Fusing Cyclopentadiene and Ferrocene by Two Silyl Bridges Stereochemically Well-Defined Building Blocks for High-Nuclear Metallocenes.

The tetrahydro-4,4,8,8-tetramethyl-4,8-disila-s-indacenyl anion (LH) reacts with or without cyclopentadienyl anion (Cp−) and Fe2Cl4(THF)3 to give the mixed ferrocenes CpFe(LH) and (LH)2Fe in 50 and 63% yield, respectively. Near room temperature and in solution CpFe(LH) exists as a pair of enantiomers (syn-3a/b). According to an X-ray analysis both enantiomers are present in a single crystal. Compound 3 consists of a ferrocene linked to a cyclopentadiene by two Me2Si groups such that the acidic Cp proton points towards the iron. Temperature-dependent NMR spectra show that syn-3a and syn-3b are non-rigid molecules which are interconverted by 1,2-silatropic bond shifts (Ea = 49.2 ± 2.9 kJmol-1). Three more isomers (anti-3a, anti-3b, and the spiro derivative 3c), though present only in low concentration, are engaged in the rearrangement. (LH)2Fe is also non-rigid and exists as a pair of diastereomers (4a and 4b). The isomers of 3 and 4 undergo hydrolysis (e.g., syn-3a/3b gives [(Me2SiOSiMe2)Cp]FeCp (5) among others) and deprotonation. The latter yields CpFe(L–) (7) and (L−)2Fe (8), which are characterized by NMR spectroscopy. 7 forms crystals with [Li(TMEDA)]+ as counterion, and an X-ray analysis shows that lithium and iron are located at opposite sides of the strongly folded ligand L. The reaction of 7 with CrCl2(THF) gives the trinuclear stacked metallocene 9 in 47% yield. 9 has two unpaired electrons which are located essentially in the center of the molecule