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

Abstract

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C₃-symmetric chiral ligands. The C₃-symmetric chiral ligands (Im^R₃Bz and Im^S₃Bz) 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 (Zn²⁺), which have a tetrahedral coordination preference, the C₃-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im^(RorS)₃Bz)₄(Zn²⁺)₃. In this structure each Zn²⁺ 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: ImH₂) to afford a coordinatively saturated assembly [(ImH₂)₃(Im^R₃Bz)₄(Zn²⁺)₃]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH₂/Zn²⁺ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH₂)_m(Im^R₃Bz)₂(Zn²⁺)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.