Synthesis, thermal and spectral studies of oxoperoxo and dioxo complexes of vanadium(V), molybdenum(VI) and tungsten(VI) with 2-(alpha-hydroxyalkyl/aryl)benzimidazole

Reaction of 2-(alpha-hydroxymethyl)benzimidazole or 2-(alpha-hydroxyethyl)b enzimidazole (LH) with the peroxovanadium(V) species, generated in situ by stirring V2O5, KOH and 30% aqueous H2O2, gives the corresponding complexes of formula K[VO(O-2)L-2]. Similar peroxo species of molybdenum and tungsten generated by stirring MoO3 or WO3. H2O with an excess of 30% aqueous H2O2 readily react with 2-(alpha-hydroxyethyl) benzimidazole in aqueous EtOH to give the peroxo complexes [MO(O-2)L-2] (M=Mo or W). The dioxo complexes of general formula [MO2L2] have also been isolated by the reaction of [MoO2(ac ac)(2)] or [WO2- (acac)(2)] (acacH=acetylacetone) with the above ligands an d with 2-(alpha-hydroxybenzyl)benzimidazole. The dioxo complexes are white, whereas peroxo complexes are light yellow to orange. The peroxo complexes generally decompose in two steps: (i) the decomposition of the peroxo group and (ii) the decomposition of the alkyl/aryl group followed by decompositi on of the complete ligand. On the other hand, decomposition of the dioxo co mplexes follows only in a later step. All the peroxo complexes exhibit thre e i.r. active vibrational modes at ca. 860 cm(-1), 760 cm(-1) and 600 cm(-1 ), characteristic of the eta(2)-coordinated peroxo group. The dioxo complex es are dominated by the presence of two sharp bands in the 900 cm(-1) regio n due to nu(sym)(O=M=O) and nu(asym)(O=M=O) modes. The nu(C=N) (ring) and n u(OH) shifts have also been measured in order to locate the coordination si tes of the ligands. A broad band at ca. 400 nm in the peroxovanadium(V) com plexes, while the absorption at ca. 350 nm in the peroxomolybdenum(VI) and tungsten(VI) complexes is assigned to the peroxo-metal charge transfer band .