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. 2020 Apr 9;59(8):5751–5759. doi: 10.1021/acs.inorgchem.0c00533

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications

Francesco Zaccaria †,‡, Peter H M Budzelaar †, Roberta Cipullo †, Cristiano Zuccaccia ‡, Alceo Macchioni ‡, Vincenzo Busico †, Christian Ehm †,*
PMCID: PMC7997381  PMID: 32271565

Abstract

graphic file with name ic0c00533_0008.jpg

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A–C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.

Short abstract

A model cluster for methylaluminoxane (MAO) was used to study the tendency of model Lewis acidic sites to release neutral Al fragments or transient aluminum cations both for unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol). Albeit only sites of lower Lewis acidity are found in TMA depleted modified MAOs, they can release cationic Al fragments when treated with certain N-donors. This shows how TMA depleted MAOs can still be potent activators.

Introduction

Methylaluminoxane (MAO)1,2 is typically used as a cocatalyst in molecular olefin polymerization catalysis for the industrial production of performance polymers,3−5 due to its remarkable combination of good impurity scavenging properties and excellent alkylating and abstracting capability.1,2,6,7 Moreover, MAO can be used to heterogenize molecular catalysts on supports like silica or alumina.2,8,9

Since its serendipitous discovery in the 1980s,2 MAO is typically produced via controlled hydrolysis of trimethylaluminum (TMA), leading to a rather complex mixture of species.1,2,6,7 Great efforts have been devoted to the structural elucidation of this aluminoxane, but the task remains—despite some considerable advancements—largely unaccomplished. The crystal structure of its higher homologue tert-butylaluminoxane (TBAO), obtained by controlled hydrolysis of tri-tert-butylaluminum, has been determined, featuring well-defined (AlOtBu)n cages with four-coordinate Al and three-coordinate O centers at the edges of four- or six-membered rings.10 Experimental and computational studies have shown that similar cages of pure (AlOMe)n are not stable enough. In the absence of the bulky tBu groups, rearrangements maximize the number of the less strained six-membered faces.11−25

Besides, some residual TMA from the synthesis (typically accounting for up to 1/3 of the total Al content)22,26 likely serves a structural, stabilizing role in MAO clusters. It is generally accepted that hydrolysis of TMA yields a complex and dynamic distribution of (AlOMe)n(AlMe3)m cages.1,11−25 The formula accounts indiscriminately for formal AlMe3 that is actually incorporated into the cage backbone and for proper AlMe3 molecules that are reversibly associated with the Al-clusters. The latter serve to stabilize residual unsaturated Al and O atoms (“structural” AlMe3) and are known to undergo exchange equilibria with residual “free” TMA molecules.11,14,24,26−28

These dynamic equilibria are particularly relevant in MAO modifications involving TMA depletion. This can be achieved, for instance, by vacuum drying (bp of TMA = 125 °C)29−31 or by addition of suitable scavengers like 2,6-di-tert-butyl-4-methylphenol (BHT, vide infra).32−34 In all cases, “free” TMA is likely removed/scavenged first, consequently inducing a partial release of “structural” AlMe3 molecules. This dissociation exposes unsaturated Al and/or O atoms on cage edges, destabilizing the cluster and inducing structural rearrangements. Formation of larger Al-cages, upon TMA depletion, has been observed experimentally by diffusion NMR spectroscopy29,33 and cryoscopy.27

Importantly, in addition to their stabilizing role, “structural” AlMe3 molecules are proposed to serve functional roles. In particular, they are thought to be responsible for the Lewis acidity of MAO via generation of transient [AlMe2]+ species relevant for precatalyst activation (Scheme 1).18,26,35−38 Experimentally, this type of reactivity has been explored via the interaction of MAO with small amounts of neutral O- or N-donors (4–10 mol %).26,39−43 Bipyridine (bipy), for instance, extracts [AlMe2]+ from MAO with high chemoselectivity in the form of the donor stabilized adduct [AlMe2(bipy)]+.41

Scheme 1. Generation of AlMe2+ species from Type A–C Sites in MAO.

Scheme 1

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See also Figure 1.

Similarly, pyridine (py), can generate the corresponding [AlMe2(py)2]+ adduct, but concomitant formation of neutral AlMe3(py) is also observed.26,28,41

Recently, analogous reactions have been reported for the phenol-modified MAO/BHT.44 It has been proposed that addition of the Brønsted acidic BHT to MAO leads to (a) conversion of “free” TMA and a fraction of “structural” AlMe3 into the relatively inert AlMe(bht)2 complex via methane generation45,46 and (b) conversion of the remaining “structural” AlMe3 in analogous “structural” AlMe2(bht) molecules, resulting in [(AlOMe)0.87(AlMe2 bht)0.13]n cages (bht = BHT phenolate).29,33,44 Addition of bipy to MAO/BHT leads to the formation of [AlMe(bht)(bipy)]+, while py selectively generates neutral AlMe2(bht)(py).44

The interaction with py has been studied also for MAO supported on silica;36,47 Brønsted acidic Si–OH groups of silica are known to scavenge TMA similarly to BHT.2,8,36,47−49 However, the complexity of these heterogeneous systems hampers straightforward connections with the structure and reactivity of unmodified MAO in solution.50

In extensive computational studies, Linnolahti and co-workers16,19 have proposed that the cage 16,6 (i.e., having n = 16 and m = 6; Figure 1a)19,51 represents the most stable model MAO cluster, in line with several other theoretical13,52,53 and spectroscopic findings.26,33,40,42,43,54,55 This 16,6 cage features four “structural” AlMe3 molecules in three different cluster environments, A, B, and C (emphasized by different colors in Figure 1).37 Assuming cage 16,6 as a realistic model for unmodified MAO, the reactivity of the proposed Lewis acidic sites A–C has been explored in some density functional theory (DFT) studies, for example, identifying C type sites as those being more prone to release [AlMe2]+ and AlMe3.37,55

Figure 1.

Figure 1

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DFT optimized structure of 16,6, proposed by Linnolahti and co-workers as the most stable cage for MAO (O atoms in red, Al atoms in gray and C atoms in black, H atoms omitted for clarity).16,19 “Structural” TMA molecules, binding different Al-sites (one type A and B, and two type C) of the cage backbone, are highlighted.

In the present paper, a systematic computational study is reported, aiming to provide further insights in the functional roles carried out by the Lewis acidic Al-sites. The representative model cluster 16,6 was considered for MAO, along with its bht-decorated analogues representing MAO/BHT. The thermodynamics of [AlMeR]+ and/or AlMe2R dissociation (R = Me or bht) was investigated both in the absence and in the presence of neutral N-donors like py and bipy, providing a close comparison with previously reported experimental studies.26,28,41,44

Results

The computational protocol M06-2X/TZ(PCM)//TPSSTPSS/DZ has been previously benchmarked56,57 and widely used for modeling of Al-species relevant to olefin polymerization33,44,58,59 and other research fields,60 and it has been shown to be suitable for studies related to MAO activators.19,33,35,41,51,61 Additional inclusion of Grimme-type long-range dispersion corrections62,63 does not affect computational results significantly (see Supporting Information).

