Dichlorovanadium(IV) diamine-bis(phenolate) complexes for ethylene (co)polymerization and 1-olefin isospecific polymerization
Graphical abstract
Introduction
The use of complexes which contain multidentate ligands as precatalysts is one of the most important trends which are observed nowadays in the catalytic polymerization of olefins. Their structures, that can be easily modified, make essential determinants for the properties of a catalytic system, inclusive of its activity, stereospecificity and ability to incorporate comonomer molecules, and for the properties of the obtained products as well. The type of the metal center makes a factor which is not less essential for the catalytic performance of a complex. Apart from the group 4 metal complexes, inclusive of those which offer a number of interesting properties – complexes with phenoxy-imine, salan and amine-bis(phenolate) ligands with an additional donor in the side arm [1], [2], [3], which are studied most frequently, the vanadium complexes attract more and more interest in the past years. The classical vanadium catalysts which are based on simple vanadium compounds (e.g. VCl4, VOCl3) play a meaningful role in the catalytic polymerization of olefins. They are capable of producing polyethylene with a very high molecular weight and narrow molecular weight distribution, syndiotactic polypropylene, amorphous ethylene-propylene copolymer with homogeneous composition, ethylene/propylene/dien (EPDM) copolymers, or ethylene/cyclic olefin (COC) copolymers [4]. The main disadvantage of this type of vanadium catalysts is their low activity due to quick deactivation through reduction which yields a catalytically inactive vanadium(II) species [4], [5]. The introduction of a mono- or multidentate ligand to a vanadium compound help to stabilize the metal oxidation state but reduction of vanadium is still observed which can be connected with migration of that ligand to the organoaluminium activator [5]. This problem can be overcome by using a mild oxidizing agent such as Cl3CCO2Et (ETA) or chlorinated hydrocarbons which reactivate the inactive vanadium(II) centers to the active vanadium(III) species [4].
In recent years a number of vanadium(III) and -(IV) complexes of O,N-chelating ligands have been developed. These complexes bearing one or two β-enaminoketonato and salicylaldiminato ligands exhibit high catalytic activities in the presence of Et2AlCl and ETA in ethylene homopolymerization and in its copolymerization with 1-olefins and norbornene [6], [7], [8], [9], [10]. The results also showed that the presence of two O,N-ligands in the complex stabilized the active species better than one ligand and maintained the single-site catalytic behavior of the vanadium catalyst, regardless of the reaction conditions [9]. Nevertheless, the amount of research works involved in investigation of catalytic properties of vanadium complexes supported by tetradentate ligands is very limited. The vanadium(II-V) complexes bearing the ancillary amino-bis(phenolate) ligand with the additional amine donor were tested in combination with EtAlCl2, in homo- and copolymerization of ethylene [11]. They produced poly(ethylene-co-1-hexene) and poly(ethylene-co-norbornene) with the moderate comonomer contents and at moderate yields. Vanadium(IV) complexes of this type tetradentate ligand and additional azido and isopropoxo groups ([V{Me2NCH2CH2N(CH2-2-O-3,5-Me2-C6H2)2}X2], where X = OiPr, N3) were obtained in direct reaction of a metal precursor, V(NMe2)2(N3)2 or V(OiPr)4, with a ligand, and the chlorovanadium complex was obtained by replacement of isopropoxo groups by chloride atoms coming from Me3SiCl [12]. Other studies involved oxovanadium(V) complexes which contained amine pyridine bis(phenolate) [13] and amine trihydroxy ligands [14]. In the presence of Et2AlCl as an activator and ETA as a reactivating agent, these complexes displayed good thermal stability and activity in ethylene polymerization, and they efficiently promoted ethylene/NBE copolymerization. The ethylene polymerization study was also conducted with the oxovanadium(V) complex which contained the trans-1,2-dithiacyclohexane-bridged bis(phenolato) ligand [15]. However, the activity of the complex with the O,S,S,O-chelating ligand was significantly lower than those of the complexes bearing tetradentate ligands with the O and N donor atoms. This might be due to both the ligand structure itself and the type of activator used which was MAO.
Taking into account that the catalytic properties of the vanadium complexes, especially of vanadium(IV), with tetradentate ligands were not actually recognized, two dichlorovanadium(IV) complexes containing ancillary diamine bis(phenolate) ligand were synthesized and their catalytic properties in ethylene homo- and copolymerization were studied. In addition, since there are only a few vanadium postmetallocene systems known which lead to higher 1-olefin polymers, their activity and stereospecificity in 1-octene polymerization were tested and the results were compared to those obtained for the corresponding group 4 transition metal complexes.
Section snippets
Materials
All manipulations of air- and/or moisture sensitive compounds were done under an inert atmosphere of argon or nitrogen, using the vacuum/argon line, a glove box and the standard Schlenk technique. Tetrahydrofuran, hexane and toluene were refluxed over and distilled from sodium/benzophenone and sodium, respectively. 1-Octene (98%, Aldrich), and C6D6 were dried under argon over 4A molecular sieves. Ethylene (3.0 grade, Linde Gas) and nitrogen (Messer) were used after having been passed through a
Synthesis of vanadium complexes
Dichlorovanadium(IV) complexes bearing diamine-bis(phenolate) ligands with the dimethylamino donors on the side-arm and two tBu groups on both phenolate rings (1) or with only one aromatic ring substituted by tBu groups (2) were synthesized by reacting a disodium salt of the ligand with one equivalent of VCl4 in dry THF at room temperature (Scheme 1). Both complexes were obtained at high yields as dark green solids. Comparison of FTIR spectra of the complexes with those of free ligands
Conclusion
Two new dichlorovanadium complexes stabilized by dianionic diamine bis(phenolate) ligands were synthesized by the reaction of deprotonated ligand with vanadium tetrachloride.
They belong to the relatively small group of vanadium complexes in the +4 oxidation state. The complexes were characterized by FTIR and 1H NMR spectroscopy and evaluated as catalyst precursors in homo- and copolymerization of olefins. The catalytic system 1/EtAlCl2 with no reactivation agent produced polyethylene with
Acknowledgement
This work was supported by the National Science Center of Poland [grant number N N209 140840].
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Vanadium
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2020, Coordination Chemistry ReviewsCitation Excerpt :A 100-fold reduction in catalytic activity was obtained with the MAO system (e.g. the activity of 39a/MAO was 2.92 × 105 g mol−1 h−1). Two vanadyl amine-bis(phenolate) complexes bearing a side-arm of amino donor, [{Me2NCH2CH2N(CH2-2-O-3,5-tBu2-C6H2)2}VCl2] (40a) and [{Me2NCH2CH2N(CH2-2-O-3,5-tBu2-C6H2)(CH2-2-O-C6H4)}VCl2] (40b) (Fig. 21) were investigated for ethylene polymerization [66]. In general, 40b was less active than 40a; it turned out that the activities of both catalysts with Al(iBu)3/Ph3CB(C6F5)4 and with MAO are significantly lower than when used with EtAlCl2.
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2020, Molecular CatalysisCitation Excerpt :Mechanism of Chain Propagation. Similar to most of the transition-metal catalyzed ethylene polymerization, chain propagation is believed to follow Cossee-Arlman mechanism in which ethylene molecule coordinates to the vanadium center and then inserts into the VC bond consecutively (Fig. 7) [49,50]. Chain propagation is primarily determined by the kinetic factors including the electronic effects and the steric effects as introduced by choosing different terminal groups in the model catalysts.