A building block strategy to access sulfur-functionalized polyoxometalate based systems using {Mo2S2O2} and {Mo3S4} as constitutional units, linkers or templates.

The present tutorial review reports on the synthetic approaches for the formation of "polyoxothiometalate" compounds with special emphasis on the unique reactivity of the preformed sulfur-containing cationic building blocks {Mo(2)O(2)S(2)}(2+) and {Mo(3)S(4)}(4+) toward polyoxometalate building blocks. Such simple chemical systems, based on chemical and structural complementarities between ionic reactive moieties have led to the synthesis of a series of relevant clusters with unrivalled large nuclearity structural arrangements, such as loops, triangles, squares and boxes. Specific reaction parameters and considerations will be pointed out showing that a deliberate pure inorganic supramolecular chemistry based on weak interactions, flexibility and dynamic is possible with polyoxometalates.

Functionalized polyoxometalates with covalently linked bisphosphonate, N-donor or carboxylate ligands: from electrocatalytic to optical properties.

The structures of two families of hybrid organic-inorganic polyoxometalates (POMs) functionalized by covalently grafted carboxylate or bisphosphonate ligands are overviewed. The first family concerns the so-called POMOF materials, built from the connection of mixed-valent ε-Keggin type polyoxomolybdates via N-donor or carboxylate organic molecules coordinated to transition metal ions (Zn(II), Co(II), Ni(II)) grafted at the surface of the POMs. The simulation of the hypothetical zeolitic-like POMOF structures is presented and compared to the experimental ones. The second family gathers the various molecular Mo(V), Mo(VI) and W(VI) POMs incorporating directly into their architecture bisphosphonate (BP) ligands. The potentiality of this family of hybrid POMs comes from the organic group grafted on the carbon atom which bears the two phosphonate groups. For both families, besides the structural description, synthetic trends and an overview of their properties are presented. Namely, the electrocatalytic properties (production of H(2) and reduction of BrO(3)(-)) of the ε-Keggin type polyoxomolybdates are described. The optical and biological activities of the POM-BP compounds as well as their ability to form nanosystems are also reported.

Electrochemical behavior of α1/α2-[Fe(H2O)P2W17O61](7-) isomers in solution: experimental and DFT studies.

The unusual redox behavior displayed by the two isomers of the Wells-Dawson phosphotungstate anion [Fe(H(2)O)P(2)W(17)O(61)](7-) is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK(a) of the electrolyte on POM-based redox potentials given the pK(a) of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers.

Manganese(III)-containing Wells-Dawson sandwich-type polyoxometalates: comparison with their manganese(II) counterparts.

We present the synthesis and structural characterization, assessed by various techniques (FTIR, TGA, UV-vis, elemental analysis, single-crystal X-ray diffraction for three compounds, magnetic susceptibility, and electrochemistry) of five manganese-containing Wells-Dawson sandwich-type (WDST) complexes. The dimanganese(II)-containing complex, [Na(2)(H(2)O)(2)Mn(II)(2)(As(2)W(15)O(56))(2)](18-) (1), was obtained by reaction of MnCl(2) with 1 equiv of [As(2)W(15)O(56)](12-) in acetate medium (pH 4.7). Oxidation of 1 by Na(2)S(2)O(8) in aqueous solution led to the dimanganese(III) complex [Na(2)(H(2)O)(2)Mn(III)(2)(As(2)W(15)O(56))(2)](16-) (2), while its trimanganese(II) homologue, [Na(H(2)O)(2)Mn(II)(H(2)O)Mn(II)(2)(As(2)W(15)O(56))(2)](17-) (3), was obtained by addition of ca. 1 equiv of MnCl(2) to a solution of 1 in 1 M NaCl. The trimanganese(III) and tetramanganese(III) counterparts, [Mn(III)(H(2)O)Mn(III)(2)(As(2)W(15)O(56))(2)](15-) (4) and [Mn(III)(2)(H(2)O)(2)Mn(III)(2)(As(2)W(15)O(56))(2)](12-) (6), are, respectively, obtained by oxidation of aqueous solutions of 3 and [Mn(II)(2)(H(2)O)(2)Mn(II)(2)(As(2)W(15)O(56))(2)](16-) (5) by Na(2)S(2)O(8). Single-crystal X-ray analyses were carried out on 2, 3, and 4. BVS calculations and XPS confirmed that the oxidation state of Mn centers is +II for complexes 1, 3, and 5 and +III for 2, 4, and 6. A complete comparative electrochemical study was carried out on the six compounds cited above, and it was possible to observe the distinct redox steps Mn(IV/III) and Mn(III/II). Magnetization measurements, as a function of temperature, confirm the presence of antiferromagnetic interactions between the Mn ions in these compounds in all cases with the exception of compound 2.

