Heteroditopic chiral uranyl-salen receptor for molecular recognition of amino acid ammonium salts

Article abstract of DOI:10.1002/ejoc.201000566

A new heteroditopic chiral uranyl-salen complex incorporating two pyrenyl groups was designed and synthesized for the recognition of ammonium salts, tetrabutylammonium (TBA) and tetramethylammonium (TMA) amino acids. UV/Vis measurements indicate the formation of 1:1 host-guest complexes with high association constants and an excellent en-antiomeric discrimination between the two enantiomers of phenylalanine-TMA. T-ROESY NMR experiments confirm cation-π and CH-π interactions between the TBA and TMA cations and the two pyrenyl arms, leading to the strong stability of the complexes. This is the first report of a uranyl receptor able to recognize chiral carboxylate guests.

Full text of DOI:10.1002/ejoc.201000566

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000566
Heteroditopic Chiral Uranyl–Salen Receptor for Molecular Recognition of
Amino Acid Ammonium Salts
Francesco P. Ballistreri,[a] Andrea Pappalardo,*[a] Gaetano A. Tomaselli,*[a]
Rosa M. Toscano,[a] and Giuseppe Trusso Sfrazzetto[a]
Keywords: Amino acids / Enantioselectivity / Host–guest systems / Molecular recognition
A new heteroditopic chiral uranyl–salen complex incorporat-
ing two pyrenyl groups was designed and synthesized for the
recognition of ammonium salts, tetrabutylammonium (TBA)
and tetramethylammonium (TMA) amino acids. UV/Vis mea-
surements indicate the formation of 1:1 host–guest com-
plexes with high association constants and an excellent en-
antiomeric discrimination between the two enantiomers of
phenylalanine–TMA. T-ROESY NMR experiments confirm
cation–π and CH–π interactions between the TBA and TMA
cations and the two pyrenyl arms, leading to the strong sta-
bility of the complexes. This is the first report of a uranyl
receptor able to recognize chiral carboxylate guests.
Recently, we have studied some chiral uranyl–salen com-
plexes acting as receptors for enantioselective molecular re-
Introduction
Enantiomeric recognition is an essential property of vari- cognition, which are able to recognize anions exploiting the
ous natural systems, which is reliant on the capacity of a ability of the Lewis acid uranyl center to coordinate equato-
chiral molecular receptor to preferentially form a dia- rially anions.[10] However, so far only few examples of chiral
stereomeric complex with one of the enantiomers of chiral uranyl–salen complexes have been described in the litera-
molecules by exploiting noncovalent weak forces including ture.[11]
hydrogen bonding and hydrophobic and electrostatic inter-
Here we report the synthesis of a new heteroditopic chi-
actions.[1] The development of synthetic receptors for this ral uranyl–salen derivative bearing two pyrenyl side arms
appealing aim is an important research area, as it can pro- and discuss its ability in the enantioselective molecular re-
vide precious information for a better understanding of the cognition of ammonium salts also containing α-amino acid
interactions between molecules in nature. Furthermore, the moieties.
study of their recognition properties may also lead to the
development of functional molecular devices and materials
useful in separation processes,[2] catalysis,[3] biochemical[4,5]
Results and Discussion
and pharmaceutical studies,[6] as well as sensing.[7]
Target chiral uranyl–salen 1 (Scheme 1) was synthesized
in four steps starting from 2-allyloxy-3-chloromethyl-5-tert-
butylbenzaldehyde (2).[11d] 1-Hydroxypyrene was treated
with 2 to yield oxypyrenyl derivative 3 (75%), which was
then deallylated to give the 3-pyrenyloxymethyl-5-tert-but-
ylsalicylaldehyde (4; 67%). Condensation of 4 with the
(1R,2R)-(+)-1,2-diphenylethylendiamine yielded salen li-
gand 5 (78%), which was finally converted into the corre-
sponding salen complex 1 (98%) by uranyl acetate. The pro-
posed structure for this new chiral uranyl–salen complex is
In recent years, there has been great interest in the de-
sign, synthesis, and investigation of heteroditopic recep-
tors.[8] Because either of the charged partners of a salt needs
a site of recognition, a receptor specifically designed for ef-
ficient ion pair complexation should consist of at least two
subunits, both of which must be capable of simultaneously
binding each partner of the ion pair. In apolar organic sol-
vents, salts typically exist as ion pairs or higher aggregates.
