Structural and Hirshfeld Surface Analyses of a Novel Hetero-Tetranuclear Cu II-Na I Bis ( Salamo )-Based Coordination Compound

The newly designed butterfly-shaped hetero-tetranuclear CuII-NaI coordination compound, [Cu3(HL)2Na]·Pic (Pic− is abbreviation of picrate) (1) which is derived from a naphthalenediol-based bis(Salamo)-type chelating ligand H4L have been synthesized and characterized by elemental analyses, UV-vis spectra, IR spectra analysis, and Hirshfeld surface analysis. X-ray crystallographic analyses revealed that the coordination compound 1 is a novel hetero-tetranuclear CuII-NaI bis(Salamo)-type coordination compound and it differs from heterotrinuclear CuII-NaI bis(Salamo)-type coordination compound reported earlier. The Cu1 and Cu3 atoms are tetra-coordinated with geometries of distorted square pyramid, while Cu2 atom are hexa-coordinated with the geometry of a distorted octahedron. The NaI atom is octa-coordinated with the geometry of a distorted hexagonal bipyramid. Furthermore, the supramolecular structure and Hirshfeld surface analyses have been discussed in detail.

Recently, our research project concentrated on the syntheses of a new naphthalenediol-based bis(Salamo)-type ligands H 4 L containing two Salamo N 2 O 2 chelate moieties and their corresponding homo-or hetero-polynuclear metal coordination compounds, which differ from Nabeshima group's research emphasis.Herein, in order to study the anion effect of coordination compounds, we have designed and synthesized a novel Cu II -Na I coordination compound [Cu 3 (HL) 2 Na]•Pic, which is different from our previously reported coordination compound [Cu 2 (L)Na(NO 3 )(CH 3 OH)] [74].

Materials and Methods
All of the chemical reagents are analytical pure reagents, which have not been purified before being used.C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analyzer (Berlin, Germany).Elemental analysis for copper was detected by IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany).The melting points were determined by microscopic melting point instrument made in Beijing Tektronix Instruments Limited Company. 1 H-NMR spectra were recorded 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 that were prepared as KBr (400-4000 cm −1 ).UV/Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrometer (Shimadzu, Tokyo, Japan).X-ray single crystal structure determination was carried out on a Bruker Smart Apex CCD diffractometer (Bruker AVANCE, Billerica, MA, USA).

Synthesis of the Coordination Compound 1
A chloroform solution (3.0 mL) of copper(II) picrate tetrahydrate (0.015 mmol, 8.84 mg) was added dropwise to the mixture solution of H 4 L (0.01 mmol, 8.80 mg) and NaOH (0.80 mg, 0.020 mmol) in methanol (3.0 mL) at room temperature, and the color changed to dark brown immediately.The mixture was filtered and the filtrate was allowed to stand at room temperature for about two weeks.The solvent was partially evaporated and brown block crystals suitable for X-ray crystallographic analysis were obtained.Yield:

Crystal Structure Determinations of the Coordination Compound 1
A crystal diffractometer provides a monochromatic beam of Mo Kα radiation (0.71073 Å) that was produced from a sealed Mo X-ray tube using a graphite monochromator and was used for obtaining crystal data for the coordination compound 1 at 291 (2) K.The LP factor and semi-empirical absorption corrections were applied using the SADABS program.The structure was solved by the direct methods (SHELXS-2016) [75].All of the hydrogen atoms were added theoretically and a difference-Fourier map revealed the positions of the remaining atoms.All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F 2 with SHELXL-2016 [76].The crystal data and experimental parameters of the coordination compound 1 are summarized in Table 1.Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1828273) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.0.0949/0.1350

