A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands

We report a new mononuclear molybdenum(iv) complex, MoOL(BIS)L(SQ), in which L(SQ) (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, L(BIS), and MoO(2)(acac)(2). The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOL(BIS)L(SQ) revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of L(BIS) and two O atoms of L(SQ). The effective magnetic moment (μ(eff)) of MoOL(BIS)L(SQ) decreased from 2.36 to 0.2 μ(B) in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOL(BIS)L(SQ) was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.