Variation of the Sterical Properties of the N-Heterocyclic Carbene

Article pubs.acs.org/Organometallics

Variation of the Sterical Properties of the N‑Heterocyclic Carbene Coligand in Thermally Triggerable Ruthenium-Based Olefin Metathesis Precatalysts/Initiators Eva Pump,† Anita Leitgeb,† Anna Kozłowska,‡ Ana Torvisco,§ Laura Falivene,∥ Luigi Cavallo,∥ Karol Grela,‡ and Christian Slugovc*,† †

Institute for Chemistry and Technology of Materials and §Institute of Inorganic Chemistry, Graz University of Technology, NAWI Graz, Stremayrgasse 9, A 8010 Graz, Austria ‡ Biological and Chemical Research Center, Department of Chemistry, University of Warsaw, Źwirki i Wigury 101, 02-089 Warszawa, Poland ∥ Kaust Catalysis Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia S Supporting Information *

ABSTRACT: A series of ruthenium complexes based on the κ2(C,N)-(2-(benzo[h]quinolin-10-yl)methylidene ruthenium dichloride fragment featuring different neutral coligands L (L = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr), 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes), 1,3-bis(2,4-dimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIXyl), and 1,3-bis(2-methylphenyl)4,5-dihydroimidazol-2-ylidene (SITol)) was prepared, characterized, and tested in the thermally induced ring-opening metathesis polymerization of dicyclopentadiene. In addition, the corresponding tricyclohexylphosphine derivative was investigated for comparison. All compounds were isolated as their trans-dichloro isomers. NMR spectroscopic features as well as structural features are, particularly within the NHC-bearing complexes, very similar, but their polymerization activity at elevated temperatures is distinctly dif ferent. While the SIMes derivative shows the desired properties, i.e., latency at room temperature and pronounced polymerization activity at elevated temperature, all other preinitiators do not. The preinitiator featuring the SIPr coligand is the most latent one, needing temperatures > 140 °C to show moderate activity in the polymerization of dicyclopentadiene. Compounds bearing the smaller N-heterocyclic carbene congeners are stable and latent at room temperature, but decompose upon heating, diminishing the polymerization activity at elevated temperatures. Density functional calculations show that the SIMes derivative is the easiest to activate and yields the most stable 14-electron intermediate. Finally calculations reveal a distinct influence of the nature of the N-heterocyclic carbene ligand on the position of the equilibrium of cis- and trans-dichloro isomers of the complexes. While the SIPr and the SIMes derivatives prefer the cisconfiguration, all other derivatives favor, at least in solvents with low dielectric constants, the trans-configuration. These computational findings were supported by the isolation and full characterization of the cis-dichloro isomer of the SIMes-bearing preinitiator obtained upon heating of its trans-isomer at 140 °C.

■

INTRODUCTION Switchable olefin metathesis catalysts/initiators based on ruthenium have attracted considerable interest during the past decade.1 An envisaged application of such species is the development of formulations containing a monomer and the initiator (as well as further additives and/or fillers) that can be cured upon exerting a proper stimulus. Several triggering principles, such as addition of acid2 or chloride,3 oxidation,4 or the application of ultrasound5 or light,6 have been disclosed so far. The most researched trigger, however, is simply applying heat.7 A heat-triggerable preinitiator is ideally designed in a way that the initiation proceeds via a considerably high energy barrier, which is high enough that it cannot be overcome at © XXXX American Chemical Society

room temperature. Furthermore, a fast propagation of the initiated species should be provided and the preinitiator should be soluble in the monomer. The realization of this design principle consists of ruthenium complexes featuring an Nheterocyclic carbene (NHC, responsible for fast propagation), two chloride ligands, and a chelating alkylidene ligand (responsible for retarded initiation); cf. Scheme 1. A large number of complexes composed in this way are known.8 Additionally, Ru complexes comprising an N-heterocyclic carbene, two chloride ligands, a nonchelating alkylidene, and a Received: August 20, 2015

A

DOI: 10.1021/acs.organomet.5b00715 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

these hypotheses hold true under high-temperature conditions needed to activate the newly disclosed thermally triggerable preinitiators for ring-opening metathesis polymerization (ROMP) of norbornene derivatives.

Scheme 1. Activation of Thermally Triggerable Preinitiators Featuring a Chelating Alkylidene Ligand

■

RESULTS AND DISCUSSION Compounds studied herein were prepared by carbene exchange reactions from the corresponding indenylidene-bearing precursors and 10-vinylbenzo[h]quinoline (3), as shown in Scheme 2. Complexes 4, 7, and 8 were obtained from tricyclohexylphosphine-coordinated precursors M1, 1, and 2, and copper(I) chloride was added as a phosphine scavenger.17 The reaction was carried out at 80 °C in toluene as the solvent. By contrast, for the preparation of complexes 5 and 6, pyridinebearing commercially available starting materials M3218 and M3119 (Umicore AG & Co. KG) were used. In these cases the reaction took place at room temperature, and dichloromethane was used as the solvent. Complexes 4−8 were obtained after appropriate purification procedures (reprecipitation procedures in the case of 4 and 7, precipitation from the reaction mixture in the case of 8, and column chromatography in the case of 5 and 6) in moderate yields in the range 41−81%. No rutheniumcontaining side products could be isolated or identified except in the case of the preparation of 6. Here a diastereomer of 6, namely, its cis-dichloro-configured congener cis-6, could be isolated in 2% yield and fully characterized. The formation of cis-6 is, on one hand, somewhat surprising since we have not encountered cis-6 during our previous work.20 On the other hand, it is known that the use of pyridine-bearing precursors favors the formation of cis-dichloro-configured isomers as long as their formation is thermodynamically feasible.21 Complexes 4−8 and cis-6 were characterized by elemental analysis, 1H and 13C NMR spectroscopy, and single-crystal Xray crystallography. 1H and 13C NMR spectra of 4, 5, and 6 are in accordance with the presence of a Cs symmetric compound (e.g., singlet with intensity 4 for the −CH2−CH2− moiety of the dihydroimidazolyl part), confirming the anticipated transdichloro geometry for those compounds. The SIXyl and SITol derivatives 7 and 8 exhibited more complicated NMR spectra

phosphite coligand are well suited as heat-triggerable preinitiators.9 Moreover, such “deactivated” catalysts/initiators, even if they show some olefin metathesis activity already at room temperature, find application in transformations where either a delayed polymerization10 or a slow release of the actual active catalyst from a catalyst precursor is needed.11 In the latter case “slow dosing” of a precatalyst that is highly active at room temperature can be avoided. Many of the mentioned precatalysts/initiators show the peculiarity that their cisdichloro isomer is thermodynamically more stable than the corresponding trans-dichloro isomer.8b This circumstance is of importance because it is generally accepted that cis-dichloro species have to isomerize to their trans-dichloro counterpart in order to be able to mediate olefin metathesis reactions.12 Energy barriers of this isomerization reaction are usually high, and thus their catalytic activity at low temperature is further decreased. However, at high temperature the initiation efficacy is governed by the relative thermodynamic stability of the cisand the trans-isomers.13 Herein we disclose a study of the impact of changing the NHC coligand ligand in a κ2(C,N)-(2-(benzo[h]quinolin-10yl)methylidene-bearing ruthenium-based latent preinitiator/ catalyst system on the olefin metathesis activity. Different NHC ligands were incorporated in the design, varying their steric load (cf. Scheme 2). As a rule of thumb, it can be assumed that the size reduction of the substituents of the N-aryl rings leads to higher conversions of sterically demanding substrates because of faster propagation.14 Size increase leads to faster initiation in the case of nonchelating alkylidene complexes15 and slower initiation in the case of chelating alkylidene complexes.16 The current work elucidates whether Scheme 2. Reaction Conditions for the Preparation of 4−8a

a For 4, M1 (1.0 equiv), 3 (1.2 equiv), Cu(I)Cl (1.0 equiv); solvent: toluene; 80 °C; 17 h; yield: 41%; for 5: M32 (1.0 equiv), 3 (1.5 equiv); solvent: CH2Cl2; 25 °C; 48 h; yield 81%; for 6: M31 (1.0 equiv), 3 (1.2 equiv); solvent: CH2Cl2; 25 °C; 48 h; yield: 65%; for 7: 1 (1.0 equiv), 3 (1.3 equiv), and Cu(I)Cl (1.2 equiv); solvent: toluene; 80 °C; 15 min; yield: 74%; for 8: 2 (1.0 equiv), 3 (1.6 equiv), Cu(I)Cl (1.3 equiv); solvent toluene: 80 °C; 20 min; yield: 50%.

