Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

Chromium-Thiolate Complex Undergoing C-S Bond Cleavage

The cleavage of C-S bonds represents a crucial step in fossil fuel refinement to remove organosulfur impurities. Efforts are required to identify alternatives that can replace the energy-intensive hydrodesulfurization process currently in use. In this context, we have developed a series of bis-thiolato-ligated CrIII complexes supported by the L2- ligand (L2- = 2,2'-bipyridine-6,6'-diyl(bis(1,1-diphenylethanethiolate), one of them displaying desulfurization of one thiolate of the ligand under reducing and acidic conditions at ambient temperature and atmospheric pressure. While only 5-coordinated complexes were previously isolated by reaction of L2- with 3d metal MIII ions, both 5- and 6-coordinated mononuclear complexes have been obtained in the case of CrIII, viz., [CrIIILCl], [CrIIILCl2]-, and [CrIIILCl(CH3CN)]. The investigation of the reactivity of [CrIIILCl(CH3CN)] under reducing conditions led to a dinuclear [CrIII2L2(μ-Cl)(μ-OH)] compound and, in the presence of protons, to the mononuclear CrIII complex [CrIII(LN2S)2]+, where LN2S- is the partially desulfurized form of L2-. A desulfurization mechanism has been proposed involving the release of H2S, as evidenced experimentally.

Boundary-Rich Carbon-Based Electrocatalysts with Manganese(II)-Coordinated Active Environment for Selective Synthesis of Hydrogen Peroxide

Coordinated manganese (Mn) electrocatalysts owing to their electronic structure flexibility, non-toxic and earth abundant features are promising for electrocatalytic reactions. However, achieving selective hydrogen peroxide (H2 O2 ) production through two electron oxygen reduction (2e-ORR) is a challenge on Mn-centered catalysts. Targeting this goal, we report on the creation of a secondary Mn(II)-coordinated active environment with reactant enrichment effect on boundary-rich porous carbon-based electrocatalysts, which facilitates the selective and rapid synthesis of H2 O2 through 2e-ORR. The catalysts exhibit nearly 100 % Faradaic efficiency and H2 O2 productivity up to 15.1 mol gcat -1  h-1 at 0.1 V versus reversible hydrogen electrode, representing the record high activity for Mn-based electrocatalyst in H2 O2 electrosynthesis. Mechanistic studies reveal that the epoxide and hydroxyl groups surrounding Mn(II) centers improve spin state by modifying electronic properties and charge transfer, thus tailoring the adsorption strength of *OOH intermediate. Multiscale simulations reveal that the high-curvature boundaries facilitate oxygen (O2 ) adsorption and result in local O2 enrichment due to the enhanced interaction between carbon surface and O2 . These merits together ensure the efficient formation of H2 O2 with high local concentration, which can directly boost the tandem reaction of hydrolysis of benzonitrile to benzamide with nearly 100 % conversion rate and exclusive benzamide selectivity.

Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent -OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N'-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(ii) center being the rate-determining step. Upon addition of the conjugate base, N,N'-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O-O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.

Kinetic and mechanistic investigations of dioxygen reduction by a molecular Cu(II) catalyst bearing a pentadentate amidate ligand

A pentadentate Cu(II) complex, [CuII(dpaq)](ClO4) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF3COOH) in acetone at 298 K. No catalytic oxygen reduction was observed in the presence of stronger Brønsted acids than CF3COOH, such as perchloric acid (HClO4) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O2 occurs by Fc* with 1 and HClO4 or HOTf in dimethylformamide (DMF). The use of CF3COOH as the proton source in DMF results in the suppression of O2 reduction under otherwise identical reaction conditions. While the O2 reduction reactions in DMF are linearly dependent on the pKa of Brønsted acids, the acid dependence on catalytic O2-reduction reactivity by 1 in acetone showed complete reversal. Cyclic voltammetry studies using p-chloranil as the probe substrates in the presence of acids in the solvents reveal that the strengths of the protonic acids increase significantly in acetone compared to that in DMF. The amidate-N in [CuII(dpaq)](ClO4) (1) undergoes protonation in the presence of HClO4 or HOTf in DMF to form [CuII(H-dpaq)]2+ (1-H+), but not in the presence of CF3COOH. Enhanced acid strength of CF3COOH in acetone, however, effectively protonates 1 and triggers O2 reduction. Protonation of 1 with HClO4 or HOTf in acetone results in the change of its coordination environment, and this protonated species does not trigger O2 reduction. Detailed kinetic studies indicate that 1-H+ undergoes reduction by two-electrons and the reduced species binds O2 to form a Cu(II)-superoxo intermediate. This is followed by a rate-determining proton-coupled electron-transfer (PCET) reduction to generate the Cu(II)-hydroperoxo intermediate. While catalytic O2 reduction in acetone occurs predominantly via a 4e-/4H+ pathway, product selectivity (H2O vs. H2O2) in DMF depends upon the concentration of the reductant (Fc*). While dioxygen reduction to H2O2 is favoured at low [Fc*], mechanistic studies suggest that O2 reduction with high [Fc*] proceeds via a [2e- + 2e-] mechanism, where the released H2O2 during catalysis is further reduced to water.

