X-ray K-absorption spectral studies of cobalt (II) hydroxamic mixed ligand complexes

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 51, December 2013, pp X-ray K-absorption spectral studies of cobalt (II) hydroxamic mixed ligand complexes Neetu Parsai 1 *, Ashutosh Mishra 2 & B D Shrivastav 3 1 Sanghvi Innovative Academy, Institute of Technology and Management, Indore School of Physics, Devi Ahilya University, Indore School of Studies in Physics, Vikram University, Ujjain, * parsai.neetu@gmail.com, neetumandloi@rediffmail.com Received 19 October 2012; revised 25 July 2013; accepted 17 October 2013 X-ray absorption fine structure (XAFS) at the K-edge of cobalt has been studied in some cobalt (II) mixed ligand complexes having hydroxamic acid as one of the ligands. The study can be used to yield useful and important information about the molecular structure of the complexes. The X-ray absorption measurements have been performed at the recently developed BL-8 Dispersive EXAFS beamline at 2.5 GeV Indus-2 Synchrotron Source at RRCAT, Indore, India. The data obtained has been processed using EXAFS data analysis program Athena. From the experimental measurements, the energies of the K absorption edge, chemical shifts, edge-widths, shifts of the principal absorption maximum in the complexes have been estimated. The data obtained from these studies have been utilized to estimate effective nuclear charge. Keywords: XAFS, Athena, Effective nuclear charge, Absorption spectra 1 Introduction The X-ray absorption spectra at the K-edge of cobalt of a series of mixed ligand cobalt complexes, having hydroxamic acid as one of the ligands, have been investigated. The X-ray K-absorption spectra of cobalt in the five mixed ligand cobalt complexes having hydroxamic as primary ligand have been studied employing the laboratory X-ray spectroscopic set-up in our own laboratory by Dave 1. The X-ray K-absorption spectra of cobalt have been studied in the present paper employing the recently developed dispersive EXAFS (DEXAFS) beamline BL-8 at the Indus-2 synchrotron source at Raja Ramanna Center for Advanced Technology (RRCAT), Indore. The five complexes are given in Table 1. The similar XAFS measurements have already been made by one of the authors on the K-edge of copper in some of its biologically important complexes involving benzimidazole (BzImH) and other inorganic anionic ligands (X), viz; [Cu(BzImH) 4 X 2 ] and [Cu(BzIm) 2 ], where X=Cl, Br, 1/2SO 4, ClO 4, NO 3 and BzIm = benzimidazolato anion 2. A series of cobalt hydroxamic acid complexes for present X-ray spectroscopic study have been chosen. The hydroxamic acid ligand constitutes an interesting class of reagents because of their varied medicine, analytical and commercial applications, viz. potential antituberculotic, antineoplastic, antidiabetic and antifertility. Moreover, it has been demonstrated that they are potential extractions and powerful drugs. Literature survey reveals that many sulpha drugs show increased biological activity when they are administered in the form of metal complexes. A number of drugs are metal-binding agents. Chelation can be considered as one of the means whereby a drug can be removed from or given to the system. These metal chelates are the active part of drugs and function effectively by penetrating biomembranes in a fully chelated form. With regards to the strong ability of hydroxamic acids to form chelates, clarification of their interaction with metal ions is of particular importance in terms of biological effects. 2 Experimental Details In the present investigation, the X-ray absorption spectra have been recorded using synchrotron radiation. The X-ray spectroscopic set-up is available at RRCAT and is called beamline. This beamline has been recently commissioned at the 2.5 GeV Indus-2 synchrotron radiation source 3-5. The beamline used in the present investigation has been designed in the dispersive mode and is called dispersive EXAFS (DEXAFS) BL-8 beamline. The energy calibration of

