Polyoxoanion- and tetrabutylammonium-stabilized, near-monodisperse, 40±6 Å Rh(0)∼1500 to Rh(0)∼3700 nanoclusters:: Synthesis, characterization, and hydrogenation catalysis

Polyoxoanion- and tetrabutylammonium-stabilized near-monodisperse 40 +/- 6 Angstrom Rh(0)(similar to 1500) to Rh(0)(similar to 3700) nanoclusters have been prepared by hydrogen reduction, in acetone, of a polyoxoanion-supported Rh(I) complex, [(n-C4H9)(4)N](5)Na-3[(1,5-COD)Rh . P2W15Nb3O62], a reaction in which the resultant Rh(0) nanoclusters serve as a cyclohexene hydrogenation catalyst. The Rh(0)(similar to 1500) to Rh(0)(similar to 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)(similar to 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.