J . Org. Chem. 1995.60, 4310-4311
4310
.. Communzcatzons N
Structural Characterization of Organocopper Complexes by EXAFS and XANES: Evidence That Cyanide Does Not Coordinate to Cu in Dimethyl Cuprate S o 1utions
THF
5 ......Li4 ..............Me Me. CU-CN Md
Terence M. Bamhart, Hui Huang, and James E. Penner-Hahn* Department of Chemistry. The University of MichiRon. Ann Arhor. Michigan 48109 Rewiwd March 14. 1995
The addition of cyanide ion to dialkyl cuprate solutions (e.g. MenCuLil leads to altered reactivity compared to cuprate solutions prepared without cyanide. This effect has been used in hundreds of syntheses and has been ascribed to formation of a three-coordinate Cu-cyanide complex1.' known as a "higher-order" cuprate, 1 (see Fimre 11. Recently. the existence of a 0-coordinated cyanide in 1 has been questioned. . NMR data have not shown the "C chemical shift differences expected for a coordinated cyanide complex.' Three-coordinate Cu complexes do not represent energetic minima in ab initio structure calculations of organocyanocuprates.' Consistent with these findings. extended X-ray absorption fine structure I EXAFSi spectra of CuCN ZBULi showed no second shell Cu...N ICUCEN.backscattering.' Moreover, the Cu-C bond length and apparent coordination numfrom the Of CuCN + 2BuLi were consistent with a two-coordinate complex, but inconsistent with a three coordinate complex. These data suggested that the dominant species in solutions of CuCN + 2BuLi was best formulated as BurCuLi.LiCN, where the precise nature of "LiCN" is not specified. Recently, Lipschutz and Jamesfihave addressed these inconsistencies by suggesting that "higher-order" cuprate effects may be due to a x-complexed, rather than a o-complexed cyanide ligand, 2 (Figure 1). While there are no crystallographically characterized examples of r-only coordinated cyanides, such a species would lack the second shell EXAFS from Cu. ..N (CuC=N) scattering and thus might be consistent with published data. Lipschutz and James further suggest that EXAFS "has not been rigorously proven for tri- and tetracoordinate Cu(I) species" which, while not true in general,' is true Author t o whom correspondence should he addressed. For reviews of cyanowprate chemistry. see: ( a i Lirhutz. B. H.: S e n ~ p t aS.. Or#. Rrocf. 1992.41. 135.ihi Lipshutz. B. H. Svnlrtt 1990. I 1,
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