Chirality and absolute helicity in a pair of enantiomeric amino acid derivatives and their complexes: Structures, chiroptical, and photoluminescent properties

Article abstract of DOI:10.1039/c3ce27028a

A pair of enantiomeric amino acid derivatives [l-H₂cala (1L) and d-H₂cala (1D), H₂cala = N-(4-carboxylbenzyl)-alanine] were synthesized by a one step substitution reaction of enantiopure l-(or d-)alanine with 4-(bromomethyl)benzoic acid. Self-assembly of 1L and 1D with divalent Cu²⁺ and Cd²⁺ ions in water produced two pairs of enantiomers formulated as [Cu(l-cala)(H₂O)]_n· n(H₂O) (2L), [Cu(d-cala)(H₂O)]_n·n(H ₂O) (2D), [Cd(l-Hcala)₂(H₂O)₂] _n·n(H₂O) (3L) and [Cd(d-Hcala)₂(H ₂O)₂]_n·n(H₂O) (3D). X-ray diffraction analyses reveal that 1L and 1D are hydrogen-bonded two-dimensional (2D) supramolecules containing one-dimensional (1D) single-stranded supramolecular helices parallel to the crystallographic b-axis. 2L and 2D are 1D single-stranded coordination helices arranged along the direction of c-axis. They are further extended into three-dimensional (3D) supramolecular frameworks by inter-helical hydrogen bonds. 3L and 3D are 1D single-stranded coordination helices parallel to a-axis. Then three neighboring single-stranded coordination helices are further tied up by hydrogen bonds, forming 1D triple-stranded supramolecular helices extending along the same direction. Interestingly, 3L and 3D also show supramolecular helicities on the directions of b- and c-axes. The chiralities and absolute helicities of these complexes are controlled by the chiralities of ligands. In addition, the solid-state circular dichroism (CD) and photoluminescent properties of them were investigated.

Full text of DOI:10.1039/c3ce27028a

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Chirality and absolute helicity in a pair of enantiomeric
amino acid derivatives and their complexes: structures,
chiroptical, and photoluminescent properties3
Cite this: CrystEngComm, 2013, 15,
3593
Xinfa Li,* Huajun Zhao and Qihua Zeng
A pair of enantiomeric amino acid derivatives [L-H2cala (1L) and D-H2cala (1D), H2cala = N-(4-
carboxylbenzyl)-alanine] were synthesized by a one step substitution reaction of enantiopure L-(or D-
)alanine with 4-(bromomethyl)benzoic acid. Self-assembly of 1L and 1D with divalent Cu2+ and Cd2+ ions in
water produced two pairs of enantiomers formulated as [Cu(L-cala)(H2O)]n?n(H2O) (2L), [Cu(D-
cala)(H2O)]n?n(H2O) (2D), [Cd(L-Hcala)2(H2O)2]n?n(H2O) (3L) and [Cd(D-Hcala)2(H2O)2]n?n(H2O) (3D). X-ray
diffraction analyses reveal that 1L and 1D are hydrogen-bonded two-dimensional (2D) supramolecules
containing one-dimensional (1D) single-stranded supramolecular helices parallel to the crystallographic
b-axis. 2L and 2D are 1D single-stranded coordination helices arranged along the direction of c-axis. They
are further extended into three-dimensional (3D) supramolecular frameworks by inter-helical hydrogen
bonds. 3L and 3D are 1D single-stranded coordination helices parallel to a-axis. Then three neighboring
single-stranded coordination helices are further tied up by hydrogen bonds, forming 1D triple-stranded
supramolecular helices extending along the same direction. Interestingly, 3L and 3D also show
supramolecular helicities on the directions of b- and c-axes. The chiralities and absolute helicities of
these complexes are controlled by the chiralities of ligands. In addition, the solid-state circular dichroism
(CD) and photoluminescent properties of them were investigated.
Received 15th December 2012,
Accepted 6th February 2013
DOI: 10.1039/c3ce27028a
www.rsc.org/crystengcomm
from L-amino acid ligands appear in left-handed (M) con-
formation more than in right-handed (P) conformation, where
Introduction
Chirality and helicity are prevalent in living and nonliving
systems at the molecular and supramolecular levels.1 However,
the relationship between chirality of molecular building units
and absolute helicity of polymeric structures is still unclear,
even if it is known unambiguously that the absolute helicity of
a-helices in the secondary structures of two most important
biopolymers (natural protein and DNA) are right-handed.2
This is also intimately related to the origin of asymmetry in the
biological process where proteins are exclusively composed of
L-(a)-amino acids while DNAs are merely made of D-sugars.3
Natural amino acids are uniformly the L-form (except the
optically inactive glycine) as a result of the chirality on the a-C.
