A Self-Assembled ZnII-NdIII Heterohexanuclear Dimer Based on a Hexadentate N2O4-Type Ligand and Terephthalic Acid: Synthesis, Structure, and Fluorescence Properties

A self-assembled ZnII-NdIII heterohexanuclear coordination compound [Zn4Nd2(L)4(bdc)2]·2NO3 based on a hexadentate Salamo-like chelating ligand (H2L = 1,2-bis(3-methoxysalicylideneaminooxy)ethane]) and H2bdc (H2bdc = terephthalic acid) has been synthesized and characterized by elemental analyses, IR and UV/Vis spectra, and X-ray crystallography. Two crystallographically equivalent [Zn2Nd(L)2] moieties lie in the inversion center linked by two (bdc)2− ligands leading to a heterohexanuclear dimer in which the carboxylato group bridges the ZnII and NdIII atoms. The heteropolynuclear 3d-4f coordination compound includes four ZnII atoms, two NdIII atoms, four completely deprotonated (L)2− units, two fully deprotonated (bdc)2− units, and two crystalling nitrate ions. All of the ZnII atoms in the ZnII-NdIII coordination compound possess trigonal bipyramidal geometries and the NdIII atoms possess distorted bicapped square antiprism coordination arrangements. In addition, the fluorescence properties of the ligand and the ZnII-NdIII coordination compound were investigated.


Introduction
Salen-like and Salamo-like ligands have been extensively investigated in modern coordination chemistry for several decades [1,2], and their transition to metal coordination compounds have been widely investigated in electrochemical fields [3], biological fields [4], magnetic [5] and luminescent materials [6], and supramolecular buildings [7]. It is important to introduce suitable functional groups into the organic moiety of the ligands in order to improve or tune the properties of these metal coordination compounds [8].
Although these Salamo-like coordination compounds have been in the process of development, the heterometallic 3d-4f coordination compounds formed by terephthalic acid auxiliary ligands are still very rare. The multidentate chelating ligands may bring new changes to the structure and simple synthesis methods will play a catalytic role in the development of these coordination compounds [9,10]. The auxiliary ligands also play an important role in the construction of coordination compounds [11]. Some superior research on heterometallic coordination compounds with various functional properties have been reported [12][13][14][15].
In order to supplement and perfect the syntheses and structures of heterometallic 3d-4f coordination compounds in this study, we have synthesized a self-assembled Zn II -Nd III hetero-metallic coordination compound [Zn4Nd2(L)4(bdc)2] by one-pot reaction of the ligand H2L with Zn II acetate, Nd III nitrate, and H2bdc in a 1:1:1:1 molar ratio. Furthermore, the fluorescence properties of the ligand H2L and its Zn II -Nd III coordination compound were also studied.

Materials and Physical Measurements
2-Hydroxy-3-methoxybenzaldehyde (>99%) was purchased from Alfa Aesar and used without further purification. The other reagents and solvents were analytical grade reagents from the Tianjin Chemical Reagent Factory.
C, H, and N analyses were obtained using a VarioEL V3.00 automatic elemental analysis instrument. Elemental analyses for Zn II and Nd III were detected by an IRIS ER/ S-WP-1 ICP atomic emission spectrometer. Melting points were obtained by using a microscopic melting point apparatus from the Beijing Taike Instrument Limited Company and the thermometer was uncorrected. IR spectra were recorded on a Vertex70 FT-IR spectrophotometer with samples prepared as KBr (400-4000 cm -1 ) pellets. UV/Vis absorption spectra were recorded on a Shimadzu UV-3900 spectrometer. Luminescence spectra in the solution were recorded on a Hitachi F-7000 spectrometer.

Synthesis of the Ligand H2L
The main reaction steps involved in the synthesis of H2L are given in Scheme 1. H2L was synthesized according to the references reported earlier [16]

Synthesis of the Zn II -Nd III Coordination Compound
A mixture of H2L (36.0 mg, 0.1 mmol), Zn(OAc)2·2H2O (21.9 mg, 0.1 mmol), and Nd(NO3)3·6H2O (45.6 mg, 0.1 mmol) in chloroform and methanol (1:2 v/v, 3 mL) was stirred for 5 min at room temperature. Afterward, a solution of H2bdc (16.6 mg, 0.1 mmol) in DMF (1 mL) was added and the mixture was stirred for another 5 min at room temperature. The mixture solution was filtered and the filtrate was kept undisturbed in the dark to avoid decomposition of the Zn II -Nd III building blocks. The yellow block-like crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of diethyl ether into the filtrate for three days at room temperature. These were separated over filtration and air-dried before undergoing X-ray diffraction analysis (Scheme 2

