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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.59 no.4 Concepción dez. 2014

http://dx.doi.org/10.4067/S0717-97072014000400024 

 

SYNTHESIS, STRUCTURAL CHARACTERIZATION AND CATALYTIC OXIDATION PROPERTY OF SCHIFF BASE COPPER(II) COMPLEXES

 

SHUANG HANa,* YUAN WANGb

a College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, P. R. China
b
Qiqihar Environmental Monitoring Central Station, Qiqihar 161005, P. R. China
* e-mail address:
hanzhq2088@163.com


ABSTRACT

Two new square planar mononuclear copper(II) complexes, [CuL1] (1) and [CuL2(NCS)] (2), where L1 is the dianionic form of the bis-Schiff base N,N'-bis(2-hydroxynaphthylmethylidene)-l,3-pentanediamine (H2L1), and L2 is the monoanionic form of 1-[(2-diethylaminoethylimino)methyl]naphthalen-2-ol (HL2), have been synthesized and characterized by physico-chemical methods and single-crystal X-ray diffraction. Complex (1) (C27H24CuN2O2) crystallizes as the orthorhombic space group Pbca, with unit cell dimensions a = 18.284(1) Å, b = 12.2373(7) Å, c = 19.775(1) Å, V = 4424.6(4) Å3, Z = 8, R1 = 0.0421, wR2 = 0.0975, S = 1.054. Complex (2) (C18H21CuN3OS) crystallizes as the monoclinic space group P2/c, with unit cell dimensions a = 12.776(1) Å, b = 24.181(2) Å, c = 11.743(1) Å, β = 98.621(1)°, V = 3586.7(7) Å3, Z = 8, R1 = 0.0520, wR2 = 0.1183, S = 1.012. X-ray analysis indicates that the Cu atoms in the complexes are in square planar coordination. The complexes were evaluated for their catalytic oxidation of cyclooctene and styrene with tert-butylhydroperoxide.

Keywords: Schiff base; copper complex; crystal structure; catalytic property.


 

INTRODUCTION

The oxidation of various hydrocarbons catalyzed by metal complexes with various ligands has received particular attention over the last few years.1-4 Copper(II) complexes have been proved to possess excellent catalytic activity towards a number of organic reactions.5-8 In particular, copper complexes derived from Schiff bases are amongst the most versatile catalysts known for oxygenation reactions.9-11 The role played by copper ions in the active sites of a large number of metalloproteins has stimulated efforts to design and characterize copper complexes as models for a better understanding of biological systems. In this paper, two new copper(II) complexes, [CuL1] (1) and [CuL2(NCS)] (2), where L1 is the dianionic form of the bis-Schiff base N,N'-bis(2-hydroxynaphthylmethylidene)-1,3-pentanediamine (H2L1; Scheme 1), and L2 is the monoanionic form of 1-[(2-diethylaminoethylimino)methyl] naphthalen-2-ol (HL2; Scheme 1), were prepared and studied their catalytic oxidation of cyclooctene and styrene with tert-butylhydroperoxide (TBHP).

 

Scheme 1: The Schiff bases.

 

EXPERIMENTAL

General Methods and Materials: Copper perchlorate, 2-hydroxy-1-naphthyaldehyde, pentane-1,3-diamine, and N,N-diethylethane-1,2-diamine were purchased Lancaster and used as received. All other reagents were of analytical reagent grade. Elemental analyses of C, H and N were carried out in a 2400 Series-II CHN analyzer. FT-IR spectra were obtained on a Jasco FT/IR-4000 spectrometer with samples prepared as KBr pellets. 1H NMR spectra in DMSO-d6 were run in a Bruker AMX 300 instrument, using TMS as internal reference. Gas chromatography analysis was performed with an Agilent Technologies 6890 N network GC system equipped with a fused silica capillary column (30 m x 0.32 mm) and a FID detector. Molar conductance was measured with a Shanghai DDS-11A conductometer. X-ray diffraction was carried out on a Bruker SMART 1000 CCD diffractometer.

Caution: Copper perchlorate is potentially explosive, only small quantity should be used and handled with great care.

