Bpytrisalen/Bpytrisaloph: A Triangular Platform That Spatially Arranges Different Multiple Labile Coordination Sites

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.

Triamine and Tetramine Edge-Length Matching Drives Heteroleptic Triangular and Tetragonal Prism Assembly

Heteroleptic metal-organic capsules, which incorporate more than one type of ligand, can provide enclosed, anisotropic interior cavities for binding low-symmetry molecules of biological and industrial importance. However, the selective self-assembly of a single mixed-ligand architecture, as opposed to the numerous other possible self-assembly outcomes, remains a challenge. Here, we develop a design strategy for the subcomponent self-assembly of heteroleptic metal-organic architectures with anisotropic internal void spaces. Zn6Tet3Tri2 triangular prismatic and Zn8Tet2Tet'4 tetragonal prismatic architectures were prepared through careful matching of the side lengths of the tritopic (Tri) or tetratopic (Tet, Tet') and panels.

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.

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

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

Metallomacrocycles with internal coordination sites have a high potential to precisely control the positions of the guest ligands and the overall shape of the assemblies by utilizing the directionality and reversibility of the coordination bonds. However, when such coordinative hosts possess multiple coordination sites, it was difficult to control to which coordination sites the incoming guest ligands bind, because such systems often result in a random, uncontrolled mixture. The metallomacrocycle that we now report, a hexanuclear palladium complex of hexapap possessing six internal coordination sites, can take two different conformations depending on the guests. One is an Alternate conformation, in which six coordination sites of pap alternatively point to Up-Down-Up-Down-Up-Down. The other is a Twisted conformation, in which the coordination sites direct Up-Middle-Down-Up-Middle-Down. Interestingly, linear ditopic α,ω-diamines are captured in three distinct cross-linking modes, and regulations between the two macrocyclic conformations have been realized by the lengths of the diamines. Furthermore, the heteroleptic site-selective bridging of two kinds of diamines has been achieved. It has been demonstrated that a slight difference in the diamine lengths leads to a significant change in the structure and selection of the produced host-guest macrocyclic complexes.

Self-assembly of a Mixed Valence Copper Triangular Prism and Transformation to Cage Triggered by an External Stimulus

A triangular prismatic metal-organic cage based on mixed valence copper ions has been designed and synthesized by using metallocycle panels and pillar ligands. The triangular prism will be quickly transformed to a 10-nuclear cage upon an external chemical stimulus, which features a bicapped square antiprism structure. This prismatic cage can act as a catalyst for oxidation of aromatic alcohols to their corresponding aromatic aldehydes with high yields at room temperature under O₂ atmosphere.

Tri-Manganese(III) Salen-Based Cryptands: A Metal Cooperative Antioxidant Strategy that Overcomes Ischemic Stroke Damage In Vivo

Oxidative stress is one of the hallmarks of ischemic stroke. Catalase-based (CAT) biomimetic complexes are emerging as promising therapeutic candidates that are expected to act as neuroprotectants for ischemic stroke by decreasing the damaging effects from H₂O₂. Unfortunately, these molecules result in the unwanted production of the harmful hydroxyl radical, HO^•. Here, we report a series of salen-based tri-manganese (Mn(III)) metallocryptands (1-3) that function as catalase biomimetics. These cage-like molecules contain a unique "active site" with three Mn centers in close proximity, an arrangement designed to facilitate metal cooperativity for the effective dismutation of H₂O₂ with minimal HO^• production. In fact, significantly greater oxygen production is seen for 1-3 as compared to the monomeric Mn(Salen) complex, 1c. The most promising system, 1, was studied in further detail and found to confer a greater therapeutic benefit both in vitro and in vivo than the monomeric control system, 1c, as evident from inter alia studies involving a rat model of ischemic stroke damage and supporting histological analyses. We thus believe that metallocryptand 1 and its analogues represent a new and seemingly promising strategy for treating oxidative stress related disorders.