Synthesis of Dinuclear Mo?Fe Hydride Complexes and Catalytic Silylation of N2

Article abstract of DOI:10.1002/chem.202000104

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N₂ activation by molecular complexes and the enzyme active site. In this study, dinuclear Mo^IV-Fe^II complexes with bridging hydrides, Cp^RMo(PMe₃)(H)(μ-H)₃FeCp* (2 a; Cp^R=Cp=C₅Me₅, 2 b; Cp^R=C₅Me₄H), were synthesized via deprotonation of Cp^RMo(PMe₃)H₅ (1 a; Cp^R=Cp*, 1 b; Cp^R=C₅Me₄H) by Cp*FeN(SiMe₃)₂, and they were characterized by spectroscopy and crystallography. These Mo?Fe complexes reveal the shortest Mo?Fe distances ever reported (2.4005(3) ? for 2 a and 2.3952(3) ? for 2 b), and the Mo?Fe interactions were analyzed by computational studies. Removal of the terminal Mo?H hydride in 2 a–2 b by [Ph₃C]⁺ in THF led to the formation of cationic THF adducts [Cp^RMo(PMe₃)(THF)(μ-H)₃FeCp*]⁺ (3 a; Cp^R=Cp*, 3 b; Cp^R=C₅Me₄H). Further reaction of 3 a with LiPPh₂ gave rise to a phosphido-bridged complex Cp*Mo(PMe₃)(μ-H)(μ-PPh₂)FeCp* (4). A series of Mo?Fe complexes were subjected to catalytic silylation of N₂ in the presence of Na and Me₃SiCl, furnishing up to 129±20 equiv of N(SiMe₃)₃ per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Full text of DOI:10.1002/chem.202000104

A Journal of
Accepted Article
Title: Synthesis of Dinuclear Mo-Fe Hydride Complexes and
Catalytic Silylation of N2
Authors: Kodai Ishihara, Yuna Araki, Mizuki Tada, Tsutomu Takayama,
Yoichi Sakai, W. M. C. Sameera, and Yasuhiro Ohki
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To be cited as: Chem. Eur. J. 10.1002/chem.202000104
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Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic
Silylation of N2
Kodai Ishihara,[a] Yuna Araki,[a] Mizuki Tada,[a,b] Tsutomu Takayama[c], Yoichi Sakai[c], W. M. C.
Sameera,*,[d] and Yasuhiro Ohki*,[a]
Dedication ((optional))
Abstract: Two transition metal atoms bridged by hydrides may
represent a useful structural motif for N2 activation by molecular
complexes and the enzyme active site. In this study, dinuclear
Mo(IV)-Fe(II) complexes with bridging hydrides, CpRMo(PMe3)(H)(µ-
H)3FeCp* (2a; CpR = Cp* = C5Me5, 2b; CpR = C5Me4H), were
synthesized via deprotonation of CpRMo(PMe3)H5 (1a; CpR = Cp*,
1b; CpR = C5Me4H) by Cp*FeN(SiMe3)2, and they were characterized
by spectroscopy and crystallography. These Mo-Fe complexes
reveal the shortest Mo-Fe distances ever reported (2.4005(3) Å for
2a and 2.3952(3) Å for 2b), and the Mo-Fe interactions were
analyzed by computational studies. Removal of the terminal Mo-H
hydride in 2a-2b by [Ph3C]+ in THF led to the formation of cationic
THF adducts [CpRMo(PMe3)(THF)(µ-H)3FeCp*]+ (3a; CpR = Cp*, 3b;
CpR = C5Me4H). Further reaction of 3a with LiPPh2 gave rise to a
phosphido-bridged complex Cp*Mo(PMe3)(µ-H)(µ-PPh2)FeCp* (4). A
series of Mo-Fe complexes were subjected to catalytic silylation of
complex,[6]
complexes,[7,8] and
neutral
Ti(µ-H)4Ti
Ti3(µ-H)4(µ3-NH)2 imide-hydride
and
Ti3(µ-H)6(µ3-H)
a
cluster,[9] which have been found to break the triple bond of
N2. The electronic effect of the hydride ligand to facilitate
the activation of N2 has been suggested by the gas-phase
reaction of a transient [Ta3N3H]– cluster anion.[10] Late
transition metal dinuclear complexes with hydrides have
been also applied in the N2 chemistry. For instance, neutral
Fe(µ-H)2Fe complexes and a dianionic Co(µ-H)2Co complex
supported by β-diketiminates have been shown to
exchange two bridging hydrides with an N2 molecule.[11,12]
Furthermore, an Fe(µ-H)2Fe complex supported by a multi-
dentate phosphine-silyl hybrid ligand has been found to
bind N2 without elimination of H2.[13] The importance of
hydrides has been also postulated for the biological N2
reduction, where a [MoFe7S9C] cluster active site (FeMo-
N2 in the presence of Na and Me3SiCl, furnishing up to 129±20 equiv. cofactor) in the resting state accumulates 4 electrons and 4
of N(SiMe3)3 per molecule of 2b. Mechanism of the catalytic cycle
protons to generate the active state with two Fe-H-Fe
hydrides prior to the activation of N2.[14]
was analyzed by DFT calculations.
The utility of hydrides in the N2 chemistry and the presence
of Mo-Fe compound in the enzyme prompted us to install Mo,
Fe, and hydrides in catalyst precursors for N2
transformation. We have recently synthesized some tetra-
nuclear [Mo2Fe2] hydride clusters (Figure 1), which are able
to catalyze the conversion of N2 into N(SiMe3)3 in moderate
activities.[15] A stoichiometric reaction of one of the [Mo2Fe2]
Introduction
Transition metal complexes supported by hydrides have
been employed as precursors in stoichiometric and catalytic
activation of various small molecules.[1-4] A seminal example
in the N2 chemistry is a dinuclear Ta2(µ-H)4 complex
supported by PhP(CH2SiMe2NPh)2 ligands, which releases
H2 and activate N2 on both tantalum atoms.[5] Analogous
roles of hydrides have been demonstrated with early
transition metal complexes such as an anionic Nb(µ-H)4Nb
clusters
A
with CNtBu resulted in the fragmentation of the
[Mo2Fe2] core into two dinuclear Mo-Fe complexes, and
therefore we speculated that in-situ generated dinuclear
Mo-Fe hydrides would work as catalytically active species.
In this study, we selectively synthesized dinuclear Mo-Fe
hydride complexes from acid-base type reactions, i.e.
deprotonation of one of the hydride ligands in
CpRMo(PMe3)H5
(
1a; CpR = Cp* = C5Me5, 1b; CpR
=
[a]
[b]
K. Ishihara, Y. Araki, Prof. M. Tada, Prof. Y. Ohki
Department of Chemistry, Graduate School of Science, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
E-mail: ohki@chem.nagoya-u.ac.jp
Si
Si
N
Prof. M. Tada
Fe
H
H
PMe3
Me3P
H
Integrated Research Consortium on Chemical Sciences (IRCCS),
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602
(Japan)
H
H
H
H
R
Mo
Mo
H
H
Mo
Fe
H
[c]
[d]
Prof. T. Takayama, Prof. Y. Sakai
Department of Chemistry, Daido University, Takiharu-cho, Minami-
ku, Nagoya 457-8530 (Japan)
Prof. W. M. C. Sameera
Institute of Low Temperature Science, Hokkaido University,
Sapporo, 060–0819 (Japan)
Me3P
Fe
H
H
N
Si
2 (R = Me, H)
Si
this study
A
E-mail: wmcsameera@lowtem.hokudai.ac.jp
previous study
Supporting information for this article is given via a link at the end of
the document.
