Ligand Effects on the Regioselectivity of Rhodium-Catalyzed

Aug 7, 2014 - Yunwen Tao , Wenli Zou , Dieter Cremer , and Elfi Kraka. The Journal of Physical Chemistry A 2017 121 (42), 8086-8096. Abstract | Full T...
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Ligand Effects on the Regioselectivity of Rhodium-Catalyzed Hydroformylation: Density Functional Calculations Illuminate the Role of Long-Range Noncovalent Interactions Manoj Kumar,*,†,‡ Raghunath V. Chaudhari,†,§ Bala Subramaniam,†,§ and Timothy A. Jackson*,†,‡ †

Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas 66045, United States Center for Environmentally Beneficial Catalysis, 1501 Wakarusa Drive, Lawrence, Kansas 66047, United States § Department of Chemical and Petroleum Engineering, University of Kansas, 1530 West 15th Street, Lawrence, Kansas 66045, United States ‡

S Supporting Information *

ABSTRACT: Density functional theory calculations have been performed to gain insight into the origin of ligand effects in rhodium (Rh)-catalyzed hydroformylation of olefins. In particular, the olefin insertion step of the Wilkinson catalytic cycle, which is commonly invoked as the regioselectivity-determining step, has been examined by considering a large variety of density functionals (e.g., B3LYP, M06-L); a range of substrates, including simple terminal (e.g., hexene, octene), heteroatom-containing (e.g., vinyl acetate), and aromatic-substituted (e.g., styrene) alkenes, and different ligand structures (e.g., monodentate PPh3 ligands and bidentate ligands such as DIOP, DIPHOS). The calculations indicate that the M06-L functional reproduces the experimental regioselectivities with a reasonable degree of accuracy, while the commonly employed B3LYP functional fails to do so when the equatorial−equatorial arrangement of phosphine ligands around the Rh center is considered. The different behavior of the two functionals is attributed to the fact that the transition states leading to the Rh−alkyl intermediates along the pathways to isomeric aldehydes are stabilized by the medium-range correlation containing π−π (ligand−ligand) and π−CH (ligand−substrate) interactions that cannot be handled properly by the B3LYP functional due to its inability to describe nonlocal interactions. This conclusion is further validated using the B3LYP functional with Grimme’s empirical dispersion correction term: i.e., B3LYP-D3. The calculations also suggest that transition states leading to the linear Rh−alkyl intermediates are selectively stabilized by these noncovalent interactions, which gives rise to the high regioselectivities. In the cases of heteroatom- or aromatic-substituted olefins, substrate electronic effects determine the regioselectivity; however, these calculations suggest that the π−π and π−CH interactions also make an appreciable contribution. Overall, these computations show that the steric crowding-induced ligand− ligand and ligand−substrate interactions, but not intraligand interactions, influence the regioselectivity in Rh-catalyzed hydroformylation when the phosphine ligands are present in an equatorial−equatorial configuration in the Rh catalyst.

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INTRODUCTION Rhodium (Rh)-catalyzed hydroformylation, where an olefin and syngas (1/1 CO/H2) are converted into the corresponding aldehyde (Scheme 1), is one of the industrially preferred routes for the production of aldehydes and their derivatives.1 Significant advancements in catalytic efficacy have been achieved through optimization of phosphine ligands as one of the reaction parameters. This approach is important, as it is

difficult to achieve the desired combination of high H2 and low CO concentrations in the reaction phase to simultaneously afford high activity and selectivity.2 Bis-phosphines and bisphosphites in particular have yielded highly active and selective catalysts and provided insights into structure−function relationships.3−6 Despite the progress witnessed in designing novel ligand structures to affect desirable catalytic properties, controlling the regioselectivity of the hydroformylation reaction remains a significant challenge in industrial and academic research. The regioselectivity of olefin hydroformylation is influenced to a considerable extent by steric and electronic attributes of

Scheme 1. Rh-Catalyzed Olefin Hydroformylation

Received: February 25, 2014 Published: August 7, 2014 © 2014 American Chemical Society

