Article pubs.acs.org/Organometallics
Oligomerization of Ethylene Using a Diphosphine Palladium Catalyst 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: The palladium diphosphine complex 4 converts ethylene to hyperbranched unsaturated oligomers (Mn ≈ 330 g/mol) at 22 °C. In addition to the high stability of this catalyst observed at temperatures up to 75 °C, an increase of the reaction temperature leads to a decrease in branching densities of the oligomers, in sharp contrast to the palladium and nickel diimine catalyst systems. One-pot oligomerizations/ hydrogenations were carried out to form hyperbranched saturated oligomers.
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INTRODUCTION Nickel-based catalysts have been extensively used for decades for olefin oligomerizations. The most notable examples are the commercial SHOP-type catalysts incorporating bidentate [N,O] ligands reported in 1968.1 Numerous other bidentate Ni-based catalysts have since been described.2 In 1995, our group reported that late metal catalysts based on Ni(II) and Pd(II) complexes bearing bidentate diimine ligands [N,N] are quite effective for polymerization of ethylene, α-olefins, and disubstituted olefins.3 The key to designing Ni(II) and Pd(II) systems that polymerize olefins is to incorporate axial bulk in these square planar systems through use of ortho-disubstituted aryl diimine ligands. Axial bulk was shown to retard the rate of chain transfer relative to chain propagation. Since these reports, late transition-metal-mediated olefin polymerizations have attracted growing interest.2b,c,4 In addition to [N,N]-based catalysts,3a,5 many palladium and/or nickel catalysts bearing other types of bidentate ligands such as [N,O],6 [P,O],4h,7 and [NHC,O]8 systems have been shown to effect ethylene homopolymerization, but remarkably only few examples of active diphosphine [P,P]-based catalysts have been reported.9 Examples of nickel diphosphine catalysts for olefin oligomerization/polymerization are limited to diphosphinomethane (1)9a,b and 1,2-phenylene-diphosphine (2)-based9c systems (Figure 1). Catalysts of type 1 with R = tBu generate linear polymers with moderate Mn (23 500 g/mol) but with very low turnover numbers (TONs < 150) at room temperature.9a Depending on the R group, catalysts of type 2 can also produce
linear polymers at room temperature but again with very modest TONs.9c Palladium diphosphine complexes have been shown to catalyze CO/ethylene alternating copolymerization10 and ethylene dimerization,11 but to our knowledge no examples of active catalysts for homopolymerization or oligomerization (>C4) of ethylene have been reported. Inspired by the success of bulky diimine palladium complexes, we have explored the possible use of palladium complexes bearing extremely bulky diphosphine ligands for polymerization. We report here the development of a diphosphine palladium catalyst that converts ethylene to branched oligomers (Mn ≈ 300 g/mol). This catalyst shows remarkable features, including high stability at 75 °C and the formation of oligomers with lower branching densities as the oligomerization temperature increases from room temperature to 75 °C, a trend opposite that of the diimine catalysts.
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RESULTS AND DISCUSSION We began our investigation by screening in situ generated palladium catalysts bearing bulky diphophine ligands for the homopolymerization of ethylene (Scheme 1). Reaction of (COD)Pd(Me)Cl with the diphosphine L1, L2, L3, L4, or L5 in CH2Cl2 followed by the addition of NaBArF and CH3CN afforded the in situ generated cationic methyl acetonitrile complexes. These catalysts were screened for polymerization/ oligomerization activity by exposure to 400 psig ethylene at rt for 4 h. While no activity was detected when using the catalyst bearing L1 as the ligand, complexes formed from diphosphines L2, L3, and L4 showed the formation of low molecular weight oligomers, C4 to ca. C10 olefins. Interestingly, tBu-substituted catalyst generated from L5 yielded ethylene oligomers with an average Mn of 330, while the formation of C4−C10 olefins was not detected. These results prompted us to study in more detail the ethylene oligomerization reactions using the palladium complex bearing
Figure 1. Example of active nickel diphosphine ethylene polymerization catalysts. © XXXX American Chemical Society
Received: November 10, 2016
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DOI: 10.1021/acs.organomet.6b00850 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
6% of the total olefins. Increasing the reaction time to 48 h results in a similar activity (TOF = 41 h−1, entry 4), highlighting the stability of 4 at room temperature. Increasing the temperature to 50 and 75 °C increases the TOF to 346 and 1343 h−1, respectively (entries 6 and 9). Catalyst 4 is stable at 75 °C for a long period of time; no decrease of oligomerization rate was observed when increasing the reaction time from 3 h to 6 h (entries 9, 11). Interestingly, by increasing the temperature from 22 °C to 50 °C and 75 °C, while no major change of Mn was observed, the branching density of the obtained oligomers decreases from 1.6 to 0.7 and 0.3 branch/chain, respectively (entries 2, 6, and 9). At 75 °C, the oligomers contain 27% terminal olefins as end groups. Such behavior is in sharp contrast to the reactions carried out with palladium and nickel diimine systems, which exhibit an increase of branching when increasing the reaction temperature.3a We also investigated the effect of ethylene pressure. At 22 °C, similar catalytic activities were observed under 400 or 600 psig of ethylene (TOF = 38 and 41 h−1, respectively, entries 2, 3), consistent with a [(P−P)Pd(alkyl)(ethylene)][BArF] complex being the catalyst resting state with migratory insertion as the turnover-limiting step. However, decreasing the ethylene pressure to 200 psig does result in a small decrease in the TOF (29 h−1, entry 1). Such behavior indicates that at this lower pressure there must be a fraction of an acetonitrile complex [(P−P)Pd(alkyl)(NCMe)][BArF] in equilibrium with the major alkyl ethylene resting state. This behavior is also observed at 50 and 75 °C (entries 5−10). After demonstrating the efficiency of catalyst 4 to oligomerize ethylene, we focused on its hydrogenation ability. While many diimine complexes [(N−N)Pd(Me)(NCMe)][BArF] are not stable under hydrogen, we observed that complex 4 upon exposure to H2 was rapidly converted in situ via hydrogenolysis of the Pd−CH3 bond to [(P−P)Pd(H)(NCMe)][BArF] (5) (31P{1H} NMR: δ = 38.8 (br), 17.8 (br); 1H NMR: −8.62 (dd, JHP = 224 Hz, JHP = 33 Hz, 1H) (Scheme 3).
