Synthetic models for the active site of the [FeFe]-hydrogenase: catalytic proton reduction and the structure of the doubly protonated intermediate

Mononuclear Fe(III) Schiff base antipyrine complexes for catalytic hydrogen generation

Mononuclear Fe(III) complexes containing an antipyrine Schiff base ligand were prepared and fully characterized, demonstrating a planar tetradentate coordination geometry. These complexes were found to be active for the hydrogen evolution reaction. Catalysis occurs at -1.4 V vs. Fc+/Fc, with an overpotential of 700 mV. The complexes are active electrocatalysts with a turnover frequency of 700 s-1. Furthermore, when paired with a chromophore and sacrificial donor, the complexes are active photocatalysts demonstrating >1700 turnovers during 40 hours of irradiation with a quantum yield of up to 5.4%. The catalysts have also been found to operate in natural water samples of varying salinity.

Hydrogen evolution driven by heteroatoms of bidentate N-heterocyclic ligands in iron(II) complexes

While Pt is considered the best catalyst for the electrocatalytic hydrogen evolution reaction (HER), it is evident that non-noble metal alternatives must be explored. In this regard, it is well known that the binding sites for non-noble metals play a pivotal role in facilitating efficient catalysis. Herein, we studied Fe(II) complexes with bidentate 2-(2'-pyridyl)benzoxazole (LO), 2-(2'-pyridyl)benzthiazole (LS), 2-(2'-pyridyl)benzimidazole (LNH), and 2-2'-bipyridyl (Lpy) ligands - by adding trifluoroacetic acid (TFA) to their acetonitrile solution - in order to examine how their reactivity towards protons under reductive conditions could be impacted by the non-coordinating heteroatoms (S, O, N, or none). By applying this ligand series, we found that the reduction potentials relevant for HER correlate with ligand basicity in the presence of TFA. Moreover, DFT calculations underlined the importance of charge distribution in the ligand-based LUMO and LUMO+1 orbitals of the complexes, dependent on the heterocycle. Kinetic studies and controlled potential electrolysis - using TFA as a proton source - revealed HER activities for the complexes with LNH, LO, and LS of kobs = 0.03, 1.1, and 10.8 s-1 at overpotentials of 0.81, 0.76, and 0.79 V, respectively, and pointed towards a correlation between the kinetics of the reaction and the non-coordinating heteroatoms of the ligands. In particular, the activity was associated with the [Fe(LS/O/NH)2(S)2]2+ form (S = solvent or substrate molecule), and the rate-determining step involved the formation of [Fe(H-H)]+, during the weakening of Fe-H and CF3CO2-H bonds, according to the experimental and DFT results.

Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Triiron Complexes Featuring Azadiphosphine Related to the Active Site of [FeFe]-Hydrogenases: Their Redox Behavior and Protonation

The design of iron clusters featuring a bimetallic core and several protonation sites in the second coordination sphere of the metal centers is important for modeling the activity of polymetallic active sites such as the H-cluster of [FeFe]-hydrogenases. For this purpose, the syntheses of complexes [Fe3(CO)5(κ2-PPh2NR2)(μ-pdt)2] (R = Ph (1), Bn (2)) and [Fe3(CO)5(κ2-PPh2NR2)(μ-adtBn)(μ-pdt)] (R = Ph (3), Bn (4)) were carried out by reacting hexacarbonyl precursors [Fe2(CO)6(µ-xdt)] (xdt = pdt (propanedithiolate), adtBn (azadithiolate) with mononuclear complexes [Fe(κ2-pdt)(CO)2(κ2-PPh2NR2)] (PPh2NR2 = (PPhCH2NRCH2)2, R = Ph, Bn) in order to introduce amine functions, through well-known PPh2NR2 diphosphine, into the vicinity of the triiron core. The investigation of the reactivity of these triiron species towards the proton (in the presence of CF3SO3H) and the influence of the pendant amines on the redox properties of these complexes were explored using spectroscopic and electrochemical methods. The protonation sites in such triiron clusters and their relationships were identified. The orientation of the first and second protonation processes depends on the arrangement of the second coordination sphere. The similarities and differences, due to the extended metal nuclearity, with their dinuclear counterparts [Fe2(CO)4(κ2-PPh2NR2)(μ-pdt)], were highlighted.

