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.

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.

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 MnIV-oxo complexes on chemoselectivity in cyclohexene oxidation. We also examine reactions of MnIV-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 MnIV-oxo unit. While MnIV-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 MnIV-oxo complexes and their reactivity towards olefin oxidation on the basis of the MnO bond dissociation free energy (BDFE). The most reactive MnIV-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.

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.

Steric control of dioxygen activation pathways for MnII complexes supported by pentadentate, amide-containing ligands.

Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf). Dioxygen titration experiments demonstrate that [MnII(dpaq)](OTf) reacts with O2 to form [MnIII(OH)(dpaq)](OTf) according to a 4 : 1 Mn : O2 stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a MnIV-oxo species and residual MnII complex to form a (µ-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the MnIII-hydroxo product. In contrast, the sterically modified [MnII(dpaq2Me)](OTf) complex was found to react with O2 according to a 2 : 1 Mn : O2 stoichiometry. This stoichiometry is indicative of a pathway in which a MnIV-oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the MnII starting complex. Isotopic labeling experiments, in which the oxygenation of the MnII complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of [MnII(dpaq)](OTf) did not result in any deuterium incorporation in the MnIII-hydroxo product, while the oxygenation of [MnII(dpaq2Me)](OTf) in d3-MeCN showed [MnIII(OD)(dpaq2Me)]+ formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.