Synthesis, Crystal Structure and Catalytic Property of a Cobalt(II) Compound Derived From 2-Bromo- N’ -(2-Hydroxy-5-Methylbenzylidene)Benzohydrazide

With a tridentate Schiff base ligand 2-bromo- N’ -(2-hydroxy-5-methylbenzylidene)benzohydrazide (HL) and cobalt nitrate, a cobalt(II) compound [ Co(L) 2 ] NO 3 · ½H 2 O ( 1 ) was prepared and characterized by elemental analysis, IR spec-troscopy and X-ray structure determination. The compound crystallizes in the monoclinic space group P 2 1 / c . Single crystal X-ray diffraction analysis reveals that the Co atom is coordinated by the NOO donor atoms of the Schiff base ligands, in an octahedral coordination. The compound show effective catalytic oxidation property on some olefins. In ge-neral, oxidation of the substrates gave the corresponding epoxides in over 80% yields for various styrene and 71% for cyclohexene.


Introduction
In recent years, the catalytic oxidation of olefins has aroused much attention in the production of chemicals and fine chemicals since epoxides are key starting materials for a wide variety of oily products. 1 Because of the environment friendly nature H 2 O 2 has been regarded as the first selected oxidant in the oxidation of olefins. Transition metal complexes with various ligands have presented interesting catalytic properties. 2 Among the complexes, cobalt species show efficient catalytic oxidation properties. 3 Hydrazones are versatile ligands in coordination chemistry. 4 Recently, we have reported the catalytic property of a molybdenum complex with hydrazone ligand. 5 As an extension of the work on the exploration of new catalytic material, in this paper, a new cobalt(II) compound derived from the Schiff base ligand 2-bromo-N'-(2-hydroxy-5-methylbenzylidene)benzohydrazide (HL) was prepared and its catalytic oxidation property was performed.

1. Materials and Methods
5-Methylsalicylaldehyde and 2-bromobenzohydrazide were purchased from Alfa Aesar. Cobalt nitrate hexahydrate and solvents are commercially available and were used without further purification. Elemental analyses for carbon, hydrogen, and nitrogen were carried out with an Elementar Vario EL. Infrared spectra were measured on KBr disks with a Hitachi I-5040 FT-IR spectrophotometer. Molar conductivity was determined in methanol with a concentration of 10 -3 M at room temperature on a DDS-11A conductometer.

Synthesis of the Compound
5-Methylsalicylaldehyde (0.136 g, 1.00 mmol) and 2-bromobenzohydrazide (0.215 g, 1.00 mmol) were mixed and stirred in methanol (20 mL) for 30 min. Then, cobalt nitrate hexahydrate (0.291 g, 1.00 mmol) dissolved in methanol (20 mL) was added, and stirred for another 30 min. The solution was kept still in air for a few days to slowly evaporate in order to give brown blockshaped single crystals of 1.

3. Catalytic Oxidation Experiment
Catalytic experiment was carried out in a 50 mL glass round-bottom flask fitted with a reflux condenser and placed in an oil bath at prearranged temperature under continuous stirring. The oxidation was carried out as follows: the compound 1 (0.032 mmol) was dissolved in 10 mL 1,2-dichloroethane. Then 10 mmol alkene was added to the reaction mixture and 30 mmol TBHP was added. The reaction mixture was refluxed for 1 h. The reaction products were monitored at periodic time intervals using gas chromatography. The oxidation products were identified by comparison with authentic samples (retention times in GC).

4. X-Ray Structure Determination
Data collection was performed with a Bruker Apex II CCD diffractometer at 298 K. The structure was solved by direct methods with SHELXS-97 and refined by fullmatrix least squares (SHELXL-97) on F 2 . 6 All non-hydrogens were refined anisotropically. Hydrogens were placed geometrically and refined with a riding model, with isotropic displacement coefficients U(H) = 1.2 U(C) or 1.5 U(C methyl ). Br1 atom is disordered over two sites, with occupancies of 0.727 and 0.273 and Br2 atom is disordered over two sites, with occupancies of 0.534 and 0.466. Atoms C12, C13, C14, C15, Br4, N9, O9, O10 and O11 were restrained using ISOR instruction.. Crystallographic data are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.

1. Chemistry
The compound 1 was prepared by the reaction of equimolar quantities of the Schiff base ligand with cobalt nitrate hexahydrate in methanol. Crystals of 1 are stable in air and soluble in methanol, ethanol, DMF and DMSO, but are insoluble in water. The molar conductance measured in methanol with a concentration of 10 -3 M is 115 Ω -1 cm 2 mol -1 , indicating the compound is a 1:1 electrolyte. 7

2. Infrared Spectra
The compound has been characterized by infrared spectroscopy. The broad band centered at 3451 cm -1 is assigned to the O-H vibrations. The sharp band at 3217 cm -1 is assigned to the N-H vibration of the amino groups. The strong band indicative of the C=N group is observed at 1605 cm -1 . 8 The bands indicative of the nitrate anions are located at 1377 and 835 cm -1 .

Crystal Structure Description of 1
The asymmetric unit of 1 contains two mononuclear cobalt(II) complex cations, two nitrate anions and one water molecule of crystallization. The Co(1) complex cation of the compound is shown in Figure 1. Since the geometries of both complex cations are very similar, the Co(2) complex cation of the compound is shown in Figure S1 as a supplementary material. The Co atoms are coordinated by two phenolate O, two imine N and two carbonyl O atoms from two Schiff base ligands, forming octahedral coordination. The distortion of the octahedral coordination can be observed from the cis and trans coordinate bond angles, viz. 79.4(3)-102.9(3)° and 166.0(3)-167.9(2)°f or Co(1), 78.8(3)-104.3(2)° and 165.8(2)-166.7(3)° for Co(2). The bond lengths related to the Co atoms are similar to each other, and also comparable to those observed in cobalt complexes with Schiff base ligands. 9 The dihedral angles between the two benzene rings of the Schiff base ligands C(1)-C(6) and C (10) (Table 3) (Table 3), to form chains, as shown in Figure 2.

4. Catalytic Property
The catalytic results are given in Table 3. As seen from the results, the products of the reactions are epoxides and the selectivities for these products are 100%. High TONs (turn over numbers = moles of substrate converted per mole of 1) obtained for the substrates suggest a very high catalytic efficiency for 1. In general, oxidation of the substrates gave the corresponding epoxides in over 80% yields for various styrene and 71% for cyclohexene. The catalytic property of the compound is comparable to the oxidovanadium(V) complex. 10

Conclusions
In summary, a new mononuclear cobalt(II) compound with a tridentate Schiff base ligand 2-bromo-N'-(2hydroxy-5-methylbenzylidene)benzohydrazide has been prepared and characterized. Single crystal structure of the compound was determined. The Co atom in the complex cation is in an octahedral coordination. Single crystal of the compound is stabilized by hydrogen bonds. The compound show effective catalytic oxidation property on some olefins. In general, oxidation of the substrates gave the corresponding epoxides in over 80% yields for various styrene and 71% for cyclohexene.

Supplementary Material
CCDC-1448088 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgments
The Department of Chemistry of Dezhou University is acknowledged for financial support. a The molar ratio of catalyst:substrate:TBHP is 1:300:1000. The reactions were performed in mixture of CH 3 OH/CH 2 Cl 2 (V:V = 6:4; 1.5 mL). b The GC conversion (%) was measured relative to the starting substrate after 1 h. c TON: turn over number = moles of substrate converted per mole of 1.