Synthesis, Crystal Structure and Catalytic Activity of a Novel Ba(II) Complex with Pyridine-2-Carboxaldehyde-2-Phenylacetic Acid Hydrazone Ligand

Abstract

A novel Ba(II) complex, [BaL₂Cl₂] (1) (L = pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone), has been synthesized using BaCl₂, pyridine-2-carboxaldehyde and 2-phenylacetohydrazide as raw materials. The structure of 1 has been determined by elemental analysis and X-ray single-crystal diffraction technique. X-ray structural analysis showed that the Ba(II) complex (1) crystallizes in monoclinic, space group P2₁/c with cell parameters: a = 12.464(3) Å, b = 13.531(3) Å, c = 8.8035(18) Å, β = 95.06(3)°. In 1, the Ba(II) atom is eight-coordinated in a distorted doubly-capped octahedral geometry through four N atoms and two O atoms from two different pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone ligands and two Cl⁻. The complex (1) forms a 3D network structure by the interaction of intermolecular N-H···Cl hydrogen bonds and π···π stacking of neighboring pyridine rings and benzene rings. The optimum conditions for the benzyl alcohol oxidation reaction using the Ba(II) complex as catalyst was investigated.

1. Introduction

The studies on structures and properties of metal complex materials have achieved much attention in the past decades [1,2,3,4,5], because they show attractive applications in many ways such as magnetic properties [6,7,8], fluorescence [9,10,11,12], catalytic activities [13,14,15], pharmacological activities [16,17,18], and so on. However, to best of our knowledge, the structure and property of alkaline earth metal complexes have not been studied extensively comparing to transition metal complex. In recent years, our group have synthesized and structurally characterized some alkaline earth metal complexes [19,20,21,22,23,24,25,26,27]. In order to further investigate the structure and property of alkaline earth metal complexes, a novel Ba (II) complex with pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone (L), [BaL₂Cl₂] (1) has been synthesized and characterized. The catalytic activity of Ba(II) complex has also been investigated. The chemical diagram of the Ba(II) complex is shown in Scheme 1.

2. Results and Discussion

2.1. Structural Description of Ba(II) Complex

The Ba(II) complex crystallizes in monoclinic system with space group P2₁/c. There are one Ba(II) ion, two pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone ligands, two Cl⁻ in the complex molecule. Figure 1 shows the coordination environment of Ba(II) ion in the complex. As shown in Figure 1, the Ba(II) ion is eight-coordinated with four N atoms and two O atoms from two different pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone ligands and two Cl⁻, forming a distorted doubly-capped octahedral geometry. The dihedral angle between the pyridine ring (N1-C1-C2-C3-C4-C5) and benzene ring (C9-C10-C11-C12-C13-C14) is 76.2°, showing that the hydrazone molecule is non-planar. In the crystal packing, the complex molecules form 1D chained structure by the intermolecular hydrogen bonds (N-H···Cl) present between the pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone ligand and coordinated Cl⁻ (Figure 2), or by the π···π stacking interactions of pyridine rings (Figure 3). Meanwhile, hydrogen bonds and π···π stacking interactions of pyridine rings and benzene rings assemble the 1D chains into a 3D network structure (Figure 4). The important bond lengths and bond angles are shown in

Table 1. Selected bond lengths (Å) and bond angles (°) for Ba(II) complex.

Bond	d	Bond	d
Ba1-Cl1	3.0214(11)	Ba1-O1	2.8330(19)
Ba1-N1	2.957(2)	Ba1-N2	2.8984(19)
C6-N2	1.268(3)	N3-C7	1.344(3)
C7-O1	1.221(3)	C1-N1	1.338(3)
C5-N1	1.343(3)	N2-N3	1.378(3)
Angle	ω	Angle	ω
O1-Ba1-Cl1	91.50(4)	O1-Ba1-Cl1A	88.50(4)
Cl1A-Ba1-Cl1	180.0	O1-Ba1-O1A	180.0
N1-Ba1-Cl1A	91.59(4)	Cl1-Ba1-N1	88.41(4)
O1A-Ba1-N1	71.15(5)	O1-Ba1-N1	108.85(5)
N1-Ba1-N1A	180.0	N2-Ba1-Cl1A	78.78(4)
N2-Ba1-Cl1	101.22(4)	N2-Ba1-O1	55.32(5)
N2-Ba1-O1A	124.68(5)	N2-Ba1-N1	55.12(5)
N2-Ba1-N1A	124.88(5)	N2-Ba1-N2A	180.0

Symmetry code: −x, 1 − y, 1 − z.

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2.2. Catalytic Performance of Ba(II) Complex

The aerobic oxidation of benzyl alcohol was catalyzed by Ba(II) complex at 130 °C with 1 MPa oxygen as the sole oxidant. The results of the catalytic activity of the Ba(II) complex are given in

Table 2. Catalytic performance of the Ba(II) complex in the oxidation of benzyl alcohol.

