Traditional and Ion-Pair Halogen-Bonded Complexes Between Chlorine and Bromine Derivatives and a Nitrogen-Heterocyclic Carbene Oscar Donoso-Tauda,* ,† Pablo Jaque, † Jose ́ Elguero, ‡ and Ibon Alkorta* ,‡ † Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Republica 275, Santiago, Chile ‡ Instituto de Química Me ́ dica (IQM-CSIC), Juan de la Cierva, 3, E-28006 Madrid Spain * S Supporting Information ABSTRACT: A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF 3 , CH 3 , 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. ■ INTRODUCTION Halogen bonding (A-X···B) is a well-known noncovalent interaction that has been defined as “a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity”. 1 It plays an important role in many fields such as drug design, crystal engineering, and material sciences. 2-8 This kind of noncovalent interaction can be explained in terms of electrostatic attraction between regions of positive electrostatic potential lying along the extension of the A-X bond axis, the so-called σ-hole, of the group VII atoms and regions of negative electrostatic potentials on B (an electron rich species, i.e., a Lewis base). 9,10 It should be mentioned that the contribution of the polarization and dispersion terms is not negligible in the stability of the A- X···B complexes. 11 In this sense, the A-X···B bond is highly directional, and the atoms are almost always linear or nearly so, giving rise to well-defined structures. The tendency to form halogen bonds with Lewis bases increases as X goes down in the periodic table group. The fluorine atom in general is unable to form halogen-bonded complexes, although it could be achieved under certain conditions. 12-15 Among the more recurrent Lewis bases as halogen acceptors are found compounds containing O, N, halogens and S atoms. 16-18 Singlet carbenes have not attracted too much attention as Lewis bases. In general, carbenes are highly reactive species with short lifetimes; consequently, not many experimental studies of complexes stabilized by either hydrogen bonding or halogen bonding involving carbenes are known. 19,20 Some theoretical studies about intermolecular complexes stabilized by hydrogen- bonding containing carbenes are found in the literature, 19,21,22 and to our knowledge, only a previous article deals with carbenes as the halogen acceptor in halogen-bonded A-X···C complexes is found. 22 In 2000, Mohand et al. 23 published a review about stable cyclic carbenes, with about 50 N-heterocyclic carbenes (NHC)s that have been structurally characterized since 1991 when the first stable structure of this type of carbene was reported by Arduengo. 24 The NHCs are cyclic diaminocarbenes, partic- ularly stable and strong σ-donors. They tend to bind more strongly to metal centers than most classical ligands. 23,25-28 The study of steric and electronic factors on these compounds has been very important in order to gain more insights into the knowledge of their reactivity. 23,29-31 The NHC-metal complexes are less sensitive to air and moisture and have proven remarkably resistant to oxidation. 27 The reactivity of NHCs with halogen derivatives is well documented. In this sense, Cole et al. 32 reported the reaction between an NHC, 1,3- bis(2,4,6-trimethylphenyl)imidazole-2-ylidene, and dibromine to produce the corresponding 1,3-bis(2,4,6-trimethylphenyl)-2- bromoimidazolium bromide as displayed in Scheme 1. Received: July 3, 2014 Revised: September 3, 2014 Published: September 4, 2014 Article pubs.acs.org/JPCA © 2014 American Chemical Society 9552 dx.doi.org/10.1021/jp506663x | J. Phys. Chem. A 2014, 118, 9552-9560