Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn3MOn Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

Interplay between protonation and Jahn-Teller effects in a manganese vanadium cubane water oxidation catalyst

Understanding the protonation behavior of metal-oxo water oxidation catalysts is essential to improve catalyst efficiency and long-term performance, as well as to tune their properties for specific applications. In this work, we explore the basicity and protonation effects of the highly active water oxidation catalyst [(Mn4O4) (V4O13) (OAc)3]3- using density functional theory. We computed the relative free energies of protonation in a systematic fashion for all symmetry-inequivalent O atoms, where the presence of multiple oxidation states from Mn4IV to Mn4III and a rich Jahn-Teller isomerism adds a significant amount of complexity. For high oxidation states, the compound behaves like some other polyoxometalates, showing protonation preferably at the terminal and μ2-bridging O atoms of the vanadate cap. However, upon reduction, eventually, the protonation preference switches to the cubane O atoms, mostly driven by a strong increase in basicity for O atoms located along the Jahn-Teller axes. Our work further evidences that protonation can potentially lead to several chemical transformations, like disproportionation and charge transfer to vanadium, dissociation of ligands, or the opening of the cubane structure. Our simulated UV/Vis absorption spectra additionally provide valuable insights about how the protonation of the catalyst could be tracked experimentally. Overall, our analysis highlights the complexity involved in the protonation of heterometallic polyoxometalate clusters.

Coordination Number in High-Spin-Low-Spin Equilibrium in Cluster Models of the S2 State of the Oxygen Evolving Complex

The S2 state of the Oxygen Evolving Complex (OEC) of Photosystem II (PSII) shows high-spin (HS) and low-spin (LS) EPR signals attributed to distinct structures based on computation. Five-coordinate MnIII centers are proposed in these species but are absent in available spectroscopic model complexes. Herein, we report the synthesis, crystal structure, electrochemistry, SQUID magnetometry, and EPR spectroscopy of a MnIIIMnIV3O4 cuboidal complex featuring five-coordinate MnIII. This cluster displays a spin ground state of S = 5/2, while conversion to a six-coordinate Mn upon treatment with water results in a spin state change to S = 1/2. These results demonstrate that coordination number, without dramatic changes within the Mn4O4 core, has a substantial effect on spectroscopy.

Regeneration and Degradation in a Biomimetic Polyoxometalate Water Oxidation Catalyst

Complete understanding of catalytic cycles is required to advance the design of water oxidation catalysts, but it is difficult to attain, due to the complex factors governing their reactivity and stability. In this study, we investigate the regeneration and degradation pathways of the highly active biomimetic water oxidation catalyst [Mn3+ 2Mn4+ 2V4O17(OAc)3]3-, thereby completing its catalytic cycle. Beginning with the deactivated species [Mn3+ 4V4O17(OAc)2]4- left over after O2 evolution, we scrutinize a network of reaction intermediates belonging to two alternative water oxidation cycles. We find that catalyst regeneration to the activated species [Mn4+ 4V4O17(OAc)2(OH)(H2O)]- proceeds via oxidation of each Mn center, with one water ligand being bound during the first oxidation step and a second water ligand being bound and deprotonated during the final oxidation step. ΔΔG values for this last oxidation are consistent with previous experimental results, while regeneration within an alternative catalytic cycle was found to be thermodynamically unfavorable. Extensive in silico sampling of catalyst structures also revealed two degradation processes: cubane opening and ligand dissociation, both of which have low barriers at highly reduced states of the catalyst due to the presence of Jahn-Teller effects. These mechanistic insights are expected to spur the development of more efficient and stable Mn cubane water oxidation catalysts.

