Dipalladium(II,II)-assembled molecular capsules that unsymmetrically encapsulate a nitrate via hydrogen bonding

Abstract

A series of capsule-like anion receptors (1: {[(bpy)2Pd2]2L12}4+, 2: {[(bpy)2Pd2]2L22}4+, 3: {[(bpy)2Pd2]3L32}6+ and 4: {[(bpy)2Pd2]3L42}6+) have been successfully self-assembled by coordinating the flexible amide based polypyrazole ligands (H2L1: N1,N4-di(1H-pyrazol-5-yl)terephthalamide, H2L2: N1,N4-bis(3-methyl-1H-pyrazol-5-yl)- terephthalamide, H3L3: N1,N3,N5-tri(1H-pyrazol-5-yl)benzene-1,3,5-tricarboxamide and H3L4: N1,N3,N5-tris(3-methyl-1H-pyrazol-5-yl)benzene-1,3,5-tricarboxamide) with dipalladium corners ([(bpy)2Pd2(NO3)2](NO3)2, where bpy = 2,2′-bipyridine) in aqueous solution. These complexes were characterized by 1H NMR, 19F NMR, 31P NMR, 2D 1H–1H NOSEY spectra, ESI-MS and single crystal X-ray diffraction. Interestingly, the NMR spectra measured in solution and structure analysis carried out in the solid state consistently confirmed that one nitrate anion was unsymmetrically accommodated in the cavity of capsules 1 and 2 via N-H···anion hydrogen bonding and other multiple weak interactions. Furthermore, 1H NMR titrations for competitive anion exchange indicated that complexes 1•4NO3− and 2•4NO3− show high selectivity for the encapsulation of NO3– in dimethyl sulfoxide.