Chiral Naphthyl-α-diimine Nickel(II) Catalysts Bearing sec-Phenethyl

Jun 28, 2013 - Complex rac-(RR/SS)-2c, bearing chiral bulky sec-phenethyl groups in the o-naphthyl position, activated by diethylaluminum chloride (DE...
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Chiral Naphthyl-α-diimine Nickel(II) Catalysts Bearing sec-Phenethyl Groups: Chain-Walking Polymerization of Ethylene at High Temperature and Stereoselective Polymerization of Methyl Methacrylate at Low Temperature Jianchao Yuan,* Fuzhou Wang, Weibing Xu, Tongjian Mei, Jing Li, Bingnian Yuan, Fengying Song, and Zong Jia Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China S Supporting Information *

ABSTRACT: A series of new naphthyl-α-diimine nickel(II) complexes, {bis[N,N′-(1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2a), {bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2b), {bis[N,N′-(2-sec-phenethyl-1-naphthyl)imino]-1,2-dimethylethane}dibromonickel (rac-(RR/SS)-2c), {bis[N,N′-(2-methyl-1-naphthyl)imino]acenaphthene}dibromonickel (2d), and {bis[N,N′-(2-naphthyl)imino]-1,2dimethylethane}dibromonickel (2e), were synthesized and characterized. The crystal structures of ligands 1b, rac-(RR/SS)-1c, 1d, 1e and their representative complexes rac-(RR/SS)-2c and 2d were determined by X-ray crystallography. These complexes, activated by diethylaluminum chloride (DEAC), were tested in the polymerization of ethylene and methyl methacrylate under mild conditions. Complex rac-(RR/SS)-2c, bearing chiral bulky sec-phenethyl groups in the o-naphthyl position, activated by diethylaluminum chloride (DEAC) shows highly catalytic activity for the polymerization of ethylene (2.81 × 106 g PE/((mol of Ni) h bar)) and produced branched polyethylene (75 methyl, 9 ethyl, 5 propyl, and 19 butyl or longer branches/1000 C at 40 °C). Interestingly, rac-(RR/SS)-2c could produce syndiotactic PMMA at low temperature (−30 °C: rr 88.75%, mr 7.26%, mm 3.99%).

1. INTRODUCTION Polymerization of α-olefin catalyzed by [NiII/PdIIX2(aryl-αdiimine)] (X = halide) containing a bulky α-diimine ligand1−14 has stimulated renewed interest. In contrast to metallocene catalysts based on early transition metals,15−18 Ni(II) and Pd(II) catalysts produced branched polyethylene with different concentrations and individual lengths of branches1,12 exclusively from the ethylene monomer and accommodated even polar monomers, such as acrylates.11,13,14 This discovery is beneficial to commercial applications and industrial production. Recently, Coates and co-workers proposed a new Brookhart type catalyst for olefin polymerization, which bears a new class of chiral anilines, and their incorporation in α-diimine Ni(II) catalysts.19,20 However, naphthyl-α-diimine Ni(II)/Pd(II) complexes have only been reported in limited cases.21 In particular, naphthyl-α-diimine Ni(II)/Pd(II) complexes bearing chiral sec-phenethyl groups have never been studied. Recently, we reported the “chain-walking polymerization” of ethylene using chiral22,23 or achiral7,8,24,25 aryl-α-diimine Ni(II) complexes. Here, we first report the synthesis and characterization of a series of new chiral or achiral naphthyl-α-diimine Ni(II) complexes of the types [NiBr2(Na-DAB)] (Na-DAB = © 2013 American Chemical Society

N,N′-dinaphthyl-1,4-diaza-1,3-butadiene) and [NiBr2(NaBIAN)] (Na-BIAN = bis(naphthylimino)acenaphthene) bearing different groups (H, methyl, and chiral sec-phenethyl group) in the o-naphthyl position, in order to study the influence of different steric effects and electron densities at the metal center on the catalyst activity, on the microstructure of polyethylene, and, in particular, on the stereoregularity structure of poly(methyl methacrylate).

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization of Ligands 1a−e and Complexes 2a−e. The general synthetic routes of the nickel(II) complexes 2a−e are shown in Scheme 1. Reaction of the naphthylamine with styrene at elevated temperature (160 °C) in the presence of CF3SO3H catalyst resulted in the corresponding o-sec-phenethyl naphthylamine compound rac-c (Scheme 1). Ligands 1a,b,d,e were prepared by the condensation of equivalents of the appropriate naphthylamine with 1 equiv of 2,3-butanedione or acenaphthenequinone, Received: May 21, 2013 Published: June 28, 2013 3960

dx.doi.org/10.1021/om400433t | Organometallics 2013, 32, 3960−3968

Organometallics

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Scheme 1. Syntheses of α-Diimine ligands 1a−ea and Their Corresponding Bis-α-diimine Nickel(II) Dibromide Complexes 2a−e

1.273(2)/1.275(3)/1.271(2) and 1.501(3)/1.497(5)/1.492(3) Å, respectively, which are very close to the values for other structurally characterized free α-diimines.26 Both C12/C19/ C11 and C1/C9/C1 possess an essentially rather planar geometry (sp2 character), as shown by the C12−N1−C1/ C19−N1−C9/C11−N1−C1 angles (120.80(16)/121.3(2)/ 120.69(15)°), which are very close to 120°. The molecular structure of complex rac-(RR/SS)-2c was also determined, and the corresponding diagram is shown in Figure 1. The structure of complex rac-(RR/SS)-2c has pseudote-