Magnetic study and DFT analysis of a doubly carboxylato-bridged Co(II) derivative anchored with a 'scorpionate' precursor as a potential electrocatalyst for heterogeneous H2 evolution

A new doubly carboxylato-bridged Co(II) dinuclear complex, [Co(bdtbpza)(NCS)]2 (1), was obtained in a satisfactory yield by employing a 'scorpionate'-type precursor, bdtbpza {bis-(3,5-di-tert-butylpyrazol-1-yl)acetate}, and was then structurally characterized. Single-crystal X-ray diffraction analysis revealed that, in 1, each Co(II) is penta-coordinated, leading to a distorted trigonal-bipyramidal geometry within the coordination environment of N3O2. Weak antiferromagnetic coupling within the Co(II) ions in 1 was found based on the isotropic spin Hamiltonian H = -J(S1·S2) for the Si = 3/2 system. For evaluating the spin density distribution and the mechanism for the magnetic exchange coupling, DFT analysis was performed, with the calculated result agreeing the experimental magnetic data. A study into electrochemical H2 evolution, involving cyclic voltammetry (CV), controlled potential electrolysis (CPE), and gas chromatographic (GC) analyses of the graphite electrode modified with the cobalt complex in a neutral aqueous solution revealed the high catalytic activity of the complex with a low overpotential toward H2O reduction. The faradaic efficiency of the catalyst was found to be 83.7% and the di-cobalt catalyst-modified electrode displayed quite an interesting H2-evolution activity compared with that of bare electrodes. These results are encouraging for the future potential application of 1 in water splitting.

Heptanuclear Mixed-Valence Co4IIICo3II Molecular Wheel─A Molecular Analogue of Layered Double Hydroxides with Single-Molecule Magnet Behavior and Electrocatalytic Activity for Hydrogen Evolution Reactions

We present a bifunctional heptanuclear cobalt(II)/cobalt(III) molecular complex formulated as [Co7(μ3-OH)4(H2L1)2(HL2)2](NO3)6·6H2O (1) (where H5L1 is 2,2'-(((1E,1'E)-((2-hydroxy-5-methyl-1,3-phenylene)bis(methanylylidene))bis(azanylylidene))bis(propane-1,3-diol)) and H2L2 is 2-amino-1,3-propanediol). Compound 1 has been characterized by single-crystal X-ray diffraction analysis along with other spectral and magnetic measurements. Structural analysis indicates that 1 contains a mixed-valence Co7 cluster where a central Co(II) ion is connected to six different Co centers (four CoIII and two CoII ions) by four μ3-OH groups, giving rise to a planar heptanuclear cluster that resembles a molecular fragment of a layered double hydroxide (LDH). Two triply deprotonated (H2L1)3- ligands form the outer side of the cluster while two singly deprotonated (HL2)- ligands are located at the top and bottom of the central heptanuclear core. Variable temperature magnetic measurements indicate the presence of weak ferromagnetic CoII···CoII interactions (J = 3.53(6) cm-1) within the linear trinuclear CoII cluster. AC susceptibility measurements show that 1 is a field-induced single-molecule magnet (SMM) with τ0 = 8.2(7) × 10-7 s and Ueff = 11.3(4) K. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 in homogeneous phase shows an overpotential of 455 mV, with a Faradaic efficiency of 81% and a TOF of 8.97 × 104 μmol H2 h-1 mol-1.

Facile electrocatalytic proton reduction by a [Fe-Fe]-hydrogenase bio-inspired synthetic model bearing a terminal CN- ligand

An azadithiolate bridged CN- bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe-Fe] H2ase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H+ to H2 between pH 0-3 at diffusion-controlled rates (1011 M-1 s-1) i.e. 108 s-1 at pH 3 with an overpotential of 140 mV. Electrochemical analysis and DFT calculations suggests that a CN- ligand increases the pKa of the cluster enabling hydrogen production from its Fe(i)-Fe(0) state at pHs much higher and overpotential much lower than its precursor bis-iron hexacarbonyl model which is active in its Fe(0)-Fe(0) state. The formation of a terminal Fe-H species, evidenced by spectroelectrochemistry in organic solvent, via a rate determining proton coupled electron transfer step and protonation of the adjacent azadithiolate, lowers the kinetic barrier leading to diffusion controlled rates of H2 evolution. The stereo-electronic factors enhance its catalytic rate by 3 order of magnitude relative to a bis-iron hexacarbonyl precursor at the same pH and potential.

