Computational Study of the Migration of Rhenium from One

Article pubs.acs.org/Organometallics

Computational Study of the Migration of Rhenium from One Enantioface of an Olefin to the Other Facilitated by (C−H)···Re Interactions Murugesan Thenraj and Ashoka G. Samuelson* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: The migration of a metal atom in a metal−olefin complex from one π face of the olefin to the opposite π face has been rarely documented. Gladysz and co-workers showed that such a movement is indeed possible in monosubstituted chiral Re olefin complexes, resulting in diastereomerization. Interestingly, this isomerization occurred without dissociation, and on the basis of kinetic isotope effects, the involvement of a trans C−H bond was indicated. Either oxidative addition or an agostic interaction of the vinylic C−H(D) bond with the metal could account for the experimentally observed kinetic isotope effect. In this study we compute the free energy of activation for the migration of Re from one enantioface of the olefin to the other through various pathways. On the basis of DFT calculations at the B3LYP level we show that a trans (C−H)···Re interaction and trans C−H oxidative addition provide a nondissociative path for the diastereomerization. The trans (C−H)···Re interaction path is computed to be more favorable by 2.3 kcal mol−1 than the oxidative addition path. While direct experimental evidence was not able to discount the migration of the metal through the formation of a η2-arene complex (conducted tour mechanism), computational results at the B3LYP level show that it is energetically more expensive. Surprisingly, a similar analysis carried out at the M06 level computes a lower energy path for the conducted tour mechanism and is not consistent with the experimental isotope effects observed. Metal−(C−H) interactions and oxidative additions of the metal into C−H bonds are closely separated in energy and might contribute to unusual fluxional processes such as this diastereomerization.

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INTRODUCTION Organometallic compounds continue to fascinate chemists. Not only are the structural aspects of organometallic compounds interesting but also their dynamic behavior challenges one’s understanding of bonding.1 The Dewar−Chatt−Duncanson (DCD) model is the most useful and successful model for explaining the interaction of metals with organic fragments.2 A key feature of this model is the synergistic bonding between the two fragments. Taking the olefin−metal interaction as an example, which is our focus in this study, the DCD model suggests two major components: donation of electron density from the olefin π system to the metal in a σ fashion and a second interaction where the d electrons on the metal are donated to the π* orbitals of the olefin in a π fashion. The simplest dynamic process observed in metal olefin complexes is rotation of the olefin about the metal−olefin bond axis (Scheme 1a).3 The σ bond between the metal and the olefin donor orbital remains unaffected on rotation of the metal fragment about the metal− olefin bond. On the other hand, back-donation from the metal to the empty orbitals of the olefin restricts the rotation of the metal fragment with respect to the olefin due to its π symmetry. Experimental studies suggest a wide range in the energy of activation required for this rotation.4 One explanation is that the extent of back-donation varies, and this is reflected in the energy required to rotate the olefin about the metal−olefin bond axis. The fact that rotation of an olefin with respect to the metal− © 2013 American Chemical Society

olefin bond axis is observed in most systems is taken as an indication of the greater stabilization due to σ interaction between the metal and olefin relative to the π interaction. The relative importance of π and σ interactions, however, has been extensively debated and studied by computational methods.5 Interestingly, a second fluxional process, the sliding movement of a metal along the CC bond of the olefin, has been less studied (Scheme 1b).6 This distortion of a metal−olefin complex disrupts the π interaction in a complex more than the σ interaction and could again indicate the importance of the backdonation or the “π bond” between olefin and metal. The energy requirements for sliding the metal about the olefin CC double bond has significant implications for the reactivity of metal olefin complexes. It is a key step in migratory insertion of a metal hydride as in hydrogenation,7 or the reverse of this reaction, βhydride elimination.8 It is also a key step in metathesis reactions, where a carbene and an olefin react.9 It could also be a prerequisite for the oxidative addition of vinylic C−H bonds. A third fluxional process is the movement of the metal from one face of the olefin to the opposite face (Scheme 2). This movement is more restrictive, as both σ and the π interactions are completely lost; understandably, it has been rarely observed.10 Received: August 26, 2013 Published: November 14, 2013 7141

dx.doi.org/10.1021/om400857n | Organometallics 2013, 32, 7141−7152

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rization of the styrene complex that could only be accounted for by two mechanisms. It was suggested that the metal’s apparent movement through the alkene’s π nodal plane is by sliding of the metal from the π bond onto the carbon−hydrogen σ bond. Subsequently, there could be an oxidative addition of the metal onto the vinylic C−H bond and movement of the metal from one face of the olefin to the other. Alternatively, the metal could utilize the M···(C−H) interaction and swing to the opposite face of the olefin, much like a gymnast on a high bar. The experimental KIE values for the cis C−H bond and geminal C−H bond were much smaller: 1.07 and 1.15, respectively. This could be due to the change in the hybridization of the coordinated olefinic carbon atoms C1 and C2 during the transition state formation or due to fewer transits through these bonds. One particularly interesting path involved the phenyl group, the substituent on the olefin studied by Gladysz. The phenyl ring can act as a transient binding site for the metal, allowing the vinyl group (−CHCH2) to rotate and thereby permit migration of the metal from one face to the other. This was called a “conducted tour mechanism”. The possibility of forming an η2arene complex could only be tested through an indirect experiment. To the best of our knowledge, the two processes favored by the experiments of Gladysz have not been explored further experimentally, nor have they been computationally investigated. Since computational methods have become useful tools to validate many experimental findings, and in some cases unravel new reaction paths, we have analyzed this diastereomerization process using the DFT method. In particular, the mechanisms which could not be distinguished by experimental methods, oxidative addition and C−H σ complex formation, were studied. The paths involving the intermediate formation of a carbene or formation of a cation were neglected, as direct NMR evidence ruled out such intermediates. We show that the experimental results are satisfactorily corroborated by computational models. Further, computational results throw light on the potential energy surface (PES) of the metal moving on the face of the styrene.

