Synthesis and Fluorescence Properties of Structurally Characterized Heterobimetalic Cu(II)–Na(I) Bis(salamo)-Based Complex Bearing Square Planar, Square Pyramid and Triangular Prism Geometries of Metal Centers

A novel heterotrinuclear complex [Cu2(L)Na(µ-NO3)]∙CH3OH∙CHCl3 derived from a symmetric bis(salamo)-type tetraoxime H4L having a naphthalenediol unit, was prepared and structurally characterized via means of elemental analyses, UV-Vis, FT-IR, fluorescent spectra and single-crystal X-ray diffraction. The heterobimetallic Cu(II)–Na(I) complex was acquired via the reaction of H4L with 2 equivalents of Cu(NO3)2·2H2O and 1 equivalent of NaOAc. Clearly, the heterotrinuclear Cu(II)–Na(I) complex has a 1:2:1 ligand-to-metal (Cu(II) and Na(I)) ratio. X-ray diffraction results exhibited the different geometric behaviors of the Na(I) and Cu(II) atoms in the heterotrinuclear complex; the both Cu(II) atoms are sited in the N2O2 coordination environments of fully deprotonated (L)4− unit. One Cu(II) atom (Cu1) is five-coordinated and possesses a geometry of slightly distorted square pyramid, while another Cu(II) atom (Cu2) is four-coordination possessing a square planar coordination geometry. Moreover, the Na(I) atom is in the O6 cavity and adopts seven-coordination with a geometry of slightly distorted single triangular prism. In addition, there are abundant supramolecular interactions in the Cu(II)–Na(I) complex. The fluorescence spectra showed the Cu(II)–Na(I) complex possesses a significant fluorescent quenching and exhibited a hypsochromic-shift compared with the ligand H4L.


IR Spectra
The main FT-IR absorption bands characteristic for H4L and its Cu(II)-Na(I) complex from 500 to 4000 cm −1 regions are shown in Table 1. Table 1. Main FT-IR absorption bands for H4L and its Cu(II)-Na(I) complex (cm −1 ). H4L  3172  1634  1241  -Complex  -1618  1232  565 Generally, the typical O-H stretching band is usually observed at the 3500-3100 cm −1 region. A broad band of medium intensity was found around 3172 cm −1 in the free ligand H4L, this is an evidence for the stretching vibration of the O-H groups. However, this band disappeared in the Cu(II)-Na(I) complex. H4L showed a typical C=N stretching band occurring at 1634 cm −1 , while the Cu(II)-Na(I) complex showed this band at 1618 cm −1 . In contrast to the free ligand H4L it is moved to a lower wavenumber by 16 cm −1 due to coordination of the N atoms of the C=N groups to the Cu(II) atoms [57]. This fact is also approved by the appearance of new bands within the wavenumber at 565 cm −1 due to v(Cu(II)-N) vibrations. The characteristic absorption band of the Ar-O group occurred at 1241 cm −1 in H4L. However, the Ar-O stretching band in the Cu(II)-Na(I) complex was moved to a lower wavenumber at ca. 9 cm −1 , exhibiting that the M-O bonds are formed between the metal atoms H 4 L

