Chiral Palladium(II) and Nickel(II) Complexes with C2-Symmetrical

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Chiral Palladium(II) and Nickel(II) Complexes with C2‑Symmetrical Tridentate Bis(oxazoline) Ligands: Synthesis, Characterization, and Catalytic Norbornene Polymerization Jianyun He,† Zhanxiong Liu,† Gaixia Du, Yinxia Fu, Shaowen Zhang,* and Xiaofang Li* Key Laboratory of Cluster Science of Ministry of Education, Department of Chemistry, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing 100081, People’s Republic of China S Supporting Information *

ABSTRACT: A series of chiral palladium(II) and nickel(II) complexes bearing a C2-symmetric monoanionic tridentate bis(oxazoline) ligand, (R2-(S,S)-BOZ)M(X) (1−6: R = CH(CH3)2, M = Pd, X = OAc (1); R = CH(CH3)2, M = Pd, X = Cl (2); R = Ph, M = Pd, X = Cl (3); R = Ph, M = Ni, X = Cl (4); R = CH(CH3)2, M = Ni, X = Cl (5); R = CH(CH3)2, M = Pd, X = OTf (6)), have been synthesized and structurally characterized. The experimental results demonstrate that such chiral palladium(II) and nickel(II) complexes bearing C2-symmetric tridentate ligands in which the monoanionic group is located inside are effective for norbornene polymerization. In the presence of various cocatalysts such as MAO, MMAO, and activator/AlR3, these chiral palladium(II) complexes exhibit much higher activities of up to 4.8 × 108 g of PNB (mol of Pd)−1 h−1 for the vinylic polymerization of norbornene, affording insoluble polynorbornenes with high packing density. In contrast, the chiral nickel(II) complexes show relatively low activities of ca. 4.5 × 107 g of PNB (mol of Ni)−1 h−1 and produce both insoluble polynorbornenes and soluble high-molecular-weight polynorbornenes with moderate molecular weight distributions.

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INTRODUCTION The development of highly efficient catalysts based on late transition metals has drawn much attention from both academic and industrial researchers in the field of metalcatalyzed olefin polymerization.1 Vinyl-type polynorbornene (PNB) is a 2,3-connected, rotationally strongly constrained cyclic olefin polymer which shows many interesting properties such as high density in the amorphous state, high chemical resistance, high optical transparency, a very high glass transition temperature (Tg), a large refractive index, and a small optical birefringence and dielectric loss and therefore has been extensively applied to many electronic and optical applications.2 So far, a large number of catalysts based on transition metals such as nickel,3 palladium,3l−r,4 iron,5 copper,6 cobalt,7 chromium,8 titanium,9 and zirconium10 have been reported for the vinylic polymerization of norbornene. Among them, late-transition-metal palladium(II) and nickel(II) complexes are two types of outstanding catalysts, and most of them exhibit very high activities above 1.0 × 106 g (mol of M)−1 h−1 in norbornene polymerization. In general, these palladium(II) and nickel(II) complexes adopt one bidentate neutral ligand or monoanionic ligand such as [NN] (e.g., anilido-imine, benzamidinato, β-diketiminato, diimine, imino-pyrrolylato, benzimidazole, bipyridine, α-dioxime), [NO] (e.g., β-ketoiminato, β-diketonato, salicylaldiminato, acylhydrazone, indanimine), [OO] (e.g., β-diketonato, acetylacetonato), [CC] (e.g., © XXXX American Chemical Society

allyl), or [PP] (e.g., bis(phosphino)alkane, diphosphane).3a−d,4a−c Recently, some palladium(II) and nickel(II) complexes bearing an unsymmetrical tridentate neutral or monoanionic ligand in which the anionic atom is often located on one side of the ligand [NNN], [NNC], [NNO], [ONO], [PNO], or [CNS] have been also reported to be active in the vinylic polymerization of norbornene.3e−r,4d In contrast, only one symmetry-neutral tridentate [NNN]-bis(imino) pyridyl nickel(II) complex has been probed to be effective for norbornene polymerization.3s To our knowledge, palladium(II) and nickel(II) complexes containing a symmetrical monoanionic tridentate ligand have not been used as efficient catalysts for the vinyl-type polymerization of norbornene. Therefore, the investigation of the catalytic potential of such palladium(II) and nickel(II) complexes in the vinylic polymerization of norbornene remains a challenge to date. A chiral monoanionic tridentate bis(oxazoline) ligand has a C2-symmetrical configuration, and it has been used widely in organic synthesis.11 Moreover, it can also provide a unique scaffold for the synthesis and isolation of organometallic reagents based on transition metals such as nickel,12 iron,13 and cobalt,13b exhibiting high activity in asymmetric reactions, including transformation, bond formation, and catalysiReceived: July 24, 2014

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dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Chiral Palladium(II) and Nickel(II) Complexes by using a Chiral (S,S)-Bis(oxazoline) Ligand

Figure 1. Monitoring of synthesis of the chiral palladium(II) complex 1 in d6-benzene using 1H NMR spectroscopy.

s.11a−g,12a,13b Recently, we reported the synthesis and structural characterization of chiral rare-earth-metal dialkyl complexes bearing this C2-symmetrical monoanionic tridentate bis(oxazoline) ligand.14 In the presence of 1 equiv of an activator such as the borate [Ph3C][B(C6F5)4] (A) or [PhMe2NH][B(C6F5)4] (B) or the borane B(C6F5)3 (C), with or without 2 equiv of AliBu3, these chiral scandium complexes served as highly efficient catalysts in the quasi-living trans-1,4-polymerization of isoprene (activity >6.8 × 105 g of PIP (mol of Ln)−1 h−1), affording pure trans-1,4-polyisoprenes (trans-1,4-selectivity: 99−100%) with moderate molecular weights (Mn = (0.2− 1.0) × 105). Such excellent results stimulated us to explore the synthesis of novel bis(oxazoline) complexes based on other transition metals and further investigate their catalytic potential in olefin polymerization. As far as we are aware, such a bis(oxazoline) palladium(II) complex has not been reported previously. Its catalytic potential for olefin polymerization has remained unexplored up to now. We report herein the synthesis and structural characterization of the series of chiral palladium(II) and nickel(II) complexes 1−6, bearing C2-symmetrical monoanionic triden-

tate bis(oxazoline) ligands. In the presence of various cocatalysts such as methylaluminoxane (MAO), modified methylaluminoxane (MMAO), or activator (perfluorinated borate or borane) together with aluminum alkyls (AlR3), these chiral palladium(II) and nickel(II) complexes showed very high activities up to 4.8 × 108 g of PNB (mol of M)−1 h−1 for the vinylic polymerization of norbornene, affording insoluble polynorbornenes with high packing density or soluble high-molecular-weight polynorbornenes with moderate molecular weight distributions, respectively.

