Oxo vs Imido Alkylidene d0-Metal Species: How ... - ACS Publications

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Oxo vs Imido Alkylidene d0‑Metal Species: How and Why Do They Differ in Structure, Activity, and Efficiency in Alkene Metathesis? Xavier Solans-Monfort,*,† Christophe Copéret,*,‡ and Odile Eisenstein*,§ †

Departament de Química, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain Department of Chemistry, ETH Zürich, Wolfgang Pauli Strasse 10 CH-8093 Zürich, Switzerland § Institut Charles Gerhardt, UMR 5253 CNRS Université de Montpellier 2, cc1501, Place E. Bataillon, F-34095 Montpellier, France ‡

S Supporting Information *

ABSTRACT: Density functional calculations have been carried out to analyze the origin of the differences in reactivity, selectivity, and stability toward deactivation in metathesis of d0 oxo alkylidene complexes vs their isoelectronic imido counterparts. DFT calculations show that the elementary steps and geometries of the extrema are similar for the oxo and imido complexes, but that the energy profiles are different, the greatest difference occurring for the deactivation pathway. For the alkene metathesis pathway, replacing the imido by an oxo ligand slightly lowers the energy barrier for alkene coordination but raises that for the [2+2]-cycloaddition and cycloreversion; it also destabilizes the trigonal bipyramidal (TBP) metallacyclobutane intermediate with respect to the separated reactants. The isomeric square-based pyramid (SP) metallacyclobutane is in general more stable, and its stability relative to the separated reactants is similar for oxo and imido systems. Consequently, the oxo complex is associated with a slightly larger energy difference between the lowest energy intermediate (SP or separated reactants) and the highest energy transition state (cycloreversion) than the imido complex, which accounts for a slightly lower activity. Changing the imido into an oxo ligand disfavors strongly the deactivation pathway by raising considerably the energy barrier of the β-H transfer at the SP metallacycle that begins the entry into the channel for deactivation and byproduct formation as well as that of the subsequent ethene insertion. This makes the oxo catalysts more selective and stable toward deactivation than the corresponding imido catalysts, when dimerization can be avoided.

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INTRODUCTION Supported group 6 and 7 transition-metal oxos (MOx/SiO2, MOx/Al2O3, etc.) belong to some of the first discovered alkene metathesis (pre)catalysts and are still at the heart of large-scale production processes such as in the conversion of 2-butenes into propene via ethenolysis.1 While metal-oxo alkylidenes are probably the key active species in such alkene metathesis processes, the mode of formation and the spectroscopic evidence for these species have remained elusive. This has triggered a tremendous research effort to generate the corresponding homogeneous catalysts based on well-defined metal-oxo alkylidene complexes (1) or precursors (2) such as metal-oxo complexes with two alkyl ligands (Scheme 1).2−5 Early reports showed the high activity of these systems, which unfortunately suffered from fast deactivation.3,4 This led to the development of the corresponding well-defined metal imido alkylidene complexes (X)(Y)M(NR)(CHR1) (3),6−9 which have been greatly improved over the years.10−13 At present, exploiting the high reactivity that is associated with dissymmetry at the metal center (X ≠ Y),14−21 the monoalkoxy pyrrolyl (MAP) family (4) have become highly robust catalysts able to selectively produce either E- or Z-alkenes22−26 or induce asymmetric ring-closing metathesis.27−30 More recently, the corresponding tungsten-oxo MAP alkylidene complexes (5) were prepared and showed high activity, selectivity, and even © 2012 American Chemical Society

stability toward deactivation, as long as large phenoxy ligands were used.31 In parallel, a recent report has shown that welldefined silica-supported metal-oxo complexes (6)32 are more stable and selective catalysts than the imido equivalents at the expense of slightly lower activity.33,34 Previous theoretical studies on metathesis have examined homogeneous catalysts based on both Ru35−51 and group 6 and 7 transition metals.52−64 For d0 Schrock-type catalysts of general formula (X)(Y)M(E)(CHR1) (M = Mo or W with E = NR and M = Re with E = CR), whether supported on silica or not (Y = siloxy or alkoxy, X = alkoxy, amido, or alkyl), we have previously shown that the elementary steps for the catalytic process are alkene coordination, [2+2]-cycloaddition, and the corresponding reverse steps (cycloreversion−decoordination), and this is regardless of the nature of the metal (Mo, W, and Re) and the E group (E = imido or alkylidyne).57,60,65 Moreover, we have shown that dissymmetry at the metal center (X ≠ Y) is key to increasing the reactivity of these systems because it both lowers the energy of the transition state (TS) for alkene coordination and raises the energy of the metallacyclobutane intermediates (Scheme 2).57 In particular, we showed that the key step was not the [2+2]-cycloaddition Received: June 27, 2012 Published: September 26, 2012 6812