Tetra- to dodecanuclear oxomolybdate complexes with functionalized bisphosphonate ligands: activity in killing tumor cells.

We report the synthesis and characterization of five novel Mo-containing polyoxometalate (POM) bisphosphonate complexes with nuclearities ranging from 4 to 12 and with fully reduced, fully oxidized, or mixed-valent (Mo(V), Mo(VI)) molybdenum, in which the bisphosphonates bind to the POM cluster through their two phosphonate groups and a deprotonated 1-OH group. The compounds were synthesized in water by treating [Mo(V)(2)O(4)(H(2)O)(6)](2+) or [Mo(VI)O(4)](2-) with H(2)O(3)PC(C(3)H(6)NH(2))OPO(3)H(2) (alendronic acid) or its aminophenol derivative, and were characterized by single-crystal X-ray diffraction and (31)P NMR spectroscopy. (NH(4))(6)[(Mo(V)(2)O(4))(Mo(VI)(2)O(6))(2)(O(3)PC(C(3)H(6)NH(3))OPO(3))(2)]·12H(2)O (1) is an insoluble mixed-valent species. [(C(2)H(5))(2)NH(2)](4)[Mo(V)(4)O(8)(O(3)PC(C(3)H(6)NH(3))OPO(3))(2)]·6H(2)O (2) and [(C(2)H(5))(2)NH(2)](6)[Mo(V)(4)O(8)(O(3)PC(C(10)H(14)NO)OPO(3))(2)]·18H(2)O (4) contain similar tetranuclear reduced frameworks. Li(8)[(Mo(V)(2)O(4)(H(2)O))(4)(O(3)PC(C(3)H(6)NH(3))OPO(3))(4)]·45H(2)O (3) and Na(2)Rb(6)[(Mo(VI)(3)O(8))(4)(O(3)PC(C(3)H(6)NH(3))OPO(3))(4)]·26H(2)O (5) are alkali metal salts of fully reduced octanuclear and fully oxidized dodecanuclear POMs, respectively. The activities of 2-5 (which are water-soluble) against three human tumor cell lines were investigated in vitro. Although 2-4 have weak but measurable activity, 5 has IC(50) values of about 10 µM, which is about four times the activity of the parent alendronate molecule on a per-alendronate basis, which opens up the possibility of developing novel drug leads based on Mo bisphosphonate clusters.

Functionalized polyoxometalates with intrinsic photochromic properties and their association with spiropyran cations.

The presence of alkylammonium groups covalently grafted on bisphosphonato ligands induces photochromic properties to ligand-coordinated polyoxomolybdate systems. Such intrinsically photoactive polyanions can be used for the preparation of materials combining photochromic polyoxometalates and functional countercations.

Influence of the heteroatom size on the redox potentials of selected polyoxoanions.

