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Structural characterization of pentadentate salen-type Schiff-base complexes of oxovanadium(IV) and their use in sulfide oxidation
Inorganica Chimica Acta 357 (2004) 1177–1184 www.elsevier.com/locate/ica
Structural characterization of pentadentate salen-type Schiff-base complexes of oxovanadium(IV) and their use in sulfide oxidation Ryuji Ando, Satsuki Mori, Miho Hayashi, 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 July 2003; accepted 11 September 2003
Abstract Pentadentate Schiff-base complexes of oxovanadium(IV), the ligands of which were derived from salicylaldehyde derivatives with a variety of substituents and two kinds of amines (2,20 -bis(aminoethyl)amine and 3,30 -bis(aminopropyl)amine), were prepared, and their coordination geometries in the solid state were determined by X-ray diffraction and IR measurements and those in CH2 Cl2 by EPR measurements. They were found to retain distorted octahedral coordination in the solid state. They showed the structural change depending on the type of the substituent. The complexes which reacted with tert-butylhydroperoxide converted methyl phenyl sulfide to the corresponding sulfoxide at lower rates of reaction than tridentate N -salicylidene-2-aminoethanolato oxovanadium(IV) ([VO(salae)]) and tetradentate (N ; N 0 -bis(salicylidene)ethylenediaminato)oxovanadium(IV) ([VO(salen)]). Ó 2003 Elsevier B.V. All rights reserved. Keywords: Oxovanadium; Pentadentate Schiff-base complex; Peroxo complex; Oxidation catalysis of sulfide; Additivity rule
1. Introduction A variety of oxovanadium(V) complexes have been shown to catalyze the oxidation of sulfides by using peroxides as oxygen atom donors [1–3]. Schiff-base ligated oxovanadium complexes have provided excellent results (e.g., 98% yield, 91% ee [5], 60% yield, 95% ee [4]) in the oxidation of disulfides. In our previous work [6,7], peroxo complexes made of tridentate and tetradentate Schiff-base oxovanadium(IV) complexes were found to yield quick progress of reaction and conversion of methyl phenyl sulfide to the corresponding sulfoxide in 80–95% yield in CDCl3 and in 30–80% yield (by tridentate Schiff-base complexes) in CD3 OD in 30 min. However, some of the *
Corresponding author. Present address: Department of Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Tel./ Fax: +81-52-735-5221. E-mail address: [email protected] (M. Maeda). 0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.09.033
tridentate Schiff-base complexes probably with low stabilities were found to be decomposed by tertbutylhydroperoxide. In the present work, pentadentate Schiff-base complexes of oxovanadium(IV), which are expected to be more stable than the previous tridentate and tetradentate Schiff-base complexes, were chosen with a view to examining the rate of oxidation of sulfide by the complexes and the extent of the protection against the decomposition by the hydroperoxide. Schiff-base complexes, within which salicylaldehyde derivatives with various substituents and two kinds of amines, were adopted. Salicylaldehyde (sal) and its derivatives, 3-methoxysalicylaldehyde (oomesal), 5-methoxysalicylaldehyde (pomesal), 5-chlorosalicylaldehyde (clsal), 3,5dichlorosalicylaldehyde (diclsal), 5-nitrosalicylaldehyde (nitsal), and 3,5-di-tert-butylsalicylaldehyde (ditbusal) were employed as aldehydes. Amines used were 2,20 bis(aminoethyl)amine (deta) and 3,30 -bis(aminopropyl)amine (dpta). Pentadentate Schiff bases derived from them are collected in Table 1.
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Table 1
R1
R1 OH
R2
C H
HO H N
N n
L1a L2a L3a L4a L5a L6a L7a L1b L2b L3b L4b L5b L6b L7b
N
R2
C H
n
Ligand
R1
R2
n
sal-dpta oomesal-dpta pomesal-dpta nitsal-dpta clsal-dpta diclsal-dpta ditbusal-dpta sal-deta oomesal-deta pomesal-deta nitsal-deta clsal-deta diclsal-deta ditbusal-deta
H OMe H H H Cl t Bu H OMe H H H Cl t Bu
H H OMe NO2 Cl Cl t Bu H H OMe NO2 Cl Cl t Bu
3 3 3 3 3 3 3 2 2 2 2 2 2 2
The following determination and examination were carried out: (1) crystal structures of the Schiff-base complexes; (2) effects of the substituents of salycilaldehyde on the structures of the complexes; (3) coordination geometries of the complexes in CH2 Cl2 and in the solid state by use of the parallel coupling constants (Ak ) and V@O stretching frequencies in the solid state, respectively; (4) effects of the substituents on the redox potentials of the complexes; (5) relations of the coordination geometries of the complexes with the stabilities of the peroxo Schiff-base complexes and the rates of oxidation of methyl phenyl sulfide.
2. Experimental 2.1. Preparation of Schiff-base oxovanadium(IV) complexes All the chemicals were of reagent grade and used without further purification. All the Schiff-base complexes were prepared at room temperature under dinitrogen according to the following procedures. Aldehyde (10 mmol) in dichloromethane (10 mL) was added to triamine (5.0 mmol) in dichloromethane (20 mL), followed by the addition of anhydrous magnesium sulfate (0.20 g). The solution was stirred for 30 min, and filtered. Bis(acetylacetonato)oxovanadium(IV) ([VO(acac)2 ]) (5.0 mmol) dissolved in dichloromethane (30 mL) was introduced slowly into the filtrate. The solution was stirred for 4 h, and then, concentrated with a rotary evaporator. Diethyl ether
was added to the concentrate to obtain the powder of the complex, which was recrystallized in dichloromethane and diethylether. Crystals of complexes [VO(sal-dpta)] (1a), [VO (pome-dpta)] (3a), and [VO(nitsal-dpta)] 2CH3 CN (4a) (the numbers indicate the complexes prepared by use of the Schiff bases with the corresponding numbers in Table 1) for X-ray diffraction measurements were prepared as follows. As for 1a and 3a, hexane (9 mL) was slowly added to a solution (1 mL) of the complex dissolved in dichloromethane to keep two solution phases, which were allowed to stand for 72 h to yield the desired crystals. As for 4a, a solution of the complex dissolved in acetonitrile (CH3 CN) was allowed to stand for 48 h to yield the desired crystal. 2.2. Measurements X-Band EPR spectra were recorded at liquid nitrogen temperature (77 K) using 5 mm o.d. quartz tubes on a JES-RE1X spectrometer (JEOL DATUM). IR spectra were recorded as KBr pellets on a FT/IR-410(JASCO). UV–Vis spectra were measured using a capped quartz cell of a light-pass length 1 mm with a UV-1600PC spectrophotometer (Shimadzu). Cyclic voltammetric measurements were carried out at 25 °C by using a potentiostat HA-151 (Hokutodenko) and a function generator HB-111 (Hokutodenko). A small-sized electrolytic cell (HX-105, Hokutodenko) for cyclic voltammetry was used, which were equipped with a platinum wire (HX-W1, 0.3 mm in diameter and 5 mm in length), a platinum coil (HX-C4), and an Ag/AgCl electrode saturated with KCl (HX-R2), respectively, as a working electrode, an auxiliary electrode, and a reference electrode. n-Tetrabutylammonium tetrafluoroborate [n Bu4 N][BF4 ] was used as a supporting electrolyte. Intensity data for X-ray diffraction of 1a, and 3a and 4a were collected on an Enraf Nonius CAD4-EXPRESS four-circle and a Rigaku CCD diffractometers, respectively, by use of graphite-monochromatized Mo Ka Crystal data and experiradiation (k ¼ 0:71070 A). mental details are listed in Table 2. The structures were solved by using the direct method. The full-matrix leastsquares refinement was employed with anisotropic displacement parameters generally employed for nonhydrogen atoms. The hydrogen atoms were placed in calculated positions. The calculations were carried out with the program teXsan on a Silicon Graphics work station. The conversion yield (%) of the substrate 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(IV) complex (0.050 mmol) was dissolved in chloroform-d (3.0 mL) and then, the internal standard of 1,3,5-trimethoxybenzene (1.25 mmol)
