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 ΔER = −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.

The [2+2]M,C addition across the MoCH2 double bond of M(O)(CH2)(CH3)2 is kinetically favored over the [3+2]C,O cycloaddition and the [2+2]M,O addition across the MO double bond for M = Cr, Mo, W. The chromium system exhibits a complex reactivity pattern where the rearrangement of the molecule to a Cr(IV) species with subsequent ethylene addition to the metal is predicted as the most favorable pathway.Figure optionsDownload as PowerPoint slide