Kinetic and catalytic studies of vanadium catechol dioxygenase model systems, plus re-investigation of a claimed adamantane dioxygenase based on a Ru-incorporated polyoxometalate

Abstract

Following a review of catechol dioxygenases and related biomimetic systems, a series of studies aimed at understanding vanadium catechol dioxygenase model systems were performed. Specific studies undertaken were: 1) spectroscopic and catalytic studies revealing a common catalyst in ten vanadium catechol dioxygenase systems; and 2) kinetic studies determining how the vanadium catechol dioxygenase precursors evolve into that common catalyst. In addition to polyoxometalate catechol dioxygenase systems, simple vanadium catecholate/semiquinone complexes were included in the study investigating the identity of the true catalyst. Selectivity, catalytic lifetime, EPR, negative ion ESI-MS and kinetic results on both polyoxometalate and simple vanadium catecholate systems all point to a common catalyst, Pierpont's crystallographically characterized complex, [VO(DBSQ)(DTBC)]2 (where DBSQ stands for 3,5-di-tert-butylsemiquinone anion and DTBC stands for 3,5-di-terf-butylcatecholate dianion). Pierpont's complex as the common catalyst explains the remarkable similarity in selectivity, catalytic lifetime and other properties for the ten vanadium catechol dioxygenase systems studied. More detailed kinetic studies were performed in order to understand how the vanadium-containing precursors evolve into Pierpont's catalyst. H2O2, a product of the autoxidation of the catechol to its corresponding benzoquinone, was found to be the key to turning on the catechol oxygenation catalysis. Finally, an important Ru2-based sandwich-type polyoxometalate, {[WZnRuIII2(OH)(H2O)](ZnW9O34)2}11-, was re-investigated. This literature compound is claimed to be a dioxygenase catalyst for adamantane hydroxylation and alkene epoxidation reactions in a Nature paper (Neumann, R.; Dahan, M. Nature 1997, 388, 353-355). However, in our hands, kinetic studies, byproduct detection, stoichiometry, and initiation / inhibition results show that the title sandwich Ru2-polyoxometalate catalyzes adamantane hydroxylation reaction via a free-radical-chain mechanism and not the dioxygenase mechanism claimed in the literature.