CO2 binding by dynamic combinatorial chemistry: An environmental selection

Article abstract of DOI:10.1021/ja909975q

We now report that a dynamic combinatorial selection approach can quantitatively provide, from trivial building blocks, an architecturally complex organic material, in which carbon dioxide is reversibly but covalently incorporated as a guest with a mass content of 20%. Solid-state analyses combined with covalent disconnection and quantization of the liberated components allowed identification of a three-component monomeric unit repeated within a range of assembled oligomeric adducts whose repartition and binding capacity can be finely tuned through the starting stoichiometries. The self-assembly of these architectures occurs through the simultaneous creation of more than 25 covalent bonds per molecular entity. It appears that the thermodynamic selection is directed by the packing efficiency of these adducts, explaining the spectacular building block discrimination between homologues differing by one carbon unit. This selectivity, combined with the reversible nature of the system, provided pure molecular building blocks after a simple chemical disconnection, promoting CO₂ as a green auxiliary to purify polyaldehyde or polyamine from mixtures of homologous structures. Moreover, the gas template could be expelled as a pure compound under thermodynamic control. This cooperative desorption process yielded back the initial libraries of high molecular diversity with a promising reduction of the energetic costs of capture and recycling.

Full text of DOI:10.1021/ja909975q

Published on Web 02/19/2010
CO2 Binding by Dynamic Combinatorial Chemistry: An
Environmental Selection
Julien Leclaire,*,† Guillaume Husson,† Nathalie Devaux,† Vincent Delorme,†
Laurence Charles,‡ Fabio Ziarelli,§ Perrine Desbois,⊥ Alexandra Chaumonnot,⊥
Marc Jacquin,⊥ Fre´de´ric Fotiadu,*,† and Ge´rard Buono†
Laboratoire Chirosciences, UMR 6263 CNRS: Institut des Sciences Mole´culaires de Marseille
´
ISM2, Ecole Centrale Marseille, UniVersite´ Paul Ce´zanne, case A62, AVenue Escadrille
Normandie-Niemen, 13397 Marseille Cedex 20, France, UniVersite´s Aix-Marseille I, II, and III -
CNRS, UMR 6264: Laboratoire Chimie, ProVence, Spectrome´tries Applique´es a` la Chimie
Structurale, F-13397 Marseille, France, Aix-Marseille UniVersite´, Fe´de´ration des Sciences
Chimiques, Spectropoˆle, case 511, aVenue Escadrille Normandie Nie´men, 13397 Marseille cedex
20, France, and IFP, IFP-Lyon, BP no. 3, 69360 Solaize, France
Received December 3, 2009; E-mail: julien.leclaire@centrale-marseille.fr;
frederic.fotiadu@centrale-marseille.fr
Abstract: We now report that a dynamic combinatorial selection approach can quantitatively provide, from
trivial building blocks, an architecturally complex organic material, in which carbon dioxide is reversibly but
covalently incorporated as a guest with a mass content of 20%. Solid-state analyses combined with covalent
disconnection and quantization of the liberated components allowed identification of a three-component
monomeric unit repeated within a range of assembled oligomeric adducts whose repartition and binding
capacity can be finely tuned through the starting stoichiometries. The self-assembly of these architectures
occurs through the simultaneous creation of more than 25 covalent bonds per molecular entity. It appears
that the thermodynamic selection is directed by the packing efficiency of these adducts, explaining the
spectacular building block discrimination between homologues differing by one carbon unit. This selectivity,
combined with the reversible nature of the system, provided pure molecular building blocks after a simple
chemical disconnection, promoting CO2 as a green auxiliary to purify polyaldehyde or polyamine from
mixtures of homologous structures. Moreover, the gas template could be expelled as a pure compound
under thermodynamic control. This cooperative desorption process yielded back the initial libraries of high
molecular diversity with a promising reduction of the energetic costs of capture and recycling.
Introduction
gentle heating and/or reduction of partial pressure. Carbon
dioxide therefore meets all requirements to be used, through
Primary and secondary amines can reversibly trap carbon
dioxide through the formation of ammonium carbamates, a
simple, quick and reversible reaction for CO2 transportation
during the respiratory process.1 This strategy is also used by
industrial units devoted to gaseous streams purification2 and is
explored in pilot installations3 aimed at reducing anthropic
emissions. Among reversible reactions explored in chemistry,
few involve a gaseous entity at room temperature.4 According
to Le Chatelier’s rules, the covalent carbamate linkage, albeit
robust and stable,5 can release CO2 and the initial amines upon
ammonium carbamate formation, as a reversible, green and
cheap agent for introducing ion pairs and hydrogen bonds into
molecular systems. A chaotic assembly of initially independent
molecules can indeed be organized, through ammonium car-
bamate pairing, into a network of controlled extension with
original and thermally reversible macroscopic properties, such
as ionic liquids,6 hybrid materials,7 organogels,8 or supramo-
lecular polymers.9
In an effort to create tailored molecular receptors for CO2,
we turned to dynamic combinatorial chemistry (DCC)10 as a
†
´
Universite´ P. Ce´zanne, Ecole Centrale Marseille - CNRS, UMR 6263.
‡ Universite´s Aix-Marseille I, II, and III - CNRS, UMR 6264.
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Pampaloni, G. Chem. ReV. 2003, 103, 3857–3897.
§ Aix-Marseille Universite´, Fe´de´ration des Sciences Chimiques, Spec-
tropoˆle.
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M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958–960. (c) Jessop,
P. G.; Heldebrant, D. J.; Li, X.; Eckert, C. A.; Liotta, C. L. Nature
2005, 436, 1102.
⊥ Institut Francais du Petrole, Separation.
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C.; Perrella, M.; Rossi-Bernardi, L.; Roughton, F. J. J. Physiol. 1972,
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3582 J. AM. CHEM. SOC. 2010, 132, 3582–3593
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CO2 Binding by Dynamic Combinatorial Chemistry
A R T I C L E S
Figure 1. Top: building blocks and reversible linkages involved in the DCL for CO2 capture: (a) transimination for oligomeric backbone formation and CO2
capture by ammonium carbamate formation (b) CO2 template (the “goose”), (c) polyamine 1-5 (the “fish”) and (d) polyaldehyde A-D (the “turtle”) building
block. Bottom: host-guest self-assembly in DCC. (e) All noncovalent. (f) Covalent self-assembly induced by non covalent interactions with a template. (g)
Mixing covalent and noncovalent template binding by a covalently assembled structure.
