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. In this study, tuning the coordination modes of pyrimidine-5-carboxylic acid (Pym-5-CA) and the assembly of [Mo₂O₂S₂]²⁺units enabled the selective construction of H₄[(Mo₂O₂S₂)₆(OH)₁₂(C₅H₃O₂N₂)₂]·6DMF·5H₂O (Mo₁₂-Pym-5-CA (1)) having an organic-templated inorganic ring architecture and theinorganic host–organic guest complex [(Mo₂O₂S₂)₅(OH)(H₂O)₅(Pym-5-CA)]·24H₂O (Mo₁₀@Pym-5-CA (2)).By employing a similar strategy except using 3-pyridine-boronic acid (3-PyBA) as the organic guest molecule, another host–guest complex, H[(Mo₂O₂S₂)₅(OH)(H₂O)₅(3-PyBA)I]·2DMF·16H₂O (Mo₁₀@3-PyBA (3)), was obtained. The host–guest structures were characterized using multiple techniques, including powder X-ray diffraction, single-crystal X-ray diffraction, and ¹H nuclear magnetic resonance spectroscopy. Proton conduction measurements at 70°C and 90% relative humidity revealed that complex 2 exhibits a proton conductivity of8.76 × 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 showed 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, this work establishes a direct correlation between proton transport and interfacial response. Beyond demonstrating the potential of polyoxothiometalates as proton-selective electrochemical interfaces, these findings provide valuable insights for the rational design of next-generation proton-conducting materials.