The a-helices of proteins are typically single-stranded and
appear in a right-handed conformation due to the selective
incorporation of L-amino acids.2 The driven force in the
formation of a-helices is hydrogen bonds between amino and
carbonyl groups of L-amino acids. Complexes constructed
the main driving forces are coordination bonds between L-
amino acid ligands and metal ions.4–10 On the contrary,
a-helices composed of D-amino acids display M conformation
and complexes containing D-amino acid ligands prefer P
conformation to M conformation. This is also the situation for
ligands derived from amino acids.11–19 Schiff base and
reduced Schiff base synthesized by the condensation of amino
acids and salicylaldehyde or pyridinecarboxaldehyde are the
most popular ligands of this kind. One of the most interesting
examples is a 1D left-handed helical coordination polymer
[(H2O)2@{Ni(L-Hsglu)(H2O)2}]?H2O [L-H3sglu = N-(2-hydroxy-
benzyl)-L-glutamic acid] encapsulating a 1D left-handed water
helix in each of its channels.13a More recently, two 1D helical
structures [Cu(L-pasp)(H2O)]?4H2O (a infinite single-stranded
M-helix) and [Cu(D-pasp)(H2O)]?4H2O (an infinite single-
stranded P-helix) [H2pasp
=
N-(2-pyridylmethyl)-aspartic
acid],14 and two enantiomeric lanthanide clusters [La7{(S)-
L}6(CO3)(NO3)6(OCH3)(CH3OH)7]?2CH3OH?5H2O (a finite tri-
ple-stranded M-helix) [(S)-H2L = N-(2-hydroxybenzyl)-L-aspara-
gine] and [La7{(R)-L}6(CO3)(NO3)6(OCH3)(CH3OH)7]?2CH3OH?
5H2O (a finite triple-stranded P-helix) [(R)-H2L = N-(2-hydro-
xybenzyl)-D-asparagine] were reported.15 It is noteworthy that
the intrinsic chirality of amino acid ligands that originated
Department of Chemistry, Zunyi Normal College, Zunyi, 563002, P. R. China.
E-mail: xflichem@163.com
Electronic supplementary information (ESI) available: Crystallographic file for
3
1L to 3D in CIF format; Fig. S1–S15 and Table S1. CCDC 916191–916194. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce27028a
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heated to reflux on an oil bath for about 40 min. After cooling
to room temperature, 6 M HCl was added dropwise to a pH
value of about 4.0. Large amounts of white precipitate thus
formed was collected by vacuum filtration, followed by
washing repeatedly with distilled water and then with EtOH.
The product was finally dried in a vacuum desiccator equipped
with allochroic silica gel. Yield: 67% [based on 4-(bromo-
methyl)benzoic acid]. The yield is significantly higher than
that of the method reported in the literature (45%).21
Calculated for C11H13NO4 (%): C, 59.19; H, 5.87; N, 6.27.
Found for 1L (%): C, 59.11; H, 5.90; N, 6.22. Found for 1D (%):
C, 59.15; H, 5.88; N, 6.25. IR (KBr pellet, cm21): 3414 (br), 3035
(m), 2450–2700 (br, w), 1705 (s), 1615 (vs), 1452 (w), 1398 (m),
1346 (w), 1258 (s), 1118 (w). 1H-NMR in deuterated DMSO
(ppm): 7.92 (d, H3 and H7), 7.51 (d, H4 and H6), 3.98 (d, H8A),
3.87 (d, H8B), 3.17 (q, H10), 1.24 (d, H11A, H11B and H11C). It
should be noted that H8A and H8B are chemically inequi-
valent due to the steric hindrance of the large –C6H4CO2H
group, so their chemical shifts are split into two adjacent
peaks in the NMR spectrum. An X-ray quality single crystal of
1L was obtained by recrystallization in H2O.
from the a-C have been transferred into these crystal
structures, and the relationship between ligand chirality and
absolute helicity is in accordance with the above-mentioned
principle. However, the principle is not absolute, and the
coordination configuration preference of metal ions also
affects the absolute helicity of coordination polymers, in
which the mechanism of chirality transfer from ligand to
complexes and crystals is rather complicated and unexpect-
able.18–20
Herein we report three pairs of enantiomeric coordination
helices and supramolecular helices based on N-(4-carboxyl-
benzyl)-L(or D)-alanine ligands: L-H2cala (1L) and D-H2cala
(1D),
[Cu(L-cala)(H2O)]
n?n(H2O)
(2L)
and
[Cu(D-
cala)(H2O)]n?n(H2O) (2D), [Cd(L-Hcala)2(H2O)2]n?n(H2O) (3L)
and [Cd(D-Hcala)2(H2O)2]n?n(H2O) (3D), concerning their
syntheses, crystal structures, chiroptical, and photolumines-
cent properties. H2cala ligands have an aromatic and an
aliphatic carboxyl group on two opposite orientations with
both rigidity and flexibility, making them prone to form
helical structures when coordinating to metal ions. These
compounds may provide fresh models for studying the
relationship between molecular chirality and supramolecular
absolute helicity in biopolymers.
Syntheses of [Cu(L-cala)(H2O)]n?n(H2O) (2L) and [Cu(D-
cala)(H2O)]n?n(H2O) (2D). Cu(CH3CO2)2?H2O (0.4 mmol) was
dissolved in 5 mL hot water and magnetically stirred, to which
L-H2cala (for 2L or D-H2cala for 2D) (0.4 mmol) was added. A
dark blue clear solution appeared immediately, which was
filtered 10 min later. The filtrate was sealed and placed in an
oven at 50 uC for about 24 h. Large prism-shaped dark blue
crystals were harvested. The crystals were collected by
Experimental
Materials and general methods
filtration
and
air-dried.
