Special issue on Polyoxometalates

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.

Recent advances of polyoxometalate-catalyzed selective oxidation based on structural classification

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.

Recent advances in isopolyoxotungstates and their derivatives

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.

Single-crystal-to-single-crystal transformations triggered by dehydration in polyoxometalate-based compounds

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.

A polyoxometalate-based metal–organic poly­hedron constructed from a {V₅O₉Cl} building unit with rhombicubocta­hedral geometry

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.

Synthesis and structural characterization of a dumbbell-like phenyl­phospho­nate-stabilized Ti₇–oxide cluster

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

Iron(II) and copper(II) paratungstates B: a single-crystal X-ray diffraction study

Two new isopolytungstates, Na₅Fe_2.5[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 Fe^II or Cu^II ions in a polydentate mode.

Synthesis, structural characterization and fluorescence enhancement of chromophore-modified polyoxometalates

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.

A novel two-dimensional layered Cu²⁺ complex based on Dawson-like [H₃BiW₁₈O₆₀]⁶⁻: synthesis, structure and magnetic properties

A two-dimensional complex based on the Dawson-like tungstobismuthate as a tetra­dentate ligand, i.e. (enH)₂[{Cu(2,2′-bipy)(en)}₂{μ₄-H₃BiW₁₈O₆₀)]·5.5H₂O, has been synthesized hydro­thermally and structurally characterized, and its electrochemical and magnetic properties have been studied.

pH-induced phase transition and crystallization of soft-oxometalates (SOMs) into polyoxometalates (POMs): a study on crystallization from colloids

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

An unprecedented Zr-containing polyoxometalate tetra­mer with mixed trilacunary/dilacunary Keggin-type polyoxotungstate units

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.

Effect of the ammonium ion on proton conduction in porous ionic crystals based on Keggin-type silicododeca­tungstate

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.

Synthesis and structural characterization of tube-type tetra­deca­vanadates

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.

Surface-grafted lanthanoid complexes of the tungstosilicate polyanion [SiW₁₂O₄₀]⁴⁻: a synthetic, structural and computational investigation

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.

Crystal structures of hybrid completely reduced phospho­molybdates and catalytic performance applied as mol­ecular catalysts for the reduction of chromium(VI)

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.

Supra­molecular inter­action-induced assemblies of polyanions and 2-amino­pyridinium in two polyoxometalate-based hybrids

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.

Zirconia-supported 11-molybdovanado­phospho­ric acid catalysts: effect of the preparation method on their catalytic activity and selectivity

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.

Direct observation of the Zr^IV inter­action with the carboxamide bond in a noncovalent complex between Hen Egg White Lysozyme and a Zr-substituted Keggin polyoxometalate

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

Synthesis and characterization of carbonate-encapsulated ytterbium- and yttrium-containing polyoxotungstates

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.

A new hexa­molybdate-based copper–2,2′-bi­imidazole coordination polymer serving as an acid catalyst and support for enzyme immobilization

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.

Two new POM-based compounds modified by lanthanide–Schiff base complexes with inter­esting NLO properties

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.

Synthesis and characterization of hybrid Anderson hexa­molybdoaluminates(III) functionalized with indometacin or cinnamic acid

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

Targeted synthesis of a polypyridyl polyoxometalate coordination complex using microwave-assisted reaction conditions

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 [B^IIICo^IIIW^VI₁₁O₃₉]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.

The mixed-valent 10-manganese(III/IV)-containing 36-tungsto-4-arsenate(V), [Mn^III₆Mn^IV₄O₄(OH)₁₂(H₂O)₁₂(A-β-AsW₉O₃₄)₄]²²⁻

The mixed-valent 10-managanese-containing 36-tungsto-4-arsenate(V) [Mn^III₆Mn^IV₄O₄(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.

Structures and energetic properties of 4-halo­benzamides

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.

Unprecedented three-dimensional hydrogen-bonded hex topological chiral lanthanide–organic frameworks built from an achiral ligand

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.

Solid-state structure and anti­microbial and cytotoxicity studies of a cucurbit[6]uril-like Cu₆L₄ constructed from 3,5-bis­[(1H-tetra­zol-5-yl)meth­yl]-4H-1,2,4-triazol-4-amine

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

Cocrystals of pyrazine and benzene polycarb­oxy­lic acids

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.

Influence of 3-{5-[4-(di­ethylamino)benzyl­idene]­rhodanine}propionic acid on the conformation of 5-(4-chloro­benzyl­idene)-2-(4-methylpiperazin-1-yl)-3H-imidazol-4(5H)-one

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.

A new twofold inter­penetrated two-dimensional cadmium(II) coordination polymer constructed from 2,2′-(benzene-1,4-dicarboxamido)­dipropionate

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.

Purification, characterization, crystal structure and NO production inhibitory activity of three new sesquiterpenoids from homalomena occulta

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.

Studies via X-ray analysis on inter­molecular inter­actions and energy frameworks based on the effects of substituents of three 4-aryl-2-methyl-1H-imidazoles of different electronic nature and their in vitro anti­fungal evaluation

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.

Order and disorder in (E)-[1-(bi­phenyl-4-yl)ethyl­idene]hydrazine: a structural, spectroscopic and theoretical study

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.

Low-dimensional compounds containing cyanido groups. Part XXXV. Structure, spectral, thermal and magnetic properties of a binuclear Cu^II–bi­quinoline complex with bridging and terminal dicyanamide ligands

The binuclear mol­ecular complex [Cu₂(biq)₂(dca)₂(μ_1,5-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.

Multifunctional aryl­sulfonamide derivatives with 5-HT₆/5-HT₇ receptor antagonistic activity: a structural study

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.

Structural and electrochemical comparison of trinuclear ruthenium oxo clusters [Ru₃(OAc)₆O(L)₃]⁺ and [Ru₃(OAc)₆O(L)₂(CO)] (L = imidazole, benz­imidazole, and 4-phenyl­pyridine)

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.

The solid-state conformation of the topical anti­fungal agent O-naphthalen-2-yl N-methyl-N-(3-methyl­phen­yl)carbamo­thio­ate

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.

Investigation of the mol­ecular structure of 4-(3-methyl-3-phenyl­cyclo­but­yl)-2-[2-(3-methyl­benzyl­idene)hydrazin­yl]thia­zole in the gas and solid phases

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.

The study of inter­actions with a halogen atom: influence of NH₂ group insertion on the crystal structures of meta-bromo­nitro­benzene derivatives

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.

Different cationic forms of (–)-cytisine in the crystal structures of its simple inorganic salts

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.

Crystal structures of racemic and enanti­opure synephrine correlated with physicochemical properties from IR spectroscopy and thermal analysis

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.

Controlled inter­penetration of 4⁴ networks: syntheses and structures of two Cu₃-cluster coordination polymers

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

Ruthenium(II)–carbonyl complexes containing two N-monodentate 1,8-naphthyridine ligands: active catalysis in transfer hydrogenation reactions

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.

Comparison of the crystal structures and Hirshfeld surface analysis of five N-(4-methyl­benzene­sulfonyl)glycine hydrazone derivatives

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.