A proton NMR study of solvent effects on the nature and reactivity of alkali metal trimethyltin derivatives

Abstract

Variable temperature NMR studies on solutions of LiSnMe, and KSnMe 3 indicate that an equilibrium between contact and solvent separated ion pairs occurs for these species. Solvents investigated, in order of decreasing ability to solvate the alkali metal ion are: hexamethylphosphoramide (HMPA) > 1,2-dimethoxyethane (DME) > bis(β-ethoxyethyl) ether (BEEE) > tetrahydrofuran (THF), with the equilibrium shifted strongly to the right by HMPA and strongly to the left by THF. The addition of two moles of HMPA per mole of lithium to a THF solution of LiSnMe 3 shifts the equilibrium strongly to the right. A shift to the right also is observed on lowering the temperature; this effect is greatest for DME solutions. Variations in 2 J(SnCH) for the MSnMe 3 species, as well as available IR data, give evidence that the structure of the SnC 3 fragment tends increasingly towards planarity with increasing solvation of the alkali metal. Unlike the carbon and silicon analogs, the trimethylstannyllithium species does not appear to attack the solvent on heating but rather rearrange to form SnMe 4 and LiSn(SnMe 3) 3. The rate of thermal decomposition of LiSnMe 3 in THF and the yield of LiSn(SnMe 3) 3 both are increased by the addition of 0.5 to 2 moles of HMPA per mole of lithium.