A New Cu(II) Metal Complex Template with 4–Tert–Butyl-Pyridinium Organic Cation: Synthesis, Structure, Hirshfeld Surface, Characterizations and Antibacterial Activity

: In this paper, we report on the chemical preparation, crystal details, vibrational, optical, and thermal behavior, and antibacterial activity of a new non-centrosymmetric compound: 4-ter-butyl-pyridinium tetrachloridocuprate. X-ray diffraction analysis shows that the structure has a 3Dnetwork made up of C–H…Cl and N–H…Cl H-bonds, and [CuCl 4 ] 2 − anions have a shape halfway between a tetrahedron and a square planar structure in this compound’s monoclinic system. Hirshfeld surface analysis was used to explain the nature and extent of intermolecular interactions, highlighting the importance of the H-bonds and the C–H ⋯ π interactions in the structure’s stabilization. Additionally, SEM/EDX experiments were conducted. The powder X-ray diffraction investigation at room temperature validated the material purity. Moreover, the different functional groups were identified using FT-IR spectroscopy. In addition, the optical properties were investigated using UV-Vis absorption. The thermal stability of (C 9 H 14 N) 2 [CuCl 4 ] was performed by TGA-DTA. The bactericidal potency of the title compound was surveyed.


Introduction
A recent hot issue in the field of crystal engineering is the design and fabrication of novel hybrid molecules. Indeed, the hybrid materials combining inorganic complex anions with organic cations have been extensively investigated for their unusual molecular arrangement and characteristics when they reach the solid stage. These includenot only the total of the contributions of both moieties, but also the type of the bonds between them within the structure, which has a significant impact on their properties [1][2][3][4]. The molecular electronic [5], interesting magnetic [6], optical [7], and metallic conductivity [8] characteristics have emerged as a consequence of the structural

Chemical Preparation
After almost two weeks of crystallization in solution at room temperature, greenish block-shaped crystals appropriate for X-ray examination were produced from the solution, with the general formula (C9H14N)2 [CuCl4]. Under continuous spinning at 373 K, 0.170 g of CuCl2·2H2O and 0.270 g of 4-ter-butyl-pyridine (4-TBP) was dissolved in 10 mL of distilled water in the proportions of 1/2. The pH was adjusted to between 2 and 3 by adding concentrated hydrochloric acid (HCl), drop by drop, until the solution became clear. The crystals were then recovered by filtering, washed with a minimum amount of ethanol, and dried at ambient atmosphere (yield: 82%). The elemental analysis method was also used: C (45.42%/45.25%), H (5.97%/5.91%) and N (5.75%/5.86%) (exp/theor).
Sigma-Aldrich supplied the chemicals required (St. Louis, MA, USA). There was no additional purification of any of the chemical reagents utilized.

Investigation Techniques
In the rest of our research, we used a variety of techniques to explore the title compound. The shape and elemental content of the title compound's crystals were studied using the SEM/EDX technique. A JEOL-JSM 6610LV (JOEL-IT 300, Tokyo, Japan) spectrometer was used to collect the SEM pictures, which were typically operated at 15 KV with an EDX detection (EDX, Oxford, UK). System from 15 mm. To avoid undesirable charge effects, the sample was partly covered with a Gold-rich tape. Furthermore, an Xray Powder Diffraction (XRD) system from Siemens D5000, equipped with a Cu anticathode (CuKα, λ= 1.54056 Å), was used to create the XRD powder pattern at room temperature.
The Atlas diffractometer (Agilent Technologies Inc., Santa Clara, CA, USA) was use to obtain (C9H14N)2[CuCl4] single-crystal X-ray diffraction data at 293(2) K using MoKα radiation (λ= 0.71073 Å). A numerical method was used to apply absorption adjustments [25][26][27]. The SHELXTL software was used to solve and refine the crystal structure using the direct technique and the full-matrix least-squares method on F 2 [28]. The structural visuals of the asymmetric unit were created using Diamond 2.0 [29] and Mercury 3.8 [30]. All non-hydrogen atoms were refined using anisotropic temperature parameters. Refinement and crystallographic data are presented in Table 1. The Hirshfeld surface analysis is useful in determining the importance of non-covalent interactions, such as hydrogen bonds and other intermolecular contacts in the crystal lattice, as well as the implication of these interactions on crystal structure and stability [31]. Crystal Explorer software 17.5 [32] uses calculated Hirshfeld surface of molecules within the crystal structure to determine intermolecular interaction between specific molecules or for the entire crystal structure. It provides information on how the crystals are packed together [33][34][35][36]. After diluting the compound sample with KBr and pressing it into pellets, the NICOLET 200 FT-IR spectrometer (SpectraLab Scientific Inc., Markham, ON, Canada) was utilized to capture the (FT-IR) spectrum between 4000 and 400 cm −1 , while the UV-Vis spectrum of the title chemical was obtained using a Perkin Elmer Lambda 11UV/Vis instrument (Waltham, MA, USA) (200-800 nm). Moreover, a PYRIS 1TGA thermogravimetric analyzer with platinum crucible air was used for the thermal examination of 11.4 mg for the TG-DTA analysis between 300 and 600K (Perkin Elmer, Waltham, MA, USA)..
The disc diffusion technique was used to test the (C9H14N)2[CuCl4] chemical for antibacterial activity against three bacterial strains. Some of the most common bacteria found in the gut are Klebsiella pneumonia and Escherichia coli, which are both Gramnegative, and Staphylococcus aureus, which is a Gram-positive bacterial strain. It was carried out in accordance with the technique established by the National Committee for Clinical Laboratory Standards (NCCLS) for disc diffusion. The Eac bacterial stain's inoculum suspension was swabbed throughout the whole Mueller-Hinton agar (MHA, Biokar-diagnostics) surface. Four dilutions of the (C9H14N)2[CuCl4] compound (100 µgmL −1 , 50 µgmL −1 , 25 µgmL −1 , and 12.5 µgmL −1 ) were ascetically put on sterile 4mm filter paper discs and then applied to the three bacterial strain plates. Prior to incubation at 37°C for 24h, the plates were kept at room temperature for 15 min to allow the excess chemical pre-diffusion. The diameters of the inhibition zones were then measured, and there were three sets of results for each experiment. Nalidixic acid (NA30), Novobiocin (NV-5), Norfloxacin (NOR-10), and Erythromycin (E-15) (ThermoFisher scientific, Waltham, MA, USA) were utilized as experimental positive controls in a microbiological susceptibility control test.