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Table 2 Crystal and experimental details
Formula Fw Crystal system Crystal size (mm3 ) a (A) b (A) c (A) b (°) 3 ) m (A Z F ð0 0 0Þ 2h range (°) Number of unique reflections Number of reflections used Number of variables Dcalc (g cm3 ) Space group T (K) Radiation/k (A) l (cm1 ) R1 Rw Goodness-of-fit
1a
3a
4a 2CH3 CN
C20 H23 N3 O3 V 404.36 monoclinic 0.38 0.38 0.63 13.261(6) 14.068(4) 10.843(7) 109.76(4) 1903(1) 4 844 5.1–52.6 7599 5147 244 1.411 P 21 =c 293 Mo Ka/0.71070 5.46 0.059 0.14 1.88
C22 H27 N3 O5 V 464.41 orthorhombic 0.06 0.13 0.15 19.166(2) 10.0196(8) 22.274(2)
C24 H27 N7 O7 V 576.46 monoclinic 0.13 0.15 0.20 25.360(2) 12.8647(7) 18.608(1) 120.630(3) 5223.6(7) 8 2392 6.2–55.0 5858 5433 352 1.466 C2=c 173 Mo Ka/0.71070 4.37 0.043 0.147 1.43
and the substrate (2.5 mmol) were added(solution(1)). The solution (2) containing the hydroperoxide of aqueous tert-butylhydroperoxide (tert-BuOOH) (70 vol%) (2.5 mmol) was prepared by its mixing in CDCl3 (3.0 mL). The two solutions (1) and (2) were mixed with each other, and stirred constantly during the reaction. The 1 H NMR spectra were measured with the passage of time. The quantitative measurements of the amounts of the sulfoxide and sulfone were carried out by comparing the integrated values of the areas under the peaks for methyl hydrogens (d 2.74 (s, 3H, CH3 ) for the sulfoxide and d 3.07 (s, 3H, CH3 ) for the sulfone) with that for methoxy hydrogen of the internal standard (d 3.87 (s, 9H, OCH3 )).
3. Results and discussion
4277(1) 8 1944 6.2–55.0 4875 3321 280 1.442 Pbca 173 Mo Ka/0.71070 5.03 0.074 0.126 1.29
tahedral coordination. Complex 4a 2CH3 CN, the crystal of which was obtained from CH3 CN, is solvated by two molecules of CH3 CN. It is apparent that the two aromatic rings are well stacked in 3a and 4a 2CH3 CN. Distances between the two aromatic rings for 3a and 4a 2CH3 CN (distances between C1 and C20) are respectively. The V@O bond 3.026(5)and 2.998(2) A, lengths (1.602(2) for 1a,1.617(3) for 3a, and 1.614(1) A for 4a 2CH3 CN) for the three complexes are a little longer than those for (N ; N 0 -bis(salicylidene)ethylenedi aminato) oxovanadium(IV) ([VO(salen)]) (1.587(1) A [9]), presumably due to the coordination [8], 1.590(1) A in the axial position within the three a complexes. However, the V@O bond lengths are not typically very sensitive to environmental factors. Preparation of crystals suitable for X-ray diffraction measurements has been unsuccessful for the other complexes.
3.1. Crystal structures of complexes 1a, 3a, and 4a The numbering systems and the crystal structures of complexes 1a, 3a, and 4a (the numbers indicate the complexes prepared by use of the Schiff bases with the corresponding numbers L1a, L3a, and L4a in Table 1) are shown in Fig. 1. Table 3 contains a list of selected bond lengths and bond angles. In the three complexes the oxovanadium(IV) is coordinated to a pentadentate ligand with a phenolate oxygen atom (O3), two Schiff-base nitrogen atoms (N1 and N3), and an amine nitrogen atom (N2) being bound in the equatorial plane and a phenolate oxygen atom (O2) in the axial position to complete the distorted oc-
3.2. Parallel coupling constants (Ak ), stretching frequencies of V@O, and redox potentials for 1a–7a EPR parameters were estimated according to the equations derived by Chasteen [10] and Casella et al. [11] by iterative calculation procedures. The EPR parameter values of Ak for 1a–7a in CH2 Cl2 and their V@O stretching frequencies in the solid state are collected in Table 4. The other parameter values of A? , gk , and g? , which are not utilized in the present discussion, are not given. It is apparent that the Ak values for 1a–6a other than 7a are in the range of (151–155) 104 cm1 . This suggests that 1a–6a are identical to each other in
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Fig. 1. ORTEP drawing and atom-numbering scheme for [VO(sal-dpta)] (1a), [VO(pomesal-dpta)] (3a), and [VO(nitsal-dpta)] 2CH3 CN (4a 2CH3 CN), the two molecules of acetonitrile in which are not depicted. Table 3 Interatomic distances and angles relevant to the vanadium(IV) coordination sphere 1a Bond lengths (A) V–O1 V–O2 V–O3 V–N1 V–N2 V–N3 Bond angles (°) O1–V–O2 O1–V–N1 O1–V–N3 O2–V–N1 O2–V–N3 O3–V–N2 N1–V–N2 N2–V–N3 O1–V–O3 O1–V–N2 O2–V–O3 O2–V–N2 O3–V–N1 O3–V–N3 N1–V–N3
1.602(2) 2.067(2) 1.967(2) 2.083(2) 2.180(2) 2.103(2) 170.5(1) 100.7(1) 93.48(10) 83.86(8) 81.86(7) 167.08(9) 80.72(8) 97.58(8) 100.4(1) 89.47(10) 87.89(9) 83.00(7) 89.24(8) 90.14(8) 165.71(9)
3a 1.617(3) 2.088(3) 1.960(3) 2.101(3) 2.176(3) 2.105(3) 171.3(1) 97.9(1) 99.2(1) 80.2(1) 83.0(1) 169.5(1) 85.5(1) 94.9(1) 100.2(1) 89.8(1) 88.3(1) 81.6(1) 89.9(1) 86.7(1) 162.9(1)
Table 4 EPR parameters Ak and V@O stretching frequencies Complex
Ak ( 104 cm1 )
m(V@O) (cm1 )
VO(sal-dpta) VO(oomesal-dpta) VO(pomesal-dpta) VO(nitsal-dpta) VO(clsal-dpta) VO(diclsal-dpta) VO(ditbusal-dpta) VO(sal-deta) VO(oomesal-deta) VO(pomesal-deta) VO(nitsal-deta) VO(clsal-deta) VO(diclsal-deta) VO(ditbusal-deta)
153.2 151.9 151.9 154.0 153.9 155.1 162.6 164.9 163.8 165.7 155.9 162.6 163.3 163.6
926 930 930 953 931 946 950 945 947 944 947 944 941 951
4a 2CH3 CN 1.614(1) 2.111(1) 1.987(1) 2.106(1) 2.163(1) 2.097(2) 173.89(6) 99.99(6) 98.49(6) 81.20(5) 80.39(5) 168.28(5) 85.06(5) 94.44(6) 100.53(6) 90.68(6) 85.45(5) 83.44(5) 89.60(5) 87.33(5) 161.52(6)
equatorial coordination environment. According to the additivity rule [10,12], which has been employed extensively as a bench mark for identification of donor atoms in the equatorial plane of oxovanadium(IV) complexes, the Ak value for 1a–6a was calculated to be 165.0 104 cm1 on the assumption that the coordination-donoratom set of the six complexes in CH2 Cl2 is identical to that of the crystals of 1a, 3a, and 4a 2CH3 CN (Fig. 1). This value is significantly larger than the observed values of (151–155) 104 cm1 . Tolis et al. [13] have recently reported that the coordination of the anionic groups of ligands in the axial position reduced Ak values by almost 10% compared to those of the corresponding ligands with non-anionic groups for coordination in the axial position. In view of these results, it is reasonable to
1a 2a 3a 4a 5a 6a 7a 1b 2b 3b 4b 5b 6b 7b
