Efficient photochemical systems based on small molecular syn-
thetic compounds with a ligand-based hydride transfer function-
ality in close proximity to a metal coordination site are still quite
rare. InFig. 17, some possible ligand architectures described in
the recent literature are shown.
A.3. Carbon dioxide docking sites
Small carbon-containing molecules such as atmospheric CO 2
are considered to be important renewable feedstocks(144,145).
In the context of mankind's increasing demand for carbon-based
materials, food, and liquid fuels, the photocatalytic reduction of
carbon dioxide under solar light irradiation is an attractive
option. Such types of artificial photosynthetic processes could
greatly enlarge the possibilities of abiotic CO 2 recycling.
To activate carbon dioxide for chemical reactions, it is advanta-
geous to fix and destabilize this rather inert molecule. An impor-
tant strategy to control and influence the reactivity of CO 2 is its
coordination to amines, metal-bound imido moieties, or metal
centers ( 144 ), which leads to a decrease of the CO bond order,
while the molecule in most cases becomes considerably bent. In
biochemical pathways, the CO 2 adduct of the coenzyme biotin
( 17 ) is involved in various carboxylation and transcarboxylation
reactions ( 146 ). Similar structural motifs could become impor-
tant functional building blocks for bioinspired photoreactions
involving carbon dioxide activation steps (Fig. 18).
Bocarsly recently described the detailed mechanism of a very
interesting photoelectrochemical process with a pyridinium-
14
NR 2
O
H
H
H
H
H
H
NH
N
R
N
H
M M
M
O
N
N
N
N
N
15 16
FIG. 17. Some examples of organic ligands acting as nicotinamide
cofactor mimetics ( 141 – 143 ) with redox-active metalsMbound toN-
heterocyclic carbene ( 14 ), 1,2-diimine ( 15 ), and amide groups ( 16 ).
264 GÜNTHER KNÖR AND UWE MONKOWIUS