Lanthanide complexes with 3,4,5-triethoxybenzoic acid and 1,10-phenanthroline: synthesis, crystal structures, thermal decomposition mechanism and phase transformation kinetics

Abstract

Three novel lanthanide complexes [Ln(3,4,5-TEOBA)₃phen]₂ (Ln = La(1), Pr(2), Eu(3); 3,4,5-TEOBA = 3,4,5-triethoxybenzoate; phen = 1,10-phenanthroline) were synthesized and characterized. Single crystal X-ray diffraction showed that the complexes are isostructural. Each complex has two center metals and each center is coordinated with seven oxygen atoms and two nitrogen atoms to form a distorted monocapped square antiprism geometry. A carboxylic group adopts three modes coordinated with center metal: bidentate chelate, bridging bidentate and bridging tridentate. The luminescence of complex 3 showed the characteristic emission of Eu³⁺ (⁵D₀ → ⁷F_0–3). The thermal decomposition mechanism of title complexes was studied by TG/DSC-FTIR technology. The heat capacities of complexes 1–3 were measured by DSC over the temperature range from 263.15 to 463.15 K. In the temperature range from 280 to 350 K, there was a solid-to-solid phase transition for each complex, which was further evidenced by four thermal circulating processes with the scanning rate of 10 K min⁻¹. A study of the phase transition of four thermal circulating processes under different heating rates revealed a fine linear relationship between the activation energy (E) and the percent conversion (α). In the heating and cooling runs, supercooling was observed and the endothermic and exothermic enthalpies behaved differently.

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Introduction

The exploration of lanthanide complexes has attracted the attention of many chemists because of their extraordinary properties, such as magnetic,¹ thermodynamics^2,3 and optical properties.^3,4 Owing to the peculiarity of the 4f shell for a rare earth element, lanthanide complexes normally possess diverse structures and fascinating coordination geometry.^5–9 The high and variable coordination numbers, however, require stricter conditions for synthesis.^10,11 Lanthanide complexes show superior luminescence properties, particularly the complexes of Eu and Tb. The f–f transition of metal center in near-infrared spectral regions makes lanthanide complexes potentially applicable for lighting, optical storage and sensors.^12–14 Lanthanide complexes are normally thermally stable.^15–17 According to a study of the thermodynamics, the service life of a material can be predicted.

Aromatic carboxylic acid ligands as linkers exhibit high affinity to lanthanide centers, and can construct lanthanide complexes with the center metal in multiple coordinated modes.^18–20 In addition, direct excitation of the lanthanide metal center is affected by the inefficient absorption of the f–f transition, so aromatic carboxylic acid and 1,10-phenanthroline can be chosen as “antenna” to sensitize the luminescence of lanthanide.^21–23

Our team has been occupied in a systematic investigation of lanthanide carboxylic complexes, particularly the thermal properties of lanthanide complexes. Recently, we reported the solid-to-solid phase transition of lanthanide aromatic carboxylic complexes, which is rarely reported in the literature.^24–27 In this paper, we report the synthesis and characterization of three lanthanide complexes [Ln(3,4,5-TEOBA)₃phen]₂ (Ln = La(1), Pr(2), Eu(3); 3,4,5-TEOBA = 3,4,5-triethoxybenzoate; phen = 1,10-phenanthroline). The thermal decomposition mechanism of the complexes was studied using TG/DSC-FTIR technology. The heat capacities of the complexes were measured by DSC, and four thermal circulating processes were implemented for complexes 1 and 2. In addition, the phase transformation kinetics of the thermal circulating processes for complex 1 were also studied.

Experimental

Materials and methods

LnCl₃·6H₂O was obtained by a reaction of Ln₂O₃ (Ln = La, Pr, Eu, Beijing Lanthanide Innovation Technology Co., Ltd, 99.9%) and hydrochloric acid in an aqueous solution, followed by the evaporation of the liquid by water bath heating. The other analytically pure chemicals were purchased and used without further purification.

