Stabilization of a vanadium(v)–catechol complex by compartmentalization and reduced solvation inside reverse micelles

Abstract

The kinetics of 1 : 1 complex formation and hydrolysis between catechol and 3-substituted catechols with aqueous vanadium(V) at pH ∼ 1 have been investigated in NaAOT (sodium bis(2-ethylhexyl)sulfosuccinate) derived aqueous reverse micelle microemulsions in isooctane. Compared with the reaction in bulk water, the forward rate constant of catechol complexation was modestly accelerated (2 fold) in the reverse micelle microemulsions for even the smallest nanosized water pools (w_o = 2). In contrast, the first order reverse (aquation) reaction was significantly suppressed in the water pools below w_o = 10. The modest rate acceleration of complex formation within the microemulsions is attributed to changes in water solvation, reaction properties and compartmentalization of the reactants. The dramatic fall off in the rate of catechol dissociation from [VO₂(cat)(OH₂)₂]⁻ is attributed to the reduction in water content as the size of the nanopools is decreased below 200 water molecules. The result is a 10 fold increase in the kinetic formation constant for [VO₂(cat)(OH₂)₂]⁻ under confinement in the RM microemulsion environment. This increase predicts that vanadium catechol complexes will be more stable in vivo and may suggest a general principle for fine-tuning efficacy of metal-based drugs.