Anion binding of tris-(thio)urea ligands

Article abstract of DOI:10.1039/c3nj00401e

Two tridentate tris-(thio)urea ligands (L² and L³) have been synthesized. Ligand L² forms a 2:2 complex (1) with the (H₂PO₄·HPO₄)^3- dimer. ¹H NMR and UV-vis titrations revealed that the mixed urea-thiourea ligand, L², shows considerable anion binding affinities, while the tris-thiourea ligand, L³, undergoes deprotonation in the presence of basic anions.

Full text of DOI:10.1039/c3nj00401e

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LETTER
Anion binding of tris-(thio)urea ligands†
Yanlei Zhang,a Rui Zhang,a Yanxia Zhao,b Liguo Ji,a Chuandong Jiab and Biao Wu*a
Cite this: NewJ.Chem., 2013,
37, 2266
Received (in Victoria, Australia)
17th April 2013,
Accepted 22nd May 2013
DOI: 10.1039/c3nj00401e
www.rsc.org/njc
Two tridentate tris-(thio)urea ligands (L2 and L3) have been synthesized.
Ligand L2 forms a 2 : 2 complex (1) with the (H2PO4ꢀHPO4)3– dimer.
1H NMR and UV-vis titrations revealed that the mixed urea–
thiourea ligand, L2, shows considerable anion binding affinities,
while the tris-thiourea ligand, L3, undergoes deprotonation in the
presence of basic anions.
Anion coordination chemistry has been an active field of
research in the past several decades.1 The design and synthesis
of abiotic receptors for biologically and/or chemically impor-
tant anions are of great importance and many elaborately
designed anion ligands have been reported.2 Among the various
anion binding functionalities, the urea and thiourea groups are
excellent candidates because they can establish two directional
hydrogen bonds with an oxoanion or chelate a spherical anion.3
Following the primary work of Wilcox4 and Hamilton,5 a variety
of anion receptors have been reported in which one or more
urea/thiourea fragments are incorporated into acyclic, cyclic or
polycyclic backbones to achieve efficient anion binding.6
We have recently reported a series of o-phenylene-bridged7
oligourea receptors, which exhibit high affinity and selectivity
toward tetrahedral anions.8 For example, the tris(urea) receptor,
L1, can form a 2 : 1 (host-to-anion) complex with the ortho-
phosphate ion in a highly complementary coordination mode.8d
In order to further explore the anion coordination behavior of
such ‘‘tridentate’’ ligands, in the present work we modified the
trisurea L1 ligand by replacing one (the central) or three of the
Scheme 1 Structures of ligands L1–L3.
urea groups by the more acidic thiourea group(s) (pKa = 21.1
for thiourea and 26.9 for urea, respectively, in DMSO)9 and
synthesized two thiourea analogues, L2 and L3 (Scheme 1).
Herein we report the synthesis of the bisurea–thiourea L2
and tris(thiourea) L3 ligands and an anion complex of L2,
(TBA)3[(L2)2ꢀHPO4ꢀH2PO4]ꢀ2.9CH2Cl2 (1), as well as their anion
binding properties in solution.
Ligands L2 and L3 were synthesized by the reaction of
p-nitrophenylisocyanate or p-nitrophenylisothiocyanate, respectively,
with 1,3-bis(2-aminophenyl)thiourea (see the ESI† for details). Crystals
of the anion complex, (TBA)3[(L2)2ꢀHPO4ꢀH2PO4]ꢀ2.9CH2Cl2 (1),
were obtained by slow diffusion of diethyl ether into a dichloro-
methane solution of ligand L2 in the presence of an excess of
(TBA)H2PO4 (TBA = tetrabutylammonium).
