Mono- and bis-templated cyclic polyoxothiomolybdates with succinate and fumarate ions

Dynamic properties of a hexadecamolybdenum wheel: studies in solution and density functional theory calculations

Variable temperature (1)H NMR studies of the host-guest complex [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(4)(PDA)(2)](4-) (1 ; PDA(2-) = phenylenediacetate) previously carried out in D(2)O have revealed a complex behavior in solution, involving a gliding motion of both parallel phenyl rings of the PDA(2-) ligands. In the present work, we present new NMR spectra carried out in the aprotic solvent CD(3)CN, which allow the observation of the proton signals associated with the bridging hydroxo groups of the inorganic host. The new spectra provide detailed information about the concerted reorganization of the guest components, that is, PDA(2-) and water molecules. The existence of an equilibrium between two distinct isomers differing in the linking mode between the inorganic host and the two equivalent PDA(2-) ligands is evidenced. This equilibrium appears strongly dependent upon the temperature, leading to a complete inversion of the distribution between 300 and 226 K. The thermodynamic data related to the isomerization reaction have been determined (Delta(r)H = -50.5 kJ mol(-1) and Delta(r)S = -215 J mol(-1) K(-1)). Furthermore, at low temperature, one of the isomers exists in two conformations, only differing in the H-bond network involving the inner water molecules. Density functional theory calculations were carried out to push ahead the interpretations obtained from experiment, identify the isomers of 1, and specify the role and the positions of the guest water molecules. Among the various structures that have been calculated for 1, three fall in a narrow energy range and should correspond to the species characterized by variable-temperature (1)H NMR experiments in CD(3)CN. Finally, this study shows how the internal disposition of the ligands affects the ellipticity of the Mo(16) ring which varies from one isomer to the other in the 0.73-1 range and highlights solvation of the ring as one of the key parameters for the conformational design of these flexible host-guest systems.

Structure, Formation, and Dynamics of Mo12 and Mo16 Oxothiomolybdenum Rings Containing Terephtalate Derivatives

The influence of rigid or semirigid dicarboxylate anions, terephtalate (TerP(2-)), isophtalate (IsoP(2-)), and phenylenediacetate (PDA(2-)) on the self-condensation process of the [Mo(2)O(2)S(2)](2+) dioxothio cation has been investigated. Three new molybdenum rings, [Mo(12)O(12)S(12)(OH)(12)(TerP)](2-) ([Mo(12)TerP](2-)), [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(4)(PDA)(2)](4-) ([Mo(16)(PDA)(2)](4-)), and [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(2)(IsoP)(2)](4-) ([Mo(16)(IsoP)(2)](4-)) have been isolated and unambiguously characterized in the solid state by single-crystal X-ray studies and in solution by various NMR methods and especially by diffusion-correlated NMR ((1)H DOSY) spectroscopy, which was shown to be a powerful tool for the characterization and speciation of templated molybdenum ring systems in solution. Characterization by FT-IR and elemental analysis are also reported. The dynamic and thermodynamic properties of both the sixteen-membered rings were studied in aqueous medium. Specific and distinct behaviors were revealed for each system. The IsoP(2-)/[Mo(2)O(2)S(2)](2+) system gave rise to equilibrium, involving mono-templated [Mo(12)IsoP](2-) and bis-templated [Mo(16)(IsoP)(2)](4-) ions. Thermodynamic parameters have been determined and showed that the driving-force for the formation of the [Mo(16)(IsoP)(2)](4-) is entropically governed. However, whatever the conditions (temperature, proportion of reactants), the PDA(2-)/[Mo(2)O(2)S(2)](2+) system led only to a single compound, the [Mo(16)(PDA)(2)](4-) ion. The latter exhibits dynamic behavior, consistent with the gliding of both the stacked aromatic groups. Stability and dynamics of both Mo(16) rings was related to weak hydrophobic or pi-pi stacking inter-template interactions and inner hydrogen-bond network occurring within the [Mo(16)(IsoP)(2)](4-) and [Mo(16)(PDA)(2)](4-) ions.

Host–guest adaptability within oxothiomolybdenum wheels: structures, studies in solution and DFT calculations

The formation of host-guest cyclic architectures, built up through the self-condensation process of [Mo(2)O(2)S(2)](2+) oxothiocations around linear dicarboxylate ions such as adipate (Adip(2-)), suberate (Sub(2-)) and azelaate (Azel(2-)) anions is reported. The complexes [Mo(12)Adip](2-), [Mo(12)Sub](2-) and [Mo(14)Azel](2-) have been characterized in the solid state by X-ray diffraction and in solution by (1)H NMR in different solvents (D(2)O, DMF, DMSO and CD(3)CN). The host-guest dynamics appear to be dependent on the nature of the system and are mainly governed by mutual adaptability between the host and the guest. (1)H NMR DOSY experiments show systematic differences, either positive or negative between the experimental and calculated molecular weights which appear to be correlated with the charge of the anion. The relative stabilities of the twelve-membered rings containing the Adip(2-), Pim(2-) (pimelate) or Sub(2-) anions were determined experimentally and decrease according to the order [Mo(12)Adip](2-) > [Mo(12)Pim](2-) > [Mo(12)Sub](2-). The host-guest adaptability depends on the length of the carbon chain and gives rise to selective encapsulation processes. Finally, theoretical DFT investigations in the gas phase yielded conformations whose symmetry and geometrical parameters proved consistent with X-ray structures and (1)H NMR spectra recorded in DMSO or DMF. Energy calculation highlights the high flexibility of the ring showing that only 3.1 kJ mol(-1) accompanies the conformational change from circular to elliptical. The host-guest bond energy (Delta E) calculated for the Mo(12)-based clusters is consistent with the experimental stability scale, major variations being due to some constraints undergone by the central alkyl chain.