TY - JOUR
T1 - Bioinspired Hydrogenase Models
T2 - The Mixed-Valence Triiron Complex [Fe-3(CO)(7)(mu-edt)(2)] and Phosphine Derivatives [Fe-3(CO)(7-x)(PPh3),(mu-edt)(2)] (x=1, 2) and [Fe-3(CO)(5)(kappa(2)-diphosphine)(mu-edt)(2)] as Proton Reduction Catalysts
AU - Rahaman, Ahibur
AU - Ghosh, Shishir
AU - Unwin, David G.
AU - Basak-Modi, Sucharita
AU - Holt, Katherine B.
AU - Kabir, Shariff E.
AU - Nordlander, Ebbe
AU - Richmond, Michael G.
AU - Hogarth, Graeme
PY - 2014/3/24
Y1 - 2014/3/24
N2 - The mixed-valence triiron complexes [Fe-3(CO)(7-x)(PPh3),(mu-edt)(2)] (x = 0-2; edt = SCH2CH2S) and [Fe-3(CO)(5)(kappa(2)-diphosphine)(mu-edt)(2)] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution 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 [Fe-3(CO)(5)(PPh3)(2)(mu-edt)(2)](+) and [Fe-3(CO)(5)(kappa(2)-diphosphine)(mu-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 [Fe-3(CO)(5)(PPh3)(2)(mu-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 [Fe-3(CO)(7)(mu-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 [Fe-2(CO)(6)(mu-edt)], [Fe-3(CO)(7)(mu-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.
AB - The mixed-valence triiron complexes [Fe-3(CO)(7-x)(PPh3),(mu-edt)(2)] (x = 0-2; edt = SCH2CH2S) and [Fe-3(CO)(5)(kappa(2)-diphosphine)(mu-edt)(2)] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution 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 [Fe-3(CO)(5)(PPh3)(2)(mu-edt)(2)](+) and [Fe-3(CO)(5)(kappa(2)-diphosphine)(mu-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 [Fe-3(CO)(5)(PPh3)(2)(mu-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 [Fe-3(CO)(7)(mu-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 [Fe-2(CO)(6)(mu-edt)], [Fe-3(CO)(7)(mu-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.
KW - IRON-ONLY HYDROGENASE
KW - DENSITY-FUNCTIONAL THEORY
KW - FE-FE HYDROGENASE
KW - H-OX STATE
KW - ACTIVE-SITE
KW - X-RAY
KW - CLOSTRIDIUM-PASTEURIANUM
KW - MOLECULAR-STRUCTURE
KW - CARBON-MONOXIDE
KW - CLUSTER
U2 - 10.1021/om400691q
DO - 10.1021/om400691q
M3 - Article
SN - 0276-7333
VL - 33
SP - 1356
EP - 1366
JO - ORGANOMETALLICS
JF - ORGANOMETALLICS
IS - 6
ER -