cis-Dioxo-molybdenum(VI) Schiff base complexes: Synthesis, crystal structure and catalytic performance for homogeneous oxidation of olefins

Inorganica Chimica Acta 386 (2012) 27–35

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cis-Dioxo-molybdenum(VI) Schiff base complexes: Synthesis, crystal structure and catalytic performance for homogeneous oxidation of olefins Saeed Rayati a,⇑, Nasim Rafiee a, Andrzej Wojtczak b a b

Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran Faculty of Chemistry, N. Copernicus University, Gagarina 7, 87-100 Torun, Poland

a r t i c l e

i n f o

Article history: Received 10 September 2011 Received in revised form 5 February 2012 Accepted 7 February 2012 Available online 15 February 2012 Keywords: Molybdenum(VI) Schiff base Crystal structure Olefin Homogeneous catalyst Oxidation

a b s t r a c t The synthesis of two Mo(VI) tetradentate Schiff base complexes derived from 2,20 -dimethylpropylenediamine and aromatic aldehydes, (MoO2{hnaphnptn} (1) and MoO2{salnptn(3-OMe)2} (2)) is reported. Full characterization of these complexes was accomplished with elemental analyses, spectroscopic studies (1H NMR, IR, and UV–Vis) and X-ray structure analysis. X-ray crystallography studies reveal that these complexes adopt a distorted octahedral six-coordinate configuration formed by tetradentate Schiff base ligand and two binding oxygen atoms. Catalytic performance of the prepared molybdenum complexes for oxidation of different olefins with tert-butyl hydroperoxide was evaluated. These complexes were found to be an efficient and selective catalyst for the homogeneous oxidation of various olefins. MoO2{salnptn(3-OMe)2} with a methoxy groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Molybdenum is an essential metal that is capable of forming complexes with many compounds of biological importance such as carbohydrates [1], amino acids [2–4], flavins [5,6] and porphyrins [7–12]. Although, molybdenum is not only much less toxic than many other metals of industrial importance but it is also an essential constituent of certain enzymes that catalyze reduction of molecular nitrogen and nitrate in plants and oxidation (hydroxylation) of xanthine and other purines and aldehydes in animals [13,14]. Epoxides are very important and versatile intermediates for manufacturing a range of important commercial products such as pharmaceuticals and polymers. In addition, it can be easily transformed to a variety of functional groups [15]. Many transition metal complexes have been reported for oxidation of olefins [16– 27], while dioxomolybdenum Schiff base complexes [28–39], or oxazoline complexes [40,41] have long been of great interest due to their ease in preparation, low cost and effortless means of tuning both sterically and electronically by the variation of corresponding amine and aldehyde ligand precursors. In this report, two new cis-dioxo-molybdenum(VI) Schiff base complexes have been prepared and characterized and their

⇑ Corresponding author. Tel.: +98 21 22850266; fax: +98 21 22853650. E-mail addresses: [email protected], [email protected] (S. Rayati). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2012.02.005

catalytic performance in the highly selective epoxidation of olefins with tert-butyl hydroperoxide (TBHP) has been investigated.

2. Experimental 2.1. Instruments and reagents Infrared spectra were recorded as KBr pellets using Unicam Matson 1000 FT-IR. Elemental analysis (C, H, N) were performed using a Heraeus Elemental Analyzer CHN-O-Rapid (Elemental-Analysesysteme KBr pellets, Gmbh, West Germany). 1H NMR spectrum was obtained in CDCl3 solutions with a Bruker FT-NMR 500 (500 MHz) spectrometer. The residual CHCl3 in conventional 99.8 at% CDCl3 gives a signal at d = 7.26 ppm, which was used for calibration of the chemical shift scale. A digital melting point measuring device (Electrothermal 9100) was used. The electronic absorption spectra were recorded on a single beam spectrophotometer (Cam SpecM330). The oxidation products were analyzed with a gas chromatograph (Shimadzu, GC-14B) equipped with a SAB-5 capillary column (phenyl methyl siloxane 30 m  320 mm  0.25 mm) and a flame ionization detector. 2,20 -Dimethylpropylenediamine, 2-hydroxy-1-naphtaldehyde, 2-hydroxy,3-methoxybenzaldehyde, molybdenyl acetylacetonate and tert-butyl hydroperoxide (solution 80% in di-tert-butyl peroxide) were used as received from commercial suppliers. Solvents were dried and distilled using standard methods before being used.

