Our variations on iron and cobalt catalysts toward ethylene oligomerization and polymerization

Wen-Hua Sun; Shu Zhang; Weiwei Zuo

doi:10.1016/j.crci.2007.09.002

Our variations on iron and cobalt catalysts toward ethylene oligomerization and polymerization

Wen-Hua Sun ¹ ; Shu Zhang ¹ ; Weiwei Zuo ¹

¹ Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 2, Beiyijie, Zhongguancun, Beijing 100080, China

Comptes Rendus. Chimie, Volume 11 (2008) no. 3, pp. 307-316.

Résumé

Iron and cobalt complexes are a new family of catalysts for ethylene oligomerization and polymerization. The extensive researches on bis(imino)pyridyl metal complexes have been carried out with the aim of synthesizing their derivatives and finding suitable reaction parameters for the optimum activity. Beyond the modification works of bis(imino)pyridyl metal complexes, several alternative models with similar coordination sphere have been developed in our group. This review article describes our experiences in innovating new models of iron and cobalt complexes as catalysts for ethylene oligomerization and polymerization.

Métadonnées

Reçu le : 2007-05-29
Accepté le : 2007-09-05
Publié le : 2007-10-25

DOI : 10.1016/j.crci.2007.09.002

Keywords: Iron complex, Cobalt complex, Ethylene oligomerization, Ethylene polymerization

Affiliations des auteurs :

Wen-Hua Sun ¹ ; Shu Zhang ¹ ; Weiwei Zuo ¹

¹ Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 2, Beiyijie, Zhongguancun, Beijing 100080, China

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     publisher = {Elsevier},
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     language = {en},
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Wen-Hua Sun; Shu Zhang; Weiwei Zuo. Our variations on iron and cobalt catalysts toward ethylene oligomerization and polymerization. Comptes Rendus. Chimie, Volume 11 (2008) no. 3, pp. 307-316. doi : 10.1016/j.crci.2007.09.002. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2007.09.002/

Version originale du texte intégral

Le texte intégral ci-dessous peut contenir quelques erreurs de conversion par rapport à la version officielle de l'article publié.

1 Introduction

Investigations on late-transition metal complexes as catalysts for olefin oligomerization and polymerization have rapidly increased in a dozen of years, which were initiated with nickel and palladium catalysts by the Brookhart group [1], as well as iron and cobalt catalysts developed simultaneously by Brookhart [2] and Gibson groups [3]. Relative research works have been focused on modifying those catalysts' models through synthesizing numerous derivatives of their complexes and also devising alternative models of complex catalysts. The progress of fast-expending researches on nickel catalysts has been nicely considered in several recent review articles [4–9]. However, there are few models of complexes' catalysts of iron and cobalt complexes performing good catalytic activities toward ethylene oligomerization and polymerization in comparison with various models of nickel complexes. There are some significant advantages for iron and cobalt catalysts, such as high catalytic activity, high α-olefin selectivity in ethylene oligomerization, and lower toxicity than other metals. Therefore intensive screening researches on new molecular models of iron and cobalt complexes are still highly attractive, and the comprehensive review papers are reported [10,11].

Most alternative models of iron and cobalt complexes showed lower catalytic activities than that of bis(imino)pyridyl metal catalogues [11]. Fortunately, the derivatives of bis(imino)pyridyl metal (Fe or Co) dihalides showed some unique properties in our results, extensively, the 6-carboxalate-2-iminopyridyl metal halides showed catalytic activities toward ethylene oligomerization. Furthermore, inspired by the coordination electronic configuration of bis(imino)pyridyl metal (Fe or Co) dihalides, the models of metal complexes bearing 2-imino-1,10-phenanthrolines and 6-organyl-2-(imino)pyridines were devised and proved to be successful in ethylene oligomerization and polymerization. Herein, our experiences were outlined with parts as derivatives of bis(imino)pyridyl iron dihalides and hypothesis of active species, endeavoring works toward metal complexes bearing 2-imino-1,10-phenanthrolines, and novel metal complexes ligated by a series of 6-organyl-2-(imino)pyridines.

2 The typical iron catalyst and the hypothesis of active species

2.1 Symmetrical and unsymmetrical 2,6-bis(imino)pyridyl iron(II) complexes

The highly active 2,6-bis(imino)pyridyl iron(II) catalysts [2,3] moved more chemists into the investigation of late-transition metal complexes as catalysts for olefin oligomerization and polymerization. In the beginning, numerous 2,6-bis(imino)pyridyl iron analogues were prepared for detail comparative investigation, and three complexes performed significantly different catalytic activities in ethylene reactivity. (Fig. 1) [12]. The complex 1 with substituted phenyl groups showed high activities toward ethylene polymerization as reported under treatment with co-catalyst of MAO. However, complexes 2 and 3 exhibited different properties: no activity for 2, and meanwhile good ethylene oligomerization for 3. Within the same coordination frame of complexes, their different catalytic performances reflected the significant influences of substituents linked on ligands, in another word, the electronic and steric effects of ligands.

Fig. 1
Symmetrical and unsymmetrical 2,6-bis(imino)pyridine iron(II) complexes.

Considering commercial catalysts in heterogeneous forms, the suitable carriers were selected in order to make supported iron catalysts for ethylene polymerization. The SiO₂-supported catalysts showed a little lower activity with acceptable morphology of resultant polyethylene particles [13]. However, the better carrier was generally found to be spherical MgCl₂ for late-transition metal catalysts [14,15]. Beyond that, simulation modeling comprising experimental works was carried out. The metal atom net charge correlation (MANCC) was employed to study the relative catalytic activities of bis(imino)pyridyl iron(II) and bis(imino)pyrimidyl iron(II) complexes for olefin oligomerization/polymerization [16]. It was concluded that the catalytic activity was more relied on the electronic configuration of the catalyst, especially the net charge on the central metal atom. Therefore, the magnitude of the net charge characterizes the activity of connecting bonds during polymerization.

