Theoretical and Crystallographic Study of the Dual σ/π Anion Binding Affinity of Quinolizinylium Cation

Intermolecular interaction potential maps from energy decomposition for interpreting reactivity and intermolecular interactions

The electrostatic potential (ESP) has been widely used to visualize electrostatic interactions about a molecule. However, electrostatic effects are often insufficient for capturing the entirety of an interaction or a reaction of interest. In this investigation, intermolecular interaction potential maps (IMIPs), constructed from the potentials derived from energy decomposition analysis (EDA) using density functional theory, were developed and applied to provide unique insight into molecular interactions and reactivity. To this end, rather than constructing a potential map from probe point charge interactions, IMIPs were constructed from probe interactions with small molecular fragments, including CH3+, CH3-, benzene, and atomic probes including alkali metals, transition metals, and halides. The interaction potentials are further decomposed producing IMIPs for each interaction component using EDA (electrostatic, orbital, steric, etc.). The IMIPs are applied to the study of various interactions including cation-π and anion-π interactions, electrophilic and nucleophilic aromatic substitution, Lewis acid activation, π-stacking, endohedral fullerenes, and select organometallics which reveal fundamental insight into the positional preferences and physical origins of the interactions that otherwise would be difficult to uncover through other surface analyses.

Placing gold on a π+-surface: ligand design and impact on reactivity

We describe a novel gold chloride complex supported by an ambiphilic phosphine/xanthylium ligand in which the AuCl moiety interacts with the π⁺ surface of the xanthylium unit as indicated by structural studies. Energy decomposition analyses carried out on a model system indicates the prevalence of non-covalent interactions in which the electrostatic and dispersion terms cumulatively dominate. The presence of these AuCl-π⁺ interactions correlates with the high catalytic activity of this complex in the cyclisation of 2-(phenylethynyl)phenylboronic acid, N-propargyl-t-butylamide, and 2-allyl-2-(2-propynyl)malonate. Comparison with the significantly less active acridinium and the 9-oxa-10-boraanthracene analogues reinforces this conclusion.

Isolation and reactivity of a gold(I) hydroxytrifluoroborate complex stabilized by anion-π+ interactions

A 9,9-dimethylxanthene-based ligand substituted at the 4- and 5-positions by a phosphine and a xanthylium unit, respectively, has been prepared and converted into an AuCl complex, the structure of which reveals an intramolecular Au-Cl⋯π⁺ interaction. This new ligand platform was also found to support the formation of an unprecedented hydroxytrifluoroborate derivative featuring a "hard/soft" mismatched Au-μ(OH)-BF₃ motif. Despite its surprising stability, this gold hydroxytrifluoroborate complex is a remarkably potent carbophilic catalyst which readily activates alkynes, without activator.

Towards design strategies for anion–π interactions in crystal engineering

This highlight article summarizes some of the fundamental aspects of the anion–π interaction leading to several design strategies for generating it in solids. In the main body we highlight some relevant examples that illustrate the viability of these strategies and the importance of anion–π interactions in crystal engineering.

Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.