Theoretical study of molecular hydrogen elimination from the undecahydrodecaborate monoanion [B₁₀H₁₁]⁻. Exopolyhedral substitution intermediates: [B₁₀H₉]⁻ monoanion and neutral [B₁₀H₁₀] cluster

Abstract

The elementary reaction of molecular hydrogen elimination from the [B₁₀H₁₁]⁻ anion, which is presumably the rate-limiting stage of acid-catalyzed reactions of substitution of exopolyhedral H atoms in the [B₁₀H₁₀]²⁻ decahydro-closo-decaborate anion, has been calculated by the density functional theory method (in the B3LYP/6-311++G** approximation). Specific transition states of H₂ elimination in which vacancies form near the boron atoms have been localized. It has been demonstrated that regioselectivity of substitution reactions can be related to the significant difference between the activation barriers for the pathways of H₂ elimination from boron atoms with different coordination numbers (CN 6 and 5). The electron density of the [B₁₀H₉]⁻ anion that forms after hydrogen molecule elimination has a characteristic shape of the lowest unoccupied molecular orbital for the interaction with nucleophilic reagents; in acid-catalyzed reactions, different anions, for example, a carboxylic acid residue, can act as such. The direct reaction of the [B₁₀H₉]⁻ intermediate with nucleophilic anions is hindered by the Coulomb charge repulsion. To overcome this hindrance, the possibility of [B₁₀H₉]⁻ protonation to form the neutral [B₁₀H₁₀] system has been considered. It has been shown that the proton affinity of the [B₁₀H₉]⁻ anion is ∼280–290 kcal/mol. For the [B₁₀H₁₀] cluster, the lowest-lying and low-lying isomers have been considered. For all the systems under consideration, the electronic chemical potential and Pearson hardness have been evaluated.