As the 50th recipient of the Stephenson prize in 1998, Professor Rolf Thauer of
the Max Planck Institute for Terrestrial Microbiology in Marburg presented an
updated understanding of the role of the different hydrogenases in the sequence of
reactions explored by Stephenson and Stickland [12]. Kindly shared by Professor
Thauer, the chemical parts of the graphic in Fig. 2 show that the [FeFe]-H 2 ase is
employed for the production of H 2 as the organic refuse is digested, while the
[NiFe]-H 2 ase is more suited to take up H 2 , using it as an electron/energy bearing
substrate in the subsequent reduction of CO 2 and the methane-producing reaction.
Note that a poor cousin of the [NiFe]-H 2 ase containing a single iron is also listed in
the latter step. It will be described in a separate dedicated section.
The organisms responsible for this remarkable reaction are archaea; other
anerobic processes requiring hydrogenases, such as sulfate, ferric and nitrate
reducers, are found in myriad bacteria or protozoa [13, 14]. With O 2 as electron
acceptor, Knallgas bacteria are, obviously, aerobes and much study has addressed
the capability of these organisms to orchestrate hydrogen chemistry under aerobic
conditions [15].
Some historical events critical to establishing the basis of organometallic
chemistry in these enzymes follow. Electron paramagnetic spectroscopic studies in
the 1980s detected various oxidation states of nickel that were used in attempts to
understand the electrochemical control of an intricate mix of ready (rapidly reactivated) and unready (slow to reactivate) states of the hydrogenase known to contain
both nickel and iron, [NiFe]-H 2 ase [16, 17]. A decade later, a team of spectroscopists discovered that infrared spectroscopy could also be a diagnostic probe of
redox levels, unexpectedly in the diatomic ligand region [18]. The v( CX) three
band IR pattern of the Ni-A unready state is almost identical to that of a classic
piano-stool complex, (η
5 -C 5 H 5 )Fe(CO)(CN) 2
−
, Fig. 4 [19]. Used initially as a
benchmark in vibrational spectroscopy, and manipulated by derivatization at the
cyclopentadienyl ring and H-bonding at cyanide [20], this small molecular anion
with its accessible spectroscopic handles encouraged synthetic chemists from the
organometallic chemistry realm to apply their expertise and imagination towards
biomimetic chemistry. Most recently it finds use as the iron unit in bimetallic
constructions of import to synthetic analogues of the [NiFe]-H 2 ase active site that
are potential electrocatalysts for proton reduction or the Hydrogen Evolution
Reaction (HER); or, alternatively, the H 2 reduction reaction, vide infra [21–23].
Certainly the decade spanning the twentieth and twenty-first century was the
most influential for engaging synthetic chemists in attempts to model the active
sites of enzymes that likely have metal hydrides as intermediates in their catalytic
cycles. Figure 5 gives examples of various bacterial or archaea sources from which
hydrogenases have been isolated, and with structures focusing on their active sites
[24–31]. Many excellent reviews have focused on these structures [32–35]. We will
explore them only as they relate to the mechanism of HER or H 2 oxidation.
Organometallic Chemistry Control of Hydrogenases
279
the Max Planck Institute for Terrestrial Microbiology in Marburg presented an
updated understanding of the role of the different hydrogenases in the sequence of
reactions explored by Stephenson and Stickland [12]. Kindly shared by Professor
Thauer, the chemical parts of the graphic in Fig. 2 show that the [FeFe]-H 2 ase is
employed for the production of H 2 as the organic refuse is digested, while the
[NiFe]-H 2 ase is more suited to take up H 2 , using it as an electron/energy bearing
substrate in the subsequent reduction of CO 2 and the methane-producing reaction.
Note that a poor cousin of the [NiFe]-H 2 ase containing a single iron is also listed in
the latter step. It will be described in a separate dedicated section.
The organisms responsible for this remarkable reaction are archaea; other
anerobic processes requiring hydrogenases, such as sulfate, ferric and nitrate
reducers, are found in myriad bacteria or protozoa [13, 14]. With O 2 as electron
acceptor, Knallgas bacteria are, obviously, aerobes and much study has addressed
the capability of these organisms to orchestrate hydrogen chemistry under aerobic
conditions [15].
Some historical events critical to establishing the basis of organometallic
chemistry in these enzymes follow. Electron paramagnetic spectroscopic studies in
the 1980s detected various oxidation states of nickel that were used in attempts to
understand the electrochemical control of an intricate mix of ready (rapidly reactivated) and unready (slow to reactivate) states of the hydrogenase known to contain
both nickel and iron, [NiFe]-H 2 ase [16, 17]. A decade later, a team of spectroscopists discovered that infrared spectroscopy could also be a diagnostic probe of
redox levels, unexpectedly in the diatomic ligand region [18]. The v( CX) three
band IR pattern of the Ni-A unready state is almost identical to that of a classic
piano-stool complex, (η
5 -C 5 H 5 )Fe(CO)(CN) 2
−
, Fig. 4 [19]. Used initially as a
benchmark in vibrational spectroscopy, and manipulated by derivatization at the
cyclopentadienyl ring and H-bonding at cyanide [20], this small molecular anion
with its accessible spectroscopic handles encouraged synthetic chemists from the
organometallic chemistry realm to apply their expertise and imagination towards
biomimetic chemistry. Most recently it finds use as the iron unit in bimetallic
constructions of import to synthetic analogues of the [NiFe]-H 2 ase active site that
are potential electrocatalysts for proton reduction or the Hydrogen Evolution
Reaction (HER); or, alternatively, the H 2 reduction reaction, vide infra [21–23].
Certainly the decade spanning the twentieth and twenty-first century was the
most influential for engaging synthetic chemists in attempts to model the active
sites of enzymes that likely have metal hydrides as intermediates in their catalytic
cycles. Figure 5 gives examples of various bacterial or archaea sources from which
hydrogenases have been isolated, and with structures focusing on their active sites
[24–31]. Many excellent reviews have focused on these structures [32–35]. We will
explore them only as they relate to the mechanism of HER or H 2 oxidation.
Organometallic Chemistry Control of Hydrogenases
279