Novel pt(ii) mono- and biscarbene complexes: synthesis, structural characterization and application in hydrosilylation catalysis

Abstract

Reactions of imidazolium and benzimidazolium halides with PtBr2 and PtI2 in DMSO afford cis-Pt(II) heterocyclic carbene DMSO mixed ligand complexes 1-3 and biscarbene complexes 4-6. The reaction between 1,3-dibenzylbenzimidazolium bromide and PtBr2 in CH3CN gives no carbene complex but a benzimidazolium hexabromoplatinate(IV) salt 7. All the products have been fully characterized by NMR and ESI-MS spectroscopy as well as X-ray single-crystal diffraction study. Both intramolecular γ-hydride interactions with metal or its coordinated bromide and intermolecular H-bonding between heterocyclic proton with bromide are evident in some of these structures. These complexes are active in the hydrosilylation of terminal alkynes (phenylacetylene and trimethylsilylacetylene) with triethylsilane and bis(trimethylsiloxy)methylsilane. When phenylacetylene is used, the monocarbene complexes (1-3) show higher catalytic activity than chelating biscarbene analogues (5 and 6). Non-chelating biscarbene complex 4 gives a different selectivity pattern favoring the β(Z) and dehydrogenative silylation products when compared to 1-3, 5 and 6, and 7. In contrast, 4 and 5 are more active than 1 and 2 toward trimethylsilylacetylene. In general, monocarbene complexes give higher conversions over a short duration in the hydrosilylation of phenylacetylene with triethylsilane and bis(trimethylsiloxy) methylsilane, whereas in the hydrosilylation of trimethylsilylacetylene with triethylsilane, the chelating and biscarbene tend to be more active. © 2009 American Chemical Society.