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Other chemicals were purchased from Merck or Fluka chemical companies.

2.2. Preparation of the ligands Tetradentate Schiff base ligands H2{hnaphnptn}) and (H2{salnptn(3-OMe)2} have been prepared according to the literature method [21].

2.3. Preparation of MoO2{hnaphnptn} (1) H2{hnaphnptn} (0.41 g, 1 mmol) was dissolved in 20 ml ethanol. An ethanolic solution of molybdenyl acetylacetonate (0.326 g, 1 mmol) was added to above solution and the reaction mixture was refluxed for 1 h. The colored solution was concentrated to yield colored powders. The products washed with warm ethanol. Yield (73%, 0.39 g), M.p.: >250. Anal. Calc. for C27H24MoN2O4 (536.427): C, 60.45; H, 4.50; N, 5.22. Found: C, 60.58; H, 4.57; N, 5.31%. Selected FT-IR data, m (cm1): 3400 (O–H), 2957 (C–H), 1601 (C@N), 1536 (C@C), 877 and 908 (Mo–O). 1H NMR (d): 0.57,1.02 (s, 6H, NCH2C( CH3)2CH2N), 3.35,3.38 (d) and 3.56, 3.58(d) (2H, NCH2C(CH3)2CH2N), 4.35, 4.37 (d) and 4.44, 4.468(d) (2H, NCH2C(CH3)2CH2N), 6.37–7.89 (m, 12H, ArH), 8.56, 8.72 (s, 2H, HC@N), 13C{1H} NMR (d): 23.18, 26.02 (NCH2C(CH3)2CH2N), 37.48 (NCH2C(CH3)2CH2N), 72.09, 74.92 (NCH2C(CH3)2CH2N), 116.73–160.37 (aromatic C), 164.53, 169.32 (C@N).

R

Y

R

O

O Mo

X

N

Me

O

Y

O N

2.4. Preparation of MoO2{salnptn(3-OMe)2} (2) H2{(salnptn(3-OMe)2} (0.37 g, 1 mmol) was dissolved in 20 ml ethanol. An ethanolic solution of molybdenyl acetylacetonate (0.326 g, 1 mmol) was added to above solution and the reaction mixture was refluxed for 1 h. The colored solution was concentrated to yield colored powders. The products washed with warm ethanol. Yield (78%, 0.38 g), M.p.: >250. Anal. Calc. for C21H24MoN2O6 (496.359): C, 50.81; H, 4.87; N, 5.64. Found: C, 50.93; H, 4.79; N, 5.57%. Selected FT-IR data, m (cm1): 3400 (O–H), 2936 (C–H), 1604 (C@N), 1543 (C@C), 880 and 914 (Mo–O). 1H NMR (d): 1.24 (s, 6H, NCH2C(CH3)2CH2N), 3.35 (s, 4H, NCH2C(CH3)2CH2N), 3.81 (s, 6H, OMe), 6.22–6.67 (m, 6H, ArH), 7.26 (s, 2H, HC@N). 13C{1H} NMR (d): 23.23, 26.17 (NCH2C(CH3)2CH2N), 36.28 (NCH2C (CH3)2CH2N), 60.85 (OMe), 73.28, 75.75 (NCH2C(CH3)2CH2N), 119.19–161.02 (aromatic C), 164.69, 169.95 (C@N). General structure of dioxo-molybdenum(VI) complexes is shown in Fig. 1.

2.5. General oxidation procedure Catalytic experiments were carried out in a 50 ml glass reaction flask fitted with a water condenser. In a typical procedure, 0.032 mmol dioxo-molybdenum(VI) complexes were dissolved in 10 ml 1,2-dichloroethane. Then 10 mmol alkene was added to the reaction mixture and 30 mmol TBHP was added. The reaction mixture was refluxed for 1 h. The reaction products were

Complex

R

MoO2{hnaphnptn}

H OMe

MoO2{salnptn(3-OMe)2}

X

X,Y XY: -CH=CH-CH=CH H

Me

Fig. 1. General structure of dioxo-molybdenum(IV) complexes.

