c2dt32578k 1..6 - pdfs.semanticscholar.org · N-donor organic ligands such as M(ATZ) 4Cl 2 (M = Co, Cu and †Electronic supplementary information (ESI) available: Crystallographic

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Cite this: DOI: 10.1039/c2dt32578k

Received 29th October 2012,Accepted 4th January 2013

DOI: 10.1039/c2dt32578k

www.rsc.org/dalton

First one-dimensional homochiral stairway-like Cu(II)chains: crystal structures, circular dichroism (CD)spectra, ferroelectricity and antiferromagneticproperties†

Xi-Li Li,*a Chun-Lai Chen,a Li-Feng Han,a Cai-Ming Liu,b You Song,c Xiao-Gang Yanga

and Shao-Ming Fang*a

The reactions of enantiopure chiral ligands (+)/(−)-4,5-pinenepyridyl-2-pyrazine (LS/LR) with CuCl2·2H2O

in CH3OH/CH2Cl2 solution led to the formations of one-dimensional homochiral enantiomeric pairs with

the formula [Cu(LR/S)Cl2]n·2H2O (R-1 and S-1, the isomers containing the LR and LS ligands, respectively).

The circular dichroism (CD) spectra verified their chiroptical activities and enantiomeric natures. Their

structures have been determined by X-ray single-crystal analyses, showing stairway-like and mirror-sym-

metric features, which represent the first examples of homochiral metal complexes with stairway-like

structures. The ferroelectric property measurement indicated that R-1 exhibits ferroelectricity with the

remnant polarization (Pr) value of 0.02 μC cm−2 under an applied electric field of 6.1 kV cm−1 at room

temperature. The magnetic investigation of R-1 showed a weak intrachain antiferromagnetic coupling

between Cu(II)–Cu(II) ions mediated by pyrazine, which can be interpreted by a spin-polarization mechan-

ism. All these results suggested that R-1 and S-1 are potential multifunctional molecule-based materials

combining optical activity, ferroelectricity and magnetism within one molecule.

Introduction

Chirality is not only an essential element of life but also animportant property of molecules. Molecule-based materialswith chirality have currently attracted intense interest owing totheir practical and potential applications in enantioselectiveseparation and catalysis, nonlinear optics and sensors.1

Furthermore, chirality can induce special functions such asmagneto-chiral dichroism (MChD), second harmonic gen-eration (SHG), triboluminescence, piezoelectricity and ferroelec-tricity in molecule-based materials, which correlate to thechiral molecular structures or noncentrosymmetric moleculararrangements.2 One target in this field is to obtain

enantiopure chiral magnets (ECM) because they may displaymagnetic properties along with intriguing MChD effects, SHGand ferroelectricity,3 thus being potential multifunctionalmaterials. However, the optically active magnets reported todate are still scarce mainly because of difficulties in control-ling the chirality of the entire magnetic system.4 Generally, aneffectual strategy to control the chirality is to use enantiopureorganic linkers, which transfer chiral information to the para-magnetic assemblies.3b,c,f

In particular, chiral magnets with the ferroelectric property,combining magnetism, optical activity and ferroelectricitywithin one molecule, have been one of the recently hot topicsof research.5 The study on these types of molecular-basedmaterials not only is of basic scientific concerns such as asym-metric magnetic anisotropy and magneto-chiral effects butalso is motivated by searching for the new multiferroics thathave potential applications in memory and read–write electricdevices.2j,3b,5d,6 Despite many efforts, limited successes havebeen fulfilled in the design of chiral molecular-based multi-ferroics due to the inherent challenge in the introduction ofmagnetic and electric order along with optical activity at amolecular level.5 However, some recent researches demon-strated that paramagnetic Cu(II) and Co(II) chloride withN-donor organic ligands such as M(ATZ)4Cl2 (M = Co, Cu and

†Electronic supplementary information (ESI) available: Crystallographic dataand additional figures. CCDC 904349 (R-1) and 904350 (S-1). For ESI and crystal-lographic data in CIF or other electronic format see DOI: 10.1039/c2dt32578k

aHenan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou

University of Light Industry, Zhengzhou 450002, China.

E-mail: [email protected], [email protected] National Laboratory for Molecular Sciences, Institution of Chemistry,

Chinese Academy of Sciences, Center for Molecular Sciences, Beijing 100190, ChinacState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical

Engineering, Nanjing National Laboratory of Microstructures, Nanjing University,

Nanjing 210093, China

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ATZ = 5-amino-1-H-tetrazole),7 Cu(Hdabco)(H2O)Cl3 (dabco =1,4-diazabicyclo[2.2.2]octane),8 display ferroelectricity, and allof them crystallize in polar point group satisfying the prerequi-site for the occurrence of the ferroelectricity. As an extensionof our studies on homochiral mononuclear metal complexesbased on enantiopure N-containing organic ligands,2f,h–j westrive to exploit new polynuclear molecular-based materialswith multifunctionality and to realize the coexistence ofoptical activity, magnetism and ferroelectricity within onemolecule.

