Kinetics of complex formation between zinc meso-tetraphenylporphyrin and some nitrogen bases in aprotic solvents, studied by an improved microwave temperature-jump method

Abstract

Rate and equilibrium constants have been determined over a range of temperature for the reactions of zinc meso-tetraphenylporphyrin with pyridine (in chlorobenzene, 1-chlorobutane and toluene), with 2-methylpyridine (in 1-chlorobutane), and with imidazole (in 1-chlorobutane). Rates were measured by means of an improved microwave temperature-jump apparatus incorporating signal-averaging, and at low temperatures by the stopped-flow method, with spectrophotometric detection. The forward rate constants are lower by about an order of magnitude than the values calculated for diffusion control from the simple Smoluchowski model. A two-step mechanism appears likely. Solvent and steric effects on the rate are discussed.