On the Nature of the Lewis Acidic Sites in “TMA‐Free” Phenol‐Modified Methylaluminoxane

Abstract

“Structural” AlMe2(bht) was found to be an integral building block of the “TMA‐free” olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6‐di‐tert‐butyl‐4‐methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N‐donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'‐bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al‐cages) not only when unmodified MAO is exploited but also in the case of “TMA‐free” BHT‐modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe2(bht) units.

Real-time analysis of methylalumoxane formation

“…11,21,28,29 Ionization in MAO comes about via neutral MAO generating the reactive Lewis acidic species [Me 2 Al] + , with the resulting bulky MAO anions being sufficiently weakly coordinating to allow high reactivity towards alkenes at the cationic metal center. [30][31][32][33][34] We have shown through mass spectrometric means that the anionic products of the activation process are dominated by a single ion, [(MeAlO) 16 (Me 3 Al) 6 Me] À henceforth [16,6] À . 35 The three dimensional structure of this anion has not been elucidated, but its unusually high abundance in the spectra of postactivation commercial MAO does raise questions about why it is so prominent, since the synthesis of MAO does not on the face of it appear to be particularly selective, being the controlled mixing of water and pyrophoric TMA.…”

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

“…The computational protocol M06-2X/TZ(PCM)//TPSSTPSS/DZ has been previously benchmarked 56 , 57 and widely used for modeling of Al-species relevant to olefin polymerization 33 , 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 corrections 62 , 63 does not affect computational results significantly (see Supporting Information ).…”

“…A Δ G R close to zero for this reaction is consistent with the experimental observation that ionization occurs with bipy but not py . 44 …”

“… 29 , 33 , 44 Addition of bipy to MAO/BHT leads to the formation of [AlMe(bht)(bipy)] + , while py selectively generates neutral AlMe 2 (bht)(py). 44 …”

“…The thermodynamics of [AlMeR] + and/or AlMe 2 R 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 …”