Iron carbonyl compounds with aromatic dithiolate bridges as organometallic mimics of [FeFe] hydrogenases

Nickel-Doped Decatungstate as a Robust Photocatalyst for Violet Light-Triggered Redox Coupling Conversion of Alcohol and Water to Aldehyde/Ketone and Hydrogen

The effective coupling of photoinduced alcohol oxidation and water reduction may economically produce hydrogen (H2) from water, which is of great significance in solving the current energy crisis. This study discloses that decatungstate (DT) and especially Ni2+ions-doped DTs are active for the photoreaction of benzyl alcohol with H2O, and under 48 h of violet light illumination, the best 1%Ni-DT yields ca. 86.1% benzoic acid and a 4.65 h-1 H2 generation efficiency (turnover frequency, TOF). Also, 1%Ni-DT is efficient for the photoredox coupling reaction of aliphatic and especially aromatic primary/secondary alcohols with water. A series of characterizations support that the doubled-reduced H2DT produced from the photoreaction plays a key role in water reduction to H2, which is accelerated by the doped Ni2+. In particular, it and the derived Ni3+ may construct a Z-type catalyst for water overall splitting, thereby hoisting the acid yield and H2 amount in the later stage of the photoreaction.

FETPY: a Diiron(I) Thio-Carbyne Complex with Prominent Anticancer Activity In Vitro and In Vivo

FETPY, an organo-diiron(I) complex, showed strong cytotoxicity across a panel of human and mouse cancer cell lines, combined with an outstanding selectivity compared to nonmalignant cells. Enhanced iron uptake in aggressive, low-differentiated cell lines, caused membrane lipid peroxidation, which resulted in ferroptosis in human ovarian cancer cells. FETPY induced significant morphological changes in murine B16-F1 and B16-F10 melanoma cells, leading to senescence and/or trans-differentiation into Schwann-like cells, thus significantly reducing their tumorigenic potential. Additionally, FETPY substantially suppressed tumor growth in low- and high-grade syngeneic melanoma models when administered in a therapeutic regimen. FETPY is featured by satisfactory water solubility (millimolar range), an amphiphilic character (Log Pow = -0.17), and excellent stability in a biological medium (DMEM). These important requisites for drug development are rarely met in iron complexes investigated so far as possible anticancer agents. Overall, FETPY holds promise as a safe and potent targeted antitumor agent.

Biomimetic Approach toward Visible Light-Driven Hydrogen Generation Based on a Porphyrin-Based Coordination Polymer Gel

There has been a widespread interest in developing self-assembled porphyrin nanostructures to mimic nature's light-harvesting processes. Herein, porphyrin-based coordination polymer gel (CPG) has been developed as a "soft" photocatalyst material for hydrogen (H₂) production from water under visible light. The CPG offers a hierarchical nanofibrous network structure obtained through self-assembly of a terpyridine alkyl-amide appended porphyrin (TPY-POR)-based low molecular weight gelator with ruthenium ions (Ru^II) and produces H₂ with a rate of 5.7 mmol g^-1 h^-1 in the presence of triethylamine (TEA) as a sacrificial electron donor. Further, the [Fe₂(bdt)(CO)₆] (dbt = 1,2-benzenedithiol) cocatalyst, which can mimic the activity of iron hydrogenase, is coassembled in the CPG and shows remarkable improvement in H₂ evolution (catalytic activity; rate ∼10.6 mmol g^-1 h^-1 and turnover number ∼1287). The significant enhancement in catalytic activity was supported by several controlled experiments, including femtosecond transient absorption (TA) spectroscopy and also DFT calculation. The TA study supported the cascade electron transfer process from porphyrin core to [Ru(TPY)₂]²⁺ center, and subsequently, the electron transfers to the cocatalyst [Fe₂(bdt)(CO)₆] for H₂ production.

