Linear free energy relationships between reaction rate constants and equilibrium constants of complex compounds: III. Kinetics and mechanisms of ternary complex formation between (5-X-1, 10-phenanthroline)copper(II) and threonine

Abstract

The kinetics of ternary complex formation involving Cu(5-X-1, 10-phen) and threonine (CuAL, A=5-X-1, 10-phen; L=threonine or represented by O-N; X=NO2, Cl, H, CH3) has been studied by temperature-jump and stopped-flow methods. The formation rate constants, kf(M−1·s−1), for the complexation reaction, CuA + L CuAL, are as follows; X=NO2, 8.68×108; X = Cl, 7.13×108; X=H, 6.12×108; X=CH3, 5.42×108. The rate constants for zwitterion attack are nil within experimental error. It has been found that a linear free energy relationship exists between the stability (logKCuACuAL) of the complexes CuAL and log kf as follows: logKCuACuAL = 0.13+0.83 logkf, r = 0.99. It suggested that the formation rate governed the stability of the ternary complexes. The rates of formation of the ternary complexes increased with decreasing electron-donating property of the substituents. A linear relationship was found to exist as expressed by the following equation: log(kRf/KOf = 0.097σ, r = 0.96. A mechanism involves a rapid equilibrium between CuA and L followed by a slow ring closure of L.