inorganic chemistry

property can also be exploited in photochemical model systems
involving such types of ligands either directly or in the first or
second coordination sphere of redox active metal centers. Some
examples of natural and synthetic ligands that can undergo
reversible or irreversible two-electron transformations are
presented above (Fig. 16).

A.2. Hydride transfer shuttles

Several enzymes such as reductases and dehydrogenases uti-
lize nicotinamide derivatives as reversible carriers of redox
equivalents. The reduced dihydronicotinamide moiety NAD(P)H
acts by donating a hydride equivalent to other molecules.
In the corresponding two-electron oxidized NAD(P)þ form, the
cofactor formally accepts a hydride ion from the substrate.
Functional models of such reversible hydride transfer processes
are of considerable interest for biomimetic chemistry, and the
strategies to regenerate nicotinamide-type cofactors are crucial
for the performance of many organic transformations involving
biocatalytic key steps(139,140).

NH

N

N

O O

O

O

OOC O

12 13

N

R

N

R

N

R

11

FIG. 16. Structures of different types of ligands acting as two-elec-
tron redox relays in natural and artificial systems: Flavins such as
( 11 ) are the essential constituents of flavodoxines and flavoproteins
( 137 ). Thea-ketoglutarate anion (a-KG, 12 ) is a typical example of a
sacrificial redox mediator which decomposes during catalysis ( 72 ). Syn-
thetic chelates such as bis-arylimino-acenaphthene (BIAN, 13 ) have
been proposed for the development of bio-inspired multielectron trans-
fer photosensitizers ( 138 ).

PHOTOSENSITIZATION AND PHOTOCATALYSIS 263