Three 2D AgI-framework isomers with helical structures controlled by the chirality of camphor-10-sulfonic acid

Article abstract of DOI:10.1039/c0dt01384f

Three 2D Ag^I-framework isomers were constructed from enantiopure camphor-10-sulfonic acids or racemic camphor-10-sulfonic acids, together with achiral 4-aminobenzoic acids. In complex 1, (+)-camphor-10-sulfonic acids bridge the single left-handed helices that are made up of Ag ions and 4-aminobenzoic acids, generating a homochiral 2D layer. In such a structure, the interweaving of triple left-handed homohelices was also found. It is worth noting that the helicity of complex 2 could be controlled by the handedness of the camphor-10-sulfonic acid. In complex 2, there are right-handed helical structures, including single right-handed and triple right-handed helical structures connected by (-)-camphor-10-sulfonic acids. For a comparative study, (±)-camphor-10-sulfonic acids were utilized to synthesize complex 3, in which equal numbers of right-handed or left-handed double-helical chains are created. All the complexes were characterized by single-crystal X-ray structure determination, powder X-ray diffraction, IR, TGA and element analysis. Circular dichroism spectra of complexes 1 and 2 were been studied to confirm the fact that enantiopure bridging ligands do not racemize.

Full text of DOI:10.1039/c0dt01384f

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PAPER
Three 2D AgI-framework isomers with helical structures controlled by the
chirality of camphor-10-sulfonic acid†
Peng Guo*
Received 13th October 2010, Accepted 9th December 2010
DOI: 10.1039/c0dt01384f
Three 2D AgI-framework isomers were constructed from enantiopure camphor-10-sulfonic acids or
racemic camphor-10-sulfonic acids, together with achiral 4-aminobenzoic acids. In complex 1,
(+)-camphor-10-sulfonic acids bridge the single left-handed helices that are made up of Ag ions and
4-aminobenzoic acids, generating a homochiral 2D layer. In such a structure, the interweaving of triple
left-handed homohelices was also found. It is worth noting that the helicity of complex 2 could be
controlled by the handedness of the camphor-10-sulfonic acid. In complex 2, there are right-handed
helical structures, including single right-handed and triple right-handed helical structures connected by
(-)-camphor-10-sulfonic acids. For a comparative study, ( )-camphor-10-sulfonic acids were utilized to
synthesize complex 3, in which equal numbers of right-handed or left-handed double-helical chains are
created. All the complexes were characterized by single-crystal X-ray structure determination, powder
X-ray diffraction, IR, TGA and element analysis. Circular dichroism spectra of complexes 1 and 2 were
been studied to confirm the fact that enantiopure bridging ligands do not racemize.
Therefore, in order to guarantee the homochirality of an individual
single-crystal and bulk materials, enantiopure bridging ligands,
which are not prone to racemization, can be utilized.16
Introduction
Increasing attention is being paid to the design and synthe-
sis of homochiral solids because of their potential application
in asymmetric catalysis,1 enantioselective separation2 and non-
linear optics.3 Recently, the flourishing research field of MOFs
(metal–organic frameworks) has provided an avenue to construct
homochiral crystalline solids by the availability of enantiopure
bridging ligands.4–10 Unfortunately, undesirable achiral crystalline
solids are often obtained owing to ligand racemization.11 Chiral
MOFs can also be assembled by achiral ligands and metal ions;
however, two phenomena hamper their potential application in
enantioselective separation and asymmetric catalysis: one is that
the single-crystals are racemic twins (Flack parameter = 0.5)
rather than homochiral ones, although they crystallize in chiral
space groups; the other is that the bulk materials tend to form
conglomerates in which homochiral right-handed and left-handed
molecules crystallize in equal proportion.12,13 In such a system,
homochiral bulk materials can be created by chiral catalysis
induction, and then be tested by a combination of X-ray single-
crystal diffraction and CD (circular dichroism) spectroscopy,
which have been extensively investigated by Bu and Morris.14,15
Simultaneously, chirality and absolute helicity are often closely
linked with each other in the same structure, although they
are two distinct concepts.17 Metal–organic frameworks also can
provide a model to better understand the relationship between
chirality and helicity. Camphorsulfonic acids are appropriate
candidates because they integrate the merits of a quite inert
chiral center with a good acceptor of hydrogen bonding (carbonyl
group C O). The latter merit possesses the potential function
to regulate helicity through hydrogen bonding. Sulfonic acid
prefers to coordinate with AgI ions rather than other transition
metals (such as Fe2+, Co2+, Ni2+), especially in the presence
of auxiliary N-containing secondary ligands, which has been
systematically investigated and elaborately elucidated by several
groups.18–25
Herein, we report two homochiral silver camphor-10-sulfonic
acid frameworks, [Ag(L2)(L1)]n (1) and [Ag(L3)(L1)]n (2) [L1 = 4-
aminobenzoic acid, L2 = (+)-camphor-10-sulfonic anion, L3 =
(-)-camphor-10-sulfonic anion] (Scheme 1), and one achiral
silver camphor-10-sulfonic acid, [Ag(L4)(L1)]n (3), involving the
racemic ( )-camphor-10-sulfonic acid ligand. It is worth noting
that the orientation of homohelices could be regulated by the
handedness of the camphor-10-sulfonic acid. Only right-handed
(P) examples are found in complex 2, unique left-handed (M)
examples exist in complex 1, while there are P and M examples in
complex 3.
