Carboxylate complexation by 1,1′-(1,2-phenylene)bis(3-phenylurea) in solution and the solid state

Article abstract of DOI:10.1039/b508144k

A simple bis-urea containing anion receptor, synthesised from ortho-phenylenediamine, has been shown to have excellent selectivity for carboxylates in solution, with a crystal structure elucidation of the benzoate complex showing four hydrogen bonds betwe

Full text of DOI:10.1039/b508144k

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COMMUNICATION
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Carboxylate complexation by 1,19-(1,2-phenylene)bis(3-phenylurea) in
solution and the solid state{
Simon J. Brooks, Philip A. Gale* and Mark E. Light
Received (in Columbia, MO, USA) 8th June 2005, Accepted 2nd August 2005
First published as an Advance Article on the web 22nd August 2005
DOI: 10.1039/b508144k
The anion complexation properties of 1–3 were studied using 1H
A simple bis-urea containing anion receptor, synthesised from
NMR titration techniques in DMSO-d6–0.5% water. Stability
constants were obtained using the EQNMR computer program.12
The results are shown in Table 1 and reveal that compound 3 is
selective for carboxylate anions under these experimental condi-
tions (binding acetate with a stability constant of 3210 M21 and
benzoate with a stability constant of 1330 M21). A comparison of
the shift of the central NH protons of compounds 1, 2 and 3 upon
addition of acetate is shown in Fig. 1 illustrating the higher affinity
of the bis-urea for acetate as compared to the bis-amidopyrrole 2
or bis-amide 1. Presumably the more open binding site offered by
the bis-urea receptor allows the receptor to form four hydrogen
bonds to the anion in solution (Fig. 2) which is possibly
ortho-phenylenediamine, has been shown to have excellent
selectivity for carboxylates in solution, with a crystal structure
elucidation of the benzoate complex showing four hydrogen
bonds between the receptor and anion in the solid state.
Receptors for carboxylate anions are important for the recognition
of a variety of biomolecules and in particular amino acids.1 An
important sub-class of these receptors are those which contain urea
(or thiourea) groups and many systems containing either one2 or
two3 have been reported and shown to be excellent carboxylate
receptors and sensors.
Over the last five years we have investigated the anion binding
properties of structurally simple receptors containing amide,4 urea5
and pyrrole6 groups. Little work has been done on anion receptors
based upon ortho-phenylenediamine or analogous diamines.7 The
potential carboxylate complexation properties of these systems has
so far yet to be exploited. In 2000, Reinhoudt and co-workers
reported the anion binding ability of cyclic and acyclic receptors
containing two ortho-phenylenediamine based bis-urea units.8 This
work showed that these receptors were selective for dihydrogen
phosphate in DMSO. We wished to ‘extract’ the bis-urea unit and
investigate the intrinsic anion binding properties of 1,19-(1,2-
phenylene)bis(3-phenylurea) (3) and those of the structurally
related compounds 1 and 2. Whilst this work was in progress,
the anion (but not carboxylate) binding properties of 2 were
reported by Cheng and co-workers.9
…
further stabilised by intramolecular CH O hydrogen bonds.
Amidopyrroles form a convergent binding site and presumably, in
the case of receptor 2, the four hydrogen bond donor NH groups
cannot efficiently coordinate the two oxygen atoms present in the
anion.6
In 1995, Umezawa and co-workers reported the carboxylate
binding properties of bis-thiourea 4 and bis-urea 5. Compound 5
was reported to bind tetrabutylammonium acetate in a 1 : 1
stoichiometry with a stability constant of 43 M21 in DMSO-d6
solution whilst the bis-thiourea 4 bound acetate with a stability
constant of 470 M21 under the same conditions.13 The higher
stability constant observed for the 3–acetate complex (measured
under slightly different experimental conditions) may be due to the
lower degree of flexibility in the ortho-phenylenediamine system as
compared to 4 and 5 and hence a higher degree of preorganization.
However it must be noted that Umezawa’s systems contain
pendant butyl groups whereas 3 contains pendant phenyl moieties
and hence this comparison must be qualified.
Compound 1 was synthesised according to literature proce-
dures10 whilst compound 2 was synthesised by the same method
reported by Cheng.9 Compound 3 was originally reported in 1966
(see the ESI for details of the synthesis and characterisation of this
compound{).11
Table 1 Stability constants (M21) of compounds 1, 2 and 3 with a
variety of putative anionic guests (added as tetrabutylammonium salts)
at 298K in DMSO2d6/0.5% water.a In all cases 1 : 1 receptor : anion
stoichiometry was observed.
Compounds
Anion
1
2b
12
—
251
113
295
—
3
Cl2
13
—
98
43
149
—
43
Br2
CH3CO2
,10
3210
1330
732
2
2
C6H5CO2
H2PO4
HSO4
2
2
10
a
b
School of Chemistry, University of Southampton, Southampton, UK
SO17 1BJ. Fax: +44 23 80596805
{ Electronic supplementary information (ESI) available: Synthesis and
characterisation of compound 3 and 1H NMR titration profiles. See http://
dx.doi.org/10.1039/b508144k
Errors estimated to be no more than ¡ 10%. We have repeated
the anion complexation studies with compound 2 under slightly
different experimental conditions from those originally reported,9
however our results are broadly similar.