Equipment and conditions of the experiment

Analyses for C, H and N were carried out on a Vario-ELIII elemental analyzer. The metal content was complexometric titrated by EDTA. The measurements of the molar conductance was implemented on a DDS-307 conductivity meter with DMSO as the solvent. The IR spectra were recorded in the range of 4000–400 cm⁻¹ on a Bruker TENSOR27 spectrometer using KBr medium pellets. The ¹H and ¹³C NMR spectra were measured on Bruker ADVANCE III 500 MHz-NMR spectrometer at room temperature with DMSO-d₆ as the solvent and TMS as the internal standard. The fluorescence spectra were measured on an F-4600 Hitachi Spectrophotometer. The data of single crystal X-ray diffraction were collected on a Smart-1000 diffractometer with graphite-monochromatic Cu Kα (λ = 1.54178 Å) for complexes 1 and 2 and Mo Kα (λ = 0.71073 Å) for complex 3 at 298(2) K. The structures were solved using the SHELXS-97 program (direct methods) and refined by the full-matrix least-squares on F² using the SHELXL-97 program.

Thermogravimetric analysis (TGA), differential thermogravimetric (DTG) analysis, differential scanning calorimetric (DSC) analysis, and Fourier transform infrared (FTIR) spectroscopy of the evolved gas of the title complexes were conducted using a TG/DSC-FTIR system, which was a Netzsch STA 449 F3 Instrument with a Bruker TENSOR 27 Fourier transform infrared spectrometer, under a simulated atmosphere (the gas flow rate of the nitrogen and oxygen was 30 mL min⁻¹ and 10 mL min⁻¹, respectively) at a heating rate of 10 K min⁻¹ from 299.15 to 973.15 K. A ∼5 mg sample was weighted into an open alumina crucible. The transfer line was used to link the Netzsch STA 449 F3 instrument and the heated gas cell of the FTIR instrument and both the transfer line and the gas cell were kept at a constant temperature of 473.15 K.

The heat capacities of the complexes were measured on a Netzsch DSC 200 F3 over the temperature range, 263.15 to 473.15 K, at a linear heating rate of 10 K min⁻¹ using an indirect measurement method. The atmosphere was nitrogen gas, and the flow rate was 20 mL min⁻¹. The baseline, reference and sample measurements were carried out under the same conditions. The sample mass was about 6 mg, and the reference standard substance sapphire mass used was 12.74 mg. The apparatus has an automatic data processing program from which we can obtain the C_p,m, curves of the sample using an indirect measurement method. In addition, four thermal circulating processes for complexes 1 and 2 were measured by DSC over the temperature range of 263.15 to 473.15 K at a scanning rate of 10 K min⁻¹. Furthermore, to study the phase transformation kinetics of complex 1, thermal circulating processes were measured by DSC under different scanning rates (7, 10, 12 and 15 K min⁻¹) over the temperature range of 263.15 to 473.15 K.

Synthesis of [Ln(3,4,5-TEOBA)₃phen]₂ (Ln = La(1), Pr(2), Eu(3)). Two ligands of 3,4,5-TEOHBA (0.6 mmol) and phen (0.2 mmol) were dissolved together in ethanol (95%), adjusting the pH of the solution to 6–7 with a NaOH solution (1 mol L⁻¹). The mixed ligands solution was then added to the LnCl₃·H₂O (0.2 mmol) aqueous solution under stirring. After stirring for six hours and depositing for twelve hours, the precipitates were filtered and dried. After the volatilization of the mother liquor, single crystals of the title complexes were collected in two weeks at room temperature. Element analysis: calcd for complex 1: C, 56.77; H, 5.51; N, 2.60; La, 12.87. Found: C, 56.39; H, 5.57; N, 2.56; La, 13.05. Calcd for 2: C, 56.67; H, 5.50; N, 2.59; Pr, 13.04. Found: C, 56.26; H, 5.65; N, 2.53; Pr, 13.07; calcd for 3: C, 56.09; H, 5.45; N, 2.57; Eu, 13.93. Found: C, 55.79; H, 5.45; N, 2.38; Eu, 14.22.