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes 1−6. The C2-symmetric tridentate (S,S)-bis(oxazoline) ligands with different substitutes were synthesized according to the literature.11b The reaction between Pd(OAc)2 and the (S,S)bis(oxazoline) ligand in benzene at 30 °C for 3 h afforded chiral (S,S)-bis(oxazoline) palladium(II) acetate complex [R2-(S,S)BOZ]Pd(OAc) (1) (R = (CH(CH3)2)2, 86%) in high yield (Scheme 1). Similarly, the combination of Pd(CH3CN)2Cl2 with the (S,S)-bis(oxazoline) ligand in THF at room temperB

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Figure 2. Monitoring of synthesis of the chiral palladium(II) complex 6 in d6-benzene using 1H NMR spectroscopy.

Single crystals of the complexes 3 and 4 suitable for an X-ray determination were grown from a mixed THF/hexane solution at −30 °C. The ORTEP diagrams and selected bond lengths and angles of these complexes are shown in Figure 3. The X-ray

ature for 2 h produced the chiral (S,S)-bis(oxazoline) palladium(II) chloride complexes [R2-(S,S)-BOZ]PdCl (2, R = CH(CH3)2, 90%; 3, R = Ph, 82%) in high yields, respectively (Scheme 1). The corresponding chiral (S,S)-bis(oxazoline) nickel(II) chloride complex [R2-(S,S)-BOZ]NiCl (4, R = Ph, 93%) was also obtained via the reaction of Ni(NMe2CH2CH2NMe2)PhCl and the (S,S)-bis(oxazoline) ligand under similar conditions (Scheme 1). When AgOTf was added to complex 2 in benzene at room temperature, the chiral palladium(II) triflate complex [R2-(S,S)-BOZ]Pd(OTf) (6; R = (CH(CH3)2)2, 90%) was immediately formed (Scheme 1). In order to thoroughly investigate the catalytic potential of the chiral palladium(II) and nickel(II) complexes in norbornene polymerization, the known chiral (S,S)-bis(oxazoline) nickel(II) chloride complex [R2-(S,S)-BOZ]NiCl (5, R = (CH(CH3)2)2, 96%) was also prepared for comparison.12a 1 H NMR spectroscopy was used to monitor the syntheses of the palladium(II) complexes 1 and 6 at room temperature. 1H NMR spectra demonstrated that the reaction of Pd(OAc)2 and the (S,S)-bis(oxazoline) ligand was easily carried out at 30 °C. After 3 h, the proton for the −NH group in the (S,S)bis(oxazoline) ligand assigned at 11.5 ppm almost completely disappeared; meanwhile, the proton for the −COOH group in the newly generated free acetic acid (AcOH) appeared at 12.9 ppm, indicating the completion of this reaction (Figure 1). In contrast, only 0.5 h was needed for the generation of complex 6 via the combination of complex 2 with AgOTf at 25 °C. By comparison with the 1H NMR spectrum of the complex 2, all of the signals changed their positions, as shown in the 1H NMR spectrum of the final product (Figure 2). The resulting complexes 1−6 are soluble in common organic solvents such as THF, toluene, and benzene and give well-resolved NMR spectra in CDCl3. 1H NMR spectra of the chiral palladium(II) and nickel(II) complexes 1−6 indicated that the two different multiplets for the methylene group in the oxazolinyl ring between 2.0 and 6.0 ppm integrating for four protons were observed, suggesting a symmetry in the chelating (S,S)bis(oxazoline) ligand.

Figure 3. ORTEP drawings of 3 and 4 with 30% thermal ellipsoids. The hydrogen atoms in 3 and 4 are omitted for clarity. Selected bond distances (Å) and angles (deg): 3, Pd−N1 1.998(6), Pd−N2 2.005(6), Pd−N3 1.996(6), Pd−Cl 2.327(2), ∠N1−Pd−N2 87.9(3), ∠N2− Pd−N3 89.1(2), ∠N1−Pd−N3 175.7(3), ∠N1−Pd−Cl 91.1(2), ∠N3−Pd−Cl 92.1(2), ∠N2−Pd−Cl 176.7(2); 4, Ni−N1 1.878(5), Ni−N2 1.884(4), Ni−N3 1.890(4), Ni−Cl 2.195(2), ∠N1−Ni−N2 89.2(2), ∠N2−Ni−N3 88.8(2), ∠N1−Ni−N3 175.1(2), ∠N1−Ni− Cl 90.6(2), ∠N3−Ni−Cl 91.7(2), ∠N2−Ni−Cl 177.2(2).

diffraction study revealed that the complexes 3 and 4 are isomorphous and isostructural (Figure 3 and Supporting Information). Complexes 3 and 4, which contain one C2symmetrical monoanionic tridentate (S,S)-bis(oxazoline) unit and one chloride group, both adopt a nearly ideal square-planar coordination geometry. Because of the ionic radius of the metal center with a trend Ni2+ (69 pm) < Pd2+ (86 pm), the distances of the chelating M−N(1), M−N(2), and M−N(3) bonds as well as the M−Cl bond decrease in the order 3 > 4. In complex 3, the Pd−N2 bond with the negatively charged oxygen atom is slightly longer at 2.005(6) Å than the Pd−N1 or Pd−N3 bonds with the neutral oxygen atom at 1.998(6) Å. In contrast, three Ni−N bonds in complex 4 increase in the order Ni−N1 (1.878(5) Å) < Ni−N2 (1.884(4) Å) < Ni−N3 (1.890(4) Å). C

