Tripodal pyrrolotetrathiafulvalene receptors for recognition of electron-deficient molecular guests

Article abstract of DOI:10.1016/j.tetlet.2013.12.011

We report the synthesis of tripodal receptors with monopyrrolo- tetrathiafulvalene arms 1a,b, based on 1,3,5-substituted 2,4,6-triethylbenzene scaffold. The three converging pyrrolotetrathiafulvalene groups form an electron rich cone-shaped binding site. Molecular hosts 1a,b are capable of binding neutral electron deficient guests in solution, as well as positively charged pyridinium species in the gas phase.

Full text of DOI:10.1016/j.tetlet.2013.12.011

Accepted Manuscript
Tripodal Pyrrolotetrathiafulvalene Receptors for Recognition of Electron-De-
ficient Molecular Guests
Michèle-Laure Lieunang Watat, Thomas Dülcks, Dorit Kemken, Vladimir A.
Azov
PII:
S0040-4039(13)02086-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.12.011
TETL 43925
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 October 2013
26 November 2013
4 December 2013
Please cite this article as: Watat, M.L., Dülcks, T., Kemken, D., Azov, V.A., Tripodal Pyrrolotetrathiafulvalene
Receptors for Recognition of Electron-Deficient Molecular Guests, Tetrahedron Letters (2013), doi: http://
dx.doi.org/10.1016/j.tetlet.2013.12.011
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Tripodal Pyrrolotetrathiafulvalene
Receptors for Recognition of Electron-
Deficient Molecular Guests
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Michèle-Laure Lieunang Watat, Thomas Dülcks, Dorit Kemken, Vladimir A. Azov*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Tripodal Pyrrolotetrathiafulvalene Receptors for Recognition of Electron-Deficient
Molecular Guests
Michèle-Laure Lieunang Watat, Thomas Dülcks, Dorit Kemken, Vladimir A. Azov*
aUniversity of Bremen, Department of Chemistry, Leobener Str. NW 2C, D-28359 Bremen, Germany
ARTICLE INFO
ABSTRACT
Article history:
Received
We report the synthesis of tripodal receptors with monopyrrolo-tetrathiafulvalene arms 1a,b,
based on 1,3,5-substituted 2,4,6-triethylbenzene scaffold. The three converging
Received in revised form
Accepted
Available online
pyrrolotetrathiafulvalene groups form an electron rich cone-shaped binding site. Molecular
hosts 1a,b are capable of binding neutral electron deficient guests in solution, as well as
positively charged pyridinium species in the gas phase.
2009 Elsevier Ltd. All rights reserved.
Keywords:
Pyrrolotetrathiafulvalenes
Tripodal receptors
Cyclic voltammetry
Molecular recognition
Mass-spectrometry
Tetrathiafulvalenes1 (TTFs) are redox active heterocyclic
compounds, which rapidly found use in the field of organic
electronics2 due to their ability to form conductive phases in the
solid-state. In later years they were extensively employed as
building blocks in diverse supramolecular systems, where they
played the role of switching units3 in different types of molecular
architectures.4 In interlocked supramolecular devices,5 TTF
moiety is employed as a redox-switchable electron donor, which
