Complexes of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III)

Abstract

The complexes of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) have been synthesized as polycrystalline hydrated solids, and characterized by elemental analysis, spectroscopy, magnetic studies and also by X-ray diffraction and thermogravimetric measurements. The analysed complexes have the following colours: violet for Nd(III), white for Gd(III) and cream for Ho(III) compounds. The carboxylate groups bind as bidentate chelating (Ho) or bridging ligands (Nd, Gd). On heating to 1173K in air the complexes decompose in several steps. At first, they dehydrate in one step to form anhydrous salts, that next decompose to the oxides of respective metals. The gaseous products of their thermal decomposition in nitrogen were also determined and the magnetic susceptibilites were measured over the temperature range of 76-303K and the magnetic moments were calculated. The results show that 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) are high-spin complexes with weak ligand fields. The solubility value in water at 293K for analysed 4-chlorophenoxyacetates is in the order of 10-4mol/dm³.

4-chlorophenoxyacetates; thermal stability; magnetic properties of Nd(III); Ho(III); Gd(III)

ARTICLE

Complexes of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III)

W.Ferenc^I,*; M.Bernat^I; J.Sarzyński^II; H.Głuchowska^I

^IFaculty of Chemistry, Maria Curie-Sklodowska University, PL 20-031 Lublin, Poland

^IIFaculty of Physics, Maria Curie-Sklodowska University, PL 20-031 Lublin, Poland

ABSTRACT

The complexes of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) have been synthesized as polycrystalline hydrated solids, and characterized by elemental analysis, spectroscopy, magnetic studies and also by X-ray diffraction and thermogravimetric measurements. The analysed complexes have the following colours: violet for Nd(III), white for Gd(III) and cream for Ho(III) compounds. The carboxylate groups bind as bidentate chelating (Ho) or bridging ligands (Nd, Gd). On heating to 1173K in air the complexes decompose in several steps. At first, they dehydrate in one step to form anhydrous salts, that next decompose to the oxides of respective metals. The gaseous products of their thermal decomposition in nitrogen were also determined and the magnetic susceptibilites were measured over the temperature range of 76-303K and the magnetic moments were calculated. The results show that 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) are high-spin complexes with weak ligand fields. The solubility value in water at 293K for analysed 4-chlorophenoxyacetates is in the order of 10^-4mol/dm³.

Keywords: 4-chlorophenoxyacetates, thermal stability, magnetic properties of Nd(III), Ho(III) and Gd(III).

Introduction

The carboxylates play an important role in inorganic and bioinorganic chemistry. Many metal cations are a component of several vitamins and drugs [1,2]. The carboxylates of d- and 4f- ion elements may be used as electric materials in the modern branches of techniques and technology. They may have also applications as precursors in superconducting ceramic and magnetic field productions and may be used as catalysts, pigments, solvents, food preservatives and plastics productions.

Metal carboxylates are applied for the productions of high degree purity of metal oxides and polycarboxylic acids are used for supermolecular compound synthesis, which in many cases, form with metal ions the molecular polymers containing in their structures, pores and channels owning to them they appear catalytic and adsorption properties. Therefore they may be used for the adsorption of inorganic gases such as: argon, nitrogen and hydrocarbons or small molecules of another inorganic compounds. Polycarboxylic acids may also form the molecules with two- and three-dimentional structures, yielding special magnetic and luminescence properties which let them be used in optical and electronic industries [3-10].

4-Chlorophenoxyacetic acid is a white solid hardly soluble in water (K=7,9 ·10^-4) and easily soluble in ether and ethanol [10,11]. It is used as growth hormone for plants [12]. In some papers the details of its molecular structure was presented [13-15]. Literature survey informs that its complexes with Li(I), Cu(II), Co(II), Mn(II) and Ni(II) ions were synthesized and their properties were studied [10,16,17].

As a continuation of our studies on chlorophenoxyacetates and also on carboxylates [10,18-30] we decided to synthesized 4-chlorophenoxyacetates with Nd(III), Gd(III) and Ho(III) and to study some of those of their properties not to have been investigated so far, such as magnetic properties in the range of 76-303K, thermal stability in air at 293-1173K, solubility in water at 293K and to record their FTIR spectra.

Thermal stability investigations give informations about the process of dehydration about the ways of decompositions, and the magnetic susceptibility measurements let study the kinds of the manner of central ion coordination. The determination of the solubility is valuable because it informs about the practical use of acid for separation of transition metal ions by extraction or ion-exchange chromatographic methods.

