Imidazole-Annelated Ferrocene Derivatives as Highly Selective and Sensitive Multichannel Chemical Probes for Pb(II) Cations

Abstract

New ferrocenyl-containing imidazopyridine and imidazophenazine receptors 2-5 show high selective affinity for Pb(II) ions over a range of other metal ions examined through different channels. Imidazopyridine-ferrocene dyad 2 behaves as a highly selective redox, chromogenic, and fluorescent chemosensor molecule for Pb(2+) cations: the oxidation redox peak is anodically shifted (DeltaE(1/2) = 150 mV), and the low energy band of the absorption spectrum is red-shifted (Deltalambda = 44 nm) upon complexation with this metal cation. This change in the absorption spectrum is accompanied by a color change from colorless to orange, which allows the potential for "naked eye" detection. The emission spectrum undergoes an important chelation-enhanced fluorescence (CHEF) effect (CHEF = 620), with an unprecedented detection limit of 2.7 microg L(-1). The presence of Zn(2+) cations also induced a perturbation of the redox potential, absorption, and emission spectra although in less extension than those found with Pb(2+) cations. Imidazophenazine-ferrocene dyad 3 has also shown its ability for sensing Pb(2+) cations through redox (DeltaE(1/2) = 120 mV), absorption (Deltalambda = 23 nm), and emission (CHEF = 133) channels, whereas the presence of Zn(2+) only has a little effect on the emission spectrum (CHEF = 74). The electrochemical changes observed in the two-armed ferrocenes 4 and 5 upon complexation show that the potential shift is higher for Zn(2+) ions (DeltaE(1/2) = 190-170 mV) than for Pb(2+) ions (DeltaE(1/2) = 180-110 mV), which is in clear contrast to those observed for the monoarmed ferrocenes 2 and 3. The recognition properties of the two-armed imidazopyridine-ferrocene triad 4 are quite similar to those exhibited by the parent monosubstituted receptor 2, and the most salient features are a strong perturbation of the redox wave (DeltaE(1/2) = 180 mV), a dramatic increasing of the fluorescent quantum yield (Phi(complex)/Phi(ligand) = 890) in the presence of Pb(2+), while the optical responses toward Zn(2+) cations were silent. The two-armed imidazophenazine-ferrocene triad 5 senses Pb(2+) cations through perturbation of the oxidation potential of the Fe(II)/Fe(III) redox couple (DeltaE(1/2) = 110 mV), important blue shift (Deltalambda = 160 nm) of the high energy band in the absorption spectrum, and a remarkable increase of the emission band (CHEF = 220), whereas smaller changes were observed in the presence of Zn(2+) cations. (1)H NMR studies as well as DFT calculations have been carried out to get information about which molecular sites are involved in the binding event.