2.2 2-(Ethylcarboxylato)-6-iminopyridyl iron and cobalt complexes

2,6-Diacetylpyridine is a basic substance in forming bis(imino)pyridines for catalyst preparation; however, it was too expensive for us to buy 2,6-diacetylpyridine. Instead, 2,6-diacetylpyridine was prepared by the reaction of 2,6-dicarbethoxypyridine with excessive amount of sodium ethanolate. Surprisingly, a byproduct of 6-acetylpyridine-2-carboxylate was often observed in small fraction or trace amount. By controlling the amount of NaOEt, 2,6-diacetylpyridine and 6-acetylpyridine-2-carboxylate could be individually obtained [17]. With our synthetic procedure, these two compounds are commercially available at reasonable price in China. In order to use 6-acetylpyridine-2-carboxylate, 2-(ethylcarboxylato)-6-iminopyridines and their metal complexes were generally prepared and investigated [17–19]. Though 2-(ethylcarboxylato)-6-iminopyridyl iron and cobalt complexes (Fig. 2) showed moderate catalytic activity (up to 10⁵ g (mol metal)⁻¹ h⁻¹) toward ethylene oligomerization and polymerization in the presence of excessive amount of MAO, the prospect of iron and cobalt catalysts was the high selectivity for α-olefins. In addition, 6-acetylpyridine-2-carboxylate could be a substance for synthesizing new heterocyclic compounds as suitable ligands for complex catalysts (see Sections 5.2 and 6).

Fig. 2
2-(Ethylcarboxylato)-6-iminopyrid iron and cobalt catalysts.

3 “Active species” and hypothesis of molecular models

The late-transition metal catalysts are also called post-metallocene catalysts; interestingly, most complex catalysts perform better catalytic activity with methylaluminoxane (MAO), though not always. As the case specialized, let us have a careful look at active species of common (iron, cobalt and nickel) complexes' catalysts compared to zirconocene catalyst. Their catalytic precursors are shown in Fig. 3. It is commonly accepted that those precursors will be alkylated into “active species”, which are dialkylated intermediates, such as metallocene [20], nickel catalyst [21], iron catalyst [22,23], with the exception of monoalkylated cobalt active species [23,24]. It is not difficult to find the 14e intermediate model for those “active species”, and such coordinative features perform the good to high catalytic activity in ethylene oligomerization and polymerization.

This hypothesis was supported by the iron dichloride- bearing 2-imino-6-pyridyl pyridines, in which the catalytic system with MAO showed good activity for ethylene oligomerization [25]. The extension to cobalt complexes bearing 6-organyl-2-iminopyridines also provided catalytic system for ethylene oligomerization [26]. However, newly designed iron and cobalt complexes generally showed lower catalytic activity than the complexes containing bis(imino)pyridines. Moreover, steric influences of ligand along with the constrained geometry will be also of importance. Based upon tridentate ligands favorable to iron catalytic models, our further explorations have been focused on devising alternative ligands of 6-organyl-2-iminopyridines and 2-imino-1,10-phenanthrolines.

4 Iron(II) and cobalt(II) complexes bearing 2-imino-1,10-phenanthrolines

Inspired by Brookhart's and Gibson's original work of the 2,6-bis(imino)pyridyl iron and cobalt complexes [2,3], we were interested in exploring new molecular models of complexes as catalysts for ethylene oligomerization and polymerization. In order to construct tridentate N^N^N ligands, the phenanthroline derivatives and their transition metal complexes were investigated through works of synthetic methodology and multi-step synthetic works. Although tremendous effort cost in multi-step syntheses, the highly active catalysts were successfully obtained for ethylene oligomerization and polymerization.

4.1 2,9-Bis(imino)-1,10-phenanthrolinyl iron(II) and cobalt(II) complexes

There are two parent chemicals commercially available, 1,10-phenanthroline and 2,9-dimethyl-1,10-phenanthroline. To construct tridentate nestle compounds as ligands for complexes, the transformation of the methyl group was potentially easy. At that stage, however, it was hard to selectively mono-functionalize 2,9-dimethyl-1,10-phenanthroline. The controlling oxidation provided 2,9-diformyl-1,10-phenanthroline, and therefore, 2,9-bis(imino)-1,10-phenanthrolines were prepared through the condensation reaction with various anilines, and then coordinated with metal halides (Fig. 4) [27]. The ligands could provide tridentate coordination feature which was confirmed by the single-crystal X-ray diffraction of their metal complexes.

Fig. 4
2,9-Bis(imino)-1,10-phenanthrolinyl metal complexes.

The original idea of designing these N^N^N tridentate compounds was to obtain the iron complexes as highly active catalysts. However, the iron complexes showed only negligible activities for ethylene polymerization. Fortunately, upon treatment with MAO, the cobalt complexes showed good activity for ethylene oligomerization and polymerization (up to 9.3 × 10⁵ g mol⁻¹ h⁻¹); meanwhile the nickel analogues showed high activity for ethylene oligomerization. The nitrogen atom of the additional imino group potentially caused this problem because the additional coordination onto the active center (iron) would occupy the sites which were necessary for the coordination of ethylene in catalytic reaction. To verify the hypothesis that the additional imino group deactivated the iron species, the additional imino group should be removed. Therefore, our further effort was the synthesis of 2-imino-1,10-phenanthrolines and their metal complexes for ethylene reactivity.

4.2 2-Imino-1,10-phenanthrolinyl iron(II) and cobalt(II) complexes

All catalysts would not be recognized without approving their high performance (activity and selectivity) in special subjects. Regarding the background of polyolefins rooted in industrial application, the synthetic procedure of catalysts is necessary to have reliable substances and manipulation process. Therefore, we spent years on their preparative procedures of 2-formal-, 2-acetyl-, and 2-benzoyl-1,10-phenanthrolines from 1,10-phenanthroline. However, some expected products could not be obtained in traditional synthetic routines. For example, the hydrolysis of 2-cyano-1,10-phenanthroline gave 1,10-phenanthroline-2-carboxylic acid, which was expected to react with methanol for its ester. The esterification reaction could not happen, however, it was necessary to form its acetyl chloride, which then reacted with methanol. The total yield of 2-acetyl-1,10-phenanthroline was about 30% on the basis of best yields of intermediate substances observed in six-step syntheses (Fig. 5). Though there are many reports on the transformation of nitrile into ketones, 2-cyano-1,10-phenanthroline did not give acetylated product on reacting with methyllithium. The trimethylaluminum is one common reagent as co-catalyst in our group, and fortunately, the trial reaction of 2-cyano-1,10-phenanthroline with trimethylaluminum could produce 2-acetyl-1,10-phenanthroline in reasonable yield (Fig. 5). In a word, 2-acetyl-1,10-phenanthrolines could be prepared from 1,10-phenanthrolines with three-step reactions instead of six-step reactions [28].

Fig. 5
The synthetic procedure of the desired substance for ligands.

With the prepared 2-formal-, 2-acetyl-, and 2-benzoyl-1,10-phenanthrolines, the various 2-imino-1,10-phenanthroline derivatives were easily formed by the condensation reaction with anilines; further coordination with iron and cobalt dihalides gave the designed 2-imino-1,10-phenanthrolinyl metal complexes (Fig. 6) [28,29].

