Insights into the Factors Controlling the H–H Bond Cleavage Reactions by Five-Membered G13/P (G13 = Group 13 Element) and B/G15 (G15 = Group 15 Element) Frustrated Lewis Pairs

The origins of reaction barriers and reactive activities for H2 activation reactions by five-membered cyclic G13/P-Rea (G13 = group 13 element) and B/G15-Rea (G15 = group 15 element) frustrated Lewis pair (FLP) molecules were explored using the density functional theory [B3LYP-D3­(BJ)/def2-TZVP]. Our theoretical examinations suggest that all G13/P-Rea FLP-type molecules are energetically feasible to undergo the H2 activation reaction from kinetic and thermodynamic viewpoints, except for the case of Tl/P-Rea FLPs. However, in B/G15-Rea FLP-type systems, only the B/P-Rea FLP species can react with H2 to generate the adduct. The theoretical analysis based on energy decomposition analysis–natural orbitals for the chemical valence (EDA–NOCV) approach revealed that the singlet–singlet interaction (or donor–acceptor interaction) rather than the triplet–triplet interaction (or electron-sharing interaction) determines the bonding natures of saddle points. The frontier molecular orbital theory and EDA–NOCV method demonstrated that the lone pair (LB) → σ*­(H2) interaction rather than the empty p-π orbital (LA) ← σ­(H2) interaction played a prominent role in determining the bonding situations between H2 and G13/P-Rea and B/G15-Rea FLP-associated molecules. The theoretical findings based on the activation strain model indicate that the origin of the barriers for the reactions of G13/P-Rea and B/G15-Rea FLP-related molecules with H2 can be attributed to the atomic radius of the LA (G13) and LB elements (G15), respectively.