Complexation of a hexameric uranium(VI) cluster by p-benzylcalix[7]arene

Article abstract of DOI:10.1039/a903289d

The reaction of uranyl nitrate hexahydrate with p-benzylcalix[7]arene (H₇L) in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) resulted in the formation of the compound (UO₂²⁺)₆(L^7-)₂

Full text of DOI:10.1039/a903289d

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Complexation of a hexameric uranium(VI) cluster by
p-benzylcalix[7]arene
Pierre Thuéry,*a Martine Nierlich,a Bahari Souley,b Zouhair Asfarib and Jacques Vicensb
a CEA/Saclay, SCM (CNRS URA 331), Bât. 125, 91191 Gif-sur-Yvette, France.
E-mail: thuery@drecam.cea.fr
b ECPM, Laboratoire de Chimie des Interactions Moléculaires Spécifiques
(CNRS UMR 7512), 25 rue Becquerel, 67087 Strasbourg, France
Received 26th April 1999, Accepted 4th June 1999
The reaction of uranyl nitrate hexahydrate with p-benzylcalix[7]arene (H7L) in the presence of 1,4-diazabicyclo-
[2.2.2]octane (DABCO) resulted in the formation of the compound (UO22ϩ)6(L7Ϫ)2(O2Ϫ)2(HDABCOϩ)6ؒ3CH3CNؒ
CHCl3ؒ5CH3OHؒ3H2O 1, whose crystal structure has been determined. In the symmetry-centred complex core a
hexanuclear uranium() cluster is surrounded by two calixarene units. Each calixarene, in a conformation different
from the usual one, encompasses two UO22ϩ moieties, which are bonded to three or four phenolic oxygen atoms and
also one to the other by one of their oxygen atoms. The two other UO22ϩ ions are located between the two calixarene
moieties and are involved, together with one of the former ions, in two pseudo-trigonal µ3-oxo-centred assemblies.
This complex, which represents one of the largest metal ion assemblies in calixarene chemistry, illustrates the ‘cluster
keeper’ character of the larger calixarenes.
The discovery by Shinkai et al.1 that some calixarene derivatives
behave as ‘uranophiles’ has attracted much interest for the last
ten years, but has left open the question of the structures of the
complexes formed. The first examples of fully characterized
actinide complexes of calixarenes were later provided by
Harrowfield et al., who reported the synthesis and crystal struc-
tures of the uranyl complex of bis(homo-oxa)-p-tert-butyl-
calix[4]arene2 and of the tetranuclear thorium complex of
p-tert-butylcalix[8]arene.3 The former compound demonstrates
that simple calixarenes, in a basic medium, are able to provide
the equatorial environment (here, four-co-ordinate, but more
frequently five-co-ordinate) suitable for uranyl ions. We further
extended this work by describing the crystal structures of the
uranyl complexes of p-tert-butylcalix[n]arenes, with n = 5–8.4–7
With n = 56 and 7,7 as in the case of bis(homo-oxa)-p-tert-
butylcalix[4]arene, the complexes are mononuclear, the uranyl
ion being in an irregular pentagonal (n = 5) or square-planar
(n = 7) equatorial environment. In the latter case the four oxy-
gen atoms of the tetrameric subunit of the calixarene only are
linked to the metal ion. In both cases, the conformation of the
complexed calixarene appears very close to that of the uncom-
plexed one. With n = 84 the calixarene is large enough to
accommodate two uranyl ions, each of them in a pentagonal
equatorial environment made of four phenolic oxygen atoms
and one bridging hydroxyl ion. The case n = 6 is peculiar since
the only complex we obtained was a dimeric species in which
each calixarene acts as a bidentate ligand towards each uranyl
ion, the uranyl axes being parallel to the calixarene mean
plane.5 In all the cases the complexes were obtained with tri-
ethylamine as a deprotonating agent. Two other studies worthy
of note were reported recently: the first deals with the extrac-
tion properties of a 1-acid-3-diethylamide substituted calix-
[4]arene and the crystal structure of its dimeric uranyl complex,
in which the ‘lower rim’ substituents provide three of the bind-
ing sites, two uranyl ions being ‘sandwiched’ between two calix-
arene moieties;8 the other demonstrates the role of caesium
ions in uranyl complexation by p-tert-butylcalix[6]arene.9
discussed in this review but not yet published in detail, some
present a particular relevance to the results reported herein.
