Self-assembly of a new cobalt complex, (C₆H₁₄N₂)₃[CoCl₄]Cl: Synthesis, empirical and DFT calculations

2.1 Materials and chemical synthesis

The monocrystal of the title complex (C₆H₁₄N)₃[CoCl₄]Cl was synthesized by adding a mixture of CoCl₂·6H₂O (0.24 g) and a few drops of HCl (37%) to the ethanol solution of cyclohexylamine (0.34 mL) (pH < 2.5). The mixture is left under magnetic stirring of 1 h hour and a half to stand in the open air. Few weeks later with slow evaporation in ambient temperature, green blue prismatic crystal was appeared and carefully selected with sufficient size of the crystals. All materials used products and solvents were commercially obtained from Sigma Aldrich Company.

2.2 XRD data collection and physical measurements

A monocrystal of sizes (0.48 × 0.32 × 0.18 mm) was selected for the crystallography analysis to the X-ray data by using Bruker-AXS APEXII diffractometer at 150 K employing monochromated MoKα radiation (λ = 0.71073 Å). Absorption improvements were effectuated using multi-scan approach with the SADABS strategy (Bruker, 2006). Amid 24,384 measured reflections we find that 6103 are independent and 4718 with intensity I > 2σ (I) on the range (2.3°-27.5°). The structure was developed in P2₁/n space group and resolved by the direct method using the SHELXT-97 program package (Sheldrick, 2015a) and refined by SHELXL (Sheldrick, 2015b) through the WINGX program (Farrugia, 2012). The DIAMOND program (Brandenburg, 1998) allows the plot of the molecular structure. Finishing enhancement on F² merged at R(F²) = 0.032 and wR(F²) = 0.071. The features used for the XRD data assortment evenly the plan for the crystal structure investigation and its results are included in Table S1. The asymmetric unit with an ORTEP drawing of (C₆H₁₄N)₃[CoCl₄]Cl is given in Fig. 1a.

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Fig. 1

Ortep structure (a) with atom numbering and optimized molecular geometrical structure (b) of (C₆H₁₄N)₃[CoCl₄]Cl.

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The infrared spectrum was reported at RT among 4000–400 cm⁻¹ with a Perkin-Elmer FT-IR 1000 spectrometer scattered with dried KBr and pressed to pellets. TG /DTA analysis was investigated by a multimodule 92 Setaram analyzer using in the range of RT to 880 K at a heating speed of 5 K min⁻¹ and a simple mass of 10.9 mg.

2.3 Computational investigations

The Gaussian 09 program (Frisch et al., 2009) was used to achieve the optimization geometry and all the theoretical calculations by the DFT/B3LYP (Becke, 1993), the 6-311++G(d,p) basis set was adopted for all atoms except the metal, the LANL2DZ basis set and effective core potentials (ECPs) have been used in order to represent the Co atom. The vibrational assignments were carried out using the VEDA4 program (Jamróz, 2004). To understanding the intermolecular interactions we use the Crystal Explorer 3.1 package (Wolff et al., 2013) imported on a CIF file to generate 3D-HS and their related 2D-FP. To establish the topological analysis of non-covalent interaction we can utilize Multiwfn package (Lu and Chen, 2012), using Bader's theory (Kumar et al., 2016), while the RDG of the compound are mapped by Multiwfn and VMD programs (Humphrey et al., 1996). HOMO-LUMO distributions and ESP and ELF analysis were performed by the optimized molecular structure.

3.1 XRD and molecular modeling

To study the geometrical and optimized structure, the title compound was investigated by XRD analysis. To establish the structural model of (C₆H₁₄N₂)₃[CoCl₄]Cl, an ORTEP drawing (Fig. 1a) was designed by the asymmetric unit whose contain three cyclohexylammonium organic cations (C₆H₁₁N₃⁺), an anionic entity which is [CoCl₄]²⁻ and an isolated Cl⁻ anion. The existence of free inorganic anions as a part of an anionic sublattice in hybrid compounds was also found in other structure (Ferchichi et al., 2010). The optimized geometric model of the structure is shown in Fig. 1b and Table 1 exhibits a comparison between experimental and calculated X-ray crystallographic parameters. The interatomic distances and the bonding angles of organic entity are in a good agreement with those crystallizing with other types of anion found in previous literature (Jomaa et al, 2021; Daghar et al. 2021).

