Three new zinc(II) complexes: design, synthesis, characterization and catalytic performance

Abstract Three new Zn(II) complexes, [Zn2L2(OAc)2]2CH3OH (1), [ZnL(3-NA)2]·H2O (2) and [ZnL(2-NA)2] (3) were synthesized by the reaction of Zn(CH3COO)2·2H2O or Zn(ClO4)2·6H2O with 2-amino-N'-(pyridin-2-ylmethylene)benzohydrazide (HL), 3-nitrobenzoic acid (3-NA) or 2-nitrobenzoic acid (2-NA). The complexes were characterized by elemental analyses, single crystal X-ray diffraction, IR, UV-Vis and 1H NMR spectroscopy. The single crystal X-ray diffraction analyses revealed that the Zn ions in the complexes are five-coordinate in tetragonal pyramid configuration. The complexes and their starting materials Zn(CH3COO)2·2H2O and Zn(ClO4)2·6H2O were studied for their catalytic performance in the selective oxidation of cyclohexane (Cy) with aqueous H2O2 as oxidant. The conditions such as time, the amount of H2O2, solvent and HNO3 additive were optimized. The results showed that 3 has the best activity under mild conditions compared to the starting zinc materials and the referenced zinc complexes. Graphical Abstract


Introduction
Selective oxidation of cyclohexane (Cy) to cyclohexanone (K, CyO) and cyclohexanol (A, CyOH), generally known as KA-oil, plays a significant role in the chemical industry since KA-oil is an important intermediate material to produce Nylon-6 and Nylon-66 polymers [1][2][3][4][5]. It is vital to find suitable selective oxidation catalysts to improve the yield of KA-oil. In the past decades, research on metal catalysts with high catalytic activity for Cy oxidation has mainly concentrated on precious metal materials for their high efficiency and stability [6][7][8][9]. However, the expensive price and toxicity limit the application and development of precious metals. Therefore, non-noble transition elements with low-cost and high catalytic activity have been committed to exploration [10][11][12]. Among them, zinc compounds have been widely studied for their excellent activity on C-H oxidation [13][14][15][16][17][18].
Vijayaraj et al. reported enhancement of the stability of aniline oxidation by Cu 0.5 Zn 0.5 Fe 2 O 4 at 300 C [13]. Hao et al. prepared Zn-Al oxide to catalyze carbonylation of 1,2-propylene glycol in the presence of urea [14]. The yield of propylene carbonate was 96.6% at 180 C. Lachheb et al. found that the conversion of styrene and selectivity of styrene oxides was 60% and 50%, respectively, when ZnO acts as a photocatalytic catalyst under H 2 O 2 /UV radiation at 250 C [15]. Based on the above research results, Zn compounds can be considered to be potential catalysts for catalytic oxidation. However, Zn salts have barely been reported in work for Cy oxidation.
Among zinc compounds, zinc complexes have often been synthesized and discussed as C-H oxidation catalysts. Shixaliyev [17]. Jadav et al. synthesized a tetranuclear zinc complex, [Zn 4 (L) 2 (LH) 2 ((CH 3 ) 2 SO) 2 ]Á2CH 3 OHÁ(CH 3 ) 2 S-OÁH 2 O(LH 3 ¼3-(E)-(2hydroxyphenylimino)methyl-4-hydroxy-5-hydroxymethylphenyl), and incorporated it into a MCM-48 mesoporous silica material to form a Zn composite modified material [18]. The morpholine conversion can reach up to 95% at 70 C in 6 h when catalyzed by this complex (20 mg Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded on a Jasco FT/IR-4000 spectrometer as KBr pellets from 4000 to 400 cm À1 . UV-Vis spectra were recorded on a Perkin-Elmer Lambda 900 spectrometer. 1 H NMR spectra were obtained by a 500 MHz Bruker Advance instrument. Molar conductivities of the complexes were measured in methanol (room temperature) using a DDS-11A conductivity meter. Single crystal structures were determined by a Bruker D8 Venture CCD diffractometer.

X-Ray crystallography
Diffraction intensities for 1-3 were collected at 298(2) K using a Bruker D8 Venture CCD diffractometer with MoKa radiation (k ¼ 0.71073 Å). The collected data were reduced with SAINT [20], and multi-scan absorption correction was performed using SADABS [21]. Structures of the complexes were solved by direct method and refined against F 2 by full-matrix least-squares method using SHELXT and SHELXL [22,23]. All non-hydrogen atoms were refined anisotropically. The amino and water H atoms in the compounds were located from difference Fourier maps and refined isotropically. The remaining hydrogen atoms were placed in calculated positions and constrained to ride on their parent atoms. In 2, the water molecule is partial occupancy (0.5) and disordered, which leads to O10-H10A bond without a hydrogen bond acceptor. The occupancy of the water molecule fixed at 0.5 can give the thermal factor 0.42, which is similar to other non-hydrogen atoms. In addition, the occupancy (0.5) can give the lowest R 1 and wR 2 values, and the better WGHT scheme. Crystallographic data for 1-3 are summarized in Table 1. Selected bond lengths and angles are given in Table 2.

