Propylene Polymerization and Deactivation Processes with Isoselective {Cp/Flu} Zirconocene Catalysts

: Industrially relevant single-site precatalysts used to produce isotactic polypropylene (iPP) include C 2 -symmetric {SBI} and C 1 -symmetric {Cp/Flu} complexes of group 4 metals. While the latter can produce iPPs with a higher degree of isotacticity, they also suffer from poor productivity compared to their {SBI} counterparts. Several causes for this trend have been suggested—2,1-Regioinsertions are frequently pointed out, as they are suspected to drive the catalyst into a dormant state. While this event does not seem to signiﬁcantly impact the productivity of {SBI} systems, the inﬂuence of these regioerror is poorly documented for isoselective {Cp/Flu} precatalysts. To address this issue, new Ph 2 X(Cp)(Flu) (Ph 2 X = Ph 2 C, FluC, Ph 2 Si) proligands ( 2a – k ) and some of the corresponding dichlorozirconocenes ( 3a – h , k ) were synthesized. These new compounds were characterized and tested in homogeneous propylene polymerization at 60 ◦ C and the amounts of regioerrors in the resulting polymers were examined by 13 C NMR spectroscopy. A possible correlation between poor productivity and a high number of regioerrors was investigated and is discussed. Furthermore, a C-H activation process in the bulky n Bu 3 C substituent upon activation of 4c (the dimethylated analog of 3c ) by B(C 6 F 5 ) 3 has been evidenced by NMR; DFT calculations support this C-H activation as a deactivation mechanism.


Introduction
Since the development of isoselective polymerization of propylene with C 1 -symmetric cyclopentadienyl-fluorenyl {Cp/Flu}-metallocene catalysts [1-4], many academic groups have set out to find "the" ideal structure that would produce polypropylene with the highest degree of isotacticity and regiocontrol, well-controlled molecular weight characteristics, and high productivity. This performance race has resulted in the synthesis of a vast number of structures with various substitution patterns [5][6][7][8][9], for the purpose of studying the influence of ligand architecture on the above parameters [10][11][12].
On the other hand, for the series of C 2 -symmetric silicon-bridged ansa-bis(indenyl) or {SBI}-metallocene catalysts, significant breakthroughs have been achieved in the last decade, and a number of new structures have emerged that exhibit significantly improved performances (Scheme 1). Among them, of note are metallocene catalysts A [13][14][15][16] and B [17] that bear bulky aromatic 4-indenyl substituents and afford astonishing productivities, with highly crystalline iPP having remarkably low amounts of stereo-and regioerrors. Very recently, zirconocene catalyst C, which incorporates conformationally rigid tripticenyl Scheme 1. Examples of highly performing C2-symmetric {SBI}-metallocene catalysts.
In a previous contribution [19], we reported the possible origin of the differing productivity and behavior when comparing isoselective {SBI}- [20] and {Cp/Flu}-type [21][22][23][24][25][26][27][28][29] propylene polymerization precatalysts (Scheme 2). Indeed, the former catalysts are typically one order of magnitude more productive than the latter systems (15·10 4 kgPP·mol −1 ·h −1 with {SBI}-1 vs. 14·10 3 kgPP·mol −1 ·h −1 with {Cp/Flu}-1 at 60 °C and 5 bar), both under homogeneous and heterogeneous (supported) conditions. Yet, the kinetic data [19] unequivocally demonstrated that {Cp/Flu}-based systems (e.g., {Cp/Flu}-2) are intrinsically more active (in terms of propagation rates) and less prone to secondary (2,1-) propylene misinsertions than the {SBI}-systems. However, the very few resulting regiodefects appeared to be much more deleterious than for their {SBI} analogs. In the present contribution, we aimed to extend the understanding of the impact of structural and/or electronic parameters on the regioselectivity of {Cp/Flu}-type isoselective catalytic systems, and decifer possible relationships between regiodefects and catalyst productivity. Thus, we considered a new series of modifications in order to: (1) vary the bulkiness of the 3-R-Cp substituents; (2) assess the impact of the nature and steric hindrance of the substituents in the 2,7-vs. 3,6-positions in the Flu platform; and (3) compare the constrained Ph2Cvs. Ph2Si-bridged {Cp/Flu} ligand platforms. This study also reports on the syntheses of these new precatalysts and their structural characterization both in solution and in the solid state. The performances of the new complexes were explored, after activation with MAO, in homogeneous propylene polymerization.