The Role of Redox-Inactive Metals in Modulating the Redox Potential of the Mn4CaO4 Model Complex

Photosynthetic oxygen-evolving center (OEC), the "engine of life", is a unique Mn₄CaO₅ cluster catalyzing the water oxidation. The role of redox-inactive component Ca²⁺, which can only be functionally replaced by Sr²⁺ in a biological environment, has been under debate for a long time. Recently, its modulating effect on the redox potential of native OEC and artificial structural OEC model complex has received great attention, and linear relationship between the potential and the Lewis acidity of the redox-inactive metal has been proposed for the MMn₃O₄ model complex. In this work, the modulating effect has been studied in detail using the Mn₄CaO₄ model complex, which is the closest structural model to OEC to date and has a similar redox potential at the S₁-S₂ transition. We found the redox-inactive metal only has a weak modulating effect on the potential, which is comparable in strength to that of the ligand environments. Meanwhile, the net charge of the complex, which could be changed along with the redox-inactive metal, has a high impact on the potential and can be unified by protonation, deprotonation, or ligand modification. Although the modulating effect of the redox-inactive metal is not very strong, the linear relationship between the potential and the Lewis acidity is still valid for Mn₄MO₄ complexes. Our results of strong modulating effects for net charge and weak modulating effects for redox-inactive metal fit with the previous experimental observations on Mn₄MO₄ (M = Ca²⁺, Y³⁺, and Gd³⁺) model complexes, and suggest that Ca²⁺ can be structurally and electrochemically replaced with other metal cations, together with proper ligand modifications.

Hemicubane topological analogs of the oxygen-evolving complex of photosystem II mediating water-assisted propylene carbonate oxidation

A series of Ca-Mn clusters with the ligand 2-pyridinemethoxide (Py-CH₂O) have been prepared with varying degrees of topological similarity to the biological oxygen-evolving complex. These clusters activate water as a substrate in the oxidative degradation of propylene carbonate, with activity correlated with topological similarity to the OEC, lowering the onset potential of the oxidation by as much as 700 mV.

Jahn-Teller Effects in a Vanadate-Stabilized Manganese-Oxo Cubane Water Oxidation Catalyst

Heuristic rules that allow identifying the preferred mixed-valence isomers and Jahn-Teller axis arrangements in the water oxidation catalyst [(Mn4 O4 )(V4 O13 )(OAc)3 ]n- and its activated form [(Mn4 O4 )(V4 O13 )(OAc)2 (H2 O)(OH)]n- are derived. These rules are based on computing all combinatorially possible mixed-valence isomers and Jahn-Teller axis arrangements of the MnIII atoms, and associate energetic costs with some structural features, like crossings of multiple Jahn-Teller axes, the location of these axes, or the involved ligands. It is found that the different oxidation states localize on different Mn centers, giving rise to clear Jahn-Teller distortions, unlike in previous crystallographic findings where an apparent valence delocalization was found. The low barriers that connect different Jahn-Teller axis arrangements suggest that the system quickly interconverts between them, leading to the observation of averaged bond lengths in the crystal structure. We conclude that the combination of cubane-vanadate bonds that are chemically inert, cubane-acetate/water bonds that can be activated through a Jahn-Teller axis, and low activation barriers for intramolecular rearrangement of the Jahn-Teller axes plays an essential role in the reactivity of this and probably related compounds.

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR-Lewis Acid)] Cores

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe₄)[Mn^V(HMPAB)(O)] [2; H₄HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn-O_term distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (Mⁿ⁺ = Li⁺, Ca²⁺, Mg²⁺, Y³⁺, and Sc³⁺) with 2 resulted in the formation of 2-Mⁿ⁺ species, which were characterized by UV-vis, ¹H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the Mⁿ⁺ ions in 2. Complex 2 revealed one-electron-reduction potential at -0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of Mⁿ⁺ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-Mⁿ⁺ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of Mⁿ⁺ ions, and a plot of log k₂ versus Lewis acidity of Mⁿ⁺ ions (ΔE) followed a linear relationship. It was observed that 2-Sc³⁺ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar₃P substrates. In contrast, 2-Ca²⁺ and 2-Sc³⁺ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-Mⁿ⁺ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of Mⁿ⁺ ions. On the basis of Eyring analysis of 2 and 2-Mⁿ⁺ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc³⁺, which is different from the reaction mechanism of 2 and 2-Ca²⁺.

