Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by Cationic Group 4 Cyclopentadienyl-Arene Active Catalysts:  Toward the Computational Design of Zirconium- and Hafnium-Based Ethylene Trimerization Catalysts

Abstract

A detailed computational exploration is presented of the catalytic abilities of heavier group 4 (M = Zr, Hf) mono(boratabenzene-arene) compounds for linear ethylene oligomerization with the cationic [(η6-BC5H5)-(bridge)-C6H5)MII(C2H4)2]+ complex as active catalyst species, employing a gradient-corrected DFT method. The influence of the boron substitution on the cyclopentadienyl moiety and the length of the boratabenzene-arene connecting bridge on the energy profile of the oxidative coupling and the competing metallacycle growth and decomposition steps has been elucidated. This allowed us to suggest promising modifications of the parent Cp-based Ti analogue, which has been described by Hessen and co-workers as a catalyst for ethylene trimerization, thereby contributing to the computer-based rational design of improved group 4 oligomerization catalysts. The boratabenzene Zr compound bearing a CMe2-bridge is indicated to be an efficient trimerization catalyst, which should exhibit an activity that exceeds what is reported for the established Ti system. The computational probing reveals for the Hf counterpart a catalytic ability that is different. This system is suggested to possess catalytic potential for production of 1-octene besides the prevalent 1-hexene oligomer product. Electronic modification of the substituent on boron can act to modulate the α-olefin product composition toward an enhanced 1-octene portion, although not as the predominant product.

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.…”

Catalytic Trimerization of Ethylene with Highly Active Half‐sandwich Titanium Complexes Bearing Pendant p‐Fluorophenyl Groups

“…However, the further research of this class of catalysts has been focused on computational DFT (Density Functional Theory) study. 11,12 To the best of our knowledge, up to now, there is no other experiments of trimerization catalyzed by group 4 metals reported, except for the discovery by our group that Cp'TiCl 3 complexes bearing pendant thienyl or ethereal groups can trimerize ethylene with high selectivity. 13,14 It should be noted that although the cyclopentadienyl group is one of the most ubiquitous ligands in organometallic chemistry, only a few such ligands substituted by fluorine have been reported.…”