Reactivity and Moessbauer spectroscopic characterization of an Fe(IV) ketimide complex and reinvestigation of an Fe(IV) norbornyl complex

Article abstract of DOI:10.1021/ic401096p

Thermolysis of Fe(Ni=C^tBu₂)₄ (1) for 8 h at 50 °C generates the mixed valent Fe(III)/Fe(II) bimetallic complex Fe₂(Ni=C^tBu₂)₅ (2) in moderate yield. Also formed in this reaction

Full text of DOI:10.1021/ic401096p

Article
pubs.acs.org/IC
̈
Reactivity and Mossbauer Spectroscopic Characterization of an
Fe(IV) Ketimide Complex and Reinvestigation of an Fe(IV) Norbornyl
Complex
Richard A. Lewis,† Danil E. Smiles,† Jonathan M. Darmon,‡ S. Chantal E. Stieber,‡ Guang Wu,†
and Trevor W. Hayton*,†
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Thermolysis of Fe(NCtBu2)4 (1) for 8 h at
50 °C generates the mixed valent Fe(III)/Fe(II) bimetallic
complex Fe2(NCtBu2)5 (2) in moderate yield. Also formed
in this reaction are tert-butyl cyanide, isobutane, and
isobutylene, the products of ketimide oxidation by the Fe4+
center. Reaction of 1 with 1 equiv of acetylacetone affords the
Fe(III) complex, Fe(NCtBu2)2(acac) (3), concomitant with
formation of bis(tert-butyl)ketimine, tert-butyl cyanide, iso-
butane, and isobutylene. In addition, the Mossbauer spectra of
̈
1 and its lower-valent analogues [Li(12-crown-4)2][Fe(N
CtBu2)4] (5) and [Li(THF)]2[Fe(NCtBu2)4] (6) were recorded. We also revisited the chemistry of Fe(1-norbornyl)4 (4) to
elucidate its solid-state molecular structure and determine its Mossbauer spectrum, for comparison with that recorded for 1.
̈
Interestingly, FeF4 is predicted by density functional theory to
exhibit a quintet ground state and a D2d geometry. Fe(1-
norbornyl)4 was isolated by Bower and Tennent in 50% yield
by reaction of 1-norbornyllithium with FeCl3·Et2O.24 In the
original report, Fe(1-norbornyl)4 was described as a diamag-
netic purple crystalline solid; however, it was not fully
characterized and the experimental details describing its
synthesis were sparse. Nonetheless, it was postulated that
Fe(1-norbornyl)4 was formed by disproportionation of a
transient Fe(III) or Fe(II) 1-norbornyl complex. Subsequently,
Thiele and co-workers reinvestigated the synthesis of Fe(1-
norbornyl)4.26 They prepared Fe(1-norbornyl)4 in modest
yield, by reaction of 1-norbornyllithium with Fe(acac)3, and
INTRODUCTION
■
Fe(IV) is both a biologically relevant and synthetically useful
oxidation state.1−5 From a synthetic standpoint, numerous
researchers have demonstrated the utility of the [FeO]2+
moiety to effect a variety of epoxidations and aliphatic C−H
activations.1,6−8 For example, Que and co-workers have shown
that [Fe(O)(TMG2dien)(MeCN)]2+ (TMG2dien = 1,1-bis{2-
[N2-(1,1,3,3-tetramethylguanidino)]ethyl}methylamine) can
rapidly oxidize 1,4-cyclohexadiene and 9,10-dihydro-
anthracene,9 while White and co-workers have used Fe-
catalyzed C−H hydroxylation to synthesize several complex
natural products.10−12 Similarly, in the aziridination of alkenes
with aryl azides, an Fe(IV) imido, [FeNR]2+, is a proposed
intermediate in the catalytic cycle.13 In a few instances, the
Fe(IV) moiety implicated in catalysis has been isolated and
structurally characterized;14−16 however, in many cases these
Fe(IV) complexes are far too reactive to isolate,7,9,17−20 in part
because of the highly oxidizing nature of these intermediates.
We recently reported the synthesis of an isolable Fe(IV)
complex, Fe(NCtBu2)4 (1), which can be prepared in good
yield by oxidation of the Fe(II) ketimide complex, [Li-
(THF)]2[Fe(NCtBu2)4], with iodine.21 Complex 1 is a rare
example of a stable Fe(IV) complex and an extremely rare
example of an FeX4-type complex. In fact, MX4-type complexes
of the late first row transition metals (Fe, Co, and Ni) are
highly uncommon.22 To our knowledge, only two other FeX4-
type complexes have appeared in the literature, namely, FeF4
and Fe(1-norbornyl)4.23−25 FeF4 was recently identified by IR
spectroscopy in Ar or Ne matrices at cryogenic temperatures.23
1
were able to record its H and 13C NMR spectra. Soon after,
Theopold and co-workers synthesized the Co analogue, Co(1-
norbornyl)4, by reaction of CoCl2·THF with 4 equiv of 1-
norbornyllithium.27−29 They characterized this complex by X-
ray crystallography, magnetometry, and cyclic voltammetry and
confirmed the low-spin ground state first proposed by Bower
and Tennent.24 Also of note is the synthesis of Ni(1-
norbornyl)3Br by oxidation of [NBu4]2[Ni(1-norbornyl)3Br]
with O2.30 Finally, we synthesized the homoleptic Co(IV)
ketimide, Co(NCtBu2)4,31 further confirming the ability of
the ketimide ligand to stabilize the 4+ ions of the late first row
transition metals.
Received: May 2, 2013
© XXXX American Chemical Society
A
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Scheme 1
Scheme 2
1
Given the above-mentioned paucity of MX4-type complexes,
we deemed it worthwhile to investigate the reactivity and
spectroscopic properties of 1 in further detail. Herein we report
a preliminary reactivity study of Fe(NCtBu2)4, along with its
evaluate the suitability of Fe(NCtBu2)4 as a general purpose
synthon for Fe(IV) chemistry. Additionally, we revisited the
synthesis of Fe(1-norbornyl)4 and report its solid-state
identified by comparison of its H NMR spectrum with that of
authentic material.32 This complex was previously isolated by
oxidation of [Li(12-crown-4)2][Fe2(NCtBu2)5] with I2. The
formation of 1 and 2 can be rationalized by transient formation
of Fe(NCtBu2)3, which is unstable under the conditions of
the experiment and disproportionates into 1 and Fe(N
CtBu2)2. Fe(NCtBu2)2 is subsequently trapped by Fe(N
CtBu2)3 to generate complex 2. The isolation of 1 by
disproportionation of the putative Fe(NCtBu2)3 intermediate
provides further evidence of the strong donating ability of the
ketimide ligand. For comparison, the formation of Cr(N
CtBu2)4 from CrCl3 is also thought to proceed via a similar
disproportionation pathway.33 Likewise, formation of Fe(1-
norbornyl)4 from FeCl3 may proceed via disproportionation of
an Fe(III) intermediate (see also below).24
Mossbauer spectroscopic characterization, in an attempt to
̈
molecular structure and Mossbauer spectrum in an effort to
̈
better understand the properties of this elusive oxidation state.