Fig. 6
Synthesis of 2-imino-1,10-phenanthrolinyl metal complexes.

Upon treatment with MAO or MMAO, these iron complexes showed high catalytic activities for ethylene oligomerization with high α-olefin selectivity. MAO was found to be a more effective co-catalyst for iron catalytic system with activity up to 4.91 × 10⁷ g mol⁻¹(Fe) h⁻¹ than MMAO [28]. The substituents' effects of ligands on the catalytic activities and the distributions of oligomer products were also investigated with various iron analogues containing different R, R^1–3 substituents. Indeed, both the R on the imino-C and the substituents on the N-aryl rings had an obvious influence on the catalytic activity, distribution of oligomers, and selectivity for α-olefins due to their steric and electronic properties.

Aldimine (R = H) and phenyl-ketimine (R = Ph) complexes showed relatively lower activities than the corresponding methyl-ketimine (R = Me) complexes. Electron-donating groups placed on the N-aryl rings increased the selectivity for α-olefins. The placement of bulky o-alkyl groups on the N-aryl rings led to reduced activity because of steric interaction. Compared with bis(imino)pyridyl iron catalyst, 2-imino-1,10-phenanthrolinyl iron catalysts performed comparable catalytic activity toward ethylene reactivity, but more preferred to form oligomers and high order polyethylene waxes. Though there are more interesting catalysts for polyethylenes, such alternative catalytic process provides new substance with unique properties, especially the vinyl-type linear polyethylene waxes. More iron analogues were reported by our group [29].

Parallel research works for their cobalt analogues [30] were carried out, and the cobalt catalysts also showed good catalytic activities, which were lower than those of their iron analogues. The cobalt catalysts produced short-chain oligomers with mainly butenes and hexenes. MMAO was found to be a more effective co-catalyst at 1 atm ethylene pressure. Increasing the ethylene pressure to 10 atm, prolonged lifetime and improved catalytic activity were observed in the presence of MAO. The ligand environment greatly affected the catalytic performance. For the complexes bearing the same alkyl groups at the ortho-positions of the imino-N-aryl ring, the catalytic activities increased in the sequence: aldimine (R = H), methyl-ketimine (R = Me) and phenyl-ketimine (R = Ph), which was different from the case of their iron analogues. Beyond our research, there were more derivative analogues of cobalt catalysts reported by the Solan group [31].

In order to directly reflect the significant differences of catalytic activities of 2-imino-1,10-phenanthrolinyl metal complexes with or without the additional imino group, typical examples in ethylene reactivity are listed in Table 1. The 2,9-bis(imino)-1,10-phenanthrolinyl iron complexes showed no oligomerization activity, though one case had a little polyethylene produced. However, all the 2-imino-1,10-phenanthrolinyl iron complexes showed very high activity for both oligomerization and polymerization. Therefore, we can get a conclusion that the additional imino group coordinated onto the active catalytic center and prevented the coordination of ethylene. Regarding their cobalt analogues (Table 1), 2-imino-1,10-phenanthrolinyl cobalt complexes showed higher oligomerization activity and no polymerization activity. Though 2,9-bis(imino)-1,10-phenanthrolinyl cobalt complexes showed lower catalytic activity, polyethylenes were obtained. The characteristic difference of cobalt and iron complexes could also be observed.

Complex	R¹	R²	A_o (g mol⁻¹ h⁻¹)	A_p (g mol⁻¹ h⁻¹)	Products
	i-Pr	H	No	No
Me	H	No	1.0 × 10⁴	PE
Me	Me	No	No

	i-Pr	H	9.42 × 10⁶	2.1 × 10⁴	C₄–C₂₈ and wax
Me	H	3.89 × 10⁷	1.02 × 10⁷	C₄–C₂₈ and wax
Me	Me	1.24 × 10⁷	1.38 × 10⁷	C₄–C₂₈ and wax

	i-Pr	H	7.0 × 10⁵	6.0 × 10³	C₄ and PE
Me	H	7.1 × 10⁵	9.6 × 10⁴	C₄, C₆ and PE

	i-Pr	H	1.26 × 10⁶	No	C₄–C₈
Me	H	6.7 × 10⁵	No	C₄–C₈

4.3 Additional remarks on complex catalysts bearing 1,10-phenanthrolinyl group

With the framework of 1,10-phenanthroline, reasonable efforts have been spent for various derivatives and their metal complexes in our group. With a view to developing complex catalysts for olefin oligomerization and polymerization, more research works including the synthesis of their derivative complexes and exploring alternative co-catalysts and optimum catalytic conditions is still worthwhile. In general, nickel complexes could be easily synthesized with various coordinative ligands. Therefore, more nickel complexes have been prepared with 1,10-phenanthroline derivatives, such as 2,9-disubstituted-1,10-phenanthrolines [32], 2-imino-1,10-phenanthrolines [33], 9-phenyl-2-imino-1,10-phenanthrolines [34], and 2-(benzimidazol-2-yl)-1,10-phenanthrolines [35] (Fig. 7). Though their nickel pre-catalysts showed good to high catalytic activities toward ethylene oligomerization, α-olefin selectivity was pretty low due to the catalytic mechanism of very fast alkyl migration on nickel active species.

Fig. 7
Nickel complexes bearing 1,10-phenanthroline derivatives.

In our experiences, the metal complexes ligated by those alternative derivatives of 1,10-phenanthrolines showed high catalytic activity toward ethylene reactivity. Moreover, some 1,10-phenanthroline derivatives could be synthesized in tens of grams of product in the laboratory. Those iron catalysts provided high activity for ethylene oligomerization, with good selectivity for α-olefins. Moreover, those catalytic systems produced less polyolefins; especially the polyolefins were polyolefin waxes without high molecular fractions. This is an advantage with regard to ethylene oligomerization process because any polyolefin with high molecular weight could potentially block the pipelines of the chemical process.

5 6-Organyl-2-(imino)ethylpyridyl iron(II) and cobalt(II) complexes

Though there are some reports regarding 6-organyl-2-(imino)pyridyl metal complexes mentioned above [17–19,25,26], the full scope of 6-organyl-2-(imino)pyridines has not been investigated yet. Driven by devising alternative tridentate metal complexes for catalysts, 6-(2-benzoimidazolyl)-2-iminopyridine and 6-(quinoxalinyl)-2-iminopyridines, as an alternative model to 6-pyridyl-2-imino-pyridines, appeared in our mind. As a matter of fact, our other challenge for iron complexes bearing 2,6-bis(2-benzoimidazolyl)pyridines was not successful within iron complex, but resulted in 2,6-bis(2-benzoimidazolyl)pyridyl chromium complexes and their catalysis for ethylene reactivity [36]. It was worthy to mention that the investigation of o-phenylenediamine was carried out in detail to form benzimidazoles, quinoxalines and 1,5-benzodiazepines by our group [37], meanwhile formation of 6-acetylpyridine-2-carboxylate as a starting substance [17] is making sense in our following research works.

