2.1.1 Rb₃(NMe₄)[Mo₁₂-succ₂]·25 H₂O (1)

The crude precursor K₂I₂Mo₁₀S₁₀O₁₀(OH)₁₀(H₂O)₅]·15 H₂O·ε NMe₄I [11] (1 g; 0.41 mmol) was dissolved in 80 ml of water at 50 °C. Succinic acid (0.1 g; 0.8 mmol) was dissolved in 10 ml of water, basified by 1.6 ml of 1 mol l^–1 KOH solution. The resultant potassium succinate solution was added to the previous one for giving an orange solution (about pH 5). The mixture was kept at 50 °C for 45 min and rubidium chloride (2.15 g; 17.7 mmol) is added. A yellow cloudy precipitate was removed by filtration and the resulting filtrate allowed to stand in air for crystallization. After several days, orange crystals suitable for X-ray diffraction are obtained, washed by water and dried in air. Yield 42%. Anal. Calc. (found) for 1: Mo 39.09 (39.53), S 13.06 (12.90). EDX atomic ratio calcd. (found): Mo/S, 1 (0.98); Mo/Rb, 4 (3.90); S/Rb, 4 (3.96). IR/cm^–1 (KBr pellet):1543(s), 1484(m), 1453(m), 1398(m), 939(s), 496(s), NMR ¹H in DMSO-d₆ per ppm: –0.70, 2.30, 3.04, 10.34.

2.1.2 Rb_1,5K_0.5[Mo₁₂-fum]·19 H₂O (2)

The crude precursor K₂I₂Mo₁₀S₁₀O₁₀(OH)₁₀(H₂O)₅]·15 H₂O·ε NMe₄I [11] (1 g; 0.41 mmol) was dissolved in 60 ml of water at 50 °C. Fumaric acid (0.076 g; 0.60 mmol) was dissolved in 10 ml of water and 1.2 ml of 1 mol l^–1 KOH solution. The resulting solution was added to the previous solution for giving a clear orange solution at pH about 4.5. A first fraction of the target compound was obtained by the addition of RbCl (0.205 g; 1.7 mmol) to the solution (yield 32%). Single crystals suitable for the X-ray diffraction study were obtained from the filtrate which was allowed to stand for 1 week (yield 30%). Anal. Calc. (found) for 1: Rb 4.91 (4.79), K 0.75 (0.85) Mo 44.14 (44.63), S 14.75 (14.66), C 1.84 (2.23). EDX atomic ratio calcd. (found): Mo/S, 1 (1.13); Mo/Rb, 8 (7.97); S/Rb, 8 (8.3). IR/cm^–1 (KBr pellet):1558(s), 1387(m), 962(m), 521(m). NMR ¹H in DMSO-d₆ per ppm: 2.99, 10.31, 3.04, 10.34.

2.1.3 Li₄[Mo₁₂^_succ₂]·n H₂O and Li₂[Mo₁₂-fum]·n H₂O

The two lithium salts were prepared from the corresponding potassium salt K₄[Mo₁₂^_succ₂]·nH₂O and K₂[Mo₁₂-fum]·n H₂O, previously dissolved in water (about 0.4 g in 30 ml of water). Each solution was passed through a cation exchanger resin in the Li⁺ form (Dowex 50WX2). The resultant solutions were evaporated until dryness, and the purity of the lithium salt checked by routine methods (¹H NMR, IR, EDX).

The potassium salts of 1 and 2 were prepared as the procedures previously described for the rubidium salt (see below). Instead RbCl, potassium chloride (4 g; 53.6 mmol) was added for the precipitation of the corresponding potassium salts. The crystalline solids of 1 and 2, as potassium salts have been characterized by routine methods (¹H NMR, IR, EDX).

