Unprecedented Fluorescent Dinuclear CoII and ZnII Coordination Compounds with a Symmetric Bis(salamo)-Like Tetraoxime

Two unprecedented homometallic CoII and ZnII coordination compounds, [M2(L)(OCH3)][M2(L)(OAc)] (MII = CoII (1) and ZnII (2)), with a novel symmetric bis(salamo)-like tetraoxime ligand H3L were synthesized and characterized by elemental analyses, infrafred (IR), ultraviolet–visible spectroscopy (UV-Vis), fluorescent spectra and single-crystal X-ray diffraction analyses. The unit cell of the two coordination compounds contains two crystallographically and chemically independent dinuclear coordination compounds. In the two coordination compounds, three metal ions are five-coordinated, formed two square pyramidal and a trigonal bipyramidal geometries, and the other metal ion is a hexacoordinate octahedral configuration. In addition, the coordination compound 1 forms a 3D supramolecular structure, and the coordination compound 2 forms a 0D dimer structure by the inter-molecular hydrogen bond interactions. Meanwhile, the fluorescence spectra of the coordination compounds 1 and 2 were also measured and discussed.

The aim of the present work is the structural characterization of the homometallic coordination compounds 1 and 2 based on a symmetric bis(salamo)-like tetraoxime ligand. Herein, the ligand H 3 L and its corresponding dinuclear coordination compounds 1 and 2 were prepared successfully. Interestingly, the obtained 2:1 (metal-to-ligand stoichiometry) type coordination compounds are unusual in the previously reported bis(salamo)-type metal coordination compounds, in which most of them possess 3:1 (metal-to-ligand stoichiometry) type of structures [43,57]. Furthermore, the supramolecular features and luminescent spectra of the coordination compounds 1 and 2 are discussed.

Crystal Structures of the Coordination Compounds 1 and 2
X-ray crystallographic analyses reveal that the structure of the coordination compound 2 is similar to that of the coordination compound 1. The coordination compounds 1 and 2 form novel dinuclear structures, which are different from the common trinuclear structures of bis(salamo)-like metal coordination compounds reported earlier [58][59][60][61][62]. The crystal structures of the coordination compounds 1 and 2 and the coordination polyhedrons of the M II atoms are shown in Figures 1 and 2. Selected bond lengths and angles are listed in Tables 1 and S1. The coordination compounds 1 and 2 crystallize in the triclinic crystal system, space group P-1, and the unit cell of the two coordination compounds contains two crystallographically and chemically independent dinuclear coordination compounds (A and B molecules) (As shown in Figures 1 and 2). In the two coordination compounds, A molecule consisting of two M II atoms, one heptadentate (L) 3− unit and one µ 2 -bridged methoxyl group, and B molecule is composed of two M II atoms, one heptadentate (L) 3− unit and one chelating acetate ion. In molecules A and B, the purpose of the acetate ion and methoxyl group is to compensate for the charge and make the whole molecule neutral. The obtained 2:1 (metal-to-ligand stoichiometry) type dinuclear coordination compounds are unprecedented in the reported bis(salamo)-like metal coordination compounds, which always possess 3:1 (metal-to-ligand stoichiometry) type of structures [43,57]. In each of the A molecules, all of the M II atoms are located in the N 2 O 2 coordination spheres of the salamo-type ligand (L) 3− unit, the µ 2 -bridged methoxyl groups bridge two M II atoms in a familiar M-O-M fashion (Figures 1a and 2a). Meanwhile, two M II atoms of the coordination compounds 1 and 2 are pentacoordinated and adopt distorted trigonal bipyramidal (Co1 and Zn1) and square pyramidal (Co2 and Zn2) geometries (Figures 1b and 2b), which were deduced by calculating the values of τ Co1 = 0.62, τ Co2 = 0.41, τ Zn1 = 0.63 and τ Zn2 = 0.41, respectively [63]. From the calculation results, we can see that the τ values of Co1 and Zn1 are greater than 0.5, forming trigonal bipyramidal configurations, and the τ values of Co2 and Zn2 are less than 0.5, forming square pyramidal configurations. The structures of the B molecules are different from those of the A molecules, the Co4 and Zn4 atoms of the coordination compounds 1 and 2 are pentacoordinated and adopt distorted square pyramidal geometries, which were deduced by calculating the values of τ Co4 = 0.49 and τ Zn4 = 0.48, respectively. The Co4 and Zn4 atoms are located in the N 2 O 2 coordination spheres of the salamo-type ligand (L) 3− unit, and coordinate to one phenoxo oxygen (O9) atom, respectively. The Co3 and Zn3 atoms coordinate to N 2 O 2 atoms of the deprotonated ligand (L) 3− units as well as two oxygen atoms from one chelating acetate ion, and have a hexacoordinated environment and adopt distorted octahedral coordination geometries (By means of continuous shape measures (CShM), when the value of CShM is the smallest, the ideal structure is the octahedron configuration, CShM = 3.03270 and 3.72885 for Co3 and Zn3 atoms) [64]. The supramolecular structures of the coordination compounds 1 and 2 are very different from each other. In the crystal structure of the coordination compound 1, there are eight significant intermolecular hydrogen bonds (C9-H9A···O13, C10-H10···O15, C40-H40A···Br8, C61-H61A···Br2, C36-H36···Br7, C8-H8A···Br8, C39-H39B···Br6 and C43-H43···O7) and one intramolecular hydrogen bond (C49-H49A···O16). The units are interlinked by the intermolecular C9-H9A···O13, C10-H10···O15, C40-H40A···Br8, C61-H61A···Br2 and C43-H43···O7 hydrogen bonds into a 2D layered supramolecular structure, which are further assembled into an infinite 3D network structure with the help of intermolecular C36-H36···Br7, C8-H8A···Br8 and C39-H39B···Br6 hydrogen bond interactions ( Figure 3). For the coordination compound 2, there is a pairs of intermolecular hydrogen bond (C55-H55···O10). The oxygen (O10) atom of the (L) 3− unit is hydrogen bonded to the C55-H55 group of another coordination compound 2 molecule, linking a 0D dimer structure ( Figure 4). In addition, A and B molecules are connected steadily by intermolecular C-H···O hydrogen bond interactions. Putative hydrogen bond interactions for the coordination compounds 1 and 2 are shown in Table 2.

