Synthesis and characterisation of permethylindenyl zirconium complexes and their use in ethylene polymerisation

Abstract

We report the synthesis of two zirconocenes, dimethylsilylbis(hexamethylindenyl) zirconium dichloride, rac-(SBI*)ZrCl_2, and ⁿbutyldimethylsilyl(hexamethylindenyl) zirconium trichloride, [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂. The complexes were characterised by NMR spectroscopy and X-ray crystallography, and the bonding was evaluated using density functional theory. rac-(SBI*)ZrCl₂ demonstrated a very high activity for solution phase polymerisation of ethylene (ca. 22 500 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹). Both rac-(SBI*)ZrCl₂ and rac-(EBI*)ZrCl₂ (EBI* = ethylenebis(hexamethylindenyl)) have been supported on MAO modified silica and AMOST layered double hydroxides (AMO-LDHs), and evaluated as catalysts in the slurry-phase polymerisation of ethylene. The highest catalytic polymerisation activities for rac-(SBI*)ZrCl₂ and rac-(EBI*)ZrCl₂ on the layered double hydroxides were 9657 and 4325 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively, for MAO modified Mg₂Al–SO₄ LDH. However, rac-(EBI*)ZrCl₂ was a three times more active catalyst than rac-(SBI*)ZrCl₂ when supported on silica.

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Introduction

Well-defined group 4 metallocenes have offered an important alternative technology to the ethylene polymerisation field dominated by Ziegler–Natta and Philips catalysts.^1,2 In contrast to the heterogeneous Ziegler–Natta systems, metallocene catalysts produce polyethylene with narrow molecular weight distributions and polydispersity indices close to two. Furthermore, the single-site nature of their active sites enables the polymer properties to be fine-tuned.³

Studies of bridged metallocene compounds have shown that slight variations of the ring substituents and bridging groups can massively influence the activity of ethylene polymerisation.^4,5 The electronic structure and spatial organisation of the active site of the catalyst, influenced by ligand structure, have profound effects on the microstructure of the polyethylene obtained and hence, the overall polymer properties.^6,7

Support materials provide a convenient way to serve as a template for the growing polymer particle.⁸ Many materials have been tested as polymerisation supports including inorganic solids such as clays,⁹ ZrO₂,¹⁰ SiO₂,¹¹ and MgCl₂.² Silica (SiO₂) is the most commonly used support for immobilising metallocene complexes.¹¹ Modified silicas, such as mesoporous SBA-15,^12,13 MCM-41,^14,15 and nanostructured silica materials,^16,17 have also been investigated. Layered double hydroxides (LDHs) are a class of hydrotalcite-like clays containing positively charged layers with intercalated anions between them.^18–24 It is most typical for the metal ions to be divalent and trivalent. The common divalent metal ions include Mg²⁺ and Ca²⁺, while the trivalent metal ion is typically Al³⁺. The intercalated anions show more variety, and while they can include large anionic organic salts, they are more typically inorganics such as CO₃²⁻, SO₄²⁻ or Cl⁻. Recently, we developed a new generation of LDHs using an aqueous miscible solvent treatment (AMOST) method.^19–21 The AMO-LDHs made in this way exhibit surface areas two orders of magnitude in excess of those previously reported (ca. 400 m² g⁻¹). We have demonstrated via an EXAFS study that they are well-ordered supports for slurry phase polymerisation of ethylene.^22,23

We report here the synthesis and characterisation of two new permethylated indenyl zirconium complexes, their reaction with both an MAO-treated layered double hydroxide and silica to form supported catalyst systems, and their use in the slurry and solution polymerisation of ethylene.

Results and discussion

Synthesis of pro-ligands

The synthesis of the dimethylsilylbis(hexamethylindene), (SBI*)H₂, ligand was previously reported by O'Hare and co-workers but the compound was not crystallographically characterised.²⁴

Single crystals of rac-(SBI*)H₂ suitable for an X-ray diffraction study were obtained from a saturated hexane solution at room temperature. The molecular structure of (SBI*)H₂ is shown in Fig. 1. Selected bond lengths and angles are given in Table S1.† The stereochemistry of the two chiral centres and the space group (P1̄) show that the crystal corresponds to a racemic mixture, rac-(SBI*)H₂.

Fig. 1 Molecular structure of (S,S)-rac-(SBI*)H₂. Ellipsoids are drawn at 50% probability level. Hydrogen atoms, except for at the chiral centres, are omitted for clarity.

