Iminopyridine-Based Cobalt(II) and Nickel(II) Complexes: Synthesis, Characterization, and Their Catalytic Behaviors for 1,3-Butadiene Polymerization

A series of iminopyridine ligated Co(II) (1a–7a) and Ni(II) (1b–7b) complexes were synthesized. The structures of complexes 3a, 4a, 5a, 7a, 5b, and 6b were determined by X-ray crystallographic analyses. Complex 3a formed a chloro-bridged dimer, whereas 4a, 5a, and 7a, having a substituent (4a, 5a: CH3; 7a: Br) at the 6-position of pyridine, producing the solid structures with a single ligand coordinated to the central metal. The nickel atom in complex 5b features distorted trigonal-bipyramidal geometry with one THF molecule ligating to the metal center. All the complexes activated by ethylaluminum sesquichloride (EASC) were evaluated in 1,3-butadiene polymerization. The catalytic activity and selectivity were significantly influenced by the ligand structure and central metal. Comparing with the nickel complexes, the cobalt complexes exhibited higher catalytic activity and cis-1,4-selectivity. For both the cobalt and nickel complexes, the aldimine-based complexes showed higher catalyst activity than their ketimine counterparts.

We have been interested in the synthesis of transition metal complexes to promote 1,3-butadiene polymerization over the past decades. For example, cis-1,4 selective polymerization of 1,3-butadiene was achieved by bis(imino)pyridine cobalt(II) complexes/MAO catalyst [9], and the incorporation of electron-withdrawing groups could enhance activity and selectivity simultaneously. Moreover, other catalyst systems usually focused on the steric and electronic effects of the substitutions at the iminoaryl rings on the catalytic activity and properties of resulting polymers. Even though some of these complexes based on the iminopyridine ligands have been reported for ethylene polymerization and oligomerization [20][21][22][23][24], iminopyridine cobalt(II) and nickel(II) complexes for 1,3-butadiene polymerization have not been investigated yet. In the present study, iminopyridine cobalt(II) and nickel(II) complexes were synthesized, and the influences of the substituents at the 6-position of the pyridine ring and in the imino bridge on catalytic activity and selectivity in 1,3-butadiene polymerization were investigated.

General Considerations and Materials
All the manipulations were carried out in a nitrogen atmosphere by using standard Schlenk techniques. 1,3-Butadiene (Jinzhou Petrochemical Company, Jinzhou, China) was purified by passing through two columns containing potassium hydroxide and active alumina. Hexane (Beijing Reagents Factory, Beijing, China) was refluxed over sodium benzophenone ketyl until the solution turned blue, and then distilled before use. Ethylaluminum sesquichloride (EASC) was purchased from Acros Chemicals (Geel, Belgium). CoCl 2 , NiBr 2 and all the anilines were obtained from Alfa Aesar (Ward Hill, UK). Other chemicals were commercially available and used without further purification.
FTIR spectra were performed on a BRUKE Vertex-70 FTIR spectrometer (Bruker Optics, Ettlingen, Germany). Elemental analysis was performed using an elemental Vario EL spectrophotometer (Elementar, Hanau, Germany). The molecular weights (M w and M n ) and molecular weight distributions (M w /M n ) of polymers were measured at 30˝C by gel permeation chromatography (GPC) equipped with a Waters 515HPLC pump, a series of four columns (HMW7THF, HMW6ETHF(two), HMW2THF) and a Waters 2414 refractive index detector (Waters, Massachusetts, UK). Tetrahydrofuran was used as an eluent at a flow rate of 1.0 mL/min. X-ray crystallography measurements. Single crystals of 3a, 4a, 5a, 7a, 5b, and 6b suitable for X-ray diffraction were obtained by a slow diffusion of diethyl ether into their THF solution. Data collections were performed at´85˝C on a BRUKE SMART APEX diffractometer (Bruker, Bremen, Germany) with a CCD area detector, using graphite monochromated Mo K radiation (λ = 0.71073 Å). The determination of crystal class and unit cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to yield the reflection data file. The structures were solved by using SHELXTL program. Refinement was performed on F 2 anisotropically for all non-hydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters.

Procedure for 1,3-Butadiene Polymerization
A typical procedure for the polymerization is as follows: a toluene solution of 1,3-butadiene (1.0 g, 1.85ˆ10´2 mol) was added to a moisture free ampoule preloaded with complex 1a (7.3 mg, 1.85ˆ10´5 mol), and EASC (1.156ˆ10´3 mol/mL, 0.4 mL) was then injected to initiate the polymerization at 20˝C. After 15 min, methanol was added to the system to quench the polymerization. The mixture was poured into a large quantity of methanol containing 2,6-di-tertbutyl-4-methylphenol (1.0 wt %) as a stabilizer. Via filtering and drying under vacuum at 40˝C, polybutadiene resulted at a constant weight (0.569 g, 56.9%).

