Lanthanide coordination polymers containing 1,3-bis(carboxymethyl) imidazolium as organic ligand: Crystal structure and luminescent properties

Inorganica Chimica Acta 497 (2019) 119075

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Research paper

Lanthanide coordination polymers containing 1,3-bis(carboxymethyl) imidazolium as organic ligand: Crystal structure and luminescent properties

T

Li-Xin Youa, Jian-Hong Haoa, Dan Qia, Shi-Yu Xiea, Shu-Ju Wanga, Gang Xionga, ⁎ ⁎ Ileana Dragutanb, , Valerian Dragutanb, Fu Dinga, Ya-Guang Suna, a b

Key Laboratory of Inorganic Molecule-Based Chemistry of Liaoning Province, Shenyang University of Chemical Technology, Shenyang 110142, China Institute of Organic Chemistry, Romanian Academy, Bucharest 060023, Romania

A R T I C LE I N FO

A B S T R A C T

Keywords: Lanthanide coordination polymers 1,3-bis(carboxymethyl) imidazolium Crystal structure Fluorescent properties

Nine lanthanide coordination polymers based on 1,3-bis(carboxymethyl) imidazolium (HBCI), [Ln(μ3-BCI) (NO3)2H2O]n (Ln = Pr (1), Nd (2), Sm (3)) and [Ln(μ4-BCI)(NO3)2H2O]n (Ln = Eu (4), Gd (5), Tb (6), Dy (7), Ho (8), Er (9)), were synthesized under hydrothermal conditions and characterized by elemental analysis, IR spectroscopy, PXRD analysis, thermogravimetry analysis and single-crystal X-ray diffraction. Crystal structures of the complexes 1–3 show a three-dimensional framework and those of 4–9 exhibit a two-dimensional layer, due to the various linking modes of HBCI. Complexes 3, 4, 6 and 7 show luminescence of the trivalent lanthanides. The fluorescence lifetime and quantum efficiency of 4 as well as the phosphorescence spectrum of 5 were measured.

1. Introduction In recent years, the construction of lanthanide coordination polymers (Ln-CPs), formed via the self-assembly of lanthanide metal nodes and organic ligands, are increasingly attracting attention from researchers in chemistry because they are used as functional crystallization materials in many fields [1], based on the characteristic luminescent emissions of trivalent lanthanide complexes in the visible and near to mid-infrared region [2]. However, the design and synthesis of Ln-CPs is a great challenge due to the unique nature of the lanthanide ions: they display a large radius, high coordination numbers, and variable coordination modes [3]. On the other hand, most lanthanide ions are difficult to be excited because of low absorption coefficients of the f excited states [4]. Organic linkers play an irreplaceable role during the synthesis of luminescent Ln-CPs, which are employed as antennas with the capability of transferring energy indirectly to lanthanide ions and result in a diversity of the structure for different applications. As a flexible ligand which is composed of the imidazolium cation and two carboxylate groups, 1,3-bis(carboxymethyl) imidazolium (HBCI) has been employed to construct CPs not only due to the various coordination modes by the carboxylate groups, but also owing to the flexible –CH2– spacers between the imidazole ring and carboxylate groups to fit the coordination conformation of the metal ions. So far, literature data on CPs with the HBCI ligand have been reported [5].

⁎

In this paper, a series of Ln-CPs, [Ln(μ3-BCI)(NO3)2H2O]n (Ln = Pr (1), Nd (2), Sm (3)) and [Ln(μ4-BCI)(NO3)2H2O]n(Ln = Eu (4), Gd (5), Tb (6), Dy (7), Ho (8), Er (9)), were obtained under hydrothermal conditions based on the HBCI ligand. Complexes 1–3 exhibit a 3D framework structure and 4–9 exhibit a 2D layer. The structures were characterized by single-crystal X-ray diffraction analysis, elemental analysis, infrared spectroscopy, powder X-ray diffraction and thermogravimetric analysis. The luminescence properties of 3, 4, 6 and 7 have also been investigated in detail. 2. Experimental section 2.1. Materials All chemicals purchased were of reagent grade and used without further purification. All syntheses were carried out in 23 mL teflon-lined autoclaves under autogenous pressure. Water used in the synthesis was distilled before use. 2.2. Physical measurements The C, H, and N analyses were carried out using a Perkin-Elmer 240 Celemental analyzer. FT-IR spectra were recorded on a Nicolet IR-470 spectrometer using KBr pellets. Thermogravimetric analysis (TGA)

