Sulfonic Group Functionalized Mixed Ligand Coordination Polymers: Synthesis, Characterization, Water Sorption, and Proton Conduction Studies

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Five sulfonic acid group functionalized mixed ligand coordination polymers (CPs), namely, {[Zn(bpeH)(5-sip)(H₂O)]·(H₂O)}n (1), {[Cu(pyz)(5-Hsip)(H₂O)₂]·(H₂O)₂}n (2), {[Cu(bpee)0.5(5-sip)(H₂O)₂]·(H₂O)₄(bpeeH₂)0.5}n (3), {[Cu(bpy)(5-Hsip)(H₂O)]·(H₂O)₂}n (4), and {[Cu(bpy)₂(5-H₂sip)₂]·(H₂O)₆}n (5) [where sip³⁻ = 5-sulfoisophthalate; bpe = 4,4'-bispyridylethane; pyz = pyrazine; bpee = 4,4'-bispyridylethylene; bpy = 4,4'-bipyridine], have been synthesized with varying different N,N'-donor linkers using slow diffusion techniques at room temperature. The CPs possess guest water filled 1D channels and noncoordinating sulfonic acid or coordinated sulfonate groups, which are interconnected by means of extended intermolecular H-bonding interaction, which supports the humidity dependent proton conductivity of the samples. Under 95% relative humidity (% RH), the CPs exhibit the temperature dependent proton conductivity which is maximum up to in the range of ∼10⁻⁵-10⁻⁶ S cm⁻¹ at 65 °C. In most of the cases, the framework shows activation energies with the value ranging from 0.35 to 0.54 eV, suggesting mostly the contribution of the Grotthuss mechanism of the proton conductivity.