Two p-tert-butylcalix[n]arenes give trinuclear uranyl complexes.
With n = 6 the complex is oxo-centred, each of the three calix-
arenes being bidentate.11 With n = 8 the complex possesses a
core similar to that of the dinuclear complex already reported,
with a third uranyl ion bonded to one oxygen atom from each
of the two former ions; the obtention of this new complex
results from the replacement of triethylammonium by tetra-
methylammonium as counter ion.12 A dinuclear complex is
also obtained in the case n = 9,13 which is reminiscent of the
dinuclear complex obtained with the acyclic analogue of p-tert-
butylcalix[6]arene.14 One conclusion of this review is that the
larger calixarenes might be regarded as receptors for uranate
clusters rather than for the uranyl ion itself. The same trend is
illustrated by the thorium complex of p-tert-butylcalix[8]arene,
which is described as a ‘hydroxothorium oligomer complex’,3
and by rare-earth-metal complexes, with the occurrence of
oxometal clusters for the largest calixarenes.13 The expression
‘cluster keeper’ was coined by Harrowfield to qualify these large
calixarenes.
The difficulties encountered in growing single crystals in this
family of compounds led us to investigate basic agents other
than triethylamine. We turned to DABCO (1,4-diazabicyclo-
[2.2.2]octane) which we thought to be able to facilitate crystal-
lization, owing to its quasi-spherical shape. We report in this
paper the synthesis and crystal structure of the hexanuclear
uranyl complex of p-benzylcalix[7]arene, which never crystal-
lized in the presence of triethylamine. This is one of the highest
nuclearity complexes of a calixarene to date, which confirms the
trends stated before.
Experimental
Synthesis
p-Benzylcalix[7]arene. The reaction was conducted according
to a procedure previously described.15 p-Benzylphenol (9.21 g,
50 mmol), paraformaldehyde (5.00 g, 160 mmol) and 14 M
KOH (0.5 ml), dispersed in tetraline (80 ml), were heated at
All the results published before 1997 and some other
unpublished ones have been reviewed by Harrowfield, with a
particular emphasis on ring-size effects.10 Among the results
J. Chem. Soc., Dalton Trans., 1999, 2589–2594
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Fig. 1 View of molecule A in complex 1. Hydrogen atoms, p-benzyl substituents and solvent molecules omitted for clarity.
180–185 ЊC under a nitrogen atmosphere for 5 h, in a 250 ml
round-bottomed flask equipped with a Dean–Stark collector.
After evaporation of tetraline under reduced pressure (10Ϫ3
mmHg), the resulting brown solid was dispersed in CHCl3 in
which 1 M HCl was added. The biphasic mixture was mag-
netically stirred during 3 h. Then the organic layer was washed
twice with water and dried over MgSO4. After filtration, the
solvents were removed to yield a brown-yellow residue which
precipitated on addition of acetone. A white solid (0.350 g) was
filtered off by suction and shown to be pure p-benzylcalix[8]-
arene by comparison with a previous sample15 (yield 3%). Then
the filtrate was evaporated and the residue triturated with
acetone. After 2 h a precipitate was formed and filtered off to
