LAN Mei-Ying LIANG Hong LU Xue-Chun DENG Jin-Qin CHENG Xing CHEN Jin-Xin, b② ZHANG Zhi-Chun

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Synthesis, Structure and Norbornene Polymerization Catalyzed by Nickel(II) Complex Bearing N,O-bis(1-(6-ethylpyridin-2-ylimino)-methylenyl)naphthalen-2-ol Ligand①

LAN Mei-YingaLIANG HongaLU Xue-ChunaDENG Jian-QinaCHENG XingaCHEN Jian-Xina, b②ZHANG Zhi-Chuna

a(350007)b(350002)

The title complex bis{1-{[(6-ethyl-2-pyridinyl)imino]methylenyl}-2-naphthale- nolato-N,O}-nickel(II) (C36H32N4O2Ni) has been synthesized by the reaction of 1-{[(6-ethyl-2- pyridinyl)imino]methylenyl}-2-naphthalenol with Ni(CH3COO)2·4H2O, and characterized by IR spectrum, elemental analysis and TG. The complicated space structure has been confirmed by single-crystal X-ray diffraction analysis. The crystal belongs to the monoclinic system, space group21/with= 11.410(4),= 14.382(4),= 18.121(6) ?,= 97.147(6)o,= 2950.5(16) ?3, C36H32N4O2Ni,M= 611.37,= 4,D= 1.376 g/cm3,= 0.698 mm-1,(000) = 1280, the final= 0.0519 and= 0.1493 (>2()). This title compound was used as precatalysts for the polymerisation of norbornene. When activated with MAO, the complex exhibited excellent catalytic activity up to 1.98 × 107g of PNB (mol of Ni)?1h?1with high monomer conversion.

nickel complex, crystal structure, catalyst, norbornene polymerization;

1 INTRODUCTION

In recent years, the polymerization of norbornene has attracted considerable attention due to its unique properties, such as good thermal stability, high che- mical resistance, low birefringence and dielectric loss in the fields of catalysis, polymer science and organometallic chemistry[1-6]. It is reported that the polymerization of norbornene can be carried out with the help of a serial of catalysts such as tradi- tional Ziegler-Natta catalyst, metallocene, late tran- sition metal catalysts,[7-9]. Compared to the for- mer transition metal catalysts (traditional Ziegler- Natta catalyst, metallocene metal catalysts), late transition metal catalysts have aroused great interest because of their weak oxophilicity and high toleran- ce against polar groups in industry and academia[10, 11]. In order to obtain industrial application value of the catalyst, we mainly enhance the catalytic activity of complexes through the modification of existing structures of ligands or the research of new ligands. The late transition metal complexes of nickel have aroused great concern because of their high cata- lytic activity toward olefin polymerization[12, 13]. In addition, the acetylacetonate complexes of nickel catalyze norbornene polymerization using MAO as the cocatalyst.

In this paper, we designed and synthesized bis{1- {[(6-ethyl-2-pyridinyl)imino]methylenyl}-2-naphthal-enolato-N,O} nickel(II) complex which was applied to norbornene. Activated by MAO, the nickel complex exhibited high catalytic activity in the norbornene polymerization. The catalytic ac- tivity up to 1.98 × 107g of PNB (mol of Ni)?1h?1was obtained.

2 EXPERIMENTAL

2. 1 Materials and measurements

All manipulations involving air- and moisture- sensitive compounds were carried out under N2atmosphere in strict accordance with the Schlenk technology standard[7]. Dichloromethane was dried by calcium hydride and steamed out under the protection of nitrogen. NB was dried by elemental sodium, and then distilled under the protection of nitrogen. The stock solution of NB was prepared in dichloromethane. All the chemicals were of analytical grade and wereused without further purification.

Fourier transform infrared (FT-IR) spectrum was recorded as KBr pellets in the range of 4000～500 cm-1using a Nicolet AV-360 spectrometer. Elemen- tal analysis was acquired on a Vario MICRO micro- analyzer. Thermogravimetric analysis (TG) mea- surement was determined in flowing N2at a heating rate of 10 ℃/min on a METTLER TGA/ SDTA851 instrument from 30 to 600 ℃. The single-crystal X- ray diffraction was performed on a SCX min MERCURY2 diffractometer.

