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

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Customizing Polyolefin Morphology by Selective Pairing of Alkali Ions with Nickel Phenoxyimine-Polyethylene Glycol Catalysts Zhongzheng Cai and Loi H. Do* Department of Chemistry, University of Houston, 4800 Calhoun Road, Houston, Texas 77004, United States S Supporting Information *

ABSTRACT: In the present work, we have prepared nickel phenoxyimine-polyethylene glycol (PEG) catalysts with sterically bulky N-aryl substituents and investigated their ethylene homo- and copolymerization behavior. We have found that different nickel catalyst and alkali ion (Na+ or K+) combinations yielded polyethylene with different branching microstructures and molecular weights. Our heterobimetallic catalysts can copolymerize ethylene and nonpolar α-olefins with high activity but are strongly inhibited in the presence of polar vinyl olefins. We demonstrate that our heterobimetallic catalysts are significantly more stable in ethylene homopolymerization in comparison to conventional nickel phenoxyimine systems on the basis of time-dependent activity studies. This work showcases the versatility of Lewis acid tunable catalyst constructs to prepare customized polyolefins and suggests that similar design strategies could be applied to other catalyst systems.

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INTRODUCTION Polyolefins are an extraordinarily versatile class of materials.1−3 For example, linear high molecular weight polyethylene (PE) gives durable solids that are useful as construction plastics and pipes, whereas branched low molecular weight polyethylene gives viscous oils that are useful as lubricants and chemical additives. The discovery that homogeneous transition-metal complexes can promote the coordination−insertion of olefins was a major breakthrough in polymerization catalysis.4−8 In general, however, most catalysts can provide only a single type of polymer under a given set of reaction conditions. In smallscale laboratory synthesis, it is usually feasible to alter the reaction temperature and/or pressure to obtain different polymer morphologies, but doing so on an industrial scale is not always possible. We propose that current olefin polymerization processes could be streamlined by devising usercustomizable methods that would allow access to different polymer products using a universal catalyst platform. For example, being able to obtain on demand either low- or medium-density PEs directly from biorenewable ethylene,9 without the need for expensive α-olefin comonomers, would be a significant advance in sustainable polymer synthesis (Scheme 1).10 Our laboratory has been exploring the concept of Lewis acid assisted coordination−insertion polymerization as a new process to prepare custom-made polyolefins.11,12 We hypothesize that non-redox-active metal cations could work cooperatively with late-transition-metal catalysts to enhance olefin polymerization processes.13−19 We based our work on nickel phenoxyimine complexes20,21 because they have been used successfully to study the effects of various design strategies on polymerization, including steric protection,22−25 bimetallic © XXXX American Chemical Society

Scheme 1. Proposed Lewis Acid Tunable Catalysts To Prepare Designer Polyethylene (PE)

active centers,26−29 and proximal Lewis acid interactions30,31 (Chart 1). We have recently reported on a family of nickel phenoxyimine complexes that feature polyethylene glycol (PEG) side chains as receptors for alkali-metal ions.12 To ensure that the alkali ions interact directly with the primary coordination sphere of the nickel center, we used the phenolate group of the supporting ligand as a metal−metal bridging unit with the PEG arms providing additional stabilization. In the present work, we describe our efforts to create bulky variants of this nickel phenoxyimine-PEG family of catalysts and report on their ethylene homo- and copolymerization behavior. We also demonstrate that the dual effects of steric shielding and Lewis acid interactions can lead to distinct reactivity patterns, which we can exploit to develop new ways to control polymer morphology in olefin polymerization. Received: July 10, 2017

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DOI: 10.1021/acs.organomet.7b00516 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

aliquots of NaBArF4 (where BArF4− = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate anion) to a Et2O solution of 3c, concomitant changes to its UV−vis absorption spectra were observed (Figure S1 in the Supporting Information). The single-wavelength plot at 340 nm suggests that more than one chemical events may be occurring, perhaps due to the formation of species with different nickel−sodium stoichiometries or dynamic ligand exchange involving the pyridine donor. Similar results were obtained when KBArF4 was used as the alkali ion source. The 1:1 nickel−sodium species NiNa(2c)(Me)(pyridine)+ (3c-Na) was grown by layering pentane over a Et2O solution containing equal concentrations of 3c and NaBArF4. X-ray crystallographic analysis of 3c-Na reveals that the complex has the expected heterobimetallic core (Figure 1). The square-

Chart 1. Nickel Phenoxyimine Olefin Polymerization Catalysts That Have Been Studied

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RESULTS AND DISCUSSION Synthesis and Characterization of Nickel Complexes. The phenoxyimine ligands 2 were prepared by acid-catalyzed imine condensation between aldehyde 1 and sterically bulky anilines in moderate yields (50−60%) (Scheme 2). The Scheme 2. Synthesis of Nickel Complexes 3a−c