C-Cl Bond Activation at Rotated vs Unrotated Dinuclear Site Related to [FeFe]-Hydrogenases

The novel dinuclear complex related to the [FeFe]-hydrogenases active site, [Fe2(μ-pdt)(κ2-dmpe)2(CO)2] (1), is highly reactive toward chlorinated compounds CHxCl4-x (x = 1, 2) affording selectively terminal or bridging chloro diiron isomers through a C-Cl bond activation. DFT calculations suggest a cooperative mechanism involving a formal concerted regioselective chloronium transfer depending on the unrotated or rotated conformation of two isomers of 1.

Linear Free Energy Relationships and Transition State Analysis of CO2 Reduction Catalysts Bearing Second Coordination Spheres with Tunable Acidity

In molecular catalysts, protic functional groups in the secondary coordination sphere (SCS) work in conjunction with an exogenous acid to relay protons to the active site of electrochemical CO2 reduction; however, it is not well understood how the acidity of the SCS and exogenous acid together determine the kinetics of catalytic turnover. To evaluate the relative contributions of proton transfer driving forces, we synthesized a series of modular iron tetraphenylporphyrin electrocatalysts bearing SCS amides of tunable pKa (17.6 to 20.0 in dimethyl sulfoxide (DMSO)) and employed phenols of variable acidity (15.3 to 19.1) as exogenous acids. This system allowed us to (1) evaluate contributions from proton transfer driving forces associated with either the SCS or exogenous acid and (2) obtain mechanistic insights into CO2 reduction as a function of pKa. A series of linear free-energy relationships show that kinetics become increasingly sensitive to variations in SCS pKa when more acidic exogenous acids are used (0.82 ≥ Brønsted α ≥ 0.13), as well as to variations in exogenous acid pKa when SCS acidity is increased (0.62 ≥ Brønsted α ≥ 0.32). An Eyring analysis suggests that the rate-determining transition state becomes more ordered with decreasing SCS acidity, which is consistent with the proposal that SCS acidity modulates charge accumulation and solvation at the rate-limiting transition state. Together, these insights enable the optimization of activation barriers as a function of both SCS and exogenous acid pKa and can further guide the rational design of electrocatalytic systems wherein contributions from all participants in a proton relay are considered.

Hybrids of [FeFe]- and [NiFe]-H2ase Active Site Models

Complexes of the type (diphosphine)Ni(μ-SR)2Fe(CO)3 are investigated with azadithiolate (adt, HN(CH2S-)2) as the dithiolate. The resulting complexes are hybrid models for the active sites of the [NiFe]- and [FeFe]-hydrogenases. The key complex (dppv)Ni(μ-adt)Fe(CO)3 (3) was prepared from the complex Ni[(SCH2)2NCbz](dppv), which contains a Cbz-protected adt ligand (Cbz = C(O)OCH2Ph, dppv = cis-1,2-(Ph2P)2C2H2). This complex combines with Fe2(CO)9 to give (dppv)Ni[(μ-SCH2)2NCbz]Fe(CO)3, which is readily deprotected to give 3. Complex 3 undergoes protonation at both Fe and N to give successively [(dppv)Ni(μ-adt)FeH(CO)3]+ ([H3]+) and [(dppv)Ni(μ-adtH)FeH(CO)3]2+ ([H3H]2+). The redox properties and dynamics of these complexes resemble previously reported analogues with propanedithiolate. Solutions of [H3]+ readily degrade to [(dppv)Ni[(μ-SCH2)2NCH2]Fe(CO)3]+ ([4]+), which features a methylene group linking N and Fe. Complex [4]+ can be made in high yield by reaction of [H3]+ with CH2O, and this conversion was also demonstrated with 13CH2O. Complex [4]+ undergoes hydrogenolysis by photochemical reaction with H2 to give [(dppv)Ni[(μ-SCH2)2NMe]FeH(CO)3]+, the N-methylated analogue of [H3]+. Upon treatment ith Me3O+, [4]+ undergoes quaternization, giving [(dppv)Ni[(μ-SCH2)2N(Me)CH2]Fe(CO)3]2+. In contrast with the lability of [H3]+, the phosphine-substituted derivative [(dppv)Ni(μ-adt)FeH(CO)2(PPh3)]+ did not degrade. Most complexes were characterized by X-ray crystallography.

