Absorption of atmospheric Co2 as carbonate inside the molecular cavity of a new tripodal hexaurea receptor

Article abstract of DOI:10.1021/ol403186k

A new hexaurea receptor has been synthesized, which absorbs atmospheric CO2 to produce an air-stable solid carbonate complex under normal conditions. Structural analysis of the carbonate complex with this receptor suggests that the carbonate is fully enca

Full text of DOI:10.1021/ol403186k

Letter
pubs.acs.org/OrgLett
Absorption of Atmospheric CO2 as Carbonate Inside the Molecular
Cavity of a New Tripodal Hexaurea Receptor
Avijit Pramanik,† Maryam Emami Khansari,† Douglas R. Powell,‡ Frank R. Fronczek,§
and Md. Alamgir Hossain*,†
†Department of Chemistry and Biochemistry, Jackson State University, Jackson, Mississippi 39217, United States
‡Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States
§Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: A new hexaurea receptor has been synthesized,
which absorbs atmospheric CO2 to produce an air-stable solid
carbonate complex under normal conditions. Structural analysis of
the carbonate complex with this receptor suggests that the
carbonate is fully encapsulated within its highly organized
intramolecular cavity via 12 strong NH···O bonds in the range of
2.703(3)−2.989(3) Å from six urea units, with each anionic oxygen
coordinated via four NH···O bonds with two urea groups.
ontinuous increase of carbon dioxide (CO2) concentration
in the atmosphere resulting from various human activities
cavity, the unique orientation of 12 binding sites provides an ideal
complementarity for the trigonal planar carbonate anion.
The new hexaurea receptor 1 was synthesized by a three-step
strategy (Scheme 1). The tris(2-aminoethyl)amine (tren) and o-
C
poses a threat to global warming.1 CO2 is also a major source of
water acidification by dissolving in water to form carbonic acid
(H2CO3).2 Therefore, there is a current need to develop new
chemical devices for the recycling of carbon dioxide to produce
chemical commodities.3 Recently epoxide-based molecules have
been used to react with CO2 in the presence of a catalyst to
produce new polycarbonates and cyclic carbonates.4 Gale et al.
reported that simple monofunctional urea-based neutral
compounds are capable of absorbing CO2 in the presence of
primary aliphatic amines to form carbamates [>N(CO2)−].5
Increasing the functional groups, tren-based tris-ureas/thiour-
eas,6 such as pentafluorophenyl-substituted tris-urea,6a m-
nitrophenyl-substituted tris-urea,6b phenyl-substituted tris-thio-
urea,6c and p-cyanophenyl-substituted tris-thiourea6d have
recently been shown to bind carbonate anions, forming 2:1
(host/guest) complexes. Although the carbonate is coordinated
via 12 hydrogen bonds (NH···O < 3.2 Å) in most cases, two
receptors are required to provide complementary binding sites
for the anion. Therefore, the complete coordination for a
carbonate anion can ideally be achieved by a receptor possessing
12 complementary binding sites around a single cavity.
Scheme 1. Synthesis of the Hexaurea 1
nitrophenyl isocyanate were reacted to give a nitro-function-
alized tris-urea 2, which was reduced with hydrazine hydrate and
Pd/C (10%) to produce an amino-functionalized tris-urea 3.8
The final coupling was achieved by reacting 3 with pentafluoro-
phenyl isocyanate 4 to form an o-phenylene-bridged hexaurea 1.
Single crystals of carbonate complex [1(CO3)](n-Bu4N)2
were obtained from slow evaporation of a DMSO solution of 1
with an excess amount of n-tetrabutyl ammonium fluoride
(TBAF) in a vial at room temperature in 2 days. The needle-like
crystals were isolated by simple decantation technique and were
found to be air-stable. Although, the primary goal was to obtain a
It is well-documented that increasing the effective binding sites
in a host leads to the enhancement of its binding ability for a
guest due to the chelate effect.7 From this viewpoint, we have
been interested in synthesizing polyurea-functionalized receptors
based on the commercially available “tren” as a core. Herein, we
report a highly organized hexaurea receptor possessing 12 H-
bond donors, which absorbs atmospheric CO2 in the form of
carbonate encapsulated in a single cavity surrounded by perfectly
arranged six urea units. Within this self-generated intramolecular
Received: November 5, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol403186k | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. H-Bonding Parameters (Å, deg) for [1(CO3)]2−
fluoride complex, crystallographic analysis revealed that, instead
of fluoride, one carbonate was encapsulated within the cavity of 1.
