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

Understanding the CO capture reaction through electronic structure analysis of four-membered-ring group-13/N- and B/group-15-based Lewis acid-base pairs

Incomplete combustion yields a significant byproduct, known for its high toxicity to humans: gas phase carbon monoxide (CO). This study utilized several advanced theoretical methods to examine the factors contributing to the activation energy involved in CO capture by a frustrated Lewis pair (FLP) and to forecast the potential success of the CO capture reaction. The current theoretical findings indicate that among the four-membered-ring Group-13/N-FLP and B/Group-15-FLP molecules, only the B/N-based FLP-type molecule effectively captures CO, considering both thermodynamics and kinetics. According to the results obtained through energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV), it can be concluded that the donor-acceptor (singlet-singlet) model, rather than the electron-sharing (triplet-triplet) model, effectively characterizes the electronic structures in the CO trapping reaction involving four-membered-ring G13/G15-FLPs. Theoretical findings, derived from EDA-NOCV and frontier molecular orbital theory, demonstrate that the CO capture reaction by G13/G15-FLP involves two distinct bonding interactions. The first interaction is characterized by FLP-to-CO forward bonding, with the lone pair of G15 (G13/G15-FLP) donating to the empty p-π* orbital of carbon (CO), which predominates. The second interaction involves CO-to-FLP backward bonding, where the empty σ* orbital of G13 (G13/G15-FLP) accepts the lone pair of carbon (CO), albeit to a lesser extent. In summary, our theoretical findings indicate that the G13-C and G15-C bonds in the G15/G15-TS species with a four-membered ring can be classified as two dative single bonds. The importance of the interaction between Lewis bases and CO surpasses that of the interaction between Lewis acids and CO. Theoretical evidences in this study demonstrate a linear connection between the G13-G15 bond length within the four-membered-ring G13/G15-FLP and the activation barrier linked to CO capture. The activation strain model analysis in this study suggests that the activation energy required for bond formation primarily depends on the geometric deformation energy of G13/G15-FLP in capturing CO. Our DFT investigation shows that Hammond's postulate is obeyed by the CO catching reaction of the four-membered-ring G13/N-FLP, meaning that an earlier transition state is associated with a lower activation barrier, but not with the CO catching reaction of the four-membered-ring B/G15-FLP.

Reactivity of the Intramolecular Vicinal Group-13/P- and B/Group-15-Based Frustrate Lewis Pairs with Sulfur Dioxide: Mechanistic Insight from DFT

The emission of SO2 gas by industrialized societies contributes to the occurrence of acid rain in natural environments. In this study, we put forward a theoretical investigation into the capture reactions of SO2. Our analysis centers on the energy profiles of intramolecular 1,2-cyclohexylene-bridged FLP-associated molecules. We will particularly examine the reactions involving G13/P-based (with G13 denoting Group 13 element) and B/G15-based (with G15 representing Group 15 element) FLP-associated molecules. Except for Tl/P-FLP, B/N-FLP, and B/Bi-FLP, our theoretical examinations indicate that the remaining six FLP-associated molecules, namely G13'/P-FLP (G13' = B, Al, Ga, and In) and B/G15 ' -FLP (G15' = P, As, and Sb), can easily undergo SO2 capture reactions due to their energetic feasibility. Particularly, our theoretical findings suggested that 1,2-cyclohexylene-bridged Al/P-FLP, Ga/P-FLP, B/As-FLP, and B/Sb-FLP are capable of undergoing a reversible reaction and returning to the initial reactant state. Our theoretical evidence indicates that the G13-G15 bond length in the 1,2-cyclohexylene-linked G13/G15-FLP can serve as a basis for evaluating the free activation barrier associated with its reaction with SO2. Two theoretical methods, namely, the frontier molecular orbital theory and the energy decomposition analysis-natural orbitals of chemical valence approach, are utilized to investigate the electronic structure and bonding nature of the reactions under consideration. Moreover, the analyses based on the activation strain model revealed that it is the geometrical deformation energies of G13/G15-FLP, which is the key factor that greatly influences the activation barriers of such SO2 capture reactions. Further, our theoretical computations indicate that such capturing reactions of SO2 by intramolecular 1,2-cyclohexylene-linked G13/G15-based FLP-type molecules obey the Hammond postulate.

