron-Hydride Reactivity

A R T I C L E S

Scheme 6

by a crystallographic inversion center. The metrical parameters

2

compare well with other complexes containing a Fe (µ-OH)₂

core.^34,63Seven paramagnetically shifted peaks are observed

in the ¹H NMR spectrum of 12a, consistent with D_2hsymmetry

in solution. No peak clearly corresponding to the bridging OH

is observed in the ¹H NMR spectrum, but O-H stretching

vibrations are seen at 3709 and 3668 cm^-1in the IR spectrum.

This binuclear complex has a solution magnetic moment of

7.5(1) µ_Bper dimer, suggesting two uncoupled high-spin Fe(II)

centers. Dihydrogen is observed by GC as another product of

the reaction. Unfortunately, the THF and H₂O peaks overlap

with the peak from the internal standard CH₄, which prevented

quantitative measurement of the H₂produced.

Figure 12. Molecular structure of 12a, showing 50% thermal ellipsoids.

Isopropyl groups removed for clarity. Selected bond distances (Å) and angles

(deg): Fe-O1, 2.082(3), 2.059(6); Fe-O1-Fe′, 98.7(2); O1-Fe-O1′,

81.3(2).

mulenes (CO₂, CS₂, carbodiimides) reacting with iron hydrides,

these reactions appear to be facile in one case.⁶⁸

Cyanide compounds (nitriles) are not usually reactive toward

insertion into the Fe-H bonds of iron-hydride complexes, aside

from one example of insertion from a dinuclear iron-carbonyl

complex.⁶⁸Acetonitrile more typically coordinates to iron.⁷⁰In

the ꢀ-diketiminate complexes described here, the iron hydrides

reduce RCN to a RC(H)dN^-ligand under mild conditions.

Interestingly, the outcome of the insertion reactions is dependent

on the steric demands of the ꢀ-diketiminate ligand and the nitrile.

With the bulkiest ꢀ-diketiminate (L^tBu) and nitrile (^tBuCN), the

product is a monomeric complex. With the smallest ꢀ-diketimi-

nate (L^Me) and nitrile (CH₃CN), the product is a dimer with

two bridging imidoyl ligands. Finally, with ^tBuCN and the less

bulky hydride complex, a single insertion occurs and the steric

bulk prevents access of the second nitrile to the remaining

bridging hydride.

The reaction of adamantyl azide with 1b gives a triazenido

complex that results from formal [1,1]-addition. A few late-

metal alkyl complexes have been observed to give [1,1]-addition

to azides.⁷⁰The lone example of a triazenido ligand on iron is

in a dinuclear iron-carbonyl complex, where the triazenide

derives from protonation of an anionic azide complex.⁷¹To our

knowledge, the only other reaction of an iron hydride complex

with an azide involves the addition of sodium azide to L₂FeH₂

Finally, we tested 2,6-diisopropylaniline (dippNH₂), which

has a similar acidity as water in organic solvents (the pK_avalues

in DMSO are 32 for H₂O and 30.6 for PhNH₂).⁶⁴A mixture of

1a and dippNH₂in C₆D₆quantitatively gives the iron amido

complex L^MeFeNHdipp within a few hours (Scheme 6), as

1

judged by H NMR spectroscopy. Dihydrogen (1.6 equiv) is

detected from the reaction, close to the 2 equiv expected. This

iron amido complex was previously synthesized from [L^Me

FeCl]₂.⁵²

-

Discussion

Addition of Fe-H across a Multiple Bond. The low coordi-

nation number at the iron atoms in these hydride complexes

enables reactions with many unsaturated organic compounds.

Rapid [1,2] additions are seen with alkenes, alkynes, imines,

ketones, nitriles, carbodiimides, and carbon dioxide. These

reactions generally place the iron fragment on the more

electronegative atom of the double bond, and the hydrogen on

the less electronegative atom. These are not uncommon reactions

of transition-metal hydride complexes,¹but they appear to be

quite rapid in this system relative to literature iron complexes

of higher coordination number. For example, a mixture of trans-

[FeHCl(dppe)₂] and TlBF₄reacts only with alkynes in which

there is an ester functionality that can chelate to the metal.⁶⁵

The reactions of phosphine-supported octahedral iron hydrides

with alkynes often give a variety of products, including acetylide

and vinylidene complexes.⁶⁶Known saturated iron hydride

complexes do not react with alkenes⁶⁶except cyclopropene.⁶⁷

Although there are not many literature examples of heterocu-

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(63) Another coordinatively unsaturated bis(µ-hydroxo)diiron(II) complex

has more acute angles at iron, presumably from steric clashes between

bulky tris(pyrazolyl)borate ligands: Kisko, J. L.; Hascall, T.; Parkin,

G. J. Am. Chem. Soc. 1998, 120, 10561-10562.

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9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6633

A R T I C L E S

Yu et al.

(L ) bidentate phosphine ligand) in methanol, which replaces

the hydride ligand with azide.⁷³A search of the CSD shows

that L^tBuFe(HNNNAd) is the ﬁrst structurally characterized

complex of a triazenido ligand bearing a hydrogen atom.³⁴

Mechanistic and spectroscopic studies indicated the formation

of hafnium, rhenium, and tungsten triazenido complexes from

the reaction of azides with hydride species.^51,74In the hafnium

complex, mechanistic studies indicated that the HNNN(aryl)^-

ligand decomposes to NH(aryl)^-through a shift of the hydrogen

atom to the carbon-bound nitrogen atom, followed by extrusion

of N₂. In the iron system studied here, a similar mechanism

can explain the conversion of the triazenido complex to

L^tBuFeNHAd.

