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Communication

Structural and Luminescent Properties of Heterobimetallic Zinc(II)-Europium(III) Dimer Constructed from N2O2-Type Bisoxime and Terephthalic Acid

by
Wen-Ting Guo
,
Xiao-Yan Li
,
Quan-Peng Kang
,
Jian-Chun Ma
and
Wen-Kui Dong
*
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(4), 154; https://doi.org/10.3390/cryst8040154
Submission received: 8 March 2018 / Revised: 29 March 2018 / Accepted: 30 March 2018 / Published: 2 April 2018
(This article belongs to the Section Crystal Engineering)
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Abstract

:
A new heterohexanuclear ZnII–EuIII dimer [{(ZnL)2Eu}2(bdc)2]·2Cl constructed from a N2O2-type chelating ligand H2L (6,6′-Dimethoxy-2,2′-[ethylenedioxybis(nitrilomethylidyne)]diphenol), Zn(OAc)2·2H2O, EuCl3·6H2O and H2bdc (terephthalic acid) was synthesized, and characterized using elemental analyses, IR (Infrared), UV-Vis (Ultraviolet–visible) spectra and X-ray single crystal diffraction method. There are two crystallographically equivalent [(ZnL)2Eu] moieties in the ZnII–EuIII complex, the two [(ZnL)2Eu] moieties are linked by two bdc2– ligand leading to a heterohexanuclear dimer, in which the carboxylato groups bridge the ZnII and EuIII atoms. Furthermore, the luminescence properties of H2L and its ZnII–EuIII complex have been studied.

Graphical Abstract

1. Introduction

Salen-type N2O2 ligands are easily obtained via the reaction of salicylaldehyde or its derivatives with diamines, can coordinated to transition metal ions in a N2O2 tetradentate chelating mode to form stable mono- or multinuclear complexes [1,2,3,4,5,6]. Salen- and salamo-type complexes have been extensively investigated in organometallic chemistry and modern coordination chemistry for several decades [7,8,9,10,11], their complexes are well-known for their potential applications in many areas, such as catalysts [12,13], biological fields [14,15,16,17,18,19,20,21], supramolecular buildings [22,23,24,25,26], molecular recognitions [27,28,29,30,31], magnetic [32,33,34,35,36,37] and luminescence [38,39,40,41,42,43,44,45] materials and so forth. Self-assembling processes of a metallohost complex with auxiliary organic ligands are usually utilized in the building of metal-organic framework (MOF) materials [46,47]. When 3-alkoxy groups are introduced of salicylidene moieties, an O4 coordination site composed of the alkoxy and phenoxo oxygen atoms are produced in addition to the N2O2 site [48,49,50,51]. The O4 site of 3-alkoxy-induced Salen-type ligand is suitable for lanthanide(III) atoms to prepare 3d–4f heteronuclear complexes. These 3d–4f heterobimetallic Salen-type complexes have been diffusely studied [14,34,37,44] in which acetate (OAc-) combine plays a key role in assembling 3d and 4f metals [52], however, 3d–4f heterobimetallic Salamo-type complexes have been rarely reported, especially the auxiliary ligands were introduced [53].
Herein, a newly 3d–4f heterobimetallic dimer, [{(ZnL)2Eu}2(bdc)2]·2Cl has been obtained by a N2O2-type chelating ligand H2L and auxiliary ligand bdc2−. In addition, the luminescence properties of H2L and its ZnII–EuIII complex have been studied.

2. Experimental Section

2.1. Materials and Measurements

2-Hydroxy-3-methoxybenzaldehyde (99%) was purchased from Alfa Aesar (New York, NY, USA), while Tianjin Chemical Reagent Factory (Tianjin, China) supplied the remaining reagents. Elemental analyses for zinc and europium were detected by IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany), elemental analyses for Carbon, hydrogen and nitrogen were analyzed using GmbH VariuoEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany). IR spectra (4000–400 cm−1) were recorded on a Vertex 70 FT-IR (Fourier Transform infrared) spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr pellets. UV-vis absorption spectra were measured on a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). 1H NMR (Nuclear Magnetic Resonance) spectra were determined by German Bruker AVANCE DRX-400/600 spectroscopy (Bruker AVANCE, Billerica, MA, USA). Fluorescence spectra were recorded on a F-7000 FL spectrophotometer (Hitachi, Tokyo, Japan). X-ray single-crystal structure was determined on a SuperNova Dual (Cu at zero) four-circle diffractometer.

