Ethylene polymerization by PN3-type pincer chromium(III) complexes

https://doi.org/10.1016/j.molcata.2014.08.005Get rights and content

Highlights

  • •

    PN3-type pincer type of chromium (III) complexes was prepared.

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    Activated by MAO, the complexes showed moderate activity for ethylene polymerization.

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    Exclusively linear solid PE without any detectable amount of oligomers was obtained.

Abstract

Chromium (III) complexes, Cr1, [2,6-(tBu2PNH)2C5H4N]CrCl3; Cr2, [2,6-(Ph2PNH)2C5H4N]CrCl3; Cr3, [2-(tBu2PNH)C5H4N]CrCl3 THF; Cr4, [6-(tBu2PNH)C5H4N-2-CH2NEt2]CrCl3; Cr5, [6-(tBu2PNH)C5H4N-2-C3H2N2]CrCl3; Cr6, [6-(tBu2PNH)C5H4N-2-(3,5-Me2)C3H2N2]CrCl3; Cr7, [6-(tBu2PNH)C5H4N-2-(3,5-iPr2)C3H2N2]CrCl3; Cr8, [6-(tBu2PNH)C5H4N-2-(3,5-Ph2)C3H2N2]CrCl3, bearing a family of neutral PN3-type pincer ligands have been prepared. The molecular structure of Cr2 was further elucidated by the X-ray crystallographic analysis, showing an octahedral geometry. Treatment of these complexes with MAO or alkylaluminum led to catalysts with moderate activities (about 105 g (PE)/Cr(mol) h) for ethylene polymerization, affording exclusively linear low molecular weight solid PE without any detectable oligomers. Among Cr1–Cr8, the highest activity was achieved for Cr1/MAO at room temperature with production of PE with highest molecular weight, indicating that replacement of both tBu groups in Cr1 with Ph groups, or one PtBu2 with the N (imine) arm, resulted in a lower catalytic activity and lower Mw.

Introduction

Polyethylene is one of the most important synthetic materials and has been playing an irreplaceable role in our modern society. The increasing market capacity for high-performance polyethylene in recent years has promoted both industrial and academic research to explore efficient ethylene polymerization catalysts. In this pursuit, chromium catalysts play central roles [1], and among them, the heterogeneous catalysts consisting of Cr/SiO2 have been utilized in the main process for commercial production of high-density polyethylene (HDPE). The real profile of the active center with respect to the catalyst initiation, metal coordination and oxidation state of these inorganic chromium catalysts supported on silica, even till today, remains an on-going debate [2]. The development of homogeneous model of chromium catalysts may provide useful information of catalyst structures and their catalytic behaviors, thus offering potential opportunities for tuning and improving the catalytic performances and also for controlling the resultant polymer properties. During the past decades, various well-defined chromium catalysts have been reported in the literature. Some of them are based on half-sandwich chromium species as homogeneous models of the Phillips catalyst [3]. The self-activated Cp supported chromium species [Cp*CrL2R]+A− (L = C5H5N, Ph2PCH2CH2PPh2, THF, MeCN; R = Me, Et; A = PF6, BPh4; Cp* = Me5Cp) have been isolated and exhibited reasonable activity for ethylene polymerization [4]. In the presence of alkylaluminum, half-sandwish chromium(III) precursor such as cyclopentadienyl-amine [5], [6], cyclopentadienyl-phosphine [7], cyclopentadienyl-amido [8], and Cp* chromium(III) ligated with moieties, such as hydroxyindanimine, β-ketoiminato, β-diketiminate or salicylaldiminato [9], [10], have also been developed for ethylene polymerization with an activity up to 107 g mol−1 bar h [3]. Meanwhile, non-metallocene systems have drawn more attention recently, for instance, anionic non-Cp ligands such as β-diketiminate [3], [11], salicylaldiminato [12], or pyrrolide-imine [13] supported chromium catalysts have been demonstrated to be efficient catalysts for the homo- and copolymerization of ethylene and α-olefins, with compatible polymerization activities to those of half-metallocene based systems.

