Polyoxometalate-based organic-inorganic hybrid materials

Hu Changwen, Wang Yonghui, Li Yangguang, Wang Enbo
(Institute of Polyoxometalate Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun, 130024, China)

Received Feb. 15, 2001; Supported by the National Natural Science Foundation of China (20071007) and the Foundation for University Key Teacher by the Ministry of Education of China.

Abstract The present paper summarizes and reviews the current developments in the area of polyoxometalate (POM)-based organic-inorganic hybrid materials including POM-based organic- or organometallic-derived hybrid materials, charge transfer (CT) salts and organic-inorganic composite films. POM-based organic- or organometallic-derived hybrids are reviewed in terms of four subclasses, namely (i) ligands as charge-compensating cations, (ii) ligands bonded directly to the POM framework, (iii) ligands bonded to the heterometal site and (iv) organometallic ligands. Moreover, POM-based CT hybrid materials described here contain different organic p-electron donors such as substituted amides, aromatic amines, or electron-rich tetrathiafulvalenes (TTF), bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) as well as decamethylferrocene (FeCp*₂). Further, it is concluded that POM-based organic-inorganic composite films can be prepared by four main strategies, namely doping POMs in electrically conductive polymers, Langmuir-Blodgett technique, layer-by-layer self-assembly method, and electrochemical growth method. Finally, this review also provides insight into the rational synthesis and potential applications of these hybrid materials.
Keywords polyoxometalate, organic-inorganic hybrid materials, structure, charge transfer salts, composite films

1. INTRODUCTION
Organic-inorganic hybrid materials are of intensive interest in the field of contemporary materials chemistry as these materials can exhibit synergetic properties such as electrical, magnetic and optical properties^[1].
    Polyoxometalates (POMs) have been found wide applications in many fields such as catalysis, medicine, magnetic properties, materials and surface chemistry, and photo- and electrochromism owing to the so-called "value-adding properties"^[2]. The number of publications and patents pertaining to POMs continues to grow. The scientific communities of the Peoples Republic of China are becoming the important contributors of the total number of worldwide publications on POMs. Figure 1 shows the countries where the most research activity exists based on publications, in which the number of publications from China occupies the third position (Russia the first and USA the second)^[2].
    It is noteworthy that POMs are receiving increasing attention in the domain of materials science ascribing to their chemical, structural and electronic versatility. A focus in this respect is the exploration of organic-inorganic hybrid materials combining POMs and organic molecules and possessing a variety of interesting conducting, magnetic, electronic and optical properties. So far, tremendous advances have been made in this rapidly growing field^[2].
    The present review, covering the years 1995 to 2000, summarizes and reviews the latest developments in the area of POM-based organic-inorganic hybrid materials. The following section summarizes the efforts in POM-based organic- or organometallic-derived hybrid materials. Section 3 presents the development of POM-based charge transfer (CT) salts. Section 4 discusses progress in POM-based organic-inorganic composite films. A summary and an outlook concerning the future of POM-based organic-inorganic hybrid materials are presented in Section 5. In addition, only structurally typical and representative POM-based hybrids are exemplified in this review owing to the length limitation of this paper.

Fig. 1 The 10 countries with the largest number of POM publications. Source: Chemical Abstracts ( till 1998)^[2].

2. POM-BASED ORGANIC-INORGANIC HYBRID MATERIALS CONTAINING ORGANIC OR ORGANOMETALLIC MOIETIES
Up till now, several synthetic methods have been adopted to prepare POM-based hybrid materials containing organic or organometallic moieties, including routine processes^[3-4], hydrothermal reactions^[5-10] and low-thermal solid phase synthesis^[11]. Low-thermal solid phase synthesis, which is a novel synthetic strategy and involves the direct reactions of POMs and organic molecules under grinding in an agate motar at room temperature, has been applied to produce POM-based hybrids^[11].
    For clarity, the POM-based hybrids containing organic ligands reviewed in the present paper are divided into three subclasses according to the role of organic ligands: (1) charge-compensating cations, (2) ligands bonded directly to the POM skeletal framework, and (3) ligands bonded to the heterometal site^[5] (see Table 1). Further, the hybrids containing organometallic moieties are listed as a fourth subclass all alone (see Table 2).

