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.

Nanostructured Manganese Oxides within a Ring-Shaped Polyoxometalate Exhibiting Unusual Oxidation Catalysis

Nanosized manganese oxides have recently received considerable attention for their synthesis, structures, and potential applications. Although various synthetic methods have been developed, precise synthesis of novel nanostructured manganese oxides are still challenging. In this study, using a structurally defined nanosized cavity inside a ring-shaped polyoxometalate, we succeeded in synthesizing two types of discrete 18 and 20 nuclear nanostructured manganese oxides, Mn18 and Mn20, respectively. In particular, Mn18 showed much higher catalytic activity than other manganese oxides for the oxygenation of alkylarenes including electron-deficient ones, and the reaction proceeded through a unique reaction mechanism due to its unusual manganese oxide structure.

Fabrication of Six Manganese Containing Polyoxometalate Modified Graphite C3N4 Nanosheets Catalysts Used to Catalyze Water Decomposition

With the increase in gas population, the demand for clean and renewable energy is increasing. Hydrogen energy has a high combustion conversion energy while water is its combustion product. In recent years, a way to convert water into hydrogen and oxygen has been found by human beings inspired by plant photosynthesis. However, water decomposition consumes a significant amount of energy and is expensive. People expect to obtain a water decomposition catalyst with low cost and high efficiency. This work selected a six-manganese containing polyoxometalate with a similar structure characteristic to photosynthesizing PSII to fabricate with graphite C3N4 nanosheets for the construction of composite film (Mn6SiW/g-C3N4NSs) electrode via layer by layer self-assembly technology, which was used for the photo-electrochemical decomposition of water under visible light conditions. The binary composite film electrode displayed good catalytic efficiency. The photoelectric density of the composite electrode is 46 μA/cm2 (at 1.23 V vs. Ag/AgCl) and 239 μA/cm2 (at 1.5 V vs. Ag/AgCl). Compared with the g-C3N4NSs electrode alone, the photoelectric density of the composite electrode increased by 1 time. The reason is attributed to the fact that Mn6SiW has a similar structure characteristic to photosynthesizing PSII and high electron transferability. The construction of the composite film containing low-cost Mn6SiW to modify g-C3N4NSs can effectively improve the photocatalytic decomposition of water, thus this study provides valuable reference information for the development of low-cost and high-performance photo-electrocatalytic materials.

Three ring-shaped Zr(iv)-substituted silicotungstates: syntheses, structures and their properties

Three Zr(iv)-substituted polyoxometalate nanoclusters, H₁₆(H₂dap)₄[{Zr₂(OH)₂(α-SiW₁₀O₃₈)}₂{Zr₂(OH)₂(β-SiW₁₀O₃₈)}₂]·8H₂O (1, dap = 1,3-diaminopropane), H₁₇Na₉[(Zr₂(OH)₂)₂(Zr₂BO(OH)₄)₂(β-SiW₁₀O₃₈)₄]·50H₂O (2) and H₂₀Na₈[(Zr₂BO(OH)₄)₂(Zr₂B₂O₂(OH)₅)₂(β-SiW₁₀O₃₈)₄]·40H₂O (3), were made by a hydrothermal reaction of trivacant [A-α-SiW₉O₃₄]^10-, Zr⁴⁺ ions, and H₃BO₃ under different alkaline conditions. Single-crystal X-ray structure analysis revealed that they are ring-shaped polyoxotungstate aggregates built by Zr₂(OH)₂ ({Zr₂}) clusters and two types of dilacunary α-/β-SiW₁₀O₃₈ ({α-/β-SiW₁₀}) fragments for 1, {Zr₂}, {β-SiW₁₀}, and Zr₂BO(OH)₄ ({Zr₂B}) clusters for 2, and {β-SiW₁₀}, {Zr₂B}, and Zr₂B₂O₂(OH)₅ ({Zr₂B₂}) clusters for 3, respectively, showing that the Zr-B-O clusters, {Zr₂B} and {Zr₂B₂}, have been introduced to the lacunary sites of polyoxometalates for the first time. The electrochemistry and electrocatalytic properties of 2 were studied, and it exhibited effective catalytic activities for the reduction of H₂O₂, NO₂^-, and BrO₃^-. Moreover, the catalysis of 2 for the oxidation reactions of various thioethers was investigated, and it was found that 2 possessed high conversion and remarkable selectivity.

