Shapeshifting Ligands Mask Lewis Acidity of Dicationic Palladium(II)
Research Poster Physical Sciences & Mathematics 2025 Graduate ExhibitionPresentation by Karli Sipps
Exhibition Number 198
Abstract
Lewis acidic palladium(II) complexes are useful for activating olefinic substrates. This mode of activation is a key step in industrially relevant catalytic reactions such as olefin isomerization, oligomerization and polymerization. However, for polar olefinic substrates, catalytic activity can be inhibited or quenched by deleterious Lewis acid-base interactions between the metal and polar group. This incompatibility limits the substrate scope and, in some cases, the utility of these catalysts. A common strategy for mitigating this limitation is to design supporting ligands which reduce the Lewis acidity of the metal. Generally, a necessary consequence of this approach is reduced catalytic performance. Advancements in catalyst design have thus focused primarily on balancing catalytic activity and polar/protic functional group tolerance. Here, we present an alternative approach to designing Lewis acidic complexes. We synthesized a series of Pd(II) complexes bearing “shapeshifting” ligands, which can bifurcate between two different coordination modes via structural rearrangements. This process imbues the complexes with two distinct electronic states – one which is highly Lewis acidic, and one which temporarily masks the metal’s Lewis acidity. We will present structural and spectroscopic data which describe the electronic features of each state, as well as evidence that suggests these two states equilibrate in solution. We will also present a comparison of the catalytic activity of our complexes vs. known Lewis acidic Pd(II) catalysts for olefin isomerization. Despite reacting like strong Lewis acids, our complexes are tolerant of polar/protic functional groups, which we attribute to the incorporation of a “masked” Lewis acidic state.
Importance
Metal complexes containing Lewis acidic palladium(II) comprise a class of catalysts used to produce a variety of specialty chemicals and plastics. A current limitation in this field is the incompatibility of these catalysts with substrates bearing polar functional groups. Efforts to address this problem have historically rendered the catalysts less effective, which limits the industrial relevance. We have designed a new type of catalyst that is both tolerant of polar functional groups and highly reactive in catalytic transformations. As a result, we can access reactivity that was previously not possible for this class of catalyst. A potential application of our system is in producing specialty plastics which may possess novel material properties or attractive features such as biodegradability.