Abstract
New bis-hydrazine lanthanide complexes with 3-acetoxybenzoic acid (3-abH) of formula, [Ln(3-ab)3(N2H4)2]·xH2O where Ln = La, Ce, Pr and Gd and ; Ln = Nd and Sm and and monohydrazine complexes of some trivalent lanthanides with 4-acetoxybenzoic acid (4-abH) of formula, [Ln(4-ab)3(N2H4)]·H2O where Ln = La, Ce, Pr, Nd, Sm and Gd have been prepared in ethanolic medium and characterized by spectroscopic techniques (IR and UV reflectance), microelemental analysis, thermoanalytical technique, powder XRD, SEM-EDS studies, and magnetic susceptibility measurements. The IR spectra of both series show bidental coordination of carboxylate ion with the metal by displaying, in the range of 1587–1602 cm−1 and the in the range of 1433–1410 cm−1, with a separation of around 200 cm−1. The ester C = O remains unaltered indicating the noninvolvement in coordination. All the complexes show absorption in the range of 929–962 cm−1 indicating the presence of hydrazine in a bridged bidentate fashion. The thermal data reveals that the hydrated compounds show endothermic dehydration followed by exothermic decomposition to leave their metal oxide as end products, and the anhydrous compounds undergo exothermic decomposition to form the respective metal oxide residues.
1. Introduction
Salicylic acid plays vital role in coordination chemistry due to its capacity for chelation. One of its acetyl derivatives, aspirin, has also been used in the preparation of complexes due to its therapeutic applications [1–3]. It has been used as an axial ligand for the preparation of tin porphyrin complexes [4]. The other two isomers of aspirin, namely, 3-acetoxybenzoic acid (3-abH) and 4-acetoxybenzoic acid (4-abH) have not been used for synthesis of complexes yet. In our laboratory, we have been synthesising metal carboxylate complexes using hydrazine as coligand [5–7]. When hydrazine is used as a coligand, it generally leads to the formation of polymeric complexes owing to its action as bridging ligand [8–11]. With the idea of preparing new hydrazine complexes, we selected the 3 and 4-acetoxybenzoic acids. In this paper, we have presented the preparation and the characterization of 3- and 4-acetoxybenzoic acid complexes of some lighter lanthanides with hydrazine as coligand by IR, uv-visible spectroscopic studies, thermoanalytical, XRD, SEM-EDX, and magnetic susceptibility measurement studies.
2. Experimental
The solvents were distilled prior to use, and double distilled water was used for the preparation and chemical analyses. The chemicals used were of AR grade. In all the reactions, 99.99% pure hydrazine hydrate was used as received.
2.1. Preparation of [Ln(3-ab)3(N2H4)2]·xH2O Where Ln = La, Ce, Pr, and Gd and ; Ln = Nd and Sm and
Lanthanum oxide (0.325 g, 1 mmol) was dissolved in a minimum quantity of 1 : 1 HNO3, evaporated to eliminate excess of acid, and dissolved in 20 mL of ethanol. This was added slowly to a freshly prepared ethanolic solution (40 mL) of the ligand containing 3-abH (1.081 g, 6 mmol) and hydrazine hydrate (0.40 g, 8 mmol), stirring the reaction mixture at pH 3. Then the reaction mixture was kept over hot water bath for 1 h 30 min. A dull white, crystalline product obtained was washed with alcohol, ether, and air dried. A similar procedure was adopted for obtaining the other lanthanides with the molar ratio Metal : Acid : Base = 1 : 6 : 8.
2.2. Preparation of [Ln(4-ab)3(N2H4)]·H2O Where Ln = La, Ce, Pr, Nd, Sm and Gd
These complexes were prepared using 4-abH, hydrazine hydrate and their respective lanthanum nitrates in alcohol medium at pH 3.5 by adopting similar procedure as mentioned above. The products obtained were washed with alcohol, ether and dried in air. All these complexes were prepared by using the molar ratio Metal : Acid : Base = 1 : 6 : 8.