Recent reports of other low-coordinate iron systems are also

indicative of high reactivity, although hydride species were not

isolable in these systems. For example, Chirik has used low-

coordinate pyridinediimine-iron and R-diimine-iron complexes

as precatalysts for catalytic alkene hydrogenations, though

hydride complexes were not isolated.⁷⁵Peters has proposed a

transient four-coordinate iron hydride complex that adds across

an aromatic CdC bond of benzene,⁷⁶and Ohki and Tatsumi

have proposed the intermediacy of Cp*FeH in the cleavage of

NdN bonds.⁷⁷These precedents imply that the high activity of

the isolable low-coordinate iron-hydride complexes described

here is likely to have broader generality to other supporting

ligands.

Reductive Elimination of Dihydrogen. Numerous polyhydride

complexes, both monometallic and multimetallic, are capable

of formal reduction through loss of H₂when a ligand is added.⁷⁸

Some synthetic hydride complexes have been observed to lose

H₂with the addition of N₂.^79,80Speciﬁcally with iron, a few

polyhydride complexes reductively eliminate H₂with ligand

binding.^25,81Iron-dihydrogen complexes have been studied in

octahedral systems.⁸²The reactions typically involve reduction

of iron(II) to iron(0), not to iron(I) as observed here. In a recent

paper, Peters has presented evidence supporting reversible

metalation of a ligand alkyl group in a low-coordinate iron(I)

complex.^23b

We see no evidence of stable H₂complexes in thermal or

photochemical elimination reactions of the diketiminate-sup-

ported iron hydride complexes, because any putative transient

“LFe” or “LFe(H₂)” is trapped by N₂(in the photochemical

reaction) or the added ligand (thermal reactions with CO or

CNR). The thermal reactions must proceed through a mechanism

in which substrate binding precedes H₂loss, because 1a and

1b are stable at room temperature in solution and under vacuum.

We infer that interaction with ligands activates the iron hydride

species toward H₂loss. As described above in the context of

alkene/alkyne insertion, the available evidence suggests that

[L^MeFeH]₂reacts through a low-population isomer in which one

iron-hydrogen bond has opened in the Fe₂H₂core. The

subsequent L^MeFe(substrate)(µ-H)Fe(H)L^Mecomplex could be

unstable with respect to breaking the remaining Fe-H bond to

give an iron(I) product and an unstable iron(III) dihydride

complex L^MeFeH₂, which could lose H₂leading to the remaining

iron(I) product. Though other mechanisms are consistent with

the data, this is a working model for further studies of these

reactions; it also rationalizes why the stronger ligands and those

more resistant to insertion reactions (isocyanides, benzo[c]cin-

noline, some azides) lead to iron(I) products. In the case of

Me₃SiN₃, the product is not the iron(I) compound itself, but

instead the result of reductive coupling of trimethylsilyl groups

in Me₃SiN₃. Because the reductive coupling product also results

from treatment of Me₃SiN₃with the iron(I) source LFeNNFeL,

iron(I) species are likely intermediates in this reaction.

The product of the reaction of 1a with benzo[c]cinnoline is

especially interesting. Despite the presence of lone pairs on the

two nitrogen atoms of this heterocycle, the iron coordinates

instead to the π-system. There are only four previous crystal

structures in which benzo[c]cinnoline is a ligand to a transition

metal: Ti and Yb complexes with the metal in the plane of the

aromatic rings,⁸³a complex with the diazene bridging the iron

atoms in a Fe₂(CO)₆fragment,⁸⁴and the cobalt(0) complex

(PMe₃)₃Co(benzo[c]cinnoline).⁸⁵The latter complex is the most

similar to the one shown here, from its formal d⁷electronic

conﬁguration to the similar angle between the MN₂and aromatic

planes (68° vs. 55° here).⁸⁵We have previously shown that

benzene coordinates in a η⁶binding mode in L^MeFe(C₆H₆),⁴⁷

but the η²binding of benzo[c]cinnoline is clearly stronger, since

the benzo[c]cinnoline complex is stable in benzene solution.

Why, then, is the binding stronger for the more hindered benzo[c]-

cinnoline? Previous studies on the binding constants of alkenes,

alkynes, and other ligands to the iron(I) L^MeFe fragment showed

that backbonding dominates the selectivity of binding.⁵⁷Because

of the greater electronegativity of nitrogen than carbon, the

stabilizing shift of electron density from the iron to the

unsaturated ligand is more effective in the iron(I) complex of

benzo[c]cinnoline.

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We also note that strongly hydridic M-H bonds like those

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2006, 128, 9472–9483. (b) Fryzuk, M. D.; MacKay, B. A.; Patrick,

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Iron-Hydride Reactivity

A R T I C L E S

drides,⁸⁶consistent with the Fe^δ+-X^δ-character of iron alkyl,

amido, and alkoxo complexes demonstrated in our earlier

studies,^30,52,57Further research will show whether the reactivity

of other low-coordinate late-metal systems will also tend to

mimic the behavior of transition metals with a lower d electron

count.

high-spin electronic conﬁguration, these factors are clearly

identiﬁed as reasonable targets for evaluation in the enzyme.

It is notable that nitrogenase inhibitors (or analogues thereof)

like CO, RNC, and RN₃typically cause the synthetic hydride

complexes 1a and 1b to lose H₂and bind the small molecule

that was added. Therefore, one may hypothesize that these small

molecules might inhibit enzymatic N₂reduction by binding

strongly to the FeMoco, causing premature loss of H₂. The

addition of 1a and 1b to small molecules that are substrates of

nitrogenase (or analogues thereof), like RCN, alkynes, RN₃, and

RNdNR, typically forms a new substrate-H bond, and no H₂

is lost. An exception is benzo[c]cinnoline: although it mimics

the nitrogenase substrate diazene, it displaces H₂like an

inhibitor. This may be attributed to the aromatic nature of

benzo[c]cinnoline,⁹⁰which perhaps makes it more resistant to

reduction than other diazenes. Azides and cyanides are inhibitors

and substrates in the enzyme, and in the synthetic compounds

they sometimes give H₂and sometimes are reduced. Therefore,

the reactions of the synthetic compounds 1a and 1b recall the

bifurcated reactivities of these molecules with the enzyme.