RESULTS AND DISCUSSION
■
As previously reported,21 Fe(NCtBu2)4 (1) can be accessed
by oxidation of [Li(THF)]2[Fe(NCtBu2)4] with elemental
iodine in Et2O. Upon crystallization from a concentrated Et2O
solution, 1 can be easily separated from the LiI byproduct,
which is soluble even in cold Et2O. Using this procedure,
complex 1 can be isolated in excellent (93%) yield. This
represents an improvement over our original synthesis of 1
which first involved selective precipitation of LiI as its DME
adduct followed by crystallization from hexanes.21 Interestingly,
we have also discovered an alternate synthetic route to this
molecule that parallels the disproportionation proposed for
Fe(1-norbornyl)4. Namely, reaction of FeCl3 with 3 equiv of
Li(NCtBu2) in THF for 4 h yields a maroon solution from
which complex 1 can be isolated in 17% yield (maximum yield
of 33% based on iron) (Scheme 1). A second product is also
formed in this transformation, Fe2(NCtBu2)5 (2), which was
Unlike many Fe(IV) coordination complexes,7,16,34 complex
1 is remarkably stable in solution. For example, in C6D6, 1
exhibits an approximate half-life of 5 days at room temperature.
The decomposition of 1 is first order with respect to Fe(N
CtBu2)4, with a rate constant of 1.54 × 10−6 s−1. This
decomposition can be accelerated by heating a solution of 1 in
C6D6 at 50 °C for 9 h, yielding a maroon solution. This results
in the disappearance of the tert-butyl resonance assignable to 1
(at 1.69 ppm) in the 1H NMR spectrum and the appearance of
a new resonance at 10.6 ppm, which we previously assigned to
be the mixed valent complex Fe2(NCtBu2)5 (2) (Scheme
2).32 Also observed in this spectrum is the formation of tert-
butyl cyanide (0.76 ppm), isobutane (1.63 and 0.84 ppm), and
B
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Scheme 3
isobutylene (4.74 and 1.59 ppm) (see the Supporting
Information). By integration against an internal standard, the
yields of tert-butyl cyanide, isobutane, and isobutylene were
determined to be 63%, 36%, and 55%, respectively, assuming
the stoichiometry suggested in Scheme 2. These products are
indicative of the ketimide ligand oxidation and have been
observed previously in the decomposition of Mn(N
CtBu2)4.35 To account for the formation of the observed
products, we suggest that homolysis of the C−C bond in the
ketimide ligand of 1 results in formation of “Fe(NCtBu2)3”,
tert-butylcyanide and the tert-butyl radical. Subsequent
disproportionation of the tert-butyl radical results in formation
of isobutylene and isobutane,35−38 while Fe(NCtBu2)3
undergoes the disproportionation described in Scheme 1. On
a preparative scale, thermolysis of 1 in Et2O at 50 °C for 8 h
provides 2 in 40% yield.32 Interestingly, complex 2 can also be
formed by heating a mixture of FeCl3 and 3 equiv of Li(N
CtBu2) in Et2O at 55 °C for 18 h. Under these conditions, 2 can
be isolated in a 72% yield.
the acac ligand. Also observed in this reaction mixture are
bis(tert-butyl)ketimine, isobutane, isobutylene, and tert-butyl
cyanide. To account for the formation of isobutane and
isobutylene, we suggest that ketimide protonation initially
generates the putative intermediate “Fe(NCtBu2)3(acac)”,
whose ligand set, with only three ketimide ligands, is unable to
support the 4+ oxidation state. Subsequent ligand oxidation
generates complex 3, tert-butyl cyanide, and the tert-butyl
radical. Alternately, the change in geometry required to form
this 5-coordinate species could induce oxidation of a ketimide
1
ligand. Monitoring the formation of 3 by H NMR spectros-
copy reveals the presence of complex 1 and Fe(acac)3 at short
reaction times. The presence of Fe(acac)3 was confirmed by
1
comparison of its H NMR spectrum with that of authentic
material. At longer reaction times (21 h) this mixture
completely converts to complex 3, suggesting that complex 1
and Fe(acac)3 undergo ligand exchange to generate the final
product. In support of this hypothesis, complex 3 can also be
formed by the reaction of 0.5 equiv of Fe(acac)3 with 1 at 25
°C in Et2O (Scheme 3).
To further support the proposed decomposition mechanism
of 1, we endeavored to trap the transient tert-butyl radical
intermediate with P4. It has been previously shown that P4 is an
effective trap for organic radicals.39,40 Thus, thermolysis of 1 at
50 °C in the presence of 0.25 equiv of P4 for 29 h in C6D6
yields PtBu3 as the sole phosphorus-containing product
(Scheme 2) in addition to the anticipated formation of 2 and
tert-butyl cyanide. The generation of PtBu3 was confirmed by
observation of a singlet in the 31P NMR spectrum at 64.2 ppm,
consistent with previously reported literature values.41 Similarly,
a doublet at 31.3 ppm (JPC = 6.2 Hz) in the 13C NMR spectrum
is also assignable to PtBu3.41 Importantly, there is no formation
Crystals of 3 suitable for X-ray crystallographic analysis were
grown from a concentrated Et2O solution at −25 °C. Complex
3 crystallizes in the triclinic space group P1 and exhibits a
̅
distorted tetrahedral environment about the Fe center [e.g.,
N1−Fe1−N2 = 113.28(7)°, N2−Fe1−O1 = 109.36(6)°, O1−
Fe1−O2 = 90.98(5)°] (Figure 1). The Fe−N bond lengths are
1.8404(16) and 1.8456(16) Å, which are slightly shorter than
those observed previously for Fe(III) ketimides, but are
nonetheless consistent with the Fe3+ oxidation-state assign-
ment.21 In addition, the Fe−N−C angles in 3 [159.53(15)° and
176.31(15)°] are suggestive of π-donation to the metal center.
Finally, the Fe−O bond lengths [Fe1−O1 = 1.9736(12) Å and
Fe1−O2 = 1.9740(13) Å] are in line with those observed
previously for Fe3+ acetylacetonate complexes.42,43
1
of isobutane or isobutylene in this sample according to H
NMR spectroscopy. This supports our hypothesis that P4
captures the tert-butyl radical before it can undergo
disproportionation.
