NMR Assignments of [6-R-nido-5,6-C2B8H10]- Anions (where R = H, Me, and n-C6H13). An Irreversible 5 → 6 Alkyl Migration via a B9 Vertex-Swing Mechanism

Abstract

Deprotonation by "proton sponge" (PS = 1,8-dimethylaminonaphthalene) of the parent dicarbaborane nido-5,6-C2B8H12 (1) and its 6-R-substituted derivatives (where R = Me and n-C6H13) leads to the anions [nido-5,6-C2B8H11]- [1]- and [6-R-nido-5,6-C2B8H10]- [6-R-1]-, respectively. In contrast, the deprotonation of the 5-substituted isomers, 5-R-nido-5,6-C2B8H11 (5-R-1), results in irreversible conversion into the 6-substituted anions [6-R-1]-, from which the neutral compounds 6-R-1 can be obtained via reprotonation. This 5 → 6 alkyl migration can be explained by the B9 vertex-swing mechanism previously proposed for the interenantiomeric fluxionality of [1]-, but now with the product dictated by the higher thermodynamic stabilities of the 6-substituted derivatives. The work has also resulted in complete assignments of 11B and 1H NMR spectra of the [nido-5,6-C2B8H11]- anion and of 11B NMR spectra of the [6-R-nido-5,6-C2B8H10]- anions.