Hydrothermal synthesis of copper (II) and Nickel (II) nano complexes with unsymmetric tetradentate Schiff base ligand. New precursors for preparation of copper (II) and nickel (II) oxides nano-particles

INTRODUCTION

Schiff bases are important ligands in coordination chemistry with extensive applications in different fields. Schiff bases are derived from aromatic carbonyl compounds and have been widely studied in connection with metalloprotien models and asymmetric catalysis, due to versatility of their steric and electronic properties [1]. Nanometer-sized particles of metal coordination supramolecules are fascinating to explore, since their unique properties are controlled by the large number of surface molecules, which experience an entirely different environment than those in a bulk crystal. Controlling the growth of materials at the submicrometer scale is of centeral importance in the emerging field of nanotechnology [2-10]. Although considerable effort has been dedicated to the controlled synthesis of nanoscale particles of metals, oxides, sulfides, and ceramic materials, little attention has been focused to date on nanoparticles of supramolecular compounds such as Schiff base complexes. Hydrothermal method is a promising synthetic method because of the low process temperature and very easy control of the particle size. The hydrothermal process has several advantages over other growth processes such as use of simple equipment, catalyst-free growth, low cost, large areauniform production, environmental friendliness and less hazard [11]. In the present work, two copper (II) and nickel (II) supramolecules with ligand {methyl-2-[N-[2-(5-choloro-2phenolate) methylidynetrilo] ethyl} aminato (-1)-1-cyclo pentenedithiocarboxylate, were synthesized by hydrothermal method. Copper (II) oxide nanoparticles has wide different applications according to the physical and chemical properties, such as superconductivity, photovoltaic properties, relative stability, and the antimicrobial activity [12]. Nowadays, application of CuO nanoparticles is of particular importance for antioxidant [13], antibacterial [14], thermal conductivity [15], battery [16], and solar cell applications [17]. CuO-NPs has been prepared via several methods such as sonochemical [18], alcohothermal and colloid-thermal synthesis [19-21], electrochemical method [22-24], and microwave radiation method [2]. Nickel oxide has a wide range of application in the manufacture of ceramic composite parts, magnetic materials, alkaline battery cathods, anti-feromagnetic layers and p-type transparent conducting films, electrochromic films, hetrogenous catalytic materials and gas sensors [26-32]. NiO nano-powder has been prepared using several methods such as ultrasonic radiation, carbonyl method, laser chemical processing, microwave pyrolysis, sol-gel technique and mechanochemical processing [33-41]. The use of coordination complexes as precursors for the preparation of inorganic nanomaterials such as copper (II) and nickel (II) oxide has not yet been investigated thoroughly. In this paper, copper (II) and nickel (II) oxide nanoparticles were obtained by direct thermal decomposition of compounds 1 and 2 at 450 ºC. The preparation of metal oxide nanostructures through thermal decomposition of complexes opens a new window for chemists to overcome challenges such as control of process condition, particle size, particle crystal structure and purity.

EXPERIMENTAL SECTION

Materials and Physical Techniques

All reagents and solvents for the synthesis and analysis were commercially available from Merck Company and used as received. Doubly distilled water was used to prepare aqueous solutions. A 30 ml Teflon-lined stainless-steel autoclave was used to do the reactions. The vessels were filled approximately to 40% capacity and heated for 4 h at 180 °C. IR spectra were recorded using Perkin-Elmer 597 and Nicolet 510P spectrophotometers. Micro analyses were carried out using a Heraeus CHN-O- Rapid analyzer. Melting points were measured on an Electrothermal 9100 apparatus. X-ray powder diffraction (XRD) measurements were performed using an X’pert diffractometer of Philips company with monochromated Cuk_α radiation (λ = 1.54056 Å). Simulated XRD powder pattern based on single crystal data were prepared using Mercury software [42]. The crystallite sizes of the selected samples were estimated using the Sherrer method. The samples were characterized with a scanning electron microscope (SEM) (Philips XL 30) with gold coating.