To achieve such a purpose, the design of ligand is a keyfactor. An ideal ligand should possess the following character-istics: (1) it must possess several coordination sites to accom-modate more than one paramagnetic metal ion, (2) thedesigned ligand must be homochiral, whose stereogeniccenters should have the same stereochemistry configurations(all R or S). In this work, we have prepared such enantiomericpairs of ligands, (+)/(−)-4,5-pinenepyridyl-2-pyrazine (LS/LR,Fig. 1a), as chiral bridging ligands for CuCl2, which not onlysatisfy the above-mentioned request for the desired ligand butalso bear a pyrazine moiety that can mediate magnetismbetween paramagnetic centers to show antiferromagneticcoupling properties.9 Based on the reactions of LS/LR andCuCl2·2H2O in CH3OH/CH2Cl2 solution, respectively, a pair ofone-dimensional Cu(II) chain enantiomers with the formula[Cu(LR/S)Cl2]n·2H2O (R-1 and S-1, the isomers containing theLR and LS ligands, respectively) have been successfully syn-thesized and structurally characterized, and feature stairwaystructures. It is important to state that only a few complexeshave been reported to have stairway-like structures in compari-son to the documented ladder-like structures in literature,10

and none of them shows optical activity. Thus, the obtainedR-1 and S-1 represent the first examples of homochiral stairway-like structures in molecular-based complexes. Herein, the

synthesis, structures, magnetic and ferroelectric propertiesalong with optical activities of R-1 and S-1 are reported.

ExperimentalMaterials and general methods

All of the chemicals were commercially available and usedwithout further purification. Elemental analyses for C, H andN were performed on a Perkin-Elmer 240C analyzer. The IRspectra were recorded on a TENSOR27 Bruker Spectropho-tometer from KBr pellets in the region of 4000–400 cm−1. Thesolid-state CD spectra were carried out with a JASCO J-810Spectropolarimeter from KBr pellets (1% wt) at room tempera-ture. The P–E hysteresis loop of R-1 was measured on a Ferro-electric Tester Precision Premier II made by RadiantTechnologies Inc. at room temperature based on the com-pressed powder sample. Magnetic susceptibility measure-ments for the polycrystalline sample of R-1 were performed ona Quantum Design MPMS SQUID magnetometer in the temp-erature range 1.8–300 K. Thermogravimetric analyses (TGA)were conducted using a Perkin-Elmer Thermal Analyzer undernitrogen atmosphere from room temperature to 520 °C at aheating rate of 10 °C min−1. Approximately 10 mg of sampleswere placed in aluminium crucibles.

Synthesis of [Cu(LR)Cl2]n·2H2O (R-1)

A solution of CuCl2·2H2O (17 mg, 0.1 mmol) in CH3OH(10 mL) was added to a solution of (−)-4,5-pinenepyridyl-2-pyra-zine (LR) (27 mg, 0.1 mmol) in CH2Cl2 (10 mL) with gentlestirring for 30 minutes at room temperature. Slow evaporationof the resulting mixture gave green block crystals of R-1 afterone week. Yield: 72% (based on Cu). Elemental analyses calcd(%) for R-1 (C16H21N3O2Cl2Cu, 421.80): C 45.56, H 5.02,N 9.96; found: C 46.71, H 4.88, N 9.71. IR (cm−1, KBr disc):νCvN = 1502, νO–H = 3449.

Synthesis of [Cu(LS)Cl2]n·2H2O (S-1)

Compound S-1 was obtained as green block crystals by amethod similar to that of R-1, except that (+)-4,5-pinenepyri-dyl-2-pyrazine (LS) was used instead of (−)-4,5-pinenepyridyl-2-pyrazine (LR). Yield: 75% (based on Cu). Elemental analysescalcd (%) for S-1 (C16H21N3O2Cl2Cu, 421.80): C 45.56, H 5.02,N 9.96; found: C 46.31, H 5.12, N 9.78. IR (cm−1, KBr disc):νCvN = 1503, νO–H = 3447.

