Lewis Acid Transition‐Metal‐Catalyzed Hydrogen Activation: Structures, Mechanisms, and Reactivities.

As a new type of bifunctional catalyst, the Lewis acid transition‐metal (LA‐TM) catalysts have been widely applied for hydrogen activation. This study presents a mechanistic framework to understand the LA‐TM‐catalyzed H2 activation through DFT studies. The mer(trans)‐homolytic cleavage, the fac(cis)‐homolytic cleavage, the synergetic heterolytic cleavage, and the dissociative heterolytic cleavage should be taken as general mechanisms for the field of LA‐TM catalysis. Four typical LA‐TM catalysts, the Z‐type κ4‐L3B‐Rh complex tri(azaindolyl)borane‐Rh, the X‐type κ3‐L2B‐Co complex bis‐phosphino‐boryl (PBP)‐Co, the η2‐BC‐type κ3‐L2B‐Pd complex diphosphine‐borane (DPB)‐Pd, and the Z‐type κ2‐LB‐Pt complex (boryl)iminomethane (BIM)‐Pt are selected as representative models to systematically illustrate their mechanistic features and explore the influencing factors on mechanistic variations. Our results indicate that the tri(azaindolyl)borane‐Rh catalyst favors the synergetic heterolytic mechanism; the PBP‐Co catalyst prefers the mer(trans)‐homolytic mechanism; the DPB‐Pd catalyst operates through the fac(cis)‐homolytic mechanism, whereas the BIM‐Pt catalyst tends to undergo the dissociative heterolytic mechanism. The mechanistic variations are determined by the coordination geometry, the LA‐TM bonding nature, the electronic structure of the TM center, and the flexibility or steric effect of the LA ligands. The presented mechanistic framework should provide helpful guidelines for LA‐TM catalyst design and reaction developments. [ABSTRACT FROM AUTHOR]