Cerium(IV)-driven water oxidation catalyzed by a manganese(V)-nitrido complex

Crystallization of Manganese(V) Oxides by Hydroflux Synthesis: Control of Anisotropic Growth and Electrochemical Stability

Despite intriguing optical, magnetic, and redox properties, inorganic materials containing pentavalent manganese (MnV) are rare and could never be designed as shape-controlled crystals, which limits the ability to tune properties. Herein, we explore alkali hydroxide mixtures with controlled water content, namely, hydrofluxes, to demonstrate phase, shape, and nanostructure control of Mn(V) oxides. We demonstrate speciation among KSrMnVO4, Sr5(MnVO4)3OH, and SrMnIVO3 with the water and strontium content and the nature of the alkali cation of the hydroxide salt. We then provide evidence of the key role of water in enabling shape and nanostructure control, which we relate to the preferential interaction of water with specific crystal facets of the hydroxyapatite Sr5(MnVO4)3OH, and to the impact of water on precursor solubility in water-poor hydrofluxes. We then show that nanostructured Mn(V) hydroxyapatite possesses an acid-base redox stability window, enabling electrochemical operation in strongly oxidative conditions. By correlating the fundamental knowledge of hydrofluxes with crystallization mechanisms, this work sheds light on the possibilities offered by hydrofluxes for crystal shape, size, and property control.

Chemical design of metal complexes for electrochemical water oxidation under acidic conditions

The development of viable, stable, and highly efficient molecular water oxidation catalysts under acidic aqueous conditions (pH < 7) is challenging with Earth-abundant metals in the field of renewable energy due to their low stability and catalytic activity. The utilization of these catalysts is generally considered more cost-effective and sustainable relative to conventional catalysts relying on precious metals such as ruthenium and iridium, which exhibit outstanding activities. Herein, we discussed the effectiveness of transition metal complexes for electrocatalytic water oxidation under acidic conditions. We focus on important aspects of 3d first-row metal complexes as they relate to the design of water oxidation systems and emphasize the importance of the fundamental coordination chemistry perspective in this field, which can be applied to the understanding of catalytic activity and fundamental structure-function relationships. Finally, we identified the scientific challenges that should be overcome for the future development and application of water oxidation electrochemical catalysts.

Energy Landscape for the Electrocatalytic Oxidation of Water by a Single-Site Oxomanganese(V) Porphyrin

A cationic manganese porphyrin, Mn^III-TDMImP, is an efficient, homogeneous, single-site water oxidation electrocatalyst at neutral pH. The measured turnover frequency for oxygen production is 32 s^-1. Mechanistic analyses indicate that Mn^V(O)(OH₂), the protonated form of the corresponding trans-Mn^V(O)₂ species, is generated from the Mn^III(OH₂)₂ precursor in a 2-e^- two-proton process and is responsible for O-O bond formation with a H₂O molecule. Chloride ion is a competitive substrate with H₂O for the Mn^V(O)(OH₂) oxidant, forming hypochlorous acid with a rate constant that is 3 orders of magnitude larger than that of water oxidation. The data allow the construction of an experimental energy landscape for this water oxidation catalysis process.

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.

Ligand Radical Mediated Water Oxidation by a Family of Copper o-Phenylene Bis-oxamidate Complexes

Understanding the reactivity landscape for the activation of water until the formation of the O-O bond and O₂ release in molecular chemistry is a decisive step in guiding the elaboration of cost-effective catalysts for the oxygen-evolving reaction (OER). Copper(II) complexes have recently caught the attention of chemists as catalysts for the 4e^-/4H⁺ water oxidation process. While a copper(IV) intermediate has been proposed as the reactive intermediate species, no spectroscopic signature has been reported so far. Copper(III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper(II) o-phenylene bis-oxamidate complexes is a function of the substitution pattern on the periphery of the aromatic ring. In-situ EPR, FTIR, and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading toward the O₂ formation selectively at the copper center. EPR and FTIR spectroelectrochemistry suggests that ligand-centered oxidations are preferred over metal-centered oxidations. rR spectroelectrochemical study revealed the accumulation of a bis-imine bound copper(II) superoxide species, as the reactive intermediate, under catalytic turnover, which provides the evidence for the O-O bond formation during OER.

