BHT-modified MAO: cage size estimation, chemical counting of

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BHT-modified MAO: cage size estimation, chemical counting of strongly acidic Al-sites and activation of a Ti-phosphinimide precatalyst Francesco Zaccaria, Cristiano Zuccaccia, Roberta Cipullo, Peter H.M. Budzelaar, Alceo Macchioni, Vincenzo Busico, and Christian Ehm ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00076 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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BHT-modified MAO: cage size estimation, chemical counting of strongly acidic Al-sites and activation of a Ti-phosphinimide precatalyst Francesco Zaccaria,a,b Cristiano Zuccaccia,b,* Roberta Cipullo,a Peter H. M. Budzelaar,a Alceo Macchioni,b Vincenzo Busicoa and Christian Ehma,*. a. Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy. b. Dipartimento di Chimica, Biologia e Biotecnologie and CIRCC, Università di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

ABSTRACT. MAO/BHT (MAO = methylaluminoxane; BHT = 2,6-di-tert-butyl-4-methylphenol) cocatalyst for olefin polymerization has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)0.1]n cages and monomeric MeAl(bht)2. Diffusion NMR indicates an average n for Al-clusters of 62-96, i.e. 2-3 times higher than that estimated for unmodified MAO under analogous conditions (n ≈ 26-41). The reactivity of MAO/BHT has been explored by monitoring the activation of the Cp*-phosphinimide titanium dichloride precatalyst Cp*(tBu3P=N)TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixtures obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=N)TiX]2(µ-Y)+ (X, Y = Me or Cl) forming ACS Paragon Plus Environment

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Outer Sphere Ion Pairs (OSIPs) with MAO/BHT-derived anions are dominant at low Al/Ti ratios, whereas mononuclear Inner Sphere Ion Pairs (ISIPs) [Cp*(tBu3P=N)Ti-X]+[MAO/BHT]- are formed at high Al/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analogue Cp*(tBu3P=N)TiBn2 results in the clean formation of [Cp*(tBu3P=N)Ti-Bn]+[MAO/BHT]- OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amount of strongly acidic sites on Al-clusters, shedding further light on the structure and properties of MAO/BHT.

KEYWORDS. Methylaluminoxane, Modified MAO, DOSY NMR, olefin polymerization catalysis, chemical shift predictions, activation, DFT, post-metallocene.

INTRODUCTION Methylaluminoxane (MAO) is the typical cocatalyst of choice in molecular olefin polymerization catalysis, due to its remarkable combination of alkylating, activating and impurity scavenging properties.1, 2 Additionally, it offers the possibility to be heterogenized by absorption on supports such as silica or alumina, expanding the application range of molecular catalysts from solution to slurry and gas phase reactors.3 It is generally accepted that MAO consists of a distribution of cagelike oligomers (AlOMe)n decorated by and in equilibrium with trimethylaluminum (TMA),4-14 although no definitive consensus has been reached on its structure, yet. The average number of Alatoms per cage (n)7, 8, 11-21 and the amount of strongly acidic Al-sites on MAO clusters8, 12, 22-25 are some of the most debated aspects. Often, the presence of TMA represents a major Achilles’ heel of typical MAO solutions. Though playing a positive role as methylating agent and impurity scavenger,9, 10, 26 ‘free' TMA can also

ACS Paragon Plus Environment

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decrease catalyst productivity and polymer molecular weight by forming rather stable heterodinuclear adducts M-(μ-X)m-Al with group IV cationic complexes (M = transition metal, X = Me or Cl, m = 1-2), suspected to be ‘dormant’ in olefin polymerization and involved in chain transfer to Al.27-35 The highly reactive TMA can also trigger catalyst deactivation.26, 36 A growing scientific and commercial interest is therefore directed at the development of different grades of Modified Methylaluminoxanes (MMAO), aiming at addressing some of its drawbacks, while preserving desirable MAO properties.1 Replacing methyl with bulkier iso-butyl or n-octyl groups,37-39 or following non-hydrolytic processes to ensure that virtually no residual TMA remains,40, 41 represent promising commercial approaches to different grades of MMAO. Alternative strategies have been proposed to modify typical commercial MAO solutions. For instance, TMA (b.p. 125 °C) can be physically removed under vacuum, although it has been debated if this removal is quantitative.20, 22, 42 An even simpler and highly effective method to trap TMA is to add a sterically hindered phenol (HP), such as 2,6-di-tert-butyl-4-methylphenol (BHT).28 TMA is thereby transformed into the far less reactive aryloxide complex MeAl(bht)2 (bht = BHT phenolate).43, 44 This chemical modification of MAO has been successfully applied in many polymerization studies,27-29, 31, 45, 46 including industrially relevant cases,47 but it has been rarely exploited for spectroscopic mechanistic investigations.20, 27 To the best of our knowledge, the sole example of an NMR study on a ternary system, catalyst/MAO/HP,20 focused on the tendency of Cp2ZrMe+ to form Inner (ISIPs) or Outer Sphere Ion Pairs (OSIPs)48, 49 with counterions and higher aggregates thereof. In the present paper, the structure and reactivity of MAO/BHT are explored in a more comprehensive manner. Detailed insights into its composition and Al-cage size have been obtained


