2.1 [Mo₂O₂O₂(H₂O)₆]²⁺

The Mo^V-oxodication was obtained by reduction of a Mo^VI-precursor, generally commercial Na₂MoO₄·2 H₂O. Depending on the synthesis conditions, two reducing agents were readily used, metallic molybdenum for hydrothermal syntheses, hydrazine for standard solution reactions at room temperature [3]. Molecular species were prepared by standard acidobasic condensation methods while one-pot hydrothermal techniques were reserved to generate solid-state materials (these solids are generally insoluble).

2.2 [Mo₂O₂S₂(H₂O)₆]²⁺

The oxothio precursor was obtained by the selective oxidation of the terminal disulfure ligands in the [(S₂)MoOS₂MoO(S₂)]^2– anion by iodine. This protocol initially described for DMF medium [4] was adapted by us to aqueous solutions [5]. The use of water as solvent is really important since it permits the control of the condensation by simple pH measurements.

3.1 Phosphates as assembling groups in hydrothermal conditions

Molybdenum phosphates containing transition metals are interesting species for combining the properties of polyoxometalates and those of the metal phosphate (magnetic properties, microporosity) [6]. In the chemistry of Mo^V and phosphates, the very stable [P₄Mo₆O_31–x(OH)_x]^12–x series is quasi systematically obtained, the degree of protonation varying from one structure to another [7]. In all these structures, see Fig. 2, a peripheral phosphate and a hydroxo group mutually connect the {Mo₂O₄} units.

The connections between each building unit are edge sharing. A central phosphato group is encapsulated in the metal ring, the central and the three peripheral phosphates lying in the same side of the six-metal plane. Dimeric sandwiched species were characterized with M²⁺ cations (M = Cr, Mn, Fe, Co, Ni, Zn, Cd) or with sodium. In the solid state, the transition-metal ions M²⁺ trapped in the sandwich are isolated from each others that do not confer any specific magnetic properties to the solid. By controlling the initial pH and the stoichiometry of the reaction mixture {Mo₂O₄}²⁺/phosphate/Co²⁺ ions, we succeeded to isolate by hydrothermal synthesis a two-dimensional cobalto-molybdenum phosphate constructed from unprecedented large structural groups [8]. The structure can be described as building units formed of sixteen molybdenum atoms and four tetramers of cobalt forming the 19 Å anionic wheel [(Mo₂O₄)₈(HPO₄)₁₄(PO₄)₁₀Co₁₆(H₂O)₂₀]^10–.

The sixteen Mo^V are arranged in four tetramers linked together by phosphates that form a hexadecamer of Mo^V. Each Mo-tetramer results from the association of two {Mo₂O₄} initial building blocks. Two phosphato groups link a tetramer of molybdenum to two tetramers of cobalt. A cobalt-tetramer consists in four distorted edge-sharing octahedra forming a square. The Co²⁺ ions are bridged by six phosphates and a water molecule, the coordination of the cobalt ions is achieved by a terminal water molecule and an oxygen atom of a {Mo₂O₄} group.

A striking detail is the presence in the center of the wheel of a floating [Co(H₂O)₆]²⁺ octahedron stabilized by hydrogen bondings with the closest phosphato groups. Each wheel is connected to four other wheels by four cobalt dimers through peripheral phosphato groups. The wheel connections ensured by dimeric Co²⁺ octahedra lead to the layer structure depicted in Fig. 3.

The magnetic behavior was studied in the range 2–300 K, and the result is given in Fig. 4 as the plot of X_MT versus T (X_M is the magnetic susceptibility per unit). The curve continuously decreases upon cooling from 300 K (X_MT = 55.54 cm³ mol^–1 K, g = 2.32) to 2 K (X_MT = 11.75 cm³ mol^–1 K).

