A Self-Assembled Hetero-Bimetallic [Ni(II)-Sm(III)] Coordination Polymer Constructed from a Salamo-Like Ligand and 4,4 (cid:48) -Bipyridine: Synthesis, Structural Characterization, and Properties

: An unusual self-assembled hetero-bimetallic [Ni(II)-Sm(III)] coordination polymer, [Ni(L)Sm(NO 3 ) 3 (4,4 (cid:48) -bipy)] n , is prepared through a hexadentate chelating ligand 2,2 (cid:48) -[1,2-ethylenedioxybis(nitrilomethylidyne)]diphenol (H 2 L). The Ni(II)-Sm(III) coordination polymer is validated through elemental analyses, Fourier-transform infrared and UV-Visible spectroscopies, and X-ray single-crystal di ﬀ raction. The Ni(II) atom forms a twisted six-coordinated octahedron, and the Sm(III) atom is ten-coordinated, adopting a twisted bicapped square antiprism. An inﬁnite three-dimensional-layer supramolecular structure is obtained through extensive π ··· π stacking and intermolecular hydrogen bonding interactions. The polymer has a good antibacterial e ﬀ ect against Staphylococcus aureus. to death. These results are similar to biological activities of related Schi ﬀ base complexes previously reported


Crystal Structure Determination of the Ni(II)-Sm(III) Polymer
The crystal structure was obtained by X-ray single-crystal diffraction on a Bruker APEX-II CCD diffractometer ( Table 1). The X-ray single-crystal diffraction data of the Ni(II)-Sm(III) polymer were recorded and collected by using a Bruker APEX-II CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å), and corrected via the Lorentz and polarization factor with a multi-scan method. The program SHELXS-2018 and Fourier difference techniques were used to solve the structure. It was corrected via the full-matrix least-squares on F 2 . The structure included a large void, and the positive or negative ions and the solvent water molecules in the void could not be confirmed, owing to it being highly disordered. Thus, SQUEEZE in the PLATON program was used to remove the highly disordered ions and solvent. (Solvent Accessible Volume = 1275, Electrons Found in S.A.V. = 133). Anisotropic displacement parameters were applied for the non-hydrogen atoms and isotropic parameters for the hydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model.

Crystal Structure Determination of the Ni(II)-Sm(III) Polymer
The crystal structure was obtained by X-ray single-crystal diffraction on a Bruker APEX-II CCD diffractometer ( Table 1). The X-ray single-crystal diffraction data of the Ni(II)-Sm(III) polymer were recorded and collected by using a Bruker APEX-II CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å), and corrected via the Lorentz and polarization factor with a multi-scan method. The program Shelxs-2018 and Fourier difference techniques were used to solve the structure. It was corrected via the full-matrix least-squares on F 2 . The structure included a large void, and the positive or negative ions and the solvent water molecules in the void could not be confirmed, owing to it being highly disordered. Thus, SQUEEZE in the PLATON program was used to remove the highly disordered ions and solvent. (Solvent Accessible Volume = 1275, Electrons Found in S.A.V. = 133). Anisotropic displacement parameters were applied for the non-hydrogen atoms and isotropic parameters for the hydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model.
CCDC 1939692 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

Results and Discussion
H 2 L and the title polymer have been gained and validated by IR spectra, UV/Vis absorption spectra, and X-ray single-crystal diffraction and fluorescence spectra. Their antimicrobial activities were investigated. The compounds were stable in air.

PXRD Analysis
A PXRD test was conducted for the title polymer to demonstrate whether the structure is truly representative. The PXRD result of the title polymer is given in Figure 1. Through detailed comparison, the experimental result is coincident with the simulation result. Therefore, the synthesized samples have high purity and can be used for further study of spectral characterization and fluorescence properties. A PXRD test was conducted for the title polymer to demonstrate whether the structure is truly representative. The PXRD result of the title polymer is given in Figure 1. Through detailed comparison, the experimental result is coincident with the simulation result. Therefore, the synthesized samples have high purity and can be used for further study of spectral characterization and fluorescence properties.

FT-IR Spectra of H2L and the Title Polymer
The FT-IR spectra of H2L and the title polymer are depicted in Figure 2. The band of H2L appearing at 3437 cm -1 is attributed to the O-H stretching band [47]. The characteristic C=N stretching band is found at approximately 1606 cm -1 in the spectrum of H2L, and moves to a lower frequency in the polymer, indicating the metal coordination at the nitrogen atoms of oxime groups [15,50]. The Ar-O stretching vibration band of H2L emerges as an intensive band at 1253 cm −1 as reported for analogous salen-based compounds [6,28,43], while the title polymer shows this band at 1210 cm −1 , again due to complexation.

