Transition Metal-substituted Polyoxometalates Research Articles

Filling Polyoxoanions into MIL-101(Fe) for Adsorption of Organic Pollutants with Facile and Complete Visible Light Photocatalytic Decomposition.

Transition metal-substituted polyoxometalates (POMs) were filled into a metal–organic framework (MOF) to construct a series of POM@MOF composites (PMo12O40@MIL-101, PMo11VO40@MIL-101, PMo10V2O40@MIL-101). The composite materials possess ultra-high adsorption ability, especially for PMo10V2O40@MIL-101, with an adsorption capacity of 912.5 mg·g−1 for cationic antibiotic tetracycline in wastewater, much higher than that of isolated MIL-101(Fe) and the commonly used adsorption materials, such as activated carbon and graphene oxide. In particular, they can be used as efficient photocatalysts for the photodegradation of antibiotics under visible light irradiation. The complete photodegradation of the adsorbed species can induce the facile reusability of these composites for multiple cycles. This work opens an avenue to introduce POMs into an MOF matrix for the simultaneous adsorption and photodegradation of antibiotics.

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Efficient Photogeneration of Hydrogen Boosted by Long-Lived Dye-Modified Ir(III) Photosensitizers and Polyoxometalate Catalyst

Mono-copper substituted phosphotungstate supported on to neutral alumina: Synthesis, characterization and detailed studies for oxidation of styrene

Mono-copper substituted phosphotungstate (PW11Cu) was supported on a neutral support (alumina) by wet impregnation method, which is the first instance of the use of alumina for supporting Transition metal substituted Polyoxometalates (TMSPOMs), characterized by various spectral techniques and the catalytic activity was evaluated for the oxidation of styrene using tert-butyl hydroperoxide (TBHP) as oxidant. Various reaction parameters were optimized and a detailed kinetic study was carried out to understand the role of each of the components of the reaction. Finally, the activity of the present catalyst was compared with zirconia supported PW11Cu, and the superiority was explained based on the presence of Lewis acidity of aluminum.

Origin of Selectivity in Protein Hydrolysis by Zr(IV)-Containing Metal Oxides as Artificial Proteases

The origin of selectivity in protein hydrolysis promoted by Zr(IV)-substituted polyoxometalates (POMs) has been investigated through a variety of computational techniques. Initially, we analyzed the reaction mechanism for the observed hydrolysis at the Asn44—Arg45 site in the hen egg-white lysozyme protein (HEWL) by the Zr-substituted Lindqvist anion [W5O18Zr(H2O)(OH)]3– (ZrL) using cluster models obtained from molecular dynamics (MD) simulations, quantum mechanics/molecular mechanics (QM/MM) calculations, and metadynamics simulations. The mechanism characterization shows that the overall activity is governed by the energy cost to reach the transition state for C–N bond cleavage from the reactants, resulting in a calculated overall free-energy barrier of 121 kJ mol–1 that is in excellent agreement with the values derived from the experimental rate constants (113–134 kJ mol–1). In addition, QM/MM metadynamics simulations on the early stages of the mechanism revealed the formation of an exergonic non-covalent POM···protein complex at the protein surface that was stabilized by positively charged amino acids maintained during the Zr coordination to the amide oxygen. For nonreactive related sites containing Arg (Asn113—Arg114, Arg45—Asn46, and Arg21—Gly22,) we found very similar overall barriers within the cluster model approach (124, 124, and 120 kJ mol–1, respectively); however, their nonbonding POM···protein interactions along the simulated coordination of Zr to the amide oxygen were significantly weaker than those for the reactive Asn44—Arg45 site. Thus, for the HEWL protein the selectivity is governed by an enzyme-like recognition of ZrL at the cleavage site that results in an overall acceleration of the reaction rate compared to those at other sites. Conversely for human serum albumin, (HSA) the observed selectivity was not directed by nonbonding POM···protein interactions but instead was controlled by the protein secondary structure. Calculations on several Arg—Leu sites placed in positive patches showed that peptide bonds in an α-helix structure have higher overall free-energy barriers, while for the active Arg114—Leu115 site in a random coil region the C–N cleavage is facilitated by the extended conformation of the protein chain. All in all, this study has identified and evaluated two complementary factors controlling the selectivity in peptide hydrolysis promoted by transition metal-substituted POMs; hydrolysis is disfavored at α-helical regions of the protein, and then specific positively charged patches can trap the POM via electrostatic-type POM···protein interactions and accelerate the reaction.