2.3. Physico chemical Methods
The hydrazine content in all complexes was determined volumetrically using 0.025 M potassium iodate solution under Andrews’ conditions [12]. The metal content was determined by EDTA complexometric titration [12] after decomposing a known weight of the sample with 1 : 1 HNO3. Magnetic measurements were carried out by the Gouy method using Hg[Co(NCS)4] as calibrant. The electronic spectra for solid-state complexes were obtained using a Varian, Cary 5000 recording spectrophotometer. Infrared spectra were recorded using KBR disc (4000–400 cm−1) on a Shimadzu FTIR-8201 (PC) S spectrophotometer. The simultaneous TG-DTA studies were done on a Perkin Elmer Diamond TG/DTA analyzer, and the curves were obtained in static air using 5–10 mg of the samples at the heating rate of 10°C/min. The XRD patterns were recorded on a Bruker AXS D8 advance diffractometer with an X-ray source Cu, wavelength 1.5406 Å using a Si (Li) PSD detector. The elemental analysis was carried out using a CHNS Elementar Vario EL III Elemental Analyzer. The SEM with EDX analysis was obtained using JEOL model JSM-6390 LV and JEOL model JED-2300 instrument.
3. Results and Discussion
All the complexes obtained were polycrystalline powders that are stable in air and insensitive to light. Lanthanum complex is sparingly soluble in water but other complexes were insoluble. They were also insoluble in organic solvents such as ethanol, ether, and benzene, but soluble in DMSO. The analytical data of the complexes is given in Table 1 and the values were consistent with the proposed formulae for them.
3.1. Electronic Spectra and Magnetic Susceptibility Measurements
The compounds were insoluble in water and organic solvents, and hence their electronic spectra were recorded for solid samples. The electronic spectral data and the assignments were summarized in Table 2. The levels assigned were and , for Nd and Pr complexes of 4-abH. Similarly the levels assigned for complexes of Nd and Pr of 3-abH were and , respectively [13, 14]. The effective magnetic moment values, 3.43, 3.48 BM corresponding to Pr and 3.51, 3.56 BM for Nd complexes of 3-abH and 4-abH, respectively, are in good agreement with the values reported [15].
3.2. IR Spectra of Complexes
The IR data of the complexes and that of the respective acids are given in Table 3. The spectrum of 3-abH acid shows absorptions at 1091 cm−1, 1207 cm−1, 1276 cm−1 due to , 1761 cm−1 due to , and 1676 cm−1 due to .
Spectra of the complexes show broad spectrum 1246–1315 cm−1 and centered at 1315 cm−1 (La), 1277 cm−1 (Ce), 1300 cm−1 (Pr), 1303 cm−1 (Nd), 1300 cm−1 (Sm), and 1300 cm−1 (Gd) due to . A medium sharp band observed at 1091 cm−1 for the acid is found at 1112 cm−1 for the complexes. These absorptions are found to shift in the same direction for all the complexes due to coordination [16].
Further, and are observed in the range of 1587–1602 and 1433–1430 cm−1. The difference being less than 200 cm−1 may be because of their bidental coordination. The similar type of bidental coordination is found to occur in Cu aspirinate complexes [16–18]. The complexes show two sharp bands in the range of 1001–1005 and 929–962 cm−1 raising ambiguity about the assignment of coordination mode of hydrazine [19]. Since lanthanides preferably set in complexes with higher coordination number greater than eight, hydrazine molecules present in the complexes reported in this work should be of bridged bidental mode [20].
Similar to 3-isomer acid, 4-isomer acid also shows peaks at 1126 cm−1, 1225 cm−1, and 1295 cm−1 due to . The peak observed at 1753 cm−1 is due to . The peak at 1680 cm−1 due to is found. In the case of complexes, is observed at 1157 cm−1, 1285 cm−1, 1326 cm−1, 1285 cm−1, 1159 cm−1, and 1239 cm−1 for La, Ce, Pr, Nd, Sm and Gd complexes, respectively. Further found at 1680 cm−1 is split into two and peaks displaying at 1600 cm−1 (La), 1598 cm−1 (Ce), 1601 cm−1 (Pr), 1601 cm−1 (Nd), 1599 cm−1 (Sm), 1600 cm−1 (Gd) and 1404 cm−1 (La), 1417 cm−1 (Ce), 1406 cm−1 (Pr), 1417 cm−1 (Gd), 1415 cm−1 (Nd), and 1416 cm−1 (Sm). The difference being less than 200 cm−1 implies the bidental coordination. The sharp peaks observed in the range of 929–932 cm−1 are assigned to N–N stretching of hydrazine which is involved in bidental coordination with the metal ion.
In addition, the hydrated complexes show broadbands observed in the range of 3400 cm−1 corresponding to OH stretching and 862 cm−1 corresponding to OH bending of water molecules present in the lattice. This broadband extends up to 3030 cm−1 may be because of the merging of N–H stretching of hydrazine molecule with of water.
