Complexation of lanthanides and other metal ions by the polypyridyl ligand quaterpyridine: Relation between metal ion size, chelate ring size, and complex stability
Metal ion complexing properties of the ligand QPY (2,2':6',2'':6'',2'''-quaterpyridine) are reported in relation to its possible use as a functional group for reagents in the extraction and separation of Ln(III) (lanthanide) ions. QPY is almost totally insoluble in water, but variation of the intense π-π* transitions in the UV spectrum of 1 × 10–5 M QPY solutions in 50% CH3OH/H2O (v/v) as a function of pH or metal ion concentration allowed for the determination of the protonation constant (pK) and log K1 values with metal ions. The pKa values of QPY are 4.54 ± 0.02 and 3.47 ± 0.02 in 50% CH3OH/H2O. The log K1 values determined by UV–visible spectroscopy for the Ln(III) ions by titration of 1:1 solutions of 1.0 × 105 M Ln(III) ion and QPY between pH 2 and 6 were: La(III), 3.79; Pr(III), 4.34; Nd(III) 4.57; Sm(III), 4.78; Gd(III) 4.69; Dy(III), 4.76, Ho(III), 4.94; Er(III), 5.36, Tm(III), 5.84, and Lu(III) 5.54, all at 25 °C and ionic strength zero. The log K1 values for the Ln(III) ions show quite strong increases from La(III) to Lu(III), with a local maximum at Sm(III), and a local minimum at Gd(III). The local maxium at Sm(III) is attributed to this ion being the best-fit size for coordinating with polypyridyl ligands. The effect of preorganization of polypyridyls such as QPY with bridging benzo groups in the ligand backbone to yield DPP (2,9-dipyrid-2-yl-1,10-phenanthroline) is to produce significant increases in log K1 for large metal ions such as Ln(III) ions, but for small metal ions such as Zn(II), there is a large decrease in log K1 in passing from QPY to DPP: this shows how rigidification of the ligand can enhance the selectivity against small metal ions of polypyridyls forming five-membered chelate rings. DFT calculations are used to analyze the variation in log K1 in passing from the La(III) complexes of QPY to DPP compared to those for the smaller Lu(III) ion.
- Research Organization:
- Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE), Energy Efficiency Office. Advanced Manufacturing Office
- Grant/Contract Number:
- AC05-00OR22725; AC02-05CH11231
- OSTI ID:
- 1607298
- Alternate ID(s):
- OSTI ID: 1635976
- Journal Information:
- Inorganica Chimica Acta, Vol. 488, Issue C; ISSN 0020-1693
- Publisher:
- ElsevierCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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