Figure 1. X-ray crystal structure of NiNa(2c)(Me)(pyridine)+ (3c-Na; ORTEP view, displacement ellipsoids drawn at the 50% probability level). Hydrogen atoms and the BArF4− anion have been omitted for clarity.

planar nickel center shows bond distances that are slightly elongated in comparison to those in nickel complexes that have similar donor groups. Most notably, the Ni−Ophenolate distance is ∼1.93 Å in 3c-Na in comparison to ∼1.90 Å in related complexes.32,33 These longer nickel−donor bond distances are most likely due to the electron-withdrawing effects of the coordinated sodium.34 The sodium ion has a highly distorted six-coordinate geometry, which is ligated by five oxygen atoms from PEG (average Na−OPEG = 2.44 Å) and one oxygen atom from the phenolate moiety (Na−Ophenolate = 2.39 Å). The solidstate structure shows that the 3,5-bis(trifluoromethyl)phenyl groups protect the axial sites of the nickel center, which is expected to promote chain growth over chain transfer processes during polymerization.33 Ethylene Homopolymerization. To evaluate whether complexes 3a−c are catalytically active, ethylene homopolymerization studies were performed. The nickel complexes were treated with 1.0 equiv of the activator B(C6F5)3 in toluene and then stirred under 100 psi of ethylene for 1 h at room temperature. The bulkier nickel complexes exhibited higher

reaction of 2 with 1.0 equiv of Ni(Me)2(pyridine)2 furnished the desired neutral Ni(phenoxyimine-PEG)(Me)(pyridine) (3) complexes as orange solids (85−99% yield). Unlike the case in our previous work,12 the Ni(Me)2(pyridine)2 precursor was used instead of Ni(Br)(Ph)(PPh3)2 because the final nickel complexes could be isolated more cleanly. A series of bulky nickel complexes was prepared for this study, including Nphenyl groups that are derivatized with isopropyl (3a), phenyl (3b), and 3,5-bis(trifluoromethyl)phenyl (3c) substituents at the 2- and 6-positions. For comparative polymerization studies, the reported nickel phenoxyimine catalyst 3d,20 which features an anthracene moiety instead of a PEG chain, was also synthesized. Consistent with our previous studies,12 complex 3 binds readily to alkali ions. For example, upon the addition of various B


Article

Organometallics Table 1. Ethylene Homopolymerization Data for 3a−ca entry

cat.

salt

polymer yield (g)

TOF (103 g/(mol h))

branchesb (/1000 C)

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

3a 3a 3a 3b 3b 3b 3c 3c 3c 3d 3d

none Na+ K+ none Na+ K+ none Na+ K+ none Na+

0.02 1.53 0.90 0.07 1.63 0.70 0.21 6.59 2.45 2.23 2.16

1.33 102 60 4.67 109 47 14 439 163 149 139

39 107 117 86 46 119 22 39 53 21f 27f

C1 (%)c C2 (%)c C3 (%)c 84 77 81 81 84 83 90 87 85 73 73

6 7 5 6 3 3 6 0 3 9 11

2 1 2 2 0.7 1 0 0 0 4 3

C4+ (%)c

Mnd (103)

Tme (°C)

Mw/Mnd

8 15 12 11 12.3 13 4 13 12 14 13

5.16 4.10 3.48 1.93 1.82 2.83 11.79 1.25 1.66 11.1

92.2

1.5 1.5 1.5 2.1 2.2 3.3 1.7 1.6 3.2 2.6

67.9 106.8 77.1 63.7

Polymerization conditions: nickel precatalyst (15 μmol), B(C6F5)3 (15 μmol), MBArF4 (15 μmol, if any), ethylene (100 psi), 10 mL of toluene, 1 h at room temperature. bThe total number of branches per 1000 carbons was determined by 1H NMR spectroscopy. cDetermined by quantitative 13C NMR spectroscopy. dDetermined by GPC in trichlorobenzene at 150 °C. eDetermined by DSC. fThe branching calculation was not corrected for chain ends. gTriethylene glycol dimethyl ether was added to solubilize the MBArF4 salt. a

