Effect of the Polyanion Structure on the Mechanism of Alcohol Oxidation with H2O2 Catalyzed by Zr-Substituted Polyoxotungstates

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Lindqvist (Bu4N)(6)[{W5O18Zr(mu-OH)}(2)] (1), Keggin (Bu4N)(8)[{PW11O39Zr(mu-OH)}(2)] (2), and Wells-Dawson (Bu4N)(11.3)K2.5H0.2[{P2W17O61Zr}(2)(mu-OH)(2)] (3) structures catalyze oxidation of alcohols using aqueous hydrogen peroxide as an oxidant. With 1 equiv of H2O2 and 1 mol % of Zr-POM, selectivity toward aldehydes and ketones varied from good to excellent, depending on the alcohol nature. Catalytic activity and attainable substrate conversions strongly depended on the Zr-POM structure and most often decreased in the order 1 > 2 >> 3. The reaction mechanism was probed using a test substrate, cyclobutanol, radical and O-1(2) scavengers, and kinetic and spectroscopic (attenuated total reflectance-Fourier transform infrared (ATR-FT-IR), P-31 NMR and electrospray ionization-mass spectrometry (ESI-MS)) tools. The results point to heterolytic alcohol oxidation in the presence of 1 and 2 and homolytic alcohol oxidation in the presence of 3. Kinetic and spectroscopic studies implicated an oxidation mechanism that involves both alcohol and peroxide binding to 2 followed by an inner-sphere heterolytic H-abstraction from the alpha-C-H bond by the Zr-hydroperoxo group, leading to a carbonyl compound. The unique capability of 1 to generate O-1(2) upon interaction with H2O2 complicates the reaction kinetics and improves the product yield. Spectroscopic studies coupled with stoichiometric experiments unveiled that dimeric monoperoxo {Zr-2(mu-eta(2):eta(2)-O-2)} and monomeric hydroperoxo {Zr(eta(2)-OOH)} species accomplish the transformation of alcohols to carbonyl compounds.