Bimetallic Rhodium Complexes: Precatalyst Activation-Triggered Bimetallic Enhancement for the Hydrosilylation Transformation

To explore how to access bimetallic catalysts that can outperform their monometallic analogues, a set of mono- and bimetallic NHC/phosphine (NHC = N-heterocyclic carbene)-ligated rhodium(I) complexes were designed, synthesized, and characterized. The complexes were used as catalysts for the intermolecular hydrosilylation of diphenylacetylene. All four catalysts displayed significantly faster catalytic rates than the related complexes reported previously containing NHC/pyrazolyl donor ligands. The bimetallic catalyst design led to enhanced catalytic activity over the monometallic analogue. The data revealed that the degree of bimetallic enhancement was highly dependent on the reaction conditions, coligands used, and the substrate addition order. The catalytic trends, in combination with mechanistic studies using individual substrates and density functional theory calculations, exposed that the bimetallic benefits were strongly linked to the catalyst activation procedure.