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Keywords:

  • Coordination polymers;
  • Copper;
  • Chiral ligands;
  • Magnetic properties

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Reaction of CuCl2·2H2O with chiral Schiff bases and sodium dicyanamide led to the formation of two chiral copper(II) coordination polymers, namely [Cu4(L1)2(dca)4]n (1) and [Cu2(L2)(μ-Cl)(dca)(H2O)]n·nH2O (2) {H2L1 = (1R, 3S)-N′,N′′-bis[salicylidene]-1,3-diamino- 1,2,2-trimethylcyclopentane, H2L2 = (1R, 3S)-N′,N′′-bis[3-ethoxysalicylidene]-1,3-diamino- 1,2,2-trimethylcyclopentane, dca = dicyanamide}. Both complexes were structurally characterized by elemental analyses, IR spectroscopy and single-crystal X-ray diffraction. Complex 1 exhibits a two-dimensional polymeric structure formed by single dca bridging tetranuclear Cu4 units. Complex 2 displays a left-handed helical chain structure constructed from Cu2 dimers with single dca bridges. The chirality of 1 and 2 was confirmed by circular dichroism (CD) measurements in solution. Both complexes exhibit strong antiferromagnetic couplings with J = –308(4) cm–1 for 1 and J = –123(1) cm–1 for 2 in 2–300 K.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Stimulated continually by wide application prospects in chiral compounds, extensive research to synthesize chiral materials has been existing for several decades. Plenty of chiral compounds were synthesized by various methods.1 Work in this field has further endured not only due to the utilization for enantioselective separation2 and asymmetric catalysts,3 but also due to other fascinating properties such as piezoelectricity, ferroelectricity and non-linear optical properties caused by chirality.4 Generally, the homochiral polymers are synthesized by self-assembly of protean chiral organic ligands and transition metals or rare-earth metals. The route is directly and effectively oriented by introduction of intrinsic enantiopure organic entities as chiral source to form homochiral coordination architectures.5

Chiral Schiff base is one kind of popular candidates to construct chiral coordination polymers, and recently, this series of ligands based on camphoric diamine attracted keen interests of some working groups. For example, Jiang et al. have reported a series of cluster complexes employed Schiff base ligands having enantiopure or racemic camphoric diamine components.6 Fan et al. synthesized six complexes bearing Schiff base ligands condensed from camphoric diamine.7 These systems have the following advantages: (i) the ligands have two chiral carbon atoms in their structures and keep themselves homochirality; (ii) the ligands are easy to synthesize and modify flexiblely by choosing different aromatic aldehydes or ketones, which lead to control well over the dimensions of the coordination compounds; (iii) camphoric diamines are rigid organic ligands, so crystallization is relatively easier than flexible organic ligands, which is convenient for purification.

Among the excellent ancillary ligands, dicyanamide (dca) has been one of the remarkable bridging ligands within polymers due to its various coordination modes and efficient mediating magnetic exchange, which favors to generate multi-dimensional frameworks8 and hold interesting long-range ferromagnetic or canted spin antiferromagnetic ordering.9

The combination of chirality and magnetism may provide interesting functional properties to materials.10 Nevertheless, keeping homochirality still has a formidable challenge in the preparation of chiral complexes, because some complexes prepared by chiral ligands lost homochirality due to internal compensation or racemization.11 In our previous studies, we have prepared complexes adopted azido groups as bridges to link partially blocked dinuclear copper with chiral Schiff base.12 To extend our works on assembling chiral complexes, chiral Schiff base ligand are prepared from camphoric diamine, which are further exploited to synthesize magnetic molecular materials. We have obtained two copper(II) coordination polymers using dicyanamide anion as linkages, including a chiral 2D network and a helical 1D chain, which result from the chiral ligands derived from the salicylidene and 3-ethoxysalicylidene (Scheme 1). Herein, we report the synthesis, crystal structures, CD spectra, and magnetic properties of the two chiral coordination polymers.

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Scheme 1. Structure of H2L (1: R = H; 2: R = OCH2CH3).

