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The unprecedented heptavanadate cluster has been isolated from reactions between trisalkoxide ligands and vanadate in water at pH = 2 as a series of alkylammonium [HxV7O18(H2O)((OCH2)3CR)](4-x)- salts (1-3, R = CH2OH; 4, R = CH3). Their structures have been determined and the partial stability of 4 in water assessed by a combination of multinuclear NMR spectroscopy and ESI-MS. The heptavanadate unit reported herein could represent an intermediate species in the formation of decavanadate that is blocked by attachment of tripodal ligands.

S. Reinoso, B. Artetxe and J. M. Gutierrez-Zorrilla. Single-crystal-to-single-crystal transformations triggered by dehydration in polyoxometalate-based compounds. En: Acta Crystallographica Section C 2018;74(11):1222-1242. https://doi.org/10.1107/S2053229618011269 Single-crystal-to-single-crystal transformations are solid-state phase transitions between different crystalline states in which the crystal integrity and the long-range structural order are retained through the whole transformation process. Such a phenomenon constitutes the structural response that some compounds afford when being exposed to a given external stimulus (temperature, pressure, light, etc.) and, therefore, its study has become a relevant focus of interest within crystal engineering because it allows for monitoring how certain properties (colour, magnetism, luminescence, porosity) of the stimuli-responsive material are modified as the structure evolves into the activated form. A range of organic, inorganic and hybrid systems have been found to undergo such phase transitions, but these examples only include a small number of compounds that incorporate polyoxometalate anions, among which the removal of guest solvent molecules (dehydration) stands out as the most common external stimulus able to induce the occurrence of a single-crystal-to-single-crystal transformation. This feature article compiles the examples of dehydration-triggered single-crystal-to-single-crystal transformation studies that have been reported to date for polyoxometalate-based compounds and reviews some of their most relevant structural aspects. © International Union of Crystallography, 2018 The following institutions are acknowledged for support: Universidad Pública de Navarra, Obra Social la Caixa and Fundación Caja Navarra (contract to SR in the framework of the program ‘Captación del Talento’); Universidad del País Vasco UPV/EHU (grants PPG17/37 and GIU17/050); Ministerio de Economía, Industria y Competitividad (grant MAT2017-89553P).

The hybrid compound [{Cu(cyclam)}3(W7O24)]·15.5H2O (1) (cyclam= 1,4,8,11-tetraaza-cyclotetradecane) was synthesized by reacting the {Cu(cyclam)}2+ complex with a tungstate source in water at pH = 8. Compound 1 exhibits an unprecedented three-dimensional covalent structure built of heptatungstate clusters linked through metalorganic complexes in a POMOF-like framework that displays water-filled channels. This dynamic architecture undergoes two sequential single-crystal-to-single-crystal transformations upon thermal evacuation of water molecules to result in the partially dehydrated [{Cu(cyclam)}3(W7O24)]·12H2O (2) and anhydrous [Cu(cyclam)]0.5[{Cu(cyclam)}2.5(W7O24)] (3) crystalline phases. These transitions are associated with cluster rotations and modifications in the CuII coordination geometries, which reduce the dimensionality of the original lattice to layered systems but preserving the porous nature. Phase 3 reverts to 2 upon exposure to ambient moisture, whereas the transition between 1 and 2 proved to be irreversible. The permanent microporosity of 3 was confirmed by gas sorption measurements (N2, CO2), which reveal a system of parallel channels made of wide cavities connected through narrow necks that limit the adsorption process, in good agreement with Grand Canonical Monte Carlo simulations.

Two new organic–inorganic hybrid compounds based on Keggin-type polyoxometalates and melaminium cations, namely (C3N6H7)3[PW12O40]·6H2O (1) and (C3N6H7)4[SiW12O40]·6H2O (2) have been synthesized at room temperature by a solvent diffusion method and characterized by IR spectroscopy, CHN analyses and single-crystal X-ray diffraction. Hybrid 1 consists of three melamonium cations per [PW12O40]3− unit with the hydration water molecules connected to each other via hydrogen bonding forming a 1D zigzag chain. In compound 2, [SiW12O40]4− polyanions and water molecules act as inorganic layers that are alternating with layers of monoprotonated organic melaminium cations.

