A SOLUTION STUDY OF THE PROTONATION AND DEPROTONATION EQUILIBRIA OF 5,10,15,20-TETRA(PARA-SULPHONATOPHENYL)PORPHYRIN - STABILITY-CONSTANTS OF ITS MAGNESIUM(II), COPPER(II) AND ZINC(II) COMPLEXES

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-degrees-C and 0.1 mol dm-3 (KClO4 + KOH) (i) tpps6- + 2H+ reversible H2tpps4- log K = 32.80 +/- 0.04 (ii) H2tpps4- + 2H+ reversible H4tpps2- log K = 3.58 +/- 0.02 (iii) H2tpps4- + Mg2+ reversible [Mg(tpps)]4- + 2H+ log K = -3.96 +/- 0.04 (iv) H4tpps2- + Zn2+ reversible [Zn(tpps)]4- + 4H+ log K = -1.80 +/- 0.04 (v) H4tpps2- + Cu2+ reversible [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 beta = 28.8), [Zn(tpps)]4-(log beta = 34.6) and [Cu(tpps)]4- (log beta = 38.1). A novel and more effective procedure for the isolation of high-purity Na4[H2tpps].4H2O is reported.