Cyclopentadienyl ring activation in organometallic chemistry and catalysis

The cyclopentadienyl (Cp) ligand is a cornerstone of modern organometallic chemistry. Since the discovery of ferrocene, the Cp ligand and its various derivatives have become foundational motifs in catalysis, medicine and materials science. Although largely considered an ancillary ligand for altering the stereoelectronic properties of transition metal centres, there is mounting evidence that the core Cp ring structure also serves as a reservoir for reactive protons (H⁺), hydrides (H^-) or radical hydrogen (H^•) atoms. This Review chronicles the field of Cp ring activation, highlighting the pivotal role that Cp ligands can have in electrocatalytic H₂ production, N₂ reduction, hydride transfer reactions and proton-coupled electron transfer.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

A cobalt complex bearing a κ-N3 P2 ligand is presented (1+ or CoI (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two-electron CoI /CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset =-1.6 V vs Fc/Fc+ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII (L)-H (22+ ), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII (L)-H (2+ ) species, which rapidly forms hydrogen and regenerates the initial CoI (L) (1+ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+ .

The hangman effect boosts hydrogen production by a manganese terpyridine complex

The manganese terpyridine complex 1 with a coordinated carboxylate in the axial position was obtained in situ. By virtue of a hangman effect, complex 1 catalyzes electrochemical hydrogen evolution from phenol in acetonitrile solution with a turnover frequency of 525 s^-1 at a low overpotential of ca. 230 mV.

Electrocatalytic hydrogen generation using tripod containing pyrazolylborate-based copper(ii), nickel(ii), and iron(iii) complexes loaded on a glassy carbon electrode

Three transition metal complexes (MC) namely, [TpMeMeCuCl(H2O)] (CuC), [TpMeMeNiCl] (NiC), and [TpMeMeFeCl2(H2O)] (FeC) {TpMeMe = tris(3,5-dimethylpyrazolyl)borate} were synthesized and structurally characterized. The three complexes CuC, NiC, and FeC-modified glassy carbon (GC) were examined as molecular electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution (0.1 M KOH). Various GC-MC electrodes were prepared by loading different amounts (ca. 0.2-0.8 mg cm-2) of each metal complex on GC electrodes. These electrodes were used as cathodes in aqueous alkaline solutions (0.1 M KOH) to efficiently generate H2 employing various electrochemical techniques. The three metal complexes' HER catalytic activity was assessed using cathodic polarization studies. The charge-transfer kinetics of the HER at the (GC-MC)/OH- interface at a given overpotential were also studied using the electrochemical impedance spectroscopy (EIS) technique. The electrocatalyst's stability and long-term durability tests were performed employing cyclic voltammetry (repetitive cycling up to 5000 cycles) and 48 h of chronoamperometry measurements. The catalytic evolution of hydrogen on the three studied MC surfaces was further assessed using density functional theory (DFT) simulations. The GC-CuC catalysts revealed the highest HER electrocatalytic activity, which increased with the catalyst loading density. With a low HER onset potential (E HER) of -25 mV vs. RHE and a high exchange current density of 0.7 mA cm-2, the best performing electrocatalyst, GC-CuC (0.8 mg cm-2), showed significant HER catalytic performance. Furthermore, the best performing electrocatalyst required an overpotential value of 120 mV to generate a current density of 10 mA cm-2 and featured a Tafel slope value of -112 mV dec-1. These HER electrochemical kinetic parameters were comparable to those measured here for the commercial Pt/C under the same operating conditions (-10 mV vs. RHE, 0.88 mA cm-2, 108 mV dec-1, and 110 mV to yield a current density of 10 mA cm-2), as well as the most active molecular electrocatalysts for H2 generation from aqueous alkaline electrolytes. Density functional theory (DFT) simulations were used to investigate the nature of metal complex activities in relation to hydrogen adsorption. The molecular electrostatic surface potential (MESP) of the metal complexes was determined to assess the putative binding sites of the H atoms to the metal complex.