Corresponding authors. E-mail addresses: [email protected] (I. Dragutan), [email protected] (Y.-G. Sun).

https://doi.org/10.1016/j.ica.2019.119075 Received 11 June 2019; Received in revised form 11 August 2019; Accepted 13 August 2019 Available online 14 August 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.

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3.1.1. Structure description of 1–3 Single-crystal X-ray structure analysis revealed that complexes 1-3 are isomorphous and belong to the monoclinic system, P21/n space group. Therefore, only the structure of complex 2 will be described in detail as a representative example. The asymmetric structural unit of 2 contains one Nd3+, one fully deprotonated BCI− ligand, one coordinating water molecule and two coordinated NO3− ions. As shown in Fig. 1a, each Nd3+ ion is surrounded by nine oxygen atoms, four from carboxyls (O8, O9A, O10B, O11C) of four BCI− ligands, one from the coordinating water (O7) and four from two NO3− (O1, O2, O4 and O5), to form a slightly distorted three-cap triangular prism polyhedral configuration. The Nd-O bond lengths range from 2.372(2) to 2.618(3) Å and the O-Nd-O bond angles vary from 49.2(8)° to 154.1(8)°. These values are in good agreement with previously reported nine- or tencoordinated Ln3+ complexes with oxygen donor ligands [11]. The BCI− ligand exhibits the same (κ1-κ1-μ2)-(κ1-κ1-μ2)-μ4 coordination mode to link four Nd3+ ions. As shown in Fig. 1b, a zigzag one-dimensional chain along the a-axis is formed through the carboxyl oxygen atoms of two BCI− ligands bridging to the adjacent Nd3+ ions. The one-dimensional chain is further connected as a three-dimensional framework through linking of the carboxyls from the ligands to the Nd3+ ions (Fig. 1c). Simplified with TOPOS 4.0 topology software [12], as shown in Fig. 1d, Nd3+ metal ions are considered to be 4-connected nodes (green balls), and BCI− ligands are considered to be 4-connected nodes (golden balls). The topology can be described as a PtS-type framework with the Schläfli symbol {42.84}.

