Equilibrium studies of triphenyltin(IV) complexes with glycine, glycyl-glycine, and glycyl-glycyl-glycine in different aqueous solutions of ethanol

摘要

The protonation equilibria of glycine (gly), glycyl-glycine (gly-gly), and glycyl-glycyl-glycine (gly-gly-gly) and their formation constants with triphenyltin(IV) chloride were studied over a wide pH range (pH 1-11), using a combination of spectrophotometric and potentiometric methods at constant temperature (25 °C), different ethanol-water mixtures (50%-80%, v/v), and constant ionic strength (0.1 mol dm~(-3) NaClO4). Least-squares regression calculations are consistent with the formation of ph3SnHL~+, ph3SnL, and ph3SnH_(-1)L~- complex species, where L~- represents the fully dissociated form of each ligand. The stability constant of the formed complexes in different media were analyzed in terms of Kamlet, Ab-boud, and Taft (KAT) parameters. Single-parameter correlations of the stability constants versus α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity), and for π* (dipolarity/polarizability) are relatively poor in all solutions, but multi-parameter correlations represent significant improvements with regard to the single- and dual-parameter models. Linear correlation is observed when the experimental logβ_(xyz) values are plotted versus the calculated ones, while all the KAT parameters are considered. Also, the stability constant values of the formed complexes are determined in zero percent of organic solvent using the Yasuda-Shedlovsky extrapolation approach. Finally, the results are discussed in terms of the effect of solvent on complexation.