Zirconium dichloride complexes with bulky ω-9-methylfluorenylalkyl substituted indenyl ligands as catalysts for homogeneous ethylene polymerization

Inorganica Chimica Acta 453 (2016) 69–73

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Research paper

Zirconium dichloride complexes with bulky x-9-methylfluorenylalkyl substituted indenyl ligands as catalysts for homogeneous ethylene polymerization Khalil Ahmad, Helmut G. Alt ⇑ Laboratorium für Anorganische Chemie, Universität Bayreuth, Postfach 101251, D-95440 Bayreuth, Germany

a r t i c l e

i n f o

Article history: Received 28 January 2016 Received in revised form 1 July 2016 Accepted 29 July 2016 Available online 30 July 2016 Dedicated to Professor Dr. Max Herberhold on the occasion of his 80th birthday (August 02, 2016).

a b s t r a c t The synthesis and characterization of four new zirconium complexes with bulky x-9-methylfluorenylalkyl substituted indenyl ligands are reported. After activation with methylaluminoxane (MAO) they can be applied as highly active catalysts for ethylene polymerization. The activities of the catalysts depend on the length of the CH2-spacer chain between the indenyl and the 9-methylfluorenyl moieties. The best results were obtained with four CH2 groups. This speaks for a comparatively free catalyst cation allowing easy coordination of the monomer to the active site and unhindered growth of the polymer chain. Ó 2016 Elsevier B.V. All rights reserved.

Keywords: Bis(indenyl)zirconium dichloride complexes x-9-Methylfluorenylalkyl substituents Homogeneous ethylene polymerization Structure-property-relationship studies

1. Introduction Metallocene complexes of group (IV) metals have been extensively studied for olefin polymerization and copolymerization [1–31]. The strong structure-property relationship in combination with the facile modification of the ligand structure has kept this field still interesting and wide open. The nature and size of substituents on metallocene complexes greatly affect the activities of such catalysts as well as the properties of the polyolefin resins produced. When an indenyl ligand of a non bridged bis(indenyl)zirconium dichloride complex is substituted with bulky groups, the free rotation of the indenyl ligand is hindered resulting in a mixture of rac and meso isomers. Several research groups studied such chiral complexes for homogeneous polymerization of ethylene and propylene [12–14] and produced elastomeric polypropylenes with specific tacticities [15–27]. Waymouth et al. investigated this system for ethylene-propylene copolymerization [28]. Coville et al. reported the effects of H, Me, Et, SiMe3, Ph, benzyl and 1-naphthyl substituents on the ethylene polymerization behavior of bis(indenyl)zirconium dichloride/MAO catalysts [29]. Recently we reported the synthesis of mono substituted bis(indenyl) complexes of ⇑ Corresponding author. E-mail address: [email protected] (H.G. Alt). http://dx.doi.org/10.1016/j.ica.2016.07.053 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.

zirconium and their potential for catalytic ethylene polymerization [30]. Herein bis(indenyl)zirconium dichloride complexes with one bulky x-9-methylfluorenylalkyl substituent on each indenyl ligand are reported for homogeneous ethylene polymerization after activation with methylaluminoxane (MAO). The length of the CH2-spacer in the indenyl ligand had a very strong influence on the performance of these catalysts.

2. Results and discussion 2.1. Synthesis of x-9-methylfluorenylalkyl substituted indenyl compounds 1–4 The synthesis of x-9-methylfluorenylalkyl substituted indenyl compounds was achieved by the reaction of x-bromo-1-indenylalkanes [30] with the lithium salt of 9-methylfluorene (Scheme 1). The synthesized compounds were characterized by GC/MS and NMR spectroscopy (see Section 3).

2.2. Synthesis of the complexes Substituted indene compounds were lithiated with n-butyllithium. The reaction of the lithiated compounds with zirconium

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K. Ahmad, H.G. Alt / Inorganica Chimica Acta 453 (2016) 69–73

Br n

n

+ -LiBr

Li

1 (n = 2), 2 (n = 3), 3 (n = 4), 4 (n = 5) Scheme 1. Synthesis of compounds 1–4.