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the ligands coordinated to the Rh center.3,10 With regard to steric influences, Casey and co-workers studied a vast array of diphosphines with systematically altered steric and/or electronic properties and showed the pronounced effect of the natural bite angle of bidentate ligands on the ratio of linear and branched aldehydes (l:b ratio); i.e., the regioselectivity of Rh-catalyzed hydroformylation.3,4 van Leeuwen et al. exclusively examined the effect of the bite angle by developing a series of xantphos-based bisphosphines with similar electronic properties and steric size.5,9 However, no clear correlation between the chelation mode and the natural bite angle in the xantphos-based [Rh(bisphosphine)(CO)2H] complexes was found, leaving the general impact of ligand sterics an open question. Clearly, a fundamental, molecular-level understanding of this catalyst system is still lacking. For example, the molecular origin of the relationship between bite angle and regioselectivity is unclear. Also, the influence of ligands on the olefin−Rh interaction and the structures of intermediates and transition states are not well understood. Quantum chemical computations have provided insights into the factors affecting activity and regioselectivity,11−21 but the size, complexity, and conformational flexibility of the phosphine ligands, olefin substrates, and corresponding Rh complexes presents a challenge even for modern DFT methods. Accordingly, theoretical treatments of Rh-catalyzed hydroformylation often employ ligand truncation (i.e., the use of PH3 as a model of PPh3), model substrates such as ethene and propene, a frozen reaction center, and/or a partitioning scheme (e.g., QM/MM and ONIOM).4,11−21 The majority of studies have employed the B3LYP functional, which, while generally quite accurate for energies and geometries, cannot properly capture nonlocal effects due to its inability to correctly describe the asymptotic −C6/R6 behavior of the interaction energy (where C6 is the averaged sixth-order dispersion coefficient for an atom pair at a distance of R).22,23 The origin of these nonlocal effects in the Rh hydroformylation could possibly be the ligand−ligand (π−π) and ligand−substrate (π−CH) interactions, as has been suggested by van Leeuwen et al.9 These interactions, while weak (∼0.3 kcal/mol for a carbon atom pair separated by 3 Å distance),24 could affect the regioselectivity substantially, as small free-energy changes give large differences in l:b ratios. Moreover, there is a significant literature precedent highlighting the importance of such nonlocal interactions in the correct modeling of chemical reactions and thermochemistry.25 Despite the fact that the use of PH3 as a phosphine model is a drastic simplification, the majority of the theoretical work in this area has been carried out using the PH3 ligand and there have been only a handful of studies4,17,19,20 where the real phosphine ligands (i.e., PPh3) have been used. For example, Hermann and co-workers applied a combined QM/MM approach to investigate the regioselectivity of Rh-catalyzed hydroformylation of propene.17 They used a frozen reaction center, which renders it challenging to determine if the steroelectronic effects of the phosphine ligands significantly affect the reaction. Casey et al. performed an MM treatment of the regioselectivity of BISBI- and DIPHOS-based Rh hydroformylation of 1-hexene where the transition states (TSs) were computed assuming a symmetric reaction coordinate.4 Carbo et al. have recently reported a very rigorous theoretical study,19 where they investigated the Rh xantphos catalyzed hydroformylation of 1-octene and 1-propene within the IMOMM method-based hybrid QM/MM scheme. It was concluded that

the nonbonding interactions between the diphenylphosphino substituents and the substrate play a key role in determining the regioselectivity of the reaction. However, such interactions could not be precisely quantified using the IMOMM approach because the ligand steric-induced electronic effects that might play an important role in determining the regioselectivity of the reaction were not completely described: i.e., only the [Rh(PH3)2(CO)H] part of the chemical system was treated at the QM level and the ligand steric was treated at the MM level, providing only an indirect treatment of ligand electronic effects. Moreover, the chemical linkages connecting the QM and MM regions, i.e., P−H and P−C bonds, were kept frozen during the geometry optimization of the minima and transition states, which could also induce significant chemical error. A solution to the problem of ligand electronic effects within the IMOMM formalism would be to expand the QM region to incorporate all electronic effects that are significant to the problem in hand: i.e., the [Rh(PPh3)2(CO)H] part of the chemical system must be considered in the QM region. Alternatively, the complete treatment could be carried out at an appropriate QM level of theory to provide an authentic description of nonlocal correlation effects. A recent study also revealed the importance of dispersion interactions in calculated structures and equilibria of [Rh(L)(CO)2H] complexes, where L is a triptycene-derived bis-phosphite ligand.26 Collectively, the present computational work reveals that the issue of hydroformylation regioselectivity must be computationally addressed at a QM level of theory: e.g., within the DFT framework using an appropriate functional that can reasonably describe nonlocal correlation interactions affecting the hydroformylation. Currently there are four dispersion-including DFT-based approaches available for studying such nonlocal effects: (1) DFT-D methods,27−29 (2) semilocal density functionals (e.g., M06-L, M06-2X),30 (3) nonlocal van der Waals density functionals (vdW-DF),31,32 and (4) dispersioncorrecting atom-centered one-electron potentials (DCACP).33 For detailed information about the dispersion problem of DFT functionals, and the current recipes available for its repair, we refer interested readers to the recent review article by Grimme.34 To the best of our knowledge, no theoretical investigation on Rh catalysts with real PPh3 ligands has so far been performed where the full complexity of the stereoelectronic properties of the ligands has been treated at the QM level. Moreover, to the best of our knowledge, the origin of regioselectivity in the hydroformylation of heteroatom-containing or aromatic-substituted substrates in the presence of PPh3 ligands has not been examined at this level of theory. Thus, to provide insights into these issues, and more directly assess the importance of nonbonding interactions on the regioselectivity of the process, we have performed a DFT investigation of Rh-catalyzed hydroformylation for a wide range of olefins where the PPh3 ligands were exclusively treated at the QM level. These results provide compelling evidence that ligand−ligand and ligand− substrate dispersion interactions have a strong influence on the olefin-insertion step and therefore should be considered in a proper understanding of experimentally observed l:b ratios.

