Generation of a silylene complex by the 1,2-migration of hydrogen from silicon to platinum

Article abstract of DOI:10.1002/(SICI)1521-3773(19981002)37:18<2524::AID-ANIE2524>3.0.CO;2-H

An extraordinary downfield-shifted ²⁹Si{¹H} NMR signal is seen at δ = 338.5 for the platinum silylene complex [(dippe)(H)Pt=SiMes₂][MeB(C₆F₅)₃] (2). This remarkably stable metal silylene complex was obtained from 1 in the first intramolecular 1,2-hydride migration from silicon to a transition metal. dippe = iPr₂PCH₂CH₂PiPr₂, Mes = 2,4,6-Me₃C₆H₂.

Full text of DOI:10.1002/(SICI)1521-3773(19981002)37:18<2524::AID-ANIE2524>3.0.CO;2-H

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[10] a) P. Beak, J. E. Hunter, Y. M. Jun, A. P. Wallin, J. Am. Chem. Soc.
1987, 109, 5403 ± 5412; b) A. Carstens, D. Hoppe, Tetrahedron 1994, 50,
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[11] The absolute configurations of 4b and 5 are opposite to our earlier
provisional assignments.[2a] The determination of the absolute config-
uration for 4b is detailed in the supporting information.
[12] H. J. Reich, J. P. Borst, M. B. Coplien, N. H. Phillips, J. Am. Chem. Soc.
1992, 114, 6577 ± 6579.
Generation of a Silylene Complex by the
1,2-Migration of Hydrogen from Silicon to
Platinum**
Figure 1. Molecular structure of 2. Selected interatomic distances [Š] and
angles [8]: Pt ± Si 2.388 (3), Pt ± P(1) 2.329 (3), Pt ± P(2) 2.285 (3), Pt ± C(1)
2.09 (1), Si ± H(36) 1.46 (3), Si ± C(16) 1.93 (1), Si ± C(25) 1.91 (1); P(1)-Pt-
P(2) 86.1 (1), P(1)-Pt-Si 177.7 (1), P(1)-Pt-C(1) 91.0 (4), P(2)-Pt-Si 96.1 (1),
P(2)-Pt-C(1) 172.0 (4), Si-Pt-C(1) 86.8 (4), Pt-Si-C(16) 119.3 (4), Pt-Si-
C(25) 117.7(4).
Gregory P. Mitchell and T. Don Tilley*
Intramolecular migrations in transition metal silicon com-
pounds have attracted considerable attention in recent
years.[1] Many of the catalytic cycles and the most interesting
transformations for metal ± silicon systems appear to feature
such migrations,[1, 2] but discrete examples of these steps have
proven difficult to observe and characterize. Silylene com-
Compound 1 is remarkably stable. Heating a solution of 1 in
[D8]toluene at 1108C for two weeks resulted in no detectable
decomposition (1H and 31P NMR spectroscopy). Also, no
reaction was observed between 1 and diphenylacetylene or 2-
butyne after heating at 1008C in [D8]toluene for three days.
However, 1 does react with H2 at 1108C over a period of one
ˆ
plexes of the type [LnM SiR2] are commonly featured as key
intermediates in mechanistic speculations on 1,2- and 1,3-
migrations.[1±3] Silylene complexes have only recently been
isolated,[4] but despite great effort the formation of a silylene
ligand through an intramolecular migration has not yet
been observed. We recently described a reversible 1,2-hy-
drogen migration which interconverts cis-[(PEt3)2Pt(H)Si-
(StBu)2][OTf] and cis-[(PEt3)2Pt(NCMe)SiH(StBu)2][OTf]
(Tf ˆ SO2CF3), probably via an intermediate silylene com-
plex.[5] Here we report the first observation of a facile 1,2-
hydride migration which generates an observable platinum
silylene complex.
[7]
month to give MeSi(H)Mes2 (GC/MS and 1H NMR
spectroscopy), presumably by reductive elimination and
formation of a Si ± C bond. These results suggest that a 1,2-
hydrogen migration from the silicon atom to produce a five-
coordinate platinum silylene species might be disfavored.