Scheme 1

Scheme 2

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COMPUTATIONAL DETAILS

All DFT calculations reported in this work were performed with the Gaussian09 program package.11 Full geometry optimization and frequency calculations were performed at the gradient-corrected DFT level using the three-parameter fit of the exchange functional suggested by Becke (B3), in conjunction with the correlation functional suggested by Lee, Yang, and Parr, (B3LYP).12 Truhlar′s hybrid meta-GGA functional M0613 was also used for comparison, since M06 has been shown to be a better functional for the noncovalent interactions in organometallic species.14 The same basis sets were used for both M06 and B3LYP functionals. The LANL2DZ15 valence-basis set with effective core potential (ECP) was used at the Re center,16 and the 631G+(d,p) basis set was used at carbon, hydrogen, oxygen, and nitrogen atoms.17 The 6-311G+(3d,3p) basis set was used for the phosphorus atom. Initial guess geometries for RS,SR and RR,SS isomers of the [CpRe(PR3)NO(PhCHCH2)]+ complex for 1 and 2 (R = Me) and 1′ and 2′ (R = Ph) were generated using the structures available for 1′ and 2′ in the Cambridge Structural Database. Appropriate initial guess structures were created for the optimization of the intermediates and transition states involved in the isomerization pathway using the Gaussview package. Geometry optimizations were carried out without symmetry and geometry constraints, unless mentioned otherwise. Transition states were located using the Berny algorithm and STQN method.18 The intermediates and transition states (TS) were characterized by the number of imaginary frequencies (NIMAG) obtained

A notable exception is a Re complex containing two optically active centers, one at Re and the other at the coordinated olefinic C having a substituent. In a carefully executed study, Gladysz and co-workers showed that the diasteromerization involves migration of the metal from one face of the olefin to the other and that it occurs without dissociation of the metal from the olefin.10 During this process, only the chirality at the olefinic carbon changes. In this paper we have computed the energy requirements for transferring the olefin from one face to the other through some of the paths suggested by Gladysz and co-workers. We show that the computational results are in accord with the experimental results. Our computational results also help in ruling out some paths that could not be ruled out on the basis of their experimental results, thus extending our understanding of a fundamental fluxional process. To put things in perspective, we first summarize the experimental findings of Gladysz and co-workers. Using isotopic labeling and NMR studies, the fluxional process was narrowed down to a few plausible paths. They observed a kinetic isotopic effect (KIE) of k(H)/k(CHDE) = 1.64 for the diastereome7142


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Table 1. Structural Parameters and Free Energies of the Different Isomers of the Complex [CpRe(PPh3)NO(PhCHCH2)]+ a RS,SR (1′)

a

RR,SS (2′)

RS,SR (1R′)

RR,SS (2R′)

param

XRD

B3LYP

M06

XRD

B3LYP

M06

B3LYP

M06

B3LYP

M06

C1−Re C2−Re C1−C2 Re−P Re−N C1−C2−Re C2−C1−Re ΔG

2.225(9) 2.258(9) 1.400(1) 2.406(2) 1.750(1) 70.6(5) 73.2(6)

2.251 2.333 1.418 2.454 1.766 68.8 75.2 0

2.230 2.278 1.417 2.428 1.763 69.9 73.5 0

2.255(7) 2.284(7) 1.410(1) 2.420(2) 1.742(7) 70.8(4) 73.0(4)

2.260 2.345 1.417 2.456 1.768 68.8 75.4 0.1

2.234 2.282 1.417 2.432 1.764 69.9 73.6 0.4

2.231 2.353 1.419 2.476 1.766 67.4 76.7 4.8

2.203 2.292 1.419 2.464 1.761 68.2 75.0 2.0

2.222 2.359 1.420 2.488 1.763 66.8 77.3 9.2

2.194 2.277 1.423 2.466 1.759 68.3 74.7 5.7

Bond lengths are given in Å, and angles are given in degrees. Relative free energies are given in kcal/mol.

from the vibrational frequency analysis. The structures reported are either minima (NIMAG = 0) or transition states (NIMAG = 1) in the potential energy surface for diastereomerization. Intrinsic reaction coordinate (IRC) calculations were carried out to confirm that TS structures connect the respective reactants and products.19 The transition states were also confirmed by optimizing the geometries along the transition vector coordinates. The gas-phase relative free energies (ΔG) are reported with respect to the most stable RS,SR isomers in both PPh3 (1′) and PMe3 (1) complexes.

