Too Persistent to Give Up: Aromaticity in Boron Clusters Survives Radical Structural Changes

Abstract

o-C2B10H12 isomerizes to m-C2B10H12 upon heating at 400 ºC. Deboronation in o-C2B10H12 is a relatively easy process, whereas it is more difficult in m-C2B10H12. These two experimental facts indicate that m-C2B10H12 is thermodynamically more stable than o-C2B10H12. On the other hand, it is widely accepted that closo boranes and carboranes are aromatic compounds. In this work, we relate difficulty in the deboronation of the carboranes with stability and aromaticity. We do this by combining lab work and by means of DFT calculations. Computationally, our 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, we conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g. ferrocene) and sandwich metallabis(dicarbollides) (e.g. [Co(C2B9H11)2]-) have traditionally been considered similar. Here it is shown that they are not. Metallabis(dicarbollides) 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 Hec 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 clusters and is comparable to the existence of diatropic currents in planar aromatic compounds. Additionally, heteroboranes with two carbon atoms have been compared to heterocycles with two nitrogen or boron atoms, C2B10H12 carboranes against planar N2C4H4 diazines or [B2C4H4]2- diboratabenzenes, proving the higher persistence of the aromaticity of the tri-dimensional compounds in heteroatom substituted species. This research accounts very well for the 'Paradigm for the Electron Requirements of Clusters' in which a closo-cluster that is aromatic upon addition of 2e- becomes also an aromatic nido species and explains the nice schemes by R.W. Rudolph and R. E. Williams