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Characterization of oxovanadium (IV)–Schiff-base complexes and those bound on resin, and their use in sulfide oxidation
Inorganica Chimica Acta 357 (2004) 2237–2244 www.elsevier.com/locate/ica
Characterization of oxovanadium (IV)–Schiff-base complexes and those bound on resin, and their use in sulfide oxidation Ryuji Ando, Takeyoshi Yagyu, Masunobu Maeda
*
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Received 9 August 2003; accepted 3 December 2003 Available online 29 January 2004
Abstract Oxovanadium (IV)–Schiff-base complexes and those bound on Merrifield resin as a polymer support were prepared for their characterization and their use as catalyst in oxidation of methyl phenyl sulfide. Schiff bases were prepared with use of salicylaldehyde and 2,4-dihydroxybenzaldehyde as the aldehydes and 2-aminoethanol, L-phenylalaninol, L-histidinol, and L-phenylalanine as the counterparts. Oxovanadium (IV) complexes made up of these Schiff bases and those bound on the resin were spectroscopically characterized. The polymer-supported Schiff-base complexes in the presence of tertiary-butylhydroperoxide converted the sulfide to the corresponding sulfoxide in 80–90% yield in CDCl3 in 90 min. They afforded slightly lower rates of oxidation than the corresponding monomeric complexes. They converted the sulfide in a stereoselective manner yielding the sulfoxide in enantiomeric excess (the highest value of 40%). The polymer-supported complexes and the corresponding monomers achieved almost the same enatiometric excesses with each other. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Polymer-supported Schiff-base complex; Sulfide oxidation; Coordination geometry in the equatorial plane; Peroxo complex
1. Introduction Oxovanadium complexes have been shown to catalyze a variety of reactions such as the oxidation of alcohols and inorganic compounds like halides and sulfur oxides, the epoxidation of alkenes and allyl alcohols, and the conversion of sulfides to sulfoxides and sulfones [1–9]. These catalytic reactions have been performed chiefly in homogeneous solution system. The homogeneous system has the advantage of the high rate of catalytic reaction, whereas some disadvantages include the difficulty in separation of the reaction product from the catalyst. Heterogeneous solid–solution phase catalyses, which take advantage of, e.g., polymer-supported *
Corresponding author. Present address: Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Tel.: +81-52-735-5221; fax: +81-52-7355442. E-mail address: [email protected] (M. Maeda). 0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.12.031
oxovanadium complexes, have been recently investigated in order to alleviate some disadvantages of the homogeneous system [10–26]. Although the heterogeneous system is generally inferior to the homogeneous one in the rate of catalysis, the former has features like the easy separation of product from catalyst by filtration and the feasibility of recycling of catalyst. The clarification of the correlation between the coordination geometry of the complex and the catalytic activity will be one of the requisites for designing and developing effectively the catalyst with high catalytic activity. However, since the insolubility in solvent characterizes the polymer-supported complex, it will be difficult to determine the coordination geometry. In the epoxidation of alkenes by [VO(salen)](N ,N 0 ethylenebis(salicylideneiminato)oxovanadium (IV)) and [VO(salen derivative)] complexes with dioxygen as an oxygen atom donor, the complexes of salen derivatives, which were derived by introduction of electron-withdrawing nitro groups into the aromatic rings of the
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ancillary ligand salen, expressed catalytic actions superior to that of the complex of salen [29]. These findings suggest that the introduction of the substituent group probably changed redox potentials of V (V)/V (IV), which resulted in improvement of the catalysis. Thus, it is worthwhile to examine the effect on the catalysis of the substituents introduced into the ligands of oxovanadium complexes. However, it was reported that the introduction of substituents like NO2 with strong electron-withdrawing abilities caused the polymerization of oxovanadium complexes by association of V@O units [9,28]. Polymerized complexes, which are probably sparingly soluble in solvent [29], are inadequate for homogeneous catalysis as well as for determination of the coordination geometry. Thus, in this work, in order to sufficiently examine the effects of substituents on catalytic actions, as well as to overcome the insolubility and to avoid the possibility of the loss of catalytic activity due to the polymerization, we have employed insoluble polymer-supported oxovanadium complexes, wherein the polymerization of the complexes probably does not occur, because the motion of polymer-bound oxovanadium complexes should be restricted and they are far apart from each other. Oxovanadium (IV)–tridentate Schiff-base complexes and the corresponding oxovanadium (IV) complexes supported on Merrifield resin as a polymer support were prepared and their coordination geometries were determined by spectroscopic methods. Catalytic actions in asymmetric oxidation reaction of sulfide such as the rate of catalysis, the extent of catalysis, and the efficiency of recycling were examined for both soluble complex- and polymer-supported complex-systems. Table 1 Schiff base ligands R2
OH
C
R3
N R1
L1a L2a L3a
R1
R2
R3
H CH2 –C6 H5
H H H
OH OH OH
H OH OH OH
CO2 H OH OH OH
OH
CO2 H
H2 C
NH N
L4a L1b L2b L3b
CH2 –C6 H5 H CH2 –C6 H5 H2 C
NH N
L4b
CH2 –C6 H5
Schiff bases derived from salicylaldehyde and 2,4-dihydroxybenzaldehyde as the aldehydes and 2-aminoethanol, L-phenylalaninol, L-histidinol, and Lphenylalanine as the counterparts are collected in Table 1. The coordination geometries and catalytic reactions of the Schiff-base complexes of oxovanadium (IV) prepared from the ligands of group a (L1a–L4a) and group b (L1b–L4b) were first determined to compare with those of the polymer-supported Schiff-base complexes. In order to prepare the polymer-supported complexes, the Schiff bases of group b, into which the OH substituent meta to the phenolate oxygen was introduced, were employed to bind their Schiff-base complexes of oxovanadium (IV) to Merrifield resin by utilizing the reaction of the substituent with ClÕs in the resin.
2. Experimental 2.1. Reagents All the chemicals were of reagent grade and used without further purification. 2.2. Preparation of oxovanadium complexes [(VO(salae))2 ] (complex 1a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) and 2-aminoethanol (0.35 g, 5.0 mmol) dissolved in dichloromethane (50 mL) was stirred for 1 h after addition of anhydrous magnesium sulfate (1.0 g) and then, filtered. A solution of [VO(acac)2 ] (acacH ¼ acetylacetone) (1.0 g, 3.8 mmol) in dichloromethane (50 mL) was introduced into the filtrate. The solution was stirred for 3 h at room temperature under nitrogen atmosphere and then, concentrated with a rotary evaporator. Diethyl ether (50 mL) was added to the concentrate to yield pale blue precipitates (yield: 0.33 g (38%); ESI-MS: m=z 460.0). [(VO(salpheol))2 ] (complex 2a). The complex was prepared with use of L-phenylalaninol (0.76 g, 5.0 mmol) according to the same procedures as those employed for 1a (yield: 0.54 g (44%); ESI-MS: m=z 640.1). [VO(salhisol)(EtOH)] (complex 3a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) in ethanol (20 mL) was introduced to a solution of L-histidinol dihydrochloride (1.1 g, 5.0 mmol) dissolved in water (10 mL) and then, sodium carbonate (0.84 g, 10 mmol) was added. The solution mixture was stirred for 3 h at room temperature. It was filtered after dehydration by addition of magnesium sulfate. A solution prepared by addition of a solution of [VO(acac)2 ] (1.0 g, 3.8 mmol) in ethanol (50 mL) to the filtrate was stirred for 5 h at room temperature under N2 . The concentration of the solution with a rotary evaporator yielded reddish brown
