Bispentiptycenyl–Diimine–Nickel Complexes for Ethene

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Bispentiptycenyl−Diimine−Nickel Complexes for Ethene Polymerization and Copolymerization with Polar Monomers Yuki Kanai, Sabine Foro, and Herbert Plenio* Organometallic Chemistry, TU Darmstadt, Alarich-Weiss-Strasse 12, 64287 Darmstadt, Germany

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S Supporting Information *

ABSTRACT: Ni 2+ coordinated within a bowl-shaped diimine ligand with two pentiptycenyl-substituents [(diimine)NiBr2] displays excellent activity for the polymerization of ethene (7 atm) with activities of up to 34 × 103 kg(mol Ni)−1 h−1 following activation with Et2AlCl. The resulting polymer is characterized by high molecular weights (Mn = 150 × 103 g·mol−1), low branching (12/1000 C), and a high melting point (Tm = 133 °C). The polymerization of ethene with polar comonomers leads to the formation of the respective polar polymers with very efficient incorporation of comonomer. The activity of the catalyst critically depends on the molar ratio of Et2AlCl activator and the polar functional group.

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narrow polar comonomer scope,43 low degree of polar group incorporation,44−46 low catalyst activities,47 or low-molecularweight polymers.13,48 Overcoming the deficits of nickel polymerization catalysts with polar comonomers is important, since nickel is much less expensive than palladium, and several strategies have been tested:49 (a) blocking the axial coordination sites at nickel with bulky ligands, to decrease the rate of chain transfer leads to increased polymer molecular weights;50,51 (b) electron-donating substituents at the ligand may lower the oxophilicity of a Ni2+ site;52,53 (c) addition of coordinating bases may compete successfully with the coordination of the polar side chain to the Ni2+ and can liberate the active center;50,54,55 (d) a second-sphere strategy for such polymerization reactions, recently reported by Chen et al., in which weak donor groups are contained in the ligand.34,56 The combination of these approaches led to a SHOP-type phosphine sulfonato nickel catalysts with excellent polar group tolerance, but it seems difficult to combine high polymer masses with high polar comonomer incorporation.57 Recent publications from Coates et al.22 and later Gao et al.58 report significant progress in the efficient polymerization of methyl 10-undecenoate with diimine-nickel complexes, utilizing bulky ligands and large amounts of cocatalyst. We recently reported the synthesis of several N-iptycenyldiimines, NHC-ligands, and the respective metal complexes possessing unique steric properties.59−62 Pentiptycene-substituted NHC-ligands were shown to be excellent catalysts for the alternating ring-opening polymerization (aROMP) of norbornene and cyclooctene62 and gold-catalyzed alkyne hydration.63 Chiral bistriptycene−NHC−copper complexes were employed in the enantioselective borylation of α,βunsaturated esters. We now wish to report on the synthesis and the application of the very bulky bispentiptycene-based diimine

INTRODUCTION Polyolefins such as polyethene (PE) and polypropene (PP) accounted for almost half of the 335 million tons of the global plastics production in 2016.1−3 This huge economic success is mainly based on advances in catalyst design, reaction engineering and polyolefin processing.4−8 Following Zieglertype catalysts for ethene polymerization,9 the utilization of diimine metal (M2+ = Fe, Co, Ni, Pd) complexes for ethene polymerization, first by Brookhart10 and later by others,11−16 led to a flurry of activities in ethene polymerization. A significant number of different diimine metal complexes (Chart 1, complexes 1a,17−19 1b,20 1c,21 1d,22 1e,23 1f,24 and others)25 as well as other bidentate ligands4 has been utilized in ethene polymerization, aiming for increased catalytic activity, high polymer mass, controlled degree of branching, low polydispersity, or certain materials properties such as high melting points.26 The properties of polyolefins can be enhanced by the introduction of polar groups into the polymer. Even a small amount of polar comonomer significantly improves adhesion and wettability of PE as well as the compatibility with other materials.27 One approach (among others)28,29 to obtain functional group containing polyolefins is the copolymerization of ethene with polar monomers, which is considered to be one of the remaining “holy grails” of polyolefin synthesis.12 However, the pronounced oxophilicity of early transition metals, which can form excellent catalysts for ethene polymerization, is incompatible with most polar monomers.30 The utilization of diimine complexes with late transition metals opened the doors for the copolymerization of ethene with polar monomers, and catalysts based on group 10 metals (Ni, Pd) appear to be superior to those based on group 8 and 9 metals.31,32 Significant progress with palladium based catalysts has been reported.24,33−41 Due to the higher oxophilicity of nickel relative to palladium, progress in the copolymerization of polar monomers remains difficult;42 typical limitations are © XXXX American Chemical Society

