Donor–acceptor intermolecular hardness on charge transfer reactions of substituted cobalt phthalocyanines

We present a theoretical study of the donor–acceptor intermolecular hardness (ηDA) associated with the interaction between substituted cobalt phthalocyanines (CoPc(X)n) and donors such as molecular oxygen (O2) or acceptor species such as 2-mercaptoethanol (OHCH2CH2SH), hydrazine (NH2NH2) and hydroxylamine (NH2OH). Semiempirical (PM3) and ab initio (ROHF/CEP-31G and ROHF/6-31G) theoretical methods were used to determine ηDA for the four charge transfer interactions. Theoretical results of ηDA correlate well with rate constants for electro-oxidation of 2-mercaptoethanol, hydrazine and hydroxylamine and for electro-reduction of O2 for processes occurring on phthalocyanines confined on a graphite electrode. The lower ηDA the higher is the reactivity. We also determined the molecular hardness (η), as defined in density functional theory, of CoPc(X)n. Linear correlations obtained between ηDA and η of CoPc(X)n for each of the charge transfer reactions studied indicate that the donor–acceptor intermolecular hardness is a reactivity index and possesses a predictive value in determining the catalytic activity of macrocyclic complexes confined on electrode surfaces.