Asymmetric Twisting and Chirality Probing Properties of Quadruple-Stranded Helicates: Coordination Versatility and Chirality Response of Na+, Ca2+, and La3+ Complexes with Octadentate Cyclen Ligand

Importance of an Axial LnIII-F Bond across the Lanthanide Series and Single-Molecule Magnet Behavior in the Ce and Nd Analogues

The recently reported compound [Dy^IIILF](CF₃SO₃)₂·H₂O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) displays a strong axial magnetic anisotropy, due to the short axial Dy–F bond, and single-molecule magnet (SMM) behavior. Following our earlier [Dy^IIILF]²⁺ work, herein we report the systematic structural and magnetic study of a family of [Ln^IIILF](CF₃SO₃)₂·H₂O compounds (Ln(III) = 1-Ce, 2-Pr, 3-Nd, 4-Eu, 5-Tb, 6-Ho, 7-Er, 8-Tm, and 9-Yb). From this series, the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization under an applied direct current magnetic field, which is modeled using a Raman process. Complete active space self-consistent field theoretical calculations are employed to understand the relaxation pathways in 1-Ce and 3-Nd and also reveal a large tunnel splitting for 5-Tb. Additional computational studies on model compounds where we remove the axial F^– ligand, or replace F^– with I^–, highlight the importance of the F^– ligand in creating a strong axial crystal field for 1-Ce and 3-Nd and for promoting the SMM behavior. Importantly, this systematic study provides insight into the magnetic properties of these lighter lanthanide ions.

The structural and magnetic properties of a family of [Ln^IIILF](CF₃SO₃)₂·H₂O (L = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane) compounds are reported. In addition to the previously reported Dy(III) analogue, we find that the Ce(III) and Nd(III) analogues show slow relaxation of the magnetization due to the strong axial magnetic anisotropy created by the axial F⁻ ligand. AC magnetic susceptibility data and CASSCF theoretical calculations are employed to understand the single-molecule magnet behavior of 1-Ce and 3-Nd.

Structural dynamism of chiral sodium peraza-macrocycle complexes derived from cyclic peptoids

A variety of cyclen and hexacyclen derivatives decorated with (S)-1-phenylethyl side chains or (S)-pyrrolidine units have been prepared via a reductive approach from the corresponding cyclic peptoids containing N-(S)-(1-phenylethyl)glycine and l-proline residues. Spectroscopic and DFT studies on their Na⁺ complexes show that point chirality and ring size play a crucial role in controlling the structural dynamism of 1,2-diaminoethylene units and pendant arms. The detection of highly symmetric C₄- and C₃-symmetric metalated species demonstrates that a full understanding of the relationship between the structure and conformational properties of peraza-macrocyclic metal complexes is possible.

Inclusion of alkyl nitriles by tetra-armed cyclens with styrylmethyl groups

A new technique for the assignment of the absolute configurations of low [α]_D alkyl-nitriles using a Ag⁺ complex with a tetra-armed cyclen is reported.

Dynamic Helicity Control of Oligo(salamo)-Based Metal Helicates

Much attention has recently focused on helical structures that can change their helicity in response to external stimuli. The requirements for the invertible helical structures are a dynamic feature and well-defined structures. In this context, helical metal complexes with a labile coordination sphere have a great advantage. There are several types of dynamic helicity controls, including the responsive helicity inversion. In this review article, dynamic helical structures based on oligo(salamo) metal complexes are described as one of the possible designs. The introduction of chiral carboxylate ions into Zn3La tetranuclear structures as an additive is effective to control the P/M ratio of the helix. The dynamic helicity inversion can be achieved by chemical modification, such as protonation/deprotonation or desilylation with fluoride ion. When (S)-2-hydroxypropyl groups are introduced into the oligo(salamo) ligand, the helicity of the resultant complexes is sensitively influenced by the metal ions. The replacement of the metal ions based on the affinity trend resulted in a sequential multistep helicity inversion. Chiral salen derivatives are also effective to bias the helicity; by incorporating the gauche/anti transformation of a 1,2-disubstituted ethylene unit, a fully predictable helicity inversion system was achieved, in which the helicity can be controlled by the molecular lengths of the diammonium guests.

