{(η5-C5Me5)Fe}2 (μ-H)2(μ-η2:η2-H2 SitBu2), a versatile precursor for bimetallic active species

Article abstract of DOI:10.1021/om0101238

Reaction of {(η⁵-C₅Me₅)Fe}₂ (μ-H)₄ with ^tBu₂SiH₂ provides the first dinuclear iron μ-silane complex, {(η⁵-C₅Me₅)Fe}₂ (μ-H)₂(μ-η²:η²-H₂ Si^tBu₂), in which two Si-H σ-bonds are coordinated to two iron centers. The coordinated Si-H bond is readily cleaved and the μ-silane ligand is eliminated to give a dinuclear iron active species which reacts in situ with organic substrates.

Full text of DOI:10.1021/om0101238

2654
Organometallics 2001, 20, 2654-2656
{(η5-C5Me5)F e}2(µ-H)2(µ-η2:η2-H2SitBu 2), a Ver sa tile
P r ecu r sor for Bim eta llic Active Sp ecies
Yasuhiro Ohki,† Takahiro Kojima, Masato Oshima, and Hiroharu Suzuki*
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology and CREST, J apan Science and Technology Corporation (J ST),
O-okayama, Meguro-ku, Tokyo 152-8552, J apan
Received February 15, 2001
t
Summary: Reaction of {(η5-C5Me5)Fe}2(µ-H)4 with Bu2-
SiH2 provides the first dinuclear iron µ-silane complex,
{(η5-C5Me5)Fe}2(µ-H)2(µ-η2:η2-H2SitBu2), in which two
Si-H σ-bonds are coordinated to two iron centers. The
coordinated Si-H bond is readily cleaved and the
µ-silane ligand is eliminated to give a dinuclear iron
active species which reacts in situ with organic sub-
strates.
Treatment of 2 with di-tert-butylsilane in toluene at
room temperature gave 4, in which two Si-H σ-bonds
are coordinated to two iron centers (eq 1).6 To our
Coordinatively unsaturated transition-metal cluster
complexes often undergo efficient and unique organic
transformations by the synergy of the adjacent metal
centers.1 We have demonstrated examples of the coop-
erative activation of organic substrates on a bimetallic
site in the dinuclear ruthenium tetrahydride complex
{(η5-C5Me5)Ru}2(µ-H)4 (1)2 and have recently synthe-
sized the dinuclear iron analogue {(η5-C5Me5)Fe}2(µ-H)4
(2).3 The latter, like the ruthenium complex 1, can
generate a reactive species. As anticipated from the
vertical trends of the transition elements, diiron tetra-
hydride 2 is much more reactive than the ruthenium
complex 1 and, also, is less stable. Several years ago,
we prepared a µ-H2SitBu2 complex of ruthenium, {(η5-
C5Me5)Ru}2(µ-H)2(µ-η2:η2-H2SitBu2) (3).4 We have now
prepared the analogous diiron complex {(η5-C5Me5)Fe}2-
(µ-H)2(µ-η2:η2-H2SitBu2) (4) and find it to be more stable
than 2; hence, it is more useful as a synthetic reagent.
Mononuclear late-transition-metal complexes having a
Si-H-M 3c-2e bond often generate unsaturated metal
intermediates by eliminating a Si-H σ-bond.5 Our novel
diiron µ-η2:η2-silane complex 4 serves as a precursor for
bimetallic active species, presumably {(η5-C5Me5)Fe}2-
(µ-H)2, by elimination of the bridging silane ligand.
