On the peroxomolybdate complexes as sources of singlet oxygen

Abstract

Peroxidation of molybdenum(VI) demonstrates that mono-, di- and tetraperoxo complexes are formed only in low quantities, triperoxomolybdate predominanting. Triperoxomolybdate may react directly with reactive reductants, whereas less reactive reductants are oxidized by indirectly formed 1 O 2 . The successive formation of peroxomolybdenum(VI) complexes is discussed and it is shown that proton-consuming steps are followed by a proton-producing one; the pH maximum (=10) appears at a [H 2 O 2 ]/[MoO 4 ] 2− ratio of 2:1. The approximated formation constants are utilized to calculate the concentrations of peroxo complexes as a function of the ligand concentration. The mono-, di- and tetraperoxo complexes are formed in only negligible quantities, whereas the oxodiperoxohydroperoxo (simply, triperoxo) complex proves to be the predominant species. The triperoxo complex may react directly if a sufficiently reactive partner is present; if not, it oxidizes H 2 O 2 to 1 O 2 , which reacts further. The activation parameters of formation of O 2 are determined as a function of pH: the enthalpy minimum (50.45 kJ mol −1 ) is found at pH 10, where the negative activation entropy reaches its lowest value (−149.5 J K −1 mol −1 ). The possible functions of the peroxo ligands are considered.