State Key Laboratory of Rare Earth Resource Utilizations, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, P. R. China. E-mail: pguo@ciac.jl.cn; Tel: +86 0431-85262941
† Electronic supplementary information (ESI) available. CCDC reference
numbers 793353–793355. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01384f
1716 | Dalton Trans., 2011, 40, 1716–1721
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X-Ray Crystallography
The X-ray intensity data of compounds 1 and 2 were collected
on a Bruker SMART APEX CCD diffractometer with graphite
˚
monochromatized Mo-Ka radiation (l = 0.71073 A) operating
at 2.0 kW (50 kV, 40 mA). The structures were solved by the
direct method of SHELXS-9726 and refined with full-matrix
least-squares techniques using the SHELXL-97 program27 within
WINGX.28 The hydrogen atoms attached to carbons and nitrogens
were generated geometrically, while those attached to oxygens were
located from difference Fourier maps and refined with isotropic
displacement parameters. All non-hydrogen atoms were refined
with anisotropic displacement parameters. Crystal data and details
of the structure determination for complexes 1–3 are listed in
Table 1. Selected bond lengths and angles are given in Table 2, and
the hydrogen bonds for complexes 1–3 are described in Table 3.
Results and discussion
Scheme 1 The structures of the ligands in this work.
Description of the structure
Experimental section
Structure descriptions of complexes 1 and 2. Complex 1
crystallizes in chiral space group P212121, and there is one
crystallographically independent AgI ion, one L2 ligand and one
L1 ligand. The AgI ion shows a distorted five coordinate square
pyramidal configuration (z = 0.044)29 with O2 and N1 atoms
from the L1 ligand, and O3 and O5 atoms from the L2 ligand
in a square configuration, and O3 from another L2 ligand in an
apical position (Fig. 1). It is of particular interest to note that
the left-handed 21 helices along the a axis are constructed from
AgI ions and L1 ligands (Fig. 2). The bond angle of Ag1–N1–C1
(112.53(15)◦) and the torsion angle of C1–N1–Ag1–O2 (28.54◦)
are mainly responsible for the construction of the left-handed 21
Materials
All the reagents and solvents employed were commercially avail-
able and used as received without further purification. All other
reagents of analytical grade were purchased and used without
further purification.
Physical measurements
IR spectra were obtained from KBr pellets on a Perkin-Elmer
580B IR spectrometer in the 400–4000 cm-1 region (SI). Elemental
analyses (C, H and N) were performed using a VarioEL analyzer.
Thermogravimetric analyses (TGA) were performed on a Perkin-
Elmer TG-7 analyzer heated from 40 to 800 C under nitrogen.
Powder X-ray diffraction data were collected on a Bruker D8-
˚
helix, with a pitch of about 6.252 A, which is reinforced through
O3 and O5 atoms from the L2 ligand. The unique m2-O3 atom
from L2 makes a great contribution to form a 2D homochiral
layer by the connection of adjacent homohelices along the b axis
(Fig. 3(a)).
◦
˚
AVANCE diffractometer equipped with Cu-Ka (l = 1.5418 A)
radiation at a scan speed of 6◦ min-1. Experimental patterns were in
good agreement with the corresponding simulated ones, indicating
the phase purity of the as-synthesized products.
