Synthesis, Structural Characterization, and Catalytic Property of A Zn(II) Complex with 5-Bromosalicylaldehyde Ligand

The study on catalytic activity of complex materials has been one of the hot spots in coordination chemistry. In order to extensively study the catalytic activity of complexes, a new six-coordination Zn(II) complex material, [ZnL2(H2O)2] (C1) (HL = 5-bromosalicylaldehyde), has been prepared with 5bromosalicylaldehyde, NaOH, and Zn(CH3COO)2·2H2O as raw materials. The structure of C1 was determined by elemental analysis, IR spectra, and single crystal X-ray diffraction. The Zn(II) complex shows a moderate catalytic activity for A3 coupling reaction of benzaldehyde, piperidine, and phenylacetylene with the benzaldehyde conversion reached 54.6 %. Furthermore, the Zn(II) complex catalyst exhibited 54.8 %, 53.8 %, and 54.4 % conversions of benzaldehyde in the second, third, and fourth cycles, respectively. In addition, the Zn(II) complex features a selectivity of 100 % to the product of propargylamine for the A3 coupling reaction. Copyright © 2017 BCREC Group. All rights reserved


Introduction
Propargylamines are versatile synthetic intermediates in organic synthesis and are also important structural elements in natural products and therapeutic drug molecules, and are used for the synthesis of polyfunctional amino derivatives [1][2][3].They were synthesized by three component coupling reaction (A 3 coupling) of aldehydes, alkynes and amines [4][5][6].How-ever, the traditionally catalysts used in the synthesis of propargylamines require very harsh reaction conditions, for example, they must be used in stoichiometric amounts, are highly moisture sensitive, and require strictly controlled reaction conditions [7].
In recent years, many chemists have been working on synthetic catalysts which could be used in mild condition.Therefore, different transition metals have been used as the catalysts for these reactions, such as: gold [4], silver [5], zinc [8], copper [9], and nickel [10].Metal complex materials have also attracted attention due to their catalytic activity for many reactions [11][12][13][14][15][16].Studies on the synthesis and catalytic activity of metal complexes have been the goal of our research group [17,18].Herein, in this paper, a new six-coordination Zn(II) complex material was synthesized and characterized by elemental analysis, IR spectra and single crystal X-ray diffraction.The catalytic activity for A 3 coupling reaction has also been investigated with Zn(II) complex as the catalysts.

Materials and measurements
The analytical grade reagents used in this e x p e r i m e n t a l w o r k s w e r e : 5-bromosalicylaldehyde, Zn(CH3COO)2•2H2O, NaOH, benzaldehyde, piperidine, phenylacetylene, and 1, 4-dioxane.Elemental analyses (C, H, and N) were measured on an Elementar Vario III EL Elemental Analyzer.IR spectra were measured on a Nicolet AVATAR 360 infrared spectrometer in the 4000-400 cm -1 region.The structural data of [ZnL2(H2O)2] were collected by a Bruker Amart CCD diffractometer.

Synthesis of [ZnL2(H2O)2] (C1)
T e n m L e t h a n o l s o l u t i o n o f 5-bromosalicylaldehyde (1.0 mmol, 0.2010 g) and NaOH (1.0 mmol, 0.040 g) was added to the solution of 5 mL ethanol/water (v:v = 1:1) containing Zn(CH3COO)2•2H2O (0.5 mmol, 0.1097 g).The mixture turned light yellow transparent solution.Then, the mixture was heated to 60︒C and maintained at this temperature for 5 h while stirring.Light yellow crystals of [ZnL2(H2O)2] was achieved by slowly evaporating the filtrate at room temperature (yield ca.82 % based on Zn(CH3COO)2•2H2O).

X-ray crystallography
The single crystal data of [ZnL2(H2O)2] (C1) with dimensions of 0.22 mm × 0.19 mm × 0.18 mm was collected on a Bruker Smart CCD diffractometer at 293 K with graphitemonochromatic Mo Kα radiation (λ = 0.71073 Å).In the range of 3.03 < θ < 25.01, a total of 12283 reflections were collected and 5532 were independent with Rint = 0.1099, of which 2136 were observed with I > 2σ(I).The structure was solved by direct methods with SHELXS-97 [19] and refined by full-matrix least-squares techniques on F 2 with SHELXL-97 [20].

