Oxidative ortho-C-N fusion of aniline by OsO₄. Isolation, characterization of oxo-amido osmium(VI) complexes, and their catalytic activities for oxidative C-C bond cleavage of unsaturated hydrocarbons

Abstract

In an unusual reaction of osmium(VIII) oxide with p-substituted aromatic amines (X-C₆H₄-NH₂, where X = Me, H, Cl) in heptane afforded the brown osmium(VI)-oxo complexes [OsO(L)₂] (1a-c, L = N-aryl-1,2-arylenediamide) in moderate yields. The ligand L is formed in situ via oxidative ortho-C-N fusion of arylamines. The reaction occurs in an inert atmosphere, and a part of Os(VIII) is used up for the oxidation of aromatic amine. Single crystal X-ray structure of a representative complex 1a is solved. The structural analysis has authenticated the ortho-C-N fusion of ArNH₂ resulting in formation of the diamide ligand, L. The complex as a whole is penta-coordinated, and the coordination sphere has a distorted square pyramidal geometry (τ = 0.26). A similar reaction of osmium(VIII) oxide with the preformed N-phenyl-1,2-phenelene diamine produced the complex 1a in nearly quantitative yield. The substituted phenazine, 5-phenyl-3-phenylimino-3,5-dihydro-phenazine-2-ylamine, is obtained as a byproduct of the latter reaction. The complexes, 1a-c, can be reduced in a reversible one-electron step, as probed by cyclic voltammetry. The one electron reduced paramagnetic Os(V) intermediate is, however, Electron Paramagnetic Resonance (EPR) silent. Solution spectra of the osmium complexes show several multiple transitions in the UV-vis region. Density functional theory calculations were employed to confirm the structural features and to support the spectroscopic assignments. The complex 1a catalyzes oxidation of a wide variety of unsaturated hydrocarbons like alkenes, alkynes, and aldehydes to the corresponding carboxylic acids in the presence of tert-butylhydroperoxide (TBHP) efficiently at room temperature.