Figure 1. Molecular structure of the catalyst precursor rac-(R,S)-2c. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for complex rac-(R,S)-2c: Ni1−N1, 2.006(4); Ni−N1A, 2.006(4); Ni−Br1, 2.3391(14); Ni−Br1A, 2.3390(14); N1− C1, 1.443(5); N1−C19, 1.279(5); N1−Ni1−N1A, 81.1(2); N1− Ni1−Br1A, 116.26(11); N1A−Ni1−Br1A, 104.83(11); N1−Ni1−Br1, 104.83(11); N1A−Ni1−Br1, 116.26(11); Br1A−Ni1−Br1, 125.30(7). Selected torsion angle (deg): C2−C1−N1−Ni1, 89.5(5).

a

trahedral geometry about the nickel center. In the solid state, the most interesting feature of ligand rac-(RR/SS)-1c is the conformation of the substituents attached to N1 and N1A. These groups are rotated about 180° from the position they must occupy to chelate Ni. The rotation has been confirmed by the crystal structure of its complex rac-(RR/SS)-2c. The X-ray structures of ligand rac-(RR/SS)-1c and complex rac-(RR/SS)2c exhibit trans and cis conformations about the central C−C bond of the backbone, respectively. The imino CN bond length of complex rac-(RR/SS)-2c (1.279 Å) is slightly larger than that of its ligand rac-(RR/SS)-1c (1.275 Å). The N1− C19−C19A bond angle of complex rac-(RR/SS)-2c (115.1°) is slightly less than that of its ligand rac-(RR/SS)-1c (116.5°) due to the N−Ni coordination. Both naphthalene rings bonded to the iminic nitrogens of the α-diimine lie nearly perpendicular to the plane formed by the nickel and coordinated nitrogen atoms. The sec-phenethyl groups in the 2-position of the naphthylamine fragments in rac-(RR/SS)-2c point toward each other above and below the plane, thus shielding the apical positions of the Ni(II) center. Its structure is similar to those reported in the literature for other similar [NiBr2(α-diimine)] compounds characterized by X-ray diffraction, {bis[N,N′-4-bromo-2,6dimethylphenyl)imino]acenaphthene}dibromonickel27 and {bis[N,N′-(2,4,6-trimethylphenyl)imino]acenaphthene}dibromonickel.28 In fact, the Ni−N bond distances in complex rac-(RR/SS)-2c (2.006 Å) are similar to those determined for these compounds (2.026 and 2.021 Å, respectively), as well as the Ni−Br bond distances (2.3390 Å for complex rac-(RR/SS)2c vs 2.3229 and 2.323 Å, respectively) and the N−Ni−Br angles (116.26° for complex rac-(RR/SS)-2c vs 113.32 and 114.4°, respectively). The free bis(2-methyl-1-naphthylimino)acenaphthene ligand 1d exhibits a cis conformation about the central C−C bond of the ligand backbone instead of the trans conformation of

Asterisks denote the chiral carbons.

usually in the presence of a formic acid. Ligand 1c was synthesized by the condensation of 2,3-butanedione with 2 equiv of the o-sec-phenethyl naphthylamine in toluene at 80− 110 °C using p-toluenesulfonic acid (p-TsOH) as catalyst. Compounds 1a−e were characterized by IR, 1H NMR, 13C NMR, and elemental analysis. The reaction of equimolar amounts of NiBr2(DME) and the naphthylamine α-diimine ligands 1a−e in CH2Cl2 led to the displacement of 1,2-dimethoxyethane and afforded the catalyst precursors 2a−e as moderately air-stable deep red microcrystalline solids in almost quantitative yields. Elemental analyses of complexes 2c,d and ligands 1b−e fit the molecular structure sobtained by X-ray structural studies. 2.2. X-ray Crystallographic Studies. Crystals of complex rac-(RR/SS)-2c, complex 2d, and ligands 1b−e suitable for Xray diffraction were obtained at −30 °C by double-layering CH2Cl2 solutions of the ligands and complexes with n-hexane. The molecular structures of the ligands 1b, rac-(RR/SS)-1c, and 1e were determined, and the corresponding diagrams are shown in Figures SI-1−SI-4 (Supporting Information). The Xray structures of the ligands 1b, rac-(R,S)-1c, and 1e exhibit a trans conformation about the central C−C bond of the ligand backbone. Bond lengths and angles are within the expected range for α-diimines; for example, the bond distances for the C12N1/C19=N1/C11N1 double bonds and the central C12−12A/C19−C19A/C11−C11A single bonds are 3961

