Chapter 3
Receptors for alkali metal salts
through cation- interactions.
3.1aIntroduction: gas-phase and computational studies.
The possibility of attractive interactions between a -donor aromatic ring and a
cation was envisaged almost thirty years ago. In 1978, Vögtle’s group designed a
diazacrown ether system 1 (Figure 1, left) whose benzenoid ring, linked with spacers
of different length, could enhance the binding of cation like Na+ and K+ to the crown
ether moiety.
O-(CH2)n-O
O
1
N
O
O
N
O
O
O
O
2
n = 2- 6
O
Figure 1: Vogle’s diazacrown 1 and Kawashima’s benzocrown 2.
In actual facts this study gave no conclusive evidence either for or against the
participation of the aromatic rings in the complex formation.1 A similar approach to the
problem was followed by Kawashima et al. in the same year.2 NMR experiments
reported down field shifts of the aromatic signals of the crown ether 2 (Fig. 1, right),
upon complexation with Li+, Na+ or K+, that, however, are not a certain proof of the
contribution of the -donor moieties in the association with the alkali metal ions, since
the possibility of a “through bond” effect cannot be excluded.
The first clear evidence of real interactions between a cation and arenes is
given in 1981 by Kebarle’s seminal paper.3 The potassium ion is found to establish
strong interactions with benzene in the gas phase. The first solvating benzene
molecule, surprisingly, gives rise to an interaction which is even stronger than that with
a water molecule (19.2 kcal mol-1 for K+ compared to 17.9 kcal mol-1 for H2O). The
Chapter 3
energy for the K(C6H6)+ complex formation has been addressed to short range
interactions (electrostatic quadrupole-ion term, ion-induced dipole and dispersion
forces). This fact, combined with the size of benzene, predicts a restricted motion for a
molecule interacting with the K+ ion. This explains why the entropy changes for further
water molecules addition after the first one are much less unfavourable than those for
benzene molecules; moreover neither K(C6H6)5+ nor K(C6H6)6+ complexes have been
detected where K(H2O)5+ and K(H2O)6+ were easily formed.
Kebarle’s work has been followed by a great amount of computational studies,
recently revisited.4 The distance between the alkali ion and the aromatic benzene
carbons depends on the level of theory chosen and whether ab-initio or DFT
calculations are used. No differences bigger than 0.1 Å were found, though. The
calculated preferred geometry which the cation would adopt on the aromatic surface is
common to the vast majority of studies: it is on the 6-fold axis normal to the benzene
plane giving a C6 symmetry, as shown in Figure 2(a). Deviations from the ideal
geometry, increasing, for example, the angle of the cation to the C6 axis normal to the
ring produces an energetic loss, as illustrated in Figure 2(b) for the Li+, Na+ and K+
cases.5
a)
b)
Figure 2: a) preferred geometry for a cation- interaction; b) energetic effects of increasing the
angle of the cation to the C6 ring normal.
When the angle is set to 30 degrees the binding energies suffer a significant decrease to
44%, 37% and 26% of their original value for Li+, Na+ and K+, respectively.
Surprisingly, K is the metal which looses more in terms of binding energy even if it is
softer than the others ions. Substitution with heteroatoms on the ring can have effects
resulting in an off-center position of the cation (an offset of 0.2 Å has been
reported).6,14
78
Receptors for alkali metal salts through cation- interactions.
In Table 1, calculated and experimental the M+ to benzene-centroid distances
and calculated H298 are reported for the series of alkali metal ion (Li+ to Cs+). Data are
taken from the most relevant works in the field.
o
Table 1: Experimental and calculated H 298
(kcal mol-1) for M-benzene interactions
and M-to arene centroid distances (Å).
M+ ion
Li+
Na+
K+
Rb+
Cs+
o
H 298
Calculated
Experimental
33.6 - 36.8±0.2
21.0 – 24.7
13.0 - 20.1
11 – 16.4±0.2
9 – 12.5±0.2
37.9 - 39.3±3.2
21.5 - 28±1.5
13.3 – 19.2
16.4±0.9
15.5±1.1
M+ to centroid
1.879-1.936
2.390-2.420
2.786-2.848
3.089-3.148
3.309-3.406
The M+ to centroid distance increases in the series from an average value of 1.90 Å for
Li+ to 3.36 (average value) for Cs+ due to the increasing dimension of the ion. The
o
enthalpy ( H 298
) decreases monotonically from Li+ to Cs+, the electrostatic term being
the principal contribution to the binding in this kind of interaction. In some cases
discrepancy among experimental values is evident.
The achieving of a rational design for receptors capable of recognition through
cation- interactions needs to consider more complex models than a benzene molecule.
Lately, computational studies are in this direction indeed. It is worth to note that
handling real receptors can be very complex and many different features have to be
taken into account. Even simple molecular receptors can have several conformations
whose populations can be altered by an interacting species. They can have different
binding sites which can be either of the same kind (homotopic) or of different kind
(heterotopic).
A recent paper dealing with calix[4]arenes (a class of macrocycles widely used
in molecular recognition) shows clearly the point.7 It reports a DFT study and second
order perturbation theory in order to identify the binding modes of Na+ and Cs+ to a
tetramethoxycalix[4]arene. Remarkably, several structures anticipated by this
computational analysis found a real correspondence in solid state structures. 8-12 The
results show how Na+ better binds partial-cone and cone conformers and that in both
cases sodium ion has strong interactions with two oxygens and two aromatics
(2O+2A). On the contrary Cs+, practically binds only to the partial cone conformation
with three aromatics and one oxygen (O+3A). The contrasting binding mode observed
79
Chapter 3
with smaller (Li+ and Na+) compared with larger (K+, Rb+, Cs+) ions is very interesting
and seems to be common in these systems.13
Cation- interactions have benefited of an increasing number of theoretical
investigations in recent years which demonstrate that their role is very important and
can be hardly overstated. Nonetheless, significant deviations from the ideal behavior
are very common in real systems. The understanding of how this kind of interactions
work in the condensed matter is still far to be reached and further experimental data are
eagerly needed.14
3.2 Introduction: solid state and solution studies.
Despite the attention given to alkali metal cations in the last three decades their
interactions with aromatic units and -donor groups have been elusive. Experimental
findings in the condensed phase have proved to be very challenging. Indeed, many of
the early proofs of the existence of cation- interactions are episodical in nature and
consist mostly in crystal structures in which an unanticipated geometry reveals close
M+ to arene contacts. The crystal structure of the K[Al(CH3)3NO3]·C6H6, dated 1978, is
a good example. It displays a K+ ion which is symmetrically placed over the benzene
centroid with an average K-C distance of 3.38(6) Å.15 A similar structure is found with
Cs+.16 Other examples of aromatic solvent molecules interacting with an alkali metal
ion via cation- interactions are found in the crystal structures reported by Michl and
Rowlings.17,18a The dissolution of alkali metal salts with bulky counteranion featuring a
very low charge density in aromatic solvents is an excellent route to develop crystal
structures with the metal ion in a very low coordinative environment. This approach
with alkali metal salts made of carborate (CB11Me12-), magnesate and zincanate
provided crystals whose X-ray analysis revealed structures with M+ to arene distances
very close to the ones reported by computational studies in gas phase.18b Notably,
Michl found that Li+ and Na+ behaved considerably different with respect to larger
metal ions (K+, Rb+ and Cs+). The latters produced isomorphous crystals in which two
benzene rings adopt a sandwich-like structure with the metal ion, thus displaying a 6bonding fashion. Metal to centroid distances are 3.14, 3.19 and 3.28 Å for K +, Rb+ and
Cs+ respectively. Four methyl of four different anions complete the coordination
sphere of 6, as shown in Figure 3(a). The structures by Rowlings are similar. Two
aromatic molecules solvate the K+ ion, with the aid of two methyls from the anion,
giving rise to infinite chains in which the cation is close to a tetrahedral geometry (Fig.
