Synthesis and Fluorescence Properties of a New Heterotrinuclear Co(II)-Ce(III)Complex Constructed from a bis(salamo)-Type Tetraoxime Ligand

[Co2(L)Ce(OAc)3(CH3CH2OH)]·1.5CH3OH∙0.5CH2Cl2, a heterotrinuclear Co(II)-Ce(III) bis(salamo)-type complex with a symmetric bi(salamo)-type ligand H4L and an acyclic naphthalenediol moiety, was designed, synthesized and characterized by elemental analyses, FT-IR, UV-Vis and fluorescence spectroscopy and X-ray crystallography. The X-ray crystallographic investigation revealed the heterotrinuclear complex consisted of two Co(II) atoms, one Ce(III) atom, one (L)4‒ unit, three μ2-acetate ions, one coordinated ethanol molecule, one and half crystallization methanol molecule and half crystallization dichloromethane molecule. Two Co(II) atoms located in the N2O2 coordination spheres, are both hexacoordinated, with slightly distorted octahedral geometries. The Ce(III) atom is nine-coordinated and located in the O6 cavity possesses a single square antiprismatic geometry. In addition, supramolecular interactions exist in the Co(II)-Ce(III) complex. Two infinite 2D supramolecular structures are built via intermolecular O–H···O, C–H···O and C–H···π interactions, respectively.

Based on these points of view, we have now designed and synthesized a symmetric bi(salamo)-type ligand H 4 L and its corresponding heterotrinuclear Co(II)-Ce(III) 3d-4f complex [Co 2 (L)Ce(OAc) 3 (CH 3 CH 2 OH)]·1.5CH 3 OH·0.5CH 2 Cl 2 . The structure of H 4 L is depicted in Figure 1. Furthermore, the structure and fluorescence properties of the Co(II)-Ce(III) complex were studied.

IR Spectra
The IR spectra of H 4 L and its corresponding Co(II)-Ce(III) complex exhibited various bands in the 4000-400 cm −1 region. The FT-IR spectrum and data of the ligand H 4 L and its corresponding Co(II)-Ce(III) complex are given in Figure 2 and Table 1.  A C=N stretching vibration band was observed at 1612 cm −1 in the IR spectrum of H 4 L. Upon complex formation, the strong characteristic C=N vibration band of the Co(II)-Ce(III) complex appeared at 1623 cm −1 , which is slightly red-shifted in comparison to the free ligand H 4 L and is attributed to coordination of the nitrogen atoms of the C=N group and the metal(II) atoms [57]. The Ar-O stretching vibration band is detected in a range of 1220-1260 cm −1 . In the free ligand H 4 L the characteristic Ar-O group absorption appeared at 1255 cm −1 , a ca. 16 cm −1 shift to a lower frequency showing that the M-O bonds are formed between the metal atoms and the oxygen atoms from methoxy and phenolic groups of the free ligand H 4 L [58]. The above facts are consistent with the results determined by X-ray diffraction.

UV-Vis Absorption Spectra
In many studies, UV-Vis absorption spectra have been utilized to study lanthanide complexes. In this study, the UV-Vis spectra of the free ligand H 4 L in CHCl 3 :CH 3 OH (1:1) (c = 2.5 × 10 −5 mol L −1 ) with its corresponding Co(II)-Ce(III) complex in CH 3 OH:H 2 O (10:1) (c = 1 × 10 −3 mol L −1 ) were collected in the range of 250-550 nm. In the absorption spectrum of the free ligand H 4 L, there are four consecutive absorption peaks at ca. 269, 342, 360 and 375 nm. The absorption peak at 269 nm can be assigned to the π-π* transition of the benzene rings. The other three absorption peaks can be attributed to the π-π* transition of the oxime groups [57].
In the UV-Vis titration experiment, it can be clearly seen that the gradual addition of Co(OAc) 2 solution caused absorption peak changes. Compared to the free ligand H 4 L, the absorption peaks are bathochromically shifted [59]. This phenomenon is due to the coordination of H 4 L with the Co(II) ions. Upon addition of Co(II) ions, the absorbance of the solution first increases. When Co(II) ions were added in excess of three equiv, the absorbance of the solution no longer changed. The spectroscopic titration clearly showed the formation of a 1:3 Co(II) complex ( Figure 3a).  The color of the solution changed unconspicuously when Ce(III) ions were added. Then, upon addition of 1 equiv of Ce(II) ions, the absorbance changed and showed three isoabsorptive points at about 313, 365 and 380 nm. The spectroscopic titration clearly exhibited that the ratio of the replacement reaction stoichiometry is 1:1 and is shown in Figure 3b.
The importance of the coordination of OAc − was confirmed by the following experiment. A 1:3:1 mixture of H 4 L, Co(NO 3 ) 2 and Ce(NO 3 ) 3 displayed an absorption spectrum identical to that of H 4 L, indicating no complexation. Spectrophotometric titration of the mixture with KOAc showed that nine equiv of KOAc were required to convert the mixture to [LCo 2 Eu(OAc) 3 ]. The nine equiv of OAc-consists of six for deprotonation and three for coordination to the trinuclear core.

