Schiff base diethyl 4,4-(pentane-2,4-diylidenebis(azanylylidene))benzoate (1) as a new ligand (L) was prepared by the reaction of acetylacetone with benzocaine in the ratio of 1 : 1. Two transition-metal complexes, [Ni(II)(LCl(HOEt))] (2) and [Zn(II)(LCl(HOEt))] (3), have been synthesized from metal salts with didentate Schiff base ligand (L) and characterized by elemental analyses, FT-IR, 1H NMR, 13C NMR UV-Vis spectroscopy, and magnetic susceptibility. The biological activity of the complexes was studied. In addition, the M06-2x density function theory method and the 6-31G(d) basic set were applied to determine the optimized structures of 13 and to determine their IR and 1H NMR, 13C NMR spectra theoretically. The data are in good agreement with the experimental results. The geometries of complexes 2 and 3 were determined to be square-planar for 2 and tetrahedral for 3.

1. Introduction

The β-diketiminate ligands generally known as “nacnac”, or [{ArNC(R)}2CH]- (where Ar = aryl and R = Me or another organic group), have emerged as popular ligands among other ancillary supports, on account of their strong binding to metals; their tunable, steric, and electronic effects; and their diversity in bonding modes [13]. The as “nacnac” ligand skeleton is analogous to the “acac” (acetylacetonate) ligand, but the oxygen atoms are replaced with nitrogen-based moieties such as NR (R = alkyl, silyl, Ar) (Scheme 1). As a result, steric protection at the metal center is provided by the substituent at the nitrogen donor atom.

The first documented cases of β-diketiminate metal complexes were reported by McGeachin 1968 [4]. Till date, the N-aryl substituted “nacnac” ligands [HN(Ar)C(Me)CHC(Me)N(Ar)] [5] and [HN(Ar)C(tBu)CHC(tBu)N(Ar)] [6] showed to be the best for stabilization of low coordinate metal sites.

These compounds are used as ligands, for example, in synthesis of heterocycles [7]. Diimines can most likely be prepared by the condensation of a dialdehyde or diketone with the respective whereby water is eliminated [8]. Diimines, as 1,2-diimine (α-diimine) [9] or 1,3-diimine (β-diimine) [10, 11], are applicable as ligands in the synthesis of a high variety of coordination complexes featuring diverse transition metals [1]. In this respect, for example, (α-diimine) ruthenium complexes can be applied in diverse areas such as solar energy conversion, sensor technology, homogeneous catalysis, biomedical research, supramolecular chemistry, and molecular electronics [12]. The β-diketiminate zinc complexes were used as catalysts for intramolecular hydroamination [13]. In addition, β-diketiminate anions were applied in lanthanide organometallic and metal-organic chemistry as they are easily accessible and show tunable steric and electronic effects [5, 14]. Tris-β-diketiminate ytterbium complexes with various β-ketiminato ligands were examined for their catalytic activity in the ring-opening polymerization of caprolactone and lactide [15] and in the addition of amines to carbodiimides, revealing that the catalytic activity of these coordination complexes is greatly affected by the steric bulk of the β-diketiminato ligands of which the bulkiest was found to be the most active one [16, 17].

Transition-metal complexes supported by β-diketiminato or β-ketiminato ligands have received increasing attention [18]. The versatile electronic properties and steric demands of β-diketiminates or β-ketiminates can be adjusted by variation of the substituents in the ligand backbone or at the nitrogen atoms to give access to transition-metal compounds that can exhibit unusual geometries and/or low coordination numbers [19]. α-Diimine nickel complexes were highly active in ethylene polymerization. For these complexes, the catalytic activities increased with polymerization temperatures and the highest activity was observed at 100°C, and these complexes represent one of the most active and thermally stable catalysts in ethylene polymerization [20]. α-Diimine Ni(II) catalysts gave poly(methyl methacrylate) with high molecular weight and narrow molecular weight distribution [21, 22].

This paper describes the synthesis of Schiff base “diethyl 4,4-(pentane-2,4- diylidenebis (azanylylidene)) benzoate” (1) and its complexation with Ni(II) (2) and Zn(II) (3), characterized by elemental analyses, FT-IR, 1H NMR, 13C NMR UV-Vis spectroscopy and magnetic susceptibility. The Gaussian 09 [23] suite of programs was used to perform geometrical optimizations for the ligand, tautomer, and complexes of Ni(II), Zn(II). All calculations including 1H NMR, 13C NMR, IR, free energy (ΔG), E(thermal), CV and entropy (S), and molecular geometry optimization were carried out at the M06-2x/6-31G(d) level of theory [23].