Figure 1. A [Mo2Fe2] hydride cluster A used in a previous
study and the Mo-Fe dinuclear complexes 2 synthesized in
this study.
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C5Me4H) with a basic amide ligand of Cp*FeN(SiMe3)2. The
resultant Mo-Fe complexes appeared to show improved
catalytic activities in the transformation of N2 into N(SiMe3)3
relative to the tetra-nuclear [Mo2Fe2] hydride clusters. It is
also notable that the Mo-Fe complexes reported herein show
short Mo-Fe bonds bridged only by hydrides.
assignment of the oxidation states in hetero-metallic complexes
is not straightforward, deprotonation of one of the hydrides in
Mo(VI) complexes by an Fe(II) amide complex is expected to
lead to the 2e-reduction of the Mo center and thereby the
Mo(IV)-Fe(II) oxidation state can be assumed for 2a and 2b,
while the Mo(III)-Fe(III) state remains as an alternative possibility.
In agreement with our assumption, the Mo-P distances of 2a
(2.4526(6) Å) and 2b (2.4357(6) Å) are within the range of
previously reported CpRMo(IV)-PMe3 distances (2.4336(5)-
2.546(5) Å)[19] and shorter than the known CpRMo(V)-PR3
Results and Discussion
distances (2.554(1)-2.687(2) Å)[20]
,
while the range for
Synthesis and Characterization of Hydride-Bridged Mo-Fe
Complexes.
CpRMo(III)-PMe3 (2.4445(5)-2.506(2) Å)[21] overlaps that of
Mo(IV) complexes. It is interesting to note that the trend of
CpRMo-PMe3 distances is opposite to the ionic radius of Mo
which should be smaller for higher oxidation states, although we
have been unable to rationalize this trend. The assignment of
Mo(IV)-Fe(II) oxidation state for 2a and 2b was supported by the
57Fe Mössbauer spectra at zero field and 78 K, where a doublet
appeared at the isomer shifts (IS) of 0.247(1) (2a) and 0.254(1)
(2b) mm/s with quadrupole splittings (QS) of 2.270(2) (2a) and
2.302(2) (2b) mm/s (Figures 3 and S4). These IS and QS values
are comparable to those reported for low-spin Cp*Fe(II) hydride
complexes supported by a bidentate [PR2NPh2] ligand (1,5-R2-
3,7-diphenyl-1,5-diphospha-3,7-diazacyclooctane), which show
doublets at IS = 0.20 mm/s and QS = 2.11 mm/s (R = Ph) and IS
= 0.23 mm/s and QS = 2.23 mm/s (R = Cy).[22] Even though the
similarity in the Mössbauer spectra indicates the low-spin Fe(II)
state of Fe atoms in 2a and 2b, we cannot rule out the possibility
of the Fe(III) state because typical range of isomer shift for
Fe(III) (S = 1/2 or 3/2) state covers the observed values.
Treatment of CpRMo(PMe3)H5 (1a; CpR = Cp*, 1b; CpR
=
C5Me4H)[16] with an equimolar amount of Fe(II) amide complex
[17]
Cp*FeN(SiMe3)2
in hexane led to the formation of hydride-
bridged Mo-Fe dinuclear complexes CpRMo(PMe3)(H)(µ-
H)3FeCp* (2a; CpR = Cp*, 2b; CpR = C5Me4H), which were
isolated as dark blue crystals in 84% (2a) and 82% (2b) yields
(Scheme 1). Their molecular structures were unequivocally
determined by means of X-ray crystallographic analysis (Figures
2 and S3). The Fe center adopts a three-legged piano-stool
geometry, while the coordination environment of Mo is
analogous to that of the precursor CpRMo(PMe3)H5 with PMe3 at
one of the equatorial positions. The Mo-Fe distances of 2a
(2.4005(3) Å) and 2b (2.3952(3) Å) are the shortest reported
thus far and significantly shorter than the previously reported
hydride-supported Mo-Fe bonds (2.7144(9)-2.9322(5) Å),[15,18]
indicating Mo-Fe bonding character (vide infra). Even though
R
Me3P
H
H
H
Si
Si
R
H
H
Mo
H
Mo
Fe
Electronic structure of 2b was further analyzed by DFT
calculations based on the BP86-D3BJ and B3LYP-D3BJ
methods (Table S4). The calculated ground state of 2b is S = 0,
which is consistent with the damagnetic nature of 2b. Also, the
key structural parameters of the complex are in agreement with
the X-ray structure. On the basis of the BP86-D3BJ method, the
optimezed S = 1 and S = 2 states were higher in energy by 14.6
kcal/mol and 20.7 kcal/mol relative to the S = 0 state. On the
other hand, the B3LYP-3DBJ functional showed the S = 2
ground state, which is not in agreement with our NMR data. Also,
+
N Fe
H
H
H
PMe3
1a (R = Me)
1b (R = H)
2a (R = Me)
2b (R = H)
Scheme 1. Synthesis of Mo-Fe hydride complexes 2a-2b.
(a) (b)
P
2a
3a-BAr4
H3
Fe
Mo
P
H4
H2
H1
H3
Fe
Mo
H1
H2
H4
–4
–2
0
2
4 –4
–2
0
2
4
Doppler Velocity (mm/s)
Doppler Velocity (mm/s)
Figure 2 Molecular structure of 2b with atomic displacement
parameters set at 50% probability. (a) Front view and (b) Side
view along the Mo-Fe axis, where methyl groups on Cp rings
are omitted. Only selected atoms are labeled, and hydrogen
atoms except hydrides are omitted for clarity. Selected
distances (Å): Mo-Fe, 2.3952(3); Mo-P, 2.4357(5); Mo-H1,
1.66(2); Mo-H2, 1.84(2); Mo-H3, 1.84(2); Mo-H4, 1.83(2); Fe-
H2, 1.58(3); Fe-H3, 1.60(2); Fe-H4, 1.63(2).
Figure 3 57Fe Mössbauer spectra of 2a (left) and 3a-BAr4
(right) at zero field and 78 K. 2a: Isomer Shift (IS, mm/s) =
0.247(1), Quadrupole Splitting (QS, mm/s) = 2.270(2). 3a-
BAr4: IS = 0.281(2), QS = 2.217(4). The doppler velocity scale
was calibrated using room-temperature Mössbauer
spectrum of a metallic iron foil.
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the key structural parameters of the B3LYP-D3BJ optimized S =
2 state showed a poor agreement with the X-ray structure. The
B3LYP-D3BJ optimized S = 0 and S = 1 states are 9.6 and 10.0
kcal/mol higher in energy, respectively. The difference between
the calculations by BP86-D3BJ and B3LYP-D3BJ indicates that
the spin state energy gaps are sensitive to the Hartree-Fock
exchange of the chosen functional. In the present study, using a
DFT method without the Hartree-Fock exchange was important,
as the ground state of 2b is a closed-shell singlet. Therefore for
this study, we have chosen the BP86-D3BJ method, from which
the optimized structure fits well with the X-ray structure in the
singlet ground state.