FT-IR Spectra of H 2 L and the Title Polymer
The FT-IR spectra of H 2 L and the title polymer are depicted in Figure 2. A PXRD test was conducted for the title polymer to demonstrate whether the structure is truly representative. The PXRD result of the title polymer is given in Figure 1. Through detailed comparison, the experimental result is coincident with the simulation result. Therefore, the synthesized samples have high purity and can be used for further study of spectral characterization and fluorescence properties.

FT-IR Spectra of H2L and the Title Polymer
The FT-IR spectra of H2L and the title polymer are depicted in Figure 2. The band of H2L appearing at 3437 cm -1 is attributed to the O-H stretching band [47]. The characteristic C=N stretching band is found at approximately 1606 cm -1 in the spectrum of H2L, and moves to a lower frequency in the polymer, indicating the metal coordination at the nitrogen atoms of oxime groups [15,50]. The Ar-O stretching vibration band of H2L emerges as an intensive band at 1253 cm −1 as reported for analogous salen-based compounds [6,28,43], while the title polymer shows this band at 1210 cm −1 , again due to complexation. The band of H 2 L appearing at 3437 cm −1 is attributed to the O-H stretching band [47]. The characteristic C=N stretching band is found at approximately 1606 cm −1 in the spectrum of H 2 L, and moves to a lower frequency in the polymer, indicating the metal coordination at the nitrogen atoms of oxime groups [15,50]. The Ar-O stretching vibration band of H 2 L emerges as an intensive band at 1253 cm −1 as reported for analogous salen-based compounds [6,28,43], while the title polymer shows this band at 1210 cm −1 , again due to complexation.

Description of the Molecular Structure of the Title Polymer
The structure of the heterobimetallic [Ni(II)-Sm(III)] salamo-based coordination polymer is depicted in Figure 3, complemented by data in Table 2.
Crystals 2020, 10, x FOR PEER REVIEW 6 of 16 The structure of the heterobimetallic [Ni(II)-Sm(III)] salamo-based coordination polymer is depicted in Figure 3, complemented by data in Table 2.   The title polymer crystallizes in the monoclinic space group P2 1 /c with Z = 4, consisting of one Sm(III) atom, one Ni(II) atom, one deprotonated (L) 2− unit, one 4,4 -bipyridine ligand, three bidentate chelating nitrate anions, and three water molecules.

Molar Conductance and Solubility and Mass Spectrometry Analysis of the Title Polymer
The [Ni(II)-Sm(III)] coordination polymer could be soluble in DMSO and DMF, slightly soluble in ethanol, methanol, THF, dichloromethane, trichloromethane, acetonitrile, and acetone, and insoluble in n-hexane, ethyl ether, and water. Molar conductance of the title polymer in DMF at 25 °C (1 × 10 −3 mol·L −1 ) is 179.6 Ω −1 ·cm 2 ·mol −1 . The result of molar conductivity is inconsistent with the 1:2 electrolyte reported previously [50].
According to the data analysis of mass spectrometry ( Figure S1

UV/Vis Absorption Spectra of H2L and the Title Polymer
The UV/Vis absorption spectra of H2L and the title polymer (in 10 -5 M ethanol solution) are depicted in the Figure 10.

Molar Conductance and Solubility and Mass Spectrometry Analysis of the Title Polymer
The [Ni(II)-Sm(III)] coordination polymer could be soluble in DMSO and DMF, slightly soluble in ethanol, methanol, THF, dichloromethane, trichloromethane, acetonitrile, and acetone, and insoluble in n-hexane, ethyl ether, and water. Molar conductance of the title polymer in DMF at 25 • C (1 × 10 −3 mol·L −1 ) is 179.6 Ω −1 ·cm 2 ·mol −1 . The result of molar conductivity is inconsistent with the 1:2 electrolyte reported previously [50].
According to the data analysis of mass spectrometry ( Figure S1

UV/Vis Absorption Spectra of H 2 L and the Title Polymer
The UV/Vis absorption spectra of H 2 L and the title polymer (in 10 −5 m ethanol solution) are depicted in the Figure 10. The characteristic peaks of H2L appear at approximately 317, 271, and 223 nm, respectively, the peaks at 271 and 223 nm could be ascribed to the π-π* transition of the benzene ring, and the peak at 317 nm could be attributed to the intraligand π-π* transition of the oxime-like group [20,47]. Compared to the peaks of H2L, the absorptions of the polymer bathochromically move to approximately 228 and 276 nm, exhibiting coordination of the (L) 2− unit to the Ni(II) atom [15,50]. Upon coordination, the intra-ligand π-π* transition of the oxime-like group disappeared again, indicating the complexation [47,50]. A new characteristic peak at approximately 350 nm is found for the coordination polymer, belonging to an n-π* charge transfer of the imino group [43,50].
In order to further prove that the polymer in the form of a solution is not affected by metal ions, 4,4′-bipy, we conducted UV spectrum experiments on its solution (Ni(OAc)2⋅4H2O, Sm(NO3)3⋅6H2O, and 4,4′-bipy in 10 -5 M EtOH). There was almost no change in the absorbance value of metal ions and no characteristic absorption peak. Further, 4,4′-bipy showed a characteristic absorption peak at 239 nm.

