Synthetic control and empirical prediction of redox potentials for Co(4)O(4) cubanes over a 1.4 V range: implications for catalyst design and evaluation of high-valent intermediates in water oxidation

The oxo-cobalt cubane unit [Co(4)O(4)] is of interest as a homogeneous oxygen-evolution reaction (OER) catalyst, and as a functional mimic of heterogeneous cobalt oxide OER catalysts. The synthesis of several new cubanes allows evaluation of redox potentials for the [Co(4)O(4)] cluster, which are highly sensitive to the ligand environment and span a remarkable range of 1.42 V. The [CoIII4O(4)](4+)/[CoIII3Co(IV)O(4)](5+) and [CoIII3Co(IV)O(4)](5+)/[CoIII2CoIV2O(4)](6+) redox potentials are reliably predicted by the pK (a)s of the ligands. Hydrogen bonding is also shown to significantly raise the redox potentials, by ∼500 mV. The potential-pK (a) correlation is used to evaluate the feasibility of various proposed OER catalytic intermediates, including high-valent Co-oxo species. The synthetic methods and structure–reactivity relationships developed by these studies should better guide the design of new cubane-based OER catalysts.