IR Spectra
IR spectra of H 3 L and its corresponding coordination compounds 1 and 2 exhibit various bands in the region of 400-4000 cm −1 . Main IR bands/cm −1 for the ligand H 3 L and its coordination compounds 1 and 2 are presented in Table 3. The free ligand H 3 L shows a characteristic C=N stretching band at 1611 cm −1 , while the C=N stretching bands of the coordination compounds 1 and 2 appear at 1619 and 1621 cm −1 , respectively [65]. For the ligand H 3 L, the Ar-O stretching band appears at 1265 cm −1 , which is observed at 1258 and 1261 cm −1 for the coordination compounds 1 and 2. The characteristic C=N and Ar-O stretching frequencies are shifted to lower frequencies, indicating that the M-N and M-O bonds are formed [66]. For the coordination compound 1, the ν(Co-O) and ν(Co-N) frequencies are observed at 447 and 512 cm −1 , respectively [67]. Meanwhile, the ν(Zn-O) and ν(Zn-N) bonds at 453 and 519 cm −1 for the coordination compound 2. As pointed out by Percy and Thornton [68], the M-O and M-N frequency assignments are at times difficult.

Ultraviolet-Visible Spectroscopy (UV-Vis) Spectra
The UV-Vis absorption spectra of H 3 L and its coordination compounds 1 and 2 were determined in 1 × 10 −5 mol·L −1 MeOH solution, as shown in Figure 5. It can be seen that the absorption peaks of the coordination compounds 1 and 2 are obviously different from those of the H 3 L upon coordination. The electronic absorption spectrum of H 3 L consists of one relatively intense peak centered at 330 nm, assigned to the π-π* transition of the oxime groups [69,70]. Compared with the absorption peak of the free ligand H 3 L, the corresponding absorption peaks of the coordination compounds 1 and 2 appear at 380 and 378 nm, which are bathochromically shifted by 50 and 48 nm, respectively, indicating the coordination of the Co II and Zn II ions with the ligand H 3 L. In the UV-Vis titration experiment of the coordination compound 1, with the increasing concentration of Co 2+ , the absorbance of the solution at 380 nm enhanced, and at 330 nm reduced. The absorption peak reached the highest value after Co 2+ was added up to 2 equiv. The spectroscopic titration indicates that the ratio of the replacement reaction was 2:1 (Co 2+ : L 3− ). Similar changes also appear in the coordination compound 2, obtaining the same conclusion ( Figure 6).

