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Synthesis and catalytic activity of a novel ruthenium(III) complex containing a sugar-based ligand
Catalysis Communications 6 (2005) 459–461 www.elsevier.com/locate/catcom
Synthesis and catalytic activity of a novel ruthenium(III) complex containing a sugar-based ligand Debabrata Chatterjee
b
a,*
, Susan Basak a, Anannya Mitra a, Ayon Sengupta a, Jean Le Bras b, Jacques Muzart b
a Chemistry Section, Central Mechanical Engineering Research Institute, Durgapur 713209, India Unite´ Mixte de Recherche Re´actions Se´lectives et Applications, CNRS-Universite´ de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France
Received 10 November 2004; accepted 6 April 2005 Available online 2 June 2005
Abstract The novel mixed-ligand complex [RuIII(TDL)(bipy)(H2O)]+ (1) (TDL = N-3,5-ditertiarybutylsalicylidine-D-glucosamine; bipy = 2,2 0 -bipyridine) has been synthesized and characterized. Complex 1 found to catalyze the epoxidation of styrene and oxidize 1-indanol to 1-indanone in dichloromethane using t-BuOOH as a terminal oxidant. Enanantiomeric induction was observed in case of styrene epoxidation. Formation of a high valent Ru(V)-oxo species is proposed to be the catalytic oxidative species. 2005 Elsevier B.V. All rights reserved. Keywords: Ruthenium complex; Sugar-based ligand; t-BuOOH; Epoxidation; Enantiomeric excess; Styrene; 1-Indanol
1. Introduction Although the chemistry of metal complexes containing Schiff-base ligands is of continued interest [1], since they have common features with metalloporphyrins with respect to their electronic structure and catalytic activities that mimic enzymatic hydrocarbon oxidation [2], reports on the studies with ruthenium complexes containing Schiff base ligands in the catalysis of hydrocarbon oxidation are few in the literature [3–6]. Recently, catalytic activity of [RuIII(TDL)(XY)(H2O)] type of complexes (where TDL = tridentate Schiff-base ligands; XY = 2,2 0 bipyridine, picolinate) towards effecting hydrocarbon oxidations in a selective manner has been explored [7– 9]. The tridentate Schiff-base ligands (TDL) are easy to prepare. Further, it could afford sterogenic as well as chiral sites in [RuIII(TDL)(XY)(H2O)] type of complexes. In the present communication we report the synthesis and *
Corresponding author. Tel.: +91 343 546818; fax: +91 343 546745. E-mail address: [email protected] (D. Chatterjee).
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.04.005
catalytic activity of a novel [RuIII(TDL)(bipy)(H2O)]Cl complex (1) (TDL = N-3,5-ditertiarybutylsalicylidine-Dglucosamine; bipy = 2,2 0 -bipyridine). Though carbohydrates are naturally occurring enantiomeric pure compounds, catalytic applications of metal centers with sugar derived ligands for hydrocarbon oxidation are quite rare [10,11]. In the present communication, we report the synthesis, characterization and catalytic ability of a novel ruthenium catalyst complex containing sugar derived Schiff-base ligand. The potential of such ruthenium catalyst complex for olefin epoxidation has not been, to the best of our knowledge, so far explored.
