Polymerization of Ethylene Catalyzed by Phosphine

Article pubs.acs.org/Organometallics

Polymerization of Ethylene Catalyzed by PhosphineIminophosphorane Palladium Complexes David Bézier, Olafs Daugulis,* and Maurice Brookhart* Center for Polymer Chemistry, Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States S Supporting Information *

ABSTRACT: Phosphine-iminophosphorane palladium complexes were synthesized and studied as catalysts for ethylene polymerization. The effect of the substituents on the phosphine and iminophosphorane moieties was investigated. Catalyst 15 promoted the conversion of ethylene to moderate molecular weight linear polyethylene (Mn ≈ 14000 g/mol, branches/1000C ≈ 7). This catalyst proved to be stable at temperatures up to 100 °C with a turnover frequency of 2100 h−1.

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INTRODUCTION The majority of polyolefins, particularly polyethylene and polypropylene, are produced using early-metal catalysts.1 In 1995, our group reported that late-metal catalysts based on bidentate diimine Ni(II) and Pd(II) complexes are highly active olefin polymerization catalysts leading to the formation of branched to hyperbranched polyolefins.2 Following this discovery, numerous catalytic systems were developed using late-transition-metal catalysts carrying diversified ligand structures. Among the most efficient neutral catalysts for olefin polymerization were a family of phosphine-sulfonate palladium complexes of type 1 shown to polymerize ethylene to linear polyethylene (Figure 1).3 The nature of the substituents (R) on

in the formation of cationic palladium complexes of type 2 (Figure 1), which showed high activity for ethylene polymerization and produced PE with moderate molecular weights.6e In order to further diversify the ligand framework associated with palladium, we chose to investigate the use of phosphineiminophosphorane ligands. Prior to our work, the group of Le Floch in 2007 reported phosphine-iminophosphorane nickel catalysts (Figure 2), which showed high activity for ethylene dimerization but no polymer formation.7

Figure 2. Nickel phosphine-iminophosphorane complexes. Figure 1. Phosphine-sulfonate palladium complexes 1 and phosphinephosphine oxide palladium complexes 2.

To develop catalysts which could potentially convert ethylene to high-molecular-weight polyethylene, we targeted the synthesis of palladium complexes bearing much bulkier phosphine-iminophosphorane ligands via incorporation of ortho-substituted aryl substituents on the nitrogen atom of the iminophosphorane moiety. These complexes (type A, Scheme 1) should provide considerably more steric bulk in the axial sites of the palladium complex in comparison to complexes of type 2. As in the case of the ortho-disubstituted

the phosphine and the lability of the ligand (L) strongly influence the catalytic activity and the polyethylene molecular weights.4 These palladium complexes also showed modest activity for the copolymerization of α-olefins with a variety of polar vinyl monomers.5 Inspired by the success of catalysts of type 1, numerous studies were carried out to modify the ligand framework.6 Among these reports, the Nozaki group replaced the anionic sulfonate group by a neutral phosphine oxide moiety, resulting © XXXX American Chemical Society

Received: May 25, 2017

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DOI: 10.1021/acs.organomet.7b00391 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. From (P,O)Pd to (P,N) Pd Complexes

aryl diimine palladium systems, the presence of steric bulk near axial sites should retard the rate of chain transfer relative to the rate of chain propagation, allowing the formation of highmolecular-weight polyethylene. We report here the synthesis of a new family of phosphineiminophosphorane palladium complexes of type A and their use as catalysts for ethylene polymerization.

Figure 3. Molecular structure of 10. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probablility level. Selected bond lengths (Å) and angles (deg): Pd1−C5 2.081(2), Pd1− N18 2.174(2), Pd1−P3 2.2374(5), Pd1−Cl2 2.3782(5), P4−N18 1.584(2); C5−Pd1−N18 170.30(8), C5−Pd1−P3 89.18(6), N18− Pd1−P3 99.08(5), C5−Pd1−Cl2 87.16(5), N18−Pd1−Cl2 87.16(5), P3−Pd1−Cl2 173.27(2).

