Highly Regioselective α-Olefin Dimerization Using Zirconium and

Jun 30, 2017 - Five new structural analogues of a previously studied group IV amine bis(phenolate) catalyst, Zr(ONNEt2O)Bn2, have been synthesized and...
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Highly Regioselective α‑Olefin Dimerization Using Zirconium and Hafnium Amine Bis(phenolate) Complexes Thilina Gunasekara,†,‡ Andrew Z. Preston,†,‡ Manhao Zeng,‡ and Mahdi M. Abu-Omar*,‡ †

Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States

‡

S Supporting Information *

ABSTRACT: Five new structural analogues of a previously studied group IV amine bis(phenolate) catalyst, Zr(ONNEt2O)Bn2, have been synthesized and tested for oligomerization activity. These structural variations encompass longer alkyl groups, propyl and butyl, on the pendant amine group and two different group IV metals, zirconium and hafnium. Remarkably, all five precatalysts, combined with tris(pentafluorophenyl)borane, oligomerized 1-hexene with almost exclusive selectivity for vinylidene end-groups with a marked preference for dimer formation. The three zirconium amine bis(phenolate) catalysts produced a Schulz−Flory distribution of oligomer products whereas the product distributions of the three hafnium analogues were seen to deviate from a Schulz−Flory distribution. The possible structures for dimers and trimers have been elucidated based on endgroup and active site distribution.

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INTRODUCTION Oligomerization of α-olefins by homogeneous single-site organometallic catalysts as a field has gained substantial momentum during the past ten years. This is primarily due to the cheap and steady supply of shale gas and petroleumderived ethylene feedstock1 and the advent of new technologies to convert bioderived compounds, such as bioethanol, to renewable fuels (C8−C22) and specialty chemicals.2,3 An important aspect of olefin oligomerization is product selectivity to desired fractions. For example, in 1-hexene oligomerization, the dimer (C12) fraction is more suitable as a component of jet fuel, whereas the trimer (C18) fraction is more suitable as a component of diesel fuel. Moreover, even in the simplest scenario of using a metal hydride to dimerize an α-olefin by means of 1,2-insertion or 2,1-misinsertion followed by β-H elimination, the reaction can produce 10 constitutional isomers including vinylidenes and vinylenesof which the properties and hence applications may vary. Of particular interest are vinylidene terminated oligomers because they are industrially important intermediate chemicals to produce surfactants, adhesives, and synthetic lubricants.4 These features exemplify the importance of careful tuning of the product regioselectivity in olefin oligomerization. Kol and co-workers have originally reported that precatalyst 1 produces both oligomer and polymer fractions.5 In an extensive mechanistic study, we uncovered that, based on activation conditions, precatalyst 1 can form selectively an oligomerization site preventing polymer formation.6 This discovery has now made it possible to harness Zr and Hf © XXXX American Chemical Society

amine bis(phenolate) complexes for selective oligomerization reactions as described in this report. Although most olefin oligomerization catalysts yield a mixture of vinylene and vinylidene terminated products, early transition metal metallocene complexes tend to propagate preferentially via 1,2-insertions and produce vinylidenes, whereas postmetallocene catalysts prefer to propagate via 2,1misinsertion and produce vinylenes.7 However, precatalyst 1 was reported to have high regioselectivity for producing vinylidenes upon activation with tris(pentafluorophenyl)borane.5 Herein we report the product and regioselectivity of a series of five postmetallocene precatalysts, 2−6 (Figure 1), which are structurally related to 1. A key feature common to all

Figure 1. 1-Hexene oligomerization using precatalysts 1−6. Bn = benzyl. Received: May 8, 2017

A

DOI: 10.1021/acs.organomet.7b00359 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Results of 1-Hexene Oligomerization Using Zr(ONNR2O)Bn2 Precatalysts 1−6a

oligomer distributionc,d (wt %) run

precatalyst

T (°C)

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

1 2 3 1 2 3 4 5 6 4 5 6

65 65 65 85 85 85 65 65 65 85 85 85

−4

kobs (×10

−1

b

s )

vinylidene selectivity (%)

dimer

trimer

tetramer

99 99 99 98 98 98 99 99 99 >99 >99 >99

73 74 77 89 88 90 86 95 94 81 97 95

22 22 20 10 11 9 9 5 5 10 3 3

5 4 3 1 1 1 5 nd 1 9 nd 2

3.2 2.7 2.8 7.0 6.7 7.0 0.8 0.4 0.5 1.7 0.6 1.0

a Oligomerization conditions: [1-hexene] = 0.9 M, [precatalyst] = 9 mM, [activator]/[precatalyst] = 1.1, in toluene. bDetermined by 1H NMR. cThe percentage represents the amount of that oligomer of a total of all oligomers detected in the GC experiment, i.e., dimer, trimer, and tetramer dFor reactions quenched at more than 80% conversion; nd = not detected; kobs = observed pseudo-first-order rate constant for monomer consumption.

