Unprecedented Dinuclear Cu II N,O-Donor Complex: Synthesis, Structural Characterization, Fluorescence Property, and Hirshfeld Analysis

: An unprecedented dinuclear Cu II complex, [Cu 2 (L 2 ) 2 ], derived from a salamo-like chelating ligand H 2 L 2 , was produced by the cleavage of a newly synthesized, half-salamo-like ligand HL 1 (2-[ O -(1-ethyloxyamide)]oxime-3,5-dichloro-phenol). This was synthesized and characterized by elemental analyses, IR, UV–Vis and ﬂuorescent spectra, single crystal X-ray di ﬀ raction analysis, and Hirshfeld surface analysis. X-ray crystallographic analysis indicated that the two Cu II (Cu1 and Cu2) ions bore di ﬀ erent (N 2 O 3 and N 2 O 2 ) coordination environments, the penta-coordinated Cu1 ion possessed a slightly twisted tetragonal pyramid geometry with the τ value τ = 0.004, and the tetra-coordinated Cu2 ion showed a slightly twisted square planar geometry. Interestingly, one oxime oxygen atom participated in the coordination reported previously. Moreover, an inﬁnite two-dimensional layered supramolecular network was formed. Compared with HL 1 , the Cu II complex possessed the characteristic of ﬂuorescence quenching. nm. Anal. Calc. for C 32 H 20 Cl 8 Cu 2 N 4 O 8 (%): C, 38.46; H, 2.02; N, 5.61; Cu, 12.72.


Preparation of the Cu II Complex
The synthetic route to the Cu II complex is depicted in Scheme 2. An acetone solution (3 mL) of HL 1 (10.60 mg, 0.04 mmol) was added to a methanol solution (3 mL) of Cu(OAc) 2 ·H 2 O (7.96 mg, 0.04 mmol). The mixed solution was stirred for 15 min and then filtered into a vial, which was sealed with aluminum foil. About two weeks later, some bright brown block-like crystals suitable for X-ray diffraction were gained and collected carefully. Yield: 54 The synthetic route to the Cu II complex is depicted in Scheme 2. An acetone solution (3 mL) of HL 1 (10.60 mg, 0.04 mmol) was added to a methanol solution (3 mL) of Cu(OAc)2·H2O (7.96 mg, 0.04 mmol). The mixed solution was stirred for 15 min and then filtered into a vial, which was sealed with aluminum foil. About two weeks later, some bright brown block-like crystals suitable for X-ray diffraction were gained and collected carefully. Yield

Crystal Structure Determination of the Cu II complex
The crystal structure determination of the Cu II complex is given in the Supplementary Materials. The key crystal data and structural parameters of the Cu II complex are summarized in Table 1. CCDC: 1959387.

Crystal Structure Determination of the Cu II Complex
The crystal structure determination of the Cu II complex is given in the Supplementary Materials. The key crystal data and structural parameters of the Cu II complex are summarized in Table 1. CCDC: 1959387.

Infrared Spectra
The infrared spectra of HL 1 and the Cu II complex exhibited various bands in the 500-4000 cm −1 range. As shown in Table 2 and Figure S1, the infrared spectrum of the ligand HL 1 exhibited a broad characteristic band at ca. 3438 cm −1 and can be attributed to the characteristic band of the ν OH group. This band is weakened in the IR spectrum of the Cu II complex, which is indicative of the fact that the phenolic OH groups of the ligand H 2 L 2 have been deprotonated and coordinated to the Cu II ions [59,60]. The ligand HL 1 showed a characteristic C=N stretching band at ca. 1615 cm −1 , while the C=N stretching band of the Cu II complex appeared at ca. 1609 cm −1 [61]. In addition, the ligand HL 1 exhibited typical aromatic C=C skeleton vibration bands at ca. 1449 and 1470 cm −1 and appeared at ca. 1443 and 1507 cm −1 for the Cu II complex [62]. Meanwhile, the free ligand HL 1 exhibited an Ar-O stretching band at ca. 1214 cm −1 and that of the Cu II complex appeared at ca. 1205 cm −1 , meaning the Ar-O stretching vibration frequency had shifted to a lower frequency. It is indicated that the Cu-O bonds are formed between the Cu II ions and the phenoxy atoms of the free ligand H 2 L 2 [63].

