The Third Hydrogenase: A Ferracyclic Carbamoyl with Close Structural Analogy to the Active Site of Hmd

Biomimetic models of [Fe]-hydrogenase featuring a 2-acylphenylthiomethyl-6-R-pyridine (R = H or OMe) ligand

A new flexible pyridine ligand (FPL)-based method is developed, by which two novel biomimetic models of [Fe]-H2ase are prepared and their enzyme-like H2/D2 activation functions are studied.

Scaffold-based [Fe]-hydrogenase model: H2 activation initiates Fe(0)-hydride extrusion and non-biomimetic hydride transfer

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor. In comparison to other non-scaffolded acyl-containing complexes, the complex described herein retains molecularly well-defined chemistry upon addition of multiple equivalents of exogenous base. Clean deprotonation of the acyl(methylene) C–H bond with a phenolate base results in the formation of a dimeric motif that contains a new Fe–C(methine) bond resulting from coordination of the deprotonated methylene unit to an adjacent iron center. This effective second carbanion in the ligand framework was demonstrated to drive heterolytic H₂ activation across the Fe(ii) center. However, this process results in reductive elimination and liberation of the ligand to extrude a lower-valent Fe–carbonyl complex. Through a series of isotopic labelling experiments, structural characterization (XRD, XAS), and spectroscopic characterization (IR, NMR, EXAFS), a mechanistic pathway is presented for H₂/hydride-induced loss of the organometallic acyl unit (i.e. pyCH₂–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O → pyCH₃+C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O). The known reduced hydride species [HFe(CO)₄]⁻ and [HFe₃(CO)₁₁]⁻ have been observed as products by ¹H/²H NMR and IR spectroscopies, as well as independent syntheses of PNP[HFe(CO)₄]. The former species (i.e. [HFe(CO)₄]⁻) is deduced to be the actual hydride transfer agent in the hydride transfer reaction (nominally catalyzed by the title compound) to a biomimetic substrate ([^TolIm](BAr^F) = fluorinated imidazolium as hydride acceptor). This work provides mechanistic insight into the reasons for lack of functional biomimetic behavior (hydride transfer) in acyl(methylene)pyridine based mimics of [Fe]-hydrogenase.

We report the synthesis and reactivity of a model of [Fe]-hydrogenase derived from an anthracene-based scaffold that includes the endogenous, organometallic acyl(methylene) donor.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H₂ oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H₂O/H₂ binding site. In [Fe]-hydrogenase, heterolytic H₂ activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H₂ cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H₄MPT⁺, thus forming methylene-H₄MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]-Hydrogenase

[Fe]-hydrogenase is an efficient biological hydrogenation catalyst. Despite intense research, Fe complexes mimicking the active site of [Fe]-hydrogenase have not achieved turnovers in hydrogenation reactions. Herein, we describe the design and development of a manganese(I) mimic of [Fe]-hydrogenase. This complex exhibits the highest activity and broadest scope in catalytic hydrogenation among known mimics. Thanks to its biomimetic nature, the complex exhibits unique activity in the hydrogenation of compounds analogous to methenyl-H₄ MPT⁺ , the natural substrate of [Fe]-hydrogenase. This activity enables asymmetric relay hydrogenation of benzoxazinones and benzoxazines, involving the hydrogenation of a chiral hydride transfer agent using our catalyst coupled to Lewis acid-catalyzed hydride transfer from this agent to the substrates.

Methanogenesis involves direct hydride transfer from H2 to an organic substrate

Certain anaerobic microorganisms evolved a mechanism to use H₂ as a reductant in their energy metabolisms. For these purposes, the microorganisms developed H₂-activating enzymes, which are aspirational catalysts in a sustainable hydrogen economy. In the case of the hydrogenotrophic pathway performed by methanogenic archaea, 8e^- are extracted from 4H₂ and used as reducing equivalents to convert CO₂ into CH₄. Under standard cultivation conditions, these archaea express [NiFe]-hydrogenases, which are Ni-dependent and Fe-dependent enzymes and heterolytically cleave H₂ into 2H⁺ and 2e^-, the latter being supplied into the central metabolism. Under Ni-limiting conditions, F₄₂₀-reducing [NiFe]-hydrogenases are downregulated and their functions are predominantly taken over by an upregulated [Fe]-hydrogenase. Unique in biology, this Fe-dependent hydrogenase cleaves H₂ and directly transfers H^- to an imidazolium-containing substrate. [Fe]-hydrogenase activates H₂ at an Fe cofactor ligated by two CO molecules, an acyl group, a pyridinol N atom and a cysteine thiolate as the central constituent. This Fe centre has inspired chemists to not only design synthetic mimics to catalytically cleave H₂ in solution but also for incorporation into apo-[Fe]-hydrogenase to give semi-synthetic proteins. This Perspective describes the enzymes involved in hydrogenotrophic methanogenesis, with a focus on those performing the reduction steps. Of these, we describe [Fe]-hydrogenases in detail and cover recent progress in their synthetic modelling.

