Ionic interactions in the oligoviologens–K4Fe(CN)6 system

• Oligoviologens were synthesized via reaction of 4,4′-bipyridine with dibromoalkanes.
• Maximum degree of polymerization is achieved at the α,ω–CnH2nBr2/BPy ratio = 1.2:1.
• Two types of interaction between oligoviologens and Fe(CN)64 − ions were established.
• Inner BPy groups form labile sphere charge-transfer complexes.
• Terminal BPy groups form stable inner-sphere complexes.

Various oligoviologen derivatives were synthesized by Menshutkin reaction of 4,4′-bipyridine (BPy) with α,ω-dibromoalkanes (α,ω–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 (α,ω–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)5L]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.

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