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COMPUTATIONAL METHODS

All calculations reported in this work were carried out using NWChem35 quantum chemical software for electronic structure and property calculations. Under the reasonable, and common, assumption that the regioselectivity of Rh-catalyzed hydroformylation is set in the 4184

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Scheme 2. Wilkinson Catalytic Cycle for [Rh(PPh3)2(CO)H]-Catalyzed Hydroformylation

platform for testing the accuracy and versatility of the chosen theoretical method. For all of the species involved in the olefin coordination and insertion step, complete geometry optimization and frequency calculations were performed at the DFT level employing a variety of functionals with different properties, including B3LYP,22,37 TPSSh,38 BHandHLYP,39 M06-L,40 M062X,40 and ωB97X-D.41 All of the resulting structures were verified to be true minima or transition states by calculating the second-order Hessian matrices. The M06-L, M062X, and ωB97X-D functionals were particularly selected, keeping in mind their ability to offer an authentic description of medium-range, nonlocal dispersive effects. The relativistic effective core potential (ECP) and valence double-ξ basis set of Hay and Wadt42 (LANL2DZ) was employed for the Rh atom, while all other atoms were treated using the 6-31G* basis set, except when explicitly stated otherwise (vide infra). The l:b ratios were calculated using a formula that only relies on the relative energies of the branched and linear transition states and is applicable only when the olefin insertion is irreversible:

olefin insertion step of the Wilkinson catalytic cycle (Scheme 2), the energetics of the olefin coordination and insertion reaction leading to the intermediates along the pathway to isomeric aldehydes (Scheme 3)

Scheme 3. General Mechanistic Pathways for Olefin Insertion in [Rh(PPh3)2(olefin)(CO)H] Leading to the Isomeric Rh−Alkyl Intermediates along the Pathways to Isomeric Aldehydes

l:b = k l:k b = e−ΔG l

⧧

/ RT −ΔG b⧧ / RT

:e

= e−ΔΔG

⧧

/ RT

≈ e−ΔΔE

⧧

/ RT

In the next step, we applied the calibrated computational protocol to evaluate the olefin insertion step using the [Rh(PPh3)2(CO)H] catalyst, and a broad set of substrates that included simple terminal olefins such as pentene, hexene, octene, decene, dodecene, heteroatom-containing vinyl acetate, and aromatic-substituted styrene. The B3LYP/LANL2DZ/6-31G* or 6-311G** and M06-L/ LANL2DZ/6-31G* or 6-311G** theoretical methods were used for calculating the l:b ratios. The reported data are corrected for basis set superposition error as well as for the zero-point vibrational effects except when stated otherwise. In order to assess the effect of basis set on the regioselectivity of [Rh(PPh3)2(CO)H]-catalyzed hydroformylation, we compared the l:b ratios calculated using the M06-L functional and LANL2DZ/6-31G* and LANL2DZ/6-311G** basis sets, where the 6-31G* or 6-311G** basis sets were used for nontransition-metal atoms. From a comparison of the values obtained with the two basis sets (Table S12, Supporting Information), it is clear that the relative activation energies, ΔΔE⧧, are negligibly affected when the larger basis set is used. There is an average increase of 0.5 kcal/mol in

were computationally studied. Even in cases where olefin insertion is not the sole step influencing regioselectivity, this present work highlights the importance of nonlocal interactions in influencing the kinetics of this important step. As we are interested in understanding the influence of ligand sterics on the hydroformylation catalyzed by the modified Rh catalyst [Rh(PPh3)2(CO)H], we first sought a computational protocol that can provide a reliable description of hydroformylation catalyzed by [Rh(PPh3)2(CO)H]. The calibration of the theoretical method was performed using an unmodified Rh catalyst, [Rh(CO)3H], and a large set of substrates that included propene, 2methylpropene, 3,3-dimethylbutene, hexene, fluoroethene, allyl methyl ether, vinyl methyl ether, and styrene. The selection of substrates was inspired by the fact that their experimentally established l:b ratios,21,36 which are distributed over a wide range, provide an appropriate 4185

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under ambient conditions; i.e., at room temperature, 25 °C. Since the l:b ratios in the present study have been calculated using DFT energies estimated at 25 °C and 1 atm, our adopted computational strategy is, therefore, reliable for studying the regioselectivity-determining olefin insertion step of the Wilkinson catalytic cycle. Moreover, the use of such a computational approach to study the Rh-catalyzed hydroformylation is well established,4,11−21 as it provides insights into the catalytically important olefin insertion step. The primary objective of this study is to understand the role of static ligand effects on the regioselectivity of the process, and for that reason, the calculations have been performed assuming a constant temperature and H2/CO ratio. Selecting an appropriate theoretical method for a given chemical process is the most crucial component of a computational investigation. In a first step, we, therefore, screened a variety of functionals by calculating the l:b ratios of the [Rh(CO)3H]-catalyzed hydroformylation for a set of substrates with a wide range of experimental l:b ratios (Tables S1−S8, Supporting Information).21,36 The B3LYP functional did an excellent job of reproducing the experimental trends (Figure 1, top left); the TPSSh and M06-L (Figure 1, bottom left) functionals also provided a reasonable description of experimental l:b ratios.

