Jacobsen's Catalyst Interaction with Polydimethylsiloxane

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J. Phys. Chem. C 2008, 112, 14830–14834

Jacobsen’s Catalyst Interaction with Polydimethylsiloxane/Tetraethoxysilane Network and Solvent Molecules: Theoretical Design of a New Polymeric Membrane Josefredo R. Pliego Jr.*,† and Marco A. Schiavon Departamento de Cieˆncias Naturais, UniVersidade Federal de Sa˜o Joa˜o del-Rei, 36301-160, Sa˜o Joa˜o del-Rei, MG, Brazil ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: July 1, 2008

Theoretical calculations were used for understanding the occlusion of the Jacobsen’s catalyst inside a polydimethylsiloxane/tetraethoxysilane (PDMS/TEOS) polymeric membrane. Our analysis indicates there is a partition equilibrium of the catalyst between the solvent and the swelled membrane. Density functional theory calculations at the B3LYP level predict interaction energies of the Jacobsen’s catalyst with benzene, chlorobenzene, dichloromethane, tetrahydrofuran, and acetone and with a model of PDMS chain in the range of 6-8 kcal mol-1. For methanol, the hydrogen bonding involving the coordinated chloride atom reaches 14 kcal mol-1. These findings were used for designing a new membrane, where a diol structure was added to the side of the polymeric chain. The modified membrane should present improved retention properties. Introduction Many organometallic compounds are useful homogeneous catalysts. An example is Jacobsen’s catalyst (Figure 1), an organometallic used for stereoselective olefin epoxidation.1 However, it is known that catalysis on homogeneous conditions have some problems, mainly related to recovery of the catalyst.2 Trying to overcome these problems, many studies on heterogenization of homogeneous catalysts has been reported.2-7 One procedure is the occlusion of the catalyst inside a polymeric membrane. In the occlusion process, the catalyst stay inside the membrane either through chemical bond7,8 or physical interactions.4-6 In the latter case, polydimethylsiloxane (PDMS)5,6 and composite membranes (PDMS/TEOS)3,9 have been used as supporting membranes (Figure 1). The PDMS is a linear polymer, whereas in the PDMS/TEOS network, the tetraethoxysilane (TEOS) is used to generate a silica cluster, which crosslinks the PDMS linear chains, generating a three-dimensional structure. The occlusion of the Jacobsen’s catalyst has been experimentally investigated in a recent paper using a PDMS/TEOS polymeric membrane.3 This heterogenization process is interesting, because it enable the exclusive recovery of the hydrophobic oxidation product in the organic phase. However, a very common problem in such system is the leaching of the catalyst out of the membrane, which is accentuated by the use of solvents with a high affinity for the catalyst and able to cause swelling of the membrane. Therefore, understanding the mechanism of the catalyst’s interaction with the membrane wall and with solvent molecules is a fundamental problem and may be important in the rational design of new membrane/solvent combinations. The aim of this paper is to provide molecular level insights on the interaction of the catalyst with a model PDMS/TEOS membrane and with some solvent molecules such as C6H6, PhCl, CH2Cl2, THF, CH3COCH3, and CH3OH. On the basis of these insights, we have designed a new polymeric membrane that

Figure 1. Jacobsen’s catalyst and polymeric membranes.

should be able to form a complex with the catalyst without loss of catalytic activity and have improved retention properties. Membrane Model

* Corresponding author e-mail: [email protected]. † Present address: Departamento de Cie ˆ ncias Exatas e da Terra, Universidade Federal de Sa˜o Paulo, 09972-270, Diadema, SP, Brazil.

In the study of the Jacobsen’s catalyst interaction with the PDMS/TEOS membrane wall, it is important to have a view of

10.1021/jp804245f CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

Jacobsen’s Catalyst Interaction with PDMS/TEOS

Figure 2. Jacobsen’s catalyst inside a PDMS/TEOS polymeric membrane cavity.

