Importance of Long-Range Noncovalent Interactions in the

Article pubs.acs.org/Organometallics

Importance of Long-Range Noncovalent Interactions in the Regioselectivity of Rhodium-Xantphos-Catalyzed Hydroformylation Manoj Kumar,*,† Raghunath V. Chaudhari,†,‡ Bala Subramaniam,†,‡ and Timothy A. Jackson*,†,§ †

Center for Environmentally Beneficial Catalysis, ‡Department of Chemical Engineering, and §Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States S Supporting Information *

ABSTRACT: M06-L-based quantum chemical calculations were performed to examine two key elementary steps in rhodium (Rh)xantphos-catalyzed hydroformylation: carbonyl ligand (CO) dissociation and the olefin insertion into the Rh−H bond. For the resting state of the Rh-xantphos catalyst, HRh(xantphos)(CO)2, our M06-L calculations were able to qualitatively reproduce the correct ordering of the equatorial−equatorial (ee) and equatorial−axial (ea) conformers of the phosphorus ligands for 16 derivatives of the xantphos ligand, implying that the method is sufficiently accurate for capturing the subtle energy differences associated with various conformers involved in Rh-catalyzed hydroformylation. The calculated CO dissociation energy from the ea conformer (ΔE = 21−25 kcal/mol) was 10−12 kcal/mol lower than that from the ee conformer (ΔE = 31−34 kcal/mol), which is consistent with prior experimental and theoretical studies. The calculated regioselectivities for propene insertion into the Rh−H bond of the eeHRh(xantphos)(propene)(CO) complexes were in good agreement with the experimental l:b ratios. The comparative analysis of the regioselectivities for the pathways originating from the ee-HRh(xantphos)(propene)(CO) complexes with and without diphenyl substituents yielded useful mechanistic insight into the interactions that play a key role in regioselectivity. Complementary computations featuring xantphos ligands lacking diphenyl substituents implied that the long-range noncovalent ligand−ligand and ligand−substrate interactions, but not the bite angles per se, control the regioselectivity of Rh-diphosphinecatalyzed hydroformylation of simple terminal olefins for the ee isomer. Additional calculations with longer chain olefins and the simplified structural models, in which the phenyl rings of the xantphos ligands were selectively removed to eliminate either substrate−ligand or ligand−ligand noncovalent interactions, suggested that ligand−substrate π-HC interactions play a more dominant role in the regioselectivity of Rh-catalyzed hydroformylation than ligand−ligand π−π interactions. The present calculations may provide foundational knowledge for the rational design of ligands aimed at optimizing hydroformylation regioselectivity.

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INTRODUCTION Rhodium (Rh)-catalyzed hydroformylation of olefins (Scheme 1) is one of the most commonly used homogeneous catalytic

syngas, CO:H2, provides a valuable tool for forming new C−C linkages via formyl group (−CHO) addition across the olefinic double bond. The processed hydroformylation products have applications in phthalate-free plasticizers,2 elastomers,3 and biodegradable lubricants.4 Given the increased availability of inexpensive shale gas in the United States, hydroformylation may also play an important role in increasing the chain length of the so-called natural gas liquids and, thus, help in the formation of detergent alcohols. Because of its importance to the chemical industry, the reaction has received significant experimental and theoretical attention over the last 30−40 years.5−16 Extensive experimental research has been performed to synthesize new ligands, allowing better chemo-, regio-, and enantioselective control of Rh-catalyzed hydroformylation.5 In recent years, DNA base-pair-type interactions6 and supra-

Scheme 1. Chemoselective Rh-Catalyzed Olefin Hydroformylation

processes in industry and accounts for more than 10 million tons annual production of bulk oxo products.1 Apart from Rh, the other transition metals used in the catalytic hydroformylation processes, in either industry or academia, are cobalt, platinum, ruthenium, and iridium. The hydroformylation reaction, which utilizes a Rh catalyst with a 1:1 mixture of © XXXX American Chemical Society

Received: December 12, 2014

A

DOI: 10.1021/om5012775 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Structures of Various Xantphos-Type Ligands Considered in the Present Worka

a

van der Veen et al. have synthesized these xantphos ligands and studied the olefin hydroformylation catalyzed by Rh-xantphos complexes.20,24

molecular7 and scaffolding8 themes have been explored to improve the reaction efficiency. Despite these advances in ligand design, a comprehensive understanding of the effect of ligand structure on catalytic activity of the hydroformylation reaction has yet to be established. The effect of stereoelectronic properties of bidentate/diphosphine ligands on the regioselectivity of the reaction has been explored in great detail.9−16 Although the natural bite angle of the ligand was initially proposed to play an important role in determining the regioselectivity,9,12 its involvement remains ambiguous. A clear understanding in this regard is also essential to delineate ligand and concentration effects on reported selectivities and thereby to guide rational process design. Quantum chemical calculations have been frequently used to gain useful insights into the origin of regioselectivity in the hydroformylation reaction.17−33 However, the large size and conformational complexity of the Rh catalyst and the olefin substrates make it challenging to perform meaningful theoretical treatments using realistic structural models, even with modern day computing resources. For these reasons, much of the computational work in this area has been performed using simplified ligand models, such as PH3. Significant insights have been gained through this approach. For example, Rocha et al. examined the propene insertion into the Rh−H bond of HRh(PH3)2CO and concluded that the relative stabilities of the linear and branched Rh-alkyl complexes could be used to explain the experimental regioselectivity of PPh3-modified Rh catalysts.25b For the HRh(CO)3-catalyzed hydroformylation of various different olefins, Alagona et al. could explain the experimental trends by comparing the transition-state barriers for the formation of Rh-alkyl isomers.27 On the other hand, there are few studies in which the realistic structural models have been used for Rh catalysts. For example, Casey et al. applied molecular mechanics calculations to study the effect of ligand sterics on the regioselectivity of HRh(BISBI)(CO)- and HRh(DIPHOS)(CO)-catalyzed hydroformylation of propene, but failed to reproduce the correct trends.9,10 Using a combined QM/MM approach and frozen reaction centers, Herrmann et al. could reproduce regio- and

enantioselectivities for Rh catalysts with monodentate PPh3 and bidentate DIPHOS, BISBI, and BINAPHOS ligands.21−23 Cundari et al. used ONIOM calculations to study ethene and propene hydroformylation and invoked the olefin orientation in the HRh(L)2(olefin)(CO) (L is a monodentate ligand) intermediate in addition to the transition-state stabilities to explain the regioselectivity trends.28 Carbó et al. performed the IMOMM-based QM(B3LYP)/MM(MM3(92)) calculations to understand the role of the ligand bite angle and steric properties of two xanthene ligands, homoxantphos and benzoxantphos, in the hydroformylation of propene.26 The nonbonding interactions between the diphenylphosphine substituents of the xanthene-based ligand and the −CH3 group of the propene substrate, rather than the bite angle, were suggested to be the main determinant of the regioselectivity. Landis et al. undertook a thorough investigation of the full catalytic cycle of the Rh-xantphos-catalyzed propene hydroformylation using the ONIOM (B3LYP/ LANL2DZ: HF/LANL2MB) calculations.29 Although their calculations provided an excellent description of CO dissociation step, the Rh−olefin binding step could not be realistically described with their chosen ONIOM method; that is, the olefin binding step was calculated to involve significant free-energy barriers of 17.1−19.3 kcal/mol. Moreover, the calculated freeenergy barriers of 26.5−34.9 kcal/mol for the olefin insertion into the Rh−H bond of HRh(xantphos)(propene)(CO) were too high to be consistent with known turnover frequencies of 100 h−1 for hydroformylation with xantphos ligands. However, their calculated l:b ratio of 42:1 for the lowest energy pathway at 80 °C was in a good agreement with the experimental value of 52:1 observed for 1-octene hydroformylation with a Rhxantphos catalyst at 80 °C.12,14 In short, although they reproduced the regioselectivity, the activity of the Rh catalyst was not replicated, and the quality of the computational approach and the limitation of modern computing resources were largely held responsible. It must be noted that in all these reported computational studies as well as in this work, it is assumed that the syngas ratio (CO:H2) in the reaction phase is 1:1, although this ratio is higher in most conventional solvents given the higher CO solubility in the liquid phase compared to B


