Spectral, Structural, and Antibacterial Study of Copper(II) Complex with N 2 O 2 Donor Schiff Base Ligand and Its Usage in Preparation of CuO Nanoparticles

bis(methanylylidene))bis(2-methoxyphenol)), through the reaction of ligand H 3 L with Cu(NO 3 ) 2 3H 2 O, in the ratio of 2 :1 in methanol solvent was prepared. The obtained ligand (H 3 L) was characterized by FT-IR, 13 C NMR, 1 H NMR and elemental analyses. Then its copper(II) complex was prepared and characterized by FT-IR spectroscopy, thermal studies, elemental analyses and single crystal X-ray di ﬀ raction. The X-ray crystallography revealed that the two H 3 L ligands in bidentate fashion coordinated to one copper center for producing Cu(H 2 L) 2 complex. We used copper(II) Schi ﬀ base complex, Cu(H 2 L) 2 , for the preparation of CuO nanoparticles via solid-state thermal decomposition. The crystalline structure of the product was studied by X-ray powder di ﬀ raction (XRD) and scanning electron microscopy (SEM). XRD indicated that the new product was copper oxide. SEM image showed that the size of CuO nanoparticles was between 46 and 53nm, and they had uniform shape. The antibacterial properties of the complex and ligand were also investigated. The results revealed that Schi ﬀ base complex showed higher biological activity than Schi ﬀ base ligand.


Introduction
Schiff base complexes have attracted many attentions in coordination chemistry because of their interesting applications, structures, and properties [1][2][3][4]. These complexes have been highlighted in coordination chemistry where they are used as catalysts for organic reactions and they also have interesting magnetic properties [5][6][7]. Moreover, metal complexes of Schiff bases received great importance in biology for their widespread applications [8,9]. In biological systems, the Schiff bases characterized by an imine group (N=CH) help to explain the process of Schiff transamination and racemization reaction [10]. Tetradentate Schiff base ligands with a N 2 O 2 donor atom are popular for coordinating with different metal ions [11][12][13]. These Schiff base complexes have attracted more attentions since they can be used as antibacterial, antifungal, and antitumor agents [14][15][16][17][18]. Because of the applications of Schiff base complexes in the preparation of CuO nanoparticles, they have received considerable attention recently [19]. The synthesis and characterization of CuO nanoparticles through thermal decomposition of complexes have been widely studied because of their unique properties and applications. There are various methods for the preparation of CuO nanoparticles such as electrochemical, hydrothermal, and thermal decomposition and evaporation. Among them, thermal decomposition is a good choice because it can control process conditions and particle crystal structure, size, and purity [20][21][22][23][24][25]. Studies on thermal decomposition of metal complex for preparation of metal oxide nanoparticles indicated its superiority over other conventional methods since it is economical, rapid, and controllable [26,27]. In the present study, we have synthesized 6,6 ′ -((1E,1 ′ E)-((azanediylbis(ethane-2,1diyl))bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol) (H 3 L) Schiff base ligand and used it for the preparation of new Cu(H 2 L) 2 complex (Scheme 1). We have also prepared a CuO nanoparticles by thermal decomposition of Cu(II) Schiff base complex, Cu(H 2 L) 2 . Antibacterial activities of Schiff base ligand and complex were tested against the Escherichia coli, Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa.

Materials and Physical
Measurements. In agreement with the published methods, the methanol was dried [28].
All materials and solvents for synthesis and analysis were commercially available. 1 H NMR and 13 C NMR spectra were collected using a Bruker Avance 400 spectrometer. The FT-IR spectra were recorded on Shimadzu (IR-60 model spectrometer) in KBr disk. Elemental analysis was carried out on a Perkin Heraeus CHN-ORAPID apparatus. Scanning electron microscopy (SEM) image was recorded on LEO 1430VP SEM.