The apparent formal potentials for the one-electron redox process of most Keggin-type heteropolytungstates, XW(12)O(40)(q-), have long been shown to linearly depend on their overall negative charges, in the absence of proton interference in the process. However, for a given overall negative charge, these formal potentials are also shown here to depend on the specific central heteroatom X. In the present work, cyclic voltammetry was used to study a large variety of Keggin-type anions, under conditions where their comparisons are straightforward. In short, apparent potential values get more negative (the clusters are more difficult to reduce) for smaller central heteroatoms within a given family of Keggin-type heteropolyanions carrying the same overall negative charge. Density functional theory calculations were performed on the same family of Keggin compounds and satisfactorily reproduce these trends. They show that internal XO(4) units affect differently the tungstate oxide cage. The electrostatic potential created by each internal anionic unit in a fragment-like approach (XO(4)(q-)@W(12)O(36)) was analyzed, and it is observed that X atoms of the same group show slight differences. Within each group of the periodic table, X atoms with lower atomic numbers are also smaller in size. The net effect of such a tendency is to produce a more negative potential in the surroundings and thus a smaller capacity to accept electrons. The case of [BW(12)O(40)](5-) illustrates well this conclusion, with the smallest heteroatom of the Keggin series with group III central elements and a very negative reduction potential with respect to the other elements of the same group. Particularly in this case, the electronic structure of the Keggin anion shows the effects of the small size of boron: the highest occupied molecular orbitals of [BW(12)O(40)](5-) appear to be approximately 0.35 eV higher than those in the other clusters of the same charge, explaining that the BO(4) unit is more unstable than AlO(4) or GaO(4) despite carrying the same formal charge.

Coordination chemistry approach for the end-to-end assembly of gold nanorods.

Gold nanorods synthesized by radiolysis were selectively end-functionalized by a fully conjugated thiol bearing a pendant terpyridine group; addition of ferrous ions led to the end-to-end 1D self-assembly of the nanorods. Similar results have been obtained when the preformed [(HStpy)Fe(tpySH)](2+) dithiol complex was directly added to the gold nanorods.

Structural, magnetic, EPR, and electrochemical characterizations of a spin-frustrated trinuclear Cr(III) polyoxometalate and study of its reactivity with lanthanum cations.

The asymmetric Cr(III) polyoxometalate complex Cs(10)[(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)] x 17 H(2)O (1) has been synthesized in water under atmospheric pressure from the trinuclear precursor [Cr(3)(CH(3)COO)(7)(OH)(2)] and the divacant ligand [gamma-SiW(10)O(36)](8-). Complex 1 is built up of two [gamma-SiW(10)O(36)](8-) Keggin units sandwiching a trinuclear {(Cr(III)(OH)(H(2)O))(3)} fragment where the paramagnetic centers are bridged by three mu-OH ligands forming a nearly isosceles triangle. The magnetic properties of this spin-frustrated system have thus been interpreted considering a 2-J Hamiltonian showing that the Cr(III) ions are antiferromagnetically coupled and that 1 possesses an S = 3/2 ground state with an S = 1/2 first excited state located at 11 cm(-1). These results have been confirmed by EPR spectroscopy measurements (Q-band), which have also enabled the quantification of the electronic parameters characterizing the quadruplet spin ground state. The magnitude of the magnetic exchange interactions and the nature of the ground state are discussed in light of previously reported isosceles triangular S = 3/2 clusters. UV-visible and electrochemical studies have shown that 1 is stable in aqueous media in a 1-7 pH range. This stability is chemically confirmed by the study of the reactivity of 1 with La(III) cations, which has allowed the isolation of the Cs(4)[(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)(La(H(2)O)(7))(2)] x 20 H(2)O compound (2). Indeed, during the synthetic process of this 3d-4f system, the integrity of the [(gamma-SiW(10)O(36))(2)(Cr(OH)(H(2)O))(3)](10-) building unit constituting 1 is maintained despite the high oxophilic character of the La(III) ions. The single crystal X-ray diffraction study of 2 has revealed that in the solid state the rare earth cations connect these subunits, affording a 3d-4f double-chain monodimensional system.

Zeolitic polyoxometalate-based metal-organic frameworks (Z-POMOFs): computational evaluation of hypothetical polymorphs and the successful targeted synthesis of the redox-active Z-POMOF1.

