Hovering above the π‐electron cloud, an anion positioned over hexafluorobenzene along the C6 axis as shown here interacts with the permanent quadrupole of the arene. Crystallographic and computational evidence demonstrate that anion–π interactions exist and are energetically favorable.

The importance of anion–π interactions in key biological processes is reported from a PDB analysis of anion–π interactions in biomolecules, also considering cooperativity effects by including other interactions.

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Several π-complexes of cations and anions with aromatic rings have been optimized at the MP2/6-31++G** level of theory. Different aspects of the cation-π interaction have been compared to those of anion-π, including changes in the aromaticity of the ring upon complexation, charge-transfer effects using the Merz-Kollman and "atoms-in-molecules" (AIM) charges, and the contribution of correlation and dispersion energies by comparing the complexation energies computed at the HF, B3LYP, and MP2 levels of theory. In this paper, we study three aromatic systems that allow direct comparisons, free from other influences, of the cation-π versus anion-π interactions, which are the 1,3,5-trifluorobenzene (TFB), s-triazine (TAZ), and 2,5-dichloropyrazine (DCP). These compounds are able to π-interact favorably with either anions or cations because of their very small quadrupole moments.

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

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In this review, we analyze the interaction of ions with aromatic rings from several points of view. We start with a short history of cation-π and anion-π interactions and continue with a description of the main forces involved in these interactions. The comprehension of these forces allows us to rationalize the requirement that both the ion and the aromatic compound should have improved the interaction. Some physical properties of both the aromatic rings and the interacting ion are directly related with the strength of the interaction. An interesting part of this review is the study of the interplay of the ion-π interactions with other noncovalent forces. The strength of the ion-π interaction is considerably influenced by the presence of hydrogen bonding or other weaker interactions. These influences can be used to tune the interaction, either weakening or strengthening it. We give some experimental examples that illustrate this point.