2.2.1 X-ray crystallography

A single crystal of 1 was mounted in a 0.3 mm diameter Lindeman tube while a single crystal of 2 was stabilized in liquid glycerin (paratone). Intensity data were collected at room temperature (298 K) for 1 and 100 K for 2 with a Brüker X8, Kappa CCD three-circle diffractometer equipped using the Mo Kα monochromatized wavelength (λ = 0.71073 Å). An empirical absorption correction was applied using the SADABS program based on the method of Blessing [16,17]. The structures were solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package [18]. Crystallographic data for 1 and 2 are reported in Table 1, selected bond distances are given in Table 2. Some Rb⁺ cations and some oxygen atoms of water molecules were found disordered. All the disordered atoms and water of crystallization were refined isotropically, the other atoms were refined anisotropically. Hydrogen atoms of the alkyl chains of the succinate and fumarate ions were located with geometrical restraints in the riding mode. Lattice water found by single crystal X-ray diffraction method slightly disagrees with those found by TGA method for 1 and 2 crystals. Such a difference is currently encountered for those ionic compounds: water molecules are often highly disordered and crystals decompose in air with loss of water.

2.2.2 Structure of the anions

In Figs. 1 and 2 are depicted the molecular structure of the [Mo₁₂-succ₂]^4– (1) and [Mo₁₂-fum]^2– (2) anions. Crystal data and selected bond distances are given in Tables 1 and 2, respectively. The two [Mo₁₂-fum]^2– and [Mo₁₂-succ₂]^4– molecular architectures consist of an inorganic cyclic neutral skeleton {Mo₁₂S₁₂O₁₂(OH)₁₂}, encapsulating two succinate ion for 1 and one fumarate ion for 2, respectively. The inorganic wheel results from {Mo₂O₂S₂}²⁺ connections through double hydroxo bridges, giving non-bonding MoLMo contacts (3.282(1)–3.383(1) Å) while sulfur atoms within the building blocks spans the MoLMo bonding contacts (2.821(1)–2.855(2) Å).

2.2.3 [Mo₁₂S₁₂O₁₂(OH)₁₂(H₂O)₄(C₄H₄O₄)₂]^4– (1)

The Mo₁₂-inorganic wheel encapsulates two succinate ions which are symmetrically arranged in the open-cavity (see Fig. 1). The four carboxylato groups exhibit the same asymmetric binding mode, each group being opposite to a single {Mo₂O₂S₂}. The O6 oxygen atom exhibits only one bonding contact with the Mo1 atom, while the O7 atom bridges the Mo2 and Mo3 atoms. The 12 molybdenum exhibit an octahedral arrangement and are arranged in a C_2v idealized local symmetry.

2.2.4 [Mo₁₂S₁₂O₁₂(OH)₁₂(OH₂)₄(C₄H₂O₄)]^2– (2)

The structure reveals the presence of one encapsulated fumarate ion [H₂C₄O₄]^2– in the Mo₁₂-ring (Fig. 2). The organic template is nearly located at the center of the elliptically distorted ring (C_2h idealized molecular symmetry) and connected through the two symmetric carboxylato groups. Four molybdenum atoms are involved in a bonding contact with the organic guest, while the coordination sphere of the other eight is filled by four inner water molecules (O14 and O15) which are doubly bonded to two adjacent molybdenum atoms. The carboxylato groups bridge two adjacent molybdenum atoms (Mo1 and Mo6), belonging to two different {Mo₂O₂S₂}. Such a mode of connection differs of those previously encountered, where the carboxylato groups are directly connected to the two adjacent molybdenum atom of the same {Mo₂O₂S₂} unit [14]. Such a result is attributed to steric constraints induced by the rigidity of the central C=C double bond of the guest, which imposes the conformational arrangement of the inorganic host around the rigid template.

2.2.5 ¹H NMR in D₂O

Besides the signal of the solvent (presence of H₂O), the spectrum of [Mo₁₂-fum]^2– (not shown) exhibits a sharp resonance at 3.02 ppm corresponding to the two equivalent protons attached to the encapsulated fumarate ion. The uncoordinated fumarate is observed at 6.4 ppm. The ¹H NMR spectrum of the [Mo₁₂-succ₂]^4– (Fig. 3) reveals more complicated, since three resonances corresponding to three different environments for the succinate ions are observed. At 295 K, the broad peak at 2.2 ppm is attributed to the uncoordinated succinate ions, while the two shielded resonances at –0.01 and –0.66 ppm correspond to encapsulated species. The presence of uncoordinated succinate ions reveals that succinate ion exchange occurs in the ring, and the simplest equilibrium (Eq. 1) can be postulated.