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RESULTS AND DISCUSSION The phosphine iminophosphorane cationic palladium complexes 13−15 were synthesized following the procedure shown in Scheme 2. The addition of mesityl azide 1 or 2,6diisopropylphenyl azide 2 to (2-bromophenyl)diphenylphosphine afforded the iminophosphorane products 5 or 6, respectively. Subsequent reaction with n-BuLi followed by the addition of diisopropylchlorophosphine or bis(2methoxyphenyl)chlorophosphine resulted in formation of ligands 7−9. Treatment of the palladium precursor (COD)Pd(Me)Cl with 7−9 afforded (P-N)Pd(Me)Cl complexes 10− 12, which were characterized by 1H and 13C spectroscopy, elemental analysis, and X-ray crystallography. The ORTEP diagrams of these complexes (Figures 3−5) show that the palladium center is coordinated to phosphorus and the nitrogen of the iminophosphorane moiety. The geometry at the palladium is square planar with the methyl ligand cis to the phosphine group. The six-membered (P-N)Pd chelate ring adopts a puckered conformation, with one P-Ph substituent occupying a pseudoaxial position and the other a pseudoequa-

torial position. Halide abstraction from 10−12 with NaBArF in the presence of DMSO afforded (P-N)Pd(Me)(DMSO)][BArF] complexes 13−15 bearing the weakly coordinated DMSO ligand.4d Next, we investigated the activity of palladium complexes 13−15 bearing different substitution patterns as catalysts for ethylene polymerization. Catalysts 13−15 were initially screened for polymerization activity by exposure to 300 psig of ethylene at 80 °C for 4 h in toluene. The results are summarized in Table 1. Polyethylene Mn and Mw values were obtained using high-temperature GPC, and the degree of branching was determined using 1H NMR spectroscopy.8 Catalyst 13, bearing isopropyl substituents on the phosphine and a mesityl substituent on the nitrogen of the phosphineimine, converts ethylene to low-molecular-weight linear polyethylene (Mn = 1800 g/mol) with moderate activity (TOF = 320 h−1, entry 1). Catalyst 14, in which the mesityl

Scheme 2. Synthesis of Phosphine-Iminophosphorane Palladium Complexes 13−15

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Organometallics

replaced by 2-methoxyphenyl groups, shows higher catalytic activity (TOF = 700 h−1) and affords polymers with higher molecular weights (Mn = 12400 g/mol, entry 2). Moving from 14 to 15, we retained the 2-methoxyphenyl substituents on the phosphine which enhanced activity and replaced the mesityl substituent by the bulkier 2,6-diisopropylphenyl group. Catalyst 15 exhibits higher activity (TOF = 1160 h−1) and affords polymers with molecular weights (Mn = 13 840 g/mol, entry 3) similar to those for 14. All three catalysts 13−15 produced nearly linear polyethylenes (branches/1000C ca. 6−9) with molecular weight distributions, Mw/Mn, of ca. 2 (see entries 1− 3), consistent with single-site behavior. Since catalyst 15 proved to be the most active catalyst and produced PE with the highest Mn, the effects of time, temperature, and ethylene pressure were further investigated for this system. When the reaction time was increased from 2 to 4 h at 80 °C, the turnover frequency increased from 730 to 1160 h−1 (Table 1, entries 4 and 3, respectively). The reason for this increase is unclear but might be due to a slow displacement of DMSO by ethylene relative to the rate of insertion,9 or it could result from slow initial insertion of the first-formed Pd(Me)(ethylene) complex relative to subsequently formed alkyl ethylene species (Pd(alkyl)(ethylene)). Activities obtained after 4, 8, and 16 h are similar (TOF ≈ 1300 h−1), demonstrating that the initial increase in TOF with time has “washed out” at long times and most importantly that this catalyst is remarkably stable at 80 °C, showing no decrease in TOF between 4 and 16 h (entries 3, 5, and 6). When the reaction temperature is increased from 80 to 100 °C, the TOF nearly doubles from 1160 to 2110 h−1 after 4 h (entries 3 and 7, respectively). This increase of temperature also affects the structure of the polymer, resulting in a decrease in Mn from 13840 to 7070 g/mol and an increase in the branching density from 6 to 12 branches/1000C (entries 3 and 7, respectively). Using catalyst 15, very low activity was observed for the copolymerization of ethylene with methyl acrylate.10 The response of catalysis by 15 to variations of ethylene pressure was probed. At 80 °C, similar catalytic activities were observed under 150, 300, or 600 psig of ethylene after 4 h (TOF = 1030, 1160, and 1300 h−1, respectively, Table 1, entries 8, 3, and 9). These results are consistent with a [(PN)Pd(alkyl)(ethylene)][BArF] complex being the catalyst resting state with migratory insertion as the turnover-limiting step. Our strategy of increasing the steric bulk close to the palladium center by replacing the phosphine oxide unit with

Figure 4. Molecular structure of 11. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Pd1−C5 2.052(2), Pd1− N40 2.191(2), Pd1−P3 2.2227(6), Pd1−Cl2 2.3639(5), P4−N40 1.587(2); C5−Pd1−N40 174.92(8), C5−Pd1−P3 85.02(7), N40− Pd1−P3 99.98(5), C5−Pd1−Cl2 87.15(7), N40−Pd1−Cl2 87.88(5), P3−Pd1−Cl2 171.93(2).

