Discrete Ionic Complexes of Highly Isoselective ... - ACS Publications

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Discrete Ionic Complexes of Highly Isoselective Zirconocenes. Solution Dynamics, Trimethylaluminum Adducts, and Implications in Propylene Polymerization Gabriel Theurkauff,† Manuela Bader,† Nicolas Marquet,† Arnaud Bondon,‡ Thierry Roisnel,§ Jean-Paul Guegan,∥ Anissa Amar,⊥ Abdou Boucekkine,⊥ Jean-François Carpentier,*,† and Evgueni Kirillov*,† †

Institut des Sciences Chimiques de Rennes, Organometallics: Materials and Catalysis Laboratories, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France ‡ Institut des Sciences Chimiques de Rennes, Ingénierie Chimique et Molécules pour le Vivant, UMR 6226 CNRS-Université de Rennes 1, PRISM, F-35042 Rennes Cedex, France § Institut des Sciences Chimiques de Rennes, Centre de diffraction X, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France ∥ Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Ecole Nationale Supérieure de Chimie de Rennes, F-35708 Rennes Cedex, France ⊥ Institut des Sciences Chimiques de Rennes, Chimie Théorique Inorganique, UMR 6226 CNRS-Université de Rennes 1, F-35042 Rennes Cedex, France S Supporting Information *

ABSTRACT: Discrete ionic complexes belonging to two main families of highly isoselective polymerization zirconocene systems, C1-symmetric {Cp/Flu} (a,b) and C2-symmetric {SBI} (c) systems, have been scrutinized. The ion pair reorganization processes for inner-sphere ion pairs (ISIPs) 3a,b-MeB(C6F5)3, 3c-MeB(C6F5)3, and 3c-MeAl(C6F5)3 and for the outer-sphere ion pair (OSIP) AlMe3 adducts [6b,c]+[B(C6F5)4]−, quantified by dynamic NMR analysis, were found to feature lower activation barriers for the {SBI}based systems in comparison to those for the {Cp/Flu}-based congeners. The higher electrophilic character of the {Cp/Flu}based cationic systems was corroborated by UV/vis spectroscopy studies coupled with TD-DFT calculations. These fundamental differences between the ionic systems of the two metallocene families are discussed in light of their respective propylene polymerization performances, and reasons for the higher productivity of {SBI} systems in comparison to {Cp-Flu} systems are proposed.

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Scheme 1. Isoselective C1- and C2-Symmetric Zirconocene Precatalysts Used in This Study

INTRODUCTION Isotactic polypropylene (iPP) is one of the landmarks of the plastics market, with a global annual production of ca. 45 MT. In addition to heterogeneous Ziegler−Natta catalytic systems, single-site group 4 metallocene systems in homogeneous or, most preferably, in heterogenized silica-supported forms are also intensively used for the production of iPP (and iPP-based olefinic copolymers). Zirconocenes 1a−c (Scheme 1), which belong to the two main families, namely C1-symmetric onecarbon-bridged cyclopentadienyl−fluorenyl ({R2R3C(Flu)(CpR1)} or {Cp/Flu}; a,b) and C2-symmetric silicon-bridged ansabis(indenyl) ({Me2Si-(2-Me-4-Ph-Ind)2} or {SBI}, c) complexes, respectively, are largely applied in industry for isoselective propylene polymerization.1 Precatalyst activation protocols for generating ionic species have a crucial impact on the efficiency and stability of active catalysts.2,3 In seminal NMR spectroscopic studies of the activation step,4−6 it has been demonstrated that combinations © XXXX American Chemical Society

of the metallocene precursor Cp2ZrX2 (Cp = η5-C5H5, X = Me, Cl) with methylaluminoxane (MAO) give rise to complex reaction mixtures, generally consisting of diamagnetic Zr(IV) Received: November 25, 2015

A

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highly active and isoselective ansa-metallocene precatalysts based on the prototype 1a23 (Scheme 1) that incorporate {Cp/ Flu}2−-type ligands.24 In that series, precursor 1b, when activated with MAO, appeared to be at least 1 order of magnitude more productive than the prototype system 1a, still providing the same level of stereocontrol. While the system based on the {SBI}-type Spaleck complex 1c25 revealed a remarkable aptitude for maintaining high activity at elevated temperatures (>90 °C),24c those derived from the fluorenylbased zirconocenes 1a,b appeared to be much less thermally robust. We have thus correlated tentatively the polymerization activity of these catalytic systems to the higher stability of the active species; the latter stability, in principle, may be determined by numerous independent factors, including sensitivity of cations toward various deactivation processes, metallocenium−anion ion-pairing strength, and propensity of the given ionic intermediates to interact e.g. with aluminum alkyls. To address the above questions, we report in the present contribution comparative studies on different metallocenium ion pairs derived from the parent metallocenes 1a−c and different cationizing agents, including E(C6F5)3 (E = B, Al), [PhNMe2H]+[B(C6F5)4]−, [Ph3C]+[B(C6F5)4]−, and commercial-grade MAO (see the Supporting Information). The activation products of metallocene 1c with MAO have been already briefly assessed by 1H NMR spectroscopy.6k In addition to these studies, the activation of the highly isoselective analogues C1-symmetric {Cp/Flu} metallocenes has never been studied before. In this article, we show that the {SBI} metallocene 1c affords the generally more stable ISIP complexes ZrMe(μ-Me)E(C6F5)3 (E = B, Al), which are more prone to anion exchange than those derived from 1a,b. We demonstrate also that the {Cp/Flu} metallocenes exhibit a high propensity to form heterometallic complexes with Zr(μMe)AlMe2 cores, which are more stable than their {SBI} metallocene congeners. The polymerization tests carried out with the ionic complexes under different conditions were in agreement with their general stabilities and reactivities. Parts of these results were recently briefly communicated.26

products of the type I−V (Scheme 2) with different electropositivities of the metal center and cation−anion Scheme 2. Putative Zr(IV) Intermediates and Products That Form upon Activation of Zirconocenes with MAO

separation: i.e., inner-sphere (ISIP) vs outer-sphere (OSIP) ion pairs.7 Concomitant formation of paramagnetic M(III) species during activation of some dichlorometallocenes (M = Ti, Zr) with MAO (or “AlMe3-depleted” MMAO) has been detected by ESR spectroscopy.8 The proportions among all these species depend, in principle, on such parameters as the nature of the metallocene precatalyst, [MAO]/[M] ratio, composition of MAO, nature of solvent, duration of activation (“aging”), etc. Among them, ISIP species IV are believed to be the immediate precursors of the “true” polymerization catalyst [Cp2ZrMe(B)]+, the OSIP species V, which forms through a reorganization or expulsion of the [Me-MAO]− anion from the inner to the outer coordination sphere. Also, the OSIP AlMe3 adduct III is the dominant component of this milieu9 and is often considered as a “dormant” species,10 acting as the reservoir of both IV and V. Alternatively, discrete metallocenium ion pairs structurally similar to those derived from MAO can be obtained by treatment of the parent neutral dialkyl metallocenes with appropriate molecular activators: that is, highly electrophilic boranes and alanes, E(C6F5)3 (E = B, Al), and borate salts of the type [R]+[B(C6F5)4]− (R = Ph3C, HNMe2Ph, HNMePh2, ...).2 In pioneering studies by Marks et al.,11 Bochmann et al.,12 Brintzinger et al.,6g,13 Landis et al.14 and others,15 two types of model complexes mimicking species IV and V (Scheme 2) have been described: the bridged ZrMe(μ-Me)E(C6F5)3 contact ion pairs and loosely associated [Cp′2ZrMe]+[B(C6F5)4]− ion pairs, respectively. Heterobimetallic metallocene complexes with the M(μ-Me)AlMe2 cores as models of species III have been also documented.16−18 More seldom, examples of dinuclear species II have been reported19,20 and models of paramagnetic Zr(III) species have been also probed.21 Yet, recent contributions by the groups of Bercaw and Brintzinger21,22 have indicated that various heterometallic metallocene-aluminum hydrido species may also be interfering in the polymerization process. Understanding the role of activation conditions, as well as ion-pairing effects and anion dynamics in the metal coordination sphere can, in principle, help identifying and developing more efficient (in terms of activity, stability, stereoselectivity, and polypropylene properties) catalytic combinations. We have recently reported a new series of

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RESULTS AND DISCUSSION The C2-symmetric racemic silicon-bridged ansa-bis(indenyl) metallocene c has two identical sites/faces (occupied by two equivalent Zr−Cl groups in complex 1c); both of them exhibit the same reactivities. Metallocenes 1a,b derived from racemic proligands are also racemates;27 however, the two sites in the C1-symmetric 1a,b feature a dissimilar accessibility (and reactivity) due to the nonsymmetric substitution at the Cp ring. These will be designated in this study as a less open (or more crowded) site for that having the 3-tBu-Cp substituent in its main area and a more open (or less crowded) site for that with a distal 5-R-Cp group (R = Me, Et) lying on its backside. Generation of Ion Pairs {R2R3C-(Flu)(Cp-R1)}ZrMe(μMe)E(C6F6)3 and rac-{Me2Si-(2-Me-4-Ph-Ind)2}ZrMe(μMe)B(C6F5)3 (E = B, Al). The dimethylzirconocenes 2a−c were selectively prepared from the parent dichloro precursors 1a−c by reactions with 2 equiv of MeLi in diethyl ether or with 2 equiv of MeMgBr in toluene. The complexes were characterized by NMR spectroscopy, X-ray crystallography (for 2b,c; Figures S48 and S49 in the Supporting Information, respectively) and elemental analysis. Like their parent dichloro precursors 1a,b,24 dimethyl complexes 2a,b also exist as C1symmetric species in benzene or toluene solution, as judged B

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Organometallics Scheme 3. Generation of Ion Pairs by Reactions of Dimethylzirconocenes 2a,b with E(C6F5)3 (E = B, Al), [PhNMe2H]+[B(C6F5)4]−, and [Ph3C]+[B(C6F5)4]−

Scheme 4. Generation of Ion Pairs by Reactions of Dimethylzirconocene 2c with E(C6F5)3 (E = B, Al), [PhNMe2H]+[B(C6F5)4]−, and [Ph3C]+[B(C6F5)4]−

from the 1H and 13C NMR spectroscopic data (see the Supporting Information). Further treatment of 2a,b with 1 equiv of B(C6F5)3, carried out on an NMR scale in toluene-d8 in the −50 to +25 °C temperature range, resulted in complete and clean generation of the corresponding ion pairs 3a-MeB(C6F5)3 and 3bMeB(C6F5)3 (Scheme 3), respectively.28 The 1H and 19F NMR spectra of 3a-MeB(C6F 5) 3 and 3b-MeB(C 6F 5)3,

recorded at room temperature (Figures S7, S9, S10, and S12 in the Supporting Information, respectively), all contained two sets of signals assigned to two distinct species found in 93:7 and 90:10 ratios, respectively. These observations are consistent with the fact that these ion pairs exist as mixtures of two isomers that do not exchange quickly on the NMR time scale.29 As evidenced from NMR data (vide infra), the more thermodynamically stable (major) isomer is the one in which C