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Table 1. Vinyl Polymerization of Norbornene by Chiral Palladium(II) and Nickel(II) Complex/Cocatalyst Systemsa

entry

cat. (amt (μmol))

metal

R

X

cocat.b

[Al]/[M]

M (amt (mmol))

Sc

T (°C)

t (min)

cat. (%)

Ad

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25e 26f 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43e 44f

1 (0.5) 6( 0.5) 2 (0.5) 2 (0.5) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (1.0) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.5) 2 (0.1) 2 (0.1) 2 (0.5) 2 (0.5) 3 (0.5) 4 (0.5) 5 (0.5) 5 (1.0) 5 (1.0) 5 (1.0) 5 (1.0) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5) 5 (0.5)

Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Pd Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni Ni

CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 Ph Ph CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2 CH(CH3)2

OAc OTf Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl Cl

MAO MAO MAO MMAO A B/AliBu3 B/AlMe3 A/AlMe3 C/AlMe3 MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO MMAO B/AliBu3 B/AlMe3 A/AlMe3 C/AlMe3 MAO MAO MAO MAO MAO MAO MAO MAO MAO MAO

8000 8000 8000 8000

NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (11) NB (32) NB (32) NB (32) NB (32) NB (32) AN (21) EN (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) NB (21) AN (21) EN (21)

T T T T T C C C C T T T T T T T T T T T C C C D T T T T T T C C C C T T T T T T T T T T

25 25 25 25 25 25 25 25 25 25 25 25 25 25 0 50 75 100 25 25 25 50 50 50 25 25 25 25 25 25 25 25 25 25 25 25 25 25 0 50 75 100 25 25

3 3 3 1 60 60 3 1 3 3 3 3 1 2 1 1 1 1 1 1 1 1 0.25 0.25 30 30 3 3 3 30 60 60 20 30 3 3 3 3 3 3 3 3 3 3

23 7 100 0.6

1.8 0.5 8.0 0.1

4 100 89 9 16 97 97 89 97 42 94 85 64 100 53 69 100 4 7 0.6 98 45 62 22 0.3 1 1 34 41 56 54 47 48 46 44

0.008 4.0 10.6 0.7 1.3 7.8 7.8 21.4 11.6 10.1 22.6 20.5 15.3 12.0 19.1 24.7 36.0 31.4 48.2 0.01 7.9 3.5 4.9 0.2 0.001 0.002 0.04 2.7 3.3 4.4 4.3 3.7 3.8 3.6 3.5

20 20 20 20 1000 5000 11000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 8000 3000 3000 3000 20 20 20 20 1000 2000 4000 5000 3000 3000 3000 3000 3000 3000

0.3

0.02

a

Conditions: [cat.]/[activator] = 1/10, total volume 5 mL. bActivator: [Ph3C][B(C6F5)4] (A), [PhMe2NH][B(C6F5)4] (B), borane B(C6F5)3 (C). Solvent: T = toluene, C = chlorobenzene, D = 1,2-dichlorobenzene. dActivity in 107 g of PNB (mol of metal)−1 h−1. eAN = 2-acetyl-5-norbornene (1.44 mL). fEN = 5-ethylidene-2-norbornene (1.44 mL). c

D

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of the molar ratio of MAO/2 expressed here as [Al]/[M] showed considerable effect on the polymer yield and catalyst activity (Table 1, entries 3 and 10−12). When the mole ratio of [Al]/[M] was 1000/1, only 16% of the norbornene was converted to PNB with a catalytic activity of 1.27 × 107 g of PNB (mol of Pd)−1 h−1 (Table 1, entry 10). When the mole ratio [Al]/[M] was gradually increased from 1000 to 8000, a significant increase of activity of up to 8.01 × 107 g of PNB (mol of Pd)−1 h−1 was observed (Table 1, entries 3, 10, and 11). A further increase of [Al]/[M] to 11000 resulted in a slight decrease of activity to 7.75 × 107 g of PNB (mol of Pd)−1 h−1 (Table 1, entry 12). In a short reaction time such as 1 or 2 min, the yields became low while the activities increased up to 2.14 × 108 g of PNB (mol of Pd)−1 h−1 (Table 1, entries 13 and 14). In addition, the temperature has a significant influence on the yield and activity (Table 1, entries 15−18). When the norbornene polymerization by the palladium(II) complex 2/ MAO system was carried out at low temperature (ca. 0 °C), the yield became lower and the activity decreased to 1.01× 108 g of PNB (mol of Pd)−1 h−1 (Table 1, entry 15). It is noteworthy that complex 2 exhibited extremely high activity of up to 2.26 × 108 g of PNB (mol of Pd)−1 h−1 when the temperature rose to 50 °C (Table 1, entry 16). A further rise in temperature to 75 or 100 °C resulted in a significant drop in final catalytic activity to 1.53 × 108 g of PNB (mol of Pd)−1 h−1 (Table 1, entries 17 and 18). In addition, with an increase of the monomer amount from 11 to 32 mmol, the activity first increased from 1.20 × 108 g of PNB (mol of Pd)−1 h−1 to 2.14 × 108 g of PNB (mol of Pd)−1 h−1 and then slightly decreased to 1.91 × 108 g of PNB (mol of Pd)−1 h−1 (Table 1, entries 13, 19, and 20). The solvent also had an effect on the yield and catalytic activity (Table 1, entries 20−24). The activities of the norbornene polymerizations in chlorobenzene (up to 3.60 × 108 g of PNB (mol of Pd)−1 h−1) are significantly higher than those in toluene (Table 1, entries 16 and 20−23). Moreover, the activity increased up to 4.82 × 108 g of PNB (mol of Pd)−1 h−1 in 1,2dichlorobenzene with a decreased catalyst concentration (Table 1, entry 24). The chiral palladium(II) chloride complex 2/ MAO system was also used as a catalyst for the polymerization of substituted norbornene derivatives such as 2-acetyl-5norbornene (AN) and 5-ethylidene-2-norbornene (EN) (Table 1, entries 25 and 26). Unfortunately, this catalytic system could not promote the polymerization of AN under similar conditions (Table 1, entry 25). If the AN monomer was replaced by the EN monomer, however, the polymerization could be catalyzed by the palladium(II) complex 2/MAO system with an activity of ca. 1.0 × 105 g of PENB (mol of Pd)−1 h−1 (Table 1, entry 26). In contrast, in the presence of MAO, the chiral palladium(II) chloride complex 3 containing the phenyl-substituted (S,S)-bis(oxazoline) ligand also exhibited high a activity of ca. 7.87 × 107 g of PNB (mol of Pd)−1 h−1 similar to that of the corresponding palladium(II) chloride complex 2/MAO system under similar conditions (Table 1, entry 27). To thoroughly investigate the structure−reactivity relationship of these chiral (S,S)-bis(oxazoline) late transition metal catalyst promoted norbornene polymerizations, the corresponding chiral nickel(II) complexes 4 and 5 were also used as catalyst precursors in the norbornene polymerization under similar conditions. Representative results are also summarized in Table 1. Similar to the case for the chiral palladium(II) complexes 1−3 and 6, cocatalysts such as MAO, MMAO, and activator together with AlMe3 or AliBu3 were also needed for

