Halogen Bonding in New Dichloride-Cobalt(II) Complex with Iodo Substituted Chalcone Ligands

Abstract

The synthesis and properties of new chalcone ligand 4I-L ((2E)-1-[4-(1H-imidazol-1-yl)phenyl]-3-(4-iodophenyl)prop-2-en-1-one) and tetracoordinate Co(II) complex [Co(4I-L)₂Cl₂], (1a), are reported in this article. Upon recrystallization of 1a, the single crystals of [Co(4I-L)₄Cl₂]·2DMF·3Et₂O (1b) were obtained and crystal structure was determined using X-ray diffraction. The non-covalent interactions in 1b were thoroughly analyzed and special attention was dedicated to interactions formed by the peripheral iodine substituents. The density functional theory (DFT), atoms in molecule (AIM) and noncovalent interaction (NCI) methods and electronic localization function (ELF) calculations were used to investigate halogen bond formed between the iodine functional groups and co-crystallized molecules of diethyl ether.

1. Introduction

In molecular crystals, the building blocks, molecules, are held together by various directional and non-directional non-covalent intermolecular interactions. Some of these interactions have been widely explored, such as hydrogen bonding, cation/anion···π or π ···π interactions [1,2,3,4,5], but non-conventional interactions such as tetrel [6], pnictogen [7], chalcogen [8] and halogen bonding [9,10] have been less extensively studied [11,12]. All the above-mentioned interactions can be used to control the supramolecular architectures [13] of the coordination compounds and thus to influence their, e.g., mechanical [14], optical [15], catalytic [16], photochemical [17], luminescent [18] and magnetic properties [19]. In our recent research we devoted a lot of attention to prominence of non-covalent interactions in properties of magnetically bistable materials. We studied magnetic exchange interactions mediated by hydrogen bonding [20], π ···π interactions [21] or their combination [22]. We also discovered a new type of magnetic exchange pathway engaging Co···π interactions [23] and revealed possible influence of hydrogen bonding on occurrence of spin-crossover phenomenon [24]. In this report we focused our attention on interplay between halogen bonding and structure of mononuclear Co(II) coordination compounds, despite being aware of unpredictable competitiveness of hydrogen and halogen bonding [25]. These have a great potential to behave as molecular nanomagnets, so-called Single Ion Magnets (SIMs), due to large magnetic anisotropy exhibited by their central atoms in various symmetries of the ligand fields [26]. An inevitable condition for preparation of SIMs exhibiting high blocking temperatures in a zero-external magnetic field is the axial type of magnetic anisotropy possessed by their metal centers. This can be achieved by careful choice of coordination number and polyhedral symmetry. It is well established that, for Co(II) compounds, very interesting results can be achieved for tetracoordinate coordination compounds exhibiting large distortion from ideal tetrahedral geometry [27]. To achieve significant distortions of coordination geometry, one can assume two viable strategies: (a) use of polydentate and sufficiently rigid ligands, (b) use of monodentate bulky ligands. Both strategies exploit steric hindrance of the ligands, which in (a) may be further supported by predefined bite angles of the chelating ligands [28]. In this work we explored approach (b) by using bulky monodentate ligands derived from chalcones of 4′-(imidazol-1-yl)acetophenone containing peripheral groups allowing formation of non-covalent interactions such as hydrogen or halogen bonding [29,30]. Such interactions can either induce or stabilize distortions of molecular shapes [31] or coordination polyhedrons [32,33]. Here, we report on the first results obtained using the above-mentioned approach. We used ligand 4I-L ((2E)-1-[4-(1H-imidazol-1-yl)phenyl]-3-(4-iodophenyl)prop-2-en-1-one) prepared by the aldol condensation [34] in reaction with CoCl₂ to obtain tetracoordinate complex [Co(4I-L)₂Cl₂] in the powder form, (1a). Following recrystallization of 1a led to isolation of hexacoordinate complex [Co(4I-L)₄Cl₂]·2DMF·3Et₂O (1b), (Et₂O stands for diethylether), the crystal structure of which was determined using single-crystal X-ray diffraction. The crystal structure of 1b contains rather weak non-covalent interactions, which were analyzed by calculating Hirshfeld surfaces and special attention was given to interactions provided by the peripheral iodine substituents. The O···I halogen bond formed by co-crystallized molecule of Et₂O was thoroughly studied by modern theoretical calculations, because reports on O···I halogen bonding involving Et₂O [35] or similar alkoxy compounds [36,37,38,39,40,41] are rare. Similarly, the crystal structures with short O···Cl (e.g., [42,43]) and O···Br (e.g., [44,45]) non-covalent interactions formed between halogen atoms and Et₂O molecules can be found in the Cambridge Structural Database (CSD version 2.0.4, [46]), but to the best of our knowledge, reports applying advanced theoretical calculations for better understanding the nature of this certain type of halogen bond are missing.