In all cases, the addition of Na+ (Table 1, entries 2, 5, and 8) or K+ (entries 3, 6, and 9) to 3 led to significant enhancements in catalyst activity (up to ∼80-fold) in comparison to polymerizations performed in the absence of additives. The most active catalyst was 3c-Na (TOF = 439 × 103 g/((mol Ni) h)), which is also our most sterically bulky nickel complex in the series. For comparison, the conventional catalyst 3d (TOF = 149 × 103 g/((mol Ni) h), Table 1, entry 10) gave about a 3-fold decrease in catalyst activity in comparison to 3c-Na. Addition of exogenous Na+, and triethylene glycol dimethyl ether to solubilize the salt, to 3d did not have any significant effect on polymerization (entry 10 vs 11). The polymerization data showed several notable trends. First, Na+ seems to accelerate polymerization to a greater extent in comparison to K+, which we propose is due to the better size match of the sodium ion for the tetrakis(ethylene glycol) side chain of 3 in comparison to the potassium ion. We had also tested LiBArF4 as an additive in polymerization, but only a trace amount of polymer was obtained. For reasons that we do not fully understand, the addition of Li+ led to inhibition of polymerization. It is possible that the Li+ cation, which is more Lewis acidic than either Na+ or K+, promotes aggregation of 3 to give extended structures that are catalytically inactive. Second, polymer branching generally increased (with the exception of 3b-Na in Table 1, entry 5) in the order 3 < 3Na < 3-K but there was no obvious trend in their molecular weights. In conventional nickel phenoxyimine systems, the presence of electron-withdrawing ligand substituents typically increase polymer branching but decrease Mn.33,35 Furthermore, the present results are not consistent with our previous observations that the addition of alkali ions to the Ni phenoxyimine-PEG catalysts can increase both polymer branching and molecular weight.12 The departure from expected trends in this study suggests that the combined effects of both steric bulk and Lewis acids are complex. Studies are ongoing to develop a more complete understanding of the factors that control polymer morphology in this family of nickel phenoxyimine-PEG catalysts. An attractive feature of our “mix and match” catalyst system is that the user has control over what polymer product is produced (Scheme 1). When one of three catalyst variants 3a− c and Na+ or K+ (or no salt) are employed, seven different PE microstructures could be obtained (Figure S45 in the

catalyst activity in comparison to the less bulky complexes (Table 1, cf. entries 1, 4, and 7). The turnover frequencies (TOF) were calculated to be 1.3 × 103, 4.7 × 103, and 14 × 103 g/((mol Ni) h) for 3a−c, respectively. The most sterically bulky complex, 3c, gave polyethylene with the highest molecular weight, with an Mn value of ∼11.8 × 103. Branching analysis by 1H/13C NMR spectroscopy showed that the polymer branches ranged from about 20 to 90 per 1000 carbon atoms, with methyl branches being the most predominant. Our results are consistent with reports by Mecking and co-workers,33,35 which showed that Ni phenoxyimine complexes with electron-deficient N-aryl substituents give polymers with higher molecular weight but lower degrees of branching (e.g., 3b vs 3c) due to the suppression of chainwalking and chain-transfer processes relative to ethylene insertion. Typically, structural or electronic tuning of a catalyst requires synthetic modifications of the supporting ligands,23,35 which can be both time consuming and synthetically challenging. The versatility of our PEGylated catalyst platforms is that we can customize its properties simply by adding or withholding external metal salts.12 We have demonstrated previously that the addition of alkali ions to our nickel phenoxyimine-PEG complexes can dramatically improve their catalytic performance. In the present work, we sought to utilize this Lewis acid responsive system to gain access to different classes of polyethylene. In a typical experiment, complex 3 was combined with 1.0 equiv of either NaBArF4 or KBArF4 in toluene to generate 3-Na or 3-K in situ, respectively (Scheme 3). The heterobimetallic complexes were then treated with B(C6F5)3 and exposed to 100 psi of ethylene for 1 h at room temperature. Scheme 3. Reaction of 3 with Alkali Ions