Computational insights into the reactivity for the [2+5] cycloaddition reactions of norbornene-linked group 14 element/P-based and Si/group 15 element-based frustrated Lewis pairs with benzaldehyde

The element effects of Lewis acid (LA) and Lewis base (LB) on the potential energy surfaces of [2+5] cycloaddition reactions of norbornene-based G14/P-based (G14 = group 14 element) and Si/G15-based (G15 = group 14 element) frustrated Lewis pair (FLP)-type molecules with benzaldehyde were theoretically examined via density functional theory and several sophisticated methods. The theoretical findings indicated that among the above nine norbornene-linked G14/G15-based FLPs, only the Si/N-Rea, Si/P-Rea, and Si/As-Rea FLP-assisted compounds can readily undergo cycloaddition reactions with doubly bonded organic systems from kinetic and thermodynamic viewpoints. The energy decomposition analysis showed that the bonding interactions between the norbornene-based G14/G15-FLPs and benzaldehyde are better described in terms of the singlet-singlet model (donor-acceptor model) rather than the triplet-triplet model (electron-sharing model). In particular, natural orbitals for chemical valence findings revealed that the forward bonding is the lone pair (G15) → p-π*(C) interaction, which is a significantly strong FLP-to-benzaldehyde interaction. However, the back-bonding is the p-π*(G14) ← lone-pair orbital(O) interaction, which is a weak benzaldehyde-to-FLP interaction. The analyses based on the activation strain model showed that the larger the atomic radius of either the G14(LA) or the G15(LB) atom, the greater the G14⋯G15 separation distance in the norbornene-based G14/G15-FLP molecule, the smaller the orbital overlaps between G14/G15-FLP and Ph(H)CO, and the higher the activation barrier during its cycloaddition reaction with benzaldehyde.

Insights into the Reactivity of the Ring-Opening Reaction of Tetrahydrofuran by Intramolecular Group-13/P- and Al/Group-15-Based Frustrated Lewis Pairs

A theoretical study concerning key factors affecting activation energies for ring-opening reactions of tetrahydrofuran (THF) by G13/P-based (G13 = B, Al, Ga, In, and Tl) and Al/G15-based (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) featuring the dimethylxanthene scaffold was performed using density functional theory. Our theoretical findings indicate that only dimethylxanthene backbone Al/P-Rea (Rea = reactant) FLP-type molecules can be energetically favorable to undergo the ring-opening reaction with THF. Our theoretical evidence reveals that the shorter the separating distance between Lewis acidic (LA) and Lewis basic (LB) centers of the dimethylxanthene backbone FLP-type molecules, the greater the orbital overlaps between the FLP and THF and the lower the activation barrier for such a ring-opening reaction. Energy decomposition analysis (EDA) evidence suggests that the bonding interaction for such a ring-opening reaction is predominated by the donor-acceptor interaction (singlet-singlet interaction) compared to the electron-sharing interaction (triplet-triplet interaction). In addition, the natural orbitals for chemical valence (NOCV) evidence demonstrate that the bonding situations of such ring-opening reactions can be best described as FLP-to-THF forward bonding (the lone pair (G15) → the empty σ*(C-O)) and THF-to-FLP back bonding (the empty σ*(G13) ← filled p-π(O)). The EDA-NOCV observations show that the former plays a predominant role and the latter plays a minor role in such bonding conditions. The activation strain model reveals that the deformation energy of THF is the key factor in determining the activation energy of their ring-opening reactions. Comparing the geometrical structures of the transition states with their corresponding reactants, a linear relationship between them can be rationally explained by the Hammond postulate combined with the respective activation barriers calculated in this work.

Understanding the Reactivity of Combination Reactions of Intramolecular Geminal Group 13 Element/Phosphorus and Gallium/Group 15 Element Frustrated Lewis Pairs with CS2

The reactions of CS₂ captured by intramolecular geminal G13/P-based (G13 = group 13 elements) and Ga/G15-based (G15 = group 15 elements) frustrated Lewis pairs have been theoretically examined by using density functional theory (DFT) computations. With regard to the nine FLP-related compounds, our DFT calculated results reveal that only Al/P-Rea and Ga/P-Rea can kinetically and thermodynamically precede the energetically feasible combination reactions with CS₂ to form the five-membered heterocyclic adducts. Our activation strain model analyses on the nine aforementioned model molecules indicate that the atomic radius of the Lewis acceptor (G13) and the Lewis donor (G15) plays a role in controlling their barrier heights to obtain good orbital overlaps among G13/P-Rea, Ga/G15-Rea, and CS₂. Our theoretical observations based on the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) approach strongly indicate that the donor-acceptor bonding (i.e., singlet-singlet bonding) rather than the electron-sharing bonding (i.e., triplet-triplet bonding) plays a central role in determining the bonding conditions of the transition states, G13/P-TS and Ga/G15-TS. In addition, the theoretical evidence obtained by the frontier molecular orbital theory and EDA-NOCV analyses reveals that the best description for the bonding natures of the combination reactions of intramolecular geminal G13/P-Rea and Ga/G15-Rea with CS₂ is the lone pair(G15) → p-π*(C) interaction rather than the p-π*(G13) ← p-π(S) interaction. Moreover, our present DFT computations concerning the calculated structures and corresponding relative energetics of the stationary points connected with the aforementioned sophisticated approaches are in accordance with the Hammond postulate.