Synthesis of a novel N,N′,N′-tetraacetyl-4,6-dimethoxyindole-based dual chemosensor for the recognition of Fe3+ and Cu2+ ions

A novel N,N'N',1-tetraacetyl-4,6-dimethoxy-1H-indole-2-carbohydrazide 5, bearing four N-acetyl groups (N-COCH3) as cation binding sites was successfully synthesized by the reaction of readily available 4,6-dimethoxy-1H-indole-2-carbohydrazide 4 with acetic anhydride in dimethyl formamide in the presence of triethylamine. The structure of the acetyl indole 5 was confirmed using H-1 NMR, C-13 NMR, FT-IR, mass spectrometry and single crystal X-ray diffraction techniques. The colorimetric sensing ability of targeted compound 5 was investigated by monitoring UV-Vis and fluorescence spectroscopy spectral changes upon addition of different cations. The compound 5 demonstrated selective recognition toward the Fe+3 and Cu+2 ions over other cations examined such as Li+, Na+, K+ Mg2+, Ca2+, Ba2+, Mn2+, Fe2+, Cr3+, Co2+, Ni2+, Ag+, Cd2+, Al3+, Hg (+), Zn2+. The complex stoichiometry of the compound 5 and Cu2+/Fe3+ ions were determined as 2:1 (ligand/metal) by fluorescence titrations (Job's plot). This is the first report showing 4,6-dimethoxyindole moiety represents a dual chemosensor capable of detecting the Cu2+ and Fe2+ ions.