Ferracyclic carbonyl complexes as anti-inflammatory agents

Reaction of Fe(CO)₄Br₂ with 2-aminopyridine and 2-aminonapthalene yields ferracyclic iron(ii) complexes bearing two CO ligands. These release CO in the light, but suppress inflammation only in the dark.

CNS and CNP Iron(II) Mono-Iron Hydrogenase (Hmd) Mimics: Role of Deprotonated Methylene(acyl) and the trans-Acyl Site in H2 Heterolysis

We report syntheses and H₂ activation involving model complexes of mono-iron hydrogenase (Hmd) derived from acyl-containing pincer ligand precursors bearing thioether (CNS^Pre) or phosphine (CNP^Pre) donor sets. Both complexes feature pseudo-octahedral iron(II) dicarbonyl units. While the CNS pincer adopts the expected mer-CNS (pincer) geometry, the CNP ligand unexpectedly adopts the fac-CNP coordination geometry. Both complexes exhibit surprisingly acidic methylene C-H bond (reversibly de/protonated by a bulky phenolate), which affords a putative dearomatized pyridinate-bound intermediate. Such base treatment of Fe-CNS also results in deligation of the thioether sulfur donor, generating an open coordination site trans from the acyl unit. In contrast, Fe-CNP maintains a CO ligand trans from the acyl site both in the parent and dearomatized complexes (the -PPh₂ donor is cis to acyl). The dearomatized mer-Fe-CNS was competent for H₂ activation (5 atm D_2(g) plus phenolate as base), which is attributed to both the basic site on the ligand framework and the open coordination site trans to the acyl donor. In contrast, the dearomatized fac-Fe-CNP was not competent for H₂ activation, which is ascribed to the blocked coordination site trans from acyl (occupied by CO ligand). These results highlight the importance of both (i) the open coordination site trans to the organometallic acyl donor and (ii) a pendant base in the enzyme active site.

Embedding a Ruthenium-Based Structural Mimic of the [Fe]-Hydrogenase Cofactor into Papain

We describe the synthesis of the ruthenacyclic carbamoyl complexes [Ru(2-NHC(O)C₅H₃NMe)(CO)₂( o,o-Me₂-C₆H₃S)(L)] (L = H₂O or MeCN), which have a labile water or acetonitrile ligand at their sixth coordination sites. Steric bulk around the ruthenium center is essential in preventing isomerization and dimerization, and embedding within papain can be achieved via coordination of its sole free cysteine residue. The observed chemistry parallels that of the natural [Fe]-hydrogenase.

Effects of Thiolate Ligation in Monoiron Hydrogenase (Hmd): Stability of the {Fe(CO)2}2+ Core with NNS Ligands

In this work, we report the effects of NNS-thiolate ligands and nuclearity (monomer, dimer) on the stability of iron complexes related to the active site of monoiron hydrogenase (Hmd). A thermally stable iron(II) dicarbonyl motif is the core feature of the active site, but the coordination features that lead to this property have not been independently evaluated for their contributions to the {Fe(CO)₂}²⁺ stability. As such, non-bulky and bulky benzothiazoline ligands (thiolate precursors) were synthesized and their iron(II) complexes characterized. The use of non-bulky thiolate ligands and low-temperature crystallizations result in isolation of the dimeric species [(NNS)₂Fe₂(CO)₂(I)₂] (1), [(N^PhNS)₂Fe₂(CO)₂(I)₂] (2), and [(^MeNNS)₂Fe₂(CO)₂(I)₂] (3), which exhibit dimerization via thiolato (μ₂-S)₂ bridges. In one particular case (unsubstituted NNS ligand), the pathway of decarbonylation and oxidation from 1 was crystallographically elucidated, via isolation of the half-bis-ligated monocarbonyl dimer [(NNS)₃Fe₂(CO)]I (4) and the fully decarbonylated and oxidized mononuclear [(NNS)₂Fe]I (5). The transformations of dicarbonyl complexes (1, 2, and 3) to monocarbonyl complexes (4, 6, and 7) were monitored by UV/vis, demonstrating that 1 and 3 exhibit longer t_1/2 (80 and 75 min, respectively) than 2 (30 min), which is attributed to distortion of the ligand backbone. Density functional theory calculations of isolated complexes and putative intermediates were used to corroborate the experimentally observed IR spectra. Finally, dimerization was prevented using a bulky ligand featuring a 2,6-dimethylphenyl substituent, which affords mononuclear iron dicarbonyl complex, [(N^PhNS^DMPh)Fe(CO)₂Br] (8), identified by IR and NMR spectroscopies. The dicarbonyl complex decomposes to the decarbonylated [(N^PhNS^DMPh)₂Fe] (9) within minutes at room temperature. Overall, the work herein demonstrates that the thiolate moiety does not impart thermal stability to the {Fe(CO)₂}²⁺ unit formed in the active site, further indicating the importance of the organometallic Fe-C(acyl) bond in the enzyme.

