The development of neutral Ni and Pd complexes with [N,O] ligands for ethene polymerisation and cross-coupling catalysis

Abstract

A series of 2-(arylamino)-5-methylcyclopent-2-en-1-one proligands, derived from the condensation of maple lactone with aryl amines, were developed. Using these [N,O] proligands, complexation experiments were carried out by deprotonation with NaH and reaction with Ni and Pd precursors to yield square planar complexes containing 5-membered [N,O] chelates. The Ni complexes featuring PPh3 and aryl (phenyl and ortho-tolyl) coligands were trialled for the polymerisation of ethene. Precatalysts with N-Dipp (Dipp = 2,6-diisopropylphenyl) substituents were found to exhibit moderate activity when activated with Ni(cod)2 or B(C6F5)3. The catalytic activity was highly dependent on temperature, with the catalysts most active at 80 °C. Significant differences in the nature of the polymer were also found when changing the activator, with polymers using Ni(cod)2 as the activator producing a highly branched viscous polymer, compared to colourless, solid polyethene with a lower number of branches produced with B(C6F5)3. The most active nickel precatalyst, featuring the N-Dipp substituent, was trialled for the copolymerisation of ethene and CO, the polymerisation of 1-hexene, and the polymerisation of methyl acrylate; no copolymerisation or polymerisation was observed. A palladium complex bearing the Dipp-substituted [N,O] ligand with PPh3 and Cl coligands was tested as a catalyst for the Suzuki-Miyaura cross-coupling. The complex was found to be highly active under mild conditions and under air. The scope and tolerance of this reaction was then investigated which showed good conversion for all aryl bromides tested that featured electron-withdrawing groups. More electron-rich substituents could be reacted successfully by increasing the temperature of the reaction and increasing the reaction time. Only boronic acids featuring strongly electron-withdrawing groups showed limited activity. Pinacol esters also showed good activity towards the Suzuki-Miyaura cross-coupling. The cross-coupling reaction of aryl chlorides with phenyl boronic acid was also achieved in mild conditions but required an elevated temperature of 40 °C for 24 hours.