Fluorometric sensing of thiocyanate ions and competitive binding of anions in a family of CdII complexes of a phenol based ligand showing diverse structures

Abstract

A family of central phenoxido and terminal alcohol bound Cd2 complexes have been synthesized and crystallographically characterized. Reaction condition and anion coordination (NO3−, NCS− and N3−) dependent changeable structures have been identified in the four complexes 1–4 following varying coordination of the binucleating Schiff base ligand H3bemp (2,6-bis[(2-hydroxy ethylimino)-methyl]-4-methyl phenol) to cadmium(II) ions. These complexes have been characterized by X-ray crystallography, spectroscopic and analytical techniques. During the formation of 2 and 3, the phenoxido bridging unit of H3bemp is transformed to non-bridging modes of binding following the coordination of NO3− and NCS−, while changing one imine function on each ligand to the zwitterionic form, generated through a kind of acid–base reaction for metal ion coordination. The H3bemp molecule registers as a highly selective and sensitive fluorescent probe for Cd2+ in a PBS buffer (5mM in pH=7.1) at 25°C. In the presence of Cd2+, the increase in fluorescence intensity is accounted for by the formation of [Cd2(H2bemp)2](NO3)2 (1) in solution. The fluorescence quantum yield of H3bemp is 0.201, and it increases more than 4-fold (0.851) in the presence of the cadmium ion. In presence of NCS−, complex 1 showed coordination induced fluorescence enhancement, whereas NO3− and N3− anions quench the emission intensities significantly.