SURFACE-MEDIATED SYNTHESIS OF METAL-CLUSTERS

Surface-mediated organometallic synthesis is a new method whereby the surface of a solid plays the role analogous to that of a solvent in conventional synthesis. The synthesis variables include the nature and loading of the metal salt or organometallic precursor adsorbed on the solid, the physical properties (e.g., surface area) of the solid, the chemical properties (e.g., acid-base properties) of the surface, the gas atmosphere, temperature, and pressure. The products synthesized on the surface are purified simply by extraction. Surface-mediated synthesis has been used to prepare numerous neutral and anionic metal carbonyl clusters and is an efficient method for preparation of [HOs3(CO)10(OH)], [H4Os4(CO)12], [Os5C(CO)14]2-, [Os10C(CO)24]2-, [Rh5(CO)15]-, and [Pt15(CO)30]2-. The latter four are prepared in one-step syntheses by reductive carbonylation taking place on the basic surface of porous MgO powder. Similar syntheses take place in the molecular-scale cages of zeolites, but removal of the products is usually hindered by their entrapment in the cages. There are parallels between surface and solution chemistry that are predicted roughly by a comparison of surface and solvent properties. For example, synthesis of some neutral metal carbonyl clusters, e.g., [H4Os4(CO)12] on SiO2 or on carbon, takes place as it does in a neutral solvent, and synthesis of some metal carbonyl cluster anions, e.g., [Os10C(CO)24]2- on MgO, takes place as it does in a basic solution in the presence of a reducing agent. Mobile mononuclear metal carbonyl fragments are postulated intermediates in the surface-mediated cluster syntheses.