A DFT investigation on structural and redox properties of a synthetic Fe6S6 assembly closely related to the [FeFe]-hydrogenases active site

[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.

Mechanisms and origins of the switchable regioselectivity of FeBr3-catalyzed [1,2]-aryl and [1,2]-alkyl shifts of α-aryl aldehydes

With the aid of DFT calculations, the FeBr3-catalyzed skeletal rearrangements of 2-cyclohexanal,2-p-C6H4OMe-propylaldehyde (1A) and 2-phenyl,2-p-C6H4OMe-propylaldehyde (1B) were investigated theoretically. As compared to mono-FeBr3 as a catalyst, the bis-FeBr3 serving as a catalyst is found to be not only enhancing the catalytic efficiency but also improving the product selectivity. For the reaction starting from 1A, the [1,2]-group shift (first step) is rate-determining, and why the Cy shift is the most favored is rationalized in comparison with the p-C6H4OMe and Me shifts. For the reaction starting from 1B, the [1,2]-H shift (second step) is rate-determining although the [1,2]-p-C6H4OMe shift is favored over the [1,2]-phenyl shift. In contrast to the experimental proposal, the newly established H2O/Br(-) joint-assisted H-shift mechanism explains the partial α-H source of the [1,2]-Cy shift product. In addition, we discussed the inherent mechanism that explains why both the [1,2]-p-C6H4OMe and [1,2]-p-C6H4CF3 shifts are more facile than the [1,2]-phenyl shift although the substituents -OMe and -CF3 have opposite electronic behaviors.

Towards [NiFe]-hydrogenase biomimetic models that couple H2 binding with functionally relevant intramolecular electron transfers: a quantum chemical study

[FeFe]- and [NiFe]-hydrogenases are dihydrogen-evolving metalloenzymes that share striking structural and functional similarities, despite being phylogenetically unrelated. Most notably, they are able to combine substrate binding and redox functionalities, which has important bearings on their efficiency. Model complexes of [FeFe]-hydrogenases that are able to couple H2 binding with a substrate-dependent intramolecular electron transfer promoting dihydrogen activation were recently shown to reproduce the complex redox chemistry of the all-iron enzyme. Notably, coupling of H2 binding and intramolecular redox events was proposed to have a key role also in [NiFe]-hydrogenases, but this feature is not reproduced in currently available nickel-iron biomimetic compounds. In the present study, we exploit dedicated density functional theory approaches to show that H2 binding and activation on a NiFe core can be favored by the installment of conveniently substituted isocyanoferrocenes, thanks to their ability to undergo intramolecular reduction upon substrate binding. Our results support the concept that a unified view on hydrogenase chemistry is a key element to direct future efforts in the modeling of microbial H2 metabolism.

A theoretical study on the enhancement of functionally relevant electron transfers in biomimetic models of [FeFe]-hydrogenases

Recent advances aimed at modeling the chemistry of the active site of [FeFe]-hydrogenases (the H-cluster, composed by a catalytic Fe(2)S(2) subcluster and an Fe(4)S(4) portion) have led to the synthesis of binuclear coordination compounds containing a noninnocent organophosphine ligand [2,3-bis(diphenylphosphino)maleic anhydride, bma] that is able to undergo monoelectron reduction, analogously to the tetranuclear Fe(4)S(4) subcluster portion of the H-cluster. However, such a synthetic model was shown to feature negligible electronic communication between the noninnocent ligand and the remaining portion of the cluster, at variance with the enzyme active site. Here, we report a theoretical investigation that shows why the electron transfer observed in the enzyme upon protonation of the catalytic Fe(2)S(2) subsite cannot take place in the bma-containing cluster. In addition, we show that targeted modifications of the bma ligand are sufficient to restore the electronic communication within the model, such that electron density can be more easily withdrawn from the noninnocent ligand, as a result of protonation of the iron centers. Similar results were also obtained with a ligand derived from cobaltocene. The relevance of our findings is discussed from the perspective of biomimetic reproduction of proton reduction to yield molecular hydrogen.