Introduction

The lanthanide compounds including N- and O-donors, as 4-bpy and carboxylate groups are very interesting because of their structural diversity and also possibility to use as new solid microporous materials. These coordination compounds have high microporosity comparing to conventional porous materials such as zeolites or activated carbon. They have various possible applications in ion exchange, gas storage (CH4, H2, N2, O2, and CO2), gas separation, heretogeneous catalysis, etc. [1]. These types of compounds are still actual in researches. 4,4′-Bipyridine is used as a potential ligand since two nitrogen donor atoms. This N-donor may create polymeric species [2–6]. Up to now, there are not many papers describing lanthanides complexes with bipyridine isomers and halogenoacetates [7–11].

This work is a continuation of our studies, and presents the synthesis, some physico-chemical properties, and thermal investigations of new complexes of Y(III) and La(III) with title ligands.

Earlier, we had obtained the compounds with N-donor (4-bpy) and dichloroacetates of Y(III) and La(III) with general formulae: Y(4-bpy)(CHCl2COO)3·H2O and [La(4,4′-bipyridine)(CCl2HCOO)3(H2O)] n were described in [6, 12]. They were characterized by elemental and thermal analysis, IR, and conductivity studies. The crystal and molecular structure of lanthanum(III) complex [6] were determinated.

Experimental

Materials, synthesis, and analysis

4,4′-Bipyridine, CCl3COOH, CHBr2COOH, Y2O3, La2O3, dimethylsulfoxide (DMSO), dimethylformamide (DMF), and methanol (MeOH) (anhydrous) p.a. were obtained from Aldrich and Lab-Scan. Water solutions of metal(III) trichloroacetates or metal(III) dibromoacetates were prepared by adding 2 mol L−1 trichloro- or dibromoacetic acid to freshly precipitated hydroxides in ca. stoichiometric quantities (in temperature ≤291 K, because lanthanide trichloro- and dibromoacetates in solution are relatively unstable; in the presence of 4-bpy, their stability arise). The contents of metal(III) ions in obtained solutions were complexometrically determined. The synthesis of complexes was analogous, as described in [13].

The carbon, hydrogen, and nitrogen contents in the prepared complexes were determined by a Carbo-Erba analyzer using V2O5 as an oxidizing agent. Obtained compounds were mineralized and metals(III) in describe complexes were determined by EDTA titration.

Methods and instruments

IR spectra were recorded using a NICOLETT 6700 Spectrometer (4000–400 cm−1 with accuracy of recording 1 cm−1) using KBr pellets. Molar conductance was measured on a conductivity meter of the OK-102/1 type equipped with an OK-902 electrode at 298 ± 0.5 K, using 1 × 10−3 mol L−1 solutions of complexes in methanol, dimethylformamide, and dimethylsulfoxide. The thermal properties of complexes in air were studied by TG-DTG techniques in the range of temperature 298–1273 K at a heating rate of 10 K min−1; TG and DTG curves were recorded on Netzsch TG 209 apparatus in flowing dynamic air atmosphere v = 20 mL min−1 using ceramic crucibles. From TG and DTG curves, some of the solid intermediate decomposition products were determined and were confirmed by the IR spectra of sinters. In sinters (prepared during heating of complexes up to temperatures defined from TG or DTG curves), the vibration modes of 4-bpy and halogenoacetates were analyzed as well as the presence of anions Cl− or Br− were also stated. The TG-FTIR coupled measurements have been carried out only for complexes II and IV using the Netzsch TG 209 apparatus coupled with Bruker FTIR spectrophotometer, in the range of temperature 293–973 K at a heating rate 10 K min−1 in flowing argon atmosphere v = 20 mL min−1 in ceramic crucibles. The X-ray powder diffraction patterns of synthesized complexes and final solid decomposition products in air were recorded on D-5000 diffractometer using Ni-filtered CuKα radiation. The measurements were carried out in the range of 2θ angles 2–80°. Obtained results were analyzed using the Powder Diffraction File [14].

