Insight into Rare Structurally Characterized Homotrinuclear Cu II Non-Symmetric Salamo-Based Complex

: A rare homotrinuclear Cu II salamo-based complex [Cu 3 (L) 2 ( µ -OAc) 2 (H 2 O) 2 ] · 2CHCl 3 · 5H 2 O was prepared through the reaction of a non-symmetric salamo-based ligand H 2 L and Cu(OAc) 2 · H 2 O, and validated by elemental analyses, UV-Visible absorption, ﬂuorescence and infrared spectra, molecular simulation and single-crystal X-ray analysis techniques. It is shown that three Cu II atoms and two wholly deprotonated ligand (L) 2 − moieties form together a trinuclear 3:2 (M:L) complex with two coordination water molecules and two bi-dentate briging µ -acetate groups ( µ -OAc − ). Besides, the Hirshfeld surface analysis of the Cu II complex was investigated. Compared with other ligands, the ﬂuorescent strength of the Cu II complex was evidently lowered, showing that the Cu II ions possess ﬂuorescent quenching effect.


Introduction
Both the salen-based ligands and their derivatives have shown strong development potential in the research of materials chemistry, coordination chemistry, and environmental monitoring for decades because of their good application prospects in organic catalytic synthesis, molecular magnetic properties, and luminescent properties [1][2][3][4][5], which, owing to their N 2 O 2 -donor structure, usually have excellent coordination ability to transition metal ions for various structural novel complexes (derivatives) [6][7][8][9][10][11].
The fluorescence on-off phenomenon in the coordination reaction of the salamo-based ligands and Cu II ions can be used to identify and detect Cu II ions in the environment [39][40][41][42]. According to a large amount of preliminary research works [43][44][45][46][47][48][49], here, a non-symmetrical salamo-derived compound H 2 L was prepared, several single crystals of its Cu II complex were obtained by natural evaporation method in chloroform/ethanol mixed solvent at room temperature in about one month, and the structures and properties of H 2 L and its Cu II complex were further characterized by various modern analytical techniques.

Materials and Instruments
All chemical solvents and raw materials were acquired from mercantile sources and could be used directly. Elemental analysis of Cu II was tested via IRISER/S-WP-1 ICP atomic All chemical solvents and raw materials were acquired from mercantile sources and could be used directly. Elemental analysis of Cu II was tested via IRISER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany), and associated elemental analyses for carbon, hydrogen, and nitrogen were carried out by GmbH VariuoEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany).The study of IR spectra were recorded according to a Bruker VERTEX70 FT-IR spectrophotometer, with samples prepared as CsI (100-500 cm −1 ) and KBr (400-4000 cm −1 ) pellets (Bruker AVANCE, Billerica, MA, USA). The UV-Visible spectra were acquired from a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). The 1 H NMR spectra were tested via German Bruker AVANCE DRX-400/600 spectrometer (Bruker AVANCE, Billerica, MA, USA). Fluorescent spectra of H2L and its Cu II complex were conducted from an F-7000FL spectrophotometer (Hitachi, Tokyo, Japan). The structure of X-ray single-crystal determination was also carried out on a SuperNova Dual (Cu at zero) four-circle diffractometer. Finally, mass spectrum was recorded using the Bruker Daltonics Esquire 6000 mass spectrometer.

Preparation of the Cu II Complex
The Cu II complex was obtained by mixing H2L (3.5 mg, 0.01 mmol) in chloroform (3 mL) with Cu(OAc)2·H2O (3.0 mg, 0.015 mmol) in ethanol (5 mL) at room temperature, and the mixed solution color turned to brownish green. The brownish green mixture was filtered, and several single crystals were acquired via natural evaporation method. About

Preparation of the Cu II Complex
The Cu II complex was obtained by mixing H 2 L (3.5 mg, 0.01 mmol) in chloroform (3 mL) with Cu(OAc) 2 ·H 2 O (3.0 mg, 0.015 mmol) in ethanol (5 mL) at room temperature, and the mixed solution color turned to brownish green. The brownish green mixture was filtered, and several single crystals were acquired via natural evaporation method. About one week later, several brownish green block-like single crystals were obtained. Yield: 42

