Five-membered cyclic metal carbyne: synthesis of osmapentalynes by the reactions of osmapentalene with allene, alkyne, and alkene.

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient.

A metal-bridged tricyclic aromatic system: synthesis of osmium polycyclic aromatic complexes.

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla-aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal-bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal-bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five-membered rings. These metalla-aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five-membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal-bridged polycyclic aromatics.

Theoretical study on the stability of osmasilabenzynes.

Metallabenzyne has attracted considerable interest from theoreticians and experimentalists since its first isolation in 2001. However, metallasilabenzyne, formed by the replacement of the carbyne carbon with a silicon atom in metallabenzyne, has never been reported either theoretically or experimentally. Here we carry out density functional theory (DFT) calculations on this system for the first time. Our results reveal a polarized and weak Os-Si triple bond in osmasilabenzyne due to the reluctance of the silicon to participate in π bonding. The effect of the ligands, substituents on the metallacycle, and bases on the stability or aromaticity of osmasilabenzyne is also discussed in detail. Specifically, an antibonding interaction between the metal and metal-bonded carbon and silicon in the HOMO of osmasilabenzyne is identified. Thus electron-donating substituents on the metallacycle can destabilize it. Because the Os-Si triple bond in osmasilabenzyne is highly polarized, a Lewis base can stabilize it by coordinating to the silicon atom. All these findings could be helpful for experimentalists to realize the first metallasilabenzyne.