Given the paucity of stable FeX4-type complexes, we revisited
the synthesis of Fe(1-norbornyl)4 in an effort to complete its
characterization. Thus, addition of 3 equiv of 1-norbornyl-
lithium to a solution of FeCl3 in 1:10 Et2O/pentane at −25 °C
generates a deep purple solution and a black precipitate after 4
h of stirring (Scheme 4). Filtration of this mixture through an
alumina column followed by extraction of the material in
MeCN and crystallization from MeCN/Et2O yields Fe(1-
norbornyl)4 (4) as a deep purple solid in 25% yield (based
upon 1-norbornyllithium). We have found that filtration
through alumina is effective for removing excess 1-norbornyl-
lithium from the reaction mixture, while recrystallization of 4
To evaluate the suitability of complex 1 as a synthon for
Fe(IV) chemistry, we undertook a series of simple ligand
exchange reactions, given that the strongly basic ketimide anion
should be readily amenable to protonation. Thus, reaction of 1
with 1 equiv of acetylacetone (Hacac) generates a purple
solution from which the Fe(III) ketimide complex, Fe(N
CtBu2)2(acac) (3), can be isolated as a purple crystalline solid
in 50% yield (Scheme 3). The 1H NMR spectrum of 3 in C6D6
consists of a very broad resonance at 59.2 ppm, assignable to
the tert-butyl groups of the ketimide ligand, while a broad
resonance at −8.2 ppm is assignable to the methyl groups of
C
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Figure 2. Solid-state molecular structure of Fe(1-norbornyl)4 (4) with
50% thermal ellipsoids for selected atoms. Selected bond lengths (Å)
and angles (deg): Fe1−C1 = 2.002(14), Fe1−C6 = 1.984(7), Fe1−
C13 = 2.002(9); C1−Fe1−C6 = 109.4(3), C6−Fe1−C13 = 109.7(3).
in Co(1-norbornyl)4, which range between 1.912(23) and
1.930(21) Å, consistent with the larger ionic radius of Fe4+. The
Fe−C bond lengths of 4 are also longer than those observed for
Ni(1-norbornyl)3Br (av. 1.93 Å).30 The nearly perfect
tetrahedral geometry of complex 4, coupled with the apparent
diamagnetism, demonstrates that 1-norbornyl is an unusually
strong field ligand, given that all four d electrons are forced into
the dz and dx −y orbitals. The strong field nature of 1-
norbornyl was previously noted by Theopold and co-workers,
who determined Δt for a series of cobalt 1-norbornyl
complexes.27
Figure 1. Solid-state molecular structure of Fe(NCtBu2)2(acac) (3)
with 50% thermal ellipsoids. Hydrogen atoms have been omitted for
clarity. Selected bond lengths (Å) and bond angles (deg): Fe1−N1 =
1.8456(16), Fe1−N2 = 1.8404(16), Fe1−O1 = 1.9736(12), Fe1−O2
= 1.9740(13); N1−Fe1−N2 = 113.28(7), N2−Fe1−O1 = 109.36(6),
O1−Fe1−O2 = 90.98(5).
2
2
2
Scheme 4
An aliquot of the crude reaction mixture, in C6D6, reveals the
presence of three identifiable species, namely, complex 4, 1-
norbornyllithium, and 1,1′-binorbornyl in an approximately
1
1:3:1 ratio, respectively, according to H NMR spectroscopy.
These materials were readily identified by their characteristic
bridgehead proton resonances, which appear at 2.33, 2.51, and
2.17 ppm for 4, 1-norbornyllithium, and 1,1′-binorbornyl,
respectively.30 The formation of 1,1′-binorbornyl can be
rationalized by the coupling of two 1-norbornyl radicals,
formed by oxidation of 1-norbornyllithium by FeCl3.44 The
presence of 1-norbornyllithium, the dominant 1-norbornyl-
containing species in solution, is more challenging to explain
given the 3:1 stoichiometry of the reaction and suggests that
unreacted iron halides are sequestered within the black
precipitate.
from MeCN/Et2O provides material that is free of 1,1′-
binorbornyl (see below).
The 1H NMR spectrum of 4 in C6D6 exhibits a characteristic
resonance at 2.33 ppm, assignable to the bridgehead CH group
at position 4 of the norbornyl skeleton, while its 13C NMR
spectrum agrees with that obtained by Thiele and co-workers.26
Single crystals of 4 suitable for X-ray diffraction analysis were
obtained from a concentrated pentane solution at −25 °C.
Fe(1-norbornyl)4 crystallizes in the orthorhombic space group
Pmn21 (Figure 2) and is isomorphous with its Co analogue.29
As was observed for Co(1-norbornyl)4, the 1-norbornyl ligands
in 4 are disordered over multiple positions, a consequence of
facile rotation about the Fe−C bond. The Fe center exhibits a
nearly idealized tetrahedral coordination environment about
the iron center [C1−Fe1−C6 = 109.4(3)°, C6−Fe1−C13 =
109.7(3)°]. The M−C bond lengths range between 1.984(7)
and 2.002(14) Å and are longer than the Co−C bond lengths
Complex 4 could also be prepared by reaction of FeCl2 with
2 equiv of 1-norbornyllithium in a 1:10 mixture of Et2O/
pentane (Scheme 4). Under these conditions, 4 can be isolated
in 21% yield. Also formed in the reaction are Fe metal, present
as a fine black powder that adheres to the stir bar, and 1,1′-
binorbornyl, which was observed in the crude reaction mixture
1
by H NMR spectroscopy (Supporting Information, Figure
S24). We also repeated the synthetic procedure reported by
Thiele,26 namely, reaction of Fe(acac)3 with 4 equiv of 1-
norbornyllithium in pentane, and were able to isolate 4 in 21%
yield (Scheme 4). We also observed formation of small
amounts of 1,1-binorbornyl in the reaction mixture, according
1
to H NMR spectroscopy (Supporting Information, Figure
S26). The choice of solvent in both reactions appears to be
crucial for success. For example, the presence of Et2O in the
D
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FeCl2 reaction is critical for the formation of 4 because in pure
pentane, the reaction is extremely slow and the yields of 4 are
greatly reduced. However, in the case of Fe(acac)3, the reaction
must be performed in pure pentane, as the presence of any
Et2O results in formation of an intractable mixture of products.