3b). Rb+, instead, is in trigonal bipyramidal geometry with three toluene molecules in
the coordination sphere (Fig. 3c). This is the only case in which more than two
80
Receptors for alkali metal salts through cation- interactions.
aromatic molecules participate to the binding of the metal center. The shortest K + to
arene contact is found in the magnesate complex with toluene (M+ to centroid: 2.884
and 2.937 Å) and in the zincanate complex with toluene for Rb+ (3.088, 3.112 and
3.113 Å).
Recently, the structure of K4[(L)2(-CO3)2-Fe(III)2)]·6H2O·2EtOH complex in
the solid state has been elucidated.19 The occurrence of very short ion to arene contacts
(2.897 and 2.930 Å) with a molecule which is clearly not designed to be a specific
receptor can be seen as a hint of the high adaptability of these interactions.
a)
c)
b)
Figure 3: a) Michl’s potassium carborate complex with benzene, M stays for K + in the picture;
b) Rowlings’s K-complex infinite chain and c) Rb+ complex: three interacting toluenes
molecules.
Cation- interactions are not only powerful but also versatile and may be observed
even if the geometry is not optimal. This structure also shows how K+ prefers the
hydrophobic part of the complex, resembling the binding modalities in proteins.
From the point of view of a rational design the studies made on calix[n]arenes
are more intriguing. Because of the ease in large scale preparation and derivatization,
81
Chapter 3
calix[n]arenes and their derivatives have emerged as an excellent scaffold for a variety
of applications in molecular recognition. Their concave molecular architecture, tunable
in size, can be a -donor site for suitable guests. Therefore, alkali cations can find
favourable -contacts with any of the n benzenoid rings of which the host is composed.
This was first highlighted by Izatt and co-workers in 1985 who showed the capability
of these compounds to transport alkali metal cations between two acqueous phases
separated by a membrane in which calixarenes were dissolved.20
Figure 4:
K4[(L)2(-CO3)2Fe(III)2)]·6H2O·2EtOH
Legenda: K=purple, Fe=green,
O=red, C=black and N=blue.
Calixarenes may exist in several different conformations. The derivatization of
upper and lower rim with bulky groups and alkoxy chains, respectively, can be useful
to block the molecule in the preferred geometry.21
The association with K+ has been deeply investigated. Shinkai found by NMR
experiments that the partial cone isomer of a tetra-t-butyl-calix[4]arene interacts with
the potassium ion exploiting three of the four aromatic rings and one oxygen offered
by the methoxy group of the inverted ring (3A+1O).22a Similarly, the 1,3 alternate
conformation in the tetrapropoxy-calix[4]arene (Fig 5a) shows high affinity for K+
(determined by extraction experiments). In this case two aromatics and two oxygens
were thought to be responsible of the binding.22b The same geometry was observed in
the X-rays structure of an 1,3 alternate tetrakis-(ethoxycarbonylmethoxy)-calix[4]arene
shown in Figure 5b. However Na+ ion seems not to be involved in any contact
consistent with the presence of weak cation- interactions with the aromatic rings.22c
82
Receptors for alkali metal salts through cation- interactions.
a)
b)
Figure 5: a) proposed geometry for the K-complex with the tetrapropoxy-calix[4]arene; b)
representations of the Na-complex with the tetrakis-(ethoxycarbonylmethoxy)-calix[4]arene:
chemical structure and X-ray determined crystal structure; sodium ion in green.
Calix[6]arenes complexed with K+ in a double partial cone conformation were reported
by Aoki.23
a)
b)
Figure 6: a) Aoki’s 8-coordinated K-complex, ORTEP view b) 6-coordinated K-complex,
ORTEP view. Only one of the two K ions present in the complexes is considered in the
discussion
Coordination number of 8 (6O+2A, close M+ to arene contacts of 3.32 and 3.35 Å) and
of 6 (5O+A, M+ to arene distances in the range 3.168-3.582 Å) are reported in Figure 6
(left and right, respectively).
The rational combination of an aromatic cavity with a crown ether moiety can
lead to enhanced binding and selectivity. Calix[n]crown[m]arenes are the product of
83
Chapter 3
this approach. The calix[4]crown[5]arene without any upper substitution reported by
Reinhoudt,24a adopts 1,3 alternate conformation which strongly binds K + with a
selectivity over Na+ even better than Valinomicyn. Obviously, the number of the
ethereal units of the crown part is very important to determine which cation is better
bound to the host. Thus, shortening the crown length by one unit, the selectivity
between K+ and Na+ is completely reversed.24b
If the target ion is Cs+, then a calix[4]crown[6] is definitely a better host.
Ungaro’s receptor, shown in Figure 7a (left), displays a remarkable selectivity for Cs +
over K+ and Na+.25 The X-ray structure clearly shows up two distinctive contacts
between the ion and the benzenoid ring (shortest Cs-carbon distances of 3.49 and 3.69
Å) together with interactions with two oxygens. The Cs+ coordination number in this
case is 9. The 1,3 alternate conformation is active in the binding of Cs+ in solution, as
documented by NMR for the related conformationally mobile compound lacking the
bulky t-butyl groups.
a)
b)
3-Cesium picrate
4-(CsSCN)2
Figure 7: a) Cs-complex with calix[4]crown-6 3; b) Cs-complex with the bis-crown 4.
To verify the presence of cation- interactions in solution, they measured the
H NMR spectra of metal ion complexes of ligand 3 in CD3OD. The downfield shifts
found were maximum for the doublet of the meta protons of the benzenoid ring and
they followed in magnitude the charge density order (K+ > Rb+ > Cs+).
