Recognition of Alkali Metal Halide Contact Ion Pairs by Uranyl−Salophen Receptors Bearing Aromatic Sidearms. The Role of Cation−π Interactions

Abstract

Hard anions have long been known to bind strongly to the uranium of uranyl-salophen complexes. Upon functionalization of the salophen framework with one or two benzyloxy substituents, efficient ditopic receptors for alkali metal ions are obtained. The solid-state structures of complexes formed by the two-armed receptor 1 with CsF and with the chlorides of K+, Rb+, and Cs+ reported here reveal the existence of dimeric supramolecular assemblies in which two receptor units assemble into capsules fully enclosing (MX)2 ion quartets. In addition to the strong coordinative binding of the anion to the uranyl center and to electrostatic cation-anion interactions, stabilizing interactions arise from coordination of each cation to six oxygens, three from each receptor, and most importantly, to two aromatic sidearms belonging to different receptors. There are marked differences in organization at the supramolecular level in the CsCl complex of the one-armed receptor 3, in that four uranyl-salophen units instead of two are assembled in a capsule-like arrangement housing a (CsCl)2 ion quartet. However, both receptors achieve the common goal of having each metal cation in close contact with carbon atoms of two aromatic rings. 1H NMR data provide strong evidence that cation-pi(arene) interactions with the sidearms participate in binding also in solution.

Crystal structure of caesium(I) hydrogen maleate, Cs(C4H3O4)

“…The anion is linked to two caesium cations, while the Cs + cation is surrounded by seven organic ligands, three of which are coordinated by employing both Oatoms and the other four are coordinated solely by Oatoms. The Cs-Odistances are in the range from 3.196 (2) to 3.466 (2) Å,which are well within the range reported in the literature for other caesium complexes [10][11][12]. In the crystal structure, intramolecular hydrogen bonds occur, linking carboxylate Oatoms.…”

Contact Ion Pairs on a Protonated Azamacrocycle: the Role of the Anion Basicity

“…This requirement cannot be easily (Figure 5b), the proton is placed on the NH group of the receptor (HN-H distance =1.09 Å) while interacting with the chloride anion (Cl-H distance =1.91 Å). This arrangement points to the existence of the contact K + Cl -ion pair over protonated M [43]. This view is consistent with the calculated K-Cl distance (3.07 Å), which is essentially equal to the sum of the K + and Cl -ionic radii (3.14 Å).…”

The Supramolecular Attitude of Metal–Salophen and Metal–Salen Complexes

“…The contribution to the binding of cation-π interactions is clearly pointed out by the increase of binding constants when the appended phenyl is replaced by a more extended aromatic structure such as a naphthyl unit. This has been confirmed also in the solid state by X-ray analysis [45,46]. Compound 31, characterized by the presence of a perfluorinated appended aromatic arm located much closer to the metal center highlights the stabilizing contribution, both in solution and in the solid state, of the weak and often elusive anion-π interaction to halide recognition.…”

“…Compound 30, endowed with aromatic pendant arms, have been used as a ditopic receptor for alkali metal [45] and quaternary ammonium halide [46] contact ion pairs in chloroform. As usually, recognition of hard anions, chloride, bromide etc., is ensured by strong binding to the hard Lewis acidic uranyl center, whereas cation-π interactions can be established between the aromatic side-arms and the cation partner of the ion pair.…”