The Classic Wells−Dawson Polyoxometalate, K6[α-P2W18O62]·14H2O. Answering an 88 Year-Old Question: What Is Its Preferred, Optimum Synthesis?

The 88-year-old problem of developing a preferred, optimized synthesis of the prototype Wells−Dawson polyoxometalate, K6[α-P2W18O62]·14H2O, is addressed herein. Specifically, six published syntheses of K6[α-P2W18O62]·14H2O are listed and discussed, with emphasis given to the two most recent syntheses, Nadjo and co-workers' 2004 synthesis and a 1997 Inorganic Syntheses procedure by Droege, Randall, Finke et al. (hereafter D-R-F). For the starting experiment, the synthesis by Nadjo and co-workers was repeated. Next, the D-R-F synthesis and then the earlier (1984) synthesis in Droege's Ph.D. thesis were repeated and reinvestigated. The results demonstrate that the Nadjo synthesis produces over 200 g of high α-isomer purity (≥97% by 31P NMR) K6[α-P2W18O62]·14H2O in four steps over 8 days in 93% yield in our hands. A recrystallization step added as part of this work (for a total of five steps over 12 days) produces an increase in purity (>99%) with a concomitant loss of 8% yield (i.e., 85% overall yield) for the Nadjo-plus-recrystallization synthesis. Next, the D-R-F Inorganic Syntheses procedure was reinvestigated to determine the cause of "failed syntheses" occasionally encountered in our laboratories, the most recent and worst example to date being when one of us (C.R.G.) found 150 g of K10[α2-P2W17O61] as an undesired side product when, as it turns out, the D-R-F Inorganic Syntheses procedure is followed rather than the earlier Droege synthesis. Specifically, it is shown that the problem in the Inorganic Syntheses procedure is that it ambiguously says to add 210 mL of HCl until a pH of 3–4 is reached when, in fact, it takes only 130–150 mL of HCL to reach a pH 3–4. Adding the full 210 mL of HCl ensures that a pH <2 is reached, as is required to produce isomerically pure K6[α-P2W18O62]·14H2O from the K10[α2-P2W17O61] intermediate. The result is K6[α-P2W18O62]·14H2O in five steps over 10 days in 82% yield and ≥97% purity. A table is provided comparing the details of the two best syntheses as reported herein: the Nadjo-plus-recrystallization synthesis and the D-R-F synthesis (with sufficient added HCl/proper pH control). That table makes apparent that the Nadjo-plus-recrystallization synthesis is improved on the basis of its better atom economy, its slightly higher product yields (85% vs 82%), slightly better purity (>99% vs >97%), and its comparable time (2 days shorter without recrystallization but 2 days longer with recrystallization) in comparison to the D-R-F synthesis with proper pH <2 control. Perhaps most importantly, some take-home messages concerning polyoxometalate synthesis illustrated by the iterative, 88 year-old quest to the best K6[α-P2W18O62]·14H2O synthesis are summarized and briefly discussed.