Seven-coordinate anion complex with a tren-based urea: Binding discrepancy of hydrogen sulfate in solid and solution states

Article abstract of DOI:10.1039/c1ob05052d

Structural characterization of a hydrogen sulfate complex with a tren-based urea suggests that the anion is coordinated with six NH...O bonds (d _N..._O = 2.857 (3) to 3.092 (3) A) and one OH...O bond (d_O..._O = 2.

Full text of DOI:10.1039/c1ob05052d

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Seven-coordinate anion complex with a tren-based urea: Binding discrepancy
of hydrogen sulfate in solid and solution states†
Avijit Pramanik,a Bethtrice Thompson,a Trina Hayes,a Kimberly Tucker,a Douglas R. Powell,b
Peter V. Bonnesen,c Erick D. Ellis,a Ken S. Lee,a Hongtao Yua and Md. Alamgir Hossain*a
Received 10th January 2011, Accepted 7th March 2011
DOI: 10.1039/c1ob05052d
12
˚
Structural characterization of a hydrogen sulfate complex
with a tren-based urea suggests that the anion is coordinated
(dO ◊ ◊ ◊ O = 2.71 and 2.79 A). Wu et al. have characterized a
multiple coordinate sulfate complex with a tris(3-pyridyl)urea
showing eleven NH ◊ ◊ ◊ O hydrogen bonds (dN ◊ ◊ ◊ O < 3.2 A).
In this communication, we report a heptacoordinated hydrogen
sulfate resulting from the coordination of three tren-based ureas
(L) via six NH ◊ ◊ ◊ O bonds (dN ◊ ◊ ◊ O = 2.857(3) to 3.092(3) A) and
one OH ◊ ◊ ◊ O bond (dO ◊ ◊ ◊ O = 2.57(2) A), and its solution binding
8b
˚
˚
with six NH ◊ ◊ ◊ O bonds (dN ◊ ◊ ◊ O = 2.857 (3) to 3.092 (3) A) and
˚
one OH ◊ ◊ ◊ O bond (dO ◊ ◊ ◊ O = 2.57 (2) A) from three receptors;
however, in solution the anion is bound within the pseudo-
cavity of one receptor.
˚
˚
studies.
Sulfate plays an important role in many biochemical processes.1
The sulfate-binding protein (SBP) was structurally identified in
1985 by Pflugrath and Quiocho, in which the amino acid residues
are involved in hydrogen bonding interactions to form a seven-
coordinate anion complex.2 In this structure, the sulfate is present
in a cleft between the two lobes of SBP and bound by seven
hydrogen bonds: five from peptide NH groups (Ala173, Asp11,
Gly131, Gly132, Ser45), one from a tryptophan side chain NH
group (Trp192), and one from a serine OH group (Ser130).
Sulfate is also of significant environmental interest.3 Therefore, an
increasing attention has recently been devoted to sulfate binding
with synthetic receptors which include polyamines,4 polyamides,5
ureas,6 indoles,7 self-assembled metal–organic hosts,8 and tria-
zolium hosts.9 The oxygen atoms in oxoanions are generally
coordinated with two hydrogen bonds, thus sulfate with four
oxygens is often found to be octacoordinated with synthetic
receptors.5a,10 Custelcean et al. have shown that a tren-based
urea linked with Ag2SO4 is capable of binding a sulfate by
The tripodal urea L was synthesized from the reaction of
tren and p-cyanophenyl isocyanate in CHCl3. Crystallographic
analysis‡ of the free urea reveals that the ligand forms a pseudo-
cavity consisting of three arms (Fig. 1). One oxygen is involved in
intermolecular hydrogen bonding interactions with two NH’s of
another arm. The bisulfate salt was obtained from the reaction of
L with H2SO4 in ethanol at room temperature. The salt crystallizes
as [HL·(HSO4)] where the tertiary amine is protonated. The proton
on the tertiary nitrogen is pointed inside the tripodal cavity formed
by the three arms, and is hydrogen-bonded with one carbonyl
˚
oxygen of a urea group (N1 ◊ ◊ ◊ O34 = 2.783(2) A). The molecule
contains a singly-charged hydrogen sulfate to balance the charge of
the cationic host. However, the anion is not encapsulated; it is held
outside the cavity via two H-bonds of N18 ◊ ◊ ◊ O1 and N21 ◊ ◊ ◊ O1
˚
(dN ◊ ◊ ◊ O = 2.981(2) and 3.020(3) A, respectively).
