Polyoxometalate-assisted synthesis of transition-metal cubane clusters as artificial mimics of the oxygen-evolving center of photosystem II

Indium-Assisted Construction of {SiNb18O54}-Based Aggregates and Their Assembly into Extended Polyoxoniobate Architectures

In this research, indium ions were introduced into polyoxoniobates (PONbs) reaction systems to facilitate the construction of different {SiNb18O54}-based aggregates, including an {In(en)2{SiNb18O54}2} (en = ethylenediamine) dimer, an {[InO2][In2(en)O3]2{SiNb18O54}3} trimer, and an {[In(en)2][InO2][In7(en)5O9]{SiNb18O54}4} tetramer. Interestingly, these aggregates were further assembled into three uncommon extended PONb architectures in the presence of [Cu(en)2]2+ complexes, namely, H3[Cu(en)2(H2O)][Cu(en)2]6[Cu(en)2]2{[In(en)2][K2{SiNb18O54}(H2O)6]2}·1.5en·16H2O, H9{[Cu(en)2]6{[Cu(en)2]3[Cu(en)2(H2O)][In(H2O)2][In2(en)(H2O)2(OH)]2{SiNb18O54}3}·5en·29H2O, and H14[Cu(en)2]0.5[Cu(en)2(H2O)]{[Cu(en)2]2{[Cu(en)2]3[Cu(en)2(H2O)]5[K(H2O)2][In(H2O)2][In(en)2][In7(OH)9(en)5]{SiNb18O54}4}·7en·39H2O. In addition, all of them have good water vapor adsorption capacities and moderate proton transport capabilities. The above results indicate that introducing suitable heteroatoms to induce the aggregation PONb building blocks and further assembling them into new structures is an effective strategy to enrich the PONbs' structural diversity and develop new functional materials.

Single Dispersion of Fe(H2O)2-Based Polyoxometalate on Polymeric Carbon Nitride for Biomimetic CH4 Photooxidation

Direct methane conversion to value-added oxygenates under mild conditions with in-depth mechanism investigation has attracted wide interest. Inspired by methane monooxygenase, the K9Na2Fe(H2O)2{[γ-SiW9O34Fe(H2O)]}2·25H2O polyoxometalate (Fe-POM) with well-defined Fe(H2O)2 sites is synthesized to clarify the key role of Fe species and their microenvironment toward CH4 photooxidation. The Fe-POM can efficiently drive the conversion of CH4 to HCOOH with a yield of 1570.0 µmol gPOM -1 and 95.8% selectivity at ambient conditions, much superior to that of [Fe(H2O)SiW11O39]5- with Fe(H2O) active site, [Fe2SiW10O38(OH)]2 14- and [P8W48O184Fe16(OH)28(H2O)4]20- with multinuclear Fe-OH-Fe active sites. Single-dispersion of Fe-POM on polymeric carbon nitride (PCN) is facilely achieved to provide single-cluster functionalized PCN with well-defined Fe(H2O)2 site, the HCOOH yield can be improved to 5981.3 µmol gPOM -1. Systemic investigations demonstrate that the (WO)4-Fe(H2O)2 can supply Fe═O active center for C-H activation via forming (WO)4-Fea-Ot···CH4 intermediate, similar to that for CH4 oxidation in the monooxygenase. This work highlights a promising and facile strategy for single dispersion of ≈1-2 Å metal center with precise coordination microenvironment by uniformly anchoring nanoscale molecular clusters, which provides a well-defined model for in-depth mechanism research.

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 μmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Efficient reduction of CO2 and inhibition of hydrogen precipitation by polyoxometalate photocatalyst modified with the metal Mn

Photocatalytic reduction of CO2 to chemical fuels is attractive for solving both the greenhouse effect and the energy crisis, but the key challenge is to design and synthesize photocatalysts with remarkable performance under visible light irradiation. Efficient catalytic carbon dioxide reduction (CO2RR) with light is considered a promising sustainable and clean approach to solve environmental problems. Herein, we found a new photocatalyst ([Mn(en)2]6[V12B18O54(OH)6]) (abbreviated as Mn6V12) based on the modifiability of polyoxometalates, in which Mn acts as a modifying unit to efficiently reduce CO2 to CO and effectively inhibit the hydrogen precipitation reaction. This Mn modified polyoxometalate catalyst has a maximum CO generation rate of 4625.0 μmol g-1 h-1 and a maximum H2 generation rate of 499.6 μmol g-1 h-1, with a selectivity of 90.3% for CO generation and 9.7% for H2 generation. This polyoxometalate photocatalyst can effectively reduce CO and inhibit the hydrogen precipitation reaction. It provides a new idea for the efficient photocatalytic carbon dioxide reduction (CO2RR) with polyoxometalate catalysts.

