Table 4 Results from quantum mechanics calculations on DMTZP.
F141-CT-96-0010). We would also like to thank the EPSRC
Geometry optimisation was carried out using the 6-31G** basis set;
and the University of Reading for funding of the image-plate
system. The use of the Origin 2000 at the University of Reading
High Performance Computer Centre (HPCC) is gratefully
acknowledged.
energies in au (= 627.509 kcal molϪ1
)
N(4)–C–C–N (py)
torsion angle/Њ
Structure
Energy
H on N(1)
1-cc
1-ct
1-tt
2-cc
2-ct
2-tt
4-cc
4-ct
4-tt
Ϫ804.11510
Ϫ804.11644
Ϫ804.11763
Ϫ804.10797
Ϫ804.12327
Ϫ804.13301
Ϫ804.11348
Ϫ804.10453
Ϫ804.08451
Ϫ6.4, Ϫ6.4
Ϫ1.1, 179.8
179.4, 179.7
46.7, 46.7
Ϫ33.9, Ϫ177.6
180.0, 180.0
0.0, 0.0
References
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H on N(4)
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complexes. The positions of the protons were not established
unequivocally but as detailed above it seems likely that one
ligand has the structure 4-cc while the other could also be 4-cc
but more likely is 1-cc.
As is apparent from Table 4 neither structure (4-cc or 1-cc)
has a particularly low energy compared to other configurations,
but given the presence of the water molecule it is easy to see
why these particular configurations are favoured in the crystal
structure. We have previously shown how the presence of a
water molecule in the cavity can stabilise a tridentate planar
ligand in the cis, cis conformation.5
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We have shown that DMTZP (and by analogy DBTZP) forms
a consistent series of structures across the lanthanide series
with variations in stoichiometry due to the decreasing size of
the metals across the series. Lanthanum as is often the case
provides the only inconsistency. The reasons for the exceptional
extraction properties of this ligand remain unclear. It has been
suggested that the basicity of the heterocyclic nitrogen ligand
has a big influence on the AmIII/LnIII separations.6 A pKa (base)
value of 3.40 has been determined for 3-methyl-5-(pyridin-2-
yl)-1,2,4-triazole17 which indicates that this type of ligand has
a lower basicity than, for example, pyridine (5.25)21 and bipyri-
dine (4.45).22 Heterocyclic nitrogen ligands with lower basicities
have been shown to give better AmIII/LnIII separations.6 It is
also clear that the addition of a weak acid, 2-bromohexanoic
acid, is crucial to the ligands extraction performance.8 There-
fore, although the solid Ln/DMTZP complexes isolated in this
work are likely to give a good indication of the type of species
which may be extracting, future work will include solution
studies with and without 2-bromohexanoic acid, to establish
better the nature of the extracting species in solution.
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Acknowledgements
We are grateful for the financial support by the European
Union Nuclear Fission Safety Programme Task 2 (Contract
Paper 9/01842E
2440
J. Chem. Soc., Dalton Trans., 1999, 2433–2440