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Desulfurization catalyzed by nickel PCP-pincer compounds
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CHAPTER 4
Desulfurization catalyzed by nickel PCP-pincer compounds J. Torres-Nieto and J.J. García Facultad de Química, Universidad Nacional Autónoma de México, México, D.F. 04510, Mexico
4.1 INTRODUCTION The composition of crude oil (petroleum) is very complex. It consists of a mixture of mainly hydrocarbons, but other components containing heteroatoms such as sulfur, nitrogen, oxygen and metals are also present,the particular constitution depending mostly on the site where the petroleum feedstock has been extracted and the class of petroleum being treated. The most abundant heteroatom present in petroleum feedstocks is sulfur, having concentrations that vary from 0.1% wt (light oil) to 5% wt (heavyweight oil), the variety of organosulfur compounds present in petroleum being very wide. These types of compounds are summarized in Fig. 4.1 [1]. Nowadays, the light oil reserves, which contain low levels of heteroatoms, are quickly decreasing, making it necessary to depend more on the use of heavyweight oil. Sulfur compounds should be removed from petroleum feedstocks because they are released into the atmosphere as sulfur oxides during fuel combustion, causing atmospheric pollution (e.g., acid rain). Moreover, organosulfur compounds poison the precious metal catalysts used in refining reactions, such as catalytic reforming and catalytic cracking [2].
Fig. 4.1. Organosulfur constituents of petroleum and its distillates. The Chemistry of Pincer Compounds D Morales-Morales and CM Jensen (Editors)
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Having in account these problems, the United States government through the Environmental Protection Agency (EPA) and the European Union have gradually established lower sulfur limits in transportation fuels. The sulfur content in gasoline and diesel fuel will be reduced from the current levels of 300–500 ppm to meet the market limits in the United States of <30 ppm in 2006 and in Europe of <10 ppm by 2008 [3].
4.2 HDS CATALYSTS Hydrodesulfurization (HDS) is the process by which sulfur is removed from organosulfur compounds through a reaction with hydrogen, releasing H2 S. This process is performed at high H2 pressures (>200 atm) and temperatures (300–450 C) (see Fig. 4.2). There are many formulations for HDS catalysts, and the selection depends on the kind of oil being treated. Still, one of the most used consists of a mixture of Mo and Co sulfides supported on alumina. Other examples include mixtures of Ni and Mo or Ni and W [4]. Several models exist, that give an explanation to the way thiophenic moieties are coordinated on the solid catalysts. The most generally accepted model is the so-called Co-Mo-S Model proposed by Topsøe. In this model the catalyst is proposed to be formed by a single layer of MoS2 , having Co or Ni at edge planes. Other phases may be present in this model, but the catalytic activity has been mainly associated to the Co-Mo-S phase having vacancies in the cobalt atom as an explanation for the HDS process [2b]. 4.2.1 Coordination Modes of Thiophenic Moieties All the thiophenic molecules, present two sites with high electronic density, these being =C bond, either of which are likely to the lone pair over the sulfur atom and the C= interact with the metal centers in the catalysts, and thus a number of coordination modes with the latter may be established. In fact, a large number of examples of metal thiophene complexes have already been identified and characterized (see Fig. 4.3) [5]. It is known that some of these coordination modes are involved in hydrogenation and hydrogenolysis processes; moreover these types of coordination can be applied to benzothiophene (BT) and dibenzothiophene (DBT). The most common coordination modes in thiophenic molecules are the 1 S and the 5 . An important observation in the 1 S coordination is that the sulfur atom presents an sp3 hybridization, all examples of this mode showing the thiophene lying in a different plane from that of the metal, a feature that is distinctive to all the thiophenic moieties [5, 6]. It has been acknowledged that the binding ability increases with the number of methyl groups over thiophene ring, even though all these compounds are weakly coordinated to the metal center. Relevant to the argument at hand, it has been observed that the thiophene (T) is actually displaced by the other methyl-substituted thiophenes. The 5 coordination is often exhibited by metals with d6 electron configurations [5, 6b, 6c], although BT and DBT in general prefer 6 coordination to 5 [7].
