Structural Characterized Homotrinuclear Zn II Bis ( Salamo )-Based Coordination Compound : Hirshfeld Surfaces , Fluorescent and Antimicobial Properties

A homotrinuclear ZnII bis(salamo) coordination compound, [LZn3(OAc)2(H2O)] of a new bis(salamo)-like ligand, has been synthesized and structurally characterized using elemental analyses, IR, UV-Vis and fluorescent spectra, and Hirshfeld surface analysis. Hirshfeld surface analyses and X-ray crystallography revealed that complexation between ZnII acetate dihydrate and the ligand H4L afforded a 3:1 (ZnII:L) type coordination compound. Moreover, the X-ray crystal structure analysis demonstrated that two μ2-acetate anions bridge three ZnII atoms in a μ2-fashion forming a homo-trinuclear structure. There were two kinds of ZnII atoms coordination geometries (strongly distorted square pyramidal (Zn1) and distorted trigonal bipyramidal (Zn2 and Zn3)) in the ZnII coordination compound. In addition, a 3D supra-molecular structure was constructed by intermolecular C-H···π and π···π interactions in the ZnII coordination compound. Most importantly, the fluorescent and antimicrobial properties of H4L and its ZnII coordination compound were investigated.

To utilize salamo units to control guest recognition, a better strategy distinguished from the macrocyclization has been proposed [38,39].Thus, we designed and prepared a new C-shaped bis(salamo)-like chelating ligand that contained a O 4 site besides the two N 2 O 2 sites, to control guest binding via using the coordination-triggered conformational changes.When the ligand is metalated, our O 4 oxygen atoms are located in an acyclic, C-shaped arrangement.Moreover, the guest binding could be more effective owing to the negatively charged phenolates of the metal coordination Crystals 2018, 8, 259 2 of 15 compounds having higher coordination ability to other metals than their phenol form.Interestingly, some studies have been devoted to research mono-, multi-, homo-or heteromultinuclear metal coordination compounds bearing salamo-type ligands or their derivatives [40,41].
Herein, on the basis of our previous studies [42][43][44][45][46], we have studied cooperative formation of a trinuclear Zn II bis(salamo)-like coordination compound, instead of the dinuclear bis(salamo)-like coordination compound reported before [47], via the metalation of bis(salamo)-like ligand H 4 L. IR, UV-Vis titration and X-ray crystallography clearly exhibited that complexation between Zn II acetate dihydrate and H 4 L can form a 3:1 [Zn 3 L] 2+ coordination compound.Meanwhile, the fluorescent and antibacterial properties of H 4 L and its Zn II coordination compound were also studied.
C, H and N analyses were gained using a GmbH VariuoEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany).Elemental analysis for Zn II was measured with an IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany).Melting points were obtained by the use of a microscopic melting point apparatus made by Beijing Taike Instrument Company Limited and were uncorrected.IR spectra (400-4000 cm −1 ) were recorded on a Vertex 70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr pellets.UV-Vis absorption spectra were recorded on a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). 1 H NMR spectra were determined by German Bruker AVANCE DRX-400/600 spectroscopy.Single crystal X-ray structure diffraction for the Zn II coordination compound was carried out a Bruker Smart Apex CCD diffractometer.Fluorescent spectra were recorded on a F-7000 FL spectrophotometer.

Synthesis of the Zn II Coordination Compound
A solution of Zn II acetate dihydrate (6.58 mg, 0.03 mmol) in methanol (1 mL) was added dropwise to a solution of H 4 L (5.88 mg, 0.01 mmol) in dichloromethane (3 mL), the color of the mixture turned to yellow immediately, the proper solvent ratio (methanol:dichloromethane = 1:3) was of utmost importance.After 0.5 h of stirring, the resulting yellow solution was filtered, and then left undisturbed.When the solution was partially evaporated, several yellow block-like single crystals suitable for X-ray crystallography were gained.Yield: 54% (4.92 mg

