An Fe6 C Core in All Nitrogenase Cofactors.

The biological process of dinitrogen reduction to ammonium occurs at the cofactors of nitrogenases, the only enzymes that catalyze this challenging chemical reaction. Three types of nitrogenases have been described, named according to the heterometal in their cofactor: molybdenum, vanadium or iron nitrogenases. Spectroscopic and structural characterization allowed the unambiguous identification of the cofactors of molybdenum and vanadium nitrogenases and revealed a central µ6 -carbide in both of them. Although genetic studies suggested that the cofactor of the iron nitrogenase contains a similar Fe6 C core, this has not been experimentally demonstrated. Here we report Valence-to-Core X-ray Emission Spectroscopy providing experimental evidence that this cofactor contains a carbide, thereby making the Fe6 C core a feature of all nitrogenase cofactors.

Determination of the iron(IV) local spin states of the Q intermediate of soluble methane monooxygenase by Kß X-ray emission spectroscopy.

Soluble methane monooxygenase (sMMO) facilitates the conversion of methane to methanol at a non-heme FeIV2 intermediate MMOHQ, which is formed in the active site of the sMMO hydroxylase component (MMOH) during the catalytic cycle. Other biological systems also employ high-valent FeIV sites in catalysis; however, MMOHQ is unique as Nature's only identified FeIV2 intermediate. Previous 57Fe Mössbauer spectroscopic studies have shown that MMOHQ employs antiferromagnetic coupling of the two FeIV sites to yield a diamagnetic cluster. Unfortunately, this lack of net spin prevents the determination of the local spin state (Sloc) of each of the irons by most spectroscopic techniques. Here, we use Fe Kß X-ray emission spectroscopy (XES) to characterize the local spin states of the key intermediates of the sMMO catalytic cycle, including MMOHQ trapped by rapid-freeze-quench techniques. A pure XES spectrum of MMOHQ is obtained by subtraction of the contributions from other reaction cycle intermediates with the aid of Mössbauer quantification. Comparisons of the MMOHQ spectrum with those of known Sloc = 1 and Sloc = 2 FeIV sites in chemical and biological models reveal that MMOHQ possesses Sloc = 2 iron sites. This experimental determination of the local spin state will help guide future computational and mechanistic studies of sMMO catalysis.

Effect of Lewis Acids on the Structure and Reactivity of a Mononuclear Hydroxomanganese(III) Complex.

The addition of Sc(OTf)3 and Al(OTf)3 to the mononuclear MnIII-hydroxo complex [MnIII(OH)(dpaq)]+ (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [MnIII(OH)(dpaq)]+. The electronic absorption spectra of [MnIII(OH)(dpaq)]+ in the presence of Sc(OTf)3 (1-ScIII) and Al(OTf)3 (1-AlIII) show modest perturbations in electronic transition energies, consistent with moderate changes in the MnIII geometry. A comparison of 1H NMR data for 1 and 1-ScIII confirm this conclusion, as the 1H NMR spectrum of 1-ScIII shows the same number of hyperfine-shifted peaks as the 1H NMR spectrum of 1. These 1H NMR spectra, and that of 1-AlIII, share a similar chemical-shift pattern, providing firm evidence that these Lewis acids do not cause gross distortions to the structure of 1. Mn K-edge X-ray absorption data for 1-ScIII provide evidence of elongation of the axial Mn-OH and Mn-N(amide) bonds relative to those of 1. In contrast to these modest spectroscopic perturbations, 1-ScIII and 1-AlIII show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10-dihydroanthracene (DHA), 1-ScIII and 1-AlIII react rapidly with DHA (k2 = 0.16(1) and 0.25(2) M-1 s-1 at 50 °C, respectively). The 1-ScIII species is capable of attacking the much stronger C-H bond of ethylbenzene. The basis for these perturbations to the spectroscopic properties and reactivity of 1 in the presence of these Lewis acids was elucidated by comparing properties of 1-ScIII and 1-AlIII with the recently reported MnIII-aqua complex [MnIII(OH2)(dpaq)]2+ ( J. Am. Chem. Soc. 2018, 140, 12695-12699). Because 1-ScIII and 1-AlIII show 1H NMR spectra essentially identical to that of [MnIII(OH2)(dpaq)]2+, the primary effect of these Lewis acids on 1 is protonation of the hydroxo ligand caused by an increase in the Brønsted acidity of the solution.