Fig. 4.2. The hydrodesulfurization reaction.
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Fig. 4.3. Common coordination modes in thiophenes.
4.2.2 Metal Insertion into C−S Bonds Usually, when coordinatively unsaturated, electron-rich metal complexes are used in reactions involving thiophenic moieties, the oxidative addition of the C−S bond to the metal center to give a metallacycle, is observed (see Fig. 4.4). The importance of the study of this type of compounds relies in that they have been proposed as key intermediaries in the HDS process. The first metallacycle was reported by Stone and co-workers in 1960 [8]. Since then, a great number of examples using metallic complexes in low oxidation states has been reported. Jones and Dong have made an extensive study on the C−S insertion using rhodium complexes [9]. T, BT and DBT and some methylated analogs were used in their study, and a C−S insertion into metal centers with or without previous coordination of the thiophenic fragment was established. Jones and Vicic have also reported the reversible formation of thiametallacycles using thiophenes and Ni-diphosphine complexes, typically dinuclear complexes. Also, they have found that the stability of the respective metallacycle complexes is given as: T > BT > DBT [10]. In addition to the systems above mentioned, numerous examples of metallacycles involving other metals such as Co [11], Ir [12], Fe [13], Ru [14], Zr [15], Mo [16] and Pt [17], have also been reported. Commonly if the thiametallacycles formed are six membered, the resultant rings are planar. However, the methyl substitution over the thiophenic ring seems to have a significant effect in the geometry of the complex, as the resultant thiametallacycles
Fig. 4.4. Schematic representation of the C−S bond cleavage reaction leading to the formation of metallacycles.
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distort from planarity as the number of methyl substituents over the thiophenic moiety increases. Additionally, molecular orbital studies on these type of complexes have found that the planarity is mainly due to steric effects rather than to electronic factors [18]. 4.2.3 Catalytic Homogeneous Desulfurization Considering the urgency of use of heavyweight oils, the study of the DBT activation along with and the more hindered analogs of it (those with alkyl groups in the 4 and 6 positions, 4-RDBT and 4,6-R2 DBT, respectively) has become a major goal, as these compounds constitute the bulk of the remaining sulfur compounds in petroleum feedstocks [19]. Recent studies have developed the first examples of catalytic homogeneous desulfurization of DBT and 4,6-Me2 DBT using nickel compounds. Hayashi and co-workers reported a variety of Ni(0) catalysts mediating the formation of chiral 1,1 -binaphthyls in the asymmetric cross-coupling of dinaphtho[2,1-b:1 ,2 -d]thiophene with Grignard reagents, which has expanded the use of 1,9-disubstituted DBTs [20]. Recently our group has developed the first catalytic example for the desulfurization of 4,6-Me2 DBT using nickel-phosphine compounds and Grignards reagents to yield the cross-coupled sulfur-free products [21]. The intermediacy of thianickelacycles, such as the ones reported by Vicic and Jones [10], participating in the catalytic cycle has been proposed (see Fig. 4.5). Additionally in this work, different types of phosphines (mono and diphosphines, containing different hindered substituents) were used. It was observed that when the bite angle in the phosphine is increased the catalytic behavior is diminished, a feature that has been detected particularly with the hindered DBT analogs. Also it has been observed that when a Grignard reagent susceptible to carry out -elimination is used, the latter process occurs.