Determination of Single-Crystal Structure of the Cu II Complex
The single-crystal of the Cu II complex with approximate dimensions of 0.22 × 0.2 × 0.18 mm 3 was mounted on goniometer head of a SuperNova Dual (Cu at zero) diffractometer. The diffraction data were collected using a graphite mono-chromated Mo-Kα radiation (λ = 0.71073 Å) at 173(2) K. The structure was solved by using the program SHELXS-97 and Fourier difference techniques, and refined by full-matrix least-squares method on F 2 using SHELXL-2017. The nonhydrogen atoms were refined anisotropically. The hydrogen and carbon atoms of the molecule (C8, H8A and H8B sites occupancy disorder 0.450, and C9 , H9 A and H9 B sites occupancy disoeder 0.550) are disordered unequally. The crystallographic parameters of the Cu II complex are listed in Table 1.  28.3010(7) b/(Å) 28.3010(7) c/(Å) 15.

IR Spectra
The main infrared spectra of H 2 L and its Cu II complex are given in Table 2. The spectrum of H 2 L showed a strong stretching vibration band at about 3216 cm −1 which indicates the presence of multi molecular association and intramolecular hydrogen bonds (ν O-H ). However, this peak disappeared in the Cu II complex, reflecting that the O−H groups of H 2 L are wholly deprotonated [51]. A new O−H stretching vibration peak in the Cu II complex was observed at approximately 3420 cm −1 that belongs to the coordination water molecules [52]. The stretching vibration bands at 1609 (ν C=N ) and 1261 cm −1 (ν Ar-O ) of the ligand H 2 L were shifted to the low frequencies via ca. 6 and 11 cm -1 upon coordination [53]. Besides, the spectrum of the Cu II complex showed absorption bands at ca. 3425, 1606, and 547 cm −1 which could be assigned to the coordination water molecules, as is substantiated by the results of elemental analyses and the crystal structure [52]. At the same time, the far-infrared spectrum of the Cu II complex was also obtained in the range Crystals 2021, 11, 113 4 of 12 of the 100~500 cm −1 region so that a distinction could be made between frequencies of the Cu-O and Cu-N bonds, and new peaks of the Cu II complexes were found at ca. 455 and 512 cm −1 [54], respectively. These results support the proposal that strong binding participations have occurred in the Cu II complex [39]. Table 2. The main IR bands for H 2 L and its Cu II complex cm −1 . Compound

UV-Visible Spectra
The UV-Visible spectra of the ligand H 2 L and its Cu II complex were tested in methanol solution (1.0 × 10 −5 mol/L) at room temperature.
As depicted in Figure 1, the spectrum of H 2 L showed four relatively strong absorption peaks at approximately 301, 312, 340, and 355 nm, the absorption peak at 301 nm belongs to the π-π* transitions of the benzene rings [55]. The peaks at 312, 340, and 355 nm can be attributed to the π-π* transitions of the C=N bonds of intra-ligand [56]. The absorption peak of the Cu II complex appeared at about 314 nm; this peak could be appointed to π-π* transitions of the C=N bonds, indicating that coordination reaction occurred between H 2 L and the Cu II atoms [56,57]. Simultaneously, two new peaks were found at about 369 and 401 nm, which could be appointed to L→M charge-transfer transitions (LMCT). This is characteristic of the metal N 2 O 2 -donor complexes [57].
ecules [52]. The stretching vibration bands at 1609 (νC=N) and 1261 cm −1 (νAr-O) of the ligand H2L were shifted to the low frequencies via ca. 6 and 11 cm -1 upon coordination [53]. Besides, the spectrum of the Cu II complex showed absorption bands at ca. 3425, 1606, and 547 cm −1 which could be assigned to the coordination water molecules, as is substantiated by the results of elemental analyses and the crystal structure [52]. At the same time, the far-infrared spectrum of the Cu II complex was also obtained in the range of the 100~500 cm −1 region so that a distinction could be made between frequencies of the Cu-O and Cu-N bonds, and new peaks of the Cu II complexes were found at ca. 455 and 512 cm −1 [54], respectively. These results support the proposal that strong binding participations have occurred in the Cu II complex [39].