Flexibility Enhances Reactivity: Redox Isomerism and Jahn-Teller Effects in a Bioinspired Mn4O4 Cubane Water Oxidation Catalyst

Understanding how water oxidation to molecular oxygen proceeds in molecular metal-oxo catalysts is a challenging endeavor due to their structural complexity. In this report, we unravel the water oxidation mechanism of the highly active water oxidation catalyst [Mn4V4O17(OAc)3]3-, a polyoxometalate catalyst with a [Mn4O4]6+ cubane core reminiscent of the natural oxygen-evolving complex. Starting from the activated species [Mn4 4+V4O17(OAc)2(H2O)(OH)]1-, we scrutinized multiple pathways to find that water oxidation proceeds via a sequential proton-coupled electron transfer (PCET), O-O bond formation, another PCET, an intramolecular electron transfer, and another PCET resulting in O2 evolution, with a predicted thermodynamic overpotential of 0.71 V. An in-depth investigation of the O-O bond formation process revealed an essential interplay between redox isomerism and Jahn-Teller effects, responsible for enhancing reactivity in the catalytic cycle. This is achieved by redistributing electrons between metal centers and weakening relevant bonds through Jahn-Teller distortions, introducing flexibility to the otherwise rigid cubane core of the catalyst. These mechanistic insights are expected to advance the design of efficient bioinspired Mn cubane water-splitting catalysts.

CaMn3 IV O4 Cubane Models of the Oxygen-Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn₃ ^IV O₄ and YMn₃ ^IV O₄ complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn₃ ^IV O₄ complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C₃ -symmetric Ca(Y)Mn₃ ^IV O₄ core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn₃ ^IV O₃ (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S₂ state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes.

Role of redox-inactive metals in controlling the redox potential of heterometallic manganese-oxido clusters

Photosystem II (PSII) contains Ca2+, which is essential to the oxygen-evolving activity of the catalytic Mn4CaO5 complex. Replacement of Ca2+ with other redox-inactive metals results in a loss/decrease of oxygen-evolving activity. To investigate the role of Ca2+ in this catalytic reaction, we investigate artificial Mn3[M]O2 clusters redox-inactive metals  [M] ([M]  = Mg2+, Ca2+, Zn2+, Sr2+, and Y3+), which were synthesized by Tsui et al. (Nat Chem 5:293, 2013). The experimentally measured redox potentials (Em) of these clusters are best described by the energy of their highest occupied molecular orbitals. Quantum chemical calculations showed that the valence of metals predominantly affects Em(MnIII/IV), whereas the ionic radius of metals affects Em(MnIII/IV) only slightly.

Bridging Polyoxometalate-Based Mn4 Cubane Clusters with Inorganic Phosphates: Structural Transformation and Magnetic Properties

The mixed-valent tetramanganese Mn^III₃Mn^IV (Mn₄) cubane clusters have been at the forefront of molecular magnetism and biomimetic catalysis research for decades. Incorporating robust polyoxometalates to Mn₄ cubanes significantly improves their stability and aqueous solubility, while providing a great platform for studying their deposition onto selected surfaces during device fabrication. In this work, we discovered that the terminal carboxylate ligands in these polyoxometalate-based [Mn^III₃Mn^IVO₄] magnetic clusters can be partially or completely replaced by inorganic phosphate/polyphosphate groups. This replacement leads to oligomeric aggregates of the Mn₄ clusters. The magnetic data of the monomeric and oligomeric Mn₄ clusters suggested that the introduction of inorganic phosphate bridges may not alter the S = 9/2 ground state of individual Mn₄ clusters, although different magnetic behaviors, especially at low temperatures, were observed primarily because of intercluster interactions.

Ligand architecture for triangular metal complexes: a high oxidation state Ni3 cluster with proximal metal arrangement

A new multidentate tetraanionic ligand platform for supporting trinuclear transition metal clusters has been developed. Two trisphenoxide phosphinimide ligands bind three Ni centers in a triangular arrangement. The phosphinimide donors bridge in μ3 fashion and the phenoxides complete a pseudo-square planar coordination sphere around each metal center. Electrochemical studies reveal two pseudo-reversible oxidation events at notably low potentials (-0.80 V and +0.05 V). The one electron oxidized species was characterized structurally, and it is assigned as a NiIII-containing cluster.

