A Well-Defined Ni(II) α-Diimine Catalyst Supported on Sulfated

Communication pubs.acs.org/Organometallics

A Well-Defined Ni(II) α‑Diimine Catalyst Supported on Sulfated Zirconia for Polymerization Catalysis Hosein Tafazolian, Damien B. Culver, and Matthew P. Conley* Department of Chemistry, University of California, Riverside, California 92521, United States S Supporting Information *

ABSTRACT: The reaction of (α-diimine)NiMe2 (α-diimine = (2,6-iPr2C6H3)NCMeMeCN(2,6-iPr2-C6H3)) with partially dehydroxylated sulfated zirconia (SZO300) in MeCN results in the formation of [(αdiimine)NiMe(NCMe)][SZO300] ([1][SZO300]) and methane. Reactions in Et2O resulted in mixtures of [(α-diimine)NiMe(OEt2)][SZO300] ([2][SZO 300]) and [(α-diimine)NiMe(OEt 2 )][MeSZO300] ([2][MeSZO300]), which were characterized by solid-state NMR spectroscopy. Contacting these solids with ethylene and monitoring the reaction by solid-state NMR showed that Ni−Me sites insert ethylene. [1][SZO300] and [2][SZO300]/[2][MeSZO300] are active ethylene polymerization catalysts and show properties similar to those of closely related homogeneous catalysts. [2][SZO300]/[2][MeSZO300] copolymerizes ethylene and methyl 10-undecenoate to form copolymers with up to 0.4% incorporation of the polar monomer.

T

he polymerization of olefins by single-site catalysts in solution forms polymers with tunable molecular weights, microstructures, and polydispersities.1 Among this family of single-site polymerization catalysts are Ni and Pd complexes containing α-diimine ligands that polymerize ethylene to give branched high-molecular-weight polymers.2 A remarkable feature of these late-metal catalysts is their ability to copolymerize ethylene and polar comonomers, such as methyl acrylate,3 which is not possible with early-metal catalysts because they are poisoned by the polar comonomer. Incorporation of a polymerization catalyst into an inorganic support is an important step for gas slurry phase applications. The synthesis of these heterogeneous polymerization catalysts involves the reaction of an early-metal precatalyst with silica pretreated with an alkylaluminum, such as methylaluminoxane (MAO). SiO2/MAO activates the precatalyst to form polymerization sites and forms ion pairs that bind the catalyst to the oxide surface.4 (α-diimine)Ni catalysts graft onto MAO/SiO2 but have low polymerization activities. However, (α-diimine)NiBr2 complexes containing hydroxyl groups react with SiO2/ MAO and are active in the polymerization of ethylene in the presence of Et3Al2Cl3 activator (Figure 1).5 The reactivity of this composition in the presence of polar monomers was not reported. The reaction of organometallic complexes with Brønsted sites on oxides forms well-defined active sites for polymerization reactions.6 For example, the reaction of (α-diimine)Ni(CH2SiMe3)2 (α-diimine = (2,6-iPr2-C6H3)NCMeMeC N(2,6-iPr2-C6H3)) with silica dehydroxylated at 700 °C forms [(SiO)Ni(α-diimine)(CH2SiMe3)], which is modestly active in the polymerization of ethylene in the presence of gaseous BF3 (Figure 1).7 This paper describes the reaction of (αdiimine)NiMe2 with sulfated zirconium oxide partially dehy© 2017 American Chemical Society

Figure 1. Examples of heterogeneous (α-diimine)Ni catalysts for olefin polymerization.

droxylated at 300 °C (SZO300) to form active [(α-diimine)NiMe(L)][SZO300] (L = MeCN, Et2O) sites (Figure 1) for polymerization of ethylene and the copolymerization of ethylene and methyl 10-undecenoate. Calcination of precipitated ZrO2 at 600 °C after soaking in a dilute sulfuric acid solution forms SZO containing Brønsted acid sites.8 Several studies showed that sulfate groups on the Received: May 30, 2017 Published: June 22, 2017 2385