2 828 INDIAN J PURE & APPL PHYS, VOL 51, DECEMBER 2013 Table 1 Cobalt (II) hydroxamic mixed ligand complexes, abbreviation and molecular formula S. No. Compound name Abbreviation Molecular formula 1. Co (II) acetohydroxamic acid Co AHA CoC 2 H 5 O 2 N 2. Co (II) 5 chloro-benzohydroxamic acid Co 5CBHA CoC 7 H 6 O 2 NCl 3. Co (II) benzohydroxamic acid Co BHA CoC 7 H 7 O 2 N 4. Co (II) acetylohydroxamic acid Co ActHA CoC 8 H 10 O 3 N 5. Co (II) mendelohydroxamic acid Co MenHA CoC 8 H 9 O 2 the beamline for a particular setting of the polychromator can be done using the method outlined by Gaur et al 6. For sample preparation the sample is mixed uniformly with a filler/binder material like cellulose acetate or boron nitride (BN). The well mixed sample can then be made into a pellet using a press (pelletizer). While preparing samples for recording X-ray absorption spectra at synchrotron beamlines, the pin hole and particle size effects must be kept in mind. 7 3 Results The intensities I 0 and I t, are obtained as the CCD outputs without and with the sample respectively. Using the relation, I t = I 0 e x, where µ is the absorption coefficient and x is the thickness of the absorber, the absorption (E) corresponding to the photon energy (E) are obtained. The experimental data has been analyzed using the available computer software package Athena. Firstly, the raw absorption spectra of the cobalt metal and the five complexes are obtained. A post-edge smooth background function (E) has been removed to isolate the XAFS. For this, a smooth polynomial sp line has been fitted to the XAFS to remove the slowly varying (low-frequency) components of (E). The absorption spectra are then normalized with the pre-edge along zero, an edge step of one and the post-edge region oscillating around one. The XAFS spectra in the region-20 < E < 60 ev, indicating positions of the absorption edges K1 and K2 and principal absorption maximum A are shown in Fig. 1(a-f). The first derivatives of the XAFS spectra in this region, indicating positions of the absorption edges K1 and K2 and principal absorption maximum A are shown in Fig. 2(a-f). First peak in the derivative spectra gives the position of first inflection point, i.e., the K1-absorption edge (E k1 ) and second peak gives the second inflection point, i.e., the K2-absorption edge (E k2 ). The position where the derivative is zero, gives the position of principal absorption maxima (E A ). The results of the energies of the K-absorption edges (E k1 and E k2 ) and the energies of principal absorption maximum A (E A ) of cobalt in metal and its five complexes are presented in Table 2. The chemical shifts (in ev) of the K-absorption edge of cobalt in the complexes are also given in Table 2. For all the complexes, the distance (in ev) of the principal absorption maximum A with respect to the respective K-absorption edge have also been computed and are collected in the Table 2. The cobalt K-edge is found to be shifted towards the high energy side in all the five complexes, as compared to the cobalt metal K-absorption edge (Table 2). 4 Discussion 4.1 Splitting of the edge The cobalt K-edge, as shown in Fig. 1(a-f), has been found to split into two components, i.e., K1 and K2 in all the complexes. The splitting is seen clearly when the first derivative of these spectra, are shown in Fig. 2(a-f), are examined. The values of E K1 and E K2 for the K-absorption edge of cobalt in its complexes are given in Table 2. These have been determined as the energies of the first two peaks in the derivative spectra. The K-edge of cobalt has been found to split into two components, i.e., K1 and K2, with a shoulder in between, in all the complexes. This feature shows that cobalt is in oxidation state +2 in these complexes. 4.2 Position of the edge As already pointed out above, in case of cobalt complexes, the absorption edge is found to split in two components K1 and K2. The inflection point of the first rise in the absorption edge corresponds to the binding energy corresponding to that edge, i.e., the inflection point on the K1 edge corresponds to the binding energy E 0 or E K of the K level. In the present work to determine the exact position of the inflection point, the first derivative of the (E) versus E curve has been computed. The first maximum on the first derivative curve gives the position of the inflection point and hence the position of the K1 absorption edge. If there is any difficulty in