Complex 1 crystallizes in the space group P2/c. The asym-
metric unit contains one L2 ligand, one CH2Cl2 molecule and
two TBA+ cations all in general positions, as well as one half
TBA+ cation lying about a twofold axis (with N9 atoms on the
twofold axis) and one phosphate anion in a general position
adjacent to a twofold axis. Notably, although complex 1 was
obtained from the dihydrogen phosphate salt (TBA)H2PO4,
it displays a 2 : 2 coordination mode in which two phosphate
a Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the
Ministry of Education, College of Chemistry and Materials Science,
Northwest University, Xi’an 710069, China. E-mail: wubiao@nwu.edu.cn
b State Key Laboratory for Oxo Synthesis & Selective Oxidation,
Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
† Electronic supplementary information (ESI) available: Experimental details,
synthesis and characterization, 1H NMR titrations, UV-vis titrations, and crystal
data. CCDC 933251. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3nj00401e
ions in different protonation states (HPO4 and H2PO4ꢁ) are
2–
c
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reduced Schiff base ligands13 and the hydrogenphosphate–
water chain bound by protonated polyamines.14
In the dimeric structure, each of the protonated phosphate
ions in the (HPO4ꢀH2PO4)3– unit is coordinated by the urea and
thiourea groups of a ligand molecule via six N–Hꢀ ꢀ ꢀO hydrogen
bonds. Thus, each phosphate ion indeed forms a total of nine
hydrogen bonds, including six N–Hꢀ ꢀ ꢀO and three P–Oꢀ ꢀ ꢀHO–P
2–
ꢁ
bonds (Table S1, ESI†). For both of the HPO4 and H2PO4
ions, the four NH donors of the two urea groups chelate one
oxygen atom (O7) of a phosphate ion, while the central thiourea
NH groups bind one oxygen (O8) through two hydrogen bonds
and this oxygen atom forms also an Oꢀ ꢀ ꢀH–O contact. The
remaining two oxygen atoms (O9 and O10) participate in a pair
of HPO4ꢀ ꢀ ꢀH2PO4 associations (Fig. 1c). The hydrogen bond
parameters of the urea and thiourea groups are comparable,
with an average Nꢀ ꢀ ꢀO distance of 2.916 Å and N–Hꢀ ꢀ ꢀO angle of
1521. In addition, the solvent CH2Cl2 molecule forms C–Hꢀ ꢀ ꢀO
and C–Hꢀ ꢀ ꢀS hydrogen bonds with the terminal nitro groups
and the sulfur atoms of the ligands (Table S1, ESI†).
Fig. 1 Crystal structure of complex 1. (a) Side view; (b) top view; (c) hydrogen
bonds around the (HPO4ꢀH2PO4)3– dimer (solvents and non-acidic hydrogen
atoms are omitted for clarity). Symmetry code (0): 1 ꢁ x, y, 1/2 ꢁ z.
1H NMR studies
The anion coordination properties of ligand L2 with different
anions in solution were studied using 1H NMR spectroscopy in
DMSO-d6–0.5% water. When 1.0 equiv. of PO43–, SO42–, or
ꢁ
sandwiched by two L2 molecules (Fig. 1a). The two phosphate H2PO4 ions (as TBA+ salts) was added, the NH protons of
ions are essentially indistinguishable due to the crystal- the urea and thiourea groups displayed significant downfield
imposed C2 symmetry, and the hydrogen atoms on them were shifts. Addition of AcOꢁ also induced some downfield shifts of
not added. The existence of the mixed states of the anions was the NH signals, due to the high basicity and complementary Y
proven by the number of TBA+ countercations (three TBA+ ions shape of AcOꢁ, as in the case of other (thio)urea ligands.15 The
per two phosphate ions) and the close contact of three pairs of very alkaline Fꢁ ions caused the NH signals to broaden (at lower
oxygen atoms belonging to two distinct anions (Oꢀ ꢀ ꢀO dis- equivalents of the anion) and disappear (at more Fꢁ ions),
tances: 2.469(6)–2.579(5) Å). This binding mode is different which has also been discussed in the literature.16 Other anions
from that in the complex of the trisurea ligand L1, which (HSO4ꢁ, Brꢁ, Iꢁ, NO3ꢁ, PF6ꢁ, and ClO4ꢁ) resulted in only slight
coordinates to the fully deprotonated phosphate (PO43–) ion or no changes (Fig. S1, ESI†).
