Pablo Ballester

Quantitative Evaluation of Anion–π Interactions in Solution

Anions included: A series of meso-tetraaryl calix[4]pyrrole receptors have been used as a model system to quantify chloride–π interactions in solution (see picture; green balls are chloride ions). The free energy values are generally indicative of a repulsive interaction; their magnitude depends on the substituent on the aromatic ring.

Hydrogen bonded supramolecular capsules with functionalized interiors: the controlled orientation of included guests

This tutorial review aims to present and describe selected examples of still quite rare endo-functionalized supramolecular capsules. Only capsules assembled through the use of neutral or polar hydrogen bonding interactions are covered. The ability of these capsules to orient and control the position of the encapsulated guest molecules represents their stand-out feature. Thus, the way in which guests interact with the functionalized interior of the capsules is discussed in detail. Several of the described capsular assemblies display chiral architectures. The manner in which chirality is manifested in these assemblies is far from trivial. For this reason, we also offer explanations of the chiral features expressed by the relevant capsules and their encapsulation complexes.

Anion–π interactions: must the aromatic ring be electron deficient?

The favorable interaction of anions with the π-cloud of aromatic derivatives has been studied theoretically using ab initio calculations and confirmed by X-ray data retrieved from the Cambridge Structural Database.

Squaramido-based receptors: Molecular recognition of carboxylate anions in highly competitive media

Abstract A new series of artificial receptors for binding carboxylates has been developed. The receptors are based on the hydrogen bonding abilities of secondary squaramides as binding subunits. The data reveal the formation of 1:1 complexes in highly competitive media. Using convenient spacers, receptors for di- and tri-carboxylates that incorporate two and three squaramide subunits have been prepared. The complexes are characterized by nmr and, when possible, by FAB spectral analysis.

Different nature of the interactions between anions and HAT(CN)6: from reversible anion-π complexes to irreversible electron-transfer processes (HAT(CN)6 = 1,4,5,8,9,12-hexaazatriphenylene).

We report experimental evidence indicating that the nature of the interaction established between HAT(CN)(6), a well-known strong electron acceptor aromatic compound, with mono- or polyatomic anions switches from the almost exclusive formation of reversible anion-π complexes, featuring a markedly charge transfer (CT) or formal electron-transfer (ET) character, to the quantitative and irreversible net production of the anion radical [HAT(CN)(6)](•-) and the dianion [HAT(CN)(6)](2-) species. The preferred mode of interaction is dictated by the electron donor abilities of the interacting anion. Thus, weaker Lewis basic anions such as Br(-) or I(-) are prone to form mainly anion-π complexes. On the contrary, stronger Lewis basic F(-) or (-)OH anions display a net ET process. The ET process can be either thermal or photoinduced depending on the HOMO/LUMO energy difference between the electron donor (anion) and the electron acceptor (HAT(CN)(6)). These ET processes possibly involve the intermediacy of anion-π complexes having strong ET character and producing an ion-pair radical complex. We hypothesize that the irreversible dissociation of the pair of radicals forming the solvent-caged complex is caused by the reduced stability (high reactivity) of the radical resulting from the anion.

Molecular recognition of pyridine N-oxides in water using calix[4]pyrrole receptors.

The incorporation of four carboxylic acids or four amino groups to the upper rim of an aryl extended calix[4]pyrrole produces water-soluble receptors which are able to effectively bind aromatic N-oxides in water by a combination of hydrogen bonding, pi-pi, CH-pi, and hydrophobic interactions.

The role of para-alkyl substituents on meso-phenyl porphyrin sensitised TiO2 solar cells: control of the eTiO2/electrolyte+ recombination reaction

We aim to investigate the effect of adding hydrophobic alkyl chains substituents to unsymmetrical free base tetra-phenyl porphyrins used for the preparation of dye sensitised solar cells (DSSC). We have used two different unsymmetrical meso-tetraphenyl substituted free base porphyrins attending to two objectives: (1) to observe how the substitution of three para positions of the meso-phenyl groups with hydrophobic alkyl chains influences the formation of molecular aggregates onto the semiconductor nanoparticles and (b) to deduce the influence that the substitution exerts over the eTiO2/electrolyte+ recombination reaction in operating devices. To achieve these goals we have focussed on the study of the electron transfer processes that take place at the different interfaces of the photovoltaic device using electrochemistry, steady-state and time resolved spectroscopic techniques.

3-Picoline mediated self-assembly of M(II)-malonate complexes (M = Ni/Co/Mn/Mg/Zn/Cu) assisted by various weak forces involving lone pair-π, π-π, and anion···π-hole interactions.

Five M(II)-malonate complexes having a common formula (C(6)H(9)N(2))(4)[M(II)(C(3)H(2)O(4))(2)(H(2)O)(2)](PF(6))(2).(H(2)O)(2) (1-5) [where C(6)H(9)N(2) = protonated 3-picoline, M(II) = Ni/Co/Mn/Mg/Zn, C(3)H(4)O(4) = malonic acid, and PF(6)(-) = hexafluorophospahte], have been synthesized and their crystal structures have been determined. Complexes 1-5 were found to be isostructural and protonated 3-picoline has primarily mediated the self-assembly process. Role of a discrete water dimer in complexes 1-5 was also studied. Weaker π-interactions have also played crucial role in stabilizing 1D chain constructed by discrete [M(II)(C(3)H(2)O(4))(2)(H(2)O)(2)] units. An additional copper complex namely, (C(6)H(9)N(2))(4)[Cu(C(3)H(2)O(4))(2)](PF(6))(2) (6) has been synthesized from the same reagents and was found to have a completely different structure from the others. Structures of all the complexes are fully described and compared here. Moreover, the lone pair-π and π-π noncovalent interactions have been analyzed by means of DFT calculations, mainly focusing our attention to the influence of the coordinating metal on the strength of the interactions and the interplay between hydrogen bonding and π-interactions. We also present here Hirshfeld surface analysis to investigate the close intermolecular contacts.

MP2 study of cooperative effects between cation??, anion?? and ??? interactions

In this manuscript we report high level ab initio calculations (RI-MP2(full)/6-31++G**) and experimental evidence that demonstrate that important non-additivity effects between three noteworthy non-covalent interactions that involve aromatic rings (benzene and hexafluorobenzene), i.e. cation–π, anion–π and π–π interactions, occur when the interactions coexist in the same complex. In some cases we have observed very high non-additivity energies (up to −20.3 kcal mol−1) that indicate that the cation–π and/or the anion–π have a strong influence on the π–π interaction and vice versa.

Thermodynamic characterization of the squaramide–carboxylate interaction in squaramide receptors

Abstract The thermodynamic characterization of squaramides is described. This fundamental information shows that the association in chloroform or DMSO is mainly exothermic. In contrast, in MeOH the equilibrium is endothermic and is entropically driven. The data shows the influence of a squaramide ring alone, modified or combined with tetraalkylammonium groups as the binding subunit for molecular recognition.