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.