Yield:
80%
(based
on
All chemicals were obtained from commercial sources and
used without further purification. Infrared (IR) spectra were
recorded on PerkinElmer Spectrum One instrument as KBr
pellets in the range of 4000–400 cm21. Elemental analyses of
C, H and N were measured on a Vario MICRO E III elemental
Cu(CH3CO2)2?H2O). Calculated for C11H15CuNO6 (%): C,
41.19; H, 4.71; N, 4.37. Found for 2L (%): C, 41.15; H, 4.75;
N, 4.39. Found for 2D (%): C, 41.14; H, 4.77; N, 4.36. IR (KBr
pellet, cm21): 3501 (br), 3148 (m), 1610 (vs), 1558 (m), 1452 (w),
1375 (s), 1092 (w).
1
analyzer. The H-NMR spectra of 1L ligand was recorded on a
Syntheses of [Cd(L-Hcala)2(H2O)2]n?n(H2O) (3L) and [Cd(D-
Hcala)2(H2O)2]n?n(H2O) (3D). L-H2cala (for 3L or D-H2cala for
3D) (0.4 mmol) and NaOH (0.4 mmol) were dissolved in 6 mL
distilled water, to which Cd(NO3)2?4H2O (0.2 mmol) was
added. The colorless clear solution thus obtained was filtered
and placed undisturbed at room temperature for 48 h.
Colorless small needle crystals were collected by filtration
and air-dried. Yield: 82% (based on Cd(NO3)2?4H2O).
Calculated for C22H30CdN2O11 (%): C, 43.25; H, 4.95; N, 4.59.
Found for 3L (%): C, 43.18; H, 4.97; N, 4.55. Found for 3D (%):
C, 43.22; H, 4.91; N, 4.58. IR (KBr pellet, cm21): 3434 (br), 3054
(w), 2450–2700 (br, w), 1600 (vs), 1542 (m), 1464 (w), 1396 (s),
1362 (s), 1106 (w).
BRUKER AVANCE III-400M NMR spectrometer in deuterated
d6-DMSO. Powder XRD patterns were collected on a Rigaku
Mini Flex II diffractometer using Cu Ka Radiation (l =
1.54056 Å) under ambient conditions. Thermogravimetric
analyses were performed on an SDT Q600 instrument at a
heating rate of 10 uC min21 under a nitrogen atmosphere.
Solid-state photoluminescent spectra of 1L and 3L were
measured at room temperature with an Edinburgh FLS920
fluorescence spectrometer. The instrument is equipped with a
Xe900 xenon arc lamp as exciting light source. Solid-state CD
spectra of 1L–3D were recorded using a BioLogic-MOS 450
spectrometer at room temperature. For each CD measurement
ca. 0.5 mg crystalline sample was taken to be mixed with 100
mg of dried and well ground KCl powder. This mixture was
then pressed into a disk by a literature method.11
Syntheses of L-H2cala (1L) and D-H2cala (1D) ligands.
Although the synthesis of 1L was previously published,21
herein we report a different but more productive method.
Enantiopure L-alanine (for 1L or D-alanine for 1D) (50 mmol)
and NaOH (60 mmol) were dissolved in 40 mL distilled water
and magnetically stirred at room temperature, to which 4-
(bromomethyl)benzoic acid (10 mmol) was added in small
portions. The mixture was stirred for about 1.5 h and then
X-ray crystallographic studies
Crystal structure determinations for 1L, 2D and 3L were
performed on an Oxford Xcalibur E CCD-based diffractometer
equipped with graphite-monochromated Mo Ka radiation (l =
0.71073 Å) at room temperature. The intensity data sets were
collected with the v-scan technique. The CrysAlisPro (Version
1.171.34.49) software was used for data reduction and
empirical absorption correction. Crystal structure determina-
tion of 2L was performed on a Rigaku SCX-mini CCD-based
diffractometer equipped with graphite-monochromated Mo
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Table 1 Crystallographic data for 1L, 2L, 2D and 3L
Table 2 Selected bond lengths [Å] for 1L, 2L, 2D and 3La
1L
3L
1L
a
Symmetry codes: for 2L #1 y + 1, 2x + 1, z + 1/4. For 3L #1 x 2 3/
2, 2y + 1/2, 2z + 2.
a
2
R = S||Fo| 2 |Fc||)/S|Fo|, wR = [Sw(Fo 2 Fc2)2/Sw(Fo2)2]1/2
.
