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- (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 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 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.