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 Å, Ir(0)∼300 nanoclusters. These nanoscopic materials are synthesized from the reduction of [(C4H9)4N]5Na3[(1,5-COD)M·P2W15Nb3O62] (M = Ir) by H2 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 = Rh precursor, and under conditions otherwise identical to the M = Ir synthesis, led to polydisperse 16 to 116 Å Rh(0) nanoclusters. The above results led, in turn, to the discovery that H2 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 Å 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.