C22H30CdN2O11
610.88
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C11H13NO4
223.22
Monoclinic
P21
O(1)–C(1)
O(3)–C(9)
2L
Cu(1)–O(3)
Cu(1)–O(1)
2D
1.311(3)
1.262(3)
O(2)–C(1)
O(4)–C(9)
1.199(3)
1.241(3)
Orthorhombic
P212121
1.919(2)
1.950(3)
Cu(1)–O(5)
Cu(1)–N(1)#1
1.922(3)
2.011(3)
7.3335(6)
5.8827(4)
12.8465(9)
99.790(7)
546.14(7)
2
5.8266(2)
17.2093(7)
23.5764(1)
90
2364.05(16)
4
Cu(1)–O(5)
Cu(1)–O(1)
3L
Cd(1)–O(9)
Cd(1)–O(5)
Cd(1)–O(1)
Cd(1)–O(6)
1.922(3)
1.949(3)
Cu(1)–O(3)
Cu(1)–N(1)
1.923(2)
2.017(3)
b/u
V/Å3
Z
2.258(4)
2.272(4)
2.327(4)
2.668(5)
Cd(1)–O(10)
Cd(1)–O(4)#1
Cd(1)–O(2)
2.269(4)
2.315(4)
2.556(4)
Dc/g cm23
1.357
1.716
m/mm21
0.104
0.0180
0.990
0.0326
Rint
Data/parameters
1533/146
0.4(14)
1.012
0.0337, 0.0585
0.0508, 0.0606
0.120/20.147
4174/327
20.04(3)
1.001
0.0378, 0.0763
0.0593, 0.0840
0.446/20.432
Flack x
GOF on F2
R1, wR2 (I . 2s(I))a
R1, wR2 (all data)a
Results and discussion
Drmin max ]
[e Å23
/
Crystal structures of L-H2cala (1L) and D-H2cala (1D)
2L
2D
As shown in Scheme 1, L-H2cala (1L) and D-H2cala (1D) are one
pair of enantiomers. They have the same structure except the
opposite chirality on C(10), which stems from amino acids
used in the synthetic process (enantiopure L-alanine for 1L
and D-alanine for 1D). So 1L is taken as representative to
depict their structures. Single crystal X-ray diffraction analysis
reveals that 1L crystallizes in the chiral P21 space group (no. 4).
Due to the lack of atoms heavier than Si in 1L, the absolute
structure cannot be determined by the anomalous dispersion
effects in diffraction measurements on the crystal.24 However,
the synthetic process does not change the chirality on C(10), so
the absolute configuration is predetermined to be the L-form.
As displayed in Fig. 1, the asymmetric unit of 1L is composed
of a zwitterionic L-H2cala molecule. It is interesting that the
deprotonated carboxyl group is C(9)O2H rather than C(1)O2H,
as suggested by C–O bonding parameters [the C(1)–O(1), C(1)–
O(2), C(9)–O(3) and C(9)–O(4) bond lengths are 1.311(3),
1.199(3), 1.262(3) and 1.241(3) Å, respectively (Table 2)]. It is
obvious that C(1)–O(1) is a single bond, C(1)–O(2) is a double
bond, while C(9)–O(3) and C(9)–O(4) are partially delocalized
bonds lying between single and double bonds. Further
evidence of the presence of both protonated and deprotonated
carboxyl groups can also be found in the IR spectrum [Fig.
Empirical formula
Formula weight
Crystal system
Space group
a = b/Å
C11H15CuNO6
320.78
Tetragonal
P43
7.5402(2)
22.7232(1)
1291.92(7)
4
1.649
1.712
0.0401
2847/173
20.004(16)
1.008
0.0356, 0.0832
0.0370, 0.0845
0.269/20.514
C11H15CuNO6
320.78
Tetragonal
P41
7.5385(1)
22.7308(1)
1291.77(7)
4
1.649
1.713
0.0264
1851/173
0.009(16)
1.004
0.0298, 0.0632
0.0330, 0.0649
0.323/20.281
c/Å
V/Å3
Z
Dc/g cm23
m/mm21
Rint
Data/parameters
Flack x
GOF on F2
R1, wR2 (I . 2s(I))a
R1, wR2 (all data)a
Drmin max ]
[e Å23
/
Ka radiation (l = 0.71073 Å) at room temperature. The
CrystalClear software was used for data reduction and
empirical absorption correction. All the structures were solved
by direct methods and successive Fourier difference syntheses,
and refined by full-matrix least-squares on F2 (SHELXL Version
5.1).22 All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms attached to C, N
and protonated carboxyl O atoms were generated theoretically
and refined by a riding-mode with isotropic thermal para-
meters fixed at 1.2 times that of the mother atoms. Positions of
hydrogen atoms bonding to water molecules were calculated
by the PLATON program,23 and then refined in the SHELXL
program with isotropic thermal parameters fixed at 1.5 times
that of the corresponding O atoms. Detailed crystallographic
data and structure refinement parameters of 1L, 2L, 2D and 3L
are summarized in Table 1. Selected bond lengths are listed in
Table 2. Hydrogen-bonding parameters are listed in Table S1
S1(a) (ESI ) 1705 cm21 (vasCO2H,), 1615 cm21 (vasCO22)]. The
3
amino group [N(1)H2] is protonated to form two charge-
assisted intermolecular hydrogen bonds with O atoms [O(3)
(ESI ).
3
Scheme 1 Schematic drawing of L-H2cala (left) and its mirror image D-H2cala
(right).
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Fig. 1 ORTEP drawing of 1L with 30% probability of thermal ellipsoids.
