Electroactive Thiazole Derivatives Capped with Ferrocenyl Units Showing Charge-Transfer Transition and Selective Ion-Sensing Properties: A Combined Experimental and Theoretical Study

Abstract

The synthesis, optical and electrochemical properties, and X-ray characterization of two thiazole derivatives capped by ferrocenyl groups (5 and 7) and their model compounds with one ferrocenyl, either at 2 or 5 position of the mono- or bis-thiazolyl rings (3, 9, 11, and 14), are presented. Bisferrocenyl thiazole 5 forms the mixed-valence species 5 ^•+ by partial oxidation which, interestingly, shows an intramolecular electron-transfer phenomenon. Moreover, the reported heteroaromatic compounds show selective ion-sensing properties. Thus, ferrocenylthiazoles linked across the 5 position of the heteroaromatic ring are selective chemosensors for Hg²⁺ and Pb²⁺ metal ions; 5-ferrocenylthiazole 3 operates through two channels, optical and redox, for Hg²⁺ and only optical for Pb²⁺, whereas 1,1‘-bis(thiazolyl)ferrocene 14 is only an optical sensor for both metal ions. Moreover, complex 3 behaves as an electrochemically induced switchable chemosensor because of the low metal-ion affinity of the oxidized 3 ^•+ species. On the other hand, ferrocenylthiazole 9, in which the heterocyclic ring and the ferrocene group are linked across the 2 position, is a selective redox sensor for Hg²⁺ metal ions, and it responds optically, as does bis(thiazolyl)ferrocene 11, to a narrow range of cations (Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, and Pb²⁺). Finally, bis(ferrocenyl)thiazole 5 is a dual optical and redox sensor for Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, and Pb²⁺, whereas bis(ferrocenyl) compound 7, bearing a bis(thiazole) unit as a bridge, is only a chromogenic sensor for Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, and Pb²⁺. The experimental data and conclusions about both the electronic and ion-sensing properties are supported by DFT calculations which show, in addition, an unprecedented intramolecular electron-transfer reorganization after the first one-electron oxidation of compound 5.