Unprecedented pseudo-zeroth-order kinetics of the catecholase-like activity of mixed-valence Mn^IIMn₂^III complexes

During the past few decades, the synthesis of coordination complexes that exhibit high catalytic efficiencies has received significant attention from bio-inorganic chemists. In this context, two trinuclear mixed-valence manganese complexes, namely [(Mn2MnII)-Mn-III(L1)(2)(OAc)(4)(H2O)(2)].5H(2)O (1) and [(Mn2MnII)-Mn-III(L2)(2)(OAc)(4)(-)(CH3OH)(2)].3CH(3)OH (2) [OAc = acetate] have been synthesized and structurally characterized. These complexes are shown to retain their nuclearity in acetonitrile medium, unveiling a new area of catecholase-like activity. In fact, both complexes showed an efficient oxidizing activity in acetonitrile using 3,5-di-tert-butylcatechol (3,5-DTBC) as the model substrate, and they were able to bring about complete oxidation of the substrate at a catalyst-to-substrate ratio of up to 1 : 600. To the best of our knowledge, the catecholase-like activity of trinuclear species is explored here for the first time. Interestingly, pseudo-zero-order kinetics with respect to the substrate are identified for both complexes, which represents the second report of this type of activity and the first report observed for a mixed-valence manganese complex. Details from experimental investigations of the mechanistic pathway revealed that the formation of oxygen radicals is believed to be the rate-limiting step, which supports the observation of the pseudo-zeroth-order kinetics.