It is also worth comparing the speciﬁc products of substrate

reduction by nitrogenase and the products from the synthetic

iron complexes. The synthetic compounds typically do not

complete the reduction and release the appropriate product, but

maintain a bond between the iron atom and the partially reduced

substrate. For example, 1a and 1b reduce alkynes to iron-vinyl

complexes, but do not release alkene. They reduce one CdO

bond of CO₂, but do not eliminate water to give CO.⁹¹They

reduce Ct N bonds to CdN bonds, but do not continue the

reduction to methylamine or methane/ammonia. These outcomes

may be attributable to the absence of protons in the synthetic

system that would release the anionic ligand, either as a product

or as an intermediate that is subsequently reduced to the ﬁnal

product. Future studies will examine the further transformations

of these small molecules at iron.

Protonation To Give Dihydrogen. Many iron-hydride com-

plexes are protonated by acids to release H₂. For example, in

iron complexes of a PNP ligand, the necessary pK_aof the acid

was determined to be less than 10.5 (a coordinated nitrogen

was protonated in preference to the hydride ligand).^87,88On the

other hand, the diketiminate-bound iron hydrides are protonated

by weak acids such as diisopropylaniline and diphenylhydra-

zine.²⁷Using literature values in DMSO,⁶⁴one can conclude

that acids with pK_avalue up to 32 (and perhaps higher) can

protonate 1a and 1b.

Protonation of a hydride ligand may be a key step in nitrogen

ﬁxation by nitrogenases. In this hypothesis, one of the metal-

bound hydride ligands is protonated and released as dihydrogen

to open a coordination site for dinitrogen binding.⁸⁹This

hypothesis is consistent with Hoffman’s ENDOR-based conclu-

sion that the N₂binding form of the iron-molybdenum cofactor

is at the E₄H₂state.⁶There are a number of acidic Arg residues

near the binding face of the iron-molybdenum cofactor.³This

hypothesis ﬁnds support here, in that weak acids are capable of

protonating the hydride ligand to form H₂.

Insight into Potential Nitrogenase Mechanisms. Some nitro-

genase researchers have explained the production of H₂by

nitrogenase during N₂reduction as the result of ligand-assisted

reductive elimination.³In this model, creation of hydrides

provides a way for the FeMoco to store electrons for N₂

reduction without reaching an unrealistically low charge and

reduction potential. Release of H₂from the FeMoco is thought

to be concurrent with N₂binding, explaining why H₂is a

competitive inhibitor of N₂reduction.⁷Other substrates are not

inhibited by H₂because the extra “boost” in reducing power

from H₂loss is not necessary. The reactivity of complexes 1a

and 1b can be viewed using this model, in that they have

sufﬁcient reducing power to react thermally with all nitrogenase

substrates except N₂. Thus, the reactivity of compounds 1

mimics not the E₄state, but a less reduced FeMoco intermediate

(E₂or E₃) that reduces alternative substrates like alkynes, CO₂,

cyanide, and azide. Photolysis of the hydride complexes 1a and

1b provides the “boost” required to bind the weak ligand N₂.

Conclusions

In summary, we have shown that the ﬁrst isolable low-

coordinate iron hydride complexes are highly reactive toward

various unsaturated small molecules. The reactions can be

categorized into three types: (1) insertions of a multiple bond

of the substrate into the Fe-H bond with two-electron reduction

of the substrate; (2) reductive elimination of H₂and coordination

(or one-electron reduction) of the substrate; (3) loss of H^-to

an acidic proton of the substrate, releasing H₂and giving an

iron complex of the conjugate base. Some of the products

isolated have novel structural features, such as a square-planar

iron(I) complex, a face-bound complex of benzo[c]cinnoline,

and a monoalkylated triazenido complex. The reaction patterns

are reminiscent of elementary steps in the proposed nitrogenase

catalytic mechanism and support the idea that hydrides on the

low-coordinate, high-spin iron atoms of the FeMoco would be

capable of some of the characteristic reactions of the FeMoco.

In addition, these studies identify some possible reasons why

certain small molecules are substrates of nitrogenases and others

are inhibitors.

Even though 1a and 1b do not reduce N₂, their reducing

power is superior to octahedral Fe-H complexes, a difference

that might arise from the low coordination number, the high-

spin electronic conﬁguration, the weak Fe-H bond strength,

or (most likely) a combination of these effects. Considering that

iron atoms in the FeMoco have a low-coordinate geometry and

(86) Hoskin, A. J.; Stephan, D. W. Coord. Chem. ReV. 2002, 233-234,

107–129.

(87) (a) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.;

DuBois, D. L.; Rakowski-DuBois, M. Organometallics 2005, 24,

2481–2491. (b) Jacobsen, G. M.; Shoemaker, R. K.; Rakowski-DuBois,

M.; DuBois, D. L. Organometallics 2007, 26, 4964–4971.

(88) (a) Dihydrogen complexes are often strong acids, for example: Jia,

G.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 875–883. (b)

Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R. H.;

Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375–3388.

(89) (a) Henderson, R. A. Chem. Commun. 1987, 1670–1672. (b) Hlatky,

G. G.; Crabtree, R. H. Coord. Chem. ReV. 1985, 65, 1–48. (c) Oglieve,

K. E.; Henderson, R. A. Chem. Commun. 1992, 441–443.

(90) Husseini, A.; Akasheh, T. S. Dirasat. UniV. Jordan 1985, 12, 65–72.

Experimental Section

General Procedures. All manipulations were performed under

a nitrogen atmosphere by Schlenk techniques or in an M. Braun

(91) We are not aware that any researchers have investigated the reduction

of formate by nitrogenase. This would be a fruitful area of investiga-

tion.

