title: Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(μ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x(PPh3)x(μ-edt)2] (x= 1, 2) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] as Proton Reduction Catalysts.
abstract: The mixed-valence triiron complexes [Fe3(CO)7-x(PPh3)x(μ-edt)2] (x= 0-2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt anantiarrangement of the dithiolate bridges, and PPh3substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)-Fe(II)-Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at -0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between -1.47 V for [Fe3(CO)7(μ-edt)2] and around -1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. 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. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.
authors.affiliation: Department of Chemistry, Jahangirnagar University , Savar, Dhaka 1342, Bangladesh ; Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and Chemical Engineering, Lund University , P.O. Box 124, SE-22100 Lund, Sweden.
authors.forename: Ahibur
authors.initials: A
authors.lastname: Rahaman
authors.affiliation: Department of Chemistry, Jahangirnagar University , Savar, Dhaka 1342, Bangladesh ; Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Shishir
authors.initials: S
authors.lastname: Ghosh
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: David G
authors.initials: DG
authors.lastname: Unwin
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Sucharita
authors.initials: S
authors.lastname: Basak-Modi
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K.
authors.forename: Katherine B
authors.initials: KB
authors.lastname: Holt
authors.affiliation: Department of Chemistry, Jahangirnagar University , Savar, Dhaka 1342, Bangladesh.
authors.forename: Shariff E
authors.initials: SE
authors.lastname: Kabir
authors.affiliation: Inorganic Chemistry Research Group, Chemical Physics, Center for Chemistry and Chemical Engineering, Lund University , P.O. Box 124, SE-22100 Lund, Sweden.
authors.forename: Ebbe
authors.initials: E
authors.lastname: Nordlander
authors.affiliation: Department of Chemistry, University of North Texas , 1155 Union Circle, Box 305070, Denton, Texas 76203, United States.
authors.forename: Michael G
authors.initials: MG
authors.lastname: Richmond
authors.affiliation: Department of Chemistry, University College London , 20 Gordon Street, London WC1H 0AJ, U.K. ; Department of Chemistry, King's College London , Britannia House, 7 Trinity Street, London SE1 1DB, U.K.
authors.forename: Graeme
authors.initials: G
authors.lastname: Hogarth
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