Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by Cationic Group 4 Cyclopentadienyl-Arene Active Catalysts:  A DFT Investigation Exploring the Influence of Electronic Factors on the Catalytic Properties by Modification of the Hemilabile Arene Functionality

Abstract

Hessen and co-workers described Ti-based precatalysts of the title class bearing a hemilabile ancillary arene functionality for ethylene trimerization. The analogues with the heavier group 4 metals Zr and Hf as the active center have been recently explored computationally. This study suggested the Zr-based system as a promising catalyst for production of an oligomer mixture containing 1-octene besides the prevalent 1-hexene. Herein, we have presented a detailed computational analysis of the influence of modifications of the hemilabile arene functionality on the catalytic abilities of cationic group 4 [(η5-Cp-(CMe2R‘-bridge)-C6H y R x )MII(C2H4)2]+ active species (M = Zr, Hf) for linear ethylene oligomerization, employing a gradient-corrected DFT method. This comprised the arene substitution with prototypical electron-releasing (R = Me) and electron-withdrawing (R = F, CF3) groups in various combinations and also the enlargement of the Cp-arene connecting bridge by an additional methylene group (R‘ = CH2). The overall barrier connected with metallacycle growth has been analyzed as being decisively determined by the strength of the MIV−arene interaction in the metallacycle precursor, with the first arene displacement by incoming ethylene as the major contribution to the relative kinetics. For catalysts bearing an extended C2-bridge or a donor-substituted arene functionality this interaction becomes increased, thereby acting to reduce the tendency for ethylene π-adduct formation and also increasing the insertion barrier. The opposite influence has been found for substituents that are electron-withdrawing. The barrier for metallacycle decomposition is essentially controlled by the ability of the arene group to stabilize the transition state coordinatively. The donor and acceptor abilities of the substituent have been described as the crucial electronic factor that acts to accelerate and retard, respectively, this process. Among the two crucial elementary processes, the ethylene insertion has been found as being distinctly affected to a larger extent by the probed modifications of the hemilabile arene functionality. The influence of these modifications, in modulating the oligomer product composition and the catalytic activity, has been elucidated.

NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d 0 group 4 metal complexes

“…Although many computational studies have been carried out on these complexes and their role in oligo-and polymerization reactions (67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77)(78)(79)(80)(81)(82)(83), the electronic structures and relative energies of relevant reactants, intermediates, and transition states are computed with the same method used for optimizing the geometries for NMR calculations for consistency. ZrEt + is more deshielded in both the β-H and α-H agostic structures, but this effect is much more pronounced in the latter (Fig.…”

Density functional studies on chromium catalyzed ethylene trimerization

“…Schemes 3 and 4). This type of concerted metal assisted hydride shift in metallacycloheptanes are well precedented in computational works on Cr, Ti, and Ta systems [13,14,49,[50][51][52][53][59][60][61].…”

“…They attempt to explain (i) preference of insertion of the third ethylene molecule into the metallacyclopentane over the liberation of 1-butene and (ii) preference of 1-hexene liberation over metallacycle growth [13,14]. The reductive 1-hexene elimination process of Scheme 2a, involving metal mediated, agostic-assisted concerted 3,7-H shift also finds support from recent computational studies [49,[50][51][52][53][59][60][61]. The concerted mechanism is found to be energetically more favorable than a stepwise mechanism for titanium, tantalum, and chromium [43,44,62] ethylene trimerization systems.…”

Oligomerization of Olefins

“…Replacement of the aryl pendant group of the ligand with a non-coordinating methyl group changes the favorable product to polyethylene, which is in agreement with the experimental results. Tobisch investigated on comparison of the possibilities for the titana(IV)cycle intermediates to cause growth or to decompose affording a-olefins as a function of their ring size, prediction of the favorable route for precatalyst activation, and exploration of the cycloalkane production as a possible side process, by using a gradient-corrected DFT method [118][119][120][121]. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion displays very similar structural and energetic characteristics for five-and seven-membered titana(IV)cycles.…”