Structural and Mechanistic Basis of the Fast Metathesis Initiation by

Article pubs.acs.org/Organometallics

Structural and Mechanistic Basis of the Fast Metathesis Initiation by a Six-Coordinated Ruthenium Catalyst Bartosz Trzaskowski*,†,‡ and Karol Grela† †

Faculty of Chemistry, University of Warsaw, 02-093 Warszawa, Poland Centre of New Technologies, University of Warsaw, 02-089 Warszawa, Poland

‡

S Supporting Information *

ABSTRACT: The N-heterocyclic carbene coordinated ruthenium benzylidene complex [(H2IMes)(PCy3)(Cl)2RuCHPh], a highly active olefin metathesis catalyst, is known to convert into a very fast initiator in the presence of pyridine or 3-bromopyridine. Computational studies presented in this work reveal the mechanistic basis of this phenomenon. Depending on the size of the olefin, the reaction follows either a dissociative or associative mechanism.

ost current efforts in the field of ruthenium-based complexes for olefin metathesis have focused on finding new applications and answering some elusive questions regarding the mechanism. The mechanism of rutheniummediated olefin metathesis catalyzed by first- and secondgeneration five-coordinated Grubbs complexes has been extensively studied by both experiment1−3 and computational modeling.4−8 From these studies it is generally accepted that the initial dissociation of the phosphine ligand is the ratedetermining step for the whole reaction of second-generation Grubbs complexes. The five-coordinated Grubbs−Hoveyda complexes have also been known for a long time, but only very recently has their initiation mechanism been proposed. On the basis of experimental work by Plenio et al.9,10 and computational investigation by Ashworth et al.11 and Nuñez-Zanur12 it has been suggested that an interchange mechanism involving simultaneous alkoxy dissociation and olefin binding is the most plausible option. It is, however, worth noting that the three possible mechanisms of initiation recognized in these studies (dissociative, interchange, and associative) are a general feature of all Grubbs-like complexes. In 2002 Grubbs et al. used the benzylidene complex [(H2IMes)(PCy3)(Cl)2RuCHPh] (1a) to obtain the sixcoordinated pyridine-modified catalyst [(H 2 IMes)(py)2(Cl)2RuCHPh] (2) or 3-bromopyridine-modified (3) system (see Figure 1).13 They showed that this synthetically accessible system is able to perform acrylonitrile crossmetathesis with high efficiency. They also noted that this species was one of the fastest initiators available at that time, with the initiation rate constant of the fastest, 3-bromopyridinemodified system being at least 3 orders of magnitude larger than the initiation rate constant of the five-coordinated Grubbs and Grubbs−Hoveyda catalysts.13 Interestingly, since then no effort has been undertaken to explain the impact of the sixth

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© XXXX American Chemical Society

Figure 1. Ruthenium-based olefin metathesis catalysts studied in this work. Mes = 2,4,6-trimethylphenyl.

ligand complexing Ru on the mechanism of the initiation of Grubbs-type systems. A similar approach of pyridine tuning has been also recently used to obtain high-efficiency catalysts for ring-closing metathesis and cross-metathesis, showing a high potential for this methodology.14 Grubbs et al. estimated the lower limit of the initiation rate constant of 3 to be >4 s−1 at 5 °C. We have used the wellknown Eyring equation for estimating the Gibbs free energy of the transition state from the reaction rate constant k and universal rate constant k*: ΔG* = − RT loge(k) + RT loge(k*)

(1)

At 278 K this equation can be rewritten as ΔG* = − 1.27 log10(k) + 16.22

(2)

Using this equation, we can estimate the upper limit of free energy of initiation reaction of 3 as 15.45 kcal/mol, which is significantly lower than the ΔG* value for 1 (equal to 19.6 ± 0.3 kcal/mol)1 as well as five-coordinated Grubbs−Hoveyda Received: March 19, 2013

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dx.doi.org/10.1021/om400233s | Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Dissociative initiation mechanism for (a) 1a and (b, c) 2 or 3. R = H for 2; R = Br for 3.

