A coordinatively saturated sulfate encapsulated in a metal-organic framework functionalized with urea hydrogen-bonding groups

Article abstract of DOI:10.1039/b511809c

A functional coordination polymer decorated with urea hydrogen-bonding donor groups has been designed for optimal binding of sulfate; self-assembly of a tripodal tris-urea linker with Ag₂SO₄ resulted in the formation of a ID metal-or

Full text of DOI:10.1039/b511809c

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COMMUNICATION
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A coordinatively saturated sulfate encapsulated in a metal–organic
framework functionalized with urea hydrogen-bonding groups{
Radu Custelcean,*a Bruce A. Moyera and Benjamin P. Hayb
Received (in Cambridge, UK) 19th August 2005, Accepted 29th September 2005
First published as an Advance Article on the web 14th October 2005
DOI: 10.1039/b511809c
and coordinatively saturates sulfate through the unprecedented
formation of 12 hydrogen bonds.
A functional coordination polymer decorated with urea
hydrogen-bonding donor groups has been designed for optimal
binding of sulfate; self-assembly of a tripodal tris-urea linker
with Ag2SO4 resulted in the formation of a 1D metal–organic
We founded our design on the tris-urea structure 1a built from
the tris(2-aminoethyl)-amine linker, which has been recently found
to act as a sulfate receptor.10 Functionalization of 1a with metal-
coordinating –CN groups afforded the analogous sulfate receptor
1b that can also act as an MOF linker.
22
framework that encapsulates SO4 anions via twelve com-
plementary hydrogen bonds, which represents the highest
coordination number observed for sulfate in a natural or
synthetic host.
Anion binding by synthetic receptors is an important and
contemporary aspect of supramolecular chemistry.1 Sulfate com-
plexation and extraction from water is particularly challenging due
to the high charge density of this anion, which translates into a
large free energy of hydration of 21080 kJ mol21.2 While Nature’s
sulfate-binding proteins use solely hydrogen bonding for this task,3
synthetic receptors have involved either hydrogen bonding alone
(in neutral hosts), or a combination of hydrogen bonding and
electrostatic interactions (in cationic hosts) for sulfate complexa-
tion.4 Recently, Bowman-James has categorized the binding of
anions based on their coordination numbers by analogy with the
understanding of cation coordination originally developed by
Werner.5 The extension of the coordination-number concept is
helpful in defining the notions of complementarity and the
maximum coordination number (saturation) for a given anion,
which can aid the design of optimal anion-binding host structures.
For the specific case of sulfate, as a prime example of interest to us,
the highest coordination number previously observed within a
natural or synthetic host was eight.3,4 However, electronic-
structure calculations by Hay et al. led to the expectation that
sulfate ideally accommodates 12 hydrogen bonds, one to each
oxygen atom in each of the six O–S–O planes.6 In the course of
examining receptors complementary for oxoanions such as sulfate,
we have started to explore metal–organic frameworks (MOFs)7 as
anion-binding hosts.8 While the anion coordination is known to
influence the self-assembly of MOFs,9 we thought that a
molecular-design approach could lead us to MOF linkers
possessing appropriately positioned arrays of hydrogen-bond
donor groups to achieve maximum complementarity between
sulfate and the MOF hosts. Herein we report an MOF
functionalized with urea binding sites9 that indeed encapsulates
While structural data for the anion binding by 1a is missing, our
molecular modeling (MMFF94)11 confirmed that this C3-symme-
trical ligand has good shape complementarity for SO422, and can
bind the anion in two distinct modes that involve all three urea
groups in chelate hydrogen bonding (Fig. 1).
aChemical Sciences Division, Oak Ridge National Laboratory, Oak
Ridge, TN 37831-6119, USA. E-mail: custelceanr@ornl.gov;
Fax: (865) 574 4939; Tel: (865) 574 5018
Fig. 1 Molecular model of 1a?SO422. a) Axial binding with the urea
groups positioned on the three edges radiating from a common oxygen
atom of sulfate. b) Facial binding with the urea groups positioned on the
three edges of the same triangular face of sulfate, which is 2.3 kcal/mol
higher in energy than the axial mode. c) Sulfate encapsulation by
concurrent axial–facial binding by two ligands.
bChemical Sciences Division, Pacific Northwest National Laboratory,
Richland, WA 99352, USA
{ Electronic supplementary information (ESI) available: synthetic proce-
dures, NMR spectroscopic data, and crystallographic details. See DOI:
10.1039/b511809c
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Chem. Commun., 2005, 5971–5973 | 5971