5.1 6-Organyl-2-(benzoimidazolyl)pyridyl metal complexes

There are alternative approaching synthetic methods in preparing various 6-organyl-2-(benzoimidazolyl)pyridines targeting to 6-(2-benzoimidazolyl)-2-iminopyridines: the simplified ways were presented in our recent paper in detail [38]. To have a whole view of various metal complexes, both bidentate and tridentate iron and cobalt complexes were prepared and well characterized (Fig. 8) [38].

Fig. 8
6-Organyl-2-(benzoimidazolyl)pyridyl iron(II) and cobalt(II) complexes.

Those iron and cobalt complexes were generally investigated for ethylene reactivity with the assistance of various organoaluminium co-catalysts. In general, the tridentate complexes showed relatively higher activity than the bidentate complexes. For the bidentate complexes, considerable catalytic activities of iron complexes were obtained with co-catalysts such as MAO, MMAO and Et₂AlCl; but the catalytic system with MAO showed relatively lower activities. However, their bidentate cobalt analogues showed very low activities toward ethylene reactivity with co-catalysts of MAO and MMAO, but good activities with Et₂AlCl.

In the presence of MAO, the tridentate iron complexes performed good to high catalytic activities, up to 1.43 × 10⁷ g mol⁻¹ h⁻¹ in producing α-olefins (C₄–C₂₈) with very high selectivity (>99%). In addition to high selectivity for α-olefins, some waxes were also obtained in some iron catalytic systems. The steric effect of ligands on the activities was easily observed when they were activated by MAO. Changing the R group on the ortho-positions of the N-aryl group, their oligomerization activity varied in the orders of Me < Et < i-Pr and Cl < Br, and the bulky group protected the metal center and consequently resulted in higher activity. The catalysts with electron-withdrawing group showed lower activity than that of catalysts with electron-donating group. The distribution of oligomers produced resembled Schulz–Flory rules. In addition, different ethylene pressure exhibited strong influence on ethylene reactivity. At ambient ethylene pressure, only C₄ and C₆ were produced. The higher pressure increased the chain propagation, leading to longer chain oligomers up to the range of C₄–C₂₈ along with the improved activity. For their tridentate cobalt analogues, however, moderate catalytic activity was observed for ethylene oligomerization, with good selectivity for α-olefins.

The reported paper contained parts of our results [38]; there are more works under progress, including further investigation for optimum catalytic conditions and derivatives along with other metal analogues [39].

5.2 2-Quinoxalinyl-6-iminopyridyl iron and cobalt pre-catalysts

This work was carried out in order to find more applications of new the organic synthon, 6-acetylpyridine-2-carboxylate [17], which is commercially available in China. The suitable methodology was developed for the preparation of 6-(2-quinoxalinyl)-2-acetylpyridine, and numerous 2-quinoxalinyl-6-iminopyridines were easily synthesized by the condensation reaction with anilines and further coordinated with iron and cobalt dichlorides to provide pre-catalysts for ethylene oligomerization (Fig. 9) [40].

Fig. 9
Iron and cobalt complexes ligated by 2-quinoxalinyl-6-iminopyridines.

Initial scanning of Et₂AlCl, MMAO and MAO as co-catalysts of the iron complexes for ethylene reactivity showed that the iron catalytic system containing MAO gave the highest activities for ethylene oligomerization, and the butenes were observed as the major products. At 10 atm of ethylene pressure, both activity and contents of longer-chain oligomers increased. In some cases, the polyethylene waxes were collected to prove good polymerization activity; one example showed 2.24 × 10⁶ g (mol Fe)⁻¹ h⁻¹ for oligomerization and 8.56 × 10⁵ g (mol Fe)⁻¹ h⁻¹ for polymerization. Different substituents on the imino-N-aryl ring exhibited some influences on their catalytic performance. The bulky substituents at the ortho-positions of the imino-N-aryl ring resulted in lower catalytic activities and the complexes with electron-withdrawing groups showed similar catalytic reactivity as their analogues with electron-donating groups. In the presence of MMAO, all cobalt(II) analogues displayed considerable catalytic activities for ethylene oligomerization, with relatively lower activity than the iron(II) complexes. In addition, their nickel analogues showed good activity for ethylene oligomerization [41]. There are more detailed investigations under progress.

In a general consideration of 6-organyl-2-(imino)pyridyl metal catalysts for olefin oligomerization and polymerization, the literature works are still limited and extensive studies will be expected for valuable results.

5.3 Comparison of 2-benzoimidazolyl- and 2-quinoxalinyl-6-iminopyridyl metal complexes

These two kinds of iron and cobalt analogues showed different catalytic behaviors toward ethylene oligomerization and polymerization. Comparing their molecular structure typically shown in Fig. 10, the coordination geometry around the metal center was a distorted trigonal-bipyramidal with the pyridyl nitrogen atom and the two chlorides forming an equatorial plane. The nitrogen atom of the imino group and the other nitrogen atom of the benzimidazolyl or quinoxalinyl group form the axial plane. However, an obvious difference about bond length was determined; in benzimidazole complexes, the bond length of Fe–N_{benzimidazole} is similar to that of Fe–N_pyridine, but the bond length of Fe–N_quinoxaline is much longer than that of Fe–N_pyridine in quinoxaline complexes, which can be explained by the high electron density on the coordinated N_{benzimidazole} atom, and Fe–N_{benzimidazole} is more stable than Fe–N_quinoxaline. The same phenomenon was also found in their cobalt analogues.

Fig. 10
Molecular structures of 2-benzoimidazolyl- and 2-quinoxalinyl-6-iminopyridyl iron complexes with thermal ellipsoids at the 30% probability level. Hydrogen atoms have been omitted for clarity.

6 Bimetallic complex catalyst

The synergic effect of multinuclear complexes or cluster was often observed in catalysis of hydroformylation. Previously we obtained dinickel complexes for ethylene oligomerization and polymerization [42]. Therefore, we have kept an eye on developing bimetallic complexes as catalyst models for ethylene reactivity. The practical way is to bridge two catalyst models in one molecule. The most popular model is the bis(imino)pyridyl metal (iron or cobalt) catalysts [2,3], meanwhile the close model could be monoiminopyridyl metal complexes [17]. The fused molecule, 3,3-dihydro-2-methyl-2,4-bis(6-acetylpyridin-2-yl)-1H-1,5-benzodiazepine, which could link the two models together, was obtained by the reaction of o-phenylenediamine with diacetylpyridine. A further step, reaction with anilines, provided 2-methyl-2,4-bis(6-iminopyridin-2-yl)-1H-1,5-benzodiazepines for preparing bimetallic complexes [43] (Fig. 11).

