Proton Conductivity and Sorption Study in Three Sulfonic Group Functionalized Mixed Ligand Coordination Polymers and...

Two new mixed ligand coordination polymers (CPs) that are sulfonic group functionalized, namely, {[Cd(bpe)_0.5(5-sip)(H₂O)]·4H₂O(bpeH₂)_0.5} (1) and {[Cd_1.5(bte)(5-sip)(H₂O)₃]·3H₂O} (2) [where 5-sip = trinegative 5-sulfoisophthalate salt, bpe = 4,4'-bispyridylethane, bte = 1,2-bis(1,2,4-triazol-1-yl)ethane] have been synthesized through the variation of the N,N'-donor connectors only, at room temperature by using a slow diffusion technique. The structural analyses of both the complexes by single crystal X-ray diffraction studies clearly revealed the formation of 2D sheets containing guest water molecules entrapped in the 1D channel. A structurally similar reported compound {[Cd_1.5(btp)(5-sip)(H₂O)₃]·2H₂O} (3) has also been synthesized to compare the property of the newly synthesized compound. Herein, all the compounds show their humidity dependent proton conductivity as well as gas sorption behavior, which are signatures of their multifunctionality in the field of not only synthetic chemistry but also in material science. It is worth mentioning that the hydrogen bond network by the guest/coordinated waters, dicarboxylic, as well as sulfonic acid group(s) are present in the interlayer spaces, which are basically responsible for showing this high degree of proton conductivity in addition to significant water adsorption. Interesting phase transformation of compound 1 during experimental study perhaps plays a crucial role for its highest conductivity value among the three reported compounds. Herein it has found that the proton conductivity values vary in the range of ∼10^-5-10^-7 S cm^-1 at 65 °C under 95% relative humidity (RH) corroborating the Grotthus mechanism of proton conduction. All the dehydrated frameworks exhibit adsorption of different gases (e.g., CO₂ and N₂) and solvents (e.g., EtOH and H₂O). In the case of compound 1 and 3, selective CO₂ uptake has been observed.