From the structure of 2b optimized by BP86-D3BJ in the S
= 0 state, the Mössbauer parameters were calculated as IS =
0.21 and QS = 2.82 mm/s, which are qualitatively in agreement
with the experimental values (Table S5). The calculated
structure of 2b in the S = 0 state reproduces well the X-ray
structure, e.g. the calculated Mo-Fe distance (2.38 Å) is in
agreement with the crystallographically determined Mo-Fe
distance of 2b (2.3952(3) Å). Computed metal-hydrogen bond
distances (Mo-H1, 1.72; Mo-H2, 1.86; Mo-H3, 1.87; Mo-H4,
1.87; Fe-H2, 1.63; Fe-H3, 1.64; and Fe-H4, 1.63 Å) are also in
an agreement with the X-ray structure. In the presence of three
bridging H atoms with short metal-hydrogen distances, the Mo
and Fe atoms are in close proximity and a Mo-Fe bond may be
formed. Kohn-Sham frontier molecular orbitals of the ground
state of 2b are shown in Figure 4, where the HOMO-2 (107) and
HOMO-3 (106) orbitals are delocalized on Fe and Mo, indicating
Mo-Fe bonding interactions. A quantum theory of atoms in
molecules (QTAIM) analysis indicated a bond critical point
(BCP) between Mo and Fe (Figure S20). Furthermore,
computed electron density [ꢀ ꢁ ] of the BCP was 0.08, and the
ellipticity [ꢂ ꢁ ] of the BCP was 0.06, suggesting a fairly ionic
with modest covalent bonding combination. The Laplacian
[∇ꢃꢀ ꢁ ] of the BCP was 0.18, indicating closed-shell interactions
in the region of the interatomic surface. Calculated Mayer bond
indices of 0.79 also indicated a Mo-Fe bonding character.
1
In the H NMR, the methyl groups of C5Me4H ligand in 2b
gave four broad 1H NMR signals at 1.70, 2.03, 2.21, and 2.35
ppm at room temperature, while these signals became sharp by
lowering the measurement temperatures below –20 °C (Figure
S8). The C5Me5 signals of 2a and 2b remained sharp in the
temperature range of -80 to 100 °C. Three bridging hydrides in
2a and 2b are inequivalent, as one of the bridging hydrides
occupy the position nearly trans to the CpR ligand with respect to
the Mo atom, while the other two are located cis and trans with
respect to the PMe3 ligand on Mo (Figure 2b). However,
exchange of hydrides in polyhydride complexes typically occurs
within or comparable to the time scale of the 1H NMR
spectrum.[23] In fact, bridging hydrides of 2a and 2b gave a 3H
1
intensity signal in the H NMR in C6D6 at room temperature at –
15.78 (2a) and –15.61 (2b) ppm, while the terminal Mo-H signal
appeared at –7.17 (2a) and –7.45 (2b) ppm (Figure S6). These
hydride signals coalesced at around 60 °C (2a) or 100 °C (2b),
indicating the slow exchange among Mo-H and three Mo-H-Fe
hydrides (Figures S7 and S8). The presence of bridging
hydrides was also supported by the IR spectra, where a band at
1743 cm-1 (both 2a and 2b) shifted to 1256 cm-1 (2a-[D4]) or
1255 cm-1(2b-[D4]) upon deuterium labeling (Figures 5 and S5).
2a
1743
2a-[D4]
1256
1256
difference
spectrum
1743
3500
2500
1500
500
Wavenumbers (cm-1)
Figure 5 IR spectra of crystalline complex 2a and
deuterated derivative 2a (obtained by using 1a-[D5]),
alongside the corresponding difference spectrum.
Figure 4 Kohn-Sham frontier molecular orbitals of the
optimized ground state structure of 2b.
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These frequencies are comparable to those of previously
reported [Mo2Fe2] hydride clusters (1712-1584 cm-1)[15] and
higher than those of [Fe4] and [Fe6] hydride clusters (1670-1498
cm-1).[24] We have been unable to rationalize the absence of
terminal Mo-H bands of 2a and 2b.
O
H2
H3
P
Electrochemical properties of 2a and 2b were investigated
by cyclic voltammetry in THF at room temperature (Figure S9).
Both 2a and 2b exhibited one each reversible reduction and
oxidation processes, at E1/2 = –2.57 (2a) and –2.61 (2b) V and
E1/2 = –0.44 (2a) and –0.52 (2b) V vs Ag/AgNO3, respectively.
The slightly less electron-donating property of the C5Me4H ligand
relative to the Cp* ligand is evident from the negative shift of
potentials of 2b relative to 2a. It should be noted that these
redox processes are reversible electrochemically but not
chemically. In fact, after scanning the reversible oxidation
process, a weak cathodic wave appeared at –1.32 (2a) and –
1.40 (2b) V, indicating partial degradation of the
electrochemically generated [2a]+ and [2b]+. This degradation
occurs possibly via loss of hydrides, as the electro-spray
ionization mass (ESI-MS) spectra of 2a and 2b in the positive
mode exhibited major signals of [2a-4H]+ at m/z = 500.11 and
[2b-4H]+ at m/z = 486.09 (Figures S10 and S11), which are
assignable as the 1e-oxidized species of dinuclear Mo-Fe
complexes without hydrides. The loss of hydrides was supported
by the ESI-MS spectrum of 2a-[D4], which exhibited the same
signal at m/z = 500.03.
Mo
Fe
H1
Figure 6 Structure of the cationic part of 3a-BAr4 with
atomic displacement parameters set at 50% probability.
Only selected atoms are labeled, and hydrogen atoms
except hydrides are omitted for clarity. Selected distances
(Å): Mo-Fe, 2.4068(9); Mo-P, 2.5007(15); Mo-O, 2.253(3);
Mo-H1, 1.81(5); Mo-H2, 1.98(6); Mo-H3, 1.84(6); Fe-H1,
1.69(5); Fe-H2, 1.41(6); Fe-H3, 1.55(6).
also furnished 3b-BAr4 in 67% yield. Whereas green crystals of
3a-PF6 furnished low-quality diffraction data sets in X-ray
crystallographic analysis, those of 3a-BAr4 allowed us a detailed
structural characterization (Figure 6). As the structural difference
between the dinuclear Mo-Fe moieties of 2a and 3a-BAr4 is one
of the terminal ligands on Mo (hydride or THF), the coordination
geometries and metrical parameters are analogous between 2a
and 3a-BAr4, as exemplified in the Mo-Fe (2.4004(4) Å (2a) vs
2.4067(10) Å (3a)) and Mo-P (2.4526(6) Å (2a) vs 2.5006(18) Å
(3a)) distances, while steric congestion around the Mo site of 3a-
BAr4 leads to the relatively long Mo-P distance. Three bridging
hydrides in 3a-BAr4 have been refined crystallographically, and
they are in accordance with the appearance of three broad 1H
NMR signals for bridging hydrides at –6.94, –15.00, and –17.20
ppm in THF-[D]8 at ambient temperature (Figure S12). Analogy
between 2a and 3a-BAr4 is also evident from the 57Fe
Mössbauer parameters. 3a-BAr4 exhibited a doublet at IS =
0.281(2) mm/s with QS = 2.217(4) mm/s (Figure 3b), and these
parameters are similar to those of 2a, in accordance with the
low-spin Fe(II) state. Thus our tentative assignment of Mo(IV)-
Fe(II) oxidation state would be also applicable to 3a-BAr4, while
an ambiguity remains to prevent us to explicitely assign the
oxidation state.
