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 1 H-and 11 B-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] 2diboratabenzenes, 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 2ebecomes also an aromatic nido species and explains the nice schemes by R.W. Rudolph and R. E. Williams.

Sunitinib-Containing Carborane Pharmacophore with the Ability to Inhibit Tyrosine Kinases Receptors FLT3, KIT and PDGFR-β, Exhibits Powerful In Vivo Anti-Glioblastoma Activity

“…Icosahedral carboranes are hydrophobic [26] and have an icosahedral shape with a depth of about 0.5 nm and a volume similar to the one of benzene in rotation with twice the number of atoms [27]. Carboranes display 3D aromaticity [28,29]. Furthermore, the inorganic 3D neutral C 2 B 10 H 12 clusters and their derivatives can produce hydrogen and dihydrogen bonds C-H [32].…”

m-Carborane as a Novel Core for Periphery-Decorated Macromolecules

“…The synthesis of 9,10-I2-1,7-closo-C2B10H10; (2) has been reported by using two different methodologies: i) the electrophilic iodination reaction of icosahedral closo m-carborane (1) by using a molar Equiv. Of iodine : monochloride, which acts as an electrophilic agent, in the presence of catalytic amounts of aluminum chloride, and ii) using iodine as an electrophilic agent in a very acidic media (HNO3:H2SO4 , 1:1).…”

“…Despite their common icosahedral geometry, they display similarities, but also important differences. Among the similarities are the high stabilities and 3D geometrical properties, their very similar 3D aromatic character [ 1 , 2 ] that leads to display great inertia to keep the original scaffold upon electrophilic substitution, their dual-mode as electron-withdrawing through carbon or electron-donating through boron vertexes [ 3 , 4 , 5 ], their molecular volume that is high compared to rotating benzene [ 6 ], and high hydrophobicity [ 7 , 8 , 9 ]. Among the differences are the dipolar moment and their different reactivity towards boron elimination [ 8 ], and the lowest unoccupied molecular orbital (LUMO) geometrical disposition that is responsible for many of the physical properties of the isomers.…”

In vitro and in vivo BNCT investigations using a carborane containing sulfonamide targeting CAIX epitopes on malignant pleural mesothelioma and breast cancer cells

“…The introduction of the carborane structure (icosahedral lipophilic cluster containing Boron atoms) in the drug design is deemed to enhance the hydrophobic interactions of biologically active compounds with their receptors and to increase their in vivo stability and bioavailability 17 – 19 . In fact, o-carborane is an icosahedral cluster that possess 3D aromatic character 20 , 21 . This property is very important because, according with docking analysis, increases the affinity for the enzyme binding region.…”