Shifting the Triangle-Square Equilibrium of Self-Assembled Metallocycles by Guest Binding with Enhanced Photosensitization

Shifting a triangle-square equilibrium in one direction is an important problem in supramolecular self-assembly. Reaction of a benzothiadiazole-based diimidazole donor with a cis-Pt(II) acceptor yielded an equilibrium mixture of a triangle ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₃≡ PtMC^T) and a square ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₄≡ PtMC^S). We report here the shifting of such equilibrium toward a triangle using a guest (pyrene aldehyde, G1). While both benzothiadiazole and pyrene aldehyde can form reactive oxygen species (ROS) in organic solvents, their therapeutic use in water is restricted due to aqueous insolubility. The enhanced water solubility of the benzothiadiazole unit and G1 by macrocycle formation and host-guest complexation, respectively, enabled enhanced ROS generation by the host-guest complex (G1' ⊂ PtMC^T) in water (G1' = hydrated form of G1). The guest-encapsulated metallacycle (G1' ⊂ PtMC^T) has shown synergistic antibacterial activity compared to the mixture of macrocycles upon white-light irradiation due to enhanced ROS generation. The mechanism for such enhanced activity was established by measuring the oxidative stress and relative internalization of PtMCs and G1' ⊂ PtMC^T.

Competition of Hydrogen Bonds and Coordinate Bonds Induces a Reversible Crystal Transformation

Achieving reversible molecular crystal transformation between coordinate aggregates and hydrogen bonded assemblies has been a challenging task because coordinate bonds are generally much stronger than hydrogen bonds. Recently, we have reported the incorporation of silver ions into the cyanuric acid-melamine (CAM) network, resulting in the formation of a 1D coordination polymer (crystal 1) through forming the κ₁N-Ag-κ₂N coordination bonds. In this work, we find crystal 1 will undergo reversible transformation to hydrogen bonded coordinate units (crystal 2) through the breaking of coordinate chains and then the addition of CAM hydrogen bonding motifs into the framework. Crystal 2 presents a pseudohexagonal arrangement comprised of the κ₁N-Ag-κ₂N units connected by two sets of the triple hydrogen bonds, which extends two-dimensionally and stacks into a layer-structured crystal. Light was shed on the tautomerization of CA and M ligands associated with the crystal transformations using single crystal X-ray diffraction and infrared spectroscopy by analyzing the bond lengths and vibrations. We also highlight that photoluminescence can be a useful tool to probe the tautomer conversions of conjugated molecules. Furthermore, crystal 1 demonstrates high flexibility and can be bent over 180° and recover to its original shape after stress release. Crystal 2, on the contrary, is brittle and shows distinct mechanical anisotropy along different crystal orientations, as unveiled by nanoindentation measurements. The elastic modulus is well correlated with the chemical bonding strength along each orientation, and it is noteworthy that the contribution of the triple hydrogen bonds is comparable to that of the coordination bonds.

Supramolecular catalysis: the role of H-bonding interactions in substrate orientation and activation

Hydrogen bonding plays significant roles in various biological processes during substrate orientation and binding and therefore assists in assorted organic transformations. However, replicating the intricate selection of hydrogen bonds, as observed in nature, in synthetic complexes has met with only limited success. Despite this fact, recent times have seen the emergence of several notable examples where hydrogen bonds have been introduced in synthetic complexes. A few such examples have also illustrated the substantial role played by the hydrogen bonds in influencing and often controlling the catalytic outcome. This perspective presents selected examples illustrating the significance of hydrogen bonds offered by the coordination and the organometallic complexes that aid in providing the desired orientation to a substrate adjacent to a catalytic metal center and remarkably assisting in the catalysis.

Hydrogen Bonding Directed Self-Assembly of a Binuclear Ag(I) Metallacycle into a 1D Supramolecular Polymer

An Ag(I) metallacycle obtained unexpectedly during the preparation of Pd(II) complexes of the bifunctional ligand 5-([2,2′-bipyridin]-5-yl)pyrimidine-2-amine (L) has been characterized using X-ray structure determination as a binuclear, metallacyclic species [Ag₂L₂](SbF₆)₂, where both the bipyridine and pyrimidine-N donors of L are involved in coordination to the metal. The full coordination environment of the Ag(I) defines a case of highly irregular 4-coordination. In the crystal, the Ag-metallacycles assemble into one-dimensional supramolecular metalladynamers linked together by hydrogen-bonding interactions.

Supramolecular Chemistry in the 3rd Millennium

The description of supramolecular chemistry as “chemistry beyond the molecule” (Jean-Marie Lehn, 1987 Nobel Lecture and Gautam R [...]

Stimuli-Responsive Self-Sorting Hybrid Hydrogen-Bonded/Metal-Coordinated Cage

The self-assembly of a unique molecular container is reported: a hybrid hydrogen-bonded/metal-coordinated cage where both hydrogen-bonding and metal-coordination form the crucial part of the topology. The hybrid cage was prepared combining hydrogen-bonded rosette motif and palladium(II)/platinum(II) coordination to a pyridine ligand. It was also shown that the hybrid cage could be prepared by integrative self-sorting from simple components. For the first time the genuine dual character of the hybrid cage was manifested as both self-assembling parts responded selectively to different stimuli (such as phosphine and cyanurate), which resulted in the disassembly of the cage.

Solid-State Structural Transformation and Photoluminescence Properties of Supramolecular Coordination Compounds

The combination of strong coordination bonds and hydrogen bonding interactions were used to generate a series of supramolecular coordination materials (SCMs), which was achieved by reacting a bis-pyridyl amide ligand, namely N-(4-pyridyl)nicotinamide (4PNA) with copper(II), zinc(II), and cadmium(II) benzoates. The SCMs were structurally characterized using X-ray diffraction and the key intermolecular interactions were identified via Hirshfeld surface analysis. The role of solvent molecules on the supramolecular architecture was analyzed by synthesizing the SCMs in different solvents/solvent mixtures. A solvent-mediated solid-state structural transformation was observed in copper(II) SCMs and we were able to isolate the intermediate form of the crystal-to-crystal transformation process. The luminescence experiments revealed that complexation enhanced the fluorescence properties of 4PNA in the zinc(II) and cadmium(II) SCMs, but a reverse phenomenon was observed in the copper(II) SCMs. This work demonstrated the tuning of supramolecular assembly in coordination compounds as a function of solvents for generating SCMs with diverse properties.

Construction of hydrogen bonding and coordination networks based on ethynylpyridine-appended nucleobases

A series of ethynylpyridine-appended nucleobases have been designed, synthesized, characterized and employed for the formation of crystalline molecular networks by hydrogen/coordination bonding.