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rings of the α-diimine Ni(II) complexes (Table 1). Complex rac-(RR/SS)-2c, bearing one bulky chiral sec-phenethyl groups in the o-naphthyl positions of the ligand, displays the highest catalytic activity (2.81 × 106 g of PE/((mol of Ni) h bar)) and produces one of the higher molecular weights (run 4, Mw = 15.5 × 104, 40 °C, [Al]/[Ni] = 800) among our five complexes. Complexes 2a (bearing a hydrogen in the o-naphthyl positions of the ligand, dimethylethane skeleton), 2b (bearing one methyl groups in the o-naphthyl positions of the ligand, dimethylethane skeleton), 2d (bearing one methyl groups in the o-naphthyl positions of the ligand, acenaphthene skeleton), and 2e (bearing a β-position in the naphthyldiimine, dimethylethane skeleton) exhibit significantly lower catalytic activity (highest activity: 2a, run 16, 0.72 × 106 g of PE/((mol of Ni) h bar); 2b, run 11, 1.10 × 106 g of PE/((mol of Ni) h bar); 2d, run 20, 1.25 × 106 g of PE/((mol of Ni) h bar); 2e, run 25, 0.41 × 106 g of PE/((mol of Ni) h bar)). In comparison with complex 2b, complex 2d produces slightly higher molecular weights and activities due to the difference in the acenaphthene and dimethylethane skeletons. In comparison with complex 2e, complex 2a produces slightly higher molecular weights and activities due to the difference in α- or β-position naphthyldiimines. These results indicate that the rate of chain propagation is greatly promoted by the bulky o-sec-phenethyl groups of the ligand’s naphthyl rings (rac-(RR/SS)-2c). As a result, the following activity trend can be summarized for our substituted precatalysts under low ethylene pressure (0.2 bar), in the range 0−60 °C: 2c > 2b ≈ 2d > 2a > 2e. In comparison with complexes 2a,b (60 °C: 2a, run 17, activity 0.31 × 106 g of PE/((mol of Ni) h bar), Mw = 4.22 × 104 g/mol; 2b, run 12, activity 0.38 × 106 g of PE/((mol of Ni) h bar), Mw = 7.42 × 104 g/mol), complex rac-(RR/SS)-2c (60 °C: run 5, activity 2.50 × 106 g of PE/((mol of Ni) h bar), Mw = 12.1 × 104 g/mol) has better thermal stability and produces higher molecular weight polyethylene; even at 60 °C, the rac(RR/SS)-2c/DEAC systems still keep high activity. The type and amount of branches formed in the polymerization of ethylene promoted by typical α-diimine nickel precatalysts depend on reaction parameters such as the reaction temperature, ethylene pressure, and ligand structure. 12 Generally, low ethylene pressure and high polymerization temperature favor the chain walking and afford highly branched polyethylenes.12 However, the effect of ligand structure on polyethylene branching is much more complicated. As shown in Table 1 and Figure 3, the catalyst system rac(RR/SS)-2c/DEAC, bearing chiral bulky o-sec-phenethyl groups, generated polyethylene with slightly lower degrees of branching. The total branching degrees of the polymer samples prepared with rac-(RR/SS)-2c/DEAC (runs 3−5, branching degrees 86, 102, and 113 branches/1000 C at 20, 40, and 60 °C, respectively) are slightly lower than those observed for 2b/ DEAC (runs 8, 11, and 12, branching degrees 93, 107, and 117 branches/1000 C at 20, 40, and 60 °C, respectively), 2a/DEAC (runs 15−17, branching degrees 100, 112, and 126 branches/ 1000 C at 20, 40, and 60 °C, respectively), 2d/DEAC (runs 19, 21, and 22, branching degrees 99, 110, and 117 branches/1000 C at 20, 40, and 60 °C, respectively). These results (Table 1) are consistent with the notion that the more electron-deficient catalyst affords a more highly branched polymer.7 In addition, 2e/DEAC systems (runs 24−26, branching degrees 80, 94, and 107 branches/1000 C at 20, 40, and 60 °C, respectively)

ligands 1b, rac-(RR/SS)-1c, and 1e due to the rigid bis(imino)acenaphthene skeleton. The different configurations of the imino units do not lead to important differences in the bond lengths (C11−N1, 1.268(8); N1−C1, 1.400(9); C11−C11A, 1.504(13) Å), being similar to those in ligands 1b, rac-(RR/SS)1c, and 1e. Both C11 and C1 possess essentially rather planar geometry (sp2 character), as shown by the C11−N1−C1 angles (121.9(6)°), which are very close to 120°. The molecular structure complex 2d was determined, and the corresponding diagram is shown in Figure 2. The imino

Figure 2. Molecular structure of the catalyst precursor 2d. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for complex 2d: Ni1−N1, 2.023(4); Ni−N1A, 2.023(4); Ni−Br1, 2.3197(9); Ni−Br1A, 2.3197(9); C12−N1, 1.227(5); O1− H1W, 0.902(18); O1−H2W, 0.910(18); N1−Ni1−N1A, 82.6(2); N1−Ni1−Br1A, 105.34(10); N1A−Ni1−Br1A, 118.31(10); N1− Ni1−Br1, 118.31(10); N1A−Ni1−Br1, 105.34(10); Br1A−Ni1−Br1, 121.09(5); N1−C12−C12A, 117.32(2); H1W−O1−H2W, 104(3).

CN bond length of complex 2d (1.227 Å) is slightly less than that of its ligand 1d (1.268 Å). The N1−C12−C12A bond angle of complex 2d (117.32°) is slightly less than that of its ligand 1d (120.3°) due to the N−Ni coordination. The Ni−N bond distances in complex 2d (2.023 Å) are similar to that determined for complex rac-(RR/SS)-2c (2.006 Å), as well as the Ni−Br bond distances (2.3197 Å for complex 2d vs 2.3391 Å for complex rac-(RR/SS)-2c) and the N−Ni−Br angles (118.31° for complex 2d vs 116.26° for complex rac-(RR/SS)2c). Both naphthalene rings bonded to the iminic nitrogens of the α-diimine lie nearly perpendicular to the plane formed by the nickel and coordinated nitrogen atoms. The methyl groups in the 2-position of the naphthylamine fragments in 2d point toward each other above and below the plane, thus shielding the apical positions of the Ni(II) center. 2.3. Polymerization of Ethylene with Nickel Complexes 2a−e. The five α-diimine nickel(II) complexes 2a−e, activated by DEAC, were tested as catalyst precursors for the polymerization of ethylene, under the same reaction conditions. The results of the polymerization experiments are shown in Table 1. It is worth noting that blank experiments carried out with DEAC alone, under similar conditions, showed its inability to polymerize ethylene on its own. At 20 °C, the activity of complex 2b increases slightly with an increase of [Al]/[Ni] ratio (runs 6−8), the maximum being reached around [Al]/[Ni] = 800 (run 8), and then the activity decreases slightly with an increase of [Al]/[Ni] ratio (runs 8 and 9). For the ratio [Al]/[Ni] = 800, the activity increases first up to 40 °C (runs 10, 8, and 11) and then decreases slightly in the range 40−70 °C (runs 11−13). The performances of the nickel precatalysts are significantly affected by the ortho-position substituents on the naphthyl 3962