Given the above evidence, we suggest that complex 4 is
formed by disproportionation of a transient Fe(III) 1-norbornyl
complex, resulting in concomitant formation of Fe(0) or other
low-valent Fe-containing products. A similar conclusion was
reached by both Bower and Theopold in their investigations of
1-norbornyllithium with metal salts.24,27 Alternately, reduction
of Fe3+ to Fe2+ by 1-norbornyllithium may be a necessary first
step along the reaction pathway. This is followed by
disproportionation of the Fe2+ ion to Fe(IV) and Fe(0)
(Scheme 5). This hypothesis is supported by the observation
In the solid state, complex 5 exhibits a squashed tetrahedral
geometry (e.g., N1−Fe1−N2 = 145.02(14)°, τ4 = 0.50)45
(Figure 3). Its Fe−N bond lengths are 1.858(4) and 1.859(3)
Å, while the ketimide ligands are bound in a distinctly nonlinear
fashion (Fe1−N1−C1 = 154.9(3)°, Fe1−N2−C10 =
156.6(3)°). The Fe−N bond lengths in 5 are ca. 0.05 Å
shorter than those observed in the isostructural Mn(III)
analogue, [Li(12-crown-4)2][Mn(NCtBu2)4],35 likely a con-
sequence of the smaller ionic radius of Fe3+. However, the
metrical parameters of 5 are nearly identical to those exhibited
by the Co(III) analogue, [Li(12-crown-4)2][Co(N
CtBu2)4].31 Finally, solid-state superconducting quantum
interference device (SQUID) magnetometry measurements
reveal that complex 5 exhibits an effective magnetic moment of
3.85 μB at 300 K, consistent with an S = 3/2 ground state. This
spin state contrasts with the high-spin S = 5/2 ground state of
[Li(DME)][Fe(NCtBu2)4], which possesses a slightly
distorted tetrahedral geometry.21 However, the intermediate S
= 3/2 spin state of complex 5 is identical to that observed for
the isoelectronic Co(IV) complex, Co(NCtBu2)4,31 suggest-
ing that complex 5 features a similar d orbital splitting pattern.
Scheme 5
̈
Mossbauer Spectroscopy. To confirm the 4+ oxidation-
state assignments in complexes 1 and 4, we recorded their zero-
field 57Fe Mossbauer spectra at 80 K. For further comparison,
̈
we also recorded the zero-field 57Fe Mossbauer spectra of
̈
complex 5 and the previously reported Fe(II) ketimide,
[Li(THF)]2[Fe(NCtBu2)4] (6).21 The zero-field 57Fe
Mossbauer spectra of 1 and 4 are shown in Figure 4, while
̈
relevant spectral parameters are summarized in Table 1.
Complex 1 features a single quadrupole doublet with
parameters of δ = −0.15 mm/s and ΔEQ = 1.62 mm/s. The
low value for the isomer shift is consistent with the 4+
oxidation-state assignment and compares well with isomer
shifts recorded for a variety of authentic Fe(IV) complexes
that FeCl2 is also a viable starting material for the formation of
4 and by the presence of 1,1′-binorbornyl in the FeCl3 reaction
mixture.
For further spectroscopic comparison, we also synthesized a
nearly isostructural Fe(III) analogue of complex 1. Thus,
reaction of [Li(DME)][Fe(NCtBu2)4]21 with 2 equiv of 12-
crown-4 yields [Li(12-crown-4)2][Fe(NCtBu2)4] (5) as a
maroon solid in 84% yield (Scheme 6). The 1H NMR spectrum
(Table 1).14,34,46−48 The Mossbauer spectrum of the Fe(III)
̈
analogue, 5, consists of two quadrupole doublets in a 93:7 ratio.
The major component is assignable to 5 and features
parameters of δ = 0.19 mm/s and ΔEQ = 3.56 mm/s (see
the Supporting Information). The minor component is
assignable to 1, which is likely formed by oxidation of 5 by
adventitious O2 during sample handling. The isomer shift of 5
is consistent with reported S = 3/2 Fe(III) compounds,
although other iron oxidation and spin states may also be found
in this range.49 The asymmetry of the spectrum is also
Scheme 6
consistent with half-integer spin.49−51 The Mossbauer spectrum
̈
of 6 also consists of two quadrupole doublets, in a 85:15 ratio.
The major component, which is assignable to 6, features
parameters of δ = 0.44 mm/s and ΔEQ = 0.85 mm/s. Again, the
minor component is assignable to complex 1. Most
importantly, a comparison of the isomer shifts of 6, 5, and 1
reveals a periodic decrease in isomer shift as the Fe oxidation
state is increased, in full accord with our proposed oxidation-
state assignments.
of 5 in py-d5 at −36 °C features a broad singlet at 31.1 ppm,
assignable to the methyl groups of the ketimide ligand, while its
7
−36 °C Li NMR spectrum consists of a broad singlet at 3.4
ppm, assignable to the [Li(12-crown-4)2] cation. When the
sample is heated to room temperature, the resonance at 31.1
ppm in the 1H NMR spectrum decreases in intensity, while new
resonances appear at 46.9 and 37.8 ppm. Similar behavior was
observed previously for the Co analogue, [Li(12-crown-
4)2][Co(NCtBu2)4],31 and was rationalized by invoking an
equilibrium between [Li(12-crown-4)2][Co(NCtBu2)4] and
a close contact ion pair formed by loss of 12-crown-4 and
coordination of Li+ by the nitrogen atoms of the ketimide
ligands.31
The Mossbauer spectrum of complex 4 exhibits an
̈
absorption modeled with spectral parameters of δ = −0.28
mm/s and ΔEQ = 0.15 mm/s. The low isomer shift of 4 is
consistent with the 4+ oxidation-state assignment,14,34,46−48
while the near zero ΔEQ is suggestive of a spherical electric field
gradient tensor.52 This latter parameter is in line with the
tetrahedral geometry of 4, as determined by X-ray crystallog-
raphy, and unambiguously identifies complex 4 as a low-spin
Fe(IV) complex.
E
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Figure 3. Solid-state molecular structure of [Li(12-crown-4)2][Fe(NCtBu2)4] (5) with 40% probability ellipsoids. [Li(12-crown-4)2]+ and
hydrogen atoms are excluded for clarity. Atoms with an asterisk are generated by symmetry. Selected bond lengths (Å) and angles (deg): Fe1−N1 =
1.859(3), Fe1−N2 = 1.858(4); N1−Fe1−N2 = 145.02(14), N1−Fe1−N1* = 94.0(2), Fe1−N1−C1 = 154.9(3), Fe1−N2−C10 = 156.6(3).
Figure 4. Solid-state zero-field 57Fe Mossbauer spectra of complexes 1 (left) and 4 (right) collected at 80 K.
̈
Table 1. Mossbauer Data for Complexes 1 and 4−6 and Selected Fe(IV) Complexes
̈
a
complex
δ (mm/s)
ΔEQ (mm/s)
oxidation state
S
ref
[Li(THF)]2[Fe(NCtBu2)4] (6)
[Li(12-crown-4)2][Fe(NCtBu2)4] (5)
Fe(NCtBu2)4 (1)
0.44
0.19
0.85
3.56
1.62
0.15
2.43
1.92
6.23
0.89
3.38
6.01
2
3
4
4
4
4
4
4
4
4
2
b
b
b
b
3/2
0
−0.15
−0.28
0.02
Fe(1-norbornyl)4 (4)
0
[Fe(F)(Me3cyclam-acetate)]2+
[Fe(N3)(Me3cyclam-acetate)]2+
Fe(N)(PhB(tBuIm)3)
1
52,53
53
0.11
1
−0.28
−0.04
−0.04
−0.34
0
48
[Fe(TAML)Cl]−
2
54
[Fe(TAML)(CNtBu)2]
1
54
[Fe(N)(PhBPiPr3)]
0
34,55
a
Ligand definitions: Me3cyclam-acetate = 4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane-1-acetate; TAML = 1,4,8,11-tetraaza-13,13-diethyl-
2,2,5,5,7,7,10,10-octamethyl-3,6,9,12,14-pentaoxocyclotetradecane; [PhB(tBuIm)3]− = phenyltris(3-tert-butylimidazol-2-ylidene)borato; PhBPiPr
=
3
b
[PhB(CH2PiPr2)3]−. This work.