Reaction of one equivalent of (SBI*)H₂ with two equivalents of ⁿbutyllithium in thf for 16 h afforded lithium dimethylsilylbis(hexamethylindenide), (SBI*)Li₂, as yellow powder in quantitative yield.²⁴

However, when the reaction was carried out in a pseudo one-pot starting from the hexamethylindene and with a shortened reaction time of 2 h (from 16 h), lithium ⁿbutyldimethylsilyl(hexamethylindenide), LiInd*SiMe₂ⁿBu, was afforded as a colourless solid in 83% yield (Scheme 1).

Scheme 1 Synthesis of lithium ⁿbutyldimethylsilyl(hexamethylindenide) (LiInd*SiMe₂ⁿBu).

The ¹H NMR spectrum of LiInd*SiMe₂ⁿBu in pyridine-d₅ (Fig. S5†) shows six singlets corresponding to the ring methyl groups at approximately 2.4–3.1 ppm and a singlet at 0.73 ppm, with intensity six, is attributed to the silicon methyl groups. A triplet at 0.92 ppm corresponds to the butyl terminal CH₃, while the three multiplets at 1.27, 1.47 and 1.72 ppm represent the methylene groups α, γ and β to the silicon respectively.

Synthesis of complexes

Stoichiometric reaction of (SBI*)Li₂ and ZrCl₄ in benzene at room temperature led to the formation of rac-(SBI*)ZrCl₂ as an orange crystalline material in 34% yield (Scheme 2). No evidence of meso-(SBI*)ZrCl₂ was seen in any reaction aliquots analysed by ¹H NMR spectroscopy – a marked change from the previous syntheses of (EBI*)ZrCl₂ and (SBI*)Fe where 50 : 50 rac- : meso-isomeric mixtures were obtained.^7,24 Reactions in toluene or thf led to yields below 10%.

Scheme 2 Synthetic pathway to rac-(SBI*)ZrCl₂.

Single crystals suitable for an X-ray crystallographic study were grown from a benzene solution at room temperature. The molecular structure is represented in Fig. 2. Selected bond lengths and angles are given in Table S3.†

Fig. 2 Molecular structure of rac-(SBI*)ZrCl₂. Hydrogen atoms are omitted for clarity. Thermal ellipsoids drawn at 50%.

rac-(SBI*)ZrCl₂ crystallises in the monoclinic space group P2₁/n, with one molecule in the asymmetric unit and four molecules in the unit cell – two of each enantiomer. The first point of note is that the value of torsion angle, TA′‡ (145.15°) is larger than 90° showing that the two indenyl rings are pointed in opposite directions as is expected for a rac-isomer. This value of TA′‡ agrees well with that reported for rac-(SBI)ZrCl₂, 145.20°,²⁵ and implies that the constraints of the dimethyl silicon bridge outweigh any steric effects from the permethylation. The tilt angle, α,‡ (58.50°), is slightly lower than that reported for either (SBI)ZrCl₂ (61.80°) or (^2,4,6-Me₃SBI)ZrCl₂ (59.18°).²⁶ The value of α is also in good agreement with that of rac-(EBI*)ZrCl₂ (57.15°).⁷ The β‡ value for rac-(SBI*)ZrCl₂ (18.89°) slightly exceeds those reported for (^2,4,6-Me₃SBI)ZrCl₂ (17.05°) or (SBI)ZrCl₂ (16.45°).²⁷ The average Zr–Cp_cent distance (2.251 Å) fits well with other values reported in the literature for ansa-bridged species including (EBI*)ZrCl₂ (2.240 Å), (SBI)ZrCl₂ (2.241 Å) and (^2,4,6-Me₃SBI)ZrCl₂ (2.231 Å), and also with unbridged examples such as (Ind₂*)ZrCl₂ (2.257 Å) and (Ind₂)ZrCl₂ (2.231 Å).^8,27

Geometry optimisation calculations were performed at the B3LYP^28,30 and BP86 ^31,32 level of DFT with those using the BP86 functional most successfully reproducing the established experimental metrical parameters of rac-(SBI*)ZrCl₂; slightly increased rotation and larger tilt angles were observed. (Table 1).

Table 1 Comparison of selected experimental and calculated bond lengths (Å), angles (°) and geometric parameters (°) of rac-(SBI*)ZrCl₂

	Experimental	Calculated B3LYP	Calculated BP86
Avg. Zr–Cpcent	2.251	2.308	2.288
Avg. Zr–Cl	2.423	2.442	2.428
C(1)–Si(1)–C(16)	96.22(7)	96.31	95.85
α	58.50	61.28	59.95
TA′	145.15	148.44	146.30

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Illustrations of the DFT-computed (BP86) HOMO and LUMO are given in Fig. 3. The HOMO primarily consists of ligand p-orbitals; localised double bond character within the benzene rings is implied. In contrast, the LUMO is primarily based on the d⁰ metal centre (29.0% Zr d_xy orbital component).