2.2. Synthesis of H2L

The ligand 6,6′-dimethoxy-2,2′-[ethylenedioxybis(nitrilomethylidyne)]diphenol (H2L) was synthesized in accordance with the earlier reported method [54]. Yield: 83.6%. m.p: 131–132 °C. 1H NMR (400 MHz, CDCl3), δ 3.91 (s, 6H), 4.49 (s, 4H), 6.83 (dd, J = 7.9, 1.9 Hz, 2H), 6.86 (t, J = 7.9 Hz, 2H), 6.91 (dd, J = 7.9, 1.9 Hz, 2H), 8.26 (s, 2H), 9.74 (s, 2H). IR (KBr, cm–1): 3131 [ν(O–H)], 1618 [ν(C=N)], 1253 [ν(Ar-O)]. UV-Vis (CH3OH), λmax (nm) (εmax): 224, 270 and 318 nm (3.0 × 10−5 M). Anal. Calcd for C18H20N2O6 (%): C 59.99; H 5.59; N 7.77. Found: C 60.29; H 5.42; N 7.56.

2.3. Synthesis of the ZnII–EuIII Complex

To a solution of H2L (7.20 mg, 0.02 mmol) in CHCl3 (3 mL) was added Zn(OAc)2·2H2O (4.39 mg, 0.02 mmol) and EuCl3·6H2O (7.33 mg, 0.02 mmol) in CH3CH2OH (2 mL). After the mixture was stirred for about 15 min at r.t., a solution of H2bdc (3.32 mg, 0.02 mmol) in DMF (1 mL) was added dropwise and continued to stir for 15 min. The mixture was filtered, and the filtrate was obtained. Several yellow block single crystals were obtained via slow evaporation of the mixture solution in open atmosphere for almost two weeks. (Scheme 1) Yield: 72.6%. IR (KBr, cm−1): 1607 [ν(C=N)], 1217 [ν(Ar–O)]. UV-Vis (CH3OH), λmax (nm) (εmax): 233, 279 and 352 nm (3.0 × 10−5 M). Anal. Calcd for C88H80Cl2Eu2Zn4N8O32 (%): C, 44.08; H, 3.36; N, 4.67; Zn, 10.91; Eu, 12.67. Found: C, 44.29; H, 4.45; N, 4.51; Zn, 11.02; Eu, 12.43.

2.4. X-ray Crystal Structure Determinations for the ZnII–EuIII Complex

The single crystal diffractometer provides a monochromatic beam of Mo-Kα radiation (0.71073 Å) produced from a sealed Mo X-ray tube using Graphite monochromator and was used for obtaining crystal data for the ZnII–EuIII complex at 291(2) K, respectively. The LP factor and semi-empirical absorption were using SADABS. The structures of the ZnII–EuIII complex were solved via the direct methods (SHELXS-2016) [55], and all hydrogen atoms were included at the calculated positions and constrained to ride on their parent atoms. All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F2 with SHELXL-2016 [56]. In the X-ray structure refinement, however, the solvent molecules of the complex could not be located because of its high thermal disorder, and the final structure model was refined without the solvent molecules by using a SQUEEZE routine of PLATON. Table 1 shows the data collection and refinements of the ZnII–EuIII complex. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1,828,198) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

