Sulfate recognition by persistent crystalline capsules with rigidified hydrogen-bonding cavities

Article abstract of DOI:10.1002/anie.200704937

Locked up: The sulfate anion is selectively encapsulated into rigid hydrogen-bonded capsules upon crystallization of a simple and flexible urea-functionalized ligand in the presence of Mg²⁺ ions and water (see structure; sulfate: space-filling).The resulting framework persists in the presence of different-shaped anions, allowing the shape selectivity of hydrogen-bonding hosts to be evaluated, and providing a simple way to separate the environmentally relevant sulfate from highly competitive mixtures of aqueous anions.

Full text of DOI:10.1002/anie.200704937

Communications
DOI: 10.1002/anie.200704937
Anion Recognition
Sulfate Recognition by Persistent Crystalline Capsules with Rigidified
Hydrogen-Bonding Cavities**
Radu Custelcean,* Priscilla Remy, Peter V. Bonnesen, De-en Jiang, and Bruce A. Moyer
Selectivity is a fundamental property of pervasive importance
in chemistry and biology as reflected in phenomena as diverse
as membrane transport, catalysis, sensing, adsorption, com-
plexation, and crystallization. Although the key principles of
complementarity and preorganization governing the binding
interactions underlying such phenomena were delineated
long ago,[1] truly profound designed selectivity has proven
elusive, in part because synthetic molecular architectures are
neither maximally complementary for binding target species
nor sufficiently rigid. Even if a host molecule possesses a high
degree of complementarity for a guest species, with almost no
exceptions it can distort its structure or even rearrange its
conformation altogether to accommodate competing guests.
One approach taken to overcome this challenge has been to
devise three-dimensional rigid molecules that bind species
within complementary cavities. Although rare examples have
been reported to demonstrate the principle,[1b] such cases are
not generally of practical utility, because of synthetic inacces-
sibility and often poor kinetics. Alternatively, more readily
synthesized flexible architectures are generally employed,[2]
but then the challenge becomes one of locking their structures
in place. This locking has been somewhat easier to accomplish
in self-assembled capsules,[3,4] though other challenges
emerge, such as finding means to internally functionalize
the cavities of the capsules.[5] Taking a cue from natural
binding agents[6] that derive their rigidity from a network of
molecular interactions, especially hydrogen bonding, we
present herein an example of a crystalline capsule that
recognizes sulfate by a highly complementary array of
rigidified hydrogen bonds. We have been employing crystal-
line host environments functionalized with anion-coordinat-
ing groups as a means to obtain maximal three-dimensional
complementarity and rigidity.[7] In the present study, we
focused on the problem of sulfate recognition and separa-
tion,[8,9] motivated in part by its high relevance to environ-
mental remediation and nuclear-waste cleanup[10] but also by
the challenge of overcoming the Hofmeister bias that
disfavors transfer of densely charged, highly hydrated
anions.[7d] The crystalline capsules described herein display
hydrogen-bonding cavities that are highly complementary to
tetrahedral divalent oxoanions, and remarkably persistent in
the presence of other oxoanions of various shapes and
basicities, despite being made from rather simple and flexible
building blocks. This system thus presents a rare opportunity
to examine anion selectivity under the ideal condition of a
constant hydrogen-bonding host environment, and, at the
same time offering a potentially practical solution for sulfate
separation from highly competitive aqueous salt solutions.
The tripodal tris-urea ligand L1 was selected for this study
based on analogous urea derivatives of tris(2-aminoethyl)-
amine that exhibited recognition of sulfate in solution and the
crystalline state.[9a,11] Notably, the m-CN-Ph substituted tris-
urea ligand encapsulated sulfate in a 2:1 complex when
incorporated into a silver coordination polymer, with the
resulting formation of 12 hydrogen bonds representing the
2ꢀ
highest coordination number reported for SO4 ions in a
synthetic receptor.[9a] However, selective and efficient sepa-
ration of sulfate from aqueous solutions with this or other
analogous crystalline frameworks proved elusive so far owing
to the low stability and relatively high solubility of the
frameworks in water. We reasoned that functionalization of
this tripodal scaffold with stronger coordinating groups, such
as pyridines, would facilitate the incorporation of this anion
receptor into more robust crystalline frameworks with
superior sulfate-separation abilities.
