Triniobium, Wells–Dawson-Type Polyoxoanion, [(<i>n</i>‑C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂: Improvements in the Synthesis, Its Reliability, the Purity of the Product, and the Detailed Synthetic Procedure

Abstract

Reproducible syntheses of high-purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ and, therefore, also the supported [(1,5-COD)­Ir^I]⁺ organometallic precatalyst, [(<i>n</i>-C₄H₉)₄N]₅Na₃(1,5-COD)­Ir­(P₂W₁₅Nb₃O₆₂), have historically proven quite challenging. In 2002, Hornstein et al. published an improved synthesis reporting 90% pure [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ in their hands. Unfortunately, 36 subsequent attempts to replicate that 2002 synthesis by four researchers in our laboratories produced material with an average purity of 82 ± 7%, albeit as judged by the improved S/N ³¹P NMR now more routinely possible. Herein we (1) verify problems in reproducing ≥90% purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, (2) determine three critical variables for the successful production of [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, (3) optimize the synthesis to achieve 91–94% pure [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂, and (4) successfully reproduce and verify the synthesis via another researcher (Dr. Saim Özkar) working only from the written procedure. The key variables underlying previously irreproducible syntheses are (i) a too-short and incomplete, insufficient volume washing step for Na₁₂[α-P₂W₁₅O₅₆]·18H₂O that (previously) failed to remove the WO₄^2– byproduct present, (ii) inadequate reaction time and the need for a slight excess of niobium­(V) during the incorporation of three niobium­(V) ions into α-P₂W₁₅O₅₆^12–, and (iii) incomplete removal of protons from the resultant [(<i>n</i>-C₄H₉)₄N]₅H₄P₂W₁₅Nb₃O₆₂ intermediate. These three insights have allowed improvement of the synthesis to a 91–94% final purity [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂ product by high S/N ³¹P NMR. Moreover, the synthesis provided both is very detailed and has been independently checked (by Dr. Özkar) <i>using only the written procedures</i>. The finding that prior syntheses of Na₁₂[α-P₂W₁₅O₅₆] are contaminated with WO₄^2– is one of the seemingly simple, but previously confounding, findings of the present work. An explicit check of the procedure is the second most important, more general feature of the present paper, namely, recognizing, discussing, and hopefully achieving a <i>level of written reporting</i> necessary to make such challenging polyoxometalate inorganic syntheses reproducible in the hands of others