Article

Organometallics H2. It should be clear, however, that a fundamental understanding of the intrinsic ligand effects on the reaction mechanism is essential to better discern concentration effects brought about by solubility and/or mass transfer limitations. We have recently performed a complete QM treatment (using the M06-L functional with the LANL2DZ basis set for Rh and the 6-31G* basis set for all other atoms) of the olefin insertion step of the hydroformylation reaction using the actual Rh catalysts supported by monodentate PPh3 as well as bidentate DIOP, TBDCP, and DIPHOS ligands with various types of olefin substrates.33 Such a theoretical description that accounts for nonlocal correlation effects and that allows for the complete relaxation of the reaction center is important for capturing the ligand-based noncovalent interactions that could play a crucial role in the reaction, as has been noted by Carbó et al.26 Using this computational protocol, we were able to reproduce the experimental regioselectivities with a reliable degree of accuracy and characterize the noncovalent interactions involving the ligand−ligand and ligand−substrate, which were crucial for determining the regioselectivity. Building upon these recent developments, we herein report our detailed QM treatment (M06-L/LANL2DZ/6-31G*) of the key elementary steps of propene hydroformylation, catalyzed by various Rh-xantphos catalysts (see Scheme 2 for a list of xantphos derivatives considered in this study). Although the reaction has been computationally studied before,26,29,31 a full QM treatment of the reaction has yet to be reported. Moreover, key mechanistic questions still remain. For example, what role, if any, does the ligand bite angle play in dictating the regioselectivity? What do ligand-based noncovalent interactions contribute toward the regioselectivity of the reaction? What are the drawbacks and potential advantages of applying simplified structural models to gain fundamental insights into the hydroformylation reaction? Is there any computational approach that could be universally applicable for studying hydroformylation catalysts for systems with considerable ligand steric bulk?

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Figure 1. Molecular structures showing potential isomers and conformers of the xanthene ligand (distal and proximal confomers; top), the HRh(xantphos)(CO)2 complex (1ee, 1ee′, and 1ea; center), and the HRh(xantphos)(CO) complex (2c, 2c′, 2t, and 2t′; bottom). distal (ee) and proximal (ee′) forms depending on the orientation of the xanthene ring relative to the axial hydride and carbonyl ligands, we have considered both situations in the present study. For the fourcoordinate HRh(xantphos)(CO) complexes formed by CO dissociation from HRh(xantphos)(CO)2, we have considered various cis and trans isomers (2c, 2c′, 2t, and 2t′ in Figure 1). The entire quantum chemical treatment reported in this work was performed using the M06-L/LANL2DZ/6-31G* level of theory, as this computational approach was recently validated for hydroformylation chemistry.33 Here the basis set LANL2DZ for Rh atoms and 6-31G* for non-Rh atoms have been used. To further validate the computational approach, we have compared our calculated results with those of previous experimental and computational studies for (i) the conformational equilibrium between the ee- and ea-HRh(xantphos)(CO)2 isomers, (ii) the CO dissociation energy, (iii) the ligand bite angles for various Rh-xantphos systems and the bite-angle ranges for a given excess strain energy of 5.0 kcal/mol, and (iv) the propene insertion into the Rh−H bond of the ee- and ea-HRh(xantphos)(propene)(CO) complex. The regioselectivity of propene insertion was calculated using the following equation, which is based on the relative energies of the transition states leading to the linear and branched Rh-propyl intermediates:

COMPUTATIONAL METHODS

All calculations reported in this work were carried out using NWChem34 quantum chemical software for electronic structure and property calculations. We examined two key elementary steps of Rhxantphos-catalyzed hydroformylation of 1-octene: (i) CO dissociation from the resting state of the Rh-xantphos catalyst, HRh(xantphos)(CO)2, and (ii) olefin insertion into the Rh−H bond of the HRh(xantphos)(olefin)(CO) complex. It is important to mention here that the present study exclusively deals with analyzing the static ligand effects on the regioselectivity of the hydroformylation process and does not take into account any variation in local CO or H2 concentration that could affect ligand substitutions. Such a computational approach may prove useful in disentangling the ligand effects from the concentration effects and thus allow a fundamental understanding of the ligand influences. To model the process, we considered a variety of Rh-xantphos catalysts with the xantphos structural models shown in Scheme 2, whereas the substrate was modeled using propene. Similar structural models have been routinely applied in previous computational investigations of Rh-catalyzed hydroformylation.26,29,31 To investigate the CO dissociation step from the HRh(xantphos)(CO)2 complex, and the propene insertion into the Rh−H bond of the HRh(xantphos)(olefin)(CO) intermediate, we considered both, equatorial−equatorial (ee) and equatorial−axial (ea), isomers for the respective complexes (Figure 1). Note that the ee and ea notations used in the present work refer to the arrangement of the phosphorus ligands around the metal center. Since the ee isomer can further exist in

l: b = kl: kb = e−ΔGl

‡

/ RT

: e−ΔGb

‡

/ RT

= e−ΔΔG

‡

/ RT

≈ e−ΔΔE

‡

/ RT

For calculating the regioselectivities, we considered only a selected set of xantphos ligands. The choice was driven by the fact that the selected ligands encompass a wide range of the regioselectivities,12,14 which offered us the opportunity to test the robustness of our computational approach. Although propene was used in the present calculations, the olefin-insertion step for selected diphosphine ligands was also studied using the longer chain olefins butene and pentene to examine the effect of substrate on the regioselectivity. To assess the basis set effect, we calculated the regioselectivities by performing single-point calculations using the M06-L/LANL2DZ/6-311G** level of theory (Table S1). However, this resulted in a modest change in the C


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Organometallics calculated results. To better mimic experimental conditions, solvation effects were implicitly accounted for using toluene as solvent and the COSMO framework (Table S1). Again, the results remained essentially unchanged. However, it is important to note that an implicit solvation model does not take into account the direct involvement of solvent molecules in the reaction. An explicit treatment of solvation effects in the hydroformylation reaction, especially at the scale required for considering solute−solvent interactions of all viable intermediates, is currently not feasible.

approaches are clearly essential to either admit or rule out such discrepancies. In the present study, we have performed M06-L/LANL2DZ/ 6-31G* calculations on the HRh(xantphos)(CO)2 complex considering a variety of xantphos-type ligands (Scheme 2). Recently, we have successfully applied this computational approach to reproduce the experimental l:b ratios for various Rh-catalyst and substrate combinations.33 The M06-L functional takes into account the medium-range correlation effects that are expected to play an important role in the hydroformylation process38 and, in principle, should be capable of providing a correct qualitative description of the ee:ea equilibrium. The calculated relative energies of the ee, ee′, and ea isomers for 16 separate HRh(xantphos)(CO)2 complexes are listed in Table 1. For all complexes considered, the ee form is found to

