Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications

Olsen, Matthew T.; Rauchfuss, Thomas B.; Wilson, Scott R.

doi:10.1021/ja103998v

Cited by 76 publications

(94 citation statements)

References 49 publications

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“…Thus, many model complexes containing an internal amine moiety have been developed. [56,57]. These results suggest that heterolytic hydrogen activation was assisted by the azadithiolate ligand.…”

Section: Diiron Complexes With An Internal Amine Moietymentioning

confidence: 77%

Hydrogen-activating models of hydrogenases

Chen

2015

Coordination Chemistry Reviews

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Section: Diiron Complexes With An Internal Amine Moietymentioning

confidence: 77%

Hydrogen-activating models of hydrogenases

Chen

2015

Coordination Chemistry Reviews

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“…19,38 The nonreactivity is understandable: loss of H 2 would generate a highly unsaturated dication, which electrochemical measurements confirm are high energy species. 44,45 In contrast to the stability of [( term -H)Fe 2 (Hadt H )(CO) 2 L 4 ] 2+ , the ammonium hydrides of nickel(II) [HNi(P 2 N 2 )(HP 2 N 2 )] 2+ eliminate H 2 spontaneously (P 2 N 2 = diaminodiphosphine). 46 Protonation of [( term -H)Fe 2 (edt)(CO) 2 (PMe 3 ) 4 ] + does give H 2 , but this reaction requires the presence of MeCN, which enables formation of [Fe 2 (edt)(CO) 2 (NCMe)(PMe 3 ) 4 ] 2+ 18.…”

Section: Hydrogen Evolution Catalyzed By Fe2(adtr)(co)2(dppv)2mentioning

confidence: 99%

“…On the other hand, oxidation of such H ox models to the diferrous state induces coordination of the amine to Fe, saturating its coordination sphere and thus ruining any chance of heterolysis. 44 …”

Section: Models For the Hox State: H2 Oxidationmentioning

confidence: 99%

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

2015

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CONSPECTUS 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 ([adtH]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 adtH, 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 adtH derivatives) or amines (to give adtR derivatives). Weak acids protonate Fe2(adtR)(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt2−) 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(HadtH)(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(adtH)(CO)2(dppv)2]+ exists in equilibrium with the ammonium tautomer [Fe2(HadtH)(CO)2(dppv)2]+. Both [(term-H)Fe2(HadtH)(CO)2(dppv)2]2+ and [(term-H)Fe2(adtH)(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(adtH)-(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(adtH)(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(adtBn)(CO)3(dppv)-(FcP*)]n+ catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

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“…1 In some cases, the [Fe-S] clusters are known to undergo proton-coupled electron transfer (PCET). 2 Biochemical and model studies are providing insight into the PCET reactivity of low-valent CO-ligated [Fe-S] clusters in hydrogenases, 3 but much less is known about the involvement of protons in the more widespread high-valent clusters. 2 The [2Fe-2S] Rieske cluster, in which one Fe is ligated by two Cys residues and the other by two His residues, is perhaps the best characterized biological [Fe-S] cluster that undergoes PCET.…”

mentioning

confidence: 99%

Protonation and Concerted Proton–Electron Transfer Reactivity of a Bis-Benzimidazolate Ligated [2Fe–2S] Model for Rieske Clusters

2012

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A model system for biological Rieske clusters that incorporates bis-benzimidazolate ligands (Prbbim)2- has been developed (PrbbimH2 = 4,4-bis(benzimidazol-2-yl)heptane). The di-ferric and mixed-valence clusters have been prepared and characterized in both their protonated and deprotonated states. The thermochemistry of interconversions of these species has been measured, and the effect of protonation on the reduction potential is in good agreement to that observed in the biological systems. The mixed-valence and protonated congener [Fe2S2(Prbbim)(PrbbimH)](Et4N)2 (4) reacts rapidly with TEMPO or p-benzoquinones to generate di-ferric and deprotonated [Fe2S2(Prbbim)2](Et4N)2 (1) and 1 equiv of TEMPOH or 0.5 equiv p-benzohydroquinones, respectively. The reaction with TEMPO is the first well-defined example of a concerted proton-electron transfer (CPET) at a synthetic ferric/ferrous [Fe-S] cluster.

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Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications

Cited by 76 publications

References 49 publications

Hydrogen-activating models of hydrogenases

Hydrogen-activating models of hydrogenases

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

Protonation and Concerted Proton–Electron Transfer Reactivity of a Bis-Benzimidazolate Ligated [2Fe–2S] Model for Rieske Clusters

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