Reactivity of immobilized cobalt phthalocyanines for the electroreduction of molecular oxygen in terms of molecular hardness

When the logarithm of the rate constant of the electroreduction of O2 is plotted versus the Co(III)/Co(II) redox potential of different substituted phthalocyanines (Co–Pcs), for graphite electrodes modified with these complexes, a straight line is obtained. Log k decreases as the driving force of the phthalocyanine increases. The same is observed when log k is plotted versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ligand. This is explained in terms of the hardness of the system, i.e. the more the separation between the energy of the frontier orbitals of the donor (Co–Pc) and the acceptor (O2), the less the reactivity. The energies of the frontier orbitals of cobalt tetraneopentoxyphthalocyanine, cobalt octamethoxyphthalocyanine, cobalt phthalocyanine, cobalt tetrasulfophthalocyanine and cobalt hexadecafluorophthalocyanine, and of the O2 molecule were calculated using the PM3 semi-empirical theoretical method. The intermolecular hardness of the Co–Pc/O2 system was estimated by taking one half the energy difference of the SOMO of the Co–Pc and the SOMO of the dioxygen molecule. A plot of log k versus the intermolecular hardness gives a straight line of negative slope, which shows that the rate constants decrease with increasing intermolecular hardness of the system.