B


Article

Organometallics indicating the coexistence of conformers adopting a syn- and an anti-orientation of the methyl groups at the corresponding aryl substituents of the NHC ligand in solution,22 but the observed signal pattern is clearly in accordance with a trans-dichloro structure of these conformers. The chemical shift of the alkylidene’s proton in compounds 5−8 is hardly influenced by the nature of the NHC ligand and appeared as a singlet in the range 19.19−19.01 ppm. Only in the case of phosphine ligated 4 was a distinct shift into the low field (19.53 ppm) noted. Complexes 6−8 featured almost the same chemical shift of the alkylidene’s carbon in the 13C NMR spectrum (290.4−289.9 ppm), and the corresponding resonances in compounds 4 and 5 were detected at somewhat lower field (295.7 ppm in the case of 4 and 306.2 ppm in the case of 5). Similarly, the peaks assigned to the coordinating NHC carbon do not show significant variations in their chemical shifts (217.7−214.7 ppm). The NMR spectra of the cis-dichloro-configured compound cis-6 exhibited the characteristic features for a C1 symmetric compound, i.e., four signals for the −CH2−CH2− moiety of the dihydroimidazolyl part, four resolved singlets for protons bonded to the mesityl moieties, and six singlets for the six methyl groups of the compound. The alkylidene’s proton gave a singlet at 19.10 ppm in the 1H NMR spectrum, and characteristic 13C NMR signals constituted the alkylidene’s carbon signal at 291.2 and the NHC’s carbon signal at 216.7 ppm. A single-crystal X-ray structure determination complemented the characterization of cis-6. Single-crystal X-ray crystallography of compounds 4, 5, 7, and 8 confirmed the presence of a trans-dichloro configuration also in the solid state, while cis-6 crystallizes in its cis-dichloro configuration (the structure of the trans-isomer 6 has been published recently20). As expected from previous studies,23 the solid-state structures of SIXyl and SITol derivatives 7 and 8 featured a syn-conformation of the N-xylyl or N-tolyl substituents of the NHC ligand. Solid-state structures of all presented compounds are characterized by displaced π-stacking of the benzo[h]quinoline moieties (cf. Supporting Information), resulting in a distinct distortion of the coordination geometry of the individual complexes. As exemplarily shown for compound 5 (see Figure 1a and b), a parallel displaced facecentered stacking arrangement of the two nonheterocyclic benzo[h]quinoline rings featuring an interplanar distance of 3.46 Å and a displacement factor (R) of 0.87 Å is dominating the solid-state structure.24 The thereof resulting distortion of the solid-state geometries of the compounds under investigations impedes a discussion of bond lengths or angles in regard to the later discussed reactivity of the compounds. Nevertheless, with the necessary precaution, the following conclusions can be drawn: (a) the Ru−alkylidene bond length in 4−8 is hardly influenced by the different coligands and is typically 1.825 ± 0.004 Å (only in 6 was a discordant value of 1.814(4) Å determined), while cis-6 displays the shortest bond length (1.811(5) Å; (b) the Ru−N bond lengths for all NHCcontaining trans-compounds are quite similar to 2.107(3) Å in 6, 2.116(2) Å in 5, and 2.118(2) Å in both 7 and 8, while in 4 the longest Ru−N distance, 2.127(2) Å, is observed. The shortest Ru−N bond is present in cis-6, 2.061(4) Å. This is consistent with bond distances observed in the diastoreomeric pair of related complexes featuring five-membered chelate rings.25 In both cases, the Ru−alkylidene and Ru−N bond lengths are slightly shorter as compared to the corresponding trans-congeners; (c) the Ru−NHC bond lengths do not follow a trend and are quite similar regardless of the bulk of the NHC

Figure 1. (a and b) Solid-state structure of 5 as determined by X-ray crystallography showing the dominant parallel face-centered stacking arrangement of the benzo[h]quinoline fragment; (c and d) superposition of the solid state (gray) and the gas-phase geometry (yellow) as obtained from DFT calculations (c: view along the RuC bond; d: view along the NHC−Ru bond).

ligand (2.039(2) Å in 5, 2.033(5) Å in cis-6, 2.039(4) Å in 6, 2.027(2) Å in 7, and 2.026(2) Å in 8). To further illustrate the influence of packing on the solidstate geometry of the compounds, DFT calculations were performed to elucidate the ground-state geometry in the gas phase. A superposition of the solid-state and the gas-phase geometry of compound 5 (see Figure 1c and d) clearly reveals the degree of distortion in the solid state. Bonding distances retrieved from DFT calculations draw a slightly different picture: The Ru−alkylidene bond is in all cases longer when compared to the X-ray results and is hardly varying in the different complexes (RuC bond lengths range from 1.840 Å in 4 to 1.836 Å in 7 and 8). The Ru−N bond is the shortest in the SIMes-bearing complex 6 (2.137 Å), but compounds 5, 7, and 8 feature only slightly longer Ru−N bonds (2.140−2.142 Å). Finally, the Ru−NHC bond lengths show the biggest variations, ranging from 2.047 Å in 6 to 2.036 Å in 5 and 2.025 Å in 8. All in all it can be assumed that all trans-configured compounds are very similar concerning the discussed parameters. Therefore, we shifted the focus on quantifying the steric demand of the different coligands by determining the buried volume (Vbur in %) of the tricyclohexylphosphine and the NHC ligands in complexes 4−8 according to the C


Article

Organometallics

Figure 2. Steric maps for the Ru-L1 fragments of compounds 4−8 (based on calculated ground-state geometries); top view on the xy-plane (definition of the coordinate system as indicated in the sketch above, left using a depiction of complex 8: origin = Ru atom, x-axis defined by the RuC bond, y-axis orthogonal to the Ru−N bond, z-axis defined by the Ru−N bond).

literature.26 Because of the distortion of the complexes in the solid state, calculated ground-state structures and not X-ray structures were used as the starting point for the evaluation (data calculated from X-ray structures are presented in the Supporting Information). As expected, Vbur decreases from SIPr (32.0%) to SIMes (31.3%) to SIXyl (31.0%) to SITol (30.9%) and to PCy3 (27.1%) Nevertheless the difference between the biggest NHC ligand SIPr and the smallest NHC ligand SITol in these examples is surprisingly small (only 2.8%). Additionally, the steric demand of the L1 ligands was visualized using sterical maps.27 Sterical maps can be considered as classical physical maps (in which various landforms are outlined by different colors, lines, tints, shading, and spot elevations). In sterical maps the origin is defined by the ruthenium atom, and x-, y-, and z-axes are defined as shown in Figure 2 (top left corner); the sterics imposed by the coligand L1 are visualized by a type of height profile. The map represents the xy-plane viewed from the top. Brown areas indicate zones where the ligand L1 protrudes like a mountain through the xy-plane, while blue areas indicate empty zones below the xy-plane. The shape of the map represents the maximal extension of L1 as an equidistant projection on the xy-plane. In complex 4 no protrusion of the PCy3 ligand through the xy-plane occurs. All NHC-bearing compounds are characterized by protrusions in the −xy and −x−y quadrants, which originate from a shielding of the sixth coordination site at ruthenium from the N-aryl parts. Interestingly, the SIMes derivative 6 exhibits the smallest elevation. Compounds 4−8 were employed as the initiator in the ROMP of three different model monomers in order to benchmark their polymerization performance. First of all, the bulk polymerization of the liquid monomer (±)-endo,exo-

bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid diethyl ester (9) with initiators 4−8 was studied by means of simultaneous thermal analysis (STA). STA experiments allow for assessing the heat dissipation from the (exothermal) polymerization via the differential scanning calorimetry data and for assessing the polymerization yield evaluating the change in sample weight (unreacted monomers are evaporated at higher temperatures). A mixture of monomer and initiator ([9]:[initiator] = 500:1) was homogenized, cooled by liquid N2, and placed in alumina pans with a hole in the lid, which were put into the measurement chamber of the machine. A heat run with a heating rate of 3 K/min was commenced at a temperature of 20 °C until 500 °C was reached. Figure 3 shows the corresponding measurements. Polymerizations with initiators 6 and 7 gave high yields of poly9, namely, 86% and 91%, respectively. The polymerization commenced under these conditions at 80 ± 2 °C in the case of both initiators. The slightly higher yield in the reaction with 7 as the initiator can be attributed to the narrower exothermic peak, indicating a faster polymerization, which, in turn, leads to a lower monomer loss (monomer loss was attributed to a concurrent retro-Diels− Alder reaction and concomitant evaporation of the resulting products). By contrast, polymerizations with initiators 4, 5, and 8 resulted in low yields of poly9. In the case of using initiators 4 and 5 no clear sign for a polymerization exotherm was observed. Nevertheless, 35% and 23% of the monomer were polymerized, while the major part of the monomers was evaporated, as indicated by the endothermic peak. These results point to an activation of the initiators at higher temperatures than 80 °C, and polymerization and evaporation of the monomer are competing processes in these two cases. Finally, on using initiator 8 the observed mass loss of 81% was the highest. Nevertheless, as distinct from tries with 4 and 5, D


Article

Organometallics

species and/or initiator 7 was deactivated after this time. By contrast, initiator 6 produced 47% poly10 after 6 days and polymerization was still ongoing. Moreover, the progress of the polymerization is different; after an induction period of approximately 2 h, relatively fast polymerization occurred, which after approximately 16 h passed into a distinctly slower polymerization progress. Accordingly it can be reasoned that initiator 6 is converted to a less active species in a concurrent reaction, and initiator 7 is deactivated under these conditions. After benchmark reactions with monomers 9 and 10 gave a general idea about the thermostability of compounds 1−5 in the presence of a substrate, the initiators were tested in the polymerization of dicyclopentadiene (DCPD). Preliminary STA measurements using 100 ppm initiator and DCPD failed. In all cases only the retro-Diels−Alder reaction of DCPD could be observed.9a However, in real life applications, e.g., in reaction injection molding, typically closed molds are used to prevent such mass loss by evaporation of the monomer. Accordingly, polymerization tests with 50 ppm initiator were carried out at 110 °C for 24 h in closed molds. The evaluation of the mechanical properties of such obtained DCPD specimens was done by compression tests.28 The commercially available catalyst M2 (see Figure 5) was utilized for obtaining a

Figure 3. STA analysis of polymerization of 9, initiated by compounds 4−8. Reaction conditions: [9]:[initiator] = 500:1. Heating rate: 3 K/ min. Big symbols: Thermogravimetric analysis; small symbols: differential scanning calorimetry.

evaporation of 9 started at higher temperatures, which might be indicative for some polymerization progress at low temperatures which is then terminated because of a low lifetime of the actually active propagating species and/or the initiator itself. In a second benchmark reaction, (±)-endo,exo-bicyclo[2.2.1]hept5-ene-2,3-dicarboxylic acid dimethyl ester (10) was employed in a solution polymerization in toluene at 110 °C. A ratio of 300 equiv of the monomer to 1 equiv of the respective initiator was chosen. The polymerization progress was monitored by taking samples over a period of 6 days and acquisition of 1H NMR spectra of these samples. Only polymerizations with initiators 6 and 7 showed conversions toward the polymer; in the case of all other initiators no evidence for polymerizations could be retrieved. Evaluating time/conversion plots of the polymerization initiated with 6 and 7 indicated different courses of the polymerization (cf. Figure 4). Initiator 7 gave 14% conversion toward poly10 after 6 days. After 18 h the progress of the polymerization stopped, indicating that the propagating Figure 5. (Top) Chemical structure of M2 (left); polymerization of DCPD showing a cartoon of the polymer’s structure consisting of a repeating unit with an unreacted cyclopentene-ring structure and a cross-linking point; P means polymer chain (right). (Bottom) Compressive strength diagram for polyDCPD specimens, prepared via initiation with M2, 5, 6, and 7.

reference material.9a Results of compression tests, initiated with 5−7 and M2, are depicted in Figure 5. Polymerizations initiated with 4 and 8 resulted in soft, jellylike specimens not suitable for mechanical testing. Initiator 7 produced a rubbery test piece characterized by an elastic modulus of 0.1 ± 0.05 MPa. Higher (140 °C) or lower (80 °C) temperatures resulted in specimens with very similar mechanical properties. Initiator 5 performed better, yielding a material with an elastic modulus of 0.3 ± 0.05 MPa at 110 °C and of 5 ± 0.05 MPa at 140 °C. Only initiators 6 and M2 gave hard solid specimens (elastic modulus exceeding the measuring range of the machine), which were further investigated by tensile testing of appropriate shoulder test bars. Using 50 ppm of initiator 6 polyDCPD test bars exhibiting an elastic modulus of 1.48 ± 0.25 GPa and an

Figure 4. Polymerization of 10. [10]:[initiator] = 300:1; [10] = 0.10 mol/L; reaction temperature = 110 °C, solvent: toluene; inert atmosphere of N2. No conversion toward poly10 was noted for the reaction with initiators 4, 5, and 8. Lines are visual aids. E


Article

Organometallics ultimate strength of 36 ± 2 MPa were obtained. By comparison, 20 ppm M2 and a temperature of 80 °C yielded a material characterized by an elastic modulus of 1.64 ± 0.2 GPa and an ultimate strength of 48 ± 1 MPa, which is within the specifications of industrially produced polyDCPD.29 Furthermore, a mixture of DCPD and initiator 6 was stored for 2 weeks at room temperature under ambient conditions, whereupon no change of the formulation’s viscosity was noted. Afterward the formulation was cured at 110 °C, yielding specimens characterized by the same mechanical properties as those prepared directly after mixing of 6 and DCPD. Accordingly, initiator 6 is particularly suited for thermally triggered polymerization of DCPD, as the monomer and the initiator can be stored together without causing a polymerization. For a better understanding of the observed phenomena the thermal stability of the initiators in the absence of a substrate was studied. For this purpose a solution of the corresponding initiator in CDCl3 (7 mM) was prepared and subsequently heated at 140 °C for 1 h in a microwave reactor. Most likely, initiator 5 underwent a trans−cis isomerization, as indicated by the formation of a second alkylidene-bearing product present in approximately 40%. The mixture of this new product and unchanged 5 could not be separated. Prolonged microwave irradiation did not change the product mixture. NMR spectroscopic data suggest that the corresponding cis-dichloro isomer cis-5 formed (cf. Supporting Information). This hypothesis was borne out by the corresponding experiment with 6. In this case conversion of 6 toward cis-6 could be observed. Upon increasing the reaction time to 8 h at 140 °C, cis-6 could be isolated in 93% yield. Accordingly, the concurrent, deactivating reaction in the case of initiator 6 observed in the polymerization of monomer 10 is most probably a trans−cis dichloro isomerization process. Initiators 7 and 8 decomposed to at least one substance that did not feature a RuCHR moiety. Although we were not able to fully characterize the products, it is likely that a similar C−H activation reaction occurred as described for similar benzylidene complexes of these coligands.30 NMR data point in the same direction (cf. Supporting Information). Therefore, the poor performance of 7 and 8 at high temperature is tentatively explained by a decomposition reaction forming olefin-metathesis-inactive products. Finally, DFT studies were carried out with the aim to learn about the relative thermodynamic stabilities of the corresponding trans- and cis-isomers. Figure 6 shows the results of simulating three different solvation situations. For compound 4 the trans-dichloro-configured isomer is thermodynamically more favored (ΔEtrans−cis in toluene = −4.4 kcal/mol). The SiPr derivative 5 and the SiMes derivative 6 prefer the cisdichloro configuration (5: ΔEtrans−cis in toluene = 1.1 kcal/mol; 6: ΔEtrans−cis in toluene = 2.7 kcal/mol), while again the transisomer is favored in the case of 7 and 8 (7: ΔEtrans−cis in toluene = −2.5 kcal/mol; 8: ΔEtrans−cis in toluene = −1.5 kcal/mol). Consequently, it can be stated that the energetic preference of the cis-dichloro isomer over its trans-dichloro counterpart is decisively determined by the nature of the NHC ligand, with polar solvent additionally stabilizing the cis-isomer.31 Additionally the energetic barrier for reaching the catalytically active 14electron species from the trans-dichloro isomer was assessed. To reach the transition state for the dissociation of the Ru−N bond, an energy of 28.8 kcal mol−1 is needed in the case of complex 6. (cf. Figure 7). In complexes 5 (29.3 kcal mol−1), 7

