Living Polymerization Caught in the Act: Direct Observation of an

Jun 13, 2019 - Therefore, most living olefin metathesis polymerizations require the ... more stable Py-coordinated Ru carbenes at room temperature. Th...
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Cite This: J. Am. Chem. Soc. 2019, 141, 10039−10047

Living Polymerization Caught in the Act: Direct Observation of an Arrested Intermediate in Metathesis Polymerization Jung-Ah Song,† Bohyun Park,‡,⊥ Soohyung Kim,† Cheol Kang,† Dongwhan Lee,† Mu-Hyun Baik,*,⊥,‡ Robert H. Grubbs,§ and Tae-Lim Choi*,† †

Department of Chemistry, Seoul National University, Seoul 08826, Republic of Korea Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ⊥ Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea § The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States

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ABSTRACT: Understanding the stability and reactivity of the propagating species is critical in living polymerization. Therefore, most living olefin metathesis polymerizations require the stabilization of the catalyst by coordination of external ligands containing Lewis basic heteroatoms, e.g., phosphines and pyridines. However, in some cases, chemists postulated that the propagating metal carbene could also be stabilized by olefin chelation. Here, we disclose that stable 16-electron olefin-chelated Ru carbenes play a key role in previously reported living/ controlled ring-opening metathesis polymerization of endo-tricyclo[4.2.2.02,5]deca-3,9-diene and cyclopolymerization of 1,8-nonadiynes using Grubbs catalysts. We successfully isolated these propagating species during polymerization and confirmed their olefin-chelated structures using X-ray crystallography and NMR analysis. DFT calculations and van ’t Hoff plots from the equilibrium between olefinchelated Ru carbenes and 3-chloropyridine (Py)-coordinated carbenes revealed that entropically favored olefin chelation overwhelmed enthalpically more stable Py-coordinated Ru carbenes at room temperature. Therefore, olefin chelation stabilized the propagating species and slowed down the propagation relative to initiation, thereby lowering polydispersity. This finding provides a deeper understanding of the olefin metathesis polymerization mechanism using Grubbs catalysts and offers clues for designing new controlled/living polymerizations.

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Ru-based Grubbs catalysts,11,20,21 because, during the polymerizations, they rapidly react with monomers or decompose via a bimolecular process.15−17 The general strategy for stabilizing these Ru carbene intermediates is to access their 16- or 18electron analogues by coordinating additional ligands, which are detectable by 1H NMR spectroscopy.11,18,19 For instance, the addition of triphenylphosphine ligands significantly improved the living ROMP of norbornene (NB),20 1,5cyclooctadiene,20 and trans-cyclooctene21 by reducing the propagation rate (kp) and suppressing secondary metathesis or chain transfer reactions. Similarly, we reported recently that living CP of various diyne monomers can be enhanced by adding 3,5-dichloropyridine (Cl2Py) to stabilize the propagating Ru carbenes and suppress chain transfer.11 Furthermore, monomers containing heteroatoms like N, O, or S can easily affect the reactivity of propagating Ru carbenes.5,19,22−25 For example, ROMP of ether-containing 3-substituted cyclobutene23 or endo-carbonyl-containing NB24 was reported to

INTRODUCTION Chain growth polymerization based on olefin metathesis is a powerful method for synthesizing well-defined complex macromolecules. The most widely used method is ringopening metathesis polymerization (ROMP, Scheme 1), which is driven by the release of the ring strain in the cycloalkene monomers. By forming stable metal carbenes that propagate the polymerization, living polymerization can be achieved to facilitate the synthesis of precisely controlled polymers and multiblock copolymers containing various useful functional groups.1−7 Another chain growth metathesis polymerization is cyclopolymerization (CP, Scheme 1), which produces soluble conjugated polymers via ring-closing metathesis of diyne monomers. This versatile polymerization technique allowed for preparing various conjugated polymers containing five- to seven-membered rings in a living and controlled manner.8−14 In these processes, a 14-electron metal alkylidene species is thought to be the propagating intermediate. Despite being the de facto standard, it has been impossible to directly observe these key propagating species for the family of © 2019 American Chemical Society