The underappreciated influence of ancillary halide on metal–ligand proton tautomerism

Syntheses of Vaska-type complexes [IrP₂X(CO)] (P = phosphine, X = halide) with all four common halides (fluoride, chloride, bromide, and iodide) was attempted using a protic and hemilabile imidazolyl di-tert-butyl phosphine ligand. In the solid-state, all four complexes were found to be ionic with the halides in the outer-sphere, and the fourth coordination site of the square plane occupied by the imidazole arm of the ligand. In solution, however, the chloride complex was found to be in equilibrium with an octahedral Ir^III–H species at room temperature. For the bromide and iodide analogs, the corresponding Ir^III–H species were also observed but only after heating the solutions. The neutral Ir^I Vaska's analogs for X = Cl, Br, and I were obtained upon addition of excess halide salt, albeit heating was required for X = Br and I. The Ir^III–H species are proposed to originate from tautomerization of minor amounts of the electron rich neutral Vaska analog (halide inner-sphere and phosphines monodentate) that are in equilibrium with the ionic species. Heating is required for the larger anions of bromide and iodide to overcome a kinetic barrier associated with their movement to an inner-sphere position prior to tautormerization. For the fluoride analog, the Ir^III–H was not observed, attributable to strong hydrogen bonding interactions of the imidazolyl proton with the fluoride anion.

Ligand protonated Ir^I bisphosphine carbonyl complexes isolated as halide salts equilibrate with their neutral Ir^III–H congeners in solution. The equilibrium constant and energy barrier to interconversion are dependent on the identity of the halide.

Biomimetics of [FeFe]-hydrogenases incorporating redox-active ligands: Synthesis, redox and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe4S4 surrogates

[FeFe]-ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H2 studied, including 1,1’-bis(diphenylphosphino)ferrocene (dppf) complexes Fe2(CO)4(-dppf)(-S(CH2)nS) (n = 2, edt; n...

Introducing Water-Network-Assisted Proton Transfer for Boosted Electrocatalytic Hydrogen Evolution with Cobalt Corrole

Proton transfer is vital for many biological and chemical reactions. Hydrogen-bonded water-containing networks are often found in enzymes to assist proton transfer, but similar strategy has been rarely presented by synthetic catalysts. We herein report the Co corrole 1 with an appended crown ether unit and its boosted activity for the hydrogen evolution reaction (HER). Crystallographic and 1H NMR studies proved that the crown ether of 1 can grab water via hydrogen bonds. By using protic acids as proton sources, the HER activity of 1 was largely boosted with added water, while the activity of crown-ether-free analogues showed very small enhancement. Inhibition studies by adding (1) external 18-crown-6-ether to extract water molecules and (2) potassium ion or N-benzyl-n-butylamine to block the crown ether of 1 further confirmed its critical role in assisting proton transfer via grabbed water molecules. This work presents a synthetic example to boost HER through water-containing networks.

Inhibition of [FeFe]-hydrogenase by formaldehyde: proposed mechanism and reactivity of FeFe alkyl complexes

The mechanism for inhibition of [FeFe]-hydrogenases by formaldehyde is examined with model complexes. Key findings: (i) CH2 donated by formaldehyde covalently link Fe and the amine cofactor, blocking the active site and (ii) the resulting Fe-alkyl is a versatile electrophilic alkylating agent. Solutions of Fe2[(μ-SCH2)2NH](CO)4(PMe3)2 (1) react with a mixture of HBF4 and CH2O to give three isomers of [Fe2[(μ-SCH2)2NCH2](CO)4(PMe3)2]+ ([2]+). X-ray crystallography verified the NCH2Fe linkage to an octahedral Fe(ii) site. Although [2]+ is stereochemically rigid on the NMR timescale, spin-saturation transfer experiments implicate reversible dissociation of the Fe-CH2 bond, allowing interchange of all three diastereoisomers. Using 13CH2O, the methylenation begins with formation of [Fe2[(μ-SCH2)2N13CH2OH](CO)4(PMe3)2]+. Protonation converts this hydroxymethyl derivative to [2]+, concomitant with 13C-labelling of all three methylene groups. The Fe-CH2N bond in [2]+ is electrophilic: PPh3, hydroxide, and hydride give, respectively, the phosphonium [Fe2[(μ-SCH2)2NCH2PPh3](CO)4(PMe3)2]+, 1, and the methylamine Fe2[(μ-SCH2)2NCH3](CO)4(PMe3)2. The reaction of [Fe2[(μ-SCH2)2NH](CN)2(CO)4]2- with CH2O/HBF4 gave [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)5]- ([4]-), the result of reductive elimination from [Fe2[(μ-SCH2)2NCH2](CN)2(CO)4]-. The phosphine derivative [Fe2[(μ-SCH2)2NCH2CN](CN)(CO)4(PPh3)]- ([5]-) was characterized crystallographically.

