Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems

Abstract

Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0°C with Pyrex-filtered UV light resulted in a rapid and complete intramolecular [2+2]cycloaddition reaction to yield the corresponding cyclobutylene-bridged ansa-zirconocene dichloride isomer (8a). This is one of the rare examples of an organic functional group chemistry that leads to carbon–carbon coupling at the framework of an intact sensitive group 4 bent metallocene complex. More sterically hindered open metallocenes that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methylethenyl functional group undergo the photochemical ansa-metallocene ring closure reaction equally facile. The metallocene systems used and obtained in this study have served as transition metal components for the generation of active metallocene propene polymerization catalysts.

Metallocene catalysis has become of great significance in olefin polymerization, especially for polyethylene, stereospecific polypropylene formation, and the production of some copolymers. The ansa-metallocenes of the group 4 metals and related systems play an essential role in this important development (1–3). Various methods have been devised and applied to construct such bent metallocene frameworks that feature a short carbon- or heteroatom-containing bridge between their substituted cyclopentadienyl, indenyl, or fluorenyl ligands. Because of the sensitive character of the organometallic group 4 metal complexes, ligand construction and variation is usually carried out at the free ligand stage before the final transmetallation step to the transition metal in the practical preparative sequences. This is a serious synthetic limitation. It would be highly desirable to have an organic functional group chemistry developed for framework variation at the actual metallocene stage. Previously, some addition reactions to metallocene frameworks had been reported, such as catalytic hydrogenation (4), hydrosilylation (5), hydroboration (6–9), or borylation (refs. 10, 11 and references therein, and 12). However, carbon–carbon coupling reactions at the intact group 4 bent metallocene frameworks were close to nonexistent before our work (13–15). Meanwhile, a few leading examples have emerged from the literature, using, e.g., olefin-metathesis (16–19) or even a variant of the Mannich reaction (20–22) for carrying out carbon–carbon coupling reactions at the reactive group 4 bent metallocene frameworks.

Intramolecular photochemical [2+2]cycloaddition reactions may become of a prime importance in this development. We had previously observed that bis(alkenyl-Cp)ZrCl₂ complexes such as, e.g., meso-1 underwent ansa-metallocene formation to yield meso-2 by intramolecular [2+2]cycloaddition when irradiated with UV light. However, this specific reaction was not synthetically useful for a clean ansa-metallocene catalyst development because of its reversibility under the photochemical conditions to result in a photostationary equilibrium mixture of the open and closed isomers of most investigated examples under practical conditions (23, 24). We later showed that the [2-(1-methylethenyl)indenyl]₂ZrCl₂ derivative (3) rapidly and completely underwent the intramolecular photolytic [2+2]cycloaddition reaction when irradiated with Pyrex-filtered light to yield 4, from which an interesting homogeneous metallocene Ziegler-Natta catalyst system was generated (25–28). This posed the question as to whether the outcome of the intramolecular [2+2]photocyclization reaction at the group 4 bent metallocenes might significantly depend on the substituent at the alkenyl functional group and that the use of, e.g., the 1-methylalkenyl moiety might actually lead to a synthetically favorable situation. We have now prepared a small series of respective (1-methylethenyl-Cp)-derived group 4 metallocenes and found that the systems tested rapidly and completely underwent the intramolecular [2+2]cycloaddition to give their cyclobutylene-bridged ansa-zirconocene isomers, which were subsequently used to generate active metallocene Ziegler-Natta catalysts (Scheme 1).

Scheme 1.

Results and Discussion

The alkenyl-functionalized group 4 bent metallocenes (7) were synthesized by fulvene-derived routes (29–31). For the preparation of the ligands the 6,6-dimethylfulvene derivatives (5) were treated with lithium diisopropylamide to give the corresponding alkenyl-Cp lithium reagents (6), which were then reacted with ZrCl₄(THF)₂ (32) to yield the (1-methylethenyl-Cp)₂ZrCl₂ complexes (7) (Scheme 2).

Scheme 2.

Photochemical ansa-Zirconocene Formation.