Received: April 14, 2019 Published: June 13, 2019 10039

DOI: 10.1021/jacs.9b04019 J. Am. Chem. Soc. 2019, 141, 10039−10047

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Journal of the American Chemical Society

an unstable 14e− complex; this leads to low ki/kp and the polymerization loses controllability.32−34 Nonetheless, we previously achieved living ROMP of endo-tricyclo[4.2.2.02,5]deca-3,9-diene (TD, Figure 1d) using HGII.34 In these studies, an unexpected carbene signal was detected in the 1H NMR spectra. Furthermore, we also observed the propagating carbene during the CP of 1,8-nonadiyne (Figure 1c) using HGII.36 It was surprising that controlled polymerization was possible, although the polymerization proceeded slowly via a challenging seven-membered ring cyclization. From these studies, we concluded that these reactions share a common feature leading to a stable propagating carbene and successful controlled living polymerization. Herein, we provided direct evidence as to why the controlled living polymerization was possible in these cases by isolating and fully characterizing the X-ray structures of the arrested propagating species caught in the act.

Scheme 1. General Mechanism of Olefin Metathesis-Based Chain Growth Polymerizations: (a) ROMP and (b) CP

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RESULTS AND DISCUSSION During our previous investigation of the CP reaction of 1,8nonadiyne (1, Figure 1c) using GIII, we observed Ru carbene peaks between 15.1 and 15.7 ppm in tetrahydrofuran (THF)d8 by 1H NMR (Figure 2a).36 These signals were significantly upfield shifted compared to general pyridine-coordinated Ru carbenes that typically show peaks in the 19−20 ppm range.11 More importantly, the same propagating carbene signals were observed from HGII without any additional ligand (Figure 2b), i.e., the 3-chloropyridine (Py) ligand did not coordinate.37 Since the possibility of oxygen coordination was ruled out based on control experiments (Figure S3), we proposed that the propagating carbene was a 14e− Ru carbene. However, the observation of 14e− Ru carbene during polymerization by 1H NMR is challenging, as explained above. Furthermore, the NMR feature at 15.1−15.7 ppm was observed both in dichloromethane (DCM)-d2 and in THF-d8, which can potentially serve as an oxygen-donor ligand and stabilize the propagating carbene intermediate (Figure S5). Thus, we questioned our original proposal and considered the alternative possibility of the adjacent olefin coordinating to the Ru center, similar to the Ru intermediate proposed by the Grubbs group 25 years ago.38

produce polymers with narrow dispersity (Đ) due to oxygen chelation, thereby increasing ki/kp. This Ru−O coordination was indirectly supported by observation of a new Ocoordinated propagating Ru carbene by NMR.22−25 Olefin coordination26−31 was also suggested to stabilize the resting state during ROMP.26 The Grubbs group first proposed this idea of olefin-chelated Ru carbene for the living ROMP of bicyclo[3.2.0]heptane.27 The proposed five-membered chelated carbene was confirmed by the Snapper group who isolated it during the ring-opening cross metathesis reaction using a first generation Grubbs catalyst (GI) (RuI, Figure 1a) and obtained the X-ray structure.28 RuII (Figure 1a) was also isolated from a reaction with a third-generation Grubbs catalyst (GIII, Figure 1b) and 1,2-divinylbenzene30,31 by the Grubbs group and showed a side-bound geometry, in contrast to the bottom-bound structure preferred by RuI. Although the importance of such chelation was proposed, direct observation from polymerization or its relevance to actual living polymerization was not fully explored. Unlike GI and GIII, it is impossible to observe the propagating Ru carbene from a second generation HoveydaGrubbs catalyst (HGII, Figure 1b) by 1H NMR because after initiation, HGII has no additional L-type ligand and becomes