[activator]/[precatalyst] = 1.1, in toluene at 45 °C) and were used as external standards for GC calibrations.

six precatalysts is the high selectivity for dimers (73−97%) with exclusive selectivity (≥98%) for vinylidene end-groups. It is worth noting that, to the best of our knowledge, few metal catalysts have been reported to have such high regioselectivity8 for vinylidene-terminated oligomers.

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RESULTS All six catalysts were found to be active in α-olefin oligomerization. 1-Hexene oligomerization results with precatalysts 1−6 activated by B(C6F5)3 are summarized in Table 1. The three zirconium catalysts (1, 2, and 3) showed comparable monomer consumption kinetics based on the observed pseudo-first-order rate constants and expected acceleration in rates upon increasing the reaction temperature. The three hafnium complexes (4, 5, and 6) were approximately an order of magnitude slower than their zirconium analogues. The monomer consumption kinetics for the Hf complexes are comparable. Even at 85 °C, monomer consumption kinetics for runs 10−12 were linear on a log scale over 4 h (ca. 90% conversion), indicating first-order dependence on monomer throughout the reaction without significant active site degradation (Figure 2).

EXPERIMENTAL SECTION

Ligand and precatalyst syntheses were performed according to literature procedures.5 Detailed synthetic procedures and 1H NMR spectra for all compounds are provided in the Supporting Information. NMR Scale Oligomerization of 1-Hexene. For a typical oligomerization, in a glovebox, 1 (0.046 g, 0.056 mmol), diphenylmethane (0.10 g, 0.625 mmol), and 1-hexene (0.473 g, 5.63 mmol) were added into a 5 mL volumetric flask and diluted to the mark with d8-toluene. A 2.00 mL portion of this solution was then added into an NMR tube and sealed with a septum. Tris(pentafluorophenyl)borane (0.038 g, 0.074 mmol, [activator]/[precatalyst] = 1.1) was dissolved in 1.50 mL of d8-toluene. A 0.50 mL portion of this solution was added into a small vial and sealed with a screw-cap septum. This vial was pierced with a 1 mL syringe and was placed in a resealable plastic bag. The remaining catalyst−internal standard−monomer solution was used to determine the initial concentration of monomer relative to internal standard. The NMR tube was submerged and allowed to equilibrate in a temperature-controlled silicone oil bath at the required temperature. The reaction was quenched at a predetermined time by injection of 0.75 mL of d4-methanol. The solution was checked for monomer consumption and end-group ratio by 1H NMR. Active Site Concentration Analysis. The reaction mixtures were filtered through a silica column to remove the quenched catalyst. The reaction sample was dried to remove some of the excess solvent and diluted in a 2 mL volumetric flask to the mark with dichloromethane. Complete drying of the solvent was not attempted as it leads to loss of some dimer (C12). d2-Dichloromethane was used as an internal standard, and the method of standard addition was used in quantification of active sites by 2H NMR. The chains terminated by d4-methanol at a primary active site, where the last insertion was a 1,2insertion, show a deuterium signal at 0.67 ppm. The chains terminated by d4-methanol at a secondary active site, where the last insertion was a 2,1-misinsertion, show a deuterium signal at 1.05 ppm. Gas Chromatographic (GC) Analysis of Oligomer Products. A detailed procedure is given in the Supporting Information. It is important to note here that the response factors of the flame ionization detector of the GC for different oligomer fractions were considerably different, and, therefore, the common practice of using a single internal standard was deemed insufficient for proper analysis. Therefore, dimers and trimers were isolated from a scaled-up reaction (total volume of 25 mL, [1-hexene] = 0.9 M, [precatalyst] = 9 mM,

Figure 2. Monomer consumption data for a 1-hexene oligomerization reaction catalyzed by precatalyst 5 (run 11; [1-hexene] = 0.9 M, [precatalyst] = 9 mM, [activator]/[precatalyst] = 1.1, in toluene at 85 °C). Observed rate constant, kobs = 0.6 × 10−4 s−1, R2 = 0.998.