Switching the O-O Bond Formation Pathways of Ru-pda Water Oxidation Catalyst by Third Coordination Sphere Engineering

Water oxidation is a vital anodic reaction for renewable fuel generation via electrochemical- and photoelectrochemical-driven water splitting or CO₂ reduction. Ruthenium complexes, such as Ru-bda family, have been shown as highly efficient water-oxidation catalysts (WOCs), particularly when they undergo a bimolecular O-O bond formation pathway. In this study, a novel Ru(pda)-type (pda^2- =1,10-phenanthroline-2,9-dicarboxylate) molecular WOC with 4-vinylpyridine axial ligands was immobilized on the glassy carbon electrode surface by electrochemical polymerization. Electrochemical kinetic studies revealed that this homocoupling polymer catalyzes water oxidation through a bimolecular radical coupling pathway, where interaction between two Ru(pda)-oxyl moieties (I2M) forms the O-O bond. The calculated barrier of the I2M pathway by density-functional theory (DFT) is significantly lower than the barrier of a water nucleophilic attack (WNA) pathway. By using this polymerization strategy, the Ru centers are brought closer in the distance, and the O-O bond formation pathway by the Ru (pda) catalyst is switched from WNA in a homogeneous molecular catalytic system to I2M in the polymerized film, providing some deep insights into the importance of third coordination sphere engineering of the water oxidation catalyst.

Molecularly Imprinted Polymer Nanoparticles: An Emerging Versatile Platform for Cancer Therapy

Molecularly imprinted polymers (MIPs) are chemically synthesized affinity materials with tailor-made binding cavities complementary to the template molecules in shape, size, and functionality. Recently, engineering MIP-based nanomedicines to improve cancer therapy has become a rapidly growing field and future research direction. Because of the unique properties and functions of MIPs, MIP-based nanoparticles (nanoMIPs) are not only alternatives to current nanomaterials for cancer therapy, but also hold the potential to fill gaps associated with biological ligand-based nanomedicines, such as immunogenicity, stability, applicability, and economic viability. Here, we survey recent advances in the design and fabrication of nanoMIPs for cancer therapy and highlight their distinct features. In addition, how to use these features to achieve desired performance, including extended circulation, active targeting, controlled drug release and anti-tumor efficacy, is discussed and summarized. We expect that this minireview will inspire more advanced studies in MIP-based nanomedicines for cancer therapy.

On the crystal chemistry of inorganic nitrides: crystal-chemical parameters, bonding behavior, and opportunities in the exploration of their compositional space

The scarcity of nitrogen in Earth's crust, combined with challenging synthesis, have made inorganic nitrides a relatively unexplored class of compounds compared to their naturally abundant oxide counterparts. To facilitate exploration of their compositional space via a priori modeling, and to help a posteriori structure verification not limited to inferring the oxidation state of redox-active cations, we derive a suite of bond-valence parameters and Lewis acid strength values for 76 cations observed bonding to N3-, and further outline a baseline statistical knowledge of bond lengths for these compounds. Examination of structural and electronic effects responsible for the functional properties and anomalous bonding behavior of inorganic nitrides shows that many mechanisms of bond-length variation ubiquitous to oxide and oxysalt compounds (e.g., lone-pair stereoactivity, the Jahn-Teller and pseudo Jahn-Teller effects) are similarly pervasive in inorganic nitrides, and are occasionally observed to result in greater distortion magnitude than their oxide counterparts. We identify promising functional units for exploring uncharted chemical spaces of inorganic nitrides, e.g. multiple-bond metal centers with promise regarding the development of a post-Haber-Bosch process proceeding at milder reaction conditions, and promote an atomistic understanding of chemical bonding in nitrides relevant to such pursuits as the development of a model of ion substitution in solids, a problem of great relevance to semiconductor doping whose solution would fast-track the development of compound solar cells, battery materials, electronics, and more.

