Reaction Mechanism of Mercury(II) with Protonated N-Substituted Porphyrin

Abstract

Abstract Kinetic and equilibrium studies on the complexation of mercury(II) ion with N-p-nitrobenzyl-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, (NO2Bz(Htpps)4−; HP4−) were carried out. The equilibrium constant, defined as KHgP = [HgP3−][H+][Hg2+]−1[HP4−]−1, was determined spectrophotometrically in the pH range 2—3 and at temperatures of between 15 and 35 °C. The value of KHgP was found to be 106.28 ± 0.04; and ΔH° and ΔS° values were 4.5 ± 0.7 kJ mol−1 and 134 ± 2 J mol−1 K−1, respectively at 25 °C. The kinetic studies were carried out under a pseudo-first order condition in an aqueous medium at 10—35 °C, I = 0.1(NaNO3), [HP4−] = 3 × 10−6, [Hg(II)] = 4.74 × 10−5 to 2.84 × 10−4, [acetate buffer] = 5 × 10−3 to 1 × 10−1 mol dm−3, and pH 4.2 to 5.6. The observed rate constant of the reaction was dependent of the total concentrations of mercury(II) and acetate buffer, but independent of the pH and ionic strength. The overall reaction was first order with respect to the total [Hg(II)] and [HP4−]. The rate expression of the reaction is d[HgP3−]/dt = (k1[Hg(OH)2] + k2[Hg(oac)2])[H2P3−]. The k1 and k2 values were found to be (1.26 ± 0.19) × 106 and (1.87 ± 0.01) × 106 mol−1 dm3 s−1 (25.0 °C), and the activation enthalpies and entropyies were ΔH‡ = 24.8 ± 3.8 kJ mol−1and ΔS‡ = −46 ± 13 J mol−1 K−1 for the k1, and ΔH‡ = 45.0 ± 2.1 kJ mol−1, ΔS‡ = 25.4 ± 7.4 J mol−1 K−1 for the k2. The kinetic data revealed that dihydroxomercury(II) directly reacts with the protonated porphyrin with a small activation enthalpy and a negative activation entropy.