Figure 6. Difference between the calculated thermodynamic stabilities of the trans-configured and the cis-configured isomers (ΔEtrans−cis); solvation model PCM.

Figure 7. Energy content of the transition state for the dissociation of the Ru−N bond (blue bar) and the 14-electron species (green bar) relative to the corresponding 16-electron species of 5, 6, 7, and 8 (calculated for CH2Cl2; solvation model PCM).

(30.4 kcal mol−1), and 8 (30.6 kcal mol−1) the barrier is slightly higher. However, substrate-induced activation via precoordination at the sixth coordination site might change the picture in practice.32 Finally, the energy contents of the corresponding 14electron species relative to the corresponding 16-electron complexes were calculated. The energy content of the 14electron species of 6 is 22.6 kcal/mol higher than that of the respective 16-electron species (cf. Figure 7). The corresponding 14-electron intermediates of 5, 7, and 8 display energy contents 26.8, 24.6, and 24.9 kcal/mol higher than their corresponding 16-electron species. In other words, the equilibrium concentration of the 14-electron species in the case of 6 is higher than in the other complexes under investigation (assuming that there is no follow-up reaction of the 14-electron species). All these data support the experimental finding that preinitiator 6, featuring the SIMes ligand, exhibits the best balance between latency at room temperature and appealing polymerization activity at elevated temperatures.

■

CONCLUSIONS In summary, we disclosed the effect of changing the steric situation at the NHC ligand in a particularly latent secondgeneration olefin metathesis (pre)initiator design on the polymerization activity at elevated temperatures. Complexes bearing a 2,6-diisopropyl (SIPr)-, 2,4,6-trimethyl (SIMes)-, 2,5F


Article

Organometallics

Netzsch STA 449C simultaneous thermal analyzer (aluminum crucibles from Netzsch) operated with a helium flow rate of 50 mL/ min in combination with a protective flow of 8 mL/min. As microwave reactor an Initiator Eight from Biotage was used. The reactions were performed in sealed microwave-assisted synthesis process vials designed exclusively for the single-mode microwave system. Tensile (and compression) tests were carried out at room temperature on an electromechanical universal testing machine (AGS-X by Shimadzu) equipped with a 50 kN load cell. Samples were tested at a test rate of 1 mm/min. The mean Young’s moduli were calculated from data obtained from the initial linear slope of the stress/strain plot with two to four samples of the same composition. Preparation of trans-Dichloro-(κ2(C,N)-(2-(benzo[h]quinolin10-yl)methylidene))(tricyclohexylphosphine)Ru (4). In a Schlenk flask, 280.0 mg of M1 (0.30 mmol, 1.0 equiv), 74.7 mg of ligand 3 (0.36 mmol, 1.2 equiv), and 30.3 mg of Cu(I)Cl (0.30 mmol, 1.0 equiv) were dissolved in toluene (14 mL). The reaction mixture was stirred and heated at 80 °C for 17 h. After 2 h a green precipitate formed, which gradually redissolved in the course of the reaction. Afterward the reaction mixture was evaporated to dryness, and the residue was redissolved in acetone. Insoluble components were removed by filtration, and acetone was removed by evaporation. The residue was column chromatographically purified (SiO2, CH2Cl2/ methanol = 10:1 (v/v) sampling the green spot at Rf = 0.9). Removal of the solvents, redissolving in CH2Cl2, and precipitation upon addition of n-pentane released pure 4 as a dark green, microcrystalline powder. Yield: 79.5 mg (41%). Anal. Calcd for C32H42Cl2NPRu (643.63): C, 59.71; H, 6.58; N, 2.18. Found: C, 59.57; H, 6.51; N, 2.13. 1H NMR (20 °C, CDCl3, 300 MHz): δ 19.53 (d, 3JHH = 10.3 Hz, RuCH), 9.60 (m, 1H, 3JHH = 5.0 Hz, Hbquin2), 8.32 (d, 1H, 3JHH = 7.6 Hz, Hbquin7), 8.30 (d, 1H, 3JHH = 7.6 Hz, Hbquin4), 8.18 (d, 1H, 3JHH = 7.7 Hz, Hbquin9), 8.05 (d, 1H, 3JHH = 8.8 Hz, Hbquin5), 7.91 (t, 1H, 3 JHH = 7.8 Hz, Hbquin8), 7.84 (dd, 1H, 3JHH = 5.0 Hz, 7.6 Hz, Hbquin3), 7.84(d, 1H, 3JHH = 8.9 Hz, Hbquin6), 2.48 (q, 3H, 3JHH = 11.3 Hz, HCy31), 2.23 (d, 6H, 3JHH = 11.5 Hz, HCy3), 2.04−1.82 (m, 12H HCy3), 1.74 (s, 3H, HCy3), 1.34 (s, 9H, HCy3). 13C NMR (22 °C, CDCl3, 75 MHz): δ 295.7 (s, 1C, RuCH), 149.0 (1C, Cbquin2), 145.1 (1Cq, Cbquin10b), 141.7 (1Cq, Cbquin4a), 137.8 (1C, Cbquin7), 136.5 (1Cq, Cbquin6a), 130.3 (1C, Cbquin8), 129.9 (1C, Cbquin4), 129.5 (1Cq, Cbquin10a), 128.7 (1C, Cbquin9), 127.4 (1C, Cbquin5), 126.1 (1C, Cbquin6), 122.7 (1Cq, Cbquin10), 121.8 (1C, Cbquin3), 53.4 (2C, C2HIm), 34.5, 30.3, 28.0, 26.4 (18C, CCy3). 31P NMR (22 °C, CDCl3, 200 MHz): δ 33.31 (RuPCy3). Preparation of trans-Dichloro-(κ2(C,N)-(2-(benzo[h]quinolin10-yl)methylidene))(1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene))Ru (5). The complex M32 (150 mg, 0.18 mmol, 1.0 equiv) was dissolved in dry, freshly degassed CH2Cl2 (9 mL), and the ligand 3 (55 mg, 0.26 mmol, 1.5 equiv) was added. The mixture was stirred for 48 h at room temperature, while the color turned from red to bright green. The reaction mixture was evaporated to dryness, and the crude product was purified via column chromatography on silica gel (cyclohexane/ethyl acetate = 3:1 (v/ v); Rf = 0.43). Yield: 110 mg (81%), light green powder. Anal. Calcd for C41H47Cl2N3Ru (753.81): C, 65.33; H, 6.28; N, 5.57. Found: C, 65.21; H, 6.20; N, 5.51. 1H NMR (20 °C, CDCl3, 300 MHz): δ 19.06 (s, 1H, RuCH); 8.61 (d, 1H, 3JHH = 5.2 Hz, Hbquin2), 8.13 (d, 1H, 3 JHH = 8.1 Hz, Hbquin7), 8.07 (d, 1H, 3JHH = 8.1 Hz, Hbquin4), 7.89 (d, 1H, 3JHH = 8.8 Hz, Hbquin5), 7.67−7.59 (m, 4H, Hbquin6,8, HSIPr), 7.52− 7.44 (dd, 1H, 3JHH = 8.1 Hz, 3JHH = 5.2 Hz, Hbquin3; m, 4H, HSIPr), 6.96 (d, 1H, 3JHH = 6.9 Hz, Hbquin9), 4.20 (s, 4H, HH2Im), 3.77 (4H, HiPr‑CH), 1.35−1.16 (d, 24H,, 3JHH = 6.9 Hz, HiPr‑CH3). 13C{1H} NMR (20 °C, CDCl3, 125 MHz): δ 306.2 (1C, RuCH), 217.7 (1C, CNN), 149.4 (1C; Cbquin2), 148.6, 145.9, 141.7, 137.2, 136.1, 129.9, 129.8, 129.5, 129.3, 128.6, 127.0, 125.4, 124.7, 122.8, 121.3 (25C, Cbquin2−10, CSIPr1−6), 54.8 (2C, CH2Im), 27.1 (4C, CiPr‑CH), 23.9 (8C, CiPr‑CH3). Preparation of cis-Dichloro-(κ2(C,N)-(2-(benzo[h]quinolin10-yl)methylidene))(1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene))Ru (cis-6). Pathway A: A Schlenk tube equipped with a stirring bar was charged with M31 (1.81 g, 2.42 mmol, 1.0 equiv) and ligand 3 (597 mg, 2.91 mmol, 1.2 equiv). The tube was

