Mechanistic Insights into H–H Bond Cleavage and [1 + 2] Cycloaddition of Ethylene Mediated by Triplet Pnictinidenes

This study presents a theoretical investigation of the mechanistic pathways for H-H bond cleavage and [1 + 2] cycloaddition with ethylene mediated by triplet monomeric pnictinidenes ([G15]3; G15 = Group 15 element). Our results demonstrate that sterically congested singlet dipnictenes ([G15]1₂) can thermally dissociate into two triplet [G15]3 monomers, with the dissociation energy decreasing systematically from N to Bi. M06-2X-D3 calculations indicate that triplet [G15]3 activates H₂ via a transition state on the triplet surface, followed by intersystem crossing to form singlet products, with the reactivity decreasing as the atomic number of G15 increases. Similarly, triplet [G15]3 undergoes [1 + 2] cycloaddition with ethylene through a triplet transition state, ultimately producing singlet three-membered heterocycles after intersystem crossing. Activation strain model (ASM) analyses reveal that the atomic radius of the central G15 atom provides a consistent explanation for the origins of the activation barriers in both H-H bond cleavage and [1 + 2] cycloaddition reactions. This study offers qualitative rationalization based on the activation barrier and assesses the feasibility of the proposed experimental observations.