strategy to readily generate a wealth of architectures from simple
building blocks through thermodynamically controlled self-
assembly. In the absence of an external selector, one potential
application of such a mixture called dynamic combinatorial
library (DCL) is to identify, within pools of molecular objects,
the most stable structure resulting from individual intramolecular
or intermolecular noncovalent interactions between library
members. Most often, a molecular partner is introduced to select
species that act as hosts or receptors. In the latter case, while
building blocks can self-assemble into molecular or supramo-
lecular entities, the selection process of the host by the guest
has so far been exclusively conducted by noncovalent means
(Figure 1e and f, bottom). We herein report for the first time
the implementation into the host-guest paradigm of DCC of a
combination of covalent and noncovalent reversible interactions,
respectively the carbamate and ammonium-carbamate linkages
(Figure 1g).
carbamate linkage while primary ones should compete for CO2
binding and imine exchange (Figure 1a). A characteristic feature
of the CdN linkage is that its formation,11a substitution,11b and
metathesis11c have all been reported to be under thermodynamic
control in chosen conditions. Transimination in organic
solvents12a,b in the absence of catalyst12c was selected to ensure
rapid regulation of the composition and connectivity within the
library during carbon dioxide introduction and subsequent
ammonium carbamate formation. Preliminary experiments on
model compounds confirmed that this exchange reaction was
essentially isoenergetic and displayed faster kinetics than
ammonium carbamate formation,13 which should serve as the
stimulus. While Schiff-base chemistry involving dicarbonyl
species has been harnessed to provide numerous macrocyclic
receptors in supramolecular chemistry,14 we deliberately turned
to tricarbonyl precursors (Figure 1d) to generate a wealth of
Polyamines (Figure 1c) appeared as appropriate building
blocks for exploring cooperative CO2 binding and simultaneous
imine exchange with carbonyl connectors. Secondary nitrogen
sites can potentially be engaged into reversible ammonium
(11) (a) Huc, I.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 2106–
2110. (b) Giuseppone, N.; Lehn, J.-M. J. Am. Chem. Soc. 2004, 126,
11448–11449. (c) Oh, K.; Jeong, K. S.; Moore, J. S. Nature 2001,
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Chem. 2005, 18, 979–985. (b) Hutin, M.; Schalley, C. A.; Bernardinelli,
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possible architectures (branched objects, macrocycles, capsules)
from which CO2 should select and amplify its best host. In
particular, trialdehydes A-C and dialdehyde D15 were chosen
as connecting units between polyamines and were, prior to use,
condensed with n-butylamine 1, yielding building blocks A13,
B13, C13, and D12 as transimination substrates (Figure 1). Upon
exchange with polyamines 2-5, the monoamine 1 released to
solution should also serve as a traceable reporter of the extent
of the transimination reaction in NMR spectroscopy.16
primary amine group on these oligomeric backbones is then
available for either CO2 binding or protonation, yielding of
virtual collection of 2.77 × 104 possible objects in the templated
system [A; 2; CO2] (Supporting Information). Introducing a
dissymmetrical polyamine such as 3 or 5 alternatively to 2 or
4 multiplies by a factor of roughly 23, 24 and 25, respectively,
the amount of expected aromatic monomers, macrocyclic and
branched dimers (a dissymmetrical polyamine generates as much
diversity as two symmetrical ones). As an illustration of this
exponential growth, these templated libraries were estimated
to contain about 7.22 × 105 members, although formed from
three trivial but chemically different precursors (Figure 2,
bottom).
Results
To first determine whether branched, macrocyclic, or capsule
architectures would be more suitable for CO2 recognition, we
generated a preliminary set of 16 libraries from simple pairs of
cores and connecting units. In a typical experiment, monomer
X1n (X ) A-C, n ) 3; X ) D, n ) 2) was combined with
two equivalents of a triamine 2-5 in methanol at room
temperature, yielding the DCL [X; n]. The composition of the
As a noticeable exception in the [X; n] series, because no
amplification could be detected in the 15 other homologous
mixtures, library [A; 2] was highly perturbed by CO2 introduc-
tion, quasi-quantitatively converting the network of scrambling
1
soluble architectures into a solid S1 (isolated yield: 90%). H
NMR (Figure 3) and, to a lesser extent, ESI(+)-MS analyses
indicated that the supernatant was mainly constituted of n-
butylamine carbamate with a few percent of the starting material
A13 and diethylenetriamine monocarbamate (2·CO2). The solid
precipitate S1 proved to be insoluble in nonaqueous solvents
(chlorinated, fluorinated, polar, coordinating, organic bases, and
acids), and its dissolution in water was accompanied by imine
hydrolysis and subsequent molecular scrambling. Despite our
numerous attempts, no suitable macroscopic monocrystals could
be obtained. Nevertheless, powder X-ray diffraction spectrum
and peak widths in 13C CP-MAS solid-state NMR revealed a
high crystalline degree characterized by an elemental lattice of
9400 Å3. n-Butylamine signals were absent from the NMR
spectrum (Figure 4a.), indicating a complete displacement of
the transimination equilibrium induced by CO2 and the formation
of a three-component material [Ax2y(CO2)z]m. CO2 incorporation
in this self-assembled system was confirmed by introducing
13CO2 into the library [A; 2], yielding a solid displaying the
very same spectral signature, two peaks corresponding to usual
carbamate chemical shifts18 being clearly amplified (Figure 4a).
Two-dimensional (2D) solid-state 15N-13C INEPT experiments
revealed that, as expected, CO2 was immobilized within the solid
network through ammonium carbamate linkages (Figure 4d.).
To determine whether the main driving force of the process
was the self-assembly of the solid adduct S1 incorporating CO2
or the favored formation of n-butylamine carbamate in solution,5
a biased library excluding 1 and formed from 2-hydroxy-1,3,5-
benzenetrialdehyde A and two equivalents of DETA 2 was
forced to complete condensation by azeotropic distillation in a
methanol/acetonitrile mixture. ESI(+)-MS analysis of this biased
imination library revealed only two single detectable species:
the capsule A223 and some remaining polyamine 2 due to the
stoichiometry initially adopted (n0(A)/n0(2) ) x0/y0 ) 1/2). As
expected from a thermodynamically controlled system, this
biased library transformed into [A; 2] upon addition of 3 equiv
of n-butylamine 1. Similarly, when submitted to CO2 uptake,
this biased system yielded a solid S2 which proved to be, from
all solid-phase analyses aforementioned, identical to S1 (Sup-
porting Information). Conversely, desorption of CO2 upon gentle
refluxing of the methanolic suspension converted S1 back to a
homogeneous solution of A223 and 2.