Preparation of Cobalt and Nickel Complexes
A mixture of L1 (0.2 g, 0.00075 mol) and anhydrous CoCl 2 (0.1 g, 0.00075 mol) were added to a flask containing 5 mL THF. The mixture was stirred at room temperature for 24 h, a green suspension was formed. Diethyl ether was added and a suspension formed. The precipitate was collected by filtration and washed with 4ˆ5 mL heptane. The desired product (0. 21 Procedure similar for 3a was adopted by using L3, and CoCl 2 produced 3a as a green powder in 52. 8% Procedure similar for 4a was adopted by using L4, and CoCl 2 produced 4a as a green powder in 92. Procedure similar for 5a was adopted by using L5, and CoCl 2 produced 5a as a green powder in 86. Procedure similar for 7a was adopted by using L7, and CoCl 2 produced 7a as a green powder in 88.1% yield. IR (KBr, cm´1)  To a suspension of NiBr 2 (dme) (0.11 g, 0.00036 mol) in 5 mL, CH 2 Cl 2 was added into the solution of ligand L1 (0.1 g, 0.00036 mol) in 5 mL CH 2 Cl 2 . The orange-red reaction mixture was formed and stirred for 24 h at room temperature, and the precipitate was collected by filtration, washed with pentane (2ˆ10 mL), and dried under vacuum to produce complex 1b as an orange-red powder. Yield: Procedure similar for 2b was adopted by using L2, and NiBr 2 (dme) produced 2b as a green powder in 91.7% yield. IR (KBr, cm´1): 2966, 1612, 1591, 1571, 1373, 940, 816, 781  Procedure similar for 5b was adopted by using L5, and NiBr 2 (dme) produced 5b as a green powder in 83. Procedure similar for 7b was adopted by using L7, and NiBr 2 (dme) produced 7b as a green powder in 85.

Synthesis and Characterization of Iminopyridine Ligands (L1-L7) and Their Cobalt(II) and Nickel(II) Complexes
The iminopyridine ligands L1, L2, L4-L7 were prepared by the condensation of 2-pyridinecarboxaldehydes or 2-pyridylketones with 2,6-di(1-methylethyl)phenylamine in the presence of a catalytic amount of formic acid in methanol (Scheme 1). Ligand L3 was synthesized with a few drops of concentrated H 2 SO 4 as catalyst, and anhydrous Na 2 SO 4 was used to remove the water formed during the reaction according to the literature [21]. All the ligands were identified by FT-IR, elemental analysis, and NMR spectra. Cobalt complexes 1a-7a were prepared by the reaction of anhydrous cobalt(II) chloride with the corresponding ligand in THF. All of the complexes were isolated as green air-stable powders in high yields. Nickel complexes 1b-7b were prepared by the reaction of the corresponding ligand with NiBr 2 (dme) in CH 2 Cl 2 . The structures of these complexes were determined by FT-IR spectra and elemental analysis. The structures of complexes 3a, 4a, 5a, 7a, 5b, and 6b were further characterized by X-ray crystallographic analysis.

