J. Phys. Chem. B 2007, 111, 4951-4962
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Rhodium-Catalyzed Hydroformylation of 1-Hexene in an Ionic Liquid: A Molecular Dynamics Study of the Hexene/[BMI][PF6] Interface† Nicolas Sieffert and Georges Wipff* Laboratoire MSM, UMR CNRS 7177, Institut de Chimie, UniVersite´ Louis Pasteur, 4 Rue B. Pascal, 67000 Strasbourg, France ReceiVed: NoVember 23, 2006; In Final Form: January 17, 2007
We report a molecular dynamics study of biphasic systems involved in the rhodium-catalyzed hydroformylation of 1-hexene in the 1-butyl-3-methyl-imidazolium hexafluorophosphate ionic liquid ([BMI][PF6] IL). We first describe the neat [BMI][PF6] interfaces with hexene (the substrate) and heptanal (the linear reaction product) as organic phases. The former interface is molecularly sharp with BMI+ cations preferentially oriented “perpendicular” (i.e., pointing their butyl chains toward the organic phase), whereas hexene molecules tend to be somewhat parallel to the interface. The interface with heptanal is approximately twice as broad, due to BMI+‚‚‚O(heptanal) attractions, and the solvent molecules are disordered at the interface. No IL ions solubilize in the organic phase(s) whereas ca. 2-3 hexene or heptanal molecules diffused into the IL phase. The presence of the CO and H2 gases does not modify the nature of the hexene/IL interface, as these gases are mainly solubilized in the organic phase, respectively, as diluted species and in the form of a “gaseous” droplet. In the IL phase, one finds a few CO monomers, whereas the less soluble H2 molecules spend only transient excursions. We next simulate the phase separation of “randomly mixed” IL/hexene liquids with the [RhH(CO)L3] precatalyst as a solute, comparing the PPh3 to the TPPTS3- ligands (L). The phases separate much more slowly than in the case of classical liquids, and the neutral complex with PPh3 ligands solubilizes in the hexene phase, displaying loose dynamical contacts with the IL interface. This contrasts with the -9 charged [RhH(CO)(TPPTS)3]9- complex that sits “immobilized” on the IL side of the interface and is mainly solvated by BMI+ cations. Finally, we characterize the solvation of -6 charged [RhH(CO)(TPPTS)2]6-, [RhH(CO)2(TPPTS)2]6-, and [RhH(CO)(TPPTS)2(hexene)]6- complexes involved as reaction intermediates in the hydroformylation reaction and of the free TPPTS3- ligand itself in the bulk IL.
Introduction Ionic liquids (ILs) are molten salts that melt below 100 °C and are generally composed of sterically mismatched ions that hinder crystal formation.1 They are described as “designer” solvents as their properties can be easily tuned for a given application, according to the choice of their constitutive ions.2 They represent a promising “green” alternative to conventional solvents in a wide range of chemical applications such as synthesis, electrochemistry, liquid-liquid extraction, and organometallic catalysis.1,3-6 For instance, the rhodium-catalyzed hydroformylation of olefins can be efficiently performed in ILs, using traditional phosphine derivatives as ligands,7 with the main advantage that the reaction products can be easily separated from the catalyst either by distillation (when using nonvolatile ILs)8 or by decantation (in the case of ILs immiscible with the organic phase).9,10 The first studies of Rh-catalyzed reactions in ILs were from Dupont et al.9 and Chauvin et al.10 who reported the hydrogenation and hydroformylation of small alkenes in the 1-butyl-3-methyl-imidazolium hexafluorophosphate IL (hereafter noted [BMI][PF6]; see Figure 1) in conjunction with triphenylphosphine (PPh3) ligands or their sulfonated derivatives. Subsequent studies focused on the improvement of the hydroformylation reaction by varying the thermodynamic conditions,11 the nature of the ligands,11-14 or the nature of the ionic liquid †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * Author to whom correspondence should be addressed. E-mail: wipff@ chimie.u-strasbg.fr.
