A Defective Linear Pentachromium Complex for the Enhanced Catalysis of the Cycloaddition of CO2 with Propylene Oxide①

WANG Wen-Zhen LIN Wei LI Lei-Lei LIU Shung ISMAYILOV Ryyt Huseyn LEE Gene-Hsing PENG Shie-Ming

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A Defective Linear Pentachromium Complex for the Enhanced Catalysis of the Cycloaddition of CO2with Propylene Oxide①

WANG Wen-Zhena②LIN WeiaLI Lei-LeiaLIU ShuangaISMAYILOV Rayyat HuseynbLEE Gene-HsiangcPENG Shie-Mingc②

a(710065)b()c()

Based on the ligand (2,7-bis(α-pyrimidylamino)-1,8-naphthyridine (H2bpmany)), a linear pentachromium complex [Cr5(μ5-bpmany)4Cl2]PF6(1, μ5-bpmany=2,7-bis(a-pyrimidyla- mino)-1,8-naphthyridine) was synthesized. The crystal structure of compound 1 has been characterized by X-ray crystallography. Interestingly, one metal atom in the center is missing in this linear chain, leading to the defective pentachromium metal string structure which is similar with the reported complex [Cr5(μ5-dpznda)4Cl2] (2, dpznda = N2,N7-di(pyrazin-2-yl)-1,8-naphthyridine-2,7-diamine). The central Cr(Ⅲ) ion of 1 is eight-coordinated and is also rare in the chromium complex.The reaction of carbon dioxide with propylene oxide that generates propylene carbonate (PC) when catalyzed by [Cr5(μ5-bpmany)4Cl2]PF6was investigated. Different reaction conditions including temperature and pressure were studied to optimize the reaction conditions.

metal string complex, pentachromium(Ⅲ) complex; extended metal atomic chain;

1 INTRODUCTION

In recent years, metal string complexes, also named extended metal atom chain complexes (EMACs), have got a great deal of attention owing to their potential applications in molecular wires, switches and single molecular transistors[1]. The typical characteristic of metal string complexes is that the metal ion bone is helically wrapped by four deprotonated oligo-a-pyridylamido ligands (Scheme 1) and all amido nitrogen atoms coordinated in a syn-form[2]. In 1997, the first tri-chromium metal string complex Cr3(dpa)4Cl2(dpa = dipyridylamide) was synthesized by Cotton's group[3]. After that, a series of tri-[4], tetra-[5], penta-[6], hexa-[7], hepta-[8]and nona-[9]nuclear EMACs have been synthesized and studied[10]. The longest structurally characte- rized metal string complex contains nine metal atoms for chromium so far[11]. In order to expand the varieties of EMACs, we developed a series of modifications to the prototypical oligo--pyridyl- amido ligands for EMACs, mainly by replacing pyridyl ring by pyrimidine or naphthyridine groups. The new ligand 2,7-bis(-pyrimidylamino)-1,8-na- phthyridine (Scheme 2) is composed of three parts including two pyrimidine units at the terminals and 1,8-naphthyridine unit at the center. The 1,8-na- phthyridine-based ligands are attractive candidates for building linear metal string complex because the 1,8-naphthyridine unit is rigid and potentially redox active[12, 13]. Here we report a pentachromium EMAC [Cr5(μ5-bpmany)4Cl2]PF6(Scheme 3) and describe its synthesis, crystal structure and its catalytic performance.

Oligo--pyridylamido ligands

Scheme 1

2,7-Bis(a-pyrimidylamino)-1,8-naphthyridine (H2bpmany)

Scheme 2

[Cr5(μ5-bpmany)4Cl2]PF6

Scheme 3

2 EXPERIMENTAL

2. 1 Synthesis

2. 1. 1 Preparation of H2bpmany

2,7-Bis(a-pyrimidylamino)-1,8-naphthyridine (H2bpmany) was synthesized following the reported procedure[14, 15].

2. 1. 2 Preparation of [Cr5(μ5-bpmany)4Cl2]PF6

The mixture of H2bpmany (313.75 mg, 1 mmol), naphthalene (30 g) and anhydrous CrCl2(162.5 mg, 1.3 mmol) was heated at ca. 170～180 °C with the protection of argon and the ButOK (0.38 g, 3.4 mmol) solution in n-butyl alcohol (10 mL) was dropwise added. The reaction lasted for 6 hours. After cooling, the mixture was transferred to hexane to wash out naphthalene. Then deep brown solution was obtained after extraction with CH2Cl2. After the CH2Cl2solution was treated with excess KPF6in MeOH (25 mL) and stirred overnight, the solvent was removed under vacuum. The residual solid was extracted with CH2Cl2(50 mL) and filtered. Deep brown crystals were obtained by slow diffusion of n-pentane vapor into the filtrate. IR (KBr)/cm–1= 1593m, 1505m, 1389m, 1272m, 989s, 779s, 653m.

