Download Supramolecular Assemblies Built from Lanthanide

Supramolecular Assemblies Built from Lanthanide
Ammoniocarboxylates and Cucurbit[6]uril
Pierre Thuéry
CEA, IRAMIS, UMR 3299 CEA/CNRS, SIS2M, LCCEf, Bât. 125, 91191 Gif-sur-Yvette, France
ABSTRACT: The reaction of lanthanide nitrates with different aminocarboxylic acids in the presence of
cucurbit[6]uril (CB6) in water at room temperature yielded five novel complexes which were crystallographically
characterized.
The
cerium
complex
involving
the
zwitterionic
form
of
β-alanine
(β-al),
[Ce(β-
al)(CB6)(NO3)(H2O)3]·2NO3·5H2O (1), is a molecular species in which the metal cation is bound to three carbonyl
groups of CB6 and one monodentate carboxylate; the ammonium-bearing chain is encapsulated in the CB6 cavity,
with the ammonium group hydrogen bonded to the uncomplexed portal. With a longer alkyl chain, 6-aminohexanoic
acid (6-ah) leads to a different, dimeric structure in [Ce(6-ah)(CB6)(H2O)3]2·6NO3·17H2O (2), in which two metal
ions connect two CB6 molecules and are further bridged by two carboxylate groups. The alkyl chains are included in
the CB6 cavities and the ammonium groups are hydrogen bonded to the uncomplexed portals. Neighbouring,
perpendicular dimers are associated through ammonium-water, water-carbonyl and CH···O(carbonyl) hydrogen
bonds. Three complexes were obtained with the enantiopure α-amino acid L-methionine (L-me), all of which are
chiral. In [Yb(L-me)(H2O)7]·CB6·3NO3·7H2O (3) and [Dy(L-me)(H2O)7]4[Dy(NO3)2(H2O)5]·4CB6·13NO3·29H2O
(4), the metal ions are bound to one monodentate carboxylate and seven water molecules. The ammonium group is
involved in hydrogen bonding with two CB6 molecules and the thiother-containing chain is included in the cavity of
one CB6, with the sulphur atom shortest contact being with a ureido carbon atom. The supramolecular arrangement is
columnar, the neighbouring chains being connected by hydrogen bonds formed by the ammonium groups. The
neodymium complex [Nd(L-me)(CB6)(NO3)(H2O)3]2[Nd(L-me)(L-me – H)(H2O)5]2·8NO3·30H2O (5) displays two
dinuclear units held by double carboxylate bridges. In one of them, the metal atoms are bound to CB6 and the L-me
molecules are excluded from the CB6 cavities while, in the other, the metal atoms are not bound to CB6, but two Lme ligands are hydrogen bonded to CB6 portals and encapsulated. Together with previous results involving other
aminocarboxylic acids, these complexes illustrate the different association modes between lanthanide ions, CB6
molecules and ammoniocarboxylates, which depend on the nature and geometry of the latter, and a balance between
CB6 complexation to the lanthanide ion and ammoniocarboxylate/CB6 association through weak interactions.
1
INTRODUCTION
Among the many remarkable properties of the cucurbit[n]uril macrocycles (CBn, Scheme
1),1 their ability to form host–guest complexes with ammonium ions2 has been put to particularly
extensive use in the design of supramolecular assemblies such as rotaxanes and pseudorotaxanes3
as well as metal-containing polyrotaxanes and molecular necklaces.4 Ion–dipole and hydrogen
bonding interactions involving the ammonium cation and the carbonyl groups of the cucurbituril
portals are the dominant forces at play, with hydrophobic interactions adding an extra
contribution when part of the ammonium-bearing molecule is included in the macrocycle cavity.
Chiral guests have also been used5 and the possibility of using the achiral CB6 and CB7 hosts
with a chiral inductor to achieve chiral recognition has been demonstrated.5a Chirality can also
arise spontaneously from achiral ligands, as observed in some helical polyrotaxanes, which are
however isolated as racemic mixtures.4c,g,h,j α-Amino acids are suitable guests and their
interaction with CBs has been investigated;6 further, in their zwitterionic form, they possess a
carboxylate group which can be used as a metal complexing site. This heterofunctionality has
been used to build chiral one-dimensional assemblies in which lanthanide ions are bound to both
L-cysteine and CB6,7 the latter being known to be a good complexant of 4f ions.8 Other
ammoniocarboxylates have been used with lanthanide ions and CB6, such as iminodiacetate,
which gives a complex displaying encapsulation of one uncoordinated carboxylic group.9
Recently, it was shown that the uranyl complexes formed with various ammoniobenzoates could
serve as assemblers for CB6 molecules.10 The results presented herein are an extension of these
previous studies to lanthanide complexes formed in the presence of CB6 with three
aminocarboxylic acids: β -alanine (β -al) and 6-aminohexanoic acid (6-ah), which differ by the
2
length of the alkyl chain, and L-methionine (L-me), which is an enantiomerically pure, sulphurcontaining α-amino acid differing from the previously used L-cysteine by the alkyl sulphurbearing chain being longer by one carbon atom, and by the replacement of the thiol by a thioether
group.