dimethyl (SiXyl)-, and 2-methyl (SITol)-substituted 1,3-bisaryl4,5-dihydroimidazol-2-ylidene were prepared. In addition, the corresponding tricyclohexylphosphine derivative was investigated for comparison. All compounds were isolated as their trans-dichloro isomers. NMR spectroscopic features as well as structural features are, particularly within the NHC-bearing complexes, very similar, but their polymerization activity at elevated temperatures is distinctly different. While 6 (the SIMes derivative) and with some limitations 7 (the SIXyl derivative) show the desired properties, i.e., latency at room temperature and polymerization activity at elevated temperature, compounds 4, 5, and 8 do not. Preinitiator 5 (the SIPr derivative) is the most latent one, needing temperatures > 140 °C to show moderate activity in the polymerization of DCPD. Compounds 7 and, in particular, 8 are stable and latent at room temperature, but upon heating, a decomposition reaction occurs, diminishing the polymerization activity at elevated temperatures. The superior performance of 6 in the investigated reactions could be rationalized with DFT calculations. Preinitiator 6 is the easiest to activate, yielding the 14-electron complex with the lowest energy content with respect to the 16-electron species. Moreover, evaluation of the steric maps of all compounds revealed that the sixth coordination site is least blocked by the SIMes ligand in 6. This feature is offering if at all the possibility of the precoordination of a substrate possibly decreasing the energy necessary for the decoordination of the N-donor ligand. Finally calculations reveal a distinct influence of the nature of the NHC ligand on the position of the equilibrium of cis- and trans-dichloro isomers. While the SIPr and the SIMes derivatives prefer in all studied solvents the cis-configuration, all other derivatives favor at least in solvents with low dielectric constant the trans-configuration. These computational findings were supported by the isolation and full characterization of cis-6 obtained upon prolonged heating of 6 at 140 °C. Preinitiator 6 is of particular interest, because it can be stored together with dicyclopentadiene for at least 14 d at room temperature without showing signs of polymerization of the monomer. Curing of this formulation at 110 °C leads to polyDCPD specimens that meet the specifications of industrially produced polyDCPD.

■

EXPERIMENTAL SECTION

General Considerations. All preparation steps were carried out under inert conditions (N2 atmosphere) with degassed, dry solvents, unless otherwise noted. Precursor complexes M1, M2, M31, and M32 were provided by Umicore AG. Complexes 1 and 2 were prepared according to the literature.23 Carbene precursor 10-vinylbenzo[h]quinoline (3) and initiator trans-dichloro-(κ2(C,N)-(2-(benzo[h]quinolin-10-yl)methylidene))(1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene))Ru (6) were prepared according to literature procedures.20 Monomers 9 and 10 were prepared according to literature protocols.7b Dicyclopentadiene (98%) was purchased from Sigma-Aldrich and was stored liquidized (upon addition of 30 μLtoluene/mLDCPD) under a N2 atmosphere. All other chemicals were purchased from commercial resources (Alfa Aesar, Sigma-Aldrich, Roth) and used as received. For polymerization toluene (Chromasolv for HPLC, 99.9%) from Sigma-Aldrich was used. NMR spectra were recorded on a Bruker Avance 300 MHz or a Varian INOVA 500 MHz spectrometer, respectively, and were referenced to SiMe4 or residual CHCl3 in CDCl3 (1H to 7.26 ppm, 13C to 77.0 ppm). Chemical shifts (δ) are given relative to TMS, and coupling constants (J) in Hz. Deuterated solvents were obtained from Cambridge Isotope Laboratories Inc. X-ray measurements were performed on a Bruker AXS Kappa APEX II diffractometer using Mo Kα radiation. Simultaneous thermal analysis measurements were performed with a G


