Me(2)CuLi*LiCN in diethyl ether prefers a homodimeric core structure [Me(2)CuLi](2) and not a heterodimeric one [Me(2)CuLi*LiCN]: (1)H, (6)Li HOE and (1)H, (1)H NOE studies by NMR

Abstract

H−Li distances and 1H−1H dipolar interactions in Me2CuLi*LiCN and Me2CuLi in diethyl ether (Et2O), obtained by NMR spectroscopy, were used to gain structural information about the contact ion pair of the salt-containing organocuprate Me2CuLi*LiCN in this solvent. The H−Li distances of Me2CuLi*LiCN and Me2CuLi in Et2O, resulting from the initial buildup rates in conjunction with the motional correlation times, are almost identical, indicating a similar homodimeric core structure [Me2CuLi]2 for both samples. However, the H−Li distances obtained for Me2CuLi*LiCN do not rigorously exclude a heterodimeric structure [Me2CuLi*LiCN] as proposed by ab initio calculations. Therefore, 1H−1H dipolar interactions were investigated by SYM-BREAK-NOE/ROE-HSQC experiments, which allow for the observation of NOEs between equivalent protons. Since these experiments showed similar 1H−1H dipolar interactions of Me2CuLi*LiCN and Me2CuLi, we propose that for Me2CuLi*LiCN a homodimeric core structure [Me2CuLi]2 indeed is predominant in Et2O.