Synthesis, characterization, redox, and Hg2+ optical ion sensing properties of ferrocenyl-containing maleo- and fumaronitrile derivatives

Mono-((Z)-2-amino-3-((E)-ferrocenylideneamino) butenedinitrile) (1) and bis-((E)-2,3-bis-((E)-ferrocenylideneamino) butenedinitrile) (2) substituted Schiff bases obtained by the reaction between ferrocenecarbaldehyde and diaminomaleonitrile were prepared and characterized by H-1 and C-13 NMR, IR, and UV-vis-NIR spectroscopy methods, elemental analysis, and X-ray crystallography. Density functional theory and time-dependent density functional theory were used to elucidate the electronic structure and the origin of observed excited states in both compounds. Electrochemical techniques were employed to investigate the possibility for the formation of a mixed-valence state of the iron centers in 2(+), which are connected by a conjugated pi-system. Two reversible oxidation potentials were observed in cyclic voltammetry, and spectroelectrochemical experiments revealed the presence of a low-energy intervalence charge transfer band in 2(+), corresponding to class II of the mixed-valence systems. Both complexes were tested for cation-recognition properties using a variety of main group and transition-metal cations. It was shown that complex 1 can be used as a selective optical sensor in recognition of Hg2+ ions. It was also shown byNMRspectroscopy and X-ray crystallography that one of the final reaction products between Hg2+ ions and complex 1 is 2-ferrocenyl-4,5-dicyanoimidazole (3).