Mono- and Bimetallic [Cp*Rh] Complexes of the 4,6-Bis(pyridyl)pyrimidine Ligand

Abstract Multinucleating polyheterocyclic platforms are intriguing ligands for half-sandwich rhodium complexes, as they could engender various modes of cooperation in electron- and proton-transfer reactions. Herein, we report the synthesis and spectroscopic, structural, and electrochemical characterization of mono- and bimetallic pentamethylcyclopentadienyl (Cp*) rhodium complexes of the 4,6-bis(pyridyl)pyrimidine (bppm) ligand. These complexes undergo two-electron reduction of each [Cp*Rh] site at potentials that depend on complex nuclearity ([Rh1] vs [Rh2]) and on the monodentate ligand at Rh (Cl– vs NCCH3). Spectroscopic and structural analysis of a chemically prepared monorhodium(I) complex confirmed significant delocalization of electron density onto the bppm platform. While [Rh-Cl] bppm complexes displayed enhanced stability vs literature compounds supported by related polyheterocyclic scaffolds, various modes of bidentate ligand exchange were observed in their solvento analogues. Electrochemical analyses revealed that dissociation of a bidentate motif from Rh could be induced by electron transfer (ET), a transformation that parallels the well-established ET-induced dissociation of monodentate ligands in these species. Investigation of complexes of the literature 2,2’-bipyrimidine (bpym) ligand demonstrated analogous bidentate ligand exchange reactivity in this system, suggesting that these processes may be more generally operative in [Cp*Rh] complexes supported by binucleating platforms.