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precipitates, which were washed with diethyl ether (2 5 mL) (yield: 0.63 g (51%); ESI-MS: m=z 356.2). [VO(salphe)(H2 O)] (complex 4a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) in ethanol (20 mL) was added to a solution of L-phenylalanine (0.83 g, 5.0 mmol) in water (20 mL) and then, sodium acetate (0.82 g, 10 mmol) was added. The solution mixture was stirred for 30 min, and then, a solution of VOSO4 (1.0 g, 3.8 mmol) in water (10 mL) was introduced slowly into the solution. The stirring of the solution thus prepared for 30 min gave pale purple precipitates (yield: 0.80 g (60%); ESI-MS: m=z 352.1). [(VO(OH salae))2 ] (complex 1b). A solution of 2,4dihydroxybenzaldehyde (2.4 g, 20 mmol) and 2-aminoethanol (1.2 g, 20 mmol) dissolved in acetone (150 mL) was stirred for 1 h under N2 after addition of magnesium sulfate (1.0 g) and filtered. A solution prepared by addition of a solution of [VO(acac)2 ] (4.0 g, 15 mmol) in dichloromethane (50 mL) to the filtrate was stirred for 2 h at room temperature under N2 . Pale purple precipitates were filtered off and washed with diethyl ether (2 5 mL) (yield: 1.8 g (49%); ESI-MS: m=z 492.0). [(VO(OH salpheol))2 ] (complex 2b). The complex was prepared with use of L-phenylalaninol (1.5 g, 10 mmol) according to the same procedures as those employed for 1b (yield: 1.6 g (62%); ESI-MS: m=z 672.1). [VO(OH salhisol)(EtOH)] (complex 3b). The complex was prepared by use of 2,4-hydroxybenzaldehyde (1.2 g, 10 mmol) according to the same procedures as those employed for 3a (yield: 1.7 g (66%); ESI-MS: m=z 372.7). [VO(OH salphe)(H2 O)] (complex 4b). The complex was prepared with use of 2,4-hydroxybenzaldehyde (1.2 g, 10 mmol) according to the same procedures as those employed for 4a (yield: 1.6 g (58%); ESI-MS: m=z 368.3). [mer-VO(salae)] (PSVC 1). A solution of N,Ndimethylformamide (DMF) (50 mL) containing VO(OHsalae) (1.03 g, 4.20 mmol), Merrifield resin (1.00 g; 1% DVB, 38–75 lm (2000–400 mesh), Cl rate (1.1 mmol/g), and sodium hydroxide (0.0800 g, 2.00 mmol) were stirred for 96 h at 110 °C under N2 . The reaction product was filtered off and washed with successive, DMF (2 5 mL), water (2 5 mL), and acetone (2 5 mL) (yield: 1.28 g, mole percentage of the substitution of complex 1a for the Cl in the resin: 71.8%). The mole percentage was estimated from the difference in weight between the raw material resin and the resin which reacted with complex 1a, based on the fact that the substitution reaction of complex 1a for the Cl in the resin occurs at a 1:1 mole ratio. [mer-VO(salpheol)] (PSVC 2) (yield: 1.41 g based on 1.41 g of 2b, mole percentage of the substitution: 84.3%); [mer-VO(salhisol)] (PSVC 3) (yield: 1.30 g based on 1.45 g of 3b, mole percentage of the substitution: 72.2%); [mer-VO(salphe)] (PSVC 4) (yield: 1.41 g based on 1.55 g of 4b, mole percentage of the substitution: 78.4%) were
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prepared according to the same procedures as those for PSVC 1. The amount of the resin was 1.00 g. 2.3. Measurements EPR spectra of the monomeric Schiff-base complexes in DMF and of the powdered polymer-supported complexes were recorded at liquid nitrogen temperature (77 K) using 5 mm o.d. quartz tubes on a JES-RE1X spectrometer (JEOL DATUM). The spectrometer was operated at X-band (8–12 GHz) with a microwave power of 1 mW. Infrared spectra of complexes in the solid state were recorded as KBr pellets using a FT/IR410 (JASCO). Infrared spectra of complexes in solution were measured by interposing the solutions between two KBr plates. UV–Vis spectra were measured by using a capped quartz cell of a light-pass length of 1 mm with a UV-1600PC spectrometer (Shimadzu). Magnetic susceptibilities of powdered samples were measured at 20 °C by the Evans method with a magnetic susceptibility balance (Sherwood Scientific). Diamagnetic corrections were applied in the usual manner by use of PascalÕs constants. ESI-mass spectra were measured with an LCT (ESI-TOF) (Micromass). The concentration of an oxovanadium complex dissolved in dichloromethane was kept at 50 lmol dm3 . The solution was sprayed at the rate of 600 lL/s with a microsyringe. Sodium iodide was used as a standard sample. Data were analyzed using MassLynx Ver 3.5. The conversion yield (%) of the substrate of methyl phenyl sulfide to the corresponding sulfoxide was determined at 18 °C with 1 H NMR spectra. The spectra were recorded on a 300 MHz GEMINI 2000 (VARIAN). An oxovanadium complex (0.050 mmol) was dissolved in chloroform-d (CDCl3 ) (2 mL) and then, the internal standard of 1,3,5-trimethoxybenzene (0.050 mmol) was added (solution (1)). A polymer-supported complex (0.050 mmol) was suspended in CDCl3 (2 mL), followed by addition of the internal standard (solution (p1)). The solution (2) containing both the substrate (0.50 mmol) and the peroxide of aqueous tertiary-butylhydroperoxide (tert-BuOOH) (70 vol%) (0.50 mmol) were prepared by its mixing in CDCl3 (2 mL). Either solution (1) or (p1), and (2) were mixed with each other, and then, stirred constantly during the reaction. The 1 H NMR spectra were measured with the elapse of time. The quantitative measurements of the amount of the sulfoxide were carried out by comparing the integrated value of the area under the peak for methyl hydrogen (d 2.74 (s, 3H, CH3 )) of the sulfoxide with that for methoxy hydrogen (d 3.87 (s, 9H, OCH3 )) of the internal standard. The enantiomeric excess (ee) was determined separately as follows. The substrate (0.50 mmol) and the peroxide (0.50 mmol) were introduced into a solution of an oxovanadium complex (0.050 mmol) dissolved in
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CHCl3 (4 mL), and the mixture was incubated at 20 °C for 1 h for the complexes and 1.5 h for the polymers. The reaction mixture was evaporated to dryness. The product from methyl phenyl sulfide was extracted from the sulfide residue with diethyl ether. The extract, which was evaporated to dryness, was diluted with the eluent of hexane/2-propanol (9:1) to obtain a 0.1% solution. The solution was used for high-performance liquid chromatography (HPLC) on a chromatograph (LC-5A, Shimadzu) equipped with a chiral column (CHIRALCEL OB-H, Daicel Chem.), a UV spectral detector (SPD-6A, Shimadzu), and a peak area calculator (Chromatopack C-R4A, Shimadzu). The column was eluted with hexane/2-propanol (9:1) at a flow rate of 0.2 mL/min. The HPLC eluate was monitored at 254 nm. The polymer-supported complexes were filtered off through a glass filter (G4) after the first oxidation cycle, washed with acetone and ethanol, and dried for the next cycle.
3. Results and discussion 3.1. Characterization of complexes (1a–4a, 1b–4b) in the solid state, dichloromethane, and N,N-dimethylformamide (DMF) IR data assigned to V@O stretching (ms (V@O)) frequencies and V–O–V antisymmetric stretching (ma (V– O–V)) frequencies for the eight complexes (1a–4a and 1b–4b) (the numbers correlate with those of the ligands (L1a–L4a and L1b–L4b in Table 1, from which the complexes were prepared), in the solid state are given in Table 2. Complexes 1a and 2a in the solid state exhibited IR bands at 892 and 869 cm1 , respectively, which are probably assigned to the ma (V–O–V) values resulting from the V–O (alkoxy oxygen)–V bond in light of the previous results that the bands around 776–898 cm1 observed for binuclear aminoethanol–Schiff-base complexes of oxovanadium (IV) in the solid state were ascribed to the corresponding frequencies for V–O (alkoxy oxygen)–V [27]. The complexes were diamagnetic. These findings probably indicate that the complexes are binuclear in the solid state. These complexes dissolved in
non-polar solvent dichloromethane were EPR-silent. It is appropriate to consider that the formation of dimers caused the EPR-silence [27,29,30] and thus, the coordination geometry in the solid state is maintained also in dichloromethane. The complexes in the solid state exhibited the ms (V@O) a little higher than 990 cm1 . It has been reported that oxovanadium complexes with coordination numbers of 5 and 6 have the ms (V@O) higher than and lower than about 990 cm1 , respectively, and this figure provides an available index for discriminating the coordination number of oxovanadium complexes [9,31,32]. In consideration of the criterion and of the fact that the complexes were prepared in non-polar dichloromethane, it is likely that no solvent is coordinated in the axial position. The experimental m=z values for 1a (460.0) and 2a (640.1) agree with the values 460.2 and 640.5, respectively, calculated by postulating these compositions and geometries. The UV–Vis spectra of 1a dissolved in polar solvent DMF were measured with the passage of time. They are depicted in Fig. 1. It is apparent that the band around 330 nm, which is probably ascribed to an LMCT resulting from the coordination of DMF due to the decomposition of the dimer to the monomer, increased in intensity with the passage of time. The weak and broad band around 500 nm, which is probably assigned to an LMCT attributable to the coordination of DMF in the axial position, increased in intensity with the elapse of time. That the spectra in-
Fig. 1. UV–Vis spectra of complex 1a in DMF measured every 20 minutes. Concentration of the complex is 1.0 103 mol dm3 .
