English

• Due to the forbidden nature of the 4f transitions and the lower molar absorption coefficients, lanthanide ions show very weak luminescence. Thus, the“ antenna”ligand is mandatory. The triplet energy level of the antenna should meet the energy level of the lanthanide ion^[1-2]. Among the wide range of antenna ligands, carboxylate and 1, 10-phenanthroline (Phen) are proven to be the widely accepted antenna. Some ternary rare earth complexes based on carboxylate and Phen have been established. Here gives some carboxylate ligands: N-((benzoylamino)thioxomethyl)glycine, N -(p-acetami-dobenzenesulfonyl)glycine, malic acid, mandelic acid, N - ((4 - methylphenyl)sulfonyl)glycine, L - proline, 2, 4 - dinitro - benzoic acid, and α - naphthyl acetic acid^[3-14]. Furthermore, the biological activity of some rare earth complexes based on 2 - ((4, 6 - dimethyl - 2 - pyrimidinyl) thio)-acetic acid has been reported^[15-16].

  In this work, considering the potential antenna property of carboxylate and Phen, the 3-((4, 6-dimethyl-2 - pyrimidinyl)thio) - propanoic acid (HL), as well as Phen, were introduced to react with lanthanide (Eu, Tb) ions giving the corresponding ternary lanthanide complexes. Meanwhile, we proved the prepared products are two isostructural lanthanide complexes. Importantly, some unique solid-state photoluminescent properties were found in this system.

  1.   Experimental

  1.1   Material and methods

  HL was prepared following the published literature^[16]. All other chemicals were analytically pure from commercial sources and used without further purifica-tion. Elemental analyses on C, H, S, and N were performed on a German Elementary Vario EL Ⅲ instrument. The IR spectra were recorded on KBr disks using a Nicolet - Avatar 370 spectrometer between 400 and 4 000 cm^-1. Thermogravimetric (TG) analyses were carried out on a NETZSCH STA 449 C unit at a heating rate of 10 ℃·min^-1. Photoluminescence studies were performed on an Edinburgh FLS920 fluorescent spectrometer in a solid state.

  1.2   Syntheses of complexes 1 and 2

  A mixture of HL (0.641 g, 3.0 mmol) and Phen (0.198 g, 1.0 mmol) was dissolved in 50 mL ethanol. Then, the lanthanide nitrate hexahydrate (1.0 mmol) was added to the mixture. NH₃·H₂O was added dropwise until the pH value was 6.5. The mixture was kept stirring overnight at room temperature. A large amount of white precipitate was formed. After filtration and washing with water and ethanol, the white solid was obtained. Single crystals suitable for single-crystal X - ray diffraction were obtained from the filtrate after ca. 8 d.

  Complex [Eu(L)₃(Phen)]₂·2H₂O (1): white solid, Yield: 0.104 g, 54%. IR (ATR, cm^-1): 3 442(m), 2 924 (w), 1 608(vs), 1 582(vs), 1 557(s), 1 478(w), 1 423(s), 1 393(s), 1 340(w), 1 269(s), 1 225(m), 1 176(w), 1 013 (w), 883(w), 764(m), 710(w), 545(w). Anal. Calcd. for C₃₉H₄₃EuN₈O₇S₃(%): C, 47.61; H, 4.41; N, 11.39; S, 9.77. Found(%): C, 48.09; H, 4.48; N, 11.98; S, 10.39.

  Complex [Tb(L)₃(Phen)]₂·2H₂O (2): white solid, 0.114 g, 59%. IR (ATR, cm^-1): 3 422(w), 3 050(w), 2 923(w), 1 604(vs), 1 580(vs), 1 551(s), 1 425(vs), 1 339(m), 1 304(m), 1 264(vs), 1 208(m), 1 139(m), 1 102(m), 1 004(w), 948(w), 845(m), 730(m), 690(m), 545(w). Anal. Calcd. for C₃₉H₄₃N₈O₇S₃Tb(%): C, 47.27; H, 4.37; N, 11.31; S, 9.71. Found(%): C, 47.39; H, 4.61; N, 11.96; S, 10.00.