give a white solid identified as p-benzylcalix[5]arene (0.220 g) by
comparison with a previous sample15 (yield 1%). Once again
the filtrate was evaporated and precipitated with acetone. After
6 h the precipitate was filtered off by suction to yield p-
glass capillary with a protecting ‘Paratone’ oil (Exxon Chemical
Ltd.) coating. The data were processed with the HKL pack-
age.17 The structure was solved by direct methods with
SHELXS 8618 and subsequent Fourier-difference synthesis and
refined by full-matrix least squares on F2 with SHELXL 93.19
Absorption effects were corrected empirically with the program
MULABS from PLATON.20 All the aromatic rings were refined
as idealized hexagons. One of the benzyl groups in molecule B
is disordered, the aromatic ring being distributed over two posi-
tions sharing two lateral carbon atoms, with occupancies
refined to 0.69(4) and 0.31(4). Owing to low crystal quality and
large number of atoms, some restraints or constraints had to be
applied on bond length and displacement parameters, par-
ticularly in the solvent molecules. The methanol molecules
appeared badly on the Fourier-difference maps and the pres-
ence of other highly disordered solvent molecules cannot be
ruled out. Uranium, oxygen, nitrogen and chlorine atoms were
refined anisotropically, as well as the carbon atoms of the
acetonitrile and DABCO moieties. The hydrogen atoms were
introduced at calculated positions (except those bonded to
nitrogen atoms in DABCO, those of water and chloroform
molecules and those in the disordered benzyl group) as riding
atoms with a displacement parameter equal to 1.2 (CH, CH2)
or 1.5 (CH3) times that of the parent atom. The nine first peaks
in the final Fourier-difference map are located near uranium
atoms as a result of residual absorption effects; the other peaks
are lower than 1 e AϪ3. Crystal data and structure refinement
parameters are given in Table 1. The molecular drawings were
done with SHELXTL.21 All calculations were performed on a
Silicon Graphics R10000 workstation.
1
benzylcalix[7]arene (2.15 g, yield 16%), mp >300 ЊC. H NMR
(CDCl3, from TMS): δ 10.33 (s, 1 H, aryl OH); 7.32–7.11 (m, 5
H, phenyl); 6.94 (s, 1 H, aromatic); 6.90 (s, 1 H, aromatic); 3.86
(s, 2 H, CH2); and 3.84 (s, 2 H, CH2). FABϩ MS: m/z 1372.6.
Calc. for C14H12O: C, 85.68; H, 6.16. Found: C, 85.70; H, 6.43%.
Uranyl complex 1. p-Benzylcalix[7]arene (300 mg, ≈0.22
mmol) was dissolved in CHCl3 (100 ml) and an excess of 1,4-
diazabicyclo[2.2.2]octane (DABCO, 470 mg, ≈4 mmol) added.
A solution of UO2(NO3)2ؒ6H2O (370 mg, ≈0.7 mmol) in
CH3CN (40 ml) was then added dropwise, yielding a dark
brownish solution that became dark red upon reflux. The result-
ing product was recrystallized from CHCl3–CH3OH (1:1), giv-
ing dark red crystals suitable for X-ray crystallography. The
crystals were unstable and decomposed rapidly, even in their
CCDC reference number 186/1493.
See http://www.rsc.org/suppdata/dt/1999/2589/ for crystallo-
graphic files in .cif format.
1
mother-solution. H NMR (CDCl3, from TMS): three multi-
plets could be detected which were not assigned, δ 7.33–6.85
(m); 4.10–3.62 (m); 2.70–1.95 (m). FABϩ MS: m/z 4318.2
[(UO22ϩ)6(C98H84O7)2 Ϫ H].
