A Ca ( II ) Coordination Polymer of 2-Carboxybenzaldehyde : Synthesis , Crystal Structure , and Catalytic Activity in Oxidation of Benzyl Alcohol

The coordination polymers constructed by carboxylate ligand have shown novel structures and excellent properties. Therefore, a new Ca(II) coordination polymer with 2-carboxybenzaldehyde ligand, [Ca(L)2(H2O)2]n (L = 2-carboxybenzaldehyde), has been synthesized. Its structure has been identified by elemental analysis, FT-IR, and single-crystal X-ray diffraction technique. The Ca(II) coordination polymer exhibits a 1D chained structure by the bridging effect of carboxylate groups. The Ca(II) coordination polymer exhibited relatively good catalytic activity in the benzyl alcohol oxidation reaction with benzyl alcohol conversion (53.8%) and moderate benzaldehyde selectivity (83.3%).


Introduction
Metal coordination polymers have attracted much attention in recent years, which is due to their novel structures and potential applications in luminescence, catalysis, magnetism, gas storage, and biological activity [1][2][3][4][5][6][7][8][9][10][11][12][13][14].Among them, the coordination polymers were mainly composed by transition metal ions and rare earth metal ions with organic ligands [15][16][17][18].The study on structure and property of alkaline earth coordination polymers are relatively few.Benzaldehyde is an important intermediate of organic synthesis and fine chemical products, widely used in medicine, dyes, spices, resins, and other industries [19].Traditionally, the benzaldehyde was prepared by oxidation of benzyl alcohol with toxic metal oxides, peroxides, halides, and so on [20][21][22].So, the development of environmentally friendly catalysts is very attractive.Our group has been working on the synthesis and catalytic properties of cheap and environmentally friendly catalysts [23][24][25][26][27].In this paper, we reported a new Ca(II) coordination polymer, [Ca(L) 2 (H 2 O) 2 ] n , and its application in aerobic oxidation of benzyl alcohol as catalyst.The chemical diagram of the Ca(II) coordination polymer is shown in Scheme 1.

Crystal Data Collection and Refinement
Single-crystal data of the Ca(II) coordination polymer were obtained at 293(2) K on a Bruker Smart CCD diffractometer (Bruker, Billerica, MA, USA) using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).The SHELXL-2014/7 program [28] was used to solve the structure by direct method.The OLEX2 [29] program was used to refine the structure.The key collection and handling information for [Ca(L) 2 (H 2 O) 2 ] n are given in Table 1.
The crystallographic data for the structure reported in this paper has been deposited with the Cambridge Crystallographic Data Centre as supplementary publication No. CCDC 1580374.Copy of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk).

Catalytic Measurements
The [Ca(L) 2 (H 2 O) 2 ] n was treated at 100 • C for 24 h under vacuum to remove the terminal water molecules before catalytic reaction, thus providing coordinatively unsaturated calcium metal sites.The procedure for the aerobic oxidation of benzyl alcohol is as follows: 0.0216 g benzyl alcohol (0.2 mmol), 1.50 g tetrahydrofuran, and the catalyst (0.06 g, containing 0.0171 mmol coordinatively unsaturated calcium metal sites) were placed into a 10 mL stainless steel autoclave.Then, the 99.999% pure O 2 was pumped to replace the atmosphere three times.The reaction mixture was kept at 130 • C with vigorous stirring.Then, the [Ca(L) 2 (H 2 O) 2 ] n catalyst was removed from the reaction mixture by centrifugation.The gas chromatography equipped with a SE-54 capillary column was used to measure the benzyl alcohol conversion and the benzaldehyde selectivity.The qualitative analysis was carried out by an external standard method.

Structural Description of [Ca(L) 2 (H 2 O) 2 ] n
The [Ca(L) 2 (H 2 O) 2 ] n crystallizes in the monoclinic space group C2/c, and its asymmetric unit contains a Ca(II) ion, two 2-carboxybenzaldehyde ligands, and two coordinated water molecules.