2. Experimental 2.1. Synthesis of [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O Sugar-based Schiff-base ligand, N-3,5-di-tertiarybutylsalicylidine-D-glucosamine (TDL) was prepared by following the literature procedure [12]. D-Glucosamine
460
D. Chatterjee et al. / Catalysis Communications 6 (2005) 459–461
(1 mmol, 0.215 g) and NaHCO3 (1 mmol, 0.084 g) were dissolved in a water–methanol mixture (2 ml of water and 8 ml of methanol). To that mixture, methanolic solution (25 ml) of 3,5-ditertiarybutyl salicylaldehyde was added dropwise and the pale yellow resultant solution was allowed to react for 2 h at 50 C. The colour of the solution turned dark which was cooled down to room temperature and volume of the solution was reduced to obtain the solid product, which was washed with cold water followed by dichloromethane. The ligand was further recrystallized from methanol and dried over CaCl2. Yield 80%. Anal. Found (Calculated): C, 63.81 (63.76); H, 8.37 (8.4); N, 3.51 (3.54). FAB-MS: M + 1 396.67 (FM 395.5). 1H NMR (CD3OD, d ppm) = 9.87 (1H, s, Ar-OH), 8.44 (1H, s, CH@N), 7.61–7.17 (Aryl protons), 5.15 (1H, H-1 of sugar unit), 4.95–3.25 (sugar ring Hs), 1.49–1.19 (H, t-Bu) 13C NMR (CD3OD, d ppm) = 170.3 (C@N), 141.5–119.6 (six aryl carbons), 97.26 (C1 of sugar), 78.3–62.9 (carbons of sugar unit), 36.02–29.89 (carbons of t-Bu). 2.2. Synthesis of [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O The [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O was synthesized in two sequential steps: methanolic solution of RuCl3 Æ 3H2O (1 mmol) and Schiff-base ligand (1 mmol) were refluxed for 2 h and subsequently bipyridine (1 mmol) was added to the hot greenish brown solution and refluxing was continued for another 2 h. A dark brown solid separated, which was filtered, washed with water and few methanol, and finally dried in desiccator over CaCl2. Yield (70%). Anal. Calculated for C31H47N3O10RuCl: Calc. C, 49.0; H, 6.2; N, 5.5. Found. C, 48.7; H, 6.1; N, 5.2 leff = 1.95 BM. KM (X1 M1 cm2) in CH3OH = 86. UV–Vis in CH3OH: kmax (emax): 479 (4282), 349 (5912), 247 (16960); IR: mC@N 1648. 2.3. Instrumentation A Perkin–Elmer 240C elemental analyzer was used to collect microanalytical (C,H,N) data. The electronic absorption spectra were measured with a GBC Cintra 10 spectrophotometer. Infrared spectra were obtained on a Perkin–Elmer (Model 783) spectrometer. Electrochemical studies were carried out with a Voltalab PGZ 301 electrochemical equipment. Magnetic susceptibility was measured by using a PAR-155 vibrating sample magnetometer. NMR studies were performed on a Bruker 300AC NMR spectrometer in CD3OD. EPR spectra of the ruthenium complex were recorded on a JEOL RE-1X EPR spectrometer at room temperature using DPPH as a marker. 2.4. Procedure of catalytic studies In a typical experiment 0.01 mmol of the catalyst complex, 1.0 mmol of 70% aqueous t-BuOOH oxidant
and 1.0 mmol of substrate in 5 ml of CH2Cl2 were rapidly magnetically stirred at room temperature for 14 h. Water (5 ml) was added, the mixture was stirred for 10 min and aqueous phase was extracted with CH2Cl2 (5 ml · 3). The organic phase was dried over MgSO4, filtered and concentrated. Flash chromatography (SiO2, 95% pet ether/5% EtOAc) of the concentrated liquid mass afforded the desired product. Estimation of enantiomertic excess (ee) of chiral epoxide was estimated by GC analysis using chiral column (Cyclodex B).
3. Results and discussion The UV–Vis spectra of 1 in CH3OH exhibited charge transfer bands. The bands appeared in the UV region are characterized by intra-ligand charge transition, whereas bands displayed in the visible region are assigned to the ligand to metal charge transfer bands (p ! t2g origin). The basis of assignment is the spectral data observed earlier [7–9] for the mixed-chelate ruthenium(III)complexes containing Schiff base and bipyridine ligands. The IR spectra of the complexes displayed bands archetypal for coordinated Schiff-base ligands (viz. 1620–1640 cm1 for C@N stretch). Conductance data established the 1:1 electrolyte nature of 1 in CH3OH solution. The magnetic moment of 1 conforms to the low-spin d5 configurations (idealized t52g , S = 1/2). EPR spectrum of 1 was found to be rhombic in nature. The corresponding g values (g1 = 2.525, g2 = 2.329 and g3 = 1.783) are in good agreement with the g values reported for distorted octahedral ruthenium(III) complexes [13]. Cyclic voltammogram of 1 in CH3OH solution displayed a quasi-reversible RuIII/RuII redox couple at E1/2 = 0.2 V (vs. SCE) with a peak-to-peak separation (DEp = 120 mV). The complex-1 catalyzed oxidation of styrene, and 1indanol was performed using t-BuOOH as a terminal oxidant as the use of alkyl hydroperoxide is suitable for such type of ruthenium catalyst complexes [14,15]. Further, safety and environmental concerns have attached special importance to the oxidation catalysis with alkyl hydroperoxides and hydrogen peroxide. In case of complex-1 catalyzed epoxidation of styrene, styrene epoxide was found to be the reaction product (57% yield) as confirmed by GC analysis. Formation of styrene epoxide was further confirmed by 1H NMR analysis. The ruthenium catalyst complex (1) has been found to be successful in bringing about enantioselective epoxidation of styrene with 37% ee. The catalytic ability of 1 was also examined with 1-indanol as substrate. Formation of 1indanone (52%) as a major product together with some amounts of the diketonic product were observed in the oxidation of 1-indanol. Based on the present experimental results and considering our earlier observations [7–9]
D. Chatterjee et al. / Catalysis Communications 6 (2005) 459–461
on the catalytic activity of mixed-chelate complexes of ruthenium(III) a following working mechanism is proposed for the overall catalytic epoxidation process.