[*] Dr. R. Custelcean, P. Remy, Dr. P. V. Bonnesen, Dr. D. Jiang,
Dr. B. A. Moyer
Chemical Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN (USA)
Fax: (+1)865-574-4939
E-mail: custelceanr@ornl.gov
[**] This research was sponsored bythe Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic EnergySciences, U.S.
Department of Energy, under contract number DE-AC05-
00OR22725 with Oak Ridge National Laboratorymanaged byUT-
Battelle, LLC.
Crystallization of L1 with various MSO4 salts (M = Zn,
Cd, Co, Mg) from 1:1 H2O/MeOH solutions afforded single
crystals with the composition MSO4(L1)2(H2O)6, (1) as
determined by X-ray diffraction and elemental analysis.
Crystal-structure determination showed that, regardeless of
M, all the solids are isostructural with the same space group
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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(P21/n) and very similar unit cell and geometrical parameters
(Supporting Information). The following study was therefore
solely focused on the Mg2+ system (1a), which appeared most
practical for sulfate separation. The structure of 1a[12] consists
of centrosymmetric SO42ꢀ(L1)2 anionic units with the sulfate
encapsulated by 12 hydrogen bonds from six urea groups
(Figure 1a), which may be considered the optimal coordina-
dependent on the formation of this crystalline framework, as
only a 1:1 SO42ꢀ/L1 complex could be detected in solution
(CH3OD/D2O, 1:1) by an NMR Jobꢀs plot analysis
(Supporting Information), in agreement with other solution
binding studies of similar receptors.[11] This result reinforces
the strength and uniqueness of the crystallization approach to
anion recognition and separation, which allows exclusive
binding modes that are otherwise difficult to achieve in
solution.
Attempts to crystallize similar frameworks under the
same conditions from monocations such as Na+ or K+ in the
presence of different monoanions of various shapes and
ꢀ
basicities, such as ClO4ꢀ, NO3ꢀ, HCO3ꢀ, HSO4ꢀ, and H2PO4
,
resulted in no crystals. Likewise, monoanionic MgX2 salts,
where X = Fꢀ, Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, and NO3ꢀ, failed to
produce any crystals in the presence of L1 under similar
conditions. It thus appears that the presence of a dication/
dianion pair, which provides a stronger electrostatic stabili-
zation, is critical for the formation of 1 from water. This
feature allowed us to separate sulfate from excess nitrate by
selective crystallization of 1a. Indeed, competitive crystalli-
zation experiments indicated that 1a could be isolated in high
yield (88%) from a 100-fold excess of NaNO3,[15] as deter-
mined by FTIR and elemental analysis (Supporting
Information). This promising result is particularly relevant
to the problem of nuclear-waste cleanup, where it is desirable
that sulfate be removed from nitrate-rich nuclear wastes prior
to their vitrification, owing to the low solubility in the glass
2ꢀ [10]
and the corrosive nature of SO4
.
It is also a striking
demonstration of the power of crystallization techniques to
effect essentially infinite selectivity for the crystal-forming
species among competing species.
Figure 1. Crystal structure of 1a. a) Sulfate encapsulation by12 hydro-
gen bonds (dashed lines) from six urea groups; yellow S, red O,
blue N. b) Hydrogen-bonded capsule assembled from two L1 (stick
2ꢀ
model), one SO4 ion (space filling model), and six bridging water
To assess the sulfate selectivity against more competitive
molecules (stick model). c) A slice of the hydrogen-bonded framework
2ꢀ
anions of similar charge, we investigated the ability of SO3
,
2+
obtained byself-assemblyof the anionic capsules with Mg
ions
CO32ꢀ, and SeO4 to form similar hydrogen-bonded frame-
2ꢀ
(green). d) 3D representation of the octahedral framework with NaCl
works in the presence of Mg2+ and L1. While the first two
topology; magenta SO42ꢀ, green Mg2+(H2O)6, gray L1.
