Association and Dissociation of Lithium Cyanocuprates in Ethereal Solvents

We use a combination of electrospray ionization mass spectrometry and electrical conductivity measurements to probe the ions present in; ethereal solutions of lithium cyanocuprates. Electrospray ionization mass spectrometry shows that solutions of LiCuR(2)center dot LiCN and Li(0.8)CuR(0.8)(CN) (R = Me, Et, (n)Bu, (s)Bu, (t)Bu, and Ph) in diethyl ether contain Li(n-1)Cu(n)R(2n)(-) and Li(n-1)Cu(n)R(n)(CN)(n)(-) anions, respectively. Analogous species are also observed for solutions of LiCu(t)Bu(2)center dot LiCN and Li(0.8)Cu(t)Bu(0.8)(CN) in 2-methyltetrahydrofuran, cyclopentyl methyl ether, and methyl tert-butyl ether and were previously found for solutions of lithium cyanocuprates in tetrahydrofuran. Although the change of solvent thus does not lead to the formation of any major new cuprate anions, it has a strong effect on the association/dissociation equilibria. As directly confirmed by the conductivity experiments, contact ion pairs strongly predominate in solutions of lithium cyanocuprates in diethyl ether, whereas the more polar tetrahydrofuran gives rise to larger amounts of solvent-separated ion pairs; a particularly high dissociation tendency is observed for the LiCu(t)Bu(2)center dot LiCN reagent. Temperature-variant conductivity measurements of LiCuPh(2)center dot LiCN solutions in tetrahydrofuran do not show a significant temperature dependence of the association/dissociation equilibria for this system. The present findings largely support the results of previous NMR spectroscopic studies and help to explain how the solvent affects the reactivity of lithium cyanocuprates.