knowledge, this is the first dinuclear iron µ-silane
complex. The µ-silane complex 4 is less reactive than 2
toward air and moisture, both in solution and in the
solid state. Its 29Si resonance occurs at δSi 71. This shift
is comparable to that observed for 3 at δSi 75.4 A broad
band was observed at 1736 cm-1 in the infrared spec-
trum of 4. This absorption was assigned as νSi-H-Fe by
subtracting the spectrum of 4-d4 from that of 4, as
shown in Figure 1. Compared with a νSi-H value of 1790
t
cm-1 in 3 and 2116 (sharp) cm-1 in free Bu2SiH2, this
indicates reduction in the Si-H bond order due to the
1
Fe-H-Si 3c-2e interaction. The H NMR of 4 at room
temperature showed three signals at δ 1.86 (30 H), 0.85
(18 H), and -16.25 (4 H) attributable to C5Me5, tBu, and
hydride ligands, respectively. The signal of the hydride
at room temperature (δ -16.25) split into two sharp
singlets at δ -5.28 and -27.12 at -110 °C. This clearly
shows that an exchange of the hydride ligands occurs
between Fe-H-Si and Fe-H-Fe in 4 by way of Si-H
bond cleavage. Line shape analysis of the variable-
temperature spectra gave the free activation energy at
the coalescence temperature ∆Gq(-50 °C) ) 8.6 kcal/
mol. This value is also similar to that of the ruthenium
analogue 3 (∆Gq(-60 °C) ) 8.5 kcal/mol).4
* To whom correspondence should be addressed. Fax: Int. code
+(81) 3-5734-3913. E-mail: hiroharu@n.cc.titech.ac.jp.
† Present address: Department of Chemistry, Graduate School of
Science, Nagoya University.
(1) (a) Su¨ss-Fink, G.; Meister, G. Adv. Organomet. Chem. 1993, 35,
41. (b) Metal Clusters in Catalysis; Gates, B. C., Guzci, L., Knozinger,
V. H., Eds.; Elsevier: Amsterdam, 1986. (c) Catalysis by Di- and
Polynuclear Metal Cluster Complexes; Adams, R. D., Cotton, F. A., Ed.;
Wiley-VCH: New York, 1998.
(2) (a) Suzuki, H.; Omori, H.; Moro-oka, Y. Organometallics 1988,
7, 2579. (b) Omori, H.; Suzuki, H.; Take, Y.; Moro-oka, Y. Organome-
tallics 1989, 8, 2270. (c) Omori, H.; Suzuki, H.; Moro-oka, Y. Organo-
metallics 1989, 8, 1576. (d) Suzuki, H.; Takao, T.; Tanaka, M.;
Moro-oka, Y. J . Chem. Soc., Chem. Commun. 1992, 476. (e) Suzuki,
H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Fukushima, M.; Tanaka, M.;
Moro-oka, Y. Organometallics 1994, 13, 1129. (f) Takao, T.; Suzuki,
H.; Tanaka, M. Organometallics 1994, 13, 2554. (g) Ohki, Y.; Suzuki,
H. Angew. Chem., Int. Ed. 2000, 39, 3463.
The structure of 4 was confirmed by an X-ray diffrac-
tion study.7 The perspective view of 4 is shown in Figure
(6) The reaction of 2 with iPr2SiH2 affords the analogous µ-silane
complex {(η5-C5Me5)Fe}2(µ-H)2(µ-η2:η2-H2SiiPr2). The preliminary result
of the X-ray diffraction study showed the dinuclear structure bridged
by the µ-η2:η2-H2SiiPr2 ligand. See the Supporting Information.
(7) X-ray structural determination of 4: crystals of 4 were grown
at -30 °C from a diethyl ether solution of the compound. Data were
collected at -40 °C on an RAXIS-II imaging plate area detector
equipped with graphite-monochromated Mo KR radiation. The com-
pound crystallizes in space group R3h, with a ) 18.6083(4) Å, R )
58.1220(7)°, V ) 4360(2) Å3, Z ) 6, dcalcd ) 1.208 g cm-3. A total of
6640 unique reflections were recorded in the range 5° e 2θ e 55°, of
which 4405 were used (F > 2σ(F)) for solution and refinement. In the
reduction of the data, Lorentz/polarization corrections were applied
to the data. The structure was solved by direct methods (SHELXS 97),
and all non-hydrogen atoms were refined anisotropically by using
SHELXL 97 on F2. The final structure of 4 was refined to R1 ) 0.076,
wR2 ) 0.149, and GOF ) 1.07 for 313 parameters.
(3) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3120.
(4) Takao, T.; Yoshida, S.; Suzuki, H. Organometallics 1995, 14,
3855.
(5) For reviews, see: (a) Schubert, U. Adv. Organomet. Chem. 1990,
30, 151. (b) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999, 99,
175.
10.1021/om0101238 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 06/01/2001

Communications
Organometallics, Vol. 20, No. 13, 2001 2655
warmed to 60 °C, a gradual decrease in the intensity of
the hydride NMR signal was observed. The intermo-
lecular H/D exchange between the hydride ligands in 4
and C6D6 was completed within 24 h at 60 °C, leading
to 4-d4. In contrast, the H/D exchange reaction between
the ruthenium analogue 3 and C6D6 required more than
13 days at 80 °C.