Catalytic oxidation reaction
The catalytic oxidation of Cy to KA-oil with H 2 O 2 as oxidant was carried out in a glass flask with a water circulated condenser. In a typical procedure, 0.3 mg of catalysts (1-3) were dissolved in 3 mL solvent and a certain amount of Cy, HNO 3 promoter and 30% H 2 O 2 added in sequence. The reaction was performed at 60 C for 4 h. For the product analyses, 0.03 g of methylbenzene (internal standard) and 1.5 mL of diethyl ether (to extract the substrate and the organic products from the reaction mixture) were added to 1.5 mL samples, respectively. After stirring for 10 min, a sample (0.1 lL) was taken from the organic phase and analyzed by the internal standard method using gas chromatography (GC). The catalytic performance was evaluated in terms of the conversion of Cy and the turnover number (TON), according to the following equations:

Chemistry
Free hydrazone HL was readily prepared by the condensation reaction of equimolar quantities of 2-pyridinecarboxaldehyde and 2-aminobenzohydrazide in methanol (Scheme 1). Complex 1 was prepared by reaction of equimolar quantities of the hydrazone and zinc acetate in methanol. Notably, it can also be obtained by reaction of HL with zinc perchlorate and sodium acetate. Complexes 2 and 3 were synthesized by reaction of HL and zinc perchlorate with 3-nitrobenzoic acid or 2-nitrobenzoic acid, respectively, in methanol (Scheme 2). Single crystals of the complexes were obtained by slow evaporation of the methanolic solutions of the complexes. Molar conductivities of 1-3 within the normal values 20-30 X À1 Ácm 2 Ámol À1 indicated their non-electrolytic nature [24].

IR spectra analyses
Infrared spectra of 1-3 are given in Figures S1-S3. A typical vibration is observed at 3450-3476 cm À1 in the IR spectrum of 1-3, which belongs to the stretching vibration band of the O-H group. The sharp bands located at 1353-1379 cm À1 indicate the N-H bending vibrations [29]. The intense band observed at 1646 cm À1 is the C ¼ O vibration [30]. The vibrations for CH ¼ N are in the range 1625-1630 cm À1 [31,32]. The absorption bands in the 427-463 cm À1 region can be attributed to the Zn-N stretching vibration, and the absorption bands in the 527-556 cm À1 region are due to the Zn-O stretching vibration [33,34].

UV-Vis spectra analyses
At room temperature, UV-Vis absorption spectra of 1-3 were measured with methanol as solvent, as shown in Figures S4-S6, which are similar. The absorption peaks observed near 400 nm could be attributed to the charge transfer transition (LMCT) [35,36]. Bands at 300, 308 and 310 nm are due to the n!p Ã transition of conjugated double bonds in the hydrazone ligand. Absorption peaks at 296, 295, and 296 nm can be attributed to p-p Ã transition in the hydrazone ligand [37].