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by diffusion ordered NMR spectroscopy (DOSY); although analogous 1D pulsed gradient spinecho (PGSE) experiments have been widely applied in this7, 8, 20, 49, 50 and other51 fields, 2D DOSY is particularly suited to analyze complex reaction mixtures52 and it has been only occasionally applied to MAO-type cocatalysts.53 The reactivity of MAO/BHT has been explored by monitoring the activation of a representative phosphinimide precatalyst Cp*(tBu3P=N)TiCl2 (1-Cl2, Figure 1) at varying Al/Ti ratios, using the P atom in the ancillary ligand as spectroscopic handle. Independently synthesized model compounds using well-defined molecular activators and DFT modelling facilitated identification of activation products in complex reaction mixtures. The catalytic activity of the observed species in 1-hexene polymerization was also evaluated, providing a rare example of a systematic study of activation of an industrially relevant catalyst class54 by a TMA-free MMAO. Finally, a simple and effective method to estimate the average number of strongly acidic sites on the Al-clusters is proposed based on the dibenzyl precatalyst 1-Bn2. Tentative structural and functional conclusions on the oligomeric fraction of MAO/BHT combining all results are presented.

Ti N tBu3P

X X

1-X2

Figure 1. Studied phosphinimide half-titanocene precatalyst. X = Me, Cl or Bn.


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RESULTS AND DISCUSSION Preparation and properties of MAO/BHT MAO/BHT was prepared by adding BHT to a diluted commercial solution of MAO in toluene, as described in ref. 28. The resulting mixture was subsequently dried under vacuum to avoid large peaks of toluene in 1H NMR spectra, providing solid-(MAO/BHT) [s(MAO/BHT)] as a white powder with good solubility properties in aromatic solvents. The phenol reacts preferentially with TMA,8, 27-29 estimated to account for 30-33% of the total Al in the starting MAO solution. However, in the late stages of this process, 1H NMR revealed that side reactions of small amounts of BHT with the (AlOMe)n cages occur, giving broad Al-phenolate signals (Figure S1).20 Relative integration of NMR peaks suggests an approximate formula of {[AlOMe0.9(bht)0.1]ˑ[MeAl(bht)2]0.3}n for s(MAO/BHT). Addition of a large excess of BHT does not affect the ratio between Al-Me and Al-bht groups on MAO-type clusters significantly,20 and does not kill the activation properties of the cocatalyst, suggesting that only the most sterically accessible Al-sites react with the phenol. s(MAO/BHT) was also analyzed by DOSY NMR. In typical 2D-DOSY plots, the chemical shift of the various species in solution is resolved according to their translational self-diffusion coefficients (Dt),52 as shown in Figure 2a for the case of s(MAO/BHT). Via the Stokes-Einstein equation, Dt values can be correlated to the hydrodynamic radius (rH) or volume (VH) of the analyte,55 providing information on molecular dimensions.56 Here, toluene solutions were analyzed at 298 K, using triphenylmethane (Ph3CH)57 as internal standard to account for temperature and viscosity fluctuations.56 Two components are found for s(MAO/BHT), having Dt equal to 7.03 and 2.37 ˑ 10-10 m2/s (Entries 2-3; Table 1), in line with previous reports.20 The larger


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Dt value of the first component is comparable to that of isolated MeAl(bht)2, which was independently synthesized by reacting TMA with BHT (Al/BHT 1:2; Entry 6, Table 1), and it is therefore diagnostic of a free-diffusing bis(phenolate) complex. The second component, having a smaller Dt, is ascribed to [AlOMe0.9(bht)0.1]n cages; as the signal attenuation for Al-Me and Al-bht fragments on these cages exhibit a similar dependence on the gradient strength (Figure 2a), it appears that the phenolate groups are rather homogeneously distributed among the Al-clusters.

Figure 2. DOSY maps (toluene-d8, 298K) of s(MAO/BHT) (a) and MeAl(bht)2-depleted washed s(MAO/BHT) (b). [AlOMe0.9(bht)0.1] ≈ 30-35 mM. Ph3CH was used as internal viscosity standard (see main text). *Traces of residual MeAl(bht)2, estimated to account for 100 eq.) to 1X+[s(MAO/BHT)]- results in a 31P NMR spectrum (Figure 4e) analogous to the one obtained with TTB activator (Figure 3g), i.e. a major sharp signal at 54.0 ppm and other minor, broader peaks at 55.5-57.2 ppm. The dormant allyl complex 1-All+[s(MAO/BHT)]- (54.0 ppm) is likely formed via CTM, and it accumulates along with some other decomposition/inactive products (55.5-57.2 ppm, probably deriving from decomposition of in situ generated Ti-H species).68 The sharpness of the peak at 54.0 ppm indicates that also 1-All+ forms OSIP with MAO/BHT anions, as the η3-allyl coordination prevents a tight cation/anion interaction.


