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

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-tert -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. H 2O2, 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, {lcub}[WZnRuIII2(OH)(H2O)](ZnW9O 34)2{rcub}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.