Polyoxometalate Anions Research Articles

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Abstract Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo12O403–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H2 is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H2 gas feeds (0, 10, 50, and 100 ppm CO in H2). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H2 gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance Rp appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H2 by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H2 fuel gases.

Assembly of a metal–organic framework by sextuple intercatenation of discrete adamantane-like cages

Metal-organic frameworks form a unique class of multifunctional hybrid materials and have myriad applications, including gas storage and catalysis. Their structure is usually achieved through the infinite coordination of metal ions and multidentate organic ligands by means of strong covalent bonds. Threaded molecules such as catenanes and rotaxanes have largely been restricted to comprising components of two-dimensional interlocking rings or polygons. There are very few examples of the catenation of polyhedral cages. Although it has been postulated that the infinite extended architecture can be obtained from the polycatenation of a discrete cage based on such threading, this has not been documented to date. Here we describe an infinite three-dimensional metal-organic framework composed of catenated polyhedral cages, in which the framework is achieved by mechanical interlocking of all of the vertices of the cages. The three-dimensional polycatenated framework shows twofold self-interpenetration in its crystal packing. The penetration of polycatenanes creates nanosized voids into which the Keggin polyoxometalate anions are perfectly accommodated as counteranions.

Detailed voltammetric and EPR study of protonation reactions accompanying the one-electron reduction of Keggin-type polyoxometalates, [XVVM11O40]4− (X = P, As; M = Mo, W) in acetonitrile

The one electron electrochemical reduction of vanadium(V) substituted-Keggin type polyoxometalate anions [XV(V)M(11)O(40)](4-) (XV(V)M(11)) where X = P, As; M = Mo or W has been investigated in CH(3)CN as a function of acid concentration. EPR studies confirm that the product is the highly basic and hence readily protonated V(IV), (XV(IV)M(11)), species. In the absence of acid or in the presence of a large concentration excess, the V(V/IV) redox couple gives one well defined chemically and electrochemically reversible process under conditions of cyclic voltammetry. In contrast, three well resolved V(V/IV) processes are observed in the presence of a moderate concentration of acid in the case of XV(V)Mo(11) but only two with XV(V)W(11). NMR and EPR spectra have been obtained as a function of acid concentration when the polyoxometalate anions are in the V(V) and V(IV) oxidation levels respectively. All voltammetric and spectroscopic data indicate that protonation reactions are coupled to the V(V) and V(IV) redox chemistry but that the reduced V(IV) state is much more basic than the V(V) one and that (XV(IV)Mo(11)) is more basic than (XV(IV)W(11)). Digital simulations of voltammograms for reduction of XV(V)Mo(11) and XV(V)W(11) have been undertaken in CH(3)CN as a function of acid concentration in order to define the thermodynamics and kinetics associated with the V(V/IV) process and the equilibrium constants that accompany the coupled acid-base chemistry. EPR spectra allow an estimation of the relative concentrations of protonated species present in frozen glasses derived from one-electron bulk electrolysis and also allow inferences to be drawn on their nature. This study provides a far more detailed analysis of the coupling of proton and electron transfer than previously available.

A 3D organic–inorganic network constructed from an Anderson-type polyoxometalate anion, a copper complex and a tetrameric [Na4(H2O)14]4+ cluster,

A rare three-dimensional organic–inorganic compound based on the Anderson-type polyoxometalate anion, [Na4(H2O)14Cu(PDA)2]H[Al(OH)6Mo6O18]·5H2O (H2PDA = pyrazine-2,3-dicarboxylic acid) (1) has been synthesized, in which each Anderson polyoxoanion [Al(OH)6Mo6O18]3− acts as a decadentate ligand, connecting four tetrameric [Na4(H2O)14]4+ clusters and two [Cu(PDA)2]2− coordination ions through terminal oxygen atoms.

Cations in control: crystal engineering polyoxometalate clusters using cation directed self-assembly

The synthetic engineering of anionic polyoxometalate (POM) clusters with predefined properties tailored to specific applications is a great challenge using routine "one-pot" POM syntheses. Under such conditions, difficulties often arise from the multitude of complex reaction pathways and self-assembly processes occurring in solution. In this respect the major role of the charge balancing cations cannot be ignored, in fact such cations are crucial, both in the assembly of the building blocks, linkage to the overall cluster, and then assembly into the bulk material. Further, the role of the cation facilitating the selective crystallization of a particular cluster type cannot be divorced from the reaction process since the crystallization process itself can help pull "virtual" building blocks into being. This perspective briefly outlines our efforts towards engineering novel POM based materials, highlighting the use of large organic cations as "Shrink-wrapping" agents to isolate new POM clusters, frameworks and cage compounds. Central to this perspective is the hypothesis that, in the case of POM cluster assembly, the mechanism and various equilibria which define the clusters can be controlled by the selective crystallisation using cation control. Consequently, this indicates that the process of crystallisation can have a profound effect on self-assembly at the molecular level. We therefore propose that the crystallization process itself may define the molecular structure of the cluster leading to the conundrum, which came first, the cluster or the crystal of the cluster?