Consequently, if the two subunits are appropriately de-
signed, a contact ion pair recognition pathway is preferred,
because this avoids the energetically unfavorable separation
of the two ions.[9]
1
consistent with the H and 13C NMR spectroscopy data as
well as the ESI mass spectrometry data.
Scheme 2 displays the ion pairs employed as guests in
the molecular recognition process by complex 1. Molecular
recognition studies were carried out by performing NMR
and UV/Vis measurements. We first investigated the binding
[a] Dipartimento di Scienze Chimiche, University of Catania,
Viale A. Doria 6, 95125 Catania, Italy
Fax: +39-095-580138
1
E-mail: gtomaselli@unict.it
properties of receptor 1 toward TBACl and TMACl by H
andrea.pappalardo@unict.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000566.
NMR titration in CDCl3 at 27 °C to understand the role of
1
the cation in the complexation process. During H NMR
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Heteroditopic Chiral Uranyl–Salen Receptor
Scheme 1. Synthesis of uranyl–salen complex 1. Reagents and con-
ditions: (a) 1-Hydroxypyrene, K2CO3, CH3CN, reflux 15 h;
(b) Pd(OAc)2, PPh3, Et3N, HCO2 H, 80% EtOH, reflux 45 min;
(c) (1R,2R)-(+)-1,2-diphenylethylenediamine, EtOH, reflux, 15 h;
(d) UO2(OAc)2·2H2O, MeOH, room temp., 12 h.
Figure 1. Selected region of the spectrum in a typical 1H NMR
titration of TBACl with receptor 1. Constant concentration of
guest TBACl (2.00ϫ10–3 ) with addition of various concentra-
tions (0 to 1.00ϫ10–2 ) of host 1. (a) TBACl; (b) [H]/[G] 1:5;
(c) [H]/[G] 1:3; (d) [H]/[G] 1:2; (e) [H]/[G] 1:1; (f) [H]/[G] 1:0.6;
(g) [H]/[G] 1:0.4; (h) [H]/[G] 1:0.2.
titration experiments we followed the upfield shift of the α-
CH2 protons signal of TBACl (2.00ϫ10–3 ), upon ad-
dition of various aliquots of host 1 (concentration range 0–
1.00ϫ10–2 ; Figure 1).
Table 1. Binding constants (Ka), limiting upfield shifts (–∆δϱ), and
Gibbs free energy changes (–∆Go) for the complexation of TBACl
and TMACl with receptor 1 in CDCl3 at 27 °C.
Guest
Ka [–1]
–∆δϱ [ppm] –∆Go [kJmol–1]
TBACl
TMACl
(3.71Ϯ1.53)ϫ104
1.02 (N–CH2)
1.33 (N–CH3)
26.2
17.9
(1.32Ϯ0.51)ϫ103
(Hb and Hc, see Figure 2) of receptor 1, which undergoes a
downfield shift, seems to indicate that the cation is sand-
wiched between the two pyrenyl arms, whereas as a conse-
quence of the coordination of the chloride anion on the
uranyl center, the Ha protons of the diphenylethane bridge
move upfield.
Scheme 2. Ammonium and amino acid salts used as guests.
Likewise, in the case of TMACl we followed the upfield
shift of the CH3 signal. Binding constants (Ka, –1) of the
complexes of receptor 1 with TBACl and TMACl, respec-
tively, and limiting upfield shifts (–∆δϱ, ppm) were obtained
as best-fit parameters by using the program Hyp NMR.[12]
Pertinent data are reported in Table 1 together with the
Gibbs free energy changes (–∆G°).[13] Job’s plots support
the 1:1 stoichiometry of the various host–guest complexes
(see the Supporting Information).[14,15]
The large upfield shift of the guest protons suggests that
the ammonium cation is strongly interacting with the pyr-
enyl arms that stabilize it by cation–π and CH–π interac-
Figure 2. Selected region of the spectra showing the downfield shift
of the diastereotopic protons as well as the upfield shift of the
Ha protons of receptor 1 after addition of TBACl. (a) Receptor 1
tions. Moreover, the signal of the diastereotopic protons (1.00ϫ10–3 ); (b) [Receptor 1]/[TBACl] 1:3.
