Synthesis and Characterization of Bioinspired [Mo2Fe2]–Hydride Cluster Complexes and Their Application in the Catalytic Silylation of N2

Article abstract of DOI:10.1002/chem.201702925

The hydride-supported [Mo₂Fe₂] cluster complex {Cp*Mo(PMe₃)}₂{FeN(SiMe₃)₂}₂(H)₈ (2 a; Cp=η⁵-C₅Me₅) and its [Mo₂Mn₂] congener 2 b were synthesized from the reactions of Cp*Mo(PMe₃)(H)₅ (1) with M{N(SiMe₃)₂}₂ (M=Fe, Mn). The amide-to-thiolate ligand-exchange reactions of complex 2 a with bulky thiol reagents (HSR; R=2,4,6-iPr₃C₆H₂ (Tip), 2,6-(SiMe₃)₂C₆H₃ (Btp)) furnished the corresponding hydride-supported [Mo₂Fe₂](SR)₂ cluster complexes. The [Mo₂Fe₂] clusters served as catalyst precursors for the reductive silylation of N₂ and yielded ≈65–69 equivalents of N(SiMe₃)₃ relative to the [Mo₂Fe₂] clusters. Treatment of complexes 2 a and b with an excess of CNtBu resulted in the formation of dinuclear Mo?Fe and Mo?Mn complexes, which indicated that the [Mo₂M₂] cores (M=Fe, Mn) split into two dinuclear species upon accommodation of substrates.

Full text of DOI:10.1002/chem.201702925

A Journal of
Accepted Article
Title: Synthesis and Characterization of Bio-Inspired [Mo2Fe2] Hydride
Clusters and Their Application to the Catalytic Silylation of N2
Authors: Yasuhiro Ohki, Yuna Araki, Mizuki Tada, and Yoichi Sakai
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201702925
Link to VoR: http://dx.doi.org/10.1002/chem.201702925
Supported by

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
Synthesis and Characterization of Bio-Inspired [Mo2Fe2] Hydride
Clusters and Their Application to the Catalytic Silylation of N2
Yasuhiro Ohki,*,[a] Yuna Araki,[a] Mizuki Tada,[a,b] and Yoichi Sakai[c]
Dedication ((optional))
Abstract:
The
hydride-supported
[Mo2Fe2]
cluster
proposed to release H2, which is generated upon
accumulation of four protons and four electrons in the
crystallographically identified resting state. The postulated
importance of hydride intermediates in the biological
reduction of N2, as well as previously reported examples on
the activation of N2 by homometallic hydride complexes of
transition metals,[9,10] prompted us to synthesize hydride-
supported heterometallic Mo/Fe clusters. Herein, we report
the synthesis and characterization of [Mo2Fe2] hydride
clusters and a [Mo2Mn2] congener. Even though the Mo-H-
Fe bridging hydrides in the [Mo2Fe2] clusters have limited
relevance to the Fe-H-Fe intermediate postulated for the
FeMo-cofactor, the [Mo2Fe2] clusters were found to serve
as catalytic precursors for the silylation of N2. Stoichiometric
reactions between the [Mo2M2] clusters (M = Fe, Mn) and
CNtBu were also investigated to gain insight into how the
[Mo2M2] clusters react with external substrates.
{Cp*Mo(PMe3)}2{FeN(SiMe3)2}2(H)8 (2a; Cp* = η5-C5Me5) and its
[Mo2Mn2] congener 2b were synthesized from the reactions of
Cp*Mo(PMe3)(H)5 (1) with M{N(SiMe3)2}2 (M = Fe, Mn). The amide-
to-thiolate ligand exchange reactions of 2a with bulky thiols (HSR; R
=
2,4,6-iPr3C6H2 (Tip), 2,6-(SiMe3)2C6H3 (Btp)) furnished the
corresponding hydride-supported [Mo2Fe2](SR)2 clusters. The
[Mo2Fe2] clusters serve as catalyst precursors for the reductive
silylation of N2, yielding 65~69 equiv of N(SiMe3)3 per [Mo2Fe2]
cluster. Treatment of 2a, b with excess CNtBu resulted in the
formation of dinuclear Mo-Fe and Mo-Mn complexes, indicating that
the [Mo2M2] cores (M = Fe, Mn) split into two dinuclear species upon
accommodation of substrates.
Introduction
The biological reduction of N2 is catalyzed by the
nitrogenase FeMo-cofactor, which has been identified as a
Mo/Fe-S cluster featuring a [MoFe7S9C] core that is
supported by an R-homocitrate as well as a histidine and a
cysteine residue.[1,2,3] The FeMo-cofactor, either in the
engineered-protein or solvent-extracted form, furthermore
catalyzes the reduction of non-native substrates such as
CO and CN–, where the C-C bond formation often leads to
C2-C4 hydrocarbons.[4] Although similar catalytic reductions
of C1 substrates have been accomplished with molecular
Mo/Fe-S and Fe-S cluster mimics,[5] the catalytic reduction
of N2 with molecular Mo/Fe clusters remains challenging.
A unique feature in the biological reduction of N2 is the
formation of at least one equivalent of H2 per N2 reduction
cycle.[6] Recent biochemical studies have suggested that
the reductive elimination of H2 occurs simultaneously to the
accommodation of N2 in the FeMo-cofactor.[7,8] Two Fe-H-
Fe moieties in the active state of FeMo-cofactor have been
Results and Discussion
Synthesis and Characterization of [Mo2Fe2] and [Mo2Mn2]
Hydride Clusters.
A prerequisite for the synthesis of the targeted hydride clusters
is the selective formation of a link between Mo and Fe with
bridging hydrides. For this purpose, we designed an acid-base
type reaction between the Mo(VI) hydride complex
Cp*Mo(PMe3)(H)5 (1, Cp* = η5-C5Me5)[11] with the Fe(II) amide
complex Fe{N(SiMe3)2}2.[12] Compared to typical hydride
complexes, the hydride ligands in 1 should be less electron rich,
as the distribution of electron density over five hydrogen atoms
by a single molybdenum center is not very efficient. Accordingly,
deprotonation of one of the hydride ligands in 1 should be
possible with a strong base, such as the amide ligands in
Fe{N(SiMe3)2}2. A similar approach using rare-earth hydrides as
bases has afforded clusters containing Mo and rare-earth
metals.[13] Indeed, the deprotonation of 1 with Fe{N(SiMe3)2}2
resulted in the formation of the [Mo2Fe2] hydride cluster
{Cp*Mo(PMe3)}2{FeN(SiMe3)2}2(H)8 (2a) in ca. 70% yield as
crystals (Scheme 1). Crystals of the [Mo2Mn2] congener (2b)
were obtained in a similar fashion from 1 and Mn{N(SiMe3)2}2 in
ca. 70% yield, while product(s) from an analogous reaction
between 1 and Co{N(SiMe3)2}2 were not identified. The presence
of eight hydrides in 2a, b is consistent with the postulated
deprotonation of 1 with an amide ligand of M{N(SiMe3)2}2 (M =
Fe, Mn) and the high crystal yields of 2a, b. Upon treatment with
bulky thiols (HSR; R = 2,4,6-iPr3C6H2 (Tip), 2,6-(SiMe3)2C6H3
(Btp)), the amide ligands in 2a were replaced by thiolates,
furnishing the crystals of [Mo2Fe2](SR)2 hydride clusters,
{Cp*Mo(PMe3)}2{Fe(SR)}2(H)8 (3a; R = Tip, 3b; R = Btp), in ca.
