EPR monitoring of in-situ catalytic oxidative assembly of MnIII-MnIV dimers via monomeric MnIV = O

Highlights: • Catalyst concentration determines the catalytic cycle since it can affect the speciation of monomeric/dimeric species formed. • The involvement of dimeric [Mn^IV-Mn^III] transients, has been invoked as active catalytic intermediate. The oxidative evolution of Mn-catalysts via transient high-oxidation state, Mn^III or Mn^IV, intermediates is often assumed in literature, however a direct in-situ evidence for cycling between high-oxidation states of Mn is scarce. Moreover, the involvement of dimeric [Mn^IV-Mn^III] transients, has been invoked as active catalytic intermediate. Herein, we present a study by Dual-Mode EPR and Low-Temperature UV–Vis spectroscopies, on the monitoring of cycling of Mn^II, Mn^III, Mn^IV states formed by a MnL catalytic system. It is found that, while starting as monomeric, the MnL catalyst, is capable to form in-situ stable oxo-bridged [Mn-Mn] dimers which are catalytically/redox-active.