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. 2015 Jul 21;48(7):2107-16.
doi: 10.1021/acs.accounts.5b00177. Epub 2015 Jun 16.

Diiron Azadithiolates as Models for the [FeFe]-hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere

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Diiron Azadithiolates as Models for the [FeFe]-hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere

Thomas B Rauchfuss. Acc Chem Res. .
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Abstract

The [FeFe] hydrogenases (H2ases) catalyze the redox reaction that interconverts protons and H2. This area of biocatalysis has attracted attention because the metal-based chemistry is unusual, and the reactions have practical implications. The active site consists of a [4Fe-4S] cluster bridged to a [Fe2(μ-dithiolate)(CN)2(CO)3](z) center (z = 1- and 2-). The dithiolate cofactor is [HN(CH2S)2](2-), called the azadithiolate ([adt(H)](2-)). Although many derivatives of Fe2(SR)2(CO)6-xLx are electrocatalysts for the hydrogen evolution reaction (HER), most operate by slow nonbiomimetic pathways. Biomimetic hydrogenogenesis is thought to involve intermediates, wherein the hydride substrate is adjacent to the amine of the adt(H), being bonded to only one Fe center. Formation of terminal hydride complexes is favored when the diiron carbonyl models contain azadithiolate. Although unstable in the free state, the adt cofactor is stable once it is affixed to the Fe2 center. It can be prepared by alkylation of Fe2(SH)2(CO)6 with formaldehyde in the presence of ammonia (to give adt(H) derivatives) or amines (to give adt(R) derivatives). Weak acids protonate Fe2(adt(R))(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt(2-)) complexes Fe2(pdt)(CO)2(PR3)4 requires strong acids. The amine in the azadithiolate is a kinetically fast base, relaying protons to and from the iron, which is a kinetically slow base. The crystal structure of the doubly protonated model [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) confirms the presence of both ammonium and terminal hydrido centers, which interact through a dihydrogen bond (dppv = cis-C2H2(PPh2)2). DFT calculations indicate that this H---H interaction is sensitive to the counterions and is strengthened upon reduction of the diiron center. For the monoprotonated models, the hydride [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) exists in equilibrium with the ammonium tautomer [Fe2(Hadt(H))(CO)2(dppv)2](+). Both [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) and [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) are highly active electrocatalysts for HER. Catalysis is initiated by reduction of the diferrous center, which induces coupling of the protic ammonium center and the hydride ligand. In contrast, the propanedithiolate [(term-H)Fe2(pdt)(CO)2(dppv)2](+) is a poor electrocatalyst for HER. Oxidation of H2 has been demonstrated, starting with models for the oxidized state ("Hox"), for example, [Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Featuring a distorted Fe(II)Fe(I) center, this Hox model reacts slowly with high pressures of H2 to give [(μ-H)Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Highlighting the role of the proton relay, the propanedithiolate [Fe2(pdt)(CO)3(dppv)(PMe3)](+) is unreactive toward H2. The Hox-model + H2 reaction is accelerated in the presence of ferrocenium salts, which simulate the role of the attached [4Fe-4S] cluster. The redox-complemented complex [Fe2(adt(Bn))(CO)3(dppv)(FcP*)](n+) catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

Conflict of interest statement

Notes

The author declares no competing financial interest.

Figures

Figure 1
Figure 1
Redox potentials of diiron hydrides (red), Fe(I)Fe(I) complexes (black), and selected reference compounds (blue). Redox couples are at least quasi-reversible, recorded in CH2Cl2 or MeCN solution with quaternary ammonium salts of BF4 or BArF4 as electrolytes.
Figure 2
Figure 2
Structure of the dication [(term-H)Fe2(HadtH)-(CO)2(dppv)2]2+.
Figure 3
Figure 3
Left: Mechanisms for H2 production with strong (solid lines, characterized by DFT calculations) and weak acids (dashed lines). (Reproduced with permission from ref . Copyright 2014 American Chemical Society.) Right: Cyclic voltammograms of a 0.5 mM solution of [(term-H)Fe2(adtH)(CO)2(dppv)2]+ (0 °C, 0.125 M [Bu4N]BArF4, CH2Cl2, scan rate =1.0 V/s, glassy carbon working electrode, Pt counter electrode, Ag wire pseudo reference electrode, Fc internal standard) recorded with increasing equivalents of CF3CO2H.
Figure 4
Figure 4
Structure of [Fe2(adtBn)(CO)4(naphthalene-1,8-(PPh2)2)]+, a synthetic model for the diiron subunit in the Hox state (Ph on adt omitted).
Figure 5
Figure 5
Catalytic cycle for H2 oxidation and production by models containing the redox complement (FcP*) and proton relay (adtBn). The central species, a resting state, arises by comproportionation.
Scheme 1
Scheme 1. Active Site of [FeFe]-H2ase in Two Catalytically Significant Statesa
aThe charges of the complexes ignore the [4Fe–4S] site.
Scheme 2
Scheme 2
Abbreviations for Some Ligands Used in Synthetic Models
Scheme 3
Scheme 3
Protonation of Fe2(adtH)(CO)2(PMe3)4 and Isomerization of the Resulting Hydrido Cations
Scheme 4
Scheme 4
Routes to Diiron Azadithiolates (adtR’s) (Reproduced with permission from ref . Copyright 2015 American Chemical Society)
Scheme 5
Scheme 5
Stereodynamics of Fe2(adtH)(CO)6
Scheme 6
Scheme 6
Competition between μ-Hydride Formation and H2 Evolution in the Protonation of Fe2(adtBn)(CO)3(dppv)(FcP*)

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