Zwitterionic Nickel(II) Catalysts for CO–Ethylene Alternating

Mar 14, 2017 - (6) Both neutral nickel catalysts with anionic N,O- and P,O-chelating ...... (b) Luo , R.; Newsham , D. K.; Sen , A. Organometallics 20...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Organometallics

Zwitterionic Nickel(II) Catalysts for CO−Ethylene Alternating Copolymerization Xiaofei Jia,†,∥ Mengru Zhang,†,∥ Maohua Li,† Fan Pan,‡ Kuiling Ding,‡ Li Jia,*,† Laura A. Crandall,§ James T. Engle,§ 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 new zwitterionic nickel(II) catalyst that comprises a partially fluorinated tetrakis(aryl)borate center in a bidentate phosphine ligand and a cationic Ni center has been developed and studied for CO−ethylene copolymerization in the context of comparison with a previously reported zwitterionic catalyst that carries a nonfluorinated borate anion. The crystal structures of several zwitterionic and related nickel compounds are characterized. Partial fluorination of the tetrakis(aryl)borate only brings a modest increase in productivity (2700 vs 1600 g of polyketone per gram of Ni, g (g of Ni)−1). Like the nonfluorinated catalyst, the fluorinated zwitterionic catalyst is extremely active at the beginning of the polymerization but deactivates rapidly. Deactivation of the two catalysts apparently follows different mechanisms. Stoichometric decomposition studies show that the partially fluorinated tetrakis(aryl)borate in the Ni compounds is stable under acidic conditions either directly introduced by addition of an acid or created by a CO atmosphere. In contrast, the nonfluorinated tetrakis(aryl)borate is readily decomposed by an acid or under acidic conditions created by CO. For the new catalyst system with the partially fluorinated tetrakis(aryl)borate anion, the deactivation likely involves initially redox processes and eventually ligand redistribution around Ni, as inferred from the stoichiometric decomposition studies. It turns out that such a process allows the deactivated catalyst to be reactivated by H2. When the polymerization is carried out in the presence of H2, the productivity of the new zwitterionic catalyst can reach 6400 g (g of Ni)−1. The zwitterionic catalyst with the nonfluorinated tetrakis(aryl)borate anion cannot be reactivated by H2. A cationic analogue of the zwitterionic catalysts is also studied for comparison. Its productivity for CO−ethylene copolymerization (230 g (g of Ni)−1) is about 1 order of magnitude lower than that of the zwitterionic catalysts, demonstrating the critical role of the zwitterionic character in attaining the aforementioned high productivity. At the productivity level of the zwitterionic catalysts, which to our knowledge is among the highest observed for Ni catalysts, an unacceptable amount of residual Ni(II) species is left in the product, causing the alternating CO−ethylene copolymer to begin to decompose near its melting temperature and hence making melt processing difficult.

■

INTRODUCTION

The cationic palladium catalysts in the commercial processes are highly active, with typical activities at ∼6000 (g of PK) (g of Pd)−1 h−1 (grams of polyketone per gram of palladium per hour), and highly productive, with productivities as high as ∼106 (g of PK) (g of Pd)−1 (grams of polyketone per gram of palladium).2 Nickel-based catalysts have been continuously investigated following the initial discovery of CO−ethylene alternating copolymerization by Reppe using K2Ni(CN)4 as the catalyst precursor.6 Both neutral nickel catalysts with anionic N,O- and P,O-chelating ligands7,8 and cationic nickel catalysts with bidentate phosphine ligands9−12 have been studied. The productivities of the best nickel catalysts are on the order of 103 (g of PK) (g of Ni)−1 (grams of polyketone per gram of nickel),

Metal-catalyzed alternating copolymerization of alkenes and carbon monoxide has received significant research attention from both academia and industry.1 The research efforts led to large-scale production of terpolymers of CO, ethylene, and propylene as engineering plastics by Shell and as packaging materials by BP in the 1990s. Although the abrupt cessation of production by Shell and BP significantly dampened research efforts, an ensemble of desirable properties that these materials possess,2,3 the low cost of the comonomers, and the low carbon footprint of the CO comonomer have provided sufficient incentives for continuous commercial and scientific endeavors in this area. Currently, the terpolymers are commercially produced by Hyosung.4 Nonalternating copolymerization has been a recent focus of research.5 © XXXX American Chemical Society

Received: December 15, 2016

A

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

2a,b with Ni(COD)2 under 1 atm of CO afforded the Ni(0) compounds 3a,b, respectively. Both were fully characterized by standard spectroscopic methods and elemental analysis, and 3a was further characterized by X-ray crystallography (Figure 1).

clearly inferior to those of the best palladium catalysts. Experimental13 and computational mechanistic studies14 have been carried out on the fundamental steps involved in the copolymerization for cationic nickel catalysts. We recently reported a zwitterionic nickel catalyst15 bearing the anionic phosphine ligand A.16,17 The zwitterionic nickel catalyst has a very high initial polymerization activity but deactivates quickly. The deactivation is attributable to instability of the tetrakis(aryl)borate moiety in A with respect to electrophilic attack. We provide here an account of continuous development of this class of zwitterionic nickel catalyst, including the design and synthesis of ligand B, the polymerization performance of the catalyst with ligand B, stoichiometric decomposition of the catalyst and related Ni compounds, and deactivation and reactivation of the catalyst during the copolymerization. The necessity of the zwitterionic character for achieving high productivities will be demonstrated by comparison of the zwitterionic catalysts with their cationic analogue.

Figure 1. X-ray single-crystal structure of the anion in 3a with 35% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−C(1) 1.766(4), Ni(1)−C(2) 1.761(4), C(1)−O(1) 1.149(4), C(2)−O(2) 1.140(5), Ni(1)−P(1) 2.1958(10), Ni(1)−P(2) 2.1980(9), C(1)−Ni(1)−C(2) 114.65(17), C(2)−Ni(1)−P(1) 110.74(12), P(1)−Ni(1)−P(2) 88.77(3), C(1)−Ni(1)−P(2) 112.91(12).

■

RESULTS AND DISCUSSION Synthesis of Ligands and Nickel Complexes. Following our initial communication, a primary goal of the ensuing study has been to improve the stability of the borate moiety in the anionic ligand. For this purpose, we initially attempted to replace the phenyl group with perfluorophenyl. Unfortunately, scrambling of the pentafluorophenyl and diphosphinophenyl groups around the boron atom invariably occurred under all reaction conditions that we attempted. We then tried to use 3,4-difluorophenyl (Ar = 3,4-F2C6H3) to replace the phenyl and successfully obtained the corresponding anionic ligand. The synthesis and subsequent access to nickel compounds used in the present study are summarized in Scheme 1. Similar to the synthesis of 2a, lithiation of 1 at −78 °C in THF followed by addition of tris(3,4-difluorophenyl)borane gave the lithium salt of the anionic diphosphine, 2b. Reaction of

The anionic part of 3a exhibits a structure similar to those seen for dicarbonylnickel(0) compounds with bis-phosphine chelates.18 The Ni−P and Ni−C bond lengths in 3a are similar to those in the known structures. For example, dicarbonyl(1,2bis(diisopropylphosphino)ethane)nickel(0) has Ni−P bond lengths of ∼2.21 Å and Ni−C lengths of ∼1.76 Å. The C− Ni−C and P−Ni−P bond angles in 3a are unexceptional and are within the expected ranges. Treatment with 2a,b with HCl in diethyl ether gave the zwitterionic phosphines 4a,b, respectively. Reaction of 4a,b with Ni(COD)2 in the presence of PPh3 afforded the hydrides 5a,b, respectively. Only one geometric isomer was observed and isolated in each case. The crystal structure of 5a, which we reported in our initial communication, allows identification of the specific geometric isomer as shown. Since the 31P−1H and 31 P−31P coupling patterns and coupling constants are nearly the

Scheme 1. Synthesis of Anionic Phosphine Ligands and Nickel Compounds

B

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics same in 5a,b, we assume that the coordination geometries are also the same for 5a,b. In the absence of PPh3, reaction of 4a and 4b with Ni(COD)2 gave intermediates that appeared to be 6a,b, which are the result of formal insertion of COD into a Ni−H bond. The intermediate 6b isomerized to the Ni-(η3-allyl) compound 7, upon standing overnight in solution at room temperature, but 6a decomposed and did not afford any isolable product under the same conditions. The identity of 7 was confirmed by X-ray crystallography (Figure 2) in addition to spectroscopic

equivalent under CO (1 atm). Again, the Ni−(η3-allyl) bond in 8 undergoes rapid dynamic η3 to η1 exchange under CO. The chemical shifts of the η3-allyl remain the same under CO or N2, indicating that the ground states are identical under CO and N2 and degenerate in the dynamic process under CO. Electron-Donating Ability of Bidentate Phosphines. The electron-donating abilities of the bidentate phosphines A, B, and 1 are ranked among themselves and against three reference bidentate phosphines by comparing the frequencies of CO stretching vibrations (νCO) of the corresponding (1,2diphosphinobenzene)Ni(CO)2 congeners. The νCO values are summarized in Table 1. The electron-donating ability of the diphosphine ligands follows the order 1,2-(PPh2)2C6H4 ≈ 1,2(PPh2)2C2H4 < 1 < B ≈ 1,2-(PEt2)2C2H4 < A: i.e., it increases from the left to the right of Table 1. Note that, although 1 is somewhat less electron donating than B, the difference is very modest. Stoichiometric Decomposition of Ni Compounds. In our preliminary communication, we reported that 5a decomposed instantaneously at room temperature in solution upon exposure to 1 atm of CO to give 9 and benzene (Scheme 2). Presumably, upon replacement of PPh3 by CO, the nickel

Figure 2. X-ray single-crystal structures of 7 (a) and the cation in 8 (b) with 35% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): 7, Ni(1)−C(1) 2.100(5), Ni(1)−C(2) 1.985(5), Ni(1)−C(3) 2.087(4), Ni(1)−P(1) 2.1494(6), Ni(1)−P(2) 2.1774(16), C(1)−C(2)−C(3) 122.2(5), P(1)−Ni(1)−P(2) 90.48(6); 8, Ni(1)−C(1) 2.076(4), Ni(1)−C(2) 1.978(3), Ni(1)−C(3) 2.078(4), Ni(1)−P(1) 2.1766(10), Ni(1)−P(2) 2.1460(10), C(1)−C(2)−C(3) 122.9(4), P(1)−Ni(1)−P(2) 90.93(4).

