σ‐Aromaticity in a Fully Unsaturated Ring

Abstract

Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both σ- and π-aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, σ-aromaticity stabilizes fully saturated rings with σ-electron delocalization whereas the traditional π-aromaticity describes the π-conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus-independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the σ-aromaticity in an unsaturated three-membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is σ-aromatic. Our findings revolutionize the fundamental knowledge of the concept of σ-aromaticity, thus opening an avenue to design σ-aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of π-aromaticity.

Adaptive σ‐Aromaticity in an Unsaturated Three‐Membered Ring

“…In addition, more organometallic complexes with σ‐aromaticity were reported [15] . It's worth noting that σ‐aromaticity can be dominant in a completely unsaturated three‐membered ring ( C‐Os in Figure 2), which is traditionally dominated by π‐aromaticity [16] . However, those previous studies were only limited to the singlet state.…”

“…[15] It's worth noting that σ-aromaticity can be dominant in a completely unsaturated three-membered ring (C-Os in Figure 2), which is traditionally dominated by πaromaticity. [16] However, those previous studies were only limited to the singlet state. σ-Aromatic species in the T 1 state were particularly rare.…”

“…Ring a in 1-Os and C-Os is σ-aromatic in the S 0 state whereas rings b and c are π-aromatic. [13,16] What is the effect of metal center on the aromaticity in C-Ru? Here, via density functional theory (DFT) calculations, we first demonstrate adaptive σ-aromaticity in an unsaturated 3MR.…”

“…[20e] In general, distinctly negative values indicate aromaticity whereas distinctly positive values suggest antiaromaticity. Following our previous strategy, [14,16] NICS(0) instead of NICS (1) zz is chosen to gain an insight into the nature of the possible σ-aromaticity in the 3MR due to σ-electrons delocalized in the ring plane. NICS (1) zz is chosen to evaluate the aromaticity in the fused 5MRs.…”

“…The CMO-NICS calculation about 1-Os.S 0 and C-Os.S 0 has been reported in our previous work. [13,16] Here, we list them in Figure 5 for facilitating a comparison with C-Ru.S 0 . Five key occupied π MOs could be located in 1-Os.S 0 whereas six π MOs are in C-Os.S 0 and C-Ru.S 0 .…”