Dialkenylmagnesium Compounds in Coordinative Chain Transfer

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation Islem Belaid,† Marie-Noel̈ le Poradowski,‡ Samira Bouaouli,‡ Julien Thuilliez,§ Lionel Perrin,*,‡ Franck D’Agosto,*,† and Christophe Boisson*,† †

Université de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), Equipe LCPP, Bat 308F, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France ‡ Université de Lyon, Université Claude Bernard Lyon 1, CPE Lyon, INSA Lyon, ICBMS, CNRS UMR 5246, Equipe ITEMM, Bât Curien, 43 Bd. du 11 Novembre 1918, 69622 Villeurbanne, France § MFP Michelin, 23 Place des Carmes Dechaux, 63040 Clermont-Ferrand, France S Supporting Information *

ABSTRACT: A range of dialkenylmagnesium compounds ([CH2CH(CH2)n]2Mg; n = 1−6) were synthesized and used as chain transfer agents (CTA) with either (C5Me5)2NdCl2Li(OEt2)2 (1) or [Me2Si(C13H8)2Nd(BH4)2Li(thf)]2 (2) neodymium precursors for the polymerization of ethylene. In all cases, the systems followed a controlled coordinative chain transfer polymerization mechanism. The intramolecular insertion of the vinyl group on the CTA in growing chains is possible and led to the formation of cyclopentyl, cyclohexyl, and possibly cycloheptyl chain ends. While the production of cyclopentyl- or cyclohexyl-capped polyethylene chains can be quantitative (n = 2−5), the integrity of this double bond can also be kept if n is higher than 6. In comparison to 1/CTA catalytic systems, 2/CTA catalytic systems showed a higher propensity to produce cycloalkyl chain ends. This was ascribed to the lower steric demand around the active site, as shown by DFT calculations. In addition, the formation of bis(cyclopentylmethyl)magnesium from dipentenylmagnesium using a catalytic amount of 2 was shown.

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INTRODUCTION Polyolefins containing cyclic units in the main chain are attractive materials. The presence of rings allows, for example, tuning the glass transition temperature and the final properties of the materials. For instance, copolymerization of olefins with cyclic olefins gives cyclic olefin copolymers that are important materials used in packaging, healthcare, or optics.1,2 Cyclic units in polyolefins can also be generated in situ by using dienes as comonomers. Cyclo(co)polymerization of nonconjugated dienes has been well documented3−6 and has led to polyolefins containing 1,3-cycloalkane units. More recently, the challenging copolymerization of olefins with butadiene has been achieved using group 47−9 or rare-earth-metal10,11 catalysts. The use of butadiene, a readily available monomer, is of high interest, and these works have allowed the discovery of new materials. Interestingly, the insertion of butadiene leads to vinyl ramifications (1,2-units) that give rise to 1,2-cycloalkane units involving three, five, or six carbon atoms. Longo et al. reported the formation of cyclopropane and cyclopentane units (Scheme 1A) via the copolymerization of ethylene with butadiene, catalyzed by zirconocene complexes, activated with methylaluminoxane (MAO).3,8,9 Under suitable copolymerization conditions, all inserted butadiene provides cyclic units. These © XXXX American Chemical Society

Scheme 1. Influence of Butadiene Insertion Regioselectivity on the Microstructure of Rings Formed by Cyclo(co)polymerization of Ethylene with Butadiene

rings are formed into the growing polymer chain by intramolecular insertion of a coordinated vinyl unit formed by a 1,2-insertion of butadiene (Scheme 1A). According to Longo et al., cyclization with group 4 metallocenes would occur when primary 1,2-insertion of butadiene takes place and if backbiting coordination and insertion of the vinyl group are favored with respect to olefin coordination and insertion. Received: March 2, 2018

