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

[FeCp*{N(SiMe)}] is a high-spin complex with four unpaired electrons according to SQUID magnetometry and Mößbauer spectroscopy. Its reactions with 4-dimethylaminopyridine (DMAP) and NPrX (X = Cl, Br) respectively afford [FeCp*{N(SiMe)}(DMAP)] (6), NBu[FeCp*Cl{N(SiMe)}] (7), and NPr[FeCp*Br{N(SiMe)}] (8). 7 and 8 react with formamidinium tetrafluoroborates fc[(NCHR)CH][BF] (fc = 1,1'-ferrocenylene; R = Ph, Mes) at room temperature in THF to furnish [FeCp*X{fc[(NCHR)C]}] (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[(NCHMes)CH][BF] and 8 takes place under the same mild conditions in toluene, instead leading to the isolation of fc[(NCHMes)CH][FeCp*Br{N(SiMe)}] (9). In contrast to its halide adducts 7 and 8, [FeCp*{N(SiMe)}] is inert towards formamidinium tetrafluoroborates. It reacts readily, however, with formamidinium halides, affording [FeCp*Cl(IPr)] (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride and [FeCp*I(BzIMe)] (2, BzIMe = 1,3-dimethylbenzimidazolin-2-ylidene) or, depending on the crystallisation conditions, [FeCp*(BzIMe)][FeCp*I] (3) with 1,3-dimethylbenzimidazolium iodide, in accord with an equilibrium of the type 2 [FeCp*X(NHC)] ⇄ [FeCp*(NHC)] + [FeCp*X]. The major product formed with fc[(NCHMes)CH]Cl is fc[(NCHMes)CH][FeCp*Cl] (5) instead of the expected NHC complex [FeCp*Cl{fc[(NCHMes)C]}] (4). Halide abstraction reactions attempted with [FeCp*X{fc[(NCHR)C]}] afforded only intractable material. [FeCp*{fc[(NCHPh)C]}][PF] (13) was serendipitously obtained in this context in trace amounts from 12 and Tl[PF]. 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 four unpaired electrons).