Triangular Triplatinum Complex with Four Bridging Si Ligands: Dynamic Behavior of the Molecule and Catalysis

A triangular triplatinum(0) complex with bridging diphenylsilylene ligands, [{(Pt(PMe3)}(3)(mu-SiPh2)(3)] (1a), reacts with H2SiPh2 to produce the 1:1 adduct, [{Pt(PMe3)}(3)(H)(2)(mu-SiPh2)(4)] (2a), which was isolated and characterized by X-ray crystallography. Two Pt-Pt bonds of the triangular Pt-3 core are bridged by a diphenylsilylene ligand, while the remaining Pt-Pt bond has two unsymmetrical bridging Si ligands. Dissolution of 1a and H2SiPh2 at a 1:3 molar ratio forms a mixture of complex 2a and unreacted la. NMR measurement of the solution at -90 degrees C revealed the structure of 2a as having two hydride ligands and four bridging silylene ligands. Two P-31{H-1} NMR signals of 2a at -90 degrees C coalesce on warming to -50 degrees C owing to facile exchange of the four Si ligands. Reversible addition of H2SiPh2 to la yielded 2a with Delta G degrees = -8.0 kJ mol(-1), Delta H degrees = -51.7 kJ mol(-1), and Delta S degrees = 146 J mol(-1) K-1. Addition of bis(4-fluorophenyl)silane and bis(4-methylphenyl)silane to the complexes, which have three bis(4-fluorophenyl)silylene and bis(4-methylphenyl)silylene ligands, respectively, also occurs reversibly in the solution, and the diarylsilane with an electron-withdrawing substituent is favored for the formation of Pt3Si4 complexes. Complex la catalyzes hydrosilyation of benzaldehyde with H2SiPh2 to produce diphenyl(benzyloxy)silane along with concurrent hydrosilyation and dehydrosilyation of phenyl(methyl)ketone. Dehydrogenative coupling of H2SiPh2 and phenol is also catalyzed to yield diphenyl(phenoxy)silane. The P-31{H-1} NMR spectra of the mixtures during the catalytic reaction show 2a as the major Pt-containing species.