Kinetics and Mechanism for the Metalation Reaction of 5,10,15,20-Tetrakis(pentafluorophenyl)porphyrin with Diaquabis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)nickel(II) in Supercritical Carbon Dioxide

Abstract

Abstract The nickel(II) ion incorporation reaction into 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (H2tpfpp) to form the [Ni(tpfpp)] complex has been kinetically investigated using diaquabis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)nickel(II), [Ni(hfac)2(H2O)2], in supercritical carbon dioxide (scCO2) medium in a spectrophotometric cell at various temperatures and pressures. The metalation reaction is first order with respect to porphyrin under the conditions where [Ni(hfac)2(H2O)2] is in a large excess relative to H2tpfpp. The saturation dependence was observed for the conditional first-order rate constants as a function of the mole fraction (xNi) of the excess [Ni(hfac)2(H2O)2]. Such dependence suggests that the metalation reaction consists of a fast pre-equilibrium of the formation of the outer-sphere association between [Ni(hfac)2(H2O)2] and H2tpfpp and a first-order rate-determining nickel(II) ion incorporation into the porphyrin core. The conditional first-order rate constants kobs are well expressed by kobs = KkxNi/(1 + KxNi), where K is the equilibrium constant for the formation of the outer-sphere association and k is the first-order rate constant for the nickel(II) ion incorporation. The thermodynamic and kinetic parameters are obtained as follows: ΔH° = 14.4 ± 1.3 kJ mol−1 at 20.5 MPa, ΔS° = 133 ± 4 J mol−1 K−1 at 20.5 MPa and ΔV° = 15 ± 1 cm3 mol−1 at 333.3 K for the pre-equilibrium; ΔH≠ = 110 ± 5 kJ mol−1 at 20.5 MPa, ΔS≠ = 22 ± 16 J mol−1 K−1 at 20.5 MPa, and ΔV≠ = 81 ± 8 cm3 mol−1 at 333.3 K for the rate-determining step. The positive change in enthalpy and volume and significant positive change in entropy strongly indicate that the outer-sphere association is derived from the desolvation of reactants in scCO2.