1a (n = 2)

2a (n = 3)

3a (n = 4)

4a (n = 5)

Scheme 2. Synthesis of complexes 1a–4a.

tetrachloride in diethyl ether yielded the bis(indenyl) complexes 1a–4a (Scheme 2). Complexes 1a–4a were characterized by 1H and 13C NMR spectroscopy and elemental analysis (see Section 3). All complexes show two sets of signals because of the existence of rac and meso isomers. The 1H NMR spectrum of complex 1a (Fig. 1) shows signals at d = 7.66–7.59 (m, 8H) and d = 7.32–6.92 (m, 40H) ppm which are assigned to the CH protons of the indenyl and fluorenyl groups (H16, H15, H14, H13, H7, H6, H5 and H4). The signals of the indenyl protons H2 and H3 arise at d = 5.74 (d, 3JH,H = 3.2 Hz, 2H), d = 5.55 (d, 3JH,H = 3.2 Hz, 2H), d = 5.53 (d, 3JH,H = 3.2 Hz, 2H) and

d = 5.21 (d, 3JH,H = 3.2 Hz, 2H) ppm. The multiplet at d = 2.16–2.02 (16H) ppm is assigned to the methylene groups H8 and H9 while the signals of the fluorenyl methyl groups (H15) appear at d = 1.38 (s, 6H) and d = 1.34 (s, 6H) ppm.

2.3. Ethylene polymerization The metallocene complexes 1a–4a were activated with MAO (M:Al = 1:2000) and transferred into a 1 L Büchi reactor filled with 250 mL n-pentane. A slurry polymerization of ethylene was performed at 65 °C under 10 bar ethylene pressure. The ethylene poly-

Fig. 1. 1H NMR spectrum of complex 1a.

K. Ahmad, H.G. Alt / Inorganica Chimica Acta 453 (2016) 69–73 Table 1 Polymerization and polymer analysis data. Complex No. of CH2 Activitya (kg DHm [J/mol] Tm [°C] Crystallinity (a) PE/mol cat h) spacer groups (n) 1a 2a 3a 4a

2 3 4 5

9600 12,300 15,800 15,700

122.0 141.3 142.1 143.4

131.8 134.3 129.3 133.3

0.42 0.49 0.49 0.49

a Polymerization conditions: M:Al = 1:2000, 250 mL n-pentane, 65 °C, 10 bar ethylene, 1 h.

merization results and DSC data of the selected polymer samples are given in Table 1. Complexes 1a–4a differ in the number of CH2-spacer groups (n) between the indenyl moiety and the 9-methyl fluorenyl substituent. Complex 1a/MAO with n = 2 shows the lowest activity (9600 kg PE/mol cat h). The activity increases by increasing the length of the spacer chain and reaches a maximum of 15,800 (kg PE/mol cat h) for n = 4. This is a higher activity than of the unsubstituted (ind)2ZrCl2 complex (3200 kg PE/mol cat h). Further increase of the length of the spacer chain results in a slight decrease in activity. Obviously, it is not only the steric bulk of the fluorenyl substituent but also the intramolecular mobility of the methylfluorenylalkyl substituent that have a strong influence on the coordination of the monomer to the active site of the catalyst and the growth of the resulting polymer chain. Also the interaction of the bulky catalyst cation and the bulky cocatalyst anion (MAO ) after the activation step could favor the good performance providing advantageous access to the active site of the catalyst. DSC results show that these complexes produce polyethylenes with high melting points and crystallinities. The effect of CH2 spacer lengths has been studied on bis(cyclopentadienyl) zirconium dichloride complexes of the type ([C13H9-(CH2)n-C5H4]2ZrCl2) [31] bearing x-fluorenylalkyl substituents and x-phenoxyalkyl substituted zirconocene dichloride complexes [32]. In the case of x-fluorenylalkyl substituted complexes, the acidic hydrogen at the fluorenyl substituent can be deprotonated by MAO and the resulting anion can compete with the active center causing a decrease of catalyst activity. An increase of the CH2 spacer length slows down such an interaction and as a consequence, the catalyst activity goes up. For x-phenoxyalkyl substituted complexes, ethylene polymerization activity increases with the length of the CH2 spacers. After reaching a maximum with four CH2 spacers, it decreases substantially with further increase of the spacer length. The reason for this behavior is the intramolecular Lewis acid/Lewis base interaction of Zr and O at smaller spacer lengths and the intermolecular coordination at a spacer length greater than four. At a spacer length of four, both intra and intermolecular coordinations have a minimum. In our case with 9-methyl fluorenylalkyl substituted complexes, the activity increases by increasing the spacer length to four CH2 groups and then it almost remains constant. Since a 9-methyl fluorenyl substituent does not bear any acidic hydrogen, the only factor which can affect the catalyst activity is the steric bulk of the 9-methyl fluorenyl group which hinders the coordination of ethylene and the growth of the polymer chain at the metal.

