Competition between phosphorus lone pairs and ring π-systems in binding to transition metals: Binuclear diphosphacyclobutadiene cobalt carbonyl derivatives

Abstract

Metal complexes of the 2,4-di-t-butyl-1,3-diphosphacyclobutadiene ligand have been synthesized by head-to-tail dimerization of t-BuCP: on transition metal sites. In this connection the geometries and energetics of the complete series of binuclear cobalt carbonyl complexes (Me2C2P2)2Co2(CO)n (n=5, 4, 3, 2, 1) of the simpler 2,4-dimethyl-1,3-diphosphacyclobutadiene ligand have been investigated by density functional theory. The lowest energy structures of these complexes have bridging η4,η1-Me2C2P2 ligands donating six electrons to the central Co2 unit through a tetrahapto ring-metal linkage to one cobalt atom and a phosphorus lone pair to the other cobalt atom. For the tetracarbonyl (Me2C2P2)2Co2(CO)4 doubly CO-bridged structures with terminal η4-Me2C2P2 ligands similar to known (η5C5H5)2Fe2(μ-CO)2(CO)2 and (η4-Me4C4)2Co2(μ-CO)2(CO)2 structures lie only ∼5kcal/mol in energy above the η4,η1-Me2C2P2 bridged structures. For the unsaturated (Me2C2P2)2Co2(CO)3 and (Me2C2P2)2Co2(CO)2 systems, structures with one or two bridging η4,η1-Me2C2P2 ligands, respectively, are preferred energetically over isomeric structures with formal CoCo double bonds and CoCo triple bonds, respectively. The lowest energy structure for the monocarbonyl (Me2C2P2)2Co2(CO) is a triplet structure in which a (Me2C2P2)2Co sandwich unit functions as a tridentate ligand to a CoCO unit through a phosphorus atom on each ring as well as the central cobalt atom.