Synthesis of the Schiff Bases: The Schiff bases were synthesized as follows. To the methanolic solution (30 mL) of 2-hydroxy-1-naphthyaldehyde (2.0 mmol, 0.34 g) was added a methanolic solution (30 mL) of pentane-1,3-diamine (1.0 mmol, 0.10 g) or N,N-diethylethane-1,2-diamine (2.0 mmol, 0.23 g) with stirring. The mixtures were stirred for 30 min at room temperature to give yellow solution. The solvent was evaporated to give yellow precipitate or gummy product, which was dried by P2O5.

For H2L1: Yield 93%. Characteristic IR data (cm-1): 3332 (m), 1633 (s), 1231 (m). Anal. Calc. for C27H26N2O2: C, 79.0; H, 6.4; N, 6.8. Found: C, 78.8; H, 6.5; N, 6.8%. 1HNMR data (CDCl3, ppm): δ = 0.91 (t, 3H), 1.65 (m, 2H), I. 96 (m, 2H), 3.09 (m, 1H), 3.57 (t, 2H), 7.20-8.10 (m, 12 H), 8.75 (s, 1H), 8.80 (s, 1H), 12.2 (d, 2H). For HL2: Yield 87%. Characteristic IR data (cm-1): 3381 (m), 1629 (s), 1227 (m). Anal. Calc. for C17H22N2O: C, 75.5; H, 8.2; N, 10.4. Found: C, 75.6; H, 8.1; N, 10.5%. 1HNMR data (CDCl3, ppm): δ = 1.03 (t, 6H), 2.75 (t, 2H), 2.99 (t, 4H), 3.66 (t, 2H), 7.20-8.10 (m, 6H), 8.67 (s, 1H), 11.8 (s, 1H).

Synthesis of Complex (1): H2L1 (82.0 mg, 0.2 mmol) and copper perchlorate (75.0 mg, 0.2 mmol) were mixed in methanol (30 mL). The mixture was refluxed for 2 h and then cooled to room temperature, and filtered. The filtrate was kept at ambient temperature. Single crystals of the complex, suitable for X-ray diffraction, were grown from the filtrate upon slow evaporation within a few days. The crystals were isolated by filtration, washed with methanol and dried in air. Yield 61%. Characteristic IR data (cm-1): 1617 (s), 1183 (m). Anal. Calc. for C27H24CuN2O2: C, 68.7; H, 5.1; N, 5.9. Found: C, 68.6; H, 5.2; N, 5.8%.

Synthesis of Complex (2): HL2 (54.0 mg, 0.2 mmol), ammonium thiocyanate and copper perchlorate (75.0 mg, 0.2 mmol) were mixed in methanol (30 mL). The mixture was refluxed for 2 h and then cooled to room temperature, and filtered. The filtrate was kept at ambient temperature. Single crystals of the complex, suitable for X-ray diffraction, were grown from the filtrate upon slow evaporation within a few days. The crystals were isolated by filtration, washed with methanol and dried in air. Yield 53%. Characteristic IR data (cm-1): 2091 (s), 1620 (s), 1185 (m). Anal. Calc. for C18H21CuN3OS: C, 55.3; H, 5.4; N, 10.7. Found: C, 55.4; H, 5.4; N, 10.9%.

X-ray Crystallography: Single-crystal X-ray diffraction data for the complexes were collected on a Bruker Smart 1000 CCD diffractometer at 298(2) K with Mo Ka radiation (ë = 0.71073 A) by ù scan mode. The program SAINT12 was used for integration of the diffraction profiles. Structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL (semi-empirical absorption corrections were applied using the SADABS program).13,14 The positions of the non-hydrogen atoms were located in difference Fourier syntheses and least-squares refinement cycles, and finally refined anisotropically. The hydrogen atoms of the complexes were placed theoretically onto the specific atoms and refined isotropically as riding atoms. Crystallographic data and experimental details for structural analyses are summarized in Table 1. Selected bond lengths and angles for the complexes are listed in Table 2.

 

Table 1 Crystallographic information for the complexes

 

Table 2 Selected bond lengths (Å) and angles (°) for the complexes
with estimated standard deviations (e.s.d.s) in parentheses

 

Catalytic Oxidation Procedure: The complexes (0.032 mmol) were dissolved in 10 mL chloroform in a 50 mL round bottom flask equipped with a condenser. Then, alkene (cyclooctene or styrene) (10 mmol) and TBHP (30 mmol) were added, respectively. The mixture was refluxed for 6 h. Samples were taken at periodic time intervals and analyzed by gas chromatography. The oxidation products were identified by comparison with authentic samples.

