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 spectra of 12 ferrocenyl-triptycenes or -barrelenes are reported.
• Protons sited directly above a cyclopentadienyl ring are shielded by about 1 ppm.
• Protons positioned adjacent to the iron atom are deshielded by about 1.5 ppm.
• The NMR-derived experimental diamagnetic anisotropy is 25% higher than for benzene.
• These data may allow comparison of the relative aromaticity of ferrocene and benzene.

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.

The 1H NMR shifts in 9-ferrocenyl-2,3-dimethytriptycene (and 11 other ferrocenyl-triptycenes or barrelenes) reveal that protons directly above a cyclopentadienyl ring are shielded by more than 1 ppm whereas those situated between the two rings are heavily deshielded by approximately 1.5 ppm.Figure optionsDownload as PowerPoint slide