Encapsulation of [(SO4)4(H2O)12]8- clusters in a metal organic framework of pyridyl functionalized cyanuric acid based tris-urea

Article abstract of DOI:10.1039/c4dt02877e

Encapsulation of hydrated sulfate in a bowl-shaped metal organic coordination polymer formed by Zn²⁺ assisted self-assembly of a 3-pyridyl terminated cyanuric acid platform based urea receptor is reported in aqueous medium. Trapping of an unusual [(SO₄)₄(H₂O)₁₂]^8- cluster in a [Zn(H₂O)₆]²⁺ capped self-assembled structure is characterized by single crystal X-ray crystallography. Furthermore, selective binding of SO₄^2- is established from the ¹H-NMR titration study.

Full text of DOI:10.1039/c4dt02877e

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Encapsulation of [(SO4)4(H2O)12]8− clusters in a
metal organic framework of pyridyl functionalized
cyanuric acid based tris-urea†
Cite this: DOI: 10.1039/c4dt02877e
Received 18th September 2014,
Accepted 17th October 2014
Ranjan Dutta, Bidyut Akhuli and Pradyut Ghosh*
DOI: 10.1039/c4dt02877e
www.rsc.org/dalton
2−
Encapsulation of hydrated sulfate in a bowl-shaped metal organic for selective recognition of SO4 via complete encapsulation.7
coordination polymer formed by Zn2+ assisted self-assembly Particularly, pyridyl–urea containing metallo-supramolecular struc-
2−
of
a 3-pyridyl terminated cyanuric acid platform based urea tures are employed for SO4 encapsulation either by discrete
receptor is reported in aqueous medium. Trapping of an unusual assembly or coordination polymer formation.7c–j,m–o In this
8−
[(SO4)4(H2O)12
]
cluster in a [Zn(H2O)6]2+ capped self-assembled context, Custelcean and Kaifer et al. have recently reported the
2−
structure is characterized by single crystal X-ray crystallography. SO4 encapsulation driven self-assembly process towards the
Furthermore, selective binding of SO4 is established from the formation of M4L6 cages in aqueous medium.7h,i However,
2−
1H-NMR titration study.
encapsulation of hydrated sulfates by synthetic receptors is
rare in the literature. Such evidence of sulfate–water cluster
recognition is reported by Das et al. via the isolation of a rugby
ball shaped [(SO4)2(H2O)3]4− cluster in the dimeric capsular
assembly of a tren based tris-urea receptor.6c
Herein we demonstrate a coordination driven self-assembly
process of a cyanuric acid platform based tris-urea L in the
presence of ZnSO4–MgSO4 in aqueous methanol (1 : 1). Impor-
tantly, encapsulation of the hydrated sulfate [(SO4)4(H2O)12]8−
Recognition of hydrated anions is of great importance due to
their presence in natural and biological environments.1 High
2−
charge density and very high hydration energy of SO4 (ΔGh =
−1080 kJ mol−1 2
) assist the formation of different sulfate–
water clusters in the aqueous environment. Association of
twelve hydrogen bonds from water molecules are predicted in
the first hydration shell of SO4 by theoretical calculation.3
2−
Sulfate–water clusters play a crucial role in many environ-
mental and biological processes such as aerosol formation,
dissolution of proteins in the human body and regulation of
cluster in the self-assembled cavity is observed in the case of
Zn2+ whereas SO4
assisted dimeric capsular assembly is
2−
observed in the case of Mg2+. Furthermore, we show selective
various metabolic processes.4 At the same time SO4 inter-
2−
2−
binding of SO4 over other anions by the 1H-NMR titration
feres in the vitrification process of nuclear waste and causes
permanent hardness of water.5 Thus, recognition of hydrated
SO4 is more relevant than naked SO4 in the area of mole-
cular recognition and self-assembly.
study.
