Reaction mechanism of mercury(II) with protonated N-substituted porphyrin

Kinetic and equilibrium studies on the complexation of mercury(II) ion with N-p-nitrobenzyl-5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin, (NO(2)Bz(Htpps)(4-); Hp(4-)) were carried out. The equilibrium constant, defined as K-HgP = [HgP3-][H+][Hg2+](-1)[HP4-](-1), was determined spectrophotometrically in the pH range 2-3 and at temperatures of between 15 and 35 degrees C. The value of K-HgP was found to be 10(6.28+/-0.4); and Delta H degrees and Delta S degrees values were 4.5+/-0.7 kJmol(-1) and 134+/-2 Jmol(-1) K-1, respectively at 25 degrees C. The kinetic studies were carried out under a pseudo-first order condition in an aqueous medium at 10-35 degrees C, I=0.1(NaNO3), [HP4-] = 3 x 10(-6), [Hg(II)] = 4.74 x 10(-5) to 2.84x 10(-4) [acetate buffer] =5x10(-3) to 1x10(-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 = (k(1)[Hg(OH)(2)] + k(2)[Hg(oac)(2)])[H2P3-]. The k(1) and k(2) values were found to be (1.26+/-0.19)x 10(6) and (1.87+/-0.01)x10(6) mol(-1) dm(3) s(-1) (25.0 degrees C), and the activation enthalpies and entropyies were Delta H-double dagger = 24.8+/-3.8 kJ mol(-1) and Delta S-double dagger = -46+/-13 Jmol(-1) K-1 for the k(1), and Delta(double dagger) = 45.0+/-2.1 Jmol(-1), Delta S double dagger = 25.4+/-7.4 Jmol(-1) K-1 for the k(2). The kinetic data revealed that dihydroxomercury(II) directly reacts with the protonated porphyrin with a small activation enthalpy acid a negative activation entropy.