η3-silapropargyl/alkynylsilyl molybdenum complexes: Synthesis, structure, and reactivity toward methanol

Article abstract of DOI:10.1021/ja9005833

(Chemical Equation Presented) Novel η³-silapropargyl/ alkynylsilyl complexes Cp*(CO)₂Mo(η³-Ph ₂SiCCR) (3a, R = ^tBu; 3b, R = ⁱPr) were synthesized by the reactions of Cp*(CO)₂(MeC

Full text of DOI:10.1021/ja9005833

Published on Web 06/11/2009
η3-Silapropargyl/Alkynylsilyl Molybdenum Complexes: Synthesis, Structure,
and Reactivity toward Methanol
Mako Yabe-Yoshida, Chizuko Kabuto, Kuninobu Kabuto, Eunsang Kwon, and Hiroyuki Sakaba*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
Received January 27, 2009; E-mail: sakaba@mail.tains.tohoku.ac.jp
Transition-metal η3-propargyl/allenyl complexes have attracted
considerable interest because of their unique structural features and
reactivity toward polar reagents.1 However, their silicon analogues,
η3-silapropargyl complexes, have not been synthesized, although
Rosenthal and co-workers2 previously reported the silapropargyl/
silaallenyl-like complexes Cp2M(η2-trans-HMe2SiCCR) (M ) Ti,
t
R ) Bu; M ) Zr, R ) SiMe2H) with an agostic Si-H-M inter-
action. In our recent attempt to synthesize the η3-silapropargyl tungsten
complex Cp*(CO)2W(η3-Ph2SiCCtBu) by the reaction of Cp*(CO)2-
(MeCN)WMe with HPh2SiCt CtBu, we obtained the novel ace-
tylide-silylene complex Cp*(CO)2W(SiPh2)(CCtBu), which is a
structural isomer of the expected silapropargyl complex.3
To clarify the bonding nature and relative stability of these two
types of complexes, Sakaki and co-workers4,5 performed detailed
theoretical studies of Cp(CO)2M(SiR2)(CCR′) and Cp(CO)2M(η3-
R2SiCCR′) (M ) W, Mo). They revealed that the metal center
controls the relative stability of the complexes: the W center favors
Cp(CO)2M(SiH2)(CCH), while the Mo center favors Cp(CO)2M(η3-
Figure 1. Molecular structures of (a) 3a, (b) 3b, and (c) 7.
H2SiCCH),5 suggesting the possibility of isolating η3-silapropargyl/
alkynylsilyl molybdenum complexes. Encouraged by this interesting
finding, we examined the reactions of the acetonitrile molybdenum
complex Cp*(CO)2(MeCN)MoMe (1) with alkynylsilanes HPh2-
SiCt CR (2a, R ) Bu; 2b, R ) Pr) and found that the reactions
indeed gave the expected complexes Cp*(CO)2Mo(η3-Ph2SiCCR)
(3a and 3b, respectively). Herein, we describe their synthesis, struc-
ture, and reactivity toward methanol.
The reaction of 1 with 2a in toluene-d8 at room temperature
resulted in the rapid, clean formation of Cp*(CO)2Mo(η3-Ph2-
SiCCtBu) (3a) along with the generation of MeCN and CH4
(Scheme 1). The corresponding reaction of 2b gave the isopropyl
studies by Sakaki and co-workers showed that the π conjugation
between the Si and C atoms of the H2SiCCH group in Cp(CO)2M(η3-
H2SiCCH) is very weak and that its electronic structure is inter-
mediate between those of η3-silapropargyl and alkynylsilyl groups,4,5
indicating that the contribution of a silaallenyl structure is minor
relative to that of an allenyl structure in an η3-propargyl com-
plex. The calculated Si-C and C-C lengths in Cp(CO)2Mo(η3-
H2SiCCtBu), 1.814 and 1.263 Å, respectively,5 are very close to
the above-mentioned Si-C1 and C1-C2 lengths in 3a and 3b.
In the 13C NMR spectra of 3a and 3b at room temperature, a
single CO signal and one set of Ph carbon signals were observed,
and these signals decoalesced into two CO signals and two sets of
Ph signals at -90 °C.8 These spectral changes indicate the existence
of the stereochemical inversion process of the metal center, which
is also evidenced by the decoalescence of a doublet (1.32 ppm,
6H, rt) due to the CHMe2 protons of 3b into two doublets (1.26
and 1.58 ppm, 3H each, -90 °C). The 13C signals at 51.4 and 129.0
ppm at -90 °C were assigned to the C1 and C2 carbons, respec-
tively, of the Si-C1-C2-iPr moiety in 3b on the basis of their
correlation with the CHMe2 proton signal in the HMBC spectrum.
Interestingly, no 29Si resonance was detected for 3b at room
temperature, while a sharp resonance appeared at 46.2 ppm upon
cooling to -90 °C. In contrast, a 29Si resonance was clearly
observed at 23.6 ppm for 3a at room temperature. These observa-
tions suggest the interconversion between 3b and a trace amount
of its isomer, for which the acetylide-silylene complex Cp*(CO)2-
Mo(SiPh2)(CCiPr) may be a candidate.
t
i
Scheme 1
analogue 3b. From preparative-scale reactions, 3a and 3b were
isolated as air-sensitive orange and yellow solids in 56 and 51%
yields, respectively. The structures of 3a and 3b were determined
by X-ray analyses (Figure 1) and showed similar bonding param-
eters for the Mo-Si-C1-C2 skeletons. The Mo1-Si1 bond
lengths [3a, 2.5249(10) Å; 3b, 2.5221(9) Å] are in the range of
typical Mo-Si single bonds (2.513-2.670 Å),6 and the C1-C2
