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)­(κ2-CHO2)­(PPh3), with an observed rate constant of [2.9(2)] × 10–4 s–1 (25 °C), which is independent of CO2 pressure. Selective formation of acrylate and propionate carbon dioxide–ethylene coupling products, (Triphos)­Mo­(H)­(κ2-C3H3O2)­(PPh3) and (Triphos)­Mo­(H)­(κ2-C3H5O2)­(PPh3), was achieved by sequential addition of olefin and heterocumulene to (Triphos)­MoH4PPh3. A formally zerovalent TriphosMo­(η2-C2H4)3 intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide–ethylene coupling.