Table 2 EPR parameters of complexes in DMF and IR frequencies of complexes in the solid state
1a 2a 3a 4a 1b 2b 3b 4b
Ak ( 104 cm1 )
A? ( 104 cm1 )
gk
g?
ms (V@O) (cm1 )
ma (V–O–V) (cm1 )
162.5 162.2 163.0 168.4 161.9 163.3 162.7 169.7
61.0 61.2 60.8 62.5 61.3 60.7 61.0 63.0
1.960 1.962 1.961 1.959 1.962 1.962 1.961 1.959
1.988 1.989 1.989 1.987 1.990 1.987 1.990 1.986
991 993 978 999 962 965 942 987
892 869
890 885
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tersecting with each other around 370 and 400 nm shifted to longer wavelengths with the elapse of time and thus, exhibited no isosbestic point indicates that the simultaneous coordination of DMF molecules in both the equatorial and axial positions did not occur. The solutions of the complexes dissolved in DMF were stirred for 1h, and their EPR spectra were measured. The solutions exhibited the EPR-activity. Coordination geometries in the equatorial plane of the complexes dissolved in DMF were deduced by comparing the observed Ak values with those calculated from model compounds [33,34] according to the additivity rule, which has been extensively employed as a bench mark for identification of donor atoms in the equatorial plane of oxovanadium (IV) complexes. Since no Ak value for DMF coordination was available in the literature, an EPR spectrum of VOSO4 dissolved in DMF was measured and the Ak value was estimated to be 176.3 104 cm1 . The Ak value of 161.7 104 cm1 calculated for these complexes by postulating the coordination of a phenolate oxygen, an imine nitrogen of the Schiff base, an alkoxy oxygen, and an oxygen of the solvent agreed with the observed values for 1a and 2a. The IR spectrum of a DMF solution containing 1a, which was stirred for 1 h at room temperature, showed the ms (V@O) of 993 cm1 . According to the criterion stated above, it is likely that no solvent is coordinated to the axial position at that time. The solution of 1a dissolved in DMF was stirred for 8 h at 50 °C under nitrogen atmosphere, and concentrated in vacuo to obtain the precipitates. Their IR spectrum in the solid state represented the ms (V@O) of 971 cm1 in addition to that at 993 cm1 . These findings suggest that the solvent began to coordinate in the axial position with the elapse of time. No observation of the change in EPR spectra both 1 h and 8 h after the dissolution indicates that the same equatorial coordination geometry is maintained regardless of the elapse of time. It is appropriate to consider on the basis of the results described above that the binuclear complex, within which two vanadium atoms are bridged in the equatorial plane, decomposed to the monomer due to the coordination of the solvent in the equatorial plane, and then, the solvent coordinated in the axial position with the elapse of time. For the reason that complexes 3a and 4a were paramagnetic (leff ¼ 1:7 lB (3a), 1.7 lB (4a)) and no ma (V–O–V) was observed, it is likely that they are monomeric in the solid state. The observed Ak value of 163.0 104 cm1 for 3a in DMF indicates that the complex retains an equatorial coordination geometry analogous to those of 1a and 2a. However, 3a represented the low ms (V@O) of 978 cm1 , suggesting the axial coordination of perhaps an imidazole nitrogen. On the other hand, the Ak value of 167.8 104 cm1 observed for 4a in DMF agreed with the value of 168.4 104 cm1 calculated on the assumption of the coordination of a phenolate oxygen, an imine nitrogen of the
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Schiff base, a carboxylate oxygen, and a solvent oxygen in the equatorial plane; the equatorial coordination geometry is different from those for the other a complexes. The observed m=z values for 3a (356.2) and 4a (352.1) are consistent with the values 356.3 and 352.2, respectively, calculated based on these compositions and geometries. Complexes 1b–4b, the Ak values whereof were almost the same as those of the corresponding a complexes, represent the same equatorial coordination geometries as those for the a complexes. However, the b complexes yielded the lower ms (V@O) than the a complexes, although the values for both kinds of complexes tended to vary in parallel. According to the aforesaid index for distinction between penta- and hexa-coordination, it is said that the b complexes maintain the axial coordination in the solid state, in contrast to that for the a complexes. However, the occurrence of the axial coordination geometry due to the introduction of only the OH substituent with the electron-withdrawing ability meta to the phenolate oxygen seems improbable. In our previous paper [27], it has been reported that the introduction of Cl (Hammett para constant rp (Cl) ¼ 0.23 [35]) and NO2 (rp (NO2 ) ¼ 0.78 [35]) substituents with electron-withdrawing abilities into the position para to the phenolate oxygen of the Schiff-base in amino acid– Schiff-base complexes of oxovanadium (IV) reduced the ms (V@O) by ca. 20 and 50 cm1 , respectively, from ca. 1000 cm1 for the amino acid–Schiff-base complex, the ligand of which has no substituent. In light of the previous results, it is likely that the decrease in ms (V@O) for the b complexes is responsible for the electron-withdrawing ability of the OH substituent (Hammett meta constant rm (OH) ¼ 0.12 [35]). 3.2. Reaction of Schiff-base complexes with tert-BuOOH Since the conversion yield and enantiomeric excess in sulfide oxidation were measured in CDCl3 and CHCl3 , respectively, the change in UV–Vis spectra of complex 1a dissolved in CHCl3 , whereto tert-BuOOH was added, was observed with the passage of time. They are shown in Fig. 2. The appearance of the band around 450 nm indicates the formation of the peroxo complex, which was completed immediately after the addition. The complex was kept stable at least for 3 h. 3.3. Characterization of polymer-supported Schiff-base oxovanadium (IV) complexes (PSVC) The support of complexes b on the resin was confirmed by the decrease in intensity of the H2 C–Cl stretching frequency (1260 cm1 ) of the resin. Table 3 collects EPR parameters (A and g) and their V@O stretching frequencies (ms (V@O)) for polymersupported Schiff-base complexes PSVC 1–4 (the
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Fig. 3. Proposed coordination geometries of polymer-supported complex and its peroxo complex for PSVC 1 and 2.
Fig. 2. UV–Vis spectra of complex 1a in CHCl3 before (- - -) and 1 min (—) after addition of tert-BuOOH. Concentrations of the complex and tert-BuOOH are 1.0 103 and 1.0 102 mol dm3 , respectively.
numbers correlate with those of the ligands in Table 1) in the solid state. Complexes PSVC 1–4 exhibited the Ak values consistent with those for the corresponding b complexes monomeric in DMF (Table 2). This indicates that the b complexes are bound on the polymers with their equatorial coordination geometries being kept as they are. Complexes PSVC 1–4 showed practically the same ms (V@O) as those for the parent b complexes. However, since the polymer-supported complexes have no longer OH substituent on their ligands, it seems unlikely that both types of complexes retain the same axial coordination geometry. As was described in Section 2.2, the polymer-supported complexes were prepared in DMF under a severe condition of heating at ca. 110 °C for ca. 100 h. As mentioned also in Section 3.1, complex 1a heated in DMF at 50 °C exhibited the ms (V@O) of 971 cm1 in the solid state, which was ascribed to the coordination of a DMF molecule in the axial position. In consideration of these results, it is most likely that DMF molecules are coordinated in the axial position for PSVC 1, 2, and 4. However, it is not certain whether 3b retains the axial coordination of the solvent or an imidazole nitrogen, the coordination of which was proposed for 3a. This will be discussed in Section 3.4. The proposed coordination geometry of PSVC 1, 2, and 4 is illustrated in Fig. 3. It is known that the powdered monomeric complex of oxovanadium (IV) give rise to a single broad EPR band because of interactions between the complexes. In contrast, the EPR spectrum of the polymer-supported
Schiff-base complexes split into 16 bands, as was the case for the monomers in DMF. This suggests that the complexes bound on the polymer are so distant from each other that they are difficult to interact with each other. Thus, the polymerization of the Schiff-base complexes caused by V@O V@O interactions within the complexes containing the electron-withdrawing substituent like NO2 , which was aforementioned, is expected to be impeded on the resin. Polymer-supported complexes suspended in CHCl3 , to which tert-BuOOH of the 100-fold concentration of the complexes was added, were stirred for 30 min, during which the suspended particles changed from brown to red. Their EPR spectra were silent, indicating the oxidation for V (IV) to V (V) by forming the peroxo complex. 3.4. Oxidation of sulfide with peracid catalyzed by monomeric and polymer-supported Schiff-base complexes Yields (%) of the conversion of methyl phenyl sulfide to the corresponding sulfoxide versus reaction time are given in Table 4. No oxidation occurred in the absence of catalyst. As for the a complexes, complex 1a with no bulky substituent yielded the quickest rate of reaction. Complex 3a, within which an imidazole nitrogen was supposed to be coordinated in the axial position, showed the lowest reaction rate over 90 min. It has been proposed by several investigators [36–38] that the substrate sulfide may directly interact with the vanadium center of the Schiff-base peroxo complex at the open axial site or by replacing the originally bound labile ligand in the axial position and then, be oxidized to sulfoxide by the activated peroxide. If this proposal is accepted, a cause for the results for 3a is likely that the weak interaction of an imidazole nitrogen with the vanadium at the axial site resisted the substitution of the
Table 3 EPR parameters and IR frequencies of polymer-supported complexes in the solid state
PSVC PSVC PSVC PSVC
1 2 3 4
Ak ( 104 cm1 )
A? ( 10 4 cm1 )
gk
g?
