Polyoxometalate Hybrid Research Articles

(Invited) Hybrid Porphyrin – Polyoxometalate Copolymer for Photo(electro)Chemical Applications: From Flat to Mesoporous ITO Electrodes

The development of hybrid polymers incorporating polyoxometalates (POMs) is a promising approach to elaborate new functional materials. To expand the practical applications, their association to a visible-light photosensitizer is needed. In this context, we have developed a method of electropolymerization of POMs based on nucleophilic attack onto the electrogenerated porphyrin dications. 1-2 The formation of hybrid POM-porphyrin copolymeric films (Fig. 1, film I) can be obtained by the electro-oxidation of porphyrin in the presence of the hybrid POM bearing two pyridyl groups (Py-POM-Py) 3 . This process is feasible for various type of POMs such as Dawson, Lindqvist or Keggin type POMs 4-6 . A second methodology is also proposed to form hybrid POM-porphyrin films (Fig. 1, films II or III): first the formation of cationic polyporphyrin electropolymer. Then, by metathesis reaction, the (partial) exchange onto the surface of the initial counter ions by the POMn- or by the nanoparticles stabilized with POMn- (POMn-@M, M = Ag, Au, Pt, etc.). 7 The photovoltaic performances of these hybrid materials have been investigated by photocurrent transient measurements under visible-light illumination 8-10 given by now good efficiency. In order to increase the efficiency, mesoporous ITO electrodes (nanofiber formed by electrospun or porous ITO electrode obtained using a colloidal co-assembly approach) have been used and will be discussed. 1. L. Ruhlmann, A. Schulz, A. Giraudeau, J.-H. Fuhrhop, J. Am. Chem. Soc., 1999, 121, 6664.2. Z. Huo, J.-P. Gisselbrecht, R. Farha, M. Goldmann, E. Saint-Aman, C. Bucher, L. Ruhlmann, Electrochimica Acta, 2014, 122, 108.3. D. Schaming, C. Allain, R. Farha, M. Goldmann, S. Lobstein, A. Giraudeau, B. Hasenknopf, L. Ruhlmann, Langmuir, 2010, 26, 5101.4. I. Azcarate, I. Ahmed, R. Farha, M. Goldmann, X. Wang, H. Xu, B. Hasenknopf, E. Lacôte, L. Ruhlmann, Dalton Trans. 2013, 42, 12688.5. Z. Huo, I. Azcarate, R. Farha, M.Goldmann, H. Xu, B. Hasenknopf, E. Lacôte, L. Ruhlmann, J. solid State Electrochemistry, 2015, 19, 2611.6. Z. Huo,D. Zang,S. Yang,R. Farha,M. Goldmann,B. Hasenknopf, H. Xu, L. Ruhlmann, Electrochimica Acta, 2015, 179, 326.7. Z. Huo,A. Bonnefont, C. Bucher, L Ruhlmann, Electrochimica Acta, 2018, 274, 177.8. I. Ahmed, R. Farha, M. Goldmann, L. Ruhlmann, Chem. Comm. 2013, 49, 496.9. I. Ahmed, R. Farha, Z. Huo, C. Allain, X. Wang, H. Xu, M. Goldmann, B. Hasenknopf, L. Ruhlmann, Electrochimica Acta, 2013, 110, 726.10. I. Azcarate, Z. Huo, R. Farha, M. Goldmann,H. Xu, B. Hasenknopf, E. Lacôte, L. Ruhlmann, Chem. Eur. J. 2015, 21, 8271. Figure 1

Novel morphologies including cowry-like crystal of polyoxometalates derivatives via coupled twinning between enantiomers

A single-sided, organically decorated Anderson-type polyoxometalates hybrid [AlMo6O18(OH)3{(OCH2)3CNH2}Cl]4- was successfully synthesized. In the presence of a chiral template, the polyoxometalates-organic derivative was crystallized in the chiral space group P21; this was confirmed via single-crystal X-ray diffraction analysis. Interestingly, Cowry-like crystal morphologies with chiral enantiomeric dimers can be formed by adjusting the ionic strength of the solvent, besides chiral template. Furthermore, the crystallization led to a series of crystals with different morphologies by adjusting the supersaturation, besides chiral template and ionic strength.

