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 (G153; G15 = Group 15 element). Our results demonstrate that sterically congested singlet dipnictenes (G1512) can thermally dissociate into two triplet G153 monomers, with the dissociation energy decreasing systematically from N to Bi. M06-2X-D3 calculations indicate that triplet G153 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 G153 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.
Zhang et al. (Tue,) studied this question.