ms (V@O) (cm1 )
163.0 162.1 161.5 170.7
61.3 60.5 61.3 63.9
1.960 1.958 1.960 1.957
1.990 1.989 1.988 1.988
974 979 975 972
R. Ando et al. / Inorganica Chimica Acta 357 (2004) 2237–2244 Table 4 Conversion yield (%) of sulfide to sulfoxide
1a 2a 3a 4a PSVC PSVC PSVC PSVC
1 2 3 4
10 min
30 min
85 42 31 40 55 35 20 32
99 80 54 75 80 62 51 59
Table 6 Enantiomeric excess (ee) of methy phenyl sulfoxide 60 min
90 min
93 78 88 88 78 69 74
96 88 96 92 85 80 81
substrate for the nitrogen, which resulted in lowering of the reaction rate. Complexes 2a and 4a with bulky substituents showed lower rates of reaction than 1a with no substituent. This is probably because it should be rather hard for the substrate to approach the vanadium center in the axial position due to interference of the bulky side chains. The bulkiness of the substituent for 3a probably led to the further lowering of the rate of the reaction. Complexes PSVC did not represent so significant inferiority to the a complexes in rate of reaction. PSVC 3 yielded the lowest rate of reaction. It was described in Section 3.3 that it is not clear whether the solvent or an imidazole nitrogen is coordinated in the axial position in PSVC 3. An imidazole nitrogen is expected to be coordinated more strongly to the axial site than the solvent. Thus, it is appropriate to consider that the lowering in rate of reaction for PSVC 3 resulted from the coordination of an imidazole nitrogen. As was the case for 1a, PSVC 1 with no substituent exhibited the highest rate of reaction. PSVC 1–4 were filtered off, washed with acetone and ethanol, and then, dried for recycling after the reaction time of 90 min. The yields (%) for run 2 (second cycle) and run 3 (third cycle) are given in Table 5. PSVC 4, whose Schiff base consists of the amino acid (L-phenylalanine), displayed a lowering of yield at run 2, whereas the other complexes, the Schiff bases of which are made up of the aminoethanols, showed much less loss of catalytic activity compared to that of PSVC 4 over the three runs. Enantiomeric excesses in oxidation catalysis of methyl phenyl sulfide in CHCl3 are given in Table 6. The polymer-supported complexes and the corresponding monomers afforded almost the same enetiomeric excesses (eeÕs) with each other. Complex 3a and the corTable 5 Recyclability of polymer-supported complexes
PSVC PSVC PSVC PSVC
1 2 3 4
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Run 1
Run 2
Run 3
92 85 80 81
90 89 81 68
85 85 73 53
ee (%) 1a 2a 3a 41 PSVC PSVC PSVC PSVC
1 2 3 4
racemic 25 42 10 racemic 22 40 13
responding polymer-supported complex PSVC 3 with six-coordinate octahedral geometry with the imidazole group being coordinated in the axial position achieved higher eeÕs than 2a and PSVC 2, and 4a and PSVC 4 having five-coordinate square-pyramidal geometry. A reason for the results may be that the bulky imidazole group coordinated in the axial position for 3a and PSVC 3 should induce some influence on the directional approach of the sulfide to the axial position of the vanadium. It is apparent that 4Õs, wherein a phenolate oxygen, an imine nitrogen of the Schiff base, a carboxylate oxygen, and a solvent oxygen are coordinated, yielded lower eeÕs than 2Õs, in which an alkoxy oxygen is coordinated in the place of a carboxylate oxygen for 4Õs. Since, in general, the basicities of carboxylate oxygens are much lower than those of alkoxy oxygens, it is appropriate to consider that the carboxylate oxygen probably coordinate more weakly to oxovanadium (IV) than the alkoxy oxygen. As a result, 4Õs are less stable than 2Õs. As was reported previously about the reactions of amino acid- and amino acid ester–Schiff-base complexes of oxovanadium (IV) with the peracid [39], the easier decomposition of 4Õs by the action of the peracid probably led to the lower eeÕs. Complex 1a and PSVC 1, the ligands wherein have no asymmetric center, yielded racemic mixtures.
4. Summary (1) Complexes 1a, 2a, 1b, and 2b were dimers both in the solid state and in non-polar solvent dichloromethane, whereas 3a, 4a, 3b, and 4b were monomers in the solid state. No axial coordination occurred in the solid state. The dimers dissolved in polar solvent DMF decomposed to the monomers. Complexes 1, 2, and 3 of both a and b in DMF represented the coordination of a phenolate oxygen, an imine nitrogen of the Schiff base, an alkoxy oxygen, and a solvent oxygen, with the possible coordination of an imidazole nitrogen in the axial position for 3a and 3b. Complexes 4a and 4b showed the coordination of a carboxylate oxygen instead of an alkoxy oxygen for the other complexes. The
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polymer-bound complexes had the same coordination geometries as those for the corresponding monomers in DMF. (2) The polymer-supported complexes in the presence of the peracid converted the sulfide to the corresponding sulfoxide in 80–90% yield in 90 min. They represented slightly lower rates of reaction than the corresponding monomeric complexes (see Table 4). Both 1a and PSVC 1 with no substituent gave the quickest rate of oxidation (see Table 4). Complex PSVC 4, the Schiff base of which was prepared from the amino acid, showed much more loss of catalytic activity than complex PSVC 1–3, the Schiff bases of which were derived from aminoethanols, over the three runs (see Table 5). Polymer PSVC 3 yielded the ennatiomeric excess of the highest value of 40% (see Table 6). The polymer-supported complexes afforded almost the same enatiomeric excesses as those of the corresponding complexes (see Table 6). References [1] R. Hiatt, in: R.L. Augustine, D.J. Trekker (Eds.), Oxidation, vol. 2, Marcel Dekker, New York, 1971 (Chapter 3). [2] G.A. Tolstilor, V.P. YurÕev, U.M. Dzhemilev, Russ. Chem. Rev. 44 (1975) 319. [3] R.A. Sheldon, J.K. Kochi, Adv. Catal. 25 (1976) 272. [4] J.E. Lyons, Aspect of Homogeneous Catalysis 3 (1977) 1. [5] K.B. Sharpless, T.R. Verhoeven, Aldrichim. Acta 12 (1979) 63. [6] R.A. Sheldon, J. Mol. Catal. 7 (1980) 107. [7] J. Sobczak, J.J. Ziolkovski, J. Mol. Catal. 13 (1981) 11. [8] R.A. Sheldon, J.K. Kochi, Metal-catalyzed Oxidation of Organic Compounds, Academic Press, New York, 1981. [9] C.J. Chang, J.A. Labinger, H.B. Gray, Inorg. Chem. 36 (1997) 5927. [10] J.W. Akitt, F.W. Kaye, B.E. Lee, A.M. North, Macromol. Chem. 56 (1963) 195. [11] S. Bhaduri, A. Gosh, H. Khwaja, J. Chem. Soc. Dalton Trans. (1980) 447. [12] B.B. De, B.B. Lohray, S. Sivaram, P.K. Dhal, Tetrahedron: Asymmetry 6 (1995) 2105. [13] Y. Kurusu, Macromol. Symp. 105 (1996) 173. [14] S. Ogunwumi, T. Bein, Chem. Commun. (1997) 901. [15] S. Skaria, C.R. Rajan, S. Ponrathnam, Polymer 38 (1997) 1699.
