English

• 1. INTRODUCTION

  The design and construction of novel lanthanide organic frameworks have become a significant research hotspot due to their potential applications in a wide variety of fields such as luminescence, magnetism, catalysis and other aspects^[1-7]. In this field, much attention has been focused on the carboxylic acid ligands system. On one hand, it is well known that -COO has versatile ligating abilities including monodentate, bridging bidentate, chelating or chelating-bridging modes; on the other hand, lanthanide ions have enhanced affinity towards oxygen donors^[8-12]. Out of such carboxylic acid ligands, 2-(4-methylbenzoyl)benzoic acid and its derivatives play an important role in constructing novel and stable lanthanide organic frameworks^[13-15]. Using them as ligands, we have also synthesized the lanthanide complexes [Tb(L)₃(2,2'-bipy)(DMF)]·(2,2'-bipy)^[16], Eu(C₁₄H₉O₃)₂(C₁₂H₈N₂)₂(NO₃)^[17] and Eu₂(C₁₅H₁₁O₃)₆(C₁₂H₈N₂)₂^[18]. With the aim of proceeding previous jobs and constructing new carboxylic acid complexes, we report herein a new lanthanide complex Tb₂(C₁₅H₁₁O₃)₆(2,2'-bipy)₂ by the assembly reaction of 2-(4-methylbenzoyl)benzoic acid and terbium(Ⅲ) nitrate. We determined its structure and further measured its properties. The complex shows three intense fluorescence emission bands arising from the transitions of Tb³⁺: ⁵D₄ → ⁷F₆ (490 nm), ⁵D₄ → ⁷F₅ (545 nm) and ⁵D₄ → ⁷F₄ (584 nm). The complex is an antiferromagnetic system in the range of 300~2 K. Thermal stability property of the complex is also reported here.

  2. EXPERIMENTAL

  2.1 Reagents and instruments

  The reagents were obtained from commercial sources and used without further purification. C, H and N analyses were conducted with a PE-2400(Ⅱ) apparatus. A fluorescence spectrum was obtained at room temperature on an F-7000 fluorescence spectrophotometer. Magnetic measurements in the range of 300~2 K were performed on a MPMS-SQUID magnetometer at a field of 2 kOe on a crystalline sample in the temperature settle mode (1 kOe = 7.96 × 10⁴ A·m^-1). Thermogravimetric analyses were recorded on a NETZSCH TG 209 F3 instrument from room temperature to 700 ℃ under air.

  2.2 Synthesis of the complex

  A mixture of 2-(4-methylbenzoyl)benzoic acid (0.32 mmol) and terbium(Ⅲ) nitrate hexahydrate (0.20 mmol) was dissolved in 12 mL mixed solvent of ethanol, methanol and water (volume ratio 5:2:5). The resultant solution was poured into 20 mL glass test-tube. In turn, 3 mL mixed solvent of water and methanol (volume ratio 1:2) and 2,2'-bipy (0.55 mmol) dissolved in 2.5 mL ethanol were added to this test-tube which, afterwards, was covered with plastic film. The mixture was put at room temperature for slow diffusion. Colorless single-crystals suitable for X-ray diffraction analysis were obtained after four weeks. Yield: 38.6%. Anal. Calcd. (%) for (C₅₅H₄₁N₂O₉Tb): C, 63.96; H, 4.00; N, 2.71. Found (%): C, 63.94; H, 3.99; N, 2.70.

  2.3 Structure determination and refinement

  X-ray diffraction measurement for the complex was carried out on a Bruker SMART APEX CCD area detector at 293(2) K by using graphite-monochromatized MoKα (λ = 0.71073 Å) radiation. The structure was solved by direct methods and refined by full-matrix least-squares techniques using the SHELXS-97^[19] and SHELXL-97^[20] programs. Corrections for Lp factors and empirical adsorption adjustment were applied and all non-hydrogen atoms were refined with anisotropic thermal parameters. The final refinement including hydrogen atoms converged to R = 0.0318 and wR = 0.0727 (w = 1/[σ²(F_o²) + (0.0390P)²], where P = (F_o² + 2F_c²)/3), (∆/σ)_max = 0.002 and S = 1.119.

  3. RESULTS AND DISCUSSION

  3.1 Structural description

  Fig. 1 shows the molecular structure of the complex and the coordination polyhedron for the central Tb(Ⅲ) ion. Selected bond lengths and bond angles are listed in Table 1.

