Syntheses, Structures, and Characterization of Two Novel Copper(II) and Cadmium(II) Compounds Based on Pyridyl Conjugated 1,2,3-Triazole

Abstract

Two new complexes with 5-methyl-1-(pyridine-3-yl)-1H-1,2,3-triazole-4-carboxylic acid (Hmptc) ligand: [$Cd(mptc)_2(H_2O)_4$] (1) and $[Cu(mptc)_4{\cdot}2H_2O]_n$ (2) were prepared and their crystal structures were determined by single crystal X-ray diffraction analyses. In complex 1, the Cd(II) ions coordinates with the pyridyl nitogen atom from the Hmptc ligand, forming a mononuclear Cd(II) compound. Complex 2 exhibits a novel two-dimensional (2D) polymer in which four Hmptc ligands stabilize the Cu(II) atom. And the coordination involves one nitrogen atom of the triazole, one oxygen atom of the carboxylic acid and the pyridyl nitrogen atom. In addition, FT-IR and solid-state fluorescent emission spectroscopy of two compounds have been determined.

Introduction

During the past decades, metal-organic coordination compounds have achieved considerable progress in the field of supramolecular chemistry and crystal engineering, because of their intriguing structural motifs and potential functional properties, such as molecular adsorption, magnetism, and luminescence.1-11 More and more people have been focused on the rational design and controlled synthesis of coordi-nation compounds using multidentate ligands such as poly-carboxylate and N-heterocyclic ligands.12-15 In recent years, there is a growing interest in the construction of coordination compounds based on triazole, tetrazole and imidazole derivates.16-20 According to the investigation of heterocyclic ligands, N-heterocyclic carboxylic acids play an important role in organic ligands.21-29 Over the past several decades, several classes of coordination compounds such as metal-caboxylates and metal-pyridyl have been synthesized and investigated.30-33 From the view point of crystal engineering, cyano, caboxylate groups, pyridyl and triazole of organic building blocks and metal centers assemble metal-organic frameworks with beautiful topologies.34-44

Transition metal salts have been studied extensively over the past two decades because of their great importance in magnetism, luminescence and porous material.45,46 Previous-ly, a series of Cu(II) ions and Cd(II) ions coordination polymers in corporating 1,2,4-triazole ligands have been documented, which show interesting structural diversity and properties.47-50 And 5-methyl-1-(pyridine-3-yl)-1H-1,2,3-tri-azole-4-carboxylic acid (Hmptc) is a multifunctional ligand containing one pyridyl group and one triazole-carboxylate group, which are both strong coordination donors to metal centers. Therefore Hmptc ligand should exhibit high-dimen-sional and highly connected coordination modes. And the crystal structures of the parent Hmptc ligand with metal ions have been rarely reported.

The prospective of this ligand to form interesting supramolecular arrays and novel coordination modes are now proved with two metal ions. We herein report two of novel coordination compounds: [Cd(mptc)2(H2O)4] (1) and [Cu(mptc)2·2H2O]n (2). In these compounds, the Hmptc molecule serves as an efficient mono- or multidentate coordinating ligand with four potential N-donors and O-donors. The crystal structures of 1 and 2 are dominated by hydrogen bonding and C-H⋯π stacking interactions.

 

Experimental

Reagents and Measurements. All the solvents and chemicals were used as received without further purifi-cation. IR spectra were obtained with KBr pellets from 4000 to 400 cm˗1 using a Nicolet 5700 spectrophotometer. 1H-NMR and 13C-NMR spectra were acquired on a Bruker 300 MHz spectrometer. The crystal structures were determined on a Bruker SMART APEX II diffractometer. Luminescence spectra for the solid samples were investigated with a Horiba Fluoromax-4 fluorescence spectrophotometer. Thermogravi-metric analysis (TGA) data were recorded on a SDT-Q600 analyser from TA instruments.

