Iron Carbonyl Clusters as Proton Reduction Catalysts

Abstract – The mixed-valence triiron complexes [Fe3(CO)7-x(PPh3)x(µ-edt)2] (x = 0, 1, 2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. In comparison to the diiron complex [Fe2(CO)6(µ-edt)], [Fe3(CO)7(µ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials, which is a significant energetic gain. In all complexes the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semi-bridging carbonyl, while the LUMO is highly delocalised in nature and is anti-bonding between both pairs of iron atoms but also contains an anti-bonding dithiolate interaction. The clusters [Fe3(CO)9(μ3-E)2] (E = S, Se, Te), [Fe3(CO)7(μ3-E)2(μ- κ2-diphosphine)] (E = S, Se, Te), [Fe3(CO)7(μ3-CO)(μ3-E)(μ-dppm)] (E = S, Se) and [Fe3(CO)8(µ3-Te)2(κ2-diphosphine)] have been prepared and examined as proton reduction catalysts. The reduction potentials for the tellurium-capped clusters occur at lower potentials than for their sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues. The 52-electron clusters [Fe3(CO)8(µ3-Te)2(κ2-diphosphine)] consist of Fe2(CO)6(µ-Te)2 “butterfly” units that are capped by a Fe(CO)2(κ2-diphosphine) moiety. Cyclic voltammetry studies reveal that their redox behaviour and properties as proton reduction catalysts largely stem from the Fe2(CO)6(µ-Te)2 entities, although computational modelling indicates that their LUMOs are centered on the briding tellurium ions and the unique “capping” iron ion. The influence of the substitution, orientation and structure of the phosphido bridges on the electrochemical and electrocatalytic properties of [Fe2(CO)6(µ-phosphido)2] clusters and bis(phosphinidene)-capped triiron carbonyl clusters, including electron-rich derivatives formed by substitution with chelating diphosphines, have been studied. The electrochemistry and electrocatalyses of the [Fe2(CO)6(µ-PR2)2] dimers show subtle variations with the nature of the bridging phosphido group(s), including the orientation of bridgehead hydrogen atoms. The reduction potential of the phosphinidene- capped clusters shift negative way to increase the electron density on the iron centers

Hydrogenase biomimetics with redox-active ligands: Electrocatalytic proton reduction by [Fe2(CO)4(κ2-diamine)(μ-edt)] (diamine = 2,2′-bipy, 1,10-phen)

Diiron complexes bearing redox active diamine ligands have been studied as models of the active site of [FeFe]-hydrogenases. Heating [Fe2(CO)6(μ-edt)] (edt = 1,2-ethanedithiolate) with 2,2′-bipyridine (2,2′-bipy) or 1,10-phenanthroline (1,10-phen) in MeCN in the presence of Me3NO leads to the formation of [Fe2(CO)4(κ2-2,2′-bipy)(μ-edt)] (1-edt) and [Fe2(CO)4(κ2-1,10-phen)(μ-edt)] (2-edt), respectively, in moderate yields. In the solid state the diamine resides in dibasal sites, while both dibasal and apical–basal isomers are present in solution. Both stereoisomers protonate readily upon addition of strong acids. Cyclic voltammetry in MeCN shows that both complexes undergo irreversible oxidation and reduction, proposed to be a one- and two-electron process, respectively. The structures of neutral 2-edt and its corresponding one- and two-electron reduced species have been investigated by DFT calculations. In 2-edt− the added electron occupies a predominantly ligand-based orbital, and the iron–iron bond is maintained, being only slightly elongated. Addition of the second electron affords an open-shell triplet dianion where the second electron populates an Fe–Fe σ* antibonding orbital, resulting in effective scission of the iron–iron bond. The triplet state lies 4.2 kcal mol−1 lower in energy than the closed-shell singlet dianion whose HOMO correlates nicely with the LUMO of the neutral species 2-edt. Electrocatalytic proton reduction by both complexes has been studied in MeCN using CF3CO2H as the proton source. These catalysis studies reveal that while at high acid concentrations the active catalytic species is [Fe2(CO)4(μ-H)(κ2-diamine)(μ-edt)]+, at low acid concentrations the two complexes follow different catalytic mechanisms being associated with differences in their relative rates of protonation