Figure 1. (a) Previously isolated olefin-chelated Ru carbenes in small molecule model studies. (b) Grubbs catalysts, (c) 1,8-nonadiynes, and (d) TD monomers used in this study. 10040

DOI: 10.1021/jacs.9b04019 J. Am. Chem. Soc. 2019, 141, 10039−10047

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These unexpected results led us to consider that the unusual living ROMP of TD catalyzed by HGII could also be due to the formation of a stable olefin-chelated Ru carbene. Our previous kinetic studies revealed that the ROMP of TD was much slower than that of NB. Previously, we attributed this observation simply to the steric bulkiness of TD.35 But indepth kinetic studies on the ROMP of 3 revealed the same 1H NMR signal (Figures 3a and 3b, 16.9 ppm in THF-d8) that we

Figure 3. Comparison of 1H NMR spectra showing Ru carbenes generated during ROMP of 3 using (a) HGII and (b) GIII in THF-d8 and (c) ROMP of 4 using HGII in DCM-d2.

assign to the propagating carbene formed by GIII and HGII. Compared to typical chemical shifts of similar Ru carbene species generated during ROMP of NB using GIII (18.3 ppm in THF-d8), these NMR signals are upfield shifted notably. These observations indicate that the carbene at 16.9 ppm was not a Py-coordinated Ru carbene. To eliminate the possibility of a carbonyl group or THF coordinating to Ru, we switched to monomer 4 containing a noncoordinating OTBS group, and we monitored its ROMP with HGII in noncoordinating DCMd2 by 1H NMR (Figure 3c). Again, a new carbene signal appeared at 17.4 ppm, suggesting that oxygen did not coordinate to Ru during the ROMP of TD. Taken together, these data lead to the conclusion that the new carbenes detected are likely the olefin-chelated Ru carbenes. To verify this hypothesis, we attempted to prepare an olefinchelated Ru carbene from the 1:1 reaction of GIII and a rigid monomer 5 containing a heavy atom, bromide, which was crucial for growing good quality crystals (Figure 4a). Fortunately, we isolated 5-Ru as a major product. Analysis of its 1H NMR spectrum revealed two interesting features in addition to the expected carbene signal at 17.1 ppm (Figure 4b). First, a total of eight olefinic protons were observed with respect to one carbene proton, which we were able to assign: olefinic protons on the cyclohexene ring (Hc, Hd, He, and Hf at 6.34, 6.15, and 5.90 ppm, respectively) and three on the acyclic backbone (Hb, Hg, and Hh at 6.48, 5.23, and 4.98 ppm, respectively). Second, the third olefinic protons on the backbone (Hi) appeared significantly shifted upfield at 3.98 ppm compared to conventional olefinic protons. From these features, we expected that this Ru carbene complex (5-Ru) to

Figure 2. 1H NMR spectra of Ru carbene (a) during CP of 1 using GIII and (b) HGII and (c) for 2-Ru at rt. (d) X-ray crystal structure of 2-Ru. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50%.

To test this hypothesis, we synthesized an olefin-chelated Ru carbene complex (2-Ru, Figure 2) from a 1:1 reaction of GIII and more crystalline 2 (the complex from 1 did not crystallize). The product 2-Ru was confirmed by 1H NMR analysis, which showed a single carbene peak at room temperature (Figure 2c, see the SI for details). After obtaining crystals via slow diffusion of hexane into the DCM solution, Xray crystallography on the structure of 2-Ru revealed a bottombound Ru-olefin complex containing two chlorides trans to each other (Figure 2d). The Ru(01)−C(23) and Ru(01)− C(22) bond lengths of 2.273(1) and 2.227(1) Å were shorter than those found in RuI, 2.362(5) and 2.339(5) Å, respectively, suggesting that the Ru-olefin interaction is stronger in 2-Ru than in RuI. Clearly, the more electrondonating N-heterocyclic carbene (NHC) ligands increase the metal-to-olefin back-donation compared to the phosphine ligand.39,40 This is reminiscent of what was seen for the second generation Grubbs catalyst (GII) that also favored the olefin coordination, thereby showing higher catalytic activity over GI.39 Such olefin-chelated Ru carbene was never observed during the CP of 1,6-heptadiyne using HGII or GIII and that is why additional ligands such as THF or pyridines were required to stabilize 14 e− Ru carbenes.11 10041

