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Metallo-deuteroporphyrins as catalysts for the oxidation of cyclohexane with air in the absence of additives and solvents
Catalysis Communications 10 (2008) 83–85
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Metallo-deuteroporphyrins as catalysts for the oxidation of cyclohexane with air in the absence of additives and solvents Bingcheng Hu *, Weiyou Zhou, Dengsheng Ma, Zuliang Liu College of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 25 July 2008 Accepted 29 July 2008 Available online 11 August 2008 Keywords: Deuteroporphyrins Cyclohexane oxidation Air Catalysis
a b s t r a c t The catalytic properties of metallo-deuteroporphyrins derived from heme have been studied in the air oxidation of cyclohexane. We have found that these complexes without meso-substituents are higher active catalysts compared to the tetraphenylporphyrins in the investigated reaction. The influence of the central metals and substituents in the macrocycle on the activity of these porphyrins is discussed. The cyclohexane oxidation catalyzed by metallo-deuteroporphyrins may undergo a different mechanism from that proposed for cytochrome P-450. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of the catalyst for selective oxidation of alkanes to corresponding alcohols and ketones with air continues to be a challenge of the industrial chemistry. A possible strategy is to build biomimetic or bioinspired catalysts [1]. During the last decades of years, a huge amount of work has shown that substituted metalloporphyrins are efficient catalysts for the direct oxidation of alkanes by air to give alcohols and/or carbonyl compounds at unprecedented rates under very mild conditions without co-reductants or stoichiometric oxidants. However, nearly all of the metalloporphyrins used as oxidation catalysts have been based on the system of synthetic meso-tetraphenylporphyrins (TPPs) [2–6]. There are few models derived from heme without substituents in the meso position [7,8]. Thanks to the difference existing between the structures of TPP and heme, the porphyrin ligand derived from heme could result in different or improved catalytic properties. In this regard, it is interesting to note that the prosthetic group of cytochrome P-450, iron-protoporphyrin 1x, contains no phenyl groups in the meso positions and in fact is unsubstituted in the positions. In this communication, we report that metallo-deuteroporphyrins dimethylester [M (DPDME)] derived from heme exhibit efficient catalytic activity in the liquid phase oxidation of
* Corresponding author. Tel./fax: +86 025 84315030. E-mail address: [email protected] (B. Hu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.07.044
cyclohexane in the absence of any cocatalyst or reductant. For the sake of comparison of the effects of substituents in the macrocycle on the reactivity, we also investigated the reactivity of meso-tetraphenylporphyrins in the same catalytic system. 2. Experimental All reagents and solvents were of analytical grade and were obtained commercially. Porphyrin ligands, tetra (p-chlorophenyl) porphyrin, TCPP, and tetra (p-methoxyl) porphyrin, TMOPP, were prepared and purified as reported [9,10]. And the DPDME was synthesized according to published procedures [11]. Cobalt, zinc, copper and nickel were then introduced by a standard metal insertion method [12]. No impurities were found in the cyclohexane by GC–MS analysis before use. GC–MS analysis of the catalytic oxidation products was accomplished with a Finnigan DSQ Quantum Mass Spectrometer with a 0.5 mm i.d. 50 m PEG2000 capillary column. The Soret bands of the metallo-porphyrins were measured by UV–Vis spectra. The catalytic oxidation of cyclohexane has been carried out in a stainless steel reactor of 2 L volume equipped with mechanical stirrer, internal thermocouple and cooling coils for regulating reaction temperature. In the typical experiment, the optimum temperature was set at 418 K, and the air pressure was 0.8 Mpa, the concentration of the metallo-porphyrin was 2 105 mol/L. During the reaction, until the yield decreased markedly, we sampled every 30 min. After the reaction, the oxidation was stopped and the products were analyzed by means of the GC–MS.
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B. Hu et al. / Catalysis Communications 10 (2008) 83–85
3. Results and discussion
16
R2 R1
R1 N
N M
R2 N
R2 N
R1
R1 R3
R2
R3
Fig. 1. Formula of the main metalloporphyrins mentioned in the text. Deuteroporphyrin dimethylester, DPDME, R1 = CH3, R2 = H, R3 = CH2CH2COOCH3; tetra (pmethoxyphenyl)porphyrin, TMOPP, R1 = H, R2 = p-methoxyphenyl, R3 = H; tetra (pchlorophenyl) porphyrin, TCPP, R1 = H, R2 = p-chlorophenyl, R3 = H. M = Co, Cu, Zn, Ni.