Article

Organometallics

Preparation of trans-Dichloro(κ2(C,N)-(2-(benzo[h]quinolin10-yl)methylidene))(1,3-bis(2-methylphenyl)-4,5-dihydroimidazol-2-ylidene))Ru (8). Precursor complex 2 (96.5 mg, 0.11 mmol, 1.0 equiv), ligand 3 (36.6 mg, 0.18 mmol, 1.6 equiv), and Cu(I)Cl (14.4 mg, 0.14 mmol, 1.3 equiv) were placed in a Schlenk flask, and dry, degassed toluene (8 mL) was added. The reaction mixture was heated to 80 °C and stirred for 20 min, whereupon a precipitate formed, which was separated by filtration and dried in vacuo. Yield: 33.0 mg (50%) of dark green microcrystals. Anal. Calcd for C31H27Cl2N3Ru (613.54): C, 60.69; H, 4.44; N, 6.85. Found: C, 60.87; H, 4.52; N, 6.76. 1H NMR (300 MHz, CDCl3, 25 °C): δ 19.01 (bs, 1H, RuCH), 8.79 (bs, 1H, Ho‑tol), 8.59 (bs, 1H, Hbquin2), 8.16 (d, 1H, 3JHH = 7.9 Hz, Hbquin4), 8.13 (d, 1H, 3JHH = 8.3 Hz, Hbquin7), 7.92 (d, 1H, 3JHH = 8.9 Hz, Hbquin5), 7.69 (d, 1H, 3JHH = 8.9 Hz, Hbquin6), 7.61 (t, 1H, 3JHH = 7.5 Hz, 7.8 Hz, Hbquin8), 7.66−7.44 (bs, 5H, Ho‑tol), 7.50 (dd, 1H, 3JHH = 5.3 Hz, 7.5 Hz, Hbquin3), 7.20 (bs, 2H, Ho‑tol), 6.92 (bs, 1H, H bquin9), 4.44, 4.09 (bs, 4H, HH2Im), 2.69 (s, 6H, Ho‑tol‑CH3). 13C{1H} NMR (75 MHz, CDCl3, 25 °C): δ 290.3 (s, 1C, RuCH), 214.7 (1Cq, CNN), 149.1 (1C; Cbquin2), 146.2 (1Cq, Cbquin10b), 142.1 (1C, Cq, Cbquin4a), 141.5, 138.3 (2C, Cq, Co‑tol) 137.2 (1C, Cbquin7), 136.0 (1Cq, Cbquin6a), 132.5 (1C, Co‑tol), 131.0 (1C, Co‑tol), 130.0 (1C, Co‑tol), 129.9 (1C, Cq, Co‑tol), 129.7 (2C, Cbquin4,8), 129.0 (1C, Co‑tol), 128.7 (1C, Co‑tol), 128.6 (1C, Cbquin5), 128.5 (1C, Cq, C bquin10a), 127.8 (1C, 1Cq, Co‑tol), 126.9 (1C, Cbquin6), 124.3 (1C, Cbquin9), 123.2 (1Cq, Cbquin10), 121.3 (1C, Cbquin3), 54.4, 51.4 (2C, CH2Im), 19.6, 18.4 (2C, Co‑tol‑CH3). Polymerization. A Schlenk flask was charged with the appropriate amount of initiator 4−8 (1.27 μmol, 1.0 equiv), and toluene (3.8 mL) was added. Subsequently, the solution was heated to 110 °C, and then monomer 9 (100.0 mg, 0.48 mmol, 300 equiv) dissolved in toluene (1 mL, overall concentration of monomer in the reaction mixture = 0.1 M) was added. The reaction mixture was monitored by TLC using cyclohexane/ethyl acetate (3:1, v/v) as eluent and KMnO4 solution for staining. Further monitoring was accomplished by 1H NMR spectroscopy. For that purpose, 200 μL of the reaction mixture was taken out, ethyl vinyl ether (50 μL) was added, and after 10 min the solvent was removed and the residue was dried in vacuo. The residue was dissolved in CDCl3, and conversion toward poly9 was determined upon integration of the peaks corresponding to the double bonds of 9 (δ = 6.28 and 6.07 ppm) and the double bonds assigned to poly9 (δ = 5.57−5.08 ppm). STA Analysis. A stock solution of the initiators 4−8 or cis-6 in CH2Cl2 was prepared (c[Ru] = 1.4 mM). A 60 μL amount of this stock solution was transferred into a 2 mL glass vial, and 100 μL of monomer 10 (ρ = 1.0 g/mL, 0.42 mmol) was added and well mixed. The solvent was removed by a N2 stream, and the residue was shock frozen by liquid nitrogen. About 8−12 mg of this mixture was transferred into a precooled DSC pan, which was then placed in the machine, and the STA run (heating rate of 3 K/min, starting at 20 °C) was commenced. Mechanical Testing. The initiator M2 (4.4 mM, 20 ppm) or 4−8 (c[Ru] = 11 mM, 50 ppm) was weighed into a 50 mL Eppendorf tube, and toluene (900 μL) was added. To the initiator solution was added 30.9 mL of liquidized monomer 11 and well mixed. Half of the solution (15.9 mL) was transferred to a 20 mL glass vial (d = 25 mm) and heated for 24 h at 80, 110, or 140 °C. The second part of the sample was stored at room temperature for 2 weeks and was then polymerized at 110 °C. For compression tests 8 mm slices of properly polymerized samples were prepared by abrading. The analysis was performed with a speed rate of 5 mm/min until either a force of 5000 N or a sample height of 2 mm was reached. Tensile testing was performed on specimen prepared with initiator 6. For this purpose a formulation as mentioned above was prepared and molded into a shouldered test bar steel mold, which was covered with a glass plate and cured for 24 h at 110 °C. Samples were tested at a test rate of 1 mm/min. The mean Young’s moduli were calculated from data obtained from the initial linear slope of the stress/strain plot with two independently prepared test specimens. Isomerization. A solution of the corresponding initiator in CDCl3 (7 mM) was prepared and subsequently heated at 140 °C for 1 h in a