Results and discussion

Molar conductance

Each complex was dissolved in DMSO at a concentration of 1 × 10⁻³ mol L⁻¹ at room temperature. The molar conductance of complexes 1–3 were 8.72, 7.65 and 7.35 S cm² mol⁻¹, respectively, which suggests that the three complexes are non-electrolyte.²⁸

IR spectrum

The IR spectrum data of ligands and complexes are listed in Table 1. The similar IR spectra of the complexes indicate that these three complexes are isostructural,²⁹ which was further proved by single crystal X-ray diffraction. Compared to the data of 3,4,5-TEOHBA, the characteristic absorption of ν_C=O (1686 cm⁻¹) was absent, and ν_sym(COO⁻) and ν_asym(COO⁻) were observed at 1570–1576 cm⁻¹ and 1407–1425 cm⁻¹.^19,30 The absorption of ν_Ln–O occurs in the vicinity of 417 cm⁻¹, indicating that the lanthanide ion is coordinated to the ligands.³¹ The ν_C=N (1645 cm⁻¹) and γ_=C–H (864 cm⁻¹, 738 cm⁻¹) of phen are red shifted to ν_C=N (1609–1618 cm⁻¹) and γ_=C–H (843–848 cm⁻¹, 730–733 cm⁻¹). This phenomenon also indicates the occurrence of coordination.³²

Table 1 Frequencies (cm⁻¹) of the absorption bands for the ligands and title complexes

Ligand/complex	νC=N	γ=C–H	νC=O	νsym(COO^−)	νasym(COO^−)	νLn–O
Phen	1645	864, 738
3,4,5-TEOHBA			1686
1	1618	843, 731		1570	1410	417
2	1609	848, 730		1576	1407	417
3	1618	848, 733		1576	1425	419

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¹H, ¹³CNMR spectra

Owing to the paramagnetic properties and the low solubility of complexes 2 and 3, only the ¹H and ¹³C NMR spectrum for the ligands and complex 1 was collected. The data is listed in Table 2.

Table 2 (a) ¹H NMR spectra data of the ligands and complex 1. (b) ¹³C NMR spectra data of the ligands and complex 1

(a) Ligands and complex	^1H NMR (δ/ppm)
	δ1	δ2	δ3	δ4	δa	δb	δc	δd
	1.34	4.06	7.20	12.87
[La(3,4,5-TEOBA)3phen]2	1.28	4.02	7.17
					9.11	7.76	8.48	7. 97
[La(3,4,5-TEOBA)3phen]2					9.13	7.80	8.52	8.02

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(b) Ligands and complex	^13C NMR (δ/ppm)
	δ1	δ2	δ3	δ4	δ5	δ6	δ7	δ8	δ9
	167.45	126.11	107.99	152.63	141.41	64.56	15.14	68.50	15.89
[La(3,4,5-TEOBA)3phen]2	176.49	127.18	107.60	152.10	139.84	64.30	15.22	68.27	15.91
	δa	δb	δc	δd	δe	δf
	150.41	123.76	136.65	128.92	146.03	127.13
[La(3,4,5-TEOBA)3phen]2	150.53	123.86	136.85	128.98	145.93	127.18

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The ¹H NMR spectrum data for ligands and complex 1 are shown in Table 2a. The ligand of 3,4,5-TEOHBA and 1,10-phen mainly show four chemical shifts, respectively. On the other hand, the chemical shift δ_H of (–COOH) disappears in the spectrum of complex 1, which indicates that the ligand of 3,4,5-TEOHBA is coordinated to the lanthanide metal. Compared to the 3,4,5-TEOHBA ligand, the chemical shift of the proton in benzene ring moves to a high magnetic field, which is due to the lower electronegativity of the lanthanide metal than a hydrogen atom. As a result, the electron cloud moves to the benzene ring and tends to equilibrate, all of which suggests that the chemical shift of the proton in the benzene ring moves to a high magnetic field for complex 1.³³ The chemical shift of δ_H for 1,10-phen almost moved to a low field after coordinating to the lanthanide metal, which is due to the decrease in density of the electron cloud.

The ¹³C NMR spectral data for the ligands and complex 1 in Table 2b shows that the chemical shift δ_C of (–COOH) moves to a low field, which indicates the group of (–COO) is coordinated to the lanthanide metal.³⁴ Because of the increase in the density of electron cloud, most δ_C in the benzene ring and (–OCH₂CH₃) moves to a high field. For 1,10-phen, due to the coordination of nitrogen atoms and lanthanide metal, the density of the electron cloud for a phen ring has decreased, which makes δ_C moves to a low field.

