A supramolecular tetranuclear zinc(II) complex constructed from an asymmetrical Salamo-type ligand: synthesis, structure and fluorescence properties

2.1 Materials and methods

2,5-Dihydroxyacetophenone and 3-hydroxylsalicylaldehyde were purchased from Sinopharm Chemical Reagent Co., Ltd., and used without further purification. The other reagents and solvents were analytical grade reagents from Tianjin Chemical Reagent Factory. Carbon, hydrogen and nitrogen were analyzed with a GmbH VariuoEL V3.00 automatic elemental analyzer. Elemental analysis for Zn was carried out with an IRIS ER/S·WP-1 ICP atomic emission spectrometer. FT-IR spectra were recorded on a VERTEX70 FT-IR spectrophotometer, with samples prepared as KBr (4000–500 cm⁻¹) pellets. UV/Vis absorption spectra in the 200–650 nm range were recorded on a Hitachi U-3900 spectrophotometer. Fluorescence spectra were taken on a Hitachi F-7000 fluorescence photometer.

2.2 Synthesis of the ligand H₃L

The synthesis of 5-methoxy-6′-hydroxy-2,2′-[ethanedioxybis(nitrilomethylidyne)]diphenol (H₃L) is shown in Scheme 1. 1,2-Bis(aminooxy)ethane and 2-[O-(1-ethyloxyamide)]oxime-5-methoxyphenol were synthesized according to the methods reported earlier [48], [53]. An ethanol solution (4 mL) of 2,3-dihydroxybenzaldehyde (256.32 mg, 2.0 mmol) was slowly added to the ethanol solution (4 mL) of 2-[O-(1-ethyloxyamide)]oxime-5-methoxyphenol (425.05 mg, 2.0 mmol), and the mixture was stirred at 52°C for 6 h. After cooling to room temperature, the precipitate was filtered and washed successively with ethanol and ethanol–hexane (1:4) (3×4 mL). The product was recrystallized from ethanol and dried in vacuo to get a yellow powder. Yield: 68%; m.p. 110–111.5°C. – Elemental analysis for C₁₇H₁₈N₂O₆ (346.19): calcd. C 59.84, H 6.21, N 7.41; found C 59.66, H 6.12, N 7.73. – ¹H NMR (400 MHz, CDCl₃): δ (ppm)=8.31 (s, 2H), 7.71 (d, J=8.9 Hz, 1H), 7.22 (d, J=8.4 Hz, 1H), 6.85 (d, J=9.5 Hz, 1H), 6.81 (d, J=7.6 Hz, 1H), 6.62 (d, J=7.2 Hz, 1H), 6.48 (s, 1H), 5.41 (s, 1H), 5.38 (s, 1H), 5.35 (s, 1H), 3.83 (s, 3H), 3.72 (d, J=7.6 Hz, 4H).

Scheme 1:

Synthetic route to ligand H₃L.

2.3 Synthesis of the Zn(II) complex

A solution of Zn(II) acetate monohydrate (2.19 mg, 0.01 mmol) in n-propanol (6 mL) was added dropwise to a solution of H₃L (3.46 mg, 0.01 mmol) in acetonitrile (2 mL) at room temperature. The mixture solution was stirred at 55°C for 5 h, and then filtered. The filtrate was allowed to stand at room temperature for about 2 weeks, and yellow single crystals were obtained suitable for X-ray crystallographic analysis. – Elemental analysis for C₄₄H₅₂N₄Zn₄O₁₈ (1211.5): calcd. C 44.52, H 4.51, N 4.70, Zn 22.07; found C, 44.54; H, 4.48; N, 4.73; Zn, 22.06.