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DOI: 10.1021/acs.organomet.8b00127 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Conversely, the copolymerization of ethylene with butadiene using neodymocene systems generates original sequences, since 1,2-cyclohexane rings are formed by intramolecular cyclization after a 2,1-insertion of butadiene, followed by the insertion of two ethylene groups (Scheme 1-B).10,11 The cyclization mechanism has been depicted at the DFT level.12 The different mechanisms involved for the cyclocopolymerization of ethylene with butadiene using group 4 and neodymium metallocene catalysts are highlighted in Scheme 1. We have recently reported on the use of bis(10-undecenyl)magnesium, [CH2CH(CH2)9]2Mg, as an original dialkenylmagnesium chain transfer agent (CTA) in association with the neodymocene complex (C5Me5)2NdCl2Li(OEt2)2 in ethylene polymerization.13 The quantitative synthesis of vinyl-terminated polyethylene has been achieved by coordinative chain transfer polymerization (CCTP) using this catalytic system. Since ethylene does not copolymerize with α-olefin (or nonconjugated diene) in the presence of this catalyst, the vinyl group brought by the chain transfer agent remains spectator. In regard to intramolecular cyclization, the integrity of the vinyl group has been maintained during the polymerization by a fine-tuning of the number of methylene groups between the magnesium metal and the vinyl group. A long alkenyl group (C11) prevents intramolecular coordination and subsequent insertion of the terminal vinyl. Conversely, using dibutenylmagnesium as a chain transfer agent leads to the quantitative formation of cyclopentyl-terminated polyethylene. On the basis of these results, we have anticipated that the use of dialkenylmagnesium with various sizes may lead to the formation of original polyethylene chains carrying a terminal ring involving different numbers of carbon atoms. We describe herein the preparation and the implementation of a range of dialkenylmagnesium complexes [CH2CH(CH 2 ) n ] 2 Mg (n = 2−6) in ethylene CCTP using (C5Me5)2NdCl2Li(OEt2)2 or [Me2Si(C13H8)2Nd(BH4)2Li(thf)]2 neodymium precursors (Scheme 2). This study not

Scheme 3. Syntheses of Dialkenylmagnesium Compounds and Their Use in CCTP of Ethylene

alkenyl bromide compounds (4-bromo-1-butene, 5-bromo-1pentene, 6-bromo-1-hexene, 7-bromo-1-heptene, 8-bromo-1octene) (Scheme 3). A THF/Bu2O solvent exchange was performed after the dialkenylmagnesium was obtained, since THF has been shown to poison ethylene polymerization.14 Aliquots of a solution of the dialkenylmagnesium compound in THF were analyzed by proton NMR (Figures S1−S6). The concentration of the dialkenylmagnesium solution and the yield were determined by pyreneacetic acid titration. High yields were obtained for all prepared dialkenylmagnesium compounds (see the Experimental Section in the Supporting Information). Nevertheless, comparison of the integrations of the signals of one vinylic proton and of the methylene bound to magnesium did not match the expected value of 2 when the number of carbons in the alkenyl moiety was higher than 6. This is ascribed to the presence of a α,ω-diene formed by the Wurtz coupling reaction,16 as shown by a GC analysis of the solution of dialkenylmagnesium compounds (Figures S14 and S15). Ethylene Polymerization. Since nonconjugated dienes are not reactive in ethylene polymerization with the employed catalytic systems, the corresponding dialkenylmagnesium compounds were further employed without purification as CTAs in ethylene homopolymerization in combination with the complexes (C 5 Me 5 ) 2 NdCl 2 Li(OEt 2 ) 2 (1) and [Me 2 Si(C13H8)2Nd(BH4)2Li(thf)]2 (2). As shown in Table 1, except for diallylmagnesium, all prepared dialkenylmagnesium compounds used in combination with 1 or 2 provided active catalysts for ethylene polymerization with stable activities. The number-average molar masses (Mn) are in agreement with the theoretical values, and the dispersities (Đ) are low, in agreement with a catalyzed chain growth mechanism. Polymerizations with Complex 1. The use of the diallylmagnesium compound does not allow the homopolymerization of ethylene with complex 1 (run 1). For this precursor, although insertion of ethylene in the Nd−allyl site is slow,17 this result is unexpected. As ethylene and butadiene do copolymerize with complex 2 in combination with a dialkylmagnesium compound,10 ethylene insertion into a Nd−allyl site when 2 is used with diallylmagnesium should be favored. However, as shown in run 2, no polymerization occurred. This lack of activity when diallylmagnesium is used is discussed in Computational Mechanistic Investigation (vide infra). Figure 1 shows the 13C NMR spectrum of the polymer obtained with 1/dibutenylmagnesium catalytic system (Table 1, run 3). The main resonance at 30.06 ppm corresponds to the methylene carbons of the polyethylene chain. The spectrum shows the absence of the resonances of terminal vinyl groups expected at 114.26 and 138.95 ppm. Signals corresponding to saturated chain ends, generated by quenching the Mg−C bond

Scheme 2. Structure of Catalyst Precursors

only depicts the use of original chain transfer agents for efficient coordinative chain transfer polymerization of ethylene14,15 but also helps to better understand the cyclization mechanism observed in the copolymerization of ethylene and butadiene mentioned above.