RESULTS AND DISCUSSION

Synthesis and Characterization: Both complexes (1) and (2) were first synthesized by the reaction of the Schiff base ligand, ammonium thiocyanate and copper perchlorate in methanol. Yet, the thiocyanate anion was not coordinate to the Cu atom of complex (1). The same product of complex (1) can also be prepared by the reaction of H2L1 and copper perchlorate in the absent of ammonium thiocyanate. Elemental analyses of the complexes are in accordance with the molecular structures proposed by the X-ray analysis. FT-IR spectra of both complexes are of similar type, except the existence of intense absorption at 2091 cm-1 of the thiocyanate ligand of complex (2). The complexes show typical C=N absorptions at about 1620 cm-1, and typical Ar-O absorptions at about 1185 cm-1. Both complexes are stable in air at room temperature. The molar conductivity of the complexes measured in acetonitrile at concentration of 10-3 M are 21.5 Ω-1 cm2 mol-1 for (1) and 25.0 Ω-1 cm2 mol-1 for (2), indicating the non-electrolytic nature of the complexes in acetonitrile [15].

Structure Description of the Complexes: The molecular structures of complexes (1) and (2) are shown in Figures 1 and 2, respectively. There are two independent molecules in the asymmetric unit of complex (2). In each of the complexes, the coordination sphere has a tetrahedrally distorted square planar geometry. The four coordinate atoms come from the phenolate O and imino N atoms of L1 for (1), and the phenolate O, imino N and amino N atoms of L2, and thiocyanate N atom for (2). The distortion of the square planar geometry can be observed from the bond values. The Cu-O/N bonds are within normal values and comparable to those observed in similar copper complexes.16,17 The cis and trans angles vary between 88.67(8) and 96.17(10)°, and 154.13(10) and 159.26(10)° for complex (1), and between 84.83(11) and 95.34(13)°, and 172.32(11) and 177.75(14)° for complex (2), respectively. In complex (1), the dihedral angle between the two naphthyl rings is 47.4(5)°.

 

Figure 1 A perspective view of complex (1) with the atom labeling scheme.
Thermal ellipsoids are drawn at the 30% probability level.

 

Figure 2 A perspective view of complex (2) with the atom labeling scheme.
Thermal ellipsoids are drawn at the 30% probability level.

 

Catalytic Oxidation Results: Selective oxidation of cyclooctene to cyclooctene oxide, catalyzed by the complexes, was carried out using TBHP as oxidant. The effect of different parameters such as the oxidant to alkene molar ratio and the nature of solvent were investigated.

It was observed that the change of TBHP/cyclooctene molar ratio from 1:1 to 3:1 increases the conversion from 51% to 72% for complex (1) and from 39% to 47% for complex (2). Addition of excess amounts of oxidant over 3:1 ratio has little effect on cyclooctene conversion. In order to investigate the effect of solvent, chloroform, dichloromethane and acetonitrile have been used and the highest conversion (68% for (1) and 45% for (2) was obtained in acetonitrile.

Catalytic oxidation of styrene with TBHP in the presence of the complexes gave benzaldehyde as major product along with moderately low yields of styrene oxide (Table 3). In order to obtain the best oxidant/styrene molar ratio, three different TBHP/styrene molar ratios (1:1, 2:1 and 3:1) were considered. A maximum of 99% conversion for complex (1) and 93% for complex (2) was obtained at the 3:1 molar ratio in acetonitrile.

 

Table 3 Catalytic oxidation results of the complexes in acetonitrilea

a Solvent: 10 mL; catalyst: 0.032 mmol; Styrene: (1.04 g, 10 mmol);
TBHP: 30 mmol.
b TON: (turnover number) moles of substrate converted per mole of
copper ion.

 

CONCLUSION

The present study reports the synthesis, characterization and crystal structures of two new mononuclear copper(II) complexes with Schiff bases. The complexes have efficient catalytic oxidation properties on alkenes with TBHP as oxidant.

APPENDIX A. SUPPLEMENTARY DATA

CCDC 892373 (1) and 614321 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

ACKNOWLEDGMENTS

This work was financially supported by the Program for Young Teachers Scientific Research in Qiqihar University (2012k-M05), and the Education Office of Heilongjiang Province (Project No. 12531757).

 

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