The Impact of Ligand Carboxylates on Electrocatalyzed Water Oxidation

Fossil fuel shortage and severe climate changes due to global warming have prompted extensive research on carbon-neutral and renewable energy resources. Hydrogen gas (H-2), a clean and high energy density fuel, has emerged as a potential solution for both fulfilling energy demands and diminishing the emission of greenhouse gases. Currently, water oxidation (WO) constitutes the bottleneck in the overall process of producing H-2 from water. As a result, the design of efficient catalysts for WO has become an intensively pursued area of research in recent years. Among all the molecular catalysts reported to date, ruthenium-based catalysts have attracted particular attention due to their robust nature and higher activity compared to catalysts based on other transition metals. Over the past two decades, we and others have studied a wide range of ruthenium complexes displaying impressive catalytic performance for WO in terms of turnover number (TON) and turnover frequency (TOF). However, to produce practically applicable electrochemical, photochemical, or photo-electrochemical WO reactors, further improvement of the catalysts' structure to decrease the overpotential and increase the WO rate is of utmost importance. WO reaction, that is, the production of molecular oxygen and protons from water, requires the formation of an O-O bond through the orchestration of multiple proton and electron transfers. Promotion of these processes using redox noninnocent ligand frameworks that can accept and transfer electrons has therefore attracted substantial attention. The strategic modifications of the ligand structure in ruthenium complexes to enable proton-coupled electron transfer (PCET) and atom proton transfer (APT; in the context of WO, it is the oxygen atom (metal oxo) transfer to the oxygen atom of a water molecule in concert with proton transfer to another water molecule) to facilitate the O-O bond formation have played a central role in these efforts. In particular, promising results have been obtained with ligand frameworks containing carboxylic acid groups that either are directly bonded to the metal center or reside in the close vicinity. The improvement of redox and chemical properties of the catalysts by introduction of carboxylate groups in the ligands has proven to be quite general as demonstrated for a range of mono- and dinudear ruthenium complexes featuring ligand scaffolds based on pyridine, imidazole, and pyridazine cores. In the first coordination sphere, the carboxylate groups are firmly coordinated to the metal center as negatively charged ligands, improving the stability of the complexes and preventing metal leaching during catalysis. Another important phenomenon is the reduction of the potentials required for the formation of higher valent intermediates, especially metal-oxo species, which take active part in the key O-O bond formation step. Furthermore, the free carboxylic acid/carboxylate units in the proximity to the active center have shown exciting proton donor/acceptor properties (through PCET or APT, chemically noninnocent) that can dramatically improve the rate as well as the overpotential of the WO reaction

<span style="font-size:11.0pt;font-family: "Times New Roman";mso-fareast-font-family:"Times New Roman";mso-bidi-font-family: Mangal;color:black;mso-ansi-language:EN-GB;mso-fareast-language:EN-US; mso-bidi-language:HI" lang="EN-GB">New tertiary phosphine derivatives of Os₃(CO)₁₂: X-ray structures of 1,2-[Os₃(CO)₁₀{PhP(<i style="mso-bidi-font-style:normal">o</i>-Tol)₂}₂], 1,2,3-[Os₃(CO)₉{(4-FC₆H₄)₃P}₃], 1,2,3-[Os₃(CO)₉{PhP(Cy)₂}₃] and [Os₃(µ-OH)₂(CO)₈{(4-FC₆H₄)₃P}₂]</span>

