Abstract
A mononuclear hexa-coordinated iron carbonyl complex [Fe(μ-bdt)(CO)2(PTA)2] 1 (bdt = 1,2-benzenedithiolate; PTA = 1,3,5-triaza-7-phosphaadamantane) with two bulky phosphine ligands in the trans position was synthesized and characterized by X-ray structural analysis coulometry data, FTIR, electrochemistry and electronic structure calculations. The complex undergoes a facilitated two-electron reduction 1/12- and shows an inverted one-electron reduction for 1/1- at higher potentials. Electrochemical investigations of 1 are compared to the closely related [Fe(bdt)(CO)2(PMe3)2] compound. A mechanistic suggestion for the hydrogen evolution reaction upon proton reduction from acid media is derived. The stability of 1 in both weak and strong acids is monitored by cyclic voltammetry.
Highlights
The metabolism of hydrogen in cyanobacteria and other microorganisms is catalyzed by hydrogenase enzymes (H2ase).[1]
Paper to form a Fe(II)–H− is thermodynamically favorable with a proton affinity of 33.1 kcal mol−1. This brings the newly formed hydride in close proximity to the protonated bdt-thiolate S atom, so that the formation and release of molecular hydrogen become possible. This observation of an ECEC mechanism was suggested for the dinuclear FeFe(PTA)[2] complex, an inverted redox potential was not reported for that complex.11b This led to smaller catalytic peak currents compared to the FeFe(bdt)(CO)[6] compound
DFT calculations showed that the reduction peaks observed in the cyclic voltammogram for complex 1 originated from inverted redox potentials
Summary
The metabolism of hydrogen in cyanobacteria and other microorganisms is catalyzed by hydrogenase enzymes (H2ase).[1]. Functional catalysts have been reported by Ott and co-workers where they have shown that complexes a and c are minimal models for the [FeFe] hydrogenase active site.[5] Complex d reported by Gao and co-workers can catalyze the production of hydrogen from acetic acid via two different pathways (ECCE and EECC) depending on the sequence of one-electron reduction events (−2.02 and −2.28 V vs Fc/Fc+).[7] The results for complex d were consistent with those obtained by Ott et al on analogous complexes (a and c).[5] Ott and co-workers have reported penta-coordinated Fe-complexes (e–j) as minimal models for the Fed-centre.[6] DuBois. Such a redox potential inversion is indicative of large structural rearrangements to facilitate the second reduction step at lower potentials than the first
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