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Stereoselective Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidenes Containing O‑Chelating N‑Heterocyclic Carbenes: Influence of Syn/Anti Interconversion and Polymerization Rates on Polymer Structure Christina Lienert,† Wolfgang Frey,‡ and Michael R. Buchmeiser*,†,§ †

Institute of Polymer Chemistry and ‡Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany § German Institutes of Textile and Fiber Research (DITF), Körschtalstr. 26, D-73770 Denkendorf, Germany S Supporting Information *

ABSTRACT: The structures of the first insertion products (syn/anti, cis/trans) in the ring-opening metathesis polymerization (ROMP) of norbornene derivatives using both neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes based on an O-chelating NHC, i.e., [Mo(N-2,6-Me2-C6H3)(N-mesityl-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe 2 Ph)(OTf)] (1), [Mo(N-2,6- i Pr 2 C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe 2Ph)(OTf)] (2), and [Mo(N-2,6-iPr2-C6H3 )(N2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CH3CN)(CHCMe2Ph)+ (B(ArF)4−)] (3), have been identified. Also, syn/anti interconversion rates of catalysts 1−3 have been determined in acetonitrile. Correlation of these values with the rate constants of polymerization revealed the importance of a balanced ratio between these two values. Disrupting that balance by changing the solvent or the monomer or by switching to a similar, but more ROMP-active catalyst leads to significant changes in the cis/trans contents of the resulting polymers. Despite the chelating and bulky nature of the ligands, a mechanism that entails inversion at the metal center of the catalyst during polymerization is proposed. Thus, highly cis-syndiotactic ring-opening metathesis polymerization-derived polymers have been prepared with the aid of 3. On the basis of our results, we propose a comprehensive mechanism for the formation of cis- and trans-configured polymers with molybdenum imido complexes containing an O-chelating NHC.

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INTRODUCTION Well-defined molybdenum and tungsten imido alkylidene complexes have been used to synthesize stereoregular ringopening metathesis polymerization (ROMP)-derived polymers over the past decades.1−8 Schrock catalysts of the general formula [M(NR)(CHR′)(OR″)2] (M = W, Mo),9 [M(NR)(CHR′)(diolate)],10 and [M(NR)(CHR′)(OR″)(pyrrolide)], also known as MAP catalysts,11,12 allow for the synthesis of tailored all-cis or all-trans polymers with either isotactic (it) or syndiotactic (st) structures.8 More recently, we developed molybdenum alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR)(NHC)(CHR′)(OR″)2],13 which displayed substantial functional group tolerance in ROMP and the cyclopolymerization of α,ω-diynes. Cationic versions thereof as well as tungsten oxo and tungsten imido alkylidene NHC catalysts, [W(O)(NHC)(CHR′)(OR″) + B(Ar F ) 4 − ] (B(Ar F ) 4 − = tetrakis(3,5-bistrifluoromethylphenyl)borate), have been prepared, too, and allow for running various types of olefin metathesis reactions with high activity and productivity, particularly when supported on silica; however, in many cases at the expense of functional group tolerance.14−21 © XXXX American Chemical Society

One particularity of ROMP is the formation of either cis- or trans-double bonds along the polymer chain. Their formation during ROMP is strongly related to the catalyst used. Schrock catalysts exist in form of two isomers: a syn-isomer with the substituent at the alkylidene pointing toward the imido ligand and an anti-isomer with the substituent at the alkylidene pointing away from the imido ligand. An established means to distinguish between syn- and anti-isomers, which are in equilibrium, is the value of the JCH coupling constant between the alkylidene carbon and hydrogen. Thus, an agostic interaction of the C−H bond with an empty orbital at the molybdenum in the syn-isomer is believed to render it more stable, explaining the finding that the equilibrium is usually far on the side of the syn-isomer and that the anti-isomer is usually considered the more reactive one.10 It has been shown for Schrock catalysts that syn- and antiisomers are crucial in terms of cis/trans linkages in ROMP derived polymers.22 Interestingly, O-chelated Mo imido Received: April 24, 2017 Revised: June 25, 2017

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DOI: 10.1021/acs.macromol.7b00841 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Different Possibilities for the [2 + 2] Cycloaddition of Norborn-2-ene to O-Chelated Mo Imido Alkylidene NHC Complexes and Structures of the First-Insertion Products

Scheme 2. Proposed Mechanism for the Formation of a Trans, Isotactic Polymer via a “Flipping” Rearrangement at the Metallacyclea

a

Ad = adamantyl; HIPTO = 2,6-(2,4,6-iPr3)2C6H3-O-.

after a syn/eneanti or an anti/enesyn insertion step. Vice versa, a cis-double bond provides evidence for a syn/enesyn or an anti/ eneanti reaction (Scheme 1).7 If more than one insertion step is considered, tacticity has to be included as well. The monomer can either add from the same side of the CNO face23 in each step to yield an it polymer or in an alternating fashion to the front and the backside of the CNO face, which leads to st polymers. The mechanism shown above is based on the assumption that any pseudorotation of the metallacycle is slower than the ring-opening of the metallacycle. However, examples have been reported where a rearrangement of metallacycle is proposed during which the configuration of the metal center is retained

alkylidene NHC complexes consist of a large fraction of antiisomer (83−85%) and thus provide an interesting opportunity to study the reactivity of both the syn- and anti-isomer and the effect on polymer structure. In case a norbornene derivative is used as monomer, there are two possibilities of insertion. The monomer can be inserted either with the bridgehead carbon pointing toward the imido ligand (enesyn) or with the bridgehead carbon pointing away from the imido ligand (eneanti).7,8 If the two alkylidene isomers are considered as well, there exist in total four different reaction pathways for insertion. Whenever there is an enesyn insertion, a syn-first insertion product is formed whereas an eneanti insertion results in an anti-insertion product. A trans double bond is obtained B


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Figure 1. Molybdenum catalysts 1−3 with O-chelating ligands.

