Synthesis and Fluorescence Properties of a Structurally Characterized Hetero-Hexanuclear Zn ( II )-La ( III ) Salamo-Like Coordination Compound Containing Auxiliary Ligands

A hetero-hexanuclear Zn(II)-La(III) coordination compound, [{(ZnL)2La}2(bdc)2](NO3)2 (H2bdc = terephthalic acid) has been synthesized with a symmetric Salamo-like bisoxime, and characterized by elemental analyses, IR, UV-Vis, fluorescent spectroscopy, and single-crystal X-ray diffraction analysis. All of the Zn(II) ions are pentacoordinated by N2O2 donator atoms from the (L)2− unit and one oxygen atom from one terephthalate anion. The Zn(II) ions adopt trigonal bipyramidal geometries (τZn1 = 0.61, τZn2 = 0.56). The La(III) ions are decacoordinated in the Zn(II)-La(III) coordination compound and has a distorted bicapped square antiprism geometry. Meanwhile, the photophysical property of the Zn(II)-La(III) coordination compound was also measured and discussed.

In order to explore the structures and optical properties of 3d-4f hetero-nuclear metal coordination compounds contained auxiliary ligands, herein the terephtalic acid was selected as a simple multidentate linker owing to its availability and application in the building of Salamo-like Zn(II)-Ln(III) coordination compound.we have successfully designed and synthesized a symmetric Salamo-like derivative H 2 L and its corresponding Zn(II)-La(III) coordination compound ([{(ZnL) 2 La} 2 (bdc) 2 ](NO 3 ) 2 ).Furthermore, the supra-molecular features and photophysical properties of the Zn(II)-La(III) coordination compound are discussed in detail.

Materials and Instrumentation
All chemical reagents were analytical pure reagents, which have not been purified before used.Carbon, nitrogen and hydrogen analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analyzer (Berlin, Germany).Elemental analyses for Zn II and La III were detected by an IRIS ER/S-WP-1 ICP atomic emission spectrometer (Berlin, Germany).Melting points were measured via a microscopic melting point apparatus (Beijing Taike Instrument Limited Company, Beijing, China). 1 H and 13 C NMR spectra were recorded in deuterated DMSO solution by German Bruker AVANCE DRX-400 spectroscopy (Bruker AVANCE, Billerica, MA, USA).Infrared spectra were measured with a VERTEX-70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr (400-4000 cm −1 ).UV-Vis absorption and fluorescence spectra were recorded on a Shimadzu UV-2550 (Shimadzu, Japan) and Hitachi F-7000 (Hitachi, Tokyo, Japan) spectrometers, respectively.Quantum yields in solid state were measured using an absolute method by integrating sphere on FLS920 of Edinburgh Instrument.X-ray single crystal structure determination was carried out on a Bruker Smart Apex CCD diffractometer (Bruker AVANCE, Billerica, MA, USA).

Preparation of the Zn(II)-La(III) Coordination Compound
Synthesis route of the Zn(II)-La(III) coordination compound is shown in Scheme.To stirring colorless transparent solution of H2L (15.8 mg, 0.02 mmol) in CHCl3 (3 mL) was added Zn(OAc)2•2H2O (4.38 mg, 0.02 mmol) and La(NO3)3•6H2O (0.02 mmol) in CH3OH (2 mL).The color of the mixture immediately turns pale yellow and then allowed to mixing with terephthalic acid (0.01 mmol) in CH3OH (1 mL) and continues stirring for about 30 min at room temperature.After the mixed solution was filtered by absorbent cotton, leaves the filtrate at room temperature for about two weeks.Finally, light-yellow and block-shaped crystals were obtained with the volatilization of solvent.Yield: 60.5%.IR (KBr, cm

