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Citation: Althobaiti, M.G.; Hermi, S.;

Alotaibi, A.A.; Alotaibi, K.M.;

Hassan, H.A.; Mi, J.-X.; Nasr, C.B.;

Mrad, M.H. A New Cu(II) Metal

Complex Template with

4–tert–Butyl-Pyridinium Organic

Cation: Synthesis, Structure,

Hirshfeld Surface, Characterizations

and Antibacterial Activity. Crystals

2022, 12, 254. https://doi.org/

10.3390/cryst12020254

Academic Editors: Marian Valko and

Venu Vangala

Received: 25 December 2021

Accepted: 10 February 2022

Published: 13 February 2022

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crystals

Article

A New Cu(II) Metal Complex Template with4–tert–Butyl-Pyridinium Organic Cation: Synthesis, Structure,Hirshfeld Surface, Characterizations and Antibacterial ActivityM. G. Althobaiti 1, Sabrine Hermi 2, Abdullah A. Alotaibi 3, Khalid M. Alotaibi 4 , Hamdy A. Hassan 5,6 ,Jin-Xiao Mi 7, Cherif Ben Nasr 2 and Mohamed Habib Mrad 2,3,*

1 Department of Physics, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia;m.althobaiti@tu.edu.sa

2 Laboratoire de Chimie des Matériaux, Faculté des Sciences de Bizerte, Université de Carthage, Zarzouna 7021,Tunisia; sabrine.hermi@fsb.ucar.tn (S.H.); cherif.bennasr@fsb.rnu.tn (C.B.N.)

3 Department of Chemistry, College of Sciences and Humanities, Shaqra University, Ad-Dawadmi 11911,Saudi Arabia; aaalotaibi@su.edu.sa

4 Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia;khalid.m@ksu.edu.sa

5 Department of Biological Science, Faculty of Science and Humanity Studies, Shaqra University,Al-Quwayiyah 11971, Saudi Arabia; hamdya@su.edu.sa

6 Department of Environmental Biotechnology, Genetic Engineering and Biotechnology Research Institute,University of Sadat City, Sadat City 32958, Egypt

7 Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005,China; jxmi@xmu.edu.cn

* Correspondence: m.mrad@su.edu.sa

Abstract: 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-tert-butyl-pyridinium tetrachloridocuprate. X-ray diffraction analysis shows that the structure has a3D network made up of C–H . . . Cl and N–H . . . Cl H-bonds, and [CuCl4]2− anions have a shapehalfway 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 sta-bilization. Additionally, SEM/EDX experiments were conducted. The powder X-ray diffractioninvestigation at room temperature validated the material purity. Moreover, the different functionalgroups were identified using FT-IR spectroscopy. In addition, the optical properties were investigatedusing UV-Vis absorption. The thermal stability of (C9H14N)2[CuCl4] was performed by TGA-DTA.The bactericidal potency of the title compound was surveyed.

Keywords: X-ray diffraction; SEM-EDX; optical absorption; vibrational study; thermal analysis;antibacterial activity

1. Introduction

A recent hot issue in the field of crystal engineering is the design and fabricationof novel hybrid molecules. Indeed, the hybrid materials combining inorganic complexanions with organic cations have been extensively investigated for their unusual moleculararrangement and characteristics when they reach the solid stage. These includenot onlythe total of the contributions of both moieties, but also the type of the bonds betweenthem within the structure, which has a significant impact on their properties [1–4]. Themolecular electronic [5], interesting magnetic [6], optical [7], and metallic conductivity [8]characteristics have emerged as a consequence of the structural integration of organiccations and their inorganic counterparts. More specifically, halogen-based cuprate hybrid

Crystals 2022, 12, 254. https://doi.org/10.3390/cryst12020254 https://www.mdpi.com/journal/crystals

Crystals 2022, 12, 254 2 of 17

compounds have received much attention, due to their structural flexibility and fascinatingfeatures, in fields such as magnetism, electrics, ferroelectrics, photoluminescence, biology,and photocatalysis [9–12]. Concerning the organic ligand, 4-tert-butylpyridine (4-TBP) isfrequently employed to enhance photovoltaic performance in solar cells. Indeed, Yang et al.demonstrated the mechanism through which 4-tert-butylpyridine (4-TBP) influences thephotovoltaic performance additively with changed concentrations [13]. Additionally, itwas discovered that the preponderance of the materials used to transport holes in n-i-pperovskite solar cells include (4-TBP). This additive appears to be necessary for excellentpower conversion efficiency and stable-state performance [14–17].

However, recent research in biology has demonstrated that certain tetrahalogenocuprateswith substituted pyridinium cations have significant gastroprotective and antiepileptic ef-ficacy [18–24]. As a part of our research into different types of hybrid materials, we reporthere the crystallographic description, spectroscopic characterization, and thermal behaviour,including the biological activities of the (C9H14N)2[CuCl4].

2. Materials and Methods2.1. Chemical Preparation

After almost two weeks of crystallization in solution at room temperature, greenishblock-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 gof CuCl2·2H2O and 0.270 g of 4-ter-butyl-pyridine (4-TBP) was dissolved in 10 mL ofdistilled water in the proportions of 1/2. The pH was adjusted to between 2 and 3 byadding 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 alsoused: 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 noadditional purification of any of the chemical reagents utilized.

2.2. Investigation Techniques

In the rest of our research, we used a variety of techniques to explore the title com-pound. The shape and elemental content of the title compound’s crystals were studiedusing the SEM/EDX technique. A JEOL-JSM 6610LV (JOEL–IT 300, Tokyo, Japan) spec-trometer was used to collect the SEM pictures, which were typically operated at 15 KVwith an EDX detection (EDX, Oxford, UK). System from 15 mm. To avoid undesirablecharge effects, the sample was partly covered with a Gold-rich tape. Furthermore, an X-rayPowder 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 toobtain (C9H14N)2[CuCl4] single-crystal X-ray diffraction data at 293(2) K using MoKα radia-tion (λ = 0.71073 Å). A numerical method was used to apply absorption adjustments [25–27].The SHELXTL software was used to solve and refine the crystal structure using the di-rect technique and the full-matrix least-squares method on F2 [28]. The structural visualsof the asymmetric unit were created using Diamond 2.0 [29] and Mercury 3.8 [30]. Allnon-hydrogen atoms were refined using anisotropic temperature parameters. Refinementand crystallographic data are presented in Table 1. The Hirshfeld surface analysis is use-ful in determining the importance of non-covalent interactions, such as hydrogen bondsand other intermolecular contacts in the crystal lattice, as well as the implication of theseinteractions on crystal structure and stability [31]. Crystal Explorer software 17.5 [32]uses calculated Hirshfeld surface of molecules within the crystal structure to determineintermolecular interaction between specific molecules or for the entire crystal structure. Itprovides information on how the crystals are packed together [33–36].

