The Upper Limit of Luminol's Amphiprotism: The Crystal Structure of 5-Ammonium-2-hydro-1,4-phthalzinediol Sulfate(IV)

Abstract

Luminol is chemically sufficiently stable to be diprotonated at high proton concentrations as provided by concentrated sulfuric acid. The luminol dication (5-ammonium-2-hydro-1,4-phthalzinediol) sulfate was isolated as macroscopic single crystals and its structure was determined and refined from single-crystal X-ray data collected at 173 K [cell parameters: a = 8.3994(17) Å, b = 6.9985(14) Å, c = 17.486(4) Å, β = 90.85(3)°, V = 1027.8(4) Å3, space group P21/c]. The structure is comprised of layers stacked along the b axis. Intralayer interactions are accomplished by strong hydrogen bonds of three luminol dications to one central [SO4]2– ion. Interlayer interactions are formed by weak hydrogen bonds of one luminol dication to two [SO4]2– ions in the adjacent layers, respectively, and alternating sandwich and parallel-displaced π-π-stacking of the 1-hydropyridazine-3,6-diol moieties of luminol dications in adjacent layers, respectively.