Mössbauer studies on ferrocene complexes: XIV. Carbenium ions and ketones of [3]ferrocenophanes

Abstract

A series of [3]ferrocenphane (FcP) derivatives have been investigated using 57Fe Mössbauer and 1H NMR spectroscopy. For the 2- and 3-acetyl-FcP in neutral and acidic media, proton resonances were assigned by analogy with the unbridged species, which themselves were analysed using specifically α deuteriated compounds. 2-Acetyl-FcP exhibited a strongly shielded methyl group due to out of plane deformation of the carbonyl function. The corresponding carbenium ions were produced from the parent alcohols in CF 3CO 2H and their NMR spectra discussed. [3]Ferrocenophan-6,8-dione (A) was unprotonated in CF 3CO 2H, monoprotonated in 80% H 2SO 4 to give the symmetrical carbenium ion. The NMR spectrum of the ion is fully discussed. Mössbauer spectra of 2- and 3-acetyl-FcP revealed the latter to have an anomalously low value of both quadrupole splitting ( QS) and isomer shift ( IS). The QS of frozen solutions of A at pH 13 show little change from those of ferrocene indicating that the carbanionic charge creates a symmetrical electron density about the iron atom by repulsion of the e 2g electrons. The carbenium ion generated at the 2 position in [3]FcP showed a higher QS than that in the 3-position due to better overlap with e 2g orbitals. The symmetrical carbenium ion derived from dehydro-[3]ferrocenophane showed exalted QS values indicative of iron e 2g orbital overlap. The effect of bridging on Mössbauer parameters is also discussed.