DOI: 10.1021/acs.organomet.7b00402 Organometallics 2017, 36, 2385−2388

Communication

Organometallics

determine the reactivity of Ni−Me* in [1*][SZO300] toward olefins, we contacted the solid with excess ethylene (20 equiv/ Ni) and recorded the 13C{1H} CPMAS NMR spectrum (Figure 2c). After contact with ethylene, the 13C{1H} CPMAS spectrum contains new signals that are characteristic of lowmolecular-weight branched alkyl products. The Ni−Me* signal is no longer present in the spectrum after contact with ethylene. This result indicates that Ni−Me* sites are consumed in this reaction and suggests that most Ni−Me* sites are active in olefin insertion reactions. The reaction of (α-diimine)NiMe2 with SZO300 in Et2O results in the formation of [(α-diimine)NiMe(OEt2)][SZO300] ([2][SZO300]), [(α-diimine)NiMe(OEt2)][MeSZO300] ([2][MeSZO300]), and methane (Scheme 1). This material contains

zirconia surface act as weakly coordinating anions to form electrophilic organometallic Zr−R+ sites for the polymerization of olefins6d,9 or electrophilic Zr−H+ sites for the hydrogenation of aromatic rings.10 We show that (α-diimine)Ni sites supported on SZO are more active and produce polymers with narrower molecular weight distributions in comparison to previously reported heterogeneous Ni polymerization catalysts. The reaction of (α-diimine)NiMe2 and SZO300 in MeCN forms [(α-diimine)NiMe(NCMe)][SZO300] ([1][SZO300]) as an orange-brown solid and methane (see the Supporting Information for details). Elemental analysis shows that [1][SZO300] contains 0.50% Ni and 0.27% N, corresponding to 2.2 ± 0.5 N/Ni. The FTIR spectrum of [1][SZO300] contains a νCN band at 2254 cm−1 (Figure S6 in the Supporting Information). The 13C{1H} cross-polarization magic angle spinning (CPMAS) spectrum of [1][SZO300] recorded at 10 kHz and −20 °C contains signals at 140 and 121 ppm assigned to aromatic residues from the α-diimine ligand (Figure S5 in the Supporting Information). Signals at 28 and 21 ppm are assigned to the isopropyl and methyl groups from the αdiimine, and the signal at 1 ppm is assigned to coordinated MeCN (Figure S4 in the Supporting Information). The Ni−Me signal was not detected in the 13C{1H} CPMAS spectrum of [1][SZO300]. Detection of M−C(α) by 13C{1H} CPMAS can be challenging and often requires isotopic enrichment. The reaction of 13C-enriched (α-diimine)Ni(Me*)2 and SZO300 forms [1*][SZO300]. The 13C{1H} CPMAS spectrum of the 13C-enriched sample is shown in Figure 2b and contains an intense signal at −5.6 ppm assigned to Ni−Me*; the shoulder at 1.0 ppm is assigned to MeCN. To

Scheme 1. Reaction of [(α-diimine)Ni(Me)2] with SZO300 in Et2O To Form [2][SZO300]/[2][MeSZO300]