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tillation over Na/K alloy. Diethyl ether was additionally distilled over lithium aluminum hydride, while toluene was additionally distilled over phosphorus pentoxide. Deuterated solvents (CDCl3 and C6D6) were purchased from Eurisotop and stored over molecular sieves (3 Å). Argon (5.0) and ethylene (3.5) were purchased from Riebner Company. Methylaluminoxane (10% in toluene) was purchased from Chemtura Europe Limited. All other starting materials were commercially available and used without further purification. 3.2. NMR spectroscopy NMR spectra were recorded with a Varian Inova (400 MHz) spectrometer. All spectra were recorded at 298 K. In the 1H NMR spectra the chemical shift of the residual proton signal of the solvent was used as a reference (d = 7.24 ppm for chloroform-d1 and d = 7.16 ppm for benzene-d6), while in the 13C NMR spectra the chemical shift of the solvent was used as a reference (d = 77.0 ppm for chloroform-d1 and d = 128.0 ppm for benzene-d6). 3.3. GC/MS GC/MS spectra were recorded with a FOCUS Thermo gas chromatograph combined with a DSQ mass detector. A 30 m HP-5 fused silica column (internal diameter 0.32 mm, film 0.25 lm and flow 1 ml/min) was used and helium (4.6) was applied as carrier gas. The measurements were recorded using the following temperature program: Starting temperature: 50 °C, duration: 2 min; Heating rate: 20 °C/min, duration: 12 min; Final temperature: 290 °C, duration: 27 min. 3.4. Elemental analysis Elemental analyses were performed with a Vario EL III CHN instrument. The values of C, H and N were calibrated using acetamide as a standard. 3.5. DSC analysis A Mettler Toledo DSC/SDTA 821e instrument was used to perform DSC analyses. The polymer samples were prepared by enclosing 4–6 mg of the polymers in standard aluminum pans. The samples were introduced into the auto sampler of the instrument and the measurements were recorded using the following temperature program: First heating phase: from 50 °C to 160 °C (10 °C/min); Cooling phase: 160 °C to 50 °C (10 °C/min); Second heating phase: from 50° C to 160 °C (10 °C/min). Nitrogen was used as a cooling medium. Melting enthalpies and melting points were taken from the second heating phase. The values were calibrated using indium as a standard (m.p. 429.78 K, Hm = 28.45 J/g). 3.6. General description of ethylene polymerization

3. Experimental section 3.1. General aspects All reactions were carried out under inert atmosphere using Schlenk technique. Argon was used as an inert gas. Methylene chloride was distilled over phosphorus pentoxide. Toluene, tetrahydrofuran, diethyl ether and n-pentane were purified by dis-

For the homogeneous polymerization of ethylene, the corresponding complex (2–3 mg) was dissolved in toluene (10 mL) and activated with methylaluminoxane (MAO). The activated complex was suspended in pentane (250 mL) and transferred into a 1 L Büchi autoclave under inert atmosphere. The temperature of the thermostat was adjusted to 65 °C and an ethylene pressure of 10 bar was applied for one hour. After releasing the pressure, the