The µ-η2:η2-H2SitBu2 group in 4 was easily replaced
by various organic substrates. Representative examples
are summarized in Scheme 1. Treatment of 4 with CO
Sch em e 1
F igu r e 1. Infrared spectra of (a) 4, (b) 4-d4, and the
difference spectrum between (a) and (b).
at atmospheric pressure afforded exclusively {(η5-C5-
t
Me5)Fe}2(CO)4 (5)9 and Bu2SiH2. Complex 4 also re-
acted with cyclopentadiene to give the µ-η2:η2-cyclopen-
tadiene complex {(η5-C5Me5)Fe}2(µ-H)2(µ-η2:η2-C5H6) (6)
t
together with free Bu2SiH2. These results indicate the
generation of a reactive bimetallic intermediate in the
reactions formed by elimination of the silane ligand. In
contrast, the diisopropylsilane analogue {(η5-C5Me5)-
Fe}2(µ-H)2(µ-η2:η2-H2SiiPr2)6 does not undergo such µ-si-
lane displacement reactions. This suggests that steric
repulsion between the tert-butyl group on the bridging
silane and the C5Me5 group most likely is responsible
for the silane displacement reactions of 4. The reactions
of the parent iron tetrahydride complex 2 with CO and
cyclopentadiene gave 5 or 6, respectively, together with
several unidentified byproducts, but the corresponding
reactions of 4 yielded 5 or 6 cleanly, without any
byproduct formation. The yields of 5 and 6 based on the
diiron tetrahydride 2 were 70% and 61%, respectively,
while the same reactions of 4 resulted in quantitative
formation of 5 and 6.
F igu r e 2. Molecular structure of {(η5-C5Me5)Fe}2(µ-H)2-
(µ-η2:η2-tBu2SiH2) (4), with thermal ellipsoids at the 30%
probability level. Selected bond lengths (Å) and angles
(deg): Fe(1)-Fe(2) ) 2.5055(8), Fe(1)-Si(1) ) 2.3820(12),
Fe(2)-Si(1) ) 2.3692(13), Fe(1)-H(3) ) 1.51(5), Fe(2)-
H(4) ) 1.51(5), Si(1)-H(3) ) 1.60(5), Si(1)-H(4) )
1.64(5); Fe(1)-Si(1)-Fe(2) ) 63.64(3), C(1)-Si(1)-C(5) )
112.5(2).
2, along with selected bond lengths and angles in the
caption. The hydride ligands of Fe-H-Si are almost on
the Fe2Si plane, in agreement with the determined
M-H-Si complexes. The Fe-Si lengths of 2.376(1) Å
(average) are longer than the usual σ-bond and typical
for the M-H-Si complexes.5b The Si-H lengths of 1.62-
(5) Å (average) are also in the range of the reported
Si-H lengths for the M-H-Si interaction. The Fe-Fe
distance of 2.5055(8) Å is indicative of an iron-iron
double bond,8 as expected from the EAN rule.
The cyclopentadiene complex 6 was identified on the
1
basis of its H NMR spectral data (Chart 1). The signal
for one of the methylene protons (endo-H) was observed
(8) For example, see: (a) Schmitt, H.-J .; Ziegler, M. L. Z. Natur-
forsch., B 1973, 28, 508. (b) J onas, K.; Koepe, G.; Schieferstein, L.;
Mynott, R.; Kru¨ger, C.; Tsay, Y.-H. Angew. Chem., Int. Ed. Engl. 1983,
22, 620. (c) Schaufele, H.; Pritzkow, H.; Zenneck, U. Angew. Chem.,
Int. Ed. Engl. 1988, 27, 1519. (d) Cotton, F. A.; Daniels, L. M.; Falvello,
L. R.; Matonic, J . H.; Murillo, C. A. Inorg. Chim. Acta 1997, 256, 269.
(e) Bo¨ttcher, H.-C.; Merzweiler, K.; Wagner, C. Z. Anorg. Allg. Chem.
1999, 625, 857.
The µ-silane complex 4 undergoes C-H bond activa-
tion with benzene (eq 2). When a C6D6 solution of 4 was
(9) Teller, R. G.; Williams, J . M. Inorg. Chem. 1980, 19, 2770.

