TAO Zho-Lin QIN Ling ZHENG He-Gen② (Bengu Medicl College, Bengu 233030, Chin) (Stte Key Lortory of Coordintion Chemistry, School of Chemistry nd Chemicl Engineering,Nnjing Ntionl Lortory of Microstructures, Nnjing University, Nnjing 210093, Chin)

1 INTRODUCTION

Over the past two decades, metal-organic frameworks have attracted great attention not only because of their intriguing aesthetic structures but also of their potential applications[1-6]. To a great extent, the structures and properties of metal-organic materials are dependent upon the molecular recognition and self-assembly methods among various components via the intermolecular interactions such as coordinate bonds, hydrogen bonds, and π-π interactions.These intermolecular interactions not only play a key role in stabilizing the complex structures but also play a guiding role, i.e. Nature’s radar guidance system in the complex assembly process from low(0D, 1D, or 2D polymers) to high dimension[7-12].Besides, in the designed synthesis of complexes, the organic ligands have significant influence on the desirable MOFs. Pyridine dicarboxylic acid compounds are excellent ligands and have been utilized widely in recent years because they have strong bridging abilities and abundant coordination modes[13-14], which can afford at most one nitrogen and four O coordination sites. Carboxylic groups of the pyridine dicarboxylic acid via participating in coordination or forming hydrogen bonds can form pro-tean coordination polymers and supramolecular structures. But only a few complexes based on H2L have been reported[15-16]. Herein, to expand our work[17], we report the syntheses and crystal structures of two new complexes 1 and 2 determined by elemental analysis, IR, TG, UV-Vis and X-ray diffraction analysis.

2 EXPERIMENTAL

2.1 Reagents and physical measurements

H2L was synthesized according to the reported method[18]. All starting chemicals were of analytical grade and used without further purification. Elemental analyses of C, H and N were carried out with Perkin-Elmer 240C elemental analyses. IR spectrum was recorded in the range of 4000～400 cm-1on a FT-IR (VECTOR-22) spectrometer with KBr pellets.Solid-state UV-vis diffuse reflectance spectra were obtained at room temperature using a Shimadzu UV-3600 double monochromator spectrophotometer,and BaSO4was used as a 100% reflectance standard for all materials. Powder X-ray diffraction (PXRD)measurements were performed on a Bruker D8 Advance X-ray diffractometer. The as-synthesized samples were characterized by thermogravimetric analysis (TGA) on a Perkin-Elmer thermogravimetric analyzer Pyris II TGA. X-ray crystallographic data were collected on a Bruker Apex Smart CCD diffractometer.

2.2 Synthesis of [Co3L2(NO2)2(H2O)2]n (1)

A mixture of H2L (27.4 mg, 0.1 mmol) and Co(NO3)2·6H2O (29.1 mg, 0.1 mmol) was dissolved in the solvent of H2O/DMF (3:1, v/v, 8 mL), which was adjusted to pH = 6.0～7.0 with 1 mol/L NaOH.The final mixture was sealed in a 25 mL PTFE-lined stainless-steel vessel under autogenous pressure and heated at 130 ℃ for 72 h. Large quantities of red block crystals were obtained. The yield of the reaction was ca. 32% based on Co. Anal. Calcd. for C28H22Co3N4O16: C, 36.69; H, 2.62; N, 6.61%.Found: C, 36.47; H, 2.71; N, 6.54%. IR (KBr, cm-1):3315(m), 1623(m), 1557(s), 1453(m), 1430(m),1381(s), 1313(m), 1298(m), 1276(m), 1134(w),1064(w), 1027(m), 966(m), 900(w), 876(w), 810(m),775(m),727(m), 621(w), 528(w), 478(w), 443(w),421(w).

2.3 Synthesis of {[Ni(L)(H2O)5]·(H2O)2·DMF}n(2)

A mixture of H2L (27.4 mg, 0.1 mmol) and Ni(NO3)2·6H2O (29.1 mg, 0.1 mmol) was dissolved in the solvent of H2O/DMF (1:3, v/v, 8 mL), which was adjusted to pH = 6.0～7.0 with 1 mol/L NaOH.The final mixture was sealed in a 25 mL PTFE-lined stainless-steel vessel under autogenous pressure and heated at 95 ℃ for 72 h. Large quantities of jadegreen block crystals were obtained. The yield of the reaction was ca. 39% based on H2L ligand. Anal.Calcd. for C17H30N2NiO13: C, 38.59; H, 5.71; N,5.29%. Found: C, 38.41; H, 5.87; N, 5.21%. IR (KBr,cm-1): 3421(vs), 1665(m), 1624(m), 1556(s),1457(m), 1431(m), 1381(s), 1272(m), 1127(w),1098(m), 1064(m), 1029(m), 957(w), 786(w),716(m), 658(w), 489(w), 418(w).

