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Spectroscopy, electrochemistry and catalytic properties of rutheniumII complexes containing the tetradentate Schiff base ligand N,N′-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane
Inorganica Chimica Acta 348 (2003) 50 /56 www.elsevier.com/locate/ica
Spectroscopy, electrochemistry and catalytic properties of rutheniumII complexes containing the tetradentate Schiff base ligand N,N?-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane Vagner R. de Souza, Genebaldo S. Nunes, Reginaldo C. Rocha, Henrique E. Toma * Instituto de Quı´mica, Universidade de Sa˜o Paulo, Caixa Postal 26077, Sa˜o Paulo, SP CEP 05513-970, Brazil Received 17 July 2002; accepted 13 November 2002
Abstract The complexes trans -[RuCl2(bpydip)] and trans -[Ru(OH2)2(bpydip)](PF6)2, where bpydip is the tetradentate Schiff base ligand, N ,N ?-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane, have been synthesized and characterized by elemental analysis, cyclic voltammetry, UV /Vis, FTIR and 1H NMR spectroscopy. The electronic spectrum of the trans -[RuCl2(bpydip)] complex has been successfully simulated on the basis of the ZINDO/S method, supporting the assignment of the absorption bands at 644, 607, 458, 418 and 374 nm to RuII(dp)0/bpydip(pp ) charge-transfer transitions, and at 282 nm, to a bpydip (p0/p*) intraligand transition. The electrochemistry of this complex is characterized by a reversible pair of waves at /0.30 and /1.70 V, ascribed to the RuIII/II and bpydip0/1 redox couples, respectively. In contact with water, the trans -[RuCl2(bpydip)] complex spontaneously and quantitatively converts into the aqua complex, leading to pronounced changes in the electronic and electrochemical behavior. A remarkable activity in the epoxidation of cyclohexene in the presence of iodosobenzene (PhIO) has been observed for the aqua complex. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium complexes; Tetradentate Schiff base; Epoxidation of cyclohexene
1. Introduction Through the years, Schiff bases have played a special role as chelating ligands in main group and transition metal coordination chemistry, due to their stability under a variety of oxidative and reductive conditions, and to the fact that imine ligands are borderline between hard and soft Lewis bases [1,2]. Transition metal complexes of tetradentate Schiff base ligands find applications in catalysis [3 /5] and as biomimetic enzyme models [6 /10]. Although a large number of compounds of the type M(N2O2), M(N2S2) and M(N4) (using the binding atom representation), have been synthesized and characterized, there are scarce reports on the N4 tetradentate Schiff base complexes of rutheniumII [1,11,12]. In contrast, similar ruthenium compounds of porphyrins, phthalocyanines and macrocyclic tetraden-
* Corresponding author. Tel.: /55-11-3091 3887; fax: /55-11-3815 5579 E-mail address: [email protected] (H.E. Toma).
tate amines are rather common [11,12]. Such complexes have been investigated because of their catalytic activity in oxygenation reactions [13,14], carrying out, for instance, the selective oxidation of organic substrates, such as olefins, alcohols, alkanes and aldehydes, under mild conditions. This property may be very attractive [15], since the oxidation products can provide important starting materials for the production of fine chemicals and polymers [16 /18]. In addition, the oxidation of organic substrates mediated by high valent rutheniumoxo species evokes much interest in the modelling of cytochrome P-450 [19]. Our interest on metal complexes with organic ligands, exhibiting variable degrees of s-basicity and p-acidity [20 /23] led us to investigate the ruthenium chemistry of a tetradentate Schiff base ligand system, N ,N ?-bis(7methyl-2-pyridylmethylene)-1,3-diiminopropane (bpydip). This ligand has two a,a?-diimine fragments and analogously to the rutheniumII-polypyridines [24], its complexes can also display charge-transfer processes. However, it should be mentioned that ruthenium com-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01480-9
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plexes possess predominantly octahedral cis geometry upon two a,a?-diimine ligands, and that only few examples of the trans complexes concerning Schiff bases are known in the literature up to the present time [11,12,25,26]. In this paper we describe the synthesis and the spectroscopic/electrochemical properties of the trans -[RuCl2(bpydip)] complex (Fig. 1) and the selective, catalytic activity of its aquation product, trans [Ru(OH2)2(bpydip)](PF6)2, in the cyclohexene epoxidation using iodosobenzene (PhIO) in 1,2-dichloroethane.
2. Experimental
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for RuC17H20N4Cl2: C, 45.13; H, 4.46; N, 12.39%). nmax (cm 1): 1590s (C /N), 262w (Ru /Cl). 2.3. Synthesis of trans -[Ru(OH2)2(bpydip)](PF6)2 trans -[RuCl2(bpydip)] (0.050 g, 0.11 mmol) was dissolved in a minimum volume of water. An excess of a saturated aqueous NaPF6 solution was added, leading to the immediate formation of a red precipitate. The solid was filtered, washed with water and diethyl ether and stored under vacuum in the dark. Yield: 0.075 g (95%). (Found: C, 28.84; H, 3.45; N, 7.90. Calc. for RuC17H24N4O2P2F12: C, 28.86; H, 3.43; N, 7.92%).
2.1. Materials 2.4. Physical measurements The tetradentate Schiff base ligand was obtained in situ by condensation of 2-acetylpyridine (Aldrich) with 1,3-diaminopropane (Aldrich) in ethanol [27]. RuCl3 ×/ 3H2O (Aldrich) was the starting material for the synthesis of the ruthenium complex. All other solvents and chemicals were reagent grade (Aldrich or Merck) and used without further purification. All manipulations were carried out under inert atmosphere (Ar) following conventional techniques. 2.2. Synthesis of trans -[RuCl2(bpydip)] RuCl3 ×/3H2O (0.26 g, 1.0 mmol) and excess LiCl (1 g) were dissolved in ethanol (25 cm3) and the mixture was refluxed for 30 min. In separate, a mixture of 2acetylpyridine (0.12 g, 1.0 mmol) and 1,3-diaminopropane (0.037 g, 0.50 mmol) in ethanol (10 cm3) was refluxed separately for 45 min on a water bath, and then added to the ruthenium red /brown solution. Immediately, a deep blue solution resulted, which was further refluxed for 2 h on a water bath. After evaporation of the solvent under reduced pressure, the trans -[RuCl2(bpydip)] complex was purified by alumina column chromatography, using acetonitrile as eluent [25]. Yield: 0.10 g (23%). (Found: C, 45.10; H, 4.49; N, 12.31. Calc.