The reaction of 1 with B(C6F5)3 in [D2]dichloromethane
resulted in the rapid generation of a clear yellow solution and
formation of primarily (>95%) one new compound (1H and
31P NMR spectroscopy). The Si ± H resonance of 1 (d ˆ 6.21)
was replaced by a Pt ± H signal at d ˆ 1.50 (1J(H,Pt) ˆ
743 Hz), suggesting that a 1,2-hydride shift had taken place
In the search for a 1,2-migration that might produce a
silylene ligand, we targeted the synthesis of an alkylsilyl
complex of the type [L2PtR(SiHR')]. It was thought that
2
ˆ
to generate the silylene complex [(dippe)(H)Pt SiMes2]-
migration of a hydrogen atom to a platinum center to produce
[MeB(C6F5)3] (2, Scheme 1). This was confirmed by observa-
ˆ
the alkyl hydride [L2Pt(R)(H)( SiR')] might result in elim-
2
ination of alkane[6] to produce a silylene complex of the type
ˆ
[L2Pt SiR2]. Thus, the reaction of [(dippe)PtMeCl] (dippe ˆ
iPr2PCH2CH2PiPr2) with [(thf)2LiSiHMes2][7] (Mesˆ 2,4,6-
Me3C6H2) in diethyl ether yielded a light brown solution,
from which the platinum silyl complex [(dippe)Pt(Me)-
SiHMes2] (1) was isolated in a 79% yield as colorless crystals
that were suitable for an X-ray diffraction study (Figure 1).[8]
The Pt ± Si distance of 2.388(3) Š is similar to that observed
for cis-[(MePh2P)2PtMe(SiPh3)] (2.381(2) Š),[9] and the sili-
con-bound hydrogen atom was located and refined at a
distance of 1.36(3) Š from the silicon atom.
[*] Prof. Dr. T. D. Tilley, G. P. Mitchell
Department of Chemistry
University of California, Berkeley
Berkeley, CA 94720 ± 1460 (USA)
Fax: (‡1)510-642-8940
E-mail: tdtilley@socrates.berkeley.edu
Scheme 1. Generation of the platinum silylene complex
2 and its
[**] This research was supported by the National Science Foundation.
conversion into 3.
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tion of the 29Si NMR chemical shift at d ˆ 338.5 (Figure 2),
which is indicative of the presence of a three-coordinate
silicon atom,[4c,d] and by the 19F NMR spectrum, which is
consistent with formation of [MeB(C6F5)3] .[10] The 1H NMR
spectrum of 2 also contains a broad signal at d ˆ 1.37, which is
spectroscopy) and multiple platinum-containing products,
but no products with silicon ± silicon bonds were observed.
Similarly, treatment of 2 with H2 generated H2SiMes2 as the
major species containing a mesityl group as well as several
uncharacterized platinum products.
This work represents the first direct detection of a 1,2-
migration from silicon to a metal atom to generate an
observable transition metal silylene complex. Of particular
interest is the fact that the four-coordinate platinum complex
1 is remarkably inert, while facile migration occurs when a
coordination site on the metal is vacated. In reactions of
square planar, four-coordinate platinum silyl complexes,
therefore, it seems that a-hydrogen migration (without prior
ligand dissociation) is an unlikely mechanistic step. Thus, for
example, the previously observed elimination of MesSiH3
from [(dmpe)Pt(SiH2Mes)2] (dmpe ˆ Me2PCH2CH2PMe2),
to form the ªdimerizedº platinum silylene complex [(dmpe)-
Pt(m-SiHMes)2Pt(dmpe)], probably proceeds through oxida-
tive addition/reductive elimination cycles rather than via the
Figure 2. 29Si{1H} NMR spectrum of 2 (99.3 MHz): d ˆ 338.1 (1J(Si,Pt) ˆ
ˆ
silylene intermediates [(dmpe)Pt( SiHMes)(H)(SiH2Mes)]
2
2
1305, J(Si,Pt)trans ˆ 187.8 Hz; J(Si,P)cis coupling was not observed).
[13]
ˆ
and [(dmpe)Pt SiHMes].
Despite abundant circumstantial evidence, the relevance of
intramolecular 1,2- and 1,3-migrations in silane polymeriza-
tion, hydrosilation, and silane redistribution mechanisms is
still largely a matter of speculation. Nevertheless, such
migrations are viable and may even be prevalent in trans-
formations of transition metal silicon complexes.