Acid-Induced Surface Functionalization of Polyoxometalate by Enclosure in a Polyhedral Silver−Alkynyl Cage

Surface functionalization of polyoxometalate anions with silver(I) tert-butylethynide in an organic solution medium has been achieved for the first time using an acid-induced facile approach. The generated neutral hybrid monomeric complexes [Ag(40)(C[triple bond]C(t)Bu)(22)(CF(3)COO)(12)(V(10)O(28))].4CH(3)OH (1) and [Ag(40)(C[triple bond]C(t)Bu)(20)(CF(3)COO)(12)(Mo(6)O(22))].2CH(3)OH (2) constitute members of a new silver(I)-polyoxometalate family in which the polyoxometalate anion is embedded in a polyhedral silver(I) cluster assembled by multiple argentophilic Ag...Ag interactions.

Molecular Conductors Based on the Mixed-Valence Polyoxometalates [SMo12O40]n− (n = 3 and 4) and the Organic Donors Bis(ethylenedithio)tetrathiafulvalene and Bis(ethylenedithio)tetraselenafulvalene

The synthesis, crystal structure, and physical characterization of two new radical salts formed by the organic donors bis(ethylenedithio)tetrathiafulvalene (ET) and bis(ethylenediseleno)tetrathiafulvalene (BETS) and the Keggin polyoxometalate (POM) [SMo(12)O(40)](n-) are reported. The salts isolated are ET(8)[SMo(12)O(40)] x 10 H(2)O (1) (crystal data: (1) monoclinic, space group I2/m with a = 13.9300(10) A, b = 43.467(3) A, c = 13.9929(13) A, beta = 107.979(6) degrees, V = 8058.9(11) A(3), Z = 2) and BETS(8)[SMo(12)O(40)] x 10 H(2)O (2) (crystal data: monoclinic, space group I2/m with a = 14.0878(2) A, b = 44.1010(6) A, c = 14.0930(2) A, beta = 106.739(3) degrees, V = 8384.8 A(3), Z = 2). Both compounds are isostructural and consist of alternating layers of the organic donors (with an alpha or theta(42+40) packing mode) and POM anions. The structural data, as well as the magnetic susceptibility, ESR measurements, and band structure calculations, indicate that the Keggin POMs have been reduced by one electron in 1 and by two electrons in 2, leading to the POMs [SMo(12)O(40)](3-) and [SMo(12)O(40)](4-) in 1 and 2, respectively. At ambient pressure 1 is a classical semiconductor with a room temperature conductivity of about 1 S cm(-1) and an activation energy of about 130 meV. Salt 2 also exhibits an activated behavior although it does not follow a classical semiconducting regime. The conductivity of 2 under applied pressure shows an enhanced conductivity, in agreement with the analysis of the electronic structure, although no metallic behavior is detected below about 10 kbar.

Photoluminescent Honeycomb Films Templated by Microwater Droplets

The europium polyoxometalate anion, [EuW(10)O(36)](9-), can be transferred from an aqueous phase into a chloroform phase through encapsulation by dioctadecyldimethylammonium (DODMA(+)). The formed (DODMA)(9)[EuW(10)O(36)] complexes can self-assemble into ordered, uniform porous honeycomb films by using a simple solvent-evaporation method at the air/water interface without any extra moist airflow. TEM, SEM, and AFM observations show porous morphologies with pores having a diameter of about 2 microm and a wall depth of about 0.8 microm. The microlamellar structure and crystalline nanoaggregates of (DODMA)(9)[EuW(10)O(36)] complexes in films are characterized by XRD measurements and high-resolution TEM observations. During self-assembly into porous honeycomb films, it is speculated that the cooled microwater droplets that are induced by the quick evaporation of chloroform act as the templates for pores and that (DODMA)(9)[EuW(10)O(36)] complexes are deposited around pores. Because of the intrinsic fluorescence of [EuW(10)O(36)], the photoluminescent porous honeycomb films of (DODMA)(9)[EuW(10)O(36)] complexes can emit fluorescence when they are excited by UV light. It is expected that this will meet more requirements of new materials for fluorescence, separation membranes, microstructured electrode surfaces, containers, and reactors.

