Infrared Spectroscopic Characterization of Cyanocuprates

The synthetically useful cyanocuprates MeCu(CN)Li ( 2 ) and “Me 2 CuLi·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 × 10 3 cm -2 M -1, while the CN stretching frequency for 3 occurs at 2115 cm -1 with ε = 4.0 × 10 2 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 × 10 3 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 [Me 2 CuCN] 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.