Tripodal urea receptor L (Scheme 1a) is synthesized by
the reaction of tripodal amine and 3-pyridyl isocyanate in
2−
2−
Various synthetic receptors having ammonium, amide,
2−
urea, and indole functionalities have been employed for SO4
recognition both in organic and aqueous media.6 Metal ion
templated self-assembled structures provide a successful strategy
Department of Inorganic Chemistry, Indian Association for the Cultivation of
Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700032, India.
E-mail: icpg@iacs.res.in
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of L and complexes 1 and 2; 1H-NMR and ITC profiles; crystallographic
tables for complexes 1 and 2; hydrogen bonding parameters of 1 and 2; TGA
and PXRD plots for 1. CCDC 1024314 (complex 1) and 1024315 (complex 2).
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt02877e
Scheme 1 (a) Molecular structure of L and (b) pictorial representation
of four hydrated SO4 encapsulation in the bowl shaped secondary
building unit.
2−
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moderate yield (Scheme 1S, ESI†). Slow evaporation of the via O–H⋯O interactions to form the [(SO4)4(H2O)12]8− cluster
MeOH–H2O (1 : 1) solution of L and ZnSO4·7H2O results in (Fig. 1c). Further insight into the coordination details of the
crystals of complex 1 with the composition [L3Zn3(SO4)4]- [(SO4)4(H2O)12]8− cluster reveals that one O atom (O6) of the
2−
[Zn(H2O)6][(H2O)18] in high yield (∼75%). However, crystals of central SO4 is solely H-bonded to three –NH groups (N4–H4)
dimeric capsules of complex 2 [L2SO4][Mg(H2O)6][(H2O)2] are of the central L unit.
obtained by slow evaporation of a MeOH–H2O solution of L
Whereas each of the three remaining O atoms (O5) are H-
and MgSO4. Complex 1 contains a large amount of solvents in bonded to one –NH group (N5–H5A) each. Furthermore, each
2−
the unit cell and crystallographic parameters are given in O5 is connected to the O atom (O2) of each side SO4 via a
Table 1S, ESI.†
bridging water molecule (O20) (Fig. 8S, ESI†). This bridging
Direct coordination of the pyridyl nitrogen donor (Py-N) water molecule is further H-bonded to the [Zn(H2O)6]2+ cation.
2−
of L to Zn2+ generates the coordination polymer 1, which O4 of the each side SO4 is H-bonded to three –NH groups
propagates in three dimensions (Fig. 1a). Each bowl shaped namely N1–H1, N10–H10 and N13–H13 of side L unit. Three
secondary building unit (SBU) of 1 contains one central remaining O atoms namely O1, O2 and O3 are H-bonded to
C3v-symmetric receptor unit (L) at the bottom of the bowl, one –NH group each namely N12–H12, N2–H2 and N9–H9
other three receptor units at the three sides of the bowl, respectively (Fig. 8S, ESI†). In addition O1 and O3 are con-
2−
three hexa-coordinated Zn2+, four encapsulated SO4 and one nected to [Zn(H2O)6]2+ through two disordered bridging water
[Zn(H2O)6]2+ cation (Scheme 1b). Each Py-N atom of the central molecules namely O23A and O22A respectively. Whereas, O2 is
L is coordinated to one Zn2+ each, thus forming a C3v-symmetric connected to [Zn(H2O)6]2+ via a bridging water molecule O19.