Table 1 Crystal data and structure refinement for 1 and 2. Identification code

1

2

Empirical formula Formula weight T (K) c (Å) Crystal system, space group Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z, Calculated density (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h Range for data collection (°) Limiting indices Reflections collected/unique Completeness to h = 27.00 (%) Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Absolute structure parameter Largest difference in peak and hole (e Å3)

C27H24MoN2O4 536.42 293(2) 0.71073 monoclinic, P2(1)/c

C21H24MoN2O6 496.36 293(2) 0.71073 orthorhombic, P2(1)2(1)2(1)

8.5113(5) 25.9488(13) 10.6777(5) 103.490(5) 2293.2(2) 4, 1.554 0.610 1096 0.12  0.10  0.05 2.46–28.58 10 6 h 6 11, 32 6 k 6 34, 13 6 l 6 13 16 122/5229 [R(int) = 0.0737] 98.9 0.9711 and 0.9317 Full-matrix least-squares on F2 5229/0/307 0.747 R1 = 0.0371, wR2 = 0.0454 R1 = 0.1074, wR2 = 0.0520

9.3097(2) 12.6177(3) 17.7238(5

0.492 and 0.517

2081.96(9) 4, 1.584 0.671 1016 0.45  0.44  0.40 2.47–28.41 12 6 h 6 12, 16 6 k 6 16, 22 6 l 6 23 14 044/4726 [R(int) = 0.0200] 99.5 0.7733 and 0.7511 Full-matrix least-squares on F2 4726/0/271 1.118 R1 = 0.0185, wR2 = 0.0481 R1 = 0.0192, wR2 = 0.0483 0.01(2) 0.307 and 0.555

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monitored at periodic time intervals using gas chromatography. The oxidation products were identified by comparison with authentic samples (retention times in GC).

Table 3 UV–Vis data for ligands and complexes. Compound

kmax (nm) (e, M1 cm1)

Assignment

MoO2{salnptn(3-OMe)2}

233 266 326 233 331 440

p ? p⁄ n ? p⁄

2.6. X-ray data collection and structure determination The orange crystals of 1 appropriate for the X-ray analysis were crystallized from DMF, while the red crystals of 2 were obtained from CH2Cl2 solution. The X-ray data for both the investigated crystals were collected with an Oxford Sapphire CCD diffractometer using Mo Ka radiation k = 0.71073 Å, at 293(2) K, by x  2h method. For 1 the 0.12  0.10  0.05 mm plate crystal was used in the diffraction experiment. The monoclinic space group P2(1)/c was determined based on the systematic absences. For 2 the 0.45  0.44  0.40 mm crystal was used and the non-centrosymmetric orthorhombic space group P2(1)2(1)2(1) was assigned based on the systematic absences, which is consistent with the E2 statistics. For both structures, the numerical absorption correction was applied [42] with the maximum and minimum transmissions being 0.9711 and 0.9317 for 1, while 0.7733 and 0.7511 for 2. The structures were solved with direct methods and refined with the full-matrix leastsquares method on F2 with the use of SHELX-97 program package [43]. No extinction correction was applied during the refinement of 1 and 2. The absolute structure of 2 was determined with the Flack method [44], the Flack parameter x = 0.01(2). For both structures, the hydrogen atoms have been located from the difference maps and constrained during the refinement. Analysis of the structures was done with the PLATON software [45]. The data collection and refinement processes are summarized in Table 1. 3. Results and discussion The Schiff base ligands H2{hnaphnptn}) and (H2{salnptn(3OMe)2} were prepared by the condensation of 2,20 -dimethylpropylenediamine with 2-hydroxy-1-naphtaldehyde and 2-hydroxy, 3-methoxybenzaldehyde respectively in ethanol and could be isolated by evaporation of the solvent. The corresponding Mo complexes (Fig. 1, MoO2{hnaphnptn} and MoO2{salnptn(3-OMe)2}) of these ligands were synthesized by the reaction of MoO2(acac)2 and the related Schiff base ligands in ethanol. All of the complexes are quite air stable as solids and also in solution. In addition to crystal data for the molybdenum complexes, spectroscopic and elemental analyses are consistent with the proposed structure in the Fig. 1. The Schiff base ligands are bounded to the MoO2+2 ions through two phenolic oxygen atoms, two imine nitrogen atoms and two oxo groups. 3.1. Characterization of the ligands and dioxo-molybdenum(VI) complexes 3.1.1. IR spectral studies A practical list of IR spectral data is presented in Table 2. Comparison of the spectra of the complexes with the ligands provides

