On the importance of cyanide in diiron bridging carbyne complexes, unconventional [FeFe]-hydrogenase mimics without dithiolate: An electrochemical and DFT investigation

A large number of catalysts for hydrogen evolution reaction (HER) that are related (structurally and/or functionally) to the active site of [FeFe]-hydrogenases (H-cluster) has been proposed in the past few decades. Very recently, a novel recipe for HER catalysts has been added to the list. This consists in the combination of a diiron core, a ligand that can act as an intramolecular weak base to support protonation (both features that are common to the H-cluster), and a peculiar carbyne ligand that bridges the two metal centers. For instance, complex [Fe2{μ-CNMe2}(μ-CO)(CO)(CN)Cp2] (4) proved to be catalytically active towards HER. The novelty related to this family of biomimicry is not only structural, but also mechanistic. Indeed, an unprecedented (in the context of hydrogenase mimicry) ligand-based mechanism of HER, which avoids protonation at metal, has been dissected by DFT. In order to increase catalytic activity of this “non dithiolate based” system, a systematic search of ligand modification is therefore desirable. In this regard, herein we present results of replacing CN− (the proton shuttle in 4) by other ligands such as CNCH2Ph (5), PPh3 (6) or azide (7). The redox behavior of these compounds has been tested, in the absence and presence of a proton source. CV experiments combined with DFT calculations have revealed and rationalized critical points entailed by modifications of stereo-electronic features of 4. However, understanding the key factors underlying the absence of catalytic activity of these new compounds can be beneficial for the design of future improved biomimicry related to HER. As an example, properly tailored hydrocarbyl complexes replacing dithiolates at the Fe2 core might allow recovering the fruitful electronic properties of cyanides, when coordinated to metal centers.