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Table 1. Ethylene Polymerizations Using Nickel Precatalysts 2a−e Activated by DEACa run

complex

[Al]/[Ni]

T (°C)

t (min)

yield (g)

activityb

TOFc (h bar)−1

Mw × 10−4 d

Mw/Mnd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

2c 2c 2c 2c 2c 2b 2b 2b 2b 2b 2b 2b 2b 2a 2a 2a 2a 2d 2d 2d 2d 2d 2e 2e 2e 2e

800 800 800 800 800 400 600 800 1000 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800 800

0 10 20 40 60 20 20 20 20 0 40 60 70 0 20 40 60 0 20 30 40 60 0 20 40 60

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

0.68 1.01 1.92 2.02 1.80 0.49 0.74 0.76 0.61 0.46 0.79 0.27 0.12 0.25 0.31 0.52 0.22 0.44 0.63 0.90 0.82 0.29 0.19 0.24 0.41 0.15

0.94 1.40 2.67 2.81 2.50 0.68 1.03 1.06 0.85 0.64 1.10 0.38 0.17 0.35 0.43 0.72 0.31 0.61 0.87 1.25 1.14 0.40 0.26 0.33 0.57 0.21

0.34 0.50 0.95 1.00 0.89 0.24 0.37 0.38 0.30 0.23 0.39 0.13 0.06 0.12 0.15 0.26 0.11 0.22 0.31 0.45 0.41 0.14 0.09 0.12 0.20 0.07

22.1 20.9 19.4 15.5 12.1 18.5 17.8 19.5 16.0 21.8 13.6 7.42 3.92 18.5 16.0 4.68 4.22 22.1 20.8 18.7 15.5 8.01 13.0 10.9 6.64 4.05

1.8 1.3 1.8 1.6 1.7 1.6 1.7 1.7 1.6 1.6 1.8 2.0 2.0 1.6 1.6 2.0 1.8 1.8 1.7 1.6 1.6 1.7 1.8 1.9 1.7 1.8

branchese/1000 C

85.7 101.6 113.3

92.7

106.8 117.4

100.1 111.9 126.4 99.3 109.9 116.9 79.6 93.9 106.7

a Polymerization conditions and definitions: n(Ni) = 3.60 μmol; ethylene relative pressure 0.2 bar, ethylene absolute pressure 1.2 bar; t = polymerization time; solvent toluene (50 mL); T = polymerization temperature. bActivity in 106 g of PE/((mol of Ni) h bar). cTurnover frequency in 105 mol of ethylene/((mol Ni) h bar). dMw in 104 g/mol, determined by GPC. eEstimated by 1H NMR:29

branches/1000 C =

1 I 3 CH3 ICH2 + ICH3 + ICH

× 1000

2

4. The number of branches was calculated according to the literature,30 and it was found that the polyethylene with 108 branches/1000 carbons (75 methyl, 9 ethyl, 5 propyl, and 19 butyl or longer branches/1000 C) was obtained at 40 °C (run 4 in Table 1). This result was consistent with that calculated from 1 H NMR. The rationale for the formation of the major types of branches (methyl, ethyl, and butyl) in the polyethylenes obtained in this work is shown in Scheme 2. 2.4. Polymerization of Methyl Methacrylate (MMA) with Nickel Complexes rac-(RR/SS)-2c and 2d. Polymerizations of MMA with complexes rac-(RR/SS)-2c and 2d in combination with DEAC were carried out at −30, 0, 25, and 50 °C, and the results are summarized in Table 2. For the ratio [Al]/[Ni] = 800, an increase in the polymerization temperature in the range −30 to +30 °C increases the activity of the precatalysts rac-(RR/SS)-2c and 2d (runs 1−3, 4−7). In addition, the rate of chain propagation is greatly promoted by the bulky o-sec-phenethyl groups of the ligand’s naphthyl rings (rac-(RR/SS)-2c). As shown in Table 2 and Figures 5 and 6, catalysts rac-(RR/ SS)-2c and 2d all gave syndiotactic-rich poly(methyl methacrylate) (PMMA). At room temperature, the tacticities of PMMA prepared with rac-(RR/SS)-2c/DEAC (run 3: rr 69.71%, mr 26.11%, mm 4.18%) are significantly higher than those observed for 2d/DEAC (run 7: rr 58.71%, mr 32.26%, mm 9.03%). The tacticities of PMMA prepared with 2d/DEAC

Figure 3. 1H NMR (400 MHz, CDCl3/1,2,4-trichlorobenzene, 1/3 v/ v) spectra of the polyethylenes catalyzed by rac-(RR/SS)-2c, 2d, 2b, 2a, and 2e/DEAC at 40 °C (Table 1s runs 4, 21, 11, 16 and 25): ICH3, integrated intensity between 0.8 and 1.0 ppm; ICH2 + ICH, integrated intensity between 1.0 and 1.5 ppm.

produced polyethylene with the lowest degrees of branching among our five complexes. The 13C NMR spectrum of the polyethylene prepared with the catalyst rac-(RR/SS)-2c/DEAC at 40 °C is shown in Figure 3963

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DEAC at high temperature (run 3, 25 °C) and for 2d/DEAC at low temperature (run 5, 25 °C). A naphthyl ligand bearing chiral bulky sec-phenethyl groups in the o-naphthyl position may better control the stereoselective of monomer insertion, which should afford a highly syndiotactic PMMA. A rationale for the formation of the syndiotactic poly(methyl methacrylate) catalyzed by rac-(RR/SS)-2c/DEAC is shown in Scheme 3.