2.4 Crystal structure determination

Crystallographic data of compounds 1 and 2 were collected on a Bruker Apex Smart CCD diffractometer equipped with a graphite-monochromatized MoKα radiation (λ = 0.71073 ?) using the φ-ω scan mode. Structure solutions were solved by direct methods and the non-hydrogen atoms were located from the trial structures and then refined anisotropically with SHELXTL using full-matrix least-squares procedures based on F2values[19]. The hydrogen atom positions were fixed geometrically at the calculated distances and allowed to ride on the parent atoms. The pertinent crystallographic data of compounds 1 and 2 are summarized in Table 1,while the selected bond lengths and bond angles are listed in Tables 2 and 3 for compounds 1 and 2,respectively.

3 RESULTS AND DISCUSSION

3.1 Crystal structure of [Co3L2(NO2)2(H2O)2]n (1)

Single-crystal X-ray analysis reveals that com-pound 1 crystallizes in monoclinic crystal system of P2/c. As shown in Fig. 1a, the asymmetric unit is comprised of three Co(II) cations, two L2-ligands,two coordinated nitrite ions, and two coordinated water molecules. The Co(II) cations are in a slightly distorted octahedral coordination geometry. Co(1)and Co(1)B cations (Symmetry code B: –1–x, 1–y,–1–z) are six-coordinated by one nitrogen atom and three oxygen atoms from L2–ligands, one oxygen atom from water molecule, and one oxygen atom from the nitrite ion[20], whereas the Co(2) cation is six-coordinated by six oxygen atoms, of which two are from two nitrite ions and four from four carboxylate groups of different L2–ligands. The Co–N bond length is 2.092(2) ? and the Co–O distances range from 2.0267(14) to 2.2534(15) ?,which are comparable to those found in similar types of Co(II) octahedral complexes[17]. The two carboxylate groups of H2L ligand are both deprotonated and take two coordination modes. One carboxylate group exhibits a bismonodentate bridge mode(μ2:η1:η1) to bridge Co(1) or Co(1)B and Co(2) centers, and the other is a bidentate-monodentate bridge(μ2:η2:η1), that is, bidentate through O(1) and O(2)toward Co(1) or Co(1)B and monodentate across O(2) toward another Co(2). Besides, one oxygen atom from nitrite ion bridges Co(1) or Co(1)B and Co(2) to form tricobalt SBUs. The SBUs are linked by the carboxylate and nitrogen atom of the L2–ligands to from an infinite 2D bilayered structure,and such adjacent 2D bilayered structures are further connected to get an infinite 3D framework by O–H··O and O–H··N hydrogen bonds (Fig. 1b). In addition, there exist weak π··π stacking interactions(centroid-to-centroid distance of 3.5678(4) ?, dihedral angle 1.795(81)°, Fig. 1c) among adjacent aromatic cycles of L2–ligands, which further stabilize the 2D layers. The detailed data of hydrogen bonds for compound 1 are shown in Table 4.

Table 1. Crystallographic Data for Compounds 1 and 2

Table 3. Selected Bond Lengths (?) and Bond Angles (°) for 2

Table 4. Hydrogen Bonds (?, o) of Compound 1

Symmetry codes: A: –1+x, 1–y, –0.5+z; B: –1–x, 1–y, –1–z; C: –x, y, –0.5–z; D: x, –1+y, z; E: –1–x, 2–y, –1–z

Fig. 1. (a) Molecular structure of compound 1 (The probability of ellipsoid is 30%);(b) Crystal packing of the 3D diagram by hydrogen bonds (The red dashed line represents hydrogen bonds);(c) π··π interactions in the 2D bilayered structures

