Kinetics and mechanism of cadmium(II) ion assisted incorporation of manganese(II) into 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrinate(4–)

Abstract

The kinetics of the reaction of 5,10,15,20-tetrakis(4-sulphonatophenyl)porphyrinate(H₂tspp) with manganese(II) were studied spectrophotometrically in the presence of cadmium(II) and in the range of pH 6–8.5 at 25 °C and at an ionic strength of 0.1 mol dm^–3(NaNO₃). The incorporation of manganese(II) into H₂tspp is catalytically assisted by cadmium(II) at concentrations as low as 10^–7 mol dm^–3. The rate equation for the reaction is expressed as –d[H₂tspp]/dt=k₁[Mn²⁺][H₂tspp]+k₂k₃[Mn²⁺][Cd²⁺][H₂tspp]/(k_–2[H⁺]²+k₃[Mn²⁺]), with k₁=(2.50 ± 0.32)× 10^–2 dm³ mol^–1 s^–1, k₂=(4.86 ± 0.16)× 10² dm³ mol^–1 s^–1, k_–2=(5.90 ± 0.17)× 10¹² dm⁶ mol^–2 s^–1, and k₃=(1.98 ± 0.10)× 10² dm³ mol^–1 s^–1. [Cd(tspp)] is formed as a reaction intermediate; it is 7 900 times as reactive as H₂tspp for the incorporation of manganese(II).