oxidation-state assignment. Upon being heated gently, Fe(N
CtBu2)4 converts into the mixed valent Fe(III/II) complex,
Fe2(NCtBu2)5. Also formed in the reaction are the products
of ketimide ligand oxidation, tert-butylcyanide, isobutane, and
CONCLUSION
We have provided further detail regarding the synthesis and
■
characterization of the homoleptic Fe(IV) ketimide, Fe(N
CtBu ) . Importantly, its Mossbauer spectrum confirms the 4+
̈
2
4
F
dx.doi.org/10.1021/ic401096p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Monitoring of the Thermal Stability of Fe(NCtBu2)4 (1) by
NMR Spectroscopy. Fe(NCtBu2)4 (8.0 mg, 0.013 mmol) and
hexamethyldisiloxane (3 μL, 14 μmol) were dissolved in C6D6 (0.7
mL), yielding a red-brown solution. The decomposition of 1 and
isobutylene. Similarly, reaction of Fe(NCtBu2)4 with
acetylacetone results in formation of a lower-valent Fe complex,
namely, Fe(NCtBu2)2(acac), along with the products of
ketimide oxidation. Because reduction of the Fe4+ center in
Fe(NCtBu2)4 is apparently quite facile, it appears unlikely
that this complex will be a useful synthon for Fe(IV) chemistry.
It is clear that the development of a ketimide-based Fe(IV)
synthon will require a reassessment of the ligand architecture as
the bis(tert-butyl)ketimide ligand is too easily oxidized. There is
no doubt that the relative stability of the tert-butyl radical plays
a role in lowering the activation barrier of the decomposition
process,56 and for future work we plan to improve the stability
of the ketimide ligand by employing alkyl substituents that are
not as likely to undergo unwanted C−C bond homolysis.
Finally, we revisited the synthesis of Fe(1-norbornyl)4 and
confirmed its formulation with X-ray crystallography and
t
formation of 2, BuCN, isobutane, and isobutylene at 25 °C were
monitored over 16 days. The yields of tBuCN, isobutane, and
isobutylene were determined by integration against the hexamethyl-
disiloxane internal standard.
Thermolysis of Fe(NCtBu2)4 (1). Fe(NCtBu2)4 (9.3 mg,
0.015 mmol) was dissolved in C6D6 (0.5 mL), yielding a red-brown
solution. The J-Young NMR tube was heated for 9 h at 50 °C, yielding
t
a maroon solution containing 2, isobutane, isobutylene, and BuCN.
1H NMR (C6D6, 25 °C, 400 MHz) δ: 10.6 (br s, Fe2(NCtBu2)5),
4.74 (s, (CH3)2CCH2), 1.63 (m, (CH3)3CH), 1.59 (s, (CH3)2CCH2),
0.84 (d, JHH = 5.6 Hz, (CH3)3CH), 0.76 (s, (CH3)3CCN).
Thermolysis of Fe(NCtBu2)4 in the Presence of P4. Fe(N
CtBu2)4 (9.0 mg, 0.015 mmol) was dissolved in C6D6 (0.5 mL),
yielding a red-brown solution. To this solution was added a solution of
P4 in C6D6 (35 μL, 0.1126 M, 0.004 mmol of P4). The J-Young NMR
tube was heated for 29 h at 50 °C, yielding a maroon solution
containing 2,32 PtBu3, and tBuCN. Formation of PtBu3 was confirmed
Mossbauer spectroscopy. Its low isomer shift and near zero
̈
quadrupolar splitting verify the low-spin d4 ground state and
provide further data for relating Mossbauer spectroscopic
̈
1
1
by H, 13C{1H}, and 31P{1H} NMR spectroscopies. H NMR (C6D6,
parameters to the electronic structure of high-valent Fe
systems.
25 °C, 400 MHz) δ: 0.76 (s, BuCN), 1.35 (s, PtBu3), 10.57 (br s,
t
Fe2(NCtBu2)5). 31P{1H} NMR (C6D6, 25 °C, 100 MHz) δ: 64.22
(s, PtBu3). 13C{1H} NMR (C6D6, 25 °C, 100 MHz) δ: 27.67 (s,
tBuCN), 31.29 (d, JPC = 6.2 Hz, P(C(CH3)3)3). The quaternary carbon
EXPERIMENTAL SECTION
■
resonances of PtBu3 and BuCN were not observed.
t
General. All reactions and subsequent manipulations were
performed under anaerobic and anhydrous conditions under an
atmosphere of nitrogen or argon. THF, hexanes, diethyl ether, and
toluene were dried using a Vacuum Atmospheres DRI-SOLV solvent
purification system. C6D6 and DME were dried over activated 3 Å
molecular sieves for 24 h before use. Li(NCtBu2) and 1-
norbornyllithium were synthesized according to the previously
reported procedures,27,57,58 while all other reagents were purchased
from commercial suppliers and used as received.
Synthesis of Fe2(NCtBu2)5 (2) via FeCl3. FeCl3 (172 mg, 1.0
mmol) was added to a stirring solution of Li(NCtBu2) (415 mg,
3.23 mmol) in Et2O (15 mL). After 24 h of being stirred, the maroon
solution was filtered through a Celite column supported on glass wool
(0.5 × 2 cm). The filtrate was dried in vacuo, and a 1H NMR spectrum
in C6D6 was recorded. 1H NMR (C6D6, 25 °C, 400 MHz) δ: 10.51 (br
s, Fe2(NCtBu2)5), 1.69 (s, Fe(NCtBu2)4). The resulting solid was
dissolved in hexanes (10 mL) and heated for 18 h at 55 °C to yield a
maroon solution. The maroon solution was filtered through a Celite
column supported on glass wool (0.5 × 2 cm). The filtrate was
concentrated in vacuo to 1 mL. Subsequent storage at −25 °C for 24 h
resulted in the deposition of maroon blocks, which were isolated by
1H, 13C{1H}, and 7Li{1H} NMR spectra were recorded on a Varian
UNITY INOVA 400, Varian UNITY INOVA 500, or Varian UNITY
1
INOVA 600 spectrometer. H NMR spectra are referenced to the
residual protio solvent peaks as internal standards. Li{1H} spectra
7
1
decanting off the supernatant (273 mg, 72% yield). H NMR (C6D6,
were referenced to external LiCl in D2O. IR spectra were recorded on
a Nicolet 6700 FT-IR spectrometer with a NXR FT Raman Module.
