Hydrogenases mimics

H2 serves as the alternative energy source relative to fossil fuels and biomass [181] because it is clean and environmentally friendly. Hence, catalytic hydrogen generation from water under mild conditions is one of the goals for the organometallic catalysis. One of the hopeful methods is the electrochemical reduction of protons by a hydrogenase mimic. 

In the same year, Evans and coworkers reported the electrochemical reduction of protons to H2 catalyzed by the sulfur-bridged dinuclear iron complex 25 as a hydrogenase mimic in which acetic acid was used as a proton source [201]. The proposed mechanism for this reaction is shown in Scheme 60. The reduction of 25 readily affords 25 via a one electron reduction product 25. Protonation... 

Heterodinuclear Ni-Fe complexes, which are not stabilized by the phosphine and NO ligands, were synthesized by Tatsumi and coworkers as [NiFe] hydrogenase mimics [208-210]. Several examples are shown in Fig. 8. However, the catalytic activities of these complexes are not ascertained. 

Smith ES, Yang YY, DuBois DL, Bullock RM. Reversible electrocatalytic production and oxidation of hydrogen at low overpotentials by a functional hydrogenase mimic. Angew Chem Int Ed. 2012 124(13) 3206-9. 

The study of model complexes as metalloenzyme mimetics has a long tradition in bioinorganic chemistry (see Chem. Rev. 2004, 104, issue 2 for a complete revision). Several Ni-Fe-based complexes have been prepared, with structural features similar to those found in the FeNi active site.98 Nevertheless, none of them has been tested in its ability to coordinate or decompose H2. Present research is mainly centered in mimicking the thiolate ligands in the Fe-Ni coordination. It has to be mentioned that Fe-only hydrogenase mimics able to electrocatalyze proton reduction have been recently reported.99... 

Biological activation of hydrogen in hydrogenases mimics organometallic chemistry (and vice versa) in that acceptor ligands such as CO are used to control... 

Organic chemists made a small CHON framework. .. S. Ogo et al. A functional [NiFe] hydrogenase mimic that catalyzes electron and hydride transfer from H2. 2013. Science 339(6120), p. 682. DOl 10.1126/science.l231345. 

The reactivity of A-heterocyclic carbenes (NHC) with iron has been presented through the various types of complexes that have been described. Their catalytic applications in cross-coupling, allylation, aziridination, hydrosilylation, and as hydrogenase mimics are detailed. A report on cross-coupling reactions catalysed by complexes of iron group metals with NHC features the reaction mechanisms. ... 

Type II molecules play the role as promoters in photocatalysis, as indicated in Fig. 4.14b. They (polyethylenimine (PEI)-grafted [FeFe]-hydrogenase mimic here) receive excited electrons that are generated via excitation of semiconductors (CdSe... 

Liang W-J, Wang F, Wen M et al (2015) Branched polyethylenimine improves hydrogen photoproduction from a CdSe quantum Dot/[FeFe]-hydrogenase mimic system in neutral aqueous solutions. Chem Eur J 21 3187-3192... 

The design of complex NiFe compounds which mimic active site of hydrogenase and have the catalytic activity comparable with pure enzyme. 

Davies, S. C., Evans, D. J., Hughes, D. L., Longhurst, S. and Sanders, J. R. (1999) Synthesis and structure of a thiolate-bridged nickel-iron complex Towards a mimic of the active site of NiFe-hydrogenase. Chem. Commun., 1999, 1935-6. 

Researchers studying the metalloenzyme hydrogenase would like to design small compounds that mimic this enzyme s ability to reversibly reduce protons to H2 and H2 to 2H+, using an active center that contains iron and nickel. Cobalamins (vitamin and its derivatives) contain an easily activated Co-C bond that has a number of biological functions, one of which is as a methyl transferase, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR). This enzyme converts homocysteine (an amino acid that has one more CH2 group in its alkyl side chain than cysteine see Figure 2.2) to methionine as methylcobalamin is converted to cobalamin. 

Proteins and antibodies are natural substrates for affinity columns because of the nature of the enzyme recognition site and the antibody-antigen interaction sites. They have a three-dimensional shape and electrical charge distributions that interact with only specific molecules or types of molecules. Once these substrate sites are identified, molecules can be isolated or synthesized with the key characteristics and used to build affinity supports. These substrates are often bound to a 6-carbon spacer so that they protrude farther away from the packing surface toward the mobile phase and are therefore more available. Certain natural and synthetic dyes have been found to serve as substrate mimics for a class of enzymes call hydrogenases and have been used to build affinity columns for their purification. 

Sun L, Akermark B, Ott S. Iron hydrogenase active site mimics in supramolecular systems aiming for light-driven hydrogen production. Coord Chem Rev 2005 249 1653-63. 

Gloaguen F, Rauchfuss TB. Small molecule mimics of hydrogenases hydrides and redox. Chem Soc Rev. 2009 38(l) 100-8. 

Darensbourg, Tom Rauchfuss, and Chris Picket saw a simple diiron organometallic molecule as an obvious mimic of the iron hydrogenase active site. Darensbourg said that this comparison was a good starting point for modeling... 

This structure mimics the resting state of the [FeFe]hydrogenase active site. The enzyme holds this conformation in position throughout proton/electron coupling/ decoupling reactions. 

The synthesis and stndy of low molecular weight model complexes that mimic the electronic structure and coordination chemistry of the hydrogenase active site is a timely area of research and has been reviewed. Typical examples of mixed Ni-Fe complexes that have been prepared include complexes (113)/ (114)/ (115)/ and (116).i i... 

It has long been established that Pt is the most efficient singlemetal electrode for the catalysis of both reactions (1) and (2). In the case of ddiydrogen activation, no metal electrocatalyst performs better than platinum. However, aside from the fact that platinum is a precious metal, a major drawback is that commercial (fossil-based) hydrogen contains residual amounts of impurities (e g., carbon monoxide) that only serve to poison the catalyst surface." To address this particular problem, present research has focused on the employment of metal additives (e.g., Ru) or of molecular catalysts that mimic the impressive activity of biological materials (e g., hydrogenase enzymes) " the use of molecular catalysts appears to be the more attractive option since such com-... 

In efforts to mimic the function of heterobimetallic hydrogenases and provide potential molecular catalysts for fuel cell technology the Fe,Ru-heterobimetallic hydride complex [Cp RuH(dppf)], dppf = l,l -bis(diphenylphosphino)ferrocene, has been reported [44b] to catalyze the elementary reaction H2 —> 2H + 2e . It was suggested [44b] that the crucial oxidation involves the ruthenium center and not... 

The chemistry and biochemistry outlined in this chapter, not only addresses the as yet unsolved problems of the mechanisms of hydrogenase and nitrogenase action, but also how chemists trying to mimic this behaviour using simple chemical models have opened up new areas of chemical research related to metal hydrides. These new research areas have little or no biological relevance, but are just as intriguing and intellectually demanding as the direct solution to... 

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