General procedure for the threecomponent coupling reaction (A 3 )
Typical procedure for the three-component (A 3 ) coupling reaction: a mixture of benzaldehyde (0.125 mmol, 0.014 g), phenylacetylene (0.165 mmol, 0.017 g), piperidine (0.150 mmol, 0.013 g), and Zn(II) complex catalyst (0.070 g) were added into 1,4-dioxane (1.500 g).The reaction mixtures were put into a closed glass reactor (8 mL) and were extensively stirred at 120 o C for 12 h.At the end of each catalytic A 3 coupling reaction, the catalysts were isolated by centrifugation from the reaction solution, dried at 50 °C under vacuum for 5 h, and then reused in the next run of each reaction.The products were analyzed using a gas chromatograph (GC-1100) equipped with a SE-54 capillary column and a hydrogen flame detector.

Structural description of [ZnL2(H2O)2] (C1)
Structural analysis suggests that the Zn(II) complex (C1) crystallizes in triclinic P-1 space group.The coordination environment of Zn(II) center in C1 is shown in Figure 2. The selected bond distances and angles of complex C1 are listed in Table 2.The Zn(II) complex (C1) consi st s o f a Zn( II) io ns , tw o 5bromosalicylaldehyde ligands with losing of proton, and two coordinated water molecules.The Zn(II) ion in C1 is surrounded by four O atoms from two 5-bromosalicylaldehyde ligands and O atoms from two coordinated water molecules, which resulting in a distorted octahedral coordination environment (Figure 3).The four coordinated O atoms of O1, O2, O3 and O4 from 5-bromosalicylaldehyde ligands are in the equatorial plane and the two coordinated O atoms of O1W and O2W from coordinated water molecules are situated axial place.The Zn-O bond distances range from 2.011(8) Å to 2.190(7) Å, which are closed to those of   Zn(II) complexes [22].The distances of C1-O1 and C14-O4 are longer than those of C7-O2 and C8-O3, which suggest that C1-O1 and C14-O4 are double bonds.In the complex C1, two sixmember rings were formed by the coordinated effect of Zn(II) ion with O1, O2, O3 and O4 atoms, and the dihedral angle between the ring 1 (C8-C9-C14-O4-Zn1-O3) and ring 2 (C1-C2-C7-O2-Zn1-O1) is 0.6 (3) o .The 1D chained structure was formed through the effect of O-H•••O hydrogen bonds between the adjacent C1 molecules (Figure 4).The perspective view of stacking is shown in Figure 5. Main crystal data and refinement details of C1 are listed in Table 1.

Catalytic studies
The catalytic activity of Zn(II) complex catalyst was tested for A 3 coupling reaction of bezaldehyde, piperidine, and phenylacetylene with 1,4-dioxane as a solvent.The results are summarized in Table 3.The Zn(II) complex catalyst shows a moderate catalytic activity, and the benzaldehyde conversion reached 54.6 % for 12 h at 120 o C. Furthermore, the Zn(II) complex catalyst exhibited 54.8 %, 53.8 %, and 54.4 % conversions of benzaldehyde at 120 o C for 12 h in the second, third, and fourth cycles, respectively.The Zn(II) complex catalyst could be recovered easily by centrifugation and re-  used up to four times without any decrease in catalytic activity.It means that an active site is a part of the Zn(II) complex catalyst and is not reduced.Moreover, the Zn(II) complex catalyst features a selectivity of 100% to the product of propargylamine for the A 3 coupling reaction.These features render the catalysts of Zn(II) complex particularly attractive in the practice of propargylamines synthesis in an environmentally friendly manner.

Conclusions
In summary, a new Zn(II) complex material has been prepared, and its structure was determined by elemental analysis, IR spectra, and single crystal X-ray diffraction.It was shown that this Zn(II) complex can catalyze the A 3 coupling reaction of benzaldehyde, piperidine, and phenylacetylene with a moderate catalytic activity. [a] Reaction conditions: aldehyde (0.125 mmol), amine (0.17 mmol), alkyne (0.15 mmol), catalyst (70 mg), dioxane (1.500 g)

Figure 2 .
Figure 2. Coordination environment of Zn(II) center in C1

Figure 5 .
Figure 5.The perspective view of stacking of Zn(II) complex C1

Table 1 .
Crystal data and refinement details for C1

Table 2 .
The selected important bond parameters of complex C1

Table 3 .
Coupling of benzaldehyde, piperidine, and phenylacetylene catalyzed by Zn(II) complex catalyst in dioxane[a]