An Organodiphosphate-Containing Polyoxoniobate Ring and Its Assembly into a Three-Dimensional Framework through Hydrogen Bonding

In this work, a novel organodiphosphate-containing inorganic-organic hybrid polyoxoniobate (PONb) ring {(PO3CH2CH2PO3H)4Nb8O16}4- (Nb8P8) has been achieved by a one-pot hydrothermal method. The ring is constructed from a tetragonal {Nb8O36} motif and four {PO3CH2CH2PO3H} ligands. Interestingly, Nb8P8 can be joined together via K-H2O clusters {K2(H2O)4(OH)2} to form one-dimensional chains {[K2(H2O)4(OH)2]Nb8P8}n and further linked by {Cu(en)2}2+ (en = ethylenediamine) complexes, resulting in a three-dimensional supramolecular framework {[Cu(en)2]2[K2(H2O)4(OH)2]Nb8P8}·3en·H2O (1). 1 exhibits good chemical and thermal stability and has a high water vapor adsorption capacity of ≤224 cm3 g-1 (22.71 mol·mol-1) at 298 K, outperforming most of the known polyoxometalate-based materials. Impedance measurements prove that 1 can transfer protons with moderate conductivity. This study not only contributes to the structural diversity of organodiphosphate-containing PONbs and PONb rings but also provides a reference for the development of PONb-based materials with unique performance.

Synthesis and Characterization of 6-Ti-Substituted Polyoxomolybdate with High Catalytic Activity for Sulfide Oxidation

A 6-Ti-substituted polyoxometalate, (NH4)5Cs7Na3H2[Cs@(Ti2GeMo10O39)3]·34H2O (1), was synthesized by reacting (NH4)6Mo7O24·4H2O, GeO2, and TiOSO4 through the conventional aqueous method. Polyanion 1a is composed of three {Ti2GeMo10} segments linked by Ti-O-Ti linkages and shows a trefoil-shaped structure. Furthermore, one Cs+ cation is encapsulated in the cavity of 1a. Notably, it possesses the highest number of Ti centers among the reported polyoxomolybdates. In addition, serving as a high-efficiency heterogeneous catalyst, 1 enables the conversion of methyl phenyl sulfide within 20 min, yielding 96.4% of the corresponding sulfoxide with good recyclability.

Reactivity of Organoiridium Tungsten Oxide Clusters with Transition Metal Aquo Cations

Organometallic-polyoxometalate (POM) complexes form a unique class of molecular organometallic oxides characterized by the dynamic behavior of the organometallic cations. Herein, we investigated the reactivity of Cp*Ir-octatungstate clusters (where Cp* represents pentamethylcyclopentadienyl, C5Me5-) with Werner-type transition-metal aquo cations. The addition of Ag+, Co2+, Ni2+, and M3+ (M = Cr, Fe, or In) cations to the aqueous solution of Cp*Ir-octatungstate clusters resulted in the formation of [{Ag(OH2)2}2{Cp*Ir(OH2)}2{Cp*IrW3O12(OH)}2(WO2)2] (1), Co1.5K0.8Na0.2[{trans-Co(OH2)2}{Cp*IrW3O12(OH)}2(WO2)1.3{cis-Co(OH2)2}0.7] (2-Co), Ni0.2K1.4Na0.2[{Ni(OH2)4}2{Cp*IrW3O12(OH)}2(WO2)1.1{cis-Ni(OH2)2}0.9] (2-Ni), and [{M(OH2)4}2{Cp*IrW3O12(OH)}2{cis-M(OH2)2}2](NO3)2 (M = Cr, 3-Cr; Fe, 3-Fe; or In, 3-In), respectively. All clusters share the same cubane-type {Cp*IrW3O12(OH)}5- building block, representing the first examples of organoiridium-POMs functionalized by transition-metal aquo cations. These compounds are insoluble in water, facilitating the evaluation of their heterogeneous water-oxidation properties. Notably, 2-Co generates the highest catalytic water oxidation current. This work provides a new synthetic method to introduce metal-aquo complexes on an organometallic oxide cluster, producing multimetallic molecules that model the catalytic sites of complex oxides.