(Scheme 2).24 This way, a trans-configured polymer is formed from a syn-propagating species. Notably, this proposed mechanism and the formation of trans, isotactic polymer that it is based on are limited to enantiomerically pure trans-2,3dicarbomethoxynorbornene that was proposed to be a “mismatch” for one of the enantiomers either (R) or (S) of the catalyst. Thus, several other monomers including the racemic mixture of trans-2,3-dicarbomethoxynorbornene have been polymerized by the same catalyst in a fashion that follows the reaction pathways of Scheme 1 without metallacycle rearrangements.25 The monomers that have been used here do not have trans-substituted substituents in 2,3-position, and therefore a “mismatch” does not seem likely. We therefore consider it rather unlikely that a metallacycle rearrangement takes place. Finally, as a note, chain end control can also play an important role, particularly in the case of bulky monomers; however, is not required here to explain our findings. Obviously, the structure of a ROMP-derived polymer, both in terms of cis-/trans configuration and in terms of tacticity, is a record of all events at the metal alkylidene, including syn/anti interconversion, approach of monomer to the metal alkylidene, any potential inversion at the metal center, etc. We were therefore strongly interested in the interplay of syn/anti interconversion and (apparent) rates of polymerization with O-chelated Mo imido alkylidene NHC complexes for different monomers in coordinating or noncoordinating solvents and about potential implications for other olefin metathesis reactionsthe more since the presence of coordinating molecules like acetonitrile has been shown to influence both kinetics and selectivity of metathesis reactions.26 Here we report our results.

Figure 2. Single-crystal X-ray structure of 3. Selected bond lengths (pm) and angles (deg): Mo(1)−N(3) 174.3(3), Mo(1)−C(34) 188.5(4), Mo(1)−O(1) 196.4(2), Mo(1)−N(4) 219.9(3), Mo(1)− C(1) 221.0(4); N(3)−Mo(1)−C(34) 107.91(15), N(3)−Mo(1)− O(1) 141.15(13), C(34)−Mo(1)−O(1) 110.27, N(3)−Mo(1)−N(4) 88.94(13), C(34)−Mo(1)−N(4) 92.93(14), O(1)−Mo(1)−N(4) 82.28(11), N(3)−Mo(1)−C(1) 97.70(13), C(34)−Mo(1)−C(1) 102.50(14), O(1)−Mo(1)−C(1) 80.96(12), N(4)−Mo(1)−C(1) 160.32(12). Solvent molecule and anion omitted for clarity.

observed. Whereas the majority of Schrock catalyst complexes exist predominantly in the syn-form,10 catalysts 1−3 all show significant amounts of the anti-isomer (83, 85, and 84%). Remarkably, for all catalysts the ratio of the syn/anti-isomers varied in different samples. In fact, the ratio of isomers was largely influenced by the way catalysts 1−3 were isolated (Figure 3). The most striking example was 2, which can be partly precipitated from the reaction mixture and is then obtained in a virtually all syn-conformation (JCH = 119 Hz). In case the same sample is recrystallized from a mixture of CH2Cl2 and n-pentane, 42% anti-isomers (JCH = 142 Hz) become visible. In another experiment, one sample could be crystallized from the same solvent mixture that contained almost solely the anti-isomer. Despite different syn/anti ratios, different samples of 1, 2, and 3 dissolved in CH2Cl2 or CHCl3 retained their given syn/anti ratio over hours before they finally started to decompose. This suggests comparably high rotational barriers, probably caused by steric overloading, between the syn- and anti-isomers in the distorted square-pyramidal complexes. However, in case these catalysts were dissolved in coordinating solvents like dimethyl sulfoxide (DMSO) or acetonitrile (MeCN), a change in the syn/anti ratio occurred within days. Thus, complexes 1−3 showed >80% anti-isomers in acetonitrile at equilibrium state (vide supra). An explanation is provided by the 19F NMR spectra of the complexes. In noncoordinating or weakly

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RESULTS AND DISCUSSION Syn/Anti Interconversion in O-Chelating Molybdenum Imido Alkylidene Complexes. The synthesis of catalysts 1 and 2 (Figure 1) has been reported earlier.15 Complex 3 was prepared from 2 in 80% isolated yield via reaction with sodium tetrakis(3,5-bistrifluoromethylphenyl)borate (NaB(ArF)4). In the solid state (Figure 2), the acetonitrile in the cationic complex 3 coordinates trans to the NHC. 3 crystallizes in the triclinic space group P-1 with a = 1223.57(10) pm, b = 1647.99(16) pm, c = 2039.46(19) pm, α = 110.963(3)°, β = 90.040(3)°, γ = 91.438(4)°, and Z = 2. The ligands adopt a strongly distorted square-pyramidal (SP) geometry (τ = 0.32).27 The imido ligand is slightly bent [C(22)−N(3)− Mo(1) = 161.2(3)°], which suggests that the π-donor abilities of the free lone pair at the nitrogen are not fully utilized. In case 3 is dissolved in acetonitrile, the signal for the coordinated acetonitrile disappears, indicative for a rapid exchange of acetonitrile. With the three O-chelating molybdenum imido alkylidene NHC complexes shown in Figure 1, some interesting features with respect to the two possible Mo−alkylidene isomers were C