Crystals 2022, 12, 254 3 of 17

Table 1. Structure refinement and crystal data.

Molecular Formula C18H28Cl4CuN2

Molecular weight 477.76

T (K) 293(2)

Crystal system/Space group Monoclinic/Pc (No. 7)

Cell parametersa = 9.4586(8) Å.

b = 21.6255(14) Å, β = 111.526(9)◦.c = 12.0806(10) Å.

V (Å3) 2298.7(3)

Z 4

Density 1.381 Mg·m−3

Absorption coefficient 1.419 mm−1

F (000) 988

Crystal dimensions 0.250 × 0.200 × 0.150 mm3

θ range 2.50 to 28.43◦

Index ranges −12 ≤ h ≤ 12, −28 ≤ k ≤ 15, −8 ≤ l ≤ 15

Reflections collected 10,159

Independent reflections 6382 [R(int) = 0.0226]

Completeness to θ = 25.242◦ 99.9%

Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 6382/8/489

Goodnessoffit on F2 1.054

Final R indices [I > 2sigma(I)] R1 = 0.0334, wR2 = 0.0732

R indices (all data) R1 = 0.0378, wR2 = 0.0781

Absolute structure parameter −0.010(11)

Extinction coefficient 0.0022(3)

Largest diff. peak and hole 0.429 and −0.454 e.Å−3

CCDC No.◦ 2,130,817

After diluting the compound sample with KBr and pressing it into pellets, the NICO-LET 200 FT-IR spectrometer (SpectraLab Scientific Inc., Markham, ON, Canada) was utilizedto capture the (FT–IR) spectrum between 4000 and 400 cm−1, while the UV-Vis spectrumof the title chemical was obtained using a Perkin Elmer Lambda 11UV/Vis instrument(Waltham, MA, USA) (200–800 nm). Moreover, a PYRIS 1TGA thermogravimetric analyzerwith platinum crucible air was used for the thermal examination of 11.4 mg for the TG-DTAanalysis between 300 and 600 K (Perkin Elmer, Waltham, MA, USA).

The disc diffusion technique was used to test the (C9H14N)2[CuCl4] chemical for an-tibacterial activity against three bacterial strains. Some of the most common bacteria foundin the gut are Klebsiella pneumonia and Escherichia coli, which are both Gram-negative, andStaphylococcus aureus, which is a Gram-positive bacterial strain. It was carried out in accor-dance with the technique established by the National Committee for Clinical LaboratoryStandards (NCCLS) for disc diffusion. The Eac bacterial stain’s inoculum suspension wasswabbed throughout the whole Mueller-Hinton agar (MHA, Biokar-diagnostics) surface.Four dilutions of the (C9H14N)2[CuCl4] compound (100 µg mL−1, 50 µg mL−1, 25 µg mL−1,and 12.5 µg mL−1) were ascetically put on sterile 4mm filter paper discs and then appliedto the three bacterial strain plates. Prior to incubation at 37 ◦C for 24 h, the plates werekept at room temperature for 15 min to allow the excess chemical pre-diffusion. The di-

Crystals 2022, 12, 254 4 of 17

ameters of the inhibition zones were then measured, and there were three sets of resultsfor each experiment. Nalidixic acid (NA30), Novobiocin (NV-5), Norfloxacin (NOR-10),and Erythromycin (E-15) (ThermoFisher scientific, Waltham, MA, USA) were utilized asexperimental positive controls in a microbiological susceptibility control test.

3. Results and Discussion3.1. 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 thesynthesized product’s purity and the crystal data utilized. However, the SEM images, takenat 100×, 500×, and 1500× magnifications, reveal porosity and concavities that correlate tofractures, 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 additionalphases in the experimental powders.

3.2. Structure Description

The title compound’s asymmetric unit is composed of two crystallographically in-dependent tetrachlorocuprate (II) anions and four non-equivalent (C9H14N))+ cations(Figure 3). The structure of (C9H14N)2[CuCl4] consists of an infinity of flat strips parallelto the [001] direction at x= 1/4, y = 0, formed by [(C9H14N)2Cu(1)Cl4] S(I) and at x =3/4,y =1/2, formed by [(C9H14N)2Cu(2)Cl4] S(II) (Figure 4). As shown in Figure 4 S(I), each[Cu(1)Cl4]2− tetrahedron links to three neighboring hexagonal rings [N41–C42–C41–C45–C44–C43] from 4-TBP molecules (named No. 4) and one counterpart from the No. 2 (4-TBP)molecule by hydrogen bonds to form strip I. Interestingly, each hexagonal ring of theNo. 4 (4-TBP) molecule also links to three neighboring [Cu1Cl4]2− tetrahedra. Moreover,the plane of the hexagonal ring of the No. 4 (4-TBP) molecule is also parallel to (100) plane,but that of the No. 2 (4-TBP) molecule is not. Similarly, each [Cu(2)Cl4]2− tetrahedron linksto three neighboring hexagonal rings from No. 1 (4-TBP) molecules and one counterpartfrom No. 3 (4-TBP) molecules by hydrogen bonds to form strip II. Strips I and II are com-bined with each other via hexagonal rings from both No. 2 and 3 (4-TBP) molecules. Thehexagonal ring from the No. 2 (4-TBP) molecule links to two [Cu(2)Cl4]2− tetrahedra andone [Cu(1)Cl4]2− tetrahedron, whereas that from No. 3 links to one [Cu(2)Cl4]2− tetrahe-dron and two [Cu(1)Cl4]2− tetrahedra. Strips I and II are crystallographically independent,leading to a non-centrosymmetric space group of Pc.