Synthesis of the Zn II -Nd III Coordination Compound
A mixture of H 2 L (36.0 mg, 0.1 mmol), Zn(OAc) 2 ·2H 2 O (21.9 mg, 0.1 mmol), and Nd(NO 3 ) 3 ·6H 2 O (45.6 mg, 0.1 mmol) in chloroform and methanol (1:2 v/v, 3 mL) was stirred for 5 min at room temperature. Afterward, a solution of H 2 bdc (16.6 mg, 0.1 mmol) in DMF (1 mL) was added and the mixture was stirred for another 5 min at room temperature. The mixture solution was filtered and the filtrate was kept undisturbed in the dark to avoid decomposition of the Zn II -Nd III building blocks. The yellow block-like crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of diethyl ether into the filtrate for three days at room temperature. These were separated over filtration and air-dried before undergoing X-ray diffraction analysis (Scheme 2

X-ray Structure Determination of the Zn II -Nd III Coordination Compound
The diffraction data was collected using graphite-monochromatized Mo Kα radition (λ = 0.71073 Å) at 173(2) K. The structure was solved by using the program SHELXS-2016 and different Fourier techniques. It was refined by using the full-matrix least-squares method on F 2 utilizing SHELXL-2016 [17,18]. The Zn II -Nd III coordination compound contained large void and the solvent and the positive or negative ions located in the void could not be identified because it was highly disordered and had a small residual peak. Therefore, the SQUEEZE in PLATON program was performed to remove the highly disordered solvent and ions (Solvent Accessible Volume = 916, Electrons Found in S.A.V. = 136) [19]. All hydrogen atoms were added theoretically. The crystal and experimental data are shown in Table 1.

Zn Zn
Zn Zn Zn Nd Nd Scheme 2. Assemble route for the Zn II -Nd III coordination compound.

X-ray Structure Determination of the Zn II -Nd III Coordination Compound
The diffraction data was collected using graphite-monochromatized Mo Kα radition (λ = 0.71073 Å) at 173(2) K. The structure was solved by using the program SHELXS-2016 and different Fourier techniques. It was refined by using the full-matrix least-squares method on F 2 utilizing SHELXL-2016 [17,18]. The Zn II -Nd III coordination compound contained large void and the solvent and the positive or negative ions located in the void could not be identified because it was highly disordered and had a small residual peak. Therefore, the SQUEEZE in PLATON program was performed to remove the highly disordered solvent and ions (Solvent Accessible Volume = 916, Electrons Found in S.A.V. = 136) [19]. All hydrogen atoms were added theoretically. The crystal and experimental data are shown in Table 1. These data can be also obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.htm.

Results and Discussion
3.1. Description of Crystal Structure of the Zn II -Nd III Coordination Compound X-ray crystallographic analysis reveals the crystal structure of the Zn II -Nd III coordination compound. Selected bond lengths and angles are given in Table S1. The molecular structure of the Zn II -Nd III coordination compound is shown in Figure 1. The Zn II -Nd III coordination compound crystallizes in the triclinic space group P-1 and consists of four Zn II atoms, two Nd III atoms, four completely deprotonated (L) 2− units, and two fully deprotonated (bdc) 2− units. Two crystallographically equivalent [Zn 2 Nd(L) 2 ] moieties lie near the inversion center and are linked by two bdc 2− ligands. Each carboxylato group acts as a bridge between the Zn II and Nd III atoms. Each Zn II atom has a penta-coordinated environment and adopts distorted trigonal bipyramidal geometries. The dihedral angles between the coordination planes and the benzene rings of terephthalic acid are 76.79(3) • and 82.49(3) • , respectively, which is shown in Figure S1. The carboxylato group bridges the Zn II and Nd III atoms in a µ 2 fashion and each Nd III atom is deca-coordinated and surrounded by ten oxygen atoms including four from the outer O 8 cavity of the deprotonated ligand (L) 2− and two from the bdc 2− ligands. Therefore, the deca-coordinated Nd III atom adopts a distorted bicapped square antiprism coordination arrangement. Moreover, this novel 4:4:2 ((L) 2− :Zn(II):Ln(III)) heterometallic Salamo-type coordination compound possesses a symmetrical structure [12][13][14][15]. This is rarely reported in comparison to the 3d-4f Salamo-type coordination compound, which has the structures of 1:1:1 [20], 2:2:1 [9], and 2:2:2 [21]. These data can be also obtained free of charge at www.ccdc.cam. ac.uk/conts/retrieving.htm.