how large the molecular cavity is inside the membrane. We have used simple molecular mechanics (MM2 force field available in the Chem3D software package) for simulating the membrane. On the basis of experimental conditions reported by Guedes et al.,3 we have estimated that the PDMS chains have, on average, 29 Si atoms. Therefore, we have used this chain length, chemically bonded to a silica cluster by TEOS hydrolysis, to create a cavity model (Figure 2). It is important to note there are many different possibilities for the membrane model, because the cross-link between the TEOS units through the PDMS chain is variable. The model presented in Figure 2 is a reasonable structure, and it was build atom-by-atom. Following the construction of the membrane, molecular dynamics calculations were done in order to allow the membrane to relax. There are many different conformations, close in energy, available for the membrane. Our objective was not to find the global minimum but only to obtain a representative low-energy structure. After the simulation, we included the catalyst inside the available hole for visualization proposes (see Figure 2). As can be seen in this simple model, there is a large space available for the catalyst, suggesting mechanical forces are not trapping the catalyst. Rather, retention of the catalyst should depend on the intermolecular forces acting among the catalyst, the membrane wall, and the solvent molecules. Jacobs and co-workers6 have studied a dimer of the Jacobsen’s catalyst occluded in PDMS, and their analysis suggests both the monomer and the dimmer have similar retention properties. Thus, it seems that intermolecular forces play the main role, and understanding the interaction of the catalyst with different solvent molecules and fragments of the membrane wall is an important goal. Quantum Chemistry Calculations Quantum mechanical calculations were done using the B3LYP hybrid functional.10 For geometry optimization, we have used the 6-31+G(d) basis set for oxygen, nitrogen, and chlorine; 6-31G(d) for silicon; and 6-31G for manganese,11 carbon, and hydrogen. The structures were fully optimized using delocalized internal coordinates and requiring the maximum gradient be below 0.0005 au. In the single point energy

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14831 calculations, we have improved the basis set for manganese and carbon, from 6-31G to 6-31G(d). The Mn atom in the Jacobsen’s catalyst has a 3d4 configuration, and it is an open shell species with a quintet ground state. Therefore, we have used the restrict open shell B3LYP method in order to avoid the spin contamination problem. Harmonic frequency calculations are very expensive for these large systems, and we have not done these calculations. The level of theory used in the calculations, for both geometry and interaction energy, deserves some attention. Density functional theory and the B3LYP functional, in particular, has been established as a reliable method for geometry prediction.12 Recent studies by Riley et al.13 have shown that when combined with the Pople 6-31+G(d) type of basis set, the B3LYP functional produces bond lengths for organic molecules with average error less than 0.01 Å. In the case of organometallic compounds, Truhlar and co-workers14 have found that their DZQ basis set (double-ζ quality) leads to a mean unsigned error less than 0.02 Å. In the present case, we are using the 6-31G basis set for C, H, and Mn and the 6-31+(d) set for O, N, and Cl. Because there are many carbon atoms, this approach saves considerable computational time without meaningful loss of the reliability. In fact, we have performed geometry optimization for the catalyst using the 6-31G(d) basis set for C and Mn (6-31+G(d) for O, N, and Cl) and compared the results with the previous optimization with the 6-31G basis set. The results have pointed out a very small variation on the geometry, with the Mn-O, Mn-N, Mn-Cl, and C-C bond lengths changing less than 0.01 Å. In the case of intermolecular forces calculations, the B3LYP method with the polarized double-ζ basis set works adequately when the interaction is dominated by electrostatic forces.13,15-17 For the present system, intermolecular forces must be mainly electrostatic and polarization, and it is expected that density functional theory will produce reasonable interaction energies. No basis set superposition error (BSSE) correction was done. The intermolecular interaction between the catalyst and the solvent (or membrane model) molecule is calculated by eq 1.

∆E(interaction) ) E(catalyst · solvent) E(catalyst) - E(solvent) (1) To provide more insights on the interaction of Jacobsen’s catalyst with solvent molecules, we have calculated atomic charges derived from the electrostatic potential using the geodesic selection point scheme developed by Spackman.18 The reactions involving the Jacobsen’s catalyst take place in solution, even inside the polymer cavity. Thus, we have included the solvent effect to evaluate the effective interaction between the catalyst and some interaction sites of the polymeric membrane. The sum of the electronic energy with the solvation free energy leads to the potential of mean force surface, a fundamental property that plays a critical role on solution phase reactivity.19,20 The solvent effects were included using the polarizable continuum model (PCM)21-23 and the integral equation formalism routines.24,25 Chlorobenzene was chosen as the solvent because it is an apolar aprotic solvent that simulates a real system. We have used a scale factor of 1.40, close to that proposed for dimethyl sulfoxide26 and other aprotic solvents.27 The following atomic radii were used: H(1.20), C(1.70), N(1.60), O(1.50), Cl(1.81), Si(2.00), and Mn(2.00). For the solvent effect computations, we have used the gas-phase optimized structures. All the calculations were done with the Gamess28 and PC Gamess29 programs.

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Pliego and Schiavon

Figure 3. Complexes involving Jacobsen’s catalyst and solvent molecules.