EXPERIMENTAL SECTION
Synthesis. Lanthanide nitrates (hexa- or penta-hydrates) were purchased from either Prolabo,
Aldrich, Strem or Fisher Scientific, β-alanine and L-methionine from Aldrich, and 6aminohexanoic acid and cucurbit[6]uril pentahydrate from Fluka. Elemental analyses were
performed by Analytische Laboratorien GmbH at Lindlar, Germany, or MEDAC Ltd. at
Chobham, UK.
[Ce(β -al)(CB6)(NO3)(H2O)3]·2NO3·5H2O (1). CB6·5H2O (11 mg, 0.01 mmol), a 10fold excess of Ce(NO3)3·6H2O (43 mg, 0.10 mmol), and a 20-fold excess of β -alanine (18 mg,
0.20 mmol) were dissolved in demineralized water (1.3 mL). The solution was then left to
evaporate slowly, giving colourless crystals of compound 1 within two weeks (8 mg, 51% yield
on the basis of CB6). Anal. Calcd for C39H59CeN28O31: C, 30.10; H, 3.82; N, 25.20. Found: C,
29.98; H, 3.98; N, 24.93%.
[Ce(6-ah)(CB6)(H2O)3]2·6NO3·17H2O (2). CB6·5H2O (11 mg, 0.01 mmol), a 10-fold
excess of Ce(NO3)3·6H2O (43 mg, 0.10 mmol), and a 20-fold excess of 6-aminohexanoic acid (26
mg, 0.20 mmol) were dissolved in demineralized water (2 mL). The solution was then left to
evaporate slowly, giving colourless crystals of compound 2 in low yield within two weeks.
3
[Yb(L-me)(H2O)7]·CB6·3NO3·7H2O (3), [Dy(L-me)(H2O)7]4[Dy(NO3)2(H2O)5]·4CB6·
13NO3·29H2O (4), and [Nd(L-me)(CB6)(NO3)(H2O)3]2[Nd(L-me)(L-me – H)(H2O)5]2·8NO3·
30H2O (5). CB6·5H2O (11 mg, 0.01 mmol), a 10-fold excess of Ln(NO3)3·xH2O (x = 5 or 6; 45,
44 and 44 mg for Ln = Yb, Dy and Nd, respectively; 0.10 mmol), and a 20-fold excess of Lmethionine (30 mg, 0.20 mmol) were dissolved in demineralized water (1.5 mL). The solution
was then left to evaporate slowly, giving colourless crystals of compounds 3–5 within about one
month. For 3: 13 mg, 74% yield on the basis of CB6. Anal. Calcd for C41H75N28O37SYb: C,
28.02; H, 4.30; N, 22.32; S, 1.82. Found: C, 27.80; H, 4.27; N, 22.15; S, 1.64%. For 5: 6 mg,
24% yield on the basis of CB6. Anal. Calcd for C102H228N64Nd4O112S6: C, 24.94; H, 4.68; N,
18.25; S, 3.92. Found: C, 25.74; H, 4.40; N, 19.36; S, 4.06%.
Crystallography. The data were collected at 150(2) K on a Nonius Kappa-CCD area
detector diffractometer11 using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The
crystals were introduced into glass capillaries with a protecting “Paratone-N” oil (Hampton
Research) coating. The unit cell parameters were determined from ten frames, then refined on all
data. The data (combinations of ϕ- and ω-scans with a minimum redundancy of 4 for 90% of the
reflections) were processed with HKL2000.12 Absorption effects were corrected empirically with
the program SCALEPACK.12 The structures were solved by direct methods with SHELXS-97,
expanded by subsequent Fourier-difference synthesis and refined by full-matrix (blocked matrix
for compound 4) least-squares on F2 with SHELXL-97.13 All non-hydrogen atoms were refined
with anisotropic displacement parameters, except for some atoms in 4 (see below), with restraints
for some atoms in the disordered parts and/or solvent molecules. Some lattice water molecules
were given partial occupancy factors in order to retain acceptable displacement parameters and/or
to account for too close contacts. Some of the hydrogen atoms bound to oxygen and nitrogen
4
atoms were found on Fourier-difference maps (see below), and the carbon-bound hydrogen atoms
were introduced at calculated positions. All hydrogen atoms were treated as riding atoms with an
isotropic displacement parameter equal to 1.2 times that of the parent atom (1.5 for CH3). Special
details are as follows:
Compound 1. The cerium atom is disordered over two sites, one at each CB6 portal, with
occupancy parameters refined to values of 0.850(3) and 0.150(3), but the β -alanine, nitrate and
water ligands bound to the main component Ce1 only were found. The hydrogen atoms bound to
N1 could not be found due to the closeness of the minor cerium component, and they were
introduced at calculated positions. The hydrogen atoms of two coordinated and two free water
molecules only were found. The value of the refined Flack parameter14 is indicative of racemic
twinning.