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molecular cage that encapsulates the sulfate in its center. The
…
cages are interlinked via CN Ag coordination bonds to form a
one-dimensional coordination polymer,13 with the Ag nodes being
tetracoordinated by two –CN groups, one urea, and a water
molecule. Compared to the molecular model, however, the ligand
deviates from the ideal C3 symmetry, apparently as a result of the
silver coordination and crystal packing effects. Furthermore, the
cage is centrosymmetric, as required by the space group symmetry,
with the sulfate sitting on the crystallographic inversion center.
Lacking a proper inversion center the tetrahedral sulfate anion is
rotationally disordered over two positions, with the eight O atoms
with half occupancy defining the corners of a cube.
Attempts to cocrystallize 1b with other soluble silver salts
containing different anions of various shape and basicity, such as
AgBF4, AgNO3, AgMeSO3, or AgOAc, under the same conditions
as in 2, failed to produce coordination polymers, and only crystals
of the free ligand could be obtained. Crystal structure analysis§
revealed that 1b forms 1D chains in the solid state, with each
Fig. 2 Crystal structure of 2. a) Sulfate encapsulation via twelve
hydrogen bonds from six urea groups. b) Coordination cage with the
two ligands related by inversion symmetry depicted in green and magenta.
The sulfate sits on the crystallographic inversion center and is rotationally
disordered over two positions. c) 1D coordination polymer with the
framework shown as stick model and the sulfate anions shown as space
filling model.
…
ligand molecule forming five urea urea hydrogen bonds (ESI).
1H NMR spectroscopy indicated that these anions interact
22
significantly more weakly with 1b compared with SO4
.
Accordingly, addition of one equivalent of AgBF4, AgNO3,
AgMeSO3, or AgOAc to a DMSO solution of 1b induced
downfield chemical shifts for the two urea NH protons of 0.00 and
0.00 ppm for BF42, 0.02 and 0.01 ppm for NO32, 0.21 and
0.08 ppm for MeSO32, and 0.18 and 0.26 ppm for AcO2, which
are significantly smaller than the observed analogous shifts caused
by sulfate (vide supra). As a better alternative to the significantly
Crystallization of 1b from water/acetone in the presence of half
equivalent of Ag2SO4 yielded a coordination polymer with the
composition [Ag2(1b)2](SO4)?(acetone)1.5?(H2O)3.7 (2), as indicated
1
by elemental analysis, H NMR spectroscopy in DMSO (ESI),
reduced interaction with BF42, NO32, MeSO3 or AcO2, 1b
2
and single-crystal X-ray diffraction.{ The framework is insoluble
in water and common organic solvents (except DMSO), and is
thermally stable up to 192 uC, at which temperature it melts with
decomposition. The urea NH protons in the NMR spectrum of 2
are shifted downfield by 0.84 and 0.48 ppm relative to 1b,
suggesting relatively strong hydrogen bonding of the sulfate, and a
Job’s plot revealed a 2 : 1 ligand to sulfate binding stoichiometry.12
Molecular modeling (MMFF94) showed that the sulfate can
accommodate two molecules of 1a, with the six urea groups
chelating the anion in a C3-symmetrical complex (Fig. 1c). Crystal
structure analysis of 2 confirmed the 2 : 1 sulfate binding by 1b,
with one ligand coordinating in the axial mode and the other in the
facial mode (Fig. 2), resulting in a total of twelve hydrogen bonds