3. Results and Discussion

3.1. IR Spectra

The IR spectra of H2L and its corresponding ZnII–EuIII complex exhibited different bands in the region of 4000–400 cm−1 region (Table 2). It is obvious that the ν(O–H) absorption band was near 3131 cm−1 in the spectrum of H2L. The free ligand exhibited a characteristic C=N stretching band at 1618 cm−1, while the ZnII–EuIII complex appeared at 1607 cm−1. The characteristic C=N stretching frequency is shifted to lower frequency, which indicating that the metal(II) atoms are bonded by oxime N atoms [57]. The Ar–O stretching frequencies appeared as a strong band within 1265–1213 cm–1 range as reported for similar Salen-type ligands [58]. Meanwhile, the free ligand H2L exhibited an Ar–O stretching frequency at 1253 cm−1, while the ZnII–EuIII complex appeared at 1217 cm−1, the Ar–O stretching frequency is shifted to lower frequency, indicating that the M–O bonds are formed [59]. In addition, characteristic absorption bands of the carboxylato ions are observed, the absorption bands νas(COO–) and νs(COO–) of the ZnII–EuIII complex are 1572 and 1468 cm–1, respectively, indicating the bidentate coordination mode of carboxylato ions [60], which is in consistent with the X-ray diffraction result obtained for the ZnII–EuIII complex.

3.2. UV–Vis Spectra

UV–Vis absorption spectra of H2L and its corresponding ZnII–EuIII complex were determined in 3.0 × 10−5 M methanol solution, as depicted in Figure 1. The absorption spectrum of the ZnII–EuIII complex is different from that of H2L. UV–Vis spectrum of H2L exhibited three absorptions at ca. 224, 270 and 318 nm. The absorptions at 224 and 270 nm can be assigned to π–π* transitions of the benzene rings, while the absorption at 318 nm can be attributed to π–π* transition of C=N groups [61]. Compared with the absorption peak of H2L, the corresponding absorption peaks at ca. 233 and 279 nm are observed in the ZnII–EuIII complex, which were bathochromically shifted, indicating the coordination of metal atoms with the ligand [62]. Meanwhile, a new absorption peak was observed at ca. 352 nm in the ZnII–EuIII complex, and assigned to L→M charge-transfer (LMCT) transitions, which is characteristic of the transition metal complexes with Salen-type N2O2 coordination spheres [63].

3.3. Description of the Crystal Structure

The crystal structure of the heterohexanuclear ZnII–EuIII complex is depicted in Figure 2 and Table 3, the crystallographic data revealed that the ZnII–EuIII complex crystallizes in the triclinic space group P-1 with Z = 1. By the reaction of H2L with Zn(OAc)2·2H2O, EuCl3·6H2O and H2bdc, a heterohexanuclear ZnII–EuIII complex is formed: [{(ZnL)2Eu}2(bdc)2]·2Cl, the molecular structure of the ZnII–EuIII complex composes of four ZnII atoms, two EuIII atoms, four deprotonated (L)2– ligands, two auxiliary ligand bdc2– moieties. Each ZnII atom (Zn1 or Zn2) is located at the N2O2 coordination spheres of Salamo moieties, they are penta-coordinated by two oxime nitrogen (N1, N2 or N3, N4) atoms, two deprotonated phenoxo-oxygen (O2, O5 or O8, O11) atoms of the (L)2− units and one oxygen (O16 or O13) atom comes from the coordinated terephthalic acid molecule. The coordination environment around all the ZnII atoms are best described as slightly distorted triangular bipyramid geometries [64], which were deduced by calculating the values of τ1 = 0.539 and τ2 = 0.6125 [65], respectively. The equatorial coordination sites with the distances of N2–Zn1 = 2.046(5) Å, O2–Zn1 = 2.000(4) Å and O16–Zn1 = 1.983(4) Å, and the axial position with the distances of O5–Zn1 = 2.052(4) and N1–Zn1 = 2.116(5) Å (Table 3). The dihedral angle between coordination planes of N1–Zn1–O2 and N2–Zn1–O5 is 51.84(5)°. The dihedral angle between coordination planes of N1–Zn2–O2 and N2–Zn2–O5 is 55.37(2)°.
In addition, the central EuIII atom is located in a deca-coordinated O10 environment, the coordination sphere of the central EuIII atom contains eight oxygen atoms (O1, O2, O5, O6 and O7, O8, O11, O12) from two deprotonated (L)2– units and two oxygen atoms (O14 and O15) comes from the coordinated bdc2– ions. Thus, the deca-coordinated EuIII atom adopts a distorted bicapped square antiprism coordination arrangement. The primary Eu–O distances are in the normal ranges of 2.378(4)–2.844(4) Å (Table 3). The dihedral angle between the O2–Zn1–O5 and O2–Eu1–O5 planes is 32.41(4)°.
As depicted in Figure 3 and Table 4, In the crystal structure of the ZnII–EuIII complex, five pairs of intramolecular C1–H1B···O8, C9–H9A···O16, C19–H19B···O2, C36–H36B···O15 and C43–H43···O13 interactions are formed [66,67,68], which plays a vital role in constructing and stabilizing the complex molecule. The donors (C1–H1B and C19–H19B) from 3-methoxy of the (L)2– units form hydrogen bonding with oxygen atoms (O8 and O2) of phenoxo-oxygen of the (L)2– units as hydrogen bonding receptors. The donors (C9–H9A and C36–H36B) from the (L)2– units form hydrogen bonding with oxygen atom (O16 and O15) of the bdc2− moiety as hydrogen bonding receptor. The donor (C43–H43) from bdc2− moiety formed hydrogen bonding with oxygen atom (O13) of bdc2− moiety as hydrogen bonding receptor.

