Half-titanocene complexes bearing dianionic 6-benzimidazolylpyridyl-2-carboximidate ligands:...

Half-Titanocene Complexes Bearing Dianionic6-Benzimidazolylpyridyl-2-carboximidate Ligands:Synthesis, Characterization, and TheirEthylene Polymerization

WEIWEI ZUO, SHU ZHANG, SHAOFENG LIU, XIJIE LIU, WEN-HUA SUN

Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences,Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

Received 20 August 2007; accepted 16 February 2008DOI: 10.1002/pola.22693Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: 6-Benzimidazolylpyridyl-2-carboximidic half-titanocene complexes, Cp0TiLCl(Cp0 ¼ C5H5, MeC5H4, C5Me5, L ¼ 6-benzimidazolylpyridine-2-carboxylimidic, C1–C13), were synthesized and characterized along with single-crystal X-ray diffraction.The half-titanocene chlorides containing substituted cyclopentadienyl groups, espe-cially pentamethylcyclopentadienyl groups were more stable, while those withoutsubstituents on the cyclopentadienyl groups were easily transformed into their di-meric oxo-bridged complexes, (CpTiL)2O (C14 and C15). In the presence of excessiveamounts of methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), allhalf-titanocene complexes showed high catalytic activities for ethylene polymeriza-tion. The substituents on the Cp groups affected the catalytic behaviors of the com-plexes significantly, with less substituents favoring increased activities and highermolecular weights of the resultant polyethylenes. Effects of reaction conditions oncatalytic behaviors were systematically investigated with catalytic systems of mono-nuclear C1 and dimeric C14. With C1/MAO, large MAO amount significantlyincreases the catalytic activity, while the temperature only has a slight effect on theproductivity. In the case of C14/MAO catalytic system, temperature above 60 8C andAl/Ti value higher than 5000 were necessary to observe good catalytic activities. Inboth systems, higher reaction temperature and low cocatalyst amount gave the poly-ethylenes with higher molecular weights. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part

A: Polym Chem 46: 3396–3410, 2008

Keywords: catalysts; oxo-bridged dimeric complexes; polyethylene (PE); Ziegler-Natta polymerization

INTRODUCTION

The production of polyolefins has reached 110 mil-lion tons in 2005 using mainly Ziegler-Natta orPhillips catalysts along with increasing share of

metallocene and other single-site catalysts. Metal-locene complexes such as titanocene Cp2TiCl2were first used as catalytic model in understand-ing the mechanism of Ziegler-Natta catalyst in1950s,1 but showed poor activity. Until early1980s, Sinn and Kaminsky found that methylalu-minoxane (MAO) could activate metalloceneCp2ZrCl2 as highly active catalyst for olefin poly-merization.2 Further researches have worked tobridged metallocenes3 and also back to titanocenes.4

Correspondence to: W.-H. Sun (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 3396–3410 (2008)VVC 2008 Wiley Periodicals, Inc.

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Later on, the bridged half-metallocene catalystsknown as constrained geometry catalysts havedrawn much attentions starting from 1990s,5

which provided suitable catalysts for industrialprocesses in Dow and Exxon Chemicals. Recently,nonbridged half-metallocene group 4 complexesbecame one hot topic with the initial study byNomura on half-titanocenes containing aryloxoligands.6 The progress of nonbridged half-metal-locene catalysts were well collected in the goodreview articles.7 Regarding to their high catalyticactivities and unique catalytic behaviors, exten-sive researches on nonbridged half-metallocenecatalysts potentially provide the promising cata-lysts for industrial processes.

Regarding to various models of half-titanocenecatalysts, the ancillary donor ligands are gener-ally mono-anionic ones acting as monodentate orbidentate bonding ways. In addition to the half-titanocene precursors of common aryloxy or keti-mido ligands (well covered in review article),7 thebidentate ancillary ligands such as acetamidi-nato,8 iminophenoxy,9 pyridinylalkyloxy,10 and N-substituted (iminomethyl)pyrrolyl11 ligands havedrawn our attentions. Furthermore, dianionic tri-dentate12 and trianionic ligands13 have also beenfound to exhibit interesting properties in theirtransition metal complexes. The hemi-liabilityfeature of multidentate ligands became attrac-tive14 because of their mixing bonding character-istics, which enhanced the stability and reactiv-ity of complexes with incoming substrates.15 Thecombined functions of pyridinylalkyloxy (A)10

and (iminomethyl)pyrrolyl (B)11 would be aninteresting strategy in designing new model half-titanocene catalysts (C) containing dianionic tri-dentate ligands for olefin polymerization.

As our work aimed at designing metal com-plexes as catalysts for ethylene oligomerizationand polymerization, imino-indolide ligands wereeffectively used in catalysts of nickel16 and tita-nium.17 Meanwhile, our recent benzimidazolyl-pyridine-based ligands have provided numeroushighly active catalysts of metal complexes forethylene oligomerization and polymerization.18

In addition, more derivatives of these benzimi-

dazolylpyridine-based compounds have also beendesigned and synthesized.19 These ligands andtheir modified derivative ligands would be gen-erally interesting for their titanium complexesand the catalytic behaviors of ethylene polymer-izations. This comes to our effort toward modelcatalyst C bearing dianionic tridentate ligands,which is a hybrid of catalysts such as amido-cyclopentadienyl titanium catalysts (CGC),5

bis(phenoxy-imine) Ti complexes (FI catalyst),20

and pyrrolide-imine ligated Ti complexes (PI cat-alyst).21 A series of 6-(benzimidazol-2-yl)-N-organylpyridine-2-carboxamide derivatives wereprepared as ancillary donor ligands to coordi-nate with titanium tetrachloride22 and (substi-tuted)cyclopentadienyltitanium trichloride. Thebis(6-benzimidazolylpyridyl-2-carboximidate)tita-nium showed good catalytic activity for ethylenepolymerization,22 meanwhile the half-titanocenecomplexes showed highly catalytic activities to-ward ethylene polymerization in the presence ofMAO or modified methylaluminoxane (MMAO).The half-titanocene complexes were thermallystable in the course of ethylene polymerizationat elevated reaction temperature. However, theirsensitivities in air gave the oxo-bridged dimericcomplexes. The dimeric complexes could performgood catalytic activity for ethylene polymeri-zation (about one-order lower activity than mono-nuclear precursor). Herein, we report the synthe-sis and characterization of 6-benzimidazolyl-N-organylpyridine-2-carboxamide derivatives andtheir half-titanocene complexes, as well as theircatalytic behaviors for ethylene polymerization.

EXPERIMENTAL

General Considerations

All manipulations of air and/or moisture-sensi-tive compounds were performed under nitrogenatmosphere using standard Schlenk technology.MAO (1.46 M in toluene) and MMAO (1.93 M inheptanes, 3A) were purchased from Akzo Nobel.Diethyl aluminum chloride (Et2AlCl, 1.7 M intoluene), trimethyl aluminum (AlMe3, 1 M inhexane), and triethyl aluminum (AlEt3, 2 Min hexane) were obtained from Acros Chemicalsand other reagents were purchased from Aldrichor Acros Chemicals. Sodium hydride (NaH), pur-chased from Beijing Regent Chemicals, waswashed with petroleum ether before use toremove contained mineral oil. Tetrahydrofuran(THF), toluene, and hexane were refluxed over so-

HALF-TITANOCENE COMPLEXES BEARING DIANIONIC LIGANDS 3397

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

dium and benzophenone, distilled, and then storedunder nitrogen atmosphere. Dichloromethane(CH2Cl2) were refluxed over calcium hydride(CaH2), distilled, and then stored under nitrogenatmosphere. Elemental analysis was performed ona Flash EA 1112 microanalyzer. 1H NMR and 13CNMR spectra were recorded on a Bruker DMX300 MHz instrument at ambient temperatureusing TMS as an internal standard. DSC traceand melting points of polyethylenes were obtainedfrom the second scanning run on a Perkin-ElmerDSC-7 at a heating rate of 10 8C/min.

