(α-Diimine)nickel Complexes That Contain Menthyl Substituents

Article pubs.acs.org/Organometallics

(α-Diimine)nickel Complexes That Contain Menthyl Substituents: Synthesis, Conformational Behavior, and Olefin Polymerization Catalysis Feng Zhai and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: We describe the synthesis and coordination chemistry of the (1R,2S,5R)-menthyl-substituted N,N′-diaryl-αdiimine ligands N,N′-(2-Men-4-Me-Ph)2-BIAN (L1, Men = menthyl, BIAN = bis(imino)acenaphthene) and N,N′-(2-Men4,6-Me2-Ph)2-BIAN (L2), the conformational properties of these ligands and their metal complexes, and the ethylene and 1hexene polymerization behavior of the corresponding (αdiimine)Ni complexes. Free ligands L1 and L2 and squareplanar (L1)PdCl2 and (L1,2)Ni(acac)+ complexes exhibit a preference for the syn conformation, in which the two menthyl units are located on the same side of the NCCN plane, while tetrahedral (L1,2)MX2 (MX2 = ZnCl2, NiBr2) complexes exhibit a preference for the anti conformation, in which the menthyl units are located on opposite sides of the NCCN plane. Both the anti and the syn conformers of [(L2)Ni(acac)][B(C6F5)4] can be activated by Et2AlCl to generate highly active ethylene polymerization catalysts (activity (2.5− 6.6) × 106 g of PE/((mol of Ni) h) at 15 psi of C2H4, room temperature). The polyethylene produced by the syn conformer (syn/anti = 91/9) has a higher molecular weight (2×) and a higher branch density (3×) in comparison to that produced by the anti conformer. The polyhexene produced by the syn conformer (syn/anti = 91/9) contains a higher level of chain straightening (syn 50%, anti 41%) and a higher percentage of Me versus Bu branches (syn 24/26, anti 6/53) in comparison to that produced by the anti isomer. These results are indicative of a greater preference for 2,1-insertion and for chain walking (versus growth) following 1,2-insertion for the syn conformer.

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INTRODUCTION The steric properties of the α-diimine ancillary ligands in (N,N′-diaryl-α-diimine)MR+ (M = Ni, Pd) olefin polymerization catalysts strongly influence the activity of the catalyst and the microstructure of the resulting polymer.1−3 Following Brookhart’s report of the benchmark catalysts A (Chart 1), which contain iPr groups in the ortho positions of the N-aryl rings and typically yield highly branched polyethylenes (PEs, ca. 100 br/1000 C),4 other (α-diimine)MR+ catalysts were found to exhibit different reactivity.5−12 Rieger and co-workers reported that terphenyl (α-diimine)Ni catalyst B is highly active and produces highly linear PE.6 In contrast, Daugulis and Brookhart and co-workers reported that 8-tolyl-1-naphthylbased “sandwich”-type α-diimine Ni and Pd catalysts C produce very highly branched PE (up to 117 br/1000 C for Pd and 150 br/1000C for Ni).7,8 Long and co-workers and Chen and coworkers showed that benzhydryl-substituted (α-diimine)M catalysts D and E exhibit high thermal stability and produce PEs with intermediate branch densities (40−60 br/1000 C for Ni and 25−30 br/1000 C for Pd).9,11 The incorporation of electron-donating or electron-withdrawing groups in the para positions of the N-aryl rings can also strongly influence catalytic performance.11,13−16 In particular, for (α-diimine)Pd catalyst A, © XXXX American Chemical Society

electron-donating X groups increase the catalyst activity and the polymer molecular weight (MW).13,14 N,N′-Diaryl-α-diimine ligands that contain unsymmetrically substituted aryl rings, e.g. two different ortho substituents on each ring, and metal complexes of such ligands may exist as syn and anti conformers. The conformer in which the more bulky ortho substituents on each aryl ring are on the same side of the NCCN plane is defined as syn, while that with the larger ortho substituents on the opposite sides of the NCCN plane is defined as anti. anti/syn isomerism of unsymmetrically substituted α-diimine Ni and Pd complexes has been reported, and the anti and syn conformers may exhibit different performance in olefin polymerization.17−21 For example, Pellecchia and co-workers reported that, at −45 °C, F-anti produces polypropylene with low stereoregularity while F-syn produces syndiotactic polypropylene.17 Additionally, Zhu and co-workers found that F-anti produces PE with higher MW and less branching in comparison to F-syn.19 Bimodal MW distributions may arise in olefin polymerizations by these catalysts when the interconversion of the anti and syn conformers of the active species is slow relative to chain Received: April 13, 2017