Synthesis
Synthesis of complex 1. An aqueous solution (10 mL) of (+)-
camphor-10-sulfonic acids (0.095 g, 0.5 mmol) was added to solid
Ag2CO3 (0.069 g, 0.25 mmol) and stirred for several minutes
until no further CO2 was given off. 10 mL of ethanol containing
4-aminobenzoic acid (0.070 g, 0.5 mmol) was added to the
aqueous solution. Colorless crystals of complex 1 were obtained
by evaporation of the solution for 5 d at room temperature (52%
yield). Anal. calc. for the complex 1 (Mr = 476.30): C, 42.87; H,
4.66; N, 2.94. Found: C, 43.10; H, 4.78; N, 3.01%.
Synthesis of complexes 2 and 3. These complexes were obtained
by the same method with (-)-camphor-10-sulfonic acids and
racemic camphor-10-sulfonic acids instead of (+)-camphor-10-
sulfonic acids, respectively. Anal. calc. for the complex 2 (Mr =
476.30): C, 42.87; H, 4.66; N, 2.94. Found: C, 42.91; H, 4.72; N,
3.12%. Anal. calc. for the complex 3 (Mr = 476.30): C, 42.87; H,
4.66; N, 2.94. Found: C, 42.75; H, 4.56; N, 2.82%.
Fig. 1 A view of the local coordination of the AgI atom in complex
1. Displacement ellipsoids are drawn at the 30% probability level. All H
1
2
atoms have been omitted for clarity [symmetry codes: (A) x + 12 , -y -
-z; (B) -x + 12 , -y + 21 , -z; (C) x + 21 , y - 12 , -z].
,
Another interesting feature of complex 1 is the interweaving of
triple left-handed homohelices, in which each single left-handed
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Table 1 Crystal data and structure refinements for complexes 1–3†
Complex 1
Complex 2
Complex3
Empirical formula
Fw
C17H22NO6SAg
476.30
C17H22NO6SAg
476.30
C17H22NO6SAg
476.30
0.20 ¥ 0.16 ¥ 0.14
Monoclinic
P21/n
15.1059(7)
6.1847(3)
18.7605(9)
90
97.099(1)
90
1739.27(14)
4
967.9
1.9–26.1
238
9821
Crystal size/mm
Crystal system
Space group
0.16 ¥ 0.15 ¥ 0.13
Orthorhombic
P212121
6.2517(2)
11.4326(4)
24.1372(9)
90
0.18 ¥ 0.15 ¥ 0.14
Orthorhombic
P212121
6.2347(4)
11.4209(7)
24.0805(15)
90
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
90
90
3
˚
V/A
1725.16(10)
4
967.9
1.7–26.0
238
9652
3398
3284
0.06(2)
0.0226
0.0205
1714.67(19)
4
967.9
2.0–26.1
238
9634
3412
3088
-0.01(3)
0.0413
Z
F(000)
q range (◦)
No. of parameters
No. of total reflections
No. of unique reflections
No. of observed [I ≥ 2s(I)]
Flack parameter
Rint
R1 [I > 2s(I)]
wR2 (all data)
Largest residuals/e A
3452
2744
0.0274
0.0280
0.0741
0.569/-0.547
0.0299
0.0565
0.434/-0.498
0.0514
0.249/-0.362
-3
˚
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 1–3
Table 3 Hydrogen bonds for complexes 1–3
Complex 1a
d(D–H/ d(H ◊ ◊ ◊ A/ d(D ◊ ◊ ◊ A/ <(DHA)
O(2)–Ag(1)
2.4732(18) O(3)–Ag(1)
1.4452(19) Ag(1)–N(1)#2
2.3240(17)
2.3123(19)
112.50(14)
157.04(7)
