Mechanistic insight into N=N cleavage by a low-coordinate iron(II) hydride complex

Article abstract of DOI:10.1021/ja069199r

The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N₂ reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin

Full text of DOI:10.1021/ja069199r

Published on Web 06/12/2007
Mechanistic Insight into NdN Cleavage by a Low-Coordinate
Iron(II) Hydride Complex
Azwana R. Sadique, Elizabeth A. Gregory, William W. Brennessel, and
Patrick L. Holland*
Contribution from the Department of Chemistry, UniVersity of Rochester,
Rochester, New York 14627
Received December 21, 2006; E-mail: holland@chem.rochester.edu
Abstract: The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N2
reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However,
almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that
the first high-spin iron hydride complex, LtBuFeH (LtBu ) bulky â-diketiminate), reacts with PhNdNPh to
completely cleave the N-N double bond, giving LtBuFeNHPh. Here, we disclose a series of experiments
that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical
mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare η2-
azobenzene complex. Therefore, this high-spin iron(II) hydride can break N-N bonds through both nonradical
and radical insertion mechanisms, a special feature that enables novel reactivity.
Introduction
Transition metal hydride complexes play a role in numerous
bond transformations.1 One of their characteristic reactions is
transfer of “H-” to substrates like alkenes and ketones, which
results in an overall two-electron reduction of that substrate.
Because nitrogenase substrates (e.g., N2, alkynes, CO2, CN-,
N2O) are all reduced by multiples of two electrons, metal
hydrides have been proposed as intermediates in the catalytic
reduction of N2 to NH3 by nitrogenase enzymes.2,3 Iron is the
only transition metal that is common to all nitrogenases,4
suggesting that an Fe-H intermediate could be an active species
during nitrogenase catalysis. Consistent with the presence of a
hydride intermediate, nitrogenase produces H2 from water at
low substrate concentrations.5 Recent ENDOR and EPR evi-
dence supports the presence of a paramagnetic iron-hydride
species upon reduction of the R-70Ile mutant of A. Vinelandii
nitrogenase.6 Although this mutant is incapable of reducing N2,
its close similarity to the wild-type nitrogenase indicates that
Fe-H species should be considered as prospective intermediates
during the nitrogenase catalytic cycle.
Figure 1. Active site “FeMoco” of nitrogenase. The “belt” region refers
to the central Fe6S3X core, where X is an unidentified anion (probably C4-
3-, or O2-).9
,
N
environments similar to those potentially present in nitrogenase.
An ideal “model” complex would act as a functional model,
reducing substrates like those reduced by nitrogenase (N2,
alkynes, NO, CO2, HCN, diazenes, hydrazine) in a manner that
is more amenable to detailed mechanistic inquiry than the
enzyme.8 A hydride complex with perfect structural analogy to
the “belt iron” sites implicated as active sites of the FeMoco
(Figure 1)3,9,10 would have three weak-field sulfur donors, a
high-spin electronic configuration, and a low coordination
number (3, 4, 5) at iron. No complex with all of these properties
is known. However, by using the bulky anionic â-diketiminate
ligand LtBu (LR ) 1,3-R2-1,3-bis(2,6-diisopropylphenylimido)-
propyl, Figure 2), we were able to synthesize the first hydride
complex of iron with a coordination number less than five,
[LtBuFe(µ-H)]2.11,12 In solution, it is in equilibrium with a three-
Our interest in synthetic analogues for nitrogenase7 motivated
us to synthesize iron hydride complexes with coordination
(1) (a) Collman, J. P.; Hegedus, L. Principles and Applications of Organ-
otransition Metal Chemistry; 1980. (b) Peruzzini, M.; Poli, R. Recent
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9
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10.1021/ja069199r CCC: $37.00 © 2007 American Chemical Society

Mechanism of N
d
N Cleavage by an Iron(II) Hydride
A R T I C L E S
Scheme 1
Figure 2. Bulky â-diketiminate ligands used in this work.
coordinate monomer, LtBuFeH, and it binds pyridine to give
LtBuFe(H)(pyridine), which has a trigonal pyramidal geometry.
The monomeric hydride complexes have the desired high-spin
(S ) 2) electronic configuration at the metal, by virtue of the
low coordination number and weak ligand field at iron. Although
they lack the characteristic three-sulfur coordination environment
of the iron atoms in the FeMoco, diketiminate-supported iron
hydrides show promise for giving hints into the relevant
reactivity patterns of hydride ligands in a weak ligand field.13
The main function of nitrogenase is the cleavage of N-N
bonds, but in Vitro it reduces numerous substrates. For example,
A. Vinelandii iron-molybdenum nitrogenase has been reported
to reduce methyldiazene and hydrazine.14 N-N single bond
cleavage is promoted by nitrogenase and model complexes,14-16
though rarely with iron.17 In a preliminary communication, we
reported that [LtBuFeH]2 reacts with azobenzene (PhNdNPh)
to afford the amido complex LtBuFeNHPh (boxed reaction in
Scheme 1).11 This reaction was the first example of complete
NdN cleavage by an iron-hydride complex, and one of the few
by any iron complex.18-21 An unusual facet of the transformation
in Scheme 1 is that the complete NdN bond cleavage of
azobenzene requires no overall change in oxidation state at the
iron. This contrasts with literature NdN cleavage reactions that
result in four-electron oxidation of a metal (or multiple metals).22
Given the importance of breaking nitrogen-nitrogen bonds
in the formation of ammonia from N2 by nitrogenase, the ability
of a low-coordinate iron hydride to completely break an NdN
bond is remarkable. This manuscript reports investigations aimed
to determine the mechanism of this unusual transformation.
Results and Discussion
Synthesis of High-Spin Iron(II) Hydride Complexes. We
have reported the synthesis of dimeric [LtBuFeH]2, and showed
that it is in equilibrium with its monomeric, trigonal-planar form
LtBuFeH.11 In order to provide an alternative ligand with
comparable electronic properties and more steric bulk, we have
prepared the new ligand abbreviated LtBu′ (Figure 2), which
differs from LtBu by the presence of an additional isopropyl
group at the para position of each aryl ring. Creation of LtBu′
necessitated the synthesis of 2,4,6-triisopropylaniline, which
came from reduction of 2,4,6-triisopropylnitrobenzene. Incor-
poration of this aniline into the â-diketiminate ligand followed
procedures analogous to those used in the synthesis of LtBu
(see the Experimental Section for details).
H
Hydride complexes were accessed by addition of KHBEt3 to
the chloride complex LtBuFeCl or LtBu′FeCl at room temperature
in toluene. An immediate color change from red to brown was
(12) A recent paper postulates a transient four-coordinate iron-hydride complex
that reacts with benzene: Brown, S. D.; Peters, J. C. J. Am. Chem. Soc.
2004, 126, 4538-4539.
(13) Studies of other promising iron-hydride complexes: (a) Brown, S. D.; Mehn,
M. P.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 13146-13147. (b) Franke,
O.; Wiesler, B. E.; Lehnert, N.; Peters, G.; Burger, P.; Tuczek, F. Z. Anorg.
Allg. Chem. 2006, 632, 1247-1256. (c) Gilbertson, J. D.; Szymczak, N.
K.; Crossland, J. L.; Miller, W. K.; Lyon, D. K.; Foxman, B. M.; Davis,
J.; Tyler, D. R. Inorg. Chem. 2007, 46, 1205-1214.
