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.
The cyclometalated Rh(III)-NHC compounds [Cp*RhI(C,C')-Triaz] (Triaz = 1,4-diphenyl-3methyl-1,2,3-triazol-5-ylidene) and [Cp*RhI(C,C')-Im] (Im = 1-phenyl-3-methyl-imidazol-2ylidene) are efficient catalysts for the hydrosilylation of terminal alkynes with complete regio-and stereoselectivity towards 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 1alkynes 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 proposed mechanism entails the Rh-CAr assisted hydrosilane activation to afford a reactive Rhsilyl intermediate that leads to a (E)-silylvinylene intermediate after alkyne insertion and a metallacyclopropene-driven isomerization. The release of the -(Z)-vinylsilane product can occur by a reversible cyclometalation mechanism involving -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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.