50% yield. Analogous reactions of 2b with thiols were not
[a]
Prof. Y. Ohki, Y. Araki, Prof. M. Tada
Department of Chemistry, Graduate School of Science and
Research Center for Materials Science, Nagoya University, Furo-
cho, Chikusa-ku, Nagoya 464-8602 (Japan)
E-mail: ohki@chem.nagoya-u.ac.jp
[b]
[c]
Prof. M. Tada
Research Center for Materials Science (RCMS) & Integrated
Research Consortium on Chemical Sciences (IRCCS), Nagoya
University, Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
Prof. Y. Sakai
Department of Chemistry, Daido University, Takiharu-cho, Minami-
ku, Nagoya 457-8530, Japan
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
the crystals of 3b was insufficient to refine the hydride ligands. In
the crystal, two pairs of Cp* and PMe3 ligands adopt a mutually
trans arrangement with respect to the planar [Mo2M2] (M = Fe,
Mn) rhombus, that is supported by eight hydrides. The edges of
the [Mo2M2] rhombus are doubly bridged by four hydrides, while
the other four hydrides occupy the front and the back faces of
the rhombus. Hydrides refined in these clusters are located
closer to Mo (Mo-H, 1.5188(3)-1.8991(3) Å) than to Fe (Fe-H,
1.9714(3)-2.3035(5) Å) or Mn (Mn-H, 2.0598(4)-2.4698(5) Å),
indicating that the Mo centers should be more electron-deficient
than the Fe or Mn centers. The Mo-Fe distances in 2a, 3a, and
3b (2.7736(4)-2.8987(5) Å) are comparable to previously
reported hydride-supported Mo-Fe bonds (2.816(2)-2.9322(5)
Å),[14] while the Mo-Mn bond lengths in 2b (2.9414(5) and
2.9859(6) Å) are slightly shorter than previously reported ones
(3.004(2)-3.135(1) Å).[15] These relatively short Mo-M (M = Fe,
Mn) distances suggest the presence of Mo-M interactions. The
Fe··Fe distances in 3a, b (2.8273(5) and 2.902(1) Å) are
indicative of weak Fe··Fe interactions that may cause magnetic
coupling to some extent between the iron atoms. In contrast, the
relatively long Fe-Fe (3.1177(6) Å) and Mn-Mn (3.3217(7) Å)
distances in 2a, b are beyond the range that suggests Fe-Fe or
Mn-Mn interactions. Although the precise assignment of the
oxidation states in hetero-metallic complexes is often difficult,
the Fe-N and Mn-N distances in 2a, b (1.981(2) Å and 2.042(2)
Å) are indicative of Fe(II) and Mn(II) oxidation states. These M-N
distances fall within the range of previously reported M(II)-
N(SiMe3)2 bond lengths (Fe, 1.885(4)-2.0646(15) Å and Mn,
1.962(4)-2.18(1) Å)[16,17] and are longer than the corresponding
M(III)-N(SiMe3)2 bonds (Fe, 1.864(5)-1.9402(12) Å and Mn,
1.890(3) Å).[18] Similarly, the Fe-S distances in 3a, b (2.3223(7)
and 2.336(2) Å) are consistent with typical Fe(II)-SR (R = Tip,
Btp) complexes (2.245(1)-2.362(2) Å),[19] supporting the
assignment of an Fe(II) state. The assignment of Fe(II) or Mn(II)
Si
Si
N
H
H
Mo
H
M
H
H
H
PMe3
H
PMe3
H
H
Mo
Mo
H
H
1
Me3P
M
H
H
Si
Si
Si
Si
N
Si
N
M
N
Si
(Fe)
(Mn)
2b
2a
(M = Fe, Mn)
R
H
M = Fe
S
Fe
H
H
PMe3
2 HSR
H
H
H
Mo
H
Mo
H
Me3P
Fe
S
Me3Si
SiMe3
R
(R = Tip)
(R = Btp)
3a
3b
(Tip)
(Btp)
Scheme 1 Synthesis of [Mo2Fe2] and [Mo2Mn] Hydride
Clusters 2a, b and 3a, b.
attempted on account of the limited relevance to the active site
of the enzyme and the low catalytic performance of 2b (vide
infra).
The molecular structures of the [Mo2Fe2] and [Mo2Mn2]
clusters 2a, b and 3a, b were determined by single-crystal X-ray
diffraction analysis (Figures 1 and S1), though that the quality of
Si
Si
Si
N
Si
N
S
H
H
H
H
Mo*
Mo*
Fe
H
H
Mn
H
Mo*
H
P
Fe
H
P
P
H
H
H
P*
P*
P*
Fe*
N*
Fe*
Mo
Mo
H
Mn*
N*
H
H
H
H
H
Mo
S*
Si
Si
Si
Si
2a
2b
3a
Figure 1 Molecular structures of 2a, 2b, and 3a with atomic displacement parameters set at 50% probability. Only selected
atoms are labeled, and hydrogen atoms, except for the hydride ligands, are omitted for clarity. Selected distances (Å) for 2a: Mo-
Fe 2.8493(5) and 2.8987(5); Fe··Fe 3.1177(6); Mo-P 2.4860(7); Fe-N 1.981(2); Mo-H 1.5188(3)-1.8991(3); Fe-H 1.9953(4)-
2.3035(5). Selected distances (Å) for 2b: Mo-Mn 2.9414(5) and 2.9859(6); Mn··Mn 3.3217(7); Mo-P 2.4847(7); Mn-N 2.042(2);
Mo-H 1.6670(3)-1.7161(3); Mn-H 2.0598(4)-2.4698(5). Selected distances (Å) for 3a: Mo-Fe 2.7736(4) and 2.7904(4); Fe··Fe
2.8273(5); Mo-P 2.4639(8); Fe-S 2.3223(7); Mo-H 1.6743(3)-1.7178(2); Fe-H 1.9714(3)-2.2433(4).
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
for 2a, b and 3a, b leads to a Mo(IV)2M(II)2 (M = Fe, Mn) state
based on their chemical formulae. Even though detailed
theoretical analyses on these clusters may provide further
support, such calculations should be conducted as a different
study, as open-shell systems with multiple interactions across d-
block metal centers require elaborate calculations on various
possible states that lie in a narrow energy range.
1698
1624
{Cp*Mo(PMe3)}2
{FeN(SiMe3)2}2(H)8 (2a)
The assignment of the iron sites in 2a and 3a as Fe(II) was
further supported by their 57Fe Mössbauer spectra at zero field
and 78 K, where a doublet appeared at IS (isomer shift) =
0.748(1) mm/s with QS (quadruple splitting) = 0.642(1) mm/s for
2a, and at IS = 0.707(2) mm/s with QS = 0.842(4) mm/s for 3a
(Figures S2 and S3). Their IS values correspond to those
reported for the hydride-supported Fe(II)-N(SiMe3)2 and Fe(II)-
SR sites of Fe4(µ-H)4(µ3-H)2{N(SiMe3)2}2(PMe2Ph)4 (0.592(3)
mm/s) and Fe4(µ-H)4(µ3-H)2(SDmp)2(PMe3)4 (0.621(3) mm/s,
Dmp = 2,6-(mesityl)2C6H3), respectively.[10] The slightly larger
QS value for thiolate-supported 3a relative to amide-supported
2a reflects the less symmetric arrangement of hydride ligands
around the Fe atoms in 3a, that is in agreement with the
molecular structures (Figure 1).
deuterated 2a
1624
1698
Clusters 2a, b and 3a, b are paramagnetic and their 1H
NMR spectra exhibited broad signals for the Cp*, PMe3, and –
N(SiMe3)2 or –SR groups, while hydride signals were not
observed (Figures S4-S7). At 297 K, small solution magnetic
moments were measured for 3a (2.7(2) µB) and 3b (3.5(5) µB)
relative to those of 2a (4.7(2) µB) and 2b (6.8(3) µB), that is in
accordance with the presence of weak Fe··Fe interactions in 3a,
b leading to some extent of anti-ferromagnetic coupling, as
discussed based on the crystal structures (vide supra). In the 1H
NMR spectra of 2a, two sets of paramagnetic signals were
observed, while the other clusters exhibited one set of signals.