  Figure 1

  

  Figure 1.  Molecular structure of the title complex and the coordination polyhedron for the Tb(Ⅲ) ion at 30% displacement ellipsoids

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  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Complex

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Tb(1)–O(1)	2.467(3)		Tb(1)–O(5A)	2.351(2)		Tb(1)–N(1)	2.539(3)
  Tb(1)–O(2)	2.399(2)		Tb(1)–O(7)	2.297(2)		Tb(1)–N(2)	2.617(3)
  Tb(1)–O(4)	2.312(2)		Tb(1)–O(8A)	2.357(3)		O(1)–C(17)	1.262(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Tb(1)–N(1)	71.22(9)		O(4)–Tb(1)–N(2)	145.34(9)		O(7)–Tb(1)–O(8A)	130.15(8)
  O(1)–Tb(1)–N(2)	101.45(9)		O(5A)–Tb(1)–O(1)	144.72(8)		O(7)–Tb(1)–N(1)	138.73(9)
  O(2)–Tb(1)–O(1)	53.81(8)		O(5A)–Tb(1)–O(2)	138.87(8)		O(7)–Tb(1)–N(2)	77.42(9)
  O(2)–Tb(1)–N(1)	94.42(9)		O(5A)–Tb(1)–O(8A)	79.52(8)		O(8A)–Tb(1)–O(1)	85.75(9)
  O(2)–Tb(1)–N(2)	70.31(9)		O(5A)–Tb(1)–N(1)	74.62(9)		O(8A)–Tb(1)–O(2)	138.34(8)
  O(4)–Tb(1)–O(1)	80.92(9)		O(5A)–Tb(1)–N(2)	69.51(9)		O(8A)–Tb(1)–N(1)	79.38(9)
  O(4)–Tb(1)–O(2)	84.56(9)		O(7)–Tb(1)–O(1)	129.77(8)		O(8A)–Tb(1)–N(2)	135.60(8)
  O(4)–Tb(1)–O(5A)	126.40(9)		O(7)–Tb(1)–O(2)	80.24(8)		N(1)–Tb(1)–N(2)	62.51(9)
  O(4)–Tb(1)–O(8A)	78.92(8)		O(7)–Tb(1)–O(4)	75.04(8).		O(1)–Tb(1)–O(2)	121.7(3)
  O(4)–Tb(1)–N(1)	145.67(10)		O(7)–Tb(1)–O(5A)	82.86(8)		O(5)–Tb(1)–O(4)	122.5(3)
  Symmetry transformation: A 1 – x, 1 – y, 1 – z

  As shown in Fig. 1, the complex consists of two Tb(Ⅲ) ions, six 2-(4-methylbenzoyl)benzoic acid anions and two 2,2'-bipy molecules. Two Tb(Ⅲ) ions are linked by four 2-(4-methylbenzoyl)benzoic acid groups. The whole molecule of the complex displays a symmetric dinuclear structure. The distance between Tb(1) and Tb(1A) is 4.136 Å, which is larger than that of 4.112 Å between Eu(1) and Eu(1A) in the complex^[18] with similar structure reported by us. In the complex, 2-(4-methylbenzoyl)benzoic acid anions are coordinated with Tb(Ⅲ) ions in bidentate chelating and bridging ways, respectively. Tb(Ⅲ) ion is coordinated by six oxygen atoms from five 2-(4-methylbenzoyl)benzoic acid anions, and two nitrogen atoms from one 2,2'-bipy molecule. The central Tb(Ⅲ) ion adopts a distorted square antiprism coordination geometry (Fig. 1). In the coordination polyhedron (TbN₂O₆), atoms O(4), O(7), O(5A) and O(8A) give the upper plane of the square antiprism, and atoms O(1), O(2), N(2) and N(1) determine the plane below, with the plane equations to be 14.103x – 3.591y – 0.612z = 5.9235 and 14.135x – 4.226y – 2.844z = 7.1990, respectively and their dihedral angle being 6.3°. The bond lengths of Tb(1)–O(1), Tb(1)–O(2), Tb(1)–O(4), Tb(1)–O(5A), Tb(1)–O(7) and Tb(1)–O(8A) are 2.467(3), 2.399(2), 2.312(2), 2.351(2), 2.297(2) and 2.357(3) Å, respectively, averaged by 2.364 Å, which falls in the normal range^[21]. The bond angles of O–Tb–O are between 53.81(8)° and 144.72(8)°, and N–Tb–O range from 69.51(9)° to 145.67(10)°.

  In addition, weak π-π stacking interactions can be observed. Weak π-π stacking interactions present between the adjacent phen and the aromatic ring of 2-(4-methylbenzoyl) benzoic acid anions (The shortest centroid-to-centroid distance is 3.6569 Å, longer than 0.3600 nm but shorter than 0.3800 nm). Weak π-π stacking interactions contribute to the stability of the title complex.

  3.2 Fluorescent property

  The title complex was dissolved in a mixed solvent of methanol and water (volume ratio is 3:2) with its concentration to be 1.0 × 10^-4 mol/L. The fluorescent properties of this solution were measured at room temperature in the range of 200~600 nm. Both excitation and emission spectra are shown in Fig. 2. As shown in Fig. 2, the excitation peak of the title complex is around 312 nm. Under 312 nm excitation, the complex shows three fluorescence emission bands at 490, 545 and 584 nm, respectively, and the fluorescence is the strongest at 545 nm. They correspond to the transitions of Tb³⁺: ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅ and ⁵D₄ → ⁷F₄, respectively.

