Electrocatalytic properties of [FeFe]-hydrogenases models and visible-light-driven hydrogen evolution efficiency promotion with porphyrin functionalized graphene nanocomposite

Biomimics of [FeFe]‑hydrogenases: Diiron aza- versus oxadiphenylpropanedithiolate complexes with mono- versus diphosphines

To extensively devolep the bioinspired chemistry of [FeFe]‑hydrogenases, this study performs an insigt into the selective substitution of all‑carbonyl diiron aza- versus oxadiphenylpropanedithiolate precursors Fe2(μ-Ph2xdt)(CO)6 (Ph2xdt = Ph2odt = (SCHPh)2O for 1 and Ph2adtNH = (SCHPh)2NH for 2) by mono- versus diphosphines P(C6H4R-p)3 (R = Me and Cl) and (Ph2P)2R' (R' = cis-CH=CH- for dppv and -CH2CH2- for dppe). With monophosphines, their monosubstituted diiron Ph2odt complexes Fe2(μ-Ph2odt)(CO)5){κ1-P(C6H4R-p)3} (R = Me for 1a and Cl for 1b) were obtained through the oxidative decarbonylating of 1 at room temperature in MeCN with Me3NO·2H2O; in contrast, analogous diiron Ph2adtNH complexes Fe2(μ-Ph2adtNH)(CO)5){κ1-P(C6H4R-p)3} (R = Me for 2a and Cl for 2b) were afforded via the photolytic decarbonlating of 2 under UV irradiation (365 nm) in toluene. With diphosphines, the dppv-chelated diiron Ph2xdt complexes Fe2(μ-Ph2xdt)(CO)4(κ2-dppv) (Ph2xdt = Ph2odt for 1c and Ph2adtNH for 2c) were prepared from the UV-irradiated decarbonylation of 1 or 2 in toluene; by contrast, the dppe-chelated diiron similar complexes Fe2(μ-Ph2xdt)(CO)4(κ2-dppe) (Ph2xdt = Ph2odt for 1d and Ph2adtNH for 2d) were synthesized from the Me3NO-assisted decarbonylation of 1 in room-temperature MeCN and that of 2 at refluxing toluene, respectively. The elemental analysis, FT-IR and NMR (1H, 31P) spectroscopy are used for the full elucidation of the molecular structures of these new diiron complexes and X-ray crystallography is applied for further confirmation of 1, 2 and 1a, 2b. The electrochemical properties of representative complexes 1, 1a, 1c and 2, 2a, 2c have been explored and compared with and without acetic acid (AcOH).

Mini-Review on Catalytic Hydrogen Evolution from Porphyrin-Graphene Structures

Porphyrin-based derivatives have been extensively investigated in photocatalytic, electrocatalytic, and photoelectrocatalytic H2 production systems as both photosensitizers and catalysts. Recently, their combination with two-dimensional materials, such as graphene oxide, reduced graphene oxide, and graphene quantum dots, has attracted significant attention for hydrogen evolution due to the advanced electronic properties, good stability, and low-cost fabrication of these materials. This mini-review summarizes the recent developments concerning the application of porphyrin-graphene ensembles in catalytic H2 generation. Current challenges concerning this application are discussed, and future perspectives are also proposed.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H₂ is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO₂ into chemical fuels can reduce the CO₂ emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H₂ production and CO₂ reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Perspective on advanced nanomaterials used for energy storage and conversion

To drive the next ‘technical revolution’ towards commercialization, we must develop sustainable energy materials, procedures, and technologies. The demand for electrical energy is unlikely to diminish over the next 50 years, and how different countries engage in these challenges will shape future discourse. This perspective summarizes the technical aspects of nanomaterials’ design, evaluation, and uses. The applications include solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOEC), microbial fuel cells (MFC), supercapacitors, and hydrogen evolution catalysts. This paper also described energy carriers such as ammonia which can be produced electrochemically using SOEC under ambient pressure and high temperature. The rise of electric vehicles has necessitated some form of onboard storage of fuel or charge. The fuels can be generated using an electrolyzer to convert water to hydrogen or nitrogen and steam to ammonia. The charge can be stored using a symmetrical supercapacitor composed of tertiary metal oxides with self-regulating properties to provide high energy and power density. A novel metal boride system was constructed to absorb microwave radiation under harsh conditions to enhance communication systems. These resources can lower the demand for petroleum carbon in portable power devices or replace higher fossil carbon in stationary power units. To improve the energy conversion and storage efficiency, we systematically optimized synthesis variables of nanomaterials using artificial neural network approaches. The structural characterization and electrochemical performance of the energy materials and devices provide guidelines to control new structures and related properties. Systemic study on energy materials and technology provides a feasible transition from traditional to sustainable energy platforms. This perspective mainly covers the area of green chemistry, evaluation, and applications of nanomaterials generated in our laboratory with brief literature comparison where appropriate. The conceptual and experimental innovations outlined in this perspective are neither complete nor authoritative but a snapshot of selecting technologies that can generate green power using nanomaterials.