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RESULTS AND DISCUSSION Dialkenylmagnesium Preparation. In order to investigate the ability of neodymium metallocene catalysts to generate rings by an intramolecular cyclization mechanism, a range of dialkenylmagnesium compounds was prepared (Scheme 3). These dialkenylmagnesium compounds act as alkylating agents and chain transfer agents (CTAs) during ethylene polymerization. Dialkenylmagnesium compounds were prepared by shifting the Schlenk equilibrium either from the corresponding commercial Grignard solution (allylmagnesium bromide) or from a solution of freshly prepared Grignard reagents from B


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Organometallics

Table 1. Ethylene Polymerization Using Neodymium Metallocene Precursors in Combination with Dialkenylmagnesium Chain Transfer Agentsa run

complex

[Nd] (μM)

CTA

[Mg]/[Nd]

activity kg mol−1 h−1

theor Mnd (g mol−1)

1 2 3 4 5 6 7b 8b 9 10 11b 12c

1 2 1 1 1 1 1 1 2 2 2 2

52 52 49 47 50 52 48 56 52 46 43 50

diallylmagnesium diallylmagnesium dibutenylmagnesium dipentenylmagnesium dihexenylmagnesium diheptenylmagnesium diheptenylmagnesium dioctenylmagnesium dihexenylmagnesium diheptenylmagnesium dioctenylmagnesium dioctenylmagnesium

78 78 83 83 81 78 84 73 78 88 93 80

none none 310 368 331 337 678 601 370 331 761 551

770 1080 1620 1500 1850 2060 1310 930 970 1060

Mn (g mol−1)

Đ

end group

selectivitye (%)

650 850 1300 1300 1530 2020 1330

1.14 1.20 1.20 1.16 1.45 1.30 1.42

1020 1220

1.34 1.53

cyclopentyl cyclohexyl cyclopentyl cyclohexyl cyclohexyl cycloheptyl cyclopentyl cyclohexyl cycloheptyl cycloheptyl

100 61 100 62 100 0 100 >97 28 39

Polymerization conditions: toluene 400 mL, temperature 75 °C; ethylene pressure 4 bar. b20 min of precontact. c1 h of precontact. dTheoretical Mn = yield/2nMg. eDetermined by 1H NMR. a

Figure 1. 13C NMR spectrum of the polyethylene obtained with 1/ Mg((CH2)2CHCH2)2.

of the final polymer by methanol (signals 1S−4S), and cyclopentyl chain ends (signals a−f in Figure 1) are fully assigned. The observed cyclopentane rings are formed by intramolecular cyclization following the insertion of an ethylene into the Nd−butenyl active site. The formation of cyclopentane rings is quantitative here, which shows that the cyclization is particularly favored over ethylene insertion. It is worth noting that, experimentally, the precursor 1 is introduced in the reactor through an injection antechamber into an ethylene-saturated solution of CTA in toluene. Cyclization to form cyclopentane rings is therefore highly selective under these conditions. In addition, no cyclopropane rings are observed. This indicates that the three-carbon-ring formation with the (C5Me5)2NdCl2Li(OEt2)2 precursor is unfavorable, as confirmed by DFT calculations (vide infra). Figure 2 shows the 13C NMR spectrum of the polyethylene obtained with the catalytic system 1/dipentenylmagnesium (Table 1, run 4). The spectrum shows unsaturated chain ends (signals 1i−4i). Nevertheless, new resonances are observed in the saturated carbon zone of the spectrum. They are attributed to a terminal cyclohexyl moiety (signals a−f in Figure 2A). The calculation by 1H NMR of the proportions of the cyclic chain ends in comparison to the vinyl ends shows 61% of ringterminated chains (Figure S7). These rings are formed after intramolecular cyclization following the insertion of an ethylene into the Nd−pentenyl active site. The presence of a terminal vinyl group indicates that the formation of cyclohexane rings is

Figure 2. 13C NMR spectrum of the polyethylene obtained with 1/ Mg((CH2)3CHCH2)2: (A) saturated carbon region; (B) unsaturated carbon region.