  1.3   X-ray crystallography

  A suitable crystal was covered in mineral oil and mounted on a glass fiber and directly transferred to a Brucker D8 advance diffractometer equipped with a sealed Mo tube and a graphite monochromator using Mo Kα radiation (λ =0.071 073 nm). All structures were solved by the direct methods using SHELXS^[17-18] and refined on F 2 with SHELXL and Olex2^[19]. All nonhydrogen atoms were refined anisotropically and hydrogen atoms were determined with theoretical calculations. Multi-scan absorption correction was applied to the intensity data using the SADABS program^[20]. SQUEEZE routine in PLATON was employed to remove the corresponding Q peaks, which may be identified as two water molecules^{[16, 21]}.

  The crystal data and structure refinement parameters for the complexes are summarized in Table 1. Images of the crystal structures were generated by Diamond, version 3.2 (software copyright, Crystal Impact GbR).

  Table 1

  

  Table 1.  Crystallographic data of complexes 1 and 2

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  Parameter	1	2
  Empirical formula	C78H86Eu2N16O14S6	C78H86N16O14S6Tb2
  Formula weight	1 967.93	1 981.85
  Temperature / K	273.15	273.15
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	C2/c
  a / nm	2.901 7(5)	2.900 61(11)
  b / nm	1.269 8(3)	1.261 20(5)
  c / nm	2.937 5(8)	2.935 21(16)
  β/(°)	117.590(5)	117.507 0(10)
  Volume / nm3	9.592(4)	9.523 9(7)
  Z	4	4
  μ / mm-1	1.486	1.665
  F(000)	3 920.0	3 936.0
  Crystal size / mm	0.497×0.129×0.07	0.499×0.078×0.053
  2θ range for data collection / (°)	2.810-28.588	2.872-28.621
  Index ranges	-37 ≤ h ≤ 37, -16 ≤ k ≤ 16, -38 ≤ l ≤ 38	-37 ≤ h ≤ 37, -16 ≤ k ≤ 16, -38 ≤ l ≤ 38
  Reflection collected	74 688	73 978
  Independent reflection	11 070 (Rint=0.133 8)	11 012 (Rint=0.117 9)
  Data, restraint, parameter	11 071, 0, 520	11 012, 0, 520
  Goodness-of-fit on F2	1.038	1.018
  Final R indexes [I≥2σ(I)]	R1=0.065 1, wR2=0.121 5	R1=0.060 5, wR2=0.110 9
  Final R indexes (all data)	R1=0.138 7, wR2=0.141 7	R1=0.124 1, wR2=0.127 6
  Largest diff. peak and hole / (e·nm-3)	1 460, -785	1 153, -677

  CCDC: 2129656, 1; 2129657, 2.

  2.   Results and discussion

  2.1   Crystal structure

  Complexes 1 and 2 are isostructural with monoclinic space group C2/c (Table 1). Selected bond lengths and bond angles are listed in Table 2 and Table 3, respectively. Both complexes feature the dimeric arrangement (Fig. 1). Two metal centers are bridged by four carboxylate ligands with Eu…Eu separation of 0.391 57(8) nm and Tb…Tb separation of 0.387 68(4) nm. The metal center is nine - coordinated with one Phen molecule (chelating η² mode) and three L- ions, in which three coordination modes can be confirmed: μ₃∶η²-η¹, μ₂∶η¹ -η¹, and η². Thus, the coordination polyhedron of Ln is a distorted mono-capped square antiprism, which can be widely observed in the published lanthanide complexes. The Eu—O bond lengths ranging from 0.236 0(3) to 0.254 5(3) nm and Tb—O bond lengths ranging from 0.232 4(3) to 0.251 9(3) nm agree with the reported single bond values in the carboxylate lanthanide complexes^[22-24]. As is apparent, the O—C —O bond angle of μ₂∶η¹-η¹ mode (126.3(5)°) is larger than those of μ₃∶η² - η¹ mode (120.6(5)°) and η² mode (121.8(5)°). It should be noted that the molecular structures of complexes 1 and 2 are highly consistent with the published complexes [RE(L)₃ (Phen)]₂·nH₂O (RE=Nd, Sm, and Y; for Sm and Nd: n= 2; for Y: n=0)^[16].

