Polyoxomolybdates and Polyoxomolybdovanadates – from Structure to Functions: Recent Results*

CROATICA CHEMICA ACTA CCACAA, ISSN-0011-1643, ISSN-1334-417X

Croat. Chem. Acta 82 (2) (2009) 345–362 CCA-3326

Authors' Review

Marina Cindrić,a,** Zorica Veksli,b and Boris Kamenara

aLaboratory of General and Inorganic Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102 a, HR-10000 Zagreb, Croatia bRuđer Bošković Institute, P. O. Box 180, HR-10002 Zagreb, Croatia

RECEIVED OCTOBER 2, 2008; REVISED APRIL 9, 2009; ACCEPTED APRIL 10, 2009

Abstract. The authors' review compiles the literature in the field of polyoxomolybdates, POMo, and polyoxomolybdovanadates, POMoV, over the past two decades. The main achievements in the field of synthesis, crystal and molecular structure determinations and applications are presented. The first section describes polyoxomolybdates; their structure starting with a smaller number of building units to the formation of new structural shapes of polyoxomolybdate clusters of supramolecular arhitecture. Polyoxo- molybdates include the compounds with various organic ligands, functionalized polyoxomolybdates and a number of organo-inorganic clusters. Recent synthetic methods such as functionalization and hydrothermal/solvothermal synthesis with a possibility to control the size and shape of the resulting structure are reviewed. The design and synthesis of inorganic-organic hybrid compounds in which POMo anions are combined with organic molecular cations appears as the most successful approach for preparing novel types of molecular based materials with functional properties. The second section covers the polyoxomolybdovanadate compounds from structure to synthesis. Interesting compounds with the molybdenum- rich region or vanadium rich-region, mixed vanadium-molybdenum polyoxoanions with various oxidation states of the metal atoms and the organic-inorganic hybrid materials are described. The last section reviews the application of the before mentioned compounds. Due to the functionalization of the complexes potential utility in biology and medicine, catalysis and material science is significantly increased. Final- ly, the importance of large polyoxometalate clusters as target nanosized molecular materials, which present a challenge for the future complex chemistry, is discussed. Keywords: polyoxomolybdates, polyoxomolybdovanadates, synthesis, crystal and molecular structure, application

INTRODUCTION pounds have been prepared starting from simple building blocks to very large complex molecular systems by linking a huge variety of basic and well defined frag- Polyoxometalates (POMs) represent a large class of 6,7 metal-oxygen cluster anions which exhibit compositi- ments. The synthesis of the wheel- and sphere-shaped onal diversity, structural versatility and remarkable giant molybdenum oxide based clusters have rised high physicochemical properties.1 Though the first poly- expectation in nanotehnology and material science. oxometalates were described by Berzelius as early as The development of a new class of functionalized 1826, the knowledge of their complex structure prog- polyoxometalates in which an organic ligand replaces a ressed very slowly.2 A number of scientists were in- terminal oxo ligand within the parent structure, have volved to solve the structure and composition of those been extensinvely studied last two decades.5,8,9 Further- compounds, but only the development of modern, more, POMs based hybrid materials namely structures sophisticated and high-resolution instrumental tech- of molybdenum and vanadium oxides contribute to the niques helped to understand fundamental chemistry and specific properties.10,11 The understanding of structure- structure of POMs.3 Due to this knowledge the polyoxo- synthesis relationship of polyoxomolybdates, hybrid metalate chemistry has moved rapidly forward.4,5 In structures of polyoxomolybdovanadates and functiona- recent years a number of new highly complex com- lized polyoxomolybdates followed to assess the desired

* Dedicated to Professor Emeritus Drago Grdenić, Fellow of the Croatian Academy of Sciences and Arts, on the occasion of his 90th birthday. ** Author to whom correspondence should be addressed. (E-mail: [email protected]) 346 M. Cindrić et al., Polyoxomolybdates and Polyoxomolybdovanadates molecular structure and morphology in order to extend a more octahedra may be accomplished by the rule of variety of important applications such as catalysis, ana- compactness. Under specific conditions it is possible to lytical and clinical chemistry, photochemistry, solid build step-wise from simple packing blocks of the octa- state devices, biochemistry and biomedical appli- hedra larger molecular systems of a size even compara- cation.5,12 ble to that of proteins.6 In the majority of structures molybdenum atoms are octahedrally coordinated by six The review covers the synthesis, structure and ap- oxygen atoms, although known are coordination poly- plications of polyoxomolybdates and molybdovanadates hedra with four (tetrahedral) and five (square pyramidal in the last decade. The achievements of the new synthet- and trigonal bipyramidal) oxygen atoms. (MoO )2 is ic methods and potential for the formation of nanoscop- 4 tetrahedral, (Mo O )2 two tetrahedra sharing common ic molecules in POMs cluster chemistry and their struc- 27 corner, trimolybdates are polymeric with infinite chains tural diversity is emphasized. of octahedra with common edges and corners (possible are additional tetrahedra or square pyramids). POLYOXOMOLYBDATES Di-, Tri-, Tetra- and Pentamolybdates A number of molybdenum oxalates of the general

Characteristic Structures formula R[MoO(CO)(HO)]225242222 were prepared and analyzed.15–17 The complexes are built up of the POMs can be described in terms of assemblages of dimeric anions containing Mo O core. Molybdenum metal-centered MO polyhedra that are linked by 25 n atoms exhibit distorted octahedral coordination. It shared corners, edges and rarely faces. According to M. should be pointed out that the angles at the bridging T. Pope the structures of polyoxoanions are the result oxygen atom in dinuclear anion differ significantly both of a favourable combination of ionic radii and depending upon the size of the cation, crystal structure charge and of accessibility of empty d orbitals for met- packing conditions and by additional hydrogen bonds. al-oxygen π-bonding. Their structures are governed by The cation of larger ionic radii may induce distortion of two general principles:3 the dimeric anion causing bending of the Mo–O–Mo 18 - Each metal atom occupies an MOn coordination bridge, and it has also the effect on the polymeric polyhedron (most commonly an octahedron or structure (Figure 1).17 square pyramid) in which the metal atoms are displaced as a result of M–O π-bonding towards those polyhedral vertices that form the surface of

the structure. Structures with MO6 octahedra that contain three or more free vertices are, in general, not observed. - The second principle known as Lipscomb restriction, has been rationalized in terms of the strong trans influence of the terminal M–O bonds,

which facilitates dissociation of neutral (MO3 ) from the cluster.13 This restriction is not absolute, particularly in aqueous solutions. In general, the polyoxoanion structures are packing of octahedra with common edges, but observed are only some of the possible packing combinations. Ac- cording to D. L. Kepert such packing is a result of the repulsion forces between Mn+···Mn+ ions.14 Following this Kepert ideas the empirical «rules of compactness» have been developed. According to these rules in the packing of the octahedra with common edges the fundamental structural motif for POM is the pair of edge- sharing octahedra. Applying such a principle the shape of poly-anions can be derived by adding additional octahedra in the edge–sharing fashion to this structural motif. Thus the third octahedra may be added in several 2 ways to generate possible trinuclear core structure. Fur- Figure 1. Polymeric structure of [Mo25 O (C 242 O ) (H 2 O) 2 ] ther expansion of the aggregates by linking forth or anions (adapted from Ref. 16).

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8 Figure 4. Structure of polyanion [(Mo5172 O ) (4,4'-bipy)] (adapted from Ref. 25).

compesation is achieved by the protonated piperazine of 2 20 Figure 2. The structure of [Mo24 O (OCOCH 36 ) ] anion the skeleton. (adapted from Ref. 19). By hydrothermal approach the alkali trimolybdates were prepared with a distorted variation of Those complexes of MoVI are of interest owing to (Mo O )2 chains as a key structural motif found their photochromic properties. According to ESR meas- 310 previously only in (C H N )[Mo O ] complex.21 The urements the exposure of the complex to UV radiation 2102 310 arrangement of the Mo–O polyhedra within the chain of changes oxidation number of Mo from +6 to +5. The alkali trimolybdates and the resulting structure strongly first structuraly characterized example of dinuclear depends upon the size of alkali cation (Rb, K, Cs).22 molybdenum(VI) acetato complex was Na224 [Mo O - 19 (OCOCH36 ) ] NaOCOCH 3 CH 3 COOH. Dinuclear The tetranuclear polyoxomolybdate(V) anion 2 2 complex anion, [Mo24 O (OCOCH 36 ) ] , is a centro- [Mo48 O (OH) 2 (H 2 O) 2 (C 242 O ) ] is built up of four symmetrical interconnected by sodium cations and edge–sharing distorted octahedra, separated in two pairs centrosymmetrical pairs of acetate-acetic acid. Two corresponding to MoV–MoV dimers with localized met- acetate ligands are monodendately bonded to each al–metal bonds. Two μ2-bridging ligands stabilize the molybdenum atom while the third one together with oxomolybdate core. Valence bond calculations have its centrosymmetrical pair acts as a bridge with Mo–O shown that two bridging oxygen atoms are hydroxo distances of 2.169(2) and 2.222(2) Å. They are the ligands and two terminal oxygen atoms belong to water longest Mo–O bonds due to the trans influence of the molecules.23 Another tetranuclear polyoxomolybdate 19 terminal oxo-oxygen atoms (Figure 2). complex [(CH34 ) N] 2 [Mo 4 O 10 (OCH 34 ) Cl 2 ] with a cen- trosymetric anion has four edge-sharing octahedra, two A very interesting Mo complex prepared solvother- MO and two MO Cl. As a result of displacement of mally (CH N)[(MoO)(MoO)(CO)]2HOVVI  6 5 41222 2 4 4 242 2 metal ions towards the polyanion surface, all four octa- reveals that the compound consists of novel molybde- hedra are distorted. All bond lengths and angles are num–oxide helical chains with oxalate groups coordi- comparable with those previously observed tetranuclear nated directly to the Mo(1) and Mo(2) sites (see Figure complexes.24 3). The 1D helical chain is composed of the dinuclear units of edge-sharing (MO6 ) octahedron linked by A dimeric pentamolybdate prepared hydrotherma-