consider that the coordination of the phenolate anion in the axial position in 1a–6a resulted in the reduction of the observed Ak values. The V@O stretching frequency (m(V@O) value) in 1a–6a ranged from 926 to 953 cm1 , which are significantly lower than those (ca. 1000 cm1 ) for penta-coordinate square-pyramidal oxovanadium(IV) complexes. The lowering is probably attributable to the electron donation from the phenolate anion, which should result in the weakening of the V@O bond. The lowering is also reflected in that the V@O bond lengths for 1a, 3a, and 4a 2CH3 CN are a little longer than that for [VO(salen)] with m(V@O) value of 989 cm1 , as was aforementioned in Section 3.1. Complexes 2a and 3a, into which the substituent OCH3 with the electrondonating ability is introduced, exhibited almost the same V@O stretching frequency as that of 1a with no substituent. The V@O stretching frequency increased with increasing electron-withdrawing ability in the order of 1a < 5a with a Cl substituent para to the salicylaldehyde oxygen atom <6a with two ClÕs at both ortho and para <4a with an NO2 at para with the much stronger electron-withdrawing ability than that of Cl. It may be said from this order that the electron-withdrawing factor,
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which should strengthen the V@O bond, reflects the V@O frequency. The Ak value of 162.6 104 cm1 for 7a larger than those for the other a complexes suggests that 7a is different in equatorial coordination geometry from the other complexes. The Ak value of 163.0 104 cm1 observed for [VO(salen)] is almost in agreement with the Ak value for 7a. Thus, it is likely that two nitrogen atoms from the Schiff base and two phenolate oxygen atoms are coordinated to oxovanadium(IV) in the equatorial plane, as is the case with [VO(salen)]. Complex 7a exhibited a shift to the significantly lower m(V@O) value (950 cm1 ) than penta-coordinate squarepyramidal [VO(salen)] (989 cm1 ). Oxovanadium complexes with coordination numbers of 5 and 6 have been shown to have m(V@O) values higher than and lower than about 990 cm1 , respectively [14–16]. According to this criterion, the frequency lowering to 950 cm1 may be attributable to the axial interaction of probably an amine (NH group) to complete the six-coordinate distorted octahedral structure different from those for the other a complexes. The proposed geometry for 7a is displayed in Fig. 2. It is likely that the structural change of 7a was caused by the repulsion between the two bulky tertiary butyl substituents on C2 and C19 (see Fig. 1). A cyclic voltammogram of 2a is depicted in Fig. 3. Redox potentials for 1a–6a measured in CH2 Cl2 by cyclic voltammetry are given in Table 5. Since the difference between oxidation (Epa ) and reduction (Epc ) potentials surpasses 0.06 V, V(V)/V(IV) redox reactions are quasi-reversible. The redox potentials for 2a and 3a with the electron-donating substituent are shifted in the cathodic direction with reference to the potentials of 1a. This indicates that the complexes become easier to oxidize due to the electron-donation. On the contrary, the potentials for 4a, 5a, and 6a with the electronwithdrawing groups are shifted in the anodic direction in the order of 5a < 6a < 4a; the complexes become
Fig. 2. Possible coordination geometry of 7a in CH2 Cl2 .
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Fig. 3. Cyclic voltammogram of 2a in CH2 Cl2 at 25 °C (1.0 103 mol dm3 complex; 0.10 mol dm3 [n Bu4 N][BF4 ]; scan speed 50 mV/s).
easier to reduce. Since the electron-withdrawing ability of the substituent is in the order of 5a with a Cl at para <6a with two ClÕs at both ortho and para <4a with an NO2 at para, the relationship between the redox potentials and the electron-withdrawing abilities of the substituents correlates well with that between the V@O stretching frequencies and the electronic properties. 3.3. Parallel coupling constants (Ak ) and stretching frequencies of V@O for 1b–7b The EPR parameter values of Ak for 1b–7b in CH2 Cl2 and their V@O stretching frequencies in the solid state are collected in Table 4. All the complexes other than 4b, the ligand of which contains an NO2 substituent para to the Schiff-base oxygen atom, yielded the Ak values of (162.6–164.9) 104 cm1 . These values were interpreted on the assumption of the equatorial coordination of two phenolate oxygen atoms and two nitrogen atoms from the Schiff base. According to the criterion for discrimination of the penta-coordination from the hexacoordination described in Section 3.2, the m(V@O) values of 945–951 cm1 for the six b complexes other than 4b suggest their hexa-coordination involving an amine (NH) in the axial position, as is the case with 7a described in Section 3.2. The ligands for a0 s (except 7a) form probably the rather flexible six-membered rings by using the three carbon atoms between the two amines, while those for b0 s (except 4b) their rigid five-membered
Table 5 Electrochemical data for the oxidation of oxovanadium(IV) complexes in CH2 Cl2
1a 2a 3a 4a 5a 6a
Complex
Epa (V)
Epc (V)
E1=2 (V)
DEp (V)
VO(sal-dpta) VO(oomesal-dpta) VO(pomesal-dpta) VO(nitsal-dpta) VO(clsal-dpta) VO(diclsal-dpta)
0.86 0.82 0.74 1.28 0.97 1.10
0.74 0.70 0.63 1.15 0.84 0.97
0.80 0.76 0.69 1.22 0.91 1.04
0.12 0.12 0.11 0.13 0.13 0.13
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rings by using the two carbon atoms. Thus, it is probable that the flexibility of the ligands forming the sixmembered ring to their change in structure probably resulted in the difference in coordination geometry between a0 s (hereafter, they are called dpta-type geometry) except 7a (deta type-geometry) and b0 s (deta-type geometry) except 4b (dpta type-geometry). The Ak value of 155.9 104 cm1 for complex 4b with the NO2 substituent on the ligand is significantly low compared to those for the other b complexes, but similar to the values for 1a–6a. These findings suggest that 4b retains a coordination geometry analogous to those of 1a–6a with dpta-type geometry. 3.4. Oxidation of sulfide with hydroperoxide catalyzed by Schiff-base complexes of oxovanadium(V) Fig. 4 shows the UV–Vis spectra of complex 1a and tert-BuOOH in CH2 Cl2 , and complex 1b and the hydroperoxide in CH2 Cl2 . As for 1b with deta-type geometry, which reacted quickly with the hydroperoxide, a band probably due to an LMCT absorption from the peroxide to vanadium was observed at about 330 nm [17], and as is shown in Fig. 5, the solution was EPRsilent. These findings indicate the formation of the peroxo oxovanadium(V) complex. The other 2b–7b complexes also manifested the analogous types of behavior to that of 1b. On the contrary, almost no change in absorption band before and after the addition of the peroxide was observed for the system of 1a with dptatype geometry. The observation of the EPR spectrum of the solution 2 h after the addition of the peroxide exhibited the EPR activity with almost the same intensity and EPR parameters as those before the addition of the peroxide (see Fig. 5). These findings are indications of
Fig. 4. UV–Vis spectra of 1a only (—) and 1b only ( ), and ) 2 h after 1a + tert-BuOOH (- - - - -) and 1b + tert-BuOOH ( addition of tert-BuOOH (1.0 103 mol dm3 complex, 0.10 mol dm3 tert-BuOOH).
