Catalytic oligomerization of ethylene to higher linear alpha-olefins promoted by the cationic group 4 [(eta 5-Cp-(CMe2-bridge)-Ph)MII(ethylene)2]+ (M = Ti, Zr, Hf) active catalysts: a density functional investigation of the influence of the metal on the catalytic activity and selectivity.

Abstract

A detailed theoretical analysis is presented of the catalytic abilities of heavier group 4 (M = Zr, Hf) metals for linear ethylene oligomerization with the cationic [(eta(5)-C(5)H(4)-(CMe(2)-bridge)-C(6)H(5))M(IV)(CH(3))(2)](+) complex as precatalyst, employing a gradient-corrected DFT method. The parent Ti system has been reported as a highly selective catalyst for ethylene trimerization. The mechanism involving metallacycle intermediates, originally proposed by Briggs and Jolly, has been supported by the present study to be operative for the investigated class of group 4 catalysts. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion is likely to occur at uniform rates for larger cycles that are furthermore comparable for Ti, Zr, and Hf catalysts. Ethylene insertion into the two smallest five- and seven-membered cycles is found to become accelerated for Zr and Hf catalysts, which is due to geometrical factors. In contrast, electronic effects act to raise the barrier for metallacycle decomposition, affording alpha-olefins upon descending group 4. This process is furthermore predicted to be kinetically more difficult for larger metallacycles. The oligomer distribution of the Zr-mediated reaction is likely to comprise predominantly 1-hexene together with 1-octene, while 1-butene and alpha-olefins of chain lengths C(10)-C(18) should occur only in negligible portions. A similar composition of alpha-olefins having C(6)-C(18) chain lengths is indicated for the Hf catalysts, but with long-chain oligomers and polymers as the prevalent fraction. Between the group 4 catalysts of the investigated type, the Zr system appears as the most promising candidate having catalytic potential for production of 1-octene, although not selectively. The influence of temperature to modulate the oligomer product composition has been evaluated.