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

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 a 3D network made up of C–H…Cl and N–H…Cl H-bonds, and [CuCl₄]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 (C₉H₁₄N)₂[CuCl₄] was performed by TGA-DTA. The bactericidal potency of the title compound was surveyed.

1. 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 integration of organic cations and their inorganic counterparts. More specifically, halogen-based cuprate hybrid compounds have received much attention, due to their structural flexibility and fascinating features, in fields such as magnetism, electrics, ferroelectrics, photoluminescence, biology, and photocatalysis [9,10,11,12]. Concerning the organic ligand, 4-tert-butylpyridine (4-TBP) is frequently employed to enhance photovoltaic performance in solar cells. Indeed, Yang et al. demonstrated the mechanism through which 4-tert-butylpyridine (4-TBP) influences the photovoltaic performance additively with changed concentrations [13]. Additionally, it was discovered that the preponderance of the materials used to transport holes in n-i-p perovskite solar cells include (4-TBP). This additive appears to be necessary for excellent power conversion efficiency and stable-state performance [14,15,16,17].

However, recent research in biology has demonstrated that certain tetrahalogenocuprates with substituted pyridinium cations have significant gastroprotective and antiepileptic efficacy [18,19,20,21,22,23,24]. As a part of our research into different types of hybrid materials, we report here the crystallographic description, spectroscopic characterization, and thermal behaviour, including the biological activities of the (C₉H₁₄N)₂[CuCl₄].

2. Materials and Methods

2.1. 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.

2.2. 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 X-ray Powder Diffraction (XRD) system from Siemens D5000, equipped with a Cu anticathode (CuK_α, 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 MoK_α 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 F² [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. 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 parameters	a = 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

. 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 600 K (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 Gram-negative, 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 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 applied to the three bacterial strain plates. Prior to incubation at 37 °C for 24 h, 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.

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 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.

3.2. Structure Description

The title compound’s asymmetric unit is composed of two crystallographically independent 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] 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 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 neighboring 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.

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) 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. [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)

). These results are analogous to those found in other Cu(II) tetrahedral complexes [37,38,39,40,41].

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 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.

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. 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)	143

Symmetry 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.

) (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 = 1/4 and 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.

The basic geometrical properties of the (C₉H₁₄N)⁺ organic cations are gathered in

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)
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)

. The values are compatible with those reported in the literature [42,43,44,45]. The pyridinium cation always has an expanded C–N–C angle compared to the parent pyridine. As reported in many structures [45,46,47], 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 pyridine forms. Figure 6 represents all the contact between the organic cations in the atomic arrangement [48,49,50]. 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.

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 [51]. 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 [52]. With an EXY greater than one, favoured contacts are more likely to make contacts, while element pairings with an E_XY < 1 are more likely to not form contacts (

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 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 d_norm 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, Figure 7b 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), 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 E_H...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 (E_C...H = 1.23), the C...H/H...C contact, attributed to the C–H…π interactions, is the third most prominent surface interaction, accounting for 10% of the Hirshfeld surface (Figure 7e). This demonstrates 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 surface areas, they are preferred and show enrichment values of E_N...H = 1.39 and E_Cu...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 E_C...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.

3.4. 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 [53,54]. 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] [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–d transition in the orbits of the metal Cu²⁺.

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 [58].

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 °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 (ΔH = 820.525 J g⁻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

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

Name of Bacteria	12.5	25	50	100
Klebsiella pneumonia	±	−	−	−
Escherichia coli	±	−	−	−
Staphylococcus aureus	±	−	−	−

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

and

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)
Klebsiella pneumonia	7	5	9	10	7
Escherichia coli	7	7.3	7	2	8
Staphylococcus aureus	6	8	0	0	0

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

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 µg mL⁻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].

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 system Pc. According to the findings of this study, the structure displays 3D networks built by C–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 significant role in the stabilization of the crystal structure. The structural data of this compound reflected the formation of two distorted tetrahedral geometries around Cu(II), presenting a shape between a tetrahedron and a square planar. Furthermore, the vibrational properties were investigated. The UV–visible absorption spectroscopy was used to measure the optical properties, showing that this compound has semiconducting properties. The thermal behavior reveals that this compound is stable until 176 °C. Finally, the antibacterial screening assay of (C₉H₁₄N)₂[CuCl₄] revealed reasonable biological activity toward the Gram-negative and Gram-positive bacteria.