Factors Controlling the Selective Hydroformylation of Internal Alkenes

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Organometallics 2009, 28, 3656–3665 DOI: 10.1021/om801166x

Factors Controlling the Selective Hydroformylation of Internal Alkenes to Linear Aldehydes. 1. The Isomerization Step Maria Angels Carvajal,* Sebastian Kozuch, and Sason Shaik* The Institute of Chemistry and the Lise Meitner Minerva Center of Computational Quantum Chemistry, The Hebrew University of Jerusalem, Givat Ram Campus, 91904 Jerusalem, Israel Received December 9, 2008

Selective hydroformylation of long-chain alkenes to linear aldehydes has long been a synthetic challenge. This process is especially important because it is the first step in the production of detergent alcohols, which are widely used as plasticizers. Major difficulties arise from the fast isomerization processes that these alkenes can undergo and thereby lead to nonlinear aldehydes. Theoretical calculations and modeling of the isomerization process may lead to a fundamental understanding that will thereby contribute toward the improvement of efficiency in the hydroformylation process. The present study uses DFT and hybrid ONIOM calculations to address the following issues: (i) the isomerization mechanism, (ii) the reason for the faster reactivity of octene compared with smaller alkenes, and (iii) the catalyst role in the kinetics of the isomerization reaction. The thus-computed catalytic cycle is used for calculating the turnover frequency of the isomerization process using the recently developed energetic span model for assessment of catalytic cycles. Introduction Hydroformylation of alkenes is one of the most important industrial processes used to functionalize hydrocarbons that are obtained, for example, from petroleum sources. This process produces each year more than 6 million tons of aldehydes and alcohols. Currently there exist efficient and selective catalytic processes for the hydroformylation of butene and propene. However, the selective hydroformylation of long-chain alkenes still needs to be improved in order to obtain the desired linear product with high selectivity. This issue has economic and environmental safety aspects, because hydroformylation is the first step in the production of detergent alcohols, which are widely used as plasticizers. Indeed, small alcohols such as butanol and propanol are progressively replaced by higher alcohols because the former have adverse environmental effects. Since linear detergent alcohols have better mechanical properties than branched ones, the selectivity of the reaction has a strong effect on the quality of the final product. On the other hand, using mixtures of long alkenes is very convenient; for instance fraction II generated in the refining process is especially rich in a mixture of octenes. For all these reasons, a highly touted goal is the finding of a catalyst capable of converting selectively a mixture of higher alkenes to linear aldehydes.1-3 *Corresponding author. E-mail: [email protected]. (1) van Leeuwen, P. W. N. M. Homogeneous Catalysis. Understanding the Art; Kluwer Academic Publishers: Dordrecht, 2004. (2) (a) van der Veen, L. A.; Kamer, P. J. C.; van Leeuven, P. W. N. M. Angew. Chem., Int. Ed. 1999, 38, 336–338. (b) Drent, E.; Budzelaar, P. H. M. J. Organomet. Chem. 2000, 593-594, 211–225. (c) Selent, D.; Wiese, :: :: K.; Rottger, D.; Borner, A. Angew. Chem., Int. Ed. 2000, 39, 1639–1641. :: (d) Selent, D.; Hess, D.; Wiese, K.; Kunze, C.; Borner, A. Angew. Chem., Int. Ed. 2001, 40, 1696–1698. (e) Klein, H.; Jackstell, R.; Wiese, K.; Borgmann, C.; Beller, M. Angew. Chem., Int. Ed. 2001, 40, 3408–3411. (3) Shi, B.; O’Brien, R. J.; Bao, S.; Davis, B. H. J. Catal. 2001, 199, 202–208. pubs.acs.org/Organometallics

The main obstacle for the achievement of such an objective is the isomerization process that higher alkenes seem to undergo at relative ease. Whereas for small alkenes such as propene and butene this does not represent a problem, for higher alkenes isomerization is much faster than hydroformylation. Hence, a thermodynamic mixture of alkenes is quickly generated (typically composed of 95% internal alkenes) and linear aldehyde yield is low (Scheme 1). Therefore, the basic requirements from a catalyst are, on one hand, to catalyze the isomerization of the internal alkenes to terminal ones, and on the other, to selectively catalyze the hydroformylation in the terminal carbon atom. A third desirable feature of the catalyst is to possess high activity, which is measured by the turnover frequency of the catalytic cycle (TOF). Fulfillment of this last condition will make the catalyst applicable for large-scale production of aldehydes. The second issue should in principle not be problematic, since generally the terminal double-bond position is the most active moiety. However, isomerization to the terminal alkene is a delicate step, since it has to overcome the thermodynamic advantage of the isomers with the internal double bonds. In this sense, effective catalysts working under mild conditions should also work under kinetic control, and thus reactivity of the terminal carbon atom will be enhanced. Attempts to improve the selectivity of the process have been made, and several Rh- and Pd-based catalysts have been proposed.2 The main feature of these catalysts is their bulky ligands, which are thought to increase the selectivity of the reaction toward the linear product. Nevertheless, the existing catalysts with high selectivity are scarcely active (having TOFs between 300 and 100 h-1 or even lower),2a whereas for catalysts with higher TOF values the selectivity is not significantly higher than that which is obtained with traditional cobalt catalysts.1 For this reason, cobalt catalysts are still the preferred ones in the industry, in spite of their low

