Direct measurement of the diamagnetic anisotropy of the ferrocenyl moiety: The origin of unusual 1H NMR shifts in ferrocenyl-triptycenes and barrelenes

The 1H NMR chemical shifts of 12 ferrocenyl-triptycenes and –barrelenes, in conjunction with their X-ray crystallographically-determined structures, provide a picture of the diamagnetic anisotropic effect of the ferrocenyl moiety. It reveals that protons sited directly above a cyclopentadienyl ring are incrementally shielded by approximately 1.2 ppm, whereas those positioned adjacent to the iron atom are deshielded by approximately 1.5 ppm. These data allow an NMR-derived evaluation of the χ value for ferrocene as of −1660 × 10−36 m3/molecule, about 25% higher than the currently accepted value for benzene. A particular feature of the present approach is that data are provided for individual protons in clearly defined geometric positions rather than for rotating methyl groups.