Oxygen sensitivity of [FeFe]-hydrogenase: a comparative study of active site mimics inside vs. outside the enzyme

[FeFe]-hydrogenase is nature's most efficient proton reducing and H2-oxidizing enzyme. However, biotechnological applications are hampered by the O2 sensitivity of this metalloenzyme, and the mechanism of aerobic deactivation is not well understood. Here, we explore the oxygen sensitivity of four mimics of the organometallic active site cofactor of [FeFe]-hydrogenase, [Fe2(adt)(CO)6-x(CN)x]x- and [Fe2(pdt)(CO)6-x(CN)x]x- (x = 1, 2) as well as the corresponding cofactor variants of the enzyme by means of infrared, Mössbauer, and NMR spectroscopy. Additionally, we describe a straightforward synthetic recipe for the active site precursor complex Fe2(adt)(CO)6. Our data indicate that the aminodithiolate (adt) complex, which is the synthetic precursor of the natural active site cofactor, is most oxygen sensitive. This observation highlights the significance of proton transfer in aerobic deactivation, and supported by DFT calculations facilitates an identification of the responsible reactive oxygen species (ROS). Moreover, we show that the ligand environment of the iron ions critically influences the reactivity with O2 and ROS like superoxide and H2O2 as the oxygen sensitivity increases with the exchange of ligands from CO to CN-. The trends in aerobic deactivation observed for the model complexes are in line with the respective enzyme variants. Based on experimental and computational data, a model for the initial reaction of [FeFe]-hydrogenase with O2 is developed. Our study underscores the relevance of model systems in understanding biocatalysis and validates their potential as important tools for elucidating the chemistry of oxygen-induced deactivation of [FeFe]-hydrogenase.

Proton transfer kinetics of transition metal hydride complexes and implications for fuel-forming reactions

Proton transfer reactions involving transition metal hydride complexes are prevalent in a number of catalytic fuel-forming reactions, where the proton transfer kinetics to or from the metal center can have significant impacts on the efficiency, selectivity, and stability associated with the catalytic cycle. This review correlates the often slow proton transfer rate constants of transition metal hydride complexes to their electronic and structural descriptors and provides perspective on how to exploit these parameters to control proton transfer kinetics to and from the metal center. A toolbox of techniques for experimental determination of proton transfer rate constants is discussed, and case studies where proton transfer rate constant determination informs fuel-forming reactions are highlighted. Opportunities for extending proton transfer kinetic measurements to additional systems are presented, and the importance of synergizing the thermodynamics and kinetics of proton transfer involving transition metal hydride complexes is emphasized.

Natural Assembly of Electroactive Metallopolymers on the Electrode Surface: Enhanced Electrocatalytic Production of Hydrogen by [2Fe-2S] Metallopolymers in Neutral Water

A molecular catalyst attached to an electrode surface can offer the advantages of both homogeneous and heterogeneous catalysis. Unfortunately, some molecular catalysts constrained to a surface lose much or all of their solution performance. In contrast, we found that when a small molecule [2Fe-2S] catalyst is incorporated into metallopolymers of the form PDMAEMA-g-[2Fe-2S] (PDMAEMA = poly(2-dimethylamino)ethyl methacrylate) and adsorbed to the surface, the observed rate of hydrogen production increases to k_obs > 10⁵ s^-1 per active site with lower overpotential, increased lifetime, and tolerance to oxygen. Herein, the electrocatalytic performances of these metallopolymers with different length polymer chains are compared to reveal the factors that lead to this high performance. It was anticipated that smaller metallopolymers would have faster rates due to faster electron and proton transfers to more accessible active sites, but the experiments show that the rates of catalysis per active site are independent of the polymer size. Molecular dynamics modeling reveals that the high performance is a consequence of adsorption of these metallopolymers on the surface with natural assembly that brings the [2Fe-2S] catalytic sites into close contact with the electrode surface while maintaining exposure of the sites to protons in solution. The assembly is conducive to fast electron transfer, fast proton transfer, and a high rate of catalysis regardless of the polymer size. These results offer a guide to enhancing the performance of other electrocatalysts with incorporation into a polymer that provides an optimal interaction of the catalyst with the electrode and solution.