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Organometallics the coordinated MeB(C6F5)3− anion occupies the less hindered lateral coordination site. Under conditions similar to those used to synthesize 3bMeB(C6F5)3, the complex 3b-MeAl(C6F5)3 was obtained from 2b and the less Lewis acidic Al(C6F5)3·0.5(toluene) adduct (Scheme 3).28 By analogy with 3a,b-MeB(C6F5)3, the product 3b-MeAl(C6F5)3 exists as a ca. 70:30 mixture of two isomers,29 as shown by 1H and 19F NMR spectroscopy (Figures S16 and S19 in the Supporting Information, respectively). The E(C6F5)3-derived (E = B, Al) {R2R3C-(Flu)(Cp-R1)} metallocenium complexes 3a,b-MeB(C6F5)3 and 3b-MeAl(C6F5)3 are extremely air- and moisture-sensitive species. Numerous attempts to obtain them pure on a larger scale in Schlenk glassware or in a glovebox were unsuccessful; the isolated materials were systematically contaminated by unidentified byproducts. Yet, when they were synthesized and kept in sealed NMR tubes, all complexes were found to be stable for days in toluene solution at room temperature and for hours upon heating to 80 °C. The remarkably higher stability of the {SBI} ionic complexes 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3 allowed generating them on a larger scale from the parent 2c (Scheme 4). After workup, both were isolated as the toluene adducts 3c-MeB(C6F5)3· 2(toluene) and 3c-MeAl(C6F5)3·(toluene), respectively. The identity of these two compounds was established by elemental analysis, 1H and 13C NMR spectroscopy in solution, and singlecrystal X-ray diffraction studies. The solution structures of 3a−c-MeB(C6F5)3 and 3cMeAl(C6F5)3 were studied by NMR spectroscopy in toluened8 (or toluene-d8/o-difluorobenzene mixtures). The PGSEderived hydrodynamic radii for 3a,b-MeB(C6F5)3 (Table 1), measured at different concentrations,30 and the aggregation number values N calculated therefrom, were all consistent with monomeric structures in solution. In addition, the differences in the chemical shifts of the meta- and para-F resonances, |Δδ(m,p-F)|, for 3a−c-MeB(C6F5)3 and 3b,c-MeAl(C6F5)3 (4.5, 4.6, 5.1, 7.6, and 7.1 ppm, respectively) were all in agreement with the ISIP nature of those species.31 Additional information on the structures of ion pairs 3a−cMeB(C6F5)3 and 3b,c-MeAl(C6F5)3 in solution was obtained from NOE NMR investigations. The room-temperature 1 H−1H NOESY spectra, recorded for complexes 3a,b-MeB(C6F5)3 and 3b-MeAl(C6F5)3, clearly showed for the major isomer NOE cross-peaks arising from close contacts between the hydrogens of the Cp and Flu moieties and those of the ZrMe and EMe (E = B, Al) groups. Careful integration of the corresponding cross-peaks allowed determination of internuclear distances in 3b-MeB(C6F5)3 (Table 2): in particular, those between the BMe and the H4 hydrogens of the Cp and Flu moieties, respectively. Similar observations were made in the 1H−1H NOESY spectrum of 3b-MeAl(C6F5)3; however, the poor quality of the NOESY data did not allow reliable integration of the cross-peaks. Strong NOE contacts between the o-F nuclei of the MeB(C6F5)3− anion and the nearby hydrogens of Flu(H4), Cp(H4), Cp(tBu), Flu(tBu), Cp(R) (where R = Me, Et), and ZrMe groups were observed in the 19 F−1H HOESY NMR spectra of 3a,b-MeB(C6F5)3 (Figure S14 in the Supporting Information) and 3b-MeAl(C6F5)3. For 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3, close contacts (98%) isomer D

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Table 2. Selected Distances (Å) between Hydrogen Atoms Determined from X-ray Diffraction (When Appropriate), NOESY, and DFT Analyses, Respectivelya 3a-MeB(C6F5)3 NOESYb

distance 4

5

Flu(H )−Flu(H ) Ph(H)−Flu(H1) Cp(H2)−Flu(H8) ZrMe−Cp(H2) ZrMe−Cp(H4) ZrMe−Flu(H5) BMe−Cp(H4) BMe−Flu(H4)

3b-MeB(C6F5)3

⟨reff⟩ DFT

⟨reff⟩ RDXb

NOESY

⟨reff⟩ DFT

2.409

2.6 2.3 3.8

2.538 2.432 2.351 3.8

3.4 3.2 3.8

3.5 3.0 4.0

2.5 3.9

2.496 3.7

2.725 2.493 2.337 3.5

3.3 3.7 4.1

3.5 3.1 3.9

3.4 3.1 3.9

[6b]+[MeB(C6F5)3]− ⟨reff⟩ DFT

[6b]+[“Me-MAO”]− NOESY

2.265 3.7 3.4

2.0 3.4 3.6

a

The mean separation of hydrogen atoms attached to the respective fragment from the ZrMe, BMe, and AlMe hydrogen atoms was averaged from the crystal or/and from the DFT optimized structures as 1/⟨r−3⟩1/3. bThe interatomic distances between the aromatic Flu(H4) and Flu(H5) hydrogen atoms (2.723−2.726 Å) in 3b-MeB(C6F5)3, as determined by X-ray crystallography, were used as reference for calibration of NOESY intensities.

Scheme 5. Formation of AlMe3 Adducts by Reaction of Complexes 3a−c-MeE(C6F5)3 (E = B, Al) and [5b,c]+[B(C6F5)4]− with AlMe3

ion pairs [5b,c]+[B(C6F5)4]−, which proceeded in toluene-d8 (or toluene-d8/o-F2-benzene mixtures) in the temperature range −50 to +25 °C (Schemes 3 and 4, respectively).26 The room-temperature 1H and 13C NMR spectroscopic data for both compounds were in agreement with the existence, in each case, of single C1-symmetric species on the NMR time scale. Reactions of {R2R3C-(Flu)(Cp-R1)}ZrMe(μ-Me)E(C6F6)3, rac-{Me2Si-(2-Me-4-Ph-Ind)2}ZrMe(μ-Me)B(C6F5)3 (E = B, Al), [{R2R3C-(Flu)(Cp-R1)}ZrMe]+[B(C6F5)4]−, and [rac{Me2Si-(2-Me-4-Ph-Ind)2}ZrMe]+[B(C6F5)4]− with AlMe3 and Formation of Heterobimetallic OSIP. Dissociation of a weakly coordinated anion (WCA) in the precatalyst ion pair to release a “naked” or solvated metallocenium alkyl cation is considered as a prerequisite step preceding olefin coordination.2 However, when exogenous alkylaluminum AlR3 reagents are deliberately employed in polymerization in large excess (with respect to the precatalyst) as in situ alkylating,

of the two possible isomers (Figures S28−S31 in the Supporting Information, respectively). This isomer is apparently that with aniline bound to Zr through the NMe2 group at the more open coordination site. The 1H and 13C NMR spectroscopic data suggest that the coordination of the PhNMe2 moiety in [4c]+[B(C6F5)4]− is similar to that in [4a,b]+[B(C6F5)4]−.35 The PGSE data (within the typical 10% experimental uncertainty for determining the translation diffusion coefficient Dt and the corresponding hydrodynamic radius rH) obtained for [4a,c]+[B(C6F5)4]− in toluene-d8/o-F2benzene mixtures (Table 1) suggest that both species are higher aggregates.36 The aniline adducts [4a,b]+[B(C6F5)4]− are stable in toluene-d8 (or toluene-d8/o-F2-benzene mixture) at room temperature over several days,12c,d while [4c]+[B(C6F5)4]− progressively decomposes to form unidentified products.37 Also, we have recently reported the reactions of 2b,c with [Ph3C]+[B(C6F5)4]− as a route to coordinatively unsaturated E

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Organometallics scavenging, and chain-transfer agents, metallocenium cations tend to afford heterobimetallic separated ion pairs of the type [Cp2M(μ-R)AlR2]+[anion]−. Especially, if a commercial-grade (obtained from Albermale Corp.; see the Supporting Information) MAO which “naturally” contains ca. 10 mol % of AlMe3 is used for activation, formation of “dormant” species of the type [Cp2M(μ-Me)AlMe2]+[“MeMAO”]− can occur. The stability of such Zr−Al heterobimetallic complexes strongly depends on the nature of the counterion and ligand. For example, different robust complexes [L 2 M(μMe)2AlMe2]+[B(C6F5)4]− and [L2M(μMe)2AlMe2]+[“MeMAO”]− have been documented.2,5,10,16,38 In addition, we have recently communicated the Zr−Al heterobimetallic [6b,c]+[B(C6F5)4]− as the products of reactions of [5b,c]+[B(C6F5)4]− with excess AlMe3 (Scheme 5), respectively.26 On the other hand, heterobimetallic complexes derived from MeB(C6F5)3-based ion pairs and AlMe3 are barely known.39,40 The latter fact can be explained, at least in part, by the high propensity of the B(C6F5)3−AlMe3 combination to undergo very rapid Me/C6F5 group exchange in hydrocarbon solvents.41 Nevertheless, from a mechanistic point of view, the combination of E(C6F5)3 (E = B, Al) as cocatalyst and AlMe3 as scavenger, which would afford dynamically rearranging ion pairs of the type [L2M(μ-Me)2AlMe2]+[MeE(C6F5)3]−, could provide a more realistic model of a polymerization catalyst. In our hands, treatment of ISIP 3a,b-MeB(C6F5)3 with AlMe3 in toluene-d8 at room temperature resulted in the very rapid formation of the AlMe3 adducts [6a,b]+[MeB(C6F5)3]− (Scheme 5), respectively. It is worth noting that Al:Zr ratios slightly above 1:1 should be used for this reaction,42 since displacement of the coordinated MeB(C6F5)3− anion by the weak donor AlMe3 appears to be thermodynamically neutral or to be disfavored.43 Both reaction products were obtained quantitatively as bright blue oily materials from toluene-d8 solution. As expected, these compounds appeared to be rather unstable at room temperature in toluene-d8 or in a toluene-d8/ o-F2-benzene (8/2 v/v) mixture, and NMR data were obtained immediately for the freshly prepared samples. The 1H and 19F NMR data confirmed the C1 symmetry of [6a,b]+[MeB(C6F5)3]− (Figures S34−S37 in the Supporting Information, respectively). In striking difference with the parent 3a,b-MeB(C6F5)3, only one species was observed for each of [6a,b]+[MeB(C6F5)3]−, respectively. The most characteristic hydrogens of the diastereotopic terminal AlMe2 and bridging Zr(μ-Me2)Al groups appear at high field: δ −0.59, −0.71 ppm and −1.30, −1.32 ppm, respectively, for [6a]+[MeB(C6F5)3]− and δ −0.56, −0.70 ppm and −1.20, −1.37 ppm, respectively, for [6b]+[MeB(C6F5)3]−. These NMR data compare well with those observed for [6b]+[B(C6F5)4]−: δ −0.55, −0.66 ppm and −1.18, −1.31 ppm, respectively.26 The differences in the chemical shifts of the meta- and para-F resonances, |Δδ(m,p-F)|, in [6a,b]+[MeB(C6F5)3]− of 2.9 ppm for both compounds suggest that these species are OSIPs.31 Diffusion NMR experiments (PGSE) allowed estimation of the translation diffusion coefficient Dt and hydrodynamic radius rH for a freshly prepared sample of [6a]+[MeB(C6F5)3]− (Table 1). The results are consistent with a monomeric species at the given concentration. Complexes [6a,b]+[MeB(C6F5)3]− slowly decompose in the −35 to 0 °C temperature range, to give products resulting from rapid scrambling of the Me/C6F5 groups, along with BMe3 (identified by 11B NMR spectroscopy; δ 86.8 ppm).41

Decomposition takes place even much more quickly at higher temperatures (>25 °C). Quite unexpectedly, similar reactions for 3b,c-MeAl(C6F5)3 and 3c-MeB(C6F5)3 conducted in the −30 to +25 °C temperature range and monitored by NMR spectroscopy, yielded instantly after mixing the reagents intractable mixtures of unidentified products; those most likely arise from scrambling of the Me/C6F5 groups and possibly other processes. Reactions of {R2R3C-(Flu)(Cp-R1)}ZrCl2 and rac-{Me2Si(2-Me-4-Ph-Ind)2}ZrCl2 with MAO. We were interested in identifying and comparing the products that form upon activation of {Cp/Flu} and {SBI} dichlorozirconocenes, 1a,b and 1c, respectively, with an excess of “regular” commercialgrade MAO (containing ca. 10−30 wt % of AlMe3, Albermarle Corp.).44 In line with previous studies,6 treatment of 1a,b with relatively small amounts of MAO (20−50 equiv vs Zr), monitored by 1H NMR spectroscopy, resulted in the formation of complex mixtures of partially alkylated/ionized products, whose identities could not be unambiguously determined. On the other hand, when a larger excess of MAO (typically, more than 100 equiv) was used, the corresponding complexes [6a,b]+[“MeMAO”]− cleanly formed (Scheme 6). Scheme 6. Activation of Dichlorozirconocenes 1a,b with MAO (90−160 equiv)

The 1H and 13C NMR spectra (toluene-d8/o-F2-benzene (8/ 2 v/v) mixture) featured the same patterns of resonances for both species, consistent with the heterobimetallic nature of the central cationic [{R2R3C-(Flu) (Cp-R1)}Zr(μ-Me)2AlMe2]+ cores of [6a,b]+[“MeMAO”]− (Figures S38−S41 in the Supporting Information, respectively). Remarkably, both the 1 H and 13C NMR resonances from ligands and from both bridging Zr(μ-Me)2Al and terminal AlMe2 groups were found to be the same as those for the corresponding discrete AlMe3 adducts [6a,b]+[MeB(C6F5)3]− and [6b]+[B(C6F5)4]−, respectively. This suggests that [6a,b]+[MeB(C6F5)3]−, [6b]+[B(C6F5)4]−, and [6a,b]+[“MeMAO”]−, though generated by different activation protocols, are all OSIPs with minimal cation−anion association/interaction.45 Similar reactions to prepare the bis(indenyl) analogue [6c]+[“MeMAO”]− starting from 1c and MAO gave rise to an incompletely identified product. The 1H NMR spectrum (Figure S43 in the Supporting Information), obtained shortly after mixing the reagents, contained broadened resonances at δ −1.18 (13C NMR δ 33.7 ppm) and −1.51 ppm (13C NMR δ −6.5 ppm), assigned to the Zr(μ-Me)2Al and AlMe2 groups, respectively. The chemical shifts of these methyl signals as well as those of signals from the ligand fragment in [6c]+[“MeMAO”]− were identical with those of [6c]+[B(C6F5)4]−,26 demonstrating identical or quite similar structures of the heterometallic cationic parts in these two complexes. Complex [6c]+[“MeMAO”]− evolves slowly with time at room F

