2,4,6-Triphenylpyridinium-Substituted Neutral NickelCatalysts: Ethylene Polymerization, Influence of Activator, CatalystDecomposition, and End-Group Analysis

The reactivity of 2,4,6-triphenylpyridinium-substituted nickel(II)-salicyliminato catalysts in ethylene polymerization has been explored. The known catalyst <b>6a</b> equipped with a 2,6-diisopropylphenyl group affords polyethylene with <i>M</i>_n = 10–12 kDa at 19 °C, while <b>8a</b> possessing the bulkier 2,6-diphenylphenyl moiety gives polymer with <i>M</i>_n = 270–310 kDa. The stability of catalyst <b>6a</b> depends on the borane as activator. If B(C₆F₅)₃ is employed, enhanced stability is observed compared with that using 3H₂O·B(C₆F₅)₃ activator. Catalyst <b>6a</b> decomposes to form a hydroxyl-bridged dimer <b>14</b> which may be reactivated, albeit inefficiently, by an excess borane activator. The polymer formed by <b>6a</b> contains ca. 3–6% of 2-trifluoromethylphenyl, pentafluorophenyl, and 3,5-bis(trifluoromethyl)phenyl end groups combined. The pentafluorophenyl end group originates from the reaction of <b>6a</b> with the borane activator and from the reactivation of hydroxyl-bridged dimer <b>14</b>. 3,5-Bis(trifluoromethyl)phenyl end groups are likely formed in a reaction sequence initiated by the protonation of one aryl group in NaBArF that creates B[C₆H₃(CF₃)₂]₃. The triarylborane can then react with <b>6a</b> or <b>14</b>, transferring the 3,5-bis(trifluoromethyl)phenyl group to nickel.