A second hydrogen bond is formed between HTBZ as a proton donor and the deprotonated HFU acid as a proton acceptor, with an H⋅⋅⋅O bond distance of 1.777(51) Å. The HTBZ cations stack head-to-tail through π-π interactions, as do the planar HFU anions. The crystal packing of the structure is shown in Figure . It should be noted that the position of the hydrogen atoms has not been determined due to the limitations of the powder diffraction method. While this position can be calculated based on geometrical constraints and differences in acidity, the interpretation should be made with care .