4.3 DESULFURIZATION CATALYZED BY NICKEL PCP-PINCER COMPOUNDS Nickel PCP-pincer compounds (see Fig. 4.6) have been largely used in a wide number of transformations, for instance to develop Heck reactions [22]. Recently our group has started to use this type of compounds to desulfurize thiophenic fragments (DBT and 4,6-Me2 DBT) due to their expected good resistance to high temperatures, obtaining encouraging results. The reactivity of the above-depicted nickel pincer compounds has been compared with the nickel phosphine-containing compounds, the desulfurization of DBT using MeMgBr found to be quite similar with both types of compounds (see Fig. 4.7). Still, when either EtMgBr or iPrMgCl is used, the reactivity of the pincer compounds is drastically diminished due to the higher sterical effect. In the case of the diphosphine compounds, the latter effect indeed occurs, but in a minor extent; and in the case of monophosphines such effect is not significative (see Fig. 4.7). The desulfurization of Me2 DBT has been found to be extremely susceptible to sterical effects, in that both the nickel-diphosphine and nickel-pincer compounds showed lower or no reactivity at all, in contrast to the case of DBT. Still, when monophosphines were
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Fig. 4.5. Mechanistic proposal for the catalytic desulfurization of DBT with nickel-phosphine compounds.
Fig. 4.6. PCP-pincer compounds used to desulfurize thiophenic fragments.
used, such as [Ni(PEt3 4 ], the extent of desulfurization was found to increase. In this process in particular, when EtMgBr and iPrMgCl were used no desulfurization was observed whatsoever (see Fig. 4.8). In the light of the latter results, it was concluded that the susceptibility of nickelpincer compounds to sterical effects during the desulfurization process is greater than that presented by nickel-phosphine compounds.
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J. Torres-Nieto and J.J. García 100
70 60 50 40 30
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90 80
20 10 Assym. pincer
Pincer
[Ni(PEt3)4]
[Ni(dtbpe)H]2
[Ni(dippe)H]2
Grignard reagent
iPrMgCl
EtMgBr
MeMgBr
0
t
talys
Ni-ca
Fig. 4.7. Desulfurization of DBT mediated by Ni compounds at 100 C.
100 90
% Desulfurization
80 70 60 50 40 30 20 Pincer lys t
10
Grignard reagent
Ni-
iPrMgCl
EtMgBr
MeMgBr
[Ni(dippe)H]2
ca ta
[Ni(PEt3)4]
0
Fig. 4.8. Desulfurization of Me2 DBT mediated by Ni compounds at 100 C.
Several experiments varying temperature conditions were carried out and in these, it was found that when temperature was increased, the reactivity of either type of nickel compounds (pincer and phosphine derivatized) diminished (see Fig. 4.9). Noteworthy, the nickel-pincer complexes are more greatly affected by the temperature than the
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Desulfurization catalyzed by nickel PCP-pincer compounds
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100 90
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80 70 60 50 40 30 Assym. pincer 20
60°C 100°C Temperature
[Ni(dippe)H]2
ys tal ca Ni-
[Ni(PEt3)4]
0
t
Pincer 10
150°C
Fig. 4.9. Temperature effect in the desulfurization of DBT.
nickel-phosphine ones. It was thus concluded that the low thermostability of both nickelpincer and nickel-phosphine compounds is due to the formation of the key metallacyclelike intermediaries, which may not be stable at the higher temperatures. It is important to mention that although pincer compounds are not better catalysts than the phosphine analogs, the versatility of this type of complexes is quite impressive, considering the increasing range of reactions in which the pincer compounds are used other than HDS itself, an area in which these type of compounds could result in assistance in promoting the development of other novel HDS catalytic systems.
