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
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Abstract
Some metal ion complexing properties of DPP (2,9-Di(pyrid-2-yl)-1,10-phenanthroline)
are reported with a variety of Ln(III) (Lanthanide(III)) ions and
alkali earth metal ions, as well as the uranyl(VI) cation. The intense
π–π* transitions in the absorption spectra of aqueous
solutions of 10<sup>–5</sup> M DPP were monitored as a function
of pH and metal ion concentration to determine formation constants
of the alkali-earth metal ions and Ln(III) (Ln = lanthanide) ions.
It was found that log <i>K</i><sub>1</sub>(DPP) for the
Ln(III) ions has a peak at Ln(III) = Sm(III) in a plot of log <i>K</i><sub>1</sub> versus 1/<i>r</i><sup>+</sup> (<i>r</i><sup>+</sup> = ionic radius for 8-coordination). For Ln(III)
ions larger than Sm(III), there is a steady rise in log <i>K</i><sub>1</sub> from La(III) to Sm(III), while for Ln(III) ions smaller
than Sm(III), log <i>K</i><sub>1</sub> decreases slightly
to the smallest Ln(III) ion, Lu(III). This pattern of variation of
log <i>K</i><sub>1</sub> with varying size of Ln(III) ion
was analyzed using MM (molecular mechanics) and DFT (density functional
theory) calculations. Values of strain energy (∑U) were calculated
for the [Ln(DPP)(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> and [Ln(qpy)(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> (qpy = quaterpyrdine) complexes
of all the Ln(III) ions. The ideal M–N bond lengths used for
the Ln(III) ions were the average of those found in the CSD (Cambridge
Structural Database) for the complexes of each of the Ln(III) ions
with polypyridyl ligands. Similarly, the ideal M–O bond lengths
were those for complexes of the Ln(III) ions with coordinated aqua
ligands in the CSD. The MM calculations suggested that in a plot of
∑U versus ideal M–N length, a minimum in ∑U occurred
at Pm(III), adjacent in the series to Sm(III). The significance of
this result is that (1) MM calculations suggest that a similar metal
ion size preference will occur for all polypyridyl-type ligands, including
those containing triazine groups, that are being developed as solvent
extractants in the separation of Am(III) and Ln(III) ions in the treatment
of nuclear waste, 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 type ligands for Am(III) will depend on the above metal
ion size-based selectivity. The selectivity patterns of DPP with the
alkali-earth metal ions shows a similar preference for Ca(II), which
has the most appropriate M–N lengths. The structures of DPP
complexes of Zn(II) and Bi(III), as representative of a small and
of a large metal ion respectively, are reported. [Zn(DPP)<sub>2</sub>](ClO<sub>4</sub>)<sub>2</sub> (triclinic, <i>P</i>1, <i>R</i> = 0.0507) has a six-coordinate Zn(II), with each of the
two DPP ligands having one noncoordinated pyridyl group appearing
to be π-stacked on the central aromatic ring of the other DPP
ligand. [Bi(DPP)(H<sub>2</sub>O)<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub>](ClO<sub>4</sub>) (triclinic, <i>P</i>1, <i>R</i> = 0.0709) has an eight-coordinate Bi, with the coordination
sphere composed of the four N donors of the DPP ligand, two coordinated
water molecules, and the O donors of two unidentate perchlorates.
As is usually the case with Bi(III), there is a gap in the coordination
sphere that appears to be the position of a 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 the side of the Bi containg the
DPP to the side where the lone pair is thought to be situated. A DFT
analysis of [Ln(tpy)(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> and [Ln(DPP)(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> complexes
is reported. The structures predicted by DFT are shown to match very
well with the literature crystal structures for the [Ln(tpy)(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> with Ln = La
and <i>n</i> = 6, and Ln = Lu with <i>n</i> =
5. This then gives one confidence that the structures for the DPP
complexes generated by DFT are accurate. The structures generated
by DFT for the [Ln(DPP)(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> complexes
are shown to agree very well with those generated by MM, giving one
confidence in the accuracy of the latter. An analysis of the DFT and
MM structures shows the decreasing O--O nonbonded distances as one
progresses from La to Lu, with these distances being much less than
the sum of the van der Waals radii for the smaller Ln(III) ions. The
effect that such short O--O nonbonded distances has on thermodynamic
complex stability and coordination number is then discussed