Fluorescence spectrum

The photoluminescence properties of complex 3 in the solid state have been investigated at room temperature. The excitation and emission spectra of complex 3 are shown in Fig. 1. The photoluminescence spectra indicate typical Eu³⁺ luminescence as well as an antenna effect of the organic ligand. The excitation spectrum was recorded from 200 to 425 nm with the emission wavelength of 620 nm. This reveals a prominent broad band from 200 to 370 nm, which can be assigned to the absorption of ligands. Other weaker signals at 395 and 465 nm can be assigned to a direct Eu³⁺ excitation.²⁹ Complex 3 exhibits an intense characteristic red emission light under UV light. The emission spectrum of complex 3 shows four typical spectral bands of the europium complex at 580, 593, 620 and 652 nm with excitation at 275 nm, which can be assigned to the characteristic f–f transitions from the emitting level ⁵D₀ to the ground multiplet (⁷F₀, ⁷F₁, ⁷F₂, and ⁷F₃) of the Eu(III) ion. Among them, the emission spectrum of complex 3 is dominated by the hypersensitive transition ⁵D₀ → ⁷F₂.

Fig. 1 Excitation spectra (a) with the emission wavelength of 620 nm and emission spectra (b) with excitation at 275 nm for complex 3.

Crystal structure

The single crystal X-ray diffraction data of the title complexes are given in Table 3. The selected bond lengths for complexes 1–3 are listed in Table 4. Single crystal X-ray crystallography analyses showed that the title complexes crystallized isostructurally in the triclinic space group of P1̄. The structure of complex 2 is described here representatively. As shown in Fig. 2, complex 2 has two asymmetric structural units, and each unit has a nine coordinated center Pr³⁺ ion. Each Pr³⁺ ion is surrounded by one 1,10-phen ligand and six 3,4,5-TEOBA ligands, which adopt three different coordinated modes: bidentate chelating (O11, O12), bridging bidentate (O6, O7), and bridging tridentate (O1, O2 and O1#). Fig. 3 shows that the nine coordinated Pr³⁺ can be described as a distorted monocapped square antiprism geometry, in which O1 acts as the capping atom. The two units are connected by two 3,4,5-TEOBA ligands adopting a bridging tridentate coordinated mode and the distance of Pr–Pr bond is 4.044 Å. From Table 4, the distance of the Pr–O bond ranges from 2.40(2) to 2.706(15) Å and the average distance of the Pr–O bond is 2.508 Å. The phen molecule binds to the center metal adopting bidentate chelating mode. The average distance of the Pr–N bond is 2.700 Å, which is longer than the distance of the Pr–O bond. Therefore, in the process of thermal decomposition, phen always tends to be lost first. For the distance of the Pr–O bond, due to the instability of the four member ring in bridging chelating, the length of the Pr–O bond in bridging bidentate (2.447 Å) is shorter than that in bidentate chelating (2.554 Å).³⁵ The binuclear molecular skeleton connects together to form a 1D chain via stacking π–π interactions between the phen rings on neighboring complex molecules, which is shown in Fig. 4. The attractive π–π interaction is formed by the centroid of phen in one binuclear molecular skeleton with the other centroid of phen in the neighboring unit with a distance of 3.777 Å.

