Structural criteria for the design of anion receptors: The interaction of halides with electron-deficient arenes

This paper refines the nature of the interactions between electron-deficient arenes and halide anions. Conclusions are based on (i) new crystal structures containing alkali halide salts with 1,2,4,5-tetracyanobenzene (TCB) and 18-crown-6, (ii) evaluation of crystal structures found in the Cambridge Structural Database, and (iii) MP2/aug-cc-pVDZ calculations of F-, Cl-, and Br- complexes with TCB, 1,3,5-tricyanobenzene, triazine, and hexafluorobenzene. When the halide lies above the plane of the pi system, the results establish that three distinctly different types of complexes are possible: strongly covalent sigma complexes, weakly covalent donor-pi-acceptor complexes, and noncovalent anion-pi complexes. When aryl C-H groups are present, a fourth type of interaction leads to C-H center dot center dot center dot X- hydrogen bonding. Characterization of the different geometries encountered with the four possible binding motifs provides criteria needed to design host architectures containing electron-deficient arenes.