Prototropic μ‑H8,9 and μ‑H9,10 Tautomers Derived from the [nido-5,6‑C2B8H11]− Anion

Reported is an unusual tautomeric behavior within the [nido-5,6-C2B8H11]− (1a–) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido-5,6-C2B8H10-μ8,9]− (2d–) and [5,6-Me2-nido-5,6-C2B8H9-μ9,10]− (3b–), which differ in the positioning of the open-face hydrogen bridge. Their structures have been determined by X-ray diffraction analyses. The 3b–structure is stabilized by intermolecular interaction involving Et3NH+ and B8–B9 and H8 atoms in the solid phase; however, its dissolution in CD3CN causes instant conversion to the more stable [5,6-Me2-nido-5,6-C2B8H9-μ8,9]− (2b–) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [nido-5,6-C2B8H11-μ8,9]− (2a–) tautomer is 3.9 kcal·mol–1 more stable than the [nido-5,6-C2B8H11-μ9,10]− (3a–) counterpart and the μ8,9 structure 2– is therefore the most stable tautomeric form in the solution, as was also demonstrated by multinuclear (1H, 11B, and 13C) NMR measurements on the whole series of C-substituted compounds.