Table 1 POM-based hybrids with organic ligands

# Wang Y H, Hu C W, Peng J Wang E B, Xu Y.  Synthesis, characterization and X-ray crystal structure of a novel   polyoxotungstoarsenate: K₆[{Cu(b-Ala)₂(H₂O)₂}₂{Cu₄(H₂O)₂(AsW₉O₃₄)₂}]·17H₂O, J. Mol. Struc., 2001, in press

2.1 Organic ligands as charge-compensating cations in POM-based hybrids 
Organic molecules such as organoamines have been extensively used as charge-compensating cations in the synthesis of POM-based hybrids and a large number of such hybrids have been prepared during these years (Table 1), as successfully exemplified by polyoxovanadate-based hybrids. One typical structural feature of this kind of hybrids is the insertion of organic molecules into the two-dimensional (2D) layered host framework of polyoxovanadates^[6-7]. A recent example is (C₆H₁₄N₂)V₆O₁₄·H₂O, which contains a new arrangement of edge-shared VO₅ square pyramids that are corner-shared with VO₄ tetrahedra to form highly puckered layers, between which the 1,4-diazabicyclo[2.2.2]octane cations are sandwiched (Fig.2)^[6]. Another example is [N(CH₃)₄]₅[V₁₈O₄₆]^[7]. The lattice is self-assembled from two distinct {V₉O₂₃} building blocks: One building block is neutral, while the other carries localized electrons and is negatively charged. The organic cations reside between the layers. During the formation of this hybrid, the organic cations act not only as a templating agent but also as a reducing agent, and complicate the dissolution and nucleation processes as well^[7].

Fig. 2 (a) Portion of the crystal structure of (C₆H₁₄N₂)V₆O₁₄·H₂O (along [100] that depicts four square-pyramidal VO₅ units linked to form a single layer: Note the pairs of edge-sharing square pyramids that have the same relative orientation; (b) the structure shown along [010].

    The structure of a one-dimensional (1D) molybdenum-containing organic-inorganic hybrid materials, (H₃N(CH₂)₄NH₃)[Mo₄O₁₃], is interesting^[8]. Its structure consists of bundles of parallel molybdenum oxide chains interspersed with organic cations that play the role of intercalating agent in the host structure.
    Although a lot of vanadium- or molybdenum- containing organic-inorganic hybrids have been prepared hydrothermally, tungsten-containing hybrids produced by hydrothermal method were scarcely documented^[10], most of which were synthesized routinely, such as [(n-C₄H₉)₄N]₆[Fe^III₄(H₂O)₂(PW₉O₃₄)₂]^[4] and [(CH₂)₃C₆H₅N]₄[(SiO₄)W₁₀Mn₂O₃₆H₆]·CH₃CN·H₂O^[3] (see Table 1). The organic agents, for example, the tetra-n-butylammonium (TBA) cations, usually act as precipitating agents or phase-transferring agents in the synthesis of these hybrids.
2.2 Organic molecules as ligands to the POM frameworks
The direct bonding of organic ligands to POMs has greatly enhanced the structural diversity of POM-based hybrid materials (Table 1).
    Recently, Zubieta et al. reported three novel hybrids, [H₂N(CH₂CH₂)₂NH₂][(VO){O₃P(CH₂)PO₃}], [H₃N(CH₂)₂NH₃][(VO){O₃P(CH₂)₂PO₃}] and [H₃N(CH₂)₂NH₃][(VO₄)(OH)₂(H₂O)₂{O₃P(CH₂)₃PO₃}₂]·4H₂O, possessing one-, two- and pillared two- or three-dimensional phase respectively^[16]. They have shown that the introduction of appropriate organic templates and modification of the length of the organic buttress of the diphosphonato ligand can allow the engineering in materials with different dimensions, specific interlayer spacings and hence tailored microporosities.
    In addition to being used as a cationic component (see Section 2.1), the organoamines also act as ligands^[12-14]. By raising the pH value of the reaction medium to prevent protonation of the pyridyl nitrogens, [MoO₃(4,4'-bpy)_0.5] (MOXI-8) and [H_xMoO₃(4,4'-bpy)_0.5] (MOXI-9) were prepared by Zubieta et al.^[12]. The structure of MOX1-8 (Fig.3) contains layers of corner-sharing {MoO₅} square pyramids linked through 4,4'-bipyridyl groups into a three-dimensional (3D) covalently bonded framework, in which alternating inorganic metal oxide layers and aromatic organic layers exist. The structure of MOX1-9 is similar to that of MOX1-8, except that the octahedra adopt a more regular 5+1 type coordination, rather than the 2+2+2 type observed for MOX1-8^[12].

