A Novel Tripyridyldiamine Nickel Complex: Synthesis, Structure and Characterization①

LI Yan WANG Wen-Zhen TANG Chang-Sheng JIA Xin-Gang FAN Wei WANG Li

(School of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China)

ABSTRACT A novel nickel(II) complex [Ni(H2tpda)(NCS)2(CH3OH)] was synthesized by using tripyridyl- diamine as ligand and Ni(NCS)2 as starting materials, and characterized by a variety of techniques including single-crystal X-ray diffraction, IR spectroscopy and TG-DSC. The single-crystal structure reveals that the complex exhibits as a neutral molecule and that the central atom Ni(II) is octahedrally coordinated by an H2tpda, two NCS- ions and a ligand molecule CH3OH. The 3D supramolecular network is formed through hydrogen bonds and π-π interactions. The complex can catalyze the addition reaction of carbon dioxide and propylene oxide.

Keywords: tripyridyldiamine, functional complexes, supramolecular chemistry, crystal structures;

1 INTRODUCTION

The research and development of functional complexes have become one of the research hotspots in the fields of coordination chemistry, material chemistry and bio-inorganic chemistry. Metal functional coordination compounds combine the characteristics of both ligands and metal ions, and that of inorganic and organic compounds. In recent years, many functional complexes with unique properties like optical, electrical, magnetic, and adsorption properties have been developed[1-5]. Because the polypyridylamine has a plurality of coordination sites, the -NH group can serve as a donor of hydrogen bonds, and the π-π stacking is easily formed between the aromatic rings, many supramolecular complexes with novel structures and peculiar functions can be synthesized by using it as a ligand[6-9]. With the ligands of these complexes mostly being the dipyridylamine, the dipyridylamine complex of nickel has a good effect on ester transfer catalysis at normal temperature and without acid[10]. In this paper, a novel nickel complex was synthesized by using tripyridine diamine as a ligand.

2 EXPERIMENTAL

2. 1 Main instruments and reagents

Bruker Smart CCD 1000 Diffractometer; Nicolet-5700 FT-IR infrared spectrometer (KBr Tablet) SMP/PF7548/ MET/400W type TG-DSC thermal analysis (in nitrogen protection). The tripyridine diamine H2tpda was synthesized according to the literature[11]. The chosen crystal (0.22mm × 0.16mm × 0.08mm) was placed on the diffractometer. Data collection was carried out at 20(2) ℃ using a MoKα radiation (λ = 0.071073 nm) with an ω-2θ mode. All calculations using SHELXS-97 and SHELXL-97 programs were completed[12,13]. Other reagents were commercially available analytical rea- gents.

2. 2 Synthesis of the complex

The experiment uses a liquid phase method, that is, a conventional solution reaction volatilization method, and the reactant molar ratio is a ligand:metal ion M = 1:1. Under magnetic stirring, Ni(NCS)2(0.2 mmol) in methanol (15 mL) was slowly added to H2tpda (0.2 mmol) in methanol (15 mL), and a yellow-brown solution was formed and stirred at room temperature for two days. Then, the color of the solution became lighter, and the pale brown liquid was obtained via filtering, and allowed to stand for about 8 days. The solvent was evaporated to precipitate a red crystal product from the solution.

2. 3 Catalytic experiment

The catalyst [Ni(H2tpda)(NCS)2(CH3OH)], co-catalyst TBAB and propylene oxide were added to an autoclave, in which was passed CO2to a certain pressure (1.5 MPa), and heated and stirred at 100 ℃ for one hour. The reaction was completed and cooled to room temperature. The conversion rate of propylene oxide was analyzed by1H NMR spectroscopy.

3 RESULTS AND DISCUSSION

3. 1 Analysis of the crystal structure of the complex

The complex [Ni(H2tpda)(NCS)2(CH3OH)] is a neutral molecular complex. The main bond lengths are shown in Table 1. The crystal crystallizes in monoclinic P21/c space group. The molecular structure is shown in Fig. 1, in which the central atom Ni(II) is six-coordinated in an octahedral configuration formed by the coordination of one ligand H2tpda, two NCS-ions and one CH3OH molecule. The angle of the axial N-Ni-N in 173.06° shows that the octahedron in which the central atom Ni(II) is located is distorted. The H2tpda is a tridentate ligand with three pyridinium nitrogen atoms as coordinating atoms that coordinate in all anti-forms. Due to the presence of steric hindrance, the three pyridines of the ligand H2tpda are not in one plane. Ni(II) is coordinated by five nitrogen and one oxygen atoms, where three pyridine nitrogen atoms from the same ligand H2tpda and an oxygen atom from the methanol molecule form an octahedral equatorial plane, with the two other nitrogen atoms from two NCS-ions located at the two axial tips of the octahedron. The nitrogen atom of the ligand is chelated with the central metal ion Ni(II) to form two stable six-membered rings, each ring consisting of two carbon and three nitrogen atoms together with one central ion Ni(II). The bond lengths of Ni-N (H2tpda) vary from 2.060 to 2.062 Å, those of Ni-N (NCS-) are 2.045 and 2.082 Å, and that of Ni-O is 2.129 Å. The N-C-S of coordinated NCS-ions has angles of 179.29° and 178.86°, which is close to a straight line.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Fig. 1. Perspective view of [Ni(H2tpda)(NCS)2(CH3OH)] with the labeling scheme (Ellipsoid probability is 50%, and hydrogen atoms are omitted for clarity)

