Hydrogenase Mimics Research Articles

Amplifying Reactivity of Bio-Inspired [FeFe]-Hydrogenase Mimics by Organic Nanotubes.

A bio-inspired FeFe hydrogenase model which catalyses hydrogen evolution reaction (HER) in acidic solutions is immobilized in polyaniline (PANI)-based nanotubes. A combination of analytical techniques reveals that this construct maintains both the molecular signatures of the bio-inspired complex and the material properties of PANI. The amine and imine-rich environment of the PANI chain amplifies the inherent HER activity of the bio-inspired complex, allowing electrocatalytic HER at neutral pH, with lower overpotentials and higher current densities compared to the bio-inspired complex alone. This construct retains the oxygen stability of the bio-inspired complex and remains stable through several hours of aerobic electrolysis, producing only 6.5 % H₂O₂ from the competing oxygen reduction reaction (ORR).

Targeted Modulation of Photocatalytic Hydrogen Evolution Activity by Nickel-Substituted Rubredoxin through Functionalized Ruthenium Phototriggers.

Light-driven hydrogen evolution is a promising means of sustainable energy production to meet global energy demand. This study investigates the photocatalytic hydrogen evolution activity of nickel-substituted rubredoxin (NiRd), an artificial hydrogenase mimic, covalently attached to a ruthenium phototrigger (RuNiRd). By systematically modifying the para-substituents on Ru(II) polypyridyl complexes, we sought to optimize the intramolecular electron transfer processes within the RuNiRd system. A series of electron-donating and electron-withdrawing groups were introduced to tune the photophysical, photochemical, and electrochemical properties of the ruthenium complexes. Our findings reveal that electron-donating substituents can increase the hydrogen evolution capabilities of the artificial enzyme to a point; however, the complexes with the most electron-donating substituents suffer from short lifetimes and inefficient reductive quenching, rendering them inactive. The present work highlights the intricate balance required between driving force, lifetime, and quenching efficiency for effective light-driven catalysis, providing valuable insights into the design of artificial enzyme-photosensitizer constructs.

Hybrid Nanoparticles: Ni and Au Decorated with [FeFe]‐Hydrogenase Mimics

Complexes [(μ‐S₂C₂H₄NHR)Fe₂(CO)₆] (R = p‐C₆H₄‐OCO(CH₂)₉Br (3a); R = p‐C₆H₄‐OCO(CH₂)₈CH₃ (3b)) were used as stabilizing agents in the synthesis of Ni@3 and Au@3 nanoparticles (NPs), which are the first reported stable metallic NPs decorated with [(μ‐S₂C₂H₄NHR)Fe₂(CO)₆] moieties. Electrochemical analysis reveals that incorporating the hydrogenase mimic into the NPs lowers the overpotential and enhances proton reduction electrocatalytic activity in organic media. The NPs act similarly to the [Fe₄S₄] cluster in natural enzymes, functioning as an electron reservoir/relay.

Molecular Dyad vs Multi‐Component Approach‐Photocatalytic Hydrogen Evolution by Combining Oligothiophene Photosensitizers with [FeFe]‐Hydrogenase Mimics

AbstractA molecular dyad (PS‐CAT) consisting of a metal‐free photosensitiser (PS) and a [FeFe]‐hydrogenase‐inspired [Fe2S2] cluster (CAT) has been synthesized for hydrogen evolution under visible light irradiation. The electrochemical properties of PS‐CAT in the absence and presence of acid were investigated by CV and IR‐SEC. The ground state and excited state photophysical properties of PS and PS‐CAT, and the non‐chemically connected reference system [PS, CAT] were investigated to elucidate the effect of a covalent linkage between PS and CAT on the electronic communication and hence on the catalytic performance. The photo‐relaxation pathway of PS‐CAT was revealed computationally by scalar‐relativistic time‐dependent density functional theory simulations. These studies are complemented by comparative analyses of photocatalytic hydrogen evolution between PS‐CAT and [PS, CAT] under visible light irradiation.