Published on Web 06/04/2009

r 2009 American Chemical Society

Article Scheme 1. Schematic Representation of the Competition between the Isomerization (a) and Hydroformylation (b) Reactions of Octene

Scheme 2. Ligands Used to Model the Isomerization of Butene and Octene Cycles by Rh Complexes

Organometallics, Vol. 28, No. 13, 2009

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higher alkenes. Subsequently, the comparison between the catalysts Cb and Cc will provide some understanding of the role of the catalyst in such a process and will specifically address the question, what factors are responsible for the faster isomerization using catalyst Cb? These comparisons are expected to be helpful for the development of new efficient catalysts. To asses the efficiency of the different catalytic systems, the turnover frequency (TOF) of the isomerization was computed using the energetic span model developed recently in the group.4 This method allows us to estimate the TOF of a catalytic reaction, and it also provides information about the main states contributing to the kinetics of the catalytic cycle: These are the TOF-determining intermediate (TDI) and the TOF-determining transition state (TDTS). The method proposes also an easy way to estimate the effective activation barrier of the catalytic cycle, so-called “energetic span of the cycle”, which determines the rate of the catalytic cycle.

Computational Details

selectivity (from 50% up to 80% depending on the ligand and the reaction conditions).1 In the cobalt-catalyzed processes, high temperature and high CO and H2 pressure are used to hinder isomerization. However, under these conditions a non-negligible amount of hydrogenation byproducts is also obtained. Additional difficulties en route toward the development of efficient catalysts originates from the lack of precise understanding of the isomerization process. This reaction is very fast, and therefore accurate experimental information about the reaction mechanism is very difficult to obtain. A mechanism in which the metal ion “runs” along the alkene chain until CO insertion takes place was proposed, but there is no experimental evidence to support it yet.1 In heterogeneous iron-catalyzed Fischer-Tropsch reaction conditions, isotopic substitution experiments agree with a mechanism involving a metal ion hydride addition-elimination mechanism.3 Computational techniques are valuable tools for understanding and rationalizing fast processes and may thereby provide useful guidelines for improving currently used processes and a rational catalyst design. The present study is a focused attempt to clarify the isomerization mechanism and to determine the role of the catalyst in the kinetics of the reaction by use of theoretical calculations and modeling of the catalytic cycle. Calculations were carried out for four catalytic systems, using the catalysts shown in Scheme 2: a small system constituting the catalyst [RhH(CO)(dbpp)] (dbpp = PH2(CH3)4-PH2), called hereafter Ca, and butene as a substrate, and the same catalyst with octene as a substrate; and finally, two catalysts with bulky ligands [RhH(CO)L] (L = substituted xanthenes Cb and Cc in Scheme 2) with octene as a substrate. Cb (L = phenoxaphosphanyl-substituted xanthene) catalyzes the hydroformylation of internal alkenes to terminal aldehydes with high selectivity (90%), although its TOF for hydroformylation is very low (15-112 h-1). The second system, Cc, catalyzes the isomerization of 1-octene to internal alkenes much slower than Cb does. Initial comparison of the small systems (Ca/butene and Ca/octene) will allow us to determine the reason for the higher isomerization rate in

For the systems Ca/butene and Ca/octene, full quantum mechanical (QM) methods have been applied, while calculations on catalytic systems Cb and Cc were done using the ONIOM method implemented in Gaussian03. Geometry optimization of the intermediates and transition states was carried out using the Gaussian 03 package.5 Frequency calculations were performed in order to check the nature of the stationary points and to estimate the zero-point energy (ZPE) correction. Although free energy is the right value to use in the TOF calculations, in the most critical states studied here there is no change in the molecularity, so we can safely neglect the entropy corrections and use the energy + ZPE landscape (see Table S3.2 in the Supporting Information (SI)). The B3LYP functional6 was used to evaluate the energy of the QM systems as well as for the QM region within the ONIOM scheme.7 TOF estimation was carried out with a program written in the group, based on a recent paper,4 and is provided in the SI. The TOF comparison between different computational levels (here QM and QM/MMONIOM) is not adequate, as small energy differences affect the TOF exponentially. Because of this, we focused first on comparing the TOFs for the butene and octene substrates in the small Ca complex, using the QM-only results, and later we compared the two bigger catalysts with the same substrate, using the ONIOM results. The basis set employed for Rh uses the relativistic effective core potentials developed in Stuttgart and the corresponding basis set,8 which has been augmented here by a polarization f function (exponent = 1.35).9 For the rest of the atoms, the 631G(d) basis set10 was used. Solvent effect was computed by means of single-point energy calculations on the gas-phase optimized structures, using the continuum polarizable conductor model (CPCM),11 implemented in Gaussian 03. The value of the dielectric constant ε = 2.379 simulates toluene. (4) (a) Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254– 278. (b) Kozuch, S.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 3355–3365. (c) Kozuch, S.; Shaik, S. J. Phys. Chem. A 2008, 112, 6032. (5) Frisch, M. J. et al. Gaussian 03 (Revision C.02); Gaussian, Inc.: Wallingford, CT, 2004. (6) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (7) (a) Maseras, F.; Morokuma, K. J. Comput. Chem. 1995, 16, 1170. (b) Vreven, T.; Morokuma, K. J. Comput. Chem. 2000, 21, 1419. :: (8) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. :: :: (9) Ehlers, A.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth, A.; :: Jonas, V.; Kohler, K.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. (10) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (11) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995.

