Synthesis, X-Ray Crystal Structure of Oxidovanadium(V) Complex Derived from 4-Bromo-N’-(2-hydroxybenzylidene) benzohydrazide with Catalytic Epoxidation Property

A new oxidovanadium(V) complex, [VOL(OCH3)(CH3OH)], where H2L = 4-bromo-N’-(2-hydroxybenzylidene)benzohydrazide, has been synthesized and fully characterized on the basis of CHN elemental analysis, FT-IR, UV-Vis, 1H and 13C NMR spectroscopy. Structures of the free hydrazone and the complex were further characterized by single crystal X-ray diffraction, which indicates that the V atom in the complex adopts octahedral coordination, and the hydrazone ligand behaves as a tridentate ligand. The catalytic epoxidation property of the complex was investigated.


Introduction
Schiff bases are a kind of interesting ligands in coordination chemistry. 1 The metal complexes with Schiff bases have attracted remarkable attention due to their facile synthesis and special biological, catalytic and industrial applications. 2 Catalytic epoxidation of olefins is an important reaction in chemistry. Many transition metal complexes are active catalysts for this process. 3 However, among the complexes, vanadium and molybdenum complexes seem more interesting because of their excellent catalytic ability in the oxidation of olefins and sulfides. 4 In this paper, a new vanadium(V) complex derived from 4-bromo-N'-(2-hydroxybenzylidene)benzohydrazide (H 2 L, Scheme 1) was prepared and studied for its catalytic epoxidation property on cyclooctene.

1. Materials and Methods
4-Bromobenzohydrazide, salicylaldehyde and VO(acac) 2 were purchased from Alfa Aesar and used as received. Reagent grade solvents were used as received. Microanalyses of the complexes were performed with a Vario EL III CHNOS elemental analyzer. Infrared spectra were recorded as KBr pellets with an FTS-40 spectrophotometer. Electronic spectra were recorded on a Lambda 900 spectrometer. 1 H and 13 C NMR spectra were recorded on a Bruker spectrometer at 500 MHz. The catalytic reactions were followed by gas chromatography on an Agilent 6890A

4. Crystal Structure Determination
Data were collected on a Bruker SMART 1000 CCD area diffractometer using a graphite monochromator Mo Kα radiation (λ = 0.71073 Å) at 298(2) K. The data were corrected with SADABS programs and refined on F 2 with Siemens SHELXL software. 5 The structures of H 2 L and the complex were solved by direct methods and difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. The amino H atom (H2) in H 2 L and the methanol H atom (H5) in the complex were located from difference Fourier maps and refined isotropically. The remaining hydrogen atoms were placed in calculated positions and included in the last cycles of refinement. Crystal data and details of the data collection and refinement are listed in Table 1.

5. Catalytic Epoxidation Process
A mixture of cyclooctene (2.76 mL, 20 mmol), acetophenone (internal reference) and the complex as the catalyst (0.05 mmol) was stirred and heated up to 80 ºC before addition of aqueous tert-butyl hydroperoxide (TBHP; 70% w/w, 5.48 mL, 40 mmol). The mixture is initially an emulsion, but two phases become clearly visible as the reaction progresses, a colorless aqueous one and a yellowish organic one. The reaction was monitored for 5 h with withdrawal and analysis of organic phase aliquots (0.1 mL) at required times. Each withdrawn sample was mixed with 2 mL of diethylether, treated with a small quantity of MnO 2 and then filtered through silica and analyzed by GC.

1. Synthesis
The hydrazone compound H 2 L and the complex were synthesized in a facile and analogous way (Scheme 2).
Electronic spectrum of the complex recorded in methanol displays strong absorption band centered at 400 nm, which is assigned as charge transfer transitions of N(pπ)-Mo(dπ) LMCT. The medium absorption band centered at 323 nm for the complex is as-Scheme 2. The synthesis of the hydrazone H 2 L and the complex The hydrazone H 2 L acts as a tridentate dianionic ONO donor ligands toward the VO 2+ core. The vanadium complex was obtained from a refluxing mixture of the hydrazone and VO(acac) 2 in 1:1 molar proportion in methanol. The complex was isolated as brown single crystals from the reaction mixture by slow evaporation at room temperature. Crystals of the complex are stable at room temperature and are found to be fairly soluble in most of the common organic solvents such as methanol, ethanol, acetonitrile, DMF and DMSO. The low molar solution conductance of the complex in methanol indicates its non-electrolyte behavior.

