2D Frameworks Self-assembled From a Hydrazone Ligand and Azide Salts: Synthesis, Structures, and Luminescent Property

Three metal-organic coordination polymers {[Cu(L)(N3)]·(H2O)0.25}n (1), {[Zn(L)(N3)]·(H2O)0.5}n (2) and [Cd2(L)2(N3)2(H2O)]n (3) have been synthesized from hydrazone ligand N’-(1-(pyrazin-2-yl)ethylidene)isonicotinohydrazide (HL), NaN3 and corresponding metal nitrates. Complexes were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. All three complexes feature 2D coordination network in which L acts as NNON tetradentate ligand and azide acts as end-on bridging ligand. In complexes 1 and 2, only intra-sheet hydrogen bonding interactions are found, while the hydrogen bonding interactions between water molecules and host framework result in 3D network for 3. In addition, complexes 2 and 3 exhibited intense fluorescent emissions in the solid state at room temperature.


Introduction
Metal compounds have attracted considerable interest because of their fascinating structures, 1 and functional applications in many research fields such as adsorption, 2 gas storage and separation, 3,4 photoluminescence, 5 and ion exchange. 6,7The organic ligands are considered to be the most important factor that affects the structures and properties of the coordination polymers. 8ydrazone ligands obtained from condensation reaction of aldehydes (or ketones) and hydrazide are an important class of Schiff bases. 9We have been interested in coordination compounds involving hydrazone ligands owning to their facile keto-enol tautomerization, the versatile coordination modes, and the strong hydrogen-bonding capability.In previous studies, we synthesized a series of complexes derived from Schiff bases with metal chloride, acetate or nitrate.][12] Azide has been demonstrated as the most common used linear ligand. 134][15] While the azide group may also function in triply or quadruply bridging modes. 16,17So, the complexes derived from hydrazone ligands and azide are good candidates allowing the access to intriguing architectures.
Wang et al.: 2D Frameworks Self-assembled From a Hydrazone ...Recently Shaabani and coworkers reported Cr(III), Mn(II) and Fe(III) complexes with 4-hydroxy-N'-((pyridin-2-yl)-methylene)benzohydrazide and azide as bridging ligands.These three complexes showed mononuclear or dinuclear structure with moderate antimicrobial activity. 18Xu and coworkers reported a series of copper complexes showing antitumor activities based on N'-(1-(pyrazin-2-yl)ethylidene)isonicotinohydrazide (HL) (Scheme 1). 19In this paper, we were prompted to study the coordination chemistry of HL in the presence of azide.Herein three complexes namely (3) were synthesized, their crystal structure and luminescence properties were also studied.
Caution! Azide compounds of metal ions are potentially explosive.Only a small amount of material should be prepared and it must be handled with care.

Synthesis of
The methanol solution (4 mL) of the ligand (9.64 mg, 0.040 mmol) was carefully layered on the top of the (4 mL) of Cu(NO 3 ) 2 • 3H 2 O (9.66 mg, 0.040 mmol).The (0.20 mol/L, 0.4 mL) of NaN 3 was gently added as the third layer.The solutions were left for 5 days at room temperature, and complex 1 (7.43

3. Synthesis of
Complex 2 (5.78 mg) was obtained by similar procedure as 1 by using Zn(NO 3 ) 2

5. X-ray crystallography
The data were collected at 298 K on a computercontrolled Bruker D8 venture diffractometer equipped with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å).The collected diffraction data were reduced using the SAINT program, 20 and multi-scan absorption corrections were performed via the SADABS program. 21he structures were solved by direct methods and refined against F 2 by full-matrix least-squares methods applying the SHELXL program package. 22All of the non-hydrogen atoms were refined anisotropically.In complexes 1 and 2, the ADPs of N6 and N7 atoms were restrained to be same within a standard deviation of 0.005 Å.In total 11 geometric restraints were used in modeling this structure.In complex 3, the atoms O3 were constrained to have the same ADPs as atoms Cd1.Total 4 restraints were used in modeling the structure.The azide ion N13 was disordered over two positions in refined ratio 0.50(4):0.50(4).All the hydrogen atoms bonded to C atoms were generated geo- Symmetry codes: (i) -x + ½, y -½, -z + 3/2; (ii) -x + 1, -y + 1, -z + 1.
metrically and refined isotropically using the riding model.The H atoms attached to water molecules were fixed by difference Fourier maps with O-H = 0.85(2) Å, H•••H = 1.44(2)Å and U iso (H) = 1.5U eq (O).The occupancy factors for water molecules in 1 and 2 were obtained by refinement of occupancy number.Details of data collection and refinements of complexes 1-3 are summarized in Table 1, selected bond distance and angles are given in Tables 2 and 3.