Crystals 2022, 12, x FOR PEER REVIEW 4 of 17

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.

3. Results and Discussion 3.1. 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 corre-late 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.

Figure 1. Experimental and theoretical powder diffractograms.

×100 ×500

Figure 1. Experimental and theoretical powder diffractograms.

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Crystals 2022, 12, x FOR PEER REVIEW 4 of 17

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.

3. Results and Discussion 3.1. 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 corre-late 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.

Figure 1. Experimental and theoretical powder diffractograms.

×100 ×500

Crystals 2022, 12, x FOR PEER REVIEW 5 of 17

×1500

Figure 2. The SEM observations and the EDX analysis of (C9H14N)2[CuCl4].

3.2. Structure Description The title compound’s asymmetric unit is composed of two crystallographically inde-

pendent tetrachlorocuprate (II) anions and four non-equivalent (C9H14N))+cations (Figure 3). The structure of (C9H14N)2[CuCl4] consists of an infinity of flat strips parallel to the [001] direction at x= 1/4, y = 0, formed by [(C9H14N)2Cu(1)Cl4] S(I)and at x =3/4, y =1/2, formed by [(C9H14N)2Cu(2)Cl4] S(II) (Figure 4). As shown in Figure 4 S(I), each [Cu(1)Cl4]2− tetrahedron links to three neighboring hexagonal rings [N41–C42–C41–C45–C44–C43] from4-TBP molecules (named No.4)and one counterpart from the No.2 (4-TBP) molecule by hydrogen bonds to form strip I. Interestingly, each hexagonal ring of the No.4 (4-TBP) molecule also links to three neighboring [Cu1Cl4]2− tetrahedra. Moreover, the plane of the hexagonal ring of the No.4 (4-TBP) molecule is also parallel to (100) plane, but that of the No.2 (4-TBP) molecule is not. Similarly, each [Cu(2)Cl4]2− tetrahedron links to three neigh-boring hexagonal rings from No.1 (4-TBP) molecules and one counterpart from No.3 (4-TBP) molecules by hydrogen bonds to form strip II. Strips I and II are combined with each other via hexagonal rings from both No.2 and 3 (4-TBP) molecules. The hexagonal ring from the No.2 (4-TBP) molecule links to two [Cu(2)Cl4]2− tetrahedra and one [Cu(1)Cl4]2− tetrahedron, whereas that from No.3 links to one [Cu(2)Cl4]2− tetrahedron and two [Cu(1)Cl4]2− tetrahedra. Strips I and II are crystallographically independent, leading to a non-centrosymmetric space group of Pc.

Figure 3. The ORTEP plot of (C9H14N)2[CuCl4]; at the 50% probability ellipsoids, the displacement ellipsoids are drawn, highlighting the atom labeling scheme. The disordered components arecircled in red, with occupancies 0.586 (13)/0.414 (13) for C27/C127, C28/C128, and C29/C129. For clarity, the H-atoms have been eliminated.

Figure 2. The SEM observations and the EDX analysis of (C9H14N)2[CuCl4].

Crystals 2022, 12, x FOR PEER REVIEW 5 of 17

×1500

Figure 2. The SEM observations and the EDX analysis of (C9H14N)2[CuCl4].

3.2. Structure Description The title compound’s asymmetric unit is composed of two crystallographically inde-

pendent tetrachlorocuprate (II) anions and four non-equivalent (C9H14N))+cations (Figure 3). The structure of (C9H14N)2[CuCl4] consists of an infinity of flat strips parallel to the [001] direction at x= 1/4, y = 0, formed by [(C9H14N)2Cu(1)Cl4] S(I)and at x =3/4, y =1/2, formed by [(C9H14N)2Cu(2)Cl4] S(II) (Figure 4). As shown in Figure 4 S(I), each [Cu(1)Cl4]2− tetrahedron links to three neighboring hexagonal rings [N41–C42–C41–C45–C44–C43] from4-TBP molecules (named No.4)and one counterpart from the No.2 (4-TBP) molecule by hydrogen bonds to form strip I. Interestingly, each hexagonal ring of the No.4 (4-TBP) molecule also links to three neighboring [Cu1Cl4]2− tetrahedra. Moreover, the plane of the hexagonal ring of the No.4 (4-TBP) molecule is also parallel to (100) plane, but that of the No.2 (4-TBP) molecule is not. Similarly, each [Cu(2)Cl4]2− tetrahedron links to three neigh-boring hexagonal rings from No.1 (4-TBP) molecules and one counterpart from No.3 (4-TBP) molecules by hydrogen bonds to form strip II. Strips I and II are combined with each other via hexagonal rings from both No.2 and 3 (4-TBP) molecules. The hexagonal ring from the No.2 (4-TBP) molecule links to two [Cu(2)Cl4]2− tetrahedra and one [Cu(1)Cl4]2− tetrahedron, whereas that from No.3 links to one [Cu(2)Cl4]2− tetrahedron and two [Cu(1)Cl4]2− tetrahedra. Strips I and II are crystallographically independent, leading to a non-centrosymmetric space group of Pc.

Figure 3. The ORTEP plot of (C9H14N)2[CuCl4]; at the 50% probability ellipsoids, the displacement ellipsoids are drawn, highlighting the atom labeling scheme. The disordered components arecircled in red, with occupancies 0.586 (13)/0.414 (13) for C27/C127, C28/C128, and C29/C129. For clarity, the H-atoms have been eliminated.

Figure 3. The ORTEP plot of (C9H14N)2[CuCl4]; at the 50% probability ellipsoids, the displacementellipsoids are drawn, highlighting the atom labeling scheme. The disordered components arecircledin red, with occupancies 0.586 (13)/0.414 (13) for C27/C127, C28/C128, and C29/C129. For clarity,the H-atoms have been eliminated.

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Figure 4. The crystal packing of (C9H14N)2[CuCl4] and therepresentation of the two flat stripsS(I) and S(II)along the a-axis.

