Synthesis and Thermal Characterization of Lanthanide ( III ) Complexes withMercaptosuccinic Acid and Hydrazine as Ligands

Reaction of hydrazine and mercaptosuccinic acid with metal ions forms complexes with general formula [Ln(N2H4)2 {CH2(COO)CH(SH)(COO)}1.5]⋅(H2O), where Ln = La(III), Pr(III), Nd(III), Sm(III), and Gd(III) at pH 5. e complexes have been characterized by elemental analysis, IR and UV-visible spectroscopic, thermal and X-ray diffraction studies. e IR data reveal that the acid moiety in the complexes is present as dianion due to the deprotonation of COOH groups by lanthanides in these complexes, leaving –SH group unionized and hydrazine as bidental neutral ligand showing absorptions in the range of 945–948 cm. e thermoanalytical data evince that the complexes are stable up to 103C and undergo complete decomposition in the range of 550–594C resulting in metal oxides. SEM images of La2O3 and Gd2O3 residues show their nano sized clusters suggesting that the complexesmay be used as precursors for nano La2O3 andGd2O3, respectively. X-ray powder diffraction patterns show isomorphism among the complexes. e kinetic parameters of the decomposition of the complexes have been computed by Coats-Redfern equation.


Introduction
Mercaptosuccinic acid, as a ligand, has been of interest because of its versatility in coordinate modes due to two carboxylic acid and sulydryl groups.It is known to form complexes with divalent transition metal ions, Mn(II), Fe(II), Co(II), and Ni(II).It is reported that, in these complexes, S-H is ionised and coordinated in addition to coordination of one of the COOH groups [1].Patil and Krishnan have reported that alkaline earth metals Mg, Sr, and Ba also form 1 : 1 complexes in which S-H group is not involved and two COOH groups involve in coordination.ese complexes are found to form precipitates of metal mercaptosuccinates with aqueous solution of zinc and cadmium salts, leaving alkaline earth metal ions in solution and hence they can be used as antidote for Zn and Cd poisoning [2].A potentiometric titration study indicates the formation of mercaptosuccinic acid complexes of Zn and Ni with and without the involvement of sulydryl group in coordination [3].Another potentiometric study of chelates formed by La 3+ , Ce 3+ , Pr 3+ , and Nd 3+ with this acid reveals that the chelates of acids containing -SH group are less stable than those with NH 2 or OH donor group [4].
A study on heterochelates of Zn 2+ with nitrilotriacetic acid and mercapto acids system explains the stability of chelates due to two factors, Π interaction in M-S bond and sigma bonding of M-S bond due to polarisation of sulfur [5].A similar type of study on heterochelates of Ni and Zn containing this acid and dipyridyl supports the above factors.However, it concludes that the greater stability of M-S bond may be due to strengthening of M-S sigma bond and the contribution of M-S Π interaction, its lower stability due to the presence of coligands.
In spite of these reports, there is no systematic study of synthesis of mercaptosuccinic acid complexes with lanthanides found in the literature.We have been studying carboxylate complexes of lanthanides and transition metals using hydrazine as coligand.ere are numerous reports on metal hydrazine complexes of formic [6], acetic [7], propionic [8], glycolic [9], salicylic [10], tri-and tetracarboxylic [11,12], and naphthoxy and hydroxy naphthoic acid [13,14] systems.In many complexes, hydrazine being a simple diamine acts as neutral monodentate, bidentatebridged, and monodentate N 2 H 5 + cation in many complexes [15,16].With the interest of understanding the nature of interaction of lanthanides with carboxylic acid containing S-H group and hydrazine together, we performed this work.We have reported the synthesis of new lanthanide complexes using mercaptosuccinic acid and hydrazine as ligands and their characterization by IR and UV-visible spectroscopic methods, simultaneous TG-DTA analysis, powder X-ray diffraction method, and magnetic measurements.Since these complexes were found to yield metal oxides of nanosize on decomposition, SEM image reports of residual oxides have also been presented.

Experimental
), Where Ln = La(III), Pr(III), Nd(III), Sm(III), and Gd(III).ese complexes were prepared by adding a ligand solution which was obtained by mixing an aqueous solution of mercaptosuccinic acid (0.3 g, 2 mmol in 60 mL of H 2 O) and hydrazine hydrate (0.2 g, 4 mmol) to a metal nitrate solution which was prepared by dissolving metal oxide (e.g., La 2 O 3 , 0.163 g, 0.5 mmol) in a minimum quantity of 1 : 1 conc.HNO 3 and evaporated to eliminate excess of acid and dissolved in distilled water at pH 5. A crystalline product formed from the turbid solution while heating over water bath at 80 ∘ C for 1 h was �ltered, washed with absolute alcohol followed by ether, and dried in a desiccator over anhydrous CaCl 2 .
IR Spectra of the complexes in the region 4000-400 cm −1 were recorded as KBr pellets using Perkin Elmer 597 spectrophotometer.Electronic re�ectance spectra of Pr(III), Nd(III), Sm(III), and Gd(III) complexes were obtained using a Varian Cary 5000 recording spectrophotometer.e magnetic susceptibility of Pr(III) and Nd(III) complexes was measured using a vibrating sample magnetometer, VSM EG & G model 155 at room temperature.e X-ray powder diffraction patterns of the complexes were recorded using Philips X-ray diffractometer (model PW 1050/70) employing Cu-K radiation with nickel �lter.e simultaneous TG-DTA experiments were carried out using SDT Q600 V8.3 instrument and Stanton 781 simultaneous thermal analyzer.ermal analyses were carried out in air at the heating rate of 10 ∘ C/min using 5 to 10 mg of the samples.Platinum cups were employed as sample holders and alumina as reference.e temperature range was ambient to 800 ∘ C. e SEM images for the �nal products of La(III) and Gd(III) complexes Transmittance (%) 4000 3200 2400 1800 1400 1000 600 were recorded using a Cambridge scanning electron microscope (JEOL model JSM-6390LV) with EDX attachment (JEOL model JED-2300).