Synthesis of Complexes C1–C15

g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dimethylphenyl)pyridyl-2-carboximidate]chlorotitanium (C1)

To a stirred solution of 6-(benzimidazol-2-yl)-N-(2,6-dimethylphenyl)pyridine-2-carboxamide (1.026g, 3.00 mmol) in dried THF (30 mL) at �78 8C,NaH (0.144 g, 6.0 mmol) was added. The mix-ture was allowed to warm to room temperatureand stirred for additional 2 h. At �78 8C, 20 mLof a CpTiCl3 (0.657, 3 mmol) solution in THFwas added dropwise over a 30-min period. Theresultant mixture was allowed to warm to roomtemperature and stirred for additional 12 h. Theresidue obtained by removing the solvent undervacuum was extracted with dried CH2Cl2 andthen the resulting mixture was filtered. The res-idue was washed with CH2Cl2 (2 3 20 mL) andthe combined filtrates were concentrated in vac-uum to reduce the volume to 10 mL. Hexane(45 mL) was layered and several days later blackcrystals were obtained (0.732 g, yield 50%).

1H NMR (CDCl3, 300 MHz): d 8.19 (t, J ¼ 7.7Hz, 1H, Py), 8.12 (d, J ¼ 7.9 Hz, 1H, Py), 8.08 (d, J¼ 7.8 Hz, 1H, Py), 7.47 (d, J ¼ 7.5 Hz, 1H, Ar),7.13 (d, J ¼ 7.2 Hz, 2H, Ar), 7.08 (t, J ¼ 6.8 Hz,1H, Ar), 6.95–7.04 (m, 2H, Ar), 6.86 (d, J ¼ 7.2 Hz,1H, Ar), 6.27 (s, 5H, C5H5), 2.38 (s, 3H, CH3), 2.23(s, 3H, CH3).

13C NMR (CDCl3, 75 MHz): d 157.9,156.6, 153.8, 149.7, 146.1, 144.3, 143.0, 139.5,127.9, 127.7, 126.7, 121.1, 120.9, 120.7, 119.6,119.2, 118.9, 118.6, 12.8. Anal. Calcd. forC26H21ClN4OTi 0.5CH2Cl2: C, 59.91; H, 4.17; N,10.55. Found: C, 59.23; H, 4.28; N, 10.13.

g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-diethylphenyl)pyridine-2-carboximidate]chlorotitanium (C2)

Using the same procedure as for the synthesisof C1, C2 was obtained as dark blue solid in56.5% yield.

1H NMR (CDCl3, 300 MHz): d 8.18 (t, J ¼ 7.8Hz, 1H, Py), 8.10 (d, J ¼ 7.6 Hz, 1H, Py), 8.08(d, J ¼ 7.7 Hz, 1H, Py), 7.50 (d, J ¼ 7.2 Hz, 1H,Ar), 7.15 (d, J ¼ 7.6 Hz, 2H, Ar), 7.10 (t, J ¼ 7.8Hz, 1H, Ar), 6.96–7.03 (m, 2H, Ar), 6.79 (d, J ¼7.5 Hz, 1H, Ar), 6.28 (s, 5H, C5H5), 2.60 (q, 4H,CH2), 1.20 (t, J ¼ 7.2 Hz, 6H, CH3).

13C NMR(CDCl3, 75 MHz): d 160.8, 154.0, 152.5, 152.3,146.7, 145.2, 142.5, 142.2, 140.8, 140.7, 140.1,135.9, 131.2, 130.4, 122.8, 116.8, 115.9, 115.2,22.6, 11.4. Anal. Calcd. for C28H25ClN4OTi: C,65.07; H, 4.88; N, 10.84. Found: C, 65.42; H,4.75; N, 10.55.

g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-diisopropylphenyl)pyridyl-2-carboximidate]chlorotitanium (C3)


1H NMR (CDCl3, 300 MHz): d 8.22 (t, J ¼ 7.8Hz, 1H, Py), 8.13 (d, J ¼ 7.8 Hz, 1H, Py), 8.06(d, J ¼ 7.7 Hz, 1H, Py), 7.49 (d, J ¼ 7.2 Hz, 1H,Ar), 7.31 (t, J ¼ 7.6 Hz, 1H, Ar), 7.16 (d, J ¼ 7.4Hz, 2H, Ar), 7.03–7.15 (m, 2H, Ar), 6.98 (d, J ¼7.2 Hz, 1H, Ar), 6.26 (s, 5H, C5H5), 3.13 (m,2H), 1.32 (d, J ¼ 6.8 Hz, 12H). 13C NMR(CDCl3, 75 MHz): d 153.0, 152.7, 146.9, 143.7,141.3, 140.6, 140.3, 134.6, 121.5, 120.6, 120.4,119.6, 119.4, 119.3, 119.0, 118.1, 116.0, 111.8,26.3, 19.8. Anal. Calcd. for C30H29ClN4OTi: C,66.13; H, 5.36; N, 10.28. Found: C, 66.01; H,5.10; N, 10.22.

g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-difluorophenyl)pyridyl-2-carboximidate]chlorotitanium (C4)


1H NMR (CDCl3, 300 MHz): d 8.20 (t, J ¼ 7.7Hz, 1H, Py), 8.14 (d, J ¼ 7.8 Hz, 1H, Py), 8.07(d, J ¼ 7.9 Hz, 1H, Py), 7.44 (d, J ¼ 7.4 Hz, 1H,Ar), 7.33 (t, J ¼ 7.8 Hz, 1H, Ar), 7.10 (d, J ¼ 7.5Hz, 2H, Ar), 7.03–7.08 (m, 2H, Ar), 6.86 (d, J ¼7.6 Hz, 1H, Ar), 6.30 (s, 5H, C5H5).

13C NMR(CDCl3, 75 MHz): d 160.3, 157.2, 153.9, 147.5,145.9, 144.9, 135.8, 135.1, 126.1, 125.3, 122.5,122.4, 121.3, 115.2, 113.9, 113.6, 108.6, 108.3.Anal. Calcd. for C24H15ClF2N4OTi: C, 58.03; H,3.04; N, 11.28. Found: C, 59.95; H, 3.11; N,11.00.

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g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dichlorophenyl)pyridyl-2-carboximidate]chlorotitanium (C5)


1H NMR (CDCl3, 300 MHz): d 8.21 (t, J ¼ 7.9Hz, 1H, Py), 8.17 (d, J ¼ 7.6 Hz, 1H, Py), 8.08(d, J ¼ 7.7 Hz, 1H, Py), 7.45 (d, J ¼ 7.2 Hz, 1H,Ar), 7.35 (d, J ¼ 7.9 Hz, 2H, Ar), 7.22 (t, J ¼ 7.8Hz, 1H, Ar), 7.01–7.06 (m, 2H, Ar), 6.95 (d, J ¼7.4 Hz, 1H, Ar), 6.33 (s, 5H, C5H5).

