Colorimetric metal ion (II) Sensors Based on imine boronic esters functionalized with pyridine

Abstract The one-step synthesis of three imine boronic esters functionalized with pyridyl groups is described (1a-1c). The presence or absence of a methyl group affects the whole conformation, being bent “L” or linear as determined by X-ray crystal diffraction. C–H⋯O, C–H⋯N, C–H⋯F and C–H … π hydrogen bond interactions support 2D supramolecular arrangements. Ligands were tested as metal ion (M2+) sensors in solution. The addition of metal cations (Fe2+, Co2+ and Cu2+) to 1a using methanol as solvent, showed significant color changes (purple, orange and green, respectively), while for metal cations Ni2+, Zn2+, Cd2+ cations no color changes were observed. The sensitivity of compound 1a towards Fe2+, Co2+ and Cu2+ was monitored by UV–Vis spectroscopy, where the presence of Fe2+ produces new bands at 360 and 566 nm. For Co2+ and Cu2+ a remarkable intensity increase was observed at 290 nm band and new bands appear at 340 and 462 nm, respectively. The stoichiometry of the complexes 1a-Fe2+, 1a-Co2+ and 1a-Cu2+ was determined by Job's plots being 1:3 (metal:ligand, complex [Fe (1a)3]2+), in contrast with the stoichiometry 1:1 observed for 1b-Fe2+. In fact, the computed ΔG formation associated value showed a strong stabilization for the complex [Fe (1a)3]2+ (−65.5 kcal/mol) in comparison with the possible complex [Fe (1b)3]2+ (−29.7 kcal/mol). Detection limits are in the mM range (determined by UV–Vis) and μM range (determined by Fluorescence). An analogous bis-bidentate derivative (2a) also shows similar behavior but stoichiometric interaction changes to 2:3 (metal:ligand) and detection limits are in the mM range obtained by UV–Vis and Fluorescence techniques.