  Table 2

  

  Table 2.  Selected bond lengths of complexes 1 and 2

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  1
  Eu1…Eu1i	0.394	61(8)	Eu1—O4	0.252 5(4)	Eu1—O2	0.235 4(4)
  Eu1—O3	0.248	1(4)	Eu1—O1i	0.260 3(4)	Eu1—O5i	0.259 5(4)
  Eu1—N1	0.258	7(5)	Eu1—N2	0.257 2(5)	Eu1—O1	0.235 0(4)
  Eu1—O6i	0.234	9(4)
  2
  Tb1…Tb1i	0.390	88(5)	Tb1—O4	0.246 5(3)	Tb1—O2	0.230 7(3)
  Tb1—O3i	0.231	9(3)	Tb1—O1	0.231 4(3)	Tb1—O5i	0.257 3(4)
  Tb1—N1	0.255	2(5)	Tb1—N2	0.255 9(4)	Tb1—O1i	0.258 0(3)
  Tb1—O6	0.249	2(4)
  Symmetry codes: i 1/2-x, 1/2-y, 1-z for 1; i 1/2-x, 1/2-y, 1-z for 2.

  Table 3

  

  Table 3.  Selected bond angles of complexes 1 and 2

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  1
  O1—Eu1—Eu1i	39.47(9)	O5i—Eu1—N1	72.31(13)	O3—Eu1—Eu1i	123.37(9)
  O1i—Eu1—Eu1i	35.02(8)	O5i—Eu1—N2	79.59(12)	O3—Eu1—O1i	155.69(15)
  O1—Eu1—O1i	74.49(13)	O6i—Eu1—Eu1i	68.32(8)	O3—Eu1—O4	52.80(11)
  O1—Eu1—O3	88.35(13)	O6i—Eu1—O1i	72.28(11)	O3—Eu1—O5i	145.18(12)
  O1—Eu1—O4	74.37(11)	O6i—Eu1—O3	83.13(12)	O3—Eu1—N1	73.42(13)
  O1i—Eu1—O5i	50.96(11)	O6i—Eu1—O4	124.92(11)	O3—Eu1—N2	79.42(12)
  O1—Eu1—O5i	124.15(11)	O6i—Eu1—O5i	95.11(12)	O4—Eu1—Eu1i	109.34(9)
  O1—Eu1—O6i	73.41(12)	O6i—Eu1—N1	77.12(13)	O4—Eu1—O1i	137.35(12)
  O1—Eu1—N1	146.76(15)	O6i—Eu1—N2	139.16(13)	O4—Eu1—O5i	139.97(12)
  O1i—Eu1—N1	110.46(12)	N1—Eu1—Eu1i	138.00(11)	O4—Eu1—N1	110.10(14)
  O1—Eu1—N2	142.93(12)	N2—Eu1—Eu1i	149.55(11)	O4—Eu1—N2	70.05(14)
  O1i—Eu1—N2	126.72(12)	N2—Eu1—N1	61.08(16)	O5i—Eu1—Eu1i	85.99(8)
  O2—Eu1—Eu1i	69.87(9)	O2—Eu1—O4	74.36(11)	O2—Eu1—N2	80.65(13)
  O2—Eu1—O1i	67.72(13)	O2—Eu1—O5i	75.85(12)	O2—Eu1—N1	133.75(15)
  O2—Eu1—O1	79.39(14)	O2—Eu1—O6i	136.43(13)	O2—Eu1—O3	128.86(13)
  2
  O1—Tb1—Tb1i	39.41(9)	O5i—Tb1—Tb1i	84.29(8)	O2—Tb1—O5i	72.57(14)
  O1i—Tb1—Tb1i	34.70 (1)	O5i—Tb1—N1	79.65(14)	O2—Tb1—O6	129.40(13)
  O1—Tb1—O1i	74.11(13)	O5i—Tb1—N2	72.58 (14)	O2—Tb1—N1	81.13(13)
  O1—Tb1—O2	78.15(13)	O6—Tb1—Tb1i	124.55(8)	O2—Tb1—N2	133.64(15)
  O1—Tb1—O3i	74.91(13)	O6—Tb1—O1i	152.44(11)	O3i—Tb1—Tb1i	68.35(9)
  O1—Tb1—O4	74.15(14)	O6—Tb1—O4	53.22(11)	O3i—Tb1—O1i	73.69(12)
  O1—Tb1—O5i	123.31(12)	O6—Tb1—O5i	144.61(11)	O3i—Tb1—O4	122.23(13)
  O1i—Tb1—O5i	51.32(13)	O6—Tb1—N1	79.56(13)	O3i—Tb1—O5i	97.21(14)
  O1—Tb1—O6	88.71(13)	O6—Tb1—N2	72.98(14)	O3i—Tb1—O6	81.89(13)
  O1—Tb1—N1	142.76(13)	N1—Tb1—Tb1i	149.39(9)	O3i—Tb1—N1	139.36(14)
  O1i—Tb1—N1	146.82(13)	N1—Tb1—N2	61.67(15)	O3i—Tb1—N2	78.34(15)
  O1—Tb1—N2	147.27(12)	N2—Tb1—Tb1i	137.48(11)	O4—Tb1—Tb1i	109.37(9)
  O1i—Tb1—N2	111.48(13)	O2—Tb1—O4	73.52(11)	O4—Tb1—O1i	137.39(12)
  O2—Tb1—Tb1i	69.11(9)	O4—Tb1—N2	110.52(14)	O4—Tb1—O5i	140.56(13)
  O2—Tb1—O1i	67.97(12)	O2—Tb1—O3i	137.39(12)	O4—Tb1—N1	70.20(13)
  Symmetry codes: i 1/2-x, 1/2-y, 1-z for 1; i 1/2-x, 1/2-y, 1-z for 2.

