Solvation and Structure of LiAlH4 in Ethereal Solvents

The nature of the solute species present in ethereal solutions of LiAlH4 is of crucial importance for understanding the mechanisms for the reduction of ketones and other functional groups by LiAlH4. We have employed a combination of theoretical and experimental techniques to investigate the structure of LiAlH4 in ethereal solutions. Using complexation agents, we measured the IR spectra of LiAlH4 and AlH4- in tetrahydrofuran (THF). Hybrid quantum-classical (QM-MM) simulations have also been carried out to compute the IR spectra of associated and dissociated LiAlH4 species and the free-energy profile for the dissociation process in solution. Our experimental estimate of the dissociation constant in THF is 0.021 (plus-minus) 0.002, while the predicted computational value corresponding to a model dimethyl ether solvent is 0.001. The free-energy profile shows only one minimum corresponding to a contact ion pair at a Li-Al separation distance of 3.0 (angstrom). These results are consistent with the fact that LiAlH4 is essentially associated in ethereal solutions forming contact ion pairs.