components14-16 or by designing supported materials17-22 and continuous flow reactions systems.23-26 One of the central questions to such biphasic reactions is the solubility and phase distribution of the components of the catalytic system (solvent liquids, substrates, products, gases, and catalysts), and in which phase the catalytic reaction is occurring is actually unclear.5 The main reaction partners of the hydroformylation reaction (e.g., alkenes,14 aldehydes, and CO/H2 gases16,27-29) are only weakly soluble in ionic liquids, a feature that is hardly consistent with a reaction occurring in the bulk IL phase as it is generally considered. Recent developments in supported ionic liquid catalysis18,30 point to the importance of the interface between the IL and the organic phase. For instance, Mehnert et al. showed that the increase of the surface area relative to the IL volume enhances the hydrogenation31 as well as the hydroformylation18 reactions, and according to Riisager et al. the reaction occurs “at the interphase or in the diffusion layer of the IL”.26 The precise nature (size, composition, and structure) of this peculiar domain of the solution remains elusive, as are the possible analogies with the corresponding more classical (e.g., water/organic liquid) interfaces. The precise solvation patterns of the complexes still need to be assessed in neat ILs as well at their interfaces. Whether the catalysts are surface-active or not is another fundamental question in the context of phase-transfer-catalyzed reactions as well as in extraction or separation processes. Insights into the nature of the classical interfaces can be experimentally obtained from spectroscopy,32 electrochemistry,33
10.1021/jp0677952 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
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Figure 2. Rhodium-catalyzed hydroformylation of 1-hexene showing the first steps of the generally accepted reaction mechanism. (Only the path leading to the linear product is represented.)
hydroformylation reactions in [BMI][PF6].10,15,18 We also analyze the solvation by the IL of selected species involved in the hydroformylation reaction (Figure 2). These are the [RhH(CO)L3] and [RhH(CO)2(TPPTS)2]6- complexes and the free TPPTS3ligand, which have been effectively observed by high-pressure NMR,15 the [RhH(CO)(TPPTS)2]6- active catalyst, and the [RhH(CO)(TPPTS)2(1-hexene)]6- reaction intermediate, which are likely to be formed by analogy with reactions involving traditional solvents.49,50 The results obtained with the [BMI][PF6] liquid should also be of interest for related halogen-free ILs developed to obtain greener catalytic systems.51 Methods Figure 1. 1-Hexene, heptanal, BMI+, and PF6- species. Bottom: Snapshot of the simulation box of the hexene-IL binary system containing three [RhH(CO)(TPPTS)3]9- complexes (system E).
kinetic studies,34 and surface tension measurements35 and, theoretically, by computer simulations, mainly molecular dynamics.36-39 We recently investigated the interfacial distribution of Rh complexes and the synergistic effect of cyclodextrins at the chloroform/water40 and decene/water41 interfaces. In this paper, we extend these studies to the IL/organic interface involved in the biphasic hydroformylation reaction, selecting hexene (the substrate of the reaction) as the organic phase and [BMI][PF6] as the ionic liquid (Figure 1). The latter has been widely used to conduct hydroformylation experiments and is rather well documented experimentally. In spite of its “non-green” characteristics,42 it possesses interesting physical properties for catalysis, as it forms biphasic systems with alkenes and with water. Moreover, it easily amenable to computer investigations.43-48 More specifically, we want to first depict the neat hexene/IL interface (without any solute) and the heptanal/IL interface whose organic components, respectively, correspond to the reactant and to the product of the hydroformylation reaction. Next, we investigate the influence of CO/H2 gases on the nature of the interface and their partitioning in the biphasic hexeneIL system. We then focus on the important question of phase separation of “randomly mixed” IL hexene liquids containing the [RhH(CO)L3] precatalyst as a solute to investigate to which extent the phases separate, and how the solute distributes, depending on the nature of its ligands L. For this purpose, we compare the complex with neutral PPh3 ligands (L) to the one with their charged tris(m-sulfonatophenyl)phosphine derivatives (hereafter noted TPPTS3-), as both have been used to conduct
Molecular Dynamics. The MD simulations were performed with the modified AMBER 7.0 software52 where the potential energy is described by a sum of bond, angle, and dihedral deformation energies and pairwise additive 1-6-12 (electrostatic + van der Waals) interactions between nonbonded atoms