A

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Organometallics Chart 1

growth. 18−21 Some (α-diimine)Ni catalysts that adopt exclusively anti conformations, such as Coates’s catalyst G, have been exploited to produce isotactic polypropylene at low temperature.22,23 However, the factors that control the relative stabilities and interconversion rates of the syn or anti isomers are not well understood. Here we describe the synthesis and coordination chemistry of the menthyl-substituted α-diimine ligands N,N′-(2-Men-4Me-Ph)2-BIAN (L1, Men = menthyl, BIAN = bis(imino)acenaphthene, Chart 1) and N,N′-(2-Men-4,6-Me2-Ph)2-BIAN (L2), the anti/syn isomerism and other conformational properties of these ligands and their metal complexes, and the ethylene and 1-hexene polymerization behavior of the corresponding (α-diimine)Ni complexes. The objective of this work was to probe how the incorporation of o-alkyl substituents that are sterically larger but electronically similar to iPr influences the olefin polymerization performance of (N,N′diaryl-α-diimine)Ni catalysts. The (1R,2S,5R)-menthyl group has been widely used as a chiral auxiliary group in asymmetric catalysts.24−28 The menthyl C6 ring is locked in a stable chair conformation due to the presence of three equatorial substituents. Recently Nozaki and co-workers showed that the replacement of the P-iPr groups in the (o-iPr2P-C6H4SO3)PdMe(py) ethylene polymerization catalyst with menthyl groups resulted in an increase in the PE MW by 2 orders of magnitude, which was ascribed to the steric profile of the menthyl group.29

Scheme 1

Scheme 2

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RESULTS AND DISCUSSION Synthesis of Menthyl-Substituted Anilines. The synthesis of o-menthylanilines 3 and 4 is shown in Scheme 1 and is based on methodology developed by Knochel.30 Transmetalation of the Grignard reagent formed from (1R,2S,5R)menthyl chloride with ZnCl2 yields a ca. 1/1 mixture of menthylzinc and neomenthylzinc reagents. Subsequent Pdcatalyzed Negishi coupling of this mixture with o-iodoanilines 1 and 2 in THF/N-ethylpyrrolidone (NEP) at room temperature yields 3 and 4 with high diastereoselectivity (>20/1).31 Anilines 3 and 4 exhibit hindered rotation of the menthyl group. 3 exists as a 12/1 equilibrium mixture of anticlinal (3-ac) and synclinal (3-sc) isomers in C6D6 at room temperature (Scheme 2).32 The room-temperature NOESY/EXSY spectrum of 3 exhibits several NOE correlations for the major isomer that enable its assignment as 3-ac, as well as exchange correlations

that establish that the two isomers undergo exchange. In the variable-temperature 1 H NMR spectra of 3 in C 6 D 6 , coalescence of the p-Me resonances was observed at ca. 35 °C (Figure S20 in the Supporting Information). The barrier to rotation of menthyl group was determined to be ΔG⧧ac→sc = 16.2(6) kcal mol−1. Similar hindered rotation of a menthyl group was observed in N,O-[2-(menthyloxy)phenyl]amido dimethylaluminum.33 4 also exists as a 12/1 mixture of 4-ac and 4-sc rotamers at room temperature. The synclinal isomers of 3 and 4 are disfavored due to the steric repulsion between the o-amino group and two axial H atoms, HI and HB (Scheme 2). The room-temperature 1H NMR spectrum of 3·HCl exhibits only one set of signals, consistent with the sole presence of the anticlinal isomer, due to the larger size of NH3+ versus NH2. B

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(Scheme 4).37 The isomer assignments were established by the symmetry implied by the NMR spectra and the 1H NMR

Synthesis of Ligands L1 and L2. Ligands L1 and L2 were prepared by ZnCl2-templated synthesis followed by demetalation (Scheme 3). The reaction of acenaphthenequinone with 2