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obtained polymer was filtered over a frit, washed with dilute hydrochloric acid, water and finally with acetone and dried under vacuum. 3.7. General synthesis procedure for x-9-methylfluorenylalkylsubstituted indenyl compounds 1–4 For the synthesis of the x-9-methylfluorenylalkyl substituted indene compounds 1–4, an amount of 5 mmol of n-butyllithium (1.6 M in hexanes) was added to 5 mmol of 9-methylfluorene dissolved in 50 mL of diethyl ether at 78 °C. The mixture was stirred for three hours at room temperature, cooled to 78 °C and an amount of 5 mmol of the desired x-bromo-1-indenylalkane was added. The mixture was stirred for 6 h at room temperature, distilled water was added and the organic layer was separated and dried over sodium sulfate. The solvent was removed to obtain the desired compound as an oil. 1: 1H NMR: (400 MHz, CDCl3, 298 K): d = 7.74–7.70 (m, 2H), 7.39–7.29 (m, 6H), 7.26-7.22 (m, 2H), 7.13–7.10 (m, 2H), 6.76 (br, 1H), 6.36 (br, 1H), 3.21 (br, 1H, Ind-H1), 2.04–1.95 (m, 2H, CH2), 1.40 (s, 3H, CH3), 1.89–1.77 (m, 2H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 151.4, 147.2, 144.4, 140.2 (Cq), 139.0, 131.0, 127.2, 127.0, 126.3, 124.5, 122.7, 120.8, 119.9, 119.8 (CH), 50.5 (Cq), 50.0 (CH-Ind), 36.4 (CH2), 26.8 (CH3), 25.3 (CH2). MS: 322 (M+, 10), 179 (1 0 0), 128 (45). 2: 1H NMR: (400 MHz, CDCl3, 298 K): d = 7.72–7.69 (m, 2H), 7.38–7.25 (m, 7H), 7.20–7.16 (m, 1H), 7.14–7.09 (m, 1H), 7.07– 7.04 (m, 1H), 5.92 (br, 1H, Ind-H2), 3.18 (br, 2H, Ind-H1), 2.31– 2.24 (m, 2H, CH2), 2.12–2.06 (m, 2H, CH2), 1.46 (s, 3H, CH3), 1.11–1.03 (m, 2H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 151.8, 145.4, 144.3, 144.0, 140.1 (Cq), 127.5, 127.1, 126.9, 125.8, 124.3, 123.5, 122.7, 119.9, 118.8 (CH), 50.7 (Cq), 40.4 (CH2), 37.5 (CH2-Ind), 27.9 (CH2), 26.8 (CH3), 22.6 (CH2). MS: 336 (M+, 6), 206 (42), 179 (100). 3: 1H NMR: (400 MHz, CDCl3, 298 K): d = 7.75–7.69 (m, 2H), 7.42–7.16 (m, 10H), 6.01 (br, 1H, Ind-H2), 3.23 (br, 2H, Ind-H1), 2.10–1.81 (m, 4H, CH2), 1.49 (s, 3H, CH3), 1.33–1.23 (m, 4H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 152.0, 145.4, 144.4, 140.1 (Cq), 145.0, 127.4, 127.1, 126.8, 126.3, 124.3, 122.7, 120.9, 119.8, 118.8 (CH), 50.7 (Cq), 40.5 (CH2), 37.5 (CH2-Ind), 35.6, 28.2 (CH2), 26.7 (CH3), 26.0 (CH2). MS: 350 (M+, 4), 220 (4), 179 (100), 129 (17). 4: 1H NMR: (400 MHz, C6D6, 298 K): d = 7.65–7.57 (m, 2H), 7.26–7.16 (m, 10H), 6.58 (br, 1H), 6.28 (br, 1H), 3.15 (br, 1H, IndH1), 2.76–2.70 (m, 2H, CH2), 1.90–1.84 (m, 2H, CH2), 1.82–1.75 (m, 2H, CH2), 1.56–1.47 (m, 2H, CH2), 1.39s (3H, CH3), 1.19–1.12 (m, 2H, CH2). 13 C NMR: (100 MHz, C6D6, 298 K): d = 152.3, 148.2, 144.6, 140.6 (Cq), 139.4, 131.0, 127.5, 127.3, 126.7, 125.0, 123.1, 122.0, 121.3, 120.0 (CH), 50.9 (Cq), 50.7 (CH-Ind), 40.9, 32.2, 31.6, 27.4 (CH2), 26.9 (CH3), 24.5 (CH2). 3.8. General procedure for the synthesis of the zirconocene dichloride complexes 1a–4a For the synthesis of the substituted bis(indenyl)zirconium dichloride complexes, n-butyllithium (2 mmol, 1.6 M in hexanes) was added to 2 mmol of the appropriate 9-methylfluorenyl substituted indene compound dissolved in 50 mL of diethyl ether at 78 °C. The mixture was stirred for three hours at room temperature. Then this mixture was transferred to a zirconium tetrachloride suspension (1 mmol) in 50 mL of diethyl ether at 78 °C and was stirred for 24 h at room temperature. Diethyl ether was removed and toluene (100 mL) was added. The mixture was fil-