8a
˚
twelve hydrogen bonds (dN ◊ ◊ ◊ O = 2.8516 to 3.1741 A), which
is consistent with Hay’s prediction that each oxygen could be
involved in a maximum of three hydrogen bonds.11 Bowman-James
et al. have reported a ten-coordinate sulfate complex stabilized in
˚
a tetramide-based host with four amides (dN ◊ ◊ ◊ O = 3.06–3.31 A),
˚
two protonated amines (dN ◊ ◊ ◊ O = 2.76 A), and four H2O molecules
aDepartment of Chemistry and Biochemistry, Jackson State University,
1325 J. R. Lynch Street, P.O. Box 17910, Jackson, MS 39212, USA.
E-mail: alamgir@chem.jsums.edu; Fax: +1 601-979-3674; Tel: +1 601-979-
3748
bDepartment of Chemistry and Biochemistry, University of Oklahoma,
Norman, OK 73019, USA
Fig. 1 Tren-based urea (L) and its X-ray structure (1).
cChemical Sciences Division, Oak Ridge National Laboratory, P.O. Box
2008, Oak Ridge, TN 37831, USA
Examining the coordination environment of hydrogen sulfate
suggests that it is coordinated with multiple hydrogen bonds
comprised of several NH ◊ ◊ ◊ O bonds and one OH ◊ ◊ ◊ O bond
in the lattice framework formed by three tren ureas (Fig. 2).
† Electronic supplementary information (ESI) available: Synthesis and
NMR studies. CCDC reference numbers 795639 and 814969. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1ob05052d
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Fig. 3 The coordination environment of bisulfate showing seven hydro-
gen bonds from three ureas.
chemical shift of NH resonances of NMR spectra, as recorded with
an increasing amount of anion solution at room temperature, gave
the best fit for a 1 : 1 binding model, yielding a binding constant
of log K = 3.0 (Fig. 4).14 The observed binding constant is much
higher than that observed for singly-charged bisulfate in an amide-
based cryptand (log K =1.83(3)).4a The solution binding mode
was further confirmed by 2D NOESY NMR experiments of both
free ligand and hydrogen sulfate complex in DMSO-d6 (Fig. 5)
As shown in Fig. 5A, the free ligand shows two strong NOESY
contacts between Hb ◊ ◊ ◊ Hc and Ha ◊ ◊ ◊ Hb, an observation which is
also supported by single crystal structure of L showing distances
Fig. 2 X-ray structure of the hydrogen sulfate complex with L showing
seven hydrogen bonds (six NH ◊ ◊ ◊ O and one OH ◊ ◊ ◊ O bonds).
The coordination environment of the anion is shown in Fig. 2,
and the hydrogen bonding parameters are listed in Table 1.
The nature of the hydrogen-bonding interactions involved in the
complex can be evaluated from a correlation of H ◊ ◊ ◊ O distance
and N–H ◊ ◊ ◊ O angle in the strong hydrogen-bonding interactions
˚
of Hb ◊ ◊ ◊ Hc = 2.30 and Ha ◊ ◊ ◊ Hb = 2.08 A. Upon the addition of
bisulfate, Hc proton on aromatic ring shifts more downfield as
compared to Hd, leading the reversal of the relative position of
these two protons (see Fig S9a in ESI). The NOESY between Ha
and Hb is absent in the complex, while the contact between Hb
and Hc becomes significantly weaker; indicating a conformational
change of the ligand due to encapsulation of a bisulfate anion
(Fig. 6). Similar changes were reported by Schneider et al. in
the optimized structure of chloride complex of a tren-based urea
ligand.15 The 1 : 1 binding stoichiometry in DMSO-d6 was further
verified by the Job’s plot showing a maximum at the 0.5 mole
fraction of L (see ESI).