Results and Discussion
3.1.IR Spectra.IR and analytical data of the complexes are listed in Table 1.In the IR spectra of the complexes, the broad bands in the region 3333-3355 cm −1 are assigned to  OH vibrations of the associated water molecule.e  SH arising from sulydryl group which appears at 2565 cm −1 in the spectrum of pure acid was found to be shied to lower frequencies, 2542-2555 cm −1 in case of complexes [18] implying that mercapto group is not involved in the coordination.is observation agrees with the reported values found in the literature [2].Further this peak appears strong, clear, and sharp for La, Pr, and Nd complexes and weak for Sm and Gd.e hydrazine complexes display a N-N stretching frequency in the range of 945-948 cm −1 showing bidentate bridging nature in the complexes [19].All the complexes show absorption in the range of 1531-1556 cm −1 and 1309-1319 cm −1 corresponding to  COO (asym) and  COO (sym), respectively, and their difference being greater than 200 cm −1 corroborates monodental coordination of carboxylate group to the metal [20].A comparison of IR spectra of lanthanide complexes is shown in Figure 1. 2 indicate that all complexes follow similar type of decomposition pattern con�rming their similar formulation indirectly.[ e thermograms (Figure 2) reveal that the complexes start losing water �rst and then hydrazine up to 290 ∘ C, showing endotherms in the range of 93 ∘ C to 105 ∘ C and broad exotherms in the range of 220-290 ∘ C, respectively.Dehydration happening around 100 ∘ C indicates that water present in the complexes is not coordinated.Endotherm and exotherm suppress each other by mutual exchange of heat resulting in display of weaker peaks.is is a commonly observed phenomenon in case of decompositions of hydrazine complexes eliminating water and hydrazine simultaneously [21].en a continuous decomposition of hydrazine complexes from 290 to 650 ∘ C corresponding to the decomposition of  SEM image of La(III) and Gd(III) complexes residue show its nanosized clusters suggesting that the complexes may be used as a precursors for nanometal oxides [23].

UV-Visible Spectroscopy and Magnetic
Susceptibility.e re�ectance data of the ��-visible electronic spectra for Pr(III), Nd(III), Sm(III), and Gd(III) complexes are summarized in Table 3.While comparing the spectral data [24] of complexes with those of aquo ion complexes, it is understood that all complexes show red shis implying the complex formation.ese magnetic moments from magnetic susceptibility measurements for Pr(III) and Nd(III) complexes are 4.10 and 3.40 BM, respectively.e variation of these values from that of free ions, 3.426 and 3.526, respectively, may be because of the in�uence of ligands on metal ions in complexes.
3.5.Kinetic Studies.Dehydration and decomposition kinetics of complexes were followed using TG.eir parameters have been computed using integral method developed by Coats and Redfern.e equation used for calculation of the  and  parameters is where  is the fraction reacted in time (), T is temperature in , A is the preexponential factor in min −1 ,  is the heating rate, E is the activation energy in KJ/mole, and  is the gas constant.Plotting  versus 1/T gives a straight line; for a parameter, , order of the reaction, where  = 1 − (1 − ) −1 /(1 − ) 2 , the activation energy  was calculated from the slope and the  factor from the intercept [25].Studies reveal that all the complexes follow the same mechanism of decomposition as inferred from their computed  values.e activation energies for dehydration of the complexes are found to be almost similar in the range of 15.0-63.2KJ/mole.Activation energies of decomposition of anhydrous complexes are found to be varying from 13.7 to 143.6 KJ/mole.Table 4 shows the computed kinetic parameters for all the complexes, and the dehydration and decomposition kinetics for La(II) complex is shown in Figures 5(a)-5(b) as a representative example.
3.6.X-Ray Diffraction.e X-ray powder diffraction data of the complexes are summarized in

T 1 :
Analytical and IR data.

5 T 5 :
X-ray diffraction of metal complexes (D spacing in Å units and intensity (Cps) in parentheses).

Table 5
. e X-ray powder diffraction data of the complexes with the formulation, [Ln(N 2 H 4 ) 2 {CH 2 (COO)CH(SH)(COO)} 1.5 ] ⋅ (H 2 O) where Ln = La(III), Pr(III), Nd(III) show similarity among them, implying isomorphism.eXRDpattern of the complexes are shown in Figures6(a)-6(e).Sm(III) and Gd(III) complexes could not be compared from their pattern showing widening of peaks with weak intensity.ismaybe because of the very small particle size.4.Conclusione reaction of metal nitrate with mercaptosuccinic acid and hydrazine hydrate yields stable complexes of formula [Ln(N 2 H 4 ) 2 {(CH 2 (COO)CH(SH)(COO)} 1.5 ] ⋅ (H 2 O), where Ln = La(III), Pr(III), Nd(III), Sm(III), and Gd(III) at pH 5. Analytical data con�rm their formulation.e IR spectroscopic data of complexes indicate the monodental coordination of carboxylic acid with metals, noninvolvement of sulydryl group in coordination and bidental bridging mode of hydrazine.