Surprising Condensation Reactions of the Azadithiolate Cofactor

Azadithiolate, a cofactor found in all [FeFe]-hydrogenases, is shown to undergo acid-catalyzed rearrangement. Fe₂ [(SCH₂ )₂ NH](CO)₆ self-condenses to give Fe₆ [(SCH₂ )₃ N]₂ (CO)₁₇ . The reaction, which is driven by loss of NH₄ ⁺ , illustrates the exchange of the amine group. X-ray crystallography reveals that three Fe₂ (SR)₂ (CO)_x butterfly subunits interconnected by the aminotrithiolate [N(CH₂ S)₃ ]^3- . Mechanistic studies reveal that Fe₂ [(SCH₂ )₂ NR](CO)₆ participate in a range of amine exchange reactions, enabling new methodologies for modifying the adt cofactor. Ru₂ [(SCH₂ )₂ NH](CO)₆ also rearranges, but proceeds further to give derivatives with Ru-alkyl bonds Ru₆ [(SCH₂ )₃ N][(SCH₂ )₂ NCH₂ ]S(CO)₁₇ and [Ru₂ [(SCH₂ )₂ NCH₂ ](CO)₅ ]₂ S.

Iron polypyridyl complex adsorbed on carbon surfaces for hydrogen generation

A series of homogeneous Fe(iii) complexes were recently reported that are active for electrocatalytic hydrogen generation. Herein we report a naphthalene-terminated Fe(iii) complex for use in the functionalization of glassy carbon surfaces for electrocatalytic hydrogen generation with retention of catalytic activity.

Computational and Experimental Investigations of the Fe2(μ-S2)/Fe2(μ-S)2 Equilibrium

Density functional theory (DFT) calculations on Fe2S2(CO)6-2n(PMe3)2n for n = 0, 1, and 2 reveal that the most electron-rich derivatives (n = 2) exist as diferrous disulfides lacking an S-S bond. The thermal interconversion of the FeII2(S)2 and FeI2(S2) valence isomers is symmetry-forbidden. Related electron-rich diiron complexes [Fe2S2(CN)2(CO)4]2- of an uncertain structure are implicated in the biosynthesis of [FeFe]-hydrogenases. Several efforts to synthesize electron-rich derivatives of Fe2(μ-S2)(CO)6 (1) are described. First, salts of iron persulfido cyanides [Fe2(μ-S2)(CO)5(CN)]- and [Fe2(μ-S2)(CN)(CO)4(PPh3)]- were prepared by the reactions of NaN(tms)2 with 1 and Fe2(μ-S2)(CO)5(PPh3), respectively. Alternative approaches to electron-rich diiron disulfides targeted Fe2(μ-S2)(CO)4(diphosphine). Whereas the preparation of Fe2(μ-S2)(CO)4(dppbz) was straightforward, that of Fe2(μ-S2)(CO)4(dppv) required an indirect route involving the oxidation of Fe2(μ-SH)2(CO)4(dppv) (dppbz = C6H4-1,2-(PPh2)2, dppv = cis-C2H2(PPh2)2). DFT calculations indicate that the oxidation of Fe2(μ-SH)2(CO)4(dppv) produces singlet diferrous disulfide Fe2(μ-S)2(CO)4(dppv), which is sufficiently long-lived as to be trapped by ethylene. The reaction of 1 and dppv mainly afforded Fe2(μ-SCH=CHPPh2)(μ-SPPh2)(CO)5, implicating a S-centered reaction.

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.

Successes, challenges, and opportunities for quantum chemistry in understanding metalloenzymes for solar fuels research

Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.

Rational Design of Fe2 (μ-PR2 )2 (L)6 Coordination Compounds Featuring Tailored Potential Inversion

It was recently discovered that some redox proteins can thermodynamically and spatially split two incoming electrons towards different pathways, resulting in the one-electron reduction of two different substrates, featuring reduction potential respectively higher and lower than the parent reductant. This energy conversion process, referred to as electron bifurcation, is relevant not only from a biochemical perspective, but also for the ground-breaking applications that electron-bifurcating molecular devices could have in the field of energy conversion. Natural electron-bifurcating systems contain a two-electron redox centre featuring potential inversion (PI), i. e. with second reduction easier than the first. With the aim of revealing key factors to tailor the span between first and second redox potentials, we performed a systematic density functional study of a 26-molecule set of models with the general formula Fe₂ (μ-PR₂ )₂ (L)₆ . It turned out that specific features such as i) a Fe-Fe antibonding character of the LUMO, ii) presence of electron-donor groups and iii) low steric congestion in the Fe's coordination sphere, are key ingredients for PI. In particular, the synergic effects of i)-iii) can lead to a span between first and second redox potentials larger than 700 mV. More generally, the "molecular recipes" herein described are expected to inspire the synthesis of Fe₂ P₂ systems with tailored PI, of primary relevance to the design of electron-bifurcating molecular devices.

