Functionalized UO2 salenes: Neutral receptors for anions

Article abstract of DOI:10.1021/ja00089a023

A novel class of neutral receptors for anions that contain a unique combination of an immobilized Lewis acidic binding site (UO₂₂₊) and additional amide C(O)NH groups, which can form a favorable H-bond with a coordinated anion guest, has been developed. X-ray analysis of free receptor 14b shows that it is organized in the solid state as dimers, the fifth position of the uranyl being occupied by an oxygen atom of the amide group of a second molecule. Anion complexation has been demonstrated by conductometry, X-ray crystallography, cyclic voltammetry, ¹H and ³¹P NMR spectroscopy, and FAB mass spectrometry. X-ray structures of 1c·H₂PO_4- and 14e·H₂PO_4- reveal that anion binding is effected by coordination to the uranyl cation and additional H-bond formation. The 1c·H₂PO_4- complex is arranged in centrosymmetric pairs, the core of the dimer consisting of two H₂PO_4- anions connected by two short H-bonds. In the case of the 14e·H₂PO_4- complex, the H₂PO_4- complexed to the uranyl forms a H-bonded associate with a second H₂PO_4- anion which itself is not complexed by the ligand. In the case of the preorganized ligands 14b, 14d, and 18, strong (K_ass > 10⁵ M^-1 in MeCN-DMSO, 99:1) and selective complexation of H₂PO_4- has been observed. Selectivities of > 10² and > 103 over Cl^- and HSO_4-, NO_2-, and SCN^-, respectively, were obtained for receptor 14b. "Naked" UO₂ salophenes 19a-c, in which the Lewis acidic binding center contains two vacant positions for guest coordination, have been designed for complexation of dianions like malonate and succinate with K values of 80-460 M^-1 in DMSO. Salophene 19a forms a 1:2 complex with H₂PO_4-. Its X-ray structure shows that the complex is organized as a H-bonded ribbon due to the facts that the H₂PO_4- anions form H-bonded dimers and, in addition, two H₂PO_4- anions are complexed by one uranyl cation.