  Figure 2

  

  Figure 2.  Both excitation and emission spectra of the complex at room temperature

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  3.3 Magnetic properties

  The magnetic susceptibility of the complex was investigated in the temperature range of 300~2 K with an applied magnetic field of 2 kOe. The temperature dependence of the molar magnetic susceptibility of the complex is revealed in Fig. 3 in the forms of X_mT and 1/X_m vs. T. The product of X_mT is 14.65098 cm³·K·mol^-1 at 300 K. Upon cooling, the X_mT gradually decreases until 22 K to get a product of 13.39389 cm³·K·mol^-1 and then quickly decreases to the corresponding product of 9.15507 cm³·K·mol^-1 at 2 K. In addition, as shown in Fig. 3, in the temperature range of 300~2 K, the data are in linear relationship in the form of 1/X_m vs. T. The linear regression equation is 1/X_m = 0.0676T + 0.1326, and the correlation coefficient is 1. According to the Curie-Weiss law, X_m = C/(T − θ), the Weiss constant (θ) can be obtained to be −1.962 K as a negative value. These magnetic behaviors show that the title complex exhibits antiferromagnetism.

  Figure 3

  

  Figure 3.  Temperature dependence of the magnetic susceptibility of the complex in the form of X_mT and 1/X_m vs. T

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  3.4 Thermal analysis

  TG-DTG analysis of the complex was performed in air atmosphere at a heating rate of 20 ℃·min^-1, and the curve is shown in Fig. 4. There are three weight-loss stages from 40 to 697.4 ℃. The first stage takes place from 239 to 332 ℃ with the weight loss of 15.17%, corresponding to the release of two 2,2'-bipy molecules (theoretical value is 15.12%). From stages 2 to 3, the weight loss of 67.11% (theoretical value: 67.17%) occurs at 332~697.4 ℃ due to the loss of 90 carbon, 66 hydrogen and 15 oxygen atoms, which are from six 2-(4-methylbenzoyl)benzoic acid anions. The final product is Tb₂O₃, with the final residual rate being 17.72% (theoretical value: 17.71%)

  Figure 4

  

  Figure 4.  TG-DTG curves of the complex

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• 

  Figure 1  Molecular structure of the title complex and the coordination polyhedron for the Tb(Ⅲ) ion at 30% displacement ellipsoids

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  Figure 2  Both excitation and emission spectra of the complex at room temperature

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  Figure 3  Temperature dependence of the magnetic susceptibility of the complex in the form of X_mT and 1/X_m vs. T

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  Figure 4  TG-DTG curves of the complex

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Complex

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Tb(1)–O(1)	2.467(3)		Tb(1)–O(5A)	2.351(2)		Tb(1)–N(1)	2.539(3)
  Tb(1)–O(2)	2.399(2)		Tb(1)–O(7)	2.297(2)		Tb(1)–N(2)	2.617(3)
  Tb(1)–O(4)	2.312(2)		Tb(1)–O(8A)	2.357(3)		O(1)–C(17)	1.262(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Tb(1)–N(1)	71.22(9)		O(4)–Tb(1)–N(2)	145.34(9)		O(7)–Tb(1)–O(8A)	130.15(8)
  O(1)–Tb(1)–N(2)	101.45(9)		O(5A)–Tb(1)–O(1)	144.72(8)		O(7)–Tb(1)–N(1)	138.73(9)
  O(2)–Tb(1)–O(1)	53.81(8)		O(5A)–Tb(1)–O(2)	138.87(8)		O(7)–Tb(1)–N(2)	77.42(9)
  O(2)–Tb(1)–N(1)	94.42(9)		O(5A)–Tb(1)–O(8A)	79.52(8)		O(8A)–Tb(1)–O(1)	85.75(9)
  O(2)–Tb(1)–N(2)	70.31(9)		O(5A)–Tb(1)–N(1)	74.62(9)		O(8A)–Tb(1)–O(2)	138.34(8)
  O(4)–Tb(1)–O(1)	80.92(9)		O(5A)–Tb(1)–N(2)	69.51(9)		O(8A)–Tb(1)–N(1)	79.38(9)
  O(4)–Tb(1)–O(2)	84.56(9)		O(7)–Tb(1)–O(1)	129.77(8)		O(8A)–Tb(1)–N(2)	135.60(8)
  O(4)–Tb(1)–O(5A)	126.40(9)		O(7)–Tb(1)–O(2)	80.24(8)		N(1)–Tb(1)–N(2)	62.51(9)
  O(4)–Tb(1)–O(8A)	78.92(8)		O(7)–Tb(1)–O(4)	75.04(8).		O(1)–Tb(1)–O(2)	121.7(3)
  O(4)–Tb(1)–N(1)	145.67(10)		O(7)–Tb(1)–O(5A)	82.86(8)		O(5)–Tb(1)–O(4)	122.5(3)
  Symmetry transformation: A 1 – x, 1 – y, 1 – z

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•