experiments were performed on a SDT Q600 instrument with a heating rate of 10 °C min−1. Powder X-ray diffraction patterns of the samples were recorded on the X-ray diffractometer (Bruker D8 Advance) using Cu-Kα radiation. Solid-state photoluminescence spectra were measured at room temperature with an Edinburgh FLS 920 fluorescence spectrometer. At 298 K, the luminescence decay curve was obtained using a FLS 920 fluorescence spectrophotometer, and the nF 900 lamp was used as an excitation resource. Quantum efficiency (QE) was measured by the integrating sphere at low temperature using a FLS 920 photoluminescence quantum yield measurement system. Then, the QE is calculated according to the method of the software provided by the manufacturer. The phosphorescence spectrum was measured by FLS 920 at a low temperature of 77 K [6]. 2.3. Preparation of complexes 1–9 For 1–3, HBCI (0.2 mmol) and Ln(NO3)3 (0.1 mmol) were mixed in deionized water (1.0 mL) and ethanol (9.0 mL). The mixture was sealed in a 23 mL teflon reactor and kept under autogenous pressure at 110 °C for 72 h, then cooled to room temperature naturally. The obtained product was washed with absolute ethanol. The yield based on the rare earth nitrate amounts to 79% for 1, 80% for 2 and 79% for 3. Anal. Calcd (%) for 1 (C7H9N4O11Pr): C 18.02; H 1.93; N 12.01. Found: C 17.99; H 1.91; N 11.99. 2 (C7H9N4O11Nd): C 17.89; H 1.91; N 11.93. Found: C 17.82; H 1.89; N 11.90. 3 (C7H9N4O11Sm): C 17.67; H 1.89; N 11.78. Found: C 17.61; H 1.87; N 11.76. Complexes 4–9 were synthesized in a similar way to that described for 1–3, except that the reaction temperature was 70 °C for 72 h. The yield based on the rare earth nitrate amounts to 83% for 4, 83% for 5, 84% for 6, 84% for 7, 84% for 8 and 83% for 9. Anal. Calcd (%). For 4 (C7H9N4O11Eu): C 17.60; H 1.88; N 11.74. Found: C 17.56; H 1.86; N 11.72. 5 (C7H9N4O11Gd): C 17.41; H 1.87; N 11.61. Found: C 17.38; H 1.84; N 11.57. 6 (C7H9N4O11Tb): C 17.35; H 1.86; N 11.61. Found: C 17.35; H 1.83; N 11.52. 7 (C7H9N4O11Dy): C 17.23; H 1.85; N 11.48. Found: C 17.20; H 1.81; N 11.46. 8 (C7H9N4O11Ho): C 17.14; H 1.84; N 11.43. Found: C 17.10; H 1.80; N 11.40. 9 (C7H9N4O11Er): C 17.06; H 1.83; N 11.37. Found: C 17.01; H 1.79; N 11.31.

3.1.2. Structure description of 4–9 X-ray single crystal diffraction analysis showed that the crystal structures of complexes 4–9 are isomorphic, and these complexes crystallized in the monoclinic system, P21/c space group. So, only the structure of complex 5 will be described in detail herein. Similar to complex 2, the asymmetric structural unit of complex 5 contains one Gd3+ ion, one BCI− ligand, a coordinating water molecule and two NO3− ions. As shown in Fig. 2a, the Gd3+ ion is coordinated by nine oxygen atoms, four from carboxyls (O8, O9, O10 and O11) of three BCI− ligands, one from the coordinating water molecule (O7) and four from two NO3− (O1, O2, O4 and O5). The Gd–O bond lengths range from 2.297(3) to 2.910(3) and the O–Gd–O bond angles vary from 50.5(9)° to 148.5(9)°, which is substantially the same as for the Gd–O bond length in other Gd3+ complexes reported [13]. The BCI− ligand exhibits (κ1-κ1-μ1)-(κ1-κ1-μ2)-μ3 coordination modes in 5 which is different from that in complex 2. As shown in the green part of Fig. 2b, the neighboring Gd3+ ions are bridged by the carboxyl from the BCI− ligands to form a 1D chain along the c axis. The adjacent 1D chains are linked by BCI− ligands with carboxyls to extend into a 2D layer (Fig. 2b). Topological analysis using the TOPOS4.0 topology software reveals that the Gd3+ metal ions (purple balls) and the BCI− ligands (yellow-green balls) are both considered to be 3-connected nodes in complex 5, which can be simplified to a fes-type network with the Schläfli symbol {4.82} (Fig. 2c).

2.4. Single crystal X-ray crystallographic study All data collections were carried out at 293 K on a Bruker SMART Apex CCD diffractometer with Mo-Kα monochromatic radiation (λ = 0.71073 Å) using the ω-2θ scan technique. An empirical absorption correction was applied [7]. The structures were solved by direct methods and refined by full-matrix least-squares against F2 using the SHELXTL crystallographic software package [8]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The hydrogen atoms were in calculated positions and refined as riding atoms with fixed isotropic thermal parameter [9]. Full crystallographic data for 1–9 are listed in Tables 1 and 2. 3. Results and discussion