Heterobimetallic [NiCo] integration in a hydrogenase mimic for boosting light-driven hydrogen evolution in CaTiO3

Heterobimetallic [NiCo] integration in a hydrogenase mimic for boosting light-driven hydrogen evolution in CaTiO3

A functional hydrogenase mimic that catalyzes robust H2 evolution spontaneously in aqueous environment

A functional hydrogenase mimic that catalyzes robust H2 evolution spontaneously in aqueous environment

Photophysics of Fe-Fe hydrogenase mimic complexes for hydrogen evolution

The photophysical and electrochemical properties of the diiron hydrogenase mimic complex, [Fe2(dpet)(CO)6] (dpet = 1,2-diphenylethene-1,2-dithiol) have been studied using a combination of spectroscopic and electrochemical techniques including cyclic voltammetry, infrared spectroelectrochemistry (IR-SEC), ultrafast transient absorption (TA) spectroscopy and time-resolved infrared spectroscopy (TRIR), with the data interpretation aided by density functional theory (DFT). The complex is a viable catalyst for electrochemical hydrogen evolution, as shown by its electrochemical reversibility and significant increase in the catalytic current upon addition of trifluoroacetic acid (TFA). IR-SEC was used to identify the first and second reduction products, where formation of a bridging μ-CO species was observed. The ultrafast spectroscopy data show that CO loss is only a minor excited state relaxation pathway, unlike other previously reported [FeFe] catalysts, in agreement with steady-state photolysis. The excited state lifetime obtained for this complex (135 ps in DCM and 237 ps in MeCN) is insufficient for [Fe2(dpet)(CO)6] to function as a photocatalyst on its own. However, the photostability of the complex opens up an opportunity for its future use in conjunction with a photosensitiser in photochemical hydrogen evolution.

FeFe]-hydrogenase H-cluster mimics mediated by ferrocenyl hetaryl thioketone derivatives

In this report, the influence of the substitution decoration in ferrocenyl selenophen-2-yl thioketone (1) and ferrocenyl furan-2-yl thioketone (2) utilized as a proligand in complexation reactions with Fe3(CO)12 was examined. As a result, three new complexes, considered [FeFe]-hydrogenase mimics, were prepared and characterized by IR, 1H, 13C{1H}, and 77Se{1H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal diffraction methods. Moreover, the obtained complexes were compared with the analogous ferrocenyl α-thienyl thioketone as a starting material. Respectively, the reaction of compound 1 with Fe3(CO)12 is found to give two complexes that have similar structures to those obtained in analogous reactions with ferrocenyl α-thienyl thioketone. Conversely, compound 2 gives only an α,β-unsaturated aldehyde moiety as a result of the furan ring being opened. Furthermore, the electrochemical properties of the resulting complexes were investigated using cylic voltammerty in the absence and presence of acetic acid (AcOH) as a proton source and compared with their structurally related thiophene derivative. Accordingly, the nature of the linkage of these synthetic approaches has a significant effect in determining its redox properties.

Mechanistic insights into consecutive 2e− and 2H+ reactions of hydrogenase mimic

•First crystal of the bridge heteronuclear [NiMnH] hydride intermediate •The full 2e−/2H+ picture of the [NiMn] heterobinuclear mimic •Distinct, acidity-dependent active Ni or Mn center Mechanistic implication of hydrogenase and mimics in variable environments. The [NiFe] heterobinuclear hydrogenase active site in nature is responsible for a typical 2e− and 2H+ transformation with extraordinarily efficient H2 evolution. However, the difficulty in isolating the [NiFe] catalytic center and the sensitivity to air pose great challenges to unravelling the detailed 2e−/2H+ process. Herein, we design and synthesize a [NiIIMnIBr] ((dppe)NiII(μ-pdt)(μ-Br)MnI(CO)3) heterobinuclear mimic to track the reaction processes. Our results reveal that [NiIIMnIBr] is reduced to form [NiIIMnIH] ((dppe)NiII(μ-pdt)(μ-H)MnI(CO)3) in the presence of a proton (H+). IR-SEC, NMR, and X-ray crystal-structure analyses identify that the key [NiIIMnIH] intermediate interacts with a strong acid for H2 evolution via Mn ([NiIIMnII/IH]), whereas in the presence of a weak acid, further 1e− reduction at the Ni site affords [NiIMnIH]−-R species to react with H+. The full 2e−/2H+ H2 evolution picture of the heterobinuclear mimic relying on different metal centers presents a unique mechanistic understanding of hydrogenase mimics. The [NiFe] heterobinuclear hydrogenase active site in nature is responsible for a typical 2e− and 2H+ transformation with extraordinarily efficient H2 evolution. However, the difficulty in isolating the [NiFe] catalytic center and the sensitivity to air pose great challenges to unravelling the detailed 2e−/2H+ process. Herein, we design and synthesize a [NiIIMnIBr] ((dppe)NiII(μ-pdt)(μ-Br)MnI(CO)3) heterobinuclear mimic to track the reaction processes. Our results reveal that [NiIIMnIBr] is reduced to form [NiIIMnIH] ((dppe)NiII(μ-pdt)(μ-H)MnI(CO)3) in the presence of a proton (H+). IR-SEC, NMR, and X-ray crystal-structure analyses identify that the key [NiIIMnIH] intermediate interacts with a strong acid for H2 evolution via Mn ([NiIIMnII/IH]), whereas in the presence of a weak acid, further 1e− reduction at the Ni site affords [NiIMnIH]−-R species to react with H+. The full 2e−/2H+ H2 evolution picture of the heterobinuclear mimic relying on different metal centers presents a unique mechanistic understanding of hydrogenase mimics.

