Stereocontrolled Self-Assembly and Self-Sorting of Luminescent Europium Tetrahedral Cages

Coordination-directed
self-assembly has become a well-established
technique for the construction of functional supramolecular structures.
In contrast to the most often exploited transition metals, trivalent
lanthanides LnIII have been less utilized in the design
of polynuclear self-assembled structures despite the wealth of stimulating
applications of these elements. In particular, stereochemical control
in the assembly of lanthanide chiral cage compounds is not easy to
achieve in view of the usually large lability of the LnIII ions. We report here the first examples of stereoselective self-assembly
of chiral luminescent europium coordination tetrahedral cages and
their intriguing self-sorting behavior. Two pairs of R and S ligands are designed on the basis of the
pyridine-2,6-dicarboxamide coordination unit, bis­(tridentate) L1 and tris­(tridentate) L2. Corresponding chiral
Eu4(L1)6 and Eu4(L2)4 topological tetrahedral cages are independently
assembled via edge- and face-capping design strategies, respectively.
The chirality of the ligand is transferred during the self-assembly
process to give either Δ or Λ metal stereochemistry. The
self-assembled cages are characterized by NMR, high-resolution ESI-TOF-MS,
and in one case by X-ray crystallography. Strict control of stereoselectivity
is confirmed by CD spectroscopy and NMR enantiomeric differentiation
experiments. Narcissistic self-sorting is observed in the self-assembly
process when two differently shaped ligands L1 and L2 are mixed. More impressively, distinct self-sorting behavior
between Eu4(L1)6 and Eu4(L2)4 coordination cages is observed for the
first time when racemic mixtures of ligands are used. We envisage
that chiral luminescent lanthanide tetrahedral cages could be used
in chiroptical probes\sensors and enantioselective catalysis.