Dissertation

|1.9 Multicomponent Systems from Fundamental Building Blocks|
potentially provide a pathway for hydrogen release from an intermediate hydride. [34] In the case
of pure redox shuttles it would also be helpful if docking groups such as -CN, COOH, -pyridy are
present which can be ligated to the involved species (chromophore, catalyst) allow for a directed
electron transfer.
Commonly uses mediators are bipyridinium ions (methylviologens like diquat and paraquat),
phenanthrolinium ions, metal ions such as Eu 3+ , V 3+ or Cr 3+ , metal complexes of Rh, Co, and
proteins like cytochrome c 3 . [29] The compounds depicted in figure 21, represent commonly used
redox shuttles with suitable redox potentials and in some cases also the property to combine
reversibly with hydrogen.
N
N
N
N N
N
N N
N +
N +
N +
Co 3+ diquat paraquat
N +
N
N
N
Rh 3+
N
N
N
[Co(sepulchrate)] 3+
[Rh(bpy) 3 ] 3+
[29, 34, 37]
Figure 21: Representative electron mediators.
Additionally, it may be necessary to use sacrificial electron donors (tertiary amines like
EDTA, triethylamine (TEA) or triethanolamine (TEOA), reduced metal complexes like Eu 2+ -
salts or other reducing agents like ascorbate, H 2 S or NADPH) as well as sacrificial electron
acceptors (peroxo species like K 2 S 2 O 8 or oxidizing complexes such as [Ce IV (NH 3 ) 2 (NO 4 ) 6 ]
or [Co III (NH 3 ) 5 Cl]Cl 2 ) to represent the second half reaction which is not relevant respective.
Sacrificial substrates are also used as fast quenchers to increase the efficiency of electron transfer
and importantly to make the back-reaction (charge recombination) less likely. In the case of
TEA/TEA − H + it is speculated that the (tertiary amine) buffer may serve both as electron donor
and proton source [32]
A quantitative theory of ET was developed by Rudolph A. Marcus, who for this achievement
was eventually honored with the Nobel Prize in 1992. [43, 44] This powerful theory describes a
number of important processes in chemistry and biology, including ground state electron transfer
reactions as well as photoinduced electron transfer processes (PET), chemiluminescence and
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