Local Structure studies of Schiff base dependent Cadmium complex

Cadmium complex with Schiff-base ligand has been receiving significant interest due to its wide variety of biological activities, such as antitumor, antibacterial, antifungal, antiviral etc properties. The present paper deals with the Synthesis of Cadmium complex prepared by Schiff base ligand in the 2:1 (ligand: metal) equimolar ratio. The ligand was derived from 2-Aminopropanoic acid. The synthesized complex was used to characterize by X-ray absorption fine structure spectroscopy at cadmium K-edge. The studies revealed its octahedral coordination geometry with + 2 oxidation state of cadmium in the complex. The processed data of the complex was used to fit with the theoretical model generated from standard crystallographic data using FEFF calculations.


Introduction
The transition metal complexes prepared from Schiff base as a ligand exhibit several beneficial biological actions, such as antibacterial, antifungal, anticancer etc and they also have properties of catalyst, dye, regularisation of plant development, etc [1].Metal complexes are appropriate for use in the pharmaceutical, medical, and industrial fields owing to such diverse range of characteristics.Today due to their usefulness, demand for the formation of Schiff-base coordination complexes with transition metal ions is rising [2,3].In this paper, a cadmium complex has been prepared using Schiff base as a ligand.Firstly, ligand was prepared by the condensation of 2-Aminopropanoic acid and then this ligand was utilized for the synthesis of Cadmium complex using metal chloride.The synthesized Cadmium complex was further used to characterise under X-ray absorption fine structure spectroscopy (XAFS) for the local structure determination at Cd K-edge.XAFS is an amazing technique used for analysing the structure around certain elements that exist inside a material at the atomic and molecular scale.It may be applied to crystals as well as materials [4].

Preparation of Ligand:
Schiff base ligand was prepared by condensation of 2-Aminopropanoic acid with equimolar 1Hindole-2, 3-Dione using methanol as a solvent and acetic acid as a catalyst.The solution was refluxed for around 8 hours.The reaction was confirmed by thin layer chromatography (TLC) and powder form of ligand was obtained by ice cube method.The obtained ligand can be formulated as C11H10N2O3.

Preparation of Cadmium complex:
The procedure used for the synthesis of cadmium complex is same as reported in the literature [5].The complex was obtained by the condensation of Cadmium chloride with the prepared Schiff base ligand in 1:2 (metal: ligand) electrolytic ratio using methanol as solvent for around 3-4 hours and acetic acid was used as a catalyst.The reaction was confirmed by TLC method.The crystals of the complex were obtained by keeping the product in open air and finally, after grinding it, powder form of cadmium complex was obtained and used for the measurement.The obtained complex can be formulated as Cd[C11H9O3N2]2

FTIR (Fourier transform infra red ) spectroscopy measurement:
For FTIR measurement, FTIR spectrometer (Bruker ALPHA) setup, which is available at Central Analytical Facilities (CAF), Manipal University Jaipur (R.J.) [6] was utilized.A little amount of powder of samples (ligand and complex) was added to few micrograms of pure KBr and then ground thoroughly.The pellets of uniform mixture were made for the samples.These pellets were characterized by FTIR spectroscopy and data were recorded in the range 4000-500cm -1 at room temperature.

XAFS spectroscopy measurement:
An X-ray absorption fine structure measurement was carried out on the Cadmium metal foil and synthesized Cadmium complex at Cd K-edge  26.7keV.For the measurement, appropriate amount of sample was systematically mixed with cellulose and pressed in the form of thin pellet which was then covered on both faces with Kepton tape.XAFS measurement was carried out by Synchrotron Radiation Source on Energy Scanning XAFS beamline (BL-9) at Raja Ramanna Centre for Advanced Technology, (RRCAT), Indore [7].

