Effect of\nthe Polyanion Structure on the Mechanism\nof Alcohol Oxidation with H<sub>2</sub>O<sub>2</sub> Catalyzed by\nZr-Substituted Polyoxotungstates

Zr-monosubstituted polyoxometalates (Zr-POMs) of the\nLindqvist\n(Bu<sub>4</sub>N)<sub>6</sub>[{W<sub>5</sub>O<sub>18</sub>Zr(μ-OH)}<sub>2</sub>] (<b>1</b>), Keggin (Bu<sub>4</sub>N)<sub>8</sub>[{PW<sub>11</sub>O<sub>39</sub>Zr(μ-OH)}<sub>2</sub>] (<b>2</b>), and Wells–Dawson (Bu<sub>4</sub>N)<sub>11.3</sub>K<sub>2.5</sub>H<sub>0.2</sub>[{P<sub>2</sub>W<sub>17</sub>O<sub>61</sub>Zr}<sub>2</sub>(μ-OH)<sub>2</sub>] (<b>3</b>) structures\ncatalyze oxidation of alcohols using aqueous hydrogen peroxide as\nan oxidant. With 1 equiv of H<sub>2</sub>O<sub>2</sub> and 1 mol %\nof Zr-POM, selectivity toward aldehydes and ketones varied from good\nto excellent, depending on the alcohol nature. Catalytic activity\nand attainable substrate conversions strongly depended on the Zr-POM\nstructure and most often decreased in the order <b>1</b> > <b>2</b> ≫ <b>3</b>. The reaction mechanism was probed\nusing a test substrate, cyclobutanol, radical and <sup>1</sup>O<sub>2</sub> scavengers, and kinetic and spectroscopic (attenuated total\nreflectance-Fourier transform infrared (ATR-FT-IR), <sup>31</sup>P\nNMR and electrospray ionization-mass spectrometry (ESI-MS)) tools.\nThe results point to heterolytic alcohol oxidation in the presence\nof <b>1</b> and <b>2</b> and homolytic alcohol oxidation\nin the presence of <b>3</b>. Kinetic and spectroscopic studies\nimplicated an oxidation mechanism that involves both alcohol and peroxide\nbinding to <b>2</b> followed by an inner-sphere heterolytic\nH-abstraction from the α–C-H bond by the Zr-hydroperoxo\ngroup, leading to a carbonyl compound. The unique capability of <b>1</b> to generate <sup>1</sup>O<sub>2</sub> upon interaction with\nH<sub>2</sub>O<sub>2</sub> complicates the reaction kinetics and improves\nthe product yield. Spectroscopic studies coupled with stoichiometric\nexperiments unveiled that dimeric monoperoxo {Zr<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-O<sub>2</sub>)} and monomeric hydroperoxo\n{Zr(η<sup>2</sup>-OOH)} species accomplish the transformation\nof alcohols to carbonyl compounds.