Halogen Bonding in Isostructural Co(II) Complexes with 2-Halopyridines

: Three complexes [Co(2-XPy) 2 Cl 2 ] (X = Cl, Br, and I) were prepared and characterized, representing a rare case of isostructurality within the Cl-Br-I row. The nature of halogen bonding (XB) in a solid state was studied by DFT calculations, revealing a tendency of XB energy growth for heavier halogens.


Introduction
Regarding all of the approaches in the field of crystal engineering, halogen bonding (XB) has attracted great interest within modern research [1,2]. This type of non-covalent interaction involving halogen atoms (according to the IUPAC-approved definition, the bond "occurs when there is evidence of 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") becomes especially prominent in a solid state, and its appearance can affect numerous physical properties of matter, such as luminescence [3][4][5], solubility [6], color [7], and so forth, therefore providing an additional tool for material design [8][9][10].
From the point of view of physical chemistry, the key parameter of XB is its energy, which mainly depends on the environment of the halogen atoms participating in the formation of noncovalent contacts. However, the effects of crystal packing can play a significant role as well [11]. Therefore, for the experimental estimation of the σ-hole-donating ability in certain building blocks (this feature is essential for XB formation [2]) of special interest are the cases where series of compounds containing different halogen atoms (Cl, Br, and I) are isostructural. This situation allows a direct comparison of energies in different X···X pairs by means of theoretical methods. However, while Cl/Br or Br/I isostructural pairs are common, the 'triple' series are very rare [12,13] especially in coordination chemistry [14,15].

Materials and Methods
All experiments were conducted in air. All reagents used were from commercial sources (Sigma-Aldrich) and used as purchased.

X-ray Diffractometry
Data for single crystals of 1-3 were obtained at 130 K on the Agilent Xcalibur diffractometer (version/model, Oxford Diffraction, UK) equipped with an area AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å, ω-scans). The integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package (CrysAlisPro 1.171.38.41. Rigaku Oxford Diffraction: The Woodlands, TX, USA, 2015). The structures were solved using the dual space algorithm (SHELXT) and refined by the full-matrix least-squares technique (SHELXL) [34] in anisotropic approximation (except hydrogen atoms). The position of the hydrogen atoms of organic ligands was calculated geometrically and refined in the riding model. The crystallographic data and details of the structure refinements are summarized in Table S1 (Supplementary Materials). CCDC 1985498-1985500 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/data_request/cif.

Results and Discussion
The formulas 1-3 are neutral. The coordination environment of the tetrahedral Co(II) consisted of two chloride and two pyridine ligands (Figure 1) 1, 2, and 3) combined with the sum of the corresponding Bondi's van der Waals radii (3.50, 3.58, and 3.73 Å for X = Cl, Br, and I [35,36]) indicates that corresponding non-covalent interactions must be present in 2 and 3 and, most likely, absent in 1. However, since it was known that "abnormally long" XB can exists, we decided to include 1 into the DFT analysis dataset. For the estimation of XB energies in 1-3, we applied an approach that was successfully used by us earlier [37][38][39][40]: atomic coordinates were extracted from XRD data and used (without optimization, see Supplementary Materials) for DFT calculations (M06/DZP-DKH) and the topological analysis of the electron density distribution [41]. Results are presented in Table 1 and in Figure 2. According to the criterion proposed by Espinosa et al., the balance between the Lagrangian kinetic energy G(r) and potential energy density V(r) at the BCPs (3, −1) indicates the nature of these interactions; if the ratio −G(r)/V(r) > 1, then the nature of the appropriate interaction is purely noncovalent; in the case of −G(r)/V(r) < 1, some covalent component occurs [42]; therefore, all considered interactions are purely non-covalent. For the estimation of XB energies, four diverse approaches were used [43][44][45]. The absolute values of Eint calculated via these methods vary, but in all cases there was a clear trend: the energy increased in the Cl-Br-I row (the difference became especially prominent in the case of the approach proposed by Bartashevich et al. [45]). Interestingly, these correlated with the contributions of corresponding interactions to the molecular Hirshfeld surfaces (see Supplementary Materials, Table S2) which increased in the 1-2-3 sequence. All these facts confirmed that 2-iodopyridine was the best XB donor in 2-XPy series. Table 1. Values of the density of all electronsρ(r), Laplacian of electron density -∇ 2 ρ(r) and appropriate λ2 eigenvalues (with promolecular approximation), energy density -Hb, potential energy density -V(r), and Lagrangian kinetic energy -G(r) (a.u.) at the bonding of critical points (3, −1), corresponding to intermolecular non-covalent interactions X···Cl (X = Cl, Br, I) in 1, 2, and 3, as well as energies for these contacts Eint (kcal/mol), defined by different approaches.  [43]. b Eint = 0.429G(r) for all types of noncovalent interactions (this correlation was initially developed for hydrogen bonding) [44]. c Eint = 0.49(−V(r)), or 0.58(−V(r)), or 0.68(−V(r)) for non-covalent interactions involving chlorine, bromine, and iodine atoms, respectively [45]. d Eint = 0.47G(r), or 0.57G(r), or 0.67G(r) for non-covalent interactions involving chlorine, bromine, and iodine atoms, respectively [45].

Conclusions
In this study, the isostructural complexes [Co(2-XPy)Cl2] represent a rare case from the point of view of crystal engineering, making direct comparisons of XB donor properties of 2-XPy (those are most prominent for 2-iodopyridine). In our opinion, these facts confirmed that halogen-substituted N-donor ligands are promising building blocks for the design of XB-based supramolecular systems (similar ideas were expressed earlier for 3-and 4-halogenated pyridines [16,19,22,26,27,46]), and this feature can be utilized in material science.

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