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

The crystal structures of the solid form of solvated 2,6-bis(1H-imidazol-2-yl)pyridine (H(2)dimpy) trihydrate, C11H9N5 center dot 3H(2)O center dot{[}+solvent], I, and its hydrate hydrochloride salt 2-{[}6-(1H-imidazol-2-yl)pyridin-2-yl]-1H-imidazol-3-ium chloride trihydrate, C11H10N5+center dot Cl-center dot 3H(2)O, 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 P3(2)21 with an atypical formation of solvent-accessible voids, as a consequence of the 3(2) 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 P2(1)/c without the formation of solvent-accessible voids. The acid-base equilibria of H(2)dimpy were studied by potentiometric and spectrophotometric titrations, and the results suggest the formation of H(3)dimpy(+) (pK(a1) = 5.40) and H(4)dimpy(2+) (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. (AU)