Experimental details

The 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) were prepared by the addition of the equivalent quantities of 0,1mol·dm^-3 ammonium 4-chlorophenoxyacetate (pH ≈ 5) to a hot solutions containing the 0,1 mol·dm^-3 Nd(III), Gd(III) and Ho(III) nitrates (V) and crystallizing at 293K (1h). The solids formed were filtered off, washed with hot water and methanol to remove ammonium ions and dried at 303K to a constant mass.

The contents of carbon and hydrogen in the complexes and in the intermediate and final products obtained from their thermal decompositions were determined by elemental analysis using a CHN 2500 Perkin-Elmer analyzer and the content of chlorine by the Schöniger method (Table 1).

The contents of M³⁺ were established gravimetrically and by XRF method using spectrometer of X-ray fluorescence with energy dispersion EDXRF-1510 (CANBERRA firm).

The IR and FIR spectra of complexes were recorded over the ranges 4000-400cm^-1 and 600-100cm^-1, respectively, using M-80 and Perkin-Elmer 180 spectrometers. Samples for IR spectra measurements were prepared as KBr discs. FIR spectra were obtained in Nujol mulls sandwiched between polyethylene plates (Table 2).

The FTIR spectra of the intermediate and final products obtained from the complex thermal decompositions were also registered.

The X-ray diffraction patterns of hydrated and dehydrated complexes, the intermediate and final products of complex thermal decompositions were taken on a HZG-4 (Carl Zeiss Jena) diffractometer using Ni filtered Cu Kα radiation. The measurements were made within the range 2Θ = 4-80º by means of the Debeye-Scherrer-Hull method. The relationships between I/I_o and 2Θ for these complexes are presented in Fig.1.

The thermal stability and decomposition of the prepared complexes were determined by Paulik-Paulik-Erday Q-1500D derivatograph with Derill converter, recording TG, DTG, and DTA curvers (Fig.2). The measurements were made at a heating rate of 10K min^-1 with a full scale. The samples (100mg) were heated in platinum crucibles in static air to 1173K with a sensitivity of TG-100mg. DTG and DTA sensitivities were regulated by a Derill computer program. The products of decomposition were calculated from TG curves and verified by the diffraction pattern registration and IR spectra. The termogravimetric analysis of DSC/TG was performed at temperature 323-723K using a differential thermoanalyzer Netzsch STA 409C 3F. The measurements were carried out under nitrogen flow (99,995% purity) and temperature increase rates of 1-12K min^-1.

Magnetic susceptibilities of polycrystalline samples of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) were measured by the Gouy method using a sensitive Cahn RM-2 balance. The samples were placed in a long cylindrical tube which was suspended from an analytical balance. The sample tube was positioned between the poles of the magnet such that one its end was in the region of homogenous field and the other end was in the region of zero field. The force exerted on the sample was a function of the volume occupied by the sample in the region of the field gradient. This force may be written in scalar form as a function of the isotropic volume susceptibility. Measurements were carried out at a magnetic field strength of 9,9 kOe. The calibrant employed was Hg[Co(SCN)₄] for which the magnetic susceptibility was assumed to be 1,644∙10^-5cm³g^-1. Correction for diamagnetism of the calibrant atoms was calculated by the use of Pascal's constants [31,32]. Magnetic moments were calculated from Eq.(1):

The solubilities of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) in water were measured at 293K. Saturated solutions of the obtained compounds were prepared under isothermal conditions. The contents of Nd(III), Gd(III) and Ho(III) were determined using ASA 880 spectrofotometer (Varian). The values of solubilities are presented in Table1.

Results and Discussion

The complexes of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) were obtained as polycrystalline products with a metal ion to ligand ratio of 1:3 and the general formula M(C₈H₆ClO₃)₃· nH₂O, where M(III)= Nd, Gd, Ho and n=2 for Nd and Gd and n=3 for Ho.Their colours are following: violet for Nd(III), cream for Ho(III), and white for Gd(III) complexes. In these compounds the f→f electron transitions of the central ions are those of the lowest energy and absorption occurs at relatively high wave lengths that depends on the nature of the metal ion [33,34].

The compounds were characterized by elemental analysis (Table 1), FTIR and FIR spectra (Table 2).

The 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) exhibit similar solid state IR spectra. The band at 1708 cm^-1 originating from the RCOOH group, presented in the acid spectrum, is replaced in the spectra of complexes by two bands at 1572-1567 cm^-1 and 1346-1328cm^-1, which can be ascribed to the asymmetric and symmetric vibrations of COO^- group, respectively [35-38].

The bands with the maxima at 3429-3410cm^-1 characteristic for ν_OH vibrations [35-40] confirm the presence of crystallization water molecules in the complexes.

