Secondary Interaction and n–π Conjugation in Ferrocenylimine Derivatives of Mercury as Probed by Hg–199 NMR

The ¹⁹⁹Hg NMR spectra of three series of 37 ortho–mercurated ferrocenylimines, (η⁵–C₅H₅)Fe [(η⁵–C₅H₃)(HgCl)C(R)= NAr] (R = H(l),CH₃(2), C₆H₅(3); Ar = substituted phenyl groups, 1–naphthyl and 2–naphthyl), have been recorded. The ¹⁹⁹Hg chemical shifts are regularly disturbed by the substituents in the N–phenyl rings, and excellent linear relationships are found to exist between the ¹⁹⁹Hg shifts (δ) and Hammett substituent constants in all three series. The directions of the substituent effect on δ are the same in the three series, i.e. electron-donating substituents cause shifts to lower field, but the sensitivities of the ¹⁹⁹Hg shift toward the substituent effect are different, ascending in the order of 1,2, and 3. These ^l99Hg NMR features support the intramolecular secondary interaction between the n-electrons of the imino nitrogen atom and the unoccupied 6p orbital of the mercury atom and the n–π conjugation between the nitrogen and the N–phenyl ring.