Synthesis, Crystal Structures and Catalytic Property of Oxidovanadium(V) Complexes with Similar Aroylhydrazones

A pair of new oxidovanadium(V) complexes, [VOL1L]·EtOH (1) and [VOL2L]·EtOH (2) (L = acetohydroxamate), derived from the aroylhydrazones N’-(5-bromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H2L) and N’-(5-bromo-2-hydroxybenzylidene)-4-methylbenzohydrazide (H2L), have been prepared and characterized by elemental analyses, FT-IR, 1H and 13C NMR spectroscopy and single-crystal structural X-ray diffraction. The complexes have octahedral structures in which the aroylhydrazone ligands behave as binegative donors. Single-crystal structure analyses reveal that the V centers are coordinated by the donor atoms of the aroylhydrazone ligands, the acetohydroxamate ligands and the oxido groups. Crystal structures of the complexes are stabilized by hydrogen bonds. The complexes function as effective olefin epoxidation catalysts.


Introduction
In recent years, remarkable attention has focused on vanadium compounds because of their biochemical significance 1 and industrial catalytic processes. 2 For instance, the use of oxovanadium complexes in asymmetric synthesis, 3 in C-C bond formation as well as in C-C, C-O and C-H bond cleavages, 4 catalytic oxidation of various olefins, 5 oxidative halogenation and selective epoxidation of unsaturated hydrocarbons and allyl alcohols. 6 Aroylhydrazones bearing typical -CO-NH-N=CH-group are interesting ligands in the preparation of various metal complexes which have considerable biological and catalytic properties. 7 A number of vanadium complexes with various types of ligands have been prepared, yet, those derived from hydrazones only few have been reported with catalytic oxidation on olefins. In this paper we are concerned about the structural investigation and catalytic activity of two vanadium complexes with hydrazone ligands, which have similar structures except for the terminal substituted groups, Me and OMe. In the present work, a pair of new vanadium(V) complexes [VOL 1 L] · EtOH (1) and [VOL 2 L] · EtOH (2) (L = acetohydroxamate), derived from the aroylhydrazones N'-(5-bromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide (H 2 L 1 ) and N'-(5bromo-2-hydroxybenzylidene)-4-methylbenzohydrazide (H 2 L 2 ; Scheme 1), are presented. Scheme 1. H 2 L 1 (X = OMe) and H 2 L 2 (X = Me)

Materials and Methods
All chemicals and solvents used were of analytical reagent grade and used as received. Micro analyses for C, H, N were carried out using a Perkin Elmer 2400 CHNS/O elemental analyzer. FT-IR spectra were recorded on a FT-IR 8400-Shimadzu as KBr discs in the range of 400-4000 cm -1 . 1 H and 13 C NMR spectra were recorded at 25 °C on the Bruker AVANCE 300 MHz spectrometer. X-ray diffraction data were collected using a Bruker Smart Apex II diffractometer.

3. Synthesis of the Complexes
An ethanolic solution (10 mL) of VO(acac) 2 (0.1 mmol, 0.026 g) was added to the ethanolic solution of acetohydroxamic acid (0.1 mmol, 0.0075 g) and H 2 L 1 (0.1 mmol, 0.035 g) for 1 and H 2 L 2 (0.1 mmol, 0.033 g) for 2, respectively, and the resulting orange mixture was refluxed for 30 min. After cooling, the solution was filtered and left to stand overnight. Orange single crystals suitable for crystallographic analysis separated after a week and dried in a vacuum desiccator over silica gel. [

6. X-Ray structure Determination
The crystal structures of the complexes were measured on a Bruker SMART Apex II CCD 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 8 and are summarized in Table 1. The structures were solved, refined, and prepared for publication using the SHEXTL package (structure solution refinements and molecular graphics), 9 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 refined isotropically, with N-H distanc- es restrained to 0.90(1) Å. Positions of the remaining hydrogen atoms were calculated from the geometry of the molecular skeleton and their thermal displacement parameters were refined isotropically on a groupwise basis. Selected bond lengths and angles are reported in Table 2. H-bonding distances and angles are shown in Table 3.
H 2 O 2 (1.1 mmol, 30% H 2 O 2 in water) as oxidant. After the reaction was over at 74.5 min, for the products analysis, the solution was subjected to multiple ether extraction, and the extract was also concentrated down to 0.5 mL by distillation in a rotary evaporator at room temperature and then a sample (2 μL) was taken from the solution and analyzed by GC. The retention times of the peaks were compared with those of commercial standards, and chlorobenzene was used as an internal standard for GC yield calculation.