2. Experimental

2.1. Materials

CoCl₂·6H₂O was purchased from Precious Metals Online, while other chemicals 4-iodobenzaldehyde, 4′-(imidazol-1-yl)acetophenone, NaOH, solvents (MeOH, diethyl ether (Et₂O), EtOH, dimethylformamide (DMF) and dichloromethane (DCM)) and deuterated solvents for NMR experiments (DMSO-d₆) were supplied by VWR International (Stříbrná Skalice, Czech Republic), Sigma-Aldrich (Prague, Czech Republic), Lach-Ner (Neratovice, Czech Republic) and Litolab (Chudobín, Czech Republic).

2.2. General Methods

Elemental analysis was performed by Flash 2000 CHNS Elemental Analyzer (Thermo Scientific, Waltham, MA, USA). Electrospray ionization mass spectrometry (ESI-MS; methanol solutions) was carried out with LCQ Fleet ion trap spectrometer (Thermo Scientific, Waltham, MA, USA; QualBrowser software, version 2.0.7) in both positive (ESI +) and negative (ESI-) ionization modes. ¹H and ¹³C NMR spectroscopy, and ¹H-¹³C gsHMQC and ¹H-¹³C gsHMBC two dimensional correlation experiments were performed using DMSO-d₆ (HA1) solution at 300 K using Varian spectrometer (Palo Alto, CA, USA) at 400.00 MHz (for ¹H NMR) and 101.00 MHz (for ¹³C NMR); gs = gradient selected, HMQC = heteronuclear multiple quantum coherence, HMBC = heteronuclear multiple bond coherence. ¹H and ¹³C NMR spectra were calibrated against the residual DMF ¹H NMR (8.03, 2.92 and 2.75 ppm) and ¹³C NMR (163.2, 34.9 and 29.8 ppm) signals. The splitting of proton resonances in the reported ¹H spectra is defined as s = singlet, d = doublet, dd = doublet of doublets, sep = septet, m = multiplet and bs = broad signal.

A Jasco FT/IR-4700 spectrometer (Jasco, Easton, MD, USA) was used for the collection of the infrared (IR) spectra of the studied ligand and complexes in the range of 400–4000 cm⁻¹ by using the attenuated total reflection (ATR) technique on a diamond plate. UV-VIS spectroscopy was performed using a Perkin-Elmer (Waltham, MA, USA) Lambda 35 spectrometer at 11,000–40,000 cm⁻¹.

2.3. Crystal Structure Determination

Data collection and cell refinement of 1b were made by Stoe StadiVari (Stoe & Cie GmbH, Darmstadt, Germany) diffractometer using Pilatus3R 300K detector and microfocused X-ray source Incoatec IμS 2.0 HB (Ag Kα radiation). The structure was solved using SHELXT [47] program and refined by the full matrix least-squares procedure with Olex2.refine [48] in OLEX2 (version 1.3) [49]. The multi-scan absorption corrections were applied using the program Stoe LANA software [50]. The molecular structures and packing diagram were drawn with MERCURY [51]. Crystal Data for C₉₀H₉₆Cl₂CoI₄N₁₀O₉ (M = 2099.19 g/mol): triclinic, space group P$\overline{1}$ (no. 2), a = 8.5546(2) Å, b = 15.2289(3) Å, c = 18.6054(4) Å, α = 110.729(2)°, β = 90.916(2)°, γ = 102.814(2)°, V = 2198.59(9) ų, Z = 1, T = 100(1) K, μ(AgKα) = 0.915 mm⁻¹, D_calc = 1.585 g/cm³, 61909 reflections measured (2.110° ≤ 2Θ ≤ 19.501°), 7765 unique (R_int = 0.0160) which were used in all calculations. The final R1 was 0.0405 (I > 2σ(I)) and wR₂ was 0.1180 (all data). The highest peak: +1.03 (0.84 Å from I1), the deepest hole −0.98 (0.90 Å from I1). Crystal structure refinement: All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed into the calculated positions and they were included into the riding-model approximation with U_iso = 1.2U_eq(C) or 1.5 U_eq (CH₃) and d(C−H) = 0.95–0.98 Å. Non-routine aspects of the refinement: one of the Et₂O molecules is disordered over two positions with the oxygen atom lying on an inversion center.