Results and discussion
The triclinic cell contains two independent and differently
oriented dimeric molecules, each with a crystallographically
imposed symmetry centre and consequently the asymmetric
unit is composed of two half-molecules. The chemical formula
of one complete unit deduced from the crystallographic investi-
gation is (UO22ϩ)6(L7Ϫ)2(O2Ϫ)2(HDABCOϩ)6ؒ3CH3CNؒCHCl3ؒ
Crystallography
The data were collected on a Nonius Kappa-CCD area detector
diffractometer16 using graphite monochromated Mo-Kα radi-
ation (0.71073 Å). The crystal was introduced in a Lindemann
2590
J. Chem. Soc., Dalton Trans., 1999, 2589–2594

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5CH3OHؒ3H2O, where H7L is p-benzylcalix[7]arene. One of the
two molecules (A) is presented in Fig. 1; the second molecule
(B) is analogous to A in its overall shape and the following
discussion applies to both of them. Selected distances and
angles are given in Table 2. The two symmetry-related calix-
arene moieties face each other, each containing two UO22ϩ ions,
one of them [U(1)] much more inclined than the other one
[U(2)] towards the calixarene mean plane. The two remaining
UO22ϩ ions [U(3), U(3Ј)] are located between the two calixarene
cores (primed atoms are related to unprimed ones by the sym-
environment, as illustrated in Fig. 2. Atom U(1) is bonded to
four calixarene phenolic oxygen atoms which separate into two
groups, the first one [O(2), O(3)] corresponding to distances
ranging from 2.18(2) to 2.21(2) [mean value 2.19(2)] Å close to
2ϩ
metry centre). Each type of UO2 ion possesses a different
Table 1 Crystal data and structure refinement details
Empirical formula
M
C244H268Cl3N15O36U6
5521.26
T/K
Crystal system
Space group
123(2)
Triclinic
¯
P1
a/Å
b/Å
c/Å
α/Њ
19.969(2)
20.071(2)
30.876(3)
82.306(5)
79.979(4)
87.709(5)
12074(3)
2
β/Њ
γ/Њ
V/Å3
Z
µ/mmϪ1
4.112
Reflections collected
Independent reflections
49631
26917
Observed reflections [I > 2σ(I)]
Rint
12634
0.10
Number of parameters
R1
1433
0.091
Fig. 2 Two views of the (UO22ϩ)6 cluster and its environment (mol-
ecule A).
wR2
0.206
Table 2 Selected bond distances (Å) and angles (Њ). Primed atoms are related to unprimed ones by the molecule symmetry centre
Uranium environment
U(1A)–O(1A)
U(1A)–O(2A)
U(1A)–O(3A)
U(1A)–O(4A)
U(1A)–O(8A)
U(1A)–O(9A)
U(1A)–O(14A)
U(1A) ؒ ؒ ؒ U(2A)
U(3A) ؒ ؒ ؒ U(3AЈ)
U(1B)–O(1B)
U(1B)–O(2B)
U(1B)–O(3B)
U(1B)–O(4B)
U(1B)–O(8B)
U(1B)–O(9B)
U(1B)–O(14B)
U(1B) ؒ ؒ ؒ U(2B)
U(3B) ؒ ؒ ؒ U(3BЈ)
2.51(2)
2.19(2)
2.21(2)
2.49(2)
1.90(2)
1.82(2)
2.20(2)
4.096(2)
3.707(2)
2.53(2)
2.18(2)
2.19(2)
2.54(2)
1.86(2)
1.81(2)
2.12(2)
4.085(2)
3.726(2)
U(2A)–O(5A)
U(2A)–O(6A)
U(2A)–O(7A)
U(2A)–O(8A)
U(2A)–O(10A)
U(2A)–O(11A)