Elemental analyses were performed at Institute Analytical Center, using a Perkin/Elmer 2400 CHN Elemental Analyser. Electronic spectra were recorded on a Hewlett /Packard model 8453 A diode array spectrophotometer. IR spectra were obtained in KBr pellets on a Shimadzu FTIR-8300 spectrophotometer. 1H NMR spectra were obtained in 5 mm NMR tube on a Varian Inova 300 MHz spectrometer in CDCl3 or acetone-d6. Cyclic voltammetry measurements were carried out using a PARC system, from EG&G Instruments, consisting of a potentiostat model 283, and a three electrode cell arrangement. A platinum disc working electrode, a platinum wire auxiliary electrode and an Ag/ AgNO3 reference electrode (E 8/0.503 V vs. NHE) [28] were employed for the measurements. The software ECHEM V. 4.30 was used. Solutions of ruthenium complexes were prepared in acetonitrile containing 0.10 mol dm 3 tetraethylammonium perchlorate (Et4NClO4). Spectroelectrochemical measurements were carried out using a PARC potentiostat, model 173, in parallel with the Hewlett/Parckard diode array spectrophotometer. The quartz spectroelectrochemical cell used has a 0.025 cm optic length. A gold minigrid working electrode and a platinum auxiliary microelectrode were employed in these measurements versus an Ag /AgNO3 reference microelectrode. 2.5. General procedure for reactions of cyclohexene with oxidants in the presence of ruthenium complex
Fig. 1. MM/ structural representation of trans -[RuCl2(bpydip)] showing the labels employed in 2D COSY 1H NMR assignment.
In a typical oxidation reaction, the catalysts (0.010 mmol) and the olefin were dissolved in 1,2-dichloroethane (5 cm3). Iodosyl benzene (0.50 mmol, unless otherwise stated) was added at once to the solution, under an argon atmosphere, with constant stirring. Aliquots were taken at timed intervals and analyzed by GC. The products were identified by comparison with authentic samples.
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2.6. Molecular calculations Semiempirical molecular orbital calculations were carried out by using the ZINDO/S method within the HYPERCHEM 6.1 program from Hypercube. The spectroscopic parameters were based on the original parameterization [29] using the universal values of ks /1.267 and kp /0.585 as interaction factors. SCF molecular orbitals were obtained at the RHF level. Geometry optimizations were carried out using the ZINDO/1 method with its resident parameters. The electronic spectra simulation was carried out using single excited configuration interaction, within an active space involving ten occupied and ten unoccupied molecular orbitals. Initial nuclear coordinates of the complex were given by molecular mechanics calculations employing the MM/ module from HYPERCHEM, with a gradient of ˚ 1 as a convergence criterion in a 1 /105 kcal A conjugate gradient algorithm. All the calculations were performed on a PC WINDOWS platform with a Pentium III microprocessor.
The 2D NMR spectrum of trans -[RuCl2(bpydip)] is displayed in Fig. 2 and the data are collected in Table 1. The 1H NMR spectra are characterized by a relatively simple pattern of resonances from the aromatic ring protons of the N ,N ?-bis(7-methyl-2-pyridylmethylene)1,3-diiminopropane ligand. The pattern consists of two doublets and two triplets. The triplets appear at a slightly higher field than the doublets due to a p electron deficiency on the rings as a consequence of coordination of the tetradentate ligand to the metal center through the azomethine function and pyridine nitrogen. The relatively simple proton spectra indicate that both pyridine moieties are equivalently coordinated, supporting the almost planar coordination geometry of the tetradentate ligand (C1 symmetry on the ligand). The two pyridine rings swing away from each other in order to evade the intramolecular repulsion between two 6position hydrogen atoms on each pyridine as found for ironII complex [30]. The methyl, a- and b-methylene resonances were observed at higher field and were assigned from their relative areas. 3.2. Electronic spectra
3. Results and discussion 3.1. 1H NMR 1
H NMR and 2D COSY spectra were used to support the structure of the ruthenium complex shown in Fig. 1.
The electronic absorption [RuCl2(bpydip)] complex was trile, as shown in Fig. 3. deconvoluted in terms of five near UV region (Fig. 3).
Fig. 2. 2D COSY spectrum of trans -[RuCl2(bpydip)] in CDCl3 at 300 MHz.
spectrum of the trans recorded in dry acetoniThe spectrum can be strong bands in the Vis-
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Table 1 1 H NMR chemical shifts (d , ppm) for the trans -[RuX2(bpydip)]n (X/Cl, H2O; n/0, 2) complexes X
H(3)
H(4)
H(5)
H(6)
H(a)
H(b)
CH3
Cl H2O
7.88 (d) 8.10 (d)
7.80 (t) 8.04 (t)
7.45 (t) 7.70 (t)
9.45 (d) 9.50 (d)
4.36 (m) 4.60 (m)
2.67 (m) 2.30 (m)
2.72 (s) 2.85 (s)
d /doublet; t/triplet; m/multiplet; s/singlet.
Fig. 3. Electronic spectra of trans -[RuCl2(bpydip)] in acetonitrile. [Ru], 5/10 5 mol dm3.
The molecular orbitals of the trans -[RuCl2(bpydip)] complex were calculated using the ZINDO/S method, and the energies and composition (in terms of molecular constituents/fragments) for the relevant ones (i.e. the frontier levels) were collected in Table 2. As one can see, the HOMO level MO(65) exhibits a large metal /ligand mixed character, i.e. Ru (57.2%) and bpydip (40.0%). This orbital mixing is also significant in MO(64) and MO(63), although in these cases the Ru orbital contribution predominates, reflecting the increasing energy separation of the metal and ligand frontier orbitals. In contrast, the LUMO MO(66) and the two next higher
energy levels exhibit essentially bpydip p*-character, while MO(69) has a significant Ru contribution (22.9%), evidencing the important contribution of p-backbonding in this system. The experimental spectra can be successfully simulated, as shown in Table 3. All the electronic transitions in the Vis-near UV region exhibit metal-to-ligand charge-transfer character, while the absorption band at 282 nm is consistent with a bpydip p 0/p* transition. On changing the axial ligand in trans -[RuX2(bpydip)] from Cl to H2O, the MLCT bands are shifted to shorter wavelengths, reflecting the perturbation of the RuII dp-orbitals, as theoretically expected for the MO(65) and MO(64) levels (Table 2), considering their relative ligand field strengths. As a matter of fact, in contact with water, trans -[RuCl2(bpydip)] spontaneously and quantitatively converts into the aqua complex. The aquation reaction can be monitored spectrophotometrically from the disappearance of the trans -[RuCl2(bpydip)] bands around 644 and 608 nm and the growth of a new band at 520 nm (o /10 120 mol 1 dm3 cm 1), ascribed to the trans [Ru(OH2)2(bpydip)]2 complex (see Fig. 4). Theses changes are consistent with loss of Cl through an aqua-chloro intermediate absorbing at lmax /590 nm, as also observed in some polypyridyl complexes of ruthenium [31,32]. The trans -[Ru(OH2)2(bpydip)]2 complex undergoes reversible deprotonation leading to the decrease of the Table 3 Experimental and ZINDO/S calculated electronic transitions in the visible region for trans -[RuCl2(bpydip)]
