Structure of 4-sila-3-platinacyclobutene and its formation via Pt-promoted γ-Si-H bond activation of 3-sila-1-propenylplatinum precursor

Article abstract of DOI:10.1021/ja017888r

4-Sila-3-platinacyclobutene was isolated from the reaction of Pt(SiHPh₂)₂(PMe₃)₂ with dimethyl acetylenedicarboxylate (DMAD) and characterized by X-ray crystallography. The formation pathway was found to involve γ-Si-H bond activation of the 3-sila-1-propenylplatinum intermediate that is formed by the insertion of a DMAD into a Pt-Si bond of Pt(SiHPh₂)₂(PMe₃)₂. Copyright

Full text of DOI:10.1021/ja017888r

Published on Web 04/06/2002
Structure of 4-Sila-3-platinacyclobutene and Its Formation via Pt-Promoted
γ-Si-H Bond Activation of 3-Sila-1-propenylplatinum Precursor
Makoto Tanabe and Kohtaro Osakada*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan
Received December 28, 2001
Complexes of group 10 metals catalyze various C-Si bond-
forming reactions of organosilicon compounds with unsaturated
molecules such as alkenes, alkynes, and dienes.1 Many Si-containing
cyclic compounds were prepared by this approach, using diorganosi-
lanes,tetraorganodisilanes,silacyclopropenes,andsilacyclopropanes.2-7
These reactions have been considered to involve 4-sila-3-metallacy-
clobutene as the common intermediate that plays an important role
in forming a new C-Si bond. This four-membered metallacycle,
however, has not been isolated or structurally characterized. Ishi-
kawa et al. reported the preparation of a 4-sila-3-nickelacyclobutene
in situ, although it is too unstable to be isolated or characterized
by crystallography.8 The Pt-complex-promoted reactions of organo-
Figure 1. ORTEP drawing of 1 with 50% thermal ellipsoidal plots. The
silanes and disilanes with alkynes were postulated to involve a sily-
lene-coordinated platinum complex9 as a precursor to the metal-
lacyclobutene intermediate.6,7 In this paper, we report the first isola-
tion and crystallographic results of a Pt-containing silacyclobutene
and demonstrate its formation pathway via an unexpected precursor.
hydrogen atoms are omitted for simplicity.
10
Pt(SiHPh2)2(PMe3)2 reacts with an equimolar mixture of
dimethyl acetylenedicarboxylate (DMAD) in THF ([Pt] ) 5.5 mM)
for 12 h to produce Pt(CZdCZ-SiPh2)(PMe3)2 (Z ) COOMe) (1)
in 85% yield as yellow crystals accompanied by the formation of
H2SiPh2 (eq 1). Figure 1 shows the molecular structure of 1 by
Figure 2. Ratios of 1 and cis-2 of the equilibrated mixtures obtained from
equimolar reactions of H2SiPh2 and 1 with [Pt] ) 1-22 mM (benzene-d6,
26 °C).
X-ray crystallography.11 The complex contains a planar four-
membered ring with an Si-Pt-C1 angle (65.8(2)°) that is smaller
than that of the saturated four-membered 2-sila-1-platinacyclobutane
(68.1(3)°).12 The 29Si{1H} NMR signal of 1 at δ -63.6 shows
coupling with 195Pt (JSiPt ) 778 Hz) and two unequivalent 31P (2JSiP
) 161 and 3 Hz) nuclei.13
The reaction of Pt(SiHPh2)2(PMe3)2 with an equimolar mixture
of DMAD in the presence of H2SiPh2 for 5 min ([Pt] ) 67 mM)
results in the isolation of cis-Pt(CZdCZ-SiHPh2)(SiHPh2)(PMe3)2
(cis-2), which is formed via insertion of DMAD into a Pt-Si bond
of Pt(SiHPh2)2(PMe3)2 (eq 2).