UV−vis/NIR experiments were performed on a UV-3600 Shimadzu
spectrophotometer. Elemental analyses were performed by the
Microanalytical Laboratory at University of California (Berkeley, CA).
Magnetism Measurements. Magnetism data were recorded
using a Quantum Design MPMS 5XL SQUID magnetometer.
Complex 5 was analyzed using 50 mg of powdered crystalline material
loaded into a NMR tube, which was subsequently flame-sealed. The
solid was kept in place with ∼45 mg of quartz wool packed on either
side of the sample. Data for complex 5 were collected using a 1 T field
between 4 and 300 K. Diamagnetic correction for 5, χdia = −7.09 ×
10−4 cm3 mol−1, was made using Pascal’s constants.59
25 °C, 400 MHz) δ: 10.59 (br s, Fe2(NCtBu2)5). This material was
spectroscopically identical to an authentic sample of Fe2(N
CtBu2)5.32
Synthesis of Fe2(NCtBu2)5 (2) by Thermolysis of Fe(N
CtBu2)4 (1). Fe(NCtBu2)4 (39.5 mg, 0.064 mmol) was dissolved in
C6D6 (0.5 mL), yielding a red-brown solution. The J-Young NMR
tube was heated for 8 h at 50 °C, yielding a maroon solution. 1H NMR
(C6D6, 25 °C, 400 MHz) δ: 10.7 (br s, Fe2(NCtBu2)5), 4.74 (s,
(CH3)2CCH2), 1.63 (m, (CH3)3CH), 1.58 (s, (CH3)2CCH2), 0.84 (d,
(CH3)3CH), 0.77 (s, (CH3)3CCN). 13C{1H} NMR (C6D6, 25 °C, 100
MHz) δ: 141.90 (s, (CH3)2CCH2), 111.11 (s, (CH3)2CCH2), 30.15 (s,
(CH3)3CCN), 27.10 (s, (CH3)3CCN), 25.86 (s, (CH3)3CH), 24.86 (s,
(CH3)3CH), 24.14 (s, (CH3)2CCH2). The solvent was removed in
vacuo, and the solid was dissolved into hexanes (1 mL). The maroon
solution was filtered through a Celite column supported on glass wool
(0.5 × 2 cm). Storage of this solution at −25 °C for 5 h resulted in the
deposition of dark blocks, which were isolated by decanting off the
supernatant (10.2 mg, 40% yield). This material was spectroscopically
identical to an authentic sample of Fe2(NCtBu2)5.32
Mossbauer Measurements. Zero-field 57Fe Mossbauer spectra
̈
̈
were collected at Princeton University on a SEE Co. Mossbauer
̈
spectrometer (MS4) with a 57Co/Rh radiation source at 80 K in
constant acceleration mode. The temperature in the sample chamber
was controlled by a Janis Research Co. CCS-850 He/N2 cryostat
within an accuracy of 0.3 K. The data were calibrated relative to an
α-iron standard at 298 K, with minimum experimental line widths of
0.23 mm/s. The fitting procedure to extract quantitative spectral
parameters uses a least-squares Lorentzian fitting method using the
WMOSS software developed by SEE Co.
Synthesis of Fe(NCtBu2)4 (1) via FeCl3. FeCl3 (24.5 mg, 0.15
mmol) was added to a stirring solution of Li(NCtBu2) (67.0 mg,
0.46 mmol) in THF (2 mL). After 4 h of being stirred, the maroon
solution was dried in vacuo, and the resulting solid was dissolved in
hexanes (2 mL). The maroon solution was then filtered through a
Celite column supported on glass wool (0.5 × 2 cm). The filtrate was
concentrated in vacuo to 1 mL. Subsequent storage at −25 °C for 24 h
resulted in the deposition of dark blocks, which were isolated by
decanting off the supernatant (12.2 mg, 17% yield, maximum yield of
33% based on iron). This material was spectroscopically identical to an
authentic sample of Fe(NCtBu2)4.21
Synthesis of Fe(NCtBu2)4 (1). To a stirring solution of
[Li(THF)]2[Fe(NCtBu2)4] (1.402 g, 1.81 mmol) in Et2O (10
mL) was added dropwise I2 (0.465 g, 1.82 mmol) as an Et2O solution
(5 mL). After 30 min of being stirred, the deep orange solution was
concentrated in vacuo to 10 mL. Subsequent storage at −25 °C for 24
h resulted in the deposition of dark crystals, which were isolated by
decanting off the supernatant (1.029 g, 93% yield). This material was
spectroscopically identical to an authentic sample of Fe(NCtBu2)4.21
G
dx.doi.org/10.1021/ic401096p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Synthesis of Fe(NCtBu2)2(acac) (3). Method A. To a stirring
red-orange solution of Fe(NCtBu2)4 (1) (103 mg, 0.17 mmol) in
Et2O (2 mL) was added an orange solution of Fe(acac)3 (28.3 mg,
0.084 mmol) in Et2O (2 mL). After 21 h of being stirred, the deep
purple solution was filtered through a Celite column supported on
glass wool (0.5 × 2 cm). The filtrate was concentrated in vacuo to 1
mL. Subsequent storage at −25 °C for 24 h resulted in the deposition
of purple blocks, which were isolated by decanting off the supernatant
(58 mg, 53% yield). Anal. Calcd for FeN2O2C23H43: C, 63.44; H, 9.95;
N, 6.43. Found: C, 63.14; H, 9.97; N, 6.26.
basic alumina column supported on glass wool (0.5 × 4 cm). The
vibrant purple filtrate was dried in vacuo, and the purple solid was
washed with acetonitrile (5 mL) (11.5 mg, 21% yield based upon 1-
1
norbornyllithium). H NMR (benzene-d6, 25 °C, 400 MHz) δ: 1.08
(br s, 24H), 1.46 (br s, 8H), 1.60 (br s, 8H), 2.33 (br s, 4H).
Synthesis of [Li(12-crown-4)2][Fe(NCtBu2)4] (5). To a brown
solution of [Li(DME)][Fe(NCtBu2)4] (115.3 mg, 0.16 mmol) in
Et2O (2 mL) was added a solution of 12-crown-4 (65.4 mg, 0.37
mmol) in pentane (3 mL). Storage of the resulting solution at −25 °C
for 24 h resulted in the deposition of a maroon solid, which was
isolated by decanting off the supernatant (132.0 mg, 84% yield). Anal.