Scheme 2. Stoichiometric Decomposition of Ni Compounds with Anionic Phosphine Ligands

methods and elemental analysis. The isopropyl groups in 7 are chemically inequivalent (i.e., four inequivalent CH3 groups and two CH groups are observed) at room temperature under N2 but become equivalent under 1 atm of CO, as observed by 1H nuclear magnetic resonance (NMR) spectroscopy. This indicates that the η3-allyl structure undergoes rapid dynamic η3 to η1 exchange under CO but not N2. The chemical shifts of the three protons in the allylic group under CO are the same as those under N2, indicating that the ground states remain the degenerate η3-allyl structures. A cationic Ni-allyl compound, 8, was also synthesized in a manner similar to that for the synthesis of 7. The neutral phosphine 1 was first protonated by H(OEt2)2+BArF4−. The protonated ligand was not isolated but was directly brought to react with Ni(COD)2 in one pot. The product, 8, was isolated in good yield (76%) and fully characterized by standard spectroscopic methods and elemental analysis. The structure of 8 was characterized by X-ray diffraction. The Ni−C distances of the Ni−(η3-allyl) bond in 8 (2.076(4), 1.978(3), and 2.078(4) Å) are somewhat shorter than those in 7 (2.087(4), 1.985(5), and 2.100(5) Å), but the Ni−P distances are almost identical in the two structures (2.1494(6) and 2.1774(16) Å vs 2.1460(10) and 2.1766(10) Å in 7 and 8, respectively). Like 7 in solution, the isopropyl groups in 8 are also chemically inequivalent in the 1 H NMR spectrum at room temperature under N2 but become

hydride becomes acidic enough to protonate the phenyl of the borate anion. No reaction occurred when 5b was exposed to 1 atm of CO. This, however, is likely due to the inability of CO to substitute PPh3 in 5b, as can be anticipated from the decreased electron-donating ability of B in comparison to A as discussed above. Indeed, upon addition of B(C6F5)3 to scavenge PPh3 and facilitate coordination of CO to Ni, decomposition of 5b instantaneously took place. Dihydrogen and 10, the structure of which was separately confirmed by X-ray crystallography (Figure 3), were immediately observed by in situ 1H and 31P NMR spectroscopy, respectively. Additional signals of broad line widths likely belonging to some paramagnetic species also arose in the 1H NMR spectrum. Eventually, 10 became the most prominent species among the decomposition products.

Table 1. Comparison of the νCO Values (cm−1) of (1,2-diphosphinobenzene)Ni(CO)2 Congeners compound

Ni(CO)2[1,2-(PPh2)2C2H4]

Ni(CO)2[1,2-(PPh2)2C6H4]

1·Ni(CO)2

3b

Ni(CO)2[1,2-(PEt2)2C6H4]

3a

νCO (cm−1)

2000, 1940

1999, 1939

1993, 1930

1988, 1927

1986, 1925

1978, 1910

C

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

for the alternating CO−ethylene copolymerization has already been reported in our preliminary communication.15 The findings will be discussed in the context of comparison with the new catalyst based on ligand B. As in the communication, the in situ generated catalyst precursor is again used here to evaluate the new catalyst. In all studies using the in situ generated catalyst, 6b was generated by reaction of 1.5 equiv of 2b with Ni(COD)2: that is, a 0.5 equiv excess of 2b was present during or at least at the beginning of the copolymerization. The same excess amount of ligand was present in the previous study using 6a as the catalyst. The isolated catalysts 7 and 5b were selectively used to augment the studies using the in situ generated catalyst 6b. All polymerization results are summarized in Table 2, including those using the cationic catalyst 8. All polymerizations were carried out at 45 °C in toluene. The effect of CO and ethylene pressure on the polymerization was examined first (entries 1−5, Table 2). The productivity of 6b increased from 50 to 300 psi of CO and then decreased upon further increasing the pressure to 500 psi (entries 2−5). This is in contrast to 6a, the productivity of which sharply declines upon increasing the CO pressure from 50 to 100 psi.15 As in the case of 6a, ethylene pressure only exerted a small effect on the polymerization.19 The optimal productivity of 6a is 1600 g (g of Ni−1), and that of 6b is about 1.5 times higher (entry 5). The productivity of the isolated catalyst 7 (entry 6) is also higher than that of 6a, although it is somewhat lower than that of the in situ generated 6b. A histogram of the polymerization activity of 6b was constructed from the yields of timed polymerization runs (Table 2, entries 7−10) and is compared to that of 6a in Figure 5. 6a,b both displayed an initial activity comparable to or better than that of a typical cationic Pd catalyst, but both polymerizations essentially stopped in 30−60 min. The higher productivity of 6b is mostly attributable to its higher activity in comparison to that of 6a rather than longevity. This is disappointing, as the stoichiometric decomposition studies suggest that the borate in 6b is resistant to strong acids and stable under CO. Interestingly, 6b can be reactivated by H2 after complete deactivation (entry 11). In contrast, 6a cannot be reactivated by H2. This difference confirms that their deactivation mechanisms are different, as suggested by the stoichiometric decomposition studies. Further, inclusion of H2 at the beginning of the polymerization catalyzed by 6b also resulted in an appreciably improved yield (entry 12). PPh3 significantly inhibited the polymerization catalyzed by 6b (Table 2, entry 13). Similarly, 5b was only able to produce a minute amount of polyketone (entry 14). The productivity was 1−2 orders of magnitude lower than that in the absence of PPh3. In contrast, the presence of PPh3 did not significantly affect the productivity of 5a. This likely reflects the difference in electron-donating ability of the ligand and hence the Lewis acidity at the cationic Ni(II) center. The strong coordination of PPh3 to the strongly Lewis acidic Ni(II) suppresses the rate of the polymerization and consequently the productivity. The zwitterionic catalysts in the present study are at least 10 times more productive than typical cationic bidentate phosphine-Ni catalysts in the literature,9,10 barring a few examples with a specific type of bidentate phosphine ligand,11,12 which will be discussed later. A critical question is whether the zwitterionic character or the specific 1-(diphenylphosphino)-2(diisopropylphosphino)benzene framework is responsible for the improved catalytic performance. To probe this question, we carried out the CO−ethylene copolymerization using both the

Figure 3. X-ray single-crystal structure of 10 with 35% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−P(1) 2.2365(16), Ni(1)−P(2) 2.2554(15), Ni(1)−P(3) 2.2322(15), Ni(1)−P(4) 2.2553(16), P(1)−Ni(1)−P(2) 86.00(6), P(2)−Ni(1)−P(3) 94.79(6), P(3)−Ni(1)−P(4) 86.04(6), P(1)−Ni(1)−P(4) 95.51(6).

No products from protonation of the borate moiety were detected. The in situ generated 6a also readily decomposed under 1 atm of CO to give 9 and benzene, accompanied by loss of COD. Exposure of 6b to 1 atm of CO merely slowed down its isomerization to 7. Once formed, 7 was stable at room temperature under CO. Prolonged refluxing of 6b and 7 for weeks under CO gave 10 and other unidentified species. Refluxing under nitrogen also gave 10 as the only recognizable product in a complex mixture. Reaction of 3a with 1 equiv of HCl in diethyl ether also gave 9 and benzene, again showing the instability of the borate in A. Reaction of 3b with 2 equiv of HCl gave 11 quantitatively in THF. The structure of 11 was crystallographically characterized (Figure 4). The reaction also afforded H2 as the byproduct, as revealed by in situ 1H NMR. Overall, Ni(0) is oxidized to Ni(II), and two protons are reduced to H2.

Figure 4. X-ray single crystal structure of 11 with 35% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−Cl(1) 2.2220(11), Ni(1)−Cl(2) 2.2113(13), Ni(1)−P(1) 2.1424(12), Ni(1)−P(2) 2.1588(11), P(1)−Ni(1)−P(2) 87.53(4), P(2)−Ni(1)−Cl(2) 91.39(4), P(1)−Ni(1)−Cl(1) 91.22(4), Cl(2)−Ni(1)−Cl(1) 91.70(5).