2.28(2)
2.22(2)
2.28(2)
2.30(2)
1.78(2)
1.83(2)
U(3A)–O(1A)
U(3A)–O(4AЈ)
U(3A)–O(12A)
U(3A)–O(13A)
U(3A)–O(14A)
U(3A)–O(14AЈ)
U(3A)–N(1A)
U(1A) ؒ ؒ ؒ U(3AЈ)
2.44(2)
2.44(2)
1.73(2)
1.75(2)
2.18(2)
2.26(2)
2.56(3)
3.844(2)
U(1A) ؒ ؒ ؒ U(3A) 3.830(2)
U(2B)–O(5B)
U(2B)–O(6B)
U(2B)–O(7B)
U(2B)–O(8B)
U(2B)–O(10B)
U(2B)–O(11B)
2.26(2)
2.12(3)
2.27(2)
2.35(2)
1.77(1)
1.76(2)
U(3B)–O(1BЈ)
U(3B)–O(4B)
U(3B)–O(12B)
U(3B)–O(13B)
U(3B)–O(14B)
U(3B)–O(14BЈ)
U(3B)–N(1B)
U(1B) ؒ ؒ ؒ U(3BЈ)
2.38(2)
2.38(2)
1.80(2)
1.77(2)
2.23(2)
2.28(2)
2.43(2)
3.841(2)
U(1B) ؒ ؒ ؒ U(3B)
3.832(2)
O(8A)–U(1A)–O(9A)
O(10A)–U(2A)–O(11A)
O(12A)–U(3A)–O(13A)
179.2(8)
179.2(9)
173.9(9)
O(8B)–U(1B)–O(9B)
O(10B)–U(2B)–O(11B)
O(12B)–U(3B)–O(13B)
179.0(8)
179.5(9)
174.2(8)
O ؒ ؒ ؒ O distances in calixarenes
O(1A) ؒ ؒ ؒ O(2A)
O(4A) ؒ ؒ ؒ O(5A)
O(7A) ؒ ؒ ؒ O(1A)
O(1B) ؒ ؒ ؒ O(2B)
O(4B) ؒ ؒ ؒ O(5B)
O(7B) ؒ ؒ ؒ O(1B)
2.79(3)
O(2A) ؒ ؒ ؒ O(3A) 2.89(3)
O(5A) ؒ ؒ ؒ O(6A) 3.05(3)
O(3A) ؒ ؒ ؒ O(4A)
O(6A) ؒ ؒ ؒ O(7A)
2.86(3)
3.05(3)
4.50(3)
4.45(3)
2.87(3)
4.55(3)
4.54(3)
O(2B) ؒ ؒ ؒ O(3B)
O(5B) ؒ ؒ ؒ O(6B)
2.83(3)
2.93(3)
O(3B) ؒ ؒ ؒ O(4B)
O(6B) ؒ ؒ ؒ O(7B)
2.89(3)
2.99(3)
Hydrogen bonds
N(2) ؒ ؒ ؒ O(5A)
N(4) ؒ ؒ ؒ O(13AЈ)
N(10) ؒ ؒ ؒ O(7B)
N(11) ؒ ؒ ؒ O(13B)
O(1) ؒ ؒ ؒ O(2)
2.62(3)
2.96(3)
2.66(3)
2.90(3)
2.84(3)
N(4) ؒ ؒ ؒ O(9AЈ)
N(6) ؒ ؒ ؒ O(7A)
N(11) ؒ ؒ ؒ O(9B)
2.97(3)
2.61(3)
2.90(3)
N(4) ؒ ؒ ؒ O(12A)
N(8) ؒ ؒ ؒ O(5B)
N(11) ؒ ؒ ؒ O(12BЈ)
2.99(3)
2.66(3)
2.95(3)
O(2) ؒ ؒ ؒ O(3)
2.87(3)
O(3) ؒ ؒ ؒ O(1)
2.90(3)
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the ideal value10 whereas the second one [O(1), O(4)] corre-
sponds to larger values ranging from 2.49(2) to 2.54(2) [mean
value 2.52(2)] Å. The latter values are close to those observed
with protonated calixarene oxygen atoms;4,6,14 however, in the
present case, the situation is quite different since the corre-
sponding oxygen atoms are bridging two UO22ϩ ions [U(1) and
U(3)] and must be deprotonated for charge equilibrium. These
distances can be compared to that obtained for a bridging
oxygen atom in the trinuclear complex of p-tert-butyl-
calix[6]arene, which is 2.54 Å.10 The co-ordination environment
of U(1) is completed by a pseudo-trigonal µ3-oxo ion O2Ϫ
[O(14)] which bridges U(1), U(3) and U(3Ј), giving rise to a
tetranuclear symmetry-centred core of a type which has been
described with various extra ligands22–27 (some oxo-centred
trinuclear species have also been reported10,28,29). The U–O(14)
distances range from 2.12(2) to 2.28(2) [mean value 2.21(6)] Å,
in good agreement with the values reported in the literature
(between 2.08 and 2.41 Å) as well as the U ؒ ؒ ؒ U distances
(mean value 3.80(6) Å, from 3.68 to 4.06 Å in the literature).
