| The reduction of the polyoxoanion-supported complex [(n-C 4H9)4N]5Na3 [(1,5-COD)RhP2W15Nb3O62] (1,5-COD = 1,5-cyclooctadiene) in acetone, under hydrogen, and in the presence of 1.65 M cyclohexene, leads to the formation of polyoxoanion- and tetrabutylammonium-stabilized, near-monodisperse, 40 ± 6 Å Rh(0) nanoclusters. The Rh(0) nanoclusters are isolable as a black powder, which may be redissolved in nonaqueous solvents such as acetone, acetonitrile, or propylene carbonate. These nanoclusters are only the second example of polyoxoanion- and tetrabutylammonium-stabilized transition metal nanoclusters.; The Rh(0) nanoclusters are characterized by transmission electron microscopy, energy dispersive X-ray spectroscopy, electron diffraction, UV-Visible spectroscopy, elemental analysis, and ion-exchange chromatography. Hydrogen gas-uptake experiments and quantitative kinetic evidence indicate that the Rh(0) nanoclusters grow by a slow nucleation, then autocatalytic surface-growth mechanism. Further kinetic and transmission electron microscopy evidence is presented, indicating that a nanocluster diffusive agglomeration growth pathway is also present, a nanocluster growth pathway that is favored under hydrogen gas-to-solution mass-transfer limiting conditions.; The polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters are active and long-lived cyclohexene hydrogenation catalysts. The solution catalytic lifetime of the Rh(0) nanoclusters is investigated and compared to the catalytic lifetimes of [RhCl(PPh3)3], [(1,5-COD)Rh(CH 3CN)2]BF4, 5% Rh/Al2O3, and polyoxoanion- and tetrabutylammonium-stabilized Ir(0) nanoclusters. The catalytic lifetimes of the Rh(0) nanoclusters, ≥193,000 total turnovers, are an order of magnitude higher than any catalytic lifetime previously reported for a transition metal nanocluster in solution; and approach those of a solid-oxide-supported heterogeneous Rh(0) catalyst. Combined, the results provide evidence suggesting that the Rh(0) nanoclusters presented in this thesis are properly designated as “soluble analogs of heterogeneous catalysts”. |