The iron and cobalt complexes were studied for their catalytic activities with various alkylaluminums as co-catalysts. The catalytic system with modified methylaluminoxane (MMAO) showed high catalytic activities for ethylene oligomerization and polymerization with very high selectivity for α-olefin. The low-molecular-weight polyethylene wax was also produced with a large percentage (up to 75.0%). The ligands greatly affected the catalytic behavior of their complexes. Under the same reaction conditions, noticeably reduced catalytic activities were observed for the sterically bulkier catalyst systems. The catalytic activity, distribution of oligomers and percentage of polyethylene wax were also greatly affected by the reaction parameters such as Al/M molar ratio and reaction temperature. In general, the iron(II) complexes showed relatively higher activity than the cobalt(II) analogues. Moreover, their nickel analogues also performed good catalytic activities toward ethylene oligomerization [44]. Therefore, the bimetallic complexes could be new targeting catalysts, and the positive results encourage us for further exploration.

7 Conclusions and prospects

The N^N^N tridentate iron(II) and cobalt(II) complexes are promising catalysts for ethylene oligomerization due to their high catalytic activity and high α-olefin selectivity, meanwhile their polymerization provided unique materials with long-chain vinyl-type linear polyethylenes. In addition to bis(imino)pyridyl metal (iron or cobalt) catalysts, there are more alternative catalysts models performing high activity and selectivity for α-olefins. Our current results will be in small portion of catalysts' models in long-term consideration. Those works are closely related to industrial application, and further investigations will result in new conceptual conclusion of structural and homogeneous theory. Surely the development of new catalytic models will rely on the master works of organic synthesis, coordination, structural and physical chemistry.

The ethylene reactivity, oligomerization and polymerization, is core with fast and multi-steps of carbon–carbon coupling reactions. The high selectivity of linear α-olefins makes way for such catalysts in industrial practice with ethylene oligomerization. The unique properties of long-chain linear polyethylenes (waxes) will be argued for new conceptual polymers, as, for example, further oligomerization provides long branched polymers or copolymerization with other olefins for high molecular-weight polymers. Those polymers with high molecular weights will be difficult to crystallize, and will be potentially rubber materials. Beyond the current consideration, iron is one of the less toxic atoms, the extension of researches using those iron complexes could be considered for organic synthesis, likely as metallocene chemistry, with great influence on organic methodology and synthesis. The cobalt catalysts are found to be oxidation catalysts; the arguable question will be whether to use cobalt catalysts with multifunctional applications in catalysis.

The less investigated iron and cobalt complexes as catalysts will give more chances to scientists. There are more flexible complexes and structures to be found, more investigations will make those complexes useful and reliable for industrial processes.

Acknowledgements

This work was supported by NSFC Nos. 20473099 and 20674089, and MOST 2006AA03Z553. We are indebted to our group members for their contributions in developing this chemistry.