Strategies for Sustainable Production of Hydrogen Peroxide via Oxygen Reduction Reaction: From Catalyst Design to Device Setup

An environmentally benign, sustainable, and cost-effective supply of H2O2 as a rapidly expanding consumption raw material is highly desired for chemical industries, medical treatment, and household disinfection. The electrocatalytic production route via electrochemical oxygen reduction reaction (ORR) offers a sustainable avenue for the on-site production of H2O2 from O2 and H2O. The most crucial and innovative part of such technology lies in the availability of suitable electrocatalysts that promote two-electron (2e-) ORR. In recent years, tremendous progress has been achieved in designing efficient, robust, and cost-effective catalyst materials, including noble metals and their alloys, metal-free carbon-based materials, single-atom catalysts, and molecular catalysts. Meanwhile, innovative cell designs have significantly advanced electrochemical applications at the industrial level. This review summarizes fundamental basics and recent advances in H2O2 production via 2e--ORR, including catalyst design, mechanistic explorations, theoretical computations, experimental evaluations, and electrochemical cell designs. Perspectives on addressing remaining challenges are also presented with an emphasis on the large-scale synthesis of H2O2 via the electrochemical route.

Evidence of Sulfur Non-Innocence in [CoII (dithiacyclam)]2+ -Mediated Catalytic Oxygen Reduction Reactions

In many metalloenzymes, sulfur-containing ligands participate in catalytic processes, mainly via the involvement in electron transfer reactions. In a biomimetic approach, we now demonstrate the implication of S-ligation in cobalt mediated oxygen reduction reactions (ORR). A comparative study between the catalytic ORR capabilities of the four-nitrogen bound [Co(cyclam)]²⁺ (1; cyclam=1,5,8,11-tetraaza-cyclotetradecane) and the S-containing analog [Co(S₂ N₂ -cyclam)]²⁺ (2; S₂ N₂ -cyclam=1,8-dithia-5,11-diaza-cyclotetradecane) reveals improved catalytic performance once the chalcogen is introduced in the Co coordination sphere. Trapping and characterization of the intermediates formed upon dioxygen activation at the Co^II centers in 1 and 2 point to the involvement of sulfur in the O₂ reduction process as the key for the improved catalytic ORR capabilities of 2.

Electrocatalytic Oxygen Reduction to Produce Hydrogen Peroxide: Rational Design from Single-Atom Catalysts to Devices

Electrocatalytic production of hydrogen peroxide (H2O2) via the 2e- transfer route of the oxygen reduction reaction (ORR) offers a promising alternative to the energy-intensive anthraquinone process, which dominates current industrial-scale production of H2O2. The availability of cost-effective electrocatalysts exhibiting high activity, selectivity, and stability is imperative for the practical deployment of this process. Single-atom catalysts (SACs) featuring the characteristics of both homogeneous and heterogeneous catalysts are particularly well suited for H2O2 synthesis and thus, have been intensively investigated in the last few years. Herein, we present an in-depth review of the current trends for designing SACs for H2O2 production via the 2e- ORR route. We start from the electronic and geometric structures of SACs. Then, strategies for regulating these isolated metal sites and their coordination environments are presented in detail, since these fundamentally determine electrocatalytic performance. Subsequently, correlations between electronic structures and electrocatalytic performance of the materials are discussed. Furthermore, the factors that potentially impact the performance of SACs in H2O2 production are summarized. Finally, the challenges and opportunities for rational design of more targeted H2O2-producing SACs are highlighted. We hope this review will present the latest developments in this area and shed light on the design of advanced materials for electrochemical energy conversion.

