Mechanistic Investigation of the Hydrogenation of O2 by a Transfer Hydrogenation Catalyst

The mechanistic details of the hydrogenation of molecular oxygen by the 18e amino-hydride Cp*IrH(TsDPEN) (1H(H)) complex to give Cp*Ir(TsDPEN-H) (1) and 1 equiv of H(2)O were investigated by means of hybrid density functional calculations (B3LYP). To comprehensively describe the overall catalytic cycle of the hydrogenation of dioxygen using H(2) catalyzed by the Ir complex 1, the potential energy surfaces for the hydrogenation process of both the catalyst 1 and the corresponding unsaturated iridium(III) amine cation ([1H](+)) were explored at the same level of theory. The results of our computations, in agreement with experimental findings, confirm that the addition of H(2) to the 16e diamido complexes 1 is favorable but is slow and is accelerated by the presence of Bronsted acids, such as HOTf, which convert 1 into the corresponding amine cation [1H](+). By deprotonation of the subsequently hydrogenated [1H(H(2))](+) complex the amine hydride catalyst 1H(H) is generated, which is able to reduce molecular oxygen. Calculations corroborate that the O(2) reduction goes through formation of an intermediate iridium hydroperoxo complex that reacts with 1H(H) to eliminate water, restore 1, and restart the catalytic cycle. From the outcomes of our computational analysis it results that the slow step of the overall O(2) hydrogenation process is the O(2) insertion into the Ir-H bond, and the highest calculated barrier along this pathway to give the hydroperoxo product shows a good agreement with the experimentally estimated value. As a consequence, unreacted 1H(H) approaches 1H(OOH) to give 1H(OH) and water according to the experimentally observed second-order kinetics with respect to [1H(H)]. Calculations were carried out to explore the possibility that H(2)O(2) is released from the hydroperoxo intermediate together with catalyst 1, and the subsequent water elimination reaction occurs by reduction of produced H(2)O(2) with 1H(H) to regenerate catalyst 1. Preliminary results concerning the O(2) reduction in acidic conditions show that the reaction proceeds by intermediate production of H(2)O(2), which reacts with 1H(H) to eliminate water, restore [1H](+), and restart the catalytic cycle. The energetics of the process appear to be definitely more favorable with respect the analogous pathways in neutral conditions.