Syntheses,Crystal Structures and Properties of Two Homochiral Co(Ⅱ)Complexes Based on N-Acetyl-L-tyrosine

MA NingSONG Hui-HuaYU Hai-Tao

(College of Chemistry and Material Sciences,Hebei Normal University,Shijiazhuang 050024,China)

Syntheses,Crystal Structures and Properties of Two Homochiral Co(Ⅱ)Complexes Based on N-Acetyl-L-tyrosine

MA NingSONG Hui-Hua＊YU Hai-Tao

(College of Chemistry and Material Sciences,Hebei Normal University,Shijiazhuang 050024,China)

Two novel homochiral coordination polymers based on N-acetyl-L-tyrosine(Hacty),{[Co(acty)(bpp)2(H2O)2] (NO3)·2H2O}n(1)and{[Co2(acty)2(bpe)3(H2O)3](ClO4)2·4H2O}n(2)(bpp=1,3-di(4-pyridyl)propane,bpe=1,2-di(4-pyridyl)ethane)have been synthesized and characterized by elemental analyses,IR,UV,TG,PXRD,singlecrystal X-ray diffraction.Complex 1 crystallizes in the monoclinic space group P21,and the six-coordinated Co(Ⅱ) ions are bridged by bpp ligands,exhibiting a 1D right-handed helical chain structure.Complex 2 belongs to the triclinic space group P1,and the six-coordinated dinuclear Co(Ⅱ)ions are bridged by bpe ligands,forming a 1D ribbon chain structure.The two different 1D chains are further extended into 3D supramolecular architectures through hydrogen-bonding interactions.The effect of different N-donor ancillary ligands on the structural assembly and diversity has been further discussed.Furthermore,circular dichroism spectra(CD)of compounds 1 and 2 were investigated.CCDC:1438882,1;1438883,2.

cobalt(Ⅱ)complex;N-acetyl-L-tyrosine;crystal structure;homochiral complex

0 Introduction

The rational design and controlled synthesis of chiralcoordinationpolymershasattractedgreat interest owing to their intriguing structures[1-3]and their potential applications in asymmetrical catalysis, chiral separation,luminescence,magnetism and nonlinear optical applications[4-8].Although a major challenge in this approach is the chance of the polymeric structures since many factors such as the solvent system,ligand-to-metal ratios,temperature,coordination geometry of the metals,nature of the ligands,pH value of solution[9-14],the internal surface functionality and topology of chiral complexes could be crafted efficiently by choosing appropriate ligands and metal ionswithdiversecoordinationgeometryunder optimum reaction conditions.Recently,numbers of chiral coordination polymers with aesthetic structural motifs and potential applications have been obtained by using multidentate ligands[15-16].

According to previously reported,the usage of chiral ligands as reactant precursors is one of the mostefficientandviableapproachesforthe homochiral coordination polymers[17-18].Chiral amino acid derivatives as good candidates for construction of chiral coordination polymers,have attracted considerable attention due to their coordination sites and rich bonding modes[19-21].N-acetyl-L-tyrosine can donate carboxylic group,which could adopt versatile coordinationmodes,includingmonodentate,bidentate chelating and bridging to give highdimensional structure.From the viewpoint of crystal supramolecular structure,N-acetyl-L-tyrosine creates countless possibilities for the formation of inter-and intramolecular interactions through aromatic interactions and hydrogen bonds to stabilize the structure of chiral coordination polymers.However,only a few examples of chiral coordination polymers with Hacty have been reported[22-23].

Apart from the amino acid derivative linker,N-donor ancillary ligands also play an important role in structural assembly and diversity of the coordination polymers.Herein,the second N-donor ligands with different conformations and lengths such as 1,3-di(4-pyridyl)propane and 1,2-di(4-pyridyl)ethane were employed respectively and we have gained two homochiral complexes,namely,{[Co(acty)(bpp)2(H2O)2](NO3)·2H2O}n(1)and{[Co2(acty)2(bpe)3(H2O)3](ClO4)2·4H2O}n(2) (bpp=1,3-di(4-pyridyl)propane,bpe=1,2-di(4-pyridyl) ethane).Bpp and bpe ligands all can be found as two geometric isomers in titled complexes,respectively (Scheme 1).More importantly,the simultaneous appearance of two different conformations of bpp or bpe ligands in one crystal structure is rarely reported. Herein we report their syntheses,crystal structures and circular dichroism spectra(CD).Furthermore,the effects of N-donor ligands on the structures of the complexes have been discussed in detail.

