Self-complementary and narcissistic self-sorting of bis-acridinium tweezers

Article abstract of DOI:10.1039/c9dt01465a

A molecular tweezer incorporating two acridinium moieties linked by a 1,3-dipyridylbenzene spacer was synthesized in three steps. The formation of its self-complementary dimer in water was demonstrated as a result of π-π stacking and hydrophobic interacti

Full text of DOI:10.1039/c9dt01465a

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Self-Complementary and Narcissistic Self-Sorting of bis-Acridinium
Tweezers
Henri-Pierre Jacquot de Rouville,*a Christophe Gourlaouena and Valérie Heitz*a
Received 00th January 20xx,
Accepted 00th January 20xx
A molecular tweezer incorporating two acridinium moieties linked by a 1,3-dipyridylbenzene spacer was synthesized in
three steps. The formation of its self-complementary dimer in water was demonstrated as the result of - stacking and
hydrophobic interactions. Moreover, a 1:1 mixture of this bis-acridinium tweezer with one build on a 2,6-diphenylpyridyl
spacer evidenced a narcissistic self-sorting behaviour in water.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Biological systems perform complex functions (transmission of sorting tweezer-shaped compounds of similar shape and size
information, chemical transformations, transport, regulation) have been reported.12
on account of reversible interactions between biomolecules.1 We recently described the synthesis of a self-complementary
The selective formation of functional aggregates relies on very bis-acridinium tweezer (1·2Cl) in a five-step synthesis (Scheme
efficient self-sorting processes arising from complex mixtures. 1). This tweezer has shown to self-assemble into an entwined
The efficiency of these self-sorting processes is encoded in the dimer in acetonitrile and water.13 More especially, two
shape and the functional groups in each components of the acridinium moieties acting as electron-deficient recognition
involved partners. In the field of supramolecular chemistry, the units were able to interact through - stacking interactions
comprehension of molecular recognition and self-assembly with the 2,6-diphenylpyridyl spacer of a second tweezer. In
phenomena2 gave rise to architectures of increasing complexity water, the complete shift of the equilibrium towards the dimer
in order to mimic various biological functions. Interestingly, the (1)2·4Cl was the result of hydrophobic interactions as
1
self-association of identical complementary units represents demonstrated by H NMR experiments (variable temperature
the simplest way to achieve complex structures endowed with and concentration studies, NOESY and DOSY experiments).
functions. Among the systems able to self-associate, those
involving – stacking interactions between complementary
units have been less reported.3
The interest in self-sorting processes in order to organize
functional systems has emerged more recently.4 According to
Isaacs et al., self-sorting was defined as the high fidelity
recognition between species within complex mixtures.5 Two
assembling processes can be foreseen for these systems: (i)
narcissistic self-sorting (if molecules self-recognize) and (ii)
social self-sorting (if molecules specifically interact with
partners). For both types of behaviour, the equilibrium is either
Scheme 1 Molecular tweezer 1·2Cl dimerizing in water.13
driven thermodynamically (thermodynamic self-sorting) or
kinetically (kinetic self-sorting). The outcome of the self-sorting
We envisioned that a new self-complementary tweezer
processes can be directed by several parameters –also called
incorporating a 1,3-dipyridylbenzene spacer (2·2Cl) could lead
molecular codes.6 Among molecular codes used in self-sorting
to interesting self-association processes governed by the
systems, geometrical complementary (size and shape),7
different electronic properties of the spacer. In comparison to
complementarity in hydrogen-bonds,8 steric effects,9
1·2Cl, the relative positions of the nitrogen atoms in the spacer
coordination sphere in metal-ligand interactions10 and charge-
transfer11 can be cited. In this context, rare examples of self-
of 2·2Cl should not alter the optimum distance for - stacking
interactions (7.1 Å) between both acridinium units.14 In the
present work, we report (i) the synthesis of a new bis-acridinium
a. Institut de Chimie de Strasbourg, UMR 7177-CNRS, Université de Strasbourg,
Institut Le Bel, rue Blaise Pascal, 67008 Strasbourg, France. E-mail:
v.heitz@unistra.fr, hpjacquot@unistra.fr
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
tweezer, (ii) the characterization of its self-association into an
entwined dimer in water and (iii) the study of a 1:1 mixture of
this new tweezer with our previously reported one (1·2Cl)13
resulting in a narcissistic self-sorting process.