the relative stabilization of TSl over TSb upon changing the basis set from 6-31G* to 6-311G** in the case of simple terminal olefins. To verify the role of noncovalent interactions, if any on the regioselectivity, additional calculations at the B3LYP-D3/LANL2DZ/ 6-31G* level of theory were performed. Furthermore, the regioselectivities for the simplified catalyst [Rh(PH3)2(CO)H] were also calculated. Over the past few years, several dispersion-inclusive DFT techniques have been developed.27−33 Among them, the DFT+D method27 is one of the most popular techniques. This approach employs the empirical, pairwise dispersion correction term of the form C6/R6 with adjustable parameters in conjunction with a conventional density functional approach. Recently, Grimme and co-workers have also reported a refined version of the DFT-D method, DFT-D3,29 which incorporates an additional C8/R8 term in the dispersion correction term and also accounts for three-body effects. This method is considered to be more reliable because the dispersion coefficients and the cutoff radii are interpolated taking into account the chemical environment of the system, which, however, was not the case with the earlier versions.28 Moreover, the dispersion coefficients are calculated using an ab initio approach, which leads to better accuracy and transferability. To model the solvent effects on the hydroformylation reaction, implicit solvation calculations, with either benzene or toluene as solvent, were performed, as these solvents have been extensively used in experimental studies.2,43−48 To account for dielectric screening effects in solvents, single-point solvation calculations were performed using the continuum solvation conductor-like screening model (COSMO), as implemented in NWChem software. The electrostatic as well as the cavitation and dispersion nonelectrostatic contributions to the solvation free energy were considered. For selected substrates, we have also performed calculations using different combinations of temperatures and pressures to model the experimental conditions. Finally, we applied our computational methodology to Rh catalysts supported by the more structurally complex bis-phosphine TBDCP, DIOP, and DIPHOS ligands.3 We used propene as a structural model of the experimental substrate hexene, as this structural simplification has been shown through experimental studies to not affect the regioselectivity significantly.49 The regioselectivity ratios for all the Rh bis-phosphine catalysts were calculated using the functional M06-L/ LANL2DZ/6-31G*.

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RESULTS AND DISCUSSION [Rh(CO)3H]-Catalyzed Hydroformylation. It is commonly assumed that regioselectivity in Rh-catalyzed hydroformylation is set at the olefin-insertion step of the catalytic cycle (Scheme 2). Under this assumption, we probed the molecular basis for experimental regioselectivities by computing the ground- and transition-state geometries and energies for this step. In this procedure, the l:b ratio of a given hydroformylation reaction is determined using the wellknown Eyring equation, for which the difference in free-energy barriers (ΔΔG⧧) between the branched (ΔGb⧧) and linear (ΔGl⧧) transition states needs to be computed (Scheme 3). Instead, we used the electronic energies that are corrected with respect to zero-point vibrational effects to estimate the l:b ratios. It is well documented21 that the use of electronic energies is preferred to that of the free energies in the case of rotation-prone olefins because the rotation, which is treated as a true vibration in the vibrational analysis, can affect the calculated thermodynamic quantities. It is also important to mention here that this procedure for calculating l:b ratios is applicable only when the olefin insertion is nonreversible. Although deuterioformylation experiments21,50−52 have established that the olefin insertion can be reversible depending upon the nature of the substrate, the type of the ligand system, and the reaction conditions, these experiments also indicate that the nonreversibility of the reaction is generally retained

Figure 1. Comparison of calculated and experimental regioselectivities for two different Rh catalysts, [Rh(CO)3H] (left) and [Rh(PPh3)2(CO)H] (right). For experimental regioselectivities for the [Rh(PPh3)2(CO)H] catalyst, the diamonds indicate either (i) the percent branched product for experiments collected at 25 °C and 1 atm (diamonds outlined in black) or (ii) in cases where such data are unavailable, an average of percent branched product for experiments collected at different temperature (blue, red, and green diamonds). In all cases the range of percent branched products in the plots on the right correspond to the range of observed (% l):(% b) ratios for experiments performed under different conditions (e.g., different temperature, pressures, solvents, etc.; see Table S9 (Supporting Information) for details). The open green diamond in the top right trace marks the B3LYP-D3 point for 1-decene, which is a notable outlier for the B3LYP-D3 computations.