161-169Reactions of 1,2-[Os3(CO)10(NCMe)2] (1) with tertiary phosphines such as tris(4-fluorophenyl)phosphine (4-FC6H4)3P, bis(<i style="mso-bidi-font-style: normal">o-tolyl)(phenyl)phosphine PhP(o-Tol)2 and dicyclohexyl(phenyl)phosphine PhP(Cy)2 have been examined at room temperature and found to yield the di- and tri-substituted products 1,2-[Os3(CO)10(PR3)2] {(2<span style="mso-bidi-font-weight: bold">), PR3 = (4-FC6H4)3P; (3<span style="mso-bidi-font-weight: bold">), PR3 = PhP(o-Tol)2; (4<span style="mso-bidi-font-weight: bold">), PR3 = PhP(Cy)2} and 1,2,3-[Os3(CO)9(PR3)3] {(5), PR3 = (4-FC6H4)3P; (6<span style="mso-bidi-font-weight: bold">), PR3 = PhP(o-Tol)2; 7, PR3 = PhP(Cy)2} as the major products, in addition to the dihydroxyl-bridged complexes 1,2-[Os3(CO)8(PR3)2(µ-OH)2] {(8<span style="mso-bidi-font-weight: bold">), PR3 = (4-FC6H4)3P; (9<span style="mso-bidi-font-weight: bold">), PR3 = PhP(o-Tol)2; (10<span style="mso-bidi-font-weight: bold">), PR3 = PhP(Cy)2} in trace amounts. Compounds (<b style="mso-bidi-font-weight: normal">2)-(10<span style="mso-bidi-font-weight: bold">) have been characterized by a combination of elemental analyses, infrared, NMR and mass spectral data together with single crystal X-ray diffraction studies for (3), (<b style="mso-bidi-font-weight: normal">5), (7<span style="mso-bidi-font-weight: bold">) and (8<span style="mso-bidi-font-weight: bold">). </span

Synthesis and molecular structures of the 52-electron triiron telluride clusters [Fe3(CO)8(μ3-Te)2(κ2-diphosphine)] - Electrochemical properties and activity as proton reduction catalysts

Heating the 50-electron cluster [Fe3(CO)9 (μ3-Te)2] (1) with the diphosphines Ph2P-R-PPh2 [R = -CH2CH2- (dppe), Z-CH=CH- (dppv), 1,2-C6H4 (dppb), -CH2CH2CH2- (dpp), ferrocenyl (dppf), naphthalenyl (dppbn)] in benzene affords the 52-electron diphosphine-containing tellurium-capped triiron clusters [Fe3(CO)8 (μ3-Te)2 (κ2-diphosphine)] (diphosphine = dppe, dppv, dppb, dpp, dppf, dppnd) (2–7) in moderate yields, resulting from both phosphine addition and carbonyl loss. With 1,2-bis(diphenylphosphino)benzene (dppb) a second product is the cubane cluster [Fe4(CO)10(μ3-Te)4 (κ2-dppb)] (8). Cyclic voltammetry measurements on 2–7 reveal that all clusters show irreversible reductive behaviour at ca. −1.85 V with a series of associated small back oxidation waves, suggesting that reduction leads to significant structural change but that this can be reversed chemically. Oxidation occurs at relatively low potentials and is diphosphine-dependent. The first oxidation appears at ca. +0.35 V for 2–6 with a small degree of reversibility but is as low as +0.14 V for the bis(diphenylphosphino)naphthalene derivative 7 and in some cases is followed by further closely-spaced oxidation. Addition of [Cp2Fe][PF6] to 2–7 results in the formation of new clusters formulated as [Fe3(CO)8(μ3-Te)2(κ2-diphosphine)]+, with their IR spectra suggesting oxidation at the diiron centre. This is supported by computational studies (DFT) of the bis(diphenylphosphino)propane cluster 5 showing that the HOMO is the Fe–Fe σ-bonding orbital, while the LUMO is centered on the diphosphine-substituted iron atom and has significant Fe–Te σ∗-anti-bonding character consistent with the irreversible nature of the reduction. Complexes 2–7 have been examined as proton reduction catalysts in the presence of para-toluenesulfonic acid (TsOH). All are active at their first reduction potential, with a second catalytic process being observed at slightly higher potentials. While their overall electrocatalytic behaviour is similar to that noted for [Fe2(CO)6{μ-E(CH2)3E}] (E = S, Se, Te), the DFT results suggest that as the added electron is localised on the unique iron atom. The mechanistic aspects of hydrogen formation are likely to be quite different from the more widely studied diiron models

