LIU Lu Malik Abaid Ullah SHU Mou-Hai

(School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China)

ABSTRACT A pair of enatiomerically pure ligands, (R-)/(S-)2-(4-carboxyphenyl)-4,5-dihydro- thiazole-4-carboxylic acid (H2LR & H2LS), have been synthesized by the reactions of 4-cyanoben- zoic acid with L- and D-cysteine, respectively. Four coordination polymers have been prepared from the ligands and structurally determined by single-crystal X-ray diffraction analysis. Complexes 1R and 1S ([NiL(Py)(H2O)]?H2O, for 1R, L = (LR)2-; for 1S, L = (LS)2-) exhibit chiral helical one-dimensional chains, and complexes 2R and 2S ({[ZnL2(H2O)3]?CH3CN}n, for 2R L = (LR)2-, for 2S L = (LS)2- ) are two-dimensional sheets. Luminescent and chir-optical properties have been investigated and compared with the free ligands. The complexes have blue-shift in luminescence spectrum compared with the free ligands.

Keywords: coordination polymer, crystal structure, chiral ligand, fluorescence;

1 INTRODUCTION

Chiral metal-organic frameworks, also known as chiral coordination polymers, are a class of crys- talline materials having chiral infinite structures built with multitopic organic ligands and metal ions. Although chiral functional coordination polymers could be obtained from achiral ligands[1], the preparations of chiral coordination polymers through the self-assembly of optically pure chiral organic ligands with metal ions are the major process[2-12]. Natural amino acids are one of the rich sources among the chiral organic linkers for the construction of chiral complexes, resulting in the development of various structures of fascinating architectures[13,14]. Retention of chirality by incorporating chiral linker to a coupling product is one of the most powerful strategies for the synthesis of chiral materials. A number of small chiral molecules are luring the great interests of material chemist to design and synthesize the new chiral product of their choice[15,16].

4,5-Dihydrothiazole-4-darboxylic acid is the chiral building block in firefly luciferin (D-(-)-luciferin). This chiral functional moiety can be synthesized by the reaction between carbonitrile and chiral cysteine. Although this kind of molecules has been applied in bioluminescence imaging and sensors[17,18], the derivatives bearing 4,5-dihydrothiazole-4-darboxylic acid has rarely been used as ligands involving in the preparation of metal complexes so far[19,20]. Due to the fact that there are multiple coordination sites in 4,5-dihydrothiazole-4-darboxylic acid and their chiral nature, it would be of great significance to design and synthesize new chiral ligands derived from 4,5-dihydrothiazole-4-darboxylic acid by the reaction between the carbonitrile and chiral cysteine.

Herein we report the synthesis of a pair of chiral ligands, namely (R)- and (S)-2-(4-carboxyphenyl)- 4,5-dihydrothiazole-4-carboxylic acid (H2LRand H2LS), and the preparation and characterization of four coordination polymers obtained from this pair of ligands.

2 EXPERIMENTAL

2. 1 Instruments and reagents

All chemicals purchased from commercial sources were of reagent grade and used as received without further purification unless otherwise stated.1H and13C NMR spectra were measured on a Bruker AVANCE III HD 400 spectrometer in DMSO-d6at 293 K with TMS as the internal standard unless otherwise specified. HRMS for the ligand were recorded on a Waters Micro mass Q-TOF Premier Mass Spectrometer in negative model. The capillary voltage, source temperature, desolvation temperature, desolvation gas, scan range, scan time and interscan time were set at 350oC, 600 l/hr, m/z 100～1500, 1s and 0.02 s, respectively, and the quoted m/z values are for the major peaks in the isotopic distribution. Circular dichroism (CD) spectra of the ligands and the complexes were recorded on a JASCO T-815 spectropolarimeter fitted with DRCD apparatus. The UV-visible (UV-Vis) absorption spectra were recor- ded using a Lambda 35 UV/Vis Spectrometer (Perkin Elmer, Inc., USA). Melting points were measured with SGW X-4 micro melting point apparatus. The fluorescence spectra were performed on a Perkin Elmer LS 55 fluorescence spectrophotometer.

