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J. Phys. Chem. C 2008, 112, 14124–14130
Comparison of Co with Mn and Fe in LDH-hosted Sulfonato-Salen Catalysts for Olefin Epoxidation Samiran Bhattacharjee,† Trevor J. Dines,‡ and James A. Anderson*,† Surface Chemistry and Catalysis Group, Department of Chemistry, UniVersity of Aberdeen, Aberdeen AB 24 3UE, Scotland, U.K. and DiVision of Electronic Engineering & Physics, UniVersity of Dundee, DD1 4HN, Scotland, U.K. ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: June 30, 2008
Sulphonato-salen complexes containing Mn(III), Co(III), and Fe(III) metal centers intercalated into Zn/Al layered double-hydroxide (LDH) host have been synthesized and tested as catalysts for the epoxidation of cyclohexene and dicyclopentadiene using pivaldehyde and molecular oxygen at atmospheric pressure and room temperature. Oxidation of cyclohexene gave cyclohexene oxide and 2-cyclohexen-1-one as reaction products, whereas dicyclopentadiene was transformed to the corresponding monoepoxide. Selectivity was found to depend on the central metal ion. Activity (TOF) increased according to LDH-[Fe(Cl)(salen)] < LDH-[Co(Cl)(salen)] < LDH-[Mn(Cl)(salen)] in both case of cyclohexene and dicyclopentadiene epoxidation. The structures of the metal(III) sulfonato-salen complexes were modeled by density functional theory calculations in order to compare differences in dimensions and geometries with differences in gallery height and catalytic behavior. 1. Introduction Heterogeneously catalyzed oxidations reactions for the production of fine and intermediate chemicals has generated a great deal of interest in recent years.1 Although, ideally, these would involve oxygen as oxidant, this is seldom employed, and more frequently, hydrogen peroxide is used as the most appropriate environmentally friendly reagent. Recently there has been growing interest in the direct manufacture of this activated oxidant from hydrogen and oxygen.2,3 There are cases such as the Prilezhaev reaction, Bayer-Villiger-oxidations, and the Rubottom oxidation for which the use of peracids are the most appropriate reagent in terms of providing high reaction selectivities, however the transport and storage of these reagents are of concern. An important breakthrough was reported by Mukaiyama et al.4 who reported the epoxidation of alkenes using a combination of molecular oxygen and pivalaldehyde to give in situ generation of peracid. This approach has been widely employed by a number of groups,5,6 including our own,7-9 for the epoxidation of chiral and prochiral substrates. Cyclohexene oxide is a valuable organic intermediate, used in the synthesis of products such as single enantiomer drugs, the pesticide propargite, epoxy paints, rubber promoter, and dyestuffs. A great number of studies have also been devoted to epoxy norbornane derivatives due to the fact that these compounds are readily produced from cyclopentadiene, which is a large scale side-product of the coke and petroleum processing industries.10 Epoxidation of cyclohexene using different metal catalysts in homogeneous11 and heterogeneous 12,13 systems has been widely investigated. The groups of Venturello14,15 and Ishii16,17 independently developed highly effective catalyst systems closely related to polyoxomatalatesbased systems that exhibit good catalytic activity for the epoxidation of cyclohexene using hydrogen peroxide. The * Corresponding author phone:+44 (0) 1224 272921; e-mail: j.anderson@ abdn.ac.uk. † University of Aberdeen. ‡ University of Dundee.
