Aromaticity

Rational Design of a Carbon–Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

Molecular nitrogen (N2) is abundant in the atmosphere and nitrogen, found in many biomolecules, is an essential element of life. The Haber–Bosch process, developed over 100 years ago, requires relatively harsh conditions to activate N2 on the iron surface and generate ammonia for use as fertilizer or to produce other chemicals, leading to consumption of more than 2% of the world’s annual energy supply. Thus, developing approaches for N2 activation under mild conditions is particularly important and urgent.

Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

Hyperconjugation and aromaticity are two of the most important concepts in chemistry. Mulliken and co‐workers combined both terms to explain the stability of cyclopentadiene. Here, we carried out DFT calculations on a series of mono‐ and disubstituted cyclopentadiene derivatives to investigate their hyperconjugative aromaticity. Our results revealed that one electropositive substituent can induce aromaticity, whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity.

Probing the Strongest Aromatic Cyclopentadiene Ring by Hyperconjugation

Hyperconjugation, an interaction of electrons in a σ orbital or lone pair with an adjacent π or even σ antibonding orbital, can have a strong effect on aromaticity. However, most work on hyperconjugative aromaticity has been limited to main-group substituents. Here, we report a thorough density functional theory study to evaluate the aromaticity in various cyclopentadienes that contain both main-group and transition-metal substituents.

An isolable catenane consisting of two Möbius conjugated nanohoops

Besides its mathematical importance, the Möbius topology (twisted, single-sided strip) is intriguing at the molecular level, as it features structural elegance and distinct properties; however, it carries synthetic challenges. Although some Möbius-type molecules have been isolated by synthetic chemists accompanied by extensive computational studies, the design, preparation, and characterization of stable Möbius-conjugated molecules remain a nontrivial task to date, let alone that of molecular Möbius strips assembling into more complex topologies.

Probing the Most Aromatic and Antiaromatic Pyrrolium Rings by Maximizing Hyperconjugation and Push–Pull Effect

Hyperconjugation, a weak interaction in organic chemistry, can have a strong effect on aromaticity, leading to the concept of hyperconjugative aromaticity, which was first proposed by Mulliken in 1939. However, most studies are limited to main group chemistry. Here we report the most aromatic and antiaromatic pyrrolium ring by maximizing the hyperconjugation caused by transition metal fragments and the push–pull effect.

Rational Design and Synthesis of Unsaturated Se‐Containing Osmacycles with σ‐Aromaticity

Isolation of the simplest 4π three‐membered heterocycles (1H‐azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π‐antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition‐metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se‐containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability.

Multiyne chains chelating osmium via three metal-carbon σ bonds

Although the formation of metal–carbon σ bonds is a fundamental principle in organometallic chemistry, the direct bonding of one organic molecule with one metal center to generate more than two metal–carbon σ bonds remains a challenge. Herein, we report an aromaticity-driven method whereby multiyne chains are used to construct three metal–carbon σ bonds in a one-pot reaction under mild conditions. In this method, multiyne chains act as ligand precursors capable of chelating an osmium center to yield planar metallapolycycles, which exhibit aromaticity and good stability.

To Be Bridgehead or Not to Be? This is a Question of Metallabicycles on the Interplay between Aromaticity and Ring Strain

Transition-metal-containing metallaaromatics have attracted considerable interest from both experimental and computational chemists because they can display properties of both organometallic compounds and aromatic organic compounds. In general, the transition metal in a metallabicycle prefers a nonbridged position to the bridgehead one because of the larger ring strain caused by the rigidity in the bridgehead position, as exemplified by metallanaphthalene and metallanaphthalyne.

Synthesis and Characterization of a Metallacyclic Framework with Three Fused Five-membered Rings

Polycyclic complexes containing a bridgehead transition metal are interesting species because the transition metal is shared by all the rings simultaneously. In this study, we present a novel osmium–bridgehead system with three fused five-membered rings. This novel framework can be viewed as a 10-atom carbon chain coordinating to the osmium center. In sharp contrast to the nonplanar organic analogue, this unique metallacycle exhibits good planarity, which was unambiguously verified by means of X-ray diffraction.

Synthesis and Characterization of Osmium Polycyclic Aromatic Complexes via Nucleophilic Reactions of Osmapentalyne

Treatment of osmapentalyne [Os{≡C-C(COOMe)=CH-C=CH-C(PPh3)=CH-}Cl(PPh3)2]+BF4- with arylamines in the presence of Cs2CO3 produced osmium-bridged polycyclic aromatic complexes. In this reaction, metal carbyne of osmapentalyne was first attacked by nucleophiles, followed by a C-H oxidative addition. The UV-Vis spectra of these osmium-bridged polycyclic aromatic complexes were measured. The result shows that these osmium-bridged polycyclic aromatic complexes have broad absorption in the UV-Vis region up to 650 nm.

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