Table 1 Oxidation of cyclohexane catalysed by different metallo-porphyrinsa Catalyst
Conversion of cyclohexane (%)
Selectivityb (%)
ol/onec
Turnover number of catalyst
Co(II) (TMOPP) Co(II) (TCPP) Co(II) (DPDME) Cu(II) (TMOPP) Cu(II) (TCPP) Cu(II) (DPDME) Zn(II) (TMOPP) Zn(II) (TCPP) Zn(II) (DPDME) Ni(II) (TMOPP) Ni(II) (TCPP) Ni(II) (DPDME)
6.08 11.04 18.58 5.63 10.21 12.08 5.70 10.41 12.99 5.57 10.15 8.18
90.35 87.20 84.63 89.43 87.95 89.17 90.10 87.11 90.73 90.67 87.03 90.42
1.27 1.05 1.13 1.18 1.07 1.20 1.13 1.06 1.06 1.00 1.05 0.97
29,676 55,131 85,147 34,558 51,241 55,798 28,037 52,297 60,135 27,162 50,602 37,506
a b c
See conditions in text, the reaction time was 4 h while 3 h for Co(II) (DPDME). Selectivity = [n (ol) + n (one)]/n (conversion). ol/one = n (ol)/n (one).
DPDME
14 12 Yield (%)
We have investigated the oxidation reaction of cyclohexane which is a rather poorly reactive substrate catalyzed by various metallo-porphyrins mentioned in the Fig. 1. The main products of the oxidation of cyclohexane by air are cyclohexanol and cyclohexanone. Table 1 summarizes the data obtained from our catalytic experiments. The complexes were active in the reaction at temperature 418 K. As shown in Table 1, the conversion of cyclohexane varies evidently with the change of the central metal, and change of the selectivity of the alcohol and ketone is very small. In the series of the DPDME, we found that Co(II) (DPDME) is the best catalyst in oxidation. When the central metal was Cu or Zn ions, the conversion was lower and for Ni ion as the central metal even a two times lower conversion was obtained. We observed the following order of reactivity: Co(II) > Cu(II) > Zn(II) > Ni(II), as same as that of the series of TMOPP and TCPP. This phenomenon may be due to the fact that the redox potential of cobalt is higher than that of other metals. In agreement with earlier observations [4,13], the catalytic activity of metallo-porphyrins increases with the increasing of the redox potential. Besides the central metals of the complexes, several other effects are usually considered in the discussion of the correlation between the properties of metallo-porphyrins and their catalytic activity in oxidation processes: electron and geometric effects of the substituents in the macrocycle. It has been found that in the case of metal porphyrins, the introduction of electron-withdrawing substituents at the macrocycle increase the redox potential of M(III)/M(II) porphyrin system and increase the catalytic activity of XM (TPP) [14,15], where X = F, Cl, Br, I. Depending on the type of the substituent, they may also exert a steric influence. Contrary to the effect, the electron-donating substituents decrease the
10 TCPP
8 6 4
TMOPP
2 0 0
1
2
3 4 Reaction time (h)
5
6
Fig. 2. Oxidation of cyclohexane using cobalt complexes with different porphyrin ligands: (j) DPDME; (d) TCPP; (N) TMOPP. See condition in text. Yield = [n (ol) + n (one)]/n (cyclohexane).