Fig. 3 Illustration of the DFT-computed (BP86) LUMO (left) and HOMO (right) of rac-(SBI*)ZrCl₂.

The ¹H NMR spectrum of rac-(SBI*)ZrCl₂ in chloroform-d₁ shows six singlets between 1.25 and 2.50 ppm. There is one resonance for the two silyl methyl groups which are in the same environment in solution and at low frequency (1.34 ppm).

Stoichiometric reaction of LiInd*SiMe₂ⁿBu with ZrCl₄ in toluene for two hours led to [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ as a red crystalline solid in 73% yield (Scheme 3).

Scheme 3 Synthesis of ⁿbutyldimethylsilyl(hexamethylindenyl) zirconium trichloride [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂.

The crystalline material was found to be suitable for analysis by X-ray diffraction, the molecular structure of [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ is depicted in Fig. 4.

Fig. 4 Molecular structure of [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂. Hydrogens omitted for clarity. Thermal ellipsoids drawn at 50%.

[(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ crystallises in the triclinic space group P1̄, with half a dimer in the asymmetric unit (Fig. 4). The Zr(1)–Cp_cent distance (2.190 Å) is comparable to that in polymeric CpZrCl₃ which has Zr–Cp_cent distances of 2.196 Å.³³ The bond lengths for the terminal chlorides (2.4037(4) and 2.3982(4) Å) are significantly shorter than those of the bridging chlorides (2.5837(4) Å). These agree well with the Zr–Cl distances reported for [CpZrCl₃]_n of 2.419(3) Å for the terminal chloride and 2.518(3) and 2.728(3) Å for the bridging ligands.

DFT calculations were performed on [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ and highlight the localised double bonds in the benzene rings in the HOMO. The LUMO is again primarily Zr-based with a 27.3% d_xy orbital component on each metal centre. (Fig. 5 and Table 2).

Fig. 5 Illustration of the DFT-computed (BP86) LUMO (top) and HOMO (bottom) of [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂.

Table 2 Comparison of selected experimental and calculated bond lengths (Å), angles (°) and geometric parameters (°) of [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂

	Experimental	Calculated B3LYP	Calculated BP86
Zr–Cpcent	2.190	2.234	2.215
Avg. Zr–Clterminal	2.4011	2.420	2.418
Avg. Zr–Clbridging	2.5835	2.656	2.643
Cl(1)–Zr(1)–Cl(2)	91.233(17)	90.74	91.59

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[(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ was analysed by ¹H NMR spectroscopy in benzene-d₆, Fig. S7,† six singlets representing the indenyl methyl groups appear in the range 1.96–2.57 ppm and there are two singlets at 0.55 and 0.65 ppm corresponding to the silyl methyl groups. Multiplets relating to the butyl substituent appear at 0.86, 1.00, 1.26 and 1.29 ppm for the δ, α, γ and β positions respectively.

Synthesis of solid ethylene polymerisation pre-catalysts

We have investigated the use of two different catalyst supports, silica and AMO-LDH, due to their proven excellent activities in the literature.^22,23,34,35 Both support materials were calcined before use: SiO₂ at 600 °C for two hours under a flow of N₂ and AMO-LDH at 150 °C for six hours under high vacuum (2 × 10⁻² mbar). The surface of both supports was then treated with 0.5 equivalents of MAO; producing MAO modified silica (SSMAO) and MAO modified AMO-LDH (LDHMAO) with a maximum loading of 15 and 13.5 wt% aluminium (from methylaluminoxane) in the resulting SSMAO and LDHMAO respectively. Immobilisation of the complex was achieved by addition of toluene to a mixture of the complex and MAO-activated support, followed by heating at 60 °C for one hour. After work-up, all the solids were afforded as coloured powders in good yield (>80%).

Scanning electron microscopy (SEM) images for the silica immobilisation illustrates an uniform shape and size distribution, Fig. S10.† The particles are not spherical but granular with an average size of approximately 10 μm. They do not change in either shape or dimension on reaction with MAO, nor on the immobilisation of rac-(SBI*)ZrCl₂.

SEM-EDX analysis demonstrated the desired elements on the surface (C, 14.3; Al, 2.6; O, 62.9; Si, 19.4 (%)) (Fig. S10†).

¹³C CPMAS NMR spectroscopy was used to characterise LDHMAO–(SBI*)ZrCl₂ (Fig. S8†). The dominant resonance at −9 ppm corresponds to the highly shielded methyl environment of MAO. Resonances between 22 and 34 ppm correspond to the methyl groups around the rings of rac-(SBI*)ZrCl₂ while a slightly weaker resonance is also observed at 13 ppm which corresponds to the low frequency silicon methyl groups. The ²⁹Si CPMAS NMR spectrum contains a resonance at −105 ppm corresponding to a single silicon environment found for the SiO₂ within SSMAO; the resonance for the silyl methyl group in the ligand was too weak to be identified (Fig. S9†).

