Fe2 and Fe4 Clusters Encapsulated in Vacant Polyoxotungstates: Hydrothermal Synthesis, Magnetic and Electrochemical Properties, and DFT Calculations

Substituent-Guided Cluster Nuclearity for Tetranuclear Iron(III) Compounds with Flat {Fe4(μ3-O)2} Butterfly Core

The tetranuclear iron(III) compounds [Fe4(μ3-O)2(μ-LZ)4] (1–3) were obtained by reaction of FeCl3 with the shortened salen-type N2O2 tetradentate Schiff bases N,N’-bis(salicylidene)-o-Z-phenylmethanediamine H2LZ (Z = NO2, Cl and OMe, respectively), where the one-carbon bridge between the two iminic nitrogen donor atoms guide preferentially to the formation of oligonuclear species, and the ortho position of the substituent Z on the central phenyl ring selectively drives towards Fe4 bis-oxido clusters. All compounds show a flat almost-symmetric butterfly-like conformation of the {Fe4(μ3-O)2} core, surrounded by the four Schiff base ligands, as depicted by both the X-ray molecular structures of 1 and 2 and the optimized geometries of all derivatives as obtained by UM06/6-311G(d) DFT calculations. The strength of the antiferromagnetic exchange coupling constants between the iron(III) ions varies among the three derivatives, despite their magnetic cores remain structurally almost unvaried, as well as the coordination of the metal ions, with a distorted octahedral environment for the two-body iron ions, Feb, and a pentacoordination with trigonal bipyramidal geometry for the two-wing iron ions, Few. The different magnetic behavior within the series of examined compounds may be ascribed to the influence of the electronic features of Z on the electron density distribution (EDD) of the central {Fe4(μ3-O)2} core, substantiated by a Quantum Theory of Atoms In Molecules (QTAIM) topological analysis of the EDD, as obtained by UM06 calculations 1–3.

Polyoxometalate-Complexed Indium Hydroxide: Atomically Homogeneous Impregnation via Countercation Exchange

Metal hydroxides catalyze organic transformations and photochemical processes and serve as precursors for the oxide layers of functional multicomponent devices. However, no general methods are available for the preparation of stable water-soluble complexes of metal hydroxide nanocrystals (NCs) that might be more effective in catalysis and serve as versatile precursors for the reproducible fabrication of multicomponent devices. We now report that In^III-substituted monodefect Wells-Dawson (WD) polyoxometalate (POM) cluster anions, [α₂-P₂W₁₇O₆₁In^IIIOH)]^8-, serve as ligands for stable, water-soluble complexes, 1, of platelike, predominantly cubic-phase (dzhalindite) In(OH)₃ NCs that after optimization contain ca. 10% InOOH. Images from cryogenic tranmsission electron microscopy reveal numerous WD ligands at the surfaces of platelike NCs, with average dimensions of 17 × 28 × 2 nm, each complexed by an average of ca. 450 In^III-substituted WD cluster anions and charge-balanced by 3600 Na⁺ countercations. Facilitated by the water solubility of 1, countercation exchange is used to stoichiometrically disperse ca. 1800 Cu²⁺ ions in an atomically homogeneous fashion around the surfaces of each NC core. The utility of this impregnation method is illustrated by using the ion-exchanged material as an electrocatalyst that reduces CO₂ to CO 15 times faster per milligram of Cu than does K₆Cu[P₂Cu^II(H₂O)W₁₇O₆₁] (control) alone. More generally, the findings point to POM complexation as a promising method for stabilizing and solubilizing reactive d-, p-, and f-block metal hydroxide NCs and for enabling their utilization as versatile components in the fabrication of functional multicomponent materials.

Arsenic(III)-Capped 12-Tungsto-2-Arsenates(III) [M2(AsIIIW6O25)2(AsIIIOH)x]n- (M = CrIII, FeIII, ScIII, InIII, TiIV, MnII) and Their Magnetic Properties

