Synthesis, Crystal Structures and Catalytic Property of Dioxidomolybdenum(VI) Complexes with Tridentate Hydrazones

New dioxidomolybdenum(VI) complexes with the formula [MoO2L(MeOH)], derived from N’-(5-chloro-2-hydroxybenzylidene)-2-methylbenzohydrazide (H2L) and N’-(3,5-dichloro-2-hydroxybenzylidene)-2-methylbenzohydrazide (H2L) were prepared. Crystal and molecular structures of the complexes were determined by single crystal X-ray diffraction method. Both complexes were further characterized by elemental analysis and FT-IR spectra. Single crystal X-ray structural studies indicate that the hydrazones L1 and L2 coordinate to the MoO2 cores through the enolate oxygen, phenolate oxygen and azomethine nitrogen. The Mo atoms in both complexes are in octahedral coordination. Catalytic properties for epoxidation of styrene by the complexes using PhIO and NaOCl as oxidant have been studied.


Introduction
The oxidation of organic compounds is an important chemical process in the chemical industry to synthesize a large variety of organic materials. Due to the slow rate of most oxidation reactions in the absence of catalysts, the catalytic oxidation of organic substrates by metal complexes has received much attention in organic synthesis. Many transition metals have been used as homogenous or heterogeneous catalysts in various oxidation systems. 1 The epoxidation products of olefins have widely applications in various fields. They are intermediates or precursors for the synthesis of pharmaceuticals, agrochemicals, and many other compounds. 2 Among various metal complexes, those with Mo centers have been attracted considerable attention due to their recently discovered biochemical significance 3 as well as their efficient catalytic properties in several organic synthesis procedures. 4 Schiff bases are widely used as ligands in the construction of metal complexes. 5 In recent years, a large number of molybdenum complexes with Schiff bases derived from salicylaldehyde and primary amines have been reported. 6 Hydrazones, bearing -C(O)-NH-N=CH-groups, are a kind of special Schiff bases, which are of particular interest in coordination chemistry and biological applications. 7 However, due to the search of the Crystallographic Structural Database, the number of molybdenum(VI) complexes with hydrazone ligands are much less than other metal complexes with such type of ligands. In this paper, two new dioxidomolybdenum(VI) complexes with the formula   N'-(5-chloro-2-hydroxybenzylidene)-2-methylbenzohydrazide (H 2 L 1 ) and N'-(3,5-dichloro-2-hydroxybenzylidene)-2-methylbenzohydrazide (H 2 L 2 ) are reported.

1. Materials and Measurements
5-Chlorosalicylaldehyde, 3,5-dichlorosalicylaldehyde and 2-methylbenzohydrazide were purchased from Aldrich and used without further purification. [MoO 2 (acac) 2 ] and other solvents and reagents were made in China and used as received. C, H and N elemental analyses were performed with a Perkin-Elmer elemental analyser. GC experiments were performed with Agilent 5977A Network GC systems. Infrared spectra were recorded on a Nicolet AVATAR 360 spectrometer as KBr pellets in the 4000-400 cm -1 region.

4. Synthesis of [MoO 2 L 1 (MeOH)] (1)
A methanolic solution (10 mL) of [MoO 2 (acac) 2 ] (0.1 mmol, 32.6 mg) was added to a methanolic solution (10 mL) of H 2 L 1 (0.1 mmol, 28.9 mg) with stirring. The mixture was stirred for 20 min to give a yellow solution. The resulting solution was allowed to stand in air for a few days. Yellow block-shaped crystals suitable for X-ray single crystal analysis were formed at the bottom of the vessel. The isolated product was washed three times with cold methanol, and dried in a vacuum over anhydrous

7. General Method for Styrene Oxidation
The oxidation reactions were carried out according to the literature method. 11 The composition of the reaction mixture was 2.00 mmol of styrene, 2.00 mmol of chlorobenzene (internal standard), 0.10 mmol of the complex (catalyst) and 2.00 mmol iodosylbenzene (PhIO) or sodium hypochlorite (NaClO) as the oxidant in 5.00 mL freshly distilled acetonitrile. When the oxidant was sodium hypochlorite, the solution was buffered to pH 11.2. The composition of reaction medium was determined by GC with styrene and styrene epoxide quantified by the internal standard method (chlorobenzene).

1. Chemistry
The complexes were prepared by mixing [MoO 2 (acac) 2 ] with the hydrazones H 2 L 1 and H 2 L 2 in methanol. Single crystal structures were obtained by slow evaporation of the complexes in methanol. The difference of the molecular packing modes of the complexes may be caused by the hindrance effects of the chloro-substituent groups. There is only one chloro-substituent group in complex 1, while two in complex 2. The complexes are soluble in methanol, ethanol, and acetonitrile. The molar conductance of the complexes 1 and 2 at the concentrations of 10 -4 M are 25 and 20 Ω -1 cm 2 mol -1 , respectively, indicating they are non-electrolytes. 12

