Activation of CH4 and H-2 by zirconium(IV) and titanium(IV) cationic complexes. Theoretical DFT study

The reactions of methane and dihydrogen molecules with the cations [(eta(5) -C5H5)(2)MCH3](+) (1) and [(eta(5)-C5H5)(2)MH](+) (3) (M = Zr, Ti) have bee n investigated by gradient-corrected density functional calculations. In th e case of CH4, the active cationic center of 1 or 3 attracts the substrate molecule first to form an agostic complex in which its C-H bond is already somewhat weakened. The sigma-bonded ligand exchange reaction in the system 1 + CH4 proceeds through a symmetric transition state with an activation ba rrier of 15.0 kcal mol(-1) (11.6 kcal mol(-1)) for the Zr (Ti) complex. Hyd rogen reacts with 1 exothermally, Delta H-0 = -7.1 kcal mol(-1) (-8.6 kcal mol(-1)) for Zr (Ti), yielding 3 and CH4 without an activation barrier. The se theoretical give insight into the mechanism of H/D exchange in methane i n the presence of Ziegler-Natta-type catalysts observed experimentally by G rigoryan et al. It is shown that organometallic cationic complexes of Zr(IV ) and Ti(IV) may prove to be promising systems for C-H and H-H bond activat ion under mild conditions. (C) 2000 Elsevier Science S.A, All rights reserv ed.