Description of Crystal Structure of the Zn II -Nd III Coordination Compound
X-ray crystallographic analysis reveals the crystal structure of the Zn II -Nd III coordination compound. Selected bond lengths and angles are given in Table S1. The molecular structure of the Zn II -Nd III coordination compound is shown in Figure 1. The Zn II -Nd III coordination compound crystallizes in the triclinic space group P-1 and consists of four Zn II atoms, two Nd III atoms, four completely deprotonated (L) 2− units, and two fully deprotonated (bdc) 2− units. Two crystallographically equivalent [Zn2Nd(L)2] moieties lie near the inversion center and are linked by two bdc 2-ligands. Each carboxylato group acts as a bridge between the Zn II and Nd III atoms. Each Zn II atom has a penta-coordinated environment and adopts distorted trigonal bipyramidal geometries. The dihedral angles between the coordination planes and the benzene rings of terephthalic acid are 76.79(3)° and 82.49(3)°, respectively, which is shown in Figure S1. The carboxylato group bridges the Zn II and Nd III atoms in a μ2 fashion and each Nd III atom is deca-coordinated and surrounded by ten oxygen atoms including four from the outer O8 cavity of the deprotonated ligand (L) 2-and two from the bdc 2-ligands. Therefore, the deca-coordinated Nd III atom adopts a distorted bicapped square antiprism coordination arrangement. Moreover, this novel 4:4:2 ((L) 2− :Zn(II):Ln(III)) heterometallic Salamo-type coordination compound possesses a symmetrical structure [12][13][14][15]. This is rarely reported in comparison to the 3d-4f Salamo-type coordination compound, which has the structures of 1:1:1 [20], 2:2:1 [9], and 2:2:2 [21].

IR Spectra of H 2 L and Its Zn II -Nd III Coordination Compound
The FT-IR spectra of H 2 L and its corresponding Zn II -Nd III coordination compound exhibits various bands in the 4000-400 cm −1 region ( Figure S4). A typical C=N stretching band of the free ligand H 2 L appears at 1601 cm −1 and the Zn II -Nd III coordination compound is observed at 1607 cm −1 . The changes in the spectrum of the Zn II -Nd III coordination compound show that H 2 L has coordinated with Zn(II) atoms. In addition, the free ligand H 2 L exhibits a typical Ar-O stretching frequency at 1258 cm −1 while the Ar-O stretching frequency of the Zn II -Nd III coordination compound appears at 1240 cm −1 . The shifts of Ar-O stretching frequencies show the Zn-O or Nd-O bond formation between Zn II or Nd III atoms and oxygen atoms of phenolic groups [23].

UV/Vis Absorption Spectra of H 2 L and Its Zn II -Nd III Coordination Compound
The UV/Vis absorption spectra of the ligand H 2 L and its Zn II -Nd III coordination compound in the dichloromethane solutions (1.0 × 10 −5 mol·L −1 ) at 298 K are shown in Figure S5. Clearly, the absorption spectrum of the free ligand H 2 L consists of three relatively intense absorption peaks centered at ca. 223 nm, 269 nm, and 317 nm. The absorption peaks at 223 nm and 269 nm can be assigned to the π-π* transition of the benzene rings and the absorption peak at 317 nm can be attributed to the intra-ligand n-π* transition of the C=N bonds [23]. Upon coordination of the ligand H 2 L, the π-π* transitions of the phenyl rings in the Zn II -Nd III coordination compound is bathochromically shifted to 233 nm and 280 nm, which indicates the coordination of Zn(II) and Nd(III) atoms with the ligand units, respectively [23]. Compared with the free ligand H 2 L, the absorption peak at 317 nm shifted to 375 nm from the UV/Vis spectrum of the Zn II -Nd III coordination compound, which indicates that the oxime nitrogen atoms are involved in coordination with the Zn(II) atoms.

