Polyoxoanion- and Tetrabutylammonium-Stabilized, Near-Monodisperse, 40 ± 6 Å Rh(0)_∼₁₅₀₀to Rh(0)_∼₃₇₀₀Nanoclusters: Synthesis, Characterization, and Hydrogenation Catalysis

Abstract

Polyoxoanion- and tetrabutylammonium-stabilized near-monodisperse 40 ± 6 Å Rh(0)∼1500 to Rh(0)∼3700 nanoclusters have been prepared by hydrogen reduction, in acetone, of a polyoxoanion-supported Rh(I) complex, [(n-C4H9)4N]5Na3[(1,5-COD)Rh·P2W15Nb3O62], a reaction in which the resultant Rh(0) nanoclusters serve as a cyclohexene hydrogenation catalyst. The Rh(0)∼1500 to Rh(0)∼3700 nanoclusters are isolated as a black powder that can be fully redispersed in non-aqueous solvents such as acetonitrile; they have been characterized by transmission electron microscopy, energy dispersive spectroscopy, electron diffraction, UV−vis spectroscopy, and elemental analysis. Ion-exchange chromatography shows that the isolated Rh(0) nanoclusters are stabilized by the adsorption of the polyoxoanion onto their outer surfaces. Hydrogen gas-uptake stoichiometry, in combination with quantitative kinetic evidence, is presented, indicating that the nanoclusters grow by the slow nucleation, then fast autocatalytic surface-growth, mechanism recently reported for their Ir(0)∼300 congeners. The isolated Rh(0) nanoclusters are also active cyclohexene hydrogenation catalysts in solution; Hg(0) poisoning experiments confirm that the Rh(0) nanoclusters are the active catalyst. These are only the second example of polyoxoanion-stabilized transition-metal nanoclusters; their availability, as well as the record stability and catalytic lifetime in solution of polyoxoanion-stabilized transition-metal nanoclusters, makes possible both fundamental and practical investigations of Rh(0) metal−particle catalysis in solution.