D–H ◊ ◊ ◊ A
(◦)
˚
A
˚
A
˚
A
O(5)–S(1)
Complex 1a
O(3)–Ag(1)–O(2)
N(1)#2–Ag(1)–O(2)
Complex 2b
N(1)–Ag(1)
O(3)–Ag(1)
C(7)–O(2)–Ag(1)#1
N(1)–Ag(1)–O(2)#2
Complex 3c
N(1)–Ag(1)
O(3)–Ag(1)
105.40(7)
94.06(7)
C(1)–N(1)–Ag(1)#1
N(1)#2–Ag(1)–O(3)
N(1)–H(1A) ◊ ◊ ◊ O(4)#3 0.92
N(1)–H(1A) ◊ ◊ ◊ O(6)#3 0.92
O(1)–H(18) ◊ ◊ ◊ O(3)#4 0.77(3)
Complex 2b
2.34
2.58
1.98(3)
3.109(3)
3.025(3)
2.720(3)
141.1
110.4
163(3)
2.309(3)
2.320(2)
127.5(2)
94.00(9)
O(2)–Ag(1)#1
Ag(1)–O(2)#2
N(1)–Ag(1)–O(3)
O(3)–Ag(1)–O(2)#2
2.464(3)
2.464(3)
157.00(10)
105.39(9)
N(1)–H(1B) ◊ ◊ ◊ O(4)#3 0.92
2.33
2.58
1.94(4)
3.101(5)
3.025(4)
2.716(3)
141.1
110.2
163(4)
N(1)–H(1B) ◊ ◊ ◊ O(6)#3 0.92
O(1)–H(1) ◊ ◊ ◊ O(3)#4
0.80(4)
Complex 3c
2.282(2)
2.305(2)
154.26(8)
O(1)–Ag(1)#1
Ag(1)–O(1)#1
N(1)–Ag(01)–O(1)#1 97.24(8)
C(7)–O(1)–Ag(01)#1 126.24(19)
2.485(2)
2.485(2)
N(1)–H(1A) ◊ ◊ ◊ O(4)#2 0.92
N(1)–H(1B) ◊ ◊ ◊ O(6)#3 0.92
O(2)–H(18) ◊ ◊ ◊ O(3)#4 0.80(4)
2.29
2.23
1.91(4)
3.150(3)
3.008(3)
2.696(3)
154.6
142.5
170(3)
N(1)–Ag(01)–O(3)
O(3)–Ag(01)–O(1)#1 103.74(7)
a Symmetry transformations used to generate equivalent atoms: #1 x -
a Symmetry transformations used to generate equivalent atoms: #3 x - 1,
1
12 , -y - 12 , -z; #2 x + 21 , -y - 2 , -z. b Symmetry transformations used to
1
y - 1, z; #4 x - 12 , -y + 2 , -z. b Symmetry transformations used to generate
generate equivalent atoms: #1 x + 12 , -y + 23 , -z + 1; #2 x - 21 , -y +
,
3
equivalent atoms: #3 x - 21 ,-y + 12 ,-z + 1; #4 x, y + 1, z. c Symmetry
2
-z + 1. c Symmetry transformations used to generate equivalent atoms: #1
-x + 1, -y, -z + 1.
transformations used to generate equivalent atoms: #2 - x + 32 , y + 21 , -z +
1
2
32 ; #3 - x + 32 , y - 21 , -z + 23 ; #4 x - 12 , -y + 21 , z -
.
helix around the crystallographic 21 axis has a repeat unit of two
AgI ions, two L1 ligands, and O3, O5 and S1 atoms from L2 with
with metal ions, which differ from the traditional strategy of helices
built from flexible ligands.30,31
˚
a pitch of about 18.755 A (Fig. 3(b)). Metal–ligand coordination
˚
interactions (O3–Ag1 2.3240(17) A), allowing the left-handed
Structure description of complex 3. For a comparative study
[Ag(L4)(L1)]n (3) was synthesized using L4 ligands. Albeit 1, 2
and 3 (with the same formula) are isomeric, 3 crystallizes in a
centrosymmetric crystal structure (P21/n) based on racemic ( )-
camphor-10-sulfonic acid ligands, rather than a chiral space group.
The coordination geometry for the five-coordinate AgI ion is close
to a distorted square pyramidal configuration (z = 0.077),26 with
helicity of the originally formed chains to be transferred uniformly,
also could lead to the formation of a homochiral 2D sheet.
