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Proton conduction studies on four porous and nonporous coordination polymers with different acidities and water uptake

Four coordination compounds {[Cd(L)1/2(H2O)2]·2H2O}n (1), {[Ni2(L)(bpyBr)2(H2O)2]·H2O}n (2), {[Ni(H2L)(bipy)(H2O)]·H2O}n (3), and [Co(H2L)(bipy)(H2O)2]n (4) (H4L = 5,5′-(pentane-1,4-diylbis(oxy))diisophthalic acid, bpyBr = 4,4′-dibromo-2,2′-bipyridyl, bipy = 4,4′-bipyridine) were synthesized and characterized. These four compounds exhibit a 3D open framework with channels, a 2D layer, a 3D supramolecular network constructed from coordination layers and a 3D supramolecular network constructed from coordination chains, respectively. The reaction temperature can influence the coordination modes of the organic multi-carboxylate ligands and the acidity of the compounds. AC impedance spectral studies suggest that the order of proton conductivity of compounds 1–4 is 3 > 4 > 1 > 2. The result reveals that the proton conductivities of low dimensional nonporous CPs 3 and 4 are higher than that of high dimensional porous CP 1. The structure–property studies reveal that the highest proton conductivity of 3 may be explained by its acidic hydrophilic groups and highest water uptake. The dimensionality and channels of the CPs can't influence proton conductivity directly. Meanwhile the amount and orientation of hydrophilic groups can decide the water absorption ability of the CPs, which is an important factor on proton conductivity. The protonation degree of hydrophilic groups, i.e. the acidity of the CPs is the first influencing factor. So, large amounts of hydrophilic groups with high protonation degrees should be considered firstly in the design and synthesis of CPs with good proton conducting performance.