SURFACE ORGANOMETALLIC CHEMISTRY ON METALS - EVIDENCE FOR A NEW SURFACE ORGANOMETALLIC MATERIAL, RH[SN(N-C4H9)X]Y/SIO2, OBTAINED BY CONTROLLED HYDROGENOLYSIS OF SN(N-C4H9)4 ON A RH/SIO2 CATALYST

Selective hydrogenolysis of Sn(n-C4H9)4 on a Rh/SiO2 catalyst has been carried out at various temperatures and at various coverages of the metallic surface. The surface reaction and the characterization of the grafted organotin complex have been followed by analytical methods, temperature-programmed reaction, electron microscopy, XPS, IR, Mossbauer spectroscopy, C-13 CP-MAS solid-state NMR. At room temperature and in the absence of metallic Rh, Sn(n-C4H9)4 is simply physisorbed on the silica surface and can be easily extracted. In the presence of metallic Rh, and provided that the amount of Sn introduced represents less than a monolayer on the metallic surface, the hydrogenolysis of Sn(n-C4H9)4 occurs exclusively on the Rh particle. Only a C-13 NMR signal corresponding to =SiOSn(n-C4H9)3 was observed on the silica surface when the amount of Sn introduced was higher than ca. one monolayer on the Rh particle. Sn remains on this metallic phase even after hydrogenolysis, as demonstrated by STEM-EDAX experiments. Gas-phase evolution (TPR) and infrared studies show that the hydrogenolysis proceeds by a stepwise cleavage of Sn-alkyl bonds. For low Sn/Rh(s) ratios (<0.5) the reaction is continuous with formation of surface tin alkyl complexes with various degrees of substitution. For higher Sn/Rh(s) ratios, there is formation of a well-defined and relatively stable surface organometallic fragment which can be formulated as Rh(s)[Sn(n-C4H9)2]y. Various possible structures have been proposed, taking into account the various results. Whereas Rh(s)[Sn(n-C4H9)2]y is the simplest possible model for this stable surface organometallic fragment, the results of molecular modeling seem to rule out such a model (at least at high coverages) on the basis of steric constraints. Another model, which apparently fits the experimental results better, corresponds to the general formula (Rh(s))2Sn[Sn(n-C4H9)3]2.