2.4 Crystal structure determination of the Zn(II) complex

Single crystal X-ray diffraction data were collected at 173 K on a Bruker Smart APEX II CCD diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å). Unit cell parameters were determined by least-squares refinement. Lp and semi-empirical absorption corrections by SADABS were applied to the intensity data. The structure was solved by Direct Methods (SHELXS-97) and subsequent difference Fourier maps revealed the positions of the remaining atoms. All hydrogen atoms were added at idealized positions. All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F² with SHELXL-97. The crystal data and experimental parameters relevant to the structure determination are listed in Table 1.

CCDC 1520025 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 The crystal structure of the Zn(II) complex

Single crystal X-ray analysis of the Zn(II) complex revealed the formation of а tetranuclear structure. The Zn(II) complex crystallizes in the triclinic system, space group P1̅ with Z=1 tetranuclear units consisting of four Zn(II) atoms, two triply deprotonated L³⁻ moieties, two μ₂-coordinated acetate ions and n-propanol molecules. To the best of our knowledge, this new 2:4 (L³⁻ to Zn(II)) Salamo-type Zn(II) complex is rarely reported in comparison to the previously reported Salamo-type Zn(II) complexes having the composition of L to Zn(II) 1:1 [53], 2:3 [54] and 4:8 [55]. The molecular structure of the Zn(II) complex is shown in Fig. 1. Selected bond lengths and angles are listed in Table 2.

Fig. 1:

(a) Molecular structure and atom numbering of the Zn(II) complex (hydrogen atoms are omitted for clarity). (b) Coordination polyhedra for the Zn(II) atoms.

In the Zn(II) complex, the terminal Zn atom (Zn1 or Zn1^#) is penta-coordinated by two nitrogen atoms (N1 and N2) and oxygen atoms (O1 and O6) of one L³⁻ unit and one oxygen atom (O8) of the coordinated μ-acetate ion. The phenolic oxygen atom (O1), the oxime nitrogen atom (N1) of the L³⁻ unit and the oxygen atom (O8) of the μ-acetate ion constitute together the basal plane (Zn1–O1, 1.956(4) Å; Zn1–O8, 1.974(4) Å and Zn1–N1, 2.113(5) Å), and one phenolic oxygen atom (O6) and oxime nitrogen atom (N2) occupy the axial positions (Zn1–O6, 2.029(4) Å and Zn1–N2, 2.079(5) Å). Thus, the Zn1 or Zn1^# atoms have a slightly distorted trigonal bipyramidal environment which is deduced from the calculated value of τ₁=0.723 [51]. The central Zn atoms (Zn2 or Zn2^#) are penta-coordinated by three phenolic oxygen atoms (O5, O5^#1 and O6), one oxygen atom (O7) of the μ-acetate ion and one oxygen atom (O9) of the coordinated n-propanol molecule. Three phenolic oxygen atoms (O5, O5^#1 and O6) and the oxygen atom (O7) of the μ-acetate ion constitute together the basal plane (Zn2–O5, 2.031(4) Å; Zn2–O5^#1, 2.002(4) Å; Zn2–O6, 2.051(4) Å and Zn2–O7, 1.985(4) Å). The oxygen atom (O9) of the coordinated n-propanol molecule occupies the axial position (Zn2–O9, 2.056(5) Å). Therefore, the Zn2 or Zn2^# atoms have a distorted tetragonal pyramidal environment which is deduced from the calculated value of τ₂=0.411 [24].

3.2 Supramolecular interaction of the Zn(II) complex

In the crystal structure of the Zn(II) complex, there exist two pairs of intramolecular hydrogen bonds C8–H8B···O8 [56], [57], [58], C8^#–H8B^#···O8^#, O9–H9A···O1 and O9^#–H9A^#···O1^# in Fig. 2. As shown in Fig. 3, the Zn(II) complex is interlinked by C17–H17C···Cg1 (C11–C16) interaction into an infinite chain structure along the b axis. Hydrogen bonding parameters for the Zn(II) complex are given in Table 3.

Fig. 2:

Intramolecular hydrogen bonds for the Zn(II) complex.

Fig. 3:

Infinite supramolecular chain structure along the b axis in crystals of the Zn(II) complex.