Table 2 Energies and composition of the valence levels (ZINDO/S) for trans [RuCl2(bpydip)]
Experimental
MO
Energy (eV)
Ru (%)
Cl (%)
bpydip (%)
lmax (nm)
o (mol1 dm3 cm 1)
70 69 68 67 66(LUMO) 65(HOMO) 64 63 62 61
0.0305 0.0283 /0.0006 /0.0089 /0.0258 /0.238 /0.242 /0.269 /0.328 /0.336
6.40 22.9 2.70 5.90 3.40 57.2 62.8 86.2 3.70 2.50
0.30 0.40 0.10 0.40 1.70 2.80 5.90 0.50 5.80 6.70
93.3 76.7 97.2 93.7 94.9 40.0 31.3 13.3 90.5 90.8
644
Calculated lmax (nm)
Transition
10 480
637
607
12 690
604
458 418 374 282
2350 4450 3700 11 090
446 408 370 290 282
MO(65)0/MO(66) MO(64)0/MO(69) MO(63)0/MO(67) MO(63)0/MO(69) MO(64)0/MO(67) MO(65)0/MO(68) MO(64)0/MO(72) MO(62)0/MO(66) MO(61)0/MO(66)
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V.R. de Souza et al. / Inorganica Chimica Acta 348 (2003) 50 /56 Table 4 pKa of some aqua-substituted polypyridine-rutheniumII complexes Complex
a
trans -[Ru(OH2)2(bpydip)]2 b [Ru(OH2)(bpy)(terpy)]2 [Ru(OH2)(phen)(terpy)]2 [Ru(pic)(OH2)(terpy)] cis -[Ru(OH2)(bpy)2(py)]2 [Ru(acac)(OH2)(terpy)] [Ru(OH2)(terpy)(tmen)]2
pKa 8.9 9.7 10.0 10.0 10.8 11.0 11.2
acac, acetylacetonate; bpy: 2,2?-bipyridine; pic, picolinate anion; phen, 1,10-phenanthroline; py, pyridine; terpy, 2,2?:6,2ƒ-terpyridine; tmen, N ,N ,N ,N -tetramethylethylenediamine. a Ref. [33], except when specified. b This work. Fig. 4. Electronic spectra of the trans -[RuCl2(bpydip)] complex (5/ 10 5 mol dm3) in acetonitrile, and successive spectral measurements for its corresponding aqueous solution recorded just after the mixture. (a) [RuCl2(bpydip)]; (b) [RuCl(OH2)(bpydip)] ; (c) [Ru(OH2)2(bpydip)]2 .
intensity of the visible charge-transfer bands at 520 nm, showing isosbestic points at 480 and 350 nm (Fig. 5(A)). The pKa was determined by spectrophotometric titration, using Handerson /Hasselbach’s equation, pH / pKa/log(A0/A )/(A/Af), where A , A0 and Af are the measured absorbances at a given pH, and at the initial (lowest pH) and final (highest pH) points, respectively (Fig. 5(B)). The calculated pKa /8.99/0.1 is significantly lower than those observed for polypyridine rutheniumII complexes [33], listed in Table 4. This result reflects the strong p-electron withdrawing effect of the bpydip ligand, removing the electronic density on the rutheniumII center via back-bonding interactions.
3.3. Electrochemistry The cyclic voltammograms for the trans -[RuCl2(bpydip)] complex consist of two reversible monoelectronic waves at /0.30 and /1.70 V (vs. NHE), ascribed to the RuIII/II and bpydip0/ redox processes, respectively. The E 8 for the RuIII/II couple is smaller than for related dichlororuthenium complexes, e.g. cis -[RuCl2(bpy)2] (0.54 V vs. NHE) and trans -[RuCl2(bpy)2] (0.52 V). However, this value is consistent with that predicted from the linear optical-electrochemical correlation between E 8(RuIII/II) and MLCT energy, previously reported for many ruthenium systems [34 /36]. In this way, a lower E 8(RuIII/II) implies a lower MLCT energy, and vice-versa. In the presence of water, the RuIII/II redox wave is shifted from E1/2 /0.30 to 0.48 V, while lmax of the main MLCT band in the visible shifts from 644 to 520 nm, in agreement with the optical/electrochemical correlations. The oxidation of the ruthenium complex leads to the decay of the MLCT band in the visible (Fig. 6), with the rise of a new band at 450 nm (o /3250 mol 1 dm3 cm 1) ascribed to a ligand-to-metal charge transfer transition.
3.4. Preliminary studies on the catalytic epoxidation of cyclohexene
Fig. 5. (A) Electronic spectra of trans -[Ru(OH2)2(bpydip)]2 (5/ 10 5 mol dm 3) in aqueous solution; (B) plot of pH vs. log(A0/ A )/(A/Af), where A , A0 and Af are the measured absorbances at a given pH, and at the initial (lowest pH) and final (highest pH) points, respectively.
Ruthenium complexes are versatile catalysts for the oxidation of alkenes and they often promote the oxidative cleavage of the double bond into aldehydes or ketones [37]. However, in transition metal catalysed oxidation reactions, the nature of the coordinative environment around the metal also plays a significant role in catalytic activity, leading to different products, depending on the reaction conditions [38 /41]. In this way, selective epoxidation can be pursued by suitable modifications of the ligand moiety.
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Fig. 6. UV /Vis spectroelectrochemistry of trans [Ru(OH2)2(bpydip)]2 in water. [Ru], 5 /10 4 mol dm3; working electrode, Pt; reference electrode, Ag/AgNO3 (0.01 mol dm 3); m , 0.1 mol dm3 Et4NClO4; room temperature.
In this work, we have observed that trans [Ru(OH2)2(bpydip)]2 is indeed an effective catalyst for cyclohexene epoxidation by iodosobenzene (PhIO) in 1,2-dichloroethane. This preliminary study showed that when a mixture of 0.01 mmol of trans [Ru(OH2)2(bpydip)]2 and 0.5 mmol of PhIO was allowed to react, at 298 K, with a solution of cyclohexene (0.5 mmol) in 1,2-dichloroethane, cyclohexene oxide formed as the major product. In addition, minor amounts of byproducts were found, among which were 2-cyclohexenone and 2-cyclohexene-1-ol. The selectivity for cyclohexene oxide formation was almost 95%. The effect of varying the catalyst concentration and temperature for the oxidation of cyclohexene is shown
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in Fig. 7. The yield of epoxide increased with the concentration of the catalyst. By increasing the temperature, the reaction rates and conversion of cyclohexene increased significantly. It should be noticed that a number of ruthenium complexes exhibit selectivity for the conversion of norbornene, cyclooctene, and linear alkenes to their respective epoxides, but in the case of cyclohexene, allylic attack is normally dominant to yield 2-cyclohexene-1-ol and 2-cyclohexene-1-one [39 /41]. However, the trans -[Ru(OH2)2(bpydip)]2 has proved to be an effective catalyst, reacting with cyclohexene via a preferential attack on the C /C bond to give mainly the epoxide, instead of the alternative attack on the C /H bond leading to allylic oxidation products [42,43]. We have also observed that the amount of PhI released during the catalytic reaction does not reflect the amount of the oxygen transferred. Consequently, for epoxidation reactions, an excess of PhIO is required [44 /46]. It was found that even in the absence of cyclohexene, the ruthenium complex reacts with PhIO to yield PhI, liberating significant amount of oxygen. Interestingly, the possible fate of the lost oxygen in related systems has already been discussed by Yang et al. [47], revealing that only a small part of the lost oxygen can be attributed to solvent oxidation. For the elucidation of this point a detailed mechanistic study is being carried out, including the characterization of the possible catalytic species, e.g. RuV /O or RuIV /O.
