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DOI: 10.17344/acsi.2015.2169
Scientific paper
Synthesis, Crystal Structure and Catalytic Activity of Dioxidomolybdenum(VI) Complex with Tridentate ONO Aroylhydrazone Ligand
Min Liang1* and Dong-Hui Zou2
1 School of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006, P. R. China. 2 College of Food and Bio-Engineering, Qiqihar University, Qiqihar 161006, P. R. China. * Corresponding author: E-mail: liangmin09@163.com Received: 09-12-2015
Abstract
A novel dioxidomolybdenum complex, [MoO2L], derived from the tridentate aroylhydrazone 2-amino-W-(2-hydroxy-3-methylbenzylidene)benzohydrazide (H2L), has been prepared and characterized by elemental analysis, FT-IR, 1H NMR, and single crystal structural X-ray diffraction. The complex has distorted square pyramidal structure in which the aroylhydrazone ligand behaves as a binegative donor, leaving the amino-N group uncoordinated towards the metal center. Single crystal structure analysis reveals that the Mo(VI) center is coordinated by the donor atoms of the binegative aroylhydrazone ligand and two oxido groups. Crystal structure of the complex is stabilized by hydrogen bonds. The complex functions as an effective olefin epoxidation catalyst with hydrogen peroxide as terminal oxidant and sodium hydrogen carbonate as a co-catalyst.
Keywords: Aroylhydrazone; Dioxidomolybdenum complex; Catalytic activity; Crystal structure
1. Introduction
In the last few years, considerable attention has focused on the coordination chemistry of molybdenum because of its biochemical significance1 as well as involvement of Mo(VI) compounds in some industrial catalytic processes.2 Molybdenum complexes are widely used as efficient catalysts in different processes such as the oxidation of olefins and sulfides.3 Aroylhydrazones are potential tridentate ligands.4 During search of literature, we found that most molybdenum complexes with aroylhydra-zones have octahedral coordination.5 The only example of square pyramidal coordination of molybdenum complexes with aroylhydrazone ligand was reported by Ghosh and coworkers.6 In the present work, a novel molybde-
Scheme 1. HL
num(VI) complex with square pyramidal coordination, [MoO2L], derived from the tridentate aroylhydrazone 2-amino-N'-(2-hydroxy-3-methylbenzylidene)benzohy-drazide (H2L; Scheme 1), was studied.
2. Experimental
2. 1. Materials and Methods
All chemicals and solvents used were of analytical reagent grade and used as received. MoO2(acac)2 was prepared as described in the literature.7 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. 1H 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.
2. 2. Synthesis of H2L
A methanolic solution (10 mL) containing 1.00 mmol of 2-hydroxy-3-methylbenzaldehyde (0.136 g) was
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added dropwise to a methanolic solution of 2-aminoben-zohydrazide (1.00 mmol, 0.151 g) with constant stirring. The mixture was refluxed for 30 min, and the resulting precipitate was filtered off, washed with cold methanol and dried in desiccator over silica gel. Yield: 0.23 g, 85%. For C15H15N3O2: anal. calcd., %: C, 66.90; H, 5.61; N, 15.60. Found, %: C, 66.73; H, 5.72; N, 15.75. FT-IR (KBr), cm1: v(NH) 3232, v(OH) 3431, v(CHar) 2830-3085, v(C=O) 1650, v(C=N) 1615, v(N-N) 1149, v(C-O) 1287. 1H NMR (300 MHz, DMSO-d6, ppm): 8 = 12.10 (s, 1H; NH), 11.05 (s, 2H, NH2), 10.76 (s, 1H; OH), 8.70 (s, 1H; CH=N), 6.80-7.15 (m, 5H, ArH), 7.43 (d, 1H, ArH), 7.69 (d, 1H, ArH), 2.13 (s, 3H, CH3).