UV-Vis Spectra
The UV-Vis absorption and titration spectra of HL 1 and the Cu II complex at room temperature are depicted in Figure 1. In the UV-Vis titration experiment (micro drop), an ethanol solution of the ligand HL 1 was prepared at a concentration of 5.0 × 10 −5 M, and an aqueous solution of Cu(OAc) 2 ·H 2 O was prepared with distilled water at a concentration of 1.0 × 10 −3 M. Meanwhile, a new absorption peak appeared at ca. 382 nm, which was attributed to the characteristic LMCT of salamo-like transition metal complexes [66,67]. When the amount of Cu 2+ droplets was added to 1.0 equivalent, the absorption peak at 382 nm reached the maximum value and did not change. The titration spectral data indicated that the ratio of displacement reaction was 1:1 ([Cu 2+ ]/[HL 1 ]).

Crystal Structure Description
The crystal structure of the Cu II complex and the coordination polyhedra of the Cu II ions are depicted in Figure 2. Significant bond lengths and angles are summarized in Table 3. Two typical absorption peaks of HL 1 at ca. 266 and 324 nm were clearly observed. The two peaks can be attributed respectively to the π-π* transitions of the benzene ring and the oxime group [64,65]. When the concentration of the Cu II ion increased gradually, the absorption peaks had evidently changed to 271 nm. Compared with the peak (266 nm) of the free ligand HL 1 , this absorption peak was red-shifted, and another absorption peak at 324 nm disappeared in the UV-Vis absorption spectrum of the Cu II complex, which indicated that the oxime nitrogen atoms of the ligand H 2 L 2 have coordinated to the Cu II ions [14,16].
Meanwhile, a new absorption peak appeared at ca. 382 nm, which was attributed to the characteristic LMCT of salamo-like transition metal complexes [66,67]. When the amount of Cu 2+ droplets was added to 1.0 equivalent, the absorption peak at 382 nm reached the maximum value and did not change. The titration spectral data indicated that the ratio of displacement reaction was 1:1 ([Cu 2+ ]/[HL 1 ]).

Crystal Structure Description
The crystal structure of the Cu II complex and the coordination polyhedra of the Cu II ions are depicted in Figure 2. Significant bond lengths and angles are summarized in Table 3. The Cu II complex crystallized in the triclinic system, space group P -1, which clearly indicated that the structure of the resulting complex was an unexpected dinuclear Cu II complex of salamo-like ligand H2L 2 , not an expected Cu II complex of half-salamo-like ligand HL 1 . Here, the half-salamo-like ligand HL 1 was converted to a salamo-like ligand H2L 2 . This phenomenon of oxime oxygen atom participating in coordination has not been previously reported in the literature [6,9,11−13] and may be due to the catalysis of the Cu II ion upon coordination [16]. Compared with the previously reported M/L as 1:2 [68], 3:2 [69], and 2:2 [70] complexes, it consisted of two Cu II ions and two wholly deprotonated (L 2 ) 2-units. The two Cu II ions (Cu1 and Cu2) have different (N2O3 and N2O2) coordination environments. The Cu II ion (Cu1) is penta-coordinated with the donor N2O2 atoms (N1, N2, O1, and O4) of the (L 2 ) 2-unit and one oxime oxygen atom (O6) from another deprotonated (L 2 ) 2unit. The four donor atoms (N1, N2, O1, and O4) formed a base plane, and the dihedral angle of the N1-Cu1-O1 and O4-Cu1-N2 planes was at 2.09(3)°. The axial position was occupied by the oxime oxygen atom (O6); thus, the Cu1 ion possessed a slightly twisted square pyramidal geometry with the τ value τ = 0.004 (τ < 0.5) [15,61]. The distance of Cu1-O6 (2.508 (2)  The Cu II complex crystallized in the triclinic system, space group P-1, which clearly indicated that the structure of the resulting complex was an unexpected dinuclear Cu II complex of salamo-like ligand H 2 L 2 , not an expected Cu II complex of half-salamo-like ligand HL 1 . Here, the half-salamo-like ligand HL 1 was converted to a salamo-like ligand H 2 L 2 . This phenomenon of oxime oxygen atom participating in coordination has not been previously reported in the literature [6,9,[11][12][13] and may be due to the catalysis of the Cu II ion upon coordination [16]. Compared with the previously reported M/L as 1:2 [68], 3:2 [69], and 2:2 [70] complexes, it consisted of two Cu II ions and two wholly deprotonated (L 2 ) 2− units. The two Cu II ions (Cu1 and Cu2) have different (N 2 O 3 and N 2 O 2 ) coordination environments. The Cu II ion (Cu1) is penta-coordinated with the donor N 2 O 2 atoms (N1, N2, O1, and O4) of the (L 2 ) 2− unit and one oxime oxygen atom (O6) from another deprotonated (L 2 ) 2− unit. The four donor atoms (N1, N2, O1, and O4) formed a base plane, and the dihedral angle of the N1-Cu1-O1 and O4-Cu1-N2 planes was at 2.09(3) • . The axial position was occupied by the oxime oxygen atom (O6); thus, the Cu1 ion possessed a slightly twisted square pyramidal geometry with the τ value τ = 0.004 (τ < 0.5) [15,61]. The distance of Cu1-O6 (2.508(2)) was considerably longer than the Cu-O and Cu-N bonds (Cu1-O1, 1.914(2); Cu1-O4, 1.921(2); Cu1-N1, 2.014(3) and Cu1-N2, 1.964(3)) in [Cu 2 (L 2 ) 2 ], indicating a weaker interaction. The lengthening of Cu1-O6 should be assigned to the involvement of O6 in a dimer bridge formation; similar elongation of the M-O bond has also been found in the dimer of [Cu 2 (L 2 ) 2 ] [71]. The Cu II ion labelled as Cu2 shows a twisted square planar coordination sphere, being the donor atoms N3, N4, O5, and O8 provided by the deprotonated (L 2 ) 2− unit, and the dihedral angle of the N3-Cu2-O5 and O8-Cu2-N4 planes was about 10.29(3) • .

