ReviewCatalysis by rhodium complexes bearing pyridine ligands: Mechanistic aspects
Introduction
Nitrogen containing aromatic heterocyclic ligands such as pyridines and related compounds offer the possibilities for the synthesis of transition metal complexes with application in catalysis [1a,b]. N-donor ligands are classified as hard donors in the Ralph G. Pearson hard–soft [Lewis] acid–base (HSAB) principle [1c–g] and they can stabilize both high and low metal oxidation states. In contrast to the phosphorus atom, the N atom has no low-lying d-orbitals available and therefore N-containing ligands have only σ-donor characteristics and weak π-acceptor properties [1h]. The metal–N bond has more pronounced ionic character than the metal–P bond.
The synthesis and characterization of rhodium(I), [Rh(cod)(amine)2](PF6), cis-[Rh(CO)2Cl(amine)] and cis-[Rh(CO)2(amine)2](PF6) complexes (Fig. 1; cod = 1,5-cyclooctadiene; amine = pyridine, 2-picoline, 3-picoline, 4-picoline, 3,5-lutidine or 2,6-lutidine (Fig. 2)) were reported by Denise and Pannetier [2]. These complexes are easy to prepare and to characterize and are stable in the air (with the exception of those bearing 2,6-lutidine ligand). One of the striking features of this type of complexes is the capability of being able to manipulate their electronic and steric factors by changing the position of the methyl groups present in the substituted pyridines. Such changes play an important role in their physical–chemical [3] and catalytic properties, as will be seen in the course of this review.
Section snippets
Water-gas shift reaction
The water-gas shift reaction (WGSR, Eq. (1)) is one pathway to produce pure hydrogen, also used to adjust the CO/H2 ratios in synthesis gas, which can be further used, as an example, in the fuel cell for power generation [4]. Recent interest in this reaction has been based also on its use on elimination of carbon monoxide in the hydrogen market. Molecular hydrogen may be probably the ideal energy carrier in the foreseeable future and can be produced from water using a variety of energy sources
Carbonylation of methanol
Dutta and co-workers [11a] reported the carbonylation of methanol to acetic acid and its ester catalyzed by the rhodium complexes cis-[Rh(CO)2Cl(amine)] (amine = 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, pyridine, 2-phenylpyridine, 3-phenylpyridine or 4-phenylpyridine) in presence of CH3I under the following reaction conditions: 4 mL of methanol, 1 mL of CH3I, 1 ml of H2O, [Rh] = 0.054 mmol, P (CO) = 20 bar at 130 °C for 1 h and the total turnover per hour for methanol conversion to carbonylated
Homogeneous systems
Results of the reduction of nitrobenzene to aniline (Eq. (4)) catalyzed by the rhodium complexes cis-[Rh(CO)2(amine)2](PF6) dissolved in the corresponding aqueous amine ligand under CO atmosphere were described by Linares et al. [13a]. The TF(aniline)/24 h values pursue the order: 2-picoline (42) > 2,6-lutidine (37) > 3-picoline (28) > 4-picoline (10) > pyridine (7) for the following reaction conditions: [Rh] = 10 mM, S/C = 2500, 8 mL of amine, 2 mL of water, P(CO) = 0.9 atm at 100 °C for 3 h, with a selectivity
Oligomerization and hydrocarboxylation of CO/ethylene
Fachinetti and co-workers [18] reported the catalysis of the hydrocarboxylation of ethylene (Eq. (6)) and the stepwise alternating oligomerization of CO/ethylene (Eq. (7)) by the rhodium complexes cis-[Rh(CO)2(amine)2](PF6) (amine = pyridine, 2-picoline, 3-picoline, 4-picoline, or 2,6-lutidine) dissolved in aqueous tetrabutyl ammonium hydrogen sulfate ([(CH3(CH2)3)4N](HSO4)) solutions. Propionic acid, penta-3-one, octane-3,6-dione and undecane3,6,9-trione were obtained as major products for the
Hydrocarbonylation of 1-hexene
