Mechanistic Insights on the Controlled Switch from Oligomerization

zirconium: reactivity and activity in the copolymerization of cyclohexene oxide with CO 2. Ralte Lalrempuia , Frida Breivik , Karl W. Törnroos , ...
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Mechanistic Insights on the Controlled Switch from Oligomerization to Polymerization of 1‑Hexene Catalyzed by an NHC-Zirconium Complex Emmanuelle Despagnet-Ayoub,*,†,‡ Michael K. Takase,§ Lawrence M. Henling,§ Jay A. Labinger,*,§ and John E. Bercaw*,§ †

CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, 31077 Toulouse Cedex 4, France Université de Toulouse, UPS, INPT, LCC, 31077 Toulouse Cedex 4, France § Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125, United States ‡

S Supporting Information *

ABSTRACT: The benzimidazolylidene zirconium complex 1 switches from an oligomerization (without additive) to a polymerization catalyst by addition of an L-type ligand such as trimethylphosphine, while larger phosphines or amines completely inhibit catalysis. On the basis of the regioselectivity of the oligomers/polymers obtained, the time profiles of reactions as a function of added ligand, and the molecular structures of several cationic zirconium complexes, we propose a mechanistic framework for interpreting this complex catalytic behavior.

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INTRODUCTION There has been intense activity toward the development of highly active catalysts for both polymerization and oligomerization of olefins during the last few decades.1 In some cases we understand fairly well why a particular catalyst leads to polymerization vs oligomerization.2 However, in many others no such interpretation is available: the factors favoring the operation of one process or the other are unpredictable, with subtle changes in ligand framework, cocatalyst, and reaction conditions all affecting reactivity and selectivity. As just one example, the titanium complex A, containing two bidentate phenoxy-imine ligands, gives a highly active (40.3 kg of PE/ mmol of Ti/h) ethylene polymerization catalyst upon activation with MAO,3 whereas under similar conditions B, with a single, closely related tridentate ligand, trimerizes ethylene to 1hexene, with even higher activity (315 kg of 1-hexene/mmol of Ti/h) and 92.3% selectivity4 (Chart 1). The vast difference in outcomes can be explained by a mechanistic switch polymerization by A proceeds via a traditional Cossee−Arlman insertion mechanism, while selective trimerization by B follows a metallacyclic mechanism that involves Ti(II) and Ti(IV) species5but the reasons for such a switch are quite unclear. The ability to induce a controlled switch from oligomerization to polymerization, or the reverse, could help elucidate some of that missing mechanistic insight, as well as offer possible opportunities for application. We recently reported a system that can be converted from an oligomerization catalyst to a © XXXX American Chemical Society

Chart 1. Phenoxy-Imine Titanium Complexes

polymerization catalyst (of 1-hexene) by the addition of a suitable ligand (Scheme 1).6 The cationic (bis-phenolate-Nheterocyclic carbene)zirconium complex 2, obtained by abstraction of a benzyl group from the neutral parent complex 1, oligomerizes 1-hexene, to yield a rough Schulz−Flory distribution with a maximum at the tetramer (Mn = 500). Addition of triphenylphosphine does not perturb the system; addition of 1 equiv of either trimethylphosphine or triethylphosphine causes it to become a polymerization catalyst (Mn = 5900 and 2300, respectively); addition of 1 equiv of tricyclohexylphosphine or 2 equiv of trimethylphosphine Special Issue: Gregory Hillhouse Issue Received: June 5, 2015

A

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Organometallics Scheme 1. Reactivity of Catalysts Based on Bis(phenolate)NHC Zirconium 1 with 1-Hexene

