Synthesis and Characterization of the Platinum-Substituted Keggin Anion α ‐ H 2 SiPtW 11 O 40 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− (1) and SiW12O40 4− (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 αH2SiPtW11O40 4− was identified as a monosubstituted Keggin anion using elemental analysis, IR spectroscopy, X-ray crystallography, electrospray ionization mass spectrometry, Pt NMR spectroscopy, W NMR spectroscopy, and W−W 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.