Hung-Low et al. [20a] described the hydrocarbonylation of 1-hexene to heptanoic acid catalyzed by rhodium(I) [Rh(cod)(amine)2](PF6) complexes (cod = 1,5-cyclooctadiene; amine = pyridine, 2-picoline, 3-picoline, 4-picoline, 3,5-lutidine or 2,6-lutidine) immobilized on P(4-VP) in contact with water under carbon monoxide atmosphere. Gaseous by-products H2 and CO2 coming from the catalysis of WGSR were also observed. Table 7 summarizes the water-gas shift and hydrocarbonylation of 1-hexene rate data
Homogeneous systems
Pardey et al. [21] reported the catalysis of the hydroesterification and hydroformylation–acetalization of 1-hexene by rhodium(I) complexes, cis-[Rh(CO)2(amine)2](PF6) (amine = pyridine, 2-picoline, 3-picoline, 4-picoline, 3,5-lutidine or 2,6-lutidine) dissolved in methanol under carbon monoxide atmosphere to give methyl-heptanoate and 1,1-dimethoxy-heptane as major products, and minor amounts of heptanal. The acetal product comes from the nucleophilic addition reaction of the methanol with the
Hydrodechlorination of dichloroethane
Trabuco and Ford [23] examined homogeneous catalysts prepared from RhCl3 in aqueous pyridine and substituted pyridines for the hydrodechlorination of 1,2-dicholoroethane to HCl, ethylene and ethane under the WGSR conditions. The conversion values (%) follows as 4-picoline = 3-picoline (34) > 2-picoline (7.5) > 2,6-lutidine (3.8) under (P(CO) = 0.9 atm at 100 °C for 22.5 h). The pattern of the hydrodechlorination of 1,2-dicholoroethane parallels the WGSR activities observed for the same solvent system [8].
Carbonylation of naphtha
The carbonylation of olefins present in naphtha constitute an alternative pathway to reduce the olefinic content in naphtha and to produce oxygenated products of greater added value which can be further used in a though be future industrial catalytic process for gasoline improving [24].
Pardey et al. [24c] reported the use of the immobilized [Rh(cod)(4-picoline)2](PF6)/P(4-VP) system in contact with methanol under carbon monoxide atmosphere as a catalyst for the carbonylation of some typical
Hydrogenation and hydroformylation of alkenes
Ford and co-workers [25a] examined the hydrogenation and hydroformylation of 1-hexene and cyclohexene catalyzed by RhCl3 immobilized on poly(4-vinylpyridine) in contact with aqueous 2-ethoxyethanol under mild WGSR conditions.
The authors reported the conversion of cyclohexene to cyclohexane and cyclohexane carboxaldehyde and 1-hexene to hexane, heptanal, 2-methylhexanal and cis- and trans-2-hexene under numerous experimental variables conditions (reaction time = 5–40 h, [cyclohexene] = 0.5–2.0 M,
Catalysis by iridium complexes bearing pyridine ligands
In this section some examples of soluble and immobilized complexes of iridium bearing pyridine ligands have shown as catalysts will be described.
Pardey and coworkers reported WGSR activity for the iridium complexes, cis-[Ir(CO)2(amine)2](PF6) [26a]. TF(H2) values follows the order 4-picoline (14) > 3-picoline (10) > pyridine (9) > 3,5-lutidine (9) > 2-picoline (4) > 2,6-lutidine (3) under 1.9 atm of CO at 100 °C; [Rh] = 10 mM, 10 mL of 80% aqueous amine. The authors suggest an electronic and steric influence.
Summary
In this short review, a survey of soluble and immobilized complexes of rhodium with pyridine ligands have shown the versatility and utility of these compounds as catalysts in reactions as varied as the WGSR, the carbonylation of methanol to acetic acid and its ester, the reduction of nitroarenes to the corresponding anilines, the oligomerization and hydrocarboxylation of CO/ethylene to ketones and propionic acid, hydrocarbonylation of 1-hexene to heptanoic acid, the hydroesterification and
Acknowledgment
The authors thank the Fonacit-Venezuela (S1-2002000260) and CDCH-UCV (PG-03-00-6928-2007) for the financial support. Special thanks to Prof. Peter C. Ford for valuable discussions.
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