C


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Organometallics

and after polymerization. Grubbs and co-workers have reported previously that the formation of nickel bis(phenoxyimine) ligand complexes is a common catalyst deactivation pathway.43 Alternatively, Mecking and co-workers have proposed that bimolecular elimination of nickel alkyl and/or nickel hydride species can also be major decomposition routes.44 Additional studies are needed to ascertain whether the greater catalyst stability of 3c-Na in comparison to 3d is due to its ability to suppress such deactivation processes. Ethylene Copolymerization. Although nickel phenoxyimine catalysts are moderately compatible with various functional groups,20 they are not active for the copolymerization of ethylene and polar vinyl olefins.8 The only examples of functionalized olefins that could be copolymerized are those which have polar moieties that are geometrically constrained,20 separated from the double bond by a long chain,27,45,46 or masked by protecting groups.47 The direct copolymerization of ethylene and polar monomers such as methyl acrylate (MA), vinyl acetate (VA), allyl acetate (AA), propyl vinyl ether (PVE), acrylamide, acrylonitrile, and vinyl chloride is still elusive.48,49 To determine whether 3c, 3c-Na, and 3c-K are competent catalysts for olefin copolymerization, we first investigated their reactions with ethylene and nonpolar α-olefins.27,50,51 When a toluene solution of 3c was activated using B(C6F5)3 in the presence of ethylene (100 psi) and 1-hexene (2.5 M) for 1 h, a semicrystalline material was obtained (Table 3, entry 1). Analysis of the PE product by NMR spectroscopy52 showed that it contained more butyl branches in comparison to polyethylene obtained from ethylene homopolymerization using 3c (Figure 2), which suggests that 1-hexene was successfully incorporated into the polymer chain. We calculated that the copolymer comprised about 1.6 mol % of 1-hexene, taking into account that a minor amount of butyl branches were formed due to chain isomerization rather than comonomer incorporation. To confirm the observation that α-olefins could be copolymerized, we conducted similar studies using 3c/ B(C6F5)3 in the presence of ethylene (100 psi) and 1-pentene (2.5 M) (Table 3, entry 4). The 13C NMR spectrum of the resulting polymer product clearly showed peaks that correspond to propyl branches (Figure 2, bottom). Because propyl branches were not observed in the NMR spectrum of homopolyethylene obtained from 3c, their presence in the copolymer must be derived from 1-pentene (1.8 mol %). The α-olefin incorporation efficiency of 3c is comparable to that of other nickel phenoxyimine catalysts reported in the literature, which is typically between 0.2 and 5 mol %.27,51 Next, the copolymerizations of ethylene and α-olefins by 3cNa (Table 3, entries 2 and 5) and 3c-K (entries 3 and 6) were performed. As expected, consistent with the trends observed in the ethylene homopolymerization studies above, copolymerization reactions by the heterobimetallic 3c-Na and 3c-K were significantly faster in comparison to that by the monometallic 3c. For example, the TOFs for 3c-Na and 3c-K were about 16and 8-fold greater than that for 3c in the copolymerization of ethylene and 1-hexene. However, this increase in catalyst activity also led to a concomitant decrease in the incorporation of α-olefins (∼0.2−0.6 mol %) as well as a decrease in polymer Mn. It is unclear whether Lewis acids increase the rate of chain transfer/termination relative to chain growth, decrease the rate of olefin insertion, or both. Finally, we tested our nickel catalysts in the copolymerization of ethylene and polar vinyl olefins (Table 3). When either 3c or 3c-Na was treated with B(C6F5)3 and then exposed to ethylene