Ring-Shaped Polyoxometalate Built by {Mn 4 PW 9 } and PO 4 Units for Efficient Visible-Light-Driven Hydrogen Evolution

A novel mixed-valence, ring-shaped multinuclear Mn-containing polyoxometalate, [H2N(CH3)2]15NaH8-[MnIII3MnIV(μ3-O)3(OAc)PO4(B-α-PW9O34)]4·36H2O (1) was made and systematically characterized using v...

Facile preparation of Co6W6C/W@NC electrocatalyst for hydrogen evolution reaction using transition metal substituted polyoxometalates as precursor

A unique strategy to synthesize bimetallic carbides-containing composites (BCCs) under mild condition is established by utilizing transition metal substituted polyoxometalates (TMSPs) as precursor. Different species of TM ions well mixed in atomic level in TMSPs render facile preparation of BCCs with multi-interfacial structure, benefiting their remarkable electrocatalytic performance towards hydrogen evolution reaction in KOH.

How do transition-metal-substituted POMs modify the photoanode of dye-sensitized solar cells? A DFT study

Herein, the electronic structures and absorption spectra of a series of transition-metal-substituted polyoxometalates (TMSPs) were systematically investigated to screen promising candidates for the POM/TiO2 nanocomposite film used in DSSCs using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods.

Visual detection of H2O2 and melamine based on PW11MO39n− (M = Cu2+, Co2+, Mn2+, Fe3+) and PW9M3O34n− (M = Cu2+, Co2+, Mn2+, Fe3+)

The peroxidase-like activity of transition metal-substituted polyoxometalates (PW11MO39n−, PW9M3O34n−, abbreviated as PW11M and PW9M3, where M = Cu2+, Co2+, Mn2+, Fe3+) in the oxidation of 3,3′,5,5′-tetramethylbenzidine by H2O2 was valuated.

Assembly Mechanism of Zr-Containing and Other TM-Containing Polyoxometalates.

The mechanism by which Zr-substituted and other transition metal-substituted polyoxometalates (POMs) form covalently linked dimers has been analyzed by means of static density functional theory (DFT) calculations with a continuous solvent model as well as Car-Parrinello molecular dynamics (CPMD) simulations with explicit solvent molecules. The study includes different stages of the process: the formation of the active species by alkalination of the solution and formation of intercluster linkages. CPMD simulations show that the Zr-triaqua precursor, [W5O18Zr(H2O)3]2-, under basic conditions, reacts with hydroxide anions to form Zr-aqua-hydroxo active species, [W5O18Zr(OH)(H2O)]3-. We computed the DFT potential energy profile for dimerization of [W5O18TM(OH)]n- [TM = ZrIV(H2O), ZrIV, TiIV, and WVI] anions. The resulting overall energy barrier is low for ZrIV, moderate for TiIV, and high for WVI. The computed thermodynamic balance favors the dibridged (μOH)2 linkages for ZrIV, the monobridged (μOH) linkage for TiIV, and the monomeric forms for WVI, in agreement with experimentally observed trends. The lowest energy barrier and largest coordination number of Zr-substituted POMs are both a consequence of the flexible coordination environment and larger radius of Zr.

