Macrocycle-Directed Construction of Tetrahedral Anion-π Receptors for Nesting Anions with Complementary Geometry

Manipulation of the emerging anion-π interactions in a highly cooperative manner through sophisticated host design represents a very challenging task. In this work, unprecedented tetrahedral anion-π receptors have been successfully constructed for complementary accommodation of tetrahedral and relevant anions. The synthesis was achieved by a macrocycle-directed approach by using large macrocycle precursors bearing four reactive sites, which enabled a kinetic-favored pathway and afforded the otherwise inaccessible tetrahedral cages in considerable yields. Crystal structure suggested that the tetrahedral cages have an enclosed three-dimensional cavity surrounded by four electron-deficient triazine faces in a tetrahedral array. The complementary accommodation of a series of tetrahedral and relevant anions including BF₄ ^- , ClO₄ ^- , H₂ PO₄ ^- , HSO₄ ^- , SO₄ ^2- and PF₆ ^- was revealed by ESI-MS and DFT calculations. Crystal structures of ClO₄ ^- and PF₆ ^- complexes showed that the anion was nicely encapsulated within the tetrahedral cavity with up to quadruple cooperative anion-π interactions by an excellent shape and size match. The strong anion-π binding was further confirmed by negative ion photoelectron spectroscopy measurements.

Orbital effect-induced anomalous anion-π interactions between electron-rich aromatic hydrocarbons and fluoride

Anion-π interactions generally exist between an anion and an electron-deficient π-ring because of the electron-accepting character of the ring. In this paper, we report orbital effect-induced anomalous binding between electron-rich π systems and F(-) through anion-π interactions calculated at the MP2/6-31+G(d,p) and ωB97X-D/6-31+G(d,p) levels of theory. We find that anion-π interactions between F(-) and electron-rich π rings increase markedly with increasing number of π electrons and size of the π rings. This is contrary to intuition because anion-π interactions would be expected to gradually decrease because of gradually increasing Coulombic repulsion between the negative charge of the anions and gradually increasing number of π electrons of the aromatic surfaces. Energy decomposition analysis showed that the key to this anomalous effect is the more effective delocalization of negative charge to the unoccupied π* orbitals of larger π rings, which is termed an "orbital effect".

On the importance of anion-π interactions in the mechanism of sulfide:quinone oxidoreductase

Sulfide:quinone oxidoreductase (SQR) is a flavin-dependent enzyme that plays a physiological role in two important processes. First, it is responsible for sulfide detoxification by oxidizing sulfide ions (S(2-) and HS(-)) to elementary sulfur and the electrons are first transferred to flavin adenine dinucleotide (FAD), which in turn passes them to the quinone pool in the membrane. Second, in sulfidotrophic bacteria, SQRs play a key role in the sulfide-dependent respiration and anaerobic photosynthesis, deriving energy for their growth from reduced sulfur. Two mechanisms of action for SQR have been proposed: first, nucleophilic attack of a Cys residue on the C4 of FAD, and second, an alternate anionic radical mechanism by direct electron transfer from Cys to the isoalloxazine ring of FAD. Both mechanisms involve a common anionic intermediate that it is stabilized by a relevant anion-π interaction and its previous formation (from HS(-) and Cys-S-S-Cys) is also facilitated by reducing the transition-state barrier, owing to an interaction that involves the π system of FAD. By analyzing the X-ray structures of SQRs available in the Protein Data Bank (PDB) and using DFT calculations, we demonstrate the relevance of the anion-π interaction in the enzymatic mechanism.