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Table 1. Equilibrium Constants (Keq) and (Apparent) Rate Constants for Syn/Anti and Anti/Syn Interconversion in Catalysts 1−3 catalyst 1 2 3 a

ks/a [s−1]

Keqa 0.21 0.18 0.19

ka/s [s−1] −6

(1.8 ± 0.1) × 10 (2.3 ± 0.1) × 10−6 (4.2 ± 0.3) × 10−6

3.8 × 10−7 4.2 × 10−7 8.0 × 10−7

Determined in CD3CN at room temperature; Keq = ka/s/ks/a.

cat‐OTfsyn ← (k1/k −1) → cat syn + + OTf − ← (ks / a , ka / s) → catanti + + OTf − ← (k′1 /k′−1) → cat−OTfanti

Depending on how fast the pre-equilibria establish, the observed values for ks/a and ka/s may additionally contain the corresponding rate constants k1, k−1, and k′1/k′−1, which are not easily accessible, if at all. For all catalysts, syn/anti interconversion proceeded faster than anti/syn interconversion by a factor of approximately 10, resulting in high anti-isomer contents. Interestingly, in MeCN values for ks/a and ka/s of the neutral complexes 1 and 2 were very similar to those of the cationic complex 3. Since both 1 and 2 become cationic in acetonitrile as well, this finding again confirms almost full dissociation of the triflate and formation of cationic species as observed by NMR (Figure 4). At this point it is worth mentioning that the weakly π-donating monomers also influence syn/anti interconversion rates of catalyst 1. While no interconversion is observed in CHCl3 (vide supra), insertion products observed by NMR in CHCl3 undergo syn/anti interconversion more readily in the presence of for example M1 and M2. Polymerization and NMR Studies. Polymerization of monomers M1−M4 (Figure 5) was carried out in CHCl3 and acetonitrile. Table 2 summarizes the cis content of all polymers prepared.

1

Figure 3. H NMR spectra of the alkylidene region of 2 in CD2Cl2: (a) >99% syn-isomer after precipitation from the reaction mixture; (b) mixture of syn- and anti-isomers after recrystallization from dichloromethane/n-pentane; (c) >99% anti-isomer after slow crystallization from dichloromethane/n-pentane.

coordinating solvents such as CHCl3, catalysts 1 and 2 show two signals for the triflate ligand corresponding to the syn- and anti-isomer, respectively, while in coordinating solvents, however, there is only one signal, which shows a chemical shift close to the one of free triflate (δ = 79.2 ppm, Figure 4).

Figure 4. 19F NMR spectra of 1 in CD3CN (top) and CDCl3 (bottom). The two distinct triflate signals for the syn- and antiisomers disappear in coordinating solvents, and only free triflate is visible.

Figure 5. Monomers M1−M4 used for ROMP with catalysts 1−3.

Table 2. Selected Polymer Data from the ROMP of M1, M3, and M4 by the Action of 1−3 in CHCl3 or MeCNa

Clearly, as proposed for monomer coordination,13−15 the presence of a coordinating solvent induces triflate dissociation and stabilizes the resulting cationic complex. As a result, syn/ anti interconversion is facilitated, and the equilibrium can be reached. To shed more light on this issue, syn/anti interconversion rates were determined by 1H NMR. In contrast to the procedure used for Schrock catalysts,22,28 irradiation with UV light to disrupt the syn/anti equilibrium was not necessary. A sample with high syn content was dissolved in CD3CN, and the rates of isomerization of the syn- to anti-isomer (ks/a) were determined (Figures S4−S6). Together with the values for the equilibrium constants (Keq = ka/s/ks/a), values for ka/s could be determined (Table 1).22,28 Notably, the rates are apparent rates. It is reasonable to assume that a 16-electron triflate complex needs to convert first into a cationic 14-electron complex according to

monomer/ catalyst

reaction time (h)

solvent

M1/1b M1/1b M1/2b M1/2b M1/3b M1/3b M3/1c M3/2b M3/3b M4/1b M4/3b

12 24 12 24 6 48 48 48 24 1 72

CHCl3 MeCN CHCl3 MeCN CHCl3 MeCN CHCl3 CHCl3 CHCl3 CHCl3 CHCl3

yield (%) 95 60 80 80 99 60 77 98 80

cis content (%) 42 62 52 48d 93 36d 27 32 75 58 11

Mn (g/mol)

PDI

18900 10600 3600

1.8 1.6 1.4

13900

1.6

9200 7600 13700 8100 25700

1.2 1.4 1.4 1.2 1.5

a With M2, no polymer was obtained with any of the catalysts. bRoom temperature. c60 °C. dDetermined by NMR.

