Abstract
We investigate theoretically the reaction mechanism where highly charged metal ions promote the methylimidazole (Mim) to perform the oxidative dehydrogenation process, which obtains the product diazafulvene. The computation is carried out at the level B3LYP density functional method with LANL2DZ basis sets, and polarized functions are augmented for carbon, nittogen and oxygen atoms. The system of a highly positive naked metal ion directly added to Mim and acetic acid is set up as a simplified model. In the calculation, it is found that the abstraction of the methyl hydrogen atom on Mim by the oxygen atom of acetate acid resulted in the formation of diazafulvene and the reduction of the highly charged metal ion into a lower one, mainly due to the strongly electron-withdrawn power of the highly charged metal ion. The highly charged metal (e.g. lead) usually exhibits as neutral compound with anionic ligands instead of being in barely ionic state so that the lead tetraacetate, Pb(OAc)4, is adequately chosen as the reactant. The theoretical simulations reveal the mechanism as Mim initially attack the central lead ion of Pb(OAc)4 to produce the adduct, followed by the dissociation of one acetate to the other side to form a hydrogen-bonded cluster. Next, the transition state is passed to yield diazafulvene, the desired product of the process. The Gibbs free energy under 1atm and 298 K increased about 4 kcal/mol. Diazafulvene is actually an unstable compound naturally. In order to stabilize diazafulvene, the phenyl group is added to the methyl group of Mim in the calculation to facilitate the formation of diazafulvene species. Consequently, the free energy of the product is nearly 10 kcal/mol lower than that of the reactant. This suggests that by stabilizing the product via the conjugation of phenyl group with Mim, the oxidative dehydrogenation reaction is promoted. In the other system, methylpyrrole (Mpy) also performs oxidative dehydrogenation assisted by Pb(OAc)4, where via the participation of phenyl group into Mpy, the free energy change from the Mpy reactant to the azafulvene product cluster is about 9.5 kcal/mol. The simulation concludes that it is likely that such processes proceed experimentally on basis of free energy decrease.