Scheme 1(a)Observed coordination mode of Hacty ligand for 1 and 2;(b)Observed coordination mode of Hacty ligand for 2;(c),(d)Observed coordination modes of bpp ligand for 1;(e),(f)Observed coordination modes of bpe ligand for compound 2

1 Experimental

1.1 Materials and methods

All reagents and solvents for syntheses were purchased from commercial sources and were used as received without further purification.Element analyses (C,H and N)were performed on an Elemental Vario EL elemental analyzer.Infrared(IR)spectra were measured on a FTIR-8900 spectrometer from 4 000 to 400 cm-1(KBr pellets).Thermogravimetric analyses (TGA)were carried out on a simultaneous STA 449F3/TENSOR 27 thermal analyzer under nitrogen with a heating rate of 10℃·min-1from room temperature to 800℃.Powder X-ray diffraction (PXRD)patterns were collected on a Bruker D8-Advance X-ray diffractometer using Cu Kα radiation(λ=0.154 2 nm,U=40 kV,I=40 mA)and ω-2θ scan mode at 293 K.The solid state circular dichroism(CD) spectra were recorded on a JASCOJ-810 spectropolarimeter with KCl pellets.Fluorescence spectra were recorded on a Hitachi F-4500 luminescence spectrometer.

1.2 Synthesis

1.2.1 {[Co(acty)(bpp)2(H2O)2](NO3)·2H2O}n(1)

Hacty(0.1 mmol,0.022 3 g)was stirred into a 10 mL aqueous solution.The solution was adjusted to pH=5.6 with the addition of 1 mol·L-1NaOH solution. Co(NO3)2·6H2O(0.1 mmol,0.029 1 g)was added to the solution,which was heated in a water bath at 60℃for about 10 min.A solution of bpp(0.2 mmol,0.039 6 g)in MeOH(5 mL)was slowly added.The resulting solution was stirred for 10 minutes,filtered off and allowed to stand for one week.The light red blockshaped transparent crystals1,suitable for X-ray analysis were obtained with 51%yield based on Co. Anal.Calcd.for C37H48CoN6O11(%):C,54.75;H,5.96; N,10.35.Found(%):C,54.88;H,5.91;N,10.36.IR (KBr,cm-1):3 246(b),1614(s),1 558(s),1 516(s),1 426 (s),1384(s),1331(s),1264(s),1227(m),1179(w),1123 (w),1 070(m),1 020(m),964(w),840(m),818(s),742 (w),616(w),521(m).

1.2.2 {[Co2(acty)2(bpe)3(H2O)3](ClO4)2·4H2O}n(2)

Compound 2 was synthesized in a procedure similar to that for 1 except that Co(ClO4)2·6H2O(0.1 mmol,0.036 6 g)and bpe(0.1 mmol,0.018 4 g)was used instead of Co(NO3)2·6H2O and bpp,respectively. The red block-shaped transparent crystals 2 were obtained with 50%yield based on Co.Anal.Calcd. for C58H74Cl2Co2N8O23(%):C,48.38;H,5.18;N,7.78. Found(%):C,49.59;H,5.30;N,7.91.IR(KBr,cm-1): 3 361(b),1 656(s),1 617(s),1 580(s),1 427(s),1 377 (m),1 295(w),1 252(m),1 100(s),1 019(m),878(w), 824(s),699(w),625(m),528(m).

1.3 X-ray crystallography

Suitable single crystals for title compounds were selected for single-crystal X-ray diffraction analyses (Crystal size/mm:0.31×0.24×0.11 for 1;0.42×0.27× 0.24 for 2).Crystallographic data were collected at 100 K for 1 and 298(2)K for 2 on a Bruker Smart Apex CCD diffractometer with graphite monochromated Mo Kα radiation(λ=0.071 073 nm).All structures were solved through direct methods and refined by fullmatrix least-squares on F2with SHELXL-97 program[24]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added theoretically and refined with a riding model.The assignment of the absolute structures for 1～2 was confirmed by the refinementoftheFlackparametertovaluesof -0.005(11)and-0.03(2),respectively[25-26].Oxygen atoms from ClO4-,carbon atoms and hydrogen atoms from bpe ligands were disordered in complex 2. Further crystallographic data and experimental details for structural analyses of complexes 1～2 are summarized in Table 1.The selected bond lengths and angles of 1～2 are given in Table 2.

CCDC:1438882,1;1438883,2.

