Quantitative comparison of the effect of methylD-glycopyranosides as cosolutes on the rates of base hydrolysis and aquation of some iron(II)-diimine Complexes

Abstract

The kinetics of base hydrolysis and of aquation of some iron(II)–diimine complexes in the presence of stereoisomeric carbohydrates were monitored spectrophotometrically at 25.0°C. In basic aqueous solution dissociation of both Fe(1,10-phenanthroline)32+ and Fe(2,2\'-bipyridine)32+ is accelerated when methyl D-glycopyranosides are present. The aquation reaction of Fe(1,10-phenanthroline)32+ in an aqueous EDTA medium is also accelerated in the presence of carbohydrates, but that of Fe(5-nitro-1,10-phenanthroline)32+ is retarded. An equation obtained from a modification of the Savage–Wood additivity of groups principle applied to kinetics is used to quantify the kinetic medium effects observed. The magnitudes of these effects can be explained in terms of the hydration characteristics of the carbohydrates, which depend on their stereochemistry, and the change in the hydration environment of the iron(II)–diimine complexes during activation. Results and their interpretation for the aquation in acidic medium of the Fe(5-bromo-1,10-phenanthroline)32+ and Fe(4,7-dimethyl-1,10-phenanthroline)32+ complexes in highly aqueous methanol and ethanol solutions are also presented. The kinetic medium effects of the carbohydrates are consistent with those of simple alcohols.