When (-)-camphor-10-sulfonic acid is used in place of (+)-
camphor-10-sulfonic acid, complex 2, with helices that are op-
posite to those in complex 1, is obtained. This demonstrates that
the absolute helicity in complexes 1 and 2 can be controlled by
the absolute chirality of camphor-10-sulfonic acid. As a result,
there are a single right-handed helix and triple right-handed
homohelices in complex 2, resembling structure of complex 1
(Fig. 4 and Fig. 5). Complexes 1 and 2 obtained here are good
illustrations of the construction of helix-integrated rigid ligands
˚
a long Ag1–O5 (2.746 A) bond that is slightly longer than the
normal Ag–O range but is similar to other reported values.19,20 It is
of great interest to note that the AgI ion and the L4 ligand generate
a closed circle, rather than a M or P helix as in complexes 1 and 2,
respectively. The closed circle is linked by m2-O3 atoms alternately
1718 | Dalton Trans., 2011, 40, 1716–1721
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Fig. 2 A single left-handed helix along the a axis.
Fig. 4 The single right-handed helix along the a axis.
Fig. 3 (a) The 2D homochiral layer of complex 1. (b) Interweaving of
triple left-handed homohelices along the a axis.
of L2 and L3 to generate an achiral 2D layer (Fig. 6(a)). O3, O5
and S1 atoms, and the closed circle based on the AgI ion and the
L4 ligand, are incorporated to form equal numbers of double-
helical chains, which are either left-handed double-stranded or
right-handed double-stranded helices (Fig. 6(b), Fig. 7).
Fig. 5 (a) The 2D homochiral layer of complex 2. (b) Interweaving of
triple right-handed homohelices along the a axis.
Influence of the chirality of the camphor-10-sulfonic acid
Although helicity and homochirality are intimately associated
with living processes, their origin remains unclear.17 The regulation
of the revolving orientation of homohelices by the handedness
of camphor-10-sulfonic acid may be attributed to the syner-
gism of such hydrogen bonds as Namino group–H ◊ ◊ ◊ Ocarbonyl oxygen and
Ocarboxyl oxygen–H ◊ ◊ ◊ Osulfonic acid. Ocarbonyl oxygen and Osulfonic acid atoms are
close to the chiral center of camphor-10-sulfonic acids, reflecting
the chiral signal and transferring it into the Ag-L1 system.
Namino group–H ◊ ◊ ◊ Ocarbonyl oxygen and Ocarboxyl–H ◊ ◊ ◊ Osulfonic acid hydrogen
bonds may play dual roles in regulating the orientation of
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Fig. 6 (a) The 2D achiral layer of complex 3. (b) Left-handed or right-handed double-stranded helices are found in complex 3.
Fig. 7 A schematic diagram of left-handed double-stranded and right-handed double-stranded helices in complex 3.
homohelices and strengthening them in this model. In complex
3, L1 ligands and Ag ions form circles, which are maintained by
Ocarboxyl–H ◊ ◊ ◊ Osulfonic acid and Namino group–H ◊ ◊ ◊ Ocarbonyl oxygen hydrogen
bonds. However, the driving force from the hydrogen bonds
is exactly the opposite for the circle because the chirality of
the camphor-10-sulfonic acids coordinating with the circles are
opposite (Fig. 8(a)). In complex 2, (+)-camphor-10-sulfonic acid
is used to break circles, leading to the formation of a single left-
handed helix, which is also reinforced by Ocarboxyl–H ◊ ◊ ◊ Osulfonic acid
and Namino group–H ◊ ◊ ◊ Ocarbonyl oxygen hydrogen bonds (Fig. 8(b)). This
single left-handed helix is the backbone of the interweaving of
triple left-handed homohelices. The single right-handed helix is
created by the same driving force in complex 2, where (-)-camphor-
10-sulfonic acid is utilized (Fig. 8(c)).
Complexes 1 and 2 were also tested by CD (Fig. 9), which is
widely used in the construction of chiral porous solids by the
availability of achiral building blocks.14 However, in this paper,
it is used to detect whether the homochiral ligands happen to
racemize or not. The CD spectra of complexes 1 and 2 in DMF
(dimethylformamide) were investigated, revealing that, in the
wavelength range l = 270–450 nm, complex 1 shows a positive
Cotton effect at l = 298 nm, while complex 2 shows a negative
one at the same wavelength. The results of CD show the fact that
homochiral camphorsulfonic acids do not racemize and that the
1720 | Dalton Trans., 2011, 40, 1716–1721
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from California State University Long Beach for his explanation
of chiral porous solids.
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The Royal Society of Chemistry 2011
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