The above studies suggest that the borate moiety in B is no longer the most reactive site in the Ni compounds. Alternative decomposition pathways begin to manifest themselves when the borate is stable. The observation of paramagnetic intermediates and production of H2 suggest that redox reactions are initially involved in the decomposition of the zwitterionic Ni(II) compounds with ligand B. Formation of 10 as the result of redistribution of the anionic phosphine ligand appears to be the eventual destiny with or without the presence of CO. Polymerization Activity and Productivity. The catalytic performance of the zwitterionic catalyst having anionic ligand A D

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Table 2. Summary of Nickel(II)-Catalyzed Copolymerization of Carbon Monoxide and Ethylenea entry b

1 2b 3b 4b 5b 6c 7b 8b 9b 10b 11d 12e 13 14c 15f 16f 17f 18c

catalyst 6b 6b 6b 6b 6b 7 6b 6b 6b 6b 6b 6b 6b/PPh3 (1 equiv) 5b Ni(COD)2/1/ H(OEt2)2+BArF4− Ni(COD)2/1/ H(OEt2)2+BArF4− Ni(COD)2/1/ H(OEt2)2+BArF4− 8

CO (psi)

ethylene (psi)

50 50 100 300 500 300 300 300 300 300 300 300 300 300 50

600 300 300 300 300 300 300 300 300 300 300 300 300 300 300

300 300 300

H2 (psi)

reaction time (min)

yield (g)

productivity (g (g of Ni−1))

120 120 120 120 120 120 5 15 30 60 120 × 2 120 120 120 240

1.20 1.08 1.51 2.05 0.50 1.35 0.97 1.40 2.04 2.05 4.22 4.80 0.06 0.02 0.07

1600 1460 2020 2740 675 1800 1290 1870 2720 2720 5620 6430 80 27 95

300

240

0.11

146

600

240

0.17

227

300

240

0.07

95

400 400

Mw (103 g mol−1)g

PDIg

155 129 91.2

3.3 5.4 4.1

130 114 121 126 49.1 33.6

3.4 3.2 2.9 3.2 3.0 2.3

Reaction conditions: 45 °C in 10 mL of toluene. In situ generated from 0.013 mmol of Ni(COD)2 and 0.019 mmol of 2b. c0.013 mmol of the isolated catalyst. dThe polymerization was carried out in the absence of H2 for 2 h. Then, H2 (400 psi) was pressed into the reactor, and the reaction was allowed to continue for another 2 h. eH2 was introduced at the beginning of the polymerization. fIn situ generated from 0.013 mmol of Ni(COD)2 and 0.019 mmol of 1. gMw (weight-average molecular weight) and PDI (polydispersity index) were measured by gel permeation chromatography. a

b

Figure 5. Histograms of catalytic activities of 6a (gray) and 6b (red).

Figure 6. 1H NMR spectrum of ethylene−CO copolymer produced by 6b from entry 4, Table 2. The solvent is CDCl3/TFA-d in a 10/1 volume ratio.

isolated cationic catalyst 8 and in situ generated cationic catalyst by reaction of 1, Ni(COD)2, and H(OEt2)2+BArF4−. Both the in situ generated cationic catalyst and 8 were decisively less productive under various pressures in comparison to those of the zwitterionic catalysts (Table 2, entries 15− 18). It is therefore of little doubt that the zwitterionic character is critical for the high productivity. Initiation, Propagation, Chain Transfer, and Chain Termination. The products of the ethylene−CO copolymerization in this study are strictly alternating polyketones. The 1H NMR spectrum of a representative polyketone sample is shown in Figure 6. All polyketones produced by the catalysts in this work and the previous communication possess an ethyl ketone end group, indicating that the polymerization is initiated by insertion of ethylene into a Ni−H bond. The Ni−H species obviously arise from β-hydrogen elimination of the cyclooctenyl group for 6a,b, 7, and 8. The following discussions are based on the investigation of polymers and polymerizations catalyzed by 6a,b although their applicability should not be limited to the specific initial form of the catalyst.

The molecular weights of the CO−ethylene copolymers produced by 6a,b (number-average molecular weight, Mn = 40000−60000 g mol−1) are higher than the typical molecular weight of those produced by cationic Pd catalysts in the commercial processes (Mn = ∼20000 g mol−1). Although the higher molecular weight may cause problems for melt processing,20 introduction of H2 to the copolymerization catalyzed by 6b reduces the molecular weight to as low as Mn = 15000 g mol−1 with an increase in the productivity at the same time. The polydispersity indices (PDIs) are significantly greater than 2, except in the case where H2 was introduced at the beginning of the polymerization catalyzed by 6b. Except for the ethyl ketone end group arising from chain initiation, no end groups attributable to chain transfer or chain termination are detectable in the 1H NMR spectrum of the products produced by 6a. For 6b, vinyl ketone from chain transfer via hydrogen elimination is observed (Figure 6). The E

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

low-molecular-weight fraction grows considerably between the first and the second hour (entry 10 vs entry 4), during which time the polymerization already has stopped (see Figure 5). The decrease in the molecular weight can be explained if a few assumptions are accepted. First, the higher molecular weight fraction is due to bimolecular chain combination that results in polymer chains twice the length of the two individual chains as the result of the catalytic chain propagation.21 Second, catalyst deactivation has two stages, and chain combination occurs predominantly during workup of the polymerization in the ambient environment after the initial but before the final stage of catalyst deactivation. Third, if the final-stage catalyst deactivation is given time to occur, chain combination no longer occurs. Consistent with the above, the molecular weight of the product from entry 11 is nearly unimodal, since the product is exposed to H2 before it is exposed to the ambient environment. The fact that H2 can reinitiate the polymerization after initial deactivation also suggests that the deactivation process has at least two stages. The dominance of the ethyl ketone end group is also explainable by the occurrence of bimolecular chain combination. Overall, the mechanisms for chain initiation, propagation, and transfer (or lack of for 6a) are straightforward for the two catalyst systems. However, the mechanisms for chain termination/catalyst deactivation are not. The results from stoichiometric decomposition studies of 5b, 6b, and 7 suggest that the partially fluorinated borate moiety is robust. We believe that redox processes and ligand redistribution observed in the stoichiometric decomposition studies are relevant to the deactivation of the catalyst with ligand B under catalytic polymerization conditions.22 Similarly as discussed in our previous communication, we believe that instability of the nonfluorinated borate observed in stoichiometric studies is responsible for the deactivation of the catalyst with ligand A. It is not immediately evident how these processes manifest themselves under the polymerization conditions in either case. However, we speculate that, in both cases, an initial deactivation analogous to the reactivities observed in the stoichiometric decomposition studies induces the bimolecular chain combination. Poisoning of the oxophilic Ni(II) catalyst by the polyketone product through formation of Ni−oxo bonds is unlikely to be responsible for the observed deactivation in the present catalyst system, since the decomposed catalyst can be reactivated by H2 under mild conditions. In general, the affinity of Ni(II) for electronegative elements does not appear to be a problem for CO−ethylene copolymerization, since the best performances of cationic Ni catalysts are obtained in methanol solution.10,11 Current Status of Ni Catalysts. Although Ni is generally known to be inferior to Pd for CO−olefin alternating copolymerization, the literature surprisingly lacks concrete discussions on the specific problems of Ni catalysts. A rudimentary question to ask at the onset of the discussion is whether the issue with nickel catalysts is turnover frequency (i.e., activity) or turnover number (i.e., productivity). The present study clearly shows that the problem for the zwitterionic Ni catalysts is not slow polymerization kinetics but lifetime of the catalyst. In the patent literature, Drent disclosed that the cationic Ni catalyst with a special bidentate ligand, 1,2-bis[bis(2methoxylphenyl)phosphino]ethane,11 displays a productivity of 5085 g (g of Ni) −1 for ethylene-CO alternating copolymerization, 30 times higher than that of the catalyst

molar ratio of ethyl ketone and vinyl ketone is about 2:1 for the polymerization that is allowed to proceed to complete deactivation. If we assume that all chains have an ethyl ketone group at one end, vinyl ketone can only account for half of the end group at the other end. The other half must be due to chain termination/catalyst deactivation. However, we were unable to identify any residual structure that can be reasonably attributed to catalyst deactivation. The only option remaining to account for the missing end groups for the polyketones produced by both 6a and 6b is bimolecular chain termination that gives ethyl ketone at both ends of the polymer chain. The trends of molecular weights and molecular weight distributions of polyketones produced by 6a are not out of usual expectations. As shown in Figure 7, the molecular weight

Figure 7. Molecular weight distribution of polyketones produced by 6a plotted as weight fraction vs log (molecular weight). The samples are the products of polymerizations run for various durations as indicated in the legend, with the corresponding entry number as in Table 1 of ref 15.

distributions are unimodal. The molecular weight moves to higher values as the polymerization proceeds until about 1 h after the onset of the polymerization. The molecular weights of the products obtained after 1 and 2 h are identical, not surprisingly, since polymerization is no longer taking place during this period of time. In contrast, the polyketones produced by 6b show bimodal molecular weight distribution unless H2 is introduced (Figure 8). The molecular weight does not change appreciably during the first 1 h of the polymerization (entries 7−10, Table 2), presumably because β-hydrogen elimination is at least partially operative as a chain transfer mechanism. Very surprisingly, the

Figure 8. Molecular weight distribution of polyketones produced by 6b plotted as weight fraction vs log (molecular weight). The samples are from various polymerization runs summarized in Table 2, as indicated by the entry numbers. F

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

temperature decomposition returned. Therefore, the lowtemperature decomposition is caused by the Ni(II) species from catalyst residuals. At the current productivity level of Ni catalysts, the polyketone product contains an unacceptable amount of Ni(II) contaminants, which diminish the meltprocessing window. Clearly, the goal for development of Ni catalysts is to increase the productivity of the catalyst to a level such that the level of Ni(II) contaminant in the polyketone product is so low that the extent of decomposition of polyketone is negligible under melt-processing conditions. The progress can be preliminarily evaluated in a synthetic laboratory using a combination of TGA and DSC measurements without access to melt extruders.