Examples of 1:2 host-guest stoichiometry are given by Tuery’s group. In the
complex between 4 and CsSCN, each cation interacts with the crown and with two
1
84
Receptors for alkali metal salts through cation- interactions.
aromatic rings (Fig. 7b, right).26 The resulting coordination number is 9 for both
cesium atoms (six oxigens, one thiocyanate molecule and two arenes). The last
interesting examples again refer to the binding of Cs. In Figure 8 a and b, the two alkali
metal complexes with calix[4]arene are shown. In both structures the Cs atom is bound
within the cup-shaped cone conformation adopted by the macrocycle ligand. In the first
structure (Fig. 8a) the large Cs+ ion allows for multiple interactions with the four
benzenoid rings and, unexpectedly, the oxigens are more than 4 Å far from the
cation.27 The complex is highly symmetrical displaying a C4 axis. Cs+ to centroid
distance is reported to be 3.57 Å (shortest contact of 3.545 Å) and an acetonitrile
molecule stays on the axis of symmetry with the nitrogen atom 3.29 Å far from the
metal. In the other Cs-complex, Figure 8b (right), however, interactions with oxygens
from solvent molecules and from the ligand itself are present. Interesting is the
133
Cs-1H HOESY spectrum which reveals that the complex is present also in
chloroform solution.28 In more polar solvent, instead, it remained undetected.
a)
b)
Figure 8: a) Ortep view of t-butylcalix[4]arene with Cs cation; b) Cs-complex of
calix[4]arene in cone conformation.
a
More recently Gokel’s group devoted a big effort in order to unequivocally
find strong evidence of cation- interactions in the solid state and in solution.29 Its
approach utilises bibrachial lariath ethers of the general structure in Scheme 1. The
principal binding contribution is demanded to the macrocycle oxygens, in close
analogy to crown ethers. The side chains attached to the nitrogens can reinforce the
binding if they posses -donor features and optimal geometry. Overcoming initial
difficulties it has been found that -donor aromatic moieties positioned two carbons
away from the flexible invertible nitrogen atoms can adopt energetically favourable
interactions with alkali metal ion (Na+ and K+) bound to the macrocycle ring. -donor
85
Chapter 3
aromatics (R groups) have been chosen appropriately. In proteins, in effect, aminoacids
like Phe, Tyr and Try are thought to be involved in several cation- interactions with
alkali metal ions (very abundant in vivo) and with positively charged side chains
belonging to residues either of the same domain (affecting secondary and tertiary
structure) or different domain.
O
O
R
N
5 R=
7 R=
6 R=
8 R=
N
R
O
O
OH
Scheme 1: Gokel’s BiBLE receptors
N
H
N
Me
Gokel’s receptors with R= benzene, phenol or indole (5, 6 and 7) are therefore
good models for the study of biological phenomena involving cation- interactions. In
Figure 9, X-ray structures of the complexes of 5-KI, 6-KI, 7-NaI, 7-KI, 7-KSCN and
7-KPF6, in their CPK models are shown.
Figure 9: Crystal structures of Gokel alkali metal salts complexes. First row from the left:
5-KI, 6-KI and 7-NaI; second row, from the left: 7-KI, 7-KSCN and 7-KPF6. Legenda: C=dark
gray, H=light gray, O=red, N=blue, S=yellow, F=green, I=purple.
Each arene occupies the apical position while the two nitrogens and the four oxygens
donor atoms of the macrocycle are in the equatorial plane of an 8-coordination ideal
86
Receptors for alkali metal salts through cation- interactions.
square bipyramidal geometry. The counterion, which can be found 5.48-6.74 Å far
from the cation, is excluded from the coordination sphere of the metal which is totally
enveloped in the “folded” sandwich-shaped host. The 7-NaI complex is the only folded
structures involving Na+. This is quite surprising in light of the experimental
measurements in the gas phase that have demonstrated the much stronger binding of
sodium over potassium ion with benzene. It can be deriving from the intrinsic
geometry of the receptor itself or from the nature of sodium ion.
Direct comparison with uncomplexed structures in the solid state can be made.
For example, in the case of receptor 7, Figure 10 below, shows how the side arms fold
back onto the macroring in order to establish cation- interactions with the alkali metal
ion upon the complexation with KPF6 (left) or NaI (right).
Figure 10: Folding of the empty host (middle) upon addition of KPF 6 and NaI. The structural
difference between empty and complexed species is evident.
The structural analysis of the complexes concerning M+ to arene distances are collected
in Table 2.
Table 2: Distances related to Gokel complexes in Figure 9.
M+ to R distances (Å) (Receptor 7)
Distances in Å
complex
5·KI
6·KI
7·NaI
7·KI
7·KPF6
7·KSCN
M+ to
arene
3.44
3.43
3.50
3.45
3.48
3.43
arenearene
6.88
6.86
6.99
6.89
6.95
6.86
M+ to
anion
6.74
6.56
6.17
6.06
5.48
5.94
R
N(indole)
C2
C3
C8
C9
Pyr centroid
NaI
3.58
3.23
3.51
4.05
4.02
3.50
KI
3.51
3.32
3.57
3.89
3.91
3.45
KPF6
3.57
3.30
3.52
3.98
3.94
3.48
KSCN
3.52
3.38
3.55
3.82
3.83
3.43
87
Chapter 3
The metal ion to arene and the arene to arene distances are very similar among the Naand K-complexes. The C2 atom of the indole ring (Figure 11), in all the Na- and K-7
complexes, is the atom which display the shortest distances with the metal ion and thus
the binding mode could be considered as 1. The shorter distances of the pyrrolo ring
compared to the benzo ring show how the former is the most favourable partner for
cation- interaction in this system (metal to centroid distances are about 1 Å shorter for
pyrrolo centroid compared to benzo centroid).
Very convincing are the data in solution which are rare in studies on this
subject. The comparison of NOE spectra of the compound 7 before and after the
complexation of NaI in acetone clearly shows that the folding process is occurring
even in solution. Titration of hosts 7 and 8 with NaI in the deuterated acetone reported
in Figure 11 demonstrated the different electronic effects that the complexation with a
cation can induce in the host.
Figure 11: The alkali metal complexation
induced shifts () on the selected host’s
signal. R=-H, left panel; R=-CH3, right
panel.
The structure with host 7 are the first ones to show an indole involved in cation-
interactions and the primary interaction is found to occur with the pyrrolo subunit of
the ring. High level calculations predicted the benzenoid ring of the indole to be the
favourite for such interactions over the pyrrole ring.30 The rationale of this unexpected
behavior can be found in steric effects which render the pyrrolo subunit more
accessible to the binding. When the side arm is connected to the indole’s 5- rather than
3-position, as in the one-armed receptors 9 and 10, the benzo unit of the indole pendant
is the one involved in the complexation.31 In these sodium and potassium complex
showed in Figure 12 below, the M+ to centroid distances are interestingly very short
88
Receptors for alkali metal salts through cation- interactions.
(2.75 and 2.97 Å for Na-9 and K-10, respectively) if compared to the bibrachial
analogues shown before.
O
O
NH
N
O
O
)n
(
9 n=1
10 n=2
Figure 12: Monobrachial artificial receptors for alkali metal ions. CPK and tube. C=black,
O=red, N=blue, Na=yellow and K=violet. H atoms are not shown.
Very similar to Gokel’s is the approach which Kochi adopts in a recent paper
dealing with the subject.32 The need of a primary binding site to constraint the metal to
interact with neighbour arenes is still ineluctable.
