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

Abstract

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Lindqvist (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ] (1), Keggin (Bu 4 N) 8 [{PW 11 O 39 Zr(μ-OH)} 2 ] (2), and Wells−Dawson (Bu 4 N) 11.3 K 2.5 H 0.2 [{P 2 W 17 O 61 Zr} 2 (μ-OH) 2 ](3) structures catalyze oxidation of alcohols using aqueous hydrogen peroxide as an oxidant. With 1 equiv of H 2 O 2 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 1 O 2 scavengers, and kinetic and spectroscopic (attenuated total reflectance-Fourier transform infrared (ATR-FT-IR), 31 P 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 Habstraction from the α−C-H bond by the Zr-hydroperoxo group, leading to a carbonyl compound. The unique capability of 1 to generate 1 O 2 upon interaction with H 2 O 2 complicates the reaction kinetics and improves the product yield. Spectroscopic studies coupled with stoichiometric experiments unveiled that dimeric monoperoxo {Zr 2 (μ-η 2 :η 2 -O 2 )} and monomeric hydroperoxo {Zr(η 2 -OOH)} species accomplish the transformation of alcohols to carbonyl compounds.