Hydrogenase Mimics in M12 L24 Nanospheres to Control Overpotential and Activity in Proton-Reduction Catalysis

Hydrogenase enzymes are excellent proton reduction catalysts and therefore provide clear blueprints for the development of nature-inspired synthetic analogues. Mimicking their catalytic center is straightforward but mimicking the protein matrix around the active site and all its functions remains challenging. Synthetic models lack this precisely controlled second coordination sphere that provides substrate preorganization and catalyst stability and, as a result, their performances are far from those of the natural enzyme. In this contribution, we report a strategy to easily introduce a specific yet customizable second coordination sphere around synthetic hydrogenase models by encapsulation inside M12 L24 cages and, at the same time, create a proton-rich nano-environment by co-encapsulation of ammonium salts, effectively providing substrate preorganization and intermediates stabilization. We show that catalyst encapsulation in these nanocages reduces the catalytic overpotential for proton reduction by 250 mV as compared to the uncaged catalyst, while the proton-rich nano-environment created around the catalyst ensures that high catalytic rates are maintained.

Metal-ligand cooperativity in the soluble hydrogenase-1 from Pyrococcus furiosus

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis. The design principles of such cooperativity in metalloenzymes are underexplored, but are critical to understand for developing efficient catalysts designed with earth abundant metals for small molecule activation. The simple substrate requirements of reversible proton reduction by the [NiFe]-hydrogenases make them a model bioinorganic system. A highly conserved arginine residue (R355) directly above the exogenous ligand binding position of the [NiFe]-catalytic core is known to be essential for optimal function because mutation to a lysine results in lower catalytic rates. To expand on our studies of soluble hydrogenase-1 from Pyrococcus furiosus (Pf SH1), we investigated the role of R355 by site-directed-mutagenesis to a lysine (R355K) using infrared and electron paramagnetic resonance spectroscopic probes sensitive to active site redox and protonation events. It was found the mutation resulted in an altered ligand binding environment at the [NiFe] centre. A key observation was destabilization of the Ni_a³⁺–C state, which contains a bridging hydride. Instead, the tautomeric Ni_a⁺–L states were observed. Overall, the results provided insight into complex metal–ligand cooperativity between the active site and protein scaffold that modulates the bridging hydride stability and the proton inventory, which should prove valuable to design principles for efficient bioinspired catalysts.

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis.

Synthetic models of hydrogenases based on framework structures containing coordinating P, N-atoms as hydrogen energy electrocatalysts – from molecules to materials

Nowadays, hydrogen has become not only an extremely important chemical product but also a promising clean energy carrier for replacing fossil fuels. Production of molecular H2 through electrochemical hydrogen evolution reactions is crucial for the development of clean-energy technologies. The development of economically viable and efficient H2 production/oxidation catalysts is a key step in the creation of H2-based renewable energy infrastructure. Intrinsic limitations of both natural enzymes and synthetic materials have led researchers to explore enzyme-induced catalysts to realize a high current density at a low overpotential. In recent times, highly active widespread numerous electrocatalysts, both homogeneous or heterogeneous (immobilized on the electrode), such as transition metal complexes, heteroatom- or metal-doped nanocarbons, metal-organic frameworks, and other metal derivatives (calix [4] resorcinols, pectates, etc.), which are, to one extent or another, structural or functional analogs of hydrogenases, have been extensively studied as alternatives for Pt-based catalysts, demonstrating prospects for the development of a “hydrogen economy”. This mini-review generalizes some achievements in the field of development of new electrocatalysts for H2 production/oxidation and their application for fuel cells, mainly focuses on the consideration of the catalytic activity of M[P2N2]2 2+ (M = Ni, Fe) complexes and other nickel structures which have been recently obtained.