9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6635

A R T I C L E S

Yu et al.

glovebox maintained at or below 1 ppm of O₂and H₂O. Glassware

was dried at 150 °C overnight. NMR data were recorded on a

Bruker Avance 500 spectrometer (500 MHz). All peaks in the NMR

spectra are referenced to residual protiated solvents (benzene δ 7.16

ppm; toluene δ 2.08 ppm; cyclohexane δ 1.38 ppm). In parentheses

are listed integrations and assignments. Resonances were singlets

unless otherwise speciﬁed. Infrared spectra (450-4000 cm^-1) were

recorded on KBr pellet samples in a Shimadzu FTIR spectropho-

tometer (FTIR-8400S) using 32 scans at 2 cm^-1resolution. UV-vis

spectra were measured on a Cary 50 spectrophotometer using screw-

cap cuvettes; descriptions of the spectra include extinction coef-

ﬁcients in parentheses. Solution magnetic susceptibilities were

determined by the Evans method.⁹²Elemental analyses were

determined by Desert Analytics, Tucson, AZ.

(C)N). µ_eff(1% Me₃SiOSiMe₃in THF-d₈, 25 °C): 5.9(1) µ_Bper

dimer. Anal. Calcd for C₆₂H₉₀N₆Fe₂: C, 72.22; H, 8.80; N, 8.15.

Found: C, 72.04; H, 8.98; N, 7.72.

L^MeFe(η¹-NdCH^tBu)(µ-H)FeL^Me(4a). A solution of 1a (100

mg, 0.11 mmol) in pentane (10 mL) was treated with ^tBuCN (12.0

µL, 0.11 mmol) and stirred at room temperature for 2 h. Volatile

materials were removed under vacuum and the residue was extracted

with toluene (30 mL), ﬁltered, and concentrated to 5 mL. Crystal-

1

lization at -35 °C gave 4a as a brown powder (97 mg, 85%). H

NMR (500 MHz, C₆D₆): The narrow range of the NMR spectrum,

the broad peaks, and the low symmetry of the molecule led to severe

overlap between peaks, which made assignment unsuccessful. The

spectrum is shown in the Supporting Information. UV-vis (pen-

tane): 332 (30 mM^-1cm^-1). IR (KBr pellet): 1629 cm^-1(CdN).

Anal. Calcd for C₆₃H₉₃N₅Fe₂: C, 73.31; H, 9.08; N, 6.79. Found:

C, 73.50; H, 9.40; N, 6.46.

Pentane, tetrahydrofuran (THF), diethyl ether, and toluene were

puriﬁed by passage through activated alumina and “deoxygenizer”

columns from Glass Contour Co. (Laguna Beach, CA). Deuterated

solvents were ﬁrst dried over CaH₂, then over Na/benzophenone,

and then vacuum transferred into a storage container. Before use,

an aliquot of each solvent was tested with a drop of sodium

benzophenone ketyl in THF solution. Celite was dried overnight

at 200 °C under vacuum. CO₂was purchased from Air Products,

dried by passing through 4Å molecular sieves, and freed from O₂

with three freeze-pump-thaw cycles. H₂O, ^tBuNC, ^tBuCN, MeCN,

and 2,6-diisopropylphenylamine were degassed before using.

Diphenylacetylene, benno[c]cinnoline, and 1-azidoadamantane

(AdN₃) were crystallized from pentane in the glovebox. Compounds

1a and 1b were prepared by published procedures.^20,21

L

^tBuFe(NdCH^tBu) (5b). A solution of 1b (91 mg, 81 µmol) in

t

diethyl ether (8 mL) was treated with BuCN (18 µL, 163 µmol)

and stirred at room temperature for 30 min. The volatile materials

were removed under vacuum. The orange residue was extracted

with toluene, ﬁltered through Celite, concentrated, and stored at

-35 °C to afford 5b as orange-red crystals (70 mg, 67%). ¹H NMR

t

(500 MHz, C₆D₆): δ 95 (9H, Bu of imide), 92 (1H, backbone

t

C-H), 36 (18H, Bu of diketiminate), 3 (4H, m-Ar), -24 (12H,

i

ⁱPr-CH₃), -99 (4H, Pr-CH), -103 (12H, Pr-CH₃), -108 (2H,

p-Ar) ppm. There was no clear peak for NCH^tBu. UV-vis

(toluene): 335 (25 mM^-1cm^-1), 397 (6.7 mM^-1cm^-1), 454 (sh),

515 (sh) nm. µ_eff(C₆D₆, 298 K): 4.6 µ_B. IR (KBr pellet): 3058 (w),

2960 (vs), 2906 (w), 2867 (m), 2717 (w), 2631 (w), 1687 (s), 1506

(s), 1462 (w), 1433 (w), 1383 (vs), 1362 (vs), 1317 (s), 1255 (w),

1217 (w), 1182 (w), 1151 (w), 1095 (m), 1024 (w) cm^-1. Anal.

Calcd for C₄₀H₆₃FeN₃: C, 74.86; H, 9.89; N, 6.55. Found: C, 74.52;

H, 9.80; N, 6.80.

