Chemical Characterization of Localized Radicals in closo-Borate Anion Derivatives

Abstract

The conceptual understanding of the reactivity of localized main group element radicals in molecules and ions has so far been strongly focused on carbon radicals in organic compounds. In this study, permanent anions with a radical site localized on the vacant vertex of the corresponding icosahedral closo-boranes/carboranes have been characterized by both various (radical) ion-molecule reactions in the gas phase and by computational investigations, including conceptual DFT, potential energy surfaces (PES) and energy decomposition analysis (EDA). The reactivity of these radical ions towards electron-deficient and electron-rich double bonds as well as their halogen and hydrogen atom abstraction reactions have been studied. The radical ions were varied with respect to their charge state, the nature of the spin-carrying atom and the substituents of the closo-borane scaffold. Additionally, their reactivity was compared with that of prototypical electrophilic and nucleophilic aryl radicals. Interestingly, not all the (di)anionic radicals are nucleophilic; particularly the ion [B₁₁I₁₁C]^•– was characterized as highly electrophilic. Hence, simple categorization based on “polarity matching” arguments is not sufficient to fully explain the reactivity of these radical ions toward allyl iodide. Element-specific spatial extension of the spin density and non-covalent interactions of the allyl iodide with the closo-borane framework determine the transition state geometries and energies and therefore strongly influence the relative rate of competing reactions. These results showcase both the transferability and the limitations of “classical” concepts (as typically applied successfully in organic chemistry) for the characterization of ionic borane cluster radicals. Our approach represents a broadly applicable, general method for understanding radical reactivity towards a broad range of reaction partners.