Table 3 Crystal data and structure refinement of the title complexes

Complex	1	2	3
Empirical formula	C102H118La2N4O30	C102H118Pr2N4O30	C102H118Eu2N4O30
Formula weight	2157.82	2161.82	2183.92
Temperature/K	298(2)	298(2)	298(2)
Wavelength/Å	1.54178	1.54178	0.71073
Crystal system	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄
Unit cell dimensions
a/Å	13.0256(13)	12.9555(12)	12.7320(11)
b/Å	14.4709(12)	14.4344(13)	14.3480(12)
c/Å	15.0513(15)	15.077(2)	15.0480(14)
α/deg	84.356(7)	84.255(9)	83.4850(10)
β/deg	73.226(9)	73.642(10)	75.328(2)
γ/deg	89.335(7)	89.269(7)	89.450(2)
Volume/Å^3	2702.8(4)	2691.4(5)	2641.6(4)
Z, calculated density/(mg m^−3)	1, 1.326	1, 1.334	1, 1.373
Absorption coefficient/mm^−1	6.647	7.485	1.253
F(000)	1112	1116	1124
Crystal size/mm	0.19 × 0.17 × 0.16	0.27 × 0.09 × 0.05	0.35 × 0.10 × 0.07
θ range for data collection/deg	3.07 to 66.05	3.07 to 66.05	2.72 to 25.02
Limiting indices	−15 ≤ h ≤ 15, −17 ≤ k ≤ 9, −17 ≤ l ≤ 17	−15 ≤ h ≤ 15, −17 ≤ k ≤ 17, −9 ≤ l ≤ 17	−15 ≤ h ≤ 15, −17 ≤ k ≤ 11, −17 ≤ l ≤ 17
Reflections collected/unique	17 460/9395 [R(int) = 0.0890]	9359/9359 [R(int) = 0.0000]	13 550/9166 [R(int) = 0.0661]
Completeness to θ = 66.05	99.7%	99.8%
Completeness to θ = 25.02			98.3%
Max. and min. transmission	0.4160 and 0.3648	0.7060 and 0.2371	0.9174 and 0.6682
Data/restraints/parameters	9395/1/632	9359/0/632	9166/1/631
Goodness-of-fit on F^2	1.039	1.099	1.024
Final R indices [I > 2σ(I)]	R1 = 0.1273, wR2 = 0.3180	R1 = 0.1423, wR2 = 0.3553	R1 = 0.1005, wR2 = 0.2479
R indices (all data)	R1 = 0.1797, wR2 = 0.3739	R1 = 0.2628, wR2 = 0.4404	R1 = 0.1684, wR2 = 0.2903
Largest diff. peak and hole/(e A^−3)	1.433 and −1.091	0.905 and −1.045	1.298 and −1.140

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Table 4 Selected bond lengths (Å) of the title complexes^a

a Symmetry transformations used to generate equivalent atoms: #1: − x + 1, −y + 1, −z + 1.
Complex 1
La(1)–O(6)	2.427(8)	La(1)–O(12)	2.563(8)
La(1)–O(1)#1	2.475(7)	La(1)–N(2)	2.709(9)
La(1)–O(7)#1	2.494(8)	La(1)–O(1)	2.737(6)
La(1)–O(2)	2.542(9)	La(1)–N(1)	2.745(9)
La(1)–O(11)	2.553(8)	La(1)–La(1)#1	4.0900(10)
Complex 2
Pr(1)–O(6)	2.40(2)	Pr(1)–O(11)	2.520(16)
Pr(1)–O(1)#1	2.455(18)	Pr(1)–N(2)	2.69(2)
Pr(1)–O(2)	2.480(18)	Pr(1)–O(1)	2.706(15)
Pr(1)–O(7)#1	2.485(17)	Pr(1)–N(1)	2.71(2)
Pr(1)–O(12)	2.510(17)	Pr(1)–Pr(1)#1	4.044(2)
Complex 3
Eu(1)–O(1)#1	2.353(10)	Eu(1)–O(12)	2.449(10)
Eu(1)–O(6)	2.360(11)	Eu(1)–N(2)	2.575(14)
Eu(1)–O(7)#1	2.377(9)	Eu(1)–N(1)	2.642(13)
Eu(1)–O(2)	2.442(9)	Eu(1)–O(1)	2.706(9)
Eu(1)–O(11)	2.449(9)	Eu(1)–Eu(1)#1	3.9746(13)

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Fig. 2 Crystal structure of complex 2.

Fig. 3 Coordination geometry of Pr³⁺ ion.

Fig. 4 Binuclear units of complex 2 are stitched together via stacking π–π interactions to form 1D chain.

Thermal behavior of title complexes

The TG, DTG and DSC curves of complexes 1–3 at a heating rate of 10 K min⁻¹ from 299.15 to 973.15 K are shown in Fig. 5. Stacked plots of the FTIR spectra of the evolved gas for complexes 1–3 are shown in Fig. 6 and the data of thermal analysis for complexes 1–3 are given in Table 5. According to DSC analysis, the enthalpies and peak temperatures for the three complexes are listed in Table 6.