Ionic liquid polyoxometalate-enhanced Pd/N,P-codoped coal-based carbon fiber catalysts for formic acid electrooxidation

Direct formic acid fuel cells have great application potential in the energy field, but their commercialization is hindered by the CO poisoning and low performance of Pd-based anode catalysts. In this study, novel composite catalysts composed of Pd, organic-inorganic hybrid imidazole ionic liquid polyoxometalates, and N,P-codoped coal-based carbon fibers with good dispersion and smaller Pd nanoparticles were synthesized successfully. Physical and electrochemical characterizations revealed that the as-prepared Pd-based composite catalysts have excellent CO poisoning resistance and enhanced electrocatalytic activity in formic acid electrooxidation. Notably, the CO oxidation onset potential of a catalyst modified by imidazole ionic liquid polyoxometalate with long alkyl chain was 0.1 V lower than that of the unmodified catalyst. The mass activity of the as-prepared Pd catalyst in formic acid oxidation was 754.1 mA mg−1Pd. It is expected that organic-inorganic hybrid polyoxometalates can be further applied to fuel cell catalysts.

Covalent hybrid materials between polyoxometalates and organic molecules for enhanced electrochemical properties

The organic functionalization of polyoxometalate (POM) clusters has exhibited increasing superiority for the rational excavation and design of advanced POM-based materials. The architectures of such inorganic–organic hybrids are just like “molecular Lego” toys by virtue of the intermolecular and/or intramolecular interactions. Although numerous novel hybrids have been reported by pioneers, the relationship between organic segments and supramolecular architectures is still ambiguous. In this paper, we focus on the influence from organic bridging ligand to the structural orientation of POM hybrid materials by X-ray diffraction, molecular simulation, and energy calculation. The results indicate that rigid organic bridging ligand possesses a much stronger tendency to form intermolecular interactions than flexible ligand and can improve the electrochemical performance for hydrogen evolution reaction.

A hybrid silicotungstate based on tri-coordination copper complex and Keggin type cluster with reactive oxygen species catalytic ability

A hybrid polyoxometalate based on binuclear copper complex and α-Keggin-type silicotungstate [Cu2(4,4′-bpy) (Phen)2][SiW12O40] (abbreviated as CSW) (4,4′-bpy = 4,4′-bipyridine; Phen = 1,10-phenanthroline) has been synthesized and structurally characterized by elemental analyses, X-ray powder diffraction (XRPD), IR spectrum, X-ray photoelectron spectrum (XPS) and single-crystal X-ray diffraction. Structural analysis shows that CSW is an ionic compound, in which the cation is a binuclear copper complex [Cu2(4,4′-bpy) (Phen)2]4+ and the anion is a α-Keggin-type silicotungstate [SiW12O40]4– cluster. Notably, the Cu ions in the [Cu2(4,4′-bpy) (Phen)2] exist in the form of tri-coordination, which the phenomenon is very rare in polyoxometalates. Moreover, CSW can catalyze the production of reactive oxygen species efficiently.

Rational regulation of transition metals in polyoxometalate hybrids without noble metal assistance for efficient light-driven H2 production.

Four new rationally designed polyoxometalate (POM)-based hybrids are reported with a maximum photocatalytic hydrogen evolution rate of 12245.59 μmol g-1 h-1, which outperform all known POM-based metal-organic photocatalysts.

Self-assembled tetramethyl cucurbit[6]uril–polyoxometalate nanocubes as efficient and recyclable catalysts for the preparation of propyl gallate

A novel class of cucurbit[n]urils–polyoxometalate (Q[n]–POM) hybrids and tetramethyl cucurbit[6]uril–phosphomolybdic acid (TMeQ[6]–PMA) nanocubes (NCs), are synthesizedviaa one-step self-assembly method.

Transition metal decorated soft nanomaterials through modular self-assembly of an asymmetric hybrid polyoxometalate.

An asymmetrically functionalised Wells-Dawson organic-inorganic hybrid polyoxometalate has been post-functionalised by Pt2+ coordination, and demonstrates self-assembly into surface-decorated micellar nanostructures. This multifunctional hybrid material is found to be a redox-active soft nanomaterial and demonstrates a new molecular design strategy with potential for applications in photo- or electro-catalysis.

