Unprecedented Hexanuclear Cobalt ( II ) Nonsymmetrical Salamo-Based Coordination Compound : Synthesis , Crystal Structure , and Photophysical Properties

A novel hexanuclear Co(II) coordination compound with a nonsymmetrical Salamo-type bisoxime ligandH4L, namely [{Co3(HL)(MeO)(MeOH)2(OAc)2}2]·2MeOH, was prepared and characterized by elemental analyses, UV–vis, IR and fluorescence spectra, and X-ray single-crystal diffraction analysis. Each Co(II) is hexacoordinated, and possesses a distorted CoO6 or CoO4N2 octahedrons. The Co(II) coordination compound possesses a self-assembled infinite 2D supramolecular structure with the help of the intermolecular C–H···O interactions. Meanwhile, the photophysical properties of the Co(II) coordination compound were studied.

The purpose of the present work is the structural characterization of polynuclear Co(II) coordination compounds derived of nonsymmetrical Salamo-type bisoxime ligands.Here, the ligand H4L and its corresponding hexanuclear Co(II) coordination compound ([{Co3(HL)(MeO)(MeOH)2(OAc)2}2]•2MeOH) was gained.In addition, the supramolecular buildings and photophysical behaviors of the Co(II) coordination compound are discussed.

Materials and Physical Measurements
All chemicals were of analytical reagent grade.Elemental analyses for C, H, and N were gained using a GmbH VarioEL V3.00 automatic elemental analysis instrument (Berlin, Germany), elemental analysis for Co was detected by an IRIS ER/S•WP-1 ICP atomic emission spectrometer (Berlin, Germany).UV-vis and fluorescence spectra were measured on a Shimadzu UV-2550 spectrometer (Shimadzu, Japan) and F-7000 FL spectrometer (Hitachi, Tokyo, Japan), respectively.Infrared (IR) spectra were performed on a VERTEX-70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr (500-4000 cm −1 ). 1 H-NMR spectra were carried out via German Bruker AVANCE DRX-400 spectroscopy (Bruker AVANCE, Billerica, MA, USA).Single-crystal X-ray structure determination was performed on a SuperNova Dual (Cu at zero) Eos four-circle diffractometer.Melting points were measured via a microscopic melting point apparatus (Beijing Taike Instrument Limited Company, Beijing, China).

X-ray Structure Determination of the Co(II) Coordination Compound
X-ray diffraction data were collected on a SuperNova Dual (Cu at zero) Eos four-circle diffractometer via graphite monochromatized Mo-K α radiation (λ = 0.71073 Å) at 298(2) K. Unit cell parameters were determined by least squares analysis.The LP factor and semi-empirical absorption corrections were applied to the intensity data.The structure was solved by the direct method (SHELXS-97), and all hydrogen atoms were added theoretically.All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F 2 with SHELXL-97.Anisotropic thermal parameters were assigned to all non-hydrogen atoms.The hydrogen atoms were generated geometrically.Some reflections with high intensities, which made the detector overflow, were automatically omitted by the diffractometer.So some reflections were missing.The crystallographic and structural refinement data for the Co(II) coordination compound are summed in Table 1.Supplementary crystallographic data for this paper have been deposited at the Cambridge Crystallographic Data Centre (1519431) and can be obtained free of charge viawww.ccdc.cam.ac.uk/conts/retrieving.html.

Description of the Crystal Structure
Asdepicted in Figure 1, the centrosymmetric neutral homomultinuclear Co(II) coordination compound has been revealedby single crystal X-raydiffraction (Table 2).It crystallizes in the monoclinic crystal system, space group C2/c, and consists of six Co(II) atoms, two (HL) 3-units, two µ 2 -acetato ions, two bidentate chelating acetate ions, two coordinated deprotonated methanol molecules, four coordinated methanol molecules, and two crystallizing methanol molecules.

IR Spectroscopy
IR spectra (Table 4) of H 4 L and its corresponding Co(II) coordination compound exhibit different bands in the region of 400-4000 cm −1 .H 4 L shows a characteristic C=N stretching band at 1630 cm −1 , while the C=N stretching band of the Co(II) coordination compound appears at 1592cm −1 [64].For the ligand H 4 L, the Ar-O stretching band appears at 1260 cm −1 , which is observed at 1255 cm −1 for the Co(II) coordination compound.The characteristic C=N and Ar-O stretching frequencies are shifted to lower frequencies, exhibiting that the Co-N and Co-O bonds are formed [69,71].The O-H stretching frequency of H 4 L appears at 3373 cm −1 , whereas the Co(II) coordination compound shows a stretching band at 3421 cm −1 , which is attributed to vibrations of the coordinated methanol molecules.For the Co(II) coordination compound, the ν (Co-O) and ν (Co-N) frequencies are observed at 463 and 519 cm −1 , respectively [74,81].