Chiral Cryptates Derived from a Hexaazamacrocycle

The reactions of hexaazamacrocycle 1 with 2,6-bis(bromomethyl)pyridine or 2,6-bis[(tosyloxy)methyl)]pyridine in the presence of appropriate carbonates result in the formation of derivatives of cryptand 6: enantiopure azacryptates of sodium and potassium. Crystal structures of these compounds indicate interaction of a metal ion with four pyridine nitrogen atoms and four tertiary amine atoms. The competition reactions monitored by NMR spectroscopy indicate preferential binding of Na⁺ over K⁺ as well as higher affinity of 6 for Na⁺ in comparison with the [2.2.1] cryptand.

Titanium(iv)-based helicates incorporating the ortho-phenylenediamine ligand: a structural and a computational investigation

We report neutral Ti(iv)-based helical architectures formed with the ortho-phenylenediamine ligand and strands bearing 2,2-biphenolato units. Experimental observations are explained through a computational study.

Chiral metal-macrocycle frameworks: supramolecular chirality induction and helicity inversion of the helical macrocyclic structures

Porous molecular solids composed of discrete macrocycles/cages have great potential for catalysis, separation and sensing techniques. Dynamic structural transformation of the host building blocks, especially a helicity inversion responsive to chemical triggers, is central to upgrading the spatial functions. Here we have achieved the syntheses of homochiral porous molecular solids composed of helical metal macrocycles through supramolecular chirality induction to both enantiomorphic forms with the aid of two different enantiopure sugar-derived lactones in the crystallization process. Moreover, we found that the helicity of the macrocyclic skeletons can be inverted in the crystalline state only by changing the type of solvent. This finding would lead to dynamic control of space chirality in connection with optical resolution, chiral amplification and asymmetric reactions.

Chirality sensing and size recognition of N-Boc-amino acids by cage-type dimeric lanthanide complexes: chirality detection of N-Boc-aspartate anions via luminescence colour change

A mixture of chiral luminescent macrotricyclic-cyclen-Tb–Eu complexes enabled naked-eye discrimination of N-Boc-d- and l-aspartates via luminescence colour change.

Quinoline-attached triazacyclononane (TACN) derivatives as fluorescent zinc sensors

TACN (1,4,7-triazacyclononane) derivatives with three 6-methoxy-2-quinolylmethyl or 1-isoquinolylmethyl moieties were examined as fluorescent zinc sensors. Upon the addition of zinc, 6-MeOTQTACN (5) exhibited a 9-fold fluorescence increase at 420 nm (λex = 341 nm, ϕZn = 0.070). Fluorescence enhancement is specific for zinc and cadmium, although cadmium induces smaller increases (ICd/I0 = 3.6 and ICd/IZn = 40%). The isoquinoline analog 1-isoTQTACN (6) exhibits minimal fluorescence enhancement upon zinc binding. TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylene-diamine) does not extract zinc from the 6-MeOTQTACN-Zn complex (5-Zn). The quantum yield, metal ion selectivity and metal binding affinity differences between TACN and ethylenediamine (EN) skeletons in quinoline-based ligands are discussed based on the X-ray crystallographic analysis of zinc and cadmium complexes, demonstrating the superiority of quinoline-TACN conjugates.

Chirality transfer in propeller-shaped cyclen-calcium(II) complexes: metal-coordinating and ion-pairing anion procedures

A series of quadruple-stranded Na(+) and Ca(2+) complexes with octadentate cyclen ligands was synthesized to produce complexes that contained four different side-arm combinations (one triazole-coumarin group and three pyridine groups (1), four pyridine groups (2), one triazole-coumarin group and three quinoline groups (3), and four quinoline groups (4)). X-ray crystallographic analysis revealed that no significant changes occurred in the stereostructure of these complexes upon replacing one pyridine group with a triazole-coumarin moiety, or by replacing Na(+) ions with Ca(2+) ions, although the coordination number of the complexes in the solid state decreased when pyridine groups were replaced by quinoline groups. In solution, all of the side arms were arranged in a propeller-like pattern to yield an enantiomer pair of Δ and Λ forms in each metal complex. The addition of a tert-butoxycarbonyl (Boc)-protected amino acid anion, that is, a coordinative chiral carboxylate anion, to the cyclen-Ca(2+) complex induced circular dichroism (CD) signals in the aromatic region by forming a 1:1 mixture of diastereomeric ternary complexes with opposite complex chirality, whilst the corresponding Na(+) complexes rarely showed any response. In complexes 1-Ca(2+) and 3-Ca(2+), this chirality-transfer process was efficiently followed by considering the induction of the CD signals at two different wavelengths, that is, the coumarin-chromophore region and the aza-aromatic region. The sign and intensity of the CD signal were significantly dependent on both the nature of the aza-aromatic moiety and the enantiomeric purity of the external anion. These Ca(2+) complexes worked as effective probes for the determination of the enantiomeric excess of the chiral anion. The cyclen-Ca(2+) complexes also interacted with the non-coordinative Δ-TRISPHAT anion through an ion-pairing mechanism to achieve chirality transfer from the anion to the metal complex; both complexes 1-Ca(2+) and 3-Ca(2+) clearly showed induced CD signals in the coumarin-chromophore region, owing to ion-paring interactions with the Δ-TRISPHAT anion. Thus, the proper combination of an octadentate cyclen ligand and a metal center demonstrated effective chirality transfer.