Compound 2. The three nitrate counterions are disordered over seven positions located
close to one another or to their image by symmetry, which were refined with occupancy
parameters of 0.5 or 0.25. The hydrogen atoms of the ammonium group and coordinated water
molecules were found on a Fourier-difference map.
Compound 3. The hydrogen atoms bound to N1, N2 and four coordinated water
molecules were found on a Fourier-difference map, but not those of the other water molecules.
Compound 4. Four nitrate ions were given 0.5 occupancy factors in order to retain
acceptable displacement parameters and to account for close contacts between them or with their
image by symmetry. All non-hydrogen atoms were refined with anisotropic displacement
parameters, but for six nitrogen and twelve oxygen atoms in badly resolved nitrate ions, and one
water oxygen atom, which were refined with fixed isotropic displacement parameters. The
hydrogen atoms bound to N1, N2 and N4 were found on a Fourier-difference map, but not those
5
of N3 and the water molecules. Some voids in the lattice likely indicate the presence of other,
unresolved water solvent molecules.
Compound 5. The monoclinic system is ruled out on the basis of the Rint value of 0.42.
Although PLATON15 suggests a missing symmetry center with 86% fit, the correct space group is
P1, due to the presence of the pure L-methionine enantiomorph. Pseudo-merohedral twinning
with a binary axis parallel to c as a twin operator was taken into account (BASF = 0.11). The
hydrogen atoms bound to N5 and N6 and those of nine coordinated water molecules were found
on a Fourier-difference map, but not those bound to the other nitrogen and oxygen atoms.
Crystal data and structure refinement parameters are given in Table 1 and selected bond
lengths in Table 2. The molecular plots were drawn with ORTEP-316 and the views of the
packings with VESTA.17
RESULTS AND DISCUSSION
The two cerium complexes 1 and 2 present the common feature of inclusion of the
ammoniocarboxylate ligand in the CB6 cavity, with differences which arise from the varying
alkyl chain length. Complex 1, represented in Figure 1, involves the zwitterionic form of β alanine which is coordinated through the monodentate carboxylate group. Isomorphous
complexes were obtained with La and Nd, but the lower quality of the crystals did not enable a
satisfying structure refinement in these cases. The coordination sphere of the cerium ion is
completed by three carbonyl groups from CB6, one bidentate nitrate ion and three water
molecules to give a nine-coordinate environment of capped square antiprismatic geometry. The
three carbonyl groups and one water molecule make one square face (O3, O5, O7, O18), while
6
the other comprises the nitrate ion and two water molecules (O15, O16, O19, O20), and the
carboxylate atom O1 is in the capping position; the dihedral angle between the two faces is
2.7(2)°. The Ce–O(carboxylate) bond length of 2.340(7) Å matches the corresponding average
value from the Cambridge Structural Database (CSD, Version 5.33),18 2.41(8) Å. There are few
examples of lanthanide ions bound to three adjacent carbonyl groups of CB6: the first to have
been reported involves the praseodymium ion, in a structure quite analogous to the present one,
but for one cation being present at each portal and the replacement of the included β-al ligand by
a nitrate ion,8d while the others involve the Ce, Pr, Yb and Lu cations with additional perrhenate
ligands.8k The average Ce–O(carbonyl) bond length in the latter cases, 2.57(10) Å, matches the
average value of 2.58(8) Å in 1, with in all cases the central bond shorter (by ca. 0.07–0.30 Å)
than the lateral ones. The metal atom is displaced by 0.669(3) Å from the average portal plane,
toward the outside. The included and disordered β-al molecule is located so that the ammonium
group is close to the uncomplexed CB6 portal, with the nitrogen atom at 0.305(10) Å on the
inside from the average portal plane. The ammonium protons were introduced at calculated
positions (see Experimental Section) and their position is therefore subject to caution; however,
they are indicative of the formation of hydrogen bonds with the uncomplexed carboxylate oxygen
atom (O2), one carbonyl group (O8) and a water solvent molecule. The β-al ligand is thus just
long enough to interact with one CB6 portal through its ammonium group while being
coordinated to a metal atom located very close to the other portal. The packing is unexceptional,
with columns of parallel molecules running along the b axis, these columns being offset with
respect to one another along the a and c axes so as to enable a more compact arrangement.
