When Identification of the Reduction Sites in Mixed Molybdenum/Tungsten Keggin-Type Polyoxometalate Hybrids Turns Out Tricky

The mixed molybdenum/tungsten Keggin-type polyoxometalate (POM) hybrid (TBA)₄[PW₉Mo₂O₃₉{Sn(C₆H₄I)}] (TBA = tert-butylammonium) has been prepared by the reaction between [α-PW₉Mo₂O₃₉]^7– and [Cl₃Sn(C₆H₄I)] in dried acetonitrile, in the presence of tetra-n-butylammonium bromide. A further coupling reaction affords the ferrocenyl derivative (TBA)₄[PW₉Mo₂O₃₉{Sn(C₆H₄)C≡C(C₆H₄)Fc}]. The POM hybrids have been thoroughly characterized by NMR and IR spectroscopies. Electrochemical analysis confirms their ease of reduction compared to the all-W analogue, albeit with a second reduction process occurring at a lower potential than in the all-Mo species. It is noteworthy that the second reduction is accompanied by an unusual red shift of the electronic absorption spectrum. Whereas there is no doubt that the first reduction deals with Mo, the location of the second electron in the bireduced species, on the second Mo or on W, has thus been the subject of a cross-investigation by spectroelectrochemistry, electron spin resonance, and theoretical calculations. Finally, it came out that the second reduction is also Mo-centered with two unpaired and antiferromagnetically coupled extra electrons.

The sites for the successive reduction processes of a mixed molybdenum/tungsten Keggin-type organotin hybrid are debated through a combination of spectroelectrochemical investigations and theoretical calculations.

Single ion magnets based on lanthanoid polyoxomolybdate complexes

Organic solubility of these POMs enhances the chemical processability into molecular spintronic devices.

Hedgehog-shaped {Mo368} cluster: unique electronic/structural properties, surfactant encapsulation and related self-assembly into vesicles and films

The hedgehog-shaped {Mo368} cluster shows unique electronic (extremely high extinction coefficient) and structural features, especially regarding its size, the high number of delocalized electrons which allows to measure the surface enhanced Raman scattering (SERS) spectrum and the option for coordination chemistry inside the cavity. Its relative instability in aqueous solution can be overcome by embedment in a hydrophobic shell of dimethyldioctadecylammonium cations. The resulting hybrid self-assembles into spherical vesicles in acetone-water mixtures, according to a process directed by hydrophobic-hydrophilic interactions. It also forms rather stable Langmuir monolayers while a second layer evolves under higher surface pressure, in accordance with a rather low alkyl surface density.

A unique fluoride nanocontainer: porous molecular capsules can accommodate an unusually high number of "rather labile" fluoride anions

The present work refers to the challenging issue of fluoride anion recognition/binding in water by taking advantage of the unique possibilities offered by the porous molecular nanocontainers of the {Mo132} Keplerate type allowing the study of a variety of new phenomena. Reaction of the highly reactive carbonate-type capsule with aqueous HF results in the release of carbon dioxide and integration of an unprecedentedly large number of fluoride anions--partly as coordinated ligands at both the pentagonal units and the linkers, partly as a disordered water/fluoride assembly inside the cavity. The internal assembly and some of the fluoride ligands are easily released, which provides interesting options for future studies regarding coordination chemistry and catalysis under confined conditions.

Synthesis, crystal structure and magnetism of new salicylamidoxime-based hexanuclear manganese(III) single-molecule magnets

Salicylamidoxime was used to synthesize 13 new polynuclear Mn(III) complexes. We present the crystallographic structures, the magnetic susceptibility and the magnetization measurements of eight of them (1-8) with the general formula [Mn(6)O(2)(H(2)N-sao)(6)(L)(2)(solvent)(4-6)] (L = carboxylate, chloride, 2-cyanophenolate; solvent = H(2)O, MeOH, EtOH, py). These complexes consist of two trinuclear {Mn(III)(3)(μ(3)-O)(H(2)N-sao)(3)}(+) cationic units linked together via two oximate and two phenolate oxygen atoms. All behave as single-molecule magnets, with the spin ground state varying from 4 to 12 and anisotropy energy barriers from 24 to 86 K, the latter being as high as the present record barrier in the Mn(6) complexes. DFT calculations were performed to compute the exchange magnetic coupling constants J between the metallic ions and to provide an orbital interpretation of exchange. Our results are in line with previously reported results with the parent salicylaldoxime derivatives. The Mn-N-O-Mn torsion angle appears as the main parameter controlling the J values. The critical angle where the exchange coupling between two Mn(III) switches from antiferromagnetic to ferromagnetic is 27°, less than the one found in related complexes with salicylaldoxime (30°). We propose a structural classification of the {Mn(6)} complexes in four classes depending on the coordination of the axial carboxylate. The work points out the structural flexibility of such systems, their sensitivity to solvent effects and their ability to achieve high anisotropy energy barriers by simple desolvation.