Preparation of CuO Nanoparticles. Crucible containing
Cu(H 2 L) 2 complex was put in an oven and heated at rate of 10°C/min in air. After 3 h, CuO nanoparticles were formed at 500°C. The product was washed with methanol and dried. FT-IR and SEM techniques were used for characterization of CuO nanoparticles.
2.5. Antibacterial Activity Determination. The antibacterial activity of ligand and complex was performed for four bacteria: Escherichia coli, Acinetobacter baumannii, Klebsiella pneumoniae, and Pseudomonas aeruginosa by using disc method [29]. The bacterial culture (10 4 -10 6 CFU/mL) was swabbed onto Muller Hinton agar plates. From each compound, 1 mg/mL in DMSO was loaded in to each disc (6 mm diameter) and then was put on the Muller Hinton agar plates. The plates were incubated at 37°C for 24 h, for determination of antibacterial activity of ligand and complex, the inhibition zone (in mm) was measured, and the results were compared with the standard drug tetracycline. Also, the MIC (minimal inhibitory concentration) and MBC (minimum bactericidal concentration) of the ligand and complex were determined [30]. MIC and MBC concentrations were recorded 24 h of incubation at 35°C.

Bond lengths (Å)
Bond angles (°) Dihedral angles (°)  Journal of Chemistry diffractometer. For Lorentz and polarization effects, data were improved but not for absorption. By a least-squares fit of 719 reflections of the highest intensity, the exact unitcell parameters were investigated. After solving the structure with SHELXT [31], it was refined with the full-matrix leastsquares method on F 2 by SHELXL97 [32]. By using ORTEX, the operations of SHELX were automated [33]. The incorporated scattering factors in SHELXL97 were employed. Anisotropically, the nonhydrogen atoms were purified. After putting the hydrogen atoms in idealized positions, they were refined as rigid groups with their Uiso's as 1.2 times Ueq of the proper carrier atom. The function ΣwðjF 0 j 2 − jF C j 2 Þ 2 was minimized, with W −1 = ½σ 2 ðF 0 Þ 2 + ð0:2000:PÞ 2 ðP = ½ max ðF 0 2 Þ + 2F C 2 /3Þ. , respectively [34]. Furthermore, a singlet peak at 8.29 ppm in the 1 H NMR spectrum of H 3 L was observed which was related to the HC=N protons. From the reaction of copper(II) nitrate salt with H 3 L ligand, Cu(H 2 L) 2 complex was produced. The ratio of the metal to ligand in complex was 1 : 2. Considering X-ray and elemental analysis, copper complex was formulated as Cu(H 2 L) 2 . The comparison of the FT-IR spectra of H 3 L and Cu(H 2 L) 2 showed distinct changes in the place of C=N and C-O stretching frequencies, which demonstrated that the ligand was coordinated to the metal ion [35]. In metal complex FT-IR, C=N vibrational frequency blue shifts by 16 cm -1 showing that imine nitrogen was coordinated.

Thermal Studies.
Under N 2 atmosphere, thermogravimetric analysis (TGA) of the Cu(H 2 L) 2 was studied with slowly increasing of the temperature (20°C per min) until 600°C (Figure 1). In the first stage, the curve showed the thermogram for Cu(H 2 L) 2 complex demonstrating that little loss of water in the lattice takes place in a range of 85°C-125°C with a mass change of 2.71%. In the next stage, the complex indicated much loss of 81.39% in the range from 125 to 568°C, and it is in agreement with the ligand decomposition and copper (II) oxide formation with a residual mass of 15.9% at 600°C.