(Table 1). The two ligands are additionally held together by
prefers to self-associate in the solid state through the formation of
…
multiple urea urea hydrogen bonds that engage all available NH
protons. We also note here that no cocrystallization was observed
when (Me4N)2SO4 was used as a sulfate source, which indicates
that silver coordination and MOF formation are critical in
stabilizing the (1b)2?SO422 complex relative to the free ligand in the
solid state.
In summary, we reported here the design of an MOF host
functionalized with urea binding sites that provide a coordinatively
saturated environment for sulfate through the unprecedented
formation of twelve complementary hydrogen bonds. Although
the exclusive encapsulation of sulfate prevented anion exchange in
this system, this study provides insight for future design of solid-
state materials for recognition and selective separation of targeted
anions.
…
…
…
CN Ag and urea(O) Ag coordination bonds, as well as p p
interactions between the phenyl rings, essentially forming a
This research was sponsored by the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. Department of Energy, under contract number
DE-AC05-00OR22725 with Oak Ridge National Laboratory
(managed by UT-Battelle, LLC), and performed at Oak Ridge
National Laboratory and Pacific Northwest National Laboratory
(managed by Battelle).
Table 1 Hydrogen bonding parameters (A, u) for SO422 binding in 2
˚
…
…
…
D A
D–H
A
H
A
, D–H–A
…
N1–H1 O1
2.28
2.24
2.09
2.27
2.08
2.37
2.25
2.12
2.11
2.24
2.17
2.28
2.9393
3.1110
2.9636
2.8516
2.9266
3.1741
3.0015
2.9901
2.9072
3.0598
3.0097
3.0440
132
170
170
124
162
152
143
168
151
156
160
146
…
N1–H1 O3
…
N2–H2 O2
…
N2–H2 O4
…
N3–H3 O1
Notes and references
…
…
N3–H3 O2
{ Crystal data for 2: C30H34N10O7S0.5Ag, M 5 770.57, colorless plate,
0.15 6 0.12 6 0.05 mm3, triclinic, space group P-1 (No. 2), a 5 10.3995(12),
N4–H4 O4
…
N4–H4 O3
˚
b 5 13.5437(15), c 5 14.6455(17) A, a 5 66.289(2), b 5 76.241(2),
…
N5–H5 O1
3
3
˚
c 5 87.576(2)u, V 5 1831.2(4) A , Z 5 2, Dc 5 1.398 g/cm , F000 5 790,
…
N5–H5 O4
˚
Bruker SMART APEX, MoKa radiation, l 5 0.71073 A, T 5 173(2) K,
…
…
N6–H6 O3
N6–H6 O2
2hmax 5 56.7u, 22524 reflections collected, 9083 unique (Rint 5 0.0326).
Final GooF 5 1.252, R1 5 0.0659, wR2 5 0.1448, R indices based on 8139
5972 | Chem. Commun., 2005, 5971–5973
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reflections with I . 2s(I) (refinement on F2), 465 parameters, 0 restraints.
Lp and absorption corrections applied, m 5 0.635 mm21. CCDC 281962.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b511809c
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§ Crystal data for 1b: C30H30N10O3, M 5 578.64, colorless block, 0.21 6
0.07 6 0.07 mm3, triclinic, space group P-1 (No. 2), a 5 13.0827(18),
˚
b 5 13.993(2), c 5 18.274(3) A, a 5 73.362(3), b 5 85.694(3), c 5 63.147(2)u,
3
V 5 2853.9(7) A , Z 5 4, Dc 5 1.347 g/cm , F000 5 1216, Bruker SMART
3
˚
˚
APEX, MoKa radiation, l 5 0.71073 A, T 5 173(2) K, 2hmax 5 50.0u,
20199 reflections collected, 10021 unique (Rint 5 0.0651). Final
GooF 5 0.997, R1 5 0.0752, wR2 5 0.1920, R indices based on 5694
reflections with I . 2s(I) (refinement on F2), 794 parameters, 0 restraints.
Lp and absorption corrections applied, m 5 0.092 mm21. CCDC 281963.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b511809c
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