Pendant bases as proton transfer relays in diiron dithiolate complexes inspired by [Fe–Fe] hydrogenase active site

Crystal structure of pentacarbonyl-(μ2-propane-1,3-dithiolato-κ4 S:S,S′:S′)-(diphenyl(o-tolyl)phosphine-κ1 P)diiron (Fe-Fe), C27H23Fe2O5PS2

C27H23Fe2O5PS2, monoclinic, P21/c (no. 14), a = 15.4423(7) Å, b = 9.8615(5) Å, c = 18.0727(8) Å, β = 90.0380(10)°, V = 2752.2(2) Å3, Z = 4, Rgt (F) = 0.0296, wRref (F 2) = 0.0703, T = 296(2) K.

Mechanism of Diiron Hydrogenase Complexes Controlled by Nature of Bridging Dithiolate Ligand

Bio-inorganic complexes inspired by hydrogenase enzymes are designed to catalyze the hydrogen evolution reaction (HER). A series of new diiron hydrogenase mimic complexes with one or two terminal tris(4-methoxyphenyl)phosphine and different μ-bridging dithiolate ligands and show catalytic activity towards electrochemical proton reduction in the presence of weak and strong acids. A series of propane- and benzene-dithiolato-bridged complexes was synthesized, crystallized, and characterized by various spectroscopic techniques and quantum chemical calculations. Their electrochemical properties as well as the detailed reaction mechanisms of the HER are elucidated by density functional theory (DFT) methods. The nature of the μ-bridging dithiolate is critically controlling the reaction and performance of the HER of the complexes. In contrast, terminal phosphine ligands have no significant effects on redox activities and mechanism. Mono- or di-substituted propane-dithiolate complexes afford a sequential reduction (electrochemical; E) and protonation (chemical; C) mechanism (ECEC), while the μ-benzene dithiolate complexes follow a different reaction mechanism and are more efficient HER catalysts.

[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.

Development of air-stable hydrogen evolution catalysts

Obtaining abundant pure hydrogen by reduction of water has an important implication in the development of clean and renewable energy. Hence research focused on the development of non-noble metal based facile and energy efficient catalysts for proton reduction is on the rise. However, for practical utilization, it is necessary that these complexes function unabated in the presence of atmospheric oxygen and other common contaminants in abundant water sources. There has been very little activity towards the development of oxygen-tolerant hydrogen producing catalysts. This article aims to draw attention to this issue of oxygen sensitivity in the HER and highlights the development of a few air-stable HER catalysts (enzymatic as well as artificial) elaborating the challenges involved and the techniques discovered to overcome this significant deterrent to large-scale hydrogen production by electrolysis from abundant water sources.

Hydrogen Evolution by FeIII Molecular Electrocatalysts Interconverting between Mono and Di-Nuclear Structures in Aqueous Phase

A new FeL/Fe₂ L₂ manifold, with HL=2-({[di(2-pyridyl)methyl](methyl)amino}methyl)phenol, was prepared in gram scale (>50 % yield) and characterized in solution and solid state. The monomer/dimer interconversion is controlled in aqueous phase, upon varying the pH conditions. The electrocatalytic hydrogen evolution reaction (HER) occurs through the FeL monomer with added trifluoroacetic acid (TFA) and through the Fe₂ L₂ μ-oxo dimer in acetate buffer (pH 4.9), with an overpotential of about 1 V and faradaic yield up to 75 %. The resulting i_cat /i_p values in the range 15-28 are among the highest reported for Fe-based electrocatalysts (i_cat is the catalytic current, whereas i_p is the current of an Fe-based redox event).

Enhancement in catalytic proton reduction by an internal base in a diiron pentacarbonyl complex: its synthesis, characterisation, inter-conversion and electrochemical investigation

An internal base bound to one of the diiron undergoes dissociation under a CO atmosphere and the freed base group as a proton relay enhances the catalysis of proton reduction.

Ligand effects on the electrochemical behavior of [Fe2(CO)5(L){μ-(SCH2)2(Ph)PO}] (L = PPh3, P(OEt)3) hydrogenase model complexes

In this paper we study the influence of substituting one CO ligand in [Fe₂(CO)₆{μ-(SCH₂)₂(Ph)PO}] (1) by better σ-donors (PPh₃(2) and P(OMe)₃(3)) in relation to the electrochemical behavior.

Terminal vs bridging hydrides of diiron dithiolates: protonation of Fe2(dithiolate)(CO)2(PMe3)4

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(4), where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C(2)-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1](+), establishing the non-intermediacy of the terminal hydride ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[μ-H1](+) indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(4)](+) ([S-H2](+)). This intermediate converts to [t-H2](+) and [μ-H2](+) by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe(3) ligands are basal.

Hydrogen generation catalyzed by fluorinated diglyoxime-iron complexes at low overpotentials

Fe(II) complexes containing the fluorinated ligand 1,2-bis(perfluorophenyl)ethane-1,2-dionedioxime (dAr(F)gH(2); H = dissociable proton) exhibit relatively positive Fe(II/I) reduction potentials. The air-stable difluoroborated species [(dAr(F)gBF(2))(2)Fe(py)(2)] (2) electrocatalyzes H(2) generation at -0.9 V vs SCE with i(cat)/i(p) ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s(-1) [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dAr(F)g(2)H-BF(2))Fe(py)(2)] (3) exhibits an improved TOF of ∼200 s(-1) (i(cat)/i(p) ≈ 8; FY = 68 ± 14%) at -0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of 2 suggest rate-limiting protonation of an Fe("0") intermediate (k(RLS) ≈ 200 M(-1) s(-1)) that undergoes hydride protonation to form H(2). Complex 3 likely reacts via protonation of an Fe(I) intermediate that subsequently forms H(2) via a bimetallic mechanism (k(RLS) ≈ 2000 M(-1) s(-1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes.