Infrared Spectroscopic Characterization of Cyanocuprates

• Published in: Journal of the American Chemical Society 1996-09, Vol.118 (37), p.8808-8816
• Main Authors: Huang, Hui, Alvarez, Khris, Lui, Qiang, Barnhart, Terence M, Snyder, James P, Penner-Hahn, James E.
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja961862w

The synthetically useful cyanocuprates MeCu(CN)Li (2) and “Me2CuLi·LiCN” (3) have been characterized by infrared spectroscopy. Titration of CuCN·2LiCl (1) with MeLi in THF solution has established that 2 has an IR absorption due to cyanide stretching at 2133 cm-1 with ε = 2.3 × 103 cm-2 M-1, while the CN stretching frequency for 3 occurs at 2115 cm-1 with ε = 4.0 × 102 cm-2 M-1. The spectra suggest that 2 and 3 are the only significant cuprate species present in the methylcyanocuprate system. The IR titration data give a formation constant of approximately 8 × 103 for 2 + MeLi ⇌ 3. The present data stand in contrast to earlier reports that 2 and 3 have identical IR spectra. The earlier conclusion appears to have been a consequence of the significantly different molar absorptivity of cyanide in 2 and 3 and the presence of small amount of 2 in equilibrium with 3 when the MeLi:CuCN ratio is 2:1. Ab initio calculations of the frequencies and intensities of the CN stretch are able to semiquantitatively reproduce the observed trends using a model in which the CN- is bound to the Cu in 2 but is not bound directly to the Cu in 3. The predicted CN stretch for a “higher order” cyanocuprate structure, with a three-coordinate [Me2CuCN]2-, is not consistent with the observed value. In contrast, the seven-membered bridged Gilman cuprate 14 appears to fulfill all of the structural and IR spectroscopic requirements for the 2115 cm-1 species.