[16] D. Pini, A. Mandori, S. Orlandi, P. Salvadori, Tetrahedron: Asymmetry 10 (1999) 3883. [17] R. Krishnan, S. Vancheesan, J. Mol. Catal. A: Chem. 142 (1999) 377. [18] M.D. Angelino, P.E. Laibinis, J. Polym. Sci. A: Polym. Chem. 37 (1999) 3888. [19] H.-X. Feng, R.-M. Wang, Y.-F. He, Z.-Q. Lei, Y.-P. Wang, C.-G. Xia, J.-S. Suo, J. Mol. Catal. A: Chem. 159 (2000) 25. [20] A. Natarajan, J.S. Madalengoitia, Tetrahedron Lett. 41 (2000) 5783. [21] S. Bhaduri, G.K. Lahiri, P. Munshi, J. Organomet. Chem. 606 (2000) 151. [22] R. Krishnan, S. Vancheesan, J. Mol. Catal. A: Chem. 157 (2000) 15. [23] B. Altava, M.I. Burguete, E. Garcia-Verdugo, S.V. Luis, M.J. Vicent, Tetrahedron 57 (2001) 8675. [24] P.K. Dhal, B.B. De, S. Sivaram, J. Mol. Catal. A: Chem. 177 (2001) 71. [25] R. Krishnan, S. Vancheesan, J. Mol. Catal. A: Chem. 185 (2002) 87. [26] T.S. Reger, K.D. Jandan, J. Am. Chem. Soc. 122 (2000) 6929. [27] R. Ando, M. Nagai, T. Yagyu, M. Maeda, Inorg. Chim. Acta, 351 (2003) 107. [28] R. Ando, H. Ono, T. Yagyu, M. Maeda, Inorg. Chim. Acta, in press. [29] A. Syamal, Coord. Chem. Rev. 16 (1975) 309. [30] U. Casellato, P.A. Vigato, M. Vidali, Coord. Chem. Rev. 23 (1977) 31. [31] S. Ooi, M. Nishizawa, K. Matsumoto, H. Kuroya, K. Saito, Bull. Chem. Soc. Jpn. 52 (1979) 452. [32] M. Mohan, R.J. Butcher, J.P. Jasinski, C.J. Carano, Inorg. Chem. 31 (1992) 2029. [33] N.D. Chasteen, in: J. Reuben (Ed.), Biological Magnetic Resonance, Plenum, New York, 1981, pp. 53–119, and references therein. [34] C.R. Cornman, E.P. Zovinka, Y.D. Boyajian, K.M. Geiser-Bush, P.D. Boyle, P. Sihgh, Inorg. Chem. 34 (1995) 4213. [35] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 91 (1991) 165. [36] D. Rehder, Coord. Chem. Rev. 182 (1999) 297. [37] K. Nakajima, M. Kojima, K. Toriumi, K. Saito, J. Fujita, Bull. Chem. Soc. Jpn. 62 (1989) 760. [38] H. Schmidt, M. Bashirpoor, D. Rehder, J. Chem. Soc., Dalton Trans. (1996) 3865. [39] R. Ando, H. Inden, M. Sugino, H. Ono, D. Sakaeda, T. Yagyu, M. Maeda Inorg. Chim. Acta, in press.
Characterization of oxovanadium (IV)–Schiff-base complexes and those bound on resin, and their use in sulfide oxidation Ryuji Ando, Takeyoshi Yagyu, Masunobu Maeda
*
Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan Received 9 August 2003; accepted 3 December 2003 Available online 29 January 2004
Abstract Oxovanadium (IV)–Schiff-base complexes and those bound on Merrifield resin as a polymer support were prepared for their characterization and their use as catalyst in oxidation of methyl phenyl sulfide. Schiff bases were prepared with use of salicylaldehyde and 2,4-dihydroxybenzaldehyde as the aldehydes and 2-aminoethanol, L-phenylalaninol, L-histidinol, and L-phenylalanine as the counterparts. Oxovanadium (IV) complexes made up of these Schiff bases and those bound on the resin were spectroscopically characterized. The polymer-supported Schiff-base complexes in the presence of tertiary-butylhydroperoxide converted the sulfide to the corresponding sulfoxide in 80–90% yield in CDCl3 in 90 min. They afforded slightly lower rates of oxidation than the corresponding monomeric complexes. They converted the sulfide in a stereoselective manner yielding the sulfoxide in enantiomeric excess (the highest value of 40%). The polymer-supported complexes and the corresponding monomers achieved almost the same enatiometric excesses with each other. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Polymer-supported Schiff-base complex; Sulfide oxidation; Coordination geometry in the equatorial plane; Peroxo complex
1. Introduction Oxovanadium complexes have been shown to catalyze a variety of reactions such as the oxidation of alcohols and inorganic compounds like halides and sulfur oxides, the epoxidation of alkenes and allyl alcohols, and the conversion of sulfides to sulfoxides and sulfones [1–9]. These catalytic reactions have been performed chiefly in homogeneous solution system. The homogeneous system has the advantage of the high rate of catalytic reaction, whereas some disadvantages include the difficulty in separation of the reaction product from the catalyst. Heterogeneous solid–solution phase catalyses, which take advantage of, e.g., polymer-supported *
Corresponding author. Present address: Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Tel.: +81-52-735-5221; fax: +81-52-7355442. E-mail address: [email protected] (M. Maeda). 0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.12.031
oxovanadium complexes, have been recently investigated in order to alleviate some disadvantages of the homogeneous system [10–26]. Although the heterogeneous system is generally inferior to the homogeneous one in the rate of catalysis, the former has features like the easy separation of product from catalyst by filtration and the feasibility of recycling of catalyst. The clarification of the correlation between the coordination geometry of the complex and the catalytic activity will be one of the requisites for designing and developing effectively the catalyst with high catalytic activity. However, since the insolubility in solvent characterizes the polymer-supported complex, it will be difficult to determine the coordination geometry. In the epoxidation of alkenes by [VO(salen)](N ,N 0 ethylenebis(salicylideneiminato)oxovanadium (IV)) and [VO(salen derivative)] complexes with dioxygen as an oxygen atom donor, the complexes of salen derivatives, which were derived by introduction of electron-withdrawing nitro groups into the aromatic rings of the
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ancillary ligand salen, expressed catalytic actions superior to that of the complex of salen [29]. These findings suggest that the introduction of the substituent group probably changed redox potentials of V (V)/V (IV), which resulted in improvement of the catalysis. Thus, it is worthwhile to examine the effect on the catalysis of the substituents introduced into the ligands of oxovanadium complexes. However, it was reported that the introduction of substituents like NO2 with strong electron-withdrawing abilities caused the polymerization of oxovanadium complexes by association of V@O units [9,28]. Polymerized complexes, which are probably sparingly soluble in solvent [29], are inadequate for homogeneous catalysis as well as for determination of the coordination geometry. Thus, in this work, in order to sufficiently examine the effects of substituents on catalytic actions, as well as to overcome the insolubility and to avoid the possibility of the loss of catalytic activity due to the polymerization, we have employed insoluble polymer-supported oxovanadium complexes, wherein the polymerization of the complexes probably does not occur, because the motion of polymer-bound oxovanadium complexes should be restricted and they are far apart from each other. Oxovanadium (IV)–tridentate Schiff-base complexes and the corresponding oxovanadium (IV) complexes supported on Merrifield resin as a polymer support were prepared and their coordination geometries were determined by spectroscopic methods. Catalytic actions in asymmetric oxidation reaction of sulfide such as the rate of catalysis, the extent of catalysis, and the efficiency of recycling were examined for both soluble complex- and polymer-supported complex-systems. Table 1 Schiff base ligands R2
OH
C
R3
N R1
L1a L2a L3a
R1
R2
R3
H CH2 –C6 H5
H H H
OH OH OH
H OH OH OH
CO2 H OH OH OH
OH
CO2 H
H2 C
NH N
L4a L1b L2b L3b
CH2 –C6 H5 H CH2 –C6 H5 H2 C
NH N
L4b
CH2 –C6 H5
Schiff bases derived from salicylaldehyde and 2,4-dihydroxybenzaldehyde as the aldehydes and 2-aminoethanol, L-phenylalaninol, L-histidinol, and Lphenylalanine as the counterparts are collected in Table 1. The coordination geometries and catalytic reactions of the Schiff-base complexes of oxovanadium (IV) prepared from the ligands of group a (L1a–L4a) and group b (L1b–L4b) were first determined to compare with those of the polymer-supported Schiff-base complexes. In order to prepare the polymer-supported complexes, the Schiff bases of group b, into which the OH substituent meta to the phenolate oxygen was introduced, were employed to bind their Schiff-base complexes of oxovanadium (IV) to Merrifield resin by utilizing the reaction of the substituent with ClÕs in the resin.