The cerium complex 2, which involves 6-ammoniohexanoate, is a centrosymmetric,
dimeric, dinuclear complex in which two CB6 molecules are held together by the two metal
7
cations (Figure 2). The latter are doubly bridged by two carboxylate groups, the corresponding 6ah molecules occupying the cavities of the CB6 molecules. Each cation is further bound to two
carbonyl groups from each CB6 and to three water molecules, thus lying in a nine-coordinate
environment of tricapped trigonal prismatic geometry. The two trigonal faces are defined by the
sets of atoms (O1, O3, O5) and (O2i, O9i, O11i) [dihedral angle 13.95(18)°], and the three water
molecules (O15, O16, O17) occupy the capping sites. The cerium atom lies at 2.0235(15) Å from
the average portal plane, while the carboxylate atoms O1 and O2 are very close to it, being
displaced outside by 0.536(3) and 0.101(3) Å, respectively. Such an assembly of two doubly
bidentate CB6 molecules held by two lanthanide ions has previously been described in the case of
[Ce(CB6)(H2O)5]2·6Br·26H2O,8a in which however the two CB6 molecules are strongly tilted
with respect to one another, while they are parallel in 2; comparable columnar, or merely dimeric
arrangements in which CB6 molecules are held together by two lanthanide ions, with however a
different denticity of CB6, were also found in cerium,8l neodymium,8e,l samarium and
gadolinium8k complexes. The average Ce–O(carbonyl) bond length of 2.61(2) Å matches that in 1
and in the related cerium complex cited above [2.50(5) Å], while the average Ce–O(carboxylate)
bond length of 2.37(4) Å is in agreement with the average value of 2.45(6) Å for similarly
bridged cerium structures reported in the CSD. The 6-ah molecule extends in the CB6 cavity,
which is indicative of hydrophobic interactions, and the ammonium group protrudes slightly out
of the cavity at the uncomplexed portal, the nitrogen atom N1 being at 1.119(4) Å from the
average portal plane. The ammonium group is involved in two hydrogen bonds with carbonyl
oxygen atoms [N1···O4 2.795(5) Å, N1–H···O4 145°; N1···O14 2.936(5) Å, N1–H···O14 162°];
the third proton points outwards from the CB6 portal and it is hydrogen bonded to the water
ligand oxygen atom O15 of a neighbouring molecule related to the first by a binary screw axis
8
[N1···O15j 3.171(4) Å, N1–H···O15j 169°; symmetry code: j = 3/2 – x, y – 1/2, 1/2 – z]. These
two molecules, perpendicular to one another, are also held together by another hydrogen bond
between the carbonyl atom O8 and the water ligand atom O17 [O17j···O8 2.935(4) Å, O17j–
H···O8 145°], as well as by five CH···O(carbonyl) hydrogen bonds with H···O distances in the
range 2.52–2.79 Å. The latter interactions are very frequent in crystal structures of CBs in which,
notwithstanding their weakness, they play a prominent role which was recently examined in
detail.1f The particularity of the present case is that each uncomplexed portal is involved in
hydrogen bonds with each of the two CB6 molecules of the neighbouring unit. As a result of
these interactions, layers parallel to the bc plane are formed, which are built from rows of
molecules at right angles to one another (Figure 3). It appears in this case that, in contrast to 1, the
longer chain of 6-ah permits the bridging of two lanthanide ions by the carboxylate group at one
CB6 portal, while bringing the ammonium moiety slightly outside the other portal, in a position
suitable for both intra- and intermolecular hydrogen bonding interactions.
The crystal structures of three complexes with L-methionine could be determined, with
the Nd, Dy and Yb cations (Eu gives a complex isomorphous to that of Nd, but the low quality of
the crystals prevented a satisfactory structure refinement; La and Ce were tried, but did not give
any crystalline material). These structures become more complicated as the size of the cation
increases, and they will thus be discussed beginning with the smallest cation. All of these
complexes crystallize in chiral space groups, the absolute configuration found from the value of
the refined Flack parameter14 being in agreement with that of the enantiomorph used. The number
of atoms in the asymmetric unit is thus quite large, resulting in 1966, 4077 and 2653 refined
parameters for 3, 4 and 5, respectively. The asymmetric unit in the ytterbium complex 3
comprises two Yb ions and two CB6 molecules, which are however nearly identical (Figure 4).
9
Both metal cations are bound to one monodentate carboxylate group and to seven water
molecules, the environment being square antiprismatic [dihedral angles of 0.6(3) and 2.7(5)°
between the two square faces]. The average Yb–O(carboxylate) bond length of 2.274(1) Å is
identical to the average value of 2.28(7) Å from the CSD. An intramolecular hydrogen bond links
one water ligand to the uncomplexed carboxylate oxygen atom. While the metal cation is not
directly bound to CB6, it is nevertheless held in its close proximity through the interactions
between the L-me and aqua ligands with CB6. The ammonium nitrogen atoms are quite far from
the CB6 portals, at 1.744(7) and 1.527(6) Å for N1 and N2, respectively, which brings the Yb1
and Yb2 atoms at distances of 4.052(4) and 4.156(4) Å. Only one hydrogen bond is formed by
each L-me ligand with a carbonyl oxygen atom of the CB6 molecule in which it is included
[N1···O19 2.742(8) Å, N1–H···O19 170°; N2···O31 2.672(8) Å, N2–H···O31 120°], but each is
