Halogen Bonds in 2,5-Dihalopyridine-Copper(I) Halide Coordination Polymers

Two series of 2,5-dihalopyridine-Cu(I)A (A = I, Br) complexes based on 2-X-5-iodopyridine and 2-X-5-bromopyridine (X = F, Cl, Br and I) are characterized by using single-crystal X-ray diffraction analysis to examine the nature of C2−X2···A–Cu and C5−X5···A–Cu halogen bonds. The reaction of the 2,5-dihalopyridines and Cu(I) salts allows the synthesis of eight 1-D coordination polymers and a discrete structure. The resulting Cu(I)-complexes are linked by C−X···A–Cu halogen bonds forming 3-D supramolecular networks. The C−X···A–Cu halogen bonds formed between halopyridine ligands and copper(I)-bound halide ions are stronger than C−X···X’–C interactions between two 2,5-dihalopyridine ligands. The C5−I5···I–Cu and C5−Br5···Br–Cu halogens bonds are shorter for C2-fluorine than C2-chlorine due to the greater electron-withdrawing power of fluorine. In 2,5-diiodopyridine-Cu(I)Br complex, the shorter C2−I2···Br–Cu [3.473(5) Å] distances are due to the combined polarization of C2-iodine by C2−I2···Cu interactions and para-electronic effects offered by the C5-iodine, whilst the long halogen bond contacts for C5−I5···Br–Cu [3.537(5) Å] are indicative that C2-iodine has a less para-electronic influence on the C5-iodine. In 2-fluoro-5-X-pyridine-Cu(I) complexes, the C2-fluorine is halogen bond passive, while the other C2-halogens in 2,5-dihalopyridine-Cu(I), including C2-chlorine, participate in halogen bonding interactions.


Introduction
Supramolecular chemistry utilizes small molecules and non-covalent interactions to self-assembly molecular aggregates with properties that are different from their individual components [1]. The design and construction of materials by self-assembly are largely predestined based on hydrogen bonding interactions due to the small size, easily polarizable and ubiquitous nature of the hydrogen atom [2]. Hydrogen bonding is observed in organic, as well as coordination, compounds. Over the last few years, orthogonal and conceptually similar to hydrogen bonds, halogen bonding has been studied as an additional important non-covalent interaction [3]. The halogen bonding is highly directional and the contact distances between donor and acceptor molecules can be modulated, due to the polarization hierarchy of different halogen atoms, in co-crystals for applications in material science [4] and solid-state research [5].

Results
Nine Cu(I)-complexes were obtained by mixing a 1:1 molar ratio of six different 2,5dihalopyridines (1-6, Figure 1) and two copper(I) halides (CuI and CuBr) in a 1:1 ratio of acetonitrile:ethanol mixture whilst heating gently. Slow evaporation of the resulting solutions provided single crystals suitable for X-ray diffraction analysis. Our attempts to grow single crystals of 5-bromo-2-iodopyridine-Cu(I) and 5-bromo-2-chloropyridine-Cu(I) complexes were unsuccessful. The Cu(I)-complexes of ligands 1-6 are labelled using the letter ''a'' for 2,5-dihalopyridine-CuI and ''b'' for 2,5-dihalopyridine-CuBr complexes. Five complexes (1a, 1b, 2a, 3a, 5a) form isostructural 1-D coordination polymers, whilst the other three complexes (2b, 4b, and 6b) show different structures and are described individually.  In complexes 1a, 1b, 2a, 3a, and 5a, two Cu(I) ions and two halide atoms form Cu 2 A 2 (A = I, Br) rhomboid-like units, where the two edges of each rhomboid are shared by two other adjacent rhomboids to yield 1-D coordination polymer ladders (Figure 2a,c,e,g). The Cu(I) ions are coordinated by one pyridine nitrogen and three halide atoms in a tetrahedral geometry with NA 3 coordination spheres. The pyridine ligands in the ladder are parallel to each other, not forming any π-π interaction between the aromatic rings, indicated by adjacent aromatic centroid-to-centroid distances of 4 . The small aromatic centroid-to-centroid distances further suggested that the interdigitation between 1-D polymers is not feasible. In the crystal lattice, C5-halogens and Cu(I)-bound halides of adjacent 1-D coordination polymer ladders manifest C5-X5···A-Cu halogen bonds, as shown in Figure 