Results and discussion

Table 1 presents results of the elemental and chemical analysis of investigated compounds. They are stable in air in solid state and the monocrystals of them have not been obtained yet. The analysis of the power diffraction patterns of these compounds reveals that they are small crystalline products. The molar conductivity values for complexes in MeOH, DMF, and DMSO are given in Table 1. La(4-bpy)(CHBr2COO)3·H2O in DMSO, complexes I–IV in MeOH, and II–IV in DMF display behaviors intermediate between those of non-electrolytes and 1:1 electrolytes. The compound III in DMSO is electrolyte type 1: 1. Y(4-bpy)2(CCl3COO)3·H2O in DMF, Y(4-bpy)2(CCl3COO)3·H2O and La(4-bpy)1.5(CCl3COO)3·2H2O in DMSO fall within the generally acceptable range for non-electrolytes. Very low molar conductance values indicate the non-electrolytic nature of them [15].

Table 1 Analytical data and molar conductivity in MeOH, DMF, and DMSO for investigated complexes
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IR spectra

IR spectra of all the obtained complexes exhibit several absorption bands characteristic for 4-bpy and carboxylate groups.

The fundamental vibration modes of 4-bpy for complexes are reported in Table 2. During coordination with lanthanide ions, the IR spectrum of free 4,4′-bipyridine changes. The most characteristic ring vibration modes ν(CC), ν(CN), ν(CCir)-A 1 symmetry, and ν(CC), ν(CC)-B 1 symmetry are at 1588 and 1530 cm−1 in the free ligand [16]. In the IR spectra of complexes, they appear at 1600–1608 and 1530–1539 cm−1, respectively. The ring deformation modes are noticed between 1000–1003 cm−1 and are shifted to higher frequencies in comparison with free 4-bpy (988 cm−1). These bathochromic shifts of principal absorption bands suggest that 4-bpy is coordinated to Y(III) and La(III) ions [16].

Table 2 IR bands for free 4- and 4-bpy in the obtained complexes
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In the IR spectra, there are also bands of vibrations of asymmetric νas(COO) and symmetric νs(COO) modes for carboxylate groups (Tables 3, 4). For complexes I and II, they are in the range 1636–1677 and 1346–1382 cm−1, respectively. In both case, the νas(COO) and for compound I also νs(COO) are split into doublet. The carboxylate groups in complexes I and II act as bidentate-chelating, non-completely equivalent [17] (more or less symmetrical [18]) ligands. For complexes I and II, vibrations νas(CCl3) are shifted to higher wave numbers according to CCl3COONa. In the case of compounds Y(4-bpy)1.5(CHBr2COO)3·3H2O III and La(4-bpy)(CHBr2COO)3·H2O IV, the bands of vibrations of νas(COO) and νs(COO) are between 1650–1663 and 1386–1389 cm−1, respectively. On the grounds of spectroscopic criteria [19–22], it can be stated that in complexes III and IV carboxylate groups are bonded as monodentate donors (the values of Δν = νas−νs of these complexes are higher than for sodium salt). The vibrations of ν(CH) appear in sodium salt at 3015 and 1191 cm−1, in complexes are at 3027 cm−1 for III, 3020 cm−1 for IV, and at 1186 cm−1 in spectra of III, 1192 cm−1 in the case of IV. The stretching modes νs(CBr2) in Y(4-bpy)1.5(CHBr2COO)3·3H2O and La(4-bpy)(CHBr2COO)3·H2O are observed at 700 cm−1. The vibrations δ(CCOO) at 1148 cm−1 in CHBr2COONa spectra, in compounds III and IV appear at 1150 cm−1.

Table 3 IR bands for CCl3COONa and COO− groups in the obtained complexes
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Table 4 IR bands for CHBr2COONa and COO− groups in the obtained complexes
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All the complexes exhibit intensive and broad band in the water stretching region (ca 3560–3350 cm−1) and only shoulder in the water bending region (ca 1730–1690 cm−1).

