Synthesis, Crystal Structure, and Luminescent Property of a Cd(II) Coordination Polymer with a N-Nicotinoylglycine Ligand

A new 1D Cd(II) coordination polymer, [{CdL2(H2O)}·(H2O)3]n (HL = N-nicotinoylglycine), has been synthesized by the assembly of Cd(NO3)2·4H2O with N-nicotinoylglycine, and its composition was determined by elemental analysis, infrared, and X-ray single-crystal diffraction analysis. In this complex, the Cd(II) is seven-coordinated by two N atoms of two N-nicotinoylglycine anions, four O atoms of two N-nicotinoylglycine anions, and one O atom of a coordinated water molecule. The complex forms a 1D looped coordination polymer by the bridging of the N-nicotinoylglycine anion. The luminescent property of the N-nicotinoylglycine ligand and the Cd(II) coordination polymer has also been studied.


Introduction
Metal-organic coordination polymers have received great attention as an important part of coordination chemistry.Many of them that are constructed by Cd(II) ions or bridging multipodal ligands show unique structures and excellent properties in gas storage, luminescence, catalysis, and magnetism [1][2][3][4][5][6][7].In the past decades, many multipodal ligands containing two -COO − functional groups and neutral ligands, such as 4,4 -bipyridine and 1,10-phenanthroline, have been chosen to build coordination polymers [8][9][10][11][12][13].Recently, our group has obtained some new Cd(II), Ca(II), Mg(II), and Zn(II) coordination polymer materials and have investigated their structures and properties [14][15][16][17][18].In order to investigate the coordination behavior of ligands containing both −COO − and pyridine bifunctional groups and the property of their metal complexes, herein, a new 1D Cd(II) coordination polymer, [{CdL 2 (H 2 O)}•(H 2 O) 3 ] n (HL = N-nicotinoylglycine) has been synthesized and its luminescent property in the solid state has also been investigated.The chemical diagram of the Cd(II) complex is shown in Scheme 1.

Results and Discussion
A comparison of the N-nicotinoylglycine ligand and Cd(II) complex was conducted with infrared data, and the ligand shows two characteristic absorption bands at 1725 cm −1 (ν (COO − )) and 1634 cm −1 (ν (C(O)-NH)), and at 1675 cm −1 (ν (COO − )) and 1636 cm −1 (ν (C(O)-NH)), respectively.The shift of COO − is 50 cm −1 , indicating that the oxygen atom of the COO − group coordinated to Cd(II).However, The shift of C(O)-NH is 2 cm −1 , indicating that the oxygen atom of the C(O)-NH group does not coordinate to Cd(II).The band at 3256 cm −1 proves that the Cd(II) complex contains water molecules [19].

Structural Description of [{CdL2(H2O)}•(H2O)3]n
The coordination environment of the Cd(II) ion in [{CdL2(H2O)}•(H2O)3]n is shown in Figure 1.Selected bond distances and bond angles are given in Table 1.In the crystal, the looped structure is formed by the bridging of N-nicotinoylglycine anion ligands (Figure 2).The 3D network is generated by π•••π stacking and hydrogen bond interactions (Figure 3).In the crystal, the looped structure is formed by the bridging of N-nicotinoylglycine anion ligands (Figure 2).The 3D network is generated by π•••π stacking and hydrogen bond interactions (Figure 3).

Luminescent Property
The luminescent behaviors of [{CdL 2 (H 2 O)}•(H 2 O) 3 ] n and the free ligand are investigated in the solid state on a multicrystalline bulk sample.The luminescent spectrum of the Cd(II) coordination polymer is shown in Figure 4.The Cd(II) coordination polymer shows a luminescent peak at 458 nm when excited at 358 nm, however, the N-nicotinoylglycine ligand does not have a luminescent peak.Thus, the luminescent emission of the Cd(II) coordination polymer may be due to the charge transfer of ligand-to-metal [20].