REFERENCES [1] R.A. Sánchez-Delgado, Organometallic Modeling of the Hydrodesulfurization and Hydrodenitrogenation Reactions. Kluwer Academic Publishers, The Netherlands, 2002. [2] (a) R.J. Angelici, Encyclopedia of Inorganic Chemistry (R.B. King, ed.). Wiley & Sons, New York, 1994, p. 1433. (b) H. Topsøe, B.S. Clausen, F.E. Massoth, Hydrotreating Technology, Catálisis, Science and Technology, vol. 11 (J.R. Anderson, M. Boudart, eds). SpringerVerlag, New York, 1996. [3] (a) U.S. Environmental Protection Agency (http://www.epa.gov/otaq/gasoline.htm). (b) European Union, EU Directive 98/70/EC, 1998. [4] (a) B.S. Clausen, B. Lengeler, H. Topsøe, Polyhedron, 5 (1986) 199. (b) R. Prins, V.H.J. de Beer, G.A. Somorjai, Catal. Rev. Sci. Eng., 31 (1989) 1. [5] R.J. Angelici, Coord. Chem. Rev., 105 (1990) 61. [6] (a) P.A. Vecchi, A. Ellern, R.J. Angelici, Organometallics, 24 (2005) 2168. (b) R.J. Angelici, Bull. Soc. Chim. Belg., 104 (1995) 265. (c) T.B. Rauchfuss, Prog. Inorg. Chem., 21 (1991) 259. (d) M. Choi, R.J. Angelici, Organometallics, 10 (1991) 2436. (e) M. Draganjac, C.J. Ruffing, T.B. Rauchfuss, Organometallics, 4 (1985) 1909.
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[13] [14] [15] [16] [17]
[18] [19] [20] [21] [22]
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J. Torres-Nieto and J.J. García J.W. Hachgenei, R.J. Angelici, Organometallics, 8 (1989) 14. R.B. King, F.G.A. Stone, J. Am. Chem. Soc., 82 (1960) 4557. W.D. Jones, L. Dong, J. Am. Chem. Soc., 113 (1991) 559. D.A. Vicic, W.D. Jones, J. Am. Chem. Soc., 119 (1997) 10855. D.A. Vicic, W.D. Jones, J. Am. Chem. Soc., 121 (1999) 7606. W.D. Jones, R.M. Chin, Organometallics, 11 (1992) 2698. W.D. Jones, R.M. Chin, J. Organomet. Chem., 472 (1994) 311. H.E. Selnau, J.S. Merola, Organometallics, 12 (1993) 1583. C. Bianchini, A. Meli, M. Peruzzini, F. Vizza, P. Frediani, V. Herrera, R.A. Sánchez-Delgado, J. Am. Chem. Soc., 115 (1993) 7505. C. Bianchini, A. Meli, M. Peruzzini, F. Vizza, P. Frediani, V. Herrera, R.A. Sánchez-Delgado, J. Am. Chem. Soc., 115 (1993) 2731. C. Bianchini, A. Meli, M. Peruzzini, F. Vizza, S. Moneti, V. Herrera, R.A. Sánchez-Delgado, J. Am. Chem. Soc., 116 (1994) 4370. T. Morikita, M. Hirano, A. Sasaki, S. Komiya, Inorg. Chim. Acta, 291 (1999) 341. J.G. Planas, M. Hirano, S. Komiya, J. Chem. Soc., Chem. Commun. (1999) 1793. G. Erker, R. Petrenz, C. Krüger, Lutz, A. Weiss, S. Werner, Organometallics, 11 (1992) 1646. W.D. Jones, R.M. Chin, T.W. Crane, D.M. Baruch, Organometallics, 13 (1994) 4448. J.J. Garcia, P.M. Maitlis, J. Am. Chem. Soc., 115 (1993) 12200. J.J. Garcia, B.E. Mann, H. Adams, N.A. Bailey, P.M. Maitlis, J. Am. Chem. Soc., 117 (1995) 2179. A. Iretski, H. Adams, J.J. Garcia, G. Picazo, P.M. Maitlis, J. Chem. Soc., Chem. Commun. (1998) 61. A. Arévalo, S. Bernés, J.J. Garcia, P.M. Maitlis, Organometallics, 18 (1999) 1680. C. Blonski, A.W. Myers, M. Palmer, S. Harris, W.D. Jones, Organometallics, 16 (1997) 3819. B.C. Gates, H. Topsøe, Polyhedron, 16 (1997) 18. T. Shimada, Y. Cho, T. Hayashi, J. Am. Chem. Soc., 124 (2002) 13396. Y. Cho, A. Kina, T. Shimada, T. Hayashi, J. Org. Chem., 69 (2004) 3811. J. Torres-Nieto, A. Arévalo, P. García-Gutiérrez, A. Acosta-Ramírez, J.J. García, Organometallics, 23 (2004) 4534. D. Morales-Morales, C. Grause, K. Kasaoka, R. Redón, R.E. Cramer, C.M. Jensen, Inorg. Chim. Acta, 300–302 (2000) 958. D. Morales-Morales, R. Redón, C. Yung, C.M. Jensen. Chem. Commun. (2000) 1619. M.E. van der Boom, D. Milstein, Chem. Rev., 103 (2003) 1759. W.J. Sommer, K. Yu, J.S. Sears, Y. Ji, X. Zheng, R.J. Davis, C.D. Sherrill, C.W. Jones, M. Weck, Organometallics, 24 (2005) 4351–4361.
