Tuning the Mn II 2 /Mn III 2 redox cycle of a phenoxo-bridged diMn catalase mimic with terminal carboxylate donors

A new phenoxo-bridged diMn complex, Na[Mn L(OH) (H O) ]·5H O (1), obtained with the ligand L = 5‑methyl‑2‑hydroxo‑1,3‑xylene‑α,α‑diamine‑N,N,N',N'‑tetraacetato, has been prepared and characterized. Mass spectrometry, conductivity, UV-visible, EPR and H NMR spectroscopic studies showed that the complex exists in solution as a monoanionic diMn complex. Complex 1 catalyzes H O disproportionation with second-order rate constant k = 305(9) M min and without a time-lag phase. Based on spectroscopic results, the catalase activity of complex 1 in methanol involves a Mn /Mn redox cycle, which distinguishes this catalyst from other phenoxo-bridged diMn complexes that cycle between Mn Mn /Mn Mn species. Addition of base stabilizes the catalyst, restrains demetallation during catalysis and causes moderate enhancement of catalase activity. The terminal carboxylate donors of 1 not only contribute as internal bases to assist deprotonation of H O but also favor the formation of active homovalent diMn species, just as observed for the enzyme.