2. Structure Description of the Complexes
The molecular structures and atom numbering schemes of complexes 1 and 2 are shown in Figures 1 and  2, respectively. The coordination geometry around the Mo atoms in both complexes is highly distorted octahedral. The dianionic hydrazones L 1 and L 2 adopt planar tridentate manner, forming five-and one six-membered chelate rings involving the MoO 2 cores. The hydrazones L 1 and L 2 in the complexes are bonded to the MoO 2 cores in planar fashion, coordinating through the phenolate O, imino N, and enolate O, and an oxo group lying trans to the nitrogen donor. In each complex, a methanol molecule completes the distorted octahedral coordination sphere which lying trans to the other oxo group. The Mo-O(methanol) bonds are significantly longer than the other Mo-O bonds, indicating that the methanol molecules are weakly bonded to the MoO 2 cores and this position holds the possibility of functioning as a substrate binding site. The atoms O1, O2, O4, and N1 show high degree of planarity from the equatorial plane, the Mo atoms displaced by 0.332(1) Å for complex 1 and 0.320(1) Å for complex 2 toward the axial oxo groups. The Mo=O bonds in the complexes are almost equal within the standard deviations, and are within previously reported ranges, 4f,13 The angular distortion in the

6. Data Collection, Structural Determination and Refinement
Diffraction intensities for the complexes were collected at 298(2) K using a Bruker Smart 1000 CCD area diffractometer with MoKa radiation (l = 0.71073 Å). The collected data were reduced using SAINT, 8 and multi-scan absorption corrections were performed using SADABS. 9 Structures of the complexes were solved by direct methods and refined against F 2 by full-matrix least-squares methods using SHELXTL. 10 All of the non-hydrogen atoms were refined anisotropically. The methanol H atoms in the complexes were located in difference Fourier maps and refined isotropically, with O-H distances restrained to 0.85(1) Å. All other H atoms were placed in idealized positions and constrained to ride on their parent atoms. The crystallographic data for the complexes are summarized in Table 1. Selected bond lengths and angles are given in Table 2.

Infrared and Electronic Spectra
The hydrazones showed stretching bands attributed to C=O, C=N, C-OH and NH at about 1654, 1629, 1150and 1225 . In addition, strong bands observed at 1612 cm -1 for H 2 L 1 and H 2 L 2 are attributed to CH=N groups. 15 Both complexes exhibit intense bands at ca. 920 cm -1 , assigned to the vibrations of the MoO 2 cores. 16 The bands due to ν C=O and ν NH are absent in the complexes, but new C-O stretches appeared at 1261 cm -1 for both complexes. Keto-imine tautomerism is present in molecules H 2 L 1 and H 2 L 2 . Upon coordination to Mo atom, enol-imine tautomerism is present in ligands L 1 and L 2 . The ν C=N absorption observed at 1629 cm -1 in the free hydrazones shifted to 1603 cm -1 for the complexes upon coordination to Mo atoms. 16 The weak peaks in the low wave numbers in the region 450-800 cm -1 may be attributed to  In the electronic spectra of the two complexes, the bands ranging from 250 to 340 nm are assigned to n → π* transitions, and those at 210-230 nm assigned to π → π* transitions. 18 The bands with the maximum absorption at 400-410 nm are due to the ligand to metal charge transfer transition. 18

4. Catalytic Property
Oxidation of styrene was carried out at room temperature with the complexes as the catalysts and PhIO and NaOCl as oxidants. The orange color of the solutions containing the complexes and the substrate was intensified after the addition of oxidant indicating the formation of oxo-metallic intermediates of the catalysts. After completion of oxidation reaction of the alkene, the solution regains its initial color which suggests that the regeneration of the catalysts takes place.
The two complexes as catalysts convert styrene most efficiently in the presence of PhIO or NaOCl. There is no obvious difference for the catalytic properties between the two complexes, as a result of similar structures. The complexes are selective towards the formation of styrene epoxide. When the reactions were carried out with PhIO, styrene conversions are 83% and 85% for 1 and 2, respectively. When the reactions were carried out with NaOCl, styrene conversions are 71% and 74% for 1 and 2, respectively. It is evident that between the oxidants PhIO and NaOCl, the former acts as a better oxidant with respect to the styrene conversion. The two complexes have similar catalytic properties on the oxidation of styrene when compared to the manganese(III) complexes with the ligand N,N'-o -phenylenebis(3-ethoxysalicylaldimine). 11 Reaction of the oxidants with the complexes would likely generate a Mo-Cl or Mo-I entity, which further exchange Cl or I for ClO or IO, and homolytic cleavage of Mo-OCl or Mo-OI bond, generate effective epoxidising agent ClO· or IO·. Then, ClO· or IO· reacts with styrene to give the styrene epoxide.

Conclusion
New dioxidomolybdenum(VI) complexes with similar hydrazones have been prepared and structurally characterized by single crystal X-ray diffraction method, as well as elemental analysis and FT-IR spectra. The hydrazones coordinate to the MoO 2 cores through the enolate oxygen, phenolate oxygen and azomethine nitrogen. Methanol is a suitable solvent for the preparation of such complexes, which readily coordinates to the Mo atom as a co-ligand. Different substituent groups in the benzene rings of the ligands can result in different molecular packing modes of the final molybdenum(VI) complexes. Both complexes have similar and effective catalytic oxidation property on styrene.