4
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The synthesis of the targeted receptor 2·2Cl bearing two species was revealed exhibiting upfield-shifts of allVpierwoAtroticnlesO(nFliinge.
acridinium moieties was achieved according to a shorter 1d). Noteworthy, chemical shifts of the DaOcrIi:d10in.1i0u3m9/Cp9rDoTto01n4s65oAf
synthetic path than for 1·2Cl (three-step vs. five-step synthesis, (2)2·4Cl ((H1/8) = 7.26, (H2/7) = 7.32, (H3/6) = 8.16 and (H4/5
)
Scheme 2).15 The bis-boronic ester derivative (3) was prepared = 8.48 ppm) are similar to (1)2·4Cl in water (Fig. 1c-d). This
in 96% yield starting from 1,3-dibromobenzene as previously observation suggests that the newly species is the result of the
reported.16 Compound 3 was then allowed to react in a self-assembling of two molecules of 2·2Cl leading to (2)2·4Cl
palladium-catalysed
Csp2-Csp2
coupling
with
2,6- with an average D2d symmetry.
dibromopyridine (4 eq.) in the presence of Pd(PPh3)4 (10%) and
K2CO3 (2N) in Toluene/EtOH/water. After purification using
column chromatography, the corresponding bis-bromopyridyl
derivative (4) was isolated in 53% yield. Finally, halogen-metal
exchange on 4 (1 eq.) using n-BuLi (2 eq.) in hexanes (2.5 M),
followed by addition of 10-methyl-9(10H)-acridone at –78°C (2
eq.) led to the corresponding bis-acridane intermediate.
Aromatization of this intermediate was performed using a
solution of HCl (37 wt. %). Purification was carried out by anion
metathesis using KPF6 and filtration. Then, the bis-acridinium
receptor (2·2PF6) was converted to the corresponding chloride
salt 2·2Cl using tetrabutylammonium chloride (TBACl) in
acetone. The desired tweezer 2·2Cl was obtained as a yellowish
solid in 30% yield from 4.17
Fig. 1 1H NMR (400 MHz, 298 K, c = 3 x 10–3 mol L–1) spectra of (a) 1·2Cl (*traces of (1)2·4Cl)
and of (b) 2·2Cl in DMSO-d6 and of (c) (1)2·4Cl and of (d) (2)2·4Cl in D2O.
This hypothesis was first corroborated by 1H–1H ROESY
experiments (Fig. 2). Cross peaks between the outer benzene
protons (Ho and Hm) and the methyl protons of the acridinium
moieties were observed. These through-space correlations can
be best interpreted in terms of the close proximity of the central
2,6-dipyridylbenzene spacer and the acridinium moieties. This
observation can be best interpreted in terms of – interactions
existing between all aromatic moieties of the molecule. As seen
for the entwined dimer formed with 1·2Cl, the cone of
anisotropy of these moieties within the dimer affect each other
Scheme 2 Three-step synthesis of 2·2Cl starting from 1,3-dibromobenzene.
1
All H NMR spectrum signals of 2·2Cl (DMSO-d6, 298 K) were
assigned using 2D NMR experiments (see ESI). Comparison of
1
the H NMR spectrum of 2·2Cl (c = 3 x 10‒3 mol L‒1) to our
1
thus explaining the observed chemical shifts in the H NMR
previously reported bis-acridinium receptor 1·2Cl showed
analogous chemical shifts for the acridinium protons ((H1/8) =
7.98 , (H2/7) = 7.88, (H3/6) = 8.45 and (H4/5) = 8.90 ppm, see
Fig. 1a-b).13 This observation suggests that the acridinium
protons of 2·2Cl are not affected by the electronic environment
of the aromatic ring (phenyl or pyridyl) at their 9-position.