D


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of the catalyst or (ii) via interconversion of the syntrans insertion product to the anti-one. If (i) was valid, then there should be a decrease of the syn-isomer together with the formation of the respective amount of antitrans insertion product, which was not the case. Presuming that the anti-isomer reacts exclusively, which is reasonable in view of its higher reactivity compared to the syn-isomer,10 and that a trans-double bond is formed, the “ene” insertion of M1 has to be syn. Consequently, syn/anti interconversion of already formed syn-insertion product is the only remaining explanation (Scheme 2). This reaction pathway also accounts for the observation that the syn-insertion product is visible first while the concentration of anti-insertion product gradually increases with time. Alternatively, one could envisage a metallacycle rearrangement, where the enesyn addition of the monomer to the anti-isomer of the catalyst leads directly to an antitrans insertion product. However, neither the sequence of appearance of syn- and anti-insertion products nor their relative intensities, especially after complete monomer consumption, support this reaction pathway. It is also worth mentioning that the 19F NMR spectrum of the reaction of 1 with M1 in CHCl3 at room temperature does not show any free triflate during the reaction. It has been suggested earlier that this type of 16electron complex has to release one anionic ligand, e.g., triflate, to form the polymerization-active species.15 In 1, the best leaving group to form a cationic 14-electron species with a free coordination site is in fact triflate. Most likely, the process of dissociation and reassociation of the triflate is fast with respect to the NMR time scale, and the equilibrium is far on the left side. Consequently, no free triflate is visible. In case the reaction of M1 with 1 was carried out in MeCN, it proceeded similarly with the formation of syn- and, subsequently, anti-insertion products. Again, only transconfigured terminal olefinic double bonds were observed. By contrast, in DMSO only the anti-insertion products (δ = 13.7, 12.7 ppm, JHa = 10−11 Hz) with a trans-double bond became visible. As evidenced by NMR, the syn-isomer of the catalyst did not react. Therefore, the most probable way to form an antiinsertion product with a trans olefin without a rearrangement of the metallacycle is by way of an enesyn insertion. As the syninsertion product was not visible by NMR, insertion must be followed by immediate syn/anti interconversion to the antiinsertion product. This process becomes comprehensible once the influence of the strongly coordinating DMSO is considered. Its coordination reduces the reaction rate by blocking the coordination site for the monomer and at the same time promotes the interconversion from the syn- to the anti-isomer by stabilizing the anti-isomer.29 In view of the finding that coordinating solvents have a great influence on syn/anti interconversion, we consider it much more likely that as discussed above a change of solvent alters the propagating species than a sudden change in polymerization mechanism that entails a metallacycle rearrangement occurs. Reaction of 1 with M2 gave similar results. Both in CHCl3 and MeCN, syn-insertion products (δ = 13.3; (CHCl3) 13.85 ppm (MeCN), JHa = 8−9 Hz) with a trans-configured olefinic double bond (δ = 5.65 (Hc); 5.30 (Hb) ppm, 3JHc = 16 Hz; 2JHb = 8 Hz; 3JHb = 16 Hz) were observed (Figure 8). In DMSO there was no reaction at room temperature but after heating the reaction mixture to 60 °C for 3 h signals for anti-insertion products (δ = 12.68 ppm, JHa = 11 Hz) appeared. At least for those initiator/monomer systems that could be investigated, initiation was significantly slower than polymerization. Values for kp/ki,30 representing the ratio of the rate

Poly-M1 prepared by the action of 1 and 2 showed cis contents around 42−62%. An illustrative spectrum is shown in Figure 6. By contrast, poly-M1 prepared by the action of 3 had a cis content of 93% (Table 2).

Figure 6. 1H NMR of the olefinic region for the polymerization of M1 with 1 (top) and 2 (bottom).

NMR studies were carried out in order to understand this finding and to shed light on the influence of the unusually high anti content in all catalysts on polymer structure. Reactions of 1 with M1 and M2 were run in CHCl3, acetonitrile, and dimethyl sulfoxide (DMSO) using an internal standard. Whereas in all solvents the integral of the syn-isomer remained roughly the same, the signal intensity for the anti-isomer decreased significantly. During the reaction of 1 with M1 in CHCl3, two overlapping new doublets (JHa = 8 Hz, syn-insertion products22,28) appeared around δ = 13.55 ppm followed by another doublet (JHa = 11 Hz, anti-insertion product22,28) around δ = 14.35 ppm (Figure 7). Notably, the signal for the

Figure 7. 1H NMR (CDCl3) spectrum of the alkylidene region in the reaction of 1 with M1.

syn-insertion product appeared first; however, signal intensities of the anti-insertion products increased over time even after all monomer was consumed, indicating syn/anti interconversion of the insertion products. The olefinic region showed several signals with coupling constants of 15−16 Hz that confirm the selective formation of trans double bonds. In principle, there are two ways an antitrans insertion product can form (Scheme 1): (i) via eneanti insertion to the syn-isomer E


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Table 3. Apparent Polymerization Rates and Cis Content of Polymers Obtained in Different Solvents with Catalysts 1−3 no.

monomer

catalyst

solvent

cisa [%]

kp(app) [s−1]

1 2 3 4 5 6 7 8 9

M1 M1 M1 M1 M1 M1 M3 M3 M4

1 1 2 2 3 3 1 3 3

CHCl3 MeCN CHCl3 MeCN CHCl3 MeCN CHCl3 CHCl3 CHCl3

43 62 52 48 93 36 27 75 11

(2.9 ± 0.05) × 10−5 b (5 ± 0.3) × 10−6 c (7 ± 0.4) × 10−6 c (4 ± 0.3) × 10−6 c (2.1 ± 0.1) × 10−3 b (1.3 ± 0.2) × 10−5 b (4 ± 0.3) × 10−6 c (3 ± 0.4) × 10−5 b,d (4 ± 0.9) × 10−7 c,d

a Determined by 1H NMR. b20 °C. cEstimated values. Rates could not be measured at 20 °C; these were therefore measured at 50 °C and extrapolated to 20 °C by dividing by 8. dAccuracy is low because of experimental error in measuring such slow polymerization rates.

