Thermodynamics of metallocene catalyst activation: alignment of theory and experiment

Abstract

Three equilibria involved in metallocene catalyst activation, including dissociation of R6Al2 (R = Me, Et or i-Bu) and related species such as [L2ZrMe2AlMe2][B(C6F5)4] (L2 = Cp2, 1,2-ethylenebis(η5-indenyl), Me2C(η5-C5H4)2) or [(L2ZrMe)2μ-Me][MePBB]...

Ionization of Cp2ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights

“…[15a] We were unable to extract the same kind of data for ion-pairs 2 and 4; in earlier work a significantly lower value of 52 M À 1 was estimated for binding of (η 5 -1,2-Me 2 C 5 H 3 ) 2 ZrMe 2 to [(η 5 -1,2-Me 2 C 5 H 3 ) 2 ZrMe][MePBB] {PBB = (o-C 6 F 5 -C 6 F 4 ) 3 B}. [27] Structures of Neutral Adducts and Contact Ion Pairs Formed from Cp 2 ZrMe 2 and 16,6…”

“…In recent work we compared the interaction energies between Me 3 Al and various species, including itself at different levels of theory. [27] That work illustrated that dimerization of Me 3 Al and binding of this unsaturated species to itself, and aliphatic and aromatic solvent molecules was well described at the M06-2X/TZVP level of theory in comparison to CCSD(T)// MP2/def2-TZVPD calculations, the highest level of theory investigated. In contrast, with saturated species like Me 6 Al 2 , the purely dispersive interactions between this molecule and solvent molecules seemed to be better modeled by MN15 [32] / def2-TZVP [33] or def2-TZVPD [34] in the few examples studied.…”

“…The presence of many low energy modes (> 30 in the case of 16,6 and the ion-pairs) and the use of the approximation invoked in earlier work for small molecules, with far fewer low energy modes, appears inappropriate in this case. [27] Of the two different approaches, only MN15/def2-TZVP correctly predicts that formation of ion-pairs 5 a (or 5 b) will be favored relative to 16,6 and OMTS-AlMe 3 in condensed phase. Note that either approach predicts that binding of Me 3 Al to OMTS is exothermic and by a similar amount (ΔH-qh = À 61.7 to À 65.7 kJ mol À 1 ).…”

“…We find formation of a simple adduct is favored from an electronic perspective by 16.7 and 15.0 kJ mol À 1 , respectively, while the most stable geometry for 16,6-CO is shown in Figure 1. We note that CO binds at the edge site in 16,6 known to be most reactive towards methide abstraction [23] but with minimal distortion of either coordinated CO or 16, 6. As discussed in detail elsewhere, [27] for bimolecular processes of this type, it is necessary to consider the enthalpy and entropy changes in gas and condensed phases, especially for large molecules like 16,6 which possess many low energy vibrational modes. The energy of these vibrations cannot be calculated with precision using the harmonic oscillator approx-…”

“…[30] However, one needs to recognize, based on our previous work, that the quasi-harmonic approach ends up systematically underestimating total entropy, while the calculation of reduced entropy in condensed phase produces an overestimate of total entropy. [27] These concerns, which are not specific for systems of interest here, carry on throughout this work, preventing quantitative estimation of reaction entropies and hence Gibbs energies.…”