redox potential of the MDPDME due to the stabilization of the high oxidation state of the metal, but the catalytic activity in the oxidation of cyclohexane was also found to increase, Fig. 2. The overall yield of alcohol and ketone after 3.5 h at 418 K has substantially reached as high as 15.72% when Co(II) (DPDME) is used as catalyst. Fig. 2 indicates the relative catalytic activities of the cobalt complexes. The frequencies of the visible absorption maxima (Soret band) are dependent on the substituents [15]. It is related to the increase in the negative charge in the macrocycle and corresponds to the lower redox potentials. The Soret band of Co(II) (TMOPP), Co(II) (TCPP) and Co(II) (DPDME) occurs at 412 nm, 414 nm and 392 nm, respectively. So the order of reduction potentials of these cobalt complexes must be TCPP > TMOPP > DPDME. It is known that the introduction of the electron-withdrawing substituents at the meso- and/or b-position remarkably increases both the catalytic activity and the redox potential. Surprisingly, in our case the situation is opposite. The metallo-dueteroporphyrins with the lower redox potential are more active catalysts in the investigated system. Apparently, the character of electronic effect does not play a pronounced influence on the catalytic properties of the deuteroporphyrins. A primary factor maybe that the reaction proceed a mechanism different from that proposed for cytochrome P-450, because in that mechanism the catalytic activity is expected to increase with the redox potential of the metallo-porphyrins. The reaction mechanism is still not well established and several pathways have been brought up [4,15,16]. From the phenomenon and the results observed in the experiments, we supposed that pathway of the oxidation of cyclohexane may be consistent with the mechanism proposed by Haber et al. [3,17]. According to the mechanism, the axial ligand participates in the reaction and initiated it by generating the radicals with simultaneous reduction of the metallo-porphyrin
XCoIII ðTPPÞ þ C6 H12 ¼ CoII ðTPPÞ þ HX þ C6 H11 The lower the redox potential of the complex, the easier is this process, as indeed observed. 4. Conclusion The dueteroporphyrins complexed with different metals have the effective behavior as catalysts in the cyclohexane oxidation
B. Hu et al. / Catalysis Communications 10 (2008) 83–85
by dioxygen. In the series of M (DPDME), where M = Co, Cu, Zn, Ni, cyclohexane being oxidized to alcohol and ketone with the relatively high yields, without the use of sacrificial co-reductant. Unlike the tetra-arylporphyrins, the catalytic activity of these complexes increases with the decreasing potential. The oxidation of cyclohexane catalyzed by metallo-deuteroporphyrins may undergo a different mechanism from that proposed for cytochrome-P-450. The nature of interactions between the reactivity and the substituents and the mechanism of the reactions is not understood, and further experiments are required for a better understanding of the detailed mechanism of alkane oxidation and the stabilization of the metallo-deuteroporphyrins in such system. References [1] D. Mansuy, C.R. Chim. 10 (2007) 392. [2] P. Tagliatesta, D. Giovannetti, A. Leoni, M.G.P.M.S. Neves, J.A.S. Cavaleiro, J. Mol. Catal. A: Chem. 252 (2006) 96.
85
[3] J. Haber, L. Matachowski, K. Pamin, J. Poltowicz, J. Mol. Catal. A: Chem. 162 (2000) 105. [4] J.E. Lyons, P.E. Ellis Jr., H.K. Myers Jr., J. Catal. 155 (1995) 59. [5] J. Poltowicz, K. Pamin, J. Haber, J. Mol. Catal. A: Chem. 257 (2006) 154. [6] Z. Gross, S. Nimri, L. Simkhovich, J. Mol. Catal. A: Chem. 113 (1996) 231. [7] J.H. Fuhrhop, M. Baccouche, H. Grabow, J. Mol. Catal. A: Chem. 7 (1980) 245. [8] E. Monzani, L. Casella, M. Gullotti, N. Panigada, F. Franceschi, V. Papaefthymiou, J. Mol. Catal. A: Chem. 117 (1997) 199. [9] H. Türk, W.T. Ford, J. Org. Chem. 56 (1991) 1253. [10] C.-C. Guo, Z.-P. Li, Chem. J. Chinese Univ. 18 (1997) 242. [11] D.-S. Ma, B.-C. Hu, C.-X. Lü, B. Cao, Chinese Appl. Chem. 23 (2006) 961. [12] A.D. Alder, F.R. Longo, F. Kampas, J. Inorg. Nucl. Chem. 32 (1970) 2443. [13] J. Haber, L. Matachowski, K. Pamin, J. Poltowicz, J. Mol. Catal. A: Chem. 198 (2003) 215. [14] H. Fujii, J. Am. Chem. Soc. 115 (1993) 4641. [15] E.R. Birnbaum, M.W. Grinstaff, Y.A. Labinger, J.E. Bercaw, H.B. Gray, J. Mol. Catal. 104 (1995) L119. [16] G.S. Nunes, I. Mayer, H.E. Toma, K. Araki, J. Catal. 236 (2005) 55. [17] J. Poltowicz, E. Tabor, K. Pamin, J. Haber, Inorg. Chem. Commun. 8 (2005) 1125.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Metallo-deuteroporphyrins as catalysts for the oxidation of cyclohexane with air in the absence of additives and solvents Bingcheng Hu *, Weiyou Zhou, Dengsheng Ma, Zuliang Liu College of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 29 May 2008 Received in revised form 25 July 2008 Accepted 29 July 2008 Available online 11 August 2008 Keywords: Deuteroporphyrins Cyclohexane oxidation Air Catalysis
