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

Abstract

High-valent metalÀ 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 MnÀ 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] 2À (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 À 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 À hydroperoxo species, [Mn II H 2 buea (OOH)] 2À using dimethylacetamide solvent as source for Keywords: high-valent MnÀ oxo • Mn III =O • Mn IV =O • H 3 buea • metalloenzyme [a] A

Mechanistic Insights into the Oxygen Atom Transfer Reactions by Nonheme Manganese Complex: A Computational Case Study on the Comparative Oxidative Ability of Manganese-Hydroperoxo vs High-Valent MnIV═O and MnIV–OH Intermediates

“…The optimized structure and its corresponding spin-density plot of 4 3 are shown in Figure d. The Mn–O bond length for 4 3 is 1.599 Å which agrees with the previously reported Mn–O bond length in high-valent Mn IV O species. ,− The spin density on the Mn atom of 4 3 is 3.053, suggesting moderate spin-polarization as the spin density is slightly larger than the expected value for three unpaired electrons. The NBO analysis of 4 3 indicates one σ bond and one π bond, i.e., MnO (see Figure S4).…”

“…However, this is expected to be barrierless from the reactant energy. The 2 TS2 d species is higher in energy by 121.7 kJmol from 4 Int1 d , suggesting that the reaction takes place solely in the quartet surface as observed earlier in several Mn IV O/Mn III O chemistries. ,− ,− The (S)­O–H bond length is found to elongate from 0.979 to 1.084 Å at the transition, whereas the (Mn)­O–H bond shrinks from 2.459 to 1.374 Å at the transition state. Also, the S–O bond length decreases from 1.684 to 1.614 Å, whereas the Mn–O bond length increases from 1.614 to 1.681 Å from the intermediate 4 Int1 d to the transition state 4 TS2 d .…”

“…We have tested the D3 dispersion at B3LYP-D3/TZVP­(all atoms)//B3LYP-D3/LanL2DZ­(Mn),6-31G*­(C, H, N, O) level of theory for selected species and only marginal differences are noted (see Table S8 in ESI). Although the chosen methodology is shown to yield a good numerical estimate of spin-state energetics and thermodynamic/kinetic data, ,,− it has been shown in several cases that when two spin states lie very close to each other, the DFT methods fail to predict correct ground state. − A limited benchmarking was undertaken with BP86, TPSSh, PBE0, M06L, and wB97XD functionals to understand the small energy gap computed, which suggests that the B3LYP method chosen is reliable if the gap is sufficiently large (see Table S9 in the ESI). The ab initio DLPNO-CCSD­(T) has recently gained popularity because of its high accuracy in predicting the correct ground state relative to experimental observations , and its affordability for larger molecules.…”

Mechanistic Insights into the Selectivity of Norcarane Oxidation by Oxo‐Manganese(V) Porphyrin Complexes

“…The high‐valent oxo‐manganese species have great potential redox activity – [17–19a] . The mechanistic insights and oxygen atom transfer of oxo‐Mn(III/IV) were also explored by DFT method, [19b–d] this study reveals a facile formation of high‐valent oxo‐Mn species using O 2 as oxidant. In particular, the oxo‐Mn(V) porphyrin complexes have been isolated and characterized by different spectroscopic techniques and show efficient catalysts for the hydroxylation and epoxidation of organic substrates [20–22] .…”