A solution study of the protonation and deprotonation equilibria of 5,10,15,20-tetra(p-sulphonatophenyl)porphyrin. Stability constants of its magnesium(II), copper(II) and zinc(II) complexes

Abstract

The protonation and deprotonation equilibria of 5,10,15,20-tetra(p-sulphonatophenyl)porphyrin, H₂tpps^4–, as well as its complex formation with Mg²⁺, Zn²⁺ and Cu²⁺ metal ions have been investigated spectrophotometrically in a dimethyl sulphoxide (dmso)–water (80:20, v/v) solution. The following equilibrium constants have been determined at 25 °C and 0.1 mol dm^–3(KClO₄+ KOH)(i) tpps^6–+ 2H⁺⇌H₂tpps^4– log K= 32.80 ± 0.04 (ii) H₂tpps^4–+ 2H⁺⇌ H₄tpps^2– log K= 3.58 ± 0.02(iii)H₂tpps^4–+ Mg²⁺⇌[Mg(tpps)]^4–+ 2H⁺ log K=–3.96 ± 0.04 (iv) H₄tpps^2–+ Zn²⁺⇌[Zn(tpps)]^4–+ 4H⁺ log K=–1.80 ± 0.04(v) H₄tpps^2–+ Cu²⁺⇌[Cu(tpps)]^4–+ 4H⁺ log K= 1.68 ± 0.04 [(i)] 0.1 mol dm^–3(KClO₄+ HClO₄)[(ii), (iv), (v)] and 0.1 mol dm^–3 KClO₄[(iii)], affording for the first time the stability constants of three metalloporphyrins, [Mg(tpps)]^4–(log β= 28.8), [Zn(tpps)]^4–(log β= 34.6) and [Cu(tpps)]^4–(log β= 38.1). A Novel and more effective procedure for the isolation of high-purity Na₄[H₂tpps]·4H₂O is reported.