Bernold Hasenknopf

Polyoxometalates: introduction to a class of inorganic compounds and their biomedical applications.

An increasing number of potential applications for polyoxometalates in human medicine have been reported in the literature. These inorganic complexes are composed of early transition metals (mainly molybdenum, tungsten and vanadium) and oxygen. The present review gives an introduction into the chemistry of these compounds, and an overview of the principal studies of their biological and biochemical effects and their therapeutic potential. The reported antitumoral and antibiotic properties of molybdates and tungstates in vitro and in vivo are compiled and discussed, as are their influences on the blood glucose level in diabetic animals. Aspects of antiviral activities and cell penetration are treated.

Anderson-Type Heteropolymolybdates Containing Tris(alkoxo) Ligands: Synthesis and Structural Characterization

Anderson-type molybdopolyanions containing tris(alkoxo) ligands [MMo6O18{(OCH2)3CR}2]3− (M = MnIII, FeIII) and [H2MMo6O18{(OCH2)3CR}2]2− (M = NiII, ZnII), (R = CH3, NO2, CH2OH), were prepared by treatment of [N(C4H9)4]4[α-Mo8O26] with tris(hydroxymethyl)methane derivatives in the presence of manganese(III) acetylacetonate, iron(III) acetylacetonate, nickel(II) acetate, or zinc(II) acetate. The complexes were structurally characterized in solution, and also by single-crystal X-ray diffraction in the cases of [N(C4H9)4]3[MnMo6O18{(OCH2)3CNO2}2], [N(C4H9)4]2[H2NiMo6O18{(OCH2)3CCH2OH}2], and [N(C4H9)4]2[H2ZnMo6O18{(OCH2)3CCH3}2]. Two tris(alkoxo) ligands replace the six hydroxo groups usually found in Anderson polyanions of formula [H6MMo6O24]n−. The complex structures may be divided into two groups: In the first one the tris(alkoxo) ligands are bound entirely to the central heteroatom, while in the second one they cap a tetrahedral cavity. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Kinetic and Thermodynamic Control in Self-Assembly: Sequential Formation of Linear and Circular Helicates

NMR and MS analysis as a function of time has shown that the self-assembly of a linear ligand with Fe2+ or Ni2+ , metal ions of octahedral coordination geometry, generates first a triple helicate and thereafter the circular helicate 1 as kinetic and thermodynamic products, respectively. The results provide insight into features of the energy hypersurface on which this self-assembly operates and point to the general role of kinetic and thermodynamic control in such processes.

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.

Chiral Recognition of Hybrid Metal Oxide by Peptides

Resolution of gram quantities of a subtly chiral polyoxometalate (POM; the chirality originates from substitution of one metal in the nanosized framework) is possible by reaction with small peptides. Functionalizable acyl polyoxotungstate (TBA)(6)[alpha(1)-P(2)W(17)O(61){SnCH(2)CH(2)C(=O)}] (see picture) can be resolved by kinetic resolution. This approach paves the way for applications of chiral POMs ranging from asymmetric catalysis to their bioactivity.

Synthesis and photocatalytic properties of mixed polyoxometalate-porphyrin copolymers obtained from Anderson-type polyoxomolybdates.

Hybrid polyoxometalate-porphyrin copolymeric films are obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) and zinc 5,15-dipyridinium octaethylporphyrin (5,15-ZnOEP(py)(2)(2+)) in the presence of the polyoxometalate [MnMo(6)O(18){(OCH(2))(3)CNHCO(4-C(5)H(4)N)}(2)](3-) (Py-POM-Py). These films allow the photocatalytic reduction of Ag(I)(2)SO(4) under visible irradiation in air in the presence of propan-2-ol at the 2D interface between water and the copolymeric films. The formation of metallic Ag(0) nanowires and triangular nanosheets is observed.

Chemoselective Catalysis with Organosoluble Lewis Acidic Polyoxotungstates

The preparation of new organosoluble Lewis acidic polyoxometalates (POMs) is reported. These complexes were prepared by the incorporation of Zr, Sc, and Y atoms into the corresponding monolacunary Dawson [P(2)W(17)O(61)](10-) and Keggin [PW(11)O(39)](7-) polyoxotungstates. The catalytic activity of these compounds was evaluated for C-C bond formation in the Diels-Alder, Mannich, and Mukaiyama-type reactions. Comparisons with previously described Lewis acidic POMs are reported. Competitive reactions between imines and aldehydes or between various imines demonstrated that fine tuning of the reactivity could be reached by varying the metal atom incorporated into the polyanionic framework. A series of experiments that employed pyridine derivatives allowed us to distinguish between the Lewis and induced Brønsted acidity of the POMs. These catalysts activate imines in a Lewis acidic way, whereas aldehydes are activated by indirect Brønsted catalysis.

Regioselective Activation of Oxo Ligands in Functionalized Dawson Polyoxotungstates

The organic side chain of tin-substituted Dawson polyoxotungstates alpha1- and alpha2-[P2W17O61{SnCH2CH2COOH}]7- can be used to direct regioselective acylations of oxo ligands in the inorganic backbone, which was examined both experimentally and computationally. Acylation of the oxo ligand gave exalted electrophilicity to the acyl moiety, and the compounds that were obtained led to direct ligation of POMs to complex organic molecules.

Discrete Covalent Organic–Inorganic Hybrids: Terpyridine Functionalized Polyoxometalates Obtained by a Modular Strategy and Their Metal Complexation

The rational design and synthesis of organic-inorganic hybrids as functional molecular materials relies on both the careful conception of building-blocks and the strategy for their assembly. Three families of trialkoxo polyoxometalates (Lindqvist 2, Anderson 3, Dawson 4) grafted with remote terpyridine coordination sites have been synthesized to extend the available building-blocks. These new units can be combined with metal complexes that play a role as (i) chromophores toward charge-separated systems in light-harvesting devices and (ii) coordination motifs for metal-directed self-assembly toward multifunctional molecular hybrid materials. The X-ray crystal structures of polyoxometalate-terpyridine hybrids indicate distances of 21 Å and 19 Å between the two terpyridyl coordination sites in 2 and 3, respectively, with angles between the coordination vectors of 180° and 177.4°, respectively. Lindqvist 2 displays a reduction at -0.52 V vs SCE while Anderson 3 exhibits one reversible oxidation attributed to Mn(III)/Mn(IV) (+0.75 V vs SCE) and a broad wave at -1.28 V vs SCE assigned to the Mn(III)/Mn(II) reduction. Dawson 4 displays several processes on a wide range of potentials (+0.5 to -2.0 V vs SCE) centered on V(V), W(VI) and the organic ligand in order of decreasing potentials. The grafted terpyridine ligands in Anderson 3 and Dawson 4 were successfully coordinated to {PdCl}(+) and {RuCl(3)} moieties, respectively. The polyoxometalates and transition metal complexes retain their intrinsic properties in the final assemblies.

Insertion of Amides into a Polyoxometalate

POM alert: The incorporation of an amide oxygen atom into the framework of the Dawson-type polyoxometalate (POM) cluster [P(2)V(3)W(15)O(62)](9-) (see picture) allows the communication of electronic effects between the organic and the inorganic parts of the molecule, including fine-tuning of the redox properties of the entire hybrid POM by the organic components, and transmission of the POMs electron-attracting properties to the organic moiety.