quenched with a 10% HCl/MeOH solution (10 mL). The oligomers were extracted with toluene, and then the solution was dried over MgSO4 and analyzed by GC. Cationic Zirconium Complex Obtained from 1/B(C6F5)3. In a J. Young NMR tube, a solution of B(C6F5)3 (4.5 mg, 0.0088 mmol) in chlorobenzene-d5 (0.4 mL) was added to a solution of complex 1 (7 mg, 0.0088 mmol) in chlorobenzene-d5 (0.3 mL). 1H NMR (400 MHz, C6D5Cl, 25 °C): δ 1.38 (s, 18H, tBu), 1.63 (s, 18H, tBu), 2.04 (s, 2H, ZrCH2), 3.44 (br s, 2H, BnB), 6.08 (d, 2H, JHH = 7.5 Hz, Ph), 6.15 (t, 2H, JHH = 7.5 Hz, Ph), 6.22 (m, 1H, Ph), 7.22 (br m, 1H, Ph), 7.31 (dd, 2H, JHH = 6.4 and 3.2 Hz, Arbackbone), 7.38 (br m, 2H, Ph), 7.54 (br m, 2H, Ph), 7.56 (d, 2H, JHH = 2.4 Hz, ArO), 7.70 (d, 2H, JHH = 2.4 Hz, ArO), 7.85 (dd, 2H, JHH = 6.4 and 3.2 Hz, Arbackbone). 19F NMR (376.16 MHz, C6D5Cl, 25 °C): δ −164.91 (br s, para-F), −160.69 (br s, meta-F), −130.78 (d, JFF = 22.2 Hz, ortho-F). 11B NMR (128.27 MHz, C6D5Cl, 25 °C): δ −10.68. General Procedure for Kinetics Experiments. For oligomerization of 1-hexene without additive, a solution of [Ph3C][B(C6F5)4] (5.7 mg, 0.0063 mmol) and complex 1 (5 mg, 0.0063 mmol) in chlorobenzene-d5 (0.6 mL) was prepared in a NMR tube with a J. Young valve. 1-Hexene (0.26 g, 3.15 mol) was added at room temperature, the tube was transferred to the NMR spectrometer, and the 1H NMR spectrum was followed over time, using the Varian parameters for “pad” at 25 °C (time delay 35s, acquisition time 25 s, number of scans 8, first pad set as 0). For polymerization of 1-hexene with trimethylphosphine, a solution of [Ph3C][B(C6F5)4] (5.7 mg, 0.0063 mmol) and complex 1 (5 mg, 0.0063 mmol) in chlorobenzene-d5 (0.6 mL) was prepared in a NMR tube with a J. Young valve. After addition of a solution of trimethylphosphine in benzene-d5 (32 μL, 0.0063 mmol), the tube was frozen in the cold well of the glovebox, and 1-hexene (0.26 g, 3.15 mmol) was added. The NMR tube was removed from the glovebox, immersed in a cold bath at −45 °C (at which temperature no reaction occurs), and transferred to the NMR spectrometer, which had been precooled to −25 °C. The evolution of the 1H NMR spectrum over time was followed as above. Generation of Complex 5. In a J. Young NMR tube, 1 equiv of 1hexene was added to a solution of [Ph3C][B(C6F5)4] (5.7 mg, 0.0063 mmol) and complex 1 (5 mg, 0.0063 mmol) in chlorobenzene-d5 (0.7 mL), and the resulting product analyzed by 1D and 2D NMR. 1H NMR (500 MHz, 298 K, C6D5Cl): δ 0.22 (dd, JHH = 13.0 Hz, JHH = 3.1 Hz, 1H, CαH2), 0.64 (t, JHH = 7.3 Hz, 3H, CH3), 0.70 (m, 1H, CγH2), 0.78 (m, 2H, CδH2), 0.94 (m, 2H, CεH2), 1.00 (m, 1H, CγH2), 1.09 (m, 1H, CαH2), 1.34 (s, 18H, tBu), 1.68 (s, 18H, tBu), 2.16 (m, 1H, CH2Bn), 2.81 (pseudosept, 1H, JHH = 5.6 Hz, CβH), 3.01 (dd, JHH

completely shuts down activity. We speculated that such dramatic changes might arise from steric influences on the orientation of the approaching monomer in the coordination/ insertion Cossee−Arlman mechanism. Here we report additional findings, including in particular crystallographic characterization of several cationic complexes that provide important information on the environment of the catalytic center with and without added ligand, and propose a unified mechanistic framework that we believe accounts for the entire gamut of observations.

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EXPERIMENTAL SECTION

General Considerations. All solutions were prepared in a glovebox under a nitrogen atmosphere. Chlorobenzene was dried over calcium hydride and then filtered through a pad of activated alumina and stored over activated 3 Å molecular sieves in the box. Methylaluminoxane (MAO) was purchased as a toluene solution from Albemarle and dried under vacuum at 150 °C overnight to remove free trimethylaluminum before use. 1-Hexene was filtered through activated alumina and stored over molecular sieves (3 Å) prior to use. C6D5Cl was purchased from Cambridge Isotopes and dried over molecular sieves (3 Å) for at least 1 day. The synthesis of complex 1, its reaction with [Ph3C][B(C6F5)4] to give 2, and the reaction of 2 with 1 and 2 equivalents of PMe3 to give 3 and 4, respectively, were all reported, along with NMR spectroscopic characterization, in our previous communication.6 Polymer molecular weights were determined from the relative intensity of end-group olefinic signals in 1H NMR spectra. General Procedure for Oligomerization/Polymerization of 1Hexene with the System 1/[Ph3C][B(C6F5)4]. A solution of [Ph3C][B(C6F5)4] (5.7 mg, 0.0063 mmol) and complex 1 (5 mg, 0.0063 mmol) in chlorobenzene (0.7 mL) was stirred for 5 min at room temperature. After addition of ligand (phosphine, amine, or none) followed by 1-hexene (0.53 g, 6.3 mmol), the solution was stirred overnight in a 10 mL Strauss flask outside the box and then quenched with 0.1 mL of methanol and 1 drop of hydrochloric acid. For oligomerization, biphenyl (50 mg) was added as internal reference, and the mixture was filtered through a plug of silica before analysis by GC. For polymerization, the solution was evaporated under vacuum overnight before analysis by NMR. Oligomerization of 1-Hexene with the System 1/MAO. A solution of complex 1 (2.5 mg, 0.003 mmol) in toluene (0.6 mL) was added to a suspension of MAO (0.18 g, 3.1 mmol) in 1-hexene (0.26 g, 3.1 mmol). After it was stirred overnight, the reaction mixture was B