Enzymatic and Bioinspired Systems for Hydrogen Production

The extraordinary potential of hydrogen as a clean and sustainable fuel has sparked the interest of the scientific community to find environmentally friendly methods for its production. Biological catalysts are the most attractive solution, as they usually operate under mild conditions and do not produce carbon-containing byproducts. Hydrogenases promote reversible proton reduction to hydrogen in a variety of anoxic bacteria and algae, displaying unparallel catalytic performances. Attempts to use these sophisticated enzymes in scalable hydrogen production have been hampered by limitations associated with their production and stability. Inspired by nature, significant efforts have been made in the development of artificial systems able to promote the hydrogen evolution reaction, via either electrochemical or light-driven catalysis. Starting from small-molecule coordination compounds, peptide- and protein-based architectures have been constructed around the catalytic center with the aim of reproducing hydrogenase function into robust, efficient, and cost-effective catalysts. In this review, we first provide an overview of the structural and functional properties of hydrogenases, along with their integration in devices for hydrogen and energy production. Then, we describe the most recent advances in the development of homogeneous hydrogen evolution catalysts envisioned to mimic hydrogenases.

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.

Adding Diversity to Diiron Aminocarbyne Complexes with Amine Ligands

The reactions of the diiron aminocarbyne complexes [Fe2Cp2(NCMe)(CO)(μ-CO){μ-CN(Me)(R)}]CF3SO3 (R = Me, 1aNCMe; R = Cy, 1bNCMe), freshly prepared from the tricarbonyl precursors 1a–b, with primary amines containing an additional function (i.e., alcohol or ether) proceeded with the replacement of the labile acetonitrile ligand and formation of [Fe2Cp2(NH2CH2CH2OR’)(CO)(μ-CO){μ-CN(Me)(R)}]CF3SO3 (R = Me, R’ = H, 2a; R = Cy, R’ = H, 2b; R = Cy, R’ = Me, 2c) in 81–95% yields. The diiron-oxazolidinone conjugate [Fe2Cp2(NH2OX)(CO)(μ-CO){μ-CN(Me)2}]CF3SO3, 3, was prepared from 1a, 3-(2-aminoethyl)-5-phenyloxazolidin-2-one (NH2OX) and Me3NO, and finally isolated in 96% yield. In contrast, the one pot reactions of 1a-b with NHEt2 in the presence of Me3NO gave the unstable [Fe2Cp2(NHEt2)(CO)(μ-CO){μ-CN(Me)(R)}]CF3SO3 (R = Me, 4a; R = Cy, 4b) as unclean products. All diiron complexes were characterized by analytical and spectroscopic techniques; moreover, the behavior of 2a–c and 3 in aqueous media was ascertained.

Hydrogenase and Nitrogenase: Key Catalysts in Biohydrogen Production

Hydrogen with high energy content is considered to be a promising alternative clean energy source. Biohydrogen production through microbes provides a renewable and immense hydrogen supply by utilizing raw materials such as inexhaustible natural sunlight, water, and even organic waste, which is supposed to solve the two problems of "energy supply and environment protection" at the same time. Hydrogenases and nitrogenases are two classes of key enzymes involved in biohydrogen production and can be applied under different biological conditions. Both the research on enzymatic catalytic mechanisms and the innovations of enzymatic techniques are important and necessary for the application of biohydrogen production. In this review, we introduce the enzymatic structures related to biohydrogen production, summarize recent enzymatic and genetic engineering works to enhance hydrogen production, and describe the chemical efforts of novel synthetic artificial enzymes inspired by the two biocatalysts. Continual studies on the two types of enzymes in the future will further improve the efficiency of biohydrogen production and contribute to the economic feasibility of biohydrogen as an energy source.

The Sulfur Rich Fluorothiophosphate Dianions [S5 P2 F2 ]2- and [S3 PF]2- : Cluster and Chelation Control of P-S Heterolysis

