Prototropic μ-H 8,9 and μ-H 9,10 Tautomers Derived from the [nido-5,6-C 2 B 8 H 11 ] - Anion.

Reported is an unusual tautomeric behavior within the [nido-5,6-C2 B8 H11 ]- (1a- ) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido-5,6-C2 B8 H10 -μ8,9 ]- (2d- ) and [5,6-Me2 -nido-5,6-C2 B8 H9 -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 Et3 NH+ and B8-B9 and H8 atoms in the solid phase; however, its dissolution in CD3 CN causes instant conversion to the more stable [5,6-Me2 -nido-5,6-C2 B8 H9 -μ8,9 ]- (2b- ) tautomer. The dynamic electron-correlation-based MP2/6-31G* computations suggest that the parent [nido-5,6-C2 B8 H11 -μ8,9 ]- (2a- ) tautomer is 3.9 kcal·mol-1 more stable than the [nido-5,6-C2 B8 H11 -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 (1 H,11 B, and13 C) NMR measurements on the whole series of C-substituted compounds.