O OH2 N HC Ru N O N HO HO
thankful to Dr. G.P. Sinha, Director, CMERI, Durgapur, India for his encouragement to this work. A.M. is thankful to CSIR for extended SRF.
HC + t-BuOOH
O O Ru N O
HO HO
O
461
N N O
OH
OH
O O C Ru N O HO HO
N
+
N
HC
O OH2 N Ru N O N
HO HO
O
O +
O OH
OH
The intermediacy of Ru(V)-oxo species is proposed on the basis of the fact that the spectral features (UV–Vis and IR) of the resultant solution that obtained upon addition of solution of t-BuOOH to an aqueous solution of 1 has close resemblance of the spectral features of oxoruthenium(V) complexes reported earlier [9,16–18].
4. Conclusion In the present communication we report for the first time the synthesis of a ruthenium complex (1) that contains a sugar-based Schiff-base ligand. Catalytic activity of such complex towards effecting asymmetric induction in aromatic alkene epoxidation has been explored. Detailed mechanistic studies including other substrates, terminal oxidants and solvents are in progress.
Acknowledgements We gratefully acknowledge the financial support obtained from Indo-French Center for Promotion of Advanced Research (IFCPAR Grant No. 2905-1). D.C. is
References [1] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189. [2] J.T. Groves, in: P. Ortiz de Montellano (Ed.), Cytochrome P-450: Structure, Mechanism and Biochemistry, 7, Plenum Press, New York, 1986, p. 1. [3] W.-H. Leung, Chi-M. Che, Inorg. Chem. 28 (1989) 4619. [4] T. Takeda, R. Irie, T. Katsuki, Synlett 7 (1999) 1166. [5] N. Nakata, T. Takeda, J. Mihara, T. Hamada, R. Ire, T. Katsuki, Eur. J. Chem. 7 (2001) 3776. [6] V.R. de Souza, G.S. Nunes, R.C. Rocha, H.E. Toma, Inorg. Chim. Acta. 348 (2003) 50. [7] D. Chatterjee, A. Mitra, Inorg. Chem. Commun. 3 (2000) 640. [8] D. Chatterjee, A. Mitra, S. Mukherjee, J. Mol. Catal. A 165 (2001) 295. [9] D. Chatterjee, A. Mitra, R.E. Shepherd, Inorg. Chim. Acta 357 (2004) 980. [10] O. Pamies, M. Diegnez, G. Net, A. Ruiz, C. Claver, J. Chem. Soc. Dalton Trans. (1999) 3439. [11] M. Diegnez, O. Pamies, A. Ruiz, C. Claver, New. J. Chem. 26 (2002) 827. [12] L.J. Theriot, G.O. Carlisle, H.J. Hu, J. Inorg. Nucl. Chem. 31 (1969) 2841. [13] O.K. Medı´, U. Agarwala, Inorg. Chem. 19 (1980) 1381. [14] T.C. Lau, C.M. Che, W.O. Lee, C.K. Poon, J. Chem. Soc. Chem. Commun. (1988) 1406. [15] R.E. Shepherd, Inorg. Chim. Acta. 209 (1993) 201. [16] K.Y. Wong, C.M. Che, F.C. Anson, Inorg. Chem. 26 (1987) 737. [17] C.M. Che, V.W.W. Yam, T.C.W. Mak, J. Am. Chem. Soc. 112 (1990) 2284. [18] G.A. Barf, R.A. Sheldon, J. Mol. Catal. A 102 (1995) 23.
