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 Me3SiCE (1 or 1Me, respectively; 1,1'-fc = 1,1'-ferrocenylene); Me3SiH and H-2 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 alpha* of 40.6 degrees. The Fe...B(sp(2)) distance is very short (2.365(4) angstrom) and the B-11 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., Et3PO and F--ion affinities) of the Lewis acidity proves that [2](+) is among the strongest boron-based Lewis acids available to date.

Automated summary

Bicyclic pyrazabole-bridged ferrocenes with BH groups at their bridgehead positions were successfully synthesized, showing a very short Fe...B(sp(2)) distance. Experimental and theoretical assessments confirmed that the compound is among the strongest boron-based Lewis acids available to date.