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1981, 20, 5140-5146. (b) Barney, B. M.; Laryukhin, M.; Igarashi, R. Y.;
Lee, H.-I.; Dos Santos, P. C.; Yang, T.-C.; Hoffman, B. M.; Dean, D. R.;
Seefeldt, L. C. Biochemistry 2005, 44, 8030-8037. (c) Barney, B. M.;
Yang, T.-C.; Igarashi, R. Y.; Dos Santos, P. C.; Laryukhin, M.; Lee, H.-I.;
Hoffman, B. M.; Dean, D. R.; Seefeldt, L. C. J. Am. Chem. Soc. 2005,
127, 14960-14961.
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Am. Chem. Soc. 2006, 128, 5302-5303.
(21) Some synthetic Fe/Mo clusters reduce diazenes, and the available evidence
indicates that the reaction takes place at molybdenum: Malinak, S. M.;
Simeonov, A.; Mosier, P. E.; McKenna, C. E.; Coucouvanis, D. J. Am.
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of monomer to dimer in a 17 mM solution in C6D6 at room
temperature is slightly larger for the LtBu′ compound (ca. 0.5)
than the LtBu compound (ca. 0.3). Therefore, a majority of the
hydride complex is present as dimer at room temperature.
However, our previous studies have shown that monomer is
formed rapidly (the reported activation parameters for addition
of alkyne to monomer extrapolate to a half-life of several
minutes at room temperature and a few seconds at 80 °C),11 so
in the following discussion the dimeric hydride complexes are
treated as being functionally equivalent to the monomers.
Reaction with Azobenzene. Reaction of [LtBuFeH]2 or [LtBu′
-
FeH]2 with 2 equiv of PhNdNPh in diethyl ether gave the
isolable hydrazido complexes LtBuFeNPhNHPh and LtBu′FeN-
PhNHPh, respectively. The reactions are complete in less than
30 min at room temperature. Although the swiftness of this
reaction has prevented kinetic studies, it is possible to propose
a mechanism based on analogous reactions we have reported
previously. We envision the first step of the reaction as
dissociation of the hydride dimer into monomeric LtBuFeH or
LtBu′FeH, which is rapid at room temperature.11 This is followed
by 1,2-insertion of the FesH bond across the NdN bond.24
We have observed [LRFeH]2 to add across CdC, CtC, CdN,
and CdO bonds, and we have used kinetic isotope effects,
kinetics, and computations to support a 1,2-insertion mecha-
nism.25 Therefore, it is reasonable to presume that reduction of
the NdN bond of diazene to the NsN bond of the hydrazido
ligand follows an analogous pathway. Note that this reaction
requires 1 equiv of azobenzene per iron, twice the amount
needed for the overall conversion of [LtBuFeH]2 and PhNdNPh
to LtBuFeNHPh.
Figure 3. Solid-state structure of [LtBu′FeH]2, with 50% probability thermal
ellipsoids. Hydrogen atoms on the diketiminate ligands are omitted for
clarity. Fe1-N11 1.988(1) Å, Fe1-N21 1.978(1) Å, Fe2-N14 2.043(1)
Å, Fe2-N24 2.010(1) Å, Fe1-Fe2 2.5292(3) Å, N11-Fe1-N21 96.41-
(5)°, N14-Fe2-N24 97.60(5)°.
observed, with dissolution of the starting material (LtBu′
complexes tend to be somewhat more soluble than their LtBu
analogues). In each case, it is important to remove the byproduct
BEt3 immediately, because the borane reacts with the hydride
complex. The details of the reactions between iron hydride
complexes and boranes will be reported separately.23
The spectroscopic characteristics of the hydrazido complexes
are unremarkable, and the characterization of LtBuFeNPhNHPh
has been presented previously.11 As an alternative to the
preparation of the hydrazido complexes from azobenzene, they
can also be accessed by addition of two equiv of PhNHNHPh
to [LtBuFeH]2, releasing H2. Samples of LtBuFeNPhNHPh from
the two preparatory methods gave similar rate constants in
subsequent reactions (see below), but the rate constants were
more reproducible when generated from the latter method.
Kinetic and Crossover Studies on the N-N Cleavage
The new hydride complex [LtBu′FeH]2 was characterized by
solution methods and by X-ray crystallography. The solid-state
structure of [LtBu′FeH]2 (Figure 3) shows a dimer similar to [LtBu
-
FeH]2.11 Each iron atom has a distorted tetrahedral geometry
around the metal center, from binding of the diketiminate ligand
in a η2 binding mode, and coordination of two bridging hydrides.
These hydrides were located in a Fourier map, and their
positional parameters were refined, but their positions must be
regarded with uncertainty, given the small electron density at
the hydrogen atom. The â-diketiminate ligands are twisted into
a distorted boat conformation, with a substantial folding along
the N‚‚‚N axis (21.79(9)° and 27.39(7)° in [LtBu′FeH]2; 24.3-
(3)° in [LtBuFeH]2). The Fe-Fe distance of [LtBu′FeH]2 is slightly
shorter (2.5292(3) Å) than that of [LtBuFeH]2 (2.624(2) Å)
despite the increased steric hindrance in the former. The nature
of the iron-iron interaction is the subject of ongoing studies.
Reaction. Benzene and toluene solutions of purified LtBu
-
FeNPhNHPh (36-48 mM) decompose at 80 °C to form 1 equiv
of LtBuFeNHPh and 0.5 equiv of PhNdNPh (Scheme 1).11,26
We followed the progress of this reaction using 1H NMR
spectroscopy in C6D6 and toluene-d8. The reaction was moni-
tored at a number of temperatures, and rate constants were
derived from exponential fits to the integrations of peaks at 32
and -72 ppm for LtBuFeNPhNHPh and 28 ppm for LtBuFeNHPh.
Integrations were calibrated to an internal capillary containing
Tp*2Co in toluene-d8. No intermediates or other products were
observed under these conditions. At most temperatures (see
below), [LtBuFeNPhNHPh] followed an exponential decay with
a rate constant near that calculated for growth of [LtBuFeNHPh].
1
The H NMR spectrum of each hydride dimer is unusually
complicated, with at least 17 relatively sharp but partially
overlapped peaks present over the range 80 to -130 ppm. The
complicated room-temperature 1H NMR spectrum is attributed
to hindered C-C and C-N bond rotations in the dimer, because
the X-ray crystal structures show that the complexes are
exceptionally crowded. Despite the complex 1H NMR spectra,
these materials are analytically pure, react with 3-hexyne to give
high yields of vinyl products,11 and convert to monomeric forms
with simple 1H NMR spectra at elevated temperatures. The ratio
(24) This step might be preceded by coordination of the azobenzene in an η1
binding mode; see ref 22o.
(25) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.;
Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L. Organometallics 2004,
23, 5226-5239.
(26) When [LtBuFeH]2 is reacted with 1 equiv of PhNdNPh (i.e., 0.5 equiv of
PhNdNPh per iron atom), then all of the PhNdNPh is consumed. In this
case, the PhNdNPh formed in the decomposition of LtBuFeNPhNHPh
immediately reacts with the excess iron-hydride complex to form more
LtBuFeNPhNHPh, eventually giving only LtBuFeNHPh as a product.
(23) Yu, Y.; Brennessel, W. W.; Holland, P. L. Organometallics 2007, 26, in
press.
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Mechanism of N
d
N Cleavage by an Iron(II) Hydride
A R T I C L E S
Figure 5. Kinetic data for the conversion of LtBuFeNPhNHPh to LtBu
FeNHPh at 315 K: (b) [LtBuFeNPhNHPh]; (×) [LtBuFeNHPh].