Supporting Information, PE classified on the basis of polymer branches and Mn). For most conventional catalysts, fine-tuning the polymer morphology usually requires changing the polymerization temperature and/or pressure or adding chain transfer agents to reduce Mn. There are recent examples of switchable catalysts that take advantage of different redox states to turn on/off polymerization36 or modulate the branching density.37,38 Our Lewis acid responsive complexes are unique because they are simple to use and only require inexpensive alkali salts for catalyst tuning. Furthermore, the relatively low molecular weight PEs obtained using 3a−c (Mn < 104) might be useful in applications such as waxes, lubricants, and specialty chemicals. As control experiments, we also performed ethylene homopolymerization studies using 3c and NaBArF4 or KBArF4 in the presence of the radical scavenger galvinoxyl (Table S1 in the Supporting Information).39,40 Our data showed that the same polymer microstructures and yields were obtained in the presence of galvinoxyl as without, suggesting that our heterobimetallic catalysts promote coordination−insertion rather than radical polymerization. All of the PEs obtained in our study are monomodal with polydispersities ranging from about 1.1 to 3.2 (Figures S29−S44 in the Supporting Information), which is typical for single-site nickel phenoxyimine catalysts.20 The PEs were estimated to have Tm values of ∼64−107 °C. Their DSC melting endotherms showed several broad peaks (Figure S28 in the Supporting Information), which we attribute to the irregular distribution of crystalline domains within their polymeric structures.41,42 Detailed DSC studies, such as using successive self-nucleation/annealing methods, are needed to provide a better understanding of the PE crystalline morphologies. Catalyst Stability. To evaluate the stability of our heterobimetallic complexes, we monitored their catalytic activity as a function of time (Table 2). When ethylene Table 2. Time-Dependent Polymerization Studiesa entry

cat.

time (min)

polymer yield (g)

TOF (103 g/(mol h))

1 2 3 4 5 6

3c-Na 3c-Na 3c-Na 3d 3d 3d

10 30 60 10 30 60

1.14 3.17 6.59 1.60 2.21 2.23

456 422 439 640 295 149

Polymerization conditions: nickel complex (15 μmol), B(C6F5)3 (15 μmol), ethylene (100 psi), 10 mL toluene, at room temperature.

a

homopolymerization reactions were performed using 3c-Na at 10, 30, and 60 min, the TOFs ((422−456) × 103 g/(mol h)) remained relatively constant. Polymerizations were stopped after 60 min because the reaction mixture became too thick to be stirred due to the substantial amount of polymer that had formed. In contrast, the standard catalyst 3d showed a significant decrease in activity with increasing time. During the first 10 min, the polymerization was highly exothermic and the TOF was measured to be 640 × 103 g/(mol h). After 60 min, however, the TOF had dropped to 149 × 103 g/(mol h), a 4-fold decrease in comparison to that at 10 min. During the course of polymerization, the reaction mixture containing 3d changed from orange to a dark yellow solution, suggesting that appreciable catalyst decomposition had occurred. In contrast, the solution of 3c-Na remained the same bright orange before D


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Organometallics Table 3. Ethylene Copolymerization Data for 3ca entry

comonomer (conc., M)

salt

polymer yield (g)

