Functionalization of Carbon Dioxide with Ethylene at Molybdenum Hydride Complexes

The molybdenum tetrahydride species (Triphos)MoH4PPh3 (Triphos = PhP(CH2CH2PPh2)(2)) generated from sodium triethylborohydride addition to (Triphos)MoCl3 was found to promote CO2 functionalization to afford acrylate, propionate, and formate species. The formation of (Triphos)MoH4PPh3 occurs via a (Triphos)Mo(H)Cl(PPh3) intermediate followed by dismutation of an unobserved six-coordinate molybdenum(II) dihydride complex. Addition of dihydrogen to the dismuation product mixture affords a nearly quantitative yield of (Triphos)MoH4PPh3. The molybdenum tetrahydride species facilitates CO2 insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(kappa(2)-CHO2)(PPh3), with an observed rate constant of [2.9(2)] X 10(-4) s(-1) (25 degrees C), which is independent of CO2 pressure. Selective formation of acrylate and propionate carbon dioxide-ethylene coupling products, (Triphos)Mo(H)(kappa(2)-C3H3O2)(PPh3) and (Triphos)Mo(H)(kappa(2)-C3H5O2)(PPh3), was achieved by sequential addition of olefin and heterocumulene to (Triphos)MoH4PPh3. A formally zerovalent TriphosMo(eta(2)-C2H4)(3) intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide-ethylene coupling.