catalysts, which have the usual values of ΔG* equal to ∼20 kcal/mol.11 Figure 2 shows the mechanism of olefin metathesis for a fivecoordinated Grubbs-type system and two possible dissociative mechanisms for the corresponding six-coordinated catalysts. Grubbs et al. noted that the binding of pyridine and its derivatives in 2 is rather weak;15 therefore, it may be possible that both five-coordinated and six-coordinated systems converge to the same structure immediately after the initiation, even before the olefin attack takes place. In such a scenario the difference in the rate of the initiation comes from the different rates of dissociation of P(Cy)3 or P(Ph)3 and one or two pyridine moieties. It is, however, also possible that a different mechanism is responsible for the fast initiation of the six-coordinated system. Due to the limited space around the Ru metal center in 2 or 3 the first step is identical to path c from Figure 2, where one of the pyridine molecules dissociates. In the second step, however, the resulting five-coordinated system complexing only one pyridine is susceptible to olefin attack. This attack can occur either via one-step interchange, where the olefin attack occurs simultaneously with pyridine dissociation (Figure 3a), or a twostep reaction of olefin association followed by pyridine dissociation (Figure 3b). The interchange mechanism hypoth-

esis for these catalysts is similar to the interchange mechanism of Grubbs−Hoveyda complexes described earlier.11 In this work we have used density functional theory (DFT) calculations to study all possible pathways of the initiation mechanism of investigated complexes and show the impact of the additional ligand on the energetic profile of the reaction. In the computational part of this study we used a well-established computational protocol. To summarize the most important points of the calculations: we have used an all-atom model for the catalyst and an ethylene molecule to model the substrate of olefin metathesis. Starting models for systems 1−3 were prepared on the basis of available crystal structures of Grubbslike catalysts as well as the recently synthesized six-coordinated Hoveyda-like catalysts.14 To compensate for the possible steric effects, we also repeated all calculations using a trans-2-butene molecule instead of ethylene. The main analysis of the results is performed for the former case, while the 2-butene results are mentioned for cases where the results are significantly different. We have modeled the initiation step of studied systems using the B3LYP geometries with the 6-31G** basis set for all atoms except the Ru atom, which was described by the Los Alamos angular momentum projected effective core potential (ECP) using the double-ζ contraction of valence functions (denoted as LACVP**).16 We have used the energy convergence criterion of 5 × 10−5 hartree. Solvation energies were calculated using B


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Figure 3. Alternative initiation mechanism for the studied Grubbs catalysts, which can follow (a) the interchange mechanism or (b) the associative mechanism. R = H for 2; R = Br for 3.

complex is approximately 20 kcal/mol higher in energy than the substrate.4 In that work, however, no entropic effects were taken into account. In 2005 Lippstreu and Straub showed that for the [(H2IMes)(PMe3)(Cl)2RuCH2] system the dissociation of PMe3 requires 9.3 kcal/mol (ΔG calculated at the B3LYP/LACV3P**+//B3LYP/LACVP* level with no solvent effects).5 Newer B3LYP/6-31G* calculations give a value of 15.8 kcal/mol for 1a,20 which is relatively close to the experimental values.1 The most recent computational study utilizing the BLYP-D density functional gives a value of 23.7 kcal/mol for the activation of 1a and 17.8 kcal/mol for the activation of 1b.21 We obtained the values of 21.4 kcal/mol for 1a and 19.2 kcal/mol for 1b (see the Supporting Information), which are also in very good agreement with the experimental data. For the third-generation complex [(H2IMes)(py)2(Cl)2Ru CHPh] (py = pyridine, 3-bromopyridine) the associative mechanism is impossible, due to lack of space around the Ru atom for olefin attachment. Nevertheless, there are four possibilities for the initiation mechanism: two employing only dissociation and two with an interchange step. In the first one, the initiation follows via simultaneous dissociation of both pyridine or 3-bromopyridine molecules, forming the 14electron complex (Figure 2b). For both systems this is an endergonic process with ΔG = 8.3 kcal/mol for 2 and ΔG = 9.0 kcal/mol for 3 (see the Supporting Information). Attachment of the olefin (modeled as ethylene) requires an additional 4.8 kcal/mol, consistent with the 5.3 kcal/mol value obtained earlier.22 This results in a total value of 13.1 kcal/mol for the ΔG value of the initiation of 2 and ethylene and 13.8 kcal/mol for 3 and ethylene. Since the olefin attack occurs here after pyridine dissociation, the size of the substrate does not affect much the energetics of this reaction path.