L^MeFe(CN^tBu)₂(2a). Reaction of 1a and ^tBuNC in C₆D₆gives

complex 2a, in a crude yield of 62% based on ¹H NMR integration

with an internal integration standard. However, it can be more

t

conveniently synthesized from L^MeFeNNFeL^Meand BuNC as

follows. A solution of L^MeFeNNFeL^Me(205 mg, 0.21 mmol) in

L^MeFe-hexyl. In a J. Young NMR tube, 1a (5.6 mg, 5.9 µmol)

and 1-hexene (1.5 µL, 12.1 µmol) were dissolved in 0.5 mL of

C₆D₆. After 1 h, a single paramagnetic species was observed by

¹H NMR spectroscopy: δ 116 (2H), 77 (1H, backbone CH), 51

(2H), 33 (6H, diketiminate CH₃), 32 (3H, hexyl CH₃), 15 (4H),

t

pentane (15 mL) was treated with BuNC (105 µL, 0.93 mmol)

with stirring. Immediate bubbling was observed and the solution

color changed from deep brown to bright orange. The reaction

solution was stirred at ambient temperature for 5 h. Volatile

materials were removed under vacuum and the residue was extracted

with pentane (20 mL) and ﬁltered through Celite. The solution was

concentrated to 3 mL and cooled to -35 °C to give bright orange

i

-19 (12H, Pr-CH₃), -21 (4H), -30 (2H), -44 (2H), -75 (12H,

ⁱPr-CH₃), -91 (2H) ppm. Peaks integrated as 4H could be aryl

m-H or ⁱPr methine CH; peaks integrated as 2H could be aryl p-H,

hexyl ꢀ-CH₂, γ-CH₂, δ-CH₂, or ꢀ-CH₂. The hexyl R-CH₂protons

were not observed in the ¹H NMR spectrum. This ¹H NMR

spectrum is very similar to the spectra of other L^MeFe(alkyl) species

we have previously reported.³⁰

1

crystals of 2a (145 mg, 54%). H NMR (500 MHz, C₆D₆): δ 9.1

i

t

(2H, aryl p-H), 5.8 (4H), 3.5 (12H, Pr-CH₃), 2.5 (18H, Bu), 1.3

(4H), -2.0 (6H, backbone CH₃), -5.8 (12H, ⁱPr-CH₃) ppm. (Peaks

integrated as 4H could be aryl m-H or ⁱPr methine CH; the backbone

CH was not observed). UV-vis (pentane): 317 (25.3 mM^-1cm^-1),

447 (3.9 mM^-1cm^-1), 808 (0.2 mM^-1cm^-1) nm. IR (KBr pellet):

2122 (vs), 2050 (m), 1969 (m), 1948 (m) cm^-1. The X-band EPR

spectrum is shown in the Supporting Information. Attempts to

measure the solution magnetic moment of 2a were not successful

because the internal standard peak was obscured. Complex 2a is

thermally unstable (decomposition of a solid sample occurred

overnight inside the glovebox), and all characterizations were done

with fresh samples. The instability prevented elemental analysis

measurements.

L^MeFe(CPh)CHPh) (6a). A mixture of 1a (88 mg, 93 µmol)

and PhCCPh (33 mg, 187 µmol) in pentane (15 mL) was stirred at

room temperature for 17 h. Volatile materials were removed under

vacuum. The product was extracted with pentane (20 mL), ﬁltered

through Celite, and concentrated to 2 mL. Cooling to -35 °C gave

6a as a yellow powder (87 mg, 75%). ¹H NMR (500 MHz, C₆D₆):

δ 123 (1H), 60 (1H), 59 (2H), 57 (6H, diketiminate CH₃), 51 (1H),

48 (2H), 33 (2H), -9 (4H), -19 (12H, ⁱPr CH₃), -78 (2H), -118

(12H + 4H, ⁱPr CH₃+ another peak, possible assignments are listed

i

L^MeFe(µ-NdCHMe)₂FeL^Me(3a).

A stirring solution of

below) ppm. Peaks integrated as 4H could be aryl m-H or Pr

[L^MeFeH]₂(1a) (146 mg, 0.15 mmol) in toluene (30 mL) was

treated with MeCN (18.0 µL, 0.34 mmol). The reaction solution

was heated at 45 °C for 18 h and the color changed from deep

brown to yellow. All volatile materials were removed under vacuum

and the residue was extracted with toluene (30 mL), ﬁltered, and

concentrated to 5 mL. Cooling to -35 °C gave orange-yellow

methine CH; peaks integrated as 2H could be aryl p-H, vinyl phenyl

m-H (2 peaks for the two phenyl groups), or phenyl o-H; peaks

integrated as 1H could be backbone CH, vinyl phenyl p-H (2 peaks

for the two phenyl groups). The vinyl CdCH and o-H from the

vinyl phenyl closer to the iron were not observed. µ_eff(C₆D₆, 298

K) ) 5.15(2) µ_B. UV-vis (pentane): 320 (37.9 mM^-1cm^-1), 495

(1.5 mM^-1cm^-1) nm.

1

crystals of 3a (155 mg, 88%). H NMR (500 MHz, C₇D₈): δ 50

L

^tBuFe(µ-N₃)₂FeL^tBu(7b). A solution of 1b (71 mg, 64 µmol)

(6H, NdCHCH₃), 20 (12H), 13 (4H), 12 (4H), -2 (12H), -7

(12H), -12 (4H), -16 (4H), -21 (12H), -22 (4H), -31 (2H,

diketiminate backbone), -40 (12H) ppm. (No peak corresponding

to the imine proton was identiﬁed. Peaks integrated as 4H could

be ⁱPr methine, aryl p-H, or aryl o-H; peaks integrated as 12H could

be ⁱPr-CH₃or backbone CH₃). UV-vis (toluene): 337 (34.2 mM^-1

cm^-1), 392 (4.8 mM^-1cm^-1) nm. IR (KBr pellet): 1637 cm^-1

in diethyl ether (10 mL) was treated with Me₃SiN₃(18 µL, 128

µmol) and stirred at room temperature for 2 h. An orange precipitate

formed within 30 min. The volatile materials were removed under

vacuum. The orange residue was extracted with hot toluene and

ﬁltered through Celite while hot to give an orange solution. This

solution was concentrated and cooled to -35 °C to afford 7b as

9

6636 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Iron-Hydride Reactivity

