Traditional and Ion-Pair Halogen-Bonded Complexes Between Chlorine and Bromine Derivatives and a Nitrogen-Heterocyclic Carbene
A theoretical study of the halogen-bonded complexes (A–X···C) formed between halogenated derivatives (A–X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug′-cc-pVDZ level of theory. Two types of A–X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C–X distances and small binding energies (8–54 kJ·mol–1). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C–X distances and large binding energies (148–200 kJ·mol–1), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A– and +[X–CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C–X bond is also identified as a significant stabilizing interaction in type-II complexes.