˚
˚
region of dN ◊ ◊ ◊ O < 3.2 A, dH ◊ ◊ ◊ O < 2.5 A and ∠N–H ◊ ◊ ◊ O >
140◦.8b,13 It is clear that all six NH ◊ ◊ ◊ O hydrogen bonds fall
within the hydrogen bonding range (see Fig. S6 in ESI). This
interpretation is further supported by the work from Ghosh et al.
for the 14-coordinate sulfate complex of a pentafluorophenyl-
substituted tripodal urea receptor (dN ◊ ◊ ◊ O = 2.852(6) to 3.217(6)
6g
˚
A), Custelcean et al. for the 12-coordinate sulfate within metal–
organic framework (dN ◊ ◊ ◊ O = 2.8516 to 3.1741 A),8a and Bowman-
˚
James for the sulfate complex of tetramide (N ◊ ◊ ◊ O = 3.06–3.31
A).12 We, therefore, conclude that hydrogen sulfate is coordinated
by a total of seven hydrogen bonds comprised of six NH ◊ ◊ ◊ O
bonds and one OH ◊ ◊ ◊ O bond (Fig. 3). While each of the three
tren ureas contributes two hydrogen-bond donors via NH groups
from a single arm, one tren unit acts as a hydrogen-bond acceptor
through its carbonyl oxygen.
1
In order to evaluate the binding affinity of L for anions, H
NMR titration studies were performed in DMSO-d6. The addition
of n-Bu4NHSO4 to L resulted in a significant downfield shift of
both NH signals (Ddmax = 0.87 and 0.60 ppm), suggesting an
interaction of NH groups with the anion. The change in the
Table 1 H-bonding parameters (A, ◦) for the bisulfate complex of L
˚
D—H ◊ ◊ ◊ O
H ◊ ◊ ◊ O
D ◊ ◊ ◊ O
∠DHO
O1—H1A ◊ ◊ ◊ O20a
N4—H4 ◊ ◊ ◊ O3b
N7—H7 ◊ ◊ ◊ O2b
N18—H18 ◊ ◊ ◊ O1
N21—H21 ◊ ◊ ◊ O1
N32—H32 ◊ ◊ ◊ O4a
N35—H35 ◊ ◊ ◊ O4a
1.79(3)
2.15(3)
2.12(3)
2.23(2)
2.20(2)
2.40(2)
2.06(3)
2.573(2)
2.979(3)
2.936(3)
2.981(2)
3.020(3)
3.092(3)
2.857(3)
178(3)
169(2)
171(2)
159(2)
153(2)
142(2)
170(2)
Fig. 4 1H NMR titration curves of L (2 mM) with n-Bu NHSO ( , ᭡)
᭹
4
4
and ZnSO4 ( , ꢀ) in DMSO-d6. Net changes in the chemical shifts of
᭺
NH are shown against the increasing amount of anion (20 mM). H1 =
CH2NHCO and H2 = CONHAr.
1
1
12 -x, -21 +y, 2 -z, b 21 -x, 12 +y, 21 -z.
a
We also performed H NMR titrations for other oxoanions:
SO42-, H2PO4 , ClO4 and NO3 in DMSO-d6.16 The results show
-
-
-
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of Energy. This work was supported by National Institutes of
Health, Division of National Center for Research Resources,
under Grant Number G12RR013459 (MAH). This material is
based upon work supported by the National Science Foundation
under CHE-0821357 (500 MHz NMR). The authors thank the
National Science Foundation (CHE-0130835) and the University
of Oklahoma for funds to acquire the diffractometer used in
this work. National Science Foundation is acknowledged for a
CAREER award (CHE-1056927) to MAH.
Notes and references
˚
‡ Crystal data for L: C30H30N10O3, M = 578.64, tricl◦inic, a = 8.7312(11) A,
Fig. 5 2D NOESY NMR experiments of L in absence (A) and presence
(B) of hydrogen sulfate (5 equiv.) in DMSO-d6.