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and allowed living ROMP of TD with narrow dispersity even when HGII, which is much slower at the initiating step, was used. So far, we confirmed the formation of 16e− olefin-chelated Ru carbenes during the ROMP of TD and CP of 1,8nonadiyne. However, Py-coordinated Ru carbenes capable of propagating the polymerization were not observed, even when GIII that exists as a monopyridine-coordinated Ru carbene in solution41 was used. In our previous report, we had observed NMR signals at 15.1−15.7 ppm in THF-d8, which we now assign to be the olefin-chelated carbene. Upon adding excess amounts of Py (10 equiv), we observed a shift of the carbene signal to 19.9 ppm, which can now be attributed to the Pycoordinated Ru carbene (Figure 6a).36 For comparison, we repeated the same experiment with the propagating carbenes at 16.9 ppm produced from the ROMP of 3, but no new signal was observed except for a small shift to 17.1 ppm with some broadening (Figure 6b), implying that olefin chelation from the ROMP of TD would be favored over Py coordination. To further understand the equilibrium between olefinchelated and Py-coordinated Ru carbene, we monitored changes in the propagating carbene from both reactions while varying temperatures by 1H NMR in DCM-d2 (Figures 6c and d). First, for the olefin-chelated propagating carbene from CP, the intensity at 15.1−15.7 ppm decreased as the temperature was reduced from 10 to −30 °C, and the intensity of the signal at 19.9 ppm gradually increased (Figure 6c), which implies that the species at 19.9 ppm is a Py-coordinated Ru carbene (Figure 6d). Second, for the ROMP case, the intensity of the original signal for the olefin-chelated carbene at 17.1 ppm started to decrease at temperatures below −15 °C with the concurrent appearance of a new broad peak at 18.8 ppm (Figure 6d). This split into another new peak at 19.4 ppm below −45 °C, and the integral ratio of these three signals at 17.1, 18.8, and 19.4 ppm was 7.3:1.6:1. Thus, the olefinchelated complex remained as the major species. The intensity of the signal at 19.4 ppm greatly increased upon addition of 10 equiv Py, and the ratio changed to 1:4.4:28.1. Upon increasing the temperature to −15 °C, the two signals at 18.8 and 19.4 ppm decreased in intensity and coalesced into a single broad peak at 19.0 ppm, and the peak at 17.1 ppm became the major peak again at −5 °C, even after adding 10 equiv of Py (Figure 6d). This observation implied that two new signals at 18.8 and 19.4 ppm corresponded to Py- and Py2-coordinated Ru carbenes, respectively. In order to understand energetic and structural relationships of the species in equilibria, quantum chemical calculations based on density functional theory (DFT) were carried out. First, we calculated the energies of plausible intermediates based on 2-Ru for the CP case, as illustrated in Figure 7. In good agreement with experimental results, olefin-chelated 2Ru was the most stable intermediate at room temperature. In this condition, 2-RuPy is the second-most stable intermediate, which is enthalpically 15.4 kcal/mol more stable but must pay an entropic penalty of 41.9 cal/mol·K compared to 2-Ru. Note that these values are calculated in gas phase. The solvation effects are implicitly included in the solvation energy corrections. As the temperature decreases, the Gibbs free energy of 2-RuPy becomes lower than that of 2-Ru due to the decrease in entropy penalty, and 2-RuPy is the major species of the equilibrium. The disubstituted species 2-Ru*Py2 requires an entropy penalty of −89.3 cal/mol·K, as two of Py coordinate to the Ru center. Hence, even with excessive

