Direct insight into the ion pair equilibria of lithium organocuprates by H-1,Li-6 HOESY experiments

An equilibrium between a monomeric solvent-separated ion pair (SSIP) and a contact ion pair (CIP) is observed directly for a representative Lithium di organocuprate, Me2CuLi . LiCN, in THF, using the H-1,Li-6 HOESY technique. Interestingly, crystal structures of related systems are also of the SSIP a nd the CIP type, whereby the latter shows a dimer as the fundamental struct ural element and the structure type depends on the Li+ solvating capability of the solvent. In crystal structures of CIPs the shortest lithium-alpha-c arbon distances are around 220 pm, which should lead to a strong dipolar in teraction. Indeed, for the salt-free Me2CuLi in Et2O a strong cross-peak be tween lithium and the CH3 groups of the cuprate is seen in the H-1,Li-6 HOE SY spectrum, indicating that the main species in solution is a CIP. In cont rast, the crystal structures of SSIPs show that the distance between lithiu m and the organic moiety of the cuprate is too long to lead to any dipolar interaction (shortest lithium-alpha-carbon distances longer than 530 pm). T his is confirmed by the H-1,Li-6 HOESY spectra of MeCu(CN)Li and t-Bu2CuLi . LiCN in THF. However, deviating from the pure SSIP structures, a weak dip olar interaction between lithium and the CH3 groups could be observed for M e2CuLi . LiCN and Me2CuLi in THF, which was attributed to a direct dipolar interaction. The magnitude of this dipolar interaction was used to identify an equilibrium between the SSIP and the CIP of Me2CuLi . LiCN in THF. At 2 13 K the dominant species in THF is the SSIP with some contributions of the CIP. As expected, this equilibrium could be shifted at lower temperatures toward the SSIP. It is demonstrated that the H-1,Li-6 HOESY technique can b e used to get direct insight into the structural features of Lithium diorga nocuprates in solution. which is of great significance for their reactivity .