Hexalacunary [α-H2 P2 W12 O48 ]12- Wells-Dawson Anion: X-ray Crystal Structural Evidence and Oligomerization to WO(OH2 )4+ -Bridged Dimer and Trimers

We report the first single-crystal X-ray structural evidence of the potassium salt of the hexalacunary [α-H2 P2 W12 O48 ]12- (abbreviated as {P2 W12 }) anion after its discovery by Contant and Ciabrini in 1977. In addition, we observed oligomerization of {P2 W12 } into a {WO(OH2 )}4+ -bridged Pacman-shaped [{WO(OH2 )}(α-HP2 W12 O48 )2 ]22- ({P4 W25 }) dimer and a cyclic [{WO(OH2 )}3 (P2 W12 O48 )3 ]30- ({P6 W39 }) trimer. The three phosphotungstate anions were synthesized through recrystallization of (NH4 )12 [α-H2 P2 W12 O48 ] from slightly alkaline (HOCH2 )3 CNH2 /KCl, CH3 NH3 Cl/KCl, and CH3 NH3 Cl/NH4 Cl solutions. The structure of {P2 W12 } is derived from [α-P2 W18 O62 ]6- that has six tungsten atoms one from each polar group and four from the belt-removed, and the center of the lacunary site is capped by a potassium cation. Structures of {P4 W25 } and {P6 W39 } are constructed by connecting two and three {P2 W12 } units with {WO(OH2 )}4+ , respectively. The isolation of a pure {P6 W39 } phosphotungstate framework without coordination with transition metal cations is unprecedented. Powder X-ray diffraction confirmed the bulk purity of these compounds, indicating that selective crystallization was achieved through the selection of countercations and pH.

Novel Mn4-Co2 nanocluster@photosensitizers/SalenCo(iii) catalyze the copolymerization of carbon dioxide and propylene oxide

In general, transition metals (TMs) often facilitate highly efficient catalysis. Herein, we synthesized a series of nanocluster composite catalysts by combining with photosensitizers and SalenCo(iii) for the first time and studied the catalytic copolymerization of CO2 and propylene oxide (PO). Systematic experiments have shown that the selectivity of copolymerization products can be improved by the nanocluster composite catalysts, and their synergistic effects significantly improved the photocatalytic performance of carbon dioxide copolymerization. At specific wavelengths, I@S1 can achieve a TON of 536.4, which is 2.26 times that of I@S2. Interestingly, in the photocatalytic products of I@R2, CPC reached 37.1%. These findings provide a new idea for the study of TM nanocluster@photosensitizers for carbon dioxide photocatalysis, and may provide guidance for exploring low cost and highly efficient carbon dioxide emission reduction photocatalysts.

Cadmium-containing windmill-like heteropolyoxoniobate macrocycle with high yield for catalyzing Knoevenagel condensation

A rare cadmium-containing windmill-like heteropolyoxoniobate macrocycle has been successfully synthesized with stable 1-D cyclic cluster aggregates. The compound exhibited promising basic catalytic ability for Knoevenagel condensation with a high yield under mild reaction conditions and high cycling stability. The theoretical calculation showed that the promising basic catalytic ability is due to the dense and stronger basic sites of the surface terminal O atoms.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Ni12 tetracubane cores with slow relaxation of magnetization and efficient charge utilization for photocatalytic hydrogen evolution

We report two Ni₁₂ multicubane topologies enclosed in the polyanions [Ni₁₂(OH)₉(WO₄)₃(PO₄)(B-α-PW₉O₃₄)₃]^21-{Ni₁₂W₃₀} and [Ni₁₂(OH)₉(HPO₄)₃(PO₄)(B-α-PW₉O₃₄)(A-α-PW₉O₃₄)₂]^21-{Ni₁₂W₂₇} that magnetically behave as Ni₁₂ units clearly distinguishing them from typical Ni₄ cubanes as shown by magnetic studies together with high field and frequency electron paramagnetic resonance (HFEPR). Beyond the unprecedented static properties, {Ni₁₂W₃₀} shows the unusual coexistence of slow relaxation of the magnetization and a diamagnetic ground state (S = 0), providing the unique opportunity of studying the essentially elusive magnetic relaxation behavior in excited states. The cubane-topology dependent activity of {Ni₁₂W₃₀} and {Ni₁₂W₂₇} as homogeneous HER photocatalysts unveils the structural key features significant for the design of photocatalysts with efficient charge utilization exemplified by high quantum yields (QY) of 10.42% and 8.36% for {Ni₁₂W₃₀} and {Ni₁₂W₂₇}, respectively.

Water Oxidation Catalyzed by a Bioinspired Tetranuclear Manganese Complex: Mechanistic Study and Prediction

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn^III ₃ Mn^IV O₃ (CH₃ COO)₃ (A-α-SiW₉ O₃₄ )]^6- . Theoretical results indicated that catalytic active species 1 (Mn₄ ^{III,III,IV,IV} ) was formed after O₂ formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn^IV -oxyl radical 4 (Mn₄ ^IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn₂ O₂ core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn^IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn₄ ^III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol^-1 . Subsequent PCET oxidation of Int1 to Int2 (Mn₄ ^III,IV,IV,IV -O₂ ⋅) enabled the O₂ release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities.