X-ray crystallography

The crystal structures of R-1 and S-1 were determined on aBruker SMART APEX CCD diffractometer with graphite mono-chromated Mo-Kα radiation (λ = 0.71073 Å) at room tempera-ture. Data reductions were made with the Bruker SAINTpackage. Absorption corrections were performed using theSADABS program. The structures were solved by directmethods and refined on F2 by full matrix least-squares usingSHELXL-97 with anisotropic displacement parameters for allnon-hydrogen atoms. H atoms were introduced in calculations

Fig. 1 (a) The structures of enantiopure ligands LR and LS; (b) the solid-stateCD spectra of LR and LS.

Paper Dalton Transactions

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using the riding model. All computations were carried outusing the SHELXTL-2000 program package. For R-1, there aretwo positions for one hydrate molecule with 0.5 and 0.5 occu-pancies for O2 and O2′ atoms, respectively. For S-1, there aretwo positions for one hydrate molecule with 0.6 and 0.4 occu-pancies for O2 and O2′ atoms, respectively. By the way, oneISOR restraint was used for preventing O1, O2, O2′ atoms frombecoming ‘non-positive-definite’ for both R-1 and S-1. Detailedcrystallographic data and structures refinement parameters forenantiomeric pairs R-1 and S-1 are summarized in Table 1.

Results and discussion

Enantiopure ligands LR and LS were synthesized according tosimilar procedures.11 Mirror-imaged CD spectra verified theirenantiomeric natures (Fig. 1b). The reaction of CuCl2·2H2Owith LR or LS (1 : 1 molar ratio) in CH3OH–CH2Cl2 affordedgreen crystals of R-1 or S-1. X-ray crystallographic analysesrevealed that R-1 and S-1 are enantiomers and crystallize inchiral space group P21 with the Flack values being 0.05(5) and0.01(3) for R-1 and S-1, respectively, suggesting that chiralityderiving from the chiral N-containing linkers has been trans-ferred successfully into the resulting solid structures. Theyshow similar physical properties. So only the results of R-1 aredescribed in detail.

Crystal structure of R-1

The crystallographic unit of R-1 contains a Cu(II) ion, a chiralligand LR, two Cl− anions and two free H2O molecules (Fig. 2).The structure of R-1 is composed of Cu(LR)Cl2 units, and theCu(II) ion is coordinated by one pyrazine nitrogen atom andone pinene-fused pyridyl nitrogen atom from a chiral ligandLR, one pyrazine nitrogen atom from another LR and two Clatoms. Each pyrazine ring connects with two neighbouringCu(II) ions to generate a uniform one-dimensional Cu(II) chainfeaturing a stairway structure (Fig. 3), whose width and heightboth are 7.116 Å. The neighboring Cu(II)–Cu(II)–Cu(II) angle in

the chain is 87.21°, and the angles of N1A–Cu1–N2 and N1A–Cu1–N3 are 89.0° and 94.5°, respectively, which are close to90° (Fig. S1, ESI†). In such a stairway-like structure, each five-coordinated Cu(II) ion exhibits a slightly distorted square pyra-midal (Sqp) CuN3Cl2 environment. A pyrazine nitrogen atom(N2) and a pinene-fused pyridyl nitrogen atom (N3) from thechiral ligand LR together with two Cl atoms form the basalplane (N2–N3–Cl1–Cl2), while a pyrazine nitrogen atom(N1*, *: −x, 0.5 + y, −z) from another LR occupies the apicalsite. Namely, one nitrogen atom of the pyrazine moiety isequatorially coordinated to the Cu(II) ion but the another nitro-gen atom is axially coordinated to the adjacent Cu(II) atom(in an eq–ax fashion). The mean deviation is 0.0582 Å for thebasal plane, and the Cl1, Cl2, N2 and N3 atoms deviate fromthe least-squares plane are −0.0507, 0.0511, 0.0644 and−0.0649 Å, respectively. The central Cu1 is shifted by 0.199 Åfrom the mean basal plane towards the apical site. TheCu1–Nequatorial bond lengths are 2.065(10) and 2.022(9) Å forCu1–N2 and Cu1–N3, respectively, which are shorter than theCu1–Naxial bond length [Cu1–N1A of 2.287(11) Å]. The (−)-4,5-pinenepyridyl-2-pyrazine (LR) ligand is close to planarity withthe dihedral angle between the pyrazine plane and the pinene-fused pyridine ring of 2.39° (0.69).

Circular dichroism (CD) spectra

To prove the chiroptical activities and the enantiomeric natureof R-1 and S-1, their solid-state circular dichroism (CD) spectrawere recorded based on a pressed KBr disk including 1% (w/w)of respective enantiopure crystal grains at room temperature.As shown in Fig. 4, R-1 and S-1 show intense Cotton effects,implying considerable chiroptical activities, and exhibitmirror-symmetrical CD spectra of one another, which are

Fig. 2 ORTEP representation (50% probability ellipsoids) of the asymmetricunits and enantiomeric pair of R-1 and S-1; for clarity, H atoms and lattice watermolecules are omitted.