with a common ligand such as 1,2-bis(diphenylphosphino)ethane. The activity of the catalyst is ∼25000 g (g of Ni)−1 h−1 in the first 7.2 min of the polymerization but also quickly decays. This initial activity is also higher than that of a typical cationic Pd catalyst. In a separate patent, a few other ligands having the bis(o-methoxyphenyl)phosphino moiety are shown to have similar productivities.12 Again, the activity of the best cationic Ni catalyst is not the problem, but the productivity is. Kläui reported a series of neutral Ni catalysts that displayed very high productivities for alternating CO−ethylene copolymerization.8 Although some conflicting values were reported, the productivity appears in the range of 3000−11000 (g of PK) (g of Ni)−1. The catalyst remains active for a few hours, but no polymer forms in the first 45 min under copolymerization conditions. It should be pointed out that the high productivity of the palladium catalyst system is in no small part attributable to an excess amount of strong acids and quinones present in the catalyst system. The strong acid and quinone reactivate the deactivated palladium species during the polymerization. This method cannot be applied to nickel systems. Challenge for Ni Catalysts. The productivities of the three best Ni catalyst systems discussed above are 2−3 orders of magnitude lower than that of the cationic Pd catalyst used in commercial processes. The price of Pd is 500−2200 times of that of Ni by weight in the last 5 years.23 On the basis of the cost of metal, the best Ni catalysts are already more competitive than the Pd catalysts. However, cost is likely not the limiting factor for nickel catalysts at this point. Rather, as shown in Figure 9, we observed that decomposition of the polyketone

■

SUMMARY The stability of the borate moiety in the anionic bidentate phosphine has been improved by replacing the B-Ph groups in A with the B-(3,4-F2C6H3) groups in B. The zwitterionic Ni catalyst based on the improved ligand, 6b, shows a modest gain in productivity for CO−ethylene copolymerization in comparison to the previous catalyst, 6a. Zwitterionic catalysts 6a,b are both more than 1 order of magnitude more productive than their cationic analogue 8, demonstrating that the zwitterionic character is critical. The new zwitterionic Ni catalyst 6b still deactivates quickly under polymerization conditions, but H2 can reactivate the deactivated catalyst and partially reinstall the activity of polymerization. The observed reactivation by H2 and the stoichiometric decomposition studies of the Ni compounds with ligand B suggest that deactivation of the new catalyst follows a mechanism different from that of the previous zwitterionic catalyst 6a. Redox processes and ligand redistribution are the possible culprits. The challenge for Ni-catalyzed alternating CO−ethylene copolymerization is catalyst lifetime, not catalytic activity. At the productivity level of the currently best Ni catalysts, an unacceptable amount of catalyst residual is left in the product, causing a decomposition process close to the melting temperature of the polyketone. The immediate goal of research in this area should be developing a catalyst that is productive enough such that decomposition near the melting temperature becomes negligible.

Figure 9. (a) TGA of polyketone from entry 4, Table 2: () without purification; (- - -) purified twice by dissolving in HFIPA and precipitating with chloroform; (···) with Ni(OAc)2 added to the purified copolymer. Weight loss below 100 °C is attributed to loss of HFIPA used for purification of the polyketone and addition of Ni(OAc)2. (b) DSC of the same polyketone: () first heating− cooling cycle, Tm = 254 °C; (- - -) second heating−cooling cycle, Tm = 237 °C.

■

EXPERIMENTAL SECTION

All manipulations were performed in a nitrogen-filled glovebox or using standard Schlenk techniques. Solvents were purchased from Sigma-Aldrich and dried using an MBraun solvent purification system. All other chemicals were purchased from Sigma-Aldrich or VWR. Syntheses of 1, 2a−4a, and 9 and were previously reported.15 H(OEt2)2+BArF4− was synthesized following a procedure in the literature.24 1 H, 13C, 31P, and 19F NMR spectra were recorded on a Varian Mercury 300 MHz spectrometer or a Varian Avance 500 spectrometer. Chemical shifts were determined using solvents as references for 1H NMR and 13C NMR, external 85% H3PO4 for 31P NMR, and hexafluorobenzene for 19F NMR. High-resolution mass spectrometry was performed on a Waters Micromass GCT Premier and a Thermo Fisher Scientific LTQ FT Ultra instrument. IR spectra were measured on an Excalibur Diglab instrument. Elemental analyses were performed with an Elementar VARIO EL apparatus or by Micro Analysis, Inc. GPC analyses were performed by Smithers Rapra & Smithers Pira Limited. Briefly, hexafluoroisopropyl alcohol was used as the eluent. The column temperature was at 40 °C. A refractive index detector was used. The molecular weights of the polyketones were determined relative to poly(methyl methacrylate) standards.

produced by the zwitterionic Ni catalysts occurred at a much lower temperature than the inherent decomposition temperature of a pure polyketone, which is well above 300 °C.3 At the melting temperature of the alternating CO−ethylene copolymer (Tm = 254 °C) according to differential scanning calorimetry (DSC), weight loss under nitrogen is about 2% according to thermogravimetric analysis (TGA). In addition, the melting temperature of the copolymer on the second heating ramp of DSC decreased to 237 °C, indicating that some decomposition had occurred. These observations suggest that the polymer will be difficult to melt-process. After the polyketone was repetitively dissolved in hexafluoroisopropyl alcohol and precipitated with chloroform, the low-temperature decomposition progressively lessened. When nickel acetate was added to the purified polyketone, the lowG

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics DSC experiments were performed on a TA Instruments Model Q2000 instrument with Advantage software version 2.8.0.382. Samples between 10.00 and 18.00 mg were loaded in aluminum pans. Experiments were performed under a nitrogen atmosphere. A temperature ramp rate of 10 °C/min was used. TGA analysis was performed on a TA Instruments Model Q500 instrument. Analysis was carried out by loading the sample on a platinum tray and performing the measurement under a nitrogen atmosphere; all samples were heated at a ramp rate of 10 °C/min to 600 °C. Synthesis of Tris(3,4-difluorophenyl)borane. Magnesium turnings (3.7 g, 156 mmol) and I2 (50 mg) were placed in a three-neck flask. 4-Bromo-1,2-difluorobenzene (11.7 mL, 104 mmol) was dissolved in Et2O (80 mL) in an addition funnel attached to the three-neck flask. One-eighth of the solution was added to the threeneck flask, and the solution was heated to reflux in an oil bath. After the reaction began, the oil bath was removed. The remaining solution of 4-bromo-1,2-difluorobenzene was added dropwise at a rate that maintained the reflux. After the addition, the reaction mixture was refluxed for another 1 h. Then, the reaction mixture was cooled to −78 °C, and BF3 (4.6 mL, 46.7% in Et2O, 34.5 mmol) was added via a syringe. The reaction mixture was warmed to room temperature overnight. The solvent was removed under vacuum. The residual solid was sublimed to give a white solid (7.2 g, 60% yield). 1H NMR (CDCl3): δ (ppm) 7.36−7.21 (m, 9H). 13C{1H} NMR (CDCl3): δ (ppm) 153.3 (dd, 1JFC = 267.8 Hz, 2JFC = 13 Hz, m-B(C6H3F2)), 150.2 (dd, 1JFC = 249.3 Hz, 2JFC = 12 Hz, p-B(C6H3F2)), 139.2 (br, iB(C6H3F2)), 135.3 (dd, 2JFC = 7.0 Hz, 3JFC = 3 Hz, m- or oB(C6H3F2)), 126.7 (dd, 2JFC = 14 Hz, 3JFC = 2 Hz, m- or oB(C6H3F2)), 117.2 (d, 3JFC = 17 Hz, o-B(C6H3F2)). 19F NMR (CDCl3): δ (ppm) −131.4 (m), −138.2 (m). HRMS (MALDI): m/z calcd for C18H9F610B 349.0738 (M)+, found 349.0742. Anal. Calcd for C18H9BF6: C, 61.76; H, 2.59. Found: C, 61.97; H, 2.74. Synthesis of 2b. Compound 1 (0.92 g, 2.0 mmol) was dissolved in THF (20 mL) and cooled to −78 °C. n-BuLi (0.8 mL, 2.5 M in hexane, 2.0 mmol) was added via a syringe. The solution was stirred for 1 h. Solid tris(3,4-difluorophenyl)borane (0.7 g, 2.0 mmol) was added to the above solution. The reaction mixture was warmed to room temperature overnight. The solvent was removed to give a pinkish foamy solid. DME (5 mL) was used to dissolve the solid, and the mixture was stirred for 10 min. After DME was removed under vacuum, Et2O (5 mL) was added to dissolve the solid. The solution was allowed to sit at −45 °C for 3 days to give a crystalline white solid. The product was collected after filtration (1.6 g, 83% yield). 1H NMR (CDCl3): δ (ppm) 7.52 (m, 1H, o-B(C6H3P2)), 7.25 (m, 10H, ArH), 7.14 (m, 1H, ArH), 7.02 (m, 6H, ArH), 6.89 (m, 3H, ArH), 6.70 (m, 1H, ArH), 3.52 (s, 12H, CH3OCH2), 3.36 (s, 18H, CH3OCH2), 1.94 (m, 2H, PCH(CH3)2), 1.07 (dd, 3JPH = 15.0 Hz, 3JHH = 7.5 Hz, 6H, PCH(CH3)2), 0.75 (dd, 3JPH = 15.0 Hz, 3JHH = 7.5 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (CDCl3, 125 MHz): δ (ppm) 161.5 (q, 1 JBC = 48 Hz, i-B(C6H3P2)), 159.7 (q, 1JBC = 50 Hz, i-B(C6H3F2)), 149.6 (d, 1JFC = 245 Hz, m-B(C6H3F2)), 146.8 (dd, 1JFC = 237.5 Hz, 2 JFC = 12 Hz, p-B(C6H3F2)), 141.2 (s, o-B(C6H3P2)), 139.5 (dd, 1JPC = 14 Hz, 2JPC = 6 Hz, m- or p- B(C6H3P2)), 137.7 (d, 1JPC = 31.8 Hz, por m-B(C6H3P2)), 135.7 (s, m-B(C6H3P2)), 134.0 (d, 1J = 18 Hz, iP(C6H5)2), 131.7 (s, o-P(C6H5)2), 130.9 (s, m-P(C6H5)2), 128.2 (d, 3 JFC = 5 Hz, o-B(C6H3F2)), 127.8 (d, 2JFC = 6 Hz, o-B(C6H3F2)), 127.5 (s, p-P(C6H5)2), 122.7 (d, 2JPC = 12 Hz, o-B(C6H3P2)), 114.0 (dd, 2JFC = 15 Hz, 3JFC = 3.8 Hz, m-B(C6H3F2)), 70.6 (s, CH3OCH2), 59.2 (s, CH3OCH2), 24.5 (dd, 1JPC = 13.8 Hz, 3JPC = 4 Hz, PCH(CH3)2), 20.2 (d, 2JPC = 17.5 Hz, PCH(CH3)2), 20.2 (d, 2JPC = 10 Hz, PCH(CH3)2)) ppm. 31P{1H} NMR (202 MHz, CDCl3): δ −2.1 (d, J = 159 Hz), −13.5 (d, J = 159 Hz). 19F NMR (CDCl3): δ −144.8 (m), −149.2 (m). HRMS (MALDI): m/z calcd for C42H3610BF6P2 726.2320 [M − Li(DME)3]+, found 726.2322. Anal. Calcd for C54H66BF6LiO6P2: C, 64.55; H, 6.62. Found: C, 64.48; H, 6.37. Synthesis of 3a. Ligand 2a (451 mg, 0.50 mmol) and Ni(COD)2 (138 mg, 0.50 mmol) were dissolved in DME (10 mL). The mixture was sitirred overnight under CO at room temperature. Et2O (25 mL) was added to the yellow solution, and a copious amount of yellow precipitate formed. The mixture was heated to reflux to redissolve the