3.3 IR spectra of H₃L and its Zn(II) complex

The FT-IR spectra of the ligand H₃L and its corresponding Zn(II) complex exhibit various bands in the 4000–500 cm⁻¹ region as listed in Table 4.

The IR spectrum of the ligand H₃L contains a C=N stretching vibration band at 1631cm⁻¹, which is shifted to 1602 cm⁻¹ in the spectrum of the Zn(II) complex indicating that the nitrogen atoms of the C=N group are coordinated with the Zn(II) atoms [59]. The Ar–O stretching vibration band of the ligand H₃L observed at 1242 cm⁻¹ appears at 1213 cm⁻¹ in the spectrum of the Zn(II) complex, indicating that Zn–O bonds are formed between Zn(II) atoms and oxygen atoms of phenolic groups [20]. The O–H stretching frequency of Salamo-type ligands is usually observed at the 3500–3300 cm⁻¹ region. However, this frequency is generally shifted to ca. 3430 cm⁻¹ due to internal hydrogen bonds (O–H···N=C) [53]. As the hydrogen bonds become stronger, the bandwidth increases, and this band sometimes is not detected. Electron-donating groups on the phenolic ring increase the electron density on the hydroxyl oxygen atom making the O–H bond stronger, and the absorption usually appears as a broad band. Here, the O–H stretching band of ligand H₃L appears at 3439 cm⁻¹. This band is disappeared in the IR spectrum of the Zn(II) complex, which is indicative of the fact that the phenolic OH groups of H₃L have been deprotonized completely and coordinated to the Zn(II) atoms. The absorption band at 3431 cm⁻¹ in the Zn(II) complex is assigned to the coordinated n-propanol molecules, which is confirmed by the crystal structure.

A far-infrared spectrum of the Zn(II) complex was also obtained in the region 560–100 cm⁻¹ in order to identify frequencies of the Zn–N and Zn–O bonds. ν(Zn–N) and ν(Zn–O) appear at 557 and 496 cm⁻¹, respectively. These assignments are consistent with literature data.

3.4 UV/Vis absorption spectra of H₃L and its Zn(II) complex

The absorption spectral data of H₃L and its Zn(II) complex were determined in a dilute aqueous ethanol solution (c=3×10⁻⁵ mol L⁻¹). The results are given in Table 5.

The absorption peaks of the Zn(II) complex are obviously different from those of the free ligand H₃L. The UV/Vis spectrum of the free ligand H₃L exhibits two absorption bands at 275 and 311 nm. The former can be assigned to the π–π* transition of the benzene rings and the latter one to the π–π* transition of the oxime groups [53]. On coordination of the ligand, the intraligand π–π* transition of the benzene ring of the salicylaldehyde group appears at ca. 289 nm in the Zn(II) complex. Compared with the free ligand H₃L, the absorption band at ca. 311 nm disappears from the UV/Vis spectrum of the Zn(II) complex, which indicates that the oxime nitrogen atoms are involved in coordination to the Zn(II) atoms. Moreover, the new absorption band is observed at ca. 340 nm for the Zn(II) complex, and assigned to the ligand-to-metal (L→M) charge transfer (LMCT) transition which is characteristic of the transition metal complexes with the N₂O₂ coordination sphere [60].

3.5 Fluorescence properties

The fluorescence properties of H₃L and its Zn(II) complex were investigated in CHCl₃ at room temperature and the results are shown in Fig. 4. The free ligand H₃L exhibits an intense emission peak at 407 nm upon excitation at 260 nm, which can be attributed to the intraligand π–π* transition [42]. The emission peak of the Zn(II) complex appears at 419 nm, exhibiting a red-shift of 12 nm of the LMCT [47]. The Zn(II) complex has a greater fluorescence intensity, and it may have the potential as a luminescent material.

Fig. 4:

Fluorescence spectra of H₃L and its Zn(II) complex.