3. CONCLUSION A series of new naphthyl-α-diimine ligands and their Ni(II) complexes have been prepared and characterized. Ligands 1a−e were modified in an attempt to change steric effects and the electronic density of the metal center and eventually to improve the activity in the polymerization of ethylene and methyl methacrylate and control the microstructure of polyethylene, in particular, the stereoregularity structure of poly(methyl methacrylate). The results obtained show that the chiral complex rac-(RR/SS)-2c, activated by DEAC, produces a highly active catalyst system for the polymerization of ethylene and highly branched polyethylene at high temperature. The rate of chain propagation is promoted by the bulky o-sec-phenethyl groups of the ligand’s naphthyl rings. Interestingly, the rac(RR/SS)-2c/DEAC catalyst system could produce syndiotactic PMMA at low temperature. Complex rac-(RR/SS)-2c, bearing chiral bulky sec-phenethyl groups in the o-naphthyl position, may better control the stereoselectivity of monomer insertion, which should afford highly syndiotactic PMMA.

Figure 4. 13C NMR (CDCl3/o-dichlorobenzene, 1/3 v/v) spectrum of polyethylene catalyzed by rac-(RR/SS)-2c/DEAC at 40 °C (Table 1, run 4). Note on labels: for xBy By is a branch of length y carbons, x is the carbon being discussed, and the methyl at the end of the branch is numbered 1. Thus, the second carbon from the end of a butyl branch is 2B4. xBy+ refers to branches of length y and longer. The methylenes in the backbone are labeled with Greek letters, which determine how far from a branch point methine each methylene is; α denotes the first methylene next to the methine. Thus, γB1+ refers to methylenes γ from a branch of length 1 or longer.

at lower temperature (run 5, −30 °C: rr 62.72%, mr 28.90%, mm 8.38%) are slightly higher than those observed at high temperature (run 7, 25 °C). Interestingly, the tacticities of PMMA prepared with rac-(RR/SS)-2c/DEAC at lower temperature (run 1, −30 °C: rr 88.75%, mr 7.26%, mm 3.99%) are significantly higher than those observed for rac-(RR/SS)-2c/

4. EXPERIMENTAL SECTION 4.1. General Considerations. All operations were carried out under an N2 atmosphere using standard Schlenk techniques unless otherwise noted. Methylene chloride and o-dichlorobenzene were predried with 4 Å molecular sieves and distilled from CaH2 under dry nitrogen. Toluene, methyl methacrylate (MMA), diethyl ether, and

Scheme 2. Rationale for the Formation of the Major Types of Branches (Methyl, Ethyl, and Butyl) in the Polyethylenes Obtained in This Worka

a

Asterisks denote chiral carbons. 3964

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Table 2. Polymerization Results for MMA Using rac-(RR/SS)-2c/DEAC and 2d/DEAC Catalytic Systemsa triad fractiond (%) b

c

entry

precatalyst

T (°C)

yield (%)

Mn

1 2 3 4 5 6 7 8

2c 2c 2c 2c 2d 2d 2d 2d

−30 0 25 50 −30 0 25 50

10.3 18.6 27.9 25.7 8.2 15.2 20.3 19.5

27.6 29.5 19.8 10.7 18.2 20.9 14.2 8.5

Mw/Mnc

rr

mr

mm

1.2 1.2 1.3 1.5 1.2 1.5 1.6 1.6

88.75

7.26

3.99

69.71

26.11

4.18

62.72

28.90

8.38

58.71

32.26

9.03

a Polymerization conditions and definitions: n(2c) = n(2d) = 3.6 μmol, [Al]/[Ni] = 800; polymerization time 12 h; VMMA = 10 mL (ρ = 0.94 g/mL); solvent toluene (30 mL); T = polymerization temperature. bDefined as (mass of dry polymer recovered)/(mass of monomer used). cDetermined by means of GPC. dObserved in 13C NMR spectra of quaternary carbon resonance, from low to high field in the spectra (δ 45.8−44.8 ppm).31