X-ray Crystallography
The single crystal of the Zn II coordination compound, with approximate dimensions of 0.38 × 0.40 × 0.48 mm was mounted on goniometer head of Bruker Smart 1000 diffractometer equipped with Apex CCD area detector.The diffraction data were collected using a graphite mono-chromated Mo Kα radiation (λ = 0.71073 Å) at 298(2) K.The structure was solved by using the program SHELXS-97 and Fourier difference techniques, and refined by full-matrix least-squares method on F 2 using SHELXL-2017.The structure contained large void, and the solvent and the positive or negative ions located 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 and ions.(Solvent Accessible Volume = 1762.9,Electrons Found in S.A.V. = 104.2).The nonhydrogen atoms were refined anisotropically.Hydrogen atoms were added in geometrical positions.Details of the data collection and refinements of the Zn II coordination compound are given in Table 1.

X-ray Crystallography
The single crystal of the Zn II coordination compound, with approximate dimensions of 0.38 × 0.40 × 0.48 mm was mounted on goniometer head of Bruker Smart 1000 diffractometer equipped with Apex CCD area detector.The diffraction data were collected using a graphite mono-chromated Mo Kα radiation (λ = 0.71073 Å) at 298(2) K.The structure was solved by using the program SHELXS-97 and Fourier difference techniques, and refined by full-matrix least-squares method on F 2 using SHELXL-2017.The structure contained large void, and the solvent and the positive or negative ions located 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 and ions.(Solvent Accessible Volume = 1762.9,Electrons Found in S.A.V. = 104.2).The nonhydrogen atoms were refined anisotropically.Hydrogen atoms were added in geometrical positions.Details of the data collection and refinements of the Zn II coordination compound are given in Table 1.Meanwhile, the free ligand H4L displayed a typical Ar-O stretching frequency at 1265 cm −1 , while the Ar-O stretching frequency of the Zn II coordination compound was observed at 1258 cm −1 .This frequency was shifted to high frequency, which can be an evidence of formation of the Zn II -O bonds between the Zn II atoms and the phenolic oxygen atoms [58][59][60].

UV-Vis Absorption Spectra
UV-Vis absorption spectra of the ligand H4L and its Zn II coordination compound were measured in 5 × 10 −5 mol/L DMF solution (Figure 2).
In the UV-Vis titration experiment of the Zn II coordination compound, the spectroscopic titration clearly showed the reaction stoichiometry ratio to be 3:1.Absorption spectra of the Zn II coordination compound were clearly different from that of H4L upon complexation.The absorption maxima at ca. 278 and 313 nm were shifted bathochromically upon coordination to the Zn II atoms, and a new absorption maxima at ca. 444 nm was absent in the spectrum of the Zn II coordination compound, which should be assigned to LMCT [61][62][63][64][65].Meanwhile, the free ligand H 4 L displayed a typical Ar-O stretching frequency at 1265 cm −1 , while the Ar-O stretching frequency of the Zn II coordination compound was observed at 1258 cm −1 .This frequency was shifted to high frequency, which can be an evidence of formation of the Zn II -O bonds between the Zn II atoms and the phenolic oxygen atoms [58][59][60].

UV-Vis Absorption Spectra
UV-Vis absorption spectra of the ligand H 4 L and its Zn II coordination compound were measured in 5 × 10 −5 mol/L DMF solution (Figure 2).
In the UV-Vis titration experiment of the Zn II coordination compound, the spectroscopic titration clearly showed the reaction stoichiometry ratio to be 3:1.Absorption spectra of the Zn II coordination compound were clearly different from that of H 4 L upon complexation.The absorption maxima at ca. 278 and 313 nm were shifted bathochromically upon coordination to the Zn II atoms, and a new absorption maxima at ca. 444 nm was absent in the spectrum of the Zn II coordination compound, which should be assigned to LMCT [61][62][63][64][65].