2. 3. Synthesis of [MoO2L]
A methanolic solution (10 mL) of MoO2(acac)2 (0.10 mmol, 0.033 g) was added to a solution of H2L (0.10 mmol, 0.027 g) in methanol (10 mL) 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 crystallography separated after a week and were dried in a vacuum desiccator over silica gel. Yield: 0.021 g, 63%. For C15H13MoN3O4: anal. calcd., %: C, 45.58; H, 3.32; N, 10.63. Found, %: C, 45.40; H, 3.41; N, 10.54. IR (KBr) cm-1: v(OH) 3466, v(NH2) 3340, v(NH) 3233, v(CH) 2810-2990, v(C=N) 1611, v(C=Cring) 1434, v(N-N) 1157, v(C-O) 1257, vsym(ris-MoO2) 949, vasym(cis-MoO2) 861. 1H NMR (400 MHz, DMSO-d6, ppm): 8 = 11.13 (s, 2H, NH2), 8.77 (s, 1H; CH=N), 6.70-7.80 (m, 7H; ArH), 2.15 (s, 3H, CH3).
2. 4. X-Ray Structure Determination
The crystal of the complex was measured on a Bru-ker SMART Apex II CCD diffractometer using Mo Ka ra-
diation (X = 0.71073 A) and a graphite monochromator at 25 °C. Unit cell and reflection data were obtained by standard methods8 and are summarized in Table 1. The structure was solved, refined, and prepared for publication using the SHEXTL package (structure solution refinements and molecular graphics).9 The structure was solved using direct methods and refined on F2 using full matrix least squares techniques with anisotropic displacement factors for all non-hydrogen atoms. The amino and methanol
Table 1. Crystal data and structure refinement for the complex
Parameter	Value
Molecular formula	C15H13MoN3O4
Formula weight	395.22
Temperature (K)	298(2)
Crystal system	triclinic
Space group	p1
a (À)	9.7363(14)
b (À)	11.7491(17)
c (À)	13.908(2)
«(°)	102.355(2)
m	94.650(2)
r(°)	104.522(2)
y (À3)	1489.1(4)
Z	4
Dcalc (g/cm3)	1.763
Absorption coefficient (mm 1)	0.905
F(000)	792
Reflections collected	12376
Independent reflection (Rint)	5427 (0.0357)
Reflections observed (I > 2o(I))	4342
Data/restraints/parameters	5427/6/429
Goodness-of-fit on F2	1.091
Final R indices (I > 2a(I))	0.0469, 0.1137
R indices (all data)	0.0612, 0.1232
Highest peak and deepest hole (e À-3)	1.072, -0.969
Table 2. Selected bond lengths (A) and angles (°) for the complex
0 Bond lengths (A)			
Mo(1)-O(1)	1.912(3)	Mo(1)-O(2)	1.992(3)
Mo(1)-O(3)	1.693(4)	Mo(1)-O(4)	1.706(4)
Mo(1)-N(1)	2.231(4)	Mo(2)-O(5)	1.905(3)
Mo(2)-O(6)	1.993(3)	Mo(2)-O(7)	1.691(4)
Mo(2)-N(4)	2.242(4)	Mo(2)-O(8)	1.700(3)
Bond angles (°)			
O(3)-Mo(1)-O(4)	106.7(2)	O(3)-Mo(1)-O(1)	99.78(18)
O(4)-Mo(1)-O(1)	101.66(17)	O(3)-Mo(1)-O(2)	97.34(17)
O(4)-Mo(1)-O(2)	96.57(16)	O(1)-Mo(1)-O(2)	150.13(15)
O(3)-Mo(1)-N(1)	98.35(18)	O(4)-Mo(1)-N(1)	153.65(17)
O(1)-Mo(1)-N(1)	81.61(15)	O(2)-Mo(1)-N(1)	71.79(14)
O(7)-Mo(2)-O(8)	106.79(18)	O(7)-Mo(2)-O(5)	100.15(17)
O(8)-Mo(2)-O(5)	101.52(16)	O(7)-Mo(2)-O(6)	96.83(16)
O(8)-Mo(2)-O(6)	97.92(16)	O(5)-Mo(2)-O(6)	149.17(14)
O(7)-Mo(2)-N(4)	96.62(16)	O(8)-Mo(2)-N(4)	155.41(16)
O(5)-Mo(2)-N(4)	81.21(14)	O(6)-Mo(2)-N(4)	71.37(13)
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H atoms were located from a difference Fourier map and refined isotropically, with N-H, O-H, and H—H distances restrained to 0.90(1), 0.85(1), and 1.43(2) À, respectively. Positions of the remaining hydrogen atoms were calculated from the geometry of the molecular skeleton and their thermal displacement parameters were refined isotropi-cally on a groupwise basis. Selected bond lengths and angles are reported in Table 2. H-bonding distances and angles are shown in Table 3.