2b,d,f,h. The higher the electronegativity of the C2-halogen, the shorter the C5-X5···I-Cu halogen bonds. For example, the C5-I5···I-Cu halogen bond (R XB = 0.92) in 1a is shorter in comparison to the C5-I5···I-Cu halogen bonds (R XB = 0.93) in 2a due to the higher electron-withdrawing nature of the C2-fluorine when compared to C2-chlorine. The electron-withdrawing effect of C2-halogen is also observed in 5a and 6a complexes for C5-Br5···I-Cu halogen bonds. The C2-halogens in 1a, 1b, 2a, and 5a are XB passive, meaning they did not function as XB-donor and acceptors. The complex formed by 2-bromo-5-iodopyridine (3) and CuI is the exotic structure of the 2-X-5iodopyridine series (X = F, Cl, Br, and I). Typically, C2−X2···I−Cu or C5−X5···I−Cu XBs are observed but in this case, C2-and C5-halogens form C2−Br2···I5−C5 halogen bonds. The C2-bromine is the XB donor and C5-iodine is the XB acceptor with ∠C2−Br2···I5 = 172(1)° ( Figure 3). The C2−Br2···I5−C5 halogen bond contacts are just below the sum of the van der Waals radii of bromine and iodine atoms (3.83 Å). This could be either due to the less electron-withdrawing power of C2-bromine or the weak a) The complex formed by 2-bromo-5-iodopyridine (3) and CuI is the exotic structure of the 2-X-5-iodopyridine series (X = F, Cl, Br, and I). Typically, C2-X2···I-Cu or C5-X5···I-Cu XBs are observed but in this case, C2-and C5-halogens form C2-Br2···I5-C5 halogen bonds. The C2-bromine is the XB donor and C5-iodine is the XB acceptor with ∠C2-Br2···I5 = 172(1) • ( Figure 3). The C2-Br2···I5-C5 halogen bond contacts are just below the sum of the van der Waals radii of bromine and iodine atoms (3.83 Å). This could be either due to the less electron-withdrawing power of C2-bromine or the weak donating ability of C5-iodine caused by crystal-packing interactions. The copper(I)-bound iodine ion is XB passive, and only forms C4-H4···I-Cu interactions with a distance of 3.10 Å (∠C4-H4···I = 148 • ). Furthermore, the C5-iodines from the adjacent 1-D polymeric structures display weak C5-I5···I5 -C5 interactions at distances of 3.936 (4)  Single crystals of complex 1b･ACN were isolated from the bulk sample 1b. The asymmetric unit contains two crystallographically independent Cu(I) ions, a pyridine ligand and coordinating acetonitrile solvent molecule. Both Cu(I) ions have tetrahedral geometry with a NBr3 coordination sphere. However, one Cu(I)-centre is coordinated by pyridine nitrogen and three µ2-Br ions while the second Cu(I)-centre is coordinated by acetonitrile nitrogen and three µ2-Br ions, as shown in Figure  4a. The CuBr cluster in 1b･ACN extends into a 1-D polymeric structure, similar to complexes 1a, 1b, 2a, and 5a, with Cu(I) coordinated pyridine ligands and acetonitrile solvents decorated on the opposite side, as depicted in Figure   Complex 4b crystallizes in the triclinic space group P-1 and is a 1-D polymeric chain with a CuBr cluster core different to 1b. The Cu(I) ions are coordinated in a trigonal planar fashion by two iodine ions and a pyridine nitrogen ( Figure 5). Although the C2-and C5-positions in ligand 4 are iodine, which is typically known for its strong XB-donor ability, the C2−I2···Br−Cu halogen bonds are shorter than the C5−I5···Br−Cu XBs due to additional polarization caused by C2−I2···Cu interactions [3.333(5) Å] and the electron-withdrawing C5-halogen. The short C2−I2···Cu contacts are similar to C2−Br2···Cu interactions in our previously reported [Cu(2,5-dihalopyridine)2Br2] complexes [31].  Complex 4b crystallizes in the triclinic space group P-1 and is a 1-D polymeric chain with a CuBr cluster core different to 1b. The Cu(I) ions are coordinated in a trigonal planar fashion by two iodine ions and a pyridine nitrogen ( Figure 5). Although the C2-and C5-positions in ligand 4 are iodine, which is typically known for its strong XB-donor ability, the C2-I2···Br-Cu halogen bonds are shorter than the C5-I5···Br-Cu XBs due to additional polarization caused by C2-I2···Cu interactions [3.333(5) Å] and the electron-withdrawing C5-halogen. The short C2-I2···Cu contacts are similar to C2-Br2···Cu interactions in our previously