3 PARSAI et al.: X-RAY K-ABSORPTION SPECTRAL STUDIES OF LIGAND COMPLEXES 829 Fig. 1 XAFS spectrum indicating position of absorption edges K1 and K2 and principal absorption maxima A for Co metal and its complexes measuring the position of the edge from the first derivative spectra, then the second derivative of the (E) versus E curve is used for this purpose. The first zero crossing point on this second derivative curve also gives the position of the K1 absorption edge correctly. For the K-absorption edge, the position of the edge is written as E K in ev. The values of E K (i.e., E K1 ) for the K-absorption edge of cobalt in its complexes are given in Table Chemical shift The K-absorption edge of cobalt has been found to be shifted towards the high energy side in all the complexes studied as compared to the K-absorption edge in the metal. The shifts of the K-absorption edge of cobalt in the complexes with respect to that of cobalt metal have been determined according to the eqn. E K = E K (complex)-e K (metal) (1) For computing the chemical shift the value of E K (Co metal) has been taken as ev. The order of the chemical shifts as indicated by their values from Table 2, has been found to be as follows: Co MenHA > Co ActHA > Co BHA > Co 5CBHA > Co AHA As is well known, an ionic bonding enhances the chemical shift, whereas a covalent bonding suppresses it. In general, the chemical shift reflects the ionic

4 830 INDIAN J PURE & APPL PHYS, VOL 51, DECEMBER 2013 Fig. 2 Derivative of XAFS spectrum indicating position of absorption edges K1 and K2 and principal absorption maxima A for Co metal foil and its complexes character of the complex, more the chemical shift, more the ionic character. Hence, the above order may also be taken as representative of the relative ionic character of the bonding in these complexes. As the complex Co MenHA is showing highest chemical shift in the series compared with other complexes, hence, it should have the maximum ionic character amongst the studied complexes. The compounds having cobalt in oxidation state in +1 show chemical shifts less than 5 ev while those having cobalt in oxidation state in +2 show chemical shifts 8,9 more than 5 ev. In Table 2, all the five

5 PARSAI et al.: X-RAY K-ABSORPTION SPECTRAL STUDIES OF LIGAND COMPLEXES 831 Table 2 XANES data for the K-absorption edge of cobalt (II) hydroxamic mixed ligand complexes Complex E k1 E k2 E A Chemical shift E k = (E complex E metal ) ENC Shift of the principal Absorption maximum Edge-width (E A E k1 ) Co metal Co AHA Co 5CBHA Co BHA Co ActHA Co MenHA Shift in B.E. of 1s electrons (in ev) Oxidation number Fig. 3 Oxidation number versus shift in B.E. of 1s electrons as given in Table 3 complexes have the values of chemical shifts between ev. Hence, on the basis of values of the chemical shifts, all the complexes are found to have cobalt in oxidation state Effective nuclear charge (ENC) Following Gianturco et al. 10, Gupta and Nigam 8,9 and Nigam and Gupta 11,12 have outlined the method of determining the effective nuclear charge on ions from the measurement of the chemical shifts. If the binding energies of K electron of cobalt in different oxidation states are determined, one can find from the difference in binding energies of the neutral atom and the ionized atom, the so called theoretical shifts in the X-ray absorption edge. In the present work, the values of binding energies have been determined from the tables of Clementi and Roetti 13 using Koopman s 14 theorem. The values of these binding energies and the theoretical shifts for cobalt are given in table. The theoretical shifts are plotted against the oxidation number. Such a graph for cobalt atom is shown in Fig. 3. This graph has been obtained by fitting a fourth order polynomial to the points. The polynomial and the parameters are given Table 3 Binding energies of 1s electrons of Co atom in different oxidation states Oxidation state Electronic configuration B.E. of 1s Shift in B.E. electrons (au) (in au) Shift in B.E. (in ev) 0 3d 7 4s d 8 4s d 7 4s d 6 4s d 5 4s Note-1 au = 2ry = The above value have been fitted to the polynomial curve y = A + B 1 x + B 2 x 2 + B 3 x 3 + B 4 x 4 The parameters are given below: Parameter Value Error A E-14 0 B B B B in Table 3. From this graph, the effective nuclear charge on cobalt in its complexes has been determined and the results have been presented in Table Principal absorption maximum In Table 2, we have also included the data for the principal absorption maximum E A for the complexes. This maximum corresponds to the 1s 4p z transition as well as to the transitions to the continuum states. It has been observed that with respect to cobalt metal, the value of E A is shifted towards the higher energy side. For the complexes mentioned in Table 2, the energy range of chemical shift in these complexes is between ev while the range for shift of principal absorption maximum is between ev. Hence, on the basis of the shift of the principal absorption maximum also it can be inferred that cobalt is in +2 oxidation state in these complexes.