Scheme 4 Scheme 3

chemical shifts of the o-An resonances (L1-E,E, δ 7.00, 2H; L1Z,E, δ 8.09, 1H; see Figure 1). When the solution was cooled to

equiv of 3 or 4 in the presence of excess ZnCl2 in AcOH yields (L1)ZnCl2 and (L2)ZnCl2, respectively.34,35 The Zn complexes were demetalated with aqueous K2C2O4 in CH2Cl2 to afford the free ligands L1 and L2.35 Terminology for Isomers of L1 and L2. Several isomers are possible for L1, L2, and their metal complexes due to imine CN bond Z,E isomerism, hindered rotation of the N-aryl groups, which gives rise to anti/syn isomerism, and hindered rotation of the menthyl groups. The E,E and Z,E isomers of free bis(imino)acenaphthene (BIAN) ligands are often observed. The E,E isomer is usually favored, and E,E/Z,E interconversion is usually fast on the laboratory time scale at room temperature.36 The E,E and Z,E isomers of N,N′-diarylBIANs can be distinguished by the 1H NMR chemical shifts of the o-acenaphthene (o-An) resonances. The two o-An H atoms of the E,E isomer lie within the shielding cones of the proximate N-aryl rings, and their 1H NMR signals (d, J = 7 Hz) appear at high field (δ 7.5−6.5). In contrast, one of the o-An H atoms of the Z,E isomer does not experience such anisotropic shielding and its 1H NMR signal appears in the range δ 8.5− 7.5. As noted above, the menthyl groups in L1 and L2 favor the anticlinal conformation for steric reasons. The anticlinal conformation should also be favored in metal complexes of L1 and L2. In the E,E isomer, the two anticlinal menthyl groups may adopt two orientations, one in which the menthyl iPr group points toward the acenaphthene backbone (endo) and one in which the menthyl iPr group points away from the acenaphthene backbone (exo). An endo menthyl group exhibits a 1H NMR doublet signal in the range δ 0 to −0.4 for one of the isopropyl Me groups due to the ring current effect of the acenaphthene backbone, while an exo menthyl group exhibits isopropyl Me signals in the normal region (δ 1.5−0.8). Isomerism of L1 and L2. L1 exists as an equilibrium mixture of C2-symmetric L1-E,E (85%) and C1-symmetric L1Z,E (13%) isomers in CD2Cl2 solution at room temperature

Figure 1. 1H NMR spectrum of L1 (CD2Cl2, room temperature, expansion of δ 8.2−6.7 region). Key assignments for aromatic 1H resonances are labeled in black for L1-E,E and in red for L1-Z,E. Note that the second o-An resonance for L1-Z,E was not located due to overlapping with other signals.

−60 °C, all of the 1H resonances of the major L1-E,E isomer split into pairs of resonances (Figure S21 in the Supporting Information). This result shows that L1-E,E favors a C1symmetric syn conformer at −60 °C and that anti/syn exchange is fast at room temperature.38,39 The barrier to exchange of the two menthyl groups of L1-E,E-syn, which corresponds to the barrier to rotation of the N-aryl units (and hence anti/syn exchange), was estimated to be 13 kcal mol−1 at 0 °C from the coalescence of benzyl CH signals of the menthyl units. Demetalation of (L2)ZnCl2-anti (vide infra) yields L2 that is highly enriched in the anti conformer (anti/syn typically >6/1). anti/syn isomerization of L2 is much slower than for L1, due to the presence of the o-Me substituents, which hinder N-aryl rotation. In contrast, the E,E/Z,E interconversions are fast on the laboratory time scale at room temperature for both L2-anti (equilibrium ratio L2-E,E-anti/Z,E-anti = 3.1/1) and L2-syn (equilibrium ratio L2-E,E-syn/Z,E-syn = 30/1). An equilibrium mixture of L2 isomers was reached after 5 days at room temperature and comprises a 12/4/81/3 mixture of L2-E,Eanti/Z,E-anti/E,E-syn/Z,E-syn (overall anti/syn = 1/5.2, Scheme C

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Organometallics 5), which is dominated by the E,E-syn isomer as in the case of L1. The composition of this mixture was determined on the

