The effect of protonation on the spectroscopic and redox properties of a series of ferrocenoyl derivatives

Five ferrocenoyl derivatives, containing pyridine (1–4) and benzene (5) moieties, were synthesised and characterised. The effect on the spectroscopic and redox properties of these compounds upon addition of H+ was studied, with NMR studies indicating that protonation took place at the pyridine nitrogens of 1–4. A crystal structure determination of the bis(amide) derivative 4 revealed the presence of two intramolecular hydrogen bonds; these remained intact in solution but were cleaved upon protonation. Protonation induced anodic shifts in the ferrocene-centred redox potential of ferrocene receptors 1, 2 and 4 and also changes in the electronic and NMR spectra of 1–4. A bathochromic shift in the lowest energy spin-allowed d–d band upon protonation was only observed for those compounds which also displayed a pronounced redox response.