also bound to a carbonyl from the other molecule [N1···O33 3.048(8) Å, N1–H···O33 142°;
N2···O29 2.842(8) Å, N2–H···O29 178°], while the third proton in each case is bound to a
solvent water molecule. This gives rise to the formation of dimers, in which two parallel
macrocycles [dihedral angle of 3.45(6)° between the proximal portals] are connected through
weak interactions mediated by the ytterbium complexes. Although the protons of some of the
water ligands were not found, the involvement of some of them (corresponding to O3, O7, O15
and O16) in hydrogen bonds with carbonyl groups (O23, O25, O37 and O39) can be inferred
from the O···O distances which are in the range 2.651(8)–2.796(8) Å. Four of the water ligands
directed away from CB6 in each complex unit, for some of which the protons were found, are
hydrogen bonded to the carbonyl groups of neighbouring molecules [O···O distances in the range
2.625(9)–2.933(9) Å], giving rise to the formation of one-dimensional assemblies running along
the b axis. The sulphur-containing chain is further included in the CB6 cavity, with the sulphur
10
atom being closer to the portal bound to the ammonium group [2.415(3) and 2.517(3) Å for S1
and S2, respectively] than to the other portal [3.701(3) and 3.596(3) Å], the terminal carbon atom
being itself completely embedded [2.337(10) and 2.216(10) Å from the nearer portal]. The
shortest contact made by the sulphur atom with the macrocycle involves an ureido carbon atom,
at ca. 3.48 Å; analogous shortest contacts, with the same carbon atoms but a shorter distance of
ca. 3.02 Å, were observed in the case of included perrhenate ions, when their position is not
constrained by metal complexation.8k It is notable that, as in the case of chlorine encapsulation,
these contacts involve the most electropositive atoms of the CB6 cavity lining.1f No other
example of inclusion of a sulphur-containing chain in CB6 seems to exist. In complex 3, the
ytterbium complexes are thus held close to the CB6 portal by ion–dipole interactions, three
hydrogen bonds and hydrophobic interactions. Further hydrogen bonding by water ligands results
in the formation of columns, which are held to one another by ammonium-carbonyl hydrogen
bonds to give sheets parallel to (1 0 ī). This is at variance with the structures obtained with Lcysteine and the cations Nd, Eu, or Tb, in which the shorter thiol-bearing chain is not included in
the CB6 cavity, but points sideways and is involved in some cases in a hydrogen bond with a
carbonyl group. In the latter compounds, the lanthanide ion is complexed to one CB6 molecule,
while the ammonium group is bound to a second molecule, thus generating a columnar onedimensional assembly. The longer thioether-bearing chain in 3 likely favours an arrangement
involving hydrophobic interactions. The packing displays a compact arrangement of sheets, with
no significant free space being present (Figure 5).
The dysprosium complex 4 has many common features with 3, but the larger asymmetric
unit comprises two dimeric units analogous to those in 3, and also two halves of dysprosium
dinitrate pentahydrate moieties located around binary axes, as well as 13 uncomplexed nitrate
11
counterions and 29 free solvent molecules, which gives 464 non-hydrogen atomic positions. The
two independent dimeric units will not be described in detail since their main features are
analogous to those in 3, although slight differences exist in the finer details, such as in the
orientation of the ammoniocarboxylates with respect to the other moieties (Figure 6). These
differences result in the Ln···Ln separation in the dimers being smaller in 4 [12.5443(5) and
12.5253(5) Å] than in 3 [14.3075(4) Å]. As in 3, the shortest contacts between sulphur atoms and
CB6 involve ureido carbon atoms, and they are in the range 3.66–3.86 Å. Each unit in the dimers
pertains to a hydrogen bonded one-dimensional assembly parallel to the c axis, and sheets parallel
to the bc plane are thus formed. In contrast to 3, these sheets are not closely packed, but are
separated by the cationic [Dy(NO3)2(H2O)5]+ moieties, other counterions and solvent molecules
(Figure 7).
The neodymium complex 5 is quite different from 3 and 4, and the arrangement of the
different ligands much less simple. This compound crystallizes in the chiral triclinic space group
P1, with the two unit cell angles α and β close to 90°. The monoclinic system can be ruled out,
but pseudo-merohedral twinning occurs, with a binary axis parallel to c as twin operator; structure
refinement proceeds smoothly when the twin law is introduced (see Experimental Section). The
asymmetric unit comprises four metal atoms, two CB6 molecules, four zwitterionic and two
anionic L-me ligands (not differentiated), ten nitrate counterions and 46 water molecules,
coordinated or free, which gives 294 non-hydrogen atomic positions. The four metal atoms are
separated into two dinuclear units; in both of them, they are bridged by two L-me ligands through
bridging bidentate carboxylates (Figure 8). In the first unit (Nd1, Nd2), each metal atom is
additionally bound to a bidentate CB6 molecule, one chelating nitrate group and three water
molecules, while, in the second (Nd3, Nd4) two extra L-me ligands, either monodentate or
12
chelating, and five water molecules complete the coordination spheres. Nd1, Nd2 and Nd4 are