not quantitative under these polymerization conditions. Insertion of ethylene is in competition with cyclization reaction and this leads to vinyl chain ends. Considering the mechanism of intramolecular cyclization, cyclopentyl chain ends should form when dihexenylmagnesium compounds were used as CTAs in combination with the neodymium precursor 1 in ethylene polymerization (Table 1, run 5). In this case, the length of the alkenyl moiety permits forming a ring immediately after the neodymium precursor has C


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Organometallics

first time using ethylene and catalytic coordination−insertion polymerization.13 Polymerizations with Complex 2. Copolymerization of ethylene with butadiene has been efficiently carried out with 2 in combination with dialkylmagnesium. The 2,1-insertion of butadiene leads to pendant vinyl bonds that have been shown to be involved in cyclization reactions, generating copolymers containing not only ethylene units and vinyl and trans-1,4butadiene units but also 1,2-cylohexane units. In the case of copolymerization of ethylene with butadiene, intramolecular cyclization has been shown to be more favorable with complex 2 than with complex 1.12 The propensity of 2 to insert the vinyl group from the growing chain during the copolymerization of ethylene with butadiene drove us to evaluate the use of 2 in combination with dihexenyl, diheptenyl, and dioctenylmagnesium for the polymerization of ethylene. As a reference experiment, 2 was first used in combination with dihexenylmagnesium (Table 1, run 9) and compared to the use of 1 (run 5). The same polymers were obtained in the two cases with a quantitative formation of cyclopentyl chain ends (Figure S12). This confirms the ability of 2 to induce the same cyclization reaction as 1. When diheptenylmagnesium was used, the superiority of 2 in run 10 versus 1 in run 6 (62% of chains carrying a cyclohexyl chain end) to induce cyclization reactions is shown by the nearly quantitative formation (>97%) of cyclohexyl chain ends featured in the 13C NMR spectrum of the polymer (Figure S13) without resorting to a precontact time (as in run 7 carried out with 1). This result confirms the efficiency of this catalyst to form rings by intramolecular cyclization, initially reported in the copolymerization of ethylene with butadiene. Eventually, the formation of a cycloheptane ring that is not observed using dioctenylmagnesium as a chain transfer agent in the presence of 1 (Table 1, run 8) was evaluated in the presence of 2. Interestingly, the 13C NMR spectrum of the obtained polymer shows new resonances in addition to the vinyl chain end resonances (Figure 3). In particular, a signal

been alkylated. Indeed, ethylene polymerization with the catalytic system 1/dihexenylmagnesium (run 5) gives a polyethylene whose 13C NMR spectrum is identical with that obtained with the system 1/dibutenylmagnesium (Figure S8). All chains have a cyclopentane ring and a saturated chain end. The absence of any other resonances strongly supports a highly selective formation of the cyclopentane structure. When diheptenylmagnesium is used as CTA (run 6), cyclohexyl chain ends are observed in the saturated carbon region of the 13C NMR spectra (Figure S9). These rings are formed by intramolecular insertion of the double bond of the heptenyl moiety. As in the case of the dipentenylmagnesium compound, the presence of terminal vinyl groups is also observed in the NMR spectrum. This indicates that the formation of cyclohexane rings with this neodymium metallocene is in competition with the insertion of ethylene under these polymerization conditions. 1H NMR analysis of the obtained polyethylene shows that 62% of the chain ends are cyclohexane rings. The same selectivity is thus observed toward formation of cyclohexane rings when dipentenylmagnesium and diheptenylmagnesium compounds such as CTA were used. To challenge the competition between the formation of rings and ethylene insertion, 1 was precontacted with the diheptenylmagnesium chain transfer agent in Table 1, run 7, for 20 min before injecting ethylene into the reactor. This run was compared to run 6 performed under exactly the same conditions without precontact between the CTA and the precursor. The 13C NMR characterization of the obtained polyethylene shows the absence of signals corresponding to terminal vinyl groups in the unsaturated carbon region of the spectrum and the quantitative formation of cyclohexylterminated polyethylene (Figure S10). Polymer chains are initiated by a cyclohexylmethyl moiety formed by cyclization of the heptenyl group on the neodymium center. The quantitative formation of cyclohexane rings indicates that the cyclization takes place during the precontact time between the neodymium precursor and the magnesium transfer agent. It also assumes that the transfer of the cyclohexylmethyl group (formed after cyclization) between neodymium and magnesium is efficient. This result shows that cyclohexane-capped polyethylene chains can be produced by fine-tuning of the polymerization conditions. This set of polymerization tests shows the superior selectivity of catalytic systems 1/Mg((CH2)nCHCH2)2 (n = 2−5) toward the formation of cylcopentane in comparison to cylcohexane rings. In spite of the high selectivity toward ring formation with metallocene catalysts, cycloheptane ring formation has not been observed so far. In order to probe the potential formation of cycloheptane rings, 1 was used in combination with dioctenylmagnesium CTA for ethylene polymerization after complying with a precontact time of 20 min. The 13C NMR spectrum of the obtained polymer (Figure S11) does not feature any signal corresponding to the formation of a terminal ring. Instead, a vinyl-terminated polyethylene was isolated. This clearly shows that the doublebond insertion is not favorable when the latter is too far from the active center. While the quantitative production of cyclopentyl- or cyclohexyl-capped polyethylene chains is possible by insertion of the double bond carried by the CTA, the integrity of this double bond can also be kept if it is spaced away enough from the Nd active center after alkylation. As mentioned in the Introduction, this last feature has indeed been valuably employed to produced telechelic polyethylene for the

