Bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) as new catalytic material

Nowadays, studies are carried out on the design and synthesis of new catalysts for olefin oligomerization and polymerization, which would contain non-toxic metals and at the same time show high catalytic activities. Complex compounds of transition metal ions such as Fe(II), Cr(III) and Zr(II) containing pyridine or quinoline as ligands show at least moderate catalytic activity in ethylene and propylene polymerizations. To investigate the catalytic activity of the complex containing pyridine ligands and quinoline derivatives, here we have synthesized the crystals of new bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) solvate. The synthesized cobalt(II) complex compound was tested in reactions of 2-chloro-2-propen-1-ol and norbornene oligomerizations. Our studies showed that bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) after activation by MMAO-12 catalyzes the formation of oligomers in nitrogen atmosphere, under atmospheric pressure and at room temperature. Bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) possesses moderate catalytic activity in the formation of norbornene oligomers process and low catalytic activity in 2-chloro-2-propen-1-ol oligomerization.

Polymer materials are produced in the global industry by the free radical polymerization method or with the use of a catalyst containing a metal atom or cation 1,2 . Organometallic complexes are known as olefin oligomerization and polymerization catalysts 3 . In industrial olefin oligomerization and polymerization, the Ziegler-Natta catalysts were used firstly, but despite high catalytic activities, they required the use of large amounts of solvents for purification of oligomerization and polymerization products 4 . The new generation of catalysts replacing Ziegler-Natta catalysts was metallocene compounds. They were metallocene complex compounds of transition metal atoms or cations, usually containing cyclopentadienyl and its derivatives as ligands 5 . Metallocene catalysts had some flaws, such as decomposition of the catalyst after activation by methylaluminoxane or an organoaluminum compound used as activator. Additionally, the oligomerization and polymerization reactions had to be carried out at high temperatures and high pressures 6 . The search for more sustainable catalysts than metallocene compounds resulted in the discovery of post-metallocene catalysts. These are complex compounds of transition metal ions, which catalyze oligomerization and polymerization reactions under low-pressure conditions 6 . Recent developments in post-metallocene catalysts have led to explore the complex compounds of transition metal ions such as Fe(II), Cr(III) and Zr(II) containing pyridine or quinoline as ligands, showing at least moderate catalytic activity in ethylene and propylene polymerizations [7][8][9] .
Among others, bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) solvate hypothetically could be applied as catalysts in polymerization processes, due to cobalt(II) complexes are known as olefin polymerization catalysts. The crystal structure of bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) ethanol solvate was described by Zhang et al. (CSD REFCODE: IVUMIX) in 2016 and his anticancer activity has been proved 10 . However, neglected area in the field of this compound is preparation in the presence of other solvents as well as studies on the catalytic activity of this complex compound.

