Transmetalation properties of Zn(II) salen-type complexes as chemosensors for recognition of Cu2+ ions in water

Copper(II) ions play a pivotal role in chemical, biological and environmental systems, therefore a deficit or an excess of this metal ion could compromise the health of plants, animals, and humans.1 In this contest, the detection of Cu2+ ions, especially in aqueous phase, is fundamental in terms of environmental and life concern. Chromogenic and fluorogenic chemosensors represent ideal tools for a simple, fast and highly sensitive detection of this important metal ion.2 Zn(II) salen-type complexes are Lewis acidic species able to form aggregates or monomeric adducts in non-coordinating or coordinating solvents, respectively.3 Moreover, they are versatile chemosensors because allow the chromogenic/fluorogenic detection of neutral and anionic Lewis bases,4-5 through the disaggregation of aggregate species and of metal ions by transmetalation of the monomeric adducts. Regarding the latter issue, we have recently demonstrated that transmetalation properties of Zn(II) salen-type complexes can be tuned in relation to the Lewis basicity of solvent and nature of metal salts.6 Here, we present the selective and sensitive detection of Cu2+ ions in water involving the transmetalation ability of the Lewis acidic Zn(II) Schiff-base complex, 1. Solutions of 1 in CH3CN show a selective and fast transmetalation of aqueous solutions of Cu2+, involving a naked-eye color change and a complete quenching of fluorescence. The limit of detection of 1 with Cu2+ is 0.6 μM, lower than the limit of the World Health Organization (WHO) and US Environmental Protection Agency (EPA) for Cu2+ in drinking water. Therefore 1 represents a useful chromogenic and fluorogenic chemosensor for the recognition of Cu2+ ions in water.