Molecular assemblies based on the template-directing effect of anionic polyoxometalate clusters and organic cationic flexibility

Molecular assembly processes by utilizing the template-directing effect of anionic polyoxometalate clusters and the flexible organic cation have achieved three hybrids: (H 2bpp)(Hbpp)[PMo 12O 40]·3DMF ( 1), (H 2bpp)(Hbpp)(bpp) 2 [PMo 9V 3O 40(VO) 2] 2 ( 2), and (H 2bpp) 2[ β-Mo 8O 26] ( 3) (bpp = 1,3-bis(4-pyridyl)propane). Three compounds were characterized using single crystal X-ray crystallography, elemental analysis, IR, XPS, EPR, voltammetric behavior and TGA. Crystal structural analyses revealed that compounds 1– 3 were all constructed from protonated organic bpp cations with different POM clusters: isolated α-Keggin P–Mo cluster in 1; dimer of bi-capped α-Keggin P–Mo–V anions linked through a {V 2O 2} unit in 2; β-octamolybdate isopolyanion in 3, respectively. All three assemblies demonstrated a higher thermal stability. The protonated bpp cations lost at temperature higher than 300 °C that the strong intermolecular interactions may account for the high initial temperature of weight loss. The electrochemistry property of compound 2 modified carbon paste electrode was also studied in 1 M H 2SO 4 aqueous.

Field emission of vertically aligned V 2O 5 nanowires on an ITO surface prepared with gaseous transport

Growing V 2O 5 nanowires (NWs) on a conducting glass substrate combines gaseous transport and pyrolytic deposition of vanadium polyoxometalate anions, and yields vertically aligned vanadium-oxide nanowires. Scanning electron and transmission electron microscopy, selected-area electron diffraction, Raman spectra and powder X-ray analyses indicate that V 2O 5 nanowires as synthesized were single-crystalline and grew anisotropically among direction [010]. NH 2OH·HCl served not only as a reducing agent to produce vanadium polyoxometalate clusters but also as a source of NH 3 gas to facilitate the vapor pyrolysis and deposition. The optical properties of V 2O 5 nanowires exhibit a character dependent on structure. Field emission (FE) measurements show a small turn-on field voltage ~8.3 V/μm, maximum current density 1.8 mA/cm 2, and a linear Fowler–Nordheim behavior.

Syntheses, crystal structures and electrochemical properties of three organic-inorganic hybrid supramolecular compounds based on copper-complex fragments and different polyoxometalates

Three new organic–inorganic hybrid compounds H[CuI(dafo)2]3{[CuI(dafo)2]2P2W18O62} · H2O (1), [CuI(dafo)2]3PMo12O40 (2) and [CuI(dafo)2]4SiW12O40 (3) (dafo = 4, 5-diazafluoren-9-one) have been hydrothermally synthesized and characterized by IR, TG and CV. Compound 1 exhibits a 3D supramolecular framework based on weak interactions between bisupported Dawson structure P2W18O626− units and copper-complex fragments. With the aid of π–π stacking and hydrogen bonding interactions, a 1D supramolecular framework accommodating polyoxometalate (POM) anions is generated in 2. Compound 3 has similar fundamental units as 2, but the units are assembled in a different way. The distinction of their structures indicates that the POMs are the key factor in determining the frameworks of the final products. The electrochemical properties of these compounds have been studied using bulk-modified carbon paste electrodes.

Immobilization of [Cu(bpy) 2]Br 2 complex onto a glassy carbon electrode modified with α-SiMo 12O 404− and single walled carbon nanotubes: Application to nanomolar detection of hydrogen peroxide and bromate