Synthesis and catalytic activity of a novel ruthenium(III) complex containing a sugar-based ligand Debabrata Chatterjee
b
a,*
, Susan Basak a, Anannya Mitra a, Ayon Sengupta a, Jean Le Bras b, Jacques Muzart b
a Chemistry Section, Central Mechanical Engineering Research Institute, Durgapur 713209, India Unite´ Mixte de Recherche Re´actions Se´lectives et Applications, CNRS-Universite´ de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France
Received 10 November 2004; accepted 6 April 2005 Available online 2 June 2005
Abstract The novel mixed-ligand complex [RuIII(TDL)(bipy)(H2O)]+ (1) (TDL = N-3,5-ditertiarybutylsalicylidine-D-glucosamine; bipy = 2,2 0 -bipyridine) has been synthesized and characterized. Complex 1 found to catalyze the epoxidation of styrene and oxidize 1-indanol to 1-indanone in dichloromethane using t-BuOOH as a terminal oxidant. Enanantiomeric induction was observed in case of styrene epoxidation. Formation of a high valent Ru(V)-oxo species is proposed to be the catalytic oxidative species. 2005 Elsevier B.V. All rights reserved. Keywords: Ruthenium complex; Sugar-based ligand; t-BuOOH; Epoxidation; Enantiomeric excess; Styrene; 1-Indanol
1. Introduction Although the chemistry of metal complexes containing Schiff-base ligands is of continued interest [1], since they have common features with metalloporphyrins with respect to their electronic structure and catalytic activities that mimic enzymatic hydrocarbon oxidation [2], reports on the studies with ruthenium complexes containing Schiff base ligands in the catalysis of hydrocarbon oxidation are few in the literature [3–6]. Recently, catalytic activity of [RuIII(TDL)(XY)(H2O)] type of complexes (where TDL = tridentate Schiff-base ligands; XY = 2,2 0 bipyridine, picolinate) towards effecting hydrocarbon oxidations in a selective manner has been explored [7– 9]. The tridentate Schiff-base ligands (TDL) are easy to prepare. Further, it could afford sterogenic as well as chiral sites in [RuIII(TDL)(XY)(H2O)] type of complexes. In the present communication we report the synthesis and *
Corresponding author. Tel.: +91 343 546818; fax: +91 343 546745. E-mail address: [email protected] (D. Chatterjee).
1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.04.005
catalytic activity of a novel [RuIII(TDL)(bipy)(H2O)]Cl complex (1) (TDL = N-3,5-ditertiarybutylsalicylidine-Dglucosamine; bipy = 2,2 0 -bipyridine). Though carbohydrates are naturally occurring enantiomeric pure compounds, catalytic applications of metal centers with sugar derived ligands for hydrocarbon oxidation are quite rare [10,11]. In the present communication, we report the synthesis, characterization and catalytic ability of a novel ruthenium catalyst complex containing sugar derived Schiff-base ligand. The potential of such ruthenium catalyst complex for olefin epoxidation has not been, to the best of our knowledge, so far explored.
