Preference of H2 as Hydrogen Source in Hydrogenation of Ketones Catalyzed by Late Transition Metal Complexes. A DFT Study

In this work, H 2 activation processes in hydrogenation of ketones catalyzed by late transition metal−ligand bifunctional catalysts have been studied using the DFT method. For systems A (RuH 2 -diphosephine/diamine complex) and B (Ru-η 5 -Cp*-1,2-diamine complex), the dihydrogen activation process in neutral and basic conditions (path 1) consisted of two steps: H 2 coordination and H−H cleavage. However, dihydrogen activations catalyzed by complexes C−F (Ru-η 6 -arene and Rh/Ir-cyclopentadiene complexes) along path 1 consist of only H−H cleavage due to the absence of H 2 coordination. Thus, systems C−F have higher energy barriers (Δ G >27 kcal/mol) for dihydrogen activation than systems A and B. However, for systems C−F under acidic conditions, dihydrogen activation (path 2) consists of the two steps involving H 2 coordination; thus the dihydrogen activation barriers decrease greatly, resulting in an easy splitting of H 2 . These results agree well with experiments. In the conversion from transfer hydrogenation to H 2 hydrogenation for C−F, the protonation of 16 e complex MN C−F changes the N 2 −M 1 −Y 3 (Y = N or O) delocalized π-bond into a M 1 −Y 3 localized π-bond. Therefore, the 16 e complexes, which can provide a vacant site for H 2 coordination, tend to perform H 2 hydrogenation.