A mononuclear hydroxomanganese(III) complex was synthesized utilizing the N5 amide-containing ligand 2-[bis(pyridin-2-ylmethyl)]amino-N-2-methyl-quinolin-8-yl-acetamidate (dpaq(2Me) ). This complex is similar to previously reported [Mn(III)(OH)(dpaq(H))](+) [Inorg. Chem. 2014, 53, 7622-7634] but contains a methyl group adjacent to the hydroxo moiety. This α-methylquinoline group in [Mn(III)(OH)(dpaq(2Me))](+) gives rise to a 0.1 Å elongation in the Mn-N(quinoline) distance relative to [Mn(III)(OH)(dpaq(H))](+). Similar bond elongation is observed in the corresponding Mn(II) complex. In MeCN, [Mn(III)(OH)(dpaq(2Me))](+) reacts rapidly with 2,2',6,6'-tetramethylpiperidine-1-ol (TEMPOH) at -35 °C by a concerted proton-electron transfer (CPET) mechanism (second-order rate constant k2 of 3.9(3) M(-1) s(-1)). Using enthalpies and entropies of activation from variable-temperature studies of TEMPOH oxidation by [Mn(III)(OH)(dpaq(2Me))](+) (ΔH(‡) = 5.7(3) kcal(-1) M(-1); ΔS(‡) = -41(1) cal M(-1) K(-1)), it was determined that [Mn(III)(OH)(dpaq(2Me))](+) oxidizes TEMPOH ∼240 times faster than [Mn(III)(OH)(dpaq(H))](+). The [Mn(III)(OH)(dpaq(2Me))](+) complex is also capable of oxidizing the stronger O-H and C-H bonds of 2,4,6-tri-tert-butylphenol and xanthene, respectively. However, for these reactions [Mn(III)(OH)(dpaq(2Me))](+) displays, at best, modest rate enhancement relative to [Mn(III)(OH)(dpaq(H))](+). A combination of density function theory (DFT) and cyclic voltammetry studies establish an increase in the Mn(III)/Mn(II) reduction potential of [Mn(III)(OH)(dpaq(2Me))](+) relative to [Mn(III)(OH)(dpaq(H))](+), which gives rise to a larger driving force for CPET for the former complex. Thus, more favorable thermodynamics for [Mn(III)(OH)(dpaq(2Me))](+) can account for the dramatic increase in rate with TEMPOH. For the more sterically encumbered substrates, DFT computations suggest that this effect is mitigated by unfavorable steric interactions between the substrate and the α-methylquinoline group of the dpaq(2Me) ligand. The DFT calculations, which reproduce the experimental activation free energies quite well, provide the first examination of the transition-state structure of mononuclear Mn(III)(OH) species during a CPET reaction.

The mononuclear Mn(IV)-oxo complex [Mn(O)(N4py)], where N4py is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine, has been proposed to attack C-H bonds by an excited-state reactivity pattern [ Cho, K.-B.; Shaik, S.; Nam, W. J. Phys. Chem. Lett. 2012 , 3 , 2851 - 2856 (DOI: 10.1021/jz301241z )]. In this model, a E excited state is utilized to provide a lower-energy barrier for hydrogen-atom transfer. This proposal is intriguing, as it offers both a rationale for the relatively high hydrogen-atom-transfer reactivity of [Mn(O)(N4py)] and a guideline for creating more reactive complexes through ligand modification. Here we employ a combination of electronic absorption and variable-temperature magnetic circular dichroism (MCD) spectroscopy to experimentally evaluate this excited-state reactivity model. Using these spectroscopic methods, in conjunction with time-dependent density functional theory (TD-DFT) and complete-active space self-consistent-field calculations (CASSCF), we define the ligand-field and charge-transfer excited states of [Mn(O)(N4py)]. Through a graphical analysis of the signs of the experimental C-term MCD signals, we unambiguously assign a low-energy MCD feature of [Mn(O)(N4py)] as the E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced Mn-oxyl character on the excited-state E surface, consistent with previous DFT calculations. Potential-energy surfaces, developed using the CASSCF methods, are used to determine how the energies and wave functions of the ground and excited states evolved as a function of Mn═O distance. The unique insights into ground- and excited-state electronic structure offered by these spectroscopic and computational studies are harmonized with a thermodynamic model of hydrogen-atom-transfer reactivity, which predicts a correlation between transition-state barriers and driving force.

