Hydrogen-Bond-Assisted Symmetry Breaking in a Network of Chiral Metal–Organic Assemblies

Herein
we elucidate the interplay of chiral, chelate, solvent,
and hydrogen–bonding information in the self-assembly of a
series of new three-dimensional metal–organic architectures.
Enantiopure ligands, each containing H-bond donors and acceptors,
form different structures, depending on the ratio in which they are
combined: enantiopure components form M4L4 assemblies,
whereas racemic mixtures form M3L3 stacks. Chiral
amplification within M3L3 enantiomers was observed
when a 2:1 ratio of R and S subcomponent
enantiomers was employed. Simply switching the solvent (from MeCN
to MeOH) or chelating unit (from bidentate to tridentate) increased
the diversity of structures that can be generated from these building
blocks, leading to the selective formation of novel M2L2 and M3L2 assemblies. The addition of
achiral ligand building blocks resulted in the formation of further
structures: When an achiral subcomponent was combined with its R and S chiral congeners, a three-layer
heteroleptic architecture was generated, with the achiral unit sitting
at the top of the stack. When combined with the S enantiomer only, however, the achiral unit assembled in the center
of the structure, thus demonstrating the selective placement of achiral
units within chiral systems. Further sorting experiments revealed
that combining R and S stereocenters
within a single ligand led to diastereoselective product generation.
These results show how geometric complementarity between different
ligands impacts upon the degree of hydrogen-bonding within the assembly,
stabilizing specific low-symmetry architectures from among many possible
structural outcomes.