Figure 4. (a) 1:1 reaction of 5 and GIII and (b) 1H NMR spectrum of 5-Ru (see the SI for specific assignment). (c) X-ray crystal structure of 5-Ru. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50%.

contain two repeating units of 5 as well as olefin chelation. After numerous attempts, we obtained crystals via slow hexane diffusion into DCM solution of 5-Ru at −20 °C, and X-ray crystallographic analysis confirmed our proposed structure, exhibiting a bottom-bound olefin-chelated Ru carbene, again with two chlorides trans to each other, containing two repeating units of the substrate (Figure 4c). This is the genuine propagating Ru carbene containing repeating units formed during polymerization. Previous examples of related systems comprise of terminal olefin- or styrene-chelated Ru carbenes that were obtained using model organic reactions.28,30,31 Thus, this is the direct evidence of the propagating carbene proposed by Grubbs and Snapper nearly three decades ago. It was exceedingly challenging to isolate and obtain crystal structures of this seemingly simple olefin-chelated propagating carbene and the same chelation would be practically impossible from the ROMP of the most widely used NB derivatives because of the restricted conformation of 1,3linkage of cyclopentanes (Figure 5). Bond lengths of Ru(01)− C(01F) and Ru(01)−C(00S) (2.451(3) and 2.395(3) Å, respectively) were longer than those found in 2-Ru and RuI, suggesting a weaker olefin−Ru interaction. Nevertheless, the higher stability of the 16e− olefin-chelated Ru carbene compared to the 14e− Ru carbene leads to slower propagation

Figure 5. Intermediate from ROMP of NB: restricted conformation of 1,3-linkage of cyclopentane for olefin chelation. 10042

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Figure 6. Changes in the 1H NMR spectra during the reaction of GIII and (a) monomer 1, (b) monomer 3 at rt (THF-d8) upon addition of 10 equiv Py, (c) monomer 1, and (d) monomer 3 at various temperatures (DCM-d2). (e) Summary of the chemical shifts corresponding to various Ru complexes observed.

amount of Py, formation of the 18e− 2-Ru*Py2 is not plausible. This result is also supported by 1H NMR spectroscopy (Figure 6a and c), which only shows two peaks at 15.1−15.7 and 19.9 ppm indicating olefin-chelated and Py-coordinated complexes, respectively. Lastly, 2-Ru*, which has a vacant binding site, was located at 6.7 kcal/mol higher enthalpically and −1.6 cal/mol· K lower in entropy value than 2-Ru, which ultimately results in 7.0 kcal/mol higher Gibbs free energy than 2-Ru at room temperature. This Gibbs free energy difference directly increases the activation energy of the propagating step and slows down the polymerization. Therefore, these computational results confirm why olefin-chelated structures are favored and how they influence reaction kinetics. Intermediate 2-Ru*Py is not considered further, as it is too high in energy to be mechanistically relevant. A similar computational analysis of the ROMP case gives an identical overall picture and agrees well with the experimental data (see the SI for details). Next, to elucidate the reason why these systems fundamentally prefer olefin chelation instead of Py coordina-

tion, we analyzed the metal−ligand interactions with secondorder perturbation theory analysis.42 In 2-Ru and 2-Ru*Py, the olefin and pyridine ligands act as the Lewis bases donating electron density to the Lewis acidic Ru-metal center. Our calculations show that the orbital perturbation associated with the electron-donation from the olefin amounts to −45.8 kcal/ mol, whereas the π back-donation from the metal to the olefin gives an additional energy of −17.3 kcal/mol to afford an electronic olefin binding energy of −63.1 kcal/mol, as illustrated in Figure 8a. The Py ligand is a much stronger σdonor, and our calculations quantify this component of binding to be −60.0 kcal/mol. Not surprisingly, the π backdonation is exceedingly weak and accounts only for −3.8 kcal/ mol to give a total perturbation energy in 2-Ru*Py of −63.7 kcal/mol, which is nominally higher than what was found for 2-Ru (Figure 8b). Interestingly, in the intermediate 2-RuPy where both the pyridyl and olefin ligands are coordinated to the Ru-center, the binding energy becomes −79.8 kcal/mol (Figure 8c). These second-order perturbation energies must be 10043