(MO4 ) tetrahedron through corner-sharing. Charge ly [4,4'-H23 bipy] [4,4'-Hbipy] 25172 [(Mo O ) (4,4'-bipy)] 8 is an example of polyanion [(Mo5172 O ) (4,4'-bipy)] with two identical pentamolybdenum ions and one 4,4'-bipyridine molecule (see Figure 4).

The anion is composed of four MO6 octahedra

and one MO4 N unit with trigonal bipyramidal coordination geometry. The five protonated 4,4'-bipyridine molecules act as counter ion to the polyanion thus giving charge-compensating and space-filling metal oxide/organic material structure.25 Hexa- and Heptamolybdates Hexamolybdate anion is well known in the structural chemistry of polyoxomolybdates. In this structure oxy- Figure 3. Different coordinations of Mo atoms in gen atoms form close packed structures (three layers) in VVI2 [(Mo24 O )(Mo O 4 )(C 242 O ) ] anion (adapted from Ref. 20). a cubic close packing feature with Mo–atoms occupying

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2 octahedral holes, imitating a fragment of the sodium phenylimido derivatives of (Mo619 O ) were prepared chloride structure. The crystal structure of bis(tetra- and characterized.9 Mono- and di- phenylimido com- 2 2 methylammonium) hexamolybdate hydrate is an exam- plexes [Mo618 O (NPh)] (1) and [Mo617 O (NPh) 2 ] (2) 2 ple of Lindquist type hexamolybdate anion (Mo619 O ) . were found in the solution and recrystalized samples. Each Mo atom is coordinated by six O atoms (one The unit cell of both 1 and 2 complexes contains four terminal, one central and four bridging) in a distorted hexamolybdate anions and eight tetrabutylammonium octahedral arrangement due to the displacement of Mo cations. Both samples are a mixture of at least two dif- atoms toward the terminal O atoms. The six MO6 octa- ferent anions. The substitution introduces a distortion of hedra share common vertex at the central O atom and the hexanuclear core of molybdenum. Some Mo–O also each octahedron shares four edges with adjacent bonds are significantlly shorter than the average of other octahedra.21 Recently most of the hexamolybdates26 and Mo–O distances which could indicate the trans influ- octamolybdates were functionalized i.e. replacing of one ence of the phenylimido ligand. The NMR results, or several oxo ligands by other ligands. By introducing Raman and electrochemistry results confirmed the suitable terminal ligands the properties of polyoxomolyb- above mentioned species and the characteristics of the dates can be tuned and their application expanded in individual components in solution. different fields. A series of Lindquist-type aryldiazemido The [Mo O ]6 anions have been confirmed as polyoxomolybdates of the type (Bu N) [Mo O - 724 43 618 the predominant species in aqueous solution at pH = 3 (N Ar)] have been investigated.27 The aryldiazemido 2 to 5.5. The heptamolybdate anion has been characte- ligands exhibit features characteristic of the singly bent rized by numerous solution studies and by the structures coordination mode with short Mo–N and N–N bonds of [NH ] [Mo O ] 4H O,28 K[MoO ]4HO, 29 for indicative of multiple–bond character. The substituents 46 7 24 2 67242 example, all of which may be crystallized from aqueous on the aromatic rings influence the physical and chemi- solution. Raman spectroscopy and X-ray investigation cal properties of the hexamolybdate which have been confirm that the structure of the anion is unchanged in examined in details by 95Mo and 17O NMR spectrosco- solution. A large number of POM-based organic and py, UV/VIS spectroscopy.27 A large class of organo- inorganic hybrid materials have been prepared. In the imido derivatives of (Mo O )2 comprising both singly 619 subclass of hybrid materials based on isopolymolybdate and multiply functionalized hexamolybdate has been cluster units, however, the polyanions are limited to prepared.8 In the case of singly functionalized imido- {Mo O }, {Mo O }, {Mo O }, and {Mo O }, clus- hexamolybdates by three ligands (butyl, cyclohexyl, 310 418 619 826 ters. Hybrid solid materials based on heptamolybdate 2,6-diisopropylphenyl), the ligand occupies a terminal are rare and therefore, it remains a great challenge to site of hexamolybdate cage. The short Mo–N bond prepare new heptamolybdate-based organic and inorganic lengths (1.711–1.739 Å) are typical of organoimido hybrid materials. One of the interesting examples is ligands bound to an octahedral d0 metal centre and are a new supramolecular compound, (Hapy) [Co(H O) - consistent with a substantial degree of M≡N triple bond 425 Mo O ] 9H O,30 isolated by hydrothermal synthe- character. Multiple functionalization has been achieved 724 2 sis. with 2,6-(diisopropyl)phenilimido ligand (NAr). The organoimido ligands, starting from 2 to 5, occupy termi- Octamolybdates and Related Compounds nal position and the metrical parameters associated with The main characteristic of the octamolybdates is their the [Mo≡NAr] units are unexceptional. Within each of diversity. The octamolybdate anions are built up from the multiply supstituent species the central oxygen is eigth condensed octahedra with 16 terminal positions. again displaced away from the [O≡Mo] site toward the The bond lengths depend again upon the nature of the trans-[Mo≡NAr] site. Mo–Ob (Ob – bridging oxygen bond as well as upon the position within the structure atom) bond lengths are generally longer at [Mo≡NAr] (e.g. cis or trans with respect to the Mo=O bond). The sites than at [O≡Mo] sites. Functionalization induces a α-, β- and γ- structures are based on the cubic close range of perturbations. Each of the organoimido ligands packed structure with Mo atoms occupying the octahe- examined furnishes more electron density to the hexa- dral sites (Figure 5). molybdate cage than does an oxo ligand rendering the 2 [Mo618 O (NR)] complexes more difficult to reduce 2 than the (Mo619 O ) parent. One of the structural conse- quences of this donation is a consistent lengthening of the Mo–O bond length at the imido bearing sites as compared to those at the terminal oxo sites. Multi- nuclear NMR results suggest that the electron density α β γ accumulates preferentially at the terminal oxo ligands rather than within the (Mo613 O ) core. In addition, Figure 5. Structures of α-, β-, γ-polyoxooctamolybdates.

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The recent expansion of coordination chemistry of centrosymmetrically edge-sharing octahedra.33 Similarly the polyoxooctamolybdates displays interesting metal- to the previously described structures, out of 16 terminal ligand reaction chemistry. Most of such octamolybdates positions 14 are occupied by oxo-oxygen atoms and two (2n 4) have a general formula [Mo8262 O (L) ] where n is a by the salycilideneiminato (1) or methioninato oxygen formal charge of L. Two sites of the Mo828 O core are (2) atoms. Of the remaining 12 oxygen atoms six are usually occupied by the ligand L. A number of octa- double-bridging two Mo atoms, four O atoms are triple- molybdates functionalized with organic ligand have bridging three Mo atoms, while two O atoms are four shown antitumor activity. This is the case with γ-octa- bridging four Mo atoms. The consequence are varying molybdates with aminoacids (DL-alanine) and peptides: Mo–O distances from 1.77 to 2.44 Å. In both cases

Na48262 [Mo O (alaO) ] 18H 2 O (1) (Figure 6), Na4 - salycilideneiminato and methioninato–oxygens are

