Beyond the active site: the impact of the outer coordination sphere on electrocatalysts for hydrogen production and oxidation.

Abstract

Redox active metalloenzymes play a major role in energy transformation reactions in biological systems. Examples include formate dehydrogenases, nitrogenases, CO dehydrogenase, and hydrogenases. Many of these reactions are also of interest to humans as potential energy storage or utilization reactions for photoelectrochemical, electrolytic, and fuel cell applications. These metalloenzymes consist of redox active metal centers where substrates are activated and undergo transformation to products accompanied by electron and proton transfer to or from the substrate. These active sites are typically buried deep within a protein matrix of the enzyme with channels for proton transport, electron transport, and substrate/product transport between the active site and the surface of the protein. In addition, there are amino acid residues that lie in close proximity to the active site that are thought to play important roles in regulating and enhancing enzyme activity. Directly studying the outer coordination sphere of enzymes can be challenging due to their complexity, and the use of modified molecular catalysts may allow us to provide some insight. There are two fundamentally different approaches to understand these important interactions. The "bottom-up" approach involves building an amino acid or peptide containing outer coordination sphere around a functional molecular catalyst, and the "top-down" approach involves attaching molecular catalyst to a structured protein. Both of these approaches have been undertaken for hydrogenase mimics and are the emphasis of this Account. Our focus has been to utilize amino acid or peptide based scaffolds on an active functional enzyme mimic for H2 oxidation and production, [Ni(P(R)2N(R('))2)2](2+). This "bottom-up" approach has allowed us to evaluate individual functional group and structural contributions to electrocatalysts for H2 oxidation and production. For instance, using amine, ether, and carboxylic acid functionalities in the outer coordination sphere enhances proton movement and results in lower catalytic overpotentials for H2 oxidation, while achieving water solubility in some cases. Amino acids with acidic and basic side chains concentrate substrate around catalysts for H2 production, resulting in up to 5-fold enhancements in rate. The addition of a structured peptide in an H2 production catalyst limited the structural freedom of the amino acids nearest the active site, while enhancing the overall rate. Enhanced stability to oxygen or extreme conditions such as strongly acidic or basic conditions has also resulted from an amino acid based outer coordination sphere. From the "top-down" approach, others have achieved water solubility and photocatalytic activity by associating this core complex with photosystem-I. Collectively, by use of this well understood core, the role of individual and combined features of the outer coordination sphere are starting to be understood at a mechanistic level. Common mechanisms have yet to be defined to predictably control these processes, but our growing knowledge in this area is essential for the eventual mimicry of enzymes by efficient molecular catalysts for practical use.