The first decavanadate-based microporous hybrid, namely, [Cu(cyclam)][{Cu(cyclam)}2(V10O28)]·10H2O (1, cyclam = 1,4,8,11-tetraazacyclotetradecane) was prepared by reaction of (VO3)− anions and {Cu(cyclam)}2+ complexes in NaCl (aq) at pH 4.6–4.7 and characterized by elemental analyses, thermogravimetry, and X-ray diffraction (powder, single-crystal) techniques. Compound 1 exhibits a POMOF-like supramolecular open-framework built of covalent decavanadate/metalorganic layers with square-like voids, the stacking of which is aided by interlamellar cementing complexes and generates water-filled channels with approximate cross sections of 10.4 × 8.8 Å2. The framework is robust enough to remain virtually unaltered upon thermal evacuation of all water molecules of hydration, as demonstrated through single-crystal X-ray diffraction studies on the anhydrous phase 1a. This permanent microporosity renders interesting functionality to 1, such as selective adsorption of CO2 over N2 and remarkable activity as heterogeneous catalyst toward the H2O2-based oxidation of the highly-stable, tricyclic alkane adamantane. This work was funded by Eusko Jaurlaritza/Gobierno Vasco (Grant No. IT477-10), MINECO (Grant No. MAT2013-48366-C2-2-P), and UPV/EHU (Grant No. UFI11/53). J.A.-M. thanks Generalitat Valenciana and FEDER (Grant No. PROMETEO2/2014/010), as well as MICINN and Plan E (Grant No. CTQ2012-31762) for financial support.

Two new Melamine-Keggin hybrid nanorods, MELH-PMo12 and MELH-PW12, for the first time are successfully synthesized by two chemical methods, one-step solid-state chemical reaction method and experimental solvent diffusion method at room temperature. One-step solid-state reactions involve grounded nanorod materials along with grinding of row material and with solvent diffusion method these nanorods synthesized in single crystals form. These compounds crystallized in trigonal space groups P-3. Elemental analysis, infrared spectroscopy, UV spectroscopy and XRD analysis results prove that the samples still possess Keggin type structure. Transmittance electron microscopy and single-crystal X-ray have shown that the samples have nanorod structures.

A series of 14 isostructural [Cu(cyclam)](2)[{Cu(cyclam)}4{(a-GeW11O39)Ln(H2O)-(OAc)(12)].18H(2)O (1-Ln, where Ln = La Lu; cyclam = 1,4,8,11-tetraazacyclotetradecane) polyoxometalate-based hybrids reported herein represent (i) the first example of a twodimensional covalent hybrid lattice involving the [{(aXWI 1039)Ln(H2O)(OAc)}2in- archetype and (ii) the first structural characterization of such a dimeric polyoxotungste for Ln = La and Pr as well as for the combination of X = Ge and Ln = Ce, Nd, Sm, or Lu. Compounds 1-Ln have been characterized by elemental analyses, infrared spectroscopy, and thermogravimetric analysis, and their thermostructural behavior has been monitored by powder and single-crystal Xray diffraction. The title compounds undergo two single-crystal-to-single-crystal transformations triggered dehydration leading to the [{Cu(cydam)}6{(alpha-GeW11O39)Ln(H2O)(OAc)}2]-4H(2)O intermediate (2-Ln, where Ln = Eu or Er) and [Cu(cyclam)]0.5[{Cu(cyclam)}55{(alpha-GeW11O39)Ln(0Ac)}2] (3-Ln, where Ln = Ce or Eu) final anhydrous phases, the latter evidencing a coordinatively unsaturated derivative of the dimeric archetype for the first time. These transitions involve formation and disruption of Cu Opom bonds that result in different {Cu(cydam)}(2+) moieties grafting onto and being released from Keggin surfaces, which reduces the dimensionality of 1-Ln to one-dimensional covalent assemblies for 2-Ln and 3-Ln. While all 3-Ln phases rehydrate fully upon exposure to air for 24 h, the kinetics governing the crystal transitions back toward 1Ln through 2-Ln depend on the nature of Ln. Under ambient moisture, the anhydrous structures fully revert back to the parent framework for Ln = La Sm, while the samples containing Eu to Lu afford mixtures of 1-Ln and 2-Ln and require immersion in water for the structural reversion to reach completion. Single-crystal X-ray diffraction analyses of the rehydrated 1R-Ln samples (Ln = Ce, Eu, and Er) support these observations. This work was funded by Eusko Jaurlaritza/Gobierno Vasco (Grant PIBA2018-59 and ELKARTEK bG18 10(2018/00054), MINECO (Grant MAT2017-89553-P), and UPV/EHU (Grants PPG17/37 and GIU17/050). S.R. thanks Obra Social la Caixa, Fundacion Caja Navarra, and UPNA for a research contract in the framework of the program "Captación del Talento". Technical and human support provided by SGIker (UPV/EHU) is gratefully acknowledged.

