INTRODUCTION

A halogen bond (XB) is a special type of non-covalent interactions in which halogen atoms (usually iodine) play an unusual role of an electrophile [1-3]. This phenomenon as well as the other “atypical” bonding modes (for example, chalcogen bonding [4-10]) has been actively studied for at least the previous decade [11-17]. This is due to both purely fundamental interest in the development of supramolecular chemistry in general and possible applications of this effect in materials science.

One of general tasks in the research area has been repeatedly noted–the search for new building blocks capable of forming XBs. Perfluorinated iodo- and bromoarenes [18-23], di- and polyhalides [24-28], hypervalent iodine compounds [29-34], etc. can be noted among the most frequently occurring objects. However, although Schiff bases represent a vast class of compounds widely used in coordination chemistry as ligands [35-44] and provide a variety of synthetic routes to introduce halogen atoms available for the XB formation, they are rarely considered from this point of view.

It is well known that in [L2MIIX2]-type complexes, where X = halide, L = halogenated pyridine, the formation of strong XBs is the rule rather than the exception [45-51]. It is logical to assume that the complexes with Schiff bases containing a sterically available iodarene substituent can exhibit similar behavior. To test this hypothesis, we synthesized E-(((4-iodophenyl)imino)methyl)phenol (L) that was reacted with cobalt dichloride to give neutral complex [L2CoCl2] (1), and its structure was studied by single crystal X-ray diffraction (XRD). In the structure of 1 there are indeed I⋯Cl XBs the energies of which were estimated by quantum chemical calculations.

Table 1 Crystallographic Characteristics and Details of the Diffraction Experiment for the Crystal of 1
Full size table

EXPERIMENTAL

The chemically pure grade reagents (or their analogues) were purchased from commercial sources and used without further purification. L was obtained from salicylic aldehyde and 4-iodoaniline by the procedure [52].

Synthesis of 1. L (50 mg, 0.155 mmol) and CoCl2·6H2O (18 mg, 0.078 mmol) were dissolved in 7 mL of acetonitrile with stirring. Gradual evaporation of the solvent results in the formation of blue-green crystals of 1. Yield: 79%.

Single crystal XRD. The structure of complex 1 was determined following the standard procedure on an Agilent Xcalibur diffractometer with an area AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å, ω-scanning). Integration was performed, absorption corrections were applied, and unit cell parameters were determined using the CrysAlisPro software. The structure was solved using SHELXT [53] and refined by the full-matrix least squares method (LSM) in the anisotropic approximation for non-hydrogen atoms by the SHELXL 2017\1 algorithm [54] in the ShelXle program [55]. The details are given in Table 1. Atomic coordinates and other parameters of single crystal XRD experiments have been deposited with the Cambridge Structural Database under No. 2329207; deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/data_request/cif).

RESULTS AND DISCUSSION

Although L (Fig. 1) can act as a N,O-donor chelate ligand, in the case of 1 there is monodentate coordination through oxygen atoms of hydroxyl groups which remain protonated. Co(II) has a tetrahedral coordination environment (Fig. 2). The Co–O and Co–Cl distances are 1.942-1.945 Å and 2.236-2.287 Å respectively.

As already mentioned, a distinctive feature of the structure of 1 is a pronounced XB between chloride ligands and iodine atoms in L. The respective distances are 3.337 Å, which is significantly less than the sum of the corresponding Bondi Van der Waals radii (3.73 Å). Due to the interactions of this type, neutral [L2CoCl2] moieties are organized into infinite 1D supramolecular chains (Fig. 3).

In order to understand the nature and estimate the energy of non-covalent I⋯Cl interactions in the crystal of 1 (these short contacts can be classified as typical XBs [3]), we performed quantum chemical calculations within density functional theory (ωB97XD/DZP-DKH) [56, 57] using the Gaussian09 software and the topological analysis of the electron density distribution by the QTAIM method [58] using the Multiwfn program [59] (version 3.7). A supramolecular dimeric cluster (idealized isolated model system for which the quantum chemistry calculations were performed) was analyzed without the geometry optimization. The results are shown in Table 2. I⋯Cl XBs in the crystal of 1 were visualized within the formalism of the non-covalent interaction analysis in the model supramolecular associate (NCI analysis [60]) (Fig. 4). Electron densities, the Laplacian of the electron density, the total energy density, the potential energy density, and the Lagrangian of the kinetic energy in the (3, –1) bond critical point corresponding to non-covalent I⋯Cl interactions in the crystal of 1, are quite typical of such supramolecular contacts involving halogen atoms. The ratio of the potential energy density and the Lagrangian of the kinetic energy (3, –1) in the bond critical point corresponding to non-covalent I⋯Cl interactions in 1 indicates that the fraction of the covalent component is not significant in these supramolecular contacts.

Fig. 1
figure 1

L ligand structure.

Full size image
Fig. 2
figure 2

Structure of 1. Hereinafter, Co is black, C is gray, Cl is green, O is red, I is purple; H atoms are omitted.

Full size image
Fig. 3
figure 3

I⋯Cl XB in 1.

Full size image
Table 2 Electron Density ρ(r), Laplacian of the Electron Density ∇2ρ(r), Total Energy Density Hb, Potential Energy Density V(r), and Lagrangian of the Kinetic Energy G(r) (a.u.) in the (3, –1) Bond Critical Point Corresponding to Non-Covalent I⋯Cl Interactions in the Crystal of 1, the Lengths of these Contacts l (Å) and their Energies E (kcal/mol)
Full size table
Fig. 4
figure 4

Visualization of I⋯Cl XBs in the crystal of 1 within the formalism of the non-covalent interaction analysis in the model supramolecular associate (NCI analysis [60]).

Full size image