Received: November 14, 2018

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DOI: 10.1021/acs.organomet.8b00836 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1. Diimine Ligands Utilized in Highly Active Ethene Polymerization Catalysts

nickel complexes for the homopolymerization of ethene and the copolymerization with polar monomers.

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RESULTS AND DISCUSSION Nickel Complexes with Pentiptycenyl Diimine. The synthesis of nickel diimine complexes 3a and 3b utilizes diimines 2a and 2b and [NiBr2(dme)] (Scheme 1, Figure 1). Scheme 1. Synthesis of Bispentiptycenyl−Diimine−NiBr2 Complexes 3a and 3ba

Figure 1. X-ray crystal structure66 of [(2a)NiBr(H2O)3Br] (or trihydrate of complex 3b) (the O-butyl groups are omitted for clarity). ORTEP and van der Waals plot; color coding: Ni green, Br yellow, O red, and N blue. Important bond lengths: Ni−N 213.1(3), 212.2(4), Ni−Br 246.61(7), Ni−O 206.1(4), 206.1(3), 207.5(3) pm.

nickel by the pentiptycene wings. On the basis of the analysis of the available space in the axial positions, the restricted geometry of the bowl-shaped ligand offers space only for very small ligands. One evidence for this is the slight tilt of Br−Ni− O axis such that the larger ligand (bromide) is further removed from the bottom plane (defined by the two N-aryl groups) of the ligand. As a consequence, the distance from the bridgehead hydrogens to the axial ligands is ca. 35 pm shorter to oxygen than it is to bromide. The binding of functional groups contained in the polar monomers appears to be less facile with a view to the steric restrictions. The two pentiptycene units form a bowl-shaped unit with the nickel center located at the bottom of this bowl−shielded from the environment. The pronounced steric bulk of the ligand can be described via the buried volume V(bur) = 45.3%, whose value is comparable to the V(bur) of diimine 1a (R = iPr).65 However, with a view to the very different catalytic activities of respective nickel complexes 3 and 4, it seems that the V(bur) is not a helpful descriptor. The backbone substituents of the diimines inhibit the rotation of the two pentiptycenyl groups around the N−

a

R, R = Me2 or naphthalene-1,8-diyl. Reagents and conditions: (a) [NiBr2(dme)], CH2Cl2, rt, 24 h.

The desired brown complexes are obtained in excellent yields according to the established procedure.10 Both complexes are paramagnetic. The 1H NMR spectrum of 3b is severely broadened (ν1/2 ca. 50 Hz), while the 1H and 13C spectra of 3a display reasonably sharp NMR peaks (ν1/2 2−15 Hz). Single crystals of nickel diimine complex 3a were grown from acetone/n-hexane. The nickel atom displays an unusual octahedral coordination sphere composed of two diimine nitrogen atoms, one bromide, and three water.64 The second bromide is not coordinated to nickel. However, the three water molecules facially bonded to nickel are linked to this second bromide via three hydrogen bonds. The existence of an octahedral complex shows that the axial positions are available to (small monatomic) ligands despite the shielding of the B

DOI: 10.1021/acs.organomet.8b00836 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Ethene Polymerization Data with 3a and 3b Activated by Et2AlCla entry

p (atm)

