Communication pubs.acs.org/Organometallics
Zwitterionic Nickel(II) Catalyst for CO−Ethylene Alternating Copolymerization Xiaofei Jia,† Mengru Zhang,† Fan Pan,‡ Ilknur Babahan,† Kuiling Ding,‡ Li Jia,*,† Laura A. Crandall,§ and Christopher J. Ziegler§ †
Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States Shanghai Institute of Organic Chemistry, 345 Ling Ling Road, Shanghai 200032, People’s Republic of China § Department of Chemistry, The University of Akron, Akron, Ohio 44325, United States ‡
S Supporting Information *
ABSTRACT: A zwitterionic nickel(II) catalyst has been discovered to display an initial catalytic activity comparable to that of cationic palladium catalysts for alternating copolymerization of carbon monoxide and ethylene. This demonstrates the absence of a severe dormant state in the present zwitterionic system, in contrast to the cationic nickel(II) catalysts. However, the highly active catalyst is short-lived. Stoichiometric decomposition of the catalyst under carbon monoxide suggests that the insufficient stability of the tetraphenylborate motif in the ligand framework with respect to electrophilic attack is likely a culprit for catalyst deactivation.
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alladium-catalyzed alternating copolymerization of alkene and carbon monoxide is a highly efficient method for synthesis of aliphatic polyketones.1−5 Simple members in this family, such as the terpolymer of CO, ethylene, and propylene, possess a plurality of desirable materials properties and are attractive semicrystalline engineering thermoplastics suitable for a variety of applications.6,7 Since the initial attempts by Shell and BP in the 1990s, efforts of large-scale production of the aliphatic polyketones have continued. However, one of the shortcomings of the current technology is the necessity of using the expensive palladium metal as the catalyst. Nickel is the first metal discovered to catalyze CO−ethylene copolymerization but is generally far inferior to palladium and has not been as widely studied. The cationic palladium catalysts in the commercial processes typically have activities of ∼6000 g (g of Pd)−1 h−1 (grams of polyketone per gram of palladium per hour) and productivities of 106 g (g of Pd)−1 (grams of polyketone per gram of palladium).2 The nickel analogues of the cationic palladium catalysts are less productive and less active than the latter by several orders of magnitude.8,9 Brookhart and his co-workers have shown from a mechanistic viewpoint that the difference between cationic Ni and Pd catalysts is the involvement of five-coordinate intermediates in the nickel system.10 The 18-electron, 5-coordinate chelate intermediate A reacts with ethylene very slowly and may be a resting state that severely impedes the catalytic process. If the 5coordinate resting state could be eliminated, theoretical work predicted that the cationic nickel catalysts should be more active than the cationic palladium catalysts.11,12 Neutral nickel compounds with anionic N,O- and P,Ochelating ligands are isoelectronic with the cationic compounds © XXXX American Chemical Society
and should be less Lewis acidic.13,14 These compounds as catalyst precursors are known to catalyze the alternating copolymerization of CO and ethylene. The neutral arylnickel(II) compounds with anionic N,O-chelating ligands appear rather active, but an induction period of at least 45 min under copolymerization conditions is required before any copolymer is produced.14a The activities of these catalysts have not been rigorously examined. Their reported productivity (∼3000 g (g of Ni)−1) is significantly lower than that of the cationic Pd catalysts. The extensive study of the coordination chemistry of phosphine ligands carrying an anionic charge in their ligand framework by Peters has established that the anionic phosphines are more electron-donating than their conventional counterparts.15 We reasoned that the enhanced electrondonating ability can be utilized to weaken the coordination of the chelating hard Lewis base in a zwitterionic analogue of A and potentially activate or eliminate the catalyst resting state.16 To probe this possibility, we designed a ligand bearing an ancillary anion with improved resistance to electrophilic attack of the cationic nickel(II) center.17 We report here the synthesis Received: July 22, 2015
A
DOI: 10.1021/acs.organomet.5b00676 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
