Structural Characterization of Organocuprate Reagents. EXAFS Spectroscopy and ab Initio Calculations

The cyanocuprate reagents, prepared from CuCN + 2 equiv of organolithium, appear to possess both unique spectroscopic and unique reactivity properties in comparison with the analogous reagents prepared from CuX (X = Cl, Br, I). We have used a combination of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, together with ab initio structure calculations, to explore the structure of the Cu in these reagents. The XANES measurements support earlier, preliminary EXAFS indications that cyanide is not coordinated to Cu in the so-called “higher-order” cuprate prepared from CuCN + 2BuLi. Multiple scattering analyses of the EXAFS data have been used to characterize both the Cu-nearest neighbor and the Cu-outer shell interactions as a function of added organolithitim. CuCN•2LiCl exists in THF as an oligomer of •••Cu-C=N-Cu••• units. The average Cu environment consists of two coordinated cyanides with the possible presence of a third ligand for some of the Cu sites. CuCN + BuLi and CuCN + 2BuLi both contain two-coordinate Cu, with average Cu—C bond lengths of 1.89 and 1.93 Å, respectively. One cyanide remains coordinated to Cu in CuCN + BuLi, while both cyanides are displaced in CuCN + 2BuLi. Ab initio structure calculations are consistent with this picture, even to the extent of predicting the correct average Cu bond lengths. The theoretical calculations demonstrate that, in the most stable structure, the cyanide is associated with the alkyl groups that are bound to the Cu. This provides an explanation for the fact that, although the cyanide in CuCN + 2RLi is not coordinated to Cu, it nevertheless possess unique NMR properties. The cyanide—alkyl interaction may play a role in controlling the reactivity of the cyanocuprate reagents.