New oxo-bridged dinuclear peroxotungsten(VI) complexes: Synthesis, stability and activity in bromoperoxidation

Two new dinuclear oxo-bridged peroxo complexes of tungsten with coordinated dipeptides of the type, Na₂W₂O₃(O₂)₄(glycyl-glycine)₂] · 3H₂O (1) and Na₂W₂O₃(O₂)₄(glycyl-leucine)₂] · 3H₂O (2) have been synthesized from the reaction of H₂WO₄, 30% H₂O₂ and the respective dipeptide at pH ca. 2.5. Synthesis of the compounds, in addition to pH, is sensitive to reaction temperature and concentrations of the components. The compounds were characterized by elemental analysis, spectral and physico-chemical methods including thermal analysis. In the dimeric complexes the two W(VI) centres with edge bound peroxo groups are bridged by an oxo group. The dipeptides occurring as zwitterions bind the metal centers through O (carboxylate) atoms leading to hepta co-ordination around each W(VI). Thermal stability of the compounds as well as their stability in solution were determined. The compounds are highly stable toward decomposition in solutions of acidic as well as physiological pH. These compounds, besides another similar dimeric compound Na₂W₂O₃(O₂)₄(cystine)] · 4H₂O (3) efficiently oxidized bromide to a bromination competent intermediate in phosphate buffer at physiological pH, a reaction in which only two of the peroxide groups of the complex species were found to be active. The complexes could also mediate bromination of organic substrate in aqueous-organic media.