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Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Crystal Structure of 1

X-ray crystallography reveals that 1 crystallizes in the monoclinic P21 space group and consists of 2D polymeric layer. Crystallographic data and structure refinement results are listed in Table 1. Selected bond lengths and angles are summarized in Table 2. A view of the asymmetric unit of 1 with atoms labeling is illustrated in Figure 1. The asymmetric unit is composed of four crystallographically independent CuII ions, two chiral tetradentate ligand L1, and four dca anions. The four central CuII atoms have distorted square pyramidal arrangement with different coordination environments. The τ parameters of geometrical distortion are 0.053, 0.300, 0.162, and 0.019 for Cu1, Cu2, Cu3, and Cu4, respectively, which clearly indicate that the coordinated arrangements of Cu1 and Cu4 are very close to square pyramid, while the coordinated arrangements of Cu2 and Cu3 display distortion towards trigonal bipyramid.13 For the square pyramidal arrangement of Cu1 and Cu4, the basal plane consists of two phenoxo oxygen atoms and two imino nitrogen atoms from the deprotonated Schiff base ligands (O1, O2, N1, and N2 for Cu1, O3, O4, N9 and N10 for Cu4). The apical site is occupied by nitrogen atom of bridging dca anion (N3 for Cu1, N8 for Cu4). For the square pyramidal arrangement of Cu2 and Cu3, the basal plane is constructed by two phenoxo oxygen atoms from the chiral ligands (O1 and O2 for Cu2, O3 and O4 for Cu3) and two nitrogen atoms from two dca groups (N6 and N16A for Cu2, N5 and N13B for Cu3). The apical site is located with nitrogen atom from bridging dca anion (N14 for Cu2, N11 for Cu3).

Table 1. Crystallographic data for 1 and 2.
 12
  1. a

    a) R1 = ∑||Fo||–|Fc||/∑|Fo|. b) wR2 = [∑w(|Fo2|–|Fc2|)2/∑w(|Fo2|)2]1/2.

FormulaC52H48Cu4N16O4C28H36ClCu2N5O6
Formula weight1215.22701.15
Crystal systemmonoclinicorthorhombic
Space groupP21P212121
a13.303(2)10.1712(10)
b10.4967(19)11.5695(11)
c18.774(3)26.835(3)
β93.366(2)90
V32617.0(8)3157.8(6)
Z24
Dc /g·cm–31.5421.475
μ /mm–11.6651.479
F (000)12401448
Reflections collected / unique19040 / 953713850 / 6126
Data / restraints / parameters9537 / 1 / 6916126 / 30 / 393
GOF (F2)1.0181.037
R1 a) [I > 2σ(I)]0.05400.0438
wR2 b) [I > 2σ(I)]0.12270.1062
Table 2. Selected bond lengths /Å and angles /° for 1.
Cu1–O11.949(5)Cu1–O21.960(4)
Cu1–N11.976(5)Cu1–N21.952(6)
Cu1–N32.400(7)Cu2–O11.988(4)
Cu2–O21.988(5)Cu2–N61.946(7)
Cu2–N142.189(7)Cu2–N16a1.966(7)
Cu3–O31.954(5)Cu3–O42.015(4)
Cu3–N51.967(7)Cu3–N13b1.980(7)
Cu3–N112.184(7)Cu4–O31.935(4)
Cu4–O41.954(5)Cu4–N82.328(8)
Cu4–N91.952(6)Cu4–N101.975(6)
Cu1–O1–Cu299.5(2)Cu1–O2–Cu299.1(2)
Cu3–O3–Cu4101.2(2)Cu3–O4–Cu498.5(2)
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Figure 1. Asymmetric unit of 1 with ellipsoids drawn at 50 % probability level. Hydrogen atoms are omitted for clarity.

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In complex 1, the bond lengths of Cu–O are in the range of 1.949(5) Å to 2.015(4) Å. The bond lengths of Cu–N range from 1.946(7) to 1.980(7) Å in basal plane, while the axial Cu–N bonds are longer [2.184(7)–2.400(7) Å]. The shortest Cu···Cu distances are 3.005, 3.006, 8.263, and 8.433 Å corresponding to the phenoxo and dca bridges, respectively. The O-bridging Cu1–O1–Cu2, Cu1–O2–Cu2, Cu3–O3–Cu4, and Cu3–O4–Cu4 angles are 99.5(2)°, 99.1(2)°, 101.2(2)°, and 98.5(2)°, respectively.

