Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Ruthenium 179
manganese subunits [328–330], with phenolate and tyrosine moieties playing
the role of an intermediate donor. In most cases, proton-coupled electron
transfer processes took place.
To achieve multielectron catalysts, more than one manganese ion was included
in the systems. Figure 16 shows some examples [329, 331, 332] of
Ru(II) species covalently linked to Mn dimers or trimers via phenolate lig-
ands. In particular, for the Ru–Mn II,II
2
complex 58 reported in the figure,
repeated flashes in the presence of a Co(III) sacrificial electron acceptor allowed
three successive one-electron oxidations of the manganese moiety by
the photooxidized Ru(III) subunit [333], as evidenced by the disappearance
of the characteristic Mn2 II,II signals and the appearance of the characteristic
Mn2 III,IV signals in EPR experiments. Manganese oxidation was suggested to
involve a ligand exchange, in which acetate is released and water molecules
are bound to form a di-µ-oxo bridge (see the reaction scheme in Fig. 16). According
to the authors, this was the first example of a light-driven, multiple
oxidation of a manganese complex attached to a photosensitizer. The ligand
exchange at the manganese sites (presumably occurring in the Mn2 III,III
state, that is, after second electron release from the initial Mn2 II,II center)
is functional to the overall process, as it allows introduction of negative
Fig. 16 Electron transfer from the manganese moiety to the photooxidized Ru(III) in a
aRu– Mn2 II,II complex and b aRu– Mn3 II,II,II complex