Surface Organometallic Chemistry of Tin: Reaction of Hydridotris(butyl)tin with the Surfaces of Partially Dehydroxylated Aluminas

Reaction of Bu₃SnH with the surface of partially dehydroxylated aluminas was followed by analysis of the evolved gases and infrared, ¹³C CP-MAS NMR, and ¹¹⁹Sn MAS NMR spectroscopies. At room temperature, the infrared and ¹³C CP-MAS NMR data suggest an initial interaction of Bu₃SnH with the hydroxyl groups of the η-alumina₍₅₀₀₎ surface via hydrogen-type bonding with the δ-CH₃ groups of the butyl ligand. The formation of the grafted entity >AlOSnBu₃ was accompanied by the release of 1 mol H₂ per mole of Sn. Data were obtained on α-, γ-, and η-aluminas dehydroxylated at either 200 or 500 °C. The various NMR data coupled with published data for molecular analogs indicate that the tin atoms can be tetra- or pentacoordinated on the alumina surface. ²⁷Al NMR is used to estimate the ratio of octahedral to tetrahedral aluminum atoms in various aluminas. Detailed study of the ¹¹⁹Sn NMR of the series of Sn/Al₂O₃ species revealed three basic types of tin coordination environments. Tin signals around 80 ppm present in some of the complexes are attributed to >Al0SnBu₃ or tetracoordinated tin. Peaks in the regions around -230 and -170 ppm are ascribed to a pentacoordinated tris(alkyl)tin fragment. The fifth ligand coordinated to tin may be either a hydroxyl group or a surface O^2- ion: formation of (> Al0)(> Al0H)Sn(n-C4H₉)₃ and of (>AlO)-(>AlO²~)Sn(n-C₄H₉)₃. The complexity of these resonances and the dependence thereof on the type of alumina used and the degree of dehydroxylation are attributed to the influence of the geometry of neighboring aluminum atoms on the tin chemical shift. These results apply the extreme sensitivity of tin chemical shifts to molecular enviroment, producing a method whereby surface organometallic complexes of tin can be used as molecular probes for determining surface structures of oxides. The ¹¹⁹Sn NMR is shown to be much more sensitive than other previously used spectroscopic techniques, such as IR, Raman, and ²⁹Al NMR.