Six arsenic(III)-capped 12-tungsto-2-arsenates(III) of the type [M₂(As^IIIW₆O₂₅)₂(As^IIIOH)_x]^n- (M = Cr^III, 1; Fe^III, 2; Sc^III, 3; In^III, 4; Ti^IV, 5; Mn^II, 6) have been synthesized in aqueous medium by direct reaction of the elements using a one-pot strategy and structurally characterized by FT-IR spectroscopy, single-crystal XRD, and elemental analysis. Polyanions 1-6 are comprised of two octahedrally coordinated guest metal ions M sandwiched between two {AsW₆} units, resulting in a structure with C_2h point-group symmetry. Polyanions 1-5 contain tri- and tetravalent metal ion guests M (M = Cr^III, Fe^III, Sc^III, In^III, and Ti^IV, respectively), and they have one {As^IIIOH} group grafted on each {AsW₆} unit, whereas the divalent Mn^II-containing derivative 6 has two such {As^IIIOH} groups grafted on each {AsW₆} unit. Magnetic studies on polyanions 3-5 over the temperature range 1.8-295 K and magnetic fields of 0-7 T confirmed that they are diamagnetic. On the other hand, polyanions 1, 2, and 6 are strongly magnetic and follow the Curie-Weiss law above 30 K. The susceptibility plots of 1 and 6 exhibit broad peaks suggesting short-range antiferromagnetic ordering, while the very weak antiferromagnetic ordering of 2 is overshadowed by traces of a paramagnetic impurity. The magnetization data of 1, 2, and 6 at 1.8 K over 0-7 T were analyzed by using the Heisenberg exchange procedure. Small (negative) values of the obtained J values help in understanding the absence of long-range antiferromagnetic ordering.

Synthesis, Anti-Tumor Activity and Apoptosis-Inducing Effect of Novel Dimeric Keggin-Type Phosphotungstate

A dimeric Keggin-type phosphotungstate (ODA)₁₀[(PW₁₁FeO₃₉)₂O]·9H₂O (abbreviated as ODA₁₀[(PW₁₁Fe)₂], ODA = octadecyltrimethylammonium bromide) was synthesized and investigated comprehensively its antitumor activity on MCF-7 and A549 cells. The dimeric structure and amorphous morphology were characterized by FT-IR, UV-vis-DRS, SEM and XRD. The in vitro MTT assay of ODA₁₀[(PW₁₁Fe)₂] showed anticancer activity on MCF-7 and A549 cells in a dose- and time-dependent manner, and the IC₅₀ values for MCF-7 and A549 cells at 48 h were 5.83 μg/ml and 3.23 μg/ml, respectively. The images of the ODA₁₀[(PW₁₁Fe)₂]-treated cells observed by inverted biological microscope exhibited the characteristic morphology of apoptosis. Flow cytometric analysis showed cell apoptosis and cycle arrested at S phase induced by ODA₁₀[(PW₁₁Fe)₂]. The above results illuminated the main mechanism of the antitumor action of ODA₁₀[(PW₁₁Fe)₂] on MCF-7 and A549 cells, indicating that this dimeric phosphotungstate is a promising anticancer drug.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na₂H₂[Mo₄O₁₂(C₈H₁₇O₅N)₂]·10H₂O (1), Na₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·10H₂O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH₄)₂[M(Bis-tris)(H₂O)]₂[Mo₇O₂₄]·6H₂O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na⁺ has higher catalytic activity than that constructed by NH₄⁺. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

A new strategy for the selective oxidation of alcohols catalyzed by a polyoxometalate-based hybrid surfactant in biphasic systems

A hybrid surfactant with catalytic oxidation activity and interfacial activity was prepared via a combination of the polyoxometalate [PW11O39]7- anion and an alkoxysilane by a covalent bond. The organic-inorganic hybrid surfactant can effectively emulsify an alcohol/water biphase system, promote the alcohol oxidation reaction by increasing the oil-water interface area, and exhibit better catalytic oxidation activity than the homogeneous polyoxometallate. The demulsification of the emulsion can be achieved by increasing the temperature, and the catalyst can be recycled several times with no apparent loss of catalytic performance.

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

An unprecedented [{Fe5O5(OH)2(OAc)2}2{W2O2(OH)}] cluster sandwiched in the tetravacant tungstophosphate

An unprecedented [{Fe₅O₅(OH)₂(OAc)₂}₂{W₂O₂(OH)}] cluster sandwiched in two tetravacant {P₂W₁₄O₅₄} units was obtained by a one-pot reaction.

Design of an inherently-stable water oxidation catalyst

While molecular water-oxidation catalysts are remarkably rapid, oxidative and hydrolytic processes in water can convert their active transition metals to colloidal metal oxides or hydroxides that, while quite reactive, are insoluble or susceptible to precipitation. In response, we propose using oxidatively-inert ligands to harness the metal oxides themselves. This approach is demonstrated by covalently attaching entirely inorganic oxo-donor ligands (polyoxometalates) to 3-nm hematite cores, giving soluble anionic structures, highly resistant to aggregation, yet thermodynamically stable to oxidation and hydrolysis. Using orthoperiodate (at pH 8), and no added photosensitizers, the hematite-core complex catalyzes visible-light driven water oxidation for seven days (7600 turnovers) with no decrease in activity, far exceeding the documented lifetimes of molecular catalysts under turnover conditions in water. As such, a fundamental limitation of molecular complexes is entirely bypassed by using coordination chemistry to harness a transition-metal oxide as the reactive center of an inherently stable, homogeneous water-oxidation catalyst.

