Direct Insight into the Ion Pair Equilibria of Lithium Organocuprates by1H,6Li HOESY Experiments

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Abstract

An equilibrium between a monomeric solvent-separated ion pair (SSIP) and a contact ion pair (CIP) is observed directly for a representative lithium diorganocuprate, Me2CuLi·LiCN, in THF, using the 1H,6Li HOESY technique. Interestingly, crystal structures of related systems are also of the SSIP and the CIP type, whereby the latter shows a dimer as the fundamental structural element and the structure type depends on the Li+ solvating capability of the solvent. In crystal structures of CIPs the shortest lithium−α-carbon distances are around 220 pm, which should lead to a strong dipolar interaction. Indeed, for the salt-free Me2CuLi in Et2O a strong cross-peak between lithium and the CH3 groups of the cuprate is seen in the 1H,6Li HOESY spectrum, indicating that the main species in solution is a CIP. In contrast, the crystal structures of SSIPs show that the distance between lithium and the organic moiety of the cuprate is too long to lead to any dipolar interaction (shortest lithium−α-carbon distances longer than 530 pm). This is confirmed by the 1H,6Li HOESY spectra of MeCu(CN)Li and t-Bu2CuLi·LiCN in THF. However, deviating from the pure SSIP structures, a weak dipolar interaction between lithium and the CH3 groups could be observed for Me2CuLi·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 213 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 1H,6Li HOESY technique can be used to get direct insight into the structural features of lithium diorganocuprates in solution, which is of great significance for their reactivity.

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