Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes Fe2(μ-xdt)(CO)4{κ2-(Ph2P)2NR} Related to [FeFe]-Hydrogenases

Covalent versus noncovalent attachments of [FeFe]‑hydrogenase models onto carbon nanotubes for aqueous hydrogen evolution reaction

In an effort to develop the biomimetic chemistry of [FeFe]‑hydrogenases for catalytic hydrogen evolution reaction (HER) in aqueous environment, we herein report the integrations of diiron dithiolate complexes into carbon nanotubes (CNTs) through three different strategies and compare the electrochemical HER performances of the as-resulted 2Fe2S/CNT hybrids in neutral aqueous medium. That is, three new diiron dithiolate complexes [{(μ-SCH2)2N(C6H4CH2C(O)R)}Fe2(CO)6] (R = N-oxylphthalimide (1), NHCH2pyrene (2), and NHCH2Ph (3)) were prepared and could be further grafted covalently to CNTs via an amide bond (this 2Fe2S/CNT hybrid is labeled as H1) as well as immobilized noncovalently to CNTs via π-π stacking interaction (H2) or via simple physisorption (H3). Meanwhile, the molecular structures of 1-3 are determined by elemental analysis and spectroscopic as well as crystallographic techniques, whereas the structures and morphologies of H1-H3 are characterized by various spectroscopies and scanning electronic microscopy. Further, the electrocatalytic HER activity trend of H1 > H2 ≈ H3 is observed in 0.1 M phosphate buffer solution (pH = 7) through different electrochemical measurements, whereas the degradation processes of H1-H3 lead to their electrocatalytic deactivation in the long-term electrolysis as proposed by post operando analysis. Thus, this work is significant to extend the potential application of carbon electrode materials engineered with diiron molecular complexes as heterogeneous HER electrocatalysts for water splitting to hydrogen.

Crystal structure of pentacarbonyl-(μ 2-ethane-1,2-dithiolato-κ 4 S:S,S′:S′)-(diphenyl(o-tolyl)phosphine-κ 1 P)diiron (Fe–Fe), C26H21Fe2O5PS2

C26H21Fe2O5PS2, orthorhombic, Pna21 (no. 33), a = 20.9559(8) Å, b = 14.1567(6) Å, c = 9.1976(4) Å, V = 2728.6(2) Å3, Z = 4, Rgt (F) = 0.0217, wRref (F 2) = 0.0570, T = 296(2) K.

Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (R = Ph and H) with chelating diphosphines

In order to develop an attractive generation of bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics with chelating diphosphines, two new series of asymmetrically diphosphine-substituted diiron model complexes [Fe₂(μ-R₂odt)(CO)₄(κ²-diphosphine)] (3-5) with bulky Ph₂odt bridge and their reference counterparts (6-8) with common odt bridge were obtained from the Me₃NO-assisted substitutions of diiron hexacarbonyl precursors [Fe₂(μ-R₂odt)(CO)₆] (R₂odt = (SCHR)₂O, R = Ph (1) and H (2)) with different diphosphines such as (Ph₂P)₂NBn (labelled PN^BnP, Bn = benzyl), (Ph₂PCH₂)₂NBn (PCN^BnCP), and (Ph₂PCH₂)₂CH₂ (DPPP)), respectively. All the as-prepared complexes have been characterized by elemental analysis, IR plus NMR spectroscopies, and particularly by X-ray crystallography for 3-8. It is interesting to note that complexes 3 and 6 chelating by small bite-angle PN^BnP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCN^BnCP or DPPP ligands possess the main apical-basal isomer in solution or in the solid state. Further, the electrochemical properties of two pairs of representative complexes 3, 6 and 5, 8 are explored and compared by cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (CF₃CO₂H) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CF₃CO₂H lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H₂).

The crystal structure of pentakis(carbonyl)-{μ-[2,3-bis(sulfanyl)propan-1-olato]}-(triphenylphosphane)diiron (Fe–Fe)C26H21Fe2O6PS2

C26H21Fe2O6PS2, monoclinic, P21/c (no. 14), a = 9.0892(6) Å, b = 27.6631(18) Å, c = 11.3409(8) Å, β = 106.409(2)°, V = 2735.4(3) Å3, Z = 4, R gt (F) = 0.0670, wR ref (F 2) = 0.1620, T = 296(2) K.

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.

[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.

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.