U)
Kr (r - req)2 + ∑ Kθ (θ - θeq)2 + ∑ bonds angles ∑ ∑ Vn (1 + cos(nω - γ)) + dihedrals n
∑ i50%). As shown by mass spectroscopy results, ILs may dissolve in polar solvents in the form of large aggregates,84,85 and these could be stabilized in the catalytic system by their interactions with other charged solutes not
considered in our study. Further experimental investigations are needed to further characterize the systems. Concerning the neutral reactant, product, and CO/H2 syngas molecules, they are found to be poorly soluble in the IL phase and to mainly partition to the organic phase. This is consistent with experimental data. For instance, Ohlin et al.16 measured maximal concentrations of CO in 1-hexene and [BMI][PF6] of 17.0 and 1.47 mM, respectively, in identical thermodynamic conditions. Concerning 1-hexene, Favre et al. reported a solubility of ca. 0.015 g per gram of [BMI][PF6] (∼0.25 M) in the conditions of the hydroformylation reaction.14 See also the related theoretical Monte Carlo or MD studies.27,86-88 In our simulations, CO is more concentrated than H2 in the IL, in agreement with experimental28,29 and simulation data.89 Since the hydroformylation reaction formally proceeds with stoichiometric amounts of CO and H2, it can be surmised that using overall higher H2/CO proportions should improve the conversion.90 The small CO and H2 molecules weakly interact with the IL and are surrounded by IL anions and cations (mainly via their butyl substituent), locally forming apolar or weakly polar microdomains.83,87,91,92 In the cases of the hexene and heptanal solutes, their polar functional group is mainly solvated by BMI+ imidazolium rings of the IL, while their apolar alkyl chain prefers the butyl(BMI) environment and, to a lesser extent, PF6- anions. Increasing the “hydrophobic character” of the IL cation (via, e.g., longer alkyl chains) as well as of the IL anion (e.g., replacing PF6- by
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Figure 10. Phase separation of the [BMI][PF6]-hexene system containing three [RhH(CO)(TPPTS)3]9- complexes (system E). Snapshots were taken at different times during the dynamics.
Tf2N-) should contribute to further solubilizing these neutral molecules, thereby facilitating the hydroformylation reaction, as observed experimentally.25 When immersed in a [BMI][PF6] solution, the neutral [RhH(CO)(PPh3)3] complex is solvated by a mixture of IL anions and cations.71 In spite of the known tendency to form π-stacking interactions with the arenes (see, e.g., [1,3-dimethylimidazolium][PF6]-benzene mixtures91,93,94 or the solubilization of imidazolium-based ILs in aromatic solvents95), the BMI+ cations do not stack with the aryl rings of the ligands but rather interact via their alkyl chains. The other studied solutes with TPPTS3- ligands are anionic in nature, and their solvation by the IL mainly results from their hydrogen-bonding interaction with the BMI+ ions, thus involving harder interactions as expected. The anions of the IL do not directly interact with these solutes but generally contribute to their second solvation shell, yielding an onion-type alternation of IL ions around the SO3- groups, as in the cases of halides, lanthanide LnCl63-, or actinide UO2Cl42- anionic solutes.96-98 Thus, depending on the charge, size, and topology of the solute, different components of the IL are involved in the solvation process, in keeping with the dual solvation capabilities of ILs, allowing us to fine-tune the choice of their components for a specific reaction while improving the “green” characteristics of the IL. Upon simulation of the randomly mixed liquids, these progressively form two distinct domains, delineating an “interface”. Note that the nanoscopic phase separation is very slow (at least 60 ns) compared to classical water-“oil” mixtures (less than 1 ns with chloroform as the organic liquid).99 Furthermore, the resulting interface is wider and less regular than that in the case of juxtaposed liquids, indicating equilibration problems with
ILs, as also observed with the IL/water mixtures.69,70 We believe that this is not a mere computational problem but that this likely reflects equilibration problems at the macroscopic level. The properties of the interface may depend on the way that it has been prepared, and in the context of the biphasic reactions, some local solvent mixing is important to enhance the interfacial area and to promote the meeting of the reaction partners that are initially in the different phases. Another important result concerns the partitioning of the precatalyst. We find that the neutral [RhH(CO)(PPh3)3] complex prefers the hexene side of the interface whereas the charged [RhH(CO)(TPPTS)3]9- one prefers the IL side.100 These results are consistent with those of hydroformylation reactions