stacks itself between two charged electon-deficient viologen units
of the cyclobis(paraquat-p-phenylene) (CBPQT4+) macrocycle.
On the other hand, supramolecular systems, employing the
reverse binding principle and comprising two or more TTF
moieties stacking electron-deficient guest between them, are
almost non-existent.
same face of the central phenyl ring, which are directed to the
other side from the three ethyl groups.13 Molecular systems
based on 1,3,5-2,4,6-trisubstituted benzenes are often
straightforward to synthesize, and, for that reason, found use in
construction of receptors for cations,14 anions,15 and polar guests
such as carbohydrates16,17 or proteins,18 as well as for the
assembly of cage molecules.17,19 In contrast, to the best of our
knowledge only one report on the employment of tripodal
receptors for recognition of non polar organic molecular guests
such as fullerenes has been published up to date.20
We have prepared tripodal receptors 1a,b21 (Scheme 1) with
high yields by reaction of deprotonated MPTTFs 2a,b with 1,3,5-
tris(bromomethyl)-2,4,6-triethylbenzene 3.
Bis-alkylthio
MPTTF 2a is very stable and easy to handle, whereas non-
substituted MPTTF 2b and its derivatives are more sensitive,
however usually affording better binding with electron-deficient
molecular guests.7,22
Being interested in designing TTF-containing supramolecular
receptors, we have prepared several bis-TTF molecular
architectures,6 and recently accomplished the construction of
original
calix[4]arene-based
molecular
tweezers
with
The new MPTTF derivatives were obtained as bright yellow
or pale yellow crystalline powders for derivatives of 1a and 1b,
respectively, displaying fairly good solubility in organic solvents,
such as CH2Cl2 or toluene. NMR spectra, measured at room
temperature, display sharp signals and concentration-independent
chemical shifts, implying the absence of molecular aggregation.23
The UV/Vis spectra of 1a (CH2Cl2, 293 K) show the common
absorption pattern for TTF derivatives24 with maximum
absorption at max of ca. 310–330 nm, as well as a shoulder at ca.
390 nm (absent in spectrum of the non-substituted TTF
derivative 1b) and a long tail with low absorption reaching to ca.
500 nm.
monopyrrolotetrathiafulvalene (MPTTF) arms, which proved to
be efficient binders of planar electron-deficient guests.7 Pyrrolo-
annealed tetrathiafulvalenes,8 used by us as molecular
recognition elements, have been also successfully employed for
the construction of TTF-calix[4]pyrrole receptors9 and
supramolecular cages for binding of neutral guests.10,11
To explore other layouts of molecular architectures, we have
turned our attention to the 1,3,5-trisubstituted benzene scaffold.
Due to ―steric gearing‖, 1,3,5-trisubstituted 2,4,6-triethylbenzene
derivatives12 exhibit a preference for an alternating, 3-up, 3-down
conformation, leading to disposition of the functional arms on the
———
* Corresponding author. Tel.: +49-421-218-63126; fax: +49-421-218-63120; e-mail: vazov@uni-bremen.de

