[3.3]Ferrocenophanes with guanidine bridging units as multisignalling receptor molecules for selective recognition of anions, cations, and amino acids.

The synthesis, electrochemical, and optical properties of a new [3.3]ferrocenophane framework in which two ferrocene subunits, with similar electronic environments, are linked through two substituted guanidine moieties, are reported. The receptors 4-7 have been prepared in good yields by the reaction of bis(carbodiimide) 3 with primary amines. This architecture is exceptionally "tunable" because a variety of "legs" may be appended to the basic [3.3]ferrocenophane scaffold to give a wide range of signaling units. These receptors show remarkable ion-sensing properties, due to the presence of a redox active unit (ferrocene), and an amphoteric binding site (guanidine). In this nitrogen-rich structural motif the guanidine bridges act as multipoint binding sites for anions, cations, and amino acids. Sensing of anions takes place both by unprecedented redox-ratiometric measurements (F-, Cl-, AcO-, NO3-, HSO4-, H2PO4-, and HP2O7(3-)), and colorimetric change (F-, AcO-, H2PO4-, and HP2O7(3-)). Sensing and discrimination of amino acids takes place by redox-ratiometric measurements, whereas the recognition of metal cations (Zn2+, Ni2+, and Cd2+) is achieved either by electrochemical or fluorescence measurements. Moreover, the reported receptors display splitting of the oxidation wave of the Fe(II)/Fe(III) redox couple, and form the mixed-valence species 4+* -7+* by electrochemical partial oxidation which, interestingly, show intervalence charge-transfer transitions associated to the appearance of absorption bands in the near infrared spectral region.