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RESULTS AND DISCUSSION To gain molecular level insight into the hydroformylation reaction catalyzed by a Rh-xantphos assembly, we computationally examined the initial elementary steps of the catalytic cycle: (i) CO dissociation and (ii) olefin insertion. These catalytic steps are particularly selected because the computational approach can be validated against kinetic data for CO dissociation24 and the experimental regioselectivities12,14 for a variety of xantphos catalysts. In the first step, we performed an extensive search of the possible conformers for the key species involved in the reaction at the full QM level. As discussed in detail by Landis and Uddin,29 there are multiple ligand conformations and coordination modes possible for realistically modeled intermediates in hydroformylation catalysis, which makes theoretical characterization a challenging task. Although the Rh-xantphos catalyst possesses a rather rigid backbone displaying pseudo-Cs symmetry, previous theoretical calculations26,29,31 indicate that it is quite possible to define several ligand conformations and coordination modes. Below is a detailed description of the geometries and relative energies of the various possible isomers. HRh(Xantphos)(CO)2 Complex. According to in situ spectroscopic studies of the hydroformylation reaction,24 the isomers of the HRh(xantphos)(CO)2 complex are the only detectable species in solution under catalytic conditions. However, for certain diphosphine ligands, dimeric Rh complexes have also been observed.9,35−37 The experimental measurements of 1JRh−H and 2JP−H coupling constants at 298 K in benzene solvent predict a rapid equilibrium between the ee and ea isomers, with the former predominating by a ratio of 7:3. Previously, Landis and Uddin performed a detailed theoretical investigation of the conformational landscape of the key hydroformylation species at the two-layered ONIOM level of theory (B3LYP/LANL2DZ:HF/LANL2MB).29 Their calculations indicated that the most stable isomer of pentacoordinated HRh(xantphos)(CO)2 has an ea arrangement of ligand P atoms; that is, the ea isomer is 1.3 kcal/mol more stable than the ee isomer, which is in contrast to the experimental findings. Similarly, the QM/MM calculations of Zuidema et al. predicted the ea isomer of the HRh(thixantphos)(CO)2 complex to be slightly more stable.31 The inability of the pure B3LYP functional to describe the noncovalent π−π interactions between the phenyl rings of the xantphos-type ligand and the use of the PH3 model in the ONIOM layer construction that leads to the underestimation of the intraligand noncovalent interactions could be one of the possible sources of discrepancy between the experimentally determined preference of the ee isomer and these computational predictions. Alternatively, the inability to identify all conformers of HRh(xantphos)(CO)2 could account for the error, as suggested by Landis and Uddin.29 As discussed earlier, possible variations in syngas ratio in the reaction phase during experimental studies have not been taken into account in these comparisons. Reliable modeling

Table 1. M06-L/LANL2DZ/6-31G*-Calculated Energies (kcal/mol) of the ea- and ee′-HRh(L)(CO)2 Complexes Relative to their ee Conformers ΔE(ΔG) L (R; X)

ea

ee′

homoxantphos (R = H; X = CH2CH2) phosxantphos (R = H; X = P(C6H5)) sixantphos (R = H; X = Si(CH3)2) xantphos (R = H; X = C(CH3)2) isopropxantphos (R = H; X = C(C(CH3)2)) benzylnixantphos (R = H; X = N(CH2C6H5)) benzoxantphos (R = H; X = C(CH)(CHCH2)) DPEphos (R = H; X = H,H) nixantphos (R = H; X = NH) thixantphos (R = H; X = S) p-dimethylamine thixantphos (R = N(CH3)2; X = S) p-methoxy thixantphos (R = OCH3; X = S) p-methyl thixantphos (R = CH3; X = S) p-fluoro thixantphos (R = F; X = S) p-chloro thixantphos (R = Cl; X = S) p-trifluoromethyl thixantphos (R = CF3; X = S)

0.3(0.2) 1.1(1.0) 1.2(1.9) 0.8(−0.4) 0.6(−0.2) 1.2(0.8) 0.4(−0.7) 0.4(0.6) 1.0(1.7) 1.1(1.4) 0.8(1.3)

1.1(0.8) 1.7(0.6) 1.4(2.0) 1.4(1.6) 1.3(1.5) 1.9(1.9) 1.2(1.4) 0.8(0.3) 1.3(1.8) 1.6(2.6) 1.2(1.6)

0.4(1.3) 1.2(0.9) 1.2(1.1) 0.9(0.6) 0.9(0.1)

1.2(2.7) 1.1(0.0) 1.4(1.5) 1.5(2.5) 1.6(1.1)

be the most stable. Furthermore, the ee isomer is found to be more stable than the ea form by 0.3−1.2 kcal/mol (Figure 2); thus, the chosen theoretical method reproduces the experimentally determined ee:ea equilibrium position that favors the ee isomer. For example, according to our M06-L calculations, the ee-HRh(xantphos)(CO)2 complex is 0.8 kcal/mol more stable than its ea isomer, which is in qualitative agreement with previous experimental findings.24 The calculated relative stability of the ee-HRh(thixantphos)(CO)2 complex is 1.1 kcal/mol. To further verify the accuracy of the method, we also analyzed the ee:ea equilibrium for Rh complexes that contain bidentate ligands other than xantphos, such as DIOP, BISBI, and CBDPP (Table S2). The ee isomers of all the complexes are again found to be more stable than their respective ea ones. CO Dissociation from the HRh(Xantphos)(CO)2 Complex. Loss of CO from the HRh(xantphos)(CO)2 complex results in the formation of a four-coordinate HRh(xantphos)(CO) species that can exist as cis and trans isomers (2c, 2c′, 2t, and 2t′; see Figure 1). The CO dissociation from the ea isomer leads to the formation of cis species, whereas that from the ee isomer results in the trans ones. Previous computational studies predict this ligand dissociation process to be barrierless.29,31 D


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Organometallics

(thixantphos)(CO)2 complex is 4.2 kcal/mol overestimated. The CO dissociation energy for thixantphos and xantphos ligands has also been previously calculated using DFT methods.29,31 Although our calculated value of 24.3 kcal/mol for thixantphos is slightly higher that the previous B3LYP value (21.4 kcal/mol) of Zuidema et al.,31 our calculations are consistent with their QM/MM calculations in a sense that their calculated CO dissociation energy from the ea pathway (21.4 kcal/mol) was 7.3 kcal/mol lower than that from the ee pathway (28.7 kcal/mol). On the other hand, our predicted dissociation energy of 23.8 kcal/mol for the xantphos ligand is marginally lower than the ONIOM-based B3LYP value (26.0 kcal/mol) of Landis and Uddin.29 P−Rh−P Bite Angle in the ee-HRh(Xantphos)(Propene)(CO) Complex. There has been considerable literature suggesting that the bisphosphine steric properties play an important role in determining the regioselectivity of Rhcatalyzed hydroformylation. Casey and co-workers studied a vast array of diphosphines with systematically varied steric/or electronic properties and demonstrated the pronounced effect of the natural bite angle of bidentate ligands on the regioselectivity of Rh-catalyzed hydroformylation.9−11 van Leeuwen et al. exclusively examined the effect of the natural bite angle by developing a series of xantphos-based bisphosphines with similar electronic properties and steric size.12 Herein we have calculated the total excess strain energy of the ee-HRh(xantphos)(propene)(CO) isomers as a function of the bite angle, P−Rh−P, for a variety of xantphos-type bisphosphine ligands. It is informative to compare our DFTcalculated bite angles for these complexes with the natural bite angles of the diphosphine ligands determined by molecular mechanics calculations.12,20,24 Although our DFT-calculated bite angles and their ranges for the excess strain energy of 5.0 kcal/mol are 0−10° underestimated as compared to the previously determined natural bite angles and their respective ranges (Figures 4 and 5 and Table S4), there is a fair degree of correlation between the natural bite angles and the DFTcalculated bite angles among the series (Figure 4). The eeHRh(benzoxantphos)(propene)(CO) and ee-HRh(benzylnixantphos)(propene)(CO) complexes are notable

Figure 2. Calculated relative stability (kcal/mol) of the ee conformers of the HRh(xantphos)(CO)2 complex for a variety of xantphos ligands considered in the present work. The zero of the scale refers to the energy of the ee conformer, whereas each bar represents the relative energy of the corresponding ea isomer.