The cyclohexylammonium cations adopt chair organization with an ammonium group in the equatorial position, with C–C average bond lengths equal to 1.520 Å and d_N-C varying from 1.492 to 1.502 Å. The average differences between the calculated geometric parameters and those of the experimental parameters were found to be approximately 0.02 Å for C–N bond lengths while for C–C bonds is approximately 0.025 Å. Besides, the bond angles (C–C–C) were observed by XRD between 110.16 and 113.50° and calculated between 110.39 and 112.17° and (N–C–C) observed by in the range of 109.58° and calculated at an average of 105.4°. The inorganic anion [CoCl₄]²⁻ adopts a tetrahedral molecular geometry, a cobalt atom Co²⁺ in the center coordinated to four chlorine atoms at the corners of a tetrahedron with an sp³ hybridization, with experimental and computed Co-Cl bond lengths ranging from 2.2605(5)–2.3081(6) Å and between 2.316 and 2.408 Å, respectively. The distinguished bond angles of Cl–Co–Cl varying from 103.92(2)° to 118.32(2)° for the experimental and from 103.52° to 129.26° for the theoretical calculations. The Cl…Cl distance among the two nearest neighbors [CoCl₄]²⁻ tetrahedra is 4.128 Å. Furthermore, the halide-halide contacts are reputed to generate weak anti-ferromagnetic interactions that decrease rapidly with increasing Cl…Cl separation (Zhou et al., 1991). It’s worth noting that these values (Table 1) are also found in other organic terachlorocobltate (Tahenti et al. 2020). This slight difference between observed and calculated parameters is due to the interatomic interactions in the crystalline state. Moreover, the τ₄ defined by Alvarez (Alvarez and Llunell, 2000) was an index parameter used for the four-coordinate transition metal complexes to explain the geometry of materials and defined by the sum of α and β angles the two biggest angles in the four-coordinate species, specified by this relation: $$\mathrm{\tau }=\frac{{360}^{°}-(\alpha +\beta )}{{141}^{°}}$$

Exercising this equation on (C₆H₁₄N₂)₃[CoCl₄]Cl reveal an τ₄ equal to 0.935 which is close to 1, so we can highlight a slightly distorted tetrahedral geometry.

As we can see in Fig. 3a, the projection of the studied compound by the side of the $\overrightarrow{b}$ axis shows the atomic organization of all the species involved in the crystal. It exhibits the arrangement of the tetrachlorocobaltate and chloride anions in ( $\stackrel{-}{1}$ 01) planes which intercalated the cyclohexylammonium groups presented in the chair conformation. Indeed, the projection of a single layer along the $\stackrel{\rightharpoonup }{a}$ axis (Fig. 2b), demonstrates that the tetrachlorocobaltate anions are located in planes y = ¼ and ¾. The packing for (C₆H₁₄N₂)₃[CoCl₄]Cl and the junction of cationic to chlorine groups of anionic entities are ensured by electrostatic interactions as well as by hydrogen bonding which summarized in Table 2 through N(C)-H…Cl with donor–acceptor length ranging from 3.198(3) Å to 3.488(6) Å for d_N…Cl, forming $R_1^2$ (4) and $R_6^3$ (12) motifs (Fig. 2c) and d_C…Cl present an average of 3.683 Å gives birth to graphs 6 $R_2^1$ graph (Fig. 2d).

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Fig. 2

The space filler of (C₆H₁₄N)₃[CoCl₄]Cl viewed along the b-axis.

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Fig. 3

The asymmetric unit (a), the Hirshfeld molecular surface mapped in d_norm (-0.480–1.090) (b) of (C₆H₁₄N)₃[CoCl₄]Cl.