Fig. 5 TG-DTG/DSC curves of complexes 1–3 at the heating rate of 10 K min⁻¹ (complex 1 = a, complex 2 = b, complex 3 = c).

Fig. 6 Stacked plots of the FTIR spectra of the evolved gases for complexes 1–3, as observed in the online TG/DSC-FTIR system at a heating rate of 10 K min⁻¹ (complex 1 = a, complex 2 = b, complex 3 = c).

Table 5 Thermal decomposition data of the title complexes (β = 10 K min⁻¹)^a

Complex	Stage	Temperature range/K	DTG Tp/K	Mass loss rate/%		Probable removed groups	Intermediate and final solid products
				Found	Calcd
a Tp is the peak temperature of DTG.b The total loss rate.
1	I	476.15–550.15	516.55	11.44		xPhen	La2(3,4,5-TEOBA)6 phen(2−x)
	II	550.15–967.25	632.15	72.48, 83.92	84.93^b	(2 − x)Phen + 6(3,4,5-TEOBA)-3O	La2O3
2	I	485.15–563.15	515.85	4.48		xPhen	Pr2(3,4,5-TEOBA)6 phen(2−x)
	II	563.15–967.45	622.15	77.67, 82.15	84.24^b	(2 − x)Phen + 6(3,4,5-TEOBA)-11/3O	11/3Pr6O11
3	I	493.15–583.15	532.75	14.12		xPhen	Eu2(3,4,5-TEOBA)6 phen(2−x)
	II	583.15–627.15	618.15	40.73		(2 − x)Phen + y(3,4,5-TEOBA)	Eu2(3,4,5-TEOBA)(6−y)
	III	627.15–967.25	638.15	27.28, 82.13	83.88^b	(6 − y)(3,4,5-TEOBA)-3O	Eu2O3

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Table 6 Enthalpies and peak temperatures for the title complexes from DSC analysis of the TG/DSC-FTIR system

Complex	Stage	Temperature rage (K)	DSC peak temperature (K)	ΔHm (kJ mol^−1)
1	I	492.25–497.15	494.85	9.3803
	II	629.45–645.65	638.55	−10 334
2	I	510.05–516.15	512.35	6.0166
	II	620.25–642.75	634.85	−13 380
3	I	509.25–519.25	514.25	30.575
	II	616.05–626.85	620.65	−1422.0
	III	632.85–649.85	642.95	−15 231

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The thermal decomposition of complexes 1–2 are similar and complex 1 will be described in detail. The thermal behavior of complex 1 was characterized by two stages with the mass percent loss of 11.44% at 476.15–550.15 K and 72.48% at 550.15–967.25 K. In the first stage, there is a small endothermic peak (T_p = 494.85 K, ΔH_m = 9.3803 kJ mol⁻¹) on the DSC curve, which was attributed to the release of a part of the phen molecule. On the other hand, corresponding to the Fig. 6a, no signal was found in this temperature range. This can be explained by the signal of gas being too weak for the instrument to detect. In Fig. 6a, the strong signals correspond to the thermal decomposition of the second stage. Fig. 7 shows the IR spectrum of the main gas products in different temperatures of the second stage. From 610.4 K to 646.95 K, the signal intensity changes gradually and reaches the maximum at 630.8 K. The strongest signal from 2362 to 2312 cm⁻¹ is corresponding to the absorption of CO₂. The absorption of CO and H₂O can be observed at 2114–2187 cm⁻¹ and 3503–3738 cm⁻¹, respectively. In addition, some gaseous organics are detected in the FTIR spectra. The weak absorption at 1653 cm⁻¹ was attributed to the ν_C=N from evolved phen, which can prove the decomposition of phen in the second stage. The band from 2989 to 2941 cm⁻¹ is considered to be the ν_C–H from evolved aliphatic or aromatic hydrocarbons. At 1731 cm⁻¹, there was a characteristic peak that was attributes to the ν_C=O of the carboxylic acid group, whereas the peak at 1211 cm⁻¹ was considered to be the ν_C–O of the carboxylic acid group. The signals of 1510 and 1558 cm⁻¹ were attributed to the ν_C=C of the benzene ring.³⁶ Overall, it can be concluded that the gaseous products contain broken and non-broken aromatic carboxylic ligands and a part of the phen ligands, which are detected in the form of gaseous small molecules (H₂O, CO₂ and CO) and molecule fragments of aromatic carboxylic ligands and phen ligands. At this stage, a strong exothermic peak can be observed on the DSC curve (T_p = 638.55 K, ΔH_m = −10 334 kJ mol⁻¹). The general thermal decomposition reaction of complex 1 can be shown as follows:

Fig. 7 FTIR spectra of the evolved gases for complex 1 at different temperatures.