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Organometallics = 6.1 Hz, JHH = 12.8 Hz, 1H, CH2Bn), 7.27 (dd, JHH = 6.3 and 3.2 Hz, 2H, Arylbackbone), 7.64 (d, JHH = 2.4 Hz, 2H, CHArO), 7.66 (t, JHH = 7.5 Hz, 1H, Ph), 7.73 (d, JHH = 2.4 Hz, 2H, CHArO), 7.86 (dd, JHH = 6.3 and 3.2 Hz, 2H, Arylbackbone), 7.88 (d, JHH = 7.5 Hz, 2H, Ph), 7.95 (d, JHH = 7.5 Hz, 2H, Ph). 13C NMR (125 MHz, 298 K, C6D5Cl): δ 14.0 (CH3), 22.6 (CεH2), 29.5 (CδH2), 30.1 (CH3tBu), 31.3 (CH3tBu), 34.8 (CtBu), 35.6 (CtBu), 40.9 (CγH2), 41.8 (CH2Bn), 57.0 (CβH), 77.4 (CαH2), 115.2 (CHAr), 118.3 (CHArO), 123.8 (CHArO), 125.3 (C), 125.5 (C), 127.0 (CHAr), 127.2 (CHPh), 130.2 (CHPh), 132.2 (C), 133.1 (CHPh), 136.8 (d, JCF = 245 Hz, CF), 138.8 (C), 140.6 (d, JCF = 245 Hz, CF), 142.2 (C), 143.2 (C), 143.7 (C), 147.9 (d, JCF = 245 Hz, CF), 156.0 (C), 190.9 (Ccarbene). Generation of Complex 6. In the glovebox, PPh3 (4.9 mg, 0.018 mmol) was added to a solution of [Ph3C][B(C6F5)4] (17 mg, 0.018 mmol) and complex 1 (15 mg, 0.018 mmol) in chlorobenzene-d5 (0.7 mL), and the resulting product was analyzed by 1D and 2D NMR. 1H NMR (400 MHz, 298 K, C6D5Cl): δ 1.29 (s, 18H, tBu), 1.35 (s, 18H, tBu), 3.37 (s, 2H, CH2Bn), 6.39 (br. m, 3H, Ph), 6.50 (br. m, 2H, Ph), 6.98 (br. m, 9H, Ph), 7.27 (br. m, 6H, Ph), 7.33 (dd, JHH = 6.3 and 3.2 Hz, 2H, Arylbackbone), 7.52 (d, JHH = 2.3 Hz, 2H, CHArO), 7.57 (d, JHH = 2.3 Hz, 2H, CHArO), 7.80 (dd, JHH = 6.3 and 3.2 Hz, 2H, Arylbackbone). 13 C NMR (100.62 MHz, 298 K, C6D5Cl): δ 30.1 (CH3tBu), 31.3 (CH3tBu), 34.7 (CtBu), 35.8 (CtBu), 69.1 (Zr-CH2), 114.1 (CHAr), 118.6 (CHArO), 123.8 (CHArO), 126.1 (CHAr), 126.3 (CHPh), 127.4 (br s, CHPh), 131.1 (CHPh), 131.3 (br s, CHPh), 132.7 (CHPh), 133.4 (CHPh), 133.6 (C), 133.8 (C), 134.5 (C), 135.8 (d, JCF = 245 Hz, CF), 138.6 (d, JCF = 245 Hz, CF), 138.7 (C), 143.7 (C), 147.0 (C), 148.3 (d, JCF = 241 Hz, CF), 149.3 (C), 195.8 (Ccarbene). A signal for one quaternary carbon is missing, probably because of overlapping with those of the chlorobenzene-d5 and the 1,1,1,2-tetraphenylethane generated by abstraction of the benzyl group. 31P NMR (161.85, 298 K, C6D5Cl): δ 11.5.

mixed methylphenylphosphines PMe2Ph (θ = 122°) and PMePh2 (θ = 136°) behave much like PMe3 (θ = 118°): addition of 1 equiv of either phosphine leads to poly(1-hexene) with Mn = 4000 and 1500, respectively. NMR was used to study the binding of phosphine to the cationic benzyl zirconium complex 2, with one ligand selected from each of the three reactivity patterns: tricyclohexylphosphine (no catalytic activity), triphenylphosphine (oligomerization), and trimethylphosphine (polymerization). Addition of 1 equiv of PCy3 to 2 yields a monophosphine complex, exhibiting a 31P{1H} NMR signal at 20 ppm (Figure 1a); upon addition of