1
ensuing polyimine living solutions was analyzed by H NMR
and ESI(+)-MS spectrometry17 comparatively before and after
CO2 introduction. Proton NMR confirmed that the resulting
libraries differed from the initial components introduced and
that n-butylamine 1 was released in the medium. ESI(+)-mass
analysis showed that diversity in terms of sizes and topologies
was indeed accessible from such precursors, although limited
to objects up to dimers of the polyaldehyde core (capsules,
macrocycles and branched adducts. See Figure 2). When CO2
was introduced into these 16 living solutions, the resulting
templated libraries [X; n; CO2] remained homogeneous without
any noticeable change in the repartition of the constitutive
polyimine species, with the exception of X ) A and n ) 2 for
which an abundant precipitate was obtained. In fact, although
carbamates were formed and detected by NMR after CO2
1
exposure, H NMR and ESI(+)-MS revealed that this did not
affect the transimination equilibria. Note that such adducts could
only be detected by ESI-MS spectroscopy in negative mode
when nonphenolic backbone C was used, presumably for
ionization requirements (Supporting Information). For the
combination (X; n) ) (A; 2), the presence of carbamate species
was confirmed by solid-state 13C NMR spectroscopy.
This preliminary set of libraries was intentionally limited to
a single building block belonging to the X and n family. Indeed,
as each of these precursors displays multiple binding sites, the
expected degree of accessible diversity shall grow exponentially
with the number of building blocks, even within collections of
low molecular mass objects. In the case of a symmetrical
polyamine combined with a D2h-symmetric aromatic core such
as in the building block pair (A; 2), iminesaminal tautomerism
and phenol regioisomerism should generate a virtual combina-
torial library11a of 120 members split into 12 different molecular
masses, each of which was actually detected by ESI(+)-MS
spectroscopy (Figure 2, top). Every remaining secondary or
(15) (a) Anderson, A. A.; Goetzen, T.; Shackelford, S. A.; Tsank, S. Synth.
Commun. 2000, 30, 3227–3232, 2000. (b) Temme, O.; Dickner, T.;
Laschat, S.; Fro¨hlich, R.; Kotila, S.; Bergander, K. Eur. J. Org. Chem.
1998, 651–659. (c) Chen, X.; Wang, J.; Sun, S.; Fan, J.; Wu, S.; Liu,
J.; Ma, S.; Zhang, L.; Peng, X. Bioorg. Med. Chem. Lett. 2008, 18,
109–113. (d) Chong, J. H.; Sauer, M.; O’Brian, P.; MacLachlan, M. J.
Org. Lett. 2003, 5, 3823–3826.
According to the reversible nature of the covalent self-
assembling process, the solid structure should, in the presence
(16) (a) Leclaire, J.; Vial, L.; Otto, S.; Sanders, J. K. M. Chem. Commun.
2005, 1959–1961. (b) Vial, L.; Ludlow, R. F.; Leclaire, J.; Perez-
Fernandez, R.; Otto, S. J. Am. Chem. Soc. 2006, 128, 10253–10257.
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Garcia-Granda, S.; Gotor, V. Eur. J. Org. Chem. 2004, 1117–1127.
(18) Georges, M.; Weiss, R. G. Langmuir 2003, 19, 8168–8176.
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Figure 2. Top: ESI(+)-MS analysis of the untemplated library [A; 2]. Bottom: correlation of the VCL size (yellow bars) and the yield in solid adduct S
(purple bars) with the number of symmetrical polyamine spacers n. Symmetricalsunsymmetrical pairs of polyamines are referred to as two symmetrical
polyamines.
desorption-homogenization could be conducted according to
this protocol, confirming that recycling was possible at moderate
temperature (refluxing methanol) in the presence or in the
absence of n-butylamine mother liquor, despite the formation
of a highly organized solid network.19
Following those preliminary results, competition experiments
between several polyamines were conducted under thermody-
namic control to evaluate the constitutional specificity of the
self-assembling process. Libraries [A; 2; n] (n ) 3 - 5) were
prepared and analyzed in the conditions described previously.
ESI(+)-MS confirmed the expected increase in molecular
diversity following the introduction of two polyamines (see
Supporting Information). Templated libraries were estimated to
reach up to 7.22 × 105 and 4.61 × 106 members respectively
in the case of symmetrical-symmetrical and symmetrical-
unsymmetrical pairs of polyamines ((2;4) and (2;3) or (2;5),
respectively). In fact, symmetrical-unsymmetrical pairs gener-
ate as much diversity as three symmetrical polyamines (see
Figure 2). Upon CO2 exposure, every templated library produced
a solid adduct with a yield that appeared to be correlated with
Figure 3. 1H NMR (200 MHz) spectra of library [A; 2] from (a) building
block A13 (240 mM), (b) after transimination with 2 equiv of 2, and (c)
filtrate of the templated library [A; 2; CO2]. * : [12 ·CO2] and O : [2·CO2].
(d) 1H NMR spectrum of [12 ·CO2] and (e) [2·CO2] as references. Spectra
were recorded at 300 K in CD3OD.
of solvent, dynamically rearrange on both covalent and su-
pramolecular levels upon CO2 departure. The reversibility of
carbon dioxide capture was confirmed by refluxing the hetero-
geneous methanolic suspension [A; 2; CO2] under a gentle
nitrogen flow. Homogeneity was reached after several minutes.
Proton NMR and ESI(+)-MS analyses confirmed return to the
state of molecular diversity previously identified as [A; 2] with
undetectable hysteresis. Several cycles of absorption-precipitation/
(19) In the presence or in the absence of n-butylamine carbamate mother
liquor reconversion into the initial libraries respectively occurs in
several minutes or more than half an hour, qualitatively illustrating
the impact of the molecular diversity on the thermodynamic and kinetic
parameters of the equilibria.
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Figure 4. Multinuclear solid-state NMR attribution of the repeating unit constituting the solid S1 (a) 13C CP-MAS (dotted red line: from 13CO2), (b) 15N
CP-MAS, (c) 1H-13C CP-MAS INEPT, and (d) 15N-13C CP-MAS INEPT. The presence or the absence of a DETA monocarbamate end group leads
1
respectively to linear or cyclic oligomers. *: carbon atoms displaying two different H-13C correlations
the estimated degree of diversity (40% and 15% respectively
for (2;4) or (2;3) and (2;5)). In each case, this adduct was
identified in 13C CP-MAS NMR analysis as the same material
[Ax2y(CO2)z]m having selectively and exclusively incorporated
diethylenetriamine 2 at the expense of homologues 3-5
(Supporting Information). This result pointed out the exceptional
constitutional selectivity of the system, allowing discrimination
of the polyamine constituents differing by a single methylene
unit. To further increase the diversity of the starting dynamic
mixtures, libraries involving three competing polyamines [A;
2; n; n′] were prepared. In the latter series, reaching a minimum
estimated degree of diversity of 1.56 × 107 species (equivalent
to 4 symmetrical polyamines. Figure 2), no solid adduct was
obtained after CO2 introduction.