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RESULTS AND DISCUSSION Computational studies on organometallic systems have often been carried out with B3LYP functionals and shown to predict covalent interactions and structural parameters accurately.20 In recent times, it is becoming increasingly clear that the Minnesota functionals are a good choice for studying organometallic reactions.14 The M06 functional has modeled activation barriers, bond dissociation energies, and noncovalent interactions better than the B3LYP functionals.21 Therefore, in our study, both B3LYP and M06 functionals are used. To assess the reliability of the methods used in this study, the X-ray crystallographic structures22 reported for RS,SR (1′) and RR,SS (2′) complexes are optimized using the two different functionals and the geometrical parameters are compared with X-ray structural parameters (Table 1). Geometry optimization of the two isomers with both M06 and B3LYP functionals produce structures similar to the crystal structures (Figure 1, Table 1). The bond lengths and bond angles obtained in the M06 calculation are closer to the experimental values than the values from B3LYP calculation for both diastereoisomers 1′ and 2′. In all cases, the Re−C1 bond is shorter than the Re−C2 bond. The difference between these two bond lengths increases in the order XRD < M06 < B3LYP, in both 1′ and 2′ isomers. A more symmetrical η2 structure is predicted by the M06 computation, just as it is found in the X-ray structure. Apart from these two isomers 1′ and 2′, their respective rotamers 1R′ and 2R′ (R denotes the rotamer) are also optimized using both functionals, and the structural parameters are compared in Table 1. The bond lengths C1−Re and C2−Re and the bond angles C1−C2−Re and C2−C1−Re suggest that the metal fragment moves slightly away from C2 in the rotamers to reduce steric repulsion. Hence, these rotamer geometries are less η2 symmetric in comparison to 1′ and 2′ in the structures computed by both functionals. To reduce the computational load, the PPh3 ligand was replaced by the PMe3 ligand and the geometry optimization was carried out with B3LYP and M06 functionals. In comparison to the PPh3 complexes, the Re−P bond length in the PMe3 complexes (optimized geometries are given in Figure 2) are

Figure 1. Optimized geometries of the different isomers of the complex [CpRe(PPh3)NO(PhCHCH2)]+. Hydrogen atoms, except those on the vinyl group, are omitted for clarity.

Figure 2. Optimized geometries of the different isomers of the complex [CpRe(PMe3)NO(PhCHCH2)]+. Hydrogen atoms, except those on the vinyl group, are omitted for clarity.

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Table 2. Structural Parameters and Free Energies of the Different Isomers of the Complex [CpRe(PMe3)NO(PhCHCH2)]+ a RS,SR (1)

a

RR,SS (2)

RS,SR (1R)

B3LYP

M06

B3LYP

M06

B3LYP

M06

B3LYP

M06

C1−Re C2−Re C1−C2 Re−P Re−N C1−C2−Re C2−C1−Re ΔG

2.250 2.319 1.421 2.424 1.768 69.2 74.5 0

2.221 2.261 1.421 2.404 1.764 70.0 73.1 0

2.252 2.337 1.420 2.425 1.768 68.7 75.3 0.7

2.223 2.278 1.421 2.406 1.764 69.5 73.7 0.5

2.225 2.346 1.421 2.430 1.768 67.3 76.6 3.4

2.194 2.287 1.422 2.419 1.762 68.0 75.1 2.1

2.223 2.349 1.420 2.443 1.766 67.1 76.8 2.5

2.192 2.287 1.422 2.430 1.762 67.9 75.2 1.4

Bond lengths are given in Å, and angles are given in degrees. Relative free energies are given in kcal/mol.

moves from the olefinic π bond to the arene π bond in the styrene, followed by rotation of the −CHCH2 group about the C(olefin)−C(aryl) bond and a reverse migration of the metal to the olefin. The experimental results, especially the KIE values, suggested that the possible pathway could be via the “trans (C−H)···Re σ bond complex” formation and/or trans C−H bond oxidative addition. While the experimental study did not distinguish between these two mechanisms, the computational study provides an opportunity to evaluate the transition state energy for these pathways. (i). C−H “σ Complex” Formation (CH). A (C−H)···Re σ complex formation was suggested by Gladysz and co-workers to precede the migration of the Re fragment from one face of the olefin to the other. Therefore, DFT calculations to locate a minimum with the (C−H)···Re σ complex formation are carried out. However, attempts to locate such a minimum failed for all three C−H bonds and always ended up with the olefin complexes 1 or 2. However, a search for the transition states connecting two diastereomers (1 and 2) with the metal in the vicinity of the C−H bonds was successful. A first -order saddle point is identified for all three C−H bonds (cis, trans, geminal) of styrene with both B3LYP and M06 functionals. The transition state TStrans‑CH involves migration of the metal from one face of the olefin to the other while interacting with the trans C−H bond. Similarly, TScis‑CH and TSgem‑CH transition states correspond to the Re migration while interacting with cis and geminal C−H bonds, respectively. In the transition state TStrans‑CH, the imaginary frequency corresponds to a swinging motion of the vinylic group (CH2) of the styrene with respect to the PhCH group. The transition state structures for the (C−H)···Re interaction pathways are given in Figure 3. In TStrans‑CH, the mode of interaction of Re with a trans C−H bond is η2 and is asymmetric in nature such that the Re−H1 bond length is shorter than the Re−C1 bond length. The trans C−H bond is lengthened by ∼0.03 Å upon coordination. The C1−H1−Re angle in the structure obtained using the M06 functional is 113° and is 10° lesser than the angle measured by the B3LYP functional. The selected structural parameters are given in Table 4. A similar Re···η2-(C−H) interaction is observed in TScis‑CH. However, the transition vector of this transition state is slightly different from that of TStrans‑CH. The imaginary mode is a swinging motion of the −CHCH2 group with respect to the Ph group of the styrene. Unlike TStrans‑CH and TScis‑CH, an η1 mode of interaction of Re with the geminal hydrogen atom (H3) is observed in the transition state TSgem‑CH. The Re−H3 bond length is ∼2.0 Å. An almost linear C2−H3−Re angle (∼160°) is obtained in the

