[1.1]Ferrocenophane-1,12-dione as a precursor of 1,12-di(cyclo-penta-2,4- dienylidene)-[1.1]ferrocenophane, a doubly bridged difulvene

An improved synthesis of [1.1]ferroce-nophane-1,12-dione (2) by oxidation of [1.1]ferroce-nophane with 2,3-dichloro-5,6-dicyano-p-benzoqui-none (DDQ) is presented. The syn conformer of dione 2 is structurally characterized. The compound undergoes various addition reactions at the bridging carbonyl groups. Attempts to add 1,1′-dilithioferro-cene result, however, in the diadduct 9, not in the symmetric trinuclear diol 8. Reaction of 2 with sodium cyclopentadienide in the presence of aluminium trichloride (A1C1₃) gives the respective difulvene 11 in excellent yield. The dynamic behavior of 11 is investigated by variable temperature ¹H NMR measurements (VT-NMR). The cyclovoltammogram of 11 indicates two reversible oxidation steps. Reduction of difulvene 11 with lithium aluminium hydride (LiAlH₄) results in the formation of [1.1]ferroce-nophane 12 with cyclopentadienyl anion substituents at either one of the two bridges. While attempts to generate a third ferrocene moiety by reaction with iron(II) choride (FeCl₂) have failed so far, the formation of 12 is established by protonolysis and methyla-tion reactions.