Differential study on the transition from a new polyhalogen-substituted unsymmetric salamo-based ligand to its Cu(II) and Co(II) complexes

Abstract Two structurally different Cu(II) and Co(II) complexes, [Cu(L)] (1) and [Co2(L)2]⋅2CH3OH (2), constructed from a new polyhalogen-substituted unsymmetric salamo-based ligand (H2L, 4-chloro-4′-bromo-6-bromo-6′-tert-butyl-2,2′-[ethylenedioxybis(nitrilomethylidyne)]diphenol) were synthesized by wet-chemical methods. Complexes 1 and 2 were characterized through elemental analyses, IR and UV-vis spectroscopies and single crystal X-ray crystallography. In addition, the differences of the structures, electronic absorption characteristics and fluorescence property transitions from the ligand to complexes 1 and 2 were studied. The largest difference is that the ligand H2L can react with M(OAc)2 (M = Cu(II) and Co(II)) to give two complexes with distinct structures and behaviors. There are two chemically identical but crystallographically independent structural units (molecules A and B) in 1. Moreover, each Cu(II) ion (Cu1 or Cu2) is four-coordinate and possesses a square planar geometry, but both Co(II) ions of 2 are bridged by phenoxide ions and possess five-coordinate trigonal bipyramidal configurations. At the same time, Hirshfeld surface analyses showed there are short-range interaction features from the ligand to complexes 1 and 2, the O···H/H···O interactions of the ligand are significantly stronger than those of 1 and 2. Graphical abstract

Among several new synthetic methods developed so far, the design and exploitation of the salamo-based ligands containing a N 2 O 2 coordination cavity are undoubtedly the most useful methods, because the tailored synthesis of these ligands can be completed through simple procedures [36][37][38]. The great majority of the complexes are based on symmetrical N 2 O 2 -donor ligands. In recent years, wide attention has been focused on functional unsymmetric salamo-based ligands [39][40][41], all of which is possible due to their unique structures and extensive applications.
As part of our long-term research, a newly designed polyhalogen-substituted unsymmetric salamo-based ligand H 2 L and its two 3d metal(II) complexes were designed and synthesized. The specificity of the new unsymmetric salamo-ligand is thanks to the introduction of polyhalogen-substituted atoms (bromine and chlorine atoms) [42,43] and a tert butyl group [44,45]. It is hoped that the special groups or atoms can influence the construction of the structures and form novel complexes. Herein, the single crystal X-ray diffraction analyses of the unsymmetric ligand H 2 L and complexes 1 and 2 were also characterized. Most importantly, the distinct structures of 1 and 2 were formed by adding two different transition metal ions into H 2 L separately. Complex 1 is a simple mononuclear compound, whereas 2 is a dimer. Moreover, the UV-vis spectra, IR spectra and fluorescence spectra of 1 and 2 were investigated to ascertain their properties.

Methods and materials
All chemicals were analytical grade reagents from Shanghai Merery Chemical Technology Co., Ltd.; solvents were analytical grade reagents from Tianjin Chemical Reagent Factory and were used without purification. C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument. Elemental analyses for Cu(II) and Co(II) were detected with an IRIS ER/S-WP-1 ICP atomic emission spectrometer. The IR spectra (4000-400 cm À1 ) of the ligand and complexes were investigated using a Vertex70 FT-IR spectrophotometer and samples were prepared as KBr disks. The UV-vis absorption spectra were recorded by a Shimadzu UV-3900 spectrometer. The 1 H NMR spectra were measured on a Bruker AV 500 MHz spectrometer. The melting points were measured by a micro melting point instrument manufactured by Beijing Tektronix Instrument Co., Ltd. A F-7000 FL spectrophotometer was used to record the fluorescence spectra. The crystal structures were determined on a four-circle diffractometer (H 2 L) or a Bruker D8 Venture diffractometer (complexes 1 and 2).

Preparation of H 2 L
3-Tert-butyl-5-bromosalicylaldehyde: to a dichloromethane solution (100 mL) of 3-tertbutyl-salicylaldehyde (178 mg, 1.00 mmol) was slowly added 3.5 mL of liquid bromine with a syringe. The mixture system was kept stable at 0 C in ice water for 2.5 h, then the solution was separated with a separatory funnel; bright yellow product 3-tertbutyl-5-bromo-salicylaldehyde was obtained by vacuum distillation.

