| Abstract
| - The reactivity toward dioxygen of two series ofdicobalt cofacial diporphyrins in solution in an aproticsolvent is described. Some of these compounds are efficientelectrocatalysts for the four-electron reduction ofdioxygenwhen adsorbed on a graphite electrode immersed in aqueous acid.Their electrochemical and spectroscopic (UV−vis, EPR) behavior in solution shows that, contrary to what is observedwith cobalt monomers, the neutral[PCoII CoIIP](1) (P stands for a porphyrin ring) form does not react withdioxygen. Uniquely the one- and two-electron-oxidizedforms of the dimer,[PCoII·CoIIP]+(1+) and[PCoII---CoIIP]2+(12+), respectively, reversiblybind dioxygen, givingtwo complexes, 2 and 3, at room temperature andin the absence of a good axial ligand. The stability constantsofthe two O2 complexes have been measuredspectrophotometrically and/or electrochemically, and prove to beremarkablyhigh. As a whole, the present O2 binding processesappear unprecedented as basically different in manyrespectsfrom the process classically described in the case of cobalt monomers.Extended Hückel molecular orbital (EHMO)calculations, based on the crystal structure of theCo2FTF4 dimer in its uncomplexed form (Co−Codistance 3.42Å), show that, in the absence of very important deformations of itsstructure, the only possible geometry for theO2complex of the two-electron-oxidized derivative[PCo−O2−CoP]2+ (3) is theμ-η2:η2-peroxo structure. Thecalculatedcorresponding electronic diagram affords a rationale for most of theexperimentally observed properties. Specifically,the O2 complex of the one-electron-oxidized form[PCo−O2•−CoP]+(2), the reduced form of complex 3, shouldbeconsidered as a species in which the O2 moiety is furtherreduced, at least partially, as compared to its peroxostatein 3, i.e., consequently in an oxidation state intermediatebetween peroxo (−1) and oxo (−2). Preliminaryresultsindicate that this species reacts with one proton, while thetwo-electron-oxidized O2 complex 3 is resistanttoprotonation. The possible implications of these specificproperties of the dicobalt dimers in the four-electronreductionmechanism of O2 are discussed, and structural andmechanistic similarities with bioinorganic dinuclear sitesappearsignificant.
|
