Formation of Chiral Heteronuclear LnIII Assemblies by Ion Pair Formation

Abstract

AbstractCationic LnIII complexes with the chiral macrocycle L, (P)‐[LnLRRRRRR]3+, interact with anionic LnIII tris(dipicolinate) complexes, [Ln′(dpa)3]3–, forming tight hydrogen‐bonded ion pairs in solution. Formation of these pairs has been documented by the DFT calculations, and a strong paramagnetic shift of the signals of the diamagnetic cation interacting with the paramagnetic anion was observed. This ion pair interaction leads to spectral enantiodifferentiation of the Λ‐[Ln′(dpa)3]3– and Δ‐[Ln′(dpa)3]3– enantiomers of the anionic complex.Although no Pfeiffer effect was observed for the (P)‐[LnLRRRRRR]3+ diastereomers of the cationic complex, the (M)‐[LnLRRRRRR]3+ diastereomers induce CD signals in the anionic [Ln′(dpa)3]3– partner. These spectra indicate a Pfeiffer effect, i.e. a shift of equilibrium between the Λ‐[Ln′(dpa)3]3– and Δ‐[Ln′(dpa)3]3– enantiomers. This effect arises from the ion pair interaction between the anionic complex [Ln′(dpa)3]3– and the intact cationic complex as well as between [Ln′(dpa)3]3– and L. The protonated macrocycle (H3LRRRRRR)3+ forms highly insoluble salts with [Ln′(dpa)3]3–. These supramolecular compounds are formed by the reaction of L, acids and [Ln′(dpa)3]3– anions. The same product can also be formed by the reaction of (M)‐[LnLRRRRRR]3+ with three equivalents of dipicolinic acid. The model structure of (H3LRRRRRR)[Ln′(dpa)3] corresponds with a linear polymer with alternate cationic macrocycles and anionic [Ln′(dpa)3]3– complexes interacting through charge interactions and multiple hydrogen bonds.