Argentivorous molecules: Structural evidence for Ag+-π interactions in solution

Article abstract of DOI:10.1021/ol3019538

Tetra-armed cyclens bearing aromatic side arms were prepared by the reductive amination of cyclen with substituted benzaldehydes. When equimolar amounts of Ag⁺ ions were added to the ligands, the aromatic rings covered the Ag⁺ ions i

Full text of DOI:10.1021/ol3019538

ORGANIC
LETTERS
2012
Vol. 14, No. 17
4576–4579
Argentivorous Molecules: Structural
Evidence for Agþꢀπ Interactions
in Solution
Yoichi Habata,*,†,‡ Mari Ikeda,† Sachiko Yamada,† Hiroki Takahashi,† Sumiko Ueno,†
Takatoshi Suzuki,† and Shunsuke Kuwahara†,‡
Department of Chemistry, Faculty of Science, and Research Center for Materials with
Integrated Properties, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510,
Japan
habata@chem.sci.toho-u.ac.jp
Received July 24, 2012
ABSTRACT
Tetra-armed cyclens bearing aromatic side arms were prepared by the reductive amination of cyclen with substituted benzaldehydes. When
equimolar amounts of Agþ ions were added to the ligands, the aromatic rings covered the Agþ ions incorporated in the ligand cavities, as if the
aromatic ring “petals” caught the Agþ ions in the way an insectivorous plant (Venus flytrap) catches insects. The ligands are called
“argentivorous molecules”. Evidence of intramolecular Agþꢀπ interactions in solution and in the solid state is reported.
Metal-cationꢀπ interactions have been a topic of much
interest in the past decade.1ꢀ5 Agþꢀπ interactions6 in
particular are one of the most extensively studied topics
in the areas of supramolecular chemistry,7,8 molecular
sensors,9,10 and functional materials.11 Many studies have
reported Agþꢀπ interactions in the solid state. A few
groups have reported the structures of Agþ complexes
with the Agþ ions incorporated in the macrocycles by
Agþꢀπ interactions between the Agþ ions and the aro-
matic side arms. Recently, we reported that two aromatic
side arms of a tetra-armed cyclam bearing 30,50-difluoro-
benzyl groups (1) interact with the Agþ ions incorporated
in the cyclam moiety in the solid state.13 In the X-ray
structure of the Agþ complex with 1, the Agþ ion is buried
in the cyclam ring. Gyr et al. reported that14 an Agþ ion
incorporated in tetrakis(2-methysulfanylethyl)cyclen is ex-
posed on the surface of the cyclen ring (see Figure S1 in the
Supporting Information (SI)). The diameter of the cavity
of cyclen, which is a 12-membered ring, is smaller than that
of cyclam, which is a 14-membered ring. These results
prompted us to focus on tetra-armed cyclens bearing
aromatic side arms. We expected that stronger interactions
between the Agþ ion incorporated in the cyclen unit and
the aromatic side arms would be observed in solution and
in the solid state. In addition, if our presumption is correct,
we would be able to make a supramolecular system with
novel functions.
1
with Agþꢀπ interactions in solution, using H NMR
spectroscopy.9
It is well-known that cyclic polyamines such as cyclen
and cyclam form stable complexes with heavy-metal
ions.12 When aromatic rings are introduced into the cyclic
polyamines, it is possible to make a new type of host
compound in which the aromatic side arms can interact
† Department of Chemistry, Faculty of Science.
‡ Research Center for Materials with Integrated Properties.
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Here we report conformational changes in Agþ com-
plexes with tetra-armed cyclens bearing aromatic side
arms, which we have named “argentivorous molecules”,
r
10.1021/ol3019538
Published on Web 08/28/2012
2012 American Chemical Society

in solution and in the solid state. Tetra-armed cyclens
(2aꢀ2b, 5aꢀ5f, and 6, Figure 1) bearing benzyl groups
were prepared by reductive amination of cyclen with
aromatic aldehydes in the presence of NaBH(OAc)3. We
also prepared cyclic triamine analogs (3) and an armed
cyclen bearing 2-phenylethyl groups as side arms (4) (see
the SI).
Titration experiments using UVꢀvis spectra were car-
ried out to confirm the Agþꢀπ interactions in solution.
Figure S4(2a) shows the Agþ-ion-induced UVꢀvis spec-
tral changes of 2a. An increase in the absorbance at λmax
(260 nm), without any absorption wavelength shift, was
observed upon the addition of Agþ ions. An inflection
point was observed at 1.0 (= [Agþ]/[2a]), showing a 1:1
Figure 1. Structures of tetra-armed cyclens and analogs.
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complex. From the UVꢀvis spectral data, the log K value
of the complex was estimated to be ca. 6.8.15 When Agþ
ions were added to a mixture of cyclen and m-difluoro-
benzene (= 1/4) (see Figure S4(mix-Ag)), no spectral
changes were observed. Similar spectral changes were
observed in Agþ complexes with calixarene derivatives,
reported by Ho9c and Prodi.10b This result strongly sup-
ports Agþꢀπ interactions between the aromatic side arms
and the Agþ ions in solution.
Figure 2. Agþ-ion-induced 1H NMR shift changes of 2a
(aromatic region) in a mixture of CD2Cl2 and CD3OD (0.75 mL/
0.02 mL). As the intensities of the doublet at δ 6.4 ppm and
the triplet at δ 6.9 ppm increase, the intensities of the doublet at δ
6.9 ppm and the triplet at δ 6.65 ppm decrease. The exchange
rate of the complex and the metal-free ligand would be slower
than the NMR time scale under the conditions used because the
cyclen units bind Agþ ions strongly.
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Org. Lett., Vol. 14, No. 17, 2012
4577