Ligand effects on structural, protophilic and reductive features of stannylated dinuclear iron dithiolato complexes

The synthesis and characterization of Fe₂(CO)₅(L){μ-(SCH₂)₂SnMe₂} (L = PPh₃ (2) and P(OMe)₃ (3)) derived from the parent hexacarbonyl complex Fe₂(CO)₆{μ-(SCH₂)₂}SnMe₂ (1) are reported.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

The electrostatic potential as a descriptor for the protonation propensity in automated exploration of reaction mechanisms

We discuss the possibility of exploiting local minima of the molecular electrostatic potential for locating protonation sites in molecules in a fully automated manner. We implement and apply this concept to exploring the mechanism of proton reduction catalyzed by a hydrogenase model complex [Orthaber et al., Dalton Trans., 2014, 43, 4537]. A large number of distinct structures arising already in the early stages of the hydrogen evolution mechanism demonstrates the need for reliable, automated algorithms for the thorough analysis of catalytic processes.

Catalytic H2 evolution/oxidation in [FeFe]-hydrogenase biomimetics: account from DFT on the interplay of related issues and proposed solutions

A DFT overview on selected issues regarding diiron catalysts related to [FeFe]-hydrogenase biomimetic research, with implications for both energy conversion and storage strategies.

Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(μ-dithiolate)(μ,κ1,κ2-triphos)

A series of Fe₂(triphos)(CO)₃(μ-dithiolate) complexes have been prepared and studied as models of the diiron centre in [FeFe]-hydrogenases.

Structural and Kinetic Studies of Intermediates of a Biomimetic Diiron Proton-Reduction Catalyst

One-electron reduction and subsequent protonation of a biomimetic proton-reduction catalyst [FeFe(μ-pdt)(CO)₆] (pdt = propanedithiolate), 1, were investigated by UV-vis and IR spectroscopy on a nano- to microsecond time scale. The study aimed to provide further insight into the proton-reduction cycle of this [FeFe]-hydrogenase model complex, which with its prototypical alkyldithiolate-bridged diiron core is widely employed as a molecular, precious metal-free catalyst for sustainable H₂ generation. The one-electron-reduced catalyst was obtained transiently by electron transfer from photogenerated [Ru(dmb)₃]⁺ in the absence of proton sources or in the presence of acids (dichloro- or trichloroacetic acid or tosylic acid). The reduced catalyst and its protonation product were observed in real time by UV-vis and IR spectroscopy, leading to their structural characterization and providing kinetic data on the electron and proton transfer reactions. 1 features an intact (μ²,κ²-pdt)(μ-H)Fe₂ core in the reduced, 1^-, and reduced-protonated states, 1H, in contrast to the Fe-S bond cleavage upon the reduction of [FeFe(bdt)(CO)₆], 2, with a benzenedithiolate bridge. The driving-force dependence of the rate constants for the protonation of 1^- (k_pt = 7.0 × 10⁵, 1.3 × 10⁷, and 7.0 × 10⁷ M^-1 s^-1 for the three acids used in this study) suggests a reorganization energy >1 eV and indicates that hydride complex 1H is formed by direct protonation of the Fe-Fe bond. The protonation of 1^- is sufficiently fast even with the weaker acids, which excludes a rate-limiting role in light-driven H₂ formation under typical conditions.

Synthesis, characterization, and H/D exchange of μ-hydride-containing [FeFe]-hydrogenase subsite models formed by protonation reactions of (μ-TDT)Fe2(CO)4(PMe3)2 (TDT = SCH2SCH2S) with protic acids

The first TDT ligand-containing μ-hydride models of [FeFe]-hydrogenases (2–7) have been prepared and the H/D exchange reactions of 7 with deuterium reagents such as D₂, D₂O, and DCl are studied.

