A Functional [NiFe]-Hydrogenase Model Compound That Undergoes Biologically Relevant Reversible Thiolate Protonation (original) (raw)
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Chemical Science, 2023
An azadithiolate bridged CN − bound pentacarbonyl bis-iron complex, mimicking the active site of [Fe-Fe] H 2 ase is synthesized. The geometric and electronic structure of this complex is elucidated using a combination of EXAFS analysis, infrared and Mössbauer spectroscopy and DFT calculations. The electrochemical investigations show that complex 1 effectively reduces H + to H 2 between pH 0-3 at diffusion-controlled rates (10 11 M −1 s −1) i.e. 10 8 s −1 at pH 3 with an overpotential of 140 mV. Electrochemical analysis and DFT calculations suggests that a CN − ligand increases the pK a of the cluster enabling hydrogen production from its Fe(I)-Fe(0) state at pHs much higher and overpotential much lower than its precursor bis-iron hexacarbonyl model which is active in its Fe(0)-Fe(0) state. The formation of a terminal Fe-H species, evidenced by spectroelectrochemistry in organic solvent, via a rate determining proton coupled electron transfer step and protonation of the adjacent azadithiolate, lowers the kinetic barrier leading to diffusion controlled rates of H 2 evolution. The stereo-electronic factors enhance its catalytic rate by 3 order of magnitude relative to a bis-iron hexacarbonyl precursor at the same pH and potential.
Inorganic chemistry, 2007
The reduction chemistry of (µ-bridge)[Fe(CO) 3 ] 2 [bridge ) propane-1,3-dithiolate (1) and ethane-1,2-dithiolate (2)] is punctuated by the formation of distinct products, resulting in a marked difference in CO inhibition of electrocatalytic proton reduction. The products formed following reduction of 2 have been examined by a range of electrochemical, spectroelectrochemical, and spectroscopic approaches. Density functional theory has allowed assessment of the relative energies of the structures proposed for the reduction products and agreement between the calculated spectra (IR and NMR) and bond distances with the experimental spectra and EXAFS-derived structural parameters. For 1 and 2, one-electron reduction is accompanied by dimerization, but the structure, stability, and reaction with CO of the dimer is different in the two cases, and this is responsible for the different CO inhibition response for electrocatalytic proton reduction. Calculations of the alternate structures of the two-electron, one-proton reduced forms of 2 show that the isomers with terminally bound hydrides are unlikely to play a significant role in the chemistry of these species. The hydride-transfer chemistry of the 1B species is more reasonably attributed to a hydride-bridged form. The combination of experimental and computational results provides a solid foundation for the interpretation of the reduction chemistry of dithiolate-bridged diiron compounds, and this will underpin translation of the diiron subsite of the [FeFe] hydrogenase H cluster into an abiological context. Figure 1. Representations of the active site structures of the crystallographically characterized (a) [NiFe]-and (b) [FeFe]H2ases together with the structure of 1, (µ-pdt)[Fe(CO)3]2 (c).
Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases
Angewandte Chemie (International ed. in English), 2017
In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H2 ) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred'. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred' in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes.
Comptes Rendus Chimie, 2008
IR spectroelectrochemical studies of bis(thiolate) and dithiolate-bridged diiron carbonyl compounds, [Fe 2 (m-SR) 2 (CO) 6 ], show that the primary reduction process results in rapid chemical reaction, leading to two-electron reduced products. When the reaction is conducted under an inert atmosphere, the major product is [Fe 2 (m-SR)(m-CO)(CO) 6 ] 1À , where in the case of dithiolate-bridged neutral compounds the product has one bridging and one non-bound sulfur atom. This product is formed in near-quantitative yield for solutions saturated with CO. Reduction of [Fe 2 (m-SR)(m-CO)(CO) 6 ] 1À occurs at potentials near À2.0 V vs. SCE to give a range of products including [Fe(CO) 4 ] 2À . Reduction of thiolate-bridged diiron compounds at mild potentials in the presence of CH 3 COOH leads to formation of [Fe 2 (m-SR)(m-CO)(CO) 6 ] 1À and this is accompanied by an acid-base reaction with the dissociated thiolate. The reaction is largely reversible with recovery of ca. 90% of the starting diiron compound and CH 3 COOH. In the presence of acid, reduction of [Fe 2 (m-SR) 2 (CO) 6 ] proceeds without generation of observable concentrations of the structurally related one-electron reduced compound. Electrocatalytic proton reduction is achieved when the potential is stepped sufficiently negative to reduce [Fe 2 (m-SR)(m-CO)(CO) 6 ] 1À , an observation in keeping with the cyclic voltammetry of the system. Since the catalytic species involved in the weak-acid reactions is structurally distinct from the starting material, and the diiron subsite of the hydrogenase H-cluster, these experiments are of dubious relevance to the biological system. To cite this article: S.
A Novel [FeFe] Hydrogenase Model with a (SCH 2 ) 2 P═O Moiety
Organometallics, 2013
A novel [FeFe]-hydrogenase model complex containing phosphine oxide in the dithiolato ligand, namely [Fe 2 (CO) 6 ][(μ-SCH 2 ) 2 (Ph)PO] (1), has been synthesized and characterized. Complex 1 was prepared via the reaction of equimolar quantities of (μ-LiS) 2 Fe 2 (CO) 6 and OP(Ph)(CH 2 Cl) 2 . The protonation properties of complex 1 have been investigated by monitoring the changes in IR (in the ν(CO) region) and 31 P{ 1 H} NMR spectra upon addition of pyridinium tetrafluoroborate, [HPy][BF 4 ], and HBF 4 ·Et 2 O, suggesting protonation of the PO functionality. In addition, high-level DFT calculations on the protonation sites of complex 1 in CH 2 Cl 2 have been performed and support our experimental observations that the PO unit is protonated by HBF 4 ·Et 2 O. Cyclic voltammetric experiments on complex 1 showed an anodic shift of the oxidation peak upon addition of HBF 4 ·Et 2 O, suggesting a CE process. Figure 1. Active site of [FeFe]-hydrogenase.
Dalton Transactions, 2013
FeFe]-hydrogenases feature a unique active site in which the primary catalytic unit is directly coordinated via a bridging cysteine thiolate to a secondary, redox active [4Fe4S] unit. The goal of this study was to evaluate the impact of a bidentate, redox non-innocent ligand on the electrocatalytic properties of the (μ-S(CH 2 ) 3 S)Fe 2 (CO) 4 L 2 family of [FeFe]-hydrogenase models as a proxy for the iron-sulfur cluster. Reaction of the redox non-innocent ligand 2,2'-bipyridyl (bpy) with (μ-S(CH 2 ) 3 S)Fe 2 (CO) 6 leads to substitution of two carbonyls to form the asymmetric complex (μ-S(CH 2 ) 3 S)Fe 2 (CO) 4 (κ 2 -bpy) which was structurally characterized by single crystal X-ray crystallography. This complex can be protonated by HBF 4 ·OEt 2 to form a bridging hydride. Furthermore, electrochemical investigation shows that, at slow scan rates, the complex undergoes a two electron reduction at −2.06 V vs. Fc + /Fc that likely involves reduction of both the bpy ligand and the metal. Electrocatalytic reduction of protons is observed in the presence of three distinct acids of varying strengths: HBF 4 ·OEt 2 , AcOH, and p-TsOH. The catalytic mechanism depends on the strength of the acid. † Electronic supplementary information (ESI) available: NMR spectra, cyclic voltammetry controls and a CIF file giving single-crystal X-ray diffraction data for 1. CCDC 905661. For ESI and crystallographic data in CIF or other electronic format see