dx.doi.org/10.1021/om300576r | Organometallics 2012, 31, 6812−6822

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(SP), where the metallacycle occupies two basal sites (Scheme 2).60,64,65 The resting state of the catalyst is thus the SP metallacyclobutane, which can either revert to the TBP structure and reenter the metathesis catalytic cycle or undergo a ring-opening via β-H transfer. This latter elementary step eventually leads to catalyst deactivation and formation of byproduct, such as alkene isomers (Scheme 2).64,65 Calculations also revealed that in these imido (E = NR) or alkylidyne (E = CR) systems, (X)(Y)M(E)(CHR1), the energy barrier for the β-H transfer reaction increases with X being a ligand with a more electronegative atom attached to M: C (alkyl) < N (amido) or O (alkoxy).65 Consequently, the previous studies have shown that the best catalysts are based on imido complexes with a pyrrolyl ligand despite the less efficient pathway because the deactivation process via β-H transfer is minimized.66 In view of the difference in reactivity, selectivity, and deactivation rate of the metal-oxo relative to the imido complexes, we use DFT calculations to determine if the proposed elementary steps found for the imido compounds also apply to the oxo systems and how the oxo ligand influences each of these elementary steps. With the aim of obtaining a deeper understanding of the generality of the effects, we have considered models of the experimentally synthesized W complexes as well as the molybdenum analogues. We thus examine first the structure of the catalysts and the energy profile of metathesis pathways, the interconversion between TBP and SP metallacyclobutanes, and finally the decomposition of the latter via β-H transfer, the key step for the catalyst decomposition and the byproduct formation in the absence of catalyst dimerization.

Scheme 1

step as often thought, but the coordination of the alkene to the metal center, which is associated with a distortion of the tetrahedral alkylidene complex into a trigonal pyramid to accommodate the incoming alkene. The influence of the dissymmetry at the metal center is already visible in the initial tetrahedral alkylidene complex since making X different from Y distorts the tetrahedron toward the TS geometry, by opening the face trans to the stronger σ-donor X ligand, resulting in lower energy barriers.57,60,65 The TS is a trigonal bipyramid with the incoming alkene at an apical site trans to X and still far from the metal (metal−alkene distance >3 Å). Consequently, the TS is best viewed as a trigonal pyramid with an empty coordination and a weakly interacting alkene, the interaction between the alkene and the metal increasing in the descent to the alkene adduct. The TS of lowest energy has the stronger σdonor X ligand trans to the incoming alkene. Moreover, we showed that the metallacyclobutane found on the reaction pathway adopts a trigonal bipyramid geometry (TBP) with the X group and the metallacyle in the equatorial plane. This TBP intermediate can interconvert with an energy barrier that depends on X (low for X = pyrrolyl and higher for X = alkyl) into a more stable square-based pyramidal metallacyclobutane

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COMPUTATIONAL DETAILS

Calculations were performed on the (X)(Y)M(E)(CHCH3) (M = W or Mo; E = O, NPh,62 or NtBu; X = CH2CH3, pyrrolyl (Pyr), or OSiH3; and Y = OSiH3 or OPh) systems, which are models of the experimental tungsten oxo homogeneous31 and heterogeneous catalysts32 as well as the corresponding Mo and W imido ones14−17,19,20 (see Scheme 3 for labeling). All calculations were carried out with the hybrid B3PW91 density functional67,68 as implemented in the Gaussian09 package.69 The Mo, W, and Si atoms were represented with the pseudopotentials developed by the Stuttgart group and the associated basis sets enlarged with a

Scheme 2

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here have simplified ligands, which describe the electronic properties of the real ligands and for which dispersive interaction is likely to be small, as shown in the literature.78 Discussion of the results in terms of E and G allows a direct comparison of data with previous studies.64,65

Scheme 3

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RESULTS Structural Features. Representative metal-oxo alkylidene complexes of general formula (X)(Y)M(E)(CHR1), shown in Scheme 3, were modeled using M = Mo and W; E = O, NPh, or NtBu; Y = OPh or OSiH3; and X = CH2CH3 or pyrrolyl (Pyr), which are simplified models of the ligands found in homogeneous (X = HIPTO, Y = 2,4-dimethylpyrrolyl)31 and heterogeneous (X = CH2tBu, Y = silica surface)32 catalysts, respectively. We also investigated the cases where X = Y = OSiH3 because the corresponding disiloxy complexes are likely active sites in classical heterogeneous catalysts79 and have been prepared for the isoelectronic homogeneous catalysts with an imido ligand.80 All compounds present a pseudotetrahedral coordination at the metal with the following specific structural features (Table 1 and Tables S5 to S9): an almost coplanar arrangement of the Ooxo−M−Calk−H atoms (162° < Ω < 180° where Ω is the dihedral Ooxo−M−Calk−H angle) and a more acute angle for Ooxo−M−Cene (between 102.2° and 105.8°) than in an ideal tetrahedron (109.4°). The distortion away from the ideal tetrahedral structure is similar to that obtained with the other alkylidene complexes.56,60,65 The most open face containing the alkylidene ligand is trans to the stronger σ-donor X ligand (X = alkyl or pyrrolyl with Y = siloxy or phenoxy). Two isomers can be distinguished depending on the position of the alkylidene substituent with respect to the oxo group. The syn isomer with the methyl pointing toward the oxo group is characterized by a M−Cene−Hα angle ranging between 108.2° and 109.6°, smaller than the ideal value of 120°, which is indicative of a weak α-