Biological systems capitalize on the redox versatility of manganese to perform reactions involving dioxygen and its derivatives superoxide, hydrogen peroxide, and water. The reactions of manganese enzymes influence both human health and the global energy cycle. Important examples include the detoxification of reactive oxygen species by manganese superoxide dismutase, biosynthesis by manganese ribonucleotide reductase and manganese lipoxygenase, and water splitting by the oxygen-evolving complex of photosystem II. Although these enzymes perform very different reactions and employ structurally distinct active sites, manganese intermediates with peroxo, hydroxo, and oxo ligation are commonly proposed in catalytic mechanisms. These intermediates are also postulated in mechanisms of synthetic manganese oxidation catalysts, which are of interest due to the earth abundance of manganese. In this Account, we describe our recent efforts toward understanding O-O bond activation pathways of Mn-peroxo adducts and hydrogen-atom transfer reactivity of Mn-oxo and Mn-hydroxo complexes. In biological and synthetic catalysts, peroxomanganese intermediates are commonly proposed to decay by either Mn-O or O-O cleavage pathways, although it is often unclear how the local coordination environment influences the decay mechanism. To address this matter, we generated a variety of Mn-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one Mn-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many Mn-peroxo model complexes that decay to oxo-bridged-MnMn dimers, decay of this Mn-peroxo adduct yielded mononuclear Mn-hydroxo and Mn-oxo products, potentially resulting from O-O bond activation of the Mn-peroxo unit. These results highlight the role of ligand sterics in promoting the formation of mononuclear products and mark an important step in designing Mn-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic Mn-oxo complexes show great potential for oxidizing substrates with strong C-H bonds, most Mn-oxo species are sluggish oxidants. Both two-state reactivity and thermodynamic arguments have been put forth to explain these observations. To address these issues, we generated a series of Mn-oxo complexes supported by neutral, pentadentate ligands with systematically perturbed equatorial donation. Kinetic investigations of these complexes revealed a correlation between equatorial ligand-field strength and hydrogen-atom and oxygen-atom transfer reactivity. While this trend can be understood on the basis of the two-state reactivity model, the reactivity trend also correlates with variations in Mn reduction potential caused by changes in the ligand field. This work demonstrates the dramatic influence simple ligand perturbations can have on reactivity but also illustrates the difficulties in understanding the precise basis for a change in reactivity. In the enzyme manganese lipoxygenase, an active-site Mn...

A combined experimental−computational study of hydrocarbon oxidation by the Mn IV -oxo complex of the neutral, pentadentate N4py ligand [N,N-bis(2-pyridylmethyl)-N-bis(2pyridyl)methylamine] offers support for a complex reaction coordinate involving multiple electronic states. Variable-temperature kinetic investigations of ethylbenzene oxidation by [Mn IV (O)(N4py)] 2+ yield experimental activation parameters that were used to evaluate computationally predicted energy barriers. Both density functional theory (DFT) and multireference complete-active-space self-consistent-field (CASSCF) computations with n-electron valence state perturbation theory (NEVPT2) corrections were employed to investigate the hydrogen-atomtransfer reaction barriers for the 4 B 1 and 4 E states. The 4 B 1 state is the ground state in the absence of substrate, and the 4 E state is related to the ground state by a one-electron Mn IV e(d xz ,3d yz ) to Mn IV b 1 (d x 2 −y 2) excitation. A comparison of the DFT, CASSCF/NEVPT2, and experimental results shows that the B3LYP-D3 method underestimates the activation barriers of both electronic states by ca. 10 kcal mol −1 . In contrast, the enthalpic barrier predicted for the 4 E state by the CASSCF/NEVPT2 method is within 2 kcal mol −1 of the experimental value. The 4 E state is early, with dominant structural distortions in the Mn− N equatorial distances and perturbations to MnO bonding that lead to strong electronic stabilization of interactions between the Mn IV -oxo unit and substrate C−H bond. While previous DFT studies were qualitatively correct in their ordering of the 4 B 1 and 4 E transition states, this combined use of experimental and CASSCF/NEVPT2 methods provides an ideal means of assessing the two-state reactivity model of Mn IV -oxo complexes.

The solution properties of Mn-hydroxo and Mn-methoxy complexes featuring N amide-containing ligands were investigated using H NMR spectroscopy. TheH NMR spectrum for one of these complexes, the previously reported [Mn(OH)(dpaq)](OTf) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate) shows hyperfine-shifted signals, as expected for this S = 2 Mn-hydroxo adduct. However, the H NMR spectrum of [Mn(OH)(dpaq)](OTf) also shows a large number of proton resonances in the diamagnetic region, suggesting the presence of multiple species in CDCN solution. The majority of the signals in the diamagnetic region disappear when a small amount of water is added to a CHCN solution of [Mn(OH)(dpaq)](OTf). Electronic absorption and Mn K-edge X-ray absorption experiments support the formulation of this diamagnetic species as the μ-oxodimanganese(III,III) complex [Mn(μ-O)(dpaq))]. On the basis of these observations, we propose that the dissolution of [Mn(OH)(dpaq)](OTf) in CDCN results in the formation of mononuclear Mn-hydroxo and dinuclear μ-oxodimanganese(III,III) species that are in equilibrium. The addition of a small amount of water is sufficient to shift this equilibrium in favor of the Mn-hydroxo adduct. Surprisingly, electronic absorption experiments show that the conversion of [Mn(μ-O)(dpaq))] to [Mn(OH)(dpaq)] by added water is relatively slow. Because this dimer to monomer conversion is slower than TEMPOH oxidation by [Mn(OH)(dpaq)], the previously observed TEMPOH oxidation rates for [Mn(OH)(dpaq)] reflected both processes. Here, we report the bona fide TEMPOH oxidation rate for [Mn(OH)(dpaq)], which is significantly faster than previously reported. H NMR spectra are also reported for the related [Mn(OMe)(dpaq)] and [Mn(OH)(dpaq)] complexes. These spectra only show hyperfine-shifted signals, suggesting the presence of only mononuclear Mn-methoxy and Mn-hydroxo species in solution. Measurements of T relaxation times and proton peak integrations for [Mn(OMe)(dpaq)] provide preliminary assignments for H NMR resonances.