Cyclic Voltammetric Studies of Chlorine-Substituted Diiron Benzenedithiolato Hexacarbonyl Electrocatalysts Inspired by the [FeFe]-Hydrogenase Active Site

Tailoring Second Coordination Sphere for Tunable Solid-Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid-liquid interface via molecular catalyst design. Specifically, the surface of a p-Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda-SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda-SO3H)2, which contains an electron-withdrawing -SO3H group in the second coordination sphere, elevates the flat-band potential of the polished p-Si photocathode and nanoporous p-Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band-edge engineering by molecular catalysts could be an important design consideration for semiconductor-catalyst hybrids toward PEC 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.

[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.

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

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.

Inter-ligand intramolecular through-space anisotropic shielding in a series of manganese carbonyl phosphorous compounds

Eight novel manganese carbonyl complexes of the type [Mn(bpy-tBu)(CO)3PR3]+ (bpy-tBu = 4,4'-di-tert-butyl-2,2'-bipyridine; R = Cy, nBu, Me, p-tol, Ph, p-F-Ph, OEt, and OMe), have been synthesized and characterized by 1H NMR, FTIR, UV/Vis, HRMS and CV. X-ray crystallographic structures of [Mn(bpy-tBu)(CO)3(PCy3)]+ and [Mn(bpy-tBu)(CO)3(PPh3)]+ were obtained. The short Mn-P bond length allows for close proximity of the bipyridine ligand and the phosphine R groups, resulting in strong anisotropic shielding of certain bipyridine protons by aryl R groups (reordering the bipyridine 1H NMR pattern in the most extreme case). Electrochemical analysis of the compound series reveals that while each is a competent precatalyst for electrochemical carbon dioxide reduction (to carbon monoxide), the lability of the PR3 ligand results in similar catalytic performance amongst the series.

Electrochemical and Computational Insights into the Reduction of [Fe2(CO)6{µ-(SCH2)2GeMe2}] Hydrogenase H-Cluster Mimic

The electrochemical reduction of the complex [Fe2(CO)6{µ-(SCH2)2GeMe2}] (1) under N2 and CO is reported applying cyclic voltammetry. Reduction of complex 1 in CO saturated solutions prevents the possible release of CO from the dianion 12−, while the latter reacts with additional CO forming a spectroscopically uncharacterized product P1. This product undergoes a reversible redox process at E1/2 = −0.70 V (0.2 V∙s−1). In this report, the structure of the neutral complex 1, isomers of dianionic form of 1, and P1 are described applying DFT computations. Furthermore, we propose reaction pathways for H2 production on the basis of the cyclic voltammetry of complex 1 in presence of the strong acid CF3SO3H.

Electrocatalytic hydrogen production using [FeFe]-hydrogenase mimics based on tetracene derivatives

The synthesis, structure, physical properties, and electrocatalytic hydrogen production of tetrathiatetracene ligand based [FeFe]-hydrogenase mimic molecules were investigated.

Proton-coupled electron transfer in the reduction of diiron hexacarbonyl complexes and its enhancement on the electrocatalytic reduction of protons by a pendant basic group

The pendant basic groups in a diiron complex acted as proton relay to ease the kinetic resistance in proton reduction and enhance proton-coupled electron transfer (PCET).

A Functional Hydrogenase Mimic Chemisorbed onto Fluorine-Doped Tin Oxide Electrodes: A Strategy towards Water Splitting Devices

A diiron benzenedithiolate hydrogen-evolving catalyst immobilized onto fluorine-doped tin oxide (FTO) electrodes is prepared, characterized, and studied in the context of the development of water splitting devices based on molecular components. FTO was chosen as the preferred electrode material owing to its conductive properties and electrochemical stability. An FTO nanocrystalline layer is also used to greatly improve the surface area of commercially available FTO while preserving the properties of the material. Electrodes bearing a covalently anchored diiron catalyst are shown to be competent for electrocatalytic hydrogen evolution from acidic aqueous media at relatively low overpotential (500 mV) with a faradaic efficiency close to unity. Compared with bulk solution catalysts, the catalyst immobilized onto the electrode surface operates at roughly 160 mV lower overpotentials, yet with similar rates.

[FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2-8): A Systematic Study

The effect of the nature of the dithiolato ligand on the physical and electrochemical properties of synthetic H-cluster mimics of the [FeFe]-hydrogenase is still of significant concern. In this report we describe the cyclization of various alkanedithiols to afford cyclic disulfide, tetrasulfide, and hexasulfide compounds. The latter compounds were used as proligands for the synthesis of a series of [FeFe]-hydrogenase H-cluster mimics having the general formulas [Fe₂(CO)₆{μ-S(CH₂)_nS}] (n = 4-8), [Fe₂(CO)₆{μ-S(CH₂)_nS}]₂ (n = 6-8), and [Fe₂(CO)₆{(μ-S(CH₂)_nS)₂}] (n = 6-8). The resulting complexes were characterized by ¹H and ¹³C{¹H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis. The purpose of this research was to study the influence of the systematic increase of n from 2 to 7 on the redox potentials of the models and the catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry.

[FeFe]-Hydrogenase H-cluster mimics mediated by naphthalene monoimide derivatives of peri-substituted dichalcogenides

Synthetic models of the active site of [FeFe]-hydrogenase containing naphthalene monoimide as bridging linker.

Facilitated carbon dioxide reduction using a Zn(II) complex

Two new Zn(II) complexes have been prepared and evaluated for their capacity to activate and reduce CO2. The electrochemical properties of dichlorobis[diphenyl-(2-pyridyl)phosphine-κ(1)-N]zinc(II) [corrected]. and dichloro[diphenyl-(2-pyridyl)phosphine-κ(1)-N]zinc(II) 2 are compared using cyclic voltammetry. Electrochemical results indicate that 2 leads to a facilitated CO2 reduction to evolve CO at a glassy carbon electrode.

Hydrogen generation: aromatic dithiolate-bridged metal carbonyl complexes as hydrogenase catalytic site models

The design, syntheses and characteristics of metal carbonyl complexes with aromatic dithiolate linkers reported as bioinspired hydrogenase catalytic site models are described and reviewed. Among these the complexes capable of hydrogen generation have been discussed in detail. Comparisons have been made with carbonyl complexes having alkyl dithiolates as linkers between metal centers.