solid. Large yellow crystals were obtained upon standing overnight. The product (420 mg, 83%) was collected after filtration. 1H NMR (CDCl3, 300 MHz): δ (ppm) 7.81 (m, 1H), 7.45−7.40 (m, 12H), 7.26 (m, 6H), 7.05 (t, J = 7.5 Hz, 6H), 6.94 (t, J = 7.5 Hz, 3H), 3.41 (s, 12H), 3.29 (s, 18H), 2.25 (m, 2H), 1.05 (dd, 3JPH = 16.5 Hz, 3JHH = 6.9 Hz, 6H), 0.72 (dd, 3JPH = 14.4 Hz, 3JHH = 6.9 Hz, 6H). 13C{1H} NMR (CDCl3): δ (ppm) 184.2 (s, Ni(CO)2), 172.5 (b, i-B(C6H3P2)), 162.2 (q, 1JBC = 50 Hz, i-B(C6H5)3), 143.5 (s, o-B(C6H3P2)), 141.8 (d, 1 JPC = 28, 2JPC = 6 Hz, m- or p-B(C6H3P2)), 140.4 (d, 2JPC = 11 Hz, mor o-B(C6H3P2)), 137.1 (dd, 1JPC = 38 Hz, 2JPC = 7 Hz, m- or oB(C6H3P2)),), 135.0 (s, o-B(C6H5)3), 133.6 (d, 2JPC = 14 Hz, oP(C6H5)2), 131.4 (d, 2JPC = 11 Hz, m- or o-B(C6H3P2)), 128.2 (s, pP(C6H5)2), 127.8 (d, 3JPC = 9 Hz, m-P(C6H5)2), 125.7 (d, 3JBC = 3 Hz, m-B(C6H5)3), 122.4 (s, p-B(C6H5)3), 71.2 (s, -O(CH2)2O−), 59.2 (s, CH3O−), 27.6 (d, 1JPC = 16 Hz, PCH(CH3)2), 19.3 (s, PCH(CH3)2), 18.2 (s, PCH(CH3)2). 31P{1H} NMR (CDCl3): δ (ppm) 74.5 (d, J = 39 Hz), 46.0 (d, J = 39 Hz). Anal. Calcd for C56H72BLiNiO8P2: C, 66.49; H, 7.17. Found: C, 66.09; H, 7.31. Synthesis of 3b. Ligand 2b (502.3 mg, 0.5 mmol) and Ni(COD)2 (137.5 mg, 0.5 mmol) were dissolved in DME (20 mL). The mixture was sitirred overnight under CO at room temperature. Solvent was removed, and Et2O (10 mL) was added to dissolve the mixture. The white solid was crystallized from diethyl ether at −45 °C and collected after filtration (482 mg, 86%). 1H NMR (CDCl3, 500 MHz): δ (ppm) 7.61 (s, 1H, o-B(C6H3P2)), 7.45−7.41 (m, 4H, ArH), 7.29−7.25 (m, 7H, ArH), 7.13−7.11 (m, 1H, ArH), 6.99−6.89 (m, 9H, ArH), 3.55 (s, 12H, CH3OCH2), 3.39 (s, 18H, CH3OCH2), 2.25−2.20 (m, 2H, PCH(CH3)2), 1.07 (dd, 3JPH = 20 Hz, 3JHH = 6.5 Hz, 6H, PCH(CH3)2), 0.72 (dd, 3JPH = 15 Hz, 3JHH = 7 Hz, 6H, PCH(CH3)2) ppm. 13C{1H} NMR (CDCl3, 125 MHz): δ (ppm) 203.2 (s, Ni(CO)2), 164.3 (q, 1JBC = 53.8 Hz, i-B(C6H3P2)), 159.2 (q, 1JBC = 48.2 Hz, i-B(C6H3F2)), 149.1 (d, 1JFC = 245.8 Hz, m-B(C6H3F2)), 146.8 (dd, 1JFC = 238.5 Hz, 2JFC = 24.3 Hz, p-B(C6H3F2)), 140.4 (dd, 1 JPC = 40.1 Hz, 2JPC = 35 Hz, m- or p- B(C6H3P2)), 139.1 (dd, 1JPC = 18.8 Hz, 2JPC = 6.4 Hz, m- or p- B(C6H3P2)), 138.9 (s, o-B(C6H3P2)), 138.3 (dd, 1JPC = 43.8 Hz, 2JPC = 43.8 Hz, o-B(C6H3P2)), 136.8 (s, mB(C6H3P2)), 132.2 (d, 1JPC = 14.3 Hz, i-P(C6H5)2), 131.3 (d, J = 10.0 Hz, o-P(C6H5)2), 130.9 (s, m-P(C6H5)2), 128.3 (s, p-P(C6H5)2), 127.8 (d, 2JFC = 15.3 Hz, m-B(C6H3F2)), 122.7 (d, 3JFC = 12.4 Hz, oB(C6H3F2)), 114.2 (d, 2JFC = 15.3 Hz, o-B(C6H3F2)), 70.4 (s, CH3OCH2), 59.3 (s, CH3OCH2), 26.8 (dd, 1JPC = 15 Hz, 3JPC = 5.1 Hz, PCH(CH3)2), 19.8 (d, 2JPC = 12.5 Hz, PCH(CH3)2), 18.6 (d, 2JPC = 12.5 Hz, PCH(CH3)2) ppm, 31P{1H} NMR (CDCl3): δ (ppm) 74.1 (d, J = 27 Hz), 46.0 (d, J = 27 Hz). 19F NMR (CDCl3): δ (ppm) −144.8 (m), −148.8 (m). Anal. Calcd for C56H66BF6LiNiO8P2: C, 60.08; H, 5.94. Found: C, 59.77; H, 5.88. Synthesis of 4b. Compound 2b (2.0 g, 2.0 mmol) was dissolved in CHCl3 (20 mL) and cooled to −78 °C. HCl (2.0 mL, 1.0 M in Et2O, 2.0 mmol) was added via a syringe to the above solution. A white precipitate formed immediately. The reaction mixture was warmed to room temperature and stirred for 1 h. The precipitate was removed by filtration through Celite. The volume of the chloroform solution was reduced to 10 mL, and hexane (50 mL) was added to the solution to precipitate the product. The product was isolated as a white solid after filtration (1.2 g, 85% yield). 1H NMR (CDCl3): δ (ppm) 7.78 (m, 1H, o-B(C6H3P2)), 7.49−7.39 (m, 7H, ArH), 7.28− 7.24 (m, 4H, ArH), 7.12−7.11 (m, 1H, ArH), 6.98−7.90 (m, 9H, ArH), 6.47−6.46 (m, 1H, P−H−P), 2.69−2.66 (m, 2H, PCH(CH3)2), 1.33 (dd, 3JPH = 19.0 Hz, 3JHH = 7.5 Hz, 6H, PCH(CH3)2), 1.05 (dd, 3 JPH = 19.0 Hz, 3JHH = 7.5 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (CDCl3): δ (ppm) 168.0 (q, 1JBC = 42.9 Hz, i-B(C6H3P2)), 157.2 (q, 1 JBC = 48.1 Hz, i-B(C6H3F2)), 149.4 (d, 1JFC = 242.5 Hz, mB(C6H3F2)), 147.3 (dd, 1JFC = 240 Hz, 2JFC = 13.8 Hz, p-B(C6H3F2)), 142.5 (s, o-B(C6H3P2)), 135.6 (d, 1JPC = 11.3 Hz, m- or pB(C6H3P2)), 135.5 (d, 1JPC = 8.8 Hz, m- or p- B(C6H3P2)), 134.6 (d, 1JPC = 7.5 Hz, o-B(C6H3P2)), 133.8 (d, 1JPC = 13.1 Hz, i-P(C6H5)2), 133.7 (d, 2JPC = 17.9 Hz, o-P(C6H5)2), 130.6 (s, m-P(C6H5)2), 129.8 (s, p-P(C6H5)2), 129.1 (d, 2JFC = 6.9 Hz, o-B(C6H3F2)), 122.7 (s, mB(C6H3P2)), 122.6 (s, o-B(C6H3F2)), 114.8 (dd, 2JFC = 14.4 Hz, 3JFC = H