the Poisson−Boltzmann self-consistent polarizable continuum method as implemented in Jaguar v.7 (Schrodinger, 2011) to represent dichloromethane (dielectric constant 8.93 and effective radius 2.33 Å) or toluene for system 1 (dielectric constant 2.38 and effective radius 2.76 Å). The solvation calculations used the B3LYP/LACVP** level of theory and the gas-phase optimized structures. For each structure frequencies were calculated to verify the nature of each stationary point. Recent calculations have established that the M06 class functional17 leads to activation energies a few kilocalories per mole more accurate than those of B3LYP.18 This method was also shown to perform particularly well for ruthenium-based catalysts, giving accurate energies for a number of Grubbs and Grubbs−Hoveyda systems.7,18 As a result, single-point energy calculations were performed using the M06 functional with a larger basis set: here Ru was described with the triple-ζ contraction of valence functions augmented with two f functions, and the core electrons were described by the same ECP; the other atoms were described with the 6-311++G** basis set. To verify that the results are not dependent on the functional, we also recalculated selected points along the reaction coordinate using the new dispersion-corrected B3LYPD3, BLYP-D3, and PBE-D3 functionals19 and the same 6-311+ +G** basis set. All energies discussed in this work are free energies, calculated as the sum of electronic energy, solvation energy, zero-point energy correction, thermal correction to enthalpy, and the negative product of temperature and entropy (at 298 K). For the sake of comparison we present the theoretical results for the second-generation Grubbs catalysts. First, it was suggested that for the similar first-generation Grubbs complexes PCy3 and PPh3 dissociation is endothermic and proceeds without any enthalpic barrier, and the resulting 14-electron C


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Figure 4. Full initiation pathway for compounds 2 and 3 with an ethylene molecule.

Figure 5. Full initiation pathway for compounds 2 and 3 with a 2-butene molecule.

coordinate for 3 confirm the finding that it is easier to dissociate the trans pyridine from the complex and the corresponding transition state of trans pyridine dissociation is approximately 2−3 kcal/mol lower in energy than for the cis dissociation (see the Supporting Information). During the dissociation process of any of the pyridines, however, the second pyridine changes its position in such a way that it is bound between the trans and cis positions, with a Ru− N(pyridine) distance of 2.26 Å and a C(carbene)−Ru− N(pyridine) angle of 166° (the trans pyridine of 3 has the corresponding angle equal to 103.1°, while that for the cis

A second, similar possibility is a mechanism where the pyridine ligands dissociate nonsimultaneously, with the intermediate structure [(H2IMes)(py)(Cl)2RuCHPh] (see Figure 2c). The overall ΔG value of this reaction is obviously identical with that of the previous case, but the intermediate structures are interesting from a mechanistic and energetic point of view. In both 2 and 3, the pyridine moiety located in a position trans to the benzylidene group has a longer Ru− N(pyridine) bond than the second pyridine by 0.2 Å (2.49 Å versus 2.29 Å), suggesting that it is only weakly bound to the complex. Potential energy scans along the Ru−N(pyridine) D


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followed by olefin association, is the most favorable energetic path for an ethylene molecule and may be plausible for certain substrates. Since the differences in energies are relatively small, it is also possible that, for some systems, the reaction may simultaneously follow two parallel pathways, similarly to other selected Grubbs−Hoveyda-type complexes.10,23 For both reaction paths the free energy barrier for this reaction agrees well with experimental data and is able to explain the high, measured values of the rate constants of 2 and 3, in comparison to 1. This work shows that ruthenium-based catalysts may be tuned not only by introducing small structural changes to their structures (e.g., electron-donating or electronwithdrawing groups)24 but also by adding new ligands/ molecules to the first coordination center of ruthenium. Such large structural changes may completely change the mechanism of the reaction and lead to lowering ΔG to yield faster and more efficient metathesis reactions. A similar computational approach using as presented in this work may be used to facilitate a cost-efficient design of a novel ligand system for ruthenium complexes as metathesis catalysts.