Scheme 2 summarizes the reactions studied in this work, using type C site as representative example (see Supporting Information for the corresponding schemes for A and B):

  • Replacement of “structural” AlMe3 with the analogous AlMe2(bht) (Scheme 2a)

  • Formation of [AlMeR]+ or AlMe2R in the absence of donors (Scheme 2b)

  • Formation of [AlMeR]+ or AlMe2R in the presence of neutral donors like py (Scheme 2c) or bipy (Scheme 2d). In the latter case, only ionization is modeled. Extraction of neutral fragments would lead to the unfavorable formation of five-coordinate AlMe2R(bipy) species41,44

Scheme 2. Reactions Studied in This Work.

Scheme 2

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(a) Replacement of “structural” AlMe3 with “structural” AlMe2(bht); (b) release of [AlMeR]+ or AlMe2R in the absence of neutral donors; (c) release of [AlMeR]+ or AlMe2R in the presence of py; (d) release of [AlMeR]+ in the presence of bipy. Type C Lewis acidic sites used as exemplifying case for the three sites shown in Figure 1. See analogous Schemes S1 and S2 in the Supporting Information for A and B type sites. “Structural” AlMe2R molecules highlighted in bold (R = Me or bht).

It should be noted here, that the experimentally found MAO anion with the highest detected abundance is also a 16,6 cluster,42 which among other potential routes can be formally derived from a 16,7 neutral cluster via [AlMe2]+ release, a 16,6 cluster via methide abstraction, or a 16,6 cluster via [AlMe2]+ abstraction to form 16,5 followed by TMA coordination to reform 16,6. The structure of the anionic 16,6 cluster is not identical with a neutral 16,6 cluster nor is the neutral 16,6 cluster considered here able to accept a methide without rearrangements. Due to the complex equilibria between different MAO clusters, we therefore decided to focus on the most abundant neutral species identified by Linnolahti.16,19 Reactions of interest are modeled neglecting any structural rearrangements that might consequently occur. The behavior of each Al-site type should be rather independent from the cage size; previous extensive DFT studies19,37 highlighted that structural characteristics of the edge sites are independent of the sizes, forms, and shapes of the MAOs.19 Corroborating this, nearly identical behaviors (within 1.5 kcal/mol) are predicted for the two inequivalent C-type sites of 16,6 studied here.

DFT estimated reaction Gibbs free energies (ΔGR) are reported in Tables 1 and 2. These values are only intended for semiquantitative comparison among the different acidic sites, since the complexity of the systems likely hampers the accurate identification of true global minima.64 Furthermore, reactions are modeled assuming naked ions in a solvent continuum, rather than the ion pairs typically formed in the low polarity solvents used (see below for further discussion on this point).7,65,66

Table 1. Calculated ΔGR (at 298 K, in kcal/mol) for the Reaction of “Structural” AlMe3 with BHT Leading to “Structural” AlMe2(bht) Incorporation (Scheme 2a).

entry Al site ΔGR
1 A –23.8
2 B –30.1
3 C –27.8
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Table 2. Calculated ΔGR (at 298 K, in kcal/mol) for the Release of [AlMeR]+ or AlMe2R from MAO or MAO/BHT (R = Me or bht; see Scheme 2b–d).

entry Al site AlMe3 loss AlMe2(bht) loss [AlMe2]+ loss [AlMe(bht)]+ loss
without donors (Scheme 2b)
1 A 24.6 14.7 104.4 91.6
2 B 35.9 32.3 96.8 90.2
3 C 7.9 2.3 75.6 67.8
with py (Scheme 2c)
4 A 0.8 –8.0 23.5 18.0
5 B –8.1 –10.7 15.9 16.7
6 C –34.1 –38.6 –5.3 –6.8
with bipy (Scheme 2d)
7 A – – 21.1 16.3
8 B – – 13.5 15.0
9 C – – –7.7 –8.5
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The ΔGR values reported in Table 1 for conversion of “structural” AlMe3 into “structural” AlMe2(bht), upon addition of BHT to MAO, clearly indicate a highly exergonic process for all the various types of acidic sites (Scheme 2a). These data support the hypothesis that no residual “structural” AlMe3 should be present on MAO/BHT-type cages, provided that enough BHT is added.33

The results concerning the generation of neutral or ionic fragments are reported in Table 2 (see also Scheme 2b–d). The reactions in the absence of stabilizing donors have been previously explored by DFT only for unmodified MAO (vide infra).37,55 Here, the release of “structural” AlMe3 is calculated to be an endergonic process for all sites with decoordination energies rising in the order C ≪ A < B (Table 2, entries 1–3). The decoordination energy of “structural” AlMe3 from 16,6 via site C (≈8 kcal/mol) is quite small and comparable to the dimerization energy of TMA (6–7 kcal/mol);56,67 appreciably higher ΔGR are estimated for sites A and B (25 and 36 kcal/mol, respectively), instead. The peculiarity of sites C lies in the “structural” AlMe3 molecules being bound to the Al site of the cage via two bridging methyl groups (similar to the TMA dimer), leading to a more labile interaction compared to sites A and B with one bridging methyl and a proximal two-coordinate O atom on the cage edge (Figure 1; compare also Scheme 1b with Schemes S1b and S2b).37

The specific structure of site A, in which the Al and O atoms involved in the interaction with AlMe3 are adjacent but not bound to each other (Figure 1), leads, upon AlMe2R detachment, to unsaturated Al and O atoms that are spatially close enough to form a new Al–O bond (Scheme 3). This is associated with small local rearrangements of the cage structure, and is not restricted to 16,6 but could also occur in other MAO clusters. The formation of this bond exemplifies the complexity of the dynamic behavior of MAO1 and partially compensates the energy loss due to TMA detachment, explaining the lower ΔGR of about 11 kcal/mol estimated for A with respect to B (Table 2).

Scheme 3. Formation of a New Al–O Bond between the Unsaturated Al and O Atoms (highlighted in blue) Generated upon AlMe3 Dissociation from Site A.

Scheme 3

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O atoms in red, Al atoms in grey and C atoms in black, H atoms omitted for clarity.

AlMe2(bht) is bound slightly weaker than AlMe3 to B and C (ΔΔGR ≈ 4–6 kcal/mol; Table 2, entries 1–3) and especially to A (ΔΔGR ≈ 10 kcal/mol). As shown in Table S3 and Figure S1, the bht fragments of “structural” AlMe2(bht) are rotated out of the optimal Al–O–Cbht binding geometry, likely due to steric congestion. This distortion is most prominent for site A and might be (co)responsible for the particularly weaker binding of the bulky Al-phenolate fragment to this site.

