Nanocluster formation synthetic, kinetic, and mechanistic studies.: The detection of, and then methods to avoid, hydrogen mass-transfer limitations in the synthesis of polyoxoanion- and tetrabutylammonium-stabilized, near-monodisperse 40±6 Å Rh(0) nanoclusters

Previously we reported P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilized, 20 +/- 3 Angstrom, Ir(0),(similar to 300) nanoclusters. These nanoscopic materials are synthesized from the reduction of [(C4H9)(4)N](5)Na-3[(1,5-COD)M . P2W15Nb3O62] (M = Ir) by H-2 in acetone and are isolable, highly catalytically active, with an unprecedented catalytic lifetime in solution. However, an initial attempt to synthesize Rh(0) nanoclusters from the analogous M = Ph precursor, and under conditions otherwise identical to the M = Ir synthesis, led to polydisperse 16 to 116 Angstrom Rh(0) nanoclusters. The above results led, in turn, to the discovery that H-2 gas-to-solution mass-transfer limitations (MTL) were responsible for the failure of the initial synthesis, an important finding since H2 is one of the most common reducing agents in syntheses of modern, near-monodisperse transition metal nanoclusters. The finding of a hydrogen MTL regime is fortified by stirring-rate-dependence data, kinetic data [demonstrating a catalyst nondependent (MTL) regime, and a catalyst-dependent (chemical-reaction rate-limiting) regime], and transmission electron microscopy used as a mechanistic probe. The results provided evidence for a growth mechanism involving parallel autocatalytic surface growth (leading to near-monodisperse nanoclusters) in competition with diffusive agglomeration (leading to polydisperse nanoclusters). The above mechanistic insights were then used, in turn, to design conditions where only the autocatalytic surface-growth pathway occurs, conditions which led to the successful synthesis of the desired near-monodisperse, 40 +/- 6 Rh(0) P2W15Nb3O629- polyoxoanion- and Bu4N+-stabilized nanoclusters. The resultant Rh(0) nanoclusters are only the second example of polyoxoanion- and Bu4N+-stabilized transition metal nanoclusters.