UV-Visible Spectra
The UV-Visible spectra of the ligand H2L and its Cu II complex were tested in methanol solution (1.0 × 10 −5 mol/L) at room temperature.
As depicted in Figure 1, the spectrum of H2L showed four relatively strong absorption peaks at approximately 301, 312, 340, and 355 nm, the absorption peak at 301 nm belongs to the π-π* transitions of the benzene rings [55]. The peaks at 312, 340, and 355 nm can be attributed to the π-π* transitions of the C=N bonds of intra-ligand [56]. The absorption peak of the Cu II complex appeared at about 314 nm; this peak could be appointed to π-π* transitions of the C=N bonds, indicating that coordination reaction occurred between H2L and the Cu II atoms [56,57]. Simultaneously, two new peaks were found at about 369 and 401 nm, which could be appointed to LM charge-transfer transitions (LMCT). This is characteristic of the metal N2O2-donor complexes [57]. In order to explain the coordination of the ligand H 2 L to Cu II ions, the UV-Vis absorption titration experiment was also performed (Figure 2). When the centration of Cu II ions were added gradually, a new absorption peak appeared between 345 nm and 460 nm, which inferred that the ligand H 2 L and Cu II ions coordinate in 1:1.5 ratio to produce a new L-Cu II complex.

Structure Analysis of the Cu II Complex
The Cu II complex crystallizes in the triclinic system, space group I4 1 /a. The bond lengths and angles are listed in Table 3. X-ray single-crystal data showed that three Cu II atoms and two completely deprotonated ligand (L) 2− moieties produce together a rare homotrinuclear 3:2 (M:L) complex with two coordination water molecules and two bi-dentate briging µ-acetate groups (µ-OAc − ). This structure differs from the usual mono-nuclear Cu II salamo-based complexes [58]. The six-coordinated terminal Cu II (Cu1) atom is sited at the N 2 O 2 cavity containing two phenolic oxygen (O4 and O1) and oxime nitrogen (N2 and N1) atoms in the ligand (L) 2− moiety, which forms a basic equatorial plane, and bound to the other two oxygen (O7 and O5) atoms coming from one coordination water molecule and the µ-OAc − group, respectively, at the end, forming a slightly distorted octahedral geometry. The central Cu II (Cu2) is located on a crystallographic center of inversion. More interestingly, the six-coordinated central Cu II (Cu2) atom is an octahedron, the Cu2 atom is surrounded by O 6 atoms, which involved two completely deprotonated ligand (L) 2− moieties and two bridged acetate (µ-OAc − ) groups (Figure 3a,b). The hydrogen bond data are summarized in Table 4. In order to explain the coordination of the ligand H2L to Cu II ions, the UV-Vis absorption titration experiment was also performed (Figure 2). When the centration of Cu II ions were added gradually, a new absorption peak appeared between 345 nm and 460 nm, which inferred that the ligand H2L and Cu II ions coordinate in 1:1.5 ratio to produce a new L-Cu II complex.

