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Communication

Synthesis and Fluorescence Properties of Asymmetrical Salamo-Type Tetranuclear Zinc(II) Complex

by
Yun-Dong Peng
,
Xiao-Yan Li
,
Quan-Peng Kang
,
Gao-Xian An
,
Yang Zhang
and
Wen-Kui Dong
*
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(2), 107; https://doi.org/10.3390/cryst8020107
Submission received: 25 January 2018 / Revised: 19 February 2018 / Accepted: 22 February 2018 / Published: 24 February 2018
(This article belongs to the Section Crystal Engineering)
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Abstract

:
A new tetranuclear zinc(II) complex with an asymmetrical Salamo-type chelating ligand, H3L (5-methoxy-6′-hydroxy-2,2′-[ethylenedioxybis(nitrilomethylidyne)]diphenol), was synthesized and characterized using FT-IR, elemental analyses, X-ray single crystal diffraction method, UV-Vis, and fluorescence spectra. The zinc(II) complex possesses the cell parameters a = 8.1960(7) Å, b = 9.8127(8) Å, c = 16.5428(15) Å, Z = 1, V = 1172.5(2) Å3, R1 = 0.0722, and wR2 = 0.1558, and crystallizes in the triclinic system, with space group P-1. X-ray crystal structure analysis reveals that Zn1 and Zn2 atoms are all pentacoordinated and adopt slightly twisted tetragonal pyramidal and trigonal bipyramidal geometries. The zinc(II) complex forms a 1D supramolecular chain via intermolecular hydrogen bonds along the b axis. Besides, the fluorescence properties have been discussed.

Graphical Abstract

1. Introduction

Over the past years, research has shown the Salen-type [1,2,3,4,5,6,7,8] and Salamo-type [9,10,11,12,13,14,15,16,17] compounds to be exceptionally good chelating ligands in in the fields of organometallic chemistry and coordination chemistry. The Salamo metal complexes are widely utilized in various fields such as industrial catalyses [18,19], biological fields [20,21], ion recognitions [22,23,24], environmental sciences [25,26,27,28], and magnetic [29,30] and luminescent materials [31,32,33,34,35,36,37,38,39]. Recently, a lot of researchers have tried many approaches to change the (–CH=N–(CH2)n–N=CH–) instead of (–CH=N–O–(CH2)n–O–N=CH–) unit in order to make the exchange reaction and hydrolysis reaction rate of the compounds greatly reduced and the balance level raised to a very great extent, so the Salamo-type compounds are more stable than the Salen-type compounds [40,41,42]. Research on Salen-type compounds has yet to be fully explored. Besides, their complexes often form supramolecular systems that have unique structures, a novel bonding pattern, a specific microstructure, and excellent macroscopic properties by the intermolecular association of the weak non-covalent bonding [43,44,45]. More ideal structural variations, drastic changes in characteristics of the previous complexes, and novel properties could be obtained through the asymmetric configuration. Herein, a new asymmetrical Salamo-type compound, 5-methoxy-6′-hydroxy-2,2′-(ethylenedioxybis(nitrilomethylidyne))diphenol (H3L) and its zinc(II) complex, were designed and synthesized and structurally characterized.

2. Experimental

2.1. Materials and Measurements

3-hydroxysalicyladehyde (99%) and 4-Methoxysalicylaldehyde (98%) were purchased from Alfa Aesar (New York, NY, USA), while Tianjin Chemical Reagent Factory supplied the remaining reagents. Elemental analysis for zinc was detected by IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany). C, H, and N were analyzed using GmbH VariuoEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany). IR spectra (400–4000 cm−1) were recorded on a Vertex 70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr pellets. UV-vis absorption spectra were recorded on a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). 1H NMR spectra were determined by German Bruker AVANCE DRX-400/600 spectroscopy (Bruker AVANCE, Billerica, MA, USA). Fluorescence spectra were recorded on an F-7000 FL spectrophotometer (Hitachi, Tokyo, Japan). X-ray single crystal structure determination of the zinc(II) complex was carried out on a SuperNova Dual (Cu at zero) four-circle diffractometer.

