X-Band Electron Paramagnetic Resonance Comparison of Mononuclear Mn(IV)-oxo and Mn(IV)-hydroxo Complexes and Quantum Chemical Investigation of Mn(IV) Zero-Field Splitting

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.

MnIV4O4 Model of the S3 Intermediate of the Oxygen-Evolving Complex: Effect of the Dianionic Disiloxide Ligand

Synthetic complexes provide useful models to study the interplay between the structure and spectroscopy of the different Sn-state intermediates of the oxygen-evolving complex (OEC) of photosystem II (PSII). Complexes containing the MnIV4 core corresponding to the S3 state, the last observable intermediate prior to dioxygen formation, remain very rare. Toward the development of synthetic strategies to stabilize highly oxidized tetranuclear complexes, ligands with increased anion charge were pursued. Herein, we report the synthesis, electrochemistry, SQUID magnetometry, and electron paramagnetic resonance spectroscopy of a stable MnIV4O4 cuboidal complex supported by a disiloxide ligand. The substitution of an anionic acetate or amidate ligand with a dianionic disiloxide ligand shifts the reduction potential of the MnIIIMnIV3/MnIV4 redox couple by up to ∼760 mV, improving stability. The S = 3 spin ground state of the siloxide-ligated MnIV4O4 complex matches the acetate and amidate variants, in corroboration with the MnIV4 assignment of the S3 state of the OEC.

Protonation structure of the closed-cubane conformation of the O2-evolving complex in photosystem II

In photosystem II (PSII), one-electron oxidation of the most stable state of the oxygen-evolving Mn4CaO5 cluster (S1) leads to the S2 state formation, Mn1(III)Mn2(IV)Mn3(IV)Mn4(IV) (open-cubane S2) or Mn1(IV)Mn2(IV)Mn3(IV)Mn4(III) (closed-cubane S2). In electron paramagnetic resonance (EPR) spectroscopy, the g = 4.1 signal is not observed in cyanobacterial PSII but in plant PSII, whereas the g = 4.8 signal is observed in cyanobacterial PSII and extrinsic-subunit-depleted plant PSII. Here, we investigated the closed-cubane S2 conformation, a candidate for a higher spin configuration that accounts for g > 4.1 EPR signal, considering all pairwise exchange couplings in the PSII protein environment (i.e. instead of considering only a single exchange coupling between the [Mn3(CaO4)] cubane region and the dangling Mn4 site). Only when a ligand water molecule that forms an H-bond with D1-Asp61 (W1) is deprotonated at dangling Mn4(IV), the g = 4.1 EPR spectra can be reproduced using the cyanobacterial PSII crystal structure. The closed-cubane S2 is less stable than the open-cubane S2 in cyanobacterial PSII, which may explain why the g = 4.1 EPR signal is absent in cyanobacterial PSII.

S3 State Models of Nature's Water Oxidizing Complex: Analysis of Bonding and Magnetic Exchange Pathways, Assessment of Experimental Electron Paramagnetic Resonance Data, and Implications for the Water Oxidation Mechanism

Broken symmetry density functional theory (BS-DFT) calculations on large models of Nature's water oxidizing complex (WOC) are used to investigate the electronic structure and associated magnetic interactions of this key intermediate state. The electronic origins of the ferromagnetic and antiferromagnetic couplings between neighboring Mn ions are investigated and illustrated by using corresponding orbital transformations. Protonation of the O4 and/or O6 atoms leads to large variation in the distribution of spin around the complex with associated changes in its magnetic resonance properties. Models for Sr²⁺ exchange and methanol addition indicate minor perturbations reflected in slightly altered spin projection coefficients for the Mn₁ and Mn₂ ions. These are shown to account for the observed changes observed experimentally via electron paramagnetic resonance methods and suggest a reinterpretation of the experimental findings. By comparison with experimental determinations, we show that the spin projections and resulting calculated ⁵⁵Mn hyperfine couplings support the open cubane form of an oxo (O5)-hydroxo (O6) cluster in all cases with no need to invoke a closed cubane intermediate. The implications of these findings for the water oxidation mechanism are discussed.

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.