3.4. Luminescence Spectra

The luminescence spectra of H2L and its ZnII–EuIII complex were measured in 3.0 × 10−5 M methanol solution (Figure 4). The ligand H2L exhibited an intense emission peak at ca. 442 nm upon excitation at 352 nm which should be assigned to intra-ligand π–π* transition [69,70]. The ZnII–EuIII complex showed slightly weak photoluminescence with maximum emission at ca. 445 nm upon excitation at 352 nm. In the luminescence spectra, only a band at ca. 375–650 nm instead of the f–f emission expected for EuIII ions [71]. In the ZnII–EuIII complex, the emission from EuIII is quenched which may due to thermal quenching of the 5D0 level of EuIII by LMCT process [72]. The concentration of H2L and its ZnII–EuIII complex used are all 3.0 × 10−5 M, indicating that the relative strengths of H2L and its ZnII–EuIII complex are independent of the concentration.

4. Conclusions

In summary, a new heterohexanuclear ZnII–EuIII dimer was synthesized and characterized. In the ZnII–EuIII complex, there are two crystallographically equivalent [(ZnL)2Eu] moieties which are linked by two bdc2– auxiliary ligand leading to a heterohexanuclear dimer. The ZnII atom possesses a penta-coordinated environment and adopts a slightly distorted triangular bipyramid geometry and the deca-coordinated EuIII atom adopts a distorted bicapped square antiprism. In addition, the luminescence spectrum of the ZnII–EuIII complex indicated that the coordination of EuIII atoms led to the fluorescence quenching of H2L.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21761018) and the Program for Excellent Team of Scientific Research in Lanzhou Jiaotong University (201706), which is gratefully acknowledged.

Author Contributions

Wen-Kui Dong conceived and designed the experiments; Quan-Peng Kang and Xiao-Yan Li performed the experiments; Jian-Chun Ma analyzed the data; Wen-Ting Guo contributed reagents/materials/analysis tools; Xiao-Yan Li and Wen-Ting Guo wrote the paper.

Conflicts of Interest

The authors declare no competing financial interests.