Similar trends to those highlighted for AlMe2R are calculated for formation of cationic [AlMeR]+, that is, ionization should occur more easily for C than for A and B. [AlMe(bht)]+ should be released somewhat more easily than [AlMe2]+ from each site (Table 2, entries 1–3). The higher propensity of sites C to release AlMe3 and [AlMe2]+ is in line with previous reports by Linnolahti and co-workers on unmodified MAO.37,55 The presence of Al-sites having different Lewis acidity has been highlighted also by some experimental studies.54,68,69

Upon ionization, the negative charge appears to be delocalized, albeit not over the whole cage, especially for sites A and B (Figure 2 and Table S4). Charge dissipation is known to be crucial for generating weakly coordinating anions and, consequently, highly active catalysts.6,7 The ability of MAO to generate large anions with a delocalized negative charge has been proposed as one of the origins of the excellent cocatalytic properties of this aluminoxane.6,7,70,71 In the presence of a neutral monodentate donor like py, all reactions are predicted to be significantly easier (Table 2, entries 4–6). The dissociation of AlMe3 becomes exergonic for B (−8 kcal/mol) and particularly for C (−34 kcal/mol), and nearly energetically neutral for A. Release of AlMe2(bht) is again favored with respect to that of AlMe3 for each site (ΔΔGR ≈ 5–7 kcal/mol). The exergonicity is enforced by two main contributions. First, py stabilizes the leaving molecular species AlMe2R by forming coordinately saturated AlMe2R(py) complexes. Second, py can bind also to the three-coordinate Al atoms on the cage edge generated by the dissociation, therefore preventing cage destabilization (compare reactions in Scheme 2b,c, left paths).72 The former contribution is cage independent, while the latter varies depending on the Lewis acidic site involved. Indeed, trends in relative ΔGR estimated for donor coordination reflect those in relative ΔΔGR for AlMe2R detachment in the absence and presence of py, as reported in Table S5. The cage stabilization due to py coordination is only in the order of ∼6 kcal/mol for A, while it is much higher for B and C (about 26 and 24 kcal/mol, respectively). The appreciably lower ΔGR in the former case is due to the formation of the aforementioned Al–O bond (Scheme 3), which represents an alternative albeit milder stabilization route for the unsaturated atoms generated upon AlMe2R dissociation from A.

Figure 2.

Figure 2

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Mapped electrostatic potential surface from total electron density for the neutral 16,6 cage (top) and the anionic cages generated therefrom upon [AlMe2]+ dissociation from sites A–C (bottom). Structure of 16,6 provided for comparison (H omitted for clarity; see also Figure 1 and Experimental Section).

Also ionization is facilitated by the presence of py (Table 2, entries 4–6), up to the point that it is calculated to be exergonic for sites C (−5 kcal/mol for [AlMe2]+ and −7 kcal/mol for [AlMe(bht)]+) but not for sites A (>16 kcal/mol) and B (>18 kcal/mol). Here, four-coordinate Al atoms are generated on the cage, and only the stabilization of the [AlMeR]+ fragments by two py molecules provides an increased driving force (Scheme 2c, right path). Different from B, sites A and C generate [AlMe(bht)]+ more easily than [AlMe2]+.

The same trends are predicted for ionization in the presence of the bidentate bipy (Table 2, entries 7–9). The stronger binding of this bidentate donor to cationic aluminum is responsible for a further drop of predicted ΔGR by a few kcal/mol compared to py.

Discussion

These results clearly indicate that sites A–C are expected to behave quite differently. Thus, considering these three as realistic models for Lewis acidic sites, the data presented here allow for some interpretation of the issues related to the properties of MAO and its modifications outlined in the Introduction. The following discussion will mainly focus on ΔGR calculated for the reactions in the presence of donors, graphically summarized in Figure 3a, since they provide a more straightforward comparison with experimental results. The features of MAO are discussed first, followed by those of MAO/BHT.

Figure 3.

Figure 3

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(a) Graphical comparison of calculated ΔGR at 298 K (in kcal/mol) for the release of [AlMeR]+ or AlMe2R from MAO or MAO/BHT (R = Me or bht; see also Table 2). (b) Structures of sites A–C relevant for MAO and MAO/BHT ionization properties.

Sites Responsible for MAO Activation Properties

It is generally accepted that only one or slightly more than one acidic site per cage should be working, on average.12,33,54,73 Type C sites are those releasing [AlMe2]+ fragments most easily and therefore likely responsible for the abstracting capability of MAO (Figure 3). This is in line with previous reports that considered ionization only in the absence of donors.37

Sites Inducing Structural Rearrangements of MAO Cages upon TMA Depletion

Type C sites also tend to release “structural” AlMe3 most easily (Figure 3). Thus, TMA depletion by either vacuum drying or addition of BHT should expose and, consequently, destabilize these sites, inducing Al-cage rearrangements. Modified TMA-free MAOs, like MAO/BHT, should therefore contain mostly acidic sites of type A and B.

Sites Binding “Structural” AlMe2(bht) in MAO/BHT

Sites A and B are predicted to bind “structural” AlMe2R rather strongly (Figure 3). Therefore, upon addition of BHT, “structural” AlMe3 molecules bound to these sites in MAO should be those transformed into “structural” AlMe2(bht) without being released by the Al cages.

Sites responsible for MAO/BHT activation properties

Based on the above considerations, DFT results suggest that MAO/BHT contains mostly sites A and B, and ionization is predicted to be similarly thermodynamically unfavorable in both cases (Figure 3). However, it is worth noting that the calculated absolute ΔGR for ionization is likely overestimated. Ion pair formation occurs in the low polarity solvents used experimentally,7,65 which, as previously demonstrated, can account for tens of kcal/mol,74 but it was not modeled here (vide supra). To prove this concept, the representative ion pair [AlMe(bht)(bipy)]+[16,5]− has been modeled (fully relaxed) for B, assuming that the cation remains in the proximity of the Al site it was generated from. Calculated ΔGR drops from 15.0 kcal/mol for the isolated ions to only 2.2 kcal/mol for the contact ion pair (Scheme 4). A ΔGR close to zero for this reaction is consistent with the experimental observation that ionization occurs with bipy but not py.44

Scheme 4. Comparison of Calculated ΔGR at 298 K (in kcal/mol) for the Release of [AlMe(bht)(bipy)]+ from Site B of MAO/BHT within the Naked Ion Approximation or Considering a Contact Ion Pair (R = Me or bht).

Scheme 4

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Why does py abstract [AlMe2]+ form MAO but not [AlMe(bht)]+ from MAO/BHT?

Computational results indicate that, for a given site, generation of [AlMe2]+ or [AlMe(bht)]+ should be similarly easy. However, as discussed in the previous sections, sites that release [AlMe2]+ in MAO (C) should not be of the same type as those releasing [AlMe(bht)]+ in MAO/BHT (A/B). The latter type is appreciably less prone to undergo ionization (Figure 3), which can explain why addition of an N-donor like py leads to (partial) ionization of MAO but not MAO/BHT.44 The difference between the two cocatalysts and the different Lewis acidic sites becomes experimentally less evident with the bidentate donor bipy, which is a more effective ionizing agent.

