Ionic interactions in the oligoviologens–K4Fe(CN)6 system
Various oligoviologen derivatives were synthesized by Menshutkin reaction of 4,4'-bipyridine (BPy), with alpha,omega-dibromoalkanes (alpha,omega-CnH2nBr2) in acetonitrile medium. 1H NMR analysis demonstrated that the degree of polymerization increases with alkyl chain length and reaction time, reaching its maximum at the molar reagent ratio (alpha,omega-CnH2nBr2/BPy) of 1.2: 1. The obtained oligomers and their interaction with [Fe(CN)(6)](4-) anions were characterized by 1H NMR, cyclic voltammetry, electronic absorption spectroscopy and EDX analysis. Two principally different types of interaction between low molecular weight oligoviologens and hexacyanoferrate ions were established: 1) fast reversible outer-sphere charge transfer complex formation and 2) slow irreversible reaction, presumably corresponding to the inner-sphere complex [Fe(CN)(5)L](q +/-) formation. Oligoviologens with medium molecular weight were found to form a poorly soluble product with [Fe(CN)(6)](4-) ions, which can be redissolved again by the addition of halide ions.