A current challenge in the development of molecular water oxidation catalysts is to overcome their inherent susceptibilities to oxidative or hydrolytic degradation under turnover conditions in water. Here, the authors design an inherently-stable water oxidation catalyst using oxidatively-inert ligands to harness a reactive metal oxide.

Synthesis, structure, spectroscopic and ferroelectric properties of an acentric polyoxotungstate containing 1:2-type [Sm(α-PW₁₁O₃₉)₂]¹¹⁻ fragment and D-proline components

An organic-inorganic hybrid mono-Sm(III) substituted phosphotungstate KNa3[HPro]₇[Sm(α-PW₁₁O₃₉)₂]·Pro·₁₈H₂O (1) (Pro=D-proline) has been synthesized in the conventional aqueous solution and structurally characterized by elemental analyses, inductively coupled plasma atomic emission spectrometry (ICP-AES) analyses, IR spectra, UV spectra, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and single-crystal X-ray diffraction. The molecule of 1 consists of a classical 1:2-type [Sm(α-PW₁₁O₃₉)₂](11-) fragment and free D-proline components. It should be pointed out that the synergistic action between the in-situ formed chiral [Sm(α-PW₁₁O₃₉)₂](11-) fragment and chiral D-proline components results in the formation of the chiral 1. The luminescence emission of 1 reveals three characteristic bands that derive from the (4)G₅/₂→(6)H₅/₂, (4)G₅/₂→(6)H₇/₂ and (4)G₅/₂→(6)H₉/₂ transitions of the Sm(III) cation as well as the synergistic contribution of the O→W transitions of [α-PW₁₁O₃₉](7-) moieties and a π(*)-n or π(*)-π transitions of Pro. Its ferroelectric behavior has been measured.

Two tetra-Cd(II)-substituted vanadogermanate frameworks

Two new tetra-Cd(II)-substituted vanadogermanate frameworks {(CdX)4Ge8V(IV)10O46(H2O)[V(III)(H2O)2]4(GeO2)4}·8H2O (X = ethylenediamine (en, 1) and 1,2-diaminopropane (dap, 2)) were hydrothermally prepared and characterized by IR spectra, elemental analysis, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and X-ray single-crystal diffraction. Both are isomorphic, and their 3-D frameworks are made up of tetra-Cd(II)-substituted {(CdX)4Ge8V(IV)10O46(H2O)}(12-) fundamental building units interconnected through planar tetra-V(III) [V(III)4O2(H2O)8](8-) clusters and tetrahedral GeO4 bridges. In the unique {(CdX)4Ge8V(IV)10O46(H2O)}(12-) cage, four [Ge2O7] dimers and four CdO4N2 trigonal prisms are alternately concatenated by μ3-O bridges to create a round {Ge8Cd4O28(X)4}(16-) fragment, five VO5 groups are linked by sharing edges to generate a pentanuclear [V5O17] subunit, and then the {Ge8Cd4O28(X)4}(16-) fragment is sandwiched by two V5O17 subunits via sharing O-atoms producing a D4h-symmetric {(CdX)4Ge8V(IV)10O46(H2O)}(12-) cage with a free water molecule located at the center. As we know, both display unprecedented 3-D organic-inorganic hybrid frameworks built up from the largest number of transition-metal-substituted vanadogermanate {(CdX)4Ge8V(IV)10O46}(12-) cluster shells linked by both GeO4 tetrahedra and rare [V(III)4O2(H2O)8](8-) clusters. Magnetic measurements reveal the antiferromagnetic couplings within the magnetic vanadium centers.

Two novel oxovanadium–organophosphonate hybrids with a 3D supramolecular structure: synthesis, crystal structures, surface photovoltage and luminescent properties

Two novel oxovanadium–organophosphonates with a 3D supramolecular structure, [{M(1,10–phen)}(VO)(OH)(hedp)]·H₂O (M = Cu (1), Zn (2)), have been hydrothermally synthesized. The surface photovoltage and luminescent properties of the two compounds have been studied.