0.41% Ni and 0.16% N, corresponding to 1.6 ± 0.6 N/Ni. The Ni sites formed under these conditions are less stable than those in [1][SZO 300 ], decomposing to methane and unidentified products at room temperature. The 13C{1H} CPMAS spectrum of [2*][SZO300]/[2*][Me*SZO300] recorded at 10 kHz and −20 °C contains the signal for Ni− Me* at 2.1 ppm and a signal at −32 ppm, indicating that some Zr−Me− sites are formed by alkyl anion transfer to the surface, resulting in [2*][Me*SZO300] (Figure S7 in the Supporting Information). The absence of Zr−Me− signals in [1][SZO300] suggests that the strong donor MeCN solvent blocks Lewis acid Zr sites necessary for alkyl transfer to form [2][MeSZO300]. The 13C{1H} CPMAS spectrum of [2*][SZO300]/[2*][Me*SZO300] after contacting the solid with ethylene (5 equiv/ Ni for 2 min) results in the consumption of the Ni−Me* peak; the Zr-Me− signal at −32 ppm is present in this spectrum (Figure S8 in the Supporting Information). This result indicates that Ni−Me* sites insert ethylene and that the Zr−Me− sites do not participate in olefin insertion reactions under these conditions. The polymerization activity of the supported nickel catalysts is summarized in Table 1. [1][SZO300] polymerizes ethylene at 45 psi and 40 °C with a turnover frequency (TOF) of 3700 h−1 to form branched polyethylene (Mn = 29000 g/mol; Đ = 2.0) containing 79 branches/1000C (Table 1, entry 1). Increasing the temperature to 60 °C results in higher TOF values (8000 h−1) and higher branch content in the polymer (89 branches/ 1000C) while high molecular weight and narrow dispersity are maintained (Table 1, entry 2). The activity of [1][SZO300] decreases at 80 °C and forms lower molecular weight polymer (Table 1, entry 3). Under 45 psi pressure of ethylene and 40 °C in toluene [2][SZO300]/[2][MeSZO300] polymerizes ethylene with a TOF of 21000 h−1 to form high-molecular-weight polyethylene (Mn = 153000 g/mol; Đ = 1.8) containing 71 branches/1000C

Figure 2. (a) Reaction of (α-diimine)Ni(Me*)2 and SZO300 to form [1*][SZO300]. (b) 13C{1H} CPMAS spectrum of [1*][SZO300]. (c) 13 C{1H} CPMAS spectrum of [1*][SZO300] after contact with 20 equiv of ethylene/Ni at 25 °C for 5 min. 13C{1H} CPMAS spectra were recorded at 10 kHz spinning speed at −20 °C (150 MHz, recycle delay 3 s, contact time 2 ms, ns = 10000 for (b) and 5000 for (c)). 2386

DOI: 10.1021/acs.organomet.7b00402 Organometallics 2017, 36, 2385−2388

Communication

Organometallics Table 1. Polymerization of Ethylene Using Supported (α-diimine)Ni Catalystsa entry

cat.

T (°C)

P (psi)

TOF (h−1)b

activity (kgPE/(molNi h))c

Mn (kg/mol)d

Đ

Be

1 2 3 4 5 6 7

1 1 1 2 2 2 2

40 60 80 40 60 80 100

45 45 45 45 45 45 45

3700 8000 7400 21000 14500 12800 10600

103 225 206 586 406 360 297

28.5 22.6 14.6 153 90.9 66.5 54.0

2.0 2.0 2.2 1.8 1.8 2.4 2.4

79 89 92 71 75 79 86

a Reaction conditions: catalyst (3.4 μmol of [1][SZO300] or 1.4 μmol of [2][SZO300]/[2][MeSZO300]), 20 mL toluene. bIn units of (mol of ethylene)/((mol of Ni) h). cDetermined after 15 min. dDetermined by GPC at 140 °C in trichlorobenzene. eNumber of branches per 1000 carbons determined by 1H NMR in tetrachloroethane-d2 at 120 °C.11

methide abstraction by Lewis acid sites to form [2][SZO300]/ [2][MeSZO300] sites. Titration experiments showed that Ni− Me sites are active in insertion of ethylene and that Zr−Me− sites formed by methide abstraction are inactive under these conditions. Grafting in MeCN results in the formation of [1][SZO300], probably due to interaction of the strong σ-donor solvent with Lewis acid surface sites in SZO300. The [(αdiimine)NiMe] sites on SZO300 show trends in activity and polymer properties similar to those of closely related homogeneous catalysts. The more active [2][SZO300]/[2][MeSZO300] catalyst incorporates up to 0.4% methyl 10undecenoate in copolymerization reactions, showing that incorporation of polar monomers is viable with supported Ni catalysts. Numerous α-diimine ligands that modify the behavior of Ni and Pd catalysts in homopolymerization of ethylene, and copolymerizations in the presence of polar comonomers, are available. The reactivity of these complexes with SZO300 may result in more active and/or selective supported catalysts for polymerization reactions.