2. Experimental 2.1. Reagents All the chemicals were of reagent grade and used without further purification. 2.2. Preparation of oxovanadium complexes [(VO(salae))2 ] (complex 1a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) and 2-aminoethanol (0.35 g, 5.0 mmol) dissolved in dichloromethane (50 mL) was stirred for 1 h after addition of anhydrous magnesium sulfate (1.0 g) and then, filtered. A solution of [VO(acac)2 ] (acacH ¼ acetylacetone) (1.0 g, 3.8 mmol) in dichloromethane (50 mL) was introduced into the filtrate. The solution was stirred for 3 h at room temperature under nitrogen atmosphere and then, concentrated with a rotary evaporator. Diethyl ether (50 mL) was added to the concentrate to yield pale blue precipitates (yield: 0.33 g (38%); ESI-MS: m=z 460.0). [(VO(salpheol))2 ] (complex 2a). The complex was prepared with use of L-phenylalaninol (0.76 g, 5.0 mmol) according to the same procedures as those employed for 1a (yield: 0.54 g (44%); ESI-MS: m=z 640.1). [VO(salhisol)(EtOH)] (complex 3a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) in ethanol (20 mL) was introduced to a solution of L-histidinol dihydrochloride (1.1 g, 5.0 mmol) dissolved in water (10 mL) and then, sodium carbonate (0.84 g, 10 mmol) was added. The solution mixture was stirred for 3 h at room temperature. It was filtered after dehydration by addition of magnesium sulfate. A solution prepared by addition of a solution of [VO(acac)2 ] (1.0 g, 3.8 mmol) in ethanol (50 mL) to the filtrate was stirred for 5 h at room temperature under N2 . The concentration of the solution with a rotary evaporator yielded reddish brown
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precipitates, which were washed with diethyl ether (2 5 mL) (yield: 0.63 g (51%); ESI-MS: m=z 356.2). [VO(salphe)(H2 O)] (complex 4a). A solution of salicylaldehyde (0.61 g, 5.0 mmol) in ethanol (20 mL) was added to a solution of L-phenylalanine (0.83 g, 5.0 mmol) in water (20 mL) and then, sodium acetate (0.82 g, 10 mmol) was added. The solution mixture was stirred for 30 min, and then, a solution of VOSO4 (1.0 g, 3.8 mmol) in water (10 mL) was introduced slowly into the solution. The stirring of the solution thus prepared for 30 min gave pale purple precipitates (yield: 0.80 g (60%); ESI-MS: m=z 352.1). [(VO(OH salae))2 ] (complex 1b). A solution of 2,4dihydroxybenzaldehyde (2.4 g, 20 mmol) and 2-aminoethanol (1.2 g, 20 mmol) dissolved in acetone (150 mL) was stirred for 1 h under N2 after addition of magnesium sulfate (1.0 g) and filtered. A solution prepared by addition of a solution of [VO(acac)2 ] (4.0 g, 15 mmol) in dichloromethane (50 mL) to the filtrate was stirred for 2 h at room temperature under N2 . Pale purple precipitates were filtered off and washed with diethyl ether (2 5 mL) (yield: 1.8 g (49%); ESI-MS: m=z 492.0). [(VO(OH salpheol))2 ] (complex 2b). The complex was prepared with use of L-phenylalaninol (1.5 g, 10 mmol) according to the same procedures as those employed for 1b (yield: 1.6 g (62%); ESI-MS: m=z 672.1). [VO(OH salhisol)(EtOH)] (complex 3b). The complex was prepared by use of 2,4-hydroxybenzaldehyde (1.2 g, 10 mmol) according to the same procedures as those employed for 3a (yield: 1.7 g (66%); ESI-MS: m=z 372.7). [VO(OH salphe)(H2 O)] (complex 4b). The complex was prepared with use of 2,4-hydroxybenzaldehyde (1.2 g, 10 mmol) according to the same procedures as those employed for 4a (yield: 1.6 g (58%); ESI-MS: m=z 368.3). [mer-VO(salae)] (PSVC 1). A solution of N,Ndimethylformamide (DMF) (50 mL) containing VO(OHsalae) (1.03 g, 4.20 mmol), Merrifield resin (1.00 g; 1% DVB, 38–75 lm (2000–400 mesh), Cl rate (1.1 mmol/g), and sodium hydroxide (0.0800 g, 2.00 mmol) were stirred for 96 h at 110 °C under N2 . The reaction product was filtered off and washed with successive, DMF (2 5 mL), water (2 5 mL), and acetone (2 5 mL) (yield: 1.28 g, mole percentage of the substitution of complex 1a for the Cl in the resin: 71.8%). The mole percentage was estimated from the difference in weight between the raw material resin and the resin which reacted with complex 1a, based on the fact that the substitution reaction of complex 1a for the Cl in the resin occurs at a 1:1 mole ratio. [mer-VO(salpheol)] (PSVC 2) (yield: 1.41 g based on 1.41 g of 2b, mole percentage of the substitution: 84.3%); [mer-VO(salhisol)] (PSVC 3) (yield: 1.30 g based on 1.45 g of 3b, mole percentage of the substitution: 72.2%); [mer-VO(salphe)] (PSVC 4) (yield: 1.41 g based on 1.55 g of 4b, mole percentage of the substitution: 78.4%) were
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prepared according to the same procedures as those for PSVC 1. The amount of the resin was 1.00 g. 2.3. Measurements EPR spectra of the monomeric Schiff-base complexes in DMF and of the powdered polymer-supported complexes were recorded at liquid nitrogen temperature (77 K) using 5 mm o.d. quartz tubes on a JES-RE1X spectrometer (JEOL DATUM). The spectrometer was operated at X-band (8–12 GHz) with a microwave power of 1 mW. Infrared spectra of complexes in the solid state were recorded as KBr pellets using a FT/IR410 (JASCO). Infrared spectra of complexes in solution were measured by interposing the solutions between two KBr plates. UV–Vis spectra were measured by using a capped quartz cell of a light-pass length of 1 mm with a UV-1600PC spectrometer (Shimadzu). Magnetic susceptibilities of powdered samples were measured at 20 °C by the Evans method with a magnetic susceptibility balance (Sherwood Scientific). Diamagnetic corrections were applied in the usual manner by use of PascalÕs constants. ESI-mass spectra were measured with an LCT (ESI-TOF) (Micromass). The concentration of an oxovanadium complex dissolved in dichloromethane was kept at 50 lmol dm3 . The solution was sprayed at the rate of 600 lL/s with a microsyringe. Sodium iodide was used as a standard sample. Data were analyzed using MassLynx Ver 3.5. The conversion yield (%) of the substrate of methyl phenyl sulfide to the corresponding sulfoxide was determined at 18 °C with 1 H NMR spectra. The spectra were recorded on a 300 MHz GEMINI 2000 (VARIAN). An oxovanadium complex (0.050 mmol) was dissolved in chloroform-d (CDCl3 ) (2 mL) and then, the internal standard of 1,3,5-trimethoxybenzene (0.050 mmol) was added (solution (1)). A polymer-supported complex (0.050 mmol) was suspended in CDCl3 (2 mL), followed by addition of the internal standard (solution (p1)). The solution (2) containing both the substrate (0.50 mmol) and the peroxide of aqueous tertiary-butylhydroperoxide (tert-BuOOH) (70 vol%) (0.50 mmol) were prepared by its mixing in CDCl3 (2 mL). Either solution (1) or (p1), and (2) were mixed with each other, and then, stirred constantly during the reaction. The 1 H NMR spectra were measured with the elapse of time. The quantitative measurements of the amount of the sulfoxide were carried out by comparing the integrated value of the area under the peak for methyl hydrogen (d 2.74 (s, 3H, CH3 )) of the sulfoxide with that for methoxy hydrogen (d 3.87 (s, 9H, OCH3 )) of the internal standard. The enantiomeric excess (ee) was determined separately as follows. The substrate (0.50 mmol) and the peroxide (0.50 mmol) were introduced into a solution of an oxovanadium complex (0.050 mmol) dissolved in
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CHCl3 (4 mL), and the mixture was incubated at 20 °C for 1 h for the complexes and 1.5 h for the polymers. The reaction mixture was evaporated to dryness. The product from methyl phenyl sulfide was extracted from the sulfide residue with diethyl ether. The extract, which was evaporated to dryness, was diluted with the eluent of hexane/2-propanol (9:1) to obtain a 0.1% solution. The solution was used for high-performance liquid chromatography (HPLC) on a chromatograph (LC-5A, Shimadzu) equipped with a chiral column (CHIRALCEL OB-H, Daicel Chem.), a UV spectral detector (SPD-6A, Shimadzu), and a peak area calculator (Chromatopack C-R4A, Shimadzu). The column was eluted with hexane/2-propanol (9:1) at a flow rate of 0.2 mL/min. The HPLC eluate was monitored at 254 nm. The polymer-supported complexes were filtered off through a glass filter (G4) after the first oxidation cycle, washed with acetone and ethanol, and dried for the next cycle.
3. Results and discussion 3.1. Characterization of complexes (1a–4a, 1b–4b) in the solid state, dichloromethane, and N,N-dimethylformamide (DMF) IR data assigned to V@O stretching (ms (V@O)) frequencies and V–O–V antisymmetric stretching (ma (V– O–V)) frequencies for the eight complexes (1a–4a and 1b–4b) (the numbers correlate with those of the ligands (L1a–L4a and L1b–L4b in Table 1, from which the complexes were prepared), in the solid state are given in Table 2. Complexes 1a and 2a in the solid state exhibited IR bands at 892 and 869 cm1 , respectively, which are probably assigned to the ma (V–O–V) values resulting from the V–O (alkoxy oxygen)–V bond in light of the previous results that the bands around 776–898 cm1 observed for binuclear aminoethanol–Schiff-base complexes of oxovanadium (IV) in the solid state were ascribed to the corresponding frequencies for V–O (alkoxy oxygen)–V [27]. The complexes were diamagnetic. These findings probably indicate that the complexes are binuclear in the solid state. These complexes dissolved in
non-polar solvent dichloromethane were EPR-silent. It is appropriate to consider that the formation of dimers caused the EPR-silence [27,29,30] and thus, the coordination geometry in the solid state is maintained also in dichloromethane. The complexes in the solid state exhibited the ms (V@O) a little higher than 990 cm1 . It has been reported that oxovanadium complexes with coordination numbers of 5 and 6 have the ms (V@O) higher than and lower than about 990 cm1 , respectively, and this figure provides an available index for discriminating the coordination number of oxovanadium complexes [9,31,32]. In consideration of the criterion and of the fact that the complexes were prepared in non-polar dichloromethane, it is likely that no solvent is coordinated in the axial position. The experimental m=z values for 1a (460.0) and 2a (640.1) agree with the values 460.2 and 640.5, respectively, calculated by postulating these compositions and geometries. The UV–Vis spectra of 1a dissolved in polar solvent DMF were measured with the passage of time. They are depicted in Fig. 1. It is apparent that the band around 330 nm, which is probably ascribed to an LMCT resulting from the coordination of DMF due to the decomposition of the dimer to the monomer, increased in intensity with the passage of time. The weak and broad band around 500 nm, which is probably assigned to an LMCT attributable to the coordination of DMF in the axial position, increased in intensity with the elapse of time. That the spectra in-
Fig. 1. UV–Vis spectra of complex 1a in DMF measured every 20 minutes. Concentration of the complex is 1.0 103 mol dm3 .
