½FeL 5 ðÞH 2 O^2 þþNO^ !½FeL 5 ðÞNO^2 þþH 2 O ð 11 ÞThe overall binding constantKNOrepresenting the ratio of the
rate constants for the “on” and “off” reactions, that is,
KNO¼kon/koff, was found to depend on the nature of L. It varied
between 1 102 (for L¼H 2 O) and 1 106 M^1 (for L 5 ¼edta^4 )
(30,41). The tendency of the FeIIcompounds to reversibly bind
NO was reported to correlate with the oxygen sensitivity of the
FeIIcomplexes, suggesting that [FeL 5 (NO)] is stabilized in the
form of [FeIIIL 5 (NO)] similar to that found for the binding of
dioxygen, viz., [FeIIIL 5 (O 2 )].
Iron complexes are involved in various steps of NO metabo-
lism; most of these complexes also contain various sulfur ligands.
Iron nitrosyls and nitrosothiols are the most relevant agents
responsible for storage and transport of NO and related com-
pounds; nitrosothiols and metal nitrosyl complexes belong to
the most important external sources of NO (NO-donors)( 42 ).
The most complex interactions are observed in ternary
iron–sulfur–nitrosyl systems. Depending on the bond nature,
the ternary iron–sulfur–nitrosyl species may be classified into
two groups: (i) iron complexes with theS-nitrosothiol ligand con-
taining the [Fe N(SR)O] moiety and (ii) RSFe NO com-
pounds, where both NO and sulfur are coordinated to the Fe-
center. Irrespective of the structural details, the mutual
interactions of all components are very strong due to consider-
able bond delocalization within both ternary systems.
The iron–sulfur clusters constitute an integral part of several
natural structures occurring in a large family of biologically rele-
vant metalloproteins. These [Fe S] units form active sites of
enzymes, which play a crucial role in living organism processes
such as: electron transfer chain, photosynthesis (photosystem I),
isomerization, respiratory chain, nitrogen fixation, and many
various catalytic reactions; they can also operate as biosensors
for oxidants and iron. The [FeS] clusters are even supposed to
be the interface between the biological and inorganic worlds,
because they catalyze redox transformations of such likely compo-
nents of the Earth's primordial atmosphere, as N 2 , CO, and H 2.
The iron–sulfur units form mono-, di-, tri-, tetra-, and hexanuclear
species in which iron atoms are connected through S bridges, with
thiolates and/or NO as terminal ligands.
The family encompasses species like [Fe 2 (m 2 -S) 2 (NO) 4 ]^2
(known as Roussin's red salt), [Fe 2 (m 2 -SR) 2 (NO) 4 ] (Roussin's red
salt ester), [Fe 4 (m 3 -S) 3 (NO) 7 ] (Roussin's black salt), [Fe 4 (m 3 -
S) 4 (NO) 4 ], and [Fe 6 (m 3 -S) 6 (NO) 6 ]. Two of these complexes,
[Fe 4 (m 3 -S) 3 (NO) 7 ]and [Fe 4 (m 3 -S) 4 (NO) 4 ], have been studied as
METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 305