Crystal Structure of the Coordination Compound 1
The crystal structure and atom numbering of the coordination compound 1 are shown in Figure 1.Selected major bond lengths and angles are listed in Table 2.
The crystallographic data revealed that the coordination compound 1 crystallizes in the monoclinic system, space group P2 1 /n.X-ray crystallography clearly showed that the coordination compound 1 is a hetero-tetranuclear butterfly-shape coordination compound, which consists of two deprotonated (HL) 3− units, three Cu II atoms, one Na I atom, and one uncoordinated picrate ion.Note worthily, this 2:3 ((HL) 3− :Cu II ) type hetero-tetranuclear Cu II -Na I coordination compound is different from the earlier reported Cu II Salamo-type coordination compounds of 1:2 [77-79], 2:2 [80-82], 2:3 [83], and 2:4 [83] type (L:Cu II ).Especially, this is the first bis(Salamo)-based metal complex containing two bis(Salamo)-type ligands.The coordination compound that is obtained is different from our previously reported coordination compound [Cu 2 (L)Na(NO 3 )(CH 3 OH)], in which the two Cu(II) atoms are located in N 2 O 2 cavities, and one Cu(II) atom is still bonded to a methanol molecule, while the other Cu(II) atom is coordinated with one oxygen atom of a µ 3 -NO 3 ion.Finally, both Cu(II) atoms adopt penta-coordinated with geometries of slightly distorted tetragonal pyramid [74].Table 2. Selected bond lengths (Å) and angles (°) for the coordination compound 1.

Angles Angles
O5-Cu1-O8 In the crystal structure of the coordination compound 1, three Cu II atoms (Cu1, Cu2, and Cu3) are located in the N 2 O 2 cavities of the deprotonated (HL) 3− units, the Cu1 and Cu3 atoms are coordinated with two phenoxy atoms and two nitrogen atoms, respectively.The Cu2 atom is coordinated by two nitrogen atoms (N2 and N6) and four oxygen atoms (O2, O4, O14, and O10) from two deprotonated (HL) 3− units.By means of continuous shape measures (CShM), the configuration of the Cu1 and Cu3 atoms is basically the same, all of which form distorted quadrilateral geometries and the Cu2 atom form a distorted octahedron configuration [84] (Figure 1c).The Na I atom is octa-coordinated with the geometry of a distorted hexagonal bipyramid (when the value of CShM is the smallest, the ideal structure is the hexagonal bipyramid configuration), which is coordinated by six oxygen (O4, O5, O8, O14, O15, and O16) atoms and two bromine (Br4 and Br5) atoms from two deprotonated (HL) 3− units (Figure 1b).
In the crystal structure of the coordination compound 1, there are three pairs of intramolecular O1-H1B•••N1, O9-H9•••N5, and C39-H39A•••O21 (Figure 2), and a pair of intermolecular C22-H22A•••O1 interactions, as summarized in Table 3.The anion picrate was linked to coordination compound molecules through an intermolecular C39-H39A•••O21 interaction, making the molecular structure more stable.In addition, a one-dimensional (1D) supramolecular structure was formed by intermolecular C22-H22A•••O1 interactions (Figure 3).In the crystal structure of the coordination compound 1, three Cu II atoms (Cu1, Cu2, and Cu3) are located in the N2O2 cavities of the deprotonated (HL) 3− units, the Cu1 and Cu3 atoms are coordinated with two phenoxy atoms and two nitrogen atoms, respectively.The Cu2 atom is coordinated by two nitrogen atoms (N2 and N6) and four oxygen atoms (O2, O4, O14, and O10) from two deprotonated (HL) 3− units.By means of continuous shape measures (CShM), the configuration of the Cu1 and Cu3 atoms is basically the same, all of which form distorted quadrilateral geometries and the Cu2 atom form a distorted octahedron configuration [84] (Figure 1c).The Na I atom is octa-coordinated with the geometry of a distorted hexagonal bipyramid (when the value of CShM is the smallest, the ideal structure is the hexagonal bipyramid configuration), which is coordinated by six oxygen (O4, O5, O8, O14, O15, and O16) atoms and two bromine (Br4 and Br5) atoms from two deprotonated (HL) 3− units (Figure 1b).