Moreover, the spectrum of 2·2Cl did not show any signals
related to the presence of its corresponding dimer as observed
in 1·2Cl thus suggesting the dimer formation is less energetically
favoured for (2)2·4Cl than for (1)2·4Cl. As observed for 1·2Cl, we
envisioned that hydrophobic interactions would conduct to the
formation of the dimer (2)2·4Cl. Solutions of 2·2Cl (c = 3 x 10‒3
mol L‒1) were recorded with an increasing percentage of D2O in
DMSO-d6 (from 0% to 100%, see Fig. S2.23). Upon increasing the
amount of D2O, the proton signals broadened out and were
upfield shifted, indicative of dynamic processes involving self-
association. In pure D2O, the spectrum of a single discrete
spectrum of (2)2·4Cl.13 Further evidence of the formation of the
dimer (2)2·4Cl was given by DOSY experiments (see the ESI, Fig.
S2.19). Experiments recorded in D2O (c = 1 x 10‒2 mol L‒1, 298 K)
showed that all protons have the same diffusion coefficient and
correspond to a unique diffusing species. From the obtained
diffusion coefficient (D) of 279 µm2 s‒1, the calculated
hydrodynamic radius (RH) was found to be 6.9 Å. The same
value was estimated for the hydrodynamic radius of the
monomer, 2·2Cl in DMSO-d6 (RH = 6.9 Å; D = 145 µm2 s‒1, see
the ESI, Fig. S2.11) attesting the good geometrical fit of the
tweezer within the dimer. Noteworthy, these hydrodynamic
radii are similar, within the experimental error of 5%, to the
calculated hydrodynamic radius obtained for 1·2Cl (RH = 7.3 Å;
D = 263 µm2 s‒1 for (1)2·4Cl)13 thus supporting the formation of
(2)2·4Cl in water.18
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cooling the sample (298 K), the previously monitored 1H NMR
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DOI: 10.1039/C9DT01465A
spectrum were recorded at both temperatures (no traces of an
hypothetical heteromeric dimer (1·2)·4Cl were observed).
Additionally, evaporation of the solvent and solubilisation of
the mixture in DMSO-d6 was performed. As demonstrated
previously, DMSO-d6 solvates each molecule in their
corresponding monomers, namely 1·2Cl and 2·2Cl (see ESI, Fig.
S3.3). After evaporation of the solvent and solubilisation in
1
D2O, the recorded spectrum was identical to the former H
NMR spectrum in D2O (see ESI, Fig. S3.4). These additional
experiments clearly demonstrate that this self-sorted mixture
is under thermodynamic control.20 Consequently, it can be
inferred that in D2O, more than 95% of the mixture is
composed of both dimers meaning that the association
constant of (1)2·4Cl is at least ten times higher than that of
(2)2·4Cl.
Fig. 2 1H–1H ROESY (500 MHz, D2O, 298 K, c = 1 x 10‒2 mol L‒1) spectrum of (2)2·4Cl.
Correlations between the m and o protons of the central pyridine with the methyl
protons are highlighted.
Further insights into the monomer–dimer equilibrium were
1
provided by H NMR variable temperature experiments. A
solution of 2·2Cl at a concentration of 1 x 10‒2 mol L‒1 in D2O
was heated from 278 K to 348 K (see ESI, Fig. S2.22). At any
1
temperature, the same H NMR spectrum of (2)2·4Cl was
observed with a slight upfield shift of all proton signals
attributed to a temperature effect. Consequently, the
monomer-dimer equilibrium is exclusively driven to the dimeric
form in water. In order to confirm this observation, variable
concentration experiments were conducted in D2O. From 10‒2
to 5 x 10‒4 mol L‒1, only signals corresponding to (2)2·4Cl were
observed at 298 K and at 348 K (see ESI, Fig. S2.20-2.21). As
previously postulated for (1)2·4Cl, the high stability of (2)2·4Cl
can be attributed to a shielding of the hydrophobic spacer from Fig. 3 1H NMR (400 MHz, D2O, 298 K, c = 1 x 10‒2 mol L‒1) spectra of (a) (1)2·4Cl, (b) a 1:1
mixture of (1)2·4Cl and (2)2·4Cl at 298 K, (c) the sample (b) after heating up at 348 K and
(d) the sample (b) after cooling back at 298 K and (e) (2)2·4Cl.
the aqueous environment by the -cationic acridinium
moieties.19
To gain deeper understanding of this narcissistic self-sorting
process, DFT investigations were undertaken on three
We were then interested in studying the mixture of the
tweezers 1·2Cl and 2·2Cl of similar shape and size.