Mono- and di-substituted [FeFe]-hydrogenase H-cluster mimics bearing the 3,4-dimercaptobenzaldehyde bridge moiety: Insight into synthesis, characterization and electrochemical investigations

This report describes the synthesis and characterization of [FeFe]-hydrogenase mimic, namely [Fe2(CO)6{μ-bdtf}] (2, bdtf = 3,4-dimercaptobenzaldehyde) obtained from the reaction of 3,4-dimercaptobenzaldehyde with Fe3(CO)12. Moreover, treatment of complex 2 with PPh3 ligand yielded both mono- and disubstituted complexes [Fe2(CO)5PPh3{μ-bdtf}] (3), [Fe2(CO)4(PPh3)2{μ-bdtf}] (4), while in the case of P(OMe)3 ligand afforded only the disubstituted complex [Fe2(CO)4(P(OMe)3)2{μ-bdtf}] (5). The molecular structures of complexes 2–5 were confirmed by X-ray diffraction analysis. Furthermore, the influence of the substituent on the benzene ring as well as at the iron center on the electrochemistry and catalysis of these mimics toward proton reduction have been evaluated by cyclic voltammetry. All complexes show their potentials toward hydrogen formation in the presence of weak acid, such as acetic acid (AcOH).

Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties.

The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe2S2] complex containing a 5,5'-diisocyanide-2,2'-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe2S2] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe2S2] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe2S2] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe2S2] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF4 salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental-computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

NMP makes the difference - facilitated synthesis of [FeFe] hydrogenase mimics.

To overcome the limitations of high reaction temperatures and long reaction times of conventional synthesis routes towards [FeFe] hydrogenase (H2ase) mimicking complexes, we introduced a more efficient synthesis route in the presence of aprotic polar co-solvents such as N-methyl-2-pyrrolidone (NMP). Versatile (di)thiol or disulfide ligands as well as selenium and tellurium analogues were converted to their corresponding complexes. While both reaction times and temperatures were reduced significantly, yields could be increased. Intensive kinetic monitoring of the formation of two [FeFe] H2ase mimics via UV-vis spectroscopy was performed, revealing an increase of the rate constant by one order of magnitude compared to that obtained in the same reaction without NMP. IR spectroscopic examination of the formation of the 1,3-propandithiole analogue (2a) revealed the appearance of a side product, analyzed by IR and UV-vis spectroscopy and mass spectrometry, which was proposed to be a NMP monosubstituted triirondodecacarbonyl (Fe3(CO)11NMP) cluster. Reacting triirondodecacarbonyl (Fe3(CO)12) with NMP in the absence of any additional ligand yielded this species as well. Quantum chemical simulations of Fe3(CO)11NMP indicated structural rearrangements including the omission of bridging carbonyls (μ-CO). Similar observations were made on utilizing other aprotic polar co-solvents.