Table 2 EPR parameters of complexes in DMF and IR frequencies of complexes in the solid state
1a 2a 3a 4a 1b 2b 3b 4b
Ak ( 104 cm1 )
A? ( 104 cm1 )
gk
g?
ms (V@O) (cm1 )
ma (V–O–V) (cm1 )
162.5 162.2 163.0 168.4 161.9 163.3 162.7 169.7
61.0 61.2 60.8 62.5 61.3 60.7 61.0 63.0
1.960 1.962 1.961 1.959 1.962 1.962 1.961 1.959
1.988 1.989 1.989 1.987 1.990 1.987 1.990 1.986
991 993 978 999 962 965 942 987
892 869
890 885
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tersecting with each other around 370 and 400 nm shifted to longer wavelengths with the elapse of time and thus, exhibited no isosbestic point indicates that the simultaneous coordination of DMF molecules in both the equatorial and axial positions did not occur. The solutions of the complexes dissolved in DMF were stirred for 1h, and their EPR spectra were measured. The solutions exhibited the EPR-activity. Coordination geometries in the equatorial plane of the complexes dissolved in DMF were deduced by comparing the observed Ak values with those calculated from model compounds [33,34] according to the additivity rule, which has been extensively employed as a bench mark for identification of donor atoms in the equatorial plane of oxovanadium (IV) complexes. Since no Ak value for DMF coordination was available in the literature, an EPR spectrum of VOSO4 dissolved in DMF was measured and the Ak value was estimated to be 176.3 104 cm1 . The Ak value of 161.7 104 cm1 calculated for these complexes by postulating the coordination of a phenolate oxygen, an imine nitrogen of the Schiff base, an alkoxy oxygen, and an oxygen of the solvent agreed with the observed values for 1a and 2a. The IR spectrum of a DMF solution containing 1a, which was stirred for 1 h at room temperature, showed the ms (V@O) of 993 cm1 . According to the criterion stated above, it is likely that no solvent is coordinated to the axial position at that time. The solution of 1a dissolved in DMF was stirred for 8 h at 50 °C under nitrogen atmosphere, and concentrated in vacuo to obtain the precipitates. Their IR spectrum in the solid state represented the ms (V@O) of 971 cm1 in addition to that at 993 cm1 . These findings suggest that the solvent began to coordinate in the axial position with the elapse of time. No observation of the change in EPR spectra both 1 h and 8 h after the dissolution indicates that the same equatorial coordination geometry is maintained regardless of the elapse of time. It is appropriate to consider on the basis of the results described above that the binuclear complex, within which two vanadium atoms are bridged in the equatorial plane, decomposed to the monomer due to the coordination of the solvent in the equatorial plane, and then, the solvent coordinated in the axial position with the elapse of time. For the reason that complexes 3a and 4a were paramagnetic (leff ¼ 1:7 lB (3a), 1.7 lB (4a)) and no ma (V–O–V) was observed, it is likely that they are monomeric in the solid state. The observed Ak value of 163.0 104 cm1 for 3a in DMF indicates that the complex retains an equatorial coordination geometry analogous to those of 1a and 2a. However, 3a represented the low ms (V@O) of 978 cm1 , suggesting the axial coordination of perhaps an imidazole nitrogen. On the other hand, the Ak value of 167.8 104 cm1 observed for 4a in DMF agreed with the value of 168.4 104 cm1 calculated on the assumption of the coordination of a phenolate oxygen, an imine nitrogen of the
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Schiff base, a carboxylate oxygen, and a solvent oxygen in the equatorial plane; the equatorial coordination geometry is different from those for the other a complexes. The observed m=z values for 3a (356.2) and 4a (352.1) are consistent with the values 356.3 and 352.2, respectively, calculated based on these compositions and geometries. Complexes 1b–4b, the Ak values whereof were almost the same as those of the corresponding a complexes, represent the same equatorial coordination geometries as those for the a complexes. However, the b complexes yielded the lower ms (V@O) than the a complexes, although the values for both kinds of complexes tended to vary in parallel. According to the aforesaid index for distinction between penta- and hexa-coordination, it is said that the b complexes maintain the axial coordination in the solid state, in contrast to that for the a complexes. However, the occurrence of the axial coordination geometry due to the introduction of only the OH substituent with the electron-withdrawing ability meta to the phenolate oxygen seems improbable. In our previous paper [27], it has been reported that the introduction of Cl (Hammett para constant rp (Cl) ¼ 0.23 [35]) and NO2 (rp (NO2 ) ¼ 0.78 [35]) substituents with electron-withdrawing abilities into the position para to the phenolate oxygen of the Schiff-base in amino acid– Schiff-base complexes of oxovanadium (IV) reduced the ms (V@O) by ca. 20 and 50 cm1 , respectively, from ca. 1000 cm1 for the amino acid–Schiff-base complex, the ligand of which has no substituent. In light of the previous results, it is likely that the decrease in ms (V@O) for the b complexes is responsible for the electron-withdrawing ability of the OH substituent (Hammett meta constant rm (OH) ¼ 0.12 [35]). 3.2. Reaction of Schiff-base complexes with tert-BuOOH Since the conversion yield and enantiomeric excess in sulfide oxidation were measured in CDCl3 and CHCl3 , respectively, the change in UV–Vis spectra of complex 1a dissolved in CHCl3 , whereto tert-BuOOH was added, was observed with the passage of time. They are shown in Fig. 2. The appearance of the band around 450 nm indicates the formation of the peroxo complex, which was completed immediately after the addition. The complex was kept stable at least for 3 h. 3.3. Characterization of polymer-supported Schiff-base oxovanadium (IV) complexes (PSVC) The support of complexes b on the resin was confirmed by the decrease in intensity of the H2 C–Cl stretching frequency (1260 cm1 ) of the resin. Table 3 collects EPR parameters (A and g) and their V@O stretching frequencies (ms (V@O)) for polymersupported Schiff-base complexes PSVC 1–4 (the
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Fig. 3. Proposed coordination geometries of polymer-supported complex and its peroxo complex for PSVC 1 and 2.
Fig. 2. UV–Vis spectra of complex 1a in CHCl3 before (- - -) and 1 min (—) after addition of tert-BuOOH. Concentrations of the complex and tert-BuOOH are 1.0 103 and 1.0 102 mol dm3 , respectively.
numbers correlate with those of the ligands in Table 1) in the solid state. Complexes PSVC 1–4 exhibited the Ak values consistent with those for the corresponding b complexes monomeric in DMF (Table 2). This indicates that the b complexes are bound on the polymers with their equatorial coordination geometries being kept as they are. Complexes PSVC 1–4 showed practically the same ms (V@O) as those for the parent b complexes. However, since the polymer-supported complexes have no longer OH substituent on their ligands, it seems unlikely that both types of complexes retain the same axial coordination geometry. As was described in Section 2.2, the polymer-supported complexes were prepared in DMF under a severe condition of heating at ca. 110 °C for ca. 100 h. As mentioned also in Section 3.1, complex 1a heated in DMF at 50 °C exhibited the ms (V@O) of 971 cm1 in the solid state, which was ascribed to the coordination of a DMF molecule in the axial position. In consideration of these results, it is most likely that DMF molecules are coordinated in the axial position for PSVC 1, 2, and 4. However, it is not certain whether 3b retains the axial coordination of the solvent or an imidazole nitrogen, the coordination of which was proposed for 3a. This will be discussed in Section 3.4. The proposed coordination geometry of PSVC 1, 2, and 4 is illustrated in Fig. 3. It is known that the powdered monomeric complex of oxovanadium (IV) give rise to a single broad EPR band because of interactions between the complexes. In contrast, the EPR spectrum of the polymer-supported
Schiff-base complexes split into 16 bands, as was the case for the monomers in DMF. This suggests that the complexes bound on the polymer are so distant from each other that they are difficult to interact with each other. Thus, the polymerization of the Schiff-base complexes caused by V@O V@O interactions within the complexes containing the electron-withdrawing substituent like NO2 , which was aforementioned, is expected to be impeded on the resin. Polymer-supported complexes suspended in CHCl3 , to which tert-BuOOH of the 100-fold concentration of the complexes was added, were stirred for 30 min, during which the suspended particles changed from brown to red. Their EPR spectra were silent, indicating the oxidation for V (IV) to V (V) by forming the peroxo complex. 3.4. Oxidation of sulfide with peracid catalyzed by monomeric and polymer-supported Schiff-base complexes Yields (%) of the conversion of methyl phenyl sulfide to the corresponding sulfoxide versus reaction time are given in Table 4. No oxidation occurred in the absence of catalyst. As for the a complexes, complex 1a with no bulky substituent yielded the quickest rate of reaction. Complex 3a, within which an imidazole nitrogen was supposed to be coordinated in the axial position, showed the lowest reaction rate over 90 min. It has been proposed by several investigators [36–38] that the substrate sulfide may directly interact with the vanadium center of the Schiff-base peroxo complex at the open axial site or by replacing the originally bound labile ligand in the axial position and then, be oxidized to sulfoxide by the activated peroxide. If this proposal is accepted, a cause for the results for 3a is likely that the weak interaction of an imidazole nitrogen with the vanadium at the axial site resisted the substitution of the
Table 3 EPR parameters and IR frequencies of polymer-supported complexes in the solid state
PSVC PSVC PSVC PSVC
1 2 3 4
Ak ( 104 cm1 )
A? ( 10 4 cm1 )
gk
g?
