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.

Graphical abstractThe 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 full-size imageDownload as PowerPoint slide