disadvantages include the use of toxic and carcinogenic chlorocarbons (chloroform and 1,2-dichloroethane) and catalyst deactivation, causing loss of catalytic activity for reuse.18 The use of homogeneous salen-based catalysts for olefin epoxidation, as introduced by Jacobsen19 and Katsuki20 in the early 1990s, led to considerable activity in the area of producing effective heterogeneous analogues as indicated in the extensive reviews by Li21,22 and Garcia.23 In the main, these reviews indicate that work has focused on the importance of the mode and method of attachment to the solid, the role of the axial ligand, and on the nature of the ligands in the vicinity of the central metal cation. Less attention was paid to the role of the active metal center, although in the homogeneous phase various metallosalen complexes including manganese(III),24 chromium(III)25 and nickel(II), 26 salen have been prepared and used for epoxidation of simple olefin, cyclohexene, or cyclooctene. Of these, the cationic manganese(III)-salen complexes showed most efficiency as catalysts.24 Sulfonato-salen-Mn(III) complexes can be successfully intercalated into a Zn/Al layered double hydroxide and have been found to show high conversion, selectivity, and de in the stereoselective oxidation of R-(+)-limonene and R-(-)-pinene using a combination of pivaldehyde and molecular oxygen or air as oxidant under mild reaction conditions.7 Other studies on the intercalation of transition metal complexes in hydrotalcite layers are few, although intercalation of a sulfonato-salen-Cr(III) into a Mg-Al layered doublehydroxide host is reported to show high selectivity for the oxidation of benzyl alcohol to benzaldehyde using hydrogen peroxide as oxidant.27 Intercalation of a dioxomolybdenum (VI) anion into a Zn/Al layered double hydroxide host is reported to show useful catalytic properties 28,29 in the oxidation of thiol and in the reduction of nitrobenzene. In this paper, we describe the synthesis of cobalt-based intercalated salen LDH materials, [Zn2.15Al0.84(OH)5.98][Co(Cl)salen]0.17[C6H5COO]0.50 · 2H2O, and compare the catalytic behavior with LDH-[Mn(Cl)salen] and LDH[Fe(Cl)salen]9 in the oxidation of cyclohexene and dicyclopentadiene using molecular oxygen in the presence of pivalaldehyde
10.1021/jp804339d CCC: $40.75 2008 American Chemical Society Published on Web 08/14/2008
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TABLE 1: Characterization of Catalyst Precursor, LDH-Host Precursor, and Hosted Cobalt Complexes analytical dataa (%) compound
C
H
N
Co
Na2[Co(Cl)(L)] · 2H2O LDH-[C6H5COO]d LDH-[Co(Cl)(L)]d
36.80 (36.69)
3.32 (3.36)
4.32 (4.28
9.40 (9.0)
a
2.21 (2.18) b
c
Zn
Al
λmax (nm)b
FTIRc
5.58 (5.67) 4.97 (4.94)
822, 587, 480, 104, 275 277, 234 824, 401
1560, 1114, 1036, 580
34.61 (34.75) 30.74 (30.64)
1117, 1029, 577
d
Calculated values are shown in parentheses. In Nujol mull. As KBr discs. XRD: 2θ/° d-spacing (Å) for LDH-[C6H5COO]: 5.8 (15.22), 11.3 (7.82), 17.0 (5.21), 22.7 (3.91); for LDH-[Co(Cl)(L)]: 4.6 (19.20), 8.2 (10.80), 19.5 (4.50).
to determine the impact that a change in the central atom may have on the catalytic behavior. DFT calculations of the structures of the different metal(III) ion sulfonato-salen complexes were carried out in an attempt to account for differences in space requirements exhibited by the different complexes and to determine the impact that the change in central metal ion had on the geometries of atoms in the immediate surroundings of the metal ion local environment of the catalytically active site. 2. Experimental Section 2.1. Preparation of Sulfonato-Salen Ligand, H2L. The chiral sulfonato-salen ligand was prepared according to our previously published method.7,8 (R,R)-1,2-Diammoniumcyclohexane mono-(+)-tartrate from trans-diaminocyclohexane (99%, Aldrich), (2R,3R)-(+)-tartaric acid (99.5%, Aldrich), sodium salicylaldehyde-5-sulfonate from