Dipyridinium-Type Anion-Exchange Extractants for Anionic Metal Complexes and Disulfonate Anions

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A new series of anion-exchange extractants, polymethylenebis[3-(N,N-dioctylcarbamoyl)pyridinium]s(abbreviated as CnBPy; n=3,4,6,8,10) were synthesized; they are lipophilic dipyridinium ions bearing two cationic centers at various separations within a molecule. The extraction behavior of metal ions from aqueous hydrohalic acid was investigated. The extractability of divalent metal ions (zinc(II) and cadmium(II)) decreased in the order: C10->C8->C6->C4->C3BPy>monocationic extractant (1-butyl-3-(N,N-dioctylcarbamoyl)pyridinium,abbreviated as BuMPy). However, in the extraction of trivalent metal ions (iron(III) and indium(III)), the extraction ability of extractants decreased in the order: BuMPy≈C10->C8->C6->C4->CnBPy. These orders were analogous to those observed in the previously reported extractions using diphosphonium extractants. The extraction of organic dianionic substrates, 1,5- and 2,6-naphthalenedisulfonates (abbreviated as NDS2-) was also investigated. The extraction of 1,5-NDS2- decreased in the order: C10-≈C8-≈C6->C4->CnBPy, while that of 2,6-NDS2- decreased in the order: C10->C8->C6->C4->C3BPy. There were no distinct maxima in the plot of extraction constant (log Kex) vs. n-value; such behavior was considerably different from that of diphosphonium extractants reported previously. The structural aspects of extractants and their relation to extraction selectivity are discussed.
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