Hydride hydrogenase

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. 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

In 2009, Rauchfuss and coworkers succeeded in the synthesis of the Fe- i-H-Ni complex [(CO)3Fe(pdt)(p-H)Ni(dppe)]BF4 28 (pdt = 1,3-propanedithiolate, dppe = 1,2-C2H4(PPh2)2) as a model for [NiFeJ-hydrogenases (Scheme 64) [212]. The structure of 28 was characterized by X-ray crystallographic analysis. This is the first example of an Fe-Ni thiolato hydride complex. Evolution of H2 by electrochemical reduction of CF3CO2H (pXa = 12.65) was observed in the presence of the catalytic amounts of 28. 

The cationic complex [CpFe(CO)2(THF)]BF4 (23) can also catalyze the proton reduction from trichloroacetic acid by formation of Fe-hydride species and may be considered as a bioinspired model of hydrogenases Fe-H Complexes in Catalysis ) [44]. This catalyst shows a low overvoltage (350 mV) for H2 evolution, but it is inactivated by dimerization to [CpFe(CO)2l2-... 

A different mechanism for reduction processes by [Fe]-hydrogenase 56 is assumed. The hydride generated by splitting dihydrogen is directly transferred to an electrophilic organic center in methenyltetrahydrocyanopterin. As no electrons need to be transferred this reaction requires only one metal center. Due to its structure the center of [Fe]-hydrogenase 56 does not count to the class of ferrates. 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

There is clearly a growing interest in photocatalysis involving metal hydrides (see Section VIII) and hydrogenase systems (see Section IX), and the search for systems to utilize solar energy to dissociate water into its elements will undoubtedly intensify further efforts. 

Figure 6.10 The catalytic site of [NiFe] and [NiFeSe] hydrogenases in oxidised inactive (top) and reduced active (bottom) states. Note the three non-protein diatomic ligands to the iron.The site bridging the Ni and Fe is occupied by an oxygen or sulfur species in the most oxidised states and probably by a hydride or molecular hydrogen in the most reduced states.

The splitting of H2 by hydrogenases is heterolytic (into H and H" ), rather than homolytic (into two H. radicals). The hydride is considered to deliver two electrons at a time to the enzyme. A [4Fe-4S] cluster in proteins can, however, accept only one electron. Other redox enzymes, e.g. flavoproteins, dealing with two-electron donors (like NADH) solve this problem by first accommodating both electrons onto the flavin, whereafter these electrons are transferred to an Fe-S cluster one at a time. 

The minimal functional module in [NiFe] hydrogenases always contains the NiFe(CN)2(CO) site plus the proximal [4Fe-4S] cluster. The active site in [Fe] hydrogenases consists of the Fe-Fe site linked to a [4Fe-4S] cluster. Oxidation of the hydride is either an action of the dinuclear site alone, or a concerted action of this site plus the proximal cluster. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

A similar reaction can be written for the [Fe] hydrogenases with a Fe-[4Fe-4S] complex replacing the nickel. Note that the nickel atom in the NiFe cluster, and the Fe-[4Fe-4S] sites are nearest to the electron carrier [4Fe-4S] clusters, indicating that electron transfer occurs through these atoms. The other atom in each of the centres is an iron atom with -CN and -CO ligands, and it seems likely that this is a binding site for hydride (Fig. 8.1). 

This mechanism easily accounts for the other reactions catalysed by hydrogenase. Exchange of the hydron and/or hydride with another hydron from the water, and reversal of step 1 would explain the H - F1 exchange reactions of hydrogenase (Chapter 5). 

The mechanism of action, and organization of the catalytic sites, in hydrogenases are different from a solid catalyst such as platinum. For a start, the reaction of H2 with hydrogenase involves heterolytic cleavage into a hydron and a hydride. This contrasts with the reaction of H2 at the surface of a metal such as platinum, which is usually considered to involve the homolytic cleavage into two hydrogen atoms. Moreover in the enzyme, the catalyst is a cluster of metal ions (with oxidation states +2 or -h3) rather than the metal (oxidation state 0). 

Hembre, R. T., McQueen, J. S. and Day, V. W. (1996) Coupling H2 to electron transfer with a 17-electron heterobimetallic hydride A Redox Switch model for the H2-activating center of hydrogenase./. Am. Chem. Soc., 118, 798-803. 

In D2O, HD was found instead of 0-H2. It is presently assumed that binding of hydrogen to a metal ion in the bimetallic active site weakens the H-H bond sufficiently to enable this reaction. Oxidation of the hydride is expected to be a two-electron process, and hydrogenases should, therefore, contain a redox unit capable of accepting these two electrons simultaneously. I assume here that the bimetallic center plus the conserved proximal Fe-S cluster perform this task. 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

As pointed out earlier, the principal requirement for an active catalyst for the heterolytic splitting of hydrogen is the presence of two suitably disposed functional groups—a metal atom to combine with the hydride ion and a base ( B) to act as a proton acceptor. In line with the evidence for the presence of a ferrous complex in hydrogenase, Rittenberg (18) has suggested the following model for the active site of the enzyme. 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

Among the chemical forms adopted by the vitamin B12 coenzyme, one is a terminal hydride of the type CoHL5 where L represents a nitrogen ligand of the corrin ring or axial base. Hydrides may be important in vivo both in this case and in that of hydrogenases. 

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