Photochemistry and Photophysics of Coordination Compounds

182 S. Campagna et al.
tained in the dark or when one of the key components (the Ru(II) chromophore,
the Pd catalyst, or the particular bridging ligand) is missing. Interestingly,
the same compound where the bridging ligand between the two metal sites is
a bipyrimidine unit does not evolve molecular hydrogen. It is also reported
that the amount of photocatalytically formed hydrogen depends strongly on
the TEA concentration and the exposure time, and chloride ions inhibit the
reaction. The amount of hydrogen produced increases steadily and levels off
after 1200 min. After about 1800 min no more hydrogen is produced. The
rate of hydrogen formation increases with increasing TEA concentration for
low TEA concentration, but becomes independent at a TEA concentration
> 0.86 mol L –1 , where it is about 1600 nmol min –1 . Although detailed mechanistic
data are not available, the authors suggest as a first step of the process
a twofold photoinduced reduction of the compound by TEA, analogously to
what was reported for the related photoinduced electron collection system 51.
Probably reduction is concomitant with proton extraction from TEA oxidation
products: TEA should therefore be the proton source. The successive step
should be reduction of the protons at the nearby Pd center. This latter step probably
passes through a temporary chloride loss. The same paper also reports the
photocatalyzed selective hydrogenation of tolane to cis-stilbene, accomplished
by the same compound. Analogously to 60, the Ru(II) chromophore of 61 acts
as the light harvester/photosensitizer (which also contains in its structure the
electron acceptor subunit, that is, the phenazine moiety of the bridging ligand),
while the role of the catalytic unit is here played by the Pd(II) center.
Molecular hydrogen evolution under visible light irradiation has also been
reported for trimetallic species like 53 [348]. Mechanistic details are not available.
5.8.2
Other Photocatalytic Systems
The catalytic potential of heterometallic species containing Ru(II) and Re(I)
chromophores for the conversion of CO2 to CO has been recently shown [349].
Compound 62 is one of the species in this regard. This investigation highlighted
the fact that the photocatalytic activity is deeply influenced by the nature of
both the bridging ligand and the peripheral ligands at the light-harvesting
Ru(II) chromophore. The proposed mechanism is that upon light irradiation
(λ > 480 nm) in DMF/triethanolamine (TEOA, acting as base) with 1-benzyl-
1,4-dihydronicotinamide (BNAH) as sacrificial donor, the initially produced
Ru-based MLCT state is reduced by BNAH. Then intrabridging ligand electron
transfer occurs, with formation of the reduced rhenium subunit. This latter
speciesisknowntoreactwithCO2 upon Cl ligand loss [350, 351]. The reduction
of CO2 is bielectronic, so it is assumed that the second electron transfer follows
a similar route. The most efficient species of this series of compounds, which is
exactly 62, exhibits a turnover number of 170.