Chloride coordination by oligoureas: From mononuclear crescents to dinuclear foldamers

Article abstract of DOI:10.1021/ol2031153

A series of acyclic oligourea receptors which closely resemble the scaffolds and coordination behavior of oligopyridines have been synthesized. Assembly of the receptors with chloride ions afforded mononuclear anion complexes or dinuclear foldamers depend

Full text of DOI:10.1021/ol2031153

ORGANIC
LETTERS
2012
Vol. 14, No. 3
684–687
Chloride Coordination by Oligoureas:
From Mononuclear Crescents to Dinuclear
Foldamers
Biao Wu,*,† Chuandong Jia,‡ Xiaolei Wang,‡ Shaoguang Li,‡ Xiaojuan Huang,‡ and
Xiao-Juan Yang‡
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, and State Key Laboratory for Oxo Synthesis & Selective Oxidation,
Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
wubiao@nwu.edu.cn
Received November 21, 2011
ABSTRACT
A series of acyclic oligourea receptors which closely resemble the scaffolds and coordination behavior of oligopyridines have been synthesized.
Assembly of the receptors with chloride ions afforded mononuclear anion complexes or dinuclear foldamers depending on the number of the
urea groups.
Foldamers are “artificial folded molecular architec-
tures” which are stabilized by a collection of noncovalent
interactions between nonadjacent monomeric units
and/or hostÀguest interactions.1 In nature, folding of the
primary sequences is a common structural feature of
many biological molecules such as proteins, peptides
and oligonucleotides.2 Foldamers have found applica-
tions in many fields, such as molecular recognition, ca-
talysis, and materials science. To gain deeper understand-
ing of folding and the functions of the folded molecules,
many artificial foldamers have been developed. Most of
the synthetic molecules studied so far resemble more or
less intramolecular H-bonds between repetitive amide
units.3 Alternatively, foldamers can also form by hostÀ
guest interactions.4 While metal ions are the most widely
used guests for this purpose,5 neutral molecule-6 and
anion-directed7 foldamers are also known. Among these
† Northwest University.
‡ Lanzhou Institute of Chemical Physics.
(1) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180. (b) Hill, D. J.;
Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev. 2001, 101,
3893–4011.
(2) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G.
Science 1997, 277, 1793–1796. (b) Oh, K.; Jeong, K.-S.; Moore, J. S.
Nature 2001, 414, 889–893. (c) Goodman, C. M.; Choi, S.; Shandler, S.;
DeGrado, W. F. Nat. Chem. Biol. 2007, 3, 252–262.
(3) (a) Horne, W. S.; Gellman, S. H. Acc. Chem. Res. 2008, 41,
1399–1408. (b) Guo, L.; Almeida, A. M.; Zhang, W.; Reidenbach, A. G.;
Choi, S. H.; Guzei, I. A.; Gellman, S. H. J. Am. Chem. Soc. 2010, 132,
7868–7869. (c) Guichard, G.; Huc, I. Chem. Commun. 2011, 47, 5933–
5941. (d) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37,
63–68. (e) Monnee, M. C. F.; Marijne, M. F.; Brouwer, A. J.; Liskamp,
R. M. J. Tetrahedron Lett. 2000, 41, 7991–7995. (f) Gillies, E. R.; Deiss,
F.; Staedel, C.; Schmitter, J.-M.; Huc, I. Angew. Chem., Int. Ed. 2007, 46,
4081–4084. (g) Li, Z.-T.; Hou, J.-L.; Li, C. Acc. Chem. Res. 2008, 41,
1343–1353. (h) Yan, Y.; Qin, B.; Ren, C.; Chen, X.; Yip, Y. K.; Ye, R.;
Zhang, D.; Su, H.; Zeng, H. J. Am. Chem. Soc. 2010, 132, 5869–5879.
(i) Khakshoor, O.; Demeler, B.; Nowick, J. S. J. Am. Chem. Soc. 2007,
129, 5558–5569. (j) Gong, B. Acc. Chem. Res. 2008, 41, 1376–1386.
(k) Corbin, P. S.; Zimmerman, S. C. J. Am. Chem. Soc. 2000, 122,
3779–3780. (l) Fischer, L.; Guichard, G. Org. Biomol. Chem. 2010, 8,
3101–3117. (m) Tew, G. N.; Scott, R. W.; Klein, M. L.; DeGrado, B. Acc.