2. 2 Preparation of the ligand and complex

The main synthetic route for the title complex was shown in Scheme 1. According to the modifi- cation of a reported procedure[14], nickel acetate tetrahydrate (0.136 g, 0.55 mmol) and 2-hydroxy- 1-naphthaldehyde (0.172 g, 1 mmol) were heated in ethanol (30 mL) along with 2-amino-6-ethylpyri- dine (0.122 g, 1 mmol) for 5 h. The solution was cooled slowly to room temperature and then remo- ved to give a green solid in 58% yield. The nickel com- plex was purified by crystallization from dichloro- methane. Anal. Calcd. (%) for C36H32N4O2Ni: C, 70.97; H, 4.93; N, 9.20. Found (%): C, 80.06; H, 5.04; N, 9.10. IR (KBr, cm-1): 3439(s), 3058(w), 1613(s), 1565(m), 1543(s), 1457(m), 1305(m), 1209(w), 1161(w), 991(w), 899(w), 838(w), 791(m), 756(m), 551(w).

Scheme 1. Synthetic route of the title compound

IR spectra of the complex show that the C=N stretching frequency appears at 1613 cm-1. Compared with the ligand, it is shifted to lower wave numbers. The results indicate that there are coordination inter- actions between the nitrogen and metal ions.

2. 3 Structure determination

A green single crystal of the nickel complex was performed with Mo-(= 0.71073 ?) on a SCX min MERCURY2 diffractometer by using an-2scan mode in the range of 3.06≤≤27.50° at 293(2) K. A total of 35660 reflections were obtained with 6728 unique ones (int= 0.0181) and used in the succeeding refinements. The structure was solved by direct methods with SHELXS-97, and refined by full-matrix least-squares techniques on2using SHELXL-97 program. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in the calculated positions.= 0.0519 and= 0.1493. Goodness-of-fit is 1.030. (Δ)max= 0.817 and (Δ)min= –0.634 e/?3. The selected bond lengths and bond angles of the title compound are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3. 1 Description of the structure

The molecular structure of the title compound is shown in Fig. 1. Selected bond lengths and bond angles are listed in Table 1.

Fig. 1 . Molecular structure of the title compound

In the molecule, the nickel atom coordinates to two phenolic oxygen atoms (O(1) and O(2)) and two azomethine nitrogen atoms (N(1) and N(3)) of two ligands to form two six-membered rings. The bond angles of O(1)–Ni(1)–O(2) and N(1)–Ni(1)– N(3) are 110.73(8) and 127.02(7)o, respectively. In other words, O(1), Ni(1) and O(2) are not in the same line and N(1), Ni(1) and N(3) are also not collinear. The Ni(1)-O(1) and Ni(1)-O(2) bonds are 1.9339(18) and 1.9244(17) ? and the Ni(1)-N(1) and Ni(1)-N(3) are 1.9869(18) and 1.9948(17) ?, respectively. The mean Ni–O and Ni–N bond lengths are smaller than the Ni–O and Ni–N bond lengths reported in literatures[15-17]. In addition, the diagonal angle O(1)–Ni(1)–N(3) is 111.67(7)° and N(1)–Ni(1)–O(2) 123.30(7)°. The other two dia- gonal angles, N(3)–Ni(1)–O(2) and N(1)–Ni(1)– O(1), are 94.08(7) and 90.25(7)°, respectively. In another word, the nickel atom coordinates to two ligands to form a tetrahedral coordination sphere around the nickel atom. This serious deformation of tetrahedral structure is probably due to steric effect.

3. 2 Homopolymerizations of NB

In the experimental process, the nickel complex, norbornene and dry CH2Cl2were added into a bottle with strong stirring under nitrogen atmosphere. After the mixture was stirred for one minute, MAO was charged into the polymerization system by means of a syringe, and the reaction was initiated. Four mi- nutes later, acidic ethanol (Vethanol:VHCl= 9:1) was added to terminate the reaction. The solid PNB was filtered, washed with ethanol, and dried in vacuum at 50 ℃. Unless otherwise stated, the total reaction volume was 15 mL.