reported [Cu(2,5-dihalopyridine) 2 Br 2 ] complexes [31]. Complex 6a crystallizes in the monoclinic space group P21/n. The asymmetric unit contains one pyridine ligand, two Cu(I) ions, and two iodide ions. The CuI and nitrogen in pyridine ligands of 6a form a 1-D honeycomb-like coordination polymer with Cu6I6N2 nodes, as shown in Figure 6. Both Cu(I) ions are tetrahedrally coordinated. One Cu(I) is coordinated by one pyridine nitrogen, one µ3and two µ4-iodide ions, and the other Cu(I) is coordinated by two µ3-and two µ4-iodide ions, leading to Cu(I)-centers with NI3 and I4 coordination spheres, respectively. The pyridine ligands are decorated parallel to each other above and below the 1-D chain, with centroid-to-centroid distances of 4.20 Å. The C5-bromine σ-hole and nucleophilic µ3-iodide ion form C5−I5···I−Cu halogen bonds whilst the C5-bromine anisotropic electron ′′belt′′ at the orthogonal directions and C2-bromide σ-hole form a C5−I5···I2−C2 type halogen bond. The µ4-iodide ion is XB passive. Complex 2b is the only example of a discrete Cu(I)-complex. The asymmetric unit consists of three crystallographically independent pyridine ligands (Py1-Py3) and two Cu(I) ions with a tetrahedral and a trigonal planar coordination geometry, respectively. In the molecular packing, the discrete structures exhibit several halogen bonds and halogen···halogen interactions; the respective distances and bond parameters are given in Table 1 Complex 6a crystallizes in the monoclinic space group P2 1 /n. The asymmetric unit contains one pyridine ligand, two Cu(I) ions, and two iodide ions. The CuI and nitrogen in pyridine ligands of 6a form a 1-D honeycomb-like coordination polymer with Cu 6 I 6 N 2 nodes, as shown in Figure 6. Both Cu(I) ions are tetrahedrally coordinated. One Cu(I) is coordinated by one pyridine nitrogen, one µ 3and two µ 4 -iodide ions, and the other Cu(I) is coordinated by two µ 3 -and two µ 4 -iodide ions, leading to Cu(I)-centers with NI 3 and I 4 coordination spheres, respectively. The pyridine ligands are decorated parallel to each other above and below the 1-D chain, with centroid-to-centroid distances of 4.20 Å. The C5-bromine σ-hole and nucleophilic µ 3 -iodide ion form C5-I5···I-Cu halogen bonds whilst the C5-bromine anisotropic electron belt at the orthogonal directions and C2-bromide σ-hole form a C5-I5···I2-C2 type halogen bond. The µ 4 -iodide ion is XB passive. Complex 6a crystallizes in the monoclinic space group P21/n. The asymmetric unit contains one pyridine ligand, two Cu(I) ions, and two iodide ions. The CuI and nitrogen in pyridine ligands of 6a form a 1-D honeycomb-like coordination polymer with Cu6I6N2 nodes, as shown in Figure 6. Both Cu(I) ions are tetrahedrally coordinated. One Cu(I) is coordinated by one pyridine nitrogen, one µ3and two µ4-iodide ions, and the other Cu(I) is coordinated by two µ3-and two µ4-iodide ions, leading to Cu(I)-centers with NI3 and I4 coordination spheres, respectively. The pyridine ligands are decorated parallel to each other above and below the 1-D chain, with centroid-to-centroid distances of 4.20 Å. The C5-bromine σ-hole and nucleophilic µ3-iodide ion form C5−I5···I−Cu halogen bonds whilst the C5-bromine anisotropic electron ′′belt′′ at the orthogonal directions and C2-bromide σ-hole form a C5−I5···I2−C2 type halogen bond. The µ4-iodide ion is XB passive. Complex 2b is the only example of a discrete Cu(I)-complex. The asymmetric unit consists of three crystallographically independent pyridine ligands (Py1-Py3) and two Cu(I) ions with a tetrahedral and a trigonal planar coordination geometry, respectively. In the molecular packing, the discrete structures exhibit several halogen bonds and halogen···halogen interactions; the respective distances and bond parameters are given in Table 1. Within the asymmetric unit, the C2-chlorine (Py1) and C5-iodine (Py3) attract electrostatically to form weak (Py1)C2−Cl2···I5−C5(Py3) halogen Complex 2b is the only example of a discrete Cu(I)-complex. The asymmetric unit consists of three crystallographically independent pyridine ligands (Py1-Py3) and two Cu(I) ions with a tetrahedral