Selective metal-cation recognition by [2.2]ferrocenophanes: the cases of zinc- and lithium-sensing.

The synthesis, electrochemical, optical, and cation-sensing properties of [2.2]ferrocenophanes, in which the two ferrocene subunits are linked through two aldiminic or iminophosphorane moieties, are reported. The new compounds show remarkably selective cation-sensing properties due to the presence of redox-active units (ferrocene) and aza-unsaturated functionalities that are able to act as putative cation-binding sites. In this structural motif, the aldimine groups act as a highly selective binding site for Zn(2+) cations, whereas the iminophosphorane bridges display an unusually strong binding affinity towards Li(+) cations, which could be explained by an additional LiFe interaction. The X-ray structure of the complex 4Li(+) as well as detailed NMR spectroscopic studies, both in solution and in the solid state, support this assessment. Experimental data and conclusions about the cation-sensing capabilities of this family of compounds are supported by DFT calculations.