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Abstract

Protonation of the [7-R-nido-7,8,10-C₃B₈H₁₀]^- (where R = PhCH₂ (1a) or R = Me (1b)) tricarbollide anion, with concentrated H₂SO₄ in a two-phase aqueous/CH₂Cl₂ system, yields the new neutral tricarbaborane: 2-R-nido-2,7,10-C₃B₈H₁₁ (where R = PhCH₂ (2a) or R = Me (2b)). The three cage-carbons of the [7-R-nido-7,8,10-C₃B₈H₁₀]^- anion are located on the open face, but spectroscopic and DFT/GIAO/NMR studies of 2a and 2b show that during the protonation reaction, isomerization of the cage framework occurs to produce the neutral tricarbaborane having a 2,7,10-structure in which only two of the carbons remain on the open face. The third (R-substituted) carbon adopts a five-coordinate vertex off of the open face, thus enabling the incoming proton to adopt a bridging position on the B-B edge of the new C₂B₃-open face. The skeletal rearrangement is reversible, since deprotonation of 2a or 2b regenerates the anions 1a and 1b, respectively, having the 7,8,10-configuration. In agreement with the experimentally observed structures of the anionic (7,8,10-structure) and neutral (2,7,10-structure) species, DFT calculations at the B3LYP/6-311G*-level show that the [7-Me-nido-7,8,10-C₃B₈H₁₀]^- anion (1b, structure 16) is 28.9 kcal/mol more stable than the [2-Me-nido-2,7,10-C₃B₈H₁₀]^- isomer (3b, structure 18), while for the neutral tricarbaborane, the 2-R-nido-2,7,10-C₃B₈H₁₁ (2b, structure 14) structure is more stable than any 7,8,10-structure (structures 7-11) which has the added proton in an endo position on the open face. Transition state calculations at the HF/6-31G*-level yielded a simple, low-energy pathway (activation barrier of only 6.5 kcal/mol for the transition state TS18/16) for the rearrangement of [2-Me-nido-2,7,10-C₃B₈H₁₀]^- (3b, structure 18) to [7-Me-nido-7,8,10-C₃B₈H₁₀]^- (1b, structure 16) requiring the movement of only one cage atom, B11, from its original position in the C7-B8-B9-C10-B11 plane of 3b, to the C7-C8-B9-C10-B11 plane of 1b.

Keywords: Boranes; Tricarbaboranes; Carboranes; Carbon rearrangement; DFT calculations; Ab initio calculations; GIAO calculations; NMR spectroscopy, ¹H, ¹³C, ¹¹B.