1
assigned to the BCH3 group. This H NMR chemical shift is
noticably downfield from that of free [MeB(C6F5)3] (d ꢀ 0.5
(br) in [D2]dichloromethane[10, 11]), and may therefore indi-
cate the presence of a tight ion pair. A similar downfield
chemical shift (d ˆ 1.67) for [MeB(C6F5)3] , was observed by
Jordan and Coles for an aluminum amidinate system and was
attributed to an Al ´´´ MeB(C6F5)3 interaction.[12] However,
the exceptional downfield-shifted 29Si NMR resonance for 2
strongly indicates that any Si ´´´ MeB(C6F5)3 interaction must
be extremely weak or nonexistent. The value of 1J(Si,Pt) for 2
(1305 Hz) is moderately smaller than the analogous value for
1 (1315 Hz), possibly reflecting the weaker s donating ability
Experimental Section
General procedures: All reactions were carried out under nitrogen using
standard Schlenk techniques. Benzene, pentane, and diethyl ether were
distilled from Na/benzophenone prior to use and stored under nitrogen.
Dichloromethane was distilled from CaH2 and degassed with two freeze-
pump-thaw cycles prior to use. The compounds [(cod)PtMeCl] (cod ˆ 1,5-
[16]
cyclooctadiene),[14] dippe,[15] [(thf)2LiSiHMes2],[7] and B(C6F5)3
were
:
of the SiMes2 ligand relative to SiHMes2. However, a much
prepared according to known prodedures. NMR spectra were recorded in
[D6]benzene at room temperature unless otherwise noted. Elemental
analyses were performed by the microanalytical facility at the University of
California, Berkeley. All IR samples were prepared as KBr pellets.
larger drop in the value of 1J(Si,Pt) was observed when trans-
[(Cy3P)2(H)PtSi(SEt)2][OTf] (1825 Hz) was converted into
trans-[(Cy3P)2(H)PtSi(SEt)2][BPh4] (1558 Hz).[4c] Initial at-
tempts to isolate 2 indicate that it is an oil at room temper-
ature. It is somewhat thermally sensitive, and decomposes at
room temperature with a half-life of approximately 12 h in
[D2]dichloromethane.
[(dippe)PtMeCl]: A procedure analogous to the preparation of [(dppe)Pt-
MeCl] (dppe ˆ 1,2-bis(diphenylphosphanyl)ethane) was used,[17] but start-
ing from [(cod)PtMeCl] and dippe in benzene. Thus, [(dippe)PtMeCl] was
crystallized from dichloromethane at 408C. 1H NMR (400 MHz): d ˆ
0.66 (dd, 3J(H,H) ˆ 7.2, 3J(H,P) ˆ 14.4 Hz, 6H, iPr), 0.70 (m, PtMe), 0.81
(dd, 3J(H,H) ˆ 7.2, 3J(H,P) ˆ 13.6 Hz, 6H, iPr), 0.92 (dd, 3J(H,H) ˆ 7.2,
3J(H,P) ˆ 16.4 Hz, 6H, iPr), 1.17 (m, CH2), 1.28 (dd, 3J(H,H) ˆ 7.2,
As with a similar silylene complex,[4c] the reaction of 2 with
p-dimethylaminopyridine (DMAP) generates the base-stabi-
lized silylene complex [(dippe)(H)PtSiMes2(DMAP)]-
[MeB(C6F5)3] (3), which was isolated in a 47% yield as a
light yellow solid (Scheme 1). The 1H NMR chemical shift of
d ˆ 0.47 ([D2]dichloromethane) for [MeB(C6F5)3] is consis-
tent with formation of the free anion, which has presumably
been completely displaced from the complex by the much
more basic DMAP ligand. Not surprisingly, compound 3 is
considerably more stable than 2, and undergoes less than 10%
decomposition over one week in [D2]dichloromethane at
room temperature.
Monitoring the formation of 2 at 708C by 31P NMR
spectroscopy provided no evidence of reaction intermediates,
such as the three-coordinate platinum species [(dippe)PtSiH-
Mes2][MeB(C6F5)3]. Addition of H2SiPh2 and HClSiPh2 to 2
resulted in the rapid formation of H2SiMes2 (1H NMR
i
3J(H,P) ˆ 15.6 Hz, 6H, iPr), 1.83 (m, 2H, Pr), 2.25 (m, 2H, iPr); 31P{1H}
NMR (161.98 MHz): d ˆ 63.19 (s with 195Pt satellites, 1J(P,Pt) ˆ 4013 Hz),
72.49 (s with 195Pt satellites, 1J(P,Pt) ˆ 1760 Hz).