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When the amount of 1-hexene used is lowered to only ~10 eq., 31P NMR evidence of the formation of 1-Bn+ via solvent rather than monomer activation was obtained (Figure S11). These observations are in line with those obtained with TTB activator,68 and provide direct NMR evidence for the formation of titanium allyl and benzyl type complexes under polymerization conditions also in the more complex case of s(MAO/BHT) cocatalyst. The rapid disappearance of starting 1-X+[s(MAO/BHT)]- broad peaks in the 31P NMR spectrum, indicates that at least the clear majority of ISIPs reacts with the monomer. These observations point to ISIPs dissociation being relatively facile, in contrast to what has been reported for metallocenes in combination with commercial MAO. In such cases, the presence of TMA leads to the preferential formation of OSIPs of bimetallic Ti-Al adducts and only a fraction of the transition metal cations giving a tight interaction with the MAO-counterions.7,

24, 34, 66

Due to the high

dissociation barriers predicted for the latter, ISIPs have been proposed as plausible dormant sites.23, 34, 78, 80

Our somewhat different observations indicate that a close binding with MAO-type anions

does not necessarily prevent monomer coordination to the cationic active species, at least for the present catalyst class. Chemical estimation of strongly Lewis acidic Al-sites The Al/Ti ratio of ~280 necessary for full activation of 1-Cl2 indicates that only a rather small fraction of Al atoms on s(MAO/BHT) actually reacts with the precatalyst, as it is generally accepted for MAO-activators.6,

12, 23

In principle, this type of reaction monitoring provides an

estimate of the stoichiometry of the activation process in terms of total Al vs Ti atoms, from which the percentage of reactive Al-center can be derived. However, this approach faces some practical hurdles, such as: A) possible side reactions like dimer formation can compete with catalyst


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activation; B) the difficulties in accurately quantifying the species in solution; C) concentrationdependent Al-cage reorganizations that might be induced upon sequential additions of cocatalyst to the reaction mixture.20 The double alkylating/abstracting role carried out by MAO in activating dichloride complexes adds further complexity. Based on the above described results concerning 1-X2 complexes, it was envisaged that quantitative monitoring of the activation of 1-Bn2 might allow to circumvent all these complications, and provide a reliable chemical estimate of the sole abstracting sites on s(MAO/BHT) cages.81 1-Bn2 is an already ‘alkylated’ species that, upon activation, does not undergo dimer formation but only yields monomeric 1-Bn+ (Figure 5). Furthermore, the Ti-Bn+ cation forms an OSIP with s(MAO/BHT) counterions, giving a sharp

31P

NMR signal and

diagnostic 1H NMR peaks (e.g. the ortho-H of the Ti-benzyl group at δH = 7.18 ppm) appearing in relatively clean regions of the spectra, making accurate integration (i.e. quantification) of this species particularly easy. Finally, experiments were designed to keep the concentration of s(MAO/BHT) relatively low ([AlOMe0.9(bht)0.1] ≈ 24 mM) and constant throughout the reaction, to prevent gelification/precipitation and avoid concentration dependent self-aggregation of Alcages. The latter should be rather negligible for phenol-modified MAO, especially at such low concentrations (see Experimental Section for further details), as previously reported.20


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Ti N

Bn

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A-

s(MAO/BHT) Ti

Bn

N tBu3 P

tBu3 P 1-Bn2

1-Bn+[s(MAO/BHT)]A-

= [s(MAO/BHT)]-

Figure 5. Exemplifying 31P NMR (toluene-d8, 298 K) spectra of the activation of 1-Bn2 after sequential additions of s(MAO/BHT), highlighting the clean formation of an OSIP of 1-Bn+ and MAO/BHT counterion as the only reaction product. The build-up of 1-Bn+ was monitored by NMR up to a constant concentration value; most of the product is formed already in the first 5-10 min (Figure 6). The initial concentration of Al-Me groups on s(MAO/BHT) cages and the final concentration of cationic Ti-benzyl complex were


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measured with respect to an external standard. In this way, it was found that 1 out of ~42 Al-sites on [AlOMe0.9(bht)0.1]n clusters is capable to abstract a benzyl group from the Ti-center.82 Analogous experiments were carried out on MeAl(bht)2-depleted s(MAO/BHT), prepared by extracting the bis(aryloxide) complex with pentane. Very similar results were obtained (1 of ~45 Al-sites), further indicating that removal of MeAl(bht)2 has only marginal effects on the oligomeric component of s(MAO/BHT). With respect to established methods using organic Lewis bases as molecular probes (e.g. TEMPO),19 the proposed procedure offers the advantage of exploiting a real abstraction reaction for the quantification of strongly acidic sites, although it cannot not contribute to the elucidation of the abstraction mechanism (e.g. involving sterically strained Al atoms on MAO cages4, 16, 83 or transient [Me2Al]+ cations17, 63, 84).