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Cité par

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Yongfeng Huang; Randi Zhang; Tongling Liang; Xinquan Hu; Gregory A. Solan; Wen-Hua Sun Selectivity Effects on N,N,N′-Cobalt Catalyzed Ethylene Dimerization/Trimerization Dictated through Choice of Aluminoxane Cocatalyst, Organometallics, Volume 38 (2019) no. 5, p. 1143 | DOI:10.1021/acs.organomet.8b00924
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Mohammad Ghashghaee Heterogeneous catalysts for gas-phase conversion of ethylene to higher olefins, Reviews in Chemical Engineering, Volume 34 (2018) no. 5, p. 595 | DOI:10.1515/revce-2017-0003
Chuanbing Huang; Shizhen Du; Gregory A. Solan; Yang Sun; Wen-Hua Sun From polyethylene waxes to HDPE using an α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chloride pre-catalyst in ethylene polymerisation, Dalton Transactions, Volume 46 (2017) no. 21, p. 6948 | DOI:10.1039/c7dt01077j
Marzieh Haghverdi; Azadeh Tadjarodi; Naeimeh Bahri-laleh; Mahdi Nekoomanesh Haghighi Cobalt complexes based on 2-(1H-benzimidazol-2-yl)-phenol derivatives: preparation, spectral studies, DFT calculations and catalytic behavior toward ethylene oligomerization, Journal of Coordination Chemistry, Volume 70 (2017) no. 11, p. 1800 | DOI:10.1080/00958972.2017.1328506
Yanjun Chen; Shizhen Du; Chuanbing Huang; Gregory A. Solan; Xiang Hao; Wen‐Hua Sun Balancing high thermal stability with high activity in diaryliminoacenaphthene‐nickel(II) catalysts for ethylene polymerization, Journal of Polymer Science Part A: Polymer Chemistry, Volume 55 (2017) no. 12, p. 1971 | DOI:10.1002/pola.28562
Youfu Zhang; Hongyi Suo; Fang Huang; Tongling Liang; Xinquan Hu; Wen‐Hua Sun Thermo‐stable 2‐(arylimino)benzylidene‐9‐arylimino‐5,6,7,8‐tetrahydro cyclohepta[b]pyridyliron(II) precatalysts toward ethylene polymerization and highly linear polyethylenes, Journal of Polymer Science Part A: Polymer Chemistry, Volume 55 (2017) no. 5, p. 830 | DOI:10.1002/pola.28433
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Shizhen Du; Wenjuan Zhang; Erlin Yue; Fang Huang; Tongling Liang; Wen‐Hua Sun α,α′‐Bis(arylimino)‐2,3:5,6‐bis(pentamethylene)pyridylcobalt Chlorides: Synthesis, Characterization, and Ethylene Polymerization Behavior, European Journal of Inorganic Chemistry, Volume 2016 (2016) no. 11, p. 1748 | DOI:10.1002/ejic.201600098
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Shizhen Du; Xinxin Wang; Wenjuan Zhang; Zygmunt Flisak; Yang Sun; Wen-Hua Sun A practical ethylene polymerization for vinyl-polyethylenes: synthesis, characterization and catalytic behavior of α,α′-bisimino-2,3:5,6-bis(pentamethylene)pyridyliron chlorides, Polymer Chemistry, Volume 7 (2016) no. 25, p. 4188 | DOI:10.1039/c6py00745g
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Wenhong Yang; Yan Chen; Wen‐Hua Sun Correlating Cobalt Net Charges with Catalytic Activities of the 2‐(Benzimidazolyl)‐6‐(1‐aryliminoethyl)pyridylcobalt Complexes toward Ethylene Polymerization, Macromolecular Reaction Engineering, Volume 9 (2015) no. 5, p. 473 | DOI:10.1002/mren.201400064
Shizhen Du; Shaoliang Kong; Qisong Shi; Jing Mao; Cunyue Guo; Jianjun Yi; Tongling Liang; Wen-Hua Sun Enhancing the Activity and Thermal Stability of Nickel Complex Precatalysts Using 1-[2,6-Bis(bis(4-fluorophenyl)methyl)-4-methyl phenylimino]-2-aryliminoacenaphthylene Derivatives, Organometallics, Volume 34 (2015) no. 3, p. 582 | DOI:10.1021/om500943u
Annie Finiels; François Fajula; Vasile Hulea Nickel-based solid catalysts for ethylene oligomerization – a review, Catal. Sci. Technol., Volume 4 (2014) no. 8, p. 2412 | DOI:10.1039/c4cy00305e
Adrien Boudier; Pierre-Alain R. Breuil; Lionel Magna; Hélène Olivier-Bourbigou; Pierre Braunstein Ethylene oligomerization using iron complexes: beyond the discovery of bis(imino)pyridine ligands, Chemical Communications, Volume 50 (2014) no. 12, p. 1398 | DOI:10.1039/c3cc47834c
Fang Huang; Qifeng Xing; Tongling Liang; Zygmunt Flisak; Bin Ye; Xinquan Hu; Wenhong Yang; Wen-Hua Sun 2-(1-Aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridyl iron(ii) dichloride: synthesis, characterization, and the highly active and tunable active species in ethylene polymerization, Dalton Trans., Volume 43 (2014) no. 44, p. 16818 | DOI:10.1039/c4dt02102a
Jing Ma; Chun Feng; Shaoli Wang; Ke-Qing Zhao; Wen-Hua Sun; Carl Redshaw; Gregory A. Solan Bi- and tri-dentate imino-based iron and cobalt pre-catalysts for ethylene oligo-/polymerization, Inorg. Chem. Front., Volume 1 (2014) no. 1, p. 14 | DOI:10.1039/c3qi00028a
Wenjuan Zhang; Shaoli Wang; Shizhen Du; Cun‐Yue Guo; Xiang Hao; Wen‐Hua Sun 2‐(1‐(2,4‐Bis((di(4‐fluorophenyl)methyl)‐6‐methylphenylimino)ethyl)‐6‐(1‐(arylimino)ethyl)pyridylmetal (iron or cobalt) Complexes: Synthesis, Characterization, and Ethylene Polymerization Behavior, Macromolecular Chemistry and Physics, Volume 215 (2014) no. 18, p. 1797 | DOI:10.1002/macp.201400140
Rong Gao; Wen-Hua Sun; Carl Redshaw Nickel complex pre-catalysts in ethylene polymerization: new approaches to elastomeric materials, Catalysis Science Technology, Volume 3 (2013) no. 5, p. 1172 | DOI:10.1039/c3cy20691b
Wenjuan Zhang; Wen-Hua Sun; Carl Redshaw Tailoring iron complexes for ethylene oligomerization and/or polymerization, Dalton Trans., Volume 42 (2013) no. 25, p. 8988 | DOI:10.1039/c2dt32337k
Shaoli Wang; Baixiang Li; Tongling Liang; Carl Redshaw; Yuesheng Li; Wen-Hua Sun Synthesis, characterization and catalytic behavior toward ethylene of 2-[1-(4,6-dimethyl-2-benzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylmetal (iron or cobalt) chlorides, Dalton Transactions, Volume 42 (2013) no. 25, p. 9188 | DOI:10.1039/c3dt00011g
Shaoli Wang; Weizhen Zhao; Xiang Hao; Baixiang Li; Carl Redshaw; Yuesheng Li; Wen-Hua Sun 2-(1-2,6-Bis[bis(4-fluorophenyl)methyl]-4-methylphenyliminoethyl)-6-[1-(arylimino)ethyl]pyridylcobalt dichlorides: Synthesis, characterization and ethylene polymerization behavior, Journal of Organometallic Chemistry, Volume 731 (2013), p. 78 | DOI:10.1016/j.jorganchem.2013.02.016
