This paper reports an interesting property of the O, N and S donor ligand 2-(2’-Hydroxy-3’-methoxyphenyl)benzothiazole which makes it switch chelating donor sites from O, S (at pH < 5.0) to O, N (above pH 5.0) while reacting with Cu(II) acetate in a 3:1 ethanol-water mixture. The authors have adopted a stopped-flow spectrophotometric technique monitored at a wavelength of 400 nm at 25 °C. The reaction is reported to proceed towards precipitation above a pH of 5.5 after some time. Although the study was carried out thoughtfully, I have some comments which would be of interest to the reader.
The Reactions of Interest:
- The procedure for synthesis of the ligand as reported by the authors is rather cumbersome. High temperatures in the range of 180 to 200 °C and polyphosphoric acid as the solvent are the drawbacks. The product further requires neutralization with NaOH.
- The data from Table 1 could be reproduced easily following the protocol mentioned by the authors. The IR and UV-vis experiments to determine the rate of reactions of formation of Cu(O, S) and Cu(O, N) complexes are closely replicable. The formation of a polymeric precipitate was also observed at pH above 5.5, which receded in an inert atmosphere, as reported by the authors.
- The synthesis of the ligand can be done in a straightforward manner which I employed. 2-aminothiophenol (0.188 g, 1.5 mM) and 2-hydroxy-3-methoxybenzaldehyde (0.228 g, 1.5 mM) were mixed in 10 mL chloroform and was heated under reflux for 3 hours. The resulting clear orange-brown solution was left for aerial evaporation. A gummy mass was obtained which was triturated with petroleum ether and dried under reduced pressure to have an orange-brown powder.
- The section “2.1.3. Kinetic Measurements” mentions an important principle which is essential for such measurements. The distinction between the actually measured pH value and the theoretically justifiable one for a 3:1 ethanol-water mixture is essential to be noted by the readers who would like to replicate the process and it has been aptly recorded as “Because the activity of a 75% alcoholic solution is not well deﬁned the concentration of H+ was used for all calculations. In order to calibrate the electrode, the measured pH values were plotted against the logarithm of several known concentrations of perchloric acid and the resulting straight line was used to calculate the electrode in terms of the H+ concentration. The resulting conversion factor used in this work was pHc= (pHmeasured + 1.085)/1.381 in which pHc= -log10[H+] is used in order to distinguish it from pH.