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Inorganics, Volume 3, Issue 3 (September 2015) – 6 articles , Pages 309-387

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5112 KiB

Open AccessArticle

Water Oxidation by Ru-Polyoxometalate Catalysts: Overpotential Dependency on the Number and Charge of the Metal Centers

by Simone Piccinin and Stefano Fabris

Inorganics 2015, 3(3), 374-387; https://doi.org/10.3390/inorganics3030374 - 2 Sep 2015

Cited by 9 | Viewed by 6999

Abstract

Water oxidation is efficiently catalyzed by several Ru-based polyoxometalate (POM) molecular catalysts differing in the number, local atomistic environment and oxidation state of the Ru sites. We employ density functional theory calculations to rationalize the dependency of the reaction overpotential on the main structural and electronic molecular properties. In particular, we compare the thermodynamics of the water oxidation cycle for single-site Ru-POM and multiple-site Ru4-POM complexes. For the Ru-POM case, we also investigate the reaction free energy as a function of the Ru oxidation state. We find that the overpotential of these molecular catalysts is primarily determined by the oxidation state of the metal center and is minimum for Ru(IV). In solution, the number of active sites is shown to play a minor role on the reaction energetics. The results are rationalized and discussed in terms of the local structure around the active sites and of the electrostatic screening due to the molecular structure or the solvent. Full article

(This article belongs to the Special Issue Polyoxometalates)

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631 KiB

Open AccessEditorial

Frontiers in Gold Chemistry

by Ahmed A. Mohamed

Inorganics 2015, 3(3), 370-373; https://doi.org/10.3390/inorganics3030370 - 24 Aug 2015

Cited by 1 | Viewed by 3670

Abstract

Basic chemistry of gold tells us that it can bond to sulfur, phosphorous, nitrogen, and oxygen donor ligands. The Frontiers in Gold Chemistry Special Issue covers gold complexes bonded to the different donors and their fascinating applications. This issue covers both basic chemistry studies of gold complexes and their contemporary applications in medicine, materials chemistry, and optical sensors. There is a strong belief that aurophilicity plays a major role in the unending applications of gold. Full article

(This article belongs to the Special Issue Frontiers in Gold Chemistry)

729 KiB

Open AccessArticle

Vanadium(V)-Substitution Reactions of Wells–Dawson-Type Polyoxometalates: From [X₂M₁₈O₆₂]⁶⁻ (X = P, As; M = Mo, W) to [X₂VM₁₇O₆₂]⁷⁻

by Tadaharu Ueda, Yuriko Nishimoto, Rie Saito, Miho Ohnishi and Jun-ichi Nambu

Inorganics 2015, 3(3), 355-369; https://doi.org/10.3390/inorganics3030355 - 14 Jul 2015

Cited by 16 | Viewed by 7291

Abstract

The formation processes of V(V)-substituted polyoxometalates with the Wells–Dawson-type structure were studied by cyclic voltammetry and by ³¹P NMR and Raman spectroscopy. Generally, the vanadium-substituted heteropolytungstates, [P₂VW₁₇O₆₂]⁷⁻ and [As₂VW₁₇O₆₂]⁷⁻, were prepared by mixing equimolar amounts of the corresponding lacunary species—[P₂W₁₇O₆₁]¹⁰⁻ and [As₂W₁₇O₆₁]¹⁰⁻—and vanadate. According to the results of various measurements in the present study, the tungsten site in the framework of [P₂W₁₈O₆₂]⁶⁻ and [As₂W₁₈O₆₂]⁶⁻ without defect sites could be substituted with V(V) to form the [P₂VW₁₇O₆₂]⁷⁻ and [As₂VW₁₇O₆₂]⁷⁻, respectively. The order in which the reagents were mixed was observed to be the key factor for the formation of Dawson-type V(V)-substituted polyoxometalates. Even when the concentration of each reagent was identical, the final products differed depending on the order of their addition to the reaction mixture. Unlike Wells–Dawson-type heteropolytungstates, the molybdenum sites in the framework of [P₂Mo₁₈O₆₂]⁶⁻ and [As₂Mo₁₈O₆₂]⁶⁻ were substituted with V(V), but formed Keggin-type [PVMo₁₁O₄₀]⁴⁻ and [AsVMo₁₁O₄₀]⁴⁻ instead of [P₂VMo₁₇O₆₂]⁷⁻ and [As₂VMo₁₇O₆₂]⁷⁻, respectively, even though a variety of reaction conditions were used. The formation constant of the [PVMo₁₁O₄₀]⁴⁻ and [AsVMo₁₁O₄₀]⁴⁻ was hypothesized to be substantially greater than that of the [P₂VMo₁₇O₆₂]⁷⁻ and [As₂VMo₁₇O₆₂]⁷⁻. Full article

