Article pubs.acs.org/Organometallics
Sterically Driven Olefin Metathesis: The Impact of Alkylidene Substitution on Catalyst Activity Justin A. M. Lummiss,† Frédéric A. Perras,† Robert McDonald,‡ David L. Bryce,*,† and Deryn E. Fogg*,† †
Centre for Catalysis Research & Innovation and Department of Chemistry and Biomolecular Sciences, University of Ottawa, Ottawa, ON, Canada, K1N 6N5 ‡ X-ray Crystallographic Laboratory, Department of Chemistry, University of Alberta, Edmonton, AB, Canada, T6G 2G2 S Supporting Information *
ABSTRACT: The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solidstate NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl2(H2IMes)(PCy3)(CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl2(PCy3)2(CHR) (GI, GIm). The compiled rate constants for PCy3 dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm′). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The 31P chemical shift (CS) tensors for the PCy3 ligand exhibited the expected changes (i.e., those characteristic of an increased Ru−P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the 13C CS tensors. Whereas calculations on truncated models predict significant differences in 13C CS tensor values for GII compared with GIIm, the experimental values are equivalent, implying a compensating effect that weakens the RuC interaction in the benzylidene complex. Published X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane by steric interactions with the benzylidene phenyl group, an effect absent in GIIm and GIm. During PCy3 loss from the [Ru]CHPh systems, established processes of alkylidene rotation transform Ph−Cl repulsion into Ph−PCy3 repulsion. Displacing the PCy3 ligand below the plane does not relieve this conflict, instead incurring steric interactions with the H2IMes ligand. Enhanced PCy3 lability in the benzylidene complexes, relative to their methylidene analogues, is hence proposed to originate in the steric pressure exerted by the Ph substituent.
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INTRODUCTION
common catalyst in current use for ring-closing metathesis (RCM)8 and continues to be widely employed for crossmetathesis (CM).9 A long-recognized, perplexing feature in this system is the dramatically reduced phosphine lability that characterizes the resting-state species, methylidene complex GIIm, relative to its benzylidene parent GII.10,11 Following development of highyield routes to GIIm,12 we were able to quantify this difference. Thus, loss of PCy3 from GIIm is 276 times slower than from GII, while for the corresponding IMes complexes, the difference increases by nearly an order of magnitude13 (see later). Because olefin binding occurs only after rate-determining loss of PCy3,10,13−18 the low lability of the methylidene species severely inhibits re-entry into the catalytic cycle, causing GIIm to accumulate during catalysis. Electronic contributions to metal−ligand bonding are central to the low lability of the PCy3 ligand in the second-generation Grubbs catalysts, relative to the first-generation catalyst GI. The strong σ-donor properties of the N-heterocyclic carbene (NHC) ligand,19−25 coupled with the π-acceptor capacity of the PCy3 ligand,26 play a key role in attenuating PCy3 lability in
The Grubbs metathesis catalysts (see GI, GII; Chart 1) have had a seminal impact on organic synthesis.1−3 Hundreds of variants on the original structure have now been reported,1,4 in a quest for improved catalyst properties. Notwithstanding the growing industrial importance of some of these derivatives,5−7 the second-generation Grubbs catalyst GII remains the most Chart 1. Trend in PCy3 Lability for the Grubbs Metathesis Catalysts and Their Methylidene Derivatives
Received: December 3, 2015
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.5b00984 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Scheme 1. Synthesis of 13C-Enriched *GIIm via CrossMetathesis (CM) of GII with β-13C-Styrene
GII and GII′.13,27 Here we sought to determine whether electronic effects are likewise responsible for the profound difference in phosphine lability between the methylidene and benzylidene complexes. This question was probed in a combined solid-state NMR, DFT, and crystallographic study. Unexpectedly, the present work reveals no electronic basis for the reactivity difference between corresponding benzylidene and methylidene complexes. Instead, the steric pressure exerted by the phenyl substituent emerges as the predominant contributor to the higher reactivity of the benzylidene precatalysts.