substitution pattern was composed of an intramolecular enamine-
quinone ion pair (δN1 ) -221 ppm),20a an imine hydrogen-
bonded to a neighboring acceptor (δN2 ) -89 ppm),20b and an
aminal group (δN4 ) -325 ppm; 1,3-dimethyl-2-phenyl-
imidazolidine was synthesized as a reference compound for
confirmation: δN ) -325 ppm).20c,d Integration from direct 13
C
solid-state NMR experiments provided an approximate repar-
tition (x; y; z) ) (1; 2; 2), with a degree of precision estimated
to be about 10% from integration of reference compounds such
as ethylenediamine monocarbamate, diethyenetriamine mono-
carbamate, and triethylenetetramine dicarbamate in the same
conditions. These reference compounds also revealed that amine-
(20) (a) Schilf, W.; Kamiensky, B.; Kolodziej, B.; Grech, E.; Rozwadowski,
Z.; Dziembowska, T. J. Mol. Struct. 2002, 615, 141–146. (b)
Kamiensky, B.; Schilf, W.; Dziembowska, T.; Rozwadowski, Z.;
Szady-Chelmieniecka, A. Solid State Nucl. Magn. Reson. 2002, 16,
285–289. (c) Clawson, J. S.; Anderson, K. L.; Pugmire, R. J.; Grant,
D. M. J. Phys. Chem. A 2004, 108, 2838–2844. (d) Bera, M.; Wong,
W. T.; Aromi, G.; Ray, D. Eur. J. Inorg. Chem. 2005, 2526–2535.
The nature of the repeating monomeric unit within the
material Ax2y(CO2)z was unambiguously identified using solid-
state multinuclear NMR spectroscopy. 13C CP-MAS experiments
(Figure 4a) revealed a single type of constituting aromatic core.
13C and 15N (Figure 4b) chemical shift attributions revealed its
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A R T I C L E S
to-carbamate conversion upon CO2 capture systematically
resulted in a 50 ppm downfield shift of the nitrogen center
concerned. Consequently, 15N signals δ ) -281.8 and -304.0
ppm were attributed respectively to the carbamates borne by
N3 and N5, which was confirmed by 2D 15N-13C correlations21a
(Figure 4d). As discussed in the following section, the only
alternative would have been a carbamate linkage involving N6.
This scenario was excluded by simulations: as opposed to the
previous scenario, it could not provide any packing mode that
agreed with powder X-ray analyses. Two-dimensional 1H-13C
spectrum revealed two types of through-bond correlations21b for
C5-H5 and C8-H8, corresponding to two distinct aromatic rings
with different magnetic environments. Such a subtle difference
impacting the chemical shifts of H5 and H8 while leaving all
carbon nuclei unaffected (Figure 4c) was attributed, in agreement
with molecular analyses, to aromatic rings occupying a terminal
or internal position on a linear oligomeric chain.
In fact, the identified monomeric unit can yield, through
repetition, either macrocycles in the absence of end group or
linear species when terminated by a diethylenetriamine mono-
carbamate unit (Figure 4, bottom). In addition to steric reasons,
capsule structures appeared to be incompatible with such a
constituting unit. The microcrystalline organization of the
material and the observation of a single pair of minor and major
monomeric aromatic units led to the conclusion that the number
of covalent morphologies within S was reduced. Macrocyclic
and branched/linear structural structures are indeed unlikely to
coexist as isolated species or as combined units, i.e. within the
material and within its constituting oligomers. Borohydride
reduction of the templated heterogeneous systems [A; 2; CO2]
was explored to provide some insight into the composition of
the material. Analysis of the supernatant revealed a mixture of
monomers and dimers of varying proportion with time. It is
well-known that such a method, although frequently used to
analyze homogeneous imine libraries,22a is valid provided that
all members display the same reactivity toward reduction.10d
The composition of the mixture shall therefore remain constant
with time if reduction does not affect the scrambling reactions.
In the present case, a small fraction of the solid adduct
underwent disconnections-reconnections of its constituting
labile linkages when suspended in methanol followed by
subsequent solubilization of the fragments generated. The latter
species are much more reactive toward hydrides than the solid
network, which explains why only reduced objects of modest
size could be detected. Moreover, while reduced capsules are
unreactive toward polyimines, reduced branched and macrocy-
clic species carry primary amines that can take part in further
transimination reactions. Similarly, freezing imine exchange in
this heterogeneous dynamic combinatorial library with Ugi
reaction22b led to the same observation and revealed that species
differed in reactivity depending on their solubility. This differ-
ence resulted in altering the initial composition of the living
mixture. More generally, these observations led to the conclusion
that such methods are inappropriate for DCLs combining
simultaneous phase-transfer processes (solid to liquid, liquid to
gas) and possibly varying topologies (capsules, macrocycles,
and branched objects). We alternatively turned to elemental
analysis of the solid material, focusing on the nitrogen content
as a key parameter to discriminate between the potential
constituting structures. Unfortunately, no satisfying results were
obtained despite rigorous screening of the combustion condi-
tions. Carbon and nitrogen values remained incompatible with
any type of structures and mixture of structures, whatever the
amount of remaining solvent (methanol) or condensation side
product (water) included in simulations (see Supporting Infor-
mation). As reference compounds, pure carbamates of polyamines
of increasing chain length evoked earlier were submitted to
elemental analyses in the same conditions and revealed a
noticeable and increasing deviation from the theoretical values,
precluding the use of this method for a quantitative constitutional
analysis.
Consequently, we decided to design a new set of experiments
to precisely determine the composition of the self-assembled
material. Constituting oligomers stacked in the solid phase
resulted from two types of reversible and acid labile connections
between three chemically distinct building blocks. Therefore, a
complete disconnection combined with an independent titration
of the resulting molecular fragments appeared as an attractive
alternative to the analysis of the flash combustion products
(elemental analysis). Acid hydrolysis in the presence of organic
solvent23 was followed by molecular ingredient quantizations
using four complementary and routinely accessible techniques.
The mass content in A was determined by UV titration. 1H liquid
NMR in the same hydrolytic conditions provided the building
block ratio x/y ) n(A)/n(2) by integration. CO2 content was
determined by volumetric measurement, and TGA coupled to
IR or MS led to the identification and quantization of a
significant amount of water in the self-assembled polyzwitte-
rionic material (Figure 5).