[Rh(PPh3)2(CO)H]-Catalyzed Hydroformylation. Of the three functionals found to perform reasonably well in modeling the regioselectivity of [Rh(CO)3H]-catalyzed hydroformylation, only M06-L properly incorporates medium-range nonlocal correlation interactions.30 Thus, we used both the B3LYP and M06-L functionals to evaluate the regioselectivity of [Rh(PPh3)2(CO)H]-catalyzed hydroformylation. The PPh3 ligands 4186

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Table 1. Calculated and Experimental l:b Ratios for the [Rh(PPh3)2(CO)H]-Catalyzed Hydroformylation of Various Substratesa calcd B3LYP/LANL2DZ/6-31G*

B3LYP-D3/LANL2DZ/6-31G*

M06-L/LANL2DZ/6-31G*

substrate

ΔΔE⧧ (kcal/mol)

l:b ((% l):(% b))

ΔΔE⧧ (kcal/mol)

l:b ((% l):(% b))

ΔΔE⧧ (kcal/mol)

l:b ((% l):(% b))

exptlb l:b ((% l):(% b))

pentene hexene heptene octene Decene dodecene styrene vinyl acetate

1.08 1.03 0.95 1.18 1.34 1.2 2.69 2.40

0.2 (14:86) 0.2 (17:83) 0.2 (18:82) 0.1 (12:88) 0.1 (9:91) 0.2 (12:88) 0.0 (1:99) 0.0 (2:98)

−0.65 −0.30 −0.62 0.04 0.43 −0.60 6.36 2.00

3.0 (75:25) 1.7 (63:37) 2.8 (74:26) 0.9 (48:52) 0.5 (33:67) 2.8 (73:27) 0.0 (0:100) 0.0 (2:98)

−0.68 −1.01 −1.16 −2.10 −0.45 −1.18 6.91 2.90

3.2 (76:24) 5.5 (85:15) 7.1 (88:12) 34.5 (97:3) 2.1 (68:32) 7.3 (88:12) 0.0 (0:100) 0.0 (1:99)

20.0 (95:5)c 11 (92:8)d 6.0 (85:15)e 4.4 (81:19)f 2.8 (74:26)g 6.6 (87:13)h 0.12 (11:89)i 0.1 (9:91)j

a

The calculated data have been corrected for the basis set superposition error and zero-point vibrational energy. bTable S10 (Supporting Information) contains more detailed information regarding the experimental regioselectivity values. Experimental temperatures and pressures are provided here for each data set. cAt 25 °C and 1 atm; see ref 43. dAverage of three data sets at 25 °C and 1 atm; see refs 43 and 44. eSee ref 45. f Average of two data sets at 80 and 90 °C and 5.6−7.0 and 19.7 atm, respectively; see ref 2. gAverage of two data sets at 50 and 13.6 atm; see ref 46. h Average of two data sets at 90 and 7.0 atm; see ref 47. iAverage of two data sets at 25 °C and 1 atm; see ref 44. jAt 100 °C and 40.5 atm; see ref 48.

Figure 2. M06-L (top) and B3LYP (bottom) calculated transition states for olefin insertion in [Rh(PPh3)2(octene)(CO)H] leading to linear and branched Rh−octyl intermediates. Key nonbonding interactions are indicated.

in the [Rh(PPh3)2(CO)H] catalyst were considered in an ee conformation around the Rh center. This ligand arrangement is inspired by an NMR study of Brown and Kent,53 showing that ee and ea conformers exist in a ratio of 85:15. For these reasons, the ee coordination mode of phosphine ligands has been commonly used in computational studies of hydroformylation.15,19 Of particular note is a hybrid QM/MM study19 by Carbo et al., where the regioselectivity-determining olefin insertion from the intermediates ee-[Rh(benzoxantphos)(olefin)(CO)H] and ee-[Rh(homoxantphos)(olefin)(CO)H] was investigated. For the systems analyzed here, the ee isomer is found to be lower in energy than the ea isomer (Table S9). See the Supporting Information for details.

For the [Rh(PPh3)2(CO)H] catalyst, the B3LYP computations predict a low l:b ratio for simple terminal olefins, in poor agreement with experimental results (Table 1, Table S10 (Supporting Information), and Figure 1, top right). In contrast, the M06-L functional properly predicts a high l:b ratio for these substrates. Low l:b ratios for styrene and vinyl acetate are predicted using both functionals, reproducing the experimental trends for aromatic- and heteroatom-containing olefins. An analysis of the transition states leading to the linear (TSl) and branched (TSb) Rh alkyl intermediates offers insight into the molecular influences of hydroformylation regioselectivity (Figure 2 and Figures S1 and S2 (Supporting Information)). Focusing on the transition states computed for [Rh4187

