An unsymmetrical mixed-valence oxidovanadium(IV/V) binuclear complex: Synthesis, characterization, DFT studies, and bromoperoxidase activity

The mixed-valence binuclear complex [{VO(acac)(bpy)}(μ–O){VO(bpy)(ox)}]·2H2O (1), where acac− = acetylacetonate, bpy = 2,2-bipyridine and ox2− = oxalate, was synthesized under mild conditions and characterized by elemental analysis and infrared (IR), electron paramagnetic resonance (EPR), and electronic (UV/Vis) spectroscopies. According to single-crystal X-ray diffraction data, the two nonequivalent oxidovanadium centers of the {(VO)2(μ-O)}3+ core adopt an anti-angular configuration and differ in the O,O-donor ligands. Density functional theory (DFT) at the ωB97X-D/def2TZVP level was used to optimize the molecular structure and describe the electronic structure of 1 in the solid state, indicating a localized mixed-valence system. Continuous-wave EPR analysis at 77 K suggests the predominance of binuclear species in dmf and dmso solutions of 1 and 1:1 mixtures of these solvents with water. This finding is supported by comparing experimental and calculated rotational radius, g-tensor and vanadium hyperfine couplings. In the presence of KBr and H2O2, complex 1 acts as a pre-catalyst in a solvent-modulated bromination of phenol red to produce bromophenol blue. Kinetic studies in dmf/H2O indicate a first-order dependence on vanadium with a reaction rate constant of 703 mol−2 L2 s−1. The better performance of 1 as a pre-catalyst in dmf/H2O than in dmso/H2O may be due to a higher formation rate for the active oxidoperoxidovanadium(V) species.