Post-functionalization through covalent modification of organic counter ions: a stepwise and controlled approach for novel hybrid polyoxometalate materials.

Post-functionalization of Class I type polyoxometalate-organic hybrids through covalent modification of the organic counter ions in a step-wise and controlled manner is reported for the first time. The properties of the post-functionalized hybrids have been studied and compared with those of their parent hybrids revealing marked differences in properties.

The corrosion inhibition of stainless steel by ferrocene–polyoxometalate hybrid molecular materials – experimental and first principles studies

The hybrid molecular materials [Fe(C5H5)2]3[PW12O40]·(H2O)4 and [Fe(C5H5)2]3[PMo12O40]·(H2O)6, denoted respectively as FcPW and FcPMo, are synthesized via a co-precipitation method under acidic conditions (pH: ∼2-3). The molecular properties of these compounds are investigated via various analytical methods. UV-vis diffuse reflectance and photoluminescence studies confirm that these compounds are charge-transfer salts. Stainless steel plates (SS, 316 grade) coated with these hybrid compounds and dipped in 0.5 M H2SO4 and Ringer's solutions show significantly inhibited corrosion. The corrosion inhibition properties of these materials are studied via weight loss, electrochemical impedance spectroscopy, and potentiodynamic polarization studies. The reproducibility of the corrosion inhibition results is ensured via repeat measurements, and error values are reported. The surfaces of the corroded steel plates are examined via scanning electron and atomic force microscopy techniques. It is found that the corrosion is of uniform type. The Tafel extrapolation method shows that the inhibition efficiency is 74% for SS@FcPMo and 65% for SS@FcPW. The corrosion inhibition mechanism is explained based on the redox nature of the hybrid materials. The experimental results are corroborated through computational studies carried out using a first principles approach with the optimized geometries and frontier molecular orbitals of the hybrid molecular materials.

Exploring the self-assembly of dumbbell-shaped polyoxometalate hybrids, from molecular building units to nanostructured soft materials.

The formation of hierarchical nanostructures using preformed dumbbell-like species made of covalent organic–inorganic polyoxometalate (POM)-based hybrids is herein described. In this system, the presence of charged subunits (POM, metal linkers, and counter ions) in the complex molecular architecture can drive their aggregation, which results from a competition between the solvation energy of the discrete species and intermolecular electrostatic interactions. We show that the nature of the POM and the charge of the metal linker are key parameters for the hierarchical nanoorganization. The experimental findings were corroborated with a computational investigation combining DFT and molecular dynamics simulation methods, which outlines the importance of solvation of the counter ion and POM/counter ion association in the aggregation process. The dumbbell-like species can also form gels, in the presence of a poorer solvent, displaying similar nanoorganization of the aggregates. We show that starting from the designed molecular building units whose internal charges can be controlled by redox trigger we can achieve their implementation into soft nanostructured materials through the control of their supramolecular organization.

The fabrication of trifunctional polyoxometalate hybrids for the cascade conversion of glycerol to lactic acid

Lactic acid (LA) has been produced with cascade reactions under non-noble metal and base-free conditions.

Dye-Sensitized Photocathodes: Boosting Photoelectrochemical Performances with Polyoxometalate Electron Transfer Mediators

The amplification effect of polyoxometalates (POMs) on the efficiency of dye-sensitized nano-ITO cathodes is disclosed. The use of hybrid polyoxometalates of the type [PW11O39{SnC6H4≡C6H4F}]4–, F s...

Photoactive Polyoxometalate/DASA Covalent Hybrids for Photopolymerization in the Visible Range.

The synthesis of TBA-DASA-POM-DASA, the first photoactive covalent hybrid polyoxometalate (POM) incorporating a donor-acceptor Stenhouse adduct (DASA) reverse photochrome, is presented. It has been evidenced that in solution the equilibrium between the colorless cyclopentenone and the highly colored triene conformers is strongly dependent not only on the nature of the solvent but also the countercations, allowing to tune its optical properties. This complex has been further associated to photochromic spironaphtoxazine cations, resulting in a material which can be activated by two distinct optical stimuli. Moreover, when combined with N-methyldiethanolamine, TBA-DASA-POM-DASA constitutes a performing photoinitiating system for polyethylene glycol diacrylate polymerization and under visible light irradiation, a promising result in a domain scarcely developed in POM chemistry.

