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

Synthesis of amphiphilic chiral salen complexes and their conformational manipulation at the air-water interface

A series of amphiphilic salen complexes, [L1a,bM] and [L2a,bM], were designed and synthesized. These complexes consist of two or four hydrophilic triethylene glycol (TEG) chains and a hydrophobic π-extended metallosalen core based on naphthalene or phenanthrene. The obtained amphiphilic complexes, [L1bM] (M = Ni, Cu, Zn), formed a monolayer at the air-water interface, while the monocationic [L1bCo(MeNH₂)₂](OTf) did not form a well-defined monolayer. The number of hydrophilic TEG chains also had an influence on the monolayerformation behavior; the tetra-TEG derivatives, [L1bNi] and [L2bNi], showed a pressure rise at a less compressed region than the bis-TEG derivatives, [L1aNi] and [L2aNi]. In addition, the investigation of their compressibility and compression modulus suggested that the tetra-TEG derivatives, [L1bNi] and [L2bNi], are more flexible than the corresponding bis-TEG analogues, [L1aNi] and [L2aNi], and that the phenanthrene derivatives [L1a,bNi] were more rigid than the corresponding naphthalene analogues, [L2a,bNi]. The Langmuir-Blodgett (LB) films of one of the complexes, [L1bNi], showed CD spectra slightly different from that in solution, which may originate from the unique anisotropic environment of the air-water interface. Thus, we demonstrated the possibility of controlling the chiroptical properties of metal complexes by mechanical compression.

Spacer-Dependent Cooperativity of Helicity in Fluorescent Bishelical Foldamers Based on L-Shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine

For the construction of helical foldamers composed of π-frameworks, the choice of appropriate π-π stacking units and π-spacers connecting them is important. The transfer of helicity between the minimal helix structural units is also an essential factor in the construction of homochiral helical foldamers. Tetramers 4 a-4 d, which have four L-shaped dibenzopyrrolo[1,2-a]naphthyridine units, were synthesized to investigate the interplay and cooperativity of the helical structures. Tetramer 4 a bridged with a biphenyl unit formed a homochiral bishelical structure with π-π stacking between the L-shaped units (3.3 Å), consisting only of (P,P)- and (M,M)-enantiomers without the (P,M)-diastereomer, owing to interplay through the axial chirality of biphenyl unit in the solid state. Similarly, in solution, thermodynamic stabilization of the two helix formations worked cooperatively to favor the bishelical form of 4 a. Furthermore, bishelical foldamer 4 a emitted intense fluorescence (Φ=0.86).

Enhancement of Alkali Metal Ion Recognition by Metalation of a Tris(saloph) Cryptand Having Benzene Rings at the Bridgeheads

A cryptand derivative, H₆L, which has three H₂saloph arms connected by two benzene ring bridgeheads, was synthesized and converted into the trinuclear metallocryptand, LNi₃. The nonmetalated host, H₆L, was found to bind to alkali metal ions (Na⁺, K⁺, Rb⁺, Cs⁺; logK_a = 3.37-6.67) in its well-defined cavity in DMSO/chloroform (1:9). The binding affinity was enhanced by 1-2 orders of magnitude upon the conversion into the metallocryptand, LNi₃, which can be explained by the more polarized phenoxo groups in the [Ni(saloph)] arms. The guest binding affinity of Na⁺ < K⁺ < Rb⁺ ≈ Cs⁺ was clearly demonstrated by the ¹H NMR competition experiments. The DFT calculations suggested that the Rb⁺ ion most suitably fit into the benzene-benzene spacing with a cation-π interaction and that only the largest Cs⁺ ion can almost equally interact with all six phenoxo oxygen donor atoms. The metallocryptand, LNi₃, also showed a strong binding affinity to Ag⁺ by taking advantage of cation-π interactions, which was confirmed by spectroscopic titrations and crystallographic analysis as well as DFT calculations. Thus, the well-defined three-dimensional cavity of LNi₃ was found to be suitable for strong binding with alkali metal ions as well as Ag⁺.

Self-Healing Metallacycle-Cored Supramolecular Polymers Based on a Metal-Salen Complex Constructed by Orthogonal Metal Coordination and Host-Guest Interaction with Amino Acid Sensing

A platinum(II) metallacycle-cored supramolecular network based on a metal-salen complex was successfully constructed by two orthogonal noncovalent interactions (host-guest interactions and metal coordination interactions). The obtained metallo-supramolecular polymer could further form gels when the concentration of metallacycle 1 was 160.0 mM. This gel exhibited multiple stimuli-responsive gel-sol phase transitions under different stimuli, such as temperature, competitive guests, etc. Moreover, it exhibited good self-healing properties and could be used as a turn-off sensor for thiol-containing amino acids.

Control of guest binding behavior of metal-containing host molecules by ligand exchange

This review describes the control of guest binding behavior of metal-containing host molecules that is driven by ligand exchange reactions at the metal centers. Recently, a vast number of metal-containing host molecules including metal-assisted self-assembled structures have been developed, and the structural transformation after construction of the host framework has now been of interest from the viewpoint of functional switching and tuning. Among the various kinds of chemical transformations, ligand exchange has a great advantage in the structural conversions of metal-containing hosts, because ligand exchange usually proceeds under mild conditions that do not affect the host framework. In this review, the structural transformations are classified into three types: (1) weak-link approach, (2) subcomponent substitution, and (3) post-metalation modification, according to the type of coordination motif. The control of their guest binding behavior by the structural transformations is discussed in detail.

Ligand Exchange Strategy for Tuning of Helicity Inversion Speeds of Dynamic Helical Tri(saloph) Metallocryptands

We developed a new strategy, ligand exchange strategy, for tuning the response speeds of helicity inversion of a metal-containing helical structure. This is based on the exchange of the two axial amine ligands of the octahedral Co³⁺ centers in the metallocryptands [LCo₃ X₆ ] (X=axial amine ligand). The response speeds of the helicity induction were controlled by using different combinations of achiral and chiral amines as the starting and entering ligands, respectively. The response speeds of the helicity inversion from P to M were also tuned by using different combinations of chiral amines.