Acknowledgements We thank FAPESP and CNPq for financial support.
Fig. 7. Time scan for the oxidation of cyclohexene with PhIO catalysed by trans -[Ru(OH2)2(bpydip)]2 . (A) effect of temperature, concentration of catalyst, 0.01 mmol; (B) varying the catalyst concentration, temperature, 308 K.
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References [1] A.D. Garnovskii, A.L. Nivorozhkin, V.I. Minkin, Coord. Chem. Rev. 126 (1993) 1. [2] R. Ziessel, Coord. Chem. Rev. 216-217 (2001) 195. [3] X.-G. Zhou, J.-S. Huang, P.-H. Ko, K.-K. Cheung, C.-M. Che, J. Chem. Soc., Dalton Trans. (1999) 3303. [4] W. Ru¨ttinger, G.C. Dismukes, Chem. Rev. 97 (1997) 1. [5] W. Odenkirk, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 114 (1992) 6392. [6] M.L.P. Santos, I.A. Bagatin, E.M. Pereira, A.M.C. Ferreira, J. Chem. Soc., Dalton Trans. (2001) 838. [7] S. Me´nage, J.-B. Galey, J. Dumats, G. Hussler, M. Seite´, I.G. Luneau, G. Chottard, M. Fontecave, J. Am. Chem. Soc. 120 (1998) 13370. [8] J.-P. Costes, F. Dahan, A. Dupuis, J.-P. Laurent, J. Chem. Soc., Dalton Trans. (1998) 1307. [9] E.-G. Ja¨ger, J. Knaudt, M. Rudolph, M. Rost, Chem. Ber. 129 (1996) 1041. [10] S.R. Collinson, D.E. Fenton, Coord. Chem. Rev. 148 (1996) 19. [11] J.W.-S. Hui, W.-T. Wong, Coord. Chem. Rev. 172 (1998) 389. [12] S.-M. Lee, W.-T. Wong, Coord. Chem. Rev. 164 (1997) 415. [13] M.J. Upadhyay, P.K. Bhattacharya, P.A. Ganeshpure, S. Satish, J. Mol. Catal. 73 (1992) 277. [14] T. Mukaiyama, T. Yamada, Bull. Chem. Soc. Jpn. 68 (1995) 17. [15] D. Riley, M. Sterm, J. Ebner, D.H.R. Barton, A.E. Martell, D.T. Sawyer, The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum, New York, 1993. [16] R.A. Sheldon, J.K. Kochi, Metal Catalysed Oxidations of Organic Compounds, Academic Press, New York, 1981. [17] K.A. Jorgensen, Chem. Rev. 89 (1989) 431. [18] D. Riley, J. Lyon, J. Chem. Soc., Dalton Trans. (1991) 157. [19] B. Meunier, Chem. Rev. 92 (1992) 1411. [20] H.E. Toma, T.E. Chavez-Gil, Spectrosc. Lett. 32 (1999) 963. [21] H.E. Toma, T.E. Chavez-Gil, R.C. Rocha, H.R. Rechenberg, J. Incl. Phenom. Macro. 34 (1999) 57. [22] H.E. Toma, T.E. Chavez-Gil, Inorg. Chim. Acta 257 (1997) 197. [23] F.S. Nunes, H.E. Toma, J. Coord. Chem. 36 (1995) 33. [24] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky, Coord. Chem. Rev. 84 (1988) 85. [25] S. Choudhury, M. Kakoti, A.K. Deb, S. Goswami, Polyhedron 11 (1992) 3183.
[26] K. Mizushima, M. Nakaura, S.-B. Park, H. Nishiyama, H. Monjushiro, K. Harada, M. Haga, Inorg. Chim. Acta 261 (1997) 175. [27] T.G. Campbell, F.L. Urbach, Inorg. Chem. 12 (1973) 1836. [28] B. Kratochvil, E. Lorah, C. Garber, Anal. Chem. 41 (1969) 1793. [29] (a) J.E. Ridley, M.C. Zerner, Theor. Chim. Acta 32 (1973) 111; (b) J.E. Ridley, M.C. Zerner, Theor. Chim. Acta 42 (1976) 223; (c) A.D. Bacon, M.C. Zerner, Theor. Chim. Acta 53 (1979) 21; (d) M.C. Zerner, G.H. Loew, R.F. Kirchner, U.T. MuellerWesterhoff, J. Am. Chem. Soc. 102 (1980) 589. [30] M. Goto, Y. Ishikawa, T. Ishihara, C. Nakatake, T. Higuchi, H. Kurosaki, V.L. Goedken, Chem. Commun. (1997) 539. [31] L. Lee, C.-H. Wang, J. Photochem. Photobiol. A: Chem. 79 (1994) 163. [32] A.R. Guadalupe, X. Chen, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 32 (1993) 5502. [33] A. Dovletoglou, S.A. Adeyemi, T.J. Meyer, Inorg. Chem. 35 (1996) 4120. [34] A.B.P. Lever, Inorg. Chem. 29 (1990) 1271. [35] E.S. Dosdsworth, A.B.P. Lever, Chem. Phys. Lett. 119 (1985) 61. [36] F. Barigelletti, A. Juris, V. Balzani, P. Belser, A. von Zelewsky, Inorg. Chem. 26 (1987) 4115. [37] M.H. Robbins, R.S. Drago, J. Chem. Soc., Dalton Trans. (1996) 105. [38] B. Cetinkaya, E. Centikaya, M. Brookhart, P.S. White, J. Mol. Catal. 142 (1999) 101. [39] G.A. Barf, D. van den Hoek, R.A. Sheldon, Tetrahedron 52 (1996) 12971. [40] G.A. Barf, R.A. Sheldon, J. Mol. Catal. 98 (1995) 143. [41] M. Bressan, A. Morvillo, Inorg. Chem. 28 (1989) 950. [42] C. Ho, W.H. Leung, C.M. Che, J. Chem. Soc., Dalton Trans. (1991) 2933. [43] E.G. Samsel, K. Srinivasan, J.K. Kochi, J. Am. Chem. Soc. 107 (1985) 7606. [44] A. Morvillo, M. Bressan, J. Mol. Catal. 125 (1997) 119. [45] A.S. Kanmani, S. Vancheesau, J. Mol. Catal. 125 (1997) 127. [46] V. Kesavan, S.J. Chandrasekaran, J. Chem. Soc., Perkin 1 (1997) 3115. [47] Y. Yang, F. Diederich, J.S. Valentine, J. Am. Chem. Soc. 113 (1991) 7195.