1 2 3 4 5 6 7 8 9 10

1 1 1 1 1 1 1 1 1 1

3a 3a 3a 3a 3a 3b 3b 3b 3b 3b

11 12 13 14 15 16 17 18

7 7 7 7 7 7 7 7

3a 3a 3a 3a 3b 3b 3b 3b

cat. (μmol) 10 10 10 10 10 10 10 10 10 10 0.2 0.2 0.2 0.2 0.05 0.05 0.05g 0.05

Et2AlCl (eq)

T (°C)

yield (g)b

100 100 100 100 100 100 100 100 100 100

0 25 40 60 80 0 25 40 60 80

2.05 2.06 2.22 1.50 0.76 1.60 1.72 1.65 1.38 0.32

500 500 500 500 1000 1000 1000 1000

25 40 60 80 25 40 60 80

0.74 1.53 0.82 0.31 0.65 0.82 0.85 0.09

activity (kg(mol Ni)−1 h−1)b

Mn (kg mol−1)c

PDI

Tm (°C)d

B/1000 Ce

410 412 444 300 152 320 344 330 276 64

8.4 3.8 4.8 3.3 1.7 103.0 30.5 26.2 17.4 7.7

4.7 3.3 3.1 2.8 2.6 3.2 3.3 2.9 2.8 2.8

130 112 113 90 f 118 99 86 62 f

28 38 59 65 68 23 37 42 66 73

7400 15300 8200 3100 26000 32800 34000 3600

22.1 12.5 11.7 11.2 148.6 138.4 117.4 57.9

3.2 3.4 3.2 4.0 3.0 3.2 2.6 2.5

139 135 134 133 133 131 119 116

36 23 24 17 21 12 12 18

a

Reaction conditions: precatalyst, Et2AlCl, toluene (50 mL), 30 min; bAverage value of two experiments; cDetermined by GPC in 1,2,4trichlorobenzene at 150 °C with PE standard; dMelting temperature was determined by differential scanning calorimetry (DSC, second heating); e Numbers of branches per 1000 carbons, as determined by 1H NMR spectrometry; fAmorphous polymer; gFor this experiment a rare earth stirring bar was used, to prevent sticky PE precipitates from blocking the stirrer

C(pentiptycenyl) bond. Consequently, the approach of the ligand CH bonds to the catalytically active center is less likely, leading to slower catalyst deactivation. These structural factors may explain the observed high polymerization activities. Ethene Polymerization. Complexes 3a and 3b activated with Et2AlCl cocatalyst show excellent performance in the polymerization of ethene (Table 1).16 Acenaphthene-based complex 3a is slightly more active at p(ethene) = 1 atm than dimethyl-substituted complex 3b. However, the molecular weight of the PE obtained from 3a is drastically lower (Table 1, Mn = 8.4 kg/mol (entry 1) vs 103 kg/mol (entry 6)). The 1 atm PE shows significant branching (determined according to the method of Galland et al.).67 At temperatures up to 40 °C, primarily methyl branches are observed for PE from 3a, while at higher polymerization temperatures, ethyl branching is more common (Figures S20 and S21). For PE from catalyst 3b, methyl branching remains dominant up to 80 °C. At p(ethene) = 7 atm, complex 3b displays an excellent level of productivity of up to 34 × 103 kg(mol Ni)−1 h−1 at T = 40 °C (Table 1, entry 17; for extended comparison with literature data see the Supporting Information and recent reviews).16,48 For comparison, the activity data for a few other representative catalysts are shown in Figure 2. At the same time, PE with high molecular weights of up M n = 138.000 g/mol and polydispersity Đ= 3.0 had a melting point approaching 140 °C. The PE melting points show very little dependence on the polymerization temperature and remain in the 129−139 °C interval with the exception of entry 18. At the same time, the 7 atm polymer from 3b is characterized by a very low degree of branching (12/1000 C) (compared to that of other Ni− diimine complexes as reviewed by Chen et al.),12 containing predominantly methyl branches. The superior performance of catalysts with methyl substituents in the backbone compared to those with an acenaphthyl backbone is in accord with observations by Brookhart et al.68

Figure 2. Comparison of activity for ethene polymerization at high ethene pressure (7 atm ethene for 3a, 3b using Et2AlCl activator; 9 atm ethene for 4a, 4b using MAO activator). Activity for 4b (denoted as 4b*) at T = 20 and 40 °C and 7 atm ethene were determined by us using Et2AlCl activator.

