This talk will describe our group’s recent efforts in the design and mechanistic evaluation of new molecular, macrocyclic Co(II) catalysts for aqueous H2 generation. These catalysts were designed to incorporate redox-active bipyridine groups which are linked by nitrogen groups, both components which can participate in electron and proton transfer steps in the catalytic cycle. In comparing two molecular catalysts that differ by only one linking nitrogen, single crystal analysis reveals a profound impact on the molecular geometry, which in turn influences their relative catalytic activity. Photocatalysis experiments show that both catalysts are highly active for aqueous proton reduction at moderate pH levels, with the closed macrocycle reaching almost 2 x 104 turnovers of H2 when photo-driven by [Ru(2,2’-bipyridine)3]2+ using ascorbate as an electron relay and a phosphine compound as the terminal electron donor. Measurements of the electrocatalytic activity were used to investigate key steps in the mechanism of proton reduction, and from a detailed analysis of these experiments we propose a mechanism for catalytic proton reduction to H2 that involves both intramolecular proton and electron transfer steps between the macrocycle ligand to the cobalt center. We will also describe how substituting pyrazine groups for the pyridyl groups in the macrocycle influence the redox properties of the macrocycle ligand, and in turn the photocatalytic activity. This work demonstrates the vital role of the second coordination sphere in the catalytic cycle, and places these relatively simple complexes on the pathway toward molecular catalysts that mimic the valuable features of enzymatic catalysis.

Molecular cobalt catalysts for H2 generation with redox activity and proton relays in the second coordination sphere

2019 Spring National Meeting

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