2−
cleft which encapsulates one SO4 in its centre via N–H⋯O Furthermore, O19 is connected to O20 via O9. Thus, the
interactions (Table 2S, ESI†). The remaining coordination sites [Zn(H2O)6]2+ capped [(SO4)4(H2O)12]8− cluster is recognized in
of Zn2+ are filled by the coordination of two Py-N atoms of two the cavity of the bowl shaped SBU. Detailed H-bonding inter-
side L units, one Py-N atom of another SBU and two water actions of [(SO4)4(H2O)12]8− with the receptor functionalities
molecules (O9 and O10) (Fig. 7S, ESI†). Thus a bowl shaped (CvO group of L) are provided in the ESI (Fig. 9Sb†). Further-
2−
SBU is generated where encapsulation of three remaining SO4
more, the packing diagram of 1 shows a porous channel
anions are achieved in the cavity formed by three side-L units. running along the c axis (Fig. 10S, ESI†). In contrast, tren
2−
In most of the sulfate assisted self-assembled structures encap- based 3-pydridyl urea is reported to form SO4 encapsulated
sulation of a single SO4 is observed in the solid state.7g–j dimeric capsular assembly irrespective of the counter cations,
2−
Thus, complex 1 represents a unique example of simultaneous where the cation exists as [M(H2O)6]2+ species [M = Zn2+, Cd2+,
encapsulation of as many as four SO42− anions in a supramole- Mg2+, Co2+, Mn2+].7c ESI-MS(negative mode) analysis of 1
cular assembly (Fig. 1b). Now each of the four encapsulated shows peaks at 715.96 and 976.91 corresponding to mono-
2−
SO4 anions are interconnected by bridging water molecules valent [L + HSO4]− and divalent [L3 + SO4]2− species respect-
ively (Fig. 11S, ESI†). The bulk purity of 1 is verified by PXRD
analysis of crystals of 1, which matches well with the simulated
pattern (Fig. 12S, ESI†). Furthermore, the solvent content of
complex 1 is verified by thermogravimetric analysis (TGA),
where a weight loss of 18.039% is observed at ∼130 °C. This
experimental weight loss corroborates with the calculated
value (18.19%) from the crystal structure of 1 (Fig. 13S, ESI†).
A comparative IR study of L and 1 also confirms the presence
of SO4 with a new peak at 1120 cm−1 (Fig. 14S, ESI†), corres-
2−
2−
ponding to the stretching frequency of SO4
.
However, crystallographic analysis of the MgSO4 complex of
2−
L i.e. 2 reveals encapsulation of SO4 in the dimeric capsular
assembly of L (Fig. 2a). Twelve strong N–H⋯O interactions are
2−
involved to encapsulate the SO4 inside the dimeric capsule
(Fig. 2b). In this case the Mg2+ cation exists as [Mg(H2O)6]2+
species which hold the capsule via H-bonding interactions
with the –CvO group of L and the Py-N atom through a bridg-
ing water molecule (O14). Similar structural features are
found in both the Mg-capsules derived from tren and cyanuric
platform based 3-pyridyl urea except the H-bonding pattern of
the [Mg(H2O)6]2+ cation. The capsular dimension of 2 is
measured as 9.71 Å compared to 9.65 Å in its tren analogue.7c
Hydrogen bonding parameters and interactions of the [SO4L2]2−
capsule with six [Mg(H2O)6]2+ cations through second-sphere
coordination are provided in the ESI (Fig. 15S and 16S, ESI†).
Fig. 1 (a) View of [Zn(H2O)6]2+ capped hydrated SO42−, [(SO4)4(H2O)12
]
8
− encapsulation in the cavity of the metal–organic coordination polymer
2−
1; (b) view of four SO4 encapsulation in the bowl shaped SBU of 1; (c)
close-up view of [Zn(H2O)6]2+ capped hydrated SO42−, [(SO4)4(H2O)12
All the hydrogens and lattice solvents are omitted for clarity.
] .
8−
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2−
Fig. 2 (a) View of complex 2 showing complete encapsulation of SO4
in the dimeric capsular assembly of L; (b) close-up view of twelve
N–H⋯O interactions in 2. Non-acidic hydrogens and countercations are
omitted for clarity.
Fig. 4 ITC profile for the titration L (0.1212 mM) with ZnSO4 (2.5 mM) in
DMSO at 298 K.
