Synthesis and Characterization of the Platinum-Substituted Keggin Anion α‑H₂SiPt­W₁₁O₄₀^4–

Acidification of an aqueous solution of K₈SiW₁₁O₃₉ and K₂Pt­(OH)₆ to pH 4 followed by addition of excess tetramethylammonium (TMA) chloride yielded a solid mixture of TMA salts of H₂SiPt­W₁₁O₄₀^4– (<b>1</b>) and SiW₁₂O₄₀^4– (<b>2</b>). The former was separated from the latter by extraction into an aqueous solution and converted into tetra-<i>n</i>-butylammonium (TBA) and potassium salts <b>TBA-1</b> and <b>K-1</b>. The α-H₂SiPtW₁₁O₄₀^4– was identified as a monosubstituted Keggin anion using elemental analysis, IR spectroscopy, X-ray crystallography, electrospray ionization mass spectrometry, ¹⁹⁵Pt NMR spectroscopy, ¹⁸³W NMR spectroscopy, and ¹⁸³W–¹⁸³W 2D INADEQUATE NMR spectroscopy. Both <b>TBA-1</b> and <b>K-1</b> readily cocrystallized with their unsubstituted Keggin anion salts, <b>TBA-2</b> and <b>K-2</b>, respectively, providing an explanation for the historical difficulty of isolating certain platinum-substituted heteropolyanions in pure form