1
H NMR in Solution and Solid State Structural Study of Lanthanide(III) Cryptates
C. Platas,
²
F. Avecilla,
²
A. de Blas,
²
C. F. G. C. Geraldes,*
,‡
T. Rodrı ´guez-Blas,*
,²
H. Adams,
§
and J. Mahı ´a
|
Departamento de Quı ´mica Fundamental e Industrial, Universidade da Corun ˜a, Campus de A. Zapateira,
s/n 15071 A Corun ˜a, Spain, Departamento de Bioquı ´mica, Faculdade de Cie ˆncias e Tecnologia da
Universidade de Coimbra, Apartado 3126, 3000 Coimbra, Portugal, Department of Chemistry,
The University, Sheffield S3 7HF, UK, and Servicios Xerais de Apoio a ´ Investigacio ´n,
Universidade da Corun ˜a, Spain
ReceiVed NoVember 13, 1998
We present here a detailed structural comparison, both in the solid state and in aqueous solution, of a complete
series of lanthanide cryptate complexes of a Schiff base axial macrobicyclic ligand L of general formula [LnL]-
[NO
3
]
3
‚xH
2
O (Ln ) La-Lu, Y); the macrobicyclic receptor L is an azacryptand N[(CH
2
)
2
NdCH-R-CHdN-
(CH
2
)
2
]
3
N (R ) m-C
6
H
2
OH-2-Me-5). The crystal structures of the Ce, Nd, and Eu complexes, chemical formulae
[CeL(NO
3
)](NO
3
)
2
‚1.5H
2
O‚0.5CH
3
CH
2
OH (3), [NdL(NO
3
)](NO
3
)
2
‚3H
2
O(5), and [EuL(NO
3
)](NO
3
)
2
‚H
2
O‚
CH
3
OH (7), as well as that of [YL(NO
3
)][Y(NO
3
)
3
(H
2
O)
2
EtOH](NO
3
)
2
.EtOH‚CH
3
CN (16), have been deter-
mined by single-crystal X-ray crystallography. The four crystals crystallize in the triclinic space group P1 h with
Z ) 2; a ) 10.853(3) Å, b ) 12.746(3) Å, c ) 17.907(5) Å, R) 98.09(2)°, ) 89.99(2)°, γ ) 96.34(2)°, for
3; a ) 10.835(2) Å, b ) 12.544(3) Å, c ) 17.701(2) Å, R) 82.220(10)°, ) 89.240(10)°, γ ) 84.45(2)° for
5; a ) 10.896(2) Å, b ) 12.566(4) Å, c ) 17.688(3) Å, R) 81.23(2)°, ) 89.500(10)°, γ ) 84.72(3)° for 7;
and a ) 12.723(2) Å, b ) 14.047(3) Å, c ) 16.943(2) Å, R) 66.07(2)°, ) 79.838(12)°, γ ) 81.616(14)° for
16. In light of their crystal structures, it can be stated that all of them adopt very similar structures, with the
nine-coordinated metal ion bound asymmetrically to seven donor atoms in the ligand cavity and also to two
oxygen atoms of a bidentate nitrate anion. The macrobicycle cavity adapts to the lanthanide contraction, while
preserving the pseudo-triple-helix conformation around the metal ion. The coordination geometry of the metal
atom is best considered as a slightly distorted monocapped dodecahedron. The aqueous solution structures of the
paramagnetic complexes were thoroughly characterized from the proton NMR LIS and LIR data, with particular
attention to the changes induced by the lanthanide contraction, and agree quite well with the crystal structures of
the Nd and Y complexes. The experimental Ln-donor distances decrease progressively along the lanthanide
series both in the solid and solution structures, but no drastic structural changes occur. The gradual contraction
and distortion of the coordination polyhedron along the series cause a variation of the crystal field parameter
A
2
°<r
2
> and the hyperfine constants A
i
of the lanthanides in the middle of the series, leading to “breaks” in the
contact-pseudo-contact shift separation plots of the proton LIS values. However, this affects only slightly the
geometric terms G
i
of the protons and not at all their R
ik
ratios. The conformational rigidity of the five-membered
chelate rings formed by the metal-bound ethylenediamino moieties of the bound cryptand increases upon lanthanide
contraction. The ∆G
q
value for the δ T λ conformational interconversion process of those rings is 70 ( 3 kJ for
the Y complex.
Introduction
Lanthanide(III) complexes with encapsulating ligands have
been the subject of numerous studies in recent years due to their
potential applications in selective extraction of metals, NMR
imaging contrast agents, fluoroimmunoassay, and diagnostic
agents.
1
Macrocyclic compartmental ligands have been systematically
investigated for the preparation of mono- and polynuclear lan-
thanide complexes,
2
but improved protection of the Ln(III) ions
from the solvent is expected to be obtained with macrobicyclic
ligands; in fact, it has been stated that this type of ligands could
enhance some interesting properties that make their lanthanide-
(III) complexes valuable for the development of technological
applications.
3
Many metal complexes with cryptands derived
from the condensation of tris(2-aminoethyl)amine and 2,6-
diformylphenols have been reported in the literature in the recent
past. These Schiff-base axial macrobicyclic
4
ligands have been
shown to form stable complexes with a wide range of metal
ions,
5
including mononuclear
6-8
and binuclear
9
Ln(III) com-
plexes.
The utility of paramagnetic lanthanide (III) complexes as an
aid in determining molecular structure and conformation in
* To whom correspondence should be addressed. Tel.: +351 39 824531.
Fax: +351 39 853607. E-mail: geraldes@cygnus.ci.uc.pt.
²
DCFI, Universidade de Corun ˜a.
‡
Universidade de Coimbra.
§
University of Sheffield.
|
SXAI, Universidade de Corun ˜a.
(1) Alexander, V. Chem. ReV. 1995, 95, 273.
(2) Guerriero, P.; Tamburini S.; Vigato P. A. Coord. Chem. ReV. 1995,
139, 17.
(3) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993,
123, 201.
(4) The term axial macrobicycle describes symmetrical molecules with
coaxial arrangement of two tripodal subunits linked by three identical
bridges. See: Lehn, J.-M. Pure Appl. Chem. 1980, 52, 2441.
3190 Inorg. Chem. 1999, 38, 3190-3199
10.1021/ic981314e CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/05/1999