The intensity and the broadness of the –0.01 ppm line should argues for its attribution to the mono-succinate species. At upper temperature than 300 K, this line (–0.01 ppm) and that at 2.2 ppm exhibits nearly equal intensities according to the equilibrium 1, while the broadness of the –0.01 ppm line (Δν_1/2 = 50 Hz) would reflect the possibility of the remaining encapsulated succinate ion to move inside the half-empty cavity of Mo₁₂-ring. The remaining line at –0.66 ppm would be then attributed to the bis succinato-containing species [Mo₁₂-succ₂]^4–, involved in the equilibrium 1. In an aprotic solvent as CD₃CN, the equilibrium 1 reveals totally cancelled since only one line is observed at –0.65 ppm, corresponding to the bis succinato compound [Mo₁₂-succ₂]^4–. The decrease of the temperature from 310 to 275 K changes drastically the distribution in the species as shown by the variable temperature ¹H NMR experiments presented in Fig. 3. A second equilibrium (Eq. 2) must be considered necessarily, involving a second exchange process of the succinate ion from the mono-succinato compound [Mo₁₂-succ]^2–.

The equilibrium 2 gives an additional species, i.e. a templated-free Mo₁₂-ring, noted [Mo₁₂]. The concentration of overall species can be calculated from the initial concentration in [Mo₁₂-succ₂]^4– (C = 3.85 × 10^–3 mol l^–1), the intensities of the ¹H NMR lines, the mass balance in succinate and in Mo₁₂-species. The results are graphically shown in Fig. 4a and reveal that as temperature decreases, the concentration in [Mo₁₂-succ₂]^4– strongly decreases for giving uncoordinated succinate ions and succinate-free Mo₁₂ compound. As a consequence of the successive equilibria, concentrations of the intermediary compound [Mo₁₂-succ]^2– do not vary significantly. Then, the equilibrium constants K₁ and K₂ could be calculated in the 310–273 K temperature range, according to the Eqs. 3 and 4, respectively.

Then, a Van’t Hoff treatment of lnK_i (i = 1, 2) versus 1/T (shown in Fig. 4B) allowed to extract the standard enthalpy for the reactions 1 and 2; the standard entropy was then calculated from the Gibb’s function. The calculated thermodynamic parameters for both successive succinate transfers are given in Table 3. The negative values found for enthalpy and entropy for Eqs. 1 and 2 could be tentatively attributed to the formation of the hydration sphere of the produced ionic species, i.e. the uncoordinated succinate ions. Such an interpretation would reflect the main contribution of the polar water molecules. In fact, the origin of the driving force governing the inclusion of the dicarboxylate within the cavity is the entropy increase.

2.2.6 Variable-temperature ¹H NMR in CD₃CN

The ¹H NMR spectra of Li₄[Mo₁₂-succ₂]·n H₂O is presented in Fig. 5. In CD₃CN, the spectra for both compounds, Li₄[Mo₁₂-succ₂]·n H₂O (not shown) and Li₂[Mo₁₂-fum]·n H₂O (Fig. 5) contain three types of signal related to the three types of protons present in the alkyl chain, in water and in the hydroxo bridges of the ring [14]. For both the compounds, at 303 K, the protons of the alkyl chain give one single sharp resonance at –0.54 and + 3.27 ppm for succinate and fumarate in Mo₁₂-ring, respectively. The deshielded domain exhibits one broad symmetric signal at about +9 ppm which corresponds to the hydroxo groups of the inorganic wheel. As the temperature decreases from 303 to 227 K, the signal of the hydroxo bridges remains single but sharpens drastically from about 50 to 5 Hz. Similarly, the peak of the water at 3.2 ppm exhibits the same type of behavior, revealing a dynamic exchange between protons of the rings and those of the water. Although those results cannot be entirely explained (only one signal for the hydroxo bridges is observed, instead of the two expected), they argues for a single host–guest conformation, likely due to impossibility of the substrate to wheel inside the cavity. In the [Mo₁₂-fum]^2– anion, the rigidity of the linear chain probably cancels any concerted hopping of both the carboxylato groups, while in the [Mo₁₂-succ₂]^4–, the presence of two guests, closely arranged in the cavity prevents any conformational change of both the succinate ions.