Each Cu(II) atom coordination geometry may be defined as a distorted [CuCl4]2-tet-rahedron. 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 [37–41].

Table 2. [Cu(1)Cl4]2− and [Cu(2)Cl4]2− bond lengths and angles.

Distances(Å) Angles (°) Cl1—Cu(1)—Cl2 140.37 (6) Cl1—Cu(1)—Cl3 98.15 (5)

Cu(1)—Cl1 2.2391 (12) Cl2—Cu(1)—Cl3 97.04 (5) Cu(1)—Cl2 2.2420 (13) Cl1—Cu(1)—Cl4 96.94 (5) Cu(1)—Cl3 2.2459 (13) Cl2—Cu(1)—Cl4 96.46 (5) Cu(1)—Cl4 2.2760 (12) Cl3—Cu(1)—Cl4 136.91 (6) Cu(2)—Cl5 2.2340 (12) Cl5—Cu(2)—Cl6 98.45 (5) Cu(2)—Cl6 2.2522 (12) Cl5—Cu(2)—Cl7 143.59 (6) Cu(2)—Cl7 2.2528 (13) Cl6—Cu(2)—Cl7 94.75 (5) Cu(2)—Cl8 2.2790 (12) Cl5—Cu(2)—Cl8 96.93 (5)

Cl6—Cu(2)—Cl8 137.91 (6) Cl7—Cu(2)—Cl8 95.59 (5)

Figure 4. The crystal packing of (C9H14N)2[CuCl4] and therepresentation of the two flat strips S(I)and S(II) along the a-axis.

Each Cu(II) atom coordination geometry may be defined as a distorted [CuCl4]2−

tetrahedron. The Cu–Cl bond lengths in this compound vary between 2.2391 (2) and2.2790 (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 tetrahe-dron (Table 2). These results are analogous to those found in other Cu(II) tetrahedralcomplexes [37–41].

Table 2. [Cu(1)Cl4]2− and [Cu(2)Cl4]2− bond lengths and angles.

Distances (Å) Angles (◦)

Cl1—Cu(1)—Cl2 140.37 (6)Cl1—Cu(1)—Cl3 98.15 (5)

Cu(1)—Cl1 2.2391 (12) Cl2—Cu(1)—Cl3 97.04 (5)Cu(1)—Cl2 2.2420 (13) Cl1—Cu(1)—Cl4 96.94 (5)Cu(1)—Cl3 2.2459 (13) Cl2—Cu(1)—Cl4 96.46 (5)Cu(1)—Cl4 2.2760 (12) Cl3—Cu(1)—Cl4 136.91 (6)Cu(2)—Cl5 2.2340 (12) Cl5—Cu(2)—Cl6 98.45 (5)Cu(2)—Cl6 2.2522 (12) Cl5—Cu(2)—Cl7 143.59 (6)Cu(2)—Cl7 2.2528 (13) Cl6—Cu(2)—Cl7 94.75 (5)Cu(2)—Cl8 2.2790 (12) Cl5—Cu(2)—Cl8 96.93 (5)

Cl6—Cu(2)—Cl8 137.91 (6)Cl7—Cu(2)—Cl8 95.59 (5)

Crystals 2022, 12, 254 7 of 17

The Cl–Cu–Cl angles depart from the perfect 109.5◦. This distortion is induced bythe interaction of the NH+ group with the chloride pairs, which affects the distortionof the [CuCl4]2− anions. To investigate this fact, the distortion indices were calculated:DI(Cl–Cu(1)) = 0.0072, DI(Cl–Cl) = 0.1016, and DI(Cl–Cu(1)–Cl) = 0.1661 for [Cu(1)Cl4]2−

anions and DI(Cl–Cu(2)) = 0.0054, DI(Cl–Cl) = 0.1210, and DI(Cl–Cu(2)–Cl) = 0.1770 for[Cu(2)Cl4]2− anions. These values indicate a high distortion of the Cl–Cu–Cl angles forboth anions when compared to Cl–Cl distances. The anions [CuCl4]2− are assumed tobe constructed by a regular arrangement of chlorine atoms, with the copper atom faintlydisplaced from the center of gravity of the tetrahedron. Moreover, the Yang parameter τmay be used to measure the geometry of the four-coordinated metal complex, with zeroindicating a perfect square planar geometry and one indicating a perfect tetrahedral geom-etry [42]. For the [Cu(1)Cl4]2− tetrahedron, the calculated τ value is 0.58, while the value ofthe [Cu(2)Cl4]2− tetrahedron is equal to 0.55 (Figure 5). These two τ values clearly indicatethat the shape of the tetrachlorocuprate anions falls between tetrahedral geometry andsquare-planar geometry.

Crystals 2022, 12, x FOR PEER REVIEW 7 of 17

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)) = 0.0072, DI(Cl–Cl) = 0.1016, and DI(Cl–Cu(1)–Cl) = 0.1661 for [Cu(1)Cl4]2− anions and DI(Cl–Cu(2)) = 0.0054, DI(Cl–Cl) = 0.1210, and DI(Cl–Cu(2)–Cl) = 0.1770 for [Cu(2)Cl4]2− anions. These values indicate a high distortion of the Cl–Cu–Cl angles for both anions when compared to Cl–Cl distances. The anions [CuCl4]2− are assumed to be con-structed by a regular arrangement of chlorine atoms, with the copper atom faintly dis-placed 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 indi-cating 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.

Figure 5. Representation of [Cu(1)Cl4]2−and [Cu(2)Cl4]2−tetrahedra.