Fig. 3 The mirror-symmetrical stairway-like structures of R-1 and S-1; Cu(II),cyan; Cl, green; N, blue; C, gray.

Table 1 X-ray crystallographic data for complexes R-1 and S-1

R-1 S-1

Chemical formula C16H21N3O2Cl2Cu C16H21N3O2Cl2CuFormula weight 421.80 421.80Crystal system Monoclinic MonoclinicSpace group P21 P21a/Å 6.3272(7) 6.3364(7)b/Å 9.8159(12) 9.8456(9)c/Å 16.2332(17) 16.314(2)β/° 96.078 96.190(10)V/Å3 1002.5(2) 1011.8(2)Z 2 2D/g cm−3 1.397 1.384μ/mm−1 1.368 1.355GOF 1.028 1.059R1

a/wR2b 0.0730/0.2029 0.0610/0.1513

Flack parameter 0.05(5) 0.01(3)

a R1 = ∑||Fo| − |Fc||/∑|Fo|.bwR2 = [∑w(Fo

2 − Fc2)2/∑w(Fo

2)2]1/2.

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indicative of their enantiomeric natures. The CD spectrum ofR-1 displays positive Cotton effects at λmax = 250 and 300 nmwith a negative one at λmax = 330 nm, while S-1 shows Cottoneffects with the opposite signals at the same wavelengths. Theresults are in accord with the structures obtained by singlecrystal X-ray diffraction.

Magnetic properties

The magnetic susceptibility measurements were performed ona SQUID magnetometer based on the polycrystalline sample ofR-1 over the temperature range of 1.8–300 K at an applied fieldof 2 kOe. The temperature dependence of the χMT product forR-1 was depicted in Fig. 5. Above 100 K, the value of χMT isnearly constant and reaches 0.39 cm3 K mol−1 at 300 K, whichis close to the expected value (0.42 cm3 K mol−1) for an iso-lated S = 1/2 Cu(II) ion assuming g = 2.1. Below 100 K, thevalue of χMT drastically drops to 0.31 cm3 K mol−1 at 1.8 K.This characteristic indicates that there is a weak intrachainantiferromagnetic coupling between Cu(II)–Cu(II) ions throughpyrazine moieties. For further investigation on the magneticbehavior of R-1, the magnetic susceptibility data were analyzed

in terms of the polynomial expression (1) for the uniformchain of S = 1/2.12

χ ¼ Ng2β2

4kTND

� �2=3ð1Þ

where N = 1.0 + 5.79799y + 16.902653y2 + 29.376885y3 +29.832959y4 + 14.036918y5, D = 1.0 + 2.7979916y + 7.0086780y2

+ 8.6538644y3 + 4.5743114y4 and y = J/2KT. Above 40 K, thebest fit gives the parameters J = −1.18 cm−1, g = 2.04 and R =1.02 × 10−6. Additionally, a good fit to the experimental databy the Curie–Weiss law χM = C/(T − θ) was obtained with aCurie constant C = 0.389 emu K mol−1 and a negative Weissconstant θ = −1.01 K. The C value corresponds to an isolatedCu(II) ion (S = 1/2) with g = 2.1. The negative value of θ furthersuggests an intrachain antiferromagnetic coupling betweenCu(II)–Cu(II) ions.

To interpret the magnetic coupling mechanism in Cu(II)complexes with pyrazine-type (pyz) bridging ligands, it iscrucial that each Pyz nitrogen atom is coordinated to a Cu(II)ion in an axial or an equatorial direction. Usually, the bridginggeometries of the pyz systems are classified into three types forCu(II) complexes: (1) one nitrogen atom of the pyz ring is co-ordinated to Cu(II) ion at an axial position, and the other iscoordinated at an equatorial site (ax–eq fashion); (2) bothnitrogen atoms of the pyz ring are coordinated to Cu(II) ionfrom the axial directions (ax–ax fashion); (3) both nitrogenatoms of the pyz ring are coordinated to Cu(II) ion at equatorialsites (eq–eq). For R-1, the coordination geometry of nitrogenatoms of the pyz ring belongs to the ax–eq fashion (Scheme 1).On the other hand, the coordination geometry of Cu(II) ion inR-1 is square pyramidal (Sqp) whose magnetic orbital is welldefined as 3dx2−y2 located on an equatorial plane. In this case,neither the dπ–pπ nor dσ–nσ orbitals overlap with the Cu(II)ion and the nitrogen atom of the pyz ring. So, the magneticexchange interactions through the σ as well as π pathways areunavailable, and the antiferromagnetic coupling in terms ofspin-polarization mechanism13 of R-1 will be reasonable.