polarization function.70−73 All other atoms (H, C, N, and O) were described by the 6-31G(d,p) basis sets.74,75 Geometry optimizations were performed without any geometrical constraint, and the nature of the extrema was verified by analytical calculation of the vibrational frequencies. The description of the results is based on electronic energies without including any zero-point energy corrections. The Gibbs energy profiles, calculated within the harmonic approximation of frequencies in standard conditions (P = 1 atm and T = 298 K), are analyzed in the Discussion section for better understanding of the nature of the rate-determining states, and they are provided in detail in the Supporting Information (Tables S1 and S2). The results with a computational method including the dispersion forces contributions are given in the Supporting Information (Tables S3 and S4). These contributions were calculated by means of the semiempirical Grimme’s correction76,77 at the B3PW91 optimized structures. Since dispersion forces have similar effects on both oxo and imido complexes, their inclusions do not have a significant influence on the results (see Tables 2 and 3 vs Tables S1 to S4). Furthermore, the systems investigated

Table 1. Selected M−L Bond Distances (Å) and L−M−L Angles (deg) for the Alkylidene Syn Isomers and Syn/Anti Energy Difference (ΔEsyn‑anti) and Interconversion Energy Barrier through Alkylidene Rotation (ΔE≠rot) (kcal mol−1) complexa

MCene

M−E

E−M−Cene

M−Cene−Hα

ΔEsyn‑antib

ΔE⧧rotc

W-N-1 W-N-1′ W-O-1 W-O-2 W-N-3 W-N-3′ W-O-3 W-O-4 W-N-5 W-O-5 Mo-N-1 Mo-N-1′ Mo-O-1 Mo-O-2 Mo-N-3 Mo-N-3′ Mo-O-3 Mo-O-4 Mo-N-5 Mo-O-5

1.894 1.899 1.900 1.899 1.892 1.894 1.895 1.897 1.894 1.896 1.874 1.878 1.882 1.882 1.869 1.872 1.878 1.878 1.872 1.876

1.752 1.741 1.700 1.700 1.748 1.735 1.698 1.699 1.746 1.696 1.725 1.717 1.675 1.676 1.725 1.710 1.673 1.674 1.724 1.669

103.8 104.1 105.6 105.8 102.7 103.3 104.5 104.8 101.6 102.9 102.6 103.1 105.2 105.4 101.5 102.3 104.2 104.6 100.5 102.2

106.1 105.7 109.2 109.5 105.8 104.6 108.2 109.3 107.0 109.0 105.7 105.4 109.3 109.6 105.4 104.2 108.9 109.2 106.5 109.1

1.6 1.5 1.5 1.3 1.8 1.7 1.5 1.2 2.0 1.9 1.4 1.2 1.4 1.2 1.6 1.5 1.3 1.1 1.9 2.0

16.8|21.4 17.5|23.0 22.7|27.9 22.7|27.8 17.4|18.0 18.2|19.3 24.2|25.5 23.9|24.8 15.4 21.6 17.1|21.4 18.0|23.3 23.6|28.8 23.8|28.9 18.2|18.5 19.2|19.9 26.1|26.9 26.0|26.5 16.3 23.4

a

See Scheme 3 for labeling definition. bA positive value means that the syn is more stable than the anti isomer. cThe two values refer to rotation of the alkylidene in two different directions. The first value refers to a rotation in which the CH3 substituent of the Cene atom becomes trans to X (X = alkyl or pyrrolyl), while the second one refers to the rotation in which the CH3 group becomes trans to the OR ligand. For W-N-5, W-O-5, Mo-N-5, and Mo-O-5 the two rotations have equal energy barriers. 6814


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Table 2. Energies (in kcal mol−1) Relative to the Separated Reactants (I + C2H4) of Intermediates and Transition States for the Reaction Pathway of Figure 1a,b

a

complexa

I + C2H4

TSI

II

TSII

TBP

TSIII

IV

TSIV

V


0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 1.4 −c −c 2.4 3.9 1.0 0.1 4.3 3.1 0.3 1.4 −c −c 2.7 3.6 1.0 0.1 5.0 3.5

−2.3 −0.7 −1.2 −2.9 −c −c −1.1 −c −c −c −1.0 0.5 −0.4 −1.8 −1.2 −0.6 0.0 −1.8 −c 1.2

−2.0 −0.7 1.2 −1.2 −c −c −c −c −c −c 0.7 1.5 4.8 2.5 −0.6 −0.6 0.7 −1.8 −c 3.1

−17.3 −15.5 −10.5 −12.2 −20.4 −17.6 −13.5 −16.8 −20.7 −14.6 −13.7 −12.0 −5.6 −7.2 −13.9 −11.8 −5.7 −9.1 −14.3 −6.4

1.5 3.9 4.2 2.4 0.2 3.3 0.9 −1.8 −c 1.1 4.4 6.5 7.5 5.9 3.4 6.1 5.2 2.7 2.2 5.6

−1.8 −c −2.7 −3.8 −0.5 1.9 −1.3 −2.0 −c −0.2 −0.1 −c −1.1 −2.0 1.5 3.3 0.4 0.2 1.7 1.9

−1.2 −c −c −c 2.0 3.7 −0.5 −c 3.2 0.9 0.2 −c −c −c 3.3 4.5 0.9 −c 4.9 2.6

−1.7 −2.1 −1.7 −1.6 −1.1 −1.4 −0.7 −0.8 −2.0 −1.7 −0.6 −1.1 −0.5 −0.4 0.1 −0.3 0.6 0.6 −0.8 −0.2

See Scheme 3 for labeling definition. bSee Figure 1a for intermediate and transition state definition. cNot located.