In the asymmetric unit of 1, there are two Cu2 dimers, where the CuII ions are connected by double phenoxo bridges. Both Cu2 dimers are linked through double μ1,5-dca anions, leading to the formation of the asymmetric tetranuclear copper unit (Cu4), which is similar to other tetranuclear complexes bridged by dca anions.14 The neighboring Cu4 units are bridged by single μ1,5-dca anion, generating a zigzag chain as shown in Figure 2a which is different from the reported in the literature.15 These zigzag chains are arranged parallel to each other, and further connected via μ1,5-dca bridges to form a 2D polymeric network (Figure 2b), in which chiral ligands are interdigitated (Figure S1, Supporting Information).

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Figure 2. View of the zigzag chain in 1 based on Cu4 units (a), and 2D polymeric network showing the interconnected chains (b). Only the dca units and the coordination sphere of the CuII ions are presented for clarity.

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Crystal Structure of 2

Complex 2 is a 1D helical chain coordination polymer. Its selected bond lengths and angles are listed in Table 3. The asymmetric unit comprises two crystallographically independent CuII ions, one chiral ligand L2, one chloride ion, one coordinated water, and one lattice water molecule (Figure 3). The Cu1 and Cu2 are both five-coordinate with distorted square pyramidal arrangement. Around Cu1, two nitrogen atoms (N1, N2) and two oxygen atoms (O1, O2) from chiral ligand L2 coordinate on the basal plane. The Cu–N bond lengths are 1.990(4), 2.011(4) Å, and the Cu–O bond lengths are 1.939(4), 1.988(3) Å. The bridging chloride atom locates at the apical position with a bond length of 2.646(2) Å. Around Cu2, the basal plane is completed by the coordination of one oxygen atom from L2, one water oxygen atom, and two nitrogen atoms from two μ1,5-dca anions. The apical position is also occupied by the bridging chloride atom. The τ parameters of geometrical distortion are 0.552 and 0.228 for Cu1 and Cu2, respectively, which is larger than those in complex 1. This indicates that the coordination arrangements around Cu1 and Cu2 are severe distortion towards trigonal bipyramid. The central Cu2(μ-O)(μ-Cl) ring is distorted from rectangle with Cu–O–Cu and Cu–Cl–Cu bond angles of 116.29(15)° and 79.24(5)°, respectively. The two copper(II) atoms are separated by 3.368 Å. All Cu–O and Cu–N bond lengths are in normal ranges, consistent with those observed for comparable copper(II) coordination environments.16 The longer Cu–Cl bond lengths are similar to those found in other pentacoordinate copper(II) complexes that contain bridging phenoxy oxygen and chlorine atoms.17

Table 3. Selected bond lengths /Å and angles /° for 2.
Cu1–O21.988(3)Cu1–O31.937(4)
Cu1–N11.990(5)Cu1–N22.011(4)
Cu1–Cl12.6456(17)Cu2–Cl12.6362(17)
Cu2–O21.978(3)Cu2–O52.002(4)
Cu2–N31.967(6)Cu2–N5a1.964(6)
Cu1–Cl1–Cu279.24(5)Cu1–O2–Cu2116.29(15)
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Figure 3. Coordination environment of CuII atoms with the thermal ellipsoids draw at 50 % probability level. The N4 atom is disordered. Hydrogen atoms and lattice water are omitted for clarity.

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Due to the effect of 3-ethoxysalicylidene group, the Schiff base ligand is distorted seriously, which results in a 52.55° dihedral angle between two benzene rings. Cu1 and Cu2 are connected by phenoxo group and chloride ion into a dimer which is obviously different from those in complex 1. The neighboring Cu2 dimers are connected by single μ1,5-dca to form a left-handed helical chain with a pitch length of 11.69(15) Å (Figure 4a). In the chiral helical chain, only Cu2 and single dca bridges construct the skeleton running parallel to the b axis (Figure 4b). It is interesting that if the Cu2 dimer can be visualized as a butterfly spreading its wings (Figure S2, Supporting Information), the chiral chain likes a bunch of butterfly perched on a pole in a certain order.