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Scheme 3. Substituted Xanthene Ligands Cb and Cc and Schematic Representation of the ONIOM Partition Employed in the Calculationsa

Table 1. Energies (in kcal/mol) of the Species Relative to the Catalyst Ca and 2-Alkenea Ca/butene speciesb ΔE

a Atoms within the dashed area belong to the QM region, whereas the rest of the ligand is treated only at the MM level.

During the ONIOM calculations, we employed the UFF force field12 for the molecular mechanical (MM) evaluation of the energy. The atoms included in the QM region were Rh, the H coordinated to the metal in the catalytically active species, the CO ligand, the six first carbon atoms and the corresponding H’s of the olefin, and part of the ligand, as outlined in Scheme 3 by the dashed lines. The cutting of the benzenes into vinyl groups in the division of the QM part may be considered not very intuitive. However, it was shown already that, apart from the steric effects, vinyl phosphines have a close behavior to phenyl phosphines.13 This partition also ensures consistency in the energy between all the stationary points of the reaction pathway.14 Another issue is whether cutting the octene that we employed (see later) can affect the results by incorrectly estimating van der Waals (vdw) interactions. While this may be so, we have to stress that the site of cutting we selected followed the advice of the ONIOM developers;14 whenever there is bond breakage and formation, one should not perform the cut closer than three bonds from the MM region. In addition, while the cut may not account properly for the vdw interactions, it can hardly affect the relative TOF of the two complexes, which is on the order of 106 (see later). Our ONIOM calculations do not include the solvent continuum model, as this is not implemented in Gaussian 03. This is not a problem for our system, where there are no large charge separations. Also ZPE corrections where not used in systems Cb and Cc since they are very expensive calculations. Neglecting the solvent and the ZPE does not change significantly the results, as can be seen from the full QM model Ca (see later Table 1; see also Table S3.2 in the SI). Energetic Span Model. The turnover frequency (TOF) of a simple catalytic cycle in a steady state regime, based on transition state theory, is given by eq 1:4

TOF ¼

e -ΔGrx -1 N P T -I -δG0i, j e i j i, j ¼1 (

δG0i, j ¼

ΔGrx 0

ð1Þ

if i > j if iej

(12) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.III; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (13) Kozuch, S.; Shaik, S.; Jutand, A.; Amatore, C. Chem. Eur. J. 2004, 10, 3072–3080. (14) Vreven, T.; Byun, K. S.; Komaromi, I.; Dapprich, S.; Montgomery, J. A.Jr.; Morokuma, K.; Frisch, M. J. J. Chem. Theory Comput. 2006, 2, 815.

TScoor(1-a) I TScoo(1-a) II TScoor(1-a) III TScoor(1-a) IV TScoor(2-a) I TScoor(2-a) II TScoor(2-a) III TScoor(2-a) IV η2(1-a) I η2(1-a) II η2(1-a) III η2(1-a) IV η2(2-a) I η2(2-a) II η2(2-a) III η2(2-a) IV TS(1-a) I TS(1-a) II TS(1-a) III TS(1-a) IV TS(2-a) I TS(2-a) II TS(2-a) III TS(2-a) IV η1(1-a) I η1(1-a) II η1(2-a) I η1(2-a) II TSir I TSir II

Ca/octene species

ΔEZPE ΔEsolvent

4.39 5.05 4.36 5.07 2.38 2.77

5.14 5.77 5.20 6.11 3.13 3.48

-3.86 -4.68 -4.46 -2.35 -3.50 -3.61

-1.62 -2.50 -2.15 -0.12 -1.53 -1.65

17.68 13.63 12.24 15.11 11.71 13.56

18.46 14.54 13.13 16.01 12.59 14.26

1.40 2.01 1.22 0.86 5.79 6.89

4.81 5.37 4.75 4.49 9.93 10.55

6.28 4.79 5.05 0.74 -0.27 0.00 2.27 0.87 0.82

17.05 15.89 15.37 16.83 8.16 8.96 8.29 8.12 12.9 13.87

ΔE 4.23 4.80 5.69 4.86 3.69 3.89 5.53 4.09 -4.02 -4.79 -3.15 -2.43 -3.80 -3.82 -2.38 -2.70 16.46 13.38 12.02 14.87 11.29 13.32 17.29 15.24 3.16 1.95 0.72 0.42 5.73 5.12

ΔEZPE ΔEsolvent 4.76 5.43 6.53 5.76 4.08 4.53 6.62 4.74 -1.86 -2.78 -0.80 -0.21 -2.11 -1.86 -0.42 -0.77 17.35 14.36 12.80 15.95 12.00 13.88 17.42 15.71 6.38 5.16 4.40 3.73 9.08 8.61