Figure 5. 13C NMR (CDCl3) spectrum of syndiotactic PMMA obtained with rac-(RR/SS)-2c/DEAC catalyst at −30 °C. 4.2. Syntheses and Characterization. The structures and numbering schemes for compounds c and 1a−e are given in Chart 1. 4.2.1. Synthesis of 2-(sec-Phenylethyl)naphthalen-1-amine (racc). CF3SO3H (0.06 g, 0.40 mmol), 1-aminonaphtalene (0.29 g, 2.00 mmol), styrene (0.31 g, 3.00 mmol), and xylene (1 mL) were placed in a 10 mL Schlenk flask and stirred at 160 °C for 5 h. The mixture was extracted three times with 15 mL of diethyl ether. Volatile materials were removed, and the residue was purified by chromatography on silica gel with petroleum ether/ethyl ester (15/1 v/v) to give 2-(1phenylethyl)naphthalen-1-amine. Yield: 0.35 g (71%). 1H NMR (400 MHz, CDCl3, ppm): δ 1.68 (t, 3H, J = 7.6 Hz, protons of C12), 3.98 (br, s, 2H, −NH2), 4.22 (q, J = 6.8 Hz, 1H, protons of C11), 7.15− 7.28 (m, 5H, protons of C15, C13, and C14), 7.34−7.40 (m, 3H, protons of C3, C4, and C7), 7.44 (t, 1H, J = 8.4 Hz, protons of C6), 7.67−7.70 (d, 1H, protons of C5), 7.76 (d, 1H, J = 6.4 Hz, protons of C8). 13C NMR (100 MHz, CDCl3, ppm): δ 21.60 (C12), 40.39 (C11), 120.95 (C4), 123.03 (C2), 123.07 (C8), 124.35 (C9), 124.44 (C6), 124.56 (C7), 126.32 (C3), 128.30 (C15), 128.68 (C5), 131.05 (C13), 137.52 (C14), 137.60 (C10), 145.67 (C1), 147.48 (C16). 4.2.2. Synthesis of the Ligands 1a−e. Synthesis of Bis[N,N′-(1naphthyl)imino]-1,2-dimethylethane (1a). Formic acid (0.50 mL) was added to a stirred solution of 2,3-butanedione (0.17 g, 2.00 mmol) and 1-naphthylamine (0.57 g, 4.00 mmol) in methanol (25 mL). The mixture was refluxed for 24 h and then cooled, and the precipitate was separated by filtration. The solid was recrystallized from EtOH/ CH2Cl2 (7/1 v/v), washed with cold ethanol, and dried under vacuum. Yield: 0.56 g (83%). 1H NMR (400 MHz, CDCl3, ppm): δ 2.31 (s, 6H, protons of C12), 6.85 (d, 2H, J = 6.8 Hz, protons of C2), 7.48− 7.55 (m, 6H, protons of C3, C6, and C7), 7.66 (d, 2H, J = 8.4 Hz, protons of C4), 7.82 (d, 2H, J = 7.6 Hz, protons of C5), 7.88 (d, 2H, J = 8.0 Hz, protons of C8). 13C NMR (100 MHz, CDCl3, ppm): δ 15.93 (C12), 112.94 (C2), 123.34 (C4), 124.10 (C6), 125.47 (C7), 125.65 (C9), 125.75 (C5), 126.28 (C8), 128.04 (C3), 134.10 (C10), 146.96 (C1), 169.08 (C11). Anal. Calcd for C24H20N2: C, 85.68; H, 5.99; N, 8.33. Found: C, 85.45; H, 6.22; N, 8.48. FT-IR (KBr): 1639 cm−1 (νCN). Synthesis of Bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2-dimethylethane (1b). Using the same procedure as for the synthesis of 1a, 1b was obtained as an orange powder. Yield: 0.59 g (81%). 1H NMR (400

Figure 6. 13C NMR (CDCl3) spectra of PMMA obtained with rac(RR/SS)-2c/DEAC and 2d/DEAC catalysts at 25 and −30 °C.

1,2-dimethoxyethane (DME) were distilled from sodium/benzophenone under an N2 atmosphere. Anhydrous NiBr2 (99%) and diethylaluminum chloride (DEAC, 0.9 M solution in toluene) were obtained from Acros. Acenaphthenequinone (98%), 2,3-butanedione (98%), 1-amino-2-methylnaphthalene (98%), 1-aminonaphthalene (98%), and 2-aminonaphthalene (98%) were purchased from Alfa Aesar and used without further purification. NiBr2 (DME) was synthesized according to the literature.32 NMR spectra were recorded at 400 (1H) and 100 (13C) MHz on a Varian Mercury plus-400 instrument with TMS as internal standard. FT-IR spectra were recorded on a Digilab Merlin FTS 3000 FTIR spectrophotometer in KBr pellets. The molecular weights and molecular weight distributions (Mw/Mn) of the polymers were determined by gel permeation chromatography/size-exclusion chromatography (GPC/SEC) via a Waters Alliance GPCV2000 chromatograph, using 1,2,4-trichlorobenzene as eluent, at a flow rate of 1.0 mL/ min and 140 °C. The elemental content of samples was determined by an elemental analyzer (Vaiio-EL106, Germany). Single-crystal structures were determined on a Bruker APEX-II CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) in the ω scan mode. 3965

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Scheme 3. Rationale for the Formation of the Syndiotactic Poly(methyl methacrylate) Catalyzed by rac-(RR/SS)-2c/DEAC

