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. 2011 Oct 30;4(1):26-30.
doi: 10.1038/nchem.1180.

Combining Acid-Base, Redox and Substrate Binding Functionalities to Give a Complete Model for the [FeFe]-hydrogenase

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Combining Acid-Base, Redox and Substrate Binding Functionalities to Give a Complete Model for the [FeFe]-hydrogenase

James M Camara et al. Nat Chem. .
Free PMC article

Abstract

Some enzymes function by coupling substrate turnover with electron transfer from a redox cofactor such as ferredoxin. In the [FeFe]-hydrogenases, nature's fastest catalysts for the production and oxidation of H(2), the one-electron redox by a ferredoxin complements the one-electron redox by the diiron active site. In this Article, we replicate the function of the ferredoxins with the redox-active ligand Cp*Fe(C(5)Me(4)CH(2)PEt(2)) (FcP*). FcP* oxidizes at mild potentials, in contrast to most ferrocene-based ligands, which suggests that it might be a useful mimic of ferredoxin cofactors. The specific model is Fe(2)[(SCH(2))(2)NBn](CO)(3)(FcP*)(dppv) (1), which contains the three functional components of the active site: a reactive diiron centre, an amine as a proton relay and, for the first time, a one-electron redox module. By virtue of the synthetic redox cofactor, [1](2+) exhibits unique reactivity towards hydrogen and CO. In the presence of excess oxidant and base, H(2) oxidation by [1](2+) is catalytic.

Figures

Figure 1
Figure 1. Structure of active site for the [FeFe]-hydrogenase and its model
Both structures contain functionality dedicated to substrate binding as well as the management of redox equivalents and proton equivalents. As shown on the left, the active site consists of a Fe2(CO)3(CN)2 centre bridged by the Brønsted-basic azadithiolate (SCH2NHCH2S) cofactor. The redox cofactor, a 4Fe–4S cluster, is attached to a single Fe centre. Substrate binding occurs at the Fe centre distal to the 4Fe–4S cluster and adjacent to the basic amine. The proposed model complex also features an Fe2(CO)3 centre bridged by a basic azadithiolate with an alkyl groups R in place of H. A redox-active ligand (Redox-L) simulates the function of the 4Fe–4S cluster, and the phosphine ligands simulate the coordinated cyanides.
Figure 2
Figure 2. Synthesis of FcP*
The route starts with the formation of the C–P bond and generation of LiC5Me4CH2PEt2. Combining this organolithium reagent with “(C5Me5)FeCl” gives FcP*.
Figure 3
Figure 3. Summary of reactions observed for [1]2+ with CO and H2
Compound numbering and relevant oxidation states are shown, using italics for the ferrocenyl iron centre. The starting compound 1 is in a fully reduced state. Initial oxidation of 1 is localized on the diiron core as shown by FT-IR and EPR measurements. The second oxidation (to [1]2+) converts the FcP* centre from ferrous to ferric. Reactions of [1]2+ with H2 and with CO involve substrate binding coupled to intramolecular electron-transfer (ET) from the diiron subunit to the oxidized FcP* ligand.
Figure 4
Figure 4. Infrared spectra probing the localization of the two one-electron oxidations of the reduced model [1]
a–d, Titration of FcBAr4F into a CH2Cl2 solution of 1 monitored by solution IR spectroscopy: [1] (a), [1] + 1 equiv. FcBAr4F (b), [1] + 1.5 equiv. FcBAr4F (c), [1] + 2 equiv. FcBAr4F (d). On conversion of [1]+ to [1]2+, an isosbestic point is observed at 1,969 cm−1.
Figure 5
Figure 5. Spectroscopic evidence confirming the reactions of [1]2+ with known hydrogenase substrates H2 and CO
a, IR spectra of dichloromethane solutions of [1]2+ before and after treatment with CO (1 atm) and H2 (1 atm) at 25 °C. In each case, the formation of a new carbonyl-containing species is indicated by the appearance of new νCO bands on the addition of the indicated substrate. b, 31P NMR spectrum (202 MHz, CD2Cl2) of [1(CO)]2+ at −60 °C showing the conversion of the 31P NMR-silent paramagnetic [1]2+ to the 31P NMR-active diamagnetic adduct [1(CO)]2+. c, 1H NMR (500 MHz, CD2Cl2) hydride resonance of [1H]+ obtained from treatment of [1]2+ with H2 and P(o-tol)3.

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