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

A series of zerovalent molybdenum complexes bearing triphosphine ligands, [Ar2PCH2CH2]2PPh, have been synthesized and evaluated for reductive functionalization of CO2 with ethylene. The ability to form dimeric triphosphine molybdenum­(II) 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-tBu-C6H3)2PCH2CH2]2PPh coupling nearly four times slower than with [(3,5-Me-C6H3)2PCH2CH2]2PPh. A considerably milder electronic effect was observed with complexes bearing [(4-F-C6H4)2PCH2CH2]2PPh reducing CO2 at approximately half the rate as with [Ph2PCH2CH2]2PPh.