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 {MoV2O4} 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]·24H2O (1a), while with the pyramidal SO32- guest, only the syn isomer (NH4)12[(MoV2O4)4(O3PCH2PO3)4(SO3)2]·26H2O (2a) was characterized. The corresponding anti isomer was obtained with Na+ as
counterions, Na12[(MoV2O4)4(O3PCH2PO3)4(SO3)2]·39H2O (2b), and with mixed Na+ and NH4+ counterions,
Na+(NH4)11[(MoV2O4)4(O3PCH2PO3)4(SO3)2]·13H2O (2d). With [O3PCH2PO3]4- extra ligands, the octanuclear wheel
Li12(NH4)2[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2]·31H2O (4a) was isolated with Li+ and NH4+ counterions and
Li14[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2]·34H2O (4c) as a pure Li+ salt. A new rectangular anion, formed by
connecting two MoV dimers and two MoVI octahedra via methylenediphosphonato ligands with NH4+ as counterions,
(NH4)10[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2(HO3PCH2PO3)2]·15H2O (3a), and Li9(NH4)2Cl[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2(HO3PCH2PO3)2]· 22H2O (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 31P 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 [(MoV2O4)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
∼2 kcal mol-1. Explicit insertion of two solvated counterions in the molecular cavity reverses this energy difference
and reduces it to less than 1 kcal mol-1, therefore accounting for the observed structural versatility.