Bis(1-methylethenyl-Cp)ZrCl₂ (7a) behaves like a conformationally rapidly equilibrating system of an averaged C_2v-symmetry in solution (33) as judged from its NMR spectra. The photolysis of 7a with Pyrex-filtered light in toluene at 0°C led to a practically quantitative conversion to the [2+2]cycloaddition product 8a within 2.5 h. Because of the lower symmetry (C_s), the product 8a now features four separate C₅H₄ ¹H NMR resonances (δ 6.57, 6.35, 5.94, and 5.74) and a single CH₃ signal at δ 1.11 (6H). The head-to-head cycloaddition has resulted in the formation of a bridging cyclobutylene ring system with each a pair of CH₃ groups as well as Cp ligands cis-1,2-attached to it. Consequently, the cyclobutane CH₂CH₂ moiety exhibits different signals of methylene hydrogens oriented cis (δ 2.16) and trans (δ 1.70) to the Cp rings (Scheme 3).

Scheme 3.

Both of the complexes 7a and 8a were characterized by x-ray diffraction. The open metallocene 7a features a conformational arrangement in the crystal where both the 1-methylethenyl substituents are C₂-symmetrically oriented at the open front side of the bent metallocene wedge (for a comparison, see ref. 34). The Zr-C(Cp) bond lengths are in a narrow range between 2.476(2) Å and 2.566(3) Å [Cp(centroid)-Zr-Cp(centroid) angle, 129.8°; Cl1-Zr-Cl1*, 96.20(4)°]. The CC double bond of the alkenyl substituent is oriented in plane with its adjacent Cp ring [C4-C5A, 1.34(2) Å] (Fig. 1).

Fig. 1.

Molecular structure of complex 7a.

The ansa-metallocene complex 8a is close to C_s-symmetric in the solid state (but not crystallographically). It features a pair of eclipsed Cp rings at zirconium [Cp(centroid)-Zr-Cp(centroid), 124.7/125.5°; Cl1-Zr-Cl2, 97.59(5)/100.80(5)°] that are linked by the newly formed cyclobutylene bridge. The bridge is located at the narrow back side of the bent metallocene wedge. Because the C₂ bridge fits the geometry of the group 4 bent metallocene framework very well, the Zr-C(Cp) bond lengths are found in a rather narrow range between 2.484(5) Å (molecule A) [2.481(5) Å, molecule B] and 2.524(5) Å [2.518(6) Å]. The structure (see Fig. 2) confirms the formation of the cyclobutane ring in 8a by head-to-head [2+2]cycloaddition. At the four-membered ring, the methyl groups are attached at adjacent carbon atoms and are oriented cis to each other. The bridging C11–C14 bond length inside the four-membered ring is 1.543(4) Å [1.552(4) Å].

Fig. 2.

Molecular structure of the cyclobutylene-bridged ansa-zirconocene complex 8a (molecule B is shown).

The reaction of the 3-substituted lithium alkenyl cyclopentadienide reagents (6b and 6c) with ZrCl₄(THF)₂ resulted in the formation of pairs of planarly chiral subunits and consequently to (≈1:1) mixtures of the respective meso- and rac-7b/7c stereoisomers. The pairs of isomers each show different spectra, but their relative stereochemical assignment cannot directly be derived from these because the averaged C_s- and C₂-symmetric species feature analogous NMR patterns. Fortunately, the different solubilities of the rac-7c and meso-7c diastereoisomers in n-heptane allowed a separation, and eventually complex meso-7c could be characterized by x-ray diffraction (Fig. 3); with supporting ¹H NMR information, this analysis allowed for tentative structural assignments of the planarly chiral meso/rac-7b/7c diastereoisomers.

Fig. 3.

A view of the molecular structure of complex meso-7c.

