What Can We Learn from a Biomimetic Model of Nature’s Oxygen-Evolving Complex?

A recently reported synthetic complex with a Mn4CaO4 core represents a remarkable structural mimic of the Mn4CaO5 cluster in the oxygen-evolving complex (OEC) of photosystem II (Zhang et al., Science 2015, 348, 690). Oxidized samples of the complex show electron paramagnetic resonance (EPR) signals at g ≈ 4.9 and 2, similar to those associated with the OEC in its S2 state (g ≈ 4.1 from an S = 5/2 form and g ≈ 2 from an S = 1/2 form), suggesting similarities in the electronic as well as geometric structure. We use quantum-chemical methods to characterize the synthetic complex in various oxidation states, to compute its magnetic and spectroscopic properties, and to establish connections with reported data. Only one energetically accessible form is found for the oxidized “S2 state” of the complex. It has a ground spin state of S = 5/2, and EPR simulations confirm it can be assigned to the g ≈ 4.9 signal. However, no valence isomer with an S = 1/2 ground state is energetically accessible, a conclusion support...