Combining acid-base, redox and substrate binding functionalities to give a complete model for the [FeFe]-hydrogenase

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.

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. 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. 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. Infrared spectra probing the localization of the two one-electron oxidations of the reduced model [1]

a–d, Titration of ${\text{FcBAr}}_4^{\mathrm{F}}$ into a CH₂Cl₂ solution of 1 monitored by solution IR spectroscopy: [1] (a), [1] + 1 equiv. ${\text{FcBAr}}_4^{\mathrm{F}}$ (b), [1] + 1.5 equiv. ${\text{FcBAr}}_4^{\mathrm{F}}$ (c), [1] + 2 equiv. ${\text{FcBAr}}_4^{\mathrm{F}}$ (d). On conversion of [1]⁺ to [1]²⁺, an isosbestic point is observed at 1,969 cm⁻¹.

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 ν_CO 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.