On the basis of the number of 1H NMR signals and their relative
intensities, we speculate at this point that in solution, a small
part of 2a transforms into a minor isomer, which features a cis-
arrangement of the Cp* and PMe3 ligands with respect to the
1239
1178
difference spectrum
3500
2500
1500
500
wavenumbers (cm–1
)
Figure 2 IR spectra of crystalline 2a and deuterated 2a
(obtained by using 1-[D]5), as well as the corresponding
difference spectrum.
1181 and 1154 cm-1 (3b) (Figures 2 and S10-13). These
stretching frequencies and the magnitudes of the shifts are
comparable to the Fe-Hbridge bands observed in our previous
study on [Fe4] and [Fe6] hydride clusters.[10] Although electro-
spray ionization mass (ESI-MS) spectrometry could be used in
our previous studies to identify the chemical formulae of Fe and
Co hydride clusters,[10,20] clusters 2a, b and 3a, b appeared to be
ESI-MS silent, suggesting either unsuccessful oxidation or
reduction in the spectrometer. These results are consistent with
the cyclic voltammograms (CVs) in THF, where irreversible
waves were observed at potentials distinct from the rest
potentials (separation: >1.0 V, Figures S14-17).
1
[Mo2Fe2] plane. Variable-temperature H NMR spectra of 2a in
toluene-d8 were measured in the range of –80 to 60 °C in order
to evaluate the equilibrium constants (Figure S8). At
temperatures higher than 60 °C, 2a decomposed. A Van’t Hoff
plot of the resulting data (Figure S9) yielded an enthalpy of –
0.26(2) kcal/mol and an entropy of –3.5(1) cal/mol·K for the
postulated equilibrium. Given that the entropy is close to zero,
this result is consistent with the proposed intramolecular
isomerization, while the slightly negative enthalpy indicates that
the population of the speculated cis-isomer should be increased
at low temperatures.
Catalytic Reduction of N2 to N(SiMe3)3 in the Presence of
[Mo2Fe2] or [Mo2Mn2] Clusters.
1
Even though the H NMR signals of the hydrides in 2a, b
The [Mo2Fe2] and [Mo2Mn2] clusters were applied as catalyst
precursors for the reductive silylation of N2 to N(SiMe3)3 under
reaction conditions that were previously reported for reactions
and 3a, b were not detected, the presence of hydrides in these
clusters was supported by the infrared (IR) spectra in
combination with deuterium-labeling experiments, wherein
deuterated 2a and 2b were prepared from Cp*Mo(PMe3)(D)5 (1-
[D]5). Broad bands assignable to the metal-hydride stretching
mode were found at 1698 and 1624 cm-1 (2a), 1673 cm-1 (2b),
1712, 1650, and 1584 cm-1 (3a), and 1641 and 1586 cm-1 (3b).
Upon deuterium labeling, these bands shifted to 1239 and 1178
cm-1 (2a), 1214 cm-1 (2b), 1230, 1186, and 1145 cm-1 (3a), and
using Fe,[10,21] Co,[22] Mo,[23] and V[24] complexes. In
a
representative reaction, Na and Me3SiCl (600 equiv/metal) were
added to a THF solution of 2a, and the mixture was stirred at
room temperature for 48 h. After addition of cyclododecane as
an internal standard, the resulting reaction mixture was analyzed
by GC/GC-MS to identify and quantify the products. Attempts
were also made to synthesize NH3 from N2 in the presence of
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
radicals are consumed predominantly for the reaction with THF
and dimerization, indicating that the accommodation of N2 onto
the Mo-Fe species or the subsequent silylation are relatively
slow. The slow silylation of N2 prompted us to consider a
possible fragmentation of clusters into mono-metallic species
during the catalysis, e.g. the generation of Cp*Mo(PMe3)-
hydrides and monomeric Fe- or Mn-amide species from 2a, b.
However, a reaction in the presence of a catalytic amount of 1
furnished only negligible amounts (< 2 equiv) of N(SiMe3)3 per 1
(Table S2). Furthermore, it has previously been reported that the
attempted catalysis using Fe{N(SiMe3)2}2 or FeCl2 was not
successful,[21a] and we found that a similar reaction in the
presence of Mn{N(SiMe3)2}2 furnished only 2 equiv of N(SiMe3)3
per the manganese amide complex (Table S2). Another
possibility to explain potential degradation is the aggregation of
the [Mo2Fe2] clusters into nanoparticles. Therefore, the silylation
of N2 catalyzed by 2a was tested in the presence of large excess
of Hg (1200 equiv, i.e. 300 equiv/metal). The reaction was not
significantly affected by the presence of Hg and provided 47
equiv N(SiMe3)3 per 2a, undermining the hypothesis that
catalytically active Mo/Fe nano-particles are generated, while
the formation of iron-mercury amalgam is difficult.[25] For further
support, a homogeneity test according to the procedure reported
by Lu et al[22a] was conducted (Figure S29). For that purpose,
the reaction mixture was separated into soluble and insoluble
fractions, which were subsequently examined for their catalytic
activity. In our variation, the reaction described in entry 1 was
carried out for 4 h, before the mixture was centrifuged to
separate the soluble and insoluble fractions. Then, the soluble
portion was split into two aliquots. One aliquot was used to
quantify the silylated products, while the other was further
subjected to the catalytic reaction and stirred for an additional 44
h (total reaction time: 48 h). The thus obtained yields of
N(SiMe3)3 were 15 equiv (4 h) and 70 equiv (48 h) per 2a, which
are comparable to yields obtained in the time-profile (Figure
S27). On the other hand, the insoluble fraction furnished merely
4 equiv of N(SiMe3)3 per 2a, suggesting that the catalytically
active species should be soluble.
Table 1 Catalytic Conversion of N2 to N(SiMe3)3 Mediated by
[Mo2Fe2] and [Mo2Mn2] Hydride Clusters 2a, b and 3a, b and a
Mo-Fe Complex 4.[a]
Cluster
(0.01 mmol)
N2 + 6 Na + 6 Me3SiCl
2 N(SiMe3)3
solvent, r.t.
48 h
(1 atm) (600 equiv / metal)
N(SiMe3)3
(equiv / cluster)
Solvent[b]
THF
Entry
1
Cluster
2a
69 ± 7
2[c]
3
THF
DME
THF
THF
THF
THF
32 ± 12
57 ± 10
12 ± 2
2a
2a
2b
3a
3b
4
(KC8)
4
65 ± 16
69 ± 9
5
6
7[d]
28 ± 5
[a] r.t. = room temperature, equiv = equivalent. Yields for all entries are averages
of three runs.
[b] THF = tetrahydrofuran, DME = 1,2-dimethoxyethane, volume = 16 mL.
[c] KC8 was used as the reductant instead of Na.
[d] See below for the synthesis and characterization of
Cp*Mo(PMe3)(H)2(CNtBu)2Fe{N(SiMe3)2} (4). For this entry, 0.02 mmol of 4 was
used to be consistent with other entries in terms of the concentration of metals.
proton sources (HBArF4⋅2Et2O (ArF
= 3,5-(CF3)2C6H3) or
trifluoromethanesulfonic acid) and reducing agents (KC8 or
Cp*2Co), however the yields of NH3 remained substoichiometric
relative to the [Mo2M2] clusters, e.g. ~1.3(1) equiv per 2a and
0.12(3) equiv per 3a (Table S4).
[Mo2Fe2] cluster 2a was used to screen the reaction
conditions. As summarized in Tables 1 and S2, 69 equiv of
N(SiMe3)3 were obtained in the presence of 2a (entry 1). The
choice of reducing agent and solvent affects the outcome in the
reported reactions.[10,21-24] In the present cases, the yield
dropped to 36 equiv when KC8 was used as the reductant (entry
2), while the reaction in DME (1,2-dimethoxyethane) was slightly
less effective and produced 57 equiv N(SiMe3)3 (entry 3). The
performance of thiolate-supported [Mo2Fe2] clusters 3a, b was
comparable to that of 2a, yielding 65-69 equiv of N(SiMe3)3 per
3a, b (entries 5-6). Conversely, [Mo2Mn2] cluster 2b merely
produced 12 equiv of N(SiMe3)3 (entry 4), which indicates that
Mn is not suitable for this catalytic system. As will be addressed
later, a dinuclear Mo-Fe complex (4) with two CNtBu ligands was
synthesized from 2a and CNtBu. This complex was also
employed as a catalyst precursor, furnishing 28 equiv of
N(SiMe3)3 per 4 (entry 7).