-
organic byproducts by mass spectrometry shows the formation
of PhNdNPh (m/z ) 182) and TolNdNTol (m/z ) 210), but
no mixed diazene PhNdNTol (m/z ) 196). This result implies
that diazene is formed before NsN bond cleavage.
Finally, we used a double crossover experiment with two
different diketiminate ligands and two different hydrazido
ligands. A mixture of LtBuFeNPhNHPh and LtBu′FeNTolNHTol
was heated to 80 °C for 2 h and the products were analyzed by
mass spectrometry. LtBuFeNHPh (m/z ) 649), LtBuFeNHTol (m/z
) 663), LtBu′FeNHPh (m/z ) 733), and LtBu′FeNHTol (m/z )
747) were observed in roughly equal amounts. Because the
amido compounds had statistically scrambled between (diketim-
inate)Fe units, we conclude that all Fe-N bonds are broken at
some point in the mechanism.
We also attempted to study the kinetic isotope effect for the
reaction. For this purpose, deuterium labeled hydrazido com-
pound, LtBuFeNPhNDPh, was synthesized from addition of
PhNDNDPh to [LtBuFeH]2. This compound was heated under
similar conditions as used with LtBuFeNPhNHPh. Unfortunately,
Figure 4. (a) Top: Plot of [LtBuFeNPhNHPh] vs time at 358 K. (b)
Bottom: Plot of ln[LtBuFeNPhNHPh] vs time at 358 K.
the IR spectrum of the product shows the presence of LtBu
-
The exponential fit implies a first-order rate dependence on [LtBu
-
FeNHPh (νN-H ) 3319 cm-1) as well as LtBuFeNDPh (νN-D
)
FeNPhNHPh], and plots of ln[LtBuFeNPhNHPh] versus time
2499 cm-1), suggesting that some of the deuterium label was
lost through an unknown mechanism. While the fate of these
deuterons remains a mystery, the complication of the proton
exchange makes it impossible to interpret the relative rates as
a genuine kinetic isotope effect for the reaction.28
were linear (Figure 4).27
Interestingly, the plots of [LtBuFeNPhNHPh] and [LtBu
-
FeNHPh] versus time at the lowest temperature used (42 °C)
show an induction period before the beginning of the exponential
decay or growth (Figure 5). This behavior, evident in three
different trials, suggests an initiation step in the reaction
pathway. The length of the induction period varied from 45 to
90 min in different trials at this temperature. The implications
of this observation will be described in more detail below.
Exploring the Possible Mechanisms. 1. Disproportionation
Mechanism. One possible mechanism for the formation of
amido product is shown in Scheme 2. A pericyclic mechanism
Scheme 2
The reaction path to the products was studied using a
crossover experiment. A mixture of two different hydrazido
compounds, LtBuFeNPhNHPh and LtBuFeNTolNHTol (Tol )
m-tolyl), was heated to 80 °C for 2 h and passed through an
activated alumina column to remove iron salts. Analysis of the
(27) A linear Eyring plot (Figure S-1) derived from the rate constants at different
temperatures gives apparent activation parameters of ∆H‡ ) 13 ( 1 kcal
mol-1 and ∆S‡ ) -36 ( 4 eu. However, the induction period suggests a
chain mechanism (discussed at length below), in which the rate constants
do not reflect elementary steps. Therefore, these apparent activation
parameters should be interpreted only with the greatest caution.
of this type was proposed for the disproportionation of N,N′-
diphenylhydrazine to azobenzene and aniline at high tempera-
ture.29 However, later kinetic studies showed a first-order
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Sadique et al.
dependence on [PhNHNHPh], ruling out this mechanism.30
According to this mechanism, the products are formed through
a bimolecular transition state as shown in Scheme 2. This
requires the rate of our reaction to have a second-order
dependence on [LtBuFeNPhNHPh]. The observed first-order
dependence on iron concentration and the induction period are
inconsistent with the disproportionation mechanism.
hydride complex [(7.1 ( 1.3) × 10-4 s-1] it is unlikely that the
hydride is an intermediate in the transformation.
One final piece of evidence argues against the mechanism in
Scheme 3: if LtBuFeH reacted rapidly with LtBuFeNPhNHPh,
as required in Scheme 3, then it would not have been possible
to isolate LtBuFeNPhNHPh during its synthesis from [LtBuFeH]2
and PhNdNPh (some LtBuFeH is present in the reaction mixture
at room temperature). These combined observations suggest that
the N-N cleavage reaction is unlikely to follow a â-hydride
elimination pathway.
2. â-Hydride Elimination Mechanism. Next, we considered
â-hydride elimination (Scheme 3) as a potential rate-limiting
step in the mechanism. This pathway has some attractive
features: it would be expected to give a first-order dependence
on [Fe]; the azobenzene is generated without N-N bond
cleavage; and the â-hydride elimination step is the reverse of
the 1,2-insertion of the Fe-H bond across the NdN bond, which
forms the hydrazido complex. If the reaction goes by this
mechanism, LtBuFeH should be formed as an intermediate of
the reaction. However, no intermediate species were detected
3. Ion-Pair and Acid-Catalyzed Mechanisms. The forma-
tion of LtBuFeNHPh, LtBuFeNHTol, LtBu′FeNHPh, and LtBu′
-
FeNHTol from heating LtBuFeNPhNHPh and LtBu′FeNTolNHTol
(see above) suggests that Fe-N bonds are disrupted as part of
the mechanism. If the reaction proceeds through heterolytic
Fe-N bond cleavage in an ion-pair mechanism as shown in eq
1, the rate of the reaction should depend on the polarity of the
solvent in which the reaction is carried out. However, the rate
constant determined in THF-d8 [k ) (4.6 ( 0.2) × 10-4 s-1] at
352 K was not faster than that in toluene-d8 [k ) (6.9 ( 0.2) ×
10-4 s-1] or benzene-d6 [k ) (5.8 ( 0.2) × 10-4 s-1] under
similar conditions, arguing against an heterolytic cleavage in
the rate-limiting step of the mechanism.
1
by H NMR spectroscopy during the course of the reaction,
and the rate of the reaction was not affected by the addition of
excess azobenzene. To account for these observations, the
â-hydride elimination would have to be the rate-limiting step:
in other words, the intermediate LtBuFeH has to react quickly
with LtBuFeNPhNHPh.
LFeNPhNHPh f LFe+ + -NPhNHPh
(1)
Scheme 3
The possibility that trace acid catalyzes the reaction was tested
by adding a non-nucleophilic base under the same reaction
conditions. When the reaction was monitored at 359 K in the
presence of lutidine (35 mM), the rate constant was (6.5 ( 0.2)
× 10-4 s-1, again showing no effect on the reaction rate.
4. Radical Mechanisms. The results discussed so far are
consistent with a pathway initiated by a homolytic cleavage to
yield radicals that react through a chain mechanism. The
crossover experiment shows that the mechanism cannot involve
NsN bond homolysis along the way to PhNdNPh, and so Fes
N bond homolysis is most strongly implicated as an initiation
reaction (eq 2).32 Light does not initiate the reaction, because
all kinetic studies were done in the dark (inside an NMR probe).
To query the intermediacy of the hydride species, the
hydrazido complex was heated in the presence of 3-hexyne in
varying amounts. The reaction of LtBuFeH with 3-hexyne to
afford the vinyl compound, LtBuFeCEtCHEt, is very fast and
irreversible at room temperature,11,31 so it is expected to be an
effective trap. However, only a small amount (<20%) of LtBu
-
LFeNPhNHPh f LFe + ‚NPhNHPh
(2)
FeCEtCHEt was formed, and the ratio of vinyl product to the
amido product was independent of the concentration of 3-hexyne
(see Supporting Information for details). This result is incon-
sistent with the intermediacy of LtBuFeH in the reaction.