Fig. 3 View of the layer structure of [MoO₃(4,4'-bpy)_0.5] (MOXI-8) parallel to the crystallographic a axis.

    In routine organic solvent synthesis, organoimide hexamolybdates constitute a new class of functionalized POMs, which permit a broad range of appended groups to be incorporated through variations in the imide substituent^[13,14]. Recently, Maatta et al. reported multifunctionalized organoimido derivatives of [Mo₆O₁₉]^2-, in which a variety of organoimido ligands are introduced at terminal oxo sites in the hexamolybdate cluster^[13,14].
    Moreover, polyvacant heteropolytungstates with organic groups anchored to their backbones are another sort of interesting organic-inorganic hybrid materials and have been receiving increasing attention during the past few decades^[15,17]. Thouvenot et al. reported the [g-SiW₁₀O₃₆(RPO)₂]^4-[17] and [g-PW₁₀O₃₆(t-BuSiOH)₂]^[3-15]. The former is built up from a [g-SiW₁₀O₃₆]^8- unit and two phenylphosphonate (PhPO²⁺) groups which are linked to two oxygen atoms of two edge-sharing WO₆ octahedra. The structure of the latter is similar to that of the former.
2.3 POM-based hybrids containing organic-ligated heterometals 
The chemistry of POM-based hybrids is significantly enriched by the introduction of heterometal coordination complexes into the hybrid systems (Table 1), either to provide charge compensation or as part of the inorganic POM framework itself^[5].
    We have hydrothermally synthesized a novel POM-based hybrid containing transition metal coordination complexes, [Cu₂(4,4'-bipy)₂{Mo₂O₇}]_n (bipy=bipyridyl)^[19]. This hybrid exhibits a 1D chain structure consisting of [Mo₂O₇]^2- units and [Cu(4,4'-bipy)]_nⁿ⁺ chains (Fig.4). The [Mo₂O₇]^2- units are covalently bridged to two [Cu(4,4'-bipy)]_nⁿ⁺ chains, forming a hybrid with a bridged double-chain structure.
    Another example is represented by [M(4,4'-bpy)₂(VO₂)₂(HPO₄)₄] (M=Co or Ni, bpy=bipyridine), possessing a novel 3D structure constructed from two subunits, namely [(VO₂)(HPO₄)]₈ helical chains and [M(4,4'-bpy)₂]²⁺ fragments (Fig.5)^[18]. Another spiral-shaped chain [{Cu(en)(OH₂)}Mo₅P₂O₂₃]^4-, consisting of [Mo₅P₂O₂₃]^6- clusters connected by {Cu(en)(OH₂)O₂}²⁺ square pyramids, was reported two years ago^[20]. Both helical hybrids containing organoamine ligands, heterometals and especially PO₄ units are reminiscent of DNA structures, and maybe provide a potential route for the rational synthesis of inorganic-organic mimetic systems^[23].

Fig. 4 A view of [Cu₂(4,4'-bipy)₂{Mo₂O₇}]_n showing 50% probability displacement ellipsoids and the atom-numbering scheme. H atoms have been omitted for clarity.

Fig.5 (a) View of the space-filling plot of the left -helical and right-helical V-P-O chains. (b) View of the structure along the (101) lane, showing the channels.