  Figure 1

  

  Figure 1.  Drawing of the molecular structure of dinuclear Eu complex 1 with an ellipsoid probability of 50%

  Hydrogen atoms and solvents are omitted for clarity; Symmetry code: ⁱ 1/2-x, 1/2-y, 1-z

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  2.2   TG analysis

  The TG properties of complexes 1 and 2 were measured from 25 to 1 000 ℃ (Fig. 2). They showcase almost identical TG curves. Thus, only complex 1 will be discussed in detail here. A slight weight loss can be observed before 225 ℃, which may indicate the removal of two water molecules (Obsd. 1.3%; Calcd. 1.8%) in the sample. These results agree with the single crystal X - ray diffraction analyses and the published isostructural complexes [RE(L)₃(Phen)]₂·2H₂O (RE=Nd, Sm)^[16]. Further weight loss can be observed at around 1 000 ℃.

  Figure 2

  

  Figure 2.  TG curves of complexes 1 and 2

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  2.3   Photoluminescent property

  The solid-state photoluminescent property of complexes 1 and 2 was measured and the emission spectra are shown in Fig. 3. After excited at 347 nm, three emission bands could be confirmed in the emission spectra of complex 1: 593 and 595 nm (⁵D₀→⁷F₁ transition); 616, 619, and 621 nm (⁵D₀→⁷F₂ transition); 689, 694, and 699 nm (⁵D₀→⁷F₄ transition). The most intense bands at 616, 619, and 621 nm corresponds to the ⁵D₀→⁷F₂ transition. The intensity ratio between ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition indicates that the Eu is not located on the centrosymmetric site in the solid state, which agrees with the single crystal diffraction analysis. The luminescent lifetime for complex 1 was 1.5 ms. The quantum yield was 87.3%. Due to the isostructural feature, the emission pattern of complex 2 was excited at 347 nm as well. Four characteristic emission bands were observed. Three weak bands at 490 nm (⁵D₄→⁷F₆ transition), 584 nm (⁵D₄→⁷F₄ transition), and 622 nm (⁵D₄→⁷F₃ transition) could be observed. The most intense band at 545 nm corresponds to the ⁵D₄→⁷F₅ transition. The luminescent lifetime of complex 2 (1.3 ms) was almost identical to complex 1. However, the quantum yield (60.4%) was lower than complex 1.

  Figure 3

  

  Figure 3.  Solid-state emission spectra of complexes 1 (left) and 2 (right) at room temperature

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  3.   Conclusions

  Two ternary lanthanide complexes of 3 - ((4, 6 - dimethyl-2-pyrimidinyl)thio)-propanoic acid (HL) and Phen, [Ln(L)₃(Phen)]₂·2H₂O (Ln=Eu (1), Tb (2)), were prepared and the molecular structures were established by single crystal X - ray diffraction analysis. Both complexes feature a dimeric arrangement, in which two lanthanide metal ions are bridged by four carboxylate ligands. Solid-state photoluminescent measurement reveals that complexes 1 and 2 manifest the characteristic emission bands of the metal center. Complex 1 possessed a high quantum yield of 87.3%.