+
…
and O(4)] from adjacent L-H2cala molecules [ N(1)–H(1B) O(3)
(x, y 2 1, z) with a D A distance of 2.759(3) Å and +N(1)–
H(1C) O(4) (2x, y 2 1/2, 2z + 1) with a D A distance of
…
Fig. 3 View of the 2D supramolecular network of 1L along a-axis.
…
…
2.783(3) Å]. As presented in Fig. 2, the protonated carboxyl
…
group acts as a hydrogen bond donor [O(1)–H(1A) O(3) (2x +
them. Adjacent helices are further connected to each other by
+
…
…
1,y 2 1/2,2z + 2) with a D A distance of 2.593(2) Å)] which
facilitates the formation of a 1D supramolecular helix along
a charge-assisted hydrogen bond N(1)–H(1C) O(4) (2x, y 2 1/
2, 2z + 1) along the c-axis, generating a 2D supramolecular
layer in the bc-plane (Fig. 3). Neighboring layers are packed to
one another by van der Waals’ interactions and there are no
interlayer hydrogen bonds and p–p interactions. The supra-
molecular structure of 1D is identical to 1L, besides 1D is just a
single-stranded M-helix. A simulated comparison of the
absolute helicities of 1L and 1D along the direction of the
+
…
with another hydrogen bond [ N(1)–H(1B) O(3) (x, y 2 1, z)].
The single-stranded supramolecular helix has right-handed
absolute helicity, consistent with that of a-helices in natural
proteins composed of L-amino acids. It is not strange since the
formation of helix in 1L is driven exclusively by hydrogen
bonds, just like the secondary structure in natural proteins.
The P-helix is arranged parallel to the b-axis with a 21 screw
axis and a pitch of 5.8827 Å (equal to a length of unit cell
parameter b). Viewed along the direction of the b-axis, the
inner sphere of the helix is a tubular channel surrounded by
b-axis is displayed in Fig. S3 (ESI ).
3
It should be noted that the conformation relating to C(8)
2
and N(1) is a gauche-form with the two larger –CH(CH3)CO2
and –C6H4CO2H groups lying adjacent to each other [Fig.
S4(a), ESI ], distinct from the staggered conformation observed
two L-H cala molecules (Fig. S2, ESI ). The shortest separation
3
3
2
in
a comparable N-(2-pyridylmethyl)-L-alanine zwitterion
between two parallel neighboring phenyl rings within the tube
is about 5.883 Å (based on the centers of the two parallel
phenyl rings), implying no p–p stacking interaction between
reported by us recently.11 It is speculated that the relatively
less stable gauche conformation in L-H2cala may be advanta-
geous to the formation of stronger intermolecular hydrogen
bonds, which may also explain the chemical inequivalence of
H8A and H8B in the NMR spectrum.
Crystal structures of [Cu(L-cala)(H2O)]n?n(H2O) (2L) and [Cu(D-
cala)(H2O)]n?n(H2O) (2D)
Single crystal X-ray diffraction analyses demonstrate that 2L
and 2D crystallize in the chiral P43 (no. 78) and P41 (no. 76)
space groups, respectively. It means that they are also a pair of
enantiomers, which is further confirmed by CD spectra. P43
and P41 are enantiomorphic space groups of each other (the
enantiomorph of 43 screw axis is 41 screw axis). If one is a
M-helix, the other should be a P-helix. 2L and 2D have the
same unit cell parameters (with a deviation lower than
0.033%) and powder XRD patterns [Fig. S5(a), ESI ]. As shown
3
in Fig. 4, the asymmetric unit of 2L consists of one Cu2+ ion, a
L-cala22 anion, an aqua ligand and one lattice H2O molecule.
L-Cala22 anion acts as a tridentate ligand by chelating to one
Cu2+ bidentately via the amino N atom and the aliphatic
carboxyl O atom to form a five-membered ring [–CuOC2N–]
and simultaneously coordinating to an equivalent Cu2+ ion
monodentately through the aromatic carboxyl group (Fig. S6,
ESI ). Cu2+ ion adopts
a distorted square coordination
3
Fig. 2 The 1D supramolecular P-helix in 1L.
geometry with a [NO3] donor set. The trans position of the N
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Fig. 4 ORTEP drawing of 2L with 30% probability of thermal ellipsoids.
atom is an aqua ligand, and the other two coordination sites
are taken up by two carboxyl O atoms. However, the five
[CuNO3] atoms are not coplanar, indicating the square is
distorted. The Cu–N and Cu–O bond lengths are 2.011(3),
1.919(2), 1.922(3) and 1.950(3) Å, respectively (Table 2). The
interconnection between Cu2+ and L-cala22 results in the
formation of a 1D single-stranded coordination helix propa-
gating along the crystallographic c-axis. The fourfold helix,
with a pitch of 22.7232 Å (equal to length of unit cell c), is left-
handed and symmetry-related by a 43 screw axis (Fig. 5). It
differs from the absolute helicity of a-helices in natural
proteins composed of L-amino acids. The difference originates
from the mode how the chirality of a-C in L-amino acids
influences the orientation of helices. a-Helices in proteins are
formed exclusively by hydrogen bonds between amino and
carbonyl groups. The orientation of a-helices is dependent on
the steric hindrance effects between side-chain groups and
main-chain moieties in proteins, which energetically prefers
the right-handed conformation. Here the orientation of [Cu(L-
cala)(H2O)]n coordination helix is directly determined by the
Fig. 6 View of the 3D supramolecular framework of 2L along c-axis.
