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, H2tpps4–, 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) tpps6–+ 2H+⇌H2tpps4– log K= 32.80 ± 0.04 (ii) H2tpps4–+ 2H+⇌ H4tpps2– log K= 3.58 ± 0.02(iii)H2tpps4–+ Mg2+⇌[Mg(tpps)]4–+ 2H+ log K=–3.96 ± 0.04 (iv) H4tpps2–+ Zn2+⇌[Zn(tpps)]4–+ 4H+ log K=–1.80 ± 0.04(v) H4tpps2–+ 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.