σ‐Aromaticity in an Unsaturated Ring: Osmapentalene Derivatives Containing a Metallacyclopropene Unit

Abstract

In general, aromaticity can be clarified as π- and σ-aromaticity according to the type of electrons with major contributions. The traditional π-aromaticity generally describes the π-conjugation in fully unsaturated rings whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. Reported herein is an example of σ-aromaticity in an unsaturated three-membered ring (3 MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3 MR in cyclopropaosmapentalene is cleaved by ethane through two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0 kcal mol(-1)). These positive values are in sharp contrast to the expected exothermicity, thus indicating aromaticity in the 3 MR. Further nucleus-independent chemical shift and anisotropy of the current-induced density calculations reveal the nature of σ-aromaticity in the unsaturated 3 MR.

CCCCC pentadentate chelates with planar Möbius aromaticity and unique properties

“…S4) ( 29 , 30 ). The results reveal that the total contribution of the NICS(1) zz value (−38.3 ppm) for the 3MR from all the σ orbitals is more negative than that (−23.5 ppm) from all the σ and π orbitals, showing σ-aromaticity in the 3MR, as previously observed in the reactant, complex 1 ( 9 ). The NICS(1) zz values for the fused 5MRs and the 6MR from all occupied MOs are −27.9, −16.7, and −6.8 ppm, indicating aromaticity in these rings.…”

“…The bond distances of Os1–C1 (2.088 Å), Os1–C4 (2.107 Å), Os1–C7 (2.090 Å), and Os1–C11 (2.025 Å) are within the range of those of osmapentalene (1.926 to 2.139 Å) reported previously ( 7 ), indicating the delocalized metal-carbon bonds. The Os1–C12 (2.253 Å) bond distance is similar to that of the Os–C( sp 3 ) bond in complex 1 (2.272 Å) ( 9 ). …”

“…In the presence of AgBF 4 , treatment of complex 1 ( 9 ) with phenylacetylene (PhC≡CH) at room temperature for 2 hours led to the formation of 2a (Fig. 1A).…”

“…Solvents were distilled from sodium/benzophenone (hexane and diethyl ether) or calcium hydride (dichloromethane) under N 2 before use. The metallapentalene derivate 1 was synthesized according to a previously published procedure, Zhu et al ( 9 ). Other reagents were used as received from commercial sources without further purification.…”

Synthesis and Characterization of Dirhenadehydro[12]annulenes

“…[23] In this regards,w en oted that the 1 HNMR signals of Re CCH of the metallacyclic complexes 15 appeared at d % 4.7 ppm, which is downfield from those of Re CCH of the related rhenium carbyne complexes 14 (d % 4.4 ppm).…”