Fluorescence Properties of the Title Polymer
The fluorescence spectra of H2L and the [Ni(II)-Sm(III)] polymer in EtOH (1.0 × 10 −5 M) are shown in Figure 11. The results show that H2L presents an intensive photoluminescence with maximum emission at approximately 395 nm at 311 nm excitation, which can be explained as the π-π* transition in H2L. In the title polymer, the emission peak appears at approximately 382 nm, and there is a slight blue shift compared to H2L [50], the fluorescence intensity of the title polymer appears to be quenched, and at the same time, it can be seen from the data of the mass spectrum that the polymer solution is almost in the form of [Ni(L)] + 4,4′-bipy + [Sm(NO3)(DMF)3], indicating that the coordinated Ni(II) ion has a heavy atom effect [62]. The high nuclear charge of the Ni(II) causes the electronic energy levels of the phosphorescent molecules to be staggered, which enhances the spin-orbit coupling of the phosphorescent molecules, thereby increasing the probability of inter-system hopping (ISC) of S1→Tl, thereby quenching the fluorescence. In the fluorescence spectra, only the band at approximately 375-650 nm instead of the f-f emission is expected for Sm(III) ions. The characteristic peaks of H 2 L appear at approximately 317, 271, and 223 nm, respectively, the peaks at 271 and 223 nm could be ascribed to the π-π* transition of the benzene ring, and the peak at 317 nm could be attributed to the intraligand π-π* transition of the oxime-like group [20,47]. Compared to the peaks of H 2 L, the absorptions of the polymer bathochromically move to approximately 228 and 276 nm, exhibiting coordination of the (L) 2− unit to the Ni(II) atom [15,50]. Upon coordination, the intra-ligand π-π* transition of the oxime-like group disappeared again, indicating the complexation [47,50]. A new characteristic peak at approximately 350 nm is found for the coordination polymer, belonging to an n-π* charge transfer of the imino group [43,50].
In order to further prove that the polymer in the form of a solution is not affected by metal ions, 4,4 -bipy, we conducted UV spectrum experiments on its solution (Ni(OAc) 2 ·4H 2 O, Sm(NO 3 ) 3 ·6H 2 O, and 4,4 -bipy in 10 −5 m EtOH). There was almost no change in the absorbance value of metal ions and no characteristic absorption peak. Further, 4,4 -bipy showed a characteristic absorption peak at 239 nm.

Fluorescence Properties of the Title Polymer
The fluorescence spectra of H 2 L and the [Ni(II)-Sm(III)] polymer in EtOH (1.0 × 10 −5 m) are shown in Figure 11. The results show that H 2 L presents an intensive photoluminescence with maximum emission at approximately 395 nm at 311 nm excitation, which can be explained as the π-π* transition in H 2 L. In the title polymer, the emission peak appears at approximately 382 nm, and there is a slight blue shift compared to H 2 L [50], the fluorescence intensity of the title polymer appears to be quenched, and at the same time, it can be seen from the data of the mass spectrum that the polymer solution is almost in the form of [Ni(L)] + 4,4 -bipy + [Sm(NO 3 )(DMF) 3 ], indicating that the coordinated Ni(II) ion has a heavy atom effect [62]. The high nuclear charge of the Ni(II) causes the electronic energy levels of the phosphorescent molecules to be staggered, which enhances the spin-orbit coupling of the phosphorescent molecules, thereby increasing the probability of inter-system hopping (ISC) of S1→Tl, thereby quenching the fluorescence. In the fluorescence spectra, only the band at approximately 375-650 nm instead of the f-f emission is expected for Sm(III) ions. Crystals 2020, 10, x FOR PEER REVIEW 12 of 16

