An Efficient Molecular Tool with Ferrocene Backbone: Discriminating Fe3+ from Fe2+ in Aqueous Media

Two novel molecular probes with ferrocene backbone have been designed and synthesized for the first time, and they were subsequently found capable of distinguishing Fe3+ and Fe2+ ion in aqueous media. The discrimination of both the oxidation states (II/III) of iron by these receptors can be established either from a striking shift in redox potential (1: ΔE1/2 ≈ 90 mV and 2: ΔE1/2 ≈ 59 mV) for Fe2+ ion or from UV–vis absorption studies (using light-absorption ratio variation approach (LARVA)). Moreover, the discrimination of Fe2+ and Fe3+ cations could be performed by naked-eye observation because of the development of different colors upon interaction with these probes which act as indicators for the in situ qualitative detection of Fe3+ and Fe2+. The limits of detection of Fe2+ and Fe3+ cations with receptor 2 were found to be as low as 30 and 15 parts per billion (ppb), respectively. The probable binding modes of these receptors with Fe2+ have also been suggested on the basis of the 1H NMR spectroscopic titration, electrospray ionization mass spectrometry (ESI-MS), Job’s plot, and computational (DFT) studies along with electrochemical and spectro-photochemical data. Single crystal X-ray diffraction analysis of 1 revealed that its solid-state structure was stabilized via intermolecular C–H/O and O–H/N hydrogen bonds and by C–H/π interactions. Interestingly, detailed theoretical calculations (DFT) indicated that hydroxymethyl (−CH2OH) group attached to naphthalene unit plays a pivotal role in sensing Fe2+/3+ ion selectively and in the stabilization of 2 in unusual eclipsed configuration through C–H···O type hydrogen bonding.