Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Revisiting the photochemical synthesis of [FeFe]-hydrogenase mimics: reaction optimization, mechanistic study and electrochemical behaviour

The photoreaction of [(μ-S)₂Fe₂(CO)₆] and alkenes or alkynes has been optimized to readily obtain functionalized [FeFe]-hydrogenase mimics. Irradiation under low CO pressure in THF produces the corresponding photo-adducts in good/acceptable (alkenes/alkynes) yields, with retention of the starting olefin stereochemistry. DFT-calculations provide plausible reaction pathways in both, singlet and triplet states. The DFT-calculation based in the singlet state is energetically more favorable. The electrochemical behavior of the synthesized compounds is also presented, including studies in acidic media. The electrochemical properties of the products vary in the presence of a double bond (cycloaddition of [(μ-S)₂Fe₂(CO)₆] to alkynes), respect to a single bond (cycloaddition to alkenes).

The photochemistry of [(μ-S)₂Fe₂(CO)₆] and alkenes or alkynes has been developed to a synthetically useful level. Computational data and electrochemistry of the photoadducts show interesting properties of these compounds.

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.

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.

The monoiron anionfac-[Fe(CO)3I3]−and its organic aminium salts: their preparation, CO-release, and cytotoxicity

The anionfac-[Fe(CO)₃I₃]⁻undergoes rapid decomposition to release CO and involve iodine radical. The CO-release can be tuned by its cations. The radical causes severe cytotoxicity which may endow the anion a great potential as an anticancer drug.

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

Diferrocenyl Thioketone: Reactions with (Bisphosphane)Pt(0) Complexes-Electrochemical and Computational Studies

Diferrocenyl thioketone reacts smoothly with (bisphosphane)Pt(0) complexes in toluene solution at room temperature yielding 1:1 adducts identified as ferrocenyl (Fc) functionalized platinathiiranes. Their structures were unambiguously confirmed by means of spectroscopic methods as well as by X-ray diffraction analysis. A unique, ferrocene-rich platinathiirane, bearing three Fc-units, was prepared starting with [bis(diphenylphosphino)ferrocene] Pt(0(η²-norbornene). For comparison, a similar platinathiirane with one Fc-unit was obtained from the reaction of the latter complex with thiobenzophenone. Quantum-chemical calculations were carried out to describe the bonding pattern and frontier molecular orbitals of the ferrocene-rich platinathiirane complexes. These calculations confirmed that the C=S bond loses its formally double-bond character upon complexation (bisphosphane)Pt(0). Cyclic voltammetry measurements were performed to characterize the obtained platinathiiranes in CH₂Cl₂ solutions. For comparison, the cyclic voltammogram for diferrocenyl thioketoneas a mixed-valent (Fe^II-Fe^III) compound was also recorded and analyzed. The results point out to a diffusion controlled electrode process in case of differocenyl thioketone and mixed diffusion and adsorption controlled electrode process in the case of the studied platinathiiranes.

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.

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.

Bond Forming Reactions Involving Isocyanides at Diiron Complexes

The versatility of isocyanides (CNR) in organic chemistry has been tremendously enhanced by continuous advancement in transition metal catalysis. On the other hand, the urgent need for new and more sustainable synthetic strategies based on abundant and environmental-friendly metals are shifting the focus towards iron-assisted or iron-catalyzed reactions. Diiron complexes, taking advantage of peculiar activation modes and reaction profiles associated with multisite coordination, have the potential to compensate the lower activity of Fe compared to other transition metals, in order to activate CNR ligands. A number of reactions reported in the literature shows that diiron organometallic complexes can effectively assist and promote most of the “classic” isocyanide transformations, including CNR conversion into carbyne and carbene ligands, CNR insertion, and coupling reactions with other active molecular fragments in a cascade sequence. The aim is to evidence the potential offered by diiron coordination of isocyanides for the development of new and more sustainable synthetic strategies for the construction of complex molecular architectures.

