Electrocatalyst For Proton Reduction Research Articles

Fast and efficient molecular electrocatalysts for H2 production: Using hydrogenase enzymes as guides

Abstract

Redox rich dicobalt macrocycles as templates for multi-electron transformations

Pyridazine-templated dicobalt macrocycles reversibly support five oxidation states with unusually positive Co(II)/Co(I) redox couples, and are also active proton reduction electrocatalysts.

The effect of bridgehead steric bulk on the ground state and intramolecular exchange processes of (μ-SCH2CR2CH2S)[Fe(CO)3][Fe(CO)2L] complexes

The flexibility of the coordination sphere in the diiron organometallic is likely an important design component in nature’s electrocatalyst for proton reduction or H2 oxidation, i.e, the active site of [FeFe]hydrogenase. A series of complexes, (m-SCH2CRR0CH2S)[Fe(CO)3][Fe(CO)2L] with steric bulk incorporated into the m-S-to-S linker was synthesized and the compounds were analyzed by infrared spectroscopy and cyclic voltammetry [(R/R0 ¼ Me/Me, Et/Et, Bu/Et), (L ¼ CO, PPh3, IMes (1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), and IMe (1,3-dimethylimidazole-2-ylidene))]. While added steric bulk at the bridgehead carbon of the m-SCH2CR2CH2S produced little change in the ground state structures (X-ray diffraction) and electronic character for the (m-SRS)[Fe(CO)3]2 complexes, monosubstitution of a CO with L produced distortions consistent with steric interference of the m-SRS with nearby ligands as compared to the similar (m-pdt)[Fe(CO)3][Fe(CO)2L] (pdt ¼ S(CH2)3S). Variable temperature NMR studies have shown that the activation barrier for CO site exchange on the sterically bulky complexes decreases in a manner predicted by theory [

Synthesis, Structure, and Electrocatalysis of Diiron C-Functionalized Propanedithiolate (PDT) Complexes Related to the Active Site of [FeFe]-Hydrogenases

New C-functionalized propanedithiolate-type model complexes (1-8) have been synthesized by functional transformation reactions of the known complex [(mu-SCH2)2CH(OH)]Fe2(CO)6 (A). Treatment of A with the acylating agents PhC(O)Cl, 4-pyridinecarboxylic acid chloride, 2-furancarbonyl chloride, and 2-thiophenecarbonyl chloride in the presence of Et3N affords the expected C-functionalized complexes [(mu-SCH2)2CHO2CPh]Fe2(CO)6 (1), [(mu-SCH2)2CHO2CC5H4N-4]Fe2(CO)6 (2), [(mu-SCH2)2CHO2CC4H3O-2]Fe2(CO)6 (3), and [(mu-SCH2)2CHO2CC4H3S-2]Fe2(CO)6 (4). However, when A is treated with the phosphatizing agents Ph2PCl, PCl3 and PBr3, both C- and Fe-functionalized complexes [(mu-SCH2) 2CHOPPh2-eta1]Fe2(CO)5 (5), [(mu-SCH2) 2CHOPCl2-eta1]Fe2(CO)5 (6), and [(mu-SCH2) 2CHOPBr2-eta1]Fe2(CO)5 (7) are unexpectedly obtained via intramolecular CO substitution by P atoms of the initially formed phosphite complexes. The simplest C-functionalized model complex [(mu-SCH2) 2CO]Fe2(CO)6 (8) can be produced by oxidation of A with Dess-Martin reagent. While 8 is found to be an electrocatalyst for proton reduction to hydrogen, starting complex A can be prepared by another method involving the reaction of HC(OH)(CH2Br)2 with the in situ generated (mu-LiS) 2Fe2(CO)6. X-ray crystallographic studies reveal that the bridgehead C atom of 8 is double-bonded to an O atom to form a ketone functionality, whereas the bridgehead C atoms of A, 1, 3, and 4 are equatorially-bonded to their functionalities and those of 5-7 axially-bonded to their functionalities due to formation of the corresponding P-Fe bond-containing heterocycles.

XAFS of short-lived reduction products of structural and functional models of the [Fe–Fe] hydrogenase H-cluster

Thiolate-bridged diiron compounds that are related to the active site of the [Fe–Fe] hydrogenase enzyme have been shown to act as electrocatalysts for reduction of protons. The use of XAFS for clarification of the structures of intermediates formed following reduction of related diiron carbonyl compounds is described. These measurements allow the determination of Fe–Fe and Fe–S bond lengths with good reliability and when used in conjunction with the standard bonding models this provides a means of validating the structures proposed for longer-lived ( t 1 / 2 > 20 s at −50 °C) reaction intermediates.

Proton Electroreduction Catalyzed by Cobaloximes: Functional Models for Hydrogenases

Cobaloximes have been examined as electrocatalysts for proton reduction in nonaqueous solvent in the presence of triethylammonium chloride. [Co(III)(dmgH)2pyCl], working at moderate potentials (-0.90 V/(Ag/AgCl/3 mol x L(-1) NaCl) and in neutral conditions, is a promising catalyst as compared to other first-row transition metal complexes which generally function at more negative potentials and/or at lower pH. More than 100 turnovers can be achieved during controlled-potential electrolysis without detectable degradation of the catalyst. Cyclic voltammograms simulation is consistent with a heterolytic catalytic mechanism and allowed us to extract related kinetic parameters. Introduction of an electron-donating (electron-withdrawing) substituent in the axial pyridine ligand significantly increases (decreases) the rate constant of the catalytic cycle determining step. This effect linearly correlates with the Hammet coefficients of the introduced substituents. The influence of the equatorial glyoxime ligand was also investigated and the capability of the stabilized BF2-bridged species [Co(dmgBF2)2(OH2)2] for electrocatalyzed hydrogen evolution confirmed.

Synthesis of the H-cluster framework of iron-only hydrogenase

The metal-sulphur active sites of hydrogenases catalyse hydrogen evolution or uptake at rapid rates. Understanding the structure and function of these active sites--through mechanistic studies of hydrogenases, synthetic assemblies and in silico models--will help guide the design of new materials for hydrogen production or uptake. Here we report the assembly of the iron-sulphur framework of the active site of iron-only hydrogenase (the H-cluster), and show that it functions as an electrocatalyst for proton reduction. Through linking of a di-iron subsite to a {4Fe4S} cluster, we achieve the first synthesis of a metallosulphur cluster core involved in small-molecule catalysis. In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analogue of the H-cluster may enable us to develop useful electrocatalytic materials for application in, for example, reversible hydrogen fuel cells. (Platinum is currently the preferred electrocatalyst for such applications, but is expensive, limited in availability and, in the long term, unsustainable.).

Making a natural fuel cell

The synthetic assembly of the active centre of hydrogen-producing enzymes adds to our understanding of their structure and function — and could produce new and useful materials that mimic these enzymes. In conventional hydrogen fuel cells the rapid interconversion of protons and electrons to hydrogen requires catalysis by expensive metals, usually platinum. In the living world enzymes catalyse this same reaction at extraordinary rates using abundant metals. Tard et al. now report the chemical synthesis of the iron–sulphur structure at the heart of the hydrogenase protein. The resulting iron–sulphur framework functions as an electrocatalyst for proton reduction, a potentially important step towards new materials to replace platinum in the anodes of fuel cells.