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(BH(3))(2)] and pyrazole or 3,5-dimethylpyrazole in the presence of Me(3)SiCl (1 or 1Me, respectively; 1,1′-fc = 1,1′-ferrocenylene); Me(3)SiH and H(2) are released as byproducts. Treatment of 1 or 1Me with 1 eq. of the hydride scavenger [Ph(3)C][B(C(6)F(5))(4)] afforded the borenium salts [2][B(C(6)F(5))(4)] (72%) and [2Me][B(C(6)F(5))(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(sp(2)) distance is very short (2.365(4) Å) and the (11)B NMR signal of the cationic B(sp(2)) 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., Et(3)PO and F(−)-ion affinities) of the Lewis acidity proves that [2](+) is among the strongest boron-based Lewis acids available to date.