2. 1. 3 General procedure for the reaction of epoxides and CO2

A 100 mL stainless-steel reactor was charged with purified propylene oxide (PO)and catalyst. Then, the reactor was pressurized with carbon dioxide and heated to the desired temperature by stirring.After reaction, the autoclave was cooled down to room temperature, and the CO2pressure was released by opening the outlet valve. The solid residue was separated from the reaction mixture by filtration. The product propylene carbonate (PC) was obtained through distillation of the filtrate under reduced pressure. A qualitative analysis of the liquid products was performed on IR spectrum. For quantitative determination, the products were analyzed on an Agilent 6890 Plus GC with flame ionization detection.

2. 2 Structure determination

Data collection was carried out at 150(2) K using a Mo-radiation (= 0.71073 ?) and a liquid nitrogen low-temperature controller on a NONIUS KappaCCD X-ray diffractometer. Cell parameters were retrieved and refined using DENZO-SMN software on all reflections. Data reduction was performed on the DENZO-SMN. The type of absorption corrections is multi-scan. Semi-empirical absorption was based on symmetry-equivalent reflections and absorption corrections were applied with the DENZO-SMN program. All structures were solved by using the SHELXL-97 and refined with SHELXL-2014 by full-matrix least-squares on2values.

3 RESULTS AND DISCUSSION

3. 1 Structure

Crystallographic data and refinement of 1 are listed in Table 1. Selected bond lengths and bond angles for 1 are listed in Table 2. The crystal structure of 1 isshownin Fig. 1. The ligand of complex 1 was obtained by introducing pyrimidine and naphthyridine groups in place of pyridine units on the basis of oligo--pyridylamido. In this ligand, there are eight nitrogen atoms which can coordinate with metal atoms, including four pyrimidine nitrogen atoms, two amido nitrogen atoms and two naphthyridine nitrogen atoms. According to the reported literature[15], it is hard for two pyrimidine nitrogen atoms in the same pyrimidine ring to coordinate at the same time, so that H2bpmany is a hexadentate ligand, and hexametal EMACs are expected to form. However, one chromium atom is missing due to the unsaturated coordination mode of naphthyridinein complex 1[16]. There are only five chromium atoms coordinated with nitrogen atoms, and this phenomenon is rare in the metal string complexes. With one metal atom in the center of metal-atom chain being lost, the pentachromium complex [Cr5(μ5-dpznda)4Cl2][16](dpznda = N2,N7- di(pyrazin-2-yl)-1,8-naphthyridine-2,7-diamine) (Scheme 4) is the first reported example of the defective chromium EMACs. Similiar to complexes based on oligo--pyridylamido ligand, complex 1 also exhibits a typical structure of EMACs in which five chromium atoms were linear including four Cr(Ⅱ) ions and one Cr(Ⅲ) ion, and the metal atom bone of complex 1 was helically wrapped by four deprotonated H2bpmany. Additionally, the linearity of molecule is also very good. The angles of Cr–Cr–Cr bond are 169.21～178.14°, and the average is 178.838°.

N2,N7-Di(pyrazin-2-yl)-1,8-naphthyridine-2,7-diamine(H2dpznda)

[Cr5(μ5-dpznda)4Cl2]

Scheme 4

Table 1. Crystal Data for Complex 1

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

Symmetry transformation: a:,+1,; b: –,, –+1/2; c: –, –+1, –

Fig. 1. ORTEP drawing of the structure of complex 1.

Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity

The bond lengths of Cr(1)–Npm(pm = pyrimidine) in complex 1 are between 2.010 and 2.088 ?, with the average value of 2.097 ?. Cr(2)–Nambond lengths are 2.022～2.088 ?, and the average is 2.047 ?. Cr(4)–Nam(am = amido) have bond lengths of 2.010～2.069 ? and an average of 2.035 ?. The bond lengths of Cr(5)–Npmare between 2.081 and 2.124 ?, averaged to be 2.101 ?. It can be seen that the average of Cr–Namdistance is shorter than the average of Cr–Npmbecause the negative charge density of the amide nitrogen atom in the ligand is higher than the pyrimidine nitrogen atom[17].

