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Synthesis and Structure of Titanatranes Containing Tetradentate Trianionic Donor Ligands of the Type [(O-2,4-R2C6H2-6-CH2)2(OCH2CH2)]N3- and Their Use in Catalysis for Ethylene Polymerization

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posted on 2020-04-03, 06:31 authored by Sudhakar Padmanabhan, Shouhei Katao, Kotohiro Nomura
Syntheses and structural characterizations of various titanatranes containing bis(aryloxo)ethanolamine ligands of the type [(O-2,4-R2C6H2-6-CH2)2(OCH2CH2)]N3- (R = Me, tBu) have been explored. The reaction of [(HO-2,4-Me2C6H2-6-CH2)2(HOCH2CH2)]N with Ti(OR‘)4 in toluene afforded dimeric titanatranes, Ti2(OR‘)2{[(O-2,4-Me2C6H2-6-CH2)22-OCH2CH2)]N}2 [R‘ = iPr (1a), tBu (1b)], in high yields. Crystallographic analyses of 1a,b indicate that these complexes have a distorted octahedral geometry around Ti, and the O atom in the alkoxo ligand is coordinated to two Ti atoms. The similar reactions of [(HO-2,4-tBu2C6H2-6-CH2)2(HOCH2CH2)]N with Ti(OR‘)4 afforded the monomeric titanatranes Ti(OR‘)[(O-2,4-tBu2C6H2-6-CH2)2(OCH2CH2)]N (2a,b) in high yields; these complexes have a rather distorted trigonal bipyramidal structure around Ti consisting of a plane formed by two aryloxo and one alkoxo ligand and an N−Ti−O (in OtBu) axis determined by the X-ray crystallographic analyses. The reaction of 1a,b with 2.0 equiv of AlMe3 (1.0 equiv per Ti) in toluene gave the Ti−Me complexes coordinated to Me2Al(OR‘), {TiMe[(O-2,4-Me2C6H2-6-CH2)22-OCH2CH2)N]}[Me2Al(μ2-OR‘)] [R‘ = iPr (3a), tBu (3b)], in exclusive yields, and these complexes were identified by 1H and 13C NMR spectra, elemental analyses, and X-ray crystallography. Complexes 1a and 2b exhibited moderate catalytic activities for ethylene polymerization at 100−120 °C in the presence of methylaluminoxane (MAO), and the activity increased upon the addition of a small amount of AlMe3. Similar catalytic activities were observed by using 3a,b in the presence of MAO, affording high molecular weight polyethylene with unimodal molecular weight distributions. Ethylene polymerization in octane by 3b took place without additional cocatalyst at 120 °C with moderate catalytic activity (94 kg PE/mol Ti·h), affording high molecular weight polymer with unimodal distribution (Mw = 1.00 × 106, Mw/Mn = 2.58). The result clearly suggests that the cationic species formed by cleavage of the Ti−O bonds plays an important role as the active species for the polymerization.

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