Platinum Bis(tricyclohexylphosphine) Silyl Hydride Complexes

A series of platinum metal silyl hydride complexes, cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ =
SiPh2H, SiEt2H, SiPh3, SiEt3, SiMe2(OSiHMe2), Si(OSiMe3)2Me, SiMe2(CH2CHCH2), SiMe2Et, SiMe2[OCH2C(Me)CH2], Si(OMe)2(CH2CHCH2), SiPh2(OSiPh2H)), have been prepared
in solution by reaction of Pt(PCy3)2 with the appropriate silane, HSiR2R‘. The complex cis-Pt(PCy3)2(H)(HSiPh2) (1-cis) has been characterized by X-ray crystallography at −100 °C.
The platinum center exhibits a distorted-square-planar geometry with angles P(1)−Pt−P(2) = 113.55(3)°, P(1)−Pt−Si = 146.83(3)°, and P(2)−Pt−Si = 99.37(3)°. The reaction of
Pt(PCy3)2 with chlorinated hydrosilanes at −78 °C yields the analogous complexes cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiMe2Cl, SiMeCl2, SiCl3), which isomerize to their trans isomers
on warming to room temperature. The complex 1-cis and several analogues convert to the
trans isomers photochemically at room temperature. Ready silane exchange is demonstrated
by the reaction of HSiPh3 with cis-Pt(PCy3)2(D)(SiPh3) and by the reaction of H2SiPh2 with
cis-Pt(PCy3)2(H)(SiPh3). These experiments also revealed the relative thermodynamic stability
of some of the platinum silyl complexes, of which the most stable was cis-Pt(PCy3)2(H)(SiPh2H). NMR spectroscopy demonstrates that the inequivalent phosphine ligands of the
cis isomers undergo intramolecular mutual exchange on the NMR time scale. In competition
with this process, the complexes undergo reversible reductive elimination of silane. Analysis
of the NMR spectra yields the thermodynamic data for dissociation of silanes for SiR2R‘ =
SiPh3, SiMe2Et. Rate constants for phosphine exchange were calculated via line-shape
analysis of 1H NMR spectra. Rate constants for reductive elimination of silane in cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiPh2H, SiMe2Et, SiPh3) were calculated via 1H EXSY
measurements. The three distinct reaction pathways, photochemical cis−trans isomerization,
intramolecular thermal phosphine site exchange, and reductive elimination, are shown to
involve three distinct transition states. The transition states for the independent processes
of phosphine site exchange and for reductive elimination must retain substantial Pt−H and
Pt−Si interactions, while there is also significant Si−H bond formation. This situation can
therefore be described as involving Pt(η2-H−SiR3) interactions.