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A New Bis(thioether)-Dipyrrin N2S2 Ligand and Its Coordination Behaviors to Nickel, Copper and Zinc

Tetradentate N2S2 coordination platforms are widespread in biological system and have endowed the metalloenzymes and metalloproteins with abundant reactivities and functions. However, there have only three types of N2S2 scaffolds...

Bioinspired mononuclear Mn complexes for O2 activation and biologically relevant reactions

A general interest in harnessing the oxidizing power of dioxygen (O₂) continues to motivate research efforts on bioinspired and biomimetic complexes to better understand how metalloenzymes mediate these reactions. The ubiquity of Fe- and Cu-based enzymes attracts significant attention and has resulted in many noteworthy developments for abiotic systems interested in direct O₂ reduction and small molecule activation. However, despite the existence of Mn-based metalloenzymes with important O₂-dependent activity, there has been comparatively less focus on the development of these analogues relative to Fe- and Cu-systems. In this Perspective, we summarize important contributions to the development of bioinspired mononuclear Mn complexes for O₂ activation and studies on their reactivity, emphasizing important design parameters in the primary and secondary coordination spheres and outlining mechanistic trends.

Catalytic Reduction of Dioxygen to Water by a Bioinspired Non-Heme Iron Complex via a 2+2 Mechanism

We report a bioinspired non-heme Fe complex with a tripodal [N₃O]^- ligand framework (Fe(PMG)(Cl)₂) that is electrocatalytically active toward dioxygen reduction with acetic acid as a proton source in acetonitrile solution. Under electrochemical and chemical conditions, Fe(PMG)(Cl)₂ selectively produces water via a 2+2 mechanism, where H₂O₂ is generated as a discrete intermediate species before further reduction to two equivalents of H₂O. Mechanistic studies support a catalytic cycle for dioxygen reduction where an off-cycle peroxo dimer species is the resting state of the catalyst. Spectroscopic analysis of the reduced complex Fe^II(PMG)Cl shows the stoichiometric formation of an Fe(III)-hydroxide species following exposure to H₂O₂; no catalytic activity for H₂O₂ disproportionation is observed, although the complex is electrochemically active for H₂O₂ reduction to H₂O. Electrochemical studies, spectrochemical experiments, and DFT calculations suggest that the carboxylate moiety of the ligand is sensitive to hydrogen-bonding interactions with the acetic acid proton donor upon reduction from Fe(III)/(II), favoring chloride loss trans to the tris-alkyl amine moiety of the ligand framework. These results offer insight into how mononuclear non-heme Fe active sites in metalloproteins distribute added charge and poise proton donors during reactions with dioxygen.

Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure

The four-electron oxygen reduction reaction (4e^--ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e^--ORR. In this study, we investigated chemical and electrochemical 4e^--ORR catalyzed by Co₂(μ-O₂) complexes bridged by xanthene (1) and anthracene (3) and by a Co₂(OH)₂ complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)₂ as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOF_init = 6.8 × 10² s^-1) and selectivity for 4e^--ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of 1-3 have the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ as an intermediate in common. In the only case that complex 1 was used as a catalyst, k_cat depended on proton concentration because the reaction rate of the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O₂ and stabilizing the Co₂(μ-O₂) structure, thus increasing both the reaction rate and selectivity for 4e^--ORR.

Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)Fe^III]^- (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH₃COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O₂ by one-electron reductants. An iron(V)-oxo complex, [(TAML)Fe^V(O)]^- (2), was produced by oxygenation of 1 with O₂ via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me₁₀Fc) and octamethylferrocene (Me₈Fc) reduced 2 to 1 by two electrons in the presence of CH₃COOH to produce decamethylferrocenium cation (Me₁₀Fc⁺) and octamethylferrocenium cation (Me₈Fc⁺), respectively. Then, 1 was oxygenated by O₂ to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br₂Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O₂ occurred.

