LITHIUM-7 AND PROTON NUCLEAR MAGNETIC RESONANCE SPECTRA OF METHYLLITHIUM/DIMETHYLMAGNESIUM (ZINC, CADMIUM) IN TETRAHYDROFURAN AND METHYLLITHIUM/DIMETHYLCADMIUM IN ETHER

Lithium-7 and proton nuclear magnetic resonance spectra of ether solutions of methyllithium with dimethylcadmium have been examined over a range of temperatures. The spectra show that, like the magnesium and zinc systems, complexes of the form Li2Cd(CH3)4 and Li3Cd(CH3)5 are formed. No evidence was obtained for or against the formation of 1/1 complex. In contrast to the magnesium and zinc systems, methyl group exchange between methyllithium and complex appears to be faster than lithium exchange; both are more rapid than in the zinc system. Lithium exchange is apparently rate-determined by the dissociation of methyllithium tetramer to dimers. The mechanism of methyl exchange is uncertain. Evidence for rapid exchange of methyl groups between molecules of Li2Cd(CH3)4 is discussed. Proton and lithium-7 nuclear magnetic resonance spectra of tetrahydrofuran (THF) solutions of methyllithium with dimethylmagnesium, dimethylzinc, and dimethylcadmium have been obtained over a wide temperature range. In contrast to ether solutions, no 3/1 complex is observed. The 2/1 complex predominates in the magnesium system, whereas spectra of the zinc system are consistent with an approximately equal molar mixture of 2/1 and 1/1 complexes. Finally, 1/1 complex predominates in the cadmium system. Lithium and methyl group exchange between methyllithium and complex in each system occur at comparable rates and activation energies; the rates increase in the order Mg < Zn < Cd. For magnesium and zinc, the exchange rates in THF and ether are similar, but for cadmium they are slower in THF. Exchange of methyl groups between (CH3)2M (M = Mg, Zn, Cd) and complex is very rapid. Evidence is presented for facile methyl group exchange between molecules of 1/1 complex in the cadmium system and between 2/1 and 1/1 complexes in the zinc system. © 1969.