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

Abstract

On the basis of our results, which concern sp³N⋯Br–R_f [R_f = per(poly)fluoroalkyl], sp³O⋯I–R_f, sp²O⋯I–R_f, sp³N⋯I–R_f, sp²N⋯I–R_f and interactions of sp²N⋯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) Å, 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 sp³O⋯X–R_f and sp²O⋯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.