Solution phase polymerisation of ethylene

The solution phase polymerisations were all carried out under ethylene at a pressure of two bar, with 50 mL hexanes, 0.2 mg of rac-(SBI*)ZrCl₂ and with an Al : Zr ratio (MAO : rac-(SBI*)ZrCl₂) of 2000 : 1. All polymerisation runs were carried out for 30 minutes, or until the stirring ceased entirely. Complete polymerisation data is reported in Tables S7 and S8.† rac-(SBI*)ZrCl₂ was used to catalyse the polymerisation of ethylene at a range of temperatures from 40–90 °C using methylaluminoxane (MAO) as the co-catalyst and scavenger (Fig. 6).

Fig. 6 Solution phase ethylene polymerisation activity dependence of rac-(SBI*)ZrCl₂ on temperature. MAO (2000 : 1); 2 bar ethylene; 0.2 mg complex loading; 50 mL hexanes; timed until cessation of stirring.

The activity achieved at 60 °C (22 622 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹) compares well with some of the highest reported values in the literature; rac-(EBI*)ZrCl₂ (61 800 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹, 70 °C);⁷ rac-(SBI)ZrCl₂, rac-(^2,4,6-Me₃SBI)ZrCl₂ and Cp₂ZrCl₂ (18 450, 44 760 and 30 450 kg_PE ⁻¹mol_Zr ⁻¹h ⁻¹ bar respectively, 30 °C).^36,37 There is a sharp increase in activity seen between 50 and 60 °C (1123 and 22 622 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively). Similar features were reported by Eskelinen et al. who attributed a sharp rise in activity between 70 and 80 °C to an increase in the solubility of the PE chain (in n-heptane), allowing for better monomer diffusion to the catalytic centre.³⁸ A marked decline in activity is also noted between 70 and 80 °C (18 703 and 8130 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹), suggesting decomposition at elevated temperatures.³⁹ These values are much higher than the recently reported bis(pentamethyl)zirconium dichloride complexes (activity around 400 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹).⁴⁰ The weight average molecular weights, M_w, were high (213 927 and 261 337 kg mol⁻¹ at 60 and 70 °C respectively).

A study was undertaken at 70 °C with rac-(SBI*)ZrCl₂ to test the effect that the Al : Zr ratio has on activity (Fig. 7).

Fig. 7 Dependence of solution phase ethylene polymerisation activity and molecular weight, M_w, for rac-(SBI*)ZrCl₂ on the Al : Zr co-catalyst (MAO) ratio. PDIs are given in parentheses. 70 °C; 2 bar ethylene; 0.2 mg catalyst loading; 50 mL hexanes; timed until cessation of stirring.

The activity at 2000 : 1 is more than four times greater than that at 200 : 1 (18 703 and 3996 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively). At an Al : Zr ratio of 200 : 1, M_w was 204 374 kg mol⁻¹ rising quickly to 271 713 kg mol⁻¹ at a ratio of 500 and plateauing on further increases. This contrasts with reports in the literature that suggest that M_w will decrease on addition of more MAO,⁴¹ or at the very least remain constant.⁴² Attempts were also made, not only to vary the Al : Zr ratio, but to vary the source of aluminium. Polymerisation attempts with 2000 equivalents of AlⁱBu₃ (TIBA) and AlMe₃ (TMA) failed to produce any polyethylene. A comparison of the solution phase polymerisation activity of rac-(SBI*)ZrCl₂ with other zirconocene catalysts is presented in Table 3.

Table 3 Comparison of selected solution phase polymerisation data

	Activity (kgPE^−1 molZr^−1 h^−1 bar^−1)	Mw (g mol^−1)	Mw/Mn	Ref.
a 60 °C, 2 bar ethylene, 50 mL hexanes.b 70 °C, 10 bar ethylene, 1.8 L isobutene.c 30 °C, 2 bar ethylene, 5 mL toluene.d 30 °C, 2 bar ethylene, 5 mL toluene.
rac-(SBI*)ZrCl2	22 622	213927	2.32	This work^a
rac-(EBI*)ZrCl2	61 800	215000	2.40	7^b
rac-(SBI)ZrCl2	18 450	260000	2.30	37^c
rac-(^2,4,6-Me3SBI)ZrCl2	44 760	250000	—	36^c
Cp2ZrCl2	30 450	62 000	2.00	37^c
rac-(Ind^#)ZrCl2	433	313893	3.07	40^d