Chem. Res. 2010, 43, 30–39. (n) Gan, Q.; Ferrand, Y.; Bao, C.; Kauffmann,
(4) (a) Juwarker, H.; Suk, J.-m.; Jeong, K.-S. Chem. Soc. Rev. 2009,
38, 3316–3325. (b) Saraogi, I.; Hamilton, A. D. Chem. Soc. Rev. 2009, 38,
1726–1743.
(5) (a) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. Rev.
1997, 97, 2005–2062. (b) Prince, R. B.; Okada, T.; Moore, J. S. Angew.
Chem., Int. Ed. 1999, 38, 233–236.
(6) (a) Inouye, M.; Waki, M.; Abe, H. J. Am. Chem. Soc. 2004, 126,
2022–2027. (b) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem.
Soc. 2000, 122, 2758–2762.
(7) (a) Juwarker, H.; Jeong, K.-S. Chem. Soc. Rev. 2010, 39,
3664–3674. (b) Gale, P. A. Chem. Soc. Rev. 2010, 39, 3746–3771.
(c) Hua, Y.; Flood, A. H. J. Am. Chem. Soc. 2010, 132, 12838–12840.
(d) Hua, Y.; Ramabhadran, R. O.; Karty, J. A.; Raghavachari, K.;
Flood, A. H. Chem. Commun. 2011, 47, 5979–5981.
ꢀ
B.; Grelard, A.; Jiang, H.; Huc, I. Science 2011, 331, 1172–1175.
r
10.1021/ol2031153
Published on Web 01/12/2012
2012 American Chemical Society

systems, the anion binding foldamers are of great
biological significance because they may potentially
mimic the functions of natural anion channels8 or anion
transporters.9 There are a few examples that fall into
this category;7,10À12 yet, information of the exact folding
dimensions and other structural features of most anion-
binding foldamers remains rare due to the lack of crystal
structures.13
In this work, the longer analogues L3 and L4 were
synthesized (SI) and the binding of L1ÀL4 with the
chloride anion was investigated. We now report four
dinuclear foldamers as well as two mononuclear cres-
cents resulted from chloride coordination with these
oligoureas. All anion complexes were structurally
characterized, and the existence of the foldamers in
solution has also been confirmed by 1D and 2D (1H,
COSY, and NOESY) NMR spectroscopy.
We have recently developed a class of oligourea
receptors14 by mimicking the scaffolds of the well-known
transition-metal ligands, oligopyridines. Inspired by the
similarities of metal coordination and anion coordina-
tion,15 we designed a bis-bisurea ligand and obtained the
first triple anion helicate from this ligand and phosphate
ions.14b As a further step to the anion-binding helical struc-
tures, we synthesized a series of o-phenyl bridged oligour-
eas with gradually increasing chain length (tris(urea) L1,
tetrakis(urea) L2, pentakis(urea) L3, and hexakis(urea) L4;
Schemes S1ÀS3, Supporting Information (SI)). The
o-phenyl group has proven to be a proper bridge to con-
nect two urea groups for effective anion binding,14aÀd,16 and
these molecules are expected to show folding conformations
when coordinating to anions. The phosphate and sulfate
binding properties of the two shorter receptors (L1 and L2)
have been reported by us, and the tetrakis(urea) L2 shows
a tendency of folding when binding a sulfate ion.14c,d
Crystals of the six anion complexes were obtained by
slow diffusion of diethyl ether to the chloroform (for L1,
L2, L4) or chloroform/acetone (10:1 v/v; for L3) solutions
of the ligands in the presence of excess (TEA)Cl, (TPA)Cl,
or (TBA)Cl (TEA = tetraethylammonium, TPA =
tetrapropylammonium, and TBA = tetrabutylammonium).
The shortest ligand L1 forms two isomeric mononuclear
crescents with a chloride ion ((TBA)[L1Cl], 1a and 1b),
while the longer ones, L2ÀL4, form dinuclear foldamers
(TBA)2[L2Cl2] (2), (TEA)2[L3Cl2] CH3COCH3 (3),
3
(TBA)2[L4Cl2] 0.5Et2O (4a), and (TPA)2[L4Cl2] (4b)
3
(Figure 1). All the complexes (except the planar molecule 1b)
are racemic, containing equimolar M- and P-helices.
