Voltammetric metal cation sensors based on ferrocene derivatives with oxazoline and imine substituents

Ferrocene derivatives 1a, 1b and 2 containing oxazoline substituents act as efficient voltammetric sensors of Mg2+ and Ca2+ ions in acetonitrile solution in concentrations as low as 10 mol%: a new redox peak appears in the cyclic voltammogram, positively shifted by 310–360 mV for 1a and 1b, and by 160–190 mV for 2, compared to E1/2 of the free ligand. For the bisferrocenyldiimine derivative 3 the shift is 150–160 mV. Compounds 1a, 1b and 3 are also responsive to Cu2+ ions (shifts of 250, 320 and 160 mV, respectively) and compound 3 is responsive to Zn2+ ions (150 mV shift) with no interference from a large excess of several other metal salts. UV–vis spectrophotometric studies and 1H-NMR titration experiments confirm that the oxazoline or imine groups are the sites of metal complexation.