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

Abstract 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 $${\text{R}}_{2}^{1}(7)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mtext>R</mml:mtext> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mn>1</mml:mn> </mml:msubsup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>7</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> [ en H 2 ]F + fragment through hydrogen bonding with the fluoride anion. Three such $${\text{R}}_{2}^{1}(7)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mtext>R</mml:mtext> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mn>1</mml:mn> </mml:msubsup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>7</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> [ en H 2 ]F + fragments, along with a fluoride anion from a fourth [ en H 2 ]F + unit, associate into a more complex $${\text{R}}_{8}^{4}(16)$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msubsup> <mml:mtext>R</mml:mtext> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mn>4</mml:mn> </mml:msubsup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mn>16</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> 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 $${\mathbf{e}\mathbf{n}\text{H}}_{2}^{2+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>e</mml:mi> <mml:mi>n</mml:mi> <mml:mtext>H</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> dication was observed and discussed.