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.

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.

Metallaaromatics have attracted considerable interest from both experimentalists and theoreticians over the past three decades. However, most studies in this field have focused on metallabenzene, in which a CH group is replaced by a transition metal fragment. In comparison with monocyclic metallabenzenes, bicyclic metallanaphthalenes are rather limited. Thus, it is urgent to explore more synthetic approaches to this less developed system. One of the difficulties in the synthesis of metallanaphthalenes could be due to its low thermodynamic stability relative to the metal indenyl complexes. Here we present a thorough theoretical investigation by quantum chemical calculations to explore the possibility of realizing traditionally “unstable” metallanaphthalenes by an isomerization of traditionally “stable” metal indenyl complexes. We systematically investigated how different substituent(s) at different position(s) on the metallacycle affect such a rearrangement. Our results indicate that although indenyl complexes are known to be thermally robust, it should be possible to shift the thermodynamic and kinetic balance toward the metallanaphthalene complexes by choosing proper ancillary substituents on the metallabicycle, which is in sharp contrast to the traditional facile isomerization of metallabenzenes to cyclopentadienyl (Cp) complexes. Therefore, our findings suggest a novel avenue to metallanaphthalenes.

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. On the contrary, the osmium atoms in recently synthesized osmapentalyne and osmapentalene are located at the bridgehead position. To probe the origin of the difference between these metallabicycles, we carried out density functional theory calculations. The metal-bridgehead osmanaphthalene and osmanaphthalyne are computed to be less stable by 17.9 and 26.3 kcal mol–1 than the non-metal-bridged ones, respectively. In addition, the metal-bridgehead osmapentalene and osmapentalyne are more stable by 11.8 and 22.8 kcal mol–1 than the non-metal-bridged isomers, respectively. Further study revealed that the ring strains in these paired isomers are comparable to each other. Thus, it is aromaticity rather than ring strain that determines the relative thermodynamic stabilities of these complexes.