1. Synthesis and Characterization
The reaction of VO(acac) 2 and acetohydroxamic acid with the tridentate aroylhydrazone ligands H 2 L 1 and H 2 L 2 in ethanol led to the formation of the complexes. Crystals of the complexes are stable at room temperature and soluble in DMSO, DMF, ethanol, acetonitrile and less soluble in other common solvents like dichloromethane, chloroform, and insoluble in benzene, n-hexane and CCl 4 .

2. 1 H NMR Spectra
1 H NMR data of the aroylhydrazone ligands when compared with the complexes reveal that the ligands serve as tridentate binegative ONO donor. The azomethine C-H signal in the complexes is shifted up-field from its original position in the free ligands upon coordination of the -CH=N-groups, on account of reduction of electron density at the azomethine C-H. The aromatic protons also show some deviation in the complexes as compared to the free ligand since in the complexes they are in direct conjugation to the coordinated O and N of the hydrazone ligands.

IR Spectra
IR spectra of the free aroylhydrazone ligands show bands at 3220-3240 cm -1 for ν(N-H), 1647 cm -1 for ν(C=O) and 3445-3455 cm -1 for ν(O-H). 10 The ν(C=O) bands are absent in the spectra of the complexes as the ligands bind in binegative mode losing protons from the carbohydrazide groups. The strong peak at about 1610 cm -1 can be assigned to ν(C=N). 11 The complexes exhibit characteristic bands at 968 cm -1 for the stretching of V=O groups. 12

4. Structure Description
The perspective views of the complexes together with the atom numbering schemes are shown in Figs. 1 and 2. The asymmetric units of the complexes contain one complex molecule and one ethanol molecule. The coordination geometry around each V atom reveals a distorted    Symmetry codes: i) x + ½, -y + 3/2, z -½; ii) ½ + x, 3/2 -y, -½ + z.

7. Catalytic Epoxidation of Olefins
To a solution of olefins (0.28 mmol), NaHCO 3 (0.11 mmol, 9.24 mg) and catalyst (9.4×10 −4 mmol) in the mixture of CH 3 OH/CH 2 Cl 2 (1.2 mL; V:V = 7:3) was added octahedral environment with an NO 5 chromophore. The ligand molecule behaves as binegative tridentate one binding through the phenolate oxygen, the enolate oxygen and the imine nitrogen and occupies three positions in the basal plane. The fourth donor of the basal plane is furnished by the hydroxyl O atom of the acetohydroxamate ligand. The oxo group and the carbonyl O atom of the acetohydroxamate ligand are located at the axial positions. The V atoms are found to be deviated from the corresponding mean basal planes by 0.283(2) Å for 1 and 0.279(2) Å for 2. The C(8)-O(2) bond lengths are closer to single bond length rather than C-O double bond length. However, the shorter length compared to C-O single bond may be attributed to extended electron delocalization in the ligand. Similarly shortening of C(8)-N(2) bond lengths together with the elongation of N(1)-N(2) lengths also supports the electron cloud delocalization in the ligand system. The ligand molecules form five-membered and six-membered chelate rings with the V centers. The bond lengths related to the V atoms are similar to those observed in other vanadium complexes. 13 In the crystal structures of the complexes, the vanadium complex molecules are linked by ethanol molecules through hydrogen bonds (Figs. 3 and 4).

5. Catalytic Property
The reactions were performed in ( lytic properties with respect to epoxidation of olefins with the complexes as catalysts are given in Table 4. Excellent epoxide yields and selectivity (> 99%) were observed for all aliphatic and aromatic substrates. The results of catalytic studies using the catalysts reveal that the efficiency of catalyst toward all the substrates is similar with maximum conversion, TON, and selectivity. When H 2 O 2 (1.1 mmol, 30% H 2 O 2 in water) was used as a sole oxidant the catalytic efficiency is not high, but when NaHCO 3 (0.11 mmol, 9.24 mg) was added as a co-catalyst the efficiency of the system increases many times. The key aspect of such a reaction is that H 2 O 2 and hydrogen carbonate react in an equilibrium process to produce peroxymonocarbonate, HCO 4 − , which is a more reactive nucleophile than H 2 O 2 and speeds up the epoxidation reaction. The catalytic properties of the presented complexes are comparable to the molybdenum and vanadium complexes reported in literature. 14 article. These data can be obtained free of charge at http:// www.ccdc.cam.ac.uk, or from Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336 033; Email: deposit@ccdc.cam.ac.uk.

Conclusion
A pair of new oxidovanadium(V) complexes with aroylhydrazone ligands have been prepared and structurally characterized using X-ray structure analysis, FT-IR and 1 H NMR spectra. The complexes have octahedral geometry with positions around the central atom being occupied with donor atoms of the aroylhydrazone ligand, the acetohydroxamate ligand and one oxo group. The complexes show effective catalytic property in the oxidation of various olefins to their corresponding epoxides.

Supplementary Material
CCDC reference numbers 1845890 and 1845891 contain the supplementary crystallographic data for this