  
  Conflicts of interest: There are no conflicts to declare.
  
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• 

  Figure 1  Drawing of the molecular structure of dinuclear Eu complex 1 with an ellipsoid probability of 50%

  Hydrogen atoms and solvents are omitted for clarity; Symmetry code: ⁱ 1/2-x, 1/2-y, 1-z

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  Figure 2  TG curves of complexes 1 and 2

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  Figure 3  Solid-state emission spectra of complexes 1 (left) and 2 (right) at room temperature

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  Table 1.  Crystallographic data of complexes 1 and 2

  Parameter	1	2
  Empirical formula	C78H86Eu2N16O14S6	C78H86N16O14S6Tb2
  Formula weight	1 967.93	1 981.85
  Temperature / K	273.15	273.15
  Crystal system	Monoclinic	Monoclinic
  Space group	C2/c	C2/c
  a / nm	2.901 7(5)	2.900 61(11)
  b / nm	1.269 8(3)	1.261 20(5)
  c / nm	2.937 5(8)	2.935 21(16)
  β/(°)	117.590(5)	117.507 0(10)
  Volume / nm3	9.592(4)	9.523 9(7)
  Z	4	4
  μ / mm-1	1.486	1.665
  F(000)	3 920.0	3 936.0
  Crystal size / mm	0.497×0.129×0.07	0.499×0.078×0.053
  2θ range for data collection / (°)	2.810-28.588	2.872-28.621
  Index ranges	-37 ≤ h ≤ 37, -16 ≤ k ≤ 16, -38 ≤ l ≤ 38	-37 ≤ h ≤ 37, -16 ≤ k ≤ 16, -38 ≤ l ≤ 38
  Reflection collected	74 688	73 978
  Independent reflection	11 070 (Rint=0.133 8)	11 012 (Rint=0.117 9)
  Data, restraint, parameter	11 071, 0, 520	11 012, 0, 520
  Goodness-of-fit on F2	1.038	1.018
  Final R indexes [I≥2σ(I)]	R1=0.065 1, wR2=0.121 5	R1=0.060 5, wR2=0.110 9
  Final R indexes (all data)	R1=0.138 7, wR2=0.141 7	R1=0.124 1, wR2=0.127 6
  Largest diff. peak and hole / (e·nm-3)	1 460, -785	1 153, -677

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  Table 2.  Selected bond lengths of complexes 1 and 2

  1
  Eu1…Eu1i	0.394	61(8)	Eu1—O4	0.252 5(4)	Eu1—O2	0.235 4(4)
  Eu1—O3	0.248	1(4)	Eu1—O1i	0.260 3(4)	Eu1—O5i	0.259 5(4)
  Eu1—N1	0.258	7(5)	Eu1—N2	0.257 2(5)	Eu1—O1	0.235 0(4)
  Eu1—O6i	0.234	9(4)
  2
  Tb1…Tb1i	0.390	88(5)	Tb1—O4	0.246 5(3)	Tb1—O2	0.230 7(3)
  Tb1—O3i	0.231	9(3)	Tb1—O1	0.231 4(3)	Tb1—O5i	0.257 3(4)
  Tb1—N1	0.255	2(5)	Tb1—N2	0.255 9(4)	Tb1—O1i	0.258 0(3)
  Tb1—O6	0.249	2(4)
  Symmetry codes: i 1/2-x, 1/2-y, 1-z for 1; i 1/2-x, 1/2-y, 1-z for 2.