Removal of the terminal Mo-H hydride in the Mo-Fe
complexes.
The electrochemical properties of Mo-Fe complexes prompted
us to investigate their chemical oxidations. In contrary to our
expectation to generate the 1e-oxidized species, treatment of 2a
with 1 equiv of [(C5H5)2Fe][PF6] in THF resulted in the formation
of a mixture of materials. From this mixture, we manually
separated
crystals
of
a
cationic
THF
adduct
[Cp*Mo(PMe3)(THF)(µ-H)3FeCp*][PF6] (3a-PF6), whose structure
was preliminary identified by means of X-ray analysis. Even
though this reaction was induced by the oxidation of 2a, the
formation of 3a-PF6 implies the removal of the terminal Mo-H
hydride followed by the coordination of THF to Mo. It is also
notable that terminal hydrides are usually more reactive than
bridging hydrides.[25] Thus, alternative reactions of 2a with
hydride-abstracting carbocation salts [Ph3C][X] (X = PF6 or
B(C6F5)4) in THF afforded 3a-PF6 and 3a-BAr4, which were
isolated as green crystals in 58 and 63% yields, respectively
(Scheme 2). An analogous reaction of 2b with [Ph3C][B(C6F5)4]
Synthesis of a Phosphido-Bridged Mo-Fe Hydride Complex.
In order to lead to a more robust Mo-Fe framework, we
examined to incorporate a bridging phosphido ligand (µ-PPh2).
Treatment of a cationic THF adduct 3a-PF6 with LiPPh2 in THF
resulted in the formation of a brown solution, from which a
phosphido-bridged Mo-Fe complex Cp*Mo(PMe3)(µ-PPh2)(µ-
H)FeCp* (4) was crystallized in 53% yield (Scheme 3). In this
reaction, a THF ligand and two hydrides were replaced by a
bridging PPh2 ligand, and thus the oxidation state of resultant
Mo-Fe complex 4 is lower by two units relative to the Mo(IV)-
Fe(II) complexes 2 or 3. In accordance with the presence of
terminal PMe3 and bridging PPh2, the 31P NMR in C6D6 exhibited
two signals at 27.9 (PMe3) and 162.8 (µ-PPh2) ppm. In the 1H
NMR, the Mo-H-Fe hydride signal was found as 1H intensity at –
X–
Me3P
H
O
Mo
Me3P
R
H
H
H
H
R
Mo
Fe
+ [Ph3C][X]
Fe
H
THF
H
3a-PF6 (R = Me, X = PF6)
3a-BAr4 (R = Me, X = B(C6F5)4)
3b-BAr4 (R = H, X = B(C6F5)4)
2a (R = Me)
2b (R = H)
Scheme 2. Removal of a Mo-H hydride from 2a or 2b by [Ph3C]-
cation in THF, furnishing cationic THF adducts.
10.54 ppm (Figure S13). The molecular structure of
4
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determined by a single-crystal X-ray analysis reveals the Mo-Fe
framework bridged by a hydride and a µ-PPh2 ligand (Figure 7).
The Mo-PMe3 distance (2.4021(19) Å) is shorter than the ranges
reported for CpRMo(III)-PMe3 (2.4445(5)-2.506(2) Å) and
CpRMo(IV)-PMe3 distances (2.4336(5)-2.546(5) Å) but within that
of CpRMo(II)-PMe3 (2.3707(5)-2.521(3) Å),[26] implying the Mo(II)-
Fe(II) oxidation state for 4. This tentative assignment was
supported by the 57Fe Mössbauer spectrum showing a doublet
at IS = 0.359(2) mm/s with QS = 2.356(4) mm/s (Figure S14).
The IS value of 4 is in between those of low-spin Fe(II) hydride
and chloride complexes Cp*FeX[PR2NPh2] (X = H, Cl; R = Ph,
Cy),[22] whose IS values were reported as 0.46 mm/s (X = Cl, R
= Ph or Cy), 0.20 mm/s (X = H, R = Ph), and 0.23 mm/s (X = H,
R = Cy). The QS value of 4 is comparable to those for
Cp*FeX[PR2NPh2] (2.11-2.23 mm/s).
equiv, entry 4) and 3b-BAr4 (8±5 equiv, entry 5) and the
phosphido-bridged complex 4 (41±11 equiv, entry 6). The
catalytic activities of 2a-2b are not as good as the best
examples reported thus far with Fe complexes (183 equiv by the
[Fe6] hydride cluster)[24] or Mo complexes (150+76 equiv for two
consecutive
runs
by
Mo(N2)2(depf)2,
depf
=
1,1-
bis(diethylphosphino)ferrocene)[28c], but the yields of more than
100 equiv of N(SiMe3)3 per 2a-2b (entries 1 and 3) are higher
than most of other reported examples.[27,28] Furthermore,
apparent improvement is suggested by comparison of the
activity with that of [Mo2Fe2] hydride cluster A (entry 9), which
yielded 69±7 equiv per molecule of A (= 35 equiv per tentative
dinuclear Mo-Fe moiety).[15] This result and the inactivity of
monomeric Cp*Mo and Cp*Fe complexes (entries 7 and 8) are
in accordance with our speculation that dinuclear Mo-Fe
hydrides work as catalytically active species in our present
and previous studies.
PF6
+ LiPPh2
Me3P
Mo
O
Mo
Me3P
In the reaction mixture, 1e-reduction of Me3SiCl by Na
occurs to generate Me3Si· radical, which not only reacts with the
metal-bound N2 to eventally furnish N(SiMe3)3 but also
undergoes dimerization to give Me3Si-SiMe3 or binds to the
oxygen atom of THF to provide silyl-ethers via ring-opening.
Thus, as demonstrated in other reports on the conversion of N2
into N(SiMe3)3 in THF, some silicon-containing side products
H
H
Fe
Fe
H
P
H
Ph2
4
3a-PF6
Scheme 3. Synthesis of a phosphido-bridged Mo-Fe complex 4.
such
as
Me3SiO(CH2)4SiMe3,
Me3Si-SiMe3,
and
Me3SiO(CH2)3CH3 were detected in the reaction mixtures. A
time-profile of the products in one of the reactions mediated by
2a revealed Me3SiO(CH2)4SiMe3 as the major product and
N(SiMe3)3 as the second major product (Figure S18), indicating
facile Si-O bond formation between Me3Si· radical and THF.
P1
Fe
Mo
P2
H1
Table 1 Conversion of N2 to N(SiMe3)3 Catalyzed by Mo-Fe
hydride complexes 2a-b, 3a-b, and 4.[a]
Mo-Fe complex
(0.01 mmol)
N2 + 6 Na + 6 Me3SiCl
2 N(SiMe3)3
solvent, r.t.
100 h
(1 atm)
(1200 equiv)
Figure
7
Molecular structure of 4 with atomic
N(SiMe3)3
(equiv / cluster)
Solvent[b]
THF
displacement parameters set at 50% probability. Only
selected atoms are labeled, and hydrogen atoms except
the hydride are omitted for clarity. Selected distances (Å):
Mo-Fe, 2.4189(12); Mo-P1, 2.4161(17); Mo-P2,
2.3135(15); Mo-H1, 1.93(6); Fe-P2, 2.2339(17); Fe-H1,
1.29(6).