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RESULTS To probe the origin of the stronger Ru−PCy3 bonding in the methylidene complexes, we set out to examine the electronic differences manifested in their chemical shift (CS) tensors, relative to the parent benzylidene species, for comparison with crystallographic parameters. CS tensor data, coupled with DFT analysis, can provide detailed insight into the origin and extent of bonding interactions. Such studies of the metal−alkylidene moiety have been used to gain insight into dynamic behavior and molecular structure in silica-bound alkylidene complexes.28 In these surface organometallic species, the orientation of the anisotropic CS tensor of the alkylidene carbon was shown to be directly related to the magnitude of the alkylidene C−H agostic interactions. Recent work on group 6 metathesis catalysts likewise demonstrated the power of combined NMR/DFT studies in relating NMR chemical shifts to electronic structure.29 Given the highly directional nature of magnetic shielding contributions from individual orbitals, we considered that the 31 P and 13C CS tensors for the phosphine and alkylidene ligands should offer detailed insight into the underlying bonding interactions that distinguish the benzylidene and methylidene complexes. While the dominant second-generation system (i.e., GII vs GIIm) was the principal focus of this work, the first-generation system was also examined, to rule out effects originating in the H2IMes ligand. Synthesis of 13C-Enriched *GIIm. Measurement of alkylidene 13C CS tensors is greatly facilitated by the accessibility of isotopologues bearing a 13C label at the key RuCHR site. We recently reported the synthesis of the >99.5%-enriched complexes RuCl2(L)(PCy3)(13CHR) (*GI, *GII, *GIm, *GIIm), via CM of GI with H2C13CHPh or 13 C2H4, followed (in the case of the second-generation derivatives) by ligand exchange with H2IMes.30 While the targets were obtained in good yield and excellent purity, a drawback to synthesis of the methylidene complexes is the need for a gaseous labeled reagent, with the attendant issues of cost, convenience, and stoichiometric precision.31 For the purposes of solid-state 13C NMR analysis, lower levels of enrichment can be sufficient. To that end, we developed an alternative route to *GIIm, involving CM of GIIm with β-13Cstyrene (Scheme 1). While competing decomposition impedes this approach for GIm,12 the improved thermal stability of the second-generation methylidene complexes11,30 facilitates application to GIIm. As minimal amounts of *GIIm were obtained at room temperature (RT), CM of GII with a 5-fold excess of β-13Clabeled styrene was carried out at 40 °C. Rate curves for NMRscale reactions are given in the SI. Preparative-scale reactions using 200 mg of GII resulted in ca. 40% conversion to *GIIm
after 24 h, at which point the reaction was terminated to limit decomposition of the product. The solvent was evaporated, and stilbene and styrene were extracted with pentane. This treatment afforded 129 mg (67%) of a fine brown powder, containing *GIIm and GII in a ratio of 47:53. The slight increase in the proportion of *GIIm relative to that seen in situ is due to the preferential solubility of GII in pentane. While substantially less pure than *GIIm prepared via the ethenolysis−ligand exchange route (see above),30 this material is of value for solid-state 13C NMR studies of GIIm because labeling is selective for the methylidene species. Although the contaminating benzylidene complex GII is present in essentially equal proportions, it contains 13C at natural isotopic abundance (1.1%) and is thus spectroscopically insignificant.32 NMR Shielding Tensor Study. Computational methods developed by Autschbach enable dissection of CS tensors into contributions arising from the individual natural localized molecular orbitals (NLMOs) that represent the familiar lone pairs, individual bonds, and core orbitals of chemical usage.33,34 We used these methods to examine the 31P and 13C CS tensors for GII and GIIm, to gain insight into the effect of the alkylidene substituent on Ru−P and RuC bonding. It should be noted that the structural similarity between the methylidene and benzylidene complexes within each catalyst generation permits direct comparison of their CS tensors. No such comparison can be made between catalyst generations, however, as the differences in the CS tensor components are then dominated by other structural changes. 