Bio-inspired, Multifunctional Metal-Thiolate Motif: From Electron Transfer to Sulfur Reactivity and Small-Molecule Activation

Sulfur-rich metalloproteins and metalloenzymes, containing strongly covalent metal-thiolate (cysteinate) or metal-sulfide bonds in their active site, are ubiquitous in nature. The metal-sulfur motif is a highly versatile tool involved in various biological processes: (i) metal storage, transport, and detoxification; (ii) electron transfer; (iii) activation of the sulfur atom to promote different types of S-based reactions including S-alkylation, S-oxygenation, S-nitrosylation, or disulfide or thiyl radicals formation; (iv) activation of small earth-abundant molecules (such as water, dioxygen, superoxide radical anion, carbon oxides, nitrous oxide, and dinitrogen).This Account describes our investigations carried out during the past 10 years on bio-inspired and biomimetic low-nuclearity complexes containing metal-thiolate bonds. The general objective of these structural, spectroscopic, electrochemical, and catalytic studies was to determine structure-properties-function correlations useful to (i) understanding the peculiar features or the mechanism of the mimicked natural systems and/or (ii) reproducing enzymatic reactivities for specific catalytic applications.By employing a unique highly preorganized N₂S₂-donor ligand with two thiolate functions, in combination with different first-row transition metals (Mn, Fe, Co, Ni, Cu, Zn, or V), we got access to a series of bio-inspired sulfur-rich complexes displaying a widespread spectrum of structures, properties, and functions. We isolated a dicopper(I) complex that, for the first time, mimicked concomitantly the key structural, spectroscopic, and redox features of the biological Cu_A center, a highly efficient electron transfer agent involved in the respiratory enzyme cytochrome c oxidase. In the field of sulfur activation, we explored (i) sulfur methylation promoted by a Zn-dithiolate complex that mimics Zn-dependent thiolate alkylation proteins and shows different selectivity compared to the Ni and Co congeners and (ii) a series of Co, Fe, and Mn complexes as the first copper-free systems able to promote thiolate/disulfide interconversion mediated by (de)coordination of halides. Concerning metal-centered reactivity, we investigated two families of metal-thiolate catalysts for small-molecule activation, especially relevant in the fields of sustainable fuel production and energy conversion: (i) two isostructural Mn and Fe dinuclear complexes that activate and reduce dioxygen selectively, either to hydrogen peroxide or water as a function of the experimental conditions; (ii) a family of dinuclear MFe (M = Ni or Fe) hydrogenase mimics active for catalytic H₂ evolution both in organic solution and on modified electrodes in water.This Account thus illustrates how the versatility of thiolate ligation can support selected functions for transition metal complexes, depending on the nature of the metal, the nuclearity of the complex, the presence and type of co-ligands, the second coordination sphere effects, and the experimental conditions.

Role of Metals and Thiolate Ligands in the Structures and Electronic Properties of Group 5 Bimetallic-Thiolate Complexes

Syntheses, structures, and electronic properties of group 5 metal-thiolate complexes that exhibit unusual coordination modes of thiolate ligands have been established. Room-temperature reaction of [Cp*VCl₂]₃ (Cp* = η⁵-C₅Me₅) with Na₅[B(SCH₂S)₄] led to the formation of [Cp*VO{(SCH₂)₂S}] (1). The solid-state X-ray structure of 1 shows the formation of six-membered l,3,5-trithia-2-vanadacyclohexane that adopted a chair conformation. In a similar fashion, reactions of heavier group 5 precursors [Cp*MCl₄] (M = Nb or Ta) with Na₅[B(SCH₂S)₄] yielded bimetallic thiolate complexes [(Cp*M)₂(μ-S){μ-C(H)S₃-κ²S:κ²S',S″}{μ-SC(H)S-κ²C:κ²S‴,S''''}] (3a: M = Nb and 3b: M = Ta). One of the key features of molecules 3a and 3b is the presence of square-pyramidal carbon, which is quite unusual. The reactions also yielded bimetallic methanedithiolate complexes [(Cp*Nb)₂(μ-S)(μ-SCH₂S-κ²S,S')(μ,η²:η²-BH₃S)] (2) and [(Cp*Ta)₂(μ-O)(μ-SCH₂S-κ²S,S')(μ-H){μ-S₂C(H)SCH₂S-κ²S″:κ²S‴,S''''}] (4). Complex 2 contains a methanedithiolate ligand that stabilizes the unsaturated niobaborane species. On the other hand, one ((mercaptomethyl)thio)methanedithiolate ligand {C₂H₄S₃} is present in 4, which is coordinated to metal centers and exhibits the {μ-κ²S″:κ²S‴,S''''} bonding mode. Along with the formation of 3b and 4, the reaction of [Cp*TaCl₄] with Na₅[B(SCH₂S)₄] yielded [(Cp*Ta)₂(μ-S){μ-(SBS)S(CH₂S)₂(BH₂S)-κ²B:κ²S:κ⁴S',S″,S‴,S''''}] (5) containing a trithiaborate unit (BS₃). Complex 5 consists of pentacoordinate boron that resides in a square-pyramidal environment. All the complexes have been characterized by multinuclear NMR, UV-vis spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies.