In a previous contribution [19], we reported the possible origin of the differing productivity and behavior when comparing isoselective {SBI}- [20] and {Cp/Flu}-type [21][22][23][24][25][26][27][28][29] propylene polymerization precatalysts (Scheme 2). Indeed, the former catalysts are typically one order of magnitude more productive than the latter systems (15·10 4 kgPP·mol −1 ·h −1 with {SBI}-1 vs. 14·10 3 kgPP·mol −1 ·h −1 with {Cp/Flu}-1 at 60 °C and 5 bar), both under homogeneous and heterogeneous (supported) conditions. Yet, the kinetic data [19] unequivocally demonstrated that {Cp/Flu}-based systems (e.g., {Cp/Flu}-2) are intrinsically more active (in terms of propagation rates) and less prone to secondary (2,1-) propylene misinsertions than the {SBI}-systems. However, the very few resulting regiodefects appeared to be much more deleterious than for their {SBI} analogs. In the present contribution, we aimed to extend the understanding of the impact of structural and/or electronic parameters on the regioselectivity of {Cp/Flu}-type isoselective catalytic systems, and decifer possible relationships between regiodefects and catalyst productivity. Thus, we considered a new series of modifications in order to: (1) vary the bulkiness of the 3-R-Cp substituents; (2) assess the impact of the nature and steric hindrance of the substituents in the 2,7-vs. 3,6-positions in the Flu platform; and (3) compare the constrained Ph2Cvs. Ph2Si-bridged {Cp/Flu} ligand platforms. This study also reports on the syntheses of these new precatalysts and their structural characterization both in solution and in the solid state. The performances of the new complexes were explored, after activation with MAO, in homogeneous propylene polymerization. In the present contribution, we aimed to extend the understanding of the impact of structural and/or electronic parameters on the regioselectivity of {Cp/Flu}-type isoselective catalytic systems, and decifer possible relationships between regiodefects and catalyst productivity. Thus, we considered a new series of modifications in order to: (1) vary the bulkiness of the 3-R-Cp substituents; (2) assess the impact of the nature and steric hindrance of the substituents in the 2,7-vs. 3,6-positions in the Flu platform; and (3) compare the constrained Ph 2 C-vs. Ph 2 Si-bridged {Cp/Flu} ligand platforms. This study also reports on the syntheses of these new precatalysts and their structural characterization both in solution and in the solid state. The performances of the new complexes were explored, after activation with MAO, in homogeneous propylene polymerization.

Results and Discussion
Syntheses of Proligands. In our previous studies [18][19][20], we used an efficient and scalable procedure for the synthesis of proligands {R 1 R 2 C-(Flu)(Cp)}H 2 via nucleophilic ad- dition of substituted fluorenyl anions onto diversely multisubstituted fulvenes. Following this protocol, an extended series of new constrained (aryl) 2 C-bridged proligands (2a-j) was here prepared in good yields (Scheme 3) and isolated as air-stable solids. The Ph 2 Si-bridged 2k was synthesized using a modified procedure reported for {R 2 Si(Flu)(Cp)}H 2 , where R = Me or Ph [30][31][32][33][34]. After optimization of the conditions and purification by column chromatography, proligand 2k was isolated with an acceptable yield (47%) as a crystalline solid. Proligands 2a-k were characterized by 1 H and 13 C NMR spectroscopy, ASAP massspectrometry, elemental analysis (see Protocol section in the Supporting Information), and X-ray crystallography (see the Supporting Information Figures S77-S87; Figure 1 for 2i). Syntheses of Proligands. In our previous studies [18][19][20], we used an efficient and scalable procedure for the synthesis of proligands {R 1 R 2 C-(Flu)(Cp)}H2 via nucleophilic addition of substituted fluorenyl anions onto diversely multisubstituted fulvenes. Following this protocol, an extended series of new constrained (aryl)2C-bridged proligands (2aj) was here prepared in good yields (Scheme 3) and isolated as air-stable solids. The Ph2Sibridged 2k was synthesized using a modified procedure reported for {R2Si(Flu)(Cp)}H2, where R = Me or Ph [30][31][32][33][34]. After optimization of the conditions and purification by column chromatography, proligand 2k was isolated with an acceptable yield (47%) as a crystalline solid. Proligands 2a-k were characterized by 1 H and 13 C NMR spectroscopy, ASAP mass-spectrometry, elemental analysis (see Protocol section in the Supporting Information), and X-ray crystallography (see the Supporting Information Figures S77-S87 Synthesis and Structure of Zirconocene Complexes. In order to prepare zirconocene dichlorides, regular salt metathesis reactions between anhydrous ZrCl4 and ligands dianions, generated in situ in Et2O, were undertaken (Scheme 4). Thus, analytically pure zirconium complexes 3a-h were isolated in reasonable yields after multiple crystallization attempts from heptane or heptane/CH2Cl2 mixtures as characteristically pink, microcrystalline materials. Following the same procedure, 3k was isolated as a yellow microcrystalline powder. The low isolated yields found for some of the metallocene products resulted from multiple crystallization attempts undertaken to achieve the satisfactory purity needed for characterization and polymerization experiments.  