ligands [59]. Most notable was the sharpening of the absorption bands, especially
upon the binding of methylene-tetrahydromethanopterin, indicating a more rigid
environment as a result of substrate binding. These changes in the shape and
frequencies of the absorption bands were greater in the presence of H 2 when
compared to Ar. These findings indicated that both the substrate and H 2 bind near to
the iron center.
The mixture of hard and soft ligand donors to iron is likely the reason for
alternate interpretations or conclusions from Mössbauer studies vs. X-ray absorption spectroscopy as to the redox states of iron. The former indicated that the active
site is a diamagnetic mononuclear iron center, either low-spin Fe
II or low-spin Fe
0 ,
with an isomer shift most consistent with Fe
0 [64]. In contrast the similarity of the
K-edge and XANES to a low spin, octahedrally coordinated Fe
II complex, which
contained two CO, suggested that the iron center in [Fe]-H 2 ase is low-spin Fe
II [76].
While mixed hard/soft ligand environments are challenging for synthetic chemists,
they are likely required for goals of eliciting noble metal-like catalysis from first
row transition metals.
4.4 From Active Site Structure to Activity
The determination of the [Fe]-H 2 ase active site structure again opened the door for
synthetic chemists to attempt to mimic nature’s design for application of heterolytic
cleavage of H 2 . It led to examples of rich heteroleptic environments in Fe
II complexes, typically based on pyridines substituted in the ortho position. The extensive
review from Rose is recommended for description of the evolution of model
complexes, demonstrating the stability of the iron(II) dicarbonyl moiety by strongly
coordinated anionic ligands, including an acyl, a thiolate, and an OH group [82].
A report from Hu and Shima showed that complex 5 in Fig. 8 can be used in hybrid
enzyme preparations [80]. That is, they found that reconstitution of the inactive
apoenzyme with this complex led to a return of enzymatic activity. While reclaimed
activity is not as impressive as was seen with the semi-synthetic [FeFe]-H 2 ase [47–
49] the lessons learned were that activity was reversible and dependent on the
presence of the pyridone/pyridinol, i.e., the methylether version was inactive, thus
confirming the pendant base role of the extraordinary cofactor [80].
5 The “Hydrogenase” that Comes from Nitrogenase
Interestingly, while H 2 is an intended product from some enzymes, it is a byproduct
from other reactions, a good example of which is the nitrogen fixation reaction by
Nitrogenase. To some degree, the “Nitrogenase” constitutes another type of
Hydrogenase. In the absence of N 2 , nitrogenases readily produce H 2 ; in its presence, at least one H 2 molecule is produced in each of the nitrogenases. The structure
286
M. Y. Darensbourg et al.
upon the binding of methylene-tetrahydromethanopterin, indicating a more rigid
environment as a result of substrate binding. These changes in the shape and
frequencies of the absorption bands were greater in the presence of H 2 when
compared to Ar. These findings indicated that both the substrate and H 2 bind near to
the iron center.
The mixture of hard and soft ligand donors to iron is likely the reason for
alternate interpretations or conclusions from Mössbauer studies vs. X-ray absorption spectroscopy as to the redox states of iron. The former indicated that the active
site is a diamagnetic mononuclear iron center, either low-spin Fe
II or low-spin Fe
0 ,
with an isomer shift most consistent with Fe
0 [64]. In contrast the similarity of the
K-edge and XANES to a low spin, octahedrally coordinated Fe
II complex, which
contained two CO, suggested that the iron center in [Fe]-H 2 ase is low-spin Fe
II [76].
While mixed hard/soft ligand environments are challenging for synthetic chemists,
they are likely required for goals of eliciting noble metal-like catalysis from first
row transition metals.
4.4 From Active Site Structure to Activity
The determination of the [Fe]-H 2 ase active site structure again opened the door for
synthetic chemists to attempt to mimic nature’s design for application of heterolytic
cleavage of H 2 . It led to examples of rich heteroleptic environments in Fe
II complexes, typically based on pyridines substituted in the ortho position. The extensive
review from Rose is recommended for description of the evolution of model
complexes, demonstrating the stability of the iron(II) dicarbonyl moiety by strongly
coordinated anionic ligands, including an acyl, a thiolate, and an OH group [82].
A report from Hu and Shima showed that complex 5 in Fig. 8 can be used in hybrid
enzyme preparations [80]. That is, they found that reconstitution of the inactive
apoenzyme with this complex led to a return of enzymatic activity. While reclaimed
activity is not as impressive as was seen with the semi-synthetic [FeFe]-H 2 ase [47–
49] the lessons learned were that activity was reversible and dependent on the
presence of the pyridone/pyridinol, i.e., the methylether version was inactive, thus
confirming the pendant base role of the extraordinary cofactor [80].
5 The “Hydrogenase” that Comes from Nitrogenase
Interestingly, while H 2 is an intended product from some enzymes, it is a byproduct
from other reactions, a good example of which is the nitrogen fixation reaction by
Nitrogenase. To some degree, the “Nitrogenase” constitutes another type of
Hydrogenase. In the absence of N 2 , nitrogenases readily produce H 2 ; in its presence, at least one H 2 molecule is produced in each of the nitrogenases. The structure
286
M. Y. Darensbourg et al.