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

Abstract

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(HPO42–) (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.