Effects of Thiolate Ligation in Monoiron Hydrogenase (Hmd): Stability of the {Fe(CO)2}2+ Core with NNS Ligands

Development of an FeII Complex Exhibiting Intermolecular Proton Shifting Coupled Spin Transition

In this study, we investigated reversible intermolecular proton shifting (IPS) coupled with spin transition (ST) in a novel FeII complex. The host FeII complex and the guest carboxylic acid anion were connected by intermolecular hydrogen bonds (IHBs). We extended the intramolecular proton transfer coupled ST phenomenon to the intermolecular system. The dynamic phenomenon was confirmed by variable-temperature single-crystal X-ray diffraction, neutron crystallography, and infrared spectroscopy. The mechanism of IPS was further validated using density functional theory calculations. The discovery of IPS-coupled ST in crystalline molecular materials provides good insights into fundamental processes and promotes the design of novel multifunctional materials with tunable properties for various applications, such as optoelectronics, information storage, and molecular devices.

Scaffold-Based Functional Models of [Fe]-Hydrogenase (Hmd): Building the Bridge between Biological Structure and Molecular Function

The well-known dinuclear [FeFe] and [NiFe] hydrogenase enzymes are redox-based proton reduction and H₂ oxidation catalysts. In comparison, the structural and functional aspects of the mononuclear nonredox hydrogenase, known as [Fe]-hydrogenase or Hmd, have been less explored because of the relatively recent crystallographic elucidation of the enzyme active site. Additionally, the synthetic challenges posed by the highly substituted and asymmetric coordination environment of the iron guanylylpyridinol (FeGP) cofactor have hampered functional biomimetic modeling studies to a large extent. The active site contains an octahedral low-spin Fe(II) center with the following coordination motifs: a bidentate acyl-pyridone moiety (C,N) and cysteinyl-S in a facial arrangement; two cis carbonyl ligands; and a H₂O/H₂ binding site. In [Fe]-hydrogenase, heterolytic H₂ activation putatively by the pendant pyridone/pyridonate-O base serving as a proton acceptor. Following H₂ cleavage, an intermediate Fe-H species is thought to stereoselectively transfer a hydride to the substrate methenyl-H₄MPT⁺, thus forming methylene-H₄MPT. In the past decade, chemists, inspired by the elegant organometallic chemistry inherent to the FeGP cofactor, have synthesized a number of faithful structural models. However, functional systems are still relatively limited and often rely on abiological ligands or metal centers that obfuscate a direct correlation to nature's design.Our group has developed a bioinspired suite of synthetic analogues of Hmd to better understand the effects of structure on the stability and functionality of the Hmd active site, with a special emphasis on using a scaffold-based ligand design. This systematic approach has contributed to a deeper understanding of the unique ligand array of [Fe]-hydrogenase in nature and has ultimately resulted in the first functional synthetic models without the aid of abiological ligands. This Account reviews the reactivity of the functional anthracene-scaffolded synthetic models developed by our group in the context of current mechanistic understanding drawn from both protein crystallography and computational studies. Furthermore, we introduce a novel thermodynamic framework to place the reactivity of our model systems in context and provide an outlook on the future study of [Fe]-hydrogenase synthetic models through both a structural and functional lens.

Methanogenesis involves direct hydride transfer from H2 to an organic substrate

Certain anaerobic microorganisms evolved a mechanism to use H₂ as a reductant in their energy metabolisms. For these purposes, the microorganisms developed H₂-activating enzymes, which are aspirational catalysts in a sustainable hydrogen economy. In the case of the hydrogenotrophic pathway performed by methanogenic archaea, 8e^- are extracted from 4H₂ and used as reducing equivalents to convert CO₂ into CH₄. Under standard cultivation conditions, these archaea express [NiFe]-hydrogenases, which are Ni-dependent and Fe-dependent enzymes and heterolytically cleave H₂ into 2H⁺ and 2e^-, the latter being supplied into the central metabolism. Under Ni-limiting conditions, F₄₂₀-reducing [NiFe]-hydrogenases are downregulated and their functions are predominantly taken over by an upregulated [Fe]-hydrogenase. Unique in biology, this Fe-dependent hydrogenase cleaves H₂ and directly transfers H^- to an imidazolium-containing substrate. [Fe]-hydrogenase activates H₂ at an Fe cofactor ligated by two CO molecules, an acyl group, a pyridinol N atom and a cysteine thiolate as the central constituent. This Fe centre has inspired chemists to not only design synthetic mimics to catalytically cleave H₂ in solution but also for incorporation into apo-[Fe]-hydrogenase to give semi-synthetic proteins. This Perspective describes the enzymes involved in hydrogenotrophic methanogenesis, with a focus on those performing the reduction steps. Of these, we describe [Fe]-hydrogenases in detail and cover recent progress in their synthetic modelling.