Anti-aromatic compounds, as well as small cyclic alkynes or carbynes, are particularly challenging synthetic goals. The combination of their destabilizing features hinders attempts to prepare molecules such as pentalyne, an 8p-electron antiaromatic bicycle with extremely high ring strain. Here we describe the facile synthesis of osmapentalyne derivatives that are thermally viable, despite containing the smallest angles observed so far at a carbyne carbon. The compounds are characterized using X-ray crystallography, and their computed energies and magnetic properties reveal aromatic character. Hence, the incorporation of the osmium centre not only reduces the ring strain of the parent pentalyne, but also converts ′ ′

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Stabilization of anti-aromatic and strained five-membered rings with a transition metal

The concept of aromaticity was initially introduced in chemistry to account for the stability, reactivity, molecular structures, and other properties of many unsaturated organic compounds. Despite that, it has been extended to other species with mobile electrons including saturated systems, transition structures, and even inorganic molecules. In this review, we focus on the aromaticity of a particular family of organometallic compounds known as metallabenzenes, which are characterized by the formal replacement of a CH group in benzene by an isolobal transition metal fragment. In addition, aromaticity of related compounds such as heterometallabenzenes is considered as well. To this end, we shall describe herein the insight gained by the available experimental data as well as by the application of the state-of-the-art computational methods developed as descriptors for aromaticity together with a critical evaluation of their performance to quantitatively estimate the strength of aromaticity of these systems.

Aromaticity of metallabenzenes and related compounds

Recent development in the chemistry of transition metal-containing metallabenzenes and metallabenzynes

Aromatic compounds, such as benzene and its derivatives, porphyrins, fullerenes, carbon nanotubes, and graphene, have numerous applications in biomedicine, materials science, energy science, and environmental science. Metalla-aromatics are analogues of conventional organic aromatic molecules in which one of the (hydro)carbon segments is formally replaced by an isolobal transition-metal fragment. Researchers have studied these transition-metal-containing aromatic molecules for the past three decades, particularly the synthesis and reactivity of metallabenzenes. Another focus has been the preparation and characterization of other metalla-aromatics such as metallafurans, metallapyridines, metallabenzynes, and more. Despite significant advances, remaining challenges in this field include the limited number of convenient and versatile synthetic methods to construct stable and fully characterized metalla-aromatics, and the relative shortage of new topologies. To address these challenges, we have developed new methods for preparing metalla-aromatics, especially those possessing new topologies. Our synthetic efforts have led to a large family of closely related metalla-aromatics known as aromatic osmacycles. This Account summarizes the synthesis and reactivity of these compounds, with a focus on features that are different from those of compounds developed by other groups. These osmacycles can be synthesized from simple precursors under mild conditions. Using these efficient methods, we have synthesized aromatic osmacycles such as osmabenzene, osmabenzyne, isoosmabenzene, osmafuran, and osmanaphthalene. Furthermore, these methods have also created a series of new topologies, such as osmabenzothiazole and osmapyridyne. Our studies of the reactivity of these osma-aromatics revealed unprecedented reaction patterns, and we demonstrated the interconversion of several osmacycles. Like other metalla-aromatics, osma-aromatics have spectroscopic features of aromaticity, such as ring planarity and the characteristic bond lengths between a single and double bond, but the osma-aromatics we have prepared also exhibit good stability towards air, water, and heat. Indeed, some seemingly unstable species proved stable, and their stability made it possible to study their optical, electrochemical, and magnetic properties. The stability of these compouds results from their aromaticity and the phosphonium substituents on the aromatic plane: most of our osma-aromatics carry at least one phosphonium group. The phosphonium group offers stability via both electronic and steric mechanisms. The phosphonium acts as an electron reservoir, allowing the circulation of electron pairs along metallacycles and lowering the electron density of the aromatic rings. Meanwhile, the bulky phosphonium groups surrounding the aromatic metallacycle prevent most reactions that could decompose the skeleton.

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The chemistry of aromatic osmacycles

Fused-ring metallabenzenes

Osmapyridine and Osmapyridinium from a Formal [4+2] Cycloaddition Reaction

Selective synthesis of osmanaphthalene and osmanaphthalyne by intramolecular C-H activation.

Synthesis and Characterization of an Air-Stable p-Osmaphenol

National Natural Science Foundation of China [20925208, 21103142]; National Basic Research Program of China (973 Program) [2012CB821600]

Synthesis of Osmapyridiniums by [4+2] Cycloaddition Reaction between Osmium Alkenylcarbyne and Nitriles

National Natural Science Foundation of China [21174115, 21202054]; National Basic Research Program of China (973 Program) [2012CB821600]; Program for Changjiang Scholars and Innovative Research Team in University

Huijuan Wang

Papers

Osmapyridine and Osmapyridinium from a Formal [4+2] Cycloaddition Reaction

Selective synthesis of osmanaphthalene and osmanaphthalyne by intramolecular C-H activation.

Synthesis and Characterization of an Air-Stable p-Osmaphenol

Synthesis of Osmapyridiniums by [4+2] Cycloaddition Reaction between Osmium Alkenylcarbyne and Nitriles

Reactivity study of a hydroxyl coordinated osmium vinyl complex OsCl2(PPh3)(2)[CH=C(PPh3)CHPh(OH)]