Design and Assembly of Chiral Coordination Cages for Asymmetric Sequential Reactions

Supramolecular
nanoreactors featuring multiple catalytically active
sites are of great importance, especially for asymmetric catalysis,
and are yet challenging to construct. Here we report the design and
assembly of five chiral single- and mixed-linker tetrahedral coordination
cages using six dicarboxylate ligands derived-from enantiopure Mn­(salen),
Cr­(salen) and/or Fe­(salen) as linear linkers and four Cp3Zr3 clusters as three-connected vertices. The formation
of these cages was confirmed by a variety of techniques including
single-crystal and powder X-ray diffraction, inductively coupled plasma
optical emission spectrometer, quadrupole-time-of-flight mass spectrometry
and energy dispersive X-ray spectrometry. The cages feature a nanoscale
hydrophobic cavity decorated with the same or different catalytically
active sites, and the mixed-linker cage bearing Mn­(salen) and Cr­(salen)
species is shown to be an efficient supramolecular catalyst for sequential
asymmetric alkene epoxidation/epoxide ring-opening reactions with
up to 99.9% ee. The cage catalyst demonstrates improved activity and
enantioselectivity over the free catalysts owing to stabilization
of catalytically active metallosalen units and concentration of reactants
within the cavity. Manipulation of catalytic organic linkers in cages
can control the activities and selectivities, which may provide new
opportunities for the design and assembly of novel functional supramolecular
architectures.