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

Abstract

The cyanocuprate reagents, prepared from CuCN + 7 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. CuCN{center_dot}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 A, 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 stablemore » structure, the cyanide is associated with the alkyl groups that are bound to the Cu. 76 refs., 6 figs., 1 tab.« less