DOI: 10.1021/jacs.9b04019 J. Am. Chem. Soc. 2019, 141, 10039−10047

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fused bridgehead moiety, a simple ligand rotation is enough for the olefin coordination. As a result, the olefin binding energy is the greatest among the carbene species considered at −7.8 kcal/mol. The same calculation was also carried out on Intermediate A, an analogue containing a five-membered ring analog of 2-Ru derived from 1,6-heptadiyne. The ligand distortion energy is found to be 11.2 kcal/mol, which reflects on the distance constraint generated by the five-membered ring, restricting the olefin from approaching the metal center. The metal−olefin interaction offers only −8.9 kcal/mol to afford an overall electronic binding energy of 2.3 and 4.5 kcal/ mol in Gibbs free energy, thereby disfavoring the coordination of the olefin. Similarly, another analogous intermediate B containing a six-membered ring prepared from the 1,7octadiyne monomer experiences a ligand distortion energy of 4.0 kcal/mol and the metal−olefin interaction gives −7.6 kcal/ mol to result in a negative olefin binding electronically, but the entropy penalty and solvation corrections raise the Gibbs free energy to 5.9 kcal/mol, thereby giving preference to B-Ru* with no olefin chelation. In summary, 2 and 5′ have no electronic penalties for olefin binding, but A and B need to pay significant structural distortion penalties and suffer from weak olefin−metal interactions, which in sum disfavor olefin coordination. Based on the analysis of NMR experiments, the thermodynamic parameters were obtained from the van ’t Hoff plot and compared with the DFT calculated values. Data points were obtained over the temperature range of +10 to −10 °C for CP and −56 to −35 °C for ROMP, where the total concentration of Ru carbenes remained constant, as shown in Figure 9. The enthalpy of olefin coordination was determined to be −9.4 and −16.3 kcal/mol for the cases of CP and ROMP, respectively. The corresponding entropy was determined as +28.5 and +62.1 cal/mol·K, respectively. These large and positive entropy terms were consistent with the release of one and two pyridyl ligands, respectively, and in reasonable agreement with the calculated values. Thus, the 18e− Py-coordinated Ru carbenes were enthalpically more stable, but 16e− olefin-chelated Ru was entropically favored because of the release of Py molecules. Therefore, we only observed olefin-chelated Ru carbenes during polymerizations at room temperature.

Figure 7. (a) Temperature−Gibbs free energy correlation. The free energy of 2-Ru is used for reference. (b) Free energy diagram for 2Ru, 2-Ru*, 2-RuPy, 2-Ru*Py, and 2-Ru*Py2. The asterisk indicates compounds with nonchelated olefins. Hydrogen atoms are omitted for clarity. Geometries are optimized at the B3LYP-D3/LACVP* level of theory.