2. Experimental 2.1. Synthesis of [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O Sugar-based Schiff-base ligand, N-3,5-di-tertiarybutylsalicylidine-D-glucosamine (TDL) was prepared by following the literature procedure [12]. D-Glucosamine
460
D. Chatterjee et al. / Catalysis Communications 6 (2005) 459–461
(1 mmol, 0.215 g) and NaHCO3 (1 mmol, 0.084 g) were dissolved in a water–methanol mixture (2 ml of water and 8 ml of methanol). To that mixture, methanolic solution (25 ml) of 3,5-ditertiarybutyl salicylaldehyde was added dropwise and the pale yellow resultant solution was allowed to react for 2 h at 50 C. The colour of the solution turned dark which was cooled down to room temperature and volume of the solution was reduced to obtain the solid product, which was washed with cold water followed by dichloromethane. The ligand was further recrystallized from methanol and dried over CaCl2. Yield 80%. Anal. Found (Calculated): C, 63.81 (63.76); H, 8.37 (8.4); N, 3.51 (3.54). FAB-MS: M + 1 396.67 (FM 395.5). 1H NMR (CD3OD, d ppm) = 9.87 (1H, s, Ar-OH), 8.44 (1H, s, CH@N), 7.61–7.17 (Aryl protons), 5.15 (1H, H-1 of sugar unit), 4.95–3.25 (sugar ring Hs), 1.49–1.19 (H, t-Bu) 13C NMR (CD3OD, d ppm) = 170.3 (C@N), 141.5–119.6 (six aryl carbons), 97.26 (C1 of sugar), 78.3–62.9 (carbons of sugar unit), 36.02–29.89 (carbons of t-Bu). 2.2. Synthesis of [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O The [RuIII(TDL)(bipy)(H2O)]Cl Æ 3H2O was synthesized in two sequential steps: methanolic solution of RuCl3 Æ 3H2O (1 mmol) and Schiff-base ligand (1 mmol) were refluxed for 2 h and subsequently bipyridine (1 mmol) was added to the hot greenish brown solution and refluxing was continued for another 2 h. A dark brown solid separated, which was filtered, washed with water and few methanol, and finally dried in desiccator over CaCl2. Yield (70%). Anal. Calculated for C31H47N3O10RuCl: Calc. C, 49.0; H, 6.2; N, 5.5. Found. C, 48.7; H, 6.1; N, 5.2 leff = 1.95 BM. KM (X1 M1 cm2) in CH3OH = 86. UV–Vis in CH3OH: kmax (emax): 479 (4282), 349 (5912), 247 (16960); IR: mC@N 1648. 2.3. Instrumentation A Perkin–Elmer 240C elemental analyzer was used to collect microanalytical (C,H,N) data. The electronic absorption spectra were measured with a GBC Cintra 10 spectrophotometer. Infrared spectra were obtained on a Perkin–Elmer (Model 783) spectrometer. Electrochemical studies were carried out with a Voltalab PGZ 301 electrochemical equipment. Magnetic susceptibility was measured by using a PAR-155 vibrating sample magnetometer. NMR studies were performed on a Bruker 300AC NMR spectrometer in CD3OD. EPR spectra of the ruthenium complex were recorded on a JEOL RE-1X EPR spectrometer at room temperature using DPPH as a marker. 2.4. Procedure of catalytic studies In a typical experiment 0.01 mmol of the catalyst complex, 1.0 mmol of 70% aqueous t-BuOOH oxidant
and 1.0 mmol of substrate in 5 ml of CH2Cl2 were rapidly magnetically stirred at room temperature for 14 h. Water (5 ml) was added, the mixture was stirred for 10 min and aqueous phase was extracted with CH2Cl2 (5 ml · 3). The organic phase was dried over MgSO4, filtered and concentrated. Flash chromatography (SiO2, 95% pet ether/5% EtOAc) of the concentrated liquid mass afforded the desired product. Estimation of enantiomertic excess (ee) of chiral epoxide was estimated by GC analysis using chiral column (Cyclodex B).
3. Results and discussion The UV–Vis spectra of 1 in CH3OH exhibited charge transfer bands. The bands appeared in the UV region are characterized by intra-ligand charge transition, whereas bands displayed in the visible region are assigned to the ligand to metal charge transfer bands (p ! t2g origin). The basis of assignment is the spectral data observed earlier [7–9] for the mixed-chelate ruthenium(III)complexes containing Schiff base and bipyridine ligands. The IR spectra of the complexes displayed bands archetypal for coordinated Schiff-base ligands (viz. 1620–1640 cm1 for C@N stretch). Conductance data established the 1:1 electrolyte nature of 1 in CH3OH solution. The magnetic moment of 1 conforms to the low-spin d5 configurations (idealized t52g , S = 1/2). EPR spectrum of 1 was found to be rhombic in nature. The corresponding g values (g1 = 2.525, g2 = 2.329 and g3 = 1.783) are in good agreement with the g values reported for distorted octahedral ruthenium(III) complexes [13]. Cyclic voltammogram of 1 in CH3OH solution displayed a quasi-reversible RuIII/RuII redox couple at E1/2 = 0.2 V (vs. SCE) with a peak-to-peak separation (DEp = 120 mV). The complex-1 catalyzed oxidation of styrene, and 1indanol was performed using t-BuOOH as a terminal oxidant as the use of alkyl hydroperoxide is suitable for such type of ruthenium catalyst complexes [14,15]. Further, safety and environmental concerns have attached special importance to the oxidation catalysis with alkyl hydroperoxides and hydrogen peroxide. In case of complex-1 catalyzed epoxidation of styrene, styrene epoxide was found to be the reaction product (57% yield) as confirmed by GC analysis. Formation of styrene epoxide was further confirmed by 1H NMR analysis. The ruthenium catalyst complex (1) has been found to be successful in bringing about enantioselective epoxidation of styrene with 37% ee. The catalytic ability of 1 was also examined with 1-indanol as substrate. Formation of 1indanone (52%) as a major product together with some amounts of the diketonic product were observed in the oxidation of 1-indanol. Based on the present experimental results and considering our earlier observations [7–9]
D. Chatterjee et al. / Catalysis Communications 6 (2005) 459–461
on the catalytic activity of mixed-chelate complexes of ruthenium(III) a following working mechanism is proposed for the overall catalytic epoxidation process.
