Sulfonate-Based 1D and 2D Coordination Polymers Featuring 2,6-di(1 <i>H</i> -imidazole-1-yl)pyridine: High Proton Conductivity and Enhanced Stability via Polyoxometalate Incorporation

Two sulfonate-based coordination polymers (CPs), CP - I and CP - II, were synthesized from 2,6-di(1 H -imidazole-1-yl)pyridine and 5-sulfoisophthalate. Single-crystal X-ray diffraction reveals that CP - I forms a 1D Zn(II) framework containing continuous sulfonate-lined channels, whereas CP - II adopts a compact and unprecedented 2D Cu/Na topology. To introduce additional Brønsted acidity and structural reinforcement, a Keggin-type phosphotungstic acid polyoxometalate (PTA, H 3 PW 12 O 40 ) was postsynthetically encapsulated into CP - I, yielding the composite PTA@CP - I . CO 2 sorption at 273–298 K confirms accessible ultramicroporosity across the series: CP - I exhibits a trimodal pore-size distribution with micropores centered at ∼4.5, 5.2, and 8.6 Å. The two smaller pores correspond to CO 2 -accessible subchannels formed by local constrictions within the hydrated, sulfonate-lined 1D cavity, while the ∼8.6 Å pore reflects the full geometric diameter of the crystallographic channel. Upon PTA incorporation, PTA@CP - I retains this trimodal character, but the micropore positions shift to 5.07, 6.11, and 8.56 Å, with the ∼8.56 Å pore becoming the dominant feature. These microporous environments strongly influence proton transport under humid conditions. CP - I achieves the highest conductivity (8.7 × 10 –3 S cm –1 at 60 °C, E a = 0.20 eV), enabled by continuous hydrophilic channels that sustain extended hydrogen-bond networks. PTA@CP - I maintains high conductivity (2.6 × 10 –3 S cm –1 at 70 °C) and shows enhanced thermal and humidity stability due to the Brønsted-acidic and structurally reinforcing PTA clusters, whereas CP - II exhibits moderate but robust proton transport consistent with its constrained 2D microporosity. These findings illustrate that combining ligand-directed channel construction with polyoxometalate reinforcement offers a general strategy for creating stable, high-performance crystalline proton conductors.