9-Ferrocenyl-9-borafluorene—A Redox-Switchable Main Group Lewis Acid

9-Ferrocenyl-9-borafluorene (FcBC 12 H 8; 3 ) is readily accessible from FcHgCl and BrBC 12 H 8 . The 9-borafluorenyl substituent is bent out of the plane of the cyclopentadienyl ring toward the iron center with corresponding dip angles of α* = 25.5° and 17.1° for the two crystallographically independent molecules in the asymmetric unit of 3 . The degree of ligand bending is one of the highest ever measured for a ferrocenylborane derivative, thereby indicating a pronounced Fe−B interaction. In line with that, oxidation of the Fe center to its Fe III state using AgBF 4 results in a substantial decrease of the dip angle in the ferricinium species [ 3 ]BF 4 (α* = 6.3°). Compound 3 can therefore be considered a rare example of a redox-switchable main group Lewis acid. The stability of the molecular framework of 3 is sufficiently high to allow for the isolation and structural characterization of its pyridine- N -oxide adduct ( 4 ). Moreover, reaction of 3 with 0.5 equiv of 1,1′-dilithioferrocene (1,1′-fcLi 2 × 2/3 tmeda) and subsequent limited access of O 2 leads to the formation of the trinuclear mixed-valence complex Li[Fc-(BC 12 H 8 )-fc-(BC 12 H 8 )-Fc], which has been structurally characterized as 12-crown-4 adduct (Li(12-c-4) 2 [ 6 ]).