Favorable Protonation of the (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging μ-H Counterpart: A Spectroscopic and DFT Study

[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.

Catalytic H2 evolution/oxidation in [FeFe]-hydrogenase biomimetics: account from DFT on the interplay of related issues and proposed solutions

A DFT overview on selected issues regarding diiron catalysts related to [FeFe]-hydrogenase biomimetic research, with implications for both energy conversion and storage strategies.

Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(μ-dithiolate)(μ,κ1,κ2-triphos)

A series of Fe₂(triphos)(CO)₃(μ-dithiolate) complexes have been prepared and studied as models of the diiron centre in [FeFe]-hydrogenases.

Brønsted-Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

Activating a Low Overpotential CO2 Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu3 tpy)(bpy)(NCCH3 )](2+) (tBu3 tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO2 to CO. The methyl group enables CO2 binding already at the one-electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO2 reduction catalysts.

Ligand effects on the electrochemical behavior of [Fe2(CO)5(L){μ-(SCH2)2(Ph)PO}] (L = PPh3, P(OEt)3) hydrogenase model complexes

In this paper we study the influence of substituting one CO ligand in [Fe₂(CO)₆{μ-(SCH₂)₂(Ph)PO}] (1) by better σ-donors (PPh₃(2) and P(OMe)₃(3)) in relation to the electrochemical behavior.

Excited state properties of diiron dithiolate hydrides: implications in the unsensitized photocatalysis of H2 evolution

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride, [Fe2(pdt)(μ-H)(CO)4dppv](+) (1(+), dppv = cis-1,2-C2H2(PPh2)2; pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe2(pdt)(μ-H)(CO)4(PMe3)2](+) (2(+)), which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; J. Am. Chem. Soc., 2012, 134, 4525). Theoretical results illustrate that the peculiar reactivity associated to the excited states of 1(+) and 2(+) is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons; (ii) reactivity could be related to cleavage of a Fe-S bond, which implies formation of a transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long enough; (iii) also in line with a Fe-S bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing S-H(δ+) and Fe-H(δ-) moieties is formed and, once reduced by a suitable electron donor, it can readily afford H2 plus an unprotonated form of the FeFe complex. This last possibility is consistent with (31)P NMR and IR solution data. All the three possibilities are compatible with the capability of 1(+) and 2(+) to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of 1(+)) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and catalyze HER. Finally, the comparison of photoexcitation behavior of 1(+) and 2(+) allows a sort of generalization about the functioning of such hydride species.