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Journal logoSTRUCTURAL
CHEMISTRY
ISSN: 2053-2296

November 2018 issue

Special issue on Polyoxometalates

Guest Editors: José Ramón Galán-Mascarós (Institute of Chemical Research of Catalonia (ICIQ), Spain) and Ulrich Kortz (Jacobs University, Germany)

Highlighted illustration

Cover illustration: The development of polyoxometalate (POM) chemistry during the last half-century or so has benefitted tremendously from single-crystal X-ray diffraction. Besides structural aspects, the study of the physicochemical properties of POMs has developed tremendously in recent decades. The multitude of attractive properties includes controllable size, composition, charge density, redox potential, acid strength, high thermal stability in the solid state, solubility in polar/nonpolar solvents and reversible electron/proton storage. Such versatility renders POMs of interest for academic and industrial applications, especially in catalysis. The contributions to this special issue on POMs provide an excellent overview of the current state of the subject, as well as providing updates to current reseach. (The cover art was prepared by Dr Rami Al-Oweini.)

editorial


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José Ramón Galán-Mascarós and Ulrich Kortz introduce this special issue of Acta Crystallographica Section C on `Polyoxometalates' marking a special anniversary of this exciting class of inorganic compounds.

polyoxometalates


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Recent advances regarding POM-catalyzed selective oxidation and work which contributes to elucidating the origin/distinction of catalytic activity, as well as reasonable mechanisms, are summarized.

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The main research progress on isopolyoxotungstates (iso-POTs) and their derivatives (iso-POT-based transition-metal derivatives, iso-POT-based rare-earth derivatives, iso-POT-based organometal derivatives and iso-POT-based heterometal derivatives) are summarized in this review and the relevant synthetic methodologies, structural types and applications are presented.

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The examples of single-crystal-to-single-crystal transformations triggered by dehydration that have been reported to date for polyoxometalate-based compounds are compiled and discussed in this feature article.

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A new polyoxovanadate-based metal–organic polyhedron with an rdo topology was constructed from a {V5O9Cl} building unit and benzene-1,3,5-tri­carb­oxy­lic acid under solvothermal conditions. The title compound displays rhombicubocta­hedral geometry when the polyoxovanadate building units and organic ligands were considered as the quadrangular and triangular faces of a polyhedron.

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A polyoxotitanium cluster (PTC) with a Ti7 core stabilized by phenyl­phospho­nate ligands has been prepared and its crystallographic structure and optical absorption behaviour studied.

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Two new isopolytungstates, Na5Fe2.5[W12O40(OH)2]·36H2O and Na4Cu3[W12O40(OH)2]·28H2O, have been prepared and structurally characterized. The compounds exhibit a three-dimensional structure, in which the paratungstate anions coordinate to FeII or CuII ions in a polydentate mode.

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Three new chromophore-modified polyoxometalates have been synthesized and characterized by single-crystal X-ray diffraction. The fluorescence properties of the resulting polyoxometalate hybrids can be boosted by tuning the solvents. We demonstrate an easy and feasible method for solving the fluorescence quenching problem of POM–chromophore dyads. Our work may open up a new pathway for the development of POM-based fluorescent sensors or switches.

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A two-dimensional complex based on the Dawson-like tungstobismuthate as a tetra­dentate ligand, i.e. (enH)2[{Cu(2,2′-bipy)(en)}24-H3BiW18O60)]·5.5H2O, has been synthesized hydro­thermally and structurally characterized, and its electrochemical and magnetic properties have been studied.

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Phase transition of colloidal {TEA–MoOx} SOM to crystalline {K·β-Mo8} POM depicting the pre-existence of the SOM phase in the crystallization of POMs.

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A novel zirconium(IV)-substituted polyoxotungstate tetra­mer has been synthesized and structurally characterized. The tetra­mer polyoxoanion consists of two dilacunary Keggin-type germanotungstate units and two trilacunary Keggin-type phospho­tungstate fragments.

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A porous ionic crystal based on Keggin-type silicododeca­tungstate with ammonium ions shows high proton conductivity and low activation energy under high relative humidity, showing that protons migrate efficiently via rearrangement of the hydrogen-bonding network formed by the NH4+ cations and the waters of crystallization.

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By controlling the hydrogen-bonding inter­actions between ammonium cations and polyoxometalates, structure conversion from anion-incorporated bowl-type dodeca­vanadates to tube-type structures was achieved.

