Analysis of solvent-accessible voids and proton-coupled electron transfer of 2,6-bis(1H-imidazol-2-yl)pyridine and...

The crystal structures of the solid form of solvated 2,6-bis(1H-imidazol-2-yl)pyridine (H₂dimpy) trihydrate, C₁₁H₉N₅·3H₂O·[+solvent], I, and its hydrate hydrochloride salt 2-[6-(1H-imidazol-2-yl)pyridin-2-yl]-1H-imidazol-3-ium chloride trihydrate, C₁₁H₁₀N₅⁺·Cl^-·3H₂O, II, are reported and analysed in detail, along with potentiometric and spectrophotometric titrations for evaluation of the acid-base equilibria and proton-coupled electron-transfer reactions. Compound I crystallizes in the high-symmetry trigonal space group P3₂21 with an atypical formation of solvent-accessible voids, as a consequence of the 3₂ screw axis in the crystallographic c-axis direction, which are probably occupied by uncharacterized disordered solvent molecules. Additionally, the trihydrated chloride salt crystallizes in the conventional monoclinic space group P2₁/c without the formation of solvent-accessible voids. The acid-base equilibria of H₂dimpy were studied by potentiometric and spectrophotometric titrations, and the results suggest the formation of H₃dimpy⁺ (pK_a1 = 5.40) and H₄dimpy²⁺ (pK_a2 = 3.98), with the electrochemical behaviour of these species showing two consecutive irreversible proton-coupled electron-transfer reactions. Density functional theory (DFT) calculations corroborate the interpretation of the experimental results and support the assignment of the electrochemical behaviour.