In the crystal, complex meso-7c attains a chiral conformation with both tert-butyl groups pointing to one side but offset by ≈1/5 Cp rotation. The C(Me)CH₂ functional groups are likewise oriented at the opposite lateral sector of the bent metallocene with the alkenyl planes oriented close to parallel to their adjacent Cp planes [dihedral angles: C7A-C6A-C1A-C2A, −168.0(2)°; C7B-C6B-C3B-C2B, −3.4(4)°] [bond lengths: C6A-C7A, 1.345(4) Å; C6B-C7B, 1.374(5) Å]. One of the C(Me)CH₂ groups (C6B-C7B) points to the front, and the other (C6A-C7A) points to the narrow back side of the bent metallocene wedge.

The rac/meso-7b complex mixture (1:1) was photolyzed in toluene solution at room temperature. Within 1 h, a complete conversion to the respective cyclobutylene-bridged ansa-zirconocene isomers (8b) was effected. Principally, a mixture of three stereoisomers could be formed in this case, namely a rac and two meso (cis-meso and trans-meso) isomers. However, we observed the formation of a 1:1 mixture of rac-8b with only one of the meso isomers. We have tentatively assigned this product the trans-meso-8b structure from ¹H NMR NOE measurements. It features a set of three ¹H NMR C₅H₃ signals of the pair of symmetry-equivalent 1,3-disubstituted Cp ligands [at δ 6.38, 6.00, and 5.56 (each m, each 2H)], two signals of the cyclobutyleneCH₂-CH₂ protons (δ 2.15/1.54), and one set of ¹H NMR signals for the pair of symmetry-equivalent isopropyl substituents [δ 1.28 (d, 6H), 1.11 (d, 6H, CH₃), δ 3.40 (sept, 2H)], as well as a singlet of the pair of CH₃ groups at the cyclobutylene bridge [δ 0.98 (s, 6H)].

The isomer rac-8b features a total of ten C₅H₃R¹R^{2 13}C NMR signals (δ 146.4, 142.2, 141.3, 139.8, 122.0, 116.5, 109.6, 109.1, 105.4, and 104.9) because of its lower symmetry. Likewise, there are a total of four cyclobutylene ¹³C NMR resonances [δ 49.8/49.5 (quart), δ 30.8/30.3 (CH₂)] and the ¹H NMR signals of a pair of attached methyl substituents [δ 1.01/1.00 (s, 3H each)]. The isopropyl substituents are spectroscopically differentiated because of their different position at the rigid rac-8b metallocene framework [δ (CH): 3.41/3.32 (¹H); 29.1/28.7 (¹³C)], and their attached methyl groups are diastereotopic [¹H NMR, δ 1.35, 1.22, 1.15, and 1.11 (each d, each 3H)].

Photolytic conversion of the 1:1 rac/meso-7c mixture to the ansa-metallocene isomers rac/meso-8c was complete after 1 h in toluene at room temperature. Again, the formation of only one of the two possible meso-8c isomers was observed. Analogous to the isopropyl-substituted system 8b, this isomer was tentatively assigned the trans-meso-8c structure according to its ¹H NMR NOE spectra (see Scheme 4).

Scheme 4.

From the mixture the meso-8c isomer is easily recognized by NMR because of its C_s-symmetry [three ¹H NMR C₅H₃ signals, δ 6.52 (H-4), 6.10 (H-5), and 5.98 (H-2), each of 2H intensity; five ¹³C NMR C₅H₃ resonances, δ 144.5 (C-3), 139.0 (C-1), 120.2 (C-4), 109.3 (C-2), and 108.5 (C-5)] and distinguished from the C₁-symmetric rac-8c isomer (six ¹H NMR C₅H₃ signals, δ 6.53, 6.30, 6.15, 6.11, 5.87, and 5.80; ten ¹³C NMR C₅H₃ signals, δ 151.0, 142.3, 141.4, 138.7, 122.9, 114.2, 113.0, 109.9, 106.8, and 103.8). The rac-8c isomer features two ¹³C NMR signals of the cyclobutylene CH₂CH₂ unit (δ 30.8 and 30.3; adjacent quaternary carbon signals at δ 50.0 and 49.3), whereas meso-8c exhibits only a single ¹³C NMR -CH₂-CH₂- resonance (at δ 30.2). Analogously, there are the signals of two different methyl groups in rac-8c (¹H, δ 1.06 and 1.04; ¹³C, δ 26.4 and 25.1) and the resonances of a pair of symmetry-equivalent CH₃ substituents at the cyclobutylene four-membered carbocycle in meso-8c (¹H, δ 1.02; ¹³C, δ 24.1).