Wen-Hua Sun Novel Polyethylenes via Late Transition Metal Complex Pre-catalysts, Polyolefins: 50 years after Ziegler and Natta II, Volume 258 (2013), p. 163 | DOI:10.1007/12_2013_212
David Schweinfurth; Cheng-Yong Su; Shi-Chao Wei; Pierre Braunstein; Biprajit Sarkar Nickel complexes with “click”-derived pyridyl-triazole ligands: weak intermolecular interactions and catalytic ethylene oligomerisation, Dalton Transactions, Volume 41 (2012) no. 41, p. 12984 | DOI:10.1039/c2dt31805a
Fan He; Weizhen Zhao; Xiao-Ping Cao; Tongling Liang; Carl Redshaw; Wen-Hua Sun 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-aryliminoethyl]pyridyl cobalt dichlorides: Synthesis, characterization and ethylene polymerization behavior, Journal of Organometallic Chemistry, Volume 713 (2012), p. 209 | DOI:10.1016/j.jorganchem.2012.05.020
Wen‐Hua Sun; Weizhen Zhao; Jiangang Yu; Wenjuan Zhang; Xiang Hao; Carl Redshaw Enhancing the Activity and Thermal Stability of Iron Precatalysts Using 2‐(1‐2,6‐bis[bis(4‐fluorophenyl)methyl]‐4‐methylphenyliminoethyl)‐6‐[1‐(arylimino)ethyl]pyridines, Macromolecular Chemistry and Physics, Volume 213 (2012) no. 12, p. 1266 | DOI:10.1002/macp.201200051
Wenjuan Zhang; Wenbin Chai; Wen-Hua Sun; Xinquan Hu; Carl Redshaw; Xiang Hao 2-(1-(Arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinoline Iron(II) Chloride Complexes: Synthesis, Characterization, and Ethylene Polymerization Behavior, Organometallics, Volume 31 (2012) no. 14, p. 5039 | DOI:10.1021/om300388m
Weizhen Zhao; Jiangang Yu; Shengju Song; Wenhong Yang; Hao Liu; Xiang Hao; Carl Redshaw; Wen-Hua Sun Controlling the ethylene polymerization parameters in iron pre-catalysts of the type 2-[1-(2,4-dibenzhydryl-6-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl] pyridyliron dichloride, Polymer, Volume 53 (2012) no. 1, p. 130 | DOI:10.1016/j.polymer.2011.11.024
Jingjuan Lai; Weizhen Zhao; Wenhong Yang; Carl Redshaw; Tongling Liang; Yongwen Liu; Wen-Hua Sun 2-[1-(2,4-Dibenzhydryl-6-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylcobalt(ii) dichlorides: Synthesis, characterization and ethylene polymerization behavior, Polymer Chemistry, Volume 3 (2012) no. 3, p. 787 | DOI:10.1039/c2py00590e
Chen Gang Cao; In Yong Ahn; Tao Jiang Synthesis of Asymmetric Tridentate Cobalt(II) Complex and Application in Ethylene Oligomerization, Advanced Materials Research, Volume 233-235 (2011), p. 1540 | DOI:10.4028/www.scientific.net/amr.233-235.1540
Tianpengfei Xiao; Jingjuan Lai; Shu Zhang; Xiang Hao; Wen-Hua Sun 2-(1-Aryliminopropylidene)quinolylcobalt(ii) dichlorides: synthesis, characterization and catalytic behaviour towards ethylene, Catalysis Science Technology, Volume 1 (2011) no. 3, p. 462 | DOI:10.1039/c1cy00028d
Jiangang Yu; Yanning Zeng; Wei Huang; Xiang Hao; Wen-Hua Sun N-(5,6,7-Trihydroquinolin-8-ylidene)arylaminonickel dichlorides as highly active single-site pro-catalysts in ethylene polymerization, Dalton Transactions, Volume 40 (2011) no. 33, p. 8436 | DOI:10.1039/c1dt10541h
Jiangang Yu; Wei Huang; Lin Wang; Carl Redshaw; Wen-Hua Sun 2-[1-(2,6-Dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylcobalt(ii) dichlorides: Synthesis, characterization and ethylene polymerization behavior, Dalton Transactions, Volume 40 (2011) no. 39, p. 10209 | DOI:10.1039/c1dt11062d
Xia Chen; Liping Zhang; Jiangang Yu; Xiang Hao; Hao Liu; Wen-Hua Sun Synthesis, characterization and ethylene oligomerization behavior of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridylnickel dihalides, Inorganica Chimica Acta, Volume 370 (2011) no. 1, p. 156 | DOI:10.1016/j.ica.2011.01.054
Shengju Song; Rong Gao; Min Zhang; Yan Li; Fosong Wang; Wen-Hua Sun 2-β-Benzothiazolyl-6-iminopyridylmetal dichlorides and the catalytic behavior towards ethylene oligomerization and polymerization, Inorganica Chimica Acta, Volume 376 (2011) no. 1, p. 373 | DOI:10.1016/j.ica.2011.06.037
Liping Zhang; Xiaohua Hou; Jiangang Yu; Xia Chen; Xiang Hao; Wen-Hua Sun 2-(R-1H-Benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridyliron(II) dichlorides: Synthesis, characterization and the ethylene oligomerization behavior, Inorganica Chimica Acta, Volume 379 (2011) no. 1, p. 70 | DOI:10.1016/j.ica.2011.09.035
Shengju Song; Tianpengfei Xiao; Carl Redshaw; Xiang Hao; Fosong Wang; Wen-Hua Sun Iron(II) and cobalt(II) complexes bearing 8-(1-aryliminoethylidene) quinaldines: Synthesis, characterization and ethylene dimerization behavior, Journal of Organometallic Chemistry, Volume 696 (2011) no. 13, p. 2594 | DOI:10.1016/j.jorganchem.2011.03.039
Shengju Song; Weizhen Zhao; Lin Wang; Carl Redshaw; Fosong Wang; Wen-Hua Sun Synthesis, characterization and catalytic behavior toward ethylene of cobalt(II) and iron(II) complexes bearing 2-(1-aryliminoethylidene)quinolines, Journal of Organometallic Chemistry, Volume 696 (2011) no. 18, p. 3029 | DOI:10.1016/j.jorganchem.2011.06.003
Deligeer Wang; Shaofeng Liu; Yanning Zeng; Wen-Hua Sun; Carl Redshaw 2-Benzimidazolyl-N-phenylquinoline-8-carboxamide Chromium(III) Trichlorides: Synthesis and Application for Ethylene Oligomerization and Polymerization, Organometallics, Volume 30 (2011) no. 11, p. 3001 | DOI:10.1021/om2001124
Tianpengfei Xiao; Shu Zhang; Gerald Kehr; Xiang Hao; Gerhard Erker; Wen-Hua Sun Bidentate Iron(II) Dichloride Complexes Bearing Substituted 8-(Benzimidazol-2-yl)quinolines: Synthesis, Characterization, and Ethylene Polymerization Behavior, Organometallics, Volume 30 (2011) no. 13, p. 3658 | DOI:10.1021/om200338b
Tianpengfei Xiao; Peng Hao; Gerald Kehr; Xiang Hao; Gerhard Erker; Wen-Hua Sun Dichlorocobalt(II) Complexes Ligated by Bidentate 8-(Benzoimidazol-2-yl)quinolines: Synthesis, Characterization, and Catalytic Behavior toward Ethylene, Organometallics, Volume 30 (2011) no. 18, p. 4847 | DOI:10.1021/om2003392
