Structural and Luminescent Properties of Heterobimetallic Zinc ( II )-Europium ( III ) Dimer Constructed from N 2 O 2-Type Bisoxime and Terephthalic Acid

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.


Synthesis of the Zn II -Eu III Complex
To a solution of H 2 L (7.20 mg, 0.02 mmol) in CHCl 3 (3 mL) was added Zn(OAc) 2 •2H 2 O (4.39 mg, 0.02 mmol) and EuCl 3 •6H 2 O (7.33 mg, 0.02 mmol) in CH 3 CH 2 OH (2 mL).After the mixture was stirred for about 15 min at r.t., a solution of H 2 bdc (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.

X-ray Crystal Structure Determinations for the Zn II -Eu III 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 Zn II -Eu III complex at 291(2) K, respectively.The LP factor and semi-empirical absorption were using SADABS.The structures of the Zn II -Eu III 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 F 2 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 Zn II -Eu III 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.

X-ray Crystal Structure Determinations for the Zn II -Eu III 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 Zn II -Eu III complex at 291(2) K, respectively.The LP factor and semi-empirical absorption were using SADABS.The structures of the Zn II -Eu III 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 F 2 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 Zn II -Eu III 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.

IR Spectra
The IR spectra of H 2 L and its corresponding Zn II -Eu III 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 H 2 L. The free ligand exhibited a characteristic C=N stretching band at 1618 cm −1 , while the Zn II -Eu III 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 H 2 L exhibited an Ar-O stretching frequency at 1253 cm −1 , while the Zn II -Eu III 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 ν as (COO -) and ν s (COO -) of the Zn II -Eu III 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 Zn II -Eu III complex.

UV-Vis Spectra
UV-Vis absorption spectra of H 2 L and its corresponding Zn II -Eu III complex were determined in 3.0 × 10 −5 M methanol solution, as depicted in Figure 1.The absorption spectrum of the Zn II -Eu III complex is different from that of H 2 L. UV-Vis spectrum of H 2 L 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 H 2 L, the corresponding absorption peaks at ca. 233 and 279 nm are observed in the Zn II -Eu III 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 Zn II -Eu III complex, and assigned to L→M charge-transfer (LMCT) transitions, which is characteristic of the transition metal complexes with Salen-type N 2 O 2 coordination spheres [63].
Figure 1.The UV-Vis spectra of H2L and its Zn II -Eu III complex (cm −1 ).

Luminescence Spectra
The luminescence spectra of H 2 L and its Zn II -Eu III complex were measured in 3.0 × 10 −5 M methanol solution (Figure 4).The ligand H 2 L 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 Zn II -Eu III 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 Eu III ions [71].In the Zn II -Eu III complex, the emission from Eu III is quenched which may due to thermal quenching of the 5 D 0 level of Eu III by LMCT process [72].The concentration of H 2 L and its Zn II -Eu III complex used are all 3.0 × 10 −5 M, indicating that the relative strengths of H 2 L and its Zn II -Eu III complex are independent of the concentration.

Luminescence Spectra
The luminescence spectra of H2L and its Zn II -Eu III 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 Zn II -Eu III 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 Eu III ions [71].In the Zn II -Eu III complex, the emission from Eu III is quenched which may due to thermal quenching of the 5 D0 level of Eu III by LMCT process [72].The concentration of H2L and its Zn II -Eu III complex used are all 3.0 × 10 −5 M, indicating that the relative strengths of H2L and its Zn II -Eu III complex are independent of the concentration.

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

Figure 1 .
Figure 1.The UV-Vis spectra of H 2 L and its Zn II -Eu III complex (cm −1 ).

Figure 2 .
Figure 2. (a) Molecular structure and atom numberings of the Zn II -Eu III complex with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity); (b) coordination polyhedrons for Zn II and Eu III atoms.

Figure 2 .
Figure 2. (a) Molecular structure and atom numberings of the Zn II -Eu III complex with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity); (b) coordination polyhedrons for Zn II and Eu III atoms.

Figure 3 .
Figure 3.View of the intramolecular hydrogen bondings of the Zn II -Eu III complex.

Figure 3 .
Figure 3.View of the intramolecular hydrogen bondings of the Zn II -Eu III complex.
Synthesis of the Zn II -Eu III complex.

Table 1 .
Crystal data and refinement parameters for the Zn II -Eu III complex.

Table 1 .
Crystal data and refinement parameters for the Zn II -Eu III complex.
GOF: Goodness-of-fit on F.

Table 2 .
The major IR spectra of H 2 L and its Zn II -Eu III complex (cm −1 ).