Entry
1
Complex
2a
113 ± 6
2[c]
3
82 ± 20
129 ± 20
15 ± 4
2a
2b
3a
3b
4
DME
THF
THF
THF
THF
4
8 ± 5
5
Catalytic Conversion of N2 into N(SiMe3)3 Mediated by the
Mo-Fe Complexes.
6
41 ± 11
7[d]
8[d,e]
9[e]
Cp*FeN(SiMe3)2
1a
THF
THF
THF
2 ± 1
1.3
The dinuclear Mo-Fe hydride complexes were used as catalyst
precursors for the silylation of N2,[27] where the previously
reported Mo- and Fe-catalyzed reactions were referred for the
conditions examined in this study.[15,24,28,29] In a typical condition,
a THF solution of Mo-Fe complex was mixed with Na and then
with Me3SiCl under N2, and the mixture was stirred for 100 h at
room temperature. As summarized in Table 1, the yields of
N(SiMe3)3 per Mo-Fe complexes 2a-2b were 113±6 equiv (2a,
entry 1) and 129±20 equiv (2b, entry 3), while the catalytic
activities were lower for the cationic THF adducts 3a-BAr4 (15±4
69 ± 7
[Mo2Fe2] (A)
[a] r.t. = room temperature, equiv = equivalent. Yields for all entries are averages
of three runs except entry 8 (average of two runs).
[b] THF = tetrahydrofuran, DME = 1,2-dimethoxyethane
[c] Yield became 99 ± 11 equiv by extending the reaction time to 200 h.
[d] 600 equiv each of Na and Me3SiCl were used (i.e. 600 equiv / metal atom).
[e] Data taken from our previous study (reference 15).
[Mo2Fe2] = {Cp*Mo(PMe3)}2{FeN(SiMe3)2}2(H)8
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Formation of silyl-ethers can be suppressed by changing the
reaction solvent from THF to DME as demonstrated in our
previous study,[24] however the corresponding reaction promoted
by 2a in DME afforded a slightly lower yield of N(SiMe3)3 (82±20
equiv, entry 2). It is also notable that the reaction in DME takes
longer to consume Na. By extension of the reaction time to 200
h, the N(SiMe3)3 yield in DME became slightly better (99±11
equiv) and comparable to that in THF.
Computed Mechanism of the Catalytic Cycle for the
Silylation of N2.
The N2-ligated species derived from the present Mo-Fe
complexes have not yet been isolated or identified
experimentally from stoichiometric reactions of Mo-Fe
complexes with N2 under heat exposure or light irradiation in the
absence or presence of additives (base, reductant, or proton
sources), and the ESI-MS detection of Mo-Fe complexes with N2
or silylated nitrogen species has not been successful by
monitoring the catalytic reaction mixtures. Therefore we used a
computational method to rationalize the reaction mechanism.
Calculated free energy profile for early part of the mechanism is
summarized in Figure 8, where the superscript numbers of the
local minima or transition states refer to the multiplicity.
Calculated spin densities of the open-shell structures are shown
in Figures S21-S23.
A notable observation for cationic complexes 3a-3b was
their immediate color change from dark green to red upon
treatment with Na in THF regardless of the atmosphere (N2 or
Ar), while THF solutions of 2a-2b did not show quick color
change in the presence of Na but without Me3SiCl. Thus,
treatment of 3a with Na probably leads to the formation of
catalytically inactive speceis. In the ESI-MS spectrum, a red
solution of 3a+Na exhibited a relatively intense signal of bis-
PMe3 species [Cp*Mo(PMe3)2H3FeCp*]+ [3a–THF+PMe3] at m/z
= 579.1 and a less intense signal of [3a-2H] at m/z = 501.1, the
latter of which appears as the major signal in the ESI-MS
spectrum of 3a, although some other unidentified signals were
also detected (Figure S15). This observation indicates facile
disproportionation of PMe3 from the reduced form of 3a,
[Cp*Mo(PMe3)(THF)H3FeCp*], to furnish bis-PMe3 species
[Cp*Mo(PMe3)2H3FeCp*], while the fate of concomitantly
generated Mo-Fe species without PMe3 remains unclear. In
contrast to 3a+Na, disproportionation of PMe3 was not
accelerated for the THF solution of 2a+Na (Figure S16), further
corroborating the quenching of catalytic activity of 3a-3b induced
by the ligand disproportionation. The Mo site of bis-PMe3
species [Cp*Mo(PMe3)2H3FeCp*] should be occupied by Cp*,
two PMe3, and hydrides, hindering the accommodation of N2 and
thereby resulting in the observed catalytic inactivity. This notion
is in accordance with the N2 binding process suggested by the
DFT calculations (see below), where the coordination of N2 to
the Mo site of mono-PMe3 species [Cp*Mo(PMe3)H3FeCp*] is
energetically favored. The low catalytic activity of phosphido-
bridged complex 4 would be also explained by the steric
congestion at the Mo site. Even though the tentatively assigned
lower Mo(II) oxidation state and the lower coordination number
of the Mo site of 4 relative to other Mo-Fe complexes may be
advantageous in the N2 activation, the steric hindrance imposed
by Cp*, PMe3, µ-PPh2, and a hydride ligands on the Mo site of 4
prevents the approach of N2 to Mo.
Computed free energy barrier for the generation of Me3Si·
radical from Na + Me3SiCl is only 7.5 kcal/mol, which is small
enough to occur under the reaction conditions. Dispite a number
of attempts, we were unable to find transition state for the
1
reaction between Me3Si· and the Mo-H moiety of 2b, where all
2
our attepts converged to I1, which is 19.3 kcal/mol stable than
the entry point. Computed relaxed potential energy surface
indicated no barrier for 12b à 2I1 transition (Figure S21).
Therefore, we suspect that the reaction between Me3Si· the Mo-
H moiety is barrierless. Afterwards, an N2 molecule approaches
2
2
to the Mo-based vacant site of I1 to yield I2 (-26.2 kcal/mol)
2
through TS1 with a barrier of 9.8 kcal/mol. The calculated free
energy barrier for N2 binding on the Fe atom is relatively higher
(TS1’, grey line), and the resulting intermediate 2I2’ is 5.7
2
kcal/mol less stable than I2. Also, the transition state for the N2
transfer from Mo to Fe (TS1’’, grey line) is 2.2 kcal/mol higher
than TS1. Therefore, we conclude that the coordination of N2 at
the Mo atom is favorable, and resulting 2I2 is the active
intermediate for the reaction. The free energy profile originates
from the 52b is relatively higher in energy, and therefore not
Recent success in the reduction of N2 to NH3 promoted by
molecular catalysts,[30] as well as a recent finding on the
activation of N2 on a Mo-containing metal-sulfur cluster,[31] also
prompted us to investigate the possible application of a Mo-Fe
complex 2a in the synthesis of NH3 from N2. However, 2a
appeared to be inactive and furnished merely negligible amounts
of NH3 and N2H4 (Table S3). Attempted conditions were the use
of Cp*2Co (reductant) and 2,6-lutidinium triflate (proton source)
at room temperature in toluene, the use of Na/Hg (reductant)
and MeOH (proton source) at room temperature in toluene, or
the use of KC8 (reductant) and [H(OEt2)2][BArF ] (proton source,
4
ArF = 3,5-(CF3)2C6H3) at –78 °C in Et2O.