A R T I C L E S

orange crystals (70 mg, 79%). 7b is insoluble in benzene, and

slightly soluble in toluene, at room temperature. ¹H NMR (500

MHz, C₇D₈, 75 °C): δ 15, 1, -3, -24, -46 ppm. The broadness

of the peaks prevented accurate integration and assignment of the

peaks. µ_eff(C₇D₈, 348 K): 4.1 µ_B. IR (KBr pellet): 2962 (s), 2929

(w), 2869 (m), 2129 (vs), 2081 (w), 1529 (w), 1490 (m), 1463

(w), 1436 (w), 1380 (m), 1357 (s), 1317 (s), 1259 (m), 1215(w),

1190 (w), 1097 (m), 1054 (w), 1024 (m) cm^-1. UV-vis (toluene):

337 (15.5 mM^-1cm^-1) nm. Anal. Calcd for C₇₀H₁₀₆N₁₀Fe₂: C,

70.10; H, 8.91; N, 11.68. Found: C, 70.04; H, 8.71; N, 11.02.

L^MeFe(µ-OCHO)₂FeL^Me(10a). A resealable ﬂask was loaded

with 1a (254 mg, 0.27 mmol) and pentane (15 mL). CO₂(256 mbar,

61.7 mL, 0.64 mmol) was condensed in the reaction ﬂask at 77 K

over 30 min. The ﬂask was warmed to room temperature and stirred

for 22 h. Volatile materials were removed from the yellow mixture

under vacuum and the residue was extracted with toluene (30 mL),

ﬁltered, and concentrated to 5 mL. Cooling to -35 °C gave yellow

1

blocks of 10a (138 mg, 50%). H NMR (500 MHz, C₆D₆): δ 18

i

(4H), 5 (12H, Pr CH₃), -11 (12H + 4H, Pr CH₃+ another 4H;

possible assignments for this 4H peak are listed below), -19 (1H,

backbone CH), -42 (2H, aryl p-H), -60 (6H, backbone CH₃) ppm.

(Peaks integrated as 4H could be aryl m-H or ⁱPr methine CH; the

two bridging formate H were not observed in the NMR spectrum).

UV-vis (pentane): 332 (ꢀ ) 29.9 mM^-1cm^-1) nm. IR: 1628 cm^-1

(C-O). µ_eff(C₆D₆, 25 °C): 8.8(1) µ_Bper dimer. Anal. Calcd for

C₆₀H₈₄N₄O₄Fe₂: C, 69.49; H, 8.16; N, 5.40. Found: C, 73.52; H,

9.52; N, 5.62. Repeated attempts at elemental analysis did not give

better agreement, indicating a possible small impurity not visible

by NMR spectroscopy.

L

^tBuFe(HNNNAd) (8b). A solution of 1b (88 mg, 79 µmol) in

diethyl ether (8 mL) was treated with a solution of AdN₃(28 mg,

157 µmol) in diethyl ether (5 mL) and stirred at room temperature

for 30 min to give an orange-red solution. The volatile materials

were removed under vacuum. The residue was extracted with

toluene and ﬁltered through Celite. This solution was concentrated

and cooled to -35 °C to afford L^tBuFe(HNNNAd) as brown crystals

(77 mg, 67%). ¹H NMR (500 MHz, C₆D₆): δ 39 (3H, Ad-R, ꢀ, or

γ), 21 (18H, ^tBu), 15 (4H, m-Ar), 10 (3H, Ad- R, ꢀ, or γ), 8 (3H,

i

Ad-R, ꢀ, or γ), 3.7 (3H, Ad-R, ꢀ, or γ), -7 (12H, Pr-CH₃), -27

L

^tBuFe(µ-OCHO)₂FeL^tBu(10b). This was identical to the

i

(1H, NH), -40 (12H, Pr-CH₃), -51 (3H, Ad-R, ꢀ, or γ), -61

synthesis of 10a, but using 1b (17 mg, 15 µmol), diethyl ether (15

mL), and CO₂(99 mbar; 7.73 mL, 30 µmol). The yield of 10b was

15 mg (82%). ¹H NMR (500 MHz, C₇D₈, 85 °C) δ 30 (1, backbone

(2H, p-H) ppm. UV-vis (toluene): 340 (18 mM^-1cm^-1), 415 (sh),

530 (sh) nm. µ_eff(C₆D₆, 298 K) 4.6 µ_B. IR (KBr pellet): 3371 (w),

3059 (w), 2960 (vs), 2926 (s), 2905 (vs), 2850 (m), 1622 (w), 1585

(w), 1526 (w), 1491 (s), 1464 (m), 1429 (w), 1380 (vs), 1356 (vs),

1311 (m), 1263 (w), 1215 (w), 1182 (w), 1153 (w), 1099 (m), 1022

(m) cm^-1. Elemental analysis could not be obtained for this

compound as 8b is thermally unstable.

t

i

C-H), 19 (18H, Bu), 16 (4H, Pr-CH), -8 (12H, Pr-CH₃), -42

(12H, ⁱPr-CH₃), -50 (2H, p-H or O₂CH), -70 (2H, p-H or O₂CH)

ppm. A 4H peak for the m-Ar protons was not observed, and is

possibly obscured by the solvent. UV-vis (toluene): 339 (43 mM^-1

cm^-1), 393 (6.0 mM^-1cm^-1), 516 (sh) nm. IR (KBr pellet): 3059

(w), 2962 (vs), 2929 (s), 2869 (m), 1626 (vs), 1529 (w), 1493 (s),

1462 (m), 1437 (m), 1381 (vs), 1360 (vs), 1317 (s), 1259 (m), 1215

(w), 1190 (w), 1157 (w), 1099 (s), 1024 (s) cm^-1. Anal. Calcd.

for C₇₂H₁₀₈Fe₂N₄O₄: C, 71.75; H, 9.03; N, 4.65. Found: 72.79; H,

8.63; N, 4.68.

