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M. Gruden et al.
be viable on the scale needed for human survival in the absence of transition metals. The chameleon-like capability of transition metals to switch between different
oxidation and spin states enables these chemical transformations to be performed at
ambient temperatures, efficiently, and often selectively. The topic of spin states has
only recently come to the forefront [1] leading to a collaborative approach within
the formation of a European network of researchers to study spin states in a variety
of chemical systems (enzymes, spin-crossover complexes, biomimetic complexes).
In all cases, a joint effort of both wet laboratories (experiments [2, 3]) and dry laboratories (theory) was needed to delve deeper into the origins of rate enhancements,
spectroscopic properties, structural changes, etc. The scientific production resulting
from this European network is too large to be highlighted in a few sentence (a representative summary can be found in a special issue in Chemistry-a European Journal
[4]). Instead, within this chapter, we will give some representative examples of how
theory and experiment go hand in hand in order to reach a deep understanding of these
transition-metal complexes, how their spectroscopic features arise, and how can we
make adjustments to further improve them. For obvious reasons, these examples are
taken mainly from our own laboratories, although we also include some from the
literature.
2 Structure of the FeMoco Cofactor of Nitrogenase
in Comparison with the Oxygen-Evolving Complex
of Photosystem II
The story of the structure of the active species in the nitrogenase enzyme is one of
brave researchers, and intense discussions, which finally was solved more than a
decade later because of a joint study by DeBeer (spectroscopy) and Neese (theory).
But, first things first. The nitrogenase enzyme catalyzes the transformation of the
inert nitrogen molecule into ammonia:
N 2 + 6 · H
+ + 6 · e
−
→ 2 · NH 3
(1)
This transformation is not an easy task and involves a large number of steps in
the catalytic cycle (>12). Interestingly enough, in the active site of the enzyme, an
iron-sulfur complex was found, but unlike the typical iron-sulfur clusters ([FeS 4 ]
2− ,
[Fe 2 S 2 ]
2+/+ , [Fe 3 S 4 ]
+/0 , [Fe 4 S 4 ]
2+/+ ) [5–7], two more exotic ones were present:
the FeMoco cofactor [Fe 7 Mo 1 S 8 (SCH 3 )]
q and the P-cluster [Fe 8 S 7 (SCH 3 ) 6 ]
q . The
FeMoco cofactor is proposed to be involved in the transformation (1), while the
P-cluster is probably involved in the electron-transfer process.
The elucidation of the crystal structure in 1992 [8] (2.7 Å resolution) showed an
empty space within the FeMoco cofactor (see Fig. 1, left). Within the 1.16 Å resolution of 2002 [9], an interstitial atom was observed (Fig. 1, middle), and proposed
to be nitrogen. This was subsequently discarded by Yang and coworkers [12] based
M. Gruden et al.
be viable on the scale needed for human survival in the absence of transition metals. The chameleon-like capability of transition metals to switch between different
oxidation and spin states enables these chemical transformations to be performed at
ambient temperatures, efficiently, and often selectively. The topic of spin states has
only recently come to the forefront [1] leading to a collaborative approach within
the formation of a European network of researchers to study spin states in a variety
of chemical systems (enzymes, spin-crossover complexes, biomimetic complexes).
In all cases, a joint effort of both wet laboratories (experiments [2, 3]) and dry laboratories (theory) was needed to delve deeper into the origins of rate enhancements,
spectroscopic properties, structural changes, etc. The scientific production resulting
from this European network is too large to be highlighted in a few sentence (a representative summary can be found in a special issue in Chemistry-a European Journal
[4]). Instead, within this chapter, we will give some representative examples of how
theory and experiment go hand in hand in order to reach a deep understanding of these
transition-metal complexes, how their spectroscopic features arise, and how can we
make adjustments to further improve them. For obvious reasons, these examples are
taken mainly from our own laboratories, although we also include some from the
literature.
2 Structure of the FeMoco Cofactor of Nitrogenase
in Comparison with the Oxygen-Evolving Complex
of Photosystem II
The story of the structure of the active species in the nitrogenase enzyme is one of
brave researchers, and intense discussions, which finally was solved more than a
decade later because of a joint study by DeBeer (spectroscopy) and Neese (theory).
But, first things first. The nitrogenase enzyme catalyzes the transformation of the
inert nitrogen molecule into ammonia:
N 2 + 6 · H
+ + 6 · e
−
→ 2 · NH 3
(1)
This transformation is not an easy task and involves a large number of steps in
the catalytic cycle (>12). Interestingly enough, in the active site of the enzyme, an
iron-sulfur complex was found, but unlike the typical iron-sulfur clusters ([FeS 4 ]
2− ,
[Fe 2 S 2 ]
2+/+ , [Fe 3 S 4 ]
+/0 , [Fe 4 S 4 ]
2+/+ ) [5–7], two more exotic ones were present:
the FeMoco cofactor [Fe 7 Mo 1 S 8 (SCH 3 )]
q and the P-cluster [Fe 8 S 7 (SCH 3 ) 6 ]
q . The
FeMoco cofactor is proposed to be involved in the transformation (1), while the
P-cluster is probably involved in the electron-transfer process.
The elucidation of the crystal structure in 1992 [8] (2.7 Å resolution) showed an
empty space within the FeMoco cofactor (see Fig. 1, left). Within the 1.16 Å resolution of 2002 [9], an interstitial atom was observed (Fig. 1, middle), and proposed
to be nitrogen. This was subsequently discarded by Yang and coworkers [12] based