Microenvironment Regulation of {Co4IIO4} Cubane for Syngas Photosynthesis

It is a great challenging task for selectivity control of both CO₂ photoreduction and water splitting to produce syngas via precise microenvironment regulation. Herein, a series of UiO-type Eu-MOFs (Eu-bpdc, Eu-bpydc, Ru_x-Eu-bpdc, and Ru_x-Eu-bpydc) with different surrounding confined spaces were designed and synthesized. These photosensitizing Ru_x-Eu-MOFs were used as the molecular platform to encapsulate the [Co^II₄(dpy{OH}O)₄(OAc)₂(H₂O)₂]²⁺ (Co₄) cubane cluster for constructing Co₄@Ru_x-Eu-MOF (x = 0.1, 0.2, and 0.4) heterogeneous photocatalysts for efficient CO₂ photoreduction and water splitting. The H₂ and CO yields can reach 446.6 and 459.8 μmol·g^-1, respectively, in 10 h with Co₄@Ru_0.1-Eu-bpdc as the catalyst, and their total yield can be dramatically improved to 2500 μmol·g^-1 with the ratio of CO/H₂ ranging from 1:1 to 1:2 via changing the photosensitizer content in the confined space. By increasing the N content around the cubane, the photocatalytic performance drops sharply in Co₄@Ru_0.1-Eu-bpydc, but with an enhanced proportion of CO in the final products. In the homogeneous system, the Co₄ cubane was surrounding with Ru photosensitizers via week interactions, which can drive water splitting into H₂ with >99% selectivity. Comprehensive structure-function analysis highlights the important role of microenvironment regulation in the selectivity control via constructing homogeneous and heterogeneous photocatalytic systems. This work provides a new insight for engineering a catalytic microenvironment of the cubane cluster for selectivity control of CO₂ photoreduction and water splitting.

Spectrophotometric Determination of Formation Constants of Iron(III) Complexes with Several Ligands

Dye-sensitized solar cells transform solar light into electricity. One commonly used dye is a ruthenium complex. However, the use of ruthenium has been shown to have several disadvantages. In this study, via singular spectrum analysis using HypSpec software, we determined the formation constants and calculated individual electronic spectra of species of iron(III) with several ligands (1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 2,2′-bipyridyl, 5,5-dimethyl-2,2′-bipyridyl, 4,4′-di-tert-butyl-2,2′-bipyridyl, 1,10-phenanthroline, and 3,4,7,8-tetramethyl-1,10-phenanthroline) in methanol solution. We present a spectral comparison of the complexes reported here to the ruthenium complex: tris-(2,2′-bipyridyl)ruthenium(II).

A Three-Dimensional (3D) Indium-Containing Polyoxoniobate Framework Based on {In5Nb71}n Helical Pillars

A rare 3D Indium-containing polyoxoniobate framework {H₉[Cu(en)₂(H₂O)₂][Cu(en)₂]₁₂[In(en)]₅[Nb₂₃-O₆₅(OH)₃(H₂O)₂]{Nb₂₄O₆₇(OH)₂(H₂O)₃]₂}·68H₂O(1), based on the In-containing polyoxoniobate cluster, {[In(en)]₅[Nb₂₃O₆₅(OH)₃(H₂O)₂][Nb₂₄O₆₇(OH)₂(H₂O)₃]₂}^35- ({In₅Nb₇₁}) and [Cu(en)₂]²⁺ linkers has been successfully synthesized. The nest-like cluster {In₅Nb₇₁} is constructed from one brand-new V-shaped {Nb₂₃O₇₀}, two triangle-shaped {Nb₂₄O₇₂} and five [In(en)]³⁺. The [In(en)] fragments link {Nb₂₄O₇₂} and {Nb₂₃O₇₀} units into unique {In₅Nb₇₁}_n helical pillars. The copper-amine complexes connect the {In₅Nb₇₁}_n helical pillars into a three-dimensional (3D) inorganic-organic hybrid In-Cu-containing framework. This material also exhibits good ionic conductivity and vapor adsorption capacity properties.

