Demetallation of α, β, γ, δ-tetra(p-sulfophenyl)porphineiron(III) in sulfuric acid-ethanol-water media

The rate of demetallation of α, β, γ,δ-tetra(p-sulfophenyl)porphineiron (III), Fe(TPPS)^3-, was determined in sulfuric acid-ethanol-water media for 8.5-10.65M sulfuric acid at different temperatures. The overall reaction was the conversion of the complex Fe(TPPS)^3- into the diacid species H₄TPPS^2- without other spectrophotometrically important species being formed to an appreciable extent, as shown by three isosbestic points at 418, 462, and 563 nm. The rate was first order in the Fe(TPPS)^3- concentration. The pseudo-first-order rate constants k were exponentially dependent on the sulfuric acid concentration, and log k was linearly dependent on the Hammett acidity function –H₀. The average ΔH? and ΔS? values for five reaction media were 18.4 ± 1.4 kcal/mol and 19 ± 3 cal/°K · mol, respectively. The linear relationship between log k and (-H₀) and the approximately constant values of ΔH? ΔS? over the acid range investigated indicated that the same mechanism of demetallation was operative over this acid range. Because of the dependence of the pseudo-first-order rate constants on the acidity of the medium, the mechanism probably involves the addition of protons to pyrrole N atoms to assist in the breaking of iron (III)-nitrogen bonds.