Keggin-type lacunary and transition metal substituted polyoxometalates as heterogeneous catalysts: A recent progress

ABSTRACTIn this article we discussed synthesis and catalytic applications of heterogeneous catalysts based on lacunary polyoxometalates (LPOMs) and transition metal substituted polyoxometalates (TMSPOMs), however exclusively the focus has been given to the mono LPOMs and mono-TMSPOMs. As the field of supported LPOMs/TMSPOMs is upcoming important field, some aspects about choice of support as well as methods of supporting have also been described. We also include a few of our new research results in order to understand the effect of support and active species as well as choice of organic transformation.

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer.

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

Three Keggin-Type Transition Metal-Substituted Polyoxometalates as Pure Inorganic Photosensitizers for p-Type Dye-Sensitized Solar Cells.

In light of the serious challenge of severe global energy shortages, p-type dye-sensitized solar cells (p-DSSCs) have attracted increasing levels of interest. The potential of three Keggin-type transition metal-substituted polyoxometalates, TBA8 Na2 [SiW9 O37 {Co(H2 O)3 }]⋅ 11 H2 O (SiW9 Co3 ), TBA4 [(SiO4 )W10 MnIII2 O36 H6 ]⋅1.5 CH3 CN⋅ 2 H2 O (SiW10 MnIII2 ), and TBA3.5 H5.5 [(SiO4 )W10 MnIII/IV2 O36 ]⋅ 10 H2 O⋅0.5 CH3 CN (SiW10 MnIII/IV2 ) has been explored as pure inorganic dye photosensitizers for p-DSSCs (TBA=(n-C4 H9 )4 N+ ). The three dyes show overall conversion efficiencies of 0.038, 0.029, and 0.027 %, respectively, all of which are higher than that of coumarin 343 (0.017 %). These polyoxometalates are the first three pure inorganic dyes reported for use with p-DSSCs and therefore demonstrate a new strategy for designing efficient dyes, especially pure inorganic dyes. Moreover, they broaden the range of applications for polyoxometalates.

The Fascination of Polyoxometalate Chemistry

We are delighted to introduce this special issue of Inorganics. [...]

Green and efficient oxidation of benzylic alcohols with hydrogen peroxide catalyzed by an inorganic–organic hybrid catalyst

Inorganic–organic hybrid catalysts 1-POM(M) were prepared by electrostatic interaction between transition metal-substituted polyoxometalates, {[PW11MO39]4− [M = Cr(III), Fe(III)], [PW11MO39]5−, [M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II)], [PW11VO40]4−}, and branched organic polyammonium, (tris[2-(dimethylammonium)ethyl]-1,3,5-benzenetricarboxylate), and characterized by elemental analyses, UV–vis and FT IR spectroscopic techniques, XRD, SEM, and Thermogravimetric-Differential thermogravimetric analyses. The hybrid material 1-POM(Zn) was an efficient and selective heterogeneous catalyst in the oxidation of benzylic alcohols to their corresponding carbonyl compounds with hydrogen peroxide. The catalyst was reused several times without significant loss of catalytic activity.

Hexa-substituted polyoxometalates made of trivacant Dawson {P2W15} fragments and {Ni6} clusters under hydrothermal conditions

Three novel polyoxometalates (POMs) containing [Ni6(μ3-OH)3](9-) and [P2W15O56](12-) units were first made, showing the first hexa-nuclearity transition-metal substituted POMs (TMSPs) based on monomeric lacunary Dawson fragments, which further indicates that the hydrothermal technique can offer an effective way for making new TMSPs through lacunary POM fragments incorporated with high-nuclear TM clusters.

Transition Metal-Substituted Polyoxometalates Supported on MCM-41 as Catalysts in the Oxidation of Cyclohexane and Cyclooctane with H2O2

The Keggin type polyoxometalates (Bu4N)4HPW11CoO39, (Bu4N)4PW11FeO39, (Bu4N)4HPW11CuO39 and (Bu4N)4PW11VO40 have been synthesized and supported on MCM-41 to be used as catalysts for the solvent-free oxidation of cyclohexane and cyclooctane with H2O2.