The bound carbonate was originated from atmospheric CO2
absorbed by the basic TBAF−receptor solution which was left
open to the atmosphere. The formation of carbonate in the
presence of TBAF is well-documented in the literature.6
Structural analysis from single-crystal X-ray diffraction of the
carbonate complex reveals that the complex crystallizes in an
orthorhombic Pna21 space group to give a molecular formula,
[1(CO3)](n-Bu4N)2, in which one divalent carbonate is fully
encapsulated.9 Two tetrabutyl ammonium ions remain outside
the cavity to balance the anionic charges. In the receptor unit, all
protons on NH groups are pointed toward the cavity and serve as
H-bond donors for the bound anion.
D−H···O
H···O
D···O
∠DHO
N4A−H4A···O1D
N7A−H7A···O1D
N14A−H14A···O2D
N17A−H17A···O2D
N7B−H7B···O2D
N4B−H4B···O2D
N14B−H14B···O3D
N17B−H17B···O3D
N4C−H4C···O3D
N7C−H7C···O3D
N14C−H14C···O1D
N17C−H17C···O1D
2.20(3)
2.01(3)
2.16(3)
1.84(3)
1.99(3)
2.19(3)
1.93(3)
2.00(3)
2.03(3)
2.21(3)
1.97(3)
1.92(3)
2.966(3)
2.852(2)
2.935(3)
2.703(3)
2.869(3)
2.949(3)
2.801(3)
2.788(3)
2.822(3)
2.989(3)
2.783(2)
2.746(3)
152(3)
165(3)
155(2)
161(3)
162(2)
154(3)
157(2)
154(3)
162(3)
153(3)
160(3)
156(3)
As shown in Figure 1a, the encapsulated carbonate is held
tightly inside the cavity by strong 12 NH···O bonds (dNH···O
=
(C1D) of CO32−, forming a pseudo-C3-symmetric complex
(Figure 1c).10
Proton NMR titrations were performed to examine the
stability and stoichiometry of the receptor−carbonate complex in
DMSO-d6 using Et4NHCO3. Upon the addition of Et4NHCO3
(20 mM) to the receptor (2 mM), a new set of NMR spectra
appeared due to slow exchange on the NMR time scale (Figure
2).11 All four NH signals were shifted significantly to downfield
1
Figure 2. Partial H NMR spectra of 1 with an increasing amount of
Et4NHCO3 (R = [Et4NHCO3]0/[1]0) in DMSO-d6.
Figure 1. (a) Single-crystal X-ray structure of the carbonate complex of
1 (countercations and hydrogen atoms on carbons are omitted for
clarity), (b) space-filling model of encapsulated carbonate complex, (c)
partial view of pseudo-C3-symmetric arrangement of tripodal cavity with
the encapsulated carbonate.
(ΔδNHa = 0.41 ppm, ΔδNHb = 1.88 ppm, ΔδNHc = 1.53 ppm,
and ΔδNHd = 1.08 ppm), indicating the interactions of all NH
groups with the anion. Presumably, the chelation of the bound
anion, as also observed in the solid-state structure of the complex
of 1, results into the formation of a kinetically stable complex on
the NMR time scale.8 The relative change in the integration
intensity of NH resonances of the 1−bicarbonate complex and
the free 1 allowed us to determine the binding constant (Figure
3).11 The experimental data provided the best fit to a 1:1 (host/
guest) binding model,12 yielding a binding constant K = 1780
M−1. The 1:1 binding in solution was further supported by a Job’s
plot analysis (Figure S17). It is noted that, because of the
2.703(3)−2.989(3) Å) from 12 NH groups of urea units (Table
1). Each oxygen of the carbonate is coordinated to four NH’s
from two different arms of the hexaurea receptor, with the two
inner NH’s from one arm and two outer NH’s from other arm.
Within a single arm of the receptor, the pentafluorophenyl and o-
phenylene rings linked with an outer urea group are almost
perpendicular to each other, while these two aromatic rings from
two different arms are stacked together via π···π or C−H···π
interactions (πc···πc = 3.602 Å, C8E−H···π = 3.841 Å, and C5E−
H···π = 3.855 Å). Such arrangements of the aromatic rings make
the receptor preorganized for the complete participation of all six
ureas in coordinating the internal anion. The space-filling view of
the complex (Figure 1b) illustrates the encapsulated carbonate
inside the cavity of 1, showing the stacking of the aromatic
groups. The trigonal planar carbonate is almost perpendicular to
the axis of the tertiary nitrogen (N4C) of 1 and the carbon
−
unavailability of a suitable DMSO soluble CO32− salt, the HCO3
as a tetraethyl ammonium (Et4N+) salt was used in the NMR
titration studies, as previously used by other groups6 for tren-
based ligands. Thus, the determined binding constant (K = 1780
M−1) is the result of the interactions of 1 with singly charged
HCO3− as opposed to CO32− observed in the crystal. The time-
dependent NMR spectra of 1 and Et4NHCO3 in DMSO-d6
−
showed no change in the NMR signals, suggesting that HCO3
B
dx.doi.org/10.1021/ol403186k | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
encapsulation of HCO3 in the receptor’s cavity.6a Figure 4d
displays the 13C NMR of [1(CO3)](n-Bu4N)2] used for single-
crystal diffraction, showing a peak more downfield at 176.7 ppm
−
−
for CO32−. This difference could be due to the charges (HCO3
vs CO32−) and the countercations (Et4N+ vs n-Bu4N+) present in
the two different samples.
