Discrimination of σ‐Bond Metathesis Pathways in H/D Exchange Reactions on [(SiO)3ZrH]: A Density Functional Theory Study

The mechanism of the H/D exchange reaction in alkane/hydrogen mixtures on silica‐supported zirconium hydride was investigated by a modelling study using density functional theory (DFT) calculations. The electronic activation enthalpy (ΔH≠elec ) for the CH bond activation step (TS3) was calculated to be around 92 kJ mol−1, whereas it would be 258 kJ mol−1 for a direct exchange process (TS1, also called the kite TS). These data clearly speak in favour of the former as a mechanism for CH bond scrambling. Moreover, the calculated enthalpy of activation (ΔH≠_ elec ) for H/D exchange in H2/D2 mixtures (TS2) is 33.5 kJ mol−1, which shows that this reaction is much faster than the H/D scrambling in alkane/H2 mixtures, as shown experimentally. Additionally, the calculated activation entropies (For TS1–4, ΔS≠_ tot ranges between −129 and −174 J mol−1 K−1) are very negative. Although the calculated activation entropies are also in full agreement with experimental data (ΔS≠_ exp =−113 J mol−1 K−1), overall, the calculated activation enthalpies are much higher than the experimental ones. This suggests that the actual catalyst is probably more electrophilic than the model chosen for the calculations.