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Crystals 08 00154 sch001 550
Scheme 1. Synthesis of the ZnII–EuIII complex.
Scheme 1. Synthesis of the ZnII–EuIII complex.
Crystals 08 00154 sch001
Crystals 08 00154 g001 550
Figure 1. The UV–Vis spectra of H2L and its ZnII–EuIII complex (cm−1).
Figure 1. The UV–Vis spectra of H2L and its ZnII–EuIII complex (cm−1).
Crystals 08 00154 g001
Crystals 08 00154 g002 550
Figure 2. (a) Molecular structure and atom numberings of the ZnII–EuIII complex with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity); (b) coordination polyhedrons for ZnII and EuIII atoms.
Figure 2. (a) Molecular structure and atom numberings of the ZnII–EuIII complex with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity); (b) coordination polyhedrons for ZnII and EuIII atoms.
Crystals 08 00154 g002
Crystals 08 00154 g003 550
Figure 3. View of the intramolecular hydrogen bondings of the ZnII–EuIII complex.
Figure 3. View of the intramolecular hydrogen bondings of the ZnII–EuIII complex.
Crystals 08 00154 g003
Crystals 08 00154 g004 550
Figure 4. Luminescence spectra of H2L and its ZnII–EuIII complex in methanol (3.0 × 10−5 M) upon excitation at 352 nm.
Figure 4. Luminescence spectra of H2L and its ZnII–EuIII complex in methanol (3.0 × 10−5 M) upon excitation at 352 nm.
Crystals 08 00154 g004
Table
Table 1. Crystal data and refinement parameters for the ZnII–EuIII complex.
Table 1. Crystal data and refinement parameters for the ZnII–EuIII complex.
ZnII–EuIII Complex
FormulaC88H80Cl2Eu2Zn4N8O32
Formula weight2397.90
Temperature (K)291(2)
Wavelength (Å)0.71073
Crystal systemtriclinic
Space groupP-1
a (Å)12.5053(15)
b (Å)16.125(3)
c (Å)16.5068(13)
α (°)68.518(13)
β (°)82.216(12)
γ (°)86.716(14)
V (Å3)3068.6(8)
Z1
Dcalc (g∙cm−3)1.298
μ (mm−1)1.885
F (000)1200
Crystal size (mm)0.18 × 0.20 × 0.22
θ Range (°)2.123–25.682
Index ranges−14 ≤ h ≤ 15
–18 ≤ k ≤ 19
0 ≤ l ≤ 20
Reflections collected11,612
Independent reflections11,611
Rint0.0726
Completeness to θ99.6% (θ = 25.242)
Data/restraints/parameters11611/0/627
GOF1.029
Final R1, wR2 indices0.0482, 0.1182
R1, wR2 indices (all data)0.0644, 0.1215
Largest differences peak and hole (e Å−3)1.696/−0.554
GOF: Goodness-of-fit on F.
Table
Table 2. The major IR spectra of H2L and its ZnII–EuIII complex (cm−1).
Table 2. The major IR spectra of H2L and its ZnII–EuIII complex (cm−1).
Compoundν(O–H)ν(C=N)ν(Ar–O)ν(C=C)νs(COO–)νas(COO–)
H2L3131161812531374
Complex-16071217131414681572
Table
Table 3. Selected bond lengths (Å) and angles (°) of the ZnII–EuIII complex.
Table 3. Selected bond lengths (Å) and angles (°) of the ZnII–EuIII complex.
BondLengthsBondLengths
N1–Zn12.116(5)N2–Zn12.046(5)
O2–Zn12.000(4)O5–Zn12.052(4)
O16–Zn11.983(4)N3–Zn22.135(5)
N4–Zn22.005(5)O8–Zn21.965(4)