4696 | Chem. Commun., 2005, 4696–4698
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Fig. 1 Shift of the resonance of the central NH groups in compounds 1
(r), 2 (&) and 3 (m) upon addition of tetrabutylammonium acetate in
DMSO-d6–0.5%.
Fig. 4 The benzoate complex of receptor 3 showing the formation of
four hydrogen bonds between the receptor and carboxylate anion in the
solid state. One of the two independent complexes in the asymmetric unit
is shown. Tetrabutylammonium counter cations and non-acidic hydrogen
atoms have been omitted for clarity.
…
intermolecular hydrogen bond to an adjacent complex (N
˚
2.874(3) A).
O
Crystals of the tetrabutylammonium benzoate complex of
receptor 3 were grown by slow evaporation of an acetonitrile
solution of the receptor in the presence of excess tetrabutylammo-
nium benzoate.§ The asymmetric unit contains two complexes with
each benzoate anion bound between the two urea groups via four
Fig. 2 A proposed hydrogen bonding array formed between compound
3 and carboxylate anions in solution.
˚
hydrogen bonds in the range 2.740(4)–2.939(4) A (Fig. 4). To the
best of our knowledge this is the first crystallographically
characterized complex of a single carboxylate bridging between
two urea groups in a bis-urea based organic anion receptor.
The crystal structure of compound 2 binding acetate shows the
anion is bound in a non-symmetrical arrangement by three
hydrogen bonds whilst the solution studies show that 2 does not
possess as high an affinity for carboxylates as compound 3. The
solution studies illustrate the selectivity of compound 3 for
carboxylate anions in DMSO-d6–0.5% water whilst the crystal
structure of the benzoate complex of 3 shows the anion is bound
to the four urea NH groups. This relatively simple yet highly
effective hydrogen bonding motif may find application in
receptors for amino acids and we are currently working towards
that goal.
Crystals of the tetrabutylammonium acetate complex of
receptor 2 were grown by slow evaporation of an acetonitrile
solution of the receptor in the presence of excess tetrabutylammo-
nium acetate.{ The structure (Fig. 3) shows the acetate anion
…
˚
bound by two amide NH groups (N O 2.751(3) and 2.731(3) A)
…
˚
and one pyrrole NH group (N O 2.781(3) A). The other
pyrrole NH group is oriented out of the cavity forming an
Fig. 3 Hydrogen bonded chains in the acetate complex of receptor 2. In the solid state the acetate anion is bound by three hydrogen bonds from the
receptor with one pyrrole group oriented to form an intermolecular hydrogen bond to an adjacent molecule. Tetrabutylammonium counter cations and
non-acidic hydrogen atoms have been omitted for clarity.
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For a seminal early paper describing the hydrogen bonding ability of
We would like to thank the EPSRC for a DTA studentship
(SJB) and the EPSRC together with Professor Mike Hursthouse
for the use of the crystallographic facilities at the University of
Southampton. PAG would like to thank the Royal Society for a
University Research Fellowship.
ureas with other hydrogen bond acceptors in the solid state see:
M. C. Etter, Z. Urban˜czyk-Lipkoska, M. Zia-Ebrahimi and
T. W. Panunto, J. Am. Chem. Soc., 1990, 112, 8415.
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4 (a) C. R. Bondy, P. A. Gale and S. J. Loeb, Chem. Commun., 2001, 729;
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K. Navakhun and G. L. Thomas, Chem. Commun., 2003, 568; (c)
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Notes and references
{ Crystal data for 2–tetrabutylammonium acetate C34H53N5O4,
Mr 5 595.81, T 5 120(2) K, monoclinic, space group P21/c, a 5 9.577(3),
3
˚
˚
b 5 19.020(8), c 5 18.309(5) A, b 5 93.73(3)u, V 5 3328.0(18) A ,
rcalc 5 1.189 g cm23, m 5 0.078 mm21, Z 5 4, reflections collected: 37336,
independent reflections: 6567 (Rint 5 0.1337), final R indices [I . 2s(I)]:
R1 5 0.0683, wR2 5 0.1406, R indices (all data): R1 5 0.1754.
wR2 5 0.1791. CCDC 278725.
5 C. R. Bondy, P. A. Gale and S. J. Loeb, J. Am. Chem. Soc., 2004, 126,
5030.
6 For an overview see: (a) P. A. Gale, Chem. Commun., 2005, 3761; (b)
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K. E. Sliwinski and B. D. Smith, Chem. Commun., 2005, 3773; (c)
I. E. D. Vega, S. Camiolo, P. A. Gale, M. B. Hursthouse and
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7 For an example of a 1,2-urea functionalised anthraquinone see: (a)
D. A. Jose, D. K. Kumar, B. Ganguly and A. Das, Org. Lett., 2004, 6,
3445; (b) For a macrocyclic system containing a ferrocene and
naphthalene see: F. Oto´n, A. Ta´rraga, M. D. Velasco, A. Espinosa
and P. Molina, Chem. Commun., 2004, 1658; (c) For a naphthalene
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§ Crystal data for 3-tetrabutylammonium benzoate C43H58N5O4,
Mr 5 708.94, T 5 120(2) K, monoclinic, space group P21/n, a 5 8.478(2),
3
˚
˚
b 5 38.654(10), c 5 24.402(7) A, b 5 93.68(3)u, V 5 7980(4) A ,
rcalc 5 1.180 g cm23, m 5 0.076 mm21, Z 5 8, reflections collected: 60181,
independent reflections: 15477 (Rint 5 0.1747), final R indices [I . 2s(I)]:
R1 5 0.0795, wR2 5 0.1442, R indices (all data): R1 5 0.2583.
wR2 5 0.2035. CCDC 278726. See http://dx.doi.org/10.1039/b508144k
for crystallographic data in CIF or other electronic format.
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4698 | Chem. Commun., 2005, 4696–4698
This journal is ß The Royal Society of Chemistry 2005