Figure 3. 13C NMR spectrum of the polyethylene obtained with 2/ Mg((CH2)6CHCH2)2.

corresponding to a CH group is observed at 41.99 ppm and suggests the formation of a new type of ring. Other unknown resonances are also observed (δ (ppm): 33.94, 33.89, 32.80, 30.56, 26.96, 26.66, 26.61), in agreement with the formation of a new ring. Considering the cyclization mechanism discussed above, we assume the formation of a terminal cycloheptyl. The accurate assignment of the signals for this type of cycle remains quite complex considering the different possible configurations for the ring. The formation of this proposed cycloheptane ring D


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Organometallics

be considered as an efficient catalyst for producing original bis(cycloalkanemethyl)magnesium compounds. The above experimental data were further backed up with a computational mechanistic investigation at the DFT level. Computational Mechanistic Investigation. Modeling Strategy. Owing to the concerted character of monomer insertion, the Nd oxidation state remains constant at its +III oxidation state during polymerization. In addition, it has been demonstrated that 4f orbitals and electrons have little to no influence on the lanthanide−ligand chemical bonds and their reactivity.18−20 As a result, the 4f shell was implicitly considered18 by using a large-core 49-electron quasi-relativistic effective core potential (ECP) and its polarized associated basis set.21,22 The choice of density functional (B3PW91), basis sets (6-311G(d,p)), and solvent (SMD-toluene) and dispersion corrections (D3-BJ) have already been discussed in previous communications.12,14,19,23 Perrin, Maron, and Eisenstein have used this strategy intensively and fruitfully in mechanistic studies involving lanthanide(III) complexes, including polymerization reaction studies.20,24,25 Notation. L_NdR refers to the neodymocene complex (C5Me5)2NdR for L = 1 and Me2Si(C13H8)2NdR for L = 2. R refers to the length of the alkenyl group; it spans from 3butenyl (Bu′) to 7-octenyl (Oct′). Adducts are defined with an underscore between the active site and the monomer to be inserted (eg 1_NdR_C2H4). Transition states (TS) are identified by an asterisk (e.g. 1_NdR_C2H4*). Cocatalyst. The polymerization reaction proceeds under reversible chain transfer conditions that involve association/ dissociation of a dialkylmagnesium with the neodymocene active catalyst.14,26,27 Thereof, the magnesium species play a crucial role in the polymerization and must be included in the chemical model. In toluene solution, the cluster Mg4(σ-nBu)2(μ-n-Bu)4(μ3-n-Bu)2, in which two alkyl chains are terminal, four are bridging, and two are facial, was calculated as the most relevant and stable aggregate.14,28,29 In this study, as an unsaturation is present on the CTA, and as Bu2O is used as a cosolvent by default, a hand sampling of the magnesium clusters has been carried out. Bu2O affect sthe magnesium cluster by displacing Mg···(CC) interactions and also by dissociating tetramers into the most stable cosolvated dimers Mg2R4·(Bu2O)2, as previously discussed (Table S1).14 Dormant Species. The magnesium cluster is bound to the active neodymocene complex L_NdR and yields the most stable and thus dormant heterobimetallic species L_NdMg2R5, in which cosolvation by Bu2O is unfavorable.14 However, depending on the length of the alkenyl group, additional Mg··· (CC) interactions were found during sampling: e.g., in the case of a butenyl group (Figure S16). Reactivity. As a general trend, two types of complexes (L_NdR and L_NdR_C) can be formed after the dissociation of the heterobimetallic complex L_NdMg2R5 (Scheme 5). L_NdR_C originates from L_NdR by the intramolecular coordination of the alkenyl group unsaturation. The relative stability of these active species depends on the length of the alkenyl group and on the ligand. In terms of reaction pathways, adduct L_NdR_C leads to the cyclized product L_NdRC via the transition state L_NdRC*. Under polymerization conditions, the active complex L_NdR can either yield L_NdR_C or coordinate ethylene to form the adduct L_NdR_E. Insertion of ethylene proceeds via transition state L_NdRE*. Additionally, Bu2O as a cosolvent can coordinate to L_NdR to form the