Figure 8. Free energy profile for N2 coordination to the Mo-
Fe species derived from 2b.
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considered in the following sections.
2I4 (red line in Figure 9) appears to be easier than the
aforementioned silylation at the distal nitrogen atom (black line).
The computed free energy barrier for the reaction between a
As shown in Figure 9, the terminal nitrogen atom of 2I2
subsequently reacts with a Me3Si· radical, and the calculated
free energy barrier for this process is 10.0 kcal/mol (1TS2). The
2
Me3Si· radical and the proximal nitrogen atom in I4 is only 12
1
2
1
resulting intermediate I3 is 29.8 kcal/mol stable than I2. Then,
another Me3Si· radical reacts with the terminal nitrogen atom of
1I3 with a free energy barrier of 6.8 kcal/mol (1TS3), giving rise to
intermediate 2I4, which is 23.2 kcal/mol more stable than 1I3. For
kcal/mol (1TS4’), giving rise to I5’ (-93.8 kcal/mol). Then, a Na
atom can react with 1I5’, as proposed by previous computational
studies on the Mo- and Co-catalyzed silylation reactions,[27c,32]
affording Na[(Me3Si)NN(SiMe3)2] and 2I1 (-133.1 kcal/mol).
2
the following silylation of I4, we have explored two possibilities
Subsequent reaction Na[(Me3Si)NN(SiMe3)2]
+
2·SiMe3 +
as to the addition of the third silyl group to the nitrogen atoms
ClSiMe3 à 2N(SiMe3)3 + NaCl (ΔG = -161.6 kcal/mol) should
proceed to furnish N(SiMe3)3.
distal and proximal to the Mo atom. The reaction between
2
Me3Si· radical and the nitrogen atom distal to Mo in I4 has a
Our DFT calculations indicated that the 2I2 à 1I3 à 2I4
reaction pathway can go through low energy barriers. The
following silylation of 2I4 appears to be the rate-determining step,
for which the silylation of the proximal nitrogen atom is kinetically
favorable. By following this rection, the nitrogen species
free energy barrier of 24.8 kcal/mol (not shown in Figure 9),
2
which is relatively high. If the PMe3 ligand dissociates from I4,
2
intermediate I4’ can be formed, which is 10.7 kcal/mol higher
than 2I4. The overall free energy barrier for the reaction between
a Me3Si· radical and the distal nitrogen atom in 2I4’ becomes
slightly lower (24.0 kcal/mol, 1TS4), leading to 1I5 (-116.8
kcal/mol) and N(SiMe3)3. Then, PMe3 can coordinate to the
1
dissociates as Na[(Me3Si)NN(SiMe3)2] from I5’, but this step is
probably slow due to insolubility of Na in THF. Although silylation
2
of the distal nitrogen atom of I4 furnishes a thermodynamically
1
1
vacant Mo site of I5 to form intermediate I6 (-148.6 kcal/mol).
Further calculations were perfomed to complete the catalytic
cycle (See Figure S25).
more stable intermediate (1I6) and N(SiMe3)3, this reaction
pathway shows a relatively large free energy barrier.
Silylation of the nitrogen atom proximal to the Mo atom in
Conclusions
In summary, we selectively synthesized two hydride-bridged
dinuclear Mo-Fe complexes 2a-2b, which were fully
characterized. The Mo-Fe distances of 2a-2b are shorter than
any other Mo-Fe distances thus far reported, and the Mo-Fe
bonding character of 2a-2b was rationalized from computational
studies. Removal of the terminal Mo-H hydride of 2a-2b in THF
led to the formation of cationic complexes 3a-3b, and the
subsequent treatment of 3a with LiPPh2 furnished a Mo(µ-
PPh2)Fe complex 4. Complexes 2a-2b were found to serve as
good catalyst precursors for the silylation of N2, while cationic
complexes 3a-b were inactive and a Mo(µ-PPh2)Fe complex 4
revealed a relatively low catalytic activity. On the basis of the
catalytic activities of Mo, Fe, Mo-Fe, and [Mo2Fe2] complexes,
we proposed the generation of dinuclear Mo-Fe active species
that performes the silylation of N2. DFT calculations indicated the
preference of N2-binding at the Mo site to furnish the active
intermediate. Subsequently, the reaction between the Mo-N2 unit
of the active intermediate and Me3Si· radicals can occur in a
stepwise manner to eventually produce N(SiMe3)3.
Experimental Section
General Procedures
All reactions were carried out under an atmosphere of N2 or Ar using
either Schlenk line or glovebox techniques. Hexane, toluene, pentane,
THF, and Et2O were purified by passing over columns of activated
alumina and a supported copper catalyst supplied by Hansen & Co. Ltd.
1
Figure 9. Free energy profile for the production of
Deuterated solvents were dried and vacuum-transferred prior to use. H,
Na[(Me3Si)NN(SiMe3)2] or the first N(SiMe3)3 starting from 2I2.
13C, and 31P NMR spectra were recorded on JEOL JNM ECS600 or
ECS500 spectrometer. Elemental analyses were carried out on
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10.1002/chem.202000104
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powdered crystalline samples sealed in tin capsules under N2, using an
Elementar Analytical vario MICRO cube elemental analyzer. Cyclic
voltammograms were measured in a single-compartment cell under a N2
atmosphere at room temperature using a BAS ALS-660A electrochemical
Ag/AgNO3). Mössbauer spectrum (mm/s, 78 K): IS (isomer shift) =
0.247(1), QS (quadruple splitting) = 2.270(2), LW (line width at the half-
hight) = 0.289(3). Anal. Calcd for C23H43FeMoP: C, 54.99; H, 8.63.
Found: C, 55.30; H, 8.28.
analyzer. The supporting electrolyte, 0.2
M
tetrabutylammonium
hexafluorophosphate ([nBu4N][PF6]), was recrystallized from THF prior to
use. All potentials are referenced to Ag/AgNO3. IR spectra were recorded
on an ATR-equipped Bruker Alpha FTIR spectrometer. Electro-spray
ionization mass spectra (ESI-MS) were acquired on a JEOL JMS-
T100CS spectrometer or a Bruker micrOTOF-II spectrometer. 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 α-iron at room temperature, and the isomer shifts are given relative to
α-iron. GC/GC-MS analyses were acquired on a Shimadzu GCMS-
2010SE. Low-temperature experiments were carried out in a glovebox
(C5Me4H)Mo(PMe3)(H)(µ-H)3FeCp* (2b). In a similar manner to the
synthesis of 2a, the reaction of 1b (0.849 g, 2.85 mmol) in hexane (50
mL) with Cp*Fe{N(SiMe3)2} (1.01 g, 2.86 mmol) in hexane (50 mL)
followed by concentration to ca. 3 mL, filtration, and cooling to –35 °C
resulted in the formation of dark blue crystals of 2b (1.14 g, 2.34 mmol,
82%). 1H NMR (C6D6, 600 MHz, r.t.): δ 2.77 (d, JPH = 6.9 Hz, 1H,
C5Me4H), 2.21-2.35 (br, 6H, C5Me4H), 2.20 (s, 15H, Cp*), 2.03 (br, w1/2
=
36 Hz, 3H, C5Me4H), 1.70 (br, w1/2 = 39 Hz, C5Me4H), –7.45 (br, w1/2 = 57
Hz, 1H, Mo-H), –15.61 (d, JPH = 12.0 Hz, 3H, Mo-H-Fe). 13C{1H} NMR
(C6D6, 125 MHz, r.t.): δ 80.9 (C5Me4H), 74.9 (C5Me5), 27.1 (d, JPC = 25.9
Hz, PMe3), 15.0 (C5Me4H), 13.0 (C5Me5). 31P{1H} NMR (C6D6, 243 MHz,
r.t.): δ 13.4 (PMe3). IR (ATR, cm-1): ν = 2956, 2894, 2854, 2715, 1743,
1453, 1418, 1369, 1323, 1298, 1279, 1174, 1069, 1024. UV-vis (toluene,
using
a
Techno
Sigma
UCR-150-GB.