0.7 0.6 0.5

[1-Bn+] (mM)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.3 0.2 0.1 0.0 0

20

40

60

t (min)

80

100

Figure 6 Concentration of 1-Bn+ as a function of 1-Bn2 + s(MAO/BHT) reaction time. Determined with respect to an external standard by using 1H NMR (toluene-d8, 298 K). 15% error assumed on concentrations.


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Assuming one ‘working’ acidic site per cage,6,

19, 23

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these results suggest that the average

[AlOMe0.9(bht)0.1]n cluster should contain n ≈ 42-45 Al atoms. This value is somewhat smaller than that estimated by diffusion NMR (n ≈ 62-96), but - considering all the approximations involved - the agreement is quite satisfactory. Alternatively, one might speculate that the average number of acidic sites per cage is actually >1; examples of dianionic MAO cages have been occasionally reported, for instance by mass spectrometry.11 Although this second explanation appears plausible,39 these results do not allow to draw definitive conclusions in this respect.

CONCLUSIONS NMR spectroscopy has been used to investigate the structure and reactivity of TMA-free solid(MAO/BHT) cocatalyst. First, some structural features have been identified by diffusion NMR, such as A) an approximate formula of {[AlOMe0.9(bht)0.1]ˑ[MeAl(bht)2]0.3}n, B) the free-diffusing nature of the MeAl(bht)2 component and C) an average cage size of 62-96 metal atoms. Then, the activation properties of s(MAO/BHT) have been tested by reacting it with the phosphinimide half-titanocene 1-Cl2. With the help of ad-hoc synthesized model species and DFT calculations, it has been found that at low Al/Ti ratios, rather stable Cl-bridged and partially methylated Ti-dimers are formed, while no indications for Ti/Al heterodinuclear adducts are observed. The dimers easily dissociate in the presence of 1-hexene, releasing a cationic alkyl complex that initiates chain growth. Addition of monomer seems to have a beneficial effect on the activation of the remaining chlorinated species, likely because it induces dissociation of the dimers, making the neutral precursors more prone to react with s(MAO/BHT). At Al/Ti ratios around 280, the abstraction process is complete, and the dimers are fully converted to ISIPs with s(MAO/BHT)


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anions. Also in this case, addition of 1-hexene rapidly leads to dissociation of the ion pair, showing that close binding of MAO-type counterions to the transition metal cation is not necessarily a significant hurdle for monomer capture. The activated species 1-X+ containing MAO-type anions exhibit a similar reactivity to that recently reported for analogous ion pairs with [B(C6F5)4]counterions; homolytic solvent and monomer activation are observed when reacting 1-X+ with suitable amounts of 1-hexene. The activation process of 1-Cl2 has been compared to that of analogous 1-Me2 and 1-Bn2. In particular, accurate monitoring of the activation of the dibenzyl complex was exploited to derive another important structural property of s(MAO/BHT), namely the fraction of strongly acidic Alcenters (1 of ~45). Under the assumption of only one strongly acidic site per cage, the method also provides an estimate for the average cage size that is not too different from the one based on diffusion NMR (n ≈ 62-96). However, it cannot be excluded that the discrepancy between these two estimations is actually due to the presence of slightly more than one reactive center on MAO/BHT clusters, on average. The higher amount of s(MAO/BHT) necessary to activate 1-Cl2 rather than 1-Bn2 is likely due to the double alkylating/abstracting role of the cocatalyst, as well as to competing dimer formation. In any case, the Al/Ti ratios used here are significantly lower than those typically used in bulk polymerization experiments to achieve optimal productivities (in the order of 103-104),1,

85

suggesting that a large part of this Al excess serves different purposes like impurity scavenging. This is in line with analogous observations on MAO17, 18, 86 and dMAO.22

EXPERIMENTAL PART


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Synthesis. All manipulations were performed under rigorous exclusion of oxygen and moisture in flame-dried Schlenk-type glassware interfaced to a high-vacuum line (< 10-5 Torr), or in a nitrogen-filled MBraun glovebox (36h. 1-MeBn. 1H NMR (400 MHz, toluene-d8): 1.91 (s, 15H, Cp- CH3), 2.80 (d, 1H, 2J = 10.6 Hz, TiCH2-Ph), 1.84 (d, 1H, Ti-CH2-Ph), 1.25 (d, 27H, JPH = 12.6 Hz, C(CH3)3), 0.53 (s, 3H, Ti-Me) ppm.