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The hydro­thermal synthesis of two inorganic–organic hybrid assemblies and their structural characterization using single-crystal X-ray diffraction reveals that they consist of [SiW12O40]4− Keggin-type polyoxometalates decorated with trimeric lanthanoid complexes and templated by a single Na+ cation. Density functional theory (DFT) calculations on both compounds were performed to analyze the polyanion–π inter­actions in the solid-state structures.

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The influence of Na, Ca and Sr on the structures of {Fe(P4Mo6O31)2}-based hybrids and the catalytic activity of the resulting POMs for the reduction of hexavalent chromium are explored.

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Two polyoxometalate-based hybrids were synthesized and characterized. Single-crystal analysis revealed that strong hydrogen bonds and electrostatic inter­actions play key roles in the formation of the final assemblies.

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The influence of the preparative method and thermal treatments on the performance of a model heterogeneous catalyst based on a zirconia-supported heteropoly acid in the oxidation of adamantane with hydrogen peroxide is investigated in this work.

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The crystal structure capturing the locked inter­mediate state of the hydrolytically active 1:1 Zr–Keggin and asparagine side chain is reported.

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A carbonate-encapsulated yttrium-containing arsenotungstate(III) has been synthesized under mild conditions and characterized by various analytical techniques. In addition, the crystal structures of two other carbonate-encapsulated germanotungstates(IV) have been reported.

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A new inorganic–organic hybrid coordination polymer, [Cu2Mo6O20(C6H6N4)2(H2O)2]n, composed of isopolyhexa­molybdate anions and copper–2,2′-bi­imidazole/H2O cations was hydro­thermally synthesized and characterized. This crystalline material exhibited good acid catalytic activity and it was firstly used as a support for horseradish peroxidase immobilization.

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Two POM-based lanthanide–Schiff base complexes have been successfully isolated under hydro­thermal conditions. Inter­estingly, they show excellent two-photon responses, suggesting that they have potential applications in the NLO field.

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A post-functionalization protocol was used for the synthesis of two new tris-hybrid Al-centred Anderson-type polyoxomolybdates with indometacin or cinnamic acid.

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The microwave-assisted synthesis of a polyoxometalate (POM)-containing coordination complex assembled from 2,4,6-tris­(4-pyridyl)-1,3,5-triazine (TPT) and [BIIICoIIIWVI11O39]6− is described. Single-crystal X-ray diffraction was used to determine the solid-state structure, revealing a trimeric mol­ecular assembly with numerous inter­molecular inter­actions evident in the solid state. The stability of the coordination complex in aqueous media was investigated using 1H NMR spectroscopy.

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The mixed-valent 10-managanese-containing 36-tungsto-4-arsenate(V) [MnIII6MnIV4O4(OH)12(H2O)12(A-β-AsW9O34)4]22− was synthesized in a one-pot reaction in aqueous medium and characterized as a mixed potassium–sodium salt in the solid state by FT–IR spectroscopy and single-crystal XRD, as well as elemental and thermogravimetric analyses.

research papers


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The crystal structures and energetic parameters of 4-halobenzamides (halo = iodo, bromo, chloro and fluoro) have been determined. To our knowledge, the results present the first systematic investigation of the effect of para-halogen substitution on the properties of the amide bond in N,N-disubstituted benzamides.

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Eight novel chiral lanthanide–organic frameworks with hex topology were synthesized from an achiral 3-nitro-4-(pyridin-4-yl)benzoate ligand with hydrogen-bonding-induced spontaneous resolution.

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Four new ligand molecules coordinate to six CuII ions to form an unexpected cucurbit[6]uril-like structure. The sulfate-templated compound shows antimicrobial properties.

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The crystal structures of four cocrystals of pyrazine with benzene polycarb­oxy­lic acids were determined. In all cases, infinite chains of alternating acid and base mol­ecules, connected by O—H⋯N hydrogen bonds, are formed.

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The aryl­idene–imidazolone derivatives are a group of compounds of great inter­est in medicinal chemistry due to their various pharmacological actions. In order to study the possible conformations of an aryl­idene–imidazolone derivative, two new crystal structures were determined by X-ray diffraction.

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A twofold inter­penetrated two-dimensional cadmium(II) coordination polymer, with a (4,4)-connected 4,4L15 net, was assembled from the designed ligand (2R,2′R)-2,2′-(benzene-1,4-dicarboxamido)­dipropionic acid. The thermal stability and photoluminescence properties of the coordination polymer were investigated.

