KOOPMHH^HOHHÂS XHMH3, 2014, moM 40, № 5, c. 269-273
yM 541.49
SYNTHESIS, STRUCTURE, AND CATALYTIC PROPERTY OF A MONONUCLEAR DIOXOMOLYBDENUM(VI) COMPLEX CONTAINING MoO5N CORE © 2014 F. M. Wang
Key Laboratory of Coordination Chemistry and Functional Materials in Universities of Shandong, Department of Chemistry,
Dezhou University, Dezhou, Shandong, 253023 P.R. China E-mail: wfm99999@126.com Received August 12, 2013
Reaction of molybdenyl(IV) oxide bis(2,4-pentanedionate) with N'-(3-ethoxy-2-hydroxybenzylidene)-2-methoxybenzohydrazide in methanol affords a mononuclear dioxomolybdenum(VI) complex containing MoO5N basic core. The complex has been characterized by various physicochemical techniques (IR and elemental analysis), and single crystal X-ray diffraction. X-ray crystal structure determination reveals that the complex crystallizes as monoclinic space group P21/c, with unit cell dimensions a = 9.251(1), b = 11.910(2), c = 17.636(3) Â, P = 103.220(2)°, V = 1891.7(5) Â3, Z = 4, R1 = 0.0693, wR2 = 0.1691. The Mo atom in the complex is octahedrally coordinated, with the tridentate ONO ligand occupying the meridional sites. Thermal stability analysis was performed. The complex shows high catalytic property for the oxidation of various olefins.
DOI: 10.7868/S0132344X14040100
INTRODUCTION
Molybdenum is an essential transition metal that of great importance in life sciences. Enzymes containing molybdenum at their active sites catalyze a wide range of reactions in carbon, sulfur, and nitrogen metabolism [1—3]. In particular the molybdenum mediated oxo transfer in enzymatic systems has attracted much attention in reactivity and coordination chemistry of cw-dioxomolybdenum(VI) complexes [4—6]. In recent years, a number of molybdenum complexes have been used as catalysts for the epoxidation and hydrox-ylation of olefines [7—9], oxidation of alcohols [10], and oxo-transfer reactions [11]. Hydrazone ligands are widely used to prepare complexes with various metal salts. Molybdenum complexes with such ligands are also reported, but the number is still less when compared to other metals. In the present work, we report the synthesis, structure, and catalytic property of a new dioxo-molybdenum(VI) complex, [MoO2L(OH2)], derived from a new hydrazone ligand N'-(3-ethoxy-2-hydro-xybenzylidene)-2-methoxybenzohydrazide (H2L).
EXPERIMENTAL
Materials and physical measurements. Reagent grade solvents were used as received. All other chemicals were of AR grade, obtained from commercial sources and used as received. The ligand H2L was prepared by using a reported procedure [12]. Elemental analyses (C, H, and N) were done with a PerkinElmer
2400 analyzer. IR spectra (as KBr pellet) were recorded using a PerkinElmer RXI FT-IR spectrophotometer. Solution electrical conductivity was measured with a DDS-11A conductivity meter. Thermal analysis of the complex was carried out by heating at a rate of 10°C per minute on a PerkinElmer TGA-4000 thermo balance. The oxidation products were analyzed with a gas chromatograph (Shimadzu, GC-14B) equipped with a SAB-5 capillary column (phenyl methyl silox-ane 30 x 320 x 0.25 mm) and a flame ionization detector.
Synthesis of the complex. The ligand H2L (0.31 g, 1.0 mmol) was dissolved in 30 mL methanol and [MoO2(Acac)2] (0.33 g, 1.0 mmol) was added to the solution and the mixture was refluxed for 1 h at room temperature. The color of the solution changed from colorless to yellow. The solution was filtered and allowed to evaporate slowly at room temperature. After a week, yellow single crystals, suitable for X-ray diffraction analysis, were obtained by filtration. The yield was 55%.
For C17H18N2O7Mo
anal. calcd., %: C, 44.55; H, 3.96; N, 6.11. Found, %: C, 44.32; H, 4.10; N, 6.27.
IR (KBr; v, cm-1): 3443 and 3341 v(O-H), 1609 v(C7=N1), 1670 v(C-N), 1262r v(C-O), 941 and 893 v(MoO2), 500-800 v(Mo-O/N).
