Reduction of protons

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

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. 

Within the past 10 years, various biomimetic Fe model complexes were prepared and their catalytic activities in the electrochemical reduction of protons to H2 were investigated (Scheme 57). 

Scheme 58 Proposed mechanism of the reduction of protons to H2 catalyzed by 22...

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... 

Scheme 61 [CpPe(CO)2]2 reduction followed by catalytic reduction of protons to H2...

Chiang and coworkers synthesized a dimer of compound 26 in which two diiron subunits are linked by two azadithiolate ligands as a model of the active site for the [FeFeJ-hydrogenase [203]. Protonation of 26 afforded the p-hydride complex [26-2H 2H ] via the initially protonated spieces [26-2H ] (Scheme 62). These three complexes were also characterized by the X-ray diffraction analyses. H2-generation was observed by electrochemical reduction of protons catalyzed by 26 in the presence of HBF4 as a proton source. It was experimentally ascertained that [26-2H 2H ] was converted into 26 by four irreversible reduction steps in the absence of HBF4. 

This process involves a series of reactions, including dissolution, hydrogen reactions and chlorine withdrawal [20], The second type of reactions include reduction of protons at the catalyst by electron transfer yielding hydrogen radicals that are consumed by reaction or give elemental hydrogen otherwise. 

ELECTROCATALYTIC REDUCTION OF PROTONS AND HYDRIDE TRANSFER REACTIONS... 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

In addition to the polymeric rhodium catalysts previously discussed, monomeric rhodium systems prepared from [Rh(CO)2Cl]2 by addition of strong acid (HC1 or HBF4) and Nal in glacial acetic acid have also been shown to be active homogeneous shift catalysts (80). The active species is thought to be an anionic iodorhodium carbonyl species, dihydrogen being produced by the reduction of protons with concomitant oxidation of Rh(I) to Rh(III) [Eq. (18)], and carbon dioxide by nucleophilic attack of water on a Rh(III)-coordinated carbonyl [Eq. (19)]. 

Of course the basis for any biological hydrogen producing system is an enzyme that is capable of carrying out what is arguably the simplest chemical reaction, the reduction of protons to hydrogen 2H+ + 2e H2. All enzymes capable of hydrogen evolution contain... 

The key enzyme hydrogenase catalyses the reversible reduction of protons to molecular hydrogen. Inhibitor experiments indicate that the ferredoxin PetF functions as natural electron donor linking the hydrogenase to the photosynthetic electron transport chain [Florin et al., 2001],... 

Eq. (8) requires determination of the two-electron oxidation potential of L M by electrochemical methods. When combined with the two-electron reduction of protons in Eq. (9), the sum provides Eq. (10), the AGh- values of which can be compared for a series of metal hydrides. Another way to determine the AGh-entails the thermochemical cycle is shown in Scheme 7.3. This method requires measurement of the K of Eq. (11) for a metal complex capable of heterolytic cleavage of H2, using a base (B), where the pK., of BH+ must be known in the solvent in which the other measurements are conducted. In several cases, Du-Bois et al. were able to demonstrate that the two methods gave the same results. The thermodynamic hydricity data (AGh- in CH3CN) for a series of metal hydrides are listed in Table 7.4. Transition metal hydrides exhibit a remarkably large range of thermodynamic hydricity, spanning some 30 kcal mol-1. 

Crampton24 has also demonstrated that for Meisenheimer complex formation, increased crowding at the reaction site caused by change from primary amines to piperidine results in rate reduction of proton transfer from the complex to the amine catalyst, and Hirst199... 

Chapter 12, Section 2). It is generally accepted that the site responsible for the oxidation of dihydrogen and the reduction of protons is the nickel-containing assembly, possibly exploiting its capacity to shuttle between the multiple oxidation states Ni(III)/Ni(II)/Ni(I)/Ni(0). 

Radicals are generated at the anode by oxidation of carbanions (Scheme lb), for example, alkoxides and carboxylates (see Chapter 5, 6), and at the cathode by reduction of protonated carbonyl compounds or onium salts (Scheme Ic) (see Chapter 7). Thereby, a wide choice of different radical structures can be mildly and simply... 

In the anodic process of another bifunctional thiol, 1,3-propanedithiol, all three forms - a neutral molecule, its mono-, and dianion undergo oxidation. The addition of HO ions to the solution or a cathodic preelectrolysis (due to the reduction of protons), shift the equilibrium toward... 

The formation of S-oxides has also been observed when oxidizing a variety of 5-substituted 2-tert-butyl-l,3-dithianes in wet acetonitrile. In an undivided cell, 4-substituted 1,2-dithiolane-l-oxides were oblained (Scheme 25) [113]. A coupled cathodic process, in this case, was the reduction of protons formed in the anodic reaction. 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

Quite recently it has also been reported that the neutral Mn(I) allenylidene [MnCp(=C=C=CPh2)(CO)(PPh3)] catalyzes the reduction of protons from HBF4 into dihydrogen via initial formation of the corresponding alkenyl-carbyne [MnCp... 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

Variation of the forward reaction rate for the reduction of protons then takes the form of Equation 1.7 or Equation 1.8. Here, n is the number of electrons transferred in the overall reaction, k" is the rate constant at the equilibrium potential and C the reactant concentration. 

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