Table 2 IR spectral data of ligands and molybdenum complexes. Compound

H2{hnaphnptn} H2{salnptn(3-OMe)2} MoO2{hnaphnptn} MoO2{salnptn(3-OMe)2}

Selective IR bands (cm1) Mo@O

C@N

– – 877, 908 880, 914

1629 1628 1601 1604

MoO2{hnaphnptn}

(29 006) (16 832) (9391) (120 071) (32 374) (10 719)

LMCT

p ? p⁄ n ? p⁄ LMCT

evidence for the coordination mode of ligand in catalysts. A sharp band appearing at 1628–1629 cm1 due to m(C@N) (azomethine), shifts to lower wave number by 24–28 cm1 and appears at 1601–1604 cm1. This indicates the involvement of azomethine nitrogen in coordination. IR spectra of the Mo complexes showed two characteristic strong cis-dioxo (MoO2) bands in the regions 877–880 and 908–914 cm1, which could be assigned to the symmetric and asymmetric stretching vibrations respectively [46–49]. 3.1.2. Electronic spectral studies Table 3 provides electronic spectral data of the molybdenum complexes along with their assignments. The electronic absorption spectrum which were measured in methanol at room temperature show three absorption bands due to the transitions p ? p⁄, n ? p⁄ and LMCT, respectively. The absorption band observed at 233 nm is attributed to the p ? p⁄ transition of the imino group [50]. The spectra of the complexes show a band at 266 and 331 nm, which is due to the n to p⁄ transition and another band at 326 and 440 nm assignable to a ligand to metal charge transfer (LMCT) due to the promotion of an electron from the highest occupied molecular orbital (HOMO) of the ligand to the lowest unoccupied molecular orbital (LUMO) of molybdenum atom [51]. 3.1.3. 1H NMR spectral studies 1 H NMR spectroscopic data, as well as IR and electronic spectroscopy, confirm the formation of compounds 1 and 2. In the 1H NMR spectra of the Schiff base ligands (H2{hnaphnptn}) and (H2{salnptn(3-OMe)2}), two methyl and two methylene groups of the 2,20 -dimethylpropylenediamine bridge are chemically equivalent [21]. But in the molybdenum complexes, they are nonequivalent. For example 1H NMR spectra of MoO2{hnaphnptn}(1) is presented in Fig. 2. Two singlet peaks at 0.57 and 1.02 ppm can be assigned to the methyl groups and two doublets in the 3.35–3.58 and 4.35– 4.64 ppm regions are due to the two methylene groups. Also it was of significance to note that each of two naphtyl moieties in complex show separately peaks in 1H NMR spectrum. In addition the signals appearing at d = 8.56 ppm and d = 8.74 ppm are related to the azomethine groups. 3.2. Crystal structure of MoO2{hnaphnptn}(1) and MoO2{salnptn(3OMe)2} (2) Structure of 1 reveals that coordination sphere of molybdenum atom is a deformed octahedron. It is formed by the tetradentate Schiff base ligand, and two oxo ligands O3 and O4 (Fig. 3). Structure of dichloromethane 1.75-solvate of the analogous complex was solved in the different space group C2/c [52]. The coordination sphere of Mo in 2 is a distorted octahedron with a geometry similar to that described for 1. The oxo ligands O5 and O6 form the pair of the shortest bonds to Mo1, the Mo1–O5 and Mo1–O6 distances being 1.7047(14) and 1.7111(15) Å, respectively (Fig. 4). Selected bond length and angles for 1 are presented in Table 3. Their trans effect results in the Mo1– O1 2.0948(12) and Mo1–N2 2.3227(16) Å longer than Mo1–O3

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Fig. 2. The 1H NMR spectrum of complex MoO2{hnaphnptn}.

