Synthesis and Characterization of β-Diketimine Schiff Base Complexes with Ni(II) and Zn(II) Ions: Experimental and Theoretical Study

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 1–3 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.


Introduction
e 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 [1][2][3].
e as "nacnac" ligand skeleton is analogous to the "acac" (acetylacetonate) ligand, but the oxygen atoms are replaced with nitrogenbased 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.
ese 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].e 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].Trisβ-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].e 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].

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 Na 2 CO 3 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.
e 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 MCl 2 (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 1-3 were characterized by IR, 1 H NMR, and 13 C NMR spectroscopy, elemental analysis and their melting points (Scheme 3).

1 H NMR-Spectra.
From the 1 H 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.e 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 Scheme 2: Preparation of 3-diketimine and its tautomer.
2 Journal of Chemistry formed as a pale orange solid and has low melting point 155 °C.In the complexes, the signals underwent small changes up to 0.11-0.18ppm, 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  Journal of Chemistry with the metal ion and quartet peak at 4.1 ppm in the spectra of complexes for 2H in CH 3 CH 2 OH 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.ese 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 1 H 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).

13 C NMR-Spectra.
e 13 C NMR spectra of the free ligand were observed in CDCl 3 .e 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 13 C NMR with the theoretical data for ligand and complexes, the theoretical data (Table 1) are comparable to the experimental data here, too.

FT-IR and UV-Vis Spectra.
e spectroscopic data for ligand and metal complexes in Table 2 are in good agreement with the expected values.e 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.erefore, 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.
e electronic spectra of Schiff base ligand and Ni(II) complexes was recorded in CH 2 Cl 2 (Figure 1).As seen, the Schiff base ligand shows a strong band at 381 nm which can be associated to n ⟶ π transition of the azomethine chromophore.
is 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 π ⟶ π * transitions of the aromatic rings and the azomethine group.
e 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 1 A 1g to 1 B 1g transition and 473 nm assigned to 1 A 1g to 1 B 2g and absorption in the region

Magnetic Susceptibility Measurements.
e 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.
e obtained structural values confirm the anticipated square-planer arrangement of 2.

Conclusion
In conclusion, we have synthesized compound 1 (new ligand) and two complexes with Zn II and Ni II metal ions and characterized the ligand and the complexes by 13 C NMR, 1 H NMR, IR, and elemental analysis and determined the optimized structures and determined the 13 C NMR, 1 H NMR,

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 CDCl 3 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 .e electronic spectra were obtained on a Shimadzu UV-1601 spectrophotometer.Elemental analyses were carried out on ermo 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.e magnetic moment of Ni(II) complex is zero which indicates that the complex has square-planar structure.

Scheme 3 :
Scheme 3: Preparation of complexes of metal ions Ni(II) and Zn(II).

Figure 1 :
Figure 1: e electronic spectra of the free β-diketimine Schiff base ligand and the related Ni(II) complexes in CH 2 Cl 2 solution.

Figure 2 :
Figure 2: Structure of the model compound used to evaluate M06-2x/6-31G(d) for the optimized structure of the β-diketimine Schiff base ligand 1 and its tautomeric equilibrium.

Table 1 :
1H and13C NMR theoretical and experimental values for the structures of 1 & 2 (in ppm).

Table 3 :
Selected bond lengths and bond angles calculated at the M062x method with the 6-31G(d) basis set for 1 and its tautomer.

Table 4 :
Selected bond lengths and bond angles calculated at the M062x method with the 6-31G(d) basis set for complexes 2 and 3.