A crystallographic and theoretical study of halogen···halogen and halogen···halide synthons.

摘要

The physical nature of halogen···halogen and halogen···halide interactions has been investigated using both ab initio calculations and crystallographic studies. Both studies show that halogen···halogen and halogen···halide interactions are electrostatic in nature. An electrostatic model is proposed to explain these interactions. This model is based on two main ideas; (a) the presence of a positive electrostatic cap on the halogen atom (except for fluorine), (b) the electronic charge is anisotropically distributed around the halogen atom. Halogen···halogen contacts in organic molecules can be represented as C-Y1˙˙˙Y2-C [where theta 1= C-Y1˙˙˙Y2, theta2 = Y1˙˙˙Y2-C; Y = Cl, Br, I]. Halogen···halogen interactions are characterized by a Y1···Y 2 separation distance less than the sum of van der Waals radii ( rvdW) of the halogen atoms. According to the electrostatic model, two preferred arrangements are possible; (a) theta1= theta 2 = ca. 150°; (b) theta1 =180° and theta 2 = 90°. The second arrangement is not investigated during our study due to the fact that at this geometry, other interactions interfere with these contacts. A population analysis of the Cambridge Structural Data Base and ab initio calculations confirm the existence of the former geometry. Closely examining this geometry shows that these contacts are influenced by three factors; (a) the type of the halogen atom; (b) hybridization of the ipso carbon; (c) the nature of the other atoms that are bonded to the ipso carbon atom apart from the halogen atom.;Halogen···halide interactions are represented by C-Y···X (X = F-, Cl-, Br -, I-). These interactions are characterized by a Y···X distance less than sum of the rvdW of the halogen atom and the ionic radius of halide anion, as well as a linear C-Y···X angle. This arrangement is expected from the electrostatic model - the halide anion should confront the positive electrostatic potential cap of the halogen. Two types of halogen···halide interactions are studied; (1) simple halogen···halide interactions. Results show that these interactions are influenced by four factors; (i) the type of the halide anion; (ii) the type of the halogen atom; (iii) the hybridization of the ipso carbon; (iv) the nature of the functional groups; (2) C-Y···X-Cu (Y = Cl, Br; X = Cl-, Br-). These interactions are studied in complexes of the type (nCP)2CuX4 (nCP+ = n-chloropyridinium; n = 2, 3, or 4; X = Cl- or Br-) and Cu(nbp)2X2, (nbp = n-bromopyridine; n = 2 and 3). A comparison of the role of these synthons in these and previously published (nBP)2CuX4 structures (nBP = n-bromopyridinium cations; X = Br- or Cl-; n = 2) shows that; (a) the heavier the halogen atom, the stronger these interactions; (b) the lighter the halide anion, the stronger these interactions and; (c) these interactions are stronger in the complexes of the ( nBP)2CuX4 in comparison to Cu(nbp) 2X2. All of these observations fit the electrostatic model. These studies show that halogen···halide interactions are directional in nature. This implies that this synthonic interaction can act as a potential crystal engineering tool in the directed architecture of supramolecular species.