[Mo826 O (glyglyO) 2 ] 15H 2 O (2) and Na4826 [Mo O - stronger donors so that Mo–O but not Mo–N or 31 (glyglyO)22 ] 12H O (3). All three crystal structures Mo–S bonds are formed. are composed of γ-octamolybdate complex anion, A new polyoxomolybdate supramolecular archi- sodium cations and water molecules. The anions consist tecture was obtained from γ-(Mo O )4 anions with of eight condensed edge-sharing MO octahedra with 826 6 nicotinato ligands (Himi) [Mo O (nic O) ].34 (imi = 16 terminal position and 14 of them occupied by oxo- 48262 imidazole, nic-OH = nicotinic acid). The nicotinic acid oxygen and two by carboxylato-oxygen anions as ligand coordinates the octamolybdate unit by its carbo- shown in Figure 6. The bond lengths in all three octa- xylic oxygen atom. The oxygen atoms within anion molybdate anions are mutually in very good agreement. clusters can be divided in four sets according to their Main structural difference in those three structures is bonding feature as in the above described structure. observed in their packing diagrams. In Na [Mo O - 4826 Each [Mo O (nic O) ]4 cluster is connected to four (alaO) ] 18H O, solvated water molecules makes these 826 2 22 adjacent clusters by two kinds of hydrogen bonds: crystals relatively unstable. The glycylglycine ligands in N–H···O and C–H···O. The hydrogen bonding contacts structures 2 and 3 have different packing mode. In the result in an extended 2D network in (101) plane, while same group of functionalized octamolybdate complex- the hydrogen bonding between imidazole cations and es displaying biological activity the complex of 32 anion clusters assembles in the 3D supramolecular struc- Na [Mo O (proO) ] 22H O is included. The DL- 48262 2 ture. Polymeric octamolybdate clusters with organic proline ligands are attached to molybdenum atoms via cations participating in bridging interactions were found monodendate carboxylate-oxygen atoms. An intramole- in the morpholinium β-octamolybdate tetrahydrate.35 cular hydrogen bond is formed between the proline A polymeric oxomolybdate(VI) (Me-NC H ) - amino hydrogen, while the free carboxyl oxygen atom 554n (Mo O ) consists of N-methylpyridinium cations and of the proline ligand is contacting oxygens of a sodium- 826n infinite anionic molybdenum oxide chains built of the water chain. eight-octahedra units and connected through pairs of

The structures of [NH32 Pr] [Mo 8224 O (OH) - Mo–O–Mo bridges into extended one dimensional 36 (OC64 H CH=NPr-2) 2 ] 6MeOH (1) and [Hmorph]4 - arrays (Figure 7).

[Mo824 O (OH) 2 (MetO) 2 ] 4H 2 O (2) consist of eight Similar chain structure is found in (C41422 H N ) - 37 (Mo826 O ) 2H 2 O, where eight (MoO6 ) octahedra 2n share both edges and vertices, forming the (Mo826 O )n 2 chains. (C4142 H N ) cations and occluded water molecules reside between chains and participate in an extensive hydrogen-bonding network thus stabilizing the structure. A number of organic-inorganic hybrid materials using POMs as building blocks have been recently suc-

cessfully prepared. The hybrid compound [Co(bpy)32 ] -

(Mo619 O )[β-(H 2 Mo 826 O )] 4H 2 O consists of racemic

4n Figure 6. Structure of anion in Na4826 [Mo O (glyglyO) 2 ] Figure 7. Infinite chains of [Mo826 O ]n anion (adapted from

15H2 O (adapted from Ref. 31). Ref. 36).

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2 complexes (– / )[Co(bpy)3 ] and both (Mo619 O ) and the left-handed units present in adjacent chains. The 4 38 β-(Mo826 O ) anions. The most remarkable structural structure of [NH32 (CH )NH 222321033 (CH ) NH ] (Mo O ) 4 2 distinction of β-(Mo826 O ) from (Mo619 O ) is that (3) is made up from the connectivity between the the former contains MO6 octahedron with two terminal (Mo826 O ) cluster units and (Mo87 O ) units. The result-

O atoms, while the latter involves MO6 octahedron ing (Mo10 O 33 ) chain has a zigzag character. The organ- with only one terminal O atom. Furthermore, ic amine molecules occupy the interchain regions. 2 [Co(bpy)3 ] complex parts are in racemic enantio- Those compounds provide a novel example of the reten- morphous architecture. Each Co2+ site is surrounded by tion of the geometry of the molecular building blocks in six nitrogen atoms from three 2,2'-bipyridine molecules the synthesis of the solid-state materials. and forms a distorted CoN octahedron. In the solid 6 On the example of the nitrosyl derivative of deca- state these three subunits are presented in the respective- molybdate [Mo O (OMe) (NO)] an important role ly independent form, though there is O···H–O hydrogen 10 25 6 of delocalization of «blue electrons» is presented.42 bonding between β-(Mo O )4 anions mediated by free 826 Several isomers differing by position of methyl groups water molecules. have shown that the electron delocalization patterns can Another polyoxomolybdate organic hybrid is a be modulated. Two Mo–O–Mo interactions are respon- compound (Bu48 N) (Mo 6192 O ) [α-Mo 826 O ] in which sible for the electron delocalization, namely within the 2 4 (Mo619 O ) coexists with α-(Mo826 O ) anions alterna- same equatorial plane and between two planes. The tively arranged in crystal architecture.39 The crystal study also demonstrated the transition from the delocali- structure is presented as layers of both anions, alterna- zation to the magnetic exchange mechanism of spin tively arranged in (011) planes. The counterions pairing in reduced polyoxomolybdates. This transition is  (Bu4 N) are regularly distributed in the interspace a consequence of the chemical bonding perturbation by formed by polyanions and show three kinds of weak the protonation of bridging oxygen atoms in Mo–O–Mo interactions with inorganic layers. Two new organic- fragments. Thus the polyoxomolybdates appears to be a inorganic hybrid compounds based on polyoxomolyb- versatile building block for the construction of mole- dates differing in their solvent content were explored; cular magnetic materials. commensurate (H DABCO) [Mo O ] 4H O (1) and 228262 The formation of new structural types of polyoxo- incommensurate (H DABCO) [Mo O ] 4.66H O (2) 228262 molybdate cluster has been recently demonstrated. (1,4-diazabicyclo[2.2.2]octane = DABCO).40 The com- Away from the symmetrical spherical structures (Lind- pound 1 is composed of self-assembly of (Mo O )4 826 quist, Keggin, Dawson) a strategy to define a new type anionic chains, (H DABCO)2 cations and water mole- 2 of nonspherical, low–symmetry and lower–nuclearity cules. The anionic chain is built up from γ-(Mo O )8 828 clusters as building blocks have been presented.43 Such octamolybdate cluster. The structure of 2 resembles that clusters restrained by «shrink–wrapping» display an of 1 except by its water molecule and (H DABCO)2 2 unprecedented topology, low symmetry and high nega- content and ordering, which generate the incommesura- tive charge: (C H N ) (H Mo O ) 34H O (1) and bility. The complex hydrogen-bond network is observed 6134102 1652 2 (C H N ) [Fe (H O) H Mo O ] 8H O (2). not only between the cations and the water molecules or 61346 2 2 8 2 1652 2 between the anionic chains and the water molecule, but The cluster 1 consists of negatively charged 10 also between some of the cations and the polyoxometa- framework of [H21652 Mo O ] and displays an unusual VVI late chains. Though this kind of hydrogen bond is a pre- flat shape with Mo centers to be Mo412 Mo . The four requisite for their photochromic properties none of the MoV centers comprise two centrosymmetrically related V compounds is photochromic. Under hydrothermal condi- Mo2 group which display a short Mo–Mo contact of tions a variation of the structures of β-octamolybdate, 2.642(7) Å, characteristic of Mo–Mo single bonds. The

(Mo826 O ), clusters were obtained. A variation of the «body» of the cluster consists of central unit with structures is mainly due to the differences in the connec- twelve Mo atoms and two «wings» each with two 41 tivity between the (Mo826 O ) clusters. The structure of molybdenum centers. In the case of 2 two[Fe224 (H O) ]

[NH32 (CH )NH 328262 ] (Mo O ) H O (1) consists of octa- units are each coordinated to the two terminal oxo ligands hedrally coordinated Mo atoms connected through their attached to MoVI and MoV. This compound also indicates 2 2 edges forming (Mo413 O ) units. Two (Mo413 O ) weak but significant intramolecular antiferromagnetic subunits give rise to β-octamolybdate clusters forming exchange interactions between FeII centers. Another 10 infinite anionic one–dimensional chains. The structure mixed–valence polyoxomolybdate [H21652 Mo O ] using  of [NH32 (CH )NH 22232930 (CH ) NH ] (Mo O ) (2) is built protonated hexamethylenetetramine (HMTAH) as up of similar β-octamolybdate cluster units. The MO6 counter ion exhibits significant nucleophilicity and can octahedra occupy a special position sitting on the inver- trap electrophiles such as divalent transition metal ions sion center. The one-dimensional chain of the formula resulting in a family of isostructural compounds.44 The

(Mo930 O ) have helicity with both the right-handed and highly reactive {Mo16 } system undergoes to rearrange-

