Role of the Azadithiolate Cofactor in Models for [FeFe]-Hydrogenase: Novel Structures and Catalytic Implications (original) (raw)
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Unsaturated, Mixed-Valence Diiron Dithiolate Model for the Hox State of the [FeFe] Hydrogenase
Angewandte Chemie International Edition, 2007
The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of protons to H 2 . The active site exists in two functional states (Scheme 1), H red , which is S =0, and H ox , which is S = 1/2. Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis and at using this information to develop protein-free bioinspired synthetic catalysts. A specific research goal is the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form. Most studies on diiron dithiolato carbonyl complexes rely on organic ligands (e.g. phosphanes) in place of the naturally occurring cyanide and μ-SR[Fe 4 S 4 ] ligands, which have complicated acid-base behavior that is often difficult to control outside of the protein. Another barrier to modeling has been the rarity of mixed-valence diiron dithiolate compounds with the appropriate structures, stability, and reactivity.
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.
Inorganic Chemistry, 2008
The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these lowmolecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2 (S 2 C 3 H 6 )(CO) 3 (L)(dppv) (L = CO, PMe 3 , Pi-Pr 3 ) oxidize at potentials ~180 mV milder than the related ethanedithiolates (Angew. Chem. Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. EPR spectra for the mixed-valence cations indicate that the unpaired electron is localized on
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
Organometallics, 2014
Active site mimics of [FeFe]-hydrogenase are shown to be bidirectional catalysts, producing H 2 upon treatment with protons and reducing equivalents. This reactivity complements the previously reported oxidation of H 2 by these same catalysts in the presence of oxidants. 2 )] 2+ ([1H] 2+ ). The species [1H] 2+ consists of a ferrocenium ligand, an Nprotonated amine, and an Fe I Fe I core. In the presence of additional reducing equivalents in the form of decamethylferrocene (Fc*), hydrogen evolution is catalytic, albeit slow. The related catalyst Fe 2 (adt Bn )(CO) 3 (dppv)(PMe 3 ) (3) behaves similarly in the presence of Fc*, except that in the absence of excess reducing agent it converts to the catalytically inactive μ-hydride derivative [μ-H3] + . Replacement of the adt in [1] 0 with propanedithiolate (pdt) results in a catalytically inactive complex. In the course of synthesizing [FeFe]-hydrogenase mimics, new routes to ferrocenylphosphine ligands and nonamethylferrocene were developed.
Dalton transactions (Cambridge, England : 2003), 2018
The mono-substituted complex [Fe2(CO)5(μ-naphthalene-2-thiolate)2(P(PhOMe-p)3)] was prepared taking after the structural principles from both [NiFe] and [FeFe]-hydrogenase enzymes. Crystal structures are reported for this complex and the all carbonyl analogue. The bridging naphthalene thiolates resemble μ-bridging cysteine amino acids. One of the naphthyl moieties forms π-π stacking interactions with the terminal bulky phosphine ligand in the crystal structure and in calculations. This interaction stabilizes the reduced and protonated forms during electrocatalytic proton reduction in the presence of acetic acid and hinders the rotation of the phosphine ligand. The intramolecular π-π stabilization, the electrochemistry and the mechanism of the hydrogen evolution reaction were investigated using computational approaches.
Molecules
The catalytic reaction occurs at the diiron site, while the [4Fe-4S] cluster functions as a redox shuttle. In the oxidized resting state (Hox), the iron ions of the diiron site bind one cyanide (CN −) and carbon monoxide (CO) ligand each and a third carbonyl can be found in the Fe-Fe bridging position (µCO). In the presence of exogenous CO, A fourth CO ligand binds at the diiron site to form the oxidized, CO-inhibited H-cluster (Hox-CO). We investigated the reduced, CO-inhibited H-cluster (Hred´-CO) in this work. The stretching vibrations of the diatomic ligands were monitored by attenuated total reflection Fourier-transform infrared spectroscopy (ATR FTIR). Density functional theory (DFT) at the TPSSh/TZVP level was employed to analyze the cofactor geometry, as well as the redox and protonation state of the H-cluster. Selective 13 CO isotope editing, spectro-electrochemistry, and correlation analysis of IR data identified a one-electron reduced, protonated [4Fe-4S] cluster and an apical CN − ligand at the diiron site in Hred´-CO. The reduced, CO-inhibited H-cluster forms independently of the sequence of CO binding and cofactor reduction, which implies that the ligand rearrangement at the diiron site upon CO inhibition is independent of the redox and protonation state of the [4Fe-4S] cluster. The relation of coordination dynamics to cofactor redox and protonation changes in hydrogen conversion catalysis and inhibition is discussed.
Diiron Dithiolato Carbonyls Related to the H ox CO State of [FeFe]-Hydrogenase
Journal of the American Chemical Society, 2008
Oxidation of the electron-rich (E 1/2 = −175 vs Ag/AgCl) ethanedithiolato complex Fe 2 (S 2 C 2 H 4 )-(CO) 2 (dppv) 2 (1) under a CO atmosphere yielded [Fe 2 (S 2 C 2 H 4 )(μ-CO)(CO) 2 (dppv) 2 ] + ([1(CO)] + ), a model for the H ox CO state of the [FeFe]-hydrogenases. This complex exists as two isomers: a kinetically favored unsymmetrical derivative, unsym-[1(CO)] + , and a thermodynamically favored isomer, sym-[1(CO)] + , wherein both diphosphines span apical and basal sites. Crystallographic characterization of sym-[1(CO)] + confirmed a C 2 -symmetric structure with a bridging CO ligand and an elongated Fe-Fe bond of 2.7012(14) Å, as predicted previously. Oxidation of sym-[1(CO)] + and unsym-[1(CO)] + again by 1e − oxidation afforded the respective diamagnetic diferrous derivatives where the relative stabilities of the sym and unsym isomers are reversed. DFT calculations indicate that the stabilities of sym and unsym isomers are affected differently by the oxidation state of the diiron unit: the mutually trans CO ligands in the sym isomer are more destabilizing in the mixedvalence state than in the diferrous state. EPR analysis of mixed-valence complexes revealed that, for [1] + , the unpaired spin is localized on a single iron center, whereas for unsym/sym-[1(CO)] + , the unpaired spin was delocalized over both iron centers, as indicated by the magnitude of the hyperfine coupling to the phosphine ligands trans to the Fe-Fe vector. Oxidation of 1 by 2 equiv of acetylferrocenium afforded the dication [1] 2+ , which, on the basis of low-temperature IR spectrum, is structurally similar to [1] + . Treatment of [1] 2+ with CO gives unsym-[1(CO)] 2+ .