when ethylene was introduced before CO (entries 1 vs 2, Table 1) and increased with decreasing CO pressure (entries 2 vs 3). Addition of PPh3 further increased the productivity to some extent (entry 4). The productivity of the catalyst generated in situ from the reaction of 1 and Ni(COD)2 was substantially higher than that of the isolated 2 regardless of the order of introduction of CO and ethylene (entry 5). Alternating copolymers with a melting temperature of 246 °C were produced in all cases without repetitive ethylene units detectable by 1H NMR. Only an ethyl ketone end group was observed in the polyketone produced by the in situ generated catalyst, indicating that a Ni−H species initiated the polymerization. Direct CO or ethylene insertion into the Ni− cyclooctenyl σ-bond was absent. The absence of olefinic resonances in the 1H NMR indicates that chain transfer via βhydrogen elimination is insignificant. The catalytic performances of the in situ generated catalyst were further studied. Again, an increase in CO pressure drastically decreased the catalyst productivity (entries 5 vs 6), while ethylene pressure exerted a comparatively small effect (entry 5 vs entry 7). Under the batch reaction conditions in a 125 mL reactor under 50 psi of CO, the maximum yield was reached at 600 psi of ethylene pressure (entry 5); i.e., the initially charged CO in the reactor was completely incorporated into the product. Repressurizing the reactor again with CO did not further increase the yield appreciably (entry 8), indicating that the catalyst was almost completely deactivated after 2 h. The presence of PPh3 did not increase the yield, either (entry 9). Thus, the best productivity is 1660 g of PK (g of Ni)−1 under the experimental conditions achievable in our laboratory. This value is on the same order of magnitude of the previously reported productivity of Ni catalyst14a and 1 order of magnitude lower than the productivity of Pd catalysts19 in the absence of benzoquinone and excess of acid, which regenerate the Pd(II) active species from deactivated Pd species at low oxidation states. Judging from the high sensitivity of the present catalyst system to CO pressure, the optimal productivity of the present catalyst system should exceed the herein reported value if the reaction were carried out under low CO pressure continuously delivered to the reactor. The most important discovery of this work is the extraordinary initial activity of the present catalyst system. A histogram of activity as a function of reaction time is constructed under the aforementioned best batch conditions (Figure 2). The activities in the first 5 and 15 min observed here are on the same order of magnitude of the activity of the Pd catalyst in the commercial process (∼6000 g (g of Pd)−1 h−1) and the highest activity (8000 g (g of Pd)−1 h−1) in the literature.2,19 However, the catalyst apparently decomposes rapidly and is completely deactivated after 2 h (entry 5 vs entry 8, Table 1). The above experimental data clearly demonstrate that the zwitterionic nickel catalyst is extraordinarily active for the alternating copolymerization of ethylene and CO. The challenge is to overcome the deactivation under practical polymerization conditions. This would require some knowledge of the deactivation process. Decomposition of 2 under CO provides some clues along this line. The solution of 2 is stable for at least 1 week under nitrogen at room temperature but decomposes under 1 atm of CO within minutes (i.e., before a 1 H NMR spectrum can be obtained) to quantitatively give 3 and benzene from protonation of a phenyl group of the borate
of a zwitterionic nickel catalyst for CO−ethylene copolymerization, the extraordinary activity of the in situ generated nickel catalyst with such a ligand, and a likely catalyst-deactivation pathway under carbon monoxide. The anionic diphosphine ligand was synthesized in two steps in a straightforward manner from (2,4-dibromophenyl)diphenylphosphine.18 Reaction of the zwitterionic protonated ligand 1 and Ni(COD)2 under a nitrogen atmosphere resulted in a mixture that appeared to consist mostly of the geometric isomers resulting from formal COD insertion into the Ni−H bond (Scheme 1). Attempts to isolate the products were Scheme 1. Formation and Synthesis of Zwitterionic Ni(II) Complexes
unsuccessful. Upon addition of PPh3 to the mixture, 2 emerged as a major product and was isolated in moderate yield. The resonance for the hydride in 2 was located at δ −9.70 ppm as a pseudopentet as the result of coupling of the three 31P nuclei with 2JP−H = 116, 58, and 58 Hz in the 1H nuclear magnetic resonance (NMR) spectrum. The geometric isomer of 2 was not detected. The structure of 2 was confirmed by X-ray crystallography (Figure 1). The isolated nickel hydride 2 was an effective catalyst for ethylene−CO copolymerization. The productivity was higher