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Table 1. Giant polyoxomolybdate clusters formulated accord- Table 2. Polyoxomolybdates formulated according to building 45 ing to metal-based building blocks {(Mo)Mo5} core and ligands Building Building block/ Ligand Synthesis Reference Ligand Synthesis Reference core anion

acetate solution, solvother- 19 4 4 Mo O (Mo O ) [Hapy] hydrothermal 29 24 oxalate mal 20 724

n  Mo25 O oxalate solution 15–17 (Mo826 O ) [Me-NC254 H ] hydrothermal 36 Mo O oxalate solution 23 (Mo O ) 2n 48 826n (C H N )2 water solution 37 (Mo O )2 4142 2-hydroxy- 619 Mo O solution 24 410 nicotinate β-(Mo O )4 826 [Co(bipy) ]2 hydrothermal 38 (racemic cop.) 3 Mo517 O 4,4'-bipyridine hydrothermal 25 (Mo O )2 methoxy 619 (Bu N) hydrothermal 39 Mo612 O solution 22 4 4 acetylacetonate α-(Mo826 O ) solution/ 4 Mo618 O aryldiazemide 26 functionalization (Mo826 O ) 2 8 (H2 DABCO) hydrothermal 40 solution/ γ-(Mo8 O) Mo619 O organoimido 8, 9 functionalization 4 (Mo826 O ) solution/ 2 2 Mo O salycilideneiminate 32 (two (Mo O ) [NH32 (CH )NH 3 ] hydrothermal 41 822 functionalization 413 units) Mo O methioninate solution 32 824 10  (H21652 Mo O ) (C6134 H N ) solution 43 (mixed valence Mo10 O 25 methoxy solution 42 (MoVVI , Mo )) (HMTAH) solution 44

alaninate 30 glycilglycinate solution/ Mo826 O functionalization prolinate 31 shapes and functionality of large clusters (Figure 8) are described in excellent papers by A. Müller and collabo- nicotinate hydrothermal 33 rators.45,46

As an arhetypical example building blocks con- ment and decomposition reaction. The crystal structure taining 17 metal atoms {Mo } units can link to form 10 17 of [H21652 Mo O ] anion is centrosymmetric with four formal MoV centers organized as two inversion sym- V metry related {Mo2 } pairs with Mo–Mo short distances of 2.612(2) Å a motif typical for mixed-valence polyoxomolybdate structures. Sixteen MO6 octahedra de- compose the cluster into a central {Mo12} building block and two {Mo2} groups. Eight Mo positions of the {Mo12} building block are arranged along the lines to form two inversion-related backbones. The other four Mo centers at the ends of the two backbones unite the two backbones through bridging ligands. The inherent reactivity of highly charged cluster anions is revealed by gradual decomposition to smaller {Mo7} units in the solution and in the presence of divalent transition metal cations pro- duce clusters of different arrangements. {Mo16 M 2 } adducts demonstrate the efficiency of polyoxomolybdates in mediating magnetic superexchange. Described polyoxometalates are summerized in Tables 1 and 2. Giant Polyoxomolybdates Complex polyoxomolybdate clusters are prepared in a step-wise manner from smaller well-defined fragments. Figure 8. Giant polyoxomolybdates (adapted from the Ref. The strategy of self-aggregation and the variety of 45).

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Table 3. Polyoxomolybdates: organo-inorganic hybrid materials Cluster Formulation based on largest building blocks Smaller building blocks of the larger groups

{Mo36 } {Mo17 } 2 {Mo 1 } 2 {Mo17 } {(Mo)Mo 5 } 2 {Mo} 5

{Mo146 } {Mo816 } {Mo 29 } {Mo852 } {(Mo)Mo }{Mo}

{Mo154 } {Mo814 } {Mo 214 } {Mo 114 } {Mo852 } {(Mo)Mo }{Mo}

{Mo176 } {Mo816 } {Mo 216 } {Mo 116 } {Mo852 } {(Mo)Mo }{Mo}

{Mo368 } {(Mo)Mo58 } {(Mo)Mo 532 }  {Mo 216 }  {Mo 28 }  {Mo 28 }  {Mo} 64

cluster anions containing two or three of these units. 12{Mo11}-polyoxomolybdate fragments of fivefold The structure of {Mo17} unit can be reduced to two symmetry and they are generated from central penta-

{Mo8}-type group linked by {Mo1}-type unit. The gonal bipyramidal {MoO7 } groups (Figure 9). {Mo } building block is found in many other large 8 The ring-shaped mixed-valence polyoxomolyb- POMo structures.47,48 Some of characteristic clusters dates of the type {Mo }, {Mo } or {Mo } are a and the corresponding formulation based on larger and 154 176 248 new class of giant clusters with nanometresized cavi- smaller building blocks are presented in Table 3. ties53 and interesting for host-guest chemistry. From the Stability of the smaller clusters, determined by the wheel-shaped {Mo176} cluster the {Mo248} cluster can solution conditions, is crucial in the formation of larger be formed by addition of further units to the inner sur- cluster such as wheel-shaped cluster {Mo154}, {Mo176} face of the {Mo176} wheel. The crystal structure of both and even more {Mo248} cluster. The ring–shaped clus- ring–shaped clusters have practically the same lattice ters can act as hosts for smaller polyoxomolybdate clus- constants and show the same type of packing of the two ter, for instance {Mo36} unit. The strategy of the forma- kinds of clusters having the same diameter. Two tion of large cluster is based on highly negatively {Mo36 O 96 (H 2 O) 4 } units incorporate in such a way that charged fragments preventing thus possible degradation the packing of the wheels is not affected and the struc- and hydrolysis. This is achieved by substitution of metal ture is extended. The peripheral ring region is built centers of lower oxidation states for ones of higher up by {Mo}{(HO)(O)MoOMo(O)(HO)},22term2 term22 oxidation state. Furthermore, increasing the molecular {Mo8} and {Mo} units each occuring 16 times. The size of the system the multi-functionality can be terminal oxygen atoms have a high electron density and achieved. For instance in the case of {Mo57} cluster it is therefore a high affinity for protonation. Thus they are possible to place or exchange paramagnetic centers like the starting point for the subsequent growth and conden- FeII/III and VOII in the linker position and control mag- sation process. netic properties.49 The cavities of the clusters filled with One of the largest polyoxomolybdates, Na - positively charged units can be emptied again by oxi- 48 [H Mo O (H O) (SO ) ]ca. 1000H O (x ≈ dation. These reactions represent a model for uptake and x 368 1032 2 240 4 48 2 16)54,55 has a shape of a hedgehog and the size of hae- release of metal centers in metal storage proteins under moglobin (diameter approximately 6 nm). This deep- redox–active conditions and for a molecular switch.50 blue compound crystallizes in the space group I4mm A new member of giant polyoxomolybdates and the unit cell contains two giant hedgehog-like clus- VI is a compound (NH42361082 )Na [Mo O (NH OH) 2 (OH) 6 - ter anions of D4 symmetry with a central ball-shaped

(H212 O) ] 35H 2 O containing bridging hydroxylamine fragment {Mo288 O 784 (H 2 O) 192 (SO 4 ) 32 } and two capping molecules. Each giant ellipse-shaped cluster anion VI {Mo36 (NO) 2 } consists of two large [Mo15 O 50 (Mo O) 2 - 5 (μ-NH2226 OH)(μ-OH)(OH) (OH ) ] units linked 2 together by two MoO2 centers. These anions

{Mo36 (NO) 2 } are connected via distorted monocapped pentagonal-pyramidal sodium in a three-dimensional structure with channels filled with lattice water mole-  51 cules and (NH4 ) ions. A. Müller and collaborators have been described the VI V salt (NH442 ) [Mo 72 Mo 60372 O (H 2 OCH 3 COO) 30 (H 2 O) 72 ] ca. 300H234 O ca. 10CH COONH , an inorganic super 52 V fullerene, with an integrated icosahedron, namely a Figure 9. Structure of the icosahedral {Mo230 } units with 12 Keplerate with more than 500 atoms and the highest regular pentagons and 20 trigonal hexagons as well as its Euclidian symmetry. The sphere consists of a total of coherence to the fullerene (adapted from Ref. 52).