Crystal Structure Description
From a 3:1 mixture of Zn II acetate dihydrate and the ligand H4L, a yellow crystalline coordination compound was obtained.X-ray crystallography obviously displayed that the Zn II coordination compound included one completely deprotonated ligand (L) 4-unit, three Zn II atoms, two μ2-acetate ligands, and one coordinated water molecule (Figure 3).
Two of the three Zn II (Zn2 and Zn3) atoms sat in the salamo N2O2 moieties, while Zn1 was located in the central O4 site.Two oxygen (O1 and O2) atoms bridged Zn1-Zn2 and Zn1-Zn3, respectively.In addition, two μ2-acetato ligands linked Zn1 to Zn2 and Zn1 to Zn3 stabilizing the homotrinuclear structure.The central Zn II (Zn1) atom was found to have an aqua molecule.Thus, two of the three Zn II (Zn2 and Zn3) atoms possess pentacoordinate distorted trigonal bipyramidal geometries (τ = 0.8038) in which the axial positions were held by N2-O1 and N4-O2, respectively.Besides, Zn1 atom possesses a strongly distorted square pyramidal (τ = 0.3128) coordination environment where the axial position was held by the O15 atom of the aqua molecule [17,23].Selected bond distances and angles are listed in Table 2.It can be seen from the data that the different positions of the substituents can lead to slightly different changes in the structure [11].

Crystal Structure Description
From a 3:1 mixture of Zn II acetate dihydrate and the ligand H 4 L, a yellow crystalline coordination compound was obtained.X-ray crystallography obviously displayed that the Zn II coordination compound included one completely deprotonated ligand (L) 4− unit, three Zn II atoms, two µ 2 -acetate ligands, and one coordinated water molecule (Figure 3).
Two of the three Zn II (Zn2 and Zn3) atoms sat in the salamo N 2 O 2 moieties, while Zn1 was located in the central O 4 site.Two oxygen (O1 and O2) atoms bridged Zn1-Zn2 and Zn1-Zn3, respectively.In addition, two µ 2 -acetato ligands linked Zn1 to Zn2 and Zn1 to Zn3 stabilizing the homotrinuclear structure.The central Zn II (Zn1) atom was found to have an aqua molecule.Thus, two of the three Zn II (Zn2 and Zn3) atoms possess pentacoordinate distorted trigonal bipyramidal geometries (τ = 0.8038) in which the axial positions were held by N2-O1 and N4-O2, respectively.Besides, Zn1 atom possesses a strongly distorted square pyramidal (τ = 0.3128) coordination environment where the axial position was held by the O15 atom of the aqua molecule [17,23].Selected bond distances and angles are listed in Table 2.It can be seen from the data that the different positions of the substituents can lead to slightly different changes in the structure [11].

Supra-Molecular Interaction
As illustrated in Table 3 and Figure [66][67][68][69][70][71][72].The weak hydrogen bonds existing in the Zn II coordination compound have been described in graph sets (Figure 5) [73].Additionally, the hydrogen bonding scheme of the Zn II coordination compound is defective due to suppression of the electron density originating from solvent molecules (used SQUEEZE) and subsequent exclusion of these solvent molecules from the refinement model.

Supra-Molecular Interaction
As illustrated in Table 3 and Figure

Hirshfeld Surfaces
The Hirshfeld surfaces of the Zn II coordination compound are depicted in Figure 6, exhibiting surfaces that have been mapped over dnorm and di [74,75].The interactions between hydroxyl oxygen in the Zn II coordination compound can be seen as bright red areas in the Hirshfeld surface in

Hirshfeld Surfaces
The Hirshfeld surfaces of the Zn II coordination compound are depicted in Figure 6, exhibiting surfaces that have been mapped over d norm and d i [74,75].The interactions between hydroxyl oxygen in the Zn II coordination compound can be seen as bright red areas in the Hirshfeld surface in Figure 7.

