Ethylene addition to group-6 transition metal oxo complexes - A theoretical study

Quantum chemical calculations using density functional theory (B3LYP) were carried out to elucidate the reaction pathways for ethylene addition to the chromium and molybdenum complexes CrO(CH₃)₂(CH₂) (Cr1) and MoO(CH₃)₂(CH₂) (Mo1). The results are compared with previously published results of the analogous tungsten system WO(CH₃)₂(CH₂) (W1). The comparison of the group-6 elements shows that the molybdenum and tungsten compounds Mo1 and W1 have a similar reactivity while the chromium compound has a more complex reactivity pattern. The kinetically most favorable reaction pathway for ethylene addition to Mo1 is the 2+2]_Mo,C addition across the MoCH₂ double bond which has an activation barrier of only 8.4 kcal/mol. The reaction is slightly exothermic with ΔE_R = −0.6 kcal/mol. The 2+2]_Mo,O addition across the MoO double bond and the 3+2]_C,O addition have much higher barriers and are strongly endothermic. The thermodynamically mostly favored reaction is the 1+2]_Mo addition of ethylene to the metal atom which takes place after prior rearrangement of the Mo(VI) compound Mo1 to the Mo(IV) isomer Mo1g. The reaction is −19.2 kcal/mol exothermic but it has a large barrier of 34.5 kcal/mol. The kinetically and thermodynamically most favorable reaction pathway for ethylene addition to the chromium homologue Cr1 is the multiple-step process with initial rearrangements Cr1 → Cr1c → Cr1g which are followed by a 1+2]_Cr addition yielding an ethylene π complex Cr1g + C₂H₄ → Cr1g-1. The highest barrier comes from the first step Cr1 → Cr1c which has an activation energy of 14.2 kcal/mol. The overall reaction is exothermic by −26.3 kcal/mol.