Because of the Mo^V–Mo^V metal–metal bond, the only magnetically active species are the Co²⁺ ions. The low-temperature X_MT is significantly lower than the theoretical value deduced for isolated Co²⁺ ions for any values of Δ/λ, where λ refers to the spin-orbit coupling parameter and Δ to the ligand-field splitting parameter. This clearly indicates relatively strong antiferromagnetic Co–Co interactions. This result differs from that reported for the tetrameric cobalto-polyoxometalate [Co₄O₁₄(H₂O)₂(PW₉O₂₇)2]^10– [9], which exhibit ferromagnetic coupling. The difference comes from the geometry of the cobalt-tetramer, here the Co–O–Co angles (94–108°) do not permit the magnetic orbital at the bridges to be non orthogonal.

By increasing the pH value of the reaction solution (from 2.0 to 3.9) and decreasing the Co-stoichiometry another layered material was isolated, both similar and different from the former. The structure of the building wheel is strictly analogous, but the wheels are differently interconnected, which leads to a drastic change in the two-dimensional structure (Fig. 5). Each wheel is connected to four other wheels by four single cobalt ions in a distorted tetrahedral environment, which imposes a non-planar arrangement.

With Ni²⁺ the sixteen-metal building unit is similar, but each wheel is connected to four others by two dimers and by two monomers of nickel, respectively. In fact, the Ni-layered structure exhibit hybrid connections reminiscent of the two former Co-compounds. The magnetic behavior of this material is complex, additional informations are available in the literature [9].

3.2 Polyphosphates as assembling groups in standard conditions

Polyoxometalates containing polyphosphato groups are rare. Kortz and Pope reported in 1994 the first X-ray structural determination of a polyoxomolybdate containing the pyrophosphato (P₂O₇)^4– ligand. Since then, only two other structures have been reported [10].The lack of examples concerning this class of compound and the absence of Mo^V pyrophosphate complex is justified by the works of Weil–Malherbe and Green [11] who noticed that molybdenum(V) catalyzes the hydrolysis of pyrophosphate into monophosphate in water. This reaction is optimal at pH 2 with a secondary maximum at pH 5.5. We have determined a domain where the hydrolysis of pyrophosphate by Mo^V is minimum. This led to the first pyrophosphate/Mo^V complex Na₂₄{Na₄(H₂O)₆⊂[(Mo₂O₄)₁₀(P₂O₇)₁₀(CH₃COO)₈(H₂O)₄]}·97 H₂O, a highly charged and highly hydrated complex formed of two nearly perpendicularly interconnected wheels, an unprecedented topology for an inorganic compound, see Fig. 6. The synthesis was performed in sodium acetate buffer. This is justified considering that the rate of hydrolysis of (P₂O₇)^4– is lowered at the pH imposed by the acetate buffer and that acetate can stabilize the [Mo₂^VO₄(μ-CH₃COO)]⁺ dimer [12] which possesses two free sites in cis-position on each Mo^V available for the formation of large rings. The largest wheel (noted {Mo₁₂}) contains twelve Mo^V centers while the smallest (noted {Mo₈}) is formed of eight Mo^V atoms. All Mo-centers are in a distorted octahedral environment with a short terminal Mo = O bond. As usually observed [11,12] the Mo^V atoms are arranged by pairs, forming the diamagnetic dinuclear unit {Mo^V₂(μ-O)₂O₂}, with an average Mo–Mo distance of 2.54 Å characteristic of a Mo–Mo bond. These dimers are linked by two types of pyrophosphates. Six pyrophosphates connect two dimers, while four others bridge three dimers ensuring the connection between the two wheels. These two wheels are nearly perpendicular with an angle of 92.8°. The coordination of four of the six dimers of the {Mo₁₂} moiety is achieved by a μ-O₂ acetato anion, the coordination of the two remaining being completed by water molecules.