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units {Mo40 O 124 (H 2 O) 24 (SO 4 ) 8 }. The cluster can be the most simple so-called T = 1 virus type containing 60 described as a structure composed by six building proteins. In both cases the 60 {MoO6 } units are packed blocks, while a giant molecular container with a huge around the fivefold axes. The above spherical cluster cavity (diameter ≈ 2.5 × 4.0 nm) offers space for a large with different pore size can act as an effective and spe- number of about 400 HO2 molecules. The {Mo368} cific nanosponge for complementary substrates. cluster can be visualized as a hybrid between the giant 56 Interesting example is a crown-shaped POMo wheel {Mo }-type and ball {Mo }-type cluster. 176 102 cluster which can accomodate large metal complexes

The {Mo(Mo5 )} and {Mo1} units are found in the with organic ligands in its nanocavity to form discrete 61 ball-type cluster, and {Mo(Mo5 )} and{Mo2} units in inorganic/organic nanocomposite material. the wheel-type cluster. The complete knowledge of the Synthesis {Mo368} cluster offers new options of handling the giant clusters as intact units for the preparation of composite A brief compilation of polyoxomolybdate synthesis is materials. presented. Mostly these are common preparative me- The linking processes follow the well-defined thods in aqueous and nonaqueous solutions. Recent rules and can occur in different phases:57 years the hydrothermal synthesis of molybdenum oxide- based materials has attracted considerable attention. The – the linking units must have appropriate well- hydrothermal method, though still in the phase of explo- defined functionalized external and internal sur- ration, due to the preparative flexibility offers to control faces the complex structure-synthesis relationship in materials – wheel and sphere-shaped nanosized molybde- chemistry. num oxide based clusters fulfil these conditions Synthesis in Aqueous and Non-aqueous Solutions – spherical clusters may be linked to form a chain, The most common synthetic method of polyoxomolyb- layers or necklace dates involves acidification of aqueous solution of simple oxoanions and the necessary heteroatoms. Isolation – crosslinking of spherical clusters produces the of the polyanions is generally achieved by addition of structure with cavities which may host large an appropriate counterion, such as alkali metal, am- anions or guest-molecules. monium or tetraalkylammonium. The recent expansion The linking of nanosized clusters present a pos- of the synthesis of polyoxomolybdates is also a conse- sibility to prepare a new class of materials with well- quence of their solubility in different organic solvents defined channel cavities. Some of those nanoclusters are displaying thus characteristic metal-ligand reaction emerging as a novel type of molecular catalysts. The chemistry. The chemistry of such polyoxomolybdates example is nanosized ring-shaped cluster unit form in non-aqueous solvents has been characterized by VI V ing the compound Na21 [Mo 126 Mo 28 O 462 H 14 (H 2 O) 54 - the replacement of peripheral oxo-groups by organic

(H227 PO ) ] 300H 2 O of 2/m symmetry in which the ligands with O– and N– donors or different inorganic units are linked through covalent Mo–O–Mo bonds to ions as ligands. give a layered structure.58 The packing of these layers Synthesis of molybdenum(VI) complexes with gives rise to nanosized channels with encapsulated oxalate ligands are of continuing interest due to their HPO ligand and water. It is also possible to replace 24 use in analytical chemistry and industry in spite of molecules/ligands at the surface or within the cavities numerous complexes known for many years. A number and subsequently change the properties. The other ex- of new molybdenum(VI) oxalate complexes with a ample is the ring-shaped structure of the compound smaller number of building blocks were prepared from a Na [Mo O H (H O) (HOC (NH )HC CH S S 21 154 462 14 2 48 3 2 mixture of molybdenum(VI) oxide, pyridine, HCl and CH CH(NH ) COO ) ] 250H O which captures 23112 equivalent amounts of oxalic acid dihydrate.18 The addi- the oxidation product of cysteine.59 tion of tetramethylammonium chloride or γ-picoline to It is possible to place molecules at the inner wall the mixture resulted in two more oxalato complexes. of the cavity of a giant metal–oxide based wheel cluster The mixing of molybdenum(VI) oxide, oxalic acid di- by replacing HO2 molecules. With the use of ambiphil- hydrate and ammonium chloride resulted in the molyb- ing and/or multiphilic ligands new surface structures denum(VI) complex with interesting intermolecular can be generated. Unusual spherical Mo-oxides hydrogen bonds within the dimeric anion.15,16 Further- 42 [{(Mo)Mo5212 O (H O) 612 } {Mo 24 O (L) 30 }] (organic mol- more, the reaction of molybdenum(VI) oxide with ox- ecule or HO)2 open up new techniques in supra- alic acid or alkali oxalate and alkali halides gives two VI molecular nanotechnology by a large number of series of Mo oxalate complexes containing Mo25 O 60 17 pores/receptor sites. The compound is analogous of and MoO3 core, respectively.

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Synthesis and characterization of acetato com- solution with an appropriate reducing agent and con- plexes of molybdenum(IV), (V) and (VI) in the mixture trolled pH ring, tube or wheel shaped self-assemlies 53,66 of appropriate molybdenum precursors, acetic acid and containing Mo154 to Mo248 clusters are synthesized. acetic acid anhydride are of interest to understand the Hydrothermal/Solvothermal Synthesis reaction of (MoO )2 with an excess of acetic an- 4 Hydrothermal method appears as a very powerful and hydride. The characterization of the products helps to flexible synthetic method in inorganic chemistry explain how the reduction of molybdenum depends (Scheme 1) ranging from nanoparticles, mesoporous upon the reaction conditions, reducing agent and start- compounds to single crystals with cm-dimensions.22,67,68 ing components.19 In order to understand the role of molybdenum as a catalyst in the esterification reactions the polyoxomolybdate complex with smaller number of building blocks (Mo410 O ) was prepared from molybdenum(VI) compound, methyl ester of 2-hydroxy- nicotinic acid in a methanol suspension.24 Hexamolyb- 2 date anion (Mo619 O ) incorporated in the structure of bis(tetramethylammonium)hexamolybdate hydrate is obtained from a mixture of MoO3 in water, glycine and Scheme 1. Systematic hydrothermal pathway to new alkali 62 tetramethylammonium salt. The same MoO3 oxide trimolybdates. was also used in the mixture with HSO24 and dimethyl- 2n 37 ethylenediamine to obtain (Mo826 O ) anion. The reaction parameters in the closed autoclave system at elevated temperatures, which determine com- A novel class of POMs with two kinds of poly- plex structure-synthesis relationship, are still in the oxomolybdate anions: (Mo O )2 and (Mo O )4 619 826 process of exploration. Polyoxomolybdates and poly- have been sythesized as a rare example with two kinds oxomolybdovanadates due to the ability to form a large of anions. The compound was prepared by the reaction number of structures, represent a good framework for of Bismarck Brown Y N,N'-dicyclohexylcarbodiimide hydrothermal synthesis. A number of POMs have been and (Bu N) [α-Mo O ] in anhydrous acetonitrile 44 826 prepared by hydrothermal and solvothermal method so under nitrogen.39 Recently a mixed valence cluster far starting with smaller building blocks to nanostruc- anions of MoV and MoVI were prepared. Such cluster tures.20,25,36,69 A novel polyoxomolybdate-based organic comprises a type of polyoxommolybdate cluster frame- inorganic complexes with Mo and Mo building blocks work that displays an unprecedent topology and high 6 8 synthesized by the same method represent the first ex- negative charge and nucleophilicity. These complexes ample of racemic complexes.38 Similarly molybdenum are obtained from the solutions by changing the reaction complex with organic ligands (e.g. nicotinic acid) with a conditions and reaction components. The building 3D supramolecular architecture was isolated.34 The blocks can link to larger aggregates.43,44,63 complexes which present novel examples of assembling Aqueous synthesis under different boundary con- the oxomolybdate under hydrothermal conditions are ditions offers a strategy for the formation of complex isolated. The variation in the structure is a consequence molecular systems by linking a large number of molyb- of the differences in the conectivity between the 41 denum oxide building blocks. The size and shape of (Mo826 O ) cluster. clusters can be controlled generally by acidity of the Functionalization of POMo reaction medium.64 Such giant polyoxomolybdate Functionalization of polyoxomolybdates i.e. the re- anions can contain bridging hydroxylamine moities.51 placement of one or several oxo ligands by other By performing the synthesis in aqueous solution of ligands, provides molecular model for selective oxi- Na MoO , hypophosphorus acid and in the presence of 24 dation and a route to their application in catalysis, medi- electrolyte salt nanosized ring-shaped layered multi- cal application and supramolecular chemistry. Organic functional structures are generated with MoVI and 126 ligands may also stabilize some unstable species. A MoV58,65 . When an aqueous solution of ammonium 28 tetranuclear oxomolybdenum(V) complex with bridging molybdate is reduced to medium ratios of MoV/MoVI at squarate ligands were synthesized from Na MoO  given pH values, MoO is generated and stabilized by 24 7 2H O in acidic water solution.23 acetate ligand and the spherical anion is formed in orga- 2 52 2 nized shape such as super fullerene. Strictly control- Functionalization of (Mo619 O ) anion was per- 2 ling (MoO4 ) solution with the acid, compounds with formed in the reaction mixture with Ph3 NPh in an- giant mixed valence cluster anions of sphere or hedge- hydrous pyridine.9 A convenient method for the syste- hog shape containing 368 Mo atoms can be pre- matic introduction of a variety of organoimido ligands 54,55 2 pared. In an acidified aqueous (MoO4 ) containing at terminal oxo sites in the hexamolybdate cluster was

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Scheme 2. Substitution of imido ligand into a hexamolybdate complex.

proposed. The reaction was performed in the mixture of The compound [N(CH344 ) ] (H 2 MoV 9 O 28 )Cl hexamolybdate ion and the various isocyanates in pyri- 6H2 O is an interesting example with only one Mo dine solution (Scheme 2).8 atom.70 The anion displays the structure of the decavanadate with the Mo atom substituted at one «capping» Several substituted octamolybdates anions vanadim site. Owing to the 2/m symmetry of the anion (Mo O L )24n with 16 terminal positions, two of 8262 the Mo atom is randomly distributed over four «cap- which are occupied by the ligands, were described start- ping» metal atom positions. All metal-to-metal dis- ing from the [MoO22 (sal) ] (salH = salicylaldehyde) and 33 tances vary from 3.065 to 3.207 while the V–O dis- MoO22 Cl in methanolic solution or water (Scheme 3). tances within VO6 octahedra are also similar to those already observed and depend upon the type of oxo ligand. The protonation was assigned to the triply- bridging oxygen atoms, while the doubly-bridging oxygens are a second choise and are protonated to a lesser

extent. The Mo–O bond number within the (Mo / V)O6 octahedron amounts 5.25, the theoretical value required for the positions occupied by vanadium and molybdenum in the ratio 3 : 1.