Antimicrobial Activity
Firstly, the Staphylococcus aureus on the plate was inoculated into Agar (2%) (LB) medium for overnight culture, and 0.1 mL of night-cultured fresh bacterial suspension was added to the LB medium after autoclaving and cooling to 50 °C. Secondly, after the mixture was mixed well, LB solid AGAR was poured into a sterile Petri dish. After being completely solidified, we punched holes in the LB medium with a perforator. Finally, DMF sample solutions were configurated with solution gradients of four various concentrations (0.35, 0.7, 1.4, and 2.8 mg mL -1 ). Then, 70 µL of samples with different concentrations were added to the samples with a pipette gun. After an eight-hour incubation period at 37 °C, diameters of the inhibition zones were measured. The experimental results are compared to Ampicillin as a reference standard with various concentrations. The diameters of the inhibition zones of H2L, nickel acetate, and the title polymer are shown in Figure 12. The title polymer shows a more enhanced antibacterial effect than H2L and the metal salt solution under the same conditions. At the same time, the complex [NiL] also has a certain antibacterial activity, but it is not as high as that of the coordination polymer, which may be due to the antibacterial activity of the dissolved polymer being the sum of the comprehensive activities of various components. It can be conjectured from the experimental results and previous literature [54], first of all, that this is the result of the destruction of the protein structure in bacteria due to the action

Antimicrobial Activity
Firstly, the Staphylococcus aureus on the plate was inoculated into Agar (2%) (LB) medium for overnight culture, and 0.1 mL of night-cultured fresh bacterial suspension was added to the LB medium after autoclaving and cooling to 50 • C. Secondly, after the mixture was mixed well, LB solid AGAR was poured into a sterile Petri dish. After being completely solidified, we punched holes in the LB medium with a perforator. Finally, DMF sample solutions were configurated with solution gradients of four various concentrations (0.35, 0.7, 1.4, and 2.8 mg mL −1 ).
Then, 70 µL of samples with different concentrations were added to the samples with a pipette gun. After an eight-hour incubation period at 37 • C, diameters of the inhibition zones were measured. The experimental results are compared to Ampicillin as a reference standard with various concentrations. The diameters of the inhibition zones of H 2 L, nickel acetate, and the title polymer are shown in Figure 12. The title polymer shows a more enhanced antibacterial effect than H 2 L and the metal salt solution under the same conditions. At the same time, the complex [NiL] also has a certain antibacterial activity, but it is not as high as that of the coordination polymer, which may be due to the antibacterial activity of the dissolved polymer being the sum of the comprehensive activities of various components. It can be conjectured from the experimental results and previous literature [54], first of all, that this is the result of the destruction of the protein structure in bacteria due to the action of heavy metal ions in coordination polymers [32]. Secondly, the ligand H 2 L destroys part of the cell membrane, making the bacteria unable to further divide and reproduce, leading to death. These results are similar to biological activities of related Schiff base complexes previously reported [65].
Firstly, the Staphylococcus aureus on the plate was inoculated into Agar (2%) (LB) medium for overnight culture, and 0.1 mL of night-cultured fresh bacterial suspension was added to the LB medium after autoclaving and cooling to 50 °C. Secondly, after the mixture was mixed well, LB solid AGAR was poured into a sterile Petri dish. After being completely solidified, we punched holes in the LB medium with a perforator. Finally, DMF sample solutions were configurated with solution gradients of four various concentrations (0.35, 0.7, 1.4, and 2.8 mg mL -1 ). Then, 70 µL of samples with different concentrations were added to the samples with a pipette gun. After an eight-hour incubation period at 37 °C, diameters of the inhibition zones were measured. The experimental results are compared to Ampicillin as a reference standard with various concentrations. The diameters of the inhibition zones of H2L, nickel acetate, and the title polymer are shown in Figure 12. The title polymer shows a more enhanced antibacterial effect than H2L and the metal salt solution under the same conditions. At the same time, the complex [NiL] also has a certain antibacterial activity, but it is not as high as that of the coordination polymer, which may be due to the antibacterial activity of the dissolved polymer being the sum of the comprehensive activities of various components. It can be conjectured from the experimental results and previous literature [54], first of all, that this is the result of the destruction of the protein structure in bacteria due to the action

Conclusions
A new heterobimetallic Ni(II)-Sm(III) polymer has been prepared and structurally validated, in which H 2 L represents a symmetric salamo-based bisoxime ligand. In the Ni(II)-Sm(III) polymer, the hexacoordinated nickel(II) atom bears a slightly twisted octahedral geometry, and the decacoordinated Sm(III) atom possesses a twisted bicapped square antiprismatic arrangement. In the crystal, the polymer forms a self-assembling infinite 2D network further linking into a 3D supramolecular structure by C-H···π, π···π, and hydrogen-bond interactions. Compared to H 2 L, the Ni(II)-Sm(III) polymer exhibits only a very slightly hypsochromically shifted fluorescence of lower intensity, indicating that the coordinated Ni(II) cation has a minor heavy atom effect. Antimicrobial experiments have shown that the title polymer demonstrates stronger antimicrobial activity than H 2 L under the same conditions.