Figure 1. (a) Intermediates and transition states of the alkene metathesis pathway. (b) The optimized structures (distances in Å) of intermediates and transition states obtained for Mo-O-3 are given as a representative example for all other catalysts.

agostic interaction. The anti isomer, with the methyl group of the ethylidene pointing away from the oxo ligand, does not have such an α-agostic interaction and lies between 1.1 and 2.0 kcal mol−1 above the syn isomer. The syn/anti energy difference

for the oxo complexes is generally slightly lower than that computed for the equivalent imido complexes (Table 1),60,65 suggesting that the α-agostic interaction is slightly weaker for the oxo systems. The interconversion between these two 6815


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Table 3. Energies (in kcal mol−1) Relative to the Separated Reactants (I + C2H4) of the Intermediates and Transition States for the Pathway of Figure 2a

a

complexa

TBP

TSA


−17.3 −15.5 −10.5 −12.2 −20.4 −17.6 −13.5 −16.8 −20.7 −14.6 −13.7 −12.0 −5.6 −7.2 −13.9 −11.8 −5.7 −9.1 −14.3 −6.4

1.3 0.9 3.7 −0.2 −8.4 −8.4 −5.2 −8.1 −10.5 −7.4 6.2 5.7 7.9 4.0 −1.0 −1.2 2.6 −0.7 −3.0 0.4

SP (ESP − ETBP) −18.3 −18.6 −18.4 −19.0 −19.8 −20.0 −20.5 −22.0 −23.0 −23.2 −16.2 −16.4 −16.2 −16.8 −16.1 −16.4 −17.0 −18.8 −18.3 −18.7

(−1.0) (−3.1) (−7.9) (−6.8) (0.6) (−2.4) (−7.0) (−5.2) (−2.3) (−8.6) (−2.5) (−4.4) (−10.6) (−9.6) (−2.2) (−4.6) (−11.3) (−9.7) (−4.0) (−12.3)

TSBb

B

TSC

C

4.9 7.1 13.5 12.8 11.4 12.6 17.6 17.4 10.4 20.5 6.3 7.9 14.3 13.4 12.1 14.4 18.8 18.5 12.4 23.7

−10.7 −11.4 −9.8 −10.0 −8.6 −9.4 −8.2 −8.3 −9.4 −8.2 −9.6 −10.2 −8.9 −8.8 −5.4 −6.2 −5.2 −5.3 −5.0 −3.5

2.4 3.4 8.4 8.8 6.6 7.0 12.8 12.4 4.7 12.4 5.1 6.2 9.5 13.2 10.7 10.9 16.6 16.5 9.8 18.0

−27.0 −27.4 −29.0 −30.3 −30.5 −30.6 −31.1 −33.4 −33.3 −33.3 −23.4 −23.5 −26.1 −26.3 −24.4 −24.6 −25.3 −27.7 −26.5 −25.0

See Scheme 3 for complex definition and Figure 2a for intermediate and transition state definition. bSee note in reference list.81

Figure 2. (a) Intermediate and transition state of the byproduct formation and catalyst deactivation pathways. (b) The optimized structures (distances in Å) of the intermediates and transition states for Mo-O-3 are shown as a representative example for all other catalysts.

reported in Table 2, but often these four steps merge in two steps when entropic contributions are added (the alkene complex becomes less stable than the transition state for alkene coordination). We focus on the comparison between the metalimido and metal-oxo species using potential energy E; Gibbs energies will be discussed when necessary. For a given set of ligands, the initial alkene coordination (TSI) is associated with a transition state of lower energy for oxo complexes, while the [2+2]-cycloaddition/cycloreversion steps (TSII and TSIII, respectively) have transition states of higher energies. While the formation of the trigonal bipyramidal (TBP) metallacyclobutane is energetically favored relative to the separated

isomers via rotation of the alkylidene ligand has an energy barrier that ranges between 21.6 and 28.9 kcal mol−1 (Table 1), values that fall right in between those computed for Realkylidyne (29.7−36.1 kcal mol−1)56 and the M-imido (M = Mo, W) complexes (16.8−21.4 kcal mol−1).60,65 The syn−anti interconversion by rotation of the alkylidene group is thus expected to be slow to very slow at room temperature. Metathesis Pathway. As found for the Re alkylidyne and the group 6 imido complexes,57,62 in terms of potential energies, the metathesis pathway involves four elementary steps (Figure 1): alkene coordination, [2+2]-cycloaddition, cycloreversion, and alkene decoordination with energetics that are 6816