Despite numerous attempts, the respective zirconium complexes derived from proligands 2i and 2j could not be isolated using the standard conditions shown in Scheme 4. Even upon using the conditions reported for the synthesis of complexes {FluC(3-tBu-  Synthesis and Structure of Zirconocene Complexes. In order to prepare zirconocene dichlorides, regular salt metathesis reactions between anhydrous ZrCl4 and ligands dianions, generated in situ in Et2O, were undertaken (Scheme 4). Thus, analytically pure zirconium complexes 3a-h were isolated in reasonable yields after multiple crystallization attempts from heptane or heptane/CH2Cl2 mixtures as characteristically pink, microcrystalline materials. Following the same procedure, 3k was isolated as a yellow microcrystalline powder. The low isolated yields found for some of the metallocene products resulted from multiple crystallization attempts undertaken to achieve the satisfactory purity needed for characterization and polymerization experiments. Despite numerous attempts, the respective zirconium complexes derived from proligands 2i and 2j could not be isolated using the standard conditions shown in Scheme 4. Even upon using the conditions reported for the synthesis of complexes {FluC(3-tBu- Despite numerous attempts, the respective zirconium complexes derived from proligands 2i and 2j could not be isolated using the standard conditions shown in Scheme 4. Even upon using the conditions reported for the synthesis of complexes {FluC(3-tBu-Cp) 2 }ZrCl 2 and {FluC(Cp) 2 }ZrCl 2 incorporating the same FluC bridge (deprotonation of the proligands with nBuLi at −40 • C, followed by the salt-metathesis reaction with ZrCl 4 at −70 • C in CH 2 Cl 2 and then under stirring for 2 h at −20 • C, see [35]), these experiments did not result in the isolation of the desired complexes, and complex mixtures of unidentified products were systematically obtained.
The solution structures of 3a-h and 3k were studied using 1 H and 13 C NMR spectroscopy and were found to be consistent with the C 1 -symmetry of these species. Analysis of the 13 C NMR data (see Figures S45, S47, S49, S51, S53, S55, S58, S60, S62 and S64 in the Supporting Information) revealed particularly upfield C9-fluorenyl carbon signals (δ C 74.5-80.8 ppm), which is indicative of a reduced coordination hapticity (η 5 →η 3 ) of the fluorenyl ligands maintained in solution [36,37]. The iASAP-MS spectra for most of the complexes are also available in the Supporting Information ( Figures S72-S79).
did not result in the isolation of the desired complexes, and complex mixtures of unidentified products were systematically obtained.
The solution structures of 3a-h and 3k were studied using 1 H and 13 C NMR spectroscopy and were found to be consistent with the C1-symmetry of these species. Analysis of the 13 C NMR data (see Figures S45, S47, S49, S51, S53, S55, S58, S60, S62 and S64 in the Supporting Information) revealed particularly upfield C9-fluorenyl carbon signals (δC 74.5-80.8 ppm), which is indicative of a reduced coordination hapticity (η 5 →η 3 ) of the fluorenyl ligands maintained in solution [36,37]. The iASAP-MS spectra for most of the complexes are also available in the Supporting Information ( Figures S72-S79).
Several trends could be specifically drawn. However, these activity data should be considered with care, since the polymerization reactions carried out with highly active systems, such as that based on {Cp/Flu}-2, were quite exothermic, even with low precatalyst loadings. As a result, the temperature of the reaction mixtures could hardly be controlled, often outreaching configured values after a few minutes (see Table 2). The trends   (6) 99.20 (6) 100.10 (5) C(7)-C(6)-C (8) 104.00 (4) 104.33 (14) 104.82 (12) 105.60 (3) 104.30 (7) 104.50 (4) 101.10 (5) Propylene polymerization. Ansa-metallocenes 3a-h,k, in combination with MAO, were evaluated in the propylene polymerization (toluene solution, 5 bar constant pressure, T pol = 60-100 • C). Each polymerization experiment was independently repeated three times under the same conditions, revealing good reproducibility in terms of the activity (gas uptake), productivity (polymer yield), and physicochemical properties (M w , M n , T m , isotacticity) of the isolated polymer. For benchmarking purposes, the performance of the reference zirconocene {Cp/Flu}-2 (Scheme 2) was explored under identical conditions. Due to a high exothermicity at [Zr] 0 = 10.0 µM, the polymerization experiments with {Cp/Flu}-2 were conducted at lower concentrations (5.0 µM). Selected polymerization results are summarized in Table 2 (and reported in Table S3).