2.2. Synthesis of H3L

The synthesis of 5-methoxy-6′-hydroxy-2, 2′-[ethanedioxybis(nitrilomethylidyne)]diphenol (H3L) is given (Scheme 1).
1,2-Bis(aminooxy)ethane and 2-[O-(1-ethyloxyamide)]oxime-5-methoxyphenol were synthesized according to an analogous method reported earlier [15,46]. A colorless 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), respectively. The product was purified with recrystallization from ethanol and dried in vacuo to get a yellow powder. Yield: 80.7%. m.p. 110–111.5 °C. Anal. calcd. for C17H18N2O6 (%): 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 9.92 (d, J = 7.8 Hz, 2H), 8.19 (d, J = 18.3 Hz, 1H), 7.05 (d, J = 8.4 Hz, 1H), 6.96 (d, J = 7.2 Hz, 1H), 6.87–6.71 (m, 2H), 6.53–6.44 (m, 2H), 4.47 (d, J = 6.7 Hz, 3H), 3.81 (s, 4H).

2.3. Synthesis of the Zinc(II) Complex

A dropwise solution of Zn(CH3COO)2·H2O (8.78 mg, 4 mmol) in ethanol (9 mL) was added to a solution of H3L (6.84 mg, 2 mmol) in chloroform (5 mL) at r.t., after which the mixed solution was stirred for 6 h at 60 °C and was then filtered off. The filtrate was allowed to stand at r.t. for several days, and yellow prismatical single crystals suitable for X-ray crystallographic analysis were obtained. Yield: 51.6%. Anal. calcd. for C42H48N4Zn4O18 (%): C, 43.54; H, 4.19; N, 4.89; Zn, 22.55. Found: C, 43.55; H, 4.18; N, 4.84; Zn, 22.58. Table 1 shows the data collection and refinements of the zinc(II) complex.

2.4. X-ray Crystallography

Single crystal X-ray diffraction data were collected at 173 K on a SuperNova Dual (Cu at zero) four-circle diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by the direct methods and all hydrogen atoms were added. All non-hydrogen atoms were refined anisotropically using a full-matrix least-squares procedure on F2 with SHELXL-2014 [47,48]. The LP factor and semi-empirical absorption correction by SADABS were applied to the intensity data. The crystal data and experimental parameters relevant to the structure determination are listed in Table 1, and the final positional and thermal parameters are available as supplementary material.
Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication, with CCDC-1519979 for the zinc(II) complex. 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: [email protected]). These data can be also obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.

3. Results and Discussion

3.1. FT-IR Spectra

The most important features in the FT-IR spectra data of H3L and its zinc(II) complex are listed in Table 2.
Data obtained from the FT-IR spectra show differences between H3L and its zinc(II) complex, and the analysis gives hint about the coordination reaction between H3L and zinc(II) atoms, and thus suggests formation of a new zinc(II) complex. The characteristic C=N stretching band of H3L is found at 1631 cm−1, and that of zinc(II) complex appears at 1600 cm−1 [49,50,51]. Upon complexation, this band shifts by a ca. 31 cm−1 to a lower frequency, and thus indicates that C=N bond order decreases owing to the binding of the zinc(II) atom to oxime nitrogen atoms [40]. The stretching frequency band of the Aromatic-O atom appears at 1283 cm−1 for H3L, while for the zinc(II) complex the band appears at 1265 cm−1 [40,52]. The frequency shift in the Aromatic-O stretching band shows that interaction between the oxygen atoms of phenolic group and zinc(II) atom results in the Zn–O bonds formation [46]. Besides, the O–H stretching band of H3L is observed at 3439 cm−1, while the absorption broad band at 3424 cm−1 in the zinc(II) complex could be ascribed to the –OH group of coordinated ethanol molecules.

