@article {32765,
	title = {Bioinspired design of redox-active ligands for multielectron catalysis: effects of positioning pyrazine reservoirs on cobalt for electro- and photocatalytic generation of hydrogen from water},
	journal = {Chem. Sci.},
	volume = {6},
	year = {2015},
	pages = {4954-4972},
	abstract = {<p>Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems{,} we present the design{,} synthesis{,} and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values{,} and comparison with analogs bearing redox-inactive zinc(ii) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution{,} under diffusion-limited conditions{,} reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink{,} resulting in high overpotentials for proton reduction{,} whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant{,} neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co{,} 2-Co{,} and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together{,} the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media{,} akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense{,} these findings highlight the significance of electronic structure considerations in the design of effective electron\&ndash;hole reservoirs for multielectron transformations.</p>},
	doi = {10.1039/C5SC01414J},
	url = {http://dx.doi.org/10.1039/C5SC01414J},
	author = {Jurss, Jonah W. and Khnayzer, Rony S. and Panetier, Julien A. and El Roz, Karim A. and Nichols, Eva M. and Head-Gordon, Martin and Long, Jeffrey R. and Castellano, Felix N. and Chang, Christopher J.}
}
