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 epsilon = 2.3 x 10(3) cm(-2) M(-1), while the CN stretching frequency for 3 occurs at 2115 cm(-1) with epsilon = 4.0 x 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 x 10(3) for 2 + MeLi reversible arrow 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.