Regioselectivity in Ligand Substitution Reactions on Diiron Complexes Governed by Nucleophilic and Electrophilic Ligand Properties

CO substitution by PPh3 in Fe2S2(CO)6 proceeds via a novel Fe2S intermediate

The reaction of Fe₂S₂(CO)₆ and PPh₃ affords Fe₂S₂(CO)₄(PPh₃)₂ by an unprecedented mechanism involving the intermediacy of SPPh₃ and Fe₂S(CO)₆(PPh₃)₂.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Mononuclear Mn complexes featuring N,S-/N,N-donor and 1,3,5-triaza-7-phosphaadamantane ligands: synthesis and electrocatalytic properties

Mononuclear Mn(i) carbonyl complexes incorporating 2-mercaptobenzothiazole or 2-mercaptobenzimidazole and phosphaadamantane ligands were evaluated as electrocatalysts for the HER both in acetonitrile and acetonitrile/water.

Synthetic Metallodithiolato Ligands as Pendant Bases in [FeIFeI], [FeI[Fe(NO)]II], and [(μ-H)FeIIFeII] Complexes

The development of ligands with specific stereo- and electrochemical requirements that are necessary for catalyst design challenges synthetic chemists in academia and industry. The crucial aza-dithiolate linker in the active site of [FeFe]-H₂ase has inspired the development of synthetic analogues that utilize ligands which serve as conventional σ donors with pendant base features for H⁺ binding and delivery. Several MN₂S₂ complexes (M = Ni²⁺, [Fe(NO)]²⁺, [Co(NO)]²⁺, etc.) utilize these cis-dithiolates to bind low valent metals and also demonstrate the useful property of hemilability, i.e., alternate between bi- and monodentate ligation. Herein, synthetic efforts have led to the isolation and characterization of three heterotrimetallics that employ metallodithiolato ligand binding to di-iron scaffolds in three redox levels, (μ-pdt)[Fe(CO)₃]₂, (μ-pdt)[Fe(CO)₃][(Fe(NO))^II(IMe)(CO)]⁺, and (μ-pdt)(μ-H)[Fe^II(CO)₂(PMe₃)]₂⁺ to generate (μ-pdt)[(Fe^I(CO)₃][Fe^I(CO)₂·NiN₂S₂] (1), (μ-pdt)[Fe^I(CO)₃][(Fe(NO))^II(IMe)(CO)]⁺ (2), and (μ-pdt)(μ-H)[Fe^II(CO)₂(PMe₃)][Fe^II(CO)(PMe₃)·NiN₂S₂]⁺ (3) complexes (pdt = 1,3-propanedithiolate, IMe = 1,3-dimethylimidazole-2-ylidene, NiN₂S₂ = [N,N'-bis(2-mercaptidoethyl)-1,4-diazacycloheptane] nickel(II)). These complexes display efficient metallodithiolato binding to the di-iron scaffold with one thiolate-S, which allows the free unbound thiolate to potentially serve as a built-in pendant base to direct proton binding, promoting a possible Fe-H^-···⁺H-S coupling mechanism for the electrocatalytic hydrogen evolution reaction (HER) in the presence of acids. Ligand substitution studies on 1 indicate an associative/dissociative type reaction mechanism for the replacement of the NiN₂S₂ ligand, providing insight into the Fe-S bond strength.

Dynamic Flexibility of Hydrogenase Active Site Models Studied with 2D-IR Spectroscopy

Hydrogenase enzymes enable organisms to use H₂ as an energy source, having evolved extremely efficient biological catalysts for the reversible oxidation of molecular hydrogen. Small-molecule mimics of these enzymes provide both simplified models of the catalysis reactions and potential artificial catalysts that might be used to facilitate a hydrogen economy. We have studied two diiron hydrogenase mimics, μ-pdt-[Fe(CO)₃]₂ and μ-edt-[Fe(CO)₃]₂ (pdt = propanedithiolate, edt = ethanedithiolate), in a series of alkane solvents and have observed significant ultrafast spectral dynamics using two-dimensional infrared (2D-IR) spectroscopy. Since solvent fluctuations in nonpolar alkanes do not lead to substantial electrostatic modulations in a solute's vibrational mode frequencies, we attribute the spectral diffusion dynamics to intramolecular flexibility. The intramolecular origin is supported by the absence of any measurable solvent viscosity dependence, indicating that the frequency fluctuations are not coupled to the solvent motional dynamics. Quantum chemical calculations reveal a pronounced coupling between the low-frequency torsional rotation of the carbonyl ligands and the terminal CO stretching vibrations. The flexibility of the CO ligands has been proposed to play a central role in the catalytic reaction mechanism, and our results highlight that the CO ligands are highly flexible on a picosecond time scale.

Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics

The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient H2-forming catalyst in nature. It comprises a diiron active site with three carbon monoxide (CO) and two cyanide (CN(-)) ligands in the active oxidized state (Hox) and one additional CO ligand in the inhibited state (Hox-CO). The diatomic ligands are sensitive reporter groups for structural changes of the cofactor. Their vibrational dynamics were monitored by real-time attenuated total reflection Fourier-transform infrared spectroscopy. Combination of (13)CO gas exposure, blue or red light irradiation, and controlled hydration of three different [FeFe]-hydrogenase proteins produced 8 Hox and 16 Hox-CO species with all possible isotopic exchange patterns. Extensive density functional theory calculations revealed the vibrational mode couplings of the carbonyl ligands and uniquely assigned each infrared spectrum to a specific labeling pattern. For Hox-CO, agreement between experimental and calculated infrared frequencies improved by up to one order of magnitude for an apical CN(-) at the distal iron ion of the cofactor as opposed to an apical CO. For Hox, two equally probable isomers with partially rotated ligands were suggested. Interconversion between these structures implies dynamic ligand reorientation at the H-cluster. Our experimental protocol for site-selective (13)CO isotope editing combined with computational species assignment opens new perspectives for characterization of functional intermediates in the catalytic cycle.

Cyanide-bridged iron complexes as biomimetics of tri-iron arrangements in maturases of the H cluster of the di-iron hydrogenase

Concepts from organometallic chemistry are used to define possibilities of cyanide as a docking unit for bioassembly processes.

Developing from certain catalytic processes required for ancient life forms, the H₂ processing enzymes [NiFe]- and [FeFe]-hydrogenase (H₂ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H₂ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent Fe^IFe^I unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.