The solid-state FT-IR analysis was also performed to examine
the interactions of the receptor with carbonate. The free receptor
shows two peaks at 1650 and 1600 cm−1 for two carbonyl groups
(−CO),13 which shift to higher wavenumbers at 1696 cm−1
(ΔνCO = 46 cm−1) and 1630 cm−1 (ΔνCO = 30 cm−1),
respectively (Figure S15). Further, the significant upward shift
(ΔνN−H = 74 cm−1) of broad NH’s stretching frequency from
3330 cm−1 (free 1) to 3404 cm−1 (complex) was observed,
suggesting the strong N−H···O interactions between NH groups
and the carbonate.
1
Figure 3. H NMR titration curve of 1 (2 mM) with an increasing
amount of Et4NHCO3 (R = [Et4NHCO3]0/[1]0) in DMSO-d6. The
titration plot was obtained from the relative change in the integration
intensity Δϕ (Δϕ = INHc/[INHf + INHc], where INHc is the intensity of
NHc signal in the carbonate−1 complex and INHf is the intensity of the
corresponding NH signal in the free 1.
The ability of 1 to uptake of the atmospheric CO2 was
examined by dissolving the receptor with TBAF in DMSO and
keeping the mixture in open air (Figure 5). The solid products
was not deprotonated to form CO32− during the titration process
(Figure S18).
The binding affinity of 1 for Et4NHCO3 was compared with its
precursor tris-urea receptor 2 (Scheme 1) under similar
conditions in DMSO-d6. The results show that this receptor
−
binds HCO3 with K = 226 M−1 (Figure S24 in Supporting
Information), which is much weaker than 1780 M−1 observed for
1. An n-butyl-functionalized tris-thiourea reported previously
was shown to bind bicarbonate with K = 564 M−1 in DMSO-d6.6c
This observation further demonstrates that the unique
orientation of the binding sites in the hexaurea 1 makes it an
excellent binder for the carbonate anions due to the possible
chelate effect observed in traditional transition metal complex-
es.7b We also performed 1H NMR titrations of 1 for other anions,
showing strong affinity for HSO4− (2250 M−1). The association
constants of 1 with Cl− and CH3COO− were 256 and 200 M−1,
respectively. However, the receptor showed no significant
Figure 5. Uptake of CO2 and the recovery of 1. Magnified 1 carbonate
crystals are shown at the right side.
deposited at the interface of the liquid and air were collected to
give almost 99% carbonate complex. The complex was
characterized by 1H NMR and also verified from X-ray diffraction
analysis (CCDC 969272), giving an identical structure described
in the preceding section. Treating the complex with water and
methanol (1:1, v/v) gave over 95% recovery of 1. In previous
reports, a pentafluorophenyl-substituted tris-urea6a and a m-
nitrophenyl-substituted tris-urea6b were shown to fix atmos-
pheric CO2, forming 98 and 90% complexes, respectively.
In conclusion, we have developed a novel receptor with six
urea groups possessing an intramolecular cavity that is suitable to
absorb atmospheric CO2 as CO32− under mild basic conditions.
The highly functionalized receptor molecule utilizes all 12 NH-
binding sites to interact with the internal carbonate anion,
forming an air-stable complex. The solid-state structure
described herein represents an exceptional example of the
encapsulated carbonate in a single molecule, providing an ideal
complementarity of binding sites for the anion. Given the
significant role of CO2 in environmental, chemical, and biological
systems,3,14 the present findings hold promise for developing
novel chemical devices for the recycling of atmospheric carbon
dioxide.
−
−
binding for I−, ClO4 , and NO3 . Determination of binding
−
constants for F− and H2PO4 was hampered due to the
broadening of NMR peaks.
The formation of the complex in DMSO-d6 was further
evaluated by a series of 13C NMR spectra (Figure 4). Partial 13C
NMR of Et4NHCO3 and free 1 are shown in Figure 4a,b,
respectively. The sharp signal at 157.19 ppm in the free
Et4NHCO3 shifted to 168.38 ppm (Δδ = 11.19 ppm) after the
addition of 1 equiv of the ligand (Figure 4c), indicating the
ASSOCIATED CONTENT
■
S
* Supporting Information
Figure 4. Partial 13C NMR spectra (125 MHz, DMSO-d6) of (a)
Et4NHCO3, (b) free receptor 1, (c) 1 in the presence of 1 equiv of
Et4NHCO3, and (d) solid carbonate complex of the receptor
([1(CO3)](n-Bu4N)2).