Several trends could be specifically drawn. However, these activity data should be considered with care, since the polymerization reactions carried out with highly active systems, such as that based on {Cp/Flu}-2, were quite exothermic, even with low precatalyst loadings. As a result, the temperature of the reaction mixtures could hardly be controlled, often outreaching configured values after a few minutes (see Table 2). The trends evidenced the flexibility and limits of the polymerization processes with these catalytic systems: - The most active precatalyst within the given series of propylene polymerization precatalysts appeared to be the reference metallocene {Cp/Flu}-2. For instance, in a typical experiment performed at 60 • C (entry 1), the productivity of the latter catalyst was found to be 1.5-10 times higher (50,700 kg PP·mol −1 ·h −1 ) than those observed for all other precursors (up to 35,500 kg PP·mol −1 ·h −1 ). - The second by productivity (35,500 kg PP·mol −1 ·h −1 ) appeared to be the less sterically constrained system incorporating 3a, which is analogous to {Cp/Flu}-2 but has no Me substituent at the position 5 of the Cp ligand. -Increasing the bulkiness of the 3-R substituent at the Cp ligand (R = tBuMe 2 C in 3b and R = 1-methylcyclohexyl in 3e) resulted in a significant drop in productivity (5300 kg PP·mol −1 ·h −1 and 16,700 kg PP·mol −1 ·h −1 ; entries 3 and 7, respectively). At the same time, metallocene systems 3c and 3d, both incorporating the bulky aliphatic 3-nBu 3 C substituent at the Cp ligand, were unexpectedly found to be very poorly or completely inactive (entries 4 and 5, respectively). The latter result can stem from deactivation involving a nBu group of the 3-nBu 3 C substituent (vide infra).  Table S4). This result cannot be explained by the higher bulkiness of the mesityl substituents. - The incorporation of a Si-bridge (entry 11) appeared to confer a negative effect on all parameters (productivity, molecular weight, stereoregularity (Table S4), and T m ), as evidenced upon comparing the results obtained with 3k and its Ph 2 C-bridged analogue {Cp/Flu}-2. In order to improve the productivity, a polymerization experiment was conducted at 100 • C (entry 12), but it resulted in an almost total loss of activity; this possibly reflects significant deactivation or even degradation of the catalyst at this temperature. The viscous material recovered from these attempts was found to be a mixture of oligomers exhibiting no melting transition. In a previous contribution, Chen and Rausch et al. [30] explained that the loss of syndioselectivity and poorer productivity of {Me 2 Si(Flu)(Cp)}ZrCl 2 with respect to the Me 2 C-bridged congener were due to steric hindrance of metal center. Thus, the larger Cp cent -Si-Flu cent bite angle, which is ca. 10 • greater than that in the carbon-bridged counterpart, makes the coordination sphere both less accessible and less selective towards the coordination of monomer. Influence of metallocene structure on regioselectivity. The deactivation of MgCl 2supported Ziegler-Natta catalyst systems used for propylene polymerization via 2,1misinsertions and reactivation of their dormant state by the deliberate introduction of H 2 as a chain-transfer agent (CTA) have been reported by Cipullo et al. [41]. In our previous contribution [19], we established a marked impact of regioirregular (2,1-, secondary) insertions on the stability and activity of propagating species in the polymerization of propylene with {Cp/Flu}-based systems. In the latter case, the formation of "dormant" Zr-sec-alkyl species by 2,1-misinsertion of the α-olefin (Scheme 5), reluctant to further insertions, has been also shown to have a critical role for deactivation process. The existence of Zr-secalkyl species can be indirectly evidenced from the 13 C NMR spectroscopic data with the presence of head-head and tail-tail sequences in the resulting polymer. The presence of nBu chain-ends in the polymers, resulting from protonolysis (transfer to H 2 or acidic hydrolysis) of "dormant" Zr-sec-alkyl species, can be hence considered as a fingerprint of massive deactivation of the catalyst. The reorganization of "dormant" Zr-sec-alkyl species though a multistep process (1,3-insertion) [42,43] comprising a β-H elimination reaction from the methyl group of the ultimate unit and then 1,2-reinsertion of the σ-olefin obtained in the previous step, could constitute a re-activation process resulting in restitution of catalytic activity (Scheme 5). Influence of metallocene structure on regioselectivity. The deactivation of MgCl2supported Ziegler-Natta catalyst systems used for propylene polymerization via 2,1misinsertions and reactivation of their dormant state by the deliberate introduction of H2 as a chain-transfer agent (CTA) have been reported by Cipullo et al. [41]. In our previous contribution [19], we established a marked impact of regioirregular (2,1-, secondary) insertions on the stability and activity of propagating species in the polymerization of propylene with {Cp/Flu}-based systems. In the latter case, the formation of "dormant" Zr-secalkyl species by 2,1-misinsertion of the α-olefin (Scheme 5), reluctant