Surface Organometallic Chemistry of Tin: Thermal Transformation, under Vacuum, of ⪫Si-O-Sn(n-Bu)₃ Supported on Silica

The reactivity of the well-defined surface organometallic fragment ⪫Si-O-Sn(n-C₄H₉)₃1 grafted on silica₂₀₀ and on silica₅₀₀ has been studied by thermal treatment of 1 at increasing temperatures in vacuo. The surface reactions have been followed by quantitative measurements of the evolved gases, infrared and Mössbauer spectroscopies, ¹³C CP-MAS and ¹¹⁹Sn NMR spectroscopy, XPS measurements, and electron microscopy (CTEM and STEM EDAX). On both types of silicas, the surface reactions are similar in nature, although differences are noticeable. First, there is formation of (⪫Si-O)₂(Sn(n-C₄H₉)₂) 2, which undergoes a second solvolysis process by silanols leading to (⪫Si-O)₃Sn(n-C₄H₉) 3 and finally surface Sn(II) and Sn(IV) atoms (as determined by XPS and Mdssbauer experiments). Although the well-defined surface organometallic compound (⪫Si-O)₂Sn(n-C₄H₉)₂ can be prepared on silica by another route, no unique surface compound can be obtained during the thermal decomposition which transforms progressively 1 into 2 and 3. A mechanism of decomposition of the various surface organometallic complexes has been deduced from a comparison of the results obtained on both solids. The alkyl groups seem to follow a β-H elimination mechanism leading to tin hydrides and 1-butene rather than a disproportionation mechanism leading to equimolar amounts of 1-butene and butane.