Synthesis of ligand cd5Clsalen, [Cu(cd5Clsalen)] (1) and [Ni(cd5Clsalen)] (2)

Methyl-2-{N-(2^׳-aminoethane)}-amino-1-cyclopentenedithiocarboxylate (Hcden) was prepared by published methods [43-46]. The ligand was prepared by addition of the equimolar amount of the 5-chlorosalicylaldehyde, to a methanolic solution of Hcden. The yellow product was recrystallized from methanol/chloroform 1:1 (V:V). To a solution of an appropriate amount of ligand (0.1 mmol) in 10 mL of chloroform/methanol 2:1 (V:V), a solution of metal (II) acetate (0.1 mmol) in 10 mL of methanol was added. The solution was stirred for 15 min and then allowed to stand at room temperature for 24 h. After filtering, the crude product was recrystallized from acetonitrile/methanol 1:1 (V:V).

Compound 1: White crystals, d.p. 260 °C. Found C: 45.98, H: 4.39, N: 6.68, S: 15.12 %; calculated for C₁₆H₁₇ N₂ClOS₂Cu; C: 46.15, H: 4.11, N: 6.73, S: 15.40 %. IR (cm^-1) selected bands: ν = 710(vs), 1166(s), 1260(vs), 1455(vs), 1629(s).

Compound 2: White crystals, m.p. 250 °C. (Found C: 46.42, H: 4.21, N: 6.83, S: 15.43 %; calculated for C₁₆H₁₇ N₂ClOS₂Ni; C: 46.69, H: 4.31, N: 6.66, S: 15.24 %). IR (cm^-1) selected bands: ν = 712(vs), 1150(s), 1260(vs), 1460(vs), 1629(s).

Synthesis of [Cu(cd5Clsalen)] (1) and [Ni(cd5Clsalen)] (2) nano-particles using hydrothermal method

To prepare nano-scale compounds 1 and 2, to a solution of an appropriate amount of ligand (0.1 mmol) in 10 mL of chloroform/methanol 2:1 (V:V), a solution of metal (II) acetate (0.1 mmol) in 10 mL of methanol was added. The solution was charged into a Teflon-lined stainless steel autoclave and heated at 450 °C for 24 h, then the autoclave was cooled to room temperature. The product was filtered, dried and characterized.

Compound 1: d.p. 253 °C. Found C: 45.49, H: 4.14, N: 7.04, S: 15.93 %; IR (cm^-1) selected bands: ν =710(vs), 1164(s), 1260(s), 1455(vs), 1629(s).

Compound 2: m.p. 244 °C. (Found C: 46.27, H: 4.13, N: 6.35, S: 15.19 %; IR (cm^-1) selected bands: ν = 715(vs), 1150(s), 1260(s), 1460(vs), 1629(s).

Synthesis of CuO and NiO nanoparticles

For preparation of CuO and NiO nano-particles calcinations of bulk powder compounds 1 and 2 were done at 450 ºC in static atmosphere of air for 4 h. IR spectrum and powder XRD diffraction shows that calcination was completed and the entire compounds decomposed.