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(THF-d8): δ (ppm) 75.6 (d, J = 5.3 Hz), 52.8 (d, J = 5.3 Hz) ppm. 19F NMR (THF-d8): δ (ppm) −144.7 (m), −149.4 (m). Anal. Calcd for C50H49BF6NiP2: C, 67.07; H, 5.52. Found: C, 67.33; H, 5.52. Synthesis of 8. Ligand 1 (326 mg, 0.713 mmol) and [H(OEt2)2]+B[3,5-(CF3)2C6H3]4− (720 mg, 0.713 mmol) were mixed in benzene to give a two-layer liquid. The liquid was transferred into a Schlenk flask containing Ni(COD)2 (196 mg, 0.713 mmol). The mixture was stirred for 2 h at room temperature. Another two-layered liquid formed. Benzene was removed. The foamy solid was dissolved in chloroform. The chloroform solution was filtered through Celite. Pentane was layered on top of the yellow solution. A crystalline yellow precipitate formed after a few days. The solid product (890 mg, 84%) was collected after filtration. 1H NMR (CDCl3): δ (ppm) 7.88 (dt, J = 1.8 and 4.8 Hz, 1H), 7.74 (dq, J = 1.5 and 8.4 Hz, 1H), 7.69 (m, 10H), 7.54−7.26 (m, 13H), 5.38 (m, 1H), 5.15 (t, J = 8.4 Hz, 1H), 5.00 (“p” or dt, J = 8.4 and 8.4 Hz, 1H), 2.58 (m, 2H), 2.33 (m, 1H), 1.76−0.92 (m, 9H), 1.30 (dd, J = 6.9 and 15.3 Hz, 6H), 1.23 (dd, J = 6.9 and 24.6 Hz, 3H), 1.06 (dd, J = 6.9 and 18.0 Hz, 3H), 0.77 (dd, J = 6.9 and 17.7 Hz, 3H). 13C{1H} NMR (CDCl3): δ (ppm) 161.7 (q, JBC = 50 Hz, iB(C6H3(CF3)2)), 141.4 (dd, 1JPC and 2JPC = 30 and 49 Hz, m- or pBr(C6H3P2)), 139.5 (dd, 1JPC and 2JPC = 36 and 39 Hz, m- or pBrC6H3P2), 135.5 (dd, 2JFC = 7 Hz, 3JFC = 1 Hz, o- or m-Br(C6H3P2)), 133.7 (bs, overlapping o- and p-B(C6H3(CF3)2)), 133.5 (dd, 2JPC = 12 Hz, 3JPC = 2 Hz, o- or m-Br(C6H3P2)), 133.2 (d, 1JPC = 15 Hz, two overlapping i-P(C6H5)2), 131.6 (d, 2JPC = 12 Hz, o-P(C6H5)2), 131.5 (d, 3JPC = 2 Hz, m-P(C6H5)2), 131.2 (d, 2JPC = 11 Hz, o-P(C6H5)2), 131.0 (d, 3JPC = 2 Hz, m-P(C6H5)2), 129.0 (s, i-Br(C6H3P2)), 128.7 (dd, 2JPC = 10 and 12 Hz, η3-C3H3(C5H10)), 128.1 (d, 3JPC = 2 Hz, o-Br(C6H3P2)), 127.9 (dd, 2JFC = 28 Hz, 3JFC = 3 Hz, mB(C6H3(CF3)2)), 124.5 (q, 1JFC = 272 Hz, BC6H3(CF3)2), 117.4 (s, p-P(C6H5)2), 112.4(s, p-P(C6H5)2), 83.9 (d, JPC = 16 Hz, η3C3H3(C5H10)), 78.2 (d, JPC = 18 Hz, η3-C3H3(C5H10)), 31.1 (dd, 1JPC = 144, 3JPC = 2 Hz, PCH(CH3)2), 27.5 (dd, 1JPC = 124, 3JPC = 3 Hz, PCH(CH3)2), 26.5 (s, η3-C3H3(C5H10)), 26.3(s, η3C 3 H 3 (C 5 H 10 )), 26.2 (s, η 3 -C 3 H 3 (C 5 H 10 )), 26.0 (s, η 3 C3H3(C5H10)), 22.1 (s, η3-C3H3(C5H10)), 19.8 (d, 2JPC = 4 Hz, PCH(CH3)2), 19.2 (d, 2JPC = 4 Hz, PCH(CH3)2),17.9 (d, 2JPC = 5 Hz, PCH(CH3)2), 17.3 (d, 2JPC = 5 Hz, PCH(CH3)2). 31P{1H} NMR (CDCl3): δ (ppm) 80.1 (d, J = 8 Hz), 55.0 (d, J = 8 Hz). 19F NMR (CDCl3): δ (ppm) −64.2 (s). Anal. Calcd for C64H52BBrF24NiP2: C, 51.64; H, 3.52. Found: C, 51.02; H, 3.40. Synthesis of 11. Compound 3b (224 mg, 0.2 mmol) was dissolved in THF (10 mL) and cooled to −78 °C. HCl (0.4 mL, 1.0 M in Et2O, 0.4 mmol) was added via a syringe. Gas bubbles were immediately observed, and the colorless solution turned red. The reaction mixture was warmed to room temperature and stirred for 2 h. The volume of the solution was reduced to 5 mL, and pentane (10 mL) was layered on top of the solution. Red crystals formed in about 1 week at −25 °C. The crystals were collected after filtration (147 mg, 86%). 1H NMR (THF-d8): δ (ppm) 7.79 (t, J = 2.4 Hz, 1H), 7.59− 7.56 (m, 2H), 7.51−7.43 (m, 9H), 6.94−6.79 (m, 10H), 2.59 (m, 2H), 1.24 (dd, J = 6.9 and 24.0 Hz, 6H), 0.76 (dd, J = 6.6 and 17.1 Hz, 6H). 13 C{1H} NMR (THF-d8): δ (ppm) 162.7 (q, 1JBC = 49 Hz, iB(C6H3F2)), 161.0 (q, 1JBC = 42 Hz, i-B(C6H3P2)), 152.9 (dd, 1JFC = 231 Hz, 2JFC = 10 Hz, m- or p-B(C6H3F2)), 150.6 (dd, 1JFC = 239 Hz, 2 JFC = 13 Hz, m- or p-B(C6H3F2)), 143.7 (dd, 1JPC = 25 Hz, 2JPC = 15 Hz, m- or p-B(C6H3P2)), 139.5 (dd, 1JPC = 19 Hz, 2JPC = 15 Hz, m- or p-B(C6H3P2)), 136.8 (d, 2JPC = 14 Hz, o-P(C6H5)2), 136.2 (d, 2JPC = 14 Hz, o- or m-B(C6H3P2)), 133.0 (bs, o-B(C6H3F2)), 133.0 (d, 2JPC = 25 Hz, o- or m-B(C6H3P2)), 132.7 (dd, 2JFC = 14 Hz, 3JFC = 10 Hz, mor o-B(C6H3F2)), 126.2 (d, 3JPC = 6 Hz, o-B(C6H3P2)), 124.8 (d, 2JPC = 12 Hz, i-P(C6H5)), 117.7 (d, 2JFC = 9 Hz, m- or o-B(C6H3F2)), 117.5 (d, 3JPC = 7 Hz, m-P(C6H5)2), 115.8 (s, p-P(C6H5)2), 33.5 (d, 1JPC = 105 Hz, PCH(CH3)2), 22.5 (d, 2JPC = 3 Hz, PCH(CH3)2), 20.6 (d, 2 JPC = 4 Hz, PCH(CH3)2). 31P{1H} NMR (THF-d8): δ (ppm) 77.5 (d, J = 27 Hz), 55.3 (d, J = 27 Hz). 19F NMR (CDCl3, 470 MHz): δ (ppm) −144.7 (m), −147.3 (m). Anal. Calcd for C50H52BCl2F6LiNiO2P2: C, 59.59; H, 5.20. Found: C, 59.13; H, 4.96. Synthesis of 10. Ligand 4b (145 mg, 0.20 mmol) and Ni(COD)2 (55 mg, 0.20 mmol) were dissolved in DME (5 mL) at room