salicylaldehyde (98%, Aldrich), and aniline (99.5%, Aldrich) were employed. A mixture of 2.97 g of (R,R)-1,2-diammonium cyclohexane mono-(+)tartrate and 3.12 g of potassium carbonate were combined with 20 mL water-ethanol (1:4) in a two-necked round-bottomed flask with reflux condenser and an addition funnel. The mixture was heated and stirred with a magnetic stirrer. A solution of sodium salicylaldehyde-5-sulfonate (5.54 g) in 20 mL of water was added dropwise to the above solution through an addition funnel with constant stirring and gentle heating. The resulting mixture was refluxed for 1 h with stirring and cooled to room temperature. The volume was reduced by 50% by rotaryevaporation until a yellow solid was separated, which was filtered off and washed with ethanol. The yellow solid was then recrystallized from a water-diethyl ether mixture and dried over silica gel. 2.2. Preparation of Na2[Co(Cl)(salen)], 1. A hot aqueous solution of ligand (2.63 g, 4.7 mmol) was added dropwise to an aqueous-ethanol solution (30 mL) of CoCl2 · 6H2O (1.19 g, 5 mmol) with stirring for 20 min. The resulting green-colored mixture was heated on a steam bath for 10 min to allow the precipitated solid to settle. The product was filtered off and washed with cold water and ethanol. The complex was purified by dissolving in water and then precipitated by adding diethyl ether, followed by drying over silica gel, to give a yield of 88%. During coordination, the cobalt(II) ion is oxidized to a cobalt(III) species with the concomitant formation of a cobalt(III) complex. Such rapid oxidation of cobalt(II) to cobalt(III) ions has also been observed in the presence of other ligands.30 2.3. Preparation of LDH-[Co(Cl)salen], 2. The LDH[Co(Cl)salen] was obtained by the partial substitution of intercalated C6H5COO ions by the [Co(Cl)salen]2- ions. The LDH-[C6H5COO] was prepared by mixing a solution of zinc(II) nitrate tetrahydrate (29.75 g) and aluminum(III) nitrate (12.50 g) in decarbonated water, together with a further separate solution prepared by dissolving benzoic acid (21.96 g) and NaOH (15.60 g) in decarbonated water under N2. The gel-like mixture was digested at 348 K for 62 h. Upon cooling, the product was isolated by filtration and dried overnight at 333 K.
Figure 1. X-ray powder diffraction pattern of (a) LDH-[Co(Cl)(L)] and LDH-[C6H5COO].
Figure 2. UV-vis spectra of (a) free chiral sulfonato-cobalt(III) complex and (b) LDH-[Co(Cl)salen]2-.
Na2[Co(Cl) salen] (1.72 g, 2.63 mmol) was dissolved in decarbonated water, LDH-[C6H5COO] (5.0 g) was added to the solution, and the mixture was stirred for 10 h at room temperature under a nitrogen atmosphere. The green product was filtered off, washed with water and ethanol, and dried at overnight 333 K. 2.4. PreparationofLDH-[Mn(Cl)salen]andLDH-[Fe(Cl)salen]. The Na2[Mn(Cl) salen] and Na2[Fe(Cl) salen] were prepared according to our published method.7,9 The LDH-[Mn(Cl salen] and LDH-[Fe(Cl) salen] were prepared in the same manner9 as described for 2, but using [Mn(Cl) salen]2- or [Fe(Cl) salen]2-, instead of the [Co(Cl) salen]2- ions. 2.5. Catalyst Characterization. FTIR spectra of samples were recorded using KBr disks on a Perkin-Elmer 1720 X spectrometer, and electronic spectra were recorded on a PerkinElmer UV/vis Lambda 16 Spectrometer. The X-ray powder diffraction patterns were recorded on a Philips PW 1010 X-ray generator with Cu KR (1.5402) radiation at 10 min-1. Aluminum, zinc, and cobalt were determined using a UNICAM 939/ 959 atomic absorption spectrometer. 2.6. Catalytic Reaction. Epoxidation of cyclohexene and dicyclopentadiene was carried out using pivalaldehyde and
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Figure 3. Two projections of the computed structures of M(III) sulfonato-salen complexes.
TABLE 2: Computed Bond Distances, Interbond Angles, and Dihedral Angles Surrounding the Central Metal Ions entry No.
