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).