Reduction of copper(ii) complexes of tridentate ligands by nitric oxide and fluorescent detection of NO in methanol and water media

Abstract

Two copper complexes, 1 and 2, with tridentate N-donor ligands, L₁ and L₂ [L₁=(1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)amine, L₂ = (2-pyridin-2-yl-ethyl)-pyridin-2 yl-methylamine] respectively, have been synthesized and characterized. On exposure to nitric oxide, the copper(II) centers in complexes 1 and 2 were found to undergo reduction in various solvents. In acetonitrile solvent the reduction was accompanied by a simultaneous N-nitrosation on the secondary amine center on the ligand frameworks. Complexes 3 and 4 were prepared with ligands L₃ and L₄, respectively. L₃ and L₄ [L₃ = 5-dimethylamino-naphthalene-1-sulfonic acid (1-methyl-1H-imidazol-2-ylmethyl)-(2-pyridin-2-yl-ethyl)-amide; L₄ = 5-dimethylamino-naphthalene-1-sulfonic acid(2-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl-amide] are the dansyl derivatives of L₁ and L₂, respectively. Complex 4, due to paramagnetic quenching, does not display any fluorescence; however, on addition of nitric oxide to a methanol or water solution of complex 4, the fluorescence intensity of the fluorophore has been found to be restored. This is attributed to the reduction of the Cu(II) center by nitric oxide to diamagnetic Cu(I). The turn-on of quenched fluorescence intensity has been observed both in methanol and water media.