Electrocatalytic proton reduction by dithiolate-bridged diiron carbonyl complexes: a connection to the H-cluster?

[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.

Unraveling the mechanism of biomimetic hydrogen fuel production – a first principles molecular dynamics study

The Fe₂(bdt)(CO)₆ [bdt = benzenedithiolato] complex, a synthetic mimic of the [FeFe] hydrogenase enzyme can electrochemically convert protons into molecular hydrogen. The free energy landscape reveals a different mechanism for the biomimetic cycle.

Electronic Communication between Dithiolato-Bridged Diiron Carbonyl and S-Bridged Redox-Active Centres

The catalytic potential of linked redox centres is exemplified by the catalytic site of [FeFe]-hydrogenases, which feature a diiron subsite linked by a cysteinyl S atom to a 4Fe4S cube. The investigation of systems possessing similarly-linked redox sites is important because it provides a context for understanding the biological system and the rational design of abiological catalysts. The structural, electrochemical and spectroscopic properties of Fe2(CO)5(CH3C(CH2S)2CH2SPhNO2, I-bzNO2 and the aniline analogue, I-bzNH2, are described and IR spectroelectrochemical studies have allowed investigation of the reduction products and their reactions with CO and protons. These measurements have allowed identification of the nitrobenzenyl radical anion, quantification of the shifts of the (CO) bands on ligand-based reduction compared with NO2/NH2 exchange and protonation of the pendent ligand. The strength of thioether coordination is related to the electronic effects, where competitive binding studies with CO show that CO/thioether exchange can be initiated by redox processes of the pendent ligand. Stoichiometric multi electron/proton transfer reactions of I-bzNO2 localised on nitrobenzene reductions occur at mild potentials and a metal-centred reduction in the presence of protons does not lead to significant electrocatalytic proton reduction.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

An oxidized active site model for the FeFe hydrogenase: reduction with hydrogen gas

Models for the oxidized form of the FeFe hydrogenase active site have been prepared. These cationic complexes contain two iron atoms, carbonyl ligands, a propanedithiolate bridge, and one other bridging group. Reduction of these complexes with hydrogen gas is demonstrated.

Theoretical studies on the redox potentials of Fe dinuclear complexes as models for hydrogenase

Density Functional calculations have been performed at the uB3LYP and uBP86 levels to calculate the one-electron redox potentials for a series of small models based on the diiron hydrogenase enzymes in the presence of acetonitrile (MeCN). The solvation effects in MeCN are incorporated via a self-consistent reaction field (SCRF) using the polarized continuum model (PCM). The calculated redox potentials reproduce the trends in experimental data with an average error of only 0.12 V using the BP86 functional, whereas comparing results with the B3LYP functional require a systematic shift of -0.82 and -0.53 V for oxidation and reduction, respectively. The bonding orbitals and d-electron populations were examined using Mulliken population analysis, and the results were used to rationalize the calculated and observed redox potentials. These studies demonstrate that the redox potential correlates with the empirical spectrochemical series for the ligands, as well as with the amount of electron density donated by the ligand onto the Fe centers.

Insights into the Mechanism of Electrocatalytic Hydrogen Evolution Mediated by Fe2(S2C3H6)(CO)6: The Simplest Functional Model of the Fe-Hydrogenase Active Site

The di-iron complex Fe2(S2C3H6)(CO)6 (a), one of the simplest functional models of the Fe-hydrogenases active site, is able to electrocatalyze proton reduction. In the present study, the H2 evolving path catalyzed by a has been characterized using density functional theory. It is showed that, in the early stages of the catalytic cycle, a neutral mu-H adduct is formed; monoelectron reduction and subsequent protonation can give rise to a diprotonated neutral species (a-muH-SH), which is characterized by a mu-H group, a protonated sulfur atom, and a CO group bridging the two iron centers, in agreement with experimental IR data indicating the formation of a long-lived mu7-CO species. H2 release from a-muH-SH, and its less stable isomer a-H2 is kinetically unfavorable, while the corresponding monoanionic compounds (a-muH-SH- and a-H2-) are more reactive in terms of dihydrogen evolution, in agreement with experimental data. The key species involved in electrocatalysis have structural features different from the hypothetical intermediates recently proposed to be involved in the enzymatic process, an observation that is possibly correlated with the reduced catalytic efficiency of the biomimetic di-iron assembly.

Powering the planet: chemical challenges in solar energy utilization

Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO(2) emissions in the atmosphere demands that holding atmospheric CO(2) levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.