Construction of inorganic macrocyclic host-guest architectures with high proton conductivity via coordination mode tuning

Constructing an inorganic macrocycle-based host-guest system remains a formidable synthetic challenge. Herein, by tuning the coordination modes of the pyrimidine-5-carboxylic acid (Pym-5-CA) and the assembly of [Mo₂O₂S₂]²⁺ units, we can obtain either an organic-templated inorganic ring architecture (H₄[(Mo₂O₂S₂)₆(OH)₁₂(C₅H₃O₂N₂)₂]·6DMF·5H₂O, denoted as Mo₁₂-Pym-5-CA, (1)) or an inorganic-host-organic-guest complex ([(Mo₂O₂S₂)₅(OH)(H₂O)₅(Pym-5-CA)]·24H₂O, Mo₁₀@Pym-5-CA (2)). By employing a similar strategy, but with another organic guest molecule, 3-pyridine-boronic acid (3-PyBA), another host-guest complex H[(Mo₂O₂S₂)₅(OH)(H₂O)₅(3-PyBA)I]·2DMF·16H₂O, Mo₁₀@3-PyBA (3) can be obtained. The host-guest structures have been characterized by multiple techniques, such as powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SC-XRD), and hydrogen nuclear magnetic resonance spectroscopy (¹H NMR). Proton conduction measurements at 70 ℃ and 90% RH reveal that complex 2 exhibits a proton conductivity of 8.76 × 10^-3 S·cm^-1, approximately 2.1 times higher than that of complex 1 and 1.7 times higher than that of complex 3. This superior performance stems from the synergistic effect of its intact carboxyl groups, which serve as strong proton donors, and its well-defined hydrogen bond network. Potentiometric measurements under varying pH conditions further reveal that complex 2 exhibits a slope of 37 mV dec^-1, higher than those of complex 1 (28 mV dec^-1) and complex 3 (32 mV dec^-1), indicating an enhanced proton-responsive behavior. By elucidating the distinct roles of supramolecular and coordination assembly in dictating the structures and proton conduction of polyoxothiometalates (POTMs), this work establishes a direct correlation between proton transport and interfacial response. Beyond demonstrating their promise as proton-selective electrochemical interfaces, these findings provide valuable insights for the rational design next-generation proton-conductive materials.