From Ru-bda to Ru-bds: a step forward to highly efficient molecular water oxidation electrocatalysts under acidic and neutral conditions

Significant advances during the past decades in the design and studies of Ru complexes with polypyridine ligands have led to the great development of molecular water oxidation catalysts and understanding on the O−O bond formation mechanisms. Here we report a Ru-based molecular water oxidation catalyst [Ru(bds)(pic)₂] (Ru-bds; bds²⁻ = 2,2′-bipyridine-6,6′-disulfonate) containing a tetradentate, dianionic sulfonate ligand at the equatorial position and two 4-picoline ligands at the axial positions. This Ru-bds catalyst electrochemically catalyzes water oxidation with turnover frequencies (TOF) of 160 and 12,900 s⁻¹ under acidic and neutral conditions respectively, showing much better performance than the state-of-art Ru-bda catalyst. Density functional theory calculations reveal that (i) under acidic conditions, the high valent Ru intermediate Ru^V=O featuring the 7-coordination configuration is involved in the O−O bond formation step; (ii) under neutral conditions, the seven-coordinate Ru^IV=O triggers the O−O bond formation; (iii) in both cases, the I2M (interaction of two M−O units) pathway is dominant over the WNA (water nucleophilic attack) pathway.

Developing efficient molecular water oxidation catalysts for artificial photosynthesis is a challenging task. Here the authors introduce a ruthenium based complex with negatively charged sulfonate groups to effectively drive water oxidation under both acidic and neutral conditions.

Cyclopentadienone–NHC iron(0) complexes as low valent electrocatalysts for water oxidation

Design and application of earth abundant iron based molecular electrocatalysts for water oxidation, an essential challenge for sustainable energy applications.

Promotion of the water oxidation activity of iridium oxide by a nitrogen coordination strategy

The water oxidation reaction is the pivotal half-reaction for photo-/electro-catalytic water splitting. Fabrication of high-efficiency and robust water oxidation is essential to realize wide-scale artificial photosynthesis. Here, we report an efficient strategy to improve the water oxidation activity of iridium oxide by a nitrogen-coordination method. Due to the coordination effect, the iridium oxide can be well dispersed to generate ultra-small nanoparticles and the intrinsic activity can be improved for the water oxidation reaction. This study suggests that high-performance water oxidation catalysts can be constructed based on a nitrogen-coordination strategy.

Structural evolution of the Ru-bms complex to the real water oxidation catalyst of Ru-bda: the bite angle matters

Ru-Based complexes have advanced the study of molecular water oxidation catalysts (WOCs) both in catalysis and mechanism. The electronic effect has always been considered as an essential factor for the catalyst properties while less attention has been focused on the bite angle effect on water oxidation catalysis. The Ru-bda ([Ru(bda)(pic)2]; bda2- = 2,2'-bipyridine-6,6'-dicarboxylate; pic = 4-picoline) catalyst is one of the most active WOCs and it has a largely distorted octahedral configuration with an O-Ru-O bite angle of 123°. Herein, we replaced the carboxylate (-COO-) groups of bda2- with two methylenesulfonate (-CH2SO3-) groups and prepared a negatively charged ligand, bms2- (2,2'-bipyridine-6,6'-dimethanesulfonate), and the Ru-bms complex [Ru(bms)(pic)2]. The O-Ru-O bite angle changed from 123° in Ru-bda to 84° in Ru-bms, leading to a dramatic influence on the catalytic behavior. Systematic analysis of the reaction intermediates suggested that Ru-bms transformed all the way to Ru-bdavia oxidative decomposition under CeIV-driven water oxidation conditions.

Dicopper(ii) tetrapyridyl complexes incorporated with ancillary ligands for effective water oxidation

Water oxidation catalysis of dicopper(ii) tetrapyridyl complexes under alkaline conditions was improved by diamine ligands.

Molecular quaterpyridine-based metal complexes for small molecule activation: water splitting and CO2 reduction

This tutorial describes recent developments in the use of metal quaterpyridine complexes as electrocatalysts and photocatalysts for water splitting and CO₂ reduction.

A dinuclear iron complex as an efficient electrocatalyst for homogeneous water oxidation reaction

A novel dinuclear iron complex of a Schiff base ligand has been exploited as a homogeneous water splitting electrocatalyst having possible real life application in renewable energy.