The product mixture near completion appeared slightly more viscous in comparison to its appearance at the beginning of the reaction. The 1H NMR spectrum of the product mixture (Figure 3) shows a sharp singlet at 4.79 ppm overlapping with one (or, in certain cases more than one) much smaller signals (4.79 to 4.87 ppm) corresponding to vinylidene end-groups.9 A B

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Organometallics

For runs 8 and 11 (Figure 4e for precatalyst 5), the tetramer amount was too low to be quantified and, therefore, a reliable R2 could not be established. Consequently, it was not possible to accurately determine whether the distribution follows a Schulz−Flory distribution. For the zirconium catalyst systems, as expected, the molecular weight distribution (MWD) shifted to lower molecular weights yielding more dimers (shown by a decreased value for α) as the reaction temperature was increased from 65 to 85 °C (black vs red data points in Figures 4a−c). In contrast, the hafnium systems did not show a significant shift of MWD to lower molecular weight oligomers, with precatalyst 4 producing higher molecular weight oligomers in run 10 than in run 7 (Figure 4d). However, compared to their zirconium analogues, the hafnium catalysts consistently yielded more dimers with the single exception of run 10. Precatalysts 1 and 4 were chosen as representative catalysts for active-site count analysis. Active sites that have undergone a 1,2-insertion before chain termination are defined as primary active sites, and sites that have undergone a 2,1-misinsertion before chain termination are defined as secondary active sites. The catalyst participation for these two precatalysts, upon activation by B(C6F5)3, was found to be 82% and 72% respectively, with about 40% of these active sites being secondary active sites (see Supporting Information). As a reminder, the chain transfer from a primary site results in formation of a vinylidene species, whereas the chain transfer from a secondary site results in formation of a vinylene species.

Figure 3. Olefinic region of the 1H NMR spectrum of the product mixture for run 9 (precatalyst 6) at 90% conversion. 600 MHz in d8toluene.

weak multiplet is also observed at 5.35 ppm which corresponds to vinylene end-groups. Interestingly, as shown in Figure 4, the zirconium catalyst systems followed a Schulz−Flory distribution, i.e., the most probable distribution of oligomer products (R2 > 0.990) according to the equation log(Wn/n) = n log(α) + constant

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(1)

DISCUSSION Previously, it has been reported that the Zr(ONNMe2O)Bn2 (a structural analogue of precatalyst 1, less bulky pendant amine) is a relatively fast polymerization catalyst compared to precatalyst 1, which is a fair oligomerization catalyst with a

where Wn is weight fraction of the n-mer and α is the probability of chain propagation (Schulz−Flory factor). In contrast, the hafnium catalyst systems (R2 < 0.880) either considerably deviated from (Figures 4d and 4f) or otherwise were not conclusively a Schulz−Flory distribution (Figure 4e).

Figure 4. Schulz−Flory plot of the product distributions obtained for 1-hexene oligomerization catalyzed by precatalysts 1−6/B(C6F5)3. Plots a−f correspond to reactions run with precatalysts 1−6, in the same order. Black data points and linear fits: reactions run at 65 °C. Red data points and linear fits: reactions run at 85 °C. Wn = weight fraction of the n-mer, α = probability of chain propagation in the Schulz−Flory equation log(Wn/n) = n log(α) + constant. Reaction conditions: [1-hexene] = 0.9 M, [precatalyst] = 9 mM, [activator]/[precatalyst] = 1.1, in toluene. Conversions >80%. Data in panel e was not fit to eq 1; the amount of tetramers and higher oligomers were not quantifiable by GC. C