2ꢀ
anions have different shapes than SO4 (trigonal-pyramidal
and trigonal-planar, respectively), SeO42ꢀ has similar tetrahe-
dral shape but is slightly larger than sulfate (rXꢀO = 1.49 and
1.65 Š for X = S and Se, respectively).[16] This series is thus
ideal for probing the shape and size selectivity of 1a (taking
into account the differences in anion basicity and hydration).
All three anions were found to form hydrogen-bonded
frameworks that are isostructural to 1a, with the same
2ꢀ
tion number for sulfate.[9a] The SO4 ion coordination
geometry deviates however from the ideal C3 symmetry
predicted by molecular modeling (one chelating urea for each
of the six O-S-O edges),[13] with the most notable difference
that one of the urea groups binds an O vertex rather than an
O-S-O edge of the sulfate. This situation is most likely the
result of the centrosymmetric geometry imposed by the
crystal symmetry, which also causes the sulfate to be rota-
tionally disordered over two positions to compensate for its
lack of an inversion center. The SO42ꢀ(L1)2 complex is
incorporated in a hydrogen-bonded capsule by 12 additional
hydrogen bonds from six water molecules, each bridging,
2ꢀ
2ꢀ
general formula of MgX(L1)2(H2O)6 (X = SO3 (1b), CO3
2ꢀ
(1c), SeO4 (1d)), same space group, and slightly different
unit-cell parameters.[17] To assess the sulfate selectivity against
these anions, we performed pairwise competitive crystalliza-
tion experiments with 1:1 molar mixtures of MgSO4 and Na2X
salts (X = SO32ꢀ, CO32ꢀ, SeO42ꢀ) in the presence of two
equivalents of L1, and analyzed the anionic compositions of
the precipitated solids by FTIR spectroscopy and elemental
analysis (Supporting Information). Qualitative analysis of the
infrared spectra (Figure 2) clearly indicates that sulfate is
selectively separated in each case. Table 1 summarizes the
selectivities observed as determined quantitatively by ele-
mental analysis.
=
through its two protons, a pyridine N and a urea C O
acceptor (Figure 1b). The resulting quasioctahedral arrange-
ment of the water molecules around the hydrogen-bonded
capsules matches the coordinating geometry of the metal
cations, present as Mg2+(H2O)6 hydrates, which leads to the
formation of a three-dimensional framework with distorted
NaCl topology (Figure 1c,d). This structure thus represents a
rare example of a hydrogen-bonded network with octahedral
connectivity.[14] We note that the sulfate encapsulation is
As can be seen from Table 1, the competition experiments
2ꢀ
established the following anion selectivity order: SO4
>
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using the experimental crystal structures (Supporting
Information). For comparison, the selectivity expected from
crystallization of the corresponding Mg salts is: MgCO3 >
MgSO3 > MgSO4,[19] which follows the increasing anion radii
order. The combined experimental and computational data
thus indicate peak selectivity for sulfate against both stronger
bases, such as carbonate and sulfite, and less hydrophilic,
larger anions, such as selenate.
2ꢀ
The remarkable selectivity for SO4 ions appears to be
determined both by the nearly ideal complementarity of the
binding site for sulfate and the rigidity of its hydrogen-
bonding environment. First, the complementarity provides
for shape and size recognition through the urea functional
groups in 1a, which, as shown in Figure 3a, completely
Figure 3. a) Anion binding sites in 1a (left), 1b (middle), and
1c (right). Hydrogen bonds are shown as black dotted lines and
repulsive interactions in 1b,c as red dotted lines. b) Overlayof
1a (magenta) and 1d (green). Mg(H2O)6 cations were removed for
clarity. c) Overlay of theL1 structures in 1a–d.
Figure 2. FTIR analysis of the competitive crystallization experiments.
