Computational study of the oxygen-oxygen bond activation and hydrocarbon oxygenation reactions in the catalytic cycles of the non-heme diiron enzymes and chiral manganese(Salen) complexes

Density functional methods have been applied to study the catalytic cycle of the (Salen)MnIII based Kochi-Jacobsen-Katsuki (KJK) catalytic system for enantioselective epoxidation of unfunctionalized olefins. In particular, comparative study of electronic and geometric properties of MnV and MnIV oxo species, as well as iso-electronic MnV nitrido species have been performed. Internal oxidation of the oxo species into stable N-oxo species has been studied. Furthermore, formation of Mn-oxo species from organic peracids and/or mixture of O 2/aldehyde during the catalytic cycle has been investigated. Various acylperoxo complexes have been identified as precursors for the oxo species formation. The O-O bond cleavage in the acylperoxo complexes has been studied at different reaction conditions. Namely, the axial ligand and acidity effects on the O-O bond cleavage, which mimic "push" and "pull" effects operating in enzymes, along with solvent effect have been elucidated. The factors determining the mode of the O-O bond cleavage have been revealed. Finally, the epoxidation by acylperoxo complexes and by oxo species has been studied with and without axial ligands. Multiple approaches of the olefin to the reactive oxygenating species have been identified even for unsubstituted Salen ligand. The obtained data advances understanding the epoxidation mechanism and the origin of the enantioselectivity in the KJK system. In addition to synthetic KJK system, the structure of the di-iron active sites of the reduced and oxidized forms of Methane Monooxygenase Hydroxylase (MMOH) and Ribonucleotide Reductase (RNR) enzymes have been elucidated in the protein with ONIOM(DFT:MM) method. For this purpose a general methodology was developed to set up large protein models. Furthermore, a new ONIOM coupling scheme for electrostatic interactions, called QM-adapted Charge Mechanical Embedding (QMC-ME), has been proposed, implemented and applied. The new scheme allows adequate study of the reactions with significant charge redistribution/transfer in the QM region. By using these new methodologies the identity of the ambiguous ligands in the oxidized active sites has been scrutinized. The mechanisms of the molecular oxygen activation by the di-iron active sites of MMOH and RNR have been investigated.