TOF (103 g/(mol h))

branchesb (/1000 C)

Mnc (103)

Tmd (°C)

ince (%)

Mw/Mnc

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

1-hexene (2.5) 1-hexene (2.5) 1-hexene (2.5) 1-pentene (2.5) 1-pentene (2.5) 1-pentene (2.5) VA (0.15) VA (0.15) AA (0.15) AA (0.15) PVE (1.86) PVE (1.86)

none Na+ K+ none Na+ K+ none Na+ none Na+ none Na+

0.28 3.11 2.26 0.20 2.33 2.98 0.02 0.06 0.003 0.01

19 311 151 13 233 199 0.08 0.25 0.013 0.04

35 48 73 34 47 71

8.21 1.31 2.39 20.86 0.86 1.16

97.7 67.5 36.5 98.3 66.6

1.6 0.4 0.2 1.8 0.3 0.6

1.7 1.5 2.2 2.1 1.9 2.4

40

12.34

110.0 86.4

0

1.1

0.08

0.89

34

0.80

102.9

0

2.9

Polymerization conditions unless specified otherwise: nickel precatalyst (15 μmol), B(C6F5)3 (15 μmol), MBArF4 (15 μmol, if any), ethylene (100 psi), 10 mL of toluene, 1 h at room temperature. Comonomer abbreviations: VA = vinyl acetate, AA = allyl acetate, PVE = propyl vinyl ether. bThe total number of branches per 1000 carbons was determined by 1H NMR spectroscopy. cDetermined by GPC in trichlorobenzene at 150 °C. d Determined by DSC. eDetermined by 1H/13C NMR spectroscopy. fThe reaction time was 40 min. gThe reaction time was 16 h. hNo B(C6F5)3 was added, and the reaction time was 6 h. a

metal chelated structures that are known to inhibit polymerization. Finally, when copolymerization reactions were performed using ethylene and propyl vinyl ether, 3c-Na (entry 12) afforded a small amount of product but 3c was completely inactive (entry 11). For these reactions, the activator B(C6F5)3 was not used to avoid PVE homopolymerization. In the absence of boranes, polymerization can still occur because the pyridine donor is hemilabile and can be displaced by olefins. Control studies using just NaBArF4 (i.e., no Ni) and ethylene/PVE did not give any products, dismissing the possibility of cationic polymerization. Analysis of the polymer obtained from 3c-Na/ethylene/PVE indicated that a homopolymer of ethylene was obtained once again. No polymers were obtained using methyl acrylate as a comonomer. For comparison, we also performed similar copolymerization studies using complex 3d.20 When polar olefins such as VA and AA were added to a solution containing 3d/B(C6F5)3, the initial bright orange solution became noticeably lighter in color. Surprisingly, upon further exposure to ethylene for up to 16 h, no polymer products were obtaind.43,44 The change in solution color and the absence of polymerization activity suggest that 3d is not stable in the presence of polar vinyl olefins.

Figure 2. Quantitative 13C NMR spectra (TCE-d2, 125 MHz, 120 °C) of polymers obtained from the reaction of complex 3c/B(C6F5)3 with (A) ethylene, (B) ethylene/1-hexene, and (C) ethylene/1-pentene. Peak assignments were made according to ref 52. Branches are given the label xBy, where y is the branch length and x is the carbon number starting from the methyl group as 1. Greek letters and “br” are used instead of x for the methylene carbons in the polymer backbone and a branch point, respectively.

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CONCLUSIONS Toward our goal of developing user-customizable catalyst platforms, we have expanded our study of nickel phenoxyiminePEG complexes to include structurally bulky derivatives. Our investigations showed that steric shielding and Lewis acid effects can both influence catalyst activity and polymer morphology. We have found that the bulkier catalysts 3b/3c are more active than the less bulky 3a and that the presence of pendant Lewis acids led to dramatic increases in TOF relative to their parent nickel complexes. The changes in polymer branching and molecular weight due to the addition of Na+ or K+ ions are difficult to fully rationalize on the basis of established trends, which highlights the need to acquire further theoretical insights into Lewis acid assisted coordination− insertion polymerization. Our heterobimetallic complexes are highly active in the copolymerization of ethylene and nonpolar α-olefins but are significantly inhibited in the presence of polar vinyl olefins. In comparison to the conventional nickel phenoxyimine catalyst 3d, the heterobimetallic complex 3c-

(100 psi) and vinyl acetate (0.15 M) or allyl acetate (0.15 M), modest amounts of polymer were obtained (entries 7−10). Consistent with other reported studies,53−55 the catalyst’s activities were significantly reduced in the presence of polar monomers in comparison to nonpolar monomers. The polymers were thoroughly washed with various solvents to remove trace impurities before analysis by NMR spectroscopy. Unfortunately, our NMR data did not show any signals corresponding to the acetate groups from VA or AA, suggesting that the isolated materials were homopolymers of ethylene. Interestingly, when either 3-butenyl-acetate (BA) or 5-hexenylacetate (HA) was tested as comonomer, no products were obtained. These results are surprising because the acetate and olefin groups of BA/HA are separated by several carbon atoms, which should make them more compatible as comonomers in comparison to VA or AA.27,45 Perhaps the carbon chain spacers in BA/HA are not long enough to prevent the formation of E