Quinol-containing ligands enable high superoxide dismutase activity by modulating coordination number, charge, oxidation states and stability of manganese complexes throughout redox cycling

Reactivity assays previously suggested that two quinol-containing MRI contrast agent sensors for H₂O₂, [Mn(H2qp1)(MeCN)]²⁺ and [Mn(H4qp2)Br₂], could also catalytically degrade superoxide. Subsequently, [Zn(H2qp1)(OTf)]⁺ was found to use the redox activity of the H2qp1 ligand to catalyze the conversion of O₂˙⁻ to O₂ and H₂O₂, raising the possibility that the organic ligand, rather than the metal, could serve as the redox partner for O₂˙⁻ in the manganese chemistry. Here, we use stopped-flow kinetics and cryospray-ionization mass spectrometry (CSI-MS) analysis of the direct reactions between the manganese-containing contrast agents and O₂˙⁻ to confirm the activity and elucidate the catalytic mechanism. The obtained data are consistent with the operation of multiple parallel catalytic cycles, with both the quinol groups and manganese cycling through different oxidation states during the reactions with superoxide. The choice of ligand impacts the overall charges of the intermediates and allows us to visualize complementary sets of intermediates within the catalytic cycles using CSI-MS. With the diquinolic H4qp2, we detect Mn(iii)-superoxo intermediates with both reduced and oxidized forms of the ligand, a Mn(iii)-hydroperoxo compound, and what is formally a Mn(iv)-oxo species with the monoquinolate/mono-para-quinone form of H4qp2. With the monoquinolic H2qp1, we observe a Mn(ii)-superoxo ↔ Mn(iii)-peroxo intermediate with the oxidized para-quinone form of the ligand. The observation of these species suggests inner-sphere mechanisms for O₂˙⁻ oxidation and reduction that include both the ligand and manganese as redox partners. The higher positive charges of the complexes with the reduced and oxidized forms of H2qp1 compared to those with related forms of H4qp2 result in higher catalytic activity (k_cat ∼ 10⁸ M⁻¹ s⁻¹ at pH 7.4) that rivals those of the most active superoxide dismutase (SOD) mimics. The manganese complex with H2qp1 is markedly more stable in water than other highly active non-porphyrin-based and even some Mn(ii) porphyrin-based SOD mimics.

Manganese complexes with polydentate quinol-containing ligands are found to catalyze the degradation of superoxide through inner-sphere mechanisms. The redox activity of the ligand stabilizes higher-valent manganese species.

Pulse EPR Spectroscopic Characterization of the S3 State of the Oxygen-Evolving Complex of Photosystem II Isolated from Synechocystis

The S₃ state is the last semi-stable state in the water splitting reaction that is catalyzed by the Mn₄O₅Ca cluster that makes up the oxygen-evolving complex (OEC) of photosystem II (PSII). Recent high-field/frequency (95 GHz) electron paramagnetic resonance (EPR) studies of PSII isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus have found broadened signals induced by chemical modification of the S₃ state. These signals are ascribed to an S₃ form that contains a five-coordinate Mn^IV center bridged to a cuboidal Mn^IV₃O₄Ca unit. High-resolution X-ray free-electron laser studies of the S₃ state have observed the OEC with all-octahedrally coordinated Mn^IV in what is described as an open cuboid-like cluster. No five-coordinate Mn^IV centers have been reported in these S₃ state structures. Here, we report high-field/frequency (130 GHz) pulse EPR of the S₃ state in Synechocystis sp. PCC 6803 PSII as isolated in the presence of glycerol. The S₃ state of PSII from Synechocystis exhibits multiple broadened forms (≈69% of the total signal) similar to those seen in the chemically modified S₃ centers from T. elongatus. Field-dependent ELDOR-detected nuclear magnetic resonance resolves two classes of ⁵⁵Mn nuclear spin transitions: one class with small hyperfine couplings (|A| ≈ 1-7 MHz) and another with larger hyperfine couplings (|A| ≈ 100 MHz). These results are consistent with an all-Mn^IV₄ open cubane structure of the S₃ state and suggest that the broadened S₃ signals arise from a perturbation of Mn4A and/or Mn3B, possibly induced by the presence of glycerol in the as-isolated Synechocystis PSII.