A key reactive species in the silylation of N2 is a ·SiMe3
radical, that is generated via the one-electron reduction of
Me3SiCl with Na. The ·SiMe3 radical not only reacts with the
metal-bound N2 to furnish N(SiMe3)3, but also dimerizes to give
Me3Si-SiMe3 and reacts with the solvent via the formation of Si-
O bonds. Accordingly, silyl-ethers such as Me3Si(CH2)4OSiMe3
and CH3(CH2)4OSiMe3 were found as byproducts. The profile of
silylated products for entry 1 was monitored as a function of the
reaction time (Figure S27), which identified Me3Si(CH2)4OSiMe3
and Me3Si-SiMe3 as the major products. Thus, the ·SiMe3
Fragmentation of the [Mo2Fe2] and [Mo2Mn2] Clusters into
Dinuclear Complexes in the Presence of CNtBu.
In order to gain further insight into the reaction patterns of 2a, b,
we examined their stoichiometric reactions, as we were unable
to identify or detect possible intermediates for the catalytic
reaction mixtures. Treatment of 2a with large excess of Me3SiCl
in THF led to the degradation into uncharacterizable species
within 1 h. The exposure of clusters 2a or 2b to 1 atm of N2 in
toluene-[D]8 at room temperature did not seem to cause
changes in the 1H NMR signals, and the products after
irradiation with a high-pressure mercury lamp have not been
identified. Therefore we subsequently examined the reactions of
2a and 2b with CNtBu, which is isoelectronic to N2 but binds
relatively strong to metals. The addition of an excess of CNtBu to
solutions of 2a or 2b led to the fragmentation into smaller Mo-Fe
and
Mo-Mn
species,
that
furnished
(4)
crystalline
and
Cp*Mo(PMe3)(H)2(CNtBu)2Fe{N(SiMe3)2}
Cp*Mo(CNtBu)(CN)(C3N3HtBu3)Mn{N(SiMe3)2} (5) in 19% and
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
N1*
N2*
tBu
Si
H
C
Si
N
N
Mo
Fe
N
Si
Mo
C
C*
C
H
N3
excess
CNtBu
Si
Me3P
Si
Fe
N1
N2
tBu
N
P
4
M
H
H
PMe3
H
H
Mo
Mo
H
H
Me3P
M
H
H
4
tBu
excess
CNtBu
N
Si
N
Si
tBu
tBu
N
N
C
C4
C2
N
(Fe)
(Mn)
2a
2b
C3
Mo
Mn
Si
N4
Mn
tBu
N
C
N2
C5
N
Si
Si
Mo
5
C1
N5
N1
Scheme 2 Reactions of 2a, b with excess CNtBu.
21% yields, respectively (Scheme 2). These results indicate that
the catalytically active dinuclear Mo-Fe (and Mo-Mn) species are
generated during the silylation of N2. This notion is supported by
the catalytic silylation of N2 mediated by 4, which furnished 28
equiv of N(SiMe3)3 per 4 (Table 1, entry 7). Even though the
yield is lower, it is not significantly different relative to that of
[Mo2Fe2] cluster 2a, which produced 35 equiv of N(SiMe3)3 per
Mo-Fe unit (69 equiv per [Mo2Fe2], entry 1). Mo-Mn complex 5
was not employed as a catalyst precursor, as the catalytic
performance of [Mo2Mn2] cluster 2b is low and thus not suitable
as a benchmark.
In relation to the postulated formation of H2 from bridging
hydrides during the activation of N2 on the nitrogenase FeMo-
cofactor,[7] some of the hydride ligands in 2a were reductively
eliminated as H2 during the reaction with CNtBu. This is evident
from a small signal for H2 at 4.5 ppm in the 1H NMR spectrum of
2a in the presence of an excess of CNtBu in toluene-[D]8. On a
preparative scale, the amount of dissociated H2 was quantified
as 1.8(1) equiv per 2a, which leaves four hydrides per [Mo2Fe2]
core of 2a, i.e. two hydrides per dinuclear Mo-Fe moiety of 4.
The formation of H2 suggests a reduction of the metals in 4.
Therefore we measured a 57Fe Mössbauer spectrum to estimate
the electronic state of iron in 4. The parameters of the observed
doublet, IS = 0.667(2) mm/s and QS = 1.590(5) mm/s (Figure
S22), are comparable to those of [Mo2Fe2] clusters 2a and 3a
which contain Fe(II) sites, and therefore we tentatively assigned
the oxidation state Mo(II)Fe(II) for 4. This assignment is
consistent with the crystallographically determined Fe-N
distance, which falls in the range of typical Fe(II)-N(SiMe3)2
distances (vide infra). The paramagnetism of 4 hampered the 1H
5
Figure 3 Molecular structures of 4 and 5 with atomic
displacement parameters set at 50% probability. The
positions for the disordered iron and carbon atoms of 4 are
omitted for clarity. Selected distances (Å) for 4: Mo-Fe
2.7144(9); Mo-P 2.4257(13); Mo-C 1.947(4); Fe-C 1.978(4);
Fe-N1,N2 2.047(11)-2.099(10); Fe-C* 2.592(4); Fe-N1*,N2*
2.904(10)-2.910(11); Fe-N3 1.949(3); C-N1,N2 1.228(11)-
1.257(11). Selected distances (Å) for 5: Mo-Mn 3.6595(6);
Mo-C1 2.126(2); Mo-C2 2.2300(19); Mo-C3 2.1077(19); Mo-
C5 2.042(2); Mo-N2 2.2904(17); Mn-N1 2.288(2); Mn-N2
2.1901(18); Mn-N4 2.1606(18); Mn-N5 2.0308(18); N2-C2
1.408(3); C2-C3 1.465(3); C2-C4 1.444(3); N4-C4 1.290(2).
C-N bands for the CNtBu ligands of 4 appeared at 1752 and
1724 cm-1. In the case of Mo-Mn complex 5, the NMR
characterization was difficult, as the broad 1H NMR signals at
4.9 and 2.2-0.8 ppm were too featureless to assign the signals
and establish the purity of 5 (Figure S19). As we have been
unsuccessful so far to obtain satisfactory elemental analysis
values for 5 (low N value), we cannot rule out with certainty the
possibility of contamination of the crystals of 5. Nevertheless,
the IR spectrum of 5 exhibited C-N bands assignable to CN,
CNtBu, and C3N3HtBu3 moieties at 2089, 2032, 1667, 1648, and
1552 cm-1 (Figure S21).