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Slurry phase polymerisation of ethylene

Slurry phase polymerisations of ethylene were carried out using methylaluminoxane derived silica (SSMAO) and AMO-LDH (LDHMAO) supported permethylindenyl catalysts. The slurry phase polymerisations were carried out similarly to the solution phase polymerisation with 10 mg of pre-catalysts. No leaching occurred during the slurry phase polymerisation. Complete polymerisation data is reported in Tables S9–S11.†

We previously reported the solution phase polymerisation of ethylene using rac-(EBI*)ZrCl₂ but no slurry polymerisations were carried out.⁷ Fig. 8 and 9 depict the ethylene polymerisation activity and the molecular weight data for SSMAO-rac-(EBI*)ZrCl₂. The activities have an optimum of 2151 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ at 60 °C and are comparatively stable (i.e. above 1500 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹) from 50–80 °C, higher than the reported results for (Ind)₂ZrCl₂ supported on silica with trimethylalumnium (400 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹).⁴³

Fig. 8 Dependence of slurry phase ethylene polymerisation activity and M_w for rac-(EBI*)ZrCl₂ on temperature. PDIs are given in parentheses. Supported on SSMAO (200 : 1 loading); TIBA co-catalyst; 2 bar ethylene; 10 mg catalyst; 50 mL hexanes; 1 hour.

Fig. 9 Dependence of slurry phase ethylene polymerisation activity and molecular weight, M_w, for rac-(EBI*)ZrCl₂ on length of run. PDIs are given in parentheses. Supported on SSMAO (200 : 1 loading); TIBA co-catalyst; 70 °C; 2 bar ethylene; 10 mg catalyst; 50 mL hexanes.

The molecular weight observed in the temperature range 50–80 °C is remarkably constant at just over 200 000 kg mol⁻¹ before dropping off to 162 273 kg mol⁻¹ at 90 °C.

The activities decreased with increasing time of polymerisation (3012 to 1414 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ for 15 minutes to two hours), consistent with literature reports.⁴¹

Data for the slurry phase polymerisation of ethylene using SSMAO-rac-(SBI*)ZrCl₂ are depicted in Fig. 10 and 11.

Fig. 10 Dependence of slurry phase ethylene polymerisation activity and M_w for rac-(SBI*)ZrCl₂ on temperature. PDIs are given in parentheses. Supported on SSMAO (200 : 1 loading); TIBA co-catalyst; 2 bar ethylene; 10 mg catalyst; 50 mL hexanes; 30 minutes.

Fig. 11 Dependence of slurry phase ethylene polymerisation activity and M_w for rac-(SBI*)ZrCl₂ on length of run. PDIs are given in parentheses. Supported on SSMAO (200 : 1 loading); TIBA co-catalyst; 70 °C; 2 bar ethylene; 10 mg catalyst; 50 mL hexanes.

The activities increase from 340 to up to 725 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ (40 to 80 °C respectively). However, as expected the molecular weight decreased with increasing temperature (around 261 002 to 160 049 kg mol⁻¹).

A comparison of two different loadings of rac-(SBI*)ZrCl₂ on SSMAO led to the observation of the opposite trend seen for rac-(EBI*)ZrCl₂. The activities obtained were lower on decreased loading at 70 °C: 725 and 611 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ (200 : 1 and 300 : 1 respectively).

These results confirmed the findings in solution phase that the ethylene bridge complex was faster the silyl bridge. However, the molecular weights were higher and the polydispersities lower for silyl based systems.

The permethylindenyl complexes rac-(EBI*)ZrCl₂ and rac-(SBI*)ZrCl₂ were also supported on an MAO-modified AMOST layered double hydroxides (AMO-LDH)^21–23 and their polymerisation was carried out in similar conditions as the silica supported (Fig. 12).

Fig. 12 Dependence of slurry phase ethylene polymerisation activity and M_w for rac-(EBI*)ZrCl₂ and rac-(SBI*)ZrCl₂ on various support. PDIs are given in parentheses. Supported on LDHMAO (200 : 1 loading); TIBA co-catalyst; 2 bar ethylene; 10 mg catalyst; 50 mL hexanes; 1 hour.

rac-(SBI*)ZrCl₂ demonstrated higher activity than its ethylene bridged analogue (rac-(EBI*)ZrCl₂) and also rac-(EBI)ZrCl₂ with activities of 9657, 4325 and 1841 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively. When the LDH support was varied, the catalyst based on Mg₂Al–SO₄ led to the highest activity, followed by Mg₂Al–CO₃ and Mg₂Al–Cl with activities of 9657, 2719 and 2373 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively. These data corroborate well with our recent studies.^21,22 There is a decrease in the polydispersity when using the permethylindenyl in comparison to the parent indenyl complex.

