Abstract
Six examples of dinuclear bis(imino)pyridine-cobalt(ii) complexes, [1,5-{2-(CMe[double bond, length as m-dash]N)-6-(CMe[double bond, length as m-dash]N(2,6-R12-4-R2-C6H2))C5H3N}2(C10H6)]Co2Cl4 (R1 = Me, R2 = H Co1; R1 = Et, R2 = H Co2; R1 = iPr, R2 = H Co3; R1 = Me, R2 = Me Co4; R1 = Et, R2 = Me Co5; R1 = CHPh2, R2 = Me Co6), have been prepared from the corresponding bis(tridentate) compartmental ligands (L1-L6) in reasonable yields. The molecular structures of Co3 and Co5 revealed two N,N,N-cobalt dichloride units to adopt anti-positions about the 1,5-naphthyl linking unit, with each cobalt center exhibiting a distorted trigonal bipyramidal geometry. On activation with either MAO or MMAO, Co1-Co6 were shown to promote both polymerization and oligomerization of ethylene with high overall activities (up to 1.03 × 107 gPE per·mol(Co) per·h for Co1/MAO at 70 °C). Curiously, on increasing the reaction temperature a larger proportion of polymer was noted, while at lower temperature an enhanced selectivity for oligomer was seen. In general, the oligomeric products displayed Schulz-Flory distributions with high selectivities for α-olefins (>99%). On the other hand, the highly linear polymers displayed narrow dispersities and comprised both fully saturated and unsaturated chain ends with the vinyl content (-CH[double bond, length as m-dash]CH2) found to rise with the reaction temperature. By modulating the steric hindrance exerted by the ortho-R1 substituents in the precatalyst, polyethylenes displaying a remarkably broad range of molecular weights could be obtained [from 4.52 kg mol-1 (R1 = Me) to 246.7 kg mol-1 (R1 = CHPh2)].
Highlights
The bis(imino)pyridine class of transition metal catalyst for ethylene polymerization and especially those involving iron and cobalt, remains a subject of enduring research activity ever since their inception over twenty years ago.[1]
Given the absence of any significant steric properties at the 2and 6-positions of the naphthyl linker in F, we considered that the two (N,N,N)CoCl2 units would display some flexibility in their relative configuration which in turn could influence the performance of the resulting catalyst
C displayed a higher optimal temperature for ethylene oligomerization than F, the activity was lower than observed for F (2.46 vs. 5.53 × 105 g·mol1(Co)·h-1atm-1).5a In terms of polymerization, F displayed its highest activity at 70 °C (MAO) and 80 °C (MMAO), suggesting that the cobalt complexes developed in this work possessed better thermal stability in terms of chain propagation when put alongside C,5a D10b and E.11a It is noteworthy that when compared to C, the higher polymerization activity of F can, in part, be attributed to the higher ethylene pressure employed in this study.1b,16 In addition when compared with D, both the activity and molecular weight of the polymer obtained using F increased
Summary
The bis(imino)pyridine class of transition metal catalyst for ethylene polymerization and especially those involving iron and cobalt, remains a subject of enduring research activity ever since their inception over twenty years ago.[1]. To investigate the effects of ligand structure on the catalytic properties, the remaining five cobalt precatalysts, Co2 - Co6, were evaluated in combination with MAO under the optimized condition established for Co1 [Al:Co ratio = 1000, run temperature = 70 oC, 10 atm C2H4, reaction time 30 min] (runs, 2 - 6, Table 3).
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