Too persistent to give up: aromaticity in boron clusters survives radical structural changes

Whereas the aromaticity of closo boranes is widely accepted, less is known about the aromaticity of nidoboranes. This work carried out by Prof. Francesc Teixidor, Prof. Clara Viñas, and Dr. Ines Bennour of the Institute of Materials Science of Barcelona (ICMAB-CSIC), Prof. Jordi Poater at the University of Barcelona (previous DiMoCat member) and Sílvia Escayola and Prof. Miquel Solà members of the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona, experimentally shows that deboronation of m-C₂B₉H₁₂ is a difficult task, whereas deboranation of o-C₂B₉H₁₂ is quite easy. Moreover, it is widely known that o-C₂B₁₀H₁₂ isomerizes to m-C₂B₁₀H₁₂ upon heating at 400 ºC. These two facts indicate that m-C₂B₁₀H₁₂ is more stable than o-C₂B₁₀H₁₂. To find a reason for the different stability of these two isomers, authors have analyzed the thermodynamic stability and aromaticity of these closo carboranes and their nido counterparts. Results show that the higher thermodynamic stability of m-C₂B₁₀H₁₂ is not related to aromaticity differences but to the location of the C atoms in the carborane structure. It is also demonstrated that the aromaticity observed in closo boranes and carboranes is also present in their nido counterparts and, consequently, authors conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g. ferrocene) and sandwich metallacarboranes (e.g. [Co(C₂B₉H₁₁)₂]^–) have traditionally been considered similar. In this work, it is shown that they are not. Metallacarboranes display global aromaticity, whereas metallocenes present local aromaticity in the ligands. Remarkable and unique is the double probe given by ¹H- and ¹¹B-NMR tracing the reciprocally antipodal endocyclic open face H_ec and B₁. These magnetic studies have permitted to correlate both nuclei and relate them to a diatropic current in the plane at the middle of the nido-[C₂B₉H₁₂]^–. This observation is the first and unique data that proves experimentally the existence of diatropic currents, thence aromaticity, in nido clusters and is comparable to the existence of diatropic currents in planar aromatic compounds. The paper was published recently in Journal of the American Chemical Society.
J. Poater, C. Viñas, I. Bennour, S. Escayola, M. Solà, and F. Teixidor
“Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes”
J. Am. Chem. Soc. 2020, ASAP-
DOI: 10.1021/jacs.0c02228