Synthesis , Crystal Structure , and Catalytic Property of Dioxomolybdenum ( VI ) Complex Derived from 4-Bromo-N ’-( 4-diethylamino-2-hydroxybenzylidene ) benzohydrazide

A novel dioxomolybdenum(VI) complex, [MoO2L(MeOH)], where L is the anion of 4-bromo-N’-(4-diethylamino-2hydroxybenzylidene)benzohydrazide, has been synthesized and characterized by physico-chemical methods and single crystal X-ray determination. In the complex the Mo atom is in octahedral coordination, with three donor atoms of the hydrazone ligand, two oxo groups, and one methanol O atom. The complex exhibits catalytic property for the sulfoxidation reactions. In the presence of the complex a 93% conversion of the sulfide to the corresponding sulfoxide was observed within 60 min and the 100% conversion was achieved after 2 h.


Introduction
Molybdenum complexes with multi-dentate ligands have received remarkable attention in recent years for their catalytic properties 1 and molecular structures. 2 Catalytic oxo transfer properties especially epoxidation and sulfoxidation by dioxomolybdenum complexes are industrially important.Many types of ligands with various functional groups have been synthesized to prepare molybdenum complexes with the aim to explore new and efficient catalytic materials. 3Salicylaldehyde and its derivatives have been widely used as ligands for the preparation of metal complexes with various applications. 4A large number of molybdenum complexes with Schiff base ligands have been reported. 5Some of the dioxomolybdenum complexes have shown oxygen atom transfer properties as they were found to oxidize thiols, hydrazine, polyketones, and tertiary phosphines. 6Recently, we reported the catalytic property of a dioxomolybdenum(VI) complex. 7As a continuation of such work, we report in this paper the synthesis, structure, and catalytic property of a new dioxomolybdenum(VI) complex, [MoO 2 L(MeOH)], where L is the anion of 4-bromo-N'-(4-dimethylamino-2-hydroxybenzylidene)benzohydrazide (Scheme 1).

1. Materials and Methods
4-Diethylaminosalicylaldehyde, 4-bromobenzohydrazide and MoO 2 (acac) 2 were purchased from Fluka.Other reagents and solvents were analytical grade and used without further purification.The hydrazone was prepared according to the literature method. 8Elemental (C, H, and N) analyses were made on a Perkin-Elmer Model 240B automatic analyzer.IR spectra were recorded on an IR-408 Shimadzu 568 spectrophotometer. 1 H NMR data was recorded on a Bruker 300 MHz spectrometer.

3. X-ray Diffraction
Crystal data of the complex were collected with a Bruker Apex II diffractometer using Mo K α radiation (0.71073 Å).The data were processed with SAINT 9 and corrected for absorption using SADABS. 10Multi-scan absorption corrections were applied with ψ scans. 11Structure of the complex was solved by direct method using SHELXS-97 and refined by full-matrix least-squares techniques on F 2 using anisotropic displacement parameters. 12All of the non-hydrogen atoms of the complex were refined anisotropically.The H atom of the methanol ligand were located from electronic density maps and refined isotropically.The other hydrogen atoms were located as riding model.Crystallographic data for the complex are summarized in Table 1.Selected bond lengths and angles are listed in Table 2.

4. Catalytic Oxidation
The dioxomolybdenum(VI) complex (0.001 M) and phenyl methyl sulfide (0.100 M) were dissolved in the mixture of CH 2 Cl 2 and CH 3 OH (6:4) with 1,3,5-trimethoxybenzene (0.100 M) as the internal standard.The solution was cooled to 283(2) K, to which was added dropwise H 2 O 2 (35% w/w, 0.125 M).An aliquot of the solution (2.0 mL) was quenched with Na 2 SO 3 (0.100 M, 5 mL), then extracted for three times with CH 2 Cl 2 (4 mL).The solvent of the organic phase was removed, and the residue was dissolved in CDCl 3 (0.6 mL) and analyzed by 1 H NMR to determine the yield.