Fig. 5. EPR spectra of 1a and 1b before (upper) and 2 h (lower) after addition of tert-BuOOH. The EPR receiver gain for 1b containing tertBuOOH was magnified to 5 times that for 1a and 1b only.
the preservation of the original valence and geometry of 1a even in the presence of the peroxide in excess, and of too high the stability of 1a for the peroxide to be allowed to coordinate to the complex. Complexes 2a–6a with dpta-type geometry other than 7a with deta-type geometry also displayed the same type of behavior as that of 1a. Complex 7a showed the same type of behavior as those of the b complexes with deta-type geometry. Yields (%) of the conversion of methyl phenyl sulfide to the corresponding sulfoxide in CDCl3 versus reaction time are given in Table 6. The results obtained using tridentate [VO(salae)] (N -salicylidene-2-aminoethanolato oxovanadium(IV)) and tetradentate [VO(salen)] are also given for comparison. No oxidation occurred in the absence of catalyst. No conversion of the substrate was observed by use of complexes 1a, 3a, and 5a which form no peroxo complex. The rate of reaction was in order of 1b ; [VO(salen)] < [VO(salae)]. Pentadentate complex 1b with the salen-type structure in the equatorial plane yielded almost the same rate of reaction as that for [VO(salen)]. Complex [VO(salae)], within which the open site in the equatorial plane for the coordination of the peroxide is present, afforded the largest rate of reaction. That complex 7a exhibited the oxidation catalysis suggests the analogous geometry of the complex to that of 1b, as was aforementioned. Complex 7a and 7b with the tert-butyl substituents brought about the significant lowering of the reaction rate, compared to the rate of 1b. This is probably because it should be rather hard for the peroxide and the substrate to approach the vanadium center due to the interference of the bulky substituents. It is apparent that 7b yielded substantially lower rates of reaction than 7a. Since 7b with the fivemembered chelate ring is probably stabler than 7a with the six-membered ring, it is likely that the former is more difficult to form the peroxo complex than the latter, which led to the further decrease in reaction rate. Complex 7b with bulky tert-butyl groups and the stable five-membered ring yielded the lowest rate of reaction at the initial stage of reaction. The b complexes showed considerably lower rates of reactions than [VO(salen)] and [VO(salae)]. Especially, the precipitation of compounds, whose composition was ambiguous, occurred
R. Ando et al. / Inorganica Chimica Acta 357 (2004) 1177–1184
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Table 6 Conversion yields (%) of sulfides to sulfoxides in CDCl3 Complex
10 min
30 min
1h
2h
1a 3a 5a 7a 1b 3b 5b 7b VO(salen) VO(salae) None
0 0 0 9.0 5.4 2.1 3.5 0 5.5 42 0
0 0 0 18 23 6.9 7.2 2.4 25 80 0
0 0 0 29 43 14 12 6.3 50 93 0
0 0 0 40 63 28 17 13 70 99 0
during the reactions by using complexes 2b, 3b, 5b, and 6b, which probably resulted in appreciable lowering of reaction rates and cessation of reactions. It was difficult to measure the rate of reaction by use of 4b because of the low solubility. Thus, the effects of the substituents on the rate of reaction were not made clear. The complexes except 1a, 3a, and 5a which afforded no sulfoxide and sulfone gave the negligibly low yield of the sulfone compared to that of the sulfoxide. Among them, [Vo(salen)] converted the sulfide to the corresponding sulfone in the highest yield of 3% after 11 h.
4. Summary (1) Crystal structure analyses of complexes 1a, 3a, and 4a 2CH3 CN revealed that in each of them, two imine nitrogen atoms, a phenolate oxygen atom, and an amine nitrogen atom of the Schiff base are coordinated in the equatorial plane with a phenolate oxygen atom being in the axial position (see Fig. 1). The V@O bond lengths of the three complexes were a little longer than that of [VO(salen)]. The V@O stretching frequencies of all the complexes in the solid state lowered significantly compared to that of [VO(salen)], which suggests the weakening of the V@O bonds due to the axial coordination of the ligands. (2) The structural change was observed due to the electronic and steric factors of the substituents. Coordination geometries of 1a–6a in CH2 Cl2 were the same as those of 1a, 3a, and 4a 2CH3 CN in crystal. In contrast, 7a with the four tert-butyl substituents exhibited a coordination geometry in which two phenolate oxygen atoms cis to each other and two nitrogen atoms of the Schiff base are coordinated in the equatorial position with an amine nitrogen atom in the axial position (see Fig. 2). Complexes 1b–7b other than 4b possessed the coordination geometries analogous to that of 7a, whereas 4b with the NO2 substituent showed
3h
6h
11 h
49 73 42 19 18 79
62 83 49 24 39 90
82 92 51 26 60 96
0
0
0
a coordination geometry analogous to those of 1a–6a. (3) The V@O stretching frequency increased with increasing electron-withdrawing ability in the order of 1a < 5a with a Cl substituent para to the salicylaldehyde oxygen atom < 6a with two ClÕs at both ortho and para <4a with an NO2 at para with the much stronger electron-withdrawing ability than that of Cl. The potentials for 4a, 5a, and 6a with the electron-withdrawing groups were shifted in the anodic direction in the order of 5a < 6a < 4a; the complexes became easier to reduce. The relationship between the redox potentials of complexes and the electron-withdrawing abilities of the substituents correlated well with that between the V@O stretching frequencies and the electronic properties. (4) Pentadentate complexes 1a–6a did not react with the peroxide. Pentadentate complexes 1b, 3b, 5b, 7b, and 7a afforded lower rates of oxidation of the sulfide than tridentate [VO(salae)] and tetradentate [VO (salen)].
5. Supporting information available Tables of atomic coordinates and isotropic thermal parameters, complete bond lengths and distances, and thermal parameter of 1a, 3a and 4a 2CH3 CN.
References [1] D. Rehder, Coord. Chem. Rev. 182 (1999) 297, and references therein. [2] A.G.J. Ligtenbarg, R. Hage, B.L. Feringa, Coord. Chem. Rev. 237 (2003) 89, and references therein. [3] C. Bolm, Coord. Chem. Rev. 237 (2003) 245, and references therein. [4] D.A. Cogan, G. Liu, K. Kim, B.J. Backes, J.A. Ellman, J. Am. Chem. Soc. 120 (1998) 8011. [5] J. Skarzewski, E. Ostrycharz, R. Siedlecka, Tetrahedron: Asymmetry 10 (1999) 3457.
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[6] R. Ando, H. Inden, M. Sugino, H. Ono, D. Sakaeda, T. Yagyu, M. Maeda, Inorg. Chim. Acta, in press. [7] R. Ando, H. Ono, T. Yagyu, M. Maeda, Inorg. Chim. Acta, 351 (2003) 107. [8] P.E. Riley, V.L. Pecoraro, C.J. Carrano, J.A. Bonadies, K.N. Raymond, Inorg. Chem. 25 (1986) 154. [9] J.C. Dutton, G.D. Fallon, K.S. Murray, Inorg. Chem. 27 (1988) 34. [10] N.D. Chasteen, in: L.J. Berliner, J. Reuben (Eds.), Biological Magnetic Resonance, vol. 3, Plenum, New York, 1981, p. 70. [11] L. Casella, M. Gullotti, A. Pintar, Inorg. Chim. Acta 144 (1988) 890.