Table 3. Comparison of Bond Lengths (?) for 1 and 2

Despite of the same synthetic conditions and similar ligand, the oxidation states of Cr ion are 3 and 2 for complexes 1 and 2, respectively. The different oxidation of the central ion might be caused by the different oxidation abilities of the pyrimidine and pyrazine groups.

3. 2 Catalytic cycloaddition of CO2 and epoxides

The catalytic activity of [Cr5(μ5-bpmany)4Cl2]PF6was first examined using the coupling of propylene oxide (PO) and CO2to produce propylene car- bonate (PC)[18-21]. The results are shown in Table 4. When the reaction was performed at 90 ℃ under 1.5 MPa CO2pressure for 1 h, n-Bu4NBr could catalyze the reaction with a low PC yield of 15.1%. The combination of [Cr5(μ5-bpmany)4Cl2]PF6and n-Bu4NBr showed high activity with PC yield of 82% under the same reaction conditions. So, we can deduce that the synergistic effect of [Cr5(μ5- bpmany)4Cl2]PF6and n-Bu4NBr can promote the cycloaddition reaction.

Table 4. Activity of Various Catalytic Systems for the Cycloaddition of CO2 with Epoxides

[a] Reaction condition: catalyst 0.03 mmol, TBAB 2 mmol under1.5 MPa at 90 ℃for 1 h

To determine the catalytic activity of 1 in the cycloaddition reaction of CO2and propylene to PC, we carried out the reaction in the temperature range of 50～110 ℃ with 1.5 MPa CO2pressure and reaction time of 1 h. Table 5 shows that the PC yield increased from 50 to 90℃, and the highest PC yield of 82% was obtained under 90 ℃for 1 h. When the temperature was further increased, the yield of PC began to decrease, indicating 90 ℃ was the optimal temperature for the cycloaddition reaction. We also examined the influence of CO2pressure in the range of 0.5～2.0 MPa on the cycloaddition reaction. As shown in Table 6, the PC yield increases with the CO2pressure from 0.5 to 1.5 MPa. Further increase of CO2pressure could decrease the yield of PC. Thus the optimal CO2pressure is 1.5 MPa with the reaction time of 1 h at 90 ℃.

Table 5. Synthesis of Propylene Carbonate Catalyzed by [Cr5(μ5-bpmany)4Cl2]PF6/TBAB in Different Temperature[a]

[a] Reaction condition: catalyst 0.03 mmol, TBAB 2 mmol under 1.5 MPa for 1 h

Table 6. Synthesis of Propylene Carbonate Catalyzed by [Cr5(μ5-bpmany)4Cl2]PF6/TBAB in Different pressure[a]

[a] Reaction condition: catalyst 0.03 mmol, TBAB 2 mmol at 90 ℃ for 1 h

Considering the different coordination environ- ments of the five Cr ions in complex 1, we suppose that the catalysis mainly occurs on the Cr(2) and Cr(4) ions. The Cr(1) and Cr(5)ions are connected with chlorine atoms and the central Cr(3) ion is eight-coordinated, so there is almost no exposed Lewis acidic site except the Cr(2) and Cr(4) ions. Partially coordinated metal ionsCr(2)and Cr(4) are readily available Lewis acid site to activate epoxide through the coordination between Cr ions and oxygen atom in the epoxide. Then the less hindered carbon atom was attacked by Br-from the n-Bu4NBr, which results in the opening of epoxy rings. This is followed by the interaction of CO2with oxygen anion of the opened epoxy ring form an alkylcarbonate anion, which is then converted into propylene carbonate through a ring closing step.

4 CONCLUSION

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27 December 2017;

16 April 2018 (CCDC 1576615)

① This project was supported by the Scientific Research Program Funded by Shaanxi Provincial Education Department(No. 17JK0606), and the Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 16JK1598), the Xi’an Shiyou University Postgraduate Innovation and Practical Ability Training Project (No.YCS16212073)

Wang Wen-Zhen, professor. Tel: 86-13389214744. E-mail: wzwang@xsyu.edu.cn;Peng Shie-Ming, professor, E-mail: smpeng@ntu.edu.tw

10.14102/j.cnki.0254-5861.2011-1937