A detailed theoretical investigation is presented of crucial elementary steps of the selective linear ethylene oligomerization to 1-hexene by the cationic [(η5-C5H4-(CMe2)-bridge)-C6H5)TiIV(CH3)2]+ precatalyst, employing a gradient-corrected DFT method. The essential aspects of the originally proposed mechanism have been confirmed and supplemented by novel insights into how the selective ethylene oligomerization operates. This includes the examination of the ability of titana(IV)cycle intermediates to grow and/or to decompose affording α-olefins as a function of their size, the prediction of the favorable route for precatalyst activation, and the exploration of the cycloalkane production as a possible side process. After the TiIV−Me2 precatalyst is smoothly converted into the active TiII−(ethylene)2 catalyst complex, the two ethylene moieties readily undergo oxidative coupling to afford first the titana(IV)cyclopentane species. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion displays very similar structural and energetic characteristics for five- and seven-membered titana(IV)cycles. Decomposition of titana(IV)cycles to α-olefins preferably takes place via a concerted transition-metal-assisted β-H transfer for conformationally flexible metallacycles beginning with the titana(IV)cycloheptane, with very similar barriers having to be overcome. This decomposition path, however, is kinetically inaccessible for the rigid five-membered titana(IV)cyclopentane. Instead, the stepwise mechanism via a metastable TiIV−alkenyl-hydride species is found to be operative in this case. A significantly raised activation barrier is connected with the stepwise path, which makes the growth of the titana(IV)cyclopentane to the seven-membered cycle the more favorable process than its decomposition to 1-butene. Cycloalkanes are less likely to be formed, due to a kinetically handicapped reductive CC elimination. On the basis of the detailed insights into the ability of titana(IV)cycles to undergo either growth or decomposition to α-olefins, the thermodynamic and kinetic aspects for the selectivity control of the linear ethylene oligomerization have been rationalized. The crucial role played by the hemilabile arene ligand for the selective oligomerization process has also been analyzed.

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.

Alkaline-earth (most prominently barium) complexes of the type [Ae{N(SiMe3 )2 }2 ⋅(THF)x ] and [{N^N}Ae{N(SiMe3 )2 }⋅(THF)x ] are very active and productive precatalysts (TON=396, TOF up to 3600 h(-1) ; Ca