Table 1Crystal data and structure refinements for complexes 1 and 2

Continued Table 1

Table 2Selected bond lengths(nm)and angles(°)for complexes 1 and 2

2 Results and discussion

2.1 Crystal structures

2.1.1 {[Co(acty)(bpp)2(H2O)2](NO3)·2H2O}n(1)

The crystallographic analysis reveals that complex 1 crystallizes in the monoclinic space group P21and features a 1D right-handed helical chain structure. There are one Co(Ⅱ)cation,one acty-anion,two bpp ligands,two coordinated water molecules,one NO3-and two lattice water molecules in the asymmetricunit.As shown in Fig.1a,each Co(Ⅱ)cation is sixcoordinated by one oxygen atom(O1)from one actyanion,two oxygen atoms(O10,O11)from two different coordinated water molecules and three nitrogen atoms (N2,N3,N6#1)from different bpp ligands.The N2, O1,O10,O11 atoms constitute the equatorial plane. N3,N6#1occupy the apical positions.The Co-O bond lengths range from 0.208 0(2)～0.215 10(2)nm,and the Co-N bond lengths are in the range of 0.214 4(2)～0.216 9(2)nm.

As shown in Fig.1b,the acty-anion acts as a decorating ligand in monodentate mode to connect one Co(Ⅱ)metal ion.The bpp ligands in 1 are of two types.One type of bpp serves as an unsymmetrical bridgingligand with TT[27](T=trans)conformation (Scheme 1c)and links two different Co(Ⅱ)cations, forming a 1D right-handed helical chain structure along the b axis.In the TT conformation of flexible bpp ligand,the dihedral angle between the two independent pyridine moieties of each bpp ligand is 66.42°.The other type of bpp acts as a monodentate ligand with TG[27](G=gauche)conformation(Scheme 1d)to connect one Co(Ⅱ)metal ion and arranges on both sides of the helical chain.The required uncoordinated NO3-anions are readily accommodated between the helical chain.In addition,extensive hydrogen bonds(Table 3)are observed and these helical chains are further linked together through hydrogen bonds to construct a 3D supramolecular structure(O(11)-H (11D)…O(9)0.177 nm;O(9)-H(9B)…O(7)#50.192 nm; O(11)-H(11E)…O(2)#70.189 nm;O(9)-H(9A)…O(3)#40.190 nm),as shown in Fig.1c.

2.1.2 {[Co2(acty)2(bpe)3(H2O)3](ClO4)2·4H2O}n(2)

Fig.1(a)Coordination environment of the Co center for 1;(b)Right-handed helical chain structure in complex 1; (c)3D supramolecular architecture through hydrogen-bonding interactions for 1

Table 3Hydrogen bond lengths(nm)and bond angles(°)for complex 1

Complex 2 presents a 1D chain structure which crystallizes in the Triclinic space group P1.Each crystallographic unit consists of two Co(Ⅱ)cations,two acty-anions,three bpe molecules,three coordinated water molecules,two ClO4-and four lattice water molecules.As shown in Fig.2a,the Co1 ion of 2 is six-coordinatedanddisplaysaslightlydistorted octahedral geometry,in which the coordination sphere of Co1 is O3N3 by three nitrogen atoms(N3,N5, N7#1)derived from three different bpe groups,one oxygen atom(O9)from one acty-anion and two oxygen atoms(O17,O18)from two different coordinated water molecules.The Co2 ion is surrounded by three nitrogen atoms(N4,N6,N8)derived from three different bpe groups,two carboxyl oxygen atoms(O13,O14)from one acty-anion and one coordinated water molecule. The Co-O bond lengths are in the range of 0.204 0(8)～0.220 2(7)nm.The Co-N bond lengths are in the range of 0.211 8(8)～0.220 7(8)nm.

The bpe ligands in 2 show bridging mode with anti and gauche[28]conformations(Scheme 1)alternatelyto link adjacent Co ions to form a 1D ribbon chain structure(the bpe ligand with anti conformation provides a N-to-N separation with the distance of 1.372 47(2) nmandthegaucheconformationseparationis 0.965 70(2)nm).Due to the flexibility of bpe ligand, the torsion angle py-C-C-py of bpe ligand for the gauche conformation is in the range of 79.7(3)°～81.83°and the dihedral angle between the two independent pyridine moieties of each bpe ligand is in the range of 46.61°～47.46°.Similar low values of the torsion angles have been found for other Co-(gauche-bpe)2-Co chains[28].Meanwhile,one half of acty-ligands adopt monodentate mode to connect Co1 ions arranged on one side of the chain,and the other half of actyligands act as chelating bidentate ligands to connect Co2 ions on the other side of the chain,as shown in Fig.2b.The adjacent 1D ribbon chains are linked to each other via O-H…O(Table 4)interactions to form a 3D supramolecular structure viewed along a axis direction,asshowninFig.2c.Significantly,the uncoordinatedanions are involved in hydrogen bondinginteractionwiththefourlatticewatermoleculesleadingtotheconstructionofa1D hydrogen-bonded supramolecular chain,as shown in Fig.2d.