Catalytic activity
The catalytic activities of several zinc salts in Cy selective oxidation are compared in Table 3. It can be seen clearly that the TON CyOH values are all less than 10, indicating   Table 4. The TON value of CyOH increased from 0 to 34 when the ratio of H 2 O 2 /Cy changed from 0 to 1:1 ( Table 4, entries 1-3), and then, decreased (entry 4). The same trend was observed for catalyst content (0.1-0.8 mg), and the maximum TON value (39) was obtained with 0.5 mg of catalyst (entries 3 and 5-7). More HNO 3 additive is not beneficial to produce CyOH; the best TON value (62) was achieved at 0.2:1 HNO 3 /Cy molar ratio (entries 6 and 8-10). The TON CyOH dropped remarkably if CH 3 CN was replaced by MeOH, EtOH or CH 2 Cl 2 (entries 11-13). In addition, the best temperature was 70 C (entries 9 and 14-17) and the optimal time was 4 h (entries 16 and 18-21). As a result, the best Cy conversion, TON value of CyOH and CyO were 88%, 78 and 19, respectively, under the optimal condition: 1:1 molar ratio of H 2 O 2 /Cy, 0.5 mg Zn(ClO 4 ) 2 Á6H 2 O, 0.2:1 molar ratio of HNO 3 /Cy with the reaction time of 4 h in CH 3 CN at 70 C.
Since 2 and 3 were both synthesized with Zn(ClO 4 ) 2 Á6H 2 O as the starting material, 3 was used as a model catalyst to compare with Zn(ClO 4 ) 2 Á6H 2 O.
It can be seen clearly that Cy conversion was continually increasing, while the TON value of CyOH showed curves that reached maxima with time ( Figure 7A) and H 2 O 2 contents ( Figure 7B). The TON of CyOH reached a maximum value of 218 at 4 h and 310 when the ratio of H 2 O 2 :Cy was 0.6:1, respectively, and then decreased. These indicate that CyOH may be transformed to other by-products [38] with time or excess H 2 O 2 [39].
A series of solvents, CH 3 Cl 3 , CH 2 Cl 2 , EtOH, CH 3 OH and CH 3 CN (3 mL), were explored while keeping fixed the amount of Cy (7.064 mmol), n catalyst (complex 3) :n Cy ¼1:14000, HNO 3 (0.942 mmol) temperature (40 C) and time (4 h), as shown in Figure 8. The order  [40]; the higher polarity of CHCl 3 than CH 2 Cl 2 make phases disperse more  uniformly, and the performance of the catalyst better, although the dielectric constant of CHCl 3 is less than that of CH 2 Cl 2 .
The influence of reaction temperature on Cy oxidation was investigated under the conditions Cy (7.064 mmol), n catalyst (complex 3) : n Cy ¼1:14000, H 2 O 2 (4.239 mmol), HNO 3 (0.942 mmol), CH 3 CN (3 mL), and 4 h, as shown in Figure 9. The Cy conversion and TON of CyOH all rose to the maxima of 83% and 218 at 40 C, respectively, and then fell with rising temperature, indicating that high temperature is not good for the Cy conversion and the formation of CyOH, and the selective Cy oxidation to CyOH with 3 can occur under very mild conditions (40 C). Noticeably, the formation of CyO could only be detected beyond 60 C, and climbed remarkably with temperature, which indicates that CyO is only produced at higher temperature when promoted by 3.
The catalytic properties of the three zinc complexes in the reaction of Cy (7.064 mmol), H 2 O 2 (4.239 mmol), catalysts (n catalyst :n Cy ¼1:14000), HNO 3 (0.942 mmol), CH 3 CN (3 mL), 40 C and 4 h are compared in Figure 10. It was found that the catalytic activities of 2 and 3 were higher than 1, which may result from their stronger electron-withdrawing effect of the ligands to expose more active metal sites. Furthermore, 3 has the highest ortho and conjugated effects of its ligand, which may be the reason why it has the best catalytic performance. The order of catalytic activity of the complexes is 3 > 2 > 1.
The catalytic performance of the zinc complexes were compared with that of their starting materials ( Table 5). The total TON value was only 6 with Zn(CH 3 COO) 2 Á2H 2 O, which could be improved to 95 when this starting material was replaced by its corresponding complex 1 (  respectively. These indicate that 1-3 are all more effective than their starting materials. Additionally, the optimal reaction temperature dropped from 60 or 70 to 40 C, and the amount of catalyst decreased from 0.0014 or 0.0013 to 0.0005 mmol when the zinc complexes were used as catalysts. This suggests that more mild reaction conditions can be used with 1-3 compared to their zinc starting material. Therefore, we have reason to believe that the synthesis of the zinc complexes can effectively promote the production of the main products under more mild and economical conditions. On the other hand, 3 shows the best catalytic activity and more mild reaction conditions compared to the catalytic results and conditions reported previously for other Zn complexes in Cy oxidation (Table 5, entries 6-9).  [a] HL ¼ 2-amino-N'-(pyridin-2-ylmethylene)benzohydrazide; AC ¼ acetate.

Probable mechanism
To detect the intermediates formed in the catalytic process, changes in the UV-Vis spectrum were monitored during successive additions of dilute H 2 O 2 solution dropwise to a 10 mL CH 3 OH solution of 1.8 Â 10 À5 M of 3, as shown in Figure 11. It can be seen that the band at 400 nm, which belongs to a ligand-to-metal charge transfer (LMCT) transition, decreased slightly in intensity. Together with the appearance of an increase in the intensity at 356 nm, we propose that active intermediates, which can transfer oxygen atom to the substrate and oxidize it to product, can be formed in the interaction of 3 with H 2 O 2 [43]. Based on the above discussion, a mechanism was proposed (Scheme 3). Zn II OL peroxido species are formed first by the reaction of H 2 O 2 with Zn II L complex (reaction 1) Figure 11. UV-vis spectral changes observed during titration of 3 with H 2 O 2 . The spectra recorded after successive addition of one drop portions of dilute H 2 O 2 [0.030 g (0.264 mmol) 30% H 2 O 2 was dissolved in 10 mL CH 3 OH] to 10 mL of 1.8 Â 10 À5 M CH 3 OH solution of 3. Scheme 3. Probable mechanism of oxidation of cyclohexane. [44], which can be supported by our results in Figure 11. Then, oxygen-centered radicals HOO and HO are produced (reaction 2). HO radical must be the crucial intermediate in the formation of KA-oil products [45], for no CyOH or CyOOH can be detected when adding t-butanol (an HO radical scavenger [46]) to our oxidation system when catalyzed by 3. HO would further form the cycloalkyl radical Cy upon Habstraction from cycloalkane CyH (reaction 3) and cyclohexylperoxyl radical CyOO by further reaction with oxygen (reaction 4). CyOOH can then be produced from CyOO (reaction 5) and decomposed to CyO and CyOO radicals (reactions 6 and 7), which would then produce CyOH and CyO (reactions 8 and 9).