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/
B. Didillon et al., 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/, Journal of the American Chemical Society, 115(21), 1993, pp. 9380-9388
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.