then the solvent was removed. The residue was purified by chromatography on silica gel with petroleum ether/ethyl ester (20/1 v/v) to give a yellow powder. Yield: 0.35 g (64%). 1H NMR (400 MHz, CDCl3, ppm): δ 1.55−1.76 (m, 12H, protons of C12 and C18), 4.23 (q, J = 7.6 Hz, 2H, protons of C11), 7.11−7.14 (m, 2H, protons of C3), 7.15−7.19 (m, 2H, protons of C16), 7.21−7.32 (m, 8H, protons of C14 and C15), 7.42−7.45 (m, 2H, protons of C6), 7.47− 7.55 (m, 4H, protons of C7 and C4), 7.73−7.76 (m, 2H, protons of C5), 7.83−7.91 (m, 2H, protons of C8). 13C NMR (100 MHz, CDCl3, ppm): δ 16.38 (C18), 22.62 (C12), 40.85 (C11), 122.19(C6), 123.28 (C4), 124.02 (C7), 124.60 (C16), 125.20 (C3), 125.89 (C9), 126.32 (C8), 127.48 (C5), 127.67 (C14), 128.18 (C2), 128.70 (C15), 132.78 (C10), 144.67 (C13), 147.06 (C1), 170.17 (C17). Anal. Calcd for C40H36N2: C, 88.20; H, 6.66; N, 5.14. Found: C, 88.37; H, 6.48; N, 5.29. FT-IR (KBr): 1633 cm−1 (νCN). Single crystals of ligand (S,R)1c suitable for X-ray analysis were obtained at −30 °C by dissolving the ligand in CH2Cl2, followed by slow layering of the resulting solution with n-hexane. Synthesis of Bis[N,N′-(2-methyl-1-naphthyl)imino]acenaphthene (1d). Formic acid (0.50 mL) was added to a stirred solution of acenaphthenequinone (0.55 g, 3.00 mmol) and 1-amino-2-methylnaphthalene (0.94 g, 6.00 mmol) in ethanol (30 mL). The mixture was refluxed for 24 h and then cooled, and the precipitate was separated by filtration. The solid was recrystallized from EtOH/CH2Cl2 (6/1 v/v), washed with cold ethanol, and dried under vacuum to give the red ligand. Yield: 1.17 g (85%). 1H NMR (400 MHz, CDCl3, ppm): δ 2.44 (s, 6H, protons of C11), 6.48 (d, 2H, J = 7.2 Hz, protons of C3), 7.22 (t, 2H, J = 7.6 Hz, protons of C7), 7.36 (t, 2H, J = 7.6 Hz, protons of C6), 7.43−7.50 (m, 4H, protons of C13 and C4), 7.72 (d, 2H, J = 8.0 Hz, protons of C5), 7.80 (d, 2H, J = 8.4 Hz, protons of C8), 7.89 (m, 4H, protons of C14 and C12). 13C NMR (100 MHz, CDCl3, ppm): δ 18.05 (C11), 119.76 (C2), 123.10 (C4), 123.70 (C14), 124.27 (C6), 124.31 (C7), 125.39 (C13), 125.44 (C3), 125.69 (C9), 127.79 (C8), 128.12 (C5), 129.18 (C15), 129.23 (C12), 130.93 (C10), 131.34 (C17), 132.81 (C16), 145.55 (C1), 162.19 (C18). Anal. Calcd for C34H24N2: C, 88.67; H, 5.25; N, 6.08. Found: C, 88.74; H, 5.19; N, 6.15. FT-IR (KBr): 1639 cm−1 (νCN). Single crystals of ligand 1d suitable for X-ray analysis were obtained at −30 °C by dissolving the ligand in CH2Cl2, followed by slow layering of the resulting solution with n-hexane. Synthesis of Bis[N,N′-(2-naphthyl)imino]-1,2-dimethylethane (1e). Using the same procedure as for the synthesis of 1a, 1e was obtained as an orange powder. Yield: 0.56 g (83%). 1H NMR (400 MHz, CDCl3, ppm): δ 2.25 (s, 6H, protons of C12), 7.05 (d, 2H, J = 8.4 Hz, protons of C3), 7.19 (s, 2H, protons of C1), 7.41−7.45 (m, 2H, protons of C6), 7.49 (t, 2H, J = 6.8 Hz, protons of C7), 7.80 (d,

Chart 1. Structure of Compounds c and 1a−e

MHz, CDCl3, ppm): δ 2.16 (s, 6H, protons of C13), 2.26 (s, 3H, protons of C11), 2.31 (s, 3H, protons of C11), 7.40 (d, 2H, J1 = 7.2 Hz, protons of C3), 7.44−7.50 (m, 4H, protons of C6 and C7), 7.56− 7.65 (m, 4H, protons of C4 and C5), 7.84−7.86 (m, 2H, protons of C8). 13C NMR (100 MHz, CDCl3, ppm): δ 16.52 (C13), 17.82 (C11), 119.76 (C2), 122.61 (C6), 123.33 (C4), 124.15 (C7), 125.48 (C8), 126.06 (C9), 128.00 (C5), 128.98 (C3), 132.62 (C10), 144.52 (C1), 169.63 (C12). Anal. Calcd for C26H24N2: C, 85.68; H, 6.64; N, 7.69. Found: C, 85.54; H, 6.78; N, 7.74. FT-IR (KBr): 1638 cm−1 (νCN). Single crystals of ligand 1b suitable for X-ray analysis were obtained at −30 °C by dissolving the ligand in CH2Cl2, followed by slow layering of the resulting solution with n-hexane. Synthesis of Bis[N,N′-(2-sec-phenethyl-1-naphthyl)imino]-1,2dimethylethane (rac-(SS/RR)-1c). 4-Methylbenzenesulfonic acid (20 mg, 0.10 mmol) was added to a stirred solution of 2,3-butanedione (0.086 g, 1.00 mol) and 2-sec-phenethyl-1-naphthylamine (0.52 g, 2.10 mmol) in toluene (20 mL). The mixture was refluxed for 16 h, and 3966