(C5Me4H)MoCl4,[33]
lmax (ε cm–1M–1)): 293 (8.9
Voltammogram (THF): E1/2
×
103), 574 (1.4
–0.44 V, –2.57
×
V
103) nm. Cyclic
(vs. Ag/AgNO3).
[34]
Cp*Mo(PMe3)H5 (1a),[16] Cp*FeN(SiMe3)2,[17] and KC8
were prepared
=
according to previously reported procedures. All other chemicals were
purchased from common commercial sources and used without further
purification.
Mössbauer spectrum (mm/s, 78 K): IS = 0.254(1), QS = 2.302(3), LW =
0.306(4). Anal. Calcd for C22H41FeMoP: C, 54.11; H, 8.46. Found: C,
53.75; H, 8.12.
(C5Me4H)Mo(PMe3)H5 (1b). PMe3 (16.8 mL, 1 M in toluene, 16.8 mmol)
was added to an Et2O (800 mL) solution of (C5Me4H)MoCl4 (6.01 g, 16.7
mmol), and the mixture was stirred overnight at room temperature to
furnish a dark brown suspension. Addition of LiAlH4 (4.32 g, 114 mol) at
0 °C caused evolution of H2 and afforded a yellow-green suspension.
The mixture was gradually warmed to room temperature and stirred for 1
h, and then carefully quenched with CH3OH (180 mL) at 0 °C. All volatile
materials were removed under reduced pressure. The residue was
extracted with hexane (500 mL) and filtered through Celite, and the
filtrate was evaporated to dryness. The residue was dissolved in THF (32
mL) and filtered through neutral alumina to collect the (nearly) colorless
solution, which was evaporated. The resultant light brown residue was
extracted with pentane (10 mL), and the solution was kept standing at –
35 °C to grow colorless crystals of 1b (1.33 g, 4.45 mmol, 27%). 1H NMR
(C6D6, 600 MHz, r.t.): δ 5.04 (s, 1H, C5Me4H), 2.14 (s, 6H, C5Me4H), 2.11
(s, 6H, C5Me4H), 1.21 (d, J = 9.2 Hz, 9H, PMe3), –3.24 (d, J = 51 Hz, 5H,
Mo-H). 13C{1H} NMR (C6D6, 125 MHz, r.t.): δ 103.5 (C5Me4H), 100.4
(C5Me4H), 82.7 (C5Me4H), 29.8 (d, JPC = 27.6 Hz, PMe3), 15.4 (C5Me4H),
13.4 (C5Me4H). 31P{1H} NMR (C6D6, 243 MHz, r.t.): δ 12.7 (PMe3). Anal.
Calcd for C12H27FeMoP: C, 48.32; H, 9.12. Found: C, 48.11; H, 9.09. Its
molecular structure, which was determined by a single crystal X-ray
diffraction study, appears in the supporting information. The deuteride
sample was prepared analogously from the reaction using LiAlD4 and
CD3OD.
[Cp*Mo(PMe3)(THF)(µ-H)3FeCp*][B(C6F5)4] (3a-BAr4). In a glove box, a
THF (6 mL) solution of 2a (49.5 mg, 0.0985 mmol) was cooled at -100 °C
and [CPh3][B(C6F5)4] (92.5 mg, 0.100 mmol) was added. The mixture was
gradually warmed to room temperature to give a green solution, which
was stirred overnight. The solvent was removed under reduced pressure,
and the residue was washed with hexane (10 mL x 2). The green solid
was dissolved in a minimal amount of THF (1 mL) and a few drops of
Et2O was added, and then the solution was stored at –35 °C. After four
days, green crystals of 3a-BAr4·Et2O were collected (78.2 mg, 0.0589
mmol, 60%). 1H NMR (THF-d8, 600 MHz, r.t.): δ 2.06 (s, Cp*(Fe)), 1.85 (s,
Cp*(Mo)), 1.82 (d, JPH = 9.24 Hz, 9H, PMe3), –6.94, –14.03, –17.76 (br,
1H each, Mo-H-Fe). 13C NMR (THF-d8, 125 MHz, r.t.): δ 148.1, 137.6,
136.0 (br, B(C6F5)4), 101.3 (s, Mo-C5Me5), 77.2 (Fe-C5Me5), 22.7 (d, JCP
= 22.8 Hz, PMe3), 12.8 (s, Mo-C5Me5), 12.0 (s, Fe-C5Me5). 31P NMR
(THF-d8, 243 MHz, r.t.): δ -4.85 (PMe3). Mössbauer spectrum (mm/s, 78
K): IS = 0.281(2), QS = 2.217(4), LW = 0.275(6). Anal. Calcd for
C51H50FeMoBOF20P: C, 48.91; H, 4.02. Found: C, 48.62; H, 4.51.
[Cp*Mo(PMe3)(THF)(µ-H)3FeCp*][PF6] (3a-PF6). In a manner similar to
the synthesis of 3a-BAr4, 2a (1.00 g, 2.00 mmol) was treated with
[CPh3][PF6] (0.776 mg, 2.00 mmol) in THF (50 mL) at –100 °C. After
workup, green crystals of 3a-PF6 (0.8312 g, 1.16 mmol, 58%) were
obtained. Crystallographic analysis of green crystals revealed severe
disorder, resulting in unsuccessful structural determination. 1H NMR
(THF-d8, 600 MHz, r.t.): δ 2.06 (s, Cp*(Fe)), 1.85 (s, Cp*(Mo)), 1.82 (d,
JPH = 9.2 Hz, 9H, PMe3), –6.94, –14.03, –17.76 (br, 1H each, Mo-H-Fe).
13C NMR (THF-d8, 125 MHz, r.t.): δ 101.4 (s, Mo-C5Me5), 77.0 (Fe-
C5Me5), 22.8 (d, JCP = 22.8 Hz, PMe3), 13.0 (s, Mo-C5Me5), 12.1 (s, Fe-
C5Me5). 31P NMR (THF-d8, 243 MHz, r.t.): δ -5.2 (PMe3), -143.4 (m, PF6).
Anal. Calcd for C27H50FeMoOF6P2: C, 45.14; H, 7.02. Found: C, 44.90; H,
6.71.