Although prepared using a trialdehyde/polyamine ratio of y0/
x0 ) 2, the material itself was characterized by a y/x stoichi-
ometry of 2.22 ( 0.03, indicating a high content in polyamine-
rich covalent structures such as linear/branched oligomers
(Figure 5). Carbon dioxide and water content was evaluated to
be respectively about 1 and 2 equiv per polyamine unit. The
mass percentage of each of the different building blocks in the
self-assembled material could be deduced independently of any
hypothesis regarding the distribution of the oligomers and fitted
the global mass of the solid analyzed with a precision of about
1%, noticeably superior to the precision obtained from elemental
analyses.
To confirm the hypothesis of a distribution of oligomers
within a material whose composition should be tuned by
equilibrium displacement, the starting stoichiometry (y0/x0) was
modified, and the resulting products were analyzed by the
aforementioned multiple molecular titrations. Increasing the
polyamine content with respect to the trialdehyde (from y0/x0
) 2.10 to 3.07) yielded materials S3-S5 which, from solid-
state NMR, powder X-ray diffraction, and elemental analysis
appeared identical to S2. Building block titration revealed, as
expected from a dynamic material, an increasing amount of
polyamine (Figure 5) and consequently carbon dioxide with
respect to the aromatic unit, through the formation of shorter
constituting oligomers. Alternatively, S3-S5 could be obtained
by suspension of S2 in methanol, thermal CO2 desorption,
addition of a controlled amount of polyamine, and reprecipitation
upon CO2 exposure.
(21) (a) Ramanathan, K. V.; Opella, S. J. J. Magn. Reson. 1990, 86, 227–
235. (b) Elena, B.; Lesage, A.; Steuernagel, S.; Bo¨ckmann, A.; Emsley,
L. J. Am. Chem. Soc. 2005, 127, 17296–17302.
(22) (a) Storm, O.; Lu¨ning, U. Chem.sEur. J. 2002, 8, 793–798. (b)
Wessjohann, L. A.; Rivera, D. G.; Leo´n, F. Org. Lett. 2007, 9, 4733–
6.
(23) Kanazawa, H.; Higuchi, M.; Yamamoto, K. Macromolecules 2006,
39, 138–144.
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Figure 5. Table: Molecular (left) and elemental (right) analyses of self-assembled material Ax2y(CO2)z S2 and S4. For each material, analyses were run in
1
triplicate on two samples prepared independently. (a) From UV titration at λ ) 340 nm in DMSO/H2O/H2SO4: 90:7:3. (b) H integration at 500 MHz in
DMSO-d6/D2O/D2SO4: 90:7:3. (c) Gasometric determination subsequent to hydrolysis with 6 M HCl. (d) From thermogravimetric analysis. (e) Independently
of any hypothesis of distribution. (f) Calculated from ∆m/m, ∆m corresponding to the differences between the mass of the sample analyzed (m) and the
individual mass contribution estimated by building block titrations. Bottom (A) Polyamine/aromatic core stoichiometries yi/xi for linear i-mers of A (bars)
and macrocycles. (B) Horizontal dotted line: y/x ratios found for solids S1-S5. Vertical dotted line: corresponding mean value of the distribution on the
population restricted to linear 2- to 5-mer. Table: ratio y0/x0 used for the preparation of solids S1-S5.
Interestingly, this disconnection method can be applied for
the recovery of pure individual building blocks in the different
phases (solid, liquid, gas) in a perspective of purification. Acidic
hydrolysis of the solid material in pure aqueous medium is
accompanied by the precipitation of pure A (the “turtle”),
desorption of pure CO2 (the “goose”), while diethylenetriamine
2 is solubilized as a salt and can be recovered as a pure liquid
(the “fish”) after flash chromatography on a DOWEX anion-
exchange resin (see Figure 1 for building block classification).
Such a result reveals that CO2, through the constitutional
selectivity of the self-assembling process, can be promoted as
a precious auxiliary to purify A or 2 from polyaldehyde and
polyamine mixtures. Conversely, A and 2 stand as selective
agents to allow the purification of CO2 from complex gaseous
streams.
Due to the thermodynamic nature of the construction, CO2
desorption should be considerably facilitated in the presence
of the supernatant comparatively to nondynamic systems. As
multiplying concurrent equilibria was proved to directly affect
the yield of the self-assembling process, it is also likely to
thermodynamically ease the expulsion of CO2 from the solid
material and its reconversion into an untemplated system of high
molecular diversity. To a lesser extent, CO2 desorption on the
isolated self-assembled solid S should also compromise the
stability and the molecular order of the whole network. In fact,
as every building block contributes to the global stabilization
of the molecular and supramolecular network, it is reasonable
to expect that the first template molecule expulsed should yield
a much less stable system, hence facilitating the departure of
the following CO2 molecules. Thermal characteristics of the
isolated solids S were first studied by Differential Scanning
Calorimetry, revealing three macroscopic endothermic events
between 25° and 130 °C (about 75, 100, and 125 °C) (Figure
6). Thermogravimetry coupled to mass and infrared gas phase
analysis on the one hand and variable temperature 13C CP-MAS
solid-state NMR spectroscopy on the other hand provided
complementary microscopic information about the evolution of
the composition of both the solid and gas phases with temper-
ature. The first transition corresponded to the expulsion of the
majority of the water content together with a few percent of
carbon dioxide globally unaffecting the solid material and
thereby providing a thermal treatment to dry the material without
decomposition. The second transition (70-125 °C) was char-
acterized by the simultaneous disappearance of the carbamate
functionalities on the macromolecular backbone and detection
of CO2 in the gas phase. With a global heat of desorption of
∆H° ) 15.7 kJ/mol(CO2), more than 95% of the carbon dioxide
was expulsed during this event, followed by the departure of a
few percent of remaining water and CO2. The resulting glassy
solid was characterized by a broad peak spectrum in 13C CP-
MAS NMR, indicating a complete loss of supramolecular
organization consecutively to CO2 departure (Supporting In-
formation). As a contrast, ethylenediamine monocarbamate24
was studied in the same conditions and underwent a unique and
sharp (130-150 °C) thermal transition at 142 °C (∆H° ) 23.6
(24) Gaines, G. L., Jr. J. Org. Chem. 1985, 50, 410–411.
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CO2 Binding by Dynamic Combinatorial Chemistry
A R T I C L E S
Figure 6. Thermal analysis of S1. (a) Top: TGA (black) and DSC (red) curves corresponding to a sequential three-step thermal scan (blue dotted line). (b)
Middle: total ion count at m/z ) 44 and m/z ) 18 corresponding to the main molecular peak of carbon dioxide and water. (c) Bottom: VT 13C CP-MAS
spectra focused on the carbamate multiplet (158 < δ < 164 ppm) of the self-assembled material between (left photograph) 20 °C and (right photograph) 130
°C.
kJ/mol(CO2)), corresponding to the sudden disappearance of the
degree of symmetry, the number of constitutional homologues
increased exponentially, providing straightforward access to
large libraries from a very small set of precursors.
single carbamate signal in 13C CP-MAS NMR (table in Figure
6).
