Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model: An Fe‐H⋅⋅⋅H‐N Dihydrogen Bond Characterized by Neutron Diffraction

Abstract

Hydrogenase enzymes in nature use hydrogen as a fuel, but the heterolytic cleavage of H-H bonds cannot be readily observed in enzymes. Here we show that an iron complex with pendant amines in the diphosphine ligand cleaves hydrogen heterolytically. The product has a strong Fe-H⋅⋅⋅H-N dihydrogen bond. The structure was determined by single-crystal neutron diffraction, and has a remarkably short H⋅⋅⋅H distance of 1.489(10) Å between the protic N-H(δ+) and hydridic Fe-H(δ-) part. The structural data for [Cp(C5F4N)FeH(P(tBu)2N(tBu)2H)](+) provide a glimpse of how the H-H bond is oxidized or generated in hydrogenase enzymes. These results now provide a full picture for the first time, illustrating structures and reactivity of the dihydrogen complex and the product of the heterolytic cleavage of H2 in a functional model of the active site of the [FeFe] hydrogenase enzyme.

Reaction Coordinate Leading to H2 Production in [FeFe]-Hydrogenase Identified by Nuclear Resonance Vibrational Spectroscopy and Density Functional Theory

“…10 Our calculations for model L -H hyd H + , which posits an Fe p (I)Fe d (II) redox level and −NH 2 + – / Fe-H coordination, finds an H h ⋯H A distance of 1.38 Å (Table S3), even shorter than in the Bullock complex, and it is thus reasonable that a similar −NH 2 + – / Fe-H ⇄ −NH– / Fe-H 2 equilibrium would occur. We propose that protonation of H hyd-A , in concert with electron transfer from [4Fe-4S] H to [2Fe] H , yields H hyd H + , which can directly yield an Fe-H 2 species.…”

Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2

“…The ligand. 52 In the structure of [4-FeH(NH)] + , the six-membered ring containing the protonated amine adopts a boat conformation. Consequently, the proton bound to N is located in an endo position with respect to the hydride ligand, and an energetically significant dihydrogen bond was found in this complex.…”

Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays

“…A single-crystal neutron diffraction study was conducted at the Spallation Neutron Source at Oak Ridge National Laboratory. 43 The short H···H separation of 1.489(10) Å between the hydridic FeH δ− and the protic NH δ+ (Figure 3) suggests strong "dihydrogen bonding". 44 The structure of this Fe complex provides an idea of how the H−H bond is heterolytically cleaved in the oxidation of H 2 by [FeFe]-hydrogenase.…”