A Remarkable Ring Opening Observed in the Reaction of Cyclopropyl Ferrocenyl Thioketone with Triiron Dodecacarbonyl Fe3(CO)12

AbstractIn continuation of the study focused on synthesis and structure of mimics of the active site of the [FeFe] hydrogenase enzyme, reaction of Fe3(CO)12 with cyclopropyl ferrocenyl thioketone 1 was carried out. Two different complexes with ring‐opened cyclopropyl fragments were isolated and identified as η4‐1‐thia‐1,3‐diene‐type mononuclear tricarbonyl iron complex 2 and η2‐acyl‐type hexacarbonyl diiron complex 3, respectively. Structures of crystalline products were unambiguously confirmed by single crystal X‐ray analysis. For the ring opening reaction of the cyclopropane moiety, leading to the formation of 2 and 3, a multistep radical mechanism was postulated. Electrochemical investigations of 3, being reminiscent of a [FeFe] hydrogenase mimic, were carried out at different scan rates.

Two Complementary Approaches to Silicon-Supported Soluble [FeFe]-Hydrogenase Mimics

Two series of silicon-supported [(μ-SCH2)2NRFe2(CO)6] (R = p-OHC6H4, 3, and (CH2)2OH, 4a, and (CH2)5OH, 4b) hydrogenase mimics, as well as octamethylsilsesquioxane (POSS)-based nanostructures containing eight [(μ-SCH2)2NRFe2(CO)6] moieties, have been prepared either by reaction of the corresponding silyl and silyloxy chlorides and compounds 3 and 4 or by CuAAC between propargyl derivatives of silyl and silyloxy chlorides and azide 14. Cycloaddition between POSS-derived azide 23b and [(μ-SCH2)2NRFe2(CO)6] (R = alkyne) complexes 24 and 26 is efficient, leading to POSS-based nanostructures containing eight [(μ-SCH2)2NRFe2(CO)6] units. All of the complexes prepared through this work were soluble in organic solvents and hence fully characterizable by spectroscopic media. The electrochemistry of the linear siloxanes 6a and 10 is similar, with a reduction wave around −1.7 V, which is characteristic of these [(μ-SCH2)2NRFe2(CO)6] entities. Complexes 6a and 6b lacking 1,2,3-triazole moieties were electrochemically stable in the presence of AcOH, showing a strong electrocatalytic wave at −2.2 V, while complexes 16 and 18 having the 1,2,3-triazole ring decomposed but were electrocatalytically active in the wave at −2.2 V. POSS-[(μ-SCH2)2NRFe2(CO)6] derivatives 25 and 27 show a strong irreversible reduction event and are deposited in the electrode either when adding AcOH or over time (successive voltammograms). 29Si NMR shows that the integrity of the silicon cage is not affected by time or AcOH addition. These synthetic protocols and electrochemical studies will be applied in the design of silicon-supported [(μ-SCH2)2NRFe2(CO)6] mimics.

Electrochemical Behavior of Mono‐Substituted [FeFe]‐Hydrogenase H‐Cluster Mimic Mediated by Stannylated Dithiolato Ligand

AbstractIn this research work we present the synthesis and characterization of asymmetrical [FeFe]‐hydrogenase mimic, [Fe2(CO)5P(OMe)3{μ‐(SCH2)2SnPh2}] (2), obtained from replacing one of the CO ligands of the parent hexacarbonyl complex [Fe2(CO)6{μ‐(SCH2)2SnPh2}] (1) with the monodentate phosphite P(OMe)3. The molecular structure of complex 2 was confirmed by X‐ray diffraction analysis. Moreover, the electrochemistry of complexes 1 and 2 was investigated to explore the influence of the steric bulk of the Ph‐ring at the bridgehead Sn atom with the nearby P(OMe)3 ligand. In addition, both complexes show their potentials toward hydrogen formation in the presence of weak acid, such as acetic acid (AcOH), suggesting an ECEC (E=electron transfer, C=chemical process) mechanism for hydrogen production.