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CHAPTER 4
Desulfurization catalyzed by nickel PCP-pincer compounds J. Torres-Nieto and J.J. García Facultad de Química, Universidad Nacional Autónoma de México, México, D.F. 04510, Mexico
4.1 INTRODUCTION The composition of crude oil (petroleum) is very complex. It consists of a mixture of mainly hydrocarbons, but other components containing heteroatoms such as sulfur, nitrogen, oxygen and metals are also present,the particular constitution depending mostly on the site where the petroleum feedstock has been extracted and the class of petroleum being treated. The most abundant heteroatom present in petroleum feedstocks is sulfur, having concentrations that vary from 0.1% wt (light oil) to 5% wt (heavyweight oil), the variety of organosulfur compounds present in petroleum being very wide. These types of compounds are summarized in Fig. 4.1 [1]. Nowadays, the light oil reserves, which contain low levels of heteroatoms, are quickly decreasing, making it necessary to depend more on the use of heavyweight oil. Sulfur compounds should be removed from petroleum feedstocks because they are released into the atmosphere as sulfur oxides during fuel combustion, causing atmospheric pollution (e.g., acid rain). Moreover, organosulfur compounds poison the precious metal catalysts used in refining reactions, such as catalytic reforming and catalytic cracking [2].
Fig. 4.1. Organosulfur constituents of petroleum and its distillates. The Chemistry of Pincer Compounds D Morales-Morales and CM Jensen (Editors)
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Having in account these problems, the United States government through the Environmental Protection Agency (EPA) and the European Union have gradually established lower sulfur limits in transportation fuels. The sulfur content in gasoline and diesel fuel will be reduced from the current levels of 300–500 ppm to meet the market limits in the United States of <30 ppm in 2006 and in Europe of <10 ppm by 2008 [3].
4.2 HDS CATALYSTS Hydrodesulfurization (HDS) is the process by which sulfur is removed from organosulfur compounds through a reaction with hydrogen, releasing H2 S. This process is performed at high H2 pressures (>200 atm) and temperatures (300–450 C) (see Fig. 4.2). There are many formulations for HDS catalysts, and the selection depends on the kind of oil being treated. Still, one of the most used consists of a mixture of Mo and Co sulfides supported on alumina. Other examples include mixtures of Ni and Mo or Ni and W [4]. Several models exist, that give an explanation to the way thiophenic moieties are coordinated on the solid catalysts. The most generally accepted model is the so-called Co-Mo-S Model proposed by Topsøe. In this model the catalyst is proposed to be formed by a single layer of MoS2 , having Co or Ni at edge planes. Other phases may be present in this model, but the catalytic activity has been mainly associated to the Co-Mo-S phase having vacancies in the cobalt atom as an explanation for the HDS process [2b]. 4.2.1 Coordination Modes of Thiophenic Moieties All the thiophenic molecules, present two sites with high electronic density, these being =C bond, either of which are likely to the lone pair over the sulfur atom and the C= interact with the metal centers in the catalysts, and thus a number of coordination modes with the latter may be established. In fact, a large number of examples of metal thiophene complexes have already been identified and characterized (see Fig. 4.3) [5]. It is known that some of these coordination modes are involved in hydrogenation and hydrogenolysis processes; moreover these types of coordination can be applied to benzothiophene (BT) and dibenzothiophene (DBT). The most common coordination modes in thiophenic molecules are the 1 S and the 5 . An important observation in the 1 S coordination is that the sulfur atom presents an sp3 hybridization, all examples of this mode showing the thiophene lying in a different plane from that of the metal, a feature that is distinctive to all the thiophenic moieties [5, 6]. It has been acknowledged that the binding ability increases with the number of methyl groups over thiophene ring, even though all these compounds are weakly coordinated to the metal center. Relevant to the argument at hand, it has been observed that the thiophene (T) is actually displaced by the other methyl-substituted thiophenes. The 5 coordination is often exhibited by metals with d6 electron configurations [5, 6b, 6c], although BT and DBT in general prefer 6 coordination to 5 [7].