   Zubieta and co-workers prepared two novel hybrid materials: [Cu(tpyor)Cu₂Mo₃O₁₁] and [{Fe(tpyor)}₃Fe(Mo₆O₁₉)₂]·xH₂O (tpyor=tetrapyridylporphyrin) by hydrothermal reactions^[21]. The structure of the former could be described as neutral [Cu(tpypor)] network linked to bimetallic {Cu₂Mo₃O₁₁} oxide chains, while the latter was constructed from a 3D cationic framework [Fe₄(tpyor)₃]_n⁴ⁿ⁺ and entrained {Mo₆O₁₉}^2- cluster anions. The two structures illustrated that molybdenum oxide structures might be readily modified by the porphyrin building blocks and appropriate selection of porphyrin ligated metal centers, which can be considered as a "ship in bottle" approach to modification of oxide microstructures^[21].
    It is noteworthy that nearly all the hybrids listed in Table 1 have been made by hydrothermal technique, except (Bu₄N)₃H₂[g-SiO₄W₁₀O₃₂(OH)Cr₂(OOCCH₃)₂(OH₂)₂]·3H₂O^[22], K₆[Cu(b-Ala)₂(H₂O)₂]₂[Cu₄(H₂O)₂(AsW₉O₃₄)₂]·17H₂O^# and [{Na(dibenzo-18-crown-6)}₃{PMo₁₂O₄₀}]·5H₂O ^[11]. The first two hybrids were prepared by routine method. In the structure of the first hybrid, the Cr₂(OOCCH₃)₂²⁺ units occupy the vacant sites of the g-SiW₁₀O₄₀ units. Recently, we have prepared a novel POM-based hybrid compound, K₆[{Cu(b-Ala)₂(H₂O)₂}₂{Cu₄(H₂O)₂(AsW₉O₃₄)₂}]·17H₂O (b-Ala=b-alanine), which can be described as two [Cu(b-Ala)₂(H₂O)₂]²⁺ fragments coordinated with the sandwich-type [Cu₄(H₂O)₂(AsW₉O₃₄)₂]^10- unit through bridging oxo groups (Fig.6).
    More recently, using low-thermal solid-phase method, You et al. obtained a yellow crystal [{Na(dibenzo-18-crown-6)}₃{PMo₁₂O₄₀}]·5H₂O through grinding the mixture of the crown ether and H₃PMo₁₂O₄₀ (Fig.7)^[11]. Its structure consists of three {Na(dibenzo-18-crown-6)} units coordinated with a triad of addenda metals of the polyoxoanion through the terminal oxo groups. To our knowledge, this is the first crystal whose Keggin-type polyoxoanion is connected with crown ether.

	Fig. 6 View of the structure of [{Cu(b-Ala)2(H2O)2}2{Cu4(H2O)2(AsW9O34)2}]6+

	Fig. 7 View of the crystal structure of   [{Na(dibenzo-18-crown-6)}3{PMo12O40}]·5H2O

2.4 POM-based hybrids containing organometallic moieties
The chemistry of POM-based hybrids containing organometallic moieties is an area of growing interest^[24-29]. Generally these hybrids can be classified into three types based on their structures: (i) organometallics supported on a POM, (ii) organometallics incorporated into a POM and (iii) integrated cubane-type clusters^[2]. As to the difference between (i) and (ii), Finke has given a clear explanation^[30]. POM-supported organometallic hybrids mean species that the organometallic moiety is firmly attached to a k₃-O site of surface oxygens of the support POM, while in POM-incorporated organometallics, the organometallic moiety is incorporated into a vacancy in the POM framework by four approximately square-planar oxo ligands.
    Quite recently, a series of new POM-based organometallic hybrids have been synthesized (Table 2). Two POM-incorporated hybrids, [PW₁₁O₃₉RhCH₂COOH]^5- and [SiW₁₁O₃₉RhCH₂COOH]^6-, were obtained by the hydrothermal method^[27].
    Owing to the fact that surface activation of POMs can be achieved by replacing the metal (Mo(VI) or W(VI)) centers by one or more lower valent metals, a number of substituted-type POM-supported organometallic hybrids have been synthesized. For example, reaction of [PW₁₁O₃₉NbO]^4- with Cp₂Zr(Otf)₂THF (Cp=C₅H₅-, Otf=O₃SCF₃-) in acetonitrile yielded the first POM-supported biscyclopentadienyl complex of Group 4, [(PW₁₁O₃₉NbO)₂ZrCp₂]^{6- [24]}.
    A series of integrated cubane-type clusters have been developed as potential models for inorganic solid surfaces, e.g. [{Ru(h⁶-p-MeC₆H₄Pri)}₄Mo₄O₁₆] ^[29]. In solid state, the hybrid displays the so-called windmill-like structure, i.e. it consists of a [Mo₄O₁₆]^8- cubic core capped by four {Ru(h⁶-p-MeC₆H₄Pri)}²⁺ groups each bound to a triply bridging and two terminal oxo ligands.

Table 2 POM-based hybrids with organometallic ligands

3. POM-BASED CHARGE TRANSFER (CT) SALTS  
A recent development of POMs in materials science deals with the use of soluble metal-oxide clusters as inorganic electron-accepting moieties in new organic-inorganic CT hybrid materials based on organic p-electron donors such as substituted amides^[31], aromatic amines^[40], or electron-rich substrates such as tetrathiafulvalenes (TTF)^[2,32], bis-(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET)^[33-36] or decamethylferrocene (FeCp*₂) ^[39]. The combination of these organic donors with POMs of various nuclearities, shapes, and charges has resulted in new types of magnetic, insulating, semiconducting, or even metallic CT salts. Table 3 gives a list of this kind of hybrid materials recently synthesized.

