Transition metal(IV) complexes of nitro-substituted 2,4,9-triazaadamantane-based tris-hydroxylamines: evidence for nitrogen α-effect on stabilization of higher oxidation states

7-Nitro-2,4,9-triazaadamantane-2,4,9-triols (NO2-TRIADs), new chelating tris-hydroxylamine ligands, were synthesized by a novel one-pot, four-component condensation involving nitromethane and nitrosoalkene precursors. Upon deprotonation, NO2-TRIADs form stable diamantane-like complexes with manganese, iron and nickel in atypical +4 oxidation state. These complexes were characterized by XRD analysis, NMR, Mössbauer spectroscopy, UV-Vis, FT-IR, HRMS, and elemental analysis. Theoretical analysis based on DFT calculations revealed a remarkable electron donating “α-effect” of the nitrogen atom in the N–O–M fragment that is responsible for the stabilization of the higher oxidation state of the metal in these complexes. Metal(IV) complexes of NO2-TRIADs exhibit high catalytic activity in oxidation reactions, in particular, in the aerobic conversion of thiols into disulfides (up to 98% yield) and oxidation of activated C–H bonds in diphenylmethanol and dihydroanthracene with mCPBA (up to 74% yield). In the oxidation of dihydroanthracene, a notable selectivity toward the formation of anthraquinone was achieved (anthraquinone/anthrone = 7).