Coordination complexes of rare earth metals with hydrazine and isomeric acetamidobenzoates as ligands– spectral, thermal and kinetic studies

2.1 Preparation of [Ln{2/3/4-C₆H₄(NHCOCH₃)₃}.(N₂H₄)], where Ln = La, Ce, Pr, Nd, Sm& Gd

Lanthanum oxide (0.325 g, 1 mmol) was dissolved in a minimum quantity of 1:1 nitric acid (HNO₃ Sp. gravity 1.42 g/mL), evaporated to eliminate excess of acid, and the residue was dissolved in 20 mL of ethanol. To a freshly prepared (1:1) ethanolic solution (40 mL) of the ligands containing 2-acetamido benzoic acid (1.075g, 6 mmol): hydrazine hydrate (0.912g, 8 mmol), the alcoholic solution of the metal was added stirring the reaction mixture vigorously. A pale yellow microcrystalline solid formed immediately at pH 3 and was kept over a hot water bath for 5 min at 90 °C. The product was cooled, filtered, and washed with ethanol and ether and then dried in a desiccator.

All the other complexes of 2, 3 & 4-acetamido benzoates were prepared by a similar procedure by adding the respective metal nitrate solution to the ligand solution in the mole ratios Metal:Acid:Base ≡ 1:6:8, 1:4:4 and 1:6:8 at pH 3, 4.5 respectively. The microcrystalline solid products formed were filtered, washed with alcohol and ether, and then they were dried over anhydrous CaCl₂ in desiccators. The preparation scheme is described below.

2.2 Physicochemical methods

The hydrazine content in all complexes was determined volumetrically using 0.025 M Potassium iodate solution under Andrews’ conditions (Vogel, 2019). The metal content was determined by EDTA complexometric titration (Vogel, 2019) after decomposing a known weight of the sample with 1:1 HNO₃. Magnetic susceptibility measurements of the complexes were carried out on a vibrating sample magnetometer, VSM EG & G model 155. The electronic spectra for solid-state complexes were obtained using a varian, Cary 5000 recording spectrophotometer. Infra-red 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 mass spectrum was obtained using TQD–WATERS liquid chromatography coupled mass spectrophotometer. The ¹³C and ¹H NMR spectras were recorded for all the samples dissolved in DMSO using JEOL-NMR device with an operating frequency of 600 MHZ.

3.1 Magnetic susceptibility and electronic spectra measurements

The effective magnetic moment values of La, Ce, Pr, Nd, Sm, and Gd complexes were obtained from the VSM graph using the following formula μ_eff = 2.828 √_χmT), where _χm (magnetic moment of the lanthanide samples) = (Molecular Weight of the Sample / Sample Wt) X Slope (Bohr Magneton) and tabulated (Table 1), which are in good agreement with reported values (Want and Shah, 2016). Besides the observed values were compared with theoretical effective values calculated using the formula μ_eff = gJ √J(J + 1) μB, where μ_B - BM respectively.

The absorptions observed in electronic spectra of Pr and Nd complexes and the levels assigned are given in Table 2. The absorptions of Nd complex are comparable to that of Nd bis-bipyridyl complexes suggesting coordination number 8 in it (Petit et al., 2006).

On comparison of electronic spectral data of Pr and Nd complexes with their aquo complexes (Parimalagandhi et al., 2016), the shifts in band positions are observed compared to those of aquo lanthanide metal ions. It is observed in case of neodymium complexes, there is decrease in the wave number by 346–1757 cm⁻¹, from its aquo complex, whereas there is no considerable change in wave number of praseodymium complex from its aquo complex. This shift, a measure of metal–ligand interaction, has been ascribed to nephelauxeatic effect (Jung et al., 2017). The involvement of f-orbital in M—O bonding is an important component of covalency (Berryman et al., 2019).

The Sinha’s covalency parameter (δ %) was calculated using the expression (Eq. (1)),

3.2 IR spectra of complexes

The analytical data and absorption frequencies of complexes are shown in Table 3 and the infrared spectra of all compounds are given in Figs. 2–4.