Thermogravimetric data in air

The thermal decompositions of described complexes have been studied in air by TG-DTG method. Pyrolysis of analyzing complexes in air is a multistage, overlapping process and complicated to interpret. Several steps of thermolysis and the solid products were determined from TG and DTG curves. Some intermediate species were verified by investigation of the sinters obtained during heating of the samples of complexes up to temperature defines from the thermal curves. The intensity of DTG peaks is different. When temperature rises, the peaks on DTG profiles are weaker. All compounds lose water molecules in one step. When temperature rises, decomposition of halogenoacetates begins and intermediate products are formed. Figures 1, 2, 3, and 4 show thermal profiles of investigated complexes. Table 5 presents thermal decomposition results of Y(III) and La(III) compounds in air and only La(III) complexes in argon atmosphere.

Fig. 1
figure 1

TG and DTG curves of thermal decomposition of Y(4-bpy)2(CCl3COO)3·H2O recorded in air atmosphere; mass sample 5.99 mg

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Fig. 2
figure 2

TG and DTG curves of thermal decomposition of La(4-bpy)1.5(CCl3COO)3·2H2O recorded in air atmosphere; mass sample 7.41 mg

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Fig. 3
figure 3

TG and DTG curves of thermal decomposition of Y(4-bpy)1.5(CHBr2COO)3·3H2O recorded in air atmosphere; mass sample 8.03 mg

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Fig. 4
figure 4

TG and DTG curves of thermal decomposition of La(4-bpy)(CHBr2COO)3·H2O recorded in air atmosphere; mass sample 9.67 mg

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Table 5 Thermal decomposition data (A) for Y(III) and La(III) complexes in air; (B) only for La(III) compounds in argon atmosphere
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Thermal decomposition of complexes I and II in air begins at 333 K. It is associated with the release of all water molecules. Mass losses calculated for dehydration processes are 1.99 % for I and 4.02 % for II, when these determined from the thermogravimetric curves are 2.2 % and 4.0 %, respectively. In the second step, the anhydrous compound Y(4-bpy)2(CCl3COO)3 decomposes probably (in temperature range 353-375 K) to Y(4-bpy)2(CCl3COO)2.5Cl0.5 (mass loss: found. 7.2 %; calc. 7.00 %). This step is clearly presented on DTG curve (strong peak at 368 K). We have also similar observations in the case of complex Y(4-bpy)(CCl2HCOO)3H2O described in [12]. Elevation of temperature induces that several peaks on DTG curve and mass losses are observed. It is connected with further decomposition of organic ligands and produces other intermediate decomposition products (which are not examined). At temperature 923 K, stoichiometry quantity equivalent for YOCl is indicated from TG curve. It connects with peak on DTG curve at 793 K. At 1038 K, the final pure product Y2O3 is obtained (found 12.5 %, calculated 12.46 %). In the case of anhydrous complex II, the DTG curve exhibits at 483 K a sharp peak indicating the maximum mass loss rate (42.5 %). It is connected with total decomposition of chloroacetates. When temperature increases, on DTG curve, peaks appear at 643 and 825 K. The TG and DTG curves suggest that at 933 K probably LaCl3 exists (found. 27.0 %; calc. 27.32 %). When temperature rises, it very slowly undergoes further destruction to La2O3.

