"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Preparation of cage-shaped hexakis(spiroborate)s

In previous research studies, various types of prismatic cage-shaped molecular containers have been prepared and evaluated in terms of their guest inclusion properties. Basically, most of these molecular cages have a cationic or electron-deficient nature, and exhibit strong affinity mainly toward electron-rich aromatic guests. On the other hand, there is no report concerning anionic prismatic cages that are expected to recognize cationic polyaromatic guests with various structures and functions. In this manuscript, we present the preparation of hexakis(spiroborate)-type molecular cages, which was achieved by the reaction of phenylene- or biphenylenebis(dihydroxynaphthalene), hexahydroxytriphenylene, and boric acid in N,N-dimethylformamide. Their triangular prismatic hollow structures were confirmed by X-ray crystallographic analysis, and it was found that both phenylene- and biphenylene-bridged spiroborate cages have internal cavities of the corresponding size. It was also revealed that tetra(n-butylammonium) cations located inside the cavity and between the two adjacent spiroborate cages resulted in the formation of a one-dimensional columnar array. The molecular recognition behavior of the spiroborate cages was evaluated using tris(pyridinium)triazines as tricationic aromatic guests. ¹H NMR measurement implied that a discrete 1 : 1 host-guest complex was formed when 1 equiv. of guest was added to the cage, whereas infinite one-dimensional aromatic stacks were constructed by the addition of 2 equiv. of guest.

Preparation of Peapod Polymer via the Supramolecular Chain Formation by Tris(spiroborate) Twin Bowl

Successive guest-containing tubular polymer was prepared by the olefin metathesis polymerization of tris(spiroborate) twin bowl after the formation of supramolecular polymer. The cationic iridium(III) complexes were topologically fixed inside the polymer to form a peapod-like structure. The polymer was evaluated by SEC, ICP-AES, and DLS analyses, and string-like structures were found in the AFM observation of the peapod polymer.

Ionic Covalent Organic Frameworks with Spiroborate Linkage

A novel type of ionic covalent organic framework (ICOF), which contains sp(3)  hybridized boron anionic centers and tunable countercations, was constructed by formation of spiroborate linkages. These ICOFs exhibit high BET surface areas up to 1259 m(2)  g(-1) and adsorb a significant amount of H2 (up to 3.11 wt %, 77 K, 1 bar) and CH4 (up to 4.62 wt %, 273 K, 1 bar). Importantly, the materials show good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when immersed in water or basic solution for two days. The presence of permanently immobilized ion centers in ICOFs enables the transportation of lithium ions with room-temperature lithium-ion conductivity of 3.05×10(-5)  S cm(-1) and an average Li(+) transference number value of 0.80±0.02. Our approach thus provides a convenient route to highly stable COFs with ionic linkages, which can potentially serve as absorbents for alternative energy sources such as H2, CH4, and also as solid lithium electrolytes/separators for the next-generation lithium batteries.

Multilayered inclusion nanocycles of anionic spiroborates

Multilayered spiroborate nanocycles were prepared from tris- or tetrakis(dihydroxynaphthalene) and tetrahydroxyanthraquinone as pillar and crossbar units via the reversible formation of a spiroborate linkage. The four-layered spiroborate nanocycle recognized two cationic aromatic guests simultaneously and exhibited the ability to form a supramolecular one-dimensional array by combining with methyl viologen dimer as the ditopic guest.

Nestable Tetrakis(spiroborate) Nanocycles

Multicomponent construction of the tetrakis(spiroborate) anionic nanocycles was achieved by reacting bis(dihydroxynaphthalene)s with tetrahydroxyanthraquinone in the presence of boric acid in a self-organized manner. These nanocycles exhibited selective molecular recognition behavior toward cationic guests such as methyl viologen derivatives. Formation of a supramolecular ring@ring and a guest@ring@ring structure was observed by combining the anionic nanocycle and the vinylogous analog of cyclobis(paraquat-p-phenylene).

Self-assembly and recognition properties of a tetraanionic macrocyclic boronate ester in aqueous medium

A tetraanionic [2 + 2] boronate ester macrocycle is self-assembled in water containing 0–5% vol DMSO and binds efficiently various cationic guests including R₄N⁺ cations, choline, acetylcholine, 1-methylnicotinamide and an alkaloid berberine.

Preparation of tris(spiroorthocarbonate) cyclophanes as back to back ditopic hosts

Twin-bowl-shaped tris(spiroorthocarbonate) cyclophanes were designed and prepared as ditopic hosts for electrically neutral or electron-rich guests. Preparation of the desired cyclophanes was achieved by cyclotrimerization of 2,2',3,3'-tetrahydroxy-1,1'-binaphthyl (THB) via the transesterification of tetraphenyl orthocarbonate or dichlorodiphenoxymethane. In those reactions, bis(spiroorthocarbonate) cyclophane containing two THB units was also formed as the kinetically favored product. The spiroorthocarbonate twin bowl exhibited ditopic molecular recognition toward fullerene C60 in the crystalline state.