Scheme 6

Scheme 5. Equilibrium Isomer Distribution for L2 at Room Temperature

characterized by X-ray diffraction (Figure 3). (L2)ZnCl2-anti isomerizes to an 11/1 anti/syn equilibrium mixture in

basis of the 1H NMR integrals of the isopropyl Me resonances in the δ 0 to −0.4 region (Figure 2), which correspond to endo

Figure 3. Molecular structure of (L2)ZnCl2-anti in (L2)ZnCl2-anti· 3C6H6. Hydrogen atoms and solvent molecules are omitted. Selected bond distances (Å) and angles (deg): Zn1−N1, 2.0884(17); Zn1− Cl1, 2.1948(5); C1−N1, 1.277(3); N1−Zn1−N1a, 81.75(9); N1− Zn1−Cl1, 105.00(5); N1−Zn1−Cl1a, 118.65(5); Cl1−Zn−Cl1a, 121.55(3).

CDCl2CDCl2 at 105 °C within 2 days. N-Aryl rotation, which effects anti/syn isomerization, is slower for (L2)ZnCl2 than for (L1)ZnCl2 due to the presence of the o-Me substituents. Tetrahedral (L1,2)NiBr2 Complexes. The reaction between L1 and (dme)NiBr2 at room temperature yields the paramagnetic, tetrahedral complex (L1)NiBr2-anti (Scheme 7). Figure 2. 1H NMR spectra (CD2Cl2, room temperature, expansion of δ 0.1 to −0.6 region) of (a) an equilibrium 12/4/81/3 mixture of L2E,E-anti/Z,E-anti/E,E-syn/Z,E-anti and (b) a 67/22/11 mixture of L2E,E-anti/Z,E-anti/Z,E-syn. Legend: #, grease.

Scheme 7

menthyl groups. In this region, the C2-symmetric conformer L2-E,E-anti exhibits one doublet (6H) for the two endo menthyl groups, while the other conformers each exhibit one doublet (3H) corresponding to one endo menthyl group. The syn conformer can be enriched by flash column chromatography due to the slow anti/syn isomerization rate at room temperature. Tetrahedral (L1,2)ZnCl2 Complexes. (L1)ZnCl2 exists as a 10/1 anti/syn equilibrium mixture in CD2Cl2 at room temperature (Scheme 6). The major isomer was assigned as (L1)ZnCl2-anti on the basis of the C2 symmetry implied by the NMR spectra and contains two endo menthyl groups on the basis of the 1H NMR spectrum. (L2)ZnCl2 was isolated as the pure anti isomer from the reaction in Scheme 3 and

The solid-state structure of (L1)NiBr2-anti was revealed by Xray diffraction to contain two endo menthyl units (Figure 4). The 1H NMR spectrum of (L1)NiBr2-anti in CD2Cl2 contains only one set of peaks consistent with a C2-symmetric structure, suggesting that (L1)NiBr2-anti retains the anti conformation in solution. Similarly, the reaction of L2 (mixture of isomers) and (dme)NiBr2 at 80 °C yields (L2)NiBr2-anti. The anti D