Generation of sigma,pi-furyl and thienyl ligands at di-iron centers via facile phosphorus-carbon bond cleavage: Synthesis and molecular structures of [Fe-2(CO)(6)(mu-eta(1),eta(2)-C4H3E){mu-P(C4H3E)(2)}] (E = O, S)

Treatment of [Fe-3(CO)(12)] with tri(2-furyl)phosphine (PFu(3)) or tri(2-thienyl)phosphine (PTh3) in dichloromethane at 40 degrees C leads to facile phosphorus carbon bond scission affording di-iron furyl- and thienyl-bridged complexes [Fe-2(CO)(6)(mu-eta(1),eta(2)-C4H3E)(mu-P(C4H3E)(2))] (1 E = O, Fu; 3 E = S, Th) in good yields, together with smaller amounts of the phosphine-substituted [Fe-2(CO)(5)(mu-eta(1),eta(2)-C4H3E){mu-P(C4H3E)(2)}[P(C4H3E)(3)}] (2E= O,4 E= S). When the same reactions were carried out at room temperature, small amounts of the tri-iron clusters [Fe-3(CO)(11){P(C4H3E)(3)}] (5 E= O, 6 E = S) were isolated in which the coordinated phosphine(s) remain intact Thermolysis of [Fe-3(CO)(11){P(C4H3E)(3)}] at 40 degrees C in dichloromethane gave [Fe-2(CO)(6)(mu-eta(1),eta(2)-C4H3E){mu-P(C4H3E)(2)}], which also undergo phosphine substitution under similar conditions. However, both of these processes are too slow to account for the reaction product ratios and yields observed in the room temperature reactions. In contrast, addition of catalytic amounts of Na+[Ph2CO](center dot) to 5 resulted in the rapid formation of 1. We therefore propose that these reactions may occur via a radical-initiated mechanism proceeding through the initial formation of the 49-electron radical anions [Fe-3(CO)(11){P(C4H3E)(3)}]center dot. The crystal structures of [Fe-2(CO)(6)(mu-eta(1),eta(2)-Fu)(mu-PFu(2))] (1), [Fe-2(CO)(5)(mu-eta(1),eta(2)-Fu)(mu-PFu(2))(PFu(3))] (2), [Fe-2(CO)(6)(mu-eta(1),eta(2)-Th)(mu-PTh2)] (3) and [Fe-3(CO)(11)(PFu(3))] (5) have been determined. The di-iron complexes all show the expected cis arrangement of three-electron donor ligands, short iron-iron distances consistent with a 34-valence electron count, and, in 2, the phosphine is coordinated to the pi-bound iron atom and lies trans to the metal-metal bond. Close inspection of the bonding parameters within the Fe2C2E core reveals that these alkenyl species are quite different to those without electron-withdrawing substituents. The tri-iron cluster 5 has two independent molecules in the asymmetric unit. Each contains two bridging carbonyls and the molecules differ in the relative positions of these carbonyls and the coordinated phosphine ligand, the latter lying in the equatorial plane in both molecules. (C) 2012 Elsevier B.V. All rights reserved

Synthesis of phosphine derivatives of [Fe2(CO)6(μ-sdt)] (sdt = SCH2SCH2S) and investigation of their proton reduction capabilities