flushed with argon, and anhydrous, degassed CH2Cl2 (30 mL) was added. The purple reaction mixture was stirred at 25 °C for 48 h, whereupon the color changed to dark green. The volume of the solvent was reduced to about 5 mL, and upon addition of Et2O (50 mL) a green precipitate formed, which was filtered off. The residue was washed with Et2O (3 × 10 mL) and dried in vacuo. Yield: 1.43 mg (88%) of a green solid. Subsequently column chromatography on silica gel (CH2Cl2/MeOH = 10:1 (v/v)) was accomplished, leading to the isolation of two compounds in pure form: trans-6, 1.05 g (65%) of a green solid (Rf = 0.9), and cis-6, 35 mg (2%) of a dark green solid (Rf = 0.6). Pathway B: 15 mg of trans-6 was dissolved in 1 mL degassed CHCl3 and heated for 8 h at 140 °C in a microwave reactor, whereupon a green precipitate formed. The solvent was evaporated, and the green microcrystalline residue was dried under vacuum. Yield: 14 mg (93%). Anal. Calcd for C35H35N3Cl2Ru (669.65): C, 62.78; N, 6.27; H, 5.27. Found: C, 62.96; N, 6.15; H, 5.41. 1H NMR (25 °C, 300 MHz, CDCl3): δ 19.19 (s, 1H, RuCH), 8.43 (d, 1H, 3JHH = 5.0 Hz, Hbquin2), 8.18 (d, 1H, 3JHH = 7.9 Hz, Hbquin5), 8.10 (d, 1H, 3JHH = 7.9 Hz, Hbquin4), 7.94 (d, 1H, 3JHH = 8.7 Hz, Hbquin7), 7.69 (d, 1H, 3JHH = 9.5 Hz, Hbquin9), 7.68 (t, 1H, 3JHH = 7.9 Hz, Hbquin6), 7.38 (d, 1H, 3JHH = 8.7 Hz, Hbquin8), 7.29 (t, 1H, 3JHH = 7.5 Hz, Hbquin3), 7.34, 7.17, 7.06, 7.01 (s, 4H, Hmes), 4.81−3.22 (m, 4H, HH2Im), 3.06, 2.72, 2.53, 2.18, 1.38, 0.37 (bs, 18H, Hmes‑CH3). 1H NMR (25 °C, 300 MHz, CD2Cl2): δ 19.10 (s, 1H, RuCH), 8.34 (ds, 1H, 3JHH = 5.4 Hz, 4JHH = 1.1 Hz, Hbquin2), 8.29 (d, 1H, 3JHH = 7.8 Hz, Hbquin7), 8.19 (d, 1H, 3JHH = 7.5 Hz, Hbquin4), 8.05 (d, 1H, 3JHH = 8.6 Hz, Hbquin5), 7.79 (d, 1H, 3JHH = 8.6, Hbquin6), 7.77 (t, 1H, 3JHH = 7.5 Hz, Hbquin8), 7.43 (d, 1H, 3JHH = 7.3 Hz, Hbquin9), 7.37 (dd, 1H, 3JHH = 7.9 Hz, Hbquin3), 7.39, 7.12, 7.03, 5.59 (s, 4H, Hmes), 4.18−3.29 (m, 1H, HH2Im), 3.00, 2.69, 2.57, 2.19, 1.39, 0.37 (bs, 18H, Hmes‑CH3). 13C{1H} NMR (20 °C, 75 MHz, CD2Cl2): δ 291.2 (1C, RuCH), 216.7 (1Cq, CNN), 154.8 (1C, Cbquin2), 146.5 (1Cq, Cbquin10b), 141.0, 140.9, 139.4, 138.8, 138.7, 136.5, 136.1, 135.8, 135.6, 134.3 (10Cq, Cmes, Cbquin4a,6a), 137.5 (1C, Cbquin4), 131.7 (1C, Cmes), 130.9 (1C, Cbquin7), 130.4 (1C, Cmes), 130.2 (1C, Cbquin8), 130.1 (1C, Cmes), 130.0 (1Cq, Cbquin10a), 129.3 (1C, Cmes), 129.1 (1C, Cbquin5), 128.8 (1C, Cbquin9), 127.8 (1C, Cbquin6), 124.2 (1Cq, Cbquin10), 122.2 (1C, Cbquin3), 51.8, 51.6 (s, 2C, CH2Im), 21.7, 21.3, 20.6, 19.4, 18.8, 17.2 (6C, Cmes‑CH3). Preparation of trans-Dichloro-(κ2(C,N)-(2-(benzo[h]quinolin10-yl)methylidene))(1,3-bis(2,5-dimethylphenyl)-4,5-dihydroimidazol-2-ylidene))Ru (7). In a Schlenk flask charged with complex 1 (96.7 mg, 0.11 mmol, 1.0 equiv) dry degassed toluene (7 mL), ligand 3 (30.0 mg, 0.15 mmol, 1.3 equiv), and Cu(I)Cl (13.2 mg, 0.13 mmol, 1.2 equiv) were added. The reaction mixture was heated to 80 °C and stirred for 15 min, whereupon the reaction mixture turned green. The solvent was removed in vacuo, and the residue was redissolved in acetone. A white residue was filtered off, and the solvent was again removed. A concentrated solution of the residue in CH2Cl2 was prepared, which was then layered with n-pentane, leading to the formation of a green microcrystalline precipitate, which was filtered off and dried under vacuum. Yield: 52.1 mg (74%) of light green crystals. Anal. Calcd for C33H31Cl2N3Ru (641.60): C, 61.78; H, 4.87; N, 6.55. Found: C, 61.82; H, 4.93; N, 6.51. 1H NMR (300 MHz, CDCl3, 25 °C): δ 19.05 (s, 1H, RuCH), 8.70 (s, 1H, Hxyl), 8.55 (s, 1H, Hbquin2), 8.16 (d, 1H, 3JHH = 8.3 Hz, Hbquin7), 8.13 (d, 1H, 3JHH = 7.7 Hz, Hbquin4), 7.93 (d, 1H, 3JHH = 8.8 Hz, Hbquin5), 7.70 (d, 1H, 3JHH = 8.7 Hz, Hbquin6), 7.62 (t, 1H, 3JHH = 7.7 Hz, Hbquin8), 7.49 (t, 1H, 3JHH = 8.0 Hz, Hbquin3), 7.42 (d, 1H, 3JHH = 7.5 Hz, Hxyl), 7.35 (d, 2H,, 3JHH = 7.9 Hz, Hxyl), 6.91 (d, 1H, 3JHH = 6.6 Hz, Hbquin9), 7.21−6.29 (bs, 2H, Hxyl), 4.54, 4.53, 4.03 (bs, 4H, HH2Im), 2.63, 2.53, 2.13 (s, 12H, Hxyl‑CH3). 13C{1H} NMR (75 MHz, CDCl3, 25 °C): δ 289.9 (s, 1C, RuCH), 214.7 (1Cq, CNN), 148.8 (1Cq, Cbquin2), 146.5 (1C, Cq, Cbquin10b), 142.1 (1C, Cbquin4a), 141.0 (1C, Cq, CXyl) 137.8 (1C, CXyl), 137.1 (1C, Cbquin7), 135.9 (1C, Cq, Cbquin6a), 134.5, 133.4 (2C, Cq, CXyl), 132.5 (1C, CXyl), 132.2 (1C, Cq, CXyl), 131.5 (1C, Cq, CXyl), 130.6 (1C, Cq, CXyl), 130.4 (2C, CXyl), 129.9 (1C, Cbquin7), 129.6 (1C, Cq, Cbquin10a), 129.5 (1C, CXyl), 129.4 (1C, Cbquin4,8), 128.6 (1C, Cbquin5), 128.4 (1C, CXyl), 126.8 (1C, Cbquin6), 123.8 (1C, Cq, Cbquin9), 123.3 (1Cq, Cbquin10), 121.2 (1C, Cbquin3), 54.2, 51.0 (2C, CH2Im), 20.8, 20.6, 19.0, 17.8 (4C, Cxyl‑CH3). H