Synthesis of 5-Methyl-1-(pyridine-3-yl)-1H-1,2,3-tri-azole-4-carboxylic acid (Hmptc). To pyridine-3-amine (4.08 g, 20 mmol) in concentrated HCl (8 mL) and water (16 mL), NaNO2 (1.28 g, 20 mmol) in water (6 mL) was added at 0 °C. The mixture was stirred at 0 °C for 20 min and then NaN3 (20 mmol, 1.30 g) in water (6 mL) was added. The reaction mixture was stirred at 0 °C for 30 min, and then extracted with CH2Cl2 (3 × 30 mL), and the combined organic layers were washed with water and dried (Na2SO4) and the solvent was removed under vacuum. To the resulting residue, ethyl acetoacetate (2.6 g, 20 mmol) was added. Then, CH3ONa (1.08 g, 20 mmol) in CH3OH (60 mL) was added dropwise to the mixture at room temperature for 24 h, and washed with 2 M hydrochloric acid to afford the white precipitate crude product. A solution of the white precipitate crude product, 25 mL of 2 M sodium hydroxide and 20 mL of CH3OH was refluxed for 3 h, and then acidified with 6 M hydrochloric acid to give 2.316 g of white, 5-methyl-1-(pyridine-3-yl)-1H-1,2,3-triazole-4-carboxylic acid; mp 209-211 °C; IR (KBr; cm˗1): 3064(m), 2446(m), 1702(s), 1599(s), 1562(s), 1488(s), 1436(vs), 1409(s), 1356(vs), 1263(vs), 1188(m), 1122(s), 1075(m), 1022(s), 1008(s), 813(s), 784(vs), 739(m), 700(s), 656(vs), 636(vs). 1HNMR (300MHz, DMSO-d6) δ 13.24 (s, 1H, COOH), 8.80-8.81 (s, 1H, Ar), 8.86-8.87 (s, 1H, Ar), 8.13-8.15 (d, 1H, Ar), 7.68- 7.72 (m, 1H, Ar), 2.35 (s, 3H, CH3). 13CNMR (300 MHz, DMSO-d6) δ 162.94, 151.46, 146.48, 140.14, 137.18, 133.78, 132.66, 124.96, and 10.05 ppm.

Synthesis of [Cd(mptc)2(H2O)4] (1). A solution (8 mL) of Cd(ClO4)2·6H2O (0.0838 g, 0.2 mmol) in water was add into test tube, after ethyl acetate (5 mL) was carefully layer-ed above, followed by another layer of a CH3OH solution (8 mL) of Hmptc (0.0408 g, 0.2 mmol) and triethylamine (0.4mL). After ten days, colorless crystal was obtained in about 47% yield based on Cd(ClO4)2. IR (KBr; cm˗1): 2958(vs), 1578(vs), 1497(vs), 1403(m), 1381(m), 1339(m), 1306(vs), 1277(s), 1234(m), 1201(m), 1142(m), 1018(w), 816(m), 707(s), 634(s), 604(m), 486(s).

Table 1.aR = Σ||Fo| ˗ |Fc||/Σ|Fo|; wR(F2)=[Σw(Fo 2 ˗ Fc 2)2/Σw(Fo 2)2]1/2

Synthesis of [Cu(mptc)2·2H2O]n (2). A solution of Hmptc (0.0408 g, 0.2 mmol) in DMF was layered onto a solution of CuCl2·2H2O (0.0341 g, 0.2 mmol) in water (10 mL). Then, the mixture was stirred and added to the test tube slowly. After 15 days, dark blue crystal was obtained in about 11% yield based on CuCl2. IR (KBr; cm˗1): 3448(vs), 3079(m), 1654(vs), 1601(vs), 1495(vs), 1445(m), 1385(vs), 1318(vs), 1251(s), 1197(s), 1140(m), 1108(m), 1043(s), 916(m), 817(vs), 702(s), 669(s), 504(m).

X-ray Crystal Structure Determination. The single crystal X-ray diffraction data collection for the complex was performed with a Bruker SMART APEX II diffractometer using Mo-Kα radiation (0.7107 Å). The structure was solved by direct methods with SHELXS-97 and refined by full-matrix least-squares on F2 using the SHELXL-97 program.51 Detailed data collection and refinements of 1 and 2 are summarized in Table 1. Selected bond lengths and angles are listed in Table 2. Relevant hydrogen bonding parameters of 1 and 2 are summarized in Table 3 and 4.

Table 2.aSymmetry transformations used to generate equivalent atoms: A: 1˗x, 2˗y, ˗z. bSymmetry transformations used to generate equivalent atoms: A: 2˗x, 1˗y, 1˗z; B: x, 1/2˗y, 1/2+z; C: 2˗x, 1/2+y, 1/2˗z.

Table 3.Symmetry code: i) 2˗x, 1/2+y, 1/2˗z; ii) ˗1+x, 5/2˗y, ˗1/2+z; iii) ˗1+x, 5/2˗y, ˗1/2+z; iiii) ˗1+x, 3/2˗y, ˗1/2+z.

Table 4.Symmetry code: i) ˗1+x, 1/2˗y, ˗1/2+z; ii) ˗1+x, 1/2˗y, ˗1/2+z; iii) 2˗x, ˗y, 1˗z.

