Highly active half-sandwich chromium(III) catalysts bearing bis(imino)pyrrole ligands for ethylene (co)polymerization

Abstract

A series of half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands, Cp′[2,5-C₄H₂N(CH=NAr)₂]CrCl [Cp′ = C₅H₅, Ar = C₆H₅ (2a), 2,6-Me₂C₆H₃ (2b), 2,6-ⁱPr₂C₆H₃ (2c), C₆F₅ (2d); Cp′ = C₅Me₅, Ar = C₆H₅ (3a), 2,6-ⁱPr₂C₆H₃ (3c)] were synthesized with good yields. The complexes were characterized by FTIR and mass spectrometry in addition to elemental analyses. X-ray structural analyses for 2a–c showed that the Cr complexes have a pseudo-octahedral coordination environment with a three-legged “piano stool” geometry. One of the imino nitrogen atoms is coordinated with the Cr metal. On activation with methylaluminoxane, the Cp-based complexes showed high catalytic activities for ethylene polymerization. High molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the nature of the polymerization was single site. The copolymerization of ethylene with norbornene by the pre-catalysts 2a–d was also explored in the presence of methylaluminoxane. The catalytic activity, co-monomer incorporation and the properties of the resultant polymers can be controlled over a wide range by tuning the catalyst structures and reaction parameters.

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Introduction

Cr-based catalysts for polymerization have an interesting position among the transition metal catalysts used for olefin polymerization. Heterogeneous Cr-based systems such as the silica-supported Phillips¹ and Union Carbide² catalysts are among the most important catalysts in the production of polyethylene materials. However, the ill-defined nature of these paramagnetic Cr catalysts has resulted in intense debate about the nature of the active species and the catalytic reaction mechanisms; this has prompted considerable efforts to develop well-defined homogeneous Cr-based catalysts.³ Among the homogeneous Cr catalysts, Cp-based Cr(III) complexes have attracted particular attention as these complexes have structural models of the active site close to those proposed for the Union Carbide heterogeneous catalyst.^4–11 The [Cp*CrL₂R]⁺A⁻ complexes (L = py, 1/2dppe, MeCN, THF; R = Me, Et; A = PF₆, BPh₄; Cp* = ç⁵-pentamethylcyclopentadienyl) were reported to catalyze ethylene polymerization in the absence of a co-catalyst. The half-sandwich Cr(III) complexes ligated with bidentate ancillary ligands also showed great potential in catalyzing ethylene polymerization. For example, half-sandwich salicylaldiminato Cr(III) catalysts, Cp*Cr[2,4-^tBu₂-6-(CH=NR)-C₆H₂O]Cl (R = ^tBu, Ph, 2,6-ⁱPrC₆H₃), showed high catalytic activity for ethylene polymerization and produced linear high molecular weight polyethylene on activation with only a small amount of AlR₃ (Al/Cr = 25).¹² The half-sandwich Cr(III) complexes bearing β-ketoiminato, β-diketiminato and hydroxyindanimine ligands showed good catalytic activity for ethylene polymerization in the presence of triethylaluminium.¹³

Nitrogen-based polydentate ligands have been widely used in olefin polymerization catalysis over the last few decades. A key attraction of these ligands is their availability and amenability to modification via straightforward Schiff-base condensation procedures.^14–23 Our group has reported the synthesis of V(III) complexes bearing bis(imino)pyrrolyl ligands and their use as catalyst precursors for olefin polymerization.²⁴ Although the bis(imino)pyrrole ligands actually acted as bidentate ligands rather than tridentate ligands, these complexes showed much higher thermal stability than mono(imino)pyrrole-based analogues under similar conditions due to the increased steric hindrance and electron-withdrawing effects in the N-aryl moiety of the ligands. We were attracted by the potential of bis(imino)pyrrole ligands in homogeneous polymerization catalysis. Considering that the atomic radii of Cr is similar to that of V, we prepared a new family of half-sandwich Cr complexes containing bis(imino)pyrrole ligands. This paper describes the synthesis and characterization of a number of novel Cr complexes with bis(imino)pyrrole chelating ligands and cyclopentadienyl or pentamethylcyclopentadienyl and explores their application as catalysts in ethylene polymerization and ethylene/norbornene (NBE) copolymerization.

