Reactions of methylmercury(II), mercury(II), and zinc(II) ions with 2-methyl-2-(2-pyridyl)thiazolidine, including stopped-flow rapid-scanning spectrophotometric studies

Abstract

Equilibrium studies of the reactions of 2-methyl-2-(2-pyridyl)thiazolidine (A) with [HgMeX](X = Cl, MeCO₂), Hg^II and Zn^II ions have been investigated by ¹H and ¹³C n.m.r. and by u.v. spectrophotometry, and kinetic studies of the reactions with Hg^II and Zn^II ions by stopped-flow rapid-scanning u.v. spectrophotometry. The n.m.r. results show that opening of the thiazolidine ring occurs with the loss of a proton, to give thiolate-bound Schiff-base chelates. In the absence of buffer, the reaction with [HgMeCl] also gives a second species in which the thiazolidine ring is not opened, and addition of acetate buffer causes this minor species to deprotonate and convert to the ring-opened product. Kinetic studies using Hg^II reveal a single, very rapid reaction, close to the limits of stopped-flow detection, and approximately first order in [Hg^II]. Two distinct kinetic stages are apparent in the reaction with zinc(II) perchlorate in methanol solution, the first stage attributed to metal binding and the pH-dependent second stage to the opening of the thiazolidine ring. Using zinc(II) acetate the first stage is too rapid for stopped-flow detection, but the second stage is kinetically simpler because of the buffering effect of the acetate ions.