31 P Solid-State NMR Study. We first examined the 31P CS tensors, in the expectation that these would report on differences in the electronic nature of the Ru−P bond (e.g., bond strength), between GII and GIIm. To determine which tensor components are most sensitive to the Ru−P bonding interaction, we carried out DFT calculations to assess the impact of computationally constrained increases in the Ru−P bond distance. Specifically, we examined the impact on each tensor component of incrementally elongating the Ru−P bond, while keeping the RuC bond length fixed. Our focus here is on the trend in computed values, rather than absolute values of the chemical shifts. This highlights the impact of the imposed structural changes, while not requiring a 1:1 correspondence between calculated and measured chemical shifts. B
DOI: 10.1021/acs.organomet.5b00984 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Given our focus on relative changes in the tensor components arising from this structural change, we used the truncated model RuCl2(IMe)(PMe3)(CH2) (Ru-1m) to reduce the computational demands of the associated NLMO analysis (vide infra). Importantly, however, calculations performed on nontruncated GII and GIIm show the same trends in magnetic shielding as a function of the systematic increase in the internuclear Ru−P distances (see SI, Table S1 and Figure S8). It should be noted that dispersion corrections, while critical for treating the energy of ligand loss and binding in these complexes,35,36 are less relevant to assessment of CS tensor values.37 The Ru−P and P−C σ-bonding NLMOs for Ru-1m are shown in Figure 1. The σ11 and σ22 components of the 31P magnetic shielding tensor prove most sensitive to the Ru−P bond lengths (Figure 2a). NLMO analysis indicated that Ru−P σ-bonding contributes strongly to changes in σ11 and σ22 (Figure 2b). The P−C σ-bonding NLMOs exert relatively little influence on these components (Figure 2c).38
Figure 3. (a) 31P MAS and static NMR spectra and their simulations for GII. (b) 31P MAS and static NMR spectra and their simulations for GIIm. (c) Two-dimensional SUPER NMR spectrum used to separate the anisotropic line shapes for GII.
Table 1. Experimental 31P NMR CS Tensor Components for RuCl2(L)(PCy3)(CHR) Complexes (in ppm) compound
δiso
δ11
PCy340 7.0 ± 0.1 34.5 ± 1 First-Generation System (L = PCy3) GIm 45.2 ± 0.2 92.5 ± 3 GI 38.5 ± 0.2 78.5 ± 2 Second-Generation System (L = H2IMes) GIIm 39.3 ± 0.2 84 ± 3 GII(A)a 33.3 ± 0.2 76 ± 3 GII(B)a 29.4 ± 0.2 86 ± 3
Figure 1. NLMOs for RuCl2(IMe)(PMe3)(CH2), Ru-1m. (a) Ru− P σ-bonding NLMOs. (b) P−C σ-bonding NLMOs.
a
δ22
δ33
17.5 ± 1
−30 ± 1
41 ± 2 38.5 ± 3
2.5 ± 3 −2.5 ± 3
39 ± 2 33 ± 2 10 ± 2
−6 ± 3 −9 ± 3 −9 ± 3
GII(A) = molecule A; GII(B) = molecule B.
The CS tensor components extracted from these spectra are listed in Table 1. Here we preserve the distinction between the calculated magnetic shielding tensors and the experimentally measured chemical shifts by showing the former as σ and the latter as δ. In both cases, it is the trend in values that is critical. In the case of the GI/GIm pair, both δ11 and δ22 are larger for GIm. Similarly, the δ11 and δ22 values are larger for GIIm than for molecule A of GII. While δ11 for molecule B of GII is indistinguishable from that of GIIm, δ22 is dramatically smaller. The fact that three of four 31P CS tensor components of GIIm are larger than those observed for GII suggests an increased Ru−P orbital interaction in the former. The increased 31P tensor components for the methylidene complexes are consistent with a stronger Ru−P orbital interaction in these species and with their attenuated phosphine lability. 13 C Solid-State NMR Study. We next assessed the CS tensors of the alkylidene carbon. As before, DFT calculations were carried out on truncated models, to determine which tensor components correlate most closely with changes in the RuC orbital interaction. These calculations predicted, first, a 15−20 ppm decrease in δ11 for the benzylidene model Ru-1, relative to its methylidene analogue Ru-1m. Second, the NLMO analysis for Ru-1m (Figure 4, Figure 5) and Ru-1 (see SI, Figure S9) revealed a sharp increase in the σ11 magnetic shielding tensor component in response to the constrained increase in RuC orbital interaction. These effects originate almost exclusively in the RuC σ- and π-bonding NLMOs; in
Figure 2. Impact on 31P magnetic shielding tensor components (σii) of constrained increases in the Ru−P bond length in Ru-1m. (a) Plot of σii vs Ru−P distance, showing the sensitivity of σ11 and σ22. (b) Contributions to σii from Ru−P σ-bonding NLMOs. (c) Contributions from P−C σ-bonding NLMOs. For line fits, see Figure S11.