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2H, J = 8.4 Hz, protons of C5), 7.84 (d, 2H, J = 8.0 Hz, protons of C8), 7.88 (d, 2H, J = 8.8 Hz, protons of C4). 13C NMR (100 MHz, CDCl3, ppm): δ 15.68 (C12), 114.94 (C3), 119.98 (C1), 124.72 (C7), 126.42 (C6), 127.43 (C4), 127.78 (C5), 128.93 (C8), 130.67 (C10), 133.95 (C9), 148.57 (C2), 168.63 (C11). Anal. Calcd for C24H20N2: C, 85.68; H, 5.99; N, 8.33. Found: C, 85.55; H, 6.15; N, 8.47. FT-IR (KBr): 1638 cm−1 (νCN). 4.2.3. Synthesis of the Catalysts 2a−e. Synthesis of {Bis[N,N′-(1naphthyl)imino]-1,2-dimethylethane}dibromonickel (2a). [NiBr2(DME)] (0.31 g, 1.00 mmol), ligand 1a (0.34 g, 1.00 mmol), and dichloromethane (30 mL) were mixed in a Schlenk flask and stirred at room temperature for 16 h. The resulting suspension was filtered. The solvent was removed under vacuum, and the residue was washed with diethyl ether (3 × 16 mL) and then dried under vacuum at room temperature to give catalyst 2a. Yield: 0.50 g (91%). Anal. Calcd for C24H20Br2NiN2: C, 51.94; H, 3.63; N, 5.05. Found: C, 51.86; H, 3.78; N, 5.29. FT-IR (KBr): 1628 cm−1 (νCN). Synthesis of {Bis[N,N′-(2-methyl-1-naphthyl)imino]-1,2dimethylethane}dibromonickel (2b). Using the same procedure as for the synthesis of 2a, 2b was obtained as a purple powder. Yield: 0.54 g (93%). Anal. Calcd for C26H24Br2NiN2: C, 53.57; H, 4.15; N, 4.81. Found: C, 53.76; H, 4.03; N, 4.59. FT-IR (KBr): 1624 cm−1 (νCN). Synthesis of {Bis[N,N′-(2-sec-phenylethyl-1-naphthyl)imino]-1,2dimethylethane}dibromonickel (rac-(SS/RR)-2c). Using the same procedure as for the synthesis of 2a, rac-(SS/RR)-2c was obtained as a purple powder. Yield: 0.65 g (86%). Anal. Calcd for C40H36Br2NiN2: C, 62.95; H, 4.75; N, 3.67. Found: C, 62.87; H, 4.96; N, 3.86. FT-IR (KBr): 1629 cm−1 (νCN). Single crystals of complex (R,S)-2c suitable for X-ray analysis were obtained at −30 °C by dissolving the complex in CH2Cl2, followed by slow layering of the resulting solution with n-hexane. Synthesis of {Bis[N,N′-(2-methyl-1-naphthyl)imino]acenaphthene}dibromonickel (2d). Using the same procedure as for the synthesis of 2a, 2d was obtained as a dark red powder. Yield: 0.63 g (90%). Anal. Calcd for C34H28Br2N2NiO2: C, 57.11; H, 3.95; N, 3.92. Found: C, 56.95; H, 4.18; N, 4.19. FT-IR (KBr): 1625 cm−1 (νCN). Single crystals of complex 2d suitable for X-ray analysis were obtained at −30 °C by dissolving the complex in CH2Cl2, followed by slow layering of the resulting solution with n-hexane. Synthesis of {Bis[N,N′-(2-naphthyl)imino]-1,2-dimethylethane}dibromonickel (2e). Using the same procedure as for the synthesis of 2a, 2e was obtained as a purple powder. Yield: 0.51 g (93%). Anal. Calcd for C24H20Br2NiN2: C, 51.94; H, 3.63; N, 5.05. Found: C, 51.86; H, 3.78; N, 5.19. FT-IR (KBr): 1627 cm−1 (νCN). 4.3. X-ray Structure Determinations. Single crystals of ligands 1b−e, complex 2c, and complex 2d suitable for X-ray analysis were obtained at −30 °C by dissolving the ligands and nickel complexes in CH2Cl2, followed by slow layering of the resulting solution with nhexane. Data collections were performed at 296(2) K on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated MoKα radiation (λ = 0.71073 A). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to yield the reflection data file. The structures were solved by using the SHELXTL program. Refinement was performed on F2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at calculated positions and were included in the structure calculation without further refinement of the parameters. Crystal data, data collection, and refinement parameters are given in Tables SI-1 and SI-2 (Supporting Information). 4.4. Ethylene Polymerization. Polymerization of ethylene was carried out in a flame-dried 250 mL crown-capped pressure bottle sealed with a neoprene septum. After the polymerization bottle was dried under an N2 atmosphere, 50 mL of dry toluene was placed to the polymerization bottle. The resulting solvent was then saturated with a prescribed ethylene pressure. The cocatalyst (DEAC) was then added in [Al]/[Ni] molar ratios in the range of 400−1000 to the polymerization bottle via a syringe. At this time, the solutions were thermostated to the desired temperature and allowed to equilibrate for

15 min. Subsequently, an o-dichlorobenzene solution of Ni catalyst was added to the polymerization reactor. The polymerization, conducted under a dynamic pressure of ethylene, was terminated by quenching the reaction mixtures with 100 mL of a 3% HCl−MeOH solution. The precipitated polymer was filtered, washed with methanol, and dried under vacuum at 60 °C to a constant weight. 4.5. Polymerization of MMA. MMA polymerizations were carried out in a Schlenk tube (100 mL) with a connection to a vacuum system. In a typical procedure, the required amounts of MMA, DEAC, and precatalyst ([Al]/[Ni] = 800) were charged into a dry Schlenk tube in this order under nitrogen flow. The polymerization was carried out at constant temperature for 12 h. The resulting solution was poured into acidified methanol (100 mL of a 5% v/v solution of HCl). The polymer was then isolated by filtration and washed with methanol before drying overnight at 60 °C. The polymer yield was determined by gravimetry.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Figures, tables, and CIF files giving crystallographic data for ligands 1b−e and complexes 2c,d. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*J.Y.: tel, +86-931-7971539; fax, +86-931-7971261; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (20964003), the Scientific Research Foundation for the Returned Overseas Chinese Scholars of State Education Ministry (2009-1341), and the Science and Technology Activities Merit-based Foundation for the Returned Overseas Chinese Scholars of State Human Resources and Social Security Ministry (2009-416) for funding. We also thank the Key Laboratory of Eco-Environment-Related Polymer Materials of the Ministry of Education, Key Laboratory of Polymer Materials of Gansu Province (Northwest Normal University), for financial support.

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REFERENCES

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