However, whereas in the cases already reported the U4O2 core
are shorter than the bridging ones (about 1.80 Å).32 In the
uranyl thiolate oxo cluster which constitutes the third reported
example, these U᎐O (terminal) and U᎐O (bridging) distances
᎐
᎐
are 1.771(7) and 1.838(7) Å, respectively, which indicates a
slight but significant lengthening upon complexation.33 A com-
parable lengthening has been reported in the case of a
Naϩ ؒ ؒ ؒ O᎐U᎐O2ϩ ؒ ؒ ؒ Naϩ complex, with U᎐O distances of
᎐
᎐
᎐
1.816(5) and 1.812(5) Å and O ؒ ؒ ؒ Na distances of 2.392(6) and
2.357(6) Å.35 A lengthening of the U᎐O bond upon complex-
᎐
ation is also apparent in the present case, with distances U(1)–
O(8) (mean value 1.88(3) Å) significantly larger than the other
U(2)᎐O and U(3)᎐O distances, which lie in the usual range
᎐
᎐
(mean value 1.77(3) Å). The U(1)–O(9) distances (mean value
1.815(7) Å) seem also slightly larger than the usual ones. The
O(8)–U(1)–O(9) angles (mean value 179.1(1)Њ) do not indicate
any deviation from linearity; this is in agreement with the
results of solid state structure determinations36 as well as rel-
ativistic extended Hückel calculations,37 which show that the
uranyl ion remains linear for U᎐O distances ranging from about
᎐
1.50 to more than 1.90 Å.
2ϩ
(or U3O in the trinuclear species) is nearly planar, the UO2
As a whole, the resulting hexanuclear cluster appears to have
a nearly planar U(1)–U(3)–U(1Ј)–U(3Ј) core, with two oxo-
bonded appendages [U(2), U(2Ј)] located on each side, above
and below the central plane. Such an arrangement is
unprecedented and provides an example of one of the highest
nuclearities obtained in calixarene complexes (an even larger
assembly, the complex of an Eu7 double-cubane cluster with
p-tert-butylcalix[9]arene, has however been obtained10). It rep-
resents also one of the highest nuclearities in dioxouranium()
assemblies, apart from the frequent polymeric chain arrange-
ment of uranyl ions; another uranyl hexanuclear complex has
been reported, which is also based on the oxo-centred trinuclear
motif but in which two such moieties are linked by two azido
bridges.29
ions being roughly parallel to each other, some deviation
from planarity is observed here (O(14) is 0.27 and 0.22 Å
from the mean uranium plane in molecules A and B, respect-
2ϩ
ively). This is likely a consequence of the U(1)O2 ion being
lengthened and strongly tilted with respect to U(3)O2, due
to the presence of U(2), as will be seen hereafter. The whole
tetranuclear assembly can be viewed as the condensation of
four pentagonal bipyramids, sharing two [U(1)] or three [U(3)]
edges. The dihedral angle between the basal planes of the
bipyramids associated with U(1) and U(3) is 21.4 and 19.1Њ in A
and B, respectively, this large value (the usual one is ≈5–10Њ)
being a consequence of the tilt of U(1)O2. The co-ordination
polyhedron of U(3) is completed by one oxygen atom from each
calixarene molecule [O(1), O(4Ј)] with distances of 2.44(2) and
2.38(2) Å in A and B, respectively, slightly lower than the dis-
tances of the same atoms with U(1) and U(1Ј), and by the
nitrogen atom of an acetonitrile molecule with U(3)–N(1) dis-
tances of 2.56(3) and 2.43(2) Å in A and B, respectively, to be
compared to 2.53(3) Å previously observed in an acetonitrile
adduct of uranyl ions.30
The complex core (UO22ϩ)6(L7Ϫ)2(O2Ϫ)2 bears a 6Ϫ charge,
which is equilibrated by six protonated DABCO molecules.