The targeted design and simulation of a new family of zeolitic metal-organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and epsilon-type Keggin polyoxometalates (POMs) as building units, named here Z-POMOFs, have been performed. A key feature is the use of the analogy between the connectivity of silicon in dense minerals and zeolites with that of the epsilon-type Keggin POMs capped with Zn(II) ions. Handling the epsilon-Keggin as a building block, a selection of 21 zeotype structures, together with a series of dense minerals were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like structure was predicted to be the most stable structure. This prediction has been experimentally validated by the targeted synthesis of the first experimental Z-POMOF structure, which was strikingly found to possess the cristobalite topology, with three interpenetrated networks. Crystals of [NBu(4)](3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)(BDC)(2)].2H(2)O (Z-POMOF1) have been isolated under hydrothermal conditions from the reduction of ammonium heptamolybdate in the presence of phosphorous acid and Zn(II) ions. Tetrabutylammonium cations play the role of counterions and space-filling agents in this tridimensional interpenetrated framework. Moreover, the electrochemistry of the epsilon-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.

Electrochemical, linear optical, and nonlinear optical properties and interpretation by density functional theory calculations of (4-N,N-dimethylaminostyryl)-pyridinium pendant group associated with polypyridinic ligands and respective multifunctional metal complexes (Ru(II) or Zn(II)).

The synthesis, linear optical and nonlinear optical properties, as well as the electrochemical behavior of a series of pro-ligands containing the 4-(4-N,N-dimethylaminostyryl)-1-methyl pyridinium (DASP(+)) group as a push-pull moiety covalently linked to terpyridine or bipyridine as chelating ligands are reported in this full paper. The corresponding multifunctional Ru(II) and Zn(II) complexes were prepared and investigated. The structural, electronic, and optical properties of the pro-ligands and the ruthenium complexes were investigated using density functional theory (DFT) and time-dependent (TD) DFT calculations. A fairly good agreement was observed between the experimental and the calculated electronic spectra of the pro-ligands and their corresponding ruthenium complexes. A quenching of luminescence was evidenced in all ruthenium complexes compared with the free pro-ligands but even the terpyridine-functionalized metal complexes exhibited detectable luminescence at room temperature. Second order nonlinear optical (NLO) measurements were performed by Harmonic Light Scattering and the contribution of the DASP(+) moieties (and their relative ordering) and the metal-polypyridyl core need to be considered to explain the nonlinear optical properties of the metal complexes.

Second-order nonlinear optical properties of polyoxometalate salts of a chiral stilbazolium derivative.

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS(+), compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)(4)[Mo(8)O(26)] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS(+) molecules, named here CHIDAMS(+), has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS(+) cation has been crystallized under two polymorphic PF(6)(-) salts, (CHIDAMS)PF(6) (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)(2)[Mo(5)O(13)(OEt)(4)(NO){Na(H(2)O)(0.5)(DMF)(0.5)}] (4) crystallizes in the noncentrosymmetric P2(1) group, but the push-pull axis of the CHIDAMS(+) cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)(3)[PW(12)O(40)].2DMF (5) associating three CHIDAMS(+) cations and a PW(12)O(40)(3-) Keggin anion crystallizes also in the P2(1) space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz-Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF(6)(-) salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature.

Heterometallic 3d-4f cubane clusters inserted in polyoxometalate matrices.

Unprecedented molecular and bidimensional compounds based on monovacant polyoxometalates capped by heterometallic 3d-4f {LnCu(3)(OH)(3)O} (Ln = La, Gd, Eu) cubane fragments have been characterized and their magnetic properties investigated.

Iron polyoxometalate single-molecule magnets.

Iron sandwich on a tungstate bun: Two new polyoxotungstates with paramagnetic iron(III) heteroatoms (see structure, W blue, Fe yellow, O red) possess S=15/2 and S=5 ground states. Both compounds are single-molecule magnets, and the hexairon species shows large hysteresis (see picture) and quantum tunneling effects at low temperature. Electrochemical studies indicate that these species are stable in solution for a wide range of pH values.

Hexa- and dodecanuclear polyoxomolybdate cyclic compounds: application toward the facile synthesis of nanoparticles and film electrodeposition.