For both complexes Y(4-bpy)1.5(CHBr2COO)3·3H2O III and La(4-bpy)(CHBr2COO)3·H2O IV, the first mass loss observed on TG curves corresponds to evaluation of all water molecules in temperature interval 328–373 and 373–418 K, respectively. Probably in temperature range 373–453 K for III the partial destruction of dibromoacetate groups takes place and intermediate species Y(4-bpy)1.5(CHBr2COO)2.25·Br0.75 is formed (mass loss found. 9.6 %; calc. 9.99 %). It is connected with very strong peak on DTG curve at 438 K. Next, further step-wise decomposition of organic ligands occurs (thermogravimetric curves show the presence of several overlap processes). In temperature range 533-975 K, mass loss observed on the thermal curve is associated with total destruction and combustion of organic ligands, YOBr is formed. Above 975 K, YOBr transforms to Y2O3. Horizontal mass level for Y2O3 begins at 1043 K (found 11.3 %, calc. 10.99 %). Anhydrous compound IV starts to decompose at 418 K. This pyrolysis is very similar to the thermal decomposition data of studied complexes. The solid product of thermolysis obtained at 1113 K is LaOBr (found. 24.9 %; calc. 24.40 %).

TG-FTIR study for La(III) complexes in argon

The combined TG-FTIR techniques was employed to study thermal decomposition and the gas generated in flowing argon atmosphere only for La(III) complexes. TG and DTG profiles of complexes II and IV are shown in Figs. 5 and 6. Thermal decomposition data are collected in Table 5. Dehydration of compound II takes place in the range 333–453 K, while elimination of water for complex IV occurs between the range 353–433 K. The mass loss from this process is expected to be 4.02 % for II and 1.87 % in the case of IV; recorded 4.7 % and 2.0 %, respectively. Anhydrous complexes start to decompose at 453 K for II and 433 K for IV with several overlapping stages. The sharp DTG peak at 507 K for II and peak at 513 K with a shoulder at 493 K for IV; in both cases they correspond to a rapid loss in mass. This decrease in mass ascribed to stepwise decomposition of halogenoacetate ligands (vide FTIR spectra). Next, degradation of organic ligands takes place. The pyrolysis for II is finished at about 953 K, residual 46.3 %. In the case of IV, residual is at 933 K and has a value 44.5 %. It is probably due to the mixture of inorganic species and solid organic fragments. The compositions of these products were not investigated.

Fig. 5
figure 5

TG and DTG curves of thermal decomposition of La(4-bpy)1.5(CCl3COO)3·2H2O recorded in argon atmosphere; mass sample 14.65 mg

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Fig. 6
figure 6

TG and DTG curves of thermal decomposition of La(4-bpy)(CHBr2COO)3·H2O recorded in argon atmosphere; mass sample 9.35 mg

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The elevation of the gaseous products during decomposition of the La(4-bpy)1.5(CCl3COO)3·2H2O and La(4-bpy)(CHBr2COO)3·H2O are shown in Figs. 7 and 8. The TG-FTIR spectra for compounds II and IV are very similar, but the peaks can be attributed to the volatile species of halogen-containing groups. The selected vibration modes are only indicated on FTIR spectra in different thermal intervals. The assignment of peaks in the FTIR spectra was attributed by comparison with literature data [1, 20, 23]. The recorded spectra confirms that the first step of thermal decomposition of compound II is dehydration process (absorption peaks between 3750–3550 cm−1). Decomposition of the desolvated form of the compound is connected with emission of CO2 molecules. It gives the characteristic double bands at 2360–2280 and 720–640 cm−1, which ascribes to the valence and deformation vibrations, respectively. The maximum elevation for compound II is observed at 508 K, which is strictly the same temperature that the sharp DTG peaks (507 K). In the case of La(4-bpy)(CHBr2COO)3·H2O, the maximum rate of forming CO2 is at ca 494 K; DTG peak at 513 K with a shoulder at 493 K. When temperature rises, the emission of CO2 decreases. It may be imply the final stage of decomposition of carboxylate groups. The double band locates at 2250–2150 cm−1 can be attributed to carbon monoxide. The absorption bands of vibrations of CH groups evolving from hydrocarbons lie between 3050–2680 cm−1 for II and 3035–2800 cm−1 for compound IV. The vibration modes of β(CH) in plane for II and IV are observed at 1265 and 1300 cm−1, respectively. However, γ(CH) out of plane are recorded in the range 800–750 cm−1 in both cases. The fragments of 4-substituted pyridine (ν(CC), ν(CN)) for investigated complexes are detected between 1600–1580 cm−1. In addition, during thermal decomposition of these complexes, the evaluations of volatile species containing halogen are produced. The FTIR spectra of the complex II shows absorption bands at ca 840 cm−1 and 745, 680 cm−1, which are assigned to the νas(CCl3) and νs(CCl3), respectively. On the other hand, for compound IV only νs(CBr2) at 695 cm−1 occurs. The absorption modes of HBr were found at ca 2355 cm−1.