IR Spectra
The main FT-IR absorption bands characteristic for H 4 L and its Cu(II)-Na(I) complex from 500 to 4000 cm −1 regions are shown in Table 1. Table 1. Main FT-IR absorption bands for H 4 L and its Cu(II)-Na(I) complex (cm −1 ). 3172  1634  1241  -Complex  -1618  1232  565 Generally, the typical O-H stretching band is usually observed at the 3500-3100 cm −1 region. A broad band of medium intensity was found around 3172 cm −1 in the free ligand H 4 L, this is an evidence for the stretching vibration of the O-H groups. However, this band disappeared in the Cu(II)-Na(I) complex. H 4 L showed a typical C=N stretching band occurring at 1634 cm −1 , while the Cu(II)-Na(I) complex showed this band at 1618 cm −1 . In contrast to the free ligand H 4 L it is moved to a lower wavenumber by 16 cm −1 due to coordination of the N atoms of the C=N groups to the Cu(II) atoms [57]. This fact is also approved by the appearance of new bands within the wavenumber at 565 cm −1 due to v(Cu(II)-N) vibrations. The characteristic absorption band of the Ar-O group occurred at 1241 cm −1 in H 4 L. However, the Ar-O stretching band in the Cu(II)-Na(I) complex was moved to a lower wavenumber at ca. 9 cm −1 , exhibiting that the M-O bonds are formed between the metal atoms and the methoxy and phenolic O atoms of H 4 L [60]. The facts mentioned above fare in agreement with the results of X ray diffraction.
It can be seen from the absorption spectrum of H 4 L, there are four continuous absorption peaks at about 269, 342, 360 and 378 nm [57]. The peak at 269 nm can be attributed to the π-π* transition of the benzene rings while the other three peaks can be assigned to the π-π* transition of the oxime groups [57,61].
In the titration experiment of the Cu(II)-Na(I) complex, the gradual addition of Cu(NO 3 ) 2 solution aroused the changes of absorption peaks. Compared to H 4 L, the peaks are bathochromically shifted [62]. This fact mentioned above is owing to the coordination of the Cu(II) ion with H 4 L. When 3 equiv of the Cu(II) ions were added to the solution of H 4 L, the absorbance of the solution no longer changed. The spectral titration obviously revealed the formation of a 1:3 (ligand-to-Cu(II) ratio) Cu(II) complex ( Figure 2a).
Molecules 2018, 23, x 3 of 14 and the methoxy and phenolic O atoms of H4L [60]. The facts mentioned above fare in agreement with the results of X ray diffraction.
It can be seen from the absorption spectrum of H4L, there are four continuous absorption peaks at about 269, 342, 360 and 378 nm [57]. The peak at 269 nm can be attributed to the π-π* transition of the benzene rings while the other three peaks can be assigned to the π-π* transition of the oxime groups [57,61].
In the titration experiment of the Cu(II)-Na(I) complex, the gradual addition of Cu(NO3)2 solution aroused the changes of absorption peaks. Compared to H4L, the peaks are bathochromically shifted [62]. This fact mentioned above is owing to the coordination of the Cu(II) ion with H4L. When 3 equiv of the Cu(II) ions were added to the solution of H4L, the absorbance of the solution no longer changed. The spectral titration obviously revealed the formation of a 1:3 (ligand-to-Cu(II) ratio) Cu(II) complex ( Figure 2a).
What is more, the gradual additions of NaOAc solution were continuously added the above solution. Upon addition of 1 equiv of Na(I) ions, the solution absorbance basically remains stable. Clearly, the spectral titration obviously indicated that the ratio of the replacement reaction stoichiometry is 1:1 and is depicted in Figure 2b.  What is more, the gradual additions of NaOAc solution were continuously added the above solution. Upon addition of 1 equiv of Na(I) ions, the solution absorbance basically remains stable. Clearly, the spectral titration obviously indicated that the ratio of the replacement reaction stoichiometry is 1:1 and is depicted in Figure 2b.

Crystal Structure Description
The structure of the Cu(II)-Na(I) complex was characterized by X-ray crystallography, and is depicted in Figure 3.