Table 3 Recently synthesized POM-based CT salts

    Coronado and co-workers have prepared the first well-characterized ET-type CT salt of the Dawson-Wells polyanion [P₂W₁₈O₆₂]^6-, (ET)₁₁[P₂W₁₈O₆₂]·3H₂O, consisting of alternating layers of anions and ET molecules in the ac plane^[35]. The Dawson-Wells polyanion has induced a b packing in the organic ET component, formed by an exotic zigzag chain arrangement of six independent ET molecules. This hybrid has very interesting electrical properties. It exhibits a metallic-like behavior characterized by increased conductivity from ~5 S cm^-1 at room temperature to 5.5 S cm^-1 at 230 K. Below this temperature the hybrid becomes a semiconductor with a very low activation energy value (Fig.8). This result illustrates how a change in the packing of the organic molecules (from a in the Keggin phases to b in the Dawson-Wells derivative) can give rise to hybrid molecular materials with higher conductivities despite the large size of the Dawson-Wells anion. The magnetic properties of this compound indicates the diamagnetic nature of this compound.

Fig. 8 Thermal variation of the dc electrical conductivity of [ET]₁₁[P₂W₁₈O₆₂]·3H₂O in the ac plane. The solid line is the fit to a semiconductor model with a very small activation energy (Ea=27 meV) in which the effect of the electron mobility on the conductivity has been taken into account.

    We have prepared a CT salt [C₄₂H₆₀C_l2O₂S₁₂]₂[SiMo₁₂O₄₀] based on p-extended bis(TTF) and a Keggin polyoxoanion^[32]. In the crystal structure, organic radical dications and silicamolybdate anions are alternatively arranged along a axis to form a 1D conducting layer. The room-temperature d.c. conductivity determined by three two-probe method is 10^-4 S cm^-1, suggesting that the compound is a semiconductor. Subsequently, we reported another POM-based CT hybrid, [ET]₈[SiMo₁₂O₄₀]·10H₂O, consisting of 2D organic and inorganic sublattices alternating along the unique axis [010] of the monoclinic unit cell. Its conductivity was lower than those of other ET salts with Keggin anions due to the longer chalcogen-chalcogen distances^[33].

Fig. 9 (a) Infinite stacks of anthracene donor and polyoxometalate acceptor with DAD/DAD/DAD sequence in the crystal structure of (Me-ANT-CH₂-Py⁺)₂[Mo₆O₁₉]^2- salt. (b) Overlap of "dimeric" anthracene nuclei in the crystal structure of (Me-ANT-CH₂-Py⁺)₂[Mo₆O₁₉]^2- salt.