These exogenous ligands are crucial since they strongly influence the curvature of the and consequently the topology of the whole edifice. This can be emphasized by the distances between the axial ligands in a {Mo^V₂O₄(H₂O)₂} dimer (d(O_t···O_t) = 2.910(9) Å and d(O_H2···O_H2) = 3.144(9) Å) and in the {Mo^V₂O₄(CH₃COO)₂} dimers (d(O_t···O_t) = 3.172(9)–3.254(10) Å and d(O_OCH3···O_OCH3) = 2.171(9)–2.207(10) Å). In the {Mo₁₂} wheel, the Mo = O groups are directed toward the outside of the cavity. The situation is quite different for the {Mo₈} wheel: each dimer is coordinated to an acetate anion and the Mo = O groups are directed toward the inner cavity. All the pyrophosphato groups are fully deprotonated, which is consistent with the presence of 28 Na⁺ cations in the lattice. Four of these cations are located inside the cavity and two others at the intersections of the wheels, connecting four {Mo^V₂O₄(CH₃COO)₂} fragments via eight oxygen atoms (d(Na–O) = 2.403(7)–2.516(7) Å). These bonded sodium play a key role in the stability of the host acting as a cryptand anion [13]. Two additional alkaline cations are linked to the water molecules of the {Mo^V₂O₄(H₂O)₂} dimers, the remaining cations being located outside the cage. Finally, a vacant site remains available at the center of the polyanion.

The ³¹P NMR spectrum of the double wheel in acetate buffer exhibits at room temperature three resonances located at δ₁ = 3.03 ppm, δ₂ = 1.66 ppm and δ₃ = –0.64 ppm with relative intensity of 2:1:2, respectively. This result is in agreement with the solid-state structure. The spectrum remains unchanged over several weeks, showing the high stability in this medium. In pure water, the ³¹P NMR spectrum presents a complex pattern, indicating decomposition in this medium (Fig. 7).

Organophosphonato ligands like [O₃PCH₂PO₃]^4– are interesting since they have a geometry quite similar to that of diphosphates, and they present the advantage to be stable toward hydrolysis. Thus, they represent an alternative way to prepare [O₃PXPO₃]^4–/molybdenum systems. To date, only two methylenediphosphonate compounds have been isolated, the fully oxidized [Mo₆O₁₈(O₃PCH₂PO₃)(H₂O)₄]^4– anion [14] and the partially reduced [Mo^V₆Mo^VIO₁₆(O₃PCH₂PO₃)₃]^8– compound [15]. In both cases, the structure consists in six coplanar molybdenum ions forming a ring.

[Mo₂O₄]²⁺ reacts at RT with methylene diphosphonate yielding Na₂₄[Na₄(H₂O)₆{(MO₂O₄)₁₀(O₃PCH₂PO₃)₁₀(CH₃COO)₈(H₂O)₄}].95H₂O. This compound [16] is isostructural to the former pyrophosphate double wheel and is stable only in a solution of acetate/acetic acid buffer. In pure water, bridging acetates exchange with water molecules resulting in other different soluble species evidenced by ³¹P NMR.

The [Mo₂O₄]²⁺ dioxocation has been condensed with [O₃PCH₂PO₃]^4– in the presence of templating exogenous ligands such as carbonate or molybdate. Some of the results obtained are given in Fig. 8, as an illustration of the versatility of the chemical system. The common point to all these structures is their cyclic arrangement resulting from the association of Mo(V)–Mo(V) dimers with diphosphonate ligands.

With molybdate [MoO₄]^2–, a wheel formed of Mo(V) octahedra, alternately linked by edges and corners, encapsulating a central tetrahedral [Mo(VI)O₄} group is formed. The overallstructure is stabilized by three peripheral diphosphonates (Fig. 8).

Carbonates led to a wheel containing four Mo(V) dimers and four [OPCH₂PO₃]^2– ligands encapsulating two carbonate ions and a central {Na(H₂O)₂}+ group (Fig. 8).

4.1 Halide recognition

The presence of a double sulfido bridge confers to the [M₂O₂S₂]²⁺ building block specific coordination properties quite different from that reported for the oxo analogue [Mo₂O₄]²⁺. The cyclic architectures described below exhibit supramolecular properties potentially relevant to anion recognition, anion-templated synthesis and 3-D structures [17].

A yellow powder was precipitated by direct addition of potassium hydroxide to a mixture of the dithiocation and potassium iodide about pH 2–3. After crystallization in pure water, the neutral dodecameric wheel, [Mo₁₂O₁₂S₁₂(OH)₁₂(H₂O)₆], noted {Mo₁₂}, was isolated [18]. The structure is that of a Mo₁₂-ring formed of six {Mo₂O₂S₂} fragments, linearly connected by double hydroxo-bridges (Fig. 9).