is not quantitative under our polymerization conditions, since only 28% of the chains feature the proposed cycloheptyl end groups. Applying a precontact time of 1 h at 75 °C between the CTA and 2 before injecting ethylene resulted in the formation of 39% of the proposed cycloheptyl chain ends. On the basis of these polymerization results, it is obvious that cyclization takes place when the alkenyl group is transferred to neodymium. Nevertheless, the exchange of the formed cycloalkylmethyl group bound to neodymium with the alkenyl group bound to magnesium has just been assumed. In order to assess the cyclization reaction and the transfer of the cyclopentylmethyl moiety between neodymium and magnesium, an experiment was performed in a NMR tube at ambient temperature (Figure 4). A solution of 2 in toluene-d8 was

Figure 4. 1H NMR investigation of the reaction of 2 with dihexenylmagnesium (Mg/Nd = 32): (A) dihexenylmagnesium in toluene-d8; (B) a mixture of 2 with dihexenylmagnesium Asterisks denote signals of di-n-butyl ether.

introduced in a Young NMR tube containing dihexenylmagnesium (Mg/Nd = 32). The 1H NMR spectrum of the reaction mixture shows the disappearance of characteristic resonances of the dihexenylmagnesium compound and full reaction of the vinyl group. The obtained product is characterized by a signal at 0.09 ppm corresponding to a methylene group bound to magnesium and five resonances corresponding to one CH and four signals integrating for two protons (Figure 4B). These signals are attributed to the formation of bis(cyclopentylmethyl)magnesium via a quantitative reaction of dihexenylmagnesium with 2. We can thus conclude that the hexenyl moiety alkylates the neodymium center, leading to the formation of a cyclopentylmethyl−neodymium complex after cyclization. Since a Mg/Nd ratio of 32 was used, it appears that cyclopentylmethyl moieties bound to neodymium can be exchanged with hexenyl moieties bound to magnesium. Thus, a reversible transfer of these moieties between neodymium and magnesium occurs, fully converting the dihexenylmagnesium compound in bis(cyclopentylmethyl) magnesium (Scheme 4). The resulting compound is in fine the proper CTA that is operating during the polymerizations. Complex 2 can thus also Scheme 4. Synthesis of Bis(cyclopentylmethyl)magnesium Compounds Catalyzed by Complex 2

E


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Organometallics

Scheme 5. Energetics of the Reaction Pathways Involved in the Initiation of Ethylene Homopolymerization Initiated by 1/ Dialkenylmagnesiuma

Standard Gibbs energies, estimated at 298.15 K, are given in kcal mol−1. Themodynamic values are written in boldface type and kinetic values in italics. a

Figure 5. 3D representation of cyclization and ethylene insertion transition states for 1 and 2. The distances are given in Å.