5.81 7.88 5.64

0.48 -0.50 1.47 2.27 0.49 1.54

16.76 15.40 14.78 16.53 10.00 8.99 7.88 7.52 12.90 12.12

ΔE is the energy without including the ZPE correction. ΔEZPE includes the ZPE correction, and ΔEsolvent includes the solvent and ZPE corrections. b For 2-butene species, values for species I and III are identical, as well for species II and IV. This is due to the symmetry of the 2-butene. For this reason only values for I and II are listed. a

Here, the energies (transition states Ti, intermediates Ij, and energy of reaction ΔGrx) are dimensionless quantities expressed in kbT units, such that

Ii ¼ GðIi Þ=kb T

ð2aÞ

Ti ¼ GðTi Þ=kb T

ð2bÞ

ΔGrx ¼ fGðProductsÞ GðReactantsÞg=kb T

ð2cÞ

where G(Ii) and G(Ti) are the free energy of the ith intermediate and transition state. The units of TOF results are s-1. The degree of TOF control (XTOF,i) specifies the sensitivity of the TOF to a change in the energy of a specific state of the cycle and is defined as follows:

1 DTOF X TOF, i ¼ TOF DEi

ð3Þ

where Ei is a dimensionless free energy of a transition state (Ti) or an intermediate (Ii). In this expression the XTOF,i varies between the limits of 1 (full dependence of the TOF on state i) and 0 (no dependence of the TOF on a change in the energy of state i). Based on eq 1, the resulting

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expression becomes

Scheme 4. Proposed Isomerization Catalytic Cycle

P XTOF, Ti ¼ P XTOF, Ij

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0

j

eTi -Ij -δG ij

ij

eTi -Ij -δG ij

0

P Ti -Ij -δG0 ij e ¼ P i T -I -δG0 i j ij ij e

ð4Þ

In many catalytic cycles, albeit not in all, we can identify two states, one intermediate and one transition state, which possess the largest TOF control indices and are, hence, the key species that determine the TOF of the catalytic cycle. The intermediate with highest XTOF,I is called the TOF-determining intermediate, TDI, while the transition state with XTOF,T closer to 1 is referred as the TOF-determining transition state, TDTS. In this manner the TOF expression becomes simple and depends only on these two states. Thus, once the TDI and TDTS of the cycle are known, the corresponding TOF can be expressed as follows:

TOF≈e

-δE

ð5Þ

where δE, the energetic span in dimensionless units, is the apparent activation energy of the cycle and is given by eq 6: ( δE ¼

TTDTS -ITDI TTDTS -ITDI þ ΔGrx

if the TDTS comes after the TDI if the TDTS comes before the TDI

ð6aÞ ð6bÞ

Note that the apparent activation energy has a different expression depending on whether the TDTS follows the TDI, eq 6a, or precedes it, eq 6b. In fact, because of the ΔGrx term in eq 6b, the TDTS and TDI are not necessarily the highest and lowest energy states of the cycle. As such, locating the TDI and TDTS requires considering all the possible pairs of transition states and intermediates. Once we identify the TDTS and the TDI, and knowing the ΔGrx of the cycle, we have (in this approximation) all the needed kinetic information to estimate the TOF of the cycle. In this work a simple program that enables one to compute the TOF from the energy landscape (using eq 1) was employed to compare the different reactions kinetics. This program permits also the identification of the determining states for a given cycle following eq 4. The program in Fortran format is given in the SI. As we explained in the Methods section, the TOF calculations were restricted to ΔErx values, since the entropic terms are not expected to play a major role given that the molecularity of the TDI and TDTS does not change. This is indeed evident from the smaller system Ca (see Table S3.2 in the SI).

Results and Discussion The catalytic cycle for the isomerization is displayed in Scheme 4. The initial step of both the isomerization and hydroformylation (not shown) reactions consists of the coordination of the alkene to the catalyst to form a (15) (a) Matsubara, T.; Koga, N.; Ding, Y.; Musaev, D. G.; Morokuma, K. Organometallics 1997, 16, 1065. (b) Carb o, J. J.; Maseras, F.; Bo, C.; van Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2001, 123, 7630. (c) Gleich, D.; Hutter, J. Chem.;Eur. J. 2004, 10, 2435. (d) Sparta, M.; Børve, K. J.; Jensen, V. R. J. Am. Chem. Soc. 2007, 129, 8487. (e) Zuidema, E.; Escorihuela, L.; Eichelsheim, T.; Carb o, J. J.; Bo, C.; Kramer, P. C.; van Leeuwen, P. W. N. M. Chem. Eur. J. 2008, 14, 1843.