Moreover, the cis isomers resulting from the dissociation were found to be lower in energy than the trans isomers by several kcal/mol. Our M06-L calculations predict the cis isomers to be 10−11 kcal/mol more stable than the trans versions, implying that the dissociation of equatorial CO from the ea form is considerably faster than that from the ee form, which qualitatively agrees with experimental findings.24 Moreover, the calculated CO dissociation energies for a variety of xantphos ligands lie in the range 22.3−24.9 kcal/mol (Figure 3; Table S3), implying that the electronic substitution at the xantphos structure does not significantly change the CO dissociation energies. The experimental activation barrier for the CO dissociation from the thixantphos ligand has been measured to be 20.1 kcal/mol. 24 Our calculated CO dissociation energy of 24.3 kcal/mol from the ea-HRh-

Figure 3. M06-L/LANL2DZ/6-31G*-calculated zero-point-corrected electronic energy (kcal/mol) for CO dissociation from the HRh(xantphos)(CO)2 complex for a variety of xantphos ligands considered in the present work.

Figure 4. Comparison of natural and M06-L/LANL2DZ/6-31G*calculated ligand bite angle in the ee-HRh(xantphos)(propene)(CO) complexes. Note that the natural bite angles were previously determined using the molecular mechanics calculations.12,20,24 E


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Organometallics

Regioselectivity of ee-HRh(L)(CO)-Catalyzed Hydroformylation. We next calculated the regioselectivity of the HRh(xantphos)(CO)-catalyzed hydroformylation by examining the propene insertion into the Rh−H bond of the rhodium propene complex HRh(xantphos)(propene)(CO). This intermediate is produced in the catalytic cycle from the HRh(xantphos)(CO)2 resting state of the catalyst by loss of one CO ligand followed by addition of propene. Gaining additional insight into the olefin insertion step of the catalytic cycle is also important in the wake of a recent combined experimental and computational study on 1-octene hydroformylation by a Rhthixantphos catalyst31 that proposed that the olefin insertion and the olefin coordination steps have similar free-energy barriers. Thus, both steps might, together, be rate-limiting. There is a large body of experimental evidence showing that the diphosphine ligands with wider bite angles lead to a high l:b ratio in the hydroformylation of terminal olefins.9,10,12,13,20 These ligands with large bite angles also favor the ee conformations of the HRh(L)(CO)2 resting complexes. For example, BISBI, a bisequatorial ligand with a natural bite angle of 112.3°, gives an l:b ratio of 66.5, while DIPHOS, an equatorial−axial ligand, which has a natural bite angle of 84.5°, gives an l:b ratio of only 2.1 in the hydroformylation of 1hexene using a Rh-diphosphine catalyst at 34 °C under 6 atm of 1:1 H2/CO.9 However, some experimental studies9,12,39 suggest that this correlation between the natural bite angle and regioselectivity is not always valid; that is, the diphosphine ligand with a wider bite angle does not necessarily give a high l:b ratio. For example, Casey et al.9 and Yamamoto et al.39 have separately shown that 2,5-dppm-nor, which has a wider natural bite angle of 126°, results in a much lower l:b ratio of 2.6 as compared to the BISBI ligand. Similarly, van Leeuwen et al. have shown that DBFphos, despite having an unusually large natural bite angle of 131.1°, gives a very modest l:b ratio of 3.4, whereas xantphos, with a natural bite angle of 111.7°, gives a much higher l:b ratio of 57.1 for the hydroformylation of 1octene at 40 °C under 9.87 atm of 1:1 H2/CO.12 The inability of diphosphine ligands with larger bite angles to form stable chelates is suggested to be the main reason for their low selectivity. Previous calculations indicate that the ee and ea isomers of the HRh(xantphos)(propene)(CO) intermediate are close in energy.26,29,31 Of particular mention are two QM/MM studies, one by Carbó et al.,26 where they have investigated the regioselectivity determining olefin insertion from the eeHRh(benzoxantphos)(propene)(CO) and ee-HRh(homoxantphos)(propene)(CO) intermediates, and the other by Zuidema,31 in which both the ee and ea isomers of HRh(thixantphos)(propene)(CO) were essentially found to be of similar energy. Therefore, we considered both ee and ea isomers of the rhodium-propene complex in the present work. We calculated and compared the energetics of ee-, ee′- (which differ in the relative orientation of the xanthene ring with respect to the axial hydride and carbonyl ligands), and eaHRh(L)(propene)(CO) isomers for a few xantphos ligands (Table S4). Irrespective of the ligand considered, the ee isomer was found to be 0.2−0.8 kcal/mol more stable than the ea form, which is consistent with prior experimental studies.24 Moreover, the ee conformer was ∼3.0−7.0 kcal/mol more favorable than the ee′ form, implying that the ee isomer is the lowest energy form of the HRh(L)(propene)(CO). Even for a given ee-HRh(xantphos)(propene)(CO) complex, multiple conformers are possible depending upon the relative

Figure 5. M06-L/LANL2DZ/6-31G*-calculated excess strain energy (kcal/mol) as a function of P−Rh−P bite angle in the eeHRh(xantphos)(propene)(CO) complex for a variety of xantphos ligands considered in the present work. The horizontal green line refers to the excess strain energy of 5.0 kcal/mol, a common reference used for estimating the bite angle range.

outliers, as the DFT-calculated bite angles of 104.8° and 104.5° are significantly smaller than the natural bite angles of 120.6° and 114.1°.12 It is interesting to compare our calculated bite angle data with the previously available computational data on the hydroformylation of Rh-xantphos-catalyzed olefins. So far, only four xantphos-based ligands, homoxantphos, benzoxantphos, thixantphos, and xantphos, have been computationally explored for their role in catalytic Rh-hydroformylation. Our calculated P−Rh−P angle of 102.8° in the ee-HRh(thixantphos)(propene)(CO) isomer matches well with 105.6° in the ee-HRh(thixantphos)(ethene)(CO) complex, predicted by the calculations of Zuidema et al.31 Carbó et al. have studied the hydroformylation catalyzed by Rh-homoxantphos and Rh-benzoxantphos catalysts.26 Their predicted P−Rh−P angle values of 101.8° and 107.4° in the eeHRh(homoxantphos)(ethene)(CO) and ee-HRh(benzoxantphos)(ethene)(CO) complexes resemble closely our calculated values of 100.9° (homoxantphos) and 104.8° (benzoxantphos). F