Consequently, a 1:1 mixture of 1·2Cl and 2·2Cl in D2O (c = 1 x
10‒2 mol L‒1, Fig. 3) was prepared. The 1H NMR spectrum of the
mixture showed the superposition of signals of both dimers
(1)2·4Cl and (2)2·4Cl (no additional peaks detected)
corresponding to a narcissistic self-sorting behaviour. Over a
period of three weeks, no changes were observed thus
suggesting the thermodynamic stability of the sample at room
temperature. In order to confirm this self-sorting process,
variable temperature experiments were performed in D2O
(see ESI, Fig. S3.1-3.2). Upon successively heating (348 K) and
4+
tetracationic dimers (1)24+, (2)2 and (1·2)4+ (Fig. 4). The
4+
optimized geometry of (1)2 is in good agreement with the
previously reported solid state structure thus validating the
method used.13 Even though similar torsion angles were found
in all dimers, the distance between the two central aromatic
4+
rings of the spacers in (2)24+ (3.962 Å) was shorter than in (1)2
(4.124 Å) and in (1·2)4+ (4.289 Å). The homomeric dimers (1)2
4+
4+
and (2)2 formation exhibit the highest driving force (G =
‒24.9 kcal mol‒1 and ‒23.0 kcal mol‒1 respectively) in
comparison to the heteromeric dimer (1·2)4+ (G = ‒22.3 kcal
4+
4+
mol‒1). Consequently, the dissociation of both (1)2 and (2)2
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dimers to form two heteromeric dimers (1·2)4+ is endergonic
(G = +3.3 kcal mol‒1) thus confirming the thermodynamic 1,3-Bis(6-bromopyridin-2-yl)benzene 4
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DOI: 10.1039/C9DT01465A
origin of the self-sorting process. Further insights into the To a degassed solution of 2,6-dibromopyridine (540 mg, 2.28
4+
4+
stabilization in (1)2 and (2)2 was given by non-covalent mmol,
3
eq.) and 1,3-bis(4,4,5,5-tetramethyl-1,3,2-
interaction analysis (Fig. 4). The observed stabilization for both dioxaborolan-2-yl)benzene16 3 (250 mg, 0.76 mmol, 1eq.) in a
homodimers in comparison to their monomers was attributed toluene/ethanol mixture (7 mL, 4:3), was added Pd(PPh3)4 (87
to van der Waals forces between the acridinium moieties of one mg, 0.07 mmol, 10%) and a degassed 2N aqueous solution of
monomer with the linker of a second monomer and to the K2CO3 (4 mL). The reaction mixture was stirred at 90°C for 16
existence of [N∙∙∙H] interactions between the inner hydrogen hours. The reaction mixture was then extracted with Et2O (3 x
atoms of the linker of one monomer and the nitrogen atoms of 35 mL) and the combined organic layers were dried (MgSO4).
the linker of a second monomer. These [N∙∙∙H] interactions are After evaporation of solvents, the crude product was purified by
partly electrostatic, tending towards the formation of hydrogen column chromatography (SiO2, cyclohexane / CH2Cl2 – 1:1). The
4+
bonds. Remarkably, these [N∙∙∙H] distances are shorter in (1)2
desired product 4 was obtained as a colorless solid in 53% yield
4+
than in (2)2 suggesting a stronger binding between linkers in (157 mg). 1H NMR (400 MHz, CDCl3, 298 K):  (ppm) = 8.57 (t, J
(1)2 in accordance with the highest driving force for the self- = 1.5 Hz, 1H, Ho’), 8.07 (dd, J = 8.0, 1.5 Hz, 2H, Ho), 7.79 (dd, J =
4+
4+
association of the homodimer (1)2
.