The two neighbouring strips, [Cu(1)Cl4(C9H14N)2]n at x= 1/4,y= 0 and [Cu(2)Cl4(C9H14N)2]n at x =3/4 y =1/2, are interconnected into a 3D supramolecular network by means of C23–H23...Cl8, C32–H32...Cl1, C33–H33...Cl4, and C(41)–H...Cl(4) H-bonds (Table 3) (Figure 4). As can be seen in Figure 4, the N atoms of each cation are directed toward each tetrahedron [CuCl4]2−, where the non-equivalent cations in each strip are locked on a triangle made up of three [CuCl4]2− anions, establishing multiple hydrogen bonds. The N21–H21A...Cl1, C23–H23...Cl1, C(44)–H(44)...Cl(1), C42–H42...Cl2, N(41)–H(41A)...Cl(2), N21–H21...Cl4, C(43)–H(43)...Cl(3),N(41)–H(41A)...Cl(3), C41–H41...Cl4, and N21–H(21A)...Cl4 are responsible for the generation of the flat strips S(I)made by [Cu(1)Cl4]2−anions and the organic cations at x = 1/4and y = 0. However, the flat strips’ S(II) cohesiveness is due to the hydrogen bonds, C11–H11...Cl5, C33–H33...Cl5,C12–H12...Cl6, C13–H13...Cl6, N11–H11A...Cl6, C13–H13...Cl7, N11–H11A...Cl7, C14–H14...Cl8, and N31–H31...Cl8 at x =3/4 and y =1/2, made by [Cu(2)Cl4]2−anions and the organic cations as listed in Table 3.

Table 3. Hydrogen bonds of (C9H14N)2[CuCl4],.

D–H...A d(D–H) d(H...A) d(D...A) <(DHA)(°) C(11)–H(11)...Cl(5)#1 0.93 2.84 3.564(5) 135 C(12)–H(12)...Cl(6)#1 0.93 2.74 3.598(5) 153 C(13)–H(13)...Cl(6) 0.93 2.93 3.462(5) 117

C(13)–H(13)...Cl(7)#2 0.93 2.68 3.434(5) 139 C(14)–H(14)...Cl(8)#2 0.93 2.89 3.815(5) 177

Figure 5. Representation of [Cu(1)Cl4]2− and [Cu(2)Cl4]2− tetrahedra.

The two neighbouring strips, [Cu(1)Cl4(C9H14N)2]n at x = 1/4, y= 0 and [Cu(2)Cl4(C9H14N)2]n at x = 3/4 y = 1/2, are interconnected into a 3D supramolecular networkby means of C23–H23...Cl8, C32–H32...Cl1, C33–H33...Cl4, and C(41)–H...Cl(4) H-bonds(Table 3) (Figure 4). As can be seen in Figure 4, the N atoms of each cation are directedtoward each tetrahedron [CuCl4]2−, where the non-equivalent cations in each strip arelocked on a triangle made up of three [CuCl4]2− anions, establishing multiple hydrogenbonds. The N21–H21A...Cl1, C23–H23...Cl1, C(44)–H(44)...Cl(1), C42–H42...Cl2, N(41)–H(41A)...Cl(2), N21–H21...Cl4, C(43)–H(43)...Cl(3),N(41)–H(41A)...Cl(3), C41–H41...Cl4,and N21–H(21A)...Cl4 are responsible for the generation of the flat strips S(I) made by[Cu(1)Cl4]2− anions and the organic cations at x = 1/4 and y = 0. However, the flatstrips’ S(II) cohesiveness is due to the hydrogen bonds, C11–H11...Cl5, C33–H33...Cl5,C12–H12...Cl6, C13–H13...Cl6, N11–H11A...Cl6, C13–H13...Cl7, N11–H11A...Cl7, C14–H14...Cl8,and N31–H31...Cl8 at x = 3/4 and y = 1/2, made by [Cu(2)Cl4]2− anions and the organiccations as listed in Table 3.

The basic geometrical properties of the (C9H14N)+ organic cations are gathered inTable 4. The values are compatible with those reported in the literature [42–45]. The pyri-dinium cation always has an expanded C–N–C angle compared to the parent pyridine. As re-ported in many structures [45–47], 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 pyri-dine forms. Figure 6 represents all the contact between the organic cations in the atomicarrangement [48–50]. We conclude from these results that the C–H· · ·π interactions, whichvary from 3.424 Å to 3.723 Å, and the H-bond help to keep the crystal packing stable andallow the formation of the three-dimensional network.

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Table 3. Hydrogen bonds of (C9H14N)2[CuCl4].

D–H...A d(D–H) d(H...A) d(D...A) <(DHA) (◦)

C(11)–H(11)...Cl(5)#1 0.93 2.84 3.564(5) 135

C(12)–H(12)...Cl(6)#1 0.93 2.74 3.598(5) 153

C(13)–H(13)...Cl(6) 0.93 2.93 3.462(5) 117

C(13)–H(13)...Cl(7)#2 0.93 2.68 3.434(5) 139

C(14)–H(14)...Cl(8)#2 0.93 2.89 3.815(5) 177

C(23)–H(23)...Cl(1) 0.93 2.84 3.389(6) 119

C(23)–H(23)...Cl(8)#2 0.93 2.77 3.571(6) 145

C(32)–H(32)...Cl(1)#3 0.93 2.66 3.590(5) 177

C(33)–H(33)...Cl(4)#4 0.93 2.79 3.541(5) 138

C(33)–H(33)...Cl(5)#1 0.93 2.91 3.515(5) 123

C(41)–H(41)...Cl(4)#2 0.93 2.80 3.631(5) 150

C(42)–H(42)...Cl(2)#2 0.93 2.64 3.509(5) 155

C(43)–H(43)...Cl(3)#1 0.93 2.77 3.674(5) 163

C(44)–H(44)...Cl(1)#1 0.93 2.83 3.558(5) 136

N(11)–H(11A)...Cl(6) 0.86 2.67 3.325(4) 133

N(11)–H(11A)...Cl(7) 0.86 2.44 3.180(4) 144

N(21)–H(21A)...Cl(1) 0.86 2.84 3.376(5) 122

N(21)–H(21A)...Cl(4) 0.86 2.32 3.099(5) 151

N(31)–H(31A)...Cl(8)#1 0.86 2.28 3.117(4) 164

N(41)–H(41A)...Cl(2)#5 0.86 2.61 3.214(4) 128

N(41)–H(41A)...Cl(3)#5 0.86 2.49 3.218(4) 143Symmetry codes: #1 x,−y+1,z−1/2; #2 x,−y+1,z+1/2; #3 x,y,z−1; #4 x+1,y,z; #5 x,y+1,z.

