Chiroptical Inversion of Europium(III) Complexes by Changing a Remote Stereogenic Center of a C-Symmetric Bispyrrolidinoindoline Manifold.

As an effort to integrate natural products chemistry and coordination chemistry, a diastereomeric pair of chiral alkaloidal manifolds composed of a bispyrrolidinoindoline (BPI) framework was designed and synthesized to generate luminescent Eu complexes with switchable chiroptical properties. The C-symmetric alkaloidal manifolds were linked with bis(benzimidazolyl)pyridine (BBIPy) as an achiral metal-binding component through substituents installed at the stereogenic 2/2' positions of the BPI manifolds. The resulting diastereomeric pair of ligands, syn-L1 and anti-L2, allow pseudomirror symmetrical presentation of the metal-binding BBIPy units due to the stereogenic centers on the alkaloidal manifold. The ligand syn-L1 induces intramolecular coordination to form the 1:1 complex Eu( syn-L1) composed of a single stranded metal helicate which exhibits a negative split Cotton effect. In contrast, the ligand anti-L2 led to a supramolecular assembly comprising the 2:2 complex Eu( anti-L2) consisting of a bimetallic double-stranded helicate which shows a positive split Cotton effect. Thus, the sp stereogenic centers in the BPI manifolds play pivotal roles in controlling both metal-ligand equilibria and chirality-switching of luminescent Eu complexes. This approach, which exploits diastereomeric natural product-based manifolds, provides a relatively unexplored means for diversifying metal coordination modes and for controlling the chiroptical properties of the resultant luminescent lanthanoid complexes.