The M−N(2) bonds and M−Cl bonds of complexes 3 and 4 divide the angles of N(1)−M−N(3) (175.7(3)−175.1(2)°) into two almost equal parts N(1)−M−N(2), N(2)−M−N(3), N(1)−M−Cl, and Cl−M−N(3) (87.9(3)−92.1(2)°), respectively. Such results suggest that the N(1), N(2), N(3), M, and Cl atoms are planar, which was also seen in the structure of [(S,S)-bis(oxazoline)]NiCl.12a These results clearly indicate that the steric hindrance around the metal center in the palladium(II) complex 3 is smaller than that in the nickel(II) complex 4. Vinyl-Type Polymerization of Norbornene by Chiral Palladium(II) and Nickel(II) Complex/Cocatalyst Systems. In the presence of various cocatalysts such as MAO, MMAO, and an activator (such as [Ph3C][B(C6F5)4] (A), [PhMe2NH][B(C6F5)4] (B), or B(C6F5)3 (C)) together with AlR3, these chiral palladium(II) and nickel(II) complexes 1−6 exhibited very high activities for the addition polymerization of norbornene, yielding insoluble vinyl-type polynorbornene materials (PNB) with high packing density or soluble highmolecular-weight polynorbornenes with moderate molecular weight distributions, respectively. Some representative results are summarized in Table 1. The neutral complexes 1−6 alone were inactive for the norbornene polymerization, suggesting that the presence of a cocatalyst is essential for the present polymerization. In the presence of MAO, all of the palladium(II) complexes 1, 2, and 6 adopting the isopropyl-substituted bis(oxazoline) ligand could promote the vinyl-type polymerization of norbornene to afford insoluble vinyl-type PNBs (Table 1, entries 1−3). Among them, the chiral palladium(II) chloride complex 2/MAO system exhibited the highest activity of up to 8.01 × 107 g of PNB (mol of Pd)−1 h−1, while the chiral palladium(II) triflate complex 6 showed the lowest activity of ca. 5.4 × 106 g of PNB (mol of Pd)−1 h−1 under the same conditions. On the basis of these results, the chiral palladium(II) chloride complex 2 was chosen as catalyst precursor to investigate the polymerization of norbornene under different conditions. For complex 2, MAO was a perfect cocatalyst (Table 1, entry 3). It only needed 3 min for the complex 2/MAO binary system to quantitatively convert 8000 equiv of norbornene into PNB in toluene at room temperature. As a cocatalyst, however, MMAO was not as good as MAO. The complex 2/MMAO binary system showed a lower activity of ca. 1.32 × 106 g of PNB (mol of Pd)−1 h−1 under similar conditions (Table 1, entry 4). If an activator such as [Ph3C][B(C6F5)4] (A) alone served as cocatalyst, the complex 2/activator binary system could not promote the polymerization of norbornene (Table 1, entry 5). In the presence of AlR3 (R = iBu or Me), however, the complex 2/activator systems were also active for the norbornene polymerization in chlorobenzene (Table 1, entries 6−9). In contrast, the 2/B/ AliBu3 ternary system (B = [PhMe2NH][B(C6F5)4], activity ca. 8.0 × 104 g of PNB (mol of Pd)−1 h−1) showed much lower activity than the 2/B/AlMe3 ternary system (4.0 × 107 g of PNB (mol of Pd)−1 h−1) under the same conditions (Table 1, entries 6 and 7). In the presence of a different activator, the 2/ A/AlMe3 system showed the highest activity of up to 1.06 × 108 g of PNB (mol of Pd)−1 h−1, which is also higher than the activity of the complex 2/MAO system under similar conditions (Table 1, entries 3 and 8). Moreover, the complex 2/C/AlMe3 ternary system (C = B(C6F5)3) was also effective for the polymerization of norbornene in chlorobenzene, with activities of up to 7.3 × 106 g of PNB (mol of Pd)−1 h−1 (Table 1, entry 9). For the complex 2/MAO binary system, variation E

dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX

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activation of the chiral nickel(II) complexes in the norbornene polymerization. In comparison with the chiral palladium(II) complex 1−3, 6/cocatalyst systems, the corresponding chiral nickel(II) complexes 4, 5/cocatalyst systems exhibited comparatively low activities in the vinylic polymerization of norbornene under similar conditions (Table 1, entries 28−44). Between them, the nickel(II) complex 4/MAO system showed a lower activity of ca. 3.5 × 107 g of PNB (mol of Pd)−1 h−1 in comparison to the nickel(II) complex 5/MAO system (activity ca. 4.9 × 107 g of PNB (mol of Pd)−1 h−1) under the same conditions, affording soluble vinyl-type PNBs which differ from those obtained by the palladium(II) catalysts (Table 1, entries 28 and 29). Similar to the case for the palladium(II) catalysts, the nickel(II) complex 5/MMAO system showed a lower activity of ca. 1.8 × 106 g of PNB (mol of Ni)−1 h−1 in comparison to the complex 5/MAO system under the same conditions (Table 1, entries 29 and 30). In contrast, the complex 5/B/AliBu3, 5/B/AlMe3, and 5/A/AlMe3 ternary systems gave inferior conversions and activities in comparison to the corresponding binary systems (Table 1, entries 29−33). Among them, the complex 5/A/AlMe3 ternary system showed the highest activity of only ca. 4.0 × 105 g of PNB (mol of Ni)−1 h−1, which was also much lower than that (1.1 × 108 g of PNB (mol of Ni)−1 h−1) of the palladium(II) complex 2/A/ AlMe3 system under similar conditions (Table 1, entries 8 and 33). The complex 5/C/AlMe3 ternary system was completely inert in the norbornene polymerization (Table 1, entry 34). In comparison with the palladium(II) complex 2/MAO binary system, the molar ratio of [Al]/[M] has less effect on the activity of the nickel(II) complex 5/MAO binary system in the norbornene polymerization under similar conditions (Table 1, entries 3, 10−12, 29, and 35−38). When [Al]/[M] was increased from 1000 to 5000, the activity first increased from 2.7 × 107 to 4.9 × 107 g of PNB (mol of Ni)−1 h−1 and then decreased to 4.3 × 107 g of PNB (mol of Ni)−1 h−1 (Table 1, entries 29 and 35−38). Moreover, the catalytic activities of the nickel(II) complex 5/MAO binary systems first increased from 3.7 × 107 g of PNB (mol of Ni)−1 h−1 to 4.9 × 107 g of PNB (mol of Ni)−1 h−1 and then decreased to 3.5 × 107 g of PNB (mol of Ni)−1 h−1 when the polymerization temperature was increased from 0 to 100 °C (Table 1, entries 39−42). Similar to the case for the palladium(II) complex 2/MAO system, the nickel(II) complex 5/MAO system was also effective for the polymerization of the substituted norbornene derivative EN with an activity of ca. 2 × 105 g of PENB (mol of Ni)−1 h−1 (Table 1, entry 44). However, the AN polymerization could not be carried out by such a binary system under similar conditions (Table 1, entry 43). The selected results of activities of complex 1−6/MAO systems in the polymerization of norbornene are exhibited in Figure 4 (compare entries 1, 2, 24, and 27−29 in Table 1). Among them, complex 2 showed the highest activity of up to 4.8 × 108 g of PNB (mol of Pd)−1 h−1. Most of the PNBs obtained by these chiral palladium(II) and nickel(II) catalysts are insoluble in common solvents and therefore cannot be characterized by NMR and GPC methods. However, the IR spectra of these PNBs demonstrate that no peaks at 1620−1680 cm−1 which are assigned to the double bonds of the ring-opening metathesis PNBs suggest the formation of the vinyl-type PNBs in the norbornene polymerization catalyzed by these chiral palladium(II) and nickel(II) catalysts.15 In contrast, some of the resulting PNBs by the chiral nickel(II) complexes have good solubility in 1,2,4-trichlorobenzene or dichlorobenzene at 145 °C. GPC curves reveal

Figure 4. Activities of complex 1−6/MAO systems in the polymerization of norbornene: comparison of the maximum activities from Table 1.

that these soluble PNBs have high molecular weights in the range 356000−1185000 and unimodal molecular weight distributions (1.94−2.75), consistent with the predominance of a homogeneous single-site catalytic species. The 13C NMR spectra of the soluble PNBs formed with the complex 4/MAO system in o-C6D4Cl2 indicate completely vinylic microstructures (vinylic selectivity 100%) and show diagnostic signals for a mm/mr/rr mixture of configurations in the ratio 29/44/27 (two bridge carbons C1, C4: mm, 38.99 ppm; mr, 40.32 ppm; rr, 41.20 ppm). The molecular weight of the PNBs obtained by the complex 4/MAO system is 904000, which was determined by GPC (see Table 2, Figures 5 and 6, and the Supporting Information). DSC curves of these vinylic PNBs show glass transition temperatures in the range 309−369 °C. Moreover, the glass transition temperatures of the soluble PNBs are higher than those of the insoluble PNBs. TGA curves exhibit that the decomposition temperatures of these PNBs are above 400 °C. These results indicate that the PNBs obtained by these chiral palladium(II) and nickel(II) catalysts have high thermostability. The wide-angle X-ray diffraction (WAXD) curves of these PNBs show two major peaks at 2θ values of 9−11 and 18−20°. As Haselwender et al. mentioned,16 the peak at low 2θ can be considered as a reflection of the interchain or intersegment distance of the polymer, which is relevant to the crystallinity and packing density of a polymer, while the peak at high 2θ reflects the intrachain distance. According to the literature, the stronger peak at the low 2θ value of 9−11° suggests the higher packing tendency of the polymer.16 From the WAXD curves, we can see that the insoluble PNBs have a stronger peak at the low 2θ value of 9−11° with intensity greater than that for the soluble PNBs, suggesting the higher packing density and higher stereoregularity of the insoluble PNBs (see the Supporting Information). Similarly, the insoluble PNBs obtained by the complex 2/A/AlMe3 ternary system also have packing densities higher than those obtained by the complex 2/MAO binary system, since their WAXD curves give stronger peaks at the low 2θ value of 9−11°. It is difficult to obtain information on true catalytically active species in the present catalyst system by experiment because of their instability; therefore, massive quantum chemistry calculations were made to investigate the mechanism of the polymerization process and catalytically active species. During the polymerization process, as we know, excess MAO or activator/AlR3 first alkylates the corresponding chloride complex and then abstracts the alkyl group from the metal F

dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Table 2. Structural Characterization of the Vinylic PNBsa entry

entry in Table1

cat.

