Surface organometallic chemistry: some fundamental features including the coordination effects of the support

Abstract

The study of the various kinds of reactions between organometallic complexes and the surfaces of inorganic oxides, metals or zeolites constitutes a new aspect of the coordination chemistry on surfaces. In this non-exhaustive and short review article, we would like to try to answer a few questions regarding this area of coordination (or organometallic chemistry) which may have some future impact in the field of catalysis. The questions that we would like to answer are the following: Are the basic rules of molecular organometallic and coordination chemistry valid when one tries to apply them to surfaces? One can wonder whether or not the functionalities which are present at the surface of an oxide, M x O y (M–OH groups, strained M–O–M groups and MO, aso) have a chemical reactivity which can be predicted on the basis of molecular chemistry. A few selected examples will be given about the reactivity of tin, rhenium or zirconium alkyls with the silanol groups of partially dehydroxylated silica. Can we obtain reliable and precise informations when some selected tools of surface science and molecular organometallic chemistry are applied simultaneously to elucidate the structure of surface organometallic fragments? One can reasonably expect that the way the surface organometallic fragments coordinate to the surface can be rationalized on the simple rules of coordination chemistry (electron counting, formal oxidation state). A few examples will be given regarding the surface structure of silica-supported zirconium hydrides or rhodium allyls. Is it possible that a well chosen surface organometallic fragment represents an intermediate in heterogeneous catalysis? If one can study the reactivity of a well chosen surface organometallic fragment, then one is in a position to demonstrate some elementary steps of heterogeneous catalysis. In this review we shall consider the surface reactivity of supported rhodium allyls or tin alkyls. What kind of mobility can we expect from surface organometallic fragments? In sharp contrast with discrete ligands of molecular chemistry, surfaces of oxides obviously provide a so called “pool of oxygens” which binds the surface organometallic fragments in a localized manner. However, due to its almost infinite structure, such a “pool” is obviously responsible for surface mobility, which is also a key parameter in certain catalytic processes (sintering, diffusion processes, reconstructions, leaching,⋯). Examples will be given on the mobility of Rh I(CO) 2 grafted onto a silica surface. The organometallic fragments are also mobile around the metal carbon bonds and this phenomenon can be evidenced by solid-state nuclear magnetic resonance (NMR) and can have applications in molecular separations and on the reactivity of the organometallic complexes. In each case, the role of the support as a coordinating ligand is a key factor of this chemistry.