Structural Characterization of a Series of N5-Ligated MnIV -Oxo Species

C-H Bond Oxidation by MnIV-Oxo Complexes: Hydrogen-Atom Tunneling and Multistate Reactivity

The reactivity of six MnIV-oxo complexes in C-H bond oxidation has been examined using a combination of kinetic experiments and computational methods. Variable-temperature studies of the oxidation of 9,10-dihydroanthracene (DHA) and ethylbenzene by these MnIV-oxo complexes yielded activation parameters suitable for evaluating electronic structure computations. Complementary kinetic experiments of the oxidation of deuterated DHA provided evidence for hydrogen-atom tunneling in C-H bond oxidation for all MnIV-oxo complexes. These results are in accordance with the Bell model, where tunneling occurs near the top of the transition-state barrier. Density functional theory (DFT) and DLPNO-CCSD(T1) computations were performed for three of the six MnIV-oxo complexes to probe a previously predicted multistate reactivity model. The DFT computations predicted a thermal crossing from the 4B1 ground state to a 4E state along the C-H bond oxidation reaction coordinate. DLPNO-CCSD(T1) calculations further confirm that the 4E transition state offers a lower energy barrier, reinforcing the multistate reactivity model for these complexes. We discuss how this multistate model can be reconciled with recent computations that revealed that the kinetics of C-H bond oxidation by this set of MnIV-oxo complexes can be well-predicted on the basis of the thermodynamic driving force for these reactions.

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.

Enhanced Understanding of Structure-Function Relationships for Oxomanganese(IV) Complexes

A series of manganese(II) and oxomanganese(IV) complexes supported by neutral, pentadentate ligands with varied equatorial ligand-field strength (N3pyQ, N2py2I, and N4pyMe2) were synthesized and then characterized using structural and spectroscopic methods. On the basis of electronic absorption spectroscopy, the [MnIV(O)(N4pyMe2)]2+ complex has the weakest equatorial ligand field among a set of similar MnIV-oxo species. In contrast, [MnIV(O)(N2py2I)]2+ shows the strongest equatorial ligand-field strength for this same series. We examined the influence of these changes in electronic structure on the reactivity of the oxomanganese(IV) complexes using hydrocarbons and thioanisole as substrates. The [MnIV(O)(N3pyQ)]2+ complex, which contains one quinoline and three pyridine donors in the equatorial plane, ranks among the fastest MnIV-oxo complexes in C-H bond and thioanisole oxidation. While a weak equatorial ligand field has been associated with high reactivity, the [MnIV(O)(N4pyMe2)]2+ complex is only a modest oxidant. Buried volume plots suggest that steric factors dampen the reactivity of this complex. Trends in reactivity were examined using density functional theory (DFT)-computed bond dissociation free energies (BDFEs) of the MnIIIO-H and MnIV ═ O bonds. We observe an excellent correlation between MnIV═O BDFEs and rates of thioanisole oxidation, but more scatter is observed between hydrocarbon oxidation rates and the MnIIIO-H BDFEs.

Electronic structure contributions to O-O bond cleavage reactions for MnIII-alkylperoxo complexes

Synthetic manganese catalysts that activate hydrogen peroxide perform a variety of hydrocarbon oxidation reactions. The most commonly proposed mechanism for these catalysts involves the generation of a manganese(III)-hydroperoxo intermediate that decays via heterolytic O-O bond cleavage to generate a Mn(V)-oxo species that initiates substrate oxidation. Due to the paucity of well-defined MnIII-hydroperoxo complexes, MnIII-alkylperoxo complexes are often employed to understand the factors that affect the O-O cleavage reaction. Herein, we examine the decay pathways of the MnIII-alkylperoxo complexes [MnIII(OOtBu)(6Medpaq)]+ and [MnIII(OOtBu)(N4S)]+, which have distinct coordination environments (N5- and N4S-, respectively). Through the use of density functional theory (DFT) calculations and comparisons with published experimental data, we are able to rationalize the differences in the decay pathways of these complexes. For the [MnIII(OOtBu)(N4S)]+ system, O-O homolysis proceeds via a two-state mechanism that involves a crossing from the quintet reactant to a triplet state. A high energy singlet state discourages O-O heterolysis for this complex. In contrast, while quintet-triplet crossing is unfavorable for [MnIII(OOtBu)(6Medpaq)]+, a relatively low-energy single state accounts for the observation of both O-O homolysis and heterolysis products for this complex. The origins of these differences in decay pathways are linked to variations in the electronic structures of the MnIII-alkylperoxo complexes.

Differences in chemoselectivity in olefin oxidation by a series of non-porphyrin manganese(IV)-oxo complexes