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Square-Planar [(L1,2)Ni(acac)][B(C6F5)4] Complexes. The reaction of L1, Ni(acac)2, and [Ph3C][B(C6F5)4] at room temperature yields air-stable [(L1)Ni(acac)][B(C6F5)4] as a 50/1 syn/anti equilibrium mixture (Scheme 9). The assignment of (L1)Ni(acac)+-syn as the major conformer is based on the C1 symmetry implied by the 1H and 13C NMR spectra, both of which contain two sets of signals for the inequivalent menthyl units. The 1H NMR spectrum contains a broad singlet at δ 1.61 for the two acac Me groups in CD2Cl2 at room temperature that splits into two singlets upon cooling to −40 °C, indicating that exchange of the two ends of the acac ligand is fast at room temperature (Figure S22 in the Supporting Information). The reaction of (L1)NiBr2-anti, Ag(acac) and Li[B(C6F5)4] generates [(L1)Ni(acac)-anti][B(C6F5)4] selectively. The (L1)Ni(acac)+-anti cation contains two endo menthyl groups and exhibits C2 symmetry on the basis of the 1H NMR spectrum. [(L1)Ni(acac)-anti][B(C6F5)4] isomerizes to the syn/anti equilibrium mixture in CD2Cl2 at room temperature (Figure S23 in the Supporting Information). The approach to equilibrium displays first-order kinetics with a half-life of 2.6 h under these conditions. Similarly, the reaction of L2-syn (95/5 syn/anti) with Ni(acac)2 and [Ph3C][B(C6F5)4] yields a 91/9 syn/anti mixture of [(L2)Ni(acac)][B(C6F5)4] (Scheme 10 and Figure 7a). The two isomers could not be separated by chromatography or crystallization, and therefore this mixture was used in further experiments. The reaction of (L2)NiBr2-anti with Ag(acac) and K[B(C6F5)4] yields [(L2)Ni(acac)-anti][B(C6F5)4] quantitatively (Figure 7b). (L2)Ni(acac)+-anti is stable at room temperature but isomerizes slowly (2 days) in CDCl2 solution at 100 °C to an 89/11 syn/anti equilibrium mixture. Summary of Conformational Behavior of L1 and L2 and Their Metal Complexes. Tetrahedral (L1,2)MX2 (MX2 = ZnCl2, NiBr2) complexes favor C2-symmetric anti (endo,endo) conformations. Steric interactions between the X ligands and the exo menthyl iPr groups disfavor the syn (endo,exo) and anti (exo,exo) conformations (Figure 8). In contrast, square-planar (L1)PdCl2 and (L1,2)Ni(acac)+ complexes favor syn conformations. This preference does not appear to be a consequence of steric interactions between the L1,2 and X ligands and thus may originate from the preference for the syn isomer in the free ligands L1 and L2. The syn conformation may be stabilized by attractive dispersive interactions between the proximal menthyl units.40 Attractive dispersion interactions between large alkyl groups provide important stabilization for a variety of organic and coordination compounds41−44 and also influence the assembly of alkyl groups in solution and in the solid state.45,46 Solvophobic effects may also contribute to the preference for the syn conformation in solution.45 Interconversion of the syn and anti conformers of both tetrahedral and square-planar (L1)MX2 and (L2)MX2 complexes is slower than that for the free ligands L1 and L2, respectively, due to the presence of the MX2 unit, which hinders N-aryl rotation. Ethylene Polymerization. The ethylene polymerization behavior of (L1)Ni complexes was studied at room temperature. Both (L1)NiBr2-anti and (L1)Ni(acac)+-syn form very active catalysts for ethylene polymerization at 70 psi ethylene pressure upon activation by Et2AlCl (2000 equiv, Table 1, entries 2 and 4) and exhibit single-site behavior.6,15,47 The activities are on the order of 106−107 g of PE/((mol of Ni) h).

Figure 4. Molecular structure of (L1)NiBr2-anti. Hydrogen atoms are omitted. Selected bond distances (Å) and angles (deg): Ni1−N1, 1.971(7); Ni1−N2, 2.014(7); Ni1−Br1, 2.3102(14); Ni1−Br2, 2.3267(15); C35−N1, 1.271(10); C46−N2, 1.263(11); N1−Ni1− N2, 82.1(3); Br1−Ni1−Br2, 134.10(6); N1−Ni1−Br1, 114.1(2); N1−Ni1−Br2, 100.8(2); N2−Ni1−Br1, 103.08(19); N2−Ni1−Br2, 110.6(2).

conformation of (L2)NiBr2-anti was assigned on the basis of its 1 H NMR spectrum, which is consistent with a C2-symmetric species. Square-Planar (L1)PdCl2 Complexes. The reaction of L1 and (MeCN)2PdCl2 in CH2Cl2 yields a 16/1 equilibrium mixture of (L1)PdCl2-syn and (L1)PdCl2-anti (Scheme 8). The Scheme 8

major conformer (L1)PdCl2-syn was isolated by recrystallization and isomerizes to the equilibrium isomer mixture in CD2Cl2 at room temperature in 3 days. The 1H NMR resonances for the two inequivalent menthyl groups of (L1)PdCl2-syn (see 1H NMR spectrum in Figure 5a) were assigned on the basis of COSY and HMQC spectra. NOESY correlations between the two menthyl groups (Figure 5b,c and Figure S15h in the Supporting Information) support the assignment of the syn conformation. The NMR spectra of (L1)PdCl2-anti confirm the C2 symmetry, and the iPr doublet at δ −0.27 (6H) indicates that the two menthyl groups are both endo (Figure 5a). The structures of (L1)PdCl2-syn and (L1)PdCl2-anti were confirmed by X-ray diffraction (Figure 6). (L1)PdCl2-syn contains one endo menthyl group and one exo menthyl group that face each other and thus completely block one axial face of the square-planar Pd center (Figure 6b). In contrast, (L1)PdCl2-anti contains two endo menthyl groups that only partially block the axial faces of the Pd center (Figure 6d). E