Functionalization and post-functionalization: a step towards polyoxometalate-based materials

Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Reduced molybenum-oxide-based core-shell hybrids: "blue" electrons are delocalized on the shell

The present study refers to a variety of reduced metal-oxide core-shell hybrids, which are unique with regard to their electronic structure, their geometry, and their formation. They contain spherical {Mo72Fe30} Keplerate-type shells encapsulating Keggin-type polyoxomolybdates based on very weak interactions. Studies on the encapsulation of molybdosilicate as well as on the earlier reported molybdophosphate, coupled with the use of several physical methods for the characterization led to unprecedented results (see title). Upon standing in air at room temperature, acidified aqueous solutions obtained by dissolving sodium molybdate, iron(II) chloride, acetic acid, and molybdosilicic acid led to the precipitation of monoclinic greenish crystals (1). A rhombohedral variant (2) has also been observed. Upon drying at room temperature, compound 3 with a layer structure was obtained from 1 in a solid-state reaction based on cross-linking of the shells. The compounds 1, 2, and 3 have been characterized by a combination of methods including single-crystal X-ray crystallography, magnetic studies, as well as IR, Mössbauer, (resonance) Raman, and electronic absorption spectroscopy. In connection with detailed studies of the guest-free two-electron-reduced {Mo72Fe30}-type Keplerate (4) and of the previously reported molybdophosphate-based hybrids (including 31P NMR spectroscopy results), it is unambiguously proved that 1, 2, and 3 contain non-reduced Keggin ion cores and reduced {Mo72Fe30}-type shells. The results are discussed in terms of redox considerations (the shell as well as the core can be reduced) including those related to the reduction of "molybdates" by FeII being of interdisciplinary including catalytic interest (the MoVI/MoV and FeIII/FeII couples have very close redox potentials!), while also referring to the special formation of the hybrids based on chemical Darwinism.

Identification of polyoxometalates as nanomolar noncompetitive inhibitors of protein kinase CK2

Protein kinase CK2 is a multifunctional kinase of medical importance that is dysregulated in many cancers. In this study, polyoxometalates were identified as original CK2 inhibitors. [P2Mo18O62](6-) has the most potent activity. It inhibits the kinase in the nanomolar range by targeting key structural elements located outside the ATP- and peptide substrate-binding sites. Several polyoxometalate derivatives exhibit strong inhibitory efficiency, with IC50 values < or = 10 nM. Furthermore, these inorganic compounds show a striking specificity for CK2 when tested in a panel of 29 kinases. Therefore, polyoxometalates are effective CK2 inhibitors in terms of both efficiency and selectivity and represent nonclassical kinase inhibitors that interact with CK2 in a unique way. This binding mode may provide an exploitable mechanism for developing potent drugs with desirable properties, such as enhanced selectivity relative to ATP-mimetic inhibitors.

Efficient Preparation of Functionalized Hybrid Organic/Inorganic Wells−Dawson-type Polyoxotungstates

Hybrid organic/inorganic Wells-Dawson polyoxotungstates have been prepared through addition of functionalized tricholorostannanes to lacunary alpha(2)- and alpha(1)-[P(2)W(17)O(61)](10-). Coupling of amines and alcohols to polyoxotungstate platforms led to new structures in good yields. Coupling of chiral amines to the previously unknown organotin-substituted alpha(1) derivatives allowed the isolation of diastereomers, which feature in some cases split (1)H, (13)C, and (31)P NMR spectra. This is the first example of NMR observation of a single pair of diastereomers in the alpha(1)-Wells-Dawson series. It opens the way to potential resolution of those chiral polyoxotungstates.

Synthesis, Characterization, and Photochemical Behavior of {Ru(arene)}2+ Derivatives of α-[PW11O39]7-: An Organometallic Way to Ruthenium-Substituted Heteropolytungstates

Reaction of [Ru(arene)Cl(2)](2) (arene = benzene, toluene, p-cymene, hexamethylbenzene) with K(7)[PW(11)O(39)].14H(2)O provided two series of organometallic derivatives of heteropolytungstates: type-1 and type-2 complexes of general formulas [PW(11)O(39){Ru(arene)(H(2)O)}](5-) and [{PW(11)O(39){Ru(arene)}}(2){WO(2)}](8-), respectively. All compounds were characterized by infrared and multinuclear NMR ((1)H, (31)P, (183)W) spectroscopies. The crystal structures of Na(4)K(4)[{PW(11)O(39){Ru(benzene)}}(2){WO(2)}].6H(2)O (NaK-2a.6H(2)O), K(7)H[{PW(11)O(39){Ru(toluene)}}(2){WO(2)}].4H(2)O (K-2b.4H(2)O), and Cs(3)K(2)[PW(11)O(39){Ru(p-cymene)(H(2)O)}].4H(2)O (CsK-1c.4H(2)O) were obtained and revealed that the {Ru(arene)} fragment is supported on the oxometallic framework. Photochemical reactivity of [PW(11)O(39){Ru(arene)(H(2)O)}](5-) (arene = toluene, p-cymene) in the presence of various ligands L (L = H(2)O, dimethyl sulfoxide, tetramethylene sulfoxide, and diphenyl sulfoxide) was investigated, and led to the formation of [PW(11)O(39){Ru(L)}](5-), in which the ruthenium is incorporated into the lacunary [PW(11)O(39)](7-) anion.

Coordination chemistry of the hexavacant tungstophosphate [H2P2W12O48]12–: synthesis and characterization of iron(iii) complexes derived from the unprecedented {P2W14O54} fragment

Clusters which display the rare cubic Fe8 topology have been obtained by reaction of the metastable hexavacant polyoxotungstate [H2P2W12O48]12- with basic trinuclear metal acetates.

Synthesis and Characterization of the First Carbene Derivative of a Polyoxometalate

Reaction of the Keggin-type polyanion [PW11O39]7- with the tetrakis-carbene ruthenium precursor [RuLMe4Cl2] (LMe = 1,3-dimethylimidazolidine-2-ylidene), in water, results in the formation of Na4K9[(PW9O34)2(cis-WO2)(cis-RuLMe2)].23H2O, which is the first carbene derivative of a polyoxometalate. The oxidation state of the ruthenium is confirmed by XANES experiments.