◦
˚
˚
b = 12.8400(17) A, c = 13.6820(18) A, a = 91.989(3) , b = 107.888(2) , g =
◦
3
¯
˚
100.753(2) , V = 1427.1(3) A , T = 100(2)K, space group P 1, Z = 2, 16034
reflections measured, 7004 independent reflections (Rint = 0.0320). The final
R1 values were 0.0495 (I > 2s(I)). The final wR(F2) values were 0.1166 (I >
2s(I)). CCDC 814969. Crystal data for HL+·HSO4-: C30H31N10O3·HO4S,
˚
˚
˚
M = 676.72, monoclinic, a = 12.696(2) A, b = 12.411(2) A, c = 20.491(4) A,
◦
3
˚
b = 103.912(9) , V = 3134.1(9) A , T = 100(2)K, space group P21/n, Z = 4,
29878 reflections measured, 7849 independent reflections (Rint = 0.0716).
The final R1 value was 0.0587 (I > 2s(I)). The final wR(F2) value was
0.1188 (I > 2s(I)). CCDC 795639.
1 (a) M. M. G. Antonisse and D. N. Reinhoudt, Chem. Commun., 1998,
443–448; (b) J. L. Atwood, K. T. Holman and J. W. Steed, Chem.
Commun., 1996, 1401–1407; (c) P. A. Gale, Coord. Chem. Rev., 2003,
240, 191–221; (d) D. P. Cormode, S. S. Murray, A. R. Cowley and P. D.
Beer, Dalton Trans., 2006, 5135–5140.
2 J. W. Pflugrath and F. A. Quiocho, Nature, 1985, 314, 257–260.
3 B. A. Moyer, L. H. Delmau, C. J. Fowler, A. Ruas, D. A. Bostick, J. L.
Sessler, E. Katayev, G. D. Pantos, J. M. Llinares, M. A. Hossain, S. O.
Kang and K. Bowman-James, Adv. Inorg. Chem., 2007, 59, 175–204.
4 (a) S. O. Kang, M. A. Hossain, D. Powell and K. Bowman-James,
Chem. Commun., 2005, 328–330; (b) G. Mani, T. Guchhait, R. Kumar
and S. Kumar, Org. Lett., 2010, 12, 3910–3913; (c) J. S. Mendy, M.
Pilate, T. Horne, V. W. Day and M. A Hossain, Chem. Commun., 2010,
46, 6084–6086; (d) M. A. Saeed, F. R. Fronczek, D. R. Powell and M. A.
Hossain, Tetrahedron Lett., 2010, 51, 4233–4236.
Fig. 6 Proposed binding mode of L for hydrogen sulfate in solution.
2-
that the host forms a strong 1 : 1 complex with SO4 giving the
binding constant of log K = 4.7 which is comparable to that
observed for a pentafluorophenyl-substituted tripodal urea (K =
4.72)6h or nitrophenyl-substituted tripodal urea log K = 4.97).6c
2-
-
Clearly, an additional charge on SO4 compared to HSO4 plays
a role for stronger electrostatic interactions, resulting in a stronger
-
binding of SO42-. The ligand also binds H2PO4 strongly with the
binding constant of log K = 4.2. On the other hand, the addition
of ClO4- and NO3- did not result in any appreciable change in the
NMR resonances (see ESI). Therefore, the binding largely depends
on the relative basicity of the anions included in this study and is
in accordance with the Hofmeister series.17
5 (a) M. A. Hossain, J. M. Llinares, D. Powell and K. Bowman-
James, Inorg. Chem., 2001, 40, 2936–2937; (b) S. O. Kang, V. W.
Day and K. Bowman-James, Org. Lett., 2009, 11, 3654–3657; (c) P. S.
Lakshminarayanan, E. Suresh and P. Ghosh, Inorg. Chem., 2006, 45,
4373–4380.
6 (a) B. Wu, X.-J. Yang, C. Janiak and P. Gerhard Lassahn, Chem.
Commun., 2003, 902–903; (b) B. Wu, X. Huang, Y. Xia, X.-J. Yang
and C. Janiak, CrystEngComm, 2007, 9, 676–685; (c) D. A. Jose, D. K.
Kumar, B. Ganguly and A. Das, Inorg. Chem., 2007, 46, 5817–5819;
(d) F. Zhuge, B. Wu, J. Liang, J. Yang, Y. Liu, C. Jia, C. Janiak,
N. Tang and X. Yang, Inorg. Chem., 2009, 48, 10249–10256; (e) R.
Custelcean, V. Sellin and B. A. Moyer, Chem. Commun., 2007, 1541–
1543; (f) R. Custelcean, J. Bosano, P. V. Bonnesen, V. Kertesz and B. P.