Hirshfeld Surfaces
The Hirshfeld surfaces of the Zn II coordination compound are depicted in Figure 6, exhibiting surfaces that have been mapped over dnorm and di [74,75].The interactions between hydroxyl oxygen in the Zn II coordination compound can be seen as bright red areas in the Hirshfeld surface in

Fluorescent Spectra
The fluorescence titration experiment of the Zn II coordination compound with H4L was studied.Figure 8 shows gradual changes in the fluorescence spectra of H4L upon addition of Zn II ions.The ligand exhibited an intense emission at ca. 525 nm upon excitation at 380 nm based on global maximum determined from three-dimensional fluorescence spectra, which could be attributed to intra-lignd π-π* transition [79][80][81].Figure 8 obviously indicates that fluorescence emission of the ligand H4L was very weak, probably owing to isomerization of C=N double bond, intramolecular hydrogen bond between azomethine and hydroxyl moieties of the aromatic group.Upon incremental addition of Zn II ions to the solution of H4L, fluorescence emission intensity at 523 nm gradually increased, and this peak remained relatively constant after the addition of 3 equiv.

Fluorescent
The fluorescence titration experiment of the Zn II coordination compound with H 4 L was studied.Figure 8 shows gradual changes in the fluorescence spectra of H 4 L upon addition of Zn II ions.The ligand exhibited an intense emission at ca. 525 nm upon excitation at 380 nm based on global maximum determined from three-dimensional fluorescence spectra, which could be attributed to Crystals 2018, 8, 259 9 of 15 intra-lignd π-π* transition [79][80][81].Figure 8 obviously indicates that fluorescence emission of the ligand H 4 L was very weak, probably owing to isomerization of C=N double bond, intramolecular hydrogen bond between azomethine and hydroxyl moieties of the aromatic group.Upon incremental addition of Zn II ions to the solution of H 4 L, fluorescence emission intensity at 523 nm gradually increased, and this peak remained relatively constant after the addition of 3 equiv.
Hirshfeld surface area of the Zn II coordination compound molecule.

Fluorescent Spectra
The fluorescence titration experiment of the Zn II coordination compound with H4L was studied.Figure 8 shows gradual changes in the fluorescence spectra of H4L upon addition of Zn II ions.The ligand exhibited an intense emission at ca. 525 nm upon excitation at 380 nm based on global maximum determined from three-dimensional fluorescence spectra, which could be attributed to intra-lignd π-π* transition [79][80][81].Figure 8 obviously indicates that fluorescence emission of the ligand H4L was very weak, probably owing to isomerization of C=N double bond, intramolecular hydrogen bond between azomethine and hydroxyl moieties of the aromatic group.Upon incremental addition of Zn II ions to the solution of H4L, fluorescence emission intensity at 523 nm gradually increased, and this peak remained relatively constant after the addition of 3 equiv.The Zn II coordination compound showed a strong and broad luminescence with maximum emission at ca. 523 nm upon excitation at 380 nm, which is moved bathochromically to that of H 4 L. Compared with the emission spectrum of H 4 L, enhanced fluorescent intensity of the Zn II coordination compound was observed, displaying that intra-ligand transition has been affected owing to the introduction of the Zn II atoms [82,83].No emissions coming from ligand-to-metal/metal-to-ligand charge-transfer or metal-centered excited states are expected for the Zn II coordination compound, since Zn II is a d 10 ion.Therefore, the emission of the Zn II coordination compound observed is tentatively assigned to the intra-ligand π-π* fluorescence.From the emission intensity by following the modified Benesi-Hidebrand equation, the association constant of compound was calculated as 1.59 × 10 4 M −1 [31-34].