2. 2 Synthesis of the ligands (H2LR) and (H2LS)[19, 20]

4-Cyanobenzoic acid (2.68 g, 20.0 mmol), L-cysteine or D cysteine (3.63 g, 30.0 mmol) and NaHCO3(2.52 g, 30.0 mmol) were mixed in a solution of CH3OH (200 mL) and H2O (120 mL), then a catalytic amount of NaOH (40.0 mg, 1.00 mmol) was added (pH ～7.5). The reaction mixture was stirred at room temperature until TLC analysis indicated the disappearance of 4-cyanobenzoic acid (48 h). Then CH3OH was removed from the mixture by evaporation under reduced pressure at 40 °C. The resulting crude mixture was acidified with dilute hydrochloride acid to pH about 2 (indicator paper) while cooling in a water/ice bath. The precipitated solid was collected by filtration and washed several times with cold water and dried in vacuum, obtaining the ligands as white powder.

H2LR, yield 3.51 g (70%); m.p. 220～222 °C.1H- NMR (400 MHz, DMSO-d6):δ= 13.29 (s, 1H), 8.06～8.04 (d, 2H), 7.92～7.90 (d, 2H), 5.35(t, 1H), 3.79～3.73 (m, 1H), 3.69～3.64 (m, 1H);13C NMR (151 MHz, DMSO-d6):δ=171.65, 167.77, 166.66, 135.86, 133.45, 129.83, 128.36, 78.48, 35.23. H RMS Calcd. for [H2LR]-m/z = 250.0179, Obs. 250.0189 (100%) with insource fragmentation peak of [H2LR-CO2]-at m/z 206.0296 (95%).

H2LS, yield 3.60 g (72%); m.p. 220～222 °C.1H NMR (400 MHz, DMSO-d6):δ= 13.19 (s, 1H), 8.05～8.03 (d, 2H), 7.91～7.89 (d, 2H), 5.35(t, 1H), 3.78～3.73 (m, 1H), 3.68～3.63 (m, 1H);13C NMR (151 MHz, DMSO-d6):δ=171.65, 167.77, 166.66, 135.86, 133.45, 129.83, 128.36, 78.48, 39.52 (DMSO), 35.23. H RMS: Calcd. for [H2LR]-m/z = 250.0179, Obs. 250.0186 (100%) with insource fragmentation peak of [H2LS-CO2]-at m/z 206.0286 (80%).

2. 3 Syntheses of the complexes 1R, 1S, 2R, and 2S

Complexes 1R and 1S Ligand (R)-2-(4-car- boxyphenyl)-4,5-dihydrothiazole-4-carboxylic acid (H2LR) (37.5 mg, 0.15 mmol) was dissolved in water (3.0 mL) in a straight test tube followed by the addition of pyridine (dropwise slowly) until the ligand was completely dissolved to get a transparent solution. A solution of Ni(ClO4)2·6H2O (54.9 mg, 0.15 mmol) in acetonitrile (1.0 mL) was carefully layered on the above solution. The test tube was covered and kept undisturbed. Needle like blue crystals of complex 1Rsuitable for X-ray diffraction analysis were obtained after a week period. Yield 27.0 mg (42% based on the ligand). Complex 1S (blue crystal) was synthesized by a similar method as that of complex 1R except that the ligand H2LRwas replaced by the ligand H2LS. Yield 28.3 mg (44% based on the ligand).