The molecular structures of 4 and 5 determined by single-
crystal X-ray diffaraction analysis (Figure 3) revealed dinuclear
Mo-Fe and Mo-Mn units bridged by isocyanides or isocyanide
derivatives, while hydrides in 4 were not determined due to the
disorder. The Mo-Fe distance in 4 (2.7144(9) Å) indicates the
retention of the Mo-Fe bonding interaction, while the Mo-Mn
interaction is not preserved in 5 (3.6595(6) Å). The Fe atom of 4
is disordered over two positions and engages in interactions only
with one of the CNtBu ligands. The Fe atom in the molecular
NMR identification of the hydrides (Figure S18), and
a
crystallographic determination of the hydrides was unsuccessful
due to the disordered structure. However, the presence of
bridging hydrides in 4 was supported by the IR spectrum, which
showed a broad metal-hydride stretching band at 1539 cm-1 that
shifted to 1108 cm-1 upon deuterium labeling (Figure S20). The
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
powdered crystalline samples sealed in tin capsules under N2, using an
Elementar Analytical vario MICRO cube elemental analyzer. Cyclic
voltammograms were measured on a single-compartment cell under a N2
atmosphere at room temperature using a BAS ALS-660A electrochemical
structure of 4 thus seems to be two-coordinated by one side-on
CNtBu and one terminal N(SiMe3)2 ligand, although hydrides are
probably bridging the Mo and Fe centers, as suggested by the
IR spectrum. The Fe-N(SiMe3)2 distance in 4 (1.949(3) Å) is
slightly shorter than that in [Mo2Fe2] cluster 2a with Fe(II) centers
(1.980(2) Å), but still in the range of typical Fe(II)-N(SiMe3)2
bonds (1.885(4)-2.0646(15) Å). It should be also noted that
1.949(3) Å is slightly outside the range of Fe(I)-N(SiMe3)2
distances reported thus far (1.882(5)-1.9219(17) Å).[26] The short
Mo-CNtBu distance (1.947(4) Å) in 4 is indicative of strong back-
donation from Mo to the isocyanide, which is in line with our
tentative assignment of a reduced Mo(II) state. In the molecular
structure of 5, three kinds of ligands derived from CNtBu are
present, i.e. terminally bound CNtBu, bridging CN, and bridging
C3N3HtBu3 ligands. The formation of bridging CN and C3N3HtBu3
ligands requires a cleavage of the CN-tBu bond and an insertion
of three CNtBu units into Mo-H moieties, respectively.
Accordingly, a complex reaction pathway should be expected for
the synthesis of 5. The C3N3HtBu3 ligand serves as an aza-η3-
allyl ligand for Mo, which is supported by the Mo-C2 (2.2300(19)
Å), Mo-C3 (2.1077(19) Å), and Mo-N2 (2.2904(17) Å) distances,
as well as by the C2-C3 (1.465(3) Å) and C2-N2 (1.408(3) Å)
distances.
analyzer. As the supporting electrolyte, 0.2
M tetrabutylammonium
hexafluorophosphate ([nBu4N][PF6]) was used, which was recrystallized
from THF prior to use. Potentials are referenced to Ag/AgNO3.
Mössbauer measurements were performed in
a
conventional
transmission mode on a Mössbauer spectrometer (Topologic Systems
Co. Model-222) with a 57Co(Rh) source (925 MBq). The spectral curve
fitting was conducted out by using MossWinn 4.0Pre on the assumption
of the sum of the Lorentzian curves. The Doppler velocity scale was
calibrated with respect to a-iron at room temperature, and the isomer
shifts are given relative to a-iron. GC/GC-MS analyses were acquired on
a Shimadzu GCMS-2010SE. Cp*Mo(PMe3)(H)5 (1),[11] Fe{N(SiMe3)2}2,[12]
Mn{N(SiMe3)2}2,[27] HSTip,[28] and HSBtp[29] were prepared according to
previously reported procedures. All other chemicals were purchased from
common commercial sources and used without further purification.
[11b]
Deuterated cluster samples were prepared by using 1-[D]5
1.
instead of
{Cp*Mo(PMe3)}2{FeN(SiMe3)2}2(H)8 (2a). Cp*Mo(PMe3)(H)5 (1, 1.50 g,
4.80 mmol) was dissolved in a mixture of hexane and THF (v/v = 9:1, 20
mL). The solution was placed at the bottom of a glass tube, and layered
with more of the hexane-THF mixture (v/v = 9:1, 40 mL). On top of these
layers, Fe{N(SiMe3)2}2 (3.62 g, 9.60 mmol) dissolved in the hexane-THF
mixture (v/v = 9:1, 20 mL) was carefully deposited. Slow diffusion
between the three layers over the course of a week at room temperature
led to the formation of black plates of 2a·THF (1.95 g, 1.73 mmol, 72%).
The crystals were collected after washing with THF (ca. 3 x 5 mL) and
drying under vacuum. 1H NMR (600 MHz, toluene-[D]8, r.t.): signals for
the major isomer appeared at δ 5.0 (w1/2 = 60 Hz, Cp*), 1.3 (w1/2 = 80 Hz,
SiMe3), –6.6 (w1/2 = 60 Hz, PMe3); signals for the minor isomer appeared
at δ 5.6 (w1/2 = 85 Hz, Cp*), –0.77 (w1/2 = 140 Hz, SiMe3 or PMe3), –2.1
(w1/2 = 90 Hz, SiMe3 or PMe3), –3.4 (w1/2 = 120 Hz, SiMe3 or PMe3).
Cyclic Voltammetry (THF): Epa = 0.51 V, Epc = –2.20 V (vs. Ag/AgNO3).
UV-vis (toluene): λmax = 299 (ε 1.0 x 104 cm-1M-1), 483 (sh, ε 1.9 x 103 cm-
1M-1) nm. Solution magnetic moment (toluene-[D]8, 297 K): 4.7(2) µB. IR
(ATR, cm-1): ν = 2961, 2945, 2899, 1698, 1624, 1457, 1426, 1375, 1302,
1282, 1245, 1235, 1069, 1028. Mössbauer spectrum (mm/s, 78 K): IS =
0.748(1) and QS = 0.642(1). Anal. Calcd for C38H92Fe2Mo2N2P2Si4: C,
43.26; H, 8.79; N, 2.66. Found: C, 43.70; H, 8.70; N, 2.69.
Conclusions
In summary, we selectively synthesized hetero-metallic [Mo2Fe2]
and [Mo2Mn2] hydride clusters via acid-base type reactions of
Cp*Mo(PMe3)(H)5 with M{N(SiMe3)2}2 (M
= Fe, Mn). The
[Mo2Fe2] cluster with amide ligands was successfully converted
into thiolate-supported derivatives through treatment with bulky
thiols. This series of [Mo2Fe2] clusters can serve as catalyst
precursors for the conversion of N2 to N(SiMe3)3, while the
catalytic synthesis of NH3 via protonation/reduction of N2 was
not successful. The [Mo2Fe2] hydride cluster reacts with CNtBu
under concomitant evolution of H2, in relation to the
accommodation of N2 at the nitrogenase FeMo-cofactor, even
though there are limited structural and electronic relevance
between the present and biological systems. The fragmentation
of the [Mo2Fe2] core into dinuclear Mo-Fe complexes in the
presence of CNtBu indicates the generation of dinuclear reactive
species during the catalytic silylation of N2.
{Cp*Mo(PMe3)}2{MnN(SiMe3)2}2(H)8 (2b). In a similar manner to the
synthesis of 2a, complex 1 (0.50 g, 1.60 mmol) in a mixture of hexane
and THF (v/v = 9:1, 7 mL), a mixture of hexane and THF (v/v = 9:1, 13
mL), and Mn{N(SiMe3)2}2 (0.60 g, 1.60 mmol) in a mixture of hexane and
THF (v/v = 9:1, 7 mL) were layered in a glass tube. Slow diffusion
between the three layers over the course of a week at room temperature
led to the formation of orange crystals of 2b·THF (0.64 g, 0.57 mmol,
71%). The crystals were collected after washing with hexane (ca. 2 x 5
mL) and drying under vacuum. 1H NMR (600 MHz, toluene-[D]8, r.t.): δ
20.8 (w1/2 = 3200 Hz, Cp*), 13.4 (w1/2 = 1100 Hz, SiMe3), 6.3 (w1/2 = 920
Hz, PMe3). Cyclic Voltammetry (THF): Epa = 0.23, –0.13 V, Epc = –1.77 V
(vs. Ag/AgNO3). UV-vis (toluene): λmax = 293 (ε 7.8 x 103 cm-1M-1), 307
(sh, ε 3.5 x 103 cm-1M-1) nm. Solution magnetic moment (toluene-[D]8,
297 K): 6.8(3) µB. IR (ATR, cm-1): ν = 2947, 2902, 1673, 1482, 1457,
1422, 1375, 1302, 1279, 1245, 1235, 1069. Anal. Calcd for
C38H92Mn2Mo2N2P2Si4: C, 43.34; H, 8.81; N, 2.66. Found: C, 43.82; H,
8.66; N, 2.28.
