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.