Experimental and Multireference ab Initio Investigations of Hydrogen-Atom-Transfer Reactivity of a Mononuclear MnIV-oxo Complex.

A combined experimental-computational study of hydrocarbon oxidation by the MnIV-oxo complex of the neutral, pentadentate N4py ligand [N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine] offers support for a complex reaction coordinate involving multiple electronic states. Variable-temperature kinetic investigations of ethylbenzene oxidation by [MnIV(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-atom-transfer reaction barriers for the 4B1 and 4E states. The 4B1 state is the ground state in the absence of substrate, and the 4E state is related to the ground state by a one-electron MnIV e(dxz,3dyz) to MnIV b1(dx2-y2) 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 4E state by the CASSCF/NEVPT2 method is within 2 kcal mol-1 of the experimental value. The 4E state is early, with dominant structural distortions in the Mn-Nequatorial distances and perturbations to Mn═O bonding that lead to strong electronic stabilization of interactions between the MnIV-oxo unit and substrate C-H bond. While previous DFT studies were qualitatively correct in their ordering of the 4B1 and 4E transition states, this combined use of experimental and CASSCF/NEVPT2 methods provides an ideal means of assessing the two-state reactivity model of MnIV-oxo complexes.

Structure and Reactivity of (µ-Oxo)dimanganese(III,III) and Mononuclear Hydroxomanganese(III) Adducts Supported by Derivatives of an Amide-Containing Pentadentate Ligand.

Mononuclear MnIII-hydroxo and dinuclear (µ-oxo)dimanganese(III,III) complexes were prepared using derivatives of the pentadentate, amide-containing dpaq ligand (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate). Each of these ligand derivatives (referred to as dpaq5R) contained a substituent R (where R = OMe, Cl, and NO2) at the 5-position of the quinolinyl group. Generation of the MnIII complexes was achieved by either O2 oxidation of MnII precursors (for [MnII(dpaq5OMe)]+ and [MnII(dpaq5Cl)]+ or PhIO oxidation (for [MnII(dpaq5NO2)]+). For each oxidized complex, 1H NMR experiments provided evidence of a water-dependent equilibrium between paramagnetic [MnIII(OH)(dpaq5R)]+ and an antiferromagnetically coupled [MnIIIMnIII(µ-O)(dpaq5R)2]2+ species in acetonitrile, with the addition of water favoring the MnIII-hydroxo species. This conversion could also be monitored by electronic absorption spectroscopy. Solid-state X-ray crystal structures for each [MnIIIMnIII(µ-O)(dpaq5R)2](OTf)2 complex revealed a nearly linear Mn-O-Mn core (angle of ca. 177°), with short Mn-O distances near 1.79 Å, and a Mn···Mn separation of 3.58 Å. X-ray crystallographic information was also obtained for the mononuclear [MnIII(OH)(dpaq5Cl)](OTf) complex, which has a short Mn-O(H) distance of 1.810(2) Å. The influence of the 5-substituted quinolinyl moiety on the electronic properties of the [MnIII(OH)(dpaq5R)]+ complexes was demonstrated through shifts in a number of 1H NMR resonances, as well as a steady increase in the MnIII/II cyclic voltammetry peak potential in the order [MnIII(OH)(dpaq5OMe)]+ < [MnIII(OH)(dpaq)]+ < [MnIII(OH)(dpaq5Cl)]+ < [MnIII(OH)(dpaq5NO2)]+. These changes in oxidizing power of the MnIII-hydroxo adducts translated to only modest rate enhancements for TEMPOH oxidation by the [MnIII(OH)(dpaq5R)]+ complexes, with the most reactive [MnIII(OH)(dpaq5NO2)]+ complex showing a second-order rate constant only 9-fold larger than that of the least reactive [MnIII(OH)(dpaq5OMe)]+ complex. These modest rate changes were understood on the basis of density functional theory (DFT)-computed p Ka values for the corresponding [MnII(OH2)(dpaq5R)]+ complexes. Collectively, the experimental and DFT results reveal that the 5-substituted quinolinyl groups have an inverse influence on electron and proton affinity for the MnIII-hydroxo unit.

