Surface organometallic chemistry of tin: Reactivity of two chiral organostannics, ((-)-menthyl)Me₃Sn and ((-)-menthyl)Me₂SnH, toward silica

Two chiral organostannics, the tetraalkyl ((-)-menthyl)Me₃Sn (1) and the hydride ((-)-menthyl)Me₂SnH (2), were synthesized. Their reactions with the surface of partially dehydroxylated silica were followed by analysis of evolved gases, IR, and ¹³C and ¹¹⁹Sn MAS NMR spectroscopic characterizations of the grafted organometallics. With both organostannics, a well-defined chiral surface organometallic compound, ≡Si-O-Sn((-)-menthyl)Me₂ (3), is obtained from the reaction of 1 mol of organotin complex with 1 mol of silica surface hydroxyl groups, with formation of 1 mol of methane (from 1) or dihydrogen (from 2). Side reactions leading to nonchiral species, such as ≡Si-O-SnMe₃ (4) or involving two surface silanol groups for one organotin molecule, giving for instance (≡Si-O)₂Sn((-)-menthyl)Me (5), also occur limiting thus the selectivity of the grafting reaction toward 3. The amount of grafted tin and the nature of the grafted tin species depend on the type of organostannic and on temperature. When using the tetraalkyl ((-)-menthyl)Me₃Sn as a precursor, temperatures higher than 100°C are required in order to graft tin onto the silica surface. Although the amount of grafted tin increases with temperature, the selectivity of the reaction toward 3 remains constant at 85% for temperatures lower than 160°C and decreases at higher temperatures. With ((-)-menthyl)Me₂SnH, tin grafting occurs already at room temperature, but a temperature of 80°C allows one to graft high tin amounts (4-6 wt % Sn) within a reasonable time, with 95-100% of the grafted tin present as 3 on the silica surface. The reaction is less selective at 100°C. The chiral surface organometallic species 3 is thermally stable up to 160-200°C.