A Mechanistic Dichotomy in Two-Electron Reduction of Dioxygen Catalyzed by N,N'-Dimethylated Porphyrin Isomers

Selective two-electron reduction of dioxygen (O₂ ) to hydrogen peroxide (H₂ O₂ ) has been achieved by two saddle-distorted N,N'-dimethylated porphyrin isomers, an N21,N'22-dimethylated porphyrin (anti-Me₂ P) and an N21,N'23-dimethylated porphyrin (syn-Me₂ P) as catalysts and ferrocene derivatives as electron donors in the presence of protic acids in acetonitrile. The higher catalytic performance in an oxygen reduction reaction (ORR) was achieved by anti-Me₂ P with higher turnover number (TON=250 for 30 min) than that by syn-Me₂ P (TON=218 for 60 min). The reactive intermediates in the catalytic ORR were confirmed to be the corresponding isophlorins (anti-Me₂ Iph or syn-Me₂ Iph) by spectroscopic measurements. The rate-determining step in the catalytic ORRs was concluded to be proton-coupled electron-transfer reduction of O₂ with isophlorins based on kinetic analysis. The ORR rate by anti-Me₂ Iph was accelerated by external protons, judging from the dependence of the observed initial rates on acid concentrations. In contrast, no acceleration of the ORR rate with syn-Me₂ Iph by external protons was observed. The different mechanisms in the O₂ reduction by the two isomers should be derived from that of the arrangement of hydrogen bonding of a O₂ with inner NH protons of the isophlorins.

Controlled O2 reduction at a mixed-valent (II,I) Cu2S core

Oxygen reduction reactions catalyzed by a mixed-valent copper complex reveal a tuneable H₂O₂/H₂O selectivity at room temperature together with high stability over several cycles.

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

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

A Mn(iv)-peroxo complex in the reactions with proton donors

Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O2-bound [PPN][MnIV(TMSPS3)(O2)] (1, PPN = bis(triphenylphosphine)iminium and TMSPS3H3 = 2,2',2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO4) in the presence of PR3 (R = phenyl or p-tolyl) results in the formation of neutral five-coordinate MnIII(OPR3)(TMSPS3) complexes (R = phenyl, 2a; p-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the MnIV-peroxo complex, leading to the isolation of a series of five-coordinate [MnIII(L)(TMSPS3)]- complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the MnIV-peroxo complex with proton donors are discussed as well.

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn₂ RNRs superoxide (O₂ ^.- ) was postulated to react with the Mn^II ₂ core to yield a Mn^II Mn^III -peroxide moiety. The reactivity of complex 1 ([Mn^II ₂ (O₂ CCH₃ )₂ (BPMP)](ClO₄ ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O₂ ^.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λ_max =440, 590 nm) characteristic of a Mn^II Mn^III -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a Mn^II Mn^III -peroxide complex. Unlike all other previously reported Mn₂ -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O₂ -activation in Class Ib Mn₂ RNRs, and the highly reactive intermediates in their catalytic cycle.

Dioxygen Reduction to Hydrogen Peroxide by a Molecular Mn Complex: Mechanistic Divergence between Homogeneous and Heterogeneous Reductants

