Structural Studies of 1-Aryl-2-aminoimidazolinium Bromides: Focus on Tautomer Preference of the 2-Aminoimidazoline Moiety in the Solid State

The crystal structures of five 1-(4-X-phenyl)-2-aminoimidazolinium bromides (where X = -OCH3, -CH3, -H, -Cl and -NO2) were determined by X-ray crystallography with the aim to investigate the tautomer preference of the aminoimidazoline moiety in the solid state. The molecular structures clearly indicate that only the ring nitrogen is protonated. The crystals are stabilized by interactions between the hydrogens of the amino group (also NH fragment of the imidazoline ring) and bromide anion. In the case of the nitro derivative additional strong hydrogen bonds are a consequence of the presence of water molecule in the crystal lattice. The methoxy and nitro derivatives were studied by 15N CP/MAS NMR and the analysis of the tautomer preference was completed by ab initio calculations at the B3LYP/6-311+G** level.