IR Spectra
The FT-IR spectral data of H4L and its corresponding Cu II -Na I coordination compound showed different bands in the 400-4000 cm −1 region in Figure 4, and the important bands that are listed in Table 4.The spectrum of the free ligand H4L showed a typical O-H stretching band at 3307 cm −1 that belongs to the phenolic hydroxyl group.For the coordination compound 1, there is a narrow typical O-H stretching band at 3303 cm −1 because of the non-deprotonated phenolic hydroxyl group, and another narrow peak may be the C-H stretching band of the benzene ring.The wide peak at about 3200 cm −1 may be the O-H stretching band in the water.The NO2 stretching of the picrate anion appeared at about 1380 cm −1 in the coordination compound 1 spectrum.The typical C=N stretching band of the free ligand appeared at 1605 cm −1 .The typical C=N stretching band of the coordination compound 1 appeared at 1601 cm −1 [85].The Ar-O stretching band of the free ligand appeared at 1244 cm −1 , while that of the coordination compound 1 is observed at 1215 cm −1 , the C=N and Ar-O stretching frequencies are shifted, indicating that the Cu II atoms are coordinated with the free ligand.

IR Spectra
The FT-IR spectral data of H 4 L and its corresponding Cu II -Na I coordination compound showed different bands in the 400-4000 cm −1 region in Figure 4, and the important bands that are listed in Table 4.The spectrum of the free ligand H 4 L showed a typical O-H stretching band at 3307 cm −1 that belongs to the phenolic hydroxyl group.For the coordination compound 1, there is a narrow typical O-H stretching band at 3303 cm −1 because of the non-deprotonated phenolic hydroxyl group, and another narrow peak may be the C-H stretching band of the benzene ring.The wide peak at about 3200 cm −1 may be the O-H stretching band in the water.The NO 2 stretching of the picrate anion appeared at about 1380 cm −1 in the coordination compound 1 spectrum.The typical C=N stretching band of the free ligand appeared at 1605 cm −1 .The typical C=N stretching band of the coordination compound 1 appeared at 1601 cm −1 [85].The Ar-O stretching band of the free ligand appeared at 1244 cm −1 , while that of the coordination compound 1 is observed at 1215 cm −1 , the C=N and Ar-O stretching frequencies are shifted, indicating that the Cu II atoms are coordinated with the free ligand.

IR Spectra
The FT-IR spectral data of H4L and its corresponding Cu II -Na I coordination compound showed different bands in the 400-4000 cm −1 region in Figure 4, and the important bands that are listed in Table 4.The spectrum of the free ligand H4L showed a typical O-H stretching band at 3307 cm −1 that belongs to the phenolic hydroxyl group.For the coordination compound 1, there is a narrow typical O-H stretching band at 3303 cm −1 because of the non-deprotonated phenolic hydroxyl group, and another narrow peak may be the C-H stretching band of the benzene ring.The wide peak at about 3200 cm −1 may be the O-H stretching band in the water.The NO2 stretching of the picrate anion appeared at about 1380 cm −1 in the coordination compound 1 spectrum.The typical C=N stretching band of the free ligand appeared at 1605 cm −1 .The typical C=N stretching band of the coordination compound 1 appeared at 1601 cm −1 [85].The Ar-O stretching band of the free ligand appeared at 1244 cm −1 , while that of the coordination compound 1 is observed at 1215 cm −1 , the C=N and Ar-O stretching frequencies are shifted, indicating that the Cu II atoms are coordinated with the free ligand.

UV-Vis Spectra
The UV-Vis absorption spectra of the free ligand H 4 L and its corresponding Cu II -Na I coordination compound in the CHCl 3 /CH 3 OH solution (CHCl 3 /CH 3 OH 3:2 v/v, 1.0 × 10 −5 mol/L) are shown in Figure 5. Obviously, the absorption maxima of the ligand H 4 L differ from those of the coordination compound 1.As shown in Figure 5, for the ligand H 4 L, the peaks at 342 nm (ε = 4.8 × 10 4 M −1 •cm −1 ), 361 nm (ε = 4.3 × 10 4 M −1 •cm −1 ), and 379 nm (ε = 3.5 × 10 4 M −1 •cm −1 ) can be assigned to the intra-ligand π-π* transitions and indicated that the ligand H 4 L contains a large conjugation system.It can be assigned to the π-π* transition of the naphthalene rings [11].When compared with the free ligand H 4 L, the three absorption peaks disappeared from the UV-vis spectrum of the coordination compound 1, and one new absorption maxima was observed at ca. 382 nm for the coordination compound 1, and is assigned to L→M (LMCT) or M→L (MLCT) charge transition which is characteristic of the transition metal coordination compounds with N 2 O 2 coordination sphere [29].
Crystals 2018, 8, x FOR PEER REVIEW 7 of

