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

Abstract

The tetrahydro‐4,4,8,8‐tetramethyl‐4,8‐disila‐ s ‐indacenyl anion (LH) reacts with or without cyclopentadienyl anion (Cp − ) and Fe 2 Cl 4 (THF) 3 to give the mixed ferrocenes CpFe(LH) and (LH) 2 Fe 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 Me 2 Si 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 ( E a = 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) 2 Fe 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 [(Me 2 SiOSiMe 2 )Cp]FeCp ( 5 ) among others) and deprotonation. The latter yields CpFe(L – ) ( 7 ) and (L − ) 2 Fe ( 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 CrCl 2 (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.