Vinu Krishnan Appukuttan; Yinshan Liu; Bong Chul Son; Chang-Sik Ha; Hongsuk Suh; Il Kim Iron and Cobalt Complexes of 2,3,7,8-Tetrahydroacridine-4,5(1H,6H)-diimine Sterically Modulated by Substituted Aryl Rings for the Selective Oligomerization to Polymerization of Ethylene, Organometallics, Volume 30 (2011) no. 8, p. 2285 | DOI:10.1021/om2000629
Tianpengfei Xiao; Shu Zhang; Baixiang Li; Xiang Hao; Carl Redshaw; Yue-Sheng Li; Wen-Hua Sun Ferrous and cobaltous chloride complexes bearing 2-(1-(arylimino)methyl)-8-(1H-benzimidazol-2-yl)quinolines: Synthesis, characterization and catalytic behavior in ethylene polymerization, Polymer, Volume 52 (2011) no. 25, p. 5803 | DOI:10.1016/j.polymer.2011.10.037
Liwei Xiao; Min Zhang; Rong Gao; Xiaoping Cao; Wen-Hua Sun 2-(1H-2-Benzimidazolyl)-6-(1-(arylimino)ethyl)pyridylnickel Complexes: Synthesis, Characterization, and Ethylene Oligomerization, Australian Journal of Chemistry, Volume 63 (2010) no. 1, p. 109 | DOI:10.1071/ch09168
Vernon C. Gibson; Gregory A. Solan Olefin Oligomerizations and Polymerizations Catalyzed by Iron and Cobalt Complexes Bearing Bis(imino)pyridine Ligands, Catalysis without Precious Metals (2010), p. 111 | DOI:10.1002/9783527631582.ch5
Su-yun Jie; Wen-hua Sun; Tianpengfei Xiao Prospects and crucial problems in oligomerization and polymerization with iron and cobalt complex catalysts, Chinese Journal of Polymer Science, Volume 28 (2010) no. 3, p. 299 | DOI:10.1007/s10118-010-0034-6
Peng Hao; Yanjun Chen; Tianpengfei Xiao; Wen-Hua Sun Iron(III) complexes bearing 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines: Synthesis, characterization and their catalytic behaviors towards ethylene oligomerization and polymerization, Journal of Organometallic Chemistry, Volume 695 (2010) no. 1, p. 90 | DOI:10.1016/j.jorganchem.2009.09.032
Shaofeng Liu; Wen-Hua Sun; Yanning Zeng; Deligeer Wang; Wenjuan Zhang; Yan Li Syntheses, Characterization, and Ethylene (Co-)Polymerization Screening of Amidate Half-Titanocene Dichlorides, Organometallics, Volume 29 (2010) no. 11, p. 2459 | DOI:10.1021/om1000748
Kefeng Wang; Miao Shen; Wen-Hua Sun Synthesis, characterization and ethylene oligomerization of nickel complexes bearing N-(2-(1H-benzo[d]imidazol-2-yl)quinolin-8-yl)benzamide derivatives, Dalton Transactions (2009) no. 21, p. 4085 | DOI:10.1039/b822441b
Qudsia Khamker; Yohan D. M. Champouret; Kuldip Singh; Gregory A. Solan Bis(imino)quaterpyridine-bearing multimetallic late transition metal complexes as ethylene oligomerisation catalysts, Dalton Transactions (2009) no. 41, p. 8935 | DOI:10.1039/b910181k
Rong Gao; Yan Li; Fosong Wang; Wen‐Hua Sun; Manfred Bochmann 2‐Benzoxazolyl‐6‐[1‐(arylimino)ethyl]pyridyliron(II) Chlorides as Ethylene Oligomerization Catalysts, European Journal of Inorganic Chemistry, Volume 2009 (2009) no. 27, p. 4149 | DOI:10.1002/ejic.200900491
Rong Gao; Kefeng Wang; Yan Li; Fosong Wang; Wen-Hua Sun; Carl Redshaw; Manfred Bochmann 2-Benzoxazolyl-6-(1-(arylimino)ethyl)pyridyl cobalt (II) chlorides: A temperature switch catalyst in oligomerization and polymerization of ethylene, Journal of Molecular Catalysis A: Chemical, Volume 309 (2009) no. 1-2, p. 166 | DOI:10.1016/j.molcata.2009.05.021
Shaofeng Liu; Jianjun Yi; Weiwei Zuo; Kefeng Wang; Deligeer Wang; Wen‐Hua Sun N‐(2‐benzimidazolylquinolin‐8‐yl)benzamidate half‐titanocene chlorides: Synthesis, characterization and their catalytic behavior toward ethylene polymerization, Journal of Polymer Science Part A: Polymer Chemistry, Volume 47 (2009) no. 12, p. 3154 | DOI:10.1002/pola.23413
Igor E. Soshnikov; Nina V. Semikolenova; Alexey N. Bushmelev; Konstantin P. Bryliakov; Oleg Y. Lyakin; Carl Redshaw; Vladimir A. Zakharov; Evgenii P. Talsi Investigating the Nature of the Active Species in Bis(imino)pyridine Cobalt Ethylene Polymerization Catalysts, Organometallics, Volume 28 (2009) no. 20, p. 6003 | DOI:10.1021/om900490b
Liwei Xiao; Rong Gao; Min Zhang; Yan Li; Xiaoping Cao; Wen-Hua Sun 2-(1H-2-Benzimidazolyl)-6-(1-(arylimino)ethyl)pyridyl Iron(II) and Cobalt(II) Dichlorides: Syntheses, Characterizations, and Catalytic Behaviors toward Ethylene Reactivity, Organometallics, Volume 28 (2009) no. 7, p. 2225 | DOI:10.1021/om801141n
Rong Gao; Liwei Xiao; Xiang Hao; Wen-Hua Sun; Fosong Wang Synthesis of benzoxazolylpyridine nickel complexes and their efficient dimerization of ethylene to α-butene, Dalton Transactions (2008) no. 41, p. 5645 | DOI:10.1039/b807604a
Andrew P. Armitage; Yohan D. M. Champouret; Hubert Grigoli; Jérémie D. A. Pelletier; Kuldip Singh; Gregory A. Solan Probing the Effect of Binding Site and Metal Centre Variation in Pentadentate Oligopyridylimine‐Bearing Bimetallic (Fe2, Co2, Ni2) Ethylene Oligomerisation Catalysts, European Journal of Inorganic Chemistry, Volume 2008 (2008) no. 29, p. 4597 | DOI:10.1002/ejic.200800650
Yanjun Chen; Peng Hao; Weiwei Zuo; Kun Gao; Wen-Hua Sun 2-(1-Isopropyl-2-benzimidazolyl)-6-(1-aryliminoethyl)pyridyl transition metal (Fe, Co, and Ni) dichlorides: Syntheses, characterizations and their catalytic behaviors toward ethylene reactivity, Journal of Organometallic Chemistry, Volume 693 (2008) no. 10, p. 1829 | DOI:10.1016/j.jorganchem.2008.02.007
Min Zhang; Rong Gao; Xiang Hao; Wen-Hua Sun 2-Oxazoline/benzoxazole-1,10-phenanthrolinylmetal (iron, cobalt or nickel) dichloride: Synthesis, characterization and their catalytic reactivity for the ethylene oligomerization, Journal of Organometallic Chemistry, Volume 693 (2008) no. 26, p. 3867 | DOI:10.1016/j.jorganchem.2008.09.046
Rong Gao; Min Zhang; Tongling Liang; Fosong Wang; Wen-Hua Sun Nickel(II) Complexes Chelated by 2-Arylimino-6-benzoxazolylpyridine: Syntheses, Characterization, and Ethylene Oligomerization, Organometallics, Volume 27 (2008) no. 21, p. 5641 | DOI:10.1021/om800647w

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