Synthesis and characterisation of novel ruthenium multi-substituted polyoxometalates: α,β-[SiW 9O 37Ru 4(H 2O) 3Cl 3] 7−

Novel ruthenium multi-substituted polyoxometalate compounds of general formulae K 6Na[SiW 9O 37Ru III 4(H 2O) 3Cl 3]· nH 2O (α- and β-SiW 9O 34 isomers) were synthesised via the reactions of the trilacunary Keggin α- and β-[SiW 9O 34] 10 − anions with RuCl 3 in aqueous solution. Characterisation was performed by analytical, spectroscopic, magnetic, and electrochemical techniques. The results are in accordance with the new polyoxometalates having the trivacant Keggin anion (α or β isomer) linked to a cubane-type Ru 4O 4 cluster. The co-ordination sphere of ruthenium is completed with water molecules and chloride anions. Magnetic measurements showed that the majority of the molecules have the Ru ions cluster in a compensated configuration constrained to an antiferromagnetic alignment at very low temperature and following a Curie-Weiss like behaviour. For both isomers, cyclic voltammetry revealed quasi-reversible redox processes at the ruthenium centres, namely Ru IV/III and Ru III/II, which were found to be pH dependent. The novel polyoxotungstates incorporate the highest content of ruthenium (W:Ru = 2.25:1) ever reported for Keggin-, Dawson or sandwich-type transition metal-substituted polyoxometalates and may find promising applications as oxidative catalysts.

Combination Chemistry of Hexa-Copper-Substituted Polyoxometalates Driven by the CuII−Polyhedra Distortion: From Tetramer, 1D Chain to 3D Framework

Reaction of CuCl(2) x 2 H(2)O and trivacant Keggin polyoxoanion Na(9)[A-alpha-PW(9)O(34)] x 7 H(2)O in the presence of ethylene-diamine/1,2-diaminopropane (en/enMe) under hydrothermal conditions led to five novel inorganic-organic hybrid polyoxotungstates (POTs):[Cu(en)(2)](2)[Cu(en)(2)(H(2)O)](2)[Cu(H(2)O)(2)][Cu(4)(H(2)O)(2)(B-alpha-PW(9)O(34))(2)] x 2 H(2)O(1), [Cu(6)(mu(3)-OH)(3)(en)(3)(H(2)O)(5)(B-alpha-PW(9)O(34))] x 6 H(2)O (2), Na(6){[Cu(6)(mu(3)-OH)(3)(en)(3)(H(2)O)(2)(B-alpha-PW(9)O(34))](2)[Cu(4)(H(2)O)(2)(B-alpha-PW(9)O(34))(2)]} x 2 H(2)en x 8 H(2)O (3), [Cu(6)(mu(3)-OH)(3)(en)(3)(H(2)O)(3)(B-alpha-PW(9)O(34))] x 4 H(2)O (4), [Cu(II)(6)Cu(I)(4)(mu(6)-O)(OH)(18)][Cu(6)(mu(3)-OH)(3)(enMe)(3)(H(2)O)(3)(B-alpha-PW(9)O(34))](4) x 4 H(3)O x 2 H(2)O (5). 1 is a new 1D chain constructed from alternate tetra-Cu(II) sandwiched units [Cu(II)(4)(H(2)O)(2)(B-alpha-PW(9)O(34))(2)](10-) and dinuclear [Cu(II)(2)(H(2)O)(6)](4+) units. 2-5 are a series of novel transition-metal substituted polyoxometalates (TMSPs) comprising hexa-Cu(II) substituted {Cu(6)PW(9)} units (Cu(6) = Cu(6)(mu(3)-OH)(3), PW(9) = B-alpha-PW(9)O(34), Scheme 1). 2 is an unusual monomer. 3 is an unprecedented tetramer built by two types of TMSP units, [Cu(6)PW(9)(en)(3)(H(2)O)(2)] and [Cu(II)(4)(H(2)O)(2)(PW(9))(2)](10-). 4 is a novel chain assembled from [Cu(6)PW(9)(en)(3)(H(2)O)(3)] units by double-bridging mode. Most interestingly, 5 is a open framework made by [Cu(6)PW(9)(enMe)(3)(H(2)O)(3)] units, in which the novel decacopper oxygen clusters, [Cu(I)(4)Cu(II)(6)(mu(6)-O)(OH)(18)](4-) formed in situ, are encapsulated into the channels, and 5 represents the first decametallic-cluster-containing open framework built by TMSP units. 2-5 all contain the hexa-Cu(II) substituted POT unit {Cu(6)PW(9)L(3)(H(2)O)(n)} (L = en/enMe, n < 6) made by {Cu(6)L(3)(H(2)O)(n)} core and trivacant Keggin PW(9) fragments. Moreover, 3-5 can be viewed as combining derivatives of 2 through substituting water ligands on hexa-Cu(II) {Cu(6)L(3)(H(2)O)(n)} units by terminal oxygen atoms of PW(9) units. Importantly, the Cu(II)-polyhedra Jahn-Tell/pseudo-Jahn-Teller distortion plays a crucial role in the formation of 1-5. The magnetic behaviors of 2-5 show antiferromagnetic interactions between the Cu(II) ions in the hexa-Cu(II) cluster.

