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The Lacunary Polyoxoanion Synthon α-P2W15O5612-:  An Investigation of the Key Variables in Its Synthesis plus Multiple Control Reactions Leading to a Reliable Synthesis

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journal contribution
posted on 23.04.2002, 00:00 by Brooks J. Hornstein, Richard G. Finke
A reliable way to determine the purity of the kinetically precipitated, noncrystalline lacunary polyoxoanion α-P2W15O5612- has been developed, namely, the conversion of α-P2W15O5612- into the tri-Nb5+- and V5+-containing polyoxoanions P2W15Nb3O629- and P2W15V3O629-, respectively, followed by quantitative analysis of their purity by 31P-NMR prior to recrystallization. With this previously unappreciated, straightforward α-P2W15O5612- purity-assessment methodology in hand, the five reported literature syntheses of α-P2W15O5612- are investigated with an emphasis on understanding the effects of the five differing variables within these syntheses (the amount of Na2CO3 base, the rate of addition of the base, the reaction temperature, the reaction scale, and the product drying method). Two methods of Nb5+ addition (Nb6O198- and NbCl5) to yield P2W15Nb3O629- are also evaluated, as is the issue of whether any purification is provided by the normally optimum strategy of first preparing a water-soluble salt and its crystallization from water (here the (CH3)4N+ salt of the Nb−O−Nb bridged anhydride, P4W30Nb6O12316-), followed by its conversion to the organic-solvent soluble, but noncrystalline, (n-C4H9)4N+ salt, [(n-C4H9)4N]9[P2W15Nb3O62]. The results yield five previously unavailable and unequivocal insights:  (1) Only the amount of added Na2CO3 base affects the purity or yield of the desired α-P2W15O5612-; the amount of added base is key, however. (2) Contant's 1990 Inorganic Syntheses procedure provides the highest-purity α-P2W15O5612- presently available. (3) All prior syntheses calling for the addition of base to P2W18O626- until pH 9 must be abandoned. (4) The purity of even Contant's α-P2W15O5612- is only 90%. (5) An identifiable impurity is the 16 tungsten polyoxoanion, α-P2W16O5912-. Also identified and summarized are multiple compounding errors in the observation of, reporting on, and thinking about the synthesis of α-P2W15O5612- historically, errors which delayed the most reliable synthesis of α-P2W15O5612- from being identified for 18 years (from the 1983 discovery of α-P2W15O5612-). However, these errors yield valuable take-home lessons for anyone interested in working in this demanding area of inorganic synthetic chemistry, where direct structural methods for identifying the products and their purity, such as the lacunary polyoxoanion synthon α-P2W15O5612-, sometimes simply do not exist.