A Novel [FeFe] Hydrogenase Model with a (SCH2)2P═O Moiety

Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x=P-substituent, y=alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise transitions, or inverted, giving bielectronic events. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Supramolecular architectures sustained by delocalised C–I⋯π(arene) interactions in molecular crystals and the propensity of their formation

A survey of delocalised C–I⋯π(chelate ring) interactions is presented.

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.

New Phosphorous-Based [FeFe]-Hydrogenase Models

[FeFe]-hydrogenases have attracted research for more than twenty years as paragons for the design of new catalysts for the hydrogen evolution reaction (HER). The bridging dithiolate comprising a secondary amine as bridgehead is the key element for the reactivity of native [FeFe]-hydrogenases and was therefore the midpoint of hundreds of biomimetic hydrogenase models. However, within those mimics, phosphorous is barely seen as a central element in the azadithiolato bridge despite being the direct heavier homologue of nitrogen. We herein synthesized three new phosphorous based [FeFe]-hydrogenase models by reacting dithiols (HSCH2)2P(O)R (R = Me, OEt, OPh) with Fe3(CO)12. All synthesized mimics show catalytic reactivity regarding HER and change their mechanisms depending on the strength of the used acid. In all presented mimics, the oxide is the center of reactivity, independent of the nature of the bridgehead. However, the phosphorous atom might be reduced by the methods we present herein to alter the reactivity of the model compounds towards protons and oxygen.

Catalytic H2 evolution/oxidation in [FeFe]-hydrogenase biomimetics: account from DFT on the interplay of related issues and proposed solutions

A DFT overview on selected issues regarding diiron catalysts related to [FeFe]-hydrogenase biomimetic research, with implications for both energy conversion and storage strategies.

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.

Sulphur–sulphur, sulphur–selenium, selenium–selenium and selenium–carbon bond activation using Fe3(CO)12: an unexpected formation of an Fe2(CO)6 complex containing a μ2,κ3-C,O,Se-ligand

Four diiron hexacarbonyl-complexes containing dithiolato (5), diselenolato (6), selenolato-thiolato (7) and μ²,κ³-C,O,Se-ligands (8), respectively have been prepared as [FeFe]-hydrogenase mimics.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN^- and amino-carbyne ligands have been elucidated. After the first reduction, CN^- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN^- that forms a Fe(CNH) moiety: i.e., the acidic source for the H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN^- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN^- to CN(Me)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN(Me)₂}(μ-CO)(CO)₂(Cp)₂]⁺ (2⁺) has been investigated and attributed to the loss of proton responsiveness caused by CN^- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe₂ core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER.

[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.

Selenium makes the difference: protonation of [FeFe]-hydrogenase mimics with diselenolato ligands

The synthetic models of the active site of an [FeFe]-hydrogenase containing a Sn atom in the bridgehead of the diselenato ligand, namely [Fe₂(CO)₆{μ-(SeCH₂Se)SnMe₂}],3and [Fe₂(CO)₆{μ-(SeCH₂)₂SnMe₂}],4have been synthesized and characterized by spectroscopic methods.

Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C-H activation, affording otherwise inaccessible derivatives. Thiones and related C═S-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C-H activation.

Mononuclear, Dinuclear, and Trinuclear Iron Complexes Featuring a New Monoanionic SNS Thiolate Ligand

The new tridentate ligand, S(Me)N(H)S = 2-(2-methylthiophenyl)benzothiazolidine, prepared in a single step from commercial precursors in excellent yield, undergoes ring-opening on treatment with Fe(OTf)2 in the presence of base affording a trinuclear iron complex, [Fe3(μ2-S(Me)NS(-))4](OTf)2 (1) which is fully characterized by structural and spectroscopic methods. X-ray structural data reveal that 1 contains four S(Me)NS(-) ligands meridionally bound to two pseudooctahedral iron centers each bridged by two thiolates to a distorted tetrahedral central iron. The combined spectroscopic (UV-vis, Mössbauer, NMR), magnetic (solution and solid state), and computational (DFT) studies indicate that 1 includes a central, high-spin Fe(II) (S = 2) with two low-spin (S = 0) peripheral Fe(II) centers. Complex 1 reacts with excess PMePh2, CNxylyl (2,6-dimethylphenyl isocyanide), and P(OMe)3 in CH3CN to form diamagnetic, thiolate-bridged, dinuclear Fe(II) complexes {[Fe(μ-S(Me)NS(-))L2]2}(OTf)2 (2-4). These complexes are characterized by elemental analysis; (1)H NMR, IR, UV-vis, and Mössbauer spectroscopy; and single crystal X-ray diffraction. Interestingly, addition of excess P(OMe)3 to complex 1 in CH2Cl2 produces primarily the diamagnetic, mononuclear Fe(II) complex, {Fe(S(Me)NS(-))[P(OMe)3]3}(OTf) (5).

Synthesis, characterization, and H/D exchange of μ-hydride-containing [FeFe]-hydrogenase subsite models formed by protonation reactions of (μ-TDT)Fe2(CO)4(PMe3)2 (TDT = SCH2SCH2S) with protic acids

The first TDT ligand-containing μ-hydride models of [FeFe]-hydrogenases (2–7) have been prepared and the H/D exchange reactions of 7 with deuterium reagents such as D₂, D₂O, and DCl are studied.

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.

Steric effect of the dithiolato linker on the reduction mechanism of [Fe2(CO)6{μ-(XCH2)2CRR′}] hydrogenase models (X = S, Se)

Investigations of the electrochemical properties of [Fe₂(CO)₆{μ-(XCH₂)₂CRR′}] (X = S, Se; R or R′ = H or Me) showed that the complex with the bulkiest CMe₂moiety undergoes reduction with potential inversion.