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via noncovalent π−π and π−CH interactions. Because benzene and toluene are routinely used in hydroformylation experiments and their effects are not explicitly accounted for in the present work, it is quite likely that the missing substrate−solvent interactions are reflected in the overestimated l:b ratio. Consequently, for styrene, the B3LYP-calculated ΔΔE⧧ value is more in line with the experimental l:b ratio, but most likely for the wrong reason. The B3LYP method underestimates the strength of the noncovalent interactions between styrene and both the Rh catalyst and the solvent, which makes the calculated value somewhat closer to the measured value. The additional hydroformylation calculations performed with the [Rh(PH3)2(CO)H] catalyst also imply that the difference in behavior of the M06-L and B3LYP functionals has its primary origin in the ligand−ligand and ligand−substrate nonlocal effects that are only adequately modeled using the full PPh3 ligands and the nonlocal interactions captured by the M06-L functional (Table S13 (Supporting Information)). Though the B3LYP-D3 calculations provide an improved description of measured l:b ratios in comparison to the B3LYP functional, it still predicts two outliers, 1-octene and 1-decene. Thus, the relative comparison of B3LYP-D3 and M06-L results with experimental data indicates that the latter method provides a better description of Rh hydroformylation. Consequently, we have performed the remaining computational investigation using the M06-L/LANL2DZ/6-31G* level of theory. Temperature−Pressure Effect. We also performed the regioselectivity calculations for selected simple terminal olefins using the temperatures and pressures that have been employed in the experimental studies (Table S14 (Supporting Information)). The calculations indicate that the implicit treatment of these experimental conditions has little effect on the regioselectivities, implying that such effects cannot be properly described using quantum chemical approaches. Explicit molecular dynamics calculations would need to be carried out to gain a comprehensive insight into the temperature and pressure dependence of the hydroformylation regioselectivity. Such computations go beyond the scope of the present study. Solvent Effects on [Rh(PPh3)2(CO)H]-Catalyzed Hydroformylation. We performed conductor-like screening modelbased (COSMO) implicit solvation calculations to account for solvent effects on the predicted regioselectivities. In a first step, we studied the dodecene hydroformylation in the presence of polar (e.g., ethanol, butanol, heptanol) and nonpolar (e.g., benzene, toluene) solvents. Our calculations indicate that the inclusion of a solvent by the COSMO method has a minor effect on these results, as the relative activation energies, ΔΔE⧧, are only changed by 0.4 kcal/mol (Table S15 (Supporting Information)). This change is independent of the choice of the solvent considered. Taking into account that benzene and toluene are commonly used solvents in hydroformylation experiments,2,43−48 we next studied the hydroformylation of other olefins using benzene as an implicit solvent. In all cases, the calculated ΔΔE⧧ values were only slightly changed (Table S16 (Supporting Information)). However, an important caveat to the use of an implicit solvation model is that the possibility of direct participation of solvent molecules in the reaction, either by coordination to the Rh center or by formation of dispersion and/or nonlocal interactions with PPh3 ligands, is not accounted for. Indeed, Schmidt et al. have recently proposed that the neglect of explicit solvent treatment when using DFT-D functionals gives rise to an unbalanced treatment of dispersion interactions.26 Nonetheless, the performance of

(PPh3)2(CO)H]-catalyzed hydroformylation of octene using the M06-L functional (Figure 2, top), TSl shows strong π−π interactions between the PPh3 ligands (distances between C atoms in two phenyl rings are ∼3.8 Å) and strong π−CH interactions between octene and the PPh3 ligands (interaction distances of 2.5−3.8 Å). In TSb, the π−CH interactions are still present, but the interligand π−π interactions are lacking. This perturbation is consistent with TSb lying 2.1 kcal/mol higher in energy than TSl. Ligand−ligand and ligand−substrate interactions are attenuated in the B3LYP-calculated transition states (Figure 2). For TSl, the weakened π−π interactions are reflected in the increased distance between interacting phenyl rings (3.8 and 4.0−4.8 Å for M06-L and B3LYP optimized structures, respectively). This attenuation of π−π and π−CH interactions results in the B3LYP-calculated TSb being more stable than TSl, leading to the erroneously low l:b ratio. In fact, according to B3LYP calculations, the product distribution trend ((% l):(% b)) in the hydroformylation of simple terminal olefins is reversed relative to experimental findings: i.e., the branched aldehyde is the preferred product. This is understandable if we take into account that the noncovalent π−π and π−CH interactions influence the regioselectivity in the case of simple terminal olefins and that the B3LYP functional is incapable of providing an accurate and reliable description of transition-metal complexes in which noncovalent (especially dispersion) interactions play a chief role.25 However, when the olefin insertion step is studied using the B3LYP-D3 level of theory, which takes into account dispersion effects, the l:b ratios are significantly enhanced and, more importantly, the (% l):(% b) trends are correctly captured for all substrates except 1-octene and 1-decene (Table 1 and Figure 1). Even the agreement between the experimental and calculated l:b ratios is appreciably improved. For example, if we consider the case of octene hydroformylation, the B3LYPpredicted TSl is 1.2 kcal/mol less stable than TSb and the l:b ratio of 0.2 is very small, erroneously predicting a dominant branched product (88%). However, with the B3LYP-D3 functional that accounts for nonlocal and three body effects, the TSl is predicted to be more stable (−0.2 kcal/mol) than TSb, and the experimental observation of a favored linear product is reproduced. Interestingly, if the relative transitionstate barriers ΔΔE⧧ are calculated using only the dispersion components of the total energies of the B3LYP-D3 estimated TSl and TSb species, there is a much better agreement between theory and experiment (Table S11 (Supporting Information)). Clearly, the incorporation of medium-range nonlocal dispersion effects is crucial to achieve a reasonable agreement between experiment and theory. Therefore, this comparative analysis of B3LYP and B3LYP-D3 data provides corroborating evidence that the regioselectivity of Rh-catalyzed hydroformylation of simple terminal olefins is affected by the noncovalent interligand π−π and ligand-substrate π−CH interactions. The importance of a carefully chosen DFT-D technique to provide a reasonable description of inter- and intramolecular interactions has also recently been reported for hydroformylation catalyzed by a triptycene-derived bis-phosphite ligand.26 Interestingly, the M06-L calculated l:b ratio for the styrene hydroformylation is significantly overestimated relative to the experimentally measured value. The calculated overestimation of the ΔΔE⧧ value for styrene hydroformylation by ∼5.0 kcal/ mol is likely due to the fact that, unlike simple terminal olefins or heteroatom-containing substrates, the aromatic ring of styrene can interact with both the Rh catalyst and the solvent 4188