Self-Assembled Supramolecular Polyoxometalate Hybrid Architecture as a Multifunctional Oxidation Catalyst.

Polyoxometalates (POMs) are widely applied as tuneable and versatile catalysts for a variety of oxidation reactions in an aqueous/organic two-phase system. However, the practical applications of POMs-based biphasic catalysis are hampered by low space-time yields and mass-transport limitation between two layers due to extremely low solubility of the organic reactants in the aqueous phase. Here, we first introduced β-cyclodextrin (β-CD) as an inverse phase transfer agent and a supramolecular nanoreactor to construct a supramolecular POM inorganic-organic hybrid framework (KCl4)Na7[(β-CD)3(SiW12O40)]·9H2O {3CD@SiW12} for various oxidation catalyses. In contrast to free CD, Keggin [SiW12O40]4- catalysts, and their mixture, the {3CD@SiW12} catalyst, efficiently catalyze oxidation reactions of alcohol, alkene, and thiophene. A comprehensive strategy of experimental, crystallographic, and density functional theory (DFT) calculations elucidates that the catalytic pathway involved three combined aspects of supramolecular recognition, phase transfer property, and POM catalysis. The strategic combination of supramolecular characteristic and POM-based catalysts to fabricate supramolecular POM hybrid materials opens up new economic and green tuning options, thus paving the way to informed catalyst design.

Dyes Adsorption Behavior of Fe3O4 Nanoparticles Functionalized Polyoxometalate Hybrid.

The magnetic adsorbent, Fe3O4@[Ni(HL)2]2H2[P2Mo5O23]·2H2O (Fe3O4@1), is synthesized by employing the nanoparticles Fe3O4 and polyoxometalate hybrid 1. Zero-field-cooled (ZFC) and field-cooled (FC) curves show that the blocking temperature of Fe3O4@1 was at 120 K. Studies of Fe3O4@1 removing cationic and anionic dyes from water have been explored. The characterization of Fe3O4@1, effects of critical factors such as dosage, the concentration of methylene blue (MB), pH, adsorption kinetics, isotherm, the removal selectivity of substrate and the reusability of Fe3O4@1 were assessed. The magnetic adsorbent displayed an outstanding removal activity for the cationic dye at a broad range of pH. The adsorption kinetics and isotherm models revealed that the adsorption process of Fe3O4@1 was mainly governed via chemisorption. The maximum capacity of Fe3O4@1 adsorbing substance was 41.91 mg g−1. Furthermore, Fe3O4@1 showed its high stability by remaining for seven runs of the adsorption-desorption process with an effective MB removal rate, and could also be developed as a valuable adsorbent for dyes elimination from aqueous system.

2‐Methyl‐1,3‐disulfoimidazolium polyoxometalate hybrid catalytic systems as equivalent safer alternatives to concentrated sulfuric acid in nitration of aromatic compounds

Ionic liquid‐derived polyoxometalate salts [mdsim]3[PM12O40] (where M = W and Mo) of two heteropolyacids H3PW12O40.nH2O and H3PMo12O40.nH2O were synthesized using 2‐methyl‐1,3‐disulfoimidazolium chloride ([mdsim][Cl]) ionic liquid and the corresponding heteropolyacids. Three equivalents of [mdsim][Cl] were treated with the respective Keggin‐structured heteropolyacids (one equivalent) in aqueous medium at room temperature to afford the water‐stable ionic polyoxometalates as acidic solids. They were completely characterized using spectroscopic and other analytical techniques including thermal analysis and Hammett acidity studies. The inherent Brønsted acidic properties of ─SO3H group of these polyoxometalate salts were studied for the nitration of aromatic compounds with 69% HNO3 at normal temperature and 80°C without use of any external concentrated sulfuric acid. These strongly acidic polyoxometalates display good recyclability and efficient reusability.