Experimental Section
General Procedures
All reactions were carried out under an atmosphere of N2 or Ar using
either Schlenk line or glove box techniques. Hexane, toluene, THF, and
pentane were purified by passing over columns of activated alumina and
a supported copper catalyst supplied by Hansen & Co. Ltd. DME and
deuterated solvents were dried over Na and vacuum-transferred prior to
use. 1H NMR spectra were recorded on JEOL JNM ECS600 or ECS500
spectrometers. IR spectra were recorded on an ATR-equipped Bruker
Alpha FT-IR spectrometer, while UV-vis spectra were measured on a
JASCO V770 spectrometer. Elemental analyses were carried out on
{Cp*Mo(PMe3)}2{Fe(STip)}2(H)8 (3a). 2a (0.402 g, 0.357 mmol) was
suspended in THF (4 mL), and HSTip (0.170 g, 0.705 mmol) was added
dropwise. The mixture was stirred overnight at room temperature,
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
causing a slight color change from brown to dark brown. The solution
was stored at –35 °C for a week to furnish dark brown crystals of
3a·2THF (0.239 g, 0.177 mmol, 50%). The crystals were collected after
washing with pre-cooled (–35 °C) pentane (ca. 3 x 2 mL) and drying
under vacuum. 1H NMR (600 MHz, toluene-[D]8, r.t.): δ 13.4 (w1/2 = 25 Hz,
p-methine), 12.9 (w1/2 = 25 Hz, m- iPr3C6H2, p- iPr3C6H2, or o-methine),
5.4 (w1/2 = 90 Hz, Cp*), 4.9 (w1/2 = 190 Hz, m- iPr3C6H2, p- iPr3C6H2, or o-
methine), 2.7 (w1/2 = 40 Hz, o- iPr3C6H2), 1.8 (w1/2 = 20 Hz, m- iPr3C6H2,
p- iPr3C6H2, or o-methine), 1.1 (w1/2 = 50 Hz, o- iPr3C6H2), 0.64 (w1/2 = 90
Hz, m- iPr3C6H2, p- iPr3C6H2, or o-methine) –0.67 (w1/2 = 130 Hz, PMe3).
Cyclic Voltammetry (THF): Epa = –0.47 V, Epc = –2.50 V (vs. Ag/AgNO3).
UV-vis (toluene): λmax = 290 (ε 2.2 x 104 cm-1M-1), 333 (sh, ε 1.0 x 104 cm-
1M-1), 389 (sh, ε 6.8 x 103 cm-1M-1) nm. Solution magnetic moment
(toluene-[D]8, 297 K): 2.7(2) µB. IR (ATR, cm-1): ν = 2955, 2901, 2889,
1712, 1650, 1584, 1479, 1461, 1446, 1418, 1375, 1353, 1308, 1302,
1277, 1254, 1245, 1163, 1106, 1055, 1033. Mössbauer spectrum (mm/s,
Cp*Mo(CNtBu)(CN)(C3N3HtBu3)Mn{N(SiMe3)2} (5). 2b (0.190 g, 0.169
mmol) was suspended in toluene (8 mL), and the solution was heated to
60 °C. A toluene (2 mL) solution of CNtBu (0.212 g, 2.55 mmol) was
added. The mixture was stirred overnight at 60 °C, causing a color
change from orange to brown. Then, the solvent was evaporated to
dryness, and the residue was dissolved in the minimal amount of
pentane (ca. 3 mL). The pentane solution was stored at –35 °C, which
furnished brownish-orange crystals of 5 (0.056 g, 0.070 mmol, 21%). 1H
NMR (600 MHz, C6D6, r.t.): δ 4.9 (w1/2 = 450 Hz), 2.2~0.8. Cyclic
Voltammetry (THF): Epa = 0.47, 0.15 V, Epc = –1.50, –2.47 V (vs.
Ag/AgNO3). UV-vis (toluene): λmax = 298 (ε 1.2 x 104 cm-1M-1), 345 (ε 9.3
x 103 cm-1M-1) nm. Solution magnetic moment (C6D6, 297 K): 5.3(3) µB.
IR (ATR, cm-1): ν = 2965, 2949, 2904, 2089, 2069, 2032, 1667, 1648
1552, 1469, 1367, 1357, 1271, 1235, 1198, 1134, 1069, 1022, 977, 957.
Anal. Calcd for C37H70MnMoN6Si2: C, 55.13; H, 8.75; N, 10.43. Found: C,
55.24; H, 8.38; N, 9.41.
78 K): IS
=
0.707(2) and QS
=
0.842(4). Anal. Calcd for
C56H102Fe2Mo2P2S2: C, 55.81; H, 8.53; S, 5.32. Found: C, 55.64; H, 8.06;
S, 5.19.
A typical procedure for the catalytic synthesis of silylamine in the
presence of [Mo2Fe2] clusters. The following procedure is analogous to
those in references [10] and [21-24]. A 100 mL round-bottom flask was
charged with Na (0.553 g, 24.1 mmol) and THF (12 mL) to furnish a
suspension. Cluster 2a (11.3 mg, 0.01 mmol) and Me3SiCl (3.05 mL, 24.1
mmol) were added sequentially. Any remaining 2a was dissolved in THF
(4 mL) and added subsequently. The flask was attached to a balloon
containing N2 (1 atm), and the mixture was stirred at room temperature
for 48 h. Cyclododecane (0.0239 g, 0.142 mmol) was added as an
internal standard for the GC analysis. After stirring for 5 min, the mixture
was centrifuged, and the supernatant was subjected to GC/GC-MS
analysis.
{Cp*Mo(PMe3)}2{Fe(SBtp)}2(H)8 (3b). 2a (0.151 g, 0.133 mmol) was
suspended in THF (1.5 mL), and a THF (1 mL) solution of HSBtp (0.070
g, 0.263 mmol) was added dropwise. The mixture was stirred overnight
at room temperature, causing a color change from brown to dark reddish
purple. The mixture was filtered and hexane (1 mL) was added to the
solution. The solution was stored at –35 °C for a week to furnish dark
reddish purple crystals of 3b (0.086 g, 0.069 mmol, 52%). Crystals were
collected after washing with pre-cooled (–35 °C) pentane (ca. 3 x 1 mL)
and drying under vacuum. 1H NMR (600 MHz, toluene-[D]8, r.t.): δ 14.9
(w1/2 = 25 Hz, m-(SiMe3)2C6H3), 6.2 (w1/2 = 100 Hz, Cp*), 5.9 (w1/2 = 50
Hz, p-(SiMe3)2C6H3), –0.65 (w1/2 = 130 Hz, PMe3), –0.98 (w1/2 = 70 Hz,
m-(SiMe3)2C6H3). Cyclic Voltammetry (THF): Epc
= –2.13 V (vs.
Acknowledgements
Ag/AgNO3). UV-vis (toluene): λmax = 298 (ε 9.5 x 103 cm-1M-1), 341 (sh, ε
7.0 x 103 cm-1M-1), 544 (ε 1.1 x 103 cm-1M-1) nm. Solution magnetic
moment (toluene-[D]8, 297 K): 3.5(5) µB. IR (ATR, cm-1): ν = 3028, 2977,
2955, 2896, 1695, 1641, 1586, 1539, 1479, 1459, 1379, 1338, 1279,
1237, 1193, 1134, 1093, 1071, 1033. Anal. Calcd for
C50H98Fe2Mo2P2S2Si4: C, 48.38; H, 7.96; S, 5.17. Found: C, 48.71; H,
7.54; S, 5.21.