Article

Organometallics

In the next step, the regioisomeric ratios (l:b ratio) were estimated using a formula that has been successfully used previously32 and that takes into account only the relative energies of the transition states leading to the branched and linear Rh-alkyl intermediates (TSb and TSl; see Computational Method section). This assumption is valid only when the olefin insertion is irreversible and when the lowest energy transition states leading to the linear and branched isomers are sampled. For estimating the regioisomeric ratios for the propene insertion into the Rh−H bond of the HRh(xantphos)(propene)(CO) complex, we have considered phosxantphos, sixantphos, thixantphos, xantphos, isopropaxantphos, benzylnixantphos, and benzoxantphos ligands. The choice of these xantphos ligands is inspired by the fact that the l:b ratios for HRh(xantphos)(CO)-catalyzed 1-octene hydroformylation span a wide distribution, 12,14 which provides a good opportunity to test the robustness of our computational protocol. The calculated zero-point energy-corrected electronic energy differences between TSbs and TSls and the corresponding l:b ratios for propene insertion into the ee-HRh(xantphos)(CO) complexes are collected in Table 3. Experimental data for 1octene hydroformylation at 80 °C and 19.7 atm are included for comparison.12,14 To ensure that the lowest pathway originating from the ee-HRh(L)(propene) complex was probed, we calculated and compared the l:b ratios for the propene insertion into the ee and ea conformers of thixantphos. For the ea-based pathway, the relative transition-state barrier, ΔΔE⧧, is −0.33 kcal/mol, which reflects an l:b ratio of 1.8, whereas for the ee-based pathway, the calculated ΔΔE⧧ value is −2.17 kcal/ mol and the l:b ratio is 38.8. The latter value is in better agreement with an experimental value of 56.6,12,14 implying that the hydride migration from the ee-HRh(L)(propene) complex is favored. For that reason, the l:b ratios for all other catalysts have been calculated assuming the ee conformation for the HRh(L)(propene)(CO) complex. Since for a given ee-HRh(L)(propene)(CO)complex, there are multiple conformations possible due to the position and orientation of the substrate alkyl chain (Scheme 3), we calculated the regioselectivities for all the pathways originating from these complexes (Table 3). From the analysis of energetic results, it is clear that, for all Rh-xantphos catalysts and all ee conformations, the barrier height for TSl formation is lower than that for the TSb formation, favoring the linear product over the branched one, which is qualitatively consistent with the experimental data. Moreover, for the propene insertion into the Rh−H bond of all the ee1-HRh(L)(propene)(CO) complexes, the relative energy differences between TSl and TSb are found to be the largest, and more importantly, the calculated l:b ratios are more consistent with experimental values. For example, the calculated l:b ratio of 38.8 for the propene insertion into the Rh−H bond of the ee1-HRh(thixantphos)(propene)(CO) is in a reasonable agreement with the experimental value of 56.6.12,14 On the other hand, for the next lowest energy complex, ee2-HRh(thixantphos)(propene)(CO), which is only 0.3 kcal/mol less stable than the analogous ee1, the calculated l:b ratio of 28.8 is appreciably underestimated. Taking into account that the lowest energy ee1HRh(L)(propene)(CO) complex leads to the most favorable reaction pathway, we specifically analyzed geometric parameters and l:b ratios calculated using the ee1 conformation for all the Rh-xantphos catalysts.

location of the propene methyl moiety (Scheme 3). We have calculated all possibilities and compared their energetics (Table Scheme 3. Various Conformations for the eeHRh(thixantphos)(propene)(CO) Complex Considered in the Present Work (Top Panel) and M06-L/LANL2DZ/631G*-Calculated Energies (kcal/mol) of Transition States for the Propene Insertion into the Rh−H Bond of Various ee-HRh(thixantphos)(CO) Complexes Leading to Linear and Branched Rh-Propyl Intermediates (Bottom Panel)

2). In all cases, the ee1 and ee2 conformers of the eeHRh(L)(propene)(CO) complex, in which the methyl group is located on the same side of the equatorial plane as the xantphos ligand backbone and the oxygen atom of the xanthene ring lies farther from the Rh center, are found to be the most stable ones. Table 2. M06-L/LANL2DZ/6-31G*-Calculated Relative Energies (kcal/mol) of Various Conformers of the eeHRh(L)(propene)(CO) Complexa ΔE(ΔG)

a

L

ee1

ee1′

ee2

ee2′

phosxantphos sixantphos thixantphos xantphos isopropxantphos benzylnixantphos benzoxantphos

0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.6(1.4) 1.5(1.4) 1.8(1.2) 1.8(1.6) 1.9(1.9) 1.3(1.3) 0.9(0.5)

−0.1(−0.8) 0.0(0.0) 0.3(0.5) 0.0(0.0) −0.1(0.3) 0.0(0.0) −0.7(−1.4)

1.9(1.7) 1.7(1.8) 1.4(0.9) 1.2(0.5) 1.2(1.4) 1.6(1.4) 1.2(1.5)

Energies of the ee conformers are relative to the ee1. G


Article

Organometallics

Table 3. Comparison of M06-L/LANL2DZ/6-31G*-Calculated and Measured l:b Ratios for the ee-HRh(L)(CO)-Catalyzed Hydroformylation of Propene calcd ee1 L phosxantphos sixantphos thixantphos xantphos isopropxantphos benzylnixantphos benzoxantphos

⧧b

ΔΔE

−1.45 −2.18 −2.17 −2.10 −2.04 −2.38 −1.94

ee1′ l:b(%l) 11.7(92) 39.5(98) 38.8(98) 34.8(97) 31.5(97) 55.6(98) 26.4(96)

⧧b

ΔΔE

−1.22 −1.68 −1.38 −1.43 −1.10 −1.78 −1.65

ee2 ⧧b

ΔΔE

l:b(%l)

−1.33 −1.28 −1.99 −2.05 −1.91 −1.59 −1.19

7.9(89) 17.0(94) 10.2(91) 11.2(92) 6.4(87) 20.0(95) 16.2(94)

expta

ee2′ ⧧b

ΔΔE

l:b(%l)

−1.30 −1.21 −0.88 −1.20 −0.95 −1.44 −0.89

9.4(90) 8.6(90) 28.8(97) 32.0(97) 25.0(96) 14.5(94) 7.5(88)

l:b(%l)

l:b(%l)

8.9(90) 7.7(89) 4.4(82) 7.6(88) 5.0(83) 11.4(92) 4.5(82)

14.6(89.7) 34(94.4) 56.6(93.7) 52.2(94.5) 49.8(94.3) 50.6(94.3) 50.2(96.5)

Experimental data used here are measured by van der Veen et al. for the hydroformylation of 1-octene at 80 °C and 19.7 atm.24 bΔΔE⧧ refers to the difference between the zero-point energy-corrected electronic barriers (kcal/mol) for the formation of linear and branched Rh-propyl intermediates due to the propene insertion into the Rh−H bond of the ee-HRh(L)(propene)CO complexes. a

Table 4. M06-L/LANL2DZ/6-31G*-Calculated Key Geometric Parameters of the Reaction Center of the Complexes and Transition States for the ee1-HRh(L)(CO)-Catalyzed Propene Hydroformylationa geometric parameters L phosxantphos

sixantphos

thixantphos

xantphos

isopropxantphos

benzylnixantphos

benzoxantphos

species Cb TSl TSb C TSl TSb C TSl TSb C TSl TSb C TSl TSb C TSl TSb C TSl TSb

Rh−P1, Rh−P2 2.38, 2.44, 2.41, 2.39, 2.42, 2.41, 2.39, 2.43, 2.40, 2.40, 2.44, 2.42, 2.43, 2.45, 2.42, 2.40, 2.47, 2.45, 2.42, 2.49, 2.47,