8.0, 1.0 Hz, 2H, H’), 7.63 (t, J = 8.0 Hz, 2H, H), 7.58 (t, J = 8.0 Hz,
1H, Hm), 7.45 (dd, J = 8.0, 1.0 Hz, 2H, H). 13C{1H} NMR (100 MHz,
CDCl3, 298 K):  (ppm) = 158.1 (s, CPy), 142.1 (s, CC-Br), 139.0 (s,
C), 138.3 (s, CPh), 129.3 (s, Cm), 128.2 (s, Co), 126.6 (s, C), 125.5
(s, Co’), 119.2 (s, C’). HRMS (ESI-TOF): for C16H11Br2N2, m/zcalc
388.9283, m/zfound = 388.9263 (100%, [M+H]+).
=
Molecular Tweezer 2·2Cl
To a solution of 1,3-bis(6-bromopyridin-2-yl)benzene (4) (100
mg, 0.26 mmol, 1 eq.) in dry THF (20 mL), was added dropwise
at ‒78°C a solution of n-BuLi (2.5 M in hexanes, 0.20 mL, 0.51
mmol, 2 eq.). After 20 minutes at ‒78°C, 10-methyl-9(10H)-
acridone (107 mg, 0.51 mmol, 2 eq.) was added dropwise. The
mixture was further stirred at ‒78°C for 2 hours, and allowed to
room temperature overnight. After addition of a 2N aqueous
solution of HCl (30 mL), the reaction mixture was poured into
an aqueous solution of KPF6 (6g in 150 mL). The solid was
washed with H2O (3 x 100 mL), solubilized in a minimum amount
of CH3CN and poured into
a saturated solution of
Fig. 4 Representations of (a) the entwined dimer of (1)24+, (b) non-covalent interaction
tetrabutylammonium chloride (TBACl) in acetone (10 mL). After
filtration, the desired product was washed with acetone and
was isolated as a yellow oil in 30% yield (55 mg).
Characterization of 2·2Cl: 1H NMR (500 MHz, DMSO-d6, 298 K):
 (ppm) = 8.90 (d, J = 8.5 Hz, 4H, H4/5), 8.77 (t, J = 1.5 Hz, 1H, Ho’),
8.49 – 8.41 (m, 6H, H3/6-’), 8.33 (t, J = 7.5 Hz, 2H, H), 8.20 (dd, J
= 8.5, 1.5 Hz, 2H, Ho), 7.98 (dd, J = 8.5, 1.5 Hz, 4H, H1/8), 7.89 (dd,
J = 8.5, 1.5 Hz, 4H, H2/7), 7.78 (dd, J = 7.5, 1.0 Hz, 2H, H), 7.64 (t,
J = 8.5 Hz, 1H, Hm), 4.96 (s, 6H, Me). 13C{1H} NMR (125 MHz,
DMSO-d6, 298 K):  (ppm) = 156.8 (s, C9), 156.5 (s, CPy), 151.9 (s,
CPy), 141.6, 138.7, 138.5 (s, C), 138.5 (s, C3/6), 129.6 (s, Cm),
129.1 (s, C1/8), 128.3 (s, Co), 128.2 (s, C2/7), 125.4 (s, Co’), 125.0
(s, C), 121.7 (s, C’), 119.4 (s, C4/5), 39.1 (s, Me). HRMS (ESI-TOF):
for C44H32N4, m/zcalc = 308.1308, m/zfound = 308.1307 (100%,
[M]2+). UV/Vis (DMSO, 298 K): max (nm) (ε (L∙mol‒1.cm‒1)) =
286 (28500), 354 (14650), 368 (22500), 408 (8300), 429 (9500),
4+
analysis in the (1)2 dimer, (c) of the entwined dimer of (2)24+, (d) non-covalent
4+
interaction analysis in the (2)2 dimer. In green are represented the van der Waals
forces, in blue the attractive electrostatic forces and in red the steric congestion.