Figure 1. Structure of compound 2 with 35% probability ellipsoids. Hydrogen atom positions other than the metal-bound hydride have been omitted for clarity. The hydride position was observed on the difference map. Selected bond lengths (Å): P1−Ni = 2.1468(8), P2− Ni = 2.1741(7), P3−Ni = 2.1856(8). The crystal is disordered and contaminated by a chloride analogue of 2, with the Ni−H group being replaced by Ni−Cl. The chloride analogue was modeled as a 15% contribution to the overall structure. The Ni−H distance is erroneous for this reason. B
DOI: 10.1021/acs.organomet.5b00676 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Zwitterionic Nickel(II)-Catalyzed Copolymerization of Carbon Monoxide and Ethylenea entry
catalyst
CO (psi)
ethylene (psi)
reacn time (min)
1 2d 3d 4d 5 6e 7e 8f 9f 10 11 12 13
2 2 2 2/PPh3 (1 equiv) Ni(COD)2/1 Ni(COD)2/1 Ni(COD)2/1 Ni(COD)2/1 Ni(COD)2/1/ PPh3 (2 equiv) Ni(COD)2/1 Ni(COD)2/1 Ni(COD)2/1 Ni(COD)2/1
50 50 30 30 50 100 50 50 50 50 50 50 50
600 600 600 600 600 600 200 600 600 600 600 600 600
120 120 120 120 120 960 960 120 × 2 120 × 2 5 15 30 60
yieldb (g) 0.20 0.30 0.41 0.52 1.22 0.24 0.89 1.19 1.24 0.56 0.92 1.03 1.19
productivity (g (g of Ni)‑1)
(0.01)
(0.02) (0.01) (0.02)
260 400 540 680 1620 320 1190 1590 1660 750 1230 1380 1590
Mw/(103 g mol−1)c PDIc
257
3.8
161 192 215 256
3.1 3.2 3.5 4.1
Reaction conditions unless otherwise noted: 0.013 mmol of Ni(COD)2 and 0.019 mmol of 1 at 45 °C in 10 mL of toluene. Carbon monoxide was introduced first unless otherwise noted. bNumbers in parentheses are the standard deviations of yields of two to four runs. Values without parentheses indicate a single run. cMw (weight-average molecular weight) and PDI (polydispersity index) were measured by gel permeation chromatography. dEthylene was introduced before CO. eIn 2 mL of toluene. fThe pressure was released after reaction for 2 h, and then the reactor was repressurized with CO (50 psi) and ethylene and heated to 45 °C for another 2 h. a
ligand framework suggests that future efforts should be directed to ligands bearing anions with further improved resistance to electrophilic attack. In view of the many examples of stable anions that have been previously developed,21,22 there is clearly ample room for improving the productivity of the zwitterionic nickel catalysts.
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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.5b00676. Details of synthesis, spectroscopic characterization, X-ray crystallography, and DSC and GPC analysis (PDF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF)
Figure 2. Histogram of catalytic activities constructed by the yields of timed polymerization runs in entries 5 and 10−13 in Table 1.
(eq 1). The identity of 3 has been confirmed by X-ray crystallography.18 In comparison, the in situ generated catalyst
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AUTHOR INFORMATION
Corresponding Author
*E-mail for L.J.:
[email protected]. Notes
The authors declare no competing financial interest.
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has a half-life of about 15 min and also decomposes to 3 nearly quantitatively.18 The decomposition is undoubtedly related to the increased acidity of the Ni+−H center upon coordination of electron-accepting CO to nickel. The Ni+−acyl intermediate during polymerization is likely less reactive than Ni+−H toward the nucleophilic phenyl on the borate, and hence substantial polymerization can occur before deactivation. However, it is conceivable that a reaction such as the Friedel−Crafts type could eventually take place to destroy the catalytically active center.20 In conclusion, we have demonstrated that the catalytic activity of nickel systems can reach a level comparable to that of the cationic palladium catalysts for alternating CO−ethylene copolymerization. However, the current zwitterionic nickel system suffers from rapid catalyst deactivation and is much less productive than the state of the art palladium catalysts. The observed decomposition of the tetraphenylborate motif in the
ACKNOWLEDGMENTS The research was supported by the National Science Foundation of the USA (CHE-1266442), the CAS/SAFEA International Partner Program for Creative Research Teams, and the National Science Foundation of China. C.J.Z. acknowledges the University of Akron for support of this research.
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REFERENCES
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DOI: 10.1021/acs.organomet.5b00676 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.5b00676 Organometallics XXXX, XXX, XXX−XXX