(Table 1, entry 4). Increasing the polymerization temperature results in lower activities, lower molecular weights, higher branch contents, and narrow dispersities (Table 1, entries 5− 7). Prolonged reaction times lead to a steady decrease in catalyst activity. For example, running the polymerization for 3 h results in an activity of 96 kgPE/(molNi h) (Table S3 in the Supporting Information). This result suggests that [2][SZO300]/[2][MeSZO300] deactivates under the reaction conditions. Under the conditions studied here, [1][SZO300] and [2][SZO300]/[2][MeSZO300] produce polymers with narrower Đ values in comparison to those obtained with (α-diimine)NiBr 2 /SiO 2 /MAO/Et 3 Al 2 Cl 3 or [(SiO)Ni(α-diimine)(CH2SiMe3)]/BF3.5,7 The catalytic activities of [1][SZO300] and [2][SZO300]/[2][MeSZO300], and the properties of the polymer produced by these catalysts, are similar to those observed for [(α-diimine)NiMe]+ catalysts in solution.2,12 We performed a polymerization reaction in an NMR tube to determine if signals for the active site (or free ligand) are present under low-pressure conditions. After 20 h, the 1H NMR spectra in Figure S15 in the Supporting Information are consistent with consumption of ethylene and the appearance of new signals for low-molecular-weight oligomeric products; signals for [(α-diimine)NiR]+ or free ligand were not detected. This result is consistent with negligible leaching of Ni sites during polymerization. As mentioned above, late-transition-metal polymerization catalysts can incorporate polar monomers in copolymerization reactions. Under 45 psi of ethylene at 40 °C [2][SZO300]/ [2][MeSZO300] copolymerizes methyl 10-undecenoate13 to form copolymer (eq 1). The copolymer has a lower molecular

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00402. Experimental procedures, spectroscopic data, and GPC data (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for M.P.C.: [email protected]. ORCID

Matthew P. Conley: 0000-0001-8593-5814 Notes

The authors declare no competing financial interest.

■

weight with a broader dispersity (Mn = 29700 g/mol; Đ = 5.17) in comparison to ethylene homopolymerizations under the same conditions. The copolymer contains 0.3% methyl 10undecenoate and 61 branches/1000C. Though [2][SZO300]/ [2][MeSZO300] copolymerizes ethylene and this monomer, the activity of the catalyst is significantly lower (TOF = 180 h−1) in the presence of the methyl 10-undecenoate. Copolymerizations at 60 °C are less active (TOF = 140 h−1) and result in polymers with more branches (72 branches/1000C; Mn = 23600 g/mol; Đ = 4.15) and similar comonomer incorporation (0.4%). The structure of [(α-diimine)NiMe] sites on SZO300 depends on the solvent used in the grafting step. Reactions in Et2O result in protonolysis of Ni−Me by Brønsted sites and

ACKNOWLEDGMENTS M.P.C. is a member of the UCR Center for Catalysis. We thank the University of California, Riverside, for funding this work. We thank Dr. Kensha Clark at Chevron Phillips Chemical Company, LP, for assistance with measuring GPC of the polymers from this study.

■

REFERENCES

(1) (a) Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem., Int. Ed. 2002, 41, 2236−2257. (b) Braunschweig, H.; Breitling, F. M. Coord. Chem. Rev. 2006, 250, 2691−2720. (c) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85−93. (d) Coates, G. W. Chem. Rev.