Fluorescent intracellular imaging of reactive oxygen species and pH levels moderated by a hydrogenase mimic in living cells

The catalytic generation of H2 in living cells provides a method for antioxidant therapy. In this study, an [FeFe]-hydrogenase mimic [Ru + Fe2S2@F127(80)] was synthesized by self-assembling polymeric pluronic F-127, catalytic [Fe2S2] sites, and photosensitizer Ru(bpy)32+. Under blue light irradiation, hydrated protons were photochemically reduced to H2, which increased the local pH in living cells (HeLa cells). The generated H2 was subsequently used as an antioxidant to decrease reactive oxygen species (ROS) levels in living cells (HEK 293T, HepG2, MCF-7, and HeLa cells). Our findings revealed that the proliferation of HEK 293T cells increased by a factor of about six times, relative to that of other cells (HepG2, MCF-7, and HeLa cells). Intracellular ROS and pH levels were then monitored using fluorescent cell imaging. Our study showed that cell imaging can be used to evaluate the ability of Ru + Fe2S2@F127 to eliminate oxidative stress and prevent ROS-related diseases.

Frontispiece: Activating a [FeFe] Hydrogenase Mimic for Hydrogen Evolution under Visible Light

Photocatalysis In their Communication (e202202079), Masaichi Saito, Stephan Kupfer, Benjamin Dietzek-Ivanšić, Wolfgang Weigand et al. utilize a precious metal-free photocatalyst consisting of a [FeFe] hydrogenase mimic for hydrogen generation under visible light.

Novel [FeFe]-Hydrogenase Mimics: Unexpected Course of the Reaction of Ferrocenyl α-Thienyl Thioketone with Fe3(CO)12.

The influence of the substitution pattern in ferrocenyl α-thienyl thioketone used as a proligand in complexation reactions with Fe3(CO)12 was investigated. As a result, two new sulfur–iron complexes, considered [FeFe]-hydrogenase mimics, were obtained and characterized by spectroscopic techniques (1H, 13C{1H} NMR, IR, MS), as well as by elemental analysis and X-ray single crystal diffraction methods. The electrochemical properties of both complexes were studied and compared using cyclic voltammetry in the absence and in presence of acetic acid as a proton source. The performed measurements demonstrated that both complexes can catalyze the reduction of protons to molecular hydrogen H2. Moreover, the obtained results showed that the presence of the ferrocene moiety at the backbone of the linker of both complexes improved the stability of the reduced species.

Noncovalent Engineering of Apoferritin with a PEGylated [FeFe] Hydrogenase Mimic for In Situ Polymerization.

Apoferritin can act as a scaffold for functionalization in the inner and outer surfaces. However, traditional covalent modification methods have a risk of disrupting the structure and physicochemical properties of apoferritin. Herein, we report a method for designing versatile apoferritin-based nanosystems through noncovalent interaction between a PEGylated [FeFe]-hydrogenase mimic (FeFe-PEG-N3) and apoferritin. FeFe-PEG-N3 can be anchored into the threefold channels of apoferritin via program injection, at a number of ∼8 per protein. We also engineered apoferritin with an FeFe-PEG-N3/ATRP initiator conjugate for in situ and noninvasive atom transfer radical polymerization (ATRP) at the apoferritin surface. This "grafting-from" method for noncovalent apoferritin engineering has the advantages of simple preparation, good controllability, and high efficiency and affords opportunities for the construction of multifunctional apoferritin-based nanosystems for broad applications such as drug delivery and catalysis.

Lewis acid protection turns cyanide containing [FeFe]-hydrogenase mimics into proton reduction catalysts.

Sustainable sources of hydrogen are a vital component of the envisioned energy transition. Understanding and mimicking the [FeFe]-hydrogenase provides a route to achieving this goal. In this study we re-visit a molecular mimic of the hydrogenase, the propyl dithiolate bridged complex [Fe2(μ-pdt)(CO)4(CN)2]2−, in which the cyanide ligands are tuned via Lewis acid interactions. This system provides a rare example of a cyanide containing [FeFe]-hydrogenase mimic capable of catalytic proton reduction, as demonstrated by cyclic voltammetry. EPR, FTIR, UV-vis and X-ray absorption spectroscopy are employed to characterize the species produced by protonation, and reduction or oxidation of the complex. The results reveal that biologically relevant iron-oxidation states can be generated, potentially including short-lived mixed valent Fe(i)Fe(ii) species. We propose that catalysis is initiated by protonation of the diiron complex and the resulting di-ferrous bridging hydride species can subsequently follow two different pathways to promote H2 gas formation depending on the applied reduction potential.