T (°C)

yield (%)

Ab

1 2 3 4 5 6 7 8 9 10

3 15 24 27 28 35 39 40 41 42

2 2 2 3 4 5 5 5 5 5

25 0 50 25 25 25 0 50 75 100

100 42 7 98 45 34 47 36 38 44

8.0 10.1 48.2 7.9 3.51 2.71 3.68 2.82 3.02 3.46

105Mnc

9.04 11.85 10.26 6.99 5.63 3.56

Mw/Mnc

Tgd (°C)

Tde (°C)

2.15 2.75 1.94 2.00 2.10 2.36

317 310 335 315 360 367 362 355 352 369

431−468 429−468 437−472 427−469 443−477 444−476 453−481 450−476 439−470 452−477

a Conditions: cat. 0.5 μmol, norbornene 21 mmol, solvent toluene, total volume 5 mL. bActivity in 107 g of PNB (mol of metal)−1 h−1. cDetermined by high-temperature GPC in 1,2,4-trichlorobenzene at 145 °C. dMeasured by DSC. eDecomposition temperatures were measured by TGA.

Figure 5. 13C NMR spectrum of PNB obtained by the chiral nickel(II) complex 4/MAO system in Table 1, entry 28.

Figure 6. GPC curve of soluble PNB obtained by the chiral nickel(II) complex 4/MAO system in Table 1, entry 28.

Figure 7. Structure of active species by quantum chemistry calculations.

center to form cationic species. After a number of attempts were made, DFT calculations indeed identified that the methyl group should be abstracted from the metal center. As a result, a monocationic species without an alkyl group was formed. However, such a monocationic species is unstable. A cationic molecule is energetically favorable to form an asymmetric binuclear compound with a neutral molecule through the −CH3 bridge, as depicted in Figure 7, in which the carbon atom is about 2.49 and 2.13 Å away from the Pd+ and Pd,

respectively. A possible mechanism of the subsequent polymerization process is shown in Figure 8 on the basis of DFT calculations. On the basis of the calculated results we speculate that, in such active species 1a, a norbornene molecule selectively coordinates to the Pd+ center of the binuclear compound at first, and then the norbornene is inserted by the −CH3 group, such as 1b. Afterward, the methyl group attacks the norbornene molecule and the cation center transfers to the other Pd atom. Then the inserted norbornene unit could bridge G

dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

Figure 8. Possible mechanism of norbornene polymerization.

the two metal centers Pd and Pd+ via its carbon anion, thus affording 1c. Next the newly incoming norbornene could also coordinate to the Pd+ center and be attacked by the carbon anion in the first norbornene molecule, as is shown in 1d. Subsequently, the newly inserted norbornene can also bridge Pd+ and Pd of the binuclear compound, leading to 1e. Consequently, the norbornene coordination and insertion process will happen alternatively on both Pd centers in the same way. As a result, the polymerization of norbornene can occur and polynorbornenes can be obtained. These DFT calculation results should shed new light on the mechanistic aspects of the (co)polymerization processes of norbornene.

3.61−3.76 (m, 4H, CH2 in oxazoline), 2.54−2.73 (m, 2H, CHMe2), 2.29 (s, 3H, −COOCH3), 0.56−0.65 (m, 12H, CHMe2). 13C NMR (100 MHz, C6D6): δ 176.14 (−COO−), 161.31, 154.18 (C in oxazoline), 132.66, 130.47, 124.14, 118.33, 115.87 (ArC), 68.96 (CH2 in oxazoline), 68.40 (CH in oxazoline), 31.03 (CHMe2), 23.57 (−COOMe), 18.59 (CHMe2), 14.54 (CHMe2). Anal. Calcd for C26H31N3O4Pd: C, 56.17; H, 5.62; N, 7.56. Found: C, 56.58; H, 5.15; N, 7.25. Synthesis of [R2-(S,S)-BOZ]PdCl (2; R = CH(CH3)2). To a colorless THF solution (15 mL) of Pd(CH3CN)2Cl2 (0.779 g, 3 mmol) was added a solution of the chiral C2-symmetrical tridentate ligand R2-(S,S)-bis(oxazoline) (R = CH(CH3)2) (1.174 g, 3 mmol) and Et3N (3.3 mmol) in THF (10.0 mL) at room temperature. The mixture was stirred at 30 °C for 2 h and then filtered. The solid was washed with THF three times (3 × 5 mL). After removal of the combined organic filtrates in vacuo, the residue was recrystallized from THF and hexane at −30 °C to give 2 as a dark red solid (1.44 g, 90% yield). 1H NMR (400 MHz, CDCl3): δ 7.62 (dd, 2H, ArH), 7.12−7.14 (m, 4H, ArH), 6.74−6.78 (m, 2H, ArH), 4.81−4.79 (m, 2H, CH2 in oxazoline), 4.51 (t, 2H, CH2 in oxazoline), 4.40 (q, 2H, CH in oxazoline), 2.55−2.60 (m, 2H, CHMe2), 0.93 (d, 6H, CHMe2), 0.78 (d, 6H, CHMe2). 13C NMR (100 MHz, CDCl3): δ 160.85, 153.07 (C in oxazoline), 132.67, 129.93, 118.23, 115.67 (ArC), 69.22 (CH2 in oxazoline), 67.94 (CH in oxazoline), 30.86 (CHMe2), 18.42 (CHMe2), 14.41 (CHMe2). Anal. Calcd for C24H28ClN3O2Pd: C, 54.15; H, 5.30; N, 7.89. Found: C, 54.44; H, 5.73; N, 7.42. Synthesis of [R2-(S,S)-BOZ]PdCl (3; R = Ph). A procedure similar to that used for the preparation of complex 2 was employed with the ligand R2-(S,S)-bis(oxazoline) (R = Ph) (230 mg, 0.5 mmol), Et3N (0.6 mmol), and Pd(CH3CN)2Cl2 (130 mg, 0.5 mmol) (246 mg, 82% yield). Single crystals of complex 3 suitable for X-ray analysis were obtained from a solution in a toluene/hexane mixture in a freezer at −33 °C. 1H NMR (400 MHz, CDCl3): δ 7.65 (dd, 2H, ArH), 7.24− 7.30 (m, 10H, ArH), 7.15−7.17 (m, 2H, ArH), 6.76 (t, 2H, ArH), 5.9 (q, 2H, CH2 in oxazoline), 4.78−4.83 (m, 2H, CH in oxazoline), 4.4 (q, 2H, CH2 in oxazoline). 13C NMR (100 MHz, CDCl3): δ 162.10, 153.39 (C in oxazoline), 141.24, 132.90, 130.17, 128.93, 128.05, 126.66, 123.54, 116.34, 115.37 (ArC), 67.99 (CH2 in oxazoline), 66.64 (CH in oxazoline). Anal. Calcd for C30H24ClN3O2Pd: C, 60.01; H, 4.03; N, 7.00. Found: C, 60.38; H, 4.45; N, 7.29. Synthesis of [R2-(S,S)-BOZ]NiCl (4; R = Ph). To a colorless THF solution (5 mL) of Ni(NMe2CH2CH2NMe2)PhCl (287 mg, 1 mmol) was added a solution of the chiral C2-symmetrical tridentate ligand R2(S,S)-bis(oxazoline) (R = Ph) (426 mg, 1 mmol) in THF (10.0 mL). The mixture was stirred at room temperature overnight and then filtered. The solid was washed with THF three times (3 × 5 mL). After removal of the combined organic filtrates in vacuo, the residue was recrystallized from THF and hexane at −33 °C to give 4 as a dark green solid (513 mg, 93% yield). Single crystals of complex 4 suitable for X-ray analysis were obtained from a THF/hexane solution in a freezer at −33 °C. 1H NMR (400 MHz, CDCl3): δ 7.60 (dd, 2H, ArH), 7.29−7.34 (m, 10H, ArH), 7.08−7.13 (m, 4H, ArH), 6.73−6.77 (m, 2H, ArH), 5.35 (q, 2H, CH2 in oxazoline), 4.63 (t, 2H, CH in