Hybrid Cu-Containing Compounds Based on Lacunary Strandberg Anions: Synthesis under Mild Conditions, Crystal Structure, and Magnetic Properties

A one-pot reaction of a copper source (metallic powder Cu⁰ or Cu²⁺ salts) and bpy (bpy = 2,2'-bipyridine) in the presence of (NH₄)₂HPO₄ and (NH₄)₆Mo₇O₂₄·4H₂O yields heterometallic hybrid compounds of the general type {[Cu(bpy)_n(H₂O)_m]_p[P₂Mo_xO_y]}. The structures exhibit a number of phosphomolybdate POMs including not only a common Strandberg anion [P₂Mo₅O₂₃]^6- but also its unprecedented bi- and trilacunary derivatives [P₂Mo₃O₁₈]^8- and [P₂Mo₂O₁₅]^8-. The structural determinants including the metal source (copper powder vs copper salts), counterion of the salts, and stoichiometry of the reagents were examined. An ex situ EPR study revealed the formation of different Cu^II complexes in the reaction mixture depending on the copper precursor. The obtained compounds have been found to possess selectivity toward the sorption of methylene blue in a mixture of organic dyes. DC magnetic measurements of 1-3 indicate rather strong antiferromagnetic metal-metal exchange interactions. Compound 1 exhibits field-induced slow magnetic relaxation in AC magnetic measurements, which is a rarely observed phenomenon among Cu(II) complexes.

Study on Catalytic Water Oxidation Properties of Polynuclear Manganese Containing Polyoxometalates

Splitting of water to produce hydrogen and oxygen is a green and effective method to produce clean energy. Finding an efficient water decomposition catalyst is the key step to realize water decomposition. In this work, by choosing from the literature, six polynuclear manganese (Mn) containing polyoxometalates (Mn-POMs) with different Mn-O clusters and oxidation states of Mn, [MnIIMnIIISiW10O37(OH)(H2O)]6− (Mn2-POM), [MnII3MnIII(H2O)2(PW9O34)2]9− (Mn4-POM), [MnII4MnIII2Ge3W24O94(H2O)2]18− (Mn6-POM-1), [MnIII2MnII4(μ3-O)2(H2O)4(B-β-SiW8O31)(B-β-SiW9O34)(γ-SiW10O36)]18− (Mn6-POM-4), [{MnIII3MnIV4O4(OH)2(OH2)}2(W6O22)(H2W8O32)2(H4W13O46)2]26− (Mn14-POM), [MnII19 (OH)12(SiW10O37)6]34− (Mn19-POM) were prepared. First, the catalytic performance towards the water oxidation of six Mn-POMs was investigated in solution for the first time. Second, six Mn-POMs were fabricated on the surface of ITO electrode using layer-by-layer self-assembly (LBL) to form the composite films, which were characterized by UV-vis spectroscopy and cyclic voltammetry, and then the catalytic water oxidation performance of the composite films was studied and compared with that in solution via a series of controlled experiments, the results indicate that the Mn-POMs with three-dimensional structures, which contain variable valence Mn-O cluster similar to the structure of photocatalytic active center (PSII) exhibit better catalytic performance.

Synergic coordination of multicomponents for the formation of a {Ni30} cluster substituted polyoxometalate and its in situ assembly

The synergic coordination of trz, en, and PW9 resulted in a {Ni30} cluster substituted POM, [Ni(trz)3]2@[Ni30(H2O)16]POM, that was discovered as the SBU of four frameworks which served as heterogeneous catalysts for HERs.

3d-4f Heterometallic cluster incorporated polyoxoniobates with magnetic properties

A series of 3d-4f heterometallic cluster incorporated polyoxoniobates (PONbs) with different magnetic properties were first made and characterized. This work not only provides a promising strategy to make new heterometallic cluster incorporated PONbs but also demonstrates an ideal model to probe how transition-metal ions influence the magnetic property of PONbs.

Design of molecular water oxidation catalysts with earth-abundant metal ions

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts

The ever-growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO₄ ^3- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na₁₅ [Ge₄ PCo₄ (H₂ O)₂ W₂₄ O₉₄ ] ⋅ 38H₂ O (Co₄ ), Na_2.5 K_17.5 [Ge₃ PCo₉ (OH)₅ (H₂ O)₄ W₃₀ O₁₁₅ ] ⋅ 45H₂ O (Co₉ ), Na₆ K₁₆ [Ge₄ P₄ Co₂₀ (OH)₁₄ (H₂ O)₁₈ W₃₆ O₁₅₀ ] ⋅ 61H₂ O (Co₂₀ ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn₄ Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT-scaffolds were active molecular WOCs, with Co₉ and Co₂₀ outperforming the well-known Na₁₀ [Co₄ (H₂ O)₂ (PW₉ O₃₄ )₂ ] {Co₄ P₂ W₁₈ } by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s^-1 Co₉ currently represents the fastest Co-GT-based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co₉ was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co₄ .