marginally shorter with both M06 and B3LYP functionals. Except for the Re−P length, key parameters (Table 2) are approximately the same as those found in the PPh3 complexes. As in the PPh3 complexes, the PMe3 complexes also have more symmetric coordination of the olefinic π bond to Re in the structures optimized by M06 in comparison to the B3LYPoptimized geometries. This suggests that both functionals are adequately suited for understanding the Re−olefin interaction. The relative free energies of 1′ and 2′ and their rotational isomers 1R′ and 2R′ are in the same order as suggested by Gladysz and co-workers, namely 1′ < 2′ < 1R′ < 2R′, using either B3LYP or M06 functionals. Our results are in accord with a previous computational study conducted by White et al., who carried out a conformational search of the same complexes.23 Comparisons of the selected structural parameters and the free energy values of the four isomers of PPh3 and PMe3 complexes are presented in Tables 1 and 2 respectively. The energy ordering for the lower energy diastereomers of PMe3 complexes is the same as that observed for the PPh3 complexes. However, the higher energy rotational isomers of the complexes having PMe3 follow a different energy ordering (1 < 2 < 2R < 1R). This difference in stability between 1R/2R and 1R′/2R′ complexes might be due to the greater steric interaction between the Ph group of styrene with that of the PPh3 ligand in the rotamers. The free energies of olefin coordination in the most stable RS,SR isomer are calculated for both PMe3 and PPh3 complexes and are given in Table 3. These energies are significantly greater Table 3. Free Energies of Olefin Coordination of the RS,SR Isomera B3LYP M06 a

RR,SS (2R)

param

PMe3 (1)

PPh3 (1′)

40.6 53.8

39.0 49.6

Energy values are given in kcal/mol.

(>15 kcal/mol) than the free energies of activation computed for the most favorable paths identified below. These results are in conformity with the reported experimental results. Computational Evaluation of the Metal Migration. The following three mechanistic pathways are considered for DFT calculations: (i) migration of the metal to the other side of the olefin via a transition state that involves an agostic (C−H)···Re interaction with any one of the three C−H bonds (trans (C1− H1), cis (C1−H2), geminal (C2−H3)) of the olefin, (ii) oxidative addition of any one of the C−H bonds and subsequent rotation of the vinyl group about the Re−Cvinyl bond and reductive elimination of the C−H bond to give the diastereomer, and (iii) a conducted tour (CT) mechanism in which the metal 7144


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Figure 3. Optimized geometries of the transition states in the (C−H)···Re interaction pathways of the PMe3 complex. Hydrogen atoms, except those on the vinyl group, are omitted for clarity.

order is different from that of the M06 calculation (TSgem‑CH > TStrans‑CH > TScis‑CH). Thus, with M06 functionals, a cis C−H σ interaction is favored over trans and geminal C−H interactions by ∼0.6 and 0.2 kcal mol−1, respectively. The M06 result does not support the experimental result that most isomerizations involved migrations through the trans C−H bond. On the other hand, calculations using the B3LYP functional suggest that the trans (C−H)···Re interaction pathway is preferred over the cis and geminal (C−H)···Re interaction pathways by ∼0.4 and 2.0 kcal/mol, respectively. (ii). Oxidative Addition (OA). Inter- and intramolecular activation of aryl, alkyl, and alkenyl C−H bonds have been observed experimentally for several Re complexes.24 Since it is possible that the isomerization can proceed through the formation of the hydrido vinyl complexes, oxidative addition of all three olefinic C−H bonds (cis, trans, geminal) has been studied computationally. The Re(III) vinyl hydride intermediates and the transition states connecting these intermediates with the isomers 1 or 2 have been computationally evaluated using B3LYP and M06 functionals. In the B3LYP optimized Re(III) intermediates, the distance between Cvinyl and H varies from 1.84 to 2.12 Å, and in the M06 optimized structures, this distance ranges from 1.92 to 2.15 Å. The Cvinyl···H distance in the transition states of the oxidative addition (TStrans‑OA1, TStrans‑OA2, TScis‑OA1, TScis‑OA2, TSgem‑OA1, TSgem‑OA2) varies from 1.60 to 1.78 Å in the B3LYP functional, whereas in the M06 functional, this range is slightly greater (1.48−1.80 Å). The imaginary frequency corresponding to these oxidative addition transition states is “rupture and formation” of the C−H bond. The important structural parameters of the trans, cis and geminal C−H oxidative addition intermediates and the corresponding transition states are represented in Tables 5 and Tables S1 and S2 (Supporting Information), respectively, and the structures are shown in Figure 5 and Figures S1 and S2 (Supporting Information), respectively. Attempts to obtain a minimum energy structure where a metal coordinates in an end-