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range of values reported before for other contact ion- pairs of this type.11−13,15 As anticipated from the presence of a more electropositive metal center in the ISIP 3b-MeB(C6F5)3 in comparison to the neutral parents 1b and 2b (Figure S46 in the Supporting Information), a slight shortening of the Zr−CpCent anf Zr−FluCent distances and a more pronounced tendency for coordination of the fluorenyl central five-membered ring to the metal atom toward a reduced η3 mode (Table S2) were the only consequences of this transformation. Large red crystals of complexes 3c-MeB(C6F5)3·2C7H8 and 3c-MeAl(C6F5)3·C7H8 were obtained from concentrated toluene solutions at room temperature. The crystal structures of both 3c-MeB(C6F5)3 (Figure 2) and 3-MeAl(C6F5)3 (see

temperature to another unidentified species and methane (see the Supporting Information). This reactivity pattern parallels previous observations made by Brintzinger et al.46 for the same 1c/MAO combination and also by us for the reaction of [5c]+[B(C6F5)4]− with excess AlMe3 (Scheme 5).26 In that latter case, the formation of unprecedented 1:1 mixed crystals of the anticipated product [6c]+[B(C6F5)4]− and [7c]+[B(C6F5)4]−, the product of a C−H activation reaction involving one of the two Zr(μ-Me)2Al bridging methyl groups in [6c]+[B(C6F5)4]− and an additional AlMe3 molecule and eliminating a molecule of CH4, was evidenced to take place in toluene-d8/o-F2-benzene solutions. The above results indicate the high propensity of the {SBI} (c) ionic complexes to undergo side reactions in the presence of excess AlMe3. The effective hydrodynamic radii of ion pairs [6a− c]+[“MeMAO”]− were estimated by PGSE NMR experiments using Si(SiMe3)4 as reference (see the Supporting Information); the corresponding values (Table 1) are indicative of a certain degree of aggregation and/or strong association of the cationic and anionic parts.47 Solid-State Structures of Inner-Sphere Ion Pairs 3b,cMeB(C6F5)3 and 3c-MeAl(C6F5)3. Deep purple crystals of the major isomer of 3b-MeB(C6F5)3·0.5(toluene)·0.5(hexane) were grown from a concentrated solution in toluene/hexane (3/1) at −30 °C. The molecular solid-state structure of 3bMeB(C6F5)3 is depicted in Figure 1, and selected crystallo-

Figure 2. Crystal structure of rac-{Me2Si-(2-Me-4-Ph-Ind)2}ZrMe(μMe)B(C6F5)3 (3c-MeB(C6F5)3.(C7H8)2). Ellipsoids are drawn at the 50% probability level. All hydrogen atoms, except those of the BMe group, and toluene molecules are omitted for clarity; only one enantiomer is depicted.

Figure S48 in the Supporting Information) show the expected zwitterionic arrangements. No substantial change is noted in the coordination of the bis(indenyl) ligand to the metal center within the series 1c-, 2c-, 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3. The Zr−MeE distances in 3c-MeB(C6F5)3 and 3cMeAl(C6F5)3 are in the range for those observed in related zirconocenium−MeB(C 6 F 5 ) 3 and zirconocenium−MeAl(C6F5)3 contact ion pairs.11−13,15 In agreement with the previously observed trend,11h some shortening of the Zr(1)− C(15) distance in 3c-MeAl(C6F5)3 vs that in 3c-MeB(C6F5)3 (2.522(3) vs 2.565(3) Å, respectively) was noticed. This is consistent with the higher Lewis acidity and higher methide abstraction character of B(C6F5)3. Propylene Polymerization Studies. A series of catalytic propylene polymerization tests were carried out in order to assess, in a uniform manner, different activation protocols for the two families of metallocene precatalysts. Homogeneous polymerizations were conducted in toluene at 5 bar of constant pressure and at 60 °C (see the Supporting Information).48 The following protocols were considered depending on the nature of (pre)catalyst: (i) activation of dichloro-zirconocenes 1a−c with MAO, (ii) in situ activation of dimethylmetallocenes 2a−c with either MAO or the [Ph3C]+[B(C6F5)4]−/TIBAl (1/200), [PhNHMe2]+[B(C6F5)4]−/TIBAl (1/200), or B(C6F5)3/TIBAl (1/200) system, (iii) direct use of the ISIP complexes 3b,cMeB(C6F5)3 and 3b,c-MeAl(C6F5)3 in combination with

Figure 1. Crystal structure of anti-{Ph(H)C-(3,6-tBu2Flu)(3-tBu-5-EtC5H2)}ZrMe(μ-Me)B(C6F5)3 (3b-MeB(C6F5)3·0.5C7H8·0.5C5H12). Ellipsoids are drawn at the 50% probability level. All hydrogen atoms, except those of the BMe group, are omitted for clarity. Only one enantiomer is depicted.

graphic and geometrical parameters are given in Tables S1 and S2 in the Supporting Information, respectively. In the solid state, 3b-MeB(C6F5)3 was found as an associated ion pair, in which the [MeB(C6F5)3]− anion is bound to the metal center via the open face of the metallocenium cation. A similar organization has been reported for a series of zwitterionic zirconium methyl complexes incorporating the MeB(C6F5)3− anion.11−13,15 The unit cell of the C1-symmetric complex 3bMeB(C6F5)3 contains two crystallographically independent enantiomeric molecules, which despite noticeable differences in the orientations of the coordinated anions feature very similar geometries and overall organizations. The essential bond distances and angles in 3b-MeB(C6F5)3 lie within the normal G

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Organometallics

comparable to those obtained with regular MAO-activated catalysts (compare entries 8/18, 8/22, and 28/34, respectively). (c) The use of AlMe3 as scavenger was detrimental in all cases, regardless of the activation protocol. The resulting systems were only marginally or not productive (see Table 3, entries 16/21/38/41). Similarly, the isolated AlMe3 adduct OSIP [6a]+[MeB(C6F5)3]− was found poorly active, even when utilized at high concentration (entries 5/6). These results parallel those obtained for the isolated heterobimetallic [6a− c]+[“MeMAO”]−and suggest that strong binding of AlMe3 onto the cationic metal center occurs during the activation step (vide infra). Hence, those propylene polymerization studies confirmed that the productivities (i.e., activities, lifetimes) of those ion pairs depend greatly on several obvious parameters: (1) the nature of the ligand system, (2) the nature of the anion and the structured of ion pairs (ISIP vs OSIP), and (3) the conditions and mode of activation. We here propose that this polymerization behavior can be accounted for through experiments addressing the stability and mobility (exchange processes) of those ion pairs in solution. Thus, the next sections discuss the solution dynamics of ion pairs and UV/vis spectroscopic studies. Solution Dynamics of Metallocenium-MeE(C6F 5) 3 Inner-Sphere Ion Pairs (E = B, Al). The dynamic behavior of metallocenium−MeE(C6F5)3 (E = B, Al) associated ion pairs in solution has been the subject of numerous studies.11−13,15d,e Two mechanisms of reorganization/exchange (site epimerization) were suggested: lateral side anion exchange (AE) of the entire MeE(C6F5)3− anion and neutral E(C6F5)3 coactivator exchange (CE) between the M−Me groups. To get a better insight into the dynamic behavior of the metallocenium ion pairs 3a,b-MeB(C6F5)3, we studied the EXSY component of the 1H−1H NOESY spectra. These EXSY spectra, recorded in the temperature range 5−45 °C and at varied mixing times (see the Supporting Information), revealed two exchange processes: one between the Zr−Me signals and another one between the B−Me signals assigned to the two isomers (3a(b)-1 and 3a(b)-2); no direct permutations between the Zr−Me and B−Me resonances of the two isomers were observed (Figure 3b). This indicates that, under the given conditions, the exchange involves predominantly a process of ion pair reorganization via lateral side exchange of the MeB(C6F5)3− anion (Scheme 7).50 For 3b-MeB(C6F5)3, a very good agreement was achieved among the rates of exchange of the fluorenyl protons at the ligand sides (Figure 3a), those for the exchange of the Zr−Me signals,13a and those for the exchange of the B−Me signals51 (Table 4), demonstrating concomitant, interconnected events. For 3a-MeB(C6F5)3, the somewhat faster rates of exchange of the Zr−Me signals with respect to those of the ligand signals are diagnostic of the contribution of the CE process in the global exchange. Additional EXSY experiments carried out for 3b-MeB(C6F5)3 revealed that the observed magnetization exchange constants are concentration-dependent. Broken reaction orders of 1.2−1.4 were calculated from the ln(kobs) vs. ln([Zr]) plots. Such fractional reaction orders were previously observed for different families of metallocenium ion pairs and were explained by more complicated exchange processes involving polynuclear species (ion quadruples or higher aggregated species).12e,13a,52 The activation parameters for these exchange reactions were determined from the corresponding first-order rate constants kobs (Table 4) using a standard Eyring-plot analysis. The values

scavengers (TIBAL, AlMe3), and (iv) direct use of the OSIP complexes [4a−c]+[B(C6F5)4]−, [6a]+[MeB(C6F5)3]−, and [6a−c]+[“MeMAO”]− in combination with TIBAL as scavenger. Representative data are summarized in Table 3. The general productivity trends for {Cp/Flu} and {SBI} systems are as follows: 1/MAO > 2/MAO ≈ [4]+[B(C6F5)4]− ≈ [6]+[“MeMAO”]− > 3-MeB(C6F5)3 (3-MeAl(C6F5)3) > [6]+[MeB(C6F5)3]− and 1c/MAO ≈ 2c/MAO ≈ [4c]+[B(C 6 F 5 ) 4 ] − > 3c-MeB(C 6 F 5 ) 3 (3c-MeAl(C 6 F 5 ) 3 ) > [6c]+[“MeMAO”]−, respectively. Several tendencies can be specifically drawn, which evidence the flexibility and limits of the polymerization processes with these catalytic systems. (a) Whatever the activation mode, utilizing molecular activators or MAO or employing isolated discrete ion pairs, the productivities of {Cp/Flu} systems are systematically inferior to those afforded by {SBI} systems. For instance, marginal productivities were observed for {Cp/Flu} complexes 2a,b when B(C6F5)3/TIBAl was used (Table 3, entry 2 and entries 10 and 11, respectively). For these systems, unusually higher amounts of precatalysts had to be used (up to 242 mM) in order to recover amounts of iPP sufficient for analyses (compare entries 10/11). In addition, the iPPs obtained in these experiments featured relatively low molecular weights (Mn = 5.5 kDa) and low stereoregularities ([m4] = 88.7%; entry 10). In contrast, the corresponding systems generated from the {SBI} complex 2c and B(C6F5)3/TIBAl were highly productive, were remarkably stereoselective ([m4] = 98.8%; entry 30), and afforded iPPs with high molecular weights and high melting/ crystallization temperatures. Similar observations were made for the catalytic systems generated from 2b,c and [PhNHMe2]+[B(C6F5)4]−/TIBAl or [Ph3C]+[B(C6F5)4]−/TIBAl, respectively. In particular, the ternary system 2c/[Ph3C]+[B(C6F5)4]−/ TIBAl (entry 33) appeared to be the highest performing system within the whole series of experiments (productivity 155770 kg mol−1 h−1, [m]4 = 97.3%, Tm = 155 °C, Mn = 47.5 kDa). For isolated ISIP complexes and OSIP PhNMe2 adducts, both scavenged with TIBAL, the respective activity trends were 3c-MeAl(C6F5)3 > 3c-MeB(C6F5)3 ≫ 3b-MeB(C6F5)3 > 3bMeAl(C6F5)3 (compare entries 39/37/40/36 and 20/24, respectively) and [4c]+[B(C6F5)4]− ≫ [4b]+[B(C6F5)4]− > [4a]+[B(C6F5)4]− (compare entries 43/26/4, respectively). All the produced iPPs exhibited monomodal, relatively narrow distributions (Mw/Mn = 2.0−3.9), consistent with single-site behavior of the corresponding catalytic systems. (b) In general, in situ activation with MAO leads to the most productive catalytic systems. The isolated MAO-derived cationic complexes [6a−c]+[“MeMAO”]− are, at least, two times less productive in comparison to their in situ generated analogues 1a−c/MAO (compare Table 3, entries 1/7, 8/27, and 28/44−46, respectively); however, the molecular weight characteristics and isotacticity levels for the iPPs obtained in these tests were very similar. The activation of dimethyl derivatives 2b,c with MAO does not lead to catalytic species as productive as those generated from the parent dichlorozirconocenes 1b,c and MAO (compare entries 8/9 and 28/29, respectively). Several attempts to use isolated ion pairs 3b,cMeB(C6F5)3 and 3b-MeAl(C6F5)3 in combination with MAO were also undertaken (entries 18/19/22/23 and 34/35). These experiments were aimed at studying the polymerization outcome of the MAO-derived systems comprising a completely cationized metallocene component.44,49 However, these multicomponent systems exhibited activities inferior to or at best H

