Catalytic Hydrogen Evolution by Fe(II) Carbonyls Featuring a Dithiolate and a Chelating Phosphine

Two pentacoordinate mononuclear iron carbonyls of the form (bdt)­Fe­(CO)­P₂ [bdt = benzene-1,2-dithiolate; P₂ = 1,1′-diphenylphosphinoferrocene (<b>1</b>) or methyl-2-{bis­(diphenylphosphinomethyl)­amino}­acetate (<b>2</b>)] were prepared as functional, biomimetic models for the distal iron (Fe_d) of the active site of [FeFe]-hydrogenase. X-ray crystal structures of the complexes reveal that, despite similar ν­(CO) stretching band frequencies, the two complexes have different coordination geometries. In X-ray crystal structures, the iron center of <b>1</b> is in a distorted trigonal bipyramidal arrangement, and that of <b>2</b> is in a distorted square pyramidal geometry. Electrochemical investigation shows that both complexes catalyze electrochemical proton reduction from acetic acid at mild overpotential, 0.17 and 0.38 V for <b>1</b> and <b>2</b>, respectively. Although coordinatively unsaturated, the complexes display only weak, reversible binding affinity toward CO (1 bar). However, ligand centered protonation by the strong acid, HBF₄·OEt₂, triggers quantitative CO uptake by <b>1</b> to form a dicarbonyl analogue <b>[1­(H)-CO]⁺</b> that can be reversibly converted back to <b>1</b> by deprotonation using NEt₃. Both crystallographically determined distances within the bdt ligand and density functional theory calculations suggest that the iron centers in both <b>1</b> and <b>2</b> are partially reduced at the expense of partial oxidation of the bdt ligand. Ligand protonation interrupts this extensive electronic delocalization between the Fe and bdt making <b>1­(H)⁺</b> susceptible to external CO binding

Sequential Oxidations of Thiolates and the Cobalt Metallocenter in a Synthetic Metallopeptide: Implications for the Biosynthesis of Nitrile Hydratase

Cobalt nitrile hydratases (Co-NHase) contain a catalytic cobalt­(III) ion coordinated in an N₂S₃ first coordination sphere composed of two amidate nitrogens and three cysteine-derived sulfur donors: a thiolate (-SR), a sulfenate (-S­(R)­O^–), and a sulfinate (-S­(R)­O₂^–). The sequence of biosynthetic reactions that leads to the post-translational oxidations of the metal and the sulfur ligands is unknown, but the process is believed to be initiated directly by oxygen. Herein we utilize cobalt bound in an N₂S₂ first coordination sphere by a seven amino acid peptide known as SODA (ACDLPCG) to model this oxidation process. Upon exposure to oxygen, Co-SODA is oxidized in two steps. In the first fast step (seconds), magnetic susceptibility measurements demonstrated that the metallocenter remains paramagnetic, that is, Co²⁺, and sulfur K-edge X-ray absorption spectroscopy (XAS) is used to show that one of the thiolates is oxidized to sulfinate. In a second process on a longer time scale (hours), magnetic susceptibility measurements and Co K-edge XAS show that the metal is oxidized to Co³⁺. Unlike other model complexes, additional slow oxidation of the second thiolate in Co-SODA is not observed, and a catalytically active complex is never formed. The likely reason is the absence of the axial thiolate ligand. In essence, the reactivity of Co-SODA can be described as between previously described models which either quickly convert to final product or are stable in air, and it offers a first glimpse into a possible oxidation pathway for nitrile hydratase biosynthesis