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

Entropy Versus Enthalpy Controlled Temperature/Redox Dual-Triggered Cages for Selective Anion Encapsulation and Release

New C₃ -symmetric imidazole ligands were designed with phosphine and phosphine oxide linkers (L^P and L^PO , respectively) to demonstrate a dual-triggered dynamic closed coordination cage. Both L^P and L^PO form discrete Zn₄ L₄ -closed cages (1^P and 1^PO , respectively) with excellent selectively for BPh₄ ^- , whereas 1^P and 1^PO encapsulate neither a slightly larger size anion [B(C₆ H₄ CH₃ )₄ ^- ] nor smaller size anions (BF₄ ^- , PF₆ ^- , SbF₆ ^- , and OSO₂ CF₃ ^- ). 1^PO exhibits more negative enthalpy and entropy changes upon anion encapsulation, thus releasing almost all of the encapsulated anions at high temperature (343 K) (trigger 1: BPh₄ ^- ⊂1^PO ← → 1^PO +BPh₄ ^- ). In contrast 1^P has less negative enthalpy and entropy changes, thus preserving the captured anion over a wide range of temperatures (298 K to 343 K). The 1^P cage can be quantitatively oxidized to the 1^PO cage by a mild oxidant (Ox.=H₂ O₂ ), and therefore the captured anion can be released by a redox triggering event (trigger 2: BPh₄ ^- ⊂1^P +Ox.→1^PO +BPh₄ ^- ).

Helix-mediated over 1 nm-range chirality recognition by ligand-to-ligand interactions of dinuclear helicates

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively) used as mediators. Both 2Th and 3Th form one-dimensional (1D) helical structures upon terminal binding of two chiral guest co-ligands (L^R or L^S). Long-range chiral self-recognition is achieved in self-assembly of 2Th with L^R and L^S to preferentially form homochiral assemblies, 2Th-L^R·L^R and 2Th-L^S·L^S, whereas there is no direct molecular interaction between the two guest ligands at the terminal edges. X-ray crystal structure analysis and density functional theory studies reveal that long-range chiral recognition is achieved by terminal ligand-to-ligand interactions between the bis-diketonate ligands and chiral guest co-ligands. Conversely, in self-assembly of 3Th with a longer helix length, statistical binding of L^R and L^S occurs, forming heterochiral (3Th-L^R·L^S) and homochiral (3Th-L^R·L^R and 3Th-L^S·L^S) assemblies in an almost 1 : 1 ratio. When phenyl side arms of the chiral guest co-ligands are replaced by isopropyl groups (L′^R and L′^S), chiral self-recognition is also achieved in the self-assembly process of 3Th with the longer helix length to generate homochiral (3Th-L′^R·L′^R and 3Th-L′^S·L′^S) assemblies as the favored products. Thus, subtle modification of the chiral guests is capable of achieving over 1.4 nm-range chirality recognition.

Long-range chirality recognition between the two chiral guest ligands can be tuned based on the helix distances (d_Ln–Ln = 11.5 and 14.0 Å) of bis-diketonate bridged dinuclear lanthanide complexes (2Th and 3Th, respectively).

Supramolecular Chiral Aggregates Exhibiting Nonlinear CD-ee Dependence

Although a linear relationship between the optical activity (normally the CD signal) and the enantiomeric excess (ee) of chiral auxiliaries has been the most commonly observed dependence in dynamic supramolecular helical aggregates, positive nonlinear CD-ee dependence, known as the "majority-rules effect" (MRE), indicative of chiral amplification, has also been well documented and to some extent understood. In sharp contrast, the negative nonlinear CD-ee dependence has been much less reported and is not well understood. Here, the state of the art of both the positive and negative nonlinear CD-ee dependence in noncovalently bound supramolecular helical aggregates is summarized, with the hope that the vast examples of supramolecular aggregates showing positive nonlinear dependence, in terms of the methods of investigations, variations in the structure of the building block (single species or multiple species), and theoretical modeling using the mismatch penalty energy and helix reversal penalty energy, would help to guide the design of building blocks to form aggregates showing negative nonlinear dependence, and thus to understand the mechanisms. The potential applications of those functional supramolecular aggregates are also discussed.