distance or angle
Mn
Fe
Co
1 2 3 4 5 6 7
θ(O24-M45-N12) θ(O24-M45-N8) θ(O24-M45-Cl46) θ(N8-C10-C11) θ(C10-N8-M45) θ(C11-N12-M45) θ(O9-M45-O24)
89.99 165.38 101.47 106.13 109.46 113.66 92.38
85.88 157.20 105.09 106.42 109.42 115.27 98.25
92.42 170.86 97.41 105.84 110.46 112.48 86.13
II)34 was used to provide an initial estimate of the geometry prior to optimization. Geometry optimization was performed with the LanL2DZ basis set, which employs Dunning-Huzinaga double-ζ (DZ) basis functions35 on C, H, N, and O atoms and Los Alamos effective core potential with DZ functions on metal, Cl, and S, atoms.36 This effective core potential basis set was judged to be a reasonable compromise in view of the large sizes of the complexes. 3. Results and Discussion
molecular oxygen at atmospheric pressure in a twin-necked round flask equipped with condenser. In a typical run, 1 mmol of alkene, 2 mmol of pivalaldehyde, 10 mL of toluene, and 0.050 g of catalyst were stirred at room temperature (25 °C) while bubbling molecular oxygen at atmospheric pressure. After allowing the reaction to proceed for a predetermined period of time, the catalyst was filtered off, and the selectivity and conversion were measured by injecting representative samples into a Hewlett-Packard GC/MS fitted with a CYDEX-B fused silica column and simultaneously employing both FID and MS detectors. 2.7. Computational Details. The structures of the chiral (sulfonato-salen)-metal(III) complexes were calculated using the Gaussian 98 program,31 with the hybrid SCF-DFT method B3-LYP. This incorporates Becke’s three-parameter hybrid functional32 and the Lee, Yang, and Parr correlation functional33 and generally gives superior results to HF-SCF calculations, which neglect the effects of electron correlation. There are no published crystal structures of these complexes for comparison, so the structure of a related complex, chloro-(1R,2R)-(-)-(1,2cyclohexanediamino-N,N′-bis(3,5-di-t-butyl-salicylidene))-Mn(I-
3.1. Synthesis of [MIII-(sulfonato-salen)(Cl)]2- and LDH-[M-salen]. The salen ligand successfully reacted with cobalt(II) chloride tetrahydrate in aqueous solution to give dianionic cobalt(III), [Co(Cl) salen]2-. The compounds were water soluble. The host hydrotalcite-like material, Zn/ Al-[C6H5COO], was prepared by coprecipitation of a solution of zinc and aluminum nitrates with an aqueous solution of NaOH and benzoic acid.7 The ion [Co(Cl) salen]2- [M ) Co], was intercalated into the Zn(II)/Al(III) layered double-hydroxide at room temperature from an aqueous medium by exchange of the benzoate ion. The extent of exchange of the Co-salen complex led to a final weight loading of 2.21% Co in the LDH, corresponding to 0.375 mmol g-1cat. This compares with values of 2.03% (0.369 mmol g-1cat) for Mn7,8 and 2.63% (0.471 mmol g-1cat) for Fe.9 The variation in extent of exchange on the basis of the nature of the central metal cation is unexpected, as it might be envisaged that the equilibrium-controlled exchange process would be dictated by the relative affinities of the sulfonate groups for the brucite layer cations relative to the benzonate anion.
LDH-hosted Sulfonato-salen Catalysts for Epoxidation
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14127
Figure 4. Labeling of atoms of the [M(Cl)salen]2- complex ions.
TABLE 3: Comparison of the Epoxidation of Cyclohexene and Dicyclopentadienea over Mn-, Fe-, and Co-based Catalysts alkene cyclohexene
catalyst
epoxide yield TOF selectivity (%) (%)b (h-1)
LDH-[Mn(Cl)(L)] LDH-[Fe(Cl)(L)] LDH-[Co(Cl)(L)] dicyclopentadiene LDH-[Mn(Cl)(L)]
74.0 70.0 64.0 A 81.0 B 19.0 LDH-[Fe(Cl)(L)] A 80.0 B 20.0 LDH-[Co(Cl)(L)] A 81.0 B 19.0
62.2 61.6 53.1 78.6 18.4 73.6 18.4 74.5 17.5
121.2 85.2 97.5 150.0 111.7 136.3
a Reaction conditions: ca. 1 mmol of substrate, 2 mmol of pivalaldehyde, 10 mL of toluene, 0.05 g of catalyst, 14.5 psi of molecular oxygen, and 298 K. b Turnover frequency calculated as (mol product)/(mol metal catalyst)/h-1.
TABLE 4: Catalytic Performance of Used LDH-[Co(Cl)(L)] Catalyst in the Epoxidation of Cyclohexenea run
conversion (%)
epoxide selectivity (%)
1 2 3
83.0 83.0 83.0
64.0 64.0 63.0
a Reaction conditions: 1 mmol of substrate, 2 mmol of pivalaldehyde, 10 mL of toluene, 0.05 g of catalyst, 14.5 psi of molecular oxygen, and 298 K.