Redox and "Antioxidant" Properties of Fe2(μ-SH)2(CO)4(PPh3)2

The chemistry of Fe₂(μ-SH)₂(CO)₄(PPh₃)₂ (2^HH) is described with attention to S-S coupling reactions. Produced by the reduction of Fe₂(μ-S₂)(CO)₄(PPh₃)₂ (2), 2^HH is an analogue of Fe₂(μ-SH)₂(CO)₆ (1^HH), which exhibits well-behaved S-centered redox. Both 2^HH and the related 2^MeH exist as isomers that differ with respect to the stereochemistry of the μ-SR ligands (R = H, Me). Compounds 2^HH, 2^MeH, and 2 protonate to give rare examples of Fe-SH and Fe-S₂ hydrides. Salts of [H2]⁺, [H2^HH]⁺, and [H2^MeH]⁺ were characterized crystallographically. Complex 2^HH reduces O₂, H₂O₂, (PhCO₂)₂, and Ph₂N₂, giving 2. Related reactions involving 1^HH gave uncharacterizable polymers. The differing behaviors of 2^HH and 1^HH reflect stabilization of the ferrous intermediates by the PPh₃ ligands. When independently generated by the reaction of 2^HH with 2,2,6,6-tetramethyl-1-piperidinyloxy, 2* quantitatively converts to 2 or, in the presence of C₂H₄, is trapped as the ethanedithiolate Fe₂(μ-S₂C₂H₄)(CO)₄(PPh₃)₂. Evidence is presented that the Hieber-Gruber synthesis of 1 involves polysulfido intermediates [Fe₂(μ-S _n)₂(CO)₆]^2- ( n > 1). Two relevant experiments are as follows: (i) protonation of [Fe₄(μ-S)₂(μ-S₂)CO)₁₂]^2- gives 1 and 1^HH, and (ii) oxidation of 1^HH by sulfur gives 1.

(Spectro)electrochemical and Electrocatalytic Investigation of 1,1'-Dithiolatoferrocene-Hexacarbonyldiiron

Hexacarbonyldiiron bridged by a 1,1'-dithiolatoferrocene, [Fe(C₅H₄S)₂{Fe(CO)₃}₂] (1), was synthesized, and the electrochemistry showed reversible oxidation at the Fe(C₅H₄S)₂ site and quasi-reversible reduction at the hexacarbonyldiiron site. Spectroelectrochemical techniques showed reduction-induced ligand isomerization, where the thiolate ligand went from bridging to terminal and one carbon monoxide ligand moved to a quasi-bridging position; this mechanism was further supported by cyclic voltammetry simulation and density functional theory calculations. Complex 1 showed electrocatalytic activity toward hydrogen-evolving reaction.

Diiron(ii) pentacarbonyl complexes as CO-releasing molecules: their synthesis, characterization, CO-releasing behaviour and biocompatibility

Four diiron(ii) carbonyl complexes, [Fe2(μ-SR)3(CO)5X] (X- = Br-, I-; R = CH2CH3, CH2CH2CH3) were facilely synthesized by reacting [Fe(CO)4X2] with monothiolates. Their potential as carbon monoxide-releasing molecules (CORMs) was systematically investigated, revealing that their CO-releasing behaviour is highly solvent-dependent. Specifically, in dimethyl sulfoxide (DMSO), the CO-releasing kinetics were fast. Intermediates with a lower oxidation state might be involved in the reaction. By contrast, in less polar solvents such as methanol, acetonitrile and dichloromethane, intermediates featuring the triiron carbonyl cation, [Fe3(μ-SCH2CH3)6(CO)6]+, were isolated. The triiron intermediate underwent further decomposition to liberate CO. One of the iodo complexes was also examined for its CO-release in PBS solution when solubilised with DMSO in the presence of deoxy-Mb and the CO-release was found to be quantitative. Furthermore, kinetic analyses were performed and the CO-release in general obeyed a first-order kinetic model. Plausible CO-releasing pathways are proposed for the parent complexes and the triiron intermediate. Assessments in cytotoxicity indicated that the cytoxicity of the diiron(ii) complexes varied with both the halide and thiolate and those bearing bromide and the thiolate with longer chains were more biocompatible.

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.

Hydrogenase Biomimetics with Redox-Active Ligands: Synthesis, Structure, and Electrocatalytic Studies on [Fe2(CO)4(κ2-dppn)(µ-edt)] (edt = Ethanedithiolate; dppn = 1,8-bis(Diphenylphosphino)Naphthalene)

Addition of the bulky redox-active diphosphine 1,8-bis(diphenylphosphino)naphthalene (dppn) to [Fe2(CO)6(µ-edt)] (1) (edt = 1,2-ethanedithiolate) affords [Fe2(CO)4(κ2-dppn)(µ-edt)] (3) as the major product, together with small amounts of a P–C bond cleavage product [Fe2(CO)5{κ1-PPh2(1-C10H7)}(µ-edt)] (2). The redox properties of 3 have been examined by cyclic voltammetry and it has been tested as a proton-reduction catalyst. It undergoes a reversible reduction at E1/2 = −2.18 V and exhibits two overlapping reversible oxidations at E1/2 = −0.08 V and E1/2 = 0.04 V. DFT calculations show that while the Highest Occupied Molecular Orbital (HOMO) is metal-centred (Fe–Fe σ-bonding), the Lowest Unoccupied Molecular Orbital (LUMO) is primarily ligand-based, but also contains an antibonding Fe–Fe contribution, highlighting the redox-active nature of the diphosphine. It is readily protonated upon addition of strong acids and catalyzes the electrochemical reduction of protons at Ep = −2.00 V in the presence of CF3CO2H. The catalytic current indicates that it is one of the most efficient diiron electrocatalysts for the reduction of protons, albeit operating at quite a negative potential.