Tri-and MonoNuclear Zinc ( II ) Complexes Based on Half-and Mono-Salamo Chelating Ligands

Two newly designed complexes, [Zn(L1)(EtOH)] (1) and [{Zn(L)(OAc)2}2Zn]·CHCl3 (2) derived from salamo and half-salamo chelating ligands (H2L and HL2) have been synthesized and characterized by elemental analyses, IR and UV-VIS spectra, fluorescence spectra, and X-ray crystallography. Complex 1 shows a slightly distorted tetragonal pyramid and forms an infinite 3D supramolecular structure. All of the Zn(II) ions in complex 2 are hexa-coordinated with slightly distorted octahedral geometries. Complex 2 possesses an infinite 2D space structure. The fluorescence titration experiments were used to characterize fluorescence properties of complexes 1 and 2. And the normalized fluorescent spectra exhibit that complexes 1 and 2 have favourable fluorescent emissions in different solvents.


Synthesis of Complex 1
To a ethanol solution (2 mL) of zinc(II) acetate dehydrate (0.01 mmol, 2.19 mg), and a solution of H 2 L 1 (0.01 mmol, 4.64 mg) in 6 mL of dichloromethane was added dropwise, and immediately the mixed solution colour changed to yellow.The mixture solution was filtered and the filtrate was allowed to stand for two weeks.Through partial solvent evaporation, single crystals suitable for X-ray diffraction analysis were obtained after two weeks.Yield:

Synthesis of Complex 2
To a methanol solution (1 mL) of zinc(II) acetate dehydrate (0.03 mmol, 6.57 mg), and a solution of HL 2 (0.02 mmol, 9.28 mg) in 2 mL of chloroform was added dropwise, The colour of the mixing solution turned to yellow immediately, then the mixture was filtered and the filtrate was obtained.The single crystals suitable for X-ray diffraction studies were obtained by vapour diffusion of diethyl ether into the filtrate for two days at room temperature.Yield:

Crystal Structure Determinations of Complexes 1 and 2
The crystal diffractometer provides a monochromatic beam of Mo Kα radiation (0.71073 Å) produced using Graphite monochromator from a sealed Mo X-ray tube was used for obtaining crystal data for complexes 1 and 2 at 173.00 (10) and 292.38 (10), respectively.The LP factor semi-empirical absorption corrections were applied using the SADABS program.The structures were solved by the direct methods (SHELXS-2014) [48].The H atoms were included at the calculated positions and constrained to ride on their parent atoms.All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F 2 with SHELXL-2014 [48].The crystal data and experimental parameters relevant to the structure determinations are listed in Table 1.
Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication, No. CCDC 1564063 and 1564062 for complexes 1 and 2. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (Telephone: (44) 01223 762910; Fax: +44-1223-336033; E-mail: deposit @ccdc.cam.ac.uk).These data can be also obtained free of charge at www.ccdc.cam.Ac.uk/conts/retrieving.html.

Results and Discussion
Complexes 1 and 2 constructed from salamo and half-salamo chelating ligands (H 2 L 1 and HL 2 ) have been synthesized, and characterized by IR spectra, UV-VIS spectra, and X-ray crystallography analyses.The fluorescence titration experiments were used to characterize fluorescence properties of complexes 1 and 2. The normalized fluorescent spectra exhibits that complexes 1 and 2 have favourable fluorescent emissions in different solvents.

IR Spectra
The FT-IR spectra of H 2 L 1 and HL 2 with their corresponding complexes 1 and 2 exhibit various bands in the 4000-400 cm −1 region (Figure 1).A typical C=N stretching band of the free ligands H 2 L 1 and HL 2 appears at 1619 and 1604 cm −1 , and that of complexes 1 and 2 at 1581 and 1597 cm −1 , respectively [49].The C=N stretching frequencies are shifted to low frequencies, indicating that the Zn(II) atoms are coordinated by azomethine nitrogen atoms of (L 1 ) 2− and (L 2 ) 1− moieties.Therefore, the conclusion could be made that H 2 L 1 and HL 2 coordinated with Zn(II) atoms [50].The typical C=O stretching band at 1728 cm −1 was exhibited by the free ligands H 2 L 1 , where at 1712 cm −1 show the C=O stretching band in complex 1.The free ligands H 2 L 1 and HL 2 exhibit Ar-O stretching frequencies at 1288 and 1249 cm −1 , while the Ar-O stretching frequencies of the complexes 1 and 2 appear at 1226 and 1242 cm −1 , respectively.The Ar-O stretching frequencies are shifted to low frequencies, which could be evidence of the Zn-O bond formation between Zn(II) atoms and oxygen atoms of phenolic groups [51].
The far-IR spectra (550-100 cm −1 ) of both complexes 1 and 2 were also obtained so as to identify the bonds of Zn-O and Zn-N frequencies.The bands at 447 and 463 cm −1 of complexes 1 and 2 can be attributed to ν (Zn-O) , while the bands at 516 and 564 cm −1 are assigned to ν (Zn-N) [52].The far-IR spectra (550-100 cm −1 ) of both complexes 1 and 2 were also obtained so as to identify the bonds of Zn-O and Zn-N frequencies.The bands at 447 and 463 cm −1 of complexes 1 and 2 can be attributed to ν(Zn-O), while the bands at 516 and 564 cm -1 are assigned to ν(Zn-N) [52].