Each chloride ion is bound by three to seven H-bonds
with the N Cl distances ranging from 3.260 to 3.385 A
and NÀH Cl angles from 144.3ꢀ to 160.5ꢀ (Tables 1
3 3 3
3 3 3
and S1, SI).
Treatment of the tris(urea) L1 with (TBA)Cl afforded
two isomeric mononuclear crescents (1a and 1b). 1a
adopts such a conformation that one of the terminal
urea subunits lies out of the plane defined by the other
two urea groups (Figure 1a). The two terminal urea
groups bind a chloride ion by four H-bonds, while the
middle urea forms two intermolecular H-bonds which
connect adjacent molecules into an infinite ribbon. In
contrast, complex 1b adopts a nearly planar conforma-
tion where the three ureas occupy three edges of a
square, binding a chloride ion in the center with five
H-bonds (Figure 1b). The remaining NH binding site is
involved in an intermolecular H-bond with the urea
carbonyl of another molecule, thus linking two planar
crescents to a dimer (Figure S1). The electronic energies
of the two isomers were evaluated by DFT calculations,
which revealed that 1b is much more stable than 1a
(by 80.3 kcal molÀ1) in the gas phase. Complex 1b has
~
(8) (a) Bianchi, A.; Bowman-James, K.; Garcıa-Espana, E., Eds.
´
Supramolecular Chemistry of Anions; Wiley-VCH, Inc.: 1997. (b) Sessler,
J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry; Royal Society of
Chemistry, Cambridge, 2006; (c) Dutzler, R.; Campbell, E. B.; Cadene, M.;
Chait, B. T.; MacKinnon, R. Nature 2002, 415, 287–294. (d) Weinstein, S.;
Wallace, B. A.; Blout, E. R.; Morrow, J. S.; Veatch, W. Proc. Natl. Acad.
Sci. U.S.A. 1979, 76, 4230–4234.
(9) (a) Feng, L.; Campbell, E. B.; Hsiung, Y.; MacKinnon, R. Science
2010, 330, 635–641. (b) Mindell, J. A. Science 2010, 330, 601–602. (c)
Smith, B. D.; Lambert, T. N. Chem. Commun. 2003, 2261–2268. (d)
Davis, J. T.; Okunola, O.; Quesada, R. Chem. Soc. Rev. 2010, 39, 3843–
3862. (e) Brotherhood, P. R.; Davis, A. P. Chem. Soc. Rev. 2010, 39,
3633–3647. (f) Gale, P. A. Acc. Chem. Res. 2011, 44, 216–226.
(10) (a) Kim, J.-i.; Juwarker, H.; Liu, X.; Lah, M. S.; Jeong, K.-S.
Chem. Commun. 2010, 46, 764–766. (b) Chang, K.-J.; Moon, D.; Lah,
M. S.; Jeong, K.-S. J. Am. Chem. Soc. 2005, 127, 12214–12215. (c) Suk,
J.-m.; Jeong, K.-S. J. Am. Chem. Soc. 2008, 130, 11868–11869.
(d) Naidu, V. R.; Kim, M. C.; Suk, J.-m.; Kim, H.-J.; Lee, M.; Sim,
E.; Jeong, K.-S. Org. Lett. 2008, 10, 5373–5376. (e) Suk, J.-m.; Jeong,
K.-S. Bull. Korean Chem. Soc. 2011, 32, 2891–2892.
ꢀ
(11) (a) Ferrand, Y.; Kendhale, A. M.; Kauffmann, B.; Grelard, A.;
Marie, C.; Blot, V.; Pipelier, M.; Dubreuil, D; Huc, I. J. Am. Chem. Soc.
2010, 132, 7858–7859. (b) Xu, Y.-X.; Wang, G.-T.; Zhao, X.; Jiang,
X.-K.; Li, Z.-T. J. Org. Chem. 2009, 74, 7267–7273. (c) Wang, Y.; Xiang,
J.; Jiang, H. Chem.;Eur. J. 2011, 17, 613–619. (d) Shi, Z.-M.; Chen,
S.-G.; Zhao, X.; Jiang, X.-K.; Li, Z.-T. Org. Biomol. Chem. 2011, 9,
8122–8129.
