Is It True Dioxygenase or Classic Autoxidation Catalysis? Re-Investigation of a Claimed Dioxygenase Catalyst Based on a Ru2-Incorporated, Polyoxometalate Precatalyst

A 1997 Nature paper (Nature 1997, 388, 353-355) and a 1998 J. Am. Chem. Soc. paper (J. Am. Chem. Soc. 1998, 120, 11969-11976) reported that a novel Ru2-incorporated sandwich-type polyoxometalate, {[WZnRuIII2(OH)(H2O)](ZnW9O34)2}11-, is an all-inorganic dioxygenase catalyst for the hydroxylation of adamantane and the epoxidation of alkenes using molecular oxygen. Specifically, it was reported that the above Ru2-containing polyoxometalate catalyzes the following reaction by a non-radical-chain, dioxygenase mechanism: 2RH + O2 (right arrow)2ROH (R = adamantane). A re-investigation of the above claim has been performed, resulting in the following findings: (1) iodometric analysis detects trace peroxides (0.5% relative to adamantane), the products of free-radical-chain autoxidation, at the end of the adamantane hydroxylation reaction; (2) a non-dioxygenase product, H218O, is observed at the end of an adamantane hydroxylation reaction performed using 18O2; (3) kinetic studies reveal a fractional rate law consistent with a classic radical-chain reaction; (4) a non-dioxygenase ~1:1 adamantane products/O2 stoichiometry is observed in our hands (instead of the claimed 2:1 adamantane/O2 dioxygenase stoichiometry); (5) adamantane hydroxylation is initiated by the free radical initiator, AIBN (2,2ʹ-azobisisobutyronitrile), or the organic hydroperoxide, t-BuOOH; (6) four radical scavengers completely inhibit the reaction; and (7) {[WZnRuIII2(OH)(H2O)](ZnW9O34)2}11- is found to be an effective catalyst for cyclohexene free-radical-chain autoxidation. The above results are consistent with and strongly supportive of a free-radical-chain mechanism, not the previously claimed dioxygenase pathway.