Polyphosphate ions encapsulated in oxothiomolybdate rings:: Synthesis, structure, and behavior in solution

Cyclic oxothiomolybdates containing polyphosphate ions were prepared by simple reactions in aqueous medium of the corresponding polyphosphate ions and the cyclic precursor K(2)l(2)Mo(10)S(10)O(10)(OH)(10)-(OH2)(5)(.)15H(2)O. K-5[Cl(P2O7}Mo12S12O12(OH)(12)(H2O)(4)](.)22H(2)O (1) was isolated from concentrated chloride solution (2.5 mol(.)L(-1)). 1 reveals a remarkable complex containing two different substrates encapsulated in a dodecanuclear ring, a H-bonded Cl- ion, and a covalently bonded {P2O7} group. The chloride ion in 1 can be selectively removed for a monohydrogenophosphate group yielding K-6[(HPO4)(P2O7)Mo12S12O12(OH)(12)(H2O)(2)](.)19H(2)O (2), a mixed species containing a {P2O7} and a {HPO4} group. The substitution is accompanied by a significant change of the ring, which adopts a "pear-shape" conformation. In the presence of triphosphate ion, the "heart-shaped" decanuclear ring Rb-3[(H2P3O10)Mo10S10O10(OH)(10)](.)17.5H(2)O (3) is formed containing a linear {P3O10} group intimately embedded in the inorganic cyclic host. The three compounds were structurally characterized by single-crystal X-ray diffraction. The behaviors of 1, 2, and 3 in solution were studied by P-31 NMR. Variable temperature experiments, supported by a two-dimensional COSY P-31 experiment, revealed that the supramolecular interaction existing between the chloride ion and the ring in solid 1 is maintained in solution. Nevertheless, 1 remains labile, and successive equilibria were evidenced and interpreted as an ion-pair association involving a halide ion (Cl, Br, or l), responsible for the conformational change of the {P2O7] group within the cavity. The influence of the nature of the halide guest (Cl-, Br-, and l(-)) on the successive equilibria was studied, and the thermodynamic constant related to the postulated equilibrium was determined. The stability of the supramolecular association decreases in the order Cl > Br > l. In solution, a phosphate exchange is observed for 2 while for 3 the absence of temperature dependence of the P-31 NMR spectrum confirms the conformation of the host-guest system is blocked. Elemental analysis and infrared characterizations are also supplied.