Thermochromism of Metal Exchange Reaction between Zinc(II) and Mercury(II) Porphyrins

Masaaki Tabata*, Masahiro Ide, Kentaro Kaneko Department of Chemistry, Faculty of Science and Engineering, Saga University, 1, Honjo-machi, Saga, 840 Japan Dedicated to Prof. Hitoshi Ohtaki on the occasion o f his 60th birthday Z. Naturforsch. 50b, 545-550 (1995); received October 26, 1994 Thermochromism, Metalloporphyrin, Equilibrium, Zinc(II) Porphyrin, Mercury(II) Porphyrin Thermochromism was observed for an aqueous solution containing zinc(II) and mercury(II) cations and N-/?-nitrobenzyl-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (NÖ2Bz(Htpps)4_) in the temperature range 10 to 70 °C. The equilibrium constants and the thermodynamic parameters of Zn (N02Bztpps)3~ and H g(N02Bztpps)3have been deter­ mined spectrophotometrically to elucidate the thermochromism at 10, 15, 20, 25 and 30 °C in 0.1 mol dm-3 NaN03. The protonation and metalation constants of N 0 2Bz(Htpps)4are defined as K2 = [H ^ H ^ ^ H P ]1, K3 = [H3P][H+]_1[H2P]_1 and K^p = [MP][H+][M2+] _1 [HP]-1, where HP and MP denote the free base form of the prophyrin and the metalloporphyrins of zinc(II) and mercury(II), respectively. Charges of the prophyrin and metalloporphyrins are omitted for simplicity. The following values were found: \ogK2 = 7.75 ±0.02 (25 °C), AH°/kJ mol“ 1 = -21.2±0.5 and J5°/Jmol-1K -1 = 77±1, logAT3 = 2.55±0.02 (25 °C), zl//°/kJmol_1 = -25±0.8 and zJS^Jmol^K-1 = -35±3 and log KXnP = 0.63±0.03 (25 °C), zl//°/kJmol_1 = 31.0±0.8 and ZlS^Jmol^K-1 = 116±3, logA:HgP = 6.22±0.03 (25 °C), AH°l kJmol“ 1 = 4.5±0.7 and zl5°/Jmol_1K _1 = 134±2. The distribution curve calculated from the thermodynamic parameters sufficiently agrees with the observed metal exchange reaction between the metalloporphyrins.