UV-Vis Spectra
The UV-Vis absorption spectra of the free ligand H4L and its corresponding Cu II -Na I coordination compound in the CHCl3/CH3OH solution (CHCl3/CH3OH 3:2 v/v, 1.0 × 10 −5 mol/L) are shown in Figure 5. Obviously, the absorption maxima of the ligand H4L differ from those of the coordination compound 1.As shown in Figure 5, for the ligand H4L, the peaks at 342 nm (ε = 4.8 × 10 4 M −1 •cm −1 ), 361 nm (ε = 4.3 × 10 4 M −1 •cm −1 ), and 379 nm (ε = 3.5 × 10 4 M −1 •cm −1 ) can be assigned to the intra-ligand π-π* transitions and indicated that the ligand H4L contains a large conjugation system.It can be assigned to the π-π* transition of the naphthalene rings [11].When compared with the free ligand H4L, the three absorption peaks disappeared from the UV-vis spectrum of the coordination compound 1, and one new absorption maxima was observed at ca. 382 nm for the coordination compound 1, and is assigned to L→M (LMCT) or M→L (MLCT) charge transition which is characteristic of the transition metal coordination compounds with N2O2 coordination sphere [29].

Hirshfeld Surface Analysis
The Hirshfeld surfaces [86] of the coordination compound 1 are illustrated in Figure 6, indicating that the surfaces have been mapped over dnorm and the corresponding location in shape index exists the complementary region of red concave surface surrounded by receptors and the blue convex surface surrounding receptors, further proving that such hydrogen bonding exists.The large and deep red spots on the three-dimensional (3D) Hirshfeld surfaces indicate the close-contact interactions, mainly responsible for the corresponding hydrogen bond contacts.As for the large amount of white region in dnorm surfaces, it is suggested that there is a weaker and farther contact between molecules, rather than hydrogen bonding.Figure 7 showed that the two-dimensional (2D) plots that were generated [87]

Hirshfeld Surface Analysis
The Hirshfeld surfaces [86] of the coordination compound 1 are illustrated in Figure 6, indicating that the surfaces have been mapped over d norm and the corresponding location in shape index exists the complementary region of red concave surface surrounded by receptors and the blue convex surface surrounding receptors, further proving that such hydrogen bonding exists.The large and deep red spots on the three-dimensional (3D) Hirshfeld surfaces indicate the close-contact interactions, mainly responsible for the corresponding hydrogen bond contacts.As for the large amount of white region in d norm surfaces, it is suggested that there is a weaker and farther contact between molecules, rather than hydrogen bonding.Figure 7 showed that the two-dimensional (2D) plots that were generated [87]

Conclusions
The unexpected hetero-tetranuclear Cu II -Na I coordination compound, [Cu3(HL)2Na]•Pic assembly from a naphthalenediol-based bis(Salamo)-type ligand (H4L) has been synthesized and characterized.For the central metals, the Cu1 and Cu3 atoms are tetra-coordinated with distorted square geometries, and the Cu2 atoms are hexa-coordinated with a distorted octahedron configuration, while the Na I atom is octa-coordinated with geometry of a distorted hexagonal bipyramid, which is coordinated with six oxygen (O4, O5, O8, O14, O15, and O16) atoms and two bromo (Br4 and Br5) atoms from two deprotonated (HL) 3− units.Furthermore, a 1D supramolecular structure was formed by intermolecular C22-H22A•••O1 interactions.In addition, the Hirshfeld surface analyses indicated that the coordination compound could be stable due to intramolecular hydrogen bonds and some weaker interactions.

Conclusions
The unexpected hetero-tetranuclear Cu II -Na I coordination compound, [Cu3(HL)2Na]•Pic assembly from a naphthalenediol-based bis(Salamo)-type ligand (H4L) has been synthesized and characterized.For the central metals, the Cu1 and Cu3 atoms are tetra-coordinated with distorted square geometries, and the Cu2 atoms are hexa-coordinated with a distorted octahedron configuration, while the Na I atom is octa-coordinated with geometry of a distorted hexagonal bipyramid, which is coordinated with six oxygen (O4, O5, O8, O14, O15, and O16) atoms and two bromo (Br4 and Br5) atoms from two deprotonated (HL) 3− units.Furthermore, a 1D supramolecular structure was formed by intermolecular C22-H22A•••O1 interactions.In addition, the Hirshfeld surface analyses indicated that the coordination compound could be stable due to intramolecular hydrogen bonds and some weaker interactions.