Figure 8. 1H NMR spectrum (CD3CN) of the insertion products in the reaction of 1 with M2. *CH2Cl2.

By contrast, polymerization of M1 by 3 must be much faster than syn/anti interconversion resulting in high cis-polymers. Vice versa, in case polymerization of M1 by the action of 3 was carried out in MeCN, a considerable lower cis content (36%) was achieved than in CHCl3 (93%). Clearly, the lower apparent polymerization rate in MeCN (1.3 × 10−5 vs 2.1 × 10−3 in CHCl3) again contributes to this result. Similar is true for M1 and its ROMP by 1, 2, and 3 in CHCl3. M1 is polymerized 70− 300 times faster by 3 than by 1 or 2 and therefore has a high cis content of 93% while 1 and 2 only afford cis/trans mixtures. For M3/1 vs M3/3, the same trend is observed. Overall, interconversion of the alkylidene during the reaction is the only reasonable explanation for the formation of this cis/trans mixture since there is no evidence for an eneanti insertion, which would lead to the formation of a cis-configured first insertion product with the anti-isomer that could not be observed in any experiment. Consequently, by increasing the rate constant for polymerization with respect to the rate of syn/anti interconversion, a polymer with high cis content is obtained whereas low apparent polymerization rates lead to larger fractions of trans-configured polymer because between every insertion step syn/anti interconversion occurs. Since for all studied catalysts (apparent) interconversion rates are similar, at least in MeCN, the considerable differences in cis/trans selectivity mostly originate from the different activities of the catalysts, i.e., apparent polymerization rates alone. Stereoselective Polymerization. So far, stereoselective ROMP is predominantly the domain of Schrock-type catalysts including MAP-type catalysts.2−8 However, as outlined recently, Mo imido alkylidene NHC catalysts also allow for stereoselective, in some cases, stereospecific polymerizations.31 As outlined above, polymerization of M1 with 3 resulted in poly-M1 with a high (93%) cis content. Additionally, based on the 13C NMR chemical shifts, a highly st (>85%) polymer structure was identified (Figure 9 and Figure S15). In view of the size of M1, chain end control is expected to play a very minor role, if any. To rationalize this finding, one has to consider the stereogenic metal center that is present both in MAP-type catalysts4 and in 3. For MAP species it has been shown that the configuration at the metal center interconverts during polymerization32 with each metathesis step.33 We proposed a similar mechanism for Mo imido alkylidene NHC complexes in the cyclopolymerization of α,ω-diynes,31 assuming that the monomer is inserted trans to the strongest σ-donor.34−36

constant of polymerization over the rate constant of insertion, could be determined for the reaction of 1 with M1 and for the reaction of 3 with M1 and were found to be 37 ± 3 and 540 ± 100, respectively. Values for reactions with initiator 2 were inaccessible because kp/ki was >1000. Consequently, even with small amounts of monomer formation of polymer was inevitable. It was therefore impossible to accumulate or even isolate the first insertion products in order to verify the coupling constants of the syn- and anti-insertion products with the respective JCH value from 13C NMR spectra or to measure rate constants for syn/anti interconversion in the different insertion products. Reaction of M1 with 2 led directly to the formation of polymer (52% cis); no insertion products were observed. Reaction of a sample of 3 with high anti content (80%) obtained via slow crystallization with 3 equiv of M1 allowed for the observation of small amounts of the syn-insertion products (δ = 14.9; 13.5 ppm, JHa = 8 Hz). For reactions of 3 with M3, the signals for the insertion products were overlapping with the parent signals, whereas for the ROMP of M4 by the action of 3 all signals were too broad to allow for any profound interpretation. The olefinic end groups, however, were visible and selectively trans configured as evidenced by NMR (Figures S38−S40). Furthermore, a decrease of the anti-isomer of 3 was observed in the reaction with M3 and M4. Table 3 summarizes the cis contents and the apparent polymerization rates kp(app) for a series of catalyst/monomer combinations. As can be seen, some trends with respect to the cis content become evident. Thus, for a given catalyst in a given solvent and at 20 °C, a lower apparent polymerization rate resulted in a lower cis content. Examples are M1/1 and M3/1 as well as M1/3, M3/3, and M4/3. In fact, a comparison of the ROMP of enantiomerically pure M4 by the action of 3, yielding a high trans-polymer (89% trans), with the ROMP of M1 by the action of 3, yielding a high cis-polymer (93% cis), deserves attention. Thus, kp(app) for M1/3 is about 5300 times larger than for M4/3 (all in CHCl3, 20 °C). Since ka/s and ks/a should be comparable even in the presence of different monomers, polymerization of M4 by the action of 3 must be slow with respect to syn/anti interconversion (kp = 4 × 10−7 s−1, Table 3). Consequently the catalyst at the growing polymer chain can rotate back into the anti-position after each insertion step before the next monomer is inserted (Scheme 4). F