Spectroscopy, electrochemistry and catalytic properties of rutheniumII complexes containing the tetradentate Schiff base ligand N,N?-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane Vagner R. de Souza, Genebaldo S. Nunes, Reginaldo C. Rocha, Henrique E. Toma * Instituto de Quı´mica, Universidade de Sa˜o Paulo, Caixa Postal 26077, Sa˜o Paulo, SP CEP 05513-970, Brazil Received 17 July 2002; accepted 13 November 2002
Abstract The complexes trans -[RuCl2(bpydip)] and trans -[Ru(OH2)2(bpydip)](PF6)2, where bpydip is the tetradentate Schiff base ligand, N ,N ?-bis(7-methyl-2-pyridylmethylene)-1,3-diiminopropane, have been synthesized and characterized by elemental analysis, cyclic voltammetry, UV /Vis, FTIR and 1H NMR spectroscopy. The electronic spectrum of the trans -[RuCl2(bpydip)] complex has been successfully simulated on the basis of the ZINDO/S method, supporting the assignment of the absorption bands at 644, 607, 458, 418 and 374 nm to RuII(dp)0/bpydip(pp ) charge-transfer transitions, and at 282 nm, to a bpydip (p0/p*) intraligand transition. The electrochemistry of this complex is characterized by a reversible pair of waves at /0.30 and /1.70 V, ascribed to the RuIII/II and bpydip0/1 redox couples, respectively. In contact with water, the trans -[RuCl2(bpydip)] complex spontaneously and quantitatively converts into the aqua complex, leading to pronounced changes in the electronic and electrochemical behavior. A remarkable activity in the epoxidation of cyclohexene in the presence of iodosobenzene (PhIO) has been observed for the aqua complex. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium complexes; Tetradentate Schiff base; Epoxidation of cyclohexene
1. Introduction Through the years, Schiff bases have played a special role as chelating ligands in main group and transition metal coordination chemistry, due to their stability under a variety of oxidative and reductive conditions, and to the fact that imine ligands are borderline between hard and soft Lewis bases [1,2]. Transition metal complexes of tetradentate Schiff base ligands find applications in catalysis [3 /5] and as biomimetic enzyme models [6 /10]. Although a large number of compounds of the type M(N2O2), M(N2S2) and M(N4) (using the binding atom representation), have been synthesized and characterized, there are scarce reports on the N4 tetradentate Schiff base complexes of rutheniumII [1,11,12]. In contrast, similar ruthenium compounds of porphyrins, phthalocyanines and macrocyclic tetraden-
* Corresponding author. Tel.: /55-11-3091 3887; fax: /55-11-3815 5579 E-mail address: [email protected] (H.E. Toma).
tate amines are rather common [11,12]. Such complexes have been investigated because of their catalytic activity in oxygenation reactions [13,14], carrying out, for instance, the selective oxidation of organic substrates, such as olefins, alcohols, alkanes and aldehydes, under mild conditions. This property may be very attractive [15], since the oxidation products can provide important starting materials for the production of fine chemicals and polymers [16 /18]. In addition, the oxidation of organic substrates mediated by high valent rutheniumoxo species evokes much interest in the modelling of cytochrome P-450 [19]. Our interest on metal complexes with organic ligands, exhibiting variable degrees of s-basicity and p-acidity [20 /23] led us to investigate the ruthenium chemistry of a tetradentate Schiff base ligand system, N ,N ?-bis(7methyl-2-pyridylmethylene)-1,3-diiminopropane (bpydip). This ligand has two a,a?-diimine fragments and analogously to the rutheniumII-polypyridines [24], its complexes can also display charge-transfer processes. However, it should be mentioned that ruthenium com-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0020-1693(02)01480-9
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plexes possess predominantly octahedral cis geometry upon two a,a?-diimine ligands, and that only few examples of the trans complexes concerning Schiff bases are known in the literature up to the present time [11,12,25,26]. In this paper we describe the synthesis and the spectroscopic/electrochemical properties of the trans -[RuCl2(bpydip)] complex (Fig. 1) and the selective, catalytic activity of its aquation product, trans [Ru(OH2)2(bpydip)](PF6)2, in the cyclohexene epoxidation using iodosobenzene (PhIO) in 1,2-dichloroethane.
2. Experimental
51
for RuC17H20N4Cl2: C, 45.13; H, 4.46; N, 12.39%). nmax (cm 1): 1590s (C /N), 262w (Ru /Cl). 2.3. Synthesis of trans -[Ru(OH2)2(bpydip)](PF6)2 trans -[RuCl2(bpydip)] (0.050 g, 0.11 mmol) was dissolved in a minimum volume of water. An excess of a saturated aqueous NaPF6 solution was added, leading to the immediate formation of a red precipitate. The solid was filtered, washed with water and diethyl ether and stored under vacuum in the dark. Yield: 0.075 g (95%). (Found: C, 28.84; H, 3.45; N, 7.90. Calc. for RuC17H24N4O2P2F12: C, 28.86; H, 3.43; N, 7.92%).
2.1. Materials 2.4. Physical measurements The tetradentate Schiff base ligand was obtained in situ by condensation of 2-acetylpyridine (Aldrich) with 1,3-diaminopropane (Aldrich) in ethanol [27]. RuCl3 ×/ 3H2O (Aldrich) was the starting material for the synthesis of the ruthenium complex. All other solvents and chemicals were reagent grade (Aldrich or Merck) and used without further purification. All manipulations were carried out under inert atmosphere (Ar) following conventional techniques. 2.2. Synthesis of trans -[RuCl2(bpydip)] RuCl3 ×/3H2O (0.26 g, 1.0 mmol) and excess LiCl (1 g) were dissolved in ethanol (25 cm3) and the mixture was refluxed for 30 min. In separate, a mixture of 2acetylpyridine (0.12 g, 1.0 mmol) and 1,3-diaminopropane (0.037 g, 0.50 mmol) in ethanol (10 cm3) was refluxed separately for 45 min on a water bath, and then added to the ruthenium red /brown solution. Immediately, a deep blue solution resulted, which was further refluxed for 2 h on a water bath. After evaporation of the solvent under reduced pressure, the trans -[RuCl2(bpydip)] complex was purified by alumina column chromatography, using acetonitrile as eluent [25]. Yield: 0.10 g (23%). (Found: C, 45.10; H, 4.49; N, 12.31. Calc.
