Models for the adsorption of dioxygen on metal chelates

Metal chelates are suitable electrocatalysts for the reduction of dioxygen. The selectivity of the reduction process (H2O2 or H2O as end product) depends on which catalyst is used. Comparable compounds, such as cobalt and iron phthalocyanine (CoPc and FePc), show different selectivities. CoPc catalyzes the dioxygen reduction pathway with H2O2 as the end product, whereas FePc gives H2O. The interaction between the catalyst and dioxygen is crucial for the selectivity of the reduction process. In this review, the most important models for the adsorption of dioxygen on metal chelates are discussed. Most of these models predict that dioxygen is adsorbed in a so-called "end-on (bent)" configuration on to the metal phthalocyanines. The models also predict that, for the dioxygen adduct of CoPc, the oxygen fragment is more negatively charged than for the adduct of FePc, and that this may be the reason for the observed difference in selectivity. Alternatively, dioxygen can bind in between two metal phthalocyanine molecules. For these dioxygen-bridged dimers, as can be derived from adsorption (bonding) models, the cleavage of the oxygen-oxygen bond occurs more easily in the case of the FePc dioxygen dimer than in the corresponding Co complex.