Table 4. The basic geometrical properties of 4-ter-butyl-pyridinium in (C9H14N)2[CuCl4].

Bond Distance (Å)

N11–C13 1.346 (6) N21–C23 1.329 (7) N31–C33 1.333 (6) N41–C43 1.333 (6)

C12–N11 1.313 (6) C21–C22 1.369 (7) C31–C32 1.367 (7) C41–C42 1.363 (7)

C11–C12 1.367 (7) C21–C25 1.392 (7) C31–C35 1.406 (6) C41–C45 1.399 (7)

C11–C15 1.405 (6) C22–N21 1.324 (7) C32–N31 1.326 (7) C42–N41 1.343 (6)

C13–C14 1.366 (7) C23–C24 1.365 (7) C33–C34 1.365 (7) C43–C44 1.360 (7)

C14–C15 1.388 (7) C24–C25 1.388 (7) C34–C35 1.392 (6) C44–C45 1.412 (7)

C15–C16 1.519 (6) C25–C26 1.517 (7) C35–C36 1.511 (7) C45–C46 1.510 (7)

C16–C18 1.527 (7) C127–C26 1.542 (14) C36–C39 1.523 (8) C46–C48 1.530 (8)

C16–C17 1.533 (7) C128–C26 1.599 (15) C36–C38 1.533 (7) C46–C49 1.531 (7)

C16–C19 1.534 (7) C129–C26 1.476 (15) C36–C37 1.539 (7) C46–C47 1.545 (7)

C26–C27 1.434 (10)

C26–C29 1.555 (11)

C26–C28 1.559 (11)

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Table 4. Cont.

Angles (◦)

C12–C11–C15 120.1 (5) C22–C21–C25 120.9 (5) C32–C31–C35 120.6 (5) C42–C41–C45 121.1 (4)

N11–C12–C11 120.9 (5) N21–C22–C21 119.7 (5) N31–C32–C31 120.0 (5) N41–C42–C41 120.0 (4)

C12–N11–C13 122.1 (4) C22–N21–C23 122.0 (5) C32–N31–C33 122.1 (5) C43–N41–C42 121.9 (4)

N11–C13–C14 118.8 (4) N21–C23–C24 120.2 (5) N31–C33–C34 120.0 (5) N41–C43–C44 119.9 (4)

N11–C13–H13 120.6 C23–C24–C25 120.5 (5) C33–C34–C35 120.9 (5) C43–C44–C45 121.2 (4)

C14–C13–H13 120.6 C24–C25–C21 116.7 (5) C34–C35–C31 116.4 (4) C41–C45–C44 115.8 (4)

C13–C14–C15 121.8 (4) C24–C25–C26 121.3 (5) C34–C35–C36 123.4 (4) C41–C45–C46 123.7 (5)

C14–C15–C11 116.2 (4) C21–C25–C26 122.0 (5) C31–C35–C36 120.2 (4) C44–C45–C46 120.5 (4)

C14–C15–C16 120.4 (4) C27–C26–C25 115.9 (6) C35–C36–C39 109.8 (4) C45–C46–C48 109.3 (4)

C11–C15–C16 123.4 (4) C129–C26–C25 119.4 (9) C35–C36–C38 111.8 (4) C45–C46–C49 107.7 (4)

C15–C16–C18 107.9 (4) C129–C26–C127 108.8 (12) C39–C36–C38 108.8 (5) C48–C46–C49 109.9 (5)

C15–C16–C17 112.0 (4) C25–C26–C127 109.8 (7) C35–C36–C37 108.3 (4) C45–C46–C47 111.2 (5)

C18–C16–C17 109.1 (5) C27–C26–C29 113.0 (8) C39–C36–C37 109.2 (5) C48–C46–C47 110.0 (5)

C15–C16–C19 108.8 (4) C25–C26–C29 106.8 (6) C38–C36–C37 108.9 (5) C49–C46–C47 108.7 (5)

C18–C16–C19 109.9 (5) C27–C26–C28 111.3 (8)

C17–C16–C19 109.1 (5) C25–C26–C28 105.0 (6)

C29–C26–C28 103.9 (8)

C129–C26–C128 109.2 (13)

C25–C26–C128 107.7 (7)

C127–C26–C128 100.1 (11)

3.3. Hirshfeld Analysis

Hirshfeld surface analysis is useful in determining the importance of the non-strongmethod for investigating intermolecular interactions and gaining insight into crystal pack-ing behaviour by giving information about the molecules’ surroundings in the crystallizedenvironment [51]. Companion techniques to the structural descriptions, HS and fingerprintplots (FP), are utilized to decode the intermolecular interactions involved in crystal packingand their magnitudes. In addition, the enrichment ratio (ER) computation in conjunctionwith the HS analysis provides insight into the likelihood that the compound under studywill interact [52]. With an EXY greater than one, favoured contacts are more likely tomake 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

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(Figure 7a), the H-bonding; C–H...Cl and N–H...Cl may be seen on the dnorm surfaces asdeep-red spots, where all the atoms inside the surface can be observed through a translucentsurface. 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 bulgingblue areas. Furthermore, Figure 7b depicts the 2D FP of all contacts that contribute to theHirshfeld surface.

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Figure 6. The interaction between the organic cations in the synthesized compound.

3.3. 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.

Table 5. The enrichment ratios for the title compound.

Atom C N Cl Cu H %S 5.85 1.55 21 1.25 69.45 C 2.33 N 0.55 0 Cl 0 0 0 Cu 0 0 0 0 H 1.23 1.39 1.43 1.43 0.86

Figure 6. The interaction between the organic cations in the synthesized compound.

Table 5. The enrichment ratios for the title compound.

