inorganic chemistry

precursors for more highly oxidized iron–oxygen species which
are formed in the course of an O-O bond cleavage process
starting from compound 0. Among those powerful oxidizing cata-
lyst species are the Fe(IV)-oxo-porphyrin cation radicals referred
to as compound I ( 156 ), and the closely related compound II
species ( 183 ), which are also acting as very versatile and strong
oxidants (Fig. 21).
Although the knowledge about metal-mediated dioxygen binding
and activation has grown immensely within the past decades, there
are not many examples of functional biomimetic systems which are
able to transfer O-atoms directly from dioxygen to substrate
molecules( 184 ). With only a few exceptions including some copper
peroxo complexes ( 185 ), most of the reported model compounds and
active species are only observable and functional at low tempera-
ture, which makes them quite impractical for synthetic oxidative
transformations. Several years ago, we have therefore started to
search for more robust photochemical model compounds that can
be applied in biomimetic dioxygen activation and light-controlled
substrate oxidations under ambient conditions ( 155 ).

N

Fe = Fe

N N

N

O O– –O O

O O O O

O HO

B B B

Oxy Ferric hydroperoxo

compound

(Compound 0)

+•

B

Fe FeIII FeIV FeIV

Compound I Compound II

FIG. 21. Typical intermediates in hemoprotein enzyme active sites.
The iron protoporphyrin IX cofactor (heme) forms dioxygen adducts ter-
med oxy-species. In the course of oxygen activation and catalytic redox
transformations, the oxy form can be consecutively converted into
hydroperoxo- and oxo-type intermediates, which are usually referred
to as compound 0, compound I, and compound II. Reproduced with per-
mission from Ref. ( 183 ). Copyright Nature Publishing Group.

272 GÜNTHER KNÖR AND UWE MONKOWIUS