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Figure 5. (a) 1H NMR spectrum of an equilibrium 16/1 mixture of (L1)PdCl2-syn and (L1)PdCl2-anti (CD2Cl2, expansion of δ 3.4 to −0.6 region) showing the aliphatic resonances. Key assignments for aromatic 1H resonances are labeled in black for the syn isomer and in red for the anti isomer. (b) Notation for 1H NMR assignments and key NOESY correlations between the menthyl units. (c) NOESY spectrum of (L1)PdCl2-syn (CD2Cl2, expansion of δ 3.2−2.6/δ 2.0 to −0.5 region). The key correlations between the menthyl units are highlighted.

ability of the B(C6F5)4− counterion in the former case in comparison to the EtAlClBr2− counterion (and its ligand redistribution products) in the latter case. Differences in activation efficiency may also contribute to the observed difference in activity, and in fact incomplete activation of (αdiimine)NiBr2/MAO systems has been reported.48−51 (L2)Ni(acac)+-anti and (L2)Ni(acac)+-syn produce polyethylene with narrower MW distributions (Mw/Mn = ca. 2.0) and higher MWs (Mn = 1.0−2.2 × 105 Da) in comparison to the o-iPr-substituted analogue {N,N′-(2-iPr-6-Me-Ph)-BIAN}NiBr2 (likely a mixture of syn and anti conformers, activated by MAO) at 20 °C under similar conditions (Mw/Mn = 2.9, Mn = 7 × 104 Da).19 These results show that the menthyl groups hinder N-aryl rotation and inhibit chain transfer relative to chain growth more effectively in comparison to iPr. Interestingly, the PE produced by (L2)Ni(acac)+-syn has a higher MW (2×) and branch density (3×) than that produced by (L2)Ni(acac)+-anti (Table 1, entries 8 and 10). These results suggest that chain transfer is inhibited relative to chain growth and chain walking by the complete blockage of one axial face by the menthyl groups in the syn conformer. For comparison, the tBu-substituted (α-diimine)Ni catalyst F-syn produces PE with lower MW and higher branch density in comparison to F-anti, highlighting the unique steric properties of menthyl groups.19 Both (L2)Ni(acac)+-anti and (L2)Ni-

(L1)Ni(acac)+-syn exhibits a higher activity (8.6×) than (L1)NiBr2-anti, but the PE produced by (L1)Ni(acac)+-syn has a lower MW (Mn = 6.2 × 103 Da) than that produced by (L1)NiBr2-anti (Mn = 3.5 × 104 Da). The activity of both catalysts decreases when the ethylene pressure is lowered to 15 psi (entries 1 and 3). The differences in the ethylene polymerization behavior of (L1)NiBr2-anti and (L1)Ni(acac)+-syn may arise from the differences in the conformations (syn vs anti) or the labile ligands (Br vs acac). These effects can be probed more easily in the (L2)Ni system because a broader range of complexes is accessible. The ethylene polymerization behavior of (L2)Ni complexes was studied at room temperature in order to minimize anti/syn isomerization. (L2)NiBr2-anti, (L2)Ni(acac)+-anti, and (L2)Ni(acac)+-syn (91/9 syn/anti) all form very active catalysts for ethylene polymerization at 15 psi ethylene pressure upon activation by Et2AlCl (200 or 2000 equiv, Table 1, entries 5−10). The Al loading has only a minor effect on the overall performance. The labile ligands (Br vs acac) have only a minor effect on the microstructure of the PE but strongly influence the catalyst activity (entries 5 and 6 vs entries 7 and 8), with the acac complex being 1.5−2 times more active than the bromide complex. The higher activity of [(L2)Ni(acac)-anti][B(C6F5)4]/Et2AlCl in comparison to (L2)NiBr2-anti/Et2AlCl may arise from the lower coordinating F