PXRD Analysis and Morphology Observations
The experimental powder X-ray diffraction (PXRD) pattern of (C9H14N)2[CuCl4] matches well with the simulated one, as depicted in Figure 1. This result still verifies the synthesized product's purity and the crystal data utilized. However, the SEM images, taken at 100×, 500×, and 1500× magnifications, reveal porosity and concavities that correlate to fractures, indicating that the microcrystals do not have complete cohesiveness. In addition, the EDX spectrum presents all the elements in the structure except the hydrogen atoms ( Figure 2). The SEM photographs and the EDX spectrum confirm the absence of additional phases in the experimental powders.

Structure Description
The title compound's asymmetric unit is composed of two crystallographically independent tetrachlorocuprate (II) anions and four non-equivalent (C9H14N)) + cations (   Each Cu(II) atom coordination geometry may be defined as a distorted [CuCl4] 2tetrahedron. The Cu-Cl bond lengths in this compound vary between 2.2391 (2) and 2.2790 (12) Å, while the bond angles around the Cu atom range from 96.46 (5)° to 140.37 (6)° for the Cu(1)Cl4 tetrahedron and from 94.75 (5)° to 137.91 (6)° for the Cu(2)Cl4 tetrahedron ( Table 2). These results are analogous to those found in other Cu(II) tetrahedral complexes [37][38][39][40][41].  (5) The Cl-Cu-Cl angles depart from the perfect 109.5°. This distortion is induced by the interaction of the NH + group with the chloride pairs, which affects the distortion of the [CuCl4] 2− anions. To investigate this fact, the distortion indices were calculated: DI(Cl-Cu (1) are assumed to be constructed by a regular arrangement of chlorine atoms, with the copper atom faintly displaced from the center of gravity of the tetrahedron. Moreover, the Yang parameter τ may be used to measure the geometry of the four-coordinated metal complex, with zero indicating a perfect square planar geometry and one indicating a perfect tetrahedral geometry [42]. For the [Cu(1)Cl4] 2− tetrahedron, the calculated τ value is 0.58, while the value of the [Cu(2)Cl4] 2− tetrahedron is equal to 0.55 ( Figure 5). These two τ values clearly indicate that the shape of the tetrachlorocuprate anions falls between tetrahedral geometry and square-planar geometry.  Table 3. The basic geometrical properties of the (C9H14N) + organic cations are gathered in Table 4. The values are compatible with those reported in the literature [42][43][44][45][46]. The pyridinium cation always has an expanded C-N-C angle compared to the parent pyridine. As reported in many structures [46][47][48], the C12-N11-C13 = 120.9 (5)°, C22-N21-C23 = 122.0 (5)°, C32-N31-C33 = 122.1 (5)°, and C43-N41-C42 = 121.9 (4)° are typical for protonated pyridine forms. Figure 6 represents all the contact between the organic cations in the atomic arrangement [49][50][51]. We conclude from these results that the C-H⋯π interactions, which vary from 3.424 Å to 3.723 Å, and the H-bond help to keep the crystal packing stable and allow the formation of the three-dimensional network. 109.2 (13) C25-C26-C128 107.7 (7) C127-C26-C128 100.1 (11) Figure 6. The interaction between the organic cations in the synthesized compound.