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polymers obtained with these catalysts we were able to gain detailed mechanistic insights into these ROMP reactions. The proposed mechanisms for the formation of cis and trans linkages are in line with our experimental results and supported by kinetic studies. Furthermore, they are coherent with studies on Schrock MAP-type catalysts.22 It has become evident that elaborate ligand tuning is not the only means to achieve high cis/trans selectivity. Complementary, significant differences between (apparent) polymerization rates and the rates of syn/anti interconversion can be used, too. This can be realized by increasing catalyst activity, e.g., by generating a cationic system, by a simple change of solvent or by application of a less reactive monomer. Notably, no matter what the contribution of the pre-equlibria to kp (apparent) is, it is this particular ratio of (apparent) kp vs (apparent) ks/a and ka/s that determines the cis/ trans ratio since it compares the overall times needed for one insertion vs the time needed for interconversion. It is fascinating to see how and why subtle changes to the reaction system lead to substantially different polymer structures. Implications for other olefin metathesis reactions such as ring-opening cross-metathesis, homometathesis, etc., in terms of tuning E/Z selectivity are currently investigated.

Figure 9. 13C NMR spectrum (CDCl3) of high cis, st poly-M1 prepared by the action of 3.

Applying the same considerations here leads to the observed st structure (Scheme 3). In view of the O-chelating NHC ligand and the substantial steric strain imposed by the other ligands it is, however, remarkable that 3 is able to undergo such a rearrangement to achieve this interconversion at the metal center. Finally, the number of signals in the 13C NMR of polyM4 prepared by the action of 3 also suggests a highly tactic polymer. The 1H, 1H COSY-NMR spectrum (Figure S28) showed no coupling of the olefinic protons, which is all in line with the formation of a trans, st polymer (Scheme 4). Consequently, a mechanism for the formation of st polymer similar to the one operative in the polymerization of M1 must be valid for this monomer as well. This formation of st polymers is another point that cannot be explained by the aforementioned metallacycle rearrangement. While it is possible that a syn propagating species forms a trans polymer, by way of an eneanti addition that is followed by a rearrangement of the metallacycle, the resulting polymer should be it rather than st as was the case in the results reported in the literature.24

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EXPERIMENTAL SECTION

General. All manipulations where carried out in a N2-filled glovebox (Lab Master 130, MBraun, Garching, Germany) or by standard Schlenk techniques. CH2Cl2, diethyl ether, toluene, pentane, and THF were dried by a solvent purification system (SPS, MBraun). Starting materials and all reagents were purchased from Sigma-Aldrich (Munich, Germany), Alfa Aesar (Karlsruhe, Germany), and ABCR (Karlsruhe, Germany), dried, and, where appropriate, distilled prior to use. [Mo(N-2,6-Me2-C6H3)(N-mesityl-N′-2-O-1-C6H4-imidazolin-2ylidene)(CHCMe2Ph)(OTf)] (1) and [Mo(N-2,6-iPr2-C6H3)(N2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (2) were prepared as described in the literature.15 Also, endo,endo-2,3-di(carbomethoxy)-5-norbornene (M2),37 exo,exo-2,3-di(pentoxymethyl)-5-norbornene (M3),38 and exo,exo-N,N-(norborn-5ene-2,3-dicarbimido)-L-valine ethyl ester (M4)39 were prepared according to the literature. NMR measurements were recorded on a Bruker Avance III 400. Chemical shifts are reported in ppm relative to the solvent signal; coupling constants are listed in hertz. IR spectra were measured on a Nicolet alpha spectrometer. GC-MS data were recorded on an Agilent Technologies device consisting of a 7693 autosampler, a 7890A GC, and a 5975C quadrupole MS. Dodecane was used as internal standard. An SPB-5 fused silica column (34.13 m × 0.25 mm × 0.25 μm film thickness) was used. The injection

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CONCLUSIONS We synthesized high cis and high trans ROMP-derived polymers using molybdenum imido alkylidene complexes containing an O-chelating ligand. By studying syn/anti interconversion, insertion products and the structures of the

Scheme 3. Formation of Different Insertion Products and Cis/Trans Double Bonds during the Reaction of 1 with M1 in CHCl3

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Macromolecules Scheme 4. Proposed Mechanism for the Formation of cis, st Poly-M1 by the Action of 3

