Adaptive σ aromaticity in the rhenacyclopropene rings

Area of Science:

• Organometallic Chemistry
• Theoretical Chemistry
• Aromaticity Studies

Background:

• Species typically display aromaticity in either the singlet (S0) or triplet (T1) electronic state, following Hückel's and Baird's rules.
• Two-state aromaticity, known as adaptive aromaticity, is uncommon, and adaptive sigma aromaticity is even rarer.
• Metallaaromatic chemistry explores aromaticity in compounds containing metal atoms within ring systems.

Purpose of the Study:

• To investigate the phenomenon of adaptive sigma aromaticity in unsaturated rhenacyclopropene rings.
• To determine if these rings exhibit aromaticity in both the ground (S0) and excited (T1) electronic states.
• To expand the understanding of adaptive aromaticity and its occurrence in metallacycles.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to study the electronic properties of rhenacyclopropene rings.
• Aromaticity was assessed using various indices, including Nuclear Independent Chemical Shift (NICS), Aromaticity Characterization by Intramolecular Dipole (ACID), and Electron-Delocalization Диаграмма (EDDB).
• Isodesmic reactions were utilized to further support the findings on aromaticity.

Main Results:

• Adaptive sigma aromaticity was identified in unsaturated rhenacyclopropene rings.
• The sigma-aromaticity present in the S0 state was found to persist in the T1 state, which arises from pi-pi* excitation.
• The aromaticity of fused ring systems involving these rhenacyclopropenes was also analyzed.

Conclusions:

• The study successfully demonstrates adaptive sigma aromaticity in unsaturated rhenacyclopropene systems.
• These findings broaden the scope of known adaptive sigma aromaticity and contribute to the field of metallaromatic chemistry.
• The persistence of sigma-aromaticity in the excited triplet state highlights unique electronic behaviors in these organometallic compounds.