Reduction of Benzonitriles via Osmium–Azavinylidene Intermediates Bearing Nucleophilic and Electrophilic Centers

The reduction of the NC bond of benzonitriles promoted by OsH₆(PⁱPr₃)₂ (1) has been studied. Complex 1 releases a H₂ molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH₄{κ¹-N-(NCC₆H₃Me₂)}­(PⁱPr₃)₂ (2), which is thermally stable toward the insertion of the nitrile into one of the Os–H bonds. In contrast to 2,6-dimethylbenzonitrile, benzonitrile and 2-methylbenzonitrile undergo insertion, via Os­(η²-NCR) intermediates, to give the azavinylidene derivatives OsH₃(NCC₆H₄R)­(PⁱPr₃)₂ [R = H (3) or Me (4)]. The analysis by means of computational tools (EDA–NOCV) of the bonding situation in these compounds suggests that the donor–acceptor nature of the osmium azavinylidene bond dominates over the mixed electron-sharing/donor–acceptor and pure electron-sharing bonding modes. The N atom is strongly nucleophilic, whereas one of the hydrides is electrophilic. In spite of the different nature of these centers, the migration of the latter to the N atom is kinetically prevented. However, the use of water as a proton shuttle allows hydride migration, as a consequence of a significant decrease in the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH₃{κ²-N,C-(NHCHC₆H₃R)}­(PⁱPr₃)₂ [R = H (5) or Me (6)]. The presence of electrophilic and nucleophilic centers in 3 confers upon it the ability to activate σ-bonds, including H₂ and pinacolborane (HBpin). The reaction with the latter gives OsH₃{κ²-N,C-[N­(Bpin)CHC₆H₄]}­(PⁱPr₃)₂ (7).