An Iminosemiquinone-Coordinated Oxidovanadium(V) Complex: A Combined Experimental and Computational Study

Ligand H(4)Sar((AP/AP)) contained two terminal amidophenolate units that were connected by a disulfane bridge. The ligand reacted with VOSO4 center dot 5H(2)O in the presence of Et3N under air and provided a mononuclear octahedral oxidovanadium complex (1). X-ray crystal structure analysis of complex 1 revealed that the oxidation state of the V ion was V and the VO3+ unit was coordinated to an iminosemiquinone radical anion. An isotopic signal at g = 1.998 in the X-band electron paramagnetic resonance (EPR) spectrum and the solution magnetic moment mu(eff) = 1.98 mu(B) at 298 K also supported the composition. The formation of complex 1 preceded through the initial generation of a diamagnetic VO2+-iminoisemiquinone species, as established by time-dependent UV-vis-near-IR (NIR), X-band EPR, and density functional theory studies. The UV-vis-NIR spectrum of complex 1 consisted of four ligand-to-metal charge-transfer transitions in the visible region, while an intervalence ligand-to-ligand charge transfer appeared at 1162 rim. The cyclic voltammogram of the complex showed four oxidation waves and one reduction wave. Spectroelectrochemical studies at fixed potentials revealed that the oxidation and reduction processes were ligand-based.