Experimental Part
Synthesis. All chemicals were of the best commercially available
grade and used without further purification. 10-Methyl-9(10H)-
acridone and 1,3-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzene were synthesized according to previously reported
procedure.16 All compounds were synthesized using schlenk
technics and were fully characterized by 1D (1H, 13C{1H}, 31P{1H}
and 19F{1H}) and 2D (COSY, HSQC and HMBC) NMR experiments
and by mass spectrometry experiments. DOSY spectra were
generated by the DOSY module of the software Topspin to build
the diffusion dimension. The diffusion coefficients were
compared to DMSO or H2O as an internal standards (η(DMSO-
d6) = 2.18 × 10−4 Pa.s and η(D2O) = 1.13 × 10−3 Pa.s). The
hydrodynamic radius (RH) was calculated from the Stokes-
Einstein equation assuming a spherical model21 and using pre-
determined solvent viscosity values.22
1
454 (6650). Characterization of (2)2·4Cl: H NMR (500 MHz,
D2O, 298 K):  (ppm) = 8.78 (br s, 1H, Ho’), 8.43 (d, J = 9.0 Hz, 4H,
H4/5), 8.11 (t, J = 9.0 Hz, 4H, H3/6), 7.49 (br s, 2H, H), 7.27 (t, J =
9.0 Hz, 4H, H2/7), 7.21 (d, J = 9.0 Hz, 4H, H1/8), 7.13 (br s, 1H, Hm),
7.01 – 6.88 (m, 4H, H-’), 6.82 (br s, 2H, ), 4.73 (s, 6H, Me).
13C NMR (125 MHz, D2O, 298 K):  (ppm) = 158.5 (s, C9), 155.3
(s, CPy), 151.9, 141.4, 139.1 (s, C3/6), 137.5 (s, C), 135.7, 129.7 (s,
4 | J. Name., 2012, 00, 1-3
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Wang, M. Frasconi, W.-G. Liu, Z. Liu, A. A. Sarjeant, M. S.
Nassar, Y. Y. Botros, W. A. Goddard III aDnOdIJ:.1F0..1S0t3Vo9ide/wCd9AarDrtitTc,l0eJ1.O4An6lmi5nAe.
Chem. Soc., 2015, 137, 876–885.
C1/8), 127.4 (s, C2/7), 127.0 (s, Co-o’), 126.4 (s, Cm), 125.7, 124.2 (s,
C/’), 119.9 (s, C/’), 118.9 (s, C4/5), 38.6 (s, Me). UV/Vis (H2O,
298 K): max (nm) (ε (L∙mol‒1.cm‒1)) = 296 (15450), 361 (15300),
408 (7650), 430 (7500), 453 (5000).
4
5
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Conclusions
In conclusion, we successfully synthesized a new molecular
tweezer incorporating two acridinium moieties bridged by a
1,3-dipyridylbenzene spacer following a three-step synthesis. In
aqueous solution, this system showed its ability to self-
assemble into an entwined dimer on account of - stacking
and hydrophobic interactions. The 1:1 mixture of this molecular
tweezer and our previously reported tweezer build on 2,6-
diphenylpyridyl spacer resulted in the exclusive formation of
homomeric dimers in water in spite of their similar shape and
6
7
size. This behaviour was the consequence of
a
thermodynamically controlled narcissistic recognition process
as demonstrated by NMR experiments (variable temperature
and competitive solvent experiments). Additionally, DFT
calculations are in perfect accordance with the estimated
association constants of both dimers. The stability of the dimers
in water is ensured by the van der Waals forces, namely -
stacking interactions between the acridinium moieties and the
spacers. Side interactions such as the [N∙∙∙H] bond network in
the core dimers also contribute to the dimer stability. Work is
now underway to synthesize and study new generations of
these bis-acridinium tweezers endowed with specific functions.
8
9
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Conflicts of interest
“There are no conflicts to declare”.
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Acknowledgements
This work was supported by the CNRS and the University of
Strasbourg. We are grateful to Dr. Lionel Allouche and to Dr
Bruno Vincent (NMR Service) of the Fédération de Chimie Le
Bel. We thanks the computation center of Strasbourg for
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DOI: 10.1039/C9DT01465A
A 1:1 mixture of two self-complementary tweezers incorporating different spacers led to a
narcissistic self-sorting process under thermodynamic control.