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).