Borane-protected cyanides as surrogates of H-bonded cyanides in [FeFe]-hydrogenase active site models

Triarylborane Lewis acids bind [Fe2(pdt)(CO)4(CN)2](2-) [1](2-) (pdt(2-) = 1,3-propanedithiolate) and [Fe2(adt)(CO)4(CN)2](2-) [3](2-) (adt(2-) = 1,3-azadithiolate, HN(CH2S(-))2) to give the 2:1 adducts [Fe2(xdt)(CO)4(CNBAr3)2](2-). Attempts to prepare the 1:1 adducts [1(BAr3)](2-) (Ar = Ph, C6F5) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C6F4-o-C6F5)3 (BAr(F)*3). By virtue of the N-protection by the borane, salts of [Fe2(pdt)(CO)4(CNBAr3)2](2-) sustain protonation to give hydrides that are stable (in contrast to [H1](-)). The hydrides [H1(BAr3)2](-) are 2.5-5 pKa units more acidic than the parent [H1](-). The adducts [1(BAr3)2](2-) oxidize quasi-reversibly around -0.3 V versus Fc(0/+) in contrast to ca. -0.8 V observed for the [1](2-/-) couple. A simplified synthesis of [1](2-), [3](2-), and [Fe2(pdt)(CO)5(CN)](-) ([2](-)) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.

Coordination and conformational isomers in mononuclear iron complexes with pertinence to the [FeFe] hydrogenase active site

6 Fe complexes of the type [Fe(X-bdt)(P^R₂N^Ph₂)(CO)] were prepared and the possibility to tune their electronic properties by ligand modification was demonstrated. IR spectroscopic and computational studies suggest that the compounds exist as a mixture of isomers in solution.

Hydrogen activation by biomimetic [NiFe]-hydrogenase model containing protected cyanide cofactors

Described are experiments demonstrating incorporation of cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H2ase) active site models. Complexes of the type (CO)2(CN)2Fe(pdt)Ni(dxpe) (dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B(C6F5)3 (BAr(F)3) to give the adducts (CO)2(CNBAr(F)3)2Fe(pdt)Ni(dxpe), (1(BAr(F)3)2, 2(BAr(F)3)2). Upon decarbonylation using amine oxides, these adducts react with H2 to give hydrido derivatives [(CO)(CNBAr(F)3)2Fe(H)(pdt)Ni(dxpe)](-) (dxpe = dppe, [H3(BAr(F)3)2](-); dxpe = dcpe, [H4(BAr(F)3)2](-)). Crystallographic analysis shows that Et4N[H3(BAr(F)3)2] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni-C/R). The Fe-H···Ni center is unsymmetrical with r(Fe-H) = 1.51(3) Å and r(Ni-H) = 1.71(3) Å. Both crystallographic and (19)F NMR analyses show that the CNBAr(F)3(-) ligands occupy basal and apical sites. Unlike cationic Ni-Fe hydrides, [H3(BAr(F)3)2](-) and [H4(BAr(F)3)2](-) oxidize at mild potentials, near the Fc(+/0) couple. Electrochemical measurements indicate that in the presence of base, [H3(BAr(F)3)2](-) catalyzes the oxidation of H2. NMR evidence indicates dihydrogen bonding between these anionic hydrides and R3NH(+) salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et4N[H3(BAr(F)3)2], strong acids such as HCl induce H2 release to give the chloride Et4N[(CO)(CNBAr(F)3)2Fe(Cl)(pdt)Ni(dppe)].

Terminal vs bridging hydrides of diiron dithiolates: protonation of Fe2(dithiolate)(CO)2(PMe3)4

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(4), where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C(2)-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1](+), establishing the non-intermediacy of the terminal hydride ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[μ-H1](+) indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(4)](+) ([S-H2](+)). This intermediate converts to [t-H2](+) and [μ-H2](+) by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe(3) ligands are basal.

Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a Di-iron Fe(μ-NH)(μ-H)Fe core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L(3)Fe(μ-NH)(μ-H)FeL(3) (L(3) = [PhB(CH(2)PPh(2))(3)](-)) complex, 3, is a novel S = (1)/(2) type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. (1,2)H and (14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H(-)-Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E(4)). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E(4). This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

Reaction of Aryl Diazonium Salts and Diiron(I) Dithiolato Carbonyls: Evidence for Radical Intermediates

Treatment of Fe(2)(pdt)(CO)(4)(dppv) (1) with aryldiazonium salts affords the 34e(-) adducts [Fe(2)(pdt)(μ-N(2)Ar)(CO)(4)(dppv)](+) (pdt(2-) = 1,3-propanedithiolate, dppv = cis-C(2)H(2)(PPh(2))(2)). Under some conditions, the same reaction gave substantial amounts of [1](+), the product of electron-transfer. Consistent with the influence of electron transfer in the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me(3)S(2)](+) and Fe(2)(pdt)(CO)(4)(dppbz) was found to give [Fe(2)(pdt)(CO)(4)(dppbz)](+) as well as Me(2)S and Me(2)S(2) (dppbz = 1,2-bis(diphenylphosphino)benzene).

Importance of the protein framework for catalytic activity of [FeFe]-hydrogenases

The active center (H-cluster) of [FeFe]-hydrogenases is embedded into a hydrophobic pocket within the protein. We analyzed several amino acids, located in the vicinity of this niche, by site-directed mutagenesis of the [FeFe]-hydrogenases from Clostridium pasteurianum (CpI) and Chlamydomonas reinhardtii (CrHydA1). These amino acids are highly conserved and predicted to be involved in H-cluster coordination. Characterization of two hydrogenase variants confirmed this hypothesis. The exchange of residues CrHydA1Met(415) and CrHydA1Lys(228) resulted in inactive proteins, which, according to EPR and FTIR analyses, contain no intact H-cluster. However, [FeFe]-hydrogenases in which CpIMet(353) (CrHydA1Met(223)) and CpICys(299) (CrHydA1Cys(169)) were exchanged to leucine and serine, respectively, showed a structurally intact H-cluster with catalytic activity either absent (CpIC299S) or strongly diminished (CpIM353L). In the case of CrHydA1C169S, the H-cluster was trapped in an inactive state exhibiting g values and vibrational frequencies that resembled the H(trans) state of DdH from Desulfovibrio desulfuricans. This cysteine residue, interacting with the bridge head nitrogen of the di(methyl)amine ligand, seems therefore to represent an essential contribution of the immediate protein environment to the reaction mechanism. Exchanging methionine CpIM(353) (CrHydA1M(223)) to leucine led to a strong decrease in turnover without affecting the K(m) value of the electron donor. We suggest that this methionine constitutes a "fine-tuning" element of hydrogenase activity.

Favorable Protonation of the (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging μ-H Counterpart: A Spectroscopic and DFT Study

The mechanism of hydrogen production in [FeFe] hydrogenase remains elusive. However, a species featuring a terminal hydride bound to the distal Fe is thought to be the key intermediate leading to hydrogen production. In this study, density functional theory (DFT) calculations on the terminal (H-term) and bridging (μ-H) hydride isomers of (μ-edt)-[Fe(2)(PMe(3))(4)(CO)(2)H](+) are presented in order to understand the factors affecting their propensity for protonation. Relative to H-term, μ-H is 12.7 kcal/mol more stable, which contributes to its decreased reactivity towards an acid. Potential energy surface (PES) calculations for the reaction of the H-term isomer with 4-nitropyridinium, a proton source, further reveal a lower activation energy barrier (14.5 kcal/mol) for H-term than for μ-H (29 kcal/mol). Besides these energetic considerations, the H-term isomer displays a key molecular orbital (MO <139>) that has a relatively strong hydride (1s) contribution (23%), which is not present in the μ-H isomer. This indicates a potential orbital control of the reaction of the hydride complexes with acid. The lower activation energy barrier and this key MO together control the overall catalytic activity of (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-term)](+). Lastly, Raman and IR spectroscopy were performed in order to probe the ν(Fe-H) stretching mode of the two isomers and their deuterated counterparts. A ν(Fe-H) stretching mode was observed for the μ-H complex at 1220 cm(-1). However, the corresponding mode is not observed for the less stable H-term isomer.