OMe
OMe
OMe
MeO
MeO
11
OMe
Figure 13: Receptor 11 and the X-ray structure of its uncomplexed host.
Receptor 11, hexakis(methoxymethyl)benzene, can be seen as a reversed lariath ether
(Fig. 13). It consists in an aromatic nucleus with ethereal substituents. Thus the
ethereal oxygens, interacting with the cation, deliver it to the benzenoid center, a
suitable site for ancillary cation- interactions with the metal ion.
Figure 14: K+ and Rb+/Cs+ complexes with the hexakis(methoxymethyl)benzene host 11.
89
Chapter 3
The crystal structures of 11 complexed with K+, Rb+ and Cs+ are shown in the Figure
14. Kochi, remarkably, reports a very detailed structural analysis which takes into
account many features of the crystal structures. Metal-ion to aromatic carbons
distances, disorder from local symmetry, distortion from benzenoid planarity and the
angular M+ coordination to the ethereal oxygens are considered. The conclusion is very
intriguing and again it stresses the different behavior of sodium over larger alkali
metals. Rubidium and cesium ions are involved in strong cation- interaction with the
aromatic ring. In particular, they show a binding mode (1) reminiscent of the classical
-arene complexes with positively charged electrophiles. The complexation of K+ ion
leads to a 2-bonding to the aromatic center, closer to the ideal computed geometry. On
the contrary, there is no significative evidence of a cation- contribution to the binding
in the Na+-complex which is formed relying exclusively on the interactions with the
ethereal oxygens.
In this section all the most notable examples, to the best of our knowledge,
have been described. In many cases cation- interactions have been found in the solid
state just as a coincidence and just few are the cases in which, on the contrary, these
interactions have been sought on purpose. Kochi’s and Gokel’s work are the best
representative of a rational research directed to this ambitious goal. These reported
examples can be considered in terms of selected distances between host and guest
carbons having in mind the coordination number (C.N.) which is important in order to
provide correct comparison. More elaborate analysis are however possible if good
X-rays data are obtained. The threshold beyond which is reasonable to admit the
occurrence of cation- interaction is obviously the sum of the vdW of the arene and the
ionic radius of the cation. On these basis designed receptors afforded excellent results:
for example the monobrachial receptor 10 by Gokel in its K-complex. In some cases
studies in solution have confirmed and enriched the solid state data. In this respect
calix[n]arenes demonstrated to be very adaptive molecular scaffolds on which it is
possible to rely for molecular recognition of alkali metal salts.
Cation- interactions have been evoked as primary binding forces between
arenes and transition metals. In this section such contributions have not been taken into
account for sake of brevity but also because these kind of interactions imply
coordinative bonds and, therefore, they cannot be considered as pure cation-
interactions (where the electrostatic term is almost 60 % of the total). An exception to
this consists in the case in which the metal is already fully coordinated by its ligands
and the arene cannot be included in its primary coordination sphere of the metal. Such
species can be considered as super cations, for example the hexaammonia complex of
cobalt [Co(NH3)6]3+ in which the metal is exhaustively coordinated to the ammonia. A
90
Receptors for alkali metal salts through cation- interactions.
very interesting and complete review on this particular subject has been recently
published. Both computational and experimental approaches are overviewed.33
3.3 Salophen-Uranyl Complexes as Receptors for Alkali Metal Salts.
The structural features inherent to alkali metals pose an extremely difficult
problem of design when the binding of such species is sought. These monovalent
cations represent featureless spheres of different sizes and none of the directionality in
the bonding exhibited by transition elements is present. Moreover, the solvation of
these cations is generally very strong and an ideal receptor should provide to the guest
ion interactions strong enough to contrast the presence of solvent molecules in the
coordination sphere of the cation. The counterion has also to be taken into account.
The energy needed to separate the ion pair can be a serious obstacle to the binding.
In this thesis we report the preparation and characterization of complexes
formed by the uranyl-salophen receptors A, F and I with halides of the larger alkali
metals (K+, Rb+, and Cs+). The results of such an investigation further substantiate the
general occurrence of cation- interactions with alkali metal ions and underscore their
role in the formation of supramolecular assemblies.
A salophen-uranyl receptor can be seen as a Lewis acid embedded in an organic frame.
The acidic properties of the UO2 center provide an excellent binding site for any Lewis
base. If the organic ligand is designed in such a way to possess a second binding site
for the recognition of a cation, salophen-uranyl complexes can be used as heteroditopic
receptors.
N
N
N
UO2
O
O
O
O
N
UO2
O
O
O
O
O
A R= H
I R=Me
R
N
N
UO2
F
C
R
Figure 15: Receptors A , F, C and I. These salophen-uranyl complexes are the protagonist of
an extensive study on their properties as heteroditopic receptors for alkali metal salts. A, F and
I are side-armed whereas C is taken as reference being it lacking of adjunct aromatic binding
sites.
91
Chapter 3
Therefore, uranyl-salophen derivatives such as A, I and F have been thought to possess
the potential to behave as receptors for alkali metal ion pairs. Molecular formulae of A,
I and F, along with the structure of C, a reference compound, are shown in Figure 15.
The aromatic sidearms of A constitute -donor moieties able to interact with the cation
through cation- interactions.
A large number of co-crystallization attempts of ligand A with KF, CsF and
the whole series of alkali metal chlorides from Li to Cs were carried out in mixed
solvents (CHCl3, MeOH, MeCN and H2O). Proportion of A to alkali metal salts
ranging from 1:1 up to 1:10 were used. The solid materials obtained upon slow
evaporation at ambient temperature were in many cases either amorphous or powder,
or contained no suitable crystal for X-ray analysis. In a number of cases, crystals of the
salt-free receptor coordinated to different solvent molecules were obtained. 34 In few
cases it has been possible to obtain the desired single crystal complex suitable for an
X-ray analysis. Thus the solid state structures of complexes between receptor A and
KCl, RbCl, CsCl and CsF are reported and discussed.
Figure 16: (A-KCl)2 complex, CPK. left) dimeric assembly; right) view of one of the two
enantiomeric halves of the complex. Legenda: H=white, C=light grey, N=blue, O=red, U=dark
grey; Cl=green; K=cyan. Solvent molecule are omitted for clarity.