Model Dimeric Manganese(IV) Complexes Featuring Terminal Tris-hydroxotetraazaadamantane and Various Bridging Ligands

A series of model dinuclear manganese(IV) complexes of the general formula [(H₃COH)(L')Mn^IV(μ-L)₂Mn^IV(L')(HOCH₃)] is presented. These compounds feature capping 4,6,10-trihydroxo-3,5,7-trimethyl-1,4,6,10-tetraazaadamantane ligands derived from a polydentate oxime compound (L'). The bridging ligands L include azide (1), methoxide (2), and oxalate (3) anions. The magnetic properties and high-field (HF) EPR spectra of 1-3 were studied in detail and revealed varying weak antiferromagnetic coupling and modest zero-field splitting (ZFS) of the local quartet spin sites. Our HF EPR studies provide insight into the dimer ZFS, including determination of the corresponding parameters by giant spin approach for methoxido-bridged complex 2. Furthermore, the physicochemical properties of 1-3 were studied using IR, UV-vis, and electrochemical (cyclic voltammetry) methods. Theoretical exchange coupling constants were obtained using broken-symmetry (BS) density functional theory (DFT). Computational estimates of the local quartet ground spins state ZFSs of 1-3 were obtained using coupled-perturbed (CP) DFT and complete active space self-consistent field (CASSCF) calculations with n-electron valence state perturbation theory (NEVPT2) corrections. We found that the CP DFT calculations which used the B3LYP functional and models derived experimental structures performed best in reproducing both the magnitude and the sign of the experimental D values. Moreover, our computational investigation of 1-3 suggests that we observe metals sites which have an increased +3 character and are supported by redox noninnocent 4,6,10-trihydroxo-3,5,7-trimethyl-1,4,6,10-tetraazaadamantane ligands. The latter conclusion is further corroborated by the observation that the free ligand can be readily oxidized to yield a NO-based radical.

Molecular Identification of a High-Spin Deprotonated Intermediate during the S2 to S3 Transition of Nature's Water-Oxidizing Complex

The identity of a key intermediate in the S₂ to S₃ transition of nature's water-oxidizing complex (WOC) in Photosystem 2 is presented. Broken-symmetry density functional theory (BS-DFT) calculations and Heisenberg-Dirac-van Vleck (HDvV) spin ladder calculations show that an S₂ state open cubane model of the WOC containing a μ-hydroxo O4 changes from an S = ⁵/₂ form to an S = ⁷/₂, form upon deprotonation of W1. Combined with X-band electron paramagnetic resonance (EPR) spectral analysis, this indicates that the g = 4.1 EPR signal corresponds to an S = ⁵/₂ form of the WOC with W1 present as a water ligand to Mn₄, while the g = 4.8/4.9 form observed at high pH values corresponds to an S = ⁷/₂ form, with W1 as a hydroxo ligand. The latter is also likely to represent the form needed to progress to S₃ in the functioning enzyme.

S = 3 Ground State for a Tetranuclear MnIV4O4 Complex Mimicking the S3 State of the Oxygen-Evolving Complex

The S₃ state is currently the last observable intermediate prior to O-O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn^IV₄ core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S₃ state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S₃ state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn^IV₄O₄ cuboidal complex as a spectroscopic model of the S₃ state. Results show that this Mn^IV₄O₄ complex has an S = 3 ground state with isotropic ⁵⁵Mn hyperfine coupling constants of -75, -88, -91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer-monomer magnetic coupling model of pseudo-octahedral Mn^IV centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S₂ state to the S₃ state. This same spin state change is observed following oxidation of the previously reported Mn^IIIMn^IV₃O₄ cuboidal complex to the Mn^IV₄O₄ complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC.