Crystal Structure of Complexes
Single crystals of 3a, 4a, 5a, 7a, 5b, and 6b suitable for X-ray diffraction analysis were obtained by crystallization from their THF or diethyl ether solutions. The crystal data together with the data collection and structure refinement parameters are presented in Table 1. Selected bond lengths and angles for cobalt complexes (3a, 4a, 5a and 7a) are given in Table 2. Selected bond lengths and angles for nickel complexes (5b and 6b) are given in Table 3.    Cobalt complexes 4a, 5a, and 7a with a substituent (4a: CH 3 ; 5a: CH 3 ; 7a: Br) at the 6-position of the pyridine ring produced the solid structures with a single ligand coordinated to the central metal. The asymmetric unit of 4a contains two independent, closely resembling molecules (Figure 1), whereas only one molecule is found in the asymmetric unit of complexes 5a ( Figure 2) and 7a (Figure 3). In complex 4a crystallizing in a monoclinic space group P2 1 /c, the ligand system adopts a cisoid conformation that allows both the nitrogen centers N1 and N2 (N3 and N4) to coordinate to the cobalt atom. The metal-nitrogen bond lengths are 2.181 Å (Co1-N1) and 2.089 Å (Co1-N2), and the Co-Cl bond lengths are 2.2769 Å (Co1-Cl1) and 2.4428 Å (Co1-Cl2). In complex 4a, the cobalt atom has distorted tetrahedral coordination geometry with N1-Co1-N2 angle of 77.69˝and Cl1-Co1-Cl2 angle of 96.61˝. The 2,6-diisopropylphenyl substituent at the imine nitrogen atom is almost perpendicular to the plane of the pyridine moiety (angle between the planes: 89.84˝). The angle of N2-C5-C6 is 120.4˝when there is a H atom in the imino bridge. Crystallizing in a monoclinic space group P2 1 /n and P2 1 /c, complexes 5a and 7a with CH 3 group in the imino bridge have a smaller N2-C7-C5 angle of 116.4˝and N2-C6-C5 angle of 116.58˝than those in complex 4a. Meanwhile, they have longer N-Co bond distances (5a: 2.0439 Å, 2.0498 Å; 7a: 2.0667 Å, 2.0514 Å), shorter Co-Cl bond distances (5a: 2.2071 Å, 2.2148 Å; 7a: 2.2079 Å, 2.2235 Å ), and bigger angles of N1-Co1-N2 (5a, 80.68˝; 7a, 80.14˝) and Cl1-Co1-Cl2 (5a, 115.44; 7a, 113.88) than complex 4a. The 2,6-diisopropylphenyl substituent at the imine nitrogen atom is also perpendicular to the plane of the pyridine moiety (angle between the planes: 5a, 89.56˝; 7a, 83.97˝). However, ligand L2 and L3 (H atom at 6-position of the pyridine ring) results in the formation of a chloro-bridged dimer. In complex 2a, reported in a previous report [22], the Co-N bond distances are 2.093 and 2.123 Å, which are longer than those in 5a and 7a but shorter than that in complex 4a; however, the Co-Cl bond distance exhibits the converse regularity (4a > 2a > 5a « 7a). In complex 3a (Figure 4), with phenyl group in the imino bridge, the coordination geometry of the central metal atom can be best described as distorted trigonal-bipyramidal with two equivalent half molecules in an overall C 2 -symmetric framework. The pyridyl nitrogen atom (N1) and Cl mark the apical centers of the distorted trigonal bipyramid. The pyridine nitrogen (N1) and the other constituent of the chloro-bridge (Cl2) occupy the axial coordination sites. Complex 3a has a bipyramidal structure most distorted towards a square-based pyramid, with the most acute angle (164.04˝) in the vertical axis (N1-Co1-Cl2) and one of the equatorial angles extended to 125.21˝.    Nickel complex 5b crystallized from THF in a monoclinic space group P2 1 /c, producing the solid structures with a single ligand and one THF molecule coordinated to the metal center. In complex 5b ( Figure 5), the nickel is five coordinate, and the coordination sphere could be best described as a distorted trigonal bypyramidal with the equatorial plane occupied by the imino N atom and the two bromine atoms. The axial positions are coordinated by the pyridine N atom and by the O atom of the THF molecule. A Br-Ni-Br angle of 141.13˝was observed within the equatorial plane and a "chelating" angle of N1-Ni-N2 is 80.22˝. The distance of Ni-O bond is 2.148 Å, which is a little longer than those of the N-Ni bond (2.0383 Å, 2.0677 Å), indicating a very strongly bound THF ligand. Complex 6b ( Figure 6) with Br at the 6-position of the pyridine ring crystallizes as centrosymmetric dimmers with two ligand nitrogen atoms, one terminal bromine and two bridging bromine atoms forming the coordination sphere around the five-coordinate nickel center. Complex 6b contains the halves of two independent and nearly identical molecules, which is similar to complex 4b reported in the literature in that both have nearly the same Co-N bond distance (