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3.2 3D-HS analysis

The analysis of molecular crystal structures based on 3D-HS was used to study and quantify the sizes and shapes of compounds (Arulraj et al., 2020a,b, 2022), also to define the space occupied by a molecule and to explore the intermolecular contact. Fig. 3 shows the asymmetric unit (a), the Hirshfeld molecular surface mapped in d_norm (−0.480–1.090) (b) of (C₆H₁₄N)₃[CoCl₄]Cl. The different results concerning hydrogen bonds discussed in the structural part (X-ray diffraction study) are summarized in an efficient way on d_norm mapping. It is observed on this map several dark red circular spots due to the contribution of H…Cl/Cl…H contacts (39%) on the surface plots of Hirshfeld, these spots corresponding to the H-bonds N(C)-H…Cl. Other spots (blue and white) appear on this map and they relate to H…H contacts. The d_norm cartography represented in Fig. 3b gives a general outline on some hydrogen bonds which participate in the stability of our crystalline structure.

Fig. S1 illustrates the 2D-FP plots of the (C₆H₁₄N)₃[CoCl₄]Cl compound. This distribution shows that the intermolecular contacts H…H occupy (59.7%) of the entire HS and these later are shown on the fingerprint plot (d_e = d_i = 1.1 Å), this value is greater than the vdW radius of the hydrogen atom (1.09 Å). The H…Cl/Cl…H contacts represent almost $1/3$ of the entire Hirshfeld surface. Indeed, the organic part has a great number of H atoms on their molecular surface (S_H = 79.45%), while the carbon atoms are hardly ever present on the surface of the molecule (S_C = 0%) as they usually form four bonds with other atoms (Fig. S2). The rest of the H…Co/Co…H and Cl…Cl contacts occupy 0.5% and 0.3% of the Hirshfeld surface, respectively. The identified intermolecular interactions were evaluated by analysis of enrichment ratios. The ratios are given in Table S2. The list of enrichment ratios highlights H…Cl/Cl…H contacts (ER_HCl = 1.24) which appear to be favored in crystal packaging with the formation of hydrogen bonds of type N–H…Cl and C–H…Cl. H… H contacts appear with an ER ratio nearly to unity (ER_HH = 0.95).

3.3 Vibrational spectroscopic study

The infrared absorption spectroscopy of our study compound has the role of determining the vibrational characteristics of atomic groups, as well the enumeration of the modes of vibration and the attribution of the bungs to the different types of vibratory movements in the anionic groups [CoCl₄]²⁻. In addition, in our study, we performed a vibrational analysis of (C₆H₁₄N)₃[CoCl₄]Cl using the experimental spectrum which was measured in the range of frequency 400–4000 cm⁻¹ and theoretical one was performed using DFT/B3LYP/6–311 + G(d,p) basis set are shown in Fig. S3. The experimental IR wavenumber and the theoretical one from DFT conception were assigned by VEDA program (Jamróz, 2004) and GaussView package (Dennington et al., 2009). The different vibration bands existing in produced compound based on bibliographic data relating to the different types of bonds in the same organic molecule (Jomaa et al., 2021) as well as the tetrachlorocobaltate anion have been assigned to all bands appeared in the infrared spectrum. Theoretical calculation presents more reasonable results from the point of view of number and positions of the bands because of the absence of imaginary frequencies. The assignments of the main groups are discussed below.

3.4 Thermal analysis

The differential thermal and thermogravimetric analysis of (C₆H₁₄N)₃[CoCl₄]Cl is carried out under argon in the temperature range [307–580 K]. This analysis is carried out on 10.9 mg of the product placed in a platinum crucible with a heating rate of 5°.min⁻¹. DTA and TG diagrams are given in Fig. S4.

The DTA thermogram reveals that (C₆H₁₄N)₃[CoCl₄]Cl undergoes a succession of endothermic peaks detected on the DTA diagram escorted by a loss of mass on the TG thermogram which can be endorsed to the pyrolysis of the cyclohexylamine entity. Then, an endothermic peak lacking loss of mass observed at 439 K which may match to a fusion. This observable fact is additional proved by heating (C₆H₁₄N)₃[CoCl₄]Cl on a Kofler bench which makes it possible to approximate the material melting temperature, this 434 K temperature is nearly that inspected on the DTA bend (Gatfaoui S. et al., 2022). Lastly, an exothermic peak with loss of mass which appears at 498 K, it can match to the total decomposition of the organic molecule. Afterwards the residual product undertakes entire decomposition which guides to vaporous gas and black remains.