 

Results and Discussion

Structure Description of Compound 1. Compound 1 crystallizes in the P21/c space group and the structure of crystal 1 is a mononuclear compound, and Cd(II) ion is six-coordinated with two pyridyl nitrogen atoms, four oxygen atoms of coordinated water. The corresponding N4-Cd-N4A angle is 180°, and the angles of O3-Cd1-N4(90.73°), O3-Cd1-N4A(89.27°), O4-Cd1-N4(90.40°), O4-Cd1-N4A(89.60°) are all different from the ideal value of 90°, indicating that the central Cd(II) ion adopts a slightly distorted octahedral geometry. Four atoms O1, O2, O3, O4 define the basal plane, while N4 and N4A are located in the apical position of the octahedral structure. The crystal structure of this compound is show in Figure 1.

In the crystal structure of 1, several strongly hydrogen bonding interactions, including O3-H3A⋯O1 (2−x, 1/2+y, 1/2−z), O3-H3B⋯O1 (−1+x, 5/2-y, −1/2+z), O4-H4A⋯N1 (−1+x, 3/2−y, −1/2+z), O4-H4A⋯O2 (−1+x, 5/2−y, −1/2+z), are found in the structure as the main action force to form a 2D structure supermolecular network (Figure 2(a)). These two-dimensional nets are not planar but undulating, which results from the hydrogen bonding of coordinated water (Figure 2(b)). And the adjacent layers are further inter-connected via C2-H2⋯π interactions, thus completing the 3D architecture (Figure 3(a)). The distances of H2 to the centroids of the pyridyl ring is 2.893 Å (Figure 3(b)), whereas the C2-H2⋯π (centriod of the pyridyl) angle is 167.84°.

Figure 1.The crystal structure of [Cd(mptc)2(H2O)4]. Symmetry code: (A) 1˗x, 2˗y, ˗z. All hydrogen atoms are omitted for clarity.

Figure 2.The 2D layer constructed by hydrogen bond 1 along a axis (a) and b axis (b), some carbon and hydrogen atoms are omitted for clarity.

Figure 3.(a) The C-H⋯π interactions between two neighboring layers constructed 3D structure in 1; (b) the distances of H2 to the centroids of the pyridyl ring was enlarge for clearly visible. Some carbon and hydrogen atoms are omitted for clarity.

Structure Description of Compound 2. X-ray analysis reveals that 2 contains one crystallographically independent Cu(II) centers, which are linked two mptc¯ to form one [Cu (mptc)2] core and two lattice H2O molecules. The title complex is coordinated by six atoms, namely, two nitrogen atoms from the pyridyl ring, two nitrogen atoms from the triazole ring, and two carboxylic oxygen atoms (Figure 4). The bond lengths of Cu1-N1C(2.0358 Å) and Cu1-N4 (1.9878 Å) are in good agreement with the values of Cu-N bond lengths reported in the literature.52 The O1-Cu1-O1A angle is 180°, whereas the N1C-Cu1-O1, N1C-Cu1-N4, N1B-Cu1-N4, O1-Cu1-N1B angles are 94.13°, 88.85°, 91.15°, 85.87°, respectively, are all different from the ideal value of 90°, indicating that the structure of complex 2 is a slightly distorted octahedral coordination polyhedron. In polymer 2, the deprotonated Hmptc is bidentate with one oxygen of carboxylate and one nitrogen of 1,2,3-triazole ring chelating to Cu atom, resulting in composition of a equatorial plane, while N1B, N1C at the axial site, forming 2D packing network (Figure 5(a)). the 2D structure may be more clearly understandable on the basis of topological approach: The structure of 2 can be simplified by each Cu atom is linked to four neighbors (by a number of Hmptc ligand) and appears to be a net point for four-connected 2D 44-net sheet (Figure 5(b)).53 The length of quadrangle is 9.103 Å, and the angle is 65.44°, 114.56°. In the crystal packing of 2, they show extensive hydrogen bonding interactions, two lattice waters hold the two neighboring layer strands together through H-bonding (Figur 6), these two-dimensional nets are con-structed a 3D structure through the intermolecular hydrogen bonding interactions among layers (Figure 7(a)). Ignoring the different in the bridge, The parallel series of 44-nets are interconnected by the second bridge (hydrogen bond) with the distance is 9.6884 Å (Figure 7(b)).

Figure 4.Molecular structure of 2 showing the coordination environment of Cu(II). Symmetry code: (a) 2-x, 1˗y, 1˗z; (b) x, 0.5˗y, 0.5+z; (c) 2˗x, 0.5+y, 0.5˗z. All hydrogen atoms are omitted for clarity.