2387

DOI: 10.1021/acs.organomet.7b00402 Organometallics 2017, 36, 2385−2388

Communication

Organometallics 2000, 100, 1223−1252. (e) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428−447. (f) Baier, M. C.; Zuideveld, M. A.; Mecking, S. Angew. Chem., Int. Ed. 2014, 53, 9722− 9744. (2) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169−1204. (3) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267−268. (b) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888−899. (c) Popeney, C. S.; Guan, Z. J. Am. Chem. Soc. 2009, 131, 12384−12393. (d) Dai, S.; Sui, X.; Chen, C. Angew. Chem., Int. Ed. 2015, 54, 9948−9953. (4) (a) Hlatky, G. G. Chem. Rev. 2000, 100, 1347−1376. (b) Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. Chem. Rev. 2005, 105, 4073−4147. (c) Stürzel, M.; Mihan, S.; Mülhaupt, R. Chem. Rev. 2016, 116, 1398−1433. (5) (a) Preishuber-Pflugl, P.; Brookhart, M. Macromolecules 2002, 35, 6074−6076. (b) Schrekker, H. S.; Kotov, V.; Preishuber-Pflugl, P.; White, P.; Brookhart, M. Macromolecules 2006, 39, 6341−6354. (6) (a) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, J.-M. Angew. Chem., Int. Ed. 2003, 42, 156−181. (b) Marks, T. J. Acc. Chem. Res. 1992, 25, 57−65. (c) Wegener, S. L.; Marks, T. J.; Stair, P. C. Acc. Chem. Res. 2012, 45, 206−214. (d) Stalzer, M.; Delferro, M.; Marks, T. Catal. Lett. 2015, 145, 3−14. (e) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P. A. Chem. Rev. 2016, 116, 323−421. (7) Dorcier, A.; Merle, N.; Taoufik, M.; Bayard, F.; Lucas, C.; de Mallmann, A.; Basset, J. M. Organometallics 2009, 28, 2173−2178. (8) (a) Hino, M.; Arata, K. J. Chem. Soc., Chem. Commun. 1980, 851− 852. (b) Arata, K., Preparation of Superacidic Metal Oxides and Their Catalytic Action. In Metal Oxide Catalysis; Wiley-VCH: Weinheim, Germany, 2009; pp 665−704. (9) (a) Williams, L. A.; Marks, T. J. ACS Catal. 2011, 1, 238−245. (b) Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 13533−13534. (c) Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 7103−7110. (d) Nicholas, C. P.; Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 4325−4331. (e) Nicholas, C. P.; Marks, T. J. Langmuir 2004, 20, 9456−9462. (f) Nicholas, C. P.; Marks, T. J. Nano Lett. 2004, 4, 1557−1559. (10) (a) Stalzer, M. M.; Nicholas, C. P.; Bhattacharyya, A.; Motta, A.; Delferro, M.; Marks, T. J. Angew. Chem., Int. Ed. 2016, 55, 5263−5267. (b) Gu, W.; Stalzer, M. M.; Nicholas, C. P.; Bhattacharyya, A.; Motta, A.; Gallagher, J. R.; Zhang, G.; Miller, J. T.; Kobayashi, T.; Pruski, M.; Delferro, M.; Marks, T. J. J. Am. Chem. Soc. 2015, 137, 6770−6780. (c) Williams, L. A.; Guo, N.; Motta, A.; Delferro, M.; Fragala, I. L.; Miller, J. T.; Marks, T. J. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 413− 418. (11) Daugulis, O.; Brookhart, M.; White, P. S. Organometallics 2002, 21, 5935−5943. (12) (a) Gates, D. P.; Svejda, S. A.; Oñate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.; Brookhart, M. Macromolecules 2000, 33, 2320−2334. (b) Hue, R. J.; Cibuzar, M. P.; Tonks, I. A. ACS Catal. 2014, 4 (11), 4223−4231. (c) Under conditions identical with those of polymerizations with [2][SZO300]/[2][MeSZO300], [(α-diimine)NiMe][B(C6F5)4] has an activity of 700 kgPE/(molNi h).. (13) (a) Long, B. K.; Eagan, J. M.; Mulzer, M.; Coates, G. W. Angew. Chem., Int. Ed. 2016, 55, 7106−7110. (b) Liu, W.; Malinoski, J. M.; Brookhart, M. Organometallics 2002, 21, 2836−2838. (c) Dai, S.; Chen, C. Angew. Chem., Int. Ed. 2016, 55, 13281−13285.

2388

DOI: 10.1021/acs.organomet.7b00402 Organometallics 2017, 36, 2385−2388