Conclusions

The structure and reactivity of MAO activators are known to be critically dependent on their Lewis acidic sites. In this work, the thermodynamics of [AlMeR]+ or AlMe2R dissociation from some established model MAO cages was systematically investigated by DFT. Different from previous reports by others,37,55 not only the reactivity of unmodified MAO but also that of TMA-free MAO/BHT was considered. Furthermore, the reactions were modeled both in the absence and in the presence of neutral N-donors, in the latter case providing a direct connection with known experimental observations.26,39−43 An interpretation of how modifications of MAO can affect acidic sites composition and, consequently, cocatalytic properties is proposed.

In unmodified MAO, type C sites should be those releasing [AlMe2]+ and AlMe3 most easily. This implies that (a) they are likely responsible for the activating capability of MAO and (b) the “structural” AlMe3 molecules bound to these sites are largely released upon TMA depletion (e.g., by vacuum drying or reaction with Brønsted acids like BHT). The consequent liberation of unsaturated three-coordinate Al-atoms on the cage can lead to structural rearrangements that “annihilate” these sites.

In TMA-depleted MAOs, “structural” AlMe2R molecules should therefore be bound primarily to the remaining sites A and B. In the case of MAO/BHT, calculations indicate that the two sites have similar tendencies to release [AlMe(bht)]+ and AlMe2(bht). Sites A and B should therefore be responsible for the properties of MAO/BHT, even though their reactivity is appreciably lower than that of sites C in unmodified MAO. These computational results explain the lower propensity of MAO/BHT with respect to MAO to undergo ionization upon interaction with py:44 this depends not only on the different stability/electrophilicity of [AlMe(bht)]+ vs [AlMe2]+ cations but also on the reactivity of B vs C-type sites they originate from.

The reactivity trends of type A–C sites should be rather independent of the size of cage they belong to, since their properties are mainly determined by their local structures. Importantly, the apparent diversity of Lewis acidic sites in MAO and its TMA depleted counterparts indicates that strongly Lewis acidic Al-sites like C are not necessarily a prerequisite for an effective abstractor.

Experimental Section

Following the protocol described in ref (57), all geometries were fully optimized using the Gaussian 09 software package75 in combination with the OPTIMIZE routine of Baker76,77 and the BOpt software package.78 All relevant structures were fully optimized at the TPSSTPSS level79 of theory employing correlation-consistent polarized valence double-ζ Dunning(DZ) basis sets (cc-pVDZ quality)80,81 from the EMSL basis set exchange library.82 All calculations were performed at the standard Gaussian 09 quality setting [Scf = Tight and Int(Grid = fine)]. Final single-point energies and natural bond orbital83 analyses were calculated at the M06-2X level of theory84 employing triple-ζ Dunning (TZ) basis sets (cc-pVTZ quality).80 Solvent corrections where included at this stage by the polarized continuum model (PCM);85 a typical solvent used in olefin polymerization, namely toluene, was considered. The density fitting approximation (resolution of identity) was used at the optimization stage as well as for final energy calculations.86−89 Enthalpies and Gibbs free energies were then obtained from TZ single-point energies and thermal corrections from the TPSSTPSS/cc-pVDZ-(PP) vibrational analyses; entropy corrections were scaled by a factor of 0.67 to account for decreased entropy in the condensed phase.90−92 The structure of 16,6, optimized by Linnolahti and co-workers, was used as starting the geometry.16,19 Electrostatic potential maps of Figure 2 were generated from cube files for electrostatic potential and electron density using the GaussView software (isosurface value = 0.01). The cubes were obtained by means of the cubegen routine of Gaussian using the formatted checkpoint files of M06-2X single point energy calculations.

Acknowledgments

Part of this work has been financially supported by PRIN 2015 (20154X9ATP_004), University of Perugia, and MIUR (AMIS, “Dipartimenti di Eccellenza - 2018-2022” program). The authors thank Dr. Giuseppe Antinucci from the University of Naples Federico II for very useful discussion. F.Z. thanks INSTM and CIRCC for a postdoctoral grant.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00533.

  • Coordinates for optimized geometries (XYZ)

  • Additional DFT results (DFT results including long-range Grimme dispersion corrections, details for steric congestion at Al-bht sites, reaction schemes for sites A and B, ranges of NPA charges, Gibbs free energies for py coordination after AlMe2R dissociation, final energies, entropy and enthalpy corrections, full Gaussian citation (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic0c00533_si_001.xyz (182.2KB, xyz)
ic0c00533_si_002.pdf (505.8KB, pdf)