O OH2 N HC Ru N O N HO HO
thankful to Dr. G.P. Sinha, Director, CMERI, Durgapur, India for his encouragement to this work. A.M. is thankful to CSIR for extended SRF.
HC + t-BuOOH
O O Ru N O
HO HO
O
461
N N O
OH
OH
O O C Ru N O HO HO
N
+
N
HC
O OH2 N Ru N O N
HO HO
O
O +
O OH
OH
The intermediacy of Ru(V)-oxo species is proposed on the basis of the fact that the spectral features (UV–Vis and IR) of the resultant solution that obtained upon addition of solution of t-BuOOH to an aqueous solution of 1 has close resemblance of the spectral features of oxoruthenium(V) complexes reported earlier [9,16–18].
4. Conclusion In the present communication we report for the first time the synthesis of a ruthenium complex (1) that contains a sugar-based Schiff-base ligand. Catalytic activity of such complex towards effecting asymmetric induction in aromatic alkene epoxidation has been explored. Detailed mechanistic studies including other substrates, terminal oxidants and solvents are in progress.
Acknowledgements We gratefully acknowledge the financial support obtained from Indo-French Center for Promotion of Advanced Research (IFCPAR Grant No. 2905-1). D.C. is
References [1] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189. [2] J.T. Groves, in: P. Ortiz de Montellano (Ed.), Cytochrome P-450: Structure, Mechanism and Biochemistry, 7, Plenum Press, New York, 1986, p. 1. [3] W.-H. Leung, Chi-M. Che, Inorg. Chem. 28 (1989) 4619. [4] T. Takeda, R. Irie, T. Katsuki, Synlett 7 (1999) 1166. [5] N. Nakata, T. Takeda, J. Mihara, T. Hamada, R. Ire, T. Katsuki, Eur. J. Chem. 7 (2001) 3776. [6] V.R. de Souza, G.S. Nunes, R.C. Rocha, H.E. Toma, Inorg. Chim. Acta. 348 (2003) 50. [7] D. Chatterjee, A. Mitra, Inorg. Chem. Commun. 3 (2000) 640. [8] D. Chatterjee, A. Mitra, S. Mukherjee, J. Mol. Catal. A 165 (2001) 295. [9] D. Chatterjee, A. Mitra, R.E. Shepherd, Inorg. Chim. Acta 357 (2004) 980. [10] O. Pamies, M. Diegnez, G. Net, A. Ruiz, C. Claver, J. Chem. Soc. Dalton Trans. (1999) 3439. [11] M. Diegnez, O. Pamies, A. Ruiz, C. Claver, New. J. Chem. 26 (2002) 827. [12] L.J. Theriot, G.O. Carlisle, H.J. Hu, J. Inorg. Nucl. Chem. 31 (1969) 2841. [13] O.K. Medı´, U. Agarwala, Inorg. Chem. 19 (1980) 1381. [14] T.C. Lau, C.M. Che, W.O. Lee, C.K. Poon, J. Chem. Soc. Chem. Commun. (1988) 1406. [15] R.E. Shepherd, Inorg. Chim. Acta. 209 (1993) 201. [16] K.Y. Wong, C.M. Che, F.C. Anson, Inorg. Chem. 26 (1987) 737. [17] C.M. Che, V.W.W. Yam, T.C.W. Mak, J. Am. Chem. Soc. 112 (1990) 2284. [18] G.A. Barf, R.A. Sheldon, J. Mol. Catal. A 102 (1995) 23.