Antimicobial Activities
The antimicrobial properties of H 4 L and its Zn II coordination compound were detected against Escherichia coli as Staphylococcus aureus and Gram-negative bacteria as Gram-positive bacteria via a punch method.The bacterial suspension was mixed in sterile LB (lysogeny broth agar) plates (2% agar), then made four holes with a hole punch, last added DMF, Zn 2+ , H 4 L, and the Zn II coordination compound into every holes.After 7 h of incubation at 37 • C, the growth-inhibitory effect was monitored and diameters of the inhibition zones were measured.The discs measuring 5 mm in diameter were dissolved in DMF.The diameters of inhibition zones of H 4 L and its Zn II coordination compound are given in Figure 9, the Zn II coordination compound proved more enhanced antimicrobial activities than the bis(salamo)-like tetraoxime H 4 L under the same concentrations.agar), then made four holes with a hole punch, last added DMF, Zn 2+ , H4L, and the Zn II coordination compound into every holes.After 7 h of incubation at 37 °C, the growth-inhibitory effect was monitored and diameters of the inhibition zones were measured.The discs measuring 5 mm in diameter were dissolved in DMF.The diameters of inhibition zones of H4L and its Zn II coordination compound are given in Figure 9, the Zn II coordination compound proved more enhanced antimicrobial activities than the bis(salamo)-like tetraoxime H4L under the same concentrations.As shown in Figure 9, the inhibitory effect of the Zn II coordination compound at different concentrations was studied, the results showed that the antibacterial effect of the Zn II coordination compound increased with increasing concentrations.The increase in the antibacterial activity of the Zn II coordination compound with increase in concentration can be explained according to the As shown in Figure 9, the inhibitory effect of the Zn II coordination compound at different concentrations was studied, the results showed that the antibacterial effect of the Zn II coordination compound increased with increasing concentrations.The increase in the antibacterial activity of the Zn II coordination compound with increase in concentration can be explained according to the chelation theory.Chelation reduces the polarity of the metal atom mainly owing to partial sharing of positive charge of Zn II atom with donor groups and possible delocalization of π-electron within the whole chelate ring.Further, it enhances the lipophilic character of the central atom.These results are similar to earlier reports of biological activities of similar salamo-like Co II coordination compounds [84].

Conclusions
A newly designed symmetric bis(salamo)-like chelating tetraoxime ligand H 4 L, possessing a C-shaped O 4 site besides the two N 2 O 2 sites, has been synthesized, and its Zn II coordination compound [LZn 3 (OAc) 2 (H 2 O)] has been determined by X-ray crystallography.The UV-Vis titration experiment clearly showed the reaction stoichiometry ratio to be 3:1.In the Zn II coordination compound, Zn1 is pentacoordinate with a strongly distorted square pyramidal geometry, while Zn2 and Zn3 possess pentacoordinates with distorted trigonal bipyramidal geometries.Furthermore, the Hirshfeld surface analysis indicated that the Zn II coordination compound could be stable due to intramolecular hydrogen bonds and some weaker interactions.Fluorescence behaviors of H 4 L and its Zn II coordination compound were investigated, compared with the ligand H 4 L, the emission intensity of the Zn II coordination compound increased obviously, which indicated that the Zn II ions possess a quality of fluorescent enhancement.Antimicrobial experiments showed that the Zn II coordination compound demonstrated more enhanced antimicrobial activities than H 4 L under the same conditions.

Figure 1 .
Figure 1.IR spectra of the ligand H4L and its Zn II coordination compound.

Figure 1 .
Figure 1.IR spectra of the ligand H 4 L and its Zn II coordination compound.

Figure 3 .
Figure 3. (a) Representation of the Zn II coordination compound structure; (b) The coordination polyhedra of Zn1 and Zn2 centers.

Figure 3 .
Figure 3. (a) Representation of the Zn II coordination compound structure; (b) The coordination polyhedra of Zn1 and Zn2 centers.
4, there were four pairs of intra-molecular O15-H15B•••O9, O15-H15B•••O5, C10-H10A•••O12 and C20-H20B•••O14 hydrogen bond interactions and a pair of C-H•••π inter-molecular hydrogen bonds in the Zn II coordination compound.The Zn II coordination compound molecules were inter-linked effectively via C-H•••π hydrogen bonds (C19-H19B•••Cg1) into a 1D supermolecular structure.Furthermore, one molecule could link four adjacent molecules into an infinite 3D net-like supramolecular structure by two pairs of intermolecular Cg1•••Cg1 and Cg3•••Cg3 interactions [66-72].The weak hydrogen bonds existing in the Zn II coordination compound have been described in graph sets (Figure 5) [73].Additionally, the hydrogen bonding scheme of the Zn II coordination compound is defective due to suppression of the electron density originating from solvent molecules (used SQUEEZE) and subsequent exclusion of these solvent molecules from the refinement model.Crystals 2018, 8, x FOR PEER REVIEW 7 of 15