2ꢀ
a) SO42ꢀ/SO3 competition: 1a is shown in green, 1b in blue, and the
solid crystallized from MgSO4/Na2SO3 in red. b) SO42ꢀ/CO32ꢀ competi-
tion: 1a is shown in green, 1c in red, and the solid crystallized from
2ꢀ
MgSO4/Na2CO3 in blue. c) SO42ꢀ/SeO4 competition: 1a is shown in
green, 1d in blue, and the solid crystallized from MgSO4/Na2SeO4 in
red.
encapsulate the sulfate with an array of favorably positioned
hydrogen bonds.[20] In direct contrast, the CO3 and SO3
2ꢀ
2ꢀ
ions, with their different shape, do not fit very well within
these hydrogen-bonding cavities, as they require a different
geometric arrangement of the urea groups for optimal
binding.[13] In fact, these anions display repulsive interactions
(shown in red in Figure 3a) between one NH proton and the
positively charged C and S atoms, with contact distances of
2.66 and 2.75 Š that are significantly shorter than the
corresponding sums of the van der Waals radii of 2.90 and
3.00 Š, respectively. Such contacts are absent in the case of
the tetrahedral SO42ꢀ and SeO42ꢀ ions because of the effective
shielding by the fourth O atom in these anions. Second, L1
shows remarkable rigidity when constrained within these
water-bridged crystalline frameworks (Figure 3b,c), despite
its otherwise flexible structure. Such rigidity prevents the
receptor from adjusting its geometry to optimize the binding
of the competing anions, which contributes to the observed
discrimination between sulfate and selenate despite their
nearly identical shape and size. This system is thus reminis-
cent of protein receptors, which despite their inherent
flexibility can achieve remarkable selectivities by forming
well-defined and rigid binding sites, whose structural integrity
is dependent on the presence of multiple noncovalent
interactions and the inclusion of water. It is also important
Table 1: Sulfate selectivityagainst SO 32ꢀ, CO32ꢀ, and SeO42ꢀ ions.
Anion (X)/crystal Sep. factor (a)[a] D(DElat)[b] PA[c]
DG0 h(X)[d]
2ꢀ
SO4 /1a
SO32ꢀ/1b
CO32ꢀ/1c
SeO42ꢀ/1d
-
372
53
0
ꢀ1889 ꢀ1080
75.3
58.7
33.8
ꢀ1971 ꢀ1295
ꢀ2042 ꢀ1315
5
ꢀ1863
ꢀ900
[a] Separation factor: a(SO42ꢀ/X)=[(mol SO42ꢀ)/(mol X)]crystal/[(mol
SO42ꢀ)/(mol X)]solution. [b] Calculated lattice energies (relative to 1a),
2+
where DE(1x)lat =E(1x)crystalꢀE(X)gasꢀ2E(L1)gasꢀE[Mg(H2O)6
]
gas
(x=
a–d), calculated at the PBE level with planewave bases (see details in
the Supporting Information). [c] Gas-phase proton affinities of anions
calculated at the B3LYP/6-31+G** level (see details in the Supporting
Information). [d] Experimental Gibbs energies of hydration.[18] All
energies are in kJmolꢀ1
.
SeO42ꢀ > CO32ꢀ > SO32ꢀ. This trend differs from the orders of
hydrogen-bond acceptor abilities of the anions (estimated
from the gas-phase proton affinities): CO32ꢀ > SO3
>
2ꢀ
SO42ꢀ > SeO42ꢀ, or of the Hofmeister series (based on DGh):
SeO42ꢀ > SO42ꢀ > SO32ꢀ > CO32ꢀ. Rather, the observed anion
selectivity order agrees with the relative crystal stabilities of
1a > 1d > 1c > 1b, as found from lattice-energy calculations
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Chemie
Rissanen, P. Cozzini, E. Marastoni, E. Fisicaro, P. Manini, R.