to further insertions, has been also shown to have a critical role for deactivation process. The existence of Zrsec-alkyl species can be indirectly evidenced from the 13 C NMR spectroscopic data with the presence of head-head and tail-tail sequences in the resulting polymer. The presence of nBu chain-ends in the polymers, resulting from protonolysis (transfer to H2 or acidic hydrolysis) of "dormant" Zr-sec-alkyl species, can be hence considered as a fingerprint of massive deactivation of the catalyst. The reorganization of "dormant" Zr-sec-alkyl species though a multistep process (1,3-insertion) [42,43] comprising a β-H elimination reaction from the methyl group of the ultimate unit and then 1,2-reinsertion of the σ-olefin obtained in the previous step, could constitute a re-activation process resulting in restitution of catalytic activity (Scheme 5). On attempting to find out the connection between the lower productivities of some {Cp/Flu}-based systems and their propensity towards affording regioirregular insertions, a closer inspection of the 13 C{ 1 H} NMR spectra of the PPs produced with 3a-c,e-h and the reference system {Cp/Flu}-2 was carried out (Table 2). For a more complete set of data, a complementary analysis was conducted with the polymers obtained with some previously reported analogous {Cp/Flu}-based systems (Scheme 6; Table 3, entries 1-7) [11,[25][26][27]. The respective resonances derived from the 2,1-and 1,3-insertions and those from the terminal nBu chain-ends were identified, quantified, and used to plot the combined bar diagram ( Figure 8). Yet, regrettably, no correlation could be identified between the productivity and the content of the titled sequences from regioirregular insertions or nBu chain-ends.
Nevertheless, it appears that minor changes (e.g., 3b or 3a vs. {Cp/Flu}-2) in the {Cp/Flu}metallocene structure can induce dramatic effects on regio-and stereo-control as well as on productivity.
In striking contrast with the {Cp/Flu}-systems, the {SBI}-based counterparts feature a much higher propensity towards affording 2,1-and 1,3-insertions, thus resulting in much larger contents of the corresponding regioerrors (Table 3, entries 8 and 9), which are apparently not deleterious for the productivity of these systems. In fact, in that case, the M-secalkyl species are still reactive and capable of either undergoing further regular 1,2-insertions of propylene or regenerating an active hydrido species upon β-H elimination [19]. trol as well as on productivity.
In striking contrast with the {Cp/Flu}-systems, the {SBI}-based counterparts feature a much higher propensity towards affording 2,1-and 1,3-insertions, thus resulting in much larger contents of the corresponding regioerrors ( Table 3, entries 8 and 9), which are apparently not deleterious for the productivity of these systems. In fact, in that case, the Msec-alkyl species are still reactive and capable of either undergoing further regular 1,2insertions of propylene or regenerating an active hydrido species upon β-H elimination [19]. Scheme 6. Structures of precatalysts used for benchmarking.  Deactivation of the catalytic system based on 3c. Complexes {Ph2C(2,7-tBu2-Flu)(3-nBu3C-Cp)}ZrCl2 (3c) and {Ph2C(3,6-tBu2-Flu)(3-nBu3C-Cp)}ZrCl2 (3d), in combination with MAO, showed particularly low productivity in the polymerization of propylene that could not be rationalized on the basis of steric factors. To address this phenomenon, we aimed at understanding the role of activation conditions as well as of ion-pairing effects. The introduction of excess MAO necessary for the activation of 3c is likely to generate a complex NMR spectrum that would not allow for observing all the phenomena involved, especially in the aliphatic region. Therefore, the generation of the corresponding ion pair was carried out upon using molecular activators such as B(C6F5)3 [27].
The dimethyl-zirconocene 4c was selectively prepared from the parent 3c by a reaction with 2 equiv. of Grignard's MeMgBr (see SI). Further treatment of 4c with 1 equiv. of B(C6F5)3, carried out on an NMR scale in toluene-d8 at −50 °C, resulted in quantitative and selective generation of the corresponding ion-pair 4c-MeB(C6F5)3 (Scheme 7). All attempts to grow crystals of 4c-MeB(C6F5)3 failed thus far. The 1 H NMR spectrum recorded at this temperature (Figure 9a) showed the complete disappearance of the Zr-Me signals (δH Deactivation of the catalytic system based on 3c. Complexes {Ph 2 C(2,7-tBu 2 -Flu)(3-nBu 3 C-Cp)}ZrCl 2 (3c) and {Ph 2 C(3,6-tBu 2 -Flu)(3-nBu 3 C-Cp)}ZrCl 2 (3d), in combination with MAO, showed particularly low productivity in the polymerization of propylene that could not be rationalized on the basis of steric factors. To address this phenomenon, we aimed at understanding the role of activation conditions as well as of ion-pairing effects. The introduction of excess MAO necessary for the activation of 3c is likely to generate a complex NMR spectrum that would not allow for observing all the phenomena involved, especially in the aliphatic region. Therefore, the generation of the corresponding ion pair was carried out upon using molecular activators such as B(C 6 F 5 ) 3 [27].