(12) Haketa, Y.; Maeda, H. Chem.;Eur. J. 2011, 17, 1485–1492.
(13) For anion binding foldamers supported by crystal structures,
see refs 10a, 11a, and 12.
(14) (a) Jia, C.; Wu, B.; Li, S.; Huang, X.; Zhao, Q.; Li, Q.-S.; Yang,
X.-J. Angew. Chem., Int. Ed. 2011, 50, 486–490. (b) Li, S.; Jia, C.; Wu, B.;
Luo, Q.; Huang, X.; Yang, Z.; Li, Q.-S.; Yang, X.-J. Angew. Chem., Int.
Ed. 2011, 50, 5721–5724. (c) Jia, C.; Wu, B.; Li, S.; Yang, Z.; Zhao, Q.;
Liang, J.; Li, Q.-S.; Yang, X.-J. Chem. Commun. 2010, 46, 5376–5378.
(d) Jia, C.; Wu, B.; Li, S.; Huang, X.; Yang, X.-J. Org. Lett. 2010, 12,
5612–5615.
(15) (a) Lehn, J.-M. Acc. Chem. Res. 1978, 11, 49–57. (b) Lehn, J.-M.
Angew. Chem., Int. Ed. Engl. 1988, 27, 89–112. (c) Bowman-James, K.
Acc. Chem. Res. 2005, 38, 671–678. (d) Kang, S. O.; Llinares, J. M.; Day,
V. W.; Bowman-James, K. Chem. Soc. Rev. 2010, 39, 3980–4003.
(16) (a) Brooks, S. J.; Gale, P. A.; Light, M. E. Chem. Commun. 2005,
4696–4698. (b) Brooks, S. J.; Edwards, P. R.; Gale, P. A.; Light, M. E.
New J. Chem. 2006, 30, 65–70.
one more NÀH Cl contact than 1a, and the solid-
3 3 3
state structure of 1a may be stabilized by the formation
of the infinite chain of intermolecular urea urea
3 3 3
H-bonds.
The tetrakis(urea) L2 forms a dinuclear foldamer
(complex 2) with two chloride ions, in which the four
urea units are arranged along a square (Figure 1c). No-
tably, single-stranded dinuclear foldamers are relatively
rare in anion coordination.11c,d,12 In complex 2, the two
anions are located on the axis of the helix and each is
bound by four H-bonds from two alternating urea
groups, with a Cl Cl distance of 3.613(9) A. Consider-
ing that the sum of their ionic radii is only 3.62 A,17 such a
3 3 3
(17) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751–767.
Org. Lett., Vol. 14, No. 3, 2012
685

Table 1. Hydrogen Bonds (A and deg) Involved in Chloride
Binding and Cl Cl Separations in the Crystal Structures of
the Six Complexes
3 3 3
Cl1
average
Cl2
average
Cl1 Cl2
3 3 3
H-bond
d(N Cl) and
H-bond
d(N Cl) and
separation
3 3 3
3 3 3
˚
˚
˚
number —NHCl [A, deg] number —NHCl [A, deg]
[A]
1a
1b
2
4
5
4
5
3
6
3.260, 160.5
3.262, 144.3
3.340, 158.4
3.349, 153.9
3.322, 154.7
3.322, 156.6
À
À
4
À
À
À
À
3.331, 160.1
3.325, 153.2
3.385, 146.4
3.298, 153.1
3.613(9)
3.826(6)
3.881(8)
4.024(5)
3
5
4a
4b
7
4
Theoretical calculations (HartreeÀFock method) were
performed to optimize the structures of the free ligands.
The results revealed that the shorter L1 and L2 adopt the
expanded conformations without a preference of folding.
For the pentakis(urea) L3, four of the urea subunits
converge to a compact conformation through intra-
molecular H-bonds, but the remaining terminal urea arm
is oriented away. The longest ligand L4 displays a fold-
ing conformation similar to its chloride complex 4b.
These results imply that there is an increasing tendency
of self-folding as the number of the urea groups ex-
tends (Figure S3). While L2 and L3 form foldamers only
with the templation of chloride ions, the hexakis(urea) L4
tends to fold itself.