13C NMR(CDCl3, 75 MHz): d 158.1, 156.0, 151.5, 151.4,142.2, 141.1, 137.8, 136.9, 126.1, 125.3, 125.1,122.9, 121.8, 121.1, 117.2, 116.3, 115.9, 113.5.Anal. Calcd. for C24H15Cl3N4OTi 0.5CH2Cl2: C,51.44; H, 2.82; N, 9.79. Found: C, 51.12; H, 2.67;N, 9.63.

g5-Cyclopentadienyl[6-(benzimidazol-2-yl)-N-butyl-pyridyl-2-carboximidate]chlorotitanium (C6)


1H NMR (CDCl3, 300 MHz): d 8.17 (t, J ¼ 7.7Hz, 1H, Py), 8.11 (d, J ¼ 7.8 Hz, 1H, Py), 8.06(d, J ¼ 7.7 Hz, 1H, Py), 7.46 (d, J ¼ 7.3 Hz, 1H,Ar), 7.02–7.07 (m, 2H, Ar), 6.88 (d, J ¼ 7.2 Hz,1H, Ar), 6.27 (s, 5H, C5H5), 3.42 (q, J ¼ 6.9 Hz,2H, CH2), 1.54 (m, 2H, CH2), 1.28 (m, 2H, CH2),0.81 (t, J ¼ 7.3 Hz, 3H, CH3).

13C NMR (CDCl3,75 MHz): d 155.2, 153.1, 149.4, 147.5, 144.7,141.9, 121.8, 121.3, 120.4, 119.5, 119.2, 118.0,117.3, 41.2, 28.0, 17.9, 10.9. Anal. Calcd. forC22H21ClN4OTi: C, 59.95; H, 4.80; N, 12.71.Found: C, 59.66; H, 4.72; N, 12.63.

g5-Methylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-diethylphenyl)pyridyl-2-carboximidate]chlorotitanium (C7)


1H NMR (CDCl3, 300 MHz): d 8.13 (t, J ¼ 7.6Hz, 1H, Py), 8.07 (d, J ¼ 7.8 Hz, 1H, Py), 8.01(d, J ¼ 7.9 Hz, 1H, Py), 7.38 (d, J ¼ 7.2 Hz, 1H,Ar), 7.17 (d, J ¼ 7.6Hz, 2H, Ar), 7.04–7.06 (m,3H, Ar), 6.90 (d, J ¼ 7.4 Hz, 1H, Ar), 6.15 (t,2H, C5H4), 5.97 (t, 2H, C5H4), 2.63 (q, 4H, CH2),2.33 (s, 3H, CH3), 1.21 (t, J ¼ 7.2 Hz, 6H, CH3).13C NMR (CDCl3, 75 MHz): d 160.3, 156.2,

152.1, 151.4, 143.7, 141.6, 140.5, 138.3, 125.1,124.7, 123.0, 122.5, 121.7, 120.6, 116.9, 120.6,116.9, 115.5, 115.1, 109.7, 24.7, 14.8, 13.5. Anal.Calcd. for C29H27ClN4OTi: C, 65.61; H, 5.13; N,10.55. Found: C, 65.47; H, 5.07; N, 10.42.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dimethylphenyl)pyridyl-2-carboximidate]chlorotitanium (C8)


1H NMR (CDCl3, 300 MHz): d 8.27 (d, J ¼ 7.2Hz, 1H, Ar), 8.08–8.13 (m, 2H, Py, Ar), 7.83 (d,J ¼ 7.9 Hz, 1H, Py), 7.77 (d, J ¼ 7.7 Hz, 1H,Py), 7.25–7.29 (m, 2H, Ar), 7.01 (t, J ¼ 6.9 Hz,1H, Ar), 6.95 (d, J ¼ 7.4 Hz, 2H, Ar), 2.17 (s,6H, CH3), 1.67 (s, 15H, C5Me5).

13C NMR(CDCl3, 75 MHz): d 153.3, 147.9, 145.5, 142.1,141.7, 140.9, 136.0, 131.5, 126.9, 126.2, 125.3,124.8, 122.5, 120.6, 119.9, 118.5, 116.9, 115.2,15.4, 9.44. Anal. Calcd. for C31H31ClN4OTi: C,66.62; H, 5.59; N, 10.02. Found: C, 66.50; H,5.44; N, 10.10.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-diethylphenyl)pyridyl-2-carboximidate]chlorotitanium (C9)


1H NMR (CDCl3, 300 MHz): d 8.28 (d, J ¼ 7.5Hz, 1H, Ar), 8.06–8.15 (m, 2H, Py, Ar), 7.83 (d, J¼ 8.0 Hz, 1H, Py), 7.78 (d, J ¼ 8.0 Hz, 1H, Py),7.24–7.30 (m, 2H, Ar), 7.14 (d, J ¼ 7.2 Hz, 2H,Ar), 7.08 (t, J ¼ 7.8 Hz, 1H, Ar), 2.78 (q, 2H,CH2), 2.47 (q, 2H, CH2), 1.76 (s, 15H, C5Me5),1.23 (t, J ¼ 7.8 Hz, 3H, CH3), 1.15 (t, J ¼ 7.8 Hz,3H, CH3).

13C NMR (CDCl3, 75 MHz): d 161.8,158.1, 156.2, 155.9, 148.6, 145.9, 143.8, 143.6,142.7, 141.8, 139.0, 137.5, 135.1, 134.6, 134.3,132.0, 125.3, 123.6, 24.7, 13.8, 12.7. Anal. Calcd.for C33H35ClN4OTi 0.5CH2Cl2: C, 63.92; H, 5.76;N, 8.90. Found: C, 63.88; H, 5.58; N, 8.73.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-diisopropylphenyl)pyridyl-2-carboximidate]chlorotitanium (C10)




1H NMR (CDCl3, 300 MHz): d 8.26 (d, J ¼ 7.2Hz, 1H, Ar), 8.07–8.11 (m, 2H, Py, Ar), 7.84 (d,J ¼ 8.1 Hz, 1H, Py), 7.75 (d, J ¼ 8.1 Hz, 1H,Py), 7.25–7.28 (m, 2H, Ar), 7.22 (d, J ¼ 7.2 Hz,2H, Ar), 7.16 (t, J ¼ 7.8 Hz, 1H, Ar), 3.19 (m,1H, CH), 3.14 (m, 1H, CH), 1.72 (s, 15H,C5Me5), 1.28 (d, J ¼ 6.8 Hz, 6H, CH3), 1.25 (d, J¼ 6.9 Hz, 6H, CH3).