Entry	Solvent	Temperature (°C)	Time (h)	Conversion (%)	Selectivity (%)
1	dimethylformamide	130	6	9	28
2	acetonitrile	130	6	11	21
3	1,4-dioxane	130	6	48	89
4	1,4-dioxane	130	7	57	17
5	tetrahydrofuran	130	3	15	80
6	tetrahydrofuran	130	4	67	53
7	tetrahydrofuran	130	6	75	35

. We set out to examine the catalytic activity of Ba(II) complex catalyst in the aerobic oxidation of benzyl alcohol in different solvent (such as dimethylformamide, acetonitrile, 1,4-dioxane, and tetrahydrofuran) and different reaction times. The conversion of benzyl alcohol and selectivity of benzaldehyde at 130 °C for 6 h in dimethylformamide and acetonitrile are 9%, 11% and 28%, 21%, respectively (Table 2, entries 1 and 2). The conversion of benzyl alcohol and selectivity of benzaldehyde in 1,4-dioxane are 48% and 89% at 130 °C for 6 h, respectively (Table 2, entry 3). The conversion of benzyl alcohol reached 57% for Ba-complex at 130 °C for 7 h (Table 2, entry 4). However, the selectivity of benzaldehyde in 1,4-dioxane at 130 °C for 7 h over Ba(II) complex catalyst was decreased to 17%. It was found that the benzyl alcohol conversions in tetrahydrofuran at 130 °C are 15%, 67%, and 75% within 3 h, 4 h, and 6 h, respectively. Meanwhile, the selectivities of benzaldehyde in tetrahydrofuran at 130 °C are 80%, 53%, and 35% within 3 h, 4 h, and 6 h, respectively (Table 2, entries 5–7). Clearly, when increasing the reaction time, it gave an increase in benzyl alcohol conversion and a decrease in benzaldehyde selectivity. The optimum conditions for the benzyl alcohol oxidation reaction over the Ba(II) complex was 130 °C, 6 h, and using 1,4-dioxane as solvent. In four successive cycles, the conversion of benzyl alcohol was 45%, 40%, 37%, and 32% at 130 °C for 6 h, respectively. In addition, the selectivity of benzaldehyde in 1,4-dioxane is only 4.5% at 130 °C for 6 h, so the Ba(II) complex catalyst shows good catalytic activity.

3. Experimental Section

3.1. Materials and Instrumentation

Pyridine-2-carboxaldehyde (A. R.), 2-phenylacetic acid hydrazone (A. R.) and BaCl₂ (A. R.) were purchased from Shanghai Chemical Reagent Company. Elemental analysis (C, H and N) was performed on an Elementar Vario III EL elemental analyzer (Hanau, Germany). Crystal data of Ba(II) complex was collected on a Bruker Smart CCD diffractometer (Bruker, Billerica, MA, USA).

3.2. Synthesis of Ba(II) Complex

Pyridine-2-carboxaldehyde (1.0 mmol, 0.1071 g) and 2-phenylacetic acid hydrazone (1.0 mmol, 0.1502 g) were dissolved in 15 mL 95% ethanol solution with stirring. Then 5.0 mL water solution containing BaCl₂ (0.5 mmol, 0.1041 g) was added to the above mixture. The reaction mixture was heated at 60 °C for 7 h with stirring and cooled to room temperature. The mixture was filtered, and the block crystals were obtained from filtrate after 40 days. Yield ca. 52%. Anal. Calcd. for C₂₈H₂₆N₆O₂Cl₂Ba: C, 48.92; H, 3.79; N, 12.23. Found: C, 48.73; H, 4.07; N, 12.38.

3.3. Crystal Data Collection and Handling

A suitable crystal of Ba(II) complex was selected on a Brucker Smart Apex CCD diffractometer (Bruker, Karlsruhe, Germany) with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293(2) K. The structure was solved by direct method using the SHELXL-2014/7 program [28], and refined on F² by full-matrix least squares with the OLEX2. refine [29] refinement package using Gauss-Newton minimisation. The data collection and handling for Ba(II) complex are given in

Table 3. Data collection and handling for Ba(II) complex.

Empirical Formula	C28H26N6O2Cl2Ba
Formula weight	686.78
Temperature/K	293(2)
Crystal system	monoclinic
Space group	P21/c
a/Å	12.464(3)
b/Å	13.531(3)
c/Å	8.8035(18)
β/°	95.06(3)
Volume/Å3	1478.9(6)
Z	2
ρcalcmg/mm3	1.542
μ/mm−1	1.559
S	1.098
F(000)	684
Index ranges	−16 ≤ h ≤ 15, −17 ≤ k ≤ 17, −11 ≤ l ≤ 11
Reflections collected	13738
Independent reflections	3353 [R(int) = 0.0755]
Data/restraints/parameters	3353/0/178
Goodness-of-fit on F2	1.098
Final R indexes [I ≥ 2σ (I)]	R1 = 0.0455, wR2 = 0.1079
Final R indexes [all data]	R1 = 0.0494, wR2 = 0.1109

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3.4. Catalytic Test of Benzyl Alcohol Oxidation

Aerobic oxidation of benzyl alcohol catalyzed by Ba(II) complex was performed in a 10 mL stainless steel autoclave under 1 MPa. In a typical reaction, benzyl alcohol (0.2 mmol, 21.6 mg), solvent (1.5 g), and catalyst (0.060 g) were added into stainless steel autoclave, and then pure O₂ (99.999%) was purged into the stainless steel autoclave. The mixture was kept at 130 °C for 3–7 h at stirring speed of 700 rpm. The mixture was centrifuged to remove the catalyst when the reaction is over. The conversion of benzyl alcohol and the selectivity of benzaldehyde were determined by gas chromatography equipped with a SE-54 capillary column (GC-1100, 0.25 mm × 0.25 mm × 30 m). The conversion of benzyl alcohol and the selectivity of products were determined by gas chromatography-massspectrometer equipped with a SE-54 column (GC-1100, 0.25 mm × 0.25 mm × 30 m). The products were identified by comparison with known authentic standards, and anexternal standard method was used for the qualitative analysis.

4. Conclusions

In summary, a novel eight-coordinated Ba(II) complex with pyridine-2-carboxaldehyde-2-phenylacetic acid hydrazone ligand has been synthesized and structural characterized. The optimum conditions for the benzyl alcohol oxidation reaction over the Ba(II) complex catalyst was 130 °C, 6 h, and using 1,4-dioxane as solvent.