2.4. Theoretical Calculations

The ORCA 4.2.1 computational package was employed for the presented calculations [52]. All the computations were based on the molecular fragments derived from the X-ray data, where only the positions of the hydrogen atoms were optimized using Density Functional Theory (DFT) with ωB97X-D3BJ functional [53] comprising the atom-pairwise dispersion correction (D3BJ) [54,55] with Sapporo-TZP-2012 basis set for all atoms [56]. The calculations exploited the resolution of identity approximation with the auxiliary basis created by the AutoAux generation procedure [57] and the chain-of-spheres approximation to exact exchange (RIJCOSX) [58,59]. The integration grids were increased by setting Grid5 and Gridx5, and the convergence criteria were set to tight SCF (self-consistent field) in all calculations. Then, the interaction energies (E_int) were calculated also taking into the account the basis set superposition error (BSSE). The further analysis was done with Multiwfn program [60,61]. The calculation of Hirshfeld [62] and shape index [63] surfaces were performed using the program Crystal Explorer 3.1 [64].

2.5. Synthesis of (2E)-1-[4-(1H-imidazol-1-yl)phenyl]-3-(4-iodophenyl)prop-2-en-1-one (4I-L)

A methanolic sodium hydroxide solution (40%; 1.2 mL) was added dropwise to a mixture of 4-iodobenzaldehyde (2 mmol, 0.464 g), 4′-(imidazol-1-yl)acetophenone (2 mmol, 0.370 g) and methanol (5 mL) over a period of 30–40 minutes with continuous stirring at room temperature until completion of the reaction (as indicated by TLC). The precipitates were filtered and washed with cold methanol–water mixture (1:10). Finally, the product was recrystallized from methanol [33].

Yellowish solid. Yield: 90%. ¹H NMR (400 MHz, DMSO-d₆, 298 K, ppm) δ 8.42 (s, 1H, C2-H), 8.26 (d, J = 8.7 Hz, 2H, C17-H, C21-H), 7.99 (d, J = 15.7 Hz, 1H, C14-H), 7.88–7.80 (m, 5H, C5-H, C7-H, C11-H, C18-H, C20-H), 7.72–7.64 (m, 3H, C8-H, C10-H, C15-H), 7.12 (s, 1H, C4-H). ¹³C NMR (101 MHz, DMSO-d₆, 298 K, ppm) δ 188.24 (C12), 143.51 (C15-H), 140.77 (C6), 138.20 (C18-H, C20-H), 136.22 (C2-H), 135.88 (C9), 134.69 (C16), 131.21 (C8-H, C10-H), 130.98 (C17-H, C21-H), 130.85 (C4-H), 123.05 (C14-H), 120.30 (C7-H, C11-H), 118.24 (C5-H), 98.14 (C19-H). ESI+MS (MeOH, m/z,): 401.17 (calc. 401.22; 100%; {4I-L+H}⁺), 423.05 (calc. 401.20; 32%; {4I-L+Na}⁺). IR (ATR, ν, cm⁻¹): 408w, 463w, 507w, 538w, 594w, 652w, 677w, 727w, 812s, 830m, 960w, 981w, 1031w, 1060w, 1118w, 1165m, 1215m, 1289m, 1337w, 1428w, 1492w, 1512m, 1588s, 1652s, 2150w, 3118w.

