Intercalation of Schiff Base Ligands with Copper (ІІ) and Nickel (ІІ) Cation-exchange Montmorillonite (MMT) Nanoclay

The Schiff bases (E)-3-{[(2-amino-5-nitrophenyl)imino]methyl}phenol and (E)-3-{[(2-amino-5-nitrophenyl)imino]methyl}benzene-1,2-diol were synthesised from 2,3-dihydroxy-benzaldehyde, salicylaldehyde and 4-nitro-o-phenylenediamine. These ligands were characterised by Fourier transform infrared (FTIR), UV-Visible, CHN analysis, and HNMR. Ni(II) and Cu(ІІ) cation-exchange montmorillonite (MMT) clay was prepared by the interactions of NiCl2, CuCl2 and MMT clay in an aqueous medium. FTIR and EDX were used to characterise the metal cation-exchange in MMT. Schiff base Cu(II) and Ni(ІІ) complexes intercalated montmorillonite were prepared by the interaction of Schiff base ligands and Cu(II) and Ni(ІІ) cation-exchange MMT. Several characterisation techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were used to characterise the modified MMT. SEM results showed that the resulting hybrid had layered structures. The XRD results indicate that the Nickel(II) Schiff base complex-MMT was partially exfoliated.


INTRODUCTION
Schiff base transition metal complexes can be easily synthesised and have been extensively studied.2][3][4][5] Tridentate oxygen-nitrogen-nitrogen (ONN) Schiff base ligands containing anionic amines (RNH) were used to prepare metal Schiff base complexes, 6,7 while tridentate metal Schiff base complexes were used as catalysts in Heck 8 and Suzuki-Miyaura cross coupling reactions. 9ong the different types of clay, montmorillonite (MMT) has been extensively used for modification due to its excellent properties, such as high cation exchange capacity, swelling behaviour, adsorption properties and its large surface area. 10,11he modified montmorillonites are versatile heterogeneous catalysts for a wide variety of organic reactions, such as Friedel-Crafts and oligomerisation reactions. 12cording to its layer structure, MMT has the potential to host Schiff base metal complexes.Several reports can be found in the literature regarding Schiff base metal complexes immobilised onto MMT clay to prepare nanohybrid materials with improved catalytic properties.For instance, a manganese(II)-Schiff base complex incorporated in the interlayer space of clay minerals exhibits significant catalytic activity. 13Parida et al. 14 have reported the preparation of Schiff base Pt(II) complex supported MMT by a cation exchange mechanism, and intercalation of the complex inside the clay matrix was confirmed by various characterisation techniques.More recently, we reported the novel nanohybrids of cobalt(III) Schiff base complexes intercalated into MMT clay by a direct exchange reaction of clay and cobalt(ІІІ) complexes. 15,16rein, we have extended our previous studies to do an ion-exchange reaction of MMT clay by interacting it with Cu(ІІ) and Ni(ІІ) cations.In a follow-up study, metal cation-exchange MMT was reacted with two different Schiff base ligands to perform in-situ synthesis of complexes within the layers of MMT clay.The metal Schiff base complexes intercalated into MMT were identified and studied via FTIR, XRD, TGA/DTG, SEM and TEM.

Chemicals
All of the chemicals and solvents used for synthesis were of commercially available reagent grade and used without further purification.Montmorillonite K-10 (MMT) with cation-exchange capacity of 119 meq/100 g was provided by Aldrich (St. Louis, MO, USA).

Synthesis of Schiff Base Ligand (L 1 )
The Schiff base ligand, (E)-

Preparation of Cu(II) and Ni(II) Cation-exchange MMT
Two saturated solutions of cupric(II) chloride and nickel(II) chloride were prepared by dissolving 10.439 g and 10.900 g in 10 ml of distilled water, respectively.The solutions were transferred into two round-bottom flasks followed by the addition of 0.500 g montmorillonite K10 clay (MMT) into each flask.The solutions were refluxed for 4 h and then filtered through a Buchner funnel and washed with plenty of distilled water.