Conclusions

We have reported the synthesis and characterisation of two new permethylindenyl complexes, rac-(SBI*)ZrCl₂ and [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂.

rac-(SBI*)ZrCl₂ demonstrated very high activity for the solution phase polymerisation (ca. 22 500 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹). However, the analogous ethylene-bridged compound demonstrated more than double this activity, albeit in a larger scale polymerisation with a higher ethylene pressure.⁷

MAO-modified AMOST layered double hydroxides support materials led to higher activity with respect to silica supported pre-catalysts. rac-(EBI*)ZrCl₂ supported on the AMO-LDH Mg₂Al–SO₄, Mg₂Al–CO₃ and Mg₂Al–Cl displayed polymerisation activities of 4325, 2719 and 2373 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹ respectively. rac-(SBI*)ZrCl₂ supported on the MAO-modified AMO-LDH Mg₂Al–SO₄ was the most active solid catalyst system at 9657 kg_PE⁻¹ mol_Zr⁻¹ h⁻¹ bar⁻¹.

However, rac-(EBI*)ZrCl₂ was three times faster than rac-(SBI*)ZrCl₂ when supported on silica.

Experimental details

Synthesis of LiInd*SiMe₂ⁿBu

Hexamethylindene (2.00 g, 10 mmol, 1 eq.) was dissolved in thf (50 mL) and ⁿBuLi (2.5 M in hexanes; 4.4 mL, 11 mmol, 1.1 eq.) was added dropwise at room temperature, effecting a change in colour to pale pink. After 2 h, Me₂SiCl₂ (0.64 g, 0.60 mL, 5 mmol, 0.5 eq.) was added causing a slight decolouration in the solution to pale yellow. After 1 h, ⁿBuLi (2.5 M in hexanes; 4.4 mL, 11 mmol, 1.1 eq.) was added to the reaction mixture and, following stirring at room temperature for 3 h, the reaction was dried under vacuum and LiInd*SiMe₂ⁿBu was obtained as an off white solid in an 83% yield (1.33 g, 4.15 mmol). ¹H NMR (300 MHz, 298 K, pyridine-d₅): δ 0.73 (s, 6H, SiMe₂), 0.92 (t, 3H, ³J_HH = 7.0 Hz, δ-Bu), 1.27 (m, 2H, α-Bu), 1.48 (m, 2H, γ-Bu), 1.72 (m, 2H, β-Bu), 2.47 (s, 3H, Ar–Me), 2.29 (s, 3H, Ar–Me), 2.82 (s, 3H, Cp–Me), 2.97 (s, 3H, Ar–Me), 3.03 (s, 3H, Cp–Me), 3.05 (s, 3H, Ar–Me). ¹³C{¹H} NMR (75 MHz, 298 K, pyridine-d₅): δ 5.96 (SiMe₂), 14.84 (δ-Bu), 16.78 (Me), 17.33 (Me), 17.99 (Me), 18.33 (Me), 18.69 (Me), 23.11 (Me), 23.30 (α-Bu), 28.04 (γ-Bu), 28.77 (β-Bu), 90.71 (C–Si), 105.11 (Cp), 119.02 (Ar), 119.48 (Ar), 122.49 (Ar), 122.97 (Ar), 132.30 (Ar), 135.69 (Cp), 138.02 (Ar). ⁷Li NMR (156 MHz, 298 K, pyridine-d₅): δ 1.54.

Synthesis of rac-(SBI*)ZrCl₂

Benzene (50 mL) was added to a mixture of (SBI*)Li₂ (2.00 g, 2.47 mmol, 1 eq.) and ZrCl₄ (0.98 g, 2.47 mmol, 1 eq.) and the reaction mixture was stirred at room temperature for 2 h. The resultant red solution was filtered away from the LiCl by-product, concentrated in vacuum by half and left overnight at room temperature to yield orange crystals. The crystals isolated by filtration and dried to afford orange crystalline material in 34% yield (0.89 g, 1.45 mmol). ¹H NMR (400 MHz, 298 K, chloroform-d₁): δ 1.34 (s, 6H, SiMe₂), 1.88 (s, 6H, Cp–Me), 2.18 (s, 6H, Ar–Me), 2.30 (s, 6H, Ar–Me), 2.39 (s, 6H, Cp–Me), 2.45 (s, 12H, Ar–Me). ¹³C{¹H} NMR (100 MHz, 298 K, chloroform-d₁): δ 10.94 (SiMe₂), 15.93 (Ar–Me), 16.09 (Ar–Me), 16.50 (Ar–Me), 17.52 (Cp–Me), 17.71 (Ar–Me), 22.12 (Cp–Me), 78.02 (Cp–Si), 127.81 (Me), 129.90 (Me), 130.17 (Me), 130.35 (Me), 130.89 (Me), 133.17 (Me), 134.31 (Me), 135.40 (Me). IR (KBr) (cm⁻¹): 2961, 2923, 1543, 1454, 1384, 1260, 1097, 1020, 850, 813, 668. MS (EI): found 615.7972; calculated 615.7908. Major fragmentation peaks noted for M−Me (600.78).