Hay, Angew. Chem., Int. Ed., 2009, 48, 4025–4029; (g) I. Ravikumar
and P. Ghosh, Chem. Commun., 2010, 46, 1082–1084; (h) I. Ravikumar,
P. S. Lakshminarayanan, M. Arunachalam, E. Suresh and P. Ghosh,
Dalton Trans., 2009, 4160–4168.
In summary, we have presented a seven coordinate complex of
hydrogen sulfate formed by three tren-based receptors in solid
state. The anion is coordinated with six NH ◊ ◊ ◊ O bonds and one
OH ◊ ◊ ◊ O hydrogen bond. In contrast, the ligand was found to
encapsulate a single anion within its cavity in solution, suggesting
an obvious discrepancy of binding mode from that observed
in solid state. While many examples exist for seven-coordinate
complexes with metal ions,18 to the best of our knowledge, there is
just one structure of seven-coordinate anion complex reported by
Bowman-James and coworkers, where one sulfate is encapsulated
by an amide-based cryptand with four hydrogen-bonds to the
cryptand and three additional hydrogen-bonds to the crystalline
water molecules.19 The seven coordinate complex in our case has
been resulted from the packing influence of the crystal, which
represents a rare example of a heptacoordinated anion with a
synthetic receptor.
7 (a) J. Kim, H. Juwarker, X. Liu, M. S. Lah and K. S. Jeong, Chem.
Commun., 2010, 46, 764–766; (b) P. A. Gale, J. R. Hiscock, C. Z. Jie,
M. B. Hursthouse and M. E. Light, Chem. Sci., 2010, 1, 215–220.
8 (a) R. Custelcean, B. A. Moyer and B. P. Hay, Chem. Commun., 2005,
5971–5973; (b) B. Wu, J. Liang, J. Yang, C. Jia, X.-J. Yang, H. Zhang,
N Tangb and C. Janiak, Chem. Commun., 2008, 1762–1764.
9 B. Schulze, C. Friebe, M. D. Hager, W. Gu¨nther, U. Ko¨hn, B. O. Jahn,
H. Go¨rls and U. S. Schubert, Org. Lett., 2010, 12, 2710–2713.
10 K. Bowman-James, Acc. Chem. Res., 2005, 38, 671–678.
11 B. P. Hay, T. K. Firman and B. A. Moyer, J. Am. Chem. Soc., 2005,
127, 1810–1811.
BT, TH and KT were supported by DOE under the Faculty
and Student Teams (FaST) Program at ORNL hosted by Dr.
Bruce A. Moyer. Participation of PVB and MAH was spon-
sored by the Division of Chemical Sciences, Geosciences, and
Biosciences, Office of Basic Energy Sciences, U.S. Department
12 S. O. Kang, V. W. Day and K. Bowman-James, Org. Lett., 2009, 11,
3654–3657.
13 B. Pirard, G. Baudoux and F. Durant, Acta Crystallogr., Sect. B: Struct.
Sci., 1995, B51, 103–107.
4446 | Org. Biomol. Chem., 2011, 9, 4444–4447
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14 H. J. Schneider, R. Kramer, S. Simova and U. Schneider, J. Am. Chem.
Soc., 1988, 110, 6442–6448.
15 F. Werner and H. J. Schneider, Helv. Chim. Acta, 2000, 83, 465–
478.
16 Titrations with Na2SO4 were hampered due to the solubility problem
in DMSO-d6. We, therefore, used ZnSO4 which was previously used by
other groups in DMSO-d6.8b.
17 F. Hofmeister, Naunyn-Schmiedebergs Arch. Pharmacol., 1888, 24, 247–
260.
18 (a) F. A. Cotton, and G. Wilkinson, Advanced Inorganic Chemistry, 5th
ed., Wiley, New York, 1988, pp. 10-22; (b) L. Duan, A. Fischer, Y. Xu
and L. Sun, J. Am. Chem. Soc., 2009, 131, 10397–10399.
19 S. O. Kang, D. Powell and K. Bowman-James, J. Am. Chem. Soc., 2005,
127, 13478–13479.
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