Fluorescence Properties of H 2 L and Its Zn II -Nd III Coordination Compound
The fluorescent properties of H 2 L and its Zn II -Nd III coordination compound were measured at room temperature ( Figure 2). The Zn II -Nd III coordination compound displays enhanced emission intensities compared to the corresponding ligand (H 2 L) when excited at 392 nm. The enhancement of fluorescence is due to the coordination of metal ions with the ligands. The ligand H 2 L exhibits a broad emission at 418 nm upon excitation at 392 nm, which should be assigned to the intraligand π-π* transition [24]. The Zn II -Nd III coordination compound shows a higher intense photoluminescence with a maximum emission at 435 nm upon excitation at 392 nm. The emission peak positions of the Zn II -Nd III coordination compound are similar to those of the free ligand H 2 L. The emission peaks in the spectrum of the Zn II -Nd III coordination compound may also rise from the intra-ligand transition. Compared with the emission spectrum of H 2 L, enhanced fluorescence intensity of the Zn II -Nd III coordination compound is observed, which indicates that the intra-ligand transition has been influenced due to the introduction of Zn(II) ions in the structure [24]. No emissions originating from metal-centered or metal-to-ligand/ligand-to-metal charge-transfer excited states are expected for the Zn II -Nd III . The emission observed in the Zn II -Nd III coordination compound is tentatively assigned to the (π-π*) intra-ligand fluorescence.

Conclusions
In conclusion, we discovered a new Zn II -Nd III heterometallic coordination compound [Zn4Nd2(L)4(bdc)2]·2NO3 with a Salamo-like chelating ligand H2L. The Zn II -Nd III coordination compound contains four Zn II atoms, two Nd III atoms, four completely deprotonated (L) 2− units, and two fully deprotonated (bdc) 2− units. All of the Zn II atoms in the Zn II -Nd III coordination compound possesses trigonal bipyramidal geometries and the Nd III atoms possesses a distorted bicapped square antiprism coordination arrangement. Furthermore, the fluorescent properties ofH2L and its Zn II -Nd III coordination compound have also been discussed.
Supplementary Materials: Figure S1: View of the dihedral angles between the benzene rings of terephthalic acid and the basal planes (N2O2 planes) of the Zn II -Nd III coordination compound. Figure S2: Intermolecular hydrogen bonding interactions of the Zn II -Nd III coordination compound (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity). Figure S3: View of the 1D supramolecular structure of the Zn II -Nd III coordination compound showing the C-H···O hydrogen bondings. Figure S4: IR spectra of H2L and its corresponding Zn II -Nd III coordination compound. Figure S5: UV/Vis absorption spectra of H2L and its Zn II -Nd III coordination compound. Table S1: Selected bond lengths (Å) and angles (°) for the Zn II -Nd III coordination compound.

Conclusions
In conclusion, we discovered a new Zn II -Nd III heterometallic coordination compound [Zn 4 Nd 2 (L) 4 (bdc) 2 ]·2NO 3 with a Salamo-like chelating ligand H 2 L. The Zn II -Nd III coordination compound contains four Zn II atoms, two Nd III atoms, four completely deprotonated (L) 2− units, and two fully deprotonated (bdc) 2− units. All of the Zn II atoms in the Zn II -Nd III coordination compound possesses trigonal bipyramidal geometries and the Nd III atoms possesses a distorted bicapped square antiprism coordination arrangement. Furthermore, the fluorescent properties ofH 2 L and its Zn II -Nd III coordination compound have also been discussed.
Supplementary Materials: Figure S1: View of the dihedral angles between the benzene rings of terephthalic acid and the basal planes (N 2 O 2 planes) of the Zn II -Nd III coordination compound. Figure S2: Intermolecular hydrogen bonding interactions of the Zn II -Nd III coordination compound (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity). Figure S3: View of the 1D supramolecular structure of the Zn II -Nd III coordination compound showing the C-H···O hydrogen bondings. Figure S4: IR spectra of H 2 L and its corresponding Zn II -Nd III coordination compound. Figure S5: UV/Vis absorption spectra of H 2 L and its Zn II -Nd III coordination compound. Table S1: Selected bond lengths (Å) and angles ( • ) for the Zn II -Nd III coordination compound.