Synthesis of [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂

To a mixture of LiInd*SiMe₂ⁿBu (1.00 g, 3.12 mmol, 1 eq.) and ZrCl₄ (0.72 g, 3.12 mmol, 1 eq.) was added toluene (50 mL). This resulted in a bright orange slurry which became vermillion in colour on stirring for a further 2 h. The slurry was allowed to settle and the supernatant was filtered off and left to stand at −35 °C for three days affording [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂ as a red crystalline solid suitable for an X-ray diffraction study in 73% yield (1.16 g, 2.28 mmol). ¹H NMR (400 MHz, 298 K, benzene-d₆): δ 0.55 (s, 3H, Si–Me₂), 0.65 (s, 3H, Si–Me₂), 0.86 (t, ³J_HH = 7.0 Hz, 3H, δ-Bu), 1.01 (m, 2H, α-Bu), 1.25 (m, 2H, β-Bu), 1.29 (m, 2H, γ-Bu), 1.96 (s, 3H, Ar–Me), 2.03 (s, 3H, Ar–Me), 2.31 (s, 3H, Cp–Me), 2.42 (s, 3H, Ar–Me), 2.48 (s, 3H, Cp–Me), 2.57 (s, 3H, Ar–Me). ¹³C{¹H} NMR (100 MHz, benzene-d₆): δ 3.37 (Si–Me₂), 3.75 (Si–Me₂), 13.98 (δ-Bu), 15.92 (Cp–Me), 16.78 (Ar–Me), 17.31 (Ar–Me), 17.31 (Ar–Me), 17.87 (Cp–Me), 19.79 (α-Bu), 21.71 (Ar–Me), 26.82 (γ-Bu), 26.89 (β-Bu), 116.31 (Cp), 126.23 (Cp), 130.46 (Ar), 132.48 (Ar), 133.23 (Ar), 134.05 (Ar), 136.57 (Ar), 137.24 (Ar), 143.54 (Cp). IR (KBr) (cm⁻¹): 3573, 2957, 1457, 1385, 1261, 1217, 1098, 814, 690, 480. HRMS (EI): found 508.0464; calculated 508.0460.

Synthesis of solid polymerisation catalysts

The quantity of catalyst immobilised on the surface of the support is given in terms of the Al : Zr ratio of the methylaluminoxane component to the organometallic complex. In the case of SSMAO and LDHMAO, 0.5 eq. MAO is used to activate the surface of each support. The ratios used in this work are 300 : 1 and 200 : 1. The molecular weights used for SiO₂ and LDH (Mg_0.75Al_0.25(OH)₂(SO₄)_0.125) are 59.97 and 70.73 kg mol⁻¹ respectively. This gives rise to molecular masses of SSMAO and LDHMAO of 177.94 and 199.46 kg mol⁻¹. SSMAO and LDHMAO are prepared by weighing out silica or LDH and the MAO into a Schlenk tube in the glovebox and adding toluene to the mixture which is then heated at 60 °C for 1 h with regular swirling, until no further effervescence is observed. The solid is allowed to settle, the solution decanted, and the product dried under vacuum with gentle heating (50 °C) to ensure all the solvent is removed. In the glovebox, the activated support and complex was weighed out into a second Schlenk tube. Toluene (50 mL) is added and the reaction mixture is swirled at 60 °C for 1 h. The coloured solid is allowed to settle from the clear, colourless solution which is decanted, and the solid is dried in vacuo (40 °C, 1 × 10⁻² mbar).

Solution phase ethylene polymerisation studies

rac-(SBI*)ZrCl₂, and the methylaluminoxane were charged into an ampoule in a glovebox and 50 mL of hexanes was added. The reactions were performed under 2 bar of ethylene in a 200 mL ampoule, with 0.2 mg of the complex. The reactions were run for 15–120 minutes at varying temperatures. The resulting polyethylene was immediately filtered under vacuum through a dry sintered glass frit. The polyethylene product was then washed with pentane (2 × 5 mL) and then dried on the frit for at least 1 h. The tests were carried out at least twice for each individual set of polymerisation conditions. The activity values were reported as an average with ±1 SD error.