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tical structures are obtained with the other anions despite the
wide variation in their size and shape. This structural
persistence is apparently a result of the hydrogen-bonded
water bridges that rigidify the framework in much the same
way as found in calixarene capsules sustained by incorporated
water molecules.[3c]
We have illustrated herein a striking example of anion
recognition and separation based on crystallization of rigid
hydrogen-bonded capsule frameworks made from a simple,
flexible ligand functionalized with complementary urea
groups, which allowed us to selectively separate the environ-
mentally relevant sulfate from highly competitive mixtures of
aqueous anions. The unusual persistence of a common
framework structure in the presence of different anions of
various sizes, shapes, and basicities provided a unique
opportunity for assessing anion selectivities in hosts with a
virtually constant hydrogen-bonding environment. While we
specifically targeted sulfate separation in this particular study,
this approach could be applicable to separation of other guest
molecules or ions through appropriate crystal design.
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Keywords: anions · crystal engineering · hydrogen bonds ·
sulfate · supramolecular chemistry
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Communications
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contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
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request/cif
[15]The yield obtained in the competitive crystallization is greater
than the 71% yield observed in the synthesis of 1a (Supporting
Information), which is likely a result of salting-out by the
NaNO3. Preliminary experiments indicate that L1 can be
recovered by crystallization, after dissolution of 1a into aqueous
HCl, sulfate removal as BaSO4, and pH adjustment to 6 – 7.
[16]Average values from Cambridge Structural Database (CSD 5.28,
November 06).
[18]Y. Marcus, J. Chem. Soc. Faraday Trans. 1991, 87, 2995 – 2999.
[19] DGcryst for the reaction Mg2+(aq) + X2ꢀ(aq)!MgX(cryst) are ꢀ29.5,
17.5, and 28.7 kJmolꢀ1 for X = CO3
, SO3 , and SO4 ,
2ꢀ
2ꢀ
2ꢀ
respectively (no data available for MgSeO4); calculated from
tabulated values in D. D. Wagman, W. H. Evans, V. B. Parker,
R. H. Schumm, I. Halow, S. M. Bailey, K. L. Churney, R. L.
Nuttal, J. Phys. Chem. Ref. Data 1982, 11, Supplement No. 2.
[20]The sulfate disorder in 1a suggests that entropy may play a role
in the strong affinity for this anion (F. P. Schmidtchen, Coord.
Chem. Rev. 2006, 250, 2918 – 2928). However, considering that
the other anions are also rotationally disordered in 1b–d (with
the sulfite and carbonate displaying somewhat larger thermal
ellipsoids than sulfate and selenate), it is unlikely that entropy is
a determining factor in the observed sulfate selectivity.
[21]a) M. D. Lankshear, P. D. Beer, Acc. Chem. Res. 2007, 40, 657 –
668; b) N. Gimeno, R. Vilar, Coord. Chem. Rev. 2006, 250, 3161 –
3189.
[17]a) Crystal data for 1b: C48H72MgN20O15S, Mr = 1225.62, mono-
clinic, space group P21/n, a = 12.5076(7), b = 18.2295(11), c =
12.9307(8) Š, b = 91.6430(10)8, V= 2947.1(3) Š3, Z = 2, 1calcd
=
1.381 gcmꢀ3, T= 173 K, R1 = 0.0620, wR2 = 0.1498; crystal data
for 1c: C49H72MgN20O15, Mr = 1205.58, monoclinic, space group
P21/n, a = 12.4253(6), b = 18.2988(10), c = 12.8511(7) Š, b =
92.3620(10)8, V= 2919.4(3) Š3, Z = 2, 1calcd = 1.371 gcmꢀ3, T=
173 K, R1 = 0.0578, wR2 = 0.1321; crystal data for 1d:
C48H72MgN20O16Se, Mr = 1288.52, monoclinic, space group
P21/n, a = 12.4822(6), b = 18.3577(10), c = 13.1051(7) Š, b =
91.4760(10)8, V= 3002.0(3) Š3, Z = 2, 1calcd = 1.425 gcmꢀ3, T=
173 K, R1 = 0.0362, wR2 = 0.0897. b) CCDC-662209 (1a),
CCDC-662210 (1b), CCDC-662211 (1c), CCDC-662212 (1d)
1870
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Angew. Chem. Int. Ed. 2008, 47, 1866 –1870