    More recently, novel CT hybrid materials have been prepared via cocrystallization of planar aromatic donors (anthracenes and pyrenes) and spherical POM acceptors such as Lindqvist-type [M₆O₁₉]^2- and Keggin-type [SiM₁₂O₄₀]^4- (M = Mo or W) through attaching a cationic "anchor" onto the arene which then clings to the POM anion by Coulombic forces^[40]. A unique arrangement of the arene donors and the hexamolybdate acceptors, namely a DAD/DAD/DAD... sequence, is observed in the stacking (see Fig.9 (a)), where two neighboring donor moieties are cofacially oriented with an interplanar distance of 0.346 nm and embedded between the spherical POM units. This allows a high degree of p-orbital overlap (see Fig.9 (b)) and results in satisfaction of the 2:1 stoichiometry based on the ionic charges and accommodation of the different shapes and sizes of donor and acceptor. Furthermore, an interesting logical structure/property relationship is observed, that is, the shorter the distance between the POM surface and the arene nucleus, the darker is the color of the CT crystal and the faster is the decay of the laser-excited charge-transfer state (due to back-electron transfer). Thus, these hybrid materials may be interesting candidates for nonlinear optical and/or ferromagnetic properties.
    Furthermore, Golhen et al. have reported several molecular assemblies of POM-based ferrocenium hybrids, including ([Fe(C₅Me₅)₂]₃[Cr(OH)₆Mo₆O₁₈]·20H₂O [FeCr(1)], Na[Fe(C₅H₅)₂]₂[Cr(OH)₆Mo₆O₁₈]·3H₂O [FeCr(2)], and [Fe(C₅Me₅)₂]₄[HPCu(H₂O)W₁₁O₃₉]·6CH₃CN (FeCu)^[39]. In the hybrid FeCr(1), the Fe(C₅Me₅)₂ units are perpendicular to each other and form a centrosymmetrical group of three cations, which alternates with the planar [Cr(OH)₆Mo₆O₁₈]^3- anion to form perpendicular mixed chains. In the hybrid FeCu, the decamethylferrocenium units form a polyhedral structure that encapsulates the Keggin polyanion. The magnetic properties indicate that the hybrids containing metallocenium radicals present weak antiferromagnetic interactions obeying the Curie-Weiss law.
    You et al. have reported a POM-based CT salt, a-H₄SiW₁₂O₄₀·4HMPA·2H₂O, composed of the heteropolytungstate a-H₄SiW₁₂O₄₀ and the organic substrate hexamethylphosphoramide (HMPA)^[31]. The complex has no photosensitivity under irradiation of sunlight, but under the near-UV light result in a charge transfer by oxidation of the HMPA and the reduction of the POM. This hybrid shows a certain second-order and third-order nonlinear optical response of I²w = 0.7I²w _KDP and c⁽³⁾ = 2.63 ×10^-11 esu, respectively.

