SPECTROSCOPIC AND ELECTROCHEMICAL STUDIES OF THE OXIDATION OF PARASUBSTITUTED TETRAPHENYLPORPHINATOMANGANESE(II) COMPLEXES

The reaction of dioxygen with a series of para-substituted meso-tetrap henylporphinatomanganese (II) complexes, Mn-II(T (p-x) PP) ((n)BuNH(2) ) where x = H, CH3, Cl, OCH3 in chloroform was studied. Oxidation rate constants were determined at room temperature using spectrophotometri c methods. Fitting the observed values for the autoxidation rate const ants to the Hammett equation (logk(b) vs 4 sigma) gave a Hammett rho(b ) value of -0.280. The effect of substitutents upon the pseudo-first-o rder rate constants is discussed. In comparison, the half-wave potenti als (E(1/2)) for the metal centred redox reaction, [(Mn-II(T(p-x)PP) ( BuNH(2))] reversible arrow [Mn-III(T(p-x)PP) (BuNH(2))](+) + e, were i nvestigated by cyclic voltammetry. A relationship between E(1/2) and t he Hammett constants was found.