3.2. UV-Vis Absorption Spectra

The data and absorption spectra of H3L and its zinc(II) complex in diluted ethanol solution were presented in Figure 1. The spectrum of the zinc(II) complex is different from that of the free ligand H3L. The UV-Vis spectrum of the free ligand H3L exhibits one absorption peak at ca. 275 nm and could be attributed to the π-π* transition of the benzene rings.
The zinc(II) complex exhibits maximum absorption peak at 289 nm, which indicates that coordination reaction occurs between the zinc(II) atoms and H3L [46]. Moreover, a new absoption peak at 340 nm was observed in the zinc(II) complex, contrary to the peak observed at 311 nm in H3L, which could be attributed to the π-π* transition of the oxime group [46].

3.3. Crystal and Supramolecular Structure of the Zinc(II) Complex

The zinc(II) complex crystal structure is depicted in Figure 2, while Table 3 shows the list of important bond distances and bond angles.
The complex [{Zn(L)(μ-OAc)Zn(CH3CH2OH)}2] belongs to the triclinic system, with space group P-1, and consists of four zinc(II) atoms and two completely deprotonated (L)3− linkers, with two ethanol molecules and two bonded acetate ions. As far as we know, this new 2:4 ((L)3− : zinc(II)) Salamo-type zinc(II) coordination complex ratio is rarely reported when compared to its counterpart complexes that have the coordination ratio of 1:1 [46], 2:3 [43], and 4:8 [45] (L: zinc(II)). The terminal zinc(II) atom (Zn2 or Zn2#) is penta-coordinated, and situated at an N2O2 site of the deprotonated ligand moiety with one O7 atom from µ-acetate ion adopting a slightly twisted trigonal bipyramidal geometry (τ2 = 0.772) [11]. The central zinc(II) atom (Zn1 or Zn1#) is also penta-coordinated via the three oxygen atoms (O1, O2 and O1#), one O9 atom from the coordinated EtOH molecule, and one O8 atom from µ-acetate ion forming a slightly distorted tetragonal pyramidal geometry (τ1 = 0.265) [11]. The Zn1 and Zn2 atoms are connected through µ-acetate ion in a familiar M-O-C-O-M fashion.
Table 4 summarizes the inter- and intramolecular hydrogen bonds in the zinc(II) complex. From Figure 3, the proton (–C9H9B or –C9#H9B#) of ethylenedioxime carbon atom (C9 or C9#) of (L)3– unit is hydrogen bonded to oxygen atom (O7) of the µ-acetate ions, and the proton (–O9H9 or –O9#H9#) of the coordinated ethanol molecule is hydrogen bonded to one of phenolic oxygen atom (O5 or O5#) of the (L)3– unit. Thus, two pairs of intramolecular hydrogen bonds C9–H9B···O7, C9#–H9B#···O7#, O9–H9···O5 and O9#–H9#···O5# [53,54,55,56,57,58,59] are formed. Under the intermolecular force of C7–H7···O6 and C7#–H7#···O6#, a 1D supramolecular chain that extends infinitely in the b axis direction is formed by the crystal of [{Zn(L)(μ-OAc)Zn(CH3CH2OH)}2], as depicted in Figure 4 [60,61,62,63,64,65,66,67].

3.4. Fluorescent Properties

As shown in Figure 5, H3L exhibits a broad emission at 406 nm upon excitation at 268 nm, while the zinc(II) complex displays an intense photoluminescence with maximum emission peak at ca. 415 nm upon excitation at 268 nm, which is considered to be bathochromically shifted when compared to that of H3L, indicating that molecular inter-atomic forces and degree of conjugation are better in the zinc(II) complex internal molecules, due to the intraligand π-π* transition [18]. The Zn(II) complex has a greater fluorescence intensity, and it may have potential as a luminescent material.

4. Concluding Remarks

In this paper, A new tetranuclear zinc(II) complex has been successfully prepared and characterized. X-ray crystal structure analysis reveals that the four zinc(II) atoms are all penta-coordinated, in which two of the zinc(II) atoms lie in the N2O2 coordination spheres of Salamo-type bis-oxime (L)3− moieties and adopt slightly distorted trigonal bipyramid geometries (τ2 = 0.772), and the remaining two zinc(II) atoms adopt slightly distorted tetragonal pyramidral geometries (τ1 = 0.265). Moreover, the zinc(II) complex molecules assemble to form an infinite 1D chain-like supramolecular structure via intermolecular C7–H7···O6 and C7#–H7#···O6# hydrogen bonding interactions. The fluorescence properties show that the Zn(II) complex may has the potential to be used as a luminescent material.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21761018) and the Program for Excellent Team of Scientific Research in Lanzhou Jiaotong University (201706), which is gratefully acknowledged.