4.3 Hz, m-B(C6H3F2)), 22.2 (dd, 1JPC = 42.5 Hz, 3JPC = 4.3 Hz, PCH(CH3)2), 17.9 (s, PCH(CH3)2), 16.8 (s, PCH(CH3)2). 31P{1H} NMR (CDCl3): δ (ppm) 26.7 (br, s), −14.6 (d, J = 67.5 Hz). 19F NMR (CDCl3, 470 MHz): δ (ppm) −142.8 (m), −147.3 (m). HRMS (MALDI): m/z calcd for C42H3610BF6P2 726.2320 (M − H)+, found 726.2324. Anal. Calcd for C42H37BF6P2: C, 69.25; H, 5.12. Found: C, 69.58; H, 5.05. Synthesis of 5b. PPh3 (524 mg, 2.0 mmol) was dissolved in toluene (10 mL), and Ni(COD)2 (137.5 mg, 0.5 mmol) was added to the toluene solution at room temperature. The reaction mixture was stirred for 10 min. COD and toluene were removed under vacuum. 4b (364 mg, 0.5 mmol) and toluene (30 mL) were added at room temperature. The solution was stirred for 10 min. Hexane (40 mL) was added to the deep red toluene solution to precipitate the product. Et2O (2 × 5 mL) was used to washed the crude product. A red powder was obtained (300 mg, 57% yield). 1H NMR (THF-d8): δ (ppm) 7.79−7.57 (m, 3H, ArH), 7.43−7.42 (m, 7H, ArH), 7.28−7.22 (m, 15H, ArH), 7.08−7.04 (m, 3H, ArH), 6.92−6.82 (m, 9H, ArH), 2.64− 2.52 (m, 2H, PCH(CH3)2), 1.15 (dd, 3JPH = 17.5 Hz, 3JHH = 6.0 Hz, 6H, PCH(CH3)2), 0.96 (dd, 3JPH = 15.5 Hz, 3JHH = 6.0 Hz, 6H, PCH(CH3)2), −9.90 (“p”, 2JPH = 116.0, 58.0, and 58.0 Hz, 1H, NiH). 13 C{1H} NMR (THF-d8): δ (ppm) 170.7 (q, 1JBC = 31.9 Hz, iB(C6H3P2)), 159.1 (q, 1JBC = 51.5 Hz, i-B(C6H3F2)), 149.6 (d, 1JFC = 249.6 Hz, m-B(C6H3F2)), 147.6 (dd, 1JFC = 239.4 Hz, 2JFC = 12.6 Hz, p-B(C6H3F2)), 141.3 (s, o-B(C6H3P2)), 140.7 (d, 1JPC = 18.1 Hz, m- or p- B(C6H3P2)), 134.5 (d, 1JPC = 11.3 Hz, m- or p- B(C6H3P2)), 133.9 (d, 1JPC = 12.5 Hz, i-P(C6H5)2), 132.9 (s, o-P(C6H5)2), 132.6 (s, mP(C6H5)2), 132.3 (s, p-P(C6H5)2), 131.8 (s, o-B(C6H3P2)), 131.7 (s, m-B(C6H3P2)), 129.9 (d, 3JFC = 10 Hz, o-B(C6H3F2), 129.7 (d, 1JPC = 10 Hz, i-P(C6H5)3), 128.6 (s, o-P(C6H5)3), 128.5 (s, m-P(C6H5)3), 128.3 (s, p-P(C6H5)3), 123.5 (d, 2JFC = 12.1 Hz, o-B(C6H3F2)), 115.0 (d, 2JFC = 14.5 Hz, m-B(C6H3F2)), 27.9 (d, 1JPC = 32.5 Hz, PCH(CH3)2), 20.2 (s, PCH(CH3)2), 19.2 (s, PCH(CH3)2). 31P{1H} NMR (THF-d8, 202 MHz): δ (ppm) 92.6−91.5 (m), 53.9−53.7 (m), 33.8−32.7 (m). 19F NMR (CDCl3, 470 MHz): δ (ppm) −142.7 (m), −147.4 (m). Anal. Calcd for C60H52BF6NiP3: C, 68.67; H, 4.99. Found: C, 67.93; H, 4.96. Synthesis of 7. Ligand 4b (218.4 mg, 0.3 mmol) was dissolved in benzene (5 mL) at room temperature, and Ni(COD)2 (82.5 mg, 0.3 mmol) was added to the benzene solution. The solution quickly turned red in a few minutes. Benzene was removed, and Et2O (5 mL) was added to dissolve the mixture. A yellow solid (185.1 mg, 69%) was crystallized from diethyl ether at −45 °C. 1H NMR (THF-d8): δ (ppm) 7.82−7.79 (s, 1H, o-B(C6H3P2)), 7.62−7.56 (s, 1H, ArH), 7.51−7.43 (m, 11H, ArH), 6.94−6.80 (m, 9H, ArH), 5.42−5.36 (m, 1H, η 3 -C 3 H 3 =(C 5 H 10 )), 5.30−5.26 (t, J = 5 Hz, 1H, η 3 C3H3=(C5H10)), 5.20−4.95 (m, 1H, η3-C3H3(C5H10)), 2.63−2.53 (m, 2H, PCH(CH3)2), 2.43−2.38 (m, 1H, η3-C3H3(C5H10)), 1.89− 1.84 (m, 1H, η 3 -C 3 H 3 (C 5 H 10 )), 1.65−1.33 (m, 8H, η 3 C3H3(C5H10)), 1.12 (dd, 3JPH = 15 Hz, 3JHH = 5 Hz, 3H, PCH(CH3)2), 1.04−0.96 (m, 6H, PCH(CH3)2), 0.79−0.74 (dd, 3JPH = 15 Hz, 3JHH = 5 Hz, 3H, PCH(CH3)2) ppm. 13C{1H} NMR (THFd8): δ ppm 169.8 (q, 1JBC = 49.4 Hz, i-B(C6H3P2)), 158.2 (q, 1JBC = 50.9 Hz, i-B(C6H3F2)), 149.2 (d, 1JFC = 249.6 Hz, m-B(C6H3F2)), 147.1 (dd, 1JFC = 239.8 Hz, 2JFC = 13 Hz, p-B(C6H3F2)), 140.0 (d, 1JPC = 43.6 Hz, m- or p- B(C6H3P2)), 139.8 (s, o-B(C6H3P2)), 135.9 (d, 1 JPC = 30 Hz, m- or p- B(C6H3P2)), 135.6 (d, 1JPC = 11.9 Hz, iP(C6H5)2), 133.2 (d, 2JPC = 12.5 Hz, m-B(C6H3P2)), 133.6 (d, 1JPC = 11.9 Hz, o-P(C6H5)2), 131.9 (d, 3JPC = 1.3 Hz, o-B(C6H3P2)), 131.0 (dd, 2JFC = 70.4 Hz, 3JFC = 2.9 Hz, o-B(C6H3F2)), 130.8 (s, mP(C 6 H 5 ) 2 ), 128.9 (dd, 2 J PC = 10 Hz, 2 J PC = 12 Hz, η 3 C3H3(C5H10)), 122.6 (d, 3JFC = 15.3 Hz, o-B(C6H3F2)), 114.1 (dd, 2JFC = 14.5 Hz, 3JFC = 4.0 Hz, m-B(C6H3F2)), 112.1 (s, pP(C6H5)2), 81.9 (d, 2JPC = 15.3 Hz, η3-C3H3(C5H10)), 76.3 (d, 2JPC = 18 Hz, η3-C3H3(C5H10)), 30.9 (d, 1JPC = 106.3 Hz, PCH(CH3)2), 27.9 (d, 1JPC = 121.3 Hz, PCH(CH3)2), 26.1 (s, η3-C3H3(C5H10)), 25.9 (s, η3-C3H3(C5H10)), 25.8 (s, η3-C3H3(C5H10)), 25.7 (s, η3C3H3(C5H10)), 24.8 (s, η3-C3H3(C5H10)), 19.4 (d, 2JPC = 3.8 Hz, PCH(CH3)2), 18.3 (d, 2JPC = 3.6 Hz, PCH(CH3)2),17.6 (d, 2JPC = 4.1 Hz, PCH(CH3)2), 16.4 (d, 2JPC = 4 Hz, PCH(CH3)2). 31P{1H} NMR I

