Mechanism of Cobalt Self-Exchange Electron Transfer

A heptanuclear cobalt cluster was synthesized in two different oxidation states, Co­(II)7 and a mixed valence Co­(III)­Co­(II)6, as a soluble model of a cobalt–phosphate/borate (Co-OEC) water splitting catalyst. Crystallographic characterization indicates similar cluster cores, distinguished primarily at the central Co atom. An anion associates to the cluster cores via hydrogen bonding. Using an isotope exchange method, an anomalously slow self-exchange electron transfer rate constant (kobs = 1.53 × 10–3 M–1 s–1 at 40 °C and 38 mM [OTf] in MeCN), as compared to that predicted from semiclassical Marcus theory, supports a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the self-repair mechanism of Co-OECs. Moreover, because H2O cannot directly bridge cobalt centers, owing to the encapsulation of the central Co within the cluster core, the observed results address a long-standing controversy surrounding the Co2+/3+ self-exchange electron transfer reaction of the hexaaqua complex.