Polyoxotungstate Archetype {P4 W27 } and its 3d Derivatives

The propensity of the new, phenylphosphonate-stabilized polyoxotungstate [(C6 H5 PV O)2 P4 W24 O92 ]16- to act as a precursor for new 3d metal-functionalized polyanions has been investigated. Reactions with MnII and CuII induce the formation of the previously unknown polyoxotungstate archetype {P4 W27 }, isolated as salts of the polyanions [Na⊂{MnII (H2 O)}{WO(H2 O)}P4 W26 O98 ]13- (1) and [K⊂{CuII (H2 O)}{W(OH)(H2 O)}P4 W27 O99 ]14- (2), which were characterized in the solid state (single-crystal X-ray diffraction, elemental and TG analyses, IR spectroscopy, SQUID magnetometry) and in aqueous solution (UV/Vis spectroscopy, cyclic voltammetry).

Arsenic(III)-Capped 12-Tungsto-2-Arsenates(III) [M2(AsIIIW6O25)2(AsIIIOH)x]n- (M = CrIII, FeIII, ScIII, InIII, TiIV, MnII) and Their Magnetic Properties

Six arsenic(III)-capped 12-tungsto-2-arsenates(III) of the type [M₂(As^IIIW₆O₂₅)₂(As^IIIOH)_x]^n- (M = Cr^III, 1; Fe^III, 2; Sc^III, 3; In^III, 4; Ti^IV, 5; Mn^II, 6) have been synthesized in aqueous medium by direct reaction of the elements using a one-pot strategy and structurally characterized by FT-IR spectroscopy, single-crystal XRD, and elemental analysis. Polyanions 1-6 are comprised of two octahedrally coordinated guest metal ions M sandwiched between two {AsW₆} units, resulting in a structure with C_2h point-group symmetry. Polyanions 1-5 contain tri- and tetravalent metal ion guests M (M = Cr^III, Fe^III, Sc^III, In^III, and Ti^IV, respectively), and they have one {As^IIIOH} group grafted on each {AsW₆} unit, whereas the divalent Mn^II-containing derivative 6 has two such {As^IIIOH} groups grafted on each {AsW₆} unit. Magnetic studies on polyanions 3-5 over the temperature range 1.8-295 K and magnetic fields of 0-7 T confirmed that they are diamagnetic. On the other hand, polyanions 1, 2, and 6 are strongly magnetic and follow the Curie-Weiss law above 30 K. The susceptibility plots of 1 and 6 exhibit broad peaks suggesting short-range antiferromagnetic ordering, while the very weak antiferromagnetic ordering of 2 is overshadowed by traces of a paramagnetic impurity. The magnetization data of 1, 2, and 6 at 1.8 K over 0-7 T were analyzed by using the Heisenberg exchange procedure. Small (negative) values of the obtained J values help in understanding the absence of long-range antiferromagnetic ordering.

Research Progress on Catalytic Water Splitting Based on Polyoxometalate/Semiconductor Composites

In recent years, due to the impact of global warming, environmental pollution, and the energy crisis, international attention and demand for clean energy are increasing. Hydrogen energy is recognized as one of the clean energy sources. Water is considered as the largest potential supplier of hydrogen energy. However, artificial catalytic water splitting for hydrogen and oxygen evolution has not been widely used due to its high energy consumption and high cost during catalytic cracking. Therefore, the exploitation of photocatalysts, electrocatalysts, and photo-electrocatalysts for rapid, cost effective, and reliable water splitting is essentially needed. Polyoxometalates (POMs) are regarded as the potential candidates for water splitting catalysis. In addition to their excellent catalytic properties and reversibly redox activities, POMs can also modify semiconductors to overcome their shortcomings, and improve photoelectric conversion efficiency and photocatalytic activity, which has attracted more and more attention in the field of photoelectric water splitting catalysis. In this review, we summarize the latest applications of POMs and semiconductor composites in the field of photo-electrocatalysis (PEC) for hydrogen and oxygen evolution by catalytic water splitting in recent years and take the latest applications of POMs and semiconductor composites in photocatalysis for water splitting. In the conclusion section, the challenges and strategies of photocatalytic and PEC water-splitting by POMs and semiconductor composites are discussed.

