A free boratriptycene-type Lewis superacid

• Published in: Chemical science (Cambridge) 2022-02, Vol.13 (6), p.168-1617
• Main Authors: Henkelmann, Marcel, Omlor, Andreas, Bolte, Michael, Schünemann, Volker, Lerner, Hans-Wolfram, Noga, Jozef, Hrobárik, Peter, Wagner, Matthias
• Format: Article
• Language: English
• Subjects: Cations; Chemistry; Crystallography; Ferrocenes; NMR; Nuclear magnetic resonance; Pyrazole; Quantum chemistry
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d1sc06404e

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. An exceptionally strong ferrocene-containing, cationic boratriptycene-type Lewis acid is stabilized by a weak Fe B through-space interaction.