4. POM-BASED ORGANIC-INORGANIC COMPOSITE FILMS
As aforementioned, POMs have been widely used in many fields such as conductivity, magnetism, electrocatalysis, photochromism and molecular electronics^[2]. However, practical applications of POMs in these areas rely largely on the successful fabrication of POM-based organic-inorganic films. Recently, several main strategies have been adopted to prepare lamellar POM-based organic-inorganic hybrids, including (i) doping POMs in electrically conductive polymers^[36,41]; (ii) Langmuir-Blodgett technique^[42-47]; (iii) layer-by-layer self-assembly method^[49-53] and (iv) electrochemical growth method^[54-56].
4.1 Composite films formed by doping POMs in electrically conductive polymers
By embedding or incorporating POMs within conducting polymers such as polypyrrole, polyaniline and polythiophene, a number of POM-based organic-inorganic composite films as well as their bulk materials have been prepared in recent years, which can be used as conducting electrodes or cation-insertion electrodes. A more commonly used incorporation method is electrochemical oxidation of a monomer under a suitable oxidation potential by using a POM as the electrolyte. Thus, the conductive polymer is deposited on the working electrodes (usually carbon or graphite) doped with the polyoxoanions.
    Gómez-Romero et al. have both chemically and electrochemically synthesized a POM-based hybrid film (C₄H₃N)₉(PMo₁₂O₄₀) formed by polypyrrole and the phosphomolybdate anion [PMo₁₂O₄₀]^{3- [41]}. The 9:1 stoichiometry agrees with the expected value for a fully oxidized polymer with the anion [PMo₁₂O₄₀]^3- balancing the charges. This hybrid maintains the good conductivity of the polymeric matrix and presents the added electrochemical activity of the inorganic clusters. Its electrochemical cycling is found to involve the insertion and de-insertion of cations during reduction and oxidation process respectively, representing a key modification of the cycling mechanism of polymeric electrodes. Thus, the present work establishes the function of these hybrid materials as electrodes taking advantage of the reversible electrochemical activity of the POM clusters.
    Clemente-León and co-workers have electrochemically prepared the organic/inorganic films consisting of the [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- polyoxoanion incorporated in polypyrrole (~ 80 pyrrole units per cobalt cluster)^[36]. The magnetic properties are very close to those observed in the K⁺ salt, indicating that the structure of the ferromagnetic cluster is maintained in the film. The electrical properties reveal a semiconducting behavior. This hybrid represents the first hybrid film formed by a high spin cluster embedded in a polypyrrole polymer in which the large magnetic moments localized on the POM coexist with a de-localized electron framework. The authors have claimed that such a strategy is general and can be extended to other kinds of magnetic POMs and conducting films.
4.2 Composite films formed by Langmuir-Blodgett deposition technique
Recently, Langmuir-Blodgett technique has been extended to create organic-inorganic LB films containing well-organized layers of polyoxoanions^[42-47]. As a matter of fact, this method has been confirmed as a useful tool to improve the order and properties of these new lamellar hybrid materials (see below).
    Using the adsorption properties of Keggin heteropolyanions along a positively charged monolayer of a lipid such as the dimethyldioctadecylammonium cation (DODA), Clemente-León et al.^[42-43] have constructed new organic/inorganic films DODA/XW₁₂O₄₀ (X=2(H⁺), P^V, Si^IV, B^III, Co^II). The control of the Keggin polyanion amount in the multilayers is made possible by mixing DODA with a negatively charged lipid, which modifies the global electrical charge of the Langmuir film. The magnetic behavior of the DODA/CoW₁₂O₄₀ LB films has been measured and found to follow the Curie law down to about 20 K, as expected for an isolated Co^II in a tetrahedral environment. Below this temperature, the magnetic moment decreases because of the zero-field splitting of the S = 3/2 Co^II ion similar to the behavior observed for the potassium salt of this anion. Thus it is demonstrated that the magnetic site of the polyanion embedded in the LB film is not sensitive to the surrounding charge of the DODA molecules.
    The same group has extended this LB method to other POMs and prepared three new LB films, that is DODA/K₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂], DODA/K₆[SiMn(H₂O)W₁₁O₃₉] and DODA/K₁₆[Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]^[44,45]. These POMs are well organized in monolayers within the LB films. The magnetic properties of these films are measured and found to be very close to those obtained in the K⁺ salts.
    Kurth et al. have fabricated the LB films of a surfactant-encapsulated cluster (SEC), (DODA)₂₀(NH₄)[H₃Mo₅₇V₆(NO)₆O₁₈₃(H₂O)₁₈]^[46]. The film thickness for the first layer is 2.6 nm and an average film thickness is 2.3 nm for subsequent layers. These values are smaller than the diameter of the SEC, but packing effects within the films can reduce the apparent thickness per layer. Deposition is observed to be very reproducible and film growth essentially linear, that is to say, in each dipping cycle equivalent amounts of SEC are transferred on the substrate (transfer ratio 0.95). In addition, the resulting LB films are completely hydrophobic, which demonstrates the efficient encapsulation of the hydrophilic cluster by the surfactant molecules. Subsequently, the LB films of another SEC, (DODA)₄₀(NH₄)₂[(H₂O)_n Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂], was reported by the same group^[47]. Since this SEC possesses a nanoporous structure, its LB films would certainly represent a completely novel type of nanoreactors, from which functional nanodevices may evolve.
    The aforementioned contributions clearly indicate that LB technique can be extended to all types of polyanions or charged clusters, giving rise to new lamellar organized materials that have particular magnetic, optical, electrochromic, or electrochemical properties. However, this still depends on the appropriate choice of both POMs and lipid molecules.
4.3 Composite films formed by layer-by-layer self-assembly method
The layer-by-layer self-assembly method originally applied to prepare multilayer assemblies of organic polymers^[48] has recently been used to produce ordered POM-based organic-inorganic films^[49-53]. This technique involves the sequential adsorption of oppositely charged species from dilute solution and is particularly attractive because of its simplicity and effectiveness in creating multilayer films on various substrates and allowing the easy control of the desired order and number of layers at the nanometer level.
    In 1996, Kuhn et al.^[49] obtained the deposits of multilayers of Os(bpy)₃²⁺-P₂Mo₁₈O₆₂^6- on different electrode surfaces by using this method. The composition of the multilayers was found to depend on the identity of the electrode surface, the pH of the deposition solution, and the electrochemical treatments applied during the deposition steps.
    Later, Ichinose et al.^[50] reported the self-assembled ultrathin multilayers formed by (NH₄)₄[Mo₈O₂₆] and poly(allylamine) hydrochloride (PAH) (Fig.10). In this work, adsorption of (NH₄)₄[Mo₈O₂₆] on PAH surfaces was found to proceed without saturation and the thickness of the oxide layer in the individual adsorption step increased with the adsorption time at a rate of ca. 0.57 nm/min, due to the condensation of the adsorbed octamolybdate and the resulting loss of negative charges. The thickness of the POM layer can be controlled in a range of 2~25 nm by the adsorption times employed, corresponding to 2~30 layers of the [Mo₈O₂₆]^4- unit. In addition, the formation of the multilayer films was found to critically depend on the pH of the polymer solutions (pH~3).