Figure 1. Crystal structures of (a) 1a, (b) 1b, (c) 2, (d) 3, (e) 4a,
and (f) 4b in top and side views (only the M-helices are shown;
shadows in 4a and 4b highlight the intramolecular H-bonds;
chloride ions are shown with the radius of 1.81 A; solvent
molecules, nonacidic H-atoms, and countercations are omitted
for clarity). Red lines: representative illustrations of the folding
conformations.
distance is quite unusual which has to overcome severe
electrostatic repulsion between the two ionic guests. In
the related oligopyrrole-based dinuclear foldamer, the
Cl Cl distance is 4.638(2)/4.632(2) A.12 In the present
3 3 3
case, the repulsion is possibly compensated by the eight
cooperative H-bonds as well as a strong π π stacking
3 3 3
interaction between the two terminal aryl groups (Table
S2 and Figure S2).
The square-like arrangement is maintained in the di-
nuclear foldamers of the pentakis(urea) L3, (TEA)2-
[L3Cl2] (3), and hexakis(urea) L4, (TBA)2[L4Cl2] 0.5Et2O
3
(4a) and (TPA)2[L4Cl2] (4b), which form 1.25 and 1.5
helical turns, respectively (Figure 1dÀf). Compared with
2, the electrostatic repulsion is released partially in the
Figure 2. Partial 1H NMR (400 MHz, CDCl3) spectra of L3
in the presence of various equivalents of (TBA)Cl (5 mM)
(indicated by black numbers).
longer analogues since the Cl Cl separation increases
3 3 3
gradually from 3.613(9) A in 2 to 4.024(5) A in 4b. On the
other hand, all NH sites in 3 participate in the binding
with the two chloride ions (each by five H-bonds), while
only ten of the twelve NH donors in 4a and 4b are
involved in anion binding. In 4a, the two chloride
ions are bound by three and seven H-bonds, while in
4b they are bound by six and four H-bonds, respectively.
The remaining two NH binding sites in 4a and 4b
form intramolecular H-bonds with the oxygen atom
The chloride binding properties of L1ÀL4 were inves-
1
tigated by H NMR experiments conducted in CDCl3.
For parallel comparison, the tetrabutyl ammonium
chloride (TBA)Cl was used in all cases. Interestingly,
though the ligands alone are hardly soluble in CDCl3,
they can dissolve in the presence of ClÀ due to the forma-
tion of the discrete chloride complexes. To completely
dissolve the receptor (5 mM), at least 1 equiv of ClÀ is
needed for L1 and L2 and 2 equiv for L3 and L4. These
solutions were used for further NMR titrations. Figure 2
shows the spectra of L3/2ClÀ which are well-resolved,
and the spectra of other receptors are given in Figure S4.
For L1/ClÀ, when more anions were added, all NH sig-
nals showed continuous downfield shifts which were not
of another urea. In both cases, the urea urea inter-
3 3 3
actions occur with one terminal urea unit, and the dif-
ference lies in its role as the H-bond donor (4a) or
acceptor (4b). DFT calculations showed that the
two isomers have almost the same energy (differing
by 2.0 kcal molÀ1).
686
Org. Lett., Vol. 14, No. 3, 2012

finished even after 25 equiv of ClÀ ions were added. The
NH signals of L2/ClÀ also showed downfield shifts, but
the main changes were completed with 2 equiv of ClÀ ions
and a sharp, saturated spectrum appeared after adding
2.5 equiv of chloride ions. Similarly, saturated spectra of
L3 (Figure 2) and L4 were achieved with approximately
2.5 and 2.0 equiv of ClÀ ions, respectively. Based on these
results, it may be concluded that, in the CDCl3 solution
with an excess of ClÀ (>2.5 equiv), L2, L3, and L4 show a
1:2 binding mode, while L1 may form multiple complexes
of higher order. In the anion binding by analogous acyclic
receptors, coexistence of multiple equilibria was also ob-
served.18 ESI-MS experiments in CHCl3 were performed.