controlled by the chirality of L-cala22 ligand. Extensive
hydrogen-bonding associations exist between L-cala22 ligands
and H2O molecules, which further joins adjacent helices
together to form a 3D supramolecular framework (Fig. 6 and
S7, ESI ). The crystal structure of 2D is the same as 2L, except
3
that 2D is a P-helix with a 41 screw axis parallel to the c-axis,
suggesting they are mirror images of each other (Fig. 5). The
Flack x parameters of 2L and 2D are 20.004 and 0.009,
respectively (Table 1), indicative of the right absolute
structures determined by the anomalous dispersion effects
during diffraction data collection.24,25
2
direction of the side-chain –CH2C6H4CO2 group, which is
The most interesting aspect in the structure of 2L and 2D is
that the H2cala ligand contains a cryptochiral N(1) atom. The
deprotonation and following coordination to Cu2+ ion bring
about two new chiral centers [N(1) and Cu(1)]. The generation
of chirality on Cu(1) is also benefited from the distortion of the
[CuNO3] coordination square.26 SC(10) (L-cala22) leads to RN(1)
DCu(1) and M-helix in 2L, while RC(10) (D-cala22) gives rise to the
,
opposite SN(1), LCu(1) and P-helix in 2D (Fig. S8, ESI ).
3
Obviously, the absolute configuration of C(10) is predeter-
mined and unchangeable, but those of N(1), Cu(1) and the
absolute helicity of the coordination helix are cryptical and
variable. It is reasonable to say that the chiralities on N(1),
Cu(1) and the helices are dependent on that of C(10), namely,
the self-assembly and crystallization process are highly
stereoselective. In other words, the chirality transfer from
ligand to complex and crystal is efficient. Similar to the case in
1L, the conformation relating to C(8) and N(1) in 2L and 2D is
also the relatively less stable gauche-form [Fig. S4(b), ESI ].
3
Crystal structures of [Cd(L-Hcala)2(H2O)2]n?n(H2O) (3L) and
[Cd(D-Hcala)2(H2O)2]n?n(H2O) (3D)
X-ray diffraction analyses, CD, elemental analyses (EA) and IR
spectra demonstrate that 3L and 3D are also a pair of
enantiomers with the same chemical formulae. According to
single crystal X-ray diffraction, 3L crystallizes in the chiral
Fig. 5 The absolute helicities of 2L (left) and its enantiomer 2D (right) on the
direction of c-axis. The mirror is drawn as a dash line. 2L is a single-stranded
M-helix and 2D is a single-stranded P-helix.
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left-handed, consistent with that in 2L since both of them are
formed by coordination bonds rather than hydrogen bonds.
The terminal L-Hcala2 ligand points to the direction of the
c-axis, engaging in hydrogen-bonding formation. It is particu-
larly interesting that three independent single-stranded
coordination helices are joined together by hydrogen bonds,
giving rise to a 1D triple-stranded supramolecular M-helix with
a pitch of 5.8266 Å (equal to length of unit cell a and one-third
of the pitch of a single-stranded coordination helix) (Fig. 8).
The triple-stranded supramolecular helix is also symmetry-
related by a 21 screw axis parallel to a-axis.
Fig. 7 ORTEP drawing of 3L with 30% probability of thermal ellipsoids.
On the direction of b-axis, the bridging L-Hcala2 ligand is
extended in a head-and-tail way by hydrogen bonds involving
the lattice [H2O(11)] and one coordinated [H2O(9)] water
molecules, forming a 1D single-stranded supramolecular helix.
The twofold helix, with a pitch of 17.2093 Å (equal to the
length of unit cell b), is symmetry-related by a 21 screw axis
P212121 (no. 19) space group with a Flack x parameter of 20.04
(Table 1), indicating the right absolute structure.24,25 As shown
in Fig. 7, the asymmetric unit of 3L is composed of a Cd2+ ion,
two L-Hcala2 anions, two aqua ligands and one lattice H2O
molecule. Cd2+ adopts a slightly distorted pentagonal bipyr-
amid coordination geometry with a [O7] donor set (Fig. S9,
[Fig. S11(a), ESI ]. In accordance with that on the direction of
3
a-axis, the absolute helicity of the helix is also left-handed,
even though its formation is driven by hydrogen bonds rather
than coordination bonds. It is not contradictory to the
principle that absolute helicity (based on L-amino acid
derivatives) exclusively driven by hydrogen bonds prefers
right-handedness whilst coordination bonds (between L-amino
acid ligands and metal ions) favor left-handedness, because
coordination bonds are stronger than hydrogen bonds. In
other words, the absolute helicity on b-axis can be seen as a
continuation of that on a-axis since both of them are
determined exclusively by the chirality of the bridging L-
Hcala2 ligand. A similar phenomenon was also observed in a
reported L-amino acid derivative complex [ZnL(H2O)2]n?n(H2O)
[H2L = N-(4-pyridylmethyl)-L-aspartic acid].12b On the direction
of c-axis, the terminal L-Hcala2 ligand is stretched by hydrogen
bonds involving the lattice [H2O(11)] and the other coordi-
ESI ). The five coordination sites on the equatorial plane are
3
occupied by one H2O molecule and four carboxyl O atoms
from two crystallographically unique L-Hcala2 ligands. The
axial positions are taken up by one H2O molecule and one
carboxyl O atom from L-Hcala2 ligand. The Cd–O bond lengths
range from 2.258(4) to 2.668(5) Å, with an average value of
about 2.381 Å (Table 2). L-Hcala2 ligands exhibit two different
coordination modes (Fig. S10, ESI ): (a) terminal mode:
3
chelates to a Cd2+ bidentately via the aromatic carboxyl group;
(b) bridging mode: chelates to a Cd2+ bidentately via the
aromatic carboxyl group and again bridges to an equivalent
Cd2+ ion monodentately through the aliphatic carboxyl group.