MnIII-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity.

The new tetradentate L7BQ ligand (L7BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support MnII and MnIII-peroxo complexes. X-ray crystallography of the [MnII(L7BQ)(OTf)2] complex shows a monomeric MnII center with the L7BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H2O2 and Et3N at -40 °C, a MnIII-peroxo adduct, [MnIII(O2)(L7BQ)]+ is formed. The formation of this new intermediate is supported by a variety of spectroscopic techniques, including electronic absorption, Mn K-edge X-ray absorption and electron paramagnetic resonance methods. Evaluation of extended X-ray absorption fine structure data for [MnIII(O2)(L7BQ)]+ resolved Mn-O bond distances of 1.85 Å, which are on the short end of those previously reported for crystallographically characterized MnIII-peroxo adducts. An analysis of the X-ray pre-edge region of [MnIII(O2)(L7BQ)]+ revealed a large pre-edge area of 20.8 units. Time-dependent density functional theory computations indicate that the pre-edge intensity is due to Mn 4p-3d mixing caused by geometric distortions from centrosymmetry induced by both the peroxo and L7BQ ligands. The reactivity of [MnIII(O2)(L7BQ)]+ towards aldehydes was assessed through reaction with cyclohexanecarboxaldehyde and 2-phenylpropionaldehyde. From these experiments, it was determined that [MnIII(O2)(L7BQ)]+ only reacts with aldehydes in the presence of acid. Specifically, the addition of cyclohexanecarboxylic acid to [MnIII(O2)(L7BQ)]+ converts the MnIII-peroxo adduct to a new intermediate that could be responsible for the observed aldehyde deformylation activity. These observations underscore the challenges in identifying the reactive metal species in aldehyde deformylation reactions.

NMR Studies of a MnIII-hydroxo Adduct Reveal an Equilibrium between MnIII-hydroxo and µ-Oxodimanganese(III,III) Species.

The solution properties of MnIII-hydroxo and MnIII-methoxy complexes featuring N5 amide-containing ligands were investigated using 1H NMR spectroscopy. The 1H NMR spectrum for one of these complexes, the previously reported [MnIII(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 MnIII-hydroxo adduct. However, the 1H NMR spectrum of [MnIII(OH)(dpaq)](OTf) also shows a large number of proton resonances in the diamagnetic region, suggesting the presence of multiple species in CD3CN solution. The majority of the signals in the diamagnetic region disappear when a small amount of water is added to a CH3CN solution of [MnIII(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 [MnIII2(µ-O)(dpaq)2)]2+. On the basis of these observations, we propose that the dissolution of [MnIII(OH)(dpaq)](OTf) in CD3CN results in the formation of mononuclear MnIII-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 MnIII-hydroxo adduct. Surprisingly, electronic absorption experiments show that the conversion of [MnIII2(µ-O)(dpaq)2)]2+ to [MnIII(OH)(dpaq)]+ by added water is relatively slow. Because this dimer to monomer conversion is slower than TEMPOH oxidation by [MnIII(OH)(dpaq)]+, the previously observed TEMPOH oxidation rates for [MnIII(OH)(dpaq)]+ reflected both processes. Here, we report the bona fide TEMPOH oxidation rate for [MnIII(OH)(dpaq)]+, which is significantly faster than previously reported. 1H NMR spectra are also reported for the related [MnIII(OMe)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+ complexes. These spectra only show hyperfine-shifted signals, suggesting the presence of only mononuclear MnIII-methoxy and MnIII-hydroxo species in solution. Measurements of T1 relaxation times and proton peak integrations for [MnIII(OMe)(dpaq)]+ provide preliminary assignments for 1H NMR resonances.

