Heptanuclear Co^II₅Co^III₂ Cluster as Efficient Water Oxidation Catalyst

Inspired by the transition-metal-oxo cubical Mn₄CaO₅ in photosystem II, we herein report a disc-like heptanuclear mixed-valent cobalt cluster, [Co^II₅Co^III₂(mdea)₄(N₃)₂(CH₃CN)₆(OH)₂(H₂O)₂·4ClO₄] (1, H₂mdea = N-methyldiethanolamine), for photocatalytic oxygen evolution. The topology of the Co₇ core resembles a small piece of cobaltate protected by terminal H₂O, N₃^-, CH₃CN, and multidentate N-methyldiethanolamine at the periphery. Under the optimal photocatalytic conditions, 1 exhibits water oxidation activity with a turnover number (TON) of 210 and a turnover frequency (TOF_initial) of 0.23 s^-1. Importantly, electrospray mass spectrometry (ESI-MS) was used to not only identify the possible main active species in the water oxidation reaction but also monitor the evolutions of oxidation states of cobalt during the photocatalytic reactions. These results shed light on the design concept of new water oxidation catalysts and mechanism-related issues such as the key active intermediate and oxidation state evolution in the oxygen evolution process. The magnetic properties of 1 were also discussed in detail.