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Fig. 2

FTIR spectra of [La{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 3

FTIR spectra of [Nd{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 4

FTIR spectra of [Gd{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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3.3 Thermal studies

Simultaneous TG-DTA data of the complexes are summarized in Table 4. The compositions of the intermediates and the final products are those which best fit with the observed mass losses in the TG studies, and are in good agreement with the corresponding DTA data. The thermograms are shown in Figs. 5–7.

Table 4A Thermal data of lanthanide complexes of 2-acetamido benzoic acids and hydrazine.

Table 4B Thermal data of lanthanide complexes of 3-acetamido benzoic acids and hydrazine.

Table 4C Thermal data of lanthanide complexes of 4-acetamido benzoic acids and hydrazine.

(+) – endo, (−) – exo, (Anal.) – Analytical& (Calcd.) – Calculated.

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Fig. 5

TG-DTA curves of Metal Hydrazine complexes: (a) [La{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d)[Nd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f)[Gd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄).

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Fig. 6

TG-DTA curves of Metal Hydrazine complexes.(a) [La{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d) [Nd{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f) [Gd{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 7

TG-DTA curves of Metal Hydrazine complexes: (a)[La{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d) [Nd{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f) [Gd{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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All the complexes undergo almost similar type of degradation pattern. The complexes show three step decompositions as recorded by TG, namely, dehydrazination, decomposition of lanthanide carboxylate to phthalate intermediate and decomposition of phthalate intermediate to metal oxides. Though the thermograms of Cerium complexes show similar pattern of decomposition, the intermediate was unstable and unidentifiable. Dehydrazination occur showing endotherms in the range of 52 to 111 °C in case of 4-acambH complexes. Exothermic dehydrazination is observed for 2-acambH and 3-acambH complexes in the range 85 to 180 °C. This may be due to stability of the formation of the former than the latter two. In case of lanthanum complexes of 4-acambH the dehydrazinated metal carboxylates undergo exothermic decomposition showing exothermic peaks in the range of 172 to 370 °C and 412 to 496 °C to metal phthalates. The mass loss percentage indicates phthalates formation. The decomposition temperature of these intermediates more or less going along with the values reported (Gorgola and Brzyska, 1999). The Neodymium complex of 3-acetamido benzoate decomposes directly into its metal oxide, whose intermediate is found to be unstable, which could not be identified. Metal phthalates finally undergo oxidative decomposition showing broad exotherms in the range of 428 to 572 °C to the metal oxide residues.

All cerium complexes show dehydrazination in the first step. As in the other cases, 4-acambH complexes show endothermic dehydrazination. After dehydrazination, cerium complexes decompose to their corresponding metal oxide with no stable intermediate formation.

Among the praseodymium complexes, thermogram of 4-isomer has registered an additional exotherm at 216 °C indicating the removal of NH group prior to the formation of the phthalate intermediate. This trend is also observed in other complexes like Nd, Sm and Gd of the same isomeric benzoate. The decomposition steps involved were given in equations 2–5. The decomposition path was followed by analyzing the decompositions of 4-acetamido samarium complex at different steps using IR spectra. The IR spectra of the intermediates of 4 - acmbH of samarium complex are shown in Figs. 8–11. The IR spectra of the intermediate (1) after heating the sample to 93 °C, intermediate (2) after heating the sample to 211 °C, intermediate (3) after heating the sample to 367 °C and the intermediate (4) after heating the sample to 452 °C were analyzed for their characteristic peaks. The spectra of the intermediates reveal that N-N stretching of the NH moiety and the NH frequency of the amide group disappear gradually by forming broad peaks. The IR results of intermediate (4) shows v_C=O(asym) band at 1582 cm⁻¹ and v_C=O(sym) band at 1495 cm⁻¹, which are the characteristic frequencies of phthalate ion (Nakamoto et al., 2009; Braibanti et al., 1968). The decomposition steps involved are given below.