Complexes 2R and 2S Ligand (R)-2-(4-carb- oxyphenyl)-4,5-dihydrothiazole-4-carboxylic acid (H2LR) (37.5 mg, 0.15 mmol) was dissolved in water (3.0 mL) in a straight test tube followed by the addition of (CH3)4NOH solution (dropwise slowly) until the ligand was completely dissolved to get a transparent solution. A solution of Zn(ClO4)2·6H2O (56.0 mg, 0.15 mmol) in acetonitrile (1.0 mL) was carefully layered on the above solution. The test tube was covered and kept undisturbed. Needle like colorless crystals of complex 2R suitable for X-ray diffraction analysis were obtained after a week. Yield 31.0 mg (57% based on the ligand). Complex 2S was prepared by a similar method as that of complex 2R except that the ligand H2LRwas replaced by the ligand H2LS. Yield 28.5 mg (52% based on the ligand).

2. 4 Crystallographic measurements and structure determination

The crystals of the complexes with suitable size were selected for X-ray single-crystal diffraction. The data were collected on a Bruker D8 VENTURE CMOS photon 100 diffractometer with Helios MX multilayer monochromator Cu-Kαradiation (λ= 1.54178 ?). The multi-scan absorption correction was applied by using the SADABS program (SMART-CCD Software, version 5.0; Siemens Analytical X-ray Instruments: Madison, WI, 1996). Data collection and reduction were performed using the SMART and SAINT software. Structures were solved by direct methods and refined with full-matrix least-squares onF2using SHELXT-2014[21]and SHELXL-2014[22]. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles, and the hydrogen atoms of organic ligands were placed in calculated sites and included as riding atoms with isotropic displacement parameters set to 1.2Ueqof the attached C atoms. All the calculations were carried out using SHELXTL program[21,22]. For complex 1R, a total of 10451 reflections were collected and 3125 were indepen- dent (Rint= 0.0275). For complex 1S, a total of 16586 reflections were collected and 3104 were indepen- dent (Rint= 0.0643); the maximum residual density during the refinement is 1.49, which is near the nickel center in complex 1S (0.66 ?). For complex 2R, a total of 12361 reflections were collected and 5132 were independent (Rint= 0.0346), and for complex 2S, out of the 16149 total reflections, 55245 were independent (Rint= 0.0246). The crystallogra- phic data and structure refinement details of the complexes are summarized in Table 1. Selected bond lengths and bond angles of complexes 1R and 1S are collected in Table 2, and bond lengths and bond angles of complexes 2R and 2S are listed in Table 3.

Table 1. Data Collection and Processing Parameters for Complexes 1 and 2

β/° 97.61(3) 97.234(4) 106.668(2) 107.0940(10) γ/° 90 90° 90 90 V/?3 872.8(4) 887.43(19) 1458.97(11) 1446.33(8) Z 2 2 2 2 Crystal size/mm 0.29 × 0.22 × 0.14 0.31 × 0.24 × 0.17 0.26 × 0.19 × 0.16 0.25 ×0.14 × 0.12 ρcalcd (Mg/m3) 1.610 1.583 1.649 1.663 μ/mm-1 3.059 3.008 3.914 3.949 F(000) 436 436 736 736 θ range/° 2.774 to 68.478°. 2.771 to 66.534°. 2.645 to 68.344°. 3.163 to 68.307°. Reflections collected 10451 16586 12361 16149 Independent reflections 3125 (0.0275) 3104 (0.0634) 5132 (0.0346) 5245 (0.0246) Data/restraints/parameter 3125/7/252 3104/7/252 5132/17/390 5245/22/422 GOF (F2) 1.045 1.020 1.046 1.046 Final Rindices (I ＞ 2σ(I)) R, 0.0227, 0.0615 0.0628, 0.1566 0.0373, 0.0897 0.0184, 0.0478 Rindices (all data) 0.0234, 0.0655 0.0647, 0.1614 0.0438, 0.0936 0.0186, 0.0480 Flack 0.04(2) 0.16(5) 0.028(14) 0.025(16) (Δρ)max, (Δρ)min (e·?-3) 0.218 and -0.308 1.491 and -0.398 0.761 and -1.648 0.302 and -0.268