It is instructive to further consider the possibility that the
hydride and hydrazido react with each other so quickly that
trapping is not possible. This possibility was experimentally
tested by adding 1 equiv of [LtBuFeH]2 to LtBuFeNPhNHPh in
C6D6. This mixture does not react under ambient conditions,
even though [LtBuFeH]2 is expected to form monomer within a
few minutes at room temperature.11 When this mixture was
heated to 358 K, LtBuFeNHPh is formed with a rate constant of
(5.3 ( 0.7) × 10-4 s-1. Because the rate constant for this
reaction is roughly the same as that in the reaction without added
Homolytic Fe-N bond cleavage would give the N,N′-
diphenylhydrazinyl radical, which could undergo spontaneous
disproportionation to PhNdNPh and PhNHNHPh (eq 3).33 The
two sides of the double bond in the PhNdNPh molecule derive
from the same hydrazido complex, assuming a pericyclic
mechanism.33 Therefore, this initial step is consistent with the
crossover experiments given above.34,35
2 ‚NPhNHPh f PhNHNHPh + PhNdNPh
(3)
(32) We cannot rule out that the initiation step is N-N bond homolysis, which
would give LtBuFeNPh (a potential chain carrier, as discussed below) and
PhNH•, which would give a small amount of PhNH2 (presumably
undetected). After initiation, the chain would proceed as in Scheme 4.
(33) (a) Shizuka, H.; Kayoiji, H.; Morita, T. Mol. Photochem. 1970, 2, 165-
176. (b) Van Beek, H. C. A.; Heertjes, P. M.; Houtepen, C.; Retzloff, D.
J. Soc. Dyers Colour. 1971, 87, 87-92. (c) The Chemistry of the Hydrazo,
Azo, and Azoxy Groups; Patai, S., Ed.; Wiley: New York, 1975.
(34) However, second-order reactions between two transient radicals are
relatively unlikely, a phenomenon known as the “persistent radical effect”.35
The implication is that the transient radical species will react primarily
with the major species in solution, LtBuFeNPhNHPh.
(28) The apparent kH/kD is 0.95 ( 0.23, and the value near unity is consistent
with loss of deuterium label.
(29) Holt, P. F.; Hughes, B. P. J. Chem. Soc. 1953, 1666-1669.
(30) Heesing, A.; Schinke, U. Chem. Ber. 1977, 110, 2867-2871.
(31) The reaction of LtBuFeCEtCHEt and PhNdNPh at room temperature does
not form LtBuFeNPhNHPh. Many other control experiments were also
performed to verify that the products do not react with each other or with
byproducts (see Supporting Information).
(35) (a) Fischer, H. Chem. ReV. 2001, 101, 3581-3610. (b) Daikh, B. E.; Finke,
R. G. J. Am. Chem. Soc. 1992, 114, 2938-43.
9
8116 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

Mechanism of N
d
N Cleavage by an Iron(II) Hydride
A R T I C L E S
Scheme 4
Figure 6. Crystal structure of LtBuFeNPhNPh. Thermal ellipsoids shown
at 50% probability. Hydrogen atoms have been omitted for clarity. Selected
metrical parameters follow: Fe-N11 1.987(1) Å, Fe-N1 1.953(1) Å, N1-
N1A 1.398(2) Å, N11-Fe-N11A 97.52(5)°, N1-Fe-N1A 41.94(5)°.
What would happen to the iron(I) LtBuFe fragment formed
in eq 2? First, it could react with PhNHNHPh (eq 4). This
hypothesis was tested by reacting a genuine iron(I) complex,
LtBuFeClK(solvent)n,36 with N,N′-diphenylhydrazine (1 equiv)
in C6D6. The amido compound LtBuFeNHPh was formed within
a few minutes at ambient temperature. Therefore, this step is a
reasonable route to the observed product of the overall reaction.
imido species could abstract a hydrogen atom from LtBu
-
FeNPhNHPh to give LtBuFeNPhNPh (eq 7), which may be
formulated either as an iron(II) complex with a radical ligand,
or an iron(I) complex with coordinated PhNdNPh.
LtBuFeNPhNPh was prepared independently by treating LtBu
-
FeNNFeLtBu with PhNdNPh at room temperature, and the solid-
state structure was determined using X-ray crystallography.
Figure 6 shows that the azobenzene ligand is coordinated
through the NdN π-bond, and backbonding considerably
weakens the NdN bond (NsN ) 1.398(2) Å) compared to free
trans-azobenzene (1.247(2) Å).40 This feature indicates that the
formal azobenzene-iron(I) formulation does not account for
substantial backbonding into the azobenzene ligand. There are
strong analogies to diketiminate-iron(I) complexes of alkenes
and alkynes recently reported by our group, in which crystal-
lographic, vibrational, Mo¨ssbauer, and theoretical evidence
support substantial backbonding into the unsaturated ligand.41
Therefore, it is reasonable to think of LtBuFeNPhNPh as an iron-
(II)-radical complex LtBuFe2+(N2Ph2)•-.
2LFe + PhNHNHPh f 2LFeNHPh
(4)
The iron(I) fragment could also abstract an NHPh radical from
LtBuFeNPhNHPh to give LtBuFeNHPh and LtBuFeNPh, an iron-
(III) imido species (eq 5). We have spectroscopically character-
ized a similar complex, LMeFeNAd (where Ad ) 1-adaman-
tyl).37,38 One characteristic reaction of LMeFeNAd is to abstract
hydrogen atoms from hydrocarbons (e.g., 9,10-dihydroan-
thracene and indene), giving an iron(II) amido complex. While
we have been unsuccessful in preparing LtBuFeNPh,39 the
analogy to LMeFeNAd suggests strongly that the transient
formation of LtBuFeNPh is feasible.
LFe + LFeNPhNHPh f LFeNHPh + LFeNPh (5)
LFeNPh + ‚NPhNHPh f LFeNHPh + PhNdNPh (6)
In turn, LtBuFeNPhNPh can lose azobenzene to regenerate
the active iron(I) fragment (eq 8), which continues the radical
chain (Scheme 4). Both sides of the azobenzene produced
LFeNPh + LFeNPhNHPh f LFeNHPh + LFeNPhNPh
through eq 8 have come from a single molecule of LtBu
-
(7)
FeNPhNHPh, so this mechanism is consistent with the crossover
experiment described above.
LtBuFeNPh should be capable of abstracting a hydrogen atom
from the hydrazinyl radical (which has a very weak NsH bond)
to form LtBuFeNHPh and PhNdNPh (eq 6). This would
represent a chain termination step.34 Alternatively, the transient
LFeNPhNPh f LFe + PhNdNPh
(8)
Effect of Radical Traps on the Rate of the Reaction.
Kinetic studies on the decomposition of LtBuFeNPhNHPh in
C6D6 at 358 K were repeated with large concentrations of the
radical traps triphenylmethane (Ph3CH) (0.46 M) and dihy-
droanthracene (DHA) (0.35 M). The rate constant for the
reaction was not affected [(9.8 ( 0.3) × 10-4 s-1 and (9.3 (
0.2) × 10-4 s-1, respectively]. The bond dissociation energy
(BDE) of the C-H bonds in Ph3CH (81 kcal/mol) and DHA
(78 kcal/mol) is higher than the BDE of the N-H bond in N,N′-
(36) 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.
