Aggregation and Association of Lithium and Magnesium Organoaluminates in Tetrahydrofuran

Transmetalation of AlCl₃ with organolithium or Grignard reagents affords organoaluminates, which are of interest not only for synthetic applications but also for their use as novel electrolyte materials. Here, we employ a combination of electrospray-ionization mass spectrometry, electrical-conductivity measurements, and ¹H NMR spectroscopy to characterize the molecular constituents of lithium and magnesium aluminates formed by the treatment of AlCl₃ with n equiv of RLi or RMgCl, n = 1–4 and R = Me, ⁱPr, Bu, Hex, Cy, Ph, and 2-thienyl, in tetrahydrofuran. In all cases, we find the formation of aluminate ions of the type [R_nAlCl_4–n]⁻. These aluminates undergo only sluggish intermolecular exchange reactions of their organyl groups, which we attribute to the stability of the tetrahedral aluminate moiety and the reluctance of the latter toward dissociative and associative processes. Nonetheless, two of the [R_nAlCl_4–n]⁻ complexes can bind a Li⁺ counterion to furnish heteronuclear aggregates [LiR_2nAl₂Cl_8–2n]⁻. Analogous aggregates containing magnesium do not form, which reflects the lack of readily available mononuclear [MgCl]⁺ ions in tetrahydrofuran. Apart from intact organoaluminates, we also observe oxygen-containing species resulting from inadvertent oxidation and hydrolysis reactions with residual traces of oxygen and moisture.