Table 4. Structural Parameters of the Transition States of the (C−H)···Re Interaction Pathways of the PMe3 Complexa TStrans‑CH

a

TScis‑CH

TSgem‑CH

param

B3LYP

M06

B3LYP

M06

B3LYP

M06

C1−Re C2−Re H1−Re H2−Re H3−Re C−H C−H−Re

2.738 3.943 1.953 2.897 4.207 1.122 123.6

2.621 3.878 1.968 2.693 4.236 1.122 113.2

2.725 3.961 2.861 1.961 4.785 1.124 121.8

2.640 3.877 2.756 1.942 4.719 1.121 116.5

3.759 3.102 3.562 4.833 2.002 1.125 164.8

3.652 3.040 3.425 4.734 1.963 1.121 159.8

Bond lengths are given in Å, and angles are given in degrees.

calculations using the M06 functional, and an even larger angle is obtained using the B3LYP functional. Although the coordination mode is different, the imaginary mode in this TS is the same as what is observed for TScis‑CH and corresponds to the rotation of the −CHCH2 moiety about the C2−C3 bond. The free energy of activation for formation of different (C− H)···Re σ complex transition states follows the order TSgem‑CH > TScis‑CH > TStrans‑CH with the B3LYP functional (Figure 4). This

Figure 4. (C−H)···Re interaction pathways of the PMe3 complex calculated using (a) B3LYP and (b) M06 levels. Relative free energies are given in kcal/mol.

Table 5. Structural Parameters of the Intermediates and Transition States of the trans C−H Bond Oxidative Addition Mechanism for the PMe3 Complexa INTtrans‑OA1

a

INTtrans‑OA2

TStrans‑OA1

TStrans‑OA2

TStrans‑OA‑rot

param

B3LYP

M06

B3LYP

M06

B3LYP

M06

B3LYP

M06

B3LYP

M06

C1−H1 Re−H1 Re−C1 C1−Re−H1 C2−C1−Re−H1

2.120 1.648 2.152 66.3 14.2

2.138 1.664 2.137 67.1 16.7

2.034 1.655 2.156 62.9 −118.1

2.110 1.670 2.143 65.8 −135.8

1.740 1.661 2.192 51.5 72.9

1.763 1.671 2.173 52.7 70.6

1.670 1.669 2.182 49.2 −97.5

1.740 1.661 2.192 51.5 −72.9

2.026 1.648 2.179 62.1 −62.6

2.000 1.664 2.157 61.6 −72.9

Bond lengths are given in Å, and angles are given in degrees. 7145


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Figure 5. Optimized geometries of the intermediates and transition states of the trans C−H bond oxidative addition mechanism of the PMe3 complex. Hydrogen atoms, except those on the vinyl group, are omitted for clarity.

Figure 6. Oxidative addition mechanisms of the PMe3 complex calculated using (a) B3LYP and (b) M06 levels. Relative free energies are given in kcal/ mol.

transition states are close in energy to the corresponding Rh(V) intermediates formed on oxidative addition.26 The RS,SR isomer 1 undergoes oxidative addition of the trans C−H bond through the transition state TStrans‑OA1 to give the Re(III) intermediate INTtrans‑OA1 with an activation barrier of 26.7 kcal mol−1 using the B3LYP functional. The intermediate INTtrans‑OA1 is converted to another Re(III) intermediate INTtrans‑OA2 by rotation of the vinylic group about the Re−C1 bond. This step requires 3.3 kcal mol−1. In these two intermediates, the CC π faces of the vinyl group with respect to the Re center are exchanged. The transition state for the Re−Cvinyl bond rotation (TStrans‑OA‑rot) has an imaginary frequency of ∼40 cm−1, and such a small frequency might be an artifact rather than a transition state. An IRC calculation for this transition state does not connect the two intermediates. However, a relaxed potential energy surface scan (RPES) confirms this to be a true transition

on fashion to the vinylic C−H bond, as suggested by Hoffmann, were unsuccessful.25 The optimization always ended up with isomer 1 or 2. The formation of the Re(III) intermediates from complexes 1 and 2 is highly endothermic. The transition states leading to the oxidative addition products (Re(III) species) are similar in energy and structure to the products. Unexpectedly, in a few cases, the relative free energies for the transition states of the oxidative addition steps are slightly lower than those of the corresponding intermediates (Figure 6). Decreasing the optimization step size does not lead to any further refinement of the energy, suggesting that the potential energy surface for the oxidative addition pathway is flat. Recently, Tilset et al. reported the DFT study of the oxidative addition of an ethylene C−H bond to a Rh(III) complex with an activation barrier of ∼30 kcal mol−1. As in our model complexes, their system also exhibits a flat PES for the oxidative addition of an ethylene C−H bond. The 7146


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Figure 7. Optimized geometries of the intermediates and transition states of the conducted tour mechanism of the PMe3 complex. Hydrogen atoms, except those on the vinyl group, are omitted for clarity.