2.6. Complex [Co(4I-L)₂(Cl)₂] (1a)

The solution of CoCl₂·6H₂O (0.5 mmol, 0.119 g) in 5 mL of methanol was heated up to 50 °C and then 2 molar equiv. of 4I-L were added (2 mmol, 0.400 g). The solution was allowed to cool down and then the reaction mixture was stirred at ambient temperature for 2 hours. The obtained blue precipitate was collected by filtration and washed with water (2 × 0.5 mL) and Et₂O (2 × 1 mL). The blue solid product was dried in desiccator under reduced pressure (overnight). Compound 1b was prepared by recrystallization of 1a from DMF/CH₃OH solvent mixture by slow diffusion of Et₂O in closed flask. Only small amounts of light blue crystals of 1b was isolated and, therefore, no technique other than single-crystal diffraction was used for characterization of this compound.

Yield: 84%. Anal. Calcd. for CoC₃₆H₂₆Cl₂N₄I₂O₂ (1a): C, 46.48; H, 2.82; N, 6.02%; found: C, 46.41; H, 2.72; N, 5.86%. ESI+MS (MeOH, m/z,): 401.14 (calc. 401,22; 30%; {4I-L+H}⁺), 567.09 (calc. 567.06; 100%; {[Co(4I-L)(Cl)₂]+2H₂O+H}⁺), 894.02 (calc. 893.91; 66%; {[Co(4I-L)(Cl)₂]+2H₂O+H}⁺). IR (ATR, v, cm⁻¹): 402w, 448w, 490w, 522w, 605w, 647w, 735w, 811s, 962w, 1002w, 1062w, 1119w, 1182w, 1213w, 1318m, 1395w, 1479w, 1519m, 1602s, 1657s, 2159w, 3131w.

3. Results and Discussion

3.1. Synthesis and Basic Characterizations

The chalcone ligand 4I-L was prepared by aldol condensation of 4′-(imidazol-1-yl)acetophenone with the 4-iodobenzaldehyd as is shown in Scheme 1. The purity and structure of 4I-L were confirmed by NMR spectroscopy and elemental analysis. The ¹H NMR (see ESI, Figure S1), ¹³C NMR (see ESI, Figure S2), spectra of 4I-L in DMSO-d₆ showed shifts characteristic for the aromatic functional groups (the list of all observed peaks can be found in the experimental chapter). The representative peak confirming the presence of the olefinic moiety resulting from aldol condensation (C14-H) was observed in ¹H NMR spectra approximately at 7.99 ppm. In the ¹³C NMR spectra corresponding peak of C14-H was observed approximately at 123.05 ppm. The C-H groups were assigned by ¹H-¹³C HMQC correlation experiments (Figure 1 left), while quaternary carbon atoms were assigned by ¹H-¹³C HMBC correlation experiments.

The complex 1a was prepared by reaction between CoCl₂·6H₂O and 4I-L (molar ratio 1:2) in methanolic solution as a blue precipitate. The elemental composition (CHN analysis) corresponds rather well with formula [Co(4I-L)₂Cl₂], and this assumption was further supported by the results of ESI+ mass spectroscopy, which revealed occurrence of peaks with suitable m/z isotopic distribution attributable to the {[Co(4I-L)₂(Cl)}⁺ species (see ESI, Figure S3). The FTIR spectra showed valence vibration of the chalcone ketone group (ν(C=O)) at ca. 1600 cm⁻¹. The UV-VIS spectroscopy of 1a measured in the solid state showed absorption bands centered at ca. 600 nm (Figure 1 right), which can be attributed to spin allowed d-d transitions characteristic for the tetracoordinate Co(II) complexes [65]. The spectral band was fitted to three Gaussian primitives having peak maxima at 518 nm, 578 nm and 631 nm (Figure 1 right). The individual bands originate from ⁴A₂ → ⁴T₁(P) transition in ideal T_d symmetry [66], although the ⁴T₁(P) term splits into three individual terms (⁴A₂(P), ⁴B₁(P) and ⁴B₂(P)) in lower symmetry (expected symmetry of coordination polyhedron of 1a is C_2v). Thus, it may be concluded that compound 1a is tetracoordinate Co(II) complex with the formula [Co(4I-L)₂Cl₂].

