The physical nature of the noncovalent interactions involved in anion recognition was investigated in the context of metalated calix[4]arene hosts, employing Kohn−Sham molecular orbital (KS-MO) theory, in conjunction with a canonical energy decomposition analysis, at the dispersion-corrected DFT level of theory. Computed data evidence that the most stable host−guest bonding occurs in ruthenium complexed hosts, followed by technetium and molybdenum metalated macrocyclic receptors. Furthermore, the guest's steric fit in the host scaffold is a selective and crucial criterion to the anion recognition. Our analyses reveal that coordinated charged metals provide a larger electrostatic stabilization to anion recognition, shifting the calixarenes cavity toward an electron deficient acidic character. This study contributes to the design and development of new organometallic macrocyclic hosts with increased anion recognition specificity.

We have quantum chemically analyzed the bonding mechanism behind the affinity of various heterocalixarenes for anions with a range of geometries and net charges, using modern dispersion-corrected density functional theory (DFT-D3BJ). The purpose is to better understand the physical factors that are responsible for the computed affinities and thus to develop principles for a more rational design of anion receptors. Our model systems comprise heterocalixarenes 1-4 as hosts, which are characterized by different bridging heteroatoms (O, N, S) as well as the anionic guests Cl, Br, I, BF, CHCO, HPO, HSO, NCS, NO, PF, and SO. We use various analysis schemes (EDA, NCI, and NBO) to elucidate the interactions between the calixarene cavity and the anions to probe the importance of the different bonding modes (anion-π, lone-pair electron-π, σ-complexes, hydrogen bonds, and others) of the interactions. Electrostatic interactions appear to be dominant for heterocalixarenes with oxygen bridges whereas orbital interactions prevail in the case of nitrogen and sulfur bridges. Dispersion interactions are however in all cases non-negligible.

Of charge and cations. Isoelectronic cation–π complexes unravel the nature of variation of the interaction.

Cucurbit­[7]­uril (CB7) catalyzes the acid hydrolysis of alkoxyphenyldioxolanes bearing both neutral and cationic alkoxy groups. The magnitude of the catalytic effect depends on the dioxolane structure, as reflected by both the CB7 binding constants and the catalysis rate constants. However, there is no clear relationship in such a way that increasing the binding affinity (cationic dioxolanes or large alkoxy groups) does not enhance the catalytic effect. The A-1 mechanism for dioxolane hydrolysis involves the protonation and formation of a carbocation by protonated dioxolane ring opening. Supramolecular catalysis takes place through the formation of the ternary complex dioxolane@CB7@H3O+, where the hydronium ion is stabilized by hydrogen bonding with the carbonyl groups of the CB7 portal. The ternary complex evolves to a binary complex by protonation of dioxolane and release of a water molecule. It is important to note that these structures are only stable in the presence of CB7 and not in bulk water. The carbocation is formed by opening the protonated dioxolane group in the rate-determining step. The distance between the carbonyl portal of CB7 and the dioxolane group in the ternary and binary complexes (protonated and carbocation) increases with the alkyl chain length, resulting in the loss of the CB7 stabilizing effect and decrease in catalytic efficiency. The existence of two recognition motifs with cationic dioxolanes results in the formation of both 1:1 and 2:1 complexes with different catalytic properties.