A μ-AsO4-Bridging Hexadecanuclear Ni-Substituted Polyoxotungstate

A novel tetrahedral μ-AsO₄-bridging hexadecanuclear Ni-substituted silicotungstate (ST) Na₂₁H₁₀[(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂(A-α-SiW₉O₃₄)₂}₂]·60H₂O (1) was made by the reactions of trivacant [A-α-SiW₉O₃₄]^10- ({SiW₉}) units with Ni²⁺ cations and Na₃AsO₄·12H₂O and characterized by IR spectrometry, elemental analysis, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). 1 contains a novel polyoxoanion [(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂(A-α-SiW₉O₃₄)₂}₂]^31- built by four trivacant Keggin [A-α-SiW₉O₃₄]^10- fragments linked through an unprecedented [(AsO₄){Ni₈(OH)₆(H₂O)₂(CO₃)₂}₂]⁹⁺ cluster, where the tetrahedral AsO₄ acts as an exclusively μ₂-bridging unit to link multiple Ni centers; such a connection mode appears for the first time in polyoxometalate chemistry. Furthermore, the electrochemical and catalytic oxidation properties of compound 1 have been investigated.

Lacunary {Se4V10} Heteropolyoxovanadate Precursor with Monometal, Metal-Richer-Sandwiched Derivatives {Se8V20M} and {Se8V20M3}: Correlations between the Synthesis, Structure, and Catalytic Property

A new family of trinuclear transition-metal (TM)-sandwiched heteropolyoxovanadates (hetero-POVs) [(SeV₁₀O₂₈(SeO₃)₃M(H₂O)₃)₂(M(H₂O)₄)]^10- (Se₈V₂₀M₃, where M = Mn²⁺, Co²⁺, and Zn²⁺) were prepared using two feasible approaches: a stepwise assembly strategy atop the POV precursor Se₄V₁₀ and a one-pot reaction approach of KVO₃, SeO₂, TM²⁺, and a proline ligand. The crystallographic studies reveal that Se₈V₂₀M₃ consist of two asymmetric [SeV₁₀O₂₈(SeO₃)₃M(H₂O)₃)]^6- units, linked by another TM²⁺ ion, thus forming an interesting staggered sandwich-type arrangement. Additionally, Se₈V₂₀M₃ and Se₈V₂₀M present strong correlations between their structures and catalytic properties. In particular, Se₈V₂₀M₃ demonstrate an analogical heterogeneous catalytic performance as Se₈V₂₀M in the sulfoxidation reaction of thioethers owing to their structural similarities.

Water-Stable Cobalt-Based MOF for Water Oxidation in Neutral Aqueous Solution: A Case of Mimicking the Photosystem II

Inspired by the highly efficient water oxidation of Mn₄CaO₅ in natural photosynthesis, development of novel artificial water oxidation catalysts (WOCs) with structure and function mimicked has inspired extensive interests. A novel 3D cobalt-based MOF (GXY-L8-Co) was synthesized for promising artificial water oxidation by employing the Co₄O₄ quasi-cubane motifs with a similar structure as the Mn₄CaO₅ as the core. The GXY-L8-Co not only shows good chemical stability in common organic solvents or water for up to 10 days but also exhibits oxygen evolution performance. It has been demonstrated that the uniform distribution of Co₄O₄ catalytic active sites confined in the MOF framework should be responsible for the good robustness and catalytic performance.

A protecting group strategy to access stable lacunary polyoxomolybdates for introducing multinuclear metal clusters

Although metal-containing polyoxomolybdates (molybdenum oxide clusters) exhibit outstanding catalytic properties, their precise synthetic method has not yet been developed. This is mainly because the very low stability of the multivacant lacunary polyoxomolybdates limited their use as synthetic precursors. Here, we present a "protecting group strategy" in polyoxometalate synthesis and successfully develop an efficient method for synthesising multinuclear metal-containing polyoxomolybdates using pyridine as a protecting group for unstable trivacant lacunary Keggin-type polyoxomolybdate [PMo₉O₃₄]^9-. Specifically, tetranuclear cubane- and planar-type manganese clusters were selectively synthesised in the polyoxomolybdates using the present method. The importance of this work is that, in addition to being the first practical way of utilizing multivacant lacunary polyoxomolybdates as precursors, this new "protecting group strategy" will make it possible to produce polyoxometalates with unexplored structures and properties.

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.