Conclusions
The unexpected hetero-tetranuclear Cu II -Na I coordination compound, [Cu 3 (HL) 2 Na]•Pic assembly from a naphthalenediol-based bis(Salamo)-type ligand (H 4 L) has been synthesized and characterized.For the central metals, the Cu1 and Cu3 atoms are tetra-coordinated with distorted square geometries, and the Cu2 atoms are hexa-coordinated with a distorted octahedron configuration, while the Na I atom is octa-coordinated with geometry of a distorted hexagonal bipyramid, which is coordinated with six oxygen (O4, O5, O8, O14, O15, and O16) atoms and two bromo (Br4 and Br5) atoms from two deprotonated (HL) 3− units.Furthermore, a 1D supramolecular structure was formed by intermolecular C22-H22A•••O1 interactions.In addition, the Hirshfeld surface analyses indicated that the coordination compound could be stable due to intramolecular hydrogen bonds and some weaker interactions.

Figure 1 .
Figure 1.(a) Molecular structure and atom numberings of the coordination compound 1 (hydrogen atoms are omitted for clarity); (b) Coordination polyhedron for the Na I atom of the coordination compound 1; and, (c) Coordination polyhedrons for Cu II atoms of the coordination compound 1.

Figure 1 .
Figure 1.(a) Molecular structure and atom numberings of the coordination compound 1 (hydrogen atoms are omitted for clarity); (b) Coordination polyhedron for the Na I atom of the coordination compound 1; and, (c) Coordination polyhedrons for Cu II atoms of the coordination compound 1.

Figure 2 .Table 3 .
Figure 2. View of the intramolecular and intermolecular hydrogen bondings of the coordination compound 1 (for clarity purpose, hydrogen atoms are omitted, except those forming hydrogen bonds).

Figure 2 .
Figure 2. View of the intramolecular and intermolecular hydrogen bondings of the coordination compound 1 (for clarity purpose, hydrogen atoms are omitted, except those forming hydrogen bonds).

Figure 3 .
Figure 3.The one-dimensional (1D) supramolecular structure of the coordination compound 1 with inter-molecular hydrogen bondings (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

Figure 3 .
Figure 3.The one-dimensional (1D) supramolecular structure of the coordination compound 1 with inter-molecular hydrogen bondings (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

Figure 5 .
Figure 5.The UV-vis spectra of the free ligand H4L and its corresponding coordination compound 1.
correspond to the O•••H, C•••H and H•••H interactions from the Hirshfeld surface of the coordination compound 1.With the help of these analysis results, different interactions can be separated from each other that would commonly overlap in full fingerprint plots.The location of H•••H interactions appeared at (1.15 Å, 1.20 Å), accounting for 22.4% of the total area of Hirshfeld surfaces.The C•••H/H•••C interactions in the range of (1.60 Å, 1.12 Å) and appeared as a pair of symmetrical wings, accounting for 6.4% of the total area of Hirshfeld surfaces.The proportion of O•••H/H•••O interactions occupies 26.3% of the total Hirshfed surfaces for each molecule of the coordination compound 1, the contribution of C•••C to Hirshfeld surfaces is 0%,

Figure 5 .
Figure 5.The UV-vis spectra of the free ligand H 4 L and its corresponding coordination compound 1.

Author
Figure Hirshfeld surfaces mapped with d norm and shape index of the coordination compound 1.

Author
Contributions: W.-K.D. and X.-Y.S. conceived and designed the experiments; L.C. and F.W. performed

Figure
Figure Fingerprint plot of the coordination compound 1: full and resolved into O•••H, C•••H and H•••H contacts showing the percentages of contacts contributed to the total Hirshfeld surface area of molecule.

Table 1 .
Crystal data and structure refinement parameters for the coordination compound

Table 4 .
The major FT-IR data of H 4 L and its Cu II -Na I coordination compound (cm −1 ).