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RESULTS AND DISCUSSION Effect of Activator. We previously reported that the choice of activation method can significantly affect the reactivity: activation of complex 1 by abstraction of benzyl with [Ph3C][B(C6F5)4] gives an active catalyst for oligomerization of 1-hexene, while protonolytic removal of benzyl by [HNMe2Ph][B(C6F5)4] completely shuts down activity, presumably a consequence of coordination of the resulting dimethylaniline.6 We have now examined two other commonly used activators. When complex 1 was added to a mixture of 1000 equiv of MAO and 1000 equiv of 1-hexene and stirred overnight, followed by quenching with acidic methanol and analysis by GC, the resulting distribution of oligomers was similar to that obtained with the system 1/[Ph3C][B(C6F5)4] (see Figure S2 in the Supporting Information). In contrast, abstraction of benzyl by 1 equiv of the neutral reagent B(C6F5)3 gave no activity at all. In this case, the 19F NMR spectrum of the cationic complex reveals a large difference (4.2 ppm) in the chemical shifts of the meta and para fluorines of the counteranion, indicating that [BnB(C6F5)3]− coordinates to the metal center,7 unlike [B(C6F5)4]−. Effect of Ligands. Several additional ligands, of varying σdonor ability and steric bulk (as measured by the cone angle θ), were examined to further investigate the relationship between ligand properties and reactivity. Tri(tert-butyl)phosphine (θ = 182°) and tri(isopropyl)phosphine (θ = 160°) completely shut down catalytic activity, as does tricyclohexylphosphine (θ = 170°). Trimesitylphosphine, a bulky (θ = 212°) but less basic phosphine, causes no perturbation, generating a similar distribution of oligomers as with triphenylphosphine (see Figures S3 and S4 in the Supporting Information) or no additive at all. Triethylamine completely deactivates the catalyst, like dimethylaniline (by inference: see above). The

Figure 1. 31P NMR spectra: (a and b) 1/[Ph3C][B(C6F5)4], 1 equiv of PCy3 and PPh3, respectively, at 25 °C; (a′ and b′) addition of another 1 equiv of PCy3 and PPh3, respectively at 25 °C; (c and c′) 1/ [Ph3C][B(C6F5)4], 1 equiv of PMe3 at 25 and −40 °C, respectively. The signal marked with an asterisk corresponds to the [Cy3P−Ph3C]+ adduct resulting from the presence of a small amount of excess trityl cation.

another 1 equiv of PCy3 a signal for free phosphine is observed, with no indication of any exchange process between coordinated and free PCy3 (Figure 1a′), implying strong P− Zr binding. One equivalent of PPh3 also gives a monophosphine adduct, but here addition of another 1 equiv gives only a single broad signal (Figure 1b,b′), implying exchange on the NMR time scale, consistent with the poorer σ-donation ability and/or smaller cone angle of this phosphine. The monophosphine adduct obtained on addition of 1 equiv of PMe3 gives a broad 31P{1H} NMR signal at room temperature; at −40 °C the signal sharpens without moving, suggesting the broadness is probably attributable to partially restricted rotations or other internal motions (Figure 1c,c′). Addition of another 1 equiv affords a bis(PMe3) adduct;6 its 31P NMR spectrum is discussed in the following section. Structural Characterization of Cationic Species. Whereas cationic organozirconium species tend to be of low stability, and structural characterizations are relatively rare, the tridentate NHC ligand used here provided access to three crystallizable complexes. The cationic monobenzyl zirconium species obtained via benzyl abstraction with [Ph3C][B(C6F5)4] C

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Organometallics Scheme 2. Formation of the Cationic Zirconium Complexes 3 and 4

Figure 2. Front (a, b, c) and side (a′, b′, c′) views of the molecular structures of complexes 2·2Et2O, 3·Et2O, and 4, respectively. The counteranion [B(C6F5)4]− and the hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (deg): (a) Zr(1)−C(1) = 2.3011(17), Zr(1)−O(1E) = 2.3940(14), Zr(1)-O(2E) = 2.308(4), Zr(1)−C(51) = 2.300(2), Zr(1)−C(52) = 2.6122(19), Zr(1)−C(51)−C(52) = 84.59(11), Zr(1)−C(1)− N(1)-C(2) = 148.19, Zr(1)−C(1)−N(2)-C(7) = 148.65; (b and c) Zr(1)−C(1) = 2.3235(16), Zr(1)−O(3B) = 2.215(7), Zr(1)−P(1) = 2.7772(5), Zr(1)−P(2) = 2.8269(10), Zr(1)−C(51) = 2.2907(17), Zr(1)−C(52) = 2.6769(17), Zr(1)−C(51)−C(52) = 88.09(10), Zr(1)−C(1)−N(1)-C(2) = 148.23, Zr(1)−C(1)−N(2)−C(7) = 149.00.

