METHANE ACTIVATION BY D(0) AND D(2) IMIDOS - EFFECTS OF D-ORBITAL OCCUPATION AND COMPARISON OF [2+2] AND OXIDATIVE ADDITION

Methane activation by oxidative and [2 + 2] C-H addition to the d2 imido W(OH)2(= NH) is investigated using effective core potential methods. The [2 + 2] C-H activation pathway is also compared with that for d0 Ti(OH)2(= NH). Several points were deduced from the calculations. Although both the d0 Ti- and d2 W-imidos have a high positive charge on the metal, the former cannot coordinate weak Lewis bases such as methane. The [2 + 2] addition of the methane C-H bond across the metal-imido linkage is slightly exothermic for Ti(OH)2(= NH), DELTAH(rxn) = -11.7 kcal mol-1, but endothermic for W(OH)2(= NH), DELTAH(rxn) = +23.4 kcal mol-1. Oxidative addition is exothermic by 9.8 kcal mol-1 for the d2 W complex. The enthalpy of activation (DELTAH(act)double dagger) for the [2 + 2] pathway is 55.2 kcal mol-1 for W(OH)2(= NH) and 13.2 kcal mol-1 for Ti(OH)2(= NH). The TS for W(OH)2(= NH) + CH4 --> W(OH)2(= NH)-(H)(CH3) is 22.3 kcal mol-1 above that of reactants, much smaller than for the [2 + 2] pathway. The C-H bond length remains constant in length along the oxidative addition trajectory, until the TS is quite imminent. Similar behavior is observed for C-H along the [2 + 2] C-H activation trajectory. The changes in the C-H distance and M-H-C angle along the reaction coordinate are similar for [2 + 2] and oxidative addition in the portion of the reaction preceding the TS; energetic discrimination between the pathways does not occur since the reactions have proceeded well into the neighborhood of the respective transition states.