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CONCLUSION In summary, a series of chiral palladium(II) and nickel(II) complexes (1−6) could be easily synthesized in high yields via one-pot reactions by use of the corresponding late-transitionmetal complexes with the chiral C2-symmetrical tridentate (S,S)-bis(oxazoline) ligand. On activation with a cocatalyst such as MAO, MMAO, and activator ([Ph3C][B(C6F5)4] (A), [PhMe2NH][B(C6F5)4] (B), or B(C6F5)3 (C)) together with AlR3 (AlMe3 or AliBu3), the chiral palladium(II) and nickel(II) complexes 1−6 showed extremely high activities of up to 4.8 × 108 g of PNB (mol of metal)−1 h−1 in the vinyl-type polymerization of norbornene, affording insoluble or soluble PNBs with different microstructures and properties. Significant influences of the metal center, the chelating ligand, cocatalyst, [Al]/[M], monomer concentration, reaction time, temperature, and solvent on the polymerization activities were also observed in these chiral catalytic systems. These are the first chiral (S,S)bis(oxazoline) palladium(II) and nickel(II) catalysts used in olefin polymerization, to our knowledge. Further studies on the detailed polymerization mechanism on the basis of DFT calculations and on chiral (S,S)-bis(oxazoline) complexes with other transition metals are in progress.

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EXPERIMENTAL SECTION

Synthesis of [R2-(S,S)-BOZ]Pd(OAc) (1; R = CH(CH3)2). To a benzene solution (3 mL) of Pd(OAc)2 (115 mg, 0.52 mmol) was added a solution of the chiral C2-symmetrical tridentate ligand R2(S,S)-bis(oxazoline) (R = CH(CH3)2) (200 mg, 0.52 mmol) in benzene (2 mL) at room temperature in the glovebox. The mixture was stirred at 30 °C for 3 h and then filtered. The filtrate was evaporated, and the solid residue was washed with pentane several times (5 × 3 mL). After removal of the combined organic filtrates in vacuo, the residue was recrystallized from THF and hexane at −33 °C to give 1 as a red solid (260 mg, 90% yield). 1H NMR (400 MHz, C6D6): δ 7.67 (dd, 2H, ArH), 7.13 (d, 2H, ArH), 6.77−6.83 (m, 2H, ArH), 6.49−6.56 (m, 2H, ArH), 4.13−4.27 (m, 2H, CH in oxazoline), H

dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX

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oxazoline), 4.15 (q, 2H, CH2 in oxazoline). 13C NMR (100 MHz, CDCl3): δ 161.93, 153.06 (C in oxazoline), 141.75, 132.48, 129.01, 128.94, 127.96, 126.48, 124.70, 118.09, 116.52 (ArC), 75.61 (CH2 in oxazoline), 65.15 (CH in oxazoline). Anal. Calcd for C30H24ClN3O2Ni: C, 65.20; H, 4.38; N, 7.60. Found: C, 64.88; H, 4.71; N, 7.46. Synthesis of [R2-(S,S)-BOZ]NiCl (5; R = CH(CH3)2). The chiral R2-(S,S)-bis(oxazoline) nickel(II) chloride complex [R2-(S,S)-BOZ]NiCl (R = CH(CH3)2) (5) was synthesized according to the literature.12a 1H NMR (400 MHz, CDCl3): δ 7.57 (dd, 2H, ArH), 7.03−7.11 (m, 2H, ArH), 6.96 (d, 2H, ArH), 6.69−6.77 (m, 2H, ArH), 4.33 (t, 2H, CH in oxazoline), 4.14−4.26 (m, 4H, CH2 in oxazoline), 2.65−2.79 (m, 2H, CHMe2), 0.91−1.02 (m, 12H, CHMe2). Anal. Calcd for C24H28ClN3O2Ni: C, 59.48; H, 5.82; N, 8.67. Found: C, 59.00; H, 5.47; N, 8.42. Synthesis of [R2-(S,S)-BOZ]Pd(OTf) (6; R = CH(CH3)2). A solution of AgOTf (52 mg, 0.20 mmol) in benzene (2 mL) was added to a solution of complex 2 (104 mg, 0.20 mmol) in benzene (3 mL) in the glovebox. The resulting solution was stirred for 1 h at room temperature, and then the white precipitate of AgCl was filtered off. The filtrate was evaporated, and the rose-colored solid residue was washed with pentane several times (5 × 3 mL). After it was dried by vacuum, the residue was recrystallized from THF and hexane at −33 °C to give 6 as a rose-colored solid (119 mg, 92%). 1H NMR (400 MHz, C6D6): δ 7.56 (dd, 2H, ArH), 6.86 (d, 2H, ArH), 6.72−6.76 (m, 2H, ArH), 6.49−6.55 (m, 2H, ArH), 4.59−4.67 (m, 2H, CH in oxazoline), 3.68−3.77, (m, 4H, CH2 in oxazoline), 2.34−2.50 (m, 2H, CHMe2), 0.72 (d, 6H, CHMe2), 0.57 (d, 6H, CHMe2). 13C NMR (100 MHz, C6D6): δ 162.23, 152.68 (C in oxazoline), 133.39, 130.52, 124.08, 119.45, 116.50 (ArC), 70.18 (CH2 in oxazoline), 68.12 (CH in oxazoline), 31.58 (CHMe2), 17.77 (CHMe2), 14.60 (CHMe2). Anal. Calcd for C25H28F3N3O5SPd: C, 46.48; H, 4.37; N, 6.50. Found: C, 46.77; H, 4.80; N, 6.11. Typical Procedure for Norbornene (NB) Polymerization by Complex 2, 5/MAO Systems (Table 1, entry 3). In a glovebox, the catalyst precursor (5.0 × 10−7 mol, 0.1 mL of 5 mmol in toluene) was placed in a 50 mL round-bottom flask with a magnetic stirrer. Then norbornene (0.021 mol, 2.00 g) and toluene (4.57 mL) were charged into this flask. After 5 min, the cocatalyst (1 mL of MAO solution (7 wt % of Al in toluene)) was added. The mixture was magnetically stirred for 3 min. Then the mixture was quickly removed from the glovebox and the reaction terminated by adding 15 mL of acidified ethanol (10/1 ethanol/HCl, v/v). The precipitate of polynorbornene was filtered off, washed with ethanol (3 × 10 mL), and dried to constant weight at 80 °C under vacuum as a white powder. Equations 1−4 give the amounts of products obtained, in which IC1 is the integration of the signals at 38.99 ppm assigned as the bridged

Typical Procedure for Norbornene (NB) Polymerization by Complex 2, 5/Activator/AlR3 Ternary Systems (Table 1, entry 8). In the glovebox, the catalyst precursor (1.0 × 10−6 mol, 0.1 mL of 10 mmol in chlorobenzene) was introduced into a 50 mL roundbottom flask with a magnetic stirrer. The reactor was charged with norbornene (0.021 mol, 2.00 g), chlorobenzene (4.88 mL), AlMe3 (20 μmL, 1 M/L in toluene), and [Ph3C][B(C6F5)4] 9.23 mg (10 μmol) in a glovebox. After 40 s, the flask was quickly removed from the glovebox and the reaction terminated by adding 15 mL of acidified ethanol (10/1 ethanol/HCl, v/v). The precipitate of polynorbornene was filtered off, washed with ethanol (3 × 10 mL), and dried to constant weight at 80 °C under vacuum as a white powder. X-ray Crystallographic Analysis. A crystal was sealed in oil under a microscope in the glovebox. Data collections were performed at 173 K on a Bruker-AXS X-ray diffractometer with a CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). 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-97 program. Refinements were performed on F2 anisotropically for all nonhydrogen atoms by the full-matrix least-squares method. The analytical scattering factors for neutral atoms were used throughout the analysis. The non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at calculated positions and were included in the structure calculation without further refinement of the parameters. The residual electron densities were of no chemical significance. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-988837 (3) and CCDC-988841 (4) and contain supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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ASSOCIATED CONTENT

* Supporting Information S

Text, figures. a table, and CIF files giving the materials and general methods, GPC, DSC, WAXD, TGA and IR spectra of representative polymer products, and crystallographic data for 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*S.Z.: e-mail, [email protected]. *X.L.: e-mail, xfl[email protected]; fax, (+) (86)10-68914780.

mm‐NB (mol %) = {IC1/(IC1 + IC2 + IC3)} × 100

Author Contributions †

(1)

These authors contributed equally to this paper.

Notes

[mr ]/(2[rr ] + [mr ]) + [mr ]/(2[mm] + [mr ]) = 1

The authors declare no competing financial interest.

(4)

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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (No. 20974014, 21274012, 21322401) and the 111 project (B07012).

methine carbons (C1, C4) of the mm stereochemistry norbornene unit and IC2 is the integration of the signals at 40.32 ppm assigned as the methine carbons of the mr stereochemistry norbornene unit, while IC3 is the integration of the signals at 41.2 ppm assigned as the methine carbons of the rr stereochemistry norbornene in the 13C NMR spectrum.

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REFERENCES

(1) (a) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85− 93. (b) Makio, H.; Fujita, T. Acc. Chem. Res. 2009, 42, 1532−1544. (c) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111, 2450−2485. (d) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587− 2598. (e) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011, 111, 2363−2449. (f) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283−315. (g) Alt, H. G.; Licht, E. H.; Licht, A. I.; Schneider, K. J. Coord. Chem. Rev. 2012, 256, 1787−1830. (h) Wu, J.Q.; Li, Y.-S. Coord. Chem. Rev. 2011, 255, 2303−2314.

rr ‐NB (mol %) = {IC3/(IC1 + IC2 + IC3)} × 100 (2)

mr ‐NB (mol %) = {IC2/(IC1 + IC2 + IC3)} × 100 (3)

I

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Organometallics

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dx.doi.org/10.1021/om5007616 | Organometallics XXXX, XXX, XXX−XXX