bond lengths [3a, 1.251(4) Å; 3b, 1.254(4) Å] lie between those
of C-C double and triple bonds, as in the case of the corresponding
bond in η3-propargyl complexes.1 The Si1-C1 bond lengths [3a,
1.806(3) Å; 3b, 1.795(3) Å] are between Si-C single- and double-
bond lengths,7 showing some double-bond character. The theoretical
A plausible mechanism for formation of 3a,b is shown in Scheme
1. The Si-H bond in 2a,b is activated by the coordinatively
unsaturated complex Cp*(CO)2MoMe generated from 1 to give A,
which undergoes reductive elimination of methane and coordination
of the alkynyl moiety to the metal center to form 3a,b. The
9
9138 J. AM. CHEM. SOC. 2009, 131, 9138–9139
10.1021/ja9005833 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
corresponding reaction sequence was shown for the formation of
the tungsten complex Cp(CO)2W(η3-H2SiCCH) in Sakaki’s theo-
retical study.4
When 3a was treated with methanol in toluene-d8 at room
temperature, a rapid reaction took place to give the stable four-
membered cyclic complex 5a, whose structure was fully character-
ized by NMR spectroscopy and X-ray analysis, which showed the
E configuration for the double bond (Scheme 2).8 In the 1H NMR
Scheme 2
C, which leads to D via a 1,2-H shift. In this system, 5a is suggested
to be thermodynamically more stable than C and D. In the case of
i
R ) Pr, the 1,2-H shift of the CHMe2 in D can open the way to
η3-allyl complex 6, which undergoes slow isomerization to 7 to
relieve steric repulsion between the silyl and methyl groups in the
allyl ligand. It has been shown that the alkyne hydride complex
[Cp*(CO)2ReH(η2-MeCCMe)]+, which is isoelectronic with 4a,b,
is transformed into the η3-allyl complex [Cp*(CO)2Re(η3-MeHC-
CHCH2)]+ via the observable 1-metallacyclopropene complex
[Cp*(CO)2Re(η2-CMeCHMe)]+.10
spectrum, a characteristic vinylic CH signal appeared at 6.60 ppm.
The reaction of 3b with methanol first gave metallacycle 5b, which
was characterized on the basis of the similarity of its spectroscopic
data with those of 5a (e.g., a vinylic CH signal at 6.25 ppm for
5b). As the reaction was monitored by 1H NMR spectroscopy, the
consecutive conversion of 5b to η3-allyl complexes 6 and 7 was
observed. The structure of the final product 7 was established by
X-ray analysis (Figure 1), which showed the exo-syn configuration
of the η3-allyl moiety. The structure of 6 was characterized by NMR
spectra, which were very similar to those of 7, and the exo con-
figuration was confirmed by the observation of an NOE between
the central allyl proton and the Cp* protons. In the reaction of 3b
with MeOD at room temperature, selective deuterium incorporation
at the vinylic position in 5b and at the terminal allylic position in
6 and 7 (∼92% D) was observed.
In summary, a novel η3-silapropargyl/alkynylsilyl complex was
successfully synthesized using molybdenum as a metal center in
accordance with the theoretical prediction,5 and its structure and
reactivity toward methanol were elucidated. Further studies of the
reactivity of the silapropargyl complex and the possibility of its
equilibration with an acetylide-silylene complex are in progress.
Supporting Information Available: Experimental details, spectro-
scopic data, and X-ray crystallographic data for 3a, 3b, 5a, and 7 (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
To obtain mechanistic information on the formation of 5b, the
reaction of 3b with methanol was monitored by low-temperature
1H NMR spectroscopy. When the temperature was increased to -60
°C from -90 °C, the formation of (methoxysilyl)alkyne hydride
complex 4b was observed as a major product (∼75%), and it was
converted to 5b upon warming to 0 °C. The structure of 4b was
characterized by low-temperature NMR spectra.8 A characteristic
hydride signal was detected at -3.86 ppm, and the signals of the
coordinated alkyne carbons were observed at 77.1 and 134.8 ppm.
The corresponding complex 4a was detected in the low-temperature
reaction of 3a with methanol, as indicated by a hydride signal at
-3.88 ppm and alkyne carbon signals at 79.6 and 143.4 ppm for
4a at -60 °C.
A possible mechanism for the reaction of 3a,b with methanol is
illustrated in Scheme 3. Nucleophilic attack of methanol at the
silicon center of 3a,b causes Si-Mo bond cleavage to form 4a,b,
which undergoes alkyne insertion into the Mo-H bond followed
by coordination of the methoxy oxygen to the resulting unsaturated
metal center to produce 5a,b. Similar four-membered metallacycle
formation was observed in the reaction of the silaallyl tungsten
complex with methanol.9 Dissociation of the methoxy ligand in
5a,b regenerates B, and coordination of the alkene moiety to the
metal center may form 1-metallacyclopropene (η2-vinyl) complex
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