The dimethyl-zirconocene 4c was selectively prepared from the parent 3c by a reaction with 2 equiv. of Grignard's MeMgBr (see SI). Further treatment of 4c with 1 equiv. of B(C 6 F 5 ) 3 , carried out on an NMR scale in toluene-d 8 at −50 • C, resulted in quantitative and selective generation of the corresponding ion-pair 4c-MeB(C 6 F 5 ) 3 (Scheme 7). All attempts to grow crystals of 4c-MeB(C 6 F 5 ) 3 failed thus far. The 1 H NMR spectrum recorded at this temperature (Figure 9a) showed the complete disappearance of the Zr-Me signals (δ H −1.09 and −0.99 ppm) and the appearance of two equal intensity resonances at δ H −0.38 and −0.22 ppm corresponding to the Zr-Me and B-Me groups, respectively. The ion pair 4c-MeB(C 6 F 5 ) 3 was found stable in the temperature range of −50-+10 • C and was characterized by multinuclear NMR spectroscopy. For example, the 1 H and 13 C{ 1 H} NMR spectra of 4c-MeB(C 6 F 5 ) 3 , recorded at −50 • C (Figures S65 and S66, respectively), each contained a single set of signals. Furthermore, the difference in the chemical shifts of the metaand para-F resonances, |∆δ (m,p-F)| = 4.8 ppm in the 19 F{ 1 H} NMR spectrum of 4c-MeB(C 6 F 5 ) 3 , is in agreement with the inner-sphere ion-pairing (ISIP) nature of that species. Typically, the corresponding |∆δ (m,p-F)| values for ISIPs are greater than 3.5 ppm, while those for OSIPs are smaller than 3.0 ppm; see: [48]. This observation is consistent with the fact that this ion pair exists as a single isomer, in which the coordinated MeB(C 6 F 5 ) 3 − anion occupies the less hindered lateral coordination site [28,29]. This is in contrast with less bulky ion-pair analogues derived from similar C 1 -symmetric metallocenes, which consist of mixtures of two isomers that feature different coordination of the MeB(C 6 F 5 ) 3 − anion (in the more open and the less hindered lateral coordination site, respectively).
Above 15 • C, a new series of signals appeared in the 1 H NMR spectrum (Figure 9b), evidencing a new species whose formation was accompanied by concomitant formation of methane (δ H 0.19 ppm). The reaction was completed after 24 h at room temperature, and the recorded 1 H NMR spectrum contained only one new series of resonances (Figure 9c). Close inspection of the aliphatic region of this 1 H NMR spectrum allowed for the identification of the nature of the product-the new cationic Zr-alkyl complex 5c-MeB(C 6 F 5 ) 3 formed by an intramolecular C-H activation reaction (Scheme 7) between one nBu group and Zr-Me, eventually releasing 1 equiv. of methane. Among the three possible products A-C arising from a C-H activation site in the alkyl chain of the nBu group, the selective formation of regioisomer C took place as judged from the NMR data. Additional 2D (COSY, HSQC) NMR spectroscopic techniques allowed for a proper assignment of all signals (Figures S70 and S71). The Zr-(2-(2-ethyl)ethyl) fragment appears in the 1 H NMR spectrum (Figure 9c) Figure S70) [49]: δ C 80.6 (Zr-CH), 32.0 (CH 2 CH 3 ), 17.1 (CH 2 CH 3 ), and 59.5 (−CCH 2 CH-Zr) ppm. Furthermore, in the 19 F{ 1 H} NMR spectrum of 5c-MeB(C 6 F 5 ) 3 ( Figure S69), the difference in the chemical shifts of the metaand para-F resonances, |∆δ (m,p-F)| = 5.0 ppm, is in line with the ISIP nature of this species [48].  (Figure 9b), evidencing a new species whose formation was accompanied by concomitant formation of methane (δH 0.19 ppm). The reaction was completed after 24 h at room temperature, and the recorded 1 H NMR spectrum contained only one new series of resonances ( Figure  9c). Close inspection of the aliphatic region of this 1 H NMR spectrum allowed for the identification of the nature of the product-the new cationic Zr-alkyl complex 5c-MeB(C6F5)3 formed by an intramolecular C-H activation reaction (Scheme 7) between one nBu group and Zr-Me, eventually releasing 1 equiv. of methane. Among the three possible products A-C arising from a C-H activation site in the alkyl chain of the nBu group, the selective formation of regioisomer C took place as judged from the NMR data. Additional 2D (COSY, HSQC) NMR spectroscopic techniques allowed for a proper assignment of all signals ( Figures S70 and S71 Theoretical studies of a possible deactivation