a b s t r a c t The catalytic properties of metallo-deuteroporphyrins derived from heme have been studied in the air oxidation of cyclohexane. We have found that these complexes without meso-substituents are higher active catalysts compared to the tetraphenylporphyrins in the investigated reaction. The influence of the central metals and substituents in the macrocycle on the activity of these porphyrins is discussed. The cyclohexane oxidation catalyzed by metallo-deuteroporphyrins may undergo a different mechanism from that proposed for cytochrome P-450. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction The development of the catalyst for selective oxidation of alkanes to corresponding alcohols and ketones with air continues to be a challenge of the industrial chemistry. A possible strategy is to build biomimetic or bioinspired catalysts [1]. During the last decades of years, a huge amount of work has shown that substituted metalloporphyrins are efficient catalysts for the direct oxidation of alkanes by air to give alcohols and/or carbonyl compounds at unprecedented rates under very mild conditions without co-reductants or stoichiometric oxidants. However, nearly all of the metalloporphyrins used as oxidation catalysts have been based on the system of synthetic meso-tetraphenylporphyrins (TPPs) [2–6]. There are few models derived from heme without substituents in the meso position [7,8]. Thanks to the difference existing between the structures of TPP and heme, the porphyrin ligand derived from heme could result in different or improved catalytic properties. In this regard, it is interesting to note that the prosthetic group of cytochrome P-450, iron-protoporphyrin 1x, contains no phenyl groups in the meso positions and in fact is unsubstituted in the positions. In this communication, we report that metallo-deuteroporphyrins dimethylester [M (DPDME)] derived from heme exhibit efficient catalytic activity in the liquid phase oxidation of
* Corresponding author. Tel./fax: +86 025 84315030. E-mail address: [email protected] (B. Hu). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.07.044
cyclohexane in the absence of any cocatalyst or reductant. For the sake of comparison of the effects of substituents in the macrocycle on the reactivity, we also investigated the reactivity of meso-tetraphenylporphyrins in the same catalytic system. 2. Experimental All reagents and solvents were of analytical grade and were obtained commercially. Porphyrin ligands, tetra (p-chlorophenyl) porphyrin, TCPP, and tetra (p-methoxyl) porphyrin, TMOPP, were prepared and purified as reported [9,10]. And the DPDME was synthesized according to published procedures [11]. Cobalt, zinc, copper and nickel were then introduced by a standard metal insertion method [12]. No impurities were found in the cyclohexane by GC–MS analysis before use. GC–MS analysis of the catalytic oxidation products was accomplished with a Finnigan DSQ Quantum Mass Spectrometer with a 0.5 mm i.d. 50 m PEG2000 capillary column. The Soret bands of the metallo-porphyrins were measured by UV–Vis spectra. The catalytic oxidation of cyclohexane has been carried out in a stainless steel reactor of 2 L volume equipped with mechanical stirrer, internal thermocouple and cooling coils for regulating reaction temperature. In the typical experiment, the optimum temperature was set at 418 K, and the air pressure was 0.8 Mpa, the concentration of the metallo-porphyrin was 2 105 mol/L. During the reaction, until the yield decreased markedly, we sampled every 30 min. After the reaction, the oxidation was stopped and the products were analyzed by means of the GC–MS.
84
B. Hu et al. / Catalysis Communications 10 (2008) 83–85
3. Results and discussion
16
R2 R1
R1 N
N M
R2 N
R2 N
R1
R1 R3
R2
R3
Fig. 1. Formula of the main metalloporphyrins mentioned in the text. Deuteroporphyrin dimethylester, DPDME, R1 = CH3, R2 = H, R3 = CH2CH2COOCH3; tetra (pmethoxyphenyl)porphyrin, TMOPP, R1 = H, R2 = p-methoxyphenyl, R3 = H; tetra (pchlorophenyl) porphyrin, TCPP, R1 = H, R2 = p-chlorophenyl, R3 = H. M = Co, Cu, Zn, Ni.