1: Et2O (15 mL) was added to a mixture of [(dippe)PtMeCl] (0.411 g,
0.810 mmol) and [(thf)2LiSiHMes2] (0.339 g, 0.810 mmol). The mixture was
stirred for 12 hours, and then the volatile compounds were removed under
reduced pressure. Extraction of the residue with pentane (5 Â 20 mL),
followed by concentration to approximately half its volume and cooling to
788C, resulted in crystallization of pure 1. Yield 79% (0.473 g).
Elemental analysis calcd for C33H58P2PtSi: C 53.57, H 7.90; found: C
53.23, H 8.12; m.p. 195-1988C (decomp); 1H NMR (400 MHz): d ˆ 0.76 (m,
12H, iPr), 0.91 (m, 12H, iPr), 1.05 (dd, 3J(Si,P) ˆ 9.6, 3J(H,P) ˆ 6.4 Hz,
PtMe), 1.13 (m, CH2 and iPr), 1.96 (m, 2H, CH2), 2.23 (s, 6H, p-Me), 2.78 (s,
12H, o-Me), 6.21 (m, 1H, SiH), 6.93 (s, 4H, ArH); 13C{1H} NMR
(100 MHz): d ˆ 5.6 (m, iPr), 8.6 (m, iPr), 13.1 (m, PtMe), 16.4 (s, Mes),
18.2 (s, Mes), 17.1 (m, CH2), 18.2 (m, CH2), 25.2 (m, iPr), 27.4 (m, iPr), 118.8
(s, Ar), 122.1 (s, Ar), 125.3 (s, Ar), 131.1 (s, Ar), 132.3 (s, Ar), 138.8 (s, Ar);
31P{1H} NMR (161.98 MHz): d ˆ 66.54 (s with 195Pt satellites, 1J(P,Pt) ˆ
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1784 Hz), 76.00 (s with 195Pt satellites, 1J(P,Pt) ˆ 1378 Hz); 29Si{1H} NMR
(99.3 MHz): d ˆ 28.70 (dd with 195Pt satellites, 1J(P,Pt) ˆ 1315,
2J(Si,Pt)trans ˆ 192, 2J(Si,P)cis ˆ 13.3 Hz); IR: 2956s, 2913s, 2051m (SiH),
1600w, 1542w, 1459s, 1405m 1384w, 1253w, 1226w, 1035m, 836s, 698m,
657m, 632m, 594m, 549w, 424 cm 1 m.
Yamashita, M. Tanaka, M. Goto, Organometallics 1992, 11, 3227; i) Y.
Tanaka, H. Yamashita, M. Tanaka, Organometallics 1995, 14, 530; j) K.
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[3] a) T. D. Tilley in The Silicon-Heteroatom Bond (Eds.: S. Patai, Z.
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2: A mixture of 1 (0.050 g, 0.067 mmol) and B(C6F5)3 (0.035 g, 0.068 mmol)
was dissolved in [D2]dichloromethane (0.700 mL), generating a bright
yellow solution of 2. 1H NMR (400 MHz): d ˆ 1.50 (dd, 2J(H,P)cis ˆ 7.1,
2J(H,P)trans ˆ 105, 1J(H,Pt) ˆ 743 Hz, 1H, PtH), 0.63 (m, 12H, iPr), 0.85 (m,
12H, iPr), 1.25 (m, CH2 and iPr), 1.36 (brs, 3H, MeB(C6F5)3), 1.62 (m, 2H,
CH2), 2.00(s, 6H, p-Me), 2.23 (s, 12H, o-Me), 6.62 (s, 4H, ArH); 31P{1H}
NMR (161.98 MHz): d ˆ 77.2 (s with 195Pt satellites, 1J(P,Pt) ˆ 1726 Hz),
81.4 (s with 195Pt satellites, 1J(P,Pt) ˆ 2523 Hz); 19F NMR (376.4 MHz): d ˆ
132.2 (brs, 2F), 165.1 (brs, 1F), 167.2 (brs, 2F); 29Si{1H} NMR (99.38
Mhz): d ˆ 338.1 (d with 195Pt satellites, 2J(Si,P) ˆ 187.8, 1J(Si,Pt) ˆ 1305 Hz).
