Resolving the Mechanism for H2O2 Decomposition over Zr(IV)-Substituted Lindqvist Tungstate: Evidence of Singlet Oxygen Intermediacy

The decomposition of hydrogen peroxide (H2O2) is the main undesired side reaction in catalytic oxidationprocessesof industrial interest that make use of H2O2 as a terminal oxidant, such as the epoxidation of alkenes. However,the mechanism responsible for this reaction is still poorly understood,thus hindering the development of design rules to maximize the efficiencyof catalytic oxidations in terms of product selectivity and oxidantutilization efficiency. Here, we thoroughly investigated the H2O2 decomposition mechanism using a Zr-monosubstituteddimeric Lindqvist tungstate, (Bu4N)(6)[{W5O18Zr(& mu;-OH)}(2)] ({ZrW ( 5 ) } ( 2 )),which revealed high activity for this reaction in acetonitrile. Themechanism of the {ZrW ( 5 ) } ( 2 )-catalyzed H2O2 degradationin the absence of an organic substrate was investigated using kinetic,spectroscopic, and computational tools. The reaction is first orderin the Zr catalyst and shows saturation behavior with increasingH(2)O(2) concentration. The apparent activationenergy is 11.5 kcal & BULL;mol(-1), which is significantlylower than the values previously found for Ti- and Nb-substitutedLindqvist tungstates (14.6 and 16.7 kcal & BULL;mol(-1), respectively). EPR spectroscopic studies indicated the formationof superoxide radicals, while EPR with a specific singlet oxygen trap,2,2,6,6-tetramethyl-piperidone (4-oxo-TEMP), revealed the generationof O-1(2). The interaction of test substrates,& alpha;-terpinene and tetramethylethylene, with H2O2 in the presence of {ZrW ( 5 ) } ( 2 ) corroborated the formationof products typical of the oxidation processes that engage O-1(2) (endoperoxide ascaridole and 2,3-dimethyl-3-butene-2-hydroperoxide,respectively). While radical scavengers (BuOH)-Bu- t and p-benzoquinone produced no effect on theperoxide product yield, the addition of 4-oxo-TEMP significantly reducedit. After optimization of the reaction conditions, a 90% yield ofascaridole was attained. DFT calculations provided an atomistic descriptionof the H2O2 decomposition mechanism by Zr-substitutedLindqvist tungstate catalysts. Calculations showed that the reactionproceeds through a Zr-trioxidane [Zr-& eta;(2)-OO(OH)] keyintermediate, whose formation is the rate-determining step. The Zr-substitutedPOM activates heterolytically a first H2O2 moleculeto generate a Zr-peroxo species, which attacks nucleophilically toa second H2O2, causing its heterolytic O-Ocleavage to yield the Zr-trioxidane complex. In agreement with spectroscopicand kinetic studies, the lowest-energy pathway involves dimeric Zrspecies and an inner-sphere mechanism. Still, we also found monomericinner- and outer-sphere pathways that are close in energy and couldcoexist with the dimeric one. The highly reactive Zr-trioxidane intermediatecan evolve heterolytically to release singlet oxygen and also decomposehomolytically, producing superoxide as the predominant radical species.For H2O2 decomposition by Ti- and Nb-substitutedPOMs, we also propose the formation of the TM-trioxidane key intermediate,finding good agreement with the observed trends in apparent activationenergies.