Crystal Structure of Complex 2
X-ray crystallographic analysis of complex 2 reveals an asymmetric trinuclear structure.It crystallizes in the triclinic system, space group P-1, consists of three Zn(II) ions, two completely deprotonated (L 2 ) 1− units, four coordinated acetate ions.Selected bond lengths and angles are listed in Table 4.

UV-VIS Spectra
The UV-VIS absorption spectra of the free ligands H 2 L 1 and HL 2 with their corresponding complexes 1 and 2 in the dichloromethane solutions (1.0 × 10 −5 mol/L) at 298 K are shown in Table 6 and Figure 7. Table 6.Absorption maxima and molar extinction coefficients for complexes 1 and 2.

UV-VIS Spectra
The UV-VIS absorption spectra of the free ligands H2L 1 and HL 2 with their corresponding complexes 1 and 2 in the dichloromethane solutions (1.0 × 10 −5 mol/L) at 298 K are shown in Table 6 and Figure 7. Table 6 Absorption maxima and molar extinction coefficients for complexes 1 and 2.
Obviously, the absorption peaks of the ligand H 2 L 1 and HL 2 differ from those of their corresponding complexes 1 and 2 .The absorption spectrum of the free salamo-type ligand H 2 L 1 consists of three relatively intense bands centered at 291, 329 and 345 nm, which may be assigned to the π-π* transitions of the phenyl rings of coumarin and the oxime group [44,65].Upon coordination of the ligand, the absorption intensities are weakened compared with the free ligand H 2 L 1 , which indicate that the oxime nitrogen atoms are involved in coordination to the Zn(II) atoms.Likewise, the absorption spectrum of the half-salamo ligand HL 2 consists of two relatively intense bands centred at 271 and 323 nm, which may be assigned to the π-π* transitions of the phenyl rings and the oxime group [45,65].On the other hand, because of complex 2 is synthesized by the half-salamo ligand HL 2 , when the Zn(II) atoms coordinated to HL 2 , the conjugate system of complex 2 not change greatly compared with complex 1, which leads to the absorption spectra were almost unchanged before and after the complexation.Upon coordination of the ligand HL 2 , the absorption intensities are weakened compared with the free ligand HL 2 , which indicate that the oxime nitrogen atoms are involved in coordination with the Zn(II) atoms [37,66].
compared with the free ligand HL , which indicate that the oxime nitrogen atoms are involved in coordination with the Zn(II) atoms [37,66].

Fluorescence Properties
The fluorescence titration experiments of H2L 1 and HL 2 were determined in DMF solution (2.0 × 10 −5 mol•L −1 ) with Zn(OAc)2•2H2O in methanol solution (1 × 10 −3 mol•L −1 ) are shown in Figure 8 and Figure 9.The free ligand H2L 1 appears as an intense emission peak at 432 nm.With the fluorescence titration experiment, upon the addition of Zn 2+ , gradual changes in the fluorescence spectra.And the fluorescence intensity increased significantly.When the added amount of Zn 2+ reached 1.0 equiv., the fluorescence emission intensity became stable, which indicates a 1:1 stoichiometry between Zn 2+ and H2L 1 .The enhancement of fluorescence is due to the coordination of metal ions with ligands [67].Likewise, Complex 2 displays enhanced emission intensities compared to the corresponding ligand (HL 2 ) when excited at 380 nm.When the added amount of Zn 2+ reached 1.5 equiv., the fluorescence emission intensity became steady.The result is corresponding to the crystal structure of complex 2 [68].For research the solvent effect in fluorescence spectra of complexes 1 and 2, the fluorescence spectra of complex 1 and 2 in a series of solvents were examined and are shown in Table 7 and Figure 10 and Figure 11.The free ligand H 2 L 1 appears as an intense emission peak at 432 nm.With the fluorescence titration experiment, upon the addition of Zn 2+ , gradual changes in the fluorescence spectra.And the fluorescence intensity increased significantly.When the added amount of Zn 2+ reached 1.0 equiv., the fluorescence emission intensity became stable, which indicates a 1:1 stoichiometry between Zn 2+ and H 2 L 1 .The enhancement of fluorescence is due to the coordination of metal ions with ligands [67].Likewise, Complex 2 displays enhanced emission intensities compared to the corresponding ligand (HL 2 ) when excited at 380 nm.When the added amount of Zn 2+ reached 1.5 equiv., the fluorescence emission intensity became steady.The result is corresponding to the crystal structure of complex 2 [68].
For research the solvent effect in fluorescence spectra of complexes 1 and 2, the fluorescence spectra of complex 1 and 2 in a series of solvents were examined and are shown in Table 7 and Figures 10 and 11.For research the solvent effect in fluorescence spectra of complexes 1 and 2, the fluorescence spectra of complex 1 and 2 in a series of solvents were examined and are shown in Table 7 and Figure 10 and Figure 11.The normalized fluorescent spectra of complexes 1 and 2 are shown in Figure 12 and Figure 13.Additionally, the fluorescence image of complexes 1 and 2 upon irradiation with a 365 nm UV lamp also indicated that the metal complexes 1 and 2 have promising applications as fluorescent materials.The solvent effect brings pivotal effect to the photoluminescence of complexes 1 and 2. The normalized fluorescent spectra of complexes 1 and 2 are shown in Figures 12 and 13.Additionally, the fluorescence image of complexes 1 and 2 upon irradiation with a 365 nm UV lamp also indicated that the metal complexes 1 and 2 have promising applications as fluorescent materials.The solvent effect brings pivotal effect to the photoluminescence of complexes 1 and 2. The normalized fluorescent spectra of complexes 1 and 2 are shown in Figure 12 and Figure 13.Additionally, the fluorescence image of complexes 1 and 2 upon irradiation with a 365 nm UV lamp also indicated that the metal complexes 1 and 2 have promising applications as fluorescent materials.The solvent effect brings pivotal effect to the photoluminescence of complexes 1 and 2.  As we know, many fluorescent complexes, especially those containing polar substituents on aromatic rings, are susceptible to solvents [27].Due to the difference in polarity of solvents, complex 1 exhibits the relatively strong maximum fluorescence emission with relatively low solvent polarity in TCM and DCM at 439 and 440 nm, respectively.Additionally, in solvents THF, DMF, and DMSO with higher solvent polarity, the maximum fluorescence emission is relatively weak at 428, 429, and 431 nm, respectively.In solvents of medium polarity, EA and CAN, the maximum fluorescence emission was at 433 and 439 nm, respectively.Meanwhile, complex 2 exhibits the relatively strong maximum fluorescence emission with relatively high solvent polarity in DMF, CAN, EtOH, and DMSO at 399, 402, 407, and 414 nm, respectively.In solvents DCM, TCM, and EA with lower solvent polarity, the maximum fluorescence emission is relatively weak at 372, 384 and 377 nm, respectively.Furthermore, the maximum fluorescence emission unusually appears at 373 nm in MeOH solvent.The influence of the solvent effect changes the luminescent properties of complexes 1 and 2, making its application areas broad [27,69].