The reactions of [Fe2(CO)6(μ-sdt)] (1) (sdt = SCH2SCH2S) with phosphine ligands have been investigated. Treatment of 1 with dppm (bis(diphenylphosphino)methane) or dcpm (bis(dicyclohexylphosphino)methane) affords the diphosphine-bridged products [Fe2(CO)4(μ-sdt)(μ-dppm)] (2) and [Fe2(CO)4(μ-sdt)(μ-dcpm)] (3), respectively. The complex [Fe2(CO)4(μ-sdt)(κ2-dppv)] (4) with a chelating diphosphine was obtained by reacting 1 with dppv (cis-1,2-bis(diphenylphosphino)ethene). Reaction of 1 with dppe (1,2-bis(diphenylphosphino)ethane) produces [{Fe2(CO)4(μ-sdt)}2(μ-κ1-dppe)] (5) in which the diphosphine forms an intermolecular bridge between two diiron cluster fragments. Three products were obtained when dppf (1,1′-bis(diphenylphosphino)ferrocene) was introduced to complex 1; they were [Fe2(CO)5(μ-sdt)(κ1-dppfO)] (6), the previously known [{Fe2(CO)5(μ-sdt)}2(μ-κ1-κ1-dppf)] (7), and [Fe2(CO)4(μ-sdt)(μ-dppf)] (8), with complex 8 being produced in highest yield. Single crystal X-ray diffraction analysis was performed on compounds 2, 3 and 8. All structures reveal the adoption of an anti-arrangement of the dithiolate bridges, while the diphosphines occupy dibasal positions. Infra-red spectroscopy indicates that the mono-substituted complexes 5, 6, and 7 are inert to protonation by HBF4.Et2O, but complexes 2, 3, 4 and [Fe2(CO)5(μ-sdt)(κ1-PPh3)] (9) show shifts of their ν(Csingle bondO) resonances that indicate that protons bind to the metal cores of the clusters. Addition of the one-electron oxidant [Cp2Fe]PF6 does not lead to any discernable shift in the IR resonances. The redox chemistry of the complexes was investigated by cyclic voltammetry, and the abilities of complexes to catalyze electrochemical proton reduction were examined.peerReviewe

Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters : tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues

Reaction of [Fe3(CO)9(µ3-Te)2] (1) with the corresponding phosphine has been used to prepare the phosphine-substituted tellurium-capped triiron clusters [Fe3(CO)9(µ3-Te)2(PPh3)] (2), [Fe3(CO)8(µ3-Te)2(PPh3)] (3) and [Fe3(CO)7(µ3-Te)2(µ-R2PXPR2)] (X = CH2, R = Ph (4), Cy (5); X = NPri, R = Ph (6)). The directly related cluster [Fe3(CO)7(µ3-CO)(µ3-Te)(µ-dppm)] (7) was isolated from the reaction of [Fe3(CO)10(µ-Ph2PCH2PPh2)] with elemental tellurium. The electrochemistry of these new clusters has been probed by cyclic voltammetry, and selected complexes have been tested as proton reduction catalysts. Each 50-electron dicapped cluster exhibits two reductive processes; the first has good chemical reversibility in all cases but the reversibility of the second is dependent upon the nature of the supporting ligands. For the parent cluster1and the diphosphine derivatives4-5this second reduction is reversible, but for the PPh3complex3it is irreversible, possibly as a result of CO or phosphine loss. The nature of the reduced products of1has been probed by DFT calculations. Upon addition of one electron, an elongation of one of the Fe-Te bonding interactions is found, while the addition of the second electron affords an open-shell triplet which is more stable by 8.8 kcal mol-1than the closed-shell singlet dianion and has two elongated Fe-Te bonds. The phosphine-substituted clusters also exhibit oxidation chemistry but with poor reversibility in all cases. Since the reduction potentials for the tellurium-capped clusters occur at more positive potentials than for the sulfur and selenium analogues, and the redox processes also show better reversibility than for the S/Se analogues, the tellurium-capped clusters1and3-5have been examined as proton reduction catalysts. In the presence ofp-toluenesulfonic acid (TsOH) or trifluoroacetic acid (TFA), these clusters reduce protons to H2at both their first and second reduction potentials. Electron uptake at the second reduction potential is far greater than the first, suggesting that the open-shell triplet dianions are efficient catalysts. As expected, the catalytic overpotential increases upon successive phosphine substitution but so does the current response. A mechanistic scheme that takes the roles of the supporting ligands on the preferred route(s) to H2production and release into account is presented