References

  1. Zijlstra H. S.; Harder S. Methylalumoxane – History, Production, Properties, and Applications. Eur. J. Inorg. Chem. 2015, 2015, 19–43. 10.1002/ejic.201402978. [DOI] [Google Scholar]
  2. Kaminsky W. Discovery of Methylaluminoxane as Cocatalyst for Olefin Polymerization. Macromolecules 2012, 45, 3289–3297. 10.1021/ma202453u. [DOI] [Google Scholar]
  3. Baier M. C.; Zuideveld M. A.; Mecking S. Post-Metallocenes in the Industrial Production of Polyolefins. Angew. Chem., Int. Ed. 2014, 53, 9722–9744. 10.1002/anie.201400799. [DOI] [PubMed] [Google Scholar]
  4. Busico V. Metal-Catalysed Olefin Polymerisation into the New Millennium: A Perspective Outlook. Dalton Trans. 2009, (41), 8794–8802. 10.1039/b911862b. [DOI] [PubMed] [Google Scholar]
  5. Klosin J.; Fontaine P. P.; Figueroa R. Development of Group IV Molecular Catalysts for High Temperature Ethylene-α-Olefin Copolymerization Reactions. Acc. Chem. Res. 2015, 48, 2004–2016. 10.1021/acs.accounts.5b00065. [DOI] [PubMed] [Google Scholar]
  6. Chen E. Y.-X.; Marks T. J. Cocatalysts for Metal-Catalyzed Olefin Polymerization: Activators, Activation Processes, and Structure-Activity Relationships. Chem. Rev. 2000, 100, 1391–1434. 10.1021/cr980462j. [DOI] [PubMed] [Google Scholar]
  7. Zaccaria F.; Sian L.; Zuccaccia C.; Macchioni A. Ion Pairing in Transition Metal Catalyzed Olefin Polymerization. Adv. Organomet. Chem. 2020, 73, 1–78. 10.1016/bs.adomc.2019.08.001. [DOI] [Google Scholar]
  8. Severn J. R.; Chadwick J. C.; Duchateau R.; Friederichs N. Bound but Not Gagged” Immobilizing Single-Site α-Olefin Polymerization Catalysts. Chem. Rev. 2005, 105, 4073–4147. 10.1021/cr040670d. [DOI] [PubMed] [Google Scholar]
  9. Tymińska N.; Zurek E. DFT-D Investigation of Active and Dormant Methylaluminoxane (MAO) Species Grafted onto a Magnesium Dichloride Cluster: A Model Study of Supported MAO. ACS Catal. 2015, 5, 6989–6998. 10.1021/acscatal.5b01697. [DOI] [Google Scholar]
  10. a Mason M. R.; Smith J. M.; Bott S. G.; Barron A. R. Hydrolysis of Tri-Tert-Butylaluminum: The First Structural Characterization of Alkylalumoxanes [(R2Al)2O]n and (RAlO)n. J. Am. Chem. Soc. 1993, 115, 4971–4984. 10.1021/ja00065a005. [DOI] [Google Scholar]; b Harlan C. J.; Bott S. G.; Barron A. R. Three-Coordinate Aluminum Is Not a Prerequisite for Catalytic Activity in the Zirconocene-Alumoxane Polymerization of Ethylene. J. Am. Chem. Soc. 1995, 117, 6465–6474. 10.1021/ja00129a008. [DOI] [Google Scholar]; c Watanabi M.; McMahon C. N.; Harlan C. J.; Barron A. R. Reaction of Trimethylaluminum with [(tBu)Al(μ3-O)]6: Hybrid tert-Butylmethylalumoxanes as Cocatalysts for Olefin Polymerization. Organometallics 2001, 20, 460–467. 10.1021/om000553i. [DOI] [Google Scholar]; d Koide Y.; Bott S. G.; Barron A. R. Reaction of Amines with [(tBu)Al(μ3-O)]6: Determination of the Steric Limitation of a Latent Lewis Acid. Organometallics 1996, 15, 5514–5518. 10.1021/om960677o. [DOI] [Google Scholar]
  11. Zurek E.; Ziegler T. Theoretical Studies of the Structure and Function of MAO (Methylaluminoxane). Prog. Polym. Sci. 2004, 29, 107–148. 10.1016/j.progpolymsci.2003.10.003. [DOI] [Google Scholar]
  12. Falls Z.; Tymińska N.; Zurek E. The Dynamic Equilibrium Between (AlOMe)n Cages and (AlOMe)n·(AlMe3)m Nanotubes in Methylaluminoxane (MAO): A First-Principles Investigation. Macromolecules 2014, 47, 8556–8569. 10.1021/ma501892v. [DOI] [Google Scholar]
  13. Zurek E.; Woo T. K.; Firman T. K.; Ziegler T. Modeling the Dynamic Equilibrium between Oligomers of (AlOCH3)n in Methylaluminoxane (MAO). A Theoretical Study Based on a Combined Quantum Mechanical and Statistical Mechanical Approach. Inorg. Chem. 2001, 40, 361–370. 10.1021/ic000845b. [DOI] [PubMed] [Google Scholar]
  14. Linnolahti M.; Severn J. R.; Pakkanen T. A. Are Aluminoxanes Nanotubular? Structural Evidence from a Quantum Chemical Study. Angew. Chem., Int. Ed. 2006, 45, 3331–3334. 10.1002/anie.200600197. [DOI] [PubMed] [Google Scholar]
  15. Linnolahti M.; Laine A.; Pakkanen T. A. Screening the Thermodynamics of Trimethylaluminum-Hydrolysis Products and Their Co-Catalytic Performance in Olefin-Polymerization Catalysis. Chem. - Eur. J. 2013, 19, 7133–7142. 10.1002/chem.201204450. [DOI] [PubMed] [Google Scholar]
  16. Linnolahti M.; Severn J. R.; Pakkanen T. A. Formation of Nanotubular Methylaluminoxanes and the Nature of the Active Species in Single-Site α-Olefin Polymerization Catalysis. Angew. Chem. 2008, 120, 9419–9423. 10.1002/ange.200802558. [DOI] [PubMed] [Google Scholar]
  17. Linnolahti M.; Luhtanen T. N. P.; Pakkanen T. A. Theoretical Studies of Aluminoxane Chains, Rings, Cages, and Nanostructures. Chem. - Eur. J. 2004, 10, 5977–5987. 10.1002/chem.200400428. [DOI] [PubMed] [Google Scholar]
  18. Hirvi J. T.; Bochmann M.; Severn J. R.; Linnolahti M. Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single-Site α-Olefin Polymerization Catalysis. ChemPhysChem 2014, 15, 2732–2742. 10.1002/cphc.201402298. [DOI] [PubMed] [Google Scholar]
  19. Linnolahti M.; Collins S. Formation, Structure, and Composition of Methylaluminoxane. ChemPhysChem 2017, 18, 3369–3374. 10.1002/cphc.201700827. [DOI] [PubMed] [Google Scholar]
  20. Oliva L.; Oliva P.; Galdi N.; Pellecchia C.; Sian L.; Macchioni A.; Zuccaccia C. Solution Structure and Reactivity with Metallocenes of AlMe2F: Mimicking Cation–Anion Interactions in Metallocenium–Methylalumoxane Inner-Sphere Ion Pairs. Angew. Chem., Int. Ed. 2017, 56, 14227–14231. 10.1002/anie.201707194. [DOI] [PubMed] [Google Scholar]
  21. Atwood J. L.; Hrncir D. C.; Priester R. D.; Rogers R. D. Decomposition of High-Oxygen Content Organoaluminum Compounds. The Formation and Structure of the [Al7O6Me16]− Anion. Organometallics 1983, 2, 985–989. 10.1021/om50002a008. [DOI] [Google Scholar]