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Table 1. Crystal data and structure refinement for the complex
Parameter Value
Formula weight 458.3
Crystal system Monoclinic
Space group P2i/c
Unit cell dimensions:
a, A 9.215(1)
b, A 11.910(2)
c, A 17.636(3)
ß, deg 103.220(2)
Volume, A3 1891.7(5)
Z 4
Pcalcd g cm-3 1.609
Absorption coefficient, mm-1 0.734
/(000) 928
Crystal size, mm 0.18 x 0.18 x 0.16
Limiting indices -10 < h < 9
-9 < k < 14
-17 < l < 20
Reflections collected/unique 4433/2332
Observed reflections (I > 2ct(I)) 1975
Parameters 252
Restraints 3
Goodness-of-fit on F 2 1.307
Final R indices (I> 2a(I)) R1 = 0.0693, Rw2 = 0.1691
R indices (all data) R1 = 0.0796, Rw2 = 0.1735
Largest difference in peak and 1.031 and -0.517
hole, e A-3
X-ray crystallography. A suitable single crystal of the complex was mounted on a thin glass fiber without protection. Cell dimensions were determined at 298(2) K from the setting angles of a Bruker SMART 1K CCD diffractometer using a graphite monochro-mated Mo^a (k = 0.71073 A) radiation source. Data collection was completed using the ®/29 scan techniques. Structure of the complex was solved by direct method, developed by successive difference Fourier synthesis and refined on F2 by a full-matrix least-squares procedure using SHELXL-97 [13]. The positions of all non-hydrogen atoms were refined with anisotropic displacement factors. The water hydrogen atoms were located from a difference map, and with O-H and H-H distances of 0.85(1) and 1.37(2) A, respectively. The remaining hydrogen atoms were geometrically calculated and isotropically fixed at positions recalculated after each cycle of refinement (C-H 0.93-0.97 A with the isotropic thermal parameters of ^iso(H) = 1.2 ^iSo(C) and 1.5 Uiso(Cmethyl)). Absorption corrections based on multi-scan using SADABS were
applied [14]. Data reduction was accomplished using the SAINT plus [15].
Pertinent cell parameters, data collection conditions, and refinement details are provided in Table 1. The important bond lengths and angles are provided in Table 2. Supplementary material for the complex has been deposited with the Cambridge Crystallographic Data Centre (no. 9506932; deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Catalytic oxidation procedure. Catalytic experiment was carried out in a 50 mL glass reaction flask fitted with a water condenser. 0.032 mmol dioxo-molyb-denum(VI) complexes were dissolved in 10 mL 1,2-dichloroethane. Then 10 mmol alkene was added to the reaction mixture and 30 mmol tert-butyl hydroperoxide (TBHP) was added. The reaction mixture was refluxed for 1 h. The reaction products were monitored at periodic time intervals using gas chromatography. The oxidation products were identified by comparison with authentic samples.
RESULTS AND DISCUSSION
The synthesis of the hydrazone ligand requires a two step reaction. In the first step the 2-methoxyben-zohydrazide was synthesized by reaction of the methyl 2-methoxybenzoate with 1.3 equivalent of hydrazine hydrate under reflux and continuous stirring for 6 h. Subsequently the hydrazone ligand was obtained via condensation reaction of 2-methoxybenzohydrazide with 3-ethoxysalicylaldehyde in methanol solution at room temperature. Stoichiometric reaction of the hydrazone ligand with MoO2(Acac)2 as molybdenum source in refluxing methanol yields the corresponding cis-dioxomolybdenum(VI) complex. The reaction progress is accompanied by an immediate color change of the solution from colorless to yellow. The complex is soluble in methanol, ethanol, acetonitrile, DMF, and DMSO. The molar conductance of a ~0.10 mmol dm-3 solution of the complex in acetonitrile at 25°C is 21.5 fi-1 cm2 mol-1. The data indicates non-electrolytic behavior in the solution [16].
An ORTEP diagram of the complex is illustrated in Fig. 1. In the complex a distorted octahedral geometry is observed for the molybdenum atom, with an [ONO] donor set from the hydrazone ligand, a complementing water O donor, and the two oxo groups of the MoO2 moiety. For the latter Mo-O bond lengths of 1.700(7) and 1.726(7) Á and OMoO bond angle of 105.7(4)° are found, which are in excellent agreement with the corresponding values observed in similar di-oxomolybdenum complexes [17, 18]. The equatorial plane at the molybdenum atom is given by the triden-tate hydrazone ligand (O(1), N(1), and O(2)) and one of the oxo groups (O(7)). The ligand coordinates in a meridional fashion forming five- and six-membered chelate rings at the MoO2 moiety with bite angles of 71.6(3)° and 82.1(3)°, respectively. The hydrazone
SYNTHESIS, STRUCTURE, AND CATALYTIC PROPERTY
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ligand is coordinated in its dianionic form. This is evident from the N(2)-C(8) and O(2)-C(8) bond lengths with values of 1.315(l3) and 1.324(12) Á, respectively, which are indicative for the presence of the enolate form of the ligand amide group. The two axial positions are occupied by the second oxo group O(6) of the MoO2 unit and an oxygen atom of a water molecule (O(5)). The rather long Mo(1)-O(5) bond and consequently weak bonding is due to the trans effect of the oxo group O(6). This is accompanied by a significant displacement of the Mo atom from the equatorial mean plane towards the axial oxygen atom O(6) by 0.324(1) Á. The coordinate bond lengths are comparable with those observed in similar dioxomolybde-num(VI) complexes [17—19]. The crystal packing of the complex is depicted in Fig. 2. The complex molecules are linked by water molecules through intermolecular O—H—O hydrogen bonds (Table 3), to form chains running down the x axis.
The ligand showed stretching bands attributed to C=O, C=N, C-OH, and NH at 1658, 1636, 1155 and 1227, and 3317 cm-1, respectively. In addition, a strong band found at 1621 cm-1 is attributed to C=N-N=C group. This IR evidence has been registered earlier for the similar class of ligands that behave as tridentate dibasic ligands upon enolization [20]. The dioxomolybdenum complex showed two prominent bands at 941 and 893 cm-1 attributed to c/s-dioxo-molybdenum (MoO2) group. The bands due to v(C=O) and v(NH) were
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