Four are located on the external sides of the molecule and are
hydrogen bonded to the phenolic oxygen atoms O(5), O(7),
O(5Ј) and O(7Ј), which are also bonded to U(2) and U(2Ј). The
other two are located between the two calixarene molecules, on
each side of the cluster mean plane and potentially hydrogen
bonded to three uranyl oxygen atoms each; the three N ؒ ؒ ؒ O
(uranyl) distances (mean value 2.97(2) Å) are rather large with
respect to the distances found with triethylamine (mean value
2.76(6) Å)4,6,7,14 since none of them corresponds to a linear
hydrogen bond. Many solvent molecules (CH3CN, CHCl3,
CH3OH, H2O) are located in the intermolecular voids, which
may explain the low stability of the crystals; the three water
molecules [O(1)–O(3)] make an isolated triangular hydrogen-
bonded assembly.
The calixarene conformation deserves some comments. Up
to now, three structures of R-calix[7]arenes only are known,
two of uncomplexed species, with R = p-ethyl38 and p-tert-butyl
(pyridine adduct)39 and that of the uranyl complex previously
cited.7 In all of them the overall calixarene conformation is the
same and can be described as the juxtaposition of a tetrameric
and a trimeric subunit, which both assume a cone shape. This is
particularly evident from the sequence of φ and χ torsion
angles, as defined by Ugozzoli and Andreetti,40 which is of the
ϩϪ ϩϪ type in each subunit and Ϫϩ at the junction between
them. In the present case the conformation is quite different, as
indicated by the torsion angles reported in Table 3. As shown
in Fig. 3, two subunits, tetrameric [O(1)–O(4)] and trimeric
[O(5)–O(7)], are still clearly defined; the trimeric one, which
encapsulates U(2), is in the cone conformation as usual, but the
tetrameric one, bonded to U(1), corresponds to a ϩϪ Ϫϩ ϩϪ
sequence, analogous to that found in the dinuclear uranyl com-
plex of p-tert-butylcalix[8]arene.4 The junction between the two
subunits corresponds to a sign reversal of the torsion angles
(one of them being very far from its ideal ‘gauche’ value) and
2ϩ
The uranyl ion U(2)O2 is in a somewhat less usual
environment. It is surrounded in its equatorial plane by three
oxygen atoms from the calixarene [O(5)–O(7)] with U–O dis-
tances ranging from 2.12(3) to 2.28(2) [mean value 2.24(6)] Å
and, more surprisingly, by one of the oxygen atoms [O(8)] from
2ϩ
the U(1)O2 ion, which is involved in a long-short U(2)–
O(8)᎐U(1) linkage and can be viewed as a dissymmetric µ-oxo
᎐
bridge. Such an arrangement exists among inorganic com-
pounds,31 and some rare examples have been reported
among metal–organic complexes.32,33 The trinuclear uranyl
complex of p-tert-butylcalix[8]arene obtained by Harrowfield
and Ogden10,12 is a recent example in a system close to the
present one. The U(2)–O(8) distances in A and B, 2.30(2) and
2.35(2) Å, respectively, are shorter than the values previously
reported (2.42(3),31 2.50(4),32 2.49(7)33 Å). The complexation of
one of the oxygen atoms of the UO22ϩ moiety is not an unlikely
event since it is known that the dipolar nature of the U᎐O bond
᎐
results in an effective charge on the uranium atom larger than
2ϩ, with the consequence of a partial δϪ charge on the oxygen
atoms.34 The hydrogen bond acceptor nature of the oxo atoms
of uranyl4,6,7,14,34 results from the presence of this negative
partial charge, which also confers a potential complexing
ability upon these atoms. A slight effect of complexation on
uranyl geometry can sometimes be detected. This is not the
case in hydrogen triuranate, in which the U᎐O distance assumes
᎐
its usual value (1.78(3) Å).31 However, in the trimeric com-
pound
uranium(), in which the three uranyl ions are bridged by three
of their oxo atoms, the terminal U᎐O distances (about 1.70 Å)
bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)dioxo-
᎐
2592
J. Chem. Soc., Dalton Trans., 1999, 2589–2594

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Table 3 Selected torsion angles (Њ)
A
B
Rings
φ
χ
φ
χ
1–2
2–3
3–4
4–5
5–6
6–7
7–1
Ϫ75
90
Ϫ122
Ϫ21
76
83
Ϫ106
118
Ϫ91
74
112
Ϫ84
Ϫ80
26
73
Ϫ118
83
Ϫ75
Ϫ109
94
77
Ϫ21
Ϫ85
119
19
Ϫ79
Ϫ91
112
by DABCO as a deprotonating agent. The former increases the
acidity of the phenolic protons, since benzyl is a less effective
electron-donor substituent than tert-butyl, which is consistent
with the degrees of deprotonation observed (four in the mono-
nuclear complex and seven in 1), whereas the pKa values of the
bases used, 11.01 for NEt3 and 2.95 and 8.60 for DABCO,
should imply a lower deprotonation degree in the second case.
