Halogen Bonds in Square Planar 2 , 5-Dihalopyridine-Copper ( II ) Bromide Complexes

Halogen bonding in self-complementary 1:2 metal-ligand complexes obtained from copper(II) bromide (CuBr2) and seven 2,5dihalopyridines are analyzed using single crystal X-ray diffraction. All presented discrete complexes form 1-D polymeric chains connected with C–X···Br–Cu halogen bonds (XB). In (2-chloro-5-Xpyridine)2·CuBr2 (X = Cl, Br and I), only the C5-halogen, and in (2bromo-5-X-pyridine)2·CuBr2 (X = Cl, Br and I) both, C2and C5halogens, form C–X···Br–Cu halogen bonds with the X acting as the XB donor and copper-coordinated bromide as the XB acceptor. The electron withdrawing C2-chloride in 2-chloro-5-X-pyridine-CuBr2 complexes has only a minor effect on the C5–X5···Br–Cu XBs, and the X5···Br distances follow expected order, I5 < Br5 < Cl5 in the RXB values of 0.91, 0.94 and 0.99, respectively. In 2-bromo-5-X-pyridineCuBr2 complexes, due to the polarization of both halogens, the C2– X2···Br–Cu and C5-X5···Br–Cu the RXB values are very similar [0.92 – 0.99] due to competition of C2and C5-halogens for XB formation. In addition to the classical halogen bonds the square planar Cu(II) complexes exhibit C2–X2···Cu (X = Cl and Br) contacts perpendicular to the Br–Cu–Br plane with shorter C2–Br2···Cu than C2–Cl2···Cu contacts. These interactions induce a pseudo-octahedral geometry for Cu(II) ions. Notably, C2–X2···Br–Cu halogen bonds and the additional C2–X2···Cu contacts are slightly enhanced by the C5halogen electronegativity.


Introduction
The importance of hydrogen bonds [1] (HB) in organic, organometallic and inorganic compounds has led to significant interest to other non-covalent interactions, most recently to the utilization of halogen bonding. The halogen bonds (XB), are predominantly studied in organic molecules and in the solid-state, as strong, specific and highly directional non-covalent interactions equivalent to HBs resulting in a rapidly developing area within supramolecular chemistry. [2] Over the past decade, C-X···A-C [C = carbon, X = halogen as XB donor, A = XB acceptor, often halide anion], halogen bonding interactions have been of growing interest as valuable crystal engineering tool. [3] The halogen bond has recently been defined [4] by IUPAC and has recently been extensively studied, [2][3] also for complex halide anions [5] using solid-state X-ray crystallography and computational methods. [2][3][4][5][6] In transition metal complexes, the importance and the role of two descriptors are well studied, viz., (a) primary coordination sphere or metal-ligand interactions, and (b) secondary coordination sphere or non-covalent interactions. [7] Structure and reactivity of metal complexes depend on metal-ligand interactions, while most chemical and physical properties are influenced by non-covalent interactions, e.g. HB and XB. In the solid-state X-ray crystal structures, the functional groups attached to the ligands in the coordination complexes gives rise to rich chemical environments allowing for variety of intra-and intermolecular non-covalent interactions from the substituents. To investigate XBs in metal complexes, halopyridines coordinated to metal ions makes it possible to study both C-X···A-C and C-X···A-M [X = halogen substituent, M = metal ion, A = XB acceptor] interactions. A few research groups, mainly, Brammer et al., [8] have crystallographically studied complexes of the type M(LX)2A2 [LX = 2-, 3-and 4-halopyridines, M = metal and A = metal bound halides], where A acts as XB acceptor and X as XB donor. Furthermore, Brammer et al. explored the cooperative and competitive nature of HBs and XBs of [LH]⁺ species as acceptors to tetrahedral MA4 anions. [9] To complement this, we have previously studied neutral pincer type viz., {2,6-bis[(di-tbutylphosphino)-methyl]phenyl}PdY (Y = Cl, Br and I) and (terpyridine)Me3PtI complexes where the metal bound halogen acts as the XB acceptor. [10] Figure 1. Representation of 2,5-dihalopyridine core numbering, and chemical structures of 2,5-dichloropyridine (1), 5-bromo-2-chloropyridine (2), 2-chloro-5iodopyridine (3), 2-bromo-5-fluoropyridine (4), 2-bromo-5-chloropyridine (5), 2,5-dibromopyridine (6) and 2-bromo-5-iodopyridine (7).
In the present study, we analyze the halogen bonds in coordination complexes formed from 2,5-dihalopyridines and copper(II) bromide (CuBr2). We utilize a series of seven 2,5dihalopyridines, three based on 2-chloropyridine (1-3) and four based on 2-bromopyridine (5-7), with a different halogen substitiuent at the 5-position of the pyridine ring [X5, Fig. 1]. We anticipated that, upon complexation with CuBr2, the X5 FULL PAPER substituent will be electronically influenced by the X2 substituent as it resides close to positive Cu(II) coordination sphere. The formally negatively-charged bromide atoms bound to the Cu(II) ion are nucleophilic enough to act as the XB acceptors. However, considering different coordination geometries of Cu(II) and the orientation of ligands influenced by crystal packing interactions, this study further aims to examine: (a) whether either X2 or X5 or both of them are able to form C-X···Br-Cu halogen bonds in the solid-state. The crystallographic studies are supplemented by qualitative analysis of the electrostatic potential surfaces of the complexes, viz. by visualizing the modulation of the σ-hole at the X2 or X5 substituent or vice versa. In the following discussion, 2chloro-, 2-bromo-, 3-chloro-and 3-bromopyridines from the literature [11] are abbreviated as 2ClPy, 2BrPy, 3ClPy and 3BrPy.

Complex
Motif Accepted Manuscript

European Journal of Inorganic Chemistry
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
This article is protected by copyright. All rights reserved.
General crystallization procedure: To a solution of CuBr2 (0.067 mmol) in acetonitrile (1.0 ml), was added respective 2,5-dihalopyridine (0.134 mmol) dissolved in acetonitrile (0.5 ml) at room temperature. In case of precipitation, the samples were sonicated to clear solutions. The solutions were left in dark at room temperature, and subjected to slow evaporation to give single crystals suitable for X-ray diffraction analysis.