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A. Pappalardo, G. A. Tomaselli et al.
SHORT COMMUNICATION
Furthermore, T-ROESY experiments reveal ROE con- Table 2. Binding constants (Ka), Gibbs free energy changes (–∆G0),
and enantioselectivities KL/KD or ∆∆G0 for the complexation of
/ amino acid derivatives and (R,S)-MBnACl with receptor 1 in
CHCl3 at 25 °C.
tacts between β-CH2 of TBACl and the pyrenyl moieties,
thus confirming the suggested spatial location of the am-
monium cation. Similar ROE interactions were also ob-
[a]
Guest
Ka [–1
]
KL/KD
–∆Go
∆∆Go
served between the CH3 protons of TMACl and the pyrenyl
aromatic framework (see the Supporting Information).
These results support the conclusion that this new uranyl–
salen complex behaves as a heteroditopic receptor in which
the metal center is able to coordinate the anion, whereas
the two pyrenyl rings act as tweezers toward the ammonium
cation stabilizing it through CH–π and cation–π interac-
tions. It is interesting to note that the cation identity affects
the binding process. In fact, TBACl displays a higher bind-
ing constant than TMACl (KTBACl/KTMACl = 28) probably
because of a larger number of and more efficient CH–π
interactions involved in the recognition process. On the
other hand, the larger TBA cation yields a looser ion pair,
whose hosting by the heteroditopic receptor requires a
lower separation energy cost.
[kJmol–1
]
[kJmol–1
]
(R)-MBnACl (5.92Ϯ0.21)ϫ105
(S)-MBnACl (2.59Ϯ0.31)ϫ106
4.4[b]
0.61
6.7
32.9
36.6
31.6
30.4
31.1
35.8
27.8
36.5
3.7[c]
-Phe-TBA
-Phe-TBA
-Trp-TBA
-Trp-TBA
(3.52Ϯ0.16)ϫ105
(2.14Ϯ0.16)ϫ105
(2.80Ϯ0.22)ϫ105
(1.88Ϯ0.30)ϫ106
1.2
4.7
-Phe-TMA (7.36Ϯ0.12)ϫ104
34.0
8.7
-Phe-TMA
(2.50Ϯ0.14)ϫ106
[a] ∆∆G0 = |∆G0() – ∆G0()|. [b] KS/KR. [c] ∆∆G0 = |∆G0(R) –
∆G0(S)|.
We also tested the enantiomeric recognition ability of re-
ceptor 1 toward (R)- and (S)-α-methylbenzylammonium
chlorides (MBnACl) and some ammonium salts of selected
amino acids (Scheme 2) by UV/Vis titrations.[16] UV/Vis ti-
tration experiments typically show an increasing ab-
Figure 4. Optimized structures of the 1:1 complexes of 1 with (S)-
sorbance value at 331 nm upon addition of increasing MBnACl (left) and (R)-MBnACl (right). Hydrogen atoms are
amounts of the guests, and a corresponding decrease in the
omitted for clarity.
absorbance at 355, 364, and 384 nm (relative to the π–π*
transitions of the pyrenyl moiety; Figure 3).
toward receptor 1 are of the same order of magnitude as
those observed for α-methylbenzylammonium chlorides.
These findings seem to indicate that the carboxylate anion
of the amino acid behaves as the hard chloride in binding
the uranyl metal center driving the molecular recognition
event.[10,17] To the best of our knowledge, this uranyl com-
plex represents the first hitherto reported chiral receptor
able to recognize and strongly bind carboxylate anions of
amino acids. NMR spectroscopic data support this conclu-
sion. In fact, even in the case of -Phe-TBA, we observed
an upfield shift of the protons of the diphenylethane bridge
(Ha, Figure 5), providing good evidence that the carboxyl-
ate anion is able to coordinate the uranyl metal center, as
the chloride anion does.
