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.