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 m etallic surface. The surface reaction and the characterization of the grafted organotin complex have been followed by analytical methods, te mperature-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 metalli c Rh, and provided that the amount of Sn introduced represents less th an a monolayer on the metallic surface, the hydrogenolysis of Sn(n-C4H 9)4 occurs exclusively on the Rh particle. Only a C-13 NMR signal corr esponding to =SiOSn(n-C4H9)3 was observed on the silica surface when t he 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 stepwi se cleavage of Sn-alkyl bonds. For low Sn/Rh(s) ratios (<0.5) the reac tion 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 organometall ic fragment which can be formulated as Rh(s)[Sn(n-C4H9)2]y. Various po ssible structures have been proposed, taking into account the various results. Whereas Rh(s)[Sn(n-C4H9)2]y is the simplest possible model fo r this stable surface organometallic fragment, the results of molecula r modeling seem to rule out such a model (at least at high coverages) on the basis of steric constraints. Another model, which apparently fi ts the experimental results better, corresponds to the general formula (Rh(s))2Sn[Sn(n-C4H9)3]2.