Synthesis and Ethylene Polymerization Capability of Metallocene-like Imido Titanium Dialkyl Compounds and Their Reactions with AlⁱBu₃

Abstract

A series of alkyl- and aryl-imido titanium dialkyl compounds Ti(NtBu)(Me3[9]aneN3)R2 (R = Me (1), CH2SiMe3 (3), CH2tBu (4), CH2Ph (5)), Ti(NR)(Me3[9]aneN3)Me2 (R = iPr (6), Ph (7), 3,5-C6H3(CF3)2 (8), 2,6-C6H3iPr2 (9), 2-C6H4CF3 (10), 2-C6H4tBu (11)), and Ti(NR)(Me3[9]aneN3)(CH2SiMe3)2 (R = iPr (12), ArF (13)) were prepared and crystallographically characterized in the case of 1, 6−9, and 11 (Me3[9]aneN3 = 1,4,7-trimethyl triazacyclononane; ArF = C6F5). These compounds, isolobal with the titanocenes Cp2TiR2, were thermally stable at elevated temperatures except for 4. Reaction of 7 with [Ph3C][BArF4] (TB) and diisopropylcarbodiimide in CH2Cl2 gave the Ti−Me insertion product [Ti(NPh)(Me3[9]aneN3){MeC(NiPr)2}][BArF4] (15-BArF4). The corresponding reaction of 7 in the absence of organic substrate gave [Ti2(μ-NPh)2(Me3[9]aneN3)2Cl2][BArF4]2 via a solvent activation reaction. The room-temperature ethylene polymerization capabilities of the dialkyl compounds were evaluated using TB cocatalyst in the presence of AliBu3 (TIBA). Among the dimethyl precatalysts, only the systems 1 and 11, with the bulkiest imido groups, showed high productivities (6230 and 1210 kg mol-1 h-1 bar-1, respectively). The productivites of the other tert-butyl imido precatalysts 3 and 4 (130 and 120 kg mol-1 h-1 bar-1, respectively) were substantially lower than that of 1. The catalyst system 1/TIBA (2500 equiv, no added TB) was also active for ethylene polymerization (225 kg mol-1 h-1 bar-1). The less productive imido dialkyl precatalysts all formed complex mixtures on exposure to TIBA. The polyethylenes produced with 1, 3, and 5−11 generally had Mw/Mn values in the range 2.6−3.0. The PE formed with 1/TB/TIBA was terminated only by methyl end groups, consistent with chain transfer to TIBA followed by subsequent β-H transfer by the resultant titanium isobutyl cation. The alkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R = Me or CH2SiMe3) reacted rapidly with TIBA in C6D5Br at −30 °C, forming isobutene. DFT calculations found that TIBA adducts of the model methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ were energetically favorable by ca. −80 to −110 kJ mol-1. Whereas 1 alone or with AlMe3 present has been shown to form only Ph3CMe on reaction with [Ph3C]+, 1:1 mixtures of 1 and TIBA gave Ph3CH as the only trityl-containing product, suggesting a key role for transient [AliBu2]+ in the activation process for these catalysts. Overall, the imido group in the Ti(NR)(Me3[9]aneN3)Me2/TB/TIBA catalysts systems appears to have two roles: to stabilize the dialkyl precatalyst toward degradation by the TIBA itself prior to activation, and to inhibit the formation of catalytically inactive hetero- or homo-bimetallic complexes.