Fig. 4.2. The hydrodesulfurization reaction.
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Fig. 4.3. Common coordination modes in thiophenes.
4.2.2 Metal Insertion into C−S Bonds Usually, when coordinatively unsaturated, electron-rich metal complexes are used in reactions involving thiophenic moieties, the oxidative addition of the C−S bond to the metal center to give a metallacycle, is observed (see Fig. 4.4). The importance of the study of this type of compounds relies in that they have been proposed as key intermediaries in the HDS process. The first metallacycle was reported by Stone and co-workers in 1960 [8]. Since then, a great number of examples using metallic complexes in low oxidation states has been reported. Jones and Dong have made an extensive study on the C−S insertion using rhodium complexes [9]. T, BT and DBT and some methylated analogs were used in their study, and a C−S insertion into metal centers with or without previous coordination of the thiophenic fragment was established. Jones and Vicic have also reported the reversible formation of thiametallacycles using thiophenes and Ni-diphosphine complexes, typically dinuclear complexes. Also, they have found that the stability of the respective metallacycle complexes is given as: T > BT > DBT [10]. In addition to the systems above mentioned, numerous examples of metallacycles involving other metals such as Co [11], Ir [12], Fe [13], Ru [14], Zr [15], Mo [16] and Pt [17], have also been reported. Commonly if the thiametallacycles formed are six membered, the resultant rings are planar. However, the methyl substitution over the thiophenic ring seems to have a significant effect in the geometry of the complex, as the resultant thiametallacycles
Fig. 4.4. Schematic representation of the C−S bond cleavage reaction leading to the formation of metallacycles.
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distort from planarity as the number of methyl substituents over the thiophenic moiety increases. Additionally, molecular orbital studies on these type of complexes have found that the planarity is mainly due to steric effects rather than to electronic factors [18]. 4.2.3 Catalytic Homogeneous Desulfurization Considering the urgency of use of heavyweight oils, the study of the DBT activation along with and the more hindered analogs of it (those with alkyl groups in the 4 and 6 positions, 4-RDBT and 4,6-R2 DBT, respectively) has become a major goal, as these compounds constitute the bulk of the remaining sulfur compounds in petroleum feedstocks [19]. Recent studies have developed the first examples of catalytic homogeneous desulfurization of DBT and 4,6-Me2 DBT using nickel compounds. Hayashi and co-workers reported a variety of Ni(0) catalysts mediating the formation of chiral 1,1 -binaphthyls in the asymmetric cross-coupling of dinaphtho[2,1-b:1 ,2 -d]thiophene with Grignard reagents, which has expanded the use of 1,9-disubstituted DBTs [20]. Recently our group has developed the first catalytic example for the desulfurization of 4,6-Me2 DBT using nickel-phosphine compounds and Grignards reagents to yield the cross-coupled sulfur-free products [21]. The intermediacy of thianickelacycles, such as the ones reported by Vicic and Jones [10], participating in the catalytic cycle has been proposed (see Fig. 4.5). Additionally in this work, different types of phosphines (mono and diphosphines, containing different hindered substituents) were used. It was observed that when the bite angle in the phosphine is increased the catalytic behavior is diminished, a feature that has been detected particularly with the hindered DBT analogs. Also it has been observed that when a Grignard reagent susceptible to carry out -elimination is used, the latter process occurs.