Organic-inorganic hybrid materials based on iron(III)-polyoxotungstates and 1-butyl-3-methylimidazolium cations

The iron(III) μ-oxo bridged dimeric polyoxometalate [(PW(11)O(39)Fe)(2)O](10-) was isolated by reacting the transition metal monosubstituted Keggin anion [PW(11)O(39)Fe(H(2)O)](4-) and the ionic liquid 1-butyl-3-methylimidazolium bromide, (Bmim)Br, at pH 5.5. The crystal structure of (Bmim)(10)[(PW(11)O(39)Fe)(2)O]·0.5H(2)O (1) (monoclinic, space group P2(1)/n, Z = 4) was determined by single crystal X-ray diffraction. By changing the reaction conditions, (Bmim)(4)[PW(11)O(39)Fe(H(2)O)]·H(2)O (2) was obtained, whilst the reaction between the Bmim(+) cation and the heteropolyanion [SiW(11)O(39)Fe(H(2)O](5-), in the pH conditions used for 1, afforded (Bmim)(5)[SiW(11)O(39)Fe(H(2)O)]·4H(2)O (3). The compounds were characterized by spectroscopic techniques, thermal analysis, cyclic voltammetry, magnetic measurements and mass spectrometry. This study contributes to the understanding of iron μ-oxo dimer formation in polyoxometalate chemistry and calls attention to the influence of the counter-cations on the stability and formation of compound 1. The combination of the cationic part of ionic liquids and iron-substituted polyoxotungstates is predicted to lead to new materials with interest to catalysis, electrocatalysis and ionic liquid based nanocomposites.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Rationalizing the molecular origins of Ru- and Fe-based dyes for dye-sensitized solar cells

As part of an effort to design more efficient dyes for dye-sensitized solar cells (DSCs), structure–property relationships are established in the world's best-performing chemical series of dyes: 2,2′-bipyridyl-4,4′-carboxylatoruthenium(II) complexes. Statistical analysis, based on crystallographic data from the Cambridge Structural Database, is used to determine common structural features and the effects of structural change to its salient molecular constituents. Also included is the report of two new crystal structures for tris(2,2′-bipyridyl)dichlororuthenium(II)hexahydrate and tris(2,2′-bipyridyl)iron(II)dithiocyanate; these add to this statistical enquiry. Results show that the metal (M) core exhibits a distorted octahedral environment with M—N π-backbonding effects affording the propensity of the metal ion towards oxidation. The same characteristics are observed in iron-based analogues. The role of carboxylic groups in this series of dyes is assessed by comparing complexes which contain or are devoid of COOH groups. Space-group variation and large molecular conformational differences occur when COOH groups are present, while such structural features are very similar in their absence. The nature of the anion is also shown to influence the structure of COOH-containing complexes. These structural findings are corroborated by solution-based UV–vis absorption spectroscopy and DSC device performance tests. The presence of COOH groups in this series of compounds is shown to be mandatory for dye-uptake in TiO2 in the DSC fabrication process. Throughout this study, results are compared with those of the world's most famous DSC dye, N3 (N719 in its fully protonated form): cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II). Overall, the molecular origins of charge-transfer in these complexes are ascertained. The findings have important implications to the materials discovery of more efficient dyes for DSC technology.

Coordination assemblies of polyoxomolybdate cluster framework: from labile building blocks to stable functional materials

Polyoxomolybdates, an important branch in polyoxometalates chemistry, present complicated solution chemistry and unmatched physicochemical properties, which endows us with both great opportunities and considerable challenges in creating new functional materials. This perspective highlights the recent development on the coordination assembly of transition-metal-substituted heteropolymolybdates by using labile lacunary heteropolymolybdates as inorganic multidentate ligands. A series of strategies have been used to stabilize the lacunary heteropolymolybdate building blocks. Finally, we introduce some researches on the modification of polymolybdates by organic groups in aqueous media, which may shed light on the green chemistry of the functionalization of polyoxomolybdates.

A Fe(bipy)2 Unit Fused to a Keggin Polyoxotungstate Anion

Two new hybrid materials [Fe(bipy)3]3[ZnW12O39(OH)Fe(bipy)2(H2O)]2 and [Fe(bipy)3]2[CoW12O40] were obtained by hydrothermal synthesis. They contain [Fe(bipy)3] n+ (bipy = 2,2′-bipyridyl; n = 2 or 3) cations and Keggin-type polyoxotungstate anions. Both have been characterized by elemental analysis, FT-IR spectroscopy, 1H-NMR spectrometry, and single crystal X-ray crystallography. The anion in the former salt features [FeII(bipy)2(OH2)]2+ units covalently bound via a W=O-Fe bridge to the protonated anion [ZnW12O39(OH)]5–. The latter salt contains discrete [CoW12O40]6– anions.