Hybrid catalysts comprised of graphene modified with rhodium-based N-heterocyclic carbenes for alkyne hydrosilylation

Abstract

Thermally partially reduced graphene oxide has been covalently modified with 3-methyl-4-phenyl-1,2,3-triazolium salts making use of the epoxy functionalities on the carbon nanomaterial. Characterization of the functionalized materials through adequate solid characterization techniques, particularly X-ray photoelectron spectroscopy (XPS), allows one to follow the stepwise building up of the triazolium fragments on the graphene oxide attached to the wall via covalent C–N linkage. The hydroxyl-triazolium-functionalized materials have been used to prepare rhodium hybrid materials containing either alkoxo or triazolylidene molecular rhodium(I) complexes depending on the protection of the hydroxyl groups present in the material. Characterization of the heterogeneous systems, especially by means of XPS and extended X-ray absorption fine structure (EXAFS) spectroscopy, has evidenced the coordination sphere of the supported rhodium(I) complexes in both rhodium hybrid materials. The graphene-oxide-supported rhodium–triazolylidene hybrid catalysts show excellent activity, comparable to that of the homogeneous [RhI(cod)(Triaz)] (Triaz = 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene) catalyst, for the hydrosilylation of terminal and internal alkynes. In addition, these catalysts have shown good selectivity to the β-(Z) vinylsilane isomers (for the not hindered terminal substrates) or syn-additions (for the internal substrates). In contrast to the rhodium(I)–alkoxo-based hybrid material, the silyl-protected rhodium(I)–triazolylidene-based hybrid catalyst can be reused in consecutive cycles without loss of activity maintaining the selectivity. The lack of leaching of active rhodium species demonstrates the strength of the C–N covalent bond of the triazolylidene linker to the graphitic wall.