Spectrometric and kinetics studies involving anionic chromogenic chemodosimeters based on silylated imines in acetonitrile or acetonitrile–water mixtures

Three chromogenic anionic chemodosimeters (1-3) based on silylated imines were synthesized and characterized. Solutions of compound 1 in acetonitrile with 4.0% (v/v) of water are colorless, but with the addition of several anions only CN-, and to a lesser extent F-, changed the color of the solutions to orange. However, compounds 2 and 3 were selective toward F- in acetonitrile. The nucleophilic attack of F- or CN- on the silicon center of the chemodosimeters, through an S(N)2@Si mechanism, released colored phenolates as leaving groups. PGSE NMR data corroborated the mechanism postulated for the reaction. Kinetics studies were carried out, revealing that a higher second-order rate constant was obtained for the reaction of 1 with F-. The addition of water to the system reduces the nucleophilicity of F-, showing a slower second-order rate constant in relation to CN-, the latter anion being less hydrated and the more reactive species for the nucleophilic attack on the silicon center of 1.