New Insights on the Mechanism of Palladium-Catalyzed Hydrolysis of Sodium Borohydride from 11B NMR Measurements

Abstract

To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH₄ in alkaline media, the kinetics of the reaction has been investigated by ¹¹B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH₄^- concentration in the range 0.05−0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03−0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH₄ and its perdeuterated analogue NaBD₄ in H₂O, in D₂O and H₂O/D₂O mixtures. When the process was carried out in D₂O, deuterium incorporation in BH₄^- afforded BH_4-nD_n^- (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a regression analysis was performed on a reaction scheme wherein the starting reactant BH₄^- is allowed both to reversibly exchange hydrogen with deuterium atoms of D₂O and to irreversibly hydrolyze into borohydroxy species B(OD)₄^-. In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.