Computational Evidence for a New Type of η~2-H2 Complex: When Main-Group Elements Act in Concert To Emulate Transition Metals

摘要

The activation of dihydrogen with transition metals is a process of major and long-standing interest on both synthetic and fundamental grounds. Most catalytic hydro-genation reactions used in industry involve transition metals, and in nature several metalloenzymes are known to efficiently promote H2 activation/liberation. Basically, two mechanistic pathways can be distinguished for dihydrogen activation at transition metals (Scheme 1). They differ by the presence or absence of an assisting base. In the absence of a base, the transition metal typically cleaves the H—H bond homolyti-cally (oxidative addition) to afford a dihydride complex (route a). Conversely, the presence of a base (external or internal to the coordination sphere) polarizes the H—H bond and induces its heterolytic cleavage. This leads to the formation of a metal hydride with protonation of the base (route b).