Elemental analyses were performed at Institute Analytical Center, using a Perkin/Elmer 2400 CHN Elemental Analyser. Electronic spectra were recorded on a Hewlett /Packard model 8453 A diode array spectrophotometer. IR spectra were obtained in KBr pellets on a Shimadzu FTIR-8300 spectrophotometer. 1H NMR spectra were obtained in 5 mm NMR tube on a Varian Inova 300 MHz spectrometer in CDCl3 or acetone-d6. Cyclic voltammetry measurements were carried out using a PARC system, from EG&G Instruments, consisting of a potentiostat model 283, and a three electrode cell arrangement. A platinum disc working electrode, a platinum wire auxiliary electrode and an Ag/ AgNO3 reference electrode (E 8/0.503 V vs. NHE) [28] were employed for the measurements. The software ECHEM V. 4.30 was used. Solutions of ruthenium complexes were prepared in acetonitrile containing 0.10 mol dm 3 tetraethylammonium perchlorate (Et4NClO4). Spectroelectrochemical measurements were carried out using a PARC potentiostat, model 173, in parallel with the Hewlett/Parckard diode array spectrophotometer. The quartz spectroelectrochemical cell used has a 0.025 cm optic length. A gold minigrid working electrode and a platinum auxiliary microelectrode were employed in these measurements versus an Ag /AgNO3 reference microelectrode. 2.5. General procedure for reactions of cyclohexene with oxidants in the presence of ruthenium complex
Fig. 1. MM/ structural representation of trans -[RuCl2(bpydip)] showing the labels employed in 2D COSY 1H NMR assignment.
In a typical oxidation reaction, the catalysts (0.010 mmol) and the olefin were dissolved in 1,2-dichloroethane (5 cm3). Iodosyl benzene (0.50 mmol, unless otherwise stated) was added at once to the solution, under an argon atmosphere, with constant stirring. Aliquots were taken at timed intervals and analyzed by GC. The products were identified by comparison with authentic samples.
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52
2.6. Molecular calculations Semiempirical molecular orbital calculations were carried out by using the ZINDO/S method within the HYPERCHEM 6.1 program from Hypercube. The spectroscopic parameters were based on the original parameterization [29] using the universal values of ks /1.267 and kp /0.585 as interaction factors. SCF molecular orbitals were obtained at the RHF level. Geometry optimizations were carried out using the ZINDO/1 method with its resident parameters. The electronic spectra simulation was carried out using single excited configuration interaction, within an active space involving ten occupied and ten unoccupied molecular orbitals. Initial nuclear coordinates of the complex were given by molecular mechanics calculations employing the MM/ module from HYPERCHEM, with a gradient of ˚ 1 as a convergence criterion in a 1 /105 kcal A conjugate gradient algorithm. All the calculations were performed on a PC WINDOWS platform with a Pentium III microprocessor.
The 2D NMR spectrum of trans -[RuCl2(bpydip)] is displayed in Fig. 2 and the data are collected in Table 1. The 1H NMR spectra are characterized by a relatively simple pattern of resonances from the aromatic ring protons of the N ,N ?-bis(7-methyl-2-pyridylmethylene)1,3-diiminopropane ligand. The pattern consists of two doublets and two triplets. The triplets appear at a slightly higher field than the doublets due to a p electron deficiency on the rings as a consequence of coordination of the tetradentate ligand to the metal center through the azomethine function and pyridine nitrogen. The relatively simple proton spectra indicate that both pyridine moieties are equivalently coordinated, supporting the almost planar coordination geometry of the tetradentate ligand (C1 symmetry on the ligand). The two pyridine rings swing away from each other in order to evade the intramolecular repulsion between two 6position hydrogen atoms on each pyridine as found for ironII complex [30]. The methyl, a- and b-methylene resonances were observed at higher field and were assigned from their relative areas. 3.2. Electronic spectra
3. Results and discussion 3.1. 1H NMR 1
H NMR and 2D COSY spectra were used to support the structure of the ruthenium complex shown in Fig. 1.
The electronic absorption [RuCl2(bpydip)] complex was trile, as shown in Fig. 3. deconvoluted in terms of five near UV region (Fig. 3).
Fig. 2. 2D COSY spectrum of trans -[RuCl2(bpydip)] in CDCl3 at 300 MHz.
spectrum of the trans recorded in dry acetoniThe spectrum can be strong bands in the Vis-
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Table 1 1 H NMR chemical shifts (d , ppm) for the trans -[RuX2(bpydip)]n (X/Cl, H2O; n/0, 2) complexes X
H(3)
H(4)
H(5)
H(6)
H(a)
H(b)
CH3
Cl H2O
7.88 (d) 8.10 (d)
7.80 (t) 8.04 (t)
7.45 (t) 7.70 (t)
9.45 (d) 9.50 (d)
4.36 (m) 4.60 (m)
2.67 (m) 2.30 (m)
2.72 (s) 2.85 (s)
d /doublet; t/triplet; m/multiplet; s/singlet.
Fig. 3. Electronic spectra of trans -[RuCl2(bpydip)] in acetonitrile. [Ru], 5/10 5 mol dm3.
The molecular orbitals of the trans -[RuCl2(bpydip)] complex were calculated using the ZINDO/S method, and the energies and composition (in terms of molecular constituents/fragments) for the relevant ones (i.e. the frontier levels) were collected in Table 2. As one can see, the HOMO level MO(65) exhibits a large metal /ligand mixed character, i.e. Ru (57.2%) and bpydip (40.0%). This orbital mixing is also significant in MO(64) and MO(63), although in these cases the Ru orbital contribution predominates, reflecting the increasing energy separation of the metal and ligand frontier orbitals. In contrast, the LUMO MO(66) and the two next higher
energy levels exhibit essentially bpydip p*-character, while MO(69) has a significant Ru contribution (22.9%), evidencing the important contribution of p-backbonding in this system. The experimental spectra can be successfully simulated, as shown in Table 3. All the electronic transitions in the Vis-near UV region exhibit metal-to-ligand charge-transfer character, while the absorption band at 282 nm is consistent with a bpydip p 0/p* transition. On changing the axial ligand in trans -[RuX2(bpydip)] from Cl to H2O, the MLCT bands are shifted to shorter wavelengths, reflecting the perturbation of the RuII dp-orbitals, as theoretically expected for the MO(65) and MO(64) levels (Table 2), considering their relative ligand field strengths. As a matter of fact, in contact with water, trans -[RuCl2(bpydip)] spontaneously and quantitatively converts into the aqua complex. The aquation reaction can be monitored spectrophotometrically from the disappearance of the trans -[RuCl2(bpydip)] bands around 644 and 608 nm and the growth of a new band at 520 nm (o /10 120 mol 1 dm3 cm 1), ascribed to the trans [Ru(OH2)2(bpydip)]2 complex (see Fig. 4). Theses changes are consistent with loss of Cl through an aqua-chloro intermediate absorbing at lmax /590 nm, as also observed in some polypyridyl complexes of ruthenium [31,32]. The trans -[Ru(OH2)2(bpydip)]2 complex undergoes reversible deprotonation leading to the decrease of the Table 3 Experimental and ZINDO/S calculated electronic transitions in the visible region for trans -[RuCl2(bpydip)]
