Dynamic recognition of metal ions by porphyrin and 18-crown-6

Abstract

Chemical properties of metalloporphyrins of zinc(II), cadmium(II) and lead(II) are significantly different in the presence of 18-crown-6 (18C6). The kinetics and mechanism of the ligand-substitution reaction of Zn-, Cd- and Pb(tpps)^4- (H₂tpps^4- = 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin) with ethylenediaminetetraacetic acid (H₄edta) has been studied to clarify the different reactivities in the presence of 18C6 at pH 8–9.5, 25°C and I = 0.1 mol dm⁻³ (LiNO₃). The rate equation for the decrease of M(tpps)^4- is expressed as -d[M(tpps)′]/dt = (k1[edta^4-] + k ₂[Hedta^3-])(1 + K ^ML[18C6])⁻¹[M(tpps)′], where M = Zn²⁺, Cd²⁺ and Pb²⁺, respectively, and [M(tpps)′] is the total concentration of metalloporphyrin. K ^ML denotes the formation constant defined by [M(tpps)^4-·18C6][M(tpps)^4-]⁻¹[18C6]⁻¹. The values of k ₁ were found to be 0 for all metalloporphyrins studied, and k ₂ = 6.4 ± 0.2 mol⁻¹ dm³ s⁻¹ and 2.9 ± 0.1 mol⁻¹ dm³s⁻¹ for Cd(tpps)^4- and Pb(tpps)^4-, respectively. The formation constants of K ^ML were 10^2.27 mol⁻¹ dm³ and 10^1.58 mol⁻¹ dm³ for Cd(tpps)^4- and Pb(tpps)^4-, respectively. The k ₂ and K _ML values for Zn(tpps)^4- were not determined because of the very slow reaction. 18-Crown-6 forms a stable intermediate of Cd(tpps)^4-·18C6 and suppressed the substitution reaction with EDTA. Hedta^3- directly attacks Pb(tpps)^4- to form Pb(edta)^2-, but cannot bind Zn(tpps)^4-. The interaction of the metalloporphyrins with 18C6 was also confirmed by ¹H-NMR and absorption spectra. The ¹NMR signal of 18C6 shifted to a higher field and the change in absorption spectra was larger for Cd(tpps)^4- than for Pb(tpps)^4-.