upon the addition of equimolar amounts of Agþ ions (see
Figures S5(1-Ag) and S5(3-Ag)). In addition, 4, bearing
2-phenylethyl groups as side arms, also showed chemical
shift changes to lower field (see Figure S5(4-Ag)). These
results indicate that the significant chemical shift changes of
theprotonsat the20-and60-positions occur only in the tetra-
armed cyclens with benzyl groups as side-arms. Arya,9a
Choi,9b and Kimura9e have reported Agþꢀπ interactions
with crown ethers or calixarenes bearing alkynic, aromatic,
or alkenic side arms in solution. They reported that the
signals from the alkynic, aromatic, and alkenic protons
connected to carbons that interacted with Agþ ions shifted
to lower field. However, in our case, the chemical shifts of
the aromatic protons connect to carbons that interacted
with Agþ ions shifted to higher field.
To see ifthe substituentsonthe aromatic side arms affect
the chemical shift changes of the protons at the 20- and
60-positions, 1H NMR titration experiments were carried out
using 5aꢀ5f (see Figures S5(5a-Ag)ꢀS5(5f-Ag)). Figure 3
shows the correlation between the chemical shift changes
of the protons at the 20- and 60-positions and the electro-
staticpotentialsofthe corresponding substitutedbenzenes.
The electrostatic potential is defined as the energy of
interaction of a positive point charge with the nuclei and
electrons of a molecule, and the value of the electrostatic
potential correlates with the electron densities on the
aromatic rings. Figure S7 shows electrostatic potential
maps of the side arm components of 2a, 2b, and
5aꢀ5f.16,17 As we expected, when the substituents on the
aromatic side arms are electron-donating groups, signifi-
cant chemical shift changes to higher field (ꢀ0.82 to ꢀ0.96
ppm) were observed. In contrast, small or no chemical shift
changes were observed for the protons at the 20- and
60-positions with electron-withdrawing groups. These results
indicate that the protons at the 20- and 60-positions are
Figure 3. Correlation of Agþ-ion-induced chemical shift
changes of the protons at the 20- and 60-positions of the side
arms, and the electrostatic potentials of the corresponding
substituted benzenes. 2a (b), 2b (9), 5a (O), 5b (]), 5c (0), 5d
(ꢁ), 5e ([), and 5f (Δ).
To examine the structure of the Agþ complex with 2a in
1
solution, titration experiments using H NMR were car-
ried out in a mixture of CD2Cl2 and CD3OD. The signals
of the protons at the 20- and 60-positions in the aromatic
rings appear as a doublet (JHF = 7.0 Hz) coupled with the
next F-atom at δ 6.9 ppm. As shown in Figure 2, unusual
higher-field shifts (ꢀ0.49 ppm) of the protons at the 20- and
60-positions were observed upon addition of equimolar
amounts of Agþ ions. When 2b, which has 30,40,50-trifluoro-
benzyl groups, was used as a ligand, the protons at the
20- and 60-positions shifted to higher field by ca. ꢀ0.14 ppm
(see Figure S5(2b-Ag)). The addition of Liþ, Naþ, Mg2þ
,
and Ca2þ ions to 2a did not give significant chemical shift
changes of the corresponding protons (see Figures S5-
(2a-Li)ꢀS5(2a-Ca)). In the case of Hg2þ ions, the protons
at the 20- and 60-positions were not shifted, although the
40-position protons of the aromatic rings shifted to lower
field (see Figure S5(2a-Hg)). The titration experiments
suggest that (i) the stoichiometry between the ligands and
Agþ ions is 1:1, (ii) the chemical shift changes vary, depend-
ing on the number of F-atoms on the aromatic rings, and
(iii) the chemical shift changes to higher field of the protons
at the 20- and 60-positions do not occur in the case of alkali-
metal, alkaline-earth-metal, and Hg2þ ions. To study the
ring-size effect on the Agþ-ion-induced chemical shift
changes, titration experiments were carried out using 1
and 3. Chemical shift changes to lower field were observed
(12) (a) Denat, F.; Brandes, S.; Guilard, R. Synlett 2000, 561. (b)
Kimura, E. Pure Appl. Chem. 1993, 65, 355. (c) Koenig, B.; Svoboda, J.
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2004, 49, 17.
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Figure 4. Side view (top left) and top view (top right) of the
ORTEP diagrams (hydrogens and anions omitted) and space-
filling diagram (bottom) of the X-ray structure of AgCF3SO3
complex with 2a. The protons at the 20-/60-positions (meshed)
are located in the shielded area in the next benzene.18
1997, 36, 2786.
(15) See SI for the logK values of 2a for Hg2þ, Cu2þ, and Zn2þ ions.
Altþhough the logK values for these metal ions are higher than that for
Ag ion,þconformational changes of the side-arms are observed only
when Ag ion was added.
4578
Org. Lett., Vol. 14, No. 17, 2012