The structures are reported of six racemic tris­(bipyrid­yl)iron salts with a range of 2-substituted-1,1,3,3-tetra­cyano­propenide anions, mostly also containing either perchlorate or tetra­fluorido­borate as co-anions. In three of the compounds the polynitrile anions are fully ordered, and in three others they are disordered, while the co-anion is also ordered in three compounds, but disordered in two others. Supra­molecular assemblies range from no continuous aggregation up to a three-dimensional hydrogen-bonded framework structure., Structures are reported for six closely related salts of tris­(bipyrid­yl)iron(II) cations, namely tris­(2,2′-bi­pyridine)­iron(II) bis­(1,1,3,3-tetra­cyano-2-meth­oxy­propenide) 0.776-hydrate, [Fe(C10H8N2)3](C8H3N4O)2.0.776H2O, (I), tris­(2,2′-bi­pyridine)­iron(II) 1,1,3,3-tetra­cyano-2-(propyl­sulfan­yl)propenide perchlor­ate, [Fe(C10H8N2)3](C10H7N4S)(ClO4), (II), tris­(5,5′-dimethyl-2,2′-bi­pyridine)­iron(II) 1,1,3,3-tetra­cyano-2-meth­oxy­propenide tetra­fluorido­borate ethanol 0.926-solvate, [Fe(C12H12N2)3](C8H3N4O)(BF4).0.926C2H2O, (III), tris­(5,5′-dimethyl-2,2′-bi­pyridine)­iron(II) 1,1,3,3-tetra­cyano-2-eth­oxy­propenide tetra­fluorido­borate, [Fe(C12H12N2)3](C9H5N4O)(BF4), (IV), tris­(5,5′-dimethyl-2,2′-bi­pyridine)­iron(II) 1,1,3,3-tetra­cyano-2-(ethyl­sufanyl)propenide tetra­fluorido­borate, [Fe(C12H12N2)3](C9H5N4S)(BF4), (V), and tris­(5,5′-dimethyl-2,2′-bi­pyri­dine)­iron(II) 1,1,3,3-tetra­cyano-2-prop­oxypropenide tetra­fluorido­borate, [Fe(C12H12N2)3](C10H7N4O)(BF4), (VI). In compound (I), one of the anions is disordered over two sets of atomic sites with equal occupancies while, in the second anion, just one of the C(CN)2 units is disordered, again over two sets of atomic sites with equal occupancies: the anionic components are linked by multiple C—H⋯N hydrogen bonds to form a three-dimensional framework. In compound (II), the polynitrile anion is disordered over two sets of atomic sites with occupancies in the approximate ratio 3:1, while the perchlorate anion is disordered over three sets of atomic sites: there are C—N⋯π inter­actions between the cations and the polynitrile anion. The polynitrile anion in compound (III) is fully ordered, but the tetra­fluorido­borate anion is disordered over two sets of atomic sites with occupancies 0.671 (4) and 0.329 (4): the cations and the tetra­fluorido­borate anions are linked by C—H⋯F inter­actions to form an inter­rupted chain. Compounds (IV) and (V) are isostructural and all of the ionic components are fully ordered in both of them: the cations and tetra­fluorido­borate anions are linked into C 2 2(12) chains. The polynitrile anion in compound (VI) is disordered over two sets of atomic sites with approximately equal occupancies, and here the chains formed by the cations and the tetra­fluorido­borate anions are of the C 2 2(13) type.

Polyoxometalates (POMs) have been revealed as interesting antitumor agents inhibiting the action of Sox2 transcription factor, which reduces the risk of metastasis during hormonal therapies. However, they have shown serious concerns to be incorporated into the cells due to their cytotoxicity. Taking this into consideration, this study aims to develop polyoxometalate-based nanocarriers to be potentially applied as new forms of anticancer therapies. Thus, the Wells-Dawson type [P2Mo18O62]6− phosphomolybdate was physically loaded into covalently crosslinked chitosan nanogels that can act as nanocarriers for local delivery. The obtained nanocomposites were extensively characterized by 31P-NMR, TEM microscopy, DLS and zeta potential measurements. This work revealed that selected chitosan nanocarriers would present great potential for POM delivery into tumoral cells due to their pH-triggered deliverability that inhibits cytotoxic drug release at physiological pH. Furthermore, the high uptakes values reported herein make prepared nanocomposites interesting candidates for future breast antitumoral treatments.