Complex 1a is only negligibly soluble in water, methanol, ethanol or acetone, but it is well soluble in DMF or DMSO. The UV-VIS spectrum measured for CH₃OH solution of 1a revealed no strong absorption bands in the range of 400–800 nm (Figure 1 right). Interestingly, upon recrystallization of 1a from CH₃OH/DMF solvent mixture, followed by slow diffusion of Et₂O, the small amount of light blue crystals precipitated. The selected single crystal was subjected to the X-ray diffraction analysis, which revealed that the prepared complex 1b is hexacoordinate (vide infra).

3.2. Crystal Structure

Compound 1b crystallizes in the triclinic crystal system with the space group P$\overline{1}$. The Co atom lies on a crystallographic inversion center (1h) and thus the asymmetric unit consists of the half of the complex molecule [Co(4I-L)₄Cl₂], one DMF molecule and one and half Et₂O molecules, which altogether gives formula of [Co(4I-L)₄Cl₂]·2DMF·3Et₂O. The complex molecule is hexacoordinate and it consists of four 4I-L and two chlorido ligands coordinated to Co(II) center. The 4I-L ligands adopt E-configuration and coordinate the Co(II) atom by nitrogen donor atoms from the imidazolyl moieties forming equatorial plane with rather dissimilar Co−N bond lengths (d(Co−N) = 2.121(3) and 2.162(3) Å). The axial coordination sites are occupied by two chlorido ligands with rather long Co−Cl bonds (d(Co−Cl) = 2.4880(9) Å). The overall geometry of the coordination polyhedron can be described best as an axially elongated octahedron (Figure 2).

The complex molecules are stabilized in the structure by plethora of weak intermolecular contacts and therefore analysis of the Hirschfeld surface was performed to identify them (Figures S4 and S5, the parameters of hydrogen bonds are listed in Table S1). The complex molecules are organized in supramolecular chains by bifurcated C−H···Cl hydrogen bonding between the aromatic C−H groups located on 1-phenyl-imidazolyl fragment of the 4I-L ligand from adjacent complex molecule (d(C···Cl) = 3.466(5) and 3.673(5), Å Figure 3a). The chlorido ligand also acts as an acceptor of another C−H···Cl hydrogen bond involving C−H group from the DMF molecule (d(C···Cl) = 3.807(4) Å). Interestingly, the DMF molecule is further stabilized in the crystal structure by set of three rather strong C−H···O hydrogen bonds between three C−H groups from the 4I-L ligand and oxygen atom of the DMF molecule (d(C···O) = 3.324(6), 3.388(5) and 3.394(5) Å, Figure 3b).

One of the chalcone ligands forms offset π-π stacking interactions with the adjacent counterpart related by operation of inversion. The shortest C···C distances are rather short (d(C···C) = 3.290(5) and 3.357(5) Å, Figure 4a). These interactions can be visualized best by calculating Hirshfeld surface followed by highlighting areas corresponding to C···C interactions (Figure 4c). Furthermore, by inspecting the shape-index surfaces we were able to confirm these interactions because of appearance of neighboring red and blue triangular map areas (red circles), which are characteristic for occurrence of π-π stacking [67,68].Both types of the supramolecular dimers, (a) and (b), are extended to chain substructures due to the inversion symmetry. The keto oxygen atoms are involved in the weak C−H···O hydrogen bonds, where one of the keto groups forms a pair of bifurcated hydrogen bonds with the C−H groups of the benzyl-imidazolyl moiety (d(C···O) = 3.193(5) and 3.420(4) Å, Figure 4a). The second keto group forms only weak C−H···O non-covalent contacts.

Two symmetry-independent iodine substituents on the 4I-L ligands form different types of the non-covalent interactions. One of them forms I···O type of the interaction, since it is capped by co-crystallized Et₂O molecule at rather short distance: d(I···O) = 3.264(4) Å (Figure 3c). The other one forms two weak contacts with two neighboring functional groups on the 4I-L ligand: one with the above-mentioned iodine substituent (d(I···I) = 4.1676(5) Å) and the second one with the neighboring C-H group (d(C···I) = 3.918(5) Å, Figure 3d).