in imidazolium-based ILs (with PF6-, BF4-, or Tf2N- as anions), showing that the use of neutral PPh3 ligands leads to an important loss of catalyst (already observed after the first cycle), whereas charged ligands allow very good catalyst recycling.10-13,15 Reaction activities in the studied IL are also higher with neutral catalysts, a feature that is also consistent with a preference for the organic phase.101 In our simulations, the Rh catalyst does not sit, however, near the center of the liquid slab where it is more soluble, but rather close to the interface. The distances between Rh and the interfaces during the last 20 ns of dynamics are represented in Figure 8, showing that the three simulated neutral [RhH(CO)(PPh3)3] complexes, although mostly surrounded by hexene molecules, display loose contacts with the ionic liquid at the interface or with the chain of IL ions immersed in the hexene phase. They are quite mobile and exchange from one interface to the other during the dynamics. Looking at a computer graphics system reveals that crossing of the bulk hexene slab is facilitated by local interactions of the complex
Rh-TPPTS Complexes in [BMI][PF6] with the IL chain connecting the two interfaces. Thus, although overall neutral, the complex displays attractive interactions with the IL, as seen above. The case of charged [RhH(CO)(TPPTS)3]9- complexes differs as, in spite of the relatively high temperature (350 K), they seem to be immobilized in the IL, closer to the interface than to the center of the IL slab. None exchanges with the bulk phase, i.e., crosses the center of the IL slab. Further explorations would require, e.g., potential of mean force calculations as a function of the z-coordinate of the complex,66 but the latter are presently precluded by the related computational costs. Interestingly, the apparent affinity of the complex for the interfacial domain is consistent with recent X-ray photoelectron spectroscopic results according to which the surface composition of a solution of the complex Pt(NH3)4Cl2 in the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtOSO3] deviates from the composition of the bulk IL and the cation Pt(NH3)42+ is enriched on the surface, while the Cl- anion is depleted.102 The Pt or the Rh complexes are bulkier than their counterions, and such species are known to be “attracted” by aqueous interfaces, as are, e.g., the Cl- anions of NaCl salts103 or ion complexes formed in assisted liquid-liquid extraction processes.35,66 The driving force seems to be the cavitation energy of the IL104 or aqueous liquids, in keeping with their high surface tensions (47 and 72 mN/M, respectively with [BMI][PF6]78 and water105). Furthermore, as seen above, the [RhH(CO)(TPPTS)3]9- complexes are as well solvated by the IL near the interface as in the bulk IL. Concerning the “immobilization” of the charged complexes near the interface, this should be favorable from a mechanistic point of view for two main reasons. First the local concentration of the substrate (hexene) should be higher than that in the bulk IL domain. Second the mass transfer limitations are somewhat reduced, thereby preventing the reduction of activity, in keeping with experimental results.10,11,13,15 Further explorations are required to investigate the surface activities of the different rhodium complexes. However, our study reveals that [RhH(CO)(TPPTS)3]9- precatalyst, when located near the interface, remains well solvated by a shell of BMI+ cations that are in contact with the organic phase. The less charged and more amphiphillic reaction intermediates like the [RhH(CO)2(TPPTS)2]6- and [RhH(CO)(TPPTS)2(hexene)]6- complexes and the TPPTS3- ligands themselves should be more surfaceactive than the precatalyst and bring the Rh center closer to the hexene phase, therefore facilitating the reaction. As pointed out by Cornils et al., referring to classical liquids, “aqueous biphasic catalytical systems are still in the infancy of their significance”. What happens at IL interfaces clearly deserves further experimental and theoretical investigations. We hope that the present results will stimulate some of them. Acknowledgment. The authors are grateful to IDRIS, CINES, Universite´ Louis Pasteur, and PARIS for computer resources and to Etienne Engler for assistance. Supporting Information Available: Number of solvent ions and CO/H2 species in the IL, interaction energies between the [RhH(CO)(TPPTS)3]9- complex and hexene molecules, charges on 1-hexene and heptanal, distribution of lifetime of hexene and heptanal at the interface, snapshots of CO and H2 in the IL, snapshots of the solvent boxes of the systems C2 and C3, and RDFs and snapshots of each rhodium complex in the IL. This material is available free of charge via the Internet at http:// pubs.acs.org.
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