Article

Organometallics

72.00, 70.86, 70.68, 70.65, 70.59, 69.69, 69.59, 59.10 ppm. 19F NMR (CDCl3, 376 MHz): δ −62.66 ppm. FT-IR: 2892 (νCHN), 1611 (νCN) cm−1. ESI−MS(+): calcd for C38H33F12NO6 [M + Na]+ 850.20080, found 850.20150. Preparation of 3a. Inside the glovebox, Ni(Me)2(pyridine)2 (49 mg, 0.19 mmol, 1.0 equiv) and 2a (97 mg, 0.19 mmol, 1.0 equiv) were dissolved in 10 mL of benzene. About 0.1 mL of excess pyridine was added to the solution. The mixture was stirred at room temperature for 2 h. The resulting dark red solution was filtered through a pipet plug and then dried under vacuum. The residue was washed with 2 mL of pentane and dried under vacuum to give a viscous red oil (0.12 g, 0.19 mmol, 99%). 1H NMR (C6D6, 400 MHz): δ 8.75 (d, JHH = 5.2 Hz, 2H), 7.53 (s, 1H), 7.09−7.04 (m, 3H), 6.83 (d, JHH = 9.2 Hz, 1H), 6.78 (t, JHH = 7.2 Hz, 1H), 6.65 (d, JHH = 8.4 Hz, 1H), 6.43 (t, JHH = 6.8 Hz, 2H), 6.36 (t, JHH = 7.6 Hz, 1H), 4.18 (m, 2H), 3.80 (t, JHH = 5.2 Hz, 2H), 3.39−3,33 (m, 12H), 3.27 (m, 2H), 3.05 (s, 3H), 1.47 (d, JHH = 6.8 Hz, 6H), 1.03 (d, JHH = 6.8 Hz, 6H), −0.70 (s, 3H) ppm. 13 C NMR (C6D6, 100 MHz): δ 165.79, 160.04, 151.94, 151.32, 150.05, 140.90, 135.95, 126.61, 126.23, 123.32, 123.18, 120.30, 118.49, 112.84, 72.08, 70.80, 70.58, 70.13, 69.04, 58.41, 28.27, 24.68, 22.95, −7.32 ppm. UV−vis (Et2O): λmax (ε, cm−1 M−1) = 351 nm (5469). FT-IR: 2868 (νCHN), 1603 (νCN) cm−1. Preparation of 3b. The same procedure was used as described above for 3a. The compounds Ni(Me)2(pyridine)2 (31 mg, 0.13 mmol, 1.0 equiv) and 2b (71 mg, 0.13 mmol, 1.0 equiv) were used. The residue was washed with 2 mL of pentane to form an orange solid (77 mg, 0.11 mmol, 87%). 1H NMR (C6D6, 400 MHz): δ 8.28 (d, JHH = 5.0 Hz, 2H), 7.76 (d, JHH = 7.0 Hz, 4H), 7.38 (s, 1H), 7.29 (d, JHH = 7.6 Hz, 2H), 7.22 (t, JHH = 7.2 Hz, 4H), 7.09−7.04 (m, 3H), 6.70 (t, JHH = 7.6 Hz, 1H), 6.65 (d, JHH = 7.6 Hz, 1H), 6.41−6.35 (m, 3H), 6.19 (t, JHH = 7.6 Hz, 1H), 3.67 (t, JHH = 5.2 Hz, 2H), 3.40−3.37 (m, 6H), 3.32−3.25 (m, 8H), 3.04 (s, 3H), −0.62 (s, 3H) ppm. 13C NMR (C6D6, 100 MHz): δ 168.20, 159.72, 151.49, 151.43, 150.40, 140.67, 136.48, 135.61, 130.59, 130.22, 128.38, 126.95, 126.30, 125.93, 122.72, 120.55, 117.83, 112.20, 72.08, 70.77, 70.58, 69.98, 68.65, 58.28, −8.22 ppm. UV−vis (Et2O): λmax (ε, cm−1 M−1) = 355 nm (6418). FT-IR: 2869 (νCHN), 1598 (νCN) cm−1. Mp: ∼74 °C dec. Anal. Calcd for C40H44N2NiO6(C2H2Cl2)0.85(C5H12)0.5: C, 64.27; H, 6.31; N, 3.39. Found: C, 64.34; H, 6.03; N, 3.12. Preparation of 3c. The same procedure was used as described above for 3a. The compounds Ni(Me)2(pyridine)2 (45 mg, 0.18 mmol, 1.0 equiv) and 2c (150 mg, 0.18 mmol, 1.0 equiv) were used. After the removal of solvent under vacuum, 4 mL of pentane was added to dissolve the residue. The product crystallized from solution as red needles (150 mg, 0.15 mmol, 85%). 1H NMR (C6D6, 400 MHz): δ 8.30 (d, JHH = 5.2 Hz, 2H), 8.16 (s, 4H), 7.70 (s, 2H), 6.92− 6.82 (m, 4H), 6.70 (t, JHH = 7.2 Hz, 1H), 6.58 (d, JHH = 9.2 Hz, 1H), 6.40 (t, JHH = 6.4 Hz, 2H), 6.26 (d, JHH = 8 Hz, 1H), 6.16 (t, JHH = 7.6 Hz, 1H), 3.58 (t, JHH = 4.8 Hz, 2H), 3.39−3.37 (m, 6H), 3.31−3.23 (m, 8H), 3.04 (s, 3H), −0.93 (s, 3H) ppm. 13C NMR (C6D6, 100 MHz): δ 167.27, 160.28, 151.05, 150.75, 141.72, 136.15, 133.43, 131.44 (q, JCF= 33 Hz), 130.60, 130.49, 126.21, 125.72, 123.77 (q, JCF= 271 Hz), 123.20, 120.92, 119.12, 118.87, 113.12, 72.05, 70.72, 70.54, 69.88, 68.74, 58.34, −8.17 ppm. 19F NMR (CDCl3, 565 MHz): δ −62.66 ppm. UV−vis (Et2O): λmax (ε/cm−1 M−1) = 370 nm (6624). FT-IR: 2872 (νCHN), 1605 (νCN) cm−1. Mp: ∼79 °C dec. Anal. Calcd for C44H40F12N2NiO6(H2O): C, 52.98; H, 4.24; N, 2.81. Found: C, 52.94; H, 4.32; N, 2.83. Ethylene Polymerization. Inside the glovebox, complex 3 (15 μmol) and MBArF4 (15 μmol) were dissolved in 10 mL of toluene and stirred for 30 min. Solid B(C6F5)3 (15 μmol) was added, the solution was transferred to a Fischer−Porter glass vessel along with a magnetic stir bar, and then the reactor was sealed. The high-pressure apparatus was removed from the glovebox and then securely fastened on top of a stir plate. The ethylene line was attached, and the reactor was purged with ethylene three times by pressurizing with ethylene and then releasing the pressure. The reactor was then pressurized to 100 psi of ethylene and stirred at room temperature for a specified amount of time. After the reaction was complete, the ethylene line was closed and the vessel was slowly vented. About 30 mL of MeOH was added,

