N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox-Leveling, High Mn Oxidation States, and a Mn ₂ O ₂ Dimer Competent for Catalytic Oxygen Evolution

The new complex [Mn(bipyalk)(H ₂ O)(µ-O)] ₂ (OTf) ₂ {bipyalk = 2-([2,2′-bipyridin]-6-yl)propan-2-olate} catalyzes oxygen evolution with a TOF of 0.0055 s ^–1 when driven by the sacrificial oxidant KHSO ₅ in water. It is proposed on the basis of EPR experiments that the catalyst proceeds through a Mn ^V (µ-O) ₂ Mn ^V =O intermediate supported by the highly donating tertiary alkoxide moiety of the ligand. The Mn(IV,IV) dimer can also be formed electrochemically from its precursor, Mn(bipyalkH)Cl ₂ . A related series of bis-ligated monomers of the type [Mn ⁿ (bipyalkH _x )(bipyalkH _y )](PF ₆ ) ₂ is also reported, where the dication features Mn ^II , Mn ^III , and Mn ^IV oxidation states and corresponding protonation states. Electrochemical data on this series underscore the importance of proton loss at the alcohol/alkoxide moiety during oxidation to maintain low overpotentials required for catalysis.