6 832 INDIAN J PURE & APPL PHYS, VOL 51, DECEMBER 2013 The order of shift of the principal absorption maximum in the studied cobalt complexes is as follows: Co BHA < Co 5CBHA < Co MenHA < Co ActHA < Co AHA The order of shift of principal absorption maximum for the complexes is in reverse order of the chemical shift in three of the complexes. However, in the remaining two complexes Co MenHA and Co ActHA, though, the order is reverse in them, yet the order does not fit in the series. This exception may be because of the small change in the values of the shifts. The reverse order represents that the shift of A is inversely proportional to ionic character for this series. 4.6 Edge-width In Table 2, the values of the edge-width (E A -E K1 ) have been reported. The order according to the edgewidth data for the five complexes is as follows: Co MenHA < Co BHA < Co ActHA < Co 5CBHA < Co AHA The edge-width of the K-absorption edges increases with the increase in covalent character of the bonds provided other factors like molecular geometry etc remain the same. In the present work, edge-width of cobalt complexes is observed to vary from 5.4 to 9.6 ev. The order of the edge-width is in the reverse order of chemical shift of the complexes with the exception of Co ActHA. This complex is also an exception in the case of principal absorption maximum. The reverse order represents that the edgewidth is inversely proportional to ionic character for this series. 5 Conclusions The conclusions drawn in this paper are as follows: (1) X-ray absorption spectra of mixed ligand cobalt complexes at the K-edge of cobalt have been recorded at the recently developed EXAFS beamline set-up at the Indus-2 synchrotron source at RRCAT, Indore. The results obtained from synchrotron are much more reliable than obtained from X-ray tube and hence the complexes on synchrotron EXAFS set-up have been investigated. (2) The K-edge has been found to split into two components, i.e., K1 and K2 in all of these complexes. The energies of K1 edge (E K1 ), K2 edge (E K2 ), and principal absorption maxima (E A ) have been reported. Hence, the shift of the K1-edge (chemical shift) has been obtained. The order of the chemical shifts has been found to be as follows: Co MenHA > Co ActHA > Co BHA > Co 5CBHA > Co AHA The above order may also be taken as representative of the relative ionic character of the bonding in these complexes. (3) The chemical shift has been used to determine the effective nuclear charge on the absorbing atom. (4) The values of the chemical shifts suggest that cobalt is in oxidation state +2 in all of the complexes. (5) The shift of the principal absorption maximum has been obtained. The order of shift of principal absorption maximum for the complexes is in reverse order of the chemical shift in three of the complexes, with the exception of the two complexes Co MenHA and Co ActHA. The reverse order represents that the shift of the principal absorption maximum is inversely proportional to ionic character for this series. (6) The edge-width has also been obtained for all the complexes. The order of the edge-width is in the reverse order of chemical shift of the complexes with the exception of Co ActHA. This complex is also an exception in the case of principal absorption maximum. The reverse order represents that the edge-width is inversely proportional to ionic character for this series. References 1 Dave M, Ph D thesis, Devi Ahilya University, Indore, Hinge V K, Joshi S K, Shrivastava B D, Prasad J & Shrivastava K, Indian J Pure & Appl Phys, 49 (2011) Bhattacharya D, Poswal A K, Jha S N, Sangeeta & Sabharwal S C, Bull Mater Sci, 32(1) (2009) Das N C, Jha S N & Roy A P, Report No. E/035 Bhabha Atomic Research Centre, Mumbai, Das N C, Jha S N, Bhattacharya D, Poswal A K, Sinha A K & Mishra V K, Sadhana, 295 (2004) Gaur A, Johari A, Shrivastava B D, Gaur D C, Jha S N, Bhattacharya D, Poswal A & Deb S K, Sadhana, 36 (2011) Kelly S D, Hesterberg D & Ravel B, Methods of Soil Analysis Part 5. Mineralogical Methods, (Soil Science Society of America, Madison, USA), 2008, Chapter Gupta M K & Nigam A K, J Phys F, 2 (1972) Gupta M K & Nigam A K, J Phys B, 5 (1972) Gianturco F A & Coulson C A, Mol Phys, 14 (1968) Nigam A K & Gupta M K, J Phys, F 3 (1973) Nigam A K & Gupta M K, J Phys, F 4 (1974) Clementi E & Roetti C, Atomic data and Nuclear data tables, 14 (1974) Koopmans T, Physica, 1 (1934) 104.