Herein we describe the preparation of hybrid polymer–inorganic interfaces by the immobilization of polyoxometalate nanoclusters on functionalized polymer surfaces. The polymeric surfaces were made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends by spin coating on a silicon wafer. The functionalization of the polymer film was obtained by interfacial migration of the amphiphilic block copolymer toward the interface upon water vapor annealing. The carboxylic acid functional groups contained in the PAA block were then employed to anchor the [LnIII(α-SiW11O39)]5− polyoxometalates (Ln: Ce, Er). This purpose was achieved by immersing the films in aqueous solutions of the in situ-formed inorganic nanoclusters. X-ray photoelectron and confocal Raman spectroscopies, together with atomic force microscopy, confirmed the immobilization of the inorganic species at the interface., This work has been financially supported by MINECO (grants MAT2010-17016 and MAT2010-21088-C03-01) and EJ/GV (IT477-10 and IT718-13; and ELKARTEK program (ACTIMAT)). The authors thank UPV/EHU for financial support (UFI11/53). S.A.-U. is indebted to UPV/EHU for her pre-doctoral fellowship. Technical and human support provided by SGIker (UPV/EHU) is gratefully acknowledged.

A new hybrid inorganic-organometallic salt has been obtained from the reaction of the Keggin-type silicotungstate anion with ferrocene in a water/methanol mixture as a result of the partial oxidation of ferrocene molecules to ferrocenium cations. Single-crystal X-ray diffraction analysis reveals the presence of four ferrocenium (FeIII) cations and one ferrocene (FeII) molecule per plenary Keggin anion in the crystal structure of [FeIII (Cp)2]4[SiW12O40]&middot, [FeII(Cp)2]&middot, 2CH3OH (1). Compound 1 thus constitutes the first example in the literature in which ferrocenium and ferrocene species coexist in the structure of a polyoxometalate-based salt. The two crystallographically independent ferrocenium species in the asymmetric unit of 1 exhibit different configurations: One displays an eclipsed conformation with ideal D5h symmetry, whereas the conformation in the other one is staggered D5d. The crystal packing of 1 can be best described as an organometallic sub-lattice of ferrocenium and ferrocene species linked by a network of &pi, &pi, interactions that generates rectangular cavities of about 14 &times, 10 &Aring, in which strings of Keggin anions and methanol molecules are hosted, further connected to each other via weak OPOM&middot, &middot, CMeOH-OMeOH&middot, OPOM type hydrogen bonds. The charge-transfer nature of the salt has been studied by solid-state diffuse reflectance UV-Vis spectroscopy and the presence of magnetically isolated FeIII/FeII centres has been confirmed by M&ouml, ssbauer spectroscopy. A topological study carried out on all of the pristine ferrocenyl species deposited in the Cambridge Structural Database (CSD) has allowed two main conclusions to be drawn: (1) these species tend to adopt extreme conformations (either eclipsed or staggered) with less than a 15% of examples showing intermediate states and (2) the oxidation state of the iron centres can be unequivocally assigned on the basis of a close inspection of the Fe&middot, Cp distances, which allows ferrocene neutral molecules and ferrocenium cations to be easily distinguished.

A new organic-inorganic compound based on octamolybdate building blocks and copper(II) complexes of tetradentate N-donor ligands has been hydrothermally synthesized, namely [Cu(cyclam)](2)[Mo8O26]center dot 1.5H(2)O (1), (cyclam: 1,4,8,11-tetraazacyclotetradecane). Compound 1 has been chemically (elemental and thermal analyses), spectroscopically (infrared spectroscopy), and structurally (single crystal and powder X-ray diffraction) characterized. The crystal packing of 1 shows a covalent framework structure formed by [Mo8O26](n)(4n-) chains running along the 100 direction which are linked to each other through the coordination spheres of {Cu(cyclam)}(2+) complexes leading to a three-dimensional open network. This structural assembly generates voids that can be described as a succession of cavities communicated through narrow bottlenecks with approximate cross section of 4 x 7 angstrom(2) where the hydration water molecules are hosted. The robust open structure of 1 remains virtually unaltered upon thermal evacuation of guest solvent molecules at 130 degrees C, resulting in the anhydrous phase [Cu(cyclam)](2)[Mo8O26] (1a) with potentially accessible micropores as demonstrated by single-crystal X-ray diffraction measurements. Electron paramagnetic resonance spectroscopy analysis of 1 has also been assessed. This work was funded by UPV/EHU (Grant PPG17/37 and GIU17/050) and University of Tunis El Manar (predoctoral fellowship to N.D., including a three-month stay at Universidad del Pais Vasco, UPV/EHU). B.A. thanks UPV/EHU for a postdoctoral fellowship within the program "Convocatorias de Ayudas para la Especializacion de Personal Investigador". Technical and human support provided by SGIker (UPV/EHU) is gratefully acknowledged.