Structural Mimics of the [Fe]-Hydrogenase: A Complete Set for Group VIII Metals

A set of structural mimics of the [Fe]-hydrogenase active site comprising all the group VIII metals, viz., [M(2-NHC(O)C₅H₄N)(CO)₂(2-S-C₅H₄N)], has been synthesized. They exist as a mixture of isomers in solution, and the relative stability of the isomers depends on the nature of the metal and the substituent at the 6-position of the pyridine ligand.

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Reversible Hydride Transfer to N,N'-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H₂ to N⁵,N¹⁰-methenyltetrahydromethanopterin (CH-H₄MPT⁺). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N'-diphenylimidazolinium cation (1⁺) as a structurally related alternative to CH-H₄MPT⁺. In connection with the enzymatic mechanism via heterolytic cleavage of H₂ at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF₄] to N,N'-diphenylimidazolidine (2) was achieved under 1 atm H₂ at ambient temperature in the presence of an equimolar amount of NEt₃ as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1⁺. The reverse reaction was accomplished by treatment of 2 with HNMe₂Ph⁺ as the proton source, where [(η⁵-C₅Me₅)Ir{(p-MeC₆H₄SO₂)NCHPhCHPhNH}] was found to catalyze the formation of 1⁺ and H₂ with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1⁺/H₂ and 2/H⁺ was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1⁺ and 2 bearing one para-substituted N-phenyl group.

Substitution reactions of iron(ii) carbamoyl-thioether complexes related to mono-iron hydrogenase

A C,N,S pincer complex has been synthesized for structural modeling of the organometallic active site of mono-[Fe] hydrogenase (HMD).

High-spin iron(II) complexes with mono-phosphorylated 2,6-diaminopyridine ligands

Abstract

Several new monophosphorylated 2,6-diaminopyridine ligands bearing PiPr₂ and PtBu₂ units (PN^NH2-iPr, PN^NH2-tBu, PN^NHMe-iPr, and PN^NHEt-iPr) are prepared by treatment of the respective 2,6-diaminopyridines with the chlorophosphines PiPr₂Cl and PtBu₂Cl in the presence of a base. Treatment of anhydrous FeCl₂ with 1 equiv of these afforded the tetracoordinated coordinatively unsaturated 14e⁻ complexes [Fe(κ²P,N-PN^NH2-iPr)Cl₂] and [Fe(κ²P,N-PN^NH2-tBu)Cl₂], while with PN^NHMe-iPr and PN^NHEt-iPr a phosphine transfer reaction of a second PN ligand took place to yield the known PNP pincer complexes [Fe(κ³P,N,P-PNP^Me-iPr)Cl₂] and [Fe(κ³P,N,P-PNP^Et-iPr)Cl₂]. The four-coordinate complexes [Fe(κ²P,N-PN^NH2-iPr)Cl₂] and [Fe(κ²P,N-PN^NH2-tBu)Cl₂] did not react with CO and the formation of iron PNC pincer complexes was not observed. The reason for the reluctance to add CO was investigated in detail by DFT calculations.

Graphical abstract

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2 -Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2-Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

A Functional Model of [Fe]-Hydrogenase

[Fe]-Hydrogenase catalyzes the hydrogenation of a biological substrate via the heterolytic splitting of molecular hydrogen. While many synthetic models of [Fe]-hydrogenase have been prepared, none yet are capable of activating H2 on their own. Here, we report the first Fe-based functional mimic of the active site of [Fe]-hydrogenase, which was developed based on a mechanistic understanding. The activity of this iron model complex is enabled by its unique ligand environment, consisting of biomimetic pyridinylacyl and carbonyl ligands, as well as a bioinspired diphosphine ligand with a pendant amine moiety. The model complex activates H2 and mediates hydrogenation of an aldehyde.