Author Contributions

Wen-Kui Dong conceived and designed the experiments; Quan-Peng Kang performed the experiments; Gao-Xian An analyzed the data; Yang Zhang contributed reagents/materials/analysis tools; Yun-Dong Peng and Xiao-Yan Li wrote the paper.

Conflicts of Interest

The authors declare no competing financial interests.

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Crystals 08 00107 sch001 550
Scheme 1. Routes to the synthesis of ligand H3L.
Scheme 1. Routes to the synthesis of ligand H3L.
Crystals 08 00107 sch001
Crystals 08 00107 g001 550
Figure 1. UV-Vis absorption spectra of H3L and its zinc(II) complex in ethanol (3.0 × 10−5 mol/L).
Figure 1. UV-Vis absorption spectra of H3L and its zinc(II) complex in ethanol (3.0 × 10−5 mol/L).
Crystals 08 00107 g001
Crystals 08 00107 g002 550
Figure 2. (a) View of the molecular structure of the zinc(II) complex with atom labeling (hydrogen atoms are omitted for clarity purpose and are drawn at the 30% probability level). (b) View of zinc(II) atoms of the zinc(II) complex showing coordination polyhedrons.
Figure 2. (a) View of the molecular structure of the zinc(II) complex with atom labeling (hydrogen atoms are omitted for clarity purpose and are drawn at the 30% probability level). (b) View of zinc(II) atoms of the zinc(II) complex showing coordination polyhedrons.
Crystals 08 00107 g002
Crystals 08 00107 g003 550
Figure 3. View of the intramolecular hydrogen bonds of the zinc(II) complex unit (for clarity’s sake, hydrogen atoms are omitted except those forming hydrogen bonds).
Figure 3. View of the intramolecular hydrogen bonds of the zinc(II) complex unit (for clarity’s sake, hydrogen atoms are omitted except those forming hydrogen bonds).
Crystals 08 00107 g003
Crystals 08 00107 g004 550
Figure 4. A 1D chain of the zinc(II) complex viewed along the b axis (for clarity’s sake, hydrogen atoms are omitted except those forming hydrogen bonds).
Figure 4. A 1D chain of the zinc(II) complex viewed along the b axis (for clarity’s sake, hydrogen atoms are omitted except those forming hydrogen bonds).
Crystals 08 00107 g004
Crystals 08 00107 g005 550
Figure 5. The Fluorescence spectra of H3L and its zinc(II) complex in methanol (3.0 × 10−5 mol/L).
Figure 5. The Fluorescence spectra of H3L and its zinc(II) complex in methanol (3.0 × 10−5 mol/L).
Crystals 08 00107 g005
Table
Table 1. Crystal data and structure refinement for the zinc(II) complex.
Table 1. Crystal data and structure refinement for the zinc(II) complex.
Molecular FormulaC42H48N4Zn4O18
Formula weight1158.32
Temperature (K)221
Wavelength (Å)0.71073
Crystal systemtriclinic
Space groupP-1
a (Å)8.1960(7)
b (Å)9.8127(8)
c (Å)16.5428(15)
α (°)106.392(8)
β (°)92.669(8)
γ (°)111.299(8)
V (Å3)1172.5(2)
Z
Dcalc (g∙cm–3)		1
1.640
µ (mm–1)2.099
F (000)592
Crystal size (mm)0.21× 0.23 × 0.25
Index range−10 ≤ h ≤ 8,
−12 ≤ k ≤ 12,
−18 ≤ l ≤ 20
Reflections collected7315
Independent reflections4569
Rint0.0620
Completeness to θ3.43 to 26.02
Data/ restraints/parameters4569/3/313
GOF1.027
Final R1, wR2 indices R1 = 0.0722, wR2 = 0.1127
R1a, wR2b indices (all data)R1 = 0.1493, wR2 = 0.1555
Largest differences peak and hole (e Å−3)0.918 and −0.493
a. R1 = Σ‖Fo| − |Fc‖/Σ|Fo|; b. wR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.
Table
Table 2. Infrared absorption spectra of H3L and its zinc(II) complex (cm−1).
Table 2. Infrared absorption spectra of H3L and its zinc(II) complex (cm−1).
Compoundν(O–H)ν(C=N)ν(Ar–O)ν(Zn–N)ν(Zn–O)
H3L343916311283--
[{Zn(L)(μ-OAc)Zn(CH3CH2OH)}2]342416001265509467
Table
Table 3. Selected bond distances (Å) and bond angles (°) of the zinc(II) complex.
Table 3. Selected bond distances (Å) and bond angles (°) of the zinc(II) complex.
BondLengthsBondLengthsBondLengths
Zn1-O12.039(5)Zn1-O22.058(5)Zn1-O81.963(6)
Zn1-O92.026(5)Zn1-O1 #2.002(5)Zn2-O22.010(5)
Zn2-N12.083(7)Zn2-O51.940(5)Zn2-O71.981(6)
Zn2-N22.127(7)
BondAnglesBondAnglesBondAngles
O1-Zn1-O279.6(2)O1-Zn1-O8140.6(2)O1-Zn1-O9111.2(2)
O1-Zn1-O1 #77.7(2)O2-Zn1-O896.3(2)O2-Zn1-O989.6(2)
O2-Zn1-O1 #156.63(2)O8-Zn1-O9107.8(2)O8-Zn1-O1 #97.9(2)
O9-Zn1-O1 #103.6(2)O2-Zn2-O594.0(2)O2-Zn2-O791.8(2)
O2-Zn2-N185.0(3)O2-Zn2-N2175.8(2)O5-Zn2-O7113.1(2)
O5-Zn2-N1117.4(3)O5-Zn2-N289.7(3)O7-Zn2-N1129.5(3)
O7-Zn2-N288.7(2)N1-Zn2-N291.5(3)
Symmetry transformations used to generate equivalent atoms: #: 2 − x, 1 − y, −z.
Table
Table 4. The intra- and intermolecular hydrogen bonds of the Zn(II) complex.
Table 4. The intra- and intermolecular hydrogen bonds of the Zn(II) complex.
D–H···Ad(D–H) (Å)d(H···A) (Å)d(D···A) (Å)∠D–H···A (°)Symmetry Code
O9–H9···O50.861.882.64 8(2)147
C9–H9B···O70.972.533.33 6(5)141
C7–H7···O60.932.553.47 3(4)171x, 1 + y, z