Catalytic Studies
The catalytic activity for the oxidation of benzyl alcohol was tested at 130 • C using the Ca(II) coordination polymer as catalyst and 99.999% pure O 2 as oxidant (Figure 4).First, the catalytic activity of Ca(II) coordination polymer catalyst was examined for the oxidation of benzyl alcohol at different reaction time.The reaction was conducted by heating Ca(II) coordination polymer catalyst (0.06 g) and benzyl alcohol (0.2 mmol) in THF (1.5 g) under 1 MPa O 2 .Figure 5 shows the catalytic activity for the oxidation of benzyl alcohol over Ca(II) coordination polymer catalyst with THF as solvent at 130 • C under 1 MPa.As shown in Figure 5, the benzyl alcohol conversion increases with the increase of reaction time.The benzyl alcohol conversion reached to 90.4% at 130 • C within 5 h.Significant differences are observed in benzaldehyde selectivity over Ca(II) coordination polymer catalyst at 130 • C using 99.999% pure O 2 as the oxidant.Upon increasing the reaction time, there is a sharp decrease of the benzaldehyde selectivity over the Ca(II) coordination polymer catalyst.Maximum selectivity of 99% was obtained within 10 min over Ca(II) coordination polymer catalyst at 130 • C.However, the selectivity was 19.6% at 130 • C for 5 h.Then, the effect of the amount of the catalyst was studied to optimize the reaction conditions.The conversion of benzyl alcohol increases upon increasing the amount of the catalyst.The conversions of benzyl alcohol were 23.8%, 38.5%, 53.8%, and 90.9% when using 20 mg, 40 mg, 60 mg, and 80 mg Ca(II) coordination polymer catalyst at 130 • C for 3 h in THF under 1 MPa, respectively.However, the selectivities of benzaldehyde were 92.5%, 85.5%, 83.3%, and 12.5%, respectively.The good benzyl alcohol conversion (53.8%) and moderate benzaldehyde selectivity (83.3%) were achieved when the oxidation reaction was carried out at 130 • C for 3 h using 60 mg Ca(II) coordination polymer catalyst in THF under 1 MPa.Wang et al. [30] synthesized a poly(divinylbenzene-co-N-vinylimidazole) (PDVB-VI-n)-supported Au catalyst by using AuCl 3 as the precursor.Benzyl alcohol conversion and benzaldehyde selectivity was 97.3% and 10.5% at 90 • C for 16 h with 1 atm oxygen as the sole oxidant and water as the solvent over 1.9 Au/PDVB-VI-0.33.Zhang et al. [31] synthesized MoO 3 /C-N hybrid materials via direct thermal treatment followed by calcination (600 • C for 2 h).The yield of oxidation of benzyl alcohol to benzaldehyde was 30% at 110 • C within 3 h using H 2 O 2 as oxidant.Philip et al. [32] synthesized gold nanoparticle-decorated halloysite nanotubes (Au-Hal) by a deposition method.3% Au-Hal-2 catalysts showed good benzyl alcohol conversion (79%) and high benzaldehyde selectivity (above 97%) in the benzyl alcohol oxidation with 10 bar of O 2 at 100 • C under magnetic stirring.Wang et al. [33] prepared palladium nanoparticles (PdNPs) supported on functional cross-linked mesoporous poly(ionic liquid)s (MPILs) (Pd@MPIL) through NaBH 4 reduction route.The obtained materials showed good benzyl alcohol (96%) and benzaldehyde selectivity (99%) by using O 2 as the oxidant and water as the sole solvent at low temperature (90 • C) with K 2 CO 3 as additive.Based on the above results, although the activity of Ca(II) coordination polymer catalyst is still less than those of Au-Hal and Pd@MPIL, the catalyst of Ca(II) coordination polymer shows higher activity than 1.9 Au/PDVB-VI-0.33 and MoO 3 /C-N hybrid materials.The reusability of the Ca(II) coordination polymer catalyst was investigated in the oxidation of benzyl alcohol in THF.The results of reusability of the Ca(II) coordination polymer catalyst are given in Table 2.The benzyl alcohol conversion and benzaldehyde selectivity at 130 • C for 3 h were 53.8% and 83.3%, respectively.The Ca(II) coordination polymer catalyst was recovered by centrifugal separation.The recovered catalyst was washed twice with THF, dried at 100 • C under vacuum for 12 h, and reused.Benzyl alcohol conversions were 51.2%, 50.8%, and 48.6% respectively when the Ca(II) coordination polymer catalyst is used in three cycles at 130 • C within 3 h.The benzaldehyde selectivities were 78.5%, 67.7%, and 59.6% in the first, second, and third runs, respectively.Ca(II) coordination polymer, O 2 gas, N 2 gas, CaCl 2 , Ca(NO 3 ) 2 , and air gas were selected to demonstrate the catalysis of Ca(II) coordination polymer catalyst in the oxidation of benzyl alcohol.The results are summarized in Table 3.The benzyl alcohol conversion and benzaldehyde selectivity were 10.3% and 89.7% without any catalyst under 1 MPa O 2 (Table 3, entry 1).The benzyl alcohol conversions and benzyldehyde selectivities were 12.3%, 53.8%, 50.9%, and 17.9%, 83.3%, 37.2% over Ca(II) coordination polymer under 1 MPa N 2 , O 2 , and air, respectively.The benzyl alcohol conversions and benzyldehyde selectivities were 94.0%, 0.0 and 7.5%, 0.0% under 1 MPa O 2 (Table 3, entries 2 and 3), respectively.The above results show that both oxygen and Ca(II) coordination polymer are needed for the oxidation of benzyl alcohol.

Conclusions
A novel eight-coordinated Ca(II) coordination polymer with 2-carboxybenzaldehyde ligand has been synthesized and structural characterized.The Ca(II) coordination polymer exhibited relatively good catalytic activity in the benzyl alcohol oxidation reaction with benzyl alcohol conversion and moderate benzaldehyde selectivity.

Scheme 1 .
Scheme 1.The chemical diagram of the Ca(II) coordination polymer.

Figure 4 .
Figure 4.The conversion of benzyl alcohol oxidation.

Figure 5 .
Figure 5.The conversion of benzyl alcohol and selectivity of the.benzaldehyde for the benzyl alcohol oxidation over Ca(II) coordination polymer catalyst with THF as solvent at 130 • C under 1 MPa O 2 .

Table 2 .
Recyclability of Ca(II) coordination polymer catalyst in the oxidation of benzyl alcohol at 130 • C a .MPa O 2 , 130 • C; b Conversion based on benzyl alcohol, and calculated by an external standard method; c The selectivity of benzaldehyde was calculated by an external standard method; d Run 1, Run 2, and Run 3 are the first, second, and third reuse of the Ca(II) coordination polymer catalyst, respectively.

Table 3 .
Effect of reaction conditions on the benzyl alcohol oxidation a .Reaction conditions: benzyl alcohol (0.2 mmol), THF (1.5 g), and Ca(II) coordination polymer catalyst (0.06 g), 1 MPa O 2 , 130 • C, 3 h; b Conversion based on benzyl alcohol, and calculated by an external standard method; c The selectivity of benzaldehyde was calculated by an external standard method. a