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Organometallics

■

temperature under CO (1 atm). The orange solution was refluxed for 2 weeks, during which time it became yellow. DME was then removed. The residual yellow solid was washed with chloroform at 0 °C. The yellow powdery product was isolated after filtration (47 mg, 31%). 1H NMR (CD2Cl2): δ (ppm) 7.73 (t, J = 2.4 Hz, 2H), 7.46−7.32 (m, 4H), 37.49 (m, 16H), 7.29 (m, 2H), 6.90 (m, 20H), 1.76 (m, 4H), 0.70 (dd, 3JPH = 18.9 Hz, 3JHH = 7.5 Hz, 6H), 0.59 (dd, 3JPH = 15.3 Hz, 3 JHH = 8.1 Hz, 6H). 31P{1H} NMR (CDCl3): δ (ppm) 72.2 (t, 3JPP = 39 Hz), 53.9 (t, 3JPP = 39 Hz). 19F NMR (CDCl3): δ (ppm) −143.6 (m), −148.0 (m). 13C{1H} NMR was not obtained because 10 is poorly soluble in common solvents. An analytically pure sample was obtained by recrystallization from acetone. Anal. Calcd for C84H72B2F12NiP4: C, 66.65; H, 4.79. Found: C, 66.53; H, 5.12. Synthesis of 1·Ni(CO)2. Ligand 1 (230 mg, 0.503 mmol) and Ni(COD)2 (140 mg, 0.509 mg) were loaded in a Schlenk flask and dissolved in THF (10 mL). The solution was exposed to CO (1 atm) and stirred for 2 h at room temperature. THF was removed, and hexane was added to dissolve the pink solid. Crystallization at −45 °C gave nearly colorless crystals (240 mg, 63%). 1H NMR (C6D6): δ (ppm) 7.40 (t, J = 1.8 Hz, 1H), 7.24−7.17 (m, 5H), 6.84−6.74 (m, 6H), 1.65−1.56 (m, 2H), 0.75 (dd, J = 6.6 and 21.6 Hz, 6H), 0.37 (dd, J = 7.2 and 14.7 Hz, 6H). 13C{1H} NMR (C6D6): δ (ppm) 201.3 (t, J = 4 Hz), 148.0 (dd, 1JPC and 2JPC = 26 and 46 Hz, m- or pBr(C6H3P2)), 145.6 (“t”, 1JPC = 2JPC = 38 Hz, m- or p-Br(C6H3P2)), 137.3 (dd, 2JPC = 28 Hz, 3JPC = 7 Hz, m- or o-Br(C6H3P2)), 134.9 (d, 1 JPC = 11 Hz, i-P(C6H5)2), 133.0 (d, 1JPC = 13 Hz, o-P(C6H5)2), 132.8 (d, 3J = 2 Hz, o-Br(C6H3P2)), 132.0 (d, 3JPC = 14 Hz, m-P(C6H5)2), 128.9 (s, p-P(C6H5)2), 128.4 (d, 2JPC = 10 Hz, m- or o-Br(C6H3P2)), 125.2 (s, i-Br(C6H3P2)), 27.3 (dd, 1JPC = 14, 3JPC = 6 Hz, PCH(CH3)2), 19.5 (d, 2JPC = 11 Hz, PCH(CH3)2), 18.8 (d, 2JPC = 4 Hz, PCH(CH3)2). 31P{1H} NMR (C6D6): δ (ppm) 56.2 (d, J = 42 Hz), 25.6 (d, J = 42 Hz). Anal. Calcd for C26H17BrNiO2P2: C, 54.59; H, 4.76. Found: C, 55.13; H, 5.23. X-ray Crystallographic Study of 3a, 7, 8, 10, and 11. Single crystals of 3a were obtained directly from the synthesis (see above). Single crystals of 7 were obtained by slow diffusion of pentane into a THF solution of 7 at −25 °C. Single crystals of 8 were obtained by slow diffusion of pentane into an ethanol solution of 8 at room temperature. Single crystals of 10 were obtained by slow diffusion of methanol into a DME solution of 10 at room temperature. Single crystals of 11 were obtained by slow diffusion of pentane into a THF solution of 11 at −25 °C. X-ray intensity data were measured on a Bruker CCD-based diffractometer with dual Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 A, Mo Kα radiation, λ = 0.71073 A). Crystals were mounted on a cryoloop using Paratone oil and placed under a steam of nitrogen at 100 K (Oxford Cryosystems). The data were corrected for absorption with the SADABS program. The structures were refined using the Bruker SHELXTL software package (Version 6.1) and were solved using direct methods until the final anisotropic full-matrix, least-squares refinement of F2 converged. Polymerization Catalyzed by in Situ Generated 6b. Ni(COD)2 (3.3 mg, 0.012 mmol) and 4b (13.1 mg, 0.018 mmol) were charged in a 125 mL Parr high-pressure reactor in a glovebox. The reactor was connected to a Schlenk line and exposed to 1 atm of CO. Under a steady flow of CO, toluene (10 mL) was added into the reactor. Next, the reactor was pressurized with CO and ethylene to the pressures specified in Table 2 and placed in a preheated oil bath at 45 °C. The reaction mixture was stirred with a magnetic stirrer and the reaction stopped by releasing the pressure after a period of time specified in Table 2. The polyketone product was isolated by suction filtration and dried in a vacuum oven overnight. A white powder was obtained and weighed. The yields are given in Table 2. Polymerization Catalyzed by 5b, 7, or 8. The isolated catalyst (0.012 mmol) was introduced into a 125 mL Parr high-pressure reactor in a glovebox. All subsequent maneuvers were identical with those described above for in situ generated catalyst 6b.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00932. 1 H, 13C{1H}, 31P{1H}, and 19F NMR spectra of all new compounds (PDF) X-ray crystallographic data for of 3a, 7, 8, 10, and 11 (CIF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail for L.J.: [email protected]. *E-mail for C.J.Z.: [email protected]. ORCID

Kuiling Ding: 0000-0003-4074-1981 Li Jia: 0000-0002-1648-1465 Christopher J. Ziegler: 0000-0002-0142-5161 Author Contributions ∥

X.J. and M.Z. made equal contributions to the work. Their names are in alphabetical order. Notes

The authors declare no competing financial interest.

■

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. We thank Dr. Eite Drent for helpful discussions and Mr. Yu Sun for his assistance in compiling the NMR spectra for the Supporting Information.

■

REFERENCES

(1) For reviews, see: (a) Sen, A. Acc. Chem. Res. 1993, 26, 303−310. (b) Drent, E.; Budzelaar, P. H. M. Chem. Rev. 1996, 96, 663−681. (c) Bianchini, C.; Lee, H. M. Coord. Chem. Rev. 2002, 225, 35−66. (d) García Suárez, E. J.; Godard, C.; Ruiz, A.; Claver, C. Eur. J. Inorg. Chem. 2007, 2007, 2582−2593. (e) Brulé, E.; Guo, J.; Coates, G. W.; Thomas, C. M. Macromol. Rapid Commun. 2011, 32, 169−185. (2) Ash, C. E.; Flood, J. E. Polymeric materials; Science and engineering. Proceedings of the ACS Division of Polymeric Materials 1997, 76, 110−114. (3) Sommazzi, A.; Garbassi, F. Prog. Polym. Sci. 1997, 22, 1547−1605. (4) http://www.plasticstoday.com/articles/polyketone-makes-acomeback-141023, accessed on December 1, 2016. (5) (a) Chen, C.; Anselment, T. M.; Frohlich, R.; Rieger, B.; Kehr, G.; Erker, G. Organometallics 2011, 30, 5248−5257. (b) Luo, R.; Newsham, D. K.; Sen, A. Organometallics 2009, 28, 6994−7000. (c) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I. Chem. Commun. 2002, 964−965. (d) Haras, A.; Michalak, A.; Rieger, B.; Ziegler, T. J. Am. Chem. Soc. 2005, 127, 8765−8774. (6) Reppe, W.; Magin, A. U.S. Patent 2,577,208, 1951. (7) (a) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123−134. (b) Klabunde, U.; Tulip, T. H.; Roe, D. C.; Ittel, S. D. J. Organomet. Chem. 1987, 334, 141−156. (8) (a) Beckmann, U.; Eichberger, E.; Rufińska, A.; Sablong, R.; Kläui, W. J. Catal. 2011, 283, 143−148. (b) Kläui, W.; Bongards, J.; Reiß, G. J. Angew. Chem., Int. Ed. 2000, 39, 3894−3896. (9) Leone, A.; Consiglio, G. Helv. Chim. Acta 2006, 89, 2720−2727. (10) Drent, E. U.S. Patent 4835250, 1989. (11) Drent, E.; De Kock, M. C. T. U.S. Patent 5688909, 1997. J

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (12) Buijink, J. K. F.; Drent, E.; Suykerbuyk, J. C. L. WO Patent 2000009584, 2000. (13) (a) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 9172−9173. (b) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001, 20, 16−18. (14) (a) Svensson, M.; Matsubara, T.; Morokuma, K. Organometallics 1996, 15, 5568−5576. (b) De Angelis, F.; Sgamellotti, A. Organometallics 2002, 21, 2036−2040. (15) Jia, X.; Zhang, M.; Pan, F.; Babahan, I.; Ding, K.; Jia, L.; Crandall, L. A.; Ziegler, C. J. Organometallics 2015, 34, 4798−4801. (16) A zwitterionic palladium catalyst for CO−ethylene copolymerization has been previously reported: Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100−5101. (17) Anionic phosphine ligands have been extensively studied. See: Peters, J. C.; Thomas, J. C.; Thomas, C. M.; Betley, T. A. ACS Symp. Ser. 2004, 885, 334−354 and references therein. (18) (a) González-Sebastián, L.; Flores-Alamo, M.; García, J. J. Organometallics 2013, 32, 7186−7194. (b) Amaike, K.; Muto, K.; Yamaguchi, J.; Itami, K. J. Am. Chem. Soc. 2012, 134, 13573−13576. (c) Ariyananda, P. W. G.; Kieber-Emmons, M. T.; Yap, G. P. A.; Riordan, C. G. Dalton Trans. 2009, 4359−4369. (19) The productivity is 1400 g (g of Ni−1) under 100 psi of ethylene and 100 psi of CO under otherwise identical conditions. (20) Private communication with Dr. Eit Drent. (21) For an example of bimolecular chain combination that results in doubling the molecular weight in a Ni-catalyzed polymerization, see Synthesis of poly(3-hexylthiophene) with a narrower polydispersity: Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. Macromol. Rapid Commun. 2004, 25, 1663−1666. (22) Deactivation through ligand redistribution has been investigated and demonstrated for neutral Ni alkene polymerization catalysts: (a) Waltman, A. W.; Younkin, T. R.; Grubbs, R. H. Organometallics 2004, 23, 5121−5123. (b) Berkefeld, A.; Mecking, S. J. Am. Chem. Soc. 2009, 131, 1565−1574. (c) Hu, X.; Dai, S.; Chen, C. Dalton Trans. 2016, 45, 1496−1503. (23) http://www.infomine.com/investment/metal-prices. (24) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920−3922.

K

DOI: 10.1021/acs.organomet.6b00932 Organometallics XXXX, XXX, XXX−XXX