The selective electrocatalytic reduction of dioxygen (O₂) to hydrogen peroxide (H₂O₂) could be an alternative to the anthraquinone process used industrially, as well as enable the on-demand production of a useful chemical oxidant, obviating the need for long-term storage. There are challenges associated with this, since the two-proton/two-electron reduction of H₂O₂ to two equivalents of water (H₂O) or disproportionation to O₂ and H₂O can be competing reactions. Recently, we reported a Mn(III) Schiff base-type complex, Mn(^tbudhbpy)Cl, where 6,6'-di(3,5-di- tert-butyl-2-phenolate)-2,2'-bipyridine = [^tbudhbpy]^2-, which is active for the electrocatalytic reduction of O₂ to H₂O₂ (ca. 80% selectivity). The less-than-quantitative selectivity could be attributed in part to a thermal disproportionation reaction of H₂O₂ to O₂ and H₂O. To understand the mechanism in greater detail, spectrochemical stopped-flow and electrochemical techniques were employed to examine the catalytic rate law and kinetic reaction parameters. Under electrochemical conditions, the catalyst produces H₂O₂ by an ECCEC mechanism with appreciable rates down to overpotentials of 20 mV and exhibits a catalytic response with a strong dependence on proton donor p K_a. Mechanistic studies suggest that under spectrochemical conditions, where the homogeneous reductant decamethylferrocene (Cp*₂Fe) is used, H₂O₂ is instead produced via a disproportionation pathway, which does not show a strong acid dependence. These results demonstrate that differences in mechanistic pathways can occur for homogeneous catalysts in redox processes, dependent on whether an electrode or homogeneous reductant is used.

Aerobic reactions of antitumor active dirhodium(II) tetraacetate Rh2(CH3COO)4 with glutathione

The aerobic reaction between glutathione (H3A) and dirhodium(II) tetraacetate, Rh2(AcO)4 (AcO- = CH3COO-), in aqueous solution (pH 7.4) breaks up the direct RhII-RhII bond and its carboxylate framework, as evidenced by UV-Vis spectroscopy. After purifying the reaction product using size exclusion chromatography, electrospray ionization mass spectrometry (ESI-MS) of the solution showed binuclear [Formula: see text] and [Formula: see text] ions. Evaporation yielded a solid compound, [Formula: see text], for which Rh K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed ~ 2 Rh-O (2.08 ± 0.02 Å) and ~ 4 Rh-S (2.33 ± 0.02 Å) bond distances around each RhIII center, and the RhIII··RhIII distance 3.11 ± 0.02 Å, close to that in dirhodium(III) complexes with three bridging thiolates connecting [Formula: see text] units. The 13C CPMAS NMR spectrum of the RhIII-glutathione complex showed a change ∆δ C > 6 ppm in the chemical shift of the COO- signal, indicating some carboxylate coordination to the Rh(III) ions. This study shows that under aerobic conditions glutathione enables oxidation of Rh2(AcO)4 and thus reduces its antitumor efficiency. The reaction of Rh2(AcO)4 with glutathione was investigated by ESI-MS, UV-Vis, 13C NMR and X-ray absorption spectroscopy, revealing that glutathione breaks down the carboxylate framework enabling oxidization of the [Formula: see text] core to Rh(III) dimeric units, bridged by three thiolates.

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular Manganese Complex with a Bipyridine-Containing Schiff Base Ligand

The synthesis and electrocatalytic reduction of dioxygen by a molecular manganese(III) complex with a tetradentate dianionic bipyridine-based ligand is reported. Electrochemical characterization indicates a Nernstian dependence on the added proton source for the reduction of Mn(III) to Mn(II). The resultant species is competent for the reduction of dioxygen to H₂O₂ with 81 ± 4% Faradaic efficiency. Mechanistic studies suggest that the catalytically active species has been generated through the interaction of the added proton donor and the parent Mn complex, resulting in the protonation of a coordinated phenolate moiety following the single-electron reduction, generating a neutral species with a vacant coordination site at the metal center. As a consequence, the active catalyst has a pendent proton source in close proximity to the active site for subsequent intramolecular reactions.

Progress Towards Direct Hydrogen Peroxide Fuel Cells (DHPFCs) as an Energy Storage Concept

This review introduces the concept of direct H2O2 fuel cells and discusses the merits of these systems in comparison with other ‘clean-energy’ fuels. Through electrochemical methods, H2O2 fuel can be generated from environmentally benign energy sources such as wind and solar. It also produces only water and oxygen when it is utilised in a direct H2O2 fuel cell, making it a fully reversible system. The electrochemical methods for H2O2 production are discussed here as well as the recent research aimed at increasing the efficiency and power of direct H2O2 fuel cells.