Pincer ligands have emerged as attractive auxiliary ligands in chromium(III)-based catalysts because the ligand backbone can be modified by varying substituents with different steric and electronic factors, and carefully changing the ligation environment can deliberately tune the catalytic behaviors of the Cr center [14], [15], [16], [17], [18], [19]. Catalysts supported by pincer ligands such as SNS [20], CNC [21], NNO [17], [22], NSN [23], and NNN [24] have been systematically investigated and it has been found that the catalytic performance was largely dependent on the coordination environment. In particular, those with bulky substitutes were beneficial for higher molecular weights, but a universal trend on how the coordinating ligands influence the product properties has not yet been established. The application of phosphorous-bridged bisphenoxy chromium (III) complexes as an efficient catalyst for highly active ethylene polymerization was demonstrated recently [25]. Another illustrative example is the bis(phosphino) amide supported chromium(III) catalyst, which shows good efficiency not only for ethylene and propylene polymerization but also for ethylene/hexene copolymerization [26]. During the course of this work, Gambarotta and co-workers [27] have reported pincer PXNXP (X = CH2, NH, NPPh2) ligand supported chromium chlorides and multi-valent chromium aluminates which show moderate activity with an order magnitude of 105 g(PE)/g(Cr), mainly producing oligomers under 40 bar of ethylene pressure at 80 or 110 °C.

We are interested in pincer ligated metal complexes [(PNX)M] (X = N, P; M = transition metal) catalyzed organic transformation [28]. When the pyridyl-supported phosphine system is adopted, the strong σ-donating phosphine donors are believed to increase the stability of cationic metal center which may facilitate the coordination interaction with incoming molecules and thus promote the activity [29]. In this contribution, we wish to present the design and synthesis of chromium complexes supported by new types of PNP and PNN ligands, which in combination of MAO or trialkylaluminum, catalyze ethylene polymerization to give sole polyethylene with moderate activity. The influence of the ligand architecture and the reaction conditions on catalytic performances and polymer properties are also discussed.

Section snippets

Synthesis and characterization of ligands and complexes

The known ligands L1–L4 were synthesized according to the literature procedures [29], [30]. L5–L8 were obtained from their corresponding compounds (L5-2)–(L8-2) by the similar procedure, and (L5-2)–(L8-2) were synthesized via the mono-substitution of 2,6-dibromopyridine with pyrazole derivatives, followed by the amination of the subsequent mono-bromide (Scheme 1) [30]. The reactions of CrCl3(THF)3 with 1.0 equiv. of the appropriate ligands in THF at room temperature under argon atmosphere

Conclusions

We have developed a series of chromium(III) trichloride complexes supported by novel PN3-type of pincer ligands (Cr1–Cr8). Upon activation with MAO or AlR3 (TIBA, TEA or TMA), the catalysts show moderate productivities for ethylene polymerization. Linear solid polyethylene of molecular weights (Mw) ranging from 12,300 to 63,200 were obtained without the formation of trace amount of oligomers, remarkably in contrast to most PNP and NNN systems. Catalyst Cr1 displayed the highest productivity

General methods

All manipulation of oxygen and/or moisture sensitive compounds was conducted under a moisture and oxygen-free argon atmosphere in a glovebox. If not specified, all reagents were purchased from commercial sources and used without further purification. PN3P ligand derivatives were synthesized according to the literature methods [29], [30]. Solvents used were treated with standard purification methods. 1H NMR (400 MHz), 13C NMR (100 MHz) and 31P NMR (162 MHz) were recorded on a Bruker AV400

Acknowledgements

We are grateful for the generous financial support from King Abdullah University of Science and Technology, the Natural Science Foundation of China (Grant Nos. 21304050 and 21302170), the Natural Science Foundation of Ningbo (Grant No. 2014A610109), K. C. Wong Magna Fund and Start-up Fund in Ningbo University.

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