Models for Hydrogenases

The NS3 ligand thus displays different behaviour with iron from that with vanadium and molybdenum this is largely dictated by the stability of the iron(ii) oxidation state. The reactions of H3(NS3) with iron compounds of very low 

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Razavet M, Artero V, Fontecave M (2005) Proton electroreduction catalyzed by cobaloximes functional models for hydrogenases. Inorg Chem 44(13) 4786-4795. doi 10.1021/ic050167z... 

Curtis CJ, Miedaner A, CiancanelU R, EUis WW, NoU BC, DuBois MR, DuBois DL. Ni(Et2PCH2NMeCH2PEt2)2 " as a functional model for hydrogenases. Inorg Chem 2003 42 216-227. 

Fig. 6 The models for the active site of [FeFe]-, [NiFe]-, and [Fe]-hydrogenases...

Fig. 9 Electrochemical H2 generation catalyzed by complex 27 as a model for the [NiFe] hydrogenase...

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-... 

The first Ni Mossbauer spectrum of nickel in a bioinorganic compound with determinable EFG and isomer shift was reported for a nickel complex compound with planar [NiSJ core and considered as a model compound for hydrogenase. This Mossbauer spectrum from the formal Ni compound is presented in Fig. 7.16. The observed quadrupolar interaction can be understood in terms of ligand field theory. In this approach, the b g and levels (d y2 and d ) are not occupied which is expected to cause a large negative EFG contribution [32]. 

Fig. 7.16 Ni Mossbauer spectrum at 4.2 K of a nickel complex compound with planar [NiS4] core known as a model compound for hydrogenase (source Nio.gsCro.is (97% enriched) activated at Mainz Microtron) (from [32])...

Structures of chemical models for the active sites of hydrogenases... 

Hoffman and collaborators (Doan et al. 1994) also performed ENDOR experiments on the oxidized [3Fe-4S] cluster in D. gigas hydrogenase. The authors detected resonances from strongly coupled protons which were assigned to the [3-CH2 of the cysteines and exchangeable protons that are probably involved in three different hydrogen bonds to the sulfurs of the cluster. Based on these data a model for the binding of the cluster to the protein was developed. 

Figure 8.5 (1-3) Structures of chemical models for the active sites of hydrogenases. For references, see text.

In conclusion, the presented dinuclear iron structure is the first example of a bio-mimetic iron compound, which can be regarded as a first generation model for the class of [Fe]-only hydrogenases. The complex incorporates both relevant carbon monoxide ligands, as well as three bridging thiolato ligands, which could be possibly present in the active site of these enzymes. 

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. 

Lyon, E. J., Georgakaki, I. R, Reibenspies, J. H. and Darensbourg, M. Y. (1999) Carbon monoxide and cyanide ligands in a classical organometallic complex model for Fe-only hydrogenase. Angew. Chem. Int. Ed. Engl., 38, 3178-80. 

Stein, M., van Lenthe, E., Baerends, E. J. and Luhitz, W. (2001a) G- and A-tensor calculations in the zero-order approximation for relativistic effects of Ni-complexes as models for the active center of [NiFej-hydrogenase./. Rhys. Chem., A 105, 416-25. 

Figure 18.4. Proposed biocorrosion model for cathodic electron depolarization of mild steel by D. vulgaris Hildenborough. CM, cytoplasmic membrane HMC, high molecular weight cytochrome [Fe] Fl2ase, iron hydrogenase [NiFe] H2ase, nickel iron hydrogenase ETS, electron transport system.

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