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Figure 4. View of the left-handed helical chain in 2 (a) and (b), only the Cu2 and dca bridges are shown for clarity.

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Spectroscopic Characterization

The infrared spectra of 1 and 2 are both consistent with their structural characteristics as determined by single-crystal X-ray diffraction. Complex 1 displays three characteristic peaks of dicyanamide ligand at 2273, 2218, and 2158 cm–1 (for 2: 2343, 2273, and 2192 cm–1), corresponding to the νas + νs(C≡N) combination modes, νas(C≡N) and νs(C≡N), respectively.18 For the L1 ligand in 1, the bands occuring at 3048, 1601, and 775 cm–1 are assigned to ν(C–H), ν(C=C) stretching vibrations, and δ(C–H) out-of-plane bending vibration of phenyl groups, respectively. The ν(C–H) stretching vibrations of methyl and methylene groups occur near 2971 cm–1. The characteristic band at 1542 cm–1 is assigned to the C=N stretching vibration of the L1 Schiff base ligand. The IR spectrum of 2 displays similar absorption band of L2 ligand.

The UV/Vis spectra of both Schiff base ligands and complexes 1 and 2 in DMF solutions are shown in Figure 5. The sharp band at 266 cm–1 of 1 (267 cm–1 for 2) is attributed to aromatic π–π* intraligand charge transfer transition, which are shifted down to 283 cm–1 (268 cm–1 for 2). Another typical bands appeared at 330 cm–1(316 cm–1 for 2) are considered as L[RIGHTWARDS ARROW]M charge transfer transition band, which are lowered to 373 cm–1 (364 cm–1 for 2) due to metal ion complexation.7 To confirm the optical activity of 1 and 2, the circular dichroism (CD) spectra were measured in DMF solution. In the CD spectra (the inset in Figure 5), negative Cotton effects were observed in the UV/Vis region, which is a good agreement between the CD and UV/Vis spectra. The CD spectra confirm the homochirality of 1 and 2.

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Figure 5. UV/Vis spectra of the Schiff-base ligands (H2L1 and H2L2) and complexes 1 (9.16 × 10–3 mM) and 2 (5.90 × 10–3 mM) in DMF solutions together with the corresponding CD spectra (the inset).

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Magnetic Properties

Temperature-dependence molar susceptibility measurements of the crystalline sample of 1 and 2 were carried out on a Quantum Design MPMS-XL7 SQUID magnetometer in an applied magnetic field of 2 kOe in the temperature range of 2–300 K. Plots of χMT vs. T for complexes 1 and 2 are depicted in Figure 6 and Figure 7, respectively, in both cases χM being the magnetic susceptibility per copper(II) dimer. At room temperature, the χMT values of 1 and 2 are 0.57 and 0.17 emu·K·mol–1, respectively, much lower than the spin-only value of 0.75 emu·K·mol–1 based on uncoupled double copper(II) atoms (SCu2+ = 1/2 and assuming gCu2+ = 2.0). When the temperature is lowered, the χMT values of both complexes decreases rapidly to 0.01 emu·K·mol–1 at 150 K for 1 and to 0.01 emu·K·mol–1 at 50 K for 2, suggesting the presence of a strong antiferromagnetic exchange interaction in both complexes.19

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Figure 6. Temperature dependence of magnetic susceptibilities of 1 in an applied field of 2 KOe. Solid line represents the best fit of the data.

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Figure 7. Temperature dependence of magnetic susceptibilities of 2 in an applied field of 2 KOe. Solid line represents the best fit of the data.