4.3 DESULFURIZATION CATALYZED BY NICKEL PCP-PINCER COMPOUNDS Nickel PCP-pincer compounds (see Fig. 4.6) have been largely used in a wide number of transformations, for instance to develop Heck reactions [22]. Recently our group has started to use this type of compounds to desulfurize thiophenic fragments (DBT and 4,6-Me2 DBT) due to their expected good resistance to high temperatures, obtaining encouraging results. The reactivity of the above-depicted nickel pincer compounds has been compared with the nickel phosphine-containing compounds, the desulfurization of DBT using MeMgBr found to be quite similar with both types of compounds (see Fig. 4.7). Still, when either EtMgBr or iPrMgCl is used, the reactivity of the pincer compounds is drastically diminished due to the higher sterical effect. In the case of the diphosphine compounds, the latter effect indeed occurs, but in a minor extent; and in the case of monophosphines such effect is not significative (see Fig. 4.7). The desulfurization of Me2 DBT has been found to be extremely susceptible to sterical effects, in that both the nickel-diphosphine and nickel-pincer compounds showed lower or no reactivity at all, in contrast to the case of DBT. Still, when monophosphines were
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Fig. 4.5. Mechanistic proposal for the catalytic desulfurization of DBT with nickel-phosphine compounds.
Fig. 4.6. PCP-pincer compounds used to desulfurize thiophenic fragments.
used, such as [Ni(PEt3 4 ], the extent of desulfurization was found to increase. In this process in particular, when EtMgBr and iPrMgCl were used no desulfurization was observed whatsoever (see Fig. 4.8). In the light of the latter results, it was concluded that the susceptibility of nickelpincer compounds to sterical effects during the desulfurization process is greater than that presented by nickel-phosphine compounds.
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J. Torres-Nieto and J.J. García 100
70 60 50 40 30
% Desulfurization
90 80
20 10 Assym. pincer
Pincer
[Ni(PEt3)4]
[Ni(dtbpe)H]2
[Ni(dippe)H]2
Grignard reagent
iPrMgCl
EtMgBr
MeMgBr
0
t
talys
Ni-ca
Fig. 4.7. Desulfurization of DBT mediated by Ni compounds at 100 C.
100 90
% Desulfurization
80 70 60 50 40 30 20 Pincer lys t
10
Grignard reagent
Ni-
iPrMgCl
EtMgBr
MeMgBr
[Ni(dippe)H]2
ca ta
[Ni(PEt3)4]
0
Fig. 4.8. Desulfurization of Me2 DBT mediated by Ni compounds at 100 C.
Several experiments varying temperature conditions were carried out and in these, it was found that when temperature was increased, the reactivity of either type of nickel compounds (pincer and phosphine derivatized) diminished (see Fig. 4.9). Noteworthy, the nickel-pincer complexes are more greatly affected by the temperature than the
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Desulfurization catalyzed by nickel PCP-pincer compounds
85
100 90
% Desulfurization
80 70 60 50 40 30 Assym. pincer 20
60°C 100°C Temperature
[Ni(dippe)H]2
ys tal ca Ni-
[Ni(PEt3)4]
0
t
Pincer 10
150°C
Fig. 4.9. Temperature effect in the desulfurization of DBT.
nickel-phosphine ones. It was thus concluded that the low thermostability of both nickelpincer and nickel-phosphine compounds is due to the formation of the key metallacyclelike intermediaries, which may not be stable at the higher temperatures. It is important to mention that although pincer compounds are not better catalysts than the phosphine analogs, the versatility of this type of complexes is quite impressive, considering the increasing range of reactions in which the pincer compounds are used other than HDS itself, an area in which these type of compounds could result in assistance in promoting the development of other novel HDS catalytic systems.
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