Considering all processes, the oxidative addition of the trans C−H bond is preferred over cis and geminal oxidative additions using either the B3LYP or M06 functional. However, the trans (C−H)···Re interaction pathway is favored over the trans C−H oxidative addition in B3LYP. In the M06 calculation, the cis (C− H)···Re interaction pathway is 1.0 kcal mol−1 more favorable than the trans C−H oxidative addition pathway. (iii). Conducted Tour Mechanism (CT). Although the conducted tour mechanism does not fit the experimental KIE values obtained for the styrene complexes, we carried out a DFT study to determine the energy requirements for shuttling the metal between olefinic and phenyl CC bonds . The migration of the metal center has precedence in other fluxional systems and has been termed “chain walking” along conjugated double and single bonds of a π system.27 Structures of the intermediates and transition states are given in Figure 7. In the RS,SR (1) isomer, the Re center moves from the olefinic double bond and coordinates to the C4−C5 carbons of the phenyl group to form a η2-arene complex (INTCT1). This complex is a minimum in the PES. The transition state TSCT1, in which the metal center interacts with the C2−C3 single bond in a η2 fashion, connects the η2-arene intermediate INTCT1 with complex 1. Although the η2-C−C σ bonding is unusual in organometallic chemistry, there are a few examples in the literature where the interaction of metal centers with C−C bonds has been characterized.28 Computational studies have also characterized transition states where metals interact with C−C σ bonds.29 The metal coordination to a C−C single bond in this transition state does not lead to a significant change in the C−C bond length. The Re−C3 and Re−C2 bond lengths suggest only a weak interaction (for TSCT1, B3LYP gives 2.94 and 3.09 Å, respectively, and M06 gives 2.75 and 2.89 Å, respectively). The imaginary mode of this transition state is the shuttling motion of the Re moiety between the aromatic ring and olefinic π bond of the styrene ligand.

state (Supporting Information, Figure S10). The reductive elimination of the C−H bond in the INTtrans‑OA2 intermediate affords isomer 2. This reductive elimination step proceeds through the transition state TStrans‑OA2 with a free energy cost of 0.5 kcal mol−1. The overall activation barrier of this isomerization pathway is 28.8 kcal mol−1, calculated with respect to 1 (Figure 6). Similarly, the isomerization can also proceed through the oxidative addition of the cis and geminal C−H bonds. The oxidative addition of the cis C−H bond from isomer 1 requires 31.1 kcal mol−1 to give the intermediate INTcis‑OA1. The Re−C1 bond rotation of this intermediate costs only 2.7 kcal mol−1 to give another intermediate, INTcis‑OA2. Finally, the reductive elimination of the C−H bond gives the diastereomer 2. The overall free energy demand for this pathway is 5.3 kcal mol−1 higher than that for the trans C−H oxidative addition mechanism. Similar types of intermediates and transition states were located for the oxidative addition of geminal C−H bond as well. Although the free energy of activation for the geminal C−H bond oxidative addition is close to that for the cis C−H, the greater energy demand for the Re−C2 bond rotation step makes the geminal C−H pathway less favorable than the cis C−H pathway by 2.1 kcal mol−1 (Figure 6). Thus, the trans C−H bond oxidative addition is the more favorable pathway in comparison to the cis and geminal C−H pathways. However, this mechanism is less favorable than the trans (C−H)···Re interaction pathway by 1.5 kcal mol−1 in B3LYP calculations. All of the intermediates and the transition states obtained using M06 functionals are higher in energy than their corresponding B3LYP optimized geometries. The overall activation barrier for the oxidative addition of a trans C−H bond is 35.1 kcal mol−1. The overall free energy demands for the cis and geminal C−H bond oxidative addition mechanisms are ∼3.0 kcal mol−1 higher than the trans C−H pathway. 7147


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Table 6. Structural Parameters of the Intermediates and Transition States of the Conducted Tour Mechanism of the PMe3 Complexa INTCT1

a

INTCT2

TSCT1

TSCT2

TSCT‑rot

param

B3LYP

M06

B3LYP

M06

B3LYP

M06

B3LYP

M06

B3LYP

M06

C1−C2 C2−C3 C4−C5 Re−C5 Re−C4 Re−C2 Re−C3 C1−C2−C3−C4

1.342 1.473 1.421 2.476 2.588

1.338 1.465 1.423 2.375 2.452

1.343 1.470 1.421 2.457 2.565

1.339 1.460 1.424 2.371 2.417

1.346 1.483 1.400

1.343 1.475 1.397

1.348 1.482 1.402

1.346 1.475 1.396

1.337 1.492 1.420 2.489 2.677

1.334 1.485 1.429 2.324 2.414

3.088 2.943 175.2

2.886 2.750 178.2

3.001 2.980 7.6

2.821 2.849 8.2

88.6

89.4

170.2

168.4

4.1

0.0

Bond lengths are given in Å, and angles are given in degrees.

9b) as the most favorable path. A comparison of the activation barriers of the three different mechanisms calculated using the B3LYP functional shows that the trans (C−H)···Re interaction pathway is more favorable than the oxidative addition of the trans C−H bond and the conducted tour mechanism by 1.5 and 4.8 kcal mol−1, respectively. These results are consistent with the experimental results of Gladysz and confirm that the trans C−H bond of the styrene is primarily responsible for the diastereomerization process without bond breaking. Although many have found M06 to be a better functional for organometallic reactions,14,21 in this instance, the most commonly used B3LYP functional is closer to the experimental results. PPh3 Complexes: B3LYP Calculation. We decided to carry out calculations for the experimental complex bearing the PPh3 ligand using the B3LYP functional, only for those paths predicted to be favorable in the model PMe3 complexes. A DFT study of the trans (C−H)···Re interaction pathway and trans C−H oxidative addition mechanisms has been carried out for the PPh3 analogues. Since the trans (C−H)···Re interaction pathway is favored over the cis C−H interaction path by a negligible margin in the PMe3 complex, the cis (C−H)···Re interaction pathway was also studied for the PPh3 complex. As in the case of PMe3 complexes, similar intermediates and transition states for both (C−H)···Re interaction pathways and oxidative addition mechanisms were obtained and are shown in Figures S3 and S4 (Supporting Information), respectively. Relative free energies of all PPh3 intermediates and transition states of the trans C−H oxidative addition and the (C−H)···Re interaction pathways are lower than the corresponding free energies of PMe3 complexes. The trans (C−H)···Re interaction pathway is more favorable than the cis (C−H)···Re interaction pathway by 1.4 kcal mol−1. The transition state for the Re−C1 bond rotation (TStrans‑OA‑rot′) is the highest energy transition state in the trans C−H oxidative addition pathway. The overall activation barrier of the trans C−H oxidative addition is 27.6 kcal mol−1, which is 2.3 kcal mol−1 higher than TStrans‑CH′. As in the PMe3 complexes, the trans (C−H)···Re interaction pathway is more favorable than the oxidative addition pathway. We were curious to see the effect of having a larger PPh3 ligand on the conducted tour mechanism. As in the case of PMe3 complexes, similar intermediates and transition states are located for the conducted tour mechanism in PPh3 complexes. However, geometry optimizations to locate the transition state TSCT1′ failed and the optimizations lead to the expulsion of the styrene group from the metal fragment! Indeed, the experimental study with a coordinated benzene also proved to be an unstable intermediate prone to loss of the ligand. Therefore, a relaxed geometry optimization with two constraints (bond lengths C2−

