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

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 [(eta(6)-BC5H5)-(bridge)-C6H5)M-II(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 alpha-olefin product composition toward an enhanced 1-octene portion, although not as the predominant product.