Calcd for FeN4C52H104O8Li: C, 63.98; H, 10.74; N, 5.74. Found: C,
63.77; H, 11.02; N, 5.64. 1H NMR (pyridine-d5, −36 °C, 500 MHz) δ:
31.1 (br s), 3.67 (s, 12-crown-4). 7Li{1H} NMR (pyridine-d5, −36 °C,
500 MHz) δ: 3.4 (br s). 1H NMR (pyridine-d5, −28 °C, 500 MHz) δ:
Method B. To a red-orange solution of Fe(NCtBu2)4 (1) (39.9
mg, 0.065 mmol) in toluene (3 mL) was added acetylacetone (7 μL,
0.09 mmol). After 16 h of being stirred, a deep purple solution was
obtained, and the solvent was removed in vacuo. The purple solid was
extracted into hexanes (1 mL). Subsequent storage at −25 °C for 24 h
resulted in the deposition of purple blocks, which were isolated by
7
57.0 (br s), 45.4 (br s), 30.9 (br s), 3.67 (s, 12-crown-4). Li{1H}
1
decanting off the supernatant (14.2 mg, 50% yield). H NMR (C6D6,
NMR (pyridine-d5, −28 °C, 500 MHz) δ: 3.4 (br s). 1H NMR
(pyridine-d5, −20 °C, 500 MHz) δ: 55.0 (br s), 45.3 (br s), 30.6 (br s),
25 °C, 400 MHz) δ: 59.2 (br s, 36H, Fe(NCtBu2)(acac)), −8.2 (br
s, 6H, Fe(NCtBu2)(O(CH3)CH(CH3)O)). IR (Et2O, cm−1): 1654
(s, ν(NC)), 1579 (m, ν(NC)).
7
3.67 (s, 12-crown-4). Li{1H} NMR (pyridine-d5, −20 °C, 500 MHz)
Monitoring the Formation of Fe(NCtBu2)2(acac) (3) by
NMR Spectroscopy. To a red-orange solution of Fe(NCtBu2)4 (1)
(8 mg, 0.02 mmol) in C6D6 (0.5 mL) was added acetylacetone (2.5
μL, 0.03 mmol). After 10 min, a deep orange solution was obtained,
δ: 3.4 (br s). 1H NMR (pyridine-d5, −3 °C, 500 MHz) δ: 51.7 (br s),
41.5 (br s), 30.0 (br s), 3.67 (s, 12-crown-4). 7Li{1H} NMR (pyridine-
d5, −3 °C, 500 MHz) δ: 3.3 (br s). 1H NMR (pyridine-d5, 11 °C, 500
MHz) δ: 49.3 (br s), 39.7 (br s), 29.4 (br s), 3.66 (s, 12-crown-4).
1
1
and a H NMR spectrum was recorded. H NMR (C6D6, 25 °C, 400
MHz) δ: 20.3 (br s, 18H, Fe(O(CH3)CH(CH3)O)3), −23.8 (br s, 3H,
Fe(O(CH3)CH(CH3)O)3). The formation of Fe(acac)3 was con-
firmed by comparison to the 1H NMR spectrum of commercially
prepared Fe(acac)3. The reaction mixture was allowed to stand, and
after 21 h the 1H NMR spectrum of the now deep purple solution was
1
7Li{1H} NMR (pyridine-d5, 11 °C, 500 MHz) δ: 3.2 (br s). H NMR
(pyridine-d5, 25 °C, 500 MHz) δ: 46.9 (br s), 37.8 (br s), 28.7 (br s),
3.66 (s, 12-crown-4). 7Li{1H} NMR (pyridine-d5, 25 °C, 500 MHz) δ:
3.2 (br s). 1H NMR (pyridine-d5, 42 °C, 500 MHz) δ: 44.5 (br s), 35.9
7
(br s), 28.0 (br s), 3.66 (s, 12-crown-4). Li{1H} NMR (pyridine-d5,
42 °C, 500 MHz) δ: 3.2 (br s). UV−vis (C4H8O, 7.05 × 10−5 M): 480
nm (ε = 3237 L mol−1 cm−1). IR (KBr, cm−1): 1650 (s, ν(NC)),
1620 (m, ν(NC)), 1479 (s), 1444 (m), 1387 (m), 1360 (s), 1302
(w), 1288 (m), 1246 (m), 1203 (s), 1136 (s), 1093 (s), 1022 (s), 945
(m), 924 (br, m), 916 (s), 843 (s), 552 (m), 484 (m).
1
recorded. H NMR (C6D6, 25 °C, 400 MHz) δ: 59.2 (br s, 36H,
Fe(NCtBu2)(acac)), −8.2 (br s, 6H, Fe(NCtBu2)(O(CH3)CH-
(CH3)O)). The γ-proton on the acetylacetonate ligand was not
1
observed in the H NMR spectra.
Synthesis of Fe(1-norbornyl)4 from FeCl3. To a cold (−25 °C),
stirring, yellow solution of FeCl3 (45.7 mg, 0.28 mmol) in a mixture of
Et2O (0.2 mL) and pentane (2 mL) was added a cold (−25 °C)
solution of 1-norbornyllithium (83.9 mg, 0.82 mmol) in pentane (4
mL). The solution immediately turned deep purple, concomitant with
the deposition of a fine black precipitate. The reaction mixture was
allowed to stir for 4 h, whereupon the mixture was filtered through a
basic alumina column supported on glass wool (0.5 × 3 cm). The
volume of the deep purple filtrate was reduced in vacuo to 1 mL and
layered on acetonitrile (12 mL). Storage at −25 °C for 24 h resulted in
the deposition of a dark purple solid. The solid was isolated by
decanting off the supernatant (22.7 mg, 25% yield based upon 1-
norbornyllithium). Anal. Calcd for FeC28H44: C, 77.05; H, 10.16.
Found: C, 76.73; H, 9.98. 1H NMR (benzene-d6, 25 °C, 600 MHz) δ:
0.98−1.2 (m, 24H), 1.45 (br s, 8H), 1.58 (br s, 8H), 2.33 (br s, 4H).
13C{1H} NMR (benzene-d6, 25 °C, 150 MHz) δ: 30.25 (C3, C5),
33.07 (C2, C6), 34.66 (C4), 42.73 (C7), 51.39 (C1). IR (KBr mull,
cm−1): 742 (m), 829 (w), 921 (w), 972 (m), 1087 (m), 1138 (m),
1206 (m), 1245 (m), 1280 (m), 1300 (m), 1314 (m).