The A-KCl complex is best described as 2:2 complex of receptor and salt due to its
evident dimeric nature (Fig. 16, left). Each uranyl strongly binds to a halide ion in the
equatorial plane (U-Cl= 2.781 Å), and two negatively charged 1·X- units are connected
through strong coordination to two metal ions. The two uranyl-salophen units in the
92
Receptors for alkali metal salts through cation- interactions.
dimer are enantiomeric and are combined in a centrosymmetrical arrangement
featuring a dimeric capsule in which an ion quartet is enclosed. Indeed the two close
contact ion pairs are completely surrounded by the receptor organic frame. In this
complex the potassium ion is ten-coordinated (6 oxygens, 2 chlorides and 2 aromatic
rings interact with each metal ion). The cutaway view of the complex shown in Figure
16 (right) represents one of the two enantiomeric halves and it enlightens the
interaction between one aromatic side arm and the potassium ion which sits on the
benzyl ring.34 Figure 17 (left) stresses another important factor: the role of the
oxygens. It cannot be underrated and it strongly contributes to the overall binding
process. Six oxygens (three from each receptor molecule) show close contacts with the
metal ion (see Table 3 for details). Peculiar is the disposition of the oxygens in the
dimeric assembly. They closely resemble a crown ether like arrangement 35 which
occurs only upon the formation of the supramoleculecule in the solid state. In respect
to this point and to the quartet complexation, the 2:1 KSCN complex with
dibenzo[24]crown-8 in Figure 17 (right) has some analogies worth to be noted.36
Figure 17: left) ball&stick version of the (A-KCl)2 complex highlighting the role of the
oxygens in the complex formation; right) crown-ether complex with two KSCN molecules.
The crystal structures of A-RbCl, A-CsCl and A-CsF obtained are shown in Figure 18
(dimeric assembly, left column and cutaway view showing the monomeric adduct,
right column). A glance to Figure 18 immediately reveals the close similarity of the
molecular structures of the four alkali metal halides complexes. RbCl, CsCl and CsF
all form structurally similar 1:1 complexes, best described as 2:2 complexes owing to
their evident dimeric nature.
93
Chapter 3
Figure 18: A-RbCl, A-CsCl and A-CsF complexes in CPK: C=light grey, H=white, N=blue,
O=red, Cl=green, F=purple Cs=yellow, Rb=light purple, U=dark gray. Left column: dimeric
assembly formed by two enantiomeric halves; right column: cutaway view showing the
monomeric 1:1 adduct, underlining the cation- interactions with the metal ion.
94
Receptors for alkali metal salts through cation- interactions.
Complexes A-RbCl and A-CsCl even crystallize with similar unit cells and
nearly isomorphous structures.
Even if theoretical studies anticipate the 6 binding mode to be the most
favourable geometry for cation- interactions with the cation sitting on the center of
the arene, situations which show deviations are common. Actually, markedly off-center
geometries are rather the rule than the exception when the cation-coordinated arenes
are part-structures of more elaborate receptors (notable examples already discussed can
be found in Section 3.2). This is clearly due to the presence of many different sources
of attractive and repulsive interactions in these systems. In our case the metal ion is
subjected to ion-dipole interactions with the oxygens, to the electric field of the anion,
to the cation- interactions by the arenes and to all the steric effects intrinsic to the
receptor structure and finally to generic van der Waals and dispersion forces. All these
terms have to be taken into account in the economy of the crystal structure formation.
A critical analysis of the reported crystal structures implies a survey of the
interatomic bond distances. The sum of van der Waals radii of two non-bonded atoms
should reveal the threshold beyond which the presence of attractive interactions is
guaranteed. The vdW radius of an aromatic carbon is widely accepted to be in the
range 1.72-1.80 Å.37 Concerning alkali metal cation, unambiguous evaluation of their
vdW radii is complicated by the lack of readily definable van der Waals contacts in
condensed phase.32 However, ionic radii instead of vdW’s can be appropriately used.
N
N
UO 2
O
10
O
26
X
O15
O31
M
C17
22C
C33
C18
38C
A
21C
C19
C
20
C34
B
37C
C35
C
36
Figure 19: left) Alkali metal ionic radius versus coordination number; right) numbering scheme
for the A-alkali metal salts complexes.
95
Chapter 3
Figure 19 (left) reports the ionic radii of the alkali metal series with respect to
their coordination number.38 In our case, a CN of ten will be considered. The
numbering scheme for receptor A is also shown (Fig. 19 right).
Table 3: Alkali Metal Halide Complexes of receptor A. Distances (Å) between the metal
cation and a given atom, with reference to the numbering scheme in Figure 19. The
apostrophe denotes an atom belonging to the enantiomeric unit (not shown)
Alkali metal salts
Atom
KCl
RbCl
CsCl
CsF
A (C17)
A (C18)
A (C19)
A (C20)
A (C21)
A (C22)
A centroid
3.297
3.576
4.004
4.109
3.820
3.418
3.454
3.743
4.534
4.959
4.656
3.845
3.338
3.986
3.601
4.056
4.322
4.135
3.730
3.437
3.644
3.583
3.818
4.195
4.349
4.140
3.768
3.737
B' (C33)
B' (C34)
B' (C35)
B' (C36)
B' (C37)
B' (C38)
B'centroid
3.643
4.413
4.797
4.509
3.724
3.269
3.796
3.578
3.373
3.790
4.285
4.451
4.083
3.703
3.830
4.525
4.857
4.568
3.841
3.446
3.975
4.213
5.204
5.635
5.159
4.159
3.608
>4.5
X (F, Cl)
X' (F, Cl)
O (UO2)
O'(UO2)
O15
O10
O26
O31
3.118
3.153
3.870
3.584
2.873
3.006
2.854
3.013
3.318
3.215
3.470
3.634
3.181
2.944
2.974
3.101
3.337
3.491
3.637
3.437
3.238
3.128
3.045
3.348
2.937
3.025
3.349
3.861
3.440
3.030
3.115
3.129
Ionic radii for a 10-coordinated K+, Rb+, and Cs+ are 1.59, 1.66, and 1.81 Å,
respectively. Using an average van der Waals radius of 1.76 Å for an aromatic carbon,
van der Waals contacts of 3.35, 3.42. and 3.57 Å are calculated for K +, Rb+, and Cs+,
respectively.
96
Receptors for alkali metal salts through cation- interactions.
As reported in Table 3, the KCl complex shows two very close contact
distances of 3.297 and 3.269 Å with C17 of ring A and with C38 of ring B' (the
apostrophe denotes the atoms belonging to the enantiomeric half of the complex not
shown in the numbering scheme in Fig 19). These contacts are clearly shorter than the
sum of vdW and ionic radii and denote a markedly eccentric 1 binding mode. Close
contacts were found in the RbCl-complex: distances of 3.338 and 3.373 Å, for C22 (A)
and C34 (B') respectively, are significatively shorter than the 3.42 Å limit. Again a 1
binding mode seems the best way to describe such interaction. The CsCl complex
displays shortest distances of 3.437 and 3.446 Å with C22 (ring A) and C38 (ring B')
respectively, well beyond the significative threshold taken as reference. An 1 binding
mode is still present even if C17 and C21 (both of ring A) are very close to the metal
ion, too. The CsF complex is somewhat different from the CsCl analogue showing a
more marked eccentric geometries in relation to the arene-Cs binding. Both of the M to
centroid distances are longer than expected (especially for ring B'). Still contacts of
3.583 and 3.608 Å are consistent with the presence of weak attractive interactions. The
role of fluoride in comparison with the bigger (and softer chloride) is to attract the
cation closer to the inorganic core of the complex, pulling the metal away from the
aromatic arms. From an electronic point of view, a greater electron density on the
whole receptor could make the oxygens-metal interaction stronger than in the CsCl
case.
