Size Matters: Computational Insights into the Crowning of Noble Gas Trioxides

In pursuit of enhancing the stability of the highly explosive and shock-sensitive compound XeO3, we performed quantum chemical calculations to investigate its possible complexation with electron-rich crown ethers, including 9-Crown-3, 12-Crown-4, 15-Crown-5, 18-Crown-6, and 21-Crown-7, as well as their thio analogues. Furthermore, we expanded our study to other noble gas trioxides (NgO3), namely KrO3 and ArO3. The basis set superposition error (BSSE) corrected binding energies for these adducts range from -13.0 kcal/mol to -48.2 kcal/mol, which is notably high for σ-hole mediated non-covalent interactions. The formation of these adducts was observed to be more favorable with the increase in the ring size of the crowns and less favorable while going from XeO3 to ArO¬3. A comprehensive analysis by various computational tools such as the mapping of the electrostatic potential (ESP), Wiberg bond indices (WBIs), Bader’s theory of atoms-in-molecules (AIM), natural bond orbital (NBO) analysis, non-covalent interaction (NCI) plots, and the energy decomposition analysis (EDA) analysis revealed that the C-H….O interactions, as well as dispersion interactions play a pivotal role in stabilizing adducts involving larger crowns. A noteworthy outcome of our study is the revelation of a coordination number of 9 for xenon in the complex formed between XeO3 and the thio analogue of 18-Crown-6, which is higher than the largest number reported to date

Mechanism of Oxygen Atom Transfer from Fe^V(O) to Olefins at Room Temperature

In biological oxidations, the intermediate Fe^V(O)­(OH) has been proposed to be the active species for catalyzing the epoxidation of alkenes by nonheme iron complexes. However, no study has been reported yet that elucidates the mechanism of direct O-atom transfer during the reaction of Fe^V(O) with alkenes to form the corresponding epoxide. For the first time, we study the mechanism of O-atom transfer to alkenes using the Fe^V(O) complex of biuret-modified Fe–TAML at room temperature. The second-order rate constant (<i>k</i>₂) for the reaction of different alkenes with Fe^V(O) was determined under single-turnover conditions. An 8000-fold rate difference was found between electron-rich (4-methoxystyrene; <i>k</i>₂ = 216 M^–1 s^–1) and electron-deficient (methyl <i>trans</i>-cinnamate; <i>k</i>₂ = 0.03 M^–1 s^–1) substrates. This rate difference indicates the electrophilic character of Fe^V(O). The use of <i>cis</i>-stilbene as a mechanistic probe leads to the formation of both <i>cis</i>- and <i>trans</i>-stilbene epoxides (73:27). This suggests the formation of a radical intermediate, which would allow C–C bond rotation to yield both stereoisomers of stilbene–epoxide. Additionally, a Hammett ρ value of −0.56 was obtained for the para-substituted styrene derivatives. Detailed DFT calculations show that the reaction proceeds via a two-step process through a doublet spin surface. Finally, using biuret-modified Fe–TAML as the catalyst and NaOCl as the oxidant under catalytic conditions epoxide was formed with modest yields and turnover numbers

Tuning the Reactivity of Fe^V(O) toward C–H Bonds at Room Temperature: Effect of Water

The presence of an Fe^V(O) species has been postulated as the active intermediate for the oxidation of both C–H and CC bonds in the Rieske dioxygenase family of enzymes. Understanding the reactivity of these high valent iron–oxo intermediates, especially in an aqueous medium, would provide a better understanding of these enzymatic reaction mechanisms. The formation of an Fe^V(O) complex at room temperature in an aqueous CH₃CN mixture that contains up to 90% water using NaOCl as the oxidant is reported here. The stability of Fe^V(O) decreases with increasing water concentration. We show that the reactivity of Fe^V(O) toward the oxidation of C–H bonds, such as those in toluene, can be tuned by varying the amount of water in the H₂O/CH₃CN mixture. Rate acceleration of up to 60 times is observed for the oxidation of toluene upon increasing the water concentration. The role of water in accelerating the rate of the reaction has been studied using kinetic measurements, isotope labeling experiments, and density functional theory (DFT) calculations. A kinetic isotope effect of ∼13 was observed for the oxidation of toluene and <i>d</i>₈-toluene showing that C–H abstraction was involved in the rate-determining step. Activation parameters determined for toluene oxidation in H₂O/CH₃CN mixtures on the basis of Eyring plots for the rate constants show a gain in enthalpy with a concomitant loss in entropy. This points to the formation of a more-ordered transition state involving water molecules. To further understand the role of water, we performed a careful DFT study, concentrating mostly on the rate-determining hydrogen abstraction step. The DFT-optimized structure of the starting Fe^V(O) and the transition state indicates that the rate enhancement is due to the transition state’s favored stabilization over the reactant due to enhanced hydrogen bonding with water