13C NMR (CDCl3, 75 MHz):d 154.0, 153.2, 148.7, 147.4, 146.3, 145.6, 143.9,142.5, 141.3, 130.9, 125.4, 120.5, 119.7, 118.4,117.7, 117.4, 115.3, 108.3, 26.3, 23.8 9.52. Anal.Calcd. for C35H39ClN4OTi: C, 68.35; H, 6.39; N,9.11. Found: C, 68.21; H, 6.24; N, 9.03.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-difluorophenyl)pyridyl-2-carboximidate]chlorotitanium (C11)


1H NMR (CDCl3, 300 MHz): d 8.30 (d, J ¼ 7.4Hz, 1H, Ar), 8.09 (t, J ¼ 7.8 Hz, 1H, Py), 8.02(d, J ¼ 7.2 Hz, 1H, Ar), 7.83 (d, J ¼ 8.1 Hz, 1H,Py), 7.75 (d, J ¼ 7.8 Hz, 1H, Py), 7.24 (d, J ¼7.6 Hz, 2H, Ar), 6.97–7.09 (m, 3H, Ar), 1.79 (s,15H, C5Me5).

13C NMR (CDCl3, 75 MHz): d159.4, 158.2, 154.7, 145.6, 142.1, 140.0, 136.2,128.2, 125.6, 121.5, 121.0, 120.2, 119.6, 119.4,117.7, 109.1, 108.8, 108.2, 9.22. Anal. Calcd. forC29H25ClF2N4OTi: C, 61.45; H, 4.45; N, 9.88.Found: C, 61.17; H, 4.24; N, 9.73.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dichlorophenyl)pyridyl-2-carboximidate]chlorotitanium (C12)


1H NMR (CDCl3, 300 MHz): d 8.33 (d, J ¼ 7.5Hz, 1H, Ar), 8.11 (t, J ¼ 7.9 Hz, 1H, Py), 8.06 (d, J¼ 7.5 Hz, 1H, Ar), 7.84 (d, J ¼ 7.7 Hz, 1H, Py),7.77 (d, J ¼ 7.9 Hz, 1H, Py), 7.32 (d, J ¼ 7.9 Hz,2H, Ar), 7.21 (t, J ¼ 7.8 Hz, 1H, Ar), 7.03–7.11 (m,2H, Ar), 1.72 (s, 15H, C5Me5).

13C NMR (CDCl3, 75MHz): d 159.3, 157.1, 152.3, 151.7, 143.4, 140.9,136.5, 136.2, 125.8, 125.3, 125.0, 122.7, 121.1,120.9, 120.7, 117.0, 116.4, 113.7, 9.35. Anal. Calcd.for C29H25Cl3N4OTi: C, 58.07; H, 4.20; N, 9.34.Found: C, 57.97; H, 4.03; N, 9.21.

g5-Pentamethylcyclopentadienyl[6-(benzimidazol-2-yl)-N-butylpyridyl-2-carboximidate]chlorotitanium (C13)


1H NMR (CDCl3, 300 MHz): d 8.26 (d, J ¼ 7.2Hz, 1H, Ar), 8.07–8.14 (m, 2H, Py, Ar), 7.84 (d,J ¼ 7.9 Hz, 1H, Py), 7.73 (d, J ¼ 7.7 Hz, 1H,Py), 7.27–7.31 (m, 2H, Ar), 3.47 (q, J ¼ 6.9 Hz,2H, CH2), 1.75 (s, 15H, C5Me5), 1.56 (m, 2H,CH2), 1.26 (m, 2H, CH2), 0.83 (t, J ¼ 7.3 Hz,3H, CH3).

13C NMR: d 153.7, 147.7, 145.7, 141.6,140.6, 131.4, 120.8, 119.8, 118.1, 117.9, 117.7,116.9, 115.1, 41.2, 30.4, 18.2, 11.1, 9.48. Anal.Calcd. for C27H31ClN4OTi: C, 63.48; H, 6.12; N,10.97. Found: C, 63.53; H, 6.03; N, 10.90.

Bis{g5-cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dimethylphenyl)pyridyl-2-carboximidate]titanium}(l-oxo) (C14)

To a stirred solution of 6-(benzimidazol-2-yl)-N-(2,6-dimethylphenyl)pyridine-2-carboxamide(1.026 g, 3.00 mmol) in dried THF (30 mL) at�78 8C, NaH (0.144 g, 6.00 mmol) was added.The mixture was allowed to warm to room tem-perature and stirred for additional 2 h. The mix-ture was cooled to �78 8C and 10 mL of CpTiCl3(0.657, 3 mmol) solution in THF was addeddropwise over a 30-min period. The resultantmixture was allowed to warm to room tempera-ture and stirred for additional 12 h. The residueobtained by removing the solvent under vacuumwas extracted with dried CH2Cl2 and then theresulting mixture was filtered. The residue waswashed with CH2Cl2 (2 3 20 mL) and the com-bined filtrates were concentrated in vacuum toreduce the volume to 10 mL. Hexane (45 mL)was layered and the solution was open to air,and 1 week later bright red crystals wereobtained (0.93 g, yield 67%).

1H NMR (CDCl3, 300 MHz): d 8.20 (t, J ¼ 7.9Hz, 1H, Py), 8.10 (d, J ¼ 7.8 Hz, 1H, Py), 8.07(d, J ¼ 7.8 Hz, 1H, Py), 7.48 (d, J ¼ 7.3 Hz, 1H,Ar), 7.21 (d, J ¼ 7.2 Hz, 2H, Ar), 7.11–7.13 (m,3H, Ar), 6.95 (d, J ¼ 7.5 Hz, 1H, Ar), 6.30 (s,5H, C5H5), 2.38 (s, 3H, CH3), 2.24 (s, 3H, CH3).13C NMR (CDCl3, 75 MHz): d 159.4, 145.7,145.2, 138.7, 136.1, 135.8, 128.5, 128.3, 127.8,126.4, 126.2, 125.7, 125.0, 123.8, 122.2, 118.4,117.8, 117.1, 13.9. Anal. Calcd. for C52H42ClN8-O3Ti2�CH2Cl2: C, 63.18; H, 4.40; N, 11.12.Found: C, 63.55; H, 4.59; N, 11.22.

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Bis{g5-cyclopentadienyl[6-(benzimidazol-2-yl)-N-(2,6-dichlorophenyl)pyridyl-2-carboximidate]titanium}(l-oxo) (C15)

Using the same procedure as for the synthesisof C14, C15 was obtained as bright red crystalsin 63.3% yield.

1H NMR (CDCl3, 300 MHz): d 8.26 (t, J ¼ 7.9Hz, 1H, Py), 8.13 (d, J ¼ 7.9 Hz, 1H, Py), 8.06(d, J ¼ 7.7 Hz, 1H, Py), 7.53 (d, J ¼ 7.6 Hz, 1H,Ar), 7.16 (t, J ¼ 7.8 Hz, 1H, Ar), 7.01–7.11(m,4H), 6.76 (d, J ¼ 7.4 Hz, 1H), 6.36 (s, 5H, C5H5).13C NMR (CDCl3, 75 MHz): d 160.2, 157.3,152.6, 151.1, 145.7, 142.3, 139.5, 137.4, 127.6,126.4, 125.5, 123.4, 122.3, 121.9, 120.7, 118.3,116.1, 111.5. Anal. Calcd. for C48H30Cl4N8O3-Ti2�CH2Cl2: C, 54.03; H, 2.96; N, 10.29. Found:C, 53.87; H, 2.99; N, 10.14.

Procedures for Ethylene Polymerization

The complex was added to a Schlenk-type flaskunder nitrogen. The flask was back-filled threetimes with N2 and twice with ethylene and thencharged with toluene and MAO solution in turnunder an ethylene atmosphere. At the prescribedtemperature, the reaction solution was vigo-rously stirred under 1 atm of ethylene pressurefor desired period of time. The polymerizationreaction was quenched by the addition of acidicethanol. The precipitated polymer was washedwith ethanol several times and dried in vacuum.