From Fig. 5c, the thermal decomposition of complex 3 is different from complexes 1 and 2, and it is characterized by three stages. Thermal decomposition of the first stage is due to the release of a part of the phen molecules. As shown in Fig. 6c, however, it is not detected in the FTIR spectra, and can be explained that the signal of gas is too weak. In the second and third stages, the remaining phen and carboxylic ligands start to decompose, which are same as the second stage for complex 1. On the other hand, the characteristic absorption of only one stage was observed in the FTIR spectra. The reactions of second and three stages occur continuously, so that only one set of signals is detected by the instrument. The general thermal decomposition reaction of complex 3 can be shown as follows:

Heat capacities

Based on the TG-DTG/DSC curve, there was no mass loss before 463.15 K for the three complexes. Therefore, the heat capacities of the title complexes were measured by DSC in the temperature range from 263.15 to 463.15 K. The average molar heat capacities for title complexes are listed in Table S1† and are plotted in Fig. 8. The experimental heat capacities were fitted to the polynomial equations by a least square method in the reduced temperature (x) and the correlation coefficient (R²) and standard deviation (SD) were obtained. The reduced temperature was calculated by the equation, x = [T − (T_max + T_min)/2]/[(T_max − T_min)/2], where T is the experimental temperature, and T_max and T_min are the upper and lower limits, respectively.^37,38 As shown in Fig. 9a, during 280–350 K, there is a peak for complexes 1–3, which can be proven to be a solid-to-solid phase transition by thermal circulating processes.^24,36,39 The curves of complexes 1 and 2 were fitted in three phases, in which the phase transition peaks were also fitted. The peak of solid-to-solid phase transition for complex 3 was weaker and the curve of complex 3 was fitted to two phases. The results are shown in Scheme 1.

Fig. 8 Relationship of the molar heat capacities varying with temperature (lines 1–3 represent complexes 1–3, respectively).

Fig. 9 DSC curves of the complexes 1 (a) and 2 (b) in the four circulating processes.

Scheme 1 Fitting results of the experimental heat capacities in the reduced temperature using the least-squares method.

Based on the fitted polynomial and thermodynamic equations, the smoothed heat capacities and thermodynamic functions of the three complexes were calculated. The thermodynamic equations are as follows:

The smoothed values of C_p,m and the thermodynamic functions relative to the standard reference temperature 298.15 K with an interval of 10 K are shown in Table S2.†

Thermal circulating and phase transformation kinetics

Four thermal circulating processes were designed for complexes 1 and 2 with a scanning rate of 10 K min⁻¹ from 263.15–463.15 K, and the DSC curves of thermal circulating are shown in Fig. 9. The temperature and the endothermic and exothermic enthalpy in every process for complexes 1 and 2 is listed in Table 7. As a result, the reversibility and repeatability of the phase transitions of the sample were verified. There was almost no change in the peak position of each circulating process, which can preferably explain the presence of a solid to solid phase transition. As shown in Table 7, in the same circulating process, the phase transition temperature in the cooling process is lower than that in the heating process, which means a supercooling phenomenon occurs in the thermal circulating processes of complexes 1 and 2. The enthalpy in the heating and cooling processes for complexes 1 and 2 decreased gradually. For example, the enthalpy for complex 1 was 8.414 kJ mol⁻¹ at the first heating circulation and with continuing circulation, the enthalpy decreased gradually to 7.488 kJ mol⁻¹ at the last heating circulation. On the other hand, there was no endothermic peak before 473.15 K in Fig. 5, which is probably due to the high sensitivity of the DSC 200 F3 instrument.