η2-complex (via TSc in Scheme 4).1,15,16 Then, the hydride coordinated to the metal is transferred to the alkene to form a [Rh(CO)(η1-CnH2n+1)L] complex (η1(1-a) complexes in Scheme 4). This species can undergo irreversible CO addition and thereby lead to the hydroformylation product (not shown).15 Alternatively, as shown in Scheme 4, a rearrangement of the intermediate η1 species is required (TSir), such that a different C-C bond (in η1(2-a)) can reach the right orientation to transfer back a hydrogen (via TS(2-a)) to the metallic center, regenerating the isomerized olefin (η2(2-a) in Scheme 4). CO coordination (which leads to the hydroformylated products) is a barrier-free process because it consists of occupying a free coordination site without any bond breaking (and the C-O bond is only slightly distorted),15 whereas the back hydride transfer via TS(2-a) implies a certain energetic penalty for breaking the C-H bond. However, due to its bimolecular character, CO coordination is entropically disfavored and the high temperatures usually used in this reaction play against it. For a catalyst of the type [Rh(CO)H(L)], in which L is a bidentate ligand, the most stable isomer has a square-planar structure. Coordination of the alkene to the catalyst leads to the formation of the η2(1-a) complexes I and II, shown in Figure 1. These are the most stable η2-intermediates, in which H and one of the P atoms of the chelating ligand occupy the axial positions, while the alkene, CO, and the remaining P atom the equatorial sites.16 The double bond in Figure 1 lies in the equatorial plane. A comparison between the geometries and energetics for the butene and octene complexes of catalyst Ca shows no noticeable differences (see Figure 1 and Table S2, SI). In both cases, upon coordination, the metal-coordinated CdC double bond undergoes elongation of about 0.1 A˚, compared to the free olefin. The rest of the distances between Rh and the ligands remain almost unchanged. The relative stability of the computed η2-complexes (relative to the free catalyst and olefin species) varies in a very similar way in all cases: between -0.1 and -2.5 kcal/mol for butene η2-complexes, and for octene between -0.2 and -2.8 kcal/mol (see Table 1 and Table S1, SI). (16) Carb o, J. J.; Lled os, A.; Vogt, D.; Bo, C. Chem. Eur. J. 2006, 12, 1457–1467.

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Figure 1. Geometries of the η2-complexes of dbpp (corresponding to catalyst Ca) showing the values of the two C-Rh bond distances, the CdC bond distance, and the Rh-H distance, all in A˚. Here and in all subsequent figures, the numbers in the upper line correspond to values for butene and the lower ones to the values for octene. The rest of the geometric details can be found in the SI (Table S3). Hereafter, R represents the rest of the alkene as a substituent on the double bond. R is naturally different for 1-alkene (1-a) or 2-alkene (2-a) species. The Roman numerals, for a given species-type, represent the orientation of R, above and below the RhCC plane.

Figure 2. Key geometric parameters of the hydride transfer transition states (TS) for the Ca (dbpp) catalyst reacting with butene and octene (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2).

The corresponding hydride transfer transition states (TS(1-a) and TS(2-a) in Scheme 4) associated with each of the η2-complexes described above are depicted in Figure 2. These transition states have a distorted trigonal-bipyramid structure, in which axial positions are occupied by the H and one of the P atoms. The reacting CdC double bond is parallel to the Rh-H bond. One of the carbon atoms (the one that will remain attached to the metal after the hydride transfer) occupies one equatorial position, while the remaining equatorial positions are occupied by the CO and the second P atom of the chelating ligand. The geometries of the transition states for the butene and octene complexes have similar features: The CdC distance of the double bond does not change substantially compared with the η2-complex, the incipient C-H bond is about 1.6 A˚ long, and the Rh-H

distance increases by about 3% with respect to its value in the η2-complexes. As shown in Table 1, the relative energy of all these transition states varies from 11 to 17 kcal/mol, both for butene and octene species. As shown in Figure 3, the resulting η1-intermediates have trigonal-bipyramidal geometries with a missing coordination site due to CdC activation. As mentioned by reference to Scheme 4, these species can undergo a back hydride transfer, generating either the original η2-complex or an η2-isomer in which the double bond has shifted one position. The rearrangement transition states are shown in Figure 4. The rotational barriers around the Rh-C bond, which lead to isomerization, were calculated and found to be only 5-6 kcal/mol (see Figure 5). The η1-intermediates are 3-9 kcal/mol less stable than 2 η -complexes, and there is a weak interaction between some

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Figure 3. Key geometric parameters of the η1-complexes of Ca, after CdC activation, showing the C-Rh, C1-C2, and the Rh 3 3 3 H distances in A˚ (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2).

Figure 4. Key geometric parameters of the internal rearrangement transition states (TSir) for the Ca complex, showing the distances (A˚) of the following bonds: C-Rh, the C-C bond involved in the isomerization, and Rh-CO (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2).

Figure 5. Calculated energetic profile for the isomerization of 1-butene/1-octene, respectively. Energies (in kcal/mol) include ZPE correction.

