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

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Abstract
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.

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