The “pogo stick” complex [FeCp*{N(SiMe3)2}]: spin state properties, adduct formation with Lewis bases, and reactivity towards weakly Brønsted acidic protonated NHCs

[FeCp<em>{N(SiMe₃)₂}] is a high-spin complex with four unpaired electrons according to SQUID magnetometry and Mößbauer spectroscopy. Its reactions with 4-dimethylaminopyridine (DMAP) and NnPr4X (X = Cl, Br) respectively afford [FeCp</em>{N(SiMe₃)₂}(DMAP)] (6), NnBu₄[FeCp<em>Cl{N(SiMe₃)₂}] (7), and NnPr₄[FeCp</em>Br{N(SiMe₃)₂}] (8). 7 and 8 react with formamidinium tetrafluoroborates fc[(NCH₂R)₂CH][BF₄] (fc = 1,1′-ferrocenylene; R = Ph, Mes) at room temperature in THF to furnish [FeCp<em>X{fc[(NCH₂R)₂C]}] (4: X = Cl, R = Mes; 10: X = Br, R = Mes; 11: X = Cl, R = Ph; 12: X = Br, R = Ph), which contain a ferrocene-based NHC ligand. No formation of 10 from fc[(NCH₂Mes)₂CH][BF₄] and 8 takes place under the same mild conditions in toluene, instead leading to the isolation of fc[(NCH₂Mes)₂CH][FeCp</em>Br{N(SiMe₃)₂}] (9). In contrast to its halide adducts 7 and 8, [FeCp<em>{N(SiMe₃)₂}] is inert towards formamidinium tetrafluoroborates. It reacts readily, however, with formamidinium halides, affording [FeCp</em>Cl(IPr)] (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride and [FeCp<em>I(BzIMe)] (2, BzIMe = 1,3-dimethylbenzimidazolin-2-ylidene) or, depending on the crystallisation conditions, [FeCp</em>(BzIMe)2][FeCp<em>I2] (3) with 1,3-dimethylbenzimidazolium iodide, in accord with an equilibrium of the type 2 [FeCp</em>X(NHC)] ⇄ [FeCp<em>(NHC)₂]+ + [FeCp</em>X₂]−. The major product formed with fc[(NCH₂Mes)₂CH]Cl is fc[(NCH₂Mes)₂CH][FeCp<em>Cl₂] (5) instead of the expected NHC complex [FeCp</em>Cl{fc[(NCH₂Mes)₂C]}] (4). Halide abstraction reactions attempted with [FeCp<em>X{fc[(NCH₂R)₂C]}] afforded only intractable material. [FeCp</em>{fc[(NCH₂Ph)₂C]}₂][PF₆] (13) was serendipitously obtained in this context in trace amounts from 12 and Tl[PF₆]. Compounds 1–13 were structurally characterised by single-crystal X-ray diffraction. Metric parameters reveal different electronic configurations for 1 and its solvate 1·0.5 benzene (two vs. four unpaired electrons).