For the complex, the tripyridinediamine ligand has two amine groups -NH as a donor of hydrogen bond, and the nitrogen and sulfur atoms of NCS-ions as the acceptors. The π-π conjugate action between pyridine rings plays a decisive role in the structure of the three-dimensional coordination polymer. The nitrogen atoms of NCS-ions and the C-H of pyridine ring form a weak C(9BA)-H(9BA)···N(5AA) intermole- cular hydrogen bond (Table 2). The molecules are connected along the b axis into a one-dimensional chain (Fig. 2). The uncoordinated -NH of ligand H2tpda and the sulfur atom of thiocyanate form the N(2AA)-H(2AA)···S(2) intermolecular hydrogen bond, forcing the one-dimensional molecular chains to be further connected around the c-axis into a two- dimensional plane (Fig. 3). The π-π conjugation around the a-axis is formed between the pyridine rings of adjacent molecules, with the distance between their surfaces to be about 3.272 Ǻ. The π-π conjugation is combined with hydrogen bond to construct a three-dimensional supramole- cular structure (Fig. 4).

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

Fig. 2. 1-D supramolecular structure of the title complex. Hydrogen bonds are represented by the dashed lines

Fig. 3. 2-D supramolecular structure of the title complex. Hydrogen bonds are represented by the dashed lines

Fig. 4. π-π interactions in the title complex. Hydrogen bonds are represented by the dashed lines

3. 2 IR spectrum analysis of complexes

The moderate-intensity absorption at 3426 cm-1represents N-H and C-H vibration bonds in the ligand tripyridinediamine. Due to the association of hydrogen bonding, the absorption peak is broadened and the width is more than 500 cm-1. The sharp peak at 2121 cm-1corresponds to the characteristic vibrational absorption on the thiocyanate coordinated on the nitrogen side. The infrared absorption of C=N of ligand H2tpda was at 1607 and 1560 cm-1, with the two peaks migrating to 1631 and 1585 cm-1, respectively. Compared with the ligand H2tpda, the infrared absorption peak of pyridine generally moves toward the high wavenumber, which means that the pyridine participates in the coordination of complex.

3. 3 Thermal analysis of complexes

The TG-DSC thermal analysis of the complex from room temperature to 800 ℃ at a heating rate of 10 ℃/min is shown in Fig. 5. In the range of 52～376 ℃, there is an obvious weight loss with the percentage of 5.91, corresponding to the departure of a methanol (percentage 6.82). In the range of 383～527 ℃, an remarkable weight loss in the percentage of 55.96 appears, which is consistent basically with losing H2tpda (percentage 55.99). Therefore, the thermal stability of this complex is excellent.

Fig. 5. TG-DSC thermal analysis for the title complex

3. 4 Catalytic performance

In this section, we designed a series of experiments to explore potential catalytic performance of the complex. The pyridyl and nickel metals in the complex could give the catalyst a high density of Lewis-basic and Lewis-acid active sites, giving it multiple functionalities, indicating that it has broad prospects in catalytic applications. Thus, we designed a series of experiments to examine their catalytic activities on the cycloaddition of CO2and propylene oxide. As shown in Table 3, the catalyst and co-catalyst had a significant effect on the cycloaddition of CO2and propylene oxide (substrate). We utilized 0.025 mol% the complex in combination with 0.10 mol% TBAB as binary catalysts to obtain solvent-free cyclic carbonates under the conditions of 100 ℃ and 1.5 MPa, which achieved good conversion (61%) and showed high catalytic activity with the initial turnover frequency (TOF) up to 2440 h-1after 1 hour reaction (Table 3, entry 4).

Table 3. Cycloaddition of CO2 and Propylene Oxide (PO) Using [Ni(H2tpda)(NCS)2(CH3OH)] (1)

4 CONCLUSION

Reaction of H2tpda with Ni(NCS)2gives [Ni(H2tpda)(NCS)2(CH3OH)] which is a neutral molecular complex whose crystal belongs to the monoclinic P21/c space group. The central atom Ni(II) is six-coordinated in a octahedral configuration formed by the coordination of one ligand H2tpda, two NCS-ions and one CH3OH molecule. Two NCS-ions are trans. The H2tpda in the complex is anti-anti configuration.

Two hydrogen bonds are formed between the complex molecules, together with the π-π conjugation between the pyridine rings, constructing a three-dimensional supramole- cular structure system.

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