Preparation of 1 and 2
4-Chloro-4 0 -bromo-6-bromo-6 0 -tert-butyl-2,2 0 -[ethylenedioxybis(nitrilomethyli-dyne)] diphenolato)copper(II) (1): 0.0100 mmol of copper(II) acetate monohydrate (2.00 mg) was stirred with 4 mL methanol solvent for 10 minutes, which was then added to a clear solution of H 2 L (5.49 mg, 0.0100 mmol) in acetone (1 mL) with stirring for 15 minutes. The mixed system was filtered and sealed with silver paper for storage. Black block-shaped single crystals of 1 suitable for X-ray crystal analysis were collected through placing it in an undisturbed and quiet environment for 14 days. Yield

Crystal structure analyses of H 2 L, 1 and 2
The single-crystal data of H 2 L were collected on a four-circle diffractometer equipped with graphite monochromated Cu-Ka radiation (k ¼ 1.54184 Å) at 99.99(10) K. The single-crystal data of 1 and 2 were collected on a Bruker D8 Venture diffractometer with graphite monochromated Cu-Ka radiation (k ¼ 1.54178 Å) at 173(2) K. The data collection and reduction of H 2 L were performed using the package CrysAlisPro [46]. For 1 and 2, data reduction was performed using the SAINT programs [47]. The data were corrected for absorption correction using the multi-scan method by SADABS software [48]. The crystal structures and integrated space-group were determined by a direct method using the ShelXT [49] in Olex 2 as a graphical interface [50]. The data were merged by SHELXL and the structures were refined using full-matrix least squares on F 2 [51]. The positions of Cu (or Co) ions are easily ascertained. The hydrogen atoms were placed on calculated positions and refined with a riding model; the coordinates of the nonhydrogen atoms were refined anisotropically.
Details of crystal data and the refinement parameters are summarized in Table 1. CCDC 2163079, 2163080 and 2163081 contain the supplementary crystallographic data for H 2 L, 1 and 2, respectively. The complete crystallographic data have been stored in the Cambridge Crystallographic Data Center as CIF files and can be provided free of charge from the website: https://www.ccdc.cam.ac.uk/structures/.
The routes to the synthesis of 1 and 2 are depicted in Scheme 2. The single crystals of 1 (or 2) were accessible from the reaction of H 2 L with copper(II) acetate monohydrate (or cobalt(II) acetate tetrahydrate) in a 1:1 molar ratio in methanol/acetone (or methanol/dichloromethane) mixed solvents (Scheme 2). After stirring for 15 minutes, the mixed system was filtered off and standing with undisturbed for about 14 (or 21) days (see Experimental).

IR spectra analyses
The IR spectra of H 2 L, 1 and 2 are shown in the range of 4000-400 cm À1 (Figure 1). In H 2 L, a strong absorption peak observed at 3428 cm À1 is due to the stretching vibration of O-H groups [53,54], and a strong sharp peak appeared at 1615 cm À1 can be assigned to the typical (C ¼ N) vibration [55]. Moreover, an absorption peak at 1260 cm À1 is a typical (Ar-O) vibration [56]. In the IR spectra of 1 and 2, the absorption peak at 3428 cm À1 has completely disappeared, which proves that the phenolic hydroxyl groups of H 2 L have been deprotonated and have coordinated to the Cu(II) or Co(II) ions. The wide O-H stretching vibration band was found at 3420 cm À1 in 2, which is in agreement with the result of elemental analysis. The corresponding O-H stretching vibration band was discovered at 3415 cm À1 in 1, which could correspond to the water molecules in the manufacturing process of KBr. The vibrational peaks of C ¼ N and Ar-O bonds were discovered at 1590 and 1221 cm À1 in 1, respectively [57]. The noteworthy shift to low wavenumber indicates that the coordination has happened. Moreover, the moderate absorption peak which appeared at 546 cm À1 is due to the Cu-N stretching vibration, and that at 479 cm À1 is due to the stretching vibration of the Cu-O bond, indicating the occurrence of coordination between the Cu(II) ion and the ligand H 2 L [58]. In 2, the absorption peak which appeared at 515 cm À1 is assigned to the Co-N absorption peak, and that at 466 cm À1 is owing due to the absorption peak of the Co-O bond, which indicates the coordination of Co(II) ions with the ligand H 2 L has occurred [59].