Results and Discussion
The complex was prepared by the reaction of the hydrazone ligand with MoO 2 (acac) 2 in a 1:1 molar ratio (Scheme 2).The crystals of the complex are soluble in  DMSO, DMF, MeOH, EtOH and MeCN.Well-shaped single crystals of the complex can be obtained from MeOH.

1. Crystal Structure Description of the Dioxomolybdenum Complex
The molecular structure of the dioxomolybdenum complex is shown in Figure 1.In the complex the Mo atom is coordinated by the phenolic oxygen, imino nitrogen, enolic oxygen of the hydrazone ligand, two oxo oxygen and one methanol oxygen, forming an octahedral coordination.The hydrazone ligand coordinates to the MoO 2 moiety by forming a five-and a six-membered chelate rings.The methanol molecule is weakly coordinated to the Mo atom, as indicated by the long bond value.The equatorial plane defined by atoms O1, O2, N1 and O5 show a high degree of planarity, with mean deviation from the plane of 0.038(3) Å.The Mo1 atom deviates from the least-squares equatorial plane by 0.346(1) Å in the direction of the axial atom O3.The coordinate bond lengths are comparable to those observed in similar complexes. 13The distortion of the octahedral coordination is indicated by the bond angles, ranging from 71.29 (8) to 105.92( 12)° for the cis angles and from 149.61 (9) to 171.34 (10)° for the trans angles.The dihedral angle between the two benzene rings of the hydrazone ligand is 0.3(5)°.
In the crystal structure of the complex, the complex molecules are linked through intermolecular hydrogen bonds of types O4-H4 4)°; symmetry code for i: -x, 1 -y, -z], to form dimers (Figure 2).

2. Infrared and Electronic Spectra
In the IR spectrum of the hydrazone compound (Figure S1) the stretching bands attributed to C=O, C=N, C-OH and NH at 1634, 1603, 1130 and 3183 cm -1 are present, respectively.In the IR spectrum of the dioxomolybdenum complex (Figure S2) two prominent bands at 948 and 850 cm -1 are present, which can be attributed to the MoO 2 group. 14The bands due to ν C=O and ν NH are absent in the dioxomolybdenum complex.This suggests occurrence of keto-imino tautomerization of the hydrazone ligand during coordination to the Mo atom.The typical C=N absorption of the dioxomolybdenum complex is observed at 1595 cm -1 . 15The weak bands in low wave numbers can be attributed to the Mo-O and Mo-N vibrations.
The electronic spectrum of the dioxomolybdenum complex recorded in MeCN display strong and medium absorption bands in the region 370-420 and 260-300 nm (Figure S3).These peaks are assigned as charge transfer transitions of the type N(pπ)-Mo(dπ) LMCT and O(pπ)-Mo(dπ) LMCT, 16 as the ligand based orbitals are either N or O donor types.The slight change of λ max values within each set of peaks may be due to the difference of electron donating capacity of the hydrazone ligand.

Catalytic Sulfoxidation
The catalytic oxidation test of the dioxomolybdenum complex as catalyst on the oxidation of sulfide under homogeneous conditions using methyl phenyl sulfide as the substrate is shown as Scheme 3. Hydrogen peroxide was used as oxidant in 1.25 equivalent based on the sulfide substrate.Reactions were run with 1 mol% of catalyst based on the substrate at 10 °C.NMR technique was used to monitor the formation of the sulfoxides with 1,3,5-trimethoxybenzene (TMB) as internal standard.The reaction was started by the addition of H 2 O 2 .A control reaction under the same condition without the complex present leads to less than 1% sulfide conversion within 4 h.In the presence of the dioxomolybdenum complex a 93% conversion of the sulfide to the corresponding sulfoxide was observed within 60 min and the 100% conversion was achieved after 2 h.http://www.ccdc.cam.ac.uk/const/retrieving.html or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033 or e-mail: deposit@ccdc.cam.ac.uk.

Figure 1 .
Figure 1.Molecular structure of the complex at 30% probability displacement.

Figure 2 .
Figure 2. The hydrogen bonds linked dimeric structure of the complex.

Table 1 .
Crystal and structure refinement data for the complex

Table 2 .
Selected bond lengths (Å) and angles (°) for the complex Scheme 2. The preparation of the complex.