2. Results and Discussion

Diethyl4,4-(pentane-2,4-diylidenebis(azanylylidene)) benzoate (1) was prepared by the synthetic methodology described by Feldman and coworkers in good yield [24] (Scheme 1). Condensation of 2,4-pentanedione with benzocaine in the presence of HCl in boiling ethanol afforded 1 HCl, which upon neutralization with Na2CO3 gave the free ligand 1 as a pale orange solid (Scheme 1). Compound 1 is stable in the normal conditions and to complexation with metal ions; however, it is advisable to store 1 under an atmosphere of inert gas.

The appropriate metal complexes of Ni(II) (2) and Zn(II) (3) were obtained by the reaction sequence shown in Scheme 2. β-Diketimine 1 was converted to the respective Na salt 1-Na by treatment of 1 with sodium acetate in ethanol in the ratio of 1 : 1. Consecutive dropwise addition of this solution to equimolecular amounts of the corresponding anhydrous metal salts MCl2 (M = Ni, Zn) dissolved in ethanol at 60°C for 4 h gave the respective transition metal complexes 2 and 3 (Scheme 2). After appropriate work-up, complex 2 was isolated as green and 3 as colorless solid in high yields. Compounds 13 were characterized by IR, 1H NMR, and 13C NMR spectroscopy, elemental analysis and their melting points (Scheme 3).

3. Characterization

3.1. 1H NMR-Spectra

From the 1H NMR spectra of compound 1, the peak at 4.95 ppm is due to the (N=CCHCNH) proton of the carbon atom between the diimine groups while these protons in acetylacetone between the two carbonyl groups appear at 3.91 ppm. The peak at 1.98 ppm arises by the 6 protons of the two N=C-CH3 segments while these protons in acetylaceton appear at 2.25 ppm. A singlet peak which appears at 12.61 ppm is assigned for the NH tautomer (25% based on the signal integration). Compound 1 is formed as a pale orange solid and has low melting point 155°C. In the complexes, the signals underwent small changes up to 0.11–0.18 ppm, which is attributed to the increased charged delocalization upon complexation. On the other hand, the singlet peak at 12.61 ppm disappeared indicating deprotonation and coordination of the nitrogen with the metal ion and quartet peak at 4.1 ppm in the spectra of complexes for 2H in CH3CH2OH and singlet peak at 3.5 ppm for proton of OH group and triplet peak at 1.1 ppm for 3H of methyl group in ethanol. These peaks did not appear in the spectra of ligand; this means the ethanol molecule is in coordination with metal ions. By comparing experimental data of 1H NMR with theoretical data for ligands and complexes, we observe that they are identical to each other, and this indicates that the theoretical data is acceptable (Table 1).

3.2. 13C NMR-Spectra

The 13C NMR spectra of the free ligand were observed in CDCl3. The peak at 166.3 ppm is due to the carbon atom of the imine group (C=N), and the peak at 153.6 ppm is due to the carbon atom (=CNH) and that at 58.9 ppm is due to the carbon atom in C=CNH (tautomer). Downfield shifting is noticed of the imine group (C=N) and C-NH from 166.3 and 153.6 ppm in the free ligand to 168 and 169 ppm in the case of Zn(II) and Ni(II) complexes, respectively. By comparing the experimental data of the 13C NMR with the theoretical data for ligand and complexes, the theoretical data (Table 1) are comparable to the experimental data here, too.

3.3. FT-IR and UV-Vis Spectra

The spectroscopic data for ligand and metal complexes in Table 2 are in good agreement with the expected values. The FT-IR spectra of two complexes compared with those of the ligands indicate that the υ(C=N) band at 11629 cm−1 is shifted to lower frequency by ∼10 and 20 cm−1 in the complexes, indicating that the ligands are coordinated to the metal ions through the nitrogen atom: υ(N-H) at 2240 cm−1, υ(N=C) at 1629 cm−1, υ(N-C(Me)) at 2998 cm−1, and υ(NC(Ar)) at 2120 cm−1. New bands were observed only in the spectra of the transition metal complexes at 512 and 519 cm−1 and not in the ligand, which are due to the nitrogen-metal stretching vibrations. Therefore, based on the FT-IR data, Schiff base ligand connects to metal as bidentate. By comparing the experimental data with the theoretical data, we find that they are identical to each other, and this indicates that the theoretical data is acceptable. As a conclusion, the theoretical data of the ligand and its metal complexes confirm the coordination of the ligand to the corresponding metal ion bidentately through β-ketiminato functionality.