UV-vis absorption spectral analyses
The UV-vis absorption spectra of H 2 L, 1 and 2 (EtOH, 1 Â 10 À5 M) are depicted in figure 2. The UV-vis absorption curve of free H 2 L is composed of two intense absorption peaks at 327 and 272 nm, which are due to the n-p Ã transitions of chromophore C ¼ N groups and the p-p Ã transitions of benzene rings [60]. Contrasted with the unsymmetric ligand H 2 L, the absorption peaks of 1 and 2 at 272 nm decreased in intensity obviously. Unexpectedly, Scheme 2. Synthetic routes to 1 and 2. a new peak of 1 was found at 382 nm (370 nm for 2), which is possibly caused by the ligand to metal charge transition (LMCT) effect [61].
The titration absorption spectra of 1 and 2 were measured (Figure 3a). With the slow addition of Cu(II) ions (1 Â 10 À3 M), the intensity of the absorption peak at 382 nm increased gradually, and the absorption spectra essentially did not change when adding additional Cu(II) ions (1 equiv.). The titration experiment results and the Job plot ( Figure S3a) reveal that the optimum binding ratio of Cu(II) ions to H 2 L is 1:1. As shown in Figure 3b, with the slow addition of Co(II) ions (1 Â 10 À3 M), the intensity of the absorption peak at 370 nm increased gradually, and the absorption spectra essentially no longer changed when continuing to add Co(II) ions (beyond 1 equiv.). The titration experiment results and the Job plot ( Figure S3b) also indicate that the optimum binding ratio of Co(II) ions to H 2 L is 1:1.

Descriptions of crystal structures
The important bond angles ( ) and lengths (Å) of H 2 L, 1 and 2 are summarized in Table 2. The hydrogen bond interactions of H 2 L, 1 and 2 are shown in Table 3. The X-HÁÁÁp interactions of 1 are shown in Table S1.  3.4.1. Crystal structure description of H 2 L H 2 L crystallizes in the triclinic space group P-1 and Z ¼ 2. There is one molecule represented in Figure 4 by its asymmetric unit. The molecular formula is C 20 H 21 Br 2 ClN 2 O 4 . As depicted in Figure 4, the molecule is an unsymmetric structure. There are four pairs of important intramolecular hydrogen bonds (C18-H18AÁÁÁO4, C19-H19CÁÁÁO4, O1-H1ÁÁÁN1 and O4-H4ÁÁÁN2) in free H 2 L (figure S4a). Attributed to the introduction of  halogen atoms (bromine and chlorine atoms), the presence of one intermolecular halogen bond (Br2 … O2 3.049 Å) is of great significance for the structural construction between the two molecules ( Figure S4b). Finally, as shown in Figure 5, a three-dimensional supramolecular structure of H 2 L is generated by different torsion angles and intermolecular hydrogen bonds.

Crystal structure description of 1
The molecular formula of 1 is C 20 H 19 Br 2 ClCuN 2 O 4 . It crystallizes in triclinic space group P-1 and Z ¼ 4. There are two molecules within the asymmetric unit (Z' ¼ 2). Complex 1 is mononuclear ( Figure 6). The unit cell includes two complex 1 molecules (A and B), which are chemically identical but crystallographically independent units. To emphasize the relationship between the two complex 1 molecules, a molecular fit (Platon) is interesting (see Figure S5). The Cu(II) ions in both molecules are fourcoordinate by the two O and N atoms of the ligand (L) 2À unit, resulting in a distorted square planar geometry. Such geometry is quite common with this kind of salmo-   [62], indicating that Cu1 (or Cu2) ion forms a distorted square planar geometry. For molecules A and B, the biggest distinction is the variety of the bond lengths and angles, as listed in Table 2.
Furthermore, the hydrogen bond interactions in 1 cannot be ignored. There are four pairs of intramolecular hydrogen bonds (C18-H18CÁÁÁO4, C19-H19AÁÁÁO4, C39-H39CÁÁÁO8 and C40-H40AÁÁÁO8) ( Figure S6a) and two pairs of important intermolecular hydrogen bonds (C9-H9AÁÁÁO6 and C29-H29BÁÁÁO2) in 1 ( Figure S6b). In addition, a C-HÁÁÁp interaction (C28-H28AÁÁÁCg12) is represented in Figure S6c. Finally, one-dimensional supramolecular structure (Figure 7) of 1 is formed by these abundant intermolecular hydrogen bond interactions. Formation of the above-mentioned intermolecular hydrogen bonds plays a non-negligible role in stabilizing the structure of 1.