Crystal Structure Description
The structure of the Cu(II)-Na(I) complex was characterized by X-ray crystallography, and is depicted in Figure 3. It is shown that the Cu(II)-Na(I) complex crystallizes in the monoclinic system with space group P 21/n (Attempting to set the space group to P21/c, the structure of the complex was not determined, which is determined by the different orientations chosen when determining the crystal cell. The structure of expected complex is obtained when the space group is set to P21/n, and the bond lengths and angles are all within the expected ranges.) [63] and contains two Cu(II) atoms, one Na(I) atom, one completely deprotonated (L) 4-moiety, one μ-NO3 ion, one crystallizing methanol and chloroform molecules, possessing a hetero-trinuclear structure. According to our design, the bis(salamo)-type ligand H4L has an O6 coordination sphere which is usually occupied by a larger radius metal ion [64]. Therefore, we consider the complex obtained to be a heterotrinuclear Cu(II)-Na(I) complex and not sodium salt of the Cu(II) complex [65]. At the same time, it is shown that the ligand-to-metal (Cu(II) and Na(I)) ratio is 1:2:1. This structure in this paper is different from the structure reported earlier, in which the two Cu(II) atoms are located in N2O2 sites, and one Cu(II) atom is still bonded to a methanol molecule, while the other Cu(II) atom is bonded to one O atom of a μ-NO3 ion. Finally, both Cu(II) atoms adopt five-coordinated and possess geometries of slightly distorted square pyramid. The capped site is occupied by Br3 with the bond length (Na1-Br3, 2.765(3) Å) and the Na-O bond length is in the ranges of 2.324(2)-3.025(3) Å [17]. It is shown that the Cu(II)-Na(I) complex crystallizes in the monoclinic system with space group P 2 1 /n (Attempting to set the space group to P2 1 /c, the structure of the complex was not determined, which is determined by the different orientations chosen when determining the crystal cell. The structure of expected complex is obtained when the space group is set to P2 1 /n, and the bond lengths and angles are all within the expected ranges.) [63] and contains two Cu(II) atoms, one Na(I) atom, one completely deprotonated (L) 4− moiety, one µ-NO 3 ion, one crystallizing methanol and chloroform molecules, possessing a hetero-trinuclear structure. According to our design, the bis(salamo)-type ligand H 4 L has an O 6 coordination sphere which is usually occupied by a larger radius metal ion [64]. Therefore, we consider the complex obtained to be a heterotrinuclear Cu(II)-Na(I) complex and not sodium salt of the Cu(II) complex [65]. At the same time, it is shown that the ligand-to-metal (Cu(II) and Na(I)) ratio is 1:2:1. This structure in this paper is different from the structure reported earlier, in which the two Cu(II) atoms are located in N 2 O 2 sites, and one Cu(II) atom is still bonded to a methanol molecule, while the other Cu(II) atom is bonded to one O atom of a µ-NO 3 ion. Finally, both Cu(II) atoms adopt five-coordinated and possess geometries of slightly distorted square pyramid. The capped site is occupied by Br3 with the bond length (Na1-Br3, 2.765(3) Å) and the Na-O bond length is in the ranges of 2.324(2)-3.025(3) Å [17].
As shown in Figure 3, the N 2 O 2 compartments of the fully deprotonated (L) 4− moiety are occupied by two Cu(II) atoms (Cu1 and Cu2), while the central Na(I) atom is located at O 6 cavity. The Cu1 atom is bonded to two N atoms (Cu1-N1, 2.002(3) and Cu1-N2, 1.950 (3)) and two O atoms (Cu1-O2, 1.907(2) Å and Cu1-O5, 1.903(2) Å) of the ligand (L) 4− moiety forming the square base. Meantime, the Cu1 atom is also coordinated with one O atom (Cu1-O12, 2.462(3) Å) of a bisdentate µ-NO 3 ion. The Cu1 atom is five-coordinated with geometry of a slightly distorted square pyramid, which was deduced by calculating the value of τ = 0.0058 [66]. Interestingly, the Cu2 atom is coordinated to the N atoms (Cu2-N3, 1.944 (3) [17]. Meanwhile, the Na(I) atom is still coordinated to one O atom (Na1-O11, 2.490(4)) of µ-NO 3 ion. In a word, the Na-O bond length is in the range of 2.355(3)-2.546(3) Å, which is in the regular range compared with previously reported data [17]. Therefore, according to our previous report, the Na-O bonds mentioned above could be regarded as coordination bonds. Finally, the Na(I) atom adopts seven-coordinated and possesses a geometry of slightly distorted single triangular prism. The crystallographic data and structure refinement parameters are summed in Table 2. Selected bond lengths and angles are summed in Table 3.  Table 3. Selected bond distances (Å) and angles ( • ) for the Cu(II)-Na(I) complex.
As shown in Figure 7, the ligand H4L exhibited one relatively strong emission at 440 nm upon excitation at 330 nm, which should be attributed to the intra-ligand π-π* transition [67]. In comparison with the H4L, fluorescent property study of the Cu(II)-Na(I) complex is shown that the fluorescent intensity weakens markedly, exhibiting a broad peak with maximum emission at 429 nm upon excitation at 330 nm. That hypsochromic-shift phenomenon can explain the complexation of H4L and metal ions (Cu(II) and Na(I)), which is attributed to ligand-to-metal charge transfer (LMCT) [68].
As shown in Figure 7, the ligand H4L exhibited one relatively strong emission at 440 nm upon excitation at 330 nm, which should be attributed to the intra-ligand π-π* transition [67]. In comparison with the H4L, fluorescent property study of the Cu(II)-Na(I) complex is shown that the fluorescent intensity weakens markedly, exhibiting a broad peak with maximum emission at 429 nm upon excitation at 330 nm. That hypsochromic-shift phenomenon can explain the complexation of H4L and metal ions (Cu(II) and Na(I)), which is attributed to ligand-to-metal charge transfer (LMCT) [68].
As shown in Figure 7, the ligand H 4 L exhibited one relatively strong emission at 440 nm upon excitation at 330 nm, which should be attributed to the intra-ligand π-π* transition [67]. In comparison with the H 4 L, fluorescent property study of the Cu(II)-Na(I) complex is shown that the fluorescent intensity weakens markedly, exhibiting a broad peak with maximum emission at 429 nm upon excitation at 330 nm. That hypsochromic-shift phenomenon can explain the complexation of H 4 L and metal ions (Cu(II) and Na(I)), which is attributed to ligand-to-metal charge transfer (LMCT) [68].