Atom C N Cl Cu H

%S 5.85 1.55 21 1.25 69.45

C 2.33

N 0.55 0

Cl 0 0 0

Cu 0 0 0 0

H 1.23 1.39 1.43 1.43 0.86

The FP decomposition reveals two prominent spikes, indicating a strong interactionbetween H...Cl/Cl...H contacts associated with the N(C)–H...Cl H-bonds (Table 4), ac-counting for (42%) of the total Hirshfeld surface. Moreover, chloride and hydrogen arepresent in large concentrations on the molecular surface (%SH = 69.45% and %S Cl = 21%),making these interactions the most prevalent with EH...Cl = 1.43 (Figure 7c). Additionally,the fingerprint pattern exhibited by H...H contacts, which contribute 41.7% to the totalHS, is interpreted as a high concentration in the center area, owing to the molecule’s highhydrogen content on the surface (%SH = 69.45%). The distribution of scattered points inthe 2D FP map reflects the H...H interactions, although they are underrepresented, with anenrichment ratio of around 0.86 (Figure 7d). Despite its significant percentage, this interac-tion’s role in structure stabilization is quite minor in magnitude, because it is between thesame species. With an enrichment ratio greater than the unit (EC...H = 1.23), the C...H/H...Ccontact, attributed to the C–H . . . π interactions, is the third most prominent surface in-

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teraction, accounting for 10% of the Hirshfeld surface (Figure 7e). This demonstrates thesignificance of this interaction on the structure of this compound. However, even thoughthe N...H/H...N and Cu...H/H...Cu interactions have relatively modest surface areas, theyare preferred and show enrichment values of EN...H = 1.39 and ECu...H = 1.43 (Figure 7f,g).There is only 0.8% of the total HS area that C...C contact, but they are more abundantwith EC...C = 2.33 (Figure 7h). Finally, the seven kinds of connections contribute greatly tothe crystal structure’s stability, and the results of this research are congruent with thosereported by X-ray diffraction analysis.

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Figure 7. 2D plots showing the title compound’s principal interactions. Hirshfeld surface mapped with dnorm and shape-index (a). Selected FP delineated into; 100% (b), H...Cl/Cl...H (c), H...H (d), C...H/H...C (e), N...H/H....N (f), Cu...H/H...Cu (g), and C…C (h).

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), accounting for (42%) of the total Hirshfeld surface. Moreover, chloride and hydrogen are present in large concentrations on the molecular surface (%SH = 69.45% and %S Cl = 21%), making these interactions the most prevalent with EH...Cl= 1.43 (Figure 7c). Additionally, the fingerprint pattern exhibited by H...H contacts, which contribute 41.7% to the total HS, is interpreted as a high concentration in the center area, owing to the molecule’s high hydrogen content on the surface (%SH= 69.45%). The distribution of scattered points in the 2D FP map reflects the H...H interactions, although they are underrepresented, with an enrichment ratio of around 0.86 (Figure 7d). Despite its significant percentage, this interaction’s role in structure stabilization is quite minor in magnitude, because it is between the same species. With an enrichment ratio greater than the unit (EC...H = 1.23), the C...H/H...C contact, attributed to the C–H…π interactions, is the third most prominent

Figure 7. 2D plots showing the title compound’s principal interactions. Hirshfeld surface mappedwith dnorm and shape-index (a). Selected FP delineated into; 100% (b), H...Cl/Cl...H (c), H...H (d),C...H/H...C (e), N...H/H....N (f), Cu...H/H...Cu (g), and C . . . C (h).

3.4. IR Spectroscopy

The infrared spectroscopy approach was utilized to understand more about the func-tional groups in the compound, verified by comparison with other compounds that areconnected with the same cation [53,54]. For the 4000–400 cm−1 range, Figure 8 shows

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a recording of this compound’s IR spectrum at ambient temperature. Asymmetric andsymmetric 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 forthe large absorption bands in the range of 3000–2900 cm−1. The C=C bonds in the pyri-dine may be identified by the shoulder band in the spectral area at 1590 cm−1, whereasthe C=N bonds can be identified by the peak at 1492 cm−1. However, at 688 cm−1, thering 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 the1102 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.

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surface interaction, accounting for 10% of the Hirshfeld surface (Figure 7e). This demon-strates the significance of this interaction on the structure of this compound. However, even though the N...H/H...N and Cu...H/H...Cu interactions have relatively modest sur-face areas, they are preferred and show enrichment values of EN...H = 1.39 and ECu...H = 1.43(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 con-tribute greatly to the crystal structure’s stability, and the results of this research are con-gruent with those reported by X-ray diffraction analysis.

3.4. IR Spectroscopy The infrared spectroscopy approach was utilized to understand more about the func-

tional 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 sym-metric 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 defor-mation 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.

Figure 8. The IR spectrum of (C9H14N)2[CuCl4] taken at room temperature. Figure 8. The IR spectrum of (C9H14N)2[CuCl4] taken at room temperature.

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− an-ion, including (C5H7N2)2CuCl4H2O, (C4H9NH3)2CuCl4, (C10H21NH3)2CuCl4, and(C6H9N2)2[CuCl4] [55,56]. 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 [57]. The second band (most intense), at 580 nm, refers to a d–dtransition in the orbits of the metal Cu2+.

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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− an-ion, 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 Cu2+.

Figure 9. The solid-state UV-Visible spectrum at room temperatureof (C9H14N)2[CuCl4] (a).Tauc’s plot giving the energy gap(b).

To obtain insight into the behavior of this material (conductor, semiconductor, or in-sulator), 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 de-termined 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 sta-

bility 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] re-mains 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.

Figure 9. The solid-state UV-Visible spectrum at room temperatureof (C9H14N)2[CuCl4] (a). Tauc’splot giving the energy gap (b).

To obtain insight into the behavior of this material (conductor, semiconductor, orinsulator), we examined the gap energy Eg of (C9H14N)2[CuCl4]. To illustrate this, weplotted the change of (αhv)2 vs. hv. As seen in Figure 9b, the optical band gap energy canbe 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. Thesegap energy values may be used to classify this material as a semiconductor [58].

