Complex formation and kinetics of phosphodiester cleavage in the hydrogen peroxide–lanthanide(III) system

Potentiometric and spectrophotometric titrations of hydrogen peroxide in the presence of La(III), Nd(III), Pr(III) and Eu(III) cations reveal formation of several types of di-, tri- and tetranuclear species. Complexes M2(O2)2(OH)2 or the respective tetranuclear complexes M4(O2)4(OH)4 as well as their monoprotonated forms are observed for all cations studied. In addition, a complex M2(O2)3 is observed with M=La and a complex M3(O2)2(OH)4 + is observed with M=Eu. Kinetics of the cleavage of bis(4-nitrophenyl) phosphate in the presence of Ln(III) and H2O2 was studied at 25 °C in the pH range 6–8. At metal concentrations below approximately 3 mM the kinetic order in metal is higher than unity, but at larger metal concentrations the reaction becomes zero-order in metal. The reaction order at low metal concentrations decreases gradually from 4 to 1.5 within the lanthanide series on going from La(III) to Eu(III). Comparisons of pH-rate profiles for all cations studied with the respective species distribution diagrams confirm high reaction orders at low metal concentrations and indicate that for each metal the protonated forms of the complexes are less reactive than the neutral species. A reaction scheme, which agrees both with concentration and pH effects on the reaction rate, is proposed, which involves the substrate-induced aggregation of di- or tetranuclear peroxide complexes affording hexa- or octanuclear active species.