Kinetics and Mechanism for the Formation and Dissociation Reactions of 21-(4-Nitrobenzyl)-5,10,15,20-tetrakis(4-sulfonatophenyl)-23H- porphyrinatozinc(II) and -cadmium(II)

Article abstract of DOI:10.1246/bcsj.70.1353

The incorporation reaction of zinc(II) and cadmium(II) into 21-(4-nitrobenzyl)-5,10,15,20-tetrakis(4-sulfonatophen- yl)-23H-porphyrin (NO₂Bz(Htpps)^4-; HP^4-) and the dissociation reaction of the metalloporphyrins ([MP]^3-) have been studied spectrophotometrically at I = 0.1 mol dm^-3 (NaNO₃). The formation rate of the metalloporphyrins is expressed by the following equation: d[MP^3-]/dt = k_M^f[M²⁺][HP^4-]+k_MOH ^f[MOH⁺][HP^4-]-k_MP ^d[MP^3-][H⁺]. The rate constants and the activation parameters of the reaction were found to be k_Zn^f = (4.86±0.06)× 10² mol^-1 dm³ s^-1 (25 °C), ΔH? = 62.1±1.6 kJ mol^-1, and ΔS? = 14.8±5.3 J mol^-1 K^-1; k_ZnOH^f = (6.39±0.14)×10³ mol^-1 dm³ s^-1 (25 °C), ΔH?=22.8±0.9 kJ mol^-1,and ΔS? = -90.2±3.1 J mol^-1 K^-1; k_ZnP^d = 122.3±2.3 mol^-1 dm³ s^-1 (25 °C), ΔH? = 19.8±2.2 kJ mol^-1, and ΔS? = -139±18 J mol^-1 K^-1 for zinc(II) and k_Cd^f = (1.17±0.02)×10⁵mol^-1 dm³ s^-1 (25 °C), ΔH? =42.5±1.3 kJ mol^-1, and ΔS? = -5.3±4.2 J mol^-1 K^-1; k_CdP^d = (2.51±0.04)×10⁷ mol^-1 dm ³ S^-1 (25 °C), ΔH? = 14.6±0.5 kJ mol^-1, and ΔS? = -54.3±1.7 J mol^-1 K^-1 for cadmium(II). Nitrobenzyl group lowers the basicity of porphyrin. The effect of the lowered basicity appeared strongly in the following order for the formation rate and equilibrium constants of the metalloporphyrins: Cd²⁺ < Zn²⁺ < [ZnOH]⁺. A hydrogen-bond formation between [ZnOH]⁺ and the pyrrole proton in NO₂Bz(Htpps)^4- is proposed for the reaction based on the larger rate constant than the water-exchange rate constant enhanced by the hydroxide ion bound to zinc(II), the small activation enthalpy, the negative activation entropy, and the dependence of porphyrin basicity.