Keto-Enol Tautomerization Triggers an Electrophilic Aldehyde Deformylation Reaction by a Nonheme Manganese(III)-Peroxo Complex

Oxygen atom transfer by high-valent enzymatic intermediates remains an enigma in chemical catalysis. In particular, manganese is an important first-row metal involved in key biochemical processes, including the biosynthesis of molecular oxygen (through the photosystem II complex) and biodegradation of toxic superoxide to hydrogen peroxide by superoxide dismutase. Biomimetic models of these biological systems have been developed to gain understanding on the structure and properties of short-lived intermediates but also with the aim to create environmentally benign oxidants. In this work, we report a combined spectroscopy, kinetics and computational study on aldehyde deformylation by two side-on manganese(III)-peroxo complexes with bispidine ligands. Both manganese(III)-peroxo complexes are characterized by UV-vis and mass spectrometry techniques, and their reactivity patterns with aldehydes was investigated. We find a novel mechanism for the reaction that is initiated by a hydrogen atom abstraction reaction, which enables a keto-enol tautomerization in the substrate. This is an essential step in the mechanism that makes an electrophilic attack on the olefin bond possible as the attack on the aldehyde carbonyl is too high in energy. Kinetics studies determine a large kinetic isotope effect for the replacement of the transferring hydrogen atom by deuterium, while replacing the transferring hydrogen atom by a methyl group makes the substrate inactive and hence confirm the hypothesized mechanism. Our new mechanism is confirmed with density functional theory modeling on the full mechanism and rationalized through valence bond and thermochemical cycles. Our unprecedented new mechanism may have relevance to biological and biomimetic chemistry processes in general and gives insight into the reactivity patterns of metal-peroxo and metal-hydroperoxo intermediates in general.

A High-Valent Non-Heme μ-Oxo Manganese(IV) Dimer Generated from a Thiolate-Bound Manganese(II) Complex and Dioxygen

This study deals with the unprecedented reactivity of dinuclear non-heme MnII -thiolate complexes with O2 , which dependent on the protonation state of the initial MnII dimer selectively generates either a di-μ-oxo or μ-oxo-μ-hydroxo MnIV complex. Both dimers have been characterized by different techniques including single-crystal X-ray diffraction and mass spectrometry. Oxygenation reactions carried out with labeled 18 O2 unambiguously show that the oxygen atoms present in the MnIV dimers originate from O2 . Based on experimental observations and DFT calculations, evidence is provided that these MnIV species comproportionate with a MnII precursor to yield μ-oxo and/or μ-hydroxo MnIII dimers. Our work highlights the delicate balance of reaction conditions to control the synthesis of non-heme high-valent μ-oxo and μ-hydroxo Mn species from MnII precursors and O2 .

Reduced thione ligation is preferred over neutral phosphine ligation in diiron biomimics regarding electronic functionality: a spectroscopic and computational investigation

Sulfur means superiority: effective electronic communication and buffering by sulfur ligation.

N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl, and tellurenyl cations (XH+, X = S, Se, Te)

NHC-stabilized parent sulfenyl (H–S⁺), selenenyl (H–Se⁺) and tellurenyl (H–Te⁺) cations have been achieved by treatment of NHC chalcogen adducts with trifluoromethanesulfonic acid.

Catalytic reduction of proton, oxygen and carbon dioxide with cobalt macrocyclic complexes

The conversion of solar energy into chemical energy by the reduction of small molecules provides a promising solution for the effective energy storage and transport. In this manuscript, we have highlighted our recent researches on the catalysis of cobalt-macrocycle complexes for the reduction of O2, proton and CO2. We have successfully clarified the reaction mechanisms of catalytic O2 reduction with cobalt phthalocyanine (Co[Formula: see text](Pc)) and cobalt chlorin (Co[Formula: see text](Ch)) based on detailed kinetic study under homogeneous conditions. The presence of proton-accepting moieties on these macrocyclic ligands enhances the electron-accepting ability, leading to the efficient catalytic two-electron reduction of O2 to produce hydrogen peroxide (H2O[Formula: see text] with high stability and less overpotential in acidic solutions. When Co[Formula: see text](Ch) is adsorbed on multi-walled carbon nanotubes (MWCNTs) and employed as an electrocatalyst, CO2 was successfully reduced to form CO with a Faradaic efficiency of 89% at an applied potential of -1.1 V vs. NHE in an aqueous solution. Finally, photocatalytic H2 evolution was attained from ascorbic acid with Co[Formula: see text](Ch) as a catalyst and [Ru(bpy)3][Formula: see text] (bpy [Formula: see text] 2,2[Formula: see text]-bipyridine) as a photocatalyst via a one-photon two-electron process.