The bands of C-H vibrations are observed at 2929-2918cm^-1. The bands of ν(C=C) ring vibrations appear at 1624-1622cm^-1, 1494cm^-1, 1177-1174cm^-1, 1111-1104cm^-1 and 1068-1066cm^-1.The valency ν(C-Cl) vibration bands occur at 734-730cm^-1 and those of the asymmetric and symmetric ν(C-O-C) at 1082-1068cm^-1 and 1060-1058cm^-1, respectively. The bands at 451-449cm^-1 confirm the ionic metal-oxygen bond [41-50]. These bands change insignificantly their positions which may suggest the different stability of these complexes.

The bands in the range of 118-106cm^-1 are connected with the O-H... O streching vibrations and they change their shapes according to the rise of atomic number of elements and the degree of hydration in 4-chlorophenoxyacetates. The bands at 138-122cm^-1 confirm the internal C-C torsion vibrations and the bands at 283-267cm^-1 arising from the aromatic ring vibrations also change their shapes.

Table 2 presents the values of the two band frequencies of asymmetrical and symmetrical vibrations for carboxylate group of analysed complexes.

The separations of the ν_as(COO^-), and ν_s(COO^-) modes in the compounds, Δν(COO^-), are smaller than that of the sodium salt (Δν_COO- = 271cm^-1) indicating a smaller degree of M-O ionic bonds in 4-chlorophenoxyacetates compared to that of the sodium salt. For the complexes the shifts of the frequencies of bands of ν_as(COO^-), and ν_s(COO^-) are lower, and lower or the same, respectively, than those for sodium 4-chlorophenoxyacetate. Accordingly, taking into account the spectroscopic criteria [36-38,49,51] the carboxylate ions appear to be bidentate groups.

In order to estimate the crystalline forms of the 4-chlorophenoxyacetates the X-ray powder diffraction measurements were done. The diffractogram values suggest them to be polycrystalline compounds with various degree of crystallinity [52] (Fig.1)

The thermal stability of Nd(III), Gd(III) and Ho(III)) 4-chlorophenoxyacetates was studied in air at 293-1273K (Table 3. Fig.2).

When heated to 1273K the Nd(III) complex is dehydrated in one step. In the temperature range of 338 - 416 K it losses two water molecules and forms anhydrous salt. The loss of mass calculated from TG curve is equal to 4,88% (theoretical value is 4,34%). The anhydrous salt at 553 - 1273 K is gradually decomposed to Nd₂O₃ that is the final product of complex decomposition. The intermediate compound formed in this range of 553-873K is NdOCl. The found mass loss is equal to 72,14% and the calculated value 73,45%. The residue mass estimated from TG curve is equal to 23,27% while that theoretically calculated 22,80%. The dehydration process is connected with endothermic effect whereas the combustion of the organic ligand is accompanied by exothermic one. The final product of complex decomposition was confirmed by IR spectra and X-ray powder diffractogram.

The dihydrate of Gd(III) 4-chlorophenoxyacetate during heating in air losses the water molecules in one stage in the range of 316 - 412 K and forms anhydrous salt. The final product of complex decomposition was identified by elemental analysis and X-ray powder diffractogram as Gd₂O₃. The intermediate product of complex decomposition is GdOCl formed at 573-923K. The found weight loss is equal to 71,42 and calculated one 72,18 %. The residue mass estimated from TG curve is 24,0% and theoretically determined 24,30%. The dehydration process is connected with the endoeffect while that of oxidation with exothermic one.

When heated in air to 1173K the trihydrate of 4-chlorophenoxyacetate of Ho(III) dehydrates in one step at 312 - 425K and forms anhydrous salt. The mass loss calculated from TG curve being equal to 6,68% corresponds to the loss of three molecules of water (theoretical value is equal to 6,96%). The anhydrous 4-chlorophenoxyacetate of Ho(III) at 573 - 1223K is decomposed to Ho₂O₃, which is the final product of thermal decomposition. The mass loss calculated from TG curve is equal to 74,29% that corresponds to the Ho₂O₃ formation (calculated value is 75,73%). The intermediate product of complex decomposition formed at 573-912K is HoOCl. The dehydration process is connected with the endothermic effect seen in DTA curve, while the combustion of the organic ligand is accompanied by exothermic one.

Considering the temperature at which the dehydration process of the complexes takes place, and the way by which it proceeds it is possible to assume that the water molecules may be in the outer sphere of complex coordination [41, 53-55].

The IR and FIR spectra recorded for analysed compounds may also suggest that the molecules of water are probably bounded by hydrogen bond in outer-sphere of complex. The detailed data obtained from the determination of the complete structures of these complexes can give fair answer concerning above assumption. However, their monocrystals have not been obtained so far, but attempts to obtain them have been made. The results indicate that the thermal decomposition of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) in air proceeds in the following way:

LnL₃ · nH₂O → LnL₃ → LnOCl → Ln₂O₃ where Ln(III)=Nd, Gd, Ho and n=2 for Nd and Gd and n=3 for Ho.