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for Complexes 1R and 1S

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complexes 1R and 1S

3 RESULTS AND DISCUSSION

3. 1 Preparation of the ligands

The ligands H2LRand H2LSwere prepared with good yields by the reactions between 4-cyano- benzoic acid (Sch. 1) andL-cysteine orD-cysteine in water/methanol mixture under a slightly basic condition (pH≈7.5) at room temperature. The1H and13C NMR spectra of the ligands indicate that they are quite pure, and the resonance signals of the protons of two ligands match each other very well. The circular dichroism (CD) spectra of the ligands H2LRand H2LSare mirror images of each other (Fig. 1), and the observation indicates their enantiomeric nature. The main peak at m/z 250.0189 in negative mode corresponding to the species (HLR)-was observed from the electrospray ionization mass spectrometry (ESI-MS) of the ligand H2LRwith insource fragmentation peak of (H2LR-CO2)-at m/z 206.0296. The main peak of ESI-MS of the ligand H2LSis at m/z 250.0186, corresponding to the species (HLS)-while the insource fragmentation peak of (H2LS-CO2)-is at m/z 206.0286. All these characterizations indicate that the ligands H2LRand H2LSare as per the design.

Scheme 1. Synthesis of ligands H2LR and H2LS

Fig. 1. UV-Vis and CD spectra of ligands H2LR and H2LS in MeOH

3. 2 X-ray single-crystal structure analysis

As shown in Table 1, the space groups of complexes 1R and 1S are the same and the cell parameters are similar, so complexes 1R and 1Sare iso-structural. Therefore, only the structure of complex 1Ris described in detail. The coordination environment of the central Ni(II) ion in complex 1R is shown in Fig. 2, where the Ni(II) ion is coor- dinated by an oxygen atom from carboxylate group of a ligand (LR)2-, a NO unit from another ligand, a nitrogen atom of pyridine, and an oxygen atom of a water molecule, forming a tetragonal pyramidal geometry. The O(1), O(4A), N(2), and N(1A) atoms are located in the basal plane, and O(5) atom occupies the axial (apical) positions. The Ni-N bond distances fall in the range of 1.995(2)～1.999(2) ?, the Ni-O bond distances are 1.981(2) and 2.206(3) ?. The bond angles vary from 82.46(9) to 173.84(10)° (Table 2). The geometric parameter τ value of the polyhedron NiN2O3is 0.450, calculated by using the literature method[23]. Each Ni(II) ion coordinates to two ligands, and each ligand bridges two nickel(II), forming a one-dimensional left hand helical chain along thebaxis, as shown in Fig. 3. The result implies that the helical configuration of the complex may be controlled by the structural feature of the chiral ligand. To confirm the influence of the ligand chirality on the helical structure in the complex, an enantiomer ligand H2LSand the Ni(II) complex 1S have been prepared successfully. Just as expected, the Ni(II) ion and ligand bridge each other to form a one-dimensional right hand helical chain along thebaxis in the crystal of complex 1S. As shown in Fig. 4, the circular dichroism (CD) spectra of complexes 1R and 1S made from R- and S-enantiomers of the ligand are mirror images of each other, which indicate their enantiomeric nature. The observations confirm that chiral helical configuration of the complexes is controlled by the structural feature of the chiral ligands. There are hydrogen bonds between O and H atoms within the helical one-dimensional chain in the crystal of complex 1R (Fig. 5), with the hydrogen bond data of complex 1R and 1S listed in Table 3.

