catalyzed multielectron reduction of carbon dioxide in aqueous
solution ( 147 ). An inner-sphere electron transfer mechanism pro-
ceeding through coordinative interactions between pyridinium
radicals and CO 2 -derived species bound to the nitrogen atom of
the aromatic heterocycle ( 18 ) is suggested. A systematic ligand
design aiming at an inclusion of this kind of catalytically compe-
tent building block might become very advantageous for biomi-
metic photocatalysis and artificial photosynthetic carbon
dioxide conversion.
A.4. Oxyl radical functionality
A widespread structural motif in many biocatalytic oxidations
is the presence of electrophilic oxygen species and reactive oxyl
radical species in close proximity to pre-organized substrate
molecules. The most prominent example is the attachment of a
Lewis acid-activated water molecule in the second coordination
sphere of a high-valent manganese oxygen center, which is sup-
posed to be involved in catalyzing the crucial first O-O bond for-
mation step in natural oxygenic photosynthesis (5,148,149).
Some monooxygenase enzymes including the versatile hemopro-
tein catalysts of the cytochrome P450 family (3,150) and the iron-
or copper-based enzymes involved in the partial oxidation of
methane to methanol are probably operating in a quite similar
way (151,152). Also many other kinds of oxidoreductases rely
on the intermediate generation of oxyl radicals to accelerate sub-
strate conversion and to overcome rate limiting steps such as
hydrogen abstraction in a controlled and efficient manner.
In earlier work( 3 ), we have already pointed out the striking
similarities of such electrophilic oxygen sites and biocatalytic
oxyl radical reagents with the electronic structures and
17 18HN NSCOOORN COO(H )
- (H )
FIG. 18. Ligand architectures for coordinative carbon dioxide activa-
tion: structure of the biotin-CO 2 coenzyme ( 17 ) involved in many
biological carboxylations ( 146 ). Proposed pyridinium-CO 2 adduct ( 18 )
accelerating electrocatalytic carbon dioxide reduction processes ( 147 ).
PHOTOSENSITIZATION AND PHOTOCATALYSIS 265