(This article belongs to the Special Issue Polyoxometalates)

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2716 KiB

Open AccessArticle

Synthesis and Characterisation of the Europium (III) Dimolybdo-Enneatungsto-Silicate Dimer, [Eu(α-SiW₉Mo₂O₃₉)₂]¹³⁻

by Loïc Parent, Pedro De Oliveira, Anne-Lucie Teillout, Anne Dolbecq, Mohamed Haouas, Emmanuel Cadot and Israël M. Mbomekallé

Inorganics 2015, 3(3), 341-354; https://doi.org/10.3390/inorganics3030341 - 13 Jul 2015

Cited by 8 | Viewed by 4695

Abstract

The chemistry of polyoxometalates (POMs) keeps drawing the attention of researchers, since they constitute a family of discrete molecular entities whose features may be easily modulated. Often considered soluble molecular oxide analogues, POMs possess enormous potential due to a myriad of choices concerning size, shape and chemical composition that may be tailored in order to fine-tune their physico-chemical properties. Thanks to the recent progress in single-crystal X ray diffraction, new POMs exhibiting diverse and unexpected structures have been regularly reported and described. We find it relevant to systematically analyse the different equilibria that govern the formation of POMs, in order to be able to establish reliable synthesis protocols leading to new molecules. In this context, we have been able to synthesise the Eu³⁺-containing silico-molybdo-tungstic dimer, [Eu(α-SiW₉Mo₂O₃₉)₂]¹³⁻. We describe the synthesis and characterisation of this new species by several physico-chemical methods, such as single-crystal X-ray diffraction, ¹⁸³W NMR and electrochemistry. Full article

(This article belongs to the Special Issue Polyoxometalates)

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(A) Mixed ball-and-stick and polyhedral representation of [Eu(SiW9Mo2O39)2]13−; (B) ball and stick representation of the {(SiW9Mo2O39)EuO4} fragment; (C) environment around the Eu3+ ion, with the Eu-O distances (Å) indicated for each bond; (D) position of the pseudo C2 axis passing through the Eu3+ ion. Blue octahedra: WO6; orange octahedra: MoO6; green tetrahedra: SiO4; plum spheres: Eu; blue spheres: W; orange spheres: Mo; red spheres: O; pink spheres: O bound to Mo ions. Full article ">Figure 2
183W NMR spectrum of the lithium salt of [Eu(SiW9Mo2O39)2]13− in D2O/H2O solution. Full article ">Figure 3
CVs of 1 (red) and of [SiW9Mo2O39]8− (blue) in 0.5 M Li2SO4 + H2SO4/pH 3. Scan rate: 10 mV.s−1. POM concentration: 2.5 × 10−4 M; working electrode: EPG; counter electrode: Pt; reference electrode: SCE. Full article ">Figure 4
CVs of 1 (red) and of [SiW9Mo3O40]4− (black) in 0.5 M Li2SO4 + H2SO4/pH 3. Scan rate: 10 mV.s−1. POM concentration: 2.5 × 10−4 M. Working electrode: EPG; counter electrode: Pt; reference electrode: SCE. Full article ">Figure 5
(A) CVs of 1 alone in an argon saturated solution (black), in a O2 saturated solution (red), and in the presence of 0.25 M H2O2 (blue) in 0.5 M Li2SO4 + H2SO4/pH 3. (B) Successive CVs of 1 in the presence of 0.25 M H2O2 in 0.5 M Li2SO4 + H2SO4/pH 3. Scan rate: 2 mV.s−1. POM concentration: 2.5 × 10−4 M. Working electrode: EPG; counter electrode: Pt; reference electrode: SCE. Full article ">

