Coordination of Aza Heterocycles with Macroheterocyclic Metal Complexes in Amphiprotic Media: II. Kinetics of Outer-Sphere Ligand Exchange in Chromium(III) Tetrphenylporphyrinate

Abstract

Rate constants and activation parameters for substitution of alcohols with aza heterocycles in the chromium(III) (acetate)tetraphenylporphyrinate complex were calculated. The nature of the leaving ligand significantly affects the activation parameters. Increased strength of the H bond in the outer coordination sphere of the macrocyclic complex decreases the substitution rate constant. The logarithms of the reaction rate constants linearly vary with heteroring basicity. An SN2 mechanism and an appreciable effect of the heteroring on the transition state are suggested.Porphyrin complexes of transition metals, such as iron(II), iron(III), chromium(III), and cobalt(III), are widely used for modeling natural biocoordination systems. Chromium(III) prophyrin complexes can be considered as the simplest analogs of natural porphyrins, in view of their structural and electronic similarity to iron-containing metal porphyrins, as well as the stability of the oxidation degree of chromium in axial coordination processes.According to our previous 1 H NMR and electronic absorption data [2], imidazole coordinates to chromium(III) (acetate)tetraphenylporphyrinate (AcO) . CrTPP in ethanol by way of consecutive substitution of two alcohol molecules in the sixth coordination position of the complex. The electron density redistributes from the second alcohol molecule (from the inner coordination sphere) to the metal cation, which results in appreciable strengthening of the Cr3 alcohol coordination bond, thus stabilizing outersphere associate A and decelerating substitution of the first alcohol molecule (from the outer coordination sphere) to form heterocycle3ethanol H complex B.