Ranking the Lacunary (Bu4N)9{H[(α2-P2W17O61]} Polyoxometalate's Stabilizing Ability for Ir(0)n Nanocluster Formation and Stabilization Using the Five-Criteria Method Plus Necessary Control Experiments

The primary goal of the present studies is to rank the monoprotonated, lacunary Wells-Dawson-type polyoxometalate {H[(alpha(2)-P2W17O61]}(9-) as a stabilizing anion for the formation and subsequent stabilization of Ir(0)(n) nanoclusters in acetone using the five-criteria method developed previously (Ozkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796). A related goal is to compare this potentially tetradentate, three W-O- plus one W-OH ligand {H[alpha(2)-P2W17O61]}(9-) system to the nanocluster formation and stabilization abilities of the present "gold standard" polyoxometalate, [P2W15Nb3O62](9-), with its established tridentate, three Nb-O-Nb ligating system. A comprehensive table of 54 references at present examining polyoxometalates (POMs) as additives/stabilizers of nanoclusters is also provided as Table S1 of the Supporting Information. To accomplish the above-noted two main goals, the organic-solvent-soluble tetrabutylammonium salts, (Bu4N)(8.4){H-1.6[alpha(2)-P2W17O61]} center dot 1.4H(2)O, (Bu4N)(9){H[alpha(2)-P2W17O61]}, and (Bu4N)(9)[P2W15Nb3O62] were prepared and their basic structures (by IR) and purity (by P-31 NMR, plus elemental analysis where appropriate) were determined. The parent Wells-Dawson POM (Bu4N)(6)[alpha-P2W18O62] was also prepared, characterized, and then used as a control of a polyoxometalate with little surface anionic charge density, one therefore expected to be a poor stabilizer for at least metal(0)(n), overall neutral core, nanoclusters. Also prepared and characterized was (Bu4N)(4){H-3[PW11O39]}, but its attempted deprotonation by (Bu4N)OH with direct P-31 NMR monitoring did not yield a clean product by P-31 NMR, so studies with this second lacunary POM were deemphasized. The resultant POMs of known composition, protonation state, and thus overall charge were then evaluated by the five criteria-the one presently available method-for their ability to promote the kinetically controlled formation, stabilization, and subsequent catalytic activity of prototype Ir(0)(n) nanoclusters. A number of additional control experiments necessary to provide confidence in the results are also reported. One main finding is that the efficacy of the POMs studied herein, as stabilizers for Ir(0), nanoclusters in acetone solvent, is [P2W15Nb3O62](9-) > {H[alpha(2)-P2W17O61)}(9-) >> [alpha-P2W18O61](6-) similar to {H-3[PW11O39]}(4-), the potentially tetradentate, lacunary {H[alpha(2)-P2W17O61]}(9-) proving somewhat less efficacious as a stabilizer than the tridentate Nb-O-Nb containing [P2W15Nb3O62]9-POM. Another finding is that the degree of protonation and the overall charge of the POM matter, the more highly charged {H[alpha(2)-P2W17O61}(9-) POM being a better stabilizer then the more protonated, less charged {H-2[alpha(2)-P2W17O61]}(8-). Two additional important findings are better insights into the inherent errors underlying three of the five criteria, and thus the use of the five-criteria method itself, and further evidence supporting the hypothesis that future studies of direct measurements of nanocluster agglomeration rate constants k(3) and k(4) (Ott, L. S.; Finke, R. G. Chem. Mater. 2008, 20, 2592-2601) should prove valuable.