New organophosphonate derivatives of monovacant Keggin-type polyoxotungstates that contain naphthyl groups have been synthesized and characterized in both solid state and solution. Single-crystal structural analysis shows that two phosphonate groups occupy the vacant position of the lacunary cluster unit in the isostructural compounds [N(C4H9)(4)](3)[H(POC11H9)(2)(-HBW11O39)] (TBA-1) and [N(C4H9)(4)](3)[H(POC11H9)(2)(-SiW11O39)] (TBA-2). Liquid-solution UV-Vis transmittance and solid-state diffuse reflectance spectroscopy studies reveal the presence of a new absorption band in the visible region, the charge transfer character of which has been further confirmed by time-dependent density functional theory (TD-DFT) calculations. The latter evidence that the charge transfer process is dominated by transitions from the highest occupied molecular orbital (HOMO), localized in the aromatic ring of the organic group, to the lowest unoccupied molecular orbital (LUMO), localized in the Keggin anion. Photoluminescence studies show that the fluorescent properties of the 1-naphthylmethylphosphonate group are quenched upon its incorporation into the inorganic oxo-tungstate skeleton. The solution stability of the hybrid clusters has been evaluated by a combination of H-1-, C-13- and P-31-Nuclear Magnetic Resonance spectroscopy and Electrospray Ionization-Mass Spectrometry. The hybrid polyanion [H(POC11H9)(2)(-HBW11O39)](3-) (1) herein constitutes the first structurally characterized organo-p-block containing borotungstate, and hence it confirms that this strategy for the organic functionalization of polyoxometalate clusters can be applied to new platforms belonging to the family of group-13 heteropolyoxotungstates.

Eleven dimetallic Zn(II)-Ln(III) complexes of the general formula [Zn(µ-L)(µ-OAc)Ln(NO3)2]·CH3CN (Ln(III) = Pr (1), Nd (2), Sm (3), Eu (4), Gd (5), Tb (6), Dy (7), Ho (8), Er (9), Tm (10), Yb (11)) have been prepared in a one-pot reaction from the compartmental ligand N,N'-dimethyl-N,N'-bis(2-hydroxy-3-formyl-5-bromo-benzyl)ethylenediamine (H2L). In all these complexes, the Zn(II) ions occupy the internal N2O2 site whereas the Ln(III) ions show preference for the O4 external site. Both metallic ions are bridged by an acetate bridge, giving rise to triple mixed diphenoxido/acetate bridged Zn(II)Ln(III) compounds. The Nd, Dy, Er and Yb complexes exhibit field induced single-ion magnet (SIM) behaviour, with Ueff values ranging from 14.12 to 41.55 K. The Er complex shows two relaxation processes, but only the second relaxation process with an energy barrier of 21.0 K has been characterized. The chromophoric L(2-) ligand is able to act as an "antenna" group, sensitizing the near-infrared (NIR) Nd(III) and Yb(III)-based luminescence in complexes 2 and 11 and therefore, both compounds can be considered as magneto-luminescent materials. In addition, the Sm(III), Eu(III) and Tb(III) derivatives exhibit characteristic emissions in the visible region.

A series of nine [Sb7W36O133Ln3M2(OAc)(H2O)8](17-) heterometallic anions (Ln3M2; Ln=La-Gd, M=Co; Ln=Ce, M=Ni and Zn) have been obtained by reacting 3 d metal disubstituted Krebs-type tungstoantimonates(III) with early lanthanides. Their unique tetrameric structure contains a novel {MW9O33} capping unit formed by a planar {MW6O24} fragment to which three {WO2} groups are condensed to form a tungstate skeleton identical to that of a hypothetical trilacunary derivative of the ɛ-Keggin cluster. It is shown, for the first time, that classical Anderson-Evans {MW6O24} anions can act as building blocks to construct purely inorganic large frameworks. Unprecedented reactivity in the outer ring of these disk-shaped species is also revealed. The Ln3M2 anions possess chirality owing to a {Sb4O4} cluster being encapsulated in left- or right-handed orientations. Their ability to self-associate in blackberry-type vesicles in solution has been assessed for the Ce3Co2 derivative.