Electronic Effects in Transition Metal Porphyrins. 2. The Sensitivity of Redox and Ligand Addition Reactions in Para-Substituted Tetraphenylporphyrin Complexes of Cobalt(II)

Abstract

Electrochemical oxidation, by cyclic voltammetry, of a series of CoII(p-X)TPP complexes yields first [CoIII-(p-X)TPP]+, followed by the π cation radical [CoIII(p-X)TPP]2+ and then the dication [CoIII(p-X)TPP]3+, in discrete reversible one-electron transfer steps. Reduction of the same series of compounds yields first [CoI(p-X)TPP]−, followed by the π anion radical [CoI(p-X)TPP]2−, again in discrete one-electron steps. In each case, a Hammett relationship is observed between the reversible E1/2 values and the σ constants for the four substituents, X, on the four phenyl rings of [Co(p-X)TPP]n±. Ring oxidations or reductions are considerably more sensitive to substituents than are metal oxidation or reduction. In addition, results of an investigation in five solvent systems indicate that the Hammett reaction constant for CoII(p-X)TPP metal reduction is less sensitive to solvent effects than is that for CoII(p-X)TPP metal oxidation, indicating that axial ligation is more important in the Co(II)/Co(III) system than in the Co(II)/Co(I) system. E1/2 values for the CoII(p-X)TPP reduction are less negative than those for Schiffs' base B12 model compounds and for vitamin B12 itself, but more negative than those for a series of cobalt(II) tetradehydrocorrins. The equilibrium constants for addition of one molecule of Lewis base (electron pair donor) to [CoII(p-X)TPP]0 in toluene solution also follow a Hammett relationship, with p = 0.168 (pyridine) and p = 0.145 (piperidine) at 25 °C. Equilibrium constants for addition of molecular oxygen to CoII(p-X)TPP·py have been measured in toluene solution as a function of temperature. Again, a Hammett relationship was observed, and p was temperature dependent. The extrapolated value of p at 20 °C is −0.056 for addition of O2 to CoII(p-X)TPP to form a dioxygen complex. In comparison, p for oxidation of CoII(p-X)TPP (calculated from ΔGox) to yield [CoIII(p-X)TPP]+ is −0.31 (Me2SO) to −0.79 (benzonitrile). The derived value of the dissociation constant for the reaction CoTPP·b·O2 ⇄ CoTPP·B + O2− is estimated to be in the range of 10−11.9 to 10−16.7M−1. © 1976, American Chemical Society. All rights reserved.