Table 3. Hydrogen bonding interactions (Â, °)
D-H-A		d(D-H)	d(H-A)	d(D-A)	Angle (D-H-A)
N(3)-H(3A>	••N(2)	0.90(1)	2.12(4)	2.795(5)	131(5)
N(6)-H(6B)"	"O(4)i	0.90(1)	2.50(3)	3.311(5)	151(5)
N(6)-H(6A>	••N(5)	0.90(1)	2.13(4)	2.810(5)	132(5)
N(3)-H(3B>'	••O(8)	0.90(1)	2.51(3)	3.308(5)	149(5)
Symmetry code: i) x, 1 + y, -1 + z.
2. 5. Catalytic Epoxidation of Olefins
To a solution of olefins (0.28 mmol), NaHCO3 (0.11 mmol) and catalyst (9.4 x 10-4 mmol) in MeCN (0.5 mL) was added H2O2 (1.1 mmol, 30% H2O2 in water) as oxidant. After the reaction was over, 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 pL) 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.
3. Results and Discussion
3. 1. Synthesis and Characterization
The reaction of MoO2(acac)2 with the tridentate aroylhydrazine ligand H2L in methanol led to the formation of ci^-MoO2 complex in a good yield (Scheme 2). The complex is stable at room temperature and soluble in DMSO, DMF, ethanol, acetonitrile and less soluble in ot-
Scheme 2. Synthesis procedure of the complex
her common solvents like dichloromethane, chloroform, and insoluble in benzene, n-hexane and CCl4.
3. 2. JH NMR Spectra
NMR data of the aroylhydrazone ligand when compared with the complex reveals that the ligand serves as tridentate binegative ONO donor with amino group remaining free. The amino residue of the ligand shows signal for N-H at 11.05 ppm, which is found unperturbed in the complex. The disappearance of the signal at 12.10 ppm in the free ligand upon complexation suggests that during complexation the [-CO-NH-] group undergoes tautomerization to [-C(OH)=N-] followed by deprotona-tion of the -OH group. The azomethine C-H signal in the complex is shifted up-field from its original position in the free ligand upon coordination of the -CH=N- group, on account of reduction of electron density at the azomethine C-H. The aromatic protons also show some deviation in the complex as compared to the free ligand since in the complex they are in direct conjugation to the coordinated O and N of the hydrazone ligands.
3. 3. IR Spectra
IR spectrum of the free aroylhydrazone ligand shows multiple bands at 3232 cm-1 for v(N-H) of amino group, 1650 cm-1 for v(C=O) and 3431 cm-1 for v(O-H).10 The v(C=O) band is absent in the spectrum of the complex as the ligand binds in binegative mode losing protons from the carbohydrazide group. The strong peak at 1611 cm-1 can be assigned to v(C=N).11 The complex exhibits two characteristic bands at 949 cm-1 and 861 cm-1 for symmetric and antisymmetric stretching of cis MoO2 core.12
3. 4. Structure Description
The perspective view of the complex together with the atom numbering scheme are shown in Fig. 1. The asymmetric unit of the complex contains two complex molecules which are linked by hydrogen bonds. The coordination geometry around each molybdenum atom in the complex reveals a distorted square pyramidal environment with an NO4 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 one oxo group. The other oxo group is located at the apical position. The Mo atoms are found to be deviated from the corresponding mean basal planes by 0.365(1) A for Mo(1) molecule and 0.354(1) A for Mo(2) molecule. The C(8)-O(2) and C(23)-O(6) 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
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may be attributed to extended electron delocalization in the ligand. Similarly shortening of C(8)-N(2) and C(23)-N(5) lengths together with the elongation of N(1)-N(2) and N(4)-N(5) lengths also supports the electron cloud delocalization in the ligand system. The ligand molecules form five-membered and six-membered chelate rings with the Mo centers.
