Since the 1930's, biologists have known that some micro-organisms produce molecular hydrogen (dihydrogen to organometallic chemists) in the course of their normal metabolism. The underlying biochemistry of the associated enzymes, called hydrogenases (H2
ases), has been actively studied since that time. Beginning with the crystallographic characterization of the [NiFe]-H2
ase from D. gigas
in 1995, the level of activity increased, in part because of the startling structure of the active site, which features an iron carbonyl cyanide. Almost five years later, the phylogenetically unrelated [FeFe]-H2
ase was also characterized crystallographically, again revealing yet another kind of iron carbonyl cyanide center.1
In an effort to understand the molecular mechanisms by which H2
ases operate, much research has been aimed at mimicking the structures of their active sites. In recent years, these models have begun to yield biochemically significant insights, although gaps remain. Perhaps most perplexing are the high rates achieved by these enzymes, especially in view of the fact that they utilize first row metals that typically display diminished affinities for dihydrogen.2
Furthermore and still more challenging, the H2
ases effect their reactions via
changes, which require odd-electron intermediates. The one-electron chemistry of metal hydrides and metal–dihydrogen complexes is lightly studied; thus, the biochemical mechanisms present opportunities for learning new organometallic chemistry relevant to dihydrogen.
The literature on the production of hydrogen in solution, homogeneous hydrogenogenesis, is not extensive,3,4
but the coordination chemistry of dihydrogen has been active for decades2
and is obviously relevant to biological processes. Although H2
itself exhibits neither redox nor any acid–base reactivity, its metal complexes exhibit both, i.e
. dihydrogen complexes can be highly acidic and, the derived metal hydrides can be oxidized. It therefore makes sense that the active sites of both the [NiFe]- and the [FeFe]-H2
ases feature metals. This generalization extends to the recently discovered hydrogen-transfer enzyme Hmd.5
Interestingly, the active sites of all three classes of “hydrogen-processing” enzymes feature thiolato iron carbonyl entities.
This review brings a particular focus on the reactivity of models for the H2ases. Reactivities of interest include protonation, binding of H2 and CO (a common inhibitor of H2ases), and redox.