The cyclic backbone {Mo₁₂O₁₂S₁₂(OH)₁₂} is neutral and delimits a central open cavity of about 11 Å, lined by six water molecules. The inner water molecules are labile enough to be exchanged for specific anions, suggesting the cavity exhibits a cationic character, induced by the presence of twelve Mo(V). This situation is quite different with that encountered with crown ether compounds where the cavity is lined by donor oxygen atoms giving the cavity an anionic character. Only the four equatorial aquo ligands of the initial dithiocation are involved in the olation reaction. The four equatorial aquo ligands are ionized below pH 2, provoking the oligomerization process, while the two axial water molecules are unaffected in the Mo₁₂-ring. The crystallization of {Mo₁₂} in DMF containing tetrabutylammonium iodide or tetraethylammonium chloride leads to Cat₂[X₂Mo₁₀S₁₀O₁₀(OH)₁₀(H₂O)₅] with Cat = Nbu₄ and X = I, or Cat = Net₄ and X = Br, respectively [19], see Fig. 10. The two molecular structures are similarly built on a neutral decanuclear ring exhibiting the same connection scheme to the previous Mo₁₂-ring.

In the solid state, two halide ions are symmetrically located on both sides of the mean plane defined by the 10 Mo atoms. The distances between the two halides and the five-oxygen atom of the ring are short enough to suggest that the stability of the supramolecular arrangement is ensured by an H–bonding network involving the protons of the inner water molecule.

In water, successive equilibria based on halide exchanges permit to obtain the iodide free neutral {Mo₁₂} cluster. In contrast, the presence of halide ions (Cl^– or I^–) and/or aprotic solvent are required to maintain the {Mo₁₀} backbone. Potentiometric titrations of the {Mo₁₀} wheel in the presence of halides show successive ionizations of the five inner water molecules. The five related acidity constants range from 4 to 9. On this occasion, within pH 6–6.5, the [ClMo₁₀S₁₀O₁₀(OH)₁₂(H₂O)₃]^3– anion was characterized. The molecular structure resembles that of the parent, but on replacing two inner aquo ligands by two hydroxo groups, the inorganic cycle interacts with only a single chloride. Halide ions can be considered as templates for imposing the nuclearity of the wheel, which can swing from 10 to 12. Thus, the ability of the cyclic rings to self-rearrange supported by the possibility of such systems to bind soft bases such as halides, open some perspectives in the field of host–guest chemistry.

4.2 Caboxylates as template

The previous results open the way to a cycle-based chemistry, the purpose being to synthesize à la demande rings of different shapes and sizes. The possibility to tune the size and shape of the ring via an adapted template is an exciting challenge. Owing to the reactivity of the cavity of the ring, the use of linear dicarboxylates, oxalate (C₂O₄^2–), glutarate (H₆C₅O₄^2–) and pimelate (H₁₀C₇O₄^2–) ions proved to be a good tool to illustrate the influence of the length of the alkyl chain on the size and shape of the wheel. Reaction of the dithiocation with oxalate, glutarate and pimelate ions, led to [Mo₈S₈O₈(OH)₈(C₂O₄)]^2– noted [Mo₈–ox]^2–, [Mo₁₀S₁₀O₁₀(OH)₁₀(H₆C₅O₄)]^2– ([Mo₁₀–glu]^2–), and [Mo₁₂S₁₂O₁₂(OH)₁₂(H₁₀C₇O₄)]^2– ([Mo₁₂–pim]^2–) anions, respectively [20]. This series of compounds [Mo₈–ox]^2–, [Mo₁₀–glu]^2– and [Mo₁₂–pim]^2–, nicely illustrates the possibility to vary the size of the ring in a very simple way. The molecular architectures given in Fig. 11, consist in a neutral inorganic ring {Mo_2nO_2nS_2n(OH)_2n}, n = 4, 5 or 6 for ox^2–, glu^2– or pim^2–, respectively, encapsulating the different guests.

