Synthesis and Thermogravimetric Behavior of Ni(II), Cu(II) and Zn(II) Complexes of Triazine-Salicyldamine Schiff Bases

Synthesis, characterization and thermogravimetric behavior of Ni(II), Cu(II) and Zn(II) complexes of Schiff bases derived from 4-phenyl-1,3,5-triazine-2,6-diamine and 5-chlorosalicylaldehyde (H2L1), 5-bromosalicylaldehyde (H2L2) have been reported. The ligands were characterized by FT-IR, H and C NMR, UV-Vis spectroscopy as well as elemental analysis. Spectral studies revealed that the ligands were acting as tetradentate chelating agents and coordinated to the metal center via deprotonated phenolate oxygen and azomethine nitrogen atoms in a 1:1 ligand to metal ratio. Thermal behavior of the complexes has been evaluated using TGA. All complexes showed similar modes of three steps weight loss upon heating to 800 C with gradual loss of organic and inorganic parts. The residues after heating corresponds to metal oxides, with copper complexes seemed to be the most stable and can be used in high-temperature catalytic cycles.

INTRODUCTION: Schiff bases are important class of organic ligands. They play an essential role in the development of coordination chemistry as they readily form stable complexes with most of the transition elements exhibiting different coordination modes and functionalities [1][2][3] . They also have the privilege of being easy to prepare, stable at ambient conditions, and do not require any especial considerations in preservation and handling 4 . Furthermore, their properties can be tuned by choosing appropriate substituents, and can stabilize many different metals in various oxidation states 5 -7 . Throughout the last decades, many researchers showed that Schiff base metal complexes have potential applications as antibacterial, anticancer, antioxidant and antiviral agents 8 -11 . They also exhibited catalytic properties in homogeneous and heterogeneous catalysis 12,13 . Numerous compounds containing 1,2,4 triazine moiety are well known in natural materials and show interesting biological applications [14][15][16] . In addition, it is reported that salicyldehyde derivatives with one or more halogen atoms in the aromatic ring reveal a variety of biological activities comprising antibacterial and antioxidant 17 . In a previous work, synthesis, characterization and antioxidant activities of Schiff bases derived from 4phenyl 1, 3,5 triazine-2,6 diamine and hydroxyl salicylaldehyde with their nickel (II) and zinc (II) complexes have been reported 18 . In this work, we expanded our research to include chloro-and bromo-substituted derivatives together with their Ni(II), Cu(II) and Zn(II) complexes. The new compounds were characterized by means of elemental analysis, IR, 1 H and 13 C NMR spectroscopy, UV-Vis and their thermal stability was examined by thermogravimetric analysis (TGA) in order to test their stability in hightemperature catalytic cycles.
Physical measurements: IR spectra were recorded with a Perkin-Elmer FT-IR spectrophotometer model Spectrum 2000 using KBr pellets as support in the range 4000 -370 cm -1 . 1 H and 13 C-NMR spectra were recorded at room temperature on a JEOL ECA-400 spectrometer, operating with a frequency of 400 MHz, using DMSO-d 6 as solvent. Electronic spectra, in DMSO solution, were obtained using a Varian 50 Conc UV-visible spectrophotometer over the wave-length range 200 -800 nm. Thermogravimetric analysis was carried out on Perkin Elmer Precisely TGA 4000 thermogravimetric analyzer. The instrument was adjusted at a heating rate of 20°C/min. The heating was performed from 50 -900 °C.

Preparation of ligands:
H 2 L1: A solution of 5-chlorosalicylaldehyde (2 g, 12.77 mmol) in ethanol (40 cm 3 ) was mixed with a solution of 4 phenyl-1,3,5 triazine-2,6-diamine (1.19 g, 6.35 mmol) in ethanol (40 cm 3 ). The mixture was stirred under reflux for 2 hours. The pale yellow powder formed was filtered and recrystallized from ethanol. It was dried in an oven (80 o C) for 30 min. The yield was 2.14 g (72%   The FT-IR spectra of L1 showed a characteristic broad band at 3444 cm -1 for intra-molecularly hydrogen bonded -OH group 19 . A strong peak due to C=N stretching at 1622 cm -1 , another strong peak at 1275 cm -1 assigned to C-O phenolic stretching, and peaks in the region 1000 -1500 cm -1 from benzene ring skeletal vibrations 20 . The peak at 825 cm -1 was due to aromatic C-H out-of-plane stretching mode. The result strongly supported the formation of the Schiff base. The IR spectra of the nickel complexes differed from that of the ligands. It was noted that the -OH peak, observed for H 2 L1 at 3444 cm -1 , was now observed at 3453 cm -1 , and was assigned to coordinated H 2 O molecules in agreement with the results from the elemental analyses 21 . The peaks for C=N at 1622 cm -1 and C-O at 1275 cm -1 observed for the ligands were shifted to 1616 cm -1 and 1319 cm -1 respectively upon complexation 22 . The Ni-O peak is observed at 542 cm -1 . These suggested that the phenolic oxygens and imino nitrogens were coordinated to Ni(II). For copper complex, the expected functional groups were noticed as previously discussed for the corresponding Ni(II) complex. The C=N and C-O peaks for CuL1.H 2 O were at 1616 cm -1 and 1317 cm -1 respectively. These were almost similar to those of the corresponding Ni(II) complex, suggesting similar bond strength. The Cu-O peak was observed at 565 cm -1 , which was higher than that of Ni-O peak (542 cm -1 ), indicating a stronger M-O bond in the copper(II) complex 23,24 . Zinc complex IR spectra, as shown in table 2, can be interpreted similarly as their Ni (II) and Cu(II) congeners.