The temperature-dependent polymerization activities of 3a and 3b were also tested (Table 1), since recently a number of high-temperature-stable Ni-diimine catalysts have been reported.58,69−72 For 3b, the polymerization activity remains extremely high up to temperatures of T = 60 °C (Table 1, entry 16), but a very pronounced drop in activity is observed at T = 80 °C (entry 18) which brings the activity to the level of the best of other thermostable catalysts.26,58,69,70,73,74 A comparison of the temperature-variable ethene polymerization of complexes 3a and 3b with that of closely related complexes 4a and 4b (Scheme 2; Figure 2) shows the pronounced effect of the pentiptycenyl group on the polymerization activity relative to the 2,6-diisopropylphenyl substituents.75 The pentiptycenyl complexes shows much better activity than the respective isopropyl-substituted complexes. The same applies to the Mn (Figure 3) as well as melting points which are higher for pentiptycenyl substituted complexes 5 than those for for isopropyl-substituted relatives C

DOI: 10.1021/acs.organomet.8b00836 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

homopolymerization of ethene. The degree of monomer incorporation is up to 4.2 mol %, translating into a very significant mass content of the polar monomer in excess of 10 wt %. It seems that comonomers with the most polar groups (R = OH, COOH, COOMe) are better incorporated than those of lower polarity monomers (R = Cl). With a view to the economic use of the relatively expensive polar comonomers, the amount of consumed monomer is also important. Typical values here are in excess of 10% of the total initial monomer (Table 2, column “consumed comonomer”), but incorporation can exceed 40%. For polymerization reactions with oxygen containing comonomers, the activity of the catalyst critically depends on the ratio of Et2AlCl activator and polar monomer and (Table 2, entries 6/7, 10/11, and 14/15). On lowering this ratio from 1 to 0.5, the polymerization reaction is efficiently suppressed when using oxygen-donor containing monomers. On the basis of the higher oxophilicity of Al compared to Ni, the functional groups in the olefins preferentially coordinate to Al, and the transition metal remains active in the olefin polymerization reaction. However, only 1 equiv of an oxygen-containing functional group relative to Et2AlCl is tolerated. The addition of a second oxygen-containing functional group leads to an inhibition of the Ni catalyst−probably because the additional donor groups inhibit the catalytically active center. The use of large amounts of Et2AlCl activator may not be a desirable strategy; however, even though this is rarely mentioned in the literature explicitly,34 it seems that previously reported nickeldiimine complexes also critically rely on using large amounts of activator for the efficient copolymerization of ethene and polar comonomers. In accord with these arguments, the oxygen-free 11-chloro-undecene has a less detrimental influence on the catalyst. At Al/comonomer = 1 the catalytic activity is close to that in the ethene homopolymer. Lowering the relative amount of activator/comonomer to 0.3, still allows the formation of a large amounts of polymer (Table 2, entry 11). This goes along with the weaker coordinating ability of chlorides toward metal compared to the coordination by oxygen. Based on this approach, the amount of incorporated polar comonomer into the final polymer can be adjusted, by increasing the amount of Et2AlCl.

Scheme 2. Closely Related Polymerization Catalysts

Figure 3. Comparison of Mn for ethene polymerization at high ethene pressure (7 atm ethene for 3a, 3b and 9 atm ethene for 4a, 4b).

4. Even longer polyethene (Mn up to 800.000 g/mol) can be obtained with classic Brookhart complexes when using modified MAO initiators at higher ethene pressure.68 The stability of the catalyst during ethene polymerization is shown in Figure 4 by plotting the total turnover number (TON) versus time. At elevated temperatures, the total TON decreases significantly, especially for reaction times in excess of 30 min.