  22. a Reddy S. S.; Sivaram S. Homogeneous Metallocene-Methylaluminoxane Catalyst Systems for Ethylene Polymerization. Prog. Polym. Sci. 1995, 20, 309–367. 10.1016/0079-6700(94)00035-Z. [DOI] [Google Scholar]; b Barron A. R. New Method for the Determination of the Trialkylaluminum Content in Alumoxanes. Organometallics 1995, 14, 3581–3583. 10.1021/om00007a070. [DOI] [Google Scholar]
  23. Benn R.; Rufińska A.; Lehmkuhl H.; Janssen E.; Krüger C. 27AI-NMR Spectroscopy: A Probe for Three-, Four-, Five-, and Sixfold Coordinated Al Atoms in Organoaluminum Compounds. Angew. Chem., Int. Ed. Engl. 1983, 22, 779–780. 10.1002/anie.198307791. [DOI] [Google Scholar]
  24. Babushkin D. E.; Semikolenova N. V.; Panchenko V. N.; Sobolev A. P.; Zakharov V. A.; Talsi E. P. Multinuclear NMR Investigation of Methylaluminoxane. Macromol. Chem. Phys. 1997, 198, 3845–3854. 10.1002/macp.1997.021981206. [DOI] [Google Scholar]
  25. Ystenes M.; Eilertsen J. L.; Liu J.; Ott M.; Rytter E.; Støvneng J. A. Experimental and Theoretical Investigations of the Structure of Methylaluminoxane (MAO) Cocatalysts for Olefin Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3106–3127. . [DOI] [Google Scholar]
  26. Ghiotto F.; Pateraki C.; Tanskanen J.; Severn J. R.; Luehmann N.; Kusmin A.; Stellbrink J.; Linnolahti M.; Bochmann M. Probing the Structure of Methylalumoxane (MAO) by a Combined Chemical, Spectroscopic, Neutron Scattering, and Computational Approach. Organometallics 2013, 32, 3354–3362. 10.1021/om4002878. [DOI] [Google Scholar]
  27. Tritto I.; Méalares C.; Sacchi M. C.; Locatelli P. Methylaluminoxane: NMR Analysis, Cryoscopic Measurements and Cocatalytic Ability in Ethylene Polymerization. Macromol. Chem. Phys. 1997, 198, 3963–3977. 10.1002/macp.1997.021981215. [DOI] [Google Scholar]
  28. Imhoff D. W.; Simeral L. S.; Sangokoya S. A.; Peel J. H. Characterization of Methylaluminoxanes and Determination of Trimethylaluminum Using Proton NMR. Organometallics 1998, 17, 1941–1945. 10.1021/om980046p. [DOI] [Google Scholar]
  29. Rocchigiani L.; Busico V.; Pastore A.; Macchioni A. Probing the Interactions between All Components of the Catalytic Pool for Homogeneous Olefin Polymerisation by Diffusion NMR Spectroscopy. Dalton Trans. 2013, 42, 9104–9111. 10.1039/c3dt00041a. [DOI] [PubMed] [Google Scholar]
  30. Pédeutour J.-N.; Radhakrishnan K.; Cramail H.; Deffieux A. Use of “TMA-Depleted” MAO for the Activation of Zirconocenes in Olefin Polymerization. J. Mol. Catal. A: Chem. 2002, 185, 119–125. 10.1016/S1381-1169(02)00071-7. [DOI] [Google Scholar]
  31. Tritto I.; Sacchi M. C.; Locatelli P.; Li S. X. Metallocenes/Methylalumoxanes: A 13C NMR Study of the Reaction Equilibria and Polymerization. Macromol. Symp. 1995, 97, 101–108. 10.1002/masy.19950970110. [DOI] [Google Scholar]
  32. Busico V.; Cipullo R.; Cutillo F.; Friederichs N.; Ronca S.; Wang B. Improving the Performance of Methylalumoxane: A Facile and Efficient Method to Trap “Free” Trimethylaluminum. J. Am. Chem. Soc. 2003, 125, 12402–12403. 10.1021/ja0372412. [DOI] [PubMed] [Google Scholar]
  33. Zaccaria F.; Zuccaccia C.; Cipullo R.; Budzelaar P. H. M.; Macchioni A.; Busico V.; Ehm C. BHT-Modified MAO: Cage Size Estimation, Chemical Counting of Strongly Acidic Al Sites, and Activation of a Ti-Phosphinimide Precatalyst. ACS Catal. 2019, 9, 2996–3010. 10.1021/acscatal.9b00076. [DOI] [Google Scholar]
  34. Ghiotto F.; Pateraki C.; Severn J. R.; Friederichs N.; Bochmann M. Rapid Evaluation of Catalysts and MAO Activators by Kinetics: What Controls Polymer Molecular Weight and Activity in Metallocene/MAO Catalysts?. Dalton Trans. 2013, 42, 9040–9048. 10.1039/c3dt00107e. [DOI] [PubMed] [Google Scholar]
  35. Kuklin M. S.; Hirvi J. T.; Bochmann M.; Linnolahti M. Toward Controlling the Metallocene/Methylaluminoxane-Catalyzed Olefin Polymerization Process by a Computational Approach. Organometallics 2015, 34, 3586–3597. 10.1021/acs.organomet.5b00394. [DOI] [Google Scholar]
  36. Velthoen M. E. Z.; Muñoz-Murillo A.; Bouhmadi A.; Cecius M.; Diefenbach S.; Weckhuysen B. M. The Multifaceted Role of Methylaluminoxane in Metallocene-Based Olefin Polymerization Catalysis. Macromolecules 2018, 51, 343–355. 10.1021/acs.macromol.7b02169. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Zijlstra H. S.; Linnolahti M.; Collins S.; McIndoe J. S. Additive and Aging Effects on Methylalumoxane Oligomers. Organometallics 2017, 36, 1803–1809. 10.1021/acs.organomet.7b00153. [DOI] [Google Scholar]
  38. Alternatively, after AlMe3 decoordination, sites A–C might directly abstract a methide/chloride group from the precatalyst, as proposed in early studies on MAO (see refs (10) and (11)).
  39. Luo L.; Sangokoya S. A.; Wu X.; Diefenbach S.; Kneale B.. Aluminoxane Catalyst Activators Derived From Dialkylalumium Cation Precursor Agents, Processes for Making Same, And Use Thereof In Catalysts And Polymerization Of Olefins. Patent US8575284, 2013.
  40. Trefz T. K.; Henderson M. A.; Wang M. Y.; Collins S.; McIndoe J. S. Mass Spectrometric Characterization of Methylaluminoxane. Organometallics 2013, 32, 3149–3152. 10.1021/om400256f. [DOI] [Google Scholar]
  41. Zijlstra H.; Joshi A.; Linnolahti M.; Collins S.; McIndoe J. S. Interaction of Neutral Donors with Methylaluminoxane. Eur. J. Inorg. Chem. 2019, 2019, 2346–2355. 10.1002/ejic.201900153. [DOI] [Google Scholar]
  42. Collins S.; Linnolahti M.; Zamora M. G.; Zijlstra H. S.; Rodríguez Hernández M. T.; Perez-Camacho O. Activation of Cp2ZrX2 (X = Me, Cl) by Methylaluminoxane As Studied by Electrospray Ionization Mass Spectrometry: Relationship to Polymerization Catalysis. Macromolecules 2017, 50, 8871–8884. 10.1021/acs.macromol.7b00933. [DOI] [Google Scholar]