Table 4Hydrogen bond lengths(nm)and bond angles(°)for complex 2

Fig.2(a)Coordination environment of the Co center for 2;(b)1D chain structure for complex 2;(c)3D supramolecular architecture through hydrogen-bonding interactions for 2;(d)Supramolecular chain through hydrogen-bonding interactions for 2

2.1.3 Effect of different N-donor ligands

The different structures of the two complexes with the same metal center of Co(Ⅱ)and acty-anion indicate that the different N-donor ancillary ligands have great influence on the connectivities of the complexesduetotheirdifferentstructuresand flexibility.In this work,we select two kinds of N-donor ligands(bpp and bpe)to observe their effects on the assembly of the coordination compounds.The bpp and bpe ligands are both N-containing linkages which can provide twisty configurations.For example, thetwistbppligandcanassumetwodifferent configurations(TT and TG)owing to the orientations of-CH2-CH2-CH2-groups.While the bpe ligand also can assume two different conformations(anti and gauche)that display different N-to-N distances owing to the orientations of-CH2-CH2-groups.The different degree of distortion is due to the different numbers of C atoms.For example,the dihedral angle between the two independent pyridine moieties is 66.42°for TG configuration of bpp ligand and in the range of 46.61°～47.46°for gauche configuration of bpe ligand.Further comparative analysis of the structures of the two complexes reveals that complex 1 shows a 1D righthanded helical chain structure and complex 2 shows a 1D ribbon chain structure.So from the results above, we can see that the different configurations and tortuosities of N-donor ligands play an important role in the formation of the crystal structures.Meanwhile, the coordination modes of the Hacty are different in the two complexes and only act as decorating ligands. The acty-ligand in monodentate mode connects one Co ion in complex 1.The acty-ligands act as monodentate and chelating bidentate ligands connect Co ions in 2.Additionally,complexes 1 and 2 selfassembletoform3Dsupramolecularstructures through hydrogen-bonding interactions.

2.2 UV-Vis spectra and IR spectra

The UV-Vis spectra of bpp,bpe,complexes 1 and 2 in H2O/DMF(1∶10,V/V)solution with the concentration of 2×10-5mol·L-1at room temperature are shown in Fig.3.Bpp and bpe show intense absorption band at 253 and 255 nm,respectively.The absorption bands of these ligands can be assigned to the π-π transition.Complexes 1 and 2 all show intense absorption bands at 255 nm and weak bands at 274 nm.The locations of intense absorption bands of complexes 1 and 2 are almost similar to the assistant ligands,while strength of intense absorption bands are stronger than the assistant ligands,which may be due to the ligands coordinate to metal ions.The different π-π transition energies may be due to the formation of Co-N and Co-O bands,which result in the weak absorption bands[29-30].

The IR spectra(Fig.4)of the complexes show broad at about 3 246 cm-1for 1,3 361 cm-1for 2, which can be ascribed to the presence of νas(O-H) stretching frequencies of Hacty.The νas(COO-)and νs(COO-)vibrations can be observed at 1 558 and1426cm-1for1,1580and 1 427 cm-1for 2,respectively. The IR bands of bpp at 1 384 and 1 614 cm-1are due to νC=Cand νC=Nvibrations for compound 1.In compound 2,νC=Cand νC=Nvibrations of bpe show stretches at 1 377 and 1 617 cm-1[27-28].

Fig.3UV-Vis absorption spectra of complexes 1 and 2

Fig.4IR spectra of complexes 1 and 2

2.3 Thermal analysis and PXRD patterns

The thermal analysis experiments are performed on solid samples consisting of numerous single crystals in the 20～800℃range under N2atmosphere(Fig.5). The TGA curve of 1 suggests that the first weight loss in the range of 56～143℃corresponds to the release of two lattice water molecules and two coordinated water molecules(Obsd.8.27%,Calcd.8.87%).Above 262℃,compound 1 releases the organic moieties to decompose.The TGA curve of 2 suggests that the first weight loss in the range of 27～136℃corresponds to the release of seven lattice water molecules(Obsd. 7.62%,Calcd.8.75%).Compound 2 continues to lose weight up 278～800℃,indicating the continuous expulsion of organic moieties even at the upper limit of the measurement range.The decomposition of organic ligands of compounds 1 and 2 occur above 262 and 278℃,respectively,indicating that the two complexes are relatively stable.