Figure 20: Ball&Stick and CPK representation of the F-CsCl complex. Left
[F4(MeOH)2(Cs)2(Cl)2] and right) half structure underlining the different roles of the two host
molecules: the methanol- and the Cl-complexed receptors. Colour code as in Fig.18.
Given that in the capsule-like dimeric complexes both sidearms of A establish
-interactions with the complexed alkali metal ions, it was of interest to investigate the
97
Chapter 3
behavior of a uranyl-salophen derivative having only one sidearm. Therefore the
one-armed receptor F was synthesized and the single crystal of a mixed complex with
CsCl having composition (F)2·(CsCl)2·(MeOH)2 was obtained. Its molecular structure
is shown in Figure 20.
There are marked differences in organization at the molecular and
supramolecular level compared to the salt complexes of the 2-armed receptor A, in that
four receptor molecules instead of two are assembled in a capsule-like arrangement in
which a (CsCl)2 ion quartet is enclosed. Moreover the ionic quartet and consequently
all the supermolecule is not centrosymmetric as in the previous case. Indeed, the two
Cl-bound hosts are identical and not enantiomers. The same occurs for the two
methanol-bound hosts. In conclusion the whole unit is chiral, therefore in Figure 20
only one of the two enantiomeric adducts present in the crystal is shown. In Figure 21
cutaway views of the complex are reported to better highlight the interactions between
Cs ion and the aromatic rings involved in the -interactions with the receptor.
Cs
Figure 21: F-CsCl: left) interaction between the Cs ion and the benzyl arm; right) second
interaction with the aromatic ring belonging to the methanol-complexed. Legenda: C=light
grey, H=white, N=blue, O=red, Cl=green, Cs=yellow, U=dark gray.
Again, each cesium cation is coordinated to six oxygens, two chlorides and two
aromatic rings (Table 4). The structure of one of the two halves (Figure 20 right) that
compose the assembly reveals van der Waals contacts of Cs+ with two different
aromatic rings in a tilted-sandwich arrangement. The two non-equivalent receptor
molecules in each half-unit play different roles. The uranyl of one of the receptors is
coordinated to chloride, whereas the other receptor is coordinated to a MeOH
molecule. The chloride-bound receptor (Figure 21 left) uses its sidearm as -donor,
98
Receptors for alkali metal salts through cation- interactions.
whereas the other receptor binds to Cs+ via its unsubstituted salicylaldehyde moiety
(ring D in numbering scheme in Fig. 22).
Figure 22:
Numbering scheme
for the F-CsCl
complex
Table 4: Cesium chloride complex of receptor F. Distances (Å) between the Cs cation and a
given atom, with reference to the numbering scheme in chart 1. The apostrophe denotes an
atom belonging to the other half unit (not shown in the numbering scheme in Fig. 22)
Aromatic ring
Ring C
(C17)B
(C18)B
(C19)B
(C20)B
(C21)B
(C22)B
C centroid
Miscellaneous
Ring D
3.589
3.607
3.933
4.187
4.193
3.886
3.655
(C25)
(C26)
(C27)
(C28)
(C29)
(C30)
D centroid
4.020
3.971
3.855
3.782
3.847
3.962
3.650
Cl
Cl'
O2
O1B,O1B'
O15B
O10B
O26B
3.567
3.601
3.576
3.237, 3.731
3.15
3.132
3.284
The M+ to centroid distances for ring C and D are similar (3.655 and 3.660 Å,
respectively), but the binding mode in the two cases are very different. Ring C shows
an eccentric binding profile similar to alkali metal complex with A, the shortest
distance being those with carbons C17B and C18B at 3.589 and 3.607 Å, respectively.
Hence the binding mode can be described as 2. In variance, the D ring belonging to
the MeOH-complex approaches a 6 binding mode, with all the distances M+ to ring’s
carbons longer than the M+ to centroid one. It appears, therefore, that both the 2-armed
receptor A and the 1-armed receptor F achieve the common goal of having each metal
ion in van der Waals contact with two aromatic -donors, but a much more elaborate
strategy is adopted by receptor F on account of its structural handicap.
99
Chapter 3
Experimental studies, in which the existence of alkali metal cation-(arene)
interactions in solution has been convincingly demonstrated, are extremely rare. A
major reason for this difficulty is the large penalty due to cation desolvation and
counterion separation, which is hardly offset by weak cation- interactions. Since
counterion separation is not a problem with our receptors (they bind to the contact ion
pair) it has been decided to ascertain whether the aromatic sidearms participate in
binding also in solution.
Solubilization into an organic solvent of an otherwise insoluble alkali metal
salt upon treatment with a suitable receptor is a well-known phenomenon since
Pedersen's discovery of crown ethers.39 Contrary to expectations, no dissolution of the
chloroform insoluble CsCl and CsF took place upon treatment with a chloroform
solution of A. Instead, the red-orange colour typical of uranyl-salophen compounds
passed from the solution to the solid phase, i.e. the formation of a precipitate occurred.
The red-orange precipitate has been analysed by SEM/EDS technique.
Figure 23: SEM analysis of sample composed of orange precipitate on solid matrix of CsCl.
The sample is prepared as follows: an excess of CsF or CsCl is added to a solution of
the receptor in chloroform. After almost complete precipitation (the initial dark red
solution became pale yellow) the solution has been removed and the solid dried by
vacuum. The picture in Figure 23 shows sample made with CsCl and receptor A. The
white big particles (yellow arrows) are made of CsCl. Nearly all the surface of the
100
Receptors for alkali metal salts through cation- interactions.
alkali metal salt is coated by microcrystals of very well defined shape. The picture in
Figure 24 is a magnification of the upper right corner of the picture in Figure 23.
Figure 24:SEM analysis: magnified portion of the picture in Figure 23. The crystallinity of the
precipitate is evident.
The crystallinity of the coating particle is clear. The most convincing hypothesis sees
the receptor extracting the CsCl from the solid (or taking it form the solution in which
to a minimal extent the Cs salt could be soluble) and forming the complex that, being
insoluble, slowly precipitates.
Element
Weight
%
C
O
F
Cs
U
45.09
14.67
2.06
13.46
24.73
Totals
100.00
Atomic

%
Weight%
1.34
1.33
1.09
0.90
1.19
75.32
18.39
2.17
2.03
2.08
Figure 25: SEM/EDS analysis on A-CsF sample with corresponding elemental percentage
analysis. A 1:1:1 ratio for the U, Cs and F elements were found.
101
Chapter 3
An EDS analysis of the samples has been thought to be a good method to
discriminate between the receptor itself and the its salt-complex. Therefore, the
Cs:alogen:U ratio (elemental percentage) has been selected to be the best candidate for
such an analysis. The complex must show a 1:1:1 ratio. After a careful choice of
suitable region of the sample, the EDS analysis gave the expected results. The picture
in Figure 25 and the corresponding EDS analysis, besides, is one example for the
CsF-complex case. The red circle enlightens the beam’s area on the targeted surface.