13C

NMR (100 MHz, toluene-d8): 118.8 (C5Me5), 68.3 (Ti-CH2-Ph), 48.5 (Ti-Me), 41.5

(C(CH3)3), 29.8 (C(CH3)3), 12.0 (Cp-CH3) ppm. 31P NMR (161 MHz, toluene-d8): 33.0 ppm. Diffusion NMR experiments. DOSY NMR measurements were performed at 298K in toluene-d8 on a Bruker Avance III HD 400 spectrometer equipped with a smart probe with a z gradient coil, by using the standard double-stimulated echo pulse sequence without spinning. The shape of the gradients was rectangular, their duration (δ) was 4 ms, and their strength (G) was varied during the experiments. All the spectra were acquired using 32K points, 256 increments, 128 scans, a spectral width of 6400 Hz, an acquisition time of 0.5 s and a relaxation delay of 1 s per transient. The DOSY spectra were processed by means of the Bruker Dynamics Center software package (version 2.5.5), using a line broadening of 1.0 Hz in the direct dimension. The DOSY maps were obtained by using the Inverse Laplace Transform routine and choosing 256 points in the vertical dimension. Since the aliphatic resonance of toluene-d8 overlaps with that MAO/BHT, selfdiffusion coefficients (Dt) were calculated with respect to that of Ph3CH, which was used as internal reference standard.58 The Dt of Ph3CH in toluene-d8 was separately calibrated using the diffusion coefficient of residual solvent resonance in a dilute solution (0.1 mM). The latter was calibrated using an external sample of HDO in D2O88 under the same exact conditions. Dt data were treated as described in the literature in order to derive the hydrodynamic dimensions.56


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Low concentrations of oligomeric Al-Me groups have been used (30-35 mM), which represent a good compromise between those typically used in catalysis1, 85 and the often higher ones used in NMR studies to guarantee a good signal to noise ratio.7,

8, 20

Furthermore, they should be

sufficiently low to prevent any gelification/precipitation and, more generally, minimize cage selfaggregation phenomena.20, 89 DFT Calculations. Following the protocol described in ref. 90, all geometries were fully optimized by using the Gaussian 09 software package91 in combination with the OPTIMIZE routine of Baker92-93 and the BOpt software package.94 All relevant minima and transition states were fully optimized at the TPSSTPSS level95 of theory employing correlation-consistent polarized valence double-ζ Dunning (DZ) basis sets (cc-pVDZ quality)96, 97 from the EMSL basis set exchange library.98 The density fitting approximation was used at the optimization stage (Resolution of Identity, RI).99 All calculations were performed at the standard Gaussian 09 quality settings [Scf=Tight and Int(Grid=Fine)]. All structures represent either true minima (as indicated by the absence of imaginary frequencies) or transition states (with exactly one imaginary frequency corresponding to the reaction coordinate). Final single-point energies were calculated at the M062X level of theory99 employing triple-ζ Dunning (TZ) basis sets (cc-pVTZ quality).96 Although M06-2X is usually recommended for main group thermochemistry, it has been found that it accurately reproduces different experimental problems concerning d0 and d1 early transition metal systems in olefin polymerization32, 45, 68, 90, 101, 102 and hydrodefluorination.103 Aluminum chemistry is similarly well described,13,

35, 104

including MAO related problems.13 Solvent effects were

included with the polarizable continuum model approach (PCM; solvent=toluene, ε=2.3741) at this stage.105 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


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corrections were scaled by a factor of 0.67 to account for decreased entropy in the condensed phase.100, 106 The GIAO method107 at the TPSSTPSS level in combination with basis sets optimized for this method (aug-cc-pVTZ-J basis set108 for Ti and IGLO-III109-110 for all other atoms) and PCM was used for 31P NMR chemical shift predictions. AUTHOR INFORMATION Corresponding Author *E-mail for C.Z.: [email protected]. *E-mail for C.E.: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional NMR and DFT details (PDF) Coordinates for optimized geometries (XYZ) 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). F.Z. thanks INSTM and CIRCC for a post-doc grant. ABBREVIATIONS


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MAO, methylaluminoxane; MMAO, modified methylaluminoxane; TMA, trimethylaluminum; BHT, 2,6-di-tert-butyl-4-methylphenol; HP, hindered phenol; ISIP, inner sphere ion pair; OSIP, other sphere ion pair; DOSY, diffusion ordered NMR spectroscopy; PGSE, pulsed gradient spin echo; DFT, density functional theory; TTB, trityl borate; CTM, chain transfer to monomer; CTS, chain transfer to solvent; TEMPO, 2,2,6,6-Tetramethylpiperidine 1-oxyl. REFERENCES 1.

Zijlstra, H. S.; Harder, S., Methylalumoxane – History, Production, Properties, and

Applications. Eur. J. Inorg. Chem. 2015, 2015, 19-43. 2.

Kaminsky, W., Discovery of Methylaluminoxane as Cocatalyst for Olefin Polymerization.

Macromolecules 2012, 45, 3289-3297. 3.

Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N., “Bound but Not Gagged”:

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46. a) Bashir, M. A.; Monteil, V.; Boisson, C.; McKenna, T. F. L., The Effect of Aluminum Alkyls and BHT-H on Reaction Kinetics of Silica Supported Metallocenes and Polymer Properties in Slurry Phase Ethylene Polymerization. J. Appl. Polym. Sci. 2018, 135, 45670; b) Collins, R. A.; Russell, A. F.; Scott, R. T. W.; Bernardo, R.; van Doremaele, G. H. J.; Berthoud, A.; Mountford, P., Monometallic and Bimetallic Titanium κ1-Amidinate Complexes as Olefin Polymerization Catalysts. Organometallics 2017, 36, 2167-2181; c) Sun, Y.; Xu, B.; Shiono, T.; Cai, Z., Highly Active ansa-(Fluorenyl)(amido)titanium-Based Catalysts with Low Load of Methylaluminoxane for Syndiotactic-Specific Living Polymerization of Propylene. Organometallics 2017, 36, 30093012; d) Cipullo, R.; Busico, V.; Fraldi, N.; Pellecchia, R.; Talarico, G., Improving the Behavior of Bis(phenoxyamine) Group 4 Metal Catalysts for Controlled Alkene Polymerization. Macromolecules 2009, 42, 3869-3872; e) Tynys, A.; Eilertsen, J. L.; Seppälä, J. V.; Rytter, E., Propylene Polymerizations with a Binary Metallocene System—Chain Shuttling Caused by Trimethylaluminium between Active Catalyst Centers. J. Polym. Sci. A: Polym. Chem. 2007, 45, 1364-1376; f) Turunen, J. P. J.; Pakkanen, T. T., Suppression of Metallocene Catalyst Leaching by the Removal of Free Trimethylaluminum from Methylaluminoxane. J. Appl. Polym. Sci. 2006, 100, 4632-4635; g) Romano, D.; Ronca, S.; Rastogi, S., Activation of a Bis-(Phenoxyimine) Titanium (IV) Catalyst Using Different Aluminoxane Co-Catalysts. Macromol. Symp. 2015, 356, 61-69; h) Tynys, A.; Eilertsen, J. L.; Rytter, E., Zirconocene Propylene Polymerisation: Controlling Termination Reactions. Macromol. Chem. Phys. 2006, 207, 295-303; i) Descour, C.; Sciarone, T. J. J.; Cavallo, D.; Macko, T.; Kelchtermans, M.; Korobkov, I.; Duchateau, R., Exploration of the Effect of 2,6-(t-Bu)2-4-Me-C6H2OH (BHT) in Chain Shuttling Polymerization. Polym. Chem. 2013, 4, 4718-4729; l) Quintanilla, E.; di Lena, F.; Chen, P., Chain Transfer to Aluminium in MAO-activated Metallocene-Catalyzed Polymerization Reactions. Chem. Commun.


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2006, 4309-4311; m) Eric P. Wasserman, E. P.; Westwood, A. D.; Yu, Z.; Oskamb, J. H.; Duenas, S. L., Synchrotron XAS Studies on a Half-Sandwich Titanium-based Polyethylene Catalyst. J. Mol. Cat. A; 2001, 172, 67-80. 47. van Doremaele, G.; van Duin, M.; Valla, M.; Berthoud, A., On the Development of Titanium κ1-Amidinate Complexes, Commercialized as Keltan ACE™ Technology, Enabling the Production of an Unprecedented Large Variety of EPDM Polymer Structures. J. Polym. Sci. A: Polym. Chem. 2017, 55, 2877-2891. 48. Macchioni, A., Ion Pairing in Transition-Metal Organometallic Chemistry. Chem. Rev. 2005, 105, 2039-2074. 49. Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A.; Marks, T. J., NOE and PGSE NMR Spectroscopic Studies of Solution Structure and Aggregation in Metallocenium Ion-Pairs. J. Am. Chem. Soc. 2004, 126, 1448-1464. 50. a) Alonso-Moreno, C.; Lancaster, S. J.; Zuccaccia, C.; Macchioni, A.; Bochmann, M., Evidence for Mixed-Ion Clusters in Metallocene Catalysts: Influence on Ligand Exchange Dynamics and Catalyst Activity. J. Am. Chem. Soc. 2007, 129, 9282-9283; b) Rocchigiani, L.; Zuccaccia, C.; Zuccaccia, D.; Macchioni, A., Self-Aggregation Tendency of Zirconocenium Ion Pairs Which Model Polymer-Chain-Carrying Species in Aromatic and Aliphatic Solvents with Low Polarity. Chem. Eur. J. 2008, 14, 6589-6592; c) Alonso-Moreno, C.; Lancaster, S. J.; Wright, J. A.; Hughes, D. L.; Zuccaccia, C.; Correa, A.; Macchioni, A.; Cavallo, L.; Bochmann, M., Ligand Mobility