Fig. 3. Molecular structure of MoO2{hnaphnptn}. (For clarity, the atomic thermal ellipsoids are plotted at 30% probability level.)

1.9436(13) and Mo1–N1 2.1379(16) Å. In particular the elongation of Mo1–N2 bond is significant. The angles within the coordination sphere involving the Schiff base atoms range from O1–Mo1–N1 79.38(6)° to O5–Mo1–O3 103.73(6)° and O3–Mo1–N1 154.98(6)°. The angles corresponding to the trans positions relative to the oxo ligands are O6–Mo1–O1 163.99(7)° and O5–Mo1–N2 170.54(7)°. Similar geometry of the coordination sphere was reported for the Mo complex with the analogous ligand lacking both methoxy groups [49], but statistically significant differences up to 2° and 0.02 Å occur. The imine bond distances C7–N1 1.297(2) and N2–C11 1.273(2) Å are similar to those in 1. Geometry of the substituted phenyl rings is typical. The methoxy groups have a typical geometry with the bond distances C2–O2 1.370(3), O2–C18 1.412(3) Å, C16–O4 1.362(3), C21–O4 1.416(3) Å. The C2–O2–C1 and C16– O4–C21 angles are 116.8(2)° and 117.3(2)°, respectively. The methoxy groups are not exactly co-planar with the aromatic rings, the

Fig. 4. Molecular structure of MoO2{salnptn(3-OMe)2}. (For clarity, the atomic thermal ellipsoids are plotted at 30% probability level.)

torsion angles C1–C2–O2–C18 and C17–C16–O4–C21 being 166.1(2)° and 167.2(2)°, respectively. The dihedral angle between the best planes of the aromatic rings is 81.75(7)°, slightly larger than that found in 1. Also the conformation of the central bridge of the Schiff base ligand is similar to that for 1, with the torsion angles C7–N1–C8–C9, N1–C8–C9–C10, C8–C9–C10–N2 and C9–C10–N2–C11 being 101.3(2), 69.5(2), 64.5(2)° and 124.66(19)°. That indicates slight difference in the central bridge conformation relative to that reported [53], with the differences in the corresponding torsion angles ranging from 1° to 10°. In the conformation reported here, the gem-dimethyl group is positioned away from the oxo O5 and O6. The conformation of the chelate rings is similar as for 1: the chair conformation of Mo1–N1–C8–C9–C10–N2, and the envelope conformation for rings

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S. Rayati et al. / Inorganica Chimica Acta 386 (2012) 27–35 Table 4 Selected bond lengths (Å) and angles (°) for 2.

Table 7 Epoxidation of cyclooctene with different oxidants catalyzed by 1 and 2.a

Mo1–O5 Mo1–O6 Mo1–O3 Mo1–O1 Mo1–N1 Mo1–N2

1.7047(14) 1.7111(15) 1.9436(13) 2.0948(12) 2.1379(16) 2.3227(16)

O1–C1 C7–N1 N1–C8 C10–N2 N2–C11

1.313(2) 1.297(2) 1.471(2) 1.476(2) 1.273(2)

O5–Mo1–O6 O5–Mo1–O3 O6–Mo1–O3 O5–Mo1–O1 O6–Mo1–O1 O3–Mo1–O1 O5–Mo1–N1 O6–Mo1–N1

103.42(8) 103.73(6) 102.19(7) 89.60(7) 163.99(7) 83.34(6) 94.15(7) 90.27(7)

O3–Mo1–N1 O1–Mo1–N1 O5–Mo1–N2 O6–Mo1–N2 O3–Mo1–N2 O1–Mo1–N2 N1–Mo1–N2

154.98(6) 79.38(6) 170.54(7) 83.96(7) 80.10(6) 82.21(5) 79.75(6)

