Dissertation

|1.10 Intramolecular Photoredoxcatalysts|
N
N
N
N N
Ru
N N
N
visible
light
visible
light
N
N N
Ru
N N
N
visible
light
O
O
O
O
N
N
N
N
Ru
N
N
O
O
N
N
N
N
Ru
N
N
Cl
N
Pd
Cl
N
O
O
e - e - e - e -
N
N
Pd
Pd
Pd Pd
Pd
Pd
N
N
Pd
Pd Pd
PdPd Pd
Pd Pd Pd Pd Pd Pd
2H + 2H + H 2
H 2
a b c 2H + H 2
Figure 34: Photoinduced degeneration of the intramolecular homogeneous catalyst (a) to an
intramolecular micro-heterogeneous system (b) and finally an intermolecular operating heterogeneous
system (c) as observed by HAMMARSTRÖM et al. clouds the true nature of the catalyst. [94]
the compromising question for the true nature of the catalyst in all supramolecular assemblies. To
answer this important question it requires the careful consideration of alternative mechanisms and
improved techniques of investigation. Accordingly, different coordination motifs need to be tested
and applied to assure the stability of the scaffold during the high energy photocatalysis processes.
Promising candidates are phosphine or N-heterocyclic carbene ligands (NHC) which can be
adapted from low valent metalorganic catalysis (e.g. Heck reaction catalyst) or the chemistry
of complex hydrides.
Another problem with the molecular bridge is the difficult determination and adjustment of
the properties relevant for interaction between the participating metal centers (compare tpphz
complexes). Using the trial-and-error method causes an intensive cost-, work- and time effort to
accomplish even little progress toward an improved catalyst.
From the economic and environmental point of view the use of expensive materials especially
noble metals for chromophores (Ru, Ir, Os) and catalysts (Pt, Pd, Rh, Au, Ag) is a major drawback
as well as low yielding multistep syntheses of molecular bridges. Screening experiments with
multimolecular and supramolecular catalysts can help to find cheap active compounds (Fe and
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