evaluated carefully, because the binding of the other ligands (e.g., SIMes) is impacted by these perturbations. Nonetheless, they are interesting to analyze separately. Expectedly, the entropies of binding in 2-RuPy and 2-Ru*Py are very similar as −41.9 and −43.8 cal/mol·K against 2-Ru, respectively, which simply reflects on the translational entropy penalty of an intermolecular reaction. But the electronic interaction energy prefers 2-RuPy by as much as 11.2 kcal/mol, which make it the lowest energy isomer at low temperature conditions, where the impact of the entropy penalty is minimized, as shown in Figure 7a. Our calculations also explain why the olefin chelation is observed during ROMP of 5 and CP of 1,8-nonadiynes such as 2, but not for the most popular monomers such as 1,6heptadiynes9−11,13,14 or 1,7-octadiynes.12,43,44 There are two energy terms to consider, namely, ligand distortion and metalolefin interaction. In 2-Ru*, a ligand distortion energy of 6.1 kcal/mol must be invested to obtain an overall electronic binding energy of −6.7 kcal/mol (Figure 8d). In complex 5′Ru*, the ligand distortion energy is negligibly small at 1.2 kcal/ mol to transition to the structure 5′-Ru, where the olefin is bound to the metal (see the SI for detailed structures). This is easy to understand, as for the olefin incorporated into a rigid

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CONCLUSION In conclusion, we demonstrated that living ROMP of TD using HGII and controlled CP of 1,8-nonadiyne using GIII was possible because of the formation of a stable 16e− olefinchelated Ru carbene. We succeeded in isolating the Ru carbene complexes that are actually generated during polymerization, and their detailed structures were determined by single crystal X-ray diffraction and 1H NMR analysis. These olefin-chelated Ru carbenes are the examples exhibiting the structure of the proposed olefin-chelated propagating carbenes during polymerization. Thermodynamic parameters describing the equilibrium between olefin-chelated carbene and Py-coordinated carbene were obtained from DFT calculations and van ’t Hoff plot. These combined studies revealed that the olefin chelation was favored over the Py coordination at room temperature because of the entropy factor. These chelated complexes vastly improved the living polymerization by stabilizing the propagating species and increasing ki/kp, thereby suppressing termination and lowering the dispersity. Based on this work, we are further investigating olefin chelation during ROMP of 10044

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Figure 8. Second-order perturbation energies of (a) 2-Ru, (b) 2-Ru*Py, and (c) 2-RuPy. (d) Electronic energy differences, ΔE, between the chelated and nonchelated Ru carbenes for 2-Ru*/2-Ru, 5′-Ru*/5′-Ru, A-Ru*/A-Ru, and B-Ru*/B-Ru. ΔEligand is the distortion energy of the repeat unit.

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Detailed experimental procedures, characterizations, NMR spectra for new compounds, and single crystal X-ray diffraction (PDF) X-ray data for 2-Ru (CIF) X-ray data for 5-Ru (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] (M.-H.B.) *[email protected] (T.-L.C.) ORCID

Jung-Ah Song: 0000-0003-4965-4462 Bohyun Park: 0000-0003-0552-8763 Dongwhan Lee: 0000-0001-6683-9296 Mu-Hyun Baik: 0000-0002-8832-8187 Robert H. Grubbs: 0000-0002-0057-7817 Tae-Lim Choi: 0000-0001-9521-6450

Figure 9. van ’t Hoff plots for the equilibria between olefin-chelated Ru propagating carbene and (a) Py-coordinated Ru carbene during CP of 1 (blue) and (b) Py2-coordinated Ru carbene during ROMP of 3 (red) using GIII.

Notes

The authors declare no competing financial interest.

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cyclooctadiene using various Ru catalysts including those having cyclic alkyl amino carbene ligands.45−47

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ACKNOWLEDGMENTS This paper is dedicated to Prof. Soon Hyeok Hong on his outstanding contribution to research and education at SNU. This research was supported financially by the Institute for Basic Science in Korea (IBS-R10-A1), Creative Research Initiative Grant, and Creative Material Discovery Program

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04019. 10045

DOI: 10.1021/jacs.9b04019 J. Am. Chem. Soc. 2019, 141, 10039−10047

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Journal of the American Chemical Society

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through NRF, Korea. We thank Prof. Soon Hyeok Hong and Mr. Seongyeon Kwon for helpful discussions.

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DOI: 10.1021/jacs.9b04019 J. Am. Chem. Soc. 2019, 141, 10039−10047