Selectivity of the Highly Preorganized Tetradentate Ligand 2,9-Di(pyrid-2-yl)-1,10-phenanthroline for Metal Ions in Aqueous Solution, Including Lanthanide(III) Ions and the Uranyl(VI) Cation

摘要

Some metal ion complexing properties of DPP (2,9-Di(pyrid-2-nyl)-1,10-phenanthroline) are reported with a variety of Ln(III) (Lanthanide-n(III)) ions and alkali earth metal ions, as well as the uranyl(VI) cation. Thenintense π−π* transitions in the absorption spectra of aqueous solutions of 10−5nM DPP were monitored as a function of pH and metal ion concentration tondetermine formation constants of the alkali-earth metal ions and Ln(III) (Ln =nlanthanide) ions. It was found that log K1(DPP) for the Ln(III) ions has a peaknat Ln(III) = Sm(III) in a plot of log K1 versus 1/r+ (r+ = ionic radius for 8-ncoordination). For Ln(III) ions larger than Sm(III), there is a steady rise in lognK1 from La(III) to Sm(III), while for Ln(III) ions smaller than Sm(III), log K1ndecreases slightly to the smallest Ln(III) ion, Lu(III). This pattern of variation ofnlog K1 with varying size of Ln(III) ion was analyzed using MM (molecularnmechanics) and DFT (density functional theory) calculations. Values of strainnenergy (ΣU) were calculated for the [Ln(DPP)(H2O)5]3+ and [Ln(qpy)(H2O)5]3+ (qpy = quaterpyrdine) complexes of all the Ln(III)nions. The ideal M−N bond lengths used for the Ln(III) ions were the average of those found in the CSD (Cambridge StructuralnDatabase) for the complexes of each of the Ln(III) ions with polypyridyl ligands. Similarly, the ideal M−O bond lengths were those forncomplexes of the Ln(III) ions with coordinated aqua ligands in the CSD. The MM calculations suggested that in a plot of ΣU versusnideal M−N length, a minimum in ΣU occurred at Pm(III), adjacent in the series to Sm(III). The significance of this result is that (1)nMM calculations suggest that a similar metal ion size preference will occur for all polypyridyl-type ligands, including those containingntriazine groups, that are being developed as solvent extractants in the separation of Am(III) and Ln(III) ions in the treatment of nuclearnwaste, and (2) Am(III) is very close in M−N bond lengths to Pm(III), so that an important aspect of the selectivity of polypyridyl typenligands for Am(III) will depend on the above metal ion size-based selectivity. The selectivity patterns of DPP with the alkali-earth metalnions shows a similar preference for Ca(II), which has the most appropriate M−N lengths. The structures of DPP complexes of Zn(II)nand Bi(III), as representative of a small and of a large metal ion respectively, are reported. [Zn(DPP)2](ClO4)2 (triclinic, P1, R = 0.0507)nhas a six-coordinate Zn(II), with each of the two DPP ligands having one noncoordinated pyridyl group appearing to be π-stacked on thencentral aromatic ring of the other DPP ligand. [Bi(DPP)(H2O)2(ClO4)2](ClO4) (triclinic, P1, R = 0.0709) has an eight-coordinate Bi,nwith the coordination sphere composed of the four N donors of the DPP ligand, two coordinated water molecules, and the O donors ofntwo unidentate perchlorates. As is usually the case with Bi(III), there is a gap in the coordination sphere that appears to be the position ofna lone pair of electrons on the other side of the Bi from the DPP ligand. The Bi-L bonds become relatively longer as one moves from thenside of the Bi containg the DPP to the side where the lone pair is thought to be situated. A DFT analysis of [Ln(tpy)(H2O)n]3+ andn[Ln(DPP)(H2O)5]3+ complexes is reported. The structures predicted by DFT are shown to match very well with the literature crystalnstructures for the [Ln(tpy)(H2O)n]3+ with Ln = La and n = 6, and Ln = Luwith n = 5. This then gives one confidence that the structuresnfor the DPP complexes generated by DFT are accurate. The structures generated by DFT for the [Ln(DPP)(H2O)5]3+ complexes arenshown to agree very well with those generated by MM, giving one confidence in the accuracy of the latter. An analysis of the DFT andnMM structures shows the decreasing O--O nonbonded distances as one progresses from La to Lu, with these distances being much lessnthan the sum of the van der Waals radii for the smaller Ln(III) ions. The effect that such short O--O nonbonded distances has onnthermodynamic complex stability and coordination number is then discussed.