eqn. (4). The extraction constant, Kex, is defined in eqns. (5) and
(6). The log Kex values are obtained by substituting the pH1/2
Ln3ϩ ϩ 3HA(o)
Kex = [LnA3]o[Hϩ]3/[Ln3ϩ][HA]o3 = D[Hϩ]3/[HA]o (5)
log Kex = log D Ϫ 3pH Ϫ 3log [HA]o (6)
LnA3(o) ϩ 3Hϩ
(4)
3
values, the pH being read from log D vs. pH plots, at which half
of the metal ions is extracted (log D = 0), in eqn. (6) and
are summarized in Table 2. Since the concentration of the
lanthanide ion is much lower than that of the extractant, the
concentration of the extractant in the organic phase ([HA]o in
eqn. (6)) can be considered equal to the initial concentration.
The extraction constant can be expressed in terms of the
stability constant, β3 ([LnA3]/[Ln3ϩ][AϪ]3), the partition con-
stants for HA, PHA ([HA]o/[HA]), and LnA3, PLnA ([LnA3]o/
[LnA3]), and the acid dissociation constant, Ka, as in eqn. (7).
3
Ϫ3
Kex = β3PLnAKa PHA
(7)
Fig. 3 Effect of the O ؒ ؒ ؒ O distance of SF(La–Yb).
Strong acidity will give enhanced extractability though Ka and
β3 compensate each other to some extent.1,3 It is also known
that PLnA and PHAϪ3 offset each other in the solid, which arises
from regular solution theory in chelate extraction systems.1,3
The linear relationship between log Kex and pKa in the extrac-
tion of tervalent europium with 4-acyl-5-pyrazolone derivatives
can be seen in a previous paper,16 demonstrating that the
extractability in the extraction of lanthanide ions is mainly
governed by the acidity of the ligands. However, the origin of
the separability is not accessible. When the nature of the target
metal ion is analogous like the lanthanide ions, the steric effect
of the ligands, and the conformational and electronic effect of
the donor atoms, could be responsible.
As seen in Table 2, for each ligand the log Kex value increases
as the atomic number of the lanthanide metal ion increases, in
other words as the ionic radius decreases. It also gradually
increases as the pKa value decreases, in other words as the acid-
ity is enhanced for HL1–HL3. The log Kex values for HL5 are
similar to those for HL1–HL3,4 for their acidity is comparable,
and those for HL6 and HL7 are much greater because of their
strong acidity.4,7 It is quite interesting that the log Kex values
for HL4 are much smaller than those for HL1–HL3 and HL5,
although its acidity is comparable. X-Ray crystallographic
studies indicate that β-diketones exist in the enol form with the
β-diketone moiety consisting of the two carbonyl groups and
the carbon in the α-position on a plane. After the release of
the enolic proton, molecular orbital calculations indicate that
the planar structure collapses and the anionic form reaches the
most stable conformation. The crystallographic studies also
demonstrate that the β-diketone moiety in the lanthanide com-
plexes as well as other metal complexes is usually kept on the
plane or close to coplanar, meaning that the anionic form in
the most stable conformation is required to adjust itself to be
planar on chelation. The energy required to rearrange the lig-
and conformation is obtained by MNDO/H calculation to be
4.19, 4.29, 5.01 and 11.40 kcal molϪ1 for HL1–HL4, respectively.
The increasing energy is attributable to the steric repulsion
between the 3-methyl and the 4-acyl groups. The steric repul-
sion for HL4 is especially large, which could be considered the
reason why the extraction constants for HL4 are much smaller
than those for HL1–HL3.
ively. As seen in Table 2, the extraction separability for the lan-
thanide metal ions improves as the O ؒ ؒ ؒ O distances becomes
shorter. The SF values are plotted against the O ؒ ؒ ؒ O distance
in Fig. 3. This tendency is also seen for other acylpyrazolones.
The SF(Yb-La) values for HL5 and HL6 are reported to be
2.944 and 3.03,6 respectively. Although the nature of the acyl
group is quite different, the separability for HL5 and HL6 is very
similar. The phenyl group in HL5 is aromatic, while the CF3
group in HL6 is aliphatic analogous to those in HL1–HL4 but
strongly electron withdrawing. This indicates that the O ؒ ؒ ؒ O
distance rather than the steric and electronic nature of the sub-
stituent is significant for the separation of the lanthanide metal
ions. HL7 is structurally analogous to acylpyrazolones and is
known as one of the most powerful extractants owing to its
extremely strong acidity. However, the separability for lanthan-
ide ions is very low. Its pKa and SF value were reported to be
1.23 and 1.47,7 respectively. It is quite interesting that the
low separability for this extractant can also be rationalized in
terms of the O ؒ ؒ ؒ O distance. A similar relationship between
the O ؒ ؒ ؒ O distance and the extraction separability for the
lanthanide metal ions is seen for trifluorothenoylacetone
(HL8, SF = 3.79),17 benzoyltrifluoroacetone (HL9, 3.75) and
benzoyltrifluoro-α-methylacetone (HL10, 4.44),14 although the
extractions were made into benzene and it is not possible simply
to compare with the separability for HL1–HL7. Here, the
O ؒ ؒ ؒ O distances for HL8 and HL9 are similar and that for
HL10 is less than that for HL9 owing to the α-methyl substitu-
ent. This also indicates that the O ؒ ؒ ؒ O distance rather than the
steric and electronic factors is responsible for the separation of
the lanthanide metal ions.
The ligand in the organic phase is distributed into the aque-
ous phase and releases hydrogen to form the anionic species
followed by complex formation with metal ions. This metal
complex is distributed to the organic phase because of the
high hydrophobicity. The heat of formation of the anionic
species was obtained by MNDO/H calculation, varying the
distance between the two donating oxygens. The β-diketone
moiety including the two carbonyl groups and the carbon in the
α position was fixed on a plane through the calculation, for
the β-diketone moiety is usually kept on the plane or close to
coplanar in the lanthanide complexes. Plots of Hf (heat of
formation) vs. the distance between the two donating oxygens
are seen in Fig. 4. The O ؒ ؒ ؒ O distances of the most stable
structure are 3.05, 3.00, 2.98 and 2.84 Å for HL1–HL4, respec-
tively. Since the ionic radius of lanthanide ions becomes smaller
as the atomic number increases, the charge density increases
as the atomic number increases. Consequently, lanthanides
Separability for lanthanide metal ions
The separation factor (SF) is defined as the difference in the
log Kex values for two metal ions and is summarized in Table 2.
The separation factors between lanthanum and ytterbium,
SF(Yb–La), are 4.47, 4.41, 4.65 and 4.97 for HL1–HL4, respect-
2790
J. Chem. Soc., Dalton Trans., 2000, 2787–2791