Supramolecular Interactions and Hirshfeld Surface Analysis
In the crystal structure of the Cu II complex, there were three intermolecular (C8−H8A···O1, C24−H24A···Cl5, and C25−H25A···O8) and one intramolecular (C24−H24A···O3) hydrogen bondings (see Table 4), which played a role in stabilizing the crystal structure of the Cu II complex [72,73]. As a result, the Cu II complex formed a self-assembled infinite two-dimensional supramolecular structure via intermolecular hydrogen bondings, as shown in Figure 3. In particular the chlorine atom (Cl5) of the ligand formed intermolecular hydrogen bondings, which gave rise to a supramolecular structure that differs from most of the salamo-like metal complexes previously reported. In fact, the latter always assembled supramolecular structures through the formation of intermolecular hydrogen bondings involving solvent molecules [22,30,[35][36][37]. The intramolecular and intermolecular interactions of molecular crystal of the Cu II complex were explored further by surface analysis using the Crystal Explorer program [14]. The interactions in molecular crystal can be clearly observed. In the Hirshfeld surface, the existence of strong interactions showed up as red spots [74], while the blue regions were related to weak contacts. The Hirshfeld surface analysis of the Cu II complex is as depicted in Figure 4a-e.
(see Table 4), which played a role in stabilizing the crystal structure of the Cu II complex [72,73]. As a result, the Cu II complex formed a self-assembled infinite two-dimensional supramolecular structure via intermolecular hydrogen bondings, as shown in Figure 3. In particular the chlorine atom (Cl5) of the ligand formed intermolecular hydrogen bondings, which gave rise to a supramolecular structure that differs from most of the salamo-like metal complexes previously reported. In fact, the latter always assembled supramolecular structures through the formation of intermolecular hydrogen bondings involving solvent molecules [22,30,35−37].  1 -x, 1 -y, 1z The intramolecular and intermolecular interactions of molecular crystal of the Cu II complex were explored further by surface analysis using the Crystal Explorer program [14]. The interactions in molecular crystal can be clearly observed. In the Hirshfeld surface, the existence of strong interactions showed up as red spots [74], while the blue regions were related to weak contacts. The Hirshfeld surface analysis of the Cu II complex is as depicted in Figure 4a-e. The short-range interaction distribution of the Cu II complex was calculated by Hirshfeld surface two-dimensional (2D) fingerprint [75]. The grey represented all regions of the fingerprint, and the blue region was used to quantify the interaction between molecules, as depicted in Figure 5  The short-range interaction distribution of the Cu II complex was calculated by Hirshfeld surface two-dimensional (2D) fingerprint [75]. The grey represented all regions of the fingerprint, and the blue region was used to quantify the interaction between molecules, as depicted in Figure 5. The proportions were as follows: C-H/H-C, O-H/H-O, H-H/ H-H, and Cl-H/H-Cl interactions, incorporating 8.4%, 6.8%, 14.7%, and 48.0%, respectively, of the total Hirshfeld surfaces for every molecule of the Cu II complex. It was clear from the above results that the intermolecular forces on the whole surface of Hirshfeld mainly came from Cl-H/H-Cl interaction. This is different from the previously reported salamo-like complexes [55,56], as the main interaction between them was from H-H/H-H interaction. In the Cu II complex, Cl-H/H-Cl was the main interaction, which was consistent with the formation of supramolecular structure.