UV-Vis Spectroscopy
The UV-vis spectra of H 4 L and its Co(II) coordination compound were measured in 1×10 −6 mol•L −1 CH 2 Cl 2 solution.It is noteworthy that the absorption peaks of the Co(II) coordination compound are evidently different from those of H 4 L (Figure 6).Electronic absorption spectrum of H 4 L composes of two relatively intense peaks centered at 275 and 299 nm, attributed to the intra-ligand π-π* transitions of the benzene rings and the C=N bonds, respectively.Compared with H 4 L, the absorption peaks of the Co(II) coordination compound appears at 277 and 311 nm, which are bathochromically shifted by ca. 2 and 12 nm, exhibiting the Co(II) ions have coordinated with H 4 L. The new peak of the Co(II) coordination compound appears at ca. 385 nm, attributed to L→M charge-transfer transition [82,83].
Crystals 2018, 8, 144 9 of 15 H4L composes of two relatively intense peaks centered at 275 and 299 nm, attributed to the intraligand π-π* transitions of the benzene rings and the C=N bonds, respectively.Compared with H4L, the absorption peaks of the Co(II) coordination compound appears at 277 and 311 nm, which are bathochromically shifted by ca. 2 and 12 nm, exhibiting the Co(II) ions have coordinated with H4L.
new peak of the Co(II) coordination compound appears at ca. 385 nm, attributed to L→M chargetransfer transition [82,83].

Fluorescence Properties
The fluorescence properties of H 4 L and its corresponding Co(II) coordination compound were studied are depicted in Figure 7. H 4 L displays strong emission peak at ca. 412 nm upon excitation at 271 nm, and it should be attributed to the intra-ligand π-π* transition.The Co(II) coordination compound displays lower photoluminescence with maximum emission at ca. 360 nm.Compared with H 4 L, emission intensity of the Co(II) coordination compound evidently reduces, showing that the Co(II) ions possess a certain degree of fluorescence quenching, which makes the conjugated system larger, and also indicates it may be a purple device.The solid-state fluorescence spectra of the ligand H 4 L and its Co(II) coordination compound are depicted in Figure 8.Compared to liquid fluorescence spectroscopy, the ligand H 4 L and its corresponding Co(II) coordination compound have strong fluorescence in solid-state fluorescence spectroscopy.

Fluorescence Properties
The fluorescence properties of H4L and its corresponding Co(II) coordination compound were studied are depicted in Figure 7. H4L displays strong emission peak at ca. 412 nm upon excitation at 271 nm, and it should be attributed to the intra-ligand π-π* transition.The Co(II) coordination compound displays lower photoluminescence with maximum emission at ca. 360 nm.Compared with H4L, emission intensity of the Co(II) coordination compound evidently reduces, showing that the Co(II) ions possess a certain degree of fluorescence quenching, which makes the conjugated system larger, and also indicates it may be a purple device.The solid-state fluorescence spectra of the ligand H4L and its Co(II) coordination compound are depicted in Figure 8.Compared to liquid fluorescence spectroscopy, the ligand H4L and its corresponding Co(II) coordination compound have strong fluorescence in solid-state fluorescence spectroscopy.

Figure 1 .
Figure 1.(a) Crystal structure and atom numberings of the Co(II) coordination compound; (b) Coordination polyhedrons for Co(II) atoms.Figure 1.(a) Crystal structure and atom numberings of the Co(II) coordination compound; (b) Coordination polyhedrons for Co(II) atoms.

Figure 1 .
Figure 1.(a) Crystal structure and atom numberings of the Co(II) coordination compound; (b) Coordination polyhedrons for Co(II) atoms.Figure 1.(a) Crystal structure and atom numberings of the Co(II) coordination compound; (b) Coordination polyhedrons for Co(II) atoms.

Figure 2 .
Figure 2. View of the intramolecular (a) and intermolecular (b) interactions of the Co(II) coordination compound.

Figure 2 . 14 Figure 2 .
Figure 2. View of the intramolecular (a) and intermolecular (b) interactions of the Co(II) coordination compound.

Figure 4 .
Figure 4. Graph set assignments for the Co(II) coordination compound.

Figure 5 .
Figure 5.View of the 2D layered structure of the Co(II) coordination compound.

Figure 4 .
Figure 4. Graph set assignments for the Co(II) coordination compound.

Figure 4 .
Figure 4. Graph set assignments for the Co(II) coordination compound.

Figure 5 .
Figure 5.View of the 2D layered structure of the Co(II) coordination compound.Figure 5. View of the 2D layered structure of the Co(II) coordination compound.

Figure 5 .
Figure 5.View of the 2D layered structure of the Co(II) coordination compound.Figure 5. View of the 2D layered structure of the Co(II) coordination compound.

Figure 8 .Figure 8 .
Figure 8. Fluorescence spectra of the ligand H4L and its Co(II) coordination compound in the solid state.
2 O 2 ligand, and obtained an unprecedented hexanuclear Co(II) coordination compound, [{Co 3 (HL)(MeO)(MeOH) 2 (OAc) 2 } 2 ]•2MeOH.X-ray crystal structure analysis of the Co(II) coordination compound revealed that each Co(II) is hexacoordinated, and possesses distorted CoO 6 or CoO 4 N 2 octahedrons.The Co(II) coordination compound possesses a 2D layered structure through intermolecular C-H•••O interactions.In addition, the fluorescence properties indicate that coordinated Co(II) ions resulted to the fluorescence quenching of H 4 L.

Table 1 .
Crystallographic and structural refinement data for the Co(II) coordination compound.

Table 4 .
Major IR bands for H 4 L and its Co(II) coordination compound(cm −1 ).