(Hmorph)645272 (Mo V O )Cl H O (morph = mor- pholine) is a first example of this type molybdovanadates obtained from aqueous solution.10 The compound is a hybrid of the decavanadate structure in the vanadium-rich region and the β-octamolybdate structure in the

molybdenum-rich region. It is built up of nine edge- Scheme 3. Substitution reactions into octamolybdate anions sharing MO octahedra but with a random distribution (adapted from Ref. 33). 6 of Mo and V atoms in two «capping» metal atom sites in the vanadium–rich region. Since the ratio Mo/V is A series of aryldiazenido compounds with 4/5, the anion is asymmetrical indicating that the crystal (Mo O )2 anion were functionalized in acetonitrile or 618 structure contains a statistical distribution of two chiral in methanol. The ligands in these isostructural com- (Mo V O )5 anions (Figure 10). pounds can act as π-donor or/and π-acceptor ligands.27 45 27 It has been shown recently that the γ-type octamolybdates coordinated by DL-alanine or glycilglycine prepared in aqueous solution of Na24 MoO depending on the acidification agent, exhibit biological activities, namely antitumor activity.31

POLYOXOMOLYBDOVANADATES

Characteristic Structures Contrary to the numerous well explored polyoxomolybdates relatively little is known about the structures of polyoxomolybdovanadates. MoVI and VV have similar ionic radii (0.73 and 0.68 Å) and coordinations, thus a variety of combinations of the compositions and structures of the oxometalate anions could be expected. They 5 are mostly molybdenum- or in some cases vanadium- Figure 10. View of (Mo45 V O 27 ) anion (adapted from Ref. rich polyoxometalates. 10).

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The distortion of the octahedra is significantly e.g. (NH46 ) [Mo 6 V 2 O 24 (C 2 O 42 ) ] 6H 2 O and (NH44 ) - 75 greater for MoO6 than for VO6 . The protonation [H222122422 Mo V O (C O ) ] 2H O. The molybdovanadate 6 scheme is also interesting. Whereas in the molybdenum- anion [Mo62 V O 242 (C O 42 ) ] is composed of six MoO6 rich region both terminal and doubly-bridging oxygen and two VO6 edge-sharing octahedra of the 4 atoms are protonated, in vanadium-rich region only γ-(Mo826 O ) type structure. The centrosymmetric 4 doubly and triply-bridging oxygen atoms are proto- tetranuclear anion [H22212242 Mo V O (C O ) ] adopts the nated. Such a hydrogen bond scheme is a result of the structure with a (M416 O ) core typical for both molyb- sequence of the negative charge on the oxygen atoms, as date-ligand and vanadate-ligand system. Bidendate well as of the steric effects of the large morpholinium oxalato ligands in both complexes are bonded to the cations and crystal structure packing. vanadium ions and in both crystal structures molybdovanadate anions are interconnected by numerous hydro- The polyoxo anion [Mo V O ]6 found in 62 26 gen bonds through ammonium ions and water mole- K[MoVO ]4HO 71 is isostructural with the 662262 cules. The third molybdovanadate compound coordi- β-octamolybdate [Mo O ]4 anion72 and with the 826 nated by oxalato ligands, K[MoVO (CO)]6HO β-hexamolybdodivanadate in the mixed potassium 662242422 73 reveals centrosymmetric molybdovanadate anion which sodium salt KNa[MoVO562262 ]4HO. consists of six MoO6 and two VO6 edge-sharing octa- IV V 4 76 The compound (Hmorph)610 [(V ,V ,Mo )- hedra to give the γ-(Mo826 O ) structure. VO ] 3H O 74 also contains morpholinium cations but 40 2 The first example of a molybdovanadate coordinat- higher ratio of Mo/V atoms. The molybdovanadate ed by an amino acid (glycine), was K[HMo-VI anion adopts the well known Keggin structure with a 26 VOV77 (NHCHCOO)]8HO . The molybdovana- VOV tetrahedron at the centre of 12 surrounding MO 22 3 2 3 2 4 6 date anion is built up of six MoO edge-sharing octahe- octahedra with a random distribution of 10MoVI, one 6 dra connected into a ring centered by a VO tetra- VIV and one VV ions between 12 metal atom positions 4 hedron. The MoO octahedra are in pairs bridged by (Figure 11). 6 glycine through its carboxylato group. Three MO octahedra sharing common corners 6 Neutral heteropolyoxometalates [MoVI Mo V V IV - form a (M O ) group. Four such groups linked together 78 313 O (PO )]M(phen) (OH) ] [M(phen) (OEt)] x H O by sharing common edges form the structure of the anion. 40 4 2 2 2 2 2 2 (M = Co, Ni) are novel windmill-like trimetalic nano- All hydrogen atoms from the morpholinium-NH groups 2 clusters.78 The mixed molybdenum-vanadium polyoxo- participate in the hydrogen bonding between cations, anion [MoVI Mo V V IV O (PO )] 4 exists in both com- molybdovanadate anions and water molecules. 78404 plexes acting as a bridge to covalently link two pairs of Several molybdovanadates with coordinatively transition metal complex fragments. Each vanadium bound oxalato ligands were described in the literature, atom exhibits a distorted VO5 square pyramidal en- vironment, and eight vanadium atoms form a central

belt by sharing edges of VO5 square pyramids. There

are two {Mo418 O } rings bonded above and below this V8 belt. The hydrogen bonds between the lattice water molecules play an important role in stabilizing the molecule in crystal structure. The complexes represent the first example of zero-dimensionality neutral mixed molybdenum-vanadium polyoxometalates with covalently bonded transition metal complex fragments. In the reaction of molybdenum(VI) oxide and ammonium IV vanadate the triclinic K88 [Mo (V )V 4402 O ] 10H O (1) IV 79 and monoclinic K88 [Mo (V )V 4402 O ] 9H O (2) were obtained. The structure of the same anion IV 8 [Mo8440 (V )V O ] in both compounds is based on the

central VO4 tetrahedron surrounded by a puckered ring

of eight edge-sharing MoO6 octahedra with four VO4 tetrahedra filling the gaps between the pairs of octahedra. The ESR spectra confirm that the compounds contain one V atom in the reduced +4 state. The bond Figure 11. Structure of the molybdovanadate strength calculations indicate that this extra electron is IV V 6 [(V ,V ,Mo10 )- VO40 ] anion with the surrounding mor- delocalized over four peripheral vanadium ions. The pholinium cations and water molecules (adapted from Ref. polyoxomolybdovanadate anions in both structures are 74).

