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3. Environmental challenges

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Our efforts in the preparation of functionnalized POMs aim to manage the peculiar features of the polyoxometallic frameworks in order to address contemporary issues of sustainable chemistry. Major applications of POMs in catalysis rely on their acidic and redox properties as well as on their thermal and oxidative robustness that make them efficient and recyclable catalysts for atom or group transfer reactions with low environmental impact (oxygenation, cyclopropanation ; G. Guillemot, A. Proust).
Owing to the unique ligand properties of lacunary POMs we are developing transition metal substituted POMs (TMSP) for the electro-assisted activation and reduction of CO2, and its conversion into usable fuels. TMSPs combine the coordinative properties of first row transition metals with the redox properties of the POM frameworks acting as multielectron and protons reservoirs (S. Blanchard, A. Proust).
Organic functionalization with long alkyl chains generates covalent amphiphilic POMs that behave as surfactant molecules. The impressive self-assembly properties of these POM-SURFs were unveiled by the formation of micelles in water and the stabilization of Winsor type microemulsions (oil/water) system that are promising microdispersed reaction media for oxidation catalysis using eco-friendly hydrogen peroxide (G. Guillemot, A. Proust).
Polyfunctional POMs are also devised for their covalent grafting onto mesoporous materials thus affording anchored homogeneous POM based catalysts aiming at fulfilling the criteria of efficiency and recyclability (R. Villanneau, A. Proust).

Keywords : POM-based catalysts for a sustainable chemistry, POMs as a non-innocent support, amphiphilic POMs, anchored homogeneous catalysis, electro-assisted activation reduction of CO2

- Oxidovanadium(V) Anchored to Silanol-​Functionalized Polyoxotungstates : Molecular Models for Single-​Site Silica-​Supported Vanadium Catalysts
G. Guillemot, E. Matricardi, L.-M. Chamoreau, R. Thouvenot, A. Proust, ACS Catalysis, 2015, 5(12), 7415-7423.

The metalation of two different types of silanol-​decorated polyoxotungstates, [XW9O34-​x(tBuSiOH)​3]​3-​ (X = P, x = 0 ; X = Sb, x = 1) and [PW10O36(tBuSiOH)​2]​3-​, by Cl3VO or (iPrO)​3VO was achieved. The characterization of the resulting oxidovanadium(V) complexes [XW9O34-​x(tBuSiO)​3VO]​3-​ (X = P, 3 ; X = Sb, 3’) and [PW10O36(tBuSiO)​2VO(iPrO)​]​3-​ (4) was fully detailed, including x-​ray anal. These compds. are present in monomeric forms and therefore represent original mol. models for tris-​grafted and bis-​grafted isolated (V=O)​3+ species dispersed onto SiO2. Their ability as precatalysts for the epoxidn. of cyclic olefins and allylic alcs. with tert-​Bu hydroperoxide (TBHP) was studied. Probably a confined tris-​grafted species, such as 3 or 3’, does not act as an efficient catalyst whereas a more labile bis-​grafted species, such as 4, does. To gain a better understanding, the authors have assessed their suitability for ligand exchange with alcs. and TBHP and performed reaction progress kinetic anal. by monitoring the epoxidn. of 3-​methyl-​2-​buten-​1-​ol by 1H and 51V NMR.

- Efficiency of Polyoxometalate-​Based Mesoporous Hybrids as Covalently Anchored Catalysts
F. Bentaleb, O. Makrygenni, D. Brouri, C. Coelho Diogo, A. Mehdi, A. Proust, F. Launay, Richard Villanneau, Inorg. Chem., 2015, 54 (15), 7607–7616

Polyoxometalate (POM) hybrids have been covalently immobilized through the formation of amide bonds on several types of mesoporous silica. This work allows the comparison of three POM-​based mesoporous systems, obtained with three different silica supports in which either the org. functions of the support (amine vs carboxylic acid) and​/or the structure of the support itself (SBA-​15 vs mesocellular foams (MCF)​) were varied. The resulting POM-​based mesoporous systems have been studied in particular by high resoln. transmission electronic microscopy (HR-​TEM) in order to characterize the nanostructuration of the POMs inside the pores​/cells of the different materials. We thus have shown that the best distribution and loading in POMs have been reached with SBA-​15 functionalized with aminopropyl groups. In this case, the formation of amide bonds in the materials has led to the nonaggregation of the POMs inside the channels of the SBA-​15. The catalytic activity of the anchored systems has been evaluated through the epoxidn. of cyclooctene and cyclohexene with H2O2 in acetonitrile. The reactivity of the different grafted POMs hybrids has been compared to that in soln. (homogeneous conditions)​. Parallels can be drawn between the distribution of the POMs and the activity of the supported systems. Furthermore, recycling tests together with catalyst filtration expts. during the reaction allowed us to preclude the hypothesis of a significant leaching of the supported catalyst.

