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Mechanistic Insights on the Formation of High‐Valent Mn III/IV =O Species Using Oxygen as Oxidant: A Theoretical Perspective
Author(s) -
Sen Asmita,
Vyas Nidhi,
Pandey Bhawana,
Jaccob Madhavan,
Rajaraman Gopalan
Publication year - 2020
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.201900142
Subject(s) - chemistry , reactivity (psychology) , deprotonation , catalysis , photosystem ii , bond cleavage , oxygen , manganese , metal , oxidation state , photochemistry , medicinal chemistry , inorganic chemistry , organic chemistry , medicine , ion , biochemistry , photosynthesis , alternative medicine , pathology
High‐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 Mn III =O and Mn IV =O species from [Mn II H 2 buea] 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 Mn II precursor using the experimental conditions. Our calculations reveal the following fundamental steps in the formation of such species: i) O 2 activation by Mn II lead to formation of Mn II −superoxide species wherein the oxidation state of the Mn II found to be intact upon O 2 binding facilitated by the deprotonated nitrogen atom present in the cavity (ii) in the second step, superoxo species is converted to Mn II −hydroperoxo species, [Mn II H 2 buea(OOH)] 2− using dimethylacetamide solvent as source for HAT reaction (iii) presence of water molecule found to aid the O−O bond cleavage in [Mn II H 2 buea(OOH)] 2− species leading to the formation of the putative Mn III =O species, [Mn III H 3 buea(O)] 2− (iv) one‐electron oxidation of Mn III =O, leads to the formation of [Mn IV H 3 buea(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 O 2 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 O 2 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.