Fig. 2. Coordination environments of the center ions in complexes 1R (left) and 1S (right)

Fig. 3. Helical chain in complexes 1R and 1S

Fig. 4. Circular dichroism (CD) spectra of complexes 1R and 1S

Fig. 5. Packing diagram of complex 1R

The fact that complexes 2R and 2S have the same space groups and similar cell parameters (Table 1) indicates theyare iso-structural. Therefore, only the structure of complex 2R is described here. As shown in Fig. 6a, considering the distances of Zn(1)-O(3) and Zn(2)-O(8A) are 2.399(3) and 2.432(3) respec- tively (Table 4), the coordination is ignored. There are two unique zinc centers with ZnO4N units in complex 2R. Zn(1) coordinates to three different ligands and a water molecule, while Zn(2) is coordinated by two ligands and two water molecules. Both of the zinc centers with ZnO4N units in 2R are tetragonal pyramidal geometry, in which the geometric parameter τ values of the polyhedral ZnNO4units calculated by using the literature method[23]are 0.302 and 0.370 for Zn(1) and Zn(2), respectively. In the crystal of complex 2R, the ligand containing N(1) and S(1) atoms connects two zinc centers while the ligand containing N(2) and S(2) atoms bridges three zinc atoms. As shown in Fig. 6c, each zinc center connects two ligands and each ligand bridges two or three zinc atoms, generating a two-dimensional sheet. The Zn(1) centers are connected by the ligands to form a left-hand chain, the Zn(2) centers are bridged by the ligands to generate another left-hand chain, and the two left-hand chains are connected via the oxygen atom O(5) to give the 2-D sheet. The circular dichroism (CD) spectra of complexes 2R and 2S (Fig. 7) made from R- and S-enantiomers of the ligand are mirror images of each other, which confirm their enantiomeric nature. There are hydrogen bonds between O or N atom and H atom in the crystal of complex 2R, as shown in Fig. 8. The neighboring two-dimensional sheets were connected into a three-dimensional structure by hydrogen bonds. The hydrogen bond data of complexes 2R and 2S are listed in Table 5.

Fig. 6. (a) Coordination environments of zinc(II) ions in complex 2R, (b) Coordination environments of zinc(II) ions in complex 2S; (c) 2-D sheet and the coordination polyhedra around the Zn(1) (in purple) and Zn(2) (in green) centers in complex 2R; (d) 2-D sheet and the coordination polyhedra around the Zn(1) (in purple) and Zn(2) (in green) centers in complex 2S

Fig. 7. Circular dichroism (CD) spectra of complexes 2R and 2S

Fig. 8. Packing diagram of complex 2R

Table 4. Selected Bond Lengths (?) and Bond Angles (°) for Complexes 2R and 2S

To be continued

Symmetry codes: #A -x+1, y+1/2, -z+2; #B -x, y-1/2, -z+3; #C -x, y+1/2, -z+3; #D -x+1, y-1/2, -z+2

Table 5. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complexes 2R and 2S

3. 3 Photoluminescence spectra

The solid state photoluminescence spectra of the ligands and the complexes have been measured at room temperature (Fig. 9). For ligands H2LRand H2LS, the main peak at 498 nm (λex= 342 nm) may account for theπ*-πtransition of phenyl linker in the ligands[24]. Emission peaks are at 446 nm (λex= 328 nm) for complexes 1R and 1S, and 449 nm (λex= 330 nm) for complexes 2R and 2S, respectively, and the emissions may be attributed to the ligand- centeredπ*-πtransitions[24]. In respect to the ligands, the emission peaks for the complexes are blue shifted by 49 to 56 nm and their photoluminescence intensi- ties are decreased significantly, which can be attributed to the coordination effect of the ligands with metal ions[25,26].

Fig. 9. Photoluminescence spectra of the ligands and the complexes

4 CONCLUSION

In summary, a pair of new enantiomeric ligands has been synthesized in one step at moderate reaction conditions, and the ligands were utilized in constructing two pairs of chiral coordination poly- mers with one-dimensional helical architectures or two-dimensional sheets. The chiral structural features of the coordination polymers were controlled by the chirality of ligands. The complexes have blue-shift in luminescent spectrum compared with the free ligands. The chiral ligands based on 4,5-dihydrothiazole-4- carboxylic acid may be a potential ligand for the design and synthesis of new chiral materials, and more detailed and systematic researches about them are under way.