The rotation of the vinyl group about the C2−C3 single bond in the INTCT1 intermediate gives rise to the intermediate INTCT2 through the transition state TSCT‑rot. These two intermediates (INTCT1, INTCT2) differ in the orientation of the olefin’s π enantioface. The dihedral angle C1−C2−C3−C4 changes from ∼170° to ∼5° in these intermediates, and in the transition state TSCT‑rot, it is ∼90° (Table 6). Thus, the diastereomer 2 is formed when the metal center traverses back to the olefinic π bond in the intermediate INTCT2 via transition state TSCT2. The key structural parameters are given in Table 6, and the free energy profile is shown in Figure 8.

Figure 8. Conducted tour mechanism of the PMe3 complex calculated using B3LYP and M06 levels. Relative free energies are given in kcal/ mol.

The diastereomerization requires three steps in the conducted tour mechanism. The least energy demanding step is the rotation of the vinyl moiety to give different η2-arene intermediates, which hardly costs ∼5.0 kcal mol−1. The relative free energies of these intermediates are lower in the M06 calculations in comparison to the B3LYP calculations. The rate-determining step is judged to be the shuttling of the Re moiety between the vinyl and the phenyl CC bonds with activation barriers of 32.1 and 33.7 kcal mol−1 with the B3LYP and M06 functionals, respectively. The conducted tour mechanism was ruled out by Gladysz et al. from indirect experimental evidence and from the fact that it is not consistent with the KIE values obtained for vinylic C−H bonds. An overall comparison of the free energy requirements for the different mechanisms of the PMe3 complex is given in Figure 9. The M06 calculations are inconsistent with the experimental results, as they suggest the conducted tour mechanism (Figure 7148


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Figure 9. Comparison of the three different mechanisms of the PMe3 complex calculated using (a) B3LYP and (b) M06 levels of theory. Relative free energies are given in kcal/mol.

Re (3.35 Å) and C3−Re (3.30 Å), which are approximately the distances observed in TSCT2′) was carried out to get an estimate of the energy requirement for migration of the metal from the olefin π bond to arene to give the intermediate INTCT1′ (Figure 10). The transition state TSCT1′ generated in this method is at 32.6 kcal mol−1, which is the highest energy structure in the conducted tour pathway. This activation barrier is 7.6 and 5.3 kcal mol−1 higher than the activation barriers of the trans (C− H)···Re interaction pathway and the trans C−H oxidative addition mechanisms, respectively. Thus, the conducted tour mechanism is the least favorable mechanism in the PPh3

complexes. These observations are completely consistent with the experimental results. On the basis of these results, a theoretical kinetic isotopic value (KIEtheor) was calculated by carrying out B3LYP calculations for the styrene and the trans deuterated styrene (trans C−H/C−D) complexes (Figure 11). The relative free energy of activation (ΔG⧧ at the experimental temperature 373 K) of the trans (C− H)···Re interaction pathway for the styrene complex is 24.63 kcal mol−1. For the trans deuterated styrene complex, it is 25.0 kcal mol−1. A KIE of 1.72 was computed from these data, which serendipitously matches the experimental KIE value. Similarly, a KIE value of 2.0 was computed for the trans C−H oxidative 7149


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Figure 10. Comparison of the three different mechanisms of the PPh3 complex calculated using the B3LYP level of theory. Relative free energies are given in kcal/mol.

Figure 11. B3LYP calculated free energy profiles (at the experimental temperature 373 K) for the trans (C−H)···Re interaction pathway (solid lines) and the trans C−H oxidative addition (dotted lines) mechanisms of the styrene (blue) and the trans deuterated styrene (red) complexes. Relative free energies are given in kcal/mol.

various reaction pathways in great detail. The metrical parameters of the ground-state structures and their relative energies are modeled correctly by the DFT method. Two pathways, one that involves migration of Re through a trans (C−H)···Re σ complex and another that involves oxidative addition of Re into the trans C−H bond, have been identified as possible paths at the B3LYP level. The migration of the metal through formation of a trans (C−H)···Re complex is more

addition mechanism. This result gives further support for migration of the Re through involvement of the trans C−H bond.

