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 H2SiPt­W11O404– (1) and SiW12O404– (2). The former was separated from the latter by extraction into an aqueous solution and converted into tetra-n-butylammonium (TBA) and potassium salts TBA-1 and K-1. The α-H2SiPtW11O404– 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 TBA-1 and K-1 readily cocrystallized with their unsubstituted Keggin anion salts, TBA-2 and K-2, respectively, providing an explanation for the historical difficulty of isolating certain platinum-substituted heteropolyanions in pure form.