Structure Analysis of the Cu II Complex
The Cu II complex crystallizes in the triclinic system, space group I41/a. The bond lengths and angles are listed in Table 3. X-ray single-crystal data showed that three Cu II atoms and two completely deprotonated ligand (L) 2− moieties produce together a rare homotrinuclear 3:2 (M:L) complex with two coordination water molecules and two bi-dentate briging μ-acetate groups (μ-OAc − ). This structure differs from the usual mono-nuclear Cu II salamo-based complexes [58]. The six-coordinated terminal Cu II (Cu1) atom is sited at the N2O2 cavity containing two phenolic oxygen (O4 and O1) and oxime nitrogen (N2 and N1) atoms in the ligand (L) 2− moiety, which forms a basic equatorial plane, and bound to the other two oxygen (O7 and O5) atoms coming from one coordination water molecule and the μ-OAc − group, respectively, at the end, forming a slightly distorted octahedral geometry. The central Cu II (Cu2) is located on a crystallographic center of inversion. More interestingly, the six-coordinated central Cu II (Cu2) atom is an octahedron, the Cu2 atom is surrounded by O6 atoms, which involved two completely deprotonated ligand (L) 2− moieties and two bridged acetate (μ-OAc − ) groups (Figure 3a,b). The hydrogen bond data are summarized in Table 4.      In addition, there are three couple of intra-molecular hydrogen bondings (C9 -H9 A· · · O5, O7-H7A· · · N2 and O7-H7B· · · O4) in the Cu II complex [59], as depicted in Figure 4. In addition, there are three couple of intra-molecular hydrogen bondings (C9-H9A⋯O5, O7-H7A⋯N2 and O7-H7B⋯O4) in the Cu II complex [59], as depicted in Figure  4.

Molecular Simulation Calculation of H 2 L and Its Cu II Complex
In order to better investigate the structures of H 2 L and its Cu II complex, the DMol 3 module of MS (Materials Studio) software was used to optimize and simulate the molecules of H 2 L and its Cu II complex [60]. The method of structural optimization (property calculation) is GGA, BP (PBE) with the base set DND (DNP), the solvent model (ethanol), the optimization precision set medium, and smooth thermal smearing to speed up the convergence of structural optimization. The molecule energies and frontier molecular orbital energies of H 2 L and its Cu II complex are shown in Table 5. For H 2 L, it could be found that the calculated energy gap between the LUMO and HOMO of the Cu II complex (0.984 ev) is lower than that of H 2 L (1.803 ev) ( Figure 5). According to the frontier orbital theory, the photoinduced electron transfer (PET) may be caused by fluorescence quenching [24].

Fluorescence Spectra
The fluorescent properties of the ligand and its Cu II complex were invested in 1 × 10 −5 M ethanol solution at 349 nm excitation wavelength. Corresponding spectra are depicted in Figure 6.
The Cu II complex underwent fluorescence quenching at 434 nm and the emission peak is red-shifted, this can be appointed to LMCT [61]. Owing to the H2L molecule's nonbonding pairs on the oxime N atoms where there is a PET (photoinduced electron transfer) process from the N atom to the benzene ring. Due to the existence of Cu II ions, the fluorescent strength of the system is quenched. This result reflects that Cu II ions interact with the system effectually and have the PET (photoinduced electron transfer) effect, which attenuates the fluorescent strength [62].

Fluorescence Spectra
The fluorescent properties of the ligand and its Cu II complex were invested in 1 × 10 −5 M ethanol solution at 349 nm excitation wavelength. Corresponding spectra are depicted in Figure 6.

Fluorescence Spectra
The fluorescent properties of the ligand and its Cu II complex were invested in 1 × 10 −5 M ethanol solution at 349 nm excitation wavelength. Corresponding spectra are depicted in Figure 6.
The Cu II complex underwent fluorescence quenching at 434 nm and the emission peak is red-shifted, this can be appointed to LMCT [61]. Owing to the H2L molecule's nonbonding pairs on the oxime N atoms where there is a PET (photoinduced electron transfer) process from the N atom to the benzene ring. Due to the existence of Cu II ions, the fluorescent strength of the system is quenched. This result reflects that Cu II ions interact with the system effectually and have the PET (photoinduced electron transfer) effect, which attenuates the fluorescent strength [62].  The Cu II complex underwent fluorescence quenching at 434 nm and the emission peak is red-shifted, this can be appointed to LMCT [61]. Owing to the H 2 L molecule's non-bonding pairs on the oxime N atoms where there is a PET (photoinduced electron transfer) process from the N atom to the benzene ring. Due to the existence of Cu II ions, the fluorescent strength of the system is quenched. This result reflects that Cu II ions interact with the system effectually and have the PET (photoinduced electron transfer) effect, which attenuates the fluorescent strength [62].

