Deciphering the mechanism of oxygen atom transfer by non-heme MnIV–oxo species: an ab initio and DFT exploration. Issue 30 (2nd July 2020)

• Record Type: Journal Article
• Authors: Sen, Asmita; Vyas, Nidhi; Pandey, Bhawana; Rajaraman, Gopalan
• Is Part Of: Dalton transactions. Volume 49:Issue 30(2020)
• Page Start: 10380
• Page End: 10393
• Publication Date: 2020-07-02
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d0dt01785j
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Our calculations reveal strong multi-reference character for Mn IV O species in OAT reactions utilising several low-lying quartet states unravelling novel multi-state reactivity hitherto unestablished. Abstract : 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, [Mn IV H3 buea(O)] − towards a variety of substrates such as PPh2 Me, 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 [Mn IV H3 buea(O)] − and its congener [Mn IV H3 buea(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 < PPh2 Me for the OAT reaction with the Mn IV O 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* x 2 − y 2 orbital of the Mn IV O facilitated by its multi-reference character. The unusual reactivity observed here has implications in understanding the reactivity of [Mn4Ca] species in photosystem II.