Materials and Methods
2-Hydroxy-3-methoxybenzaldehyde (99%), pyridinium chlorochromate (98%), methyl trioctyl ammonium chloride (90%) and borontribromide (99.9%) were bought from Alfa Aesar (New York, NY, USA). Hydrobromic acid 33 wt % solution in acetic acid was purchased from J&K Scientific Ltd. (Beijing, China). All chemicals and solvents used for the synthesis were of the best available analytical reagent grade, without further purification in the preparation of the free ligand and its complex. Elemental analyses for C, H, and N were carried out using a GmbH Vario EL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany). Elemental analyses for Cu and Na atoms were detected with an IRIS ER/S-WP-1 ICP atomic emission spectrometer (IRIS, Elementar, Berlin, Germany). IR spectra were measured on a VERTEX70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr (500-4000 cm −1 ) pellets. Melting points were recorded on use of a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company and were uncorrected (Beijing, China). UV-Vis absorption spectra were recorded on a Shimadzu UV-3900spectrometer (Hitachi, Shimadzu, Tokyo, Japan). The fluorescence spectra were taken on Hitachi F-7000 spectrometer (Hitachi, Tokyo, Japan). X-ray single crystal structure determinations were carried out on a Super Nova Dual (Cu at zero) Eos four-circle diffractometer (Bruker, Billerica, MA, USA).

Materials and Methods
2-Hydroxy-3-methoxybenzaldehyde (99%), pyridinium chlorochromate (98%), methyl trioctyl ammonium chloride (90%) and borontribromide (99.9%) were bought from Alfa Aesar (New York, NY, USA). Hydrobromic acid 33 wt % solution in acetic acid was purchased from J&K Scientific Ltd. (Beijing, China). All chemicals and solvents used for the synthesis were of the best available analytical reagent grade, without further purification in the preparation of the free ligand and its complex. Elemental analyses for C, H, and N were carried out using a GmbH Vario EL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany). Elemental analyses for Cu and Na atoms were detected with an IRIS ER/S-WP-1 ICP atomic emission spectrometer (IRIS, Elementar, Berlin, Germany). IR spectra were measured on a VERTEX70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr (500-4000 cm −1 ) pellets. Melting points were recorded on use of a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company and were uncorrected (Beijing, China). UV-Vis absorption spectra were recorded on a Shimadzu UV-3900spectrometer (Hitachi, Shimadzu, Tokyo, Japan). The fluorescence spectra were taken on Hitachi F-7000 spectrometer (Hitachi, Tokyo, Japan). X-ray single crystal structure determinations were carried out on a Super Nova Dual (Cu at zero) Eos four-circle diffractometer (Bruker, Billerica, MA, USA).