(37) Eckert, N. A.; Vaddadi, S.; Stoian, S.; Lachicotte, R. J.; Cundari, T. R.;
Holland, P. L. Angew. Chem., Int. Ed. 2006, 45, 6868-6871.
(38) Other recently isolated iron(III) species with a terminal imido ligand: (a)
Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125,
322-323. (b) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126,
4538-4539. (c) Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127,
1913-1923. (d) Mehn, M. P.; Peters, J. C. J. Inorg. Biochem. 2006, 100,
634-643. (e) Thomas, C. M.; Mankad, N. P.; Peters, J. C. J. Am. Chem.
Soc. 2006, 128, 4956-4957.
(39) All attempts to create (diketiminate)FeIIIdN(aryl) species have given
(diketiminate)FeII-NH(aryl) products, consistent with powerful H-atom
abstraction ability of the putative imido. Eckert, N. A. Ph.D. Thesis,
University of Rochester, 2005.
(40) Bouwstra, J. A.; Schouten, A.; Kroon, J. Acta Crystallogr., Sect. C 1983,
39, 1121-1123.
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8117

A R T I C L E S
Sadique et al.
diphenylhydrazine (69 kcal/mol),42 and lower than the BDE of
N-H in aniline (92 kcal/mol), so this observation is com-
patible with the presence of hydrazinyl radicals and argues
against amido radicals. Because LtBuFeNPhNHPh is decomposed
by TEMPO and nitrosobenzene, these radical traps are not
informative.
Although the details are tentative, the amassed experimental
data are most supportive of a chain mechanism of the type
shown in Scheme 4. It remains only to explain how addition of
3-hexyne to the hydrazido complex could yield a small,
concentration-independent amount of LtBuFeCEtCHEt (see
above). To rationalize this observation, we note that LtBuFeH
could be formed by abstraction of a hydrogen atom from the
Which Species Are Chain Carriers? The decomposition
of solutions of LtBuFeNPhNHPh at 42 °C in C7D8 shows an
induction period, as described above (Figure 5). This reaction
was repeated with different reagents added (12-14 mM), to
weak N-H bond in the hydrazinyl radical ‚NPhNHPh by “LtBu
-
Fe” (eq 9; Scheme 4). Because the hydride complex is off the
main reaction pathway, the amount of vinyl complex formed is
independent of [3-hexyne].
learn whether they eliminate the induction period. We used LtBu
-
FeCl as a representative of iron(II) compounds, LtBuFeClK-
(solvent)n as a proxy for the “LtBuFe” intermediate in the
catalytic cycle, and the azobenzene complex LtBuFeNPhNPh,
in separate experiments. Addition of LtBuFeCl did not eliminate
the induction period, while addition of LtBuFeClK(solVent)n or
LtBuFeNPhNPh resulted in formation of LtBuFeNHPh without
an induction period at 42 °C. These observations support the
chain mechanism including LtBuFe and LtBuFeNPhNPh as chain
carriers. In a different experiment, we added benzophenone (2
equiv) to LtBuFeNPhNHPh in order to trap transient iron(I).43
Heating this mixture afforded unidentified paramagnetic com-
plexes and no LtBuFeNHPh was formed. Other traps for iron(I)
gave ambiguous results.43
LFe + ‚NPhNHPh f LFeH + PhNdNPh
LFeH + EtCCEt f LFeCEtCHEt
(9)
(10)
Conclusions
The reaction of LtBuFeH with 0.5 equiv of PhNdNPh cleanly
leads to LtBuFeNHPh, formally cleaving the diazene into “NPh”
units that insert into the Fe-H bond. Although the reaction
stoichiometry is simple, the mechanism is complex. Fortunately,
a combination of mechanistic experiments can be used to rule
out most mechanisms, and to support a chain mechanism. The
first step of this chain process consists of adding the Fe-H
bond across the NdN bond to give a hydrazido complex LtBu
-
FeNPhNHPh. This intermediate is susceptible to Fe-N bond
homolysis to give iron(I) fragments that are capable of breaking
N-N bonds. The picture that emerges is a radical chain
mechanism that has odd-electron iron species as chain carriers.
One lesson to be learned from this mechanism is that iron
complexes with weak ligand fields are capable of [1,2]-addition
reactions through both nonradical (in the addition of Fe-H
across the NdN bond) and radical (in the N-N cleaving chain
reaction) pathways.45,46 Although there are clear differences
between the coordination sphere of the diketiminate-iron
complexes here and the important iron sites in the FeMoco of
nitrogenase, one can use these conclusions to speculate about
the numerous N-N cleavage and N-H bond forming steps that
lie along the path from N2 to ammonia. Previous workers have
understandably favored simpler nonradical pathways as in the
Chatt cycle,47 and these are representative of the mechanisms
of reactions at molybdenum and other second- and third-row
metal ions. However, given the recent evidence supporting high-
spin iron as the reactive site on the FeMoco of nitrogenase,10 it
is important to consider one-electron reactivity in the enzymatic
system.
Which Is the Rate-Limiting Step after Initiation? This is
a subject that must be approached with caution. As stated by
Huyser, “Rate laws for free-radical chain reactions are of limited
value in determining the general mechanism of the reaction.”46
This is especially true when considering Scheme 4, in which
many of the propagation and termination steps give the same
products. However, the following arguments may be advanced.
The post-initiation kinetics follow a first-order rate dependence
on [Fe], which suggests that eq 8 (top arrow in the catalytic
cycle of Scheme 4) may be rate-limiting. When LtBuFeNPh-
NHPh was heated in the presence of added PhNdNPh (45 mM)
in C6D6 at 358 K, there was no effect on the rate constant [(7.0
( 0.1) × 10-4 s-1]. This result suggests that eq 8 is effectively
irreversible: every time LtBuFe is formed it is trapped by
PhNHNHPh or LtBuFeNPhNHPh rather than by PhNdNPh. In
order to test this idea, 1 equiv of PhNdNPh was added to a
C6D6 solution of LtBuFeClK(solvent)n with and without 1 equiv
of LtBuFeNPhNHPh. Without LtBuFeNPhNHPh, only LtBu
-
1
FeNPhNPh was observed in the H NMR spectrum, but in the
presence of the hydrazido species no LtBuFeNPhNPh was evident
(instead, there was slow conversion to LtBuFeNHPh). Therefore,
our experiments suggest that loss of PhNdNPh from the
formally iron(I) center in LtBuFeNPhNPh is the slow step of
the chain.44
Experimental Section
General Procedures. All manipulations were performed under a
nitrogen atmosphere by standard Schlenk techniques or in an M. Braun
glove box maintained at or below 1 ppm of O2 and H2O. Glassware
was dried at 150 °C overnight. NMR data were recorded on a Bruker
(41) (a) Stoian, S. A.; Yu, Y.; Smith, J. M.; Holland, P. L.; Bominaar, E. L.;
Mu¨nck, E. Inorg. Chem. 2005, 44, 4915-4922. (b) Yu, Y.; Smith, J. M.;
Flaschenriem, C. J.; Holland, P. L. Inorg. Chem. 2006, 45, 5742-5751.
(42) Zhao, Y.; Bordwell, F. G.; Cheng, J.-P.; Wang, D. J. Am. Chem. Soc. 1997,
119, 9125-9129.
(44) Azobenzene loss may require an isomerization of the η2-bound ground state
(Figure 6) to an η1 isomer, a process that can be slow: see ref 22o. We are
unable to reconcile this rate-limiting step with the apparent negative value
of ∆S‡ in footnote 27.
(45) Recent review on radical organometallic reactions in diketiminate chem-
istry: Smith, K. M. Organometallics 2005, 24, 778-784.