H substituents of the alkene and the metal (the Rh 3 3 3 H distance is about 2.1 A˚, hence showing the presence of one agostic interaction). The Rh-C distance is about 0.1 A˚ longer than in the η2-complexes because the carbon atom now occupies one axial position. The C-C bond length increases by 0.1 A˚, in line with the single-bond character of the C-C interaction after the H transfer. The η1-species I


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and II would be enantiomers if the Rh(CO)(dbpp) moiety had a symmetry plane. However, since the structure of the dbpp ligand is not symmetric, the energy of the isomers is slightly different. The comparison between butene and octene derivatives does not reveal any clear energetic trend that distinguishes the complexes depending on the length of the hydrocarbon coordinated to the metal (consult Table 1). Butene vs Octene: What Property of the Substrate Enhances Isomerization Activity? As already argued, and as shown clearly in Figure 5, a simple comparison of the relative activation energies in the butene and octene reactions does not provide much insight into the relative rates of the isomerization processes. Considering all the possible isomers, energetic differences are never larger than 2 kcal/mol for the analogous species in the two paths. Furthermore, in some steps, the activation energy is larger for octene than for butene, whereas in other steps the opposite behavior is true; for instance, activation energy for the first hydride transfer involving η2(1-a) III and TS(1-a) III is 0.6 kcal/mol larger for butene, whereas for the second hydride transfer involving η1(2-a) II and TS(2-a) II the activation energy is 0.4 kcal/mol larger for octene. For this reason, it was necessary to calculate and compare the TOFs of the two cycles.4 A TOF calculation considers the catalytic cycle as a whole, and it is expected to be useful for understanding the differences between the two alkenes. Moreover, this analysis yields information about the states that contribute the most to the rate of the cycle. In order to apply the model to the reaction, we added the final step in the catalytic cycle in Scheme 4. In this last step, the isomerized alkene must dissociate in order to recover the catalyst and restore the catalytic cycle for a new turnover. As will be shown however, this step is not relevant for the kinetics of the reaction. The isomers taken into consideration for the TOF calculation are TSc(1-a) I, η2(1-a) II, TS(1-a) III, η1(1-a) I, TSir I, η1(2-a) II, TS(2-a) II, η2(2-a) II, and TSc (2-a) I. We selected these species assuming that isomerization between the η2-isomers is faster than the hydride transfer reaction (since this can require just coordination-decoordination, a process with a lower barrier than the hydride transfer), so the most abundant η2(1-a) intermediate and the most probable TSc(1-a) transition state are the lowest in energy among all the possible isomers. After hydride transfer, the isomers cannot interconvert easily since they are stereoisomers. Therefore we considered as the TDIs the η1(1-a) and η1(2-a) intermediates, TSir and TS(2-a), the only ones coming from the reaction of the lowest energy TS(1-a), which is isomer III. Finally, since η2(2-a) complexes can in principle easily isomerize;for the same reason that η2(1-a) species can;we considered the lowest energy η2(2-a) intermediate and TScoor(2-a) transition state in the TOF calculation. The TOF calculations for the explored reaction pathways are collected in Table 2 (see also Table S3.1, SI). The data show the following trends: (i) the TOF for isomerization is always higher for octene than for butene, in agreement with experimental evidence, and (ii) the most relevant states that contribute to the rate of the reaction are the hydride transfer transition states as the TDTSs and the η2(1-a) intermediates as TDIs. Interestingly, both of the hydride transfer transition states contribute significantly to the TOF, although the contribution of one of them is more dominant (see X values in Table 2). For instance, using gas-phase energies

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Table 2. Calculated ΤOF Values, Degrees of TOF Control (X), and Energies (with ZPE correction) Relative to Reactants of the TDI and TDTS Species, in the Gas Phase and in Solution, at T = 398 K TOF gas phase solvent a

butene octene butene octene

4.1 10 4.6 103 2.5 103 2.9 103 3

TDI

XTDI

E(TDI)

TDTS

XTDTSa

E(TDTS)

η (1-a) η2(1-a) η2(1-a) η2(1-a)

1.00 1.00 0.99 0.99

-2.5 -2.7 -0.3 -0.5

TS(2-a) TS(2-a) TS(2-a) TS(2-a)

0.80 0.80 0.76 0.80

14.3 13.9 16.8 16.5

2

For the sake of clarity, only the value of the degree of TOF control for the dominant TS is shown.

Figure 6. Key geometric parameters of the three catalysts considered in this study (Ca, Cb, and Cc). All distances are in A˚ units. The different representation in ball and sticks and tubes for different parts of the ligands corresponds to the two layers in the ONIOM calculations.

