Mechanistic Insights on the Formation of High‐Valent MnIII/IV=O Species Using Oxygen as Oxidant: A Theoretical Perspective

Abstract

AbstractHigh‐valent metal−oxo species are of great interest as they serve as a robust catalyst for various organic transformations, and at the same time, they offer significant insight into the reactivity of various metalloenzymes. Formation of Mn−Oxo species is of great interest as they are involved in the Oxygen Evolving Complex of Photosystem II, and various bio‐mimic models were synthesized to understand its reactivity. In this context, using urea decorated amine ligands, Borovik et al. have reported the facile formation of MnIII=O and MnIV=O species from [MnIIH2buea]2− (here H2buea=tris[(N′‐tert‐butylureayl)‐N‐ethyl]amine) precursor complex using oxygen as the oxidant. While reactivity of these species is thoroughly studied, mechanism of formation of such species is scarcely explored. In this work, we have attempted to establish the formation of these species from the MnII precursor using the experimental conditions. Our calculations reveal the following fundamental steps in the formation of such species: i) O2 activation by MnII lead to formation of MnII−superoxide species wherein the oxidation state of the MnII found to be intact upon O2 binding facilitated by the deprotonated nitrogen atom present in the cavity (ii) in the second step, superoxo species is converted to MnII−hydroperoxo species, [MnIIH2buea(OOH)]2− using dimethylacetamide solvent as source for HAT reaction (iii) presence of water molecule found to aid the O−O bond cleavage in [MnIIH2buea(OOH)]2− species leading to the formation of the putative MnIII=O species, [MnIIIH3buea(O)]2− (iv) one‐electron oxidation of MnIII=O, leads to the formation of [MnIVH3buea(O)]− species and this step is endothermic and need some external oxidants for its formation. While various spin‐states and their roles are explored, our calculations reveal that the Mn atom prefers to be in the high‐spin state across the potential energy surface studied. However, the nature of the formation is strongly correlated to the spin state arising from the radical nature present in the O2 moiety and also in the deprotonated nitrogen atom. This offers a unique multistate reactivity channel for the formation these species easing various kinetic barriers across the potential energy surface. Further, we have also computed the spectral parameters for the experimentally observed species, which are in agreement with the reported data offering confidence on the mechanism established. To this end, our study unveils a facile formation of high‐valent Mn−Oxo species using O2 as oxidant and role of water molecules in the formation of such species, and these mechanistic insights are likely to have implications beyond the example studied here.