Synthesis and Characterization of some Hippurato Rare Earth Metal Complexes

INTRODUCTION

The study of the reaction between metals and hippuric acid, 4-amino hippuric acid, nucleoside or related base type is a topic of increasing interest due to their presence in biological system. It is produced in the renal metabolism of P-amino benzoic and it is also involved in sulfate transport in human neutrophils.1-3 From the chemical studies for the hippuric acid derivatives such as 4-amino hippuric acid indicate that the presence of an NH2 group on the phenyl ring could, in principle, be a potential donor group in metal coordination.4-8 In these studies the ligand is reported to act as an N, O bridging ligand besides its normal monodentate behavior through the carboxylate oxygen.

Hippurates of Mn(II), Cd(II), Ag(I), Zn(II) and Hg(II) were isolated from solutions as crystal solids. 8-12 The infrared spectra for all studied hippurate complexes and X–ray crystal structure of the Cd(II) hippurate complex8 indicate that metal -hippurate coordination involves only the carboxylate group via the two oxygen atoms as a chelating ligand.

The aim of this work is to synthesize and examine systematically, in the solid state, the new hippurate complexes formed on the reaction of lanthanum(III) nitrate and cerium(III), Samarium(III) chlorides with hippuric acid shown in (I) in alkaline media. The products complexes were characterized by their elemental analysis, IR, 1HNMR and electronic spectral as well as thermo gravimetric analysis to elucidate the coordination properties of hippuric acid with these metal ions.

 

EXPERIMENTAL

All of the reagent employed in this investigation was of analytical grade. The white solid complex [La(hip)3]·4H2O was prepared by adding lanthanum(III) nitrate hexahydrate (0.866 g, 2 mmole) in 10 ml bidistilled water dropwisly to a stirred mixture of hippuric acid (1.074 g , 6 mmol) in 5 ml ethanol and sodium hydroxide (0.240 g, 6 mmol) in 50 ml bidistilled water. The reaction mixture was heated to about 60 ℃ for about 3 h., the resulting precipitate was filtered, washed several times with hot water, and dried over phosphorous pentoxide. The hippurate complexes [Ce(hip)3]·7H2O and [Sm(hip)3]·8H2O were prepared in a similar way to that described above by the reaction of, CeCl3·7H2O and SmCl3·6H2O with hippuric acid in a molar ratio of 1:3, respectively.

Elemental C, H, N analysis were carried out on a Perkin Elmer CHN 2400. Lanthanum, cerium and samarium contents were determined gravimetrically by transforming the product into the corresponding oxides. The obtained analytical data are summarized in Table 1. Infrared spectra (4000-400 cm−1) were recorded as KBr pellets on a Gensis II FT IR spectrometer and the electronic spectra were registed on a Shimadzu UV -spectrophotometer model 1601 PC in the region of 700-200 nm. 1HNMR spectra were recorded on a Varian Gemini 200 MHZ, at room temperature.

Table 1.Analytical data

Thermegravimetric (TG) was carried out under N2-atmosphere using detectors model Shimadzu TGA-50 H.

 

RESULTS AND DISCUSSION

Hippuric acid reacts with La (III) nitrate and Ce (III), Sm (III) chlorides in alkaline aqueous media at about 60 ℃ to form the obtained solid hippurate complexes formulated as [La(hip)3]·4H2O, [Ce(hip)3]·7H2O and [Sm(hip)3]·8H2O, respectively. The infrared spectra of hippuric acid and its complexes are given in Fig. 1, and their band assignments are given in Table 2. In the infrared spectra of the La(III), Ce(III) and Sm(III) complexes some bands disappear at 1758 and 1750 cm−1, arising from the free carboxylic acid (-COOH) group as shown in the spectrum of hippuric acid. This indicates that the hydrogen ion in the hippuric acid molecule is substituted by the metal ions. However, the IR spectra of the prepared hippurates show strong absorption bands in the region of 1578-1537 cm−1 due to the asymmetrical vibration of the carboxylate anion, νas(COO−). The corresponding symmetric vibration νs(COO−) is observed in the region 1426-1405 cm−1. The shift of the absorption band of νas(COO−) to lower frequencies, suggests that carboxylate anion in the complexes under study behaves as a chelating ligand13 (Table 2). The coordination of the metal ions via oxygens of the carboxylate is confirmed by observing M-O band stretching vibrations, ν(M-O) at 538 and 494 cm−1 for La (III), at 588, 530 and 499 cm−1 for Ce (III) and at 588 and 446 cm−1 for Sm (III) complex. These bands are not observed in the spectrum of hippuric acid. A new broad band is also observed in the complexes spectra in the region 3400-3470 cm−1 due to the vibration ν(OH) of lattice water in the hydrated complexes. The infrared spectra of M(III) hippurates display a group of bands due to N-H vibrations at 3365-3070 cm−1 and 1650-1598 cm−1. This last band exhibits both bendbending motion, δ(N-H) and ring vibrational character. The assignment of those bands agrees quite well with those for related complexes containing typical hippurate ligands.8,13-16 The complexes were also investigated by 1HNMR in DMSO-d6 as a solvent Fig. 2, where the data obtained are in agreement with the proposed coordination through the carboxylic group (disappearance of the H(1) signal) in a symmetrical geometry and the peaks characteristic for water molecules were observed around δ 3.52 ppm. The 1HNMR data for free hippuric acid: δ 12.5 (H, H(1), COOH), 8.84, 8.80 [H, H(3)], 7.91 [2H, H(6)], 7.53[3H, H(7.8)], 3.97, 3.94 [2H, H(2)], for [La(hip)3]·4H2O: δ 8.34 [1H, H(3)], 7.88 [2H, H(6)], 7.45 [3H, H(7,8)], 3.85 [2H, H(2)], 3.25, 3.17 [H, (H2O)], for [Ce(hip)3]·7H2O: δ 9.24 [1H, H(3)], 7.96 [2H, H(6)], 7.51 [3H, H(7, 8)], 4.93 [2H, H(2)], 3.76 [H, (H2O) and for [Sm-(hip)3]·8H2O: δ 8.8 (1H, H(3), 7.88 (2H, H(6), 7.50 3H , H (7.8)] , 3.94 [2H, H (2)], 3.31 [H, (H2O)]. The aromatic signals H(6), H(7) and H(8) nearly do not shift significantly, thus showing that the magnetic environment of the aromatic ring has not changed significantly with coordination.