2.38 2.33 2.36 2.38 2.33 2.37 2.38 2.33 2.36 2.39 2.33 2.37 2.43 2.33 2.37 2.40 2.34 2.37 2.41 2.35 2.39

Rh−H

Rh−CO

1.64 1.67 1.67 1.63 1.67 1.67 1.63 1.67 1.67 1.63 1.66 1.67 1.63 1.67 1.67 1.63 1.67 1.67 1.63 1.67 1.67

1.93 1.90 1.91 1.93 1.90 1.91 1.93 1.90 1.91 1.93 1.90 1.91 1.94 1.90 1.91 1.93 1.90 1.91 1.93 1.90 1.90

Rh−C 2.21, 2.22 2.23 2.21, 2.22 2.23 2.22, 2.22 2.23 2.21, 2.22 2.23 2.19, 2.22 2.23 2.22, 2.22 2.23 2.21, 2.21 2.22

2.20

2.19

2.20

2.20

2.18

2.20

2.19

H−C 2.60, 1.73 1.63 2.58, 1.72 1.63 1.63 1.72 1.63 2.58, 1.72 1.63 2.57, 1.73 1.64 2.58, 1.74 1.63 2.58, 1.74 1.63

2.58

2.57

2.58

253

2.58

2.57

CC

P−Rh−P

H−Rh−C−C

1.41 1.41 1.42 1.41 1.41 1.42 1.41 1.41 1.42 1.41 1.41 1.42 1.42 1.41 1.42 1.41 1.41 1.42 1.41 1.41 1.42

102.9 104.0 104.0 102.2 103.9 103.7 102.8 105.0 104.8 103.9 106.0 105.3 102.5 105.4 105.2 104.5 106.0 105.7 104.8 106.0 105.7

85.8 11.5 26.4 86.1 14.3 26.4 86.7 13.2 26.1 86.7 14.8 26.1 88.9 12.6 26.1 86.5 12.8 27.7 87.1 13.4 26.3

a

L = a bidentate xantphos ligand. All bond distances are in angstroms, while bond angles are in degrees. bC refers to the ee1-HRh(L)(propene)CO complex.

Carbó et al. have previously applied the rigorous IMOMMbased hybrid QM(B3LYP)/MM(MM3) calculations to study the propene insertion into the Rh−H bond of the HRh(L)(propene)(CO) complex, L = homoxantphos and benzoxantphos.26 Our M06-L-calculated l:b ratio of 26.4 for benzoxantphos is significantly higher than their value of 4.9 and is in better agreement with the experimental value of 50.2.12,14 A better correlation between the results of our calculations and the experimental data could be rooted in the fact that the geometry optimization of the minima and transition states has been exclusively performed at the quantum mechanics level that provides an authentic description of ligand- and substrate-based nonlocal correlation effects. Our calculated l:b ratio of 34.8 for the xantphos ligand is slightly lower than the previous value of 42 for the lowest energy pathway, predicted by ONIOM (B3LYP+HF) calculations of Landis and Uddin.29

Table 4 contains the key geometric parameters of the reaction centers of the ee1-HRh(xantphos)(propene)(CO) complexes and the transition states leading to the formation of branched (TSb) and linear (TSl) Rh-propyl intermediates due to the propene insertion into the Rh−H bond via the lowest energy pathway. Previous experimental studies9,10,12 suggest that the regioselectivity of HRh(L)(CO)-catalyzed olefin hydroformylation linearly correlates with the natural bite angle of the bidentate L ligand. Qualitatively, our calculations reproduce this experimental observation. In spite of the narrow range of calculated bite angles in the HRh(L)(propene)(CO) complexes (101−105°), the benznixantphos (104.5°) and benzoxantphos (104.8°) ligands, which are experimentally more selective for the linear isomer, have wider bite angles, whereas the sixantphos (102.2°) and phosxantphos (102.9°), which are less selective for the linear isomer, have relatively smaller bite angles. H


Article

Organometallics

Figure 6. M06-L/LANL2DZ/6-31G*-calculated transition-state structures for the ee1-HRh(L)(CO)-catalyzed hydroformylation of propene.

Table 5. Comparison of M06-L/LANL2DZ/6-31G*-Calculated and Measured l:b Ratios for the ee1- and ee2-HRh(L)(CO)Catalyzed Hydroformylation of Propene Using PPh2- and PH2-Type Structural Models of Rh-Catalysta calcd ee1 PPh2-type L phosxantphos sixantphos thixantphos xantphos isopropxantphos benzylnixantphos benzoxantphosf

ΔΔE

‡e

−1.45 −2.18 −2.17 −2.10 −2.04 −2.38 −1.94

ee2

c

l:b(%l) 11.7(92) 39.5(98) 38.8(98) 34.8(97) 31.5(97) 55.6(98) 26.4(96)

PH2-type ΔΔE

⧧e

1.62 1.67 1.44 1.72 1.51 1.45 −3.18

d

PPh2-type ⧧e

ΔΔE

l:b(%l)

−1.33 −1.28 −1.99 −2.05 −1.91 −1.59 −1.19

0.1(6) 0.1(6) 0.1(8) 0.1(5) 0.1(7) 0.1(8) 215.3(100)

c

l:b(%l) 9.4(90) 8.6(90) 28.8(97) 32.0(97) 25.0(96) 14.5(94) 7.5(88)

PH2-typed ⧧e

ΔΔE

1.74 1.67 1.43 1.72 1.51 1.45 −3.12

exptb

l:b(%l)

l:b(%l)

0.1(5) 0.1(6) 0.1(8) 0.1(5) 0.1(7) 0.1(8) 215.0(100)

14.6(89.7) 34(94.4) 56.6(93.7) 52.2(94.5) 49.8(94.3) 50.6(94.3) 50.2(96.5)

All energies are in kcal/mol. bExperimental data used here are measured by van der Veen et al. for the hydroformylation of 1-octene at 80 °C and 19.7 atm.24 cPPh2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus intact. dPH2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus substituted by hydrogens. eΔΔE⧧ refers to the difference between the zero-point energy-corrected electronic barriers for the formation of linear and branched Rh-propyl intermediates due to the propene insertion into the Rh−H bond of the ee1and ee2-HRh(L)(propene)CO complex. fOne of the Rh−P bonds in the calculated TSl for ee1-HRh(L)(CO)-catalyzed hydroformylation of propene for the PH2-type model is significantly lengthened (∼3.73 Å), which dominantly favors the formation of a linear Rh-propyl intermediate. a

(π-HC) interactions and will help in uncoupling their effect on the regioselectivity. The following structural models are examined: (1) PH2-type models, in which both the phenyl rings on each of ligand phosphorus atoms were substituted by hydrogens, but the xanthene backbone was retained; (2) PHLtype models, in which the phenyl rings engaged in ligand− ligand π−π interactions were substituted by an H group, but the xanthene backbone and the other Ph rings were retained; and finally, (3) PHS-type models, in which the phenyl rings engaged in ligand−substrate π-HC interaction were substituted by a H group, but the xanthene backbone and the other Ph rings were retained. PH2-type structural models have been used in a recent computational study to assess the role of similar interactions in the regioselectivity of propene hydroformylation by Rhhomoxantphos and Rh-benzoxantphos catalysts.26 Thus, comparing the l:b ratios calculated for the ee1- and ee2HRh(L)(propene)CO complexes with and without phenyl substituents would provide useful insight into the importance of interactions between the aromatic rings of the xantphos