2912 KiB

Open AccessCommunication

Activity and Stability of the Tetramanganese Polyanion [Mn₄(H₂O)₂(PW₉O₃₄)2]^10— during Electrocatalytic Water Oxidation

by Sara Goberna-Ferrón, Joaquín Soriano-López and José Ramón Galán-Mascarós

Inorganics 2015, 3(3), 332-340; https://doi.org/10.3390/inorganics3030332 - 8 Jul 2015

Cited by 13 | Viewed by 5096

Abstract

In natural photosynthesis, the oxygen evolving center is a tetranuclear manganese cluster stabilized by amino acids, water molecules and counter ions. However, manganese complexes are rarely exhibiting catalytic activity in water oxidation conditions. This is also true for the family of water oxidation catalysts (WOCs) obtained from POM chemistry. We have studied the activity of the tetranuclear manganese POM [Mn₄(H₂O)₂(PW₉O₃₄)2]^10—(Mn₄), the manganese analog of the well-studied [Co₄(H₂O)₂(PW₉O₃₄)2]^10— (Co₄), one of the fastest and most interesting WOC candidates discovered up to date. Our electrocatalytic experiments indicate that Mn₄ is indeed an active water oxidation catalysts, although unstable. It rapidly decomposes in water oxidation conditions. Bulk water electrocatalysis shows initial activities comparable to those of the cobalt counterpart, but in this case current density decreases very rapidly to become negligible just after 30 min, with the appearance of an inactive manganese oxide layer on the electrode. Full article

(This article belongs to the Special Issue Polyoxometalates)

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Cyclic voltammetry in a pH 7 sodium phosphate buffer (50 mM) water solution as electrolyte: 1.0 mM Co4 (blue line); 1.0 mM Mn4 (green line); blank (black line). E reported vs. Ag/AgCl (3 M) reference electrode. Full article ">Figure 2
(a) Bulk water electrolysis under an applied anodic potential of 1.40 V /vs. NHE) with an fluorine-doped tin oxide (FTO) anode and Pt mesh cathode in a pH 7 sodium phosphate buffer (50 mM) water solution as electrolyte with 1.0 mM Mn4 (red line); a consecutive experiment with the as-used electrode in a Mn4-free electrolyte solution (blue); blank (black line). (b) analogous experiment with addition of bpy(10 mM). Full article ">Figure 3
Molecular structure of the [Mn<math display="inline"> <msub> <mrow/> <mn>4</mn> </msub> </math>(H<math display="inline"> <msub> <mrow/> <mn>2</mn> </msub> </math>O)<math display="inline"> <msub> <mrow/> <mn>2</mn> </msub> </math>(PW<math display="inline"> <msub> <mrow/> <mn>9</mn> </msub> </math>O<math display="inline"> <msub> <mrow/> <mn>34</mn> </msub> </math>)<math display="inline"> <msub> <mrow/> <mn>2</mn> </msub> </math>]<math display="inline"> <msup> <mrow/> <mrow> <mn>10</mn> <mo>–</mo> </mrow> </msup> </math> (Mn4) polyoxometalate. Full article ">

10215 KiB

Open AccessReview

Structural and Electronic Properties of Polyoxovanadoborates Containing the [V₁₂B₁₈O₆₀] Core in Different Mixed Valence States

by Patricio Hermosilla-Ibáñez, Karina Muñoz-Becerra, Verónica Paredes-García, Eric Le Fur, Evgenia Spodine and Diego Venegas-Yazigi

Inorganics 2015, 3(3), 309-331; https://doi.org/10.3390/inorganics3030309 - 3 Jul 2015

Cited by 9 | Viewed by 5588

Abstract

This review summarizes all published data until April 2015 related to crystalline lattices formed by the [V₁₂B₁₈O₆₀] core, which generates polyanionic clusters with different degrees of protonation and mixed-valence ratios. The negative charge of this cluster is counterbalanced by different cations such as protonated amines, hydronium, and alkaline, and transition metal ions. The cluster is shown to form extended 1D, 2D, or 3D frameworks by forming covalent bonds or presenting hydrogen bond interactions with the present secondary cations. These cations have little influence on the solid state reflectance UV-visible spectra of the polyanionic cluster, but are shown to modify the FT-IR spectra and the magnetic behavior of the different reported species. Full article

(This article belongs to the Special Issue Polyoxometalates)

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