RESULTS AND DISCUSSION

Reaction between tetradentate Schiff base ligand cd5Clsalen and copper (II) or nickel (II) acetate yielded crystalline materials formulated as [Cu(cd5Clsalen)] (1) and [Ni(cd5Clsalen)] (2), respectively. Nano-particles of compounds 1-2 were obtained by hydrothermal method while the bulk powder of compounds 1-2 were obtained using reflux method. Scheme 1 gives an overview of the methods used for the synthesis of compounds 1-2 using the two different routes. The elemental analysis of the nanoparticles and single crystalline materials of compounds 1 and 2 are indistinguishable. The FT-IR spectra of two complexes 1-2 compared with those of the corresponding ligand indicates that the ν (C=N) band around 1600 cm^–1_, ν (C-O) bands at 1220-1286 cm^–1_, ν (C=C) band in the region of 1456-1496 cm^–1 _, ν (C-S) band at 700-790 cm^–1 and ν (C-S) + ν (C-N) cm^–1 bands between 1099 and 1180 cm^–1 are shifted to lower energies. These results showed that the Cu (II) and Ni (II) ions are coordinated through the nitrogen atoms of the amine group, oxygen atom of the phenolic group and sulfur of the C=S group for two complexes. Figs. 1 and 2 show the comparison of XRD patterns, simulated from single crystal X-ray data against the nano powder of compounds 1-2 prepared by the hydrothermal method, respectively. The comparison between these XRD patterns indicates acceptable matches with slight differences in 2θ. Estimated from the Sherrer formula for the calculation of particle sizes from the broadening of the XRD peaks (D = 0.891λ/βcosθ, where D is the average grain size, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full-width at half maximum of an observed peak, respectively), the average size of the particles was found to be around 92.4 for compound 1 and 76.3 for compound 2. Figs. 3 and 4 show the SEM images of the compounds 1 and 2. The structures of compounds [Cu(cd5Clsalen)] (1) and [Ni(cd5Clsalen)] (2) were previously analyzed and reported [47-48]. In compound 1, the Cu (II) atom is coordinated by S, O and two N atoms in a nearly planar environment. The coordination geometry around Cu center is nearly planar with the dihedral angle of 7.65(3)° between planes of NCuO and NCuS (. The x-ray structure of compound 2 revealed that the complex has a N₂OS coordination sphere, with the two nitrogen atoms coordinated in a cis configuration (Figs. 5 and 6). The coordination around nickel is essentially square-planar with a small tetrahedral distortion. (Dihedral angle of 5.43(3)° between coordination planes NNiO and SNiN (Figs. 7 and 8).

Nano-particles of CuO and NiO have been generated by calcination of compounds 1 and 2. The final product upon calcination of compounds 1-2 at 400°C, based on their IR and XRD patterns are CuO and NiO. The IR spectrum of CuO and NiO nanoparticles after calcinations of compounds 1-2 shows absorption bands at about 500 cm^-1 that can be attributed to the stretching modes of M-O (M=Cu and Ni) and the weak bands in the range of 1380-3425 cm^-1 are probably attributed to the presence of water in the KBr matrix. Figs. 9 and 10 show X-ray powder diffraction pattern of CuO and NiO nanoparticles after calcination of compounds 1 and 2. The XRD patterns of CuO and NiO nanoparticles after calcinations of compounds 1 and 2 are in agreement with the typical CuO diffraction (monoclinic phase, space group C2/c, with lattice constants a = 4.5850 Å, b=.4230 Å and c = 5.2000 Å, Z = 4, ICSD No. 00-041-0254) and NiO diffraction (cubic phase, space group Fm3m, with lattice constant a,b,c=4.1771 and z=4, ICSD No. 00-047-1049). No characteristic peaks of impurities are detected in XRD patterns. Figs. 11 and 12 show the SEM images and the corresponding particle size distribution histogram of CuO and NiO nanoparticles obtained from calcinations of nano compounds 1-2 at 450 ºC.

CONCLUSION

Nano particles of copper (II) and nockel (II) complexes with a tetredentate sciff base ligand, [[Cu(cd5Clsalen)] (1) and [Ni(cd5Clsalen)] (2), have been synthesized by hydrothermal method. Compounds 1-2 were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), elemental analyses and IR spectroscopy. Nano-particles of copper (II) and nickel (II) oxide have been prepared by calcinations of two different Schiff base complexes, compounds 1-2. The nano oxides were characterized by scanning electron microscopy (SEM) images, X-ray powder diffraction (XRD) and IR spectroscopy. This study demonstrates that the Schiff base complexes may be suitable precursors for the preparation of nanoscale materials with interesting morphologies.

ACKNOWLEDGEMENT

This work was supported by the Islamic Azad University of Firoozabad.

CONFLICT OF INTEREST

The author declare that there is no conflict of interests regarding the publication of this manuscript.