Results and discussion

Synthesis and characterization of half-sandwich Cr(III) complexes

A general synthetic route for the half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands is shown in Scheme 1. The bis(imino)pyrrole ligands 1a–d were deprotonated by 1.0 equiv. of n-BuLi, followed by treatment with Cp′CrCl₂(THF) in THF at −78 °C. The bidentate Cr complex containing the iminopyrrolide chelate ligand 2f was also prepared for comparison. The pure complexes were then isolated by recrystallization from a mixture of dichloromethane and hexane at room temperature. The severe line broadening in the ¹H-NMR spectra indicates that these complexes are paramagnetic species.

Scheme 1 General synthetic route for the Cr complexes used in this study.

These complexes were identified by FTIR and mass spectrometry as well as by elemental analyses. The N–H stretching at about 3452 cm⁻¹ disappeared in the IR spectra. The decrease in the signal intensity of ν(C=N) around 1622 cm⁻¹ and the appearance of a low frequency signal around 1559 cm⁻¹ indicate that one of the imino nitrogen atoms coordinated with Cr metal. To further confirm the structures of these new complexes, crystals of 2a–c suitable for X-ray diffraction were grown from the chilled concentrated mixed toluene–hexane solution. The ratio of toluene to hexane was adjusted in the range 1 : 3–1 : 6 according to the solubility of the complexes. The crystallographic data, together with the collection and refinement parameters, are summarized in Table S1† of the ESI. Selected bond distances and angles for complexes 2a–c are listed in Table 1.

Table 1 Selected bond distances (Å) and angles (degrees) for complexes 2a–c

	2a	2b	2c
Bond distances (Å)
Cr(1)–N(1)	2.0304(16)	2.036(3)	2.017(4)
Cr(1)–N(2)	2.0766(17)	2.070(3)	2.080(4)
Cr(1)–Cl(1)	2.206(2)	2.2883(11)	2.2885(11)
Cr(1)–Cp(centroid)	1.876	1.889	1.889
N(2)–C(11)	1.435(2)	1.445(4)	1.439(5)
N(3)–C(17)	1.280(3)	1.256(5)	1.274(5)
Bond angles (°)
N(1)–Cr(1)–Cl(1)	97.74(5)	92.81(9)	92.77(9)
N(1)–Cr(1)–N(2)	80.95(6)	80.59(11)	80.60(11)
N(2)–Cr(1)–Cl(1)	96.97(5)	99.20(8)	99.19(8)
Cp(centroid)–Cr(1)–Cl(1)	122.91	122.01	122.02
Cp(centroid)–Cr(1)–N(2)	121.94	121.81	121.85
Cp(centroid)–Cr(1)–N(1)	126.04	129.96	129.94
N(1)–C(6)–C(17)	127.20(18)	124.0(3)	122.0(4)
N(3)–C(17)–C(6)	126.47(18)	124.5(4)	121.8(4)

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The structure of 2a is shown in Fig. 1. Complex 2a adopted a three-legged “piano stool” geometry with the N, N and Cl atoms representing the three legs and the Cp ring representing the seat, which is similar to the structures of the half-sandwich salicylaldiminato¹² and β-ketoiminato¹³ Cr complexes. The Cr–N(1) bond length is 2.0304(16) Å, suggesting that the N atom in the pyrrole group formed a σ-bond with the Cr. The Cr–N(2) bond distance in 2a is 2.0766(17) Å, indicative of significant coordination of this imino nitrogen atom with the metal center in the solid state. The distance from N(3) to Cr is 3.686 Å, therefore the interaction between the N(3) atom and the Cr in 2a is rather weak.

Fig. 1 Molecular structure of 2a with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity.