In summary, we have demonstrated a unique example of
hydrated sulfate cluster [(SO4)4(H2O)12]8− recognition in a self-
assembled metal–organic coordination polymer derived from a
3-pyridyl attached cyanuric acid platform based tripodal urea
and Zn2+ in aqueous medium. Encapsulation of as many as
Fig. 3 (a) 1H-NMR spectral changes of L with added ZnSO4 in DMSO-
d6 ([L] is varied from 4.43 mM to 3.28 mM by the addition of aliquots of
34.08 mM ZnSO4). Ratio of concentration [L]/[SO42−]: (i) 0, (ii) 0.12, (iii)
0.37, (iv) 0.62, (v) 0.86, (vi) 1.11, (vii) 1.35, (viii) 1.72, (ix) 2.22, and (x) 2.71.
(b) Job’s plot for L with ZnSO4.
2−
four SO4 anions is demonstrated in the cavity of the bowl
shaped secondary building unit of the coordination polymer.
PG thanks the Department of Science and Technology
(DST), India for financial support through a Swarnajayanti
Fellowship. R. D. would like to acknowledge IACS, Kolkata for
a research fellowship. B. A. acknowledges CSIR, India for a
SRF. The X-ray crystallography study is performed at the DST-
funded National Single Crystal X-ray Diffraction Facility at the
Department of Inorganic Chemistry, IACS.
Comparative IR spectra of L and 2 show a characteristic peak
of SO42− at 1095 cm−1 (Fig. 17S, ESI†).
A solution state complexation study of L and ZnSO4 is per-
formed by titrating a solution of L with ZnSO4 in DMSO-d6. A
significant downfield shift of NHa (Δδ ∼ 1.2 ppm) and NHb
(Δδ ∼ 1.4 ppm) protons are observed upon addition of ∼1
equiv. of ZnSO4 (Fig. 3a, Fig. 18S, ESI†). Job’s plot analysis by
monitoring the chemical shift of NHa protons reveals 1 : 1
Notes and references
2−
association between L and SO4 in solution (Fig. 3b). This
2−
1 : 1 association of L and SO4 in solution is the same as that 1 (a) A. Lee, P. A. Dawson and D. Markovich, Int. J. Biochem.
of the solid state binding pattern in 1. Binding constant calcu-
lation shows a log K value of 5.47 for L with ZnSO4 in DMSO-
Cell Biol., 2008, 37, 1350; (b) D. T. Richens, The Chemistry of
Aqua Ions, Wiley, Chichester, West Sussex, England, 1997.
d6. However, no such changes in chemical shift are observed 2 Y. J. Marcus, Chem. Soc., Faraday Trans., 1991, 87, 2995–
for L with Zn(AcO)2, Zn(NO3)2, ZnCl2 and Zn(ClO4)2 as evident
2999.
1
from qualitative H-NMR analysis in DMSO-d6 (Fig. 19S, ESI†). 3 B. Gao and Z. Liu, J. Chem. Phys., 2004, 121, 8299.
Moreover, solution state binding of L and ZnSO4 is verified by 4 (a) F. Hofmeister, Arch. Exp. Pathol. Pharmacol., 1887, 24,
the ITC study by titrating a solution of L with ZnSO4 in DMSO
(Fig. 4). The observed exothermic binding profile with 1 : 1
(host–guest) stoichiometry (n = 0.87 0.01) further supports
247; (b) J. W. Pflugrath and F. A. Quiocho, Nature, 1985, 314,
257–260; (c) G. Y. Stokes, A. M. Buchbinder, J. M. Gibbs-
Davis, K. A. Scheidt and F. M. Geiger, Vib. Spectrosc., 2009,
50, 86–98.
1
the H-NMR data. The estimated kinetic and thermodynamic
parameters (K, TΔS, ΔH and ΔG) of the above experiment are 5 (a) The Problem with Anions in the DOE Complex. In Funda-
provided in the ESI (Fig. 20S†).
mentals and Applications of Anion Separations, ed. B. A. Moyer
and R. P. Singh, Kluwer Academic, New York, 2004,
p. 107114; (b) B. A. Moyer, R. Custelcean, B. P. Hay,
J. L. Sessler, K. Bowman-James, V. W. Day and S.-O. Kang,
Inorg. Chem., 2013, 52, 3473–3490.