Fig.5TGA curves of complexes 1 and 2

To check the phase purity of the products,the powder X-ray diffraction(PXRD)experiments were carried out for 1～2 at room temperature.The main peaks of simulated patterns of 1～2 are basically consistentwiththeirexperimentalpatterns, demonstrating that the bulk synthesized materials and the measured single crystals are the same.The differences in intensity may be due to the preferred orientation of the crystal samples(Fig.6).

2.4 Solid-state circular dichroism(CD)spectra

To further examine the chiroptical activities,the solid-state CD(circular dichroism)spectra of the ligand Hacty and complexes 1～2 were measured in KCl pellet,as shown in Fig.7.

Chiral Hacty exhibits two positive bands centered at 228 and 287 nm.The CD spectrum of 1 shows two positive Cotton effect around 230 and 284 nm and a negative signal centered around 241 nm,and the spectrum of 2 shows two positive Cotton effect around 233 and 269 nm.Compounds 1 and 2 show obvious Cotton effect in the CD spectra,which confirm the chirality of the bulk materials[31].The formation of the chiral structures of 1 and 2 have influence on the direction(+or-)of the CD signals except for a slightshift of the absorption peaks position compared with Hacty,which indicates the CD chromophores maybe affected by the ligand Hacty.The outcomes are in accord with the structures obtained by single crystal X-ray diffraction.

Fig.6XRD patterns of complexes 1 and 2

Fig.7Solid-state CD spectra of bulk samples of the ligand Hacty and complexes 1 and 2

3 Conclusions

In summary,we have successfully synthesized two homochiral compounds using N-acetyl-L-tyrosine under same condition.Compounds 1 and 2 show 1D right-handed helical chain and 1D ribbon chain structure,respectively.The results show that different numbers of C atoms have critical effect on the flexibilities and tortuosities of N-donor ligands(bpp and bpe),thereby different lengths and configurations of N-donor ligands directly influence the final crystal structures.3D supramolecular structures of 1 and 2 are formed finally through hydrogen-bonding interactions to stabilize their homochiral networks.Meanwhile, N-acetyl-L-tyrosineexhibitsdifferentcoordination modes.Compounds 1 and 2 are chiral complexes,and theCDspectrademonstratedthattheyareall homochiral.This study also reveals the significant effects of auxiliary ligands on the self-assembly of chiral compounds.Future research will further focus onthechiralsynthesis,whichmightleadto breakthroughs in synthesis of more chiral materials.

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基于N-乙酰-L-酪氨酸構筑的兩例純手性鈷(Ⅱ)配合物的合成、結構及性質

馬寧宋會花＊于海濤
(河北師范大學化學與材料科學學院，石家莊050024)

利用手性配體N-乙酰-L-酪氨酸(Hacty)與鈷鹽通過溶液法合成了2例純手性配合物{[Co(acty)(bpp)2(H2O)2](NO3)·2H2O}n(1)和{[Co2(acty)2(bpe)3(H2O)3](ClO4H2O}n(2)(bpp=1，3-聯(lián)(4-吡啶)丙烷，bpe=1，2-聯(lián)(4-吡啶)乙烷)，并對它們進行了元素分析(EA)、紅外光譜(IR)、紫外光譜(UV)、熱重(TG)、粉末X射線衍射(PXRD)及X射線單晶衍射測定。配合物1屬于單斜晶系P21空間群，六配位的Co(Ⅱ)離子被bpp配體連接形成一維右手螺旋鏈結構。配合物2屬于三斜晶系P1空間群，六配位的雙核Co(Ⅱ)離子被bpe配體連接形成一維帶狀鏈結構。在氫鍵的作用下，它們均形成三維超分子結構，深入討論了不同構型的含N輔助配體對配合物結構的影響。此外，測定了2例手性配合物的圓二色(CD)光譜。

鈷配合物；N-乙酰-L-酪氨酸；晶體結構；純手性配合物

O614.81+2

A

1001-4861(2016)06-1078-11

2015-12-17。收修改稿日期：2016-04-15。

10.11862/CJIC.2016.127

國家自然科學基金(No.21141002)和河北省自然科學基金(No.B2012205040)資助項目。

＊通信聯(lián)系人。E-mail：songhuihua@mail.hebtu.edu.cn