H and 13 C NMR spectra:
The 1 H NMR spectra for H 2 L1 was consistent with the expected structural formula of the ligand (Figure 1).

Figure 1: Proposed chemical structure of the ligands
A singlet at 10.19 ppm was due to phenolic hydrogen; a singlet at 8.21 ppm was due to imino hydrogen; and a multiplet in the range 6.72 -7.70 ppm was due to the aromatic hydrogens. The integration ratio for these hydrogens was 1:1:5.7 respectively (expected ratio = 1:1:5.5), and supported the molecular symmetry for the Schiff base 25 . The 1 H NMR spectra of H 2 L2 was closely similar and can be explained in the same way. The replacement of bromine atom in the 5 th position of the salicyldehyde moiety didn't impose significant impact in the chemical shift values. 13 C NMR spectra for both ligands show 12 peaks. Compared to H 2 L1, Br atom in H 2 L2 causes the chemical shift of the carbon atom directly attached to it to move towards lower energy (more shielded). At the same time, the two ortho-carbon atoms were deshielded, and insignificant effects on the other carbon atoms were observed.

UV-Vis spectra:
The UV-Vis spectral data of the ligands and their complexes in DMSO were listed in table 3. The UV-Vis spectrum of a solution of H 2 L1 in DMSO showed a high intensity broaden absorption band at about 270 nm (ε = 1.1x10 4 M -1 cm -1 ) assigned to π-π * transition of the aromatic ring 26 . The n-π * transition of the azomethine chromophore was observed as a shoulder within the high intensity peak at about 300 nm (ε = 1.4x10 4 M -1 cm -1 ). These values were in agreement with other Schiff bases reported in the literatures 27 . For example, the π-π * and n-π * transitions were observed at 255 nm and 308 nm respectively 27 . For H 2 L2, however, The UV-Vis spectrum in DMSO showed a high intensity broaden absorption band at about 279 nm (ε = 1.5x10 4 M -1 cm -1 ) assigned to ππ * transition of the aromatic ring. The nπ * transition of the azomethine chromophore was observed as a shoulder at the high intensity peak at about 345 nm(ε = 1.6x10 4 M -1 cm -1 ) 28 . Thus, compared to H 2 L1 (270 nm, 378 nm), there was no significant effect for the π π * transition, while the nπ * transition was shifted to higher energy when Cl was replaced by Br. For Ni(II) complex, the UV-Vis spectra showed weak d-d bands 1060 (ε max = 273 M -1 cm -1 ), 1010 nm (ε max = 318 M -1 cm -1 ), and 899 nm (ε max = 400 M -1 cm -1 ). These are consistent with an octahedral configuration at Ni(II) 29 . These bands were assigned to the transitions 3 A 2g  3 T 2g , 3 A 2g  3 T 1g (F) and 3 A 2g  3 T 1g (P), respectively, and the value of Δ o was 13,643 cm -1 .
The peak at 407 nm (ε = 1.5 x 10 4 M -1 cm -1 ) was assigned to metal-ligand charge transfer (MLCT) 30 . The spectrum was also compared with that of H 2 L1. It was noted that the ππ * band observed for H 2 L1 (270 nm) remained almost unshifted in the complex 271 nm(ε = 1.9 x 10 4 M -1 cm -1 ). However, the nπ * band may be hidden under the strong MLCT band at 407 nm. Thus, this band was significantly red-shifted from about 300 nm to about 400 nm as a result of coordination to the Ni(II). UV-Vis spectra of Cu (II) complex showed a broad dd peak at 700 nm (ε max = 200 M -1 cm -1 ). Thus, [CuL1(H 2 O)] was a mononuclear square pyramidal complex 31 . The ππ * and MLCT bands were at 268 nm(ε = 1.7x10 4 M -1 cm -1 ) and 396 nm(ε = 0.8x10 4 M -1 cm -1 ) respectively, which were almost the same as for the corresponding Ni(II) complex (271 nm, 407 nm), and may be similarly explained. The UV-Vis spectrum of Zn (II) complex showed that the MLCT and ππ * peaks 390 nm(ε =  TGA of CuL1: From the TGA curve of Cu(II) complex of L1, shown in figure 3, it was evident that the complex was more stable than its Ni(II) counterpart.   CONCLUSION: New Schiff bases of 4-phenyl-1,3,5-triazine-2,6-diamine were prepared via condensation reaction of the triazine with halo-substituted salicylaldehydes. Their nickel (II), copper (II) and zinc (II) complexes were also prepared. Spectroscopic and analytical data revealed that the complexes have the general formula ML with a 1:1 metal to ligand ratio.
Copper complexes appeared to be more thermally stable than their Ni(II) and Zn(II) analogues. In general, thermal stability of the chloro-ligand complexes followed the order: Cu complex > Ni complex > Zn complex The same trend was almost observed by the complexes of the bromo-ligand, with copper and nickel complexes being nearly the same, where zinc complex being the least stable. All complexes showed three steps decomposition pattern ends up with the metal oxide. Copper complex can serve in further studies in high-temperature catalytic cycles.