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CONCLUSIONS We have synthesized new bispentiptycene-diimine nickel complexes, which (following activation with Et2AlCl) are characterized by outstanding catalytic activities in PE synthesis while at the same producing polymers with high molecular weight, very low branching, and high melting points. This is most likely due to the nickel being located in a highly shielded, hemispherical environment of the two pentiptycene units. Polar monomers and ethene can be copolymerized efficiently leading to polymers with up to 4.2 mol % of incorporated polar monomer. This reaction depends very much on the amount of activator relative to the amount of polar comonomer. Using equimolar amounts of Et2AlCl activator and polar comonomer in the copolymerization with ethene results in to excellent polymerization activities and high incorporation of the polar comonomer. Lowering the ratio Et2AlCl: polar comonomer to 0.5 stalls polymerization activity for oxygen-containing monomers, while 11-chloro-1-undecene is still polymerized efficiently. For future work, the steric bulk of the bispentiptycenyl ligands can be extended by installing additional substituents at the pentiptycene wings providing

Figure 4. Comparison of time-dependent polymerization catalyst activity (turnover number, TON) at 1 atm ethene for catalysts 3a, 3b, and 1a (CHPh2) at T = 25 and 80 °C using Et2AlCl activator.

Ethene/Polar Comonomer Polymerization. Next, the copolymerization of ethene with polar comonomers (methyl 10-undecenoate, 10-undecenoic acid, 11-chloroundecene, and 5-hexene-1-ol) (Table 2) was studied. Three of the four comonomers possess oxygen-containing functional groups with a high affinity toward cationic nickel, while 11-chloroundecene contains a weakly coordinating halide.76,77 Complex 3b shows excellent properties concerning polymer yield, catalytic activity, the amount of incorporated monomer, molecular weight Mn, and the degree of branching, when using a large amount of activator (ratio Al/comonomer = 1:1) with activities in the range of 100−352 kg(mol Ni)−1 h−1. Polymer molecular weights Mn can be even higher than those in the related D

DOI: 10.1021/acs.organomet.8b00836 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 2. Ethene−Polar Monomer Copolymerizationa comonomer (mol L−1)

entrya 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

COOMe

g

COOH

Cl

OH

0.05 0.1 0.05 0.02 0.06 0.1 0.1 0.02 0.04 0.06 0.06 0.06 0.08 0.1 0.1

Et2AlCl Al/Ni (Al/comon.) 200 400 100 100 300 500 250 100 200 300 100 300 400 500 250

(0.8) (0.8) (0.4) (1) (1) (1) (0.5) (1) (1) (1) (0.3) (1) (1) (1) (0.5)