1. IR Spectroscopy
The IR spectra of complexes 1-3 (Figure S1) display broad band at about 3300∼3400 cm -1 due to the stretching band of water molecules. 23The absence of typical ν(C=O) band of HL (1662 cm -1 ) and the appearance of ν(C-O) absorption bands (1616 cm -1 for 1, 1610 cm -1 for 2, and 1613 cm -1 for 3) support the coordination of HL in the enol form. 24The stretching vibration of the azomethine bands for complexes 1-3 are found at 1562, 1564, and 1568 cm -1 , respectively.Whereas the same band in the free l igand HL was observed at 1632 cm -1 .On complexation the shifts of azomethine C=N band towards lower wavenumbers indicates coordination of the azomethine to the metal center. 12,25The sharp band of the azide ions are found at 2047 cm -1 for 1, 2056 cm -1 for 2, and 2054 cm -1 for 3.
Complexes 1 and 2 crystallize in the monoclinic system, space group P2 1 /n.As shown in Figures 1 and S2, the two complexes possess very similar coordination environment; so only the structure of 1 is described in detail herein.
The asymmetric unit of compound 1 is composed of one Cu II ion, one deprotonated ligand L 1-, one azide anion N 3 -, and one lattice water molecule.As presented in Figure 1, the coordination polyhedron around the Cu(II) center is distorted octahedral.The equatorial plane is surrounded by one nitrogen donor N1 from pyrazine ring, one nitrogen donor N3 from azomethine, one enolate oxygen donor O1 and one nitrogen donor N6 from azide ion.The sum of the four equatorial angles (≈358.09°) is very close to the ideal value (360.00°), which ensures the planarity of equatorial plane.The copper ion is only 0.0984 Å out of the basal plane.The axial positions are occupied by the pyridyl nitrogen donor N5 i from another ligand and azide nitrogen N6 ii (symmetry codes: (i) -x + ½, y -½, -z + 3/2; (ii) -x + 1, -y + 1, -z + 1).The average bond length of the equatorial plane is 1.9839(17) Å, while the average axial bond length is 2.5313(18) Å, thus the coordination sphere for Cu ion in complex 1 is a stretched octahedron.In complex 1, the azide group adopts the asymmetric end-on bridging mode.The Cu-N azide bond length, especially the bond length of Cu1-N6 ii (2.7748(19) Å), is somewhat longer than those reported in Cu-hydrazoneazide analogues, 25,28 while it is shorter than that in Cubipyridine-(N 3 ) 2 being 2.849(4) Å. 29 The Cu(II) centers are bridged by two μ 1,1 -N 3 to form a planar Cu-(μ 1,1 -N 3 ) 2 -Cu ring.The Cu•••Cu distance within the four-membered cyclic units is 3.5837(5) Å.The adjacent (CuL) 2 -(μ 1,1 -N 3 ) 2 units are connected with each other via the pyridine N5 atoms forming the 2D sheet structure of 1 (Figure 2).In earlier work, Xu et al. have reported a series of mono-, bi-, tetra-nuclear and 1D chain Cu(II) complexes derived from HL and copper salts. 19HL was also in enolic form in those complexes, and acts as NNO donor in mono-, bi-, and tetranuclear complexes.While in the 1D chain complex {[Cu 2 (L) 2 (NO 3 )(H 2 O) 2 ] • (NO 3 )} n , L acts as NNON donor as that in complex 1, however the monodentate coordination mode of the nitrate anion limits the further extension of the structure.Therefore the counter anions influence the structures of the complexes efficiently. 19n complex 1, the free water molecules are linked to the sheet via hydrogen-bonding interactions O2-H2A••• N7, O2-H2A•••N8, and O2-H2B•••O1 (Figure S3).
Crystal structure study reveals that 3 crystallizes in triclinic system, space group P1 -.The asymmetric unit of 3 consists of two crystallographically independent Cd 2+ , two deprotonated ligands L 1-, two counter anion N 3 -, and one coordinated water molecule.

Conclusions
In this paper, three complexes with 2D layered structures based on multidentate hydrazone ligand HL and azide salts have been isolated.The asymmetric end-on bridging mode of N 3 -was found in complexes 1-3.The monoanionic ligand L 1-coordinated to the metal centers in an enolic form and served as tetradentate NNON type bridging ligand.In complexes 1 and 2 only intra-sheet hydrogen bonds were observed.The hydrogen bonding interactions between the water molecules and host framework construct a 3D supramolecular network for 3.In addition, complexes 2 and 3 exhibit green emission fluorescence behaviors owing to the rigidity of structure.

Luminescence Study
The d 10 transition metal based complexes with no dd transition have intrinsic electronic properties.They are potential candidates for photoactive materials.Here the luminescent properties of HL, 2 and 3 were investigated in the solid state at room temperature.
As shown in Figure 7 upon excitation at 402 nm the free ligand HL exhibits fluorescent emission centered at 468 nm, which can be attributed to the intra-ligand π*-π and π*-n transitions.Complexes 2 and 3 exhibit a little less intense photoluminescence, with emissions around 541 and 551 nm, respectively.As Zn 2+ or Cd 2+ ions are difficult to oxidize or reduce owning to their closed shell structure, 30,31 thus the luminescent emissions of the corresponding complexes 2 and 3 can be ascribed to the intra-ligand transitions.The red shifts of their emission spectra may be due to

Figure 2 .
Figure 2. View of the 2D structure of 1.
This work was supported by the National College Students' Innovative and Entrepreneurial Training Plan of China (201610433115 and 201610433157).

Figure 5 .
Figure 5.The 3D framework connected by H-bonding interactions in complex 3.

Figure 6 .
Figure 6.Fluorescence emission properties of the HL and complexes 2, 3 in the solid state.

Table 1 .
Crystallographic data for 1