In Search of the Perfect Triple BB Bond: Mechanical Tuning of the Host Molecular Trap for the Triple Bond B≡B Fragment

The coordination of the B2 fragment by two σ-donor ligands L: could lead to a diboryne compound with a formal triple bond L:→B≡B←:L. σ-Type coordination L:→B leads to an excess of electrons around the B2 central fragment, whereas π-back-donation from the B≡B moiety to ligand L has a compensation effect. Coordination of the σ-donor and π-acceptor ligand is accompanied by the lowering of the BB bond order. Here, we propose a new approach to obtain the perfect triple BB bond through the incorporation of the BB unit into a rigid molecular capsule. The idea is the replacement of π-back-donation, as the principal stabilization factor in the linear NBBN structure, with the mechanical stabilization of the BB fragment in the inert molecular capsule, thus preserving the perfect B≡B triple bond. Quantum-chemical calculations show that the rigid molecular capsule provided a linear NBBN structure and an unusually short BB bond of 1.36 Å. Quantum-chemical calculations of the proposed diboryne adducts show a perfect triple bond B≡B without π-back-donation from the B2 unit to the host molecule. Two mechanisms were tested for the molecular design of a diboryne adduct with a perfect B≡B triple bond: the elimination of π-back-donation and the construction of a suitable molecular trap for the encapsulation of the B2 unit. The second factor that could lead to the strengthening or stretching of a selected chemical bond is molecular strain produced by the rigid molecular host capsule, as was shown for B≡B and for C≡C triple bonds. Different derivatives of icosane host molecules exhibited variation in BB bond length and the corresponding frequency of the BB stretch. On the other hand, this group of molecules shows a perfect triple BB bond character and they all possess a similar level of HOMO.

Crystal structure of [Th3(Cp*)3(O)(OH)3]2Cl2(N3)6: a discrete molecular capsule built from multinuclear organothorium cluster cations

An unusually large and structurally complex charge-neutral polynuclear cluster, hexa-μ2-azido-di-μ3-chlorido-hexa-μ2-hydroxido-di-μ3-oxido-hexakis(pentamethylcyclopentadienyl)hexathorium–diethyl ether–tetrahydrofuran (1/0.56/1.44), [Th3(C10H15)6Cl3(N3)6(OH)6O2]·0.56C4H10O·1.44C4H8O or [Th3(Cp*)3(O)(OH)3]2Cl2(N3)6·0.56C4H10O·1.44C4H8O (Cp* = [pentamethylcyclopentadienyl])−, has been crystallized as a mixed tetrahydrofuran/diethyl ether solvate and structurally characterized. The molecule contains a number of unusual features, the most notable being a finite yet exceptionally long cyclic metal-azido chain. These rare features are the consequence of both sterically protecting Cp* ligands and highly bridging oxide and hydroxide ligands in the same system and illustrate the interesting new possibilities that can arise from combining organometallic and solvothermal f-block element chemistry.

Hydrogen Bond Synchronized Dual Activation Enables the Unified β-Selective O-Glycosylation Inside a Molecular Capsule

<div>Carbohydrates are of central importance in biology. The selective chemical synthesis of carbohydrates, however, still poses a challenge; particularly, the selective formation of the</div><div>thermodynamically labile b-glycosidic bond is difficult and depends on the substrate’s substitution pattern. We here demonstrate that a molecular capsule catalyzes the highly</div><div>challenging selective formation of b-glycosides independent of the substrate’s substitution pattern and configuration. We demonstrate the versatility of the catalyst by synthesizing small to medium sized 1,2-cis, 2-deoxy, and 1,2-trans b-glycosides in very high selectivity and good yield. The confined space inside the molecular capsule naturally limits the scope concerning the size of reactants. Interestingly, the proposed mechanism involves the synchronized activation of the glycosyl donor and acceptor inside the supramolecular capsule via a relay involving seven hydrogen bonds. Such an activation is known for enzymes, however, to our knowledge, is unprecedented for man-made catalysts.</div>

Hydrogen Bond Synchronized Dual Activation Enables the Unified β-Selective O-Glycosylation Inside a Molecular Capsule

<div>Carbohydrates are of central importance in biology. The selective chemical synthesis of carbohydrates, however, still poses a challenge; particularly, the selective formation of the</div><div>thermodynamically labile b-glycosidic bond is difficult and depends on the substrate’s substitution pattern. We here demonstrate that a molecular capsule catalyzes the highly</div><div>challenging selective formation of b-glycosides independent of the substrate’s substitution pattern and configuration. We demonstrate the versatility of the catalyst by synthesizing small to medium sized 1,2-cis, 2-deoxy, and 1,2-trans b-glycosides in very high selectivity and good yield. The confined space inside the molecular capsule naturally limits the scope concerning the size of reactants. Interestingly, the proposed mechanism involves the synchronized activation of the glycosyl donor and acceptor inside the supramolecular capsule via a relay involving seven hydrogen bonds. Such an activation is known for enzymes, however, to our knowledge, is unprecedented for man-made catalysts.</div>

Synthesis of A Chiral Metallo-capsule Composed of Concave Molecules and Chirogenesis upon Fullerene Binding

An enantiopure molecular capsule was synthesized quantitatively using complexation of four phosphangulenes as concave molecules with four Zn2+ ions and applied to fullerene binding and chirogenesis. The capsule encapsulated selectively...

Molecular Capsule Catalysis: Ready to Address Current Challenges in Synthetic Organic Chemistry?

Self-assembled molecular capsules, host structures that form spontaneously when their building blocks are mixed, have been known since the 1990s. They share some basic similarities with enzyme pockets, as they feature defined hydrophobic binding pockets that are able to bind molecules of appropriate size and shape. The potential to utilize such host structures for catalysis has been explored since their discovery; however, applications that solve current challenges in synthetic organic chemistry have remained limited. In this short article, we discuss the challenges associated with the use of molecular capsules as catalysts, and highlight some recent applications of supramolecular capsules to overcome challenges in synthetic organic chemistry.

A metal–peptide capsule by multiple ring threading

Cavity creation is a key to the origin of biological functions. Small cavities such as enzyme pockets are created simply through liner peptide folding. Nature can create much larger cavities by threading and entangling large peptide rings, as learned from gigantic virus capsids, where not only chemical structures but the topology of threaded rings must be controlled. Although interlocked molecules are a topic of current interest, they have for decades been explored merely as elements of molecular machines, or as a synthetic challenge. No research has specifically targeted them for, and succesfully achieved, cavity creation. Here we report the emergence of a huge capsular framework via multiple threading of metal–peptide rings. Six equivalent C4-propeller-shaped rings, each consisting of four oligopeptides and Ag+, are threaded by each other a total of twelve times (crossing number: 24) to assemble into a well-defined 4 nm-sized sphere, which acts as a huge molecular capsule.