pathway. To gain further insight into this deactivation phenomenon, DFT calculations were conducted using as model the putative cationic methyl-zirconocene complex [4c-Me] + (Scheme 8) obtained upon activation with MAO or B(C 6 F 5 ) 3 , namely [{Ph 2 C(2,7-tBu 2 Flu)(3-nBu 3 -C 5 H 2 )}Zr-Me] + (B3PW91 level, see the Supporting Information for details). The objectives of these non-exhaustive computations were to assess the total electronic energy profiles for two possible concurrent processes, that is: (a) regular coordination/insertion of propylene into the Zr-Me bond of [4c-Me] + to give the respective insertion species [4c-iBu] + , vs. (b) intramolecular C-H activation in the pendant nBu 3 C group by the cationic metal center generating metallacyclic species (most possible isomers B and C, Scheme 7), and consecutive reactions of the latter species with propylene. Despite metallocene species with fully substituted Cp and Flu rings were considered in the computations, some simplifications/assumptions were made to reduce the calculation costs: (i) the influence of the counter anion was disregarded, and only "naked" cationic species were considered; and (ii) the methyl or iso-butyl group (representing the polymeryl chain) resides in the opposite direction to the bulky nBu 3 C substituent. In addition, when reactions with propylene were conceived, it was assumed that (iii) the insertion of propylene proceeds in a primary ("pr") fashion, and (iv) finally, as usually considered for isospecific polymerization of propylene mediated by C 1 -symmetric {Cp/Flu}-metallocene catalysts, the molecule of monomer coordinates to the cationic zirconium center with its methyl group positioned "head down" into the free space in the central region of the fluorenyl ligand, while the methylene group is directed toward the less-congested quadrant (opposite to the methyl or iso-butyl groups/polymeryl chain). Theoretical studies of a possible deactivation pathway. To gain further insight into this deactivation phenomenon, DFT calculations were conducted using as model the pu- The regular propagation pathway was assessed at the first insertion step, that is, conversion of the π-complex [4c-Me] + (Scheme 8) to isobutyl species [4c-iBu] + . As expected, this process was found to be favorable on both kinetic and thermodynamic grounds, showing the reasonable calculated activation barrier ∆E = TS1 prsi = 12.1 kcal·mol −1 for the primary insertion of propylene with si face, which should result in isotactic polymer. Barriers in the range 7.1-8.6 and 10.0-15.0 kcal·mol −1 for the first and second primary insertions of propylene into the Zr-C bond, respectively, were calculated for the cationic species [{R 1 2 C(3,6-tBu 2 Flu)(3-tBu-5-R 2 -C 5 H 2 )}Zr-(polymeryl)] + (R 1 = H, Ph; R 2 = Me, Ph); see reference [10].
The metallation (C-H activation) of the nBu group in the nBu 3 C substituent, requiring a proximal Zr-R function and accompanied by a concomitant alkane (methane or isobutane) elimination, was computed to occur through somewhat higher but still accessible transition states both for R = Me in [4c-Me] + (∆E = TS2 = 19.7 and 20.2 kcal·mol −1 ) and for R = iBu in [4c-iBu] + species (∆E = TS2 = 18.4 and 19.4 kcal·mol −1 ). While the C-H activation reaction from [4c-iBu] + species was computed to be essentially endothermic (8.7 and 6.5 kcal·mol −1 ), the same process from [4c-Me] + species appeared to be thermodynamically favored (−3.9 and −1.7 kcal·mol −1 ). This suggests that the C-H activation phenomenon leading to isomers B and C can preferably occur for the cationic complex initially formed at the precatalyst activation stage and, especially, in the absence of propylene. In line with the experimental results, the formation of isomer C was found computationally to be more favored both kinetically (∆∆E = = 0.5-1.0 kcal·mol −1 ) and thermodynamically (∆∆E = 2.2 kcal·mol −1 ), regardless the nature of [4c-R] + precursor. Further propylene insertions in the metallacyclic isomers B and C (to give the corresponding products P) were computed to be favored thermodynamically (∆E = −11.4-−5.5 kcal·mol −1 ), and connected through accessible transition states (∆E = = 12.9-17.3 kcal·mol −1 ).