the neutral complex 1.6 The benzyl group adopts a pronounced η2 bonding mode (Zr(1)−C(51) = 2.30 Å; Zr(1)−C(52) = 2.61 Å; Zr(1)−C(51)−C(52) = 84°), also suggested by the large 1JCH coupling constant (142 Hz) observed in the 13C NMR spectrum. The Zr−Oether bond lengths are not identical: Zr(1)−O(E1) (2.39 Å) is longer than Zr(1)−O(E2) (2.30 Å), suggesting that the η2 benzyl group exhibits a higher trans influence than the carbene moiety. The complex 2·2Et2O is

from the dibenzyl parent complex 1, previously characterized as 2 by NMR (Scheme 2),6 was crystallized as a bis(ether) adduct by slow diffusion of pentane into a chlorobenzene/ether solution at low temperature. The complex 2·2Et2O (Figure 2a) features distorted-octahedral geometry with two ether molecules coordinated in a cis fashion. The distortion is mainly due to the nonplanarity of the benzimidazolylidene moiety, the C6N2 grouping of which is bent out of the OAr−Zr−OAr plane by 148° (see side view in Figure 2a′), as previously observed for D

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Figure 3. (a) Dynamic 31P NMR experiments on complex 4. (b) Eyring plot.

Scheme 3. Mechanistic Steps for Olefin Oligomerization/Polymerization via the Cossee−Arlman Pathwaya

a

P: polymer.

The phosphine is located opposite to the direction of bending of the benzimidazolylidene moiety, presumably the less crowded side of the molecule. Complex 4 has essentially the same structure as 3·Et2O, with the ether molecule replaced by a second phosphine, and similar parameters (Figure 2c,c′). Its 31P NMR spectrum at room temperature is broad and indistinct but at 0 °C resolves into an AB quartet signal, with δ −28 and −32 ppm and JPP = 99 Hz. The large coupling constant is consistent with the trans arrangement of phosphines revealed by the crystal structure, while the chemically inequivalence presumably reflects the asymmetry resulting from the benzimidazolylidene bending and/or the orientation of the η2-benzyl. Dynamic 31P NMR was carried out over the range of −20 to 50 °C, showing a coalescence temperature at around 35 °C (Figure 3a); simulated spectra were fit to experimental data at the different temperatures with Topspin 1.3, and the corresponding rate

inactive for 1-hexene oligomerization, reflecting strong binding of the ether molecules to the cationic zirconium species. While several attempts to crystallize monophosphine complex 3 were unsuccessful, cooling an ether-containing solution of the bisphosphine complex 4 surprisingly gave mixed crystals, whose structure could nonetheless be successfully solved and refined, proving to consist of 72% of the expected (PMe3)2 complex 4 (Figure 2c) and 28% of a PMe3 Et2O complex, the ether adduct 3·Et2O (Figure 2b). The latter has PMe3 in an equatorial position trans to the ether molecule, with the benzyl ligand located in the axial position trans to the carbene moiety. Despite the presence of six good σ-donor ligands, the benzyl ligand still adopts the η2 coordination mode (Zr(1)−C(51) = 2.29 Å; Zr(1)−C(52) = 2.68 Å; Zr(1)− C(51)−Zr(52) = 88°). As with 2·2Et2O, the benzimidazolylidene moiety is nonplanar with a similar bending angle (Zr(1)− C(1)−N(1/2)−C(2/7) = 148°, see side view in Figure 2b′). E

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Figure 4. (a) Reaction profile of 1-hexene polymerization with 1/[Ph3C][B(C6F5)4]/PMe3. Conditions: 500 equiv of 1-hexene, 1/ [Ph3C][B(C6F5)4]/PMe3 1/1/1, chlorobenzene (0.6 mL), −25 °C. (b) Plot of ln [1-hexene] vs time.

insertion, with the much less frequent 2,1-insertion events resulting in chain termination. In the absence of added PMe3, the oligomerization of 1hexene catalyzed by 1/[Ph3C][B(C6F5)4] proceeds considerably more slowly (a generally counterintuitive observation) and can be monitored at room temperature by 1H NMR (Figure 5). The rate of consumption of monomer does not follow clean first-order kinetics over the entire reaction, probably due to deactivation/decomposition of the catalyst. During the first 20 min of the reaction, only two signals for olefinic products are observed, at 4.9 and 5.5 ppm, attributable respectively to vinylidene and vinylene end groups, with the latter favored by about a 70/30 ratio. After 20 min, the vinylidene signal begins to decrease, with the growth of a new broad signal at 5.3 ppm, assigned to trialkyl-substituted alkenes with support of 2D HSQC spectroscopy (see the Supporting Information). Such species would result from isomerization, via reinsertion of the vinylidene end group followed by successive β-H elimination/reinsertion, a chain-walking process that allows the formation of more stable internal olefins (Scheme 3). (The 1-hexene monomer itself undergoes this isomerization sequence: cis-2-, trans-2-, and 3-hexenes are detected by GC at the end of the reaction.) Figure 5 indicates that vinyleneterminated alkenes do not reinsert, so that the ultimate ratio of vinylene to vinylidene end groups is around 85/15, as reported previously.6 The 13C NMR spectrum exhibits a number of signals in the range of 10−45 ppm (see Figure S30 in the Supporting Information) that would result from these isomerizations, but a complete assignment would be very difficult. The disubstituted terminal olefin 2-ethyl-1-hexene, a model vinylidene-terminated oligomer, is neither oligomerized or isomerized by 1/[Ph3C][B(C6F5)4], suggesting that these olefins undergo insertion with the cationic zirconium hydride intermediate generated during the course of the reaction but not with the cationic monobenzylzirconium complex 2, presumably reflecting steric hindrance with the benzyl ligand. The product distribution for reactions carried out at higher dilution is very similar, suggesting that bimolecular processes