Structure Description of the Zn(II)-La(III) Coordination Compound
Crystal data of the Zn(II)-La(III) coordination compound were collected on a Bruker Smart Apex CCD diffractometer at 173(2) K (Mo-Kα radiation (λ = 0.71073 Å)).The LP factor and Semi-empirical absorption corrections were applied to the intensity data.The structure was solved by the direct methods and refined anisotropically using full-matrix least-squares methods on F 2 with the SHELX-2018 program package.The hydrogen atoms were positioned geometrically and refined isotropically using the 'riding' model (SHELXL-2018).In addition, DELU and AFIX were applied in the structure refinement.The structure contained large in the void couldn't be identified because it was highly disordered and had so small residual peak.Therefore, SQUEEZE in PLATON program was performed to remove the highly disordered solvent.(Solvent Accessible Volume = 914, Electrons Found in S.A.V. = 484).The nonhydrogen atoms were refined anisotropically.Crystal data and structure parameters for the Zn(II)-La(III) coordination compound are given in Table 1.Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1434632) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Structure Description of the Zn(II)-La(III) Coordination Compound
Crystal data of the Zn(II)-La(III) coordination compound were collected on a Bruker Smart Apex CCD diffractometer at 173(2) K (Mo-K α radiation (λ = 0.71073 Å)).The LP factor and Semi-empirical absorption corrections were applied to the intensity data.The structure was solved by the direct methods and refined anisotropically using full-matrix least-squares methods on F 2 with the SHELX-2018 program package.The hydrogen atoms were positioned geometrically and refined isotropically using the 'riding' model (SHELXL-2018).In addition, DELU and AFIX were applied in the structure refinement.The structure contained large in the void couldn't be identified because it was highly disordered and had so small residual peak.Therefore, SQUEEZE in PLATON program was performed to remove the highly disordered solvent.(Solvent Accessible Volume = 914, Electrons Found in S.A.V. = 484).The nonhydrogen atoms were refined anisotropically.Crystal data and structure parameters for the Zn(II)-La(III) coordination compound are given in Table 1.Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1434632) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Infrared Spectra
The infrared spectra of H 2 L and its Zn(II)-La(III) coordination compound exhibited various bands in the 400-4000 cm −1 range (Figure 1).The free ligand H 2 L exhibited an obvious characteristic band at 3137 cm −1 and can be attributed to the characteristic bands of the OH group.This band was dispeared in the infrared spectrum of the Zn(II)-La(III) coordination compound, indicating the interaction between the OH group and the Zn(II) ion leads to hydroxyl deprotonation [34].In addition, the free ligand H 2 L showed an individual C=N stretching band at 1601 cm −1 , while the C=N stretching band of the Zn(II)-La(III) coordination compound appeared at 1557 cm −1 .For the ligand H 2 L, the Ar-O stretching band appeared at 1255 cm −1 , which was observed at 1220 cm

UV-Vis Spectra
The UV-Vis spectra of H2L and its Zn(II)-La(III) coordination compound were measured in 1 × 10 −5 mol•L −1 CH2Cl2 solution in freshly prepared solution are obtained in the range of 200-450 nm at room temperature, as shown in Figure 2. As can be seen from the diagram, the absorption peak of the Zn(II)-La(III) coordination compound is evidently different from the ligand before coordination.The absorption spectrum of H2L showed that two relatively strong absorption peaks appeared at ca. 266 nm (ε = 1.8 × 10 4 M −1 •cm −1 ) and 322 nm (ε = 6.1 × 10 3 M −1 •cm −1 ), which can be attributed to π-π*

UV-Vis Spectra
The UV-Vis spectra of H 2 L and its Zn(II)-La(III) coordination compound were measured in 1 × 10 −5 mol•L −1 CH 2 Cl 2 solution in freshly prepared solution are obtained in the range of 200-450 nm at room temperature, as shown in Figure 2. As can be seen from the diagram, the absorption peak of the Zn(II)-La(III) coordination compound is evidently different from the ligand before coordination.The absorption spectrum of H 2 L showed that two relatively strong absorption peaks appeared at ca. 266 nm (ε = 1.8 × 10 4 M −1 •cm −1 ) and 322 nm (ε = 6.1 × 10 3 M −1 •cm −1 ), which can be attributed to π-π* transitions of the benzene rings and the C=N bonds [17].Compared with the free ligand H 2 L, the corresponding absorption peak of the Zn(II)-La(III) coordination compound appeared at ca. 278 nm (ε = 4.1 × 10 4 M −1 •cm −1 ) was remarkably red shifted upon coordination to metal ions.The absorption peak at ca. 322 nm is absent in the Zn(II)-La(III) coordination compound.Meanwhile, a new absorption peak appeared at ca. 349 nm (ε = 1.2 × 10 4 M −1 •cm −1 ) in the Zn(II)-La(III) coordination compound that might be owing to M→L (MLCT) charge-transfer transition, which is characteristic of the transition metal coordination compound with N 2 O 2 coordination spheres [27].

