Consequences of Protonation of Diamines in the Gas Phase and the Solid State

An interesting model class of compounds are proton sponges—aromatic diamines characterized by exceptional basicity, such as 1,8-bis(dimethylamino)naphthalene (DMAN). For these compounds, protonation causes substantial redistribution of electron density which may be traced by observing the changes of structural parameters as well as of the properties of electron density at bond critical points. On protonation, electron density in DMAN goes from the terminal atoms towards the center of the molecule. This redistribution can also be confirmed by good quality neutron structural data. An ionic complex of DMAN with triformylmethane (TFM) acid has been studied by X-ray diffraction. We propose a multicenter model of hydrogen bonding {[Me₂N–H^0.5….^0.5H–NMe₂]⁺… X⁻} in this complex. The influence of weak intermolecular interactions of the DMANH⁺ cation with the nearest electronegative atom from the counterion on the strong intramolecular [N–H···N]⁺ hydrogen bonding has been demonstrated. Such interactions affect the localization of the proton in the intramolecular [N–H···N]⁺ hydrogen bridge.