These results indicate that the formation of metallacyclic species by intramolecular C-H activation is a smooth process and likely to take place during the very first steps of polymerization. However, this process seems to be not deleterious for the posterior reactivity with propylene and could lead to the formation of larger metallacyclic propagating species. Apparently, deactivation takes effect on later steps through a series of intramolecular C-H activation processes successively involving all the nBu groups of the nBu 3 C substituent and eventually resulting in a steric obstruction of the active site. The regular propagation pathway was assessed at the first insertion step, that is, conversion of the π-complex [4c-Me] + (Scheme 8) to isobutyl species [4c-iBu] + . As expected, this process was found to be favorable on both kinetic and thermodynamic grounds, showing the reasonable calculated activation barrier ΔE ≠ TS1 prsi = 12.1 kcal·mol -1 for the primary insertion of propylene with si face, which should result in isotactic polymer. Barriers in the range 7.1-8.6 and 10.0-15.0 kcal·mol -1 for the first and second primary insertions of propylene into the Zr-C bond, respectively, were calculated for the cationic species [{R 1 2C(3,6-tBu2Flu)(3-tBu-5-R 2 -C5H2)}Zr-(polymeryl)] + (R 1 = H, Ph; R 2 = Me, Ph); see reference [10].
The metallation (C-H activation) of the nBu group in the nBu3C substituent, requiring a proximal Zr-R function and accompanied by a concomitant alkane (methane or iso-butane) elimination, was computed to occur through somewhat higher but still accessible transition states both for R = Me in [4c-Me] + (ΔE ≠ TS2 = 19.7 and 20.2 kcal⋅mol -1 ) and for R = iBu in [4c-iBu] + species (ΔE ≠ TS2′ = 18.4 and 19.4 kcal⋅mol -1 ). While the C-H activation reaction from [4c-iBu] + species was computed to be essentially endothermic (8.7 and 6.5 kcal⋅mol -1 ), the same process from [4c-Me] + species appeared to be thermodynamically favored (−3.9 and −1.7 kcal⋅mol -1 ). This suggests that the C-H activation phenomenon leading to isomers B and C can preferably occur for the cationic complex initially formed at the precatalyst activation stage and, especially, in the absence of propylene. In line with the experimental results, the formation of isomer C was found computationally to be more favored both kinetically (ΔΔE ≠ = 0.5-1.0 kcal⋅mol -1 ) and thermodynamically (ΔΔE = 2.2 kcal⋅mol -1 ), regardless the nature of [4c-R] + precursor. Further propylene insertions in the metallacyclic isomers B and C (to give the corresponding products P) were computed to be favored thermodynamically (ΔE = −11.4-−5.5 kcal⋅mol -1 ), and connected through accessible transition states (ΔE ≠ = 12.9-17.3 kcal⋅mol -1 ).
These results indicate that the formation of metallacyclic species by intramolecular C-H activation is a smooth process and likely to take place during the very first steps of polymerization. However, this process seems to be not deleterious for the posterior reactivity with propylene and could lead to the formation of larger metallacyclic propagating species. Apparently, deactivation takes effect on later steps through a series of intramolecular C-H activation processes successively involving all the nBu groups of the nBu3C substituent and eventually resulting in a steric obstruction of the active site. Scheme 8. Energy profiles computed at the B3PW91 level for propylene coordination/insertion and possible decomposition routes for active species generated from the [4c-Me] + cation (energies in kcal·mol −1 relative to [4c-Me] + + propylene).

Conclusions
New se C 1 -symmetric ansa-zirconocenes with multisubstituted {R 2 E-(Flu)(Cp)} 2− ligands were prepared. In most cases, the activation of these complexes with MAO gave highly active and isoselective propylene polymerization catalytic systems. The highest productivity, comparable to that of the {Cp/Flu}-2 reference system, was observed with 3a and 3e, with both incorporating bulky 3-R-Cp substituents (tBuCMe 2 C and MeCy, respectively). The complete inactivity of the 3-nBu 3 C-Cp substituted 3c and 3d in propylene polymerization was rationalized from NMR spectroscopic studies on a model system incorporating the [4c] + [MeB(C 6 F 5 ) 3 ] ion pair and from DFT calculations. Both indicated a concurrent decomposition route involving intramolecular C-H activation in the nBu 3 Cgroups. The most isoselective system within the whole series appeared to be 3f, bearing an octamethyloctahydrodibenzofluorenyl-based ligand. Quite unexpectedly, the 2,7-Mes 2 -Flu substitution in 3h appeared to be detrimental for stereocontrol and resulted in much poorer isotacticity. The impact of different substitution patterns on the occurrence of regioerrors involving both 2,1-and 1,3-insertions was sought in this study. However, no distinct correlation could be identified thus far between them nor between the observed productivities and the content of the regioirregular sequences in the obtained polymers. Further investigations aimed at clarifying the origin of the different catalytic behaviors among this series of metallocene catalysts are underway in our laboratories.