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CONCLUSION

The trusted workhorse of DFT computational studies, the B3LYP functional, and the increasingly popular M06 functional have been used for exploring the unusual migration of the metal from one face of an olefin to the other in a Re−styrene complex. Use of the smaller PMe3 ligand allows one to quickly evaluate the 7150


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(4) Friedman, L. A.; Meiere, S. H.; Brooks, B. C.; Harman, W. D. Organometallics 2001, 20, 1699−1702. (5) (a) Zhao, H.; Ariafard, A.; Lin, Z. Inorg. Chim. Acta 2006, 359, 3527−3534. (b) Kim, C. K.; Lee, K. a; Kim, C. K.; Lee, B.-S.; Lee, H. W. Chem. Phys. Lett. 2004, 391, 321−324. (c) Pidun, U.; Frenking, G. J. Organomet. Chem. 1996, 525, 269−278. (d) Guo, B. C.; Castleman, A. W. Chem. Phys. Lett. 1991, 181, 16−20. (e) Nelson, J. H.; Wheelock, K. S.; Cusach, L. C.; Jonassen, H. B. J. Am. Chem. Soc. 1968, 91, 7005− 7008. (6) (a) Chang, T. C. T.; Foxman, B. M.; Rosenblum, M.; Stockman, C. J. Am. Chem. Soc. 1981, 103, 7361−7362. (b) Eisenstein, O.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4308−4320. (7) (a) Church, T. L.; Rasmussen, T.; Andersson, P. G. Organometallics 2010, 29, 6769−6781. (b) Rowley, C. N.; Foucault, H. M.; Woo, T. K.; Fogg, D. E. Organometallics 2008, 27, 25−27. (c) Coussens, B. B.; Buda, F.; Oevering, H.; Meier, R. J. Organometallics 1998, 17, 795−801. (d) Han, Y.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 5939− 5945. (8) (a) Theofanis, P. L.; Goddard, W. A. Organometallics 2011, 30, 4941−4948. (b) Tellmann, K. P.; Humphries, M. J.; Rzepa, H. S.; Gibson, V. C. Organometallics 2004, 23, 5503−5513. (9) (a) Tia, R.; Adei, E. Dalton Trans. 2010, 39, 7575−7587. (b) Suresh, C. H.; Koga, N. Organometallics 2004, 23, 76−80. (c) Vyboishchikov, S. F.; Michael, B.; Thiel, W. Chem. Eur. J. 2002, 8, 3962−3975. (d) Monteyne, K.; Ziegler, T. Organometallics 1998, 17, 5901−5907. (e) Eisenstein, O.; Hoffmann, R.; Rossi, A. R. J. Am. Chem. Soc. 1981, 103, 5584−5586. (10) Peng, T. S.; Gladysz, J. A. J. Am. Chem. Soc. 1992, 114, 4174− 4181. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, aÖ .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (12) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R, G. Phys. Rev. B 1988, 37, 785−789. (13) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101− 194118. (14) (a) Zhao, Y.; Truhlar, D. G. Chem. Phys. Lett. 2011, 502, 1−13. (b) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. (15) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284−298. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310. (16) On the basis of a reviewer’s suggestion, we verified energies of olefin complexes (RS,SR and RR,SS) and key intermediates and transition states using a triple-ξ valence basis set with an additional polarization function (LANL2TZ(f)) on Re at the B3LYP level. However, the extended basis set failed to reproduce the energy difference of RS,SR and RR,SS isomers correctly (Supporting Information, Figures S8 and S9), and so we base our arguments on the LANL2DZ basis sets only. (17) (a) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1979, 939−946. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (18) (a) Li, X.; Frisch, M. J. J. Chem. Theory Comput. 2006, 2, 835−839. (b) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449−454. (19) Hratchian, H. P.; Schlegel, H. B. J. Chem. Theory Comput. 2005, 61−69.

favorable than the oxidative addition, although by a small amount. Even though the ground-state structures of the olefin complexes are correctly reproduced by the M06 calculations, the favored pathway for migration is identified to be the conducted tour mechanism, which is not consistent with the experimental result. This computational study has reaffirmed the experimental result that the migration of the metal from one face of the olefin to the other can be carried out without dissociation. It also brings to light that there is a surprisingly small energy difference between the paths involving oxidative addition and a simple (C− H)···Re interaction. Variations in the metal and ligands might tilt the reaction in favor of oxidative addition. This suggests that metal migrations such as that observed with this Re complex is probably a more common phenomenon than is currently recognized.

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ASSOCIATED CONTENT

* Supporting Information S

Figures and tables giving structures and key structural parameters of the intermediates and transition states of the cis and geminal oxidative addition mechanisms of the PMe3 complex and all of the PPh3 structures, relaxed potential energy surface (RPES) for the Re−C bond rotation of the trans C−H bond oxidative addition mechanism of the PMe3 complex, and relative electronic energies, enthalpies, free energies, Cartesian coordinates, and absolute energies for all minima and transition states. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.G.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Supercomputer Education and Research Centre (SERC) of the Indian Institute of Science for computational facilities and Prof. G. Frenking for valuable suggestions. M.T. acknowledges a Senior Research Fellowship (SRF) from the Council of Scientific and Industrial Research (CSIR). A.G.S. thanks the Department of Science and Technology (DST) for the award of a research grant.

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REFERENCES

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Computational Study of the Migration of Rhenium from One

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