Competing Allosteric Mechanisms for Coordination-Directed Conformational Changes of Chiral Stacking Structures with Aromatic Rings

Abstract

This work revealed that significant asymmetric nonlinear effects can be found in a coordination-directed conformational alteration through competing allosteric mechanisms. Toward this aim, we have prepared new chiral bridging ligands [(S,S)-and (R,R)-Im 2 An] containing an anthracene ring as a spacer with two ethynyllinked chiral imidazole groups at the 9,10-positions. The (S,S)-and (R,R)-Im 2 An ligands (L) spontaneously form the assemblies with Zn 2+ ions (M) in solution phase, giving L 4 M 2 -type assemblies with a general formula [(S,S)-or (R,R)-Im 2 An] 4 (Zn 2+ ) 2 . NMR studies revealed that the [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 assembly has an anthracene dimer structure with a parallel-displaced geometry, leading to relatively small circular dichroism (CD) signals, as expected for nonchiral objects. Conversely, subsequent addition of chiral coligands [(R)-or (S)-Ph-box] to [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 afforded an alternative Zn 2+ assembly with general formula [(R)-or (S)-Phbox] 2 [(S,S)-Im 2 An] 2 (Zn 2+ ) 2 , where the chiral coligands expel two of the (S,S)-Im 2 An ligands that were singly bound to the Zn 2+ ions in the original [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 assembly. This ligand-exchange reaction causes conformational alteration from a parallel-displaced structure to a twisted stacking between the anthracene rings inside the Zn 2+ assembly, which results in a significant enhancement of CD signals due to excitonic interactions of the chiral anthracene dimer. Dissymmetry factor (g CD ) for CD due to chiral stacking structures shows a significant inverse sigmoidal response to the enantiomeric excess of the chiral coligands. The observed nonlinear phenomena are results of the two conflicting mechanisms, homochiral cooperative association (homochiral self-sorting) to form CD-active assemblies [(S)-or (R)-Ph-box] 2 [(S,S)-Im 2 An] 2 (Zn 2+ ) 2 versus heterochiral cooperative dissociation of [(S,S)-Im 2 An] 4 (Zn 2+ ) 2 by sequestering of Zn 2+ inside the assembly through formation of a heterochiral 2:1 Zn 2+ complex ([(R)-Ph-box][(S)-Ph-box]Zn 2+ ). The presented mechanisms provide a new strategy for generating switch-like OFF/ON states in chiral systems.

Supramolecular Chiral Aggregates Exhibiting Nonlinear CD–ee Dependence

“…In addition to the supramolecular polymers, examples of negative nonlinear dependence have also been reported in small molecule host–guest systems [ 91,92 ] and in asymmetric synthesis. [ 93–95 ] These systems might be of help for the understanding of the negative nonlinear CD–ee dependence in supramolecular polymeric systems.…”

Dynamic Open Coordination Cage from Nonsymmetrical Imidazole–Pyridine Ditopic Ligands for Turn‐On/Off Anion Binding

“…Besides the pyridine-based ditopic ligands,w eh ave extensively developed imidazole-based ditopic ligands (type 2) that include ethynyl spacers for supramolecular coordination systems which we describe as "metal-ion clipping (MIC)". [17,18] During the course of our studies on MIC,w e realized the intrinsic benefits of the imidazole-based ditopic ligands:s ynthetic accessibility of the imidazole nitrogen side chain allows easy introduction of chiral groups at the metal- coordination sites,w hich gives rise to overall chirality in the resulting self-assembly. [18] Moreover,b ecause of the unique coordination vectors and various conformations of the ligand, the output self-assembly is often highly dynamic.…”

“…[17,18] During the course of our studies on MIC,w e realized the intrinsic benefits of the imidazole-based ditopic ligands:s ynthetic accessibility of the imidazole nitrogen side chain allows easy introduction of chiral groups at the metal- coordination sites,w hich gives rise to overall chirality in the resulting self-assembly. [18] Moreover,b ecause of the unique coordination vectors and various conformations of the ligand, the output self-assembly is often highly dynamic. [17,18] Thus, herein, we describe new imidazole/pyridine-based ditopic ligands (type 3, 1 S and 1)t hat are essentially nonsymmetric (C 1 )when the imidazole unit has achiral pendant group (1 S ).…”

“…[18] Moreover,b ecause of the unique coordination vectors and various conformations of the ligand, the output self-assembly is often highly dynamic. [17,18] Thus, herein, we describe new imidazole/pyridine-based ditopic ligands (type 3, 1 S and 1)t hat are essentially nonsymmetric (C 1 )when the imidazole unit has achiral pendant group (1 S ). At least three typical orientations (perpendicular,p arallel, and antiparallel) exist in 1 S and 1, depending on the rotation around the ethynyl spacers (Scheme 1c).…”