3.2. Infrared spectroscopy 3.1. Description of crystal structures Infrared spectra of HBCI, NaBCl and 1–9 are shown in Fig. S1 of ESI. Complexes 1–3 exhibit a similar pattern with a broadband centered around 3428 cm−1 that may be assigned to the OeH stretching vibration, indicating the presence of water molecules in the structure. There are no C]O absorption bands of HBCI at 1730 cm−1. Instead, the asymmetric and symmetric stretching vibrations of the carboxylate groups are observed around 1617 and 1383 cm−1, indicating that the ligand is completely deprotonated when it is coordinated with lanthanide ions. For 4–9, the spectra exhibit a similar pattern. The peak around 3460 cm−1 was attributed to the OeH stretching mode from the coordinating water molecules. The asymmetric and symmetric stretching vibrations of carboxylate around 1617 and 1385 cm−1

Complexes 1–9 were obtained by the reaction of lanthanide nitrate with the HBCI ligand under similar hydrothermal conditions. Although they have the same chemical formula, 1–3 show a three-dimensional framework and 4–9 exhibit a two-dimensional layer, because of the different coordination modes of HBCI at the different reaction temperatures. It should be noted that the products are amorphous when Eu3+, Gd3+, Tb3+, Dy3+,Ho3+ and Er3+ were used under the same synthetic conditions as those of 1–3. The reason may be related to the effect of metal ions, because the larger ionic radius of Pr, Nd and Sm compared to Eu, Gd, Tb, Dy, Ho and Er results in different crystal structures [10]. 2

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Table 1 Crystal data and structure refinement for 1–5.

A

Complex

1

2

3

4

5

Formula Fw Temp(K) Crystal system Space group a(Å) b(Å) c(Å) β/° V(Å)3 Z ρc(g cm−3) μ/mm−1 F(0 0 0) Rec/unique data/restraints/params GOF on F2 R1A [I ≥ 2σ(I)] wR2B [I ≥ 2σ(I)]

C7H9N4O11Pr 466.09 298.15 monoclinic P21/n 9.3739(13) 12.7518(16) 12.1983(18) 101.657(6) 1428.0(3) 4 2.168 3.479 904.0 10381/3246 3246/0/209 1.092 0.0409 0.0891

C7H9N4O11Nd 469.42 298.15 monoclinic P21/n 9.342(5) 12.741(7) 12.174(7) 101.600(5) 1419.5(14) 4 2.197 3.726 908.0 14095/3265 3265/0/210 1.068 0.0227 0.0535

C7H9N4O11Sm 475.53 298.15 monoclinic P21/n 9.284(5) 12.679(6) 12.132(6) 101.602(7) 1398.9(12) 4 2.258 4.267 916.0 14033/3221 3221/0/209 1.113 0.0287 0.0594

C7H9N4O11Eu 477.14 298.15 monoclinic P21/c 10.746(6) 14.053(9) 8.874(6) 96.816(7) 1330.7(14) 4 2.382 4.786 920.0 13243/3057 3057/0/210 1.067 0.0231 0.0521

C7H9N4O11Gd 482.43 298.15 monoclinic P21/c 10.739(5) 14.054(6) 8.850(4) 96.862(7) 1326.2(11) 4 2.416 5.074 924.0 12111/3062 3062/2/217 1.080 0.0280 0.0707

R1 = Σ||Fo| − |Fc||/Σ|Fo|. BwR2 = {Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]}1/2.

indicated the deprotonation of the carboxylate groups and their coordination to the lanthanide ions.

Table 2 Crystal data and structure refinement for 6–9.