Optical purity, enantiomeric excess and the Horeau effect

The optical purity of an enantiomeric mixture deduced from specific rotation measurements was found by Horeau to be different from its enantiomeric excess, which came to be known as the Horeau effect. This observation had important implications in the practical use of specific rotations and has led to investigations on homochiral and heterochiral aggregation processes. In this review, dedicated to the Horeau principle, the theoretical basis for the observance of the Horeau effect and a survey of the specific rotation studies investigating the Horeau effect are provided, and possible future investigations are suggested.

Zinc-Ion-Stabilized Charge-Transfer Interactions Drive Self-Complementary or Complementary Molecular Recognition

Here, we show that charge-transfer interactions determine whether donor and acceptor ditopic ligands will associate in a complementary or self-complementary fashion upon metal-ion clipping. Anthracene-based (^9,10L_D and ^1,5L_D) and anthraquinone-based (^1,5L_A) ditopic ligands containing two imidazole side arms as zinc coordination sites were designed. The ^9,10L_D and ^1,5L_A systems associated in a complementary fashion (L_A/L_D/L_A) upon clipping by two zinc ions (Zn²⁺) to form an alternating donor-acceptor assembly [(^9,10L_D)(^1,5L_A)₂-(Zn²⁺)₂]. However, once the charge-transfer interactions were perturbed by subtle modifications of the imidazole side arms (^9,10L_D'^(S) and ^1,5L_A'^(S)), self-complementary association (L_D'/L_D'/L_D'/L_D' and L_A'/L_A'/L_A'/L_A') between the donor (^9,10L_D'^(S)) and acceptor (^1,5L_A'^(S)) ligands predominantly occurred to form homoassemblies [(^9,10L_D'^(S))₄-(Zn²⁺)₂ and (^1,5L_A'^(S))₄-(Zn²⁺)₂]. As in the case of a homochiral pair (^9,10L_D'^(S) and ^1,5L_A'^(S)), self-complementary association (narcissistic self-sorting) occurred in the Zn²⁺ assembly with heterochiral combinations of the donor and acceptor ligands (^9,10L_D'^(S)/^1,5L_A'^(R) and ^9,10L_D'^(S)/^1,5L_A'^(R)/^1,5L_A'^(R)). Narcissistic self-sorting also took place between the positional isomer of the donor ligands (^9,10L_D and ^1,5L_D) to form individual homoligand assemblies [(^9,10L_D)₄-(Zn²⁺)₂ and (^1,5L_D)₄-(Zn²⁺)₂]. Conversely, statistical association took place in the Zn₂L₄ assembly process of an isomorphous pair of the donor and acceptor ligands (^1,5L_D and ^1,5L_A).

Luminescent Zn(ii) and Cd(ii) complexes with chiral 2,2′-bipyridine ligands bearing natural monoterpene groups: synthesis, speciation in solution and photophysics

Coordination of chiral ligands containing a 2,2′-bipyridine moiety and natural terpene (+)-limonene or (+)-3-carene groups to zinc(ii) and cadmium(ii) leads to excitation wavelength dependent emission.

Remarkable self-sorting selectivity in covalently linked homochiral and heterochiral pairs driven by Pd2L4 helicate formation

Imidazole-based ditopic ligands bearing two chiral alkyl groups (LRR, LSS, and LRS) were synthesized. The ligands formed Pd2L4 helicates with palladium ions (Pd2+). Self-sorting occurred between LRR and LRS to form (Pd2+)2(LRR)4 and (Pd2+)2(LRS)4 homoligand assemblies, whereas mixing of LRR and LSS with Pd2+ gave a near statistical mixture.

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

This work demonstrates a new nonconventional ligand design, imidazole/pyridine-based nonsymmetrical ditopic ligands (1 and 1^S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd²⁺ at a 1:4 molar ratio, 1 and 1^S initially form mononuclear PdL₄ complexes (Pd²⁺ (1)₄ and Pd²⁺ (1^S )₄ ) without formation of a cage. The PdL₄ complexes undergo a stoichiometrically controlled structural transition to Pd₂ L₄ open cages ((Pd²⁺ )₂ (1)₄ and (Pd²⁺ )₂ (1^S )₄ ) capable of anion binding, leading to turn-on anion binding. The structural transitions between the Pd₂ L₄ open cage and the PdL₄ complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1^S to the (Pd²⁺ )₂ (1^S )₄ open cage holding a guest anion ((Pd²⁺ )₂ (1^S )₄ ⋅G^- ) enables the structural transition to the Pd²⁺ (1^S )₄ complex, which does not have a cage and thus causes the release of the guest anion (Pd²⁺ (1^S )₄ +G^- ).