Both the 1:1 and 1:2 (host/guest) chloride complexes of
L4 were observed, while only the 1:1 complex of L1, L2,
and L3 was detected (Figure S5).
clear assignment of the protons. We have also tested
other anions (FÀ, BrÀ, NO3À, AcOÀ, SO42À, 3 equiv, as
TBA salts), which could aid the dissolution of the ligands
(L3 and L4) in CDCl3 but showed poor dispersion of the
spectra (Figure S8). Hence, the system L3/3ClÀ was used
for 2D NMR (600 MHz, COSY and NOESY, in CDCl3)
studies. In the NOESY spectrum, cross-peaks are formed
between all adjacent NH protons, which is consistent with
the crystal structure of complex 4, wherein all NH pro-
tons point to the inside of the foldamer. Additional
supports for the foldamer are the cross-peaks between
NHe-NHc, NHb-CH9, and NHc-CH9 which are caused
by the through-space coupling. These cross-peaks are not
found in the COSY spectrum, thus confirming that they
result from the spatial effect (Figures S9 and S10).
Efforts were made to determine the binding affinity as
well as the binding stoichiometry (by the Job’s plot) by
UV/vis titrations in CHCl3-0.5% DMSO (DMSO was
used to dissolve the ligands, Figure S11). Unfortunately,
the colorimetric changes in DMSO are not large enough
to allow accurate determination. Nevertheless, the two-
step changes of UVÀvis spectra provided evidence for the
1:2 (host/guest) binding mode between L2ÀL4 and ClÀ.
Upon addition of ClÀ, the charge transfer bands showed
a continuous bathochromic shift until 2 equiv of ClÀ were
added. During the addition of 1 equiv of ClÀ, clear isos-
bestic points were formed indicating the formation of
only one single complex, possibly the 1:1 binding mode.
As more ClÀ ions were added, the newly emerged band
shifted away gradually from the isosbestic points and
reached saturation after addition of 2 equiv of ClÀ.
In summary, a series of dinuclear chloride-binding
foldamers have been obtained based on o-phenyl-bridged
oligoureas. A growing tendency for dinuclear foldamers
was elucidated with the increasing number of urea units.
This current work further proves the strategy for design-
ing anion ligands by simply translating the well developed
transition-metal ligands to anion binding scaffolds, which
have been successful in the construction of novel anion-
based architectures.
Partial conformational information of the complexes
can be obtained by comparing the NMR spectra of
L1ÀL4 alone and in the presence of ClÀ ions. The spectra
of free L1ÀL4 determined in DMSO-d6 displayed very
similar, highly overlapped signals (Figure S6), indicating
that the free ligands possibly adopt similar expanded
conformations. After 2 equiv of ClÀ ions were added,
the CH protons on the terminal p-nitrophenyl groups of
L1 shifted slightly downfield (Δδ: 0.05, 0.08 ppm). In
contrast, these CH protons of L2ÀL4 showed upfield
shifts (0.05À0.18 ppm) (Figure S7). The differences were
also observed in the spectra of L/2ClÀ recorded in CDCl3.
We suppose that L2ÀL4 might adopt folded conforma-
tions on binding chloride ions, which can result in shield-
ing effects on the terminal CH protons. However, there is
no such shielding in the crescent complexes of L1. On the
other hand, the chemical shifts in both DMSO and CDCl3
are better resolved in the presence of chloride ions. A high
1
dispersion of H NMR signals is usually thought to be
typically characteristic of a well-ordered solution confor-
mation.5b Thus these observations are consistent with the
putative folding conformations of the complexes 2, 3, and
4 in solution.
For further evidence of the folding in solution, 2D
NMR (in CDCl3) investigations have been performed. In
the case of L3/3ClÀ (an excess of chloride ions was added
to ensure the formation of the dinuclear foldamer) the
spectrum is well-resolved, but the signals for other ligands
and 3 equiv of ClÀ are not dispersed enough to allow
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20872149).
Supporting Information Available. Experimental de-
tails, X-ray data, NMR and UVÀvis titrations, and DFT
computations. This material is available free of charge via
the Internet at http://pubs.acs.org.
(18) Caltagirone, C.; Hiscock, J. R.; Hursthouse, M. B.; Light, M. E.;
Gale, P. A. Chem.;Eur. J. 2008, 14, 10236–10243.
Org. Lett., Vol. 14, No. 3, 2012
687