To obtain the relevant anisotropic tensor components, we measured the static and MAS 31P solid-state NMR spectra. The spectra for GII and GIIm, and their fits, are shown in Figure 3a and b, respectively; those for GI and GIm are provided in the SI (Figure S6). Anisotropic line shapes consistent with a single crystallographic phosphorus site were observed for each of GI, GIm, and GIIm. For GII, two resonances were evident, in both commercial and 13C-enriched samples. We attribute this to the presence of two independent conformers in the asymmetric unit (identified below as molecules A and B). The breadth of the 31P MAS NMR resonances hampers extraction of the CS tensor components for the individual conformers. To resolve the two static powder patterns, a two-dimensional SUPER NMR experiment was carried out (Figure 3c; SUPER = separation of undistorted powder patterns by effortless recoupling).39 C
DOI: 10.1021/acs.organomet.5b00984 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. NLMOs for model system Ru-1m. (a) Ru−C σ-bonding NLMO. (b) Ru−C π-bonding NLMO. (c) C−H σ-bonding NLMOs.
Figure 6. 13C NMR spectra and their simulations for the 13C-labeled metathesis catalysts shown. (a) For *GII. (b) For *GIIm. Top: MAS spectra; bottom: static spectra. Peaks due to natural-abundance carbon sites were not modeled in the simulations.
Table 2. Experimental 13C Chemical Shift Tensor Components (ppm) for the RuC Moiety in RuCl2(L)(PCy3)(CHR) complex
δiso
δ11
First-Generation System (L = PCy3) GIm 289.8 ± 0.5 660 ± 3 GI 293.7 ± 0.2 658 ± 3 Second-Generation System (L = H2IMes) GIIm 292.8 ± 0.2 663 ± 3 GII 295.3 ± 0.2 668 ± 5
13
Figure 5. Impact on C shielding tensor components (σii) of constrained increases in the Ru−C bond length in Ru-1m. (a) Plot of σii vs Ru−C distance, showing the sensitivity of σ11. (b) Contributions to shielding from the Ru−C σ-bonding NLMO. (c) Contributions from the Ru−C π-bonding NLMO. (d) Contributions from the C−H σ-bonding NLMO. For line fits, see Figure S13.
δ22
δ33
224 ± 2 233 ± 2
−15 ± 5 −11 ± 5
232 ± 2 241 ± 10
−17 ± 5 −24 ± 10
weakened Ru−C interaction in the benzylidene vs the methylidene systems. Thus, these data imply a counterinfluence in the benzylidene complex GII, but absent in methylidene GIIm, that weakens the Ru−C orbital interaction for RuCHPh. The origin of this behavior is treated in the Discussion. Crystallographic Analysis of GIm. To augment the insights attainable by DFT and solid-state NMR analysis, we turned to X-ray crystallography. Crystallographic data have been reported for all three benzylidene complexes GI,41 GII,42 and GII′43 and for the second-generation methylidene complexes GIIm44 and GIIm′.13 We wished to complete this series with the first-generation complex GIm, to exclude the potential influence of the NHC ligand in the benzylidene− methylidene comparison. Crystallization of GIm has long proved elusive, owing to the instability of this species. Even at 35 °C, the maximum half-life of GIm is just 6.6 h in benzene, a figure that drops to 2.5 h in dichloromethane or 1.1 h in THF.30 We were able to obtain Xray quality crystals by rapid crystallization from pentane at RT (glovebox; 25−27 °C). The limited solubility of GIm in this solvent, coupled with the volatility of pentane, enabled deposition of crystals within
