Gauche and trans ethylenediammonium in the structures of ethylenediammonium tetrafluoroborate and tetrafluoroborate-fluoride crystals

The possible stereochemical conformations of the isolated ethylenediammonium dication were discussed, along with an assessment of their relative stabilities. Crystals of ethylenediammonium tetrafluoroborate-fluoride and tetrafluoroborate salts were prepared and structurally characterised. The crystal packing and hydrogen bonding interactions involving the ethylenediammonium dication were also described and analysed. In the structure of [ en H 2 ](BF 4 )F, the ethylenediammonium dication adopts a superposition of left-handed and right-handed C 2 -symmetrical gauche ( synclinal ) conformations, with an NCCN torsion angle of ± 75.49(10)°. These conformers form a cyclic R₂¹(7) R 2 1 ( 7 ) [ en H 2 ]F + fragment through hydrogen bonding with the fluoride anion. Three such R₂¹(7) R 2 1 ( 7 ) [ en H 2 ]F + fragments, along with a fluoride anion from a fourth [ en H 2 ]F + unit, associate into a more complex R₈⁴(16) R 8 4 ( 16 ) ring motif within the hydrogen-bonded network of [ en H 2 ](BF 4 )F. In the [ en H 2 ](BF 4 ) 2 salt, the ethylenediammonium dications adopt a trans configuration, corresponding to C 2 h point group symmetry. Raman and IR spectra were recorded and tentatively interpreted. A clear spectroscopic distinction between the trans and gauche conformations of the enH₂²⁺ e n H 2 2 + dication was observed and discussed.