2
Tetrahedron Letters
calibration purposes.
b Weak oxidation wave
We have tested several compounds (Figure 2) as possible
molecular guests for receptors 1a,b. Qualitative experiments
with TCNQ and TNF in CH2Cl2 solution gave evidence for
formation of charged-transfer (CT) complexes, manifesting itself
in the build up of greenish (with TCNQ) or brownish (with TNF)
color hue. CT bands of the complexes were centered at ca. 850
nm (TCNQ) or stretched into the near-IR region with the
maximum above 1000 nm (TNF)27 Coloration was noticeable for
1b, whereas 1a did not afford any visible color change, although
CT bands were detectable by means of the UV/Vis spectrometer.
Charged pyridinium salts, taken as iodides (NMPI, NMMPI,
NEMPI), as well as fullerene, did not show formation of CT
complexes with 1a,b in a solution. UV/Vis binding titrations28 in
CH2Cl2 afforded a binding constant Ka of 780 M-1 (850 = 400 M-
1cm-1) for the 1b·TCNQ complex. Reliable determination of
constants for the other complexes was not possible, either due to
fast precipitation of the host-guest complex in case of 1b and
TNF, or due to low binding constants in case of 1a. We can only
speculate about possible orientation of an extended planar
molecule, such as TCNQ or TNF, upon its binding with a
tripodal receptor. Due to relatively small binding pocket size and
symmetry mismatch we suppose that such a guest can be stapled
between two neighboring TTF moieties.
Scheme 1. Synthesis of tripodal receptors 1a,b.
The electrochemical properties of 1a,b MPTTF derivatives
were investigated by cyclic voltammetry (CV) in
CH2Cl2/Bu4NClO4 solutions (Figure 1 and Table 1). Receptor
1a, comprising three SPr-substituted TTF units, displays the
classical electrochemical behavior of tetrathiafulvalene
derivatives,2 showing two quasi-reversible electrochemical
processes on the cathodic scan, the first one leading to the TTF
radical cation, and the second affording the dication. Value of
the first and second half-wave oxidation potentials E1/2ox, as well
as the peak separation Ep are similar to the previously reported
values for the N-substituted bis-alkyltho-MPTTF derivatives in
CH2Cl2,8b implying lack of direct electronic communication
between the tetrathiafulvalene moieties. On the other hand, the
CV of 1b is poorly reversible, representing one strong low-lying
oxidation potential, presumably leading to mono-oxidations of
the MPTTF groups, whereas only a small wave is observed at the
position of the expected second oxidation wave.25 Although the
nature of this irreversibility has not yet been clarified, we
suppose that this effect may be due to an intramolecular reaction
between two spatially proximal MPTTF moieties26 or due to
irreversible adsorption of the oxidized species to the electrode
surface.11
Figure 2. Structures of molecular guests tested in this study: 7,7,8,8-
tetracyano-quinodimethane (TCNQ), 2,5,7-trinitro-9-
dicyanomethylenefluorene (TNF), tetraethylammonium bromide (TEA+),
tetrapropylammonium bromide (TPA+), tetrabutylammonium bromide
(TBA+), 1-methylpyridinium iodide (NMP+), 1-methyl-4-
(methoxycarbonyl)pyridinium iodide (NMMP+), 1-ethyl-4-
(methoxycarbonyl)pyridinium iodide (NEMP+).
ESI-MS spectra measured during the characterization of 1a,b
revealed their tendency to form stable positively charged
molecular complexes not only with alkali metal cations, but also
with organic species, such as [Et3NH]+. Taking into account
recent advances in the field of mass-spectrometry of
supramolecular aggregates,29 such as MS characterization of
host-guest complexes with molecular capsules30 or molecular
tweezers,31 we decided to conduct a set of competition
experiments using ESI-MS as the probe. Thus, the binding
affinities as well as the selectivities of two families of charged
organic molecular guests (Figure 2), trialkylammonium bromides
(TEA+, TPA+, TBA+) and pyridinium iodides (NMP+, NMMP+,
NEMP+), to receptors 1a and 1b were tested. Both families of
molecular guests were taken as equimolar mixtures at ca. 25 M
concentrations in CH2Cl2/MeCN (1:10), mixed with the equally
concentrated host solution and analyzed using ESI-MS.
Figure 1. Cyclic voltammograms of monopyrrolo-TTF derivatives: 1a and
1b. CH2Cl2/0.1 M Bu4NClO4, scan rate 100 mV/s, potentials are plotted vs.
SCE.
Table 1. Electrochemical data of monopyrrolo-TTF derivatives: 1a and 1b.a
Compound
E1/2ox1 (V)
E1/2ox2 (V)
1a
1b
0.40
0.81
(0.74)b
Experiments with tetraalkylammonium ions displayed the
formation of host-guest complexes with 1a with slight preference
for binding of TEA+, whereas 1b showed virtually no complex
formation.27 Additionally, the build up of trialkylammonium
bromide clusters with masses rising as high as 1,5 kDa was
0.23
a Data were obtained using a one-compartment cell in CH2Cl2/0.1 M
Bu4NClO4, Pt as the working and counter electrodes and a non-aqueous
Ag/Ag+ reference electrode; scan rate 100 mV/s. Values given at room
temperature vs. SCE; the Fc/Fc+ couple (0.480 V vs. SCE) was used for