Thermal Conversion of 8b to L^tBuFeNHAd. A sample of 8b

(∼5 mg) in C₆D₆was heated to 80 °C for 5 h. The ¹H NMR

spectrum was identical to a sample prepared independently through

the following method. A mixture of L^tBuFeCl (257 mg, 0.433

mmol), LiNHAd (68 mg, 0.43 mmol), and diethyl ether (10 mL)

was stirred at room temperature for 2 h. The yellow-orange solution

was ﬁltered through Celite, and the volatile materials were removed

under vacuum. The orange residue was dissolved in n-hexane (3

mL), and cooling the solution to -45 °C afforded L^tBuFeNHAd as

L

^tBuFe(ⁱPrNCHNⁱPr) (11b). A solution of 1b (57 mg, 51 µmol)

in diethyl ether (8 mL) was treated with N,N′-diisopropylcarbodi-

imide (15.8 µL, 102 µmol) and stirred for 1 h. The reaction mixture

turned orange within a couple of minutes. The volatile materials

were removed under vacuum. The resultant orange solid was

extracted with toluene, ﬁltered through Celite, concentrated, and

stored at -35 °C to afford orange-red crystals of 11b (62 mg, 89%).

¹H NMR (500 MHz, C₆D₆): δ 142 (2H, p-Ar), 120 (1H, backbone

C-H of diketiminate), 19 (4H, m-Ar or ⁱPr-CH), 10 (18H, ^tBu), 5

(12H, ⁱPr-CH₃), 2.9 (12H, ⁱPr-CH₃), 2.0 (4H, m-H or ⁱPr-CH), -9

(12H, ⁱPr-CH₃), -36 (2H, ⁱPr-CH of amidinate), -85 (1H, backbone

C-H of amidinate) ppm. UV-vis (toluene): 349 (14 mM^-1cm^-1),

405 (10.2 mM^-1cm^-1), 475 (sh) nm. µ_eff(C₆D₆, 298 K) 4.2(3)

µ_B. IR (KBr pellet): 3057 (w), 2962 (vs), 2929 (w), 2867 (m), 1657

(w), 1547 (s), 1479 (m), 1460 (m), 1431 (w), 1382 (vs), 1360 (s),

1317 (m), 1257 (vs), 1191 (w), 1097 (m), 1020 (s) cm^-1. Anal.

Calcd for C₄₂H₆₈FeN₄: C, 73.66; H, 10.01; N, 8.18. Found: C, 73.80;

H, 10.42; N, 8.10.

1

bright orange crystals in 2 crops (230 mg, 75%). H NMR (500

MHz, C₆D₆): δ 95 (6H, Ad-R), 67 (3H, Ad-ꢀ or γ), 42 (3H, Ad-ꢀ

t

or γ), 37 (18H, Bu), 31 (3H, Ad-ꢀ or γ), -1 (4H, m-Ar), -24

(12H, ⁱPr-CH₃), -93 (4H, ⁱPr-CH), -102 (2H, p-Ar), -118 (12H,

ⁱPr-CH₃) ppm. The N-H and backbone C-H protons were not

located. µ_eff(C₆D₆, 298 K): 4.6(3) µ_B. IR (KBr pellet): 3277 (w,

ν

_N-H), 3052 (w), 3013 (w), 2959 (vs), 2925 (s), 2903 (vs), 2845

(m), 1534 (m), 1502 (s), 1459 (m), 1443 (m), 1430 (m), 1385 (vs),

1364 (vs), 1319 (s), 1303 (m), 1253 (w), 1218 (m), 1197 (m), 1131

(m), 1095 (s), 1055 (m), 1029 (m), 948 (w), 933 (w), 890 (w) cm^-1

.

UV-vis (pentane): 340 (14 mM^-1cm^-1), 510 (sh, ∼0.4 mM^-1

cm^-1) nm.

L^MeFe(benzo[c]cinnoline) (9a). Although reaction of 1a and

benzo[c]cinnoline gives complex 9a (yield 90% by ¹H NMR

spectroscopy), it is more easily synthesized from L^MeFeNNFeL^Me

and benzo[c]cinnoline. A vial was loaded with L^MeFeNNFeL^Me

(94 mg, 96 µmol) and benzo[c]cinnoline (35 mg, 190 µmol).

Pentane (15 mL) was added, causing an immediate color change

from brown to green. The reaction solution was stirred at room

temperature for 5 h, ﬁltered through Celite, concentrated to 3 mL,

and cooled to -35 °C to give dark green plates of 9a (113 mg,

91%). ¹H NMR (500 MHz, C₆D₁₂): δ 140 (4H), 111 (4H), 17 (12H,

[L^MeFe(µ-OH)]₂(12a). To a solution of 1a (189 mg, 0.20 mmol)

in THF (10 mL), a solution of H₂O in THF (0.28 mM, 1.45 mL,

0.41 mmol) was added dropwise with stirring. Bubbling was

observed and the solution turned to green. The solution was stirred

at room temperature for 20 h. Volatile materials were removed

under vacuum with heating at 120 °C. The residue was extracted

with pentane (20 mL), ﬁltered, and concentrated to 5 mL. Cooling

to -35 °C gave a green powder, which was further dried under

vacuum with heating at 120 °C for 10 h to give dry 12a (131 mg,

i

ⁱPr-CH₃), -17 (6H, backbone CH₃), -21 (4H), -52 (12H, Pr-

1

i

CH₃), -78 (2H, p-Ar) ppm (peaks integrated as 4H could be m-Ar,

ⁱPr CH, or overlapped peaks for benzo[c]cinnoline ligand). UV-vis:

305 (16.3 mM^-1cm^-1), 325 (13.1 mM^-1cm^-1), 388 (9.7 mM^-1

cm^-1), 419 (7.0 mM^-1cm^-1), 585 (2.3 mM^-1cm^-1) nm. µ_eff

(C₆D₆, 25 °C): 3.1(1) µ_B. Anal. Calcd for C₄₁H₄₉N₄Fe: C, 75.33;

H, 7.55; N, 8.57. Found: C, 73.31; H, 7.68; N, 7.98. Repeated

attempts at elemental analysis did not give better agreement,

indicating a possible small impurity not visible by NMR spectroscopy.