in the 2 : 1 mode,8ꢁd although both complexes were isolated from
1H NMR titration experiments were subsequently carried out
the initial H2PO4 ion. Attempts to grow crystals of the analo- for L2 and selected anions, and the association constants were
gous 2 : 1 complex of the orthophosphate ion by using other determined using the WinEQNMR program (errors are less
phosphate salts, such as Na3PO4, K3PO4, {K([18]crown-6)}3PO4, than 10% in all cases).17 The association constant of L2 to
and (TBA)3PO4, have not yet been successful.
acetate ions was calculated to be log K = 3.06 Mꢁ1 based on the
In complex 1, the two anions are in close proximity, joined 1 : 1 binding mode (Fig. S2–S4, ESI†). For SO42ꢁ, the association
by three hydrogen bonds between HPO42– and H2PO4ꢁ to form constant (log K = 3.54 Mꢁ1) was also obtained by fitting the
a (HPO4ꢀH2PO4)3– dimer with a P–P distance of 3.586 Å (Fig. 1a titration curves to a 1 : 1 binding mode (Fig. S5 and S6, ESI†)
and c). The dimerization (or oligomerization and polymeriza- which was further confirmed from the Job plot. This binding
tion) of phosphate ions in their different protonation states has affinity is somewhat smaller than that of the trisurea L1 ligand
been observed in many cases.10 Very recently, an unusual for SO42ꢁ (log K = 4.60 Mꢁ1 in DMSO). The binding constant for
ꢁ
phosphate dimer, [H3PO4ꢀPO4]3–, was obtained by Jurczak H2PO4 could not be calculated because it is unable to deter-
et al., in which the two phosphate anions are also linked by mine the binding mode due to the coexistence of multiple
three hydrogen bonds and the P–P distance is 3.68 Å.11 We equilibria, possibly caused by the different protonation states
recently reported a [(HPO4)2]4ꢁ dimer coordinated by two bis- of the phosphate ion.
bisurea ligands, wherein the phosphate anions are dimerized
Notably, when 0.1 equiv. of PO43– ions (as Na+ salt, in DMSO-
by two hydrogen bonds.12 The sandwich structure of 1 is similar d6–5% water) was added to the solution of L2, a slow exchange
to that of this bis-bisurea complex of [(HPO4)2]4ꢁ, with the two process was observed in the H NMR spectrum. A new set of
1
ligand molecules being arranged in a head-to-tail manner signals appeared besides those of the ligand L2.8d The intensity
(Fig. 1b). On the other hand, the aggregation of phosphate of the new signals increased gradually with the addition of
ions with water molecules has also been reported, such as the PO4 ions, while the signals of free L2 decreased and disap-
3–
1D dihydrogenphosphate–water polymers (H2PO4ꢀH2O)nnꢁ with peared completely after 0.5 equiv. of the PO43– ions were added.
c
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Letter
Table 1 Association constants Ka (Mꢁ1) for receptors L2 and L3 toward various
anions in DMSO-d6-0.5% water at 298 K (errors o 15%)
AcOꢁ
Clꢁ
2ꢁ
ꢁ
3ꢁ
SO4
H2PO4
PO4
a
L2
L3
3.54
3.17
—
—
5.85b
—
3.06
—
2.47
—
a
c
c
d
a
b
Data could not be fitted to any model. Data for DMSO-25% water by
UV-vis titration (the data for DMSO-d6-0.5% water could not be fitted).
c
d
Peak broadening. No significant shift of NH peaks.