Counterion Dependence of Dinitrogen Activation and Functionalization by a Diniobium Hydride Anion

We report the synthesis of anionic diniobium hydride complexes with a series of alkali metal cations (Li⁺ , Na⁺ , and K⁺ ) and the counterion dependence of their reactivity with N₂ . Exposure of these complexes to N₂ initially produces the corresponding side-on end-on N₂ complexes, the fate of which depends on the nature of countercations. The lithium derivative undergoes stepwise migratory insertion of the hydride ligands onto the aryloxide units, yielding the end-on bridging N₂ complex. For the potassium derivative, the N-N bond cleavage takes place along with H₂ elimination to form the nitride complex. Treatment of the side-on end-on N₂ complex with Me₃ SiCl results in silylation of the terminal N atom and subsequent N-N bond cleavage along with H₂ elimination, giving the nitride-imide-bridged diniobium complex.

"Canopy Catalysts" for Alkyne Metathesis: Molybdenum Alkylidyne Complexes with a Tripodal Ligand Framework

A new family of structurally well-defined molybdenum alkylidyne catalysts for alkyne metathesis, which is distinguished by a tripodal trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based catalysts with a significantly improved functional group tolerance. They are easy to prepare on scale; the modularity of the ligand synthesis allows the steric and electronic properties to be fine-tuned and hence the application profile of the catalysts to be optimized. This opportunity is manifested in the development of catalyst 1f, which is as reactive as the best ancestors but exhibits an unrivaled scope. The new catalysts work well in the presence of unprotected alcohols and various other protic groups. The chelate effect entails even a certain stability toward water, which marks a big leap forward in metal alkylidyne chemistry in general. At the same time, they tolerate many donor sites, including basic nitrogen and numerous heterocycles. This aspect is substantiated by applications to polyfunctional (natural) products. A combined spectroscopic, crystallographic, and computational study provides insights into structure and electronic character of complexes of type 1. Particularly informative are a density functional theory (DFT)-based chemical shift tensor analysis of the alkylidyne carbon atom and ⁹⁵Mo NMR spectroscopy; this analytical tool had been rarely used in organometallic chemistry before but turns out to be a sensitive probe that deserves more attention. The data show that the podand ligands render a Mo-alkylidyne a priori more electrophilic than analogous monodentate triarylsilanols; proper ligand tuning, however, allows the Lewis acidity as well as the steric demand about the central atom to be adjusted to the point that excellent performance of the catalyst is ensured.

Templated syntheses of heterometal tungsten-cobalt-sulfur clusters with different nuclearities

A series of new W-Co-S clusters have been synthesized utilizing the complex [Et₄N][(Tp*)WS₃] (Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate(1-)) as the synthetic template, by rational control of the stoichiometry of reactants and by employment of a reductant or precipitant as an auxiliary reagent. By solubility control or addition of a precipitant to avoid the involvement of Cl^- in the reaction, the cluster [(Tp*)WS₃]₂Co (2) could be obtained. With different ratios of the Co source to the template, together with the use of an appropriate amount of reducing reagent, clusters [{(Tp*)WS₃}₂Co]^1- (3) and [{(Tp*)WS₃}₂Co₂Cl]^1- (4) could be synthesized as their Et₄N⁺ salts, and [{(Tp*)WS₃}₂Co₄Cl₂(DMF)]^1- (5) could be isolated as its [Co(DMF)₆]²⁺ salt. A common structural feature of these clusters is that they all have a pair of [(Tp*)WS₃] templates chelating different numbers of Co atoms to form W-Co-S clusters with distinct nuclearities. The clusters [(Tp*)WS₃]₂Co (2) and [{(Tp*)WS₃}₂Co]^1- (3) both have one Co atom chelated by two [(Tp*)WS₃] templates, and the difference lies in not only the oxidation states but also the fact that the Co atom in 2 adopts an octahedral coordination environment while that in 3 bears a tetrahedral coordination sphere, which means that the [(Tp*)WS₃] template is tri-dentate in 2 and bi-dentate in 3. The cluster [{(Tp*)WS₃}₂Co₂Cl]^1- (4) has two Co atoms chelated by a pair of [(Tp*)WS₃] templates, in which one of the Co atoms is also terminally bound by a Cl^- ligand. There are four Co atoms chelated by two [(Tp*)WS₃] templates in the cluster [{(Tp*)WS₃}₂Co₄Cl₂(DMF)]^1- (5), two of which are bound terminally by the Cl^- ligands. Studies of redox properties by cyclic voltammetry indicate the existence of two reversible redox pairs related to clusters 2 and 3, respectively.

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.

