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

The reactions of methane and dihydrogen molecules with the cations [(η5-C5H5)2MCH3]+ (1) and [(η5-C5H5)2MH]+ (3) (M=Zr, Ti) have been investigated by gradient-corrected density functional calculations. In the 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 σ-bonded ligand exchange reaction in the system 1+CH4 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, ΔH0=−7.1 kcal mol−1 (−8.6 kcal mol−1) for Zr (Ti), yielding 3 and CH4 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.