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Macchioni, A.; Zuccaccia, D.; Zuccaccia, C., Synthesis, Characterization, Interionic Structure, and Self-Aggregation Tendency of Zirconaaziridinium Salts Bearing Long Alkyl Chains. Organometallics 2011, 30, 100-114. 51. a) Cohen, Y.; Avram, L.; Frish, L., Diffusion NMR Spectroscopy in Supramolecular and Combinatorial Chemistry: An Old Parameter—New Insights. Angew. Chem. Int. Ed. 2005, 44, 520-554; b) Pregosin, P. S., Applications of NMR Diffusion Methods with Emphasis on ion Pairing in Inorganic Chemistry: a Mini-review. Magn. Res. Chem. 2017, 55, 405-413; c) Rocchigiani, L.; Macchioni, A., Disclosing the Multi-faceted World of Weakly Interacting Inorganic Systems by Means of NMR Spectroscopy. Dalton Trans. 2016, 45, 2785-2790; d) Macchioni, A.; Romani, A.; Zuccaccia, C.; Guglielmetti, G.; Querci, C., Experimental Evidence for the Aggregation of [(Phen)2Pd2(μ-H)(μ-CO)]+ in Solution. Organometallics 2003, 22, 15261533; e) Zuccaccia, D.; Pirondini, L.; Pinalli, R.; Dalcanale, E.; Macchioni, A., Dynamic and Structural NMR Studies of Cavitand-Based Coordination Cages. J. Am. Chem. Soc. 2005, 127, 7025-7032; f) Pettirossi, S.; Bellachioma, G.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D.; Macchioni, A., Diffusion and NOE NMR Studies on Multicationic DAB-Organoruthenium Dendrimers: Size-Dependent Noncovalent Self-Assembly to Megamers and Ion Pairing. Chem. Eur. J. 2009, 15, 5337-5347; g) Rocchigiani, L.; Bellachioma, G.; Ciancaleoni, G.; Crocchianti, S.; Laganà, A.; Zuccaccia, C.; Zuccaccia, D.; Macchioni, A., Anion-Dependent Tendency of DiLong-Chain Quaternary Ammonium Salts to Form Ion Quadruples and Higher Aggregates in Benzene. ChemPhysChem 2010, 11, 3243-3254. 52. a) Li, D.; Keresztes, I.; Hopson, R.; Williard, P. G., Characterization of Reactive Intermediates by Multinuclear Diffusion-Ordered NMR Spectroscopy (DOSY). Acc. Chem. Res. 2009, 42, 270-280; b) Macchioni, A.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D., Diffusion


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Ordered NMR Spectroscopy (DOSY). In Supramolecular Chemistry, Gale, P. A.; Steed, J. W., Eds. John Wiley & Sons, Ltd.: Hoboken, New Jersey, USA, 2012; pp 319-330. 53. Eilertsen, J. L.; Hall, R. W.; Simeral, L. S.; Butler, L. G., Tools and Strategies for Processing Diffusion-Ordered 2D NMR Spectroscopy (DOSY) of a Broad, Featureless Resonance: an Application to Methylaluminoxane (MAO). Anal. Bioanal. Chem. 2004, 378, 15741578. 54. Stephan, D. W., The Road to Early-Transition-Metal Phosphinimide Olefin Polymerization Catalysts. Organometallics 2005, 24, 2548-2560. 55. Edward, J. T., Molecular Volumes and the Stokes-Einstein Equation. J. Chem. Educ. 1970, 47, 261. 56. Macchioni, A.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D., Determining Accurate Molecular Sizes in Solution Through NMR Diffusion Spectroscopy. Chem. Soc. Rev. 2008, 37, 479-489. 57. The stability of s(MAO/BHT) in the presence of Ph3CH has been tested over >16h, observing no significant changes in the 1H NMR spectrum. 58. Zuccaccia, D.; Macchioni, A., An Accurate Methodology to Identify the Level of Aggregation in Solution by PGSE NMR Measurements: The Case of Half-Sandwich Diamino Ruthenium(II) Salts. Organometallics 2005, 24, 3476-3486. 59. This strategy has been occasionally adopted to simplify interpretation of NMR spectra (see Supporting Information).


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60. Bellachioma, G.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D.; Macchioni, A., NMR Investigation of Non-covalent Aggregation of Coordination Compounds Ranging from Dimers and Ion Pairs up to Nano-aggregates. Coord. Chem. Rev. 2008, 252, 2224-2238. 61. Benn, R.; Janssen, E.; Lehmkuhl, H.; Rufińska, A.; Angermund, K.; Betz, P.; Goddard, R.; Krüger, C., Drei- oder Vierfach-Koordination des Aluminiums in Alkylaluminiumphenoxiden und deren Unterscheidung durch 27Al-NMR-Spektroskopie. J. Organomet. Chem. 1991, 411, 37-55. 62. Upper limit for VH0: 0.9*80 + 0.1*460 = 118 Å3; lower limit (assuming 430 Å3 for AlO(bht)): 0.9*67 + 0.1*430 = 103.3 Å3. The 1.8 correction factor for VH proposed by Stellbrink, Linnolahti and Bochmann (ref. 8) is considered for estimating n. 63. 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. 64. Zijlstra, H.; Collins, S.; McIndoe, J. S., Oxidation of Methylalumoxane Oligomers. Chem. Eur. J. 2018, 24, 5506-5512. 65. a) von Lacroix, K.; Heitmann, B.; Sinn, H., Behaviour of Differently Produced Methylalumoxanes in The Phase Separation with Diethyl Ether and Molecular Weight Estimations. Macromol. Symp. 1995, 97, 137-142; b) Eilertsen, J. L.; Rytter, E.; Ystenes, M., In Situ FTIR Spectroscopy During Addition of Trimethylaluminium (TMA) to Methylaluminoxane (MAO) Shows No Formation of MAO–TMA Compounds. Vib. Spectrosc. 2000, 24, 257-264.