The cyclic skeleton of [Mo₈–ox]^2– contains eight octahedral molybdenum atoms, each oxygen atom of the two equivalent carboxylate groups is doubly bonded to two adjacent Mo atoms. The structures of [Mo₁₀–glu]^2– and [Mo₁₂–pim]^2– are distorted but can be considered as formally deriving from those of their perfectly circular Mo₁₀ and Mo₁₂ parents. The encapsulated dicarboxylate groups diametrically coordinate two Mo atoms. No inner water is present in the cavity of [Mo₁₀–glu]^2– and [Mo₁₂–pim]^2–. This illustrates the versatility of molybdenum, which can adopt either an octahedral, either a square pyramidal coordination. The possibility for Mo^V to be sixfold or fivefold coordinated gives a great flexibility to these architectures illustrated by the values of the Mo–Mo–Mo angles which can vary, in the same structure, from 135° to 180°. The weaker values are observed at the diametrically opposite carboxylate groups. The organic carboxylate acts as a pincer which distorts the ring. The cycle adapts its geometry to that of the encapsulated molecule. Aqueous solution ¹H NMR studies confirmed the presence of the encapsulated organic chain, while electrospray mass spectrometry investigations confirm the crystal structures are maintained in solution.

With trimesate (1,3,5 methylcarboxylato benzene), the perfectly circular dodecameric ring Mo₁₂ is obtained relatively to the symmetrical shape of the trimesate. The encapsulated trimesate lies in the mean plane defined by the twelve molybdenum atoms. The ring ideally fits with the shape of the guest giving the circular wheel represented in Fig. 12.

Now, we are convinced that the nuclearity of the cycle can be tuned via the control of the steric strains imposed by the shape and size of the template. An additional and critical parameter is the number of templates, which can be introduced within the ring.

4.3 Phosphates as template

Phosphates are interesting species for their chelating and acidobasic properties, small size and tetrahedral shape. [(HPO₄)Mo₁₀S₁₀O₁₀(OH)₁₀(H₂O)₃]^2– has been prepared [21] in nearly stoichiometric conditions in phosphate from dilute aqueous solutions. The structure is that of a neutral decanuclear ring {Mo₁₀O₁₀S₁₀(OH)₁₀} encapsulating a single hydrogeno-phosphate [HPO₄]^2– anion. [(HPO₄)₂Mo₁₂S₁₂O₁₂(OH)₁₂(H₂O)₂]^4– was obtained from more phosphate concentrated solutions [21]. The twelve metal ring encapsulates two hydrogeno-phosphato groups [HPO₄]^2–. The chelating effect resulting from the presence of two phosphate groups strongly distorts the ring from circular to elliptical as represented in Fig. 13.

The flattening of the ring shape is attributed to electrostatic repulsions occurring between the diametrically opposed phosphate groups and to their coordination pincer effects. Internal hydrogen bonds between the deprotonated P = O bonds and terminal inner water molecules bring additional stability to the elliptical shape. Solutions containing [PMo₁₀]^2– and [P₂Mo₁₂]^4– have been thermodynamically studied by ³¹P NMR at various temperatures, revealing fast dynamical phosphate exchanges accompanied with the rapid change of geometry between [PMo₁₀]^2– and [P₂Mo₁₂]^4– [21].

A striking feature was discovered by variable temperature ³¹P NMR and phosphate–arsenate exchange experiments: the phosphate groups rotate in the cavity, hopping on the vacant adjacent coordination sites. In addition, fast dynamical exchanges take place involving uncoordinated and coordinated phosphate ions in the Mo₁₀ and Mo₁₂ wheels. The phosphate-containing rings system confirms our first hypotheses about the relationship between the ring size and the number of encapsulated template-agent: a single phosphate stabilizes a ten-metal ring, while the insertion of the second one provokes the ring expansion to twelve metals.