Scheme 5. Proposed Mechanism for the Formation of trans, st Poly-M4 by the Action of 3

temperature was set to 150 °C. The column temperature ramped from 45 to 250 °C within 8 min and was then held for further 5 min. The column flow was 1.05 mL/min. Molecular weights of polymers were determined by GPC in CHCl3 at 40 °C (1 mL/min) using an Agilent 390-MDS GPC system with PLgel Mixed D-type columns (separation range 500−3 000 000 g/mol) and refractive index/UV (254 nm) detection. Calibration vs polystyrene standards (800 < Mn < 5 800 000 g/mol) was used. [Mo(N-2,6-iPr2-C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2ylidene)(CH 3 CN)(CHCMe 2 Ph) + (B(Ar F ) 4 − )] (3). [Mo(N-2,6- i Pr 2 C 6 H 3 )(N-2,6- i Pr 2 -C 6 H 3 -N′-2-O-1-C 6 H 4 )imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (2) (61 mg, 0.0670 mmol) was dissolved in CH2Cl2, and a few drops of acetonitrile were added. Subsequently, sodium tetrakis(3,5-bistrifluoromethylphenyl)borate, NaB(ArF)4 (62 mg, 0.0670 mmol), was added, and the reaction mixture was stirred for 3 h at room temperature. The solvent was removed in vacuo, and the residue was dissolved in a minimum amount of CH2Cl2. The solid was filtered through Celite, a few drops of n-pentane were added, and the product was crystallized at −35 °C (85 mg, 0.0536 mmol, 80%). 1H NMR (400 MHz, CD2Cl2): δ 15.02 (s, 1H, anti-isomer, JCH = 144 Hz), 13.39 (s, 1H, syn-isomer, JCH = 118 Hz), 7.73 (s, 8H), 7.57 (s, 4H), 7.49 (t, J = 7.8 Hz, 1H), 7.37 (t, J = 7.7 Hz, 2H), 7.30−7.05 (m, 12H), 4.67−4.54 (m, 1H), 4.50−4.38 (m, 1H), 4.21−4.07 (m, 1H), 4.07−3.93 (m, 1H), 3.53−3.31 (m, 2H), 2.83−2.62 (m, 2H), 1.92 (s, 3H), 1.39 (s, 3H), 1.17 (d, J = 6.9 Hz, 3H), 1.11 (d, J = 6.8 Hz, 3H), 1.03 (d, J = 6.6 Hz, 6H), 1.00 (d, J = 9.0 Hz, 6H), 0.87 (d, J = 6.8 Hz, 6H), 0.67 (d, J = 6.7 Hz, 3H). 19F NMR (376 MHz, CD2Cl2): δ

−62.84 (s); 13C NMR (101 MHz, CD2Cl2): δ 321.9 (CHCMe2Ph), 204.6 (CNcarbene), 162.2 (q, 1JB−C = 50 Hz, B(ArF)4), 151.8, 151.2, 147.6, 146.0, 145.6, 136.9, 135.3 (B(ArF)4), 130.9, 129.6, 129.3 (q, J = 31.1 Hz, B(ArF)4), 129.0, 127.2, 127.1, 126.7, 126.4 (B(ArF)4), 125.8, 125.6, 123.8, 123.7, 121.7, 121.0, 119.8, 118.2, 117.9 (B(ArF)4), 56.0, 54.7, 49.6, 31.3, 29.6, 29.0, 28.9, 28.1, 26.5, 26.2, 24.4, 23.9, 23.7, 22.3, 3.1. Elemental analysis (%) calcd for C77H70F24MoN4O: C 56.73, H 4.33, N 3.44. Found: C 56.81, H 4.42, N 3.59. General Procedure for NMR Studies of Insertion Products. The catalyst was dissolved in the corresponding deuterated solvent (4−5 mg in 0.4−0.5 mL), and a blank sample was measured to determine the syn/anti ratio in the given sample. Monomer (2−12 equiv) was then transferred directly into the NMR tube, and the sample was measured in certain time intervals depending on reaction rates. Alkylidene signals were integrated with respect to an internal standard (CH2Cl2, diethyl ether, n-pentane, or tetramethylsilane). Determination of kp/ki. Reactions were carried out in an NMR tube in CDCl3 for [I]0:[M]0 = 1:3, and [I]/[I]0 was determined from the integrals of the alkylidene signals in the 1H NMR for the parent catalyst and the sum of the insertion products. Values for kp/ki were calculated using the following equation:30 kp/ki = (1 − [M]0/[I]0 − [I]/[I]0)/(1 + ln([I]/[I]0) − [I]/[I]0). General Procedure for Polymerizations. The catalyst (1 equiv) was dissolved in about 0.5 mL and added to a solution of the monomer (15−50 mg, 25−50 equiv) in 3−15 mL of solvent. Reactions were conducted at room temperature for 1−72 h, quenched with ferrocene aldehyde, and stirred for another 30 min. The reaction H


Article

Macromolecules mixture was concentrated in vacuo, and the polymer precipitated from methanol or n-pentane and dried in vacuo. General Procedure for Determination of kp(app). To 1 mL of monomer stock solution, dodecane was added as internal standard. A blank sample was taken before addition of the respective amount of catalyst. At given time intervals, samples were taken and quenched immediately in n-pentane. Consumption of monomer was then determined by GC-MS analysis. Determination of (Apparent) Rate Constants for Syn/Anti Interconversions. A sample of the catalyst, which was not at equilibrium, was dissolved in CD3CN, and interconversion was monitored by 1H NMR through integration of the respective alkylidene signal (relaxation time 1 s). Relaxation times of 0.5, 1, and 3 s were measured with no visible change of the alkylidene integrals, indicating full relaxation of all isomers.