X-ray Crystallography. Data for 3 and 4 were collected on a
Bruker KAPPA APEX II diffractometer equipped with an APEX II
CCD detector using a TRIUMPH monochromator with a Mo Kα X-
ray source (α = 0.71073 Å). The crystals of 3 and 4 were mounted on
a cryoloop under Paratone-N oil, and the data were collected at
100(2) K using an Oxford nitrogen gas cryostream system. Data for 5
were collected on a Bruker 3-axis platform diffractometer equipped
with a SMART-1000 CCD detector using a graphite monochromator
with a Mo Kα X-ray source (α = 0.71073 Å). The crystal of 5 was
mounted on a glass fiber under Paratone-N oil, and data were collected
at 150(2) K using an Oxford nitrogen gas cryostream system. A
hemisphere of data was collected using ω-scans with 0.5° frame widths
for 3 and 4 and 0.3° frame widths for 5. Frame exposures of 25 and 10
s were used for 3 and 5, respectively, while frame exposures of 5 s (low
angle) and 15 s (high angle) were used for 4. Data collection and cell
parameter determinations were conducted using the SMART
program.60 Integration of the data frames and final cell parameter
refinements were performed using SAINT software.61 Absorption
correction of the data for 3 and 4 was carried out using the multiscan
method SADABS,62 while the absorption correction of the data for 5
was carried out empirically based on reflection ψ-scans. Subsequent
calculations were carried out using SHELXTL.63 Structure determi-
nation was done using direct or Patterson methods and difference
Fourier techniques. All hydrogen atom positions were idealized and
rode on the atom of attachment. Structure solution, refinement,
graphics, and creation of publication materials were performed using
SHELXTL.63
Synthesis of Fe(1-norbornyl)4 from FeCl2. To a cold (−25 °C)
stirring suspension of FeCl2 (39.2 mg, 0.31 mmol) in a mixture of
Et2O (1 mL) and pentane (4 mL) was added a cold (−25 °C) solution
of 1-norbornyllithium (63.3 mg, 0.62 mmol) in pentane (6 mL). The
solution was allowed to stir for 45 min, whereupon the color became
deep purple and a fine black solid was deposited. This solid adhered to
the stir bar once the stirring was stopped. The mixture was filtered
through a basic alumina column supported on glass wool (0.5 × 3 cm).
The solvent was removed in vacuo, and the purple solid was washed
with acetonitrile (5 mL) (14.5 mg, 21% yield based upon 1-
1
norbornyllithium). H NMR (benzene-d6, 25 °C, 400 MHz) δ: 1.08
Complex 4 exhibits positional disorder about each quaternary
carbon bound to iron. Alternate positions for the 1-norbornyl groups
were not assigned. Only the iron center and the α-carbon atoms were
refined anisotropically. Idealized hydrogen atoms were not assigned to
the isotropic carbon atoms. A summary of the relevant crystallographic
data for 3−5 are presented in Table 2.
(br s, 24H), 1.46 (br s, 8H), 1.598 (br s, 8H), 2.33 (br s, 4H).
Synthesis of Fe(1-norbornyl)4 from Fe(acac)3. To a cold (−25
°C) stirring solution of 1-norbornyllithium (51.8 mg, 0.51 mmol) in
pentane (10 mL) was added Fe(acac)3 (44.0 mg, 0.12 mmol). This
was allowed to stir for 3 h, during which time the solution darkened
and a white solid was deposited. The mixture was filtered through a
H
dx.doi.org/10.1021/ic401096p | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(6) Mas-Balleste,
́
R.; Que, L. J. Am. Chem. Soc. 2007, 129, 15964.
Table 2. X-ray Crystallographic Data for Complexes 3−5
(7) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
Stubna, A.; Kim, J.; Munck, E.; Nam, W.; Que, L. J. Am. Chem. Soc.
2004, 126, 472.
(8) Nam, W. Acc. Chem. Res. 2007, 40, 522.
(9) England, J.; Guo, Y.; Van Heuvelen, K. M.; Cranswick, M. A.;
Rohde, G. T.; Bominaar, E. L.; Munck, E.; Que, L. J. Am. Chem. Soc.
2011, 133, 11880.
(10) White, M. C. Science 2012, 335, 807.
(11) Chen, M. S.; White, M. C. Science 2010, 327, 566.
(12) Chen, M. S.; White, M. C. Science 2007, 318, 783.
(13) Cramer, S. A.; Jenkins, D. M. J. Am. Chem. Soc. 2011, 133,
19342.
3
4
5
empirical formula FeN2C23O2H43
crystal habit, color block, purple
FeC28H44
FeN4C52H104O8Li
block, maroon
block, purple
crystal size (mm) 0.40 × 0.10 ×
0.40 × 0.40 ×
0.10
0.60 × 0.50 × 0.40
̈
0.05
crystal system
space group
vol (Å3)
a (Å)
triclinic
orthorhombic
Pmn21
monoclinic
C2/c
P1
̅
1247.52(13)
8.4491(5)
11.0002(6)
15.0329(9)
68.873(4)
84.357(4)
73.185(4)
2
1160.91(8)
11.7710(5)
10.0996(4)
9.7652(4)
90.00
5982(3)
19.260(6)
19.428(6)
16.840(6)
90.00
b (Å)
c (Å)
(14) Meyer, S.; Klawitter, I.; Demeshko, S.; Bill, E.; Meyer, F. Angew.
Chem., Int. Ed. 2013, 52, 901.
(15) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.;
Bukowski, M. R.; Stubna, A.; Munck, E.; Nam, W.; Que, L. Science
α (deg)
β (deg)
γ (deg)
Z
90.00
108.316(6)
90.00
90.00
̈
2
4
2003, 299, 1037.
fw (g/mol)
435.44
436.48
976.18
(16) England, J.; Guo, Y.; Farquhar, E. R.; Young, V. G., Jr.; Munck,
E.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 8635.
(17) Grapperhaus, C. A.; Mienert, B.; Bill, E.; Weyhermuller, T.;
Wieghardt, K. Inorg. Chem. 2000, 39, 5306.
(18) Xue, G.; Pokutsa, A.; Que, L. J. Am. Chem. Soc. 2011, 133,
16657.
density (calcd)
1.159
1.249
1.084
(Mg/m3)
abs coeff (mm−1
)
0.623
0.661
476
0.300
2148
F000
474.00
10797
total no. of
reflections
13440
19556
(19) Li, F.; Meier, K. K.; Cranswick, M. A.; Chakrabarti, M.; Van
no. of unique
4265
1902
4368
Heuvelen, K. M.; Munck, E.; Que, L. J. Am. Chem. Soc. 2011, 133,
7256.
(20) White, M. C.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc.
̈
reflections
Rint
0.0260
0.0222
0.1438
final R indices
[I > 2σ(I)]
R1 = 0.0312
R1 = 0.1339
R1 = 0.0755
2001, 123, 7194.
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wR2 = 0.0767
wR2 = 0.3659
wR2 = 0.2000
largest diff peak
0.284 and
−0.250
2.461 and
−0.791
0.628 and −0.463
and hole
ed.; John Wiley & Sons: New York, 1999.
(e− Å−3
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1.020
1.113
1.175
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic details (as CIF files) and spectral data for
complexes 1−5. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(26) Thiele, K.-H.; Dimitrov, V.; Schenke, D.; Kruger, A.; Rusina, A.
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AUTHOR INFORMATION
Corresponding Author
*Email: hayton@chem.ucsb.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (NSF) (CHE
1059097) and Alfred P. Sloan Foundation for financial support
of this work. The research carried out here made use of the
SQUID Magnetometer at the Materials Research Laboratory,
an NSF Materials Research Science and Engineering Center,
supported by NSF Grant DMR 1121053. We also thank Prof.
Paul Chirik for helpful discussions.
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