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 structura 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 approximate to 4.9 and 2, similar to those associated with the OEC in its S-2 state (g approximate to 4.1 from an S = 5/2 form and g approximate to 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 S-2 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 approximate to 4.9 signal. However, no valence isomer with an S = 1/2 ground state is energetically accessible, a conclusion supported by a wide range of methods, including density matrix renormalization group with full valence active space. Alternative candidates for the g approximate to 2 signal were explored, but no low-spin/low-energy structure was identified. Therefore, our results suggest that despite geometric similarities the synthetic model does not mimic the valence isomerism that is the hallmark of the OEC in its S-2 state, most probably because it lacks a coordinatively flexible oxo bridge. Only one of the observed EPR signals can be explained by a structurally intact high-spin one-electron-oxidized form, while the other originates from an as-yet-unidentified rearrangement product. Nevertheless, this model provides valuable information for understanding the high-spin EPR signals of both the S-1 and S-2 states of the OEC in terms of the coordination number and Jahn-Teller axis orientation of the Mn ions, with important consequences for the development of magnetic spectroscopic probes to study S-state intermediates immediately prior to O-O bond formation.