3.2. Characterization of [CoIII(Cl)salen]2- and LDH[Co(Cl)salen] Catalysts. As observed in the cases of [Mn(Cl)salen]2-7 and [Fe(Cl) salen]2-,9 the FTIR spectra of [Co(Cl)(L)]2- showed strong bands at ca. 1114 and 1036 cm-1 due to the antisymmetric and symmetric stretching modes, respectively, of the SO3- unit,37 which are slightly red-shifted in comparison to the free ligand (1110 and 1035 cm-1). Complex
1 shows a band at 1560 cm-1 due to ν(C-O) vibrations, which is blue-shifted (38 cm-1) with respect to the free ligand (1522 cm-1), indicating that the Co(III) is bonded through the two N and O donor atoms of ligand as a tetradentate ONNO functionalities.9 Selected details (Table 1) from the FTIR spectra of the intercalated compound shows bands at ca. 1117 and 1029 cm-1, due to the presence of the sulfonato group, and at 577 cm-1 for ν(Co-O) vibrations, which is not apparent in the spectrum of the LDH-[C6H5COO], thereby qualitatively confirming the successful exchange of the Co-salen complex into the layered double-hydroxide. Note that absorption features arising from the benzoate anion are expected to appear in spectra of the solid before and after exposure to the salen complex due to the incomplete nature of the exchange process. Consequently, bands due to, for example, the carbonyl features of the benzoate anion were not used to monitor the efficiency of the exchange process. Analysis of the XRD pattern of the exchanged material (Figure 1) shows that the basal spacing increased from 15.22 Å for LDH-[C6H5COO] to 19.20 Å after intercalation of the Co-salen complex. This infers a gallery height of 14.5 Å following subtraction of the thickness of the brucite layers (4.7 Å). This increase in the gallery height provides further evidence for the successful intercalation of the Co-salen complex. It is interesting to note that the gallery height exhibited by the LDH-[Co(Cl)salen] was greater than the 14.1 Å shown by LDH-[Mn(Cl)salen] 7,8 but was less than the 15.83 Å found for the LDH-[Fe(Cl)salen].9 Choudary et al.,38 using methodology developed in our laboratory7,8 for the intercalation of sulfanato Mn-salen, found that the LDH diffraction pattern in the 2θ ) 3-65° range was largely unaffected by the incorporation of the Mn-salen complex and suggested that the intercalation proceeded homogeneously by localized structural distortion of the layers. The extent of exchange into the LDH was 0.076 mmol g-1,38 which is ca. 10 times less than the loadings we report here.
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Elemental analysis (Table 1) for the intercalated Co-salen catalyst suggests a unit formula [Zn2.15Al0.84(OH)5.98][Co(Cl)salen]0.17[C6H5COO]0.50 · 2H2O. The ratio [Al/(Zn + Al)] in both the final catalyst and the LDH-[C6H5COO] are almost identical, indicating that leaching did not occur of either ZnII or AlIII during the exchange procedure. Electronic spectra (in Nujol mulls) of the free Co-salen and LDH-hosted compounds are shown in Figure 2. The [Co(Cl)salen]2- displays bands at 822, 587, 480, 401, and 275 nm (Figure 2a). The electronic spectrum (Nujol mull) of the [Co(Cl)salen]2- displays a band at ca. 820 nm, which can be assigned as the 1T1g r 1A1g transition in the Oh point group.39 The LDH-[C6H5COO] shows only two band maxima at 277 and 234 nm. After exchange, the LDH hosted compound showed bands at 824 and 401 nm (Figure 2b). The detection of similar features in the hosted complex as in the free complex indicates that during intercalation no significant changes of the cobalt(III) co-ordination center took place. 3.3. Structure Calculations of [M(Cl)salen]2- Complexes. Two projection views of the [M(Cl)salen]2- complex ions for Mn(III), Fe(III), and Co(III) from the B3-LYP/LanL2DZ calculation are shown in Figure 3. A selection of the computed bond distances, interbond angles, and dihedral angles surrounding the central metal ions is provided in Table 2 using the labeling of atoms provided in the structure shown in Figure 4. A complete listing of data is provided in the Supporting Information. These calculations reveal that the free M(III) complexes have dimensions that can be described as a box with sides of lengths 16.064 × 9.460 × 4.994 Å (Mn), 16.164 × 9.429 × 5.329 Å (Fe), and 15.631 × 9.538 × 3.997 Å (Co). Given that the minimum gallery height exhibited by the complexes as determined by XRD was 14.1 Å, it is clear that the complexes are not orientated parallel (i.e., flat lying), nor are they expected to be placed perpendicular to the brucite layers with the sulfonato groups orientated into the gallery space (i.e., with the longest axes parallel to the galleries), which would not maximize interactions between the anions of the complex and the positive charges of the brucite layers. The latter scenario would be predicted to lead to gallery height spacings less than 10 Å, but the data of which is inconsistent with the information provided by the XRD patterns (Figure 1(a).7-9 Assuming that the complex ions fit into the galleries in an orientation that maximizes interactions between the anionic sulfonato groups and the brucite layers, then it would be expected that the gallery height would increase in the order Co(III) < Mn(III) < Fe(III). This is inconsistent with the order of gallery heights obtained from XRD data, that is, Mn (III) < Co(III) < Fe(III), and suggests that these complex ions may adopt an orientation in which they lie diagonally within the gallery spacing, at an angle less than 90° to the layers. This is supported by the fact that in each case the calculated molecular dimensions are greater than the dimensions of the gallery heights.
