A free boratriptycene-type Lewis superacid

Bicyclic pyrazabole-bridged ferrocenes with BH groups at their bridgehead positions were prepared from [Li(thf)]₂[1,1'-fc(BH₃)₂] and pyrazole or 3,5-dimethylpyrazole in the presence of Me₃SiCl (1 or 1Me, respectively; 1,1'-fc = 1,1'-ferrocenylene); Me₃SiH and H₂ are released as byproducts. Treatment of 1 or 1Me with 1 eq. of the hydride scavenger [Ph₃C][B(C₆F₅)₄] afforded the borenium salts [2][B(C₆F₅)₄] (72%) and [2Me][B(C₆F₅)₄] (77%). According to X-ray crystallography, [2Me]⁺ contains one trigonal-planar borenium cation, the cyclopentadienyl (Cp) rings of the 1,1'-fc fragment remain parallel to each other, but the Cp-B bond vector is bent out of the Cp plane by an unprecedentedly large dip angle α* of 40.6°. The Fe⋯B(sp²) distance is very short (2.365(4) Å) and the ¹¹B NMR signal of the cationic B(sp²) center is remarkably upfield shifted (23.4 ppm), suggesting a direct Fe(3d) → B(2p) donor-acceptor interaction. Although this interpretation is confirmed by quantum-chemical calculations, the coupling between the associated orbitals corresponds to an energy of only 12 kJ mol^-1. Accordingly, both the experimental (e.g., Gutmann-Beckett acceptor number AN = 111) and theoretical assessment (e.g., Et₃PO and F^--ion affinities) of the Lewis acidity proves that [2]⁺ is among the strongest boron-based Lewis acids available to date.