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Fig. 8

FTIR analysis of intermediate −1 of [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 9

FTIR analysis of intermediate-2 of [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 10

FTIR analysis of intermediate- 3 of [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 11

FTIR analysis of intermediate- 4 of [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Reaction scheme 1:

Reaction scheme 2:

The remaining IR spectras of 15 isomeric complexes are presented in the supplement index.

3.4 X-Ray diffraction studies

The powder XRD patterns along with their d-spacings are given in Table 5. The comparison of XRD patterns of the lanthanides are shown in Figs. 12–14. The XRD patterns of acetamidobenzoate lanthanides series reveal that each set of acetamidobenzoic acid complexes has similarities in their structures implying similar compositions. The powder XRD patterns of the 2 and 3-isomeric complexes exhibit more or less sharp peaks indicating polycrystalline nature whereas the X-ray pattern of the 4-isomeric lanthanide complexes display wide and weak intensity peaks may be due to smaller particle sizes. All substances are found to be crystalline and complexes of 2, 3 isomeric acids show more crystallinity. Correlation studies of the X-ray patterns were subsequently used to confirm that the above products were single-phase materials. However, they display similar patterns implying isomorphism. Their particle sizes were substantiated by SEM- EDAX analysis (Gaye, et al., 2003). With the justification of analytical, magnetic, spectral, thermal and XRD characterizations the eight coordination sites were ascribed around the Ln³⁺ ions. Among the eight coordinated sites six were occupied by three bidentate acetamido benzoate moieties and the rest of two sites were inhabited by two bridging bidentate hydrazine groups.

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Fig. 12

XRD patterns of Metal Hydrazine complexes: (a) [La{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d)[Nd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f) [Gd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 13

XRD patterns of Metal Hydrazine complexes. (a) [La{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d)[Nd{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f) [Gd{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄).

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Fig. 14

XRD patterns of Metal Hydrazine complexes: (a) [La{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (b) [Ce{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (c) [Pr{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (d) [Nd{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (e) [Sm{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)]; (f) [Gd{4-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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3.5 SEM–EDAX studies

It is generally observed that the hydrazine complexes yield metal oxides of nano scale on decomposition, due to their explosive nature (ref). Similar study was undertaken by decomposing the complexes in muffle furnace at their decomposition temperatures (observed from thermal analysis), keeping them at the same temperature for 30 min., and analyzing for their morphology and particle size of the residual oxides using SEM EDAX studies. Owing to the breaking of hydrazine and exothermic decomposition of organic moiety, the residues were of irregular shapes as evidenced by their SEM images. The SEM–EDAX pictures of Nd₂O₃ and Pr₆O₁₁ confirmed the existence of corresponding metals presented in Figs. 15–20. A comparison between the SEM–EDAX pictures of residues with the reported nano-neodymium and praseodymium oxides (Figs. 16, 18), shows that they display similar spectra, supporting the existence of the corresponding rare earth metal in the compounds as reported (Lembang et al., 2018; Pourmortazavi et al., 2017). The size of metal oxide particles lie in the range of 1–10 µm due to the insufficient quantity of hydrazine in the complex to decompose to oxides of nano scale.

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Fig. 15

SEM images of Nd₂O₃ residue of [Nd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 16

SEM-EDAX image of (a) Nd₂O₃ nanoparticles (Reported); (b): Nd₂O₃ residue of [Nd{2-C₆H₄(CH₃CONH)COO}₃.(N₂H₄).

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Fig. 17

SEM images of Pr₆O₁₁ residue of [Pr{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 18

SEM-EDAX image of (a)Pr₂(WO₄)₃Nanoparticles (reported; (b)Pr₆O₁₁ residue of [Pr{3-C₆H₄(CH₃CONH)COO}₃.(N₂H₄)].

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Fig. 19

SEM images of Pr₆O₁₁ residue of[Pr{4-C₆H₄(CH₃CONH)COO}₃(N₂H₄)].

