Stereochemistry and transformations of NH-N hydrogen bonds - Part II. Proton stability in the monosalts of 1,4-diazabicyclo[2.2.2]octane

Crystal structures of three monosalts of 1,4-diazabicyclo[2.2.2]octane, perchlorate, tetrafluoroborate and bromide, have been studied by X-ray diffraction. The orthorhombic perchlorate and tetrafluoroborate salts are isostructural, while the bromide salt crystallizes in the hexagonal system. Varying stabilities of the acidic proton in the NH-N hydrogen bonds, linking the cations into chains, have been observed: in the perchlorate salt the H-atom is ordered, in the tetrafluoroborate it is 0.75:0.25 disordered at two N-sites, and in the hydrobromide the H-atom is 50:50 disordered between the symmetry-related sites. Possible mechanisms leading to the disorder of the protons and their stabilizing factors are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.