These results suggest that reaction 1 involves cis-2 as the initial
product, which is turned into 1 during the reaction. Dissolution of
an equimolar mixture of H2SiPh2 and 1 in benzene-d6 causes the
partial formation of cis-2 within 1 h. Figure 2 plots the ratios of 1
and cis-2 in the equilibrium mixtures starting from equimolar mix-
tures of H2SiPh2 and 1 with [Pt] ) 1-22 mM. Formation of 1 is
thermodynamically favored with a low concentration of the com-
plexes. These results suggest that the two complexes are converted
easily and reversibly via H2SiPh2-promoted ring opening of 1 and
γ-Si-H bond activation of cis-2, followed by elimination of
H2SiPh2 (eq 3). Slow formation of trans-Pt(CZdCZ-SiHPh2)-
Dissolution of once-isolated cis-2 in benzene-d6 causes its partial
conversion into 1, producing a mixture of 1, cis-2, and H2SiPh2.
(SiHPh2)(PMe3)2 (trans-2) in the reaction mixture prevented the
precise determination of the thermodynamic parameters of the above
reaction.
* Corresponding author. E-mail: kosakada@res.titech.ac.jp.
9
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J. AM. CHEM. SOC. 2002, 124, 4550-4551
10.1021/ja017888r CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
In summary, a stable 4-sila-3-platinacyclobutene (1) was char-
acterized in both the solid state and solution. The complex is
transformed into cis-2 in an equilibrium reaction involving H2SiPh2.
Facile and reversible conversion between 1 and cis-2 indicates that
cis-2 is the real precursor of 1 in reaction 1.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Supporting Information Available: Experimental procedures for
1
the synthesis of the complexes and H, 13C{1H}, 29Si{1H}, and 31P-
{1H} NMR spectra of 1, cis-2, trans-2, 3, and an equilibrated mixture
of 3 and 4, and results of X-ray crystallography for 1, trans-2, and 3
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. ORTEP drawing of 3 with 50% thermal ellipsoidal plots. The
crystal contains solvated THF molecules. Hydrogen atoms, except for SiH,
and solvent molecule are omitted for simplicity.
References
Complex Pt(CZdCZ-SiHPh2)(SiHPh2)(dmpe) (3) (dmpe ) 1,2-
bis(dimethylphosphino)ethane), which has an analogous structure
to cis-2, is obtained by the reaction of DMAD with Pt(SiHPh2)2-
(dmpe) (eq 4). The molecular structure of 3 (Figure 3)14 shows an
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(8) 4-Sila-3-nickelacyclobutene, prepared in situ from the reaction of a
silacyclopropene with Ni(PEt3)4, was characterized by NMR in solution.
See: Ishikawa, M.; Ohshita, J.; Ito, Y.; Iyoda, J. J. Am. Chem. Soc. 1986,
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agostic interaction of the γ-hydrogen of the 3-sila-1-propenyl ligand.
The hydrogen is located at an apical position of the Pt center. The
Pt‚‚‚H distance, 2.43 Å, is less than the 2.95 Å distance predicted
from the sum of van der Waals radii of Pt and H.15
1
Temperature-dependent change of JSiH (198 Hz at 70 °C, 196
Hz at 25 °C, 195 Hz at -30 °C; cis-2, 197 Hz at 25 °C) of 3-sila-
1-propenyl ligand in the NMR spectra of 3 and lower ν(SiH) values
of cis-2 (2098 cm-1), trans-2 (2078 cm-1), and 3 (2116 cm-1) than
Ph2Si(CHdCH2)H (2124 cm-1 16
) indicate the presence of a weak
interaction between the SiH hydrogen and Pt in the solid state and
in solution. The structure with the cis CdC bond is suited for further
strengthening of the interaction, which leads to γ-Si-H bond cleav-
(9) (a) Grumbine, S. D.; Tilley, T. D. J. Am. Chem. Soc. 1993, 115, 7884.