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

Analysis of extended X-ray absorption fine structure (EXAFS) data for the MnIV -oxo complexes [MnIV (O)(DMM N4py)]2+ , [MnIV (O)(2pyN2B)]2+ , and [MnIV (O)(2pyN2Q)]2+ (DMM N4py=N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B=(N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, and 2pyN2Q=N,N-bis(2-pyridyl)-N,N-bis(2-quinolylmethyl)methanamine) afforded Mn=O and Mn-N bond lengths. The Mn=O distances for [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ are 1.72 and 1.70 Å, respectively. In contrast, the Mn=O distance for [MnIV (O)(2pyN2Q)]2+ was significantly longer (1.76 Å). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [MnIV (O)(2pyN2Q)]2+ . The Mn=O distances for [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ could only be well-reproduced using DFT-derived models that included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggest an important role for the 2,2,2-trifluoroethanol solvent in stabilizing MnIV -oxo adducts. The DFT methods were extended to investigate the structure of the putative [MnIV (O)(N4py)]2+ ⋅(HOTf)2 adduct. These computations suggest that a MnIV -hydroxo species is most consistent with the available experimental data.

MnIV-Oxo complex of a bis(benzimidazolyl)-containing N5 ligand reveals different reactivity trends for MnIV-oxo than FeIV-oxo species

Using the pentadentate ligand (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, 2pyN2B), presenting two pyridyl and two (N-methyl)benzimidazolyl donor moieties in addition to a central tertiary amine, new MnII and MnIV-oxo complexes were generated and characterized. The [MnIV(O)(2pyN2B)]2+ complex showed spectroscopic signatures (i.e., electronic absorption band maxima and intensities, EPR signals, and Mn K-edge X-ray absorption edge and near-edge data) similar to those observed for other MnIV-oxo complexes with neutral, pentadentate N5 supporting ligands. The near-IR electronic absorption band maximum of [MnIV(O)(2pyN2B)]2+, as well as DFT-computed metric parameters, are consistent with the equatorial (N-methyl)benzimidazolyl ligands being stronger donors to the MnIV center than the pyridyl and quinolinyl ligands found in analogous MnIV-oxo complexes. The hydrogen- and oxygen-atom transfer reactivities of [MnIV(O)(2pyN2B)]2+ were assessed through reactions with hydrocarbons and thioanisole, respectively. When compared with related MnIV-oxo adducts, [MnIV(O)(2pyN2B)]2+ showed muted reactivity in hydrogen-atom transfer reactions with hydrocarbons. This result stands in contrast to observations for the analogous FeIV-oxo complexes, where [FeIV(O)(2pyN2B)]2+ was found to be one of the more reactive members of its class.

Manganese-Hydroxido Complexes Supported by a Urea/Phosphinic Amide Tripodal Ligand

Hydrogen bonds (H-bonds) within the secondary coordination sphere are often invoked as essential noncovalent interactions that lead to productive chemistry in metalloproteins. Incorporating these types of effects within synthetic systems has proven a challenge in molecular design that often requires the use of rigid organic scaffolds to support H-bond donors or acceptors. We describe the preparation and characterization of a new hybrid tripodal ligand ([H₂pout]^3-) that contains two monodeprotonated urea groups and one phosphinic amide. The urea groups serve as H-bond donors, while the phosphinic amide group serves as a single H-bond acceptor. The [H₂pout]^3- ligand was utilized to stabilize a series of Mn-hydroxido complexes in which the oxidation state of the metal center ranges from 2+ to 4+. The molecular structure of the Mn^III-OH complex demonstrates that three intramolecular H-bonds involving the hydroxido ligand are formed. Additional evidence for the formation of intramolecular H-bonds was provided by vibrational spectroscopy in which the energy of the O-H vibration supports its assignment as an H-bond donor. The stepwise oxidation of [Mn^IIH₂pout(OH)]^2- to its higher oxidized analogs was further substantiated by electrochemical measurements and results from electronic absorbance and electron paramagnetic resonance spectroscopies. Our findings illustrate the utility of controlling both the primary and secondary coordination spheres to achieve structurally similar Mn-OH complexes with varying oxidation states.

Structurally characterized terminal manganese(iv) oxo tris(alkoxide) complex

First structurally characterized Mn(iv) oxo.