Hirshfeld Surface Analysis
Hirshfeld surface supplies a 3-D figure of inter-molecular inter-actions in the Cu II complex (Figure 7) [63], which could clearly indicate that the surfaces have been mapped over d norm and the corresponding location in shape index exists in the complementary region of red concave surface surrounded by receptors and the blue convex surface surrounding receptors, further proving that such hydrogen bonding exists. The large and deep red spots on the three-dimensional (3D) Hirshfeld surfaces indicate close-contact interactions, which are mainly responsible for the corresponding hydrogen bond contacts. As for the large amount of white region in the d norm surfaces, it is suggested that there is a weaker and farther contact between molecules, rather than hydrogen bonding. The red zone expresses the O-H between the H and O atoms in the Cu II complex. In the interaction intensity figure, the heavier the red area color is, the stronger O-H inter-actions are. As illustrated, the shallower areas mostly represent the spread of influences such as H-H and C-H. As illustrated in the figure, the spread of the approximated hydrogen bonds among the Cu II complex could also be analyzed. This is conducive of investigating inherent elements of the steady existence among the Cu II complex [64].

. Hirshfeld Surface Analysis
Hirshfeld surface supplies a 3-D figure of inter-molecular inter-actions in the Cu II complex (Figure 7) [63], which could clearly indicate that the surfaces have been mapped over dnorm and the corresponding location in shape index exists in the complementary region of red concave surface surrounded by receptors and the blue convex surface surrounding receptors, further proving that such hydrogen bonding exists. The large and deep red spots on the three-dimensional (3D) Hirshfeld surfaces indicate close-contact interactions, which are mainly responsible for the corresponding hydrogen bond contacts. As for the large amount of white region in the dnorm surfaces, it is suggested that there is a weaker and farther contact between molecules, rather than hydrogen bonding. The red zone expresses the O-H between the H and O atoms in the Cu II complex. In the interaction intensity figure, the heavier the red area color is, the stronger O-H inter-actions are. As illustrated, the shallower areas mostly represent the spread of influences such as H-H and C-H. As illustrated in the figure, the spread of the approximated hydrogen bonds among the Cu II complex could also be analyzed. This is conducive of investigating inherent elements of the steady existence among the Cu II complex [64]. In addition, the proportion of C-H/H-C, O-H/H-O, and H-H in the Cu II complex can also be acquired by Hirshfeld surfaces analyses [65][66][67][68][69]. Here, we theoretically calculated the percentages of connects devoted to the total Hirshfeld surface region of the Cu II complex.
As shown in Figure   Hirshfeld surface supplies a 3-D figure of inter-molecular intercomplex (Figure 7) [63], which could clearly indicate that the surfaces over dnorm and the corresponding location in shape index exists in the gion of red concave surface surrounded by receptors and the blue c rounding receptors, further proving that such hydrogen bonding ex deep red spots on the three-dimensional (3D) Hirshfeld surfaces indica teractions, which are mainly responsible for the corresponding hydro As for the large amount of white region in the dnorm surfaces, it is sugg weaker and farther contact between molecules, rather than hydrogen zone expresses the O-H between the H and O atoms in the Cu II comple intensity figure, the heavier the red area color is, the stronger O-H in illustrated, the shallower areas mostly represent the spread of influence C-H. As illustrated in the figure, the spread of the approximated hydr the Cu II complex could also be analyzed. This is conducive of investig ments of the steady existence among the Cu II complex [64].

Conclusions
In summary, we prepared the non-symmetric salamo-derived ligand H 2 L and several single crystals of its Cu II complex. [Cu 3 (L) 2 (µ-OAc) 2 (H 2 O) 2 ]·2CHCl 3 ·5H 2 O were cultured by slow evaporation method and various test methods were characterized. Interestingly, the single crystal structure analysis showed that H 2 L and Cu II ions form a symmetric trinuclear Cu II complex. The UV-Visible titration clearly showed that the radio of H 2 L to Cu II ions has a 2:3 stoichiometry. Hirshfeld surface analysis indicated that the Cu II complex could be stable due to intra-molecular hydrogen bond interactions.