Synthesis, X-Ray Crystal Structures and Catalytic Epoxidation of Oxidovanadium(V) Complexes with Aroylhydrazone and Ethyl Maltolate Ligands.

Two oxidovanadium(V) complexes, [VOL1L] (1) and [VOL2L] (2) (L = ethyl maltolate), derived from the aroylhydrazones 4-bromo-N'-(2-hydroxy-5-methylbenzylidene)benzohydrazide (H2L1) and N'-(3,5-dibromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H2L2), respectively, have been synthesized and characterized by elemental analysis, infrared and electronic spectroscopy. Structures of the complexes were further confirmed by single crystal X-ray determination. The V atoms in the complexes are coordinated by the ONO donor atoms of the aroylhydrazone ligand, OO donor atoms of the ethyl maltolate ligand, and one oxido O atom, forming octahedral coordination. The complexes function as effective olefin epoxidation catalysts with hydrogen peroxide as terminal oxidant and sodium hydrogen carbonate as a co-catalyst.


Introduction
Schiff base complexes have gained remarkable attention due to their interesting applications in the development of new materials like catalysts, and biological applications like DNA cleavage, antibacterial, antiviral and antifungal agents. 1 Metal complexes of hydrazone type Schiff bases were used as catalysts for the organic synthesis, such as olefin polymerization and epoxidation reactions. 2 Among the various metal ions, the complexes of vanadium have received considerable interest in their biochemical significance and industrial catalytic processes. 3 For instance, the use of vanadium complexes in asymmetric synthesis, in C-C bond formation as well as in C-C, C-O and C-H bond cleavages, catalytic oxidation of various olefins, oxidative halogenation and selective epoxidation of unsaturated hydrocarbons and allyl alcohols. 4 Aroylhydrazones bearing typical -CO-NH-N=CHgroup are interesting ligands in the preparation of various metal complexes which have considerable biological and catalytic properties. 5 To date, a number of vanadium complexes have been obtained. However, the vanadium complexes with hydrazones are rarely reported with catalytic oxidation of olefins. Recently, our research group has reported some vanadium complexes and their catalytic epoxidation property. 6 As a continuation of such work, we report in this paper two new vanadium(V) complexes [VOL 1 L] (1) and [VOL 2 L] (2) (L = ethyl maltolate), derived from the aroylhydrazones 4-bromo-N'-(2-hydroxy-5-methylbenzylidene)benzohydrazide (H 2 L 1 ) and N'-(3,5-dibromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H 2 L 2 ).

5-Methylsalicylaldehyde,
3,5-dibromosalicylaldehyde, 4-bromobenzohydrazide and 4-methoxybenzohydrazide were purchased from Sigma-Aldrich. VO(acac) 2 and the solvents with analytical reagent grade were purchased from Xiya Chemicals Co. Ltd. Microanalyses for C, H, N were carried out using a Perkin Elmer 2400 CHNS/O elemental analyzer. 1 H NMR spectra were recorded on a Bruker AVANCE 500 MHz spectrometer. FT-IR spectra were recorded on a FT-IR 8400-Shimadzu as KBr discs in the range of 400-4000 cm -1 . UV-Vis spectra were recorded on a Lambda 35 spectrometer. X-ray diffraction data were collected using a Bruker Smart 1000CCD diffractometer.

4. Synthesis of the complexes [VOL 1 L] (1) and [VOL 2 L] (2)
The aroylhydrazones H 2 L 1 (0.10 mmol, 33 mg) or H 2 L 2 (0.10 mmol, 43 mg) was dissolved in ethanol (15 mL). To each solution an ethanolic solution (10 mL) of VO(acac) 2 (0.10 mmol, 26 mg) and ethyl maltol (0.10 mmol, 14 mg) was added with stirring. Mixtures were stirred at room temperature for 30 min to give deep brown solution. Brown block-shaped single crystals suitable for X-ray analysis were obtained after slow evaporation of the solvent over a few days. The crystals were isolated by filtration.

