Fluorine-containing donor-acceptor complexes: crystallographic study of the interactions between electronegative atoms (N, O, S) and halogen atoms (I, Br)

On the basis of our results, which concern sp(3)N...Br-R-f [R-f = per(poly)fluoroalkyl], sp(3)O...I-R-f, sp(2)O...I-R-f, sp(3)N...I-R-f, sp(2)N...I-R-f and interactions of sp(2)N...I-R-f with different substituents, a brief profile of the interactions between electronegative atoms (N, O, S) and halogen atoms (I, Br) of fluorine-containing alkylates is determined. The first example of an aliphatic fluorine-containing donor-acceptor supramolecule that is based on the N...Br-R-f interaction is reported in this paper. Our X-ray structure shows that 1,2-dibromotetrafluoroethane and 1,4-dimethylpiperazine alternately form endless chains depending on the N...Br-R-f interactions. The distance between Br and N is 2.864(3) Angstrom, which is considerably longer than the average covalent bond between N and Br, but it is also definitively shorter than the sum of the corresponding van der Waals radii of N and Br. From two other crystals, we successfully obtained precise data on the sp(3)O...X-R-f and sp(2)O...X-R-f interactions. Furthermore, an investigation of the substituent group effect is presented. We also report a valuable method to recrystallize and collect X-ray data of co-crystals that are easily disordered.