Importance of Nonclassical σ-Hole Interactions for the Reactivity of λ3-Iodane Complexes.

Key for the observed reactivity of λ3-iodanes, powerful reagents for the selective transfer of functional groups to nucleophiles, are the properties of the 3-center-4-electron bond involving the iodine atom and the two linearly arranged ligands. This bond is also involved in the formation of the initial complex between the λ3-iodane and a nucleophile, which can be a solvent molecule or a reactant. The bonding in such complexes can be described by means of σ-hole interactions. In halogen compounds, σ-hole interaction was identified as a force in crystal packing or in the formation of supramolecular chains. More recently, σ-hole interactions were also shown to affect the reactivity of the iodine-based hypervalent reagents. Relative to their monovalent counterparts, where the σ-hole is located on the extension of the sigma-bond, in the hypervalent species our DFT calculations reveal the formation of a nonclassical σ-hole region with one or even two maxima. This observation is also made in fully relativistic calculations. The SAPT analysis shows that the σ-hole bond between the λ3-iodane and the nucleophile is not necessarily of purely electrostatic nature but may also contain a significant covalent component. This covalent component may facilitate chemical transformation of the compound by means of reductive elimination or other mechanisms and is therefore an indicator for its reactivity. Here, we also show that the shape, location, and strength of the σ-holes can be tuned by the choice of ligands and measures such as Brønsted activation of the iodane reagent. At the limit, the tuning transforms the nonclassical σ-hole regions into coordination sites, which allows us to control how a nucleophile will bind and react with the iodane.

Diverging effects of isotopic fractionation upon molecular diffusion of noble gases in water: mechanistic insights through ab initio molecular dynamics simulations.

Atmospheric noble gases are routinely used as natural tracers to analyze gas transfer processes in aquatic systems. Their isotopic ratios can be employed to discriminate between different physical transport mechanisms by comparison to the unfractionated atmospheric isotope composition. In many applications of aquatic systems molecular diffusion was thought to cause a mass dependent fractionation of noble gases and their isotopes according to the square root ratio of their masses. However, recent experiments focusing on isotopic fractionation within a single element challenged this broadly accepted assumption. The determined fractionation factors of Ne, Ar, Kr and Xe isotopes revealed that only Ar follows the prediction of the so-called square root relation, whereas within the Ne, Kr and Xe elements no mass-dependence was found. The reason for this unexpected divergence of Ar is not yet understood. The aim of our computational exercise is to establish the molecular-resolved mechanisms behind molecular diffusion of noble gases in water. We make the hypothesis that weak intermolecular interactions are relevant for the dynamical properties of noble gases dissolved in water. Therefore, we used ab initio molecular dynamics to explicitly account for the electronic degrees of freedom. Depending on the size and polarizability of the hydrophobic particles such as noble gases, their motion in dense and polar liquids like water is subject to different diffusive regimes: the inter-cavity hopping mechanism of small particles (He, Ne) breaks down if a critical particle size achieved. For the case of large particles (Kr, Xe), the motion through the water solvent is governed by mass-independent viscous friction leading to hydrodynamical diffusion. Finally, Ar falls in between the two diffusive regimes, where particle dispersion is propagated at the molecular collision time scale of the surrounding water molecules.

Exploring the role of the 3-center-4-electron bond in hypervalent λ(3)-iodanes using the methodology of domain averaged Fermi holes.

Hypervalent iodine compounds, in particular λ(3)-iodanes, have become important reagents in organic synthesis for the electrophilic transfer of substituents to arenes and other nucleophiles. The structure and reactivity of these compounds are usually described based on a 3-center-4-electron bond model, involving the iodine central atom and its two trans substituents. The goal of this computational study is to explore Fermi correlation in view of a more advanced description of bonding in these compounds. For that matter, we apply the analysis of Domain Averaged Fermi Holes (DAFH). The DAFH analysis reveals a relationship between the occurrence of multicenter bonding and structural parameters which cannot be easily observed based on simple MO theory. Whereas for λ(3)-iodanes carrying electron-rich ligands pairing of electrons over three centers is indeed observed, compounds with electron-withdrawing substituents fall into a different category: the pairing of electrons is restricted to extend over two centers only, thus challenging the multicenter bonding picture in this case. Accordingly, a drastic reduction of the DAFH three center bond index is observed. The establishment of the multicenter bond in λ(3)-iodanes is driven by a pseudo Jahn-Teller (PJT) effect, whose extent is tightly coupled to the reactivity of the corresponding compound. The PJT stabilization scales with the degree of s-p hybridization of the central atom, which, in return, depends on the electron-withdrawing power of the ligands in the trans position. The response of the multicenter bond to the iodine "ligand field" can be expressed quantitatively in terms of DAFH bond indices. These show, for example, that the activation of the reacting hypervalent species by means of protonation results in a weaker 3-center-4-electron bond, thus making the reagent more reactive. In this work we explain a number of experimentally known facts concerning the reactivity of these compounds. We also show that the DAFH analysis offers a more complete understanding of hypervalency in λ(3)-iodanes, and that it is a tool to assist the search for novel reagents.

Breaking down the reactivity of λ(3)-iodanes: the impact of structure and bonding on competing reaction mechanisms.

The functionalization of arenes via diaryliodonium salts has gained considerable attention in synthesis, as these compounds react under mild conditions. Mechanistic studies have shown that the formation of corresponding λ(3)-iodane intermediates takes a key role, as they determine the course and selectivity of the reaction. Bridged diaryliodonium salts, featuring a heterocyclic moiety involving the iodine atom, were shown to exhibit a distinctly different reactivity, leading to different products. These products are not just the result of reductive elimination reactions but may also arise via radical mechanisms. Our quantum chemical calculations reveal that the λ(3)-iodane intermediate is also the "gateway" for reactions that are observed only for strained bridged systems. At the same time, we find a remarkable affinity of the hypervalent region to planarity for all reaction mechanisms. This also explains the correlation between the size of the bridge connecting the aryl groups and the reaction products observed. Furthermore, the energetics of these competing reactions are examined by analysis of the mechanisms. Finally, using model compounds, some of the basic features governing the reactivity of λ(3)-iodanes are discussed.

Reductive eliminations from λ3-iodanes: understanding selectivity and the crucial role of the hypervalent bond.

This computational study investigates the factors governing the selectivity of the reductive eliminations from rapidly equilibrating isomeric λ(3)-iodanes derived from a diaryl iodonium salt and a nucleophile. The chemoselectivity is mainly determined by the partial charge at the ipso-carbon atom involved in the 3-center-4-electron bond.