Conclusion
In summary, we have reported the successful syntheses and characterizations of two newlydesigned complexes, [Zn(L 1 )(EtOH)] (1) and [{Zn(L 2 )(OAc)2}2Zn]•CHCl3 (2), derived from salamo and half-salamo chelating ligands (H2L 1 and HL 2 ).Complex 1 includes one Zn(II) ion, one completely deprotonated (L 1 ) 2− unit and one coordinated ethanol molecule, which shows a slightly distorted As we know, many fluorescent complexes, especially those containing polar substituents on aromatic rings, are susceptible to solvents [27].Due to the difference in polarity of solvents, complex 1 exhibits the relatively strong maximum fluorescence emission with relatively low solvent polarity in TCM and DCM at 439 and 440 nm, respectively.Additionally, in solvents THF, DMF, and DMSO with higher solvent polarity, the maximum fluorescence emission is relatively weak at 428, 429, and 431 nm, respectively.In solvents of medium polarity, EA and CAN, the maximum fluorescence emission was at 433 and 439 nm, respectively.Meanwhile, complex 2 exhibits the relatively strong maximum fluorescence emission with relatively high solvent polarity in DMF, CAN, EtOH, and DMSO at 399, 402, 407, and 414 nm, respectively.In solvents DCM, TCM, and EA with lower solvent polarity, the maximum fluorescence emission is relatively weak at 372, 384 and 377 nm, respectively.Furthermore, the maximum fluorescence emission unusually appears at 373 nm in MeOH solvent.The influence of the solvent effect changes the luminescent properties of complexes 1 and 2, making its application areas broad [27,69].

Figure 2 .
Figure 2. (a) Molecular structure and atom numberings of complex 1 with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity).(b) Coordination polyhedron for Zn(II) ion of complex 1.

Figure 2 .
Figure 2. (a) Molecular structure and atom numberings of complex 1 with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity).(b) Coordination polyhedron for Zn(II) ion of complex 1.

2 Figure 7 .
Figure 7.The UV-VIS spectra of the free ligands H2L 1 and HL 2 with their corresponding complexes 1 and 2 (cm −1 )

Figure 12 . 18 Figure 12 .
Figure 12.The normalized fluorescent spectra of complex 1. Inset image: the fluorescence picture of complex 1 in various solvents upon irradiation with a 365 nm UV lamp.

Figure 13 .
Figure 13.The normalized fluorescent spectra of complex 2. Inset image: the fluorescence picture of complex 1 in various solvents upon irradiation with a 365 nm UV lamp.

Figure 13 .
Figure 13.The normalized fluorescent spectra of complex 2. Inset image: the fluorescence picture of complex 1 in various solvents upon irradiation with a 365 nm UV lamp.

Table 1 .
Crystal data and structure refinement parameters for complexes 1 and 2.

Table 7 .
The maximum fluorescence emission in difference solvents for complexes 1 and 2.

Table 7
The maximum fluorescence emission in difference solvents for complexes 1 and 2.