On the other hand, a strong downfield shift of the signals
for the diastereotopic protons of receptor 1 (Hb and Hc) is
observed, confirming therefore the presence of the cation
Figure 3. UV/Vis titration between receptor 1 and -Trp-TMA.
In Table 2 we report pertinent affinity values (Ka), Gibbs nearby the π-electron rich pyrene framework. A ROESY
free energy changes (–∆G0), and enantioselectivities KS/KR experiment displays also in this case a ROE contact be-
and KL/KD (or ∆∆G0) ratios for the complexation of (R)- tween the CH3 protons of the TBA cation and the pyrenyl
and (S)-α-methylbenzylammonium chlorides and / arms, therefore supporting the spatial location of the cation
amino acid ammonium salts, respectively, with receptor 1.
The observed enantioselectivity value (KS/KR = 4.4) for
(see the Supporting Information).
As Table 2 shows, selectivity values of 0.61, 6.7, and 34.0
the two chiral salts (S)-MBnACl and (R)-MBnACl is re- for the ,-Phe-TBA, ,-Trp-TBA, and ,-Phe-TMA,
lated to the opposite configurations of the benzylic carbon respectively, support a very efficient recognition ability of
atom stereocenter. In fact, in the (S)-MBnACl salt, and dif- receptor 1 toward these amino acids salts.[18] The observed
ferently from the (R)-MBnACl, the phenyl group is oriented selectivity shows a quite complex dependence on the config-
to develop π–π interactions with the pyrenyl unit, increasing uration of the anion (amino acid carboxylate) and cation
therefore the affinity for receptor 1 (Figure 4).
identities. The possibility to develop interactions among the
When the amino acids are taken in consideration as aromatic moieties of the amino acid anions [which occupy
guests, it is interesting to note that pertinent affinities values the fifth equatorial coordination site of the uranyl(VI)
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Heteroditopic Chiral Uranyl–Salen Receptor
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Conclusions
We have synthesized a new chiral uranyl–salen receptor
bearing two pyrenyl arms and evaluated the enantiomeric
recognition properties of this complex toward ammonium
salts containing α-amino acid moieties by 1H NMR and
UV/Vis titrations. This receptor is readily prepared by a
short synthetic sequence and displays a very high selectivity
toward an , pair of amino acid salts, in particular for the
two enantiomers of Phe-TMA. T-ROESY experiments have
shown that the two pyrenyl groups play a key role in the
complexation phenomena interacting through CH–π and
cation–π with the ammonium cations, leading to high bind-
ing affinities. The identity of the cation, TMA versus TBA,
and the configuration of the amino acid are crucial in de-
termining the enantiomeric selectivity for the  and  pair.
[9]
Supporting Information (see footnote on the first page of this arti-
cle): General experimental methods, characterization of all com-
pounds, DOSY experiments, Job Plots, and UV/Vis and H NMR
titrations.
[10]
[11]
1
Acknowledgments
We thank the University of Catania for financial support, Professor
Peter Gans for helpful discussion in determining UV/Vis binding
constants, and Dr. Anna Notti of the University of Messina for
DOSY NMR experiments.
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SHORT COMMUNICATION
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[18] ()- and ()-Trp-TMA salts were also prepared according to a
general procedure (see the Supporting Information), but their
use was precluded by their scanty solubility in chlorinated sol-
vents.
[19] A pentagonal bipyramidal coordination geometry for the uran-
yl(VI) ion with two axial oxo groups and with the fifth equato-
rial site available for complexation with anionic monodentate
ligands X– has been previously indicated in these uranyl chiral
macrocyclic complexes.[11d,17]
Received: April 22, 2010
Published Online: June 9, 2010
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Eur. J. Org. Chem. 2010, 3806–3810