Nickel alpha-keto-beta-diimine initiators for olefin polymerization.

Late transition metal initiators for olefin polymerization and oligomerization are of interest owing to their low oxophilicity and high functional group tolerance. Nickeland palladiumbased initiators provide several benefits. These include control over molecular weight characterisitics, backbone stereochemistry, and comonomer incorporation. 4] “Chain walking” reactions, in which the metal center migrates along the growing polymer chain through a series of b-hydride eliminations and reinsertions, allow for the generation of a wide variety of novel materials. New ligand structures continue to appear and offer further optimization of reactivity and polymer properties. 4b, 5c,6] Scheme 1 shows representative cationic and zwitterionic nickel active sites. Example 1a (charge compensating anion is not shown) corresponds to the broadly studied class of adiimine complexes, for which the resting state is a cationic alkyl olefin species. These species can be generated from a dihalide precursor in the presence of olefin and an appropriate activator, such as methylaluminoxane (MAO). Also shown are two prototypical zwitterionic initiators (1b and 1c ; important resonance contributions are not shown). A relevant perspective for understanding the reactivity of zwitterionic species is that Lewis acid complexation onto a basic ligand site reduces the electron density of the metal center in a neutral complex. This concept is more easily appreciated by examination of 1c. Bulky substituents that occupy axial sites are important structural components for minimizing chain-growth termination in both cationic and zwitterionic systems: for example, 1b produces high-molecular-weight polyethylene (PE), whereas 1c yields a distribution of 1-alkenes. Herein, we report on the synthesis, characterization, and reactivity of a ligand–metal combination that was designed to be cationic and also to benefit from removal of electron density from the metal center by the action of a Lewis acid on the ligand framework. We targeted a nickel complex containing a bulky a-keto-b-diimine ligand with 2,6-diisopropylphenyl substituents (2, Scheme 2). The exocyclic carbonyl functionality was anticipated to provide an electronically delocalized conduit extending from the potentially cationic metal center to a site of Lewis acid interaction through the oxygen lone pairs. The previously reported b-diimine complex Scheme 1. Examples of cationic and zwitterionic polymerization/oligomerization active sites. R = alkyl.