To understand the molecular basis for the high l:b ratios, we next analyzed the geometries of the transition-state structures. There are no appreciable differences in the calculated bite angles between the TSl and TSb geometries, which, however, does not rule out the possibility that a wide bite angle intrinsically favors the formation of a linear product. Although the bite angle in TSl is slightly larger, the difference between the TSl and TSb bite angles is no more than 1°. A comparison of the optimized TSl and TSb structures reveals that the phenyl rings on the phosphine ligands engage in noncovalent interactions with the adjacent phenyl rings on the other phosphine ligand (π−π-type) as well as with the substrate (πHC-type; Figure 6). However, these noncovalent interactions in both TSls and TSbs are of similar strength, as judged from their distances of separation, which makes it difficult to quantify their impact on the regioselectivity. Thus, we next studied the propene insertion into the Rh−H bond of the HRh(L)(propene)CO complex considering three types of simplified structural models that allow selective turning on and/or off of the ligand−ligand (π−π) and ligand−substrate I


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Organometallics

Table 6. Comparison of M06-L/LANL2DZ/6-31G*-Calculated and Measured l:b Ratios for the ee1-HRh(L)(CO)-Catalyzed Hydroformylation of Propene Using Various Structural Models of Rh-Catalyst calcd ee1(PPh2-type) L phosxantphos sixantphos thixantphos xantphos isopropxantphos benzylnixantphos benzoxantphosg

⧧f

ΔΔE

−1.45 −2.18 −2.17 −2.10 −2.04 −2.38 −1.94

b

l:b(%l) 11.7(92) 39.5(98) 38.8(98) 34.8(97) 31.5(97) 55.6(98) 26.4(96)

ee1(PH2-type) ⧧f

ΔΔE

1.62 1.67 1.44 1.72 1.51 1.45 −3.18

c

ee1(PHL-type)d ⧧f

l:b(%l) 0.1(6) 0.1(6) 0.1(8) 0.1(5) 0.1(7) 0.1(8) 215.3(100)

ee1(PHS-type)e ⧧f

expta

ΔΔE

l:b(%l)

ΔΔE

l:b(%l)

l:b(%l)

−0.60 −0.60 −1.02 −0.86 −0.98 −1.09 −1.18

2.8(73) 2.7(73) 5.6(85) 4.3(81) 5.2(84) 6.3(86) 7.3(88)

1.50 1.19 1.26 0.70 1.00 1.20 0.0

0.1(7) 0.1(12) 0.1(11) 0.3(23) 0.2(16) 0.1(10) 1.0(50)

14.6(89.7) 34(94.4) 56.6(93.7) 52.2(94.5) 49.8(94.3) 50.6(94.3) 50.2(96.5)

Experimental data are measured by van der Veen et al. for the hydroformylation of 1-octene at 80 °C and 19.7 atm.24 bPPh2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus intact. cPH2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus substituted by hydrogens. dPHL-type model refers to a xantphos ligand that has one of the phenyl rings on the phosphorus that was engaged in ligand−ligand π−π interactions, substituted by hydrogens. ePHS-type model refers to a xantphos ligand that has one of the phenyl rings on the phosphorus that was engaged in ligand−substrate π-HC interactions, substituted by hydrogens. fΔΔE⧧ is the difference between the zero-point energy-corrected electronic barriers (kcal/mol) for the formation of linear and branched Rh-propyl intermediates due to the propene insertion into the Rh−H bond of the ee1- and ee2-HRh(L)(propene)CO complex. gOne of the Rh−P bonds in the calculated TSl for ee1-HRh(L)(CO)-catalyzed hydroformylation of propene for the PH2-type model is significantly lengthened (∼2.70 Å), which dominantly favors the formation of linear Rhpropyl intermediate. a

incorporated in the IMOMM-based hybrid QM/MM partitioning scheme used by Carbó et al.26 In a recent study,33 we have also shown the dominant role of such noncovalent effects in the regioselectivity of HRh(PPh3)2CO-catalyzed hydroformylation of terminal olefins, where the %l:%b ratios were completely reversed upon substituting the phenyl rings of PPh3 ligands by hydrogens. For the propene insertion into the Rh−H bond of the PHLtype models, where the ligand−ligand π−π interactions are eliminated, the calculated relative stability of TSl’s was lowered by ∼1.0 kcal/mol, which resulted in a signficant reduction in the l:b ratios (Table 6). For example, the calculated ΔΔE⧧ for the PHL-type thixantphos model is −1.02 kcal/mol, which is 1.16 kcal/mol lower than that for the PPh2-type model. This corresponds to an l:b ratio of 5.6 and %l of 85. However, in contrast to what was observed for the PH2-type models, this structural simplification does not reverse the product distribution trends, as the linear Rh-propyl intermediate is still predicted to be the dominant product. This clearly shows that, although the ligand-based π−π interactions make an important contribution of ∼1.2 kcal/mol toward the relative stabilization of TSl, these interactions are not the key determinants of regioselectivity. On the other hand, for PHS-type models, where the ligand− substrate π-HC interactions are eliminated, the calculations indicate that the TSl is less stable than the TSb by ∼1.2−1.3 kcal/mol, thus favoring the formation of a branched product (Table 6). For PHS-type thixantphos models, ΔΔE⧧ is +1.26 kcal/mol, which reflects a significant TSl destabilization of 3.43 kcal/mol, relative to original PPh2-type models, and results in a very small l:b ratio and %l of only 11. The comparative analyses of results obtained for PHL- and PHS-type models imply that the ligand−substrate π-HC interactions contribute significantly more toward the l:b ratios than the ligand−ligand π−π interactions. The findings of our present calculations are supported by previous experimental and computational studies. In a recent experimental study on Rh-catalyzed hydroformylation by Zuidema et al.,37 the lower regioselectivities observed for the phenoxaphosphine-based diphosphine ligands were correlated with the decreased steric interactions between

ligand and the substrate in determining the regioselectivity. Because in the PHL-type and the PHS-type models the respective ligand−ligand and ligand−substrate interactions are selectively turned off, these PH-type models provide a unique opportunity to gain additional insight into the key determinants of regioselectivity. The calculated relative energies of transition states and the l:b ratios for the propene insertion into the Rh−H bond of the PH2-type model ee1- and ee2-HRh(L)(propene)CO complexes are listed in Table 5. Predicted l:b ratios for the PH2-type xantphos ligands are dramatically reduced in comparison to their diphenylphosphine-containing analogues, i.e., the original PPh2-type models. Irrespective of the model ligand considered, the predicted l:b ratio is 0.1, which favors the dominant formation of the branched Rh-propyl intermediate, with the linear Rh-propyl intermediate only constituting 5−8% of the total product distribution. For example, the %l for phosxantphos and sixantphos ligands is reduced from 92% and 98% to 6% and 8% on going from the PPh2-type to the PH2-type models. Clearly, the noncovalent ligand−ligand and ligand− substrate interactions mediated by the phenyl rings exert a major influence on the regioselectivity. Moreover, the calculated bite angles for both PPh2- and PH2-type models are nearly the same (Table S6), implying that the orbital effects arising from the bite angle do not play an important role in determining the hydroformylation regioselectivity. Carbó et al. have also calculated and quantified the effect of substituting phenyl rings by hydrogens on the regioselectivity for homoxantphos and benzoxantphos ligands.26 Their calculations predicted relatively smaller reduction in the calculated %l for this substitution. For example, the %l for homoxantphos and benzoxantphos were reduced from 73% and 83% to 63% and 74% on going from PPh2-type to PH2-type models. However, our M06-L calculations indicate that removing phenyl substituents rather dramatically reduces the %l for the benzoxantphos ligand from 96% to 8%. This quantitative difference between our and previous calculations is possibly due to the fact that the ligand steric-induced electronic effects in the present study have been directly treated at the quantum mechanics level, whereas those effects were indirectly J