A 250-mL autoclave stainless steel reactorequipped with a mechanical stirrer and a tem-perature controller was heated in vacuum for atleast 2 h over 80 8C, which was allowed to coolto the required reaction temperature underethylene atmosphere and then charged withtoluene, the desired amount of cocatalyst and atoluene solution of the catalytic precursor tomake a total volume of 100 mL. After reachingthe reaction temperature, the reactor was sealedand pressurized to 10 or 30 atm of ethylenepressure. The ethylene pressure was kept con-stant during the reaction time by feeding the re-actor with ethylene. After a period of desiredreaction time, the polymerization reaction wasquenched by addition of acidic ethanol. The pre-cipitated polymer was washed with ethanol sev-eral times and dried in vacuum.

X-ray Structure Determinations

Single crystals of C9 and C13 suitable for X-raydiffraction were obtained by slow diffusion of

hexane into their CH2Cl2 solutions. Crystals ofC14 and C15 suitable for single-crystal X-rayanalysis were obtained by slowly laying hexaneon CH2Cl2 solutions of complex C1 and C5,respectively, and then opening these solutions toair for 1 week. Single-crystal X-ray diffractionfor complex C13 was performed on a BrukerSmart CCD diffractometer with graphite-mono-chromated Mo Ka radiation (k ¼ 0.71073 A) at293(2) K, while the intensity data for complexesC9, C14, and C15 were collected on RigakuRAXIS Rapid IP diffractometer with graphite-monochromated Mo Ka radiation (k ¼ 0.71073A) at 571293(2) K. Cell parameters wereobtained by global refinement of the positions ofall collected reflections. Intensities were cor-rected for Lorentz and polarization effects andempirical absorption. The structures were solvedby direct methods and refined by full-matrixleast-squares on F2. All nonhydrogen atomswere refined anisotropically. Structure solutionand refinement were performed by using theSHELXL-97 package.23 Crystal data collectionand refinement details for all compounds aregiven in Table 1. CCDC-656980 (C9), -656981(C13), -656982 (C14), and -656983 (C15) containthe supplementary crystallographic data for thisarticle, which could be obtained free of chargefrom the Cambridge Crystallographic DataCentre via www.ccdc.cam.ac.uk/data_request/cif.

RESULTS AND DISCUSSION

Synthesis and Characterization ofHalf-Titanocene Complexes

Usually the carboxamides can be easily trans-formed into their 2-carboximidic forms by pre-treatment with one equivalent NaH.24 Becauseof the presence of benzimidazole group, the cur-rent 6-(benzimidazol-2-yl)-N-organyl-pyridine-2-carboxamides would consume two equivalents ofNaH to form dianionic sodium intermediate inTHF at �78 8C. Treating the obtained salt withone equivalent of Cp0TiCl3 in THF at �78 8C,followed by stirring at room temperature for12 h, afforded purple solutions. Purple solidand black crystals for pure half-titanocene com-plexes with components of Cp0LTiCl [Cp0 ¼C5H5, MeC5H4, C5Me5, L ¼ 6-benzimidazolylpyr-idine-2-carboxylimidic, C1–C13] (Scheme 1)were obtained after purification. The elementaland NMR spectroscopic analyses confirmed their



Table

1.

Crystallog

raphic

Data

andRefi

nem

entDetailsforC9,C13,C14andC15

C9�0.

5CH

2Cl 2

C13

C14�CH

2Cl 2

C15�CH

2Cl 2

Formula

C33H

35ClN

4OTi�0

.5CH

2Cl 2

C27H

31ClN

4OTi

C52H

42N

8O

3Ti 2�CH

2Cl 2

C48H

30Cl4N

8O

3Ti 2�CH

2Cl 2

Formula

weight

629.43

510.88

1007.66

1089.33

T(K

)293(2)

293(2)

293(2)

293(2)

Wavelen

gth

(A)

0.71073

0.71073

0.71073

0.71073

Cryst

system

Orthorhom

bic

Orthorhom

bic

Mon

oclinic

Mon

oclinic

Space

group

P21212

P212121

C2/c

C2/c

a(A

)14.178(3)

7.8091(6)

25.352(5)

25.756(5)

b(A

)29.925(6)

16.0035(2)

12.479(3)

12.322(3)

c(A

)7.2671(2)

20.0819(2)

16.337(3)

16.167(3)

b(deg

)90.00

90.00

104.27(3)

105.48(3)

V(A

3)

3083.3(2)

2509.7(4)

5008.8(2)

4944.8(2)

Z4

44

4D

calcd(g/cm

3)

1.356

1.352

1.336

1.463

l(m

m�1)

0.485

0.475

0.477

0.698

F(000)

1,316

1,072

2,080

2,208

hrange(8)

1.36–27.4

2.03–28.31

1.66–27.48

1.85–27.48

Lim

itingindices

�18�

h�

18

�8�

h�

10

�31�

h�

32

�33�

h�

33

�38�

k�

38

�15�

k�

21

�15�

k�

16

�15�

k�

16

�9�

k�

9�2

4�

k�

26

�21�

k�

21

�20�

k�

20

No.

ofreflection

scollected

22,018

14,754

10,388

10,315

No.

ofuniquereflection

s4,012

6,105

5,668

5,649

Com

pletenessto

h(%

)99.7

(h¼

27.408)

100.0

(h¼

28.318)

98.6

(h¼

27.488)

99.5

(h¼

27.488)

Abscor

Empirical

Empirical

Empirical

Empirical

No.

ofparameters

373

287

308

305

Goo

dnessof

fiton

F2

0.934

1.050

1.103

1.087

FinalR

indices

(I>

2r(I))

R1¼

0.0495;wR2¼

0.1215

R1¼

0.0797;wR2¼

0.1761

R1¼

0.0584;wR2¼

0.1482

R1¼

0.0672;wR2¼

0.1628

Rindices

(alldata)

R1¼

0.0865;wR2¼

0.1459

R1¼

0.1424;wR2¼

0.2049

R1¼

0.0893;wR2¼

0.1706

R1¼

0.0968;wR2¼

0.1794

Largestdifference

pea

k,hole(e/A

3)

0.467and�0

.424

0.606and�0

.438

0.460and�0

.512

0.609and�0

.476

3402 ZUO ET AL.


structures, and several complexes were furthercharacterized by X-ray analysis to determinetheir molecular structures.

The half-titanocene complexes could be rela-tively stable in solid state, however, for C1–C6the color of their solutions would slowly changefrom purple to bright red after 1 week. To knowwhat the final stable products could be, CH2Cl2/hexane solutions of C1 and C5 were open to airfor 1 week. Bright red crystals were obtainedand analyzed by single crystal X-ray diffraction.The newly formed complexes were oxo-bridgeddimeric complexes after losing chlorides (C14and C15, Scheme 2). The phenomena of trans-forming titanium halide complexes into oxo-bridged dimeric titanium complexes were alsoobserved in other complexes due to the betterstability of final products.25 In contrast, com-plexes containing pentamethylcyclopentadienylgroups (C8–C13) were robust even in contactwith air for long time regardless in solution or

solid state. The improved stability when com-pared with its Cp analog was attributed to theintroduction of methyl groups on the Cp ring,which prevents the metal center from beingattacked. However, as will be discussed later,such steric hindrance at the metal center canalso reduce olefin coordination and insertion inthe polymerization process, and as a resultlower catalytic activities will be observed.