3: Dichloromethane (10 mL) was added to a mixture of 1 (0.300 g,
0.405 mmol) and B(C6F5)3 (0.207 g, 0.405 mmol) to generate 2. After all of
the reactants had dissolved, a solution of DMAP (0.049 g, 0.405 mmol) in
dichloromethane (5 mL) was added by cannula. This resulted in the
immediate formation of a colorless solution. Removal of the volatile
material under reduced pressure gave a light yellow oil, to which Et2O
(5 mL) was added. Cooling the mixture to 788C for 12 h resulted in
precipitation of 3 as a light yellow powder. Yield 47% (0.225 g). Elemental
analysis calcd for C58H68BF15N2P2PtSi: C 58.83, H 5.79; found: C 58.53, H
5.56; m.p. 105 ± 1078C (dec). 1H NMR (400 MHz, [D2]dichloromethane):
d ˆ 3.57 (dd with 195Pt satellites, 2J(H,P)cis ˆ , 2J(H,P)trans ˆ 148,
1J(H,Pt) ˆ 918 Hz, 1H, PtH), 0.47 (brs, MeB(C6F5)3), 0.65 (m, 12H, iPr),
0.94 (m, 12H, iPr), 1.13 (m, CH2 and iPr), 1.96 (m, 2H, CH2), 2.32 (s, 6H, p-
Me), 2.36 (s, 12H, o-Me), 3.09 (s, 6H, NMe2), 6.56 (d, 3J(HH) ˆ 7.6 Hz),
6.81 (s, 1H, ArH), 8.32 (d); 13C{1H} NMR (100 MHz, [D2]dichloro-
methane): d ˆ 7.1 (m, iPr), 8.2 (m, iPr), 10.1 (br s, MeBAr3), 19.1 (s, Mes),
19.9 (s, Mes), 23.3 (m, CH2), 28.4 (m, CH2), 32.1 (s, NMe2), 117.5 (s, Ar),
119.1 (s, Ar), 121.3 (s, Ar), 124.1 (s, Ar), 127.1 (s, Ar), 130.1 (s, Ar), 131.6 (s,
Ar), not all of the aryl carbon atoms were observed; 31P{1H} NMR
(161.98 MHz, [D2]dichloromethane): d ˆ 71.87 (d with 195Pt satellites,
2J(P,P) ˆ 3.4, 1J(P,Pt) ˆ 2075 Hz), 92.37 (d with 195Pt satellites, 1J(P,Pt) ˆ
1636 Hz); IR: 2971s, 2916s, 2072m (PtH), 1572w, 1489s, 1426m 1401w,
1319w, 1251w, 1092m, 913s, 852m, 741m, 695m, 525 cm 1 m.
[5] G. P. Mitchell, T. D. Tilley, J. Am. Chem. Soc., in press.
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[8] Crystal data for 1: C33H57PtP2Si, 0.10 Â 0.15 Â 0.10 mm, tetragonal,
space group P21c, a ˆ 22.0264 (3), c ˆ 14.2660 (2) Š, Vˆ 6947.3 (1) Š3,
Z ˆ 8; m(MoKa) ˆ 41.69 cm 1, Tˆ 158 K. Of 28993 data collected
(2q < 46.58), 3551 were independent and used in the refinement of 334
variables. The data were corrected for Lorentz and polarization
effects, but no absorption correction was applied. All non-hydrogen
atoms were refined with anisotropic thermal parameters. All hydro-
gen atoms were treated as idealized contributions, except H(36),
which was located and refined isotropically . R(F) ˆ 0.029; R(wF) ˆ
3
0.029. Max./min. peaks in final difference map: 0.55/ 0.57 e Š
.
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-102839. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
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[11] C. L. Gillis, M-J. Tudoret, M. C. Baird, J. Am. Chem. Soc. 1993, 115,
2543.
Received: March 24, 1998 [Z11632IE]
German version: Angew. Chem. 1998, 110, 2602 ± 2605
Keywords: platinum ´ silicon ´ silylene ´ rearrangements
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