Figure 5. Molecular structure of 12. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probablility level. Selected bond lengths (Å) and angles (deg): Pd−C1 2.107(2), Pd− N1 2.184(2), Pd−P1 2.2268(6), Pd−Cl1 2.4058(6), P2−N1 1.595(2); C1−Pd−N1 175.81(9), C1−Pd−P1 84.06(7), N1−Pd−P1 99.85(6), C1−Pd−Cl1 85.61(7), N1−Pd−Cl1 90.65(6), P1−Pd−Cl1 168.23(3).

substituent on the nitrogen of the iminophosphorane is retained but the isopropyl substituents on the phosphine are

Table 1. Ethylene Polymerization Catalyzed by Phosphine-Iminophosphorane Palladium Complexes 13−15 entrya

cat.

T (°C)

time (h)

ethylene (psig)

yield (g)

TON

TOF (h−1)

Mnb (103 g/mol)

Mw/Mnb

branches (/1000C)c

1 2 3 4 5 6 7 8 9

13 14 15 15 15 15 15 15 15

80 80 80 80 80 80 100 80 80

4 4 4 2 8 16 4 4 4

300 300 300 300 200 300 300 150 600

0.36 0.79 1.30 0.41 2.91 5.95 2.36 1.15 1.45

1280 2805 4643 1464 10393 21250 8429 4107 5179

320 700 1160 730 1300 1330 2110 1030 1300

1.80 12.40 13.84 13.88 12.03 12.12 7.07 12.60 12.30

2.2 2.2 2.1 1.9 2.1 2.1 2.2 2.1 2.2

7 9 6 6 8 8 12 8 6

Conditions: cat. 13−15 10 μmol, 48 mL of toluene, 2 mL of dichloromethane. bMolecular weight and polydispersity were determined by GPC in trichlorobenzene at 140 °C using polyethylene standards. cNumber of branches per 1000C was determined by 1H NMR spectroscopy in C6D5Br at 120 °C.

a

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aryl-substituted iminophosphorane moieties did not result in an increase in catalytic activity or PE molecular weight relative to (P,O)Pd catalyst 2. Catalyst 2 is ca. 50 times more active and produces PE with molecular weights ca. 2 times higher than does catalyst 15.6e It is not clear why this is the case, but perhaps part of the explanation lies in the fact that the weaker donor ability of the phosphine oxide renders the Pd(II) center more electrophilic in complexes of type 2. Increased electrophilicity is known to result in lower barriers to migratory insertion.

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CONCLUSIONS In summary, a family of palladium complexes 13−15 carrying phosphine-iminophosphorane ligands were synthesized and investigated as catalysts for ethylene polymerization. The effect of substituents on the phosphine and on the nitrogen of the iminophosphorane moieties was studied. Catalyst 15 showed the highest catalytic activity of systems investigated (TOF of 2110 h−1 at 100 °C) and afforded moderate molecular weight linear polyethylene (Mn ≈ 14000 g/mol, branches/1000C ≈ 7 at 80 °C). The thermal stability of 15 at 100 °C is remarkable in comparison to cationic (diimine)Pd complexes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00391. Synthetic procedures, polymerization methods, NMR spectra, and X-ray crystallography data for complexes 10−12 (PDF) Accession Codes

CCDC 1552423−1552424 and 1552428 contain 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 [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 Authors

*E-mail for O.D.: [email protected]. *E-mail for M.B.: [email protected]. ORCID

David Bézier: 0000-0002-2514-3070 Olafs Daugulis: 0000-0003-2642-2992 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Welch Foundation (Grant E-1893 to M.B. and Chair E-0044 to O.D.). Dr. Peter S. White and Dr. James D. Korp are thanked for solving the X-ray structures of 10−12.

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REFERENCES

(1) Mülhaupt, R. Macromol. Chem. Phys. 2003, 204, 289−327. (2) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414−15. (b) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267−268. (c) Mecking, S.; Johnson, L. K.; Wang, L.; Brookhart, M. J. Am. Chem. Soc. 1998, 120, 888−899. D


Article

Organometallics (9) Treating complex 15 with 20 equiv of ethylene at 35 °C in CD2Cl2 results in precipitation of polyethylene with complex 15 remaining in solution. This observation is consistent with slow initiation followed by rapid chain growth. (10) The copolymerization of ethylene with methyl acrylate was attempted using the following conditions: total volume of toluene and methyl acrylate 50 mL, methyl acrylate (1 M), 200 mg of BHT, ethylene (300 psig), catalyst 15 (50 μmol), 80 °C, 16 h. Results: yield, 50 mg; TON = 36; TOF = 2 h−1; MA incorporation, 0.9%; Mn = 1960 g mol−1; PDI = 2.2.

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Polymerization of Ethylene Catalyzed by Phosphine

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