5. X-Ray Structure Determination
Crystal structures of complexes were measured on a Bruker SMART 1000CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å) and a graphite monochromator at 25 °C. Unit cell and reflection data were obtained by standard methods and are summarized in Table 1. 7 The structures were solved, refined, and prepared for publication using the SHEXTL package (structure solution refinements and molecular graphics), 8 and using full-matrix least-squares techniques by using F 2 with anisotropic displacement factors for all non-hydrogen atoms. The amino H atoms were located from difference Fourier maps and

1. Synthesis and Spectral Characterization
The two complexes were readily prepared from the reaction of the corresponding aroylhydrazone ligands and VO(acac) 2 . The single crystals of the complexes are stable at ambient condition.
The ν(C=N) absorptions are observed at 1611 cm -1 for 1 and 1608 cm -1 for 2. 9 The intense bands indicative of the C=O vibrations and the sharp bands indicative of the N-H vibrations are absent in the complexes, indicating the enolization of the aroylhydrazone ligands. The weak peaks in the low wave numbers in the region 450-700 cm -1 may be attributed to V-O and V-N bonds in the complexes. The complexes exhibit typical bands at 971-972 cm -1 , which are assigned to the V = O vibrations. 10 The UV-Vis spectra of the complexes were recorded in 10 -5 mol L -1 in ethanol, in the range 200-500 nm. The weak bands centered at 325-332 nm for the complexes are attributed to intramolecular charge transfer transitions from the p π orbital on the nitrogen and oxygen to the empty d orbitals of the metal. 10 The intense bands observed at 265-270 nm are assigned to intraligand π-π* transition. The bands centered at about 410 nm are attributed to the ligand-to-metal charge transfer transitions (LMCT). 11
In the crystal structure of complex 1, the vanadium complex molecules are linked through C−H•••O hydrogen bonds (Table 3) to form layers along the ab plane (Fig. 3). In the crystal structure of complex 2, the vanadium complex molecules are linked through C−H•••O and C−H•••Br hydrogen bonds (Table 3) to form three-dimensional network (Fig. 4).

Catalytic Property
The catalytic experiment was carried out according to the literature method. 6b A mixture of CH 3 OH/CH 2 Cl 2 (V:V = 7:3, 1.2 mL) was used for the reactions at 25 °C. The molar ratios for the catalyst:substrate:NaHCO 3 :H 2 O 2 are 1:298:117:1170. The conversion was measured after 74.5 min. Both vanadium complexes have good property in the olefin oxidation processes with epoxides as the products. The results are summarized in Table 4. Interestingly, both complexes have similar catalytic properties with high epoxide yields and good selectivity toward the aliphatic and aromatic substrates. However, when H 2 O 2 was used as single oxidant the catalytic efficiency is not good. When NaHCO 3 was added as a co-catalyst to the above reactions,   (4) 156 (5) Symmetry codes: ii) 1 + x, y, z; iii) 1 -x, 1 -y, 1 -z; (iv) -x, -y, 1 -z; (v) 1 -x, -y, 1 -z; (vi) -x, 1 -y, 1 -z;

Conclusion
Two new similar oxidovanadium(V) complexes with aroylhydrazone ligands have been prepared and structurally characterized using X-ray structure analysis. The complexes have octahedral geometry with positions around the central atom being occupied with donor atoms of the aroylhydrazone ligand, the ethyl maltolate ligand and one oxido group. The complexes show effective catalytic property in the oxidation of various olefins to their corresponding epoxides.

Supplementary Material
CCDC reference numbers 2043121 and 2043122 contain the supplementary crystallographic data for this article. These data can be obtained free of charge athttp:// www.ccdc.cam.ac.uk, or from Cambridge Crystallographic Data Center, 12Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336 033; Email:deposit@ccdc.cam.ac.uk.