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position and byproduct formation for silica-supported catalysts have also been shown to occur via a unimolecular β-H transfer process.64,65 We have thus investigated the β-H transfer, which forms an allyl hydride intermediate, as well as the energetically favorable step, in which ethene inserts into the metal−hydride bond. The results for the oxo complexes are shown in Table 3 and compared to those obtained for the corresponding imido systems. The β-H transfer can take place via three pathways that differ by the nature of the ligand trans to the hydride ligand at the transition state: trans to the oxo ligand, the weakest σ-donor ligand, siloxy or phenoxy (Y), or to the alkyl or pyrrolyl ligand (X). Of the three different possibilities, all attempts to locate a transition state in which the β-hydride transfer occurs trans to the oxo ligand failed, as they evolve to a transition state in which H is transferred trans to Y. This is probably due to the strong trans influence of both H and the oxo ligand that disfavors them from becoming trans to each other. Within the other two possibilities, the transfer trans to Y (siloxy or phenoxy) is generally associated with a slightly lower energy barrier when compared to values for the process trans to X, but the differences are much less pronounced than those calculated for the imido species.65 The energies of the transition states for the hydride transfer trans to Y are reported in Table 3; the energies (E and G) and Cartesian coordinates for the other transfers are given in the Supporting Information. For M-oxo complexes, the β-hydride transfer energy barriers range between 30.2 and 44.0 kcal mol−1 with respect to the SP metallacycle, indicating that this process is significantly more energy demanding than the alkene metathesis pathway (TSIII at least 6.8 kcal mol−1 lower in energy than TSB). The highest values are obtained for tungsten complexes with an electronegative X ligand (with a decrease of energy barriers siloxy, pyrrolyl, and alkyl), in agreement with previous studies on Mimido species.65 The substitution of the imido ligand for an oxo group increases the energy barrier for the β-hydride transfer by at least 6.9 kcal mol−1, showing that the M-oxo metallacycles are less likely to engage in this reaction. The β-hydride transfer trans to Y leads also to hydride complexes that are more stable than those obtained with the hydride trans to X. The resulting hydride complex (B) has a square-based pyramidal geometry with an apical oxo group. Therefore, there is a vacant site trans to the oxo ligand perfectly adapted for the insertion of an incoming ethene into the basal M−H bond. This insertion step is equivalent to the reverse reaction of the previous step, β-H transfer, the only difference arising from the fact that it is here intramolecular and thus involves different entropic contributions. We focus in this section on the potential energies, which reflect in particular the differences between the oxo and imido complexes. The energies of the transitions states of the intermolecular insertion, TSC, are lower than the transition state for intramolecular insertion by 0.2 to 8.1 kcal mol−1, and they vary with the ancillary ligands in a similar manner to TSB (Table 3). Consequently, the difference in energy between the SP complexes and TSC in the case of the oxo complexes, which ranges from 17.9 to 22.0 kcal mol−1, is larger than that calculated for the imido complexes (13.1 to 16.1 kcal mol−1). The entropic contribution raises the transition state of the ethene insertion in a similar manner for the oxo or imido groups. As a result, the TSC transition state has a Gibbs energy slightly higher than that of TSB, the transition state of the β-H elimination, and is always higher for the oxo species than for the imido ones. The analogies between

reactants, it is less so for oxo than for imido complexes. These effects are obtained for any set of X and Y ancillary ligands and are thus determined by the oxo/imido ligand. For the metaloxo species, it then appears that the highest transition state corresponds to the [2+2]-cycloreversion step and not the alkene coordination or decoordination, in contrast to what was obtained for most of the metal-imido complexes. TBP−SP Interconversion of Metallacyclobutanes. While the TBP metallacyclobutane is the intermediate on the pathway of alkene metathesis, it can also interconvert into an SP isomer via a turnstile process.65 The SP metallacyclobutane, which is not on the metathesis pathway, is usually more stable than the TBP isomer. The TBP−SP equilibrium thus reduces the concentration in the TBP metallacycle and the productive rate of alkene metathesis. Therefore, the relative energies of these isomers and the energy barriers of the TBP to SP isomerization are important factors for the efficiency of alkene metathesis reaction.62,65 The energy characteristics of the TBP−SP isomerization are also important because the formation of the SP metallacycle opens the way to deactivation and byproduct formation (Figure 2).64,65 Table 3 shows the energies of the TBP and SP energies relative to the separated reactants as well as the energetics of the transition state for the TBP−SP conversion (TSA). In all cases, the SP metallacyclobutane is significantly more stable than the corresponding TBP isomer by 6.8 to 11.3 kcal mol−1. It is noteworthy that this energy difference is larger in oxo systems than that in the corresponding M-imido (M = W and Mo) and Re-alkylidyne complexes, for which the TBP−SP relative stabilities vary between −0.6 (TBP more stable) and 7.0 kcal mol−1 for Mimido (Table 3) and between 2.2 and 7.7 for Re-alkylidyne.62,65 For a given X,Y ligand pair, the stability of the SP isomer with respect to the separated reactants does not depend on the nature of E (oxo, aryl imido, or alkyl imido), and thus the TBP−SP energy difference is mainly driven by the destabilization of the TBP isomer with respect to the reactants. Moreover, the M−oxo and M−Y bond distances, which lengthen by about 0.02 and 0.06 Å, respectively, on going from the initial alkylidene to the TBP intermediate, shorten by almost the same amount in the SP isomer when comparing with the TBP metallacycle (Figure 2), trends that were already observed for the M-imido complexes. The interconversion between the TBP and SP isomers involves a turnstile process, associated with energy barriers ranging from 6.8 to 14.2 kcal mol−1. The highest values of the energy barrier are obtained when X is a strong σ-donor such as alkyl vs pyrrolyl or siloxy. It is noteworthy that the TBP−SP interconversion has a lower energy barrier than the cycloreversion step, with the exceptions of Mo-N-1 and Mo-O-1. As a consequence, the TBP to SP interconversion is faster than (or competitive with) the productive metathesis process, and the reaction rates for metathesis are thus, in good part, determined by the stability of the SP isomer and the energy barrier for cycloreversion. Overall, comparing oxo and imido complexes, the former should display slightly lower activity because the energy difference between the highest transition state (here cycloreversion) and the more stable intermediate (SP metallacyclobutane or separated reactants) is calculated to be slightly higher. Catalyst Decomposition and Byproduct Formation. While the deactivation process can be dominated by dimerization for molecular catalysts,11,82,83 catalyst decom6817