[12] C.R. Cornman, E.P. Zovinka, Y.D. Boyajian, K.M. Geiser-Bush, P.D. Boyle, P. Sihgh, Inorg. Chem. 34 (1995) 4213. [13] E.J. Tolis, V.I. Teberekidis, C.P. Raptopoulou, A. Terzis, M.P. Sigalas, Y. Deligiannakis, T.A. Kabanos, Chem. Eur. J. 7 (2001) 2698. [14] S. Ooi, M. Nishizawa, K. Matsumoto, H. Kuroya, K. Saito, Bull. Chem. Soc. Jpn. 52 (1979) 452. [15] M. Mohan, R.J. Butcher, J.P. Jasinski, C.J. Carrano, Inorg. Chem. 31 (1992) 2029. [16] C.J. Chang, J.A. Labinger, H.B. Gray, Inorg. Chem. 36 (1997) 5927. [17] M.J. Clague, N.L. Keder, A. Butler, Inorg. Chem. 32 (1993) 4754.
Structural characterization of pentadentate salen-type Schiff-base complexes of oxovanadium(IV) and their use in sulfide oxidation Ryuji Ando, Satsuki Mori, Miho Hayashi, 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 July 2003; accepted 11 September 2003
Abstract Pentadentate Schiff-base complexes of oxovanadium(IV), the ligands of which were derived from salicylaldehyde derivatives with a variety of substituents and two kinds of amines (2,20 -bis(aminoethyl)amine and 3,30 -bis(aminopropyl)amine), were prepared, and their coordination geometries in the solid state were determined by X-ray diffraction and IR measurements and those in CH2 Cl2 by EPR measurements. They were found to retain distorted octahedral coordination in the solid state. They showed the structural change depending on the type of the substituent. The complexes which reacted with tert-butylhydroperoxide converted methyl phenyl sulfide to the corresponding sulfoxide at lower rates of reaction than tridentate N -salicylidene-2-aminoethanolato oxovanadium(IV) ([VO(salae)]) and tetradentate (N ; N 0 -bis(salicylidene)ethylenediaminato)oxovanadium(IV) ([VO(salen)]). Ó 2003 Elsevier B.V. All rights reserved. Keywords: Oxovanadium; Pentadentate Schiff-base complex; Peroxo complex; Oxidation catalysis of sulfide; Additivity rule
1. Introduction A variety of oxovanadium(V) complexes have been shown to catalyze the oxidation of sulfides by using peroxides as oxygen atom donors [1–3]. Schiff-base ligated oxovanadium complexes have provided excellent results (e.g., 98% yield, 91% ee [5], 60% yield, 95% ee [4]) in the oxidation of disulfides. In our previous work [6,7], peroxo complexes made of tridentate and tetradentate Schiff-base oxovanadium(IV) complexes were found to yield quick progress of reaction and conversion of methyl phenyl sulfide to the corresponding sulfoxide in 80–95% yield in CDCl3 and in 30–80% yield (by tridentate Schiff-base complexes) in CD3 OD in 30 min. However, some of the *
Corresponding author. Present address: Department of Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan. Tel./ Fax: +81-52-735-5221. E-mail address: [email protected] (M. Maeda). 0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.09.033
tridentate Schiff-base complexes probably with low stabilities were found to be decomposed by tertbutylhydroperoxide. In the present work, pentadentate Schiff-base complexes of oxovanadium(IV), which are expected to be more stable than the previous tridentate and tetradentate Schiff-base complexes, were chosen with a view to examining the rate of oxidation of sulfide by the complexes and the extent of the protection against the decomposition by the hydroperoxide. Schiff-base complexes, within which salicylaldehyde derivatives with various substituents and two kinds of amines, were adopted. Salicylaldehyde (sal) and its derivatives, 3-methoxysalicylaldehyde (oomesal), 5-methoxysalicylaldehyde (pomesal), 5-chlorosalicylaldehyde (clsal), 3,5dichlorosalicylaldehyde (diclsal), 5-nitrosalicylaldehyde (nitsal), and 3,5-di-tert-butylsalicylaldehyde (ditbusal) were employed as aldehydes. Amines used were 2,20 bis(aminoethyl)amine (deta) and 3,30 -bis(aminopropyl)amine (dpta). Pentadentate Schiff bases derived from them are collected in Table 1.
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Table 1
R1
R1 OH
R2
C H
HO H N
N n
L1a L2a L3a L4a L5a L6a L7a L1b L2b L3b L4b L5b L6b L7b
N
R2
C H
n
Ligand
R1
R2
n
sal-dpta oomesal-dpta pomesal-dpta nitsal-dpta clsal-dpta diclsal-dpta ditbusal-dpta sal-deta oomesal-deta pomesal-deta nitsal-deta clsal-deta diclsal-deta ditbusal-deta
H OMe H H H Cl t Bu H OMe H H H Cl t Bu
H H OMe NO2 Cl Cl t Bu H H OMe NO2 Cl Cl t Bu
3 3 3 3 3 3 3 2 2 2 2 2 2 2
The following determination and examination were carried out: (1) crystal structures of the Schiff-base complexes; (2) effects of the substituents of salycilaldehyde on the structures of the complexes; (3) coordination geometries of the complexes in CH2 Cl2 and in the solid state by use of the parallel coupling constants (Ak ) and V@O stretching frequencies in the solid state, respectively; (4) effects of the substituents on the redox potentials of the complexes; (5) relations of the coordination geometries of the complexes with the stabilities of the peroxo Schiff-base complexes and the rates of oxidation of methyl phenyl sulfide.