Crystal Structure of Cu(H 2 L) 2 .
Complex Cu(H 2 L) 2 crystallizes in the triclinic space group Pī. Table 1 indicates the details of the X-ray analysis and crystal data for Cu(H 2 L) 2 . Table 2 also shows selected angles and bond lengths, and the crystal structure of Cu(H 2 L) 2 is illustrated in Figure 2.
In the crystal structure of Cu(H 2 L) 2 , two ligand H 3 L acts as a bis(bidentate) ligands which are connected to one Cu(II) atom. From the two different ligands, nitrogen atoms N(1) and N(6) and also oxygen atoms O(3) and O(4) are coordinated to Cu(II) atom. In the coordination environment in which metal center has a seesaw (CuN 2 O 2 ), the geometrical parameter τ 4 is defined as ½360 − ðα + βÞ/141, where α and β are the two largest coordination angles [36].   [36][37][38][39][40][41][42]. A geometrical analysis has been done on pipi stacking in Cu(H 2 L) 2 complex. In the crystal structure of complex, there are two types of pi-pi interactions between phenyl rings (Cg1…Cg2 and Cg3…Cg4). In the crystal lattice of Cu(H 2 L) 2 complex, linking of N3H1…O5 (2.482 Å), N3H3…O7 (2.307 Å), and C29H29…O8 (2.510 Å) hydrogen bonds led to forming S (20), S (6), and S(17) motifs ( Figure 3) [43]. The centroid-centroid distances are 3.792 and 3.958 Å for Cg1…Cg2 and Cg3…Cg4, respectively ( Figure 4). The interaction of phenyl rings in this structure is in a parallel displaced mode, and the noncovalent pi-pi interaction has an important effect on the orientation of the ligand during the crystallization process.
3.5. Hirshfeld Surface Analysis. One of the significant and complementary procedures for crystal development pattern by visualization and comparing ratio of the intermolecular interactions is Hirshfeld surface analysis (HS) [44,45]. For the complex, HS and fingerprint plots (FPs) were prepared by using Crystal Explorer 3.1 according to Crystallographic Information Files [46]. By applying a standard surface resolution with a fixed color scale of -0.172 to 2.698 Å, the HS is mapped over d norm . The 3D d norm surface of the complex is indicated in Figure 5(a). The prominent red areas on the d norm Hirshfeld surfaces of complex around the metal ion are due to the significant intermolecular N-H…C interactions. The shape index of the HS is a means for visualizing the pi-pi stacking by the presence of adjacent red and blue triangles. As illustrated in Figure 5(b), pi-pi interactions are observed between adjacent molecules in the complex. The curvedness plan ( Figure 5(c)) of the complex also indicates the big green area and blue region. The first shows a relatively planar surface area, while the last represents areas of curvature. The planar regions around the rings on the HS mapped over curvedness indicate the presence of pi-pi stacking interactions. Figure 6 demonstrates the contribution percentage of the different interactions to the HS area. Figure 6 shows that all contribution contacts of H atoms with N and O atoms are 1.6 and 1.4, respectively. In the complex, the molecules are held together mainly through C•••H and H•••H contacts (Figure 7). Figure 8 illustrates fingerprint plots (2D representation) of HS for Cu(H 2 L) 2 complex. According to the fingerprint plots, percentage contributions of intermolecular interactions are remarkably noticeable in H•••H and C•••H contacts. All in all, the interactions of these two groups form nearly 92% of the HS.     3.7. Antibacterial Activity. The antibacterial activity of ligand (H 3 L) and Cu(II) complex were tested against E. coli, A. baumannii, K. pneumoniae, and P. aeruginosa by making disc diffusion method ( Figure 12). Findings of antibacterial activity studies are reported in  [48,49]. The increasing of the antibacterial activity of the complex comparing to ligand can be justified by Tweedy's theory and Overtone's concept [50]. Cell permeability concept of Overtone claims that liposolubility is a significant factor that governs the activity of antibacterial agents. By chelation, the polarity of the metal ions is decreased because of the overlap of the ligand orbitals and partial sharing of positive charge of metal atom with donor groups. Furthermore, it enhances the delocalization of pielectrons over the whole chelate ring and increases the lipophilicity of the complexes, and this leads to the increasing of the penetration of the complexes into the lipid membrane and increases their antibacterial activities. Remarkably, Cu(H 2 L) 2 complex exposes higher antibacterial activity on A. baumannii. Also, the MIC and MBC of the ligand and complex are measured, and the results are presented in Table 4.

Conclusion
The compounds were synthesized in good yields. The calculated τ 4 index showed that the copper complex is distorted square planar. In the crystal lattice of Cu(H 2 L) 2 complex, linking of N3H1…O5 (2.482 Å), N3H3…O7 (2.307 Å), and C29H29…O8 (2.510 Å) hydrogen bonds led to forming S (20), S (6), and S(17) motifs. In the crystal structure, the pi-pi interactions between phenyl rings at adjacent molecules were remarkable. By thermal decomposition of Cu(II)

Journal of Chemistry
Schiff base complex, CuO nanoparticles with the size of 46-53 nm were synthesized. The Schiff base ligand and complex were screened for their antibacterial activity against Gramnegative bacteria. The zones of inhibition were determined. Schiff base complex showed potent antibacterial activity than Schiff base ligand.

Data Availability
The [DATA TYPE] data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.