Mono-/Multinuclear Water Oxidation Catalysts

Water splitting, in which water molecules can be transformed into hydrogen and oxygen, is an appealing energy conversion and transformation strategy to address the environmental and energy crisis. The oxygen evolution reaction (OER) is dynamically slow, which limits energy conversion efficiency during the water-splitting process and requires high-efficiency water oxidation catalysts (WOCs) to overcome the OER energy barrier. It is generally accepted that multinuclear WOCs possess superior OER performances, as demonstrated by the CaMn₄ O₅ cluster in photosystem II (PSII), which can catalyze the OER efficiently with a very low overpotential. Inspired by the CaMn₄ O₅ cluster in PSII, some multinuclear WOCs were synthesized that could catalyze water oxidation. In addition, some mononuclear molecular WOCs also show high water oxidation activity. However, it cannot be excluded that the high activity arises from the formation of dimeric species. Recently, some mononuclear heterogeneous WOCs showed a high water oxidation activity, which testified that mononuclear active sites with suitable coordination surroundings could also catalyze water oxidation efficiently. This Review focuses on recent progress in the development of mono-/multinuclear homo- and heterogeneous catalysts for water oxidation. The active sites and possible catalytic mechanisms for water oxidation on the mono-/multinuclear WOCs are provided.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

Copper-containing hybrid compounds based on extremely rare [V2Mo6O26]6– POM as water oxidation catalysts

Hybrid Cu/V/Mo compounds with rare [α-V₂Mo₆O₂₆]^6– and oxides prepared by their thermal degradation were used as catalysts for water oxidation.

Why nature chose the Mn4CaO5 cluster as water-splitting catalyst in photosystem II: a new hypothesis for the mechanism of O-O bond formation

Resolving the questions, namely, the selection of Mn by nature to build the oxygen-evolving complex (OEC) and the presence of a cubic Mn3CaO4 structure in OEC coupled with an additional dangling Mn (Mn4) via μ-O atom are not only important to uncover the secret of water oxidation in nature, but also essential to achieve a blueprint for developing advanced water-oxidation catalysts for artificial photosynthesis. Based on the important experimental results reported so far in the literature and on our own findings, we propose a new hypothesis for the water oxidation mechanism in OEC. In this new hypothesis, we propose for the first time, a complete catalytic cycle involving a charge-rearrangement-induced MnVII-dioxo species on the dangling Mn4 during the S3 → S4 transition. Moreover, the O-O bond is formed within this MnVII-dioxo site, which is totally different from that discussed in other existing proposals.

Identifying MnVII-oxo Species during Electrochemical Water Oxidation by Manganese Oxide

Identifying surface active intermediate species is essential to reveal the catalytic mechanism of water oxidation by metal-oxides-based catalysts and to develop more efficient catalysts for oxygen-oxygen bond formation. Here we report, through electrochemical methods and ex situ infrared spectroscopy, the identification of a Mn^VII = O intermediate during catalytic water oxidation by a c-disordered δ-MnO_x with an onset-potential-dependent reduction peak at 0.93 V and an infrared peak at 912 cm⁻¹. This intermediate is proved to be highly reactive and much more oxidative than permanganate ion. Therefore, we propose a new catalytic mechanism for water oxidation catalyzed by Mn oxides, with involvement of the Mn^VII = O intermediate in a resting state and the Mn^IV−O−Mn^VII = O as a real active species for oxygen-oxygen bond formation.

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A reactive Mn^VII-oxo intermediate was identified during water oxidation by a MnO_x

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The Mn^VII-oxo species was proved to be much more oxidative than permanganate ion

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The Mn^IV−O−Mn^VII = O moiety is a real highly active state for O–O bond formation

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A new mechanism for Mn oxide-catalyzed electrocatalytic water oxidation is proposed

New ultrasonic assisted co-precipitation for high surface area oxide based nanostructured materials

A non-system specific method for the synthesis of metal oxide nanoparticles with high homogeneity, spherical morphology and high surface areas is proposed based on an aqueous precipitation.

Cu(ii) coordination polymers with nitrogen catenation ligands for efficient photocatalytic water oxidation

Cu(ii) coordination polymers with nitrogen catenation ligands can photocatalyze water oxidation with the highest TOF (1.68 s⁻¹) among copper-based photocatalysts.

An Epitope-Imprinted Biointerface with Dynamic Bioactivity for Modulating Cell-Biomaterial Interactions

In this study, an epitope-imprinting strategy was employed for the dynamic display of bioactive ligands on a material interface. An imprinted surface was initially designed to exhibit specific affinity towards a short peptide (i.e., the epitope). This surface was subsequently used to anchor an epitope-tagged cell-adhesive peptide ligand (RGD: Arg-Gly-Asp). Owing to reversible epitope-binding affinity, ligand presentation and thereby cell adhesion could be controlled. As compared to current strategies for the fabrication of dynamic biointerfaces, for example, through reversible covalent or host-guest interactions, such a molecularly tunable dynamic system based on a surface-imprinting process may unlock new applications in in situ cell biology, diagnostics, and regenerative medicine.