thus in nine-coordinate capped square antiprismatic environments and Nd3 is in an eightcoordinate square antiprismatic one. The average Nd···O(carboxylate) bond lengths of 2.42(2)
and 2.61(10) Å, for monodentate and chelating coordination, respectively, are in agreement with
the average values of 2.44(6) and 2.54(6) Å from the CSD. Two parallel CB6 molecules are thus
held together by the dinuclear unit comprising Nd1 and Nd2, each being bound to one metal only,
with an average Nd···O(carbonyl) bond length of 2.48(2) Å, in agreement with the value of
2.450(6) Å in the neodymium complex with L-cysteine. The two bridging L-me ligands point
sideways and are not included in the CB6 cavities; one of the ammonium/amine groups likely
forms a hydrogen bond with one carbonyl oxygen atom [N1···O57 2.843(16) Å], but the other
bonds formed involve solvent water molecules or nitrate counterions. The other dinuclear unit is
surrounded by four L-me ligands, either zwitterionic or anionic. As in the first unit, two L-me
ligands are not included in CB6 molecules, and their ammonium/amine groups form hydrogen
bonds with lattice water molecules. The two bridging L-me molecules have their thioethercontaining chain included in CB6 cavities and their ammonium groups, for which the protons
have been found, are involved in two (N5) or one (N6) hydrogen bonds with carbonyl groups
[N5···O38 2.786(13) Å, N5–H···O38 170°; N5···O40 2.752(14) Å, N5–H···O40 134°;
N6···O48i 2.883(12) Å, N6–H···O48i 155°, symmetry code: i = x, y – 1, z – 1], while the other
hydrogen bonds are with complexed or free water molecules. The nitrogen atoms N5 and N6 are
respectively located at 1.625(11) and 1.828(9) Å from the average portal planes, and the sulphur
atoms S3 and S4 at 2.414(6) and 2.352(5) Å from the same portals. As in complexes 3 and 4, the
shortest sulphur-CB6 contacts involve ureido carbon atoms, at ca. 3.49–3.54 Å. The alternation of
CB6 molecules with neodymium dinuclear units, either bonded or associated through weak
13
interactions, results in the formation of chains parallel to [0 1 1], with the CB6 molecules being
tilted with respect to the chain axis (Figure 9); these chains are stacked so as to form layers
parallel to the bc plane. The CB6-complexed neodymium atoms Nd1 and Nd2 are at 1.966(5) and
1.967(5) Å from the corresponding average portal plane, while the uncomplexed Nd3 and Nd4
are at distances in the range 4.260(6)–4.377(5) Å, comparable to those in 3 and 4. It is interesting
to note that only the latter cations appear to be suitably located so as to enable encapsulation of
the bridging L-me ligand, whose position with respect to the CB6 host does not change much
throughout this series, with only second-sphere hydrogen bonding between the lanthanide aqua
ligands and carbonyl groups. In contrast, the CB6-coordinated cations are seemingly too close to
the portal to allow the formation of the specific L-me/CB6 interactions. This is reminiscent of the
situation encountered in the lanthanide ion complexes with iminodiacetic acid and CB6,9 in
which the metal ion is kept out of bonding distance of the carbonyl portal due to the interactions
between the latter and the ammonium group. These results, which evidence some competition
between metal and ammonium complexation, are to be compared to the recent findings that the
ion–dipole and hydrogen bonding interactions between tryptophan or tryptamine and CB6 are
sufficiently strong to displace coordinated magnesium ions.6g
CONCLUSION
The results reported herein are part of an investigation of the structures of the complexes
formed by 4f or 5f element ions with ammoniocarboxylates in the presence of CB6.7,9,10 As
expected, the most obvious variation in the series of complexes obtained is between uranyl and
lanthanides. Although uranyl-lanthanide heterometallic complexes of CB6 with both cations
14
bound to carbonyl groups have been reported,8j recent results obtained with 1,2-ethanedisulfonate
as an additional ligand have shown that CB6 preferentially binds the lanthanide cation, while the
uranyl ion prefers sulfonate coordination.8m Although no complex with both cations and
ammoniocarboxylates exists as yet, those obtained with each cation separately, although quite
heterogeneous as to the nature and geometry of the ligands used, seem to confirm the lesser
affinity of uranyl for CB6. Indeed, in the family of uranyl complexes with ammoniobenzoates,10
the uranyl ion is never bound to CB6, but only to the carboxylate groups (with the occurrence of
additional oxo, hydroxo, formato or aqua ligands in some cases). The geometry of the complexes
is thus determined by the arrangement of the ammoniocarboxylates around the uranyl mono-, dior tetranuclear central units, the CB6 molecules being held at the periphery through ion–dipole
and hydrogen bonding interactions. In contrast, although the lanthanide ions are always bound to
the carboxylate ligand, they can also be coordinated to CB6, as in the case of L-cysteine,7 β-al, 6ah and L-me, or the related case of 2-pyridylacetate.9 With iminodiacetate9 and L-me, they are
only involved in second-sphere tethering to CB6 through the aqua ligands. The key factor in
determining the specificities of the structure is the possibility for the ammoniocarboxylate
molecule to bring the lanthanide ion close enough to CB6 for carbonyl coordination while
retaining a position suitable for ammonium complexation and pendant group encapsulation.