Nevertheless, simulations and experimental measures26a have
recently proved that relative levels of amplifications may not
necessarily be correlated with the binding affinities, especially
when the size of the library is increased.26b A breakdown in
the correlation may additionally occur when an excess of guest
is used (here p(CO2) ) 1 represents an obvious stoichiometric
excess)27 and when several library members (trialdehyde and
CO2) compete for a scarce building block (the polyamine). These
conditions were indeed all fulfilled in most of our soluble
collections [X; n; CO2].
Our adaptative chemical system was designed to generate
molecular receptors through a CO2-driven shift in the distribution
of the libraries’ constituents toward the best binder. The selector
being a gas in standard conditions, temperature and partial
pressure were considered as potentially influencing the distribu-
tion of library members in solution. In this context of high
molecular diversity and excess of gaseous template, no isolated
molecular receptor could be identified in solution. Alternatively,
our experimental conditions led to quantitatively and selectively
obtaining a self-assembled three-component material as a
Discussion
The results described above are evidence that simple building
bocks (linear polyamines, tricarbonyl cores, and CO2) displaying
multiple sites for several reversible competitive connections
(imine, aminal, and ammonium carbamate) can generate DCLs
with complex network architectures from which original struc-
tures can be identified and isolated. In recent years, DCLs of
increased architectural and compositional diversity have been
explored, mostly by simultaneously utilizing several types of
coupling chemistry for the construction of the libraries. Dynamic
covalent bonds have been used in combination with orthogonal
noncovalent,25a,b orthogonal labile covalent,25c or intricated
covalent and noncovalent linkages.25d Some of us recently
demonstrated that this approach could be extended to competi-
tive reversible covalent linkages such as disulfides and
thioesters.16a Within the frame of DCC, we managed herein to
access a wide variety of topologies, each class including
oligomers of modest but various sizes. When increasing the
number of building block of one type and/or reducing their
(26) (a) Corbett, P. T.; Sanders, J. K. M.; Otto, S. Chem.sEur. J. 2008,
14, 2153–2166. (b) Corbett, P. T.; Otto, S.; Sanders, J. K. M. Org.
Lett. 2004, 6, 1825–1827.
(25) (a) Kolomiets, E.; Lehn, J.-M. Chem. Commun 2005, 1519–1521. (b)
Sreenivasachary, N.; Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2005,
102, 5938–5943. (c) Christinat, N.; Scopelliti, R.; Severin, K. Angew.
Chem., Int. Ed. 2008, 47, 1848–1852. (d) Hutin, M.; Bernardinelli,
G.; Nitschke, J. R. Chem. Eur. J., 2008, 14, 4585–4593.
(27) (a) Saur, I.; Severin, K. Chem. Commun. 2005, 1471–1473. (b) Corbett,
P. T.; Sanders, J. K. M.; Otto, S. J. Am. Chem. Soc. 2005, 127, 9390–
9392.
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singularity among the building blocks screened. Its mass content
in CO2 reaches 20%, which approaches the loading capacity of
monoethanolamine currently used in pilot plants (diluted in
water, 20% massic, see ref 3). As previously described by Lehn
et al. on linear chains25a,b and more recently by Iwasawa and
colleagues on capsules,28 obtaining a structured phase drives
the selection of the components that make up the dynamic
constituent producing the most highly organized and most stable
assembly. The formation of an organized phase, not necessarily
from the most stable covalent architecture, drives the selection
process. This therefore explains the spectacular amplification
obtained in the context of high molecular diversity, despite the
use of an excess of CO2.
spacers. The bicyclic dihedral angle inevitably induces an
intermolecular pairing along the x axis involving the primary
ammonium-aminal carbamate (N6-C11; Figure 7a). Simulations
on the alternative associations between tertiary aminal am-
monium and aminal carbamate (N4-C11) never yielded any
regular and efficient three-dimensional packing. This intermo-
lecular binding mode was therefore ruled out. The remaining
aminal ammonium-amine carbamate pairing appears to occur
following a chelating pattern (C′10-N4-C10) along the z axis.
Such a binding mode requires an extended conformation of the
bridging diethylenetriamine spacers, inducing an alternating
anti-anti orientation of the aforementioned anionic-cationic-
anionic binding sites on the same backbone. The relative
positions of the aminal ring and the N5-carbamate molecular
moieties were herein defined as either syn or anti with respect
to the N1(CH2)nN5(CH2)n′N2 backbone (Figure 7a). These
intermolecular binding modes and their conformational require-
ments provide a coherent and satisfying explanation of the
polyamine selectivity experimentally observed. As expected
from this model, building blocks 3, 4, and 5 should respectively
yield anti-syn, syn-anti, and syn-syn orientations of the
charged residues, each of which is unfavorable for intermo-
lecular ion pairing along z and therefore precluding any efficient
packing (Figure 7b, c, and d). This ionic binding mode along
the z axis also underlines the importance of the third aldehyde
function on the aromatic backbone for optimizing the intermo-
lecular interactions. The electrostatic surface generated from the
corresponding conformer of A429(CO2)9 reveals a “double comb”
profile, confirming the electrostatic and steric self-complemen-
tarities along the x and z axes of this family of oligomers
(Figure 7e). As an experimental confirmation of this proposed
packing scenario, the volume of this envelope correlates finely
with the dimensions of the crystal lattice determined from
the powder X-ray spectrum.31 Presumably, y axis interactions
occur through the unpaired diethylenetriammonium carbam-
ate-terminating groups acting as “sticky ends”. This inter-
pretative model provides a coherent picture of the structure-
selectivity relationship of the system yielding, for a very
single set of building blocks, as in Escher’s famous symmetry
drawing,32 a three-component (“turtle-fish-goose”) self-
complementary class of oligomeric objects with unique
packing ability (Figure 7f).
The combinatorial screening provided by the dynamic
combinatorial strategy yielded a structure-activity pattern of
building blocks and molecular species that requires interpreta-
tion. In fact, the set of libraries analyzed revealed the compo-
sitional requirement for a stable self-assembled material: one
set of building blocks appeared appropriate, and within the
library thereby generated, one type of oligomer was amplified.