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Three new sesquiterpenoids were isolated from homalomena occulta, and their structures were confirmed based on extensive spectroscopic analyses and single-crystal X-ray diffraction. The three compounds were evaluated for their activity against the LPS-induced production of nitro­gen oxide (NO) in RAW 264.7 cells.

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The inter­molecular ineractions of three 4-aryl-2-methyl-1H-imidazoles show an inter­esting dependence on the electronic nature of the mol­ecules induced by the substituent group in the para site of the aryl group. Apparently, the influence on the acid–base properties of the mol­ecules not only affects the crystal structure, but also has an important role in their anti­fungal activity.

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Ordered and disordered single-crystal structures of (E)-[1-(biphenyl-4-yl)ethyl­idene]hydrazine were investigated to examine the subtle nature of rotations and glides in the solid state.

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The binuclear mol­ecular complex [Cu2(biq)2(dca)21,5-dca)2] (biq is 2,2′-bi­quinoline and dca is dicyanamide) was isolated and characterized by structural analysis, and by spectroscopic, thermal and magnetic measurements.

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The X-ray crystal structures of four 5-HT6R and 5-HT7R antagonists have been determined and compared with pharmacophore models. The role of selected pharmacophore points in inter­molecular inter­actions was studied by Hirshfeld surface analysis, confirming the important role of the sulfonyl group in C—H⋯O inter­actions and the involvement of aromatic fragments in C—H⋯π contacts.

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The triruthenium oxo clusters [Ru3(OAc)6O(L)3]+ and [Ru3(OAc)6O(L)2(CO)] possess unique electronic characteristics that vary based on the ligands L. We report an investigation of the structural, electrochemical, and optical properties of clusters with imidazole, benzimidazole, and 4-phenyl­pyridine ligands.

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Large flat colorless needles of tolnaftate have been obtained from Tinactin, and have been shown to contain a 50:50 mixture of the (+ap,−sp,+ac,−ac) and (−ap,+sp,−ac,+ac) conformers. This finding fills a long-standing void in the structural chemistry of this classic anti­fungal compound, and suggests that prior theoretical models used to explain the spectroscopy, bioactivity, and mode of binding of tolnaftate to squalene epoxidase are probably inappropriate or suspect.

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The crystal structure of the Schiff base 4-(3-methyl-3-phenyl­cyclo­but­yl)-2-[2-(3-methyl­benzyl­idene)hydrazin­yl]thia­zole is stabilized by classical inter­molecular N—H⋯N hydrogen bonding. The compound can exist in two different tautomeric forms and experimental and theoretical studies were carried out on these forms.

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In the crystal structures of m-bromo­nitro­benzene and its amine derivatives, different inter­actions with the Br atom were found. The position of the amino group influences the σ-hole potential value that enables a switching of the Br-atom character from electrophilic to nucleophilic.

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The crystal structures of 13 simple salts of cytisine, an alkaloid isolated from the seeds of Laburnum anagyroides, have been determined. Cytisine has two potential protonation sites, i.e. the N atom of the piperidine ring and the carbonyl O atom of the pyridone ring. Three forms of the cytisinium cation were identified, namely the monocation, which is always protonated at the N atom, the dication, which utilizes both protonation sites, and the third form, which contains two cytisine moieties connected by very short and linear O⋯H⋯O hydrogen bonds.

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The proto-alkaloid synephrine is found to crystallize as a neutral mol­ecule in the racemate and as a zwitterion in the pure enanti­omer, in which the phenolic H atom has been transferred to the amino group.

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This work explores the controlled inter­penetration of 44 networks in two Cu3-cluster coordination polymers.

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cis-[RuCl2{2-(4-meth­oxy­phen­yl)-1,8-naphthyridine-κN8}2(CO)2] and the 2-meth­oxy­phen­yl derivative exhibit moderate catalytic activity with 100% selectivity for the hydrogen-transfer reaction to aceto­phenone in the presence of a base.

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A new series of N-acyl­hydrazone derivatives has been synthesized through the reactions of N-(4-methyl­benzene­sulfon­yl)glycinyl hydrazides and para-substituted aromatic aldehydes. The compounds adopt an E configuration about the C=N bond, while the NMR data indicate the presence of two isomers in solution.

Special and virtual issues

Acta Crystallographica Section C has recently published special issues on

NMR Crystallography (March 2017)

Scorpionates: a golden anniversary (November 2016)

Full details are available on the special issues page.

The latest virtual issue features Coordination polymers, with an introduction by Len Barbour.

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