Synthesis and characterization of multinuclear manganese-containing tungstosilicates

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.

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.

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).

Synthesis, structure, electrochemistry and magnetism of cobalt-, nickel- and zinc-containing [M4(OH)3(H2O)2(α-SiW10O36.5)2]13- (M = Co2+, Ni2+, and Zn2+)

Interaction of the trilacunary 9-tungstosilicate [A-α-SiW₉O₃₄]^10- with cobalt(ii), nickel(ii) and zinc(ii) ions in pH 9 aqueous medium at room temperature led to the formation of the respective M₄-containing heteropolytungstates [M₄(OH)₃(H₂O)₂(α-SiW₁₀O_36.5)₂]^13- (M = Co²⁺ (1), Ni²⁺ (2), and Zn²⁺ (3)). Polyanions 1-3 were characterized in the solid state by single-crystal XRD, FT-IR spectroscopy, and thermogravimetric and elemental analyses. Electrochemical studies showed that the Co²⁺ ions in 1 can be oxidized to Co³⁺ and the CVs of the W^VI centers of the polyanions feature well-defined and chemically reversible reduction waves. Magnetic measurements on 1 and 2 showed paramagnetism with complex ferromagnetic and antiferromagnetic interactions. A model was presented for extracting the exchange constants for the magnetic exchange interaction.

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.

Assembly, disassembly and reassembly: a "top-down" synthetic strategy towards hybrid, mixed-metal {Mo10Co6} POM clusters

Polyoxometalates (POMs) are commonly prepared using a "bottom-up" synthetic procedure. The alternative "top-down" approach of disassembling a pre-formed POM unit to generate new synthetic intermediates is promising, but relatively comparatively underused. In this paper, a rationale for the top-down method is provided, demonstrating that this approach can generate compounds that are fundamentally inaccessible from simple bottom-up assembly. We demonstrate this principle through the synthesis of a series of 10, new, mixed-metal, hybrid compounds with the general formula [TBA]2[MoVI10CoII6O30(RpPO3)6(RcCOO)2(L)x(H2O)6] (TBA = tetrabutylammonium, Rp = phosphonate moiety, Rc = carboxylate moiety, L = pyridyl ligand, and x = 2-4), including a one-dimensional polyoxometalate-based coordination polymer. We propose that these structures are generated from {MoxO3x-1} fragments that cannot be accessed from bottom-up assembly alone. The POM clusters are stabilised by three distinct classes of organic ligand - organophosphonate, carboxylate and pyridyl ligands - which can each be substituted independantly, thus providing a controlled route to ligand functionalisation.

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.

Redox active mixed-valence hexamanganese double-cubane complexes supported by tetravanadates

A double-cubane-type hexamanganese complex that is reminiscent of the structure of an oxygen evolution center was synthesized using a polyoxovanadate ligand.

Synthesis, structure and electrochemical characterization of an isopolytungstate (W4O16) held by MnII anchors within a superlacunary crown heteropolyanion {P8W48}

The first example of an isopolyanion (W₄O₁₆) held by redox-active Mn^II anchors within an archetypal superlacunary heteropolyanion {P₈W₄₈}.

The mixed-valent 10-manganese(III/IV)-containing 36-tungsto-4-arsenate(V), [MnIII 6MnIV 4O4(OH)12(H2O)12(A-β-AsW9O34)4]22−

Interaction of the mixed-valent 12-manganese coordination complex [MnIII 8MnIV 4O12(CH3COO)16(H2O)4] with the lacunary 9-tungstoarsenate(V) [A-α-AsW9O34]9− resulted in the 10-manganese(III/IV)-containing 36-tungsto-4-arsenate(V), [MnIII 6MnIV 4O4(OH)12(H2O)12(A-β-AsW9O34)4]22− (1). Polyanion 1 was isolated as a hydrated mixed potassium–sodium salt, K14Na8[MnIII 6MnIV 4O4(OH)12(H2O)12(A-β-AsW9O34)4]·104H2O, which crystallizes in the orthorhombic space group Pbcn and was characterized by FT–IR spectroscopy and single-crystal X-ray diffraction, as well as elemental and thermogravimetric analyses. The title polyanion contains a unique [MnIII 6MnIV 4O4(OH)12(H2O)12]14+ core stabilized within the 36-tungsto-4-arsenate(V) framework.