The values of enthalpy of dehydration process were determined with the use of DSC/TG system under nitrogen atmosphere. They are in the range of 156,80 - 90,81 kJ·mol^-1 and 68,25 - 45,4 kJ·mol^-1 per one molecule of water. These values indicate that the water molecule is the strongest bounded in the Gd(III) complex while that the weakest in Nd(III) compound. Taking into account the temperatures of dehydration process it appears that Nd(III) complex is the most thermally stable while Ho(III) has the least thermal stability.

The solubility of analysed compounds was measured (Table 1). It is in the order of 10^-4 mol·dm^-3. The Ho(III) 4-chlorophenoxyacetate is the most soluble salt, while that of Nd(III) the least soluble one. These compounds are relatively hardly soluble in water and the order value is not sufficient to use 4-chlorophenoxyacetic acid for the separation of some metal ions by ion-exchange chromatography method [10].

The magnetic susceptibility of 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) was measured over the range of 76-303K (Table 4). The values of the Weiss constant, Θ, for analysed complexes were found to have a negative sign which probably arises from the antiferromagnetic spin interaction or from a crystal field splitting of the paramagnetic spin state [56-61]. The analysed complexes obey the Curie-Weiss law.

The magnetic moment values experimentally determined at 76-303K for Nd(III), Gd(III) and Ho(III) compounds change from 2,95 µ_B (at 76K) to 3,47 µ_B (at 303K) for Nd(III) complex, from 7,70 µ_B (at 76K) to 7.82 µ_B (at 303K) for Gd(III), and from 10.07 µ_B (at 76K) to 10,56 µ_B (at 303K) for Ho(III) 4-chlorophenoxyacetates.

In 4-chlorophenoxyacetates of Nd(III), Gd(III) and Ho(III) the paramagnetic central ions remain practically unaffected by diamagnetic ligands coordinated around them. The f-electrons causing their paramagnetism are well separated from outside influences and they do not participate in the formation of M-O bond. They are in an inner shell characterized by radius 0,35Å [62] to be very small in comparison with the radius of the 5s²5p⁶ closed shell (~1 Å).Their energy levels are the same as in the free ions. For most of the lanthanide ions the ground state is separated by several hundred of cm^-1 from the next higher lying state. Therefore the magnetic properties can be taken as those of the ground state alone. Taking into account this fact lanthanide ions in the compound act in the same way as the free ions. The values of µ_eff determined for 4-chlorophenoxyacetates are close to those calculated for Ln³⁺ ions by Hund and Van Vleck (Table 5). Their values at room temperature for analysed complexes are following: 3,74 µ_B for Nd; 7,23 µ_B for Gd and 10,60µ_B for Ho [63].

From the values of magnetic moments determined for the complexes it appears that energies of 4f electrons in the central ions are not changed compared to those in the free lanthanide ions. Thus, the colours of the complexes stay the same as those in the free lanthanide ions. The electron density in the molecules makes the f-f electronic transitions of the central ions to be those of the lowest energy and the absorption occurs at relatively high wavelengths. The 4f orbitals of lanthanide ions effectively shielded by the 5s²5p⁶ octet. Therefore the metal ligand bonding in lanthanide complexes is mainly electrostatic in nature [64,65].

Conclusions

On the basis of the results obtained it appears that 4-chlorophenoxyacetates of Nd (III), Gd(III), and Ho(III) were synthesized as hydrated complexes. Their colours are typical for the particular Ln(III) ion, i.e. violet for Nd, white for Gd, and cream for Ho, having their origin in the lowest energy of f-f electronic transitions of the central ions. Their energies are not radically changed in comparison with the free lanthanide ions, therefore the colours of the complexes are the same as those for the free lanthanide ions. The Ln-O band is mainly electrostatic in nature. The complexes are crystalline compounds that on heating in air to 1173K decompose in three steps. In the first step they dehydrate to form anhydrous complexes that next decompose to the oxides of appropriate metals with intermediate formation of LnOCl. The values of µ_eff calculated for the compounds ale close to those obtained for Ln³⁺ by Hund and Van Vleck. There is no influence of the ligand field of 4f electrons on lanthanide ions.

The solubilites of Nd(III), Gd(III) and Ho(III) 4-chlorophenoxyacetates in water at 293K are of the order of 10^-4mol·dm^-3.

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Publication Dates

• Publication in this collection
  09 June 2010
• Date of issue
  2010