A

Complex

6

7

8

9

Formula Fw Temp(K) Crystal system Space group a(Å) b(Å) c(Å) β/° V(Å)3 Z ρabc(g cm−3) μ/mm−1 F(0 0 0) Rec/unique data/restraints/ params GOF on F2 A R1 [I ≥ 2σ(I)] wR2B [I ≥ 2σ(I)]

C7H9N4O11Tb 484.1 298.15 monoclinic P21/c 10.762(6) 14.038(8) 8.861(5) 96.837(7) 1329.1(13) 4 2.419 5.394 928.0 11071/3021 3032/0/209

C7H9N4O11Dy 487.68 298.15 monoclinic P21/c 10.726(7) 14.007(9) 8.874(6) 96.713(8) 1320.9(13) 4 2.452 5.730 932.0 13160/2239 3024/0/209

C7H9N4O11Ho 490.11 298.15 Monoclinic P21/c 10.755(4) 14.093(6) 8.882(4) 96.959(5) 1336.4(9) 4 2.436 5.993 936.0 13326/3073 3073/0/209

C7H9N4O11Er 492.44 298.15 monoclinic P21/c 10.725(6) 13.945(8) 8.788(5) 96.572(6) 1305.7(13) 4 2.505 6.502 940.0 10113/2255 2255/0/209

0.971 0.0447 0.0942

1.078 0.0229 0.0557

1.090 0.0283 0.0783

1.026 0.0466 0.1001

3.3. Powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) In order to ensure the purity of the obtained crystals, the powder Xray diffraction (PXRD) test was carried out, as shown in Fig. S2 of ESI. The results show that the experimental patterns for1-9 are in good agreement with the calculated ones obtained from the single-crystal structures, confirming the purity of bulk materials. For the thermal stability of 1–9, thermogravimetric analysis (TGA) was carried out at a heating rate of 10°C min−1 under a N2 atmosphere at 40 to 800 °C. As shown in (Fig. S3), complexes 1–3 have similar TGA curves, hence complex 2 was selected for the detailed description herein. The first weight loss from 130 to 210 °C accounted for 4.0% and was assigned to the loss of the coordinating water molecules (calculated value: 3.8%). At more than 300 °C, the curves began to decay. For complexes 4–9, the TGA curves are substantially the same, and complex 5 is described in detail. The initial weight loss occurred at 150–250 °C, and the weight loss of 3.8% was assigned to the loss of coordinated water molecules (calculated value: 3.7%). Above 300 °C, the frame structure began to

R1 = Σ||Fo| − |Fc||/Σ|Fo|. BwR2 = {Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]}1/2.

Fig. 1. (a) Coordination environment of Nd3+ ion in 2. Symmetry code: A 1 − x, 1 − y, 1 − z; B 1.5-x, 0.5 + y, 0.5-z; C 0.5 + x, 0.5-y, 0.5 + z. (b) 1D chain Viewed along the a-axis; (c) View of 3D structure along the a-axis. (d) The topology of 2. The green balls represent the Nd3+ centers and the golden balls represent the ligands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3

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Fig. 2. (a) Coordination environment of Ga3+ in complex 5. Symmetry code: A −x, 2 − y, −z; B x, 2.5 − y, 0.5 + z. (b) View of 2D layer of 5. (c) Topology of 5. The purple balls represent the Gd3+ centers and the yellow-green balls represent the ligands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Solid-state fluorescent spectra of 3, 4, 6 and 7.

Fig. 4. Fit of the attenuation curve for solid composite 4. The sample was excited at 365 nm. Fitted by Fit = A + B1 × exp(−t/τ1) + B2 × exp(−t/τ2).

Fig. 5. Phosphorescence spectra of 5 in the solid-state at low-temperature (77 K). The excitation spectrum monitored the emission at 435 nm and the emission spectrum were recorded upon excitation at 310 nm, respectively.

4

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functional crystallization materials in many fields.

collapse. The TGA results show that 1–9 exhibit relatively good thermal stability.

Acknowledgements 3.4. Photoluminescence properties This work was supported by the National Natural Science Foundation of China (21671139), the Distinguished Professor Project of Liaoning province (2013204), the Natural Science Foundation of Liaoning Province (20170540713) and the Science and Technology Innovation Program for Middle-aged and Youth Talents of Shenyang (RC180086).

