Functionalization of Carbon Dioxide with Ethylene at Molybdenum Hydride Complexes

Abstract

The molybdenum tetrahydride species (Triphos)MoH 4 PPh 3 (Triphos = PhP(CH 2 CH 2 PPh 2 ) 2 ) generated from sodium triethylborohydride addition to (Triphos)MoCl 3 was found to promote CO 2 functionalization to afford acrylate, propionate, and formate species. The formation of (Triphos)MoH 4 PPh 3 occurs via a (Triphos)Mo(H)Cl(PPh 3 ) 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)MoH 4 PPh 3 . The molybdenum tetrahydride species facilitates CO 2 insertion into a metal hydride to produce a formate complex, (Triphos)Mo(H)(κ 2 -CHO 2 )(PPh 3 ), with an observed rate constant of [2.9(2)] × 10 –4 s –1 (25 °C), which is independent of CO 2 pressure. Selective formation of acrylate and propionate carbon dioxide–ethylene coupling products, (Triphos)Mo(H)(κ 2 -C 3 H 3 O 2 )(PPh 3 ) and (Triphos)Mo(H)(κ 2 -C 3 H 5 O 2 )(PPh 3 ), was achieved by sequential addition of olefin and heterocumulene to (Triphos)MoH 4 PPh 3 . A formally zerovalent TriphosMo(η 2 -C 2 H 4 ) 3 intermediate was characterized by NMR spectroscopy and computational analysis along the pathway for carbon dioxide–ethylene coupling.