This striking selectivity toward both the building blocks and
the class of oligomers must then be exploited through a
Structure-ActiVity Relationship approach to provide some
potential insights into the highly favorable packing mode
between the molecular constituents. As mentioned earlier, this
efficient packing indeed constitutes the driving force and
therefore the selector in the templating process. The substitution
pattern of the aromatic unit, the length of the polyamine spacer,
and the topology of the constitutive oligomers are discriminating
criteria. Indeed, selection processes within soluble DCL gener-
ally favor small and cyclic objects for entropic reasons.27b The
unexpected amplification of linear oligomers of high molecular
masses (up to 2240 Da) should originate in their packing
efficiency in the condensed phase. The substitution pattern (G1,
G3, G4, G5) (Figure 7a) of the aromatic unit seems to essentially
account for intramolecular oligomeric stabilization. In fact, the
G1 ) OH substituent induces an imine-enamine tautomerism
driven by an intramolecular stabilization, which is well-known
on salicylic derivatives.20a Alternatively, the imidazolidine
aminal cycle in the C4 position requires both the presence of a
carbonyl group (G4 ) CHO) and the absence of sterically
hindering substituents in C3 and C5 positions (G3 ) G5 ) H).
This binding pattern is preferred to an imine linkage and
therefore contributes to the stability of the oligomeric backbone.
Interestingly, six-membered ring aminal cycles have been
reported to be more stable than the corresponding five-
membered analogues.29 Therefore, polyamine selectivity favor-
ing the shortest chain length that bears ethylene spacers
presumably originates in the interoligomer noncovalent interac-
tions. Among noncovalent interactions, ion pairs were shown
from simulations to provide the main contributions in this
polyzwitterionic material. Browsing crystallographic databases
revealed that the X-ray structures of 2-phenylimidazolidine
derivatives systematically display a common feature of fixed
dihedral angle between the connected cycles, averaging 78° with
a very sharp distribution.30 Because ammonium centers are the
only tetrahedral nitrogen sites, most of the conformational
degrees of freedom in the three-dimensional structure of the
oligomers are thus gathered in the aforementioned ethylene
The dynamic facet of the combinatorial strategy provided a
three-component system with unique and unexpected thermo-
dynamic properties. As a critical concentration of the molecular
precursors of the templated architecture should be reached to
trigger the precipitation event, increasing the size of the library
by introducing alternative building blocks produces an expo-
nential number of new species competing for the selected
constituents. This reduction in amplification predicted by
simulations27 is here embodied by the decrease in yield in S
when the number of competing symmetrical polyamines is
increased (90, 40, 15, and 0% obtained when introducing
respectively 0, 1, 2, and 3 concurrent building blocks).
Alternatively, converting back to a state of high molecular
diversity [A; 2] through reversible disconnection-reconnections
thermodynamically facilitates the desorption step. In the pilot
(30) Cambridge Crystallographic Database; 2009.
(31) Pepe, G.; Perbost, R.; Courcambeck, J.; Jouanna, P. J. Cryst. Growth
2009, 311, 3498–3510. The electrostatic envelope was estimated using
the same genetic algorithm, GenMol.
(28) (a) Iwasawa, N.; Takahagi, H. J. Am. Chem. Soc. 2007, 129, 7754–5.
(b) Takahagi, H.; Fujibe, S.; Iwasawa, N. Chem.sEur. J. 2009, 15,
13327–30.
(32) With courtesy of the M. C. Escher foundation: http://www.mcescher.
com.
(29) Go¨blyo¨s, A.; Lazar, L.; Fu¨lhop, F. Tetrahedron 2002, 58, 1011–1016.
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CO2 Binding by Dynamic Combinatorial Chemistry
A R T I C L E S
Figure 7. Top: Correlation between polyamine length and relative disposition of the ionic centers on a linear oligomer in extended conformation: (a)
anti-anti for 2, (b) anti-syn for 3, (c) syn-anti for 4, and (d) syn-syn for 5. (e) Electrostatic surface of the corresponding oligomer A429(CO2)9 displaying
a “double comb” profile and a self-complementarity for (f) efficient packing in the solid phase.
process CASTOR devoted to postcombustion capture currently
tested,3 this desorption step occurs through consecutive preheat-
ing of the amine aqueous solution and subsequent CO2 stripping
by a stream of vapor in a gas-liquid exchanger. Therefore,
vapor generation and amine solution heating account for most
of the energetic costs of the process. In this context, a decrease
in the temperature required for CO2 desorption comparatively
to conventional molecular carbamates appears promising for
rendering the process of “capture and release” economically
attractive.33 This reduction in the temperature of regeneration
experimentally observed for [A; 2] in refluxing methanol is
partly explained by the “entropic” driving force consisting in
multiple concurrent equilibria. It also accounts for the reduction
in the yield of the most stable organized system during the
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A R T I C L E S
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absorption step. Nevertheless, it could appear contradictory that
the self-assembled material S should also be the most stable
supramolecular pattern that can be obtained within the molecular
network formed from three types of precursors. When the
number of building blocks is limited to a polyamine such as 2
and CO2, the complexity of the network is considerably lowered,
and the most stable adduct is the molecular carbamate.34 This
entity may indeed be more stable than our self-assembled
material but it does not form in the presence of an additional
ingredient such as a polycarbonyl connector A. This modifica-
tion of the molecular context explains the paradox that S can
simultaneously be the best binder within the context of three-
component systems (A/2/CO2) while being less energy-demand-
ing for recycling than a two-component adduct, such as a simple
molecular ammonium carbamate (2/CO2). The reduction of the
free energy of the capture/regeneration step indeed occurs on
both entropic and enthalpic bases. Reduction in enthalpy reaction
is first due to the cooperativity of the process: carbamate bond-
breaking/-forming is facilitated as the structure progressively
dissociates or forms. Additionally, as mentioned earlier, the
energetic levels of both the “untemplated starting resting state”
(three reactants: A + 2 + CO2) and the templated heterogeneous
final system (S) are altered compared to a simple amine/CO2
pair (undergoing the transmformation. 2 + CO2 f 2·CO2).
Hence, the gap between both states and the corresponding
enthalpy of reaction is modified. Both entropic and enthalpic
contributions impact the temperature of desorption in the liquid
phase. Solid-phase analyses only illustrate the enthalpy-based
cooperative self-assembly. In this particular case, the absence
of mother liquor composed of solvent and concurrent amine(s)
prevents any entropically- and enthalpically favored molecular
scrambling that should yield the initial untemplated state of
diversity [A; 2] (Figure 6 photographs). Nevertheless, the
synergistic nature of the three-component construction is well
embodied by the desorption from the solid material displaying
a broad and cooperative transition, as opposed to molecular
carbamates. As a result of this synergy, the temperature of
desorption drops from 140 to 100 °C, and the corresponding
average heat is lowered by a factor of 0.65 which, as mentioned
earlier, only accounts for a fraction of the total reduction
expected in the liquid phase.
