A Hafnium-Substituted Lindqvist Polyoxotungstate: Synthesis, Characterization, and Catalysis of H₂O₂-Based Oxidations

In this work, we first synthesized a Hf-substituted Lindqvist-type dimeric polyoxotungstate, (ⁿBu₄N)₆[{HfW₅O₁₈(μ-OH)}₂] ({HfW₅}₂), and compared its synthetic availability, structure, and catalytic performance in H₂O₂-based oxidations with the zirconium analogue (ⁿBu₄N)₆[{ZrW₅O₁₈(μ-OH)}₂] ({ZrW₅}₂). The {HfW₅}₂ 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 {ZrW₅}₂. Despite the similar structures of {HfW₅}₂ and {ZrW₅}₂, 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 {ZrW₅}₂. In singlet-oxygen probe reactions (oxidation of tetramethylethylene and α-terpinene), {HfW₅}₂ 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.