Figure 7 .
The light red spots are owing to C-H•••O interactions, and other visible spots correspond to C•••H and H•••H contacts on the surface.
Figure 7  shows the 2D plots generated[76][77][78] which correspond to the C•••H, O•••H and H•••H interactions from the Hirshfeld surface of the Zn II coordination compound.To provide context, the overview of the full fingerprint is depicted in grey and the blue area showing the separate contact.The proportions of O•••H/H•••O, C•••H/H•••C and H•••H interactions are composed of 22.3, 12.1 and 49.7% of the all Hirshfed surfaces for each Zn II coordination compound molecule, respectively.It is because of the existence of these hydrogen bondings that the Zn II coordination compound can be stable.

Figure 5 .
Figure 5. (a) Graph set assignments for the Zn II coordination compound; (b) Partial enlarged drawing of hydrogen bonds.
spots are owing to C-H•••O interactions, and other visible spots correspond to C•••H and H•••H contacts on the surface.Figure 7 shows the 2D plots generated [76-78] which correspond to the C•••H, O•••H and H•••H interactions from the Hirshfeld surface of the Zn II coordination compound.To provide context, the overview of the full fingerprint is depicted in grey and the blue area showing the separate contact.The proportions of O•••H/H•••O, C•••H/H•••C and H•••H interactions are composed of 22.3, 12.1 and 49.7% of the all Hirshfed surfaces for each Zn II coordination compound molecule, respectively.It is because of the existence of these hydrogen bondings that the Zn II coordination compound can be stable.

Figure 7 .
The light red spots are owing to C-H•••O interactions, and other visible spots correspond to C•••H and H•••H contacts on the surface.Figure 7 shows the 2D plots generated [76-78] which correspond to the C•••H, O•••H and H•••H interactions from the Hirshfeld surface of the Zn II coordination compound.To provide context, the overview of the full fingerprint is depicted in grey and the blue area showing the separate contact.The proportions of O•••H/H•••O, C•••H/H•••C and H•••H interactions are composed of 22.3, 12.1 and 49.7% of the all Hirshfed surfaces for each Zn II coordination compound molecule, respectively.It is because of the existence of these hydrogen bondings that the Zn II coordination compound can be stable.

Figure 6 .
Figure 6.Hirshfeld surfaces mapped with (a) dnorm and (b) di of the Zn II coordination compound.The surfaces are depicted as transparent to allow visualization of the molecule structure.

Figure 6 .
Figure 6.Hirshfeld surfaces mapped with (a) d norm and (b) d i of the Zn II coordination compound.The surfaces are depicted as transparent to allow visualization of the molecule structure.Crystals 2018, 8, x FOR PEER REVIEW 9 of 15

Figure 7 .
Figure 7. Fingerprint plot of the Zn II coordination compound: full and resolved into full and resolved into O•••H, C•••H and H•••H contacts exhibiting the percentages of contacts contributed to the total Hirshfeld surface area of the Zn II coordination compound molecule.

Figure 7 .
Figure 7. Fingerprint plot of the Zn II coordination compound: full and resolved into full and resolved into O•••H, C•••H and H•••H contacts exhibiting the percentages of contacts contributed to the total Hirshfeld surface area of the Zn II coordination compound molecule.

Figure 9 .
Figure 9. (a) The diameters of inhibition zones of E. coli and S. aureus at different concentrations; (b) the diameter of inhibition zones of E. coli and S. aureus in different concentrations.

Figure 9 .
Figure 9. (a) The diameters of inhibition zones of E. coli and S. aureus at different concentrations; (b) the diameter of inhibition zones of E. coli and S. aureus in different concentrations.

Table 1 .
Crystallographic data and collection parameters for the Zn II coordination compound.

Table 1 .
Crystallographic data and collection parameters for the Zn II coordination compound.