The structures of 2b and 2c are shown in Fig. S1 and S2,† with the selected bond distances and angles summarized in Table 1. These complexes also adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry. The Cr–Cl bond distance in 2b and 2c is longer than that in 2a (2a, 2.206(2) Å; 2b, 2.2883(11) Å; 2c, 2.2885(11) Å), in line with the Cr–Cp (centroid) distances in 2a–c (2a, 1.876 Å; 2b, 1.889 Å; 2c, 1.889 Å). These bond distances might be related to the catalytic activity of these complexes towards ethylene polymerization. The N(1)–Cr(1)–N(2), N(1)–Cr(1)–Cl(1) and N(2)–Cr(1)–Cl(1) bond angles in these complexes occur in the ranges 80.59–80.95, 92.77–97.74 and 96.97–99.20°, respectively, with the variation in the imino groups. These complexes have similar dihedral angles between the Cp ring and the plane through Cr, N(1) and N(2) atoms in the range 46.30–49.73°. In comparison, the impact of the imino groups on the dihedral angle between the Cp ring and the pyrrole ring in the bis(imino)pyrrole ligand is much more remarkable for these complexes, which makes the Cp-pyrrolyl ring dihedral angle change over a large range from 44.76 to 60.92°. In addition to the steric effect of the substituents, in some cases the packing force may also be an important factor affecting the bond parameters.

Ethylene polymerization

All the complexes are inactive for ethylene polymerization in the presence of alkylaluminium compounds or halogen-containing alkylaluminium compounds. Interestingly, on activation with a small amount of methylaluminoxane (MAO), these complexes showed notable activity towards ethylene polymerization (Table 2).

Table 2 Ethylene polymerization by Cr complexes/MAO catalytic systems^a

Entry	Catalyst	Al/Cr (molar ratio)	Ethylene (atm)	Time (min)	Yield (g)	Activity (kg molCr^−1 h^−1 bar^−1)	Mw^b (10^4)	Mw/Mn^b
a Reaction conditions: 3 μmol catalyst; MAO as the co-catalyst; 25 °C; Vtotal = 80 mL.b Weight-average molecular weights and polydispersity indexes of the polymer samples determined by high temperature GPC at 150 °C in 1,2,4-C6Cl3H3 versus narrow polystyrene standards.c Insoluble in 1,2,4-C6Cl3H3 at 150 °C.
1	2a	250	5	10	2.93	1170	23.6	2.2
2	2b	250	5	10	3.31	1320	32.4	2.5
3	2c	250	5	10	4.43	1770	42.7	2.6
4	2d	250	5	10	3.23	1290	24.6	2.6
5	2e	250	5	10	0.58	230	6.7	3.8
6	3a	250	5	10	0.77	310	—^c	—
7	3c	250	5	10	Trace	—	—^c	—
8	2c	60	5	10	2.38	950	58.6	2.7
9	2c	125	5	10	3.96	1580	52.5	3.0
10	2c	500	5	10	2.66	1060	22.9	2.9
11	2c	1000	5	10	1.56	620	10.3	2.7
12	2c	250	5	5	2.81	2250	25.8	2.3
13	2c	250	5	30	9.14	1220	60.1	2.9
14	2c	250	2	10	1.12	1120	30.8	2.5

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Complex 2a showed high catalytic activity (1170 kg mol_Cr⁻¹ h⁻¹ bar⁻¹) for ethylene polymerization. Analogue 2b, containing a dimethylphenyl-substituted bis(imino)pyrryl ligand, exhibited a slightly higher catalytic activity (1320 kg mol_Cr⁻¹ h⁻¹ bar⁻¹) than 2a under the same conditions. Further improvement was observed when complex 2c, with a di(isopropyl)phenyl-substituted bis(imino)pyrryl ligand was used (1770 kg mol_Cr⁻¹ h ⁻¹ bar⁻¹). The resultant polymers prepared by 2a–c had high molecular weights and unimodal molecular weight distributions. The molecular weight of the polymers also increased in the following order: 2c, 42.7 kg mol⁻¹ > 2b, 32.4 kg mol⁻¹ > 2a, 23.6 kg mol⁻¹. These results suggest that the bulky substituents at the ortho-positions in the N-aryl moiety (ⁱPr > Me > H) are important for these complexes to show high catalytic activity and produce high molecular weight polyethylenes. This is because a bulky substituent may not only provide efficient protection of the metal center from inactivation, but also restrains the chain transfer reaction. The catalytic activity of complex 2d was between that of 2a and 2b (entries 1 and 2 versus entry 4). These data suggested that the electron-donating effect of the aliphatic group was not important, but the steric bulk effect played a key role in the enhanced catalytic activity.