A simple procedure has been used for preparation of modified glassy carbon electrode with carbon nanotubes and copper complex. Copper complex [Cu(bpy) 2]Br 2 was immobilized onto glassy carbon (GC) electrode modified with silicomolybdate, α-SiMo 12O 40 4− and single walled carbon nanotubes (SWCNTs) . Copper complex and silicomolybdate irreversibly and strongly adsorbed onto GC electrode modified with CNTs. Electrostatic interactions between polyoxometalates (POMs) anions and Cu-complex, cations mentioned as an effective method for fabrication of three-dimensional structures. The modified electrode shows three reversible redox couples for polyoxometalate and one redox couple for Cu-complex at wide range of pH values. The electrochemical behavior, stability and electron transfer kinetics of the adsorbed redox couples were investigated using cyclic voltammetry. Due to electrostatic interaction, copper complex immobilized onto GC/CNTs/α-SiMo 12O 40 4− electrode shows more stable voltammetric response compared to GC/CNTs/Cu-complex modified electrode. In comparison to GC/CNTs/Cu-complex the GC/CNTs/α-SiMo 12O 40 4− modified electrodes shows excellent electrocatalytic activity toward reduction H 2O 2 and BrO 3 − at more reduced overpotential. The catalytic rate constants for catalytic reduction hydrogen peroxide and bromate were 4.5(±0.2) × 10 3 M −1 s −1 and 3.0(±0.10) × 10 3 M −1 s −1 , respectively. The hydrodynamic amperommetry technique at 0.08 V was used for detection of nanomolar concentration of hydrogen peroxide and bromate. Detection limit, sensitivity and linear concentration range proposed sensor for bromate and hydrogen peroxide detection were 1.1 nM and 6.7 nA nM −1, 10 nM–20 μM, 1 nM, 5.5 nA nM −1 and 10 nM–18 μM, respectively.

Correlating Acid Properties and Catalytic Function: A First-Principles Analysis of Alcohol Dehydration Pathways on Polyoxometalates

Density functional theory calculations and reactivity data were used to examine the mechanism of alcohol dehydration on Keggin-type polyoxometalate (POM) catalysts and the influence of the POM composition and the degree of substitution of the alcohol on kinetically relevant elimination steps. Dehydration was found to proceed through E1 pathways in which the alcohol C—O bond is cleaved heterolytically via a carbenium-ion transition state. Dehydration rates were found to depend on the elimination rate constant and the equilibrium constant for the formation of unreactive alcohol dimers. E2-type elimination transition states, involving concerted C—H and C—O bond cleavage, were not found. The extent of substitution at the α-carbon on the alcohol was found to lead to marked effects on elimination barriers, because substitution increases the proton affinity of the alcohol and the stability of the carbenium-ion transition state. Changes in the central and addenda atoms of the POM cluster and the presence of n-donors, a support, vicinal POM clusters, or charge-compensating cations were found to lead to changes in the deprotonation energy (DPE) of the POM cluster, activation barriers to dehydration, and the stability of the unreactive dimer. These effects are all captured in a general linear relation between activation barriers and deprotonation energy, a rigorous measure of acid strength. The explicit dependence of the E1 activation barrier on the acid deprotonation energy is much weaker than that on reactant proton affinity. This results from the more effective compensation between the acid deprotonation energy and the interaction energy between the cationic hydrocarbon fragment and the anionic POM cluster at the transition state. The direct interactions between the POM protons and the support, other POM clusters, n-donors, base probe molecules, and charge-compensating cations increased the negative charge of the oxide shell of the W12O40 conjugate base, which increased the DPE and decreased the POM acid strength. This decrease in acidity is not fully compensated by the concomitant increase in the interaction energy, leading to elimination barriers that generally increase with increasing DPE.

Self-Process-Programmed Structural Diversity in Supramolecular Assembly Based on Polyoxometalate Anion and Halogensubstituted Bipyridine Cation

To investigate the relationship between network connectivity and the types of anions and cations, we synthesized a series of supramolecular assemblies based on different polyoxometalate (POM) anions and halogen-substituted bipyridine cations, namely, (Hcpy)6H2[PMo12O40] (1), (Hcpy)3[PMo12O40]·2.2H2O (2), (Hcpy)2(Hbppy)2[PMo12O40]·2H2O (3), (Hcpy)2(Hbppy)H2[PMo12O40]·H2O (4), (Hbppy)4[GeW12O40]·3H2O (5), (Hbppy)4[β-Mo8O26] (6), and (H6/7bppy)7[P2W18O60]·5H2O (7) (cpy = 4-(5-chloropyridin-2-yl)pyridine and bppy = 4-(5-(4-bromophenyl)pyridin-2-yl)pyridine). These compounds have been structurally characterized by elemental analyses, IR spectra, thermogravimatric analysis, and X-ray crystallography. All supramolecular frameworks of 1−7 are guided by nonconvalent interactions, especially directional hydrogen bonds and halogen bonds. Research results reveal that decorating nitrogen heterocycles with inductive electron withdrawal halogen groups increases the tendency to form face-to-face π-stacking interactions between them. Anionic clusters also present different and interesting structural features: in 1 and 5 Keggin-type anions form a one-dimensional pillar-like structure, while forming a two-dimensional waved sheet in 4, and only isolated clusters in 2 and 3. Compounds 1−5 consist of Keggin-type clusters with different organic cpy and bppy cations. Complex 6 is a supramolecular assembly based on [β-Mo8O26]4− and bppy, whereas complex 7 is an assembly containing Dawson-type clusters in which anions allow the assembly of bppy cations into distorted-wave organic layers. These results show that the types and ratios of cations and anions play significant roles in fabricating inorganic−organic arrangements. The thermal and electrochemistry properties of these compounds are discussed.