Bhattacharjee et al.
Varying the central metal ion also impacted upon the position of atoms in the near vicinity of the active catalytic center which might be expected to influence the activity and or selectivity of the catalyst. For example, in addition to the role of the two stereogenic carbon centers, asymmetric induction is believed to result from catalyst twist due to the puckered M(NCC′N′) ring40 and asymmetric fold along the NO edges of the catalyst due to the axial-ligand strain.41 It might be envisaged that this sensitivity to local geometry might also be reflected in differences in the manner in which substrate molecules are accommodated by the active center and consequently reflected in differences in activity and selectivity. Entries 4, 5, and 6 in Table 2, indicate that replacing the central metal ion had little effect on the bond angles forming the pentagon containing the metal ion and the two chiral centers. However, the dihedral angles across the metal cation, as illustrated by entry 2 in Table 2, indicate a variation in the extent of puckering of the central atom as determined by the electronic properties of the metal ion and which results in a flattening of the unit in the order Fe < Mn < Co. The increasing approximation to 180° is particularly clear in the case of Co as seen in the terminal projection as shown in Figure 3. 3.4. Epoxidation over LDH-[M(Cl)salen] Catalysts. The epoxidation of cyclohexene and dicyclopentadiene was studied in the presence of dioxygen and pivaldehyde over LDH-[M(Cl) salen] {M ) Mn, Co, or Fe} catalysts at room temperature with results summarized in Tables 3 and 4. Blank tests carried out under the same reaction conditions confirmed that cyclohexene and dicyclopentadiene conversion were negligible (5.5% and 4.5%, respectively) in the presence of the LDH-[C6H5COO] (i.e., in the absence of the metal-salen complex). Results (Table 3) confirm that all catalysts were active for the epoxidation of cyclohexene and dicyclopentadiene under the conditions employed. Under these reaction conditions, cyclohexene gave only two products (eq 1), cyclohexene oxide and 2-cyclohexen-1-one, and these were produced over all LDH-[M(Cl)(L)] catalysts, independent of the selection of central metal ion. No evidence was found for the presence of 2-cyclohexen-1-ol or the diols that may have arisen from hydrolysis of the epoxide. The best selectivity toward cyclohexene oxide was 74% for the case of the LDH-[MnCl)(L)], with the Co-based sample showing the poorest (64%) epoxide selectivity (and yield). Because the conversion levels were similar in both cases (ca. 83%), this suggest that Co is intrinsically less selective than Mn toward epoxidation and, as indicated by the values for the TOF (Table 3), is also less active for reaction of the cyclohexene. The epoxidation of dicyclopentadiene (a mixture of endo and exo) using LDH-[M(Cl)(L)] as catalyst and molecular oxygen as oxidant in combination with pivalaldehyde formed a mixture of monoepoxides, A and B (eq 2). In the presence of LDH-[M(X)(L)], the epoxides were formed in a ratio of ca.