A new octa-Mn-substituted poly(polyoxotungstate)

A novel octa-Mn-substituted silicotungstate (ST) tetramer [H₂N(CH₃)₂]₈H₁₉Na₄{[K(H₂O)₄WO₄] [SiMnII2W₁₀O₃₈]₃}[SiMnIII2W₁₀O₃₈]·27H₂O (1) was prepared, and its electrochemistry, thermal stability and magnetic properties were investigated.

Chain length effect in the functionalization of polyoxometalates with α,ω-alkyldiphosphonates

Varying the alkylene tethers in hybrids of polyoxometalates and α,ω-alkyldiphosphonates, even just by a single methylene group, has met with strong structural and magnetic responses.

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions of trinuclear mixed-valence Mn(ii)/Mn(iii) complexes

The effect of the orientation of the Jahn–Teller distortion on the magnetic interactions in two new mixed-valence trinuclear Mn(iii)–Mn(ii)–Mn(iii) complexes has been investigated.

In situ redox reactions facilitate the assembly of a mixed-valence metal-organic nanocapsule

C-alkylpyrogallol[4]arenes (PgCs) have been studied for their ability to form metal-organic nanocapsules (MONCs) through coordination to appropriate metal ions. Here we present the synthesis and characterization of an Mn^II/Mn^III-seamed MONC in addition to its electrochemical and magnetic behavior. This MONC assembles from 24 manganese ions and 6 PgCs, while an additional metal ion is located on the capsule interior, anchored through the introduction of bridging nitrite ions. The latter originate from an in situ redox reaction that occurs during the self-assembly process, thus representing a new route to otherwise unobtainable nanocapsules.

New approaches are required to access metal-organic assemblies with unusual structural properties. Here, the authors use an in situ redox reaction to obtain a mixed-valence, Mn(II)/Mn(III)-containing metal-organic nanocapsule with an odd number of metal ions.

A high-nuclearity isopolyoxotungstate based manganese cluster: one-pot synthesis and step-by-step assembly

A manganese(iii,iv)-tungsten(vi) supercluster based on 72 manganese ions (48 MnIV and 24 MnIII) and 48 tungsten(vi) centers [{MnIV24MnIII12O28(H2O)23}2(W24O120)2]40- has been prepared from the carboxylic Mn12 cluster. Its structure comprises two unprecedented cage-like Mn36W24 cores linked via two Mn-O-W bonds, leading to a Mn72W48 assembly. The inorganic synthetic mechanism was investigated through different synthesis methods and comprehensive ESI-MS tests.

Two Unprecedented POM-Based Inorganic-Organic Hybrids with Concomitant Heteropolytungstate and Molybdate

Two novel POM-based inorganic-organic hybrids, [Cu₆^II(2,2'-bipy)₆(Mo₆O₂₂)(SiW₁₂O₄₀)]_n (1), and {[Cu₆^II(ppz)₆(H₂O)₅(MoO₄)(SiW₁₂O₄₀)]·4H₂O}_n (2) (2,2'-bipy = 2,2'-bipyridine, Hppz = 3-(pyrid-2-yl)pyrazole), have been constructed from heteropolytungstates and molybdates. Two compounds have been identified by single crystal X-ray diffraction, elemental analysis, and FT-IR. Compound 1 shows a 1D (one-dimensional) chain structure constructed from classical Keggin heteropolytungstate [SiW₁₂O₄₀]^4- clusters and [Cu₆(2,2'-bipy)₆] modified isopolymolybdates [Mo₆O₂₂]^8-. Compound 2 also represents a 1D chain-like motif built from classical Keggin heteropolytungstate [SiW₁₂O₄₀]^4- clusters and [Cu₈(ppz)₆(H₂O)₅] modified molybdates MoO₄^2-. Compound 1 represents the first example of POM-based inorganic-organic hybrid with mixed heteropolytungstates and isopolymolybdates. ESI-MS (electrospray ionization mass spectrometry) technique was employed to reveal the species and their evolutions in the hydrothermal reaction, whereby trivacant [SiW₉] building block gradually transforms to classical Keggin [SiW₁₂] during assembly process. Furthermore, the electrocatalytic and magnetic properties were discussed in details.

