Search

The electrochemical reduction of complexes [Fe2(CO)4(κ2‐phen)(μ‐xdt)] (phen=1,10‐phenanthroline; xdt=pdt (1), adtiPr (2)) in MeCN‐[Bu4N][PF6] 0.2 m is described as a two‐reduction process. DFT calculations show that 1 and its monoreduced form 1− display metal‐ and phenanthroline‐centered frontier orbitals (LUMO and SOMO) indicating the non‐innocence of the phenanthroline ligand. Two energetically close geometries were found for the doubly reduced species suggesting an intriguing influence of the phenanthroline ligand leading to the cleavage of a Fe−S bond as proposed generally for this type of complex or retaining the electron density and avoiding Fe−S cleavage. Extension of calculations to other complexes with edt, adtiPr bridge and even virtual species [Fe2(CO)4(κ2‐phen)(μ‐adtR)] (R=CH(CF3)2, H) or [Fe2(CO)4(κ2‐phen)(μ‐pdtR)] (R=CH(CF3)2, iPr) showed that the relative stability between both two‐electron‐reduced isomers depends on the nature of the bridge and the possibility to establish a remote anagostic interaction between the iron center {Fe(CO)3} and the group carried by the bridged‐head atom of the dithiolate group. [ABSTRACT FROM AUTHOR]

Catalytic H2 oxidation has been dissected by means of DFT into the key steps common to the Fe2 unit of both the [FeFe]‐hydrogenase cofactor and selected biomimics. The aim was to elucidate the molecular details underlying the very different performances of the two systems. We found that the better enzyme performance is based on a single iron atom that is maintained electron‐poor, favoring H2 binding, although embedded within a highly electron‐rich cofactor, ensuring a facile oxidation of the Fe2–H2 adduct. This is due to 1) CN− coordinating to both iron atoms, due to their amphipathic Lewis acid/base properties, and 2) the 4Fe4S subunit further withdrawing electrons from the Fe2 core. Preserving a moderate electron deficiency at a single iron also helps the cofactor preserve hydride affinity, which favors H2 cleavage. Such valuable characteristics allow the biocatalyst to turnover close to equilibrium conditions. All previous biomimicry has shown, in contrast, the impossibility to properly balance the two apparently contrasting aforementioned requisites, although evident progress has been made by the H2‐ase community. Disclosure of the differences identified could inspire the design of novel biomimics, for instance, reconsidering the use of CN− in the catalyst architecture. Indeed, in the presence of bases normally employed in oxidative catalysis, undesired stable protonation at coordinated CN−, which affects the opposite process (proton reduction), could be overcome. Nature versus biomimicry: Dissecting H2 oxidation by the [FeFe]‐H2ase core in comparison with a series of Fe2S2 models has revealed that CN− ligands and the 4Fe4S subunit are the key to the higher catalytic performance of the enzyme compared with the synthetic compounds. Indeed, cyanides and cubane provide a single electron‐poor iron (which assists H2 binding) embedded within an electron‐rich environment, which in turn ensures a facile subsequent oxidation. [ABSTRACT FROM AUTHOR]

Electrochemical oxidation of the complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] (adtBn=(SCH2)2NCH2C6H5, dmpe=Me2PCH2CH2PMe2) (1) has been studied by cyclic voltammetry (CV) in acetonitrile and in dichloromethane in the presence of various substrates L (L=MeCN, trimethylphosphite, isocyanide). The oxidized species, [1‐MeCN](PF6)2, [1‐(P(OMe)3)2](PF6)2 and [1‐(RNC)4](PF6)2 (R=tert‐butyl, xylyl), have been prepared and characterized by IR and NMR spectroscopies and, except [1‐MeCN](PF6)2, by X‐ray diffraction analysis. The crystallographic structures of the new FeIIFeII complexes reveal that the association of one additional ligand (P(OMe)3 or RNC) occurs and, according to the nature of the substrates, further substitutions of one or three carbonyl groups, by P(OMe)3 or RNC, respectively, arise. Density functional theory (DFT) calculations have been performed to elucidate and discriminate, in each case, the mechanisms leading to the corresponding oxidized species. Moreover, the different degree of ligand substitution in the diiron core has been theoretically rationalized. Modeling [FeFe] hydrogenases: Cyclic voltammetry analyses and density functional theory calculations give new insights into the behavior of the redox key stage FeIFeII of diiron molecules related to the active site of [FeFe] hydrogenases. [ABSTRACT FROM AUTHOR]

It is successfully shown that photocatalytic proton reduction to dihydrogen in the presence of a sacrificial electron donor, such as trimethylamine (TEA) and ascorbate, can be driven by compact sensitizer-catalyst dyads, that is, dithiolate-bridged [FeFe] hydrogenase models tethered to organic sensitizers, such as fluorenes and silafluorenes ( 1 a- 4 a). The sensitizer-catalyst dyads 1 a- 4 a show remarkable and promising catalytic activities as well as enhanced stabilities during photocatalysis performed under UV-light irradiation. The photocatalysis was carried out both in non-aqueous and aqueous media. The latter experiments were performed by solubilizing the photocatalysts within micelles formed by either sodium dodecyl sulfate (SDS) or cetyltrimethylammonium bromide (CTAB). In this study a turnover number of 539 (7 h) is achieved under optimized conditions, which corresponds to an exceptionally high turnover frequency of 77 h−1. Theoretical investigations as well as emission decay experiments were performed to understand the observed phenomena together with the mechanisms of photocatalytic H2 generation. [ABSTRACT FROM AUTHOR]

Rotated geometry: The first example of a dinuclear iron(I)–iron(I) complex featuring a fully rotated geometry related to the active site of [Fe–Fe] hydrogenase is reported (see figure). [ABSTRACT FROM AUTHOR]

A lucky break: A combination of steric pressure and electron asymmetry has provided the first example of a diiron dithiolate that is both rotated and basic. The present work establishes the feasibility of a hydride free rotated structure for the Hred state of the enzyme (see figure). [ABSTRACT FROM AUTHOR]

The behavior of [Fe2(CO)4(κ2-PNPR)(μ-pdt)] (PNPR=(Ph2PCH2)2NR, R=Me ( 1), Ph ( 2); pdt=S(CH2)3S) in the presence of acids is investigated experimentally and theoretically (using density functional theory) in order to determine the mechanisms of the proton reduction steps supported by these complexes, and to assess the role of the PNPR appended base in these processes for different redox states of the metal centers. The nature of the R substituent of the nitrogen base does not substantially affect the course of the protonation of the neutral complex by CF3SO3H or CH3SO3H; the cation with a bridging hydride ligand, 1 μH+ (R=Me) or 2 μH+ (R=Ph) is obtained rapidly. Only 1 μH+ can be protonated at the nitrogen atom of the PNP chelate by HBF4⋅Et2O or CF3SO3H, which results in a positive shift of the proton reduction by approximately 0.15 V. The theoretical study demonstrates that in this process, dihydrogen can be released from a η2-H2 species in the FeIFeII state. When R=Ph, the bridging hydride cation 2 μH+ cannot be protonated at the amine function by HBF4⋅Et2O or CF3SO3H, and protonation at the N atom of the one-electron reduced analogue is also less favored than that of a S atom of the partially de-coordinated dithiolate bridge. In this situation, proton reduction occurs at the potential of the bridging hydride cation, 2 μH+. The rate constants of the overall proton reduction processes are small for both complexes 1 and 2 ( kobs≈4-7 s−1) because of the slow intramolecular proton migration and H2 release steps identified by the theoretical study. [ABSTRACT FROM AUTHOR]