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Figure 3. Energy and Gibbs energy profiles in parentheses (in kcal mol−1) for the alkene metathesis (left) and byproduct and catalyst deactivation (right) pathways for the reaction of Mo-N-3 (blue curve) and Mo-O-3 (red curve) complexes with ethene.

the key point is that using either the potential or the Gibbs energies, the oxo complexes are predicted to be slightly less active than the corresponding imido catalysts. The entropic contribution destabilizes the β-H transfer in a similar manner as it does the metallacycle and the cycloreversion transition state. However, the entropy disfavors further the intermolecular insertion of ethene in the M−H bond (via TSC), making this step highly disfavored. This intermolecular reaction becomes the rate-determining step for deactivation in most cases, which is consistent with available experimental information showing that the deactivation of silica-supported imido catalysts and of some of the related oxo systems is first order in ethene.64,85 The entropic contribution is similar for the imido and oxo complexes, which leads to similar conclusions when using either the potential energies or the Gibbs energies for comparing the reactivities of the two sets of complexes. A consequence of the parallel behaviors of potential energies and Gibbs energies as a function of oxo/imido groups is that the difference in the reactivity of the oxo/imido alkylidene complexes in alkene metathesis originates from their electronic properties. Consequently, the potential energy profiles will be used hereafter for analyzing the influence of the E group (oxo vs imido) in the alkene metathesis productive pathway and in the catalyst deactivation. This is necessary for carrying out an energy-partitioning scheme of the energy barriers. Alkene Metathesis and Deactivation Pathways for the Metal-Oxo and Metal-Imido Complexes. The effects induced by the imido for oxo substitution are (i) the alkene coordination transition step, which is slightly more favorable for the oxo systems; (ii) the [2+2]-cycloaddition and cycloreversion steps, which are slightly disfavored for the oxo complexes; and more importantly (iii) a strong destabilization of the TBP metallacyclobutane intermediate for the oxo species, while the SP metallacycle isomers have similar stability with respect to separated reactants and (iv) a strong destabilization of the transition state for the β-hydride transfer

TSB and TSC are also visible in their geometries. In both transition states the M−C bond that is cleaved (TSB) and formed (TSC) is essentially trans to the oxo or the imido ligand, hence the higher energies of TSB and TSC for the former. Overall, the β-H transfer, which opens the channel for the deactivation of the catalyst, and the ethene insertion are associated with higher energy barriers for the oxo than for the equivalent M-imido species, which suggests that the formation of side products and catalyst deactivation via this pathway are more disfavored for oxo systems. This is consistent with experimental data on silica-supported isostructural complexes.32,33

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DISCUSSION Figure 3 shows the energy and Gibbs energy profiles for the metathesis (entry channel) and deactivation pathways with the molybdenum monoalkoxy pyrrolyl oxo (Mo-O-3) and imido (Mo-N-3) complexes as selected examples. The reaction starts by association of two molecules, and the molecularity is maintained for all extrema but TSC and C, where an additional molecule (ethene) is needed to describe the reaction pathway. As a consequence, adding the entropic contribution destabilizes the energy of all extrema by about 10 to 15 kcal mol−1 relative to the separated alkylidene complex I and ethene with the exception of TSC and C, which are destabilized by about 20 to 28 kcal mol−1.84 This leads to metallacycles that are not more stable than the separated reactants and are not necessarily isolable intermediates. In the case of molybdenum particularly, the metallacycles are less stable than the separated reactants, which is consistent with the fact that they are typically not detected as intermediates. The consequence for the catalytic cycle of the metathesis reaction is that the largest difference in energy between the lowest energy intermediate and the highest energy transition state is not necessarily that between SP and the TS for cycloreversion, but could be that between the separated reactants and the TS for cycloreversion. In all cases, 6818


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as well as that of the subsequent alkene insertion for the oxo species, the two key processes in the byproduct formation and catalyst deactivation. Overall, this would lead to a slightly less active but more stable catalyst, as evidenced by a larger energy difference between the lowest intermediate (SP resting state or the separated reactants) and the highest transition state for the alkene metathesis (cycloreversion) and the much higher energy barrier for deactivation (β-hydride and ethene insertion process). a. Ethene Coordination and Metallacycle Formation. The coordination of the alkene (ethene) occurs with barriers that are generally lower than with the imido ligands. At the coordination/decoordination step, the energy barrier is controlled by the energy needed to distort the metal complex (Edist‑M) to open a vacant site for the coordination of the incoming alkene (Table 4) combined with the interaction