Cellulose nanocrystal shelled with poly(ionic liquid)/polyoxometalate hybrid as efficient catalyst for aerobic oxidative desulfurization

Immobilization of polyoxometalates (POMs) plays significant roles in their industrial applications. In this work, POM-supported catalyst has been developed with cellulose nanocrystal (CNC) as a sustainable carrier and employed in the aerobic oxidative desulfurization (AODS) of fuels. The catalyst was constructed by grafting a poly(ionic liquid) (PIL) onto CNCs and subsequently exchanging the counterions of the PIL into [Co(OH)6Mo6O18]3−, a kind of POM anionic clusters. The resulting CNC@PIL@POM was systematically characterized and utilized as a catalyst in the AODS of thiophenic compounds including benzothiophene (BT), dibenzothiophene (DBT), and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT), with oxygen contained in air as oxidizing agent. The catalyst could achieve complete conversion of DBT in model diesel under mild condition and be reused five times without apparent losing of activity. As for the desulfurization of a real diesel, it was demonstrated that almost all of the original sulfides were completely converted. Based on these results, it is expected that the core-shell POM-supported catalyst may serve as an efficient, separable and sustainable catalyst for AODS of fuels.

A PHBA-functionalized organic-inorganic hybrid polyoxometalate as a luminescent probe for selectively sensing chromium and calcium in aqueous solution

Detection of trace metal ions in drinking as well as irrigation water is crucial in understanding their roles among living humans and animals. Herein, an organic PHBA-functionalized polyoxometalate (POM) derivative [N(CH3)4]3K2Eu(PHBA)(H2O)2(α-PW11O39)]·7H2O (1) (PHBA = p-hydroxybenzoic acid) was successfully synthesized which acts as a luminescent probe for selective sensing of chromium and calcium in aqueous solution. The aqueous solution photoluminescence properties of 1 were carefully investigated, indicating PHBA ligand can sensitize the luminescence of Eu3+ ion in 1. The time-resolved emission spectroscopy was also performed to authenticate the energy transfer from π-π* transition of PHBA group to Eu3+ ion. The sensing properties of 1 were performed on Cd2+, Cr3+, Ba2+, K+, Li+, Pb2+, Zn2+, Al3+, Ag+, Hg2+, Co2+, Sr2+, Ca2+, Na+, Mg2+ ions in aqueous solution, and the photoluminescence intensity for Cr3+ ion display a quenching phenomenon, whereas, Ca2+ ion displays an enhancement signal in their photophysical behavior. The detection limits for detecting Cr3+ and Ca2+ ion were 1.423 μM and 0.676 mM, respectively. In the detection process of Cr3+ ion, the values of quenching rate constant (Kq) were varied from 4.34 × 106 L (mol s)−1 to 4.29 × 109 L (mol s)−1, which were lower than the value of the maximum scatter collision quenching constant (2.0 × 1010 L (mol s)−1). The results reveal that the quenching mechanism between 1 and Cr3+ ion can be attributed to dynamic collision quenching mechanism. Furthermore, the active sites for sensing Cr3+ and Ca2+ ions also have been investigated, determining the different interaction mechanisms exist in detection of Cr3+ and Ca2+ ions in solution.

Toward Advanced Functional Systems: Honeycomb-Like Polymeric Surfaces Incorporating Polyoxovanadates with Surface-Appended Copper-Cyclam Complexes.

In this work the immobilization of hybrid polyoxometalates (POMs) onto functional polymeric surfaces is exposed and discussed. Thus, various hybrid polymer‒inorganic films were prepared by anchoring selected hybrid POMs onto tailored polymeric surfaces that consisted of breath figures (BFs) made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends. Functionalization of the BF films was performed by selective arrangement of acrylic acid groups of the amphiphilic block copolymer on the surface pores because of their affinition for the water condensed during breath figure formation. These carboxylic acid functional groups contained within the PAA blocks were then employed to anchor [Cu(cyclam)][{Cu(cyclam)}2(V10O28)]·10H2O (1-CuV10) and [{Cu(cyclam)}(VO3)2]·5H2O (1-CuV1), hybrid POMs by immersing the films into aqueous solutions of the in situ formed hybrid clusters, resulting in the hybrid films BF1 and BF2, respectively. Superficial analysis of these hybrid polymeric films was carried out by the sophisticated ion beam-based technique time-of-flight secondary ion mass spectrometry (ToF-SIMS) that was revealed to be an excellent method for the superficial compositional mapping of patterned surfaces.