DOI: 10.1021/acs.organomet.5b00965 Organometallics XXXX, XXX, XXX−XXX

I

33 34 35 36 37 38 39 40

28 29 30 31 32

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1a 2a 3a-MeB(C6F5)3 f [4a]+[B(C6F5)4]− f [6a]+[MeB(C6F5)3]− f

1 2 3 4 5 6 7

3c-MeAl(C6F5)3f

3c-MeB(C6F5)3f

1c 2c

[4b]+[B(C6F5)4]− f [6b]+[“Me-MAO”]− f

3b-MeAl(C6F5)3 f

3b-MeB(C6F5)3 f

1b 2b

[6a]+[“Me-MAO”]− f

prec

entry

2 2 5 10g 10 10 10 10g

2 2 10 5 10

10 12 11 242 11 11 11 11 11 10 10 313 300 300 10 313 313 300 10 10

11 27 25 25 25 367 25

[Zr] (μmol L−1)

TMA (200) TIBAL (200)

TIBAL (200)

[Ph3C]+[B(C6F5)4]−/TIBAl (1/1000) MAO (500)

MAO (21000) MAO (4000) B(C6F5)3/TIBAl (1/200) [PhNMe2H]+[B(C6F5)4]−/TIBAl (1/400) [PhNMe2H]+[B(C6F5)4]−/TIBAl (1/200)

TIBAL (100) TMA (100) MAO (500) MAO (500) TIBAl (100) TMA (100) TIBAl (200) TIBAl (200)

[PhNMe2H]+[B(C6F5)4]−/TMA (1/200) [Ph3C]+[B(C6F5)4]−/TIBAl (1/200) MAO (500)

MAO (4600) MAO (4100) B(C6F5)3/TIBAl (1/240) B(C6F5)3/TIBAl (1/240) [PhNMe2H]+[B(C6F5)4]−/TIBAl (1/200)

MAO (4800) B(C6F5)3/TIBAl (1/200) TIBAL (200) TIBAL (200) TIBAL (200) TIBAl (45) TIBAL (100)

activator/scavenger (amt (equiv))

Table 3. Propylene Polymerization Experimentsa 60 (65) (65) (65) (44) (64) (65)

60 60 60 60 60 60 60 60

(87) (65) (92) (65) (80) (65) (73) (68)

60 (88) 60 (72) 60 (65) 60 (96) 60 (102)

60 60 (65) 60 (60) 60 (79) 40 (45) 50 (55) 60 (65) 80 (82) 60 (60) 60 (65) 60 (64) 60 (100) 60 (72) 60 (65) 60 (65) 60 (100) 60 (64) 60 (65) 60 (65) 60 (65)

60 60 60 40 60 60

Tpolymb (°C)

23.4 0.6 30.0 0.6 8.3 0 13.2 4.2

21.6 9.1 5.0 33.0 38.9

10.5 9.3 0.2 12.4 5.0 4.8 3.7 0.2 0 3.1 1.4 50.8 3.2 0.3 5.2 62.5 traces 0.6 7.1 4.8

1.4 0.2 0.2 0.9 0.7 2.0 1.8

miPP (g)

155770 4040 85620 780 11100 0 17600 5620

135750 58420 6660 88060 5,810

14000 9900 230 680 6190 5970 4650 310 0 4100 1920 2160 150 14 6930 2660 2 26 9500 6340

1700 95 100 480 380 70 950

prod (kg mol−1 h−1)

157 158

155 160 160 144 137 148 155 157 153 158 157

149 149 no 135 132 149 no nd 147 150 150

152 148 147 145 158 154 149 nd

152 141 143 146 154 142 147

Tmc (°C)

116 116

116 119 117 115 117

108 117 121 110 106

113 114 no 103 103 112 no nd 114 112 111

104 104 106 nd 111 113 111 nd

nd 109 113 112 112 111 111

Tcristc (°C)

85.9 nd

47.5 nd 33.0 56.3 nd

41.0 78.6 145.7 25.9 nd

23.4 nd 7.1 3.2 nd 96.1 nd nd nd 24.2 115.2

75.0 nd 5.5 nd 79.7 38.9 22.7 nd

69.1 nd nd 16.3 20.7 13.8 67.7

Mnd (×103)

2.4 nd

3.0 nd 3.1 3.5 nd

2.0 2.4 2.2 3.9 nd

2.1 nd 2.3 2.5 nd 2.2 nd nd nd 2.1 2.2

2.0 nd 2.0 nd 2.7 2.6 2.3 nd

2.4 nd nd 2.5 2.2 3.6 2.6

Mw/Mnd

98.5 nd

97.3 nd nd 98.4 nd

97.7 nd 98.8 92.5 nd

nd nd 50.8 68.9 nd 91.2 nd nd nd 91.7 91.9

92.0 nd 88.7 nd 95.2 nd 90.2 nd

95.2 nd nd 88.4 95.1 85.8 90.2

[m4]e (%)

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[6c]+[“Me-MAO”]−f

(ΔH⧧298 = 9−27 kcal mol−1; ΔS⧧298 = −25 to +4 cal mol−1 K−1) are in the range of those typically reported for similar exchange processes (ΔH⧧298 = 9−27 kcal mol−1; ΔS⧧298 = −26− to +23 cal mol−1 K−1) involving metallocenium ion pairs.11−13 The anion exchange for 3a-MeB(C6F5)3 appeared to proceed slightly faster than that for 3b-MeB(C6F5)3 (Table 4), as evidenced by the difference in activation parameters, especially in the enthalpy values (ΔH⧧298(12) = 10.7(1) kcal mol−1 for 3a-MeB(C6F5)3 vs 17.1(1) kcal mol−1 for 3bMeB(C6F5)3 (toluene-d8)). It should be noted that, as the temperature increased, additional series of very low intensity cross-peaks could be detected in the 1H EXSY spectra, which eventually resulted from the exchange of Zr−Me and B−Me resonances of the two isomers. The latter process, associated with the neutral B(C6F5)3 borane exchange, seems to be less kinetically favored, as it becomes distinguishable only at higher temperatures (typically above 40 °C). Unfortunately, gradual decomposition of the ionic complexes 3a-MeB(C6F5)3 and 3bMeB(C 6F5) 3 precluded accurate measurements of rate constants at higher temperatures (≥35 °C for 3a-MeB(C6F5)3 and ≥45 °C for 3b-MeB(C6F5)3). No line broadening could be detected for these species in the given temperature range. The alane-based ion pair 3b-MeAl(C6F5)3 did not exhibit any visible exchange in toluene-d8 even at elevated temperatures (>60 °C) at which significant decomposition started. Unexpectedly, 3b-MeAl(C6F5)3 also rapidly decayed in the more polar toluene-d8/o-F2-benzene mixture (8/2 v/v) at room temperature, giving unknown products that precluded further investigations. In contrast with the {Cp/Flu} systems, both types of exchange processes were found operational for the {SBI} ion pair 3c-MeB(C6F5)3 (Scheme 8).13a Thus, the cocatalyst exchange process is represented both by signal permutation between the Zr−Me and B−Me groups and by that between hydrogens from the corresponding groups on the ligand sides, whereas only the latter type of exchange contributes to the anion exchange process. These two processes could be efficiently distinguished only if the corresponding signals from the Zr−Me and B−Me group exchange were sufficiently resolved (Table 5). The borane exchange process constituted 20−60% of the global exchange process (e.g., compare kobsLig(275 K) = 0.67(3) s−1 vs kobsZr−Me/B−Me(275 K) = 0.14(2) s−1; [Zr], 28 mM; toluene-d8/o-F2-benzene) with the activation parameters ΔH⧧298 = 22.9(3) kcal mol−1 and ΔS⧧298 = 21(4) cal mol−1 K−1, calculated in the 275−298 K temperature range. As expected for a slower process, these values are somewhat larger than the activation parameters for the overall exchange calculated from the kappLig values in the 275−308 K temperature range (Table 5). Actually, these data are in good agreement with the values ΔH⧧298 = 22(1) kcal mol−1 and ΔS⧧298 = 8(2) cal mol−1 K−1, previously reported for the cocatalyst exchange in rac-{EBI}ZrMe(μ-Me)B(C6F5)3 (toluene-d8; [Zr] = 2 and 10 mM).11h Interestingly, also in contrast with the {Cp/Flu} systems, it appears that for 3c-MeB(C6F5)3 the exchange process is not significantly affected either by the [Zr] concentration or by polarity of the media.11 For instance, nearly the same rates were measured for 3c-MeB(C6F5)3 in toluene-d8 at 298 K at lower concentrations ([Zr] = 10.0 mM). A remarkably different behavior was observed for the alanebased analogue 3c-MeAl(C6F 5)3. First, under identical conditions (298 K; [Zr] = 10.0 mM; toluene-d8), the exchange process was found to be 2 orders of magnitude slower (kobs(298

a Polymerization conditions: 300 mL high pressure glass reactor; solvent toluene, 150 mL; P(propylene) = 5 bar; time 30 min. bData in parentheses refer to the maximum temperature (exotherm) reached in the reactor. cDetermined by DSC from second run. dDetermined by GPC. eDetermined by 13C NMR spectroscopy. fFreshly generated in glovebox; clean formation of precatalyst was verified by 1H NMR spectroscopy. gIsolated crystalline product was used.

97.8 nd nd 99.2 nd 2.7 nd nd 2.2 2.4 50.6 nd nd 219.4 45.4 116 111 119 123 120 156 149 158 160 156 0 114460 40920 440 11070 41380 0 17.5 30.7 0.3 3.9 14.5 (65) (80) (90) (65) (65) (80) 60 60 60 60 60 60 TMA (200) TIBAl (1000) TIBAl (200) TIBAl (1000) TIBAl (400) TIBAl (200) 10 2 10 2 5 10 [4c]+[B(C6F5)4]−f

41 42 43 44 45 46

prec entry

Table 3. continued

[Zr] (μmol L−1)

activator/scavenger (amt (equiv))

Tpolymb (°C)

miPP (g)

prod (kg mol−1 h−1)

Tmc (°C)

Tcristc (°C)

Mnd (×103)

Mw/Mnd

[m4]e (%)

Organometallics

J

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Figure 3. Details of the 1H−1H EXSY NMR spectra for the ion pair 3b-MeB(C6F5)3 (T = 298 K, [Zr] = 57 mM): (a) aromatic region, 298 K, τm = 0.5 s, kAE(obs)21 = 1.92(8) s−1; (b) high-field region, τm = 0.5 s, kAE(obs)21 = 2.3(1) s−1.

polymerization for the former system with respect to the latter system. In other words, we surmise that the faster reorganization process for the {SBI}-based system may lay at the origin of its better polymerization performance in comparison to that of the {Cp/Flu}-based analogue. This more rapid process for the {SBI}-based systems may be responsible for maintaining, at a time, a greater amount of active centers available for coordination of monomer. Stable μ-Me bridged dinuclear cationic metallocenium species have been previously documented by Bochmann,16 Brintzinger,20 and Marks11d,53 to form when substoichiometric amounts (vs the neutral dimethylmetallocene) of a cocatalyst are used. We also attempted to generate similar dinuclear cationic complexes via the reactions of 2a (also 2b,c) with B(C6F5)3 using [Zr]/[B] ratios above 1.5/1.54 However, only selective formation of mixtures of 3a-MeB(C6F5)3 and unreacted 2a (or 3b-MeB(C6F5)3 and 2b, 3c-MeB(C6F5)3 and 2c, respectively) was observed by 1H and 19F NMR spectroscopy. These observations are in line with some previously reported results testifying that complexes with sterically demanding ligands can be reluctant to afford binuclear species.20,55 On the other hand, in addition to the direct, rapid exchange between the two isomers in each ion pair, slow exchange between the ionic and the neutral components for the binary mixtures 3a-MeB(C6F5)3/2a, 3b-MeB(C6F5)3/2b, and 3cMeB(C6F5)3/2c was evidenced by 1H−1H EXSY NMR spectroscopy, with cross-peaks between the corresponding Zr−Me signals (see Figure 4 for the 3b-MeB(C6F5)3/2b mixture). In addition, direct permutations between the resonances of the Zr−Me and B−Me groups of the two isomers and those of the neutral metallocene were observed for these three binary systems. This suggests the occurrence, under the given conditions, of a parallel process involving free borane exchange between the ionic and the neutral components (Scheme 9). The latter process is sluggish for the {Cp/Flu} systems (Table S3 in the Supporting Information), while for the 3c-MeB(C6F5)3/2c system (Figure S27 in the Supporting Information) it is the principal exchange process.

