Polynuclear catalysts have long been recognized to offer unique advantages over mononuclear variants for certain transformations: For mechanisms wherein multiple oxidative addition or reductive elimination steps must occur, polynuclear systems provide redox leveling, and provide additional coordination sites. Many small molecule activations may proceed via unfavorable pathways if a suitable catalyst is not present. Coordination-driven self-assembly offers a novel route to construct cofacial and related multi-centered catalysts. The key to this strategy is the selection of complementary building blocks spanning Lewis-acid acceptors and basic donors with controllable orientations. This approach has the potential to greatly simplify the design and synthesis of new catalysts with multiple, proximal active sites since each component can be prepared independently and then unified via self-assembly reactions which typically furnish thermodynamic products in high yields. We will present the self-assembly of cofacial electrocatalysts for oxygen reduction reactions of relevance to fuel cells with an emphasis on improving the selectivity and kinetics of porphyrin-based designs. Two catalytically active metalloporphyrins may be held in a cofacial arrangement using arene ruthenium “molecular clips” that control metal-metal separation, influence redox behavior, and tune substrate access to the internal cavity of the prismatic metallacages. By selecting the proper clip, a preference for four-electron four-proton reduction may be enforced with enhanced kinetics over mononuclear systems.

Small molecule activations with self-assembled polynuclear catalysts

256th National Meeting (2018)

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