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

Unravelling the Effect of Acid-Driven Electron Transfer in High-Valent FeIV =O/MnIV =O Species and Its Implications for Reactivity

The electron transfer (ET) step is one of the crucial processes in biochemical redox reactions that occur in nature and has been established as a key step in dictating the reactivity of high-valent metal-oxo species. Although metalloenzymes possessing metal-oxo units at their active site are typically associated with outer-sphere electron transfer (OSET) processes, biomimetic models, in contrast, have been found to manifest either an inner-sphere electron transfer (ISET) or OSET mechanism. This distinction is clearly illustrated through the behaviour of [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) complexes, where complex 1 showcases an OSET mechanism, while complex 2 exhibits an ISET mechanism, especially evident in their reactions involving C-H bond activation and oxygen atom transfer reactions in the presence of a Lewis/Bronsted acid. However, the precise reason for this puzzling difference remains elusive. This work unveils the origin of the perplexing inner-sphere vs outer-sphere electron transfer process (ISET vs OSET) in [(N4Py)MnIV (O)]2+ (1) and [(N4Py)FeIV (O)]2+ (2) species in the presence of Bronsted acid. The calculations indicate that when the substrate (toluene) approaches both 1 and 2 that is hydrogen bonded with two HOTf molecules (denoted as 1-HOTf and 2-HOTf, respectively), proton transfer from one of the HOTf molecules to the metal-oxo unit is triggered and a simultaneous electron transfer occurs from toluene to the metal centre. Interestingly, the preference for OSET by 1-HOTf is found to originate from the choice of MnIV =O centre to abstract spin-down (β) electron from toluene to its δ(dxy ) orbital. On the other hand, in 2-HOTf, a spin state inversion from triplet to quintet state takes place during the proton (from HOTf) coupled electron transfer (from toluene) preferring a spin-up (α) electron abstraction to its σ* (dz 2 ) orbital mediated by HOTf giving rise to ISET. In addition, 2-HOTf was calculated to possess a larger reorganisation energy, which facilitates the ISET process via the acid. The absence of spin-inversion and smaller reorganisation energy switch the mechanism to OSET for 1-HOTf. Therefore, for the first time, the significance of spin-state and spin-inversion in the electron transfer process has been identified and demonstrated within the realm of high-valent metal-oxo chemistry. This discovery holds implications for the potential involvement of high-valent Mn-oxo species in performing similar transformative processes within Photosystem II.

DFT insights into the mechanism of O2 activation catalyzed by a structural and functional model of cysteine dioxygenase with tris(2-pyridyl)methane-based ligand architecture

Cysteine dioxygenation is an important step in the metabolism of toxic L-cysteine (Cys) in the human body, carried out by cysteine dioxygenase enzyme (CDO). The disruption of this process is found to elicit neurological health issues. This work reports a computational investigation of mechanistic aspects of this reaction, using a recently reported tris(2-pyridyl)methane-based biomimetic model complex of CDO. The computed results indicate that, the initial SO2 bond formation process is the slowest step in the S-dioxygenation process, possessing an activation barrier of 12.7 kcal/mol. The remaining steps were found to be downhill requiring very small activation energies. The transition states were found to undergo spin crossover between triplet and quintet states, while the singlet surface remained unstable throughout the entire reaction. In essence, the mechanistic scheme and multistate reactivity pattern together with the relatively small computed rate-limiting activation barrier as well as the exothermic formation energy demonstrate that the model complex is an efficient biomimetic CDO model. In addition, the study also substantiates the involvement of Fe(IV)oxido intermediates in the mechanism of S-dioxygenation by the chosen model complex. The insights derived from the O₂ activation process might pave way for development of more accurate CDO model catalysts that might be capable of even more efficiently mimicking the geometric, spectroscopic and functional features of the CDO enzyme.

Mechanistic insights into the selectivity of norcarane oxidation by oxoMn(V) porphyrin complexes

OxoMn(V) porphyrin complexes perform competitive hydroxylation, desaturation, and radical rearrangement reactions using diagnostic substrate norcarane. Initial C-H cleavage proceeds through the two hydrogen abstraction steps from the two adjacent carbon on the norcarane, then the selective reaction is performed to generate various products. Using density functional theory calculations, we show that the hydroxylation and desaturation reactions are triggered by a rate-determining H-abstraction step, whereas the rate-determining step for the radical rearrangement is located at the rebound step ( TS2 ). We find that the  endo- 2  reaction is favorable over other reactions, which is consistent with the experimental result. Furthermore, the competitive pathways for norcarane oxidation depend on the non-covalent interaction between norcarane and porphyrin-ring, and orbital energy gaps between donor and acceptor orbitals because of stable or unstable acceptor orbital. The stereo- and regio-selectivities of norcarane oxidation are hardly sensitive to the zero-point energy and thermal free energy corrections.

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

Understanding the comparative oxidative abilities of high-valent metal-oxo/hydroxo/hydroperoxo species holds the key to robust biomimic catalysts that perform desired organic transformations with very high selectivity and efficiency. The comparative oxidative abilities of popular high-valent iron-oxo and manganese-oxo species are often counterintuitive, for example, oxygen atom transfer (OAT) reaction by [(Me₂EBC)Mn^IV-OOH]³⁺, [(Me₂EBC)Mn^IV-OH]³⁺, and [(Me₂EBC)Mn^IV═O]²⁺ (Me₂EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane) shows extremely high reactivity for Mn^IV-OOH species and no reactivity for Mn^IV-OH and Mn^IV═O species toward alkyl/aromatic sulfides. Using a combination of density functional theory (DFT) and ab initio domain-based local pair natural orbital coupled-cluster with single, double, and perturbative triples excitation (DLPNO-CCSD(T)) and complete-active space self-consistent field/N-electron valence perturbation theory second order (CASSCF/NEVPT2) calculations, here, we have explored the electronic structures and sulfoxidation mechanism of these species. Our calculations unveil that Mn^IV-OOH reacts through distal oxygen atom with the substrate via electron transfer (ET) mechanism with a very small kinetic barrier (16.5 kJ/mol), placing this species at the top among the best-known catalysts for such transformations. The Mn^IV-OH and Mn^IV═O species have a much larger barrier. The mechanism has also been found to switch from ET in the former to concerted in the latter, rendering both unreactive under the tested experimental conditions. Intrinsic differences in the electronic structures, such as the presence and absence of the multiconfigurational character coupled with the steric effects, are responsible for such variations observed. This comparative oxidative ability that runs contrary to the popular iron-oxo/hydroperoxo reactivity will have larger mechanistic implications in understanding the reactivity of biomimic catalysts and the underlying mechanisms in PSII.

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, [Mn^IVH₃buea(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 [Mn^IVH₃buea(O)]^- and its congener [Mn^IVH₃buea(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 Mn^IV[double bond, length as m-dash]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*_x²-y² orbital of the Mn^IV[double bond, length as m-dash]O facilitated by its multi-reference character. The unusual reactivity observed here has implications in understanding the reactivity of [Mn4Ca] species in photosystem II.