  43. Trefz T. K.; Henderson M. A.; Linnolahti M.; Collins S.; McIndoe J. S. Mass Spectrometric Characterization of Methylaluminoxane-Activated Metallocene Complexes. Chem. - Eur. J. 2015, 21, 2980–2991. 10.1002/chem.201405319. [DOI] [PubMed] [Google Scholar]
  44. Zaccaria F.; Zuccaccia C.; Cipullo R.; Budzelaar P. H. M.; Macchioni A.; Busico V.; Ehm C. On the Nature of the Lewis Acidic Sites in ‘TMA-Free’ Phenol-Modified Methylaluminoxane. Eur. J. Inorg. Chem. 2020, 2020, 1088–1095. 10.1002/ejic.201901035. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Stapleton R. A.; Al-Humydi A.; Chai J.; Galan B. R.; Collins S. Sterically Hindered Aluminum Alkyls: Weakly Interacting Scavenging Agents of Use in Olefin Polymerization. Organometallics 2006, 25, 5083–5092. 10.1021/om060474s. [DOI] [Google Scholar]
  46. Stapleton R. A.; Galan B. R.; Collins S.; Simons R. S.; Garrison J. C.; Youngs W. J. Bulky Aluminum Alkyl Scavengers in Olefin Polymerization with Group 4 Catalysts. J. Am. Chem. Soc. 2003, 125, 9246–9247. 10.1021/ja030121+. [DOI] [PubMed] [Google Scholar]
  47. Zakharov V. A.; Panchenko V. N.; Semikolenova N. V.; Danilova I. G.; Paukshtis E. A. IRS Study of Ethylene Polymerization Catalyst SiO2/MAO/Zirconocene. Polym. Bull. 1999, 43, 87–92. 10.1007/s002890050537. [DOI] [Google Scholar]
  48. Tanaka R.; Kawahara T.; Shinto Y.; Nakayama Y.; Shiono T. An Alternative Method for the Preparation of Trialkylaluminum-Depleted Modified Methylaluminoxane (DMMAO). Macromolecules 2017, 50, 5989–5993. 10.1021/acs.macromol.7b01003. [DOI] [Google Scholar]
  49. Bashir M. A.; Vancompernolle T.; Gauvin R. M.; Delevoye L.; Merle N.; Monteil V.; Taoufik M.; McKenna T. F. L.; Boisson C. Silica/MAO/(n-BuCp)2ZrCl2 Catalyst: Effect of Support Dehydroxylation Temperature on the Grafting of MAO and Ethylene Polymerization. Catal. Sci. Technol. 2016, 6, 2962–2974. 10.1039/C5CY01285F. [DOI] [Google Scholar]
  50. It should be emphaized here that MAO likely undergoes major modification upon interaction with the support (e.g., scrambling leading to Si-Me groups; ref (49)) which significantly complicates structural understanding.
  51. Endres E.; Zijlstra H. S.; Collins S.; McIndoe J. S.; Linnolahti M. Oxidation of Methylalumoxane Oligomers: A Theoretical Study Guided by Mass Spectrometry. Organometallics 2018, 37, 3936–3942. 10.1021/acs.organomet.8b00587. [DOI] [Google Scholar]
  52. Zurek E.; Ziegler T. A Combined Quantum Mechanical and Statistical Mechanical Study of the Equilibrium of Trimethylaluminum (TMA) and Oligomers of (AlOCH3)n Found in Methylaluminoxane (MAO) Solution. Inorg. Chem. 2001, 40, 3279–3292. 10.1021/ic001444z. [DOI] [PubMed] [Google Scholar]
  53. Boudene Z.; De Bruin T.; Toulhoat H.; Raybaud P. A QSPR Investigation of Thermal Stability of [Al(CH3)O]n Oligomers in Methylaluminoxane Solution: The Identification of a Geometry-Based Descriptor. Organometallics 2012, 31, 8312–8322. 10.1021/om300916q. [DOI] [Google Scholar]
  54. Talsi E. P.; Semikolenova N. V.; Panchenko V. N.; Sobolev A. P.; Babushkin D. E.; Shubin A. A.; Zakharov V. A. The Metallocene/Methylaluminoxane Catalysts Formation: EPR Spin Probe Study of Lewis Acidic Sites of Methylaluminoxane. J. Mol. Catal. A: Chem. 1999, 139, 131–137. 10.1016/S1381-1169(98)00194-0. [DOI] [Google Scholar]
  55. Zijlstra H.; Collins S.; McIndoe J. S. Oxidation of Methylalumoxane Oligomers. Chem. - Eur. J. 2018, 24, 5506–5512. 10.1002/chem.201705458. [DOI] [PubMed] [Google Scholar]
  56. Ehm C.; Antinucci G.; Budzelaar P. H. M.; Busico V. Catalyst Activation and the Dimerization Energy of Alkylaluminium Compounds. J. Organomet. Chem. 2014, 772–773, 161–171. 10.1016/j.jorganchem.2014.09.013. [DOI] [Google Scholar]
  57. Ehm C.; Budzelaar P. H. M.; Busico V. Calculating Accurate Barriers for Olefin Insertion and Related Reactions. J. Organomet. Chem. 2015, 775, 39–49. 10.1016/j.jorganchem.2014.10.019. [DOI] [Google Scholar]
  58. Ehm C.; Cipullo R.; Budzelaar P. H. M.; Busico V. Role(s) of TMA in Polymerization. Dalton Trans. 2016, 45, 6847–6855. 10.1039/C5DT04895H. [DOI] [PubMed] [Google Scholar]
  59. Zaccaria F.; Vittoria A.; Correa A.; Ehm C.; Budzelaar P. H. M.; Busico V.; Cipullo R. Internal Donors in Ziegler-Natta Systems: Is Reduction by AlR3 a Requirement for Donor Clean-Up?. ChemCatChem 2018, 10, 984–988. 10.1002/cctc.201701422. [DOI] [Google Scholar]
  60. Jaeger A. D.; Ehm C.; Lentz D. Organocatalytic C-F Bond Activation with Alanes. Chem. - Eur. J. 2018, 24, 6769–6777. 10.1002/chem.201706061. [DOI] [PubMed] [Google Scholar]
  61. Zijlstra H. S.; Joshi A.; Linnolahti M.; Collins S.; McIndoe J. S. Modifying Methylalumoxane via Alkyl Exchange. Dalton Trans. 2018, 47, 17291–17298. 10.1039/C8DT04242J. [DOI] [PubMed] [Google Scholar]
  62. Grimme S.; Antony J.; Ehrlich S.; Krieg H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104. 10.1063/1.3382344. [DOI] [PubMed] [Google Scholar]
  63. Grimme S.; Ehrlich S.; Goerigk L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456–1465. 10.1002/jcc.21759. [DOI] [PubMed] [Google Scholar]
  64. For further considerations on this aspect, see ref (41).
  65. Macchioni A. Ion Pairing in Transition-Metal Organometallic Chemistry. Chem. Rev. 2005, 105, 2039–2074. 10.1021/cr0300439. [DOI] [PubMed] [Google Scholar]
  66. Sian L.; Macchioni A.; Zuccaccia C. Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal–Carbon Bond. ACS Catal. 2020, 10, 1591–1606. 10.1021/acscatal.9b04929. [DOI] [Google Scholar]