All of the four carboxyl groups are deprotonated. Both of the
amino groups [N(1)H2 and N(2)H2] are protonated as hydrogen
bond donors and for charge-balance. The interlinkage between
Cd2+ ion and the bridging L-Hcala2 ligand leads to the
formation of a 1D single-stranded coordination helix as shown
in Fig. 8(a). The twofold helix is arranged parallel to the a-axis
with a 21 screw axis and a pitch of 17.4798 Å (three times of the
length of unit cell a). The absolute helicity of the helix is also
nated [H2O(10)] water molecules in
a head-to-tail way,
generating a 1D single-stranded supramolecular helix. It is
also a twofold helix with a 21 screw axis and a pitch of
23.5764 Å (equal to the length of unit cell c) [Fig. S11(b), ESI ].
3
However, the absolute helicity of this helix, which is merely
decided by the chirality of the terminal L-Hcala2 ligand, is
right-handed, very similar to the situation in 1L. Therefore, the
final structure of 3L is a 3D supramolecular framework with
Ma–Mb–Pc absolute helicities on the directions of the a, b and
c-axes, respectively. The 3D supramolecular structure of 3L is
presented in Fig. S12 (ESI ). The conformation relating to C(8)
3
and N(1) in the bridging ligand and C(19) and N(2) in the
terminal ligand adopted the more stable staggered-form,
different from those observed in 1L and 2L (Fig. S4, ESI ).
3
The enantiomorph of 21 screw axis is itself. Consequently, it
is expected that 3D is also crystallized in the P212121 space
group and have the same crystal structure as that of 3L, even if
we do not get its single crystal structure due to the inferior
quality of 3D crystals (the needle crystals are too small to
collect diffraction data). This is further supported by the fact
that 3L and 3D have the same as-synthesized powder XRD
patterns [Fig. S5(b), ESI ]. It is evident that 3D should have just
3
the opposite RC(10), RC(21) absolute configurations and Pa–Pb–
Mc absolute helicities, which is confirmed by CD spectra. A
simulated comparison of the absolute helicities of 3L and 3D
Fig. 8 Absolute helicity of 3L on the direction of a-axis. (a) A single-stranded
coordination polymeric M-helix. (b) A triple-stranded supramolecular M-helix.
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on the directions of a, b and c-axes is displayed in Fig. S13
(ESI ).
3
IR spectroscopy and thermogravimetric analysis
1L and 1D, 2L and 2D, 3L and 3D have very similar IR spectra,
respectively (Fig. S1, ESI ). In the IR spectra of 1L and 1D, the
3
broad absorption band in the region 2450–2700 cm21 is due to
the stretching vibration of NH2 group.27 A similar absorption
+
was also observed in a comparable N-(2-pyridylmethyl)-L-
alanine zwitterion.11 The strong absorption peak at 1705
cm21 is attributed to the asymmetric stretching vibration (vas)
of the protonated carboxyl group. The vas of the deprotonated
carboxyl group occurs at 1615 cm21, showing a 90 cm21 red-
shift as compared to that of the protonated carboxyl group.
These indicate the zwitterionic state of H2cala molecule, which
is consistent with the result of X-ray single crystal structure
analysis. The broad band centered at 3414 cm21 is ascribed to
the stretching vibration of hydrogen-bonded OH groups. As a
+
comparison, the characteristic absorption peaks of NH2
group at 2450–2700 cm21 and protonated carboxyl group at
1705 cm21 do not appear in the IR spectra of 2L and 2D. The
peaks at 1610 cm21 and 1375 cm21 are attributed to the vas
and vs (symmetric stretching vibration) of monodentate
carboxyl groups of the cala22 ligand, respectively. The broad
band centered at 3501 cm21 is due to the stretching vibration
of hydrogen-bonded water molecules. In the IR spectra of 3L
+
and 3D, the typical absorption band of NH2 group at 2450–
2700 cm21 is also observed. However, the vas of protonated
carboxyl group does not exist, which suggests the zwitterionic
states of Hcala2 ligands, in accordance with the result of X-ray
single crystal structure analysis. The peaks at 1600 cm21 and
1396 cm21 are ascribed to the vas and vs of deprotonated
carboxyl groups of Hcala2 ligands, respectively. Similar to
those in 2L and 2D, the broad band centered at 3434 cm21 is
also attributed to the stretching vibration of hydrogen-bonded
water molecules.