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According to the crystal structure, 1 consists of Cu2 dimers, where the Cu2+ ions are linked by double phenoxo bridges connecting two equatorial positions of the square pyramidal central copper atoms. The Cu2 dimers are further connected through double dicyanamide bridges to tetranuclear copper(II) units (Cu4). These Cu4 units are bridged by single dicyanamide groups to construct a 2D sheet-like polymer. In general, all the phenoxo-bridged complexes present stronger antiferromagnetic coupling, since they show a strong overlap with the magnetic orbitals dinline image on both copper(II) ions.15 Taking into account that this coupling is the most important one, we can neglect the coupling though the dca groups. Based on the isotropic Hamiltonian H = –2JS1S2, the experimental data in the whole temperature range were fitted to the Bleaney-Bowers equation modified by Kahn and co-workers to take into account some paramagnetic impurity for an isotropically coupled pair of S = 1/2 ions.20

  • equation image((1))

where χM is the molar magnetic susceptibility of the Cu2 dimer, ρ is the percentage of paramagnetic impurity, and other symbols have their usual meanings. The best-fit parameters reproducing satisfactorily magnetic properties of 1 as shown in Figure 6, are g = 2.26(4), J = –308(4) cm–1 and ρ = 0.0033(4), with an agreement factor of R = ∑[(χMT)calc–(χMT)obs]2/∑(χMT)obs2 = 6.4 × 10–6.

As presented in Figure 6, the Cu2 dimer model reproduces very satisfactorily the magnetic behavior in the whole temperature range. The result is logical taking into account that the coupling though dca groups is negligible compared to the strong magnetic exchange through double phenoxo bridges. According to the density functional calculations, the coupling parameter J is involved in the structural parameters, such as the Cu–O–Cu angle and the distance of Cu···Cu. The double phenoxo bridges in the basal planes mediate the antiferromagnetic exchange interaction due to the Cu–O–Cu bond angles larger than 98°.21

Complex 2 is a 1D chain coordination polymer based on copper(II) dimers connected by single phenoxo and halide bridges. Therefore, the 1D chain can be treated as alternating uniform Cu2 dimers with the different intradimeric constant Jd and intrachain exchange constant Jc to estimate the magnetic coupling interaction between CuII ions.22

  • equation image((2))
  • equation image((3))
  • equation image((4))
  • equation image((5))

where u = coth(JcSd(Sd+1)/kT)–kT/JcSd(Sd+1).

Using this rough model, the magnetic susceptibilities of 2 was simulated, giving the best fit with parameters g = 2.29(6), Jd = –123(1) cm–1, Jc = –16(7) cm–1, with R = ∑[(χMT)calc–(χMT)obs]2/∑(χMT)obs2 = 2 × 10–5.

The exchange coupling parameter Jd is –123(1) cm–1, which corresponds with that of complexes with a phenoxide/chloride bridging unit exhibiting antiferromagnetic couplings in the range of –80 to –190 cm–1.13a,23 Owning to the weaker antiferromagnetic coupling in chloride bridged,24 the Jd value is smaller than the J value of 1, in which two CuII ions are linked by two phenoxide bridges. As for the 1D systems with μ1,5-dca bridges, the magnetic coupling constant is normally very samll due to the weak overlap between the magnetic orbitals dinline image of copper(II) ions.25 Thus, the Jd value is much larger than the Jc value. This magnetic analysis showing a dominant coupling through the phenoxide/chloride bridge, also further confirms that it is reasonable to neglect the coupling though the dca groups in complex 1.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Two chiral copper(II) complexes based on Schiff base H2L1 and H2L2 ligands were synthesized and characterized. The substitution of phenyl in the ligand results in a structural transformation of the copper(II) complexes, from a 2D network to a 1D helical chain. Complex 1 is a 2D coordination polymer formed by single dca bridging Cu4 units, whereas complex 2 is a left-handed helical chain polymer constructed from copper(II) dimers with single dca bridges, attributed to the effect of 3-ethoxysalicylidene group of H2L2. The chirality of both complexes was confirmed by CD spectroscopy in DMF solution. Both complexes 1 and 2 showed strong antiferromagnetic interactions with J = –308(4) cm–1 for 1 and J = –123(1) cm–1 for 2.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