pyridine is 177.3°). From an energetic point of view, dissociation of the first pyridine from 2 is almost isoenergetic, while dissociation of the first 3-bromopyridine from 3 requires 4.3 kcal/mol. Since the intermediate structure with one pyridine or 3bromopyridine moiety is reasonably stable, one can suggest that further steps of initiation follow a different, nondissociative mechanism. Hence, there are two more possibilities of the initiation mechanism. One of the possibilities is an interchange mechanism, where the dissociation of the pyridine (or its derivative) occurs simultaneously with olefin attack (Figure 3a). In the second scenario the reaction occurs via a two-step rearrangement, where the olefin attack/association occurs first, followed by pyridine or 3-bromopyridine dissociation. DFT calculations suggest that when an ethylene molecule is used as the substrate model the second scenario is more plausible from an energetic point of view, since its free energy barrier is ∼2−3 kcal/mol less than for the interchange case (Supporting Information). The rate-determining step of the whole initiation is the last step of this reaction, namely pyridine or 3bromopyridine dissociation (see Figure 4), which proceeds via a transition state. The obtained values of 11.4 kcal/mol (for 3) and 12.4 kcal/mol (for 2) are within the expected range of the ΔG of the limiting step of this reaction. It is also interesting to note that the ΔG of this particular step is smaller for 3bromopyridine with respect to pyridine (by 1.0 kcal/mol), which could explain the faster initiation of the former compound, as seen experimentally (>4 s−1 for 3 versus >0.2 s−1 for 2, both at 5 °C).13 It is, however, important to notice that replacing the ethylene molecule by the larger 2-butene molecule has a non-negligible impact on the energetics of some reaction paths. We should expect that the steric effects caused by the size of the 2-butene or any larger olefin may strongly affect those transition states, in which the substrate can interact with the pyridine moiety: namely the interchange mechanism (Figure 3a) and the twostep rearrangement (Figure 3b). This is indeed the case, and in both cases the transition states with a 2-butene molecule are now 2−5 kcal/mol higher on the free energy scale. As a result, both reaction paths now have higher energy barriers than the reaction path where both pyridines dissociate to form the active complex (Figure 5). The obtained energy barriers for this reaction path, equal to 14.4 kcal/mol for 3 and 13.7 kcal/mol for 2, are also in perfect agreement with the expected values. Results obtained using the B3LYP-D3, BLYP-D3, and PBE-D3 functionals, although giving slightly higher energy barriers, are consistent with these conclusions (see the Supporting Information). In conclusion, compounds 2 and 3 are interesting examples of tuning the electronic properties of a ruthenium complex to obtain a high-efficiency olefin catalyst, and our calculations show that the DFT approach is able to accurately predict the mechanism of its initiation. The attachment of the additional electron-deficient ligand to the Ru-based complex results in a very fast catalyst initiation. This phenomenon may be fully explained by proposing two possible reaction mechanisms of [(H2IMes)(py)2(Cl)2RuCHPh] (py = pyridine, 3-bromopyridine) catalyst, which may occur depending on the size of the substrate. DFT calculations suggest that the initiation occurs most likely via the dissociation of both pyridine (or 3bromopyridine) moieties to give the active 14-electron complex. The three-step mechanism which starts with dissociation of one pyridine or 3-bromopyridine moiety,

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ASSOCIATED CONTENT

S Supporting Information *

Schematic representations of initiation pathways of 1a,b, 2, and 3 and tables giving calculated DFT energies and coordinates. 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 B.T.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Centre for Research and Development (grant SImPLeCAT to B.T.) and Foundation for Polish Science (Team Program cofinanced by the EU European Regional Development Fund, Operational Program Innovative Economy 2007-2013 to K.G.). All calculations were performed at the Interdisciplinary Center for Mathematical and Computational Modeling (University of Warsaw) under a G539 computational grant.

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REFERENCES

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Structural and Mechanistic Basis of the Fast Metathesis Initiation by

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