Compared with 2a, complex 2e containing the mono(iminopyrrolyl) chelate ligand showed a low catalytic activity for ethylene polymerization (2e, 230 kg mol_Cr⁻¹ h⁻¹ bar⁻¹). In addition, 2e produced low molecular weight polymers with broad molecular weight distributions under the same conditions (entry 5 in Table 2). These results provide further evidence of the advantage of using these novel half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands for ethylene polymerization.

The Cp*-based complexes displayed much lower activities and produced much higher molecular weight polymers than the Cp analogues. The polymers produced by 3a and 3c were insoluble in 1,2,4-C₆Cl₃H₃ at 150 °C. Moreover, complex 3a showed higher activity than 3c. The observed result was an interesting contrast to that found with 2a and 2c, in which 2c showed a higher activity than 2a. It may be assumed that the bulky steric hindrance around the metal center prevented ethylene insertion in this case. These results suggest that both a cyclopentadienyl ligand and an anionic donor ligand could affect the polymerization behavior.

As a result of the better performance observed with the catalysts containing 2,6-ⁱPr substituents at the imine group, pre-catalyst 2c was investigated in detail by changing the reaction parameters, such as the co-catalyst concentration and the ethylene pressure. It was found that the Al/Cr molar ratio strongly influenced the catalytic activity. Note that even at an extremely low amount of MAO (Al/Cr molar ratio = 60), complex 2c still showed very high activity towards ethylene polymerization (entry 8). The molecular weights of the resultant polymers decreased with the Al/Cr molar ratio, suggesting that chain transfer to Al occurred during the polymerization (Fig. 2).

Fig. 2 Plots of the catalytic activity of catalyst 2c and the molecular weight of the resultant polyethylenes versus the Al/Cr molar ratio. Reaction conditions: 3 μmol; 10 min; 25 °C; 5 atm ethylene.

Increasing the ethylene pressure resulted in an increase both in the polymer yields and in the molecular weight of the resultant polymers, which also supported the suggestion that the dominant chain transfer would be Al transfer from the propagating metal alkyl species. Note that these catalysts showed a long life for ethylene polymerization. The polymer yields continued to increase for 30 min, although the activity decreased with time. The decrease in activity might be caused by mass transport limitations as large amounts of polymers were produced during the polymerization (Fig. 3). The obtained polyethylene samples were all analyzed by differential scanning calorimetry (DSC) and the melting temperature (T_m) was in the range 134–139 °C, which is the classical T_m for high-density polyethylene. This is in full agreement with the ¹³C-NMR studies of the polyethylenes, which showed no branches on the polymer backbone.

Fig. 3 Kinetic profiles of ethylene polymerization using catalysts 2a. Reaction conditions: 3 μmol; Al/Cr = 250; 25 °C; 5 atm ethylene.

The half-sandwich type β-ketoiminato and β-diketiminate Cr(III) catalyst systems are inactive for ethylene polymerization in the presence of MAO, but they displayed high catalytic activities on activation with a small amount of AlEt₃.^12,13 The formation of a hetero-bimetallic Cr–Al intermediate was thus assumed to occur. In contrast, the complexes used in this study showed rather low activities on activation with AlEt₃. Attempts to isolate an intermediate by the treatment of 2c with 10 equiv. MAO in toluene for 2 h were unsuccessful; no exact information was obtained from the IR spectrum. Based on these results, it has been shown that the catalyst structures strongly affect the formation of the active species, although the true active species remains unknown at this time.

Ethylene/NBE copolymerization

One of the greatest advantages of these novel pre-catalysts for olefin polymerization is that they could promote the copolymerization of ethylene with NBE. Typical results for this copolymerization are summarized in Table 3. It was very interesting that the trend of catalytic activity for the copolymerization was similar to that for ethylene homopolymerization. Except for 2d, the catalysts 2a–e showed high activities towards ethylene/NBE copolymerization. The resultant polymers were poly(ethylene-co-NBE)s with high molecular weights and unimodal molecular weight distributions. However, the incorporation of NBE is dependent on the imino substituent used (2a > 2b > 2c). The steric bulk of the aryl moiety in the bis(imino)pyrrole ligands might be responsible for this property.