Interaction of the Catalyst with the Solvent Molecules Jacobsen’s catalyst has a Mn3+ atom at its center and has a pyramidal structure. This means that the Mn atom is exposed and should be able to interact with electron-rich species such as π electron systems and negatively charged atoms such as oxygen and nitrogen. In addition, the chlorine atom bonded to the Mn has a negative charge and could interact with Lewis acids through hydrogen bonding. To support this picture, a simple calculation of atomic charges derived from the electrostatic potentials was done. These calculations show the Mn atom has a charge of 1.08 e whereas the Cl atom has a charge of -0.60 e. Therefore, these are adequate centers for electrostatic interactions. Although Jacobsen’s catalyst is a relatively large molecule and there are different positions for interaction with the solvent, more important interactions must take place in the region around the Mn/Cl atoms. In this way, we have only analyzed complexes involving direct interaction of the solvent molecules with the Mn and Cl atoms. The structure of the most stable complexes formed between Jacobsen’s catalyst and solvent molecules are presented in Figure 3. Table 1 shows the calculated interaction energies. The complex involving benzene interacts through the π system, and we notice that one C-C axis of the benzene moves toward the Mn atom. The interaction energy is 7.25 kcal mol-1. For chlorobenzene, a similar structure was found with interaction energy of 7.45 kcal mol-1. We have investigated the direct interaction of the Cl atom from chlorobenzene with the Mn center of the catalyst, but we have found a very small interaction. Thus, the π system interaction is also the most important structure for chlorobenzene-catalyst complex. The CH2Cl2 molecule has an important dipole moment due to a polar C-Cl bond, and we could expect a strong interaction. Our calculations point out the interaction of the Cl from the chloromethane with the Mn atom from the catalyst, but the interaction energy is not large, only 4.70 kcal mol-1. Another possibility is the interaction with the chloride bonding to the

TABLE 1: Interaction Energy between Jacobsen’s Catalyst and Different Moleculesa Cat-BZ Cat-PhCl Cat-CH2Cl2 Cat-THF Cat-Acet Cat-MeOH Cat-PDMS-Mn Cat-PDMS-Cl Cat-cis-diol Cat-trans-diol

B3LYP/6-31(+)G(d)

∆∆Gsolvb

∆Wc

-7.25 -7.45 -8.49 -7.23 -8.46 -14.02 -6.16 -6.34 -11.62 -16.31

2.63 4.84

-9.00 -11.48

a Units of kcal mol-1. Complexes presented at Figures 3, 4, and 6. b Solvation (chlorobenzene) contribution to the potential of mean force. c Variation of the potential of mean force for complex formation in chlorobenzene.

Mn. In this case, the hydrogen atoms of the CH2Cl2 molecule point to the chloride, as can be seen in Figure 3. This is the most stable structure, with an interaction energy of 8.49 kcal mol-1. On the other hand, the tetrahydrofuran (THF) molecule has an interaction energy as large as the π system of benzene and chlorobenzene: 7.23 kcal mol-1. The charge on the oxygen suggests the interaction in this system is mainly electrostatic, and we note that the oxygen is close to acidic hydrogen. Therefore, both interactions with the hydrogen and the Mn seem to be important. Acetone is other molecule that has a π system. Our calculations show that the interaction with the Mn atom takes place through the π electrons rather than the oxygen atom (Figure 3), and the interaction energy, 8.46 kcal mol-1, is greater than any other investigated π system. In the case of methanol, the most stable structure involves a hydrogen bond of the hydroxyl group with the chloride coordinated to the Mn atom (Figure 3). The interaction energy is 14.49 kcal mol-1, the strongest interaction among the studied solvent molecules. Another possibility is the

Jacobsen’s Catalyst Interaction with PDMS/TEOS

Figure 4. Complexes involving Jacobsen’s catalyst and a model of PDMS.

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14833

Figure 6. Complexes involving Jacobsen’s catalyst and modified membranes.

This view is supported by experimental findings, where no difference is observed in the electronic spectrum of the catalyst in solution and occluded in the membrane.3 Design of a New Membrane

Figure 5. Modified membranes for improved retention of Jacobsen’s catalyst.

direct interaction of the oxygen with the Mn atom, but in this case the stabilization energy is only 7.24 kcal mol-1. Interaction of the Catalyst with a PDMS Membrane Model The PDMS/TEOS membrane must compete with the solvent molecules for the catalyst. The membrane has oxygen atoms, which are potential interaction sites. We have used a membrane model for understanding the importantce of the catalyst-PDMS interaction and evaluating the role of both Mn and Cl interaction sites. Figure 4 presents the optimized structures. We can see the oxygen atoms of the membrane are not exposed and they are unable to interact with the catalyst. Rather, the methyl groups are in close contact with the Mn and Cl atoms, and we have calculated that the interaction energies are 6.16 and 6.34 kcal mol-1, respectively. These values are slightly smaller than the interaction with the investigated solvent molecules. On the basis of the present results, our view is that the swelled PDMS/TEOS membrane works like a solvent, and the occlusion of the catalyst is a partition equilibrium between the solvent and the membrane.