3.2 Crystal structure of{[Ni(L)(H2O)5]·(H2O)2·DMF}n (2)

Single-crystal X-ray analysis reveals that compound 2 crystallizes in monoclinic crystal system of P21/n. As shown in Fig. 2a, the asymmetric unit consists of one Ni(II) cation, one L2-ligand, five coordinated water molecules, one solvated DMF molecule, and two solvated water molecules. Each Ni(II)ion is six-coordinated by one nitrogen atom from L2-ligand and five oxygen atoms from water molecules,generating a distorted octahedral coor- dination geometry. The Ni(1)–N bond length is 2.0628(17) ?,and the Ni(1)–O distances range from 2.0318(16) to 2.0784(17) ?. The Ni–O and Ni–N bond lengths are comparable to that found in similar types of Ni(II)octahedral complexes[21–22]. It is noteworthy that the two carboxylate groups of H2L ligand are both deprotonated and uncoordinated. There are a lot of intermolecular hydrogen bonds among carboxyl groups, coordinated water molecules, and solvated molecules in the complex (Fig. 2b). The structure of compound 2 from 0D to 3D depends upon intermolecular hydrogen interaction between the coordinated water molecules and carboxylate oxygen atoms. At the same time, the centroid-to-centroid distance of neighboring benzene and pyridine rings is 3.6427(1) ?, with a dihedral angle of 1.78(7)°,indicating a π-π stacking interaction between the aromatic rings (Fig. 2b). Obviously, compound 2 is stabilized by intramolecular hydrogen bonds and π-π stacking interactions. The detailed data of hydrogen bonds for complex 2 are shown in Table 5.

Table 5. Hydrogen Bonds (?, o) of Compound 2

Fig. 2. (a) Molecular structure of compound 2 (The probability of ellipsoid is 30%);(b) Crystal packing of the 3D diagram by hydrogen bonds and π··π interactions(All hydrogen atoms are omitted for clarity. The green dashed line represents the hydrogen bonds)

3.3 IR spectra

The infrared spectra of compounds 1 and 2 have been recorded and some important assignments are shown in the experimental section. The strong peaks at 1556 and 1557 cm-1were assigned to the asymmetrical stretching vibration of νas(COO-), while the absorption band at 1381 cm-1corresponds to the symmetric stretching of νs(COO-). Furthermore, the strong broad bands at 3315 and 3421 cmˉ1can be assigned to the O–H stretching vibration of water molecules. The above analyses are finally confirmed by the X-ray diffraction analysis.

3.4 Thermal analysis and XRD results

To estimate the stability, their thermal behaviors were carried out by TGA in flow N2in the temperature range of 20～700 ℃ (Fig. 3), with a heating rate of 20 ℃·min-1. For compound 1, a little weight loss of 4.98% is observed from 165 to 210 ℃due to the release of two coordinated water molecules (calcd. 4.72%). The second weight loss of 10.06% is observed from 210 to 360 ℃, which is attributed to the departure of two coordinated nitrite ions (calcd. 10.62%). Then the structure begins to decompose at 365 ℃. For compound 2, the lost weight is 37.84% owing to the release of two solvated water molecules, five coordination water molecules, and one solvated DMF molecule from 45–315 ℃ (calcd. 37.65%). Further weight loss indicates the collapse of coordination framework from 315 ℃. The thermal decomposition feature of the complexes is in good agreement with their crystal structures. As shown in Fig. 4, XRD experiment was carried out for compounds 1 and 2. The experimental and simulated XRD patterns are in agreement with each other, confirming the purity of the complexes.

Fig. 3. TG curves of compounds 1 and 2

Fig. 4. Experimental and simulated XRD spectra of compounds 1(a) and 2(b)

3.5 UV-Vis spectra

As can be seen in Fig. 5, the H2L, compounds 1 and 2 show intense absorption peaks at 200–340 nm,which can be ascribed to π-π* transitions of the ligands. In addition, for compound 1, we observe two peaks at 480 nm (4T1g(F) →4T2g(F)) and 530 nm (4T1g(F) →4T1g(P)). Besides, two additional peaks are also observed for compound 2, of which one small peak center is located at 385 nm (3T1g(P)→3A2g), and another site is at 656 nm (3T1g(F) →3A2g). Those are typical for octahedrally coordinated Co(II) and Ni(II) complexes[23]. UV-Vis spectra of the complexes are in complete agreement with the coordination environment revealed by X-ray singlecrystal structure analysis.

Fig. 5. Solid-state ultraviolet absorption spectra of H2L and compounds 1 and 2 at room temperature

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