Na exhibits longer catalyst lifetime and better performance in ethylene homopolymerization. Building on our prior work, we have demonstrated that the introduction of pendant Lewis acids to olefin polymerization catalysts is a useful strategy to enhance their catalytic properties. Although we have so far focused only on the PEG chain as the secondary metal ion receptor, we can replace it with other metal-binding groups to expand the range of Lewis acid cations that are accessible. Furthermore, it should be possible to apply similar design strategies to other well-known catalyst systems, such as palladium complexes that are capable of copolymerizing ethylene and polar vinyl olefins. We anticipate that this work will open up new research opportunities in heterobimetallic catalysis and provide novel synthetic routes to designer polyolefins.

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EXPERIMENTAL SECTION

General Procedures. Commercial reagents were used as received. All air- and water-sensitive manipulations were performed using standard Schlenk techniques or under a nitrogen atmosphere using a glovebox. Anhydrous solvents were obtained from an Innovative Technology solvent drying system saturated with argon. High-purity polymer grade ethylene was obtained from Matheson TriGas without further purification. Compound 2a,12 NaBArF4,56 KBArF4,57 and Ni(Me)2(pyridine)258 were prepared using literature procedures. Elemental analyses were performed by Atlantic Microlab. Trace levels of solvents in elemental analysis samples were quantified by 1H NMR spectroscopy. NMR spectra were acquired using JEOL spectrometers (ECA-400, 500, and 600) and referenced using residual solvent peaks. 19F NMR spectra were referenced to CFCl3. IR spectra were measured using a Thermo Nicolet Avatar FT-IR spectrometer. High-resolution mass spectra were obtained from the mass spectral facility at the University of Texas at Austin. Gel permeation chromatography (GPC) data were obtained using a Malvern high-temperature GPC instrument equipped with refractive index, viscometer, and light scattering detectors at 150 °C with 1,2,4trichlorobenzene (stabilized with 125 ppm of BHT) as the mobile phase. Synthesis. Preparation of 2b. Solid 2,6-diphenylaniline59 (0.19 g, 0.77 mmol, 1.0 equiv) and 3-(tetraethylene glycol)salicylaldehyde (1;12 0.25 g, 0.77 mmol, 1.0 equiv) were dissolved in 5 mL of MeOH. The mixture was treated with a few drops of acetic acid and then stirred under reflux for 12 h. The yellow solution was evaporated to dryness to give a yellow oil. The crude product was then purified by silica gel column chromatography (3/7 hexane/ethyl acetate) to afford a viscous yellow oil (0.23 g, 0.42 mmol, 54%). 1H NMR (CDCl3, 400 MHz): δ 12.73 (s, 1H), 7.94 (s, 1H), 7.41−7.29 (m, 11H), 7.22 (m, 2H), 6.91 (d, JHH = 8.0 Hz, 1H), 6.63 (t, JHH = 7.9 Hz, 1H), 6.47 (d, JHH = 7.6 Hz, 1H), 4.15 (t, JHH = 4.8, 2H), 3.87 (t, JHH = 5.4 Hz, 2H), 3.71 (m, 2H), 3.67−3.62 (m, 8H), 3.53 (m, 2H), 3.36 (s, 3H) ppm. 13 C NMR (CDCl3, 100 MHz): δ 168.98, 151.49, 147.31, 145.27, 139.53, 135.01, 130.41, 129.91, 128.45, 127.06, 125.89, 124.34, 118.98, 118.17, 117.16, 72.04, 70.92, 70.73, 70.63, 69.77, 68.73, 59.16 ppm. FT-IR: 2870 (νCHN), 1615 (νCN) cm−1. ESI−MS(+): calcd for C34H37NO6 [M + Na]+ 578.25130, found 578.25280. Preparation of 2c. The same procedure was used as described above for the preparation of 2b. The compounds 2,6-bis(3,5bis(trifluoromethyl)phenyl)aniline59 (0.34 g, 0.66 mmol, 1.0 equiv) and 3-(tetraethylene glycol)salicylaldehyde (0.21 g, 0.66 mmol, 1.0 equiv) were used. The pure product was collected as a yellow solid (0.38 g, 0.46 mmol, 55%). 1H NMR (CDCl3, 400 MHz): δ 11.65 (s, 1H), 8.00 (s, 1H), 7.85 (m, 4H), 7.77 (s, 2H), 7.52−7.44 (m, 3H), 6.99 (d, JHH = 6.8 Hz, 1H), 6.69 (t, JHH = 7.6 Hz, 1H), 6.56 (d, JHH = 9.2 Hz, 1H), 4.16 (t, JHH = 4.8 Hz, 2H), 3.84 (t, JHH = 5.2 Hz, 2H), 3.71 (m, 2H), 3.66−3.62 (m, 8H), 3.53 (m, 2H), 3.36 (s, 3H) ppm. 13 C NMR (CDCl3, 100 MHz): δ 169.87, 151.78, 147.31, 145.95, 141.03, 132.13, 131.82 (q, JCF= 34 Hz), 131.39, 130.10, 126.64, 124.47, 123.19 (q, JCF= 271 Hz), 121.16, 119.81, 118.92, 118.43, F


Article

Organometallics

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followed by the addition of 1 mL of HCl(aq). The white precipitate was collected by filtration. The resulting material was washed with MeOH and CH2Cl2 and then dried under vacuum at 60 °C. If no precipitate had formed, the solution was evaporated to dryness under vacuum. The residue was washed with MeOH and CH2Cl2 and then dried under vacuum at 60 °C. Ethylene Copolymerization. Inside the glovebox, complex 3 (15 μmol) and MBArF4 (15 μmol) were dissolved in 10 mL of toluene and stirred for 30 min. The comonomer (0.15−2.50 M) and solid B(C6F5)3 (15 μmol) were added to the catalyst solution, and the mixture was then transferred to a Fischer−Porter glass vessel. A magnetic stir bar was added, and the reactor was sealed before taking it outside of the glovebox. The high-pressure apparatus was removed from the glovebox and then securely fastened on top of a stir plate. The ethylene line was attached, and the reactor was purged with ethylene three times by pressurizing with ethylene and then releasing the pressure. The reactor was pressurized to 100 psi of ethylene and stirred at room temperature for a specified amount of time. After the reaction was complete, the ethylene line was closed and the vessel was slowly vented. About 30 mL of MeOH was added, followed by the addition of 1 mL of HCl(aq). The white precipitate was collected by filtration. The resulting material was washed with MeOH and CH2Cl2 and then dried under vacuum at 60 °C.

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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.7b00516. Polymer characterization and analysis, NMR spectra, and crystallographic data (PDF) Accession Codes

CCDC 1562643 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for L.H.D.: [email protected]. ORCID

Loi H. Do: 0000-0002-8859-141X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Welch Foundation (Grant No. E-1894), ACS Petroleum Research Fund (Grant No. 54834-DNI3), and the University of Houston New Faculty Startup Grant for funding. We thank Dawei Xiao (UH) for providing us with the NaBArF4 and KBArF4 salts and Dr. Tatyana Makarenko (UH) for performing DSC measurements.

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REFERENCES

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