Occlusion of the catalyst and leaching minimization depends on the interaction of the catalyst with the membrane and with the solvent molecules. The Mn and Cl atoms in the catalysts are the main interaction sites because they have a substantial charge and are more exposed for interacting. The previous calculations point out that different functional groups have very similar interaction energy with the catalyst, around 8 kcal mol-1. An exception is the methanol. In this case, the hydrogen bonding with the coordinated chloride reaches almost 15 kcal mol-1. This is an important property that could be explored in the design of a new polymeric membrane. On the basis of this idea, we have conjectured that a vicinal diol added to some positions in the chain of the polymer should be able to complex with the catalyst through the chloride site (see Figure 5). If the solvent is apolar, the catalyst should stay trapped inside the polymeric membrane, resulting in an efficient retention of the catalyst. We have investigated the importance of the interaction of Jacobsen’s catalyst with the diol, considering both cis and trans diol structures. Figure 6 shows the optimized complexes. In the case of cis-diol, only one hydrogen bond between the catalyst and the membrane takes place. The other hydrogen bonding involves an internal O-H · · · O interaction. Our calculations show that the stabilization energy is 11.62 kcal mol-1, smaller than that involving methanol. On the other hand, in the case of trans-diol, the interaction is more favorable, reaching 16.31 kcal mol-1 and taking place through two intermolecular hydrogen bonds. This is substantial interaction energy, suggesting this modified membrane could have improved retention properties. The previous analysis is qualitative, and a full quantitative prediction of the partition of the catalyst between the solvent and the swelled membrane would require the calculation of the solvation free energy inside the membrane. Other possibility is using an ideal lattice gas model30 for computing the adsorption of the catalyst in the diol sites. However, it is not easy to apply these calculations for this system. On the other hand, considering that the membrane is apolar and the solvents used also are

14834 J. Phys. Chem. C, Vol. 112, No. 38, 2008 apolar, we can make a rough evaluation on the importance of the medium effect on the diol-catalyst interaction using a simple dielectric continuum solvation model such as PCM. This approach will provide the effective interaction energy, known as potential of mean force, between the diol and the catalyst inside the swelled membrane. We have considered chlorobenzene as the solvent, and the results are in Table 1. In the case of cis-diol, the effective interaction energy becomes 9.00 kcal mol-1, whereas for trans-diol it becomes 11.48 kcal mol-1. These values suggest that, even in solution, the interaction remains strong, and this modified membrane should have improved retention properties. In addition, it could be useful for avoiding formation of the µ-oxo dimmer during the catalytic cycle. Another issue that deserves attention is the reaction mechanism. At present, it is believed the reaction mechanism involves the formation of an oxo intermediate, the active species in the oxidation step.1 Recent theoretical studies have addressed these mechanistic questions and indicate that different spin states of the Mn atom are involved in the reaction.31-35 In the occlusion process, it is important that the Mn atom stay uncoordinated and available for the catalysis. Therefore, the suggested approach should produce minimal modifications on the catalytic activity. Conclusion Jacobsen’s catalyst has a weak interaction with different solvent molecules, not overcoming ∼ 8 kcal mol-1. One exception is the interaction with methanol. In this case, a hydrogen bonding with the coordinated chloride atom reaches 14 kcal mol-1. This fact was used for designing a new polymeric membrane with a diol structure added to the side of the polymeric chain and able to form two hydrogen bonds. Our analysis suggests this new membrane should minimize the leaching of the catalyst without decreasing its activity. Acknowledgment. The authors thank the Fundac¸a˜o de Amparo a` Pesquisa do Estado de Minas Gerais (FAPEMIG) for the support. References and Notes (1) McGarrigle, E. M.; Gilheany, D. G. Chem. ReV. 2005, 105, 1563. (2) Baleizao, C.; Garcia, H. Chem. ReV. 2006, 106, 3987. (3) Guedes, D. F. C.; Mac Leod, T. C. O.; Gotardo, M. C. A. F.; Schiavon, M. A.; Yoshida, I. V. P.; Ciuffi, K. J.; Assis, M. D. Appl. Catal. A: Gen. 2005, 296, 120.

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