Electrocatalytic Water Oxidation by a Homogeneous Copper Catalyst Disfavors Single-Site Mechanisms

Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)Cu(II)]₂-(μ-OH)₂}(OTf)₂ (L = Me₂TMPA = bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s^-1 at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (k_H/k_D = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)Cu(II)(OH)]}⁺ adduct (K_eq = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu(III)]₂-(μ-O)₂}²⁺, which avoids formation of terminal Cu(IV)═O/Cu(III)-O^• intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu(III)]₂-(μ-O)₂}²⁺ are energetically accessible pathways for O-O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn.

On the applicability of density functional theory to manganese-based complexes with catalytic activity toward water oxidation

The present contribution assesses the performance of several popular and accurate density functionals, namely B3LYP, BP86, M06, MN12L, mPWPW91, PBE0, and TPSSh toward manganese-based coordination complexes. These compounds show promising properties toward application to catalytic water oxidation. Although manganese with N- and O-biding ligands tends to give rise to high spin complexes, the results show that BP86, mPWPW91, and specially MN12L, tend to yield low-spin complexes. The usage of these functionals for such compounds is, thus, discouraged. All the functionals considered deliver accurate geometries. The present results show, however, that B3LYP delivers geometries deviating from experimental values when compared to the other functionals of the set. M06, PBE0, and TPSSh deliver geometries of similar accuracy, PBE0 outstanding slightly with respect to the other two. © 2017 Wiley Periodicals, Inc.

Modulation of redox potentials utilizing the flexible coordination sphere of a penta-coordinate complex in the solid state

Slight changes in the coordination structure of the manganese(v)-nitrido anionic complex, [Mn^V(N)(CN)₄]^2-, induced by using a "lipid package" approach markedly made an impact on the corresponding redox potentials. The single crystals of four lipid assemblies, [dabco-(CH₂)_n-1-CH₃]₂[Mn(N)(CN)₄(H₂O)]·xH₂O (n = 15, 16, 17 and 18; dabco = 1,4-diazabicyclo[2,2,2]octane), were synthesized and solid-state cyclic voltammetric studies demonstrated that the [Mn^V(N)(CN)₄]^2- anions with smaller "cross" NC-Mn-CN bond angles exhibit higher redox potentials. The observed trend reflects the energy change associated with the structural transformation from [Mn^V(N)(CN)₄]^2- to [Mn^VI(N)(CN)₄]^2- and is supported by the results of DFT calculations. The NC-Mn-CN bond angles in the flexible [Mn(N)(CN)₄]^2- structure exhibit excellent correlation with the redox potentials of the complexes in the solid state.

Mechanism of Water Oxidation Catalyzed by a Mononuclear Manganese Complex

The design and synthesis of biomimetic Mn complexes to catalyze oxygen evolution is a very appealing goal because water oxidation in nature employs a Mn complex. Recently, the mononuclear Mn complex [LMn^II (H₂ O)₂ ]²⁺ [1, L=Py₂ N(tBu)₂ , Py=pyridyl] was reported to catalyze water oxidation electrochemically at an applied potential of 1.23 V at pH 12.2 in aqueous solution. Density functional calculations were performed to elucidate the mechanism of water oxidation promoted by this catalyst. The calculations showed that 1 can lose two protons and one electron readily to produce [LMn^III (OH)₂ ]⁺ (2), which then undergoes two sequential proton-coupled electron-transfer processes to afford [LMn^V OO]⁺ (4). The O-O bond formation can occur through direct coupling of the two oxido ligands or through nucleophilic attack of water. These two mechanisms have similar barriers of approximately 17 kcal mol^-1 . The further oxidation of 4 to generate [LMn^VI OO]²⁺ (5), which enables O-O bond formation, has a much higher barrier. In addition, ligand degradation by C-H activation has a similar barrier to that for the O-O bond formation, and this explains the relatively low turnover number of this catalyst.

Electrocatalytic water oxidation by Cu(ii) ions in a neutral borate buffer solution

Herein we report that a Cu(ii) salt can efficiently catalyze water oxidation in a neutral borate buffer.