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Organometallics remarkable versatility.5,10,11 This difference in reactivity can be attributed to the difference in pendant bond binding affinity to the metal center (Zr−NMe2 = 2.59 Å, Zr−NEt2 = 2.76 Å). In fact, it is well documented that the pendant arm has a significant effect on the activity and lifetime of these polymerization catalysts.11−13 To better understand the effect of changing the pendant amine group, we designed two other new ligands which have longer alkyl groups, namely, n-propyl and n-butyl, attached to the pendant nitrogen atom. We tested these ligands with both zirconium and hafnium metals. Zirconium and hafnium (in their +4 oxidation state) are similar in that they have the same number of outer shell delectrons and the same ionic radii due to the lanthanide contraction. Therefore, most analogous complexes of zirconium and hafnium have virtually identical structures.14 However, probably due to the larger metal−carbon bond enthalpy of the hafnium systems compared to the analogous zirconium systems,15 hafnium catalysts often have lower activities compared to their zirconium analogues.16,17 For the zirconium catalyst systems, as shown in Table 1, the activity and the product distribution do not change appreciably among the three catalysts. This is expected, as the increase in length of the alkyl group on the pendant amine would not have a significant effect on the active alkyl chain, which is trans to the amine group. For the zirconium catalyst systems, the monomer consumption rate increased by more than 2-fold as the reaction temperature was increased by 20 °C; however, no catalyst deactivation was observed. As expected, the hafnium catalyst systems are slower than their zirconium analogues under the same reaction conditions. Similar to zirconium analogues, the reactions catalyzed by hafnium showed an approximately 2-fold increase in monomer consumption rate upon increasing the reaction temperature by 20 °C. For a polymerization reaction, assuming monomer-independent chain transfer, the correlation between rate constants and the number-average molecular weight (Mn) is given by the equation ⎛ k pCM ⎞ M n = mM ⎜ ⎟+1 ⎝ kt ⎠

vinylidene protons are equivalent (Figure 5). The only structure that is consistent with this feature is a dimer of 1-

Figure 5. 1H NMR of the isolated dimer of 1-hexene; d6-benzene, 600 MHz.

hexene formed by two 1,2-insertions (Figure 6). A 2,1misinsertion followed by a 1,2-insertion and a β-H elimination would have resulted in a structure that has three methyl groups with only two of them having about the same chemical shift (this structure is not shown in Figure 6 for clarity). However, the 1H NMR spectrum of the dimer fraction isolated from a larger scale reaction showed only one methyl peak, of which the peak area indicates two methyl groups. This is indicative and also consistent with the literature for a dimer formed by two 1,2-insertions.19 For isolated trimers, however, the 1H NMR showed two singlets for the two vinylidene protons, which could be consistent with either of the two possible vinylidene trimer structures shown in Figure 6, or a mixture of these two isomers (see Figure S10). However, it is also clear that, as no (or very little) isomerized 1-hexene species are observed, the first monomer insertion most frequently occurs through a 1,2insertion. Given that a significant amount of secondary active sites were also observed, the second insertion of monomer could be either 1,2-insertion or 2,1-misinsertion. If the second insertion was a 1,2-insertion, given that these catalysts possess high kt/kp ratios, it is reasonable to assume that these lead to chain termination into a vinylidene dimer rather than to another propagation event. On the other hand, if the second insertion is a 2,1-misinsertion, the probability that it would insert another monomer is much higher than that it would chain terminate. Therefore, these regioerrors result in extending the growing oligomer chain by one monomer unit, until it is recovered by a 1,2-insertion. This phenomenon eventually leads to a shift in the molecular weight distribution to higher molecular weights, where all the higher oligomers presumably contain at least one regioerror. An unexpected feature of the product distributions for precatalysts 4 and 6 is their deviation from Schulz−Flory (or the most probable) chain length distribution. Typically, most transition metals promote α-olefin polymerization/oligomerization via a Cossee mechanism, which results in a Schulz−Flory distribution.20 The basis for this distribution is the Flory principle of equal reactivity, which states that, regardless of the length of the growing polymeryl chain, the reactivity of every chain for a subsequent propagation or termination is the same.

(2)

where mM is the molecular weight of the monomer; CM is the concentration of monomer; kp is the rate constant for chain propagation; kt is the rate constant for chain termination.18 For the zirconium catalyst systems, since the product distributions do not differ significantly, it is clear that the kp/kt ratio is similar for all three precatalysts. Among the hafnium catalysts, reactions catalyzed by precatalyst 4 show the highest amount of higher oligomers. If the catalyst participation is similar for the three hafnium catalysts, the monomer consumption rate (kobs) reflects the chain propagation rate (kp). Therefore, precatalyst 4 presumably has the highest chain propagation rate among the three hafnium catalysts and, consequently, results in more trimers and higher oligomers than the other two hafnium catalysts. Remarkably, all six catalysts were highly selective for vinylidene end-groups. This suggests that the oligo(1-hexenes) were almost always produced by a β-hydrogen elimination subsequent to a 1,2-insertion. However, the presence of a considerable amount of secondary active sites (about 40%) suggests that 2,1-misinsertion does occur, but chain termination after a 2,1-misinsertion is unlikely. Interestingly, the predominant vinylidene peak is a singlet, indicating that the two D