The electronic spectra of Schiff base ligand and Ni(II) complexes was recorded in CH2Cl2 (Figure 1). As seen, the Schiff base ligand shows a strong band at 381 nm which can be associated to nπ transition of the azomethine chromophore. This band disappears in complexes after bonding Schiff base ligand to metal center [25]. All the bands in the 200–300 nm region are attributed to the ππ transitions of the aromatic rings and the azomethine group. The ligand bands shift to longer wavelengths in the metal complexes as compared to their position in the free ligand which indicates the bond between Schiff base and metal center [26]. Complexes (2) shows absorption in the region 500–530 nm assigned to 1A1g to 1B1g transition and 473 nm assigned to 1A1g to 1B2g and absorption in the region 243–276 nm assigned to intraligand charge transfer band, which again suggests square-planar geometry. Generally, Zn(II) complexes do not exhibit any d-d electronic transition due to its completely filled d10 electronic configuration but often exhibit charge transfer spectra. The Zn(II) complex shows an absorption band at 414 nm attributed to the (L to M) charge transfer transition, which is compatible with this complex having a tetrahedral geometry [27].

3.4. Magnetic Susceptibility Measurements

The magnetic moments of Zn(II) complex is zero; its diamagnetic properties and Ni(II) complex are zero (diamagnetic), which indicates that the complex has square-planar structure.

3.5. Geometry Optimization

The M06-2x/6-31G(d) level of theory was used for structure optimization of 13. Figures 24 show the optimization geometry of the β-diketimine ligand 1, the β-diketimine tautomer (Figure 2) and coordination complexes 2 and 3 (Figures 3 and 4). The bond lengths and bond angles for 13 are summarized in Tables 3 and 4. The corresponding thermodynamic parameters (free energy (ΔG), E (thermal), CV, and entropy (S)) are presented in Table 5.

The optimized molecular structure of the Ni(II) β-diketimine complex 2 is depicted in Figure 3, the bond lengths and angles are given in Table 4. The bond distances of the (Ni-N), (Ni-Cl), and (Ni-O) bonds were calculated to (1.867 Å), (2.226 Å), and (1.845 Å), respectively, and experimentally the distance range of (Ni-N) bond is (1.860–1.857 A°) [28, 29], (Ni-Cl) bond is (2.196 A°) [30], and (Ni-O) bond is (1.844–1.851 A°) [28, 29]. The respective bond angles of 2 were found to be 88.05° for (Cl-Ni-O), 93.50° for (5N-Ni-6N), 173.52° for (5N-Ni-O), 89.88° for (6N-Ni-O), 96.06° for (Cl-Ni-5N), and 172.04° for (Cl-Ni-6N). The calculated results are in good agreement with experimental data characteristic for Ni(II) Schiff base complexes [2830]. The obtained structural values confirm the anticipated square-planer arrangement of 2.

The optimized molecular structure of the appropriate Zn(II) β-diketimine complex 3 is shown in Figure 4, the bond lengths and angles are summarized in Table 4. The bond distances of the (Zn-N), (Zn-Cl), and (Zn-O) bonds were calculated to (1.979 Å–1.998 Å), (2.228 Å), and (2.119 Å), respectively, and experimentally the distance range of (Zn-N) is (2.165–2.247 Å) [31], (Zn-Cl) is (2.333 Å) [32], and (Zn-O) is (2.064–2.277 Å) [31]. The bond angles of (Cl-Zn-O), (6N-Zn-7N), (6N-Zn-O), (7N-Zn-O), (Cl-Zn-6N), and (Cl-Zn-7N) are 102.2°, 99.0°, 96.3°, 98.7°, 102.1°, and 100.8°, respectively. The found bond distances and angles agree well with the ones typical for similar Zn(II) Schiff base complexes [32, 33]. The overall structure of 3 is tetrahedral.

4. Conclusion

In conclusion, we have synthesized compound 1 (new ligand) and two complexes with ZnII and NiII metal ions and characterized the ligand and the complexes by 13C NMR, 1H NMR, IR, and elemental analysis and determined the optimized structures and determined the 13C NMR, 1H NMR, and IR values by the M062x/6-31G(d) level of theory. The theory confirms that the structure of Zn(II) complex is tetrahedral and square planar for the Ni(II) complex. It was found that they have no effect on the growth of any microorganism taken.