67%). H NMR (500 MHz, C₆D₆): δ 9 (4H), 4 (12 H, Pr-CH₃),

-5 (1H, backbone CH), -6 (6H, diketiminate CH₃), -8 (4H), -33

i

(2H, aryl p-H), -38 (12H, Pr-CH₃) ppm (peaks integrated as 4H

i

could be Pr-CH or aryl o-H; the bridging OH protons are not

observed.). UV-vis (pentane): 326 (27.1 mM^-1cm^-1) nm. IR (KBr

pellet): 3709, 3668 cm^-1(O-H). µ_eff(C₆D₆, 25 °C): 7.5(1) µ_Bper

dimer. Anal. Calcd for C₅₈H₈₄N₄O₂Fe₂: C, 71.01; H, 8.63; N, 5.71.

Found: C, 71.11; H, 8.72; N, 5.53.

9

J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008 6637

A R T I C L E S

Yu et al.

Detection of H₂Using Gas Chromatography.²⁸The reaction

solution from a given reaction in 3 mL of toluene was performed

in a 25 mL round-bottom ﬂask. The ﬂask was capped by an adapter

with a stopcock leading to a rubber septum. Using a syringe, 8 mL

of the gas inside was removed and 8 mL of CH₄(1043 mbar) was

injected into the ﬂask as an internal standard. An aliquot (20 µL)

of the gas was withdrawn and injected into a GC (Shimadzu GC-

17A) with a 5 Å molecular sieve column (30 m × 0.25 mm) at 26

°C, carrier gas N₂, 600 kPa. The ratio of integrated H₂/CH₄

responses were compared to a calibration plot previously deter-

mined²⁸by injecting known amounts of H₂into the same ﬂask with

3 mL of toluene.

Mo¨ssbauer Spectroscopy. Mo¨ssbauer data were recorded on a

spectrometer with alternating constant acceleration. The minimum

experimental line width was 0.24 mm/s (full width at half-height).

The sample temperature was maintained constant either in an

Oxford Instruments Variox or an Oxford Instruments Mo¨ssbauer-

Spectromag cryostat. The latter is a split-pair superconducting

magnet system for applied ﬁelds up to 8 T where the temperature

of the sample can be varied in the range 1.5-250 K. The ﬁeld at

the sample is perpendicular to the γ-beam. The ⁵⁷Co/Rh source

(1.8 GBq) was positioned at room temperature inside the gap of

the magnet system at a zero-ﬁeld position. Isomer shifts are quoted

relative to iron metal at 298 K. Magnetic Mo¨ssbauer spectra for

the paramagnetic contamination of 1a were simulated by using a

spin-Hamiltonian description of the electronic ground state:

Computational Methods. The Gaussian 03 package⁹⁴was used

for all calculations described herein. Hybrid quantum mechanics/

molecular mechanics (QM/MM) calculations were used to study

full experimental models of L^MeFe(CN^tBu)_2.The QM/MM calcula-

tions utilized the ONIOM⁹⁵methodology. The MM region was

modeled with the universal force ﬁeld (UFF)⁹⁶and included the

Ar and Me substituents of L^Me,Arand the methyl groups of ^tBuNC.

The remainder of L^MeFe(CN^tBu)₂was modeled at the B3LYP/6-

311+G(d) level of theory. Geometries were fully optimized using

gradientmethodsunlessotherwisenoted.TheunrestrictedKohn-Sham

formalism was used for the description of all open-shell species.

The energy Hessian was calculated for all stationary points and

thus conﬁrmed the calculated stationary points as minima (no

imaginary frequencies). All reported enthalpies are calculated at 1

atm and 298.15 K.

Acknowledgment. This work was supported by grants from

the National Institutes of Health (Grant GM065313 to P.L.H.) and

the National Science Foundation (Grant CHE-0701247 to T.R.C.),

as well as a Sloan Research Fellowship (P.L.H.) and a Weissberger

Fellowship (Y.Y.). We thank Bernd Mienert for collection of

Mo¨ssbauer data. Calculations employed the UNT computational

chemistry resource, whose purchase was supported by a NSF CRIF

grant (CHE-0342824).

Supporting Information Available: Complete ref 94; spectra

(PDF) and crystallographic data (CIF). This material is available

free of charge via the Internet at http://pubs.acs.org.

H_e) D[S_t,z²- S_t(S_t+ 1)/3 + (E/D)(S_t,x²- S_t,y²)] + µ_BB · S_t

(1)

where S_tis the total spin of the system, and D and E/D are the

axial and rhombic zero-ﬁeld parameters. The hyperﬁne interactions

for ⁵⁷Fe were calculated by using the usual nuclear Hamiltonian.⁹³

For 1a only the nuclear Hamiltonian was used.

JA710669W

(94) Frisch, M. J. et al. Gaussian 03; Gaussian Inc.: Pittsburgh, PA, 1998.

(95) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber,

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Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024–10035. (b) Casewit,

C. J.; Colwell, K. S.; Rappe´, A. K. J. Am. Chem. Soc. 1992, 114,

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Bonding (Berlin) 1991, 78, 1–95.

9

6638 J. AM. CHEM. SOC. VOL. 130, NO. 20, 2008

Article Doi

The reactivity patterns of low-coordinate iron-hydride complexes

DOI: 10.1021/ja710669w

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Article abstract of DOI:10.1021/ja710669w

Full text of DOI:10.1021/ja710669w

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