Several cases are thus possible: (i) with β -al and 6-ah, metal and ammonium complexation to
CB6, and encapsulation, all coexist, with either one portal or both being involved; (ii) with Lcysteine, complexation to CB6 results in the ammonium being bound to the portal of a second
CB6 molecule, thus giving a columnar assembly; (iii) in the case of iminodiacetate, ammonium
coordination is only compatible with metal second-sphere bonding; (iv) finally, compound 5
illustrates two different situations and it is the only case in which some of the
15
ammoniocarboxylate ligands do not interact with CB6, which suggests that, in this case, there
may exist a delicate balance between lanthanide complexation by CB6, associated with
ammoniocarboxylate complete exclusion, and lanthanide second sphere tethering to CB6,
associated with ammoniocarboxylate complexation and encapsulation.
ACKNOWLEDGMENT. The Direction de l’Energie Nucléaire of the CEA is thanked for its
financial support through the Basic Research Program RBPCH.
ASSOCIATED CONTENT
Supporting Information. Tables of crystal data, atomic positions and displacement parameters,
anisotropic displacement parameters, and bond lengths and bond angles in CIF format. This
information is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author E-mail: [email protected].
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19
Table 1. Crystal Data and Structure Refinement Details
1
chemical formula
M (g mol−1)
cryst syst
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Dcalcd (g cm−3)
µ(Mo Kα) (mm−1)
F(000)
reflns collcd
indep reflns
obsd reflns [I > 2σ(I)]
Rint
params refined
R1
wR2
S
∆ρmin (e Å−3)
∆ρmax (e Å−3)
Flack parameter
C39H59CeN28O31
1556.26
monoclinic
P21
15.2093(5)
11.5727(4)
15.7952(4)
90
93.599(2)
90
2774.67(15)
2
1.863
0.947
1590
94585
10499
10172
0.021
912
0.072
0.196
1.075
−1.00
3.39
0.43(2)
2
3
C84H144Ce2N56O69
3322.79
monoclinic
C2/c
34.5703(11)
21.2363(6)
21.7315(5)
90
127.1909(15)
90
12709.4(7)
4
1.737
0.837
6832
173673
16369
13649
0.031
1100
0.055
0.171
1.079
−1.54
2.84
−
20
C41H75N28O37SYb
1757.39
monoclinic
P21
14.7871(1)
26.7248(3)
16.7471(2)
90
90.3591(5)
90
6618.03(12)
4
1.764
1.566
3596
197611
25021
23801
0.042
1966
0.057
0.154
1.098
−1.15
3.81
0.015(6)
4
5
C164H312Dy5N115O163S4
7444.03
monoclinic
P2
25.4155(2)
18.8099(2)
29.8228(3)
90
93.6325(6)
90
14228.5(2)
2
1.738
1.458
7598
417600
53848
50057
0.032
4077
0.067
0.190
1.060
−2.94
2.96
0.058(6)
C102H228N64Nd4O112S6
4912.80
triclinic
P1
14.2583(7)
15.0998(4)
25.1467(12)
90.052(3)
90.186(2)
116.190(3)
4858.2(4)
1
1.679
1.239
2520
182415
36063
32731
0.041
2653
0.061
0.161
1.030
−1.33
2.88
0.005(11)
Table 2. Environment of the Metal Atoms in Compounds 1–5: Selected Bond Lengths (Å)
1a
2b
3
a
Ce1–O1
Ce1–O3
Ce1–O5
Ce1–O7
Ce1–O15
Ce1–O16
Ce1–O18
Ce1–O19
Ce1–O20
Ce–O1
Ce–O2i
Ce–O3
Ce–O5
Ce–O9i
Ce–O11i
Ce–O15
Ce–O16
Ce–O17
Yb1–O1
Yb1–O3
Yb1–O4
Yb1–O5
Yb1–O6
Yb1–O7
Yb1–O8
Yb1–O9
Yb2–O10
Yb2–O12
Yb2–O13
Yb2–O14
Yb2–O15
Yb2–O16
Yb2–O17
Yb2–O18
2.340(7)
2.659(10)
2.468(6)
2.602(7)
2.667(8)
2.549(7)
2.492(8)
2.542(7)
2.541(8)
2.325(3)
2.410(3)
2.650(3)
2.606(3)
2.607(3)
2.585(3)
2.582(3)
2.539(3)
2.562(3)
2.273(5)
2.332(6)
2.314(6)
2.314(6)
2.315(5)
2.282(6)
2.405(6)
2.346(5)
2.275(5)
2.282(7)
2.419(7)
2.305(9)
2.218(12)
2.526(12)
2.285(7)
2.254(8)
4
Dy1–O1
Dy1–O3
Dy1–O4
Dy1–O5
Dy1–O6
Dy1–O7
Dy1–O8
Dy1–O9
Dy2–O10
Dy2–O12
Dy2–O13
Dy2–O14
Dy2–O15
Dy2–O16
Dy2–O17
Dy2–O18
Dy3–O19
Dy3–O21
Dy3–O22
Dy3–O23
Dy3–O24
Dy3–O25
Dy3–O26
Dy3–O27
Dy4–O28
Dy4–O30
Dy4–O31
Dy4–O32
Dy4–O33
Dy4–O34
Dy4–O35
Dy4–O36
Dy5–O37
Dy5–O38
Dy5–O40
Dy5–O41
Dy5–O42
Dy6–O43
Dy6–O44
Dy6–O46
Dy6–O47
Dy6–O48
2.311(5)
2.355(6)
2.378(6)
2.373(6)
2.420(6)
2.373(7)
2.401(6)
2.423(7)
2.322(5)
2.399(7)
2.376(6)
2.376(6)
2.390(6)
2.389(5)
2.403(6)
2.371(7)
2.294(6)
2.372(6)
2.365(6)
2.363(7)
2.403(6)
2.400(6)
2.402(6)
2.381(6)
2.300(5)
2.357(7)
2.342(6)
2.375(6)
2.395(6)
2.413(7)
2.381(6)
2.380(6)
2.438(12)
2.453(10)
2.432(13)
2.353(7)
2.410(10)
2.452(6)
2.526(9)
2.275(12)
2.378(6)
2.397(6)
Values for the main cerium position only are given. b Symmetry code: 2: i = 3/2 – x, 1/2 – y, –z.