Investigating the Transformations of Polyoxoanions Using Mass Spectrometry and Molecular Dynamics

The reactions of [γ-SiW10O36](8-) represent one of the most important synthetic gateways into a vast array of polyoxotungstate chemistry. Herein, we set about exploring the transformation of the lacunary polyoxoanion [β2-SiW11O39](8-) into [γ-SiW10O36](8-) using high-resolution electrospray mass spectrometry, density functional theory, and molecular dynamics. We show that the reaction proceeds through an unexpected {SiW9} precursor capable of undertaking a direct β → γ isomerization via a rotational transformation. The remarkably low-energy transition state of this transformation could be identified through theoretical calculations. Moreover, we explore the significant role of the countercations for the first time in such studies. This combination of experimental and the theoretical studies can now be used to understand the complex chemical transformations of oxoanions, leading to the design of reactivity by structural control.

pH-Controlled assembly of two novel Dawson-sandwiched clusters involving the in situ reorganization of trivacant α-[P2W15O56](12-) into divacant α-[P2W16O57](8.)

Modified classical trivacant Wells-Dawson α-[P2W15O56](12-) and the assembly of related sandwiched transition metal clusters are of interest, but surprisingly few reports of these materials exist because of the sensitivity of α-[P2W15O56](12-) to the assembly environment. Herein, we describe the pH-controlled assembly of two novel Dawson-sandwiched clusters, (H2bpz)6[Co2(P2W16O57)2]·22H2O (1, bpz = 3,3',5,5'-tetramethyl-4,4'-bipyrazole) and (H2bpz)6[Co3H2(P2W16O57)(P2W15O56)(H2O)]·12H2O (2), involving the in situ transformation of α-[P2W15O56](12-). Both clusters were characterized by X-ray single-crystal diffraction, FT-IR spectroscopy, UV-Visible spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and elemental analyses. X-ray crystallography showed that both heteropolytungstates become divacant α-[P2W16O57](8-) and symmetrically encapsulate two edge-shared CoO6 octahedra in the interior in 1, while only one divacant α-[P2W16O57](8-) is observed in 2, which combined with another trivacant α-[P2W15O56](12-) to asymmetrically clamp three edge-shared CoO6 octahedra. The α-[P2W16O57](8-) heteropolytungstate should be generated in situ from α-[P2W15O56](12-)via self-decomposition equilibria in solution. Their electrochemical behaviors reveal characteristic multi-electron redox processes related to W(VI) centers. The electrocatalytic reduction performances toward nitrite, hydrogen peroxide, chlorate, bromate and iodate were fully measured and discussed; among these species, both clusters exhibit the best electrocatalytic activity towards the reduction of bromate. Magnetic measurements indicate weak ferromagnetic exchange interactions between Co atoms sandwiched by vacant polyoxometalates.

Catalysis of "outer-phase" oxygen atom exchange reactions by encapsulated "inner-phase" water in {V15Sb6}-type polyoxovanadates

A water molecule encapsulated inside water-soluble {V₁₅Sb₆} antimonato polyoxovanadate cages accelerates oxygen-exchange reactions in the cluster periphery.

Antimonato polyoxovanadate (POV) cluster compounds {M(en)₃}₃[V₁₅Sb₆O₄₂(H₂O)_x]·nH₂O (M = Fe^II, Co^II, Ni^II and x = 0 or 1) obtained under solvothermal conditions exhibit unusual high water solubility making these compounds promising synthons for generation of new POV structure types. Electrospray ionization mass spectrometry provides evidence (i) for a water molecule encapsulated inside the cavity of a fraction of the spherical cluster shells, (ii) for a post-functionalization in water, namely a slow exchange of VO against Sb₂O, (iii) for the inner-phase reactivity of the encapsulated water that is capable of opening an oxo-bridge, and (iv) for a significant acceleration of the ¹⁶O/¹⁸O exchange reactions of oxygen atoms in the cluster periphery with surrounding H₂¹⁸O, when encapsulated water is present. To the best of our knowledge, this is the first example in polyoxovanadate chemistry for the transduction of inner-phase reactivity of an encapsulated guest molecule into changes in the outer-phase reactivity of the cluster. Magnetic susceptibility measurements reflect the individual contributions of the frustrated {V₁₅} spin polytope and the {M(en)₃}²⁺ complexes, with very weak coupling between these groups.

The main progress over the past decade and future outlook on high-nuclear transition-metal substituted polyoxotungstates: from synthetic strategies, structural features to functional properties

In this article, we discuss the synthetic methodologies, structural diversities and relevant properties of the high-nuclear TMSPTs reported in the past decade.