Fig. 2 1H NMR titration of L2 (5 mM) with Na3PO4 (DMSO-d6-5% water, 400 MHz).
addition of about 10 equiv. of the anion. Meanwhile, the
thiourea NHa protons also began to change after 1.0 equiv. of
Clꢁ ions was added, indicative of a second chloride ion being
bound by the thiourea group. The change in the thiourea was
not complete even after addition of 100 equiv. of the chloride
ions. This binding mode is similar to that of the oligo(thiourea)
receptors designed by Pfeffer et al.18
There were no further changes with the addition of more anions,
3–
indicating that the ligand (L2) and PO4 ions adopt a 2 : 1
binding mode (Fig. 2). This is similar to the phosphate coordina-
tion behavior of the analogous ligand L1. However, the thiourea
(NHa) protons disappeared during the titration process, suggest-
ing that the more acidic thiourea may undergo deprotonation in
the presence of the basic anion. This phenomenon was observed
in a number of thiourea receptors, e.g., Dehaen et al. reported
a series of oxacalix[2]arene[2]-pyrimidine-based bis(thio)ureido
receptors, where the more acidic NH proton resonance disap-
peared when 1.0 equiv. of H2PO4ꢁ or AcOꢁ ions was added to the
host solution.6f Fabbrizzi et al. reported that a thiourea receptor
led to the formation of complexes which are intrinsically stabꢁle
The changes in the urea NHb and NHc protons were fitted to
the 1 : 1 binding mode, with a binding constant of log K1
=
2.47 Mꢁ1 (Table 1, Fig. S7 and S8, ESI†). The titration results are
consistent with the preliminary crystal structure of a dichloride
complex of L2, in which one chloride ion is coordinated by the
two urea groups while the other chloride is bound by the
central thiourea group. Unfortunately, the crystal data are too
poor to allow for further refinement (Fig. S18, ESI†). Moreover,
this 1: 2 binding is different from that of the trisurea analogue L1,
which binds only one chloride ion by using all of the three
urea groups. The difference may be attributed to the weaker
tendency of the thiourea sulfur to form hydrogen bonds with
the aryl rings, thus leading the NH donors to twist away and
bind another Clꢁ ion. On the other hand, the results also
demonstrated the interesting binding behavior of the mixed
urea–thiourea ligand L2 toward different anions, which shows
considerable affinity to less basic anions but deprotonation of
thiourea with strongly basic anions.
but are unstable with respect to HX (X = Fꢁ, CH3COOꢁ, H2PO4
,
C6H5COOꢁ) release and deprotonation, while for urea the situa-
tion is caused only by fluoride.6g
1H NMR titration of Clꢁ ions to ligand L2 indicated inter-
esting interactions between the ligand and the anion. When up
to 1.0 equiv. of chloride ions were added to L2, the NH signals
of the two terminal urea groups showed significant downfield
shifts (Fig. 3), while the central thiourea NH peaks displayed
almost no change, suggesting that the Clꢁ ion was firstly bound
by the two urea groups (NHb and NHc). Then, when more
chloride ions were added, the urea NHb and NHc signals
continued to move to a lower field and reached a plateau after
The anion binding behavior of the tristhiourea ligand L3 was
also tested (Fig. S9, ESI†). When 1.0 equiv. of AcOꢁ, H2PO4ꢁ, or
Fꢁ ions (as TBA+ salt) was added to the solution of L3, the NH
signals of ligand L3 broadened or disappeared. During the
3–
titration of PO4 ions, the NHa, NHb and NHc proton signals
broadened, and these signals disappeared completely after
0.5 equiv. of PO43– ions (as TBA+ salt) were added (Fig. S10, ESI†),
suggesting that the acidic thiourea NH protons were deproto-
nated in the presence of the basic anion. Meanwhile, the H3, H6
and H7 protons on the aromatic rings (see Scheme 1 for the
numbering of the protons) showed significant downfield shifts,
which may result from the enhanced intramolecular C–Hꢀ ꢀ ꢀS
interactions caused by the increased electron density on sulfur
upon the deprotonation of the thiourea groups. Besides, there
is an obvious color change from yellow to red (Fig. S11, ESI†).
Addition of sulfate ions (as TBA+ salt) to ligand L3 resulted in
2–
downfield shifts of the NH signals up to 1.0 equiv. of SO4
(Fig. S12 and S13, ESI†). However, it is noticeable that the NH
protons of L3 also showed complete deprotonation after 2.0 equiv.