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the ideal approach of explicit treatment of solvent effects was not thoroughly evaluated in that study. Treatment of solvation at the explicit level, which is required for considering nonelectrostatic solute−solvent interactions, is very demanding due to a huge increase in the number of degrees of freedom. Such an approach goes beyond the scope of the present study, which is focused on evaluating and understanding the impact of ligand−ligand and ligand−substrate nonlocal interactions on the olefin insertion step and the corresponding effects on hydroformylation regioselectivity. [Rh(L)(CO)H]-Catalyzed Hydroformylation. Finally, we applied the validated M06-L/LANL2DZ/6-31G* methodology to study [Rh(L)(CO)H]-catalyzed hydroformylation where L is a bis-phosphine ligand. Experimental data, collected at 34 °C and 6 atm, are available for testing the robustness of the method. Specifically, we selected the TBDCP, DIOP, and DIPHOS ligands,3,4 as the corresponding [Rh(L)(CO)2H] catalysts gave a range of l:b ratios (12, 8.5, and 2.2, respectively) that correlated with the natural bite angles of the ligands (107, 102, and 85°, respectively). In our computations, we used propene as a model for hexene, as a previous experimental study established that this substitution does not significantly affect the regioselectivity.49 Although the calculations reported in the previous sections indicate that the ligand−substrate interactions also contribute to the regioselectivity, the structural complexity of the bis-phosphine ligands renders it computationally challenging to study the hydroformylation using the real substrate in combination with Rh bis-phosphine catalyst. In fact, we were unable to locate the appropriate transition states for the hexene hydroformylation with these catalysts using our computational methodology. The selection of propene as a substrate is also motivated by the work of Casey et al.,4 where they have used the same substrate model to study Rh bisphosphine catalyzed hydroformylation. Importantly, we have been able to reproduce the experimental l:b ratios using propene as a surrogate for hexene. Our calculated P−Rh−P bite angles for [Rh(L)(C3H6)(CO)H] are in excellent agreement with the experimental values for the [Rh(L)(CO)H] catalysts (Table 2). We also

Figure 3. Calculated excess strain energy of [Rh(L)(C3H6)(CO)H] complexes (where L = TBDCP, DIOP, DIPHOS) as a function of the P−Rh−P bite angle. The horizontal line marks 5.0 kcal/mol strain energy.

(Table 3). The agreement in absolute l:b ratios is not as good as that achieved for the [Rh(PPh3)2(CO)H] catalyst, although the deviations between experimental and computed l:b ratios amount to corresponding differences in ΔΔG⧧ of less than 1.0 kcal/mol (Table 3). As far as the basis set effect is concerned, the use of a larger basis set has a negligible effect on the calculated l:b ratios (Table S17 (Supporting Information)). We also performed implicit solvation calculations, with toluene as solvent, to investigate the role of implicit solvent in influencing the l:b ratios. However, the l:b ratios remained largely unperturbed upon implicit solvation (Table S18 (Supporting Information)). Taking into account the ability of the M06-L/ LANL2DZ/6-31G* method to provide a reasonably reliable description of Rh bis-phosphine regioselectivity, we next analyzed the transition states involved in the reactions to gain insight into the factors governing the l:b ratios. The consideration of intraligand and ligand−substrate interactions once again provides an explanation for the regioselectivity patterns. The TS l for [Rh(TBDCP)(C 3 H 6 )(CO)H] is preferentially stabilized due to improved π−π and π−CH interactions. Specifically, the propene substrate has a greater number of favorable π−CH interactions with the TBDCP ligand (Figure 4), resulting in a stabilization of TSl relative to TSb and, consequently, a high l:b ratio (73.5). Though the π−π interactions are absent in the calculated transition states for [Rh(DIOP)(C3H6)(CO)H], the greater number of strong π− CH interactions in TSl ensures the dominant formation of linear product (l:b = 17.2). For [Rh(DIPHOS)(C3H6)(CO)H], the calculated l:b ratios for the ea and ee isomers were not significantly different (l:b = 9.0 and 6.3, respectively), indicating that the electronic effects mainly determined the regioselectivity in this case. Finally, we note that, for a given [Rh(L)(CO)H] catalyst, the bite angle in the TSb and TSl transition states differs by only ∼1° (Table 2). This is consistent with observations by van Leeuwen and co-workers, who noted that “the chelation mode in the [Rh(diphosphine)(CO)2H] complexes per se is not the key parameter controlling the regioselectivity.”9