Crystals of C9 and C13 suitable for single-crystal X-ray analysis were obtained by layinghexane on their CH2Cl2 solutions. The molecularstructures of them are illustrated in Figures 1and 2, and the selected bonds and angles areshown in Table 2.

Figure 1 shows the molecular structure of C9with titanium coordinating to a chlorine atom, aCp* in g5-binding fashion, and a dianionic tri-dentate 6-(benzimidazol-2-yl)-N-diethylpyridyl-2-carboximidate ligand. A similar coordinationstyle could be observed in several relatedcomplexes.12b–e Four molecules of C9 and two

Scheme 2. Formation of oxo-bridged dimeric com-plexes.

Figure 1. Molecular structure of complex C9. Ther-mal ellipsoids are shown at the 30% probability level.Hydrogen atoms and the dichloromethane moleculehave been omitted for clarity.

Figure 2. Molecular structure of complex C13.Thermal ellipsoids are shown at the 30% probabilitylevel. Hydrogen atoms have been omitted for clarity.

Scheme 1. Synthesis of complexes C1–C13.



molecules of dichloromethane were incorporatedin one lattice. The 6-(benzimidazol-2-yl)-N-dieth-ylpyridyl-2-carboximidate ligand adopts a puck-ered chelating style with the dihedral angledefined by the two coordinated chelating rings(Ti1-N1-C7-C8-N2, Ti1-N2-Cl2-Cl3-O1) at 17.38.The angles N1-Ti1-N2 and N2-Ti1-O1 areapproximately same at 71.85(2)8 and 72.95(1)8with a sum of 137.64(2)8, which is much widerthan the corresponding angles found in Ti{2,20-S(OC6H2-4-Me-6-tBu)2}(g

5-C5H5)Cl and Ti(g5-C5H5)(g

2-MBMP)Cl.12a,b This may be caused bythe rigid nature of 6-(benzimidazol-2-yl)-N-organylpyridyl-2-carboximidate ligand. The twoTi–N bonds (Ti1–N1 ¼ 2.012(1) A; Ti1–N2 ¼2.166(2) A) are slightly different due to the dif-ferent chemical environments of the nitrogenatoms. The Ti1–O1 bond length is 1.967(3) A,which is much longer than those in Ti{2,20-S(OC6H12-4-Me-6-tBu)2}(g

5-C5H5)Cl and Ti(g5-C5H5)(g

2-MBMP)Cl. The C13–O1 bond length(1.307(2) A) is longer than C¼¼O bond (1.19–1.23A) but shorter than C–O single bond (1.42–1.46A), while C13–N3 bond (1.27 A) is shorter thantypically C–N single bond, displaying the doublebond character. These indicate that the electronon the amide nitrogen partly delocalizes on thechelate ring Ti1-N2-Cl2-Cl3-O1. In addition, therigid nature of the ligand and the resultantstrain from the chelating ring might also makecontributions to the elongation of Ti–O (amide)bond. Such elongated Ti–O bond was assumedto be the weakest linkage among those of tita-nium center and the ligated groups, and it is

possible that in the reactions such weak bondwill be easily broken.

Complex C13 (Fig. 2) possesses a similarcoordination feature as that in C9, despite ofthe substituents of the amide group. Similarly,elongated C13–O1 (1.352(7) A) and shorter C13–N3 (1.257(8) A) bonds can also be observed incomplex C13, indicating the presence of electrondelocalization phenomenon.

Crystals of C14 and C15 suitable for single-crystal X-ray analysis were obtained by layinghexane on CH2Cl2 solutions of complex C1 andC5, respectively, and then opening these solu-tions to air for 1 week. The molecular structuresare illustrated in Figures 3 and 4, and theselected bond lengths and angles are shown inTable 3. The molecular structure of C14 (Fig. 3)shows a binuclear species (CpTiL) bridged by an

Figure 3. (a) Molecular structure of complex C14.(b) View of the half part of the whole molecule. Ther-mal ellipsoids are shown at the 30% probability level.Hydrogen atoms and one dichloromethane moleculehave been omitted for clarity.

Table 2. Selected Bonds (A) and Angles (8) for C9and C13

C9 C13

Bond LengthsTi1-O1 1.967(3) 1.941(4)Ti1-Cl1 2.274(2) 2.2809(2)Ti1-N1 2.112(3) 2.112(5)Ti1-N2 2.139(3) 2.140(4)C13-O1 1.307(2) 1.352(7)C13-N3 1.271(5) 1.257(8)

Bond AnglesO1-Ti1-N1 137.64(2) 135.49(2)O1-Ti1-N2 72.95(1) 73.44(2)N1-Ti1-N2 71.85(2) 71.93(2)Cl1-Ti1-O1 94.14(1) 92.27(1)Cl1-Ti1-N1 90.59(1) 92.09(1)Cl1-Ti1-N2 129.69(9) 133.98(1)

3404 ZUO ET AL.


oxygen atom (O2). The bond lengths of Ti1–O2and Ti1A–O2 were identical at 1.805(2) A, whichis common in titanium complexes.12a,b In eachunit, the coordination geometry around the tita-nium center can also be described as four-leggedpiano-stool. The methyl groups on the Cp ringshave exerted some influences on the complexstructure. Because of the less steric require-ments of Cp ligand than its Cp* analog, thelength between the Cp centroid and titaniumatom is 2.044 A in C14 instead of length 2.065 Ain C9. The Cp ligand offered less steric coverageto the Ti than Cp*; therefore, complex ligated byCp confer less robustness. For example, com-plexes C9 and C13 as solid state are stable incontact with air, and survive for several daysin their solution; however, complexes C1 and C5in CH2Cl2 solution would easily be convertedto C14 and C15 through a Ti–Cl hydrolysisprocess. Comparing with the data in Table 2,however, other structural parameters includingbond lengths and bond angles are similar asthose found in C9 and C13. The solid statestructures of C14 and C15 (Fig. 4) are verysimilar, apart from the phenyl substituents.The earlier findings suggest that the cyclo-pentadienyl groups have exerted some influ-ences on the complex structures and theymay also affect the catalytic behaviors in olefinpolymerizations.

Ethylene Polymerization Behaviors ofMononuclear Complexes

Effect of Cocatalyst

The titanium complexes C1–C7 were first stud-ied for their catalytic activities in ethylene poly-merization using various organoaluminum com-pounds as cocatalysts. In terms of AlMe3, AlEt3,or Et2AlCl as cocatalyst, there was no ethyleneuptake observed, although the color of reactionmixture has changed after the addition of coca-talysts. Variation of the Al/Ti ratios (10 to 1500)and increasing the temperature also did notwork to initiate polymerization. However, when500 equivalents of MAO were added, the ethyl-ene polymerization occurred and the activitieswere increased along with the amount of MAO.Similar results were obtained when usingMMAO as cocatalyst and better catalytic activ-ities could be observed. In the presence of MAOor MMAO, complexes C1–C7 showed good activ-ities for ethylene polymerization [105 g/(mol(Ti)h)] at ambient pressure of ethylene, andtheir results are summarized in Table 4. Theactivities are higher than those of reportedhalf-titanocenes9b bearing tridentate dianionicligands.