2–
Fig. 3 Change in the chemical shifts of NH signals upon titration of L2 with
Clꢁ ions.
of the SO4 ions were added, which is accompanied by the
downfield shifts of the H3, H6 and H7 protons as in the case of
c
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the phosphate ion. The association constant of L3 in the and solution binding modes of the two ligands (and the
binding of the first equivalent of SO4 (1 : 1 binding mode) trisurea L1 ligand reported previously) show significant differ-
2ꢁ
was calculated to be log K = 3.17 Mꢁ1 (Table 1).
ences. The crystal structure of the 2 : 2 anion complex 1 with
ligand L2 shows an unusual (H2PO4ꢀHPO4)3– dimer sandwiched
between two ligand molecules. 1H NMR and UV-vis titration
experiments demonstrated that the mixed urea–thiourea ligand
L2 can form a 2 : 1 complex with an orthophosphate ion as in
the case of the tris(urea) analogue. However, the tris(thiourea)
L3 ligand undergoes deprotonation in the presence of basic
anions accompanied by color change of the solution, while the
urea analogues show deprotonation only in the presence of an
excess of the strongly basic anions (e.g. PO43–).
UV-vis studies
The anion binding properties of the ligands L2 and L3 were
further studied using UV-vis titration experiments in DMSO.
Among the various anions examined (added as TBA+ salts), only
ꢁ
PO43–, SO42–, Fꢁ, H2PO4 and AcOꢁ induced bathochromꢁic
shifts. Other anions (Clꢁ, Brꢁ, Iꢁ, PF6ꢁ, NO3ꢁ, HSO4ꢁ, ClO4
)
resulted in slight or no changes (Fig. 4, Fig. S14, ESI†), which is
in agreement with the NMR results.
The interaction between L2 and PO4 (as Na+ salt) was
3ꢁ
investigated using the UV-vis titration method. The Job plot
curve indicates a 2 : 1 (host–guest) binding mode (Fig. S15,
ESI†), which is consistent with the results of NMR titration
(in DMSO-d6–5% water). However, in a more competitive
environment (DMSO–25% water), the spectrum reached a
Experimental section
Synthesis of ligand L2
A solution of 1,3-bis(2-aminophenyl)thiourea (1b, see ESI† for
the synthesis) (0.26 g, 1.0 mmol) in 100 mL THF was added
dropwise into a 130 mL refluxing THF solution of p-nitrophenyl
isocyanate (0.36 g, 2.2 mmol). The mixture was refluxed for 5 h
and the precipitate was filtered off and washed several times
with toluene and diethyl ether and then dried under vacuum to
yield pure L2 as a yellow solid (0.46 g, 78%). M.p.: 227 1C.
1H NMR (400 MHz, DMSO-d6, ppm): d 10.01 (s, 2H, Hc), 9.37
(s, 2H, Ha), 8.323 (s, 2H, Hb), 8.16 (d, J = 8.0 Hz, 4H, H8), 7.81
(d, 2H, H3), 7.64 (d, J = 8.0 Hz, 4H, H7), 7.43 (d, J = 8.0 Hz, 4H,
H6), 7.23 (t, J = 4.0 Hz, 4H, H4), 7.23 (t, J = 4.0 Hz, 4H, H5).
13C NMR (100 MHz, DMSO-d6): d 157.7 (CS), 151.7 (CO), 146.4
(C), 139.7 (C), 134.0 (C), 132.4 (C), 130.5 (CH), 129.0 (CH), 127.7
(CH), 122.9 (CH). Anal. calcd for C27H22N8O6S: C, 55.28; H, 3.78;
N, 19.10. Found: C, 55.18; H, 3.66; N, 19.02%. ESI-MS: m/z 585.1
(17%), [L2 ꢁ H]ꢁ.
3ꢁ
plateau after 1.0 equiv. of PO4 ions (as Na+ salt) were added
(Fig. S16, ESI†), with an association constant of log K = 5.85 Mꢁ1
by fitting the titration curve to a 1 : 1 binding mode (Table 1).