Spectroscopic and Structural Characterization of Mn(III)-Alkylperoxo Complexes Supported by Pentadentate Amide-Containing Ligands.

Manganese-alkylperoxo species have been proposed as important intermediates in certain enzymatic pathways and are presumed to play a key role in catalytic substrate oxidation cycles involving manganese catalysts and peroxide oxidants. However, structural and spectroscopic understanding of these intermediates is very limited, with only one series of synthetic MnIII-alkylperoxo complexes having been reported. In the present study, we describe the formation and properties of two new MnIII-alkylperoxo complexes, namely, [MnIII(OO tBu)(dpaq)]+ and [MnIII(OO tBu)(dpaq2Me)]+, which utilize the anionic, amide-containing pentadentate dpaq ligand platform. These complexes were generated by reacting the corresponding MnII precursors with a large excess of tBuOOH at -15 °C in MeCN. In both cases, the corresponding mononuclear MnIII-hydroxo complexes [MnIII(OH)(dpaq)]+ and [MnIII(OH)(dpaq2Me)]+ are observed as intermediates en route to the MnIII-alkylperoxo adducts. These new MnIII-alkylperoxo complexes were characterized by electronic absorption, infrared, and Mn K-edge X-ray absorption spectroscopies. Complementary density functional theory calculations were also performed to gain insight into their bonding and structural properties. Compared to previously reported MnIII-alkylperoxo adducts, the MnIII centers in these complexes exhibit significantly altered primary coordination spheres, with a strongly donating anionic amide nitrogen located trans to the alkylperoxo moiety. This results in MnIII-alkylperoxo bonding that is dominated by σ-interactions between the alkylperoxo πip*(O-O) orbital and the Mn d z2 orbital.

Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes.

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [MnIII(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79 Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2 eV from the MnII to the MnIII-OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by ~ 0.3% from the MnII to MnIII-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive MnIII-OH intermediates, such as those proposed in enzyme active sites.

Manganese-Oxygen Intermediates in O-O Bond Activation and Hydrogen-Atom Transfer Reactions.

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 MnIII-peroxo adducts and hydrogen-atom transfer reactivity of MnIV-oxo and MnIII-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 MnIII-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one MnIII-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many MnIII-peroxo model complexes that decay to oxo-bridged-MnIIIMnIV dimers, decay of this MnIII-peroxo adduct yielded mononuclear MnIII-hydroxo and MnIV-oxo products, potentially resulting from O-O bond activation of the MnIII-peroxo unit. These results highlight the role of ligand sterics in promoting the formation of mononuclear products and mark an important step in designing MnIII-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic MnIV-oxo complexes show great potential for oxidizing substrates with strong C-H bonds, most MnIV-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 MnIV-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 MnIII/IV 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 MnIII-hydroxo adduct initiates substrate oxidation by abstracting a hydrogen atom from a C-H bond. Precedent for this chemistry from synthetic MnIII-hydroxo centers is rare. To better understand hydrogen-atom transfer by MnIII centers, we developed a pair of MnIII-hydroxo complexes, formed in high yield from dioxygen oxidation of MnII precursors, capable of attacking weak O-H and C-H bonds. Kinetic and computational studies show a delicate interplay between thermodynamic and steric influences in hydrogen-atom transfer reactivity, underscoring the potential of MnIII-hydroxo units as mild oxidants.

Spectroscopic and Computational Investigations of a Mononuclear Manganese(IV)-Oxo Complex Reveal Electronic Structure Contributions to Reactivity.

The mononuclear Mn(IV)-oxo complex [MnIV(O)(N4py)]2+, 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 4E 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 [MnIV(O)(N4py)]2+ 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 [MnIV(O)(N4py)]2+. Through a graphical analysis of the signs of the experimental C-term MCD signals, we unambiguously assign a low-energy MCD feature of [MnIV(O)(N4py)]2+ as the 4E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced MnIII-oxyl character on the excited-state 4E 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.

Steric and Electronic Influence on Proton-Coupled Electron-Transfer Reactivity of a Mononuclear Mn(III)-Hydroxo Complex.

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.