Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Abstract

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

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

“…1,8,11,12,35 Past proposals for the catalytic mechanism can be divided into one set involving a bridging hydride between Fe p and Fe d 36 (Mechanism I as described by Trohalaki and Pachter 11 ) and another set involving a terminal hydride on Fe d (Mechanism II). 35 We see no evidence that H hyd involves a bridging hydride.…”

“…4-6 Fe d , the site at which hydrogen binds, is the Lewis acid. 7,8 Many previous experimental and theoretical studies 9-12 implicate a Fe d -H h ⋯H A -N ADT species as the key intermediate leading to H-H bond formation (Figure 1A). Despite its mechanistic importance, however, experimental characterization of such HLH interactions in the enzyme has not been available so far.…”

Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States

“…In contrast to other diiron complexes with terminal hydride ligands, 3 [ 1 ( t -H)] 0 shows no tendency to isomerize. Heating to the point of its decomposition (hours at 50 °C in THF solution) gave none of the isomeric bridging hydride [ 1 ( μ -H)] 0 , which we have prepared as described below.…”

“…Its 1 H NMR spectrum features a triplet at δ −4.95 ( t , J P–H = 76 Hz), a chemical shift that is diagnostic of terminal iron hydrides. 3 …”

“…Iron hydrides play important roles in biology, in industry, and even in geosciences. 1 The Fe–H bond been invoked not only in all three hydrogenases 2,3 but also in nitrogenase. 4,5 Molecular iron hydrides are the basis of emerging “earth-abundant” catalytic technologies.…”