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(6) Jeong, H.; Ng, V. W. L.; Börner, J.; Schrock, R. R. Stereoselective Ring-Opening Metathesis Polymerization (ROMP) of Methyl-N-(1phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate by Molybdenum and Tungsten Initiators. Macromolecules 2015, 48, 2006−2012. (7) Schrock, R. R. Synthesis of stereoregular ROMP polymers using molybdenum and tungsten imido alkylidene initiators. Dalton Trans. 2011, 40 (29), 7484−7495. (8) Schrock, R. R. Synthesis of Stereoregular Polymers through RingOpening Metathesis Polymerization. Acc. Chem. Res. 2014, 47 (8), 2457−2466. (9) Schrock, R. R. High Oxidation State Multiple Metal−Carbon Bonds. Chem. Rev. 2002, 102 (1), 145−180. (10) Schrock, R. R.; Hoveyda, A. H. Molybdenum and Tungsten Imido Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts. Angew. Chem., Int. Ed. 2003, 42 (38), 4592−4633. (11) Schrock, R. R. Recent Advances in High Oxidation State Mo and W Imido Alkylidene Chemistry. Chem. Rev. 2009, 109 (8), 3211− 3226. (12) Hock, A. S.; Schrock, R. R.; Hoveyda, A. H. Dipyrrolyl Precursors to Bisalkoxide Molybdenum Olefin Metathesis Catalysts. J. Am. Chem. Soc. 2006, 128 (50), 16373−16375. (13) Buchmeiser, M. R.; Sen, S.; Unold, J.; Frey, W. N-Heterocyclic Carbene, High Oxidation State Molybdenum Alkylidene Complexes: Functional-Group-Tolerant Cationic Metathesis Catalysts. Angew. Chem., Int. Ed. 2014, 53 (35), 9384−9388. (14) Sen, S.; Schowner, R.; Imbrich, D. A.; Frey, W.; Hunger, M.; Buchmeiser, M. R. Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica. Chem. - Eur. J. 2015, 21 (39), 13778−13787. (15) Buchmeiser, M. R.; Sen, S.; Lienert, C.; Widmann, L.; Schowner, R.; Herz, K.; Hauser, P.; Frey, W.; Wang, D. Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Structure−Productivity Correlations and Mechanistic Insights. ChemCatChem 2016, 8 (16), 2710−2723. (16) Imbrich, D. A.; Frey, W.; Naumann, S.; Buchmeiser, M. R. Application of Imidazolinium Salts and N-Heterocyclic Olefins for the Synthesis of Anionic and Neutral Tungsten Imido Alkylidene Complexes. Chem. Commun. 2016, 52, 6099−6102. (17) Imbrich, D. A.; Elser, I.; Frey, W.; Buchmeiser, M. R. First Neutral and Cationic Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes. ChemCatChem 2017, DOI: 10.1002/ cctc.201700189. (18) Schowner, R.; Frey, W.; Buchmeiser, M. R. Cationic TungstenOxo-Alkylidene-N-Heterocyclic Carbene Complexes: Highly Active Olefin Metathesis Catalysts. J. Am. Chem. Soc. 2015, 137, 6188−6191. (19) Pucino, M.; Mougel, V.; Schowner, R.; Fedorov, A.; Buchmeiser, M. R.; Copéret, C. Cationic Silica-Supported N-Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis. Angew. Chem., Int. Ed. 2016, 55, 4300−4302. 2016, 128 (13), 4372−4374. (20) Liao, W.-C.; Ong, T.-C.; Gajan, D.; Casano, G.; Yulikov, M.; Pucino, M.; Schowner, R.; Schwarzwälder, M.; Buchmeiser, M. R.; Jeschke, G.; Ouari, O.; Tordo, P.; Lesage, A.; Emsley, L.; Copéret, C. Dendritic Polarizing Agents for DNP SENS. Chem. Sci. 2017, 8, 416− 422. (21) Elser, I.; Schowner, R.; Frey, W.; Buchmeiser, M. R. Molybdenum and Tungsten Imido Alkylidene N-Heterocyclic Carbene Catalysts Bearing Cationic Ligands for Use in Biphasic Olefin Metathesis. Chem. - Eur. J. 2017, 23, 6398. (22) Oskam, J. H.; Schrock, R. R. Rotational isomers of molybdenum(VI) alkylidene complexes and cis/trans polymer structure: investigations in ring-opening metathesis polymerization. J. Am. Chem. Soc. 1993, 115 (25), 11831−11845. (23) Schrock, R. R. The alkoxide ligand in olefin and acetylene metathesis reactions. Polyhedron 1995, 14 (22), 3177−3195. (24) Flook, M. M.; Börner, J.; Kilyanek, S. M.; Gerber, L. C. H.; Schrock, R. R. Five-Coordinate Rearrangements of Metallacyclobutane Intermediates during Ring-Opening Metathesis Polymerization of 2,3-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00841. Details on single-crystal X-ray data, 1H NMR, 13C NMR, and 19F NMR spectra of compound 3; NMR spectra of insertion products, plots for the determination of polymerization rates, and syn/anti interconversions; NMR and IR spectra of selected polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.R.B.). ORCID

Michael R. Buchmeiser: 0000-0001-6472-5156 Funding

Financial support was provided by the Deutsche Forschungsgemeinschaft (DFG, project BU 2174/19-1). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS ROMP, ring-opening metathesis polymerization; DMSO, dimethyl sulfoxide; NaB(ArF)4, sodium tetrakis(3,5-bistrifluoromethylphenyl)borate.

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REFERENCES

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