Mo1–O1–C1–C6–C7–N1 and Mo1–O2–C17–C12–C11–N2, with Mo1 ion positioned out of the ring plane. The slight twist of the imine moieties relative to the aromatic ring system is found similar to the structure of 1, with the dihedral angles between C1- -C6 and C6- -C8, as well as C12- -C17 to C10- -C12 being 29.3(2)° and 21.4(2)°, respectively. Analysis of the intermolecular interactions reveals the presence of C–H. . .Cg interactions involving both the phenolic rings and C19 methyl group of the central bridge, the C19–H19A. . .Cg(C12- -C17) [1/2 + x, 1/2  y, 1  z] of 2.96 Å and C19–H19C. . .Cg(C1- -C6)[x, 1/2 + y, 1/2  z] of 2.82 Å. There are some intermolecular contacts between the C–H groups and O atoms: C20–H20C. . .O5[x, 1/ 2 + y, 1/2  z] of 2.44 Å, the C–H. . .O angle of 161°, and C7– H7A. . .O5[x, 1/2 + y, 1/2  z] of 2.53 Å and C–H. . .O angle being 141°. Selected bond length and bond angle for 2 are presented in Table 4.

3.3. The catalytic oxidation of alkenes The catalytic performance of cis-MoO2{hnaphnptn} (1) and cisMoO2{salnptn(3-OMe)2} (2) was investigated in the epoxidation of cyclooctene, as a model substrate, and tert-butyl hydroperoxide as

Oxidant

H2O2 UHP n-Bu4NIO4 TBHP

1

2

Conversion (%)

Conversion (%)

1.1 1.3 2.1 91.2

1.6 1.9 2.4 96.5

a Conditions: the molar ratios for using cat.cyclooctene:oxidant are 0.032:10:30. The reactions were run for 1 h at 80 °C in 1,2-dichloroethane.

Fig. 5. The influence of catalyst concentration on the oxidation of cyclooctene using TBHP catalyzed by 1 and 2. The reactions were run under air in DCE and the molar ratio of cyclooctene:TBHP:catalyst was 10:30:X.

the oxygen donor. In the absence of catalyst, the reactions did not proceed even under reflux. In order to find the optimum reaction conditions, the effect of various reaction parameters that may affect the conversion and selectivity of the reaction was investigated. Solvent, catalyst concentration and the nature of oxidant are the factors that have been evaluated.

Table 5 The effect of various solvents on the epoxidation of cyclooctene with TBHP using 1.a

a b c

Solvent

Boiling point

Donor number

Conversion (%)b

Selectivity (%) epoxide

TOF (h1)c

1,2-Dichloroethane Chloroform Acetonitrile Dichloromethane Methanol

84 61 82 39 65

0 0 14.1 1 19

91.2 65.4 40.7 10.8 3.7

100 100 100 100 100

285.1 204.5 127.2 34.2 11.7

Conditions: the molar ratios for using cis-MoO2{salnptn(3-OMe)2}:cyclooctene:oxidant are 0.032:10:30. The reactions were run for 1 h at 80 °C. GC yields based on the starting olefin. Turnover frequency (TOF) = [mol of epoxide/(mol of catalyst)]/h.

Table 6 The effect of various solvents on the epoxidation of cyclooctene with TBHP using 2.a

a b c

Solvent

Boiling point

Donor number

Conversion (%)b

Selectivity (%) epoxide

TOF (h1)c

1,2-Dichloroethane Chloroform Acetonitrile Dichloromethane Methanol

84 61 82 39 65

0 0 14.1 1 19

96.5 86.1 46.7 34.5 5.6

100 100 100 100 100

301.5 269.2 146.1 107.8 17.5

Conditions: the molar ratios for using cis-MoO2{salnptn(3-OMe)2}:cyclooctene:oxidant are 0.032:10:30. The reactions were run for 1 h at 80 °C. GC yields based on the starting olefin. Turnover frequency (TOF) = [mol of epoxide/(mol of catalyst)]/h.

