Coordination of Distal Carboxylate Anion Alters Metal Ion Specific Binding in Imidazo[1,2-a]pyridine Congeners.

Imidazo[1,2-a]pyridine derivatives have excellent potential for chelation with transition metal ions. Two new imidazo[1,2-a]pyridine-8-carboxylates were synthesized and characterized by H NMR, C NMR, HRMS, and single crystal-XRD techniques. Methyl carboxylate (probe 1) turns on fluorescence upon coordination with Zn, while sodium carboxylate (probe 2) turns off its fluorescence upon coordination with Co or Cu ions present in aqueous acetonitrile medium. C NMR study revealed that the change in metal ion specific binding was due to the involvement of carboxylate anion in complex formation with Co or Cu ions. The carboxylate anion at 8-position also enhanced the sensitivity of detection of probe 2 by an order of magnitude (detection limits: 3.804 × 10 M, probe 1/Zn; 0.420 × 10 M, probe 2/Co and 0.304 × 10 M, probe 2/Cu). The detection limits of probes 1 and 2 comply well with the World Health Organization (WHO) and US Environmental Protection Agency (US-EPA) guidelines for detection of heavy metal ions present in drinking water and ground water. Both the probes form a 1:1 complex with Zn, Co or Cu, and the stoichiometry was verified by Job plot and ESI-mass analysis. The sensing mechanism is explained using C NMR experiments, ESI-mass analytical data and theoretical DFT calculations. The suitability of probes 1 and 2 for on-site detection and quantitative determination of Zn, Co and Cu ions present in biological, environmental and industrial samples is demonstrated. In addition, both 1 and 2 are used for detection of intracellular contamination of Zn, Co or Cu ions in onion epidermal cells.