Ancillary Ligand Effects on Carbon Dioxide-Ethylene Coupling at Zerovalent Molybdenum

A series of zerovalent molybdenum complexes bearing triphosphine ligands, [Ar2PCH2CH2](2)PPh, have been synthesized and evaluated for reductive functionalization of CO2 with ethylene. The ability to form dimeric triphosphine molybdenum(U) acrylate hydride species from CO2-ethylene coupling was found to be highly sensitive to steric encumbrance on the phosphine aryl substituents. Trapping of triphosphine molybdenum(II) acrylate hydride species using triphenylphosphine afforded isolable monomeric CO2 functionalization products with all ancillary ligands studied. Kinetic analysis of the acrylate formation reaction revealed a first-order dependence on molybdenum, but no influence from CO2 pressure or the triphenylphosphine trap. Systematic attenuation of steric and electronic features of the triphosphine ligands showed a strong CO2 functionalization rate influence for ligand size with [(3,5-Bu-t-C6H3)(2)PCH2CH2](2)PPh coupling nearly four times slower than with [(3,5-Me-C6H3)(2)PCH2CH2](2)PPh. A considerably milder electronic effect was observed with complexes bearing [(4-F-C6H4)(2)PCH2CH2](2)PPh reducing CO2 at approximately half the rate as with [Ph2PCH2CH2](2)PPh.