Scheme 7. Predominant Site-Epimerization Processes Observed for the {Cp/Flu} ISIPs 3a-MeB(C6F5)3 and 3bMeB(C6F5)3

K) = 0.040(5) s−1) than that observed with the borane congener 3c-MeB(C6F5)3 (Table 5). Addition of o-F2-benzene (20% v/v) resulted in a significant acceleration of the exchange (kobs(298 K) = 1.48(7) s−1). In addition, increasing the [Zr] concentration from 10 mM up to 28 mM afforded a ca. 5-fold acceleration of the exchange (kobs(298 K) = 7.1(2) s−1). In these experiments, the rates of permutation of the Zr−Me and Al−Me groups and those for the hydrogens from the corresponding groups on the ligand sides appeared to be remarkably close; this indicates that only one process, namely, via cocatalyst exchange, is operative for 3c-MeAl(C6F5)3. In contrast to the borate system 3c-MeB(C6F5)3, the alane-based analogue 3c-MeAl(C6F5)3 implies a largely negative activation entropy (ΔS⧧298 = −30(1) cal mol−1 K−1; Table 5), indicative of an associative process. The faster exchange observed for the {SBI}-based system 3cMeB(C6F5)3 with respect to the {Cp/Flu}-based counterparts 3a-MeB(C6F5)3 and 3b-MeB(C6F5)3 (kobs(3c-MeB(C6F5)3)/ kobs(3a-MeB(C6F5)3 or 3b-MeB(C6F5)3) = 1.5−5 s−1) is in good agreement with previous observations by Bochmann et al. on the related metallocenium systems {Me2Si(Ind)2}Zr(CH 2 SiMe 3 )(Me)/coactivator and {Me 2 C(Flu)(Cp)}Zr(CH2SiMe3)(Me)/coactivator (where coactivator = B(C6F5)3, [Ph3C]+[B(C6F5)4]−).12e This increase in exchange rates on going from {SBI}- to {Cp/Flu}-based systems can be correlated with the higher productivity observed in propylene K

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Table 4. EXSY-Derived Apparent Rate Constants and Activation Parameters for Exchanges in Complexes 3a-MeB(C6F5)3 and 3b-MeB(C6F5)a rate constant ligand complex 3aMeB(C6F5)3

[Zr] (mM)

T (K)

56.4

278 288 298 308

3bMeB(C6F5)3

57.3

318 278 288 298 308 318

41.8

298

28.0

298

28.0e

298

kobs21, kobs12 (s−1) 0.63(4), 0.077(4) 1.5(3), 0.17(4) 3.6(3), 0.53(4) 4.9(3), 0.61(4) ndd 0.184(8), 0.027(1) 0.63(1), 0.097(3) 1.92(8) 0.30(2) 5.6(4), 0.81(6) 9.8(7), 1.41(9) 1.65(7), 0.254(8) 1.19(1), 0.183(2) 2.9(6), 0.38(8)

Zr−Me kapp b (M−1 s−1) 11.5(7) 27(6) 66(6), 89(6)

ndd 3.2(1) 11.0(2) 34(1) 98(7) 171(12) 40(2) 42.5(4) 104(21)

kobs21, kobs12 (s−1)

kapp b (M−1 s−1)

1.14(3), 0.092(5) 2.9(1), 0.29(2) 5.7(4), 0.59(4) 7.6(5), 0.71(4) 15(1), 1.8(1) 0.194(6), 0.023(2) 0.68(1), 0.093(7) 2.3(1) 0.325(6) 6.2(4), 0.95(7) 12.4(9), 1.8(1) 1.81(1), 0.268(2) 1.22(3), 0.184(3) 3.8(6), 0.58(5)

20.7(6)

ndc

53(2)

ndc

104(7)

ndc

138(9)

ndc

E−Me kobs21, kobs12 (s−1)

273(18) 3.4(1)

ndd 0.14(2), 0.020(1)

11.9(2)

ndc

40(2)

ndc

108(7)

ndc

216(16)

10.2(7), 2.1(2)

43.3(2)

ndc

44(1)

ndc

136(21)

ndc

ΔH⧧f (kcal mol−1), ΔS⧧ (cal mol−1 K−1), ΔG⧧298 (kcal mol−1) 10.7(1), −19(2), 16.5(1)

17.6(1), 2(1), 17.1(1)

a See the Supporting Information for details. bValues derived from kapp = kobs21/[Zr] = kobs12/[Zr]. cIntegration was hampered due to overlapping of diagonal peaks. dSome degradation was observed at higher temperatures during EXSY experiments. eToluene-d8/o-F2-benzene was used as solvent. f Calculated from kobs using Eyring plot analysis.

for the binary systems 3a−c-MeB(C6F5)3/2a−c are also concentration-dependent, as are those for the discrete counterparts 3a−c-MeB(C6F5)3.57 UV/Vis Spectroscopy Studies and TD-DFT Calculations. We also focused our attention onto UV/vis spectroscopy, which is a convenient technique for assessing the nature, dynamics, and stability of ionic metallocene complexes in solution.5,6,44 In addition, the relationship between activities of some {Cp/Flu}-zirconocenes catalyst and their UV/vis absorption bands λmax has been reported.58 Activation of dichlorozirconocene 1b (λmax 505 nm) with MAO (300 equiv) in toluene at room temperature afforded within 30 s a highly electrophilic green species, exhibiting a ligand-to-metal charge transfer (LMCT) band with λmax 668 nm (Figure 5). This bathochromic shift is, in terms of energy (ΔE = −13.8 kcal mol−1), somewhat larger than those reported for syndioselective {Cp/Flu}-metallocene/MAO systems (ΔE = −8 to −6 kcal mol−1).58,59 Within several minutes, the reaction mixture turned deep blue, i.e. the band at λmax 668 nm decreased with concomitant increase of a new one at λmax 573 nm, which is an energy change ΔE = −6.7 kcal mol−1 with respect to the initial band for 1b. This new absorption band, unambiguously attributed to [6b]+[“MeMAO”]−,60 remained unchanged over the course of several hours. A kinetic monitoring of the activation process ([1b]0 = 0.45 mM, [MAO]/[Zr] = 500, room temperature) by UV/vis spectroscopy (via the decrease and increase of the intensities of bands at λmax 668 and 573 nm, respectively; Figure S51 in the Supporting Information) allowed estimating the first-order rate constant kapp = [1.78(2)] × 10−3 s−1 and half-lifetime t1/2 =

Scheme 8. Site-Epimerization Processes Observed for the {SBI} ISIPs 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3, Respectively

Overall, the kinetic and thermodynamic data obtained for these three binary systems (Tables S3 and S4 in the Supporting Information, respectively) allowed us to draw the following conclusions: (a) the exchange for 3c-MeB(C6F5)3/2c is faster than that for the 3a-MeB(C6F5)3/2a and 3b-MeB(C6F5)3/2b analogues; (b) the anion exchange is the predominant process for the {Cp/Flu} systems,56 whereas for the {SBI} systems reorganization occurs essentially via the cocatalyst exchange; (c) the anion exchange for binary systems 3a−c-MeB(C6F5)3/ 2a−c proceeds with nearly the same rates in comparison with discrete complexes 3a−c-MeB(C6F5)3; (d) the exchange rates L

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Organometallics

Table 5. EXSY-Derived Apparent Rate Constants and Activation Parameters for Exchanges in Complexes 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3a ligand rate constant

complex 3cMeB(C6F5)3

[Zr] (mM) 28.0

d

T (K) 275

10.0 28.0d

298 298

0.67(3), 0.35e 2.43(2), 1.3e 6.9(2), 6.6e 24.7(3), 20e 54.1(1), 59e 6.1(1) 7.1(2)

9.8d

275 288 298 308 318 298

0.32(1) 0.71(9) 1.48(7) 2.0(2) 2.6(2) 0.040(5)

288 298 308 318

3cMeAl(C6F5)3

kobs (s−1)

10.0

kappb (M−1 s−1) 24(1)

Zr/E−Me rate constant ΔH⧧ f (kcal mol−1), ΔS⧧ (cal mol−1 K−1), ΔG⧧298 (kcal mol−1) 17.6(1), 5(1), 16.2(1)

87(1)

kobs (s−1)

kapp b (M−1 s−1)

ΔH⧧ f (kcal mol−1), ΔS⧧ (cal mol−1 K−1), ΔG⧧298 (kcal mol−1)

0.14(2)

5 (1)

22.9(3), 21(4), 16.7(2)

0.7(3)

25(11)

246(7)

4(4)

143(143)

882(11)

ndc

ndc

1932(4)

ndc

ndc

610(10) 254(7)

4(2)c 11.3(1)

400(200) 404(4)

8.3(1), −30(1), 17.3(1)

33(1) 73(9) 151(7) 204(20) 265(20) 4.0(5)

0.31(3) 0.76(5) 1.48(7) 2.1(2) 2.7(3) 0.098(1)

32(3) 78(5) 151(7) 214(20) 276(31) 9.8(1)

-

8.1(1), −31(1), 17.3(1)

a

See the Supporting Information for details. bValues derived from kapp = kobs/[Zr]. cIntegration was hampered due to overlapping of diagonal peaks. d Toluene-d8/o-F2-benzene was used as solvent. eCalculated from line-shape analysis of the corresponding resonance. fCalculated from kobs using Eyring plot analysis.

stantly forms, which then slowly rearranges intramolecularly to the new OSIP heterobimetallic complex [6b]+[“MeMAO”]−. A quite similar observation has been reported by Miller et al. for the related system {Ph2C-(Oct)(Cp)}ZrCl2/MAO (Oct = octamethyloctahydrodibenzofluorene),58 which is a rapid red shift from λmax 521 nm (neutral dichlorozirconocene) to λmax 622 nm (a new species with half-life t1/2 = 29−32 min) followed by a relaxation to λmax 590 nm;62 in comparison, the nonsubstituted system {Ph2C-(Flu)(Cp)}ZrCl2/MAO appeared to be less electron deficient and, upon activation, exhibited a transition from λmax 499 to 557 nm without further evolution.58 The authors proposed a different explanation for this phenomenon that is apparently similar to ours: it was interpreted in terms of the larger bulkiness of the {Cp/Oct} system, which would in turn induce a greater metallocenium cation−[“MeMAO”]− anion separation and, thus, a more electrophilic cationic center. This was proposed to account for the (at least) 1 order of magnitude higher polymerization productivity of the bulkier {Ph2C-(Oct)(Cp)}ZrCl2/MAO system in comparison to that of {Ph2C-(Flu)(Cp)}ZrCl2/ MAO.58 To shed more light on the electrophilic nature of the cationic species that form during the course of activation with MAO (vide supra), we performed an additional set of UV/vis experiments for the b zirconocene series using molecular activators instead of MAO. Hence, methylation of the dichlorozirconocene 1b to give 2b resulted in a hypsochromic shift to λmax 454 nm (ΔE1b→2b = 6.4 kcal mol−1) (Figure S55 in the Supporting Information). The activation of dimethylmetallocene 2b with 1 equiv of B(C6F5)3 or Al(C6F5)3, which formed the electron-deficient complexes 3b-MeB(C6F5)3 and 3b-MeAl(C6F5)3, respectively, resulted in red-shifted bands at λmax 522 nm (ΔE2b→3b‑MeB(C6F5)3 = −8.2 kcal mol−1) and 533 nm (ΔE2b→3b‑MeAl(C6F5)3 = −9.3 kcal mol−1), respectively. Then,

Figure 4. Details of the 1H−1H EXSY NMR spectrum of the 3bMeB(C6F5)3/2b mixture showing cross-peaks arising from the exchange between the Zr−Me groups of the two isomers of the ion pair and those of the neutral metallocene and between the Zr−Me and B−Me groups (toluene-d8, T = 298 K, [Zr] = 55.2 mM, [B] = 36.0 mM); τm = 0.5 s, kAE21 = 1.63(5) s−1.