A novel poly(polyoxometalate) built by {Cu9}/{Cu5} clusters and {PW9}/{PW10}/{PW11} lacunary fragments

The hydrothermal reactions of [A-α-PW9O34](9-) with CuCl2·2H2O in the presence of ethanediamine (en) and acetate afforded a novel poly(polyoxotungstate) containing different lacunary fragments: monolacunary {α-PW11O39}, dilacunary {α-PW10O37}, trilacunary {B-α-PW9O34}, etc. In addition, a cockhorse-like nona-copper {Cu9} slab is incorporated. Magnetic investigation indicates overall antiferromagnetic interactions in this compound.

A CO32−-containing, dimanganese-substituted silicotungstate trimer, K9[H14{SiW10MnIIMnIIIO38}3(CO3)]·39H2O

An unprecedented dimanganese-substituted silicotungstate trimer K₉[H₁₄{SiW₁₀Mn^IIMn^IIIO₃₈}₃(CO₃)]·39H₂O was synthesized, in which the CO₃²⁻ resides inside the three Keggin {SiW₁₀Mn^IIMn^IIIO₃₈} units.

What can electrospray mass spectrometry of paratungstates in an equilibrating mixture tell us?

[W₇O₂₄]⁶⁻, the main species in an aqueous solution of Na₂WO₄at pH ≤ 7 proved by NMR and Raman spectroscopy, fails to be detected in ESI-MS due to its ESI-induced dissociation into Lindqvist [W₆O₁₉]²⁻.

Synergistic combination of multi-Zr(IV) cations and lacunary Keggin germanotungstates leading to a gigantic Zr24-cluster-substituted polyoxometalate

Synergistic directing roles of six lacunary fragments resulted in an unprecedented Zr24-cluster substituted poly(polyoxotungstate) Na10K22[Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4(GeW8O31)2]·85H2O (Na10K22·1·85H2O), which contains the largest [Zr24O22(OH)10(H2O)2] (Zr24) cluster in all the Zr-based poly(polyoxometalate)s to date. The most remarkable feature is that the centrosymmetric Zr24-cluster-based hexamer contains two symmetry-related [Zr12O11(OH)5(H2O)(W2O10H)(GeW9O34)2(GeW8O31)](16-) trimers via six μ3-oxo bridges and was simultaneously trapped by three types of different segments of B-α-GeW9O34, B-α-GeW8O31, and W2O10. The other interesting characteristic is that there are two pairs of intriguing triangular atom alignments: one is composed of the Zr(2,4,6,8,11) and W21 atoms and the other contains the Ge(1-3), Zr(3,5,7,9,10,12) and W26 atoms, and the Zr5 atom is inside the triangle; a linking mode is unobserved. The oxygenation reactions of thioethers by H2O2 were evaluated when Na10K22·1·85H2O served as a catalyst. Results show that it is an effective catalyst for oxygenation of thioethers by H2O2. The unique redox property of oxygen-enriched polyoxotungstate fragments and Lewis acidity of the Zr cluster imbedded in Na10K22·1·85H2O provide a sufficient driving force for the catalytic conversion from thioethers to sulfoxides/sulfones.

Synthesis and characterization of multinuclear manganese-containing tungstosilicates

The five manganese-containing, Keggin-based tungstosilicates [Mn(II)3(OH)3(H2O)3(A-α-SiW9O34)](7-) (1), [Mn(III)3(OH)3(H2O)3(A-α-SiW9O34)](4-) (2), [Mn(III)3(OH)3(H2O)3(A-β-SiW9O34)](4-) (3), [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-) (4), and [Mn(III)3Mn(IV)O3(CH3COO)3(A-β-SiW9O34)](6-) (5) were synthesized in aqueous medium by interaction of [A-α-SiW9O34](10-) or [A-β-SiW9O34H](9-) with either MnCl2 (1) or [Mn(III)8Mn(IV)4O12(CH3COO)16(H2O)4] (2-5) under carefully adjusted reaction conditions. The obtained salts of these polyanions were analyzed in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. The salts of polyanions 1, 2, and 4 were further characterized in the solid state by magnetic studies, as well as in solution by electrochemistry.

A polyoxometalate-based single-molecule magnet with a mixed-valent {Mn(IV)2Mn(III)6Mn(II)4} core

A polyoxometalate (POM)-based {Mn(IV)2Mn(III)6Mn(II)4} complex with single-molecule magnet (SMM) behaviour was prepared, exhibiting the largest known Mn nuclearity and the most Mn valence states in the POM-based SMM family.