High valent metal-oxo intermediates are versatile oxidants known to facilitate both oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions in nature. In addition to performing essential yet challenging biological reactions, these intermediates are known for their selectivity in favoring the formation of one oxidation product. To understand the basis for this selectivity, we explore the role of equatorial ligand field perturbations in Mn^IV-oxo complexes on chemoselectivity in cyclohexene oxidation. We also examine reactions of Mn^IV-oxo complexes with cyclohexene-d₁₀, cyclooctene, and styrene. Within this series, the product distribution in olefin oxidation is highly dependent on the coordination environment of the Mn^IV-oxo unit. While Mn^IV-oxo complexes with sterically encumbered, and slightly tilted, MnO units favor CC epoxidation products in cyclohexene oxidation, a less encumbered analogue prefers to cleave allylic C-H bonds, resulting in cyclohexenol and cyclohexenone formation. These conclusions are drawn from GC-MS product analysis of the reaction mixture, changes in the UV-vis absorption spectra, and kinetic analyses. DFT computations establish a trend in thermodynamic properties of the Mn^IV-oxo complexes and their reactivity towards olefin oxidation on the basis of the MnO bond dissociation free energy (BDFE). The most reactive Mn^IV-oxo adduct from this series oxidizes cyclohexene-d₁₀, cyclooctene, and styrene to give corresponding epoxides as the only detected products. Collectively, these results suggest that the chemoselectivity obtained in oxidation of olefins is controlled by both the coordination environment around the MnO unit, which modulates the MnO BDFE, and the BDFEs of the allylic C-H bond of the olefins.

Structure and Reactivity of Nonporphyrinic Terminal Manganese(IV)-Hydroxide Complexes in the Oxidative Electrophilic Reaction

High-valent transition metal-hydroxide complexes have been proposed as essential intermediates in biological and synthetic catalytic reactions. In this work, we report the single-crystal X-ray structure and spectroscopic characteristics of a mononuclear nonporphyrinic Mn^IV-(OH) complex, [Mn^IV(Me₃-TPADP)(OH)(OCH₂CH₃)]²⁺ (2), using various physicochemical methods. Likewise, [Mn^IV(Me₃-TPADP)(OH)(OCH₂CF₃)]²⁺ (3), which is thermally stable at room temperature, was also synthesized and characterized spectroscopically. The Mn^IV-(OH) adducts are capable of performing oxidation reactions with external organic substrates such as C-H bond activation, sulfoxidation, and epoxidation. Kinetic studies, involving the Hammett correlation and kinetic isotope effect, and product analyses indicate that 2 and 3 exhibit electrophilic oxidative reactivity toward hydrocarbons. Density functional theory calculations support the assigned electronic structure and show that direct C-H bond activation of the Mn^IV-(OH) species is indeed possible.

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(III)-hydroxo and Mn(III)-aqua complexes

Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

The most reactive iron and manganese complexes with N-pentadentate ligands for dioxygen activation—synthesis, characteristics, applications

The iron and manganese complexes that activate oxygen atom play multiple role in technologically relevant reactions as well as in biological transformations, in which exist in different redox states. Among them, high-valent oxo intermediate seems to be the most important one. Iron, and/or manganese-based processes have found application in many areas, starting from catalysis and sustainable technologies, through DNA oxidative cleavage, to new substances useful in chemotherapeutic drugs. This review is not only the latest detailed list of uses of homogeneous N-pentadentate iron and manganese catalysts for syntheses of valuable molecules with huge applications in green technologies, but also a kind of "a cookbook", collecting "recipes" for the discussed complexes, in which the sources necessary to obtain a full characterization of the compounds are presented. Following the catalytic activity of metalloenzymes, and taking into account the ubiquity of iron and manganese salts, which in combination with properly designed ligands may show similarity to natural systems, the discussed complexes can find application as new anti-cancer drugs. Also, owing to ability of oxygen atom to exchange in reaction with H₂O, they can be successfully applied in photodriven reactions of water oxidation, as well as in chemically regenerated fuel cells as a redox catalyst.

Mechanistic insight into oxygen atom transfer reactions by mononuclear manganese(IV)-oxo adducts

High-valent metal-oxo intermediates are well known to facilitate oxygen-atom transfer (OAT) reactions both in biological and synthetic systems. These reactions can occur by a single-step OAT mechanism or by a stepwise process initiated by rate-limiting electron transfer between the substrate and the metal-oxo unit. Several recent reports have demonstrated that changes in the metal reduction potential, caused by the addition of Brønsted or Lewis acids, cause a change in sulfoxidation mechanism of MnIV-oxo complexes from single-step OAT to the multistep process. In this work, we sought to determine if ca. 4000-fold rate variations observed for sulfoxidation reactions by a series of MnIV-oxo complexes supported by neutral, pentadentate ligands could arise from a change in sulfoxidation mechanism. We examined the basis for this rate variation by performing variable-temperature kinetic studies to determine activation parameters for the reactions of the MnIV-oxo complexes with thioanisole. These data reveal activation barriers predominantly controlled by activation enthalpy, with unexpectedly small contributions from the activation entropy. We also compared the reactivity of these MnIV-oxo complexes by a Hammett analysis using para-substituted thioanisole derivatives. Similar Hammett ρ values from this analysis suggest a common sulfoxidation mechanism for these complexes. Because the rates of oxidation of the para-substituted thioanisole derivatives by the MnIV-oxo adducts are much faster than that expected from the Marcus theory of outer-sphere electron-transfer, we conclude that these reactions proceed by a single-step OAT mechanism. Thus, large variations in sulfoxidation by this series of MnIV-oxo centers occur without a change in reaction mechanism.

Direct C-H bond halogenation and pseudohalogenation of hydrocarbons mediated by high-valent 3d metal-oxo species

Late-stage direct functionalization of the C-H bond is synthetically desirable. Metalloenzymes having metal-oxo active sites are well known to selectively catalyze hydroxylation and halogenation reactions with high efficiency. This review highlights the recent developments in the field of direct C-H halogenation and pseudohalogenation reactions catalyzed by the functional models of metalloenzymes.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

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.