Table 2 Energies and composition of the valence levels (ZINDO/S) for trans [RuCl2(bpydip)]
Experimental
MO
Energy (eV)
Ru (%)
Cl (%)
bpydip (%)
lmax (nm)
o (mol1 dm3 cm 1)
70 69 68 67 66(LUMO) 65(HOMO) 64 63 62 61
0.0305 0.0283 /0.0006 /0.0089 /0.0258 /0.238 /0.242 /0.269 /0.328 /0.336
6.40 22.9 2.70 5.90 3.40 57.2 62.8 86.2 3.70 2.50
0.30 0.40 0.10 0.40 1.70 2.80 5.90 0.50 5.80 6.70
93.3 76.7 97.2 93.7 94.9 40.0 31.3 13.3 90.5 90.8
644
Calculated lmax (nm)
Transition
10 480
637
607
12 690
604
458 418 374 282
2350 4450 3700 11 090
446 408 370 290 282
MO(65)0/MO(66) MO(64)0/MO(69) MO(63)0/MO(67) MO(63)0/MO(69) MO(64)0/MO(67) MO(65)0/MO(68) MO(64)0/MO(72) MO(62)0/MO(66) MO(61)0/MO(66)
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V.R. de Souza et al. / Inorganica Chimica Acta 348 (2003) 50 /56 Table 4 pKa of some aqua-substituted polypyridine-rutheniumII complexes Complex
a
trans -[Ru(OH2)2(bpydip)]2 b [Ru(OH2)(bpy)(terpy)]2 [Ru(OH2)(phen)(terpy)]2 [Ru(pic)(OH2)(terpy)] cis -[Ru(OH2)(bpy)2(py)]2 [Ru(acac)(OH2)(terpy)] [Ru(OH2)(terpy)(tmen)]2
pKa 8.9 9.7 10.0 10.0 10.8 11.0 11.2
acac, acetylacetonate; bpy: 2,2?-bipyridine; pic, picolinate anion; phen, 1,10-phenanthroline; py, pyridine; terpy, 2,2?:6,2ƒ-terpyridine; tmen, N ,N ,N ,N -tetramethylethylenediamine. a Ref. [33], except when specified. b This work. Fig. 4. Electronic spectra of the trans -[RuCl2(bpydip)] complex (5/ 10 5 mol dm3) in acetonitrile, and successive spectral measurements for its corresponding aqueous solution recorded just after the mixture. (a) [RuCl2(bpydip)]; (b) [RuCl(OH2)(bpydip)] ; (c) [Ru(OH2)2(bpydip)]2 .
intensity of the visible charge-transfer bands at 520 nm, showing isosbestic points at 480 and 350 nm (Fig. 5(A)). The pKa was determined by spectrophotometric titration, using Handerson /Hasselbach’s equation, pH / pKa/log(A0/A )/(A/Af), where A , A0 and Af are the measured absorbances at a given pH, and at the initial (lowest pH) and final (highest pH) points, respectively (Fig. 5(B)). The calculated pKa /8.99/0.1 is significantly lower than those observed for polypyridine rutheniumII complexes [33], listed in Table 4. This result reflects the strong p-electron withdrawing effect of the bpydip ligand, removing the electronic density on the rutheniumII center via back-bonding interactions.
3.3. Electrochemistry The cyclic voltammograms for the trans -[RuCl2(bpydip)] complex consist of two reversible monoelectronic waves at /0.30 and /1.70 V (vs. NHE), ascribed to the RuIII/II and bpydip0/ redox processes, respectively. The E 8 for the RuIII/II couple is smaller than for related dichlororuthenium complexes, e.g. cis -[RuCl2(bpy)2] (0.54 V vs. NHE) and trans -[RuCl2(bpy)2] (0.52 V). However, this value is consistent with that predicted from the linear optical-electrochemical correlation between E 8(RuIII/II) and MLCT energy, previously reported for many ruthenium systems [34 /36]. In this way, a lower E 8(RuIII/II) implies a lower MLCT energy, and vice-versa. In the presence of water, the RuIII/II redox wave is shifted from E1/2 /0.30 to 0.48 V, while lmax of the main MLCT band in the visible shifts from 644 to 520 nm, in agreement with the optical/electrochemical correlations. The oxidation of the ruthenium complex leads to the decay of the MLCT band in the visible (Fig. 6), with the rise of a new band at 450 nm (o /3250 mol 1 dm3 cm 1) ascribed to a ligand-to-metal charge transfer transition.
3.4. Preliminary studies on the catalytic epoxidation of cyclohexene
Fig. 5. (A) Electronic spectra of trans -[Ru(OH2)2(bpydip)]2 (5/ 10 5 mol dm 3) in aqueous solution; (B) plot of pH vs. log(A0/ A )/(A/Af), where A , A0 and Af are the measured absorbances at a given pH, and at the initial (lowest pH) and final (highest pH) points, respectively.
Ruthenium complexes are versatile catalysts for the oxidation of alkenes and they often promote the oxidative cleavage of the double bond into aldehydes or ketones [37]. However, in transition metal catalysed oxidation reactions, the nature of the coordinative environment around the metal also plays a significant role in catalytic activity, leading to different products, depending on the reaction conditions [38 /41]. In this way, selective epoxidation can be pursued by suitable modifications of the ligand moiety.
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Fig. 6. UV /Vis spectroelectrochemistry of trans [Ru(OH2)2(bpydip)]2 in water. [Ru], 5 /10 4 mol dm3; working electrode, Pt; reference electrode, Ag/AgNO3 (0.01 mol dm 3); m , 0.1 mol dm3 Et4NClO4; room temperature.
In this work, we have observed that trans [Ru(OH2)2(bpydip)]2 is indeed an effective catalyst for cyclohexene epoxidation by iodosobenzene (PhIO) in 1,2-dichloroethane. This preliminary study showed that when a mixture of 0.01 mmol of trans [Ru(OH2)2(bpydip)]2 and 0.5 mmol of PhIO was allowed to react, at 298 K, with a solution of cyclohexene (0.5 mmol) in 1,2-dichloroethane, cyclohexene oxide formed as the major product. In addition, minor amounts of byproducts were found, among which were 2-cyclohexenone and 2-cyclohexene-1-ol. The selectivity for cyclohexene oxide formation was almost 95%. The effect of varying the catalyst concentration and temperature for the oxidation of cyclohexene is shown
55
in Fig. 7. The yield of epoxide increased with the concentration of the catalyst. By increasing the temperature, the reaction rates and conversion of cyclohexene increased significantly. It should be noticed that a number of ruthenium complexes exhibit selectivity for the conversion of norbornene, cyclooctene, and linear alkenes to their respective epoxides, but in the case of cyclohexene, allylic attack is normally dominant to yield 2-cyclohexene-1-ol and 2-cyclohexene-1-one [39 /41]. However, the trans -[Ru(OH2)2(bpydip)]2 has proved to be an effective catalyst, reacting with cyclohexene via a preferential attack on the C /C bond to give mainly the epoxide, instead of the alternative attack on the C /H bond leading to allylic oxidation products [42,43]. We have also observed that the amount of PhI released during the catalytic reaction does not reflect the amount of the oxygen transferred. Consequently, for epoxidation reactions, an excess of PhIO is required [44 /46]. It was found that even in the absence of cyclohexene, the ruthenium complex reacts with PhIO to yield PhI, liberating significant amount of oxygen. Interestingly, the possible fate of the lost oxygen in related systems has already been discussed by Yang et al. [47], revealing that only a small part of the lost oxygen can be attributed to solvent oxidation. For the elucidation of this point a detailed mechanistic study is being carried out, including the characterization of the possible catalytic species, e.g. RuV /O or RuIV /O.