Solution Polymerization of 1,3-Butadiene
Cobalt complexes could polymerize 1,3-butadiene activated by ethylaluminum sesquichloride (EASC). Herein, we mainly discuss the influences of the R 1 group (H, CH 3 and Br) at the 6-position of the pyridine ring and R 2 group (H, aldimine; CH 3 , ketimine) in the imino bridge on catalytic activity and selectivity of the complexes. The polymerization results are summarized in Table 4. The substituent (R 1 ) at the 6-position of the pyridine ring significantly influence the catalytic performances of the complexes. For the aldimine-and ketimine-based cobalt complexes, the introduction of halogen atom (Br) at 6-position of the pyridine ring afforded polymers in higher yields but with relatively lower molecular weight than the complexes without substituent. The complexes with CH 3 group produced polymers in lower yield but with higher molecular weight and narrower molecular weight distribution. For instance, the polymerization of 1,3-butadiene with aldimine cobalt complex 6a (R 1 = Br) produced polymer in 92.8% yield, which is much higher than complexes 1a (R 1 = H, 56.9%) and 4a (R 1 = CH 3 , 42.9%). Similar to aldimine cobalt complexes, ketimine cobalt complex 7a (R 1 =Br) displayed higher catalyst activity (polymer yield: 90.8%) than 2a (R 1 = H, 51.6%) and 5a (R 1 = CH 3 , 31.9%). On the other hand, the molecular weights of polybutadienes obtained by these cobalt complexes exhibited the converse regularity (in the orders of 6a < 1a < 4a and 7a < 2a < 5a). These results could be explained by the idea that the electron-withdrawing group (Br) at the 6-position reduces the electron density of central metal, and the increased Lewis acidic character facilitates the coordination of 1,3-butadiene molecule, leading to the increment of chain propagation rate. Moreover, due to the electron-contributing and steric hindrance of the CH 3 group, the coordination reaction between 1,3-butadiene and the cobalt center was retarded [10] . The CH 3 group can serve to raise polymer molecular weight (4a, 31.5ˆ10 4 ; 5a, 33.8ˆ10 4 ) and regulate molecular weight distribution (4a, 1.9; 5a, 1.7), suggesting that electron-contributing substituent at 6-position of the pyridine ring retard chain transfer reaction to some extent.
The substituent (R 2 ) in the imino bridge of the iminopyridine cobalt(II) complex was also investigated. Complex 3a with a phenyl group exhibited much higher activity than 1a (R 2 = H) and 2a (R 2 = CH 3 ). This result indicates that the conjugation of phenyl and imine groups can stabilize the active centers and cause the increased chain propagation rate. In olefin catalysts based on bisiminopyridine transition metal complexes, replacing a ketimine (R 2 = CH 3 ) with an aldimine (R 1 =H) usually leads to difference in molecular weight and molecular weight distribution, particularly the productivity, and usually the catalysts based on an ketimine ligand are approximately one order of magnitude more active than those based on an aldimine ligand [25]. In 1,3-butadiene polymerization, bisiminopyridine cobalt complexes based on ketimine ligand also showed higher catalytic activity than their aldimine counterparts [9]. In the present study, however, iminopyridine cobalt(II) complexes based on aldimine produced the polybutadiene in a higher polymer yield, smaller molecular weight and broader molecular weight distribution than the ketimine complexes, and the results were as follows: 1a (R 1 =H, 56.9%) > 2a (R 1 = H, 51.6%), 4a (R 1 = CH 3 , 42.9%) > 5a (R 1 = CH 3 , 31.9%) and 6a (R 1 = Br, 92.8%) > 7a (R 1 = Br, 90.8%). This is due to the fact that, during the activation with EASC, the -CH=N groups of the complexes can be deprotonated to yield an anionic amide ligand. This anionic amide ligand is in the form of an ion pair or in the free form, resulting in higher polymerization activity [12,26].
The nature of the metal center had a large influence on the catalytic performance. In general, cobalt-based catalysts were more active than the corresponding nickel-based analogs; under the same conditions, except for a prolonged polymerization time by 3h employed here, iminopyridine-based nickel complexes produced polymers in 29.2% to 79.3% yields. All the nickel complexes 1b-7b yielded polymers with much lower molecular weights (7000-9900 g/mol), narrower molecular weight distributions (1.72-2.67), and lower cis-1,4 contents (79.3%-90.7%) than their cobalt counterparts (Entry 8-14 in Table 4). Similarly, as for the nickel complexes, the aldimine (R 2 = H) complexes showed higher catalyst activity than the corresponding ketimine (R 2 = CH 3 ) complexes in the orders: 1b (R 1 = H, 69.7% conv.) > 2b (R 1 = H, 35.8% conv.), 4b (R 1 = CH 3 , 75.4% conv.) > 5b (R 1 = CH 3 , 47.1% conv.) and 6b (R 1 = Br, 79.3% conv.) > 7b (R 1 = Br, 72.8% conv.). Moreover, nickel complexes (6b, 7b) with Br at 6-position of pyridine ring exhibited a higher catalyst activity than the cobalt complexes described. However, 3b with phenyl in the imino bridge showed the lowest catalyst activity, quite different from 3a with the same ligand, indicating that phenyl group in the complex could retard the chain propagation and chain transfer reaction to produce the polymer with lower yield, higher molecular weight, and narrower molecular weight distribution (Entry 10 in Table 5). The results in Table 5 demonstrated that the ligand environment did not significantly influence molecular weight and molecular weight distribution of the obtained polymers, but remarkably influenced the stereoregularity of the polybutadienes. The aldimine complexes produced polymers with lower cis-1,4 content, more trans-1,4 content and nearly invariable 1,2 content compared with ketimine complexes. The 13 C NMR spectra of the polymers obtained with 6a and 6b are shown in Figure 7. It is clear that the nickel complex produced the polymers with higher trans-1,4 content than the cobalt complex sharing the same corresponding ligand L6.