tered to remove the insoluble lithium chloride and the volume of the filtrate was reduced. The complex was precipitated by adding pentane. The precipitate was filtered, washed with pentane and dried under vacuum to obtain the desired complex as yellow powder in 50% yield. These complexes exist as a mixture of rac and meso isomers. In the 13C NMR data only one set of signals is listed. 1a: 1H NMR: (400 MHz, CDCl3, 298 K): rac/meso d = 7.66–7.59 (m, 8H), 7.32–6.92 (m, 40H), 5.74 (d, J = 3.2 Hz, 2H), 5.55 (d, J = 3.2 Hz, 2H), 5.53 (d, J = 3.2 Hz, 2H), 5.21 (d, J = 3.2 Hz, 2H), 2.16–2.02 (m, 16H, CH2), 1.38 (s, 6H, CH3), 1.34 (s, 6H, CH3). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 151.2, 140.3, 126.2, 125.7, 123.2 (Cq), 127.3, 127.1, 126.0, 125.3, 125.0, 123.1, 122.7, 120.9, 120.0, 98.3 (CH), 50.6 (Cq), 39.7 (CH2), 23.0 (CH3), 22.7 (CH2). Elemental analysis: Found: C 73.59, H 6.41 Calc.: C 74.79, H 5.79%. 2a: 1H NMR: (400 MHz, CDCl3, 298 K): rac/meso d = 7.74–7.62 (m, 8H), 7.39–7.04 (m, 40H), 5.68 (d, J = 3.2 Hz, 2H), 5.62 (d, J = 3.2 Hz, 2H), 5.56 (d, J = 3.2 Hz, 2H), 5.30 (d, J = 3.2 Hz, 2H), 2.59–2.26 (m, 8H, CH2), 2.11–1.93 (m, 8H, CH2), 1.43 (s, 6H, CH3), 1.40 (s, 6H, CH3), 1.36–1.17 (m, 8H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 151.9, 140.2, 126.7, 125.7, 123.2 (Cq), 127.2, 127.0, 126.1, 125.4, 125.2, 123.6, 122.7, 120.9, 119.9, 99.4 (CH), 50.7(Cq), 40.2, 28.0 (CH2), 27.0 (CH3), 24.8 (CH2). 3a: 1H NMR: (400 MHz, CDCl3, 298 K): rac/meso d = 7.64–7.59 (m, 8H), 7.31–7.05 (m, 40H), 5.95 (d, J = 3.2 Hz, 2H), 5.72 (d, J = 3.2 Hz, 2H), 5.55 (d, J = 3.2 Hz, 2H), 5.37 (d, J = 3.2 Hz, 2H), 2.63–2.45 (m, 8H, CH2), 2.41–2.15 (m, 8H, CH2), 1.92–1.83 (m, 8H, CH2), 1.35 (s, 6H, CH3), 1.37 (s, 6H, CH3), 1.25–1.10 (m, 8H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 151.9, 140.1, 125.9, 125.8, 123.2 (Cq), 127.1, 126.9, 126.0, 125.6, 125.2, 123.7, 122.7, 121.8, 119.8, 99.1 (CH), 50.6 (Cq), 40.3, 30.2, 27.4 (CH2), 26.6 (CH3), 24.3 (CH2). Elemental analysis: Found: C 74.59, H 6.61. Calc.: C 75.47, H 6.33%. 4a: 1H NMR: (400 MHz, CDCl3, 298 K): rac/meso d = 7.74–7.65 (m, 8H), 7.39–7.13 (m, 40H), 6.10 (d, J = 3.2 Hz, 2H), 5.90 (d, J = 3.2 Hz, 2H), 5.73 (d, J = 3.2 Hz, 2H), 5.53 (d, J = 3.2 Hz, 2H), 2.82–2.62 (m, 8H, CH2), 2.56–2.34 (m, 8H, CH2), 2.00–1.85 (m, 8H, CH2), 1.44 (s, CH3), 1.38–1.16 (m, 8H, CH2), 1.12–0.99 (m, 8H, CH2). 13 C NMR: (100 MHz, CDCl3, 298 K): d = 152.0, 140.1, 126.1, 125.8 (Cq), 127.2, 126.9, 126.0, 125.6, 125.3, 123.8, 122.7, 121.7, 119.8, 99.1 (CH), 50.7 (Cq), 40.5, 29.7, 29.6, 27.6 (CH2), 26.7 (CH3), 24.0 (CH2). 4. Conclusions Bis(indenyl) zirconium dichloride complexes with alkyl substituents at the indenyl ligand having a bulky 9-fluorenyl end group can be activated with MAO and be applied for catalytic ethylene polymerization. For structure-property-relationship studies only the length of the CH2-spacer group was changed; all other parameters were constant. The activity of the catalysts depends very much on the length of the corresponding alkyl substituent. Four CH2-spacer groups between the fluorenyl moiety and the indenyl ligand gave the best performance (15,800 kg PE/mol cat h). These results prove the strong influence of steric conditions in the catalyst cation on the performance of these catalysts. Acknowledgements We are grateful to Higher Education Commission of Pakistan (HEC Pakistan) and Deutscher Akademischer Austauschdienst (DAAD) for financial support.

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