Article

Organometallics microwave reactor. Conversions were then determined by 1H NMR spectroscopy. Computational Details. The DFT geometry optimizations were performed at the GGA level with the Gaussian09 package,33 using the BP86 functional of Becke and Perdew.34 No symmetry constraint was used in the geometry optimizations, and the final geometries were confirmed to be minimum potential energy structures through frequency calculations. The electronic configuration of the molecular systems was described with the standard split valence basis set with a polarization function of Ahlrichs and co-workers for H, C, N, P, and Cl (SVP keyword in Gaussian09),35 and for Ru we used the small-core, quasirelativistic Stuttgart/Dresden effective core potential, with the associated valence basis set (standard SDD keywords in Gaussian09).36 The reported energies have been obtained through singlepoint calculations with the M06 functional of Truhlar. In these singlepoint calculations the electronic configuration of the molecular systems was described by a triple-ζ basis set for main group atoms (TZVP keyword in Gaussian09). 37 Solvent effects including contributions of nonelectrostatic terms have been estimated in single-point calculations on the gas-phase-optimized structures, based on the polarizable continuum solvation model PCM, using either toluene, chloroform, or dichloromethane as the solvent.38

■

Modicom, F.; Dumas, A.; Borré, E.; Toupet, L.; Baslé, O.; Mauduit, M. Beilstein J. Org. Chem. 2015, 11, 1541−1546. (3) Zirngast, M.; Pump, E.; Leitgeb, A.; Albering, J. H.; Slugovc, C. Chem. Commun. 2011, 47, 2261−2263. (4) Savka, R.; Foro, S.; Gallei, M.; Rehahn, M.; Plenio, H. Chem. Eur. J. 2013, 19, 10655−10662. (5) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Nat. Chem. 2009, 1, 133−137. (6) (a) Wang, D.; Wurst, K.; Knolle, W.; Decker, U.; Prager, L.; Naumov, S.; Buchmeiser, M. R. Angew. Chem., Int. Ed. 2008, 47, 3267−3270. (b) Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2038−2039. (c) Vidavsky, Y.; Lemcoff, N. G. Beilstein J. Org. Chem. 2010, 6, 1106−1119. (7) (a) Ung, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y. Organometallics 2004, 23, 5399−5401. (b) Gstrein, X.; Burtscher, D.; Szadkowska, A.; Barbasiewicz, M.; Stelzer, F.; Grela, K.; Slugovc, C. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3494−3500. (8) (a) Vidavsky, Y.; Anaby, A.; Lemcoff, N. G. Dalton Trans. 2012, 41, 32−43. (b) Pump, E.; Cavallo, L.; Slugovc, C. Monatsh. Chem. 2015, 146, 1131−1141. (9) (a) Leitgeb, A.; Wappel, J.; Urbina-Blanco, C. A.; Strasser, S.; Wappl, C.; Cazin, C. S. J.; Slugovc, C. Monatsh. Chem. 2014, 145, 1513−1517. (b) Urbina-Blanco, C. A.; Bantreil, X.; Wappel, J.; Schmid, T. E.; Slawin, A. M. Z.; Slugovc, C.; Cazin, C. S. J. Organometallics 2013, 32, 6240−6247. (c) Bantreil, X.; Cazin, C. S. J. Monatsh. Chem. 2015, 146, 1043−1052. (10) (a) Kovačič, S.; Krajnc, P.; Slugovc, C. Chem. Commun. 2010, 46, 7504−7506. (b) Kovačič, S.; Preishuber-Pflügl, F.; Pahovnik, D.; Ž agar, E.; Slugovc, C. Chem. Commun. 2015, 51, 7225−7228. (11) (a) Songis, O.; Slawin, A. M. Z.; Cazin, C. S. J. Chem. Commun. 2012, 48, 1266−1268. (b) Bantreil, X.; Poater, A.; Urbina-Blanco, C. A.; Bidal, Y. D.; Falivene, L.; Randall, R. A. M.; Cavallo, L.; Slawin, A. M. Z.; Cazin, C. S. J. Organometallics 2012, 31, 7415−7426. (c) Abbas, M.; Slugovc, C. Tetrahedron Lett. 2011, 52, 2560−2562. (d) Abbas, M.; Slugovc, C. Monatsh. Chem. 2012, 143, 669−673. (e) Leitgeb, A.; Abbas, M.; Fischer, R. C.; Poater, A.; Cavallo, L.; Slugovc, C. Catal. Sci. Technol. 2012, 2, 1640−1643. (f) Abbas, M.; Leitgeb, A.; Slugovc, C. Synlett 2013, 24, 1193−1196. (12) (a) Benitez, D.; Tkatchouk, E.; Goddard, W. A., III Chem. Commun. 2008, 46, 6194−6196. (b) Credendino, R.; Poater, A.; Ragone, F.; Cavallo, L. Catal. Sci. Technol. 2011, 1, 1287−1297. (13) (a) Pump, E.; Poater, A.; Zirngast, M.; Torvisco, A.; Fischer, R.; Cavallo, L.; Slugovc, C. Organometallics 2014, 33, 2806−2813. (b) Strasser, S.; Pump, E.; Fischer, R. C.; Slugovc, C. Monatsh. Chem. 2015, 146, 1143−1151. (c) Wappel, J.; Grudzień, K.; Barbasiewicz, M.; Michalak, M.; Grela, K.; Slugovc, C. Monatsh. Chem. 2015, 146, 1153−1160. (14) Stewart, I. C.; Ung, T.; Pletnev, A. A.; Berlin, J. M.; Grubbs, R. H.; Schrodi, Y. Org. Lett. 2007, 9, 1589−1592. (15) Urbina-Blanco, C. A.; Leitgeb, A.; Slugovc, C.; Bantreil, X.; Clavier, H.; Slawin, A. M. Z.; Nolan, S. P. Chem. - Eur. J. 2011, 17, 5045−5053. (16) Courchay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules 2003, 36, 8231−8239. (17) Rivard, M.; Blechert, S. Eur. J. Org. Chem. 2003, 2003, 2225− 2228. (18) Urbina-Blanco, C. A.; Manzini, S.; Perez Gomes, J.; Doppiu, A.; Nolan, S. P. Chem. Commun. 2011, 47, 5022−5024. (19) Burtscher, D.; Lexer, C.; Mereiter, K.; Winde, R.; Karch, R.; Slugovc, C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 4630−4636. (20) Szadkowska, A.; Gstrein, X.; Burtscher, D.; Jarzembska, K.; Wozniak, K.; Slugovc, C.; Grela, K. Organometallics 2010, 29, 117− 124. (21) Aharoni, A.; Vidavsky, Y.; Diesendruck, C. E.; Ben-Asuly, A.; Goldberg, I.; Lemcoff, N. G. Organometallics 2011, 30, 1607−1615. (22) (a) Borguet, Y.; Zaragoza, G.; Demonceau, A.; Delaude, L. Dalton. Trans. 2015, 44, 9744−9755. (b) Perfetto, A.; Costabile, C.; Longo, P.; Grisi, F. Organometallics 2014, 33, 2747−2759.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00715. NMR spectra of 4−8, isomerization and degradation studies, details on the packing of the complexes in the solid state, details on mechanical testing and DFT calculations (PDF) X-ray structural data for 4, 5, cis-6, 7, and 8 (CCDC 1419405−1419409) (CIF) File containing all calculated structures (XYZ)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail (C. Slugovc): [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Authors acknowledge the OeAD for the WTZ PL10/2014 exchange grant. E.P. gratefully acknowledges the receipt of the “Chemical Monthly Fellowship” financed by Springer Verlag, the Austrian Academy of Sciences (Ö AW), and the Gesellschaft Ö sterreichischer Chemiker (GÖ CH). L.C. thanks the King Abdullah University of Science and Technology for supporting this research. The Polish authors thank the Foundation for Polish Science for the “Team”-program cofinanced by the European Regional Development Fund−Operational Program Innovative Economy 2007/2013.

■

REFERENCES

(1) (a) Naumann, S.; Buchmeiser, M. R. Macromol. Rapid Commun. 2014, 35, 682−701. (b) Monsaert, S.; Vila, A. L.; Drozdzak, R.; Van der Voort, P.; Verpoort, F. Chem. Soc. Rev. 2009, 38, 3360−3372. (c) Schanz, H.-J. Curr. Org. Chem. 2013, 17, 2575−2591. (2) (a) Opstal, T.; Verpoort, F. Angew. Chem., Int. Ed. 2003, 42, 2876−2879. (b) Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2038−2039. (c) Keitz, B. K.; Bouffard, J.; Bertrand, G.; Grubbs, R. H. J. Am. Chem. Soc. 2011, 133, 8498−8501. (d) Kozłowska, A.; Dranka, M.; Zachara, J.; Pump, E.; Slugovc, C.; Skowerski, K.; Grela, K. Chem. - Eur. J. 2014, 43, 14120−14125. (e) Schmid, T. E.; I


Article

Organometallics (23) (a) Torborg, C.; Szczepaniak, G.; Zieliński, A.; Malińska, M.; Woźniak, K.; Grela, K. Chem. Commun. 2013, 49, 3188−3190. (b) Grela, K.; Torborg, C.; Szczepaniak, G.; Zielinski, A. Patent WO2014027040 2014-02-20. (24) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885−3896. (25) Barbasiewicz, M.; Szadkowska, A.; Bujok, R.; Grela, K. Organometallics 2006, 25, 3599−3604. (26) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 2009, 1759−1766. See: https://www.molnac.unisa.it/OMtools/sambvca.php, July 28, 2015. (27) Poater, A.; Falviene, L.; Urbina-Blanco, C. A.; Manzini, S.; Nolan, S. P.; Cavallo, L. Procedia Comput. Sci. 2013, 18, 845−854. (28) Compression tests with the equipment at our disposal are limited by a maximum force of 5000 N, giving a maximum achievable stress of 10 ± 1 MPa for the test cylinder. Hence, this method is suited for only incompletely cured, elastic materials. Fully cured polyDCPD samples are subsequently tested by tensile strength analysis. (29) Slugovc, C. Industrial Applications of Olefin Metathesis Polymerization. In Olefin Metathesis, Theory and Practice; Grela, K., Ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2014. (30) (a) Hong, S. H.; Chlenov, A.; Day, M. W.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46, 5148−5151. (b) Vehlow, K.; Gessler, S.; Blechert, S. Angew. Chem., Int. Ed. 2007, 46, 8082−8085. (31) Benitez, D.; Goddard, W. A., III J. Am. Chem. Soc. 2005, 127, 12218−12219. (b) Correa, A.; Cavallo, L. J. Am. Chem. Soc. 2006, 128, 13352−13353. (32) (a) Szadkowska, A.; Ż ukowska, K.; Pazio, A. E.; Woźniak, K.; Kadyrov, R.; Grela, K. Organometallics 2011, 30, 1130−1138. (b) Thiel, V.; Hendann, M.; Wannowius, K.-J.; Plenio, H. J. Am. Chem. Soc. 2012, 134, 1104−1114. (c) Trzaskowski, B.; Grela, K. Organometallics 2013, 32, 3625−3630. (d) Ż ukowska, K.; Szadkowska, A.; Trzaskowski, B.; Pazio, A.; Pączek, Ł.; Woźniak, K.; Grela, K. Organometallics 2013, 32, 2192−2198. (e) Ż ukowska, K.; Pump, E.; Pazio, A. E.; Woźniak, K.; Cavallo, L.; Slugovc, C. Beilstein J. Org. Chem. 2015, 11, 1458−1468. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2009. (34) (a) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (b) Perdew, J. P. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 33, 8822−8824. (c) Perdew, J. P. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34, 7406−7406. (35) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (36) (a) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105, 1052−1059. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535− 7535. (c) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 78, 1211−1224. (37) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (38) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027−2094.

J


Variation of the Sterical Properties of the N-Heterocyclic Carbene

Recommend Documents