Competitive crystallization of L with ZnSO4·7H2O in the
presence of 10 equivalents of NaNO3 in MeOH–H2O (1 : 1)
yields crystals of 1 in high yield as determined by FTIR
(Fig. 21S, ESI†) and X-ray structural studies.
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6 (a) S. O. Kang, M. A. Hossain, D. Powell and K. Bowman-
James, Chem. Commun., 2005, 328–330; (b) D. Curiel,
A. Cowley and P. D. Beer, Chem. Commun., 2005, 236–238;
(c) D. A. Jose, D. K. Kumar, B. Ganguly and A. Das, Inorg.
Chem., 2007, 46, 5817–5819; (d) I. Ravikumar,
P. S. Lakshminarayanan, M. Arunachalam, E. Suresh and
P. Ghosh, Dalton Trans., 2009, 4160–4168; (e) J. S. Mendy,
M. Pilate, T. Horne, V. W. Day and M. A. Hossain, Chem.
Commun., 2010, 46, 6084–6086; (f) I. Ravikumar and
P. Ghosh, Chem. Soc. Rev., 2012, 41, 3077–3098.
7 (a) C. R. Bondy, P. A. Gale and S. J. Loeb, J. Am. Chem. Soc.,
2004, 126, 5030–5031; (b) R. Custelcean, B. A. Moyer and
B. P. Hay, Chem. Commun., 2005, 5971–5973;
(c) R. Custelcean, P. Remy, P. V. Bronnesen, D. Jiang and
B. A. Moyer, Angew. Chem., Int. Ed., 2008, 47, 1866–1870;
(d) B. Wu, J. Liang, J. Yang, C. Jia, X.-J. Yang, H. Zhang,
N. Tang and C. Janiak, Chem. Commun., 2008, 1762–1764;
(e) R. Custelcean, A. Bock and B. A. Moyer, J. Am. Chem. Soc.,
2010, 132, 7177–7185; (f) A. Rajbanshi, B. A. Moyer and
R. Custelcean, Cryst. Growth Des., 2011, 11, 2702–2706;
(g) R. Custelcean, J. Bosano, P. V. Bronnesen, V. Kertesz and
B. P. Hay, Angew. Chem., Int. Ed., 2009, 48, 4025–4029;
(h) R. Custelcean, P. V. Bronnesen, N. C. Duncan, X. Zhang,
L. A. Watson, G. V. Berkel, W. B. Parson and B. P. Hay, J. Am.
Chem. Soc., 2012, 134, 8525–8534; (i) S. Yi, V. Brega,
B. Captain and A. E. Kaifer, Chem. Commun., 2012, 48,
10295–10297; ( j) R. Custelcean, P. V. Bronnesen,
B. D. Raoch and N. C. Duncan, Chem. Commun., 2012, 48,
7438–7440; (k) G. Mezei, P. Baran and R. G. Raptis, Angew.
Chem., Int. Ed., 2004, 43, 574–577; (l) I. R. Fernando,
S. A. Surmann, A. A. Urech, A. M. Poulsen and G. Mezei,
Chem. Commun., 2012, 48, 6860–6862; (m) Z. Yang, B. Wu,
X. Huang, Y. Liu, S. Li, Y. Xia, C. Jia and X.-J. Yang,
Chem. Commun., 2011, 47, 2880–2882; (n) S. Banerjee,
N. N. Adarsh and P. Dastidar, Eur. J. Inorg. Chem., 2010,
3770–3779; (o) B. Akhuli and P. Ghosh, Dalton Trans., 2013,
42, 5818–5825; (p) R. Custelcean, Chem. Commun., 2013,
49, 2173–2182; (q) J. Pang, F. Jiang, D. Yuan, J. Zheng,
M. Wu, G. Liu, K. Su and M. Hong, Chem. Sci., 2014, 5,
4163–4166.
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