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 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 [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.
Highlights
Over the past decade there has been intense interest in the chemistry of dithiolate-bridged diiron complexes,1−5 since they closely resemble the two-iron unit of the H-cluster active site of iron-only hydrogenases.6−8 As a result of these studies important advances have been made in our understanding of how this enzyme site functions;9−12 many challenges remain
That at 221.5 ppm is assigned to the carbonyl bound to the central iron atom, while the other resonances are attributed to apical and basal carbonyls bound to the other iron atoms
The IR spectrum shows a series of strong absorptions between 2073 and 1975 cm−1 together with a weak absorption at 1904 cm−1, which we attribute to the carbonyl on the central iron atom
Summary
Over the past decade there has been intense interest in the chemistry of dithiolate-bridged diiron complexes,− since they closely resemble the two-iron unit of the H-cluster active site of iron-only hydrogenases.− As a result of these studies important advances have been made in our understanding of how this enzyme site functions;− many challenges remain. In 2005, Pickett and co-workers reported the serendipitous isolation of the tetrairon cluster [Fe4(CO)8{μ3(SCH2)3CMe}2] (A) formed upon reaction of Bosnich’s thiol with [Fe3(CO)12].13. This is a rare example of a linear 66electron cluster being characterized by three metal−metal bonds− and is formally a mixed-valence complex comprised. With respect to functional modeling of the hydrogenase active site, while the one-electron-reduction product A− was shown to be only a moderate catalyst for proton reduction, addition of a second electron resulted in formation of A2−, which was shown to be an excellent electrocatalyst, dihydrogen elimination being at least 500 times greater than that found in related [Fe2(CO)6(μ-dithiolate)] complexes.. Scheme 2 Scheme 3 of a chain of Fe(I)−Fe(II)−Fe(II)−Fe(I) centers. Importantly with respect to functional modeling of the hydrogenase active site, while the one-electron-reduction product A− was shown to be only a moderate catalyst for proton reduction, addition of a second electron resulted in formation of A2−, which was shown to be an excellent electrocatalyst, dihydrogen elimination being at least 500 times greater than that found in related [Fe2(CO)6(μ-dithiolate)] complexes. Later detailed electrochemical and DFT studies shed some light onto the high activity of A2−.20,21 it is proposed that upon addition of two electrons the central iron−iron bond of A is cleaved, which in turn leads to rotation of the iron tricarbonyl groups and formation of bridging carbonyls and vacant coordination sites, the latter being able to bind protons efficiently and leading to high electrocatalytic ability (Scheme 1).
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