Data in Table 4 reveal that for all the com-plexes, using MMAO as cocatlyst, can lead to alittle higher catalytic activity and produce poly-mers with slightly increased Tm values. Theresultant polyethylenes possess melting temper-

Figure 4. Molecular structure of complex C15.Thermal ellipsoids are shown at the 30% probabilitylevel. Hydrogen atoms and one dichloromethane mole-cule have been omitted for clarity.

Table 3. Selected Bonds (A) and Angles (8) for C14and C15

C14 C15

Bond LengthsTi1-O1 1.983(2) 2.002(2)Ti1-O2 1.805(1) 1.7974(9)Ti1-N1 2.101(4) 2.101(3)Ti1-N2 2.166(2) 2.159(3)C13-O1 1.315(3) 1.302(4)C13-N3 1.276(4) 1.284(4)

Bond AnglesO1-Ti1-N1 139.23(9) 139.63(1)O1-Ti1-N2 73.13(9) 72.93(1)N1-Ti1-N2 71.52(9) 71.93(4)O2-Ti1-O1 93.87(8) 92.90(9)O2-Ti1-N1 91.76(1) 92.39(1)O2-Ti1-N2 127.43(9) 126.66(1)O2-Ti1-N2 127.43(9) 126.6(1)Ti1-O2-Ti2 162.7(2) 162.5(4)



atures in the range of 133.5–134.5 8C, typical

for high-density polyethylene. IR analysis con-

firmed the linear feather of the produced poly-

ethylenes. Because of the higher price of

MMAO, the detail investigation was carried out

using MAO as cocatalyst. Further investigations

at 30 atm with complexes C1–C13 were also

carried out in the presence of MAO, and their

data were collected in Table 5.

Effect of Ligand Environment on theCatalytic Behavior

According to Table 5, the substituents on Cp0

groups had significant influences on catalyticproperties. Complexes C1–C6 containing Cpgroups without additional substituents exhibitedvery high ethylene polymerization activity up to107 g/(mol(Ti) h) (entries 1–6 in Table 5). Com-plex C7 with a methyl substituent on the Cp

Table 5. Ethylene Polymerization with C1–C13/MAOa

Entry Complex Polymer (g) Activityb Mwc (kg/mol) Mw/Mn

c

1 C1 3.90 1.87 24.6 4.582 C2 3.90 1.87 23.0 4.723 C3 4.10 1.97 21.6 4.774 C4 3.30 1.58 27.6 4.835 C5 3.20 1.54 25.9 4.566 C6 3.20 1.54 26.4 4.537 C7 3.00 1.448 C8 1.50 0.72 7.55 3.669 C9 1.60 0.77 7.48 3.74

10 C10 1.90 0.91 7.23 3.5711 C11 1.10 0.53 12.8 3.9512 C12 1.30 0.62 9.93 3.6513 C13 0.90 0.43 9.88 3.84

a Conditions: 2.5 lmol, Al/Ti ¼ 10,000, 30 atm, 100 8C, 5 min, total volume 100 mL.b 107 g/(mol(Ti) h).c Determined by GPC.

Table 4. Ethylene Polymerization at 1 atma

Entry Complex Cocatalyst Polymer (g) Activityb Tm (8C)c

1 C1 MAO 0.42 3.36 133.92 C2 MAO 0.38 3.04 134.13 C3 MAO 0.80 6.40 133.74 C4 MAO 0.21 1.68 133.85 C5 MAO 0.26 2.08 133.86 C6 MAO 0.19 1.52 133.77 C7 MAO 0.36 2.88 133.58 C1 MMAO 0.67 5.36 134.39 C2 MMAO 0.69 5.52 134.1

10 C3 MMAO 0.83 6.64 134.511 C4 MMAO 0.59 4.72 134.312 C5 MMAO 0.60 4.80 134.413 C6 MMAO 0.49 3.92 134.314 C7 MMAO 0.64 5.12 134.2

a Conditions: 5 lmol complex, total volume 50 mL, 15 min, 20 8C, Al/Ti ¼ 1500.b 105 g/(mol(Ti) h).c Determined by DSC.

3406 ZUO ET AL.


ring showed a little lower catalytic activity thanits Cp analog (C2). The reason may be that thesteric hindrance has reduced the monomeraccess to the active sites and slow down thepropagation rate. This steric effect is more evi-dent as shown in complexes C8–C13, whichreveals that polymerization with Cp* complexesshow much lower catalytic activities. Taking allthe earlier results into account, it is clear thatthe substituents on the cyclopentadienyl groupplay an important role in determining the activ-ities of the titled complexes, with bulking sub-stituents reducing activity. This is in agreementwith the improved stability found in Cp* ana-logs, in which the metal center was efficientlyprotected from the attack by the impurities.

Beyond variations of Cp0 groups, the variousparts of dianionic tridentate ligands are organylgroups in 6-(benzimidazol-2-yl)-N-organylpyr-idyl-2-carboximidates. According to the X-rayanalysis, these substituents are located far awayfrom the metal center. As a result, the variationof the catalytic activities may be due to the elec-tronic effect rather than the steric one. FromTable 5, it can be found that the halogen-con-taining complexes usually exhibit a little loweractivity than their alkyl analogs (entries 4, 5 vs.1, 2, and 3; entries 11, 12 vs. 8, 9, and 10),although such differences are not very apparent.

In terms of the molecular weights of resultantpolyethylenes, the complexes containing substi-tuted Cp0 groups produced polyethylenes withmuch lower molecular weights due to steric hin-drance. Similarly, less bulk of substituents onaryl group linked to nitrogen of carboximidatelead to higher molecular weights of polyethy-lenes obtained (entries 1–3 and 8–10). It can

also be found that the halogen-containing com-plexes generally produce polymers with highermolecular weights (entries 4, 5 vs. 1, 2, and 3;entries 11, 12 vs. 8, 9, and 10).

When compared with titanium bis(6-benzimi-dazolylpyridyl-2-carboximidate),22 the influenceof tridentate dianionic ligands on catalyticbehaviors was not so remarkable. Noteworthy,the half-titanocenes exhibit higher catalyticactivities and better stability than the bis(6-ben-zimidazolylpyridyl-2-carboximidate)22 and bis-(imino-indolide) dichlorides.17

Effect of Al/Ti Molar Ratio andReaction Temperature

To study the effect of reaction conditions oncatalytic behavior of C1/MAO system, ethylenepolymerizations were carried out at various co-catalyst concentrations and different polymer-ization temperatures using 10 atm of ethylenepressure. The results are shown in Table 6.