Fluorescence Properties
The fluorescence properties of HL 1 and the Cu II complex were studied in ethanol solution (5.0 × 10 −5 M) at room temperature at ca. 390 nm excitation wavelength, as shown in Figure 6. The strong fluorescence of the ligand HL 1 at ca. 460 nm can be attributed to the π-π* transition. The fluorescence quenching of the Cu II complex at 460 nm can be observed at the same excitation wavelength, indicating the strong coordination of Cu II ions with nitrogen and oxygen atoms of the ligand H2L 2 [76,77]. The binding affinity between Cu II ions and HL 1 was determined further by fluorescence titration. With the increase of Cu II ions (c = 1.0 × 10 −3 M), the fluorescence intensities decreased linearly. When the concentration of Cu II ions increased to 1.0 equivalent, the fluorescence completely quenched and reached the end of titration. Through the determination of a job curve (illustration), it was found to be consistent with the titration result. Furthermore, according to the corrected Benesi-Hildebrand formula, the bonding constant of the Cu II ions to the ligand HL 1 calculated in the Supplementary Materials was estimated to be 5.09 × 10 10 M −1 [37].

Fluorescence Properties
The fluorescence properties of HL 1 and the Cu II complex were studied in ethanol solution (5.0 × 10 −5 M) at room temperature at ca. 390 nm excitation wavelength, as shown in Figure 6. The strong fluorescence of the ligand HL 1 at ca. 460 nm can be attributed to the π-π* transition. The fluorescence quenching of the Cu II complex at 460 nm can be observed at the same excitation wavelength, indicating the strong coordination of Cu II ions with nitrogen and oxygen atoms of the ligand H 2 L 2 [76,77]. The binding affinity between Cu II ions and HL 1 was determined further by fluorescence titration. With the increase of Cu II ions (c = 1.0 × 10 −3 M), the fluorescence intensities decreased linearly. When the concentration of Cu II ions increased to 1.0 equivalent, the fluorescence completely quenched and reached the end of titration. Through the determination of a job curve (illustration), it was found to be consistent with the titration result. Furthermore, according to the corrected Benesi-Hildebrand formula, the bonding constant of the Cu II ions to the ligand HL 1 calculated in the Supplementary Materials was estimated to be 5.09 × 10 10 M −1 [37].
titration. With the increase of Cu II ions (c = 1.0 × 10 −3 M), the fluorescence intensities decreased linearly. When the concentration of Cu II ions increased to 1.0 equivalent, the fluorescence completely quenched and reached the end of titration. Through the determination of a job curve (illustration), it was found to be consistent with the titration result. Furthermore, according to the corrected Benesi-Hildebrand formula, the bonding constant of the Cu II ions to the ligand HL 1 calculated in the Supplementary Materials was estimated to be 5.09 × 10 10 M −1 [37].

Conclusion
In this work, we have designed and synthesized an unprecedented dinuclear Cu II complex [Cu2(L 2 )2], and a series of characterizations were made in detail. The crystal structure analysis of the Cu II complex indicated that the penta-coordinated Cu1 ion possessed a slightly twisted square pyramidal geometry, and the tetra-coordinated Cu2 ion had a twisted square planar one. The Cu II complex was self-assembled by intermolecular C-H···O hydrogen-bonding interactions to form a 2D layered supramolecular network. Interestingly, one oxime oxygen atom participated in the coordination reported previously in the salamo-like metal complexes. The Cu II complex possessed the characteristic of fluorescence quenching.
Supplementary Materials: The following are available online at www.mdpi.com/2073-4352/9/12/607/s1, Figure  S1: The infrared spectra of the ligand HL 1 and the Cu II complex. Acknowledgments: In this section you can acknowledge any support given which is not covered by the author

Conclusions
In this work, we have designed and synthesized an unprecedented dinuclear Cu II complex [Cu 2 (L 2 ) 2 ], and a series of characterizations were made in detail. The crystal structure analysis of the Cu II complex indicated that the penta-coordinated Cu1 ion possessed a slightly twisted square pyramidal geometry, and the tetra-coordinated Cu2 ion had a twisted square planar one. The Cu II complex was self-assembled by intermolecular C-H···O hydrogen-bonding interactions to form a 2D layered supramolecular network. Interestingly, one oxime oxygen atom participated in the coordination reported previously in the salamo-like metal complexes. The Cu II complex possessed the characteristic of fluorescence quenching.