2. Experimental 2.1. Preparation of Schiff-base oxovanadium(IV) complexes All the chemicals were of reagent grade and used without further purification. All the Schiff-base complexes were prepared at room temperature under dinitrogen according to the following procedures. Aldehyde (10 mmol) in dichloromethane (10 mL) was added to triamine (5.0 mmol) in dichloromethane (20 mL), followed by the addition of anhydrous magnesium sulfate (0.20 g). The solution was stirred for 30 min, and filtered. Bis(acetylacetonato)oxovanadium(IV) ([VO(acac)2 ]) (5.0 mmol) dissolved in dichloromethane (30 mL) was introduced slowly into the filtrate. The solution was stirred for 4 h, and then, concentrated with a rotary evaporator. Diethyl ether
was added to the concentrate to obtain the powder of the complex, which was recrystallized in dichloromethane and diethylether. Crystals of complexes [VO(sal-dpta)] (1a), [VO (pome-dpta)] (3a), and [VO(nitsal-dpta)] 2CH3 CN (4a) (the numbers indicate the complexes prepared by use of the Schiff bases with the corresponding numbers in Table 1) for X-ray diffraction measurements were prepared as follows. As for 1a and 3a, hexane (9 mL) was slowly added to a solution (1 mL) of the complex dissolved in dichloromethane to keep two solution phases, which were allowed to stand for 72 h to yield the desired crystals. As for 4a, a solution of the complex dissolved in acetonitrile (CH3 CN) was allowed to stand for 48 h to yield the desired crystal. 2.2. Measurements X-Band EPR spectra were recorded at liquid nitrogen temperature (77 K) using 5 mm o.d. quartz tubes on a JES-RE1X spectrometer (JEOL DATUM). IR spectra were recorded as KBr pellets on a FT/IR-410(JASCO). UV–Vis spectra were measured using a capped quartz cell of a light-pass length 1 mm with a UV-1600PC spectrophotometer (Shimadzu). Cyclic voltammetric measurements were carried out at 25 °C by using a potentiostat HA-151 (Hokutodenko) and a function generator HB-111 (Hokutodenko). A small-sized electrolytic cell (HX-105, Hokutodenko) for cyclic voltammetry was used, which were equipped with a platinum wire (HX-W1, 0.3 mm in diameter and 5 mm in length), a platinum coil (HX-C4), and an Ag/AgCl electrode saturated with KCl (HX-R2), respectively, as a working electrode, an auxiliary electrode, and a reference electrode. n-Tetrabutylammonium tetrafluoroborate [n Bu4 N][BF4 ] was used as a supporting electrolyte. Intensity data for X-ray diffraction of 1a, and 3a and 4a were collected on an Enraf Nonius CAD4-EXPRESS four-circle and a Rigaku CCD diffractometers, respectively, by use of graphite-monochromatized Mo Ka Crystal data and experiradiation (k ¼ 0:71070 A). mental details are listed in Table 2. The structures were solved by using the direct method. The full-matrix leastsquares refinement was employed with anisotropic displacement parameters generally employed for nonhydrogen atoms. The hydrogen atoms were placed in calculated positions. The calculations were carried out with the program teXsan on a Silicon Graphics work station. The conversion yield (%) of the substrate 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(IV) complex (0.050 mmol) was dissolved in chloroform-d (3.0 mL) and then, the internal standard of 1,3,5-trimethoxybenzene (1.25 mmol)
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Table 2 Crystal and experimental details
Formula Fw Crystal system Crystal size (mm3 ) a (A) b (A) c (A) b (°) 3 ) m (A Z F ð0 0 0Þ 2h range (°) Number of unique reflections Number of reflections used Number of variables Dcalc (g cm3 ) Space group T (K) Radiation/k (A) l (cm1 ) R1 Rw Goodness-of-fit
1a
3a
4a 2CH3 CN
C20 H23 N3 O3 V 404.36 monoclinic 0.38 0.38 0.63 13.261(6) 14.068(4) 10.843(7) 109.76(4) 1903(1) 4 844 5.1–52.6 7599 5147 244 1.411 P 21 =c 293 Mo Ka/0.71070 5.46 0.059 0.14 1.88
C22 H27 N3 O5 V 464.41 orthorhombic 0.06 0.13 0.15 19.166(2) 10.0196(8) 22.274(2)
C24 H27 N7 O7 V 576.46 monoclinic 0.13 0.15 0.20 25.360(2) 12.8647(7) 18.608(1) 120.630(3) 5223.6(7) 8 2392 6.2–55.0 5858 5433 352 1.466 C2=c 173 Mo Ka/0.71070 4.37 0.043 0.147 1.43
and the substrate (2.5 mmol) were added(solution(1)). The solution (2) containing the hydroperoxide of aqueous tert-butylhydroperoxide (tert-BuOOH) (70 vol%) (2.5 mmol) was prepared by its mixing in CDCl3 (3.0 mL). The two solutions (1) and (2) were mixed with each other, and stirred constantly during the reaction. The 1 H NMR spectra were measured with the passage of time. The quantitative measurements of the amounts of the sulfoxide and sulfone were carried out by comparing the integrated values of the areas under the peaks for methyl hydrogens (d 2.74 (s, 3H, CH3 ) for the sulfoxide and d 3.07 (s, 3H, CH3 ) for the sulfone) with that for methoxy hydrogen of the internal standard (d 3.87 (s, 9H, OCH3 )).
3. Results and discussion
4277(1) 8 1944 6.2–55.0 4875 3321 280 1.442 Pbca 173 Mo Ka/0.71070 5.03 0.074 0.126 1.29
tahedral coordination. Complex 4a 2CH3 CN, the crystal of which was obtained from CH3 CN, is solvated by two molecules of CH3 CN. It is apparent that the two aromatic rings are well stacked in 3a and 4a 2CH3 CN. Distances between the two aromatic rings for 3a and 4a 2CH3 CN (distances between C1 and C20) are respectively. The V@O bond 3.026(5)and 2.998(2) A, lengths (1.602(2) for 1a,1.617(3) for 3a, and 1.614(1) A for 4a 2CH3 CN) for the three complexes are a little longer than those for (N ; N 0 -bis(salicylidene)ethylenedi aminato) oxovanadium(IV) ([VO(salen)]) (1.587(1) A [9]), presumably due to the coordination [8], 1.590(1) A in the axial position within the three a complexes. However, the V@O bond lengths are not typically very sensitive to environmental factors. Preparation of crystals suitable for X-ray diffraction measurements has been unsuccessful for the other complexes.
3.1. Crystal structures of complexes 1a, 3a, and 4a The numbering systems and the crystal structures of complexes 1a, 3a, and 4a (the numbers indicate the complexes prepared by use of the Schiff bases with the corresponding numbers L1a, L3a, and L4a in Table 1) are shown in Fig. 1. Table 3 contains a list of selected bond lengths and bond angles. In the three complexes the oxovanadium(IV) is coordinated to a pentadentate ligand with a phenolate oxygen atom (O3), two Schiff-base nitrogen atoms (N1 and N3), and an amine nitrogen atom (N2) being bound in the equatorial plane and a phenolate oxygen atom (O2) in the axial position to complete the distorted oc-
3.2. Parallel coupling constants (Ak ), stretching frequencies of V@O, and redox potentials for 1a–7a EPR parameters were estimated according to the equations derived by Chasteen [10] and Casella et al. [11] by iterative calculation procedures. The EPR parameter values of Ak for 1a–7a in CH2 Cl2 and their V@O stretching frequencies in the solid state are collected in Table 4. The other parameter values of A? , gk , and g? , which are not utilized in the present discussion, are not given. It is apparent that the Ak values for 1a–6a other than 7a are in the range of (151–155) 104 cm1 . This suggests that 1a–6a are identical to each other in
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Fig. 1. ORTEP drawing and atom-numbering scheme for [VO(sal-dpta)] (1a), [VO(pomesal-dpta)] (3a), and [VO(nitsal-dpta)] 2CH3 CN (4a 2CH3 CN), the two molecules of acetonitrile in which are not depicted. Table 3 Interatomic distances and angles relevant to the vanadium(IV) coordination sphere 1a Bond lengths (A) V–O1 V–O2 V–O3 V–N1 V–N2 V–N3 Bond angles (°) O1–V–O2 O1–V–N1 O1–V–N3 O2–V–N1 O2–V–N3 O3–V–N2 N1–V–N2 N2–V–N3 O1–V–O3 O1–V–N2 O2–V–O3 O2–V–N2 O3–V–N1 O3–V–N3 N1–V–N3
1.602(2) 2.067(2) 1.967(2) 2.083(2) 2.180(2) 2.103(2) 170.5(1) 100.7(1) 93.48(10) 83.86(8) 81.86(7) 167.08(9) 80.72(8) 97.58(8) 100.4(1) 89.47(10) 87.89(9) 83.00(7) 89.24(8) 90.14(8) 165.71(9)
3a 1.617(3) 2.088(3) 1.960(3) 2.101(3) 2.176(3) 2.105(3) 171.3(1) 97.9(1) 99.2(1) 80.2(1) 83.0(1) 169.5(1) 85.5(1) 94.9(1) 100.2(1) 89.8(1) 88.3(1) 81.6(1) 89.9(1) 86.7(1) 162.9(1)
Table 4 EPR parameters Ak and V@O stretching frequencies Complex
Ak ( 104 cm1 )
m(V@O) (cm1 )
VO(sal-dpta) VO(oomesal-dpta) VO(pomesal-dpta) VO(nitsal-dpta) VO(clsal-dpta) VO(diclsal-dpta) VO(ditbusal-dpta) VO(sal-deta) VO(oomesal-deta) VO(pomesal-deta) VO(nitsal-deta) VO(clsal-deta) VO(diclsal-deta) VO(ditbusal-deta)
153.2 151.9 151.9 154.0 153.9 155.1 162.6 164.9 163.8 165.7 155.9 162.6 163.3 163.6