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Figure 6. Possible structures of dimers and trimers of 1-hexene. Based on the end-group and active site data, the pathways shown with a dashed arrow are much less likely to be formed compared to those shown in green. For vinylene oligomers, the structure corresponding to only one of the two β-H termination pathways is shown.

example, Cs and C1. However, at −35 °C, the spectra become more complicated with the appearance of extra peaks. This observation would then imply that, at lower temperature, the solution structure is C1-symmetric. (The possibility that at lower temperatures the extra peaks are due to the presence of two species is not considered here as the odds of these two species being in an exactly 1:1 ratio for all three hafnium catalysts is unlikely.) Although it is not conclusive, it is possible that the hafnium catalyst structures are in fast exchange at room temperature and, as the temperature is decreased, the equilibrium shifts in the direction of a C1-symmetric structure. This is consistent with our previous hypothesis that the active sites are in exchange between two (or possibly more) different conformations, all of which are active toward α-olefin oligomerization. If the transfer of ligand steric interactions to the active site is facile enough, this implies that different conformations have different rate constants. (It is important to note here that these different forms are not the same as primary vs secondary sites which arise due to the orientation of the inserting monomer.) This may be the reason why the reactions catalyzed by hafnium catalysts do not follow a Schulz−Flory distribution, nor do they show a significant increase in the dimer amounts with the increase of the reaction temperature. Of course, this is based on the assumption that similar behaviors are expected for the activated catalysts whose spectra are increasingly complicated to interpret. However, since activated catalysts have one less ligand coordination compared to the corresponding precatalyst, the conformational freedom can only be expected to be even greater. Surprisingly, the zirconium analogues, which produce Schulz−Flory distributions of oligomers, show all sharp peaks in the NMR at room temperature as well as at low temperatures. This would imply that either only one conforma-

The only imaginable exception occurs when the chain is too short. However, these deviations are generally minor and cannot account for the deviations we observe for the hafnium catalysts. Although we cannot conclusively exclude a metallacyclic mechanism,21,22 due to the relative instability of the intermediate metallacycles, such a mechanism would generally favor trimerization of α-olefins; the hafnium catalysts in this study produce >80% dimers of 1-hexene. Another possible explanation is if the active catalytic species is in equilibrium between two forms for which the rate constants are not identical. Consider the validity of the equal reactivity principle for the propagation step for two different forms of active sites which are in dynamic equilibrium with each other. This would mean that any given growing chain would experience two different forms of the active species which would have different rate constants. As a result, the growing chain is composed of monomers that were inserted at two different rates, which constitutes a violation of the Flory principle of equal reactivity. The same effect applies to termination steps as well. Consequently, the product distribution deviates from the Schulz−Flory distribution. For salan-type complexes similar to the ones examined in this study, Bercaw and co-workers report the observation of conformational changes in solution by means of phenolate ring twisting.23,24 They suggested that if these conformational changes are slower than insertion rates and the ligand steric effects are efficiently transferred to the metal site, enantiomorphic site control can be used to obtain stereoblock polymers. We noticed in our 1H NMR studies that the hafnium catalysts exhibit sharp peaks at room temperature with a pair of singlets corresponding to the two benzyl groups. This would imply that the solution structure is either Cs-symmetric or in fast exchange between different geometries of different symmetries, for E