Scheme 1. Ethylene Oligomerization Reactions Catalyzed by in Situ Generated Diphosphine Palladium Complexes
L5. Complex (P−P)Pd(Me)Cl (3) was synthesized by treating L5 with Pd(COD)MeCl in dichloromethane (Scheme 2). Addition of NaBArF and CH3CN afforded [(P−P)Pd(Me) (NCMe)][BArF] (4) (31P NMR: δ = 43.2 (d, JPP = 46 Hz), 15.8 (d, JPP = 46 Hz)). Scheme 2. Synthesis of (P−P)Pd(Me)Cl (3) and [(P− P)Pd(Me)(NCMe)][BArF] (4)
The effect of time, temperature, and ethylene pressure was investigated for ethylene oligomerization reactions in toluene using pure isolated catalyst 4 (25 μmol) (Table 1). The reactions were first carried out using 400 psig of ethylene (entries 2, 4, 6, 9, 11, and 12). The number of branches/chain12 and the average molecular weight (Mn) were determined by 1H NMR spectroscopy. Carrying out the reaction at 22 °C, 0.64 g of oligomer was obtained after 24 h (turnover frequency (TOF) = 38 h−1, entry 2). The oligomer is branched (1.6 branches/chain) with an average molecular weight, Mn, of 328 g/mol and end groups composed mainly of internal olefins. Terminal olefins represent
Scheme 3. In Situ Generation of [(P− P)Pd(H)(NCMe)][BArF], 5
Following this observation, one-pot ethylene oligomerization/ hydrogenation reactions using catalyst 4 were carried out to
Table 1. Ethylene Oligomerization Catalyzed by 4 entrya
T (°C)
ethylene (psig)
time (h)
yield (g)
TON
TOF (h−1)
branches/chainb
Mnc (g/mol)
1 2 3 4 5 6 7 8 9 10 11
22 22 22 22 50 50 50 75 75 75 75
200 400 600 400 200 400 600 200 400 600 400
24 24 24 48 24 24 24 3 3 3 6
0.48 0.64 0.68 1.38 4.10 5.81 6.25 2.21 2.82 2.93 5.35
687 914 974 1971 5857 8300 8929 3157 4029 4186 7643
29 38 41 41 244 346 372 1052 1343 1395 1274
1.6 1.6 1.7 1.6 0.7 0.7 0.7 0.3 0.3 0.3 0.3
310 328 345 314 251 253 259 361 307 265 314
Conditions: 4 = 25 μmol, V(toluene) = 50 mL, ethylene: Matheson purity. bNumber of branches per chain were determined after hydrogenation by H NMR spectroscopy in o-C6D4Cl2 at 100 °C.12 cMolecular weight was determined by 1H NMR spectroscopy in o-C6D4Cl2 at 100 °C.
a
1
B
DOI: 10.1021/acs.organomet.6b00850 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 2. One-Pot Ethylene Oligomerization/Hydrogenation Reactions Catalyzed by 4
entrya
T (°C)
time (h)
yield (g)
TONb
TOFb (h−1)
Me/1000Cc
Mnd (g/mol)
Me/chaine
branches/chainf
1 2 3
22 50 75
24 24 3
0.71 5.78 2.81
1014 8257 4014
42 344 1338
154 151 105
328 253 307
3.6 2.7 2.3
1.6 0.7 0.3
a Conditions: 4 = 25 μmol, V(toluene) = 50 mL, (1) ethylene (400 psig), (2) release of ethylene followed by addition of H2 (200 psig), 24 h. bTON and TOF are calculated based on the oligomerization reaction. cMe/1000C determined by 1H NMR spectroscopy in o-C6D4Cl2 at 100 °C. d Molecular weight obtained from unsaturated oligomers using data from Table 1. eMe/chain obtained from Me/1000C and Mn. fBranches/chain obtained by subtracting 2 from the number of methyls/chain.