Table 2. Calculated P−Rh−P Angles in the Key Species Involved in the Regioselectivity Determining Step for [Rh(L)(CO)H]-Catalyzed Hydroformylation of Propene (Where L = TBDCP, DIOP, DIPHOS)a P−Rh−P bite angle (deg) calcd bis-phosphine ligand

Rh olefin complex

TSb

TSl

exptl3,4

TBDCP (ee) DIOP (ee) DIPHOS (ea) DIPHOS (ee)

104.9 102.2 84.2 86.1

96.7 95.2 86.1 83.1

97.4 95.8 86.1 83.5

107 (93−131) 102 (90−120) 85 (70−95)

a The values in parentheses correspond to the flexibility range for the bite angle.

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reproduced the established bite angle flexibility ranges for these chelates within an excess strain energy of 5.0 kcal/mol (Figure 3). Moreover, computations employing the M06-L functional qualitatively provide a correct description of the experimentally observed trends in hydroformylation regioselectivity: i.e., the DIOP- and DIPHOS-supported catalysts are the most and least selective, respectively, for the formation of linear aldehyde

CONCLUSIONS In summary, we have performed quantum chemical calculations to gain insight into the role of ligand effects in determining the regioselectivity of Rh-catalyzed hydroformylation of olefins. The present calculations indicate that the ligand−ligand and ligand−substrate nonbonding interactions play an important 4189

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Table 3. Comparison of M06-L Calculated and Experimental l:b Ratios for [Rh(L)(CO)H]-Catalyzed Hydroformylation of Propene (Where L = TBDCP, DIOP, DIPHOS)a regioselectivity exptl3,4

calcd bis-phosphine ligand

ΔΔE⧧ (kcal/mol)

TBDCP (ee) DIOP(ee) DIPHOS (ea) DIPHOS (ee)

−2.55 −1.69 −1.30 −1.09 −1.21b

l:b ((% l):(% b)) 73.5 17.2 9.0 6.3 7.7

ΔΔE⧧ (kcal/mol)

l:b ((% l):(% b))

−1.47 −1.27 −0.50

12 (91:9) 8.5 (90:10) 2.2 (68:31)

(99:1) (95:5) (90:10) (86:14) (88:12)b

a The calculated data have been corrected for basis set superposition error as well as for the zero-point vibrational effects. bData are referenced with regard to the ea conformer of DIPHOS.

the use of the M06-L/LANL2DZ/6-31G* level of theory, suggests that the subtle medium-range nonlocal dispersive interactions account for, or at least contribute to, these small free energy changes, dramatically affecting the predicted regioselectivity. In that regard, Rh-catalyzed hydroformylation can provide an appropriate avenue for exploring DFT functionals developed to provide a proper description of nonlocal interactions.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and xyz files giving additional computational details, transition states involved in the olefin insertion reaction, calculated thermodynamic data and l:b ratios, and Cartesian coordinates for the calculated structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.K.: [email protected]. *E-mail for T.A.J.: [email protected]. Author Contributions

Figure 4. Calculated transition states for olefin insertion in [Rh(TBDCP)(propene)(CO)H] (top) and [Rh(DIOP)(propene)(CO)H] (bottom) leading to linear and branched Rh propyl intermediates.

The authors declare no competing financial interest. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by USDA grant no. 2011-1000630362. We thank Dr. B. Sarkar and Prof. W. H. Thompson at the University of Kansas for helpful conversations.

role in determining the regioselectivity in the case of simple terminal olefins when the ee conformation of [Rh(PPh3)2(olefin)(CO)H] is considered. These noncovalent interactions selectively stabilize transition states leading to the linear Rh alkyl intermediates along the pathway to linear aldehydes, consistent with the experimentally observed high l:b ratios for these substrates. On the other hand, though the regioselectivity of Rh-catalyzed hydroformylation of heteroatom-containing vinyl acetate and aromatic-substituted styrene is chiefly decided by the nature of electronic effects, these noncovalent interactions are found to exert a noticeable influence. Taking into account that the ligand effect on the regioselectivity of the hydroformylation process has been studied assuming a constant H2/CO composition, the present modeling approach may aid in uncoupling the ligand effects from the concentration effects and thus allow a fundamental understanding of the ligand influences. Reproducing changes in regioselectivity has been a major challenge in computational chemistry, as large changes in the l:b ratios reflect small changes in free energy. Our computational methodology, which involves

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