Gas-Phase Fragmentation of Polyoxotungstate Anions

A series of phospho-polyoxotungstate anions was transferred to the gas phase via electrospray ionization (ESI), and the anions' fragmentation was examined by collision-induced dissociation (CID). The anions included [PW12O40]3-, [P2W18O62]6-, and {Co4(H2O)2][PW9O34]2}10- as well as lacunary and metal-substituted derivatives such as [PW11O39]7- and [MPW11O39]5- (M = Co(II), Ni(II), Cu(II)). Common species observed in the mass spectra arose from protonation and alkali metal cationization of the precursor ions. Additional species arising from the formal loss of oxide from the precursor species were also observed, presumably formed via protonation and the loss of an oxo ligand as water. These processes of protonation/cationization and the loss of water both led to species with reduced gas-phase anionic charges, and their formation appears to be driven by the enhanced effects of Coulombic repulsion in the desolvated species generated during transfer to the gas phase via ESI. Fragmentation of selected species was examined by multistage mass spectrometry experiments employing CID. Fragmentation occurred via multiple reaction channels, leading to pairs of complementary product anions whose total stoichiometry and charge matched those of the precursor anion. For example, [PW12O40]3- fragmented to give pairs of product ions of general formulas [W(x)O(3x+1)]2- and [PW(12-x)O(39-3x)]- (x = 6-9), with the most intense pair being [W6O19]2- and [PW6O21]-. Similar ions were also observed for fragmentation of [P2W18O61]4- (derived from the loss of water from [P2W18O62]6-). The lacunary and M(II)-substituted lacunary systems fragmented via related pathways, with the latter generating additional fragment ions due to the presence of M(II). These results highlight the usefulness of ESI-MS in the characterization of complex polyoxometalate anion clusters.

Synthesis and Characterization of Intercluster Compounds Consisting of Various Gold Clusters and Differently Charged Keggin Anions

The crystal structures of five intercluster compounds consisting of gold clusters and Keggin anions have been determined by single-crystal x-ray diffraction: [Au9(PPh3)8][PMo12O40] (P4/n, Z = 2, a = 24.0195(13), c = 13.6818(10) A), [Au9(PPh3)8][HSiMo12O40] (P4/n, Z = 2, a = 24.270(3), c = 13.752(2) A), [Au9(PPh3)8][H3CoW12O40] (P4/n, Z = 2, a = 24.4776(16), c = 13.7759(13) A), [Au8(PPh3)8]2[SiMo12O40] (Pbca, Z = 4, a = 36.269(4), b = 24.488(3), c = 38.612(4) A) and (nBu4N)[Au9(Ptol3)8][SiMo12O40] (Cc, Z = 4, a = 24.832(5), b = 24.955(5), c = 40.096(8) A, β = 106.744(3)°). In these compounds, the charges of the building blocks were varied. Altering the charge on the Keggin anion in combination with the [Au9(PPh3)8]3+ cluster gave rise to isostructural compounds as the charge is compensated by protonation of the polyoxometalate anion. Varying the charge or the ligand type of the gold cluster led to compounds with a completely different packing principle.

Investigation of hybrid molecular material prepared by ionic liquid and polyoxometalate anion

A solid hybrid molecular material containing 1-butyl 3-methyl imidazolium cations and Keggin anions of phosphotungstic acid has been synthesized. It is fully characterized by CHN analysis, FTIR, XRD, UV-Vis-NIR DRS, 31P MAS NMR, TGA and SEM. The FTIR spectrum of the compound shows the fingerprint vibrational bands of both Keggin molecular anions and imidazolium cations. The aromatic C-H stretch region (2700–3250 cm−1) of imidazolium cation is split due to the interaction between the ring C-H and bulky Keggin anion. The red-shift in the UV-Vis spectra and the downfield 31P MAS NMR chemical shift also confirm the electrostatic interaction between the ions in the compound. Near IR spectral region (1000–2500 nm) shows the elimination of water in the compound which is hydrophobic.

