The Hydrogen Evolving Reaction HER Hydrogen Evolution Catalysts

The vast majority of hydrogen evolving catalysts are still mostly limited to noble-metal colloids or solids.257-260 Early studies on solar or photo-driven hydrogen production used these colloids along with a sensitizer such as Rubpy and electron relays (e.g., MV2+) with modest success.15,16,261-268 The development of good molecular catalysts for HER remains a surprisingly elusive goal excepting the biologically produced iron-only and iron-nickel hydrogenases.269 These enzymes are able to catalyze proton-reduction or hydrogen oxidation essentially at the thermodynamic potential and do so with rapid turnover.270 With the recent X-ray crystallographic structural elucidation of the active sites of these enzymes,271-273 shown schematically in Fig. 13, considerable progress has been made in understanding their mechanism of action.

Numerous groups have models the active site of the FeNi274,275 and Fe-only276-279

hydrogenases in small molecules, however, the functional activity of these model complexes is limited by the large overpotentials required for function. 280 281 Similarly, dyads in which a Rubpy sensitizer is covalently linked to a biomimetic diiron complex have been prepared but photodriven H2 is still elusive.282,283

An early review by Koelle on transition metal catalyzed proton reduction nicely developed the various chemical steps involved in hydrogen evolution including metal hydride formation, hydride acidity (basicity) and protonation and requisite redox potentials.284 The complexes review here have little structural relevance to the hy-drogenase active sites but many show promising catalytic activity. More recently

Iron Iron Hydrogenase Structure

Fig. 13. Biologically produced iron-only and iron-nickel hydrogenases used in the catalysis for


Fig. 13. Biologically produced iron-only and iron-nickel hydrogenases used in the catalysis for


(2005), Artero and Fontecave reviewed the most efficient catalysts for hydrogen evolution from an applications perspective and nicely defined the general principles for the design of robust and economically viable catalysts for the HER.285 A number of the catalysts highlighted by Artero and Fontecave are shown in Fig. 14 and while a detailed description of their action would be repetitive, the general properties desired are good basicity of the metal center (either as is or after reduction), open sites for hydride coordination, and finally an accessible redox n+2 state for heterolytic cleavage upon protonation. Homolytic pathways are also possible but require close juxtaposition of two metal centers or unfavorable bimolecular reactions between catalytic metal centers. Collman and coworkers prepared co-facial Ru porphyrins to address this possibility but saw little improvement over monomeric porphyrin com-


The cobalt diglyoximate complexes like 35a-c were first investigated as HER catalysts in the early 1980's by Espenson and coworkers290-292 and have recently been revisited by Peters, Lewis and coworkers.293 In particular, the difluoroboryl-dioxime complexes 36a and 36b seem promising as the macrocylic structure imparts good stability in acidic solution and the potential at which H2 is evolved electrochemically is surprisingly high. Complex 36b evolves H2 at -0.28 V vs. SCE in acetonitrile solutions with HClEt2O as the proton source.

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