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

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 [(η 5 -Cp-(CMe 2 R‘-bridge)-C 6 H y R x )M II (C 2 H 4 ) 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, CF 3 ) groups in various combinations and also the enlargement of the Cp-arene connecting bridge by an additional methylene group (R‘ = CH 2 ). The overall barrier connected with metallacycle growth has been analyzed as being decisively determined by the strength of the M IV −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 C 2 -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.