Hindering the formation of ferrocenes: mono(cyclopentadienyl)halo iron complexes [Fe(C5R5)X] containing a sterically bulky cyclopentadienyl ligand

The reaction of lithium 1,2,4-tris(trimethylsilyl)cyclopentadienide Li(Si3Cp) with ferrous halide FeX2 (X=Cl, Br, I) in THF at −95 °C gives a thermally extremely sensitive compound that can be regarded as a functional equivalent for the 14-electron fragment [Fe(Si3Cp)X]. Experiments to trap this species with TMEDA gives thermally sensitive, colorless, paramagnetic crystals of the composition [LiFe(Si3Cp)Cl2(TMEDA)3]. Trimethylphosphite gives isolable 18-electron half-sandwich complexes [Fe(Si3Cp){P(OMe)3}2X]. [Fe(Si3Cp)X] is also formed by reducing the ferric derivative [Fe(Si3Cp)Cl2] with zinc. Analogous reactivity towards ferrous halides is observed for the lithium derivatives of the similarly bulky 1,2-bis(trimethylsilyl)-4-tert-butyl- and 2,4-di-tert-butyl-1-trimethylsilylcyclopentadienide, Li(BuSi2Cp) and Li(Bu2SiCp). Trapping with carbon monoxide gives complexes of the type [Fe(BunSi3−nCp)(CO)2X].