926 930 930 953 931 946 950 945 947 944 947 944 941 951
4a 2CH3 CN 1.614(1) 2.111(1) 1.987(1) 2.106(1) 2.163(1) 2.097(2) 173.89(6) 99.99(6) 98.49(6) 81.20(5) 80.39(5) 168.28(5) 85.06(5) 94.44(6) 100.53(6) 90.68(6) 85.45(5) 83.44(5) 89.60(5) 87.33(5) 161.52(6)
equatorial coordination environment. According to the additivity rule [10,12], which has been employed extensively as a bench mark for identification of donor atoms in the equatorial plane of oxovanadium(IV) complexes, the Ak value for 1a–6a was calculated to be 165.0 104 cm1 on the assumption that the coordination-donoratom set of the six complexes in CH2 Cl2 is identical to that of the crystals of 1a, 3a, and 4a 2CH3 CN (Fig. 1). This value is significantly larger than the observed values of (151–155) 104 cm1 . Tolis et al. [13] have recently reported that the coordination of the anionic groups of ligands in the axial position reduced Ak values by almost 10% compared to those of the corresponding ligands with non-anionic groups for coordination in the axial position. In view of these results, it is reasonable to
1a 2a 3a 4a 5a 6a 7a 1b 2b 3b 4b 5b 6b 7b
consider that the coordination of the phenolate anion in the axial position in 1a–6a resulted in the reduction of the observed Ak values. The V@O stretching frequency (m(V@O) value) in 1a–6a ranged from 926 to 953 cm1 , which are significantly lower than those (ca. 1000 cm1 ) for penta-coordinate square-pyramidal oxovanadium(IV) complexes. The lowering is probably attributable to the electron donation from the phenolate anion, which should result in the weakening of the V@O bond. The lowering is also reflected in that the V@O bond lengths for 1a, 3a, and 4a 2CH3 CN are a little longer than that for [VO(salen)] with m(V@O) value of 989 cm1 , as was aforementioned in Section 3.1. Complexes 2a and 3a, into which the substituent OCH3 with the electrondonating ability is introduced, exhibited almost the same V@O stretching frequency as that of 1a with no substituent. The V@O stretching frequency increased with increasing electron-withdrawing ability in the order of 1a < 5a with a Cl substituent para to the salicylaldehyde oxygen atom <6a with two ClÕs at both ortho and para <4a with an NO2 at para with the much stronger electron-withdrawing ability than that of Cl. It may be said from this order that the electron-withdrawing factor,
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which should strengthen the V@O bond, reflects the V@O frequency. The Ak value of 162.6 104 cm1 for 7a larger than those for the other a complexes suggests that 7a is different in equatorial coordination geometry from the other complexes. The Ak value of 163.0 104 cm1 observed for [VO(salen)] is almost in agreement with the Ak value for 7a. Thus, it is likely that two nitrogen atoms from the Schiff base and two phenolate oxygen atoms are coordinated to oxovanadium(IV) in the equatorial plane, as is the case with [VO(salen)]. Complex 7a exhibited a shift to the significantly lower m(V@O) value (950 cm1 ) than penta-coordinate squarepyramidal [VO(salen)] (989 cm1 ). Oxovanadium complexes with coordination numbers of 5 and 6 have been shown to have m(V@O) values higher than and lower than about 990 cm1 , respectively [14–16]. According to this criterion, the frequency lowering to 950 cm1 may be attributable to the axial interaction of probably an amine (NH group) to complete the six-coordinate distorted octahedral structure different from those for the other a complexes. The proposed geometry for 7a is displayed in Fig. 2. It is likely that the structural change of 7a was caused by the repulsion between the two bulky tertiary butyl substituents on C2 and C19 (see Fig. 1). A cyclic voltammogram of 2a is depicted in Fig. 3. Redox potentials for 1a–6a measured in CH2 Cl2 by cyclic voltammetry are given in Table 5. Since the difference between oxidation (Epa ) and reduction (Epc ) potentials surpasses 0.06 V, V(V)/V(IV) redox reactions are quasi-reversible. The redox potentials for 2a and 3a with the electron-donating substituent are shifted in the cathodic direction with reference to the potentials of 1a. This indicates that the complexes become easier to oxidize due to the electron-donation. On the contrary, the potentials for 4a, 5a, and 6a with the electronwithdrawing groups are shifted in the anodic direction in the order of 5a < 6a < 4a; the complexes become
Fig. 2. Possible coordination geometry of 7a in CH2 Cl2 .
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Fig. 3. Cyclic voltammogram of 2a in CH2 Cl2 at 25 °C (1.0 103 mol dm3 complex; 0.10 mol dm3 [n Bu4 N][BF4 ]; scan speed 50 mV/s).
easier to reduce. Since the electron-withdrawing ability of the substituent is in the order of 5a with a Cl at para <6a with two ClÕs at both ortho and para <4a with an NO2 at para, the relationship between the redox potentials and the electron-withdrawing abilities of the substituents correlates well with that between the V@O stretching frequencies and the electronic properties. 3.3. Parallel coupling constants (Ak ) and stretching frequencies of V@O for 1b–7b The EPR parameter values of Ak for 1b–7b in CH2 Cl2 and their V@O stretching frequencies in the solid state are collected in Table 4. All the complexes other than 4b, the ligand of which contains an NO2 substituent para to the Schiff-base oxygen atom, yielded the Ak values of (162.6–164.9) 104 cm1 . These values were interpreted on the assumption of the equatorial coordination of two phenolate oxygen atoms and two nitrogen atoms from the Schiff base. According to the criterion for discrimination of the penta-coordination from the hexacoordination described in Section 3.2, the m(V@O) values of 945–951 cm1 for the six b complexes other than 4b suggest their hexa-coordination involving an amine (NH) in the axial position, as is the case with 7a described in Section 3.2. The ligands for a0 s (except 7a) form probably the rather flexible six-membered rings by using the three carbon atoms between the two amines, while those for b0 s (except 4b) their rigid five-membered
Table 5 Electrochemical data for the oxidation of oxovanadium(IV) complexes in CH2 Cl2
1a 2a 3a 4a 5a 6a
Complex
Epa (V)
Epc (V)
E1=2 (V)
DEp (V)
VO(sal-dpta) VO(oomesal-dpta) VO(pomesal-dpta) VO(nitsal-dpta) VO(clsal-dpta) VO(diclsal-dpta)
0.86 0.82 0.74 1.28 0.97 1.10
0.74 0.70 0.63 1.15 0.84 0.97
0.80 0.76 0.69 1.22 0.91 1.04
0.12 0.12 0.11 0.13 0.13 0.13
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rings by using the two carbon atoms. Thus, it is probable that the flexibility of the ligands forming the sixmembered ring to their change in structure probably resulted in the difference in coordination geometry between a0 s (hereafter, they are called dpta-type geometry) except 7a (deta type-geometry) and b0 s (deta-type geometry) except 4b (dpta type-geometry). The Ak value of 155.9 104 cm1 for complex 4b with the NO2 substituent on the ligand is significantly low compared to those for the other b complexes, but similar to the values for 1a–6a. These findings suggest that 4b retains a coordination geometry analogous to those of 1a–6a with dpta-type geometry. 3.4. Oxidation of sulfide with hydroperoxide catalyzed by Schiff-base complexes of oxovanadium(V) Fig. 4 shows the UV–Vis spectra of complex 1a and tert-BuOOH in CH2 Cl2 , and complex 1b and the hydroperoxide in CH2 Cl2 . As for 1b with deta-type geometry, which reacted quickly with the hydroperoxide, a band probably due to an LMCT absorption from the peroxide to vanadium was observed at about 330 nm [17], and as is shown in Fig. 5, the solution was EPRsilent. These findings indicate the formation of the peroxo oxovanadium(V) complex. The other 2b–7b complexes also manifested the analogous types of behavior to that of 1b. On the contrary, almost no change in absorption band before and after the addition of the peroxide was observed for the system of 1a with dptatype geometry. The observation of the EPR spectrum of the solution 2 h after the addition of the peroxide exhibited the EPR activity with almost the same intensity and EPR parameters as those before the addition of the peroxide (see Fig. 5). These findings are indications of
Fig. 4. UV–Vis spectra of 1a only (—) and 1b only ( ), and ) 2 h after 1a + tert-BuOOH (- - - - -) and 1b + tert-BuOOH ( addition of tert-BuOOH (1.0 103 mol dm3 complex, 0.10 mol dm3 tert-BuOOH).