Fig. 10 Schematic illustration of a molybdenum oxide/PAH multilayer. The first two oxide layers are composed of three layers of [Mo₈O₂₆]^4- units corresponding to the 4- and 7-min adsorptions of (NH₄)₄[Mo₈O₂₆]. PAH layers have a constant thickness of ca. 3 nm.

    Moriguchi et al.^[51] prepared the ultrathin films self-assembled from poly (diallyldimethylammonium) chloride (P) and sodium decatungstate (W) on various substrates (S) and also studied the electrochromic properties of these films. The amount of W adsorbed on quartz in one self-assembly step is ca. 2~3 times greater than that evaluated from the geometric area of this molecule, suggesting that W₁₀O₃₂^4- penetrates into and adsorbs onto an underlying P layer to form a P-W ion complex. In addition, the concentration of W in S-(P/W)_n films was almost constant irrespective of the species of the substrates. The author also confirmed the stable electrochromic property of the self-assembled S-(P/W)_n/P films.
    Also in 1998, Caruso et al.^[52] produced ultrathin multilayer films of a novel molybdenum (VI) POM cluster (NH₄)₂₁[H₃Mo₅₇V₆(NO)₆O₁₈₃(H₂O)₁₈] (Mo₅₇) and PAH by this method. The Mo₅₇ density in the film could be readily controlled by varying the polyelectrolyte interlayer separation between each Mo₅₇ layer, and the total film thickness could be controlled by altering the number of adsorption cycles.
    More recently, the same group^[53] has reported another kind of stable ultrathin composite films consisting of polyallylamine hydrochloride (PAH) and the nanoporous isopolyoxomolybdate [(H₂O)_n Ì Mo₁₃₂O₃₇₂(CH₃COO)₃₀(H₂O)₇₂]^42- (Mo₁₃₂) fabricated by the layer-by-layer self-assembly method. The deposition of the Keplerate Mo₁₃₂ was found to be highly reproducible and the average effective film thickness for a Mo₁₃₂/PAH layer pair was 2.2±0.2 nm. The measured film thickness of a single Mo₁₃₂ layer amounts to ~1 nm. The remaining 1.2-nm film thickness is, therefore, attributed to PAH.
    On the whole, in contrast to LB technique, the layer-by-layer self-assembly method appears more facile, versatile and promising in fabricating POM-based organic-inorganic composite films with nanocontrol. However, it is crucial to understand the factors controlling the film growth and precise thickness, such as the adsorption time, the solution pH value and the polycations employed.
4.4 Composite films formed by electrochemical growth method
Dong et al. have developed a novel electrochemical growth method involving cyclic potential sweeps (CPS) over a suitable potential range in modifier solutions to produce POM-containing organic-inorganic films^[54-56]. A successful example is composite films based on SiMo₁₁VO₄₀^5- and a cationic polymer of quaternized poly(4-vinylpyridine) partially complexed with osmium bis(2,2'-bipyridine) chloride^[54]. These films not only are uniform and homogeneous, but also exhibit well-defined surface waves characteristic of the POMs' redox reactions. This electrochemical strategy has also been successfully popularized by the same group to other structurally different POMs^[55-56].
    By comparing this electrochemical method with the immersion method, the authors conclude that the electrochemical growth may provide at least two advantages in favor of the multilayer growth^[54]. First, a positive electric field is formed in electrode interface by CPS in a positive potential window. It must accelerate and suit the deposition of highly negatively charged polyanions. On the other hand, by electrostatic repulsion, it avoids the drawback of overdeposition. Second, redox reactions between heteropolyacids and heteropolyblues (HPAÛHPB) during CPS should be more adaptable to loading polyanions, since the resulting HPBs are known as complex colloidal species, bearing much higher negative charges.
    In short, the electrochemical growth method is claimed to be more advantageous than the immersion growth and suitable for anchoring nearly all kinds of POMs, including those unable to be modified by the previous strategies^[54].

5. SUMMARY AND OUTLOOK
In this review, we have presented the latest developments in the newly emerging area of POM-based organic-inorganic hybrid materials. So far, the number of these hybrid materials has been continuously and rapidly growing owing to their potential applications in catalysis, medicine and materials science. However, the rational design and synthesis of this kind of hybrid materials with desired structures and specific properties is still a challenge to synthetic chemists. Furthermore, new synthetic routes and strategies need development in order to enrich the chemistry of POM-based organic-inorganic hybrid materials.

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