Two new compounds based on O(3)PCH(2)PO(3)(4-) ligands and {Mo(V)(2)O(4)} dimeric units have been synthesized and structurally characterized. The dodecanuclear Mo(V) polyoxomolybdate species in (NH(4))(18)[(Mo(V)(2)O(4))(6)(OH)(6)(O(3)PCH(2)PO(3))(6)] x 33 H(2)O (1) is a cyclohexane-like ring in a chair conformation with pseudo S(6) symmetry. In the solid state, the wheels align side by side, thus delimiting large rectangular voids. The hexanuclear anion in Na(8)[(Mo(V)(2)O(4))(3)(O(3)PCH(2)PO(3))(3)(CH(3)AsO(3))] x 19 H(2)O (2) has a triangular framework and encapsulates a methylarsenato ligand. (31)P NMR spectroscopic analysis revealed the stability of 2 in various aqueous media, whereas the stability of 1 depends on the nature of the cations present in solution. It has been evidenced that the transformation of 1 into 2 occurs in the presence of CH(3)AsO(3)(2-) ions. This behavior shows that 1 can be used as a new precursor for the synthesis of Mo(V)/diphosphonate systems. The two complexes were very efficient both as reductants of Pt and Pd metallic salts and as capping agents for the resulting Pt(0) and Pd(0) nanoparticles. The size of the obtained nanoparticles depends both on the nature of the polyoxometalate (POM; i.e., 1 or 2) and on the [metallic salt]/[POM] ratio. In all cases, X-ray photoelectron spectroscopy (XPS) measurements have revealed the presence of Mo(VI) species that stabilize the nanoparticles and the absence of Mo(V) moieties. Diffuse-reflectance FTIR spectra of the Pt nanoparticles show that the capping Mo(VI) POMs are identical for both systems and contain the diphosphonato ligand. The colloidal solutions do not show any precipitate and the nanoparticles remain well-dispersed for several months. The electrochemical reduction of Mo(V) species was studied for 2. Cyclic voltammetry alone and electrochemical quartz crystal microbalance coupled with cyclic voltammetry show the deposition of a film on the electrode surface during this reduction.

1,10-Phenanthroline and 1,10-phenanthroline-terminated ruthenium(II) complex as efficient capping agents to stabilize gold nanoparticles: application for reversible aqueous-organic phase transfer processes.

1,10-Phenanthroline (phen) and 1,10-phenanthroline-terminated ruthenium(II) complex [Ru-Lphen](2+) have been used to stabilize and functionalize gold nanoparticles (Au-NPs). The strong interaction between the nitrogen atoms of phen and the surface of Au-NPs allowed for the phase transfer of Au-NPs from toluene to aqueous phase containing [Ru-Lphen](2+). Reverse phase transfer of these Au-NPs from water to acetonitrile by substituting the Cl(-) counter anion by PF(-)(6) has also been demonstrated. Such facile post-functionalization, phase transfer and solvent transfer processes using metallic complexes bearing a terminal phenanthroline pendant group constitute a prerequisite for further studies of the electronic and optical properties of these NCs in various media.

Octa- and nonanuclear nickel(II) polyoxometalate clusters: synthesis and electrochemical and magnetic characterizations.