constants were used to construct an Eyring plot (see the Supporting Information and Figure 3b), giving ΔH⧧ = 15.2 kcal/mol, ΔS⧧ = 6.59 cal/mol, and ΔG⧧(293 K) = 13.3 kcal/ mol. Addition of another 1 equiv of PMe3 results in a signal for free ligand which does not exchange with the coordinated PMe3; that observation and the small ΔS⧧ value suggest that the fluxional behavior is due to an intramolecular process: a “flip-flop” of the benzimidazolylidene moiety or rotation of the benzyl ligand. Unfortunately, the 1H NMR spectrum does not allow us to discriminate between them. Mechanistic Studies. The mechanistic steps for olefin oligomerization/polymerization via the Cossee−Arlman pathway are well-documented, consisting of the following: initiation, propagation via 1,2-insertion (“normal insertion”) positioning the alkyl R group away from the metal center to limit steric interaction with the ligands or via 2,1-insertion (“misinsertion”) placing the alkyl R group in close proximity with the metal center, chain termination via β-H elimination generating a zirconium hydride species, and isomerization via chain walking by reversible β-H elimination (Scheme 3).8 The reaction of 500 equiv of 1-hexene with 1/[Ph3C][B(C6F5)4]/PMe3 proceeds too rapidly to follow at room temperature; therefore, it was monitored by 1H NMR at low temperature (−25 °C) (Figure 4). The reaction is cleanly first order in 1-hexene, with a rate constant of 1.5 × 10−2 min−1. The 13C{1H} NMR spectrum of the poly-1-hexene obtained exhibits the anticipated six resonances, with broad peaks for the C(1)−C(4) signals, characteristic of a regioirregular atactic polymer (see the Supporting Information), with no evidence of misinsertion (2,1-insertion) or chain walking.9 However, both vinylidene (RCHCH2) and vinylene (RCHCHR′) end groups are observed by 1H and 13C NMR; these result from βH elimination following 1,2- or 2,1-insertion, respectively. Thus, as no regio errors are observed in the main polymer chain, this suggests that the 2,1-insertion process is systematically followed by chain transfer, probably because of steric crowding around the metal center. Since polymers, not oligomers, are obtained, the dominant mode must be 1,2F

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Figure 6. Reaction profile of 1-hexene oligomerization with 1/ [Ph3C][B(C6F5)4]/PPh3. Conditions: 500 equiv of 1-hexene, 1/ [Ph3C][B(C6F5)4]/PPh3 1/1/1, chlorobenzene (0.6 mL), 25 °C.

the resulting oligomers contain both Bn and H end groups, including the “monomer” C4H9C(Bn)CH2. These observations imply that there are two competing pathways by which initiation is completed (Scheme 4): insertion of a second 1hexene into the Zr−C bond, which probably requires decoordination of the phenyl arm, leading to oligomers with Bn end groups (upper path), and generation of a hydride zirconium complex via β-H elimination, followed by successive insertions, to give oligomers with H end groups (lower path). When a similar experiment is carried out for the system 1/ [Ph 3C][B(C 6F 5 ) 4]/PPh3 , which gives a distribution of oligomers similar to that of the ligand-free system (see above), a 1H NMR signal characteristic of a Zr-bound benzyl group carbon persists through the addition of first 1 and then 10 equiv of 1-hexene. The 31P NMR spectrum is very similar to that seen in the absence of added 1-hexene (Figure 1b), although somewhat broader, with no visible free phosphine signal. The situation thus appears closely analogous to that for the phosphine-free catalyst: initiation is again very slow relative to propagation, with most of the Zr in solution remaining inactive, here primarily as the PPh3 adduct of 2 (complex 6). In contrast, the system 1/[Ph3C][B(C6F5)4]/PMe3 behaves quite differently: the 1H NMR signal for Zr−Cbenzyl of the PMe3 adduct 3 disappears rapidly and completely upon addition of 1−10 equiv of 1-hexene, with no persistent signals for any precatalyst, thus indicating that initiation is at least as fast as propagation in this polymerization process. Again, no 31P NMR signal corresponding to free phosphine was observed over the course of the reaction. Mechanistic Interpretation. Our preliminary study6 on catalysis on the basis of bis(phenolate)NHC zirconium complex 2, with and without added ligands, reported several unusual findings. Some ligands effect a switch from oligomerization to polymerization of 1-hexene, others completely inhibit activity, while still others have little or no apparent effect. There are also modulations, most probably related to the switch, on the microstructure of the resulting oligomers/polymers, particularly preferences in regioselectivity

Figure 5. Reaction profile of 1-hexene oligomerization with 1/ [Ph3C][B(C6F5)4]. Conditions: 500 equiv of 1-hexene, 1/[Ph3C][B(C6F5)4] 1/1, chlorobenzene (0.6 mL), 25 °C.