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In oxidation reactions the solvent usually plays an important role and can affect conversion and product selectivity [54]. Therefore, the influence of different solvents on the oxidation of cyclooctene were studied using 1 and 2 and as catalyst and results are presented in Tables 5 and 6. Chloroform, acetonitrile, dichloromethane, methanol and 1,2-dichloroethane (DCE) were used as solvent and the highest conversion (91.2% in 1 and 96.5% in 2) was obtained in 1,2-dichloroethane. The higher conversions in DCE relative to the other solvents possibly are due to the higher boiling point and non-coordinating behavior of the former. Coordinative solvents (such as acetonitrile and methanol) compete with TBHP to bind to the molybdenum center and inhibited the reaction [55,56]. A very low conversion was observed in methanol with respect to the acetonitrile is due to the higher donor ability and lower boiling point of the former. In order to investigate the effect of oxidizing agent in the oxidation reaction, two other peroxides (hydrogen peroxide and urea hydrogen peroxide (UHP)) and also tetrabutylammonium periodate (n-Bu4NIO4) were used, and results show that no significant product formation was observed in the case of UHP, n-Bu4NIO4 and hydrogen peroxide. Therefore, TBHP is a more efficient oxidant with respect to the other solvents (Table 7).

Different catalyst concentrations have been used in the oxidation of cyclooctene (Fig. 5). It was observed that oxidation of cyclooctene required 0.033 mmol of catalyst for completion. In order to establish the general applicability of the method, under the optimized conditions, oxidation of various olefins were subjected in the presence of the catalytic amount of 1 and 2 and the results are presented in Tables 8 and 9. Oxidation of 1-octene, 4-octene, indene, 4-Cl-styrene, 4-Mestyrene, 4-OMe-styrene and cyclooctene gave the corresponding epoxide as the sole product, while in the oxidation of styrene some benzaldehyde, in the oxidation of cyclohexene some cyclohexene1-ol and cyclohexene-1-one and also in the oxidation of a-methyl styrene, acetophenone were detected as by product. Terminal double bonds (Table 8, entry 1) are less reactive than the conjugated double bonds or non-terminal double bond (Table 8, entry 5). Double bonds conjugated with aryl groups (Table 8, entries 6–16) are less reactive than the others. Higher reactivity of amethyl styrene with respect to the styrene seems to be due to the electron donation effects of methyl group to the double bond, while a-methyl styrene gives 30% acetophenone as the by-product. High reactivity observed in the case of 4-chlorostyrene in comparison with styrene seems to be due to the p-electron donation effect of

Table 8 Epoxidation of olefins using TBHP catalyzed by 1 in DCE.a Entry

a b c d e

Alkene

Conversion (%)b

Product

Selectivity (%) epoxide

Time

TOF (h1)

1

5.6

100

1

17.5

2 3 4 5

20 31 50 31

100 100 100 100

2 3 4 2

62.5 96.8 156.2 96.8

6

36

100

1

112.5

7 8

73 20

100 60c

2 1

228.1 62.5

9 10

100 30

81 100

2 1

312.5 93.7

11 12

100 11

100 100

2 1

32.5 34.4

13 14

58 5

100 100

2 1

181.2 15.6

15 16

44 100

100 74d

2 1

137.5 312.5

17

100

72e

1

312.5

18

91

100

1

285.1

The molar ratio of catalyst:alkene:TBHP was 0.032:10:30. Conversions and selectivities were determined by GC based on the starting alkene. Benzaldehyde is the by-product. Acetophenone is the by-product. Cyclohexene-1-ol and cyclohexene-1-one are the by-products.

