Kinetic and equilibrium studies of the protonation of meso-tetraphenylporphyrin in dimethyl sulphoxide–water

Abstract

The equilibrium between meso-tetraphenylporphyrin (B) and its diprotonated form (BH₂²⁺) has been studied spectrophotometrically in 98, 90, and 80% Me₂SO–H₂O (v/v). Visible spectra of solutions of the porphyrin containing varying concentrations of hydrochloric acid show good isosbestic points. The equilibrium involves the free base and the diprotonated species; the monoprotonated porphyrin (BH⁺) is not present in sufficiently high concentrations to be detected. The position of equilibrium and the rates of equilibration between B and BH₂²⁺ have been measured in 95, 90, and 85% Me₂SO–H₂O (v/v). The reaction is slow, with relaxation times (measured by the temperature-jump method) in the range 40–5 000 µs. Values of the rate coefficient for proton removal by water from BH₂²⁺ to give BH+ were obtained for each solvent mixture from the acid dependence of the reciprocal relaxation time. In comparison with other proton transfers from nitrogen acids, proton transfer from the diprotonated form of meso-tetraphenylporphyrin is exceptionally slow. This may arise because the reaction is strongly disfavoured thermodynamically; other possible factors are discussed.