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incubation time (the time period before adding monomer) leads to a slow transformation of the oligomer sites to a polymer site. Spectroscopic evidence suggets that the metal−pendant amine bond of the initially formed oligomer site slowly dissociates during the incubation time to form the polymer site and that the counteranion is outersphere for the oligomer site and innersphere for the polymer site. Therefore, the key to avoid the formation of polymer sites is to have no incubation time, i.e., the precatalyst (or the activator) should be added after the addition of the monomer. (7) Lian, B.; Beckerle, K.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2007, 46, 8507−8510. (8) Dagorne, S.; Bellemin-Laponnaz, S.; Romain, C. Organometallics 2013, 32, 2736−2743. (9) Janiak, C.; Blank, F. Macromol. Symp. 2006, 236, 14−22. (10) Switzer, J. M.; Travia, N. E.; Steelman, D. K.; Medvedev, G. A.; Thomson, K. T.; Delgass, W. N.; Abu-Omar, M. M.; Caruthers, J. M. Macromolecules 2012, 45, 4978−4988. (11) Steelman, D. K.; Xiong, S.; Pletcher, P. D.; Smith, E.; Switzer, J. M.; Medvedev, G. A.; Delgass, W. N.; Caruthers, J. M.; Abu-Omar, M. M. J. Am. Chem. Soc. 2013, 135, 6280−6288. (12) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Chem. Commun. 2000, 379−380. (13) Oakes, D. C. H.; Kimberley, B. S.; Gibson, V. C.; Jones, D. J.; White, A. J. P.; Williams, D. J. Chem. Commun. 2004, 2174−2175. (14) Steelman, D. K.; Pletcher, P. D.; Switzer, J. M.; Xiong, S.; Medvedev, G. A.; Delgass, W. N.; Caruthers, J. M.; Abu-Omar, M. M. Organometallics 2013, 32, 4862−4867. (15) Simoes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629− 688. (16) Kissounko, D. A.; Zhang, Y.; Harney, M. B.; Sita, L. R. Adv. Synth. Catal. 2005, 347, 426−432. (17) Pletcher, P. D.; Switzer, J. M.; Steelman, D. K.; Medvedev, G. A.; Delgass, W. N.; Caruthers, J. M.; Abu-Omar, M. M. ACS Catal. 2016, 6, 5138−5145. (18) Janiak, C. Coord. Chem. Rev. 2006, 250, 66−94. (19) Cahiez, G.; Gager, O.; Habiak, V. Synthesis 2008, 2008, 2636− 2644. (20) Osakada, K. Organometallic Reactions and Polymerization; Springer: 2014; Vol. 85. (21) McGuinness, D. S. Chem. Rev. 2011, 111, 2321−2341. (22) Sattler, A.; Labinger, J. A.; Bercaw, J. E. Organometallics 2013, 32, 6899−6902. (23) Agapie, T.; Henling, L. M.; DiPasquale, A. G.; Rheingold, A. L.; Bercaw, J. E. Organometallics 2008, 27, 6245−6256. (24) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27, 6123−6142.

tional isomer is present or, if a fast exchange occurs, its energy barrier is small such that the effects on the oligomer distribution are subtle.

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CONCLUSION In summary, we have synthesized five new group IV amine bis(phenolate) catalysts that are structurally related to the salan-type oligomerization catalyst Zr(ONNEt2O)Bn2. All six precatalysts, upon activation by tris(pentafluorophenyl)borane, form active oligomerization catalysts which have exceptional selectivity for vinylidene terminated products (>99%). While all these precatalysts produced dimer of 1-hexene as the major product, hafnium catalysts were especially selective for dimers (up to 97%). Surprisingly, it was found that the molecular weight distribution of hafnium catalysts do not follow a typical Schulz−Flory distribution. It is possible that the hafnium catalysts are in two (or more) conformationally different active forms as indicated by the fluxional behavior in the spectroscopic studies. The different rate constants associated with each form cause the product distribution to deviate from a Schulz−Flory distribution.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00359. Syntheses, experimental procedures, spectroscopic data, GC chromatographs, and active site concentration data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mahdi M. Abu-Omar: 0000-0002-4412-1985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Keith Steelman, Dr. Grigori A. Medvedev, Baoyuan Liu, and Jungsuk Kim for helpful discussion. Financial support was provided by the U.S. Department of Energy by Grant No. DE-FG02-03ER15466. T.G. was supported by a Henry B. Hass Fellowship. The NMR facility at UCSB is supported by NIH shared instrument grant No. 1S10OD012077-01A1.

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REFERENCES

(1) Wang, Q.; Chen, X.; Jha, A. N.; Rogers, H. Renewable Sustainable Energy Rev. 2014, 30, 1−28. (2) Harvey, B. G.; Meylemans, H. A. Green Chem. 2014, 16, 770− 776. (3) Markham, J. N.; Tao, L.; Davis, R.; Voulis, N.; Angenent, L. T.; Ungerer, J.; Yu, J. Green Chem. 2016, 18, 6266−6281. (4) Londaitsbehere, A.; Cuenca, T.; Mosquera, M. E. G.; Cano, J.; Milione, S.; Grassi, A. Organometallics 2012, 31, 2108−2111. (5) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2001, 20, 3017−3028. (6) Gunasekara, T.; Abu-Omar, M. M.; et al. Unpublished results. The activation of precatalyst 1 with tris(pentafluorophenyl)borane instantaneously forms oligomer active sites. However, further F

DOI: 10.1021/acs.organomet.7b00359 Organometallics XXXX, XXX, XXX−XXX