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 Mn^III-hydroxo complex [Mn^III(OH)(dpaq)]⁺ (1) gives rise to new intermediates with spectroscopic properties and chemical reactivity distinct from those of [Mn^III(OH)(dpaq)]⁺. The electronic absorption spectra of [Mn^III(OH)(dpaq)]⁺ in the presence of Sc(OTf)₃ (1-Sc^III) and Al(OTf)₃ (1-Al^III) show modest perturbations in electronic transition energies, consistent with moderate changes in the Mn^III geometry. A comparison of ¹H NMR data for 1 and 1-Sc^III confirm this conclusion, as the ¹H NMR spectrum of 1-Sc^III shows the same number of hyperfine-shifted peaks as the ¹H NMR spectrum of 1. These ¹H NMR spectra, and that of 1-Al^III, 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-Sc^III 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-Sc^III and 1-Al^III show greatly enhanced reactivity toward hydrocarbons. While 1 is unreactive toward 9,10-dihydroanthracene (DHA), 1-Sc^III and 1-Al^III react rapidly with DHA (k₂ = 0.16(1) and 0.25(2) M^-1 s^-1 at 50 °C, respectively). The 1-Sc^III 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-Sc^III and 1-Al^III with the recently reported Mn^III-aqua complex [Mn^III(OH₂)(dpaq)]²⁺ ( J. Am. Chem. Soc. 2018, 140, 12695-12699). Because 1-Sc^III and 1-Al^III show ¹H NMR spectra essentially identical to that of [Mn^III(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.

Oxygen-atom vacancy formation and reactivity in polyoxovanadate clusters

Overview of recent work detailing oxygen-deficient polyoxovanadate clusters as models for reducible metal oxides: toward gaining a fundamental understanding the consequences of vacancy formation on metal oxide surfaces during catalysis.

Molybdenum Alkylidyne Complexes with Tripodal Silanolate Ligands: The Next Generation of Alkyne Metathesis Catalysts

A new type of molybdenum alkylidyne catalysts for alkyne metathesis is described, which is distinguished by an unconventional podand topology. These structurally well-defined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups; even certain protic substituents were found to be compatible. The new catalysts were characterized by X-ray crystallography and by spectroscopic means, including 95 Mo NMR.

Oxygen atom transfer with organofunctionalized polyoxovanadium clusters: O-atom vacancy formation with tertiary phosphanes and deoxygenation of styrene oxide

We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes followed by OAT from styrene oxide to the reduced scaffold, resulting in the formation of styrene.

We report a rare example of oxygen atom transfer (OAT) from a polyoxometalate cluster to a series of tertiary phosphanes. Addition of PR₃ (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃) to a neutral methoxide-bridged polyoxovanadate-alkoxide (POV-alkoxide) cluster, [V₆O₇(OMe)₁₂]⁰, results in isolation of a reduced structure with phosphine oxide datively coordinated to a site-differentiated V^III ion. A positive correlation between the steric and electronic properties of the phosphane and the reaction rate was observed. Further investigation of the steric influence of the alkoxy-bridged clusters on OAT was probed through the use of POV clusters with bridging alkoxide ligands of varying chain length ([V₆O₇(OR′)₁₂]; R′ = Et, ⁿPr). These investigations expose that steric hinderance of the vanadyl moieties has significant influence on the rate of OAT. Finally, we report the reactivity of the reduced POV-alkoxide clusters with styrene oxide, resulting in the deoxygenation of the substrate to generate styrene. This result is the first example of epoxide deoxygenation using homometallic polyoxometalate clusters, demonstrating the potential for mono-vacant Lindqvist clusters to catalyze the removal of oxygen atoms from organic substrates.

Stabilization of reactive Co4O4 cubane oxygen-evolution catalysts within porous frameworks

A major challenge to the implementation of artificial photosynthesis (AP), in which fuels are produced from abundant materials (water and carbon dioxide) in an electrochemical cell through the action of sunlight, is the discovery of active, inexpensive, safe, and stable catalysts for the oxygen evolution reaction (OER). Multimetallic molecular catalysts, inspired by the natural photosynthetic enzyme, can provide important guidance for catalyst design, but the necessary mechanistic understanding has been elusive. In particular, fundamental transformations for reactive intermediates are difficult to observe, and well-defined molecular models of such species are highly prone to decomposition by intermolecular aggregation. Here, we present a general strategy for stabilization of the molecular cobalt-oxo cubane core (Co₄O₄) by immobilizing it as part of metal-organic frameworks, thus preventing intermolecular pathways of catalyst decomposition. These materials retain the OER activity and mechanism of the molecular Co₄O₄ analog yet demonstrate unprecedented long-term stability at pH 14. The organic linkers of the framework allow for chemical fine-tuning of activity and stability and, perhaps most importantly, provide "matrix isolation" that allows for observation and stabilization of intermediates in the water-splitting pathway.

