Bridged ferrocenes. 10. Structural phenomena

Abstract

The structures of 1,1',2,2'-bis(tetramethylene)ferrocene, I, 1,1',2,2',4,4'-tris(tetramethylene)ferrocene, II, and 1,1',2,2',4,4'-tris(pentamethylene)ferrocene, III, are given and are compared to the previously determined structure of 1,1',2,2',4,4'-tris(trimethylene)ferrocene, IV. The iron-to-ring distances are consistent with the reported Moessbauer spectra and redox potentials. The cyclopentadienyl rings are eclipsed in the compounds with trimethylene and pentamethylene bridges but are staggered by 12-14 in the compounds with tetramethylene bridges. In compounds with tetra- and pentamethylene bridges the presence of staggering or eclipsing is attributed to the need to avoid eclipsing of the protons in the bridges. In the compound with three trimethylene bridges, the shortness of the bridges is of primary importance. The staggered conformation observed in I and II may be the source of the apparent anomalies observed in the ring-proton region of the NMR spectra of bridged ferrocenes. The bridge-proton region of the NMR spectra at 360 MHz are given for I and other bridged ferrocenes. Disorder of the bridge carbons observed in the crystals are correlated with the flipping of the bridges observed in the NMR spectra. 30 references, 8 tables.