3
noted. Much higher complex stability and selectivity was
observed for pyridinium derivatives. Although the NEMP+
signal was the strongest in the ESI-MS spectrum of the equimolar
mixture of the pyridinum guests, which was to be expected from
considerations of hydrophobicity,32 both receptors 1a and 1b
displayed a marked selectivity for binding of the NMMP+ cation
(Figure 3). Moreover, almost no free hosts (i.e., radical cations
or sodiated molecular ions, as they were observed in the mass
spectra of the pure host solutions) could be detected in the MS
spectra of the mixtures, indicating stability of the host-guest
complexes in the gas phase.
process in ESI, and specific receptor-guest complex
formation.29b We suppose that in case of pyridinium cations
specific host-guest complex formation with a guest ion residing
within the binding pocket of the receptor takes place. This can be
concluded from the high degree of selectivity of the binding of
NMMP+ as discussed above, the high intensity of the complex
signal in relation to the signal of the free receptor, and from the
results of molecular modeling. However, under the experimental
conditions chosen the obtained mass spectra to large extent
reflect solution-phase binding properties.
The detailed
investigation of gas-phase binding properties would require gas-
phase dissociation studies,34 such as skimmer-CID or MS/MS
experiments, which will be performed in due course.
a)
a)
b)
Figure 4. Energy-minimized (PM3) structures of (a) receptor 1b; (b) host-
guest copmplex of 1b with NMP+.
We should note that MS signals of the complex ions show a
noticeable broadening, which was found to be persistent
throughout the measurements. Since the signals of the free
pyridinium ions did not show this broadening, we believe that
this may be due to a gradual decay of the complex ions within the
ion trap of the mass spectrometer during the measurement
interval, which is on the order of several milliseconds.
b)
To conclude, two tripodal receptors with TTF arms described
in the paper were shown to bind electron-deficient species in
solution and such complexes remained stable upon ionization
under ESI-MS conditions. When compared to more rigid
calixarene-based molecular tweezers recently reported by us,7
receptors 1a,b reveal limitations of flexible architecture with
three freely rotating TTF moieties and their symmetry mismatch
with planar guests such as TCNQ and TNF, leading to their
inferior guest-binding capability in solution. On the other hand,
the affinity to positively charged pyridinium derivatives, revealed
by ESI-MS studies, requires more detailed examination of these
receptors as organic cation binders in the gas phase.
Figure 3. ESI-MS+ spectra of (a) mixture of pyridinium iodides plus receptor
1a; (b) mixture of pyridinium iodides plus receptor 1b.
Molecular modeling27 exhibits a very good size match of the
binding pocket of 1b and the N-methylpyridinium cation
(Figure 4), whereas the bulkier and bent out-of-plane ethyl
substituent of NEMP+ makes the molecule less compact and
impedes the formation of a tight host-guest complex. Thus, the
selectivity of 1a,b for the binding of NMMP+ can be explained
by the following factors. First, NMP+ and NMMP+ display a
better geometrical fit to the binding pocket of 1 compared to
NEMP+. Second, the electron-withdrawing effect of the 4-
methoxycarbonyl group, which is located opposite to the methyl
substituent and does not interfere with the binding process,
provides additional stabilization to the host-guest complexes of
NMMP+ with the electron-rich receptors 1a,b compared to
NMP+, making NMMP+ the most favorable guest cation.
Molecular modeling also showed a size disparity between 1b and
fullerene, which is slightly larger than the binding pocket of the
receptor, thus accounting for the absence of the binding affinity
of 1a,b to C60.33
Acknowledgments
We are indebted to Mr. J. Stelten (NMR) and to Dr. Uli Papke
(HR-MS, TU Braunschweig) for their help with the
characterization of the new compounds.
Supplementary Data
Supplementary data associated with this article can be found,
in the online version, at:
References and notes
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4
Tetrahedron Letters
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32. In this context the term ―hydrophobicity‖ describes the decreasing
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solvation shell) ions with unpolar hydrocarbon chains of
increasing length. In ESI-MS, where polar and often protic
solvents are used, it is a common experience that ions with larger
unpolar residues show relatively higher ionisation efficiencies.
33. Monopyrrolo-TTF derivatives with two or more MPTTF moieties
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21. Receptor 1a: bright yellowcrystalline powder. M.p. 121125 °C.
Rf = 0.9 (CH2Cl2). 1H NMR (360 MHz, toluene-d6): δ 0.61 (t, 3J =
7.2 Hz, 9H), 0.77 (t, 3J = 7.2 Hz, 18H), 1.42 (sext, 3J = 7.2 Hz,
12H), 2.24 (q, 3J = 7.2 Hz, 6H), 2.48 (t, 3J = 7.2 Hz, 12H), 4.49 (s,
6H), 5.84 (s, 6H). 13C NMR (90 MHz, toluene-d6): δ 13.1, 15.2,
23.3, 23.5, 38.2, 47.4, 111.4, 111.9, 119.9, 120.2, 130.9, 145.7.
UV/Vis (CH2Cl2): λmax (ε) 290 nm (42300 L mol-1 cm-1), 327
(37000), 390 sh (3800), 450 sh (1000). MS (ESI+): m/z (%) 1371
(monoisotopic) (30) [M]+·, 685.5 (monoisotopic) (100) [M]2+.
HRMS (ESI+): m/z [M]+ Calcd. for C57H69N3S18+: 1371.04587;
ox2
Found 1371.04590. CV (vs. SCE, CH2Cl2): E1/2ox1 = 0.40 V, E1/2
= 0.81 V.

Highlights
 We report the synthesis of tripodal monopyrrolo-tetrathiafulvalene receptors
 We examine their binding ability towards electron-deficient substrates
 In solution, our receptors bind non-charged molecular guests
 Our receptors display binding to pyridinium ions in the gas phase