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

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 outersphere 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: (delta)H = 14.4 (plus-minus) 1.3 kJ mol^-1 at 20.5 MPa, (delta)S = 133 (plus-minus) 4 J mol^-1 K^-1 at 20.5 MPa and (delta)V = 15 (plus-minus) 1 cm^3 mol^-1 at 333.3 K for the pre-equilibrium; (delta)Hnot equal) = 110 (plus-minus) 5 kJ mol^-1 at 20.5 MPa, (delta)Snot equal) = 22 (plus-minus) 16 J mol^-1 K-1 at 20.5 MPa, and (delta)Vnot equal) = 81 (plus-minus) 8 cm^3 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.