β‑(Z) Selectivity Control by Cyclometalated Rhodium(III)–Triazolylidene Homogeneous and Heterogeneous Terminal Alkyne Hydrosilylation Catalysts

The cyclometalated Rh­(III)–NHC compounds [Cp*RhI­(C,C′)-Triaz] (Triaz = 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene) and [Cp*RhI­(C,C′)-Im] (Im = 1-phenyl-3-methyl-imidazol-2-ylidene) are efficient catalysts for the hydrosilylation of terminal alkynes with complete regio- and stereoselectivity toward the thermodynamically less stable β-(Z)-vinylsilane isomer at room temperature in chloroform or acetone. Catalyst [Cp*RhI­(C,C′)-Triaz] shows a superior catalytic performance in terms of activity and has been applied to the hydrosilylation of a range of linear 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMePh2, HSiMe2Ph, HSiEt3, and the bulkier heptamethylhydrotrisiloxane (HMTS), to afford the corresponding β-(Z)-vinylsilanes in quantitative yields. The graphene-based hybrid material TRGO-Triaz-Rh­(III), featuring cyclometalated [Cp*RhI­(C,C′)-Triaz] (Triaz = 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene) rhodium­(III) complexes covalently immobilized through the triazolylidene linker, has been prepared by metalation of the trimethylsilyl-protected 3-methyl-4-phenyl-1,2,3-triazolium iodide functionalized graphene oxide material, TRGO-Triaz, with [Cp*RhCl2]2 using sodium tert-butoxide as base. The coordination sphere of the supported rhodium­(III) complexes has been determined by means of XPS and extended X-ray absorption fine structure (EXAFS) spectroscopy, showing the replacement of the iodido ligand by O-functionalities on the carbon wall. In sharp contrast with the homogeneous catalyst, the heterogeneous hybrid catalyst TRGO-Triaz-Rh­(III) is not active at room temperature although it shows an excellent catalytic performance at 60 °C. In addition, the hybrid catalyst TRGO-Triaz-Rh­(III) has shown an excellent recyclability, allowing at least six catalytic runs in the hydrosilylation of oct-1-yne with HSiMePh2 in acetone with complete selectivity to the β-(Z)-vinylsilane product. The reaction mechanism for the molecular catalyst [Cp*RhI­(C,C′)-Triaz] has been explored by means of DFT calculations, pointing to a metal–ligand bifunctional mechanism involving reversible cyclometalation that is competitive with a noncooperative pathway. The proposed mechanism entails the Rh–CAr assisted hydrosilane activation to afford a reactive Rh–silyl intermediate that leads to a (E)-silylvinylene intermediate after alkyne insertion and a metallacyclopropene-driven isomerization. The release of the β-(Z)-vinylsilane product can occur by a reversible cyclometalation mechanism involving σ-CAM with the CAr–H bond or, alternatively, the Si–H bond of an external hydrosilane. The energy barrier for the latter is 1.2 kcal·mol–1 lower than that of the CAr–H bond, which results in a small energy span difference that makes both pathways competitive under catalytic conditions.