Protonic Conductivity and Hydrogen Bonds in (Haloanilinium)(H₂PO₄) Crystals

Brønsted acid-base reactions between phosphoric acid (H₃PO₄) and haloanilines in alcohols formed 1:1 proton-transferred ionic salts of (X-anilinium⁺)(H₂PO₄^-) and 2:1 ones of (X-anilinium⁺)₂(HPO₄^2-) (X = F, Cl, Br, and I at o, m, and p positions of anilinium). Only the former 1:1 single crystals showed proton conductivity under the N₂ condition, and the latter 2:1 crystals became protonic insulators. In crystals, diverse hydrogen-bonding structures from 1D to 2D networks were achieved by modification of the molecular structure of X-anilinium cations. The protonic conductivity was associated with the connectivity of H₂PO₄^- anions in the hydrogen-bonding networks. The hydrogen-bonding ladder chains in (o-cloroanilinim)(H₂PO₄^-) and (o-bromoanilinim)(H₂PO₄^-) resulted in the highest protonic conductivity of 10^-3 S cm^-1. The protonic conductivity of the ladder-chain (H₂PO₄^-) arrangements was higher than that of 2D sheets. The motional freedom of protons was analyzed by difference Fourier analysis of the single-crystal X-ray structure. The 2D layer, including (H₂PO₄^-)₂ dimers and (H₂PO₄^-)₄ tetramers, showed relatively low protonic conductivity, and the activation energy for proton conductivity was lowered by increasing the hydrogen-bonding connectivity and uniformity between H₂PO₄^- anions.