ms (V@O) (cm1 )
163.0 162.1 161.5 170.7
61.3 60.5 61.3 63.9
1.960 1.958 1.960 1.957
1.990 1.989 1.988 1.988
974 979 975 972
R. Ando et al. / Inorganica Chimica Acta 357 (2004) 2237–2244 Table 4 Conversion yield (%) of sulfide to sulfoxide
1a 2a 3a 4a PSVC PSVC PSVC PSVC
1 2 3 4
10 min
30 min
85 42 31 40 55 35 20 32
99 80 54 75 80 62 51 59
Table 6 Enantiomeric excess (ee) of methy phenyl sulfoxide 60 min
90 min
93 78 88 88 78 69 74
96 88 96 92 85 80 81
substrate for the nitrogen, which resulted in lowering of the reaction rate. Complexes 2a and 4a with bulky substituents showed lower rates of reaction than 1a with no substituent. This is probably because it should be rather hard for the substrate to approach the vanadium center in the axial position due to interference of the bulky side chains. The bulkiness of the substituent for 3a probably led to the further lowering of the rate of the reaction. Complexes PSVC did not represent so significant inferiority to the a complexes in rate of reaction. PSVC 3 yielded the lowest rate of reaction. It was described in Section 3.3 that it is not clear whether the solvent or an imidazole nitrogen is coordinated in the axial position in PSVC 3. An imidazole nitrogen is expected to be coordinated more strongly to the axial site than the solvent. Thus, it is appropriate to consider that the lowering in rate of reaction for PSVC 3 resulted from the coordination of an imidazole nitrogen. As was the case for 1a, PSVC 1 with no substituent exhibited the highest rate of reaction. PSVC 1–4 were filtered off, washed with acetone and ethanol, and then, dried for recycling after the reaction time of 90 min. The yields (%) for run 2 (second cycle) and run 3 (third cycle) are given in Table 5. PSVC 4, whose Schiff base consists of the amino acid (L-phenylalanine), displayed a lowering of yield at run 2, whereas the other complexes, the Schiff bases of which are made up of the aminoethanols, showed much less loss of catalytic activity compared to that of PSVC 4 over the three runs. Enantiomeric excesses in oxidation catalysis of methyl phenyl sulfide in CHCl3 are given in Table 6. The polymer-supported complexes and the corresponding monomers afforded almost the same enetiomeric excesses (eeÕs) with each other. Complex 3a and the corTable 5 Recyclability of polymer-supported complexes
PSVC PSVC PSVC PSVC
1 2 3 4
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Run 1
Run 2
Run 3
92 85 80 81
90 89 81 68
85 85 73 53
ee (%) 1a 2a 3a 41 PSVC PSVC PSVC PSVC
1 2 3 4
racemic 25 42 10 racemic 22 40 13
responding polymer-supported complex PSVC 3 with six-coordinate octahedral geometry with the imidazole group being coordinated in the axial position achieved higher eeÕs than 2a and PSVC 2, and 4a and PSVC 4 having five-coordinate square-pyramidal geometry. A reason for the results may be that the bulky imidazole group coordinated in the axial position for 3a and PSVC 3 should induce some influence on the directional approach of the sulfide to the axial position of the vanadium. It is apparent that 4Õs, wherein a phenolate oxygen, an imine nitrogen of the Schiff base, a carboxylate oxygen, and a solvent oxygen are coordinated, yielded lower eeÕs than 2Õs, in which an alkoxy oxygen is coordinated in the place of a carboxylate oxygen for 4Õs. Since, in general, the basicities of carboxylate oxygens are much lower than those of alkoxy oxygens, it is appropriate to consider that the carboxylate oxygen probably coordinate more weakly to oxovanadium (IV) than the alkoxy oxygen. As a result, 4Õs are less stable than 2Õs. As was reported previously about the reactions of amino acid- and amino acid ester–Schiff-base complexes of oxovanadium (IV) with the peracid [39], the easier decomposition of 4Õs by the action of the peracid probably led to the lower eeÕs. Complex 1a and PSVC 1, the ligands wherein have no asymmetric center, yielded racemic mixtures.
4. Summary (1) Complexes 1a, 2a, 1b, and 2b were dimers both in the solid state and in non-polar solvent dichloromethane, whereas 3a, 4a, 3b, and 4b were monomers in the solid state. No axial coordination occurred in the solid state. The dimers dissolved in polar solvent DMF decomposed to the monomers. Complexes 1, 2, and 3 of both a and b in DMF represented the coordination of a phenolate oxygen, an imine nitrogen of the Schiff base, an alkoxy oxygen, and a solvent oxygen, with the possible coordination of an imidazole nitrogen in the axial position for 3a and 3b. Complexes 4a and 4b showed the coordination of a carboxylate oxygen instead of an alkoxy oxygen for the other complexes. The
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polymer-bound complexes had the same coordination geometries as those for the corresponding monomers in DMF. (2) The polymer-supported complexes in the presence of the peracid converted the sulfide to the corresponding sulfoxide in 80–90% yield in 90 min. They represented slightly lower rates of reaction than the corresponding monomeric complexes (see Table 4). Both 1a and PSVC 1 with no substituent gave the quickest rate of oxidation (see Table 4). Complex PSVC 4, the Schiff base of which was prepared from the amino acid, showed much more loss of catalytic activity than complex PSVC 1–3, the Schiff bases of which were derived from aminoethanols, over the three runs (see Table 5). Polymer PSVC 3 yielded the ennatiomeric excess of the highest value of 40% (see Table 6). The polymer-supported complexes afforded almost the same enatiomeric excesses as those of the corresponding complexes (see Table 6). References [1] R. Hiatt, in: R.L. Augustine, D.J. Trekker (Eds.), Oxidation, vol. 2, Marcel Dekker, New York, 1971 (Chapter 3). [2] G.A. Tolstilor, V.P. YurÕev, U.M. Dzhemilev, Russ. Chem. Rev. 44 (1975) 319. [3] R.A. Sheldon, J.K. Kochi, Adv. Catal. 25 (1976) 272. [4] J.E. Lyons, Aspect of Homogeneous Catalysis 3 (1977) 1. [5] K.B. Sharpless, T.R. Verhoeven, Aldrichim. Acta 12 (1979) 63. [6] R.A. Sheldon, J. Mol. Catal. 7 (1980) 107. [7] J. Sobczak, J.J. Ziolkovski, J. Mol. Catal. 13 (1981) 11. [8] R.A. Sheldon, J.K. Kochi, Metal-catalyzed Oxidation of Organic Compounds, Academic Press, New York, 1981. [9] C.J. Chang, J.A. Labinger, H.B. Gray, Inorg. Chem. 36 (1997) 5927. [10] J.W. Akitt, F.W. Kaye, B.E. Lee, A.M. North, Macromol. Chem. 56 (1963) 195. [11] S. Bhaduri, A. Gosh, H. Khwaja, J. Chem. Soc. Dalton Trans. (1980) 447. [12] B.B. De, B.B. Lohray, S. Sivaram, P.K. Dhal, Tetrahedron: Asymmetry 6 (1995) 2105. [13] Y. Kurusu, Macromol. Symp. 105 (1996) 173. [14] S. Ogunwumi, T. Bein, Chem. Commun. (1997) 901. [15] S. Skaria, C.R. Rajan, S. Ponrathnam, Polymer 38 (1997) 1699.
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