Intramolecular Electron Transfer in a Series of Mixed-Valence Copper(II)-Copper(I) Complexes

Article abstract of DOI:10.1021/ja00344a018

A series of seven mixed-valence binuclear copper(II)-copper(I) complexes of macrocyclic ligands has been prepared to study systematically the factors affecting intramolecular electron transfer.The mixed-valence copper complexes are similar to one prepared by Gagne et al.,^3b except the methyl groups on the phenolic residue have been replaced by tert-butyl substituents.In addition, the diamine linkages B and B' have been varied in the molecule .Four of the complexes have both copper sites equivalent with B = B' = propylene (I), 2,2-dimethylpropylene (II), butylene (III), or 2,2'-biphenylene (IV).Three of the complexes have two different copper sites: B = propylene, B' = 2,2-dimethylpropylene (V); B = propylene, B' = 2,2'-biphenylene (VI); and B = propylene, B' = butylene (VII).Chemical reduction of the binuclear copper(II) complexes with sodium dithionite was used to prepare the mixed-valence complexes.Each binuclear copper(II) complex exhibits two quasi-reversible one-electron reduction waves.Variable-temperature EPR data, taken for acetone solutions from room to liquid-nitrogen temperature, are presented.In the low-temperature glass medium, each species shows an EPR spectrum characteristic of the single unpaired electron localized on one copper center.At room temperature in solution, four of the molecules (I, II, III, and V) are EPR delocalized, whereas the other three molecules each show four-line copper hyperfine spectra.Approximate temperatures of conversion in solution from EPR localized to EPR delocalized are noted for several of the complexes.Intervalence transfer (IT) bands are seen in the electronic absorption spectra for the seven complexes.