Table 3 Copolymerization of ethylene with NBE using complexes 2a–e with MAO^a

Entry	Catalyst (μmol)	Al/Cr	NBE (mol L^−1)	Yield (g)	Activity (kg molCr^−1 h^−1 bar^−1)	NBE incorporated (mol%)	Mw^b (10^4)	Mw/Mn
a Reaction conditions: 1.5 μmol catalyst; MAO as the co-catalyst; ethylene 5 atm; 25 °C; Vtotal = 50 mL; co-polymerization for 10 min.b Weight-average molecular weights and polydispersity indexes of the copolymer samples determined by high temperature GPC at 150 °C in 1,2,4-C6Cl3H3 versus narrow polystyrene standards.
1	2a	1000	0.5	0.23	180	32.8	13.4	2.4
2	2b	1000	0.5	0.31	250	26.7	19.8	2.1
3	2c	1000	0.5	0.56	450	18.1	25.8	2.3
4	2d	1000	0.5	0.26	210	22.5	22.1	2.6
5	2c	1000	1.0	0.34	2700	31.5	19.9	2.1
6	2c	1000	1.5	0.11	90	38.8	9.8	2.3
7	2c	500	0.5	Trace	—	—	—	—
8	2c	2000	0.5	1.21	970	18.8	16.7	2.7

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The effect of the reaction conditions on the ethylene/NBE copolymerization were also investigated using catalyst 2c. Both the observed catalytic activities and the molecular weights for the resultant copolymers decreased with increasing initial NBE concentration (entries 3, 5 and 6 in Table 3). The incorporation of NBE increased at higher NBE concentrations; the level of NBE incorporation approached 40 mol% in the resultant copolymer. A certain excessive amount of MAO was required to give a high catalytic activity (entries 3, 7 and 8) in this catalyst system. In addition, the increase in the molar ratio of Al/Cr led to a decrease in the molecular weight of the copolymer, indicating that chain transfer to aluminum alkyls is also the dominant chain transfer pathway in the E/NBE copolymerization under these conditions. The NBE contents of the resultant poly(ethylene-co-NBE)s were independent of the Al/Cr molar ratio.

The microstructures of the ethylene/NBE copolymers were established by ¹³C-NMR in o-C₆D₄Cl₂ at 125 °C with the assignment of the microstructure following previous work.²¹ As shown in Fig. S3,† the resultant copolymer predominantly possessed an isolated NBE inserted unit ([ENE] assigned as 47.7 ppm) among the repeated ethylene insertions when the NBE incorporation of the copolymer was not high (entry 3). However, when NBE incorporation reached 38.8 mol.% (entry 6), the resultant copolymer possessed an isolated NBE inserted unit ([ENE] assigned as 47.7 ppm) among the repeated ethylene insertions and the alternating sequence ([NEN] assigned as 48.4 and 47.9 ppm) was also present. No resonance ascribed to repeated NBE insertion was observed.

Conclusions

A series of half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands, Cp′[2,5-C₄H₂N(CH=NAr)₂]CrCl [Cp′ = C₅H₅, Ar = C₆H₅ (2a), 2,6-Me₂C₆H₃ (2b), 2,6-ⁱPr₂C₆H₃ (2c), C₆F₅ (2d); Cp′ = C₅Me₅, Ar = C₆H₅ (3a), 2,6-ⁱPr₂C₆H₃ (3c)] were synthesized, characterized and investigated as efficient catalysts for olefin polymerization. These complexes adopt a pseudo-octahedral coordination environment with a three-legged piano stool geometry in the solid state. The bis(imino)pyrrole ligands acted as a bidentate ligand, with one of the imino nitrogen atoms coordinated with Cr metal. Both the cyclopentadienyl group and the anionic donor ligand affected the polymerization behavior. High molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the nature of the polymerization was single site. In addition, complexes 2a–d showed high catalytic activities for ethylene/NBE copolymerization. The level of NBE incorporation reached 38.8 mol% in the resultant copolymers using catalyst 2c. We believe that these results provide important information for designing efficient transition metal catalysts for olefin polymerization.

