Charge transfer in the viologen (1,1′-dialkyl-4,4′-bipyridinium ion) salts of cyanocuprate(I)

The viologen salts of cyanocuprate(I), HV2+[Cu(CN)3]2-·2H2O and MV2+[Cu3(CN)5]2- (HV2+=1,1′-diheptyl- 4,4t - ́bipyridinium ion; MV2+=1,1′-dimethyl-4,4′-bipyridinium ion), were obtained by mixing a solution of the appropriate alkylviologen halide with a solution containing CuCN and NaCN. The resulting red-brown materials exhibited charge-transfer bands with maxima at 370 and 500 nm in the solid state, and at 350 nm in aqueous solution. The presence of CuI ions and viologen dications was deduced from the X-ray photoelectron spectra of the solid compounds. An X-ray crystal analysis of HV[Cu(CN)3]·2H2O showed that a copper atom was located on a triangular plane formed by three CN carbon atoms. A [Cu(CN)3]2- ion and an HV2+ ion were stacked face-to-face to form an ion pair with a close CuC contact of 3.2 Å. This CuC contact is responsible for the cation-anion charge transfer. The ion pairs are so stable that the charge-transfer bands are found even in solution. © 1992.