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66. Bryliakov, K. P.; Talsi, E. P.; Bochmann, M., 1H and 13C NMR Spectroscopic Study of Titanium(IV) Species Formed by Activation of Cp2TiCl2 and [(Me4C5)SiMe2NtBu]TiCl2 with Methylaluminoxane (MAO). Organometallics 2004, 23, 149-152. 67. Ma, K.; Piers, W. E.; Parvez, M., Competitive ArC−H and ArC−X (X = Cl, Br) Activation in Halobenzenes at Cationic Titanium Centers. J. Am. Chem. Soc. 2006, 128, 3303-3312. 68. Zaccaria, F.; Zuccaccia, C.; Cipullo, R.; Budzelaar, P. H. M.; Macchioni, A.; Busico, V.; Ehm, C., Toluene and α-Olefins as Radical Scavengers: Direct NMR Evidence for Homolytic Chain Transfer Mechanism Leading to Benzyl and “Dormant” Titanium Allyl Complexes. Organometallics 2018, 37, 4189-4194. 69. Bochmann, M.; Lancaster, S. J., Base-free Cationic Zirconium Benzyl Complexes as Highly Active Polymerization Catalysts. Organometallics 1993, 12, 633-640. 70. The high field shifted ipso carbon of the benzylic phenyl group in 1-Bn+[B(C6F5)4]- (δC = 146.4 ppm) with respect to 1-Bn2 (δC = 152.8 ppm) is in line with the hypothesis of an η2-benzyl coordination (see ref. 69), although no significant broadening of the Ti-CH2Ph signal is observed even after cooling a dichloromethane-d2 solution to 213 K. 71. a) Cabrera, L.; Hollink, E.; Stewart, J. C.; Wei, P.; Stephan, D. W., Cationic Methyl- and Chlorotitanium Phosphinimide Complexes. Organometallics 2005, 24 1091-1098; b) Smith, J. C.; Ma, K.; Piers, W. E.; Parvez, M.; McDonald, R., A New Weakly Coordinating Aluminate Anion Incorporating a Chelating Perfluoro-bis-anilido Ligand. Dalton Trans. 2010, 39, 10256-10263. 72. Based on 2D NMR characterization, the two most stable dimer configurations seem to be independent of the nature of the counterion (B(C6F5)4- vs [s(MAO/BHT)]- anions, Figure S4).


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79. Xu, Z.; Vanka, K.; Firman, T.; Michalak, A.; Zurek, E.; Zhu, C.; Ziegler, T., Theoretical Study of the Interactions between Cations and Anions in Group IV Transition-Metal Catalysts for Single-Site Homogeneous Olefin Polymerization. Organometallics 2002, 21, 2444-2453. 80. a) Zurek, E.; Ziegler, T., A theoretical Study of the Insertion Barrier of MAO (Methylaluminoxane)-Activated, Cp2ZrMe2-Catalyzed Ethylene Polymerization: further Evidence for the Structural Assignment of Active and Dormant Species. Faraday Discuss. 2003, 124, 93109; b) 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. 81. Unfortunately, this approach cannot be easily extended to unmodified MAO since 1) phosphinimide precatalysts tend to decompose in the presence of TMA (ref. 36); 2) the presence of 'free' TMA would likely lead to more rapid Me/Bn scrambling competing with Bn abstraction; 3) Ti-Me+ ISIP give broad NMR signals complicating accurate quantification. 82. Strictly speaking, the results are not transferable to 1-Me2 activation, for which is more difficult to obtain a reliable estimation due to competing dimer formation and, possibly, solvent activation (Figure 3a). Nevertheless, it appears that the amount of MAO/BHT needed for full activation of 1-Me2 and 1-Bn2 are comparable, i.e. acidic sites abstract Bn and Me groups similarly easily. 83. a) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R., Hydrolysis of Tri-tertbutylaluminum: the First Structural Characterization of Alkylalumoxanes [(R2Al)2O]n and (RAlO)n. J. Am. Chem. Soc. 1993, 115, 4971-4984; b) Falls, Z.; Zurek, E.; Autschbach, J.,


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TOC Graphic


BHT-modified MAO: cage size estimation, chemical counting of

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