(including ZPE correction), the TOF for butene isomerization is 4.1 103 (s-1) and that for octene is 4.6 103 (s-1). The solvent does not affect the relative rates significantly; solvation lowers the TOFs, but increases the relative difference between the values (2.5 103 for butene and 2.9 103 for octene; so in this case octene isomerizes 1.16 times faster than butene, while in the gas phase the ratio is 1.12). Although isomerization for higher alkenes is known to be “faster” than for the analogous reactions of butene and propene, there are no accurate experimental data about the isomerization step alone. A higher TOF means that the energetic span of the corresponding reaction is smaller; the energetic span is approximately the energetic difference between hydride transfer transition states (the TDTSs) and η2(1-a) intermediates (the TDIs). The reason for this different behavior can be tentatively traced to the stabilization of the TDTS, associated with the stronger πCdC/σ*MH and σMH/π*CdC orbital interactions.17 Concerning the TDIs; the relative energies for the butene and octene complexes show a small advantage for the octene complex. According to the Dewar-Chatt-Duncanson model and other studies,18 one could have expected that the back-donation in the octene case would be slightly more favorable than for butene, and since back-donation is more important in this kind of TDI complexes, this would have stabilized the corresponding octene complex.18c However, looking at the overall TOF results, one can see that the stabilization of the TDTSs is more pronounced and has a stronger effect on the full catalytic cycle than the stabilization of the TDI. Role of the Catalyst. The effect of large ligands with rigid backbone and large substituents has been explored by means of ONIOM calculations. Since analysis of the catalytic cycle (17) Koga, N.; Jin, S. Q.; Morokuma, K. J. Am. Chem. Soc. 1988, 110, 3417–3425. (18) See for instance: (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71. (b) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (c) Nunzi, F.; Sgamelotti, A.; Re, N.; Floriani, C. J. J. Chem. Soc., Dalton Trans. 1999, 3487. (d) Schlappi, D. N.; Cede~ no, D. L. J. Phys. Chem. A 2003, 107, 8763.

with the dbpp ligand demonstrates that the relevant states controlling the rate of the cycle are the η2-complexes and the hydride transfer transition states, only these species will be discussed in detail. Information about the rest of species is shown in the SI. As explained before, solvent effects are not calculated for these catalysts, because ONIOM is incompatible with the continuum solvent models. This however is not a serious problem, since only the η1-species, which are not important for the TOF, develop significant localized charge, while the TOF-determining species do not develop significant charge separation. It is expected therefore that solvent effects will not affect the main conclusions of this study. This lack of solvent effect on TOF is evident by looking at the Ca catalysts, which have the same energetic spans in the gas phase and from the calculations including the solvent (Table S3.2, SI). The optimized geometries of the square-planar catalysts are shown in Figure 6 together with Ca analogous species. The geometric features are very similar to the Ca catalyst, with slight differences: the Rh-P distances are about 0.1 A˚ longer and the P-Rh-P0 angle is wider (116.2°, 106.7°, and 99.6° for Cb, Cc, and Ca respectively); the P0 -Rh-CO angle (P0 is the phosphorus atom cis to CO) is very similar in all the cases (98.6°, 95.0°, and 97.6° for Cb, Cc, and Ca, respectively). These differences can be ascribed to the wider bite angle of the xanthene ligands together with their rigid backbone, compared with dbpp. Concerning η2-complexes, a comparison between Figures 1 and 7 shows that the geometric features are similar to the corresponding Ca complexes, and the only significant differences are the P-Rh-P angle (which is wider again: 111-113°, 103-106°, and ∼ 98° for Cb, Cc, and Ca, respectively) and the longer Rh-P distances (2.5-2.6 vs ∼2.4 A˚ for Ca). In contrast to the above small differences, of the η2(2octene) complexes vis- a-vis the corresponding species for Ca (average relative energy of about -4 kcal/mol, see Table 3), the η2(1-octene) complexes are significantly more stable than the analogous Ca compounds (average relative energy about

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-10 kcal/mol). A comparison between 1-octene and 2-octene η2 species shows that, while in Ca species the stability and geometrical parameters of 1-octene and 2-octene complexes are quite similar, the xanthene ligand differentiates energetically the terminal and internal η2-alkene complexes. Specifically, this ligand stabilizes the 1-alkene species much more than the 2-alkene species (compare energies for η2-species in Tables 2 and 3). This can be related to the ∼0.1 A˚ elongation of the Rh-P distances in the η2 2-alkene complexes compared with the 1-alkene species, probably due to steric crowding in the proximity of the metal sphere. This trend is observed in all the η2-isomers (see Table 3 and Table S1, SI). Since important geometric differences between catalyst Cb and catalyst Cc species are not found, the different Table 3. Relative Energiesa of the η2, TS, and η1 Species by Reference to the Catalyst and 2-Alkene, in kcal/mol, for the Reaction Involving Cb and Cc Catalysts η (1-a) I η2(1-a) II η2(1-a) III η2(1-a) IV η2(2-a) I η2(2-a) II η2(2-a) III η2(2-a) IV TS(1-a) II TS(1-a) III TS(2-a) I TS(2-a) II η1(1-a) I η1(1-a) II η1(2-a) I η1(2-a) II 2

a

Without ZPE correction.

Cb/octene

Cc/octene

-7.9 -10.1 -9.8 -9.8 -3.6 -7.4 -3.8 -8.9 3.9 0.1 0.2 5.4 -12.0 -9.8 -10.1 -4.8

-13.3 -17.7 -15.1 -14.3 -0.4 -6.6 -4.5 -8.4 -2.7 0.0 5.4 0.8 -12.9 -12.4 -3.5 -10.4