Dynamic cyclen-metal complexes for molecular sensing and chirality signaling

Structural dynamism plays important roles in artificial and biological systems, because it controls structures and functions of various molecules and assemblies. In this review, molecular recognition and self-assembling behavior of dynamic armed cyclen-metal complexes are discussed at the molecular and supramolecular levels. These metal complexes provide useful platforms for molecular receptors, supramolecules, and molecular assemblies that can respond rapidly to guest molecules and environments. Since armed cyclens have many structural and geometrical variations, they form a wide variety of metal complexes having specific sensing and signaling functions. The Lewis acidity of the metal cations plays an essential role in anion binding and in hydrolytic catalysis of phosphate esters. Characteristic luminescence and magnetic properties of lanthanides also enable techniques for effective bio-imaging. They also serve as chiral building blocks for self-assembled architectures, which offer chirality integration effective for chirality sensing and signaling at the supramolecular level.

Coordination chemistry strategies for dynamic helicates: time-programmable chirality switching with labile and inert metal helicates

'Chirality switching' is one of the most important chemical processes controlling many biological systems. DNAs and proteins often work as time-programmed functional helices, in which specific external stimuli alter the helical direction and tune the time scale of subsequent events. Although a variety of organic foldamers and their hybrids with natural helices have been developed, we highlight coordination chemistry strategies for development of structurally and functionally defined metal helicates. These metal helicates have characteristic coordination geometries, redox reactivities and spectroscopic/magnetic properties as well as complex chiralities. Several kinds of inert metal helicates maintain rigid helical structures and their stereoisomers are separable by optical resolution techniques, while labile metal helicates offer dynamic inversion of their helical structures via non-covalent interactions with external chemical signals. The latter particularly have dynamically ordered helical structures, which are controlled by the combinations of metal centres and chiral ligands. They further function as time-programmable switches of chirality-derived dynamic rotations, translations, stretching and shape flipping, which are useful applications in nanoscience and related technology.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

Enantio- and chemoselective differentiation of protected α-amino acids and β-homoamino acids with a single copper(II) host

The association between an achiral copper(II)-containing host 1 and chiral carboxylates has been expanded beyond previous studies to new chiral carboxylate guests, both α-amino acids and β-homoamino acids. The observed exciton-coupled circular dichroism (ECCD) signals for the enantiomers of each carboxylate were equal and opposite, and these signals differed in size and shape between the individual amino acids. Linear discriminant analysis (LDA) was applied as a statistical analysis technique to differentiate the amino acids, both enantioselectively and chemoselectively, giving the absolute configuration and identity of the amino acid. The identity of each of the α-amino acids and β-homoamino acids were determined independently by LDA, and then the two were considered together. Each of these analyses showed good differentiation of the amino acid guests with the use of only one host molecule.

Identifying three-way DNA junction-specific small-molecules

Three-way junction DNA (TWJ-DNA, also known as 3WJ-DNA) is an alternative secondary DNA structure comprised of three duplex-DNAs that converge towards a single point, termed the branch point. This point is characterized by unique geometrical properties that make its specific targeting by synthetic small-molecules possible. Such a targeting has already been demonstrated in the solid state but not thoroughly biophysically investigated in solution. Herein, a set of simple biophysical assays has been developed to identify TWJ-specific small-molecule ligands; these assays, inspired by the considerable body of work that has been reported to characterize the interactions between small-molecules and other higher-order DNA (notably quadruplex-DNA), have been calibrated with a known non-specific DNA binder (the porphyrin TMPyP4) and validated via the study of a small series of triazacyclononane (TACN) derivatives (metal-free or not) and the identification of a fairly-affinic and exquisitely TWJ-selective candidate (a TACN-quinoline construct named TACN-Q).