(46) Despite the conceptual complexity of radical processes, they can lead to
high yields and clean products. See, for example: Huyser, E. S. Free-
Radical Chain Reactions; Wiley: New York, 1970.
(47) (a) Chatt, J.; Pearman, A. J.; Richards, R. L. Nature 1975, 253, 39-40.
(b) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76-78. (c) Betley,
T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782-10783. (d) Betley,
T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252-6254.
(43) Diketiminate-iron(I) complexes react with ketones to give pinacol coupling
products: see ref 36. We have obtained ambiguous results when using other
traps for iron(I) in the conversion of LtBuFeNPhNHPh to LtBuFeNHPh, as
follows. PPh3 and PEt3 bind weakly to the LtBuFe fragment (see refs 36
and 41b) and do not affect the rate of conversion. CO does bind to LtBuFe
(see ref 36), but does not affect the decomposition of LtBuFeNPhNHPh:
this is consistent with the observation that addition of a small amount of
LtBuFe(CO)2 to the LtBuFeNPhNHPh abolishes the induction period for its
decomposition. Therefore, it is likely that CO binds reversibly to the LtBu
Fe fragment, and does not substantially affect the chain reaction.
-
9
8118 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

Mechanism of N
d
N Cleavage by an Iron(II) Hydride
A R T I C L E S
Avance 400 (400 MHz) or Bruker Avance 500 spectrometer (500 MHz).
All peaks in the NMR spectra are referenced to residual C6D5H at 7.16
ppm. In parentheses are listed integrations and assignments. In some
cases, it was not possible to determine integrations because of peak
overlap. UV-vis spectra were measured on a Cary 50 spectrophoto
meter, using screw-cap cuvettes. Solution magnetic susceptibilities were
determined by the Evans method,48 monitoring the shift in the residual
solvent peak relative to a capillary of solvent. Elemental analyses were
determined by Desert Analytics, Tucson, AZ.
solvent and byproducts were distilled off, and the product was
distilled at 110 °C (3 mbar) to yield 1-chloro-1-(2,4,6-triisopropylphe-
nylimino)-2,2-di(methylpropane) (17.8 g, 77%) as a colorless gel. This
1
product was handled under N2. H NMR (400 MHz, CD2Cl2) δ 1.16
(12, d, J ) 8 Hz, o-iPr-CH3), 1.18 (6, d, J ) 8 Hz, p-iPr-CH3), 1.42
(9, s, tBu), 2.73 (2, septet, p-iPr-CH), 2.95 (1, septet, o-iPr-CH), 7.0
(2, s, m-H).
Step d. A round-bottom flask was charged with 1-chloro-1-(2,4,6-
triisopropylphenylimino)-2,2-dimethylpropane (14.8 g, 46 mmol) and
pentane (80 mL) inside a glovebox and cooled in a cold well
(-60 °C). To this solution was added 1.6 M methyllithium (29 mL,
46 mmol) in diethyl ether. The turbid yellow reaction mixture was
stirred at room temperature for 18 h, and then brought out of the glove
box. Water (50 mL) was added to the reaction mixture slowly and very
carefully. The aqueous layer was separated and extracted with diethyl
ether. The organic fractions were combined and dried with anhydrous
magnesium sulfate and filtered, and solvent was removed. The product
was dried under vacuum to yield 2-(2,4,6-triisopropylphenylimino)-
3,3-dimethylbutane (10.7 g, 77%) as a pale yellow solid. 1H NMR (400
MHz, CD2Cl2) δ 1.2 (12, br, o-iPr-CH3), 1.3 (24, br, p-iPr-CH3,tBu),
1.7 (3, br, CH3), 2.6 (2, br, o-iPr-CH), 2.8 (1, br, p-iPr-CH), 6.9
(2, s, m-H).
Pentane, diethyl ether, and toluene were purified by passage through
activated alumina and “deoxygenizer” columns from Glass Contour
Co. (Laguna Beach, CA). Deuterated benzene and toluene 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. The
compounds nitro-2,4,6-triisopropylbenzene,49 LtBuFeCl,50 KBEt3H,51
[LtBuFeH]2,11 LtBuFeNPhNHPh,11 LtBuFeCEtCHEt,11 LtBuFeNNFeLtBu 36
,
LtBuFe(Cl)K(solvent)x36 were prepared by published procedures. 3-Hex-
yne, N,N′-diphenylhydrazine, and azobenzene were obtained from
Aldrich, and m-azotoluene was obtained from TCI America.
3-Hexyne was degassed, vacuum transferred to a new container, and
stored in the glovebox at -35 °C. Azobenzene and m-azotoluene
were dissolved in pentane, dried over molecular sieves, filtered through
Celite, and then dried under vacuum. 1,3,5-Triisopropylbenzene,
lutidine, n-butyllithium, methyllithium, TMEDA, Pd/C, phosphorus
pentachloride, pivaloyl chloride, and triphenylmethane were purchased
from Aldrich Chemical Co. and used as received. 9,10-Dihydroan-
thracene and Ph3CH were crystallized from ethanol, and the crystals
were vacuum-dried. D2O was purchased from Cambridge Isotope
Laboratories, Inc.
Step e. A round-bottom flask was charged with 2-(2,4,6-triisopro-
pylphenylimino)-3,3-dimethylbutane (10.7 g, 35 mmol) and diethyl
ether (60 mL). To this was added TMEDA (5.5 mL, 36 mmol), and
the reaction mixture was cooled in a cold well (-60 °C). n-Butyllithium
(14.6 mL, 2.5 M in hexanes, 36 mmol) was added slowly while stirring.
The resultant dark orange reaction mixture was stirred in the cold well
for 1 h and then stirred at room temperature for 18 h. The dark orange
reaction mixture with yellow precipitate was cooled in the cold well,
and then 1-chloro-1-(2,4,6-triisopropylphenylimino)-2,2-dimethylpro-
pane (11.4 g, 35 mmol) in 20 mL of pentane was added dropwise.
Once the addition was complete, the reaction mixture was stirred at 40
°C for 6 h. Water (50 mL) was added; the aqueous layer was separated
and extracted with diethyl ether. All the organic fractions were
combined and dried with magnesium sulfate and filtered, and the volatile
components were removed under vacuum. The pale yellow solid was
Preparation of LtBu′H. Step a. A 200 mL Paar bomb was charged
with nitro-2,4,6-triisopropylbenzene (20.0 g, 80.2 mmol) dissolved
in isopropanol (80 mL) and Pd/C (3.0 g). H2 gas (∼ 600 psi) was
introduced, and the mixture was stirred at 60 °C for 18 h. The contents
were filtered in air after cooling to room temperature. Isopropanol was
removed by distillation, and the product was distilled at 80 °C (3 mbar)
to afford 2,4,6-triisopropylaniline (16.5 g, 94%) as a pale yellow oil.
1H NMR (400 MHz, CDCl3) δ 1.1 (12, d, J ) 8 Hz, o-iPr-CH3), 1.3
(6, d, J ) 8 Hz, p-iPr-CH3), 2.9 (1, septet, p-iPr-CH), 3.0 (2, septet,
o-iPr-CH), 3.8 (2, br, NH), 6.9 (2, s, m-H).
dissolved in hot pentane, filtered, and stored at -25 °C to afford LtBu′
H
1
(15.3 g, 72%) as pale yellow crystals. H NMR (400 MHz, CDCl3) δ
1.1 (24, br, o-iPr-CH3), 1.3 (18, br, tBu), 1.7 (12, br, p-iPr-CH3), 2.7
(1, br, p-iPr-CH), 2.9 (2, br, o-iPr-CH), 3.3 (2, br, backbone CH), 6.9
(4, s, m-H). LCMS: m/z ) 587. Anal. Calcd for C41H66N2: C 83.89,
H 11.33, N 4.77. Found: C 82.62, H 11.33, N 4.65.