located in the shielded area of the next aromatic side arm in
solution.
X-ray crystallography was performed on the Agþ com-
plexes with 2a and 5aꢀ5f (see Figures S6(2a-Ag)ꢀS6-
(5f-Ag)). Figure 4 shows the X-ray structure of the AgCF3SO3
complex with 2a. The Agþ ion is four-coordinated by the
four ring N-atoms. Interestingly, the four aromatic rings
cover the Agþ ion incorporated in the cyclen moiety, with
the aromatic ring “petals” catching the Agþ ions in the
same way as an insectivorous plant (Venus flytrap) catches
insects. The N1ꢀAg1, N2ꢀAg1, N3ꢀAg1, and N4ꢀAg1
bond distances are in the range 2.408ꢀ2.497 (average 2.447)
Figure 5. Isosurfaces (at 0.032 au) of the LUMO of the X-ray
structure (left) and the virtual structure (right) of 5a from the
B3LYP/3-21G(*) calculation.
˚
A; the C10ꢀAg1, C17ꢀAg1, C24ꢀAg, and C31ꢀAg1 dis-
˚
tances are in the range 3.499ꢀ3.631 (average 3.555) A; and
the C11ꢀAg1, C18ꢀAg1, C25ꢀAg, and C32ꢀAg1 distances
˚
are in the range 3.234ꢀ3.574 (average 3.405) A. The distances
are comparable with those of anthracene-cryptandꢀAgþ and
anthracene-diphosphineꢀAgþ systems.8 The X-ray structure
suggests that the Agþ ions interact with the aromatic side
arms with η2-hapticity. The X-ray structure strongly sup-
ports the higher-field shift of the protons at the 20- and
60-positions (meshed protons in Figure 4). It is important
tonote that the 40-nitrobenzyl sidearmsof 5ealsocover the
Agþ ions in the solid state (see Figure S6(5e-Ag)), although
no chemical shift changes of the protons at the 20- and
B3LYP/3-21G(*) theoretical level.16 Figure 5 shows the
LUMO of the X-ray structure and the virtual structure of
the 5a/Agþ complex (see Figure S8 for LUMOs of the Agþ
complexes with 2a and 5aꢀ5f, and Figure S9 for LUMOs
and HOMOs of the Agþ complexes with 2a and 5aꢀ5f). In
the X-ray structure, the LUMO of the Agþ ion is distorted
by interaction with the HOMOs of the aromatic rings. In
contrast, the LUMO of the Agþ ion in the virtual structure
without any interaction with aromatic side arms is a
sphere-shaped orbital. These graphics clearly support
Agþꢀπ interactions between the Agþ ion and the aromatic
side arms.
In conclusion, we have demonstrated that tetra-armed
cyclens with aromatic side arms behave similarly to an
insectivorous plant (Venus flytrap) when they form Agþ
complexes. Weconfirmed thatthe conformationalchanges
are the result of Agþꢀπ interactions between the Agþ ions
and the side arms in solution and in the solid state.
Investigation of functionalized compounds using the un-
ique conformational changes of the argentivorous mole-
cules is now in progress.
1
60-positions were observed in the H NMR titration (see
Figure S4(5e-Ag)). This means that the Agþꢀπ interaction
occurs even if the electron density on the aromatic rings is
very low. Several groups have reported the X-ray struc-
tures of Co2þ, Ni2þ, Cu2þ, and Zn2þ complexes with tetra-
armed cyclens.19 Aromatic side arms in the complexes with
armed cyclens, however, never cover the metal ions incorpo-
rated in the cyclen unit. We named the tetra-armed cyclens
bearing aromatic side arms argentivorous molecules.
To visualize the Agþꢀπ interactions, the isosurfaces of
the LUMO of the Agþ complexes were calculated using the
(16) Spartan ’10, Wavefunction, Inc.: Irvine, CA.
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Molecular Mechanics and Quantum Chemistry Calculations; Wavefunc-
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Acknowledgment. This research was supported by
Grants-in-Aid (08026969 and 11011761), a High-Tech
Research Center project (2005ꢀ2009), and the Supported
Program for Strategic Research Foundation at Private
Universities (2012ꢀ2016) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan, and the
Futaba Memorial Foundation, for Y.H.
(18) The protons at the 20- and 60-positions are equivalent in the Agþ
complexes because rotation rate of the aromatic side-arms would be
faster than the NMR time-scale under the condition.
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Supporting Information Available. Experimental pro-
cedures and physicochemical properties, titration experi-
ments, X-ray structures, and molecular calculations
(PDF). Crystallographic data of ligands and complexes
(CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 17, 2012
4579