Synthesis and characterization of octa- and hexanuclear polyoxomolybdate wheels:: Role of the inorganic template and of the counterion

Eight new compounds based on [O3PCH2PO3](4-) ligands and {(Mo2O4)-O-V} dimeric units have been synthesized and structurally characterized. Octanuclear wheels encapsulating various guests have been isolated with different counterions. With NH4+, a single wheel was obtained, as expected, with the planar CO32- guest, (NH4)(12)[(MoV2O4)(4)-(O3PCH2PO3)(4)(CO3)(2)]center dot 24H(2)O (1a), while with the pyramidal SO32- guest, only the syn isomer (NH4)(12)[(MoV2O4)(4)-(O3PCH2PO3)(4)(SO3)(2)]center dot 26H(2)O (2a) was characterized. The corresponding anti isomer was obtained with Na+ as counterions, Na-12[(MoV2O4)(4)(O3PCH2PO3)(4)(SO3)(2)]center dot 39H(2)O (2b), and with mixed Na+ and NH4+ counterions, Na+(NH4)(11)[(MoV2O4)(4)(O3PCH2PO3)(4)(SO3)(2)]center dot 13H(2)O (2d). With [O3PCH2PO3](4-) extra ligands, the octanuclear wheel Li-12(NH4)(2)[(MoV2O4)(4)(O3PCH2PO3)(4)(HO3PCH2PO3)(2)]center dot 31H(2)O (4a) was isolated with Li+ and NH4+ counterions and Li-14[((Mo2O4)-O-V)(4)(O3PCH2PO3)(4)(HO3PCH2PO3)(2)]center dot 34H(2)O (4c) as a pure Li+ salt. A new rectangular anion, formed by connecting two Mo-V dimers and two Mo-VI octahedra via methylenediphosphonato ligands with NH4+ as counterions, (NH4)(10)[((Mo2O4)-O-V)(2)((MoO3)-O-VI)(2)(O3PCH2PO3)(2)(HO3PCH2PO3)(2)]center dot 15H(2)O (3a), and Li-9(NH4)(2)Cl[((Mo2O4)-O-V)(2)((MoO3)-O-VI)(2)(O3PCH2PO3) 2(HO3PCH2PO3)(2)]center dot 22H(2)O (3d) as a mixed NH4+ and Li+ salt have also been synthesized. The structural characterization of the compounds, combined with a study of their behavior in solution, investigated by P-31 NMR, has allowed a discussion on the influence of the counterions on the structure of the anions and their stability. Density functional theory calculations carried out on both isomers of the [((Mo2O4)-O-V)(4)(O3PCH2PO3)(4)(SO3)(2)](12-) anion (2), either assumed isolated or embedded in a continuum solvent model, suggest that the anti form is favored by similar to 2 kcal mol(-1). Explicit insertion of two solvated counterions in the molecular cavity reverses this energy difference an reduces it to less than 1 kcal mol(-1), therefore accounting for the observed structural versatility.