Step b. A round-bottom flask was charged with 2,4,6-triisopropyl-
aniline (16.5 g, 75.2 mmol) and dichloromethane (50 mL). To the flask
was added triethylamine (7.7 g, 75 mmol) and then a solution of pivaloyl
chloride (9.3 mL, 75 mmol) in 50 mL of dichloromethane slowly (∼5
drops/s). The reaction mixture was refluxed for 2 h at 40 °C, and the
solvent was removed using a rotory evaporator. The resultant pink
solid was washed with water and diethyl ether to yield N-(2,4,6-
triisopropylphenyl)-2,2-dimethylpropionamide as a white microcrys-
talline powder which was dried under vacuum (21.8 g, 96%). 1H NMR
(400 MHz, CDCl3) δ 1.17 (12, d, J ) 8 Hz, o-iPr-CH3), 1.22 (6, d,
J ) 8 Hz, p-iPr-CH3), 1.33 (9, s, tBu), 2.86 (1, septet, p-iPr-CH), 2.97
(2, septet, o-iPr-CH), 6.73 (1, s, NH), 6.98 (2, s, m-H).
Preparation of LtBu′Li(THF). A Schlenk flask was charged with
LtBu′H (12.1 g, 20.6 mmol) and THF (100 mL) and cooled to -60 °C.
A solution of n-butyllithium (9.0 mL, 2.5 M in hexanes, 22.5 mmol)
was added slowly with stirring. The reaction mixture was warmed to
room temperature and heated at 60 °C for 2 h. The volatile components
were removed under vacuum, and the product was dissolved in warm
pentane and stored at -35 °C to afford pale yellow crystals of LtBu′
-
Li(THF) (9.6 g, 70%). 1H NMR (400 MHz, C6D6) δ 0.9 (4, br, THF),
1.14 (12, d, iPr-CH3), 1.20 (12, d, iPr-CH3), 1.39 (18, s, tBu), 1.42
(12, d, iPr-CH3), 2.3 (4, br, THF), 2.8 (2, septet, iPr-CH), 3.5 (4, septet,
iPr-CH), 5.24 (1, s, backbone-CH), 6.96 (4, s, m-H).
Step c. Phosphorus pentachloride (16 g, 73 mmol) was added in
small portions to a slurry of N-(2,4,6-triisopropylphenyl)-2,2-dimeth-
ylpropionamide (21.8 g, 71.8 mmol) in benzene (33 mL) under N2 flow.
During the course of the addition, HCl gas was liberated, and it was
passed through aqueous Na2CO3. The resultant yellow reaction
mixture was stirred for 18 h under the continuous flow of N2. The
Preparation of LtBu′FeCl. A bomb flask was charged with FeCl2-
(THF)1.5 (1.3 g, 5.6 mmol), LtBu′Li(THF) (3.7 g, 56 mmol), and toluene
(70 mL). The red reaction mixture was heated at 100 °C for 18 h. The
volatile components were removed under vacuum, and the red residue
was transferred to a glass thimble, which was placed in a Soxhlet
extractor. The residue was extracted continuously with boiling diethyl
ether, until the extracting solvent was clear (24 h). The red extract was
concentrated under vacuum, and the resultant red solid was isolated
(48) (a) Baker, M. V.; Field, L. D.; Hambley, T. W. Inorg. Chem. 1988, 27,
2872. (b) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(49) Newton, A. J. Am. Chem. Soc. 1943, 65, 2434-2439.
(50) (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. Organo-
metallics 2002, 21, 4808-4814.
(51) Fryzuk, M. D.; Lloyd, B. R.; Clentsmith, G. K. B.; Rettig, S. J. J. Am.
Chem. Soc. 1994, 116, 3804-3812.
(2.04 g, 54%). The mother liquor was stored at -35 °C to afford LtBu′
-
9
J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8119

A R T I C L E S
Sadique et al.
1
FeCl as red crystals (1.2 g, 32%). Total yield 3.2 g (86%). H NMR
(400 MHz, C6D6) δ 105 (1, backbone-CH), 42 (18, 0.3, tBu), 35 (2,
0.7, iPr-CH), 2.1 (4, m-H), -5 (12, 0.7, iPr-CH3), -28 (12, 0.5, iPr-
CH3), -111 (12, 0.1, iPr-CH3), -115 (4, iPr-CH). LCMS: m/z ) 676.
Anal. Calcd for C41H65N2FeCl: C 71.71, H 9.67, N 4.14. Found: C
70.81, H 10.25, N 3.99.
solution of n-butyllithium (0.95 mL, 2.5 M in hexanes) was added while
stirring. The thick yellow suspension was stirred for 18 h at room
temperature. The volatile components were removed under reduced
pressure. The contents were cooled in an ice bath, and D2O (10 mL)
was added under nitrogen flow to give a white precipitate. The slurry
was stirred for 2 h, and the reaction mixture was allowed to settle. The
yellow solution was decanted using a cannula, and the white solid was
dried under reduced pressure. The solid was extracted with pentane/
diethyl ether solvent mixture, filtered through Celite, concentrated, and
stored at -35 °C to afford PhNDNDPh as colorless flaky crystals
Preparation of [LtBu′FeH]2. A Schlenk flask was charged with LtBu′
-
FeCl (0.884 g, 1.30 mmol) and toluene (30 mL). To this red solution
was added a clear solution of KBEt3H (0.180 g, 1.30 mmol) in toluene
(10 mL). The dark red reaction mixture was stirred at room temperature
for 45 min, and the volatile materials were removed under vacuum.
The residue was extracted with pentane and filtered through Celite to
give a dark red solution. This solution was concentrated, warmed to
dissolve the product, and then cooled to -35 °C to afford very dark
red crystals (501 mg, 78%). It is critical that the reaction is not stirred
1
(∼98% deuteration by NMR). H NMR (400 MHz, C6D6) δ 6.7 (4,
o-H), 6.8 (2, p-H), 7.2 (4, m-H).
Preparation of LtBuFeNPhNDPh. A solution of PhNDNDPh (8.3
mg, 44 µmol) in Et2O (2 mL) was added to a dark red slurry of [LtBu
-
FeH]2 (25 mg, 22 µmol) in Et2O (8 mL). A red solution was formed
over the course of an hour. The volatile materials were removed under
vacuum. The residue was extracted with pentane, filtered through Celite,
concentrated, and stored at -35 °C to give LtBuFeNPhNDPh as red
crystals (56 mg, 60%). 1H NMR (400 MHz, C6D6) δ 112 (1,
backbone-CH), 43 (18, tBu), -7 (2, diketiminate m-H), -8 (2,
diketiminate m-H), -23 (6, iPr-CH3), -28 (6, iPr-CH3), -45 (1,
hydrazido p-H), -87 (6, iPr-CH3), -94 (2, diketiminate p-H), -171
(8, iPr-CH3 and iPr-CH).
for too long because [LtBu′FeH]2 reacts with the BEt3 byproduct. Unlike
its LtBu analogue, [LtBu′FeH]2 is soluble in pentane. H NMR ([LtBu′
1
-
FeH]2, 22 °C, C6D6) 72, 42, 22, 21, 14, 13, 1.2, -1.3, -5, -8, -10,
-19, -26, -29, -42, -51, -120. H NMR (LtBu′FeH, 80 °C, C6D6)
1
δ 45 (1, backbone-CH), 31 (2, p-iPr-CH), 30 (18, tBu), 9 (4, m-H),
-3 (12, iPr-CH3), -17 (12, iPr-CH3), -81 (4, o-iPr-CH), -82 (12,
iPr-CH3). Anal. Calcd for C82H132N4Fe: C 76.61, H 10.35, N 4.36.