All chemicals were of analytical grade and obtained from commercial sources without further purification. Schiff base ligands H2L1 and H2L2 (Scheme 1) were prepared according to the literature methods using D-(+)-camphor as the starting materials.26 FT-IR spectra were recorded with a Nicolet Avatar A370 spectrometer using KBr pellets in the 400–4000 cm–1 region. Elemental analyses for carbon, hydrogen, and nitrogen were carried out with a Vario EL III elemental analyzer. UV/Vis spectra were performed with a Puxi TU-1900 spectrometer with a 1.0 cm quartz cell in DMF solvent. Circular dichroism spectra were measured with a JASCO J-815 spectropolarimeter using the same solutions as those for the UV/Vis determination. Variable-temperature magnetic susceptibility measurements were taken at an applied field of 2 kOe with a Quantum Design MPMS-XL7 SQUID magnetometer working in the temperature range of 300–2 K. The molar magnetic susceptibilities were corrected for the diamagnetism estimated from Pascal's tables and for sample holder by previous calibration.

Synthesis of [Cu4(L1)2(dca)4]n (1): To a 3 mL yellow methanol solution of H2L1 (18.4 mg, 0.050 mmol) was added dropwise a 5 mL methanol solution of CuCl2·2H2O (17.6 mg, 0.10 mmol) with gently stirring, which resulted in a dark brown solution. Afterwards a 2 mL methanol solution of NaN(CN)2 (27.8 mg, 0.30 mmol) was slowly added into the mixture. After filtration, the brown filtrate was kept undisturbed and slowly evaporated for 2 d. Brown strips of crystals were collected by filtration and washed with methanol (yield 56 % based on Cu). C52H48N16O4Cu4: calcd. C 51.39; H 3.98; N 18.44 %; found: C 51.58; H 3.97; N 18.59 %. IR (KBr, selected data): equation image = 3048 w, 2971 m, 2273 s, 2218 s, 2158 s, 1601 s, 1542 s, 1466 m, 1316 s, 1302 s, 865 m, 775 s, 754 m, 611 m cm–1.

Synthesis of [Cu2(L2)(μ-Cl)(dca)(H2O)]n·nH2O (2): This complex was synthesized with a similar procedure as described for the synthesis of 1, except using H2L2 instead of H2L1. Brown crystals were obtained with 62 % yield based on Cu. C28H36ClCu2N5O6: calcd. C 47.96; H 5.18; N 9.99 %; found: C 48.14; H 5.21; N 10.13 %. IR (KBr, selected data): equation image = 3428 s, 3059 w, 2978 m, 2343 m, 2273 m, 2192 s, 1611 s, 1551 m, 1467 s, 1394 m, 1291 m, 1244 s, 1218 s, 1046 m, 743 s, 534 w cm–1.

X-ray Crystallography: The perfect crystals of complexes 1 and 2 were carefully chosen to determine the X-ray diffraction. The crystal data were collected with a Bruker Smart Apex-II CCD diffractometer at room temperature. Intensities were collected with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å), using the φ and ω scan technique. The data reduction was made with SAINT package. Absorption corrections were performed using SADABS program. The structures were solved by the direct methods and refined on F2 by full-matrix least-squares using SHELXTL-2000 program package with anisotropic displacement parameters for all non-hydrogen atoms.27 Hydrogen atoms were introduced in calculations using the riding model. The crystal data and structural refinement results are summarized in Table 1.

Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-935817 http://www.ccdc.cam.ac.uk/data_request/cif and CCDC-935800 http://www.ccdc.cam.ac.uk/data_request/cif (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).

Supporting Information (see footnote on the first page of this article): Space-filling model of complex 1 showing helical axis. The dimer in 2 spreading its wings like a butterfly. FT-IR spectra of complexes 1 and 2.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

We cordially thank Mr. Kun Liu from College of Materials Science and Engineering (Shanghai University) for assistance with the Circular dichroism spectrometry experiments. This work was supported by the National Natural Science Foundation of China (No. 20901049 and 21171115), and the Opening Foundation of Zhejiang Provincial Top Key Discipline (No. 100061200130). Xiao-Kang Hou is supported by the Graduate Student Creative Foundation of Shanghai University (No. SHUCX120122).

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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