Propene Polymerization Reactions.

Propene polymerization reactions were carried out in toluene solution at 20°C. Catalyst activation was achieved by treatment of the zirconocene dichloride complexes with a 500- to 1,000-fold excess of methylalumoxane (MAO). The catalysts derived from both the open metallocene 7a as well as its ansa-metallocene isomer 8a were quite active. Both gave atactic polypropylenes of a rather low molecular weight (Table 1). The chiral metallocenes 7/8 (b, c) were used as 1:1 meso/rac mixtures. The open metallocene 7b/MAO system showed a moderate catalyst activity yielding an atactic polymer, whereas the catalyst derived from the 8b isomer was more active and gave a slightly isotactic polypropylene. The meso/rac-7c/MAO system was inactive under the applied reaction conditions, probably because of steric hindrance. Consistently, the isomeric meso/rac-8c/MAO catalyst was much less active than the related less hindered 7a/MAO and 7b/MAO systems but produced a low-molecular-weight polypropylene of markedly higher isotacticity. It remains to be clarified which of the respective substituted catalyst components, rac- or meso-7/8, are predominantly responsible for the formation of the rather narrow polydispersity polypropylenes.

Table 1.

Propene polymerization with the metallocene/MAO catalyst systems

No.	Catalyst, mg (mmol)	Polypropylene, g	Activity†	% mmmm	Mn	Mw	PDI
7a	14 (0.037)	34.3	464	6	1,520	2,690	1.78
8a	22 (0.059)	36.6	309	2	3,490	7,990	2.29
7b	20 (0.044)	11.5	131	4	3,830	7,680	2.01
8b	24 (0.053)	66.8	630	24	1,910	3,580	1.87
7c	14 (0.028)	—	—	—	—	—	—
8c	14 (0.028)	2.9	52	42	680	1,070	1.58

In toluene solution, MAO activated, 20°C, 2 bar propene, 1-h reaction time. 7b, 8b, 7c, and 8c were employed as 1:1 meso/rac-mixtures. PDI, polydispersity index.

^†In g polypropylene/mmol [Zr]·h·(bar propene).

Concluding Remarks.

Our study has shown that the intramolecular photochemical [2+2]cycloaddition reaction can in some cases reliably be used for the preparation of ansa-metallocenes that serve as transition-metal components to generate active olefin polymerization catalysts. This easily performed carbon–carbon coupling reaction thus represents one of the few examples of a useful functional group chemistry at the backbone of a sensitive early metal bent metallocene. It seems that the 1-methylethenyl substituent was a good choice because it resulted in a very rapid photochemical ring-closure reaction. Under the applied reaction conditions, this substitution pattern achieved a very favorable photostationary equilibrium that was lying practically completely on the ansa-metallocene side, quite different from most early examples reported of this general reaction type at the bent metallocene nucleus (23, 24). The interesting observation that the [2+2]photocyclization is not adversely affected by the presence of even very bulky alkyl substituents at the Cp rings makes us hope for a broad application spectrum of this coupling method for ansa-metallocene formation and general carbon–carbon coupling at the stage of the intact early metal bent metallocene frameworks and other related sensitive organometallic systems.

Materials and Methods

Preparation of the (1-Methylethenyl-Cp)₂ZrCl₂ Complexes.