Scheme 4

having the oxo ligand in the equatorial plane together with the alkylidene and Y at the TS for ethene coordination, suggesting that the oxo ligand has a larger trans influence than the imido ligand.62 This destabilization is, however, fully compensated by a larger interaction energy between the metal fragment and the alkene (−1.9 and −2.8 kcal mol−1 for the oxo vs −0.6 and −1.6 kcal mol−1 for the imido complexes), which is consistent with a stronger Lewis acidic metal center for the alkylidene oxo complexes, as evidenced by the higher positive charge on the metal (see Tables S10 to S11 in the Supporting Information). Therefore, the larger electrophilicity of the metal center seems to be a key factor controlling the small decrease of the energy barrier for alkene coordination in the oxo systems when comparing with the imido species. For the [2+2]-cycloaddition and cycloreversion steps, where the alkylidene ligand is lost or formed again, the associated transition states are higher relative to the separated reactants for the oxo than for the imido complexes. This result can be traced back to the stronger alkylidene π-bond, as already suggested by the higher alkylidene rotation energy barrier (vide supra). The TBP metallacycle intermediate has the Y ligand trans to the E group (oxo or imido) and the X ligand and the two M−C bonds of the metallacycle ring in the equatorial plane. The energies of the oxo TBP metallacycles are lower than the sum of the energy of the separated reactants (I + C2H4) for any set of X and Y, but the difference in energy with respect to separated reactants is smaller than that for the corresponding imido analogues. This can be related to two factors: (i) the higher positive charge at the metal center in oxo systems (around +0.2 e− compared to the imido complexes), which strengthens all metal−ligand bonds,86 thereby enhancing trans influences, and (ii) the oxo group itself has a larger trans influence than the imido group. This was not expected in view of the higher electronegativity of oxygen compared to nitrogen, but it is consistent with previous finding concerning the weaker agostic interaction and the higher distortion energy of the pseudotetrahedral alkylidene intermediate for alkene coordination (vide supra). The importance of the trans influence of the E ligand can be further probed by comparing the energy of formation of the TBP metallacycle relative to the separated reactants (I + C2H4) for an alkyl imido ligand system (R = tBu, −11.8 to −17.6 kcal mol−1), an arylimido (R = aryl, −13.7 to −20.4 kcal mol−1), and an oxo complex (−5.6 to −13.5 kcal mol−1). The TBP metallacycle is less stable with an alkyl imido than with the aryl imido, a weaker electron donor, which indicates that the low energy of formation of the metallacycle with an oxo ligand is, at least in part, due to its stronger trans influence. This effect disappears at the SP metallacycles, because the axial oxo or imido is trans to an empty coordination site. This rationalizes why the energy of formation of the SP metallacycle is similar for the oxo, alkyl imido, and aryl imido ligands. This also translates into similar M−E bond distances in

Table 4. Energy Partitioning Scheme (in kcal mol−1) for the Analysis of the Energy Barrier of Ethene Coordination (ΔE≠): Distortion Energies of the Metal Fragment (ΔEdist‑M) and Ethene (ΔEdist‑(C2H4)) and the Interaction Energy between the Two Fragments at the Transition State (ΔEint) complexa

ΔE⧧

ΔEdistM

ΔEdist(C2H4)

ΔEint

∑αb


0.1 1.4

1.5 2.0

0.0 0.0

−1.4 −0.6

2.4 3.9 1.0 0.1 4.3 3.1 0.3 1.4

3.9 5.1 3.7 1.6 5.9 5.7 1.6 2.2

0.0 0.0 0.1 0.3 0.1 0.1 0.0 0.0

−1.5 −1.2 −2.8 −1.9 −1.7 −2.7 −1.3 −0.8

2.7 3.6 1.1 0.1 5.0 3.5

4.0 4.9 3.8 2.3 6.6 6.2

0.0 0.0 0.1 0.0 0.0 0.1

−1.3 −1.3 −2.8 −2.2 −1.6 −2.8

311.9 310.3 310.9 309.5 324.0 324.2 323.3 320.3 333.4 331.2 310.2 309.7 309.9 308.5 324.8 324.7 320.3 318.7 333.4 331.4

See Scheme 3 for labeling definition. bSum of the three Lb−M−Lp angles in the alkylidene complex (Scheme 4). a

energy (Eint) between the alkene and metal fragment in their respective geometry at the transition states.57,62 The calculations show that the energy to distort ethene (Edist‑(C2H4)) at the transition state is small and that the metal···ethene distance is larger than 3 Å. The long metal−ethene distance indicates a weak interaction between the two fragments. This interaction is limited to the donation of the electron density from the ethene π-orbital to a low-lying empty d metal orbital of appropriate directionality since a d0 metal is not involved into any backdonation to the ethene. The distortion and interaction energies at the TS of ethene coordination are given in Table 4. The distortion energy is slightly higher for the oxo than the imido complexes despite a structure of the starting alkylidene slightly closer to the TS in the case of the oxo complexes. This probably originates from 6819


