Preference of H₂ as hydrogen source in hydrogénation of ketones catalyzed by late transition metal complexes. A DFT study

In this work, H₂ activation processes in hydrogenation of ketones catalyzed by late transition metal-ligand bifunctional catalysts have been studied using the DFT method. For systems A (RuH₂diphosephine/ diamine complex) and B (Ru-η⁵-Cp^*-l,2-diamine complex), the dihydrogen activation process in neutral and basic conditions (path 1) consisted of two steps: H₂ coordination and H-H cleavage. However, dihydrogen activations catalyzed by complexes C-F (Ru- η⁶-arene and Rh/ Ir-cyclopentadiene complexes) along path 1 consist of only H-H cleavage due to the absence of H₂ coordination. Thus, systems C-F have higher energy barriers (△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 H₂ coordination; thus the dihydrogen activation barriers decrease greatly, resulting in an easy splitting of H₂. These results agree well with experiments. In the conversion from transfer hydrogenation to H₂ hydrogenation for C-F, the protonation of 16e complex MN _C-F changes the N²-M¹ -Y³ (Y = N or O) delocalized jr-bond into a M¹-Y³ localized jr-bond. Therefore, the 16e complexes, which can provide a vacant site for H₂ coordination, tend to perform H₂ hydrogenation.