For the preparation of the parent compound (7a), 6,6-dimethylfulvene (5a) was treated with lithium diisopropylamide to give 6a. The alkenyl-Cp lithium reagent (6a, 10.5 g, 94.0 mmol) was reacted with ZrCl₄(THF)₂ (17.7 g, 47 mmol) in toluene (−78°C to room temperature) to yield 9.45 g (54%) of 7a after recrystallization from dichloromethane. ¹H NMR (500 MHz, 298 K, CD₂Cl₂): δ 6.51, 6.32 (each m, each 4H, 2-H to 5-H), 5.38/5.14 (4H, CH₂), 2.05 (dd, J = 1.5 Hz/0.9 Hz, 6H, CH₃); ¹³C NMR (150.8 MHz, 298 K, CD₂Cl₂): δ 137.0 (C6), 131.4 (C1), 115.2/114.7 (C2–5), 113.7 (C7), 21.5 (C8); mp 180°C [decomp 273°C, differential scanning calorimetry (DSC)]; anal. calculated for C₁₆H₁₈Cl₂Zr (372.4): C 51.60%, H 4.81%; found: C 51.14%, H 4.95%. X-ray crystal structure analysis (single crystals from a concentrated solution in dichloromethane and storage at +4°C): orthorhombic, space group Fmm2 (no. 42), a = 11.805(1), b = 19.140(1), c = 6.773(1) Å, V = 1,530.3(3) ų, ρ_calc = 1.616 g·cm⁻³, μ = 1.051 mm⁻¹, empirical absorption correction (0.710 ≤ T ≤ 0.979), Z = 4, λ = 0.71073 Å, T = 198 K, ω and ϕ scans, 1,818 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.66 Å⁻¹, 887 independent reflections (R_int = 0.032) and 882 observed reflections [I ≥ 2σ(I)], 60 refined parameters, R = 0.020, wR² = 0.050, CH₂ (C5A) and CH₃ (C5B) group refined independently with occupancy 0.5 for each; hydrogen atoms calculated and refined as riding atoms.

The substituted derivatives were prepared analogously. In theses cases, ≈1:1 mixtures of meso/rac-7b and meso/rac-7c were obtained. The reaction of 6b (10.3 g, 66.8 mmol) with ZrCl₄(THF)₂ (12.6 g, 33.4 mmol) in 150 ml of toluene and recrystallization from dichloromethane gave 11.5 g (76%) of meso/rac-7b (1:1). Anal. calculated for C₂₂H₃₀Cl₂Zr (456.6): C 57.87%, H 6.62%; found: C 58.41%, H 6.94%; mp 107°C (decomp 205°C, DSC). ¹H NMR (meso-7b, 600 MHz, 298 K, d₆-benzene): δ 6.37 (m, 2H, H-2), 5.95 (m, 2H, H-5), 5.69 (m, 2H, H-4), 5.17 (br d, J = 2.2 Hz, 2H, H-7_Z), 4.89 (br d, J = 2.2 Hz, 2H, H-7_E), 3.17 (sept, J = 6.9 Hz, 2H, CHMe₂), 1.86 (dd, J = 1.6 Hz/0.8 Hz, 6H, H-8), 1.17 (d, 6H) and 1.09 [d, J = 6.9 Hz, 6H, CH(CH₃)₂]; ¹H NMR (rac-7b): δ 6.50 (m, 2H, H-2), 5.98 (m, 2H, H-5), 5.57 (m, 2H, H-4), 5.13 (dd, J = 2.2 Hz/1.5 Hz, 2H, H-7_Z), 4.86 (dd, J = 2.2 Hz/0.8 Hz, 2H, H-7_E), 3.31 (sept, J = 6.9 Hz, 2H, CHMe₂), 1.84 (dd, J = 1.5 Hz/0.8 Hz, 6H, H-8), 1.27 (d, J = 6.9 Hz, 6H), and 1.10 [d, 6H, CH(CH₃)₂]. For additional data, see Supporting Appendix, which is published as supporting information on the PNAS web site.