Croat. Chem. Acta 82 (2009) 345–362 M. Cindrić et al., Polyoxomolybdates and Polyoxomolybdovanadates 357 interconnected by potassium ions and water molecules tains two types of heteropoly anions: tetra-capping resulting in a very complicated three-dimensional net- Keggin anions and hexadeca-metal host shell anions.83 work in the crystal structure. Another two-dimensional mixed polyoxomolybdovanadate complex [PMoVI Mo V O (V IV O) {Co(phen) } ]- Polyoxomolybdovanadates in a form of α-Keggin 84402 22 (H O) (phen = phenanthroline) exhibits two distinct fragments can be linked to chains and the compound 2 [PMo V O ] clusters.84 One is covalently linked by [(H en) H O][Mo V O (VO )(VO) ] 4H O (en = 12 2 42 233 84364 2 2 [Co(phen) ]2 cation to form 2D cationic sheets ethylendiamine) is one example of this bicapped 2 [PMoVI Mo V O (V IV O) {Co(phen) } ] , the other sup- α-Keggin fragment. The {Mo V O } units linked to- 84402 22 87 42 ports two [Co(phen) ({Co(phen) }H O)] complexes as gether through V–O–V bonds forming an infinite 1D 222 heteropolyoxo anion [PMoVI Mo V O (V IV O) {Co- structure.80 Its structure consists of one (Mo V O )7 48402 87 42 (phen)} (H O) ]3 forming 3D supramolecular frame- polyanion, three ethylenediammonium cations, one 222 work. The basic building unit (PMo V O ) is built on proton hydrate and four water molecules. The polyanion 12 2 42 the well-known α-Keggin structure with two additional of the chain is composed by trans-vanadium capped five-coordinating terminal {VO} units. Each basic α-Keggin unit. The Keggin structure is based on a cen- building unit acts as multidendate ligand covalently tral VOV tetrahedra surrounded by 12MO (M = Mo 4 6 bonded to {Co(phen) (H O)} or to {Co(phen) } sub- or V) octahedra arranged in four groups of three edge– 22 2 units, respectively. All V atoms are in the +4 oxidation shared octahedral (M O ). These groups are linked by 313 states, 4 Mo atoms within the cluster are in the +5 oxi- sharing corners to each other and to the central VOV 4 dation states and the remanent 8 Mo atoms in the +6 tetrahedron. The analogous chain structure with the oxidation states, while in the cluster anion 8 Mo atoms unusual Mo/V polyoxometalate shows the compound are in the +5 oxidation states and the remainder are the Na K [MoVI V IV O (V V O )(V IV O) ] 12.5H O. 81 The 0.5 6.5 8 4 36 4 2 2 +6 oxidation states. The compound exhibits antiferro- structure of basic anionic unit is described as follows: VI VI magnetic interactions. four Mo centers of an α-Keggin type [Mo12 O 36 - V3 IV (V O44 ) ] species are replaced by V centers giving a Among the organic-inorganic hybrid materials «layered» type structure formed by the different metal polyoxomolybdovanadate bearing transition metal com- atoms. The VIV···VIV distance between atoms within plexes reveal interesting crystal structures and magnetic each layer is 5.1 Å and ca. 11 Å between layers in ad- properties.85 For example, these are the bi- and the tetra- joining units. This enables exchange interactions inter- capped Keggin anion-supported cobalt-phenanthroline VI IV esting in the magnetochemistry. complexes [{Co(phen)22 (H O)} 2 PMo 10442 V O ][{Co - (phen) } PMoVI Mo V V IV O ] 4H O 86 (1) and [Co - The compounds K (VMo O ) 19H O and 22 8 2 4 42 2 2 312402 (phen) (OH) (H O) ] [{Co(phen) (H O)} AsMoVI - [Ni(H O) ](H PMoVVI Mo O ) 30H O also contain 22240.5 2226 26 3 11402 2 MoVIV V O ] 2H O (2) and the bicapped Keggin anion- Keggin-type heteropolyanions.63 In the first molybdo- 28 44 2 based nickel-phenanthroline compound [Ni(phen) - vanadate each Keggin-type polyanion is capped and 2 HO][AsMoMo(VO)VI V IV ]2HO (3). The com- linked by six potassium ions to form extended three– 22 11 20.52 pound 1 consists of a bicapped anion (PMoVI V IV O ) 3 dimensional polymeric structure by the build up of 10 4 42 supported by two [Co(phen) (H O)] fragments through {K [VMo O ]} . This confirmed that the templating 22 31240n its terminal oxygen atoms of the two capping V atoms at effects of cations play an important role in the formation the opposite sites. All polyanions constitute an anionic of polymers. The second molybdovanadate, [Ni(H O) ]- 26 layer, while all the polycations are linked to form a (H PMoVVI Mo O ) 30H O, exhibits discrete Keggin- 3114022 cationic layer by the hydrogen bonding. The compound type structure, in which each heteropolyanion contains 2 has two types of free complex cation [Co (phen) - one reduced MoV atom. NiII coordinated by six water 22 (OH) (H O) ]2 in different orientations in the crystal molecules as the counter cation balances the negative 224 structure and these free cations coexist in the space charge of the molecule. between polyoxometalate clusters. In the similar com-

The compound [NH(CH22 CH OH) 3621862 ] (V Mo O )  pound 3 the Keggin unit is bicapped instead of tetra- VI V IV 5 ca. 3H2 O is the first example of polyoxomolybdovana- capped. Each (AsMo62844 Mo V O ) unit has strong date with the unpredicted composition of V/Mo = 2 : covalent interactions to two {Co(phen)22 (H O)} units in 82 6 V 18, and (V21862 Mo O ) anion based on VO4 tetra- 2. Two types of free complex cations of the large vol- hedra reveals a conventional Dawson structure. The ume packed in the space between polyoxoanion clusters VI polyanion consists of eighteen Mo O6 octahedra form- are attributed to the supramolecular interactions be- ing two (Mo313 O ) and six (Mo210 O ) units which are tween phenanthroline rings. Furthermore, 2 contains linked to each other by point-sharing and together sur- strong hydrogen bonding interactions between the ter- V round two VO4 tetrahedra. Recently obtained and minal oxygen atoms of polyanions and the oxygen structurally characterized polyoxomolybdovanadate atoms from ligand water molecule coordinated to the Co VVIIV [Co(C22834 N H ) ] [Mo Mo 141680 V O (PO 42 ) ] 10H 2 O con- atoms. The extensive hydrogen bonding leads to the

Croat. Chem. Acta 82 (2009) 345–362 358 M. Cindrić et al., Polyoxomolybdates and Polyoxomolybdovanadates formation of a chain structure. The structure of 3 con- VI V IV 4 sists of bicapped polyoxoanion [AsMo11 Mo (V 0.5 O)] 2 supported by two [Ni(phen)22 H O] cations and two VI V IV lattice water molecules. The [AsMo11 Mo (V 0.5 O)] unit as in other bicapped structure contains twelve MoO6 octahedra and a AsO4 tetrahedron at the center with two {VO} square pyramidal capping on the opposite VI V sites of the α-Keggin core {AsMo11 Mo O 40 }. Two Scheme 4. Proposed reaction mechanism of conversion ascorbic bridge oxygen ligand shared between two CoII ions, and or tartaric acid to oxalate (R = tartaric or ascorbic residues). MoV and VIV atoms also linked by bridge oxygen ligands offer a possible route for antiferromagnetic coup- polyoxomolybdovanadate coordinated by glycinato 77 ling interactions. ligand. In acidified aqueous solution of Na24 MoO , NH VO and a rather high concentration triethanol- Synthesis 43 amine a conventional Dawson structure polyoxomolybdo-

Polyoxomolybdovanadates are prepared in a similar vanadate cluster anion based on VO4 tetrahedra was 682 manner such as polyoxomolybdates. Oxo-anions of both isolated, [Mo18 O 54 (VO 4 ) 2 ] . molybdenum and vanadium are mixed in aqueous or Prolongated photolysis of aqueous solution con- nonaqueous solutions in the presence of acid or base. taining (V O )4 and KMoO at pH = 5.5 resulted in The complexes are mostly molybdenum-rich molybdo- 412 24 the mixed-valent polyoxomolybdovanadate cluster, vanadates. In a typical reaction between molybde- K H [VIV V V O (MoO )]. 87 These type of reactions num(VI) oxide and ammonium vanadate ion in aqueous 62 8 1454 4 allows the preparation processes to be easily controlled solution by adding potassium hydroxide two compounds IV 8 and provides many opportunities to prepare mixed with [Mo (V )V O ] anion were precipitated. The 8440 valence PMOs, such as spherical polyoxomolybdovana- polyoxomolybdovanadate anions in both structures are 88 79 dates described by M. T. Pope. interconected by potassium ions and water molecules.

In the aqueous mixture of MoO3 and NH43 VO a mor- The hydrothermal synthesis enables preparation of pholine is added. The compound with central tetrahedral a number of complex organic-inorganic hybrid materials VV atom is formed, while the other two vanadium atoms of polyoxomolybdovanadates with different Mo oxi- are crystallographycally disordered over 12 MoO6 dation states and a formation of new 3D supramolecular IV V 84,85 octahedral sites (Hmorph)61040 [(V ,V ,Mo )VO ] structures. The preparative flexibility of hydrother- 74 3H2 O. Molybdovanadates with vanadium-rich polya- mal synthesis was demonstrated in the preparation of nion have been synthesized from ammonium vanadate polyoxomolybdovanadate clusters with various complex and molybdenum(VI) oxide in acidic aqueous solution structures. A mixture of VO25 , MoO3 , ethylenedi- by adding tetramethylammonium chloride. This is the amine and water was treated hydrothermaly at 140 °C in first example of molybdovanadate that is dominant in the autoclave. The reaction resulted in the polyanion aqueous solution.70 The vanadium–rich compounds with chains constructed by trans-vanadium capped α-Keggin 5 7 80 the asymmetric anion (Mo45 V O 27 ) was prepared from fragment {[Mo84 V O 3 (VO 4 )(VO 2 )]n } anion. During the same precursors in acidic aqueous solution and mor- the hydrothermal synthesis many factors such as stoi- pholine addition.10 chiometry, acidity, temperature, pressure and time have a great influence on the compounds. By mixing Although many different species of molybdovana- NH VO and H MoO 2H O with the addition of dates exist in aqueous solution their formation and crys- 43 242 1,10-phenanthroline and ethanol, depending on the tal composition strongly depend upon the metal ions reaction condition two windmill-complexes with concentration, the range of pH and the time and temper- VI V IV 4 [Mo78404 Mo V O (PO )] polyoxoanion were ob- ature rather than the relative stability of the species in 84 V V tained. Mixed Mo/V metal-oxygen cluster compound solution. Water suspension of Mo and V oxides in the containing the two types of typical heteropoly anions presence of ammonium oxalate results in two complex- 75 was prepared from VO , CoCl , Na MoO , ethy- es with coordinatively bound oxalato ligands. Simi- 25 2 24 lene-diamine and water.83 lary polyoxomolybdovanadate complex with oxalato ligands is obtained in the suspension with ascorbic and tartaric acid. Both acids were converted into oxalate APPLICATION OF POLYOXOMETALATES indicating thus the catalytic role of molybdenum (Scheme 4).76 Heteropoly- and isopolyoxometalates offer extreme

