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

9-Ferrocenyl-9-borafluorene (FcBC12H8; 3) is readily accessible from FcHgCl and BrBC12H8. 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 FeIII state using AgBF4 results in a substantial decrease of the dip angle in the ferricinium species [3]BF4 (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′-fcLi2 × 2/3 tmeda) and subsequent limited access of O2 leads to the formation of the trinuclear mixed-valence complex Li[Fc-(BC12H8)-fc-(BC12H8)-Fc], which has been structurally characterized as 12-crown-4 adduct (Li(12-c-4)2[6]).