A Mn(iv) complex featuring a terminal oxo ligand, [Mn^IV(O)(ditox)₃][K(15-C-5)₂] (3; ditox = ^tBu₂MeCO^–, 15-C-5 = 15-crown-5-ether) has been isolated and structurally characterized. Treatment of the colorless precursor [Mn^II(ditox)₃][K(15-C-5)₂] (2) with iodosobenzene affords 3 as a green free-flowing powder in high yields. The X-ray crystal structure of 3 reveals a pseudotetrahedral geometry about the central Mn, which features a terminal oxo (d(Mn–O_term = 1.628(2) Å)). EPR spectroscopy, SQUID magnetometry, and Evans method magnetic susceptibility indicate that 3 consists of a high-spin S = 3/2 Mn(iv) metal center. 3 promotes C–H bond activation by a hydrogen atom abstraction. The [Mn^IV(O)(ditox)₃]^– furnishes a model for the proposed terminal oxo of the unique manganese of the oxygen evolving complex of photosystem II.

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 Mn^III-peroxo adducts and hydrogen-atom transfer reactivity of Mn^IV-oxo and Mn^III-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^III-peroxo adducts with varied ligand environments. Using parallel-mode EPR and Mn K-edge X-ray absorption techniques, the decay pathway of one Mn^III-peroxo complex bearing a bulky macrocylic ligand was investigated. Unlike many Mn^III-peroxo model complexes that decay to oxo-bridged-Mn^IIIMn^IV dimers, decay of this Mn^III-peroxo adduct yielded mononuclear Mn^III-hydroxo and Mn^IV-oxo products, potentially resulting from O-O bond activation of the Mn^III-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^III-peroxo complexes that convert cleanly to high-valent Mn-oxo species. Although some synthetic Mn^IV-oxo complexes show great potential for oxidizing substrates with strong C-H bonds, most Mn^IV-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^IV-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^III/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 Mn^III-hydroxo adduct initiates substrate oxidation by abstracting a hydrogen atom from a C-H bond. Precedent for this chemistry from synthetic Mn^III-hydroxo centers is rare. To better understand hydrogen-atom transfer by Mn^III centers, we developed a pair of Mn^III-hydroxo complexes, formed in high yield from dioxygen oxidation of Mn^II 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 Mn^III-hydroxo units as mild oxidants.

Manganese-Cobalt Oxido Cubanes Relevant to Manganese-Doped Water Oxidation Catalysts

Incorporation of Mn into an established water oxidation catalyst based on a Co(III)₄O₄ cubane was achieved by a simple and efficient assembly of permanganate, cobalt(II) acetate, and pyridine to form the cubane oxo cluster MnCo₃O₄(OAc)₅py₃ (OAc = acetate, py = pyridine) (1-OAc) in good yield. This allows characterization of electronic and chemical properties for a manganese center in a cobalt oxide environment, and provides a molecular model for Mn-doped cobalt oxides. The electronic properties of the cubane are readily tuned by exchange of the OAc^- ligand for Cl^- (1-Cl), NO₃^- (1-NO₃), and pyridine ([1-py]⁺). EPR spectroscopy, SQUID magnetometry, and DFT calculations thoroughly characterized the valence assignment of the cubane as [Mn^IVCo^III₃]. These cubanes are redox-active, and calculations reveal that the Co ions behave as the reservoir for electrons, but their redox potentials are tuned by the choice of ligand at Mn. This MnCo₃O₄ cubane system represents a new class of easily prepared, versatile, and redox-active oxido clusters that should contribute to an understanding of mixed-metal, Mn-containing oxides.

Equatorial Ligand Perturbations Influence the Reactivity of Manganese(IV)-Oxo Complexes

Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic Mn^IV -oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of Mn^IV -oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the Mn^IV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these Mn^IV -oxo complexes, the rate enhancements are correlated with both 1) the energy of a low-lying ⁴ E excited state, which has been postulated to be involved in a two-state reactivity model, and 2) the Mn^III/IV reduction potentials.

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

The mononuclear Mn(IV)-oxo complex [Mn^IV(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 [Mn^IV(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 [Mn^IV(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 [Mn^IV(O)(N4py)]²⁺ as the ⁴E excited state predicted to be involved in hydrogen-atom-transfer reactivity. The CASSCF calculations predict enhanced Mn^III-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.

Determination and prediction of the magnetic anisotropy of Mn ions

EPR spectroscopy combined with quantum chemistry for the investigation of the magnetic anisotropy of Mn^II, Mn^III and Mn^IV.