Suspension of MoO3 , VO25 and glycine in water variabilities of molecular compositions, size, shape and at elevated temperature resulted in a new complex of charge density which determine their properties and

Croat. Chem. Acta 82 (2009) 345–362 M. Cindrić et al., Polyoxomolybdates and Polyoxomolybdovanadates 359

6 applications. Generally, the application of polyoxo- while the other domaines reoxidize [Mo723 O (OH)] 6 metalates is centered on their redox properties and their back to (Mo724 O ) . high charges. The majority of the literature and patents (Mo O )66  1e H [Mo O (OH)]  of about 85 % are found in the area of catalysis.89 The 724 723 remaining applications cover analytical chemistry, elec- Reduced form MoV is highly toxic leading to the trochemistry, clinical analysis, sensors, medicine and specifity of tumor cell killing. This results indicated that 6 other fields. The application of POMs in different areas the structure of the (Mo724 O ) is important for anti- of chemistry, medicine and technology is well and com- tumor activity. Polyoxomolybdate, namely the structure 6 prehensively surveyed up to 1998 in Chemical Re- of (Mo724 O ) has been recognized as one of significant views.5 Polyoxomolybdates and polyoxomolybdovana- antitumoral polyoxomolybdate (Scheme 5). dates are part of this application. The research in this Polyoxomolybdate (NH Pri ) (Mo O ) 3H O, field is still growing due to the new synthesis of com- 36 7242 with the characteristic heptamolybdate anion inhibited plex structures with controlled properties. Therefore, in the cell growth of human pancreatic cancer. It is sug- this presentation only recent results including polyoxo- gested that the anti-tumor activity of the complex results molybdates and polyoxomolybdovanadates are re- from the activation of the apoptotic pathway and that viewed. Polyoxomolybdovanadates are of special inter- the mechanism differs from that of conventional anti- est as redox catalysts. Their reoxidation by molecular tumor agents and thus may be effective in treating pan- oxygen resulted in their use in a variety of oxidation creatic cancer.93 The same heptamolybdate anion was reactions.76 used to perform anti-tumor tests in MKN45 cells (hu- Recently obtained polyoxomolybdates containing man gastric cancer line in nude mice).94 tris(hydroxymethyl)aminoethane have shown substan- Above mentioned polyoxomolybdate did not in- tial catalytic activity toward the reduction of bromate duce any adverse effects such as body weight loss in the and weak activity toward chlorate.90 In order to apply host mice. It has also shown the growth suppression eco-friendly, recyclable and inexpensive catalyst vana- against several tumors, for example Co-4, human colon dium(V)-substituted polyoxomolybdates (series of cancer, MX-1, human brest cancer and OAT, human [H PMo V O ], x = 1–3) have been used as a 31240xxx lung cancer.95 The compound is water soluble and struc- catalyst for regioselective nitration of phenol.91 The turally stable at pH = 5–7. All described characteristics effects of various parameters such as concentration of and further analysis of cytotoxicity of polyoxomolyb- phenol, solvent, temperature and time reaction on the date in vivo model of anti-tumor growth offer a novel catalytic process were examined in details considering development of drug delivery systems using this class of the yield and selectivity. compounds. One of the most attractive application of POMs is Photoreduced compound of (NH Pri ) (Mo O ) in medicine. They have been known for their antitumor- 36 724 3H O gives the polyoxomolybdate, (Me NH) - al, antiviral and antibiotic activity. This activity is as- 2 36 [H MoVVI O (OH) (Mo O ) ] 2H O. Anti–tumor tests cribed to the advantageous feature of POMs that nearly 2122812 342 have shown that this polyoxomolybdate is a novel effec- every molecular property that impacts the recognition tive reagent in treating pancreatic cancers. The research and reactivity of those complexes with target biological continues in in vivo models.96 Polyoxomolybdates are macromolecules can be altered. A number of new compounds functionalized with organic or biological groups increase bioavailibilities and enhance the recognition of biological targets. However, POMs have a principal e- disadvantage in the medical use since they are not organic species which dominate in the pharmaceutical Tumor cell 6- e- [Mo O (OH)]6- industry. Though the number of papers on potential [Mo7O24] 7 23 applications of POMs have been published to date including excellent reviews,5,12 further knowledge regard- e- ing structure–activity behaviour in vivo is needed. This Cell killing information is crucial in the design of better drugs. Last Antitumor activity few years there is a growing interest in biological activi- Host Cell ty of polyoxomolybdates and their relevance in medi- Toxicity cine. The first report describing the antitumoral activity of heptamolybdate and the corresponding mechanism was proposed by Yamase.92 Some domaines in tumor Scheme 5. Proposed mechanism of the antitumor activity of 6 cells reduce heptamolybdate(VI) to heptamolybdate(V), (Mo724 O ) (adapted from Ref. 4).

Croat. Chem. Acta 82 (2009) 345–362 360 M. Cindrić et al., Polyoxomolybdates and Polyoxomolybdovanadates known to hydrolyze adenosine triphosphate, ATP, and and may show magnetic properties.99 Recently discrete the reactivity towards ATP has been implicated as a key inorganic-organic nanocomposite material has been in their anti-cancer activity. ATP hydrolysis in the pres- reported.90,100,101 Spherical-shaped POMo structures are ence of polyoxomolybdates at different pH levels has designed so that they behave as an artificial been investigated with a help of 31P NMR, 1H NMR cell/nanochromatograph (AC/NC) towards a wide range measurements, high pressure liquid chromatography and of counter cations. The AC/NC can be used to study the isothermal titration calorimetry. Systematic analysis of complexation behaviour under confined conditions with the ATP hydrolysis products in wide range of pH at 20 the possibility to mimic cation transport through bio- °C has revealed at pH levels of 6 and 4 a dominant logical ion channels including general informations formation of adenosine diphosphate, ADP, while at pH = about the transport processes through molecular 2 ATP was decomposed mainly to adenosine mono- pores.102 Further research in the application of POMs phosphate, AMT. ATP hydrolysis occurs through the presents a challenge for complex chemistry. interaction of phosphate sites in the ATP side-chain Acknowledgement. Financial support for this research was with molybdate forming [(PO ) Mo O ]6 -like ATP 42 5 15 provided by Ministry of Science, Education and Sports of the molybdate complex as an intermediate. 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SAŽETAK Polioksomolibdati i polioksomolibdovanadati – od strukture do funkcije: noviji rezultati

Marina Cindrić,a Zorica Vekslib i Boris Kamenara

aLaboratorij za opću i anorgansku kemiju, Kemijski odsjek, Prirodoslovno-matematički fakultet, Sveučilište u Zagrebu, Horvatovac 102a, HR-10000 Zagreb, Hrvatska bInstitut "Ruđer Bošković", p. p. 180, HR-10002 Zagreb, Hrvatska

Autorski revijalni članak daje literaturni pregled važnijih radova u području kemije polioksomolibdata i poliokso- molibdovanadata posljednjih dvadesetak godina. Navedena su najznačajnija postignuća u sintezi, određivanju kris- talne i molekulske strukture kao i primjeni ove klase spojeva. U prvom su dijelu opisani polioksomolibdati građeni od manjeg broja strukturnih jedinica do novih strukturnih oblika polioksomolibdatnih klustera supramolekulske arhitekture. Nove sintetske metode kao funkcionalizacija i hidrotermalna/solvotermalna sinteza omogućuju kon- trolu oblika i veličine nastalih struktura. Sinteza organsko-anorganskih hibridnih spojeva pokazala se najuspješnijom u pripravi novih vrsta materijala. U drugom je dijelu obuhvaćena sinteza i struktura polioksomo- libdovanadata s poliokso jezgrama bogatim molibdenom ili vanadijem, miješani vanadij-molibden polioksoanioni s različitim oksidacijskim stanjima metalnih iona i organsko-anorganski hibridni materijali. U trećem je dijelu opi- sana primjena navedenih spojeva. Funkcionalizacija polioksometalata organskim ligandima čini ove spojeve važnim u području biologije, medicine, katalize i znanosti o materijalima. Razmotren je značaj velikih poliokso- metalatnih klustera u kemiji nanomaterijala i izazov koji oni predstavljaju za budućnost kemije kompleksa.

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