The 1:1 meso/rac-7c complex mixture was prepared analogously from 500 mg (2.97 mmol) of 6c and 555 mg (1.48 mmol) of ZrCl₄(THF)₂ to yield 539 mg (75%) of product. Anal. calculated for C₂₄H₃₄Cl₂Zr (484.7): C 59.48%, H 7.07%; found: C 59.58%, H 7.00%; mp 183°C (decomp 294°C, DSC). X-ray crystal structure analysis of meso-7c (single crystals from dichloromethane at +4°C): monoclinic, space group P2₁/n (no. 14), a = 11.746(1), b = 14.878(1), c = 14.452(1) Å, β = 110.35(1)°, V = 2,368.0(3) ų, ρ_calc = 1.359 g·cm⁻³, μ = 6.97 mm⁻¹, empirical absorption correction (0.873 ≤ T ≤ 0.979), Z = 4, λ = 0.71073 Å, T = 198 K, ω and ϕ scans, 25,904 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.66 Å⁻¹, 5,624 independent (R_int = 0.068) and 4,446 observed reflections [I ≥ 2σ(I)], 252 refined parameters, R = 0.036, wR² = 0.077. For spectroscopic data, see Supporting Appendix.

Photolysis Reactions.

The photolysis of a suspension of 7a (4.25 g, 11.4 mmol) in 30 ml of toluene at 0°C (Philips HPK 125-W, Pyrex filter) was carried out for 2.5 h. Solvent was removed in vacuo, and the product was washed with pentane to yield 4.12 g (97%) of the ansa-metallocene product 8a, mp 207°C (DSC). Anal. calculated for C₁₆H₁₈Cl₂Zr (372.4): C 51.60%, H 4.87%; found: C 51.56%, H 5.00%. X-ray crystal structure analysis (single crystals from a concentrated solution in toluene at room temperature): monoclinic, space group P2₁/n (no. 14), a = 10.404(1), b = 21.667(1), c = 13.758(1) Å, β = 98.16(1)°, V = 3,070.0(4) ų, ρ_calc = 1.612 g·cm⁻³, μ = 1.048 mm⁻¹, empirical absorption correction (0.744 ≤ T ≤ 0.818), Z = 8, λ = 0.71073 Å, T = 198 K, ω and ϕ scans, 27,782 reflections collected (±h, ±k, ±l), [(sinθ)/λ] = 0.66 Å⁻¹, 7,333 independent reflections (R_int = 0.055) and 5,603 observed reflections [I ≤2σ(I)], 347 refined parameters, R = 0.054, wR² = 0.168, max. residual electron density 1.60 (−1.07) e Å⁻³ in the region of the four-membered rings, indicating some unsolved disorder that was refined with geometrical constraints (SADI); two almost identical independent molecules in the asymmetric unit; hydrogen atoms calculated and refined as riding atoms. For spectral data, see the text and Supporting Appendix.

Photolysis of a solution (2.00 g, 4.38 mmol) of meso/rac-7b (1:1) in 20 ml of toluene under similar conditions (1 h, room temperature) gave meso/rac-8b (1.56 g, 78%) as a 1:1 mixture. Anal. calculated for C₂₂H₃₀Cl₂Zr (456.6): C 57.87%, H 6.62%; found: C 56.51%, H 6.77%. Analogously, a 1:1 meso/rac-8c mixture (1.85 g, 82% yield) was isolated from photolysis of 2.00 g (34.13 mmol) of meso/rac-7c (1:1) dissolved in 100 ml of toluene, mp 143°C (decomp 214°C, DSC). Anal. calculated for C₂₄H₃₄Cl₂Zr (482.1): C 59.48%, H 7.07%; found: C 59.40%, H 7.04%. For spectral data, see Supporting Appendix.

Propene polymerization reactions were carried out in a thermostated Büchi glass autoclave system, which was charged with toluene (200 ml) and 20 ml of a 10.5 wt % MAO solution in toluene. At 20°C propene [2 bar (1 bar = 100 kPa)] was added for 30 min with stirring and then a solution of 10 mg of the Zr-catalyst precursor in 20 ml of toluene. After 1 h of reaction time, the reaction was stopped by careful addition of a 1:1 mixture of 2 M aqueous HCl and methanol. Excess monomer was vented, and the polymer was extracted with toluene, stripped, and dried in vacuo. Characterization of the obtained polypropylene samples was carried out by GPC and ¹³C NMR pentad analysis (refs. 35–38; and ref. 39 and references therein); for details, see Table 1 and Supporting Appendix.

Abbreviations

DSC

differential scanning calorimetry

MAO

methylalumoxane.