Beyond the proof-of-concept that CO2 may be used as a
building block in DCC, implementing system chemistry in the
paradigm of CO2 capture seems to be a promising route toward
the reduction of the costs of capture, showing how interactions
between molecules lead to the emergence of a cooperative
behavior.35 This process may one day become economically
more attractive than direct emission. Such a multicomponent
strategy also reveals that carbon dioxide can be promoted as a
synthetic auxiliary and building block, giving access to adapta-
tive, recyclable, and architecturally complex materials. Providing
a complete molecular picture of the self-assembly in the solid
phase proved a challenging task. Solid-state NMR allowed us
to identify the main repeating molecular unit formed from the
three building blocks A, 2, and CO2. As an alternative to
elemental analysis, building block titration appeared as a
straightforward and powerful method of analysis of multicom-
ponent dynamic materials. This new analytical approach allowed
us in some ways to straddle two rather different areas: that of
discrete molecular species, where the traditional tools of analysis
are usually available, and that of polymer chemistry, where
rather different tools are applied and with different goals. It
unambiguously provided the length and topology of the
constitutive oligomers obtained by the repetition of the identified
monomeric unit. Such a disconnection strategy, based on the
reversible nature of the construction, leads to two essential
observations. Building blocks can thereby easily be recovered
as pure compounds, promoting CO2 as a valuable and green
auxiliary for the purification of polyamine and polyaldehyde
from homologous mixtures. Additionally, modulating the build-
ing block stoichiometry results in finely tuning the constitutive
population of self-assembled covalent oligomers within a
structural family (centered here between linear dimers and
pentamers). More importantly, the loading capacity can thereby
be modulated within what appears to be a new type of adaptative
and recyclable organic material.
Methods
Generation of Combinatorial Libraries. In a typical experi-
ment, a di- or tributylimine core (0.182 mmol; A13: 62.6 mg; B13:
59.7 mg; C13: 68.3 mg; D12: 50.0 mg) is diluted in methanol (10
mL). Two equivalents of triamine n (0.365 mmol) or of an
equimolar mixture of triamines are then added, and the clear yellow
solution is stirred at room temperature for 30 min, yielding library
[X; n; n′]. CO2 is then introduced by gentle bubbling for 15 min
(during which precipitation occurs in the case (X, n) ) (A, 2)),
yielding library [X; n; n′; CO2]. At this stage, nitrogen can be
introduced by gentle bubbling while refluxing the resulting solution
for 30 min for complete CO2 desorption (yielding library [X; n;
n′]).
Material Ax2y(CO2)z. From Library [A; 2]. CO2 is introduced
by gentle bubbling for 15 min, during which precipitation occurs,
yielding library [A; 2; CO2]. S1 can be isolated: the heterogeneous
solution is centrifuged and filtered. The resulting pale-yellow solid
is then washed with methanol (2 × 5 mL) and dried under vacuum
pressure. It can be further dried by heating at 70 °C under nitrogen
flux for 1 h.
From Direct Imination Library. In a typical experiment,
2-hydroxy-1,3,5-benzenetricarbaldehyde A (0.182 mmol; 32.4 mg)
and diethylenetriamine 2 (S2: 0.365 mmol; 37.7 mg. S3: 0.383
mmol; 39.6 mg. S4: 0.411 mmol; 42.4 mg. S5: 0.560 mmol; 57.9
mg) are dissolved in an acetonitrile-methanol mixture(4 + 8 mL)
in a RB-flask equipped with Dean-Stark apparatus. The acetoni-
trile-methanol-water ternary azeotrope is continuously distilled
for 4 h while fresh methanol is reintroduced every 15 min, keeping
the concentration constant. After cooling back to room temperature,
CO2 is bubbled in the solution until precipitation is complete.
Purification is accomplished by using the the previous procedure.
Yield: 90%; 13C NMR: 177.6 (C-OH), 167.1 (CHdNH+), 163.2
(CHdN), 163.2 (N-COO-), 159.1 (N-COO-) 142.5 (CHdC),
135.4 (CdC), 127.1 (CHdC), 118.7 (CdC), 114.6 (CdC), 72.3
(N-CH-N), 59.2 (CH2-N), 53.2 (CH2-N), 50.3 (CH2-N), 47.5
(CH2-N), 44.3 (CH2-N), 39.0 (CH2-N); 15N NMR: -89.0
(CHdN), -221.4 (CHdNH+), -281.8 (N), -304.0 (N), -325.0
(N, NH), -352.4 (NH3+); decomposition (DSC): 70.6, 101.6, 125.7
°C.
Poly(n-butyl)imine Cores Synthesis. Di- or trialdehyde (3.12
mmol) (2-hydroxy-1,3,5-benzenetricarbaldehyde: 0.55 g; 1,3,5-
benzenetricarbaldehyde: 0.51 g; 2,4,6 trihydroxy-1,3,5-benzenet-
ricarbaldehyde: 0.66 g; 2-hydroxy-5-methyl-1,3-benzenedicarbal-
dehyde: 0.51 g) and n-butylamine 1 (A13: 0.98 g; 13.42 mmol;
B13: 0.98 g; 13.42 mmol; C13: 0.98 g 13.42 mmol; D12: 0.65 g;
8.91 mmol)) are diluted in a suspension of 4 Å molecular sieves in
(33) European Climate Exchange: http://www.ecx.eu.
(34) This adduct is unfortunately liquid at room temperature and therefore
could be used as a reference in the set of thermal solid-phase analyses.
(35) Nitschke, J. R. Nature 2009, 462, 736–8.
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CO2 Binding by Dynamic Combinatorial Chemistry
A R T I C L E S
THF (10 mL). The mixture is refluxed for 19 h after which the
solution is cooled to room temperature and filtered. The solvent is
removed by evaporation and the residual n-butylamine by distillation
under reduced pressure.
Acknowledgment. We thank Dr. V. Heresanu for powder X-ray
diffraction, Dr. Benedicte Elena for 2D 1H-13C NMR analysis, and
Dr. G. Pepe for preliminary calculations. Financial support from
the TGE RMN THC and IFP for conducting the research is
gratefully acknowledged.
Summary
Supporting Information Available: Experimental procedures,
characterization data for all new compounds and libraries, and
evidence for dynamic interconversion of Ax2y(CO2)z.This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
As a proof of concept, a dynamic combinatorial selection
approach has provided a self-assembled multicomponent organic
material incorporating carbon dioxide, depicting selectivity for
the constitutional building blocks required down to one carbon
unit and paving the way toward a promising reduction of
energetic costs of recycling.
JA909975Q
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