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Organometallics

Table 7. Comparison of M06-L/LANL2DZ/6-31G*-Calculated and Measured l:b Ratios for the ee1-HRh(L)(CO)-Catalyzed Hydroformylation of Propene, Butene, and Pentene Using Various Structural Models of Rh-Catalyst calcd ee1(PPh2-type) L thixantphos

xantphos

isopropxantphos

olefin propene butene pentene propene butene pentene propene butene pentene

⧧e

ΔΔE

−2.17 −2.40 −3.42 −2.10 −2.01 −2.94 −2.04 −2.66 −3.28

a

l:b(%l) 38.8(98) 57.4(98) 319.0(100) 34.8(97) 29.6(97) 144.0(99) 31.5(97) 89.4(99) 251.9(100)

ee1(PH2-type) ⧧e

b

ee1(PHL-type)c ⧧e

ΔΔE

l:b(%l)

ΔΔE

1.44 2.28 2.92 1.72 2.51 2.76 1.51 2.50 2.79

0.1(8) 0.0(2) 0.0(1) 0.1(5) 0.0(1) 0.0(1) 0.1(7) 0.0(1) 0.0(1)

−1.02 0.19 −0.40 −0.86 −0.65 −0.46 −0.98 −0.38 −0.48

ee1(PHS-type)d

l:b(%l)

ΔΔE⧧e

l:b(%l)

5.6(85) 0.7(42) 2.0(66) 4.3(81) 3.0(75) 2.2(68) 5.2(84) 1.9 (65) 2.2(69)

1.26 0.91 1.15 0.70 1.27 1.10 1.00 1.28 0.77

0.1(11) 0.2(18) 0.1(13) 0.3(23) 0.1(10) 0.2(14) 0.2(16) 0.1(10) 0.3(21)

a PPh2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus intact. bPH2-type model refers to a xantphos ligand that has phenyl rings on the phosphorus substituted by hydrogens. cPHL-type model refers to a xantphos ligand that has one of the phenyl rings on the phosphorus that was engaged in ligand−ligand π−π interactions, substituted by hydrogens. dPHS-type model refers to a xantphos ligand that has one of the phenyl rings on the phosphorus that was engaged in ligand−substrate π-HC interactions, substituted by hydrogens. eΔΔE⧧ is the difference between the zero-point energy-corrected electronic barriers (kcal/mol) for the formation of linear and branched Rh-propyl intermediates due to the propene insertion into the Rh−H bond of the ee1- and ee2-HRh(L)(propene)CO complex.

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CONCLUSION In this study we have reported the first computational treatment of rhodium (Rh)-xantphos-catalyzed hydroformylation in which the ligand-steric-induced electronic effects have been exclusively considered at the quantum mechanics level, M06-L/LANL2DZ/6-31G*. This level of theory provides an appropriate description of noncovalent interactions arising from the diphenyl diphospine substituents that could play an important role in the regioselectivity of hydroformylation. Using this computational protocol and realistic structural models for the Rh-xantphos catalyst assembly and octene substrate, we are able to reproduce (i) the correct ordering of the equatorial−equatorial and equatorial−axial conformers of the resting state of the Rh catalyst, HRh(xantphos)(CO)2, (ii) the dissociation energy for removing the CO ligand, (iii) the bite angle ranges in the HRh(xantphos)(olefin)(CO) complexes, and (iv) the experimental regioselectivities for the HRh(xantphos)(CO)-catalyzed octene hydroformylation. Qualitatively, the calculations predict the more and less selective xantphos ligands to have wider and shorter bite angles, respectively. However, the bite angle preference is not evident in the calculated transition states; that is, for a given xantphos ligand, the transition states leading to the linear (TSl) and branched (TSb) rhodium-propyl intermediates differ by no more than 1°. The comparative analysis of the optimized TSl and TSb geometries as well as of the regioselectivities for the propene insertion into the Rh−H bond of the HRh(xantphos)(CO) catalyst with and without diphenyl substituents imply that the noncovalent ligand−ligand (π−π) and ligand− substrate (π-HC) interactions play a very important role in governing the regioselectivity. Additional calculations involving longer chain olefins, butene and pentene, and selective substitution of phosphorus phenyl rings by hydrogens further imply that π-HC interactions between the xantphos ligands and the substrate contribute more significantly than intraxantphos π−π interactions toward the regioselectivity of olefin hydroformylation. The stronger ligand−substrate π-HC interactions may indirectly reflect the effect of the wider bite angle and, thus, may help in the molecular understanding of the correlation between the selectivity and the natural bite angle. These results could be used to guide both ligand and

the diphosphine ligand and the olefin substrate. The hybrid QM/MM calculations of Carbó et al. on the Rh-diphosphinecatalyzed hydroformylation have also suggested a dominant role for these nonbonding interactions in the regioselectivity determination.26 Since the stronger π-HC interactions between the diphosphine ligand and the olefin substrate could be the indirect manifestation of the wider bite angle, our calculations may prove useful in explaining the experimentally observed correlation between the selectivity and the natural bite angle. Finally, for three selected diphosphine ligands, we also calculated the l:b ratios for the hydroformylation of the longer chain alkenes butene and pentene. The objective was to assess the effect of the alkene chain length on the strength of the ligand−substrate π-HC interactions, as well as to better mimic the actual substrate, octene. The calculated ΔΔE⧧s, the l:b ratios, and the relevant experimental data for the olefin insertion into the Rh−H bond of the ee1-HRh(L)(propene)(CO), ee1-HRh(L)(butene)(CO), and ee1- HRh(L)(pentene)(CO) complexes for three diphosphine ligands are collected in Table 7. The calculations with PPh2-type models indicate that increasing the length of the alkyl chain preferentially stabilizes the TSl, which results in larger l:b ratios for the longer chain olefins. Although the calculated l:b ratios for butene are fairly close to the experimental l:b ratio observed for 1-octene (∼50), the calculated ratios for pentene are significantly overestimated. There are several potential explanations for this discrepancy. For example, explicit interactions between solvent (toluene) and the Rh complexes, which are not included in our computational treatment, could weaken the π−π and π-HC interactions within the complex. This could tend to reduce the l:b ratios. In any case, it is important to note that the calculated data for different ligand and olefin structural models lead to qualitatively similar conclusions; that is, although the ligandbased π−π interactions make an appreciable contribution, the ligand−substrate π-HC interactions play a larger role in determining the regioselectivity. This comparative analysis also implies that the use of propene as a model substrate can provide useful insights into the molecular basis for hydroformylation reaction steps. K


Article

Organometallics

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experimental design to optimize the regioselectivity of rhodium-catalyzed hydroformylation.

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ASSOCIATED CONTENT

S Supporting Information *

Tables containing calculated geometric parameters and thermodynamic data. 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: [email protected] (M. Kumar). *E-mail: [email protected] (T. A. Jackson). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by USDA/NIFA grant no. 2011-1000630362.

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REFERENCES

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