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intermediate or separated reactants (lowest intermediates) and the TS for cycloreversion (highest transition state), which is only slightly higher for oxo than for imido, accounting for the slightly lower activity of the former. It is noteworthy that the channel for deactivation and byproduct formation, which is initiated from the SP metallacycle, is essentially shut down in the case of the oxo complex because of the higher energy barrier associated with the key β-H transfer as a result of the stronger trans influence of the oxo ligand. Thus, even though the oxo may not improve the activity of the catalyst, it can still lead to a more efficient catalyst because it shuts down deactivation and byproduct formation via β-H transfer. This is consistent with experimental evidence for welldefined silica-supported tungsten systems for which the oxo catalysts32 are more efficient than the corresponding imido systems.33 However, as suggested by the recent work of Schrock et al.,31 one of the main challenges is to generate stable molecular oxo systems, because they are probably more susceptible to bimolecular decomposition facilitated by the decrease in steric protection (the oxo is smaller than the imido ligand). Additionally, it may be worth reiterating that if it were possible to stop the formation of SP metallacyclobutane, oxo complexes could in principle become more active than the imido analogues. Both arguments speak for the development of metal-oxo alkylidene with larger ligands and in particular supported complexes, which avoid bimolecular decomposition.

I and the SP isomer and an increase of this bond length by ca. 0.02 Å in the less stable TBP isomer. b. Selectivity and Catalyst Deactivation. The energy barrier for the β-H transfer at the SP metallacycle, one of the key elementary steps for byproduct formation and catalyst deactivation, is raised by at least 4.8 kcal mol−1 for the metaloxo systems relative to the analogous imido complexes. This effect originates from the same two factors that control the destabilization in the TBP isomer. The larger positive charge at the metal center (around + 0.2 e− in the SP) for metal-oxo systems leads to stronger metal−ligand bonds, which disfavors the decomposition of the metallacyclobutane via β-H transfer. At the transition state TSB, which has a quasi-octahedral coordination, the M···C bond that is cleaved during the formation of the allyl hydride B is almost trans to the E group (Figure 2). The geometrical parameters show that the M···C interaction (M···C distance ranging from 2.3 to 2.4 Å) is still present at the transition state, whose associated energy is thus raised by having a ligand with larger trans influence like oxo. This also explains why the activation barrier for this process is also higher for alkylimido (N-tBu) vs aryl imido (N-Ph). It is also consistent with an increase of reaction barrier for the subsequent ethene insertion step for the oxo system since the alkene coordinates and inserts trans to the E ligand. It is remarkable that overall the energy differences between the TBP metallacyclobutane and the transition state for βhydride transfer are almost not affected by the nature of E (E = oxo and imido). This suggests again that for both cases the destabilization results from the presence of the oxo ligand trans to another ligand. This destabilizing effect is not present at the key transition state of the alkene metathesis process ([2+2]cycloaddition/cycloreversion), because there is no ligand really trans to the E group.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 to S4, containing the Gibbs energies and G + D values of all intermediates and transition states associated with the alkene metathesis and deactivation pathways. Tables S5 to S9, reporting the M−L bond distances and L−M−L angles of the initial alkylidene complexes. Tables S10 and S11, with the NPA charges and AIM analysis for the alkylsiloxy, pyrrolyl siloxy, and bisiloxy imido and oxo systems. List of Cartesian coordinates and absolute energies and Gibbs energies (au) of all extrema. This material is available free of charge via the Internet at http://pubs.acs.org

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CONCLUSION According to DFT calculations, oxo and imido alkylidene d0 Schrock alkene metathesis catalysts (X)(Y)M(E)(CHR1) have similar structures. They both have pseudo-tetrahedral coordination around the enantiotopic metal center, in which the face trans to the strong σ-donor X ligand is more opened and is ready to coordinate the alkene. Oxo and imido catalysts also share the same reaction pathways for alkene metathesis with similar potential energy surfaces (PES), the main difference residing in the fact that, for the oxo ligand, coordination/decoordination of the substrate is slightly easier. However, the transition states for the [2+2]-cycloaddition and cycloreversion, which are slightly higher in energy for the oxo than for the imido complexes, become the highest transition states of the metathesis pathway (Figure 3). This can be traced back to the presence of a more electrophilic metal center for oxo complexes, which facilitates alkene coordination, and to a stronger π-alkylidene MC bond, which disfavors the [2+2]cycloaddition step. This leads to a rather unstable TBP metallacyclobutane intermediate by comparison with the imido catalysts because the oxo ligand, which has a stronger trans influence, is trans to a ligand (Y). However, this intermediate is connected to a much more stable square-based pyramidal metallacycle (SP) of similar energy for oxo and imido complexes since, in this geometry, no ligand is trans to the E ligand. This TBP−SP interconversion takes place via a turnstile process with an energy barrier lower than that for cycloreversion in most cases, so that the activity of the catalyst is determined by the difference in energy between the SP

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.S.M. is thankful for the support of the Generalitat de Catalunya (Project SGR2009-638) and the MEC/MICINN of Spain (CTQ2011-24846/BQU). O.E. thanks the CNRS and the Ministère de la Recherche et de l’Enseignement Supérieur for funding. X.S.M. and C.C. thank the ETH for funding.

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Organometallics

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