Fig. 9 Solid-state CD spectra of (a): 1L, 1D, 3L and 3D; (b) 2L and 2D.
the same wavelengths (a weak peak at 211 nm, a medium peak
at 230 nm, and two strong peaks at 245 and 282 nm) with
almost equal intensities, suggesting they are one pair of
enantiomers. Similarly, the spectra patterns of 3L and 3D are
inverted from one another with a strong and broad band
centered at 216 nm and a medium band at 287 nm. As
compared to the free ligands (1L and 1D), the CD signals of the
Cd2+ complexes (3L and 3D) in the range of 265–400 nm show
5 nm red-shift (282 A 287 nm), while the Cotton effects from
200 to 265 nm are significantly different. It corresponds to the
results of single crystal structure analysis that the free ligands
exhibit absolute helicity only on the direction of b-axis, while
the Cd2+ complexes show absolute helicities on three direc-
tions parallel to the a, b and c-axes.
In the ultraviolet and visible regions 200–800 nm [Fig. 9(b)],
the spectra patterns of 2L and 2D are also inverted from each
other with two medium peaks at 206 and 225 nm, a strong
peak at 259 nm, a weak and broad band centered at 585 nm,
and a strong and broad band centered at 717 nm. By
comparison with the free ligands, the CD signals of the
Cu(II) complexes in the range of 200–235 nm exhibit 5 nm
blue-shift (211 A 206 nm and 230 A 225 nm), but the Cotton
effects from 235 to 400 nm are distinct. It means that the two
new chiral centers [N(1) and Cu(1)] that arise from the
stereoselective assembly of ligands and Cu2+ ion may exhibit
chiral vicinal effects on the phenyl and carboxyl chromo-
The thermal stability of coordination polymers 2L, 2D, 3L
and 3D were examined by thermogravimetric analysis (TGA) in
a nitrogen atmosphere from 40 to 900 uC (Fig. S14, ESI ). 2L
3
and 2D, 3L and 3D show similar thermal decomposition
behaviors respectively, so 2L and 3L are described here as
representatives. The TGA curve of 2L undergoes two main
stages of weight loss. The weight loss from 40 to 70 uC is
observed to be 5.58%, corresponding to the release of the
lattice water molecule (theoretical value 5.61%). Then the
curve shows a plateau in the temperature range of 70–180 uC.
Upon further heating, the weight losses are due to the
decomposition of organic ligand and collapse of the 1D
coordination polymer. 3L also undergoes two main steps of
weight loss. The lattice and two coordinated water molecules
are gradually lost in the temperature range of 40–180 uC
(theoretical/found: 8.84%/8.80%). The second step of weight
loss occurs after 235 uC, which is attributed to the decom-
position of organic ligand and collapse of the 1D coordination
polymer.
Chiroptical and photoluminescent properties
In the CD spectra covering the ultraviolet region 200–400 nm
[Fig. 9(a)], 1L and 1D demonstrate opposite Cotton effects at
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phores. More interestingly, Cu2+-based d A d electronic
transitions in the visible region are also observed (585 and
717 nm), further proving that the chiralities of the divalent
Cu2+ centers in 2L and 2D are opposite.
Besides, the photoluminescent properties of 1L and 3L were
examined in the solid-state at room temperature. Upon
excitation at 310 and 320 nm, 1L and 3L exhibit broad
emission bands centered at 361 nm and 368 nm, respectively
6 Y. X. Tan, Y. P. He and J. Zhang, Inorg. Chem., 2011, 50,
11527.
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(Fig. S15, ESI ). Both of them can be assigned to ligand
centered fluorescence.
3
12 (a) X. L. Yang, M. H. Xie, C. Zou and C. D. Wu, CrystEngComm,
2011, 13, 6422; (b) X. L. Yang, M. H. Xie, C. Zou, F. F. Sun and C.
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Conclusions
In conclusion, three pairs of enantiomeric coordination
helices and supramolecular helices based on N-(4-carboxyl-
benzyl)-L(or D)-alanine ligands were synthesized and charac-
terized. The absolute helicities of 1L and 1D controlled by the
chirality on the a-C are similar to the situation of a-helices in
natural proteins, where L-amino acids result in P-helices. The
chiralities and absolute helicities of 2L and 2D are dependent
on the chirality of the ligands, indicating the self-assembly
process is highly stereoselective. 3L and 3D show ligand-
controlled Ma–Mb–Pc and Pa–Pb–Mc helicities on the directions
of the a, b and c-axes, respectively. Therefore, the chirality
transfer from ligand to complex and crystal exhibited here is
efficient. This work provides new perspectives on the rational
design of models for researching the relationship between
molecular chirality and supramolecular absolute helicity in
biopolymers.
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