Deciphering the mechanism of oxygen atom transfer by non-heme MnIV–oxo species: an ab initio and DFT exploration†
Oxygen atom transfer (OAT) reactions employing transition metal–oxo species have tremendous significance in homogeneous catalysis for industrial use. Understanding the structural and mechanistic aspects of OAT reactions using high-valent metal–oxo species is of great importance to fine-tune their reactivity. Herein we examine the reactivity of a non-heme high-valent oxo-manganese(IV) complex, [MnIVH3buea(O)]− towards a variety of substrates such as PPh2Me, PPhMe2, PCy3, PPh3, and PMe3 using density functional theory as well as ab initio CASSCF/NEVPT2 methods. We have initially explored the structure and bonding of [MnIVH3buea(O)]− and its congener [MnIVH3buea(S)]−. Our calculations affirm an S = 3/2 ground state of the catalyst with the S = 5/2 and S = 1/2 excited states predicted to be too high lying in energy to participate in the reaction mechanism. Our ab initio CASSCF/NEVPT2 calculations, however, reveal a strong multi-reference character for the ground S = 3/2 state with many low-lying quartets mixing significantly with the ground state. This opens up various reaction channels, and the admixed wave-function evolves during the reaction with the excited triplet dominating the ground state wave-function at the reactant complex. Our calculations predict the following pattern of reactivity, PCy3 < PMe3 < PPh3 < PPhMe2 < PPh2Me for the OAT reaction with the MnIVO species which correlates well with the experimental observations. Detailed electronic structure analysis of the transitions states reveal that these substrates react via an unusual low-energy δ-type pathway where a spin-up electron from the substrate is transferred to the δ*x2−y2 orbital of the MnIVO facilitated by its multi-reference character. The unusual reactivity observed here has implications in understanding the reactivity of [Mn4Ca] species in photosystem II.