Prismatic and Antiprismatic [42]Ferrocenophanes ([4](1,1′)[4](3,3′)- and [4](1,1′)[4](3,4′)Ferrocenophanes)

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Abstract
[4](1,1′)[4](3,4′)Ferrocenophane (2a), also trivially called antiprismatic [42]ferrocenophane, which possesses a C2 symmetric configuration has been synthesized via intramolecular oxidative coupling of the dienolate ion derived from 3,4′-diacetyl[4](1,1′)ferrocenophane. The antiprismatic phane (2a) and the corresponding prismatic [4](1,1′)[4](3,3′)ferrocenophane (1a), which is bridged by a C2ν symmetric mode, has been characterized by X-ray analysis and 1H NMR and electronic spectrometries. The crystal data of the phanes are as follows: 1a, orthorhom-bic, Pbca, a = 18.469(4), b = 19.994(4), c = 7.610(2) Å; Z = 8; 2a, triclinic, , a = 7.547(1), b = 8.540(1), c = 5.886(1) Å, α = 96.60(1), β = 107.10(1), γ = 101.71(1)°, Z = 1. Antiprismatic phane 2a has a completely staggered conformation about the two cyclopentadienyl (Cp) rings, while prismatic 1a has an intermediary conformation of eclipsed and staggered forms. The Cp–Fe–Cp distance of 2a (3.260 Å) is shorter than that of 1a (3.275 Å). The hypsochromic shift of the d–d* absorption band of 2a in the electronic spectrum is more remarkable than that of 1a (ferrocene, 443 nm; 1a, 420 nm; 2a, 403 nm in THF). Those results reveal that an antiprismatic bridging mode with tetramethylene chains brings about a higher strain in the molecule compared with a prismatic one.