Table 1 Oxidation of cyclohexane catalysed by different metallo-porphyrinsa Catalyst
Conversion of cyclohexane (%)
Selectivityb (%)
ol/onec
Turnover number of catalyst
Co(II) (TMOPP) Co(II) (TCPP) Co(II) (DPDME) Cu(II) (TMOPP) Cu(II) (TCPP) Cu(II) (DPDME) Zn(II) (TMOPP) Zn(II) (TCPP) Zn(II) (DPDME) Ni(II) (TMOPP) Ni(II) (TCPP) Ni(II) (DPDME)
6.08 11.04 18.58 5.63 10.21 12.08 5.70 10.41 12.99 5.57 10.15 8.18
90.35 87.20 84.63 89.43 87.95 89.17 90.10 87.11 90.73 90.67 87.03 90.42
1.27 1.05 1.13 1.18 1.07 1.20 1.13 1.06 1.06 1.00 1.05 0.97
29,676 55,131 85,147 34,558 51,241 55,798 28,037 52,297 60,135 27,162 50,602 37,506
a b c
See conditions in text, the reaction time was 4 h while 3 h for Co(II) (DPDME). Selectivity = [n (ol) + n (one)]/n (conversion). ol/one = n (ol)/n (one).
DPDME
14 12 Yield (%)
We have investigated the oxidation reaction of cyclohexane which is a rather poorly reactive substrate catalyzed by various metallo-porphyrins mentioned in the Fig. 1. The main products of the oxidation of cyclohexane by air are cyclohexanol and cyclohexanone. Table 1 summarizes the data obtained from our catalytic experiments. The complexes were active in the reaction at temperature 418 K. As shown in Table 1, the conversion of cyclohexane varies evidently with the change of the central metal, and change of the selectivity of the alcohol and ketone is very small. In the series of the DPDME, we found that Co(II) (DPDME) is the best catalyst in oxidation. When the central metal was Cu or Zn ions, the conversion was lower and for Ni ion as the central metal even a two times lower conversion was obtained. We observed the following order of reactivity: Co(II) > Cu(II) > Zn(II) > Ni(II), as same as that of the series of TMOPP and TCPP. This phenomenon may be due to the fact that the redox potential of cobalt is higher than that of other metals. In agreement with earlier observations [4,13], the catalytic activity of metallo-porphyrins increases with the increasing of the redox potential. Besides the central metals of the complexes, several other effects are usually considered in the discussion of the correlation between the properties of metallo-porphyrins and their catalytic activity in oxidation processes: electron and geometric effects of the substituents in the macrocycle. It has been found that in the case of metal porphyrins, the introduction of electron-withdrawing substituents at the macrocycle increase the redox potential of M(III)/M(II) porphyrin system and increase the catalytic activity of XM (TPP) [14,15], where X = F, Cl, Br, I. Depending on the type of the substituent, they may also exert a steric influence. Contrary to the effect, the electron-donating substituents decrease the
10 TCPP
8 6 4
TMOPP
2 0 0
1
2
3 4 Reaction time (h)
5
6
Fig. 2. Oxidation of cyclohexane using cobalt complexes with different porphyrin ligands: (j) DPDME; (d) TCPP; (N) TMOPP. See condition in text. Yield = [n (ol) + n (one)]/n (cyclohexane).