Single-step catalytic synthesis of diphenyl carbonate over transition-metal-substituted Keggin-type tungstophosphoric acid

Abstract Single-step catalytic synthesis of diphenyl carbonate (DPC) through the transesterification of dimethyl carbonate (DMC) and phenol could be firstly arrived by using the transition-metal-substituted polyoxometalates (TMS-POMs) K 5 [PW 11 O 39 M(H 2 O)]/TiO 2 system (M = Mn II , Co II , Ni II , Cu II , Zn II ). The catalytic activity of this transesterification was strongly influenced by the type of transition metal in the TMS-POMs. Among them, the K 5 [PW 11 O 39 Zn(H 2 O)]/TiO 2 catalytic system showed the highest activity and selectivity for DPC. The conversion of phenol reached 12.8%, and the selectivity to DPC was 83.6%. The turnover number (TON) reached 6.44 × 10 4 mol/mol [PW 11 O 39 Zn(H 2 O)] 5− /TiO 2 . A possible mechanism that the transition metal in the polyoxometalates anchored DMC to form intermediate compound in the transesterification was proposed.

Heterogeneous selective oxidation catalysts based on coordination polymer MIL-101 and transition metal-substituted polyoxometalates

Titanium- and cobalt-monosubstituted Keggin heteropolyanions, [PW 11CoO 39] 5− and [PW 11TiO 40] 5−, were electrostatically bound to the chromium terephthalate polymer matrix MIL-101. The MIL-supported polyoxometalate (POM) catalysts were characterized by elemental analysis, XRD, N 2 adsorption, and FT-IR-spectroscopy. The catalytic performance of both MIL-101 and the novel composite materials M-POM/MIL-101 was assessed in the oxidation of three representative alkenes— α-pinene, caryophyllene, and cyclohexene—using molecular oxygen and aqueous hydrogen peroxide as oxidants. Ti-POM/MIL-101 demonstrated fairly good catalytic activity and selectivity in α-pinene allylic oxidation and caryophyllene epoxidation with hydrogen peroxide, whereas Co-POM/MIL-101 catalyzed α-pinene allylic oxidation by molecular oxygen. Both composite materials are stable to POM leaching, behave as true heterogeneous catalysts, and can be used repeatedly without sustaining a loss of activity and selectivity in oxidations with O 2 and H 2O 2, provided that rather mild reaction conditions ( T < 50 °C, [H 2O 2] < 0.2 M) are used with the latter oxidant.