A Hafnium-Substituted Lindqvist Polyoxotungstate: Synthesis, Characterization, and Catalysis of H 2 O 2 -Based Oxidations

In this work, we first synthesized a Hf-substituted Lindqvist-type dimeric polyoxotungstate, (nBu4N) 6HfW5O18 (μ-OH) 2 (HfW52), and compared its synthetic availability, structure, and catalytic performance in H2O2-based oxidations with the zirconium analogue (nBu4N) 6ZrW5O18 (μ-OH) 2 (ZrW52). The HfW52 dimer was characterized by single-crystal and powder X-ray diffraction analysis, elemental analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy (17O, 183W), and high-resolution electrospray ionization mass spectrometry, and benchmarked against ZrW52. Despite the similar structures of HfW52 and ZrW52, the former revealed a ca. 5 times higher rate of thioanisole oxidation with aqueous H2O2 and a lower rate of unproductive H2O2 dismutation as compared to ZrW52. In singlet-oxygen probe reactions (oxidation of tetramethylethylene and α-terpinene), HfW52 generated products typical of 1O2 involvement but with modestly lower yields and rates than those of the Zr analogue under identical conditions. These results demonstrate that replacing Zr (IV) with more oxophilic Hf (IV) in the Lindqvist tungstate meaningfully impacts the catalytic performance in H2O2-based oxidations.