The earlier research indicated that the highly conjugatedlinkers such as pyrazine and pyrimidine can induce the spinpolarization between the metal ions.9,14 According to the spinpolarization mechanism, an alternative spin density with theopposite sign always displays in a conjugated linker becausethe unpaired electrons of one atom polarize the electron cloudof the neighbouring atom, as shown in Fig. 6. Thus the spinsjust exhibit the antiferromagnetic arrangement between two

Fig. 4 Solid-state CD spectra of R-1 and S-1.

Fig. 5 Temperature dependence of χMT (○) and χM−1 (□) for R-1, the solid red

lines represent the best fits to the data.

Scheme 1 Coordination geometry of R-1 and definition of local x, y, z coordi-nations around Cu(II) ion.

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Cu(II) ions mediated by para linkage of the pyrazine moiety inR-1. Besides, the reported Cu(II) complexes with pyrazine-typebridging ligands in ax–eq fashion also display antiferromag-netic coupling with small J values (Table 2).

Ferroelectric properties

Recent research confirmed that CuCl2 combining withN-donor organic ligands with acentric structures display ferro-electricity.7,8 Since R-1 crystallizes in the chiral space groupP21 belonging to the polar point group 2, it satisfies the basicrequirement for ferroelectric properties. Fig. 7 clearly showsthe well-shaped P–E hysteresis loop of R-1 based on a com-pressed powder sample at room temperature, obviously

indicating its ferroelectricity. The remnant polarization (Pr)value is about 0.02 μC cm−2 at an applied electric field of6.1 kV cm−1. The remnant polarization value is comparable tothose reported Cu(II) chlorides containing N-donor organicligands such as Cu(ATZ)4Cl2 (Pr = 0.015 μC cm−2)7 and Cu-(Hdabco)(H2O)Cl3 (Pr = 0.045 μC cm−2).8 The saturation valueof spontaneous polarization (PS) is about 0.035 μC cm−2 forR-1, which is smaller than that of classical organic–inorganicferroelectrics NaKC4H4O6·4H2O (PS = 0.25 μC cm−2). Theferroelectricity of R-1 may arise from the off-centering ofcharges between Cu(II) cation and two chlorine anions, whichresults in spontaneous electric dipolar moments.7,15 Sub-sequently, these dipolar moments are aligned in the samedirection under an applied electric field, which leads to theoccurrence of ferroelectricity in R-1.

Conclusions

In this study, a new pair of Cu(II) one-dimensional chain enan-tiomers (R-1 and S-1) has been successfully prepared based onenantiopure N-donor linkers. Their structures exhibit stairway-like and mirror-symmetric features, representing the firstexamples of chiral complexes bearing a stairway-like structure.The investigations of ferroelectric and magnetic propertiesindicated that the homochiral Cu(II) complexes are potentialmolecule-based multifunctional materials combining opticalactivity, ferroelectricity and antimagnetic properties withinone molecule. Such a stereoselectively synthetic method provesto be useful for the preparation of molecule-based materialswith fascinating multifunctionality.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (20971112, 91022014 and21171089).

Notes and references

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Fig. 6 Representation of the spin polarization mechanism in R-1 between twoCu(II) ions bound by pyrazine.

Table 2 Magnetic coupling parameters (J), g values and the distances (d)between Cu–Cu for ax–eq Cu–μ-pyz–Cu compounds

Compounds J (cm−1) g d (Å) Ref.

R-1 −1.18 2.04 7.116 This work[Cu(hfac)2]2(μ-ibupyz) −0.07 2.23 7.16 16[Cu(hfac)2]3(μ-trpyz)2 −0.121 2.23 7.185 17[Cu(hfac)2]3(μ-iprpyz)2 −0.27 2.21 7.191 16[Cu(hfac)2(μ-prpyz)]n −0.086 2.14 7.308 13[Cu(hfac)2(μ-ibupyz)]n −0.83 2.21 7.068 16

hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione, ibupyz = 2-isobutyl-pyrazine, trpyz = 2,3,6-trimethylpyrazine, iprpyz = 2-isopropylpyrazine,prpyz = 2-propylpyrazine.

Fig. 7 P–E hysteresis loop of R-1 based on a compressed powder sample atroom temperature.

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Paper Dalton Transactions

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