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

Laxson, William W.
Özkar, Saim
Finke, Richard G.
Reproducible syntheses of high-purity [(n-C4H9)(4)N](9)P2W15Nb3O62 and, therefore, also the supported [(1,5-COD)Ir-I](+) organometallic precatalyst, [(n-C4H9)(4)N](5)Na-3(1,5-COD)Ir(P2W15Nb3O62), have historically proven quite challenging. In 2002, Hornstein et al. published an improved synthesis reporting 90% pure [(n-C4H9)(4)N](9)P2W15Nb3O62 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 P-31 NMR now more routinely possible. Herein we (1) verify problems in reproducing >= 90% purity [(n-C4H9)(4)N](9)P2W15Nb3O62, (2) determine three critical variables for the successful production of [(n-C4H9)(4)N](9)P2W15Nb3O62, (3) optimize the synthesis to achieve 91-94% pure [(n-C4H9)(4)N](9)P2W15Nb3O62, and (4) successfully reproduce and verify the synthesis via another researcher (Dr. Saim Ozkar) 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 Na-12[alpha-P2W15O56]center dot 18H(2)O that (previously) failed to remove the WO42- 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 alpha-P2W15O5612-, and (iii) incomplete removal of protons from the resultant [(n-C4H9)(4)N](9)P2W15Nb3O62 intermediate. These three insights have allowed improvement of the synthesis to a 91-94% final purity [(n-C4H9)(4)N](9)P2W15Nb3O62 product by high S/N P-31 NMR. Moreover, the synthesis provided both is very detailed and has been independently checked (by Dr. Ozkar) using only the written procedures. The finding that prior syntheses of Na-12[alpha-P2W15O56] are contaminated with WO42- 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 level of written reporting necessary to make such challenging polyoxometalate inorganic syntheses reproducible in the hands of others.