Supramolecular assemblies built from lanthanide ammoniocarboxylates and cucurbit[6]uril

Abstract

The reaction of lanthanide nitrates with different aminocarboxylic acids in the presence of cucurbit[6]uril (CB6) in water at room temperature yielded five novel complexes which were crystallographically characterized. The cerium complex involving the zwitterionic form of β-alanine (β-Ala), [Ce(β-Ala)(CB6)(NO₃)(H₂O)₃]·2NO₃·5H₂O (1), is a molecular species in which the metal cation is bound to three carbonyl groups of CB6 and one monodentate carboxylate; the ammonium-bearing chain is encapsulated in the CB6 cavity, with the ammonium group hydrogen bonded to the uncomplexed portal. With a longer alkyl chain, 6-aminohexanoic acid (6-ah) leads to a different, dimeric structure in [Ce(6-ah)(CB6)(H₂O)₃]₂·6NO₃·17H₂O (2), in which two metal ions connect two CB6 molecules and are further bridged by two carboxylate groups. The alkyl chains are included in the CB6 cavities and the ammonium groups are hydrogen bonded to the uncomplexed portals. Neighbouring, perpendicular dimers are associated through ammonium-water, water-carbonyl and CH⋯O(carbonyl) hydrogen bonds. Three complexes were obtained with the enantiopure α-amino acid L-methionine (L-Met), all of which are chiral. In [Yb(L-Met)(H₂O)₇]·CB6·3NO₃·7H₂O (3) and [Dy(L-Met)(H₂O)₇]₄[Dy(NO₃)₂(H₂O)₅]·4CB6·13NO₃·29H₂O (4), the metal ions are bound to one monodentate carboxylate and seven water molecules. The ammonium group is involved in hydrogen bonding with two CB6 molecules and the thioether-containing chain is included in the cavity of one CB6, with the sulphur atom shortest contact being with a ureido carbon atom. The supramolecular arrangement is columnar, the neighbouring chains being connected by hydrogen bonds formed by the ammonium groups. The neodymium complex [Nd(L-Met)(CB6)(NO₃)(H₂O)₃]₂[Nd(L-Met)(L-Met – H)(H₂O)₅]₂·8NO₃·30H₂O (5) displays two dinuclear units held by double carboxylate bridges. In one of them, the metal atoms are bound to CB6 and the L-Met molecules are excluded from the CB6 cavities while, in the other, the metal atoms are not bound to CB6, but two L-Met ligands are hydrogen bonded to CB6 portals and encapsulated. Together with previous results involving other aminocarboxylic acids, these complexes illustrate the different association modes between lanthanide ions, CB6 molecules and ammoniocarboxylates, which depend on the nature and geometry of the latter, and on a balance between CB6 complexation to the lanthanide ion and ammoniocarboxylate/CB6 association through weak interactions. Short contacts between the sulphur atoms of L-Met and ureido carbon atoms in 3–5 are likely indicative of the presence of weak dipole–dipole interactions.