Synthesis, Structure, Bonding, and Ligand Exchange of a Bimetallic Rhodium Dimetallocyclobutane Complex

We report the synthesis of a 2-phosphinoimidazole-scaffolded bimetallic Rh complex (7) that contains a unique dimetallocyclobutane structure resulting from addition to a cyclooctene (COE) ligand. This bridging COE ligand is labile in the presence of 2-butyne and readily transforms into dimetallocyclobutene complex 8. Density functional theory (DFT) calculations and bonding analyses indicate that the dimetallocyclobutane structure 7 has significant Rh–Rh single σ bond character and two Rh–C covalent σ bonds. DFT calculations also suggest that ligand exchange occurs via an associative mechanism without formation of a RhRh cycloreversion intermediate. Complex 7 was oxidized to a halide-bridged bimetallic Rh(III,III) complex 9 by treatment with PhICl2, which resulted in destruction of the Rh–Rh bond.