Crystal Structure of Zn(II)-La(III) Coordination Compound
Crystal structure of the Zn(II)-La(III) coordination compound exhibited a symmetric hexanuclear structure, which is different from the common trinuclear bis(salamo)-type coordination compounds reported earlier [43,46].The crystal structure of the Zn(II)-La(III) coordination compound and the coordination polyhedra of metal atoms are shown in Figure 3. Essential bond lengths and angles are listed in Table 2.

Crystal Structure of Zn(II)-La(III) Coordination Compound
Crystal structure of the Zn(II)-La(III) coordination compound exhibited a symmetric hexanuclear structure, which is different from the common trinuclear bis(salamo)-type coordination compounds reported earlier [43,46].The crystal structure of the Zn(II)-La(III) coordination compound and the coordination polyhedra of metal atoms are shown in Figure 3. Essential bond lengths and angles are listed in Table 2.In the crystal structure of the Zn(II)-La(III) coordination compound, each Zn(II) ion is located in the N2O2 coordination cavity, which have pentacoordinate environments, and the axial position is occupied by one oxygen atom of terephthalic acid.The four Zn(II) ions assumes a distorted trigonal bipyramidal geometries, which were inferred by calculating the value of τZn1 = 0.61, τZn2 = 0.56, respectively [50].The La(III) ions have a decacoordinate environment, consisting of eight oxygen atoms (O1, O2, O5, O6, O7, O8, O11, and O12) come from two deprotonation (L) 2-units, two oxygen atoms (O13 and O16) of two terephthalic acid.Thus, all of the La(III) ions adopt a distorted bicapped square antiprism geometry.
The hydrogen bonding interactions are listed in Table 3.In the crystal structure of the Zn(II)-La(III) coordination compound, the molecular structure is stabilized through intramolecular 4).The Zn(II)-La(III) coordination compound crystallizes in the triclinic crystal system, space group P − 1 and the unit cell contains four Zn(II) ions, four (L) 2− units, two (bdc) 2− ions, two La(III) ions, and two free NO 3 − ions.The Zn(II)-La(III) coordination compound was assembled by two trinuclear units [(ZnL) 2 La] and two terephthalic acid, similar to the Zn(II)-Ln(III) coordination compounds reported [49].
In the crystal structure of the Zn(II)-La(III) coordination compound, each Zn(II) ion is located in the N 2 O 2 coordination cavity, which have pentacoordinate environments, and the axial position is occupied by one oxygen atom of terephthalic acid.The four Zn(II) ions assumes a distorted trigonal bipyramidal geometries, which were inferred by calculating the value of τ Zn1 = 0.61, τ Zn2 = 0.56, respectively [50].The La(III) ions have a decacoordinate environment, consisting of eight oxygen atoms (O1, O2, O5, O6, O7, O8, O11, and O12) come from two deprotonation (L) 2− units, two oxygen atoms (O13 and O16) of two terephthalic acid.Thus, all of the La(III) ions adopt a distorted bicapped square antiprism geometry.

Fluorescence Properties
The fluorescence properties of H2L and its Zn(II)-La(III) coordination compound were Table 3. Putative hydrogen bonding interactions (Å, • ) for the Zn(II)-La(III) coordination compound.

