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(κ ‐diphosphine)(μ- edt)2] as Proton Reduction Catalysts

The mixed-valence tr i iron complexes [Fe3(CO)7−x(PPh3)x(μ-edt)2] (x = 0−2; edt = SCH2CH2S) and [Fe3(CO)5(κ -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(κ -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. ■ INTRODUCTION 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; however, many challenges remain. Most notably, while it is relatively easy to prepare complexes which bear a close structural resemblance to the Hcluster site, the preparation of good functional models has not yet been achieved. Thus, while virtually all diiron(I) dithiolatebridged complexes are able to act as catalysts for the reduction of protons to hydrogen under reducing conditions, they are generally characterized by high overpotentials and poor turnover numbers and frequencies. In 2005, Pickett and co-workers reported the serendipitous isolat ion 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 Received: July 15, 2013 Published: March 5, 2014 Article pubs.acs.org/Organometallics © 2014 American Chemical Society 1356 dx.doi.org/10.1021/om400691q | Organometallics 2014, 33, 1356−1366 Terms of Use CC-BY