  67. Smith M. B. The Monomer-dimer Equilibria of Liquid Aliminum Alkyls: III. Trimethylaluminum: The Monomer-dimer Equilibria of Liquid and Gaseous Trimethylaluminum and Triethylaluminum. J. Organomet. Chem. 1972, 46, 31–49. 10.1016/S0022-328X(00)90473-X. [DOI] [Google Scholar]
  68. Babushkin D. E.; Naundorf C.; Brintzinger H. H. Distinct Methylalumoxane(MAO)-Derived Me–MAO– Anions in Contact with a Zirconocenium Cation - a 13C-NMR Study. Dalton Trans. 2006, 38, 4539–4544. 10.1039/B611028M. [DOI] [PubMed] [Google Scholar]
  69. Wieser U.; Schaper F.; Brintzinger H.-H. Methylalumoxane (MAO)-Derived MeMAO– Anions in Zirconocene-Based Polymerization Catalyst Systems – A UV-Vis Spectroscopic Study. Macromol. Symp. 2006, 236, 63–68. 10.1002/masy.200690068. [DOI] [Google Scholar]
  70. Eilertsen J. L.; Støvneng J. A.; Ystenes M.; Rytter E. Activation of Metallocenes for Olefin Polymerization As Monitored by IR Spectroscopy. Inorg. Chem. 2005, 44, 4843–4851. 10.1021/ic0482638. [DOI] [PubMed] [Google Scholar]
  71. Laine A.; Linnolahti M.; Pakkanen T. A. Alkylation and Activation of Metallocene Polymerization Catalysts by Reactions with Trimethylaluminum: A Computational Study. J. Organomet. Chem. 2012, 716, 79–85. 10.1016/j.jorganchem.2012.06.006. [DOI] [Google Scholar]
  72. Please note that here reactions of MAO with 4–10 mol % of py are considered. Larger amounts of donor would lead to cage decompositon independently from reactivity of the Lewis acidic sites (see ref (41)).
  73. Zurek E.; Ziegler T. Toward the Identification of Dormant and Active Species in MAO (Methylaluminoxane)-Activated, Dimethylzirconocene-Catalyzed Olefin Polymerization. Organometallics 2002, 21, 83–92. 10.1021/om010812j. [DOI] [Google Scholar]
  74. Chan M. S. W.; Vanka K.; Pye C. C.; Ziegler T. Density Functional Study on Activation and Ion-Pair Formation in Group IV Metallocene and Related Olefin Polymerization Catalysts. Organometallics 1999, 18, 4624–4636. 10.1021/om9903285. [DOI] [Google Scholar]
  75. Frisch M. J.et al. Gaussian 09, Revision B.1; Gaussian, Inc.: Wallingford, CT, 2009.; Full Citation in the Supporting Information.
  76. Baker J.PQS, Version 2.4; Parallel Quantum Solutions: Fayetteville, AR, 2001.
  77. Baker J. An Algorithm for the Location of Transition States. J. Comput. Chem. 1986, 7, 385–395. 10.1002/jcc.540070402. [DOI] [Google Scholar]
  78. Budzelaar P. H. M. Geometry Optimization Using Generalized, Chemically Meaningful Constraints. J. Comput. Chem. 2007, 28, 2226–2236. 10.1002/jcc.20740. [DOI] [PubMed] [Google Scholar]
  79. Tao J.; Perdew J. P.; Staroverov V. N.; Scuseria G. E. Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91, 146401. 10.1103/PhysRevLett.91.146401. [DOI] [PubMed] [Google Scholar]
  80. Balabanov N. B.; Peterson K. A. Systematically Convergent Basis Sets for Transition Metals. I. All-Electron Correlation Consistent Basis Sets for the 3d Elements Sc–Zn. J. Chem. Phys. 2005, 123, 064107. 10.1063/1.1998907. [DOI] [PubMed] [Google Scholar]
  81. Balabanov N. B.; Peterson K. A. Basis Set Limit Electronic Excitation Energies, Ionization Potentials, and Electron Affinities for the 3d Transition Metal Atoms: Coupled Cluster and Multireference Methods. J. Chem. Phys. 2006, 125, 074110. 10.1063/1.2335444. [DOI] [PubMed] [Google Scholar]
  82. Schuchardt K. L.; Didier B. T.; Elsethagen T.; Sun L.; Gurumoorthi V.; Chase J.; Li J.; Windus T. L. Basis Set Exchange: A Community Database for Computational Sciences. J. Chem. Inf. Model. 2007, 47, 1045–1052. 10.1021/ci600510j. [DOI] [PubMed] [Google Scholar]
  83. Foster J. P.; Weinhold F. Natural Hybrid Orbitals. J. Am. Chem. Soc. 1980, 102, 7211–7218. 10.1021/ja00544a007. [DOI] [Google Scholar]
  84. Zhao Y.; Truhlar D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Function. Theor. Chem. Acc. 2008, 120, 215–241. 10.1007/s00214-007-0310-x. [DOI] [Google Scholar]
  85. Tomasi J. Thirty Years of Continuum Solvation Chemistry: A Review, and Prospects for the near Future. Theor. Chem. Acc. 2004, 112, 184–203. 10.1007/s00214-004-0582-3. [DOI] [Google Scholar]
  86. Whitten J. L. Coulombic Potential Energy Integrals and Approximations. J. Chem. Phys. 1973, 58, 4496–4501. 10.1063/1.1679012. [DOI] [Google Scholar]
  87. Baerends E. J.; Ellis D. E.; Ros P. Self-Consistent Molecular Hartree—Fock—Slater Calculations I. The Computational Procedure. Chem. Phys. 1973, 2, 41–51. 10.1016/0301-0104(73)80059-X. [DOI] [Google Scholar]
  88. Feyereisen M.; Fitzgerald G.; Komornicki A. Use of Approximate Integrals in Ab Initio Theory. An Application in MP2 Energy Calculations. Chem. Phys. Lett. 1993, 208, 359–363. 10.1016/0009-2614(93)87156-W. [DOI] [Google Scholar]
  89. Vahtras O.; Almlöf J.; Feyereisen M. W. Integral Approximations for LCAO-SCF Calculations. Chem. Phys. Lett. 1993, 213, 514–518. 10.1016/0009-2614(93)89151-7. [DOI] [Google Scholar]
  90. Tobisch S.; Ziegler T. Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by the Cationic Group 4 [(H5-Cp-(CMe2-Bridge)-Ph)MII(Ethylene)2]+ (M = Ti, Zr, Hf) Active Catalysts: A Density Functional Investigation of the Influence of the Metal on the Catalyt. J. Am. Chem. Soc. 2004, 126, 9059–9071. 10.1021/ja048861l. [DOI] [PubMed] [Google Scholar]
  91. Zaccaria F.; Ehm C.; Budzelaar P. H. M.; Busico V. Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity. ACS Catal. 2017, 7, 1512–1519. 10.1021/acscatal.6b03458. [DOI] [Google Scholar]
  92. Zaccaria F.; Cipullo R.; Budzelaar P. H. M.; Busico V.; Ehm C. Backbone Rearrangement during Olefin Capture as the Rate Limiting Step in Molecular Olefin Polymerization Catalysis and Its Effect on Comonomer Affinity. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2807–2814. 10.1002/pola.28685. [DOI] [Google Scholar]

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