The question arises as to whether the coordination polyhedra around the five-coordinated Mo atoms can be described as distorted square pyramid or distorted trigonal bipyramid. Further information can be obtained by determining the structural index t which represents the relative amount of trigonality (square pyramid, t= 0; trigonal bipyramid, t= 1); t= (fi- a)/60°, a and fi being the two largest angles around the central atom.13 The values of t are 0.059 for Mo(1) and 0.104 for Mo(2). The coordination geometries of the Mo atoms in the complex can therefore be described as distorted square pyramids. Molecules are linked through intermolecular hydrogen bonds to form 2D sheets (Fig. 2).
3. 5. Catalytic Property
The complex showed effective catalytic property in the oxidation of various olefins to their corresponding epoxides. The details of catalytic properties with respect to epoxidation of olefins with the complex as catalyst are given in Table 4. Excellent epoxide yields and selectivity were observed for all aliphatic and aromatic substrates. The results of catalytic studies using dioxomolybdenum catalysts reveal that the efficiency of catalyst toward all the substrates is similar with maximum conversion, TON, and selectivity. The mechanism for the epoxidation of various olefins to epoxides using the two complexes as catalysts has been proposed by comparison with the literature report (Scheme 3).14 When H2O2 was used as a sole oxidant the catalytic efficiency is not high, but when NaHCO3 was added as a co-catalyst the efficiency of the system increases many times. The key aspect of such a reaction is that H2O2 and hydrogen carbonate react in an equilibrium process to
Figure 1. An ORTEP diagram of the complex with atom labeling scheme and 30% probability thermal ellipsoids for all non-hydrogen atoms. Hydrogen bonds are shown as dashed lines.
Scheme 3. Proposed mechanism of catalytic oxidation process
Figure 2. Packing diagram of the complex, viewed along the x axis. Hydrogen bonds are shown as dashed lines.
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Table 4. The catalytic oxidation results3
Substrate
Product
Conversion (%)b (TON)c
Selectivity (%)d
^<1
100 (321) 100 (305)
91 (287) 93 (292)
100
100
100
100
a The molar ratios for catalyst: substrate: NaHCO3:H2O2 are 1:298:117:1170. The reactions were performed in (70:30) mixture of CH3OH/CH2Cl2 (1.2 mL) under air at room temperature. b The GC conversion (%) are measured relative to the starting olefin after 74.5 min. c TON = (mmol of product)/mmol of catalyst. d Selectivity to epoxide = (epoxide%/(epoxide% + aldehyde%)) X 100.
produce peroxymonocarbonate, HCO4, which is a more reactive nucleophile than H2O2 and speeds up the epoxida-tion reaction.15 The basic principle of the catalytic reaction is the conversion of oxoperoxo-complex to dioxo-complex transferring oxo species to the olefins and the conversion of dioxo complex to the oxoperoxo complex reacting with HCO4- to regain the catalytic activity.
4. Conclusion
A novel dioxidomolydenum(VI) complex with aroylhydrazone ligand 2-amino-W-(2-hydroxy-3-methyl-benzylidene)benzohydrazide has been prepared and structurally characterized using X-ray structure analysis, FT-IR and 1H NMR spectra. The complex has square pyramidal geometry with positions around the central atom being occupied with donor atoms of the aroylhydrazone ligand, and two oxo groups. The complex showed effective catalytic property in the oxidation of various olefins to their corresponding epoxides.
5. Supplementary Material
CCDC reference number 1435811 contains the supplementary crystallographic data for this 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.
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Povzetek
Sintetizirali smo nov dioksidomolibdenov kompleks, [MoO2L], z vezavo troveznega aroilhidrazona 2-amino-V-(2-hi-droksi-3-methilbenziliden)benzohidrazida (H2L) in ga okarakterizirali z elementno analizo, FT-IR, NMR in mono-kristalno rentgensko difrakcijo. Kompleks ima popačeno kvadratno piramidalno strukturo, kjer je aroilhidrazonski lig-and dinegativni donor, medtem ko amino-N skupina ni koordinirana na kovinski center. Monokristalna strukturna analiza razkrije, da je Mo(VI) center koordiniran z donorskimi atomi dinegativnega aroilhidrazonskega liganda in dveh ok-sido skupin. Kristalna struktura je stabilizirana z vodikovimi vezmi. Kompleks je učinkovit katalizator za epoksidacijo olefinov z vodikovim peroksidom kot oksidantom in z natrijevim hidrogenkarbonatom kot kokatalizatorjem.
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