Site-Selective Ligand Bridging among Multiple Internal Coordination Sites of a Metallomacrocycle and Its Conformational Regulation

Selective synthesis of a heterotetranuclear complex from a macrocyclic ligand with multiple identical chelating units

Selective complexation of specific units in homooligomers is usually challenging due to their similar reactivity. We now report a heteronuclear complex using a macrocyclic ligand with four identical ONO chelating units, which was synthesized by selective coordination to molybdenum at two diagonal units, followed by palladium at the remaining units.

External Phosphate Binding Triggered Formation of Coordination Bridged Dimer of Macrocyclic Trinuclear Complex

Macrocyclic complexes with spatially-arranged coordination sites have the potential to create unique supramolecular systems that combine the precise switching of self-assembled structures with the coordinative recognition of specific molecules. We now report a macrocyclic trinuclear Zn complex that undergoes a unique dimerization by molecular binding from the outside. The multinuclear complex was synthesized from a trispap ligand, a designed macrocyclic trimer of an N,N,O-type tridentate chelating unit pap. This macrocyclic ligand is capable of fixing the metal centers and retaining their exchangeable coordination sites. The macrocyclic trimer reacts with zinc acetate to form the trinuclear complex, in which the coordination sites of the three zinc atoms are aligned perpendicular to the macrocyclic plane and capped with the acetates. Subsequently, phosphate anion, PO4 3-, coordinates from one side of the macrocycle in a tripodal manner. This phosphate binding produced an electrically-neutral complex moiety with available coordination sites on the other side of the macrocycle which leads to the formation of the coordination-bridged dimer.

Pseudo[3]rotaxane and [3]Rotaxane of Per(5-carboxy-5-dehydroxymethyl)-Cyclodextrins Utilizing Carboxylic Acid Dimer as Recognition Units

The formation of pseudo[3]rotaxane using dimers of per(5-carboxy-5-dehydroxymethyl)-cyclodextrins as ring components and secondary ammoniums as an axle is presented. This supramolecule is formed by the unique interaction of circularly arranged carboxylic acid dimers acting as hydrogen bonding acceptors covering the axle moiety. On these grounds, we could achieve the synthesis of the [3]rotaxane by capping the terminal groups through thiol-ene reactions.

Single-Atom Tuning of Pyridine-Strapped Pillar[5]arene Capsules for Specific Guest Binding

Pyridine bis(carboxamide)-strapped pillar[5]arene capsules were synthesized with the serendipitous formation of macrotricyclic products. The structural integrity of the supramolecular capsules, determined by the specific orientation of a single nitrogen atom, controls the electronic properties of the confined binding cavity, facilitating length-selective recognition of aliphatic organic guests with nitrile, isocyanide, and amine functional groups with exceptional host-guest binding affinity and selectivity for 1,2-diaminoethane (Ka > 104 M-1) in a polar organic solvent.

Stereoselective Sequential Diels-Alder Reactions on a Cyclic Furan Trimer for the Construction of an Asymmetric Scaffold

Multiple Diels-Alder reactions are a powerful method to construct large asymmetric scaffolds, but these reactions often produce numerous isomers. We now report a triple Diels-Alder reaction between a cyclic furan trimer and N-substituted maleimides, achieving selective synthesis of a single asymmetric tris-adduct. The stereoselectivity of cycloaddition to π-extended furan derivatives was clarified by the experimental analysis of intermediates and theoretical calculations. Furthermore, the potential of the method to synthesize a chiral, optically active molecule was demonstrated.

Synthesis of macrocyclic salen rare-earth complexes and their framework conversion regulated by coordination sphere engineering

We now report a macrocyclic rare-earth complex whose frameworks have been regulated between a trimer and a tetramer by an external capping ligand. Coordination sphere engineering of the rare earth metals with flexible and variable coordination numbers (CN > 6) offers a unique way to control the self-assembled structures.

Formation of a Metallomacrocycle Possessing Nitrogen Donors Assembled in the Cavity and Unique Water Clusters

2,2'-Bipyridyl (bpy) is widely used as a chelating unit for metal complexation but is not usually considered as a hydrogen-bonding unit. This is because the metal-free bpy units are usually in a transoid conformation, and the two nitrogen lone pairs are pointed to the opposite sides. We now report a metallomacrocycle whose three metal-free bpy units are in a cisoid conformation and are fixed in the cavity. The complexation of nickel(II) only at the salen units of the triangular bpytrisalen ligand produced this rigid and planar macrocycle. Its cavity is surrounded by hydrogen-bond acceptors (N of bpy and O of salen), and it was found that unique pentagonal prism clusters of water molecules templated by the cavity were formed in the crystal. This study has not only increased the variation of the synthetic methodologies of multinuclear complexes but has also provided the structural platform on which multiple bpy units exert hydrogen-bonding functions.

Synthesis of a macrocyclic oligomer of pyridylbenzoxazole utilizing dynamic covalent bonds and its unsymmetric conversion

A new unsymmetric macrocycle was synthesized by an effective and high yield 3-step synthesis, which was composed of macrocyclization, irreversible conversion, and desymmetrization. The dynamic nature of imine bonds as well as selective conversion based on strain release were utilized. Moreover, the unsymmetric macrocycle formed an interesting bivalve-like 2 : 1 complex with zinc.