Characterisations
Infrared (FTIR) spectra were recorded as KBr discs on a PERKIN ELMER 2000 FTIR spectrophotometer in the range of 4000-400 cm -1 .The elemental analysis was determined using a CHN Analyzer, model PERKIN ELMER II.UV-Vis spectra were recorded on a UV-2600 Shimadzu spectrophotometer in the 190-900 nm range.The 1 H NMR spectra were recorded on a Bruker 500 MHz ultrashield TM spectrometer.The X-ray diffraction (XRD) was recorded on a high resolution X-ray diffractometer system model PAnalytical X'PERT PRO MRD PW3040.Transmission Electron Microscopy (TEM) study was performed using a FEI/PHILIPS CM12, thickness and silicate layer spacing were recorded using Olympus image analysis software.The thermal stability of specimens was tested using a TGA/SDTA851 Thermo Gravimetric analyser from ambient temperature to 900°C with a heating rate of 20°C min -1 under nitrogen gas.Scanning Electron Microscopy (SEM) was recorded on a QUANTA FEG 650 2012 SEM system.Energy Dispersive X-ray Analysis (EDX) (EDAX Falcon System) was conducted to analyse the presence of elements in the specimens.

FTIR Characteristics
The C=N stretching frequency for L 1 and L 2 exists at 1620 and 1614 cm -1 , respectively.The NO 2 vibrational frequency presence at 1325 cm -1 is for L 1 and 1344 cm -1 is for L 2 .Both metal(II) cation-exchange MMT products showed peaks related to their functional groups (Figure 1).A broad peak was observed in the range of 3600-3200 cm -1 , which can be attributed to the interlayer absorbed water molecules.A weak band appeared at 1635 cm -1 , which is related to the OH bending of these water molecules.The Si-O stretching peak appears in the range of 1300-900 cm -1 .The peaks at 474 and 528 for Cu(II) cation-exchange MMT and 527 for Ni(II) cation-exchange MMT are due to Mg-O and Al-O, respectively.The FTIR spectra for the intercalation of Schiff base ligands into metal(II) cation-exchange MMT showed some differences compared to the pure MMT (Figure 2).Some peaks that are related to the metal Schiff base complexes were observed in the new intercalation compounds.The C=N band appears at approximately 1600 cm -1 .Theoretically, the C=N band is predicted to shift to a lower frequency (blue shift) relative to the free Schiff base.From the experiment, the imine band of the Ni(II) Schiff base intercalated MMT is shifted to a lower frequency at 1609 cm -1 .However, the imine band of the Cu(II) Schiff base intercalated MMT shifted to a higher frequency (red shift) at 1619 cm -1 due to a decrease in repulsion between the nitrogen lone pair of electrons as a result of their participation in coordination with the metal ion. 17

XRD Analysis
The XRD analysis confirmed that the intercalation and partial exfoliation of the MMT were successful with the cationic complexes.

Morphology
The SEM images of the intercalated Ni(II) and Cu(II) Schiff base complexes in MMT were observed using SEM technique (Figure 4).The SEM images for Ni(II) and Cu(II) Schiff base intercalated MMT showed layered structures.6.
The d-spacing of unmodified MMT (d = 1.006 nm) is calculated from the peak position at 2θ = 8.78 using Bragg's equation.The diffraction peak of Ni(II) Schiff base intercalated MMT nanohybrid decreased and shifted to a new position at 2θ = 8.073 (d = 1.095 nm).The Cu(II) Schiff base intercalated MMT shifted to a new position at 2θ = 7.448 (d =1.187 nm) after the ion exchange reaction between MMT and the cationic complexes (Figure3).The Ni(II) Schiff base intercalated MMT was partially exfoliated since the d basal distance almost disappeared, but it still had layered structures.The intercalated reaction and partial exfoliation of MMT occurred because of increases in the interlayer distance, which leads to a shift of the diffraction peak towards lower angles followed by increasing basal distances.

Figure 3 :
Figure 3: The XRD patterns of the hybrid materials of (A) Ni(II) Schiff base complex intercalated MMT, (B) pure MMT, (C) Cu(II) Schiff base complex intercalated MMT.

Figure 5 (
i) and Figure5(ii) show the EDX spectrum of Ni(II) and Cu(II) cationexchange MMT particles, respectively.In the EDX spectrum, the following six elements can be observed: oxygen (O), magnesium (Mg), aluminium (Al), potassium (K), silicon (Si), nickel (Ni) and copper (Cu) in Ni(II) cation-exchange MMT or Cu(II) cation-exchange MMT.The presence of elemental nickel and copper confirmed that the cation-exchange of the MMT clay with metal ions had occurred successfully.

Figure 9 :
Figure 9: High magnification TEM image of Cu(II) Schiff base intercalated MMT clay.