O11–Zn22.056(3)O13–Zn21.986(3)
Eu1–O52.383(4)Eu1–O22.385(3)
Eu1–O152.418(3)Eu1–O82.445(4)
Eu1–O112.452(4)Eu1–O12.637(3)
Eu1–O72.670(4)Eu1–O122.832(4)
Eu1–O62.844(4)Eu1–O142.378(4)
BondAnglesBondAngles
O16–Zn1–O2111.62(15)O16–Zn1–O597.39(15)
O2–Zn1–O576.97(14)O16–Zn1–N2118.07(18)
O2–Zn1–N2129.54(18)O5–Zn1–N288.3(2)
O16–Zn1–N197.88(17)O2–Zn1–N188.36(17)
O5–Zn1–N1161.88(16)N2–Zn1–N193.1(2)
O8–Zn2–O13114.67(16)O8–Zn2–N4125.97(18)
O13–Zn2–N4118.96(19)O8–Zn2–O1179.89(15)
O13–Zn2–O1197.49(15)N4–Zn2–O1187.20(18)
O8–Zn2–N386.80(19)O13–Zn2–N398.07(17)
N4–Zn2–N391.8(2)O11–Zn2–N3162.72(17)
O14–Eu1–O5107.44(12)O14–Eu1–O2151.04(12)
O5–Eu1–O263.88(13)O14–Eu1–O1572.43(11)
O5–Eu1–O1571.56(12)O2–Eu1–O1578.67(12)
O14–Eu1–O877.38(12)O5–Eu1–O8117.81(13)
O2–Eu1–O8131.54(12)O15–Eu1–O8149.80(12)
O14–Eu1–O1170.71(11)O5–Eu1–O11177.52(11)
O2–Eu1–O11116.88(12)O15–Eu1–O11106.14(12)
O8–Eu1–O1163.68(13)O14–Eu1–O1138.67(11)
O5–Eu1–O1113.38(12)O2–Eu1–O160.79(11)
O15–Eu1–O1126.52(11)O8–Eu1–O178.26(11)
O11–Eu1–O168.66(11)O14–Eu1–O7125.52(12)
O5–Eu1–O769.27(12)O2–Eu1–O779.07(12)
O15–Eu1–O7140.37(12)O8–Eu1–O760.58(12)
O11–Eu1–O7113.11(12)O1–Eu1–O766.58(12)
O14–Eu1–O1297.83(12)O5–Eu1–O12121.21(12)
O2–Eu1–O1268.48(12)O15–Eu1–O1267.04(12)
O8–Eu1–O12119.14(12)O11–Eu1–O1257.94(11)
O1–Eu1–O1266.45(11)O7–Eu1–O12131.59(12)
O14–Eu1–O665.87(13)O5–Eu1–O659.08(12)
O2–Eu1–O6120.77(13)O15–Eu1–O697.08(12)
O8–Eu1–O669.06(12)O11–Eu1–O6120.81(12)
O1–Eu1–O6132.51(11)O7–Eu1–O667.58(12)
O12–Eu1–O6160.75(11)
Table
Table 4. Hydrogen bonding interactions (Å, °) of the ZnII–EuIII complex.
Table 4. Hydrogen bonding interactions (Å, °) of the ZnII–EuIII complex.
D–H···AD–HH···AD···AD–H···A
C1–H1B···O80.962.563.281(6)132
C9–H9A···O160.972.413.311(10)154
C19–H19B···O20.962.553.280(8)133
C36–H36B···O150.962.543.148(7)122
C43–H43···O130.932.462.773(7)100

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Guo, W.-T.; Li, X.-Y.; Kang, Q.-P.; Ma, J.-C.; Dong, W.-K. Structural and Luminescent Properties of Heterobimetallic Zinc(II)-Europium(III) Dimer Constructed from N2O2-Type Bisoxime and Terephthalic Acid. Crystals 2018, 8, 154. https://doi.org/10.3390/cryst8040154

AMA Style

Guo W-T, Li X-Y, Kang Q-P, Ma J-C, Dong W-K. Structural and Luminescent Properties of Heterobimetallic Zinc(II)-Europium(III) Dimer Constructed from N2O2-Type Bisoxime and Terephthalic Acid. Crystals. 2018; 8(4):154. https://doi.org/10.3390/cryst8040154

Chicago/Turabian Style

Guo, Wen-Ting, Xiao-Yan Li, Quan-Peng Kang, Jian-Chun Ma, and Wen-Kui Dong. 2018. "Structural and Luminescent Properties of Heterobimetallic Zinc(II)-Europium(III) Dimer Constructed from N2O2-Type Bisoxime and Terephthalic Acid" Crystals 8, no. 4: 154. https://doi.org/10.3390/cryst8040154

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