and a trigonal planar coordination geometry, respectively. In the molecular packing, the discrete structures exhibit several halogen bonds and halogen···halogen interactions; the respective distances and bond parameters are given in Table 1. Within the asymmetric unit, the C2-chlorine (Py1) and C5-iodine (Py3) attract electrostatically to form weak (Py1)C2-Cl2···I5-C5(Py3) halogen bonds with distances of 3.688(3) [∠C2-Cl2···I5 = 176.8(4); R XB = 0.99]. The C2-chlorine (Py1) is an XB-donor to the C5-iodine (Py3) XB-acceptor (Figure 7b). These halogen-bond contacts are weak, just below the sum of the van der Waals radii of Cl-and I-atoms [3.73 Å]. The C5-iodine substituents of Py1 and Py2 form relatively strong halogen bonds between two crystallographically independent copper-bound bromines (Figure 7b,c). The C5-iodine of Py3 exhibits two other halogen···halogen contacts to C2-chlorines of (Py3 ') [R XB = 0.95] and (Py2 ) [R XB = 0.98] to neighboring complexes, as depicted in Figure 7c. The strongest interactions are observed between C5-iodine substituents and nucleophilic Cu(I)-bound bromine ions.

Conclusions
The present study shows the application of rarely used 2,5-dihalopyrdine ligands to prepare copper(I)-coordination polymers that are stabilized by halogen bond interactions in the solid-state. The copper(I) complexes extend into 3-D supramolecular networks through C2−X2···A-Cu and C5−X5···A-Cu halogen bonds (X = Cl, Br, I; and A = Br, I) between the X2-or X5-halogen substituent (donor) and the copper(I)-bound halide anion (acceptor). With the exception of the C2-fluorine, all C2-and C5-halogen substituents, including the more electronegative C2-chlorine, participate in halogen bond interactions. Strong halogen bonds are formed between the C5-halogen substituent and the nucleophilic halide ions coordinated to copper(I), with C5-halogen always acting as XB donor and the halide anion as an XB acceptor. The flexible copper(I) coordination sphere allows the 2,5dihalopyridine halogen substituents to function as a halogen bond acceptor for C−X···X'-C interactions, in addition to C2−X2···A-Cu, C5−X5···A-Cu halogen bonds. This feature is not promoted
General synthesis of complexes 1a-6a: The solid 2,5-dihalopyridine (0.1046 mmol) was added to the solution of CuA (A = Br, I) (0.1046 mmol) in acetonitrile/ethanol (2.0 mL). If needed, the solutions were heated to dissolve the components. Single-crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of the corresponding solutions.
Crystal structure determination: The X- ray data for 1b, 2a, 2b, 4b and 5a were obtained by using a Bruker-Nonius Kappa CCD diffractometer with an APEX-II CCD detector utilizing graphite-monochromated Mo-Kα (λ = 0.71073 Å) radiation. Those of 1b·ACN, 3a, and 6a data were collected with a Rigaku Oxford Diffraction SuperNova instrument with an EoS CCD detector. The used Mo-Kα (λ = 0.71073 Å) radiation was monochromatized using multi-layer optics. The data for 1a was collected using the Rigaku SuperNova dual-source Oxford diffractometer equipped with an Atlas detector using mirror-monochromated Cu-Kα (λ = 1.54184 Å) radiation. The CrysAlisPro program and Gaussian face-index absorption correction method were used for data collection and reduction for 1a, 1b·ACN, 3a, and 6a. For the data collection and reduction for 1b, 2a, 2b, 4b and 5a, the programs COLLECT and HKL DENZO AND SCALEPACK [38] were used. The intensities were multi-scan absorption-corrected using SADABS [39]. Direct methods (SHELXS) [40] and full-matrix least squares on F 2 using the OLEX2 software [41] with SHELXL-2013 module [40] were used for all structures.
Crystal Author Contributions: The project was designed by R.P., and the single-crystal X-ray diffraction analysis for copper (I) complexes was performed by C.v.E and R.P. The manuscript was written by R.P., and edited from contributions of all co-authors C.v.E., R.P., and K.R.

Funding:
The authors kindly acknowledge the Academy of Finland (Project numbers RP: 298817) and the University of Jyväskylä for financial support.

Conflicts of Interest:
The authors declare no conflict of interest.