Synthesis of the Cu(II)-Na(I) complex
A mixture solution of Cu(NO3)2 . 2H2O (4.82 mg, 0.02 mmol) in methanol (2 mL) and NaOAc (0.82 mg, 0.01 mmol) in methanol (2 mL) was added to a stirring solution of H4L (6.32 mg, 0.01 mmol) in chloroform (2 mL). The color of the mixing solution immediately turned dark green. The mixture was filtered, and the filtrate was allowed to stay for slow evaporation. After about three weeks, clear light green single-crystals suitable for X-ray diffraction studies were gained. Yield: 40

Crystal Structure Determination and Refinement
Single crystal of dimensions 0.15 × 0.21 × 0.24 mm for the heterotrinuclear Cu(II)-Na(I) complex was mounted on a glass rod. The crystal data were collected with a Super Nova (Dual, Cu at zero, Eos) diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 154.89(10) K. Multiscan absorption corrections were applied. The structure was solved by direct methods and refined anisotropically using full-matrix least-squares methods on F 2 with the SHELX-2014 program package. Nonhydrogen atoms of the compound were refined with anisotropic temperature parameters. The positions of H atoms were calculated and isotropically fixed in the final refinement. Supplementary

Conclusions
Overall, we have designed and synthesized a novel heterobimetallic Cu(II)-Na(I) complex derived from a symmetric bis(salamo)-type ligand H4L. X-ray crystallographic investigation of the heterotrinuclear Cu(II)-Na(I) complex revealed there is a 1:2:1 ligand-to-metal (Cu(II) and Na(I)) ratio. The Cu1 atom is five-coordinated and possesses the geometry of slightly distorted square pyramid and the Cu2 atom is four-coordinated and possesses the geometry of square planar. The Na(I) atom adopts seven-coordinated with a slightly distorted single triangular prism geometry. The UV-Vis titration experiment clearly displayed the coordination ratio between the ligand H4L and the Cu(II) and Na(I) ions. The fluorescence spectra showed the complex possesses a significant fluorescent quenching, and exhibited a hypsochromic-shift compared with the ligand H4L.

Synthesis of the Cu(II)-Na(I) Complex
A mixture solution of Cu(NO 3 ) 2 ·2H 2 O (4.82 mg, 0.02 mmol) in methanol (2 mL) and NaOAc (0.82 mg, 0.01 mmol) in methanol (2 mL) was added to a stirring solution of H 4 L (6.32 mg, 0.01 mmol) in chloroform (2 mL). The color of the mixing solution immediately turned dark green. The mixture was filtered, and the filtrate was allowed to stay for slow evaporation. After about three weeks, clear light green single-crystals suitable for X-ray diffraction studies were gained. Yield: 40.3%. Anal. Calcd. (%) for C 34

Crystal Structure Determination and Refinement
Single crystal of dimensions 0.15 × 0.21 × 0.24 mm for the heterotrinuclear Cu(II)-Na(I) complex was mounted on a glass rod. The crystal data were collected with a Super Nova (Dual, Cu at zero, Eos) diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 154.89(10) K. Multiscan absorption corrections were applied. The structure was solved by direct methods and refined anisotropically using full-matrix least-squares methods on F 2 with the SHELX-2014 program package. Nonhydrogen atoms of the compound were refined with anisotropic temperature parameters. The positions of H atoms were calculated and isotropically fixed in the final refinement.
Supplementary crystallographic data have been deposited at Cambridge Crystallographic Data Centre (CCDC No. 1815945) and can be gained free of charge via www.ccdc.cam.ac.uk/conts/ retrieving.html.

Conclusions
Overall, we have designed and synthesized a novel heterobimetallic Cu(II)-Na(I) complex derived from a symmetric bis(salamo)-type ligand H 4 L. X-ray crystallographic investigation of the heterotrinuclear Cu(II)-Na(I) complex revealed there is a 1:2:1 ligand-to-metal (Cu(II) and Na(I)) ratio. The Cu1 atom is five-coordinated and possesses the geometry of slightly distorted square pyramid and the Cu2 atom is four-coordinated and possesses the geometry of square planar. The Na(I) atom adopts seven-coordinated with a slightly distorted single triangular prism geometry. The UV-Vis titration experiment clearly displayed the coordination ratio between the ligand H 4 L and the Cu(II) and Na(I) ions. The fluorescence spectra showed the complex possesses a significant fluorescent quenching, and exhibited a hypsochromic-shift compared with the ligand H 4 L.