Hence, ethylene insertion will lead to the formation of hexenyl and heptenyl groups. These two cases are discussed below. Hexenyl and Heptenyl Groups. In the case of 1, the most stable active species are L_NdR. After dissociation of the heterobimetallic species, cyclization transition states 1_NdRC* are more favorable by 7.0 kcal mol−1 for R = Hex′ (5.2 for R = Hept′) than the transition state for ethylene insertion 1_NdRE* (Scheme 5). The formation of the cyclopentane and cyclohexane rings is exergonic by ca. 7−13 kcal mol−1, respectively, and is thus thermodynamically favored. Hence, cyclizations leading to cyclopentyl and cyclohexyl moieties is kinetically favored over ethylene insertion. Cyclopentyl chain ends are preferentially formed relative to other cyclic chain

adduct L_NdR_Bu2O. In all cases, the formation of this adduct is endergonic and will not be further explored. In the following sections, we will discriminate our calculation cases depending on the length of the alkenyl groups. Butenyl and Pentenyl Groups. For 1, after dissociation of the heterobimetallic species the most stable active species is L_NdR_C. Cyclization transition state 1_NdRC* is lower in energy by 5.0 kcal mol−1 for R = Bu′ (2.9 for R = Pent′) than the transition state of ethylene insertion 1_NdRE* (Scheme 5). However, as cyclization is endergonic by 6−9 kcal mol−1 relative to the heterobimetallic species, the formation of a cyclic chain end is prevented, most likely due to cyclic strain. F


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Organometallics

Figure 6. Free energy profile for the alkyl/allyl exchange between Nd and Mg centers. For the sake of clarity, the methyl groups of the (C5Me5)2Nd metallocene fragment have been omitted.

ends, as shown by ΔΔrG(L_NdRE*- L_NdRC*) (Scheme 5), and do not require a precontact step to be quantitively formed. With a lower ΔΔrG(L_NdRE*- L_NdRC*) value, the chemoselectivity associated with the formation of cyclohexyl chain ends is less pronounced and requires a precontact step in order to be quantitative. Interestingly, the experimental chemoselectivity of 60% in favor of cyclization would be in line with close energy barriers for 1_NdHeptE* and 1_NdHeptC* cyclizations. As the Gibbs energy difference ΔΔrG(1_NdHeptE* − 1_NdHeptC*) = 5.2 kcal mol−1 and the enthalpy difference ΔΔrH(1_NdHeptE* − 1_NdHeptC*) = −4.2 kcal mol−1, this suggests an overestimation of the computed entropy contribution (TΔS at 298,15 K), within the harmonic oscillator approximation, of intermolecular reaction of ca. 5 kcal mol−1. Though not demonstrated, this value is coherent with various observations30,31 and corrections previously reported.32−37 The negative ΔΔrG(1_NdRE* − 1_NdRC*) value for R = Oct′ shows that cycloheptyl chain ends cannot be formed by 1. Due to the higher value of ΔΔrG(2_NdRE* − 2_NdRC*), 2 enables the formation of cycloheptyl chain ends (Scheme 5). In general, higher values of ΔΔrG(2_NdRE* − 2_NdRC*) highlight the chemoselectivity of 2 in favor of cyclization reactions relative to 1. The absence of an ansa bridge and the presence of 10 methyl groups on the neodymocene scaffold 1 raise the steric hindrance around the active site in comparison to 2. This is illustrated by the geometry analysis reported in Figure 5. Indeed, for 1, carbon atoms of the alkenyl chain are farther from the Nd center than for 2 but these atoms are closer to the ligand in the case of 1 in comparison to those of 2. When the length of the chain increases, the steric demand to undergo cyclization at the active site is increased, whereas ethylene insertion is not affected. For the hindered (C5Me5)2 ligand, the

scope of cycles, in terms of size, is lower than that offered by the Me2Si(C13H8)2 ligand. As an additional illustration of the steric hindrance within the complexes, noncovalent interaction analyses are available in Figure S17.38 Diallylmagnesium. In an attempt to explain the absence of reactivity observed while diallylmagnesium was used as both an allylating and a chain transfer agent, we have computationally assessed the energetics of Mg to Nd allyl transfer. Indeed, joint experimental/theoretical studies of ethylene−butadiene copolymerization revealed that the reverse transfer does not take place.12 In order to model the chain exchange reaction between [Mg]−allyl and [Nd]−alkyl sites, we have considered 1_NdBu as a model for the active catalyst and the magnesium dimer Mg2(C3H5)4 as the most relevant diallylmagnesium cluster under polymerization conditions. Starting from 1_NdBu and Mg2(C3H5)4, the allyl/alkyl exchange is exergonic by 11.5 kcal mol−1. This exchange cannot be performed by a one-step concerted mechanism and requires four rearrangements and two association/dissociation steps to proceed. The Gibbs energy profile shown in Figure 6 depicts the two isoenergic most stable states: the bimetallic complex BM2 and exchanged products 1_Nd(C3H5) and Mg2(Bu)(C3H5)3. In terms of mechanism, this chain exchange mechanism reveals some interesting features. Starting from 1_NdBu and Mg2(C3H5)4, the first step of this mechanism corresponds to the coordination between the neodymocene alkyl complex and the diallylmagnesium to form a doubly bridged bimetallic complex BM1 in which the bridging allyl group is μ,η1-η1 with respect to Mg centers and η1 relative to Nd. Then, three successive steps are needed to complete the sliding of the allyl motif in order to form the bimetallic complex BM4, in which the bridging allyl motif is η3 coordinated to Nd G


