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 K2I2Mo10S10O10(OH)10(OH2)5·15H2O. K5[Cl(P2O7}Mo12S12O12(OH)12(H2O)4]·22H2O (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 K6[(HPO4)(P2O7)Mo12S12O12(OH)12(H2O)2]·19H2O (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 Rb3[(H2P3O10)Mo10S10O10(OH)10]·17.5H2O (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 31P NMR. Variable temperature experiments, supported by a two-dimensional COSY 31P 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 I), responsible for the conformational change of the {P2O7} group within the cavity. The influence of the nature of the halide guest (Cl-, Br-, and I-) 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 > I. In solution, a phosphate exchange is observed for 2 while for 3 the absence of temperature dependence of the 31P NMR spectrum confirms the conformation of the host−guest system is blocked. Elemental analysis and infrared characterizations are also supplied.