Crystal Structure Description
The crystal structure of the Co(II)-Ce(III) complex was determined by X-ray crystallography and is shown in Figure 4.  X-ray crystallographic analysis revealed that the Co(II)-Ce(III) complex crystallizes in a triclinic system, space group of P-1, possessing a symmetric trinuclear structure. The Co(II)-Ce(III) complex consists of two Co(II) atoms, one Ce(III) atom, one (L) 4unit, three µ 2 -acetate ions, one coordinated ethanol molecule, one and half crystallization methanol molecules and half crystallization dichloromethane molecule. As shown in Figure 4, we can see that the coordination ratio of the ligand (L) 4− unit to metal atoms (Co(II) and Ce(III)) in the Co(II)-Ce(III) complex is 1:2:1 [60,61]. Meanwhile, the terminal Co(II) atoms (Co1 and Co2) are located in the N 2 O 2 compartments and they are both hexa-coordinated with slightly distorted octahedral geometries [62][63][64]. Differently, the Co1 atom is bonded to two µ 2 -acetate ions (O14 and O16), the nitrogen atoms (N1 and N2) and oxygen atoms (O1 and O2) of the oxime and phenolic groups. The Co2 atom is coordinated the nitrogen atoms (N3 and N4) and oxygen atoms (O6 and O7) of the oxime and phenolic groups, one µ 2 -acetate oxygen atom (O11) and one oxygen atom (O13) from the coordinated ethanol molecule. The central O 6 site (O1, O2, O3, O6, O7 and O8) was occupied by one Ce(III) atom coordinating to three oxygen atoms (O12, O15 and O17) of three µ 2 -acetate ions. Hence, the Ce(III) atom is nine-coordinated with a single square antiprismatic geometry. In the crystal structure of the Co(II)-Ce(III) complex, three µ 2 -acetate ions bridge Co(II) and Ce(III) atoms in a familiar µ 2 -fashion mode. The high coordination number of Ce(III) atom is determined by its longer ionic radius and smaller winding angles. The Ce-O bond distances of the four phenolic oxygen (O1, O2, O6 and O7) atoms from the completely deprotonated (L) 4− unit are in the range of 2.421(4)-2.523(3) Å and the Ce-O bond distances of the methoxy groups are about Ce1-O3, 2.688(4) and Ce1-O8 2.655(4) Å, which are clearly shorter than later. The crystallographic data and structural refinement paramenters are summarized in Table 2. Selected bond lengths and angles of the Co(II)-Ce(III) complex are listed in Table 3.  R 1 = 0.0519, wR 2 = 0.1377 R indices (all data) b R 1 = 0.0693, wR 2 = 0.1537 Goodness-of-fit for F 2 c 1.020 Largest differences peak and hole (e Å −3 ) 1.130 and −1.300

Fluorescence Tests
Recently, synthesis of several examples of lanthanide complexes of salamo-type ligands to study their luminescence properties have been reported [75,76]. The fluorescence emission spectra of H 4 L in CHCl 3 :CH 3 OH solution and its corresponding Co(II)-Ce(III) complex in methanol solution were investigated at room temperature. In the fluorescence titration experiment, the emission spectrum of the free ligand H 4 L exhibited broad visible photoluminescence with maximum emission at ca. 416 nm upon excitation at 340 nm (Figure 9a). When Co(II) ions was added, the fluorescence emission intensity quenches. The fluorescence emission intensity of the solution no longer changed after the Co(II) ions was added in excess of three equiv. The spectroscopic titration indicated that the stoichiometric ratio between Co(II) and the ligand H 4 L is 3:1 (Figure 9b). Then, the fluorescence intensity enhances upon addition of Ce(III) ions and showed a maximum at ca. 433 nm upon excitation at 341 nm (Figure 10a). When the amount of Ce(III) ions reaches one equiv, the fluorescence emission intensity of the solution becomes stable. Obviously, the spectroscopic titration indicated that the ratio of the replacement reaction was 1:1 (Figure 10b), which obtained the same conclusion with UV-Vis titration experiments.