Organometallics

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and η1 coordinated to Mg. This mechanism is enabled by the versatility of σ and/or π bonding of the allyl ligand.39 Kinetically, the energy barriers involved in this exchange reaction are low in energy and suggest that the exchange between [Mg]−allyl and [Nd]−alkyl sites should proceed and thus cannot account for the lack of reactivity observed while diallylmagnesium is used as a chain transfer agent. Thus, we assume that the formation of the allyl-neodymium complex is the limiting step toward the formation of active species when 1 or 2 is combined with diallylmagnesium.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for L.P.: [email protected]. *E-mail for F.D.: [email protected]. *E-mail for C.B.: [email protected]. ORCID

Lionel Perrin: 0000-0002-0702-8749 Franck D’Agosto: 0000-0003-2730-869X Christophe Boisson: 0000-0002-7909-901X

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Notes

CONCLUSION A range of dialkenylmagnesium compounds ([CH2CH(CH2)n]2Mg; n = 2−6) were synthesized and used as chain transfer agents (CTA) with neodymium precursors 1 and 2 for the polymerization of ethylene. These two precursors were chosen on the basis of their ability to lead in the presence of dialkylmagnesium compounds to coordinative chain transfer polymerization (CCTP) of ethylene. In all cases, CCTP was effective when dialkenylmagnesium compounds were employed as well. The additional presence of a vinyl group on the CTA allowed the reinsertion of this double bond to generate cycloalkyl-capped polyethylene chains. Cyclopentyl and cyclohexyl chain ends were either obtained quantitatively or obtained together with vinyl-capped polyethylene chains. The preparation of α-vinyl-ω-cyclohexane-PEs is in progress in order to assess their applicability as macromonomers in isomerization polymerization.40 The selectivity toward the formation of cycloalkyl chain ends was dictated by the number of carbons in the CTA, the nature of the precursor employed, and the polymerization conditions complying with sometimes a precontact time between the CTA and the precursor. Quantitative production of cyclopentyl-capped polyethylene chains was observed when n = 2, 4 and in the presence of 1. When n = 5, a precontact time between 1 and the CTA was required to quantitatively produce cyclohexyl chain ends. The superiority of 2 in comparison to 1 to produce these rings was shown by producing quantitatively cyclohexyl-capped polyethylene with 2/[CH2CH(CH2)5]2Mg while 1/[CH2 CH(CH2)5]2Mg only led to 62% of cyclohexyl-capped polyethylene chains. With the dihexenylmagnesium chain transfer agent, it was shown that the CTA first alkylates the Nd precursor, the cyclization occurs, and the resulting cyclopentylmethyl moiety can be transferred back to Mg, the newly formed (cyclopentylmethyl)2Mg acting as the proper CTA. The integrity of the double bond carried by the CTAs can also be kept if n is higher than 6. The use of dialkenylmagnesium chain transfer agents is thus a powerful tool to provide information on the ability of neodymium metallocenes to form cycles in CCTP.

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Article

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Manufacture Michelin is acknowledged for scientific and financial support of this project. M.-N.P., S.B. and L.P. thank the CCIR of ICBMS and P2CHPD of Université Lyon 1 for providing computational resources and technical support. The authors thank the NMR Polymer Center of Institut de Chimie de Lyon (FR5223) for assistance and access to the NMR facilities and Olivier Boyron and Manel Taam (C2P2) for HTSEC analyses.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00127. Experimental details, NMR of dialkenylmagnesium compounds, additional NMR of polymers, and computational details (PDF) Cartesian coordinates and associated energies of calculated structures (XYZ) H


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Dialkenylmagnesium Compounds in Coordinative Chain Transfer

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