Crystallographic file in CIF format, and synthetic procedures and
NMR titration spectra in pdf format. This material is available
free of charge via the Internet at http://pubs.acs.org.
C
dx.doi.org/10.1021/ol403186k | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: alamgir.hossain@jsums.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The National Science Foundation is acknowledged for a
CAREER award (CHE-1056927) to M.A.H. NMR core facility
at JSU was supported by the National Institutes of Health
(G12RR013459).
REFERENCES
■
(1) (a) Hansen, J.; Nazarenko, L.; Ruedy, R.; Sato, M.; Willis, J.; Genio,
A. D.; Koch, D.; Lacis, A.; Lo, K.; Menon, S.; Novakov, T.; Perlwitz, J.;
Russell, G.; Schmidt, G. A.; Tausnev, N. Science 2005, 308, 1431.
(b) Climate Change 2007: Synthesis Report, International Panel on Climate
Change; Cambridge University Press: Cambridge, UK, 2007.
(2) National Research Council, Ocean Acidification: A National Strategy
To Meet the Challenges of a Changing Ocean; The National Academies
Press: Washington, DC, 2010; p 1.
́
(3) Das, C.; Gomes, N.; Jacquet, O.; Villiers, C.; Thuery, P.;
Ephritikhine, M.; Cantat, T. Angew. Chem., Int. Ed. 2012, 51, 187.
(4) (a) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388. (b) Sakakura,
T.; Kohno, K. Chem. Commun. 2009, 1312.
(5) (a) Edwards, P. R.; Hiscock, J. R.; Gale, P. A. Tetrahedron Lett.
2009, 50, 4922. (b) Edwards, P. R.; Hiscock, J. R.; Gale, P. A.; Light, M.
E. Org. Biomol. Chem. 2010, 8, 100.
(6) (a) Ravikumar, I.; Ghosh, P. Chem. Commun. 2010, 46, 1082.
(b) Dey, S. K.; Chutia, R.; Das, G. Inorg. Chem. 2012, 51, 1727.
(c) Busschaert, N.; Gale, P. A.; Haynes, C. J. E.; Light, M. E.; Moore, S.
J.; Tong, C. C.; Davis, J. T.; Harrell, J. W. A. Chem. Commun. 2010, 46,
6252. (d) Bose, P.; Dutta, R.; Santra, S.; Chowdhury, B.; Ghosh, P. Eur. J.
Inorg. Chem. 2012, 5791.
(7) (a) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic
Chemistry; University Science Books: Mill Valley, CA, 2006. (b) Smith,
R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York,
1975.
(8) Jia, C.; Wu, B.; Li, S.; Huang, X.; Zhao, Q.; Li, Q.-S.; Yang, X.-J.
Angew. Chem., Int. Ed. 2011, 50, 486.
(9) Crystal data for [1(CO3)][n-Bu4N]2: M = 1720.82, a =
41.2339(11) Å, b = 13.6303(4) Å, c = 15.0005(4) Å, α = 90°, β =
90°, γ = 90°, V = 8430.7(4) Å3, T = 100(2) K, space group Pna21, Z = 4,
μ(Mo Kα) = 0.112 mm−1, 85 226 reflections measured, 19 877
independent reflections (Rint = 0.0294). The final R1 value was 0.0360
(I > 2σ(I)). Flack parameter = 0.06(10). CCDC 963688.
(10) Isi̧ klan, M.; Saeed, M. A.; Pramanik, A.; Wong, B. M.; Fronczek, F.
R.; Hossain, M. A. Cryst. Growth Des. 2011, 11, 959.
(11) Hossain, M. A.; Kang, S. O.; Llinares, J. M.; Powell, D. R.;
Bowman-James, K. Inorg. Chem. 2003, 42, 5043.
(12) (a) Schneider, H. J.; Kramer, R.; Simova, S.; Schneider, U. J. Am.
Chem. Soc. 1988, 110, 6442. (b) Mendy, J. S.; Pilate, M. L.; Horne, T.;
Day, V. W.; Hossain, M. A. Chem. Commun. 2010, 46, 6084.
(13) Custelcean, R.; Remy, P.; Bonnesen, P. V.; Jiang, D.-E.; Moyer, B.
A. Angew. Chem., Int. Ed. 2008, 47, 1866.
(14) Aresta, M.; Dibenedetto, A. Dalton Trans. 2007, 2975.
D
dx.doi.org/10.1021/ol403186k | Org. Lett. XXXX, XXX, XXX−XXX