Electrocatalytic dihydrogen evolution mechanism of [Fe2(CO)4(κ2-Ph2PCH2CH2PPh2)(μ-S(CH2)3S)] and related models of the [FeFe]-hydrogenases active site: a DFT investigation

Abstract

A DFT study of protonation thermodynamics in H₂-evolving biomimetic catalysts related to [FeFe]-hydrogenases active site is presented here. Taking as a reference system the electrocatalytic dihydrogen evolution mechanism recently proposed for the synthetic assembly [Fe₂(CO)₄(κ²-Ph₂PCH₂CH₂PPh₂)(μ-S(CH₂)₃S)] (a, which is able to release H₂ after having undergone monoelectron reduction steps and three sequential protonation reactions), we show how the reduction of model complexes to oxidation states lower than those observed in [FeFe]-hydrogenases cofactor leads to a protonation regiochemistry that has no counterpart in the enzymatic mechanism of H₂ production. In particular, double protonation of the metal centers turned out to be disfavored in a by up to 12.5 kcal mol⁻¹ with respect to alternative protonation paths; as for the regiochemistry of triple protonation, the formation of η²-H₂ adducts is disfavored by at least ∼25 kcal mol⁻¹. Structural analysis of the theoretical models also revealed that over-reduction of synthetic complexes, though necessary for observing H₂ evolution from the currently available biomimetic electrocatalysts, can generally impair their structural integrity. Possible approaches for the modulation of protonation regiochemistry are then proposed; in particular, it turned out that a targeted use of σ-donating ligands showing low basicity can favor double protonation of iron centers.