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stability of the η2(alkene) species may well be related to the presence of a strong dipolar moment in the Cb ligand due to the presence of the oxo group in this catalyst; specifically, η2(1-octene) Cc species are considerably more stable than η2(1-octene) Cb species, because the dipole present in Cb interacts unfavorably with the alkene chain. However, the η2(2-octene) species have similar energies for both catalysts Cb and Cc (see Table 3 and Table S1, SI). This could be explained by the steric effects in the 2-octene species, since the presence of the terminal methyl group next to the double bond;which is absent in the 1-octene complexes; increases the steric crowding in the proximity of the metal (see Figure 7). Regarding the hydride transfer transition sates, only TS(1-o) II and III and TS(2-o) I and II have been calculated, since according to the calculations for Ca/octene those isomers yield the most stable TSs (see Table 1 and Table S2, SI). The geometrical differences between the dbpp (Ca) compound and the xanthene analogues (Cb and Cc) are not very significant, except for the slightly longer Rh-P distances and the P-Rh-P angle (compare Figures 2 and 8 and see Table S2, SI). Relative energies are in general higher for transition states involving 2-octene species than for 1-octene, most likely due to the steric crowding in the first isomer (see Table 3 and Table S2, SI). However, the activation energies for the hydride transfer from η2(1-octene) and η2(2-octene) with respect to the corresponding precursor η2-complex are always higher for the 1-octene species due to the extra stability of the η2(1-octene) complexes. This is in contrast with the findings for the smaller Ca ligand, where activation energies are similar in all the cases. Consequently, isomerization of 2-octene to a terminal alkene can take place much faster than the isomerization of 1-octene to 2-octene. Hence, the concentration of η2(1-alkene) will be always higher than η2(2-alkene) concentration, so one can expect that

Figure 7. Key geometric parameters for the most stable η2-complexes (isomers II and IV for the 1-octene and 2-octene species, respectively) of the catalysts Cb and Cc, in A˚ units (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2). Hereafter, the different representation in ball and sticks and tubes for different parts of the ligands corresponds to the two layers in the ONIOM calculations. One of the ligands is shown by lines for the sake of clarity.

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Figure 8. Key geometric parameters of the TS (isomers III and II for the 1-octene and 2-octene species, respectively) for Cb and Cc; bond distances are in A˚ units (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2). The different representation in ball and sticks and tubes for different parts of the ligands corresponds to the two layers in the ONIOM calculations. However, one of the ligands is shown by lines for the sake of clarity.

stability features: the xanthene ligand stabilizes the complexes more than dbpp, and in some cases these η1-complexes are even more stable than the η2-complexes (see Tables 2 and 3), probably due to a minor steric crowding (see Figure 9 and compare with Figure 4). To estimate the energetic span of the isomerization reaction, the isomers we have considered are the most stable η2(1-a) complex and the lowest TS(2-a), because these are the TDI and the most important TDTS according to the TOF calculations done on the catalyst Ca species. In the case of the catalysts Cb and Cc, those isomers are η2(1-a) II and TS(2-a) II, and the energetic span considering the energy without ZPE correction (since we assume that ZPE will not change trends) is, respectively, 10.3 and 18.6 kcal/mol. This difference corresponds to a relative TOFrel(Cb/Cc) of 106 (see eq 5), thus accounting for the experimental fact that Cc is almost inactive toward isomerization, while Cb is comparatively more active. This 8 kcal/mol mainly arises from the different stability of the η2(1-a) complexes of the two catalysts (7 kcal/mol), most likely because of the ligand-octene interactions, as discussed above. Thus, the TOF analysis enables the identification of the best catalyst for a given reaction cycle. Figure 9. Key geometric parameters of the η1-complexes (isomers I and II for the 1-octene and 2-octene species, respectively) for Cb and Cc; distances are in A˚ units (for the meaning of the various labels see Figure 1). The rest of the distances can be found in the Supporting Information (Table S2).

hydroformylation of the terminal alkene will occur more easily. Finally, the features of the η1-complexes with catalysts Cb and Cc are very similar to those with Ca, except for the

Conclusions DFT calculations show that a possible mechanism for the isomerization of octene consists of a series of hydride transfer steps. Calculations of the turnover frequency for the various cycles4 gave results in accord with experimental data. Thus, for the small catalyst, Ca, TOF(octene) > TOF (butene), whereas for the large catalysts Cb and Cc with octene, TOF(Cb) .TOF(Cc). These trends are determined by the electron-donor property of the alkyl chain, which stabilizes the transition states in which the Rh-H bond is

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broken. Concerning Cb and Cc, ONIOM calculations on complexes bearing large xanthene ligands show that the main effect of such a ligands is to stabilize the η2(1-octene) intermediates over the corresponding η2(2-octene) complexes, since η2-coordination to Cb and Cc of 2-octene yields sterically more crowded complexes. This probably increases the concentration of terminal alkene derivatives, which lead to the corresponding linear product aldehyde after the hydroformylation takes place. However, this greater stability makes the η2(1-octene) intermediates also less reactive.


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Acknowledgment. This research is supported by the Ministry of Education and Research within the Framework of the German-Israeli Project Cooperation DIP (DIP-F 7.1). M.A.C. is supported by Generalitat de Catalunya (Autonomous Catalan Government). Supporting Information Available: Energies of the computed species, key geometrical parameters, full data on TOF calculation, Cartesian coordinates of the computed species, and Fortran program for TOF and XTOF calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

Factors Controlling the Selective Hydroformylation of Internal Alkenes

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