Found: C 76.59, H 10.42, N 4.22.
Preparation of LtBu′FeNPhNHPh. From [LtBu′FeH]2: An orange
solution of azobenzene (14 mg, 77 µmol) in Et2O (2 mL) was added
to a dark red slurry of [LtBu′FeH]2 (50 mg, 38 µmol) in Et2O (8 mL).
A red solution was formed over the course of an hour. The volatile
materials were removed under vacuum. The residue was extracted with
pentane, filtered through Celite, concentrated, and stored at -35 °C to
give LtBu′FeNPhNHPh as red crystals. From LtBu′FeCl: A 20 mL
scintillation vial was charged with LtBu′FeCl (172 mg, 253 µmol),
azobenzene (46 mg, 253 µmol), and Et2O (10 mL). A solution of
KBEt3H (35 mg, 253 µmol) in Et2O (2 mL) was added to give a
dark red solution. After stirring at room temperature for 2 h, the
volatile components were removed under reduced pressure. The
resultant residue was extracted with pentane and filtered through
Celite to give a red solution. The solution was concentrated and
stored at -35 °C to afford LtBu′FeNPhNHPh as red crystals (126 mg,
Preparation of LtBuFeNPhNPh. A 20 mL scintillation vial was
charged with LtBuFeNNFeLtBu (98 mg, 86 µmol) and Et2O (10 mL). A
solution of azobenzene (31 mg, 170 µmol) in Et2O (2 mL) was added
to yield a brown solution, which turned dark green upon stirring for
30 min. After stirring at room temperature for 2 h, the volatile
components were removed under reduced pressure. The resultant residue
was extracted with toluene and filtered through Celite to give a dark
green solution. The solution was concentrated and stored at -35 °C to
1
afford LtBuFeNPhNPh as dark green crystals (51 mg, 40%). H NMR
(400 MHz, C6D6) δ 119 (4, br, Ph o-H), 72 (1, Ph p-H), 42 (1, Ph
p-H), 24 (18, tBu), -11 (12, iPr-CH3), -15 (4, diketiminate m-H),
-17 (2, diketiminate p-H), -22 (2, Ph m-H), -50 (12, iPr-CH3), -70
(2, br, iPr-CH). Anal. Calcd for C47H63N4Fe: C 76.30, H 8.58, N 7.57.
Found: C 76.51, H 8.42, N 8.00.
1
60%). H NMR (400 MHz, C6D6) δ 111 (1, backbone-CH), 43 (18,
Crystallography. Crystals were placed onto the tip of a 0.1 mm
diameter glass capillary tube or fiber and mounted on a Bruker SMART
APEX II CCD Platform diffractometer52 for data collection at 100.0-
(1) K using Mo KR radiation (graphite monochromator). A randomly
oriented region of reciprocal space was surveyed: four major sections
of frames were collected with 0.30° steps in ω at four different φ settings
and a detector position of -28° in 2θ. The intensity data were corrected
for absorption.53 Final cell constants were calculated from the xyz
centroids of >3500 strong reflections from the actual data collections.54
The structures were solved using direct methods and refined using
SHELXL-97.55 The space groups (C2/c in each case) were determined
on the basis of systematic absences and intensity statistics. All non-
hydrogen atoms were refined with anisotropic displacement parameters.
The bridging hydrogen atoms in [LtBu′FeH]2 were found from the
difference Fourier map, and their positions were refined independently
from the iron atoms, but with relative (based on Fe1) isotropic
displacement parameters. It is understood that these positions are
approximate. All other hydrogen atoms were placed in ideal positions
and refined as riding atoms with relative isotropic displacement
tBu), 23 (2, p-iPr-CH), -7.8 (12, p-iPr-CH3), -8.6 (4, diketiminate
m-H), -23 (6, o-iPr-CH3), -29 (6, o-iPr-CH3), -45 (1), -82 (6, o-iPr-
CH3), -170 (6, 0.09, o-iPr-CH3), -181 (br). Due to the thermal
instability of the solid, we have not been able to obtain successful
elemental analysis.
Preparation of LtBuFeNTolNHTol. A 20 mL scintillation vial was
charged with LtBuFeCl (102 mg, 172 µmol), azo-m-toluene (36 mg,
172 µmol), and Et2O (10 mL). A solution of KBEt3H (24 mg, 172
µmol) in Et2O (2 mL) was added to yield a dark red solution. After
stirring at room temperature for 2 h, the volatile components were
removed under reduced pressure. The resultant residue was extracted
with pentane and filtered through Celite to give a red solution. The
solution was concentrated and stored at -35 °C to afford LtBu
-
FeNTolNHTol as red crystals (82 mg, 62%). 1H NMR (400 MHz, C6D6)
δ 112 (1, backbone-CH), 43 (18, tBu), -7 (2, diketiminate m-H), -8
(2, diketiminate m-H), -23 (6, iPr-CH3), -28 (6, iPr-CH3), -44 (1,
hydrazido p-H), -83 (6, iPr-CH3), -91 (2, diketiminate p-H), -172
(8, iPr-CH3 and iPr-CH).
Preparation of LtBu′FeNTolNHTol. An orange solution of azo-m-
toluene (16 mg, 76 µmol) in Et2O (2 mL) was added to a dark red
slurry of [LtBu′FeH]2 (49 mg, 38 µmol) in Et2O (8 mL). A red solution
was formed over the course of an hour. The volatile materials were
removed under vacuum. The residue was extracted with pentane, filtered
parameters. The final full-matrix least-squares refinement for [LtBu′
-
FeH]2 converged to R1 ) 0.0467 (F2, I > 2σ(I)) and wR2 ) 0.1328
(F2, all data). The final full-matrix least-squares refinement for
(52) APEX2 V1.0-22; Bruker Analytical X-ray Systems: Madison, WI,
2004.
through Celite, concentrated, and stored at -35 °C to give LtBu′
-
(53) SADABS V2.10: Blessing, R. Acta Crystallogr. 1995, A51, 33-38.
(54) SAINT V7.06A; Bruker Analytical X-ray Systems: Madison, WI, 2003.
(55) SHELXTL V6.14; Bruker Analytical X-ray Systems: Madison, WI,
2000.
FeNTolNHTol as red crystals (39 mg, 60%).
Preparation of PhNDNDPh. A Schlenk tube was charged with
N,N′-diphenylhydrazine (215 mg, 1.17 mmol) and pentane (10 mL). A
9
8120 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007

Mechanism of N
d
N Cleavage by an Iron(II) Hydride
A R T I C L E S
LtBuFe(N2Ph2) converged to R1 ) 0.0384 (F2, I > 2σ(I)) and wR2 )
0.1035 (F2, all data).
for helpful discussions. Financial support was provided by the
National Institutes of Health (GM-065313).
Acknowledgment. This paper is dedicated to the memory of
Ian Rothwell, who taught the chemical community about
transition-metal-mediated NdN bond breaking, and who pro-
vided useful discussions in the early part of this work. We also
thank Joseph Dinnocenzo, William Jones, and Richard Finke
Supporting Information Available: Eyring plot and NMR
data (pdf) and crystallographic data (cif). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA069199R
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J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007 8121