- Electro-​Assisted Reduction of CO2 to CO and Formaldehyde by (TOA)​6[α-​SiW11O39Co(_)​] Polyoxometalate
M. Girardi, S. Blanchard, S. Griveau, P. Simon, M. Fontecave, F. Bedioui, A. Proust, Eur. J. Inorg. Chem., 2015, 22, 3642–3648

We report here on the multiproton-multielectron electrochemical reduction of CO2 in homogeneous solution by using (TOA)​6[α-​SiW11O39Co(_)​] (TOA = tetraoctyl ammonium ; _ = vacant position in the coordination sphere of Co) as an electrocatalyst. First, the electrochemical behavior of (TOA)​6[α-​SiW11O39Co(_)​] was analyzed in detail by cyclic voltammetry in dichloromethane, studying the influence of the presence of protons and/or CO2. These preliminary results were further used to optimize the conditions of electrolysis in terms of reduction potentials. Analysis of the electrolysis products in the gas and liquid phases show the formation of CO and HCHO without formation of H2. Our results tend to show that the (TOA)​6[α-​SiW11O39Co(_)​] polyoxometalate is a catalyst for CO2 electroreduction, with unique selectivity.

- Covalent amphiphilic polyoxometalates for the design of biphasic microemulsion systems
V. Jallet, G. Guillemot, J. Lai, P. Bauduin, V. Nardello-Rataj, A. Proust, Chem. Commun., 2014, 50, 6610-6612.

Covalent amphiphilic polyoxometalates generated from alkylphosphonic acids have been synthesized, characterized and monitored by multinuclear NMR spectroscopy. Among them, K3H[γ–SiW10O36(C12H25PO)2], has been successfully used as surfactant for the stabilization of a Winsor I type microemulsion system.

- Covalent Grafting of Organic-Inorganic Polyoxometalates Hybrids onto Mesoporous SBA-15 : A Key Step for New Anchored Homogeneous Catalysts
R. Villanneau, A. Marzouk, Y. Wang, A. Ben Djamaa, G. Laugel, A . Proust, F. Launay, Inorg. Chem., 2013, 52, 2958-2965.

Covalent grafting of heteropolyanions hybrids B,α-[AsIIIW9O33{P(O)(CH2CH2CO2H)}2]5- on 3-aminopropyl functionalized SBA-15 has been achieved through the formation of peptide bonds. The covalent link has been confirmed by using IR and 13C CP MAS NMR spectroscopies. Electrostatic interactions between carboxylate and protonated amines have been discarded on the basis of the retention of POMs after repeated washings of the resulting material by ionic liquid (bmimCl). This is the first example of anchored monovacant polyoxometalates (POM) in which nucleophilic oxygen atoms are still available after incorporation into mesoporous supports. Further characterization of the textural properties of grafted materials has shown that they still retain an important mesoporosity, which is compatible with their potential use in heterogeneous catalysis. Such systems are thus interesting candidates for the preparation of anchored homogeneous catalysts in which the POMs would play the role of polydentate inorganic ligands for the active centers.

- Surface pressure induced 2D-crystallization of POM-based surfactants : preparation of nanostructured thin films
L. de Viguerie, A. Mouret, H.-P. Brau, V. Nardello-Rataj, A. Proust, P. Bauduin, Cryst. Eng. Comm., 2012, 14, 8446-8453.

A polyoxometalate based surfactant shows the coexistence between monodisperse micrometric hexagonal 2D-crystallites with a liquid state monolayer at the air–water interface.

- Bifunctional Polyoxometalates for planar gold surfaces nanostructuration.
D. Mercier, S. Boujday, S. Annabi, R. Villanneau, C.-M. Pradier, A. Proust, J. Phys. Chem. C 2012, 116,13217-13224.

Surface nanostructuration was successfully achieved by binding the polyoxometalate (POM) (NBu4)3[PW11O39(SiC6H4NH2)2O] covalently onto planar gold surfaces. To do so, POMs functionalized with two terminal amino groups were synthesized and reacted with a mercaptoundecanoic acid self-assembled monolayer adsorbed on gold. These amine-terminated POM macroanions proved to be remarkably efficient as nanostructuring agents. Using polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), conditions were optimized to elaborate a dense and well-dispersed layer of POMs, leaving a “free” amine function for the further linkage of proteins. Antirabbit immunoglobulins (anti-rIgGs) were thus grafted on the POM-structured layer, and the recognition of their specific target, rabbit immunoglobulin (rIgGs), was tested by using a quartz crystal microbalance with dissipation measurement (QCM-D). The recognition was good and highly specific, indicating that an efficient, nanostructured biosensor has been constructed. The method reported herein can be easily applied, not requiring any sophisticated experimental setup, and is promising for the patterning of any molecular probes bearing amine groups.

- Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles : A New Effective Medium for Oxidation Reactions
L. Leclercq, A. Mouret, A. Proust, V. Schmitt, P. Bauduin, J.-M. Aubry, V. Nardello-Rataj, Chem. Eur. J., 2012, 18, 14352-14358.

Mixing it up a bit : Spherical nanoparticles of decyl- and dodecyltrimethylammonium cations with [PW12O40]3− formed stable Pickering emulsions in the presence of an aromatic solvent and water. This system is an efficient medium for epoxidation.

- Addition of N-Heterocyclic-Carbenes to Ruthenium(VI)-Nitrido Polyoxometalate : a new route to Cyclic Guanidines
C. Besson, J.-H. Mirebeau, S. Renaudineau, S. Roland, S. Blanchard, H. Vezin, C. Courillon, A. Proust, Inorg. Chem., 2011, 50, 2501-2506.

The scope of N-atom transfer from the electrophilic ruthenium(VI) nitrido containing polyoxometalate [PW11O39RuVIN]4− has been extended to the N-heterocyclic carbene {CH2(Mes)N}2C and the coupling product {CH2(Mes)N}2CNH2+ characterized by 1H NMR and high-resolution mass spectrometry. Because guanidines display many fields of applications ranging from biology to supramolecular chemistry, this could afford an original route to the synthesis of cyclic guanidines. This also enlarges the potential of nitrido complexes in the synthesis of heterocycles, mainly illustrated in the literature through the formation of aziridines through N-atom transfer to alkenes. In the course of the reaction, the ruthenium(III)-containing polyoxometallic intermediate [PW11O39RuIII{NC{N(Mes)CH2}2}]5− has been thoroughly characterized by continuous-wave and pulsed electron paramagnetic resonance, which nicely confirms the presence of the organic moiety on the polyoxometallic framework, Ru K-edge X-ray absorption near-edge structure, and electrochemistry.

- Polyoxometalates : Powerful Catalysts for Atom-Efficient Cyclopropanations
I. Boldini, G. Guillemot, A. Caselli, A. Proust, E. Gallo, Adv. Synth. Catal. 2010, 352,2365-2370.

The polyoxometalate-based catalytic cyclopropanation of olefins by EDA (EDA = ethyldiazoacetate) is reported. The outstanding catalyst productivity (TON up to 100.000) and the use of equimolar EDA/olefin ratio confer to the methodology a high sustainability. Preliminary mechanistic investigations are also discussed.

- Cs9[(γ-PW10O36)2Ru4O5(OH)(H2O)4], a new all-inorganic, soluble catalyst for the efficient visible-light-driven oxidation of water
Claire Besson, Zhuangqun Huang, Yurii V. Geletii, Sheri Lense, Kenneth I. Hardcastle, Djamaladdin G. Musaev, Tianquan Lian, Anna Proust and Craig L. Hill
Chem. Commun., 2010, 46, 2784-2786

The tetraruthenium-substituted polyoxometalate Cs9[(γ-PW10O36)2Ru4O5(OH)(H2O)4] was synthesized and structurally, spectroscopically and electrochemically characterized ; it was shown to be a catalyst for visible-light-induced water oxidation.

- Structural, Physicochemical, and Reactivity Properties of an All-Inorganic, Highly Active Tetraruthenium Homogeneous Catalyst for Water Oxidation
Y. V. Geletii, C. Besson, Y. H., Q. Yin, D. G. Musaev, D. Quinonero, R. Cao, K. I. Hardcastle, A. Proust, P. Kogerler, C. L. Hill,
J. Am. Chem. Soc., 2009, 131, 17360–17370

Several key properties of the water oxidation catalyst Rb8K2[{RuIV4O4(OH)2(H2O)4}(γ-SiW10O36)2] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{RuVRuIV3O6(OH2)4}(γ-SiW10O36)2]11− has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O2) indicate these two complexes contain [RuIV4O6] and [RuVRuIV3O6] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H2O/O2 potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)3]3+ has been examined in detail. There is a hyperbolic dependence of O2 yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O2 yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H2O oxidation/O2 evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).