Acknowledgements We thank FAPESP and CNPq for financial support.
Fig. 7. Time scan for the oxidation of cyclohexene with PhIO catalysed by trans -[Ru(OH2)2(bpydip)]2 . (A) effect of temperature, concentration of catalyst, 0.01 mmol; (B) varying the catalyst concentration, temperature, 308 K.
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References [1] A.D. Garnovskii, A.L. Nivorozhkin, V.I. Minkin, Coord. Chem. Rev. 126 (1993) 1. [2] R. Ziessel, Coord. Chem. Rev. 216-217 (2001) 195. [3] X.-G. Zhou, J.-S. Huang, P.-H. Ko, K.-K. Cheung, C.-M. Che, J. Chem. Soc., Dalton Trans. (1999) 3303. [4] W. Ru¨ttinger, G.C. Dismukes, Chem. Rev. 97 (1997) 1. [5] W. Odenkirk, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 114 (1992) 6392. [6] M.L.P. Santos, I.A. Bagatin, E.M. Pereira, A.M.C. Ferreira, J. Chem. Soc., Dalton Trans. (2001) 838. [7] S. Me´nage, J.-B. Galey, J. Dumats, G. Hussler, M. Seite´, I.G. Luneau, G. Chottard, M. Fontecave, J. Am. Chem. Soc. 120 (1998) 13370. [8] J.-P. Costes, F. Dahan, A. Dupuis, J.-P. Laurent, J. Chem. Soc., Dalton Trans. (1998) 1307. [9] E.-G. Ja¨ger, J. Knaudt, M. Rudolph, M. Rost, Chem. Ber. 129 (1996) 1041. [10] S.R. Collinson, D.E. Fenton, Coord. Chem. Rev. 148 (1996) 19. [11] J.W.-S. Hui, W.-T. Wong, Coord. Chem. Rev. 172 (1998) 389. [12] S.-M. Lee, W.-T. Wong, Coord. Chem. Rev. 164 (1997) 415. [13] M.J. Upadhyay, P.K. Bhattacharya, P.A. Ganeshpure, S. Satish, J. Mol. Catal. 73 (1992) 277. [14] T. Mukaiyama, T. Yamada, Bull. Chem. Soc. Jpn. 68 (1995) 17. [15] D. Riley, M. Sterm, J. Ebner, D.H.R. Barton, A.E. Martell, D.T. Sawyer, The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum, New York, 1993. [16] R.A. Sheldon, J.K. Kochi, Metal Catalysed Oxidations of Organic Compounds, Academic Press, New York, 1981. [17] K.A. Jorgensen, Chem. Rev. 89 (1989) 431. [18] D. Riley, J. Lyon, J. Chem. Soc., Dalton Trans. (1991) 157. [19] B. Meunier, Chem. Rev. 92 (1992) 1411. [20] H.E. Toma, T.E. Chavez-Gil, Spectrosc. Lett. 32 (1999) 963. [21] H.E. Toma, T.E. Chavez-Gil, R.C. Rocha, H.R. Rechenberg, J. Incl. Phenom. Macro. 34 (1999) 57. [22] H.E. Toma, T.E. Chavez-Gil, Inorg. Chim. Acta 257 (1997) 197. [23] F.S. Nunes, H.E. Toma, J. Coord. Chem. 36 (1995) 33. [24] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Zelewsky, Coord. Chem. Rev. 84 (1988) 85. [25] S. Choudhury, M. Kakoti, A.K. Deb, S. Goswami, Polyhedron 11 (1992) 3183.
[26] K. Mizushima, M. Nakaura, S.-B. Park, H. Nishiyama, H. Monjushiro, K. Harada, M. Haga, Inorg. Chim. Acta 261 (1997) 175. [27] T.G. Campbell, F.L. Urbach, Inorg. Chem. 12 (1973) 1836. [28] B. Kratochvil, E. Lorah, C. Garber, Anal. Chem. 41 (1969) 1793. [29] (a) J.E. Ridley, M.C. Zerner, Theor. Chim. Acta 32 (1973) 111; (b) J.E. Ridley, M.C. Zerner, Theor. Chim. Acta 42 (1976) 223; (c) A.D. Bacon, M.C. Zerner, Theor. Chim. Acta 53 (1979) 21; (d) M.C. Zerner, G.H. Loew, R.F. Kirchner, U.T. MuellerWesterhoff, J. Am. Chem. Soc. 102 (1980) 589. [30] M. Goto, Y. Ishikawa, T. Ishihara, C. Nakatake, T. Higuchi, H. Kurosaki, V.L. Goedken, Chem. Commun. (1997) 539. [31] L. Lee, C.-H. Wang, J. Photochem. Photobiol. A: Chem. 79 (1994) 163. [32] A.R. Guadalupe, X. Chen, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 32 (1993) 5502. [33] A. Dovletoglou, S.A. Adeyemi, T.J. Meyer, Inorg. Chem. 35 (1996) 4120. [34] A.B.P. Lever, Inorg. Chem. 29 (1990) 1271. [35] E.S. Dosdsworth, A.B.P. Lever, Chem. Phys. Lett. 119 (1985) 61. [36] F. Barigelletti, A. Juris, V. Balzani, P. Belser, A. von Zelewsky, Inorg. Chem. 26 (1987) 4115. [37] M.H. Robbins, R.S. Drago, J. Chem. Soc., Dalton Trans. (1996) 105. [38] B. Cetinkaya, E. Centikaya, M. Brookhart, P.S. White, J. Mol. Catal. 142 (1999) 101. [39] G.A. Barf, D. van den Hoek, R.A. Sheldon, Tetrahedron 52 (1996) 12971. [40] G.A. Barf, R.A. Sheldon, J. Mol. Catal. 98 (1995) 143. [41] M. Bressan, A. Morvillo, Inorg. Chem. 28 (1989) 950. [42] C. Ho, W.H. Leung, C.M. Che, J. Chem. Soc., Dalton Trans. (1991) 2933. [43] E.G. Samsel, K. Srinivasan, J.K. Kochi, J. Am. Chem. Soc. 107 (1985) 7606. [44] A. Morvillo, M. Bressan, J. Mol. Catal. 125 (1997) 119. [45] A.S. Kanmani, S. Vancheesau, J. Mol. Catal. 125 (1997) 127. [46] V. Kesavan, S.J. Chandrasekaran, J. Chem. Soc., Perkin 1 (1997) 3115. [47] Y. Yang, F. Diederich, J.S. Valentine, J. Am. Chem. Soc. 113 (1991) 7195.