When receptor A was replaced by the parent compound C, which does not
posses any side arms, no precipitation took place. It is worth noting the fact that
receptor A, that is the one which precipitates, is also much more soluble (in
chloroform) than C which remains in solution. This markedly different behavior is well
illustrated by the competition experiment reported in Figure 26. An equimolar mixture
of A and C in CDCl3 was exposed to the action of excess CsCl in an NMR tube. After
two days, about 3/4 of A (●) were transformed into the insoluble CsCl complex,
whereas the concentration of C () in the solution remained unchanged.
Figure 26: Plot of the
concentration in time;
host A (●), host C ().
All the concentrations reported in the plot above have been calculated with respect to
an internal standard, namely 1,1,2,2-tetrachloroethane. The same experiment was
repeated with CsF and the same behavior was observed even if on a shorter time
interval (since this phenomenon is surface dependent it was possible to ascertain
whether the different rates are due to the anion identity or due to different surface
accessibility, granularity, or hygroscopicity of the salts). The different behavior of the
two receptors is remarkable and more compelling evidences of the contribution of the
side arms to what already reported have been sought.
A convincing assessment of the presence of cation- interactions with alkali
metals ion in solution has proven to be difficult to obtain. This difficulty explains why
102
Receptors for alkali metal salts through cation- interactions.
in literature only relatively few data in solution are reported. Alkali metal halides are
insoluble in apolar and weakly donor solvents where weak interactions can be more
easily detected. On the other hand a solvent which solubilizes the salt prevents weak
interactions to occur. The use of anions with a delocalised charge (PF6-, for instance) is
generally the way to overcome this problem. It has to be noted, however, that such
anionic species are not bound to the hard uranyl center thus preventing any successful
binding in salophen-uranyl based systems. These features require, therefore, a step-bystep method, briefly described. Addition of a halide salt with a bulky organic cation
(like tetrabutylammonium) is the first step to form the salophen-UO2-anion complex.
The further addition of an alkali metal salt with an appropriate counteranion (CsPF 6
will be used) represents the next step for the formation of the desired M-halogen-host
complex. Clearly, all this depends on the solvent. Weakly polar solvents maintain the
ion pair associated even with TBA salts and the solubility of any alkali metal salts (for
CsPF6 also) is not enough for most of the purposes. On the contrary, strongly polar
solvents are effective competitors for the desired association. A screening of
experimental condition to choose the appropriate solvent is therefore necessary. This
was carried out and acetone has been chosen as the best compromise considering its
polarity and its donor number.
A portion of the 1H NMR spectrum of A in CD3COCD3 is shown in Fig 27a.
Addition of about 1/2 molar equivalent of solid tetrabutylammonium chloride caused
the appearance of a second species (Fig. 27b), whose imine and aromatic protons were
upfield shifted relative to free A, whereas the benzyl protons were strongly downfield
shifted.
Figure 27: NMR spectra in acetone at 25°C a) Host A, b) spectrum a after the addition of
TBACl c) spectrum b after addition of CsPF6.
103
Chapter 3
These results are interpreted in terms of a virtually quantitative formation of a
complex of A with chloride anion, whose complexation-decomplexation rate is slow
on the 1H NMR timescale. The chemical shifts of the tetrabutylammonium cation (not
shown) were practically the same as those of free tetrabutylammonium chloride, which
indicates that the chloride-bound species A·Cl- is not the counteranion of an ion pair,
but a free species in acetone.
Addition of excess solid CsPF6 caused precipitation of the red-orange complex
of A with CsCl, leaving, after few minutes, receptor A as the sole detectable species in
solution. However, in a spectrum taken immediately after the addition of CsPF6 (Fig
27c), besides free A, a second species is clearly visible, which is interpreted as a
Cs+-associated A-Cl- entity, present in solution as a transient species before
precipitation. The use of chloride as the anion is motivated by a slower precipitation
rate than of fluoride, thus allowing for the recording of the spectra.
Figure 28: NMR spectra in acetone at 25°C a) Host F, b) spectrum a after the addition of
TBACl c) spectrum b after addition of CsPF6.
The behavior of the one-armed receptor F is similar to receptor A with a sole
significant difference: no precipitation occurs. The spectra in Figure 28 confirm the
formation of a new species after the addition of solid CsPF6 (the Cs-Cl-F complex).
These experiments demonstrate the formation of a CsCl complex with the receptors A
and F. No information can be obtained about the geometry of the complex and whether
the cation is held in the complex by other forces than the simple electrostatic
interaction with the anion is not clear. Therefore, the role of the lateral arms has to be
elucidated. Thus, the direct comparison with receptor C has been made, following the
same procedure adopted for receptors A and F. The relevant portion of the 1H NMR
spectrum of C in CD3COCD3 is shown in Fig 29a. Addition of approx. 1 eq. of TBACl
transforms the receptor in its Cl-complex (b). Upfield shifts on iminic and aromatic
protons are evident. The further addition of excess solid CsPF6 causes the appearance
104
Receptors for alkali metal salts through cation- interactions.
of a new peak in the iminic region of the spectrum, the reappearance of the free
receptor peak and the precipitation of CsCl as a white solid (c).40 The occurrence of the
salt precipitation is a clear suggestion about the strength of the binding. It can be stated
that the affinity of CsCl towards A and F is much higher than that towards the parent
receptor C and that the observed increase in binding affinity is a manifestation of
cation- interactions with the sidearms of A and F. The process can be repeated with
the addition of TBACl (in two portions Fig 29 d,e) which leads to the formation of the
Cl-bound species. The further addition of the Cs salts generates the free species and the
CsCl-complex.
a)
b)
c)
d)
e)
f)
Figure 29: left: a) Host C in acetone; b) spectrum a after the addition of TBACl; c) spectrum b
after the addition of CsPF6; d) and e) further addition of TBACl to solution c and f) spectrum e
after further addition of CsPF6; right) schematic representation of the experiment with the
precipitation of CsCl (encircled CsCl) underlined when occurring, H=host.
The solid state structures of the alkali metal-complex with A suggest the need of
considering their occurrence even in the solution. Even if a clear concentration
dependent analysis has not been possible for the very limited solubility of the transient
species, it has been concluded that the presence of the dimeric assembly is unlikely.
The structural rigidity needed to guarantee close contacts, especially in the case of the
aromatic arms would make the dimeric complex too insoluble. Indeed, the analysis of
the obtained crystal structures clearly addresses to the cation- interactions a
considerable contribution to the formation of the assembly. Thus, the presence of the
side arms in A renders energetically available the dimeric species which, then,
precipitates from the solution. The lack of appropriate binding sites for the cation in
receptor C avoids the formation of any dimer and the CsCl-complex remains in
105
Chapter 3
solution even if maintained at –20 °C. Receptor F is a kind of compromise between the
two situations: it forms the CsCl complex which is still more stable than the one
formed with C (no CsCl precipitation upon the addition of CsPF6 has been observed),
but the presence of only one arm requires a more elaborate organization in the solid
state (see the solid state structures), therefore it remains in solution as a “monomeric”
complex species. The situation is summarized in Scheme 3 where the receptors are
schematically represented:
Receptor
A
Receptor
F
Receptor
C
Scheme 3: Summary on the different behaviors among hosts A, B and C.
