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

Abstract

The reduction of the N-C triple bond of benzonitriles promoted by OsH6(P i Pr3)2 (1) has been studied. Complex 1 releases a H2 molecule and coordinates 2,6-dimethylbenzonitrile to afford the tetrahydride OsH4{κ 1-N-(NCC6H3Me2)}(P i Pr3)2 (2), which is thermally stable towards 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(η 2-NCR) intermediates, to give the azavinylidene derivatives OsH3(=N=CC6H4R)(P i Pr3)2 (R = H (3), 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 the hydride migration, as a consequence of a significant decrease of the activation barrier. The resulting phenylaldimine intermediates evolve by means of orthometalation to give OsH3{κ 2-N,C-(NH=CHC6H3R)}(P i Pr3)2 (R = H (5), Me (6)). The presence of electrophilic and nucleophilic centers in 3 confers it ability to activate σ-bonds, including H2 and pinacolborane (HBpin). The reaction with the latter gives OsH3{κ 2-N,C-[N(Bpin)=CHC6H4]}(P i Pr3)2 (7). Supporting Information. The following files are available free of charge. General information, crystallographic data, computational details, NMR and IR spectra (PDF) Cartesian coordinates of calculated structures (XYZ) Accession Codes CCDC 1907065-1907067 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_

Osmium-Promoted Transformation of Alkyl Nitriles to Secondary Aliphatic Amines: Scope and Mechanism

“…Spectroscopic features of 2 are: a triplet ( 2 JH-P = 13.2 Hz) at -9.98 pmm due to the hydride ligands, in the 1 H NMR spectrum, which are involved in a thermally activated position exchange process, in agreement with that previously observed for the related compound OsH4{κ 1 -N-[NC(2,6-C6H3Me2)]}(P i Pr3)2 34 and a singlet at 43.1 ppm in the 31 P{ 1 H} NMR spectrum. In contrast to 2, the hydride ligands of 3 give rise to three resonances at -9.86, -11.57, and -13.56 ppm in the 1 H NMR spectrum, whereas the 31 P{ 1 H} NMR spectrum displays a singlet at 37.3 ppm.…”

“…The structure (Figure 1), which confirms the trihydrideazavinylidene nature of the molecule, displays CS symmetry with trans-phosphines (P(1)-Os-P(2) = 173.31(3)º), as expected for a six-coordinate d 4 OsH3XL2 species. 34,37 The reaction of 1 with 2-methoxyacetonitrile shows significant differences with regard to that with pivalonitrile, which are consistent with a faster hydrogenation of the nitrile and a higher tendency to undergo the C(sp)-C(sp 3 ) bond rupture. It was performed in closed NMR tubes and was followed by 1 H and 31 P{ 1 H} NMR spectroscopy at 50 and 80 ºC (Figures S69-S72).…”

“…41 Previous DFT calculations on the formation of the phenylazavinylidene derivative OsH3(=N=CHPh)(P i Pr3)2 revealed a much higher activation energy for the 1,3-hydrogen shift than for the 1,2hydrogen migration (64.4 versus 17.2 kcal mol -1 ) even though the intermediate OsH4(η 2 -N≡CPh)(P i Pr3)2 is 14.9 kcal mol -1 less stable than the species OsH4(κ 1 -N≡CPh)(P i Pr3)2. 34 The replacement of the phenyl substituent of the nitrile by an ethyl group modifies the situation. Although the relative stability of the κ 1 -N and η 2 -C≡N forms does not significantly change, the activation energy for the 1,3-hydrogen shift dramatically decreases with regard to that reported for the aromatic nitrile.…”

Osmium- and Iridium-Promoted C–H Bond Activation of 2,2′-Bipyridines and Related Heterocycles: Kinetic and Thermodynamic Preferences

“…The resulting unsaturated OsH 4 (P i Pr 3 ) 2 (A) species has been trapped with pyridines 20 and 2,6-dimethylbenzonitrile. 21 The interaction between the coordinated C−H bond and the metal center of A involves σ-donation from the σ-orbital of the C−H bond to empty orbitals of the metal and back bonding from the metal to the σ*(C−H) orbital. 22 The d 4 -ion center is scarcely basic enough to provide back-donation.…”

Sigma-bond activation reactions induced by unsaturated Os(IV)-hydride complexes

“…However, this situation is markedly different in 140; for it, the mixed electronsharing/donor-acceptor bonding better describes the Os-N interaction. 77 In this context, it should be mentioned that quantum chemical calculations using DFT at the BP86- ∆E orb values similar to those of neutral fragments in their electronic doublet state. 78 For 139 and 140, a pure electron-sharing bonding with the fragments in the quartet state afforded the highest ∆E orb and was therefore discarded.…”