Fluorescence Spectra
The fluorescence spectra of H 3 L and its corresponding coordination compounds 1 and 2 were investigated at room temperature and are shown in Figure 7. The free ligand H 3 L exhibits a relatively strong emission peak at ca. 462 nm upon excitation at 370 nm, and it should be assigned to the intraligand π-π * transition. The coordination compound 1 shows lower photoluminescence with maximum emission at ca. 454 nm. Compared with the ligand H 3 L, emission intensity of the coordination compound 1 reduces obviously, indicating that the Co II ions have a quality of fluorescent quenching, which makes the conjugated system larger and also indicates it may be a purple device. On the other hand, the coordination compound 2 shows an obvious fluorescence enhancement at ca. 460 nm. The intense peak is likely due to the coordination of H 3 L with the Zn II ions, which breaks the intramolecular hydrogen-bonding interactions of H 3 L and increases the coplanarity of the conjugated system. In addition, the fluorescence titration experiment of the coordination compound 2 is shown in Figure 8. The fluorescence intensity of the solution hardly changed after the Zn II ions were added up to 2 equiv, which shows the same conclusion compared with the UV-Vis titration experiment. Meanwhile, coordination of the Zn II ions evidently increases the fluorescence intensity of the ligand H 3 L.

Materials and Physical Measurements
All chemicals were of analytical reagent grade and were used without further purification. C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument (Berlin, Germany). Elemental analyses for Co and Zn were detected by an IRIS ER/S·WP-1 ICP atomic emission spectrometer (Berlin, Germany). Melting points were measured via a microscopic melting point apparatus (Beijing Taike Instrument Limited Company, Beijing, China). 1 H-NMR spectra were determined by German Bruker AVANCE DRX-400 spectroscopy (Bruker AVANCE, Billerica, MA, USA). Infrared (IR) spectra were recorded with a VERTEX-70 FT-IR spectrophotometer, with samples prepared as KBr (400-4000 cm −1 ) (Bruker, Billerica, MA, USA). Ultraviolet-visible spectroscopy (UV-Vis) absorption and fluorescence spectra were recorded on a Shimadzu UV-2550 spectrometer (Shimadzu, Tokyo, Japan) and F-7000 FL spectrometer (Hitachi, Tokyo, Japan), respectively. X-ray single crystal structure determinations were carried out on a Bruker APEX-II CCD diffractometer (Bruker AVANCE, Billerica, MA, USA). Supplementary crystallographic data for this paper have been deposited at the Cambridge Crystallographic Data Centre (1562395 and 1562396 for the coordination compounds 1 and 2) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Syntheses of the Coordination Compounds 1 and 2
The coordination compounds 1 and 2 were synthesized by the reaction of H 3 L with Co(OAc) 2 ·4H 2 O and Zn(OAc) 2 ·2H 2 O, respectively. A solution of Co(OAc) 2 ·4H 2 O (4.98 mg, 0.02 mmol) in methanol (2 mL) was added dropwise to a solution of H 3 L (8.8 mg, 0.01 mmol) in dichloromethane (3 mL). The color of the mixing solution turned to bronzing immediately, and then continuous stirring for 0.5 h at room temperature. The mixture was filtered and the filtrate was allowed to stand at room temperature for about two weeks. The solvent was partially evaporated and obtained brown, block-shaped single crystals suitable for X-ray crystallographic analysis with a yield of 76.4%. Anal. Calcd. for [Co 2 (L)(OCH 3  3.4. X-ray Structure Determination of the Coordination Compounds 1 and 2 X-ray diffraction data were collected on a Bruker APEX-II CCD diffractometer (296(2) K) for the coordination compounds 1 and 2 using graphite monochromatized Mo-K α radiation (λ = 0.71073 Å). Unit cell parameters were determined by the least-squares analyses. The LP factor and Semi-empirical absorption corrections were applied to the intensity data. The structures were solved by the direct method (SHELXS-2016), 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-2016 (Bruker AVANCE, Billerica, MA, USA). Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Contributions to scattering due to these highly disordered solvent molecules were removed using the SQUEEZE routine of PLATON, the structures were then refined again using the data generated. The hydrogen atoms were generated geometrically. Crystallographic data and refinement parameters for the coordination compounds 1 and 2 are given in Table 4.

Conclusions
We have designed and synthesized a novel symmetric bis(salamo)-like tetraoxime ligand H 3 L, and two unusual dinuclear coordination compounds 1 and 2, [M 2 (L)(OCH 3 )][M 2 (L)(OAc)] (M II = Co II and Zn II ). X-ray crystal structure analyses of the coordination compounds 1 and 2 reveal that the unit cell of the two coordination compounds contains two crystallographically and chemically independent dinuclear metal coordination compounds. The supramolecular structures of the coordination compounds 1 and 2 are different from each other, the coordination compound 1 forms a 3D supramolecular structure and the coordination compound 2 forms a 0D dimer structure by the inter-molecular hydrogen bond interactions. Furthermore, the fluorescence spectra of the coordination compounds 1 and 2 indicates that the coordination of Co II and Zn II ions leads to the fluorescence quenching and enhancing of H 3 L, respectively, which can be further studied as a new type of fluorescent material.