Figure 5.(a) The 2D coordination network of 2, all of H atoms are omitted for clarity. (b) The 2D 44-net topology of 2, turquiose particle: Cu; pink line: Hmptc bridge.

Figure 6.Hydrogen bond interactions by two lattice waters along b axis. Some H atoms are omitted for clarity.

Figure 7.(a) Two neighboring layers interconnected by the hydrogen bond through two lattice water constructed 3D structure in 2, some H atoms are omitted for clarity; (b) turquiose particle: Cu; pink line: Hmptc bridge; blue line: hydrogen bond.

IR Spectra. In complex 1, broad bands at 3100 cm−1 suggest that coordination waters are present. According to the Hmptc’s IR, there is no band at 1700 cm−1 show in complexes 1 and 2, because of the coordinated water and carboxylate group form hydrogen bonds in complex 1, while complex 2 is deprotonated. The pyridyl ring normally ex-hibit two different bands in the region of 1600-1450 cm˗1 (band I), 1350-1100 cm˗1 (band II). And around 1410, 1380 cm˗1 are C=N stretching of triazole group and CH3 group, respectively. This analysis is also supported by X-ray diffr-action measurements.

Luminescent Properties. Luminescent properties of compounds with d10 metal centers have attracted much interest due to their potential applications in electrolumine-scent display, chemical sensors, photochemistry.54-60 There-fore, the luminescence of 1 and 2 as well as the free ligands was investigated in the solid state at room temperature (Figure 8), since all three compounds are virtually insoluble in most common solvents such as ethanol, acetone, chloro-form, benzene, water, etc. At the room temperature, the free Hmptc ligand exhibits a narrowly emission with maximum at 436 nm upon excitation at 384 nm in the solid state, On comparison with the free ligand, compound 1 show the emission bands centered at 433 nm (λex = 380 nm), and complex 2 show a blue fluorescent emission maximum at 349 nm, when excited with 368 nm light. The emissions of 1 and 2 may be assigned to intraligand (π−π*) fluorescence which is modified by the Cu(II) and Cd(II) ions. The emission peak of 1 is near to that of the free Hmptc, and the effective enhancement of emission bands in 1 should be due to the coordination of to Cd(II) ion. However, compound 2 exhibits obviously different luminescent behavior, in contrast to the compound 1. It is revealed that complex 2 adopts a rigid 2D layer structure while can offer less advantage of energy transfer than zero-dimensional structure of 1, as a result, a weaker emission with shorter wavelength is observed for 2. The varying coordination modes of Hmptc have great potential for synthesizing novel frameworks with intriguing structure and unique properties.

Figure 8.Solid-state fluorescence spectra of 1, 2 and the free Hmptc ligand at room temperature.

Figure 9.TGA curves of the complexes.

Thermal Analysis. To examine the thermal stablilities of compounds 1-2, their thermal behaviors were studied by thermogravimetric analyses (TGA) technique under N2 atmosphere (Figure 9). As TGA curves show, the weight loss of 1 from room temperature to 250 °C (15.74%), corre-sponding to the escape of four water molecules and two –CH3 (caled. 17.21%). The second weight loss of 52.75% from 250 °C to 590 °C results from the decomposition of the triazole molecule and the pyridyl molecule, the total weight loss at 590 °C is 68.49%. Compound 2 displays a small gradual weight loss of 10.52% between 50 and 160 °C, which is attributed to the two water molecules and four –CH3 (caled. 9.32%). And then a sharp continual weight loss occurred at 240 °C, which is attributed to the decomposition of the triazole molecule and the pyridyl molecule. The total weight loss at 595 °C is 85.03%.

 

Conclusion

In conclusion, this study demonstrates the synthesis of the free Hmptc ligand and two new coordination compounds under different solvents. Comparison of 1 and 2 from crystal structures has revealed that the same main groups display two different coordinative behavior. The complex 2 proves that the triazole carboxylic acid ligands are valuable multi-dentate chelating ligands because of 3-N nitrogen atom of the triazole ring is electron donor. The varying coordination modes of Hmptc with metal ions display two kind of fluore-scence intensity and blue-shifted. And TGA analysis shows that the complexes are thermally stable up to 150-250 °C.

On the basis of this study, more structures and properties studies of the Hmptc ligand with other metals are under way in our laboratory. Thus, we anticipate that this type of organic ligand will result in a variety of novel coordination complexes with properties.