Synthetic and Structural Studies of 2-Acylmethyl-6-R-Difunctionalized Pyridine Ligand-Containing Iron Complexes Related to [Fe]-Hydrogenase

As active site models of [Fe]-hydrogenase, tridentate 2-acylmethyl-6-methoxymethoxy-difunctionalized pyridine-containing complexes η(3)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(L1) (4, L1 = I; 5, SCN; 6, PhCS2) were prepared via the following multistep reactions: (i) etherification of 2-MeO2C-6-HOC5H3N with ClCH2OMe to give 2-MeO2C-6-MeOCH2OC5H3N (1), (ii) reduction of 1 with NaBH4 to give 2-HOCH2-6-MeOCH2OC5H3N (2), (iii) esterification of 2 with 4-toluenesulfonyl chloride to give 2-TsOCH2-6-MeOCH2OC5H3N (3), (iv) nucleophilic substitution of 3 with Na2Fe(CO)4 followed by treatment of the resulting Fe(0) intermediate Na[(2-CH2-6-MeOCH2OC5H3N)Fe(CO)4] (M1) with I2 to give complex 4, and (v) condensation of 4 with KSCN and PhCS2K to give complexes 5 and 6, respectively. In contrast to the preparation of complexes 4-6, bidentate 2-acylmethyl-6-methoxymethoxy-difunctionalized pyridine-containing model complexes η(2)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(I)(L2) (7, L2 = PPh3; 8, Cy-C6H11NC) and η(2)-(2-COCH2-6-MeOCH2OC5H3N)Fe(CO)2(L3) (9, L3 = 2-SC5H4N; 10, 8-SC9H6N) were prepared by ligand exchange reactions of 4 with PPh3, Cy-C6H11NC, 2-KSC5H4N, and 8-KSC9H6N, respectively. Particularly interesting is that the tridentate 2,6-bis(acylmethyl)pyridine- and 2-acylmethyl-6-arylthiomethylpyridine-containing model complexes η(3)-[2,6-(COCH2)2C5H3N]Fe(CO)2(L4) (11, L4 = PPh3; 12, CO) and η(3)-2-(COCH2-6-ArSCH2C5H3N)Fe(CO)2(ArS) (13, ArS = PhS; 14, 2-S-5-MeC4H2O) were obtained, unexpectedly, when 2,6-(TsOCH2)2C5H3N reacted with Na2Fe(CO)4 followed by treatment of the resulting mixture with ligands PPh3 and CO or disulfides (PhS)2 and (2-S-5-MeC4H2O)2. Reactions of ligand precursors 3 and 2,6-(TsOCH2)2C5H3N with Na2Fe(CO)4 were monitored by in situ IR spectroscopy, and the possible pathways for producing complexes 4 and 11-14 via intermediates Na[(2-CH2-6-MeOCH2OC5H3N)Fe(CO)4] (M1), Na[(2-CH2-6-TsOCH2C5H3N)Fe(CO)4] (M2), and (2-COCH2-6-CH2C5H3N)Fe(CO)3 (M3) are suggested. New compounds 1-14 were characterized by elemental analysis, spectroscopy, and, for some of them, X-ray crystallography.

Synthesis, structural characterization, and chemical properties of pentacoordinate model complexes for the active site of [Fe]-hydrogenase

Four pentacoordinate iron dicarbonyl with bulky NHC ligands were synthesised as model of [Fe]-hydrogenase active site, which exhibited different protonation reactivity due to the variable electronic and steric effects of introduced ligands.

A novel iron complex for highly efficient catalytic hydrogen generation from the hydrolysis of organosilanes

Hydrolytic oxidation of organosilanes based on an iron catalyst is described for the first time. The novel iron complex, [Fe(C6H5N2O)(CO)(MeCN)3][PF6], exhibits excellent mediating power in the catalytic hydrolysis of organosilanes to produce dihydrogen and organosilanols with turnover numbers approaching 10(4) and turnover frequencies in excess of 10(2) min(-1) under ambient conditions.

Toward functional type III [Fe]-hydrogenase biomimics for H2 activation: insights from computation

The chemistry of [Fe]-hydrogenase has attracted significant interest due to its ability to activate molecular hydrogen. The intriguing properties of this enzyme have prompted the synthesis of numerous small molecule mimics aimed at activating H2. Despite considerable effort, a majority of these compounds remain nonfunctional for hydrogenation reactions. By using a recently synthesized model as an entry point, seven biomimetic complexes have been examined through DFT computations to probe the influence of ligand environment on the ability of a mimic to bind and split H2. One mimic, featuring a bidentate diphosphine group incorporating an internal nitrogen base, was found to have particularly attractive energetics, prompting a study of the role played by the proton/hydride acceptor necessary to complete the catalytic cycle. Computations revealed an experimentally accessible energetic pathway involving a benzaldehyde proton/hydride acceptor and the most promising catalyst.