Crystal structure description of 2
Complex 2 crystallizes in the trigonal space group R-3c with Z ¼ 18 with the molecular formula C 42 H 46 Br 4 Cl 2 Co 2 N 4 O 10 , which means that the asymmetric unit contains half of the dinuclear unit (Z 0 ¼ 0.5).   [63].

Hirshfeld surface analyses
In recent years, Hirshfeld surfaces have been used as a tool for molecular crystal structure analysis [64]. It is a technique for obtaining the information of crystal stacking trend, which provides a convenient and intuitive method for quantitative analysis of the interactions in single crystal structures through the derivation of the Hirshfeld surfaces and the decomposition of two-dimensional fingerprints [65]. Hirshfeld surface analyses of H 2 L, 1 and 2 are shown in Figure 10. The Hirshfeld surfaces are mapped to d n , d i and d e . The differences between H 2 L, 1 and 2 directly show the level of intermolecular bindings. For H 2 L, 1 and 2, the red region usually represents the interactions of OÁÁÁH/HÁÁÁO in the map d n . On the surface, the white region is caused by HÁÁÁH interactions. Meanwhile, there is no interaction on the blue area, which indicates that the distance between the atoms is large.

Fluorescence prosperities
The fluorescence spectral measurements were carried out at excitation wavelength 373 nm and the solvent used was absolute ethanol. The fluorescence emission spectra of H 2 L, 1 and 2 (5 Â 10 À5 M) are shown in Figure 12. The unsymmetric ligand H 2 L has an intense emission peak at 475 nm, which is probably attributed to the p-p Ã intraligand transition [66]. The emission peaks of 1 and 2 decrease significantly. The reduction of the fluorescence intensity of 1 and 2 is assigned to the interaction of the Cu(II) and Co(II) ions. Impressively, compared with H 2 L the emission peaks of 1 and 2 also have notable blue shifts compared to 475 nm and appear at 460 and 463 nm, which may be due to the LMCT effect [67].
As depicted in Figure 13, the fluorescence titration experiments of 1 and 2 were carried out in ethanol solution. The fluorescence intensity was monitored and a  gradual decrease was observed (Figure 13a) with the addition of Cu(II) ions (1 Â 10 À3 M) at 475 nm. The emission peak decreased to a minimum at 475 nm when 1 equivalent Cu(II) ions was added. Thereafter, with continuous addition of the Cu(II) ions, the emission intensity did not change. In all cases, a typical saturation binding curve can be observed when the fluorescence intensity value was plotted against the Cu(II) ions concentration, which demonstrates the optimal coordination ratio of the unsymmetric ligand H 2 L to Cu(II) ions is 1:1. In Figure 13b, a similar phenomenon occurred and the fluorescence intensity decreased gradually with the addition of Co(II) ions (1 Â 10 À3 M) until the amount of Co(II) ions reached 1.0 equivalent. A typical saturation binding curve can be observed when the fluorescence intensity value was plotted against the Co(II) ions concentration. It shows that the optimal coordination ratio of the unsymmetric ligand H 2 L to Co(II) ions is 1:1, which agrees with the single crystal structure.

Conclusion
Two new 3d metal(II) complexes, [Cu(L)] (1) and [Co 2 (L) 2 ]Á2CH 3 OH (2), were obtained by the reactions of a new polyhalogen-substituted unsymmetrical salamo-based ligand with different metal(II) acetates. The current study was performed using various spectral techniques ( 1 H NMR, IR, fluorescence and UV-vis spectra). Single crystal X-ray diffraction analysis showed that the ligand H 2 L is a simple linear molecule. The Cu(II) ion locates in N 2 O 2 coordination sphere and forms a square planar geometry for 1, while each Co(II) ion is bridged by phenoxide ions (O1 and O1 #1 ), which leads to the formation of a five-coordinate distorted trigonal bipyramidal geometry for 2. The binding ratio between the ligand and metal(II) ions was verified by UV-Vis titration experiments and Job plots. The result of Hishfield surface analysis suggests that the difference of interactions is that the OÁÁÁH/HÁÁÁO interaction in the ligand is significantly stronger than those of complexes 1 and 2. Moreover, H 2 L has good fluorescence based on a strong emission band at 475 nm, but 1 and 2 show intense fluorescence quenching behaviors, indicating that the Cu(II) and Co(II) ions have coordinated with the ligand.