Fig. 7
figure 7

Typical FTIR spectra in argon of gaseous species produced during decomposition of La(4-bpy)1.5(CCl3COO)3·2H2O in different temperatures

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Fig. 8
figure 8

Stacked plot of FTIR spectra of the evolved gases for La(4-bpy)(CHBr2COO)3·H2O complex in argon

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Conclusions

Here, we describe new complexes with formulae: Y(4-bpy)2(CCl3COO)3·H2O, La(4-bpy)1.5(CCl3COO)3·2H2O, Y(4-bpy)1.5(CHBr2COO)3·3H2O, and La(4-bpy)(CHBr2COO)3·H2O. It courses, that La(4-bpy)(CHBr2COO)3·H2O has the same stoichiometric formula as La(4-bpy)(CHCl2COO)3·H2O [6]. They are small, crystalline substances. On the base of IR spectra of these compounds, it may be stated that 4,4′-bipyridine coordinates to metal(III) ions [16]; carboxylate groups in complexes I and II act as bidentate-chelating, non-completely equivalent [17] (more or less symmetrical [18]) ligands; in case of compounds III and IV carboxylate groups are linked with metal(III) as monodentate donors [19–22]. The thermal decomposition of the described complexes in flowing dynamic air atmosphere is a multistage and overlapping process. In all cases, dehydration is the first step of pyrolysis. Next, partial and total decomposition of organic ligands takes place. The final solid products of thermolysis are: Y2O3 for I and III. In the case of lanthanum(III) complexes were not observed horizontal mass level.

Comparing complex Y(4-bpy)(CHCl2COO)3·H2O [12] with Y(4-bpy)2(CCl3COO)3·H2O I and Y(4-bpy)1.5(CHBr2COO)3·3H2O III, we notice that their thermal stability in air is very similar: (328–333 K). It changes for anhydrous species: Y(4-bpy)(CHCl2COO)3 [12] (403 K) >Y(4-bpy)1.5(CHBr2COO)3 III (373 K) >Y(4-bpy)2(CCl3COO)3 I (353 K).

In the case of lanthanum(III) complexes, the thermal stability in air presents in the following line:

  • La(4-bpy)(CHBr2COO)3·H2O IV (373 K) = La(4-bpy)(CHCl2COO)3·H2O [6] (373 K) > La(4-bpy)1.5(CCl3COO)3·2H2O II (333 K).

and after dehydration:

  • La(4-bpy)(CHCl2COO)3 [6] (433 K) > La(4-bpy)(CHBr2COO)3 IV (418 K) > La(4-bpy)1.5(CCl3COO)3 II (413 K).

The TG-FTIR study also suggests several steps of thermolysis in argon atmosphere. The first process is dehydration containing elevation of water. When temperature rises, partial and total decomposition of organic ligands takes place. The gas evolved contains mainly H2O, CO2, CO, hydrocarbons, and mixture of volatile organic fragments coming from decomposition of appropriate halogenoacetates.

In summary, the investigation in this paper complete the information of the solid species containing 4,4′-bipyridine and dichloro- or trichloroacetates in the lanthanide series: Y, La → Lu without Pm (obtained in similar manner), additionally some information about Ln(III) species with 4,4’-bipyridine and monochloroacetates [13].