Effect of the Polyanion Structure on the Mechanism of Alcohol Oxidation with H 2 O 2 Catalyzed by Zr-Substituted Polyoxotungstates.

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 H-abstraction 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.