Fig. 5. EPR spectra of 1a and 1b before (upper) and 2 h (lower) after addition of tert-BuOOH. The EPR receiver gain for 1b containing tertBuOOH was magnified to 5 times that for 1a and 1b only.
the preservation of the original valence and geometry of 1a even in the presence of the peroxide in excess, and of too high the stability of 1a for the peroxide to be allowed to coordinate to the complex. Complexes 2a–6a with dpta-type geometry other than 7a with deta-type geometry also displayed the same type of behavior as that of 1a. Complex 7a showed the same type of behavior as those of the b complexes with deta-type geometry. Yields (%) of the conversion of methyl phenyl sulfide to the corresponding sulfoxide in CDCl3 versus reaction time are given in Table 6. The results obtained using tridentate [VO(salae)] (N -salicylidene-2-aminoethanolato oxovanadium(IV)) and tetradentate [VO(salen)] are also given for comparison. No oxidation occurred in the absence of catalyst. No conversion of the substrate was observed by use of complexes 1a, 3a, and 5a which form no peroxo complex. The rate of reaction was in order of 1b ; [VO(salen)] < [VO(salae)]. Pentadentate complex 1b with the salen-type structure in the equatorial plane yielded almost the same rate of reaction as that for [VO(salen)]. Complex [VO(salae)], within which the open site in the equatorial plane for the coordination of the peroxide is present, afforded the largest rate of reaction. That complex 7a exhibited the oxidation catalysis suggests the analogous geometry of the complex to that of 1b, as was aforementioned. Complex 7a and 7b with the tert-butyl substituents brought about the significant lowering of the reaction rate, compared to the rate of 1b. This is probably because it should be rather hard for the peroxide and the substrate to approach the vanadium center due to the interference of the bulky substituents. It is apparent that 7b yielded substantially lower rates of reaction than 7a. Since 7b with the fivemembered chelate ring is probably stabler than 7a with the six-membered ring, it is likely that the former is more difficult to form the peroxo complex than the latter, which led to the further decrease in reaction rate. Complex 7b with bulky tert-butyl groups and the stable five-membered ring yielded the lowest rate of reaction at the initial stage of reaction. The b complexes showed considerably lower rates of reactions than [VO(salen)] and [VO(salae)]. Especially, the precipitation of compounds, whose composition was ambiguous, occurred
R. Ando et al. / Inorganica Chimica Acta 357 (2004) 1177–1184
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Table 6 Conversion yields (%) of sulfides to sulfoxides in CDCl3 Complex
10 min
30 min
1h
2h
1a 3a 5a 7a 1b 3b 5b 7b VO(salen) VO(salae) None
0 0 0 9.0 5.4 2.1 3.5 0 5.5 42 0
0 0 0 18 23 6.9 7.2 2.4 25 80 0
0 0 0 29 43 14 12 6.3 50 93 0
0 0 0 40 63 28 17 13 70 99 0
during the reactions by using complexes 2b, 3b, 5b, and 6b, which probably resulted in appreciable lowering of reaction rates and cessation of reactions. It was difficult to measure the rate of reaction by use of 4b because of the low solubility. Thus, the effects of the substituents on the rate of reaction were not made clear. The complexes except 1a, 3a, and 5a which afforded no sulfoxide and sulfone gave the negligibly low yield of the sulfone compared to that of the sulfoxide. Among them, [Vo(salen)] converted the sulfide to the corresponding sulfone in the highest yield of 3% after 11 h.
4. Summary (1) Crystal structure analyses of complexes 1a, 3a, and 4a 2CH3 CN revealed that in each of them, two imine nitrogen atoms, a phenolate oxygen atom, and an amine nitrogen atom of the Schiff base are coordinated in the equatorial plane with a phenolate oxygen atom being in the axial position (see Fig. 1). The V@O bond lengths of the three complexes were a little longer than that of [VO(salen)]. The V@O stretching frequencies of all the complexes in the solid state lowered significantly compared to that of [VO(salen)], which suggests the weakening of the V@O bonds due to the axial coordination of the ligands. (2) The structural change was observed due to the electronic and steric factors of the substituents. Coordination geometries of 1a–6a in CH2 Cl2 were the same as those of 1a, 3a, and 4a 2CH3 CN in crystal. In contrast, 7a with the four tert-butyl substituents exhibited a coordination geometry in which two phenolate oxygen atoms cis to each other and two nitrogen atoms of the Schiff base are coordinated in the equatorial position with an amine nitrogen atom in the axial position (see Fig. 2). Complexes 1b–7b other than 4b possessed the coordination geometries analogous to that of 7a, whereas 4b with the NO2 substituent showed
3h
6h
11 h
49 73 42 19 18 79
62 83 49 24 39 90
82 92 51 26 60 96
0
0
0
a coordination geometry analogous to those of 1a–6a. (3) The V@O stretching frequency increased with increasing electron-withdrawing ability in the order of 1a < 5a with a Cl substituent para to the salicylaldehyde oxygen atom < 6a with two ClÕs at both ortho and para <4a with an NO2 at para with the much stronger electron-withdrawing ability than that of Cl. The potentials for 4a, 5a, and 6a with the electron-withdrawing groups were shifted in the anodic direction in the order of 5a < 6a < 4a; the complexes became easier to reduce. The relationship between the redox potentials of complexes and the electron-withdrawing abilities of the substituents correlated well with that between the V@O stretching frequencies and the electronic properties. (4) Pentadentate complexes 1a–6a did not react with the peroxide. Pentadentate complexes 1b, 3b, 5b, 7b, and 7a afforded lower rates of oxidation of the sulfide than tridentate [VO(salae)] and tetradentate [VO (salen)].
5. Supporting information available Tables of atomic coordinates and isotropic thermal parameters, complete bond lengths and distances, and thermal parameter of 1a, 3a and 4a 2CH3 CN.
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