3.3. Thermal Data of Complexes
The thermal data of the complexes were given in Table 4.
3.3.1. Thermal Analysis of Lanthanides of 3-abH
The lanthanides of 3-abH show two steps of decomposition in their TG curve, namely, dehydrazination and decomposition to their corresponding metal oxides without forming intermediates. Among them, Sm and Nd complexes being hydrated show endothermic dehydration in the temperature range of 40–45°C showing the weight loss temperature of 30–100°C in the TG. All the complexes show exothermic dehydrazination between 180 and 197°C and undergo oxidative decomposition to form their corresponding metal oxides, displaying the exotherms in the range of 437–640°C in DTA.
The TG-DTA of [Pr(3-ab)3(N2H4)2], [Sm(3-ab)3(N2H4)2]·H2O, and [Pr(4-ab)3(N2H4)]·H2O, [Sm(4-ab)3(N2H4)]·H2O are shown in Figures 1, 2, 3, and 4 as representative examples.
The sequence of reactions proposed for the decompositions is: where Ln = Sm and Nd where Ln = Ln, Pr and Gd where Ln = Ln, Pr and Gd
3.3.2. Thermal Analysis of Lanthanides of 4-abH
La, Pr, Nd, and Sm complexes with 4-abH show almost similar type of decomposition pattern in their thermograms. They undergo dehydration showing endotherms in the range of 50–71°C, dehydrazination showing exotherms in the range of 166–175°C and oxidative decomposition showing many exotherms in the range of 213–550°C to metal oxide residue. La and Pr complexes show the formation of metal carbonate intermediate [21] before final decomposition to metal oxide, whereas the other two do not.
Ce and Gd complexes, though found to follow similar type of decomposition to form metal oxide residue, the TG traces are not clear to identify dehydrazination and the intermediate formation. The thermal degradation patterns are indicated by the following reactions: where Ln = La, Ce, Pr, Nd, Sm and Gd where Ln = La, Pr, Nd and Sm where Ln = La and Pr where Ln = Nd and Sm
3.4. X-Ray Diffraction Analysis
The powder XRD patterns along with their d spacings are given in Table 5. The comparison of XRD patterns of the lanthanide complexes is shown in Figures 5 and 6. The XRD patterns of lanthanides of 4-abH and 3-abH reveal that each set of complexes has similarity in their structures implying similar compositions.
3.5. SEM-EDX Studies
The complexes were calcined in muffle furnace at their decomposition temperature, heating subsequently at the same temperature, and analyzed for their morphology and particle size. The SEM-EDX images of residues obtained from [Gd(3-ab)3(N2H4)2] and [Nd(4-ab)3(N2H4)]·H2O are shown in Figures 7, 8, 9, and 10. From the images, it is understood that the residues are microsized metal oxides with irregular shapes.
4. Conclusion
The isomeric acetoxybenzoic acids and hydrazine hydrate yield the complexes of formulae, [Ln(3-ab)3(N2H4)2]·xH2O where Ln = La, Ce, Pr and Gd and ; Ln = Nd and Sm and and [Ln(4-ab)3(N2H4)]·H2O where Ln = La, Ce, Pr, Nd, Sm and Gd resulting from their reaction with the respective metal nitrates. The compositions of the complexes were confirmed by elemental data. The presence of hydrazine in bidental bridging mode is inferred from their N–N stretching frequencies. The complexes of 3-abH release hydrazine exothermally in the temperature range 166–178°C whereas 4-abH complexes undergo exothermic dehydrazination at higher temperature range 180–197°C indicating that the two molecules of hydrazine are tightly held between the metal atoms in four directions. All the complexes of 3-abH undergo oxidative decomposition in the range of 437–477°C excepting for lanthanum, and the complexes of 4-abH in the range of 490–550°C. The lower-temperature decomposition of 3-abH complexes may be due to fuelling effect of two hydrazine molecules. The magnetic and electronic data indicate the presence of metal in the complexes. A comparable XRD data of complexes imply that they have similar type of structures. However, single crystal XRD can only confirm their structures. In our case since the complexes are insoluble in any solvent due to their polymeric nature, single crystals could not be prepared. SEM-EDX analysis indicated the oxides formation in microsize.
Acknowledgment
The authors wish to acknowledge the All India Council for Technical Education (AICTE) for sponsoring this work (Grant in aid no./8023/BOR/RID/RPS-2, 2008-2009).