Hirshfeld Analysis
Hirshfeld surface analysis is useful in determining the importance of the non-strong method for investigating intermolecular interactions and gaining insight into crystal packing behaviour by giving information about the molecules' surroundings in the crystallized environment [52]. Companion techniques to the structural descriptions, HS and fingerprint plots (FP), are utilized to decode the intermolecular interactions involved in crystal packing and their magnitudes. In addition, the enrichment ratio (ER) computation in conjunction with the HS analysis provides insight into the likelihood that the compound under study will interact [53]. With an EXY greater than one, favoured contacts are more likely to make contacts, while element pairings with an EXY< 1 are more likely to not form contacts ( Table 5). The Hirshfeld surface was produced for the compound's asymmetric components (Figure 7a), the H-bonding; C-H...Cl and N-H...Cl may be seen on the dnorm surfaces as deep-red spots, where all the atoms inside the surface can be observed through a translucent surface. Moreover, there are multiple bright-red spots associated to C-H⋯π interactions, which are well confirmed by the Shape-index function as hollow orange areas and bulging blue areas. Furthermore, Figure7b depicts the 2D FP of all contacts that contribute to the Hirshfeld surface. The FP decomposition reveals two prominent spikes, indicating a strong interaction between H ... Cl/Cl ... H contacts associated with the N(C)-H ... Cl H-bonds (Table 4) (Figure 7f,g). There is only 0.8% of the total HS area that C...C contact, but they are more abundant with EC...C = 2.33 ( Figure 7h). Finally, the seven kinds of connections contribute greatly to the crystal structure's stability, and the results of this research are congruent with those reported by X-ray diffraction analysis.

IR Spectroscopy
The infrared spectroscopy approach was utilized to understand more about the functional groups in the compound, verified by comparison with other compounds that are connected with the same cation [54,55]. For the 4000-400 cm −1 range, Figure 8 shows a recording of this compound's IR spectrum at ambient temperature. Asymmetric and symmetric N-H stretching vibrations are responsible for the band between 3300-3000 cm −1 in the IR spectrum of (C9H14N)2[CuCl4]. The C-H stretching modes are responsible for the large absorption bands in the range of 3000-2900 cm −1 . The C=C bonds in the pyridine may be identified by the shoulder band in the spectral area at 1590 cm −1 , whereas the C=N bonds can be identified by the peak at 1492 cm −1 . However, at 688 cm −1 , the ring deformation band δ(py) is observed. The out-of-plane ring deformation band (py) develops at 550 cm −1 as well. C-N stretching bands were found at 1340 and 1150 cm −1 . Asymmetric and symmetric C-C stretching modes were shown to be responsible for the 1102 cm −1 band. Twisting and rocking modes of NH + groups occur at 1014 and 930 cm −1 , respectively. C-H out-of-plane deformation is detected at 801 cm −1 , whereas δ(C-C-C) and δ(C-C-N) are detected at 670 cm −1 .

3.5.UV-Visible Spectrum
The UV-Visible spectrum of (C9H14N)2[CuCl4] was examined in the 200-800 nm range (Figure 9a). This finding is consistent with other compounds containing the [CuCl4] 2− anion, including (C5H7N2)2CuCl4H2O, (C4H9NH3)2CuCl4, (C10H21NH3)2CuCl4, and (C6H9N2)2[CuCl4] [56,57]. Two distinct bands were observed in the UV-Vis spectrum. The band at 350 nm (the least intense) is associated with the π-π* transition, owing to the (C9H14N) + organic group [58]. The second band (most intense), at 580 nm, refers to a d-d transition in the orbits of the metal Cu 2+ . To obtain insight into the behavior of this material (conductor, semiconductor, or insulator), we examined the gap energy Eg of (C9H14N)2[CuCl4]. To illustrate this, we plotted the change of (αhv) 2 vs. hv. As seen in Figure 9b, the optical band gap energy can be determined by linear interpolation of the x-axis of the graphical representation (αhν) 2 (direct band gap) and (αhν) 1/2 (indirect band gap), which is predicted to be 1.75 eV. These gap energy values may be used to classify this material as a semiconductor [59].

3.6.Thermal Analysis of (C9H14N)2[CuCl4]
The TGA/DTA thermograms of (C9H14N)2[CuCl4] were used to assess the thermal stability of the synthesized compound, and it was accomplished on 11.5 mg, with heating at a rate 10 °Cmin −1 . As shown in Figure 10,the synthesized compound (C9H14N)2[CuCl4] remains stable up to 176 °C. The DTA thermogram presents an intense endothermic peak, observed at 250°C (ΔH= 820.525 Jg −1 ), with 87.02% experimental weight loss observed in the TGA thermogram (86.69% calculated weight loss). This almost global loss of mass may correspond to the decomposition and degradation of the organic part in the first and the pyrolysis of the inorganic anion in the second. This good thermal stability is adequate for the applications in electronic and optoelectronic devices. At the end of the experiment, the obtained solid is a black residue that represents 6% of the initial compound, which is a mixture of carbon and copper.

3.7.Antibacterial Assay
Tables 6 and 7 detail the antibacterial properties of the extracts in terms of MICs (minimum inhibitory concentrations) and inhibition zone widths. The antibacterial activity of (C9H14N)2[CuCl4] compound at 25 µgmL −1 (MIC) was shown to be notably effective against the three tested bacteria strains. Indeed, comparable inhibitory efficacy against Gram-negative and Gram-positive bacteria was shown by the tested compound. However, there was a distinct preference for Gram-positive and Gram-negative bacteria in the antibacterial activity of (C9H14N)2[CuCl4].