6.5 min (Figure S52 in the Supporting Information).61 These data suggest that, in the first activation step, the highly electrophilic OSIP complex [{Ph(H)C-(3,6-tBu2-Flu)(3-tBu-5Me-Cp)}Zr(Me)]+[“Me-MAO”]− ([5b]+[“MeMAO”]−) inM

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Organometallics Scheme 9. Different Exchange Processes Proposed for the ISIP {R1R2-(Flu)(Cp)}metallocenium-MeB(C6F5)3

Figure 5. Time course of UV−vis absorbance changes for the reaction of 1b (λmax 505 nm) with MAO (500 equiv) (toluene, 25 °C, [Zr] = 0.45 mM) toward [6b]+[“MeMAO”]− (λmax 573 nm); isosbestic points at λ 480 and 620 nm.

{Me2Si(Ind)2}Zr(Me)(μ-“MeMAO”) with λmax 458 nm, which red-shifted to λmax 493 nm upon addition of excess AlMe3. The latter absorption value was assigned to the heterobimetallic OSIP [{rac-{Me2Si-(Ind)2}Zr(μ-Me2)AlMe2]+[“MeMAO”]−. In our case, the treatment of 1c with MAO (300 equiv) in toluene at room temperature resulted instantaneously in a species with λmax 523 nm (ΔE1c→6c = −6.7 kcal mol−1; Figure S55 in the Supporting Information). The latter band is redshifted with respect to that of [{rac-{Me2Si-(Ind)2}Zr(μMe2)AlMe2]+[“MeMAO”]− by 30 nm,6h which is consistent with the more electrophilic character of the 4,4′-diphenylsubstituted 1c in comparison to its nonsubstituted analogue. Further “aging” of this mixture over more than 9 h at room temperature or upon addition of AlMe3 (25 equiv vs Zr) did not result in any visible change. An electrophilicity trend quite similar to that found for the b series was observed for the c series counterparts when replacing MAO by molecular activators (Figure S56 in the Supporting Information). Thus, a hypsochromic shift from λmax 466 to 410 nm, associated with the generation of 2c (ΔE1c→2c = 8.4 kcal

the addition of an excess of AlMe3 (5 equiv) to 3b-MeB(C6F5)3 afforded again a bathochromic shift at λmax 534 nm (ΔE3b→6b = −1.2 kcal mol−1) (Figure S55), which is consistent with the formation of the OSIP [6b]+[MeB(C6F5)3]−.5a,44 The above observations enable us to propose a more unequivocal conclusion on the dynamics and structure of the ion pairs that form upon activation of {Cp/Flu}- and closely related {Cp/Oct}-based metallocenes with MAO (and molecular activators as well): the degree of the cation−anion separation responsible for the polymerization ability of ion pairs is determined by the propensity of the “naked” metallocenium cation [{Cp/Flu}ZrMe]+ (kinetic product) to bind a “free” AlMe3; the bulkier the metallocenium core, the slower the reaction leading to the corresponding heterobimetallic OSIP AlMe3 adduct [{Cp/Flu}Zr(μ-Me)2AlMe2]+ (thermodynamic product). Regarding {SBI}-type species, in earlier studies by Brintzinger and Bochmann,6h,63 it has been shown that the activation of rac-{Me2Si-(Ind)2}ZrCl2 with AlMe3-depleted MAO ([Al]/[Zr] > 120) afforded the contact ion pair racN

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Organometallics

Table 6. Absorption Bands (λmax) of LMCT (Experimental and TD-DFT Calculated), Oscillator Strengths f, Main Transitions, HOMO−LUMO Gaps, and Dipole Moments for Neutral and Ionic Metallocenesa λmax (nm) complex

exptl

calcd

f

[5a]+[“Me-MAO”]−

585

1b 2b 3b-MeB(C6F5)3 3b-MeAl(C6F5)3 [6b]+[MeB(C6F5)3]−

505 454 522 533 534

425 500 483 538

0.05 0.05 0.06 0.04

[6b]+[B(C6F5)4]−

580

561

0.04

[6b]+[“Me-MAO”]−

573

1c 2c 3c-MeB(C6F5)3 3c-MeAl(C6F5)3 ([6c]+[MeB(C6F5)3]−)

466 408 493 480 410d

397 457 446 510

0.14 0.10 0.10 0.09

([6c]+[MeAl(C6F5)3]−)

410d

505

0.09

546

524

0.09

505

0.08

[6c]+[B(C6F5)4]− [7c]+[B(C6F5)4]− [6c]+[“Me-MAO”]−

EHOMO (eV)

ELUMO (eV)

HOMO−LUMO gap (eV)

dipole moment (D)

HOMO → LUMO (97%) HOMO → LUMO (97%) HOMO → LUMO (98%) HOMO-3 → LUMO (95%) HOMO-3 → LUMO (97%)

−5.26 −5.97 −5.95 −6.18 (−6.46)b

−1.30 −2.36 −2.25 −2.97

3.96 3.61 3.70 3.21 (3.47)c

1.3 14.7 13.9 29.9

−6.24 (−6.46)b

−3.05

3.19 (3.41)c

31.4

HOMO → LUMO (95%) HOMO → LUMO (94%) HOMO → LUMO (94%) HOMO-3 → LUMO (94%) HOMO-2 → LUMO (94%) HOMO-4 → LUMO (94%) HOMO-4 → LUMO (94%)

−5.58 −6.12 −6.09 −6.15 (−6.50)b

−1.43 −2.38 −2.31 −3.05

4.15 3.74 3.78 3.10 (3.45)c

1.9 14.3 13.6 29.3

−6.34 (−6.49)b

−3.02

3.32 (3.47)c

29.2

−6.22 (−6.60)b

−3.20

3.02 (3.42)c

32.8

−6.20 (−6.62)b

−3.05

3.15 (3.55)c

33.7

main transition (weight (%))

523

a

See the Supporting Information for the details of TD-DFT calculations. bEnergy level of the corresponding (HOMO-n) orbital is given. cThe corresponding (HOMO-n)−LUMO gap is given. dVery fast decomposition was observed.

mol−1), was followed by strong bathochromic shifts at λmax 493 nm (ΔE2c→3c‑MeB(C6F5)3 = −11.8 kcal mol−1) and 480 nm (ΔE2c→3c‑MeAl(C6F5)3 = −10.2 kcal mol−1) when electrophilic 3cMeB(C6F5)3 and 3c-MeAl(C6F5)3, respectively, were generated. Further treatment of both 3c-MeB(C6F5)3 and 3cMeAl(C6F5)3 with AlMe3 (5 equiv) led instantly to unidentified products apparently resulting from the scrambling reaction of Me/C6F5 groups (vide supra), whose bands relaxed back to λmax 410 nm (Figure S56). No stable analogues of [6b]+[MeB(C6F5)3]− (namely, [6c]+[MeB(C6F5)3]− and [6c]+[MeAl(C6F5)3]−, respectively) or other intermediates could be detected for this process. By analogy with the b series, the λmax absorption bands for the OSIP [6c]+[B(C6F5)4]− and [6c]+[“MeMAO”]− (546 and 523 nm, respectively; Figure S56) again suggest their structural and electronic similarity. TD-DFT Computations. We then sought additional clues about the electronic structure of the above ionic species and carried out time-dependent DFT (TD-DFT) computations to identify unambiguously the nature of the observed absorption bands.64 The methodology used is reported in the Supporting Information In general, the calculated LMCT absorption bands λmax for all complexes (Table 6) are in good agreement with the experimental results. The values calculated for the neutral 2b,c and the ion pairs 3b,c-MeB(C6F5)3 and 3b,c-MeAl(C6F5)3 are due to the HOMO → LUMO transitions. As expected, the HOMOs of both neutral and ionic complexes of the b and c series (Figures S57 and S59 in the Supporting Information, respectively) are mainly localized on the fluorenyl and indenyl ligands, respectively, with some minor contribution from zirconium (8−15%). On the other hand, the LUMOs are

largely centered on the Zr metal centers (31−58%) with some participation from the ligands. In line with the higher electrophilic character of the metal centers in ithe onic complexes 3b,c-MeB(C6F5)3 and 3b,c-MeAl(C6F5)3, their calculated HOMO−LUMO gaps are ca. 0.3 eV narrower than those for the parent neutral zirconocenes. In addition, frontier orbital calculations (Table 6) demonstrated stabilization of both the HOMO and LUMO in the ionic complexes 3b,cMeB(C6F5)3 and 3b,c-MeAl(C6F5)3 with respect to the corresponding neutral parent complexes. However, the LUMOs of 3c-MeB(C6F5)3 and 3c-MeAl(C6F5)3 are slightly more stabilized (0.02−0.06 eV) in comparison to those of 3bMeB(C6F5)3 and 3b-MeAl(C6F5)3, and the HOMO−LUMO gaps are somewhat larger for 3c-MeB(C6F5)3 and 3cMeAl(C6F5)3 in comparison to those calculated for 3bMeB(C6F5)3 and 3b-MeAl(C6F5)3. These trends indicate the higher electrophilicity of the latter b {Cp-Flu} species in comparison to the c {SBI} series. The electronic structures of the heterobimetallic complexes [6b]+[MeB(C6F5)3]− and [6b]+[B(C6F5)4]− are of particular interest. Unlike the case for 2b and 3b-MeB(C6F5)3, the computed λmax bands for [6b]+[MeB(C6F5)3]− and [6b]+[B(C6F5)4]− at 538 and 561 nm, respectively, result from more energetic HOMO-3 → LUMO excitations (also assigned to LMCT), which are responsible for the deep blue color of these species. From these results in the {Cp-Flu} series, we can establish a general trend that the contribution of zirconium in the LUMO increases on going from 2b to 3b-MeB(C6F5)3 and to [6b]+[MeB(C6F5)3]− and [6b]+[B(C6F5)4]−. This tendency is also in line with the growing electrophilicity of the metal center within this series, associated with stabilization of both O

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Organometallics the HOMO and LUMO and narrowing of the HOMO− LUMO gaps. The same trend is characteristic for the analogous complexes of the {SBI} series (including the putative [6c]+[MeB(C6F5)3]− and [6c]+[MeAl(C6F5)3]−).65 Of note, the heterobimetallic [6c]+[B(C6F5)4]− and its degradation product, the heterotrimetallic [7c]+[B(C6F5)4]−, appeared to feature the same calculated main transitions and very close calculated λmax bands; thus, both could hardly be distinguished experimentally using UV/vis spectroscopy. One more observation is that the calculated dipole moments for the above complexes (Table 6) are in agreement with their increasing ionic character on going from neutral metallocenes 2b,c to ISIPs 3b,c-MeB(C6F5)3 and 3c-MeAl(C6F5)3 and, then, to OSIPs [6b] +[MeB(C6F5)3]−, [6b]+[B(C6F5)4]−, and [6c] + [B(C 6 F 5 ) 4 ] − /[7c] + [B(C 6 F 5 ) 4 ] − (and putative [6c]+[MeB(C6F5)3]− and [6c]+[MeAl(C6F5)3]−).

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AUTHOR INFORMATION

Corresponding Authors

*J.-F.C.: e-mail, [email protected]. *E.K.: fax, (+33)(0)223-236-939; e-mail, evgueni.kirillov@ univ-rennes1.fr. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Total Raffinage-Chimie (Ph.D. grants to G.T., M.B., and N.M.). We thank Dr. J.-M. Brusson, Dr. O. Miserque, Dr. A. Vanthomme, and Dr. A. Welle (Total Research) for fruitful discussions. C. Orione and Dr. S. Sinbandhit (CRMPO, UR1) are gratefully acknowledged for technical assistance in some NMR spectroscopy experiments. We thank S. Boyer (London Metropolitan University) for elemental analyses.

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CONCLUSIONS The factors controlling the formation and stability of ionic complexes of both ISIP and OSIP types, derived from isoselective metallocene precatalysts of the two different {Cp/ Flu} and {SBI} families, have been investigated by a combination of experimental and theoretical methods. Although all the investigations were carried out in the absence of monomer (e.g., propylene) and can be considered to eventually model only the first activation step, the results unveil the possible origin of the higher polymerization productivity of the {SBI}-based catalysts. UV/vis spectroscopy studies and TD-DFT calculations suggest that the cations derived from the {Cp/Flu} metallocenes exhibit a more electrophilic character and hence tend to form more robust ionic adducts (both of ISIP and OSIP nature) with AlMe3 as compared to their {SBI}based congeners. In particular, heterobimetallic complexes with a Zr(μ-Me)AlMe2 core incorporating various anions ([MeB(C6F5)3]−, [B(C6F5)4]−, [“Me-MAO”]−), generated by different protocols implying AlMe3, were systematically found to be much more stable than their {SBI}-zirconocene congeners. More generally speaking, we assume this is likely to be true also with different Lewis basic substrates (monomer, impurities). On the other hand, the study by NMR techniques of ISIP 3b,c-MeE(C6F5)3 (E = B, Al) and OSIP [6b,c]+[B(C6F5)4]− complexes allowed us to demonstrate that the {SBI}-based ionic adducts are significantly more dynamic in solution than the {Cp/Flu} analogues. These exchange phenomena, implying rearrangement of ligands in the coordination sphere of the metal center and presumably operating as a part of a global polymerization mechanism, may be responsible for generation of larger amounts of active initiating species in the case of {SBI}-based catalysts. More detailed investigations on the polymerization kinetics for both metallocene systems are currently under way in our laboratories.