The sulfur rich difluoropentathiodiphosphate dianion [S₅ P₂ F₂ ]^2- , from fluoride addition to P₄ S₁₀ , has a somewhat checkered history and proves to be the main product of the reaction in acetonitrile. Its optimized synthesis, and structural characterization, as either a tetraphenylphosphonium or a tetrapropylammonium salt, [Nⁿ Pr₄ ]₂ [S₅ P₂ F₂ ] allows for the first coordination chemistry for this dianion. Reactions of [S₅ P₂ F₂ ]^2- with d¹⁰ metal ions of zinc(II), and cadmium(II), and d⁹ copper(II) resulted in a surprising diverse array of binding modes and structural motifs. In addition to the simple bis-chelate coordination of [S₅ P₂ F₂ ]^2- with zinc, cleavage of the P-S bond resulted in complexes with the unusual [S₃ PF]^2- fluorotrithiophosphate dianion. This was observed in two cluster complexes: a trinuclear cadmium complex with mixed [S₅ P₂ F₂ ]^2- /[S₃ PF]^2- ligands, [Cd₃ (S₅ P₂ F₂ )₃ (S₃ PF)₂ ]^4- as well as an octanuclear copper cluster, [Cu₈ (S₃ PF)₆ ]^4- which form rapidly at room temperature. These new metal/sulfur/ligand clusters are of relevance to understanding multimetal binding to metallothionines, and to potential capping strategies for the condensed nanoparticulate cadmium chalcogenide semiconductors CdS and CdSe.

[Fe2S2-Agx]-Hydrogenase Active-Site-Containing Coordination Polymers and Their Photocatalytic H2 Evolution Reaction Properties

Three [Fe₂S₂-Ag_x]-hydrogenase active-site-containing coordination polymers (CPs), {[Fe₂S₂-Ag₁](4-cpmt)₂(CO)₆(ClO₄^-)}_n (1), {[Fe₂S₂-Ag₂](4-cpmt)₂(CO)₆(OTf^-)₂(benzene)}_n (2), and {[Fe₂S₂-Ag₂](3-cpmt)₂(CO)₆(ClO₄^-)₂}_n (3), were obtained by a direct synthesis method from ligands [FeFe](4-cpmt)₂(CO)₆ [L1; 4-cpmt = (4-cyanophenyl)methanethiolate] and [FeFe](3-cpmt)₂(CO)₆ [L2; 3-cpmt = (3-cyanophenyl)methanethiolate] with silver salts. 1-3 represent the first examples of [FeFe]-hydrogenase-based CPs. It was worth noting that the Ag-S bonding between the Ag centers and S atoms of a [Fe₂S₂] cluster produced a novel [Fe₂S₂-Ag_x] (x = 1 or 2) catalytic site in all three polymers. The results of photochemical H₂ generation experiments indicated that 2 and 3 containing [Fe₂S₂-Ag₂] active sites showed obviously improved catalytic performances compared with ligands L1 and L2 and [Fe₂S₂-Ag₁]-containing 1. This work provides a pioneering strategy for the direct synthesis of [Fe₂S₂]-based CPs or metal-organic frameworks.

A novel preparation process of straw-based iron material for enhanced persulfate activation of reactive black 5 degradation

In this study, a new straw-iron composite material (ST@Fe) was synthesized through impregnation and freeze-drying process for persulfate (PS) activation to degrade reactive black 5 (RB5). Scanning electron microscope, Brunauer-Emmett-Teller, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy confirmed that straw owns huge pore structure and varieties of organic functional groups, including hydroxyl carboxyl groups, which could effectively adsorb and complex iron ions. The interaction between the active iron particles in ST@Fe and straw generated Fe²⁺ for PS activation, effectively degrading over 94.80% of RB5 at an initial concentration of 20 ppm in 100 min with a specific degradation capacity of 18.97 min^-1 per unit of iron ions. ST@Fe/PS system demonstrated high tolerance in a wide initial pH range, which could gradually attack the RB5 molecular structure and significantly reduce the mineralization of water. Quenching experiments and electron paramagnetic resonance demonstrated the efficient generation of ROS including sulfate radicals, hydroxyl radicals, and singlet oxygen, and confirmed the dominance of sulfate radicals in the degradation process. The continuous degradation capacity and reusability of ST@Fe were also evaluated, which proved that the contaminant could be effectively degraded even after multiple cycles in the simulated textile wastewater, indicating its potential for use in practical remediation. This work provided a new method for the preparation of modified functional materials for the degradation of organic pollutants in textile wastewater and posed a novel strategy for the utilization of waste biomass.

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.