3.5 AIM topological analysis

Various theoretical investigations, particularly AIM (Atoms In Molecules) approach allows to distinguish the type and the stability of inter and intra-atomic interaction bonds, precisely the hydrogen bonds which is verified by the presence of the critical point (BCP), proposed by Bader's theory (Bader and Austen, 1997). Among the topological parameters we can found: the electron density ρ(r), the Laplacian ${\mathrm{\nabla }}^2\rho \left(r\right)$ , the eigenvalues (λ₁, λ₂, λ₃), the λ1/λ3 ratio, the kinetic energy densities G(r), the total energy densities H(r), the potential V(r) and the bond energy E offered several information on the properties of RCPs and BCPs of the title compound. Therefore, Multiwfn program (Lu and Chen, 2012) with LANL2DZ level is used to represent the AIM graphical analysis which is shown in Fig. 4 while the computed topological parameters of (C₆H₁₄N)₃[CoCl₄]Cl are tabulated in Table 3.

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Fig. 4

AIM molecular diagram viewing the different BCPs of (C₆H₁₄N)₃[CoCl₄]Cl.

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As stated by Rozas et al. (Rozas et al., 2000), we can classify H-bonds to 3 classes: we have strong H-bonds when ${\mathrm{\nabla }}^2\rho \left(r\right)$  < 0 and the value of H(r) < 0, moderate H-bonds when ${\mathrm{\nabla }}^2\rho \left(r\right)$  > 0 and H(r) < 0 and weak H-bonds when ${\mathrm{\nabla }}^2\rho \left(r\right)$  > 0 and H(r) > 0.

Concerning Table 3 analysis, it can be observed that AIM analysis present 10H-bonds that meet up the Koch and Popelier criteria (Popelier et al., 2000) and that Cl₆₆…H₂₄, Cl₆₇…H₂, Cl₆₈…H₄ and Cl₆₉…H₂₃ interactions present symmetrical topological properties to Cl₆₆…H_61, Cl₆₇…H₂₅, Cl₆₈…H₆₂ and Cl₆₉…H₂₂ interactions. Also, the G/ρ ratio is used to distinguish the type of interaction which must be >1 to affirm the existence of H-bonds interactions. Positive Laplacian values are observed in the range of 0.01657 a.u and 0.08747 a.u at the CP of binding, is indicate that the distance between the interacting atoms is bigger than the sum of the vdW rays of these atoms, so is indicative of the existence of hydrogen weak bonds. This basis reveals that the title compound is stabilized by four Cl…H hydrogen bonds which are considered to be weak since the Laplacien and H(r) values are positive with bond energies in a range of −8.230 kcal mol⁻¹ to 73.681 kcal mol⁻¹. While the moderate are located at the rest of H-bonds were confirmed by the negative value of the total energy densities H(r) with energy varying between 75.276 kcal mol⁻¹ and 115.12 kcal mol⁻¹. These studies reveal clearly, that hydrogen bonds observed by XRD and HS analysis were investigated and confirmed by AIM approach.

3.6 RDG analysis

The non-covalent interaction (NCI) or RDG is a topological technique improved by Johnson et al. (2010) and Contreras-García et al. (2011) based on electronic density and used to examine the various types of interactions present in the title compound especially weak interactions like H-bond, vdW interaction and steric effect. This approach have been performed and mapped by VMD and Multiwfn softwares (Humphrey et al., 1996; Lu and Chen, 2012). From Fig. 5 we can distinguish the variation of RDG as a function of (ρ*sign λ₂). Generally, λ₂ is an essential parameter of the determination of the nature of interactions, it plays a very important role as indicator of stabilization or of destabilization of inter and intra interactions. As clearly seen in Fig. 6 significant by the evolution between RDG (a.u) versus sign (λ₂)ρ, which we employed a color code in order to indicate intensity and type bonds, sign (λ₂)ρ > 0 for repulsive interaction and characterized by red color spots, sign (λ₂)ρ < 0 for attractive interaction and have a blue color and sign (λ₂)ρ nearly zero for vdW interaction (weak interaction) described by green color. As we can see, the RDG peaks around −0.05, −0.02 u.a with sign (λ2)ρ correspond to a strong attractive NCI also by the low-density values (Akman F. et al., 2020). We can notice the light blue spots are located between the hydrogen and chlorine atoms, just to explain the strong attractive non covalent interaction type N–H…Cl. Also, green patches around the anion entity are accredited to vdW interactions. While the strong steric cyclic effect attached to repulsive interactions is associated by the elliptical red plaque situated at the center of the cycle.