In summary, receptor A strongly binds the ion pair and this complex is in equilibrium
with the supramolecular assembly which, once formed, quickly precipitates; F strongly
binds the ion pair but it is unable to access to the second equilibrium. C binds CsCl but
the complex formed is weaker than those with A and F and this is revealed by the
formation of solid CsCl upon addition of CsPF6. The binding of fluoride to the uranyl
center is even stronger than that of chloride in acetone. The complexation with F induces greater shifts on 1H-NMR than Cl, as shown in Figure 30. Spectrum (a)
represents the receptor F in acetone. The addition of 2/3 eq. of F- (as TBA salt) creates
the fluoride-bound complex (in slow exchange with the free receptor, broad signals,
spectrum b). The addition of TBACl (ca. 1/3 eq.) converts the rest of free receptor in
its Cl-complex (spectrum c). Thus, in solution there are two species, namely the F- and
Cl- complexes. The presence of relatively sharp signals reveal that the two species may
be not in exchange. The addition of the soluble CsPF6 salt induces the appearance of a
broad signal in the iminic and in the benzylic part of the spectrum belonging to the two
Cs-halide-complexes in the fast exchange on the NMR time scale (spectrum d). Only
the exchange of the ion pair (and not the exchange of the anion alone) seems to be a
favourable process in acetone.41
106
Receptors for alkali metal salts through cation- interactions.
Figure 30: NMR spectra in acetone at 25°C a) Host F, b) spectrum a after the addition of TBAF
c) spectrum b after addition of TBACl, d) spectrum c after the addition of CsPF6.
To better highlight the interactions between the aromatic pendant arms and the
cation, the receptor I has been synthesized. The substitution on the para carbon of the
benzyl rings should render signals belonging to that part more readable in the 1H NMR
spectrum (an AA'XX' pattern should be evident).
Figure 31:Portion of NMR spectra in acetone: a) I 5x10-3 M-1, b) spectrum a after the addition
of TBACl 0.65 eq., c) b after the addition of CsPF 6. Magnification of the aromatic signals is
shown on the right hand side.
107
Chapter 3
Unfortunately, the analysis of the aromatic signals in an analogous step-by-step
experiment made with receptor I turned to be ambiguous and hardly rationalized also
due to peak broadening. However, receptor I behaved in complete accordance with
receptor A (see Figure 31, spectrum a to c)
It was possible to obtain single crystal suitable for X-ray analysis for the CsCl
complex of I. The structure is similar of those reported before for similar receptors as
shown in Figure 32.
Figure 32: CsCl-I complex. Legenda: see colour code in Figure 18.
Table 5 reports all the relevant distances in accordance to the numbering scheme in
Figure 19. M+ to arene carbon distances of 3.528 and 3.490 Å (C18 and C38,
respectively) are consistent with the presence of cation-interactions in a 1 binding
mode.
Table 5: Cesium chloride complex of receptor I. Distances (Å) between the Cs cation and a
given atom, with reference to the numbering scheme in Figure 19. The apostrophe denotes an
atom belonging to the other half unit (not shown)
Aromatic ring
Ring A
(C17)
(C18)
(C19)
(C20)
(C21)
(C22)
A centroid
108
3.729(4)
3.528(4)
3.921(5)
4.456(2)
4.599(3)
4.272(5)
3.865
Miscellaneous
Ring B'
B' (C33)
B' (C34)
B' (C35)
B' (C36)
B' (C37)
B' (C38)
B' centroid
3.79(1)
4.44(1)
4.81(2)
4.58(2)
3.93(1)
3.490(9)
3.967
Cl
Cl'
O1,O1'
O10
O15
O26
O31
3.367(2)
3.473(3)
3.78(1), 3.73(1)
3.247(5)
3.325(2)
3.299(5)
3.340(1)
Receptors for alkali metal salts through cation- interactions.
Geometries are quite eccentric being the M-to centroid distances more than 3.8 Å. Is
worth to note that four out of six aromatic carbons of the salycilaldehyde ring of the
receptor I face the p-substituted methyl in the supramolecular assembly with distances
in the range 3.595-4.021 Å. Weak interactions of CH···type can be supposed to exist.
This small energetic contribution may contrast the destabilization of the whole
superstructure due to the increased steric hindrance of the methyl groups.
Finally, ESI-TOF mass-spectrometric analysis (exact mass) of a mixture of A
and F with CsCl. The mass spectra show peaks due to A·Cs+, A·CsCl·Cs+,
(A)2·CsCl·Cs+, and (A·CsCl)2·Cs+ and F·Cs+, F·CsCl·Cs+, (F)2·CsCl·Cs+ and
(F·CsCl)2·Cs+. No additional source of protons has been used due to the instability of
the receptors in acidic media. We suggest that the structure of the (A·CsCl)2·Cs+
species is a dimeric assembly similar to that found in the solid state (Fig 18, left
column, middle) complexed with a Cs+ cation in exo-mode.
The experimental part concerning the synthesis of the receptors employed in
this chapter has been already reported (see chapter 2.6).
3.4 Conclusions.
Our search for contact ion-pair recognition resulted in the discovery of ion-quartet
recognition. In all of the isolated complexes of receptor and salt an ion quartet having
composition of a dimeric ion pair (MX)2 constitutes the core of a supramolecular
assembly in which either two 2-armed or four 1-armed receptor molecules assemble
into capsules fully enclosing the ion quartets. Thus, the objective of alkali metal halide
recognition without any penalty arising from counteranion separation was fully
achieved.
The results presented here, added to the results of organic quaternary
ammonium salts complexation (see Chapter 2), show that uranyl-salophen A is quite
adaptive a receptor, in that upon guest complexation its conformationally flexible
sidearms adapt to achieve a stable arrangement, in which multiple cation- interactions
are established. The sidearms positions and, consequently, the molecular and crystal
structures of the resulting complexes are controlled by interactions of the aromatic
sidearms with the guest.
1
H NMR solution studies indicate that in CD3COCD3 Cs+ ion binds to A·Cland F·Cl- much more strongly than to C·Cl-. Therefore, solution data indicate that the
structure of the above complexes correlate with the solid state structures, thus
reinforcing the view that the sidearms participate in binding both in solution and in the
solid state.
109
Chapter 3
In conclusion, although the primary interaction between receptors A and F
with alkali metal halides is the anion binding to the uranyl and, in turn, the primary
interaction between uranyl-salophen-complexed halide anion and the cation is
electrostatic in nature, the appropriately positioned sidearms reinforce the binding
through cation- interactions.
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Chapter 3
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112