Recent developments in electrochemical investigations into iron carbonyl complexes relevant to the iron centres of hydrogenases

In this brief review mainly based on our own work, we summarised the electrochemical investigations into those iron carbonyl complexes relevant to the iron centres of [FeFe]-and [Fe]-hydrogenases in the following aspects: (i) electron transfer (E) coupled with a chemical reaction (C), EC process, (ii) two-electron process with potential inversion (EC_isoE), and (iii) proton-coupled electron transfer (PCET) and the role of an internal base group in the non-coordination sphere. Through individual examples, these processes involved in the electrochemistry of the iron carbonyl complexes are discussed. In probing the complexes involving a two-electron process with potential inversion, the co-existence of one- and two-electron for a complex is demonstrated by incorporating intramolecularly a ferrocenyl group(s) into the complex. Our studies on proton reduction catalysed by three diiron complexes involving the PCET mechanism are also summarised. Finally, perspectives of the electrochemical study in iron carbonyl complexes inspired by the iron-containing enzymes are mentioned in the sense of developing mimics of low overpotentials for hydrogen evolution through exploiting the PCET effect.

Ferro- vs. antiferromagnetic exchange between two Ni(II) ions in a series of Schiff base heterometallic complexes: what makes the difference?

Three new NiII/ZnII heterometallics, [NiZnL'2(OMe)Cl]2 (1), [NiZnL''(Dea)Cl]2·2DMF (2) and [Ni2(H3L''')2(o-Van)(MeOH)2]Cl·[ZnCl2(H4L''')(MeOH)]·2MeOH (3), containing three-dentate Schiff bases as well as methanol or diethanolamine (H2Dea) or o-vanillin (o-VanH), all deprotonated, as bridging ligands were synthesized and structurally characterized. The Schiff base ligands were produced in situ from o-VanH and CH3NH2 (HL'), or NH2OH (HL"), or 2-amino-2-hydroxymethyl-propane-1,3-diol (H4L'''); a zerovalent metal (Ni and Zn in 1, Zn only in 2 and 3) was employed as a source of metal ions. The first two complexes are dimers with a Ni2Zn2O6 central core, while the third compound is a novel heterometallic cocrystal salt solvate built of a neutral zwitterionic ZnII Schiff base complex and of ionic salt containing dinuclear NiII complex cations. The crystal structures contain either centrosymmetric (1 and 2) or non-symmetric di-nickel fragment (3) with NiNi distances in the range 3.146-3.33 Å. The exchange coupling is antiferromagnetic for 1, J = 7.7 cm-1, and ferromagnetic for 2, J = -6.5 cm-1 (using the exchange Hamiltonian in a form Ĥ = Jŝ1ŝ2). The exchange interactions in 1 and 2 are comparable to the zero-field splitting (ZFS). High-field EPR revealed moderate magnetic anisotropy of opposite signs: D = 2.27 cm-1, E = 0.243 cm-1 (1) and D = -4.491 cm-1, E = -0.684 cm-1 (2). Compound 3 stands alone with very weak ferromagnetism (J = -0.6 cm-1) and much stronger magnetic anisotropy with D = -11.398 cm-1 and E = -1.151 cm-1. Attempts to calculate theoretically the exchange coupling (using the DFT "broken symmetry" method) and ZFS parameters (with the ab initio CASSCF method) were successful in predicting the trends of J and D among the three complexes, while the quantitative results were less good for 1 and 3.

Switching Site Reactivity in Hydrogenase Model Systems by Introducing a Pendant Amine Ligand

