Electron Pool Enrichment of Polyhedral Carboranes: A Key to Isomerization and Nucleophilicity

Carboranes are inorganic clusters exhibiting 3D aromaticity, which imparts exceptional thermal stability, low electrophilicity, and a large HOMO-LUMO gap. The isomerization of carboranes at ambient temperature remains, in many cases, a synthetic challenge. In this work, we present a new method based on the reactivity of <i>closo</i>-1,2-C₂B₈H₁₀ with various types of carbenes. Such reactions yield <i>arachno</i>-shaped carboranes, which undergo facile dihydrogen elimination at low temperatures, accompanied by C-C bond cleavage and migration of the carbon atoms. These compounds react in the reverse manner compared to common neutral boranes, and their basicity is governed by the electronic parameters of the coordinated carbene molecules. Treatment with hydrogen chloride can thus leave the neutral carborane intact or protonate it once or even twice. The doubly protonated carboranes immediately undergo chlorination by the chloride anion. Thus, the carbene-carborane adducts undergo hydrogen-induced redox changes.