3.6. Thermal Analysis of (C9H14N)2[CuCl4]

The TGA/DTA thermograms of (C9H14N)2[CuCl4] were used to assess the thermalstability of the synthesized compound, and it was accomplished on 11.5 mg, with heating ata rate 10 ◦C min−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 (∆H = 820.525 J g−1), with 87.02% experimental weight loss observedin the TGA thermogram (86.69% calculated weight loss). This almost global loss of massmay correspond to the decomposition and degradation of the organic part in the first andthe pyrolysis of the inorganic anion in the second. This good thermal stability is adequatefor 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 amixture 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 activityof (C9H14N)2[CuCl4] compound at 25 µg mL−1(MIC) was shown to be notably effectiveagainst the three tested bacteria strains. Indeed, comparable inhibitory efficacy againstGram-negative and Gram-positive bacteria was shown by the tested compound. How-ever, there was a distinct preference for Gram-positive and Gram-negative bacteria in theantibacterial activity of (C9H14N)2[CuCl4].

Crystals 2022, 12, 254 14 of 17Crystals 2022, 12, x FOR PEER REVIEW 14 of 17

Figure 10. TGA-DTA curves for (C9H14N)2[CuCl4].

3.7.Antibacterial Assay

Tables 6 and 7 detail the antibacterial properties of the extracts in terms of MICs (min-imum 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-neg-ative 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 antibacte-rial activity of (C9H14N)2[CuCl4].

Table 6. MIC of different dilutions of (C9H14N)2[CuCl4].Growth in nutrient agar containing different concentrations (C9H14N)2[CuCl4] (µgmL−1).

Name of Bacteria 12.5 25 50 100

Klebsiella pneumonia ± - - -

Escherichia coli ± - - -

Staphylococcus aureus ± - - -

All determinations were made in triplicate; ‘+’ =Growth;‘-’= No growth.

Figure 10. TGA-DTA curves for (C9H14N)2[CuCl4].

Table 6. MIC of different dilutions of (C9H14N)2[CuCl4].Growth in nutrient agar containing differentconcentrations (C9H14N)2[CuCl4] (µg mL−1).

Name of Bacteria 12.5 25 50 100

Klebsiella pneumonia ± − − −

Crystals 2022, 12, x FOR PEER REVIEW 14 of 17

Figure 10. TGA-DTA curves for (C9H14N)2[CuCl4].

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].

Table 6. MIC of different dilutions of (C9H14N)2[CuCl4].Growth in nutrient agar containing different concentrations (C9H14N)2[CuCl4] (µgmL−1).

Name of Bacteria 12.5 25 50 100

Klebsiella pneumonia ± - - -

Escherichia coli ± - - -

Staphylococcus aureus ± - - -

All determinations were made in triplicate; ‘+’ =Growth;‘-’= No growth.

Escherichia coli ± − − −

Crystals 2022, 12, x FOR PEER REVIEW 14 of 17

Figure 10. TGA-DTA curves for (C9H14N)2[CuCl4].

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].

Table 6. MIC of different dilutions of (C9H14N)2[CuCl4].Growth in nutrient agar containing different concentrations (C9H14N)2[CuCl4] (µgmL−1).

Name of Bacteria 12.5 25 50 100

Klebsiella pneumonia ± - - -

Escherichia coli ± - - -

Staphylococcus aureus ± - - -

All determinations were made in triplicate; ‘+’ =Growth;‘-’= No growth.

Staphylococcus aureus ± − − −

Crystals 2022, 12, x FOR PEER REVIEW 14 of 17

Figure 10. TGA-DTA curves for (C9H14N)2[CuCl4].

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].

Table 6. MIC of different dilutions of (C9H14N)2[CuCl4].Growth in nutrient agar containing different concentrations (C9H14N)2[CuCl4] (µgmL−1).

Name of Bacteria 12.5 25 50 100

Klebsiella pneumonia ± - - -

Escherichia coli ± - - -

Staphylococcus aureus ± - - -

All determinations were made in triplicate; ‘+’ =Growth;‘-’= No growth.

All determinations were made in triplicate; ‘+’ = Growth; ‘−’ = No growth.

Table 7. Diameters of inhibition zones produced by (C9H14N)2[CuCl4], Nalidixic acid (NA30),Novobiocin (NV-5), Norfloxacin (NOR-10), and Erythromycin (E-15).

Name of Bacteria (C9H14N)2[CuCl4](25 µg)

Nalidixic Acid(NA30)

Novobiocin(NV-5)

Norfloxacin(NOR-10)

Erythromycin(E-15)

Klebsiellapneumonia 7 5 9 10 7

Escherichia coli 7 7.3 7 2 8

Staphylococcusaureus 6 8 0 0 0

Inhibition zone included the diameter of the filter paper disc (5 mm).

Crystals 2022, 12, 254 15 of 17

4. Conclusions

In conclusion, we reported the preparation of a novel 3D, structure of 4-ter-butyl-pyridium tetrachloridocuprate, (C9H14N)2[CuCl4], with the monoclinic crystal systemPc. According to the findings of this study, the structure displays 3D networks built byC–H . . . Cl and N–H . . . Cl H-bonds.The chlorine atoms are hydrogen bond acceptors.The HS analysis confirms that the H-bonding and C–H· · ·π interactions have a significantrole in the stabilization of the crystal structure. The structural data of this compoundreflected the formation of two distorted tetrahedral geometries around Cu(II), presenting ashape between a tetrahedron and a square planar. Furthermore, the vibrational propertieswere investigated. The UV–visible absorption spectroscopy was used to measure theoptical properties, showing that this compound has semiconducting properties. Thethermal behavior reveals that this compound is stable until 176 ◦C. Finally, the antibacterialscreening assay of (C9H14N)2[CuCl4] revealed reasonable biological activity toward theGram-negative and Gram-positive bacteria.

Author Contributions: Writing original draft, M.G.A.; Writing original draft, S.H.; Investigation,A.A.A.; Project administration, K.M.A.; Resources, H.A.H.; Investigation, J.-X.M.; Validation, C.B.N.;Validation, M.H.M. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Acknowledgments:

• The authors thank Taif University Researchers Supporting Project number (TURSP-2020/272)Taif University, Taif, Saudi Arabia.

• The authors thank Researchers Supporting Project number (RSP-2021/242), King Saud Univer-sity, Riyadh, Saudi Arabia.

• We would like to thank Werner Kaminsky from the University of Washington (USA) for com-pleting the structure resolution.

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

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