Capture of the [Mo₃S₄]⁴⁺ Cluster within a {Mo₁₈} Macrocycle Yielding a Supramolecular Assembly Stabilized by a Dynamic H-Bond Network

The use of the [Mo3S4(Hnta)3]2− complex (nta3− = nitrilotriacetate) as structuring agent toward the self-condensation process of the [Mo2O2S2(OH2)6]2+ cation leads to the largest oxothiomolybdenum ring. In the solid state, X-ray diffraction analysis reveals the presence of the targeted molecular compound (noted 1a), which consists of the {Mo18O18S18(OH)18} host templated by the [Mo3S4(Hnta)3]2− guest. Nevertheless, the structure shows an additional molecular moiety corresponding to a dinuclear unit {Mo2O2S2} coordinated to two nta3− ligands, mutually arranged in a cis fashion (1b). In the solid state, both entities interact through two short hydrogen bonds to give a striking supramolecular adduct, noted {1a−1b}. Synthetic procedures to prepare the individual species as pure compounds were reported. 1a was obtained as a pure mixed Cs+/NMe4+ salt while the dinuclear unit [Mo2O2S2(Hnta)2]2− was obtained as mixed K+/Na+ crystals. X-ray diffraction study of the latter reveals a trans isomer (noted 1b′), characterized by the specific coordination of both nta3− ligands. All the compounds were characterized in solution (D2O or DMSO) by multiexperiment 1H NMR (1D, COSY, NOESY, and DOSY). The overall results were consistent with the retention of the adduct {1a−1b} which exhibits a supramolecular reactivity. The dinuclear individual species in solution gave rise to cis−trans equilibrium, while in the presence of the oxothiomolybdenum ring 1a, the dinuclear unit is maintained as a frozen cis complex. DOSY NMR provides a definitive argument for the integrity of the supramolecular assembly. In addition, preliminary electrochemical study of 1a is also reported.