LDH-hosted Sulfonato-salen Catalysts for Epoxidation 4:1 in favor of A, with selectivities for A falling in the range 78.6-74.5% and for B between 18.4 and 17.5%. In terms of selectivity there was virtually no difference between Mn-, Fe-, and Co-containing samples; however, the order of activity was Mn > Co > Fe, as indicated by the TOF values (Table 3). This order was identical to that found in the epoxidation of cyclohexene. The stability of the catalysts was studied by performing repeat epoxidation reaction cycles using the same conditions as described above but using cyclohexene as substrate. At the end of each reaction cycle, the catalyst was recovered by filtration, washed with toluene, dried, and reused. The results are shown in Table 4 for Co-based catalyst after reuse up to three times. Similar results were found for both Mn- and Fe-based catalysts tested for the same reaction. The conversion and selectivity were almost identical irrespective of the number of cycles performed. No evidence of leaching or decomposition of the catalyst complex was observed for Mn or Fe or Co during the catalysis reaction, and metal ions were not detected by atomic absorption spectroscopic measurement of the liquid reaction mixture after the reaction. The FTIR spectrum of the solid catalyst after reuse was indistinguishable from the fresh catalyst. 3.5. Influence of Central Metal Substituents on the Structural and Catalytic Activity Aspects of the LDH-hosted Compound. The ions [M(Cl)(L)]2- [M ) Co], were intercalated into the Zn(II)/Al(III) layered double-hydroxide at room temperature from an aqueous medium by exchange of the benzoate ion. This method results in a partial substitution of benzoate ions by M-salen complexes. The X-ray power diffraction of LDH-[M(X)(L)] showed that the basal spacing of the [M(Cl)(L)]2- [M ) Mn, Fe, and Co] ions differed from each other. The basal spacing of the LDH-[M(Cl)(L)] catalyst increased in the order LDH-[Mn(Cl)(L)] < LDH-[Co(Cl)(L)] < LDH-[Fe(Cl)(L)]. Results indicate that the physical dimensions of the different complexes in the LDH host, as indicated by the range of gallery heights, is not directly related to the catalytic activity for epoxidation reaction (Tables 3 and 4) and, in fact, give an inverse correlation. This would suggest that the rate of reaction is not being controlled by diffusion limitations of the substrates into the exchanged salen units held between the layers of the LDH. Despite clear differences in the geometries of atoms surrounding the central catalytically active metal ion (Figure 3), selectivities in reactions involving dicyclopentadiene were relatively insignificant (Table 3) and, in fact, were more pronounced for reactions involving the structurally simpler cyclohexene as substrate. Interestingly, the TOF values are greater for the bulkier dicyclopentadiene than for the cyclohexene. These finding would suggest that the exact arrangement of atoms surrounding the active metal may have a lesser influence in determining the nature of the catalyst-substrate intermediate than is envisaged in cases where prochiral molecules are involved.40,41 Differences in reaction rates appear to be directly related to the selection of metal ion, independent of the nature of the substrate, with the order Mn > Co > Fe, and probably reflects a more appropriate orbital availability arising from occupancy by d electrons due to spin states that facilitate interaction with the olefinic double bond. 4. Conclusions The sulfonato-salen-MIII (M ) Mn, Fe, Co) complexes after intercalation into Zn/Al-LDH have been characterized and tested for their epoxidation activity using cyclohexene and dicyclopentadiene using dioxygen at atmospheric pressure and at