Redox Tuning via Ligand-Induced Geometric Distortions at a YMn3O4 Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn₄O₄] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle.

The redox properties of [YMn₃O₄] and [Mn₄O₄] complexes featuring bridging ligands of different basicity and chelating properties are reported. In the absence of ligand-induced geometric distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. A chelating ligand results in contractions of Y-oxo distances by ∼0.1 Å, which correlates with an increase of cluster reduction potential even in the presence of strongly basic donors.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn₃ and Fe₃ cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn₃MO₄], [Mn₃MO(OH)], and [Fe₃MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Tetranuclear [MnIIIMn3IVO4] Complexes as Spectroscopic Models of the S2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S₂ state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure-electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^IIIMn₃^IVO₄] cuboidal complexes as spectroscopic models of the S₂ state of the OEC. Resembling the oxidation state and EPR spectra of the S₂ state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed.

Uncoupled Redox-Inactive Lewis Acids in the Secondary Coordination Sphere Entice Ligand-Based Nitrite Reduction

Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(15bz5PDI)(CO)2 was shown to encapsulate the redox-inactive alkali ion, Na+, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na+, can entice the corresponding counterion, NO2-, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.

A CaMn4O2 model of the biological oxygen evolving complex: synthesis via cluster expansion on a low symmetry ligand

Using a new multinucleating ligand featuring two dipyridyl alkoxide moieties and a carboxylate moiety, low symmetry tetranuclear complexes 1-M (M = Mn, Fe, and Co) have been synthesized. Complex 1-Mn was used as a precursor for the synthesis of a pentanuclear CaMn₄O₂ cluster (3) with the same metal stoichiometry as the biological OEC.

Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers

X-ray absorption spectroscopy at the L-edge of 3d transition metals provides unique information on the local metal charge and spin states by directly probing 3d-derived molecular orbitals through 2p-3d transitions. However, this soft x-ray technique has been rarely used at synchrotron facilities for mechanistic studies of metalloenzymes due to the difficulties of x-ray-induced sample damage and strong background signals from light elements that can dominate the low metal signal. Here, we combine femtosecond soft x-ray pulses from a free-electron laser with a novel x-ray fluorescence-yield spectrometer to overcome these difficulties. We present L-edge absorption spectra of inorganic high-valent Mn complexes (Mn ∼ 6-15 mmol/l) with no visible effects of radiation damage. We also present the first L-edge absorption spectra of the oxygen evolving complex (Mn₄CaO₅) in Photosystem II (Mn < 1 mmol/l) at room temperature, measured under similar conditions. Our approach opens new ways to study metalloenzymes under functional conditions.

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.

Design of porphyrin-based ligands for the assembly of [d-block metal : calcium] bimetallic centers

A secondary binding-site for alkaline-earth cations is introduced on a porphyrin platform to obtain competent bitopicN,O-ligands.

Effect of the Mn Oxidation State on Single-Molecule-Magnet Properties: Mn(III) vs Mn(IV) in Biologically Inspired DyMn3O4 Cubanes

Inspired by the ferromagnetic coupling in the cubane model CaMn(IV)3O4 of the oxygen-evolving complex of photosystem II, 3d-4f mixed-metal DyMn3O4 clusters were prepared for investigation of the magnetic properties. For comparison, YMn(IV)3O4 and YMn(IV)2Mn(III)O4 clusters were investigated as well and showed ferromagnetic interactions, like the calcium analogue. DyMn(IV)3O4 displays single-molecule-magnet properties, while the one-electron-reduced species (DyMn(IV)2Mn(III)O4) does not, despite the presence of a Mn(III) center with higher spin and single-ion anisotropy.