FTIR study:
Figure 1 Shows the FTIR spectrum of Cd complex along with Schiff base ligand.Because of the (NH2) group of alanine and (C=O) group of isatin, the bands at 3250 and 1742 cm -1 were found to be missing [8].A sharp band emerged in the ligand at position 1664cm -1 owing to the azomethine (C = N) group [9][10], signifying the condensation of ketone portion of the isatin as well as the amino group of alanine and leading to the production of Schiff base ligand.The comparison of the FTIR spectra of the Schiff base ligand with the Cd complex showed a shift to 22cm -1 in azomethine group (C=N) suggestive of the association of the azomethine nitrogen with the corresponding metal ion [11][12].The new band, which appeared at position 555cm -1 and 745cm -1 in the spectra of complex, is assigned to stretching frequency of (NCd) bond formation [13].The spectrum of ligand which contain absorption band appeared at position 2781cm -1 and 3090cm -1 assigned to hydrogen-bonded (OH).This position got vanishes after preparation of the complex, suggesting chelation of the oxygen to the metal atom [14].Cadmium complex showed the asymmetric (COO -) and symmetric (COO -) absorption peaks at postion 1529cm -1 and 1318cm -1 respectively which indicate that the coordination took place via the oxygen atom [14].

XAFS study:
From these spectra both XANES (X-ray absorption near edge structure) and EXAFS (Extended X-ray absorption fine structure) features have been estimated using computer software package Demeter contain Athena and Artemis.

XANES study:
Figure 2 shows the XANES or derivative spectra of XAFS data of metal foil and complex.Cadmium complex is found to be splitted in two components and the energies EK1 and EK2 corresponding to these components are reported in Table 1.The chemical Shift is found to be  11.4eV.The principal absorption maximum is found to be shifted towards lower energy side.The Edge width value is found to be 10.1eV.The Splitting of edges and value of chemical shift of Cadmium complex are suggestive of +2 oxidation state of Cd in the complex [15].The absence of pre-edge feature in the XAFS spectrum and the value of edge width are suggestive of octahedral coordination geometry of Cadmium complex [16]

EXAFS study:
The local structural informations about the Cadmium complex have been extracted out with the help of FEFF calculations.In order to generate theoretical models and to fit it, crystallographic data [17] analogous to the studied complex has been utilized.The Rbkg was set to 1.0Å and the data was processed in Athena for the removal of the background using the Spline range.The Fourier transformation was implemented in the k-range of 3.0 -7.5 Å -1 with dk=1.0.Using kw=2, the modeled data was applied to the experimental data and fitting was done in the R range of 1.0-5.0Å.The FEFF computation was done by using two single oxygen scattering paths O12.1 and O12.2.The method by Kelly et-al [18] was pursued to produce the theoretical model.A common value for S0 2 and ∆E0 was utilized for all paths but different values of ∆R were specified different in both paths.The reduced chisquare (ν 2 ) is found to 12.3 and R-factor so obtained is 0.01.Both are found to be in reasonable range.The value of S0 2 , ∆E0 and ∆R are also found to be reasonable.The theoretically fitted data by these two paths with the experimental results are seen in Figure 3 and fitted data are reported in the Table 2.For the paths Cd-O12.1 and Cd-O12.2 the bridging distance (R) between Cd and Oxygen are found to be 2.34 Å and 2.94 Å respectively.The Debye wall factors (σ 2 ) so obtained are 0.003Å 2 and 0.01Å 2 for paths O12.1 and O12.2 respectively.

Conclusions:
The distinctive FTIR band, revealed the Functional groups of the Cd complex prepared from Schiff base ligand and the spectrum of Cadmium complex is found to be different from ligand, suggesting the formation of complex from the ligand.The splitting of edge of derivative spectra of XAFS and the value of chemical shift revealed the +2 oxidation state of Cd in the complex.The principal absorption maximum for cadmium complex is found to be shifted towards lower energy side.This shift is due the overlapping of ligand orbitals with metal ion.The edge width values and non existence of pre-edge peak feature confirms the octahedral coordination geometry of the complex.After the generation of theoretical model from the standard similar crystallographic data using FEFF calculation the coordinating neighbours, the bond lengths of neighbours from the central atom, Debye wall factor etc estimated.All the parameters estimated for Cd complex will be useful during the preparation of new drugs because Cd complex may have antifungal, antibacterial etc properties.