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computational details), NMR and UV−vis spectra, additional characterization data, and kinetic plots (PDF)

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00965. Crystallographic data for CCDC 1433492−1433496 (2b,c, 3b,c-MeB(C6F5)3, and 3c-MeAl(C6F5)3, respectively) (CIF) Experimental section (general considerations, instruments, methods and measurements, synthetic and P

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Organometallics Brintzinger, H. H. Chem. - Eur. J. 2007, 13, 5294−5299. (l) Song, F.; Cannon, R. D.; Lancaster, S. J.; Bochmann, M. J. Mol. Catal. A: Chem. 2004, 218, 21−28. (m) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A.; Brand, J.; Alonso-Moreno, C.; Bochmann, M. J. Organomet. Chem. 2007, 692, 859−868. (7) Macchioni, A. Chem. Rev. 2005, 105, 2039−2073. (8) (a) Bryliakov, K. P.; Babushkin, D. E.; Talsi, E. P.; Voskoboynikov, A. Z.; Gritzo, H.; Schroder, L.; Damrau, H.-R. H.; Wieser, U.; Schaper, F.; Brintzinger, H. H. Organometallics 2005, 24, 894−904. (b) Bryliakov, K. P.; Semikolenova, N. V.; Panchenko, V. N.; Zakharov, V. A.; Brintzinger, H.-H.; Talsi, E. P. Macromol. Chem. Phys. 2006, 207, 327−225. (c) Lyakin, O. Y.; Bryliakov, K. P.; Panchenko, V. N.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 2007, 208, 1168−1175. (9) In a few cases, species III appeared to be inaccessible due to steric reasons and activation reaction afforded prevailingly the AlMe3-free species IV. See: (a) Reference 6g. (b) Bryliakov, K. P.; Talsi, E. P.; Bochmann, M. Organometallics 2004, 23, 149−152. (10) The intermediacy of heterobimetallic species III in chaintransfer polymerization reactions has been reviewed. See: Valente, A.; Mortreux, A.; Visseaux, M.; Zinck, P. Chem. Rev. 2013, 113, 3836− 3857. (11) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623−3625. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015−10031. (c) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 842−857. (d) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1772−1784. (e) Chen, Y.X. E.; Metz, M. W.; Li, L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 6287−6305. (f) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A. S.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448−1464. (g) Chen, M.-C.; Roberts, J. A. S.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 4605−4625. (h) Stahl, N. G.; Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10898−10909. (12) (a) Bochmann, M.; Lancaster, S.; Hursthouse, M. B.; Abdul Malik, K. M. Organometallics 1994, 13, 2235−2243. (b) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223−237. (c) Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey, S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics 2005, 24, 1315−1328. (d) Alonso-Moreno, C.; Lancaster, S. J.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. J. Am. Chem. Soc. 2007, 129, 9282− 9283. (e) Alonso-Moreno, C.; Lancaster, S. J.; Wright, J. A.; Hughes, D. L.; Zuccaccia, C.; Correa, A.; Macchioni, A.; Cavallo, L.; Bochmann, M. Organometallics 2008, 27, 5474−5487. (13) (a) Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H.H. J. Am. Chem. Soc. 2001, 123, 1483−1489. (b) Beck, S.; Prosenc, M.H.; Brintzinger, H.-H. J. Mol. Catal. A: Chem. 1998, 128, 41−52. (14) (a) Landis, C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem. Soc. 2003, 125, 1710−1711. (b) Moscato, B. M.; Zhu, B.; Landis, C. R. J. Am. Chem. Soc. 2010, 132, 14352−14354. (c) Moscato, B. M.; Zhu, B.; Landis, C. R. Organometallics 2012, 31, 2097−2107. (15) (a) Horton, A. D. Organometallics 1996, 15, 2675−2677. (b) Mohammed, M.; Nele, M.; Al-Humydi, A.; Xin, S.; Stapleton, R. A.; Collins, S. J. Am. Chem. Soc. 2003, 125, 7930−7941. (c) Razavi, A.; Bellia, V.; De Brauwer, Y.; Hortmann, K.; Peters, L.; Sirole, S.; Van Belle, S.; Marin, V.; Lopez, M. J. Organomet. Chem. 2003, 684, 206− 215. (d) Al-Humydi, A.; Garrison, J. C.; Youngs, W. J.; Collins, S. Organometallics 2005, 24, 193−196. (e) Al-Humydi, A.; Garrison, J. C.; Mohammed, M.; Youngs, W. J.; Collins, S. Polyhedron 2005, 24, 1234−1249. (f) Theurkauff, G.; Roisnel, T.; Waassenaar, J.; Carpentier, J.-F.; Kirillov, E. Macromol. Chem. Phys. 2014, 215, 2035−2047. (g) Chen, C.-H.; Shih, W.-C.; Hilty, C. J. Am. Chem. Soc. 2015, 137, 6965−6971. (16) Bochmann, M.; Lancaster, S. J. Angew. Chem. 1994, 106, 1715− 1718. (17) Shaughnessy, K. H.; Waymouth, R. M. Organometallics 1998, 17, 5728−2745.

(18) (a) Petros, R. A.; Norton, J. R. Organometallics 2004, 23, 5105− 5107. (b) Camara, J. M.; Petros, R. A.; Norton, J. R. J. Am. Chem. Soc. 2011, 133, 5263−5273. (19) (a) Chen, Y.-X.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 12451−12452. (b) See ref 11e. (20) Beck, S.; Prosenc, M.-H.; Brintzinger, H.-H.; Goretzki, R.; Herfert, N.; Fink, G. J. Mol. Catal. A: Chem. 1996, 111, 67−69. (21) Lenton, T. N.; Bercaw, J. E.; Panchenko, V. N.; Zakharov, V. A.; Babushkin, D. E.; Soshnikov, I. E.; Talsi, E. P.; Brintzinger, H.-H. J. Am. Chem. Soc. 2013, 135, 10710−10719. (22) Use of Al(iBu)2H in the activation process has been shown to give rise to various zirconocene-aluminum hydrido intermediates and products: (a) Baldwin, S. M.; Bercaw, J. E.; Brintzinger, H.-H. J. Am. Chem. Soc. 2008, 130, 17423−17433. (b) Baldwin, S. M.; Bercaw, J. E.; Brintzinger, H.-H. J. Am. Chem. Soc. 2010, 132, 13969−13971. (c) Baldwin, S. M.; Bercaw, J. E.; Henling, L. M.; Day, M. W.; Brintzinger, H.-H. J. Am. Chem. Soc. 2011, 133, 1805−1813. (23) (a) Razavi, A.; Thewalt, U. Coord. Chem. Rev. 2006, 250, 155− 169. (b) Razavi, A.; Thewalt, U. J. Organomet. Chem. 2001, 621, 267− 276 and references cited therein. (24) (a) Kirillov, E.; Marquet, N.; Razavi, A.; Belia, V.; Hampel, F.; Roisnel, T.; Gladysz, J. A.; Carpentier, J.-F. Organometallics 2010, 29, 5073−5082. (b) Kirillov, E.; Marquet, N.; Bader, M.; Razavi, A.; Belia, V.; Hampel, F.; Roisnel, T.; Gladysz, J. A.; Carpentier, J.-F. Organometallics 2011, 30, 263−272. (c) Bader, M.; Marquet, N.; Kirillov, E.; Roisnel, T.; Razavi, A.; Lhost, O.; Carpentier, J.-F. Organometallics 2012, 31, 8375−8387. (d) Castro, L.; Kirillov, E.; Miserque, O.; Welle, A.; Haspeslagh, L.; Carpentier, J.-F.; Maron, L. ACS Catal. 2015, 5, 416−425. (25) Spaleck, W.; Kuber, F.; Winter, A.; Rohrmann, J.; Bachmann, B.; Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954− 963. (26) Theurkauff, G.; Bondon, A.; Dorcet, V.; Carpentier, J.-F.; Kirillov, E. Angew. Chem. 2015, 127, 6441−6444. (27) Coordination of the ligand in 1b, having a nonsymmetrically substituted cyclopentadienyl moiety and a monosubstituted Ph(H)C bridge, grants two elements of chirality. Of the two possible diastereoisomers for 1b, which differ by the relative positioning of the Et substituents on the Cp moiety and Ph group of the bridge, i.e. anti-Phbridge,EtCp and syn-Phbridge,EtCp isomers, the former featuring minimized repulsions between the Ph and Et groups has been found to form selectively; see ref 24c. (28) (a) A small (ca. 5 mol %) excess of E(C6F5)3 (E = B, Al) was used in each case; otherwise, substoichiometric amounts of activators might result in formation of binuclear ionic species of the type [Cp#2ZrMe(μ-Me)Zr(Me2)Cp#2]+[anion]−; see ref 13. (b) In no case, the small excess of coactivator induced double activation of both zirconium-methyl groups and formation of dicationic products. See: Chen, E. Y.-X.; Kruper, W. J.; Roof, G.; Wilson, D. R. J. Am. Chem. Soc. 2001, 123, 745−746. No attempts to purposely obtain such species have been conducted. (29) The molar ratio between these isomers allowed calculating the energy difference (0.2−06 kcal mol−1) between the ground states using Maxwell−Boltzmann statistics: N1/N2 = (g1/g2)e−ΔE/RT, where N1/N2 is the molar isomeric ratio and g1 = g2 = degeneracy. (a) Goodman, J. M.; Kirby, P. D.; Haustedt, L. O. Tetrahedron Lett. 2000, 41, 9879−9882. (b) McQuarrie, D. A.; Simon, J. D. Molecular Thermodynamics; University Science Books: Sausalito, CA, 1999. The energy differences between the computed isomers were relatively high (2.7−3.8 kcal mol−1 at the M06 level) yet were within the uncertainty of these DFT calculations. (30) Due to the high solubility of metallocenium ion pairs 3a,b, derived from the parent multisubstituted metallocenes, their concentrations can be maintained 10−50 times higher than those typically used for similar purposes for the lesser soluble unsubstituted analogues. See refs 13a and 11f−h. (31) Typically, the corresponding |Δδ(m,p-F)| values for ISIPs are greater than 3.5 ppm, while those for OSIPs are smaller than 3.0 ppm. See: Ciancaleoni, G.; Fraldi, N.; Budzelaar, P. H. M.; Busico, V.; Q