Figure 2. 13C NMR spectrum of the hyperbranched oligomer obtained after oligomerization/hydrogenation one-pot reaction at 22 °C. The oligomer was dissolved in 1,2-dichlorobenzene at 100 °C with 0.05 M Cr(acac)2. Relaxation delay = 20 s. Assignments are numbered according to the literature.6c,13 Chain ends are assigned with S1−S4. Branches are labeled as xBy, where y is the branch length and x is the carbon, starting from the methyl end with 1. The methine groups for the different branch lengths are labeled with *By. A and B are the methyl groups of a sec-butyl branch.
convert ethylene to saturated oligomers (Table 2). These onepot reactions were carried out by exposing catalyst 4 to 400 psig of ethylene in toluene. Following the oligomerization reactions, ethylene was replaced by hydrogen (200 psig) and the reaction mixtures were stirred for 24 h. Results are summarized in Table 2. At 22 °C, following oligomerization (24 h) and hydrogenation (24 h), 0.71 g of saturated oligomer was obtained (entry 1). This polymer contains 154 Me/1000C (determined by 1H NMR spectroscopy). On the basis of the average molecular weight determined previously from the unsaturated oligomer (Mn = 328, Table 1, entry 2), the number of Me/1000C corresponds to 3.6 Me groups/chain. Considering that each saturated chain possesses two methyl chain ends, this oligomer possesses 1.6 branches/chain (3.6−2). The oligomer microstructure was analyzed by 13C NMR spectroscopy (Figure 2), which revealed a hyberbranched structure with the presence of a “branch on branch” motif; specifically a sec-butyl branch is prominent (Figure 2). By increasing the temperature of these one-pot reactions to 50 and 75 °C, the number of branches/chain of the saturated polymers decreases to 0.7 and 0.3, respectively (Table 2, entries 2 and 3). The decrease of branches/chain was confirmed by 13C NMR spectroscopy (Figure 3), which shows that the amount of methyl, ethyl, propyl, and sec-butyl branches decreases when increasing reaction temperature. At 75 °C, the 13C
Figure 3. 13C NMR spectra of the oligomers obtained after oligomerization/hydrogenation one-pot reactions. The oligomers were dissolved in 1,2-dichlorobenzene at 100 °C with 0.05 M Cr(acac)2. Relaxation delay = 20 s.
spectrum of the oligomer shows a nearly linear structure (Figure 3, top). Interestingly, the oligomer formed at 22 °C is a viscous liquid, while the oligomer formed at 75 °C is a solid. C
DOI: 10.1021/acs.organomet.6b00850 Organometallics XXXX, XXX, XXX−XXX
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A few mechanistic details can be gleaned from these observations. As noted above, the fact that the turnover frequency is independent of the ethylene pressure shows that the catalyst resting state is the alkyl ethylene complex and that migratory insertion is turnover-limiting. The calculated ΔG⧧ at 22 °C for migratory insertion is based on the TOF (k = 41 h−1) of ca. 19 kcal/mol, which compares closely to the barrier for migratory insertion in (dimine)Pd(R)(ethylene)+ complexes of ca. 18 kcal/mol.3a As in the palladium diimine systems, chainwalking must be responsible for branching, and the fact that branch-on-branch structures are observed implies that palladium can walk through tertiary centers (i.e., Pd−C(R)(R′)(R″) species are energetically readily accessible). Branching must occur via insertion of ethylene into a secondary palladium alkyl bond. One plausible explanation for decreasing branching with increasing temperature is that the entropy of activation of ethylene insertion into a secondary Pd−alkyl bond (ksec) is much more negative than that for insertion into a primary alkyl bond (kprim). This would result in an increase in kprim/ksec with temperature, and, coupled with a rate of chain-walking occurring faster than insertion (Curtin−Hammett kinetics), the net result would be more insertion into primary bonds at higher temperatures and thus decreased branching.
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CONCLUSIONS
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ASSOCIATED CONTENT
REFERENCES
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To summarize, the palladium diphosphine complex, 4, catalyzes ethylene oligomerization to generate olefins with an Mn of ca. 300. This catalyst exhibits unique features, including excellent stability at temperatures of 75 °C and the production of hyperbranched oligomers (69 branches/1000C) when the reaction is carried out at 22 °C. Furthermore, increasing the reaction temperature leads to an unexpected decrease of branching with a nearly linear oligomer formed at 75 °C and no loss in molecular weight. Complex 4 is a hydrogenation catalyst; thus one-pot oligomerization/hydrogenation reactions could be carried out to obtain saturated oligomers. A more detailed mechanistic study of this system and the development of nickel analogues are currently under way in our laboratory.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00850.
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Synthetic procedures, polymerization methods, and NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
David Bézier: 0000-0002-2514-3070 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Welch Foundation (Grant E1893 to M.B. and Chair E-0044 to O.D.). D
DOI: 10.1021/acs.organomet.6b00850 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.6b00850 Organometallics XXXX, XXX, XXX−XXX
