Irene P. Georgakaki

Carbon Monoxide and Cyanide Ligands in a Classical Organometallic Complex Model for Fe‐Only Hydrogenase

The Fe(I) organometallic complex [(µ-SCH(2)CH(2)CH(2)S)Fe(2)(CO)(6)] provides a structural model for the cyano-carbonyl diiron site of Fe-only hydrogenase as characterized by X-ray crystallography (the picture shows the structure (black) of the model overlaid with that of the Fe-Fe dimetallic site in the hydrogenase isolated from Desulfovibrio desulfuricans). Cyanide substitution of CO occurs readily and provides spectroscopic references for the active site.

Electrocatalysis of hydrogen production by active site analogues of the iron hydrogenase enzyme: structure/function relationships

A series of binuclear FeIFeI complexes, (μ-SEt)2[Fe(CO)2L]2 (L = CO (1), PMe3 (1-P)), (μ-SRS)[Fe(CO)2L]2 (R = CH2CH2 (μ-edt): L = CO (2), PMe3 (2-P); R = CH2CH2CH2(μ-pdt): L = CO (3), PMe3 (3-P); and R = o-CH2C6H4CH2 (μ-o-xyldt): L = CO (4), PMe3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The redox levels for H2 production were established by spectroelectrochemistry to be Fe0Fe0 for the all-CO complexes and FeIFe0 for the PMe3-substituted derivatives. As electrocatalysts, the PMe3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H2. An (η2-H2)FeII–Fe0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

The organometallic active site of [Fe]hydrogenase: Models and entatic states

The simple organometallic, (μ-S2)Fe2(CO)6, serves as a precursor to synthetic analogues of the chemically rudimentary iron-only hydrogenase enzyme active site. The fundamental properties of the (μ-SCH2CH2CH2S)[Fe(CO)3]2 compound, including structural mobility and regioselectivity in cyanide/carbon monoxide substitution reactions, relate to the enzyme active site in the form of transition-state structures along reaction paths rather than ground-state structures. Even in the absence of protein-based active-site organization, the ground-state structural model complexes are shown to serve as hydrogenase enzyme reaction models, H2 uptake and H2 production, with the input of photo- or electrochemical energy, respectively.

Fundamental properties of small molecule models of Fe-only hydrogenase: computations relative to the definition of an entatic state in the active site

Abstract Well-studied organometallic complexes (μ-SRS)Fe 2 (CO) 6 that serve as structural models of the active site of Fe-only hydrogenases have been employed in DFT computational studies with the goal of understanding the fundamental nature of the active site of this biological catalyst. Intramolecular CO site exchange processes, experimentally observable in variable temperature (VT) NMR studies were modeled. The transition state structure of the Fe(CO) 3 unit rotation looks very similar to the structure that the active site has adopted in the protein environment. That is, a semi-bridging CO is formed upon Fe(CO) 3 rotation partially disrupting the FeFe bonding interaction and leaving an open site trans to this semi-bridging CO. The CN − /CO substitution reaction of these complexes which yields the disubstituted derivatives, (μ-SRS)[Fe(CO) 2 (CN)] 2 2− , was also examined as experimental results found a complicated, R-dependent, reactivity pattern for the second CN − addition. The connection of the above rotation process to the CN − /CO substitution was supported by the fact that an intermediate with a μ-CO group, like that resulting from the Fe(CO) 3 unit rotation, is formed upon CN − attack. The assumption that the Fe(CO) 3 rotational barrier is an important contributor to the overall activation energy of CN − attack, explains the experimental observation that generally the second CN − addition finds a lower Fe(CO) 3 rotational barrier due to the presence of the already coordinated CN − ligand.

SYNERGY BETWEEN THEORY AND EXPERIMENT AS APPLIED TO H/D EXCHANGE ACTIVITY ASSAYS IN [Fe]H2ase ACTIVE SITE MODELS

Publisher Summary This chapter discusses synergy between theory and experiment as applied to H/D exchange activity assays in [Fe] H2ase active site models. The growing importance of computational chemistry in mechanistic inorganic chemistry may be ascribed to the broad accessibility and application of density functional theory and related techniques to large molecules, in this case a diiron complex with 10 to 12 coordination sites filled with diatomic or larger ligands. Hydrogenases are biological catalysts responsible for H 2 uptake or production in which the required H 2 cleavage is established to occur in a reversible and heterolytic manner (H + /H – ). This activity is typically assayed by H/D exchange reactivity in H 2 /D 2 O or H 2 /D 2 / H 2 O mixtures. The observation of inhibition of the H/D exchange reaction by CO and CH 3 CN implicates coordinatively unsaturated intermediates in the H 2 capture process. New experiments are also carried out to test the hypotheses implied by some of the individual steps of the proposed mechanism, which are calculated to be energetically feasible.