Hydrogenases are versatile enzymatic catalysts with an unmet hydrogen evolution reactivity (HER) from synthetic bio-inspired systems. The binuclear active site only has one-site reactivity of the distal Fed atom. Here, binuclear complexes [Fe2(CO)5(μ-Mebdt)(P(4-C6H4OCH3)3)] 1 and [Fe2(CO)5(μ-Mebdt)(PPh2Py)] 2 are presented, which show electrocatalytic activity in the presence of weak acids as a proton source for the HER. Despite almost identical structural and spectroscopic properties (bond distances and angles from single-crystal X-ray; IR, UV/vis, and NMR), introduction of a nitrogen base atom in the phosphine ligand in 2 markedly changes site reactivity. The bridging benzenedithiolate ligand Mebdt interacts with the terminal ligand's phenyl aromatic rings and stabilizes the reduced states of the catalysts. Although 1 with monodentate phosphine terminal ligands only shows a distal iron atom HER activity by a sequence of electrochemical and protonation steps, the lone pair of pyridine nitrogen in 2 acts as the primary site of protonation. This swaps the iron atom catalytic activity toward the proximal iron for complex 2. Density-functional theory (DFT) calculations reveal the role of terminal phosphines ligands without/with pendant amines by directing the proton transfer steps. The reactivity of 1 is a thiol-based protonation of a dangling bond in 1- and distal iron hydride mechanism, which may follow either an ECEC or EECC sequence, depending on the choice of acid. The pendant amine in 2 enables a terminal ligand protonation and an ECEC reactivity. The introduction of a terminal nitrogen atom enables the control of site reactivity in a binuclear system.

Phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6−n(PPh3)n (R = Ph, Me, H and n = 1, 2) featuring desymmetrized oxadithiolate bridges: structures, protonation, and electrocatalysis

The influence of desymmetrized dithiolates (Rodt) and phosphine coordination modes (PPh3) on the structural, protophilic, and electrocatalytic features of diiron complexes 4–6 and 7–9 is described.

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.

Diiron azadithiolate clusters supported on carbon nanotubes for efficient electrocatalytic proton reduction

The first example of diiron azadithiolate clusters supported on carbon nanotubes (1-f-SWCNTs) was constructed via covalent attachment. This nanohybrid shows efficient electrocatalytic proton reduction with a TOF of 9444 s⁻¹ in 0.2 N aqueous H₂SO₄.

Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes

Electrocatalytic generation of H2 is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H2 generation by 50 mA/cm2 at current densities over 100 mA/cm2 compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]+) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure-activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

Electrochemical Sensor Based on Prussian Blue Electrochemically Deposited at ZrO2 Doped Carbon Nanotubes Glassy Carbon Modified Electrode

In this work, a new hydrogen peroxide (H₂O₂) electrochemical sensor was fabricated. Prussian blue (PB) was electrodeposited on a glassy carbon (GC) electrode modified with zirconia doped functionalized carbon nanotubes (ZrO₂-fCNTs), (PB/ZrO₂-fCNTs/GC). The morphology and structure of the nanostructured system were characterized by scanning and transmission electron microscopy (TEM), atomic force microscopy (AFM), specific surface area, X-ray diffraction (XRD), thermogravimetric analysis (TGA), Raman and Fourier transform infrared (FTIR) spectroscopy. The electrochemical properties were studied by cyclic voltammetry (CV) and chronoamperometry (CA). Zirconia nanocrystallites (6.6 ± 1.8 nm) with cubic crystal structure were directly synthesized on the fCNTs walls, obtaining a well dispersed distribution with a high surface area. The experimental results indicate that the ZrO₂-fCNTs nanostructured system exhibits good electrochemical properties and could be tunable by enhancing the modification conditions and method of synthesis. The fabricated sensor could be used to efficiently detect H₂O₂, presenting a good linear relationship between the H₂O₂ concentration and the peak current, with quantification limit (LQ) of the 10.91 μmol·L^-1 and detection limit (LD) of 3.5913 μmol·L^-1.

Unraveling the mechanism of biomimetic hydrogen fuel production – a first principles molecular dynamics study

The Fe₂(bdt)(CO)₆ [bdt = benzenedithiolato] complex, a synthetic mimic of the [FeFe] hydrogenase enzyme can electrochemically convert protons into molecular hydrogen. The free energy landscape reveals a different mechanism for the biomimetic cycle.

A DFT study of structure and electrochemical properties of diiron-hydrogenase models with benzenedithiolato and benzenediselenato ligands

The chalcogen atom substitution in the Fe₂(bdt)(CO)₆ complex results in higher and lower proton affinities of iron and chalcogen atoms, respectively.

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.