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Fig. 5

Visualization of different interactions in (C₆H₁₄N)₃[CoCl₄]Cl using Multiwfn and VMD softwares.

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Fig. 6

Electrostatique Surface Potentiel (ESP) of (C₆H₁₄N)₃[CoCl₄]Cl.

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3.7 Electrostatic surface potential (ESP)

ESP is an effective property for understanding reactivity in a molecule (A. Sagaama and N. Issaoui, 2020). This method allows the investigation of molecular interactions; it has been approved by the calculation of electrostatic properties derived from the charge density of a molecule (D. Romani et al., 2020). Fig. 7 indicates the electrostatic potential of our structure, the color of which depends on the value of ESP. The red color is specific for the negative electrostatic potential (chloride and the tetrachlorocobaltate anion), the yellow color slightly rich in electrons (nitrogen atoms) while the green color indicates where the potentials are close to zero and the blue color for the positive electrostatic potential.

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Fig. 7

Shaded surface plot with projection effect of ELF in XY plane of (C₆H₁₄N)₃[CoCl₄]Cl compound.

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It can be seen at the end that there is an overall electrostatic attraction between the cations and the anions.

3.8 ELF analysis

ELF is a new topological descriptor allows us to predict and understand the description of chemical structure (Sagaama A. et al., 2021). It makes possible to identify the nature of the weak interactions and the reactivity of the compound. Fig. 7 shows the ELF maps of (C₆H₁₄N)₃[CoCl₄]Cl, which characterized by two regions: regions of maximum Pauli repulsion (well-localized electrons), when ELF values close to 1 while the areas of minimum Pauli repulsion present an ELF values near 0 by using Multiwfn program (Lu and Chen, 2012). In addition, the blue color zone and especially dark blue areas located in the range (0–0.4) proved a reduction in ELF in the corresponding regions whereas the average ELF values appeared in the region of (0.5–0.8) are justified by color series goes from yellow to green. The higher first of the scale identified by the red color revealed a decrease in the ELF. The main conclusion that can be drawn from this part, ELF is related to the consequence of Pauli’s repulsive effect (A. Sagaama and N. Issaoui, 2020). We can see that the strongest regions with maximum Pauli repulsion are represented by red area surrounded by single electron hydrogen atoms, while the regions characterized by the blue color are occupied by the chlorine atom of the tetrachlorocobaltate group and the carbon atoms of the organic entity allow us to understand that it belongs to the lowest regions with minimal Pauli repulsion.

3.9 Frontier molecular orbital analysis

The theoretical calculation was carried out using data from the CIF file studied using the DFT method. In the subject to study chemical reactivity and the stability and of the title compound, frontier molecular orbital (FMO) analyzes were investigated (Ghalla H. et al., 2014). The FMO orbitals of our studied compound are shown in Fig. S5. Examination of this figure shows clearly that HOMOs are located on the inorganic part of the crystal, while LUMOs are located on the organic part. The energy difference ΔE between two border orbitals is one of the first parameters to confirm the stability of our crystalline structure. The values of the HOMO and LUMO energies of our compound are −6.02 eV and −3.45 eV respectively and other electronic properties electro negativity (χ), softness (S), etc….… are computed and grouped in Table S3. The band gap value is equal to Eg = 2.57 eV, close to 3 eV, this value implies that our prepared material behaves as a semiconductor, which is similar to other organic tetrachlorocobaltate(II) compounds (Tahenti et al., 2020).

To conclude, from the overhead parameters, we can notice that the gap energy of our studied compound is signed that is possessed a high kinetic stability (high-lying LUMO) and low reactivity criterion (low-lying HOMO).