Three high-nuclearity NiII-substituted polyoxometalate compounds functionalized by exogenous ligands have been synthesized and characterized. The octanuclear complexes in Na15[Na{(A-R-SiW9O34)Ni4(CH3COO)3(OH)3}2] . 4NaCl . 36H2O (1) and Na15[Na{(A-R-SiW9O34)Ni4(CH3COO)3(OH)2(N3)}2] . 32H2O (2) can be described as two {Ni4} subunits connected via a {Na(CH3COO)6} group, with the acetato ligands also ensuring in each subunit the connection between the paramagnetic centers. In 2, two azido groups replace two of the six mu-hydroxo ligands present in 1. The nonanuclear complex K7Na7[(A-R-SiW9O34)2Ni9(OH)6(H2O)6(CO3)3] . 42H2O (3) exhibits a double cubanestructure with two [(A-R-SiW9O34)Ni4(OH)3]5- subunits linked by three carbonato ligands. A ninth NiII center connected to one subunit via a carbonato ligand and a O=W group completes this asymmetric polyoxometalate.Electronic spectroscopy and electrochemical studies indicate that, while compounds 1-3 decompose in a pure aqueous medium, these complexes are very stable in a pH 6 acetate medium. The cyclic voltammetry pattern of each complex is constituted by a first eight-electron reduction wave followed by a second large-current intensity wave. The characteristics of the first waves of the complexes are clearly distinct from those obtained for their lacunary precursor [A-R-SiW9O34]10-, a feature that is due to the Ni centers in the complexes. Such observations of electroactive, stable, and highly nickel-rich polyoxometalates are not common. Measurements of the magnetic susceptibility revealed the occurrence of concomitant ferromagnetic and antiferromagnetic interactions in 1 and 3.For both of these compounds, the extension of the magnetic exchange has been determined by means of a spin Hamiltonian with three and four J constants, respectively.

Square versus tetrahedral iron clusters with polyoxometalate ligands.

Two new insoluble transition metal substituted phosphotungstates, (C2N2H10)11[{(B-alpha-PW9O34)Fe3(OH)3}4(PO4)4Fe].38H2O(1) and K4(C2N2H10)12[(alpha-PW10Fe2O39)4].30H2O(2), have been isolated by the hydrothermal reaction of [A-alpha-PW9O34]9-, Fe(III) ions and ethylenediamine. Compound 1 has a tetrahedral symmetry and contains a Fe13 core built from the assembly of four Fe(III) trisubstituted [B-alpha-PW9O34]9- anions around a central disordered iron ion via four phosphato ligands. The anion in 2 can be described as a square of disubstituted [PW10O37]9- anions linked by Fe(III)-O-Fe(III) bridges. Magnetic measurements performed on 1 and 2 have shown the occurrence of antiferromagnetic interactions between the iron ions and have allowed the coupling constants between the magnetic centers to be determined.

Water substitution on iron centers: from 0D to 1D sandwich type polyoxotungstates.

Four novel polyoxotungstates have been synthesized by reaction of the sandwich type compound [Fe (III) 4(H 2O) 10(B-beta-SbW 9O 33) 2] (6-) (noted Fe 4(H 2O) 10Sb 2W 18) with ethylenediamine (en) and/or oxalate (ox) ligands under various conditions. The one-dimensional (1D) compound [enH 2] 3[Fe (III) 4(H 2O) 8(SbW 9O 33) 2].20H 2O ( 1) is isolated at 130 degrees C and results from the elimination of two water molecules and the condensation of the polyoxotungstate precursor. The reaction of Fe 4(H 2O) 10Sb 2W 18 with oxalate ligands affords the molecular complex Na 14[Fe (III) 4(ox) 4(H 2O) 2(SbW 9O 33) 2].60H 2O ( 2) where two organic ligands substitute four water molecules, while the same reaction in the presence of en molecules at 130 degrees C leads to the formation of the functionalized 1D chain [enH 2] 7[Fe (III) 4(ox) 4(SbW 9O 33) 2].14H 2O ( 3) with protonated ethylenediamine counterions. Finally, at 160 degrees C a rearrangement of the Fe 4(H 2O) 10Sb 2W 18 polyoxotungstate is observed, and the sandwich type compound [enH 2] 5[Fe (II) 2Fe (II) 2(enH) 2(Fe (III)W 9O 34) 2].24H 2O ( 4) crystallizes. In 4, the heteroelement is a Fe (III) ion, and the water molecules on the two outer Fe (II) centers are bound to pendant monoprotonated en ligands. The four compounds have been characterized by IR spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. A detailed study of the magnetic properties of the mixed-valent hexanuclear iron complex in 4 shows evidence of an S = 5 ground-state because of spin frustration effects. A quantification of the electronic parameters characterizing the ground state ( D = +1.12 cm (-1), E/ D = 0.15) confirms that polyoxotungstate ligands induce large magnetic anisotropy.