Oxovanadium(IV) and -(V) Complexes of Dithiocarbazate-Based Tridentate Schiff Base Ligands: Syntheses, Structure, and Photochemical Reactivity of Compounds Involving Imidazole Derivatives as Coligands

The tridentate dithiocarbazate-based Schiff base ligands H2L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate, R = NO2, L = L2; R = Br, L = L3) react with [VO(acac)2] in the presence of imidazole derivatives as coligands to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the products are oxovanadium(IV) complexes, viz. [VOL3(BzIm)]·0.5CH3CN (1a) and [VOL(N-MeIm)2] (L = L3, 1b; L = L2, 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo group (in the “equatorial” plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand, the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO2L(R‘-ImH)] (L = L3 and L2, R‘ = H and Me, 2a − d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray crystallographic investigations of 2a,d, involving an alternating array of LVO2 - species and the imidazolium counterions held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a − d are stable in water or methanol. In aprotic solvents, viz. CH3CN, DMF, or DMSO, however, they undergo photochemical transformation when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic 1H NMR experiments.