Mechanistic Insight into Electrocatalytic H₂ Production by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

Abstract

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe(CN){μ-CN(Me)}(μ-CO)(CO)(Cp)] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN and amino-carbyne ligands have been elucidated. After the first reduction, CN works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN that forms a Fe(CNH) moiety: i.e., the acidic source for the H generation. The hydride (formally 2e/H), necessary to the heterocoupling with H is thus provided by the μ-CN(Me) ligand and not by Fe centers, as occurs in typical LFeS derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN to CN(Me) is highlighted. The incompetence for the HER of the related species [Fe{μ-CN(Me)}(μ-CO)(CO)(Cp)] (2) has been investigated and attributed to the loss of proton responsiveness caused by CN replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe scaffold, could be exploited to develop more active and robust catalysts for the HER.

H2 Activation in [FeFe]‐Hydrogenase Cofactor Versus Diiron Dithiolate Models: Factors Underlying the Catalytic Success of Nature and Implications for an Improved Biomimicry

“…[62] Recently,aCN À ,o nce installed in aF e 2 complex with ac arbyne ligand in place of dithiolate, has provede ffectivea saprotonshuttleinc atalytic H 2 production. [63] Heterolytic cleavage is predicted to be exergonic at the Fe II Fe I stage in synthetic models, whereas in the H-cluster it becomesf avorable only after the first electron removal, which suggestst hat the iron affinity forH À in biomimics is higher than in the H-cluster.H owever,a ne fficient and completeH 2 activation has been experimentally detectedo nly when the heterolytic cleavage is sufficiently favorable to compensatet he (usuallye nergetically unfavored) H 2 binding( such as in 9(adtN Bn ) 2 + + , 9 2 + + ,a nd 10 2 + + ). Indeed, DFT calculations indicate that only in these cases the process entailing H 2 binding and cleavage is overall exergonic.…”

“…Alternatively, the basicity of CN − could be dampened by changing the nature of the other ligands coordinated to the iron atoms . Recently, a CN − , once installed in a Fe 2 complex with a carbyne ligand in place of dithiolate, has proved effective as a proton shuttle in catalytic H 2 production …”

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] Related to the Active Site of [FeFe] Hydrogenases

“…DFT calculations have been performed with the pure functional BP86 and an all‐electron triple‐ζ basis set with polarization on all atoms (TZVP), as implemented in the TURBOMOLE 7.1.1 suite . This level of theory has provided good and consistent results in the reproduction of structural and reactivity features in bioinspired compounds and also very recently in other diiron catalysts for hydrogen evolution . In addition, the BP86/TZVP scheme was previously used to study the oxidative behavior of complex 1 in the absence and in the presence of CO .…”

“…[41] This level of theory has provided good and consistent results in the reproduction of structural and reactivity features in bioinspired compounds [33,42,43] and also very recently in other diiron catalysts for hydrogen evolution. [44] In addition, the BP86/TZVP scheme was previously used to study the oxidative behavior of complex 1 in the absence and in the presence of CO. [23] When dealing with associative versus dissociative steps (i.e.,i ntermolecular interactions involved in ligand exchange processes), the method was further refined by including Grimme'se mpirical dispersion corrections (BP86-D3/TZVP). [45,46] The resolution of identity (RI) technique was adopted to speed up computations.…”

Constructing Organometallic Architectures from Aminoalkylidyne Diiron Complexes

“…Thus, a huge number of dithiolate [FeFe]compounds has been designed to mimic biological systems, and investigated for various catalytic applications. [5b], It is worthy of notice that diiron complexes resembling the [Fe 2 Cp 2 (CO) 4 ] structure, and lacking of dithiolate groups, may exhibit electrocatalytic properties which are comparable to those of closer [FeFe]‐hydrogenase models . A wide range of diiron complexes containing CO and/or stabilizing Cp ligands have progressively emerged as versatile platforms for the building of uncommon organic fragments, exploiting unusual reaction pathways, assisted by the cooperative interaction of the two iron centers.…”