Materials and Methods
2-Hydroxy-3-methoxybenzaldehyde (99%), methyltrioctylammonium chloride (90%), pyridinium chlorochromate (98%) and boron tribromide (99.9%) were bought from Alfa Aesar (New York, NY, USA) 33 wt % hydrobromic acid solution in acetic acid was purchased from J&K Scientific Ltd. (Beijing, China). The other general reagents and solvents in this work were used directly without further purification in the preparation of the free ligand and its complex. Elemental analyses (C, H and N) were carried out using a VarioEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany).Elemental analyses for metals were performed on an ER/S·WP-1 ICP atomic emission spectrometer (IRIS, Elementar, Berlin, Germany).Melting points were measured using a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company(Beijing, China) and were uncorrected. Infrared spectra were recorded between 500 and 4000 cm −1 on a VERTEX 70FT-IR spectrophotometer (Bruker, Billerica, MA, USA)for samples prepared as KBr pellets. UV-Vis spectra in the 250-550 nm range were recorded by a U-3900H spectrometer (Hitachi, Shimadzu, Tokyo, Japan). Fluorescence spectra were taken on a Hitachi F-7000 fluorescence photometer.(Hitachi, Tokyo, Japan). 1 H-NMR spectra were determined with a German Bruker AVANCE DRX-400 spectrometer (Bruker AVANCE, Billerica, MA, USA). X-ray single crystal structure determination was carried out on a SuperNova Dual (Cu at zero) four-circle diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 293.66(10) K (Bruker, Billerica, MA, USA). Reflection data were corrected for Lorentzian and polarization effects and for absorption using the multi-scan method.

Synthesis of the Heterotrinuclear Co(II)-Ce(III) Complex
A mixed solution of Co(OAc) 2 ·4H 2 O (4.98 mg, 0.02 mmol) in ethanol (2 mL) and Ce(OAc) 3 ·H 2 O (3.35 mg, 0.01 mmol)) in water/methanol (1:1, 2 mL) was added to a solution of the ligand H 4 L (6.32 mg, 0.01 mmol) in dichloromethane (2 mL). Then the resulting mixed solution immediately turned yellow and was stirred at room temperature for 30 min. The mixture was filtered and the filtrate was allowed to stand at room temperature for approximately four weeks, giving colorless prismatic single crystals suitable for X-ray crystallographic analysis. Yield: 37%. Anal. Calcd. (%) for C 42

Crystal Structure Determination and Refinement
The structure was solved by direct methods (SHELX-2014) and refined anisotropically using full-matrix least-squares methods on F 2 with the SHELX-2014 program package. Lp and semi-empirical absorption corrections by SADABS were applied to the intensity data. The non-hydrogen atoms were refined anisotropically except for the solvent molecules of the crystal of the Co(II)-Ce(III) complex. All hydrogen atoms were added in calculated positions. Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1814394) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Conclusions
The synthesis, structural characterization, and fluorescence properties of the bis(salamo)-type ligand H 4 L and its corresponding heterotrinuclear Co(II)-Ce(III) complex were described. The X-ray crystal structure revealed that two Co(II) atoms are located in the N 2 O 2 coordination environment and are both hexacoordinated with slightly distorted octahedral geometries. Simultaneously, the O 6 cavity of the completely deprotonated (L) 4− unit is occupied by the Ce(III) atom which is nine-coordinated with a single square antiprismatic geometry. The UV-Vis titration experiments revealed the ratio of the heterotrinuclear Co(II)-Ce(III) complex is 1:2:1 (ligand/Co(II)/Ce(III)). Ethanol as a coordinating solvent participates in the coordination in the Co(II)-Ce(III) complex.
The different peak wavelength variation of the heterotrinuclear complex clearly showed the success of transformation from homotrinuclear to heteronuclear complex, which could be used in host-guest systems. Furthermore, the Co(II)-Ce(III) complex showed weak photoluminescence and exhibiting a hypsochromic-shift.