Table 7 Enthalpies and peak temperatures for complexes 1 and 2 in the four circulating processes^a

Complexes	Step	Heating				Cooling
		T0 (K)	Tt (K)	Tp (K)	Endothermic (kJ mol^−1)	T0 (K)	Tt (K)	Tp (K)	Exothermic (kJ mol^−1)
a T0 is the initial temperature of the phase transition peak by a DSC extrapolation; Tt is the ending temperature of the phase transition peak by a DSC extrapolation; Tp is the peak temperature of the phase transition peak.
1	1	324.52	329.29	326.62	8.414	293.45	302.87	298.70	−5.936
	2	324.46	329.15	326.61	8.114	293.38	302.68	298.83	−5.703
	3	324.39	329.19	326.53	7.779	293.47	302.83	298.66	−5.604
	4	324.38	329.19	326.51	7.488	293.29	302.84	298.44	−5.336
2	1	312.70	323.67	318.02	5.483	278.82	288.92	283.45	−1.668
	2	312.64	323.40	317.8	5.476	280.09	288.84	282.50	−1.492
	3	312.94	323.93	317.72	5.433	277.94	288.68	283.37	−0.871
	4	312.58	323.63	317.48	5.243	277.71	288.53	282.90	−0.855

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The enthalpy of the heating processes for complex 1 is larger than others, so the accuracy of the measurement is higher. Only the heating processes of complex 1 were studied for the phase transformation kinetics. The activation energy (E) of the four circulating processes for complex 1 was calculated using the iso-conversional method: M. J. Starink⁴⁰ and Madhusudanan–Krishnan–Ninan.⁴¹ The equations are as follows:

M. J. Starink:

Madhusudanan–Krishnan–Ninan:

Eqn (2) can be changed into:

The relationship between the activation energy (E) and percent conversion (α) for different circulating processes with two methods are displayed in Fig. 10. The activation energies (E) in different percent conversion calculated using the two methods were similar and in four circulations, the activation energy (E) and percent conversion (α) showed a fine linear relationship. The curves in Fig. 10 were linear fitted and the equations are listed in Table 8. The activation energy (E) of the first circulation deviated from the other circulation, which may be due to the uncertainty of the initial waiting period for DSC.

Fig. 10 The relationship of activation energy (E) and percent conversion (α) of the heating processes for complex 1. (M. J. Starink (a); Madhusudanan–Krishnan–Ninan (b)).

Table 8 The linear fitting results of E and α of the heating processes for complex 1

Circulation process	Equation	R	SD
M. J. Starink
1	E = 439.29087 − 212.28831α	−0.99716	4.08506
2	E = 380.61997 − 169.18766α	−0.99176	5.56670
3	E = 379.85463 − 168.48214α	−0.98944	6.28491
4	E = 373.61061 − 162.53506α	−0.98642	6.89242
Madhusudanan–Krishnan–Ninan
1	E = 439.21608 − 212.25162α	−0.99716	4.08456
2	E = 380.55899 − 169.16656α	−0.99175	5.56668
3	E = 379.79702 − 168.46039α	−0.98944	6.28456
4	E = 373.55174 − 162.51526α	−0.98642	6.89120

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Conclusions

In summary, we reported the synthesis and characterization of the title complexes. The three complexes were isostructural and all binuclear molecules. The neighboring complex molecules were connected together to form a 1D chain via a stacking π–π interactions. Complex [Eu(3,4,5-TEOBA)₃phen]₂ shows the characteristic emission of Eu³⁺. According to the study of the thermal decomposition data and stacked plots of the FTIR spectra of the evolved gas, we have obtained the thermal decomposition mechanism of the title complexes were obtained. The heat capacities of the complexes revealed a solid-to-solid phase transition for each complex. The thermal circulating of solid-to-solid phase transition for complexes 1and 2 was measured, and phase transformation kinetics of complex 1 was studied. The activation energy (E) and the percent conversion (α) showed a linear relationship.

Acknowledgements

The research was supported by the National Natural Science Foundation of China (no. 21073053, 21473049) and the Natural Science Foundation of Hebei Province (no. B2012205022).

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Footnote

† Electronic supplementary information (ESI) available: Experimental molar heat capacities of complexes 1–3. Smoothed molar heat capacities and thermodynamic functions of complexes 1–3. CCDC 1024382, 1023816 and 1023820. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12063a

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