2656 Organometallics, Vol. 20, No. 13, 2001
Communications
Ch a r t 1
at δ 3.52 coupled with the hydride signal at δ -43.19
(J ) 6.3 Hz). The structure of 6 was confirmed by an
X-ray diffraction study (Figure 3).10 The two Cp* ligands
tilt to the same side with respect to the dinuclear
framework. The cyclopentadiene ligand is coordinated
to the two iron atoms in an µ-η2:η2 fashion. The iron-
iron distance of 2.483(1) Å lies in the range of iron-
iron double bonds.8
F igu r e 3. Molecular structure of {(η5-C5Me5)Fe}2(µ-H)2-
(µ-η2:η2-C5H6) (6), with thermal ellipsoids at the 30%
probability level. Selected bond lengths (Å): Fe(1)-Fe
(2) ) 2.483(1), Fe(1)-C(2) ) 2.075, Fe(1)-C(3) ) 2.095,
Fe(2)-C(4) ) 2.173(6), Fe(2)-C(5) ) 2.067(4), C(1)-
C(2) ) 1.487(8), C(1)-C(5) ) 1.468(8), C(2)-C(3) ) 1.385-
(8), C(3)-C(4) ) 1.434(8), C(4)-C(5) ) 1.377(7).
The reactions of 4 with diphenylsilane and diphen-
ylphosphine proceeded at room temperature, resulting
in quantitative formation of the known3 µ-silylene
complex 7 and bis(µ-diphenylphosphido) complex 8 via
Si-H and P-H bond cleavage, respectively (Scheme 1).
µ-silane complex 4 are mutually cis with respect to the
Fe-Fe vector. In the case of the bulky R group, the
transformation from µ-silane complex to µ-silylene
complex generates steric repulsion between Cp* and R.
Thus, the transformation of the iPr2SiH2 complex needs
to be heated,6 and complex 4 cannot afford the corre-
sponding µ-silylene complex.
The results illustrated in Scheme 1 show that the
silane complex 4 can substitute well for the unstable
diiron tetrahydride 2. This is the first example of a
dihydrosilane acting as a labile bridging ligand.
t
In contrast to Bu2SiH2, the Si-H bond of Ph2SiH2 is
cleaved easily to afford the µ-silylene complex 7. The
reactivity of the coordinated hydrosilane is dominated
by the steric properties of the SiR2 group. The Cp*
ligands in the µ-silylene complex 7 are almost perpen-
dicular to the Fe-Fe vector, while these ligands in the
(10) X-ray structural determination of 6: crystals of 6 were grown
at -30 °C from a diethyl ether solution of the compound. Data were
collected at -50 °C on an RAXIS-II imaging plate area detector
equipped with graphite-monochromated Mo KR radiation. The com-
pound crystallizes in space group P21/n, with a ) 16.0682(3) Å, b )
19.6255(8) Å, c ) 16.2646(5) Å, â ) 117.618(2)°, V ) 4544.6(3) Å3, Z )
8, dcalcd ) 1.316 g cm-3. A total of 9554 unique reflections were recorded
in the range 5° e 2θ e 55°, of which 5662 were used (F > 3σ(F)) for
solution and refinement. In the reduction of the data, Lorentz/
polarization corrections were applied to the data. The structure was
solved by the Patterson method (DIRDIF92 PATTY), and all non-
hydrogen atoms were refined anisotropically by using full-matrix least-
squares techniques on F. The final structure of 6 was refined to R )
0.049, Rw ) 0.044, and GOF ) 2.12 for 487 parameters. Crystal-
lographic data for 6 have been deposited with the Cambridge Crystal-
lographic Data Centre as Supplementary Publication No. CCDC-
153550. Copies of the data can be obtained free of charge on application
to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44)-
1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
Ackn owledgm en t. We are grateful to Kanto Chemi-
cal Co., Inc., for a generous gift of pentamethylcyclo-
pentadiene.
1
Su p p or tin g In for m a tion Ava ila ble: A table of H, 13C,
and29Si{1H} NMR spectral assignments for 4, a table of 1H
and 13C NMR spectral assignments for 6, ORTEP diagrams,
text describing X-ray procedures, and tables of X-ray data,
positional and thermal parameters, and distances and angles
for 4 and 6. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM0101238