This research was financially supported by the PRESTO
program of the Japan Science and Technology Agency
(JPMJPR1342),
Grants-in-Aid
for
Scientific
Research
(15H00936, 15K13655, and 16H04116) from the Japanese
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), and the Hori Sciences and Arts Foundation. We are
grateful to Prof. K. Tatsumi for supporting us with access to
instruments, and Profs S. Muratsugu and H. Matsui for generous
advice on electrochemical and GC/GC-MS measurements.
Cp*Mo(PMe3)(H)2(CNtBu)2Fe{N(SiMe3)2} (4). 2a (0.405 g, 0.359 mmol)
was suspended in toluene (12 mL), and a toluene (1 mL) solution of
CNtBu (0.297 g, 3.57 mmol) was added. The evolution of gas (H2) was
visible for ca. 10 min after addition of CNtBu. The mixture was stirred
overnight at room temperature, causing a color change from brown to
black. The solvent was evaporated to dryness, and the residue was
dissolved in the minimal amount of pentane (ca. 5 mL). The filtered
pentane solution was stored at –35 °C, which furnished black crystals of
4 (0.095 g, 0.137 mmol, 19%). The crystals were collected after washing
with pre-cooled (–35 °C) pentane (ca. 2 x 1 mL) and drying under
vacuum. 1H NMR (600 MHz, C6D6, r.t.): δ 17.5 (w1/2 = 230 Hz, PMe3 or
SiMe3 or tBu), 14.4 (w1/2 = 420 Hz, PMe3 or SiMe3 or tBu), 13.0 (w1/2 = 50
Hz, Cp*), 2.5 (w1/2 = 100 Hz, PMe3 or SiMe3 or tBu), –14.2 (w1/2 = 350 Hz,
Keywords: Mo • Fe • Hydride • Clusters • N2 reduction
[1]
[2]
[3]
M. L. Kirik and B. Stein, in Comprehensive Inorganic Chemistry Second
Edition (Eds.: J. Reedijk and K. Poeppelmeier), Elsevier, Amsterdam,
2013, Vol. 3, ch. 3.11.
a) M. W. Ribbe, Y. Hu, K. O. Hodgson, B. Hedman, Chem. Rev. 2014,
114, 4063-4080; b) M. Can, F. A. Armstrong, S. W. Ragsdale, Chem.
Rev. 2014, 114, 4149−4174.
t
t
PMe3 or SiMe3 or Bu), –15.7 (w1/2 = 480 Hz, PMe3 or SiMe3 or Bu).
Cyclic Voltammetry (THF): E1/2 –1.21 (vs. Ag/AgNO3). UV-vis
a) T. Spatzal, M. Aksoyoglu, L. Zhang, S. L. A. Andrade, E. Schleicher,
S. Weber, D. C. Rees, O. Einsle, Science 2011, 334, 940; b) L. M.
Lancaster, M. Roemelt, P. Ettenhuber, Y. Hu, M. W. Ribbe, F. Neese, U.
Bergmann, S. DeBeer, Science 2011, 334, 974-977.
=
V
(toluene): λmax = 340 (sh, ε 6.4 x 103 cm-1M-1), 456 (sh, ε 7.0 x 103 cm-1M-
1) nm. Solution magnetic moment (C6D6, 297 K): 4.8(1) µB. IR (ATR, cm-
1): ν = 2965, 261, 2906, 2875, 1877, 1752, 1724, 1539, 1457, 1426, 1414,
1377, 1359, 1897, 1277, 1234, 1194, 1067, 1000, 941. Mössbauer
spectrum (mm/s, 78 K): IS = 0.667(2) and QS = 1.590(5). Anal. Calcd for
C29H62FeMoN3PSi2: C, 50.35; H, 9.03; N, 6.07. Found: C, 50.19; H, 8.73;
N, 6.23.
[4]
a) Y. Hu, C. C. Lee, M. W. Ribbe, Science 2011, 333, 753-755; b) Z. –Y.
Yang, D. R. Dean, L. C. Seefeldt, J. Biol. Chem. 2011, 286, 19417-
19421; c) C. C. Lee, Y. Hu, M. W. Ribbe, Angew. Chem. Int. Ed. 2012,
51, 1947-1949; d) Z. –Y. Yang, V. R. Moure, D. R. Dean, L. C. Seefeldt,
Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 19644-19468; e) J. G.
Rebelein, Y. Hu, M. W. Ribbe, Angew. Chem. Int. Ed. 2014, 53, 11543-
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
11547; f) C. C. Lee, Y. Hu, M. W. Ribbe, Angew. Chem. Int. Ed. 2015,
54, 1219-1222.
Ellison, P. P. Power, S. C. Shoner, J. Am. Chem. Soc., 1989, 111,
8044-8046.
[5]
a) K. Tanifuji, C. C. Lee, Y. Ohki, K. Tatsumi, Y. Hu, M. W. Ribbe,
Angew. Chem. Int. Ed. 2015, 54, 14022-14025; b) K. Tanifuji, N.
Sickerman, C. C. Lee, T. Nagasawa, K. Miyazaki, Y. Ohki, K. Tatsumi,
Y. Hu, M. W. Ribbe, Angew. Chem. Int. Ed. 2016, 55, 15633-15636; c)
N. Sickerman, K. Tanifuji, Y. Ohki, K. Tatsumi, M. W. Ribbe, Y. Hu, J.
Am. Chem. Soc. 2017, 139, 603-606.
[19] a) U. Bierbach, W. Saak, D. Haase, S. Pohl, Z. Naturforsch. B., 1991,
46, 1629-1634; b) U. Bierbach, W. Barklage, W. Saak, S. Pohl, Z.
Naturforsch. B., 1992, 47, 1593-1601; c) R. Hauptmann, R. Klib, J.
Schneider, G. Henkel, Z. Anorg. Allg. Chem., 1998, 624, 1927-1936; d)
S. Pohl, M. Harmjanz, J. Schneider, W. Saak, G. Henkel, Inorg. Chim.
Acta, 2000, 311, 106-112; e) Y. Ohki, Y. Ikagawa, K. Tatsumi, J. Am.
Chem. Soc., 2007, 129, 10457-10465; f) S. Ohta, Y. Ohki, Y. Ikagawa,
R. Suizu, K. Tatsumi, J. Organomet. Chem., 2007, 692, 4792-4799.
[20] Y. Ohki, Y. Shimizu, R. Araake, M. Tada, W. M. C. Sameera, J. Ito, H.
Nishiyama, Angew. Chem. Int. Ed., 2016, 55, 15821-15825.
[21] a) M. Yuki, H. Tanaka, K. Sasaki, Y. Miyake, K. Yoshizawa, Y.
Nishibayashi, Nat. Commun. 2012, 3, 1254; b) G. Ung, J. C. Peters,
Angew. Chem. Int. Ed. 2015, 54, 532-535.
[6]
[7]
a) D. J. Lowe, R. N. F. Thorneley Biochem. J., 1984, 224, 895-901; b)
R. N. F. Thorneley, D. J. Lowe, Biochem. J., 1984, 224, 903-909.
a) Z. –Y. Yanga, N. Khadkaa, D. Lukoyanovb, B. M. Hoffman, D. R.
Dean, L. C. Seefeldt, Proc. Natl. Acad. Sci. U.S.A., 2013, 110, 16327-
16332; b) D. Lukoyanov, Z. –Y. Yang, N. Khadka, D. R. Dean, L. C.
Seefeldt, B. M. Hoffman, J. Am. Chem. Soc. 2015, 137, 3610-3615.
B. M. Hoffman, D. Lukoyanov, Z. –Y. Yang, D. R. Dean, L. C. Seefeldt,
Chem. Rev., 2014, 114, 4041-4062.
[8]
[9]
[22] a) R. B. Siedschlag, V. Bernales, K. D. Vogiatzis, N. Planas, L. J.
Clouston, E. Bill, L. Gagliardi, C. C. Lu, J. Am. Chem. Soc. 2015, 137,
4638-4641; b) R. Imayoshi, H. Tanaka, Y. Matsuo, M. Yuki, K.
