Terminal Mo Carbide and Carbyne Reactivity: H2 Cleavage, B–C Bond Activation, and C–C Coupling

Abstract

Transition-metal carbides have been posited as intermediates in the upgrading of C1 feedstocks, including the Fischer–Tropsch synthesis of higher olefins from CO and H2. Still, molecular examples remain rare, and their reactivity is poorly understood. In a molecular platform supported by a flexible terphenyl diphosphine ligand, the important C–C coupling step was previously demonstrated on the reaction of Mo carbide and carbene species with CO. Methylidyne and methylidene complexes were accessible on sequential treatment of the carbide with sources of H+ and H–, surrogates for heterolyzed H2. Herein, we demonstrate that the terminal carbide complex is also capable of directly activating H2, yielding a P–C-bonded ylide complex. The carbide moiety also inserts into the B–C bond of triphenylborane, yielding an unusual example of a borylcarbene with a direct Mo–B contact. C–C coupling from the terminal methylidyne, a potential intermediate formed on heterolytic H2 cleavage and H+ transfer to the carbide, yields a Mo ketenyl complex, thus giving information for the first time on the relative rates of coupling from carbide, carbyne, and carbene species in the same molecular platform.