21
5
Nd1–O1
Nd1–O3
Nd1–O5
Nd1–O6
Nd1–O11
Nd1–O12
Nd1–O13
Nd1–O35
Nd1–O37
Nd2–O2
Nd2–O4
Nd2–O8
Nd2–O9
Nd2–O14
Nd2–O15
Nd2–O16
Nd2–O47
Nd2–O49
Nd3–O17
Nd3–O19
Nd3–O21
Nd3–O25
Nd3–O26
Nd3–O27
Nd3–O28
Nd3–O29
Nd4–O18
Nd4–O20
Nd4–O23
Nd4–O24
Nd4–O30
Nd4–O31
Nd4–O32
Nd4–O33
Nd4–O34
2.384(8)
2.421(7)
2.605(9)
2.558(8)
2.445(7)
2.461(7)
2.533(8)
2.451(7)
2.502(8)
2.428(8)
2.444(7)
2.563(7)
2.551(9)
2.475(7)
2.468(8)
2.523(8)
2.483(7)
2.494(8)
2.479(8)
2.379(8)
2.363(10)
2.544(8)
2.474(8)
2.465(8)
2.458(10)
2.478(8)
2.381(8)
2.512(7)
2.504(7)
2.709(8)
2.497(8)
2.459(8)
2.469(8)
2.477(9)
2.551(9)
Figure Captions
Figure 1. View of the cerium complex 1. Displacement ellipsoids are drawn at the 30%
probability level. Counterions, solvent molecules and carbon-bound hydrogen atoms are omitted.
Hydrogen bonds are shown as dashed lines. Only one position of the disordered parts is
represented.
Figure 2. View of the cerium complex 2. Displacement ellipsoids are drawn at the 40%
probability level. Counterions, solvent molecules and carbon-bound hydrogen atoms are omitted.
Hydrogen bonds are shown as dashed lines. Symmetry code: i = 3/2 – x, 1/2 – y, –z.
Figure 3. View of the packing in complex 2. Counterions, solvent molecules and hydrogen atoms
are omitted. The cerium coordination polyhedra are represented.
Figure 4. View of the two independent units in the ytterbium complex 3. Displacement ellipsoids
are drawn at the 30% probability level. Counterions, solvent molecules and carbon-bound
hydrogen atoms are omitted. Hydrogen bonds are shown as dashed lines.
Figure 5. View of one sheet in the packing of complex 3. Counterions, solvent molecules and
hydrogen atoms are omitted. The ytterbium coordination polyhedra are represented.
22
Figure 6. View of one dimeric unit in the dysprosium complex 4. Displacement ellipsoids are
drawn at the 50% probability level. Carbon-bound hydrogen atoms are omitted. Hydrogen bonds
are shown as dashed lines.
Figure 7. View of the packing in complex 4, with the sheets viewed edge-on. Counterions,
solvent molecules and hydrogen atoms are omitted. The dysprosium coordination polyhedra are
represented.
Figure 8. View of the neodymium complex 5. Displacement ellipsoids are drawn at the 30%
probability level. Counterions, solvent molecules and carbon-bound hydrogen atoms are omitted.
Hydrogen bonds are shown as dashed lines.
Figure 9. View of the packing in complex 5. Counterions, solvent molecules and hydrogen atoms
are omitted. The neodymium coordination polyhedra are represented.
23
Scheme 1. Cucurbit[n]uril
Figure 1
24
Figure 2
Figure 3
25
Figure 4
Figure 5
26
Figure 6
Figure 7
27
Figure 8
Figure 9
28
For Table of Contents Use Only
Supramolecular Assemblies Built from Lanthanide
Ammoniocarboxylates and Cucurbit[6]uril
Pierre Thuéry
Different association modes of lanthanide ions, cucurbit[6]uril, and ammoniocarboxylate
molecules are observed, depending upon the nature and geometry of the latter. While the metal
ion is always bound to the carboxylate group, the association of the ammoniocarboxylate to CB6
through weak interactions is not always compatible with carbonyl coordination.
29