3.3. Theoretical Studies

The DFT theory was employed together with ORCA program to evaluate the interaction energies for selected non-covalent contacts affecting the crystal packing with range-separated functional ωB97X-D3BJ. First, the I···O type of the interaction between 4I-L ligand and co-crystallized Et₂O molecule was dealt with (Figure 3c). Thus, for the molecular fragment {(4I-L)···(Et₂O)} shown in Figure 5, the computed interaction energy is E_int = −4.401 kcal/mol. Next, the I···I type of the interaction (Figure 3d) between two 4I-L ligands {(4I-L)···(4I-L)} from different complex moieties was inspected and the calculation resulted in E_int = −2.212 kcal/mol (Figure 5). Moreover, these interactions were analyzed with the noncovalent interaction (NCI) method [69] with the help of Multiwfn software. This method is based on the analysis of the electron density (ρ) by defining the reduced density gradient (RDG) function which helps to identify the weak intra/inter molecular interactions and the nature of these interactions, thus attractive vs repulsive, is analyzed by the sign of eigenvalue λ₂ of the electron density Hessian matrix. These interactions were successfully visualized with VMD program [70] as sign (λ₂)ρ values in Figure 5. It is evident that in {(4I-L)···(4I-L)} molecular fragment (Figure 5 top) two types of weak interactions are present, the hydrogen bond C−H···I and halogen bond I···I, whereas the second fragment {(4I-L)···(Et₂O)}, (Figure 5 bottom), is dominated by O···I halogen bond. Indeed, the topological analysis based on the atoms in molecules (AIM) method [71] implemented in Multiwfn program identified the bond critical points (BCP) of the type (3,−1) as depicted in Figure 6 together with the bond paths, which follow the maximal gradient path connecting two BCPs. Next, the NCI sign(λ₂)ρ values were calculated in selected BCPs as −0.00529 for I···I, −0.0103 for C-H···I and −0.0102 for O···I contacts.

Thus, we can conclude that all these interactions are attractive and sort them by the increasing strength as I···I < C-H···I ≈ O···I, which agrees with the DFT calculated energies. It well known that halogen bonds are characteristic by formation of so-called σ-holes on heavy halogen atoms, which can interact with an electron pair of the electron donor atom. To better understand and visualize this phenomenon, Multiwfn package was used to calculate Electron Localization Function (ELF). The ELF was introduced by Becke and Edgecombe [72] and its application to the study of electronic structure is discussed by Lu and Chen [73]. Thus, the colored mapped figures were calculated as shown in Figure 7 to elucidate the distribution of electron density by ELF. Indeed, in both cases of I···I (Figure 7 left) and O···I (Figure 7 right) halogen bonds, the presence of σ-holes on I atoms is evident and the complementary donor electron pair of I or O atoms is present too. The detailed view of ELF along the particular AIM calculated bond paths (Figure 7) is also depicted in Figure S6.

4. Conclusions

In this article, we reported on the synthesis of the chalcone ligand (2E)-1-[4-(1H-imidazol-1-yl)phenyl]-3-(4-iodophenyl)prop-2-en-1-one (4I-L), which was further used for synthesis of tetracoordinate complex [Co(4I-L)Cl₂], (1a), which after recrystallization transformed into hexacoordinate complex [Co(4I-L)₄Cl₂]·2DMF·3Et₂O, (1b). Ligand and complex 1a were thoroughly characterized by spectroscopic techniques and for 1b, the crystal structure was determined by X-ray diffraction analysis. It was revealed that 1b is hexacoordinate Co(II) complex with chlorido ligands in the axial positions. The crystal structure of 1b is stabilized by a plethora of weak non-covalent interactions, mainly by C−H···O, C−H···Cl and π−π stacking of aromatic rings. Furthermore, the peripheral iodine substituents on the 4I-L ligands form I···O, C−H···I and I···I non-covalent interactions. These were theoretically studied by the DFT, NCI, AIM and ELF calculations and all confirmed formation of halogen bond of significant strength in the case of the I···O contact. The weak nature of the C−H···I hydrogen bonding and I···I interaction was confirmed.