Luminescent Property
The luminescent behaviors of [{CdL2(H2O)}•(H2O)3]n and the free ligand are investigated in the solid state on a multicrystalline bulk sample.The luminescent spectrum of the Cd(II) coordination polymer is shown in Figure 4.The Cd(II) coordination polymer shows a luminescent peak at 458 nm when excited at 358 nm, however, the N-nicotinoylglycine ligand does not have a luminescent peak.Thus, the luminescent emission of the Cd(II) coordination polymer may be due to the charge transfer of ligand-to-metal [20].

Crystal Structure Determination
Crystal data of [{CdL2(H2O)}•(H2O)3]n were collected on a Bruker Smart APEX CCD diffractometer at 293(2) K using graphite-monochromated MoKα radiation (λ = 0.71073 Å).The structure was solved by direct method using the SHELX program [21], and refined by full-matrix least squares on F 2 using the SHELXL program [22].Technical details of the crystal structure solutions and refinements are listed in Table 2.

Synthesis of [{CdL
solution of N-nicotinoylglycine (0.1802 g, 1.0 mmol) and NaOH (0.040 g, 1.0 mmol) was stirred to clarify, then a water solution (3 mL) of Cd(NO 3 ) 2 •4H 2 O (0.3085 g, 1.0 mmol) was added to the above mixture.The mixture was stirred for 5 h at 60

Crystal Structure Determination
Crystal data of [{CdL 2 (H 2 O)}•(H 2 O) 3 ] n were collected on a Bruker Smart APEX CCD diffractometer at 293(2) K using graphite-monochromated MoKα radiation (λ = 0.71073 Å).The structure was solved by direct method using the SHELX program [21], and refined by full-matrix least squares on F 2 using the SHELXL program [22].Technical details of the crystal structure solutions and refinements are listed in Table 2.

Conclusions
In summary, [{CdL 2 (H 2 O)}•(H 2 O) 3 ] n has been prepared and characterized by elemental analysis, infrared, and X-ray single-crystal diffraction analysis.The Cd(II) complex forms a 1D helix coordination polymer by the bridging of the N-nicotinoylglycine anion.The 3D network is generated by π•••π stacking interactions.The luminescent property of the N-nicotinoylglycine ligand and the Cd(II) coordination polymer has also been studied.

Scheme 1 .
Scheme 1.Chemical diagram of the Cd(II) complex.
Its crystal structure shows that each Cd(II) ion is seven-coordinated by four O atoms from two carboxyl group of two different N-nicotinoylglycine anion ligands, two N atoms from two different N-nicotinoylglycine anion ligands, and one O atom from a coordinated water molecule.Each Cd(II) ion adopts a distorted pengonal bipyramidal coordination environment.The carboxyl groups of N-nicotinoylglycine anion ligands adopt a didentate coordination mode in the complex molecule.The distances of Cd•••Cd separated by N-nicotinoylglycine anion ligands are 10.064Å and 9.933 Å, respectively.

Figure 1 .Scheme 1 .
Figure 1.The coordination environment of the Cd(II) ion.
Its crystal structure shows that each Cd(II) ion is seven-coordinated by four O atoms from two carboxyl group of two different N-nicotinoylglycine anion ligands, two N atoms from two different N-nicotinoylglycine anion ligands, and one O atom from a coordinated water molecule.Each Cd(II) ion adopts a distorted pengonal bipyramidal coordination environment.The carboxyl groups of N-nicotinoylglycine anion ligands adopt a didentate coordination mode in the complex molecule.The distances of Cd•••Cd separated by N-nicotinoylglycine anion ligands are 10.064Å and 9.933 Å, respectively.

Figure 1 .
Figure 1.The coordination environment of the Cd(II) ion.Figure 1.The coordination environment of the Cd(II) ion.

Figure 1 .
Figure 1.The coordination environment of the Cd(II) ion.Figure 1.The coordination environment of the Cd(II) ion.

Figure 2 .
Figure 2. One-dimensional chained structure by the bridging of ligands.

Figure 2 .
Figure 2. One-dimensional chained structure by the bridging of ligands.

Figure 2 .
Figure 2. One-dimensional chained structure by the bridging of ligands.

Figure 4 .
Figure 4.The luminescent spectrum of the Cd(II) coordination polymer.

Figure 4 .
Figure 4.The luminescent spectrum of the Cd(II) coordination polymer.