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  Table 3.  Selected bond angles of complexes 1 and 2

  1
  O1—Eu1—Eu1i	39.47(9)	O5i—Eu1—N1	72.31(13)	O3—Eu1—Eu1i	123.37(9)
  O1i—Eu1—Eu1i	35.02(8)	O5i—Eu1—N2	79.59(12)	O3—Eu1—O1i	155.69(15)
  O1—Eu1—O1i	74.49(13)	O6i—Eu1—Eu1i	68.32(8)	O3—Eu1—O4	52.80(11)
  O1—Eu1—O3	88.35(13)	O6i—Eu1—O1i	72.28(11)	O3—Eu1—O5i	145.18(12)
  O1—Eu1—O4	74.37(11)	O6i—Eu1—O3	83.13(12)	O3—Eu1—N1	73.42(13)
  O1i—Eu1—O5i	50.96(11)	O6i—Eu1—O4	124.92(11)	O3—Eu1—N2	79.42(12)
  O1—Eu1—O5i	124.15(11)	O6i—Eu1—O5i	95.11(12)	O4—Eu1—Eu1i	109.34(9)
  O1—Eu1—O6i	73.41(12)	O6i—Eu1—N1	77.12(13)	O4—Eu1—O1i	137.35(12)
  O1—Eu1—N1	146.76(15)	O6i—Eu1—N2	139.16(13)	O4—Eu1—O5i	139.97(12)
  O1i—Eu1—N1	110.46(12)	N1—Eu1—Eu1i	138.00(11)	O4—Eu1—N1	110.10(14)
  O1—Eu1—N2	142.93(12)	N2—Eu1—Eu1i	149.55(11)	O4—Eu1—N2	70.05(14)
  O1i—Eu1—N2	126.72(12)	N2—Eu1—N1	61.08(16)	O5i—Eu1—Eu1i	85.99(8)
  O2—Eu1—Eu1i	69.87(9)	O2—Eu1—O4	74.36(11)	O2—Eu1—N2	80.65(13)
  O2—Eu1—O1i	67.72(13)	O2—Eu1—O5i	75.85(12)	O2—Eu1—N1	133.75(15)
  O2—Eu1—O1	79.39(14)	O2—Eu1—O6i	136.43(13)	O2—Eu1—O3	128.86(13)
  2
  O1—Tb1—Tb1i	39.41(9)	O5i—Tb1—Tb1i	84.29(8)	O2—Tb1—O5i	72.57(14)
  O1i—Tb1—Tb1i	34.70 (1)	O5i—Tb1—N1	79.65(14)	O2—Tb1—O6	129.40(13)
  O1—Tb1—O1i	74.11(13)	O5i—Tb1—N2	72.58 (14)	O2—Tb1—N1	81.13(13)
  O1—Tb1—O2	78.15(13)	O6—Tb1—Tb1i	124.55(8)	O2—Tb1—N2	133.64(15)
  O1—Tb1—O3i	74.91(13)	O6—Tb1—O1i	152.44(11)	O3i—Tb1—Tb1i	68.35(9)
  O1—Tb1—O4	74.15(14)	O6—Tb1—O4	53.22(11)	O3i—Tb1—O1i	73.69(12)
  O1—Tb1—O5i	123.31(12)	O6—Tb1—O5i	144.61(11)	O3i—Tb1—O4	122.23(13)
  O1i—Tb1—O5i	51.32(13)	O6—Tb1—N1	79.56(13)	O3i—Tb1—O5i	97.21(14)
  O1—Tb1—O6	88.71(13)	O6—Tb1—N2	72.98(14)	O3i—Tb1—O6	81.89(13)
  O1—Tb1—N1	142.76(13)	N1—Tb1—Tb1i	149.39(9)	O3i—Tb1—N1	139.36(14)
  O1i—Tb1—N1	146.82(13)	N1—Tb1—N2	61.67(15)	O3i—Tb1—N2	78.34(15)
  O1—Tb1—N2	147.27(12)	N2—Tb1—Tb1i	137.48(11)	O4—Tb1—Tb1i	109.37(9)
  O1i—Tb1—N2	111.48(13)	O2—Tb1—O4	73.52(11)	O4—Tb1—O1i	137.39(12)
  O2—Tb1—Tb1i	69.11(9)	O4—Tb1—N2	110.52(14)	O4—Tb1—O5i	140.56(13)
  O2—Tb1—O1i	67.97(12)	O2—Tb1—O3i	137.39(12)	O4—Tb1—N1	70.20(13)
  Symmetry codes: i 1/2-x, 1/2-y, 1-z for 1; i 1/2-x, 1/2-y, 1-z for 2.

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