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Figure 6. (a) Molecular structure of one independent molecule of (L1)PdCl2-syn in (L1)PdCl2-syn·0.85CH2Cl2. The structures of the other independent molecules are similar but are disordered. Hydrogen atoms and solvent molecules are omitted. Selected bond distances (Å): Pd1−N1, 2.037(6); Pd1−N2, 2.023(7); Pd1−Cl1, 2.277(2); Pd1−Cl2, 2.266(2). (b) Top view of space-filling model of (L1)PdCl2-syn. (c) Molecular structure of (L1)PdCl2-anti in (L1)PdCl2-anti·2.5CH2Cl2. Hydrogen atoms and solvent molecules are omitted. Selected bond distances (Å): Pd1− N1, 2.054(4); Pd1−N2, 2.038(4); Pd1−Cl1, 2.2767(13); Pd1−Cl2, 2.2692(13). (d) Top view of space-filling model of (L1)PdCl2-anti.

Scheme 9

Scheme 10

(acac)+-syn generate PEs with similar MWs but much lower branch densities in comparison to that formed by the iPrsubstituted catalyst A (106 br/1000 C at 1 atm at 35 °C) under similar conditions.4 G

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1-Hexene Polymerization. (α-Diimine)nickel catalysts exhibit chain-straightening reactivity in α-olefin polymerization.50,52−56 As shown in Scheme 11, consecutive 1,2Scheme 11

Figure 7. 1H NMR spectra (CD2Cl2, room temperature, expansion of δ 1.5 to −0.5 region) of (a) [(L2)Ni(acac)-syn][B(C6F5)4] (91/9 syn/ anti) and (b) [(L2)Ni(acac)-anti][B(C6F5)4]. Key assignments are labeled in black for the syn isomer and in red for the anti isomer. Legend: Me, equatorial methyl groups on menthyl units; iPr, methyl groups in isopropyl units; #, grease.

insertions of 1-hexene generate butyl branches. The catalyst may also undergo 1,2-insertion followed by chain walking to the chain-end C6 carbon and subsequent chain growth to form a methyl branch (6,2-enchainment). Finally, 2,1-insertion, subsequent chain walking to the chain-end C6 carbon, and further chain growth will result in 6,1-enchainment, which does not create a branch. As a result, the resulting polyhexenes produced by (α-diimine)Ni catalysts are usually less branched than expected. The steric profile of α-diimine ligand influences the regioselectivity of 1-hexene insertion and has a major effect on the microstructure of the resulting polyhexene.50,55 Activation of (L2)NiBr2-anti, (L2)Ni(acac)+-anti, and (L2)Ni(acac)+-syn (91/9 syn/anti) with Et2AlCl generates 1-hexene polymerization catalysts that produce polyhexenes with relatively narrow MW distributions (Table 2, entries 1, 2, and 5). The labile ligand (Br or acac) influences the catalyst activity and the MWs of the produced polyhexene. (L2)Ni(acac)+-anti is ca. twice as active and produces polyhexene with a higher MW in comparison to (L2)NiBr2-anti. (L2)Ni(acac)+-syn (91/9 syn/anti) produces polyhexene with a bimodal MW distribution, in which the minor high-MW

Figure 8. Possible conformations of and steric interactions within tetrahedral (L1,2)MX2 complexes. R = H (L1), Me (L2).

Table 1. Ethylene Polymerization by (L1,2)Ni Catalystsa entry

catalyst

Al/Ni

Pb

yield (g)

act.c

TOF (103 h−1)

Mn (103)

Mw/Mn

Bd

Tm (°C)

1 2 3 4 5 6 7 8 9 10

(L1)NiBr2-anti (L1)NiBr2-anti (L1)Ni(acac)+-syn (L1)Ni(acac)+-syn (L2)NiBr2-anti (L2)NiBr2-anti (L2)Ni(acac)+-anti (L2)Ni(acac)+-anti (L2)Ni(acac)+-syne (L2)Ni(acac)+-syne

2000 2000 2000 2000 200 2000 200 2000 200 2000

15 70 15 70 15 15 15 15 15 15