Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C3-symmetric ligands

摘要

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C _(3) -symmetric chiral ligands. The C _(3) -symmetric chiral ligands ( Im ~(R) _(3) Bz and Im ~(S) _(3) Bz ) contain three chiral imidazole side arms (Im ~( R ) ?and Im ~( S ) ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn ~(2+) ), which have a tetrahedral coordination preference, the C _(3) -symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula ( Im ~(() ~(R) ~(or) ~(S) ~()) _(3) Bz ) _(4) (Zn ~(2+) ) _(3) . In this structure each Zn ~(2+) ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH _(2) ) to afford a coordinatively saturated assembly [(ImH _(2) ) _(3) ( Im ~(R) _(3) Bz ) _(4) (Zn ~(2+) ) _(3) ]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH _(2) /Zn ~(2+) molar ratio of ～1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH _(2) ) _( m ) ( Im ~(R) _(3) Bz ) _(2) (Zn ~(2+) ) _(3) ] ( m = 4–6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.