Cation-Directed Synthetic Strategy Using 4f Tungstoantimonates as Nonlacunary Precursors for the Generation of 3d-4f Clusters

The first synthetic pathway using a series of four nonlacunary 4f-heterometal-substituted polyoxotungstate clusters Na₂₁[(Ln(H₂O)(OH)₂(CH₃COO))₃(WO₄)(SbW₉O₃₃)₃]·nH₂O (NaLnSbW₉; Ln = Tb^III, Dy^III, Ho^III, Er^III, Y^III) as precursors for the directed preparation of nine new 3d–4f heterometallic tungstoantimonates K₅Na₁₂H₃[TM(H₂O)Ln₃(H₂O)₅(W₃O₁₁)(SbW₉O₃₃)₃]·nH₂O (KTMLnSbW₉; TM = Co^II, Ni^II; Ln = Tb^III, Dy^III, Ho^III, Er^III, Y^III) has been developed. Systematic studies revealed an increased K content in the aqueous acidic reaction mixture to be the key step in the cation-directed preparation of 3d–4f compounds; among those, the Co-containing members represent the first examples of KCoLnSbW₉ (Ln = Tb^III, Dy^III, Ho^III, Er^III, Y^III) heterometallic tungstoantimonates exhibiting the SbW₉ building block. All 13 compounds have been characterized thoroughly in the solid state by powder and single-crystal X-ray diffraction (XRD), revealing a cyclic trimeric polyoxometalate architecture with three SbW₉ units encapsulating a planar triangle of Ln^III ions in the case of NaLnSbW₉ and a heterometallic core of one TM^II and three Ln^III for KTMLnSbW₉ (TM = Co^II, Ni^II; Ln = Tb^III, Dy^III, Ho^III, Er^III, Y^III). The results obtained by XRD are supplemented by complementary characterization methods in the solid state such as IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by UV–vis spectroscopy. Detailed magnetic studies on the representative compounds KTMDySbW₉ (TM = Co^II, Ni^II) and KCoYSbW₉ of the series revealed field-induced slow magnetic relaxation.

The first step-by-step synthetic protocol using preformed 4f tungstoantimonate clusters as nonlacunary precursors for the controlled preparation and thorough characterization of a family of nine new 3d−4f heterometallic polyoxometalates [TM(H₂O)Ln₃(H₂O)₅(W₃O₁₁)(SbW₉O₃₃)₃]^20- (KTMLnSbW₉) (TM = Co^II, Ni^II; Ln = Tb^III, Dy^III, Ho^III, Er^III, Y^III) is reported. Magnetic studies on the Dy^III-containing representatives [TM(H₂O)Dy₃(H₂O)₅(W₃O₁₁)(SbW₉O₃₃)₃]²⁰⁻ (TM = Co^II, Ni^II) show single-molecule-magnet behavior.

Discovery of a Fullerene-Polyoxometalate Hybrid Exhibiting Enhanced Photocurrent Response

A new C₆₀-polyoxometalate compound was synthesized by a C₆₀ derivative with a {SiW₁₁Mn} cluster connecting through a coordination bond and characterized. The photocurrent response test showed great improvement of this new compound compared to C₆₀ and the polyoxometalate precursor, which could be used as a potential photoelectric material.

Ionic strength effect on regulating the synthetic assembly of polyoxometalate clusters with slow magnetic relaxation behavior

Three novel polyoxometalate (POM) clusters of K10Na10[Dy3(H2O)6Ni(H2O)(W3O11)(B-α-SbW9O33)3]·(H2O)45 (1), (NH4)5K4Na8[Dy3(H2O)6SbV(H2O)(W3O11)(B-α-SbW9O33)3]·(H2O)50 (2), and (NH4)9Na[Ni2(H2O)6(WO2)2(B-β-SbW9O33)2]·(H2O)24 (3) were successfully obtained using the same precursor under different ionic strength conditions. Structural analysis showed that compounds 1-3 possess discrepant structural characteristics in 1 M KCl, 1 M NH4Cl, and saturated NH4Cl, respectively. Among them, 2 is the first reported lanthanide cluster including both Sb3+ and Sb5+ in POM derivatives. Furthermore, the analysis of ac magnetic data proved that 1 and 2 embodied dramatic field-induced slow magnetic relaxation, and the effective barrier of 2 was estimated as Ueff/KB = 21.10 K based on the Orbach process. It was found that the structure and magnetic properties of POM materials can be regulated using an effective and satisfactory ionic strength-controlled strategy.

An exceptionally stable octacobalt-cluster-based metal-organic framework for enhanced water oxidation catalysis

Extensive efforts have been devoted to developing efficient and durable catalysts for water oxidation. Herein, we report a highly stable metal-organic framework that shows high catalytic activity and durability for electrically driven (an overpotential of 430 mV at 10 mA cm-2 in neutral aqueous solution) and photodriven (a turnover frequency of 16 s-1 and 12 000 cycles) water oxidation, representing the best catalyst for water oxidation reported to date. Computational simulation and isotope tracing experiments showed that the μ4-OH group of the {Co8(μ4-OH)6} unit participates in the water oxidation reaction to offer an oxygen vacancy site with near-optimal OH- adsorption energy.