Redox–state Dependent Activation of Silanes and Ammonia with Reverse Polarity (PCcarbeneP)Ni Complexes: Electrophilic vs. Nucleophilic Carbenes

A rigidified PC_alkylP ligand allowed for the synthesis and charactarization of cationic and radical PC_CarbeneP nickel complexes in which the carbene anchor of the pincer framework is electrophilic rather than nucleophilic. Alpha-hydride abstraction from (PC_alkylP)nickel halide complex readily leads to the cationic carbene complex, which furnishes the radical carbene complex by one electron reduction. The reactivity of these reverse polarity carbene complexes towards small molecules (H₂, CO, CO₂, R₃SiH, NH₃) reveals different modes of activation when compared to previously reported nucleophilic nickel carbene complexes, and a clear dependence on the redox state of the complex. For H₂, CO and CO₂, no reaction is observed, but silanes react via hydride transfer and formation of solvated silylium ions. Ammonia is activated in a novel way, wherein it coordinates the carbene carbon and is deprotonated to form a robust C-N bond. This is not only a rare example of ammonia activation by a first row transition metal but also evidence of the intermediacy of group 10 carbenes in direct C-N bond forming reactions.