Results and discussion
The complex compound bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt is known in the literature 10 , but its polymorphic form has been synthesized by our group and its physicochemical and catalytic properties have been described in this report. The bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt water solvate (1:2) has been structurally characterized using single-crystal XRD method. The molecular structure of title compound has been shown in Fig. 1. The crystallographic data for title compound have been collected in Table 1. Title compound crystallize in the orthorhombic Pnna space group with half bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) molecule and one disordered water molecule in asymmetric unit ( Fig. 1) and is isostructural with bis(5-bromoquinolin-8-olato)-bis(pyridine)-cobalt(II) water solvate (1:2) (CSD REFCODE: EKISUO) 11 . In the crystal structure of the title compound, the geometric parameters (bond lengths and angles) characterized structure of cobalt(II) complex are similar to those observed in the crystal structure of EKISUO ( Fig. 1). The Co(1) atom is six-coordinated in an octahedral coordination geometry, in which two 5-chloro-8-hydroxyquinolinium and two pyridine ligands. In the crystals, neighboring molecules of cobalt(II) complex are linked via weak C-H⋯π and π⋯π interactions to form porous metal-organic framework in voids of which disordered solvent molecules are located (Fig. 2). The polymorph obtained by our group of the compound crystallizes in triclinic crystal system and has a molecular weight of 574.32 g mol −110 , immediately the compound described in this study in orthorhombic crystal system and has a molar mass of 610.34 g mol −1 . Whereas bis(5-bromoquinolin-8-olato)-bis(pyridine)cobalt(II) water solvate (1:2) crystallizes in the orthorhombic crystal system and has the Pnna space group as well as the compound studied by our group. The cobalt(II) complex with pyridine and 5-chloro-8-hydroxyquinoline described by Zhang et al. 10 is a polymorph of the complex described in our report. It is the same chemical substance-one cobalt(II) cation bonded with two pyridine molecules and two 5-chloro-8-hydroxyquinoline anions but this complex occurs as different crystallographic form. These two polymorphs differ in unit cell dimensions (a, b, c) and crystallize in a different crystal system. Additionally, the complex described in our report has two crystallization water molecules. Therefore, the compound reported by Zhang et al. 10 has a different molar mass than that described in this work. Elemental analysis of synthesized bis (   The porous structure of the bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) can clearly be seen from its scanning electron micrograph (SEM) as shown in Fig. 3. As observed, the complex is composed of aggregated flake-like particles with a size of several tens of micrometers. Moreover, the sample possesses not only macropores but also mesopores with a considerable volume. The hierarchically porous structure of the complex could be advantageous for a higher efficiency of the catalytic reactions. SEM studies of other cobalt(II) complex compounds, for example cobalt(II) complex compound with triphenylphosphine ligands, show that this material exists in micrometer-sized grains similar to the complex compound described in this paper 13 .
For the first time, the catalytic properties of bis(5-chloroquinolin-8-olato)-bis(pyridine)-cobalt(II) have been studies for 2-chloro-2-propen-1-ol and norbornene oligomerizations. Bis(5-chloroquinolin-8-olato)bis(pyridine) -cobalt(II) as a new catalyst is characterized by the following features: it is a coordinating compound where the center of coordination is a cobalt(II) cation with a coordination number equal to 6, the catalyst has a molar mass equal to 610.34 g mol −1 , the morphological structure of the new catalyst is flakes several dozen micrometers in size, it is well soluble in an equivolume mixture (1: 1) of DMSO and toluene, it must be activated by MMAO-12 for oligomerization of 2-chloro-2-propen-1-ol and norbornene, and after the reaction it cannot be reused due to the structure change by reaction with MMAO-12. The analysis of the MALDI-TOF-MS spectra www.nature.com/scientificreports/ showed that the oligomerization reaction of 2-chloro-2-propen-1-ol formed oligomers with 3-5 mers in the chain. The oligomerization of 2-chloro-2-propen-1-ol usually proceeds in such a way that a sample is produced containing oligomer chains of 5-15 mers 14 . In contrast, in the case of the norbornene oligomerization reaction, the product chains contain from 3 to 9 mers. Thermal analysis of the oligomerization products allowed to study their thermal stability in the range from 20 to 1000 °C. The products of 2-chloro-2-propen-1-ol oligomerization thermally decompose within 4 steps while at 838.4 °C the product mass does not degrade further. During thermal decomposition, molecules of CO, CO 2 , HCl  Comparing the determined values of catalytic activity with other cobalt(II) complex compounds, it can be concluded that the new polymorph described in this work will produce relatively low rates of catalytic activity. Taking into account the general classification of catalysts in terms of their efficiency, it can be concluded that the new polymorph belongs to the catalysts with moderate or low activity 15 . Cobalt(II) complexes of bis(aryliminophosphoranyl)methane after activation by methylaluminoxane, exhibit moderately activity for ethylene polymerization at 20 °C 16 , while cobalt(II) complexes with bis(2,6-iminophosphoranyl)pyridine ligands after methylaluminoxane and triisobutylaluminum activation exhibit catalytic activity equal to 62 g mmol −1 h −1 bar −117 . During the polymerization reaction catalyzed by bis(2,6-iminophosphoranyl)pyridine cobalt(II) complexes, ethylene is introduced at a pressure of 10 bar at 25 °C 17 . Iminodiacetate cobalt(II) complex compound is highly active catalyst for 2-chloro-2-propen-1-ol oligomerization undergoing at normal pressure and at room temperature 14 . This complex compound exhibit catalytic activity equal to 759.04 g mmol −1 h −1 bar −114 . To our best knowledge, there are no cobalt(II) complex compounds described in the literature, which would show higher catalytic activity in olefin oligomerization or polymerization than iminodiacetate cobalt(II) complex compound. By analyzing the effect of the type of metal center in the post-metallocene catalyst i.e. Fe(II), Cr(III) and Zr(II) containing pyridine, lactones, proline derivatives or quinoline as ligands, on the catalytic activity, it  Then the mixture was poured with the solution prepared by mixing ethanol and pyridine in the 28: 1 volume ratio. The mixture has been heated at 80 °C in a round-bottomed flask until the color of the mixture changed. After cooling, brown crystals were obtained (Fig. 5). 2-Chloro-2-propen-1-ol oligomerization (Fig. 6) was performed under a nitrogen atmosphere, under atmospheric pressure and at room temperature according to the following procedure: 3 µmol of the synthesized complex was placed in a glass cell, then dissolved in 1 mL of toluene and 1 mL of DMSO. Then, while stirring vigorously, 3 mL of MMAO-12 solution (MMAO-12, 7 wt% aluminum in toluene) was dropped into the cell. The molar ratio cobalt(II) compound:MMAO-12 was equal to 1:1000. The solution turned brown. In the next step, 1 mL of 2-chloro-2-propen-1-ol (purchased from Merck, 90 % purity) was dropped into the mixture of complex compound and MMAO-12. The molar ratio 2-chloro-2-propen-1-ol:cobalt(II) compound was equal to 37,667:1. The solution has been stirred over a magnetic stirrer until a gel was formed. Then the mixture was washed with hydrochloric acid 1M and methanol in 1:1 molar ratio.
Norbornene oligomerization (Fig. 6) was performed under a nitrogen atmosphere, under atmospheric pressure and at room temperature as follows: 3 µmol of the synthesized complex dissolved in the mixture 1 mL of toluene and 1 mL of DMSO. In the next step, 3 mL of MMAO-12 solution (MMAO-12, 7 wt% aluminum in toluene) was dropped into the cell. The molar ratio cobalt(II) compound:MMAO-12 was equal to 1:1000. As a result the solution turned brown. Next, 1.2 g of norbornene (purchased from Merck, 99 % purity) was dissolved in 1 mL of toluene. The molar ratio 2-chloro-2-propen-1-ol:cobalt(II) compound was equal to 4233:1. Then, 1 mL of the solution of norbornene was dropped into the cell. The solution has been stirred over a magnetic stirrer until a gel was formed. Then the mixture was washed with hydrochloric acid 1 M and methanol in 1:1 molar ratio.