For the tristhiourea L3 ligand, addition of PO43–, Fꢁ, H2PO4
,
ꢁ
SO42–, and AcOꢁ ions resulted in a clear red-shift of the low-
wavelength band, from 357 nm to about 402 nm (Fig. 4), with a
concomitant color change. This is consistent with the deproto-
nation of the thiourea groups by the basic anions. Such
deprotonation processes of the acidic thiourea protons have
been reported for many thiourea-based receptors.6f,g In contrast,
no deprotonation was observed for the bisurea–monothiourea
ligand L2 even in the presence of the strongly basic phosphate
anion. Fig. S17 (ESI†) shows [A(335 nm)/A(410 nm)] upon the
addition of 1.0 equiv. of various anions into the DMSO-d6–0.5%
water solution of ligands L2 and L3.
Synthesis of ligand L3
In summary, the tridentate monothiourea–bisurea ligand, L2,
and tris(thiourea) ligand, L3, have been synthesized and their
anion coordination properties investigated. Both the solid-state
1,3-Bis(2-aminophenyl)thiourea (0.10 g, 0.4 mmol) was added
to a solution of p-nitrophenyl isothiocyanate (0.17 g, 0.9 mmol)
in dry CH3CN (130 mL). The mixture was stirred at r.t. for about
30 h. The precipitate was filtered off and washed several times
with toluene and diethyl ether and then dried under vacuum to
yield pure L3 as a yellow solid (0.20 g, 81%). M.p.: 184 1C.
1H NMR (400 MHz, DMSO-d6, ppm): d 10.57 (s, 2H, Hc), 9.65
(s, 2H, Ha), 9.44 (s, 2H, Hb), 8.18 (d, J = 8.8 Hz, 4H, H8), 7.88
(d, J = 8.8 Hz, 4H, H7), 7.47 (m, 4H, H3 + H6), 7.25 (m, 4H,
H4 + H5). 13C NMR (100 MHz, DMSO-d6): d 180.7 (CS), 180.1
(CS), 148.1 (C), 142.7 (C), 134.4 (C), 134.0 (C), 128.3 (CH), 128.0
(CH), 126.9 (CH), 126.7 (CH), 124.5 (CH), 122.0 (CH). Anal.
calcd for C27H22N8O4S3: C, 52.41; H, 3.58; N, 18.11. Found: C,
52.23; H, 3.43; N, 18.05%. ESI-MS: m/z 619.1 (12%), [L3 + H]+;
641.1 (100%), [L3 + Na]+.
Synthesis of (TBA)3[(L2)2ꢀHPO4ꢀH2PO4]ꢀ2.9CH2Cl2 (1)
L2 (10 mg, 0.02 mmol) was mixed with (TBA)H2PO4 (0.05 mmol)
in CH2Cl2 (4 mL). After stirring for 1 h at room temperature,
a clear light yellow solution was obtained. Slow vapor diffu-
sion of diethyl ether into this solution provided yellow crystals
Fig. 4 UV-vis spectra of L3 (1.0 ꢂ 10–5 M) in DMSO-0.5% water (v/v), alone and in
the presence of 1.0 equiv. of various anions (added as TBA salts, PO43– as Na+ salt). of complex 1 within 3 days (11 mg, 51%). Anal. calcd for
c
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(TBA)3[(L2)2ꢀHPO4ꢀH2PO4]ꢀ2CH2Cl2 (C104H159Cl4N19O20P2S2): C,
55.19; H, 7.08; N, 11.76%. Found: C, 55.10; H, 7.02; N, 11.68%.
Crystal data for 1: C104.9H160.8Cl5.8N19O20P2S2, M = 2339.80,
monoclinic, space group P2/c, a = 17.251(3), b = 12.4954(19), c =
28.007(4) Å, b = 97.612(3)1, V = 5984.0(15) Å3, Z = 2, T = 293(2) K,
Dcalc = 1.299 g cmꢁ3, F(000) = 2488, m = 0.272 mmꢁ1, R1 [I > 2s(I)] =
0.0845, wR2 [I > 2s(I)] = 0.2508.
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