Nakajima, K. Yoshizawa, Y. Nishibayashi, Chem. Eur. J. 2015, 21,
8905-8909.
For examples: a) M. D.Fryzuk, S. A. Johnson, B. O. Patrick, A. Albinati,
S. A. Mason, T. F. Koetzle, J. Am. Chem. Soc. 2001, 123, 3960-3973;
b) F. Akagi, T. Matsuo, H. Kawaguchi, Angew. Chem. Int. Ed. 2007, 46,
8778–8781; c) Y. Yu, A. R. Sadique, J. M. Smith, T. R. Dugan, R. E.
Cowley, W. W. Brennessel, C. J. Flaschenniem, E. Bill, T. R. Cundari,
P. L. Holland, J. Am. Chem. Soc. 2008, 130, 6624-6638; d) K. Ding, W.
W. Brennessel, P. L. Holland, J. Am. Chem. Soc. 2009, 131, 10804-
10805; e) T. Shima, S. Hu, G. Luo, X. Kang, Y. Luo, Z. Hou, Science
2013, 340, 1549-1552; f) J. Rittle, C. C. L. McCrory, J. C. Peters, J. Am.
Chem. Soc. 2014, 136, 13853-13862.
[23] a) K. Komori, H. Oshita, Y. Mizobe, M. Hidai, J. Am. Chem. Soc., 1989,
111, 1939-1940; b) H. Tanaka, A. Sasada, T. Kouno, M. Yuki, Y.
Miyake, H. Nakanishi, Y. Nishibayashi, K. Yoshizawa, J. Am. Chem.
Soc., 2011, 133, 3498-3506; c) Ogawa, T.; Kajita, Y.; Wasada-Tsutsui,
Y.; Wasada, H.; Masuda, H. Inorg. Chem. 2013, 52, 182-195; d) Q.
Liao, N. Saffon-Merceron, N. Mézailles, Angew. Chem. Int. Ed., 2014,
53, 14206-1420.
[10] R. Araake, K. Sakadani, M. Tada, Y. Sakai, Y. Ohki, J. Am. Chem. Soc.
2017, 139, 5596-5606.
[24] R. Imayoshi, K. Nakajima, Y. Nishibayashi, Chem. Lett. 2017, 46, 466-
468.
[11] a) F. Abugideiri, M. A. Kelland, R. Poli, Organometallics 1993, 12,
2388-2389; b) C. A. Bayse, M. B. Hall, B. Pleune, R. Poli,
Organometallis, 1998, 17, 4309-4315.
[25] S. Linderoth, S. Morup, J. Phys. Condens. Matter, 1992, 4, 8627-8634.
[26] a) C. G. Werncke, P. C. Bunting, C. Duhayon, J. R. Long, S. Bontemps,
S. Sabo-Etienne, Angew. Chem. Int. Ed., 2015, 54, 245-248; b) A. A.
Danopoulos, P. Braunstein, K. Y. Monakhov, J. van Leusen, P.
Kögerler, M. Clémancey, J. –M. Latour, A. Benayad, M. Tromp, E.
Rezabal, G. Frison, Dalton Trans., 2017, 46, 1163-1171.
[12] a) R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, M. F. Lappert,
W. –P. Leung, K. Rypdal, Inorg. Chem. 1988, 27, 1782–1786; b) Y.
Ohki, S. Ohta, K. Tatsumi, Inorg. Synth., 2010, 35, 137-143.
[13] a) T. Shima, Y. Luo, T. Stewart, R. Bau, G. J. Mclntyre, S. A. Mason, Z.
Hou, Nat. Chem. 2011, 3, 814-820; b) T. Shima, Z. Hou, Chem. Eur. J.
2013, 19, 3458-3466.
[27] a) D. C. Bradley, M. B. Hursthouse, K. M. Abdul Malik, R. Möseler,
Transition Met. Chem. 1978, 3, 253-254; b) C. R. Hamilton, M. R. Gau,
R. A. Baglia, S. F. McWilliams, M. J. Zdilla, J. Am. Chem. Soc. 2014,
136, 17974-17986.
[14] For the shortest/longest Mo-Fe distances in hydride-supported Mo/Fe
clusters listed in the Cambridge Structural Database, see: a) P. Mathur,
S. Ghose, M. M. Hossain, C. V. V. Satyanarayana, J. E. Drake, J.
Organomet. Chem., 1998, 557, 221-225; b) A. J. Bridgeman, M. J.
Mays, A. D. Woods, Organometallics, 2001, 20, 2932-2935.
[28] P. J. Blower, J. R. Dilworth, J. P. Hutchinson, J. A. Zubieta, J. Chem.
Soc., Dalton Trans. 1985, 1533–1541.
[29] E. Block, V. Eswarakrishnan, M. Gernon, G. Ofori-Okai, C. Saha, K.
Tang, J. Zubieta, J. Am. Chem. Soc. 1989, 111, 658–665.
[15] For the shortest/longest Mo-Mn distances in hydride-supported Mo/Mn
clusters listed in the Cambridge Structural Database, see: a) M. J.
Doyle, T. J. Duckworth, L. Manojlovic-Muir, M. J. Mays, P. R. Raithby, F.
J. Robertson, J. Chem. Soc., Dalton Trans., 1992, 2703-2714; b) C. M.
Alvarez, M. A. Alvarez, M. E. García, A. Ramos, M. A. Ruiz, C. Graiff, A.
Tiripicchio, Organometallics 2007, 26, 321-331.
[16] For the shortest/longest Fe(II)-N(SiMe3)2 distances listed in the
Cambridge Structural Database, see: a) R. Pryadun, R. H. Holm, Inorg.
Chem., 2008, 47, 3366-3370; b) B. A. Frazier, V. A. Williams, P. T.
Wolczanski, S. C. Bart, K. Meyer, T. R. Cundari, E. B. Lobkovsky, Inorg.
Chem., 2013, 52, 3295-3312.
[17] For the shortest/longest Mn(II)-N(SiMe3)2 distances listed in the
Cambridge Structural Database, see: a) H. –J. Gosink, H. W. Roesky,
H. –G. Schmidt, M. Noltemeyer, E. Irmer, R. Herbst-Irmer,
Organometallics, 1994, 13, 3420-3426; b) H. Chen, R. A. Bartlett, R.
Dias, M. M. Olmstead, P. P. Power, Inorg. Chem., 1991, 30, 2487-2494.
[18] For the shortest/longest Fe(III)-N(SiMe3)2 distances, see: a) Y. Ohki, Y.
Sunada, K. Tatsumi, Chem. Lett., 2005, 34, 172-173; b) C. R. Sharp, J.
S. Duncan, S. C. Lee, Inorg. Chem., 2010, 49, 6697-6705; c) J. L.
Wong, R. H. Sanchez, J. G. Logan, R. A. Zarkesh, J. W. Ziller, A. F.
Heyduk, Chem. Sci. 2013, 4, 1906-1910. For the only
crystallographically determined Mn(III)-N(SiMe3)2 distance, see: d) J. J.
This article is protected by copyright. All rights reserved.

10.1002/chem.201702925
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
FULL PAPER
Hydride-Supported
Ar
Fe
Yasuhiro Ohki,* Yuna Araki, Mizuki
Tada, Yoichi Sakai
N
Mo2Fe2
Fe
Page No. – Page No.
Mo
Mo
Synthesis and Characterization of Bio-
Inspired [Mo2Fe2] Hydride Clusters and
Their Application to the Catalytic
Silylation of N2
S
Si
N2
N(SiMe3)2
Text for Table of Contents
Inspired by a process postulated for the biological reduction of N2, hydride-
supported [Mo2Fe2] clusters were synthesized and characterized. The [Mo2Fe2]
clusters were found to serve as catalyst precursors for the reductive silylation of N2.
This article is protected by copyright. All rights reserved.