Water oxidation activity and magnetic properties of a Mn2IIICa^II[jls-end-space/]core analogous to the Oxygen-Evolving Complex (OEC) of photosystem II

This manuscript demonstrates the synthesis, structure, catalytic activity and magnetism of a trinuclear MnIII−CaII[jls-end-space/]complex which is compositionally relevant to the Oxygen Evolving Complex (OEC) of Photosystem II. The μ–O bridged hetero–metallic complex containing redox–inactive metal ion CaII was synthesized using a tetradentate ligand framework. The complex exhibits water oxidation activity, assessed through electrochemical studies. The i_cat/i_p ratio calculated at 1000 mM H₂O and 100 mM OH− was 9.97 and 12, respectively. The turnover frequency (TOF) of the complex was assessed with both H₂O and OH− providing the values of 2.41 s−1 and 3.5 s−1 respectively. Magnetic susceptibility measurements reveal antiferromagnetic interactions between MnIII centers; within direct current magnetic susceptibility (dc) measurements there was no indication of slow magnetic relaxation. Fitting the full curve with a Heisenberg–Dirac–van Vleck dimer model for two S₁ = S₂ = 2 centers affords g = 1.834 and an antiferromagnetic exchange constant J = 0.117 cm−1. X–band CW–EPR at 77 K exhibits a broad derivative profile; EasySpin simulation reproduces the spectrum with g = 1.834, axial zero–field splitting (ZFS) D₂ = −1.00 cm˗1 and negligible rhombicity, together with an effective isotropic exchange J = 0.336 cm˗1. The unresolved 55Mn hyperfine sextet is attributed to strong ZFS, exchange, and line broadening. These results contribute to the understanding of biomimetic water oxidation systems and provide a structural framework for designing OEC–inspired functional materials.