Systematics in NH⁺···N-Bonded Monosalts of 4,4′-Bipyridine (44′biPy) with Mineral Acids

Despite significantly different crystal symmetry and packing, the crystal structures of NH⁺···N hydrogen-bonded salts of 4,4′-bipyridine (44′biPy) with mineral acids H<i>A</i> = HCl, HBr, HI, HClO₄, HBF₄ and H₂SiF₆ exhibit close analogies in the hydration, aggregation of the cations and their twisted conformation, as well as proton disordering. All monosalts have been synthesized and, at normal conditions, form crystals of general formula [44′biPyH]⁺<i>A</i>^<i>–</i>·<i>x</i>H₂O (<i>x</i> = 0.5, 1, or 2), and [44′biPyH]⁺₂SiF₆^2–·5H₂O. In the structures, the 44′biPyH⁺ cations are NH⁺···N bonded into linear chains, and in most [44′biPyH]⁺<i>A</i>^<i>–</i>·<i>x</i>H₂O crystals the protons are disordered, similarly as in anisotropic relaxors 1,4-diazabicyclo[2.2.2]­octane hydroiodide and hydrobromide (dabcoHI and dabcoHBr, respectively). The proton disorder implies generation of point defects of neutral 44′biPy molecules and [44′biPyH₂]²⁺ dications in all these structures. In all [44′biPyH]⁺<i>A</i>^<i>–</i> structures investigated, the acid anions are hydrogen bonded to water molecules and interact with pyridine hydrogen atoms. Two polymorphs of [44′biPyH]⁺I^–·H₂O differ in color: the orthorhombic polymorph α is yellow, and the triclinic polymorph β is orange