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.