Synthesis, Crystal Structure, Hirschfeld Surface Analysis and Catalytic Activity of a New Binuclear Zn(II) Complex Based on Homophthalic Acid and 2,2'-Bipyridine Ligands

A new binuclear Zn(II) complex, [Zn 2 L 2 (BIPY) 2 (H 2 O) 2 ] (1) (H 2 L = homophthalic acid, BIPY = 2,2'-bipyridine) has been synthesized by one-pot method of homophthalic acid, 2,2'-bipyridine, zinc acetate dihydrate, and NaOH in water/ethanol (v:v = 1:1) solution. The structure of complex (1) was characterized by IR and X-ray single-crystal diffraction analysis. The results show that each Zn(II) ion is five-coordinated with two carboxylic O atoms from two homophthalate ligands (O2, O3 or O2a, O3a), two N atoms from two 2,2'-bipyridine ligands (N1, N2 or N1a, N2a) and one O atom from coordinated water molecule (O5 or O5a), and forms a distorted trigonal bipyramid coordination geometry. Complex (1) forms 1D chained structure and 3D network structure by the  -  interaction of 2,2'-bipyridine ligands. The Hirschfeld surface analysis of complex (1) was calculated. The catalytic performance of complex (1) has also been investigated for the oxidation of benzyl alcohol under O 2 atmosphere. The optimal reaction temperature and pressure were 100 °C and 0.3 MPa for complex (1).


Materials and Measurements
The materials of homophthalic acid, 2,2'bipyridine, zinc acetate dihydrate, NaOH were purchased from Jilin Chinese Academy of Sciences-Yanshen Technology Co., Ltd.. IR spectra were carried on a Nicolet AVATAR 360 FTIR spectrophotometer with KBr discs (Nicolet Instrument Inc., Madison, WI, USA) (range 4,000-400 cm −1 ). The crystal data of complex (1) were obtained at 100 K on a SuperNova diffractometer (Bruker, Billerica, MA, USA). Selective oxidation of benzyl alcohol was conducted in a 10 mL stainless-steel high-pressure reactor equipped with magnetic stirring and a temperature controller at 90-110 °C under 0.1 MPa-0.5 MPa O2 pressure mild conditions.

Synthesis of Complex (1)
An amount of 0.0900 g homophthalic acid (0.5 mmol), 0.0781 g 2,2'-bipyridine (0.5 mmol) and 0.040 g NaOH (1.0 mmol) were added to added to the solution of 20 mL ethanol-water (v:v = 3:2) and stirred at R.T. After the solid was dissolved, 0.1097 g zinc(II) acetate dihydrate (0.5 mmol) solid was added. Then the mixture was stirred and kept at 70 °C for 4 h. After the mixture was cooled and filtered, the colourless crystals of bis[homophthalic acid-2,2'-bipyridine-aquazinc(II)] were received from the filtrate in two weeks with yield 66%.

Crystal Structure Determination
A suitable crystal (0.12 mm × 0.11 mm × 0.10 mm) of complex (1) was selected to collect data using Olex2 [35] on a SuperNova, Dual (Cu at zero) diffractometer with an Atlas detector at 100(10) K. The structure was solved with the SHELXT [36] structure solution program using Intrinsic Phasing and refined with the SHELXL [37] refinement package. Crystallographic data of complex (1) are shown in Table  1. The Hirschfeld surface analysis of complex (1) was calculated by the CrystalExplorer software 21.5 [38]. The crystallographic data for the structure reported in this paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC 2207232. The CIF file can be obtained conveniently from the website: https:// www.ccdc.cam.ac.uk/structures.

Selective Oxidation of Benzyl Alcohol
Selective oxidation of benzyl alcohol was conducted in a 10 mL stainless-steel highpressure reactor equipped with magnetic stirring and a temperature controller at 90-110 °C under 0.1 MPa-0.5 MPa O2 pressure mild conditions. In a typical experiment, 1.0 mmol (108.1 mg) benzyl alcohol, 15 mg complex (1), and 7.0 mL tetrahydrofuran (THF) were mixed in the stainless-steel high-pressure reactor. After the reactor was sealed, the reactor was heated to reaction temperature (90-110 °C) and maintained 2 h. Then the reactor was cooled naturally to room temperature. The mixture was centrifuged to remove the complex (1) completely. The remaining solution was analyzed with a gas chromatograph (GC-6890) equipped with SE-54 capillary column and flame ionization detector. The vaporizing chamber and detector temperatures were 250 °C. The GC analysis program was as follows: initial column temperature of 50 °C to 250 °C at 10 °C/min, and held for 10 min.

The Hirschfeld Surface of the Complex (1)
The Hirschfeld surface of the complex (1) was analyzed by the CrystalExplorer software 21.5. As shown in Figure 6, the Hirschfeld surfaces mapped over dnorm, di and de of the crystal (a-c), and the two-dimensional (2D) fingerprint plots represented overall and the top three interactions (H‧‧‧H, C‧‧‧H/H‧‧‧C and

Catalytic Studies
The catalytic activities of the as-prepared complex (1) for benzyl alcohol oxidation were determined with O2 as a green oxidant using THF as solvent. The results are shown in Table  3. To avoid producing benzoic acid and benzyl benzoate, a series of reactions to obtain optimal reaction conditions to produce benzaldehyde were conducted. The effects of reaction temperature and reaction pressure on benzyl alcohol conversions, benzaldehyde selectivity, and yields of complex (1) were studied. The benzyl alcohol conversion (9.3%) and benzaldehyde yield (3.2%) were low for the oxidation of benzyl alcohol at 100 °C within 2 h under 0.3 MPa of O2. The conversion of benzyl alcohol increased with the increasing of reaction temperature and pressure. However, the selectivity of benzaldehyde decreased with the increasing of reaction temperature and reaction pressure. The optimal reaction temperature and pressure were 100 °C and 0.3 MPa for complex (1), respectively. The highest yield (39.0%) of benzaldehyde were obtained at 100 °C under 0.3 MPa of O2. The yields of benzaldehyde were 50.8% for complex [Zn3(L1)4(L2)2(CH3COO)2] (HL1 = 6-phenylpyridine-2-carboxylic acid, L2 = bis(4-pyridyl)amine) at 90 °C with THF as solvent under 0.5 MPa O2 within 3 h [16]. The benzyl alcohol conversion and benzaldehyde yield over ZnL4(Phen)2 (HL=3-bromo-2h y d r o x y b e n z a l d e h y d e -p y r i d i n e -2carbohydrazone) were 37   the benzaldehyde yield was lower than that of [Zn3(L1)4(L2)2(CH3COO)2] (50.8%), complex (1) catalyst produces higher yields (39.0%) than ZnL4(Phen)2 (1.9%) catalyst.

Conclusions
In summary, a new binuclear Zn(II)homophthalate complex has been successfully synthesized by one-pot reaction and structurally characterized by IR and X-ray single-crystal diffraction analysis. The Hirschfeld surface analysis of complex (1) shows that the H‧‧‧H contacts represented the largest contribution (43.5%) to the Hirschfeld surface and the π-π stacking interactions play a subordinate role in forming the crystal, whereas selective oxidation of benzyl alcohol with complex (1) as catalyst shows that the optimal reaction temperature and pressure were 100 °C and 0.3 MPa.

Acknowledgments
This project was supported by National Natural Science Foundation of China (No. 21171132), Science Foundation of Weifang (2020ZJ1054) and Science Foundation of Weiyuan Scholars Innovation Team.