Obviously, the nature of the base is not the principal factor that
makes the degree of calixarene deprotonation different in the
two cases and the increase in calixarene acidity seems to be
predominant (it has been noted that uranyl ion complexation
may enhance the phenolic groups acidity;2,6 however, this effect
must be of comparable importance in the two present cases). It
is likely that a higher deprotonation degree, which affects the
trimeric subunit in p-benzylcalix[7]arene, makes more easy
the complexation of multiple species. As a second-order effect,
the replacement of tert-butyl by benzyl groups may also
increase the hydrophobicity of the reacting medium, which
could increase the stability of uranyl clusters in solution. As a
confirmation of the predominant effect of substituent replace-
ment, it may be noted that, when DABCO is used during the
synthesis of the p-tert-butylcalix[8]arene complex, it results in a
dinuclear species whose structure is the same as that obtained in
the presence of NEt3,† the two compounds only differing by the
solvent molecules and counter ions that surround the complex
core, with 6.5 DABCO molecules making an extended hydrogen
bonding network with calixarenes, nitrate ions and other sol-
vent molecules in the second case. It is dubious, however, that a
higher deprotonation degree in p-tert-butylcalix[8]arene could
lead to completely different species, the ditopic potential of this
ligand being fulfilled in all the cases. Harrowfield10 previously
reported that, when replacing NEt3 by NMe4OH during the
synthesis of the uranyl complex of p-tert-butylcalix[8]arene, a
trinuclear complex was obtained instead of the dinuclear one,
but, since the third uranyl ion has no contact with the calix-
arene moiety, this effect is not directly comparable to the
one described here. Unfortunately, it has been impossible up
to now to grow single crystals of the cross-species (p-tert-
butylcalix[7]arene with DABCO and p-benzylcalix[7]arene with
NEt3) which would help to make this point clearer. Obviously,
even if the ring-size effects begin to be correctly understood,
much work remains to be done to get the complexing properties
of these systems under satisfactory control.
Fig. 3 Three views of the calixarene moiety, without (a, b) or with (c)
uranyl ions (molecule A). Hydrogen atoms and p-benzyl substituents
omitted for clarity.
also by a large O ؒ ؒ ؒ O separation (Table 2). The whole calix-
arene conformation is derived from the usual one by the upside-
down reversing of the units 1 and 4. The dihedral angle between
the planes defined by O(1)–O(4) and O(5)–O(7) is 63Њ in both A
and B, to be compared to 90.5–90.8Њ in the usual conform-
ation.7,39 The present complex can be viewed as half a dinuclear
p-tert-butylcalix[8]arene complex associated with a trimeric
cone complex. The two uranyl ions are so close and so tilted
with respect to each other that a direct linkage between them
ensues. In the mononuclear uranyl complex of p-tert-butyl-
calix[7]arene previously reported7 the tetrameric subunit only is
active in complexation, which leaves an unoccupied space in
the molecule. Both subunits are complexed in 1, owing to a
conformation change. The present result shows that the calix-
[7]arene core is not large enough to accommodate two nearly
parallel ions like the calix[8]arene one. It likely represents the
border between mono- and di-nuclear complexation, whereas
calix[6]arene is definitely too small to encapsulate two uranyl
ions.
† Crystal data: C261H421Cl3N30O52U4, M = 5869.75, monoclinic, space
group P21/c, a = 22.491(3), b = 37.201(4), c = 34.568(3) Å,
β = 90.960(4)Њ, V = 28919(4) Å3, T = 123 K, Z = 4.41
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