At a fixed Al/Ti ratio of 3000, the C1/MAOsystem was studied by changing reaction tem-perature from 20 to 100 8C (entries 1–5, Table 6).As shown in Table 6, elevating temperaturefrom 20 to 100 8C slightly increases the poly-merization activity. The reason may be that thehigher reaction temperature is helpful to formmore active sites and increase the propagationrate in polymerization process. Noteworthy, thecatalytic activity at elevated temperature suchas 100 8C still remain stable. The thermal sta-bility of these complexes is one of the importantvariables for their future industrial applications.In addition, the resultant polyethylenes showedhigher molecular weights and narrower molecu-

Table 6. Condition Effects on Catalytic Behaviors of C1/MAOa

Entry Al/Ti T (8C) Polymer (g) Activityb Mwc (kg/mol) Mw/Mn

c

1 3,000 20 0.80 1.922 3,000 40 0.90 2.163 3,000 60 1.00 2.40 25.1 3.724 3,000 80 1.30 3.12 28.4 2.855 3,000 100 1.30 3.12 33.3 2.706 750 80 Trace /7 1,500 80 1.10 2.64 30.2 2.608 5,000 80 2.10 5.04 25.5 2.809 10,000 80 4.40 10.6 24.8 3.15

10 15,000 80 4.50 10.8

a Conditions: 2.5 lmol, MAO as cocatalyst, 10 atm, total volume 100 mL, 10 min.b 106 g/(mol(Ti) h).c Determined by GPC.



lar weight distributions along with elevatingtemperatures. These observations are in con-trast to the traditional metallocene catalysts,which usually exhibited lower activity and re-sulted in lower molecular weights at elevatedtemperatures.2

The amount of MAO applied has a strongeffect on the catalytic activity, with higher Al/Tiratio improving the productivity efficiently(entries 4, 6–10, Table 6). When the Al/Ti ratiowas 750, C1/MAO was found to be inactive forethylene polymerization, while when 1500equivalent MAO was applied, the catalytic activ-ity was considerable at 2.64 3 106 g/(mol h).Further increasement improves the catalytic ac-tivity accordingly, and noteworthy when suchratio reached 10,000, a remarkable activityhigher than 107 g/(mol h) was observed. GPCanalysis reveal that the obtained polymers pos-sess Mw value in the range of 24.8–30.2 kg/moland molecular weight distribution ranging from2.60 to 3.15, depending on Al/Ti molar ratios.The use of large amount of MAO gives polyethy-lenes having reduced Mw values and wider mo-lecular weight distributions, which may be dueto the increased rates of chain transfers alongwith increasing MAO concentrations. In the catalyticsystem based on titanium bis(6-benzimidazolyl-pyridyl-2-carboximidate),22 similar correlationcatalytic activities on reaction temperature andratios of cocatalyst were also observed. There-fore, it can be concluded that for catalysts deriv-ing from 6-benzimidazolylpyridyl-2-carboximi-dates, changing the reaction parameters is alsoan efficient method to control both catalytic ac-tivity and polymer properties.

It has already been recognized that the activespecies of olefin polymerization catalysts includeda methyl alkyl and a cis-located vacant coordi-nation site for monomer binding.26 The activespecies can be commonly formed by alkylationand abstraction of halogen or alkyl ligands ofthe transition metal complexes using cocatalystincluding alkylaluminum or alkylaluminoxane.

Regarding to the current half-titanocene cata-lysts, there is only one Ti–Cl bond for alkylationto form the titanium alkyl complex. As a result,it is thought that the introduction of MAO hasremoved one of the ligated atoms to open a sitefor the binding of the approaching monomer.26a–c

As revealed by X-ray analysis, the elongatedTi–O(amine) bond was assumed to be the weak-est linkage among those of titanium center andthe ligated groups, and it is possible that suchweak bond was broken by MAO to open up asite for further interaction with the approachingmonomers. The plausible active species wasformed as described later (Scheme 3).

Therefore, it is not surprising to observe thestrong dependence of catalytic activity on Al/Timolar ratios, as more amounts of MAO may beadvantageous to break the Ti–O(amide) bondand favorable in generating more active species.In addition, the elevated reaction temperaturemay accelerate the activation process and behelpful to increase the catalytic activity.

Polymerization Behavior of DimericTitanium Complexes

Complexes C14 and C15 were found to be inac-tive at 20 and 40 8C at various cocatalyst con-centrations. This indicates that for currentdimeric complexes higher temperature is requiredto form the polymerization active sites. Whenthe reaction temperature was elevated to 80 8C,ethylene uptake was observed although in gen-eral the catalytic activity was much lower thantheir mononuclear analogues (C1 and C5).Interestingly, the formed active species couldlast longer than 5 h without apparent activitydecrease. This is consistent with previous obser-vation that, complexes C14 and C15 are morestable than their monomeric analogs. To getmore information about the catalytic behaviorsof these dimeric titanium complexes, ethylenepolymerization using C14/MAO system at differ-

Scheme 3. Plausible mechanism for forming active species.

3408 ZUO ET AL.


ent conditions were also carried out and theresults are illustrated in Table 7.

Similar to those found in C1/MAO system,the employment of more MAO led to higher cat-alytic activities and lower polymer molecularweights (entries 1–5, Table 7). Slightly differentfrom that of C1/MAO system, the catalyticbehavior of C14/MAO was found to be stronglytemperature dependent, with higher tempera-ture increasing the productivity efficiently(entries 3, 6–9, Table 7). In addition, highermolecular weight and narrow molecular weightdistributions were observed at elevated tempera-tures. When compared with C1/MAO system,the C14/MAO one need more cocatalyst andhigher temperature to initiate the polymeriza-tion process and exhibited lower activity, butproduced polyethylene with higher molecularweights. These differences should be related tothe substitution of a chlorine ligand with an ox-ygen one, which may change the structures ofthe active sites. The earlier observations alsoreveal that for those titanium complexes whichdo not contain any halogen and alkyl ligands,under suitable conditions they can also be acti-vated to be polymerization active species.

CONCLUSIONS

The 6-benzimidazolylpyridyl-2-carboximidic half-titanocene complexes were synthesized and wellcharacterized. The CpTi analogs were easilytransformed to the dimeric derivatives(CpTiL)2O (C14 and C15) as a result of Ti–Clbond hydrolysis. The introduction of substitu-ents on the cyclopentadienyl groups played an

important role in improving the stability of thecomplexes. The Cp*Ti analogs were found to berobust even in contact with air for a long time.The obtained complexes showed remarkablyhigh activities for ethylene polymerization inthe presence of MAO or MMAO as cocatalyst.The catalytic activity and polymer propertiescan be easily controlled either by modification ofligand structure or changing reaction conditions.The substituents on the Cp0 groups had signifi-cant influences on catalytic properties, withbulky substituents leading to decreased activ-ities and lower polymer molecular weights. Forboth mononuclear and dimeric complexes, moreMAO amount led to higher catalytic activitiesand lower molecular weights of polyethylene.Elevating reaction temperature could not onlyincrease catalytic activity but also improve mo-lecular weight of resultant polymers. Theunique properties of these catalytic systemswould be favorable for the further considerationof industrial process.

This work was supported by MOST No. 2006AA03Z553.The authors thank S. A. Amolegbe (the CAS-TWASPostgraduate Fellow from Nigeria) for the Englishcorrections.

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Table 7. Ethylene Polymerization of C14/MAOa

Entry Al/Ti T (8C) Polymer (g) Activityb Mwc (kg/mol) Mw/Mn

c

1 1,000 80 Trace /2 3,000 80 Trace /3 5,000 80 1.30 3.12 32.2 3.614 7,500 80 1.70 4.085 10,000 80 2.10 5.04 27.1 4.306 5,000 20 0 07 5,000 40 0 08 5,000 60 0.20 0.48 25.9 6.529 5,000 100 1.40 3.36 48.2 3.45

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Half-titanocene complexes bearing dianionic 6-benzimidazolylpyridyl-2-carboximidate ligands:...

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