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14129 298 K. All catalysts showed good activity and selectivity. In the reaction of cyclohexene, the epoxide selectivity increased as follows: Co < Fe ≈ Mn, whereas, in the case of dicyclopentadiene, the epoxide selectivity was almost identical and showed no dependence on the nature of the metal center. The Mn-based catalysts displayed the highest activity (TOF) irrespective of the substrate molecule studied. Density functional theory calculations show that the nature of the metal ion significantly modifies the geometry of the local environment surrounding the catalytically active site, although the impact of these differences was not significant for the reactions studied. The differences in the calculated dimensions of the complexes containing different metal ions were not consistent with the gallery height spacings as indicated by XRD patterns, suggesting that the exchange complexes take up an orientation that is not perpendicular to the brucite layers within the LDH. Supporting Information Available: A detailed atom number scheme and calculated bond angles and distances for the salen complexes are provided. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Hoelderich, W. F.; Kollmer, F. In SPR, Catalysis; RSC: London, 2002; vol 16, p 43. (2) Hutchings, G. J. Chem. Commun. 2008, 1148. (3) Liu, Q.; Bauer, J. C.; Schaak, R. E.; Lunsford, J. H. Appl. Catal. A 2008, 130, 339. (4) Yamada, Y.; Imagawa, K.; Nagata, T.; Mukaiyama, T. Chem. Lett. 1992, 2231. (5) Kureshy, R. I.; Khan, N. H.; Abdi, S. H. R.; Iyer, P. J. Mol. Catal. A: Chem. 1997, 91, 124. (6) Watanabe, Y.; Yamamoto, K.; Tatsumi, T. J. Mol. Catal. A: Chem. 1999, 145, 281. (7) Bhattacharjee, S.; Anderson, J. A. Chem Commun., 2004, 554. (8) Bhattacharjee, S.; Dines, T. J.; Anderson, J. A. J. Catal. 2004, 225, 398. (9) Bhattacharjee, S.; Anderson, J. A. J. Mol. Catal. A: Chem. 2006, 249, 103. (10) Kasyanm, L. I. Russ. Chem. ReV. 1998, 67, 263. (11) (a) Kotov, S. V.; Kolev, T. M.; Georgieva, M. G. J. Mol. Catal. A: Chem. 2003, 195, 83. (b) Wang, G. W.; Chen, G.; Luck, R. L.; Wang, Z.; Mu, Z.; Evans, D. G.; Duan, X. Inorg. Chim. Acta 2003, 195, 3223. (c) Ding, Y.; Gao, Q.; Li, G.; Zhang, H.; Wang, J.; Yan, L.; Suo, J. J. Mol. Catal. A: Chem. 2004, 218, 161. (d) Qi, J.; Li, Y.; Zhaou, Z.; Yeung, C.; Chan, A. S. C. AdV. Syth. Catal. 2005, 347, 45. (12) Raja, R.; Sankar, G.; Thomas, J. M. Chem. Commun. 1999, 829. (13) Fraile, J. M.; Garcia, J. I.; Mayoral, J. A.; Vispe, E. Appl. Catal., A 2003, 245, 363. (14) Venturello, C.; Aloisio, R. D’.; Bart, J. C. J.; Ricci, M. J. Mol. Catal. 1985, 32, 107. (15) Venturello, C.; Aloisio, R. D’J. Org. Chem. 1998, 53, 1553. (16) Matoba, Y.; Inone, H.; Akagi, J.; Okabayashi, T.; Ishii, Y.; Ogawa, M. Synth. Commun. 1984, 14, 865. (17) Ishii, Y.; Yamawaki, K.; Ura, T.; Yamada, H.; Yoshida, T.; Ogawa, M. J. Org. Chem. 1998, 53, 3587. (18) Duncan, D. C.; Chambers, R. C.; Hecht, E.; Hill, C. L. J. Am. Chem. Soc. 1995, 117, 681. (19) Jacobsen, E. N.; Zhang, W.; Muci, A. R.; Ecker, J. R.; Dent, L. J. Am. Chem. Soc., 1991, 113, 7063. (20) Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T. Tetrahedron: Asymmetry, 1991, 2, 481. (21) Li, C. Catal. ReVs, 2004) , 46, 419. (22) Li, C.; Zhang, H.; Jiang, D.; Yang, Q. Chem. Commun, 2007, 547. (23) Balezao, C.; Garcia, H. Chem ReVs., 2006, 106, 3987. (24) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc. 1986, 108, 2309. (25) Samsel, E. G.; Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc. 1985, 107, 7606. (26) Yoon, H.; Burrows, C. T. J. Am. Chem. Soc. 1988, 110, 4087. (27) Wu, G.; Wang, X.; Li, J.; Zhao, N.; Wei, W.; and Sun, Y. Catal. Today, 2008, 131, 402. (28) Cervilla, A.; Corma, A.; Fornes, V.; Llopis, E.; Palanca, P.; Rey, F.; Ribera, A. J. Am. Chem. Soc. 1994, 116, 1595. (29) Cervilla, A.; Corma, A.; Fornes, V.; Llopis, E.; Perez, F.; Rey, F.; Ribera, A. J. Am. Chem. Soc. 1995, 117, 6781.
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