Organic Functionalized Carbon Nanostructures for Solar Energy Conversion

This review presents an overview of the use of organic functionalized carbon nanostructures (CNSs) in solar energy conversion schemes. Our attention was focused in particular on the contribution of organic chemistry to the development of new hybrid materials that find application in dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs), and perovskite solar cells (PSCs), as well as in photocatalytic fuel production, focusing in particular on the most recent literature. The request for new materials able to accompany the green energy transition that are abundant, low-cost, low-toxicity, and made from renewable sources has further increased the interest in CNSs that meet all these requirements. The inclusion of an organic molecule, thanks to both covalent and non-covalent interactions, in a CNS leads to the development of a completely new hybrid material able of combining and improving the properties of both starting materials. In addition to the numerical data, which unequivocally state the positive effect of the new hybrid material, we hope that these examples can inspire further research in the field of photoactive materials from an organic point of view.

Mononuclear nickel(II) dithiolate complexes with chelating diphosphines: Insight into protonation and electrochemical proton reduction

Inspired by the metal active sites of [FeFe]- and [NiFe]‑hydrogenases, a series of mononuclear Ni(II) ethanedithiolate complexes [{(Ph₂PCH₂)₂×}Ni(SCH₂CH₂S)] (X = NCH₂C₅H₄N-p (2a), NCH₂C₆H₅ (2b), NCH₂CHMe₂ (2c), and CH₂ (2d)) with chelating diphosphines were readily synthesized through the room-temperature treatments of mononuclear Ni(II) dichlorides [{(Ph₂PCH₂)₂×}NiCl₂] (1a-1d) with ethanedithiol (HSCH₂CH₂SH) in the presence of triethylamine (Et₃N) as acid-binding agent. All the as-prepared complexes 1a-1d and 2a-2d are fully characterized through elemental analysis, nuclear magnetic resonance (NMR) spectrum, and by X-ray crystallography for 1b, 2a-2d. To further explore proton-trapping behaviors of this type of mononuclear Ni(II) complexes for catalytic hydrogen (H₂) evolution, the protonation and electrochemical proton reduction of 2a-2c with aminodiphosphines (labeled PCNCP = (Ph₂PCH₂)₂NR) and reference analogue 2d with nitrogen-free diphosphine (dppp = (Ph₂PCH₂)₂CH₂) are studied and compared under trifluoroacetic acid (TFA) as a proton source. Interestingly, the treatments of 2a-2d with excess TFA resulted in the unexpected formation of dinuclear Ni(II)-Ni(II) dication complexes [{(Ph₂PCH₂)₂×}₂Ni₂(μ-SCH₂CH₂S)](CF₃CO₂)₂ (3a-3d) and mononuclear Ni(II) N-protonated complexes [{(Ph₂PCH₂)₂N(H)R}Ni(SCH₂CH₂S)](CF₃CO₂) (4a-4c), which has been well supported by high-resolution electrospray ionization mass spectroscopy (HRESI-MS), NMR (³¹P, ¹H) as well as fourier transform infrared spectroscopy (FT-IR) techniques, and especially by X-ray crystallography for 3d. Additionally, the electrochemical properties of 2a-2d are investigated in the absence and presence of strong acid (TFA) by using cyclic voltammetry (CV), showing that the complete protonation of 2a-2d gave rise to dinuclear Ni₂S₂ species 3a-3d for electrocatalytic proton reduction to H₂.

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.

Phosphine-substituted Fe–Te clusters related to the active site of [FeFe]-H2ases

Mono-, di-, and tetranuclear phosphine-substituted Fe/Te clusters 1–6 were described.

Catalytic Metallopolymers from [2Fe-2S] Clusters: Artificial Metalloenzymes for Hydrogen Production

Reviewed herein is the development of novel polymer-supported [2Fe-2S] catalyst systems for electrocatalytic and photocatalytic hydrogen evolution reactions. [FeFe] hydrogenases are the best known naturally occurring metalloenzymes for hydrogen generation, and small-molecule, [2Fe-2S]-containing mimetics of the active site (H-cluster) of these metalloenzymes have been synthesized for years. These small [2Fe-2S] complexes have not yet reached the same capacity as that of enzymes for hydrogen production. Recently, modern polymer chemistry has been utilized to construct an outer coordination sphere around the [2Fe-2S] clusters to provide site isolation, water solubility, and improved catalytic activity. In this review, the various macromolecular motifs and the catalytic properties of these polymer-supported [2Fe-2S] materials are surveyed. The most recent catalysts that incorporate a single [2Fe-2S] complex, termed single-site [2Fe-2S] metallopolymers, exhibit superior activity for H₂ production.

Photocatalytic Hydrogen Evolution by a Synthetic [FeFe] Hydrogenase Mimic Encapsulated in a Porphyrin Cage

The design of a biomimetic and fully base metal photocatalytic system for photocatalytic proton reduction in a homogeneous medium is described. A synthetic pyridylphosphole-appended [FeFe] hydrogenase mimic was encapsulated inside a supramolecular zinc porphyrin-based metal-organic cage structure Fe4 (Zn-L)6 . The binding is driven by the selective pyridine-zinc porphyrin interaction and results in the catalyst being bound strongly inside the hydrophobic cavity of the cage. Excitation of the capsule-forming porphyrin ligands with visible light while probing the IR spectrum confirmed that electron transfer takes place from the excited porphyrin cage to the catalyst residing inside the capsule. Light-driven proton reduction was achieved by irradiation of an acidic solution of the caged catalyst with visible light.