Fluorescence Properties
The fluorescence properties of H 2 L and its Zn(II)-La(III) coordination compound were researched at room temperature (Figure 6).With excitation at 360 nm, the free ligand H2L showed strong emission peak at about 419 nm, which can be attributed to the intra-ligand π-π * transition.Similarly, the Zn(II)-La(III) coordination compound also exhibited an intense luminescence with maximum emission at ca. 433 nm and the emission quantum yield Φ = 0.19% [49].Compared with the ligand H2L, the fluorescence intensity of the Zn(II)-La(III) coordination compound showed a marked reduction, indicating that the addition of metal ions induced the change of fluorescence characteristics of the ligand; it is further explained that the (Zn/L)-center has absorbed and transferred energy to La(III) ion as a type of metal-organic antenna [43].

Conclusions
We have designed and synthesized a symmetric Salamo-like bioxime ligand H2L, and obtained a hetero-hexanuclear Zn(II)-La(III) coordination compound [{(ZnL)2La}2(bdc)2](NO3)2. The crystal structure of the Zn(II)-La(III) coordination compound showed that all of the Zn(II) ions have pentacoordinate environments and adopt distorted trigonal bipyramidal geometries.The La(III) ions adopt a distorted bicapped square antiprism geometry.The fluorescence behavior of H2L and its Zn(II)-La(III) coordination compounds was studied, compared with the ligand H2L, the fluorescence intensity of the Zn(II)-La(III) coordination compound showed a marked reduction, indicating that the addition of Zn(II)-La(III) ions induced the change of fluorescence characteristics.With excitation at 360 nm, the free ligand H 2 L showed strong emission peak at about 419 nm, which can be attributed to the intra-ligand π-π * transition.Similarly, the Zn(II)-La(III) coordination compound also exhibited an intense luminescence with maximum emission at ca. 433 nm and the emission quantum yield Φ = 0.19% [49].Compared with the ligand H 2 L, the fluorescence intensity of the Zn(II)-La(III) coordination compound showed a marked reduction, indicating that the addition of metal ions induced the change of fluorescence characteristics of the ligand; it is further explained that the (Zn/L)-center has absorbed and transferred energy to La(III) ion as a type of metal-organic antenna [43].

12 Figure 1 .
Figure 1.The infrared spectra of the ligand H2L and its Zn(II)-La(III) coordination compound.

Figure 1 .
Figure 1.The infrared spectra of the ligand H 2 L and its Zn(II)-La(III) coordination compound.

Figure 3 .
Figure 3. (a) Crystal structure of the Zn(II)-La(III) coordination compound (hydrogen atoms are omitted in structure).(b) Coordination polyhedra for metal atoms of the Zn(II)-La(III) coordination compound.

Figure 3 .
Figure 3. (a) Crystal structure of the Zn(II)-La(III) coordination compound (hydrogen atoms are omitted in structure).(b) Coordination polyhedra for metal atoms of the Zn(II)-La(III) coordination compound.

Crystals 2018 ,
10, x 8 of 12suppression of the electron density originating from solvent molecules (used SQUEEZE) and subsequent exclusion of these solvent molecules from the refinement model.

Figure 4 .
Figure 4. Intramolecular hydrogen bonding interactions of the Zn(II)-La(III) coordination compound (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

Figure 5 .
Figure 5.The one-dimensional structure of the Zn(II)-La(III) coordination compound with intermolecular hydrogen bondings (hydrogen atoms have been omitted except those formation of hydrogen bonds).

Figure 4 .
Figure 4. Intramolecular hydrogen bonding interactions of the Zn(II)-La(III) coordination compound (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

Figure 4 .
Figure 4. Intramolecular hydrogen bonding interactions of the Zn(II)-La(III) coordination compound (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

Figure 5 .
Figure 5.The one-dimensional structure of the Zn(II)-La(III) coordination compound with intermolecular hydrogen bondings (hydrogen atoms have been omitted except those formation of hydrogen bonds).

Figure 5 .
Figure 5.The one-dimensional structure of the Zn(II)-La(III) coordination compound with intermolecular hydrogen bondings (hydrogen atoms have been omitted except those formation of hydrogen bonds).

Table 1 .
Crystal data and structure parameters for the Zn(II)-La(III) coordination compound.
Scheme 1. Synthesis routes of the Salamo-like derivative H 2 L and its Zn(II)-La(III) coordination compound.

Table 1 .
Crystal data and structure parameters for the Zn(II)-La(III) coordination compound.