2. IR and Electronic Spectra
The IR spectrum of the hydrazone H 2 L shows bands centered at 3220 cm -1 for ν (N-H), 3445 cm -1 for ν(O-H), and 1643 cm -1 for ν(C=O). 6 The peaks attributed to ν(N-H) and ν(C=O) are absent in the spectrum of the complex as the ligand binds in dianionic form resulting in losing proton from carbohydrazide group. Strong band observed at 1607 cm -1 for the complex is attributed to ν(C=N), which is located at lower frequencies as compared to the free hydrazone ligand, viz. 1612 cm -1 . 7 The complex exhibits characteristic band at 953 cm -1 for the stretching of V=O bond. 8 Based on the IR absorption, it is obvious that the hydrazone ligand exists in the uncoordinated form in keto-amino tautomer form and in the complex in imino-enol tautomeric form. This is not uncommon in the coordination of the hydrazone compounds. 9 signed as charge transfer transitions of O(pπ)-Mo(dπ) LMCT. 10

Description of the Structure of H 2 L
The perspective view of H 2 L together with the atom numbering scheme is shown in Figure 1. Selected bond lengths and angles are given in Table 2. The molecule adopts an E configuration with respect to the methylidene unit. The methylidene bond, with the distance of 1.264(4) Å, confirms it a typical double bond. The shorter distance of the C8-N2 bond (1.343(4) Å) and the longer distance of the C8-O2 bond (1.225(4) Å) for the amide group than usual, suggests the presence of conjugation effects in the hydrazone molecule. The presence of intramolecular O1-H1•••N1 hydrogen bond, as well as the conjugation effects, result in the two benzene rings form a dihedral angle of 6.8 (5)

4. Description of the Structure of the Complex
The perspective view of the complex together with the atom numbering scheme is shown in Figure 3. The coordination geometry around the V atom reveals a distorted octahedral environment with NO 5 chromophore. The hydrazone ligand behaves as a dianionic tridentate ligand binding through the phenolate oxygen, the enolate oxygen and the imine nitrogen, and occupies three positions in the equatorial plane. The fourth position of the equatorial plane is occupied by the deprotonated methanol ligand. The neutral methanol ligand occupies one axial position of the octahedral coordination, and the other axial position is defined by the oxido group. The vanadium atom is found to be deviated from the mean equatorial planes defined by the four donor atoms by 0.310(2) Å. The V1-O5 bond length is longer than the normal single bond lengths (2.358(5) Å against 1.9-2.0 Å). This shows that the neutral methanol ligand is loosely attached to the V center. This is due to the trans effect generated by the oxido group. The remaining V-O bond lengths of 1.58-1.95 Å and the V-N bond length of 2.12 Å are similar to the corresponding bond values observed in other vanadium(V) complexes. 11 The C8-O2 bond length in the complex is 1.311(7) Å, which is 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. 12 Similarly shortening of C8-N2 bond length (1.308(7) Å instead of normal 1.38 Å) together with the elongation of N1-N2 bond length (1.398(6) Å) also supports the electron cloud delocalization in the ligand system. The hydrazone ligand forms a five-membered and a six-membered chelate rings with the V center. The five-membered metallacycle ring is thus rather planar, but the six-membered metallacycle ring is clearly distorted. The two benzene rings form a dihedral angle of 4.3(5)°. The trans angles are in the range 154.2(2)-173.7(2)°, indicating considerable    (7) Symmetry codes: (i) -½ + x, ½ -y, -½ + z; (ii) -x, 1 -y, 1 -z.  distortion of the coordination octahedron around the V center.

5. Catalytic Epoxidation Results
Before addition of aqueous TBHP at 80 °C, the complex dissolved completely in the organic phase. The aqueous phase of the solution was colorless and the organic phase was brown, indicating that the catalyst is mainly confined in the organic phase. TBHP is mainly transferred into the organic phase under those conditions, and for that reason the reactant and products in the organic layer were analyzed.
Cyclooctene and cyclooctene oxide are not significantly soluble in water, therefore the determination of the epoxide selectivity (epoxide formation/cyclooctene conversion) is expected to be accurate. For the cyclooctene epoxidation by using aqueous TBHP, with no extra addition of organic solvents, the present study shows effective activity. Kinetic profiles of the complex as catalyst are presented in Figure 5. No induction time was observed. The cyclooctene conversion for the complex is 93% after 5 h, and the selectivity towards cyclooctene oxide is 67%. Possible mechanistic consideration involves coordination of TBHP as a neutral molecule, with the hydrogen bond O-H···O (Scheme 3).

Conclusion
In summary, a new hydrazone compound 4-bromo-N'-(2-hydroxybenzylidene)benzohydrazide was prepared and structurally characterized. With the hydrazone Scheme 3. Proposed mechanism for the catalytic process of the complex.