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Feldman, J. D.; Mitchell, G. P.; Nolte, J. O.; Tilley, T. D. J. Am. Chem.
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1
age promoted by Pt. The H NMR spectrum of 3 shows its slow
conversion into the corresponding 4-sila-3-platinacyclobutene
(10) Kim, Y.-J.; Park, J.-I.; Lee, S.-C.; Osakada, K.; Tanabe, M.; Choi, J.-C.;
Koizumi, T.; Yamamoto, T. Organometallics 1999, 18, 1349.
(11) Crystal data of 1: C24H34SiPtP2O4, Mr ) 671.65, orthorhombic, P212121
(No. 19), a ) 15.697(2) Å, b ) 17.690(5) Å, c ) 16.442(2) Å, V )
Pt(CZdCZ-SiPh2)(dmpe) (4) with concomitant formation of
H2SiPh2 (eq 5). Although isolation of 4 from the equilibrium
2719.8(9) Å3, Z ) 4, µ(Mo KR) ) 5.326 mm-1, Dc ) 1.640 g cm-3
,
F(000) ) 1328, 3358 unique reflections, 289 variables, R ) 0.030, Rw
0.024, GOF ) 1.67, using 2846 reflections with I > 3σ(I).
)
(12) Tanabe, M.; Yamazawa, H.; Osakada, K. Organometallics 2001, 20, 4451.
(13) The 4-sila-3-nickelacyclobutene of ref 8 showed the Si NMR peak at δ
-125.3 (corrected as the chemical shift from SiMe4). The higher magnetic
field position than 1 may be ascribed to a SiMe3 substituent at the Si
atom.
(14) Crystal data for 3‚THF: C36H44Si2PtP2O4‚C4H8O, Mr ) 926.06, mono-
clinic, P21/n (No. 14), a ) 9.120(2) Å, b ) 22.878(4) Å, c ) 98.43(2) Å,
â ) 98.43(2)°, V ) 4195(1) Å3, Z ) 4, µ(Mo KR) ) 3.504 mm-1, Dc )
1.466 g cm-3, F(000) ) 1872, 10444 unique reflections, 451 variables, R
) 0.045, Rw ) 0.035, GOF ) 1.72, using 5094 reflections with I > 3σ(I).
(15) Bondi, A. J. Phys. Chem. 1964, 68, 441.
mixture of 3 and 4 was not feasible,17 similar NMR parameters
between 1 and 4 clearly indicate the 4-sila-3-platinacuclobutene
structure of 4.18
Thus, complexes cis-2 and 3 cause intramolecular γ-Si-H bond
activation by Pt to afford mixtures with the corresponding 4-sila-
3-platinacyclobutenes. Reaction 1 with a low concentration of Pt
complex rendered the isolation of 1 possible. Another possible route
to 4-sila-3-platinacyclobutene from the reaction of an alkyne with
disilylplatinum complexes, involving the initial formation of
silylene-platinum complexes,7 is less plausible for explaining the
formation of silaplatinacyclobutene in the reaction of this paper.
(16) Stefanac, T. M.; Brook, M. A.; Stan, R. Macromolecules 1996, 29, 4549.
(17) The solution of 3 is partially converted into 4 and forms an equilibrated
mixture of the complexes (75:25) in 2 h at room temperature. Upon raising
the temperature to 90 °C, 3 is further converted to 4 to produce the mixture
of 3 and 4 in a ratio of 30:70.
(18) 31P{1H} NMR in toluene-d8: δ 21.2 (JPPt ) 2117 Hz) and 37.3 (JPPt
1345 Hz) for 4 at 90 °C, δ -31.9 (JPPt ) 2334 Hz) and -17.5 (JPPt
)
)
1368 Hz) for 1 at 24 °C. 1H NMR in toluene-d8: δ 3.40 and 3.68 for 4
at 90 °C, δ 3.25 and 3.73 for 1 at 24 °C.
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