33

S. Rayati et al. / Inorganica Chimica Acta 386 (2012) 27–35 Table 9 Epoxidation of olefins using TBHP catalyzed by 2 in DCE.a Entry

a b c d e

Alkene

Conversion (%)b

Product

Selectivity to epoxide%

Time

TOF (h1)

1

8.2

100

1

25.6

2 3 4 5

36 42 67 42

100 100 100 100

2 3 4 2

112.5 131.2 209.4 131.2

6

42

100

1

131.2

7 8

86 29

100 62c

2 1

268.7 90.6

9 10

100 34

91 100

2 1

3125 106.2

11 12

100 26

100 100

2 1

312.5 81.2

13 14

63 14

100 100

2 1

196.8 43.7

15 16

46 100

100 70d

2 1

143.7 312.5

17

100

78e

1

312.5

18

96.5

100

1

301.6

The molar ratio of catalyst:alkene:TBHP was 0.032:10:30. Conversions and selectivities were determined by GC based on the starting alkene. Benzaldehyde is the by-product. Acetophenone is the by-product. Cyclohexene-1-ol and cyclohexene-1-one are the by-products.

Cl-group to the double bond, while lower reactivity on the 4-methoxy styrene may be due to the r-electron accepting effect of the methoxy group. Comparison of the catalytic performance of two different catalyst in the epoxidation of various olefins show that the catalytic activity of the MoO2{salnptn(3-OMe)2} is a little higher than MoO2{hnaphnptn}. As expected, an electron donating group in the salicylidene ring should decrease the activity of the catalyst, resulting from the decreasing Lewis acidity of the molybdenum center and therefore the deduction of TBHP activation [57,58]. While higher conversion was obtained in the case of 2 with an electron donating methoxy group in the salicylidene ring. Presumably, existence of methoxy group near to the coordination sphere of molybdenum center in 2 led to the formation of the hydrogen bonding between TBHP and methoxy group (Fig. 6). It seems that formation of such hydrogen bonding might be responsible for this behavior. This interaction is expected to contribute to a facile coordination/activation of TBHP the molybdenum center. On the other hand, the presence of two XY groups in 1, may increase the steric hindrance around the active site of the catalyst and thus reduce reactivity of 1 with respect to 2.

3HC

O

O O

CMe3

H

O

O

O

Mo N

OMe

O

N Me Me

Fig. 6. Hydrogen bonding formation between TBHP and 2.

In the proposed catalytic cycle [19,21,59–60,34,40], at first step, TBHP will be activated by coordination to the molybdenum center and formation of hepta-coordinated molybdenum intermediate (I, Scheme 1) [61,62]. It seems that heterolytic cleavage of the TBHP occurs. Therefore catalysts with higher lewis acidity character increase the efficiency of the coordinated peroxo group. Then olefin as a nucleophile will attack to the electrophile oxygen atom of the coordinated TBHP [63].

34

S. Rayati et al. / Inorganica Chimica Acta 386 (2012) 27–35 R Y X N

R H CMe3 R O O O O Mo O

O

N

Y

Y

O

N

TBOH

X

Me

Y +TBHP

Me

R

R CMe3 H O R O O O O

X

R

O

Mo

Me

Y

O

N

X

Me

O

X

Mo N N Me

Y O

X

H O CMe3 R O O O Mo

O

N

X

Y

N Me

O X

Y

Me

Me

I Scheme 1. Proposed catalytic cycle.

A comparison of the results obtained for our present catalytic system with those reported in the literature which used TBHP as oxidizing agent [19,21–23,64–70] reveals that reaction time is lower than other systems and higher conversion for epoxide formation will be achieved. 4. Conclusions In summary, two molybdenum(VI) complexes (MoO2{hnaphnptn} and MoO2{salnptn(3-OMe)2}) were synthesized by the reaction of two tetradentate N2O2 Schiff base ligands with MoO2(acac)2. These complexes are employed as catalyst for the epoxidation of olefins with tert-butyl hydroperoxide. Excellent selectivity (100%) for epoxide formation was obtained in the case of cyclooctene, 1-octene, indene, 4-Cl/4-Me/4-OMe-styrene and a-methyl styrene. Moreover the epoxide yields is strongly depend on the nature of oxidizing agent, solvent and temperature, and the best yields were obtained in 1,2-dichloroethane at reflux in the presence of tert-butly hydroperoxide as oxidant. Acknowledgment The financial support of this work by K.N. Toosi University of Technology research council is acknowledged.

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Appendix A. Supplementary material CCDC 836325 and 836326 contain the supplementary crystallographic data for complexes 1 and 2, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2012.02.005.

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