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MDPI and ACS Style

Peng, Y.-D.; Li, X.-Y.; Kang, Q.-P.; An, G.-X.; Zhang, Y.; Dong, W.-K. Synthesis and Fluorescence Properties of Asymmetrical Salamo-Type Tetranuclear Zinc(II) Complex. Crystals 2018, 8, 107. https://doi.org/10.3390/cryst8020107

AMA Style

Peng Y-D, Li X-Y, Kang Q-P, An G-X, Zhang Y, Dong W-K. Synthesis and Fluorescence Properties of Asymmetrical Salamo-Type Tetranuclear Zinc(II) Complex. Crystals. 2018; 8(2):107. https://doi.org/10.3390/cryst8020107

Chicago/Turabian Style

Peng, Yun-Dong, Xiao-Yan Li, Quan-Peng Kang, Gao-Xian An, Yang Zhang, and Wen-Kui Dong. 2018. "Synthesis and Fluorescence Properties of Asymmetrical Salamo-Type Tetranuclear Zinc(II) Complex" Crystals 8, no. 2: 107. https://doi.org/10.3390/cryst8020107

APA Style

Peng, Y. -D., Li, X. -Y., Kang, Q. -P., An, G. -X., Zhang, Y., & Dong, W. -K. (2018). Synthesis and Fluorescence Properties of Asymmetrical Salamo-Type Tetranuclear Zinc(II) Complex. Crystals, 8(2), 107. https://doi.org/10.3390/cryst8020107

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