In more concentrated phosphate solutions, the hexanuclear [(HPO₄)₄Mo₆S₆O₆(OH)₃]^5– anion is exclusively formed in solution (³¹P NMR) and characterized in the solid-state by X-ray diffraction methods [22]. The overall geometry of the sulfido-{P₄Mo₆} anion is quite similar to that of the oxo-{P₄Mo₆} represented in Fig. 2, but its reactivity is drastically different. A central phosphate is encapsulated in a six-metal ring that yields a quite close compact arrangement of oxo and sulfido ions. The arsenate analogue [(HAsO₄)₄Mo₆S₆O₆(OH)₃]^5– was also structurally characterized.

The three peripheral phosphates are labile; they can be easily replaced by other functional groups. The mechanism of the substitution reaction was established by ³¹P NMR spectroscopy by replacing phosphate for acetate groups. The exchange steps are successive with formation of mixed acetate-phosphate species. The quantitative treatments of the data pointed out that the relative affinity of the two substituting groups is essentially related to their own acidity constants. In Fig. 14 the resulting diagram of speciation is given.

The selective replacement of sulfur atoms within the {Mo₂S₂O₂} fragment by oxygen atoms was obtained by controlled hydrolysis. The reaction takes place in drastic conditions (80–100 °C) and in agreement with ³¹P NMR results, the nucleophilic substitution revealed to be regioselective. In a first step, only the three coplanar sulfur atoms located at the opposite side of the four phosphate groups are concerned by the substitution. The other three sulfur atoms are protected by the four bulky phosphates located nearby. The mechanism of substitution was confirmed by the resolution of the structure of the half-desulfurated [(HPO₄)₄Mo₆S₃O₉(OH)₃]^5– anion, which supports the previous ³¹P NMR observations in solution.

4.4 Metalates as template

At this step, we know that organic and p-block oxo-anions can serve as ring templating groups. In fact, more sophisticated templates, like transition-metal derivatives have been also used as structuring guests. With [MO₄]^2– (M = Mo or W) tetra-oxometalate ions, the self-condensation of the dithiocation [Mo₂O₂S₂]²⁺ gives the mixed metal [Mo₈S₈O₈(OH)₈(HMO₅(H₂O))]^3– compounds [23] containing a central {M^VIO₆} octahedron encapsulated in an octanuclear ring {Mo₈S₈O₈(OH)₈}. This arrangement resembles that previously described for oxalate ion, see § 4.2. The geometry of the encapsulated Mo(VI)-moiety has changed from tetrahedral to octahedral. Although the anion consists in a mixed valence Mo^VI/Mo^V system, the electronic spectrum does not exhibit any intervalence charge transfer (IVCT) from peripheral Mo^V atoms toward the central Mo^VI. The eight Mo(V) electrons are firmly trapped as pairs in the metal–metal bonds, and in addition the junctions between the Mo^V and Mo^VI centers are ensured through long Mo–O bond (2.2–2.4 Å) in trans position to the Mo=O double bond (trans effect). Thus, such compounds exhibit strongly trapped mixed valence for ranging in the class I of Robin and Day. The tungsten analogue was obtained and characterized.

A family of octanuclear wheels has been obtained by crystallization of [Mo₈S₈O₈(OH)₈(HMO₅(H₂O))]^3– and [Mo₈S₈O₈(OH)₈(C₂O₄)]^2– in an aqueous solution of LiCl, NaCl, KCl, and RbCl. Single crystals X-ray diffraction studies on these solids showed that the alkali salts exhibit a similar 3-D structure represented in Fig. 15.

Alkali cations are located in columns to which the anionic wheels are anchored [24]. Ionic conductivity measurements on pressed pellets revealed two different behaviors. The lithium salts of {HMo₈M₃}^– (M = Mo, W) are moderately good proton conductors at room temperature, σ = 10^–5 S cm^–1. The profile of conductivity as a function of relative water pressure showed that the conductivity is related to surface motion.

In contrast, the lithium salt of {Mo₈ox}^2– competes with the best solid lithium conductors at room temperature, σ = 10^–3 S cm^–1. The mobility of lithium cations along 1D channels was confirmed by ⁷Li NMR experiments [24].