Experimental

General procedures and materials

All work involving air- and/or moisture-sensitive compounds was carried out under a dry nitrogen atmosphere using standard Schlenk techniques or under a dry argon atmosphere in an MBraun glove box, unless stated otherwise. All solvents used were purified from an MBraun SPS system. The NMR data of ligands were obtained on a Bruker 400 MHz spectrometer at ambient temperature with CDCl₃ as the solvent. The NMR analyses of the polymer samples were performed on a Bruker 400 MHz spectrometer at 135 °C using o-C₆D₄Cl₂ as the solvent. The IR spectra were recorded on a Bio-Rad FTS-135 spectrophotometer. Elemental analyses were carried out on an elemental Vario EL spectrometer. Mass spectra were obtained using electron impact mass spectrometry (EI-MS) on an LDI-1700 instrument (Linear Scientific Inc.). Differential scanning calorimetry (DSC) measurements were performed with a Perkin-Elmer Pyris 1 DSC differential scanning calorimeter at a rate of 10 °C min⁻¹. The molecular weight and the polydispersity indices of the polymer samples were determined at 150 °C by a PL-GPC 220 high-temperature chromatograph equipped with three PLgel 10 μm Mixed-B LS type columns. 1,2,4-Trichlorobenzene was used as the solvent at a flow-rate of 1.0 mL min⁻¹. Calibration was carried out using the polystyrene standard EasiCal PS-1 (PL Ltd). The 2.20 M n-butyllithium solution in hexane was purchased from Acros. CrCl₃(THF)₃ was purchased from Aldrich. Methylaluminoxane was purchased from Akzo Nobel Chemical Inc. Commercial ethylene was used without further purification. The ligands used were synthesized according to previously published methods.²³

Synthesis of half-sandwich Cr(III) complexes

A suspension of CpLi (0.15 g, 2.00 mmol) in THF (10 mL) was slowly added to a purple suspension of CrCl₃(THF)₃ (0.75 g, 2.00 mmol) in THF (20 mL) at −20 °C. The mixture was warmed to room temperature and stirred overnight to obtain a blue solution. In another flask, a solution of n-BuLi (2.20 M, 2.00 mmol) in hexane was slowly added to a solution of compound 1a (0.55 g, 2.00 mmol) in dried THF (15 mL). The reaction mixture was allowed to warm to room temperature and stirred for 2.5 h, then added slowly to the above reaction mixture at −78 °C. The obtained reaction mixture was allowed to warm to room temperature and stirred overnight. During the reaction, the color of the reaction mixture changes from blue to green. The evaporation of the solvent under reduced pressure yielded a crude product, followed by extraction with toluene (20 mL) to remove the insoluble impurities. The filtrate was concentrated and recrystallized in toluene–hexane and yielded 601 mg of the pure complex as a dark green block solid of 2a. Complexes 2b–e, 3a and 3c were synthesized in the same manner as complex 2a with different starting materials.

Cp[2,5-(CH=NC₆H₅)₂C₄H₂N]CrCl (2a). Yield: (71%). FTIR (KBr, cm⁻¹): ν_C=N 1558, 1621. EI-MS (70 eV): m/z = 423 [M⁺]. Analysis calculated for C₂₃H₁₉ClCrN₃: C, 65.02; H, 4.51; N, 9.89. Found: C, 64.84; H, 4.46; N, 9.83.

Cp[2,5-(CH=N-2,6-MeC₆H₃)₂C₄H₂N]CrCl (2b). Yield: (75%). FTIR (KBr, cm⁻¹): ν_C=N 1561, 1629. EI-MS (70 eV): m/z = 479 [M⁺]. Analysis calculated for C₂₇H₂₇ClCrN₃: C, 67.42; H, 5.66; N, 8.74. Found: C, 67.27; H, 5.61; N, 8.68.

Cp[2,5-(CH=N-2,6-ⁱPrC₆H₃)₂C₄H₂N]CrCl (2c). Yield: (64%). FTIR (KBr, cm⁻¹): ν_C=N 1559, 1620. EI-MS (70 eV): m/z = 592 [M⁺]. Analysis calculated for C₃₅H₄₃ClCrN₃: C, 70.87; H, 7.31; N, 7.08. Found: C, 71.03; H, 7.39; N, 7.01.