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Fig. 20

SEM-EDAX image of Pr₆O₁₁ residue.

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3.6 Conductance studies

Conductance of hydrazine complexes lanthanide metal complexes of isomeric acetamido benzoic acids was measured by solution method using DMSO as solvent. Solutions of complexes were prepared in DMSO and specific conductance was measured first and then molar conductance was calculated as in the case of transition metal complexes the values are given in Table 6. The molar conductance values are found to be in the range of 8–20 O⁻¹cm²mol⁻¹ indicating that these compounds are neutral complexes (Geary, 1971; Ramalingam and Soundararajan, 1967).

3.7 ¹³C and ¹H - NMR spectral studies

To ascertain the presence acid moiety in the complexes, ¹³C – NMR spectra were recorded and given in Fig. 22. In the spectra of the complexes, [La{2-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)], [Gd{3-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)] & [La{4-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)], the important features noticed are, ¹³C- NMR (600 MHz, DMSO) On analyzing ¹³C - NMR spectra, the important characteristic peaks exhibited by the complexes are listed in Table 7 which are similar to that shown by the 4-acetamido benzoic acid Fig. 21 (SDBS No.6318 CDS-01-576,). The observations indicate that the complexes contain acid moiety, which has not undergone any degradation during the preparation of complexes (Abraham and Loftus, 1980).

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Fig. 21

4-acetamido benzoic acid with carbon numbering.

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Fig. 22

¹³C – NMR Spectra of (a): [La{2-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)]; (b): [Gd{3-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)]; (c): [La{4-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)].

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On evaluating the ¹H - NMR(SDBS No.6318 HSP- 47–33) spectrum of 4-acetamido benzoic acid, majority of the peaks observed are found to be compatible with those of the complexes, which are given in Table 7. ¹H -NMR (600 MHz, DMSO. Since the value of δ 12.8 is ascribed to C1H in the 4-acetamido benzoic acid is vanishes in the complexes implying that the acid moiety is involved in complexation (Milenkovic et al., 2012).

3.8 Mass spectral studies

The mass Spectrum of 4 - acetamido complex was recorded to find out the molecular mass of the complex, as a representative example. Since the attempt made to obtain the single crystals of the synthesized compounds was not fruitful, the mass spectroscopic characterization was carried out to find the molecular mass of the complexes (Pavia et al., 2015).

The mass Spectrum of 4 - acetamido benzoic acid (NIST number 375119) is correlated with that of the corresponding complex [La{x-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)] shown in Fig. 23. The prominent peaks identified in 4- acetamido benzoic acid with m/z values are 43, 120 and 135 representing the fragments of acyl radical, benzoate radical and phenyl acetamido moieties respectively. The various characteristic fragments of the above-mentioned complex, such as anions, cations and neutral species are listed in Table 7. The prominent peak identified in the spectrum at m/z value of 673, which represents the formula La{C₆H₄(CH₃CONH)COO}₃ is corresponding to M⁺ ion. The m/z value of 59 represents the acetamide (CH₃CONH₂) moiety. The benzoate radical (C₆H₅COO) is shown by the peak at 121(m/z). Lanthanum metal cation is observed at m/z of 136. The phthalate radical {C₆H₄(COO)₂} ascribed for the m/z value of 166 in the spectrum. A fragment of {La(C₆H₄(COO)₂}is observed at m/z 303. Besides, the base peak (most abundant) is corresponding to [(C₇H₄O₂)_3.N₂H₄] moiety is exhibited at m/z 392. As per the spectrum, the molecular ion peak at m/z 1380 accounts for the molecular formula [La₂{C₆H₄(NHCOCH₃)COO}₆] which was the dimer of the target compound, due its instability the abundance is found to be minimum.

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Fig. 23

¹H – NMR Spectra of (a): [La{2-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)]; (b): [Gd{3-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)]; (c): [La{4-C₆H₄(NHCOCH₃)COO}₃(N₂H₄)].

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