A new mechanically-interlocked [Pd2L4] cage motif by dimerization of two peptide-based lemniscates

Most metallo-supramolecular assemblies of low nuclearity adopt simple topologies, with bridging ligands spanning neighboring metal centers in a direct fashion. Here we contribute a new structural motif to the family of host compounds with low metal count (two) that consists of a pair of doubly-interlocked, Figure-eight-shaped subunits, also termed “lemniscates”. Each metal is chelated by two chiral bidentate ligands, composed of a peptidic macrocycle that resembles a natural product with two pyridyl-terminated arms. DFT calculation results suggest that dimerization of the mononuclear halves is driven by a combination of 1) Coulomb interaction with a central anion, 2) π-stacking between intertwined ligand arms and 3) dispersive interactions between the structure's compact inner core bedded into an outer shell composed of the cavitand-type macrocycles. The resulting cage-like architecture was characterized by NMR, MS and X-ray structure analyses. This new mechanically bonded system highlights the scope of structural variety accessible in metal-mediated self-assemblies composed of only a few constituents

Toward unidirectional switches: 2-(2-Hydroxyphenyl)pyridine and 2-(2-methoxyphenyl)pyridine derivatives as pH-triggered pivots

The pH-induced switching process of 2-(2-hydroxyphenyl)pyridine and 2-(2-methoxyphenyl)pyridine derivatives was investigated with the help of UV spectroscopy. Quantum chemical calculations at the B3LYP/6-31G* level of theory were performed to show that in the case of 2-(2-methoxyphenyl)-3-methylpyridine and 2-(2-hydroxyphenyl)-3-methylpyridine the rotation during the switching process proceeds unidirectionally at the molecular level. If a 2-(2-methoxyphenyl)pyridine derivative is fixed to a chiral cyclopeptidic scaffold, a unidirectional progress of the rotation is achieved macroscopically

Long Chalcogen–Chalcogen Bonds in Electron-Rich Two and Four Center Bonds: Combination of π- and σ‑Aromaticity to a Three-Dimensional σ/π-Aromaticity

Quantum chemical calculations were carried out by applying density functional theory to study the two center-three electron (2c-3e) bonds between the sulfur centers of cyclic dithio­ethers. Calculated were the S–S distance, the stabilization energy, and the energy of the σ → σ* transition. The extension of the calculations to two (2c-3e) bonds in one molecule shows that a rearrangement to one σ bond and two lone pairs on sulfur is usually more favorable. Exceptions are [H₂S₂ ⁺]₂, the dimer of the 1,2-dithia-3,5-diazolyl radical (<b>27a</b>), the dimer of the 1,2,4-trithia-3,5-diazolyl radical cation (<b>26a</b> ²⁺), and its Selena congeners and derivatives. In the case of [H₂S₂ ⁺]₂, the (4c-6e) bond between the chalcogen centers is a good description of this dimer. To describe the binding situation in the dimer <b>26a</b> ²⁺ and <b>27a</b>, the concept of a “simple” (4c-6e) bond was extended. Our calculations reveal a strong σ-aromaticity within the plane of the four sulfur centers in addition to a strong π-conjugation within the five-membered rings. The whole phenomenon can best be described as a three-dimensional σ/π-aromaticity within the 14π dimers

Interplay between 1,3-Butadien-1,4-diyl and 2‑Buten-1,4-dicarbene Derivatives: The Quest for Nucleophilic Carbenes

By means of high level quantum chemical calculations, the influence of electron-donating heteroatomic groups (O, NH) was investigated on the 1,6-transannular ring closure of 1,6-cyclodecadiyne (<b>8a</b>). In the case of <b>8a</b>, the bicyclo[4.4.0]­deca-1,6-dien-2,7-diyl biradical <b>12</b> is generated. It was found that oxygen centers or NH groups next to the triple bond reduce the activation energy of the ring closure considerably. For the intermediate, a 2-buten-1,4-dicarbene derivative is predicted. The extension of the model calculations to two hydroxyl- or aminoacetylenes predicts the formation of the corresponding 1,3-butadien-1,4-diyl intermediates or the 2-buten-1,4-dicarbene derivatives, a member of the nucleophilic carbene family. Moreover, the calculations predict that two separated dimethoxyacetylenes are more than 7 kcal/mol less stable than the corresponding biradical and dicarbene, respectively. Possible reactions of the dicarbenes with transition metal compounds are discussed

Switching Process Consisting of Three Isomeric States of an Azobenzene Unit

Azobenzene and its derivatives are among the most commonly used switching units in organic chemistry. The switching process consists of two states, in which the <i>trans</i> isomer has a stretched and the <i>cis</i> isomer a compact form. Here, we have designed a system in which all isomeric states of an azobenzene moiety (<i>trans</i> → <i>cis</i>-(<i>M</i>) → <i>cis</i>-(<i>P</i>)) are passed step by step. The first step involves a change in the distance between the benzene units, which is common for azobenzene derivatives. In the second step an inversion of the helicity (<i>M</i>→<i>P</i>) of the <i>cis</i> azobenzene unit takes place. The third step leads back to the stretched <i>trans</i> isomer. This switching cycle is achieved by coupling the azobenzene unit with two chiral clamps and with a further azobenzene switching unit

Enediyne Dimerization vs Bergman Cyclization

High-level quantum chemical calculations reveal that the dimerization of enediynes to 1,3-butadiene-1,4-diyl diradicals is energetically more favored than the corresponding Bergman cyclization of enediynes. Moreover, the activation barrier of both reactions can be drastically reduced by the introduction of electron-withdrawing substituents like fluoro groups at the reacting carbon centers of the triple bonds