NMR Spectroscopic Studies of Interactions in Solution during the Synthesis of MoVTeNb Oxide Catalysts

AbstractAqueous solutions used for the synthesis of MoVTeNb oxide catalysts have been studied with 125Te, 95Mo, 51V, and 17O NMR spectroscopy. Polyoxometalate (POM) anion V9TeO285– (I), which has the decavanadate structure, containing Te6+ in one of its two central positions, has been identified for the first time in VV‐TeVI solutions. In ternary MoVI‐VV‐TeVI solutions, the POMs TeMo6O246– (II) (pH ca. 7.8–2) as well as previously unknown complexes of the proposed compositions TeV3Mo3O249– (III) (pH ca. 8–5) and TeMo3V5O275– (IV) (pH ca. 6–1) were observed. In the structure of IV, half of the decavanadate anion is combined with half of the octamolybdate anion, in which the central Mo atom is replaced with Te6+. There is an equilibrium, IV [rlarr2] I + II, in the ternary solutions. Despite its stability in solution, polyanion IV could not be isolated in the solid state. During concentration under ambient conditions, less soluble polyanion II begins to crystallize first, and the equilibrium in solution gradually shifts to I. Upon dissolving the precipitate, complex IV forms again. The distribution diagrams have been derived from the NMR spectra for a solution of the cationic composition Mo1V0.3Te0.23 with NH4+ as a counterion, acidified with HClO4 in the pH range 7.25–1. In addition to the above‐mentioned complexes, VO2+ and MoO22+ oxalates form in the presence of oxalic acid. Upon adding niobium oxalate, Nb2O5 gel forms without any Nb‐containing species in solution. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Self-Assembly of Organic−Inorganic Hybrid Amphiphilic Surfactants with Large Polyoxometalates as Polar Head Groups

Mn-Anderson-C6 and Mn-Anderson-C16, A type of inorganic-organic hybrid molecules containing a large anionic polyoxometalate (POM) cluster and two C6 and C16 alkyl chains, respectively, demonstrate amphiphilic surfactant behavior in the mixed solvents of acetonitrile and water. The amphiphilic hybrid molecules can slowly assemble into membrane-like vesicles by using the POM clusters as polar head groups, as studied by laser light scattering and TEM techniques. The hollow vesicles have a typical bilayer structure with the hydrophilic Mn-Anderson cluster facing outside and long hydrophobic alkyl chains staying inside to form the solvent-phobic layer. Due to the rigidity of the POM polar heads, the two alkyl tails have to bend significantly for the vesicle formation, which makes the vesicle formation more difficult compared to some conventional surfactants. This is the first example of using hydrophilic POM macroions as polar head groups for a surfactant system.

Tuning the Dimensionality of the Coordination Polymer Based on Polyoxometalate by Changing the Spacer Length of Ligands

Through tuning the spacer length of flexible bis(triazole) ligands, three Keggin anion-based coordination polymers with different dimensionalities, [Cu4(H2O)4(bte)2(HPMoVI10MoV2O40)]·2H2O (1) (bte = 1,2-bis(1,2,4-triazol-1-yl)ethane), [Cu(btb)][Cu2(btb)2(PMo12O40)] (2) (btb = 1,4-bis(1,2,4-triazol-1-y1)butane), and [Cu5(btx)4(PMoVI10MoV2O40)] (3) (btx = 1,6-bis(1,2,4-triazol-1-y1)hexane), were synthesized and structurally characterized. Compound 1 exhibits a ladder-like chain, in which the polyoxometalate (POM) anions act as the “middle rails” of the ladder. The bte ligand with shortest spacer length acts as not only the bridging linker but also the chelator, which terminates the dimensional extension. Compound 2 exhibits a 2D POM-based framework, containing POM/Cu/btb grid-like layers. In compound 3, there exist three sets of (123)(121)2 2D Cu-btx frameworks to generate a 3-fold interpenetrating structure, into which the hexadentate POM anions are inserted to construct a 3D structure. The structural analyses reveal that the spacer length of flexible bis(triazole) ligands has influence on the dimensionality of these compounds.