yield (g)b

activityb

XM (mol %)c

consumed comon. (%)c

Mnd

PDI

Tm (°C)e

Bf

0.40 0.38 trace 0.64 0.97 0.55 0.05 1.72 1.35 1.76 0.70 0.84 1.01 0.93 0.005

80 76

1.2 2.8

7.1 7.5

61.9 14.1

3.1 2.2

99 94

27 52

128 194 110 9.4 344 270 352 140 168 202 186 1

1.7 3.8 4.2

39.5 44.0 16.5

39.6 9.1 19.4

5.8 8.9 5.3

86 76 77

46 42 39

0.4 0.7 1.0 1.2 1.4 2.6 3.1

24.5 17.3 21.4 9.8 11.6 23.6 20.5

24.6 21.1 25.5 18.8 71.6 27.8 47.0

3.8 3.7 3.5 3.8 3.0 5.8 2.5

98 100 98 101 87 84 81

31 27 16 29 34 41 38

a

Monomer: COOMe, methyl 10-undecenoate; COOH, 10-undecenoic acid; Cl, 11-chloro-1-undecene; OH, 5-hexene-1-ol. Reaction conditions: 3b (10 μmol), Et2AlCl, total volume of toluene + comonomer (50 mL), ethene (1 atm), 30 min, 25 °C. bAverage value of two experiments in kg(mol Ni)−1 h−1. cComonomer incorporation determined by 1H NMR spectrometry. dDetermined by GPC in 1,2,4-trichlorobenzene at 150 °C with PE standard in kg·mol−1. eDetermined by differential scanning calorimetry, second heat. fNumbers of branches per 1000 carbons, as determined by 1H NMR spectrometry. gThe polymers were dissolved and reprecipitated several times to remove unreacted polar monomer. 123.67, 122.85, 122.78, 122.48, 122.16, 121.00, 75.63, 64.91, 49.15, 47.98, 47.02, 32.49, 32.22, 19.65, 19.29, 13.71. HRMS (ESI) calcd for C90H67BrN3NiO2 (M + CH3CN) 1358.37646. Found 1358.37535. IR (KBr) 1618 cm−1 (imine). Synthesis of Nickel Complex 3b. A flame-dried Schlenk flask containing diimine 2a (500 mg, 0.461 mmol) and [NiBr2(DME)] (142 mg, 0.461 mmol) was evacuated and backfilled with nitrogen three times. CH2Cl2 (25 mL) was added via syringe. The reaction mixture was stirred at room temperature for 24 h and filtered through Celite. The filtrate was concentrated under reduced pressure to a half volume. To this mixture was added pentane (50 mL) to obtain nickel complex 3a as a brown powder (480 mg, 0.368 mmol, 80%). 1 H NMR (paramagnetic complex) (500 MHz, CDCl3) δ 11.20 (br), 7.83 (br), 7.71 (br), 7.12 (br), 6.93 (br), 6.81 (br), 6.74 (br), 6.47 (br), 6.28 (br), 5.78 (br), 5.31 (br), 4.13 (br), 4.03 (br), 2.69 (br), 2.38 (br), 2.17 (br), 2.08 (br), 1.78 (br), 1.51 (br), 1.19 (br), 0.08 (br). HRMS (APCI) calcd for C82H67BrN3NiO2 (M + CH3CN − Br) 1262.376460. Found 1262.375152. IR (KBr) 1631 cm−1 (imine). Ethene Polymerization (Atmospheric Pressure). Under an inert atmosphere, a Schlenk flask (100 mL) was charged with a magnetic stirring bar, nickel complex (10 μmol), and toluene (50 mL). Ethene gas was bubbled through the reaction mixture for 30 min to saturate. The flask was placed in an oil bath at the desired temperature. Then the mixture was injected Et2AlCl (100 equiv, 1 M in hexane). After 30 min, the reaction was quenched by the addition of MeOH (50 mL). The precipitated polymer was filtered and washed with MeOH (100 mL). The polymer was dried in vacuo overnight at 60 °C. Ethene Polymerization (High Pressure). Under an inert atmosphere, a pressure reactor (200 mL) was charged with a magnetic stirring bar, nickel complex, and toluene (50 mL). The reactor was evacuated and backfilled with ethene gas and pressurized to 7 atm and the reaction mixture stirred for 10 min to saturate the solution with ethene. The reactor was placed in an oil bath at the desired temperature. Then, Et2AlCl (1 M in hexane) was injected into the mixture. After 30 min, the reaction was quenched by releasing the ethene pressure and the by addition of MeOH (50 mL). The precipitated polymer was filtered and washed with MeOH (100 mL). The polymer was dried in vacuo overnight at 60 °C.

the chance to obtain even more shielded metal centers with potentially higher catalytic activities.