Fig. 1.Infrared spectra of: (A): Hippuric acid, (B): [La(hip)3]·4H2O, (C): [Ce(hip)3]·7H2O, (D): [Sm(hip)3]·8H2O.

Table 2.(a): s=strong, w=weak, m=medium, sh=shoulder, v=very, br=broad. (b): ν, stretching; δ and δr correspond to bending and rocking motions, respectively.

Fig. 2.1HNMR Spectra of: (A): Hippuric acid, (B): [La(hip)3] ·4H2O, (C): [Ce(hip)3]·7H2O (D): [Sm(hip)3]·8H2O.

To make sure about the proposed structure for our hippurate complexes the electronic spectra were carried out in dimethyl sulfoxide, which absorbed around 260 nm, Fig. 3. The absorption spectra of M(III) hippurate are compared with the absorption maxima of hippuric acid, there are evident that the increasing in the absorbance (hyperchromic effect) clasified in all of the three mentioned hippuric acid complexes attributed to the complexation behaviour of hippuric acid towards metal ions.

Fig. 3.Electronic spectra of: (A): Hippuric acid, (B): [La(hip)3] ·4H2O, (C): [Ce(hip)3]·7H2O (D): [Sm(hip)3]·8H2O.

Thermogravimetric (TG) was carried out under a N2 atmosphere, Fig. 4. Decomposition mechanisms have been shown in Table 3. Decomposition of the complexes starts at different temperatures and exhibits two degradation stages. The hydrated complexes of Ce(III) and Sm(III) lose upon heating some water molecules in one step within the temperature range of 50-150 ℃ with an accompanying a weight loss of 9.12% and 6.22%, corresponding with the loss of four and three water molecules in agreement with the theoretical values of 9.00% and 6.52%, respec-tively, while the complex [La(hip)3]·4H2O is dehydrated completely in one step at a maximum temperature of 89 ℃. The relatively low value of temperature of this step may indicate that these water molecules undergoes less H-bonding with hippurate anion. The trihydrated Ce(III) hippurate and penta hydrated Sm(III) complex are simultaneously dehydrated and decomposed to the corresponding oxide at the temperature range 150-800℃ with intermediate formation of very unstable products13 which were not identified. The dehydrated La(III) complex is stable within the temperature range of 110-250 ℃ and then decompose to oxide, La2O3 within the temperature range of 250-800 ℃. The proposed structural formula on the basis of the results discussed in our paper located as follows:

Fig. 4.TG diagrams of: (A): [La(hip)3]·4H2O, (B): [Ce(hip)3]·7H2O, (C): [Sm(hip)3]·8H2O.

Table 3.Thermal data of the decomposition reactions of La(III), Ce(III) and Sm(III) hippurates

The infrared spectra of the final products Fig. 5, show the absence of all bands associated with the hippurate anion and water molecules and instead the characteristic spectra for oxides are appeared. According to the above discussion, the mechanisms proposed for the thermal decomposition of hippurato complexes are summarized as follow:

Fig. 5.The final decomposition products of: (A): [La(hip)3]·4H2O, (B): [Ce(hip)3]·7H2O (C): [Sm(hip)3]·8H2O.