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Organometallics Macchioni, A. Dalton Trans. 2009, 8824−8827 and references cited therein. (32) For examples of structurally and/or spectroscopically authenticated aniline adducts of metallocenium complexes, see: (a) Wilson, P. A.; Wright, J. A.; Oganesyan, V. S.; Lancaster, S. J.; Bochmann, M. Organometallics 2008, 27, 6371−6374. (b) Varga, V.; Pinkas, J.; Cisarova, I.; Horacek, M.; Mach, K. Organometallics 2009, 28, 6944−6956. (c) Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics 1991, 10, 1501−1505. (33) For examples of structurally and/or spectroscopically authenticated aniline adducts of post-metallocene ionic complexes, see: (a) Tjaden, E. B.; Swenson, D. C.; Jordan, R. F. Organometallics 1995, 14, 371−386. (b) Horton, A. D.; De With, J. Organometallics 1997, 16, 5424−5436. (c) Flores, M. A.; Manzoni, M. R.; Baumann, R.; Davis, W. M.; Schrock, R. R. Organometallics 1999, 18, 3220−3227. (d) Schrock, R. R.; Casado, A. L.; Goodman, J. T.; Liang, L.-L.; Bonitatebus, P. J.; Davis, W. M. Organometallics 2000, 19, 5325−5341. (e) Basuli, F.; Clark, R. L.; Bailey, B. C.; Brown, D.; Huffman, J. C.; Mindiola, D. J. Chem. Commun. 2005, 2250−2252. (34) The protonolysis reaction of 2c with [PhNMe2H]+[B(C6F5)4]− appeared to be less selective in comparison to those of 2a,b. Simultaneous formation of unidentified minor byproducts was evidenced by 29Si{1H} NMR spectroscopy (two additional lowintensity signals at δ −11.9 and −13.4 ppm in toluene-d8/o-F2benzene). (35) (a) The upfield-shifted 1H and 13C NMR aromatic signals of the PhNMe2 moiety in [4c]+[B(C6F5)4]− (δ 5.74, 5.60, and 5.29 ppm for m-H, p-H, and o-H, respectively, and δ 135.4, 115.6, and 107.1 ppm for Cmeta, Cortho, and Cpara, respectively) likely stem from the magnetic anisotropy of the phenyl-substituted bis(indenyl) ligand. (b) These changes in chemical shifts may also be explained by η6 coordination of the PhNMe2 ligand in [4c]+[B(C6F5)4]− in solution. Similar spectroscopic observations have been reported for yttrium alkyl cations. See: (c) Kenward, A. L.; Piers, W. E.; Parvez, M. Organometallics 2009, 28, 3012−3020. (d) For the crystal structure of a cationic scandium complex with an η6-coordinated PhNMe2, see: Yu, N.; Nishiura, M.; Li, X.; Xi, Z.; Hou, Z. Chem. - Asian J. 2008, 3, 1406−1414. (36) The reaction of {Me2C-(Flu)(Cp)}MMe2 (M = Ti, Zr, Hf) with 1 equiv of [PhNMe2H]+[B(C6F5)4]− afforded the bimetallic complex [({Me2C-(Flu)(Cp)}MMe)2(μ-Me)]+ as the main product, which converts to the corresponding [{Me2C-(Flu)(Cp)}MMe(PhNMe2)]+ in the presence of an excess of [PhNMe2H]+[B(C6F5)4]− and PhNMe2; see ref 32a. (37) Some N,N-dimethylaniline-bound metallocenium ion pairs, generated from the corresponding dimethylzirconocenes and [PhNMe2H]+[B(C6F5)4]−, are known to undergo intramolecular C− H activation of the NMe group, leading to stable zirconaaziridinium products. See: (a) Rocchigiani, L.; Bellachioma, G.; Zuccaccia, C.; Macchioni, A. J. Organomet. Chem. 2012, 714, 32−40. (b) Rocchigiani, L.; Bellachioma, G.; Ciancaleoni, G.; Macchioni, A.; Zuccaccia, D.; Zuccaccia, C. Organometallics 2011, 30, 100−114. (c) Rocchigiani, L.; Ciancaleoni, G.; Zuccaccia, C.; Macchioni, A. Angew. Chem., Int. Ed. 2011, 50, 11752−11755. (38) Another example of a crystallographically characterized ionic AlMe3 adduct, namely [(Me3[9]aneN3)(tBuN)Ti(μMe)2AlMe2]+[B(C6F5)4]−, has been described. See: (a) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. Chem. Commun. 2005, 3313−3315. (b) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. J. Am. Chem. Soc. 2006, 128, 15005−15018. (39) Complete conversion of (EBTHI)Zr(Me)-MeB(C6F5)3 upon reaction with AlMe3 after 8 h in CH2Cl2 to provide the stable heterobimetallic product [(EBTHI)Zr(μMe)2AlMe2]+[MeB(C6F5)3]− has been reported by Waymouth et al.; see ref 17. The identity of the product was established by NMR spectroscopy. (40) Mathis, D.; Couzijn, E. P. A.; Chen, P. Organometallics 2011, 30, 3834−3843.

(41) (a) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908−5912. (b) Klosin, J.; Roof, G. R.; Chen, E. Y.-X. Organometallics 2000, 19, 4684−4686. (c) Hair, G. S.; Cowley, A. H.; Gorden, J. D.; Jones, J. N.; Jones, R. A.; Macdonald, C. L. B. Chem. Commun. 2003, 424−425. (d) Janiak, C.; Lassahn, P.-G. Macromol. Symp. 2006, 236, 54−62. (42) Originally, [AlMe3]/[M] ratios of (1.00−1.05)/1 were employed by Bochmann et al.12,16 for the synthesis of AlMe3 adducts of general composition [Cp2M(μ-Me)2AlMe2]+[B(C6F5)4]−, whereas a much larger excess of AlMe3 (5−20 equiv vs metallocene) was subsequently used to quantitatively generate the corresponding heterobimetallic complexes.6 In a regular polymerization experiment cocatalyzed with commercial-grade MAO, the [AlMe3]/[M] ratios are greater than 50−100. (43) In general, anion substitution reactions in metallocene alkyl cations by Lewis bases are thermodynamically favored. However, kinetic and thermodynamic preferences for these reactions can be influenced by the bulkiness of both the metallocenium cation and base. See: (a) Reference 13b. (b) Schaper, F.; Geyer, A.; Brintzinger, H. H. Organometallics 2002, 21, 473−483. (44) Recent investigations by Bochmann et al. on metallocene activation with various MAO grades (obtained via different synthetic protocols and containing different amounts of AlMe3) highlighted catalytic systems with different productivities and transfer kinetics. See: Ghiotto, F.; Pateraki, C.; Severn, J. R.; Friederichs, N.; Bochmann, M. Dalton Trans. 2013, 42, 9040−9048. (45) 1H−1H ROESY NMR studies performed on [6b]+[“MeMAO”]− (Figure S42 in the Supporting Information and Table 2) revealed close contacts within the heterobimetallic cationic fragment [{Ph(H)C(3,6-tBu 2 -Flu)(3-tBu-5-Et-Cp)}Zr(μMe)2AlMe2]+ ([6b]+), while no interionic contacts could be seen. No exchange process was observed for [6b]+[“MeMAO”]− at room temperature at variable relaxation delays. (46) On the basis of limited 1H NMR spectroscopic data,6k it was surmised that, upon activation of 1c with MAO, formation of the putative methylidene species [rac-{Me2Si-(2-Me-4-Ph-Ind)2}Zr(μCH2)(μ-Me)AlMe2]+[“Me-MAO”]− takes place from the corresponding parent {rac-[{Me2Si-(2-Me-4-Ph-Ind)2}Zr(μ-Me)2AlMe2]+[“MeMAO”]−, accompanied by concomitant evolution of methane. (47) The effective hydrodynamic radius (rH = 12.5−14.4 Å) of the [Cp2Zr(μ-Me)2AlMe]+[“MeMAO”]− ion pair, derived from the Cp2ZrMe2/MAO system in benzene-d6, has been estimated by Babushkin and Brintzinger using bis-α,α′-(trityl)-4.4′-dimethylbiphenyl as the bulky reference compound; see ref 6b. (48) Each polymerization experiment was repeated independently at least twice under the same conditions, revealing good reproducibility in terms of activity (gas uptake) and productivity (polymer yield), as well as physicochemical properties (Mw, Mn, Tm, isotacticity) of the isolated polymer. (49) For different catalytic systems based on a neutral metallocene precursor and MAO, the molar fraction of active sites varies between 8 and 25%. See: (a) Cipullo, R.; Mellino, S.; Busico, V. Macromol. Chem. Phys. 2014, 215, 1728−1734. (b) Bochmann, M.; Cannon, R. D.; Song, F. Kinet. Catal. 2006, 47, 160−169. (50) (a) Note, however, that a possible contribution of a competing process related to the neutral borane exchange, which would result in appearance of intense cross-peaks between Zr−Me and B−Me resonances in the EXSY spectra, should not be completely discarded. Dynamic effects resulting in substantial broadening of B−Me signals in the 1H NMR spectra of complexes 3a,b-MeB(C6F5)3 may be also responsible for masking the corresponding cross-peaks in the EXSY spectra.. (b) Similarly, ion pairs generated from complexes rac{Me2Si(2-Me-Benz[e]Ind)2}ZrMe2 and rac-{Me2Si(2-Me-4-tBu-Cp)2} ZrMe2 and B(C6F5)3 did not show cross-peaks resulting from the neutral borane exchange process in the EXSY NMR spectra; see ref 13a. (51) Due to important overlapping of diagonal peaks from the B−Me group of the minor isomer with those from aliphatic (tBu and Me) R

DOI: 10.1021/acs.organomet.5b00965 Organometallics XXXX, XXX, XXX−XXX

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Organometallics groups, only in a few cases were we able to make careful determination of peak volumes and calculate the corresponding rates. (52) The exchange rates were reported to be independent of the {Me2C(Flu)(Cp)}Zr(Me)(μ-Me)B(C6F5)3 ion pair concentration; see ref 11g. (53) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840−842. (54) It should be noted that the equilibrium ratios between the two isomers of ion pairs 3a,b-MeB(C6F5)3 did not change when a default of B(C6F5)3 was used. (55) The formation of putative dinuclear OSIP intermediates [3b2b]+[MeB(C6F5)3]− and [3c-2c]+[MeB(C6F5)3]− is computed to be thermodynamically disfavored by 5.4−9.2 and 12.5 kcal mol−1, respectively. (56) (a) Additional 19F−19F EXSY NMR derived kinetic data, obtained for binary systems 3a-MeB(C6F5)3/2a and 3b-MeB(C6F5)3/ 2b at 318 K (Figure S15 in the Supporting Information), are in agreement with that obtained for the exchange processes of both the aromatic (Flu) and ZrMe signals; see Table S3 in the Supporting Information, footnotes g and h, respectively. (b) For steric reasons, the borane migration pathway is not operative for ion pairs {Me2C(Flu)(Cp)}Zr(CH2SiMe3)(μ-Me)B(C6F5)3 derived from the parent mixed-alkyl metallocene; see ref 12e. (57) These rates of exchange are to be compared with the rate of the pseudo-first-order reaction (k = 3 × 10−4 s−1 at 298 K in C6D6) of the dinuclear ionic complex [{rac-{Me 2 Si(Ind) 2 }Zr(Me)} 2 (μMe)]+[B(C6F5)4]− with excess [Ph3C]+[B(C6F5)4]− to afford [rac{Me2Si(Ind)2}Zr(Me)]+[B(C6F5)4]−; see ref 12b. (58) Price, C. J.; Chen, H.-Y.; Launer, L. M.; Miller, S. A. Angew. Chem., Int. Ed. 2009, 48, 956−959. (59) Direct comparisons with other metallocene/MAO systems should be done with care, since speciation (ion pair composition) for such binary combinations may depend greatly on the MAO grade, notably its AlMe3 content; see ref 44. (60) In a separate experiment, complex [6b]+[“MeMAO”]−, generated and authenticated by NMR spectroscopy, afforded an identical UV−vis spectrum. (61) When varying concentrations of 1b were employed (0.12−0.45 mM), the plot of ln kapp vs ln [1b]0 was linear, affording a 0 slope and showing that the reaction overall is first order in [1b]. (62) A similar behavior has been observed with the unilaterally substituted system {Ph2C(Tet)(Cp)}ZrCl2/MAO (Tet = tetramethyltetrahydrodibenzofluorene) showing a red shift from λmax 510 to 597 nm and further to 570 nm with a half-life t1/2 of 2.2−2.9 min. (63) A similar study has been conducted for the metallocene rac{Me2Si-(2-Me-Benz[e]Ind)2}ZrCl2; see ref 44. (64) For ab initio and TD-DFT calculations of neutral metallocenes and ionic systems, see: (a) Makela, N. I.; Knuuttila, H. R.; Linnolahti, M.; Pakkanen, T. A. J. Chem. Soc., Dalton Trans. 2001, 91−95. (b) Makela, N. I.; Knuuttila, H. R.; Linnolahti, M.; Pakkanen, T. A.; Leskela, M. A. Macromolecules 2002, 35, 3395−3401. (c) MakelaVaarne, N. I.; Linnolahti, M.; Pakkanen, T. A.; Leskela, M. A. Macromolecules 2003, 36, 3854−3860. (d) Belelli, P. G.; Damiani, D. E.; Castellani, N. J. Chem. Phys. Lett. 2005, 401, 515−521. (65) In the putative [6c]+[MeB(C6F5)3]− and [6c]+[MeAl(C6F5)3]−, the HOMO-3 and HOMO-2, respectively, remained delocalized on the indenyl ligands and the LUMOs are largely centered on the Zr atoms with a weight of 61% for both species. The calculated absorption at λ max 510 nm for [6c]+[MeB(C6F5)3]− (505 nm for [6c]+[MeAl(C6F5)3]−) is shifted by 113 nm (108 nm for [6c]+[MeAl(C6F5)3]−) with respect to 2b and resulted from the HOMO-3 → LUMO transition for [6c]+[MeB(C6F5)3]− (HOMO-2 → LUMO for [6b]+[MeAl(C6F5)3]−).

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DOI: 10.1021/acs.organomet.5b00965 Organometallics XXXX, XXX, XXX−XXX