are not significantly involved in chain transfer (see the Supporting Information). Addition of triphenylphosphine to the system 1/[Ph3C][B(C6F5)4] does not significantly perturb the distribution of oligomers obtained, as mentioned previously, or the rate of 1hexene consumption; however, the selectivity for oligomers featuring vinylene end groups is significantly higher (vinylene/ vinylidene 95/5), and no significant isomerization to trisubstituted olefins is observed (Figure 6). We were also able to probe the first insertion of 1-hexene into the Zr−Bn bonda key step of initiation leading to the propagating speciesby the addition of just 1 equiv of 1hexene. For the ligand-free system 1/[Ph3C][B(C6F5)4] a single product is thus produced, identified by 1D and 2D NMR spectroscopy as the 1,2-inserted product 5 (Scheme 4). In particular, the Zr-bonded carbon Cα, with its characteristic downfield chemical shift (77.4 ppm), appears as a methylene group, not a methine. The methylene protons of both the benzyl and the Cα groups are diasterotopic, suggesting a locked geometry such as that shown in Scheme 4, which is supported by NOESY experiments: correlations between the phenyl group of the benzyl and the ortho tert-butyl groups on the tridentate NHC ligand imply close proximity, indicating coordination of the phenyl arm to the metal center. Upon addition of a further 10 equiv of 1-hexene to the solution of complex 5, oligomerization takes place but the NMR signals of 5 do not disappear to an observable extent, implying that formation of 5 does not complete initiation, that initiation is slow relative to propagation, and that a relatively small fraction of the total Zr species in solution accounts for all the catalytic activity. Furthermore, GC-MS analysis shows that G

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Organometallics Scheme 4. Stepwise Addition of 1-Hexene to 1/[Ph3C][B(C6F5)4] System

not have the stabilizing coordinated phenyl arm feature of the ligand-free 5, so that subsequent insertions can proceed without requiring an additional and unfavorable dissociation. Initiation from precatalyst 3 is hence fast, at least comparable to propagation, thus affording a high concentration of active species, unlike the ligand-free catalyst where only a small fraction of Zr species becomes active. This accounts for the apparently paradoxical finding that ligand coordination accelerates overall catalysis: in fact, the putative chain-carrying Zr hydride complex probably is inherently more reactive in the absence of a ligand, but there is much less of it present, and so the observed activity is lower. The switch from oligomerization to polymerization can be explained by increased steric crowding brought about by coordination of PMe3, leading to a much stronger preference for 1,2-insertion, as shown by the highly regioregular microstructure of the poly-1-hexene obtained: almost no 2,1enchainment is observed. Since 2,1-enchainment appears (from the behavior of the ligand-free catalyst) to lead directly to chain termination, termination should become relatively much slower in comparison to propagation when 2,1-enchainment is sterically disfavored, resulting in polymer instead of oligomer. As noted above, there are three classes of ligand behavior. Those that effect the oligomerization/polymerization switch are relatively small, strongly basic phosphines: PMe3 (θ = 118°), PMe2Ph (122°), PEt3 (132°), PMePh2 (136°), PEtPh2 (140°). These all presumably follow the mechanistic pattern for PMe3: the ligand remains coordinated throughout catalysis, providing access to incoming monomer, but exerts sufficient steric influence to strongly favor 1,2- over 2,1-insertion. The molecular structure of the PMe3 adduct 3·Et2O (Figure 2b′) suggests an attractive model for that effect: a vacant site on the side of the molecule opposite to the coordinated ligand (occupied by the ether used for crystallization, which is of course absent in catalysis) is open for incoming monomer but strongly influenced by the bent C6 ring of the benzimidazo-

of insertion. Perhaps most remarkably, upon coordination of PMe3 the resulting polymerization catalysis is considerably faster than the oligomerization catalysis without ligand, an observation for which we could offer no satisfying explanation. With the additional work reported here, however, we are able to construct a unified mechanistic picture that can account for essentially all of the observed behavior. For the case of oligomerization of 1-hexene without additive, the experiments with low loading of monomer show that complex 2 undergoes rapid and quantitative 1,2-insertion with one molecule of 1-hexene, generating the metallacyclic precatalyst 5. However, further insertion of monomer is slow, presumably requiring decoordination of the phenyl arm, so that overall initiation is quite slow, and only a relatively small fraction of the initially present 2 ever becomes active. After initiation has been completed, though, the resulting cationic zirconium hydride is quite active and also appears to be relatively unconstrained sterically, as manifested in the observed isomerization of vinylidene-terminated oligomeric olefins. As a consequence, the normal steric preference for 1,2- as opposed to 2,1-insertion is not too pronounced: both processes occur with comparable frequency.10 However, the fact that the majority (70%) of the oligomers obtained have vinylene end groups suggests that termination via β-H elimination is particularly facile following a 2,1-enchainment. This, in turn, explains why this system catalyzes oligomerization, not polymerization: the generation of an oligomeryl−Zr structure prone to termination is much more common than would be the case for a catalyst that strongly favors 1,2-insertion. Addition of 1 equiv of trimethylphosphine completely changes the catalysis, to yield poly(1-hexene) instead of oligomers, and also the initiation pattern: no persistent precatalyst can be detected by NMR, and polymerization proceeds rapidly (Figure 4), even though the phosphine remains coordinated throughout. This would suggest that in the presence of coordinated PMe3 the first insertion product does H