Cp[2,5-(CH=NC₆F₅)₂C₄H₂N]CrCl (2d). Yield: (66%). FTIR (KBr, cm⁻¹): ν_C=N 1561, 1617. EI-MS (70 eV): m/z = 603 [M⁺]. Analysis calculated for C₂₃H₉ClCrF₁₀N₃: C, 45.68; H, 1.50; N, 6.95. Found: C, 45.83; H, 1.55; N, 6.89.

Cp[2-^tBu-5-(CH=NC₆H₅)C₄H₂N]CrCl (2e). Yield: (70%). FTIR (KBr, cm⁻¹): ν_C=N 1620. EI-MS (70 eV): m/z = 376 [M⁺]. Analysis calculated for C₂₀H₂₂ClCrN₂: C, 63.57; H, 5.87; N, 7.41. Found: C, 63.32; H, 5.93; N, 7.53.

Cp*[2,5-(CH=NC₆H₅)₂C₄H₂N]CrCl (3a). Yield: (77%). FTIR (KBr, cm⁻¹): ν_C=N 1545, 1618. EI-MS (70 eV): m/z = 493 [M⁺]. Analysis calculated for C₂₈H₂₉ClCrN₃: C, 67.94; H, 5.91; N, 8.49. Found: C, 67.74; H, 5.96; N, 8.60.

Cp*[2,5-(CH=N-2,6-ⁱPrC₆H₃)₂C₄H₂N]CrCl (3c). Yield: (74%). FTIR (KBr, cm⁻¹): ν_C=N 1560, 1621. EI-MS (70 eV): m/z = 661 [M⁺]. Analysis calculated for C₄₀H₅₃ClCrN₃: C, 72.43; H, 8.05; N, 6.33. Found: C, 72.61; H, 8.15; N, 6.54.

General procedures for ethylene polymerization

High-pressure polymerization experiments were performed in a mechanically stirred 200 mL stainless-steel reactor equipped with an electric heating mantle controlled by a thermocouple dipping into the reaction mixture. The reactor was heated under a flow of nitrogen for 12 h at 150 °C and subsequently cooled to the temperature of polymerization. The reagents were transferred via a gas-tight syringe to the evacuated reactor. Ethylene was introduced into the reactor and the reactor pressure was maintained at the prescribed ethylene pressure throughout the polymerization run by continuously feeding ethylene gas. After proceeding for 10 min, the polymerization was stopped by turning the ethylene off and relieving the pressure. The reaction mixture was poured into a solution of hydrochloric acid–ethanol to precipitate the resultant polymer. The polymer was isolated by filtration, washed with ethanol and dried under vacuum at 60 °C for 10 h in a vacuum oven.

Copolymerization of ethylene with norbornene

A typical procedure was performed as follows: the prescribed amounts of toluene, NBE and MAO were added to the reactor (200 mL, stainless steel) in the autoclave and the apparatus was purged with ethylene. The reaction mixture was then pressurized to the prescribed ethylene pressure soon after the addition of a toluene solution containing the Cr complex. The reaction mixture was poured into a solution of hydrochloric acid–ethanol (10 vol%) to precipitate the polymer. The polymer was isolated by filtration, washed with ethanol and dried under vacuum at 60 °C for 10 h in a vacuum oven.

X-ray crystallography

Single crystals of complexes 2a–c suitable for X-ray structural determination were grown from a hexane solution at −20 °C in a glove box, thus maintaining a dry O₂-free environment. The intensity data were collected in the ω scan mode (186 K) on a Bruker Smart APEX diffractometer with a CCD detector using Mo Kα radiation (λ = 0.71073 Å). Lorentz polarization factors were calculated for the intensity data and the absorption corrections were determined using the SADABS program. The crystal structures were solved using the SHELXTL program and refined using full matrix least squares. The positions of the hydrogen atoms were calculated theoretically and included in the final cycles of refinement in a riding model along with the attached carbons.

Acknowledgements

The authors are grateful for financial support provided by the National Natural Science Foundation of China (nos 21234006, 21274144).

References

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Footnote

† Electronic supplementary information (ESI) available: Crystal data and structure refinements of complexes 2a–c; the structures for 2b and 2c and X-ray diffraction data for 2a–c as cif; ¹³C-NMR spectra of E/NBE copolymer with different NBE incorporations produced by 2c. CCDC 979517–979519. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02039a

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