Cp*Mo(PMe3)(H)(µ-H)3FeCp* (2a). In a glove box, a hexane (40 mL)
solution of Cp*Fe{N(SiMe3)2} (1.03 g, 2.93 mmol) was added to a hexane
(40 mL) solution of 1a (0.890 g, 2.85 mmol) at room temperature, and the
mixture was stirred overnight. The mixture was concentrated to ca. 3 mL
under reduced pressure and filtered through a disposable Whatman 25
mm filter. The filtrate was cooled to –35 °C to give dark blue crystals of
2a (1.20 g, 2.34 mmol, 84%). 1H NMR (C6D6, 600 MHz, r.t.): δ 2.19 (s,
15H, Fe-Cp*), 1.92 (s, 15H, Mo-Cp*), 1.39 (d, JPH = 9.2 Hz, 9H, PMe3), –
7.17 (br, w1/2 = 390 Hz, 1H, Mo-H), -15.78 (br, w1/2 = 140 Hz, 3H, Mo-H-
Fe). 13C{1H} NMR (C6D6, 125 MHz, r.t.): δ 93.8 (Mo-C5Me5), 74.4 (Fe-
C5Me5), 26.2 (d, JPC = 98 Hz, PMe3), 13.4 (Mo-C5Me5), 12.6 (Fe-C5Me5).
31P NMR (C6D6, 243 MHz, r.t.): δ 9.7 (PMe3). IR (ATR, cm-1): ν = 2960,
2892, 2851, 2715, 1743, 1577, 1478, 1450, 1414, 1373, 1295, 1275,
1068, 1025. UV-vis (toluene, lmax (ε cm–1M–1)): 290 (1.0 × 104), 586 (1.4
× 103) nm. Cyclic Voltammogram (THF): E1/2 = –0.52 V, –2.61 V (vs.
[(C5Me4H)Mo(PMe3)(THF)(µ-H)3FeCp*][B(C6F5)4] (3b-BAr4). In
a
manner similar to the synthesis of 3a-BAr4, 2b (200 mg, 0.409 mmol)
was treated with [CPh3][B(C6F5)4] (379 mg, 0.411 mmol) in THF (5 mL) at
–100 °C. After workup, black powder of 3b-BAr4 (339 mg, 0.273 mmol,
67%) was obtained. 1H NMR (THF-d8, 600 MHz, r.t.): δ 4.35 (d, JPH = 5.5
Hz, 1H, C5Me4H), 2.12 (s, 3H, C5Me4H), 2.07 (s, 15H, Cp*), 2.05 (s, 3H,
C5Me4H), 1.81 (d, JPH = 9.2 Hz, PMe3),1.80 (s, 3H, C5Me4H), 1.44 (s, 3H,
C5Me4H), –7.04, –13.43, -17.65 (br, 1H each, Mo-H-Fe). 13C NMR (THF-
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d8, 125 MHz, r.t.): δ 148.2, 137.7, 136.1 (br, B(C6F5)4), 113.4, 105.3, 99.8,
93.8, 85.6 (s, C5Me4H), 77.4 (C5Me5), 23.3 (d, JCP = 22.8 Hz, PMe3), 12.1
(s, C5Me5), 16.7, 15.2, 12.5, 12.1 (C5Me4H). 31P{1H} NMR (THF-d8, 243
MHz, r.t.): δ 0.8 (PMe3). Anal. Calcd for C50H48FeMoBOF20P: C, 48.92; H,
3.94. Found: C, 48.61; H, 4.26.
electrons, where the relativistic Scalar ZORA approach was used for Fe
and Mo.[50]
Acknowledgements
Cp*Mo(PMe3)(µ-H)(µ-PPh2)FeCp* (4). A THF (5 mL) solution to HPPh2
(48.5 µL, 0.279 mmol) was treated with a hexane solution of nBuLi (2.65
M, 105 µL, 0.278 mmol) at 0 oC, and the resultant orange solution was
stirred for 90 min. This orange solution was added dropwise to a THF (5
mL) solution of 3a (189 mg, 0.263 mmol) at –30 oC, and the temperature
was kept during stirred overnight. The mixture was gradually warmed to
room temperature and stirred for 3 h, giving a brown solution. All volatile
materials were removed under reduced pressure, and the brown residue
was extracted with hexane (2 mL). After filtration, the extract was cooled
at –35 oC for three weeks to grow brown crystals of 4 (95.3 mg, 0.139
mmol, 53%).1H NMR (C6D6, 600 MHz, r.t.): δ 7.12-8.02 (m, 10H, µ-PPh2),
1.94 (s, 15H, Fe-Cp*) , 1.61 (s, 9H, Mo-Cp*) ,1.37 (d, JPH = 6.6 Hz, 9H,
PMe3,), –10.54 (br, w1/2 = 41.4 Hz, 1H, Mo-H-Fe). 13C NMR (C6D6, 125
MHz, r.t.): δ 135.5 (d, JCP = 7.5 Hz, PPh2), 132.5 (d, JCP = 9.6 Hz, PPh2),
127.5 (m, PPh2), 125.9 (s, PPh2), 99.3 (s, Mo-C5Me5), 86.1 (s, Fe-C5Me5),
27.6 (d, JCP = 21.6, PMe3), 12.2 (s, Mo-C5Me5), 12.0 (s, Fe-C5Me5)
31P{1H} NMR (C6D6, 243 MHz, r.t.): δ 162.8 (s, μ-PPh2), 27.9 (s, PMe3).
Mössbauer spectrum (mm/s, 78 K): IS = 0.359(2), QS = 2.356(4), LW =
0.421(6). Anal. Calcd for C23H43FeMoP: C, 61.41; H, 7.36. Found: C,
61.08; H, 7.04.
This work was financially supported by Grant-in-Aids for
Scientific Research (19H02733, 18H04246, and 17H06445)
from the Japanese Ministry of Education, Culture, Sports,
Science and Technology (MEXT) and the Takeda Science
Foundation and the Yazaki Memorial Foudation. W.M.C.S.
thanks to MEXT KAKENHI for grant 17H06445. Supercomputing
resources at the Institute of Molecular Science (IMS) is
acknowledged. We are grateful to Prof. Satoshi Muratsugu for
generous advice on electrochemical measurements.
Keywords: Molybdenum • Iron • Hydride • Dinuclear Complex •
Silylamine
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A typical procedure for the catalytic conversion of N2 into N(SiMe3)3
in the presence of Mo-Fe complexes. The following procedure is
analogous to those in references [15,24,27,28]. A 100 mL round-bottom
flask was charged with Na (0.267 g, 11.6 mmol) and THF (10 mL) to
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solid to the suspension, and any remaining 2b was dissolved in THF (6
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Conditions for GC analysis and calibration curves for silicon-containing
reaction products are shown in the supporting information.
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7.4257). Vibrational frequency calculations were performed at 298.15 K
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basis sets with one polarization function (TZP) were applied for all
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Entry for the Table of Contents (Please choose one layout)
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Kodai Ishihara, Yuna Araki, Mizuki Tada,
Tsutomu Takayama, Yoichi Sakai, W. M.
C. Sameera,* and Yasuhiro Ohki*
N2
P
H
Fe
+ 6 Na
+ 6 Me3SiCl
Mo
Mo Fe
Page No. – Page No.
H
Synthesis of Dinuclear Mo-Fe Hydride
Complexes and Catalytic Silylation of N2
2.3952(3) Å
N(SiMe3)3
~ 129 eq.
Mo-Fe
Text for Table of Contents
Dinuclear Mo-Fe complexes supported by bridging hydrides were synthesized and
fully characterized. These dinuclear complexes served as good catalyst precursors
for the reduction of N2 into N(SiMe3)3. Mechanism of catalytic cycle was rationalized
by DFT calculations.
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