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presence of PPh3 does appear to change the end group distribution somewhat, even though the rate and size distribution remain roughly the same, suggests that this ligand may play some role during the oligomerization process; at present we do not have a model to account for these apparently contradictory observations.) In conclusion, we have shown how catalysis by the system 1/ [Ph3C][B(C6F5)4]/Lone of the rare examples of carbeneligated early-transition-metal complexes active for catalytic transformations of α-olefins12can be “tuned” to oligomerization, polymerization, or inactivity by the appropriate choice (or omission) of added ligand L, even to the point of controlling the average molecular weight of polymers by adjusting the size of the ligand. Furthermore, we offer a mechanistic interpretation for the entire range of behaviors, on the basis of the size and binding strength of the ligand, the interaction of ligand coordination with the detailed molecular structure of the complex to modulate the regioselectivity of monomer enchainment, and the influence of the latter upon the relative rates of propagation and termination. Finally, the surprising finding that ligand binding appears to increase catalytic activity is explained by ligand perturbation of initiation: the ligand-free system probably is inherently more reactive, but its extremely slow initiation means that only a small fraction of the total Zr species in solution enters the catalytic cycle, as is clearly demonstrated by NMR, whereas in the ligated system initiation is fast and essentially all the Zr participates. We envision extending these kinds of considerations to additional systems, with the ultimate goal of elucidating and controlling catalytic behavior that currently is only poorly understood.

lylidene group. A molecule of ether or a second PMe3 can instead occupy the site, in which case catalysis is completely inhibited. It is notable that the molecular weight of the polymer obtained decreases smoothly with the size of the phosphine ligand (Figure 7). These polymers are predominantly

Figure 7. Plot of Mn of poly(1-hexene) vs cone angle of phosphine.

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terminated by vinylidene groups,6 and so presumably the increased steric crowding tends to favor β-H elimination even following a 1,2-insertion; since there are some vinylene end groups but no detectable misinsertions within the polymer chain, 2,1-insertion must essentially always be followed immediately by termination, as was postulated for oligomerization in the absence of added ligand. Next we have the ligands that completely inhibit catalysis: these also will bind strongly to a cationic Zr center (basic phosphines, amines) but are considerably larger than those in the previous group: i-Pr3P (θ = 160°), PCy3 (170°), t-Bu3P (182°), dimethylaniline (produced in situ by using the anilinium salt as protonolytic activator). Presumably their larger size results in still further structural distortion, resulting in a site that is too crowded to accommodate incoming monomer. It is noteworthy that a close analogue of 2, with a saturated central NHC ring and no fused benzo group, does catalyze 1-hexene polymerization when activated by the anilinium salt;11 for that structure there is no large group that can be pushed over into the vacant site by the large coordinated ligand, and so inhibition is not observed. Finally, the large ligands triphenylphosphine (θ = 145°) and trimesitylphosphine (212°) lead to the same distribution of oligomers as the ligand-free system 1/[Ph3C][B(C6F5)4]. In these cases the phosphine-coordinated analogues of 3 are persistent, retarding initiation as does phenyl coordination in 5, but since they do bind considerably less strongly than those in the previous group, being less basic, they can dissociate (albeit slowly) and lead to a small amount of active species. This difference in lability can be seen in the 31P NMR spectra (Figure 1): addition of another 1 equiv of PPh3 to 2 results in a single broad signal, whereas with PCy3 distinct signals for coordinated and free ligand are observed. (The fact that the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00472. Crystallographic data of complexes 2−4 (CIF) GC traces of the oligomers, NMR spectra (1/B(C6F5)3/ coordination of the L ligand, complexes 5 and 6, oligomers/polymers obtained), reaction profile at higher dilution, dynamic NMR study of complex 4, and crystallographic data for complexes 2−4 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for E.D.-A.: [email protected]. *E-mail for J.A.L.: [email protected]. *E-mail for J.E.B.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from King Fahd University of Petroleum and Minerals (Dhahran, Saudi Arabia) and the USDOE Office of Basic Energy Sciences (Grant No. DE-FG03−85ER13431). The Bruker KAPPA 30 APEXII X-ray I

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(g) Zhao, N.; Hou, G.; Deng, X.; Zi, G.; Walter, M. D. Dalton Trans. 2014, 43, 8261−8272.

diffractometer was purchased via an NSFCRIF:MU award to the California Institute of Technology (CHE-0639094).

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DEDICATION A tribute to Greg Hillhouse, a warm and generous human being, a creative scholar, and self-proclaimed “favorite former postdoc” of J.E.B.

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REFERENCES

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