Slurry phase ethylene polymerisation studies

The silica-supported zirconocene catalysts were tested for their ethylene polymerisation activity under slurry conditions in the presence of tri(isobutyl)aluminium (TIBA), an aluminium-based scavenger. The reactions were performed under 2 bar of ethylene in a 200 mL ampoule, with 10 mg of the catalyst suspended in 50 mL of hexanes. The polyethylene work-up was similar to the solution phase polymerisation.

X-ray crystallography

Crystals were mounted on MiTeGen MicroMounts using perfluoropolyether oil, then transferred to a goniometer head on the diffractometer and cooled rapidly to 150 K in a stream of cold nitrogen using an Oxford Cryosystems CRYOSTREAM unit. For rac-(SBI*)ZrCl₂ and [(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]_2, raw frame data were collected at 150 K using a Enraf-Nonius Kappa CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å),⁴⁴ reduced using DENZO-SMN⁴⁵ and corrected for absorption using SORTAV.⁴⁶ The structures were solved using direct methods (SHELXS)⁴⁷ or a charge-flipping algorithm (SUPERFLIP)⁴⁸ and refined by full-matrix least-squares procedures using the Win-GX program suite.⁴⁹ The data collection for (SBI*)H₂ was carried out on an Oxford Diffraction Supernova diffractometer using mirror-monochromated Cu Kα radiation (λ = 1.54178 Å) and data were processed using CrysalisPro⁵⁰ and refined using full matrix least-squares procedures using CRYSTALS.^51,52

rac-(SBI*)H2. C₃₂H₄₄Si, triclinic (P1̄), a = 9.2291(7) Å, b = 9.7804(7) Å, c = 15.8873(12) Å, α = 89.600(6)°, β = 81.392(6)°, γ = 73.234(6)°, V = 1356.64(18) ų, Z = 2, λ = 1.54180 Å, T = 150(2) K, μ = 0.87 mm⁻¹, 5617 independent reflections, R_int = 0.081; R₁ = 0.097, wR₂ = 0.279. CCDC 1001294.

rac-(SBI*)ZrCl2. C₃₂H₄₂SiCl₂Zr, monoclinic (P21/n), a = 14.1078(1) Å, b = 14.2840(2) Å, c = 14.3753(2) Å, α = γ = 90°, β = 96.8940(5)°, V = 2875.91(6) ų, Z = 4, λ = 0.71073 Å, T = 150(2) K, μ = 0.63 mm⁻¹, 6558 independent reflections, R_int = 0.013; R₁ = 0.030, wR₂ = 0.073. CCDC 1001295.

[(Ind*SiMe₂ⁿBu)Zr(μ-Cl)Cl₂]₂. C₂₁H₃₃SiCl₃Zr, triclinic, P1̄, a = 9.52710(10) Å, b = 11.12740(10) Å, c = 11.36160(10) Å, α = 78.1654(5)°, β = 87.3403(5)°, γ = 77.9022(5)°, V = 1152.67(2) ų, Z = 2, λ = 0.71073 Å, T = 150(2) K, μ = 0.88 mm⁻¹, 5248 independent reflections, R_int = 0.013; R₁ = 0.030, wR₂ = 0.073. CCDC 1419346.

Solid-state NMR spectroscopy data. LDHMAO-rac-(SBI*)ZrCl₂

¹³C CPMAS NMR: δ −9.32 (AlOMe), 12.87 (SiMe2), 22.44 (Ar–Me), 24.57 (Ar–Me), 29.54 (Ar–Me), 31.10 (Ar–Me), 74.97 (Cp), 128.39 (Ar). ²⁷Al CPMAS NMR: δ −527, −28, 470. SSMAO-EBI*ZrCl₂. ¹³C CPMAS NMR: δ −9.03 (AlOMe). ²⁷Al CPMAS NMR: δ −309, −113, 3, 182, 336. ²⁹Si CPMAS NMR: δ −106.

Acknowledgements

The authors would like to thank SCG Chemicals Ltd, Thailand for financial support, Dr. Nicholas H. Rees (University of Oxford) for the solid state NMR spectroscopy experiments and Chemical Crystallography (University of Oxford) for the use of the diffractometers. Z.R.T gratefully acknowledges Trinity College, Oxford for a Junior Research Fellowship.

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Footnotes

† Electronic supplementary information (ESI) available: General details, NMR spectroscopy, X-ray crystallography, computational details and definitions of structural parameters. CCDC 1001294, 1001295 and 1419346. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra20465h

‡ Definitions of the structural parameters (TA, RA, α, β) are detailed in the ESI.†

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