redox potential of the MDPDME due to the stabilization of the high oxidation state of the metal, but the catalytic activity in the oxidation of cyclohexane was also found to increase, Fig. 2. The overall yield of alcohol and ketone after 3.5 h at 418 K has substantially reached as high as 15.72% when Co(II) (DPDME) is used as catalyst. Fig. 2 indicates the relative catalytic activities of the cobalt complexes. The frequencies of the visible absorption maxima (Soret band) are dependent on the substituents [15]. It is related to the increase in the negative charge in the macrocycle and corresponds to the lower redox potentials. The Soret band of Co(II) (TMOPP), Co(II) (TCPP) and Co(II) (DPDME) occurs at 412 nm, 414 nm and 392 nm, respectively. So the order of reduction potentials of these cobalt complexes must be TCPP > TMOPP > DPDME. It is known that the introduction of the electron-withdrawing substituents at the meso- and/or b-position remarkably increases both the catalytic activity and the redox potential. Surprisingly, in our case the situation is opposite. The metallo-dueteroporphyrins with the lower redox potential are more active catalysts in the investigated system. Apparently, the character of electronic effect does not play a pronounced influence on the catalytic properties of the deuteroporphyrins. A primary factor maybe that the reaction proceed a mechanism different from that proposed for cytochrome P-450, because in that mechanism the catalytic activity is expected to increase with the redox potential of the metallo-porphyrins. The reaction mechanism is still not well established and several pathways have been brought up [4,15,16]. From the phenomenon and the results observed in the experiments, we supposed that pathway of the oxidation of cyclohexane may be consistent with the mechanism proposed by Haber et al. [3,17]. According to the mechanism, the axial ligand participates in the reaction and initiated it by generating the radicals with simultaneous reduction of the metallo-porphyrin
XCoIII ðTPPÞ þ C6 H12 ¼ CoII ðTPPÞ þ HX þ C6 H11 The lower the redox potential of the complex, the easier is this process, as indeed observed. 4. Conclusion The dueteroporphyrins complexed with different metals have the effective behavior as catalysts in the cyclohexane oxidation
B. Hu et al. / Catalysis Communications 10 (2008) 83–85
by dioxygen. In the series of M (DPDME), where M = Co, Cu, Zn, Ni, cyclohexane being oxidized to alcohol and ketone with the relatively high yields, without the use of sacrificial co-reductant. Unlike the tetra-arylporphyrins, the catalytic activity of these complexes increases with the decreasing potential. The oxidation of cyclohexane catalyzed by metallo-deuteroporphyrins may undergo a different mechanism from that proposed for cytochrome-P-450. The nature of interactions between the reactivity and the substituents and the mechanism of the reactions is not understood, and further experiments are required for a better understanding of the detailed mechanism of alkane oxidation and the stabilization of the metallo-deuteroporphyrins in such system. References [1] D. Mansuy, C.R. Chim. 10 (2007) 392. [2] P. Tagliatesta, D. Giovannetti, A. Leoni, M.G.P.M.S. Neves, J.A.S. Cavaleiro, J. Mol. Catal. A: Chem. 252 (2006) 96.
85
[3] J. Haber, L. Matachowski, K. Pamin, J. Poltowicz, J. Mol. Catal. A: Chem. 162 (2000) 105. [4] J.E. Lyons, P.E. Ellis Jr., H.K. Myers Jr., J. Catal. 155 (1995) 59. [5] J. Poltowicz, K. Pamin, J. Haber, J. Mol. Catal. A: Chem. 257 (2006) 154. [6] Z. Gross, S. Nimri, L. Simkhovich, J. Mol. Catal. A: Chem. 113 (1996) 231. [7] J.H. Fuhrhop, M. Baccouche, H. Grabow, J. Mol. Catal. A: Chem. 7 (1980) 245. [8] E. Monzani, L. Casella, M. Gullotti, N. Panigada, F. Franceschi, V. Papaefthymiou, J. Mol. Catal. A: Chem. 117 (1997) 199. [9] H. Türk, W.T. Ford, J. Org. Chem. 56 (1991) 1253. [10] C.-C. Guo, Z.-P. Li, Chem. J. Chinese Univ. 18 (1997) 242. [11] D.-S. Ma, B.-C. Hu, C.-X. Lü, B. Cao, Chinese Appl. Chem. 23 (2006) 961. [12] A.D. Alder, F.R. Longo, F. Kampas, J. Inorg. Nucl. Chem. 32 (1970) 2443. [13] J. Haber, L. Matachowski, K. Pamin, J. Poltowicz, J. Mol. Catal. A: Chem. 198 (2003) 215. [14] H. Fujii, J. Am. Chem. Soc. 115 (1993) 4641. [15] E.R. Birnbaum, M.W. Grinstaff, Y.A. Labinger, J.E. Bercaw, H.B. Gray, J. Mol. Catal. 104 (1995) L119. [16] G.S. Nunes, I. Mayer, H.E. Toma, K. Araki, J. Catal. 236 (2005) 55. [17] J. Poltowicz, E. Tabor, K. Pamin, J. Haber, Inorg. Chem. Commun. 8 (2005) 1125.