The activities of catalysts in norbornene polyme- rization have been proved to be influenced by several parameters, including MAO dosage, reaction temperature, monomer concentration and reaction time. So, we have extensively investigated the reac- tion parameters affecting polymerization of norbor- nene by MAO dosage, reaction temperature, and monomer concentration[18].

The variation of ratio of MAO/nickel complex has a significant influence on the polymer yields and catalytic activities. As shown in Table 2, with the increase of Al/Ni ratio from 1000 to 3000, the polymer yields and catalytic activities first increase rapidly, and then tend to be steady when it reaches 2000/1. Similar results are observed with other nickel catalysts reported previously[18-22].

Table 2. Influence of the MAO Amount on the Activity of the Title Complex

Polymerization condition: complex, 0.5 μmol; solvent, methylene chloride; T, 30 ℃; NB/Ni, 20000; Time, 5 min; total volume,15 mL.

aIn units of 107g of PNB (mol of Ni)?1h?1

The reaction temperature also influences the ca- talytic activities of the complex as well as the mole- cular weights of the polymers. A series of polymeri- zations catalyzed by the complex were carried out at different reaction temperature from 10 to 50 ℃. The polymer yields and catalytic activities first increase and then decrease from 10 to 50 ℃, as shown in Table 3. The optimal polymerization temperature was 30℃[18].

Table 4 shows the catalytic activity of nickel complex versus the molar ratios of NB/Ni. Ap- parently, with the increase of NB/Ni molar ratios, the catalytic activity was enhanced dramatically. The catalytic activity of 1.44 × 107g of PNB (mol of Ni)?1h?1was obtained at a NB/Ni ratio of 20000:1 while a higher catalytic activity of 1.98 ×107g of PNB (mol of Ni)?1h?1was obtained at a NB/Ni ratio of 30000:1. This result implies that higher NB /Ni ratio facilitates the complexation of monomers with the activated nickel complexes[18].

Table 3. Influence of the Reaction Temperature on the Activity of the Title Complex

Polymerization condition: complex, 0.5 μmol; solvent, methylene chloride; Al/Ni, 2000; NB/Ni, 20000; Time, 5 min; total volume, 15 mL.aIn units of 107g of PNB (mol of Ni)?1h?1

Table 4. Influence of the Molar Ratios of NB/Ni on the Activity of the Title Complex

Polymerization condition: complex, 0.5 μmol; solvent, methylene chloride; T, 30 ℃; Al/Ni, 2000; Time, 5 min; total volume, 15 mL.

aIn units of 107g of PNB (mol of Ni)?1h?1

3. 3 IR spectrum and thermal analysis

As shown in Fig. 2, the absence of signals at 1620～1680 cm-1in FT-IR spectra revealed that the catalytic reaction belongs to vinyl-type polymeriza- tion rather than ring-opening metathesis polymeriza- tion (ROMP)[23-25]. IR (KBr, cm-1): 3733(w), 34459(m), 2947(s), 2869(s), 1452(m), 1380(w), 1295(w), 1259(w), 1149(w), 1108(w), 943(w), 892(w), 805(w).

Thermal analysis curves (TG) of PNBs, which were catalyzed by the Ni complex, are shown in Fig. 3. TG curves show that PNBs have good thermal stability up to 445.28 ℃and begin to decompose at higher temperature[25-27]. The results indicate that PNBs possess excellent thermal stability in the N2atmosphere.

Fig. 2 . IR spectra of polynorbornene

Fig. 3 . TG of polynorbornene

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22 September 2014; accepted25 November 2014 (CCDC 946797)

① Supported by the Natural Science Foundation of Fujian Province (2010J01026), the Ministry of Education of China (No. 208066), the Education Department of Fujian Province (JA07029), and the State Key Laboratory of Structural Chemistry (No. 20130013)

. Born in 1964, professor, majoring in catalytic chemistry. Tel: 0591-83448669, E-mail: jxchen_1964@163.com

10.14102/j.cnki.0254-5861.2011-0519