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EXPERIMENTAL SECTION

Synthesis of Butylated Diimine 2a. To the solution of diimine 1a (2.33 g, 2.18 mmol) in DMF (100 mL) at 70 °C was added K2CO3 (3.62 g, 26.16 mmol). The mixture was stirred 1 min and BuI (989 μL, 8.72 mmol) was added. The reaction mixture was stirred for 24 h at 70 °C. The orange suspension was cooled to room temperature and poured into 500 mL of water. The orange precipitate was collected by filtration, washed with water (200 mL), methanol (200 mL), and pentane (200 mL). After drying at 60 °C, the product was obtained as an orange powder (2.51 g, 97%). 1 H NMR (500 MHz, CDCl3) δ 7.58 (d, J = 8.2 Hz, 2H), 7.49 (dd, J = 5.5, 3.0 Hz, 4H), 7.41 (dd, J = 5.5, 2.9 Hz, 4H), 7.36 (d, J = 7.3 Hz, 4H), 7.02−6.96 (m, 8H), 6.81 (ddd, J = 7.3, 5.4, 3.2 Hz, 4H), 6.46−6.39 (m, 10H), 5.82 (s, 4H), 5.61 (s, 4H), 5.32 (d, J = 7.3 Hz, 2H), 4.16 (t, J = 6.6 Hz, 4H), 2.15 (dt, J = 14.5, 6.6 Hz, 4H), 1.87 (sep, J = 7.4 Hz, 4H), 1.24 (t, J = 7.4 Hz, 6H). 13C NMR (126 Hz, CDCl3) δ 164.84, 146.74, 145.56, 145.46, 145.43, 145.11, 141.25, 139.74, 136.63, 132.57, 130.48, 128.74, 128.36, 127.57, 125.38, 125.34, 124.73, 124.67, 124.43, 123.60, 123.52, 123.24, 76.40, 49.94, 48.75, 32.98, 20.04, 14.42. HRMS (APCI) calcd for C88H65N2O2 (M + H) 1181.50406. Found 1181.50396. IR (KBr) 1631 cm−1 (imine). Synthesis of Nickel Complex 3a. A flame-dried Schlenk flask containing diimine 2a (500 mg, 0.423 mmol) and [NiBr2(DME)] (131 mg, 0.423 mmol) was evacuated and backfilled with nitrogen three times. CH2Cl2 (25 mL) was added via syringe. The reaction mixture was stirred at room temperature for 24 h and filtered through Celite. The filtrate was concentrated under reduced pressure to half the volume. To this mixture was added pentane (50 mL) to obtained nickel complex 3a as a brown powder (525 mg, 0.375 mmol, 88%). Xray quality crystals were obtained by slow diffusion of hexane into an acetone solution of the complex at 5 °C. 1 H NMR (paramagnetic complex) (500 MHz, CDCl3) δ 21.44 (s, 2H), 17.43 (d, J = 5.4 Hz, 2H), 7.67 (d, J = 6.7 Hz, 4H), 7.56 (d, J = 6.6 Hz, 4H), 7.50 (s, 4H), 6.78 (s, 4H), 6.60−6.45 (m, 8H), 6.32 (s, 4H), 6.12 (br, 3H), 5.63 (s, 4H), 4.29 (d, J = 6.2 Hz, 2H), 3.87 (s, 4H), 2.49 (s, 4H), 2.25 (q, J = 7.2 Hz, 4H), 1.40 (t, J = 7.2 Hz, 6H). 13 C NMR (126 MHz, CDCl3) δ 194.29, 189.82, 180.59, 148.85, 144.76, 144.67, 144.62, 144.56, 144.33, 139.52, 139.46, 127.55, 126.81, 125.47, 125.27, 124.91, 124.72, 124.59, 124.04, 123.97, E

DOI: 10.1021/acs.organomet.8b00836 Organometallics XXXX, XXX, XXX−XXX

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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.8b00836. Full experimental, NMR spectra, GPC, X-ray and DSC data (PDF) Cartesian coordinates (XYZ) Accession Codes

CCDC 1811399 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuki Kanai: 0000-0001-9561-861X Herbert Plenio: 0000-0002-2257-983X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the TU Darmstadt and the DFG Pl 178/13-2. We thank Ms. D. Mahr and Prof. Dr. M. Busch (Technische Chemie III, TU Darmstadt) for the gpc of PE; Mr. T. Macko, Dr. R. Brüll, and Dr. R. Pfändner (Fraunhofer Institute for Structural Durability and System Reliability LBF, Darmstadt) for the gpc of polar PE, and Dr. M. Lepple and Prof. Dr. B. Albert for support with DSC measurements.

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REFERENCES

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