Insight into the reactivity of in situ formed {(NbO2)3SiW9}: synthesis, structure, and solution properties of a trimeric polytungstosilicate trapping a {MnNb9} core

The first example of the trimeric polyoxometalate cluster, Cs₁₀Na₃[(SiW₉Nb₃O₃₈)₃MnO₃(H₂O)₃]·18H₂O (1a), has been synthesized using an in situ synthetic strategy and thoroughly characterized.

A novel SWCNT-polyoxometalate nanohybrid material as an electrode for electrochemical supercapacitors

A novel nanohybrid material that combines single-walled carbon nanotubes (SWCNTs) with a polyoxometalate (TBA)5[PVMoO40] (TBA-PV2Mo10, TBA: [(CH3(CH2)3)4N](+), tetra-n-butyl ammonium) is investigated for the first time as an electrode material for supercapacitors (SCs) in this study. The SWCNT-TBA-PV2Mo10 material has been prepared by a simple solution method which electrostatically attaches anionic [PV2Mo10O40](5-) anions with organic TBA cations on the SWCNTs. The electrochemical performance of SWCNT-TBA-PV2Mo10 electrodes is studied in an acidic aqueous electrolyte (1 M H2SO4) by galvanostatic charge/discharge and cyclic voltammetry. In this SWCNT-TBA-PV2Mo10 nanohybrid material, TBA-PV2Mo10 provides redox activity while benefiting from the high electrical conductivity and high double-layer capacitance of the SWCNTs that improve both energy and power density. An assembled SWCNT-TBA-PV2Mo10 symmetric SC exhibits a 39% higher specific capacitance as compared to a symmetric SC employing only SWCNTs as electrode materials. Furthermore, the SWCNT-TBA-PV2Mo10 SC exhibits excellent cycling stability, retaining 95% of its specific capacitance after 6500 cycles.

A cascade approach to hetero-pentanuclear manganese-oxide clusters in polyoxometalates and their single-molecule magnet properties

Hetero-pentanuclear clusters {MMn₄} with large uniaxial magnetic anisotropy and SMM behavior were successfully synthesized by sequential introduction of metal cations into the trivacant lacunary polyoxometalates.

Antimony-dependent expansion for the Keggin heteropolyniobate family

Antimony-dependent syntheses leading to the expansion of the Keggin heteropolyoxoniobate family involved nine new members.

A silver-alkynyl cluster encapsulating a fluorescent polyoxometalate core: enhanced emission and fluorescence modulation

A fluorescent polyoxometalate-based silver(i)-alkynyl cluster exhibits sensitivity towards high energy UV irradiation and selectivity for detection of S²⁻.

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.

Synergistic effect of sandwich polyoxometalates and copper–imidazole complexes for enhancing the peroxidase-like activity

Two inorganic–organic hybrids based on copper(ii)–imidazole complex modified sandwich-type tungstobismuthate or tungstoantimonite, have been synthesized, demonstrating higher peroxidase-like activity using TMB as a peroxidase substrate.

Two new members of the niobium-substituted polytungstophosphate family based on hexalacunary [H2P2W12O48]12− building blocks

Herein we present two structurally novel nanoscale clusters [{Nb₆(O₂)₄P₂W₁₂O₅₇}₂]²⁰⁻ (1) and [{P₂W₁₂Nb₇O₆₃(H₂O)₂}₄{Nb₄O₄(OH)₆}]³⁰⁻ (2) formed by controlling the reaction parameters.

Two types of novel tetra-iron substituted sandwich-type arsenotungastates with supporting lanthanide pendants

Two classes of novel tetra-iron substituted sandwich-type arsenotungastates with supporting lanthanide pendants have been synthesized and characterized.

Photocatalytic water oxidation by a mixed-valent Mn(III)₃Mn(IV)O₃ manganese oxo core that mimics the natural oxygen-evolving center

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redox Mn(III)/Mn(IV) manifold that evolves through five electronic states (S(i), where i=0-4). The synthetic model system of this catalytic cycle and of its S0→S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-)(Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 state; it features a hybrid organic-inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)3](2+) and S2O8(2-) is obtained by nanosecond laser flash photolysis; its S0→S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7%.