5. Experimental Section

All chemicals were purchased from Aldrich and used as received. NMR spectra were recorded on Bruker 300 MHz and AC 75 MHz spectrometer with CDCl3 as a solvent and standard. IR spectra were recorded as KBr disks with a Matsson 5000-FT-IR spectrophotometer within the range of 4000–500 cm−1. The electronic spectra were obtained on a Shimadzu UV-1601 spectrophotometer. Elemental analyses were carried out on Thermo Scientific OEA Flash 2000 Analyzer. Magnetic susceptibility measurements of the complexes in the solid state were determined by a Gouy balance at room temperature using mercury(II) tetrathiocyanatocobaltate(II) as the calibrant.

5.1. Synthesis of the Diethyl 4,4-(Pentane-2,4-diylidenebis(azanylylidene)) Benzoate Compound 1

A mixture of benzocaine (1.65 g, 0.01 mol) and 2,4-pentanedione (1.00 g, 0.010 mol) was heated at 60°C in the presence of HCl and ethanol for 4 h. Then the mixture was neutralized by addition of Na2CO3 in methanol. The title compound 1 was obtained as a pale orange solid after recrystallization from ethylacetat: n-hexane (3 : 7) (2.84 g, 7.16 mmol) based on (72% yield).

Mp.: 155°C. 1H NMR (CDCl3, 300 MHz) δ 1.98 (s, 6H) 2CH3C = N, 12.61(s, 1H, NH, tautomer) 4.95 (s, H, CH) N=CCH=CNH, (7.38 (d, 4H), 8.07(d, 4H)) (benzene rings), 4.3 (q, 4H) CH2CO, 1.34–1.45 (t, 6H) 2CH3 CH2O. 13C NMR (CDCl3, 75 MHz) δ 153.6, 166.3, 61.0, 48, 153, 156, 122–135, 24, 16. FT-IR (KBr, cm−1): 2240.79, 1629, 2998.66, 1667, 2122, and 2120. UV-vis in CH2Cl2, λ, (nm): 401, 352, 268, 235. Anal. C23H26N2O4, calcd. C 70.03, H 6.64, N 7.10, found C 69.84, H 6.52, N 6.98%.

5.2. Synthesis of the Ni(II) β-Diketimine Complex 2

Compound 1 (3.94 g, 0.01 mol) and 1.297 g, 0.01 mol of anhydrous NiCl2 were mixed dropwise in presence of sodium acetate in ethanol and refluxed for 4 hours. A green solid was formed (4.16 g, 7.8 mmol) based on 78% yield. Mp: 239°C. 1H NMR (CDCl3, 300 MHz) δ 7.25–8.4(benzene rings), 4.8 (q, 4H) 2(-CH2OCO), 5.2 (s, 1H), 4.15 (q, 2H) –OCH2CH3, 1.1(t, 3H) –OCH2CH3, 3.51 (s, 1H) OH group. 13C NMR (CDCl3, 75 MHz) δ 25, 20, 71, 63, 153, 125–137, 169, 158. UV-vis in CH2Cl2, λ, (nm): 528, 473, 276, 243. Anal. C25H31ClN2O5Ni (523.31): calcd (C 56.26; H 5.85; Cl 6.64; Ni 11.00; N 5.25), found (C 56.39; H 5.68; Cl 6.29; Ni 11.27; N 5.19). FT-IR (KBr, cm−1): 1619, 1723, 2093, and 512. The magnetic moment of Ni(II) complex is zero which indicates that the complex has square-planar structure.

5.3. Synthesis of the Zn(II) β-Diketimine Complex 3

3.94 g, 0.01 mol of compound 1 and 1.36 g, 0.01 mole of ZnCl2 anhydrous were mixed dropwise in presence sodium acetate in ethanol and refluxed for 4 hours, a white diamagnetic solid was formed (3.44 g, 6.4 mmol) based on 64% yield. Mp: 287°C. 1H NMR (CDCl3, 300 MHz) δ 7.3–8.2(benzene rings), 4.72 (q, 4H) 2(-CH2OCO), 5.1 (s, 1H), 4.1 (q, 2H) –OCH2CH3, 1.1(t, 3H) –OCH2CH3, 3.45 (s, 1H) OH group. 13C NMR (CDCl3, 75 MHz) δ 24, 18, 70, 61, 154, 124–135, 168, 157. Anal. C25H31ClN2O5Zn (538.12): calcd (C 55.57; H 5.78; Cl 6.56; N 5.18; Zn 12.10), found (C 55.76; H 5.44; Cl 6.88; N 5.42; Zn 12.26). FT-IR (KBr, cm−1): 1612, 1745 and 528.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.