Activation of CH4 and H2 by zirconium(IV) and titanium(IV) cationic complexes. Theoretical DFT study

Abstract The reactions of methane and dihydrogen molecules with the cations [(η 5 -C 5 H 5 ) 2 MCH 3 ] + ( 1 ) and [(η 5 -C 5 H 5 ) 2 MH] + ( 3 ) (M=Zr, Ti) have been investigated by gradient-corrected density functional calculations. In the case of CH 4 , 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 σ-bonded ligand exchange reaction in the system 1 +CH 4 proceeds through a symmetric transition state with an activation barrier of 15.0 kcal mol −1 (11.6 kcal mol −1 ) for the Zr (Ti) complex. Hydrogen reacts with 1 exothermally, Δ H 0 =−7.1 kcal mol −1 (−8.6 kcal mol −1 ) for Zr (Ti), yielding 3 and CH 4 without an activation barrier. These theoretical results give insight into the mechanism of H/D exchange in methane in the presence of Ziegler–Natta-type catalysts observed experimentally by Grigoryan 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 activation under mild conditions.