The solid-state fluorescence properties of complexes 3, 4, 6 and 7 are discussed for 298 K. They all exhibit the characteristic transitions of the corresponding lanthanide ions (Fig. 3). The emission spectrum of complex 3 was obtained upon excitation with a wavelength of 368 nm, in which four emission peaks at 565, 596, 642 and 704 nm correspond to the characteristic 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2 and 4 G5/2 → 6H11/2 transitions of the Sm3+ ion [14], as shown in Fig. 3a. At the excitation wavelength of 368 nm, the three fluorescence emission peaks of complex 4 at 591, 617, 698 nm belong to the three energy level transitions of 5D0 → 7FJ (J = 1, 2, 4, respectively) (Fig. 3b). The 5D0 → 7 F2 transition (induced electric-dipole transition) is much more intense than the 5D0 → 7F1 transition (magnetic-dipole transition) and dominates the red emission light. The intensity ratio of 5D0 → 7F2/5D0 → 7F1 is 5.1, which indicates the absence of inversion symmetry at the site of the Eu3+ ion [15]. The solid-state emission spectrum of complex 6 was obtained upon excitation at a wavelength of 368 nm. The emission bands at 490, 544, 588, 621 and 647 nm are generated by the transitions from the excited state 5D4 to the ground state 7FJ (J = 6, 5, 4, 3, 2) (Fig. 3c). The relative intensity of the emission bands at 490 nm and 544 nm is larger because of the efficient energy transfer from the ligand to the Tb3+ ion for the 5D4 → 7F6 and 5D4 → 7F5 transitions. At an excitation wavelength of 368 nm, complex 7 exhibits a characteristic emission of Dy3+ at 481, 575, 662, 751 nm, which is mainly attributed to the 4F9/2 → 6HJ (J = 15/2, 13/2, 11/2, 9/2) transition [16], as shown in Fig. 3d. In addition to the steady-state emission, we also determined the fluorescence lifetime and quantum yield of complex 4. The fluorescence decay curve is shown in Fig. 4. The luminescence decay curve was tested at room temperature to fit the double exponential decay function. The emission decay lifetime was τ1 = 450 μs (68.9%), τ2 = 510 μs (31.1%). At 298 K, barium sulfate was chosen as a reference, and the quantum efficiency of 4 was measured to be 4.13%. The emission phosphorescence spectrum of the Gd3+ complex allows an identification of the lowest ligand triplet in the complex. The lowest triplet energy level (T1) of HBCI is 22,989 cm−1 (435 nm) (Fig. 5), which is significantly higher than the 4G5/2 level of Sm3+ (17,900 cm−1), the 5D0 level of Eu3+ (17,300 cm−1), the 5D4 level of Tb3+ (20,500 cm−1) and the 4F9/2 level of Dy3+ (21,000 cm−1), indicating that the ligand can act as an antenna for the photosensitization of the trivalent lanthanide ions and can transfer energy in a nonradiative process through the T1 state to excited states of the lanthanide ions. In addition, the singlet energy level (S1) of the HBCI ligand is 32258 cm−1 (310 nm) (Fig. S4, ESI). The energy gap ΔE(S1-T1) is 9269 cm−